Petrology of Ultramafic and Related Rocks from Mt

PETROLOGY OF ULTRAMAFIC AND RELATED ROCKS FROM MT. LIGHTNING, NEAR GUNDAGAI, NEW SOUTH WALES, AUSTRALIA.

By A. S. Ray, M.Sc.

A THESIS SUBMITTED TO THE UNIVERSITY OF NEW SOUTH WALES FOR THE DEGREE OF DOCTOR OF PHILOSOPHY, 1977. ABSTRACT

A rock assemblage in the Mt. Lightning area of the Coolac Serpentine Belt in south-eastern New South Wales has been mapped and studied by means of petrographic and chemical techniques.

The principal lithologies include massive to locally foliated, partially serpentinized, olivine-rich harzburgites, with deformed and recrystallized textures, containing consistently Mg-rich olivine {Fo89 _92} and enstatite {En87_90 } with subordinate diopside and chrome-spine!. These rocks are associated with blocky and sheared lizardite- chrysotile serpentinites. Massive and variolitic varieties of low-potash spilites containing tholeiitic pyroxenes as inferred from chemical compositions, occur within and flanking the serpentinites. Smaller rock masses enclosed by, or flanking the ultramafic rocks, include low-iron rodingites of two genetic groups, a series of low-potash feldspathic rocks ranging from quartz-rich trondhjemites to quartz-poor albitites, and both er-rich and Al-rich chromitites.

Group 1 rodingites containing grossularite, vesuvia nite, chlorite and relict diopside, and characterized by high Cr {> 200 ppm} and Ni {> 150 ppm} contents, and high Cr/V ( > 2) and Mg/Fe (> 2) ratios, are considered to be metasomatized mafic rocks. Group 2 rodingites are reaction zone products that formed at contacts of spilite and ultramafic rocks. They contain zoisite, prehnite, chlorite and grossularite but lack vesuvianite and relict pyroxene, have low er ( <. 40 ppm) and Ni ( <. 50 ppm) contents, and low Cr/V

( < 1) and Mg/Fe (<1.5') ratios.

The trondhjemitic rocks are devoid of k-feldspar and are typically K2o-poor ( <. 0. 8 wt%) ; they are considered to be alteration products of dioritic parents.

The ultramafic-mafic-sedimentary rock assemblage at Mt. Lightning accords with that of an ophiolite in which the harzburgite represents the basal member. The mineral chemistry and textures of the harzburgite also accord with a status as upper mantle residue. It is therefore possible that the Mt. Lightning rock assemblage represents a fragment of Lower Palaeozoic oceanic lithosphere with part of the crustal section missing.

A partial melting model of upper mantle is tentatively proposed for the generation of the principal and minor rock types at Mt. Lightning. The ultramafic rocks are considered to be refractory residues from partial melting processes. At lease two stages of partial melting might have taken place - a low degree (~ 5% by volume) of partial melting producing the gabbroic parent (s) of Group 1 rodingites and a relatively high amount (~ 20% by volume) of melting giving rise to the tholeiitic parent of the spilites. Differentiation of the latter tholeiitic magma is believed to have resulted in dioritic rocks from which the trondh jemites and albitites were derived. ACKNOWLEDGEMENTS

I have received much assistance from a large number of people. To one and all I express my thanks, However, I would like to pay special tribute to the following people who assisted me greatly:-

• Dr. H.G. Golding who acted as my Supervisor. I am most indebted to him for all his personal help, encouragement and constant guidance during the course of my present study. The numerous discussions that I had with him on . this topic have all provided me with an invaluable help. With all confidence I can say that without his supervision this work could never have been completed.

• Dr. B.J. Hensen, Dr. P.C. Rickwood and Dr. M.B. Katz with whom I had helpful discu ssions from time to time.

• Prof. T.G. Vallance of the University of Sydney with whom I had valuable discussions on spilitic rocks.

• Dr. B.J. Franklin who provided the facili ties to take microphotographs at the Institute of Technology. • Dr. B. Chappell and Mr. R. Freeman of the Australian National University, Dr. R. Flood and Mr. G. Pooley of the Macquarie Univer- sity for X-ray fluorescence analys~s, and from the University of New South Wales:

Mr. F. Scott for electron microprobe analyses, Ms G. Chorley and Mr. M. Walker for wet chemical analyses, Mr. G. Small for assistance with photography and Ms M. Clark for typing the tables.

My list of acknowledgements would not be complete without my payins tribute to the late Professor J.J. Frankel who provided me with the initial inspira tion to commence this thesis. CONTENTS

CHAPTER 1. INTRODUCTION

Page No.

1.1 PREAMBLE 1 1.2 LOCATION, PHYSIOGRAPHY AND MAPPING 1.21 Location and Physiography 4 1.22 Mapping 7 1.3 PREVIOUS REFERENCES 8 1.4 REGIONAL GEOLOGICAL SETTING 1.41 Introduction 11 1.42 The Coolac Serpentine Belt 11 1.43 Honeysuckle Beds 17 1.44 Young Granodiorite 19

CHAPTER 2. ULTRAMAFIC ROCKS

2.1 INTRODUCTION 23 2.2 GENERAL STATEMENT 25 2.3 PETROGRAPHY AND TEXTURE 29 2.31 Microtexture 29 2.32 Mineralogy and mineral chemistry 42 2.321 Olivine 42 2.322 Orthopyroxene 44 2.323 Clinopyroxene 48 2.324 Chrome-spinel 51 2.325 Serpentine-group 53 2.326 Other secondary minerals 59 2.4 WHOLE ROCK CHEMISTRY OF MT. LIGHTNING PERIDOTITES 60 Page No.

2.5 PETROGENESIS OF PRIMARY ULTRAMAFIC ROCKS 64 2.51 Temperature-pressure estimation based.on CaSiOrl and Al2o3 contents of clinopyroxe e . ' 64 2.52 Temperature estimation based on cation distribution in co-existing· orthopyroxene and clinopyroxene p6 2.53 Temperature estimation based on cation distribution in co-existing olivine and ·orthopyroxene 67 2.54 Summary 69 2.55 Classification of Mt. Lightning ultramafic rocks 71 2.56 Origin of Alpine-type ul tramafic . rocks 73 2.57 Origin of Mt. Lightning ultramafic rocks 80 2.6 SERPENTINIZATION OF COOLAC ULTRAMAFIC ROCKS 82

CHAPTER 3. SPILITES

3.1 INTRODUCTION 88 3.2 GENERAL STATEMENT 90 3.3 PETROGRAPHY 94 3.4 MINERALOGY 3 .41£. fAlbi te 95 3. 42 r•" 0 hroup Minerals 97 3.43,. Chlorite 100 3.44 Pyroxene 102 3.45 Other minerals 103 3.5 TEXTURE 106 3.6 WHOLE ROCK CHEMISTRY 110 3.7 MT. LIGHTNING SPILITES AS SECONDARY ROCKS 124 Page No.

3.8 DETERMINATION OF SPILITIC PARENTAGE 3.81 Using major element chemistry 127 3.82 Using trace element data 131 3.83 Using clinopyroxene composition 134 3.9 ORIGIN OF MT. LIGHTNING SPILITES 138

CHAPTER 4. RODINGITES

4.1 INTRODUCTION 141 4.2 NOMENCLATURE 142 4.3 CLASSIFICATION OF RODINGITES 143 4.4 MODE OF OCCURRENCE 4.41 Group 1 rodingites 145 4.42 Group 2 rodingites 146 4.5 PETROGRAPHY AND TEXTURE 4. 51 Group 1 rodingi tes 149 4.52 Group 2 rodingites 152 4.6 MINERALOGY 4.61 Group 1 rodingites 4.611 Garnet 156 4.612 Vesuvianite 158 4.613 Clinopyroxene 164 4.614 Chlorite 168 4.615 Other minerals 169 4.62 Group 2 rodingites 4.621 Zoisite 173 4.622 Prehnite 179 4.623 Chlorite 182 4. 624 G\rnet 185 4. 625 SpAene 186 4.626 Tremolite-actinolite 186 Page No.

4.7 CHEMISTRY OF RODINGITES 4.71 Group 1 rodingites 187 4.711 Variation of composition within Group 1 rodingite bodies at Mt. Lightning 197 4.72 Group 2 rodingites 200 4.721 Variation of composition across Group 2 rodingite bodies 206 4.8 GENESIS OF RODINGITE 210 4.81 Summary of various hypothesis for the origin of rodingites 211 4.82 Origin of Group 1 rodingites from Mt. Lightning 216 4.821 Rodingitization and Serpentinization 224 4.83 Origin of Group 2 rodingites from Mt. Lightning 230 4.84 Temperature-pressure estimation 234

CHAPTER 5. TRONDHJEMITES AND ALBITITES

5.1 INTRODUCTION 236 5.2 GENERAL STATEMENT 238 5.3 PETROGRAPHY 242 5.4 MICROTEXTURE 5.41 Granitic texture 249 5.42 Gneissic or Banded texture 250 5.43 Mylonitic texture 250 5.44 Microbrecciated texture 252 5.45 Other textures 252 5.5 WHOLE ROCK CHEMISTRY 255 5.6 GENESIS 264 CHAPTER 6. CHROMITITES Page No.

6.1 INTRODUCTION 270 6.2 GENERAL FEATURES OF THE MT. LIGHTNING CHROMITE PODS 276 6.21 Distribution of Pods 276

6.22 Shape, dimension and orientation J of Pods 276 6.23 Internal structure, textures and contacts of Pods 277 6.24 Rodingite-chromitite relationships 279 6.3 MINERALOGY OF CHROMITITES 281 6.4 CHEMISTRY OF CHROMITES 282 6.41 Location and brief description of analysed samples 282 6.42 Analytical results 286 6.5 DISCUSSION 6.51 Conclusions based on the author's investigations 291 6.52 Aspects of the genesis of the Podiform chromitites 292

CHAPTER 7. DISCUSSION

7.1 OPHIOLITES AND THE MT. LIGHTNING ROCKS 297 7.2 MT. LIGHTNING ROCKS AS PART OF A LAYERED OCEANIC CRUST-UPPER MANTLE SEQUENCE 301 7.3 TECTONIC EVOLUTION OF THE AREA STUDIED 304 7.31 Marginal Basins and emplacement of ophiolites 309 7-4 CONCLUSIONS 312 BIBLIOGRAPHY 316 APPENDIX 1

C H A P T E R 1

I N T R O D U C T I O N

1.1 PREAMBLE:

Ultramafic rocks that occupy extens~ve fault bounded belts have received increasing attention in recent years largely as a consequence of plate tectonic concepts and the hypothesis that such rock masses are solid-emplaced, on-land fragments of pre-existing oceanic sub-crust, but broad generalizations concerning such ultramafic belts have tended to outstrip knowledge of their detailed constitution. Initial investigations along some belts have revealed a high degree of internal lithologic diversity that precludes a rapid appraisal of their detailed petrology. For such occurrences a number 2

of restricted or "type" areas may be selected for detailed study, each study contributing to the overall picture.

This thesis records the results of one such study.

This work deals with the nature and geneses of ultramafic and related rocks that occur in the immediate vicinity of Mt. Lightning, near Coolac in south-eastern

New South Wales.

Mt. Lightning is one of the most readily acce ssible areas within a prominent belt of ultramafic rocks in south-eastern New South Wales. This belt has been variously termed as 'Coolac Ultramafic Belt', 'Coolac

Serpentine Belt', 'Coolac-Goobarragandra Ultramafic Belt',

'Coolac Serpentinite Belt' and 'Coolac Serpentinite'.

Throughout the thesis these terms have been used synonymously although the name 'Coolac Serpentine Belt' has been used more often than the others.

The principal claim of the Mt. Lightning area for specific study is the abundance, and relatively good exposure, of diverse minor rock masses that occur within, and closely associated with, the predominating harzburgi tes and serpentinites. These minor rock masses may be grouped into:

i) Spilites

ii) Rodingites

iii) Trondhjemites and Albitites, and

iv) Chromitites. 3

The peridotites and serpentinites together with these enclosures and some flanking rocks comprise an ultramafic mafic-felsic association that is conveniently referred to below as a "mafic-ultramafic" association.

Interest in the minor bodies lies in the possi bility that they may provide clues to the origin of the mafic-ultramafic association. The fact that in studies of mafic-ultramafic associations elsewhere such minor rock bodies have received inadequate attention was a further incentive to their study. Field observations, and mineralogical and chemical work have provided the basis for the present sutdy.

The aim of the present study is to investigate the nature, relationships and origin of the ultramafic and associated rocks, and particularly to characterize and elucidate aspects of the genesis of rodingitic rocks, in the Mt. Lightning area. 4

1.2 LOCATION, PHYSIOGRAPHY AND MAPPING:

1.21 Location and Physiography:

Mt. Lightning is located in the northern half of

the Coolac Serpentine Belt and is about 395 km west-south-

. west of Sydney, New South Wales. The nearest settlements

are the villages of Coolac and Adjungbilly, 9 km to the

north-west and 10 km to the south-east respectively.

The nearest town is Gundagai, about 20 km to the south

west (see Fig. 1, p 5 ).

Mt. Lightning forms a prominent hill (540 m A.S.L.)

between two pronounced topographic discontinuities - the

Murrun~iagee Valley to the north and the Adjungbilly.

Valley to the south. Fig. 2 (p 6 ) shows Mt. Lightning

looking south across the Murrumbidgee Valley. Mt.

Lightning separates a low-lying plateau to the east,

formed on the Young Granodiorite, from a terrain of low

undulating hills (average elevation of about 400 m A.S.L.)

composed mainly of Palaeozoic volcanic and sedimentary

rocks.

To the north of the Murrumbidgee River is the

Mooney Mooney Range with a general level of about 500 m

A.S.L. and to the south of Adjungbilly Creek the country

forms a terrain of ridges and plateaus known as the

Honeysuckle Range with a general level of about 700 m A.S.L. 5

I I QUEENSLAND ,-... ,/- 1".:" ------I I NEW SOUTH WALES I •

,.. "' I '-' I I ' '.._-.,..,~ I ......

1 I

AOJUNGBILL Y '

Murrumbidgee

Fig .1. Locality map of the Mt.Lightning area. Fig.2. A view of Mt.Lightning looking southward across the Murrumbidgee River valley.

L 2 2 Mapping:

The Mt. Lightning area was mapped on a scale of 1:8000 using enlarged aerial photographs and military maps. The aerial photographs used are portions of the

Cootamundra Series, Run 11, published by the N.S.W.

Department of Lands in 1969.

The area studied is covered by portions of four adjoining military maps (1:50,000) compiled in 1955, which are designated as follows:

Coolac Sheet 8528 III Series R 753

Jugiong II II II II 11 11

Gundagai II 8527 IV II II II

Tumorrama II II I II II II

Grid references quoted in this thesis refer to the above military maps. All the rock specimens and thin sections referred to are located in the School of

Applied Geology, University of New South Wales. 8

1.3 PREVIOUS REFERENCES:

Carne (1893) was the first to record occurrences of ultramafic rocks and chromite deposits in the Coolac

Serpentine Belt. Most of the early reports by the

Geological Survey of N.S.W. (Carne, 1896, 1897, 1908;

Jacquet, 1917; Harper, 1917, 1920; Raggatt, 1925) on the belt concern the chromite and copper deposits.

Brief references to ultramafic rocks of southern

N.S.W. were made by Card (1896), Benson (1926), David

{1950), Joplin {1962), Brown et al {1968), Vallance {1969c) and Crook and Felton (1975).

In recent years a large portion of the Coolac

Serpentine Belt and its surrounding areas has provided topics for theses by a number of students from the

University of Sydney, the University of N.S.W. and the

Australian National University. These studies include those by:

Fraser (1961, 1967), Veeraburus {1963), Ashley

(1967), Chenhall (1967), Irving {1967), Boots {1967),

Clift (1967), Cremer (1968) Thomson (1970), Wright (1971),

Thrum {1972), Winward (1972), Brown (1973), Clark {1974) and Gibbings (1974).

Outstanding contributions to the geology of the 9

belt and its surrounding region were made by Golding,

Ashley and Franklin. Golding (1961, 1962, 1963, 1966, with Bayliss, 1968a and b, 1969, with Johnson, 1971,

1975 and with Ray 1975a and b)is mainly responsible for initiating interest in the geology of the area. He was the first to record the occurrence of rodingite (Golding,

1962), to name and describe the North Mooney Complex

(Golding, 1969), and has worked extensively on the chromite deposits of the belt (Golding, 1963, 1966, 1975; Golding and Bayliss, 1968a; Golding and Johnson, 1971).

Over the last decade Ashley (1967, 1968, 1969a and b, 1970, 1973, 1974, 1975a and b, with Irving, 1976, with Chenhall, 1976) has discussed in considerable detail the petrology and sulphide deposits of the southern half of Coolac Serpentine Belt. In 1971, Ashley et al summarised the geology of the area east of Tumut and in

1973, Ashley and Basden introduced the term 'Young

Granodiorite' for granitic intrusions at Burrinjuck, Young and Cowra. Ashley (1973 and 1975b) is also one of the first to propose the tectonic setting of the Coolac

Serpentine Belt in terms of current plate tectonic theory.

Franklin (1972) was the first to establish the

layered nature of the North Mooney Complex. In 1975,

she completed a comprehensive study of the North Mooney

Complex. She recognized two distinct divisions within

the complex based on structural and textural evidences - 10

a predominantly ultramafic layered sequence containing magmatic accumulation features, mineral grading and

cyclic units, and a massive gabbroic portion overlying

and/or intruding the layered sequence. She concluded

that rocks of North Mooney Complex and its associated

area belong to typical ophiolitic assemblage.

At present P. Brown, H. Wallis and P. Hanna

from the University of N.S.W. are working in the northern

half of the Coolac Serpentine Belt. P. Brown is studying

the geochemistry of basic and acidic volcanic rocks in the

vicinity of the North Mooney Complex. H. Wallis and

P. Hanna are investigating areas south of Adjungbilly

Creek as part of their undergraduate thesis.

Geological maps of the area covering the Coolac

Serpentine Belt have been published by the Geological

Survey of N.S.W. These include the Wagga Wagga 1:250,000

Geological Sheet by Adamson and Loudon (1966),the

Cootamundra 1:250,000 Geological Sheet by Rose (1967)

and the Macquarie 1:500,000 Geological Sheet by Brunker

et al (1970). Recently, Basden et al (1976) have

published the Cootamundra Geological Sheet on a larger

scale (1:100,000), which includes the northern part of

the area mapped by the author. At present the Geological

Survey of N.S.W. is mapping the southern part of the Coolac

Serpentine Belt and surrounding areas on a scale of

1:100,000 (Tumut Geological Sheet). 11

1. 4 REGIONAL GEOLOGICAL SETTING:

1.41 Introduction:

A series of linear ultramafic masses are exposed in South-eastern and central New South Wales. These discrete sub-parallel masses crop out discontinuously from Kiandra in the south to as far north as Girilambone

(see Fig. 3, p 12) and Rayner (1961) suggested the name

'Gundagai Serpentine Belt' for the whole group. However, the concept of one distinct ultramafic belt is probably• incorrect since the series includes ultramafic rocks of different ages and origins (Scheibner, 1972c, 1974b, 1976).

The Coolac Serpentine Belt is the largest linear ultramafic body in south-eastern N.S.W., and is comparable in size to the larger masses of the Great Serpentine

Belt of north-eastern N.S.W. (Fig. 3, p 12 ). Diverse rock types occur associated with the Coolac Serpentine

Belt (Golding, 1969; Vallance, 1969c; Ashley et al

1971; Ashley, 1973 and Franklin, 1975). In the following sub-sections a summary of the regional geology of the por tion pertinent to the area studied by the author, is given ( see Fi9. 4, p 13 ).

1.42 The Coolac Serpentine Belt:

The Coolac Serpentine Belt forms a continuous exposure of variably serpentinized ultramafic and 12

I~--,.._.._. ,-__ ,__ .. l , ------_,,,, '" ,... J . ; · "' \ Eastern Serpentinite I 1I Great Serpentine., ~ ; Belt . I " TAMWORTH•'-'-'' GIRILABONE• MACQUARIE I• - ••'

' Coolac Serpentine ~ .... -.. / Belt ,..,. -, 'e • \ Gundaga,. ,\~ : , Scrpentinite ---.\ 1 CANBERR ... ,, Belt • ' ,-~ \ '~· ~--,~-'·-;('''. I Tumut Pond \ • Scrpcntinitc Belt ', ......

FIGURE 3

SKETCH MAP OF N.S.W. SHOWING PRINCIPAL ULTRAMAFIC BELTS 13

N + + + + + l + 3: + + 0 NORTH 0 z MOONEY _ _!__.L.--P;,,:~~ m COMPLEX -< + +

COO LAC •I'- - + + I

,- -, YOUNG \ + \ -· ' ~ \ ' \ ,- -- '---"• 1 ' I - J+ + + 'MURRUMBIDGEE 35°00 1 RIVER I - \ ADJUNGBILL I \ / CREEK

0 2 3 km '\ +

MID - LATE Granodiorite f::!J Dacite SILURIAN Siltstone and Dacitic Tuff

Basalt 1 S pil ite, Gobbro, Serpentinite, and Intermediate Volcanics. EARLY Dunite 1 Wehrlite, Clinopyroxenite, SILURIAN Gabbro and Minor Diorite. [s ~ Peridotite and Serpentinite

Greenschist, Serpentinite, Quartz - CAMBRIAN Mica Schist , Chert and Sandstone.

FIG.4 Regional Geological Setting of Mt. Lightning and North Mooney Complex (modifitc:l ca.fter Fni.nklin, 1975): 14

associated rocks extending from the Hume Highway near Coolac in the north to Patten's Ridge near Goobarragandra in the South (see Fig. 3 , p 12 ) • The total length is approximately 55 km. The width of the belt varies from a maximum of 3.5 km near Red Hill to a minimum of 10 m where it crosses the Goobarragandra River. At Mt.

Lightning it is approximately 2 km wide.

The Coolac Serpentine Belt trends north- north- westerly, the average strike being 340° azimuth. This is revealed by the predominant shearing planes of serpentinites, the foliation planes of metasedimentary rocks in the west and those in the eastern granodiorite, and by the contacts between ultrarnafics and adjacent rocks. The foliation planes dip steeply to the east at

70° to 90°.

Apart from minor gabbros, diorites and trondh jemitic rocks, the Coolac Serpentine Belt consists almost entirely of ultrarnafic rocks, the latter being predomi nantly serpentinized harzburgites and serpentinites. Minor dunites, lherzolites, wehrlites, chromitites and rodingites occur locally. Except in chromitites, the predominant primary mineral in all these ultrarnafic rocks is olivine. Enstatite, diopside and chrome-spine! form the subsidiary minerals. Serpentinization of the ultrarnafic rocks ranges from partial to complete. Lizardite and chrysotile are the most common serpentine 15

minerals while antigorite occurs locally in the vicinities of the North Mooney Complex and the Bogong Granite

(Franklin, 1975; Ashley 1973a).

A characteristic feature of the Coolac Serpentine

Belt is that both serpentinization and shearing increase from the eastern towards the western side of the belt.

Indeed a mappable distinction exists between an eastern sector of predominantly massive, blocky peridotite and serpentinite, and a western sector of predominantly sheared and completely serpentinized peridotite. Most chromite pods occur in the eastern sector of the belt

(Golding, 1966). Serpentinization and shearing also increase near the southern end where the width of the belt decreases.

The ultramafic rocks of the Coolac Serpentine

Belt are flanked on the east by the Young Granodiorite except in the extreme south near Goobarragandra and at

Red Hill. The term 'Young Granodiorite' was introduced by Ashley and Basden (1973). The granodiorite is believed to be of Upper Silurian age (see Richards et al, 1972,

Ashley and Basden, 1973). The granodiorite is a massive to foliated rock. the intensity of foliation increasing towards the contact with the ultramafic rocks. Mylonitic varieties are found close to the contact.

The contact between ultramafic rocks and the 16

granodiorites is sharp and is believed to be entirely a fault contact. This is indicated by the microbrecciated nature of the granodiorites near the contact, by the linearity of the contact and by the absence of thermal metamorphic effects.

At Red Hill the ultramafic- granodiorite contact is not exposed, the area being covered by Tertiary alkali basaltic capping (Golding, 1966; Ashley, 1973) •. Near the southern end of the Coolac Serpentinite Belt at Patten's Ridge, the ultramafic rocks are separated from the granodiorite by amphibolites.

The western contact between serpentinite and the Honeysuckle Beds is also believed to be a tectonic contact. However, near Mt. Lightning the contact is not well-defined. For instance, to the north of Murrumbidgee River serpenti nites various sedimentary and spilitic rocks of the Honeysuckle Beds show an intertonguing relationship. Outcrops of trondhjemite rocks occur along this contact. This portion of the area may be called a 'melange'.

the The age of generation of~Coolac Serpentine Belt is uncertain. Judging from the tectonic history of the Cowra Trough, in which the ultramafic mass lies (see Chapter 7, Section 7.3), it seems likely that generation of the ultramafic- mafic association of the Coolac-Tumut region took place in the Late Ordovician - Early Silurian 17

time during the opening of the Cowra Trough.

The relative age of emplacement of the Coolac

Serpentine Belt may be tentatively placed at the Uppermost

Silurian to Early Devonian time. This is based on the tectonic contact relationship between Coolac Serpentine

Belt and the Late Silurian Young Granodiorite, and on the intrusive relationship between the ultramafic Belt and the

Early-Middle Silurian Bogong Granite (see Ashley and

Basden, 1973). However, due to possible repeated movements of serpentinites, it is difficult to determine the actual age of emplacement of serpentinites (Wilkinson,

1969).

1.43 Honeysuckle Beds:

In the vicinity of Mt. Lightning the Honeysuckle

Beds consist predominantly of spilites with interbedded metasedimentary rocks. Close to the Ultramafic belt only minor amounts of metasediments occur except at

Adjungbilly Creek where phyllites abut serpentinites.

The volume of spilitic rocks decreases with a corresponding increase in metasediments towards the western edge of the area mapped by the author.

The metasedimentary rocks include phyllite, chert, quartzite, metagreywacke and intermediate tuffs. All these rocks, except for the tuffs, show a foliation direction 18

sub-parallel to the general trend of the Coolac Serpentine

Belt, ie, NNW-SSE, the foliation being more prominent in phyllites and cherts than i~ quartzites and metagrewackes.

The phyllites are greenish grey to yellowish brown in colour and contain fine-grained quartz and albite fragments (0.01 to 0.05 mm in maximum diameter), which are commonly angular. The matrix is composed of sericite, quartz, albite, epidote and haematite.

Both reddish brown and grey cherts occur in the area. They consist essentially of sub-rounded to polygonal grains of quartz (average diameter - 0.02 mm) with minor haematite. The haematite content increases

(up to 20 vol.%) in the reddish brown variety which resembles jasper.

Quartzites are of restricted occurrence and are

found only in the south-western portion of the area mapped by the author. They are medium- to fine-grained,

the grain size ranging from 0.05 to 0.4 mmr Quartz makes

up approximately 90% by volume of the rock and is

associated with subordinate albite, epidote, chlorite, biotite, sericite, sphene and opaques. Preferred

orientation of lenticular quartz grains is common.

Ashley (1973) reported occurrences of pink garnet in

quartzites of the Honeysuckle Beds from the southern part

of the Coolac Serpentine Belt. Quartzites from the 19

Mt. Lightning area, however, do not contain garnet.

Greywackes are also of restricted occurence and crop out near the western margin of the area mapped by the writer. They consist essentially of quartz, plagio clase (of albite- oligoclase composition) and k-feldspar with minor chlorite, epidote, tremolite-actinolite, sphene, calcite and opaques, the average grain size being 0.3 mm. They also contain dacitic and andesitic clasts and with increasing amount of pyroclastic content greywackes grade into tuffs. The tuffs are of dacitic to andesitic composition and lack any sign of penetrative deformation.

The spilitic members of the Honeysuckle Beds are described in Chapter 3.

1.44 Young Granodioiite:

The Mt. Lightning ultramafic mass is flanked on the east by the Young Granodiorite and as mentioned previously the contact is a fault contact. This tectonic contact is probably a southern extension of the Mooney Thrust System as described by Basden (1974) and Franklin ( 19 7 5) •

In the area mapped by the author the Young Granodiorite is a massive to foliated rock. The foliation increases in intensity towards the contact with ultramafic 20

rocks and is parallel to the contact. In places, especially to the north of the Murrumbidgee River, the granodiorite is highly sheared and crushed resembling 'mylonite'.

The foliation is defined by alternate bands of dark biotite and light coloured quartz-feldspar. The highly deformed specimens show sheared and stretched biotite crystals, lenticular quartz grains and bent grains of plagioclase. The massive varieties show hypidiomorphic to allotriomorphic textures. Micrographic texture is also common in some specimens.

The granodiorite is composed essentially of quartz, plagioclase (An 15-An35 ) and biotite. Subordinate perthite is also common. Typical accessories include muscovite, sphene and apatite.

Inclusions of Young Granodiorites (up to 4m in maximum diameter) occur within the ultramafic mass close to the contact. It should be pointed out that these inclusions are quite different from the trondhjemtic bodies that also occur near the contact between ultramafic rocks and granodiorite (see Chapter 5). The essential differenceis that the Young Granodiorite contains k-feldspar, intergrown with albite in perthites and with quartz forming micrographic texture, while the trondhjemites typically lack k-feldspar (see Section 5.3 ). 21

Chemical analyses of four Young Granodiorite specimens collected from the vicinity of Mt. Lightning were carried out by the author (Table 1 , p 22 ) • The granodiorites contain more than 2 wt.% K20 whereas the trondhjemitic rocks are typically K20-poor with less than 0.8 wt.% K2o (see Section S.5 ) • TABLE 1

YG-1 (GR 245786 Jugiong 1:50,000 Sheet): Foliated quartz plagioclase-biotite-granodiorite with minor muscovite and sphene. YG-2 (GR 262765 Jugiong 1:50,000 Sheet) : Foliated quartz plagioclase- perthite-biotite-granodiorite with minor sphene. YG-3 (GR 248783 Jugiong 1:50,000 Sheet) : Foliated quartz plagioclase-perthite- biotite-granodiorite with minor muscovite, apatite and sphene. YG-4 (GR 271758 Tummorama 1:50,000 Sheet) : Non-foliated quartz-plagioclase-biotite-granodiorite with minor muscovite. 22

TABLE l CHEMICAL ANALYSES A,~D NORMS OF YOUNG GRANODIORITES FROM THE VICINITY OF ~IT.LIGHTNING. (in weight per cent)

YG-1 YG-2 YG-3 YG-4

SiO2 68. 72 7i.. 15 67.39 70,52 TiO2 0.51 0.32 0.68 0.41 Al 2o3 13.63 13.81 14.62 13.55 Fe2o3 1.20 o.38 1.15 0.76 FeO 3.91 3.80 4.06 2.95 MnO 0.05 0.11 0.07 0.02 MgO 2.21 1.76 2.46 1.91 Cao 1.92 2.45 2.73 2.12 Na2o 3.28 2.31 2,87 3.11 K20 2.59 3.38 3.14 2.26 P205 0.07 0.10 0,12 0,08 H2o+ 1.10 0.76 0.89 1.21 Ho- 2 0.16 0.16 0.29 0.07 Co2 0.08 0.00 0.10 0.06

Total 99.43 100.49 100.57 99.03

NORMS (C. I . P. W.)

q 30.28 34.32 27.28 36.14 or 15.30 19.97 18.55 13.35 ab 27.74 19.54 24.27 26.3 an 8.56 11.50 12.12 9.61 C 2.30 2.14 2.06 2.47 hy 10.94 10. 72 11.63 7,67 mt 1. 74 0.55 1.67 1.1 hm - - - - i1 0.97 0.61 1.29 0,78 ap 0.16 0.23 0.28 0.19 cc 0.18 0.00 0.23 0.14 H2o 1.26 0.92 1.18 1.28

Analr.ses by the author • • 23

C H A P T E R 2

U L T R A M A F I C R O C K S

2.1 INTRODUCTION:

More than 70% of the rocks exposed at Mt. Light ning are ultramafic silicate rocks. The petrography, mineral and bulk chemistry of these rocks and their origin as magmatic or non-magmatic products are considered in this chapter. The areas of predominant ultramafic silicate rocks at Mt. Lightning enclose minor masses of chromitite and of mafic and felsic rocks or their deriva tives namely, rodingites, trondhjemites and albitites.

These minor rock types will be considered in other chapters.

The ultramafic rocks of the Coolac serpentine belt are all more or less serpentinized. At Mt. Lightning the least serpentinized rocks contain about 30% by volume of 24

serpentine minerals and some are completely serpentinized.

Despite serpentinization, the essential minerals of the primary rocks can usually be recognized both in the field and under the microscope. The primary rocks are principally harzburgites. Dunites and lherzolites occur locally. These terms are used in accordance with the classification of Jackson (1968) except that limiting values have been changed from 10 to 5% by volume. The term "peridotite" has been used to include both harzburgite and lherzolite and the term "serpentinite" for rocks containing more than 80% by volume of serpentine minerals. 25

2.2 GENERAL STATEMENT:

The ultramafic rocks of Coolac serpentine belt are exposed over a length of 56 km and a width of up to 2 km between Coolac and Goobarragandra (Golding, 1969). The geological setting of the belt is described in Chapter 1 •

In the area mapped by the author the ultramafic rocks are composed almost entirely of serpentinized harzburgites. Lherzolites and dunites occur locally.

Distinction between harzburgite and lherzolite can only be made under the microscope. At Mt. Lightning dunites are always completely serpentinized and occur either intermixed with serpentinized peridotites in the western sector of the belt or as veins associated with chromite pods.

Serpentinization ranges from moderate to complete. Serpentinization and shearing increase from east to west across the belt with the development of a highly sheared serpentinite sector, 400 to 500 m wide, to the west. The eastern sector (about 1500 m wide) is composed predominantly of massive and blocky, variably serpentinized peridotite with minor lenses of weakly foliated serpentinites especially near the contact with granite. As mentioned by Golding (1966), these eastern and western sectors can be separately mapped although the boundary 26

between the two is gradational.

Ultramafic rocks at Mt. Lightning are flanked by granodiorite to the east and the contact is believed to be a fault contact (see Section 1.4 ). The western contacts of ultramafic rocks vary from south to north as follows: In the Adjungbilly Valley sheared serpentinites abut phyllites. On the northern fall of Mt. Lightning sheared serpentinites abut spilites. North of the Murrumbidgee River the contact is ill-defined being represented by a zone of intertonguing sheared serpentinite and spilite.

Ultramafic rocks can be readily distinguished in the field by their physical appearance. Massive or blocky serpentinized peridotite commonly shows a brown limonitic weathered coating which rarely exceeds 1 cm in thickness. The weathered surface usually has a rough and pitted appearance due to the persistence of relatively more resistant serpentinized orthopyroxene grains as compared with the easily weathered olivine grains. The colour of fresh broken surfaces varies according to the degree of serpentinization. Thus, the least serpentinized samples show a green to grey colour becoming darker as serpentinization increases. Golding (1966) suggested that harzburgitic rocks from the Coolac serpentine belt containing less than 40% serpentine minerals are medium grey and finely granular while those containing more than 27

70% serpentine minerals are black and dense.

Serpentinized peridotites at Mt. Lightning usually occur as lenses and angular blocks, the width of the lenses and blocks decreasing with increasing serpentinization.

A narrow zone (up to 5 m wide) of sheared serpentinite often separates outcrops of blocky serpentinized peridotite. Jointing is common in the massive serpentini tes and serpentinized peridotite and is orthogonal in places. However, in most outcrops joints appear to be randomly oriented. The close-spaced jointing in sheared serpentinites and serpentinized peridotite is commonly parallel to the strike of the belt.

The western sector of sheared serpentinite shows a prominent foliation parallel to the strike of the ultramafic belt (NNW - SSE). Lenses of these schistose serpentinites are smaller in dimension than lenses of the massive and blocky ultramafics to the east, reach a metre wide and form resistant ridges. Schistose serpentinites show variable colours, pale grey, pale green and pale brown predominating. Greenish black slickensided surfaces are common. Weathered schistose and massive serpentinites · commonly show an anastomosing texture formed by intersect ing ,veinlets of variably coloured serpentine minerals and iron oxides. These have been referred to as serpentine

"pseudo-breccias" by Golding (1966). Fig. 5 An outcrop of blocky serpentinized peridotite

showing rough and pitted surface.

_/ Fig. 6 Outcrops of highly sheared serpentinites

with steeply dipping foliation planes. 28 29

2.3 PETROGRAPHY AND TEXTURE:

Ultramafic rocks from Mt. Lightning contain up to 75% by volume of primary minerals. Modal analyses of ten peridotite specimens, which have also been chemically analysed, are given in Table 2 , p 30 . Olivine, orthopyroxene, clinopyroxene and chrome-spine! form the primary minerals assemblage in Mt. Lightning peridotites. Serpentine, magnetite, chlorite, talc and magnesite constitute the secondary minerals. The mineralogy of these rocks is described in sub-section 2.32 of this chapter.

Megascopically, serpentinized peridotites usually have a finely granular groundmass in which larger grains (up to 3 mm wide) of orthopyroxene occur as "pseudo phenocrysts". Serpentinites, in general, are aphanitic and commonly show mesh texture both in hand specimen and in thin section. The mesh texture is believed to be controlled by pre-serpentinization fractures (Coleman and Keith, 1971).

2.31 Microtexture:

Microtextures of the Mt. Lightning peridotites are complex and present a number of problems for interpretation. Broadly microtextures of Mt. Lightning peridotites can be divided into two categories as follows: TABLE 2

MODALANALYSES OF ULTRAMICROCKS FROM MT. LIGHTNINGAREA {in volume per cent)

1 2 3 4 5 6 7 8 9 10 Olivine 28.9 30.7 52.6 50.4 55.4 33.9 26.7 31.5 7.5 8.1 Orthopyroxene 6.2 9.0 10.1 9.7 10.0 8.4 6.3 11.9 1.6 2.0 Clinopyroxene 5.4 8.2 4.2 4.5 3.3 3.7 4.1 5.6 1.5 2.3 Chromite 1.4 0.8 1.0 1.1 1.2 1.2 0.6 1. 7 1.2 1.4 Magnetite 3.2 3.6 2.2 1.8 1.1 3.4 4.2 2.7 3.8 4.9 Serpentine 54.5 47.6 29.8 32.4 28.8 49.2 58.1 46.4 84.3 81.0 Chlorite/Talc 0.4 0.1 0.1 o.o 0.2 0.2 0.0 0.2 0.1 0.3

Samples as in Table 7 , p 61.

w 0 31

(a) High temperature textures characterized by

kink bands and internal and peripheral

recrystallization of primary minerals.

(b) Low temperature textures resulting from

cataclasis and serpentinization.

(a) High Temperature Textures:

Because olivine forms the bulk of the rocks, olivine textures approximate to rock textures. Two textural groups have been distinguished as follows:

Group 1: In a few thin sections equant, interlocking grains (2 - 3 nun in diameter) of olivine with sinuous mutual boundaries are present. Apart from occasional undulose extinction, signs of deformation are lacking in these grains. Contiguous individuals in grain clusters differ only slightly in orientation and may have recrystallized from pre-existing coarser grains

(Mercier and Nicolas, 1975).

Group 2: In most thin sections deformed olivine grains (2 - 3 nun in diameter) with jagged boundaries are surrounded by smaller (0.05 - 0.5 nun), strain-free, recrystallized grains (Fig. 7, p 33 ).

Similar texture has been reported from alpine-type peridotites elsewhere (Ragan, 1969; Nicolas et al, 1971). 32

The large relict olivine grains (porphyroclasts) display sharply defined sub-parallel bands (up to 0.3 mm wide) Of differing extinction. Such bands have been termed "kink bands" by Raleigh (1965) and Davies (1971). The presence of kink bands suggests deformation temperature in excess of 300 0 C (Carter, 1971). ·

Thin sections of some specimens display numerous aligned, elongated olivine porphyroclasts up to 6.0 x 0.5 mm (Fig. 15 , p 40 ) which, viewed between crossed polarise rs, extinguish together within a stage rotation of 5 to 20°. This feature suggests penetrative deformation where olivine grains have re-oriented under stress (Davies, 1971).

These two textural groups approximate to the "protogranular" and "porphyroclastic" textures of perido tite xenoliths from basalts and peridotites of some European massifs described by Mercier and Nicolas (1971). These authors also recognized a third type of· texture ("equigranular" texture) and regarded the three textures as indicative of stages in the sequence precursor grains_,,. recrystallization~ plastic deformation...=,. recrystallization. Textures of Mt. Lightning peridotites may be interpreted in the same way except that the equigranular texture, which corresponds to the last state of recrysta llization, is not present in Mt. Lightning peridotites.

Enstatite. Both isolated grains and grain- Fig. 7 Kinked olivine porphyroclast (centre)

surrounded by smaller strain-free grains

of olivine and minor serpentine. Specimen

number 1744B.

Between crossed polars. Frame length: 2.5mm.

Fig. 8 A large enstatite grain (grey) containing

narrow lamellae of exsolved diopside (orange

yellow and white). Specimen number 3/28.

Between crossed polars. Frame length: 0.9mm. 33 34

clusters of enstatite are to be found. Grain shapes range from equant and rectangular to highly elongated and irregular, the largest grains being 2-3 mm wide. All large enstatite grains contain narrow lamellae of exsolved diopside (Fig. 8, p 33).

In a few grain-clusters the individuals differ only slightly in extinction and show sinuous mutual boundaries partly occupied by other minerals. Such indivi duals rarely show kinking and are believed to pre-date deformation. Like ·Group 1 olivine grains, these enstatite grains may have recrystallized from coarser pre-existing grains possibly during a partial melting event (see Mercier and Nicolas, 1975).

Kinking is commonly observed in large (2-3 mm wide) porphyroclasts of enstatite which have jagged boundaries and sometimes gives rise to herring-bone patterns of exsolution lamellae. These kinked porphyroclasts are surrounded by small grains of olivine and enstatite. According to Carter {1971) kinking of enstatite suggests temperatures of deformation in excess of soo 0 c.

Enstatite,like olivine, shows the various stages in a metamorphic development as follows: coarser precursor grains~ recrystallization~ deformation. Fig.9. Photomicrograph showing large porphyroclastic grains of enstatite (en) and of bastite (bs) in a groundmass formed by olivine and serpen tine minerals. Crossed polars. Frame length: 2.5mm.

Fig.10. Photomicrograph of an enstatite porphyroclast surrounded by shells of olivine and serpentine minerals in a highly serpentinized harzburgite. Specimen No; 426. Ordinary light. Frame length: 3.6mm. 35" 36

In addition to exsolution lamellae in enstatite, diopside occurs as discrete grains in most samples. These discrete grains (up to 0.5 mm wide) are angular, pellucid and completely unaltered and display diallage parting (Fig. 11, p 37). Such grains are presum ably fragments of pre-existing larger grains. Occasionally, in lherzolitic variants of the peridotite, grain clusters of diopside accompany enstatite grains.

Deformation effects are less apparent in diopside than in olivine and in enstatite. Kinking is rare seen in diopside grains. However, bent diopside grains are not uncommon in these peridotites.

Chrome-spinel. Two principal groups of chrome spinel have been observed in Mt. Lightning peridotites:

Group 1: Cr-spinel grains or grain-clusters are amoeboid or vermiform with blocky portions from which "arms" protrude. Cr-spinel is usually intergrown with enstatite (Fig. 12, p 37) and is commonly translucent and brownish yellow.

Group 2: Cr-spinel grains have "holly leaf" shapes (Mercier and Nicolas, 1975) with cuspate margins (Fig. 13, p 39). They may have formed by disruption of intergrowths, dismemberment of arms (Fig. 14, p 39) and by minor grain boundary adjustments of Cr-spinel against silicates. This essentially brittle deformation of Cr spinel probably accompanied plastic deformation of surrounding silicates. This type of Cr-spinel grains tends to Fig.11. A discrete grain of diopside (centre) showing pronounced parting. The grain is surrounded by serpentine minerals (grey and white, low relief) relict olivine (high relief). Specimen No: 3/28. Ordinary light. Frame length: 2.2mm.

Fig. 12. Photomicrograph showing intergrowth of amoeboid Cr-spinel (black) with enstatite (showing cleavage) Specimen No: 449A. Ordinary light. Frame length: 2mm. 31 38

occur in "strings'' ~li9ned with the ;foliation (Fig. 15, p 40) and is more opaque than those of Group 1 type.

The two cr-spinel textural variants represent stages in a crystallization (or recrystallization) brittle deformation sequence and are similar to those in the respec tive mantle peridotite textures of Mercier and Nicolas (1975).

Intergrowth of Cr-spinel and pyroxene, varying in detail but broadly similar to those in Mt. Lightning peridotites, are widespread in mantle-derived xenoliths from basalts and kimberlites (Reid and Dawson, 1972; Basu and McGregor, 1975; Wallace, 1975; Dawson and Smith, 1975;

Suva et al, 1975). They also occur in peridotites of European massifs and Newfoundland ophiolites (Mercier and

Nicolas, 1975) and in alpine-type peridotites of the Great

Serpentine Belt of Eastern Australia (Benson, 1913; White,

1964; Wilkinson, 1953, 1969; and, Murray, 1969). The origin of such intergrowth is currently under debate. b) Low Temperature Textures:

Examination of thin sections indicates that an .,., earlier relatively coarse-grained texture has been overp~nted, and commonly obliterated, by a subsequent fine-grained texture. The superimposed texture gives the imrression of having resulted from:

(i) Pervasive randomly oriented fracturing of • olivine grains. Fig.13. Photomicrograph of 'holly leaf' Cr-spinel grains (black) associated with olivine (high relief) and bastite and chlorite (white). Specimen No: 1744. Ordinary light. Frame length: 2.2mm.

Fig.14. Photomicrograph of 'holly leaf' Cr-spinel grains (black) showing dismemberment of arms. Other minerals in the section are olivine (high relief) and bastite and chlorite (white). Ordinary light. Frame length: 2.2mm. 39 Fig.15. Photomicrograph showing Group 2 Cr-spinel grains occurring in "strings" aligned with the foliation of the rock formed by olivine (variegated interference colours) and altered enstatite (white). Crossed polars. Frame length: 3.5mm:

Fig.16. Photomicrograph of a dunitic serpentinite showing cross-cutting veinlets of serpentine minerals and disseminated, euhedral, nearly opaqu opaque Cr-spinels. Specimen No: 1622. Partly crossed polars. Frame length: 3.2mm. 40 41

(iil ya,,ri~le ,replacement of olivine by serpentine minerals along the fractures, leaving residual volumes of unaltered olivine grains which are

0.05 - 0.3 mm wide, with more or less rectangular or triangular shapes. These fragments are associated with a net-work of serpentine veinlets some of which contain magnetite granules.

Much of postulated fragmentation of olivine seems to have accompanied serpentinization and is therefore regarded as a low temperature process cioo 0 - 300°c, see Section 2.6). Hence, the term "cataclasis" is more appropriate than "protoclasis 11 , the latter implying the presence of some magma (see Ragan, 1967). It appears that at low temperatures olivine fractures and alters more readily than enstatite, Cr-spinel becomes Fe-enriched but diopside commonly persists unchanged.

Transitions between high-temperature (pre-serpent inization) and low temperature textures occur in moderately serpentinized harzburgites. Abraded enstatite grains, and ovoid clusters of diopside grains, are surrounded by alternate shells of serpentine and granulated olivine (Fig. 10, p 35), suggesting differential movement of pyroxenes within less competent matrix prior to and/or during serpentinization. A similar occurrence in peridotite from the Mid-Atlantic Ridge was described by Aurnento and Loubat (1971). 42

2.32 Mineralogy and Mineral Chemistry:

This sub-section deals with compositions of the major silicate constituents and gives descriptions of some of the commonly occurring secondary and minor minerals.

2.321 Olivine:

Chemical compositions, structural formulae and optical data of five olivines from Mt. Lightning peridotites are presented in Table 3, p 43 • Electron microprobe technique was used to determine olivine compositions. Electron microprobe scans across olivine grains revealed that they are compositionally homogeneous.

Olivines from Mt. Lightning peridotites range in

composition from Fo90 _5 to Fo92 _2 . This restricted compositional range is similar to that obtained by Ashley (1973) and Franklin (1975) from other parts of the Coolac Serpentine belt. Green (1964) and Challis (1965) have shown by comparisons of olivine compositions from various ultramafic rocks that those from alpine-type peridotites are typically magnesium-rich and have a

restricted compositional range from Fo88 to Fo93 in contrast to those from stratiforrn intrusions. The latter olivines have a wider compositional range

extending from Fo80 to compositions as magnesium-rich as those of alpine-type. TABLE 3 43

ELECTRON MICROPROBE ANALYSES, STRUCTURAL FORMULAE AND 2V OF OLIVINES FROM MT. LIGHTNING PERIDOTITES

(in weight per cent)

1 2 3 4 5

Si02 40.92 41.69 40.32 38.63 41.47

Al 2o3 0.07 tr. tr. tr. tr. FeO* 7.67 8. 72 8.28 8.09 8.93 MnO o. 13 0.15 0.09 0.11 0.12 MgO 50.94 48.01 50.30 51.54 47.94 NiO 0.57 0.39 0.39 0.53 0.24

Total 100.30 98.96 99.32 98.90 98.70

Structural formulae: 4(0)

Si 0.993 1.025 0.990 0.958 1.023 Al 0. 001' - - - - Fe2+ 0.156 0.179' 0.170 0.168 0.184 2.010 1.949 2.019 2.083 1.954 Mn 0.003 0.003 0.002 0.002 0.003 Mg 1.842 1.760 1.841 1.907 1.762 Ni 0.008., 0.007 0.006 0. 008_, 0 .005~

Fo 92.2 90.8 91.6 91.9 90.6

2V 85° 88° 86° 88° 90°

* total Fe was determined as FeO tr. = trace amounts

1. Specimen 2/10 2. Specimen 3/17 3. Specimen 3/28 4. Specimen 3/32 s. Specimen 4/22 Refer to Table 7, p 61 , for sample data.

Analyses by the author. 44

The composition of the Mt. Lightning olivine accords with that of other alpine-type peridotites. Scrne examples are given below:

Troodos Complex, Cyprus, (Fo92 ) Gass (1969) Great serpentine Belt of N.S.W. Wilkinson (1969) Rockharnpton District, Queensland

(Fo92) Murray (1962) Burro Mountain, California, Loney et al - (1971)

Vulcan Peak, Oregon, (Fo90 _7) Hirnrnelberg and Loney (1973) and,

Papuan Ultrarnafics, (Fo91 _6 _ 93 _6 > England and Davies (1973)

2.322 Orthopyroxene:

Orthopyroxene crystals in the Coolac harzburgite are inhomogeneous by reason of enclosed exsolution larnellae of clinopyroxene. This inhomogeneity is optically apparent and is also revealed by electron microprobe scans across the grains. Electron microprobe analyses and structural formulae of four orthopyroxenes from Mt. Lightning harzburgites, are reported in Table 4 , p 46 • These analyses have been made on areas of grains free from clinopyroxene larnellae. Consequently, the analyses do not report bulk compositions of the grains but compositions of the orthopyroxene hosts. 45

The analysed orthopyroxenes have a restricted compositional range from En 88 _2 to En90 _8 • The Mg/Mg+ Fe ratio varies from 0.91 to 0.92 and the Mg x 100/My +

Fe+ Mn ratio varies from 0.90 to 0.91. These values are similar to those of orthopyroxenes from other alpine type peridotites (Challis, 1965; Loney et al, 1971;

Jackson and Thayer, 1972; Himmelberg and Loney, .1973).

The Mt. Lightning orthopyroxen1:: s contain 2. 2 0

This range of values is fairly general for orthopyroxenes in other alpine-type peridotites.

Orthopyroxenes in New Zealand peridotites contain 1.43 to

4.09 wt.% Al2o3 (Challis, 1965), those from the Burro Mountain peridotite contain 1.4 to 3.0 wt.% (Loney et al,

1971) and those from the Vulcan Peak peridotite contain

1.8 to 2.0 wt.% Al2o3 (Himmelberg and Loney, 1973). The

Al2o3 content of orthopyroxenes from high temperature - high pressure peridotites, however, attains higher values.

Thus orthopyroxenes in the Lizard peridotite contain up to 6.5 wt.% Al2o3 (Green, 1964) and those from peridotites in South Western Oregon described by Medaris (1972) contain

2.61 to 5.60 wt.% Al2o3 .

In Fig. 17, p 47, Al2o3 contents are plotted against Mg x 100/(Mg +Fe+ Mn) for orthopyroxenes from various ultramafic rocks. The Mt. Lightning orthopyroxenes show a close correlation with those from other alpine type peridotites and plot below the high temperature - high 46 TABLE 4

ELECTRON MICROPROBE ANALYSES AND STRUCTURAL FORMULAE OF ORTHOPYROXENES FROM MT. LIGHTNING PERIDOTITES

(in weight per cent)

1 2 3 4

Si02 56.34 54.96 56.25 55.68 Ti02 tr. tr. tr. tr. Al 2o3 2.20 2.48 2.69 3.04 FeO* 4.96 5.47 5.17 5.86 MnO 0.10 0.08 0.12 0.12 MgO 33.43 33.89 32.96 33.40 CaO 0.84 1.06 0.98 1.68 Cr2o3 0.86 0.89 1.06 0.97

Total 98.73 98.83 99.23 100.75

Structural formulae: 6(0)

Si 1. 960} 2. 00 1,92212 .oo l.9SOJ2.oo 1.916}2,oo Aliv 0.040 0.078 0.050 0.084 Alvi 0.050 0.024'', o. 040' 2 0.06~1 Fe + 0.144 0.160 0.150 0.168 Mn 0.0021 0.002 0.004 1.984 2.018 0.004 1.986 1.012 Mg 1.732 1.768 1, 706j 1. 712 Ca 0.032 0.040 0.036 0.062 Cr 0.024, 0.024. 0.030 0.026, En 90.8 89.8 90.2 88.2 Fe 7.5 8.1 7.9 8.7 Wo 1. 7 2.1 1.9 3.1 2 Mg/Mg+Fe + 0.92 0.92 0.92 0.91 MgxlOO Mg + Fe 2+ + Mn 92.20 91.61 91. 71 90.90

* total Fe was determined as FeO tr = trace 1. Specimen 2/10 2. Specimen 3/28 3. Specimen 3/32 4. Specimen 4/22 Refer to Table 7 , p 61 , for sample data. Analyses by the author. 47

7-----"W

+ + 6 @) Mt. Lightning

\ + Li-zard, England l( X 5 l( x Southwestern Oregon X )0( ! • Alpine peridotihs 4 -# ~ l( • X ...; 3 X 0 3 o.. 1 .. ~ .... Skacrgaard <( ® • 2 • ' Stillwahr __ • .---~.::::~-=--- '· ----Bushvcld • •• • 1 • • •

0'-----1,,JW 100 90 100 90 80 70 60 50

100 Mg ) ( Mg+Fc +Mn

Fig.17. Al 2o3 contents of orthopyroxenes from Mt.Lightning peridotites compared with those from other occurrences (data from Medaris,1972). 48

pressure peridotites. They differ from orthopyroxenes of stratiform basic intrusions by having a very restricted Mg x 100,..{Mg +Fe+ Mn)ratio.

Loney et al (1971) reported similar Si, Al and er contents for host orthopyroxene and included clinopyroxene lamellae from Burro Mountain peridotites. However, electron microprobe analyses indicate differences between orthopyroxene host and included clinopyroxene lamellae. Thus Al2o3 and Sio2 are lower and cr2o3 is higher in the lamellae compared with those in the orthopyroxene host (Anal. 1 & 2 in Table 4, p. 46 ' and Anal. 3 & 4 in Table 5, p. 50 ).

2.323 Clinopyroxene:

In the Mt. Lightning harzburgites clinopyroxene occurs both as discrete grains and as exsolution lamellae within orthopyroxene hosts. Electron microprobe analyses of discrete grains in two samples and exsolution lamellae in two other samples are given in Table 5 , p. 50 • Analyses of the discrete grains represent bulk chemical compositions because the analysed clinopyroxenes from Mt. Lightning harzburgites lack visible orthopyroxene exsolution lamellae and electron probe scans across the grains do not reveal any compositional inhomogeneity. Clinopyroxenes from South Western Oregon peridotites (Medaris, 1972) and from Vulcan Peak harzburgites 49

(Himmelberg and Loney, 1973) are also devoid of exsolution lamellae.

The clinopyroxenes show a compositional range of

Ca46.3 - 49.1 Mg47.3 - 49.6 Fe2.2 - 4.1 ie, they are all diopsides according to the classification of Poldervaart and Hess ( 1951) . The discrete grains and the exsolution lamellae in orthopyroxene hosts have essentially the same composition.

Compared with the associated orthopyroxenes, the clinopyroxenes are consistently richer in cr2o 3 (1.19 - 1.61 wt.% in clinopyroxene compared with 0.86 - 1.06 wt.% in orthopyroxene) and in Tio2 (0.08 - 0.12 wt.% in clinopyroxene compared with 0.03 - 0.04 wt.% in ortho pyroxene) and contain substantial amounts of Na 2o (0.22 - 0.38 wt.%) which is not present in detectable amounts in the orthopyroxene. The cr2o 3 content of the Mt. Light- ning clinopyroxenes permits their designation as Cr- bearing diopsides. Similar cr2o 3 contEmts are recorded for diopside from South Western Oregon peridotites (Medaris, 1972), from the Coolac harzburgite (Ashley, 1973) and from pyroxenit1:.~s of the Gosse Pile layered intrusion, Central Australia (Moore, 1971). Diopsides of Burro Mountain peridotite, however, contain only 0.42 - 0.7 wt.%

Cr2o3 (Loney et al, 1971) and those of Vulcan Peak peridotite contain 0.63 - 0.88 wt.% cr2o 3 (Himmelberg and Loney, 1973). 50 TABLE 5

ELECTRON MICROPROBE ANALYSES AND STRUCTURAL FORMULAE OF CLINOPYROXENES FROM MT. LIGHTNING PERIDOTITES

(in weight per cent)

1 2 3+ 4+

Si02 52.48 53.07 53.42 53.61 Ti02 0.11 0.08 0.12 0.09 A1 203 1.09 1.84 1.76 2.11 FeO* 1.39 2.21 2.32 2.46 MnO 0.08 0.10 0.12 0.11 MgO 17.23 16.94 16. 77 17.21 CaO 24.11 23.68 24.21 22.32 Cr2o3 1.61 1.40 1. 19 1.28

Na2o 0.26 0.31 0.22 0.38 NiO tr. 0.00 0.00 0.00

Total 98.36 99.63 100.13 99.69

Structural formulae: 6(0)

Si 1. 942] 2 .00 1.9so} 2_00 1.950}2,oo Aliv l.94J o.~44 1.994 0.058 0.050 0.050 Alvi 0.20 ' 0. 024' 0 .042' Ti 0.002 0.002 0.002 0.002 Fe2+ 0.042 0.006 0.070 0,077 Mg 0.953 0.924 0.911 0.933 ">l,943 1.997 1.997 Mn 0.002 0.002 0.002 2.014 0.002 Ca 0.957 0.928 0.946 0,869 Cr 0.044 0.039 0.030 0.040 Na 0.016_. 0.022 0.012 0.032.

atoms%

Ca 49.0 48.4 49.1 46.3 Mg 48.8 48.2 47.3 49.6 2 Fe + 2.2 3.4 3.6 4.1

tr. = trace * total Fe was determined as FeO + exsolution lamella iR orthopyrox·ene l, Specimen 3/17. 3. Specimen 2/10 Refer to Table 7 , p 61 , for sample data 2• Specimen 4/22. 4. Specimen 3/28. .. Analyses by the author 51

The Al2o3 content of clinopyroxenes ranges from 1.09 - 2.11 weight% and is consistent with the Al2o3 content of clinopyroxenes reported from other alpine-type peridotites (Challis, 1965; Loney et al, 1971; Himrnelberg and Loney, 1973). Green (1964) and Medaris (1972)

reported much higher A1 2o3 contents (up to 6.8 weight%) for high temperature and high pressure peridotites, respectively.

In Fig. 18, p 52, A1 2o3 contents are plotted against MgO x 100/(Mg +Fe+ Mn) ratio of clinopyroxenes from various ultramafic rocks. Clinopyroxenes from Mt. Lightning peridotites show a close correlation with those from other alpini=-type peridotites and plot below the high temperature-high pressure peridotites. They differ from clinopyroxenes from stratiform intrusions by having a restricted MgO x 100/(Mg +Fe+ Mn) ratio.

2.324 Chrome-Spinel:

The nature of chrome-spinel in Mt. Lightning peridotites has been described under microtextures (Sub- Section 2. 31) . Chrome-spinels in the dunitic serpentinites, that are generally associated with chromite pods, differ from those in the peridotites. The dunitic spinels are usually euhedral to subhedral and opaque (Fig. 17, p 40 ) as against the anhedral types in peridotites. In many serpentinites that may be of 52

7 + " Ht. Lightning ~ \ + Lizard, England x Southwcstcrrn •• Orcagon ' •+ • Alpine pcridotitu

5 I~~ ...,. •= + l t ' •• 0-. • --~/.Stillwatcr ..... l et ....,,.... ,· "·, ,,, ', ·, ,skacrgaard ,, ,_ - -<" 8 2 ,( Bushvcldj 8~

, 8 • •

0 100 90 100 90 10 70 60 50

( 100 Mg ) Mg+Fc +Mn

Fig.18. Al 203 contents of clinopyroxenes from Mt.Lightning peridotites compared with those from other occurrences (data from Medaris,1972). 53

dunitic parentage, however, deformation has resulted in granulation of chromite into "dust".

The accessory chromites in Mt. Lightning ultrarnafic rocks have not been chemically analysed by the author. The composition of chromite from chromite segregations at Mt. Lightning will be discussed in Chapter 6 •

2.325 Serpentine-Group:

A combination of X-ray analysis differential thermal analysis, thin section study and chemical analysis was used to distinguish the different types of serpentine minerals in the Mt. Lightning ultrarnafic rocks. Lizardite and chrysotile are the two main species present although chrysotile is subordinate to lizardite. Antigorite is rare and has been found only at the eastern contact with granite and in one specimen from the schistose serpentinite zone in the west. Golding (1966, 1969, 1971) and Franklin (1975) have reported abundant antigorite from rocks occurring at North Mooney Ridge to the north of the area studied. The serpentine replacing enstatite is usually called bastite and is believed to be lizardite (Page 1967a) and not antigorite (Winchell, 1951). No X-ray diffraction analysis of bastite from Mt. Lightning rocks was done by the author. However, an electron-probe analysis 54

of bastite is given in Table 6 , p 58 •

Differential thermal ;1nalyses curves for Mt. Lightning serpentinites are shown in Fig. 19, p 55 • Antigorite can be distinguished from chrysotile, lizardite and mixtures of both because the former gives rise to an endothermic peak at about soo 0 c compared to an endothermic peak at about 100°c for the latter (Faust and Fahey, 1962). Faust and Fahey (1962) also found that a difference of 12°c or less between exothermic and endother

mic peaks ( l:::. Ex - En) is indicative of predominant antigorite. However, differential thermal analysis cannot be used to distinguish between lizardite and . chrysotile as they yield similar curves.

Mumpton and Thompson (1975) devised an identi fication scheme for various serpentine minerals based on X-ray data of Whittaker and Zussman (1956) and on the X-ray patterns of standard chrysotile, lizardite and antigorite. According to these authors critical "d

spacing" values (in A0 ) for antigorite are 2.52, 2.16, 1.562 and 1.536, for lizardite - 3.88, 2.49, 2.15, 1.531 and 1.500; and, for chrysotile - 2.45, 2.09 and 1.535.

In the present study a-values of 2.16 A0 (for antigorite)

and 2.15 A0 (for lizardite) were not used as it was found difficult to distinguish between such closely spaced peaks on the X-ray diffraction chart. For the same reason, a-values 1.536 A0 (for antigorite), 1.531 A0 Fig.19. Differential Thermal Analysis curves for selected serpentinite samples from Mt.Lightning: A- Lizardite-chrysotile serpentinite with (Ex-En) =98°. Specimen No: 4/18. B- Lizardite-chrysotile serpentinite with (Ex-En) = 77~ Specimen No: 4/21. C- Lizardite-chrysotile bearing serpentinized harzburgite with (Ex-En) = 84~ Specimen No: 4/22. The sample also contains minor chlorite (Ex= 605°). D- Lizardite-chrysotile serpentinite with (Ex-En) = 75~ Specimen No: 2/15A. E- Lizardite-chrysotile serpentinite with (Ex-En) = 80°. Specimen No: 2/56. F- Lizardite-chrysotile bearing serpentinized harzburgi te with (Ex-En) = 72°. Specimen No: 3/ 32. The sample also contains minor chlorite (Ex= 610°). 55

200 400 600 800 1000

Tempera t ure ( oc) 56

(for lizardite) and 1.535 A0 (for chrysotile) were also discarded. The following are the most useful "d-spacings" for rapid interpretation of diffractometer charts:

Lizardite 3.88, 2.49 and 1.500 Chrysotile 2.45 and 2.09

Antigorite 2. 52 and 1. 562

Page (1967a) used the intensities of the (202) reflection of lizardite and the (202) reflection of chrysotile, which corresponds to 2.49 A0 and 2.45 A0 , respectively (Mumpton and Thompson, 1975), to estimate qualitatively the relative amounts of the two minerals. This method was found useful to determine that lizardite predominates over chrysotile in Mt. Lightning serpentinites.

Page (1968) suggested that the three principals serpentine mineral species differ in chemical composition as follows:

• Antigorite contains more Sio2 but less MgO and less H2o than either lizardite or chrysotile.

• Lizardite has the highest Fe2o3/FeO ratio.

• Chrysotile has the lowest Al2o 3 content.

Electron microprobe analyses of four serpentine samples, including one bastite grain, together with average chemical analyses of lizardite, chrysotile and antigorite 57

(taken from Coleman, 1971) are given in Table 6 , p 58

The Fe2o3/Fe0 ratio of the analysed serpentines is not obtainable from the microprobe analyses but, an attempt to apply the other criteria of Page (1968) for the distinc tion of serpentine minerals has been made.

The chemical analyses of the Mt. Lightning serpentines show them to have lower Sio2 and higher MgO content than those listed for the average antigorite (Table 6, p 58 ). Analysis 1 is similar to that of average chrysotile while the other three analyses, including that of bastite closely correspond to the average lizardite analysis.

In thin section serpentines are colourless to pale green with very low birefringence ( ~ 0.005). Some sections are almost isotropic. They are usually turbid as compared with colourless primary minerals (olivine and orthopyroxene) and have length slow orientation. Chrysotile, when present in cross-cutting veins, shows a· characteristic fibrous habit but when mixed with lizardite .to form mesh texture serpentine it is difficult to distinguish from lizardite. Felted antigorite texture was not encountered in any Mt. Lightning section and the presence of minor antigorite in a few samples was recognized with the help of X-ray diffraction analysis (see above).

, ,

VI VI

00 00

(a (a

p61 p61

the the

7, 7,

(1971, (1971,

by by

taken taken

3/32 3/32

3/28 3/28

3/17 3/17

data. data.

2/10 2/10

4 4

Lizardite Lizardite

7 7

Antigorite Antigorite

Chrysotile Chrysotile

grain) grain)

microprobe microprobe

900) 900)

Table Table

to to

1 1

5 5

to to

Coleman Coleman

p. p.

sample sample

anal.) anal.)

author author

Average Average

Specimen Specimen

bastite bastite

from from

(electron (electron

for for

Anal. Anal.

7. 7.

6. 6.

5. 5.

3. 3.

Refer Refer

Anal. Anal.

2. 2.

4. 4.

1. 1.

I I

6.007 6.007

}4,010 }4,010

.. ..

.15 .15

3.73 3.73

1.17 1.17

1.64 1.64

7 7

12.10 12.10

38.37 38.37

99 99

42.14 42.14

7.680 7.680

0.083 0.083

0.297 0.297

0.184 0.184

5.443 5.443

to to

1 1

3.976 3.976

•5.824 •5.824

" "

.J .J

72 72

724 724

o. o.

0.62 0.62

0. 0.

18(0,0H). 18(0,0H).

13.54 13.54

98.06 98.06

40.93 40.93

41.53 41.53

6 6

5. 5.

0.051 0.051

0.079 0.079

0,049 0,049

3,897} 3,897}

Analyses Analyses

of of

988 988

basis basis

MINERALS MINERALS

5.815 5.815

3. 3.

~ ~

the the

.., ..,

834} 834}

1.40 1.40

4.10 4.10 0.42 0.42

13.29 13.29

39.44 39.44

99.67 99.67

5 5

41.02 41.02

on on

8.283 8.283 8.476

5.494 5.494

0.154 0.154

3. 3.

0.288 0.288

0.033 0.033

equivalents.in equivalents.in

SERPENTINE SERPENTINE

OF OF

formula formula

oxygen oxygen

'6.096 '6.096

7 7

14 14

cent) cent)

to to

6 6

of of

formulae+ formulae+

0.31 0.31

0.09 0.09

1.94 1.94

4.12* 4.12*

5 5

0.02~ 0.02~

8.00 8.00

0.325 0.325

0.011 0.011

5.730 5.730

0.007 0.007

0.205 0.205

3.795}4,00 3.795}4,00

40.68 40.68

40.16 40.16

4 4

per per

FOID,flJLAE FOID,flJLAE

1:87.30 1:87.30

ht ht

· ·

basis basis

TABLE TABLE

on on

Analyses Analyses

6.163 6.163

Structural Structural

sub-total sub-total

in in

STRUCTURAL STRUCTURAL

1: 1:

1.08 1.08

0.09 0.09

1.88* 1.88*

0.09 0.09

0.007, 0.007,

8.00 8.00

5.997 5.997

0.008 0.008

3.827}3.95 3.827}3.95

AND AND

0.123 0.123

3 3 0.151 0.151

39.51 39.51

41.86 41.86

I:84.81 I:84.81

calculated calculated

determined; determined;

were were

ANALYSES ANALYSES

6.009 6.009

~ ~

not not

3.69* 3.69*

1.66 1.66

0.12 0.12

0.06 0.06

0.009 0.009

8.00 8.00

5.559 5.559

0.041 0.041

0.005 0.005 2 2

0.292 0.292

39.41 39.41

41.51 41.51

0.144 0.144

FeO FeO

3.959}4,00 3.959}4,00

was was

formulae formulae

1:86.45 1:86.45

CHEMICAL CHEMICAL

o o

as as

Fe Fe

since since

-5.994 -5.994

4 4

total total

Structural Structural

.s1 .s1

039., 039.,

1.58 1.58

2.39* 2.39*

092~ 092~

0.11 0.11

0.51 0.51

1 1

+ +

* *

39.84 39.84

41.08 41.08

0. 0.

8.00 8.00

0.009 0.009

3.918}4,00 3.918}4,00

5.663 5.663 0. 0.

0.082 0.082

0.191 0.191

ms ms

3 3

2 2

+ +

o+ o+

(OH) (OH)

FeO FeO

Si0

NiO NiO

Mn Mn Ni Ni

Al Al

Mg Mg

MgO MgO

MnO MnO Total Total

Al Al

Si Si

H r r 59

2.326 Other Secondary Minerals:

Chlorite is a minor constituent ( < 1% by volume). It is usually associated with chromite and serpentine and as borders to serpentinized enstatite grains. Golding and Bayliss (1968a, 1968b) suggested that chlorite originates from alumina metasomatism of lizardite-chrysotile serpentinite that accompanies release of alumina from chromite. In thin section chlorite occurs as colourless or pale green non-pleochroic flakes which show length fast orientation. This length fast character of chlorites in Mt. Lightning peridotite distinguishes it from antigorite. Birefringence of the chlorite is usually <. 0.006 and anomalous interference colours were rarely observed.

Talc is rare in the Mt. Lightning ultramafic rocks and is only found to occur as an alteration product of enstatite in strongly serpentinized peridotites. Golding (1966) reported that in places bastite is altered to finely aggregated talc. A post-serpentinization origin for talc is therefore likely. A higher birefring ence (up to 0.02) distinguishes talc from chlorites and serpentines. 60

2.4 WHOLE ROCK CHEMISTRY OF MT. LIGHTNING PERIDOTITES:

Chemical compositions of variably serpentinized peridotite samples from the Coolac serpentine belt are listed in Table 7, p. 61 Twelve samples from the vicinity of Mt. Lightning were analysed - Nos. 1 to 10 by the author and Nos. 11 and 12 by others (see Golding,

1966) • Four analyses of harzburgites from other parts of the Coolac serpentine belt have also been included in the Table to obtain a general idea of the composition of peridotites from the belt. Analyses 13 and 14 are of

samples from the south of Mt. Lightning (Ashley, 1973), while the last two analyses of the Table (Nos. 15 and 16)

show compositions of harzburgites from the northern extremity of the belt (Franklin, pers. comm.). These

comparison analyses include examples of the least

serpentinized, as well as some of the most serpentinized

harzburgites.

The bulk chemistry of the ultramafics is strongly

influenced by the degree of serpentinization. Therefore,

direct comparison of analyses is not possible. Controversy

still exists as to whether serpentinization involves

volume change or not. This will be discussed in section 2.6.

The H2o+ content in peridotite can be directly TABLE 7

1. Serpentinized peridotite (lherzolitic). Specimen No: 2/10, GR 257763 Jugiong 1:50,000 Sheet. 2. Serpentinized peridotite (lherzolitic). Specimen No: 2/19, GR 258766 Jugiong 1:50,000 Sheet. 3. Serpentinized harzburgite. Specimen No: 3/17, GR 242773 Coolac 1:50,000 Sheet. 4. Serpentinized harzburgite. Specimen No: 3/27, GR 255756 Tummorama 1:50,000 Sheet. 5. Serpentinized harzburgite. Specimen No: 4/22, GR 237783 Coolac 1:50,000 Sheet. 6. Serpentinized harzburgite. Specimen No: 4/17, GR 253766 Jugiong 1:50,000 Sheet. 7. Serpentinized harzburgite. Specimen No: 3/32, GR 253760 Tummorama 1:50,000 Sheet. 8. Serpentinized harzburgite. Specimen No: 3/28, GR 257751 Tummorama 1:50,000 Sheet. 9. Serpentinite. Specimen 'No: 4/21, GR 247753 Tummorama 1:50,000 Sheet. 10. Serpentinite. Specimen No: 4/18, GR 260768 Jugiong 1:50,000 Sheet. 61 TABLE 7

OiEMICALANALYSES OF PARTLYTO COMPLETELYSERPENTINIZED PERIDOTITES FROM TI-IE COOLAC SERPENTINE BELT

1 2 3 4 5 6 7 8 9 10 1111. 12.a. 13~ 14~ 15• 16c

Si02 38,79 41.64 40.81 41.43 40.52 39.64 41.04 39,84 39.75 40.09 41. 79 40.88 43.51 40.27 43.80 39.96 Ti02 0.02 o.oo 0.02 0.02 o.oo 0.02 0.04 0.01 0.04 0.01 0.05 0.02 0.01 0.00 0.05 0.04 Al203 1.30 1.22 2,11 1.89 1.56 1,41 1,26 1.37 0.91 1.02 2.28 1.30 1.14 0.52 2.12 1.27 Fe2o3 2,73 2,73 1.62 1.80 4,63 3.29 2.:n 2.48 5,66 5.36 1,40 2,18 2.57 7.15 1.35 3.08 PeO 4,86 S,14 5,S8 '1,21 S,41 &,11 s,so &,43 1.71 2.49 6,2S 11,77 6,U 0,62 4,H 5,06 MnO' 0.52 0.59 0,22 . 0,20 0.17 0,33 0.24 0.18 0.31 0.11 0.11 0.15 0.14 0.07 0.10 0,04 MgO 40,20 41.11 39,96 38.52 40,96 40,09 39,66 40.38 37.14 38.08 39.52 41.49 41.28 37.41 40.90 38.80 CaO 1,39 1.21 1.61 0.91 0.66 0.81 1.46 1.56 0.22 0.70 2.35 1.51 2.52 0.01 1.52 0.94 Na20 0,06 0.09 0,14 0,05 0,08 0.08 0.10 0.06 0.01 0,00 0.05 . 0.10 0.01 0.00 0.05 0.23 K20 o.oo 0.02 0.02 0.03 0.01 0.04 0.03 0.00 o.oo 0.00 0.04 0.06 0.02 0.00 0,03 0.00 P205 0.00 o.oo 0.01 0.00 0.02 0.02 0.00 o.oo 0.01 0.00 trace trace 0.01 0.01 0.00 0.04 H2o+ 8.97 7.74 5.88 6.11 5,18 6.62 8.01 7.67 12.48 11.93 5.45 S.66 3.79 13.12 3.56 10.65 H2o- 0.18 0.41 0.21 0.31 0.19 0.14 0.09 0.24 0.54 0.35 0.03 0.32 0.09 0.80 0.21 0.24 Co2 .... 0.08 .. 0.06 .0.15 0.09 0.23 0.08 0.11 .0,07 0.09 0.14 0.09 0.00 0.02 0.17 ' 0.22 0,12 Total 99.10 100.03 98.34 .98.57 99.62 . 98.34 99.55 99.29 98.87 100.28 100.06 100.04 99.42 100.15 98.86 100. 71 Trace .elements (in ppm) -

er 2510 2473 2880 2805 2204 2566 2902 2448 2681 2362 3800 3500 3013 2333 2880 2365 Cu 16 5 12 6 10 6 19 5 lZ 8 21 9 l 2 Ni 2124 2205 2410 1980 2056 1925 2167 2224 1869 1941 1700 2500 2169 3161 1374 1886 V 40 47 28 56 62. 29 38. 36. 32. 12 41 5 20 24

Molar ratio MgO/(Mgo+ FeO*+MnO) 0.90 0.92 0,91 0.90 0.89 0,90 0.91 0.90 0.90 o·.9o 0.90 0.90 0.90 0,90 0.92. 0.83

S.G. 2.82 2.85 3.00 2.92. 3.10 .2.88 2.80. 2.90 2.40 2.46 3.03 3.00 3.09 2,33 3.14 2.81

\Serp. 0 63 60 40 52 .27 56 68 52 100 100 36 40 28 100 16 67

• total iron as FeO •• calculated from specific gravity measUTements (Page 1967a Coleman and Keith, 1971).

a Analyses from Golding (1966) b .'ulalyses from Ashley (1973) c Analyses from Franklin (1975)

Analyses 1 to 10 by the author, 62

attributed to the amount of serpentine minerals present. + A range of 5.18 - 12.48 weight% H2o has been obtained for Mt. Lightning peridotites. constituent of the primary minerals in these rocks. So the range in Fe2o3 content (1.80 - 5.66 weight%) is largely due to the presence of secondary magnetite, which is a product of serpentinization process. Assuming no

Fe was added or removed during alteration of primary minerals, and calculating total Fe as FeO, a comparison of the molar ratio MgOAMgO + FeO + MnO) can be made for various samples. Variations in the proportions of olivine and orthopyroxene in these harzburgitic rocks do not significantly affect the MgO/(MgO + FeO + MnO), since these minerals have very similar(MgO/MgO + FeO) ratios

(see Table 7, p. 61). This ratio varies from

0.89 - 0.91 for Mt. Lightning peridotites and is comparable with those obtained from other parts of the belt (see

Analyses 13 - 16, Table 7 , p. 61 ). A similar molar ratio has been reported for alpine-type peridotites elsewhere (Loney et al, 1971 - 0.91 for Burro Mountain peridotites; Himmelberg and Loney, 1973 - 0.91 for Vulcan

Peak peridotites). England and Davies (1973) reported slightly higher values (0.92 - 0.93) for Papuan peridotites.

Concentrations of alkali-elements are usually very

low in alpine-type ultramafic rocks (Stueber and Murthy,

1966). Mt. Lightning peridotites are no exceptions.

Hamilton and Mountjoy (1965) reported a range of 0.001 - 63

0.19 weight% for Na 2o and 0.001 - 0.031 weight% for K20 in alpine-type ultramafic rocks. from various localities. For Mt. Lightning peridotites only two samples show a slightly higher K2o content (0.04 and 0.06 weight% for analyses 11 and 12 respectively, see Table

7 , p. 61 ) while K2o in the other samples and the Na2o content in all samples are in the ranges described by Hamilton and Mountjoy. The er content of Mt. Lightning ultramafics is largely dependent on the amount of accessory chromite and to a less extent, on the modal proportion of pyroxenes since pyroxenes (mainly clinopyroxenes) are the major er-bearing minerals apart from chromite. A range of 2204 to 2902 ppm er with an average of 2582 ppm er, is found for Mt. Lightning ultramafic rocks. This average er value is closely comparable to those recorded by other authors (2400 ppm

Cr for ultramafic rocks - Stueber and Goles, 1966; and,

2500 ppm er for serpentinites - Hess and Otalora, 1964 cited in Goles, 1967). 64

2.5 PETROGENESIS OF PRIMARY ULTRAMAFIC

ROCKS:

2.51 Temperature-pressure Estimation based on

casio3 and A1 2o3 contents of Clinopyroxene:

O'Hara (1967b) devised a petrogenetic grid based on the composition of clinopyroxene co-existing with olivine, orthopyroxene and an aluminous phase, either plagioclase, spinel or garnet. This petrogenetic grid enables a temperature-pressure estimation to be made from a chemical analysis of clinopyroxene. ·Two parameters

ac*and Be*, related to casio3 and R2o3 contents of clinopyroxene respectively, are calculated (O'Hara, 1963 and 1967b) and are plotted on the grid in relation to two intersecting sets of contours. Thus a specific point is located on the petrogenetic grid for each clinopyroxene analysis. Four chemical analyses (Table 5 , p 50) of diopsides from Mt. Lightning peridotites were used to deten,iine a.c and Be values. All diopside compositions plot in the spinel lherzolite field of O'Hara's petrogenetic grid (see Fig. 20 ,p 65 ) at

temperatures between 1000°c and 11so0 c. The estimated

pressure range is between 8 to 11 K-bars.

Total Fe was expressed as FeO in the electron microprobe analyses of Mt. Lightning diopsides. As a result, values of a. and B obtained from these analyses C C

• CX:c = wt%CaSiO3 x 100 - wt%(CaSiO3 +MgSiO3) & 65

I I I l

------_------_-_ ------1500 ------·-- ...,.,------· :::::,:,------~-;~~------u - 0 ___ .,. __ ,--------// G'i;:..,. I I ..J,.\C, _,, ,,.,,...... -- ____ ,,-- /,' I),." / ,,,,,,, - _ _. J' ',// ot~,,,.,., ,, .,,.. .,,. .,,. ; : • SPINE L //,,,,,,,, GARNET .,,- .,, : ,'•LHERZOLITE,:,,,, ..'./-"' LHERZOLITE /,,, -1000 (/) ,_ ,,. ,,,,,,, ,q_"' "' :: . ,(,' ,,,, -' -' 1, /1 I / 00 :,1 I /II ,,,,.,,. / 0 "' ,,,,,, - Q: / I ,' ' / c,:, "' I I / I / ,q :i: I I / I / -'..., ;' 12. I:I I / / / ,' / ,, / 1, .,,.,. ;,/ ,, / / //. ,,,,..,, I ,,- - 500 / ,,,. I _,,/ I / I ; I / I / I /,,, // t/ /1 . . 10 20 30 40 P,-e.ssu.n (I< b.ars)

Fig.20. Compo!--.ition!'- of clinopyroxenes from Mt.Lightning pcridotites plotted on a Pressure-Temperature

will plot on the grid at slightly higher temperatures and lower pressures than would be the case if Fe2o3 values were determined separately (Medaris, 1972).

2.52 Temperature Estimation based on Cation Distribution in co-existing Orthopyroxene and Clinopyroxene:

Where chemical analyses of co-existing pyroxene phases are available, equilibration temperatures of clinopyroxene-orthopyroxene pairs can be calculated from an equation derived by Wood and Banno (1973, p. 119): - 10202 T = Cpx Mg 2si2o6 2 ln - 7.65 XOpx + 3.88 XOpx - 4.6 Opx Fe Fe Mg 2si2o6 where, T = temperature (in K) Cpx = clinopyroxene Opx = orthopyroxene Fe2+ XOpx = Fe Fe2+ + Mg2+ in orthopyroxene Mg2+ Mg 2si2o6 = Ca 2+ + Mg2+ + Fe2+ + Mn 2+ + Na 2+ M2 Mg2+

Fe3+ + Fe2+ + A1 3+ Ti4+ + cr3+ Mg2+ Ml 67

Ml and M2 being two sites in pyroxene 2+ 2+ 2+ structure - Ca , Na and Mn are assigned to M2 site, while Al3+, cr3+, Ti. 4+ an d Fe 3+ are assigne . d to M 1 site..

Equilibration temperatures of Mt. Lightning peridotites calculated from analyses of co-existing orthopyroxene-clinopyroxene pairs range from

1032° to 1oso0 c, are similar to those obtained for other alpine peridotites, eg, 975° to 101s0 c for South-West Oregon peridotites and 1062° to 1081°c for Vulcan peak peridotites (Wood and Banno, 1973, p 121).

2.53 Temperature Estimation based on Cation Distribution in co-existing Olivine and Orthopyroxene:

The theoretical distribution functions of cations in co-existing phases that crystallized in equilibrium have been discussed by Kretz (1961). Accorqing to Nafziger and Muan (1967) a reaction expressing Mg 2+ - Fe2+ exchange between co-existing olivine and orthopyroxene can be written as follows:

Fe Sio3 + Mg Si0.502 in pyroxene in Olivine

= Mg Si03 + Fe Si0.502 in pyroxene in Olivine 68

In terms of distribution of cations between olivine and orthopyroxene, the equilibrium distribution function (K) for the above reaction is:

X0l 01 X0px 0px Fe Fe Mg Mg K = X0l 01 X0px 0px Mg Mg Fe Fe

01 where XMg and are the mole fractions of the end members MgSi0 • 5o2 and Fesi0 _5o2 respectively in olivine, xOpx and XOpx are the mole fractions of the end members Mg Fe MgSi03 and FeSio3 respectively in orthopyroxene, and i values are the appropriate activity co-efficients.

Co-existing olivine and orthopyroxene in

Mt. Lightning peridotites yield K = 1.08 to 1.15, using the activity co-efficient data of Nafziger and Muan (1967). This range of K values is closely comparable with 'the

K value (1.07 + 0.16) determined experimentally by Nafziger and Muan (1967) in their equilibrium study of co-existing olivine and orthopyroxene, over a temperature range of 1200° to 12so0 c. Therefore, Mt. Lightning peridotites may also have equilibrated in the temperature range of 1200° to 12so0 c. Ashley (1973) has recorded similar K values (range 1.07 to 1.26) for co-existing olivine and orthopyroxene from peridotites occurring elsewhere in the Coolac serpentine belt. 69

2. 54 summary:

Compositions of co-existing pyroxenes provide valuable information regarding the temperature of formation of mineral assemblages in alpine-type peridotites. From the various methods discussed above, it seems that the primary mineral assemblages of Mt. Lightning peridotites formed at a temperature in the range of 1000° to 12so0 c.

Estimation of pressure from mineral assemblages of ultramafic rocks appears to be less reliable. Most experimental works indicate that compositions of co-exist ing olivine and pyroxenes are independent of pressure, for example· below 14oo0 c the solubility of enstatite in diopside is essentially the same at 30 K-bars pressure as at 1 atm. pressure, (Davis and Boyd, 1966), while Kretz (1961) has shown that the effect of pressure is negligible in the case of cation distribution in co-existing olivine and orthopyroxene. McGregor (1967) reported that A1 2o3 content of pyroxenes varies with pressure at constant temperature. Thus, for a spinel peridotite mineral assemblage, the

Al2o3 content of pyroxenes decreases with increasing pressure and A1 2o3 goes into the spinel phase. O'Hara (1967a), on the other hand, has suggested that A12o3 content of clinopyroxene increases with increasing pressure at constant temperature. However, O'Hara's petrogenetic grid is experimentally controlled only for the garnet peridotite stability field. Since no chemical 70

analysis of chromian spinel co-existing with pyroxenes has been carried out by the author, the variation in

A1 2o3 content of pyroxenes with pressure cannot be determined for Mt. Lightning peridotites.

Using McGregor's (1967, p 388) temperature pressure diagram for peridotite mineral assemblages, the estimated pressure for the formation of Mt. Lightning mineral assemblages will be a minimum of 7 to 8 kbars at a temperature of formation in the range 1000°c to 1250°c. This range of estimated minimum pressure corresponds to a depth of about 25 km. The maximum estimated pressure of formation, using the same diagram, will range from 13 to 19 kbars, corresponding to a depth of between 45 km and 62 km. This maximum estimated pressure of formation, up to 19 kbars, is higher than that obtained by using O'Hara's petrogenetic grid. However, O'Hara's petrogenetic grid is provisional (Medaris, 1972, and Himmelberg and Loney, 1973) and the experimental control of the grid is more precise for the garnet lherzolite field than for the spinel lherzolite or plagioclase lherzolite field.

Recently Wiltshire and Jackson (1975) have discussed the problems in determining temperature and pressure from pyroxene compositions of ultramafic rocks. According to them alpine peridotites show apparent pressure ranges far in excess of those appropriate to their 71

thickness. Igneous and metamorphic processes operating in the crust and mantle will affect the bulk chemical composition of primary ultramafic rocks. As a result, calculated temperatures and pressures from pyroxene compositions could give inaccurate results. Bearing these problems in mind, estimated physical conditions of equilibration for Mt. Lightning peridotites should be treated with caution.

2.55 Classification of Mt. Lightning

Ultramafic Rocks:

Before a discussion on the origin of Mt. Light ning peridotites is presented, it is desirable to classify these rocks into a certain category, eg, stratiform-, concentric-, or alpine-type. Some of the characteristic features shown by Mt. Lightning peridotites are listed below:

(1) They are exposed as a steeply dipping linear

sheet-like body with serpentinite margins and

fault contacts.

(2) Harzburgite is the predominant rock type

present with minor lenses of dunite and

lherzolite.

(3) Olivine (Fo90 _6 _ 92 _2 ), orthopyroxene 72

(En88 • 2 _ 90 • 8 ) clinopyroxene (Cct.u.3-1t9,Mg41_3 _49_6 Fe1 _,. ••_,) and chromite form the primary mineral assemblage. Plagioclase and hornblende are absent.

(4) Recognizable cumulate textures are absent. The textures of Mt. Lightning peridotites are metamorphic with abundant evidences of solid-state deformation and partial recrystallization followed by cataclasis.

(5) They appear to be tectonically emplaced. There is no evidence of chilled border zones or contact metamorphism.

{6) Associated magmatic mineral deposits include podiform chromite ore bodies. Compositions of chromite ores range from high chromium to high aluminium types {Chapter 6 ).

Recently Jackson and Thayer (1972} have critically reviewed the differences between stratiform, concentric and alpine peridotite - gabbro complexes and suggested some criteria for distinguishing between them.

Although gabbros have not been encountered in the area studied by the author, they have been recorded in association with peridotites from other parts of Coolac 73

serpentine belt (Golding, 1966, 1969, 1971; Ashley et al 1971; Ashley,1973; Franklin, 1975; and, Brown, 1973). The various features mentioned above indicate that Mt. Lightning peridotites belong to the group designated as alpine-type complex and more precisely, to the harzburgite sub-type according to the classification of Jackson and Thayer (1972).

2.56 Origin of Alpine-type Ultramafic Rocks:

Various authors including Wyllie (1967, 1969, 1970), Den Tex (1969), Maxwell (1969), Thayer (1967, 1969), Moores and Vine (1971), Jackson (1971) and Ringwood (1975) have recently discussed the petrogenesis of alpine-type ultramafic rocks. Controversy still exists regarding the source material and conditions and nature of crystallization of these rocks. However, there is a general agreement that alpine-type peridotites have been derived from the upper mantle.

Benson (1926) and Hess (1939) are among the earlier workers who supported a mantle origin for these rocks. The worldwide occurrences of alpine-peridotites in deformed mountain belts and the evidence of solid state deformation led Hess (1955) to believe that these rocks represent fragments of upper mantle material. Another feature in favour of an upper mantle origin is 74

that unaltered alpine peridotites possess the required uppermost mantle seismic velocities and densities (Ringwood, 1975).

The various processes proposed for the origin of alpine-type peridotites can be broadly divided into three groups:

(a) Crystallization from an ultramafic magma.

(b) Crystal accumulation from a mafic magma.

(c) Crystalline residue as a result of partial melting of primitive mantle material together with extraction of basaltic material.

(a) Crystallization from an ultramafic magma:

Hess (1938) proposed that the parent magma of peridotites is an ultramafic liquid which has a composition similar to serpentine. However, on the basis of experimental results Bowen and Tuttle showed that a serpentine magma cannot exist (Turner and Verhoogen, 1960, p. 315).

Recently, Loney et al (1971) suggested that crystallization from an ultramafic magma would be the most likely origin for Burro Mountain peridotite. According 75

to them cross-cutting dunite dykes and sills in harzburg ites represent products of crystallization from an ultramafic magma, and the harzburgites being closely similar in composition to dunites also crystallized from such a magma. The partial melting hypothesis was discounted by Loney et al due to the lack of associated mafic rocks. Himmelberg and Loney (1973) concluded that the Vulcan peak peridotite, which is in many ways similar to Burro Mountain peridotite, may also have formed by crystallization from an ultramafic or picritic magma on the basis of possible relict igneous textures.

It appears unlikely, that Mt. Lightning perido tites were formed as a result of magmatic crystallization from an ultramafic magma. There is little or no evidence of dunite dykes intruding Mt. Lightning peridotites. Moreover, experimental work on the forsterite-diopside-iron oxide system by Presnall (1966) strongly militates against the process of crystallization from an ultramafic magma. Forsteritic olivine-rich rocks would crystallize at an extremely high temperature ( > 1, aoo 0 c) according to the investigation of Presnall. 76

(b) Crystal Accumulation from a Mafic Magma:

Layered structures often found in alpine-type peridotites have led many authors to believe that these rocks were formed in a similar way to the stratiform intrusions, eg, Bushveld, Stillwater and Great Dyke. Thayer (1960, 1963 a and b, 1967 a, 1969 a and b), in particular, has emphasized the close association between alpine-type ultramafic rocks and associated gabbros and has advocated an origin for these rocks from a mafic magma by crystal accumulation process. Relict cumulate texture has been cited as an additional evidence in favour of cumulus process. Textural differences between stratiform and alpine-type peridotites have been explained as resulting either from the intrusion of an unconsolidated, settled crystal mush and/or from solid state deformation of cumulates (Thayer, 1960, 1967 a, 1969 a and b).

Alpine-type peridotites in New Zealand have also been interpreted as products of crystal settling from mafic magma (Challis, 1965). Whereas Thayer regarded alpine-peridotites as mantle cumulates, Challis regarded them as products of differentiation in crustal sub-volcanic magma chambers. Smith (1958) had earlier proposed that alpine-peridotites may originate by remobilisation of ultramafic portions of stratiform mafic intrusions. 77

Mt. Lightning peridotites, like other alpine type peridotites, differ from peridotites of stratiform intrusions in many ways. Contact metamorphism and chilled border zones, which are principal features of stratiform complexes, are absent at Mt. Lightning. Also, compositions of individual minerals (olivine and orthopyroxene), determined by the author, show restricted ranges compared with those obtained for the same minerals from stratiform-type peridoti tes (see Section 2. 32). Therefore, it is unlikely that Mt. Lightning peridotites represent remobilised ultramafic portions of a stratiform mafic intrusion.

Perhaps, the best evidence in favour of or against crystal accumulation theory can be obtained from textures and s true ture s . Cumulate texture has not been encountered in Mt. Lightning peridotites. However, due to later deformation and serpentinization any original cumulate texture of these rocks would have been destroyed (Davies, 1971).

Mt. Lightning peridotites also lack any evidence of cryptic or rhythmic layering. Although rhythmic layering like cumulate texture may not be preserved, variation in chemical compositions of individual minerals would be expected if they were formed in a similar manner to the stratiform intrusions. 78

In recent years, the idea of partial melting of upper mantle to yield basaltic magmas and a refractory ultramafic residue has become increasingly accepted. Alpine-type peridotites are believed to represent the refractory residue, while associated gabbros and basalts are thought to have originated from the partial melt product {Green, 1969, 1970).

Green and Ringwood (1967) developed a model to explain the partial melting process operating in the upper mantle. Their model requires a peridotitic upper mantle composition known as "pyrolite", which is capable of generating about 30% by volume of Hawaiian type olivine- rich tholeiitic magma. The residue will consist of predominant olivine and lesser enstatite. Green and Ringwood (1967) suggested that diapiric movement of a "pyrolite" body from low velocity zone to higher levels will result in partial melting within the diapir due to a decrease in pressure. The low velocity zone in upper mantle exists between 70 and 200 km depth (Green, 1972).

Green (1970) suggested that ultramafic complexes the. of different types may originate from processes operative I\ below mid-ocean ridges. He described the relationship between the partial melt and the residue at various 79

stages during the upward movement of a "pyrolite" body from the low velocity zone. For instance, at 25 to 30 kbar pressure and at temperatures between 1250° and 13S0°c the "pyrolite" body will consist of olivine+ orthopyroxene (with about 4% Al2o3) + clinopyroxene (with about 6% A1 2o3) + minor garnet and 1 to 2% hydrons nephelinite liquid. Compositions of crystalline fractions and that of liquid will change continuously with diapiric ascent depending on the amount of partial melting and pressure-temperature conditions. As the diapir moves to lower pressure areas (at about 70 km depth) the initial garnet peridotite will convert to a

spinel peridotite at about 1300°c. Al2o3 contents of pyroxenes will increase; thus, orthopyroxene will now

contain about 6% A1 2o3 and clinopyroxene about 8% A1 2o3• Partial melting will continue until the proportion of basaltic liquid becomes large enough to segregate from the peridotite residue. The liquid fraction presumably moves rapidly to higher levels while the residue lags behind. Initially the peridotite diapir will be at a temperature above that of the enclosing upper mantle and may cool to the temperature of the environment in the absence of external forces. However, due to the lower viscosity of the peridotite diapir, it may intrude at higher levels, if horizontal stress is applied. Such peridotite would be expected to show structures and textures indicative of solid flow, various signs of deformation and recrystallization or cataclasis or both. 80

England and Davies (1973) favour the idea that ultramafic tectonites (ie, noncumulus ultramafic rocks) of eastern Papua were formed as a refractory residue during partial melting of primitive mantle. Menzies and Allen (1974) also attribute the formation of noncumulus harzburgites of Troodos, Cyprus, and those of Othris, northern Greece, to the same process. According to them, noncumulus harzburgites represent depleted upper mantle produced by partial melting of aluminous upper mantle peridotite.

2.57 Origin of Mt. Lightning Ultramafic Rocks:

The theory of partial melting of "pyrolite" material whereby ultramafic rocks are formed as refractory residues, seems appropriate for Mt. Lightning peridotites. The difficulties involved in explaining the origin of Mt. Lightning peridotites as products of ultramafic magmas or as crystal accumulates from mafic magmas have been outlined above. Features of the Mt. Lightning peridotites believed by the author to be of special significance are:

i) The rocks display deformation and recrystallizat ion textures which are similar to textures in some mantle-derived xenoliths (see Sub-Section2.31) and which are compatible with solid-state flow in the upper mantle, although such textures considered in isolation might also be interpreted 81

as products of metamorphism in some crustal conditions.

ii) The restricted Mg-rich compositional range of olivine in Mt. Lightning peridotites (see Sub Section2.32) is to be expected for bodies that represent refractory residues resulting from partial melting process (Ringwood, 1975).

These features have influenced the author to favour the view that Mt. Lightning peridotites are residual mantle materials. The problem of origin and emplacement of such peridotites, however, is bound up with the nature and genesis of the rock association of which it forms a part and with the geological evolution of the terrain in which it cocurs. These matters are considered in Chapter 7. 82

2.6 SERPENTINIZATION OF COOLAC ULTRAMAFIC

ROCKS:

Controversy still exists regarding the process of serpentinization. Whether serpentinization involves volume change or not remains debatable despite arguments by many workers (Thayer, 1966, 1967 b; Page, 1967 b). Not only is it difficult to determine whether serpentini zation takes place at constant volume or at constant composition but it is also difficult to calculate actual chemical changes involved unless the relationship between chemical changes and volume changes that accompany the process is clearly understood (Greens, 1967).

Coleman (1966) attempted to calculate the gains and losses of various elements during serpentinization of , a dunite to a pure antigorite, assuming firstly, a constant volume process and secondly, a volume expansion process. Losses in FeO, MgO, Cao, MnO, Co and Ni contents and gains in A1 2o3 and H2o contents were recorded in both processes. However, the former process, where volume is assumed to be constant, involved a loss of Sio2 while the latter process involved gains in Sio2 and Fe2o3 • Coleman also noted that losses in FeO and MgO are greater for the constant volume process than for the volume expansion process.

Lack of extensive iron and magnesium metasomatisrn 83

in the vicinity of Mt. Lightning may indicate that the volume expansion process was perhaps more important than the constant volume process during the serpentinization of Mt. Lightning peridotites.

Coleman and Keith (1971) concluded from the chemical data of Burro Mountain ultramafic rocks that serpentinization was accomplished by introduction of water and removal of only Cao, all the other components remaining unchanged. These conclusions were based on the constancy of the RO'/Sio2 ratio over a broad range of serpentinization, where RO' represents the molecular sum of the amounts of MgO, total Fe as FeO, Cao, MnO and

NiO reduced by the molecular amounts of A1 2o3 and cr2o3 (Coleman and Keith, 1971, p. 318). These authors favour the idea that expansion accompanied serpentinization.

For Mt. Lightning peridotites, which represent a range in serpentinization from 28 to 100%, the RO'/Sio2 ratio varies from 1.57 to 1.73. This restricted range in RO'/Sio2 ratio may reflect a variation in primary mineral contents, in which case it is possible that Sio2 , FeO (total Fe as FeO) and MgO contents remained unchanged during progressive serpentinization (Coleman and Keith,

1971, p. 319). Alternatively, the variation in RO'/SiO2 ratio may indicate movements of Sio2 , FeO (total Fe as FeO) and MgO, as suggested by Thayer (1966, 1967b},who favours the theory of constant-volume serpentinization. 84

As mentioned earlier, due to lack of recognizable iron and magnesium metasomatism in the area under study, serpentinization at Mt. Lightning may have been accomplished by a volume expansion, the volume expansion being accommo dated by associated faults and fractures.

Studies of serpentinization indicate that reactions may be constructed to explain the nature of serpentiniza tion process (Page 1967 a; Coleman, 1966, 1971; Coleman and Keith, 1971). Some of these reactions show that brucite accompanies serpentine minerals as an end product of alteration. Since brucite was not encountered in

Mt. Lightning rocks, the following unbalanced reactions, which do not involve brucite, may be considered for

Mt. Lightning serpentinites:

(1) Olivine (Mg2sio4 ) + Enstatite (MgSio3 ) +

H2o ~ Serpentine Mg 3si2o5 (OH) 4 •••••• • • • . (Coleman, 1966).

(2) Olivine (Mg 2sio4 ) +H2o+ co2 ~

Serpentine Mg 2 si2o 5 (OH) 4 + Magnesite

(Mgco3 ) ••••.•.••. (Coleman, 1966).

2+) . (3) Olivine (Mg, Fe 2sio4 + Enstatite 2 (Mg, Fe +) Sio3 + H2o+ 02 ~ Chrysotile 2+> s· (Mg, Fe . 3 1 20 5 (OH) 4 + Lizardite F 2+ (Mg, e I Fe3+)3 Si2o 5 (OH) 4 + Magnetite 85

Awaruite (FeNi) •••• (Coleman, 1971).

Reactions (1) and (3) may be applicable to harzburgitic rocks. Minor amounts of awaruite have been recorded by previous authors from the Coolac serpentine belt (Golding, 1966, 1969; Ashley 1973). Reaction (3) may be applicable to minor dunitic rocks in the area. The nature of serpentine minerals produced during the process of serpentinization depends on factors such as temperature, oxygen fugacity, H2o - activity, co2 - activity, compositions of primary minerals and many others.

Recent studies indicate that low temperature meteoric and connate waters are responsible for the formation of lizardite-chrysotile assemblage (Barnes and O'Neil, 1969; Wenner and Taylor, 1971). Wenner and Taylor (1971) attribute the formation of antigorite to the reaction of peridotite with deep-seated metamorphic waters.

Estimation of physical conditions of formation of serpentinites at Mt. Lightning can be made from the mineral assemblages. Coleman (1971) suggested that lizardite-chrysotile assemblage form in the temperature

range 100°c to 300°c, while antigorite represents temperatures in excess of 3oo 0 c and perhaps as high as sso 0 c. Since lizardite and chrysotile are the two main serpentine species encountered at Mt. Lightning, most of 86

the serpentinization process is believed to have taken place between 100°c and 300°c. Rare antigorite has been identified only in highly schistose serpentinite in tectonic zones. Therefore, fo:anation of antigorite in Mt. Lightning ultramafic rocks may have been facilitated by shearing stress. Golding (1966, 1969) suggested that fo:anation of antigorite in serpentinized dunite and harzburgite to the north of Mt. Lightning may be related to the intrusion of gabbros or associated late magmatic fluids, but at Mt. Lightning there is little or no evidence of similar intrusions.

Pressure estimations of the serpentinization process at Mt. Lightning can only be made by indirect methods. Metamorphic rocks of blueschist facies are sometimes associated with alpine type peridotites, eg, at Port Macquarie, N.S.W., and at Burro Mountain, California. A pressure range of 6 to 9 kbars is indicated by blueschist metamorphism (Taylor and Coleman, 1968). In the area under study, no blueschist mineral assemblage has been recognized in the country rocks associated with serpentinized ultramafic rocks. Therefore, serpentinization of Mt. Lightning ultramafic rocks may have taken place at pressures less than 6 kbars.

It is possible that serpentinization of Mt. Lightning ultramafic rocks may be continuing today. Recent studies of inter alia Barnes et al (1967), Barnes 87

and O'Neil (1969) and Campbell (1975), suggest that serpentinization can be a low temperature, present-day process. Campbell (1975) concludes that alpine-type peridotites are subject to at least two stages of serpentinization process, an early stage and a present-day stage. He believes that if unaltered olivine is brought in contact with H2o and co2 , near surface present-day serpentinization will take place. 88

CHAPTER 3

S P I L I T E S

3.1 INTRODUCTION:

The occurrence of spilites in association with sedimentary and ultramafic rocks, as found at Mt. Lightning, is by no means uncommon. It is widely accepted that spilitic rocks are an integral part of an ophiolite association. Indeed a large volume of chemical data, that are available for these rocks, has recently led many authors to focus their attention on spilitic rocks in an attempt to understand the tectonic setting of the ophiolite association. This will be discussed in Section 3.8.

Next to ultramafic rocks, spilite is the most abundant rock type in the area mapped by the author. The term "spilite" has been variously defined in the 89

literature (see Vallance, 1960). Vallance (1974a) has suggested spilite be used as a group name "for those rocks which are analogous to basalts in thei.r mode of occurrence and broad fabric elements, but differ from basalts in consisting largely of mineral phases of the greensch 1st' f ac1es • type ••••...II Cann (1969) suggested a similar definition for Carlsberg Ridge spilites. Mt. Lightning spilites also satisfy such a definition. 90

3.2 GENERAL STATEMENT:

Spilites occur to the west of the ultramafic mass in the area and belong to the Honeysuckle Beds. The Honeysuckle Beds are believed to be of Middle Silurian - basal Devonian age (Ashley et al, 1971), and form undula ting hills of low relief that flank the ultramafic rocks. Close to the contact with ultramafic rocks spilites are interbedded with narrow lenses of greywackes, phyllites and cherts. The relative volume of sedimentary rocks increases westward until the Honeysuckle Beds become predominantly sedimentary. Ultramafic rocks in the area are invariably separated from the sedimentary rocks by spilites except at Adjungbilly Valley to the south of Mt. Lightning where phyllites abut sheared serpentinites.

The contact between spilites and ultramafic rocks is extremely variable and complex, and is believed to be tectonic or faulted. To the north of the Murrumbidgee River sheared serpentinites and spilites show inter tonguing relationships. Minor pockets of trondhjemite outcrop sporadically along this zigzag contact. On the southern side of the Murrumbidgee River sheared serpenti nite is in contact with both variolitic as well as massive spilite. Dykes or lenses of rodingites, albitites, trondhjemi tes, chlori te rocks and tremol.i te :rocks are abundant at or near the contact. Or.e pocket of varioli tic spilite, about 200 m long and up to 80 m wide occurring 91

to the west of Haystack Creek, appears to be enclosed within serpentinites, and the contact between the two rock types is ill-defined, often brecciated and is complicated by the presence of numerous rodingites and trondhjemites. This pocket of variolitic spilite with associated minor rock bodies is believed to be a co~posite inclusion within the sheared serpentinites.

Two distinct types of spilites can be recognized - a variolitic type and a massive fine grained basaltic type, the former being subordinate to the latter in abundance. Occurrences of variolitic spilites are confined to junctions with sheared serpentinites at Haystack Creek and at Ellamatta Creek. As mentioned earlier the Haystack Creek occurrence forms an inclusion within the ultramafic rocks while the Ellamatta Creek variolites grade into the massive non-variolitic type. Variolitic spilites usually outcrop as small blocks up to 2 m long and 1 m across. Massive varieties, on the other hand, tend to form larger bouldery outcrops (Fig. 21 , p 9 3 ) up to 4 m long and 3 m wide. The relatively smaller outcrops of variolitic types may be due to their occurren ce in a tectonic zone, where pre-existing masses have been sliced up and/or brecciated.

Variolitic spilites er variolites are relatively conspicuous in the field due to the prominence of the varioles which attain a maximum size of 1. 5 cm in diameter 92

and contrast in colour with the rock matrix on weathered surfaces. In fresh samples both varioles and the matrix are greenish grey (Fig. 22 , p 9 3 ) while in weathered material varioles become pale yellowish brown in contrast with the darker matrix (Fig. 23 , p 93 ) . Massive spilites are greenish grey to dark grey when fresh but weathered specimens are coated with brown ferrugincus material. Fig. 21. Field photograph of a large bouldery outcrop of massive spilite from the Mt.Lightning area.

Fig.22. Sawn surface of a hand specimen of a variolitic spilite. Specimen No: 2/9.

Fig.23. Weathered surface of a variolitic spilite specimen, Specimen No: 2/7. 93 94

3.3 PETROGRAPHY:

About 50 thin sections cut from randomly collected spilite samples were studied. Most of the samples come from the area south of Murrumbidgee River while the rest wc.,-Q. collected from outcrops immediately north of the River.

Mt. Lightning spilites consist principally of plagioclase (predominantly albite), epidote - clinozoi site, chlorite and relict pyroxene with minor amounts of sphene, ilmenite, tremolite-actinolite, carbonate minerals and quartz. Olivine, prehnite and purnpellyite are absent. The presence of potash feldspar is doubtful since in extremely fine grained varieties it is difficult to be certain that all the feldspars are plagioclase.

However, low K2o contents of these rocks (see Section 3.6, p 113) indicate that appreciable amounts of potash feldspar are absent. The mine:.:-al assemblage of Mt. Lightning spilites is in close agreement with the usually diverse mineralogy recorded for spilites elsewhere (Vallance, 1960, 1969b; Amstutz, 1968}. 95

3.4 MINERALOGY:

3.41 Albite:

One of the essential differences between spilites and basalts is that alkaline feldspar is characteristic of spilite, albi te being the most common type present. (Vallance, 1960). Mt. Lightning spilites are no exceptions. Albite is the predominant feldspar; only on rare occasions has oligoclase been identified. In fine-grained varieties of Mt. Lightning spilites feldspar crystals are usually clear. Clear feldspars have also been recorded in fine-grained spilites from Nundle (Vallance, 1960). Some feldspar crystals in coarser- grained (doleritic) spilites at Mt. Lightning contain irregular inclusions of epidote suggesting replacement of an originally more calcic plagioclase by albite (Dewey and Flett, 1911). Battey (1956) reported inclusions of chlorite flakes in feldspars of New Zealand spilites. Although chlorite is abundant in Mt. Lightning spilites inclusions of chlorite in feldspars are absent.

Feldspars in Mt. Lightning spilites show a compositional range from An4 to An8• As mentioned before, oligoclase (up to An16 ) is rare. Using a 4-axis Universal Stage, compositions were determined for feldspar sections which are normal to both (001) and (010) cleavages, and show albite twin lamellae on (010). 96

Extinction angles were determined in such sections and a graph (shown in Deer, Howie and Zussman, 1966, p 333) was used to determine Ab contents.

The optic axial, angle (2V) of albite grains in Mt. Lightning spilites varies from 80 to 84 and the optic sign is always positive. A low temperature structure is thus indicated from this optical data (Fig. 24, p 98 ). Low temperature albite is, in fact, characteristic of many reported spilites (Vallance, 1960).

Chemistry of Feldspars:

Two albite grains occurring in association with relict clinopyroxenes were analysed using the electron rnicroprobe technique. For each grain two analyses were made - one at the margin of the grain and one in the centre. These analyses and their averages are shown in Table 8 , p 99 • Compositions determined from chemical analyses are in good agreement with those obtained from optical methods. There is little variation in composi tion between the margin and the centre of a grain. K2o contents are consistently low. Cao shows a slight incre ase {from 1.:2.2. to i.:t9 weight%) from the margin to the centre in one specimen (Anal.la&b, Table 8, p 99 ). Whether this change in Cao content is due to the presence of lime-bearing submicroscopic inclusions or reflects the compositional variation in the original feldspar is not 97

known.

3.42 Epidote Group Minerals:

Monoclinic members of the epidote group (epidote clinozoisite) are extremely common in Mt. Lightning spilitic rocks. Rarely they comprise up to 70% by volume of the rock. More commonly they constitute up to 20% by volume.

Epidote minerals occur as disseminated grains in the groundmass. They also occur as the predominant constituent in veins and in amygdules. Such diversity in their distribution has been noted in spilites elsewhere (Vallance, 1960). The grain size of epidote minerals is also quite variable. In veins and vesicles the grains are in general coarser than those in the groundmass. They attain a size of up to 1 mm in length in vesicles and veins, while in the groundmass they rarely exceed 0.2 mm in length.

Iron-poor orthorhombic zoisite is absent in Mt. Lightning spilites although it is abundant in associated rodingites. Ferriferous epidote, which shows variegated high interference colours and has a negative optic sign, is by far the most common type present. Clinozoisite, which is optically positive and shows first order grey/white interference colours (often masked by anomalous blue colours) occurs only as a vesicle filling 98

-40°------, ', -so• ' ' ' ' ' \ -60° \ ' ~, -70° 1\ I \ High 2V : 1\ Tempuature

-so• I ' \ ' ' \ 90° \ \ / / / /' so• I I ,,,,,, , _ _J.-1--

10·------0 20 40 60 80 100 Mol. per cent anorthih

Fig.24. Graph showing the variations in 2V with composition for high temperature and low temperature-plagioclases (curve from Smith,J.R., 1955-56). Plagioclases from Mt.Lightning spilites are shown by dots. 99

TABLE 8

ELECTRON MICROPROBE ANALYSES AND STRUCTURAL FORMULAE OF PLAGIOCLASE GRAINS FROM MT. LIGHTNING SPILITES

(in weight per cent)

1 2 a b Average a b Average

Si02 65.83 66.12 65.98 64.82 65.04 64.93

Al 203 19.35 19.61 19.48 21.42 21.57 21.53 Cao 1.22 1.29 1.26 1.54 2.02 1. 78 Na2o 12 .11 11.88 12.00 10.12 9.93 10.03 K20 0.16 0.16 0 .16 0.12 0.09 0.11

Total 98.87 99.06 98.02 98.65

Structural formulae: 32 (o)

Si 11. 759 Si 11. 569 Al 4.091 Al 4.516 Ca 0.246 Ca 0.343 Na 4.155 Na 3.468 K 0.043 K 0.021 z 15.850 z 16.085 X 4.444 X 3.832

Ab 93.5 Ab 90.5 An 5.5 An 9.0 Or 1.0 Or 0.5

a = grain margin b = grain centre

1. Specimen 4/41 2. Specimen 4/46

Refer to Table 10, p 1.11 for sample data. Analyses by the author. 100

associated with epidote, chlorite, quartz and sometimes calcite.

3.43 Chlorite:

Like the epidote minerals, chlorite is also extremely variable in its distribution. It occurs as finely fibrous aggregates in the groundmass, in vesicles and in veins. Little rounded pools of chlorite, similar to those reported by Battey (1956) for New Zealand occurr ences and by Vallance (1960) for Nundle spilites, are also common in Mt. Lightning spilites (Fig. 25, p 101). Battey, on the basis of textural evidence, regarded the chlorite as a primary mineral in New Zealand spilites. On the other hand, Hughes (1973) suggested that the sub calcic augite of original basaltic rocks is usually completely replaced by chlorite in spilites. Chlorite replacing olivine, pyroxene or hornblende is also reported (Vallance, 19 60).

Chlorite usually constitutes 15 to 30% by volume in Mt. Lightning spilites. Individual grain size is difficult to estimate because of its tendency to form granular aggregates. In plane polarized light chlorite usually shows feeble pleochroism from pale green to colourless. In polarized light both anomalous brown and anomalous blue colours are common. However, when chlorite shows normal interference colours, the bire- Fig.25. Photomicrograph of a massive spilite showing subrounded pools of chlorite in a groundmass containing mainly plagioclase (greyish white) and minor epidote (golden yellow). Specimen No: 4/47. Crossed polars. Frame length: 1.7mm.

Fig.26. Variolitic spilite showing plumose aggregates of albite (greyish white and yellowish white) and radiating needles of clinopyroxene containing magnetite (black). Specimen No: 1684. Crossed polars. Frame length: 2.1mm. 101 102

fringence is usually 0.00G. Vallan~e (1960) has listed the various types of chlorite that have been recorded in spilites and concluded that the composition of chlorites commonly encountered ranges from si2 _5A1 1 _5 to si3 _5Al0 _5 (for a formula with 18 o OH), total Fe/Fe+ Mg values from nearly zero to nearly unity and

Fe2o3 up to at least 12.5 wt.%.

3.44 Pyroxene:

Fresh pyroxene does not occur in every spilite sample from Mt. Lightning. It is more common in variolitic than in massive types. In variolitic spilites pyroxenes usually occur as long slender prisms and also as radiating acicular crystals in association with albite. On the other hand, pyroxenes in massive spilites take the form of subhedral grains or short prisms.

Pyroxene in Mt. Lightning spilitic rocks is usually fresh and colourless in plane polarized light. 2V ranges from 45° to s2° and the extinction angle (ZAC) varies from 43° to 48°. The optic sign is positive. Judging from optical data pigeonite is not present in Mt. Lightning spilites. Occurrences of pigeonite in spilites have been reported from North Borneo and Southern Urals (cited in Vallance, 1960), but as pointed out by Vallance pigeonite rarely occurs in spilite. 103

Electron microprobe analyses of 3 pyroxene grains from Mt. Lightning spilites are shown in Table 9 ,p 106'. Electron microprobe scans across grains show that these pyroxenes are chemically homogeneous. The compositional range may be expressed as ca39 _1 _ 39 _5Mg 49 _8 _ 50 .8

Fel0.2 - 11.2° In other words these pyroxenes are augites according to the classification of Poldervaart and Hess (1951). Pyroxenes of similar composition have been reported from Nundle spili tes by Vallance (1969b), except that the Nundle pyroxenes are moderately titani ferous types. Analysed pyroxenes from Mt. Lightning spilites show low Tio2 contents ranging from0.30 to0.48wt%.

A further discussion of pyroxene compositions is given in Section 3. &pp 134-136.

3.45 Other Minerals:

Tremolite-actinolite is present in accessory amounts in some Mt. Lightning spilite samples. It is usually pale green to colourless with indistinct pleochroism and occurs as fibrous disseminated grains in the groundmass. Near the contact_between spilites and serpentinites, and close to rodingites, tremolite- actinolite becomes more abundant. Small pockets of arnphibole rock containing more than 70% by volume of tremolite-actinolite have been encountered in Haystack Creek close to the ultramafic mass. 104

Quartz has been reported from many spilitic rocks (see Vallance, 1960). At Mt. Lightning a few samples (?andesitic) contain fine grained quartz in the groundmass associated with albite. In these samples quartz rarely exceeds 5% by volume. However, veins and vesicles that penetrate the Mt. Lightning .spilites, commonly contain quartz together with epidote, clinozoi site, chlorite and carbonate.

Carbonate minerals are present mainly in veins and vesicles, but are less common than other minerals. They rarely occur interstitially in Mt. Lightning spilites. Treatment with dilutedHCl has indicated that calcite is the predominant carbonate mineral in the vesicles. However, other carbonate minerals such as, dolomite, ferriferous carbonates, manganese-bearing carbonates and aragonite, have been recorded in addition to calcite in spilites elsewhere (see Vallance, 1960, p 30).

Sphene is a common accessory in the variolitic types, but is less common in massive varieties. It occurs in the groundmass as tiny anhedral grains showing typically high relief and high birefringence. 105 TABLE 9

ELECTRON MICROPROBE ANALYSES AND STRUCTURAL FORMULAE OF CLINOPYROXENES FROM MT. LIGHTNING SPILITES

(in weight per cent)

1 2 3

Si02 52.20 50.79 50.24 Ti02 0.30 0.41 0.48 A1 2o3 3.14 3.36 4.06 FeO* 6.83 6.41 6.31 MnO 0.36 0.53 0.24 MgO 17 .11 17.38 17.62 CaO 18.66 19.07 18.85 Na 2o 1.01 1.12 1.05

Total 99.61 99.06 98.85

Structural formulae: 6(0)

Si 1.923} 2.00 1. 888} 2.00 1.808] Aliv 0.077 0.112 o.~73 1.994 Alvi 0.059" 0.036" Ti 0.009 0.012 0.013"' Fe2+ 0.211 0.199 0.190 2.038 2.069 Mn 0.011 0.017 0.007 Mg 0.939 0.963 0.945 1.943 Ca 0.737 0.761 o. 727 Na 0.072, 0.081 0.074.. atoms % Ca 39.1 39.5 39.0 Mg 49.8 50.1 50.8 Fe2+ 11.1 10.4 10.2

* total Fe was determined as FeO 1. Specimen 4/41 2. Specimen 4/44 3. Specimen 4/46 Refer to Table 10 , p 111 for sample data. Analyses by the author. 106

3.5 TEXTURE:

Mt. Lightning spilites show a great variety of textural features. As previously mentioned the rocks can be divided into two main categories on the basis of macrotexture - a variolitic type and a non-variolitic or massive type.

In variolitic spilites, varioles are nearly spherical in shape with diameters of 5 to 15 mm. The varioles consist of radiating crystals of feldspar together with slender and curved threadlike pyroxene crystals (Fig. 26, p 101). Granules of ilmenite or leucoxene are often associated with feldspars and pyroxe nes in the varioles (cf. Battey, 1956). The term

"plumose growth" has been applied to such textures.

Battey (1956) noted plumose outgrowths of feldspars and pyroxenes springing from tips of a prominent feldspar lath or microlite in New Zealand variolites. Varioles in Mt. Lightning spilites show similar plumose outgrowths.

The dark green/grey coloured dense matrix that occurs between varioles contains chlorite, epidote, sphene, opaques and glass, chlorite being the most common cons ti tuen t. This matrix is extremely fine grained and resembles the groundrnass of a basalt. In some variolites closely packed aggregates of varioles, consisting of two or more coalesced varioles (cf. Furnes, 1973), make up 107

the entire mass of a portion of the rock as has been described by Harker (1935, p 181). As a result, the proportion of dense interstitial matrix is greatly reduced in such samples.

Textural similarities between spilites and basalts have long been reco9nized. Non-variolitic or massive spilites at Mt. Lightning resemble basalts in textural features both in hand specimens and under the microscope; some coarser varieties are similar to dolerites (Fig. 27, p 108). Microphenocrysts of albite, relict pyroxene and epidote in a finer grained groundmass often produce a porphyritic texture. Glomeroporphyritic texture formed by seggregation of feldspar laths is less frequently observed. Epidote and chlorite usually fill spaces between plagioclase laths forming intersertal fabric in these rocks. Mineral lineation or flow texture has not been encountered in Mt. Lightning spilites.

Pale grey to greenish grey coloured amygdu~.es are common in the massive varieties, but ara absent in the groundmass of variolitic types. Epidote-clinozoisite, chlorite, quartz and less frequently calcite make up the cavity-filling minerals. In these amygdules epidote clinozoisite and calcite, when present, commonly exhibit a fan-shaped, a feather-like or a radial arrangement

(Fig. 28, pl08 ). Quartz, on the other hand, occurs as single discrete grains or as aggregate of grains, while Fig.27. Thin section of medium to fine grained(?doleritic) massive spilite showing intersertal fabric with epidote (high interference colours) filling in spaces between plagioclase laths (grey and white), Specimen No: 4/70A. Crossed polars. Frame length: 2.5mm.

Fig.28. Amygdaloidal massive spilite showing radial arrangement of epidote-clinozoisite (variegated interference colours) associated with quartz (white) in the amygdules. Specimen No: 4/67. Crossed polars. Frame length: 2.1mm. 108 109

chlorite forms patchy fine grained aggregates. Unfilled cavities or vesicles have not been observed in these spilites. Veins up to 1 cm wide occasionally appear and are filled by the same minerals as those mentioned above, although chlorite tends to be less frequent in veins than in arnygdules • 110

3.6 WHOLE ROCK CHEMISTRY OF MT. LIGHTNING SPILITES:

Whole rock analyses of 20 spilite samples are listed in Table lQ,p 111. It is evident from these analyses that Mt. Lightning spilites show a wide range of chemical compositions. Variations in mineral constitution and in their respective modal percentages have already been mentioned. Such heterogeneity in these spilites will certainly be reflected in their chemical compositions. For example, a sample containing a large concentration of albite will have a high Na 2o content. Amstutz (1968) has pointed out that there is a certain amount of bias involved in selecting samples for analyses. As a result such conclusions as "Na- enrichment" in spilites, could be due to a "human error".

The chemical analyses of Mt. Lightning spilites show that major oxides vary over ranges roughly as follows:

Sio2 10% cao 8.5%

Al2o 3 5.5% Na 2o 4% Fe2o 3 2% TiO2 1%

FeO 3% K20 1% HO+ MgO 7% 2 2%.

All the other oxides vary over a range of less than 1% TABLE 10

1. Variolitic spilite. Specimen No: 2/6, GR 242765 Coolac 1:50,000 Sheet. 2. Variolitic spilite. Specimen No: 2/7, GR 243764 Coolac 1:50,000 Sheet. 3 . Variolitic spilite. Specimen No: 2/8, GR 238761 Coolac 1:50,000 Sheet. 4. Variolitic spilite. Specimen No: 2/9, GR 240764 Coolac 1:50,000 Sheet. 5. Massive spilite. Specimen No: 4/41, GR 241759, Gundagai ' 1:50,000 Sheet. 6. Massive spilite. Specimen No: 4/42, GR 242752, Gundagai 1:50,000 Sheet. 7 . Massive spilite, Specimen No: 4/43, GR 245755 Gundagai 1:50,000 Sheet. 8. Massive spilite. Specimen No: 4/44, GR 238760 Gundagai 1:50,000 Sheet. 9 • Massive spilite. Specimen No: 4/46A, GR 232755 Gundagai 1:50,000 Sheet. 10. Massive spilite. Speecimen No: 4/46B, GR 221762 Gundagai 1:50,000 Sheet. 11. Massive spilite, Specimen No: 4/47, GR 228763 Gundagai 1:50,000 Sheet. 12. Massive spilite. Specimen No: 4/48, GR 215750 Gundagai 1:50,000 Sheet. 13. Massive spilite. Specimen No: 4/49, GR 215752 Gundagai 1:50,000 Sheet. 14. Massive spilite. Specimen No: 4/50, GR 211763 Gundagai 1:50,000 Sheet. 15. Massive spilite. Specimen No: 4/65, GR 220763 Gundagai 1:50,000 Sheet. 16. Massive spilite. Specimen No: 4/67, GR 240755 Gundagai 1:50,000 Sheet. 17. Massive spilite. Specimen No: 4/71, GR 216768 Coolac 1:50,000 Sheet. 18. Massive spilite. Specimen No: 4/72, GR 220765 Coolac 1:50,000 Sheet. 19. Massive spilite. Specimen No: 4/8, GR 215770 Gundagai 1:50,000 Sheet. 20. Massive spilite, Specimen No: 4/9, GR 215767 Gundagai 1:50,000 Sheet. .11.1 TABLE 10

CHEMICALCOMPOSITIONS AND NORMS ·op SPILITIC ROCKSFROM MT. LIGHTNINGAREA

l 2 3 4 s 6 7 8 9 10 11 12 13 . · 14 15 16 17 18 19 20

Si02 54.62 51.82 57.34 55.16 50.10 48.21 51.22 52.72 51.43 55.96 49.57 52.92 50.00 47.27 48.01 51.75 57.69 50.86 52.86 53.02 Ti02 1.31 0.80 0.64 0.79 0.69 1.14 0.96 0.62 0.39 0.28 0.82 1.08 0.80 1.02 0.41 0.70 0.98 0.77 0.82 0.66 Ali3 15.63 15.33 16.11 15.67 14.82 15. 72 16.52 13.86 15.13 11. 74 17.22 14.79 16.23 15.50 15.12 14.69 14.71 15.. 77 16,34 16.32 Fe2o3 1.27 2.16 2.20 1.22 1.81 3.47 2.12 1.66 2.21 1.09 1.86 2.38 3.49 2.64 3.32 1.89 2.34 2.70 2.17 2.30 FeO 4. 14 5.20 4.07 4.28 5.93 4.82 5.90 6.16 6.72 3.94 6.00 5.04 5.12 6.73 5.69 5.52 3.17 6.08 5.65 5.60

MnO 0.24 0.32 0.12 0.04 0,35 0.41 0.12 0.07 o. 72 0.04 0.28 0.12 0.16 0,41 0, I 7 0.68 0,09 0,09 0,09 0.03

MgO 6.43 7.06 5.00 5.53 6.67 9,06 8.16 7.63 7.02 6.80 12.21 6.64 9.13 8.23 8. 13 6.69 6, 13 6.21 4.02 4.?o cao 5.67 6.32 6. 11 7.01 12.84 9.26 8.92 10.86 10.07 12 .43 12.21 10. I 7 8.56 10.40 10.26 9.80 5.82 8.79 13.82 14.10 Na20 6.18 5.89 6.02 6.40 3.30 3.26' 4.07 2.65 3.70 3.30 2.90 2.10 1. 88 3.50 6.07 3,84 5.82 5.06 1.92 1.85 K20 0.88 0. 18 0.49 0.06 0.12 0.93 0.21 0.06 0,04 0.25 0.09 0.53 0.62 0.21 0.39 0 .13 1.13 0.04 o.os 0.03 P205 0.08 0.22 0.04 0 .13 0.14 0.09 0.17 0.06 0.06 0.05 0.18 0.14 0.20 a. 13 o. 16 0.13 0.11 0,14 0,06 o.os H2o+ 3.07 3.18 2.11 2.25 2.64 2.51 2.28 2.59 3.27 3.93 2.55 2.62 3.42 3.17 2.38 3.34 2.07 2 .16 1.93 1.98 H2o- 0. 71 0.31 0.16 0.13 d.18 0.33 0.14 0.21 0.15 0.22 0.12 0.21 o. 13 0.11 0.12 0.22 0.40 0.24 0.22 0.20 Co2 0.08 0.26 0.10 0.38 0.09 0.06 0.06 0.10 0.08 0.10 0.08 0.27 0,39 0.07 0.42 0.06 0.11 0.10 0.09 0.12

Total 100.31 99,03 100.51 99.05 99.68 99.27 100.83 99.23 100.99.100,11 .. 99.50 .. 99.01 100.13. 99,39 100.65 99 . 44 100 • S 7 99. 01 100. 04 99.96

Norms (CIPW) CIPW Notms Q - - 1.13 0,00 - - - 4.73 - 6.32 - 9.52 4.84 - -- 1.06 - 9,82 9.03 or 5.20 1.06 2.90 0.35 0,71 5.49 1.24 0.18 0.24 1.48 0.53 3.13 3.66 1.24 2.30 0.77 6.68 0.24 0.30 0.18 ab SO.SI 48.23 Su.~2 54.06 27.91 27.57 34.42 22.41 31.29 27.91 21.47 17.76 15.90 26.97 22.61 32.48 49.22 40.80 16.24 15.65 • ne 0.79 0.86 - 0.04 - - - -. -. ~ 1.66 - -. 1.43 15.56 - - 1.08 - - an 12.32 14.87 15. so 13. 86 25.27 25.52 26.19 25.84 24.56 16.49 33. 71 29.37 34.29 25.97 12.87 22.47 10.69 20:21 35.82 36.41 di 11.81 10.78 11.19 15.85 29.96 15.44 13.31 21.83 19.97 35.47 20.10 14.80 3.50 19.64 27.56 20.19 13.26 17.69 26.02 26.42 hy - - 11.89 - 2.15 1.85 8.04 17.48 12.73 5.86 -. 15.16 26,56 .. - 15.30 11.44 - 4.64 6.23 ol 10.90 14.00 - 8.48 6.40 13.01 9.79 - 4.51 - 21.10 - - 14.64 10.32 0.17 - 10.67 - - mt 1.84 3.13 3.19 1. 77 2,62 5.03 3,07 2.41 3.20 1.58 2.70 3.45 5.06 3.83 4.81 2, 74 3.39 3.91 3.15 3,33 il 2.49 1.52· 1.22 1.5U '. 11 2.17 1.82 1.18 0.74 0.53 1.56 2.05 1.52 1.94 0.78 1. 33 1.(16 1.46 1.56 1.24 ap 0.19 0.51 0.09 0,30 0.32 0.21 0.39 0.14 a .14 0.12 0.42 0.32 0.46 0.30 0.37 0.30 0.25 0.32 0.14 0.12 cc 0.18 0.59 0.23 0.77 0.20 0.14 0.14 0.23 0.18 0,23 0.18 0.61 0.89 0,16 0.96 0 .14 0.25 0.23 0,20 0.27 H2o 3.78 3.49 2.27 2.38 2.82 2.84 2.42 2.80 3.42 . .4.15 .. :l.67., 2.,83 ~- }.,55-._ ,) .• 28 2.40. 3.56 2.47 2.40. 2.15 2.18 Fe0*(total Fe as FeO)/ MgO 0.82 1 .01 1.21 0.97 1.13 0.88 0.96 1.00 1.24 0.72 0.63 1.08 0,90 1.11 1.07 1.08 0.86 1.37 1.89 1.63 Na2o/ K2 0 7.02 32.72 12.29 106.67 27.50 3.51 19.38 44.17 92.50 13.20 32.22 .3.96 3.03. 16.67 15.56 29.54 5,15 126.50 38.40 61.67 . ·,• .• .. ·,·.

Analyses by the autl,or. 112

by weight. In short, these spilites have a wide range of major element distribution. Nevertheless, it is noteworthy that some elements sho•\1 cow.;iderzble variations from one sample to another while others tend to remain constant. For instance, silica varies from 47.27 wt.% to 57.69 wt.% while alumina values (in wt.%) are not far removed from an average value of 15.36. Only one sa~ple

(No.4/46 B) gives a comparitively low Al2o 3 value

( 11. 7 4 wt. %• ) •

Most spilites at Mt. Lightning show a Sio2 content similar to that of basalts. Only a few correspond to andesite with regard to Sio2 content (eg., samples 4/71 and 2/8). Other chemical characteristics of Mt. Light ning spilites include consistently low values of K2o

( < 1. 14 wt.%) and TiO2 ( "'- 1. 32 wt.%) as well as high

Na 2o;K2o ratio.

Cao values of these spilites merit special mention. In general variolitic spilites contain less lime than massive spilites as can be seen from analyses in Table 10, p 111. The first four analyses (Nos.l to 4) are those of variolites while the rest represents massive spilites. It is significant that certain rodingite bodies consisting mainly of hydrated calcium aluminium silicates, occur associated with variolites at Haystack Creek. Nothing definite can be said, however, about other elements as far as the two different textural types 113

of spilites are concerned.

The average oxide values of 20 Mt. Lightning spilites together with those of 92 spilites given by

Vallance (1960), and those of alkali and tholeiitic basalts of Nockolds (1954), are listed in Table 11, p 114 for comparison.

Compared with Vallance's average spilites the average Mt. Lightning spilite is similar in Al2o 3 and Na2o contents, but differs in having distinctly lower Tio2, K2o and co2 values. It also has more lime, silica and magnesia and less total iron content than Vallance's average s pi li te. Vallance (1960) pointed out that lime~ rich spilites are not necessarily rich in co2 . Data for some Mt. Lightning spilites which are rich in Cao contents confirm this opinion.

Compared with Nockolds' basalts the average

Mt. Lightning spilite has considerably less Tio2 , K2O, FeO, and P2o 5 and more Na 2o and combined H2o contents. As far as silica and magnesia are concerned the average Mt. Lightning spilite is somewhat similar to Nockolds' average tholeiite.

The average FeO* (total Fe as FeO)/MgO ratio (1.02) of Mt. Lightning spilite is distinctly lower than those of ·Vallance's average spilite (1.87), and Nockolds' tholeiitic (1.83) and alkali (1.21) basalts. Therefore, 114

TABLE 11

AVERAGE CHEMICAL COMPOSITIONS OF SPILITES AND BASALTS

(in weight per cent)

1 2 3 4

SiOz 52.13 49.65 50.83 45.78 Ti02 0.78 1.57 2.03 2.63 Al 2o3 15.36 16.00 14.07 14.64 Fe 2o3 2.22 3.85 2.88 3. 16 Fe0 5.29 6.08 9.00 8.73 Mn0 0.23 0.15 0.18 0.20 Mg0 7.07 5.10 6.34 9.39 Ca0 9.67 6.62 10.42 10.74 Na2o 3.97 4.29 2.23 2.63

K20 0.32 1.28 0.82 0.95

Pzo5 0.12 0.26 0.23 0.39 HO+ 2.67 2 3.49 0.91 0.76 H2o 0.23 coz 0.15 1.63

1. The average Mt. Lightning spilite (the average of Analyses 1 to 20. of Table 10 , p 111).

2. The average spilite (Vallance, 1960).

3. The average tholeiitic basalt (Nockolds, 1954).

4. The average alkali basalt (Nockolds, 1954). 115

sundius' (1930) claim that spilites have higher Fe/Mg ratios than basalts is not valid for Mt. Lightning spili tes. Vallance (1960) noted similarity in Fe/Mg ratios between tholeiitic basalts and many spilitic rocks. Mt. Lightning spilites, on the other hand, are closer, although not quite similar, to the average alkali basalt of Nockolds as far as Fe/Mg ratio is concerned (see Fig. 29 , p 116 ) •

Plots of normative Qz-Hy-O1-Di-Ne for analysed Mt. Lightning spilites are shown in Fig. 31, p 117 . It is clear, even from only 20 analyses that these spilites have a wide range of normative compositions like common basalts (see Fig. 32 , p 117 , for comparison}. Diversity in. normative types of spilites have already been reported by Yoder (1967) who concluded that nonns of spili tes "run the gamut of the normative limits of normal basalts". Vallance (1974a) in comparing the norms of Nundle spilites with those of normal basalts reported that Nundle spilites tend to show more variety in normative Di than basalts but otherwise the normative ranges of both are of the same order. Mt. Lightning spilites do not display any greater variety in normative Di than that of normal basalts.

Macdonald and Katsura's (1964) plot of weight per cent SiO2 against the weight per cent total alkali (Na 2o + K2O) has often been used to distinguish Fig.29. Diagram showing variation in Na 2o+K2o -total Fe-MgO in spilites and average basalts. • Mt.Lightning spilites . x The average Mt.Lightning spilite • Average spilite (Vallance,1960)

0 Average tholeiitic basalt (Nockolds,1954) e Average alkali basalt (Nockolds,1954)

Fig.30. Plots of normative feldspars of spilites ...... Mt.Lightning spilites • Other spilites (data from Yoder,1967) 116

• • • . • ,' • •• • • e e,• • •

F . M (MgO} (Feo• = total Fe as FeO)

•• • • • • ..:...... • • • • , •••.. •• • ...... • • . "• • .. . •• Fig.31. Plots of normative Ne-Di-01-Hy-Qz for spilitic rocks from Mt.Lightning.

Fig.32. Plots of normative Ne-Di-01-Hy-Qz for basalts (data from Vallance,1974c). -- · 117

• • • • • • • • •• • • • • • • • • • • .. • • •• • • • •• 118

Hawaiian alkali basalts from tholeiites. The boundary which separates the two rock types can also be used as a dividing line to separate rocks having Hy from Ne in their norms (Yoder, 1967). Yoder clai~ed that spilites plot for the most part in the field of Hawaiian alkali basalts, but, as can be seen from Fig. 33, p 119, more than half the analysed spilites from Mt. Lightning lie in the tholeii tic field. Therefore, the use of an alkali-silica diagram to determine the relationship of spilites to either tholeiitic or alkali basalts is questionable.

By using normative An-Ab-Or data for spilites (compiled mainly by Vallance, 1960), Yoder (1967) emphasized the more sodic nature of the normative feldspar of spilites, and remarked that the modal feldspar in these rocks is not in agreement with the n.::;rmative feldspar. Mt. Lightning spilites also have a moderate to high normative An content (up to 36.41), although albite is the predominant plagioclase member encountered in modal analyses. In Fig. 30, p 116, variation in normative feldspars for Mt. Lightning spili tes is compared with that for spilites shown in Yoder's (1967, p 271) diagram. The two groups of spilites show similar variation except that Mt. Lightning spilites tend to have less variety in normative Or. This is not unexpected in view of the characteristically low K2o content of the Mt. Lightning spilites. 119

/ /

0 ,..N 7 z • / . . + • 0 N :-: / M • C (I" I.. II 4 • / ~ • 4J • • .c /. • U.· 'ii • 31: • / • • ~3/ ' ~~~ ~f)' ~ /

40 45 50 55 60 Welght per cent Si02

Fig.33. Plots of Na20+K2o against Sio2 for Mt.Lightning spilites (after MacDonald and Katsura.1964). 120

It is reasonable to expect that the range of variation in trace element composition of spilites will be of the same order as that for the major elements. Trace element data for 20 samples from Mt. Lightning, which were also analysed for major elements, are given in Table 12, p 122. It is clear that most of tr.e trace elements analysed show a wide range of variation from one sample to other. For example er varies over a range of nearly 300 parts per million. Approximate ranges of other elements are as follows:

Ti 5,500 ppm y 20 ppm

V 150 ppm Zr 115 ppm

Ni 150 ppm Nb 8 ppm Cu 70 ppm Ba 120 ppm

Rb 6 ppm La 20 ppm Sr. 280 ppm

The average trace el~ment. concentrations of 20 Mt. Lightning spili tes togeth•.!T with the average values of four Carlsberg Ridge spilites (Nicholls and Islam, (1971) are shown in Table 13, p 123. The average trace element contents of oceanic tholeiitic and alkali basalts from Engel et al (1965) are also included in the same table for comparison.

Compared with average Carlsberg Ridge spilites, average Mt. Lightning spilites are notable for their 121

distinctly high Sr content. They also have higher er, Ni, Ba and La concentrations. Nb and Rb contents are similar to the average Carlsberg Ridge spilite while v, Cu and Zr values are lower in the average Mt. Light ning spilite. 122 TABLE 12

TRACE ELEMENT CONTENTS OF SPILITIC ROCKS FROM Mr; LIGHTNING AREA

(in ppm)

1 2 3 4 5 6 7 8 9 10

Ti 8438 5106 4225 5222 3945 8152 5816 3860 3012 2869 V 219 247 255 156 163 198 205 172 140 104 Cr 323 327 370 298 622 576 346 374 421 300 Ni 150 180 142 240 160 208 229 312 175 234 Cu 85 72 36 65 60 80 85 64 102 77 Rb 4 7 4 3 8 8 2 4 7 3 Sr 277 201 196 142 345 222 323 276 304 162 y 22 35 32 27 17 19 15 27 31 37 Zr 86 81 80 175 60 63 72. 108 62 114 Nb 7 4 8 9 4 5 6 4 7 2 Ba 5 30 19 44 126 10 3 24 52 3 La 17 4 5 12 24 10 2 16 7 4

11 12 13 14 15 16 17 18 19 20

Ti 5207 -7420 5067 7209 3549 4762 6125 5156 5220 4986 V 235 244 226 236 170 182 193 220 175 190 Cr 189 490 173 536 614 320 242 319 380 534 Ni 155 134 152 210 216 256 203 305 267 148 Cn 82 75 54 110 so 46 94 92 70 70 Rb 3 5 5 7 2 2 3 3 3 2 Sr 420 174 257 210 168 270 219 287 242 312 y 24 28 26 29 27 30 27 21 29 21 Zr 79 110 116 107 74 70 66 82 92 102 Nb 3 3 4 10 9 9 5 7 8 7 Ba 23 12 24 5 3 11 21 16 8 15 La 18 4 5 3 10 1.7 3 18 6 14

Samples as in Table 10 , p 111 Analyses by the author, and Dr. B. Chappell, A.N.U. 123

TABLE 13

AVERAGE TRACE ELEMENT CONCENTRATIONS IN SPILITES AND BASALTS

(in ppm)

1 2 3 4

Ti 5267

V 201 350 292 252

Cr 388 315 297 67

Ni 204 164 97 51

Cu 73 102 77 36

Rb 4 4 10 33

Sr 250 48 130 815

y 26 43 54

Zr 90 136 95 333

Nb 6 5 30 72

Ba 22 · 10 14 498

La 10 4 80 90

1. The average Mt. Lightning spilite (the average of Analyses 1 to 20 of Table 10 , p 111) •

2. The average spili te from the Carlsberge Ridge at 5!.2°N (The average of Analyses 5 to 8 in Table 5 of Nicholls and Islam, 1971).

3. The average oceanic tholeiite (Engel et al, 1965).

4. The average alkali basalt (Engel et al, 1965). 124

3,7 MT. LIGHTNING SPILITES AS SECONuARY ROCKS:

There is considerable confusion regarding the origin of spilitic rocks. Opinion is sharply divided as to whether spilites are primary or secondary rocks and a number of theories have been proposed for the genera

tion of spilites. A summary of the various schemes proposed is not attempted here as it is well documented

in Vallance (1960, 1969b} and in Amstutz (1968).

The present writer favours a secondary

(metamorphic) origin for Mt. Lightning spilites. The greenschist facies mineralogy, the evidence of pseudo morphic replacement of originally calcic plagioclase by albite (see Sub-Section 3.-41}, the low temperature optics of albite and its association with relict augite suppert a secondary origin for Mt. Lightning spilites. Moreover,

the association of spilites with deep-seated or mantle

derived harzburgitic rocks in the area studied fits well with the theory of metamorphic origin (Vaugnat, 1974).

Perhaps a further check on whether or not

Mt. Lightning spilites are metamorphic can be made by

using the method of Hughes (1973). Hughes has shown by

plotting total alkali content against potash/total alkali

ratio of common volcanic rocks that they plot in a field

which he calls an "igneous spectrum". Hughes is of the

opinion that most spilites and keratophyr~s lie outside 125

Most of the so called II igneous spectrum". the analysed spilites from Mt. Lightning plot outside the "igneous spec t rum " (Fig. 34, p 126) and are thus metamorphic according to Hughes' scheme. It is also interesting to note that a majority of Mt. Lightning spilites lie outside the "spilite field" drawn by Hughes. This is due to extremely low K2o values of Mt. Lightning spilites (see Table 10, p 111) .

Having decided that Mt. Lightning spilites are secondary in origin the problem then is to identify the parental material from which these spilites were derived.

Three different methods have been attempted by the present writer to solve this task:

i) Using major element chemistry.

ii) Using trace element chemistry.

iii) Using clinopyroxene composition. 126

14 .------I I ' 12

I I 10 I 0 I N C1l z 8 + 0 N • • ~ • •• 6 ~------' ' 'i.., .:~;~,~- -~~ •• 2 -.• ,,'

10 30 so 70 90

Fig.34. Plots of (Na20+K20) against (K20/K20+Na20)xl00 of Mt.Lightning spilites(dots). The field enclosed by solid lines with dashed ends represent the 'igneous spectrum', while that enclosed by dashed lines includes analyses of various spilites (see Hughes,1972). 127

3.8 DETERMINATION OF SPILITIC PARENTAGE:

3.81 Using Major Element Chemistry:

As shown above, normative compositions of Mt. Lightning spilites fail to reveal any definite relationship with either tholeiitic or alkali basalt types. Spili tes in general shm.., such a wide variety of normative compositions (Yoder, 1967; Vallance, 1974a) that one must seek their affinities from other evidence.

Amongst the major elements Si cannot be used as a reliable guide to original composition, because of its erratic variation in Mt. Lightning rocks. In view of evidence for Ca-metasomatism associated with the variolitic types in the area, Ca is also considered to be unreliable.

Investigations of various spilitic rocks suggest that Al perhaps shows least variation (Vallance, 1965;

Smith, 19 6 8) • In general, the Al2o3 values in Mt. Lightning spilites are reasonably close to the average value of 15.36 wt.% except for one sample (see Section3.6).

However, comparison of average A1 2o3 content of Mt. Lightning spilites with those of average basalts of Nockolds' (1954) does not conclusively indicate whether they were derived from tholeiitic or alkaline type. On the other hand, the average Mt. Lightning spilite 128

has similar alumina content to t.l1~t of .Mans'.Jn and

Poldervaart's (1964) average alkali-basalt (A1 2o 3 = 15.44 wt.%), but is lower than that of Manson and

Poldervaart's average tholeiite (A1 2o 3 = 16.26 wt.%).

In his discussion°lhe tectonic setting of ·" Troodos ophiolite complex Miyashiro (1975a) remarked that Fe, M~ and Ti are generally immobile elements while Na and Kand sometimes Ca and Si are relatively mobile in metasomatic alteration. Although both total Fe and Mg in Mt. Lightning spili tes show wide variations, it is, nevertheless, interesting to note that the FeO*/MgO ratio (total Fe as FeO) in these rocks is always less than 2.0 and is similar to that of abyssal tholeiites from mid-oceanic ridges (Miyashiro, 1975a). 2\lso, most of the Mt. Lightning spilites fall in the abyssal tholeiite field of Miyashiro and Shido (197S) in the FeO* versus FeO*/MgO diagram (Fig. 35, p 129).

On the basis of their study of altered pillow lavas from Cape Verde Islands Paepe et al (1974) emphasised the constancy of Tio2 and P2o 5 contents. They have shown that these two oxides are hardly affected by low grade metamorphism and by incipient weathering, and are therefore, according to these authors, the most diagnostic oxides for distinguishing between tholeiitic and alkali basalts. Paepe et al (1974), however, have not stated any limiting values for these oxides which 129

'Abyssal tholeiite ff~ld ' ( Miyashira and Shido, 1975) .

8 • •

•

2------r------,------1 3

------

Fig.35. Plots of FeO * (total iron as FeO) versus FeO * /MgO of Mt.Lightning spilites. The field of abyssal tholeiites shown is after Miyashiro and Shido(1975). 130

would enable the distinction between the two basalt types. Tio2 and P2o 5 values of Mt. Lightning spilites are consistently low, being less than 1.32 and 0.23 weigh_t percentages respectively. These low Ti02 and P2o5 values as well as low K2o values and high Na/K ratio (Section3.6) recorded for Mt. Lightning spilites are in accordance with those published for ocean floor basalts and ocean ridge tholeiites (Engel et al, 1965; Kay et al, 1970; and Cann, 1971).

From the above discussion it may be said that Mt. Lightning spilites have certain peculiarities, such as low K2o, Tio2 and P2o 5 contents as well as high Na 20/K2o ratio, which characterize ocean-floor basalts and mid-ocean ridge tholeiites. The term 'oceanic tholeiite' has often been used to designate ocean-floor basalts (see Engel et al, 1965). Hcwe,rer, rec1..':mt investigations suggest that although some oceanic rocks are definitely tholeiitic in character, there are transitional and alkaline varieties in some areas (Kay et al, 1970; Cann, 1971). In fact, Cann suggested that the term 'oceanic tholeiite' should not be used as a general name for ocean-floor basalts.

Therefore it may be concluded that major element data for Mt. Lightning spilites fail to conclusively prove that these rocks were derived from tholeiitic, alkali basalt or transitional types. Also, it should 131

be emphasised that effects of alteration and metamorphism (and metasomatism) on the major elements must be consider ed before any method involving these elements can be applied fruitfully to the task of determining the original character.

3,82 Using Trace Element Data:

Recently, attempts have been made to distinguish volcanic rocks of different tectonic settings on the basis of trace element data (Pearce and Cann, 1971, 1973). Some authors, including Gast {1968), Hart {1971) and Nicholls and Islam (1971), consider trace element patterns as being more useful than major element data in tracing the origin of ocean-floor rocks.

Pearce and Cann (1971, 1973) have used Ti, Zr, Y, Sr and Nb to classify various magma types correlated with tectonic setting. According to them these elements, except for Sr, are generally stable particularly to zeolite and greenschist facies metamorphism (Pearce and Cann, 1973, p 298). Since greenschist facies mineral assemblages are ubiquitous in Mt. Lightning spilites, Sr has not been considered in the present discussion.

In Figures 36 and 37 , p 133, trace element data for Mt. Lightning spilites are plotted on a Ti-Zr-Y triangular diagram and on a Ti-Zr binary diagram 132

respectively. These plots clearly show that these of spili tes fall in three different fields/\ Pearce and Cann (1973) . namely their ocean-floor basalt., low K- fi.e..lcLs. tholeiite and calc-alkali basalt A The Ti-Zr-Y diagram indicates that a majority of the analyses occupy the ocean-floor basalt field, while the Ti-Zr diagram shows that a majority lie in the calc-alkali basalt field. In other words, these plots yield conflicting results, and it seems unlikely that the parent magma from which Mt. Lightning spilites are derived would have such diversity in composition. Vallance (1974b) has also reported conflicting results obtained from Ti-Zr-Y ratios for spilites elsewhere, while Miyashiro (1975) commented that these trace elements in altered volcanic rocks from Troodos fail to distinguish between mid-oceanic ridge and island arc varieties.

Pearce and Cann (1973) from their observation that alkali rocks have greater concentrations of Nb with respect to Y or Zr than tholeiitic rocks, suggested that Y/Nb ratio can be used to distinguish between alkali and tholeiitic basalts. This ratio, accor 3. On Fig.36. Discrimination diagram using Ti,Zr,and Y(after Pearce and Cann,1973). Within-plate basalts plot in field D, · ocean-floor basalts infield B, low-potassium tholeiites in fields A and B,and calc-alkaline basalts in fields C and B. o Mt.Lightning spilites.

Fig.37. Discrimination diagram using Ti and Zr (after Pearce and Cann,1973). Ocean-floor basalts plot in fields Band D, low-potassium tholeiites in fields A and B, and calc-alkali basalts in fields C and B. o Mt.Lightning spilites. 133

·--...... •", _.:

10,000 Ti (ppm)

5,000

so 100 150 200 Zr (ppm) 134

this basis a tholeiitic parentage seems likely for Mt. Lightning spilites.

Amongst Ti, Zr and Y, Zr is considered to be least flexible in its behaviour (Vallance, 1974b). The striking similarity in Zr concentration between oceanic t..holeiite of Engel et al (1965) and Mt. Light- ning spilite has already been noted. This also favours the idea of an original tholeiitic magma.

Again a word of caution is necessary before these trace elements can be successfully employed to seek parentage of altered rocks. Vallance (1974b) has produced evidences of anatase, brookite and zircon occurring as authigenic phases in diagenetically adjusted sediments while Y has been detected in authigenic carbonates and epidotes. Therefore, the irr.mobili ty of these trace elements is certainly questionable. In fact, epidote minerals are abundant in rodingitic bodies associated with Mt. Lightning spilites.

3.83 Using Clinopyr:xene Composition:

It is clear from previous discussions that whole rock chemistry of spilites is of questionable genetic value. Vallance (1969a) suggested that study of relict clinopyroxenes in spilites may provide clues as to the nature of the parental material. More recently 135

Vallance (1974a} emphasised the compositional similarity between basaltic pyroxenes and spilitic pyroxenes. Vallance (1974b} also remarked " .•...• pyroxene norms are the most useful discriminants".

Three clinopyroxenes from Mt. Lightning spilit es were analysed using the electron microprobe technique (Table 9 , p 104 ) . However, electron microprobe analysis does not permit calculation of norms. Electron rnicroprobe analyses of clinopyroxenes can, nevertheless, be used to determine the pristine character of altered rnafic rocks (Hashimoto, 1972). By plotting the atomic proportions of Al against those of Ti (after Kushiro,

1960) and weight percentages of Sio2 against those of A1 2o3 (after LeBas, 1962) from electron microprobe analyses of clinopyroxenes, Hashimoto has shown that Palaeozoic greenstones from Tamba District, South-West Japan, were derived from various magma types while greenstones from the Mikabu area originated from tholeiitic magma.

The Ti and Al atomic contents of all three clinopyroxenes from Mt. Lightning spilites (Table 9 , page 104} when plotted on a diagram after Kushiro (1960}, lie in the tholeiitic field (Fig. 38, p 137). Thus a tholeiitic parentage for Mt. Lightning spilites is indicated by these pyroxenes. However, it is not possible to be certain of the nature of the tholeiitic magma. 136

Two of the host rocks in which these pyroxenes occur are olivine normative while the other host rock is quartz normative.

The analysed clinopyroxenes from Mt. Lightning spilites when plotted on a LeBas'-type diagram (A1 2o 3 wt% against Sio2 wt.%) also indicate that they lie in the non-alkaline field (Fig. 39 , p 137 ) However, it is not possible to distinguish between tholeii tic, high alumina and calc-alkaline parent rocks from LeBas' diagram.

Thus the Al2o 3 and Sio2 weight percentages of these pyroxenes, al though according with a tholeii tic pa1.·ent is not conclusive. Fig.38. Plots of Al against Ti (after Kushiro,1960) of clinopyroxenes from Mt.Lightning Spilites(Aot0 ~~cl f.,.o,... G.ro"-f 1. ,,-o.:li-'J\j"l:~s (cros-se..s).

Fig.39. Plots of Si02 against Al 2o3 (after LeBas,1962} of clinopyroxenes from Mt.Lightning spilites(dot~ a:A.o\. fro"1'"\ (

Alk. basalt Cfeldsparthofd• :bearing> ,,,.---- .....

Al Alk. basalt Cfeldsparthold free> \ .,\ • \ • \ \ 04 \

)( \ Tholeffte .. X

005 0·10 TI

X X

• Tholellte

Alkali basalt ,, ,, ,,.,,,.,

Perat_kall basalt

43"-.....L.-2.1.-.....a...___,,4~------6~...... ~8----~10 At2"3 Wt" 138

3~ ORIGIN OF MT. LIGHTNING SPILITES:

The tectonic environment of Mt. Lightning region and the similarity in rock association at Mt. Lightning to that of oceanic lithosphere (see Chapter 7) suggest that Mt. Lightning spilites might be closely related to basaltic provinces in oceanic areas.

Although bulk chemical composition of Mt. Lightning spilites does not conclusively prove that they were derived from tholeiitic.basalts, in terms of whole rock chemistry these spilites are similar to oceanic tholeiites (see Section

3.6 ). A tholeiitic parentage is also indicated by the compositions of.relict clinopyroxenes (Sub-section 3.83). Furthemore, MT. Lightning spilites,like other oceanic spilites, lack modal biotite.

the Based on above observations, the present writer ~ favours the idea that Mt. Lightning spilites wree derived from abyssal tholeiites. The origin of oceanic tholeiites has been discussed by a number of authors including Aumento (1969), Gast(l968), Kushiro(l968,1973), Kay et al. (1970), Green(l969,1971,1973) and Karig(1971). Collectively it has been suggested that variation in bulk chemistry of ocean-floor basalts is the result of different degrees of partial melting of upper mantle material and the segregation of magma at different depths. The partial melting model of upper mantle, as has been discussed previously in section 2.5 , to produce tholeiitic magmas is believed to offer satisfactory explanation 139

for the origin of Mt; Lightning mafic-ultramafic association.

If the pyrolite composition is accepted as the composition of the upper mantle then, as Green{l973) suggested, the oceanic crust may result from the separation of the upper mantle pyrolite into an overlying basaltic layer and an underlying residual peridotitic lithosphere. A 20-30% melting of the upper mantle pyrolite will produce basalts of olivine tholeiite composition. Such a high degree of partial melting may account for the large volume of mafic rocks present in the area. However, whether or not the spilites and the associated ultramafic rocks at Mt. Lightning represent complementary products of the same partial melting event is difficult to prove.

The present writer is of the opinion that the greenschist facies mineralogy of Mt. Lightning spilites was produced by low grade metamorphism of abyssal tholeiites. Lack of fresh tholeiites in the area studied, however, does not permit the study of chemical changes involved during metamorphism.

Vallance(1974a) suggested that both local hydro thermal and regional burial metamorphism could convertsolid basalts to spilites. Smith's{l968) work on an Ordovician marine sequence exposed in the Central West of New South Wales clearly demonstrates that regional burial metamorphism of basic lavas can produce a spilitic lithology. Smith{l968) 140

noted two diverging trends in the alteration process - one leading to the development of spilitic rocks and the other producing a Ca-enriched lithology. In the area studied by the present author the occurrence of Ca-enriched Group 2 rodingites in association with variolitic spilites at Haystack Creek suggests that a somewhat similar alteration mechanism might have taken place.

The term 'ocean floor metamorphism' may also be applicable to the alteration proceas that converted abyssal tholeiites to Mt. Lightning spilites. According to Miyashiro (1973), ocean floor metamorphism is a kind of burial meta morphism whereby basalts that occur in oceanic regions are metamorphosed to the zeolite and greenschist facies and a majority of them are non-schistose, preserving their original igneous textures.

In view of the close association of variolitic spilites and Group 2 rodingites at Haystack Creek (see Chapter 4) local hydrothermal alteration process should also be considered. Recent experimental investigation by Hajash (1975) shows that reaction between seawater and basalts between 200°-soo0 c and at 500-800 bars can produce rocks similar to metamorphosed oceanic basalts. The alteration product includes, according to Hajash, such minerals as albite, trernolite-actinolite and prehnite. The abundance of albite in Mt. Lightning spilites and its presence in some albite prehnite rocks (see Chapter 4), the occurrence of tremolite actinolite both in spilites and along outer rims of Group 2 rodingites and that of prehnite in Group 2 rodingites suggest that seawater- basalt reaction might have taken place locally. 141

C H A P T E R 4

R O D I N G I T E S

4.1 INTRODUCTION:

Occurrences and descriptions of rodingitic rocks in the Coolac Serpentine Belt were recorded by Golding (1962, 1966, 1969), Ashley et al (1971), Ashley (1973) and Golding and Ray (1975a and b), but further data were essential to enable a critical reappraisal of these controversial rocks. Mt. Lightning is particularly favourable for their detailed study in view of the abundance and diversity of rodingites in the area. Such a study by the writer is reported in this chapter, together with a discussion on their origin. 142

4.2 NOMENCLATURE

The term 'rodingite' was introduced by Bell et al (1911) for coarse-grained gabbro-like rocks that penetrate serpentinites of the Roding River Valley, New Zealand, and which contain diallage, grossularite and prehnite. The term has since been extended by various authors to include rocks containing other calc-silicate minerals such as vesuvianite, zoisite and clinozoisite, as well as chlorite, with the stipulation that such rocks are intimately associated with serpentinite. Such a stipulation precludes the use of the term 'rodingite' for rocks of· somewhat similar mineralogy, such as calc silicate hornfelses, that are commonly not associated with serpentinites and the nature, classification and origin of which, in general, are far less problematic than is the case for rodingites.

The term 'rodingite'is used in this thesis to denote those rock associates of serpentinite in the Mt. Lightning area, in which calc-silicate minerals predomin ate. The writer accepts the commonly held view that an intimate field association with serpentinite is an essential attribute of rodingite but, contrary to many such views, does not accept that rodingitization is necessarily dependent on serpentinization. This matter will be discussed in the appropriate sub-section of this chapter. 143

4.3 CLASSIFICATION OF RODINGITES:

Golding (1969) proposed two main groups of rodingites in the Coolac serpentine belt as follows:

(a) Group 1 rodingites which are completely enclosed within ultramafic rocks. They contain garnet and/or vesuvianite and chlorite with or without diopside and minor tremolite.

(b) Group 2 rodingites which occur at junctions between dissimilar rocks, particularly variolitic spilites and serpentinites. These contain variable amounts of zoisi te, preh: .ni te, garnet, chlorite and sphene.

Excellent examples of both groups are to be found in the vicinity of Mt. Lightning. Detailed study by the present author not only confirms the classification of Golding, but has revealed further distinctions between the two groups. The major differences between these two groups are outlined in Table 14 , p 144 . In addition to these major differences between the two groups there are various other dissimilarities which will be discussed in the following relevant sections. 144

TABLE 14 Essential differences between Group 1 and Group 2 rodingites

GROUP 1 RODINGITE GROUP 2 RODINGITE

Mode of Occurrence: Enclosed completely within Occurs at junctions of serpentinites and serpentinized variolitic spilites and peridotites. Commonly forms serpentinites. Selvedges, tabular, dyke-like bodies. veins and pods are common, Contacts with ultramafic rocks contacts with serpentinites sharp. sharp but that with vario litic spilites is not expo sed and is probably grada tional. Mineralogy: Garnet and/or vesuvianite + Zoisite and/or prehnite + chlorite + diopside + minor chlorite +garnet+ trem tremolite-+ minor zoTsite and olite + sphene. Diopside rare accessory chromite. and vesuvianite absent. Prehnite and sphene absent. Relict Texture: Coarse-grained varieties have Variolitic appearance in gabbroic appearance and some prehnite-rich types medium to fine-grained types both in hand specimens and resemble dolerite in hand in thin sections. specimens. Relict microtextures resemble When present relict micro gabbroic, doleritic or texture resembles varioli basaltic depending on grain tic spilite. size. Bulk Chem is try: MgO/FeO* (total Fe as FeO) MgO/FeO*ratio < 2. 3. er ratio is > 2. 3. er and Ni and Ni contents are low contents are high ( > 250 ( < 38 and < 100 ppm and > 200 respectively). respectively}. Cr/V ratio Cr/V ratio is high ( > 2) • is low ( < 0 • 6 ) • 145

4.4 MODE OF OCCURRENCE:

4.41 Group 1 Rodingites:

Apart from chromitites and rarely trondhjemitic rocks Group 1 rodingite is the only rock-type that forms rock masses completely enclosed within ultramafic rocks at Mt. Lightning. It occurs within both massive serpentinized ho.rzburgite and sheared and massive serpen tini te.

Group 1 rodingites form tabular dyke-like bodies generally up to 15 m long and less than 2 m wide. The strike is quite variable. Only nineteen of thirty-one dykes measured trend parallel or sub-parallel to the general strike of the belt (NNW-SSE).

The contact between Group 1 rodingite and ultramafic rocks is invariably sharp. Dark outer shells of dense black chlorite in rodingitic tectonic inclusions reported elsewhere (Coleman, 1966, 1967; De, 1962) are absent in Group 1 rodingite bodies at Mt. Lightning. Only rarely have marginal slickensides been observed.

Group 1 rodingites appear to have intrusive rather than tectonic relationships to their host rocks. Many Group 1 rodingite bodies display one or more median joints parallel to their length (Fig. 40 , p 14 7) 146

similar to those in basaltic dykes elsewhere (Frankel, 1967). Transverse fractures also occur. These features favour an in situ intrusive origin or suggest consolida tion of magma pockets between blocks of peridotite as proposed by Golding (1966).

4.42 Group 2 Rodingites:

Golding pointed out that zoisite and prehnite rocks occur in diverse settings in the Coolac serpentine belt but are particularly well represented at Haystack Creek, Mt. Lightning. Haystack Creek follows the junction between a westerly mass of variolite, about 100 m long and 20 m wide, and serpentinites to the east.

Group 2 rodingites form dyke-like bodies or sub-rounded pods (Fig. 41, p 147) which vary in size from 0.5 m to 3 m in maximum width or diameter. The general trend of the bodies is similar to that of the ultramafic mass (NNW-SSE). Rare prehnite-rich veins are narrower, about 20 cm wide.

Group 2 rodingites sharply abut serpentinite, the junction being commonly marked by a narrow zone, about 10 cm wide, of grey coloured tremolite-actinolite -rock. The junction of rodingite and variolite, however, is gradational or obscure due to lack of exposure. At Fig.40. Field photograph of a Group 1 rodingite dyke showing characteristic median jointing.

Fig.41. A dyke-like body of Group 2 rodingite from the Haystach Creek. 147 148

three localities along Haystack Creek outcrops of these rodingites are separated from variolites by monomineralic chlorite rocks and at another locality the intervening rock consists of prehnite, albite and chlorite. The junction of Group 2 rodingite and variolite along the creek is further complicated by the sporadic occurrence of trondhjemitic rocks. Bodies of Group 2 rodingite lack longitudinal joints but transverse joints are commonly present. Group 2 rodingites are regarded as metasomatic selvedges not relict intrusive dykes. 149

4.5 PETROGRAPHY AND TEXTURE:

4.51 Group 1 Rodingites:

Some 75 specimens of Group 1 rodingites were examined for this study. All specimens are extremely tough and dense, the specific gravity ranging from 3.13 to 3.5"5' • These rocks have a reddish brown skin about 2 mm thick due to weathering and are often pitted. Fresh surfaces show these rocks to be generally pale coloured in tints of grey, white and cream. Pyroxene rich varieties are commonly speckled in green. Dyke centres tend to be more altered, poorer in relict pyroxene and paler than dyke margins. Pyroxenes are the only original constituents that are recognizable in hand specimens. Such pyroxene grains, up.to 2 x 1.5 cm in size, occur in coarse gabbroic variants (Fig. 42, pl53). The other constituents form a fine sugary or chalk-like aggregate rarely resolvable into well-defined grains or discrete minerals without the aid of the microscope.

Microscopic examination of Group 1 rodingites reveals a simple mineralogy. They consist of garnet and/or vesuvianite, clinopyroxene (+ minor tremolite), and chlorite with small amounts of magnetite. Zoisite was observed in only two samples and chromite in only one sample. The relative proportions of these constituents however vary considerably as shown in Table 15, 150

p 151. The frequently encountered mineral assemblages are the following: (i) Clinopyroxene, garnet and chlorite, (ii) Clinopyroxene, vesuvianite and chlorite, (iii) Garnet and chlorite.

The assemblage clinopyroxene, garnet, vesuvianite and chlorite was less commonly found and this suggests that equilibrium was not attained in some specimens because this assemblage showed a cross-cutting tie line relationship on an ACF diagram.

Thin sections of sub-samples across several single Group 1 rodingite dykes indicate systematic mineralogical variations from the margins towards the centres of the dykes as follows:

(i) Heterogeneous margins tend to become monomineralic at centres. (ii) Vesuvianite increases relative to garnet towards centres. (iii) Relict clinopyroxene decreases towards centres.

Some specimens of Group 1 rodingite retain unequivocal textures of precursor basic igneous rocks. Such relict textures accord with gabbroic, doleritic and basaltic parents for different samples. In these specimens garnet and/or vesuvianite occur as subhedral TABLE 15

Modal Analyses of Analysed Group 1 Rodingite Specimens From Mt.Lightning

(in volume per cent)

1 2 3 4 5 6 7 8.- 9 10 11 12 13 14

Diopside (including 22.7 30.6 33.0 26.5 20.6 - 35.6 .24 .1 19.3 28.4 31.5 tremolite alteration) Garnet 34.2 32.1 - 43.6 19.5 52.0 25.2 39.4 35.3 - 36.8 83.0 80.6

Vesuvianite 11.1 - 21.4 - 26.7 12.4 7.2 - - 36.0 5.4 - - 86.6

Chlorite 32.0 37.3 45.6 29.9 33.2 35.6 32.0 36.5 45.4 35.6 26.3 17.0 19.4 13.4

Samples as in Table 25, p 188.

,-.. V, ,-.. 152

crystals or crystal aggregates apparently pseudomorphous after plagioclase. (Fig. 43 ,pl53) whilst pyroxene commonly persists unchanged. More altered variants of

Group 1 rodingite include heterogeneous pyroxene-bearing types and nearly monomineralic variants in which a little chlorite is associated with predominant garnet and/or vesuvianite. When relict texture is not present

Group 1 rodingites usually show a porphyroclastic texture where larger grains of clinopyroxene are embedded in a granoblastic matrix composed of garnet and/or vesuvianite.

4.52 Group 2 Rodingites:

Some 40 specimens of Group 2 rodingites were examined for this study. Hand specimens of these rocks are commonly pale pink to pale grey and are commonly indistinguishable from some Group 1 rodingites. A few handspecimens are porcellanous in appearance. Often a narrow pale grey coloured (tremolite) zone is present at the margins of Group 2 rodingite bodies.

Most specimens are fine grained and grain bounda ries are indistinguishable macroscopically. Rarely, medium-grained varieties are encountered where interlocki ng grains (up to 2mm wide) produce a saccharoidal texture.

It should be noted that none of the constituent minerals could be identified with certainty in hand specimens. ~ig.42. Hand specimen of a coarse-grained gabbroic rodingite (Group 1 type) containing large grains of clinopyroxene (greenish grey) and garnet (pink). Specimen No: 3/15.

Fig.43. Photomicrograph of a doleritic Group 1 rodingite showing relict intergranular texture with laths of original plagioclase being replaced by garnet (isotropic) and clinopyroxene occupying the spaces between garnet laths. Specimen No: 3/13. Crossed polars. Frame length: 1.5mm.

Fig.44. Photomicrograph of a medium-grained (doleritic) Group 1 rodingite showing fibrous tremolite (tr) replacing diopside (di). Other minerals in the slide include garnet (gt) and chlorite (chl). Specimen No: 3/14. 153 154

Microscopic examination of Group 2 rodingites

reveals a simple mineralogy. They consist essentially of zoisite and/or prehnite with or without garnet, chlorite, tremolite-actinolite and sphene. Rarely a garnet-rich variety (garnet-chlorite rock) is to be

found. A notable feature of these rocks is almost total absence of opaque minerals. The relative proportions of the mineral constituents are given in Table 16, plSS.

These modal analyses clearly show that most specimens are monomineralic.

Since Group 2 rodingites are largely monomineralic rocks, the microtextures are similar to those of the predominating constituents. These microtextures are described in a later section (Section 4.6 ). ,_,

V'I V'I

1.2

87.0 11.8

0.6

4.8 1.

13.9

65.0

14.0

AREA

7.2

3.6

0.5

88.7

7 8

3.4 0.2

0.3

4.3

LIGHTNING

91.8

201.

MT.

6 - 5.4

5.6 0.8

1.2

1.1

85.9

29,

FROM

5 - -

5.2 5.7

2.5

86.6

cent)

Table

per

2.2

0.4 6.3 2.0 4.4

84.7

RODINGITES

TABLE

volume

1.3

1.9 0.9 5.6 0.2

90.1

GROUP

(in

Samples

3.1 2.5

0.4

12.8

80.7

8.6

1.5 0.8

0.6 0.5

88.5

ANALYSES

MODAL

actinolite

Zoisite

Garnet

Prehnite

Chlorite

Sphene

Tremolite- Albite 156

4. 6 MINERALOGY:

4.61 Group 1 Rodingite:

4. 611 Garnet:

Garnet is by far the most common constituent of Group 1 rodingites, in which it occurs associated with diopside, chlorite and vesuvianite. Commonly it forms aggregates of sub-rounded to nearly polygonal grains, 0.1

to O. 3 mm wide (Fig. 45 , p 157). Less frequently, lath-shaped garnet grains, presumably grain-aggregates pseudomorphs after plagioclase, occur (Fig. 46 , p 157 ). Some grains are clear, colourless and isotropic, others are, turbid, pale pink or grey and are weakly anisotropic. Zoning is not apparent.

Compositions of garnet in Group 1 rodingites were determined using physical properties. Refractive indices vary from 1.722 to.1.739 {Table 17,p 159).

Unit cell values range from 11.849 to 11.890 A0 , although a large majority of these values do not signi

ficantly depart from 11. 855 A0 (Table 17 , p 159 ) . A plot of R.I. values against unit cell values enables the determination of garnet composition. Fig. 4 7 , P 160, clearly shows that garnets from Group 1 rodingites are close to pure grossularite in composition. Fig.45. Photomicrograph of a monomineralic garnet rodingite (Group 1 type) showing granoblastic aggregates of garnet. Specimen No: 3/20. Ordinary light. Frame length: 4.2mm.

Fig.46. Photomicrograph of a garnet-chlorite rodingite (Group 1 type) showing laths of garnet (high relief) and chlorite. Specimen No: 3/40. Ordinary light. Frame length: 2.1mm. 157 158

Differential thermal curves confirm that these garnets are largely anhydrous. The d.t.a. curve of strongly hydrous grossularite shows an endothermic peak at 650 - 690°c and two exothermic peaks at 870° and 940°C (Deer et al, 1962 a). Heflik and Zabinski (1969) reported two endothermic peaks at 860° and 980°c for hydrogrossularite.

D.T.A. curves of garnet-rich Group 1 rodingites containing less than 10% by volume of chlorite or vesuvianite lack peaks (Fig. 48 , pl62) suggesting an anhydrous composition.

Rarely an emerald green chromium-rich variety

of garnet can be found in veinlets Group 1 rodingite penetrating chromite ore deposits. Zoning is common in this variety with a green core (r.i. = 1.82) and a

pale pink rim (r. i. = 1. 79) (Golding and Ray, 1975b).

4.612 Vesuvianite:

Apart from garnet, vesuvianite is the only Ca-Al

silicate frequently encountered in Group 1 rodingites. Commonly it forms patches, streaks and veinlets surrounded

by grossularite, chlorite and diopside. In some rodingites, vesuvianite occurs to the exclusion of grossulari te. In.others, where both grossularite and vesuvianite are present, the ratio between the two 159 TABLE 17

Refractive Index and Unit Cell Values of Garnets and Vesuviani tes from -Mt, Lightning Rodingites.

Group 1 Rodingites

§.Recimen Number R.I. Unit Cell (A0 ) 1/40 1.733 11.849 1/57 1.732 11.853 2/14 1.729 11.866 2/15 1.732 11.851 2/16 1.740 11.862 2/36 1.730 11.859 2/39 1.730 11.880 3/5 1.728 11.876 3/13 1.726 11.868 3/15 1.724 11.861 3/20 1.720 11.886 3/22 1.738 11.859 3/23 1.722 11.875 3/30 1.725 11.890 3/36 1.728 11.865

Group 2 Rodingites

2/26 1. 723 11.891 4/2 1.716 11.930 4/31 1.714 11.926

VESUVIANITE (From Group 1 Rodingites only)

Specimen Number R.I. (nmax) Unit Cell (A0 ) a C 2/34 1.718 15.573 11.860 3/15 1.720 15.602 11.852 3/36 1.713 15.596 11.871 3/40 1.714 15.611 11.855 4/19 1.710 15.548 11.864 I-' 0 °'

12·65

rodingitu

Group Group

12-45

from

cell

from values

rodingites.

Garnets unit

X • cell

taken

12-25

unit are

against

and

Mt.Lightning

(A)

12·05

index

Andraditc

77.

from

cell

indices

compositions

unit

11·85

garnets refractive

al,1962,p

member

et for

refractive

end

11·65

(a)

of The Deer Plots

11-45

Fig.47.

·65

1·90 1·851-

1·75 1•80 1·70

1 1·60

er 161

minerals is quite variable and often vesuvianite predominates over garnet towards the centre of the roding ite body.

Microscopic examination shows vesuvianite to occur in aggregates of polygonal and short prismatic grains

(up to O. 5 mm wide) • The grains are almost colourless or very pale pink in plane polarized light and commonly show anomalous brown interference colours. Normal first order grey/white interference colours are less frequently observed. Both uniaxial and biaxial grains (with 2V < 15°) have been observed, the former type being more common. The optic sign is always negative.

The refractive indices and unit cell sizes of selected samples are given in Table 17, p 159. It has been suggested that an increase in refractive indices of vesuvianite may be related to increasing amounts of iron and titanium (Deer et al, 1962 a), and

according to Treger (1956) w ~ 1. 720 indicates ~ 5 wt%

(Tio2 + Fe2o 3 + FeO). The maximum refractive index (~ in the uniaxial variety and Yin the biaxial variety) of vesuviani tes from Group 1 rodingi tes varies from 1.715 to 1.720 and chemical analyses (see below) reveal

(Tio2 + total Fe as FeO) values to be consistently well :under 5 wt.%. However, simple correlations between optical properties, cell dimensions and chemical compositions of vesuvianites cannot be made because of Fig.48. Differential Thermal Analysis curves for selected rodingite samples: A- Monomineralic garnet-rock (Group 1 rodingite, sample 3/20). B- Monomineralic garnet-rock (Group 1 rodingite, sample 3/23). c- Garnet-chlorite-rock (Group 2 rodingite, sample 4/2). D- Garnet-vesuvianite-diopside-rock (Group 1 rodingite, sample 3/15). E- Garnet-vesuvianite-diopside-rock Group 1 rodingite, sample 2/36). F- Zoisite-rock (Group 2 rodingite, sample 4/26). G- Garnet-vesuvianite-chlorite-rodingite, (Group 1 rodingite, sample 3/13). H- Zoisite-rock (Group 2 rodingite, sample 4/5). I- Prehnite-rock (Group 2 rodingite, sample 4/23). J- Prehnite from Prospect Intrusion, New South Wales. 162

·'

B C

500 600 700 800 900 1000 1100 163

extensive possibilities of ionic substitution (Ito and

Arem, 1970).

Differential thermal curves of vesuvianites from

Group 1 rodingites show an endothermic peak at ,_ 1050°c

followed by an exothermic peak at .,....._,, 1090°c (Fig. 48,

p 162). Peters (1961) noted similar thermal characteristics of vesuvianite from the Totalp-Serpentinmasse, near Davos, Switzerland, and suggested that the endothermic reaction is due to lattice disruption and dehydration while the exothermic reaction registers recrystallization to garnet, melilite and doubtful anorthite. An X-ray diffraction trace of vesuvianite from Mt. Lightning

rodingite pre-heated to 1200°c however, failed to yield any recognizable peak suggesting little recrystallization.

A d.t.a. of brown vesuvianite from a contact metamorphic aureole near Marulan, N.S.W., d8termined for

comparison \ showed a distinct endother- ' ' ' mic reaction at a much lower temperature (- 930°c) than that obtained for Mt. Lightning Vesuvianite. The refractory thermal behaviour of vesuvianites from ophio litic assemblages may be due to relative deficiencies of iron and titanium as compared with those from thermally

metamorphosed rocks (Golding and Ray, 1975).

Electron microprobe analyses of selected samples reveal that both compositionally zoned and unzoned 164

crystals of vesuvianite are present in Mt. Lightning rodingites. For example, two analyses carried out on one single grain of vesuvianite (Anal. 1 a+ b, Table 18, p 165) show a remarkable difference in MgO content between the core and the rim. Other minor differences exist in silica and total iron contents. In fact the composition of the core, which has less magnesia and slightly more silica and iron oxide, approaches that of a garnet (grossularite or hydrogrossularite, cf. Deer

et al, 1962 a, Anal. 2. p 94 , and Anal. 4 , p 105' ) • This occurrence of possible garnet cores to vesuvianite crystals may indicate that the development of vesuvianite post-dated that of grossularite in the Group 1 rodingites. A possible occurrence (? intergrowth) of vesuvianite in garnet was reported by Zabinski (1964).

4.613 Clinopyroxene:

Clinopyroxene is an important constituent of most Group 1 rodingites other than the monomineralic garnet - and vesuvianite-rich varieties. It usually forms discrete tabular grains of variable size ranging from 0.2 to 1 mm wide, and rarely up to 8 mm, in length.

Optical characters of some clinopyroxenes from Group 1 rodingites are listed below in Table 19. 2V varies from 51° to 56° and extinction angle (ZA C) ranges from 41° to 45°. Only coarse-grained varieties

were selected for refractive index ( ~ } determinations 165

TABLE 18 ELECTRON MICROPROBE ANALYSES OF VESUVIANITES FROM GROUP 1 RODINGITES

i 1 2

a b a b

- 35.46 30.91 34.81 34.94 : SiO I 2 i tr tr tr i Ti02 - 18. 72 22.16 20.43 20.36 !12°3 * 1.16 FeO 0.81 1.83 1.24 I O.ll tr tr I MnO - I !~ 2.94 tr 3.12 3.21 I 36.52 36.64 1 CaO 37 .63 35.26 I

£ 95.66 98.16 96.12 96.31

I ' . I Structural formulae . 72(ot I

Si 17 .067} 17 .938} 16.655} 16.660} 18.00 18.00 18.00 IV 18.00 Al 0.933 0.062 1.345 1.340 . AlVI 9.712 11.952 10.161 10.130 Fe2+ 0.318 0.692 0.489 0.459 \ ~ 12. 200 \ 12.644 12.865 12.883 Mn 0.058 - - - Mg 2.ll2 - 2. 215 2.294 -Ca 19.410 17 .412 18. 726 18. 724

a = grain margin b = grain centre tr = trace * total Fe was calculated as FeO. t = sub-total + structural formulae have been calculated on the basis of 72 oxygen equivalents ignoring H2o+. l, Specimen No. 2/34 2• Specimen No. 3/40 Refer to Table 25, p 188 for sample data. Analyses by the author. 166

because of separation difficulties encountered with fine grained specimens. R..I.(n13)varies from 1.675 to 1.680. These optical data accord with a diopsidic composition, but are of little diagnostic value for distinguishing members of the diopside-hedenbergite series (Deer et al,

1962).

TABLE 19

Optical characters of clinopyroxenes from Group ·1 rodingites

Optic Specimen No. Sign 2V ZAC R. I. (n13)

2/14 + 53° 42° 1.677 2/15 + 56° 44° 1.680 2/16 + 51° 44° * 3/13 + 55° 45° * 3/30 + 51° 41° 1. 680 3/36 + 56° 44° 1.675 3/37 + 51° 45° *

Refer to Table 25, p 188, for sample data. * not determined

Four electron microprobe analyses of these clinopyroxenes are given in Table 20, p 167. The samples were selected so as to cover the wide variation in grain size but no significant difference that can be correlated TABLE 20 167

ELECTRON MICROPROBE ANALYSES AND STRUCTURAL FORMULAE OF CLINOPYROXENES FROM GROUP 1 RODINGITES

(in weight per cent)

1 2 3 4 . . ..

Si02 54.39 55.21 53.10 54.30 Ti02 tr 0.11 0.16 tr A1 203 1.09 0.68 1.64 1.85 FeO* 2.62 3.53 3.84 3.38 MnO 0.17 0.11 0.10 0.08 MgO 16.51 16.07 15.69 15.80 cao 23.98 24.21 23.84 22.34 Cr2o3 1.02 0.84 0.79 0.91 Na2o 0.10 0.11 tr 0.16 Total 99.88 100.87 99.16 98.82

Structural formulae: 6(0)

Si 1.982} 2.00 1,998] 2.00 1.961]2.oo 1.992}2 _00 Aliv 0.018 0.002 0.039 0.008 Alvi 0.029 0.027 0.032 0.072"' Ti - 0.004 0.006 - Fe 2+ 0.080 0.107 0.118 0.104 Mn 0.005 0.003 0.003 0.002 > 1.984 >- 1. 979 1.988 1.961 Mg 0.897 0.867 0.863 0.868 Ca 0.936 0.939 0.943 0.878 Cr 0.029 0.024 0.023 0.026

Na 0.008., o.oo; - J 0.011... atoms % Ca 48.9 49.1 49.0 47.5 Mg 46.9 45.3 44.9 46.9 2 Fe + 4.2 5.6 6.1 5.6

* total Fe was determined as FeO tr = trace amount 1, Specimen 2/14 2 Specimen 3/36 Refer to Table 25 , p 188 for sample data. 3 Specimen 2/16 Analyses by the author. 4 Specimen 3/13 168 with the grain size is apparent from these analyses.

The range of composition may be expressed as ca47 _49 _ 49 _09

Mg 44.85 _ 46.89 Fe4.lS _ 6 _13. Therefore, these clino pyroxenes are diopsides according to the classification of Poldervaart and Hess (1951).

It is noteworthy that these diopsides are similar in composition to those in the surrounding peridotites except that the former are slightly more iron-rich and chromium-poor than the latter. Relict clinopyroxenes encountered in the nearby spilites (see Chapter 3,Sec. 3.44) have a different composition from those in Group 1 rodingites. The possibility that these rodingite dykes represent feeders to the overlying extrusives is therefore considered remote.

4.614 Chlorite:

Chlorite is an essential minor constituent of Group 1 rodingites. In thin sections fine-grained aggregates ( ~0.05 mm in diameter) of chlorite surround grains of garnet, vesuvianite and diopside, and often occupy intersertal areas between garnet euhedra. Whether or not chlorite has formed as a secondary replacement of original mafic minerals remains inconclusive. Chlorite pseudomorphing pyroxenes is not apparent and in rocks which contain unaltered pyroxene chlorite is just as common as in rocks devoid of pyroxene. 169

Chlorite in Group 1 rodingites is commonly colour less and rarely pale green in plarepolarized light. Between crossed polars it shows normal first order grey colours or, less frequently, anomalous blue/brown colours. Optic axial angles of five chlorite grains measured are very low ( < 10°), the optic sign being positive in each

case. Refractive indices (nmax ), determined in three samples, was found to be ~ 1.57 (sample 2/36A - n max = 1.568; sample 3/23 - nmax = 1.568; and sample 2/34 - nmax = 1.570). These optical characteristics accord with an iron-poor composition.

Three electron microprobe analyses of chlorite are given in TabJ.e 21, p170 (Anal. 1 to 3). These analyses clearly indicate that chlorites in Group 1 rodingites are typically low in total·iron and high in magnesia. The low total iron content ( < 4 wt.%) suggests that these chlorites are unoxidised. Since + H2o was not determined for these analyses, structural formulae were calculated on the basis of 28(0)~ Compositions of these chlorites, when plotted on a diagram

(Fig. 49, p 171) after Hey (1954), lie in the fieldsof

clinochlore, and. du.r~clani.te..

4.615 Other Minerals:

Tremolite is a minor constituent in many Group 1 rodingi tes. It replaces diopside and commonly occurs as ,.,. TABLE 21 ELECTRON MICROPROBE ANALYSES OF CHLORITES ,

1 2 3 4 5 6

Si02 32.04 31.22 30.45 30.59 29.62 28.36 * total Fe was determined as Ti02 0.0 tr. tr. 0.06 tr. 0.08 FeO Al 2o3 16. 71 18.15 20 .16 18.44 21.27 20.55 tr = trace amount FeO* 3.51 3.89 2.27 4.08 2.96 12.94 1: = sub-total MnO tr. 0.0 0.11 0.0 0.0 tr. + Since H2o+ was not determined MgO 35.42 32.85 37.01 34. 24 32.16 25.69 structural formulae have been Cao 0.0 0.10 0.08 0.0 0.11 0.0 calculated on the basis of Cr 2o3 0.10 0.08 tr. 0.0 0.0 tr. 28 oxygen equivalents, NiO 0.08 0.08 0.0 0.0 0.0 tr. ignoring H2o+. E 87.85 86.37 90.07 87.41 86 .12 87.62 Anal. 1 to 3 are those of +Structural formulae: 28(0) chlorites from Gpl rodingites. 1. Specimen 2/14. Si 5. 999}B. O s.916Js.o 5.544}8.0 s. 776}s.o 5.624}8.0 s .ss1Js.o Al 2.001 2.084 2.456 2.224 2.376 2.419 2. Specimen 3/36. Al 1.687 1. 970' 1.873 1.881- 2.387 2. 348' 3. Specimen 3/23. Ti - - - 0.007 - 0.012 Refer to Table 25 , p 188 Fe2+ 0.548 0.761 0.346 0.644 0.470 2.130 for sample data. Mn - 12.148 - 12.052 0.015 12.291 - --12.166 - 11. 981 - 12.024 Anal. 4 to 6 are those of Mg 9.885 9.276 10.042 9.634 9.101 7.534 chlorites from Gp2 rodingites Ca - 0.021 0.015 - 0.023 - and from a chlorite rock. Ni 0.012 0.013 - - -- 4. Specimen 2/20. 5. Specimen 4/2. 6. Specimen 4/6. Cr 0.016 o.oq -- .., --. .. - Refer to Table 29, p 201, for sample data. ,... -.J 0 171

Thuringite Chamositc Delessite , .A ii .>.. I I I I I I I I I I I I 10·0 I I I 1 ·0 8·0

0·9 .... :! ·5,"' C :21 ·;: > 0·8 :, "'C Q) .s::..... :, 6·0 u. 0 ... 'Cl m 0-7 :, _, "' ...ro ll."' ..."' 0 0·6 0 t- 'Cl 0, 'ii ~ ~ ::E ... +i 0·5 ii 0 4·0 Q) + 1:. fil u. u ..0 0 ,,, _, C if 0·4 u 0 ro _, >, b n:J .. a. I- ... :~ 0 0·3 .s::. I- 0. 0 !! 2·0 't:J ·.::. C .. 0 0·2 ::, ...... "' ...... 0 ·c 1: 0 :c ·c u u u C I 0·1 0 u .c ll."' ;;; ··-u• I- 0 0 4·0 5·0 5·6 6·2 7·0 8·0 Si -

Fig.49. Analyses of chlorites from Mt.Lightning rodingites plotted on a Hey-type diagram (after Hey,1954). 2+ Total Fe was calculated as Fe . 172

fringes on clinopyroxene grains (Fig. 44, p153 ). Less frequently tremolite replaces diopside along the cleavage directions of the latter. Under the microscope tremolite is easily distinguished from diopside by its fibrous habit and lower interference colours. The shape of individual tremolite grains is usually needle-like.

Zoisite although extremely common in Group 2 rodingites, is rare in Group 1 type and has been found only in two thin sections. In both sections zoisite occurs closely associated with grossularite and occurs as discrete rectangular grains (0.04 mm long) with normal

1st order grey/white interference colour. Optically these zoisites are similar to those obtained in Group 2 rodingites.

Chrome-spinel and Magnetite Opaque minerals are rare in Group 1 rodingites. Chrome-spinel has been found in only two rodingite samples (Specimen Nos. 3/13 and 3/45), where it occurs as anhedral grains up to

0.2 mm in maximum diameter. Chrome-spinel grains have darker outer rims with reddish brown cores. Magnetite occasionally occurs as inclusions in diopside grains.

It forms anhedral subrounded grains up to 0.05 mm in maximum diameter. It should be noted that accurate identification of fine-grained opaque accessory minerals in these rodingites is extremely difficult. Thus distinction between magnetite and chrome-spinel was 173

based on detectable internal reflection. Low Ti02 content of these rocks excludes the possibility of significant amount of ilmenite.

4.62 Group 2 Rodingites:

4. 621 Zoisite:

Zoisite is by far the most common constituent of

Group 2 rodingites. Considerable variation exists in its crystal form. In well-crystallized rocks, zoisite forms felted aggregates of euhedral crystals, up to 1 mm long and 0.5 mm wide, showing a decussate texture

(Fig. SO, p 174). The habit of these grains results in three principal types of sections; (i) narrow, elongate, length-fast sections parallel to cleavage (100),

(ii) broad plates parallel to (001} giving Bxa interfer ence figures and showing length-slow cleavage traces and (iii) doubly terminated, length-fast sections parallel to (010) and optic axial plane. In addition to the well-crystallized variety, zoisite forms masses of interlaced prisms or plumose acicular aggregates

(Fig. 51, p 17 4) .

Euhedral zoisite grains are generally clear, the plumose acicular aggregates are turbid. Cross fractures are common in most crystals. Zoisite in

Mt. Lightning rocks displays normal low first order Fig.SO. Photomicrograph of a zoisite rodingite (Group 2 type) containing euhedral zoisite crystals (grey and white) with intersertal prehnite (orange-yellow). Specimen No: 4/5. Crossed polars. Frame length: 2.4mm.

Fig,51. Photomicrograph showing plumose aggregate of zoisite in a Group 2 rodingite. Specimen No: 2/26, Crossed polars. Frame length: 2mm. 174 175 interference colour (birefringence< O.O08). Zoning and multiple twinning are lacking.

Other optical characteristics of Mt. Lightning zoisites include refractive index ( f3 ) ~ 1. 700 and dispersion T >v • The optic axial angle, however, is quite variable ranging from 16° to 36° (Table 22, p 176).

It is often difficult to distinguish between zoisite and clinozoisite from optical characters. In the present study orthorhombic zoisite has been disting uished from monoclinic clinozoisite using the X-ray diffraction data of Seki (1959), the diagnosti·c X-ray reflections for zoisites being d (in A0 ) =-3.67, 3.62, 3.15, 3.08, 2.72 and 2.33.

Considerable confusion still exists as regards the terminology of the orthorhombic members of the epidote group. They may be sub-divided into two groups according to optical properties: ~ -zoisite, in which the optic axial plane is parallel to (100) (Deer et al, 1962a). However, Myer (1966) proposed that the terms -zoisite and~ -zoisite be discarded in favour of ferrian zoisite and zoisite respectively. According to Myer, ferrian zoisite has> 0.14 atoms of Fe3+, optic axial plane parallel to cleavage (100) and dispersion r < v, while zoisite has up to 0.14 atoms of Fe3+, OAP normal to cleavage (100) and dispersion r > v. The Mt. Lightning 176

TABLE 22

Optical characters of zoisites and prehnites from Group 2 rodingites

Zoisite

Specimen No. p 21{ Dispersion

2/31 1.697 36° r>V 2/s 1.695 33° r>V

3/31 1.700 200 n.d.

4/5 1.692 27° n.d. 4/33 1.700 16° r'? V

n.d. = not determined

Prehnite

Specimen No. 2V Refractive Index

0(. 'Y"

2/20 64° 1.609 1.639

2/54 62° 1.615 1.640 3/81 62° 1.614 1.640

4/4 66° 1.600 1.635 4/23 67° 1.611 1.632 4/24 62° 1.610 1.635 177

zoisites have OAP normal to cleavage (100) (see Golding and Ray, 1975, Fig. 5, p 126) and two out of four analysed samples have dispersion r > v (in the other two sections dispersion was very weak). Hence, they are zoisites according to Myer's classification.

Although there are numerous records of rodingites containing zoisites chemical data on such zoisites are scanty. Zoisites in four specimens of Group 2 rodingites have been analysed using the electron micro probe technique. The microprobe analyses (Table -z-3, p17 8) clearly indicate that the zoisite crystals are composition ally zoned with respect to total iron. In samples 2/31 and 4/5 broad platy sections parallel to (001) showing length-slow cleavage traces were analysed; in each case analyses were carried out at three points on a traverse from the core to the rim of a grain. A doubly terminated section was chosen in sample 3/31 while in sample 4/33 a narrow, elongated length-fast grain parallel to cleavage (100) was selected and in each grain two points were analysed, one at the core and the other at the rim.

The electron microprobe analyses clearly indicate that zoisite crystals in Group 2 rodingites have a Fe-rich core. This compositional zoning could not be detected in the optical study. A similar trend of iron depletion towards the margins of zoisite grains has been recently recorded by Ackermand and Raase (1973) in biotite schists from Hohe Tauern, Austria.

I-' I-'

~ ~

4.254 4.254

.019) .019)

tr tr

b b

0 0

O.llf O.llf

0.13 0.13

4.124 4.124 0.097 0.097

5.903}6,00 5.903}6,00

5.846 5.846

0.82 0.82

data. data.

24.87 24.87

96.78 96.78

32.69 32.69

38.27 38.27

4 4

00 00

sample sample

4.161 4.161

centre. centre.

for for

96216. 96216.

056' 056'

tr tr

a a

0.022 0.022

0. 0.

0.16 0.16

0.038 0.038

5. 5.

grain grain

5.883 5.883

0.41 0.41

.201 .201

96.15 96.15

32.44 32.44

38.52 38.52

,p ,p

b: b:

RODINGITES RODINGITES

29 29

2 2

00 00

4.331 4.331

Table Table

GROUP GROUP

015, 015,

80716. 80716.

determined. determined.

margin, margin,

tr tr

b b

0.028 0.028

0.082' 0.082'

4.206 4.206 4.083

0.193 0.193

0. 0.

to to

5. 5.

0.09 0.09

5.830 5.830

0.11 0.11

0.65 0.65

98.37 98.37

25.81 25.81 24.62

33.55 33.55

38.16 38.16

not not

FROM FROM

grain grain

Refer Refer

was was

a: a:

3 3

o o

4/5 4/5

ZOISITES ZOISITES

OF OF

785}6.00 785}6.00

tr tr

a a

0.12 0.12

0.09 0.09

0.39 0.39

Specimen. Specimen. since since

0.019 0.019

0.018J4.344 0.018J4.344

0.018 0.018

0.0551 0.0551 33.43 33.43

25.92 25.92

37.77 37.77

4.252 4.252

0.215 0.215 5.823 5.823

5. 5.

+ +

cent) cent)

3 3

2 2

4. 4.

per per

sub-total sub-total

FORMULAE FORMULAE

formulae formulae

4.342 4.342

TA.BLB TA.BLB

3/31:, 3/31:,

E E

equivalents equivalents

022, 022,

754}6.00 754}6.00

weight weight

b b

0.028 0.028

0.17 0.17

0.246 0.246

o. o.

5. 5.

0.87 0.87

0.111' 0.111'

0.11 0.11

0.18 0.18

4.153 4.153

5.833 5.833

25.23 25.23

37.47 37.47

33.58 33.58

97.61 97.61 97.72

(in (in

STRUCTURAL STRUCTURAL

oxygen oxygen

Structural Structural

Specimen. Specimen.

AND AND

2 2

25 25

6.00 6.00

trace trace

3. 3.

4.392 4.392

of of

. .

~} ~}

• •

; ;

a a

= =

ANALYSES ANALYSES

0.065 0.065

4.218 4.218

0.015 0.015

0.27 0.27

0.12 0.12

0.56 0.56

0.019 0.019

0.075' 0.075'

0.11 0.11

0.228 0.228 5.801 5.801

basis basis

32.93 32.93

37.15 37.15

96.47 96.47

25.33 25.33

4/5 4/5

on on

tr tr

author, author,

4.362 4.362

6.00 6.00

the the

. .

MICROPROBE MICROPROBE

Specimen. Specimen.

749} 749}

by by

tr tr

calculated calculated

b b

0.019 0.019

0.251 0.251

0.121 0.121

0.14 0.14

4.203 4.203

5.824 5.824

0.09 0.09

0.94 0.94 s. s.

33.36 33.36

97.16 97.16

25.41 25.41

37.22 37.22

2. 2.

; ;

ELECTRON ELECTRON

FeO FeO

Analyses Analyses

1 1

00 00

formulae formulae

2/31 2/31

6. 6.

4.339 4.339

as as

., .,

Fe Fe

762} 762}

a a

0.13 0.13

0.238 0.238

0.019 0.019

0.28 0.28

5.836 5.836

0.22 0.22

0.0 0.0

0.056 0.056

0.037' 0.037'

4.227 4.227

s. s.

33.46 33.46

37.43 37.43

25.62 25.62

97.14 97.14

Specimen. Specimen.

total total

Structural Structural

3 3

2 2

* *

1. 1.

+ +

E E

p p

FeO* FeO*

Fe Fe

Ca Ca

Al Al

cao cao

Si0

Al Al

MnO MnO Si Si

MgO MgO

Mg Mg

Mn Mn

P2°5 P2°5 , , 179

Apart from the variation noted in Fe-content two of the zoisite samples (specimens 2/31 and 4/5) analy sed by the author show a slight increase in Mg-content towards crystal margins. In other words, a negative correlation exists between the iron and magnesium contents from the margin to the core in these two specimens. Specimens 3/31 and 4/33, however, do not show any notable difference in Mg-content between the core and the rim.

The typically low iron content ( <- 0._95 wt.% total Fe as FeO) of zoisites from Group 2 rodingites confirms the optical determinations that they are zoisites and not ferrian zoisites. ( > 0.05 wt.%) obtained from these analyses accord with those for zoisites (Myer, 1966).

4,622 Prehni te:

Except in monomineralic garnet-rich varieties prehnite is an essential constituent of Group 2 rodingites. It forms fine to medium grains (0.5 to 2 mm in length) and is frequently associated with zoisite. In zoisite- rich rodingites prehnite usually occupies the angular areas between zoisite euhedra and occasionally forms fine vein1ets (up to 1 mm wide). Prehnite-rich rodingites contain interlocking grain aggregates which exhibit diverse textures such as radiating, polygonal granular, banded and sutured (Fig. 5l, f 181) . Prehnite 180

euhedra are absent in Mt. Lightning rocks.

In plane polarized light prehnite grains appear slightly turbid and show two sets of cleavages. Effects of· deformation are well exhibited by pronounced undulatory extinction of the grains (Fig. 53, p 181). The optic axian angle (2Vy) of six samples examined varies from 62° to 67° (Table 22, p 176), a range slightly lower than that reported in Deer et al (1962a). Refractive indices of prehnite increase with increasing substitution of Fe3+ for A1 3+ (Hashimoto, 1964). Prehnites in Group 2 rodingites have o<.. < 1.616 and "'t < 1.641 (Table 22, p176), these values indicating a low iron content. However, the chemical composition of prehnite cannot be accurately determined from optical properties (Hashimoto

Ibid)

The d.t.a. curves for prehnites from Mt. Light ning show double endothermic peaks corresponding to dehydration in two stages at --. 780° and '-"" 870°c (Fig. 48 , p 162 ) and accord with that reported for prehnite by McLaughlin (1957). The intensity of the peaks depend mainly on the amounts of associated minerals (zoisite and chlorite) in the sample. Golding and Ray (1975) noted that the d.t.a. curve of prehnite from Prospect Intrusion near Sydney is notably different from that obtained for Mt. Lightning prehnites. This difference may indicate two structural varieties of Fig.52. Photomicrograph of a prehnite rodingite (Group 2 type) showing spherulitic texture. Specimen No; 4/23. Crossed polars. Frame length: 4.2mm.

Fig.53. Photomicrograph showing undulose extinction in prehnite in a prehnite rodingite (Group 2 type). Specimen No: 2/20. Crossed polars. Frame length: 1mm. 181 182

prehnite formed under different environments.

Recent studies of prehnites from rocks in prehnite pumpellyite metagreywacke facies (Surdam, 1969) and from contact metamorphic rocks (Robinson, 1973) have shown a diversity in composition from one grain to the other with some grains containing up to 30 mole per cent of the ferrian end-member ca2Fe23+si3o10 (0H) 2 . Electron microprobe analyses of Mt. Lightning prehnites, however, do not reveal any pronounced variation in iron content (see Table 24 -, p 183). In fact, three grains analysed in each of two samples (Nos.4/4 and 2/20) show a variation of only 0.5 wt.% of total iron oxide. Prehni- tes in Group 2 rodingites are typically iron poor and are therefore close to the end-member ca2Al2si3o10 (oH) 2 in composition.

4. 623 Chlori te:

In Group 2 rodingites chlorite usually occurs as a minor constituent associated with prehnite and garnet and, less commonly, with zoisite. It is extremely difficult to recognize chlorite in hand specimens except in garnet-rich rocks where it forms grey patches in an otherwise white or cream coloured specimen. In thin sections chlorite occurs either as subrounded aggregates of fine granules ( ~ 0.05 mm in diameter) in prehnite and zoisite rocks or as fibrous clusters up to 2 mm in length.

CX) CX)

,_. ,_.

w w

data. data.

3 3

71 71

determined. determined.

72 72

tr tr

RODINGITES RODINGITES

0.037 0.037

0.16 0.16

3.745}3.782 3.745}3.782

6.203 6.203

0.31 0.31

0.044 0.044

3,917}3,961 3,917}3,961

26.15 26.15

22. 22.

93. 93.

44.37 44.37

Grain Grain

2 2

sample sample

not not

for for

was was

GROUP GROUP

o o

2/20 2/20

}3.924 }3.924

2 2

FROM FROM

871

tr tr tr

0.025 0.025

3.952}3.977 3.952}3.977

0.053 0.053

since since

0.09 0.09

6.087 6.087

0.0 0.0

0.45 0.45

26.47 26.47

23.56 23.56

94.26 94.26

43.69 43.69

201 201

Grain Grain

GRAINS GRAINS

p p

Specimen Specimen

· ·

29, 29,

065 065

1 1

equivalents equivalents

PREHNITE PREHNITE

027}4. 027}4.

+ +

Table Table

OF OF

0.038 0.038

0.14 0.14 0.043 0.043

3.769}3.812 3.769}3.812

tr tr

0.37 0.37

tr tr

6.125 6.125

4. 4.

to to

23.08 23.08

27.13 27.13

94.96 94.96

44.24 44.24

Grain Grain

oxygen oxygen

22 22

formulae formulae

Refer Refer

cent) cent)

FORMULAE FORMULAE

of of

per per

24 24

3 3

basis basis

3.944}3.978 3.944}3.978

0.0 0.0

0.034 0.034

0.0 0.0

5.962 5.962

0.56 0.56

0.13 0.13 0.068 0.068

TABLE TABLE

4.021}4.089 4.021}4.089

Structural Structural

26.17 26.17

on on

24.31 24.31

42.42 42.42

93.59 93.59

weight weight

Grain Grain

STRUCTURAL STRUCTURAL

(in (in

author. author.

AND AND

3.99 3.99

the the

2 2

4/4 4/4

~}4,001 ~}4,001

calculated calculated

by by

' '

tr tr

0.034 0.034

3.914} 3.914} ANALYSES ANALYSES

6.035 6.035

0.0 0.0

0.076 0.076

0.65 0.65

23.68 23.68

43.05 43.05

26.39 26.39

Grain Grain

were were

Specimen Specimen

04 04

. .

Analyses Analyses

FeO FeO

1 1

MICROPROBE MICROPROBE

formulae formulae

973} 973}

tr tr

as as

0.58 0.58

0.017 0.017

0.08 0.08 0.11

4.090}4.107 4.090}4.107

0.067 0.067

3. 3.

0.0 0.0

5.938 5.938

24.16 24.16

27.34 27.34

42.58 42.58

94.74 94.74 93.88

Fe Fe

Grain Grain

.ELECTRON .ELECTRON

3 3

total total

sub-total sub-total

Structural Structural

2 2

0 0

+ +

* *

E E

+ +

FeO* FeO*

Ca Ca

MnO MnO

Si0

Al Al

MgO MgO

Na Na

CaO CaO

Na Si Si 184

It is commonly colourless and rarely pale green and shows both no:anal (1st order grey) and anomalous (brown and blue) · interference colours. In short, these chlorites are very similar under the microscope to those from Group 1 roding i te s •

Apart from being associated with calc-silicate minerals in Group 2 rodingites, chlorite also fo:ans the predominant constituent of a grey coloured rock in Haystack Creek (see Sub-Section 4.42). In the mono mineralic rock chlorite occurs as platy grains, 0.05 to 0.2 mm long and up to 0.1 mm wide fo:aning a decussate texture and is weakly pleochroic from pale green to colourless ( ex. = colourless, P = ~ = pale green; absorption: oc < f3 = r >- Abnormal Berlin blue interference colour is commonly observed and the optic sign is negative with a very low c, 5°) 2V. Refractive index (nmax), measured in one sample (Specimen No.4/6) gives a value of 1.610 + 0.002.

Three specimens, two Group 2 rodingites and one associated chlorite, rock, were chosen for electron microprobe analyses of chlorite (given in Table 21, p 170,

Anal. 4-6). Chlorites from the rodingites (Anal. 5 and 6) are distinctly iron-poor compared with that from the monomineralic chlorite rock and are similar to those from Group 1 rodingites (cf., Anal. 1 - 3, Table 21, p 170). When plotted on Hey's diagram (Fig. 49, p 171) ', the 185

analyses lie in the fields of clinochlore .a:ncl shel"~da.:ni.te..

4.624 Garnet:

Commonly garnet occurs in veins up to 0.5 mm wide in prehnite and zoisite rocks. In addition, it also forms monomineralic garnet rocks where it is associated with minor chlorite. Both in veins and in monomineralic rocks garnet usually forms granoblastic aggregates of sub-rounded to polygonal grains, 0.06 to 0.2 mm wide. In thin sections both clear and turbid grains are observed. As in Group 1 rodingites the clear grains are isotropic while the turbid grains are weakly anisotropic.

Refractive indices and unit cell values, determined on three monomineralic garnet rocks to obtain the composition of garnets were found to vary from 1.714 to

1.723 and from 11.89 to 11.93 A0 respectively (Table 17, p 159). Plots of refractive indices against correspond ing unit cell values (Fig. 4 7, p 160) indicate that two out of three are comparatively more hydrous than those from Group 1 rodingites. Nevertheless, these garnets plot reasonably close to the grossularite end-member and hence, may be called grossularite.

Differential thermal analyses were carrjed out on these samples to check if any of the characteristic peaks of hydrogrossularite is present. However, apart from 186

. 0 a peak at,_ 560 for chlorite which is present in these samples, no other peak was recorded in the d.t.a. curves thus indicating a mainly anhydrous composition for these garnets.

4. 625 Sphene:

Sphene is a connnon accessory mineral in Group 2 rodingi tes. It usually occurs as small grains (-' 0. 02 nnn in maximum diameter) and rarely as grain-aggregates often associated with chlorite. It is found to be more common in prehnite-rich varieties than in either zoisite-rich or garnet-rich varieties. It is easily r~cognized under the microscope for its very high relief and very high interference colour.

4.626 Tremolite-Actinolite:

Monomineralic tremolite-actinolite rock often occurs along the margins of Group 2 rodingite bodies near their contact with serpentinites. Tremolite-actinolite constitutes approximately 90 vol.% of the rock and is commonly associated with minor chlorite and zoisite and/or prehnite.

In hand specimen the amphibole is pale grey to greyish white in colour. Under the microscope it usually occurs as fibrous elongate grains (up to 1 mm in length) and is weakly pleochroic from colourless to pale green colourless, '( = pale green) • Bulk chemical analyses of two tremolite-actinolite rocks from the area are given in Sub-Section 4.722. 187

4. 7 CHEMISTRY OF RODINGITES:

4.71 Group 1 Rodingites:

Chemical. ' analyses of 14 Group 1 rodingite samples are given in Table 25, p 188. Whole rock analyses of four serpentini te samples which occur in the immediate vicinity of rodingite bodies are also included in Table

(Anals. 15 to 18} .

It is clear from these analyses that Group 1 rodingites show considerable variation in chemical composition. For instance, Cao varies over a range of approximately 14 wt.%. Ranges of other major oxidep are roughly as follows;

Si02 7%, Al2o3 9%, Fe2o 3 1%

FeO 4%, MgO 13%, H 0+ 4% 2 Ti02 , MnO, Na2o, -K20, P2o5 and co2 vary over a range of less than 1% by weight.

Variations in mineral constituents and in their respective modal percentages have been discussed in section 4. 5 • Such heterogeneity in Group 1 rodingites will certainly be reflected in their chemical composition. Thus, extremely high lime contents of certain samples are mainly due to a very high content of garnet and/or vesuviani te and to a lesser extent due to high diopside content; analyses 12, 13 and 14 (Table 25 , p 188 ) TABLE 25

1. Di-gt-ves-chl rodingite. Specimen No: 2/13, GR 242763 Coolac 1:50,000 Sneet. 2. Di-gt-chl rodingite. Specimen No: 2/14, GR 242761 Coolac 1:50,000 Sheet. 3. Di-ves-chl rodingite. Specimen No: 2/15, GR 243765 Coolac 1:50,000 Sheet. 4. Di-gt-chl rodingite. Specimen No: 2/16, GR 244761 Coolac 1:50,000 Sheet. 5. Di-gt-ves-chl rodingite. Specimen No: 2/37, GR 245760 Coolac 1:50,000 Sheet. 6. Gt-ves-chl rodingite. Specimen No: 3/13, GR 242758 Gundagai 1:50,000 Sheet. 7. Di-gt-ves-chl rodingite. Specimen No: 3/15, GR 245756 Gundagai 1:50,000 Sheet. 8. Di-gt-chl rodingite. Specimen No: 3/30, GR 250750 Tumorrama 1:50,000 Sheet. 9. Di-gt-chl rodingite. Specimen No: 3/36, GR 248752 Tumorrama 1:50,000 Sheet. 10. Di-ves-chl rodingite. Specimen No: 3/40, GR 251755 Tumorrama 1:50,000 Sheet. 11. Di-gt-ves-chl rodingite. Specimen No: 3/42, GR 256753 Tumorrama 1:50,000 Sheet. 12. Gt-chl rodingite. Specimen No: 3/20, GR 243757 Gundagai 1:50,000 Sheet. 13. Gt-chl rodingite. Specimen No: 3/23, GR 241760 Gundagai 1:50,000 Sheet. 14. Ves-chl rodingite. Specimen No: 2/34, GR 243761 Gundagai 1:50,000 Sheet. 15. Serpentinite. Specimen No: 2/15A, Grid reference s-ame as No: 3 above. 16. Serpentinite, Specimen No: 3/30A, Grid reference same as No: 8 above. 17. Serpentinite. Specimen No: 3/23A, Grid reference same as No: 13 above. 18. Serpentinite. Specimen No: 2/34A, Grid reference same as No:14 above.

Di= diopside Gt= garnet Ves= vesuvianite Chl= chlorite

188 188

72 72

5'8 5'8

1.03 1.03

0,3:! 0,3:!

0.02 0.02

0.04 0.04

6.81 6.81

4.11 4.11

0.28 0.28

o.i1 o.i1

0.14 0.14

0.18 0.18

1.02 1.02

2.03 2.03

0.01 0.01

0.00 0.00

0.02 0.02

0.10 0.10

1.82 1.82

5, 5,

18 18

0.02 0.02

2 2

5.19 5.19

34.96 34.96

40.44 40.44

10.42 10.42

37.22 37.22

98.77 98.77

40.07 40.07

10.60 10.60

14 14

0, 0,

0,07 0,07

4.55 4.55

2.65 2.65

0.06 0.06

0.62 0.62

0.03 0.03

0.25 0.25

8.38 8.38

0.41 0.41

0.01 0.01

0.06 0.06

0.03 0.03

0,65 0,65

0.21 0.21

3.14 3.14

4.91 4.91

1.65 1.65

0.00 0.00

17 17

53.06 53.06 30.13 30.13

39.58 39.58

41.26 41.26

J,80 J,80

S.11 S.11

8,79 8,79

100.32 100.32

l8 l8

13 13

---

- 77 77

17 17

.19 .19

O. O.

2.05 2.05

0,11 0,11

6.21 6.21

3.02 3.02

0.23 0.23

0.08 0.08

o.oo o.oo

0.12 0.12

0.24 0.24

o.oo o.oo

o. o.

0.13 0.13

1.85 1.85

1.13 1.13

6.33 6.33

0.06 0.06

1-·H 1-·H

29. 29.

27.31 27.31

S S

11.22 11.22

39. 39.

99.82 99.82

38.82 38.82

11,46 11,46

' '

---

74 74

0,25 0,25

0,06 0,06

6,77 6,77

1. 1.

0.81 0.81

o.i7 o.i7

0.11 0.11

0.38 0.38

9.53 9.53

0.00 0.00

0.00 0.00 0.02

0.49 0.49

3.11 3.11

0,02 0,02

0,06 0,06

1.48 1.48

0.03 0.03

15 15 16

4.67 4.67

.?..69 .?..69

S,26 S,26 9,81 9,81

36.24 36.24

43.11 43.11

38.45 38.45

99.06 99.06

40.73 40.73

. .

hm hm

C C

ab ab

21 21

14 14

3,37 3,37

3.39 3.39

2,30 2,30

0,07 0,07

o. o. 1.39 1.39

0.09 0.09

0.03 0.03

2.16 2.16

0,14 0,14

0.04 0.04

0.00 0.00

1.12 1.12

o.oo o.oo

6.73 6.73 0.17 0.17

0.96 0.96

0.11 0.11

34.86 34.86

12.82 12.82

47.86 47.86

32.44 32.44

99.28 99.28

17.54 17.54

37,84 37,84

79 79

10 10

:3.46 :3.46

2,50 2,50

4.82 4.82

1. 1.

3.13 3.13

0.14 0.14

0.04 0.04

0,80 0,80

2.32 2.32

0.18 0.18

0.00 0.00

0.16 0.16

0.00 0.00

0.18 0.18

0.04 0.04

0.82 0.82

0.55 0.55

!).06 !).06

0.02 0.02

13 13

6.34 6.34

27.32 27.32

11.04 11.04

52.37 52.37

30. 30.

99.31 99.31

19.26 19.26

39.46 39.46

· ·

79 79

3-48 3-48

3,80 3,80

1,8S 1,8S

0.02 0.02

2.15 2.15

8.33 8.33

2.62 2.62

0.09 0.09

0.90 0.90

1. 1.

0.04 0.04

0.01 0.01

0.00 0.00

4.90 4.90 0.00 0.00

0.73 0.73

0.62 0.62

12 12

25.31 25.31

59,37 59,37

30,22 30,22

21.76 21.76

40.SO 40.SO

100.64 100.64

· ·

77 77

11 11

ASSOCIATED ASSOCIATED

3.13 3.13

3.76 3.76

8,90 8,90

6.20 6.20 0,36 0,36

o.os o.os

0.23 0.23

1.55 1.55

5. 5.

1.07 1.07

0.43 0.43 0.06

0.00 0.00

0.05 0.05

2.72 2.72

0.02 0.02

23.20 23.20

12.02 12.02 47.44 47.44

39.84 39.84

18.67 18.67

.100.18 .100.18

AND AND

74 74

AREA AREA

10 10

3.:2.0 3.:2.0

3.03 3.03

0,23 0,23

S.7S S.7S

0,17 0,17

1. 1.

0.18 0.18 0.23

0.14 0.14

5.55 5.55

5.49 5.49

0.09 0.09

0.10 0.10 0.16 0.00

0.09 0.09 0.12

0.06 0.06 0.02

0.02 0.02

0.06 0.06

1.20 1.20 3.36 3.36

10.08 10.08

25.59 25.59

49.75 49.75

99.56 99.56

18.32 18.32 17.47

13.46 13.46 13.84

38.88 38.88

RODINGITES RODINGITES

g_ g_

3.17 3.17

S,S2 S,S2

2.06 2.06

4.53 4.53

0,05 0,05

0.09 0.09

0.27 0.27

0.14 0.14

0.28 0.28 0.26

5.24 5.24

0.00 0.00

0.10 0.10

1.42 1.42

0.02 0.02

0.04 0.04

LIGH1NING LIGH1NING

1 1

2.79 2.79

(CIPW) (CIPW)

12.93 12.93

35.57 35.57

47.78 47.78

36.29 36.29 18.11 18.11 18.22

17.56 17.56

1S-

100.45 100.45

MI'. MI'.

GROUP GROUP

.11 .11

----

NORMS NORMS

8 8

3.5'5 3.5'5

3.60 3.60

author, author,

l.64 l.64

4,69 4,69

0,05 0,05

o.os o.os

0.17 0.17

0.05 0.05

0.02 0.02

3.36 3.36

4.53 4.53 0:02 0:02

0.16 0.16

1.13 1.13

TABLI! TABLI!

15.28 15.28

31.52 31.52

4:i.68 4:i.68

19.43 19.43

15.76 15.76 18.43

17 17

FROM FROM

OF OF

100.13 100.13

the the

ii!h.· ii!h.·

70 70

by by

7 7

NORMS NORMS

J.13 J.13

2.61 2.61

0,20 0,20

2.46 2.46

S,22 S,22

0.02 0.02

0.28 0.28

2.76 2.76

0.27 0.27

0.00 0.00 0.00

1. 1.

4.96 4.96

0.15 0.15 0.09 0.14

0.06 0.06 0.00 0.03 0.04

4.84 4.84

10.85 10.85

33.93 33.93

99.48 99.48

43.47 43.47

37.92 37.92 38.44

16.66 16.66

16.68 16.68

AND AND

(' ('

72 72

3.4i 3.4i

•. •.

6.01 6.01

2.01 2.01

0,18 0,18

1.33 1.33 0.23 0.23

----

0,25 0,25 -

0.31 0.31

6 6

SERPENTINITES SERPENTINITES

Analyses Analyses

19.81 19.81

16.04 16.04

23.45 23.45

36.10 36.10

1.69 1.69

0.11 0.11 0.01

0.08 0.08 0.09

0.12 0.12

1.32 1.32 0.06 0.06 0.12 0.08

0.92 0.92

0.00 0.00

99 99

13.23 13.23 16.03

42.34 42.34

26,62 26,62

12.91 12.91

. .

~ ~

.. ..

75 75

s s

......

3,2.3 3,2.3

2.38 2.38

4,SO 4,SO

0.09 0.09

0.23 0.23

2.54 2.54

0,93 0,93

0.10 0.10

0.19 0.19

0.04 0.04

0.14 0.14 0.32 0.26

0.49 0.49

0.04 0.04

1. 1.

0.12 0.12

4.30 4.30

. .

16.58 16.58

25.10 25.10

11.16 11.16

38.80 38.80

14.46 14.46

21.65 21.65

14.00 14.00

39.17 39.17

lQ0.66, lQ0.66,

.. ..

CHEMICAL.ANALYSES CHEMICAL.ANALYSES

75 75

73 73

·,· ·,·

4 4

J.2.0 J.2.0

6.47 6.47

0.20 0.20

5.01 5.01

0.23 0.23

1.25 1.25 0.40 0.40

2.12 2.12

0.40 0.40

0.18 0.18 0.55

0.14 0.14

0.09 0.09

1. 1.

4.64 4.64 4.36

0.09 0.09

0.21 0.21

0.10 0.10

0.03 0.03

0.86 0.86

0.04 0.04

19.47 19.47

29.35 29.35

'.l'.1.43 '.l'.1.43

15.14 15.14

21. 21.

16.21 16.21

41.04 41.04

38.14 38.14

.-36 .-36

3.37 3.37

6.00 6.00

4.10 4.10

0.21 0.21

3.94 3.94

0.09 0.09

1.65 1.65

0.65 0.65

0.50 0.50

0.00 0.00

0.16 0.16

0.05 0.05

0.09 0.09

0.11 0.11

1.20 1.20

1.14 1.14

18(26 18(26

18.77 18.77

.!3.47 .!3.47

34.40 34.40

26.49 26.49

12 12

99.11 99.11

12.79 12.79

40.64 40.64

l•i l•i

2 2 3

3,24 3,24

5.08 5.08

S,44 S,44

0,09 0,09

I.SS I.SS

0,16 0,16

0.15 0.15

5.23 5.23

0.10 0.10

1.07 1.07

0.08 0.08

0.02 0.02 0.00

0.00 0.00

2.44 2.44

0.23 0.23 0.14

0.04 0.04

0.21 0.21

IS.SS IS.SS

43.98 43.98

38.0S 38.0S

19. 19.

17.29 17.29

100.01 100.01

72 72

.16 .16

1 1

5.18 5.18

3.41 3.41

4.30 4.30

0,07 0,07

0,09 0,09

1.78 1.78 0.25 0.25

0.03 0.03

0.14 0.14

0.27 0.27

0.20 0.20

1.23 1.23

3.50 3.50

0.03 0.03

0.06 0.06

4 4

0.04 0.04 0.07

o.oo o.oo

23,82 23,82

31.74 31.74 33.17

99,52 99,52

38,86 38,86

36.52 36.52

14.34 14.34 16.14

23.44 23.44

15. 15.

Fe Fe

3 3

PeO) PeO)

2 2

o o

0 0

2 2

o o

(total (total

as as

S.G, S.G,

cc cc

ap ap

il il

MgO/FeO* MgO/FeO*

le le

Si0 FeO FeO

MgO MgO

MnO MnO Cao Cao CS CS

di di P205 P205

Ti0

an an

mt mt

or or

hy hy

WO WO

Total Total

ol ol

"20+ "20+ ne ne 189

represent monomineralic vesuvianite- and garnet-rocks and consequently have very high ( > 30% by weight) lime contents. Similarly, modal variations in chlcrite and diopside control the magnesia value of an analysis.

It has long been known that rodingites are characterized by low silica and high lime contents.

Group 1 rodingites at Mt. Lightning are no exceptions.

They are all undersaturated with respect to silica with an average of 38. 63 wt.% Sio2 (Anal. 1, Table 26 , p190 ). The lowest CaO value obtained for these rodin gites is 18.13 wt.% (the average Cao of 14 rodingite analyses being 22.SS wt.%). In the monomineralic garnet and vesuvianite rocks lime accounts for almost a third of the total weight per cent oxides (Anals. 12,

13 and 14, Table 25,. p 188).

Mt. Lightning Group 1 rodingites are also characterized by very low ferrous and ferric iron contents

( < 5 Wt.% and < 2 wt.% respectively). This is not unexpected since all the major mineral constituents are also typically iron poor. Chemical analyses of rodingitic rocks from elsewhere indicate a considerable variation in iron content. For instance, Honnorez and

Kirst (1975) reported two analyses of dredged rodingites from the Mid-Atlantic Ridge, one containing 4.68 wt.%, the other 11.87 wt.%, of total iron as FeO. Rodingites from California show a variation of 6.20 to 15.46 wt.% 190

TABLE 26

AVERAGE CHEMICAL COMPOSITIONS OF RODINGITES

1 2 3 4 5 6 7

Si02 38.93 41.63 34.84 43.15 38.57 41.94 35 .85 Ti02 0.13 0.52 0.45 0.74 1.35 0.45 2 .10 Al 2o3 16.51 26.64 16.04 10.23 14.84 14.36 14.04 ** Fe2o3 1.12 0.91 1.06 1. 78 7.37 2.36 FeO 2.45 0.99 10.32 * 4.58 7 .19 6.14 MnO 0.11 0.10 0.26 0.24 0 .12 0.25 0.18 MgO 13.33 1.62 4.22 10.99 10.35 8.42 13.34 CaO 22.85 22.60 29.24 26.05 18.67 20.79 14.18 Na 2o 0.04 0.39 0.56 0.02 0.96 1.64 0.44 K20 <0.01 0.01 0.04 0 .03 · 0.06 P205 0.06 0.07 0.10 0.07 1. 70 H o+ 4.16 3.99 2.67 5.19 3.97 9.33 2 ] 3.13-i- H2o- 0.22 0.21 0.28 0.19 0.32 Co2 0.05 0.03 0.15 0.07 0.81 0.38

* total Fe as FeO ** total Fe as Fe 2o3 + the average loss on ignition

1. The average Group 1 rodingite from Mt. Lightning - average of Anal. 1-14, Table Z5 , p 188 . 2. The average Group 2 rodingite from Mt. Lightning - average of Anal. 1-10, Table 2.9, p 2.01. 3. The average rodingite of Bell et al. (1911) - average of Anal. 4. The average garnetised gabbro of Miles (1950) - average of A.... a.!. A ~ B . T3.ble. 13, p 125' S. The average rodingite of Bilgrami and Howie (1959) - average of Ana 1. 1 , 2 " 3 , Tab/... 1 , f' 7 95 • 6. The average rodingite of Coleman (1967) - average of six rodingite analyses from Table~, p 13 , in Coleman(ibid: I 7, The average rodingite of Honnorez and Kirst (1975) - average of · two analyses in Table 4 , p 2.48 , in Honnore and Kirst (Ibid.). 191

total iron as FeO (Coleman, 1967).

Like iron oxides, magnesia shows considerable variation in rodingites reported elsewhere (Suzuki, 1952; Bilgrami and Howie, 1959; Coleman, 1967; Honnorez and Kirst, 1975). Mt. Lightning Group 1 rodingites also reveal a wide range of MgO contents. Monomineralic garnet and vesuvianite rocks contain less MgO ( < 7 wt.%) than the heterogeneous types (in which MgO is> 12 wt.%). Nevertheless, the MgO/FeO* (total Fe as FeO) ratio of these rodingites is always> 2 and is notably higher than that in most rodingites reported in literature available to the author. In fact, only the following four published analyses have MgO/FeO* ratios comparable with Mt. Lightning Group 1 rodingites - Miles (1951, Anal. A, Table 13) - 2. 4, Bloxam (1954, Anal. A, Table 1 ) - 1.1, Coleman (1967, Anal. 6, Table 2) - 2.4, and Honnorez and Kirst (1975, Anal. 1, Table 4) - 3.4. The high MgO/FeO* ratio of the rodingites under study may indicate a high MgO/FeO* ratio of the precursor rock at Mt. Lightning.

An interesting feature of Mt. Lightning Group 1 rodingites is that in general there is a tendency for the higher Cao values to be associated with lower MgO values. At Mt. Lightning this is clearly shown by the monominera lic variants (Anals. 12, 13 and 14, Table 25 , p 188 ) which are believed to represent the extreme alteration 192

products of precursor rocks. Table 26 , p 190, compares analyses of rodingitic rocks from other locali- •ties with the average of 14 Group 1 rodingite analyses and shows that such a negative correlation between cao and MgO values exists in rodingitic rocks elsewhere. In fact, Miles (1950) concluded from two chemical analyses of garnetised gabbros (believed to be similar to rodingites) that the more altered specimen showed a distinct increase in lime and alumina at the expense largely of magnesia and silica. Study of Group 1 rodingites at Mt. Lightning accords with Miles' findings as regards lime and magnesia although nothing definite can be said about the distribution of alumina and silica.

The amount of alumina in Group 1 rodingites is dependent not only on the modal garnet and vesuvianite but also on that of chlorite. It should be noted that alumina contents of these rodingites do not depart much from the average value of 16.51 wt.% (Anal. 1, Table 26,

p 190 . Although the monomineralic garnet rocks (Anal.12 and 13, Table 25, p 188) contain the highest alumina, Miles' (1950) claim that progressive rodingitization results in a distinct increase in alumina content is not generally valid for Mt. Lightning rodingites. This is

because vesuvianite, which contains less Al2o3 than grossularite often forms the predominant mineral in some _extremely rodingitized specimens. Thus, a positive correlation between lime and alumina, as implied by 193

Miles (1950) for garnetized gabbros from Western Australia, is only applicable in garnet-rich varieties of Mt. Lightning rodingites.

Other peculiar features of Group 1 rodingites include low Tio2 contents (an average of 0.13 wt.%, Anal. I, Table 26, p 190), almost total absence of alkalies and very low co2 contents. H2o+ is somewhat variable, the average value being 3.88 wt.%.

Table ~2'6, p 190, shows averages of rodingite analyses from the original locality in New Zealand (Bell et al, 1911), from Western Australia (Miles, 1950), from Pakistan (Bilgrami and Howie, 1959), from the Pacific Coast in United States (Coleman, 1967) and from the Mid-Atlantic Ridge (Honnoroz and Kirst, 1975). The average of 14 Mt. Lightning Group 1 rodingite analyses is also given in the same Table for comparison. It is apparent that apart from having a higher MgO/FeO ratio the average Group 1 rodingite is not strikingly different from any of the other averages.

Normative compositions of Group 1 rodingites (Table 25, p 188) show that these rocks are markedly undersaturated with respect to silica. This is evident from the presence of olivine and calcium orthosilicate in all of the calculated norms. The generally high lime contents of these rocks are reflected in the considerable 194

amounts of calcium orthosilicate and in the notably high values ( > 34%) of anorthite. The deficiency in alkalies is indicated by the absence of albite and orthoclase.

Analyses of Group 1 rodingites are plotted on an

ACF diagram (Fig. 54 A, p 195), and are compared with rodingites from other localities (Fig. 54 B, p 195). comparisons of these two ACF plots indicates a close correspondence with minor overlap.

Although a considerable volume of major element data for rodingites has accumulated in recent years, little information on their trace element chemistry is available. In fact, the published data seems to be restricted to 6 analyses of rodingites from the West Pacific region (Coleman, 1967). Eight trace elements were analysed for Group 1 rodingites from Mt. Lightning and are listed in Table 27, p 196 .

Apart from the monomineralic garnet and vesuvia nite rocks, these rodingites are characterized by very high er ( > 700 ppm), high er/V ratio ( > 10. 0) , high Ni ( > 400 ppm) , low Y ( < 13 ppm) and low Zr ( < 11 ppm). The monomineralic rocks (Anal. 12,13 and 14, Table 27, p 196) have lower er and Ni ( < 400 ppm for both but higher Zr and Y ( > 70 and > 50 ppm respectively) compared with the heterogeneous types. They also have Fig.54A. ACF diagram showing plots of analysed rodingites from the Mt.Lightning area. Circles= Group 1 rodingites, Crosses= Group 2 rodingites.

Fig.54B. ACF diagram showing plots of rodingites described in literature (data from Coleman,1967). 195

• •• •• •.... 0 .. 0 ••

C F

• • • 0 • o OOo,p 0 . Oo •••• 0 • -~ .,. 0 0 • 0 •- 0 • C F TABLE27

TRACEELEMENT CONTENTS OF GROUP1 RODINGITESFROM MT. LIGHTNING

(in ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

V 64 86 123 116 82 116 76 84 90 93 88 140 115 101 Cr 1557 1420 1275 1163 984 1460 764 1120 992 1614 1425 354 214 224 Ni 615 576 520 709 790 535 773 802 750 490 584 120 184 380 Rb * * * * * 2 2 * * * 1 * 3 2 Sr 33 27 so 36 18 39 23 20 21 219 195 21 15 22 y 3 5 9 4 12 7 8 8 7 7 6 56 61 64 Zr 2 6 16 8 5 9 8 4 5 10 4 73 88 308 Ba * * * * * * s * 2 * * * * * . . . . .

Cr/V 24.3 16.5 10.4 10.0 12.0 12.6 10.0 13.3 11.0 17.3 16.2 2.5 1.9 2.2

* not detected Samples as in Table 25, p 18 8 . Analyses by the author and Dr. B. Chappell, A.N.U. .... \0 O'I 197

a lower Cr/V ratio ( < 2.5).

The Cr content of Mt. Lightning Group 1 roding ites depends mainly on modal clinopyroxene and to a less extent on the amount of accessory spine! (magnetite and chromite). Thus, the monomineralic garnet and vesuvia nite rocks and the diopside-poor varieties have lower concentrations of Cr than the diopside-rich rodingites.

Four serpen tini tes, collected from the imrnedi.ate vicinity (within 30 cm from the dyke margins) of four different Group 1 rodingite dykes, were chemically analysed

(Anal. 15 to 18, Table 25, p 188 ) to check whether they show any remarkable compositional difference from those collected elsewhere in the area. Coleman (1967, p 12) observed that serpentinites associated with rodingites from the Pacific coast area in the United States show increases in both calcium and aluminium. However, serpentinites associated with Mt. Lightning Group 1 rodingites do not show any notable difference in chemical composition (cf. Anal.. 1 to 12, Table 7 , p 61 ) •

4.711 Variation of Composition within Group 1

Rodingite Bodies at Mt. Lightning

A decrease in diopside content from the margin towards the centre of some Group 1 rodingite dykes was noted in both hand specimens and in thin sections (see 198

Sub Section 4. 51). Two of these dykes were sampled for chemical analyses in order to ascertain the nature of compositional variation within each dyke. Table 'Z-8, p 199, reports chemical and modal analyses of these samples.

Analyses la, lb and le are of sub-samples from the margin, intermediate portion and centre, respectively, of a Group 1 rodingite dyke, 1.7 m wide, enclosed within highly schistose serpentinite. Analyses 2a and 2b are of sub-samples from the margin and centre respectively of a rodingite dyke, 0.8 m wide, surrounded by brecciated serpen tini te. These analyses show a drastic increase in lime and decrease in magnesia from margin towards the centre for both dykes. In one dyke {Anal. 1) A1 2o 3 increased and Fe2o 3 + FeO decreased while in the other dyke {Anal. 2) Al2o 3 and Fe2o 3 +FeO remained virtually unchanged and Sio2 decreased from the margin to the centre. The trend of Sio2 in Anal. 1, however is erratic.

In addition to major oxide variation, there is a pronounced but selective minor element variation. For instance, V, Cr and Sr decrease from the margin towards the centre in both dykes. The Cr/V ratio shows a similar trend. The dyke centres are also depleted in Ni compared with the margins although lb indicates that the inter mediate portion of one dyke contains the lowest amounts of Ni. 199 TABLE 28

COMPOSITIONAL VARIATIONS WITI-IIN GROUP 1 RODINGITE DYKES FROM MT. LIGHTNING Major elements (in weight per cent)

1 2

a b C a b

Si02 40.54 37.81 38.42 39.61 36.83 ; Ti02 0.36 0.22 0.21 0.12 0.14 A1 203 13.79 16. 73 . 17 .11 20.14 20.57 Fe2o3 1.71 1.45 0.82 0. 72 0.62 FeO 2.43 2.38 1.02 0.86 1.05 MnO 0.28 0.20 0.09 0.15 0.09 MgO 17.65 12.32 10.63 12.22 5.98 cao 19.47 24.57 27.39 23.69 30.14 Na2o 0.12 0.14 0.05 0.05 0.12 K20 0.00 0.02 0.00 0.00 0.00 Pz05 0.08 0.04 0.12 0.10 0.06 H2o+ 3.20 4.28 3.46 2.64 3.86 "20- 0.31 0.23 0.19 0.14 0.20 Co2 0.06 0.04 0.07 0.08 0.08

Total 100.00 99.43 99.58 100.52 99.74

Trace elements (in ppm)

V 82 60 52 106 81 Cr 1350 884 740 1165 479 Ni 560 324 361 456 235 sr· 45 31 28 23 18 y 8 4 7 9 14 Zr 10 7 10 16 26

Cr/V 16.5 14.7 14.2 10.9 5.9 MgO/FeO* 4.4 3.3 6.0 8.1 3.7 (total Fe as FeO) 1. Specimen 2/46: GR 251750, Tumorrama 1:50,000 Military map a - dyke margin , b - 40 cm from dyke margin , c - dyke centre. 2. Specimen 3/61: GR 245773, Coolac 1:50,000Militarymap a - dyke margin , b - dyke centre.

Major element analyses by the author, and trace element analyses by the author and Dr. B. Chappell, A.N.U. 200

There is a striking similarity in chemical composition between the monomineralic variants of Group 1 rodingites (see Anal. 12, 13 & 14, Table 25, p188) and the central portions of the dykes analysed.

4.712 Group 2 Rodingites:

Table 29, p 201, reports the chemical analyses of

10 Group 2 rodingites together with their respective norms.

Variations in major oxide contents in these rocks merely' reflect their mineralogy. Thus, prehnite-rich varieties

(Anal. 5 - 9) contain more Sio2 then either zoisite- or garnet-rich types. Variation ranges of major oxides in

Group 2 rodingites are roughly as follows (given in weight percentages) :

Sio2 12% Fe2o 3 - 2% cao 13%

Ti02 1% FeO - 2% Na2o - 2% MgO 4% HO+ - 4% A1 2o 3 - 16% - 2

Other oxides vary over a range of less than 1%.

Like other rodingites, Mt. Lightning Group 2 rodingites are characterized by low silica and high lime contents. In addition, they are also typically high in alumina {the average being 26. 64 wt.%, see Table 26, p 19 O) • TABLE 29

1. Zoisite rodingite. Specimen No: 2/26, GR 247770 Coolac 1:50,000 Sheet. 2. Zoisite rodingite. Specimen No: 2/31, GR 247769 Coolac 1:50,000 Sheet. 3. Zoisite rodingite. Specimen No: 3/31, GR 247769 Coolac 1:50,000 Sheet. 4. Zoisite rodingite. Specimen No: 4/5, GR 248765 Jugiong 1:50,000 Sheet. 5. Prehnite rodingite. Specimen No: 2/20, GR 247769 Coolac 1:50,000 Sheet. 6. Prehnite rodingite. Specimen No: 4/4, GR 246770 Coolac 1:50,000 Sheet. 7. Prehnite rodingite. Specimen No: 4/23, GR 248765 Jugiong 1:50,000 3heet. 8. Prehnite rodingite. Specimen No: 4/80, GR 248768 Coolac 1:50,000 Sheet. 9. Prehnite-chlorite-albite rock. Specimen No: 3/29, GR 247765 Jugiong 1:50,000 Sheet. 10. Garnet-chlorite rodingite. Specimen No: 4/2, GR 246771 Coolac 1:50,000 Sheet. 201 TABLE 29

CHEMICAL ANALYSES AND NORMS OF GROUP 2 RODINGITES FROM MT. LIGHTNING AREA

Major Elements (in weight per cent)

- 1 2 3 4 5 6 7 8 9 10 -- 39.95 39.14 38.51 42.69 43.17 42.31 43.78 49.25 37.94 Si02 40.01 0.18 0.34 0.33 1. 24 0.07 0.19 1. 49 0.76 0.31 Ti02 0.26 32.49 31.06 30.44 25.31 · 26. 77 27.53 24.53 16.13 22.31 I A1 203 29.84 1. 21 0.62 0.70 0.64 1. 22 2.37 0.76 Fe 2o3 0.52 0.64 0.46 FeO 0.22 0.24 0.29 1.42 1.04 0.36 1.00 1.13 3.14 1.10 t,mO 0.09 0.02 0.22 0.10 0.11 0.04 0.08 0.16 0.28 0.01 MgO 0.18 0.53 0.27 1. 28 1.69 1. 36 0.76 1. 84 4.23 4.09 CaO 25.00 24.31 25.46 22.33 20.73 22.19 22.81 20.21 15 .00 27.90 2.64 0.10 Na 20 0.17 0.10 0.12 0.15 0.21 0.31 0.09 0.09 0.04 0.00 K20 0.01 0.00 0.02 0.02 0.02 0.00 0.02 0.02 0.10 0.18 P205 0.06 0.04 0.03 0.07 0.08 0.07 0.02 0.08 4.47 H2o+ 2.64 2.18 2.50 3.24 5.14 4. 70 3.49 5.46 6 .10 H2o- 0.24 0.12 0.21 0.34 0.12 0.08 0.23 0.15 0.26 0.40 co2 0.02 0.00 0.02 0.02 0.10 0.04 0.00 0.03 0.05 0.03

Total 99.26 101.08 100.14 99.46 99.10 99.86 99.17 100.19 100.35 99.60

NORMS (CIPW)

Qz - - -- 2.09 0.21 - 4.00 4.05 - Or - -- - 0.12 - - 0.12 0.24 - Ah - - -- 1. 78 2.62 - 0.76 22.33 - Ne 0.78 0.50 0.55 0.69 - - 0.41 - - 0.46 An 80.63 88.15 84.15 82.35 68.06 71. 65 74.65 66.47 32.05 60.42 Di 0.02 - - - 9.08 7.38 6.23 9.88 28.48 0.27 Wo 0.27 - - - 9.43 11.84 12.27 8.42 2.39 0.22 Ot 0.31 0.92 0.49 3.04 - - - - - 7.74 Cs 13.06 9.95 12.98 8.61 - - 0.56 -- 23.44 Mt 0.25 0.32 0.66 1. 75 0.11 1.01 0.93 - 3.44 1.10 11m 0.35 0.38 0.01 - 0.54 - - 1.22 -- It 0.49 0.34 0.65 0.63 2.35 0.13 0.36 2 •. 72 1.44 0.59 Ap 0.14 0.09 0.07 0.16 0.19 0.16 0.05 0.19 0.23 0.42 Ce 0.05 - - 0.05 - 0.05 - 0.07 0.11 0.07 ..__H 2o 2.88 2-30 2. 71 3.58 5.26 4.78 3. 72 5.61 6.36 4.87 llgO/FeO* 0.26 0.68 0.40 0.51 1.06 1. 37 0.48 0.83 0.80 2.29 (total Fe as ~O) liii....:: 202

Mineral chemistry has shown that the major

constituents of Group 2 rodingites have extremely low

total iron and magnesium contents (see Section 4. 6 ,pp 173-185).

This is clearly reflected in the bulk chemistry of these

rocks. The MgO/FeO* (total Fe as FeO) ratio is also very low and is less than 2 in all but one sample (Anal.10,

Table 29, p 201) which contains a significant amount (12% by vol.) of magnesium-rich chlorite (see Anal.10, Table 16, p 1 SS) • It may be mentioned that the MgO/FeO* ratio of

Group 2 rodingites differs notably from that ( > 2) of

Group 1 rodingites.

Apart from one sample (Anal.9, Table 29, p 201)

all analysed Group 2 rodingites show a deficiency in

alkali content. The relatively high Na2o value of ~al1aes 9 (2.64 wt.%) is due to the modal albite present in the rock. Tio2 contents of these rodingites are also low ( < 1. 50 wt.%} and may be attributed to the accessory sphene. H2o+ varies from 2.18 to 6.10 wt.%, the zoisite rich types (Anal. 1 to 4, Table 29) being relatively poor

in H2o+ compared with the prehni te-and garnet-rich varieties.

Unlike Group 1 rodingites, where certain corre~

lations between major oxides seem to exist (see Sub-Sec

tion 4.71), Group 2 rodingites do not reveal any such

trend in major oxides.

The average of 10 analysesof Group 2 ro a·1ng1tes . t\ 203

is given in Table 26, p 190, where it is compared with the average Group 1 rodingite and with the averages of rodingites from elsewhere. The average Group 2 rodingite differs chiefly from other averages in having considerably more alumina and less iron and magnesia. Compared with Bell et al' s rodingi te (Anal. 3, Table 26, p 190) the average Group 2 rodingite is depleted in lime but contains more silica. Average lime contents of Group 1 and Group 2 rodingi tes are similar.

Normative compositions reveal that most of the Group 2 rodingites contain calcium orthosilicate and nepheline thus indicating undersaturation with respect to silica. Normative quartz is present only in prehnite- rich varieties although one prehnite-rock has small amounts of calcium orthosilicate and nepheline. The generally high lime content is reflected in considerable amounts of normative anorthite.

Analyses of Group 2 rodingites, when plotted on an ACF diagram, show that most of them lie close to the A-C join (Fig.54 A, p 195), and in fact close to the ideal compositions of zoisite and prehnite. This diagram also reveals that t..~e concentration of Group 2 rodingites is quite distinct from that of Group 1 rodingites. This is because of the relatively high alumina and low magnesia and iron oxide contents of the former compared with those of the latter. When compared with rodingites from other 204

localities (Fig. 54 B, p 195), Group 2 rodingites show

a minor overlap on an ACF diagram.

Group 2 rodingites show a considerable variation

in most of the trace elements analysed. These analyses

are presented in Table 30, p205. Apart from the garnet chlorite rock (Anal. 10, Table 30, p 205), all the other analysed specimens are characterized by high Sr (ranging

from 143 to 1446 ppm). It is also noteworthy that the

zoisite rodingites (Anal. 1 to 4) have a distinctly higher concentration of Sr than the prehnite rodingites

(Anal. 5 to 9). Perhaps, Sr enters more readily in the structure of epidote minerals than in that of either prehni te or garnet. Indeed a Sr-rich epidote ( "hancocki te"} is reported in the literature (Deer et al, 1966).

The Ba content of these rodingites is puzzling.

It shows a wide range from 11 to 396 ppm. The highest concentration is found in the prehnite-albite rock

(396 ppm - Anal. 9, Table 30, p 205) and in general prehnite rodingites are enriched in Ba although one sample

(Anal. 6) contains only 7 ppm of Ba. The zoisite rocks

(Anal. 1 to 4) and the garnet-chlorite rock (Anal.10) are poor in Ba (concentration< 50 ppm).

Compared with Group 1 rodingites, Group 2 types

are depleted in er (8 to 59 ppm) and Ni (19 to 80 ppm)

and are enriched in Y (10 to 67 ppm) and Zr (60 to 168 ppm}. 0

....,

11 35

148

0.19

396

501

168

0.51

179

465

0.33

AREA

250

755

0.28

LIGHTNING

A.N.U.

MT.

7 2

127

143

FROM

0.44

Chappell,

202

368

0.40

RODINGITES

Dr.

201

pfm)

and

TABLE

29,

39 48

GROUP

(-i..n

0.44

1115

author

Table

the

1446

CONTENTS

Samples

Analyses

ELEMENT

0.56

1361

TRACE

118

976

106

Cr/v

y Ba

! 206

The cr/V ratio is also low ( < 0.60) in Group 2 rodingites as compared with that ( > 2. O} in Group 1 types.

4.721 Variations in chemical compositions across Group 2 rodingite bodies.

In order to show the chemical variations along variolite-serpentinite contact where Group 2 rodingites occur, samples were collected across the rodingite bodies. Two such rodingite outcrops were selected in this study. The chemical analy~es of the samples are presented in Table 31,p 208. The first set of analyses (Anal. 1 to 5) include those of a variolite, a chlorite rock, a prehnite rock, a trmolite rock and two serpentinites (collected 10 cm and 30 cm away from the tremolite rock contact). The second set of analyses include those of a variolite, a chlorite rock, a zoisite rock, a tremolite rock and two serpentinites (collected 10 cm and 30 cm away from the tremolite rock contact). The analyses are arranged in the same order as they occur in the field from the variolites towards the serpentinites.

The chemical changes observed across the rodingite bodies are plotted on a variation diagram (Fig.56,p 209) and are summarised as follows:

i) A1 2o3 shows an increase from the variolites through the chlorite rocks to the prehnite and the zoisite rocks, and decreases sharply in the vemolite rocks while Cr,Ni and

V show an opposite trend. The serpentinites are consistently poor in A1 2o3 . 207

ii) Cao, MgO, FeO, Sio2 , Sr and Y show considerable mobility during alteration. Cao, Sr and Y are concen trated in the Group 2 rodingites (the prehnite and the zoisite rocks) which are also notably iron-poor. The chlorite rocks are distinctly iron-rich. The behaviour of sio2 is erratic. Coleman(1967) observed that serpentinites, which occur near the contacts with reaction zones in the Pacific coast area, are enriched in both lime and alumina. However, at Haystack Creek, Mt.Lightning, there is no noticeable difference in chemical compositions between serpentinite samples collected at and away from the contacts. Moreover, the compositions of these serpentinites are similar to those from the study area in general (cf. Table 7,p 61, and Table 25,p 188). TABLE 31

1. Variolitic spilite. Specimen No: 3/80A, GR 249770 Coolac 1:50,000 Sheet. 2. Chlorite rock. Specimen No: 3/80B, Grid reference same as number 1. 3. Prehnite rock. Specimen No: 3/80C, Grid reference same as number 1. 4. Tremolite rock. Specimen No: 3/80D, Grid same as number 1.r SA. Serpentinite (collected 10 cm away from tremolite rock contact). Specimen No: 3/80E, Grid reference same as number 1. SB. Serpentinite (collected 30 cm away from tremolite rock contact). Specimen No: 3/80F, Grid reference same as number 1.

6. Variolitic spilite. Specimen No: 2/54A, GR 250768 Jugiong 1:50,000 Sheet. 7. Chlorite rock. Specimen No: 2/54B, Grid reference same as number 6. 8. Zoisite rock. Specimen No: 2/54C, Grid reference same as number 6. ~- Tremolite rock. Specimen No: 2/54D, Grid reference same as number 6. l0A. serpentinite (collected lo cm away from tremolite rock contact). Specimen No: 2/54E, Grid reference same as number 6. l0B. Serpentinite (collected 30 cm away from tremolite rock contact). Specimen No: 2/54F, Grid reference same as number 6.

N N

00 00

0 0

2 2

6 6

22 22

2043 2043

2459 2459

0.01 0.01

0.35 0.35

0.03 0.03

0.07. 0.07.

lOB lOB

o.oo o.oo

0.28 0.28

0.44 0.44

0.04 0.04

1.42 1.42

1.56 1.56

11.61 11.61

41.50 41.50

Chappell,A.N.U. Chappell,A.N.U.

100.93 100.93

2 2

5 5 3

4 4

B. B.

28 28

2634 2634

1981 1981

0.07 0.07

0.00 0.00

0.04 0.04

0.36 0.36

5.92 5.92 5.81

1.37 1.37

o.oo o.oo

0.51 0.51

0.06 0.06

1.65 1.65

lOA lOA

11.43 11.43

Dr. Dr.

100.74 100.74

0 0 0 0

24 24

27 27

72 72

and and

268 268

146 146

148 148

9 9

2.01 2.01

0.82 0.82

2.74 2.74

0.02 0.02

0.06 0.06

0.29 0.29

6.93 6.93 0.43 0.43

0.04 0.04

1.93 1.93

Bodies Bodies

12.66 12.66

99.55 99.55 48.68 48.68 41.39

36 36

20 20

45 45

52 52

12 12

90 90

author author

8 8

1270 1270

2.65 2.65

0.33 0.33 0.10 0.22

0.17 0.17

0.06 0.06

o.oo o.oo

0.31 0.31

0.35 0.35

0.22 0.22

0.70 0.70

38.95 38.95

99.87 99.87

32.63 32.63

22.58 22.58

the the

Rodingite Rodingite

7 7

0 0 2 2 0 0

10 10

66 66

2 2

by by

257 257

102 102

125 125

180 180

0.10 0.10

0.20 0.20

0.03 0.03 0.06

0.00 0.00

0.06 0.06

2.78 2.78

0.08 0.08

1.19 1.19

0.19 0.19

28.42 28.42

12.79 12.79

18.62 18.62

15.67 15.67

cent) cent)

101.03 101.03

4 4

Group Group

22 22

19 19

85 85

per per

345 345

164 164

182 182

152 152

6 6 7

0.10 0.10

5.02 5.02

5.59 5.59

3.06 3.06

2.40 2.40

0.23 0.23

0.07 0.07

0.28 0.28

2.52 2.52

0.12 0.12

analyses analyses

56.71 56.71

14.32 14.32

ppm) ppm)

Across Across

weight weight

3 3

0 0

31. 31.

(in (in

39 39

01 01

.. ..

SB SB

element element

(in (in

2256 2256

2138 2138

0.00 0.00

0.05 0.05 0.93

0.02 0.02

4.10 4.10

4.61 4.61

9.96 9.96

0 0

0.04 0.04

0.23 0.23

99.40 99.40 99.49

TABLE TABLE

3 3 1

0 0

2 2

32 32

Collected Collected

trace trace

SA SA

2137 2137

1963 1963

elements elements

3.96 3.96

0.02 0.02

0.05 0.05

0.01 0.01

4.63 4.63

0.23 0.23 0.28

1.14 1.14 1.21

oxides oxides

39.61 39.61 39.55

10.64 10.64

and and

6 6 0

4 4 0

42 42

18 18

83 83

104 104

205 205

172 172

Samples Samples

Trace Trace

Major Major

3.49 3.49

0.86 0.86 0.06

2.25 2.25

2.76 2.76

7.29 7.29

0.91 0.91

0.07 0.07

0.02 0.02

12.91 12.91

46.30 46.30

of of

author author

48 48

75 75

26 26

13 13

12 12

14 14

146 146

670 670

3 3 4

the the

0.18 0.18

0.03 0.03 0.27 0.19

0.59 0.59

4.16 4.16

0.08 0.08 0.04

0.31 0.31

0.15 0.15 0.16 0.40 0.42

0.12 0.12

0.62 0.62

26.02 26.02

23.11 23.11

44.03 44.03

100.89 100.89 100.35 100.06

by by

Analyses Analyses

30 30

47 47

78 78

12 12

342 342

158 158

140 140

1.05 1.05 2.12 2.12

0.08 0.08

2.31 2.31

0.15 0.15

0.18 0.18

0.06 0.06

0.10 0.10

0.15 0.15 0.00

30.01 30.01

99.92 99.92

11.26 11.26

16.34 16.34

2 2 0

analyses analyses

Cemical Cemical

26 26

13 13

278 278

327 327

525 525

112 112

209 209

1 1 2

2.25 2.25

0.14 0.14

1.23 1.23

0.08 0.08

5.13 5.13 8.07 8.07

0.21 0.21

0.19 0.19

1.92 1.92

0.43 0.43

6.20 6.20

52.85 52.85

16.04 16.04 16.83

100.05 100.05

oxide oxide

3 3

2 2

o o

2 2

o o

Ba Ba

Zr Zr

TiO TiO

Sr Sr

Rb Rb Ni Ni

Si0

HdO HdO

Total Total

P205 P205

K2 K2

er er y y

Fe Fe

Al/>

cao cao

V V

C C

MnO MnO

fe Major Major Fig.S6. Semilogarithmic plot of chemical variation of samples collected across Group 2 rodingite bodies from the Haystack Creek area, Mt.Lightning. Sample numbers are those given in Table 31, p 208, except for numbers Sand 10 which represent the averages of analyses SA &SB, and l0A & 10B respectively. The abcissa does not represent horizontal distances between samples. 209

V C p T 5 V C z T s ~ 100 ~-~, ~ ----2_02 .,,,..,, ~/ , // ' .,,.,,,,~ MgO ,,,/ ,,,. MgO / I I \ '· .,. I ,, I I // I \ I 10 / I \ \ I ' \ \, \ \ \ \~ \ / ,, \ \\Al 2 0 3 " \ I / \'fl20J \ \ \ I \ \ \ \ \ I \ \ \ \ V \ \ \ \ 1 \ \ \ \ 'cao \CaO \ \ \ I/\ \ \ \ I 0 1 V

0·01 ,/ 1 3 4 s i 10

V Variolitic spilite C Chlorite ro'ck T Tremolite rock S Serpentinite P Prehnite roe k Z Zoisitll rock . 210

4.8 GENESIS OF RODINGITE:

Having established t:.he differences between the two groups of rodingites at Mt. Lightning, it is felt that their origins should also be discussed separately.

In fact, field, petrographic and chemical evidences point to two distinct parentages for these rocks. But before discussions on the genesis of Mt. Lightning rodingites are presented, it seems pertinent to summarise some of the hypotheses relating to the formation of rodingites in general.

Since rodingites were first defined by Bell et al in 1911, these rocks have been examined by a number of authors in various localities. As a result it is only to bi? expected that disagreement regarding their origin would arise. Indeed, various theories have been advanced Ca--,.i.ch to account for the origin of these unusual rocks. Broadly, I\ the proposed hypotheses can be divided into two groups:

iJ The lime needed to form rodingitic assemblages

is attributed to the release of Ca during

serpentinization of ultramafic wall rocks.

ii) The lime needed to form rodingitic assemblages

originated elsewhere either within the precursor

rocks themselves, or from other external sources. 211

It should be noted that there is virtually an unanimous agreement as to the secondary origin of rodingites although some doubt on this question seems to be implied by Bell et al (1911). In their description of rodingite dykes from the Roding Valley area in New

Zealand, they proposed that grossularite was the last mineral to crystallize in these dykes. However, they also recognized the dissimilarity between rodingites and common igneous rocks and remarked " •••...•. they (the grossularite rocks) contain nearly twice as much lime as any other igneous rocks of which analyses are available."

That the mineral assemblages, apart from a few relict constituents, of rodingites are typically metamorphic and the striking difference in chemical compcsition between rodingites and known igneous rocks are sufficient proofs of a secondary origin for these rocks. Also, the general gradations from unmetmnorphosed precursors to pure rodingitic rocks, as have been noted by various authors including Benson (1918), Miles (1950) and Coleman (1967). confirm that rodingites are secondary in origin.

4.81 Summary of various hypotheses for the origin

of Rodingites:

i) Ca derived from serpentinization of associated ultramafic rocks: 212

This is by far the more widely accepted theory.

In view of the world-wide occurrence of rodingites within or close to serpentinized ultramafic rocks, many authors believe that rodingitization and serpentinization processes are not only genetically related but that the latter is directly responsible for the former.

Graham (1917) was the first author to suggest that rodingite dykes in the Black Lake - Thetford Area,

Quebec were formed during serpentinization of the host peridotite and pyroxenite bodies. He concluded that heated siliceous magmatic waters emanating from intrusive granitic magma would cause the breakdown of the clino pyroxene during serpentinization thus producing a lime- rich solution. This lime-rich solution was believed to have produced the lime silicate minerals in the rodingites.

A somewhat similar origin for the New Zealand rodingites was proposed by Grange (1927) who regarded them as products of garnetization of original gabbroic rocks. According to Grange the lime silicate minerals in these rodingites were formed by the action of late magmatic waters on the calcic minerals in the gabbros with additional lime derived from monoclinic pyroxenes of associated ultrabasic rocks during serpentinization.

Miles (1950) concluded that garnetized gabbros 213

from the Eulaminna District, Western Australia, which are similar to the rodingites described by Bell et al (1911) from New Zealand, were derived by the alteration of original gabbro. The sources of lime and alumina required for such alteration process were ascribed to the break down of monoclinic pyroxene and calcic plagioclase felds par in the original ultrabasic host rock during its serpentinization.

Bilgrami and Howie {1959) suggested a similar origin for the rodingites from Hindubagh, Pakistan, which were believed to have formed from dolerites. Like Miles {1950), these authors considered that the lime required for the alteration of dolerite came from the breakdown of pyroxene and the small amounts of calcic plagioclase during serpentinization of the adjacent peridoti tes.

From the literature available to the author, it seems that the most comprehensive study on rodingites was carried out by Coleman {1966, 1967}. Although a great majority of theories put forward for the origin of rodingites indicate a gabbroic or a doleritic parental rock and a few indicate a dioritic or a granitic precursor rock {Arshinov and Merenkov, 1930; De, 1972), Coleman {1967} is the first author to point out that rodingitic _assemblages can be produced by metasomatism of sedimentary rocks in addition to the metasomatism of a wide range of 214

igneous rocks. From the chemical data of Poldervaart

(1955) and of Faust and Fahey (1962), Coleman (1967) concluded that a considerable amount of Cao had been removed from dunite-peridotites during serpentinization.

The calcium released during serpentinization of pyroxenes can migrate to a variety of rock types which will be metasomatically altered to rodingites by enrichment of lime and a concomitant loss of silica.

Recently, Honnorez and Kirst (1975) have proposed that rodingites dredged from the Equatorial Mid-Atlantic

Ridge were generated from gabbro-norites by a metasomatic process involving enrichment in lime and water, and a loss of silica and alkalies. They have also suggested that the lime was released during serpentinization of the associated ultrHmafic rocks.

The list could be extended. It is sufficient to say that proponents of this hypothesis essentially agree on two points - a) that monoclinic pyroxene in the associated ultramafic rocks is the main source of Ca, and, b) serpentinization and rodingitization are related

and perhaps, complementary processes, and in general that rodingitization is a consequence of serpentinization.

ii) Ca derived from a source other than associated

ultramafic rocks:

Bloxam (1954) acknowledged that lime had to be 215

introduced into the gabbros of Girvan-Ballantrae complex,

Ayrshire, to transform them to rodingites. He recognized that although some lime might have been available from the alteration of pyroxene and plagioclase within the gabbro, it would be insufficient in amount. However, he argued that the lime could not have been derived from the serpent inization of associated ultramafics the latter being dominantly harzburgites (presumably diopside-poor).

Bloxam concluded that "lime-rich hydrothermal solutions and co2 , which immediately post-dated serpentinization, were responsible for the alteration of the gabbro •.....• "

De (1972) proposed that rodingites from South

Eastern Quebec originated from the crystallization of gabbro, diorite, or less commonly granite, and by modification of the crystallizing minerals under the high water pressure and the silica-deficient environment of serpentinized ultramafic rocks. He found that the ultramafic rocks in the area were dominantly serpentinized harzburgites and dunites and concluded from chemical analyses that only a fraction of a per cent CaO might have been lost during serpentinization of harzburgites while in the case of dunites the loss of Cao would not be appreciable. In addition, De also noted that calcium rich pyroxenes in the ultramafic rocks were comparitively resistant and persisted until the advanced stages of serpentinization. 216

All the theories discussed above point to a conclusion that rodingitization and serpentinization are directly or indirectly related phenomena irrespective of the source of lime. However, it should be made clear that such a relationship is not universally accepted.

For instance, Phemister (1964) disagreed that rodingitic transfo.rmation of gabbro from Shetland Islands, Scotland, was genetically related to the serpentinization process and proposed that lime-rich solutions were probably related in origin to those which were effective in the saussuritization of the gabbro. Kolesnik (1974) in his discussion on the origin of rodingites from ultrabasic massifs remarked that the lower limit of thermal stability

(400°c) of rodingitic assemblages did not conclusively prove that calcium silicate rocks and serpentinites were formed simultaneously. Kolesnik also pointed out that rodingites are well known in harzburgite massifs with virtually no calcium. From the hydrogen and oxygen isotope analyses, Wenner (1975) has recently suggested that much of the metasomatism associated with the formation of rodingites, except for late stage monomineralic veins of xonotlite and possibly pectolite, did not occur during the formation of lizardite and chrysotile in the associated· serpen tin i tes.

4. 82 Origin of Group 1 Rodingites from Mt. Lightning:

The characteristic features of Group 1 rodingites 217

are summarized below:

i) They generally form dyke-like bodies completely

enclosed within the serpentinized ultramafic rocks and

show intrusive relationships with the host ultr&~afics.

ii) Their mineralogy is dominated by calcsilicate minerals, of which diopside is the only relict mineral present.

iii) They show relict gabbroic and doleritic textures

and less commonly basaltic textures.

iv} Their bulk chemical composition is characterized by typically high cao, high H2o, high er and Ni, and high

Cr/V ratio, and low co2 and lm·1 alkalies.

v} They show a decrease in modal diopside and a

corresponding increase in grossularite and vesuvianite

from the margin towards the centre of a dyke.

The present author favours a secondary origin for

these rodingites. No primary rocks of similar

composition, either in mineralogy or in chemistry, to that

of a rodingite is known. Moreover the actual gradation

from a precursor rock to a rodingite, as has been noted

by a number of authors including Miles (1950}, Bloxam (1954},

Bilgrami and Howie (1959}, Coleman (1967} and De (1972}, 218

indicates a secondary origin for rodingites.

Ona task then is to identify the pristine characters of Group 1 rodingites. As has been mentioned in Sub-

Section 4. 81, roding i tes may be derived from a number of rock types. However, nowhere in the area mapped by the author has a Group 1 rodingite body been found to have a gradational contact with any other rock type.

Determination of the precursor of Group 1 rodingites thus becomes extremely difficult.

Diopside in Group 1 rodingites is considered by the present author to be a relict mineral. It should be mentioned that diopside of both igneous and metamorphic origins has been reported in literature

(Honnorez and Kirst, 1975) altl,ough most studies show that clinopyroxene is a relict mineral in rodingitic rocks. Textural evidences suggest that diopside in

Group 1 rodingites is of primary origin. The high cr2o3 content of diopsides from these rodingites (see Table 20, p 167) is also consistent with a primary origin since chemical analyses of diopsides (see Deer et al

1962b) indicate that metamorphic diopsides lack cr2o3 • -

Apart from diopside, no other relict mineral has been identified in Group 1 rodingites. In some specimens diopside shows partial alteration to colourless tremolite while samples collected from the central portions of dykes 219

contain very little or no diopside. Therefore, it is possible that some of the other major constituents

(grossularite, vesuvianite and chlorite) may owe their origin, at least partly, to the breakdown of diopside.

Relict textures perhaps provide the best clues to the nature of the original rock types. The unequivocal gabbroic and doleritic, and, less commonly, basaltic textures that are present in some specimens indicate a close connection between basic igneous rocks and Group 1 rodingites. Similar relict textures in rodingitic rocks have been reported from many localities elsewhere and have led a number of authors to reach the conclusion that many rodingites are altered or metamorphos ed basic igneous rocks.

The relict textures in Group 1 rodingites also suggest a pseudomorphous replacement of original plagio clase by garnet where rectangular or lath-shaped outlines of original plagioclase grains are still preserved.

The presence of relict clinopyroxene in the interstitial spaces of garnet pseudomorphs indicates that the pristine rock contained both plagioclase and clinopyroxene. Thus the textures point to a gabbroic parent rock.

Bulk chemical compositions of rodingites are of little genetic value. It is to be expected that during alteration or metasomatism of the parent rock most, if not 220

all, oxide values would change. Thus the low silica content of Group 1 rodingite does not necessarily indicate an ultramafic parentage since ultrabasic compositions can also be obtained from the alteration of basic rocks

(Vallance, 1969b). Nevertheless, it should be pointed out that the generally high Cr and Ni contents of Group 1 rodingites are consistent with a basic parentage.

Therefore it may be concluded mainly from textural evidences that basic igneous rock is the most likely parent for Group 1 rodingites. The immediate question to arise is the magmatic affinity of such a parent basic igneous rock. Once again it may be said that whole rock chemistry or normative composition of

Group 1 rodingi tes provide no evidence to establish the original magmatic character. As is the case for spilites from Mt. Lightning (see Section3.8) one has to seek other evidence such as the composition of relict diopside.

Four such diopside grains were analysed using the electron microprobe technique (Table 20, p 167). When plotted on a Kushiro-type diagram (Fig. 38, p 137) these diopsides lie in the tholeiitic field, and a LeBas-type diagram

(Fig. 39, p 137) shows that they all lie in the non- alkaline field. Thus it may be said that the parent material of Group 1 rodingite was derived from a non alkaline and, presumably, from a tholeii tic magma.

Although it was previously pointed out from 221

textural and mineralogical evidence that the precursor rock contained clinopyroxene and plagioclase, it is not possible to establish the modal percentages of these minerals. In fact a variation in the relative proportion of diopside and plagioclase cannot be precluded since some of the monomineralic varieties of Group 1 rodingites contain very little or no diopside. Two possible origins, among others, fer the monomineralic rocks can be suggested:

i) The monomineralic rodingites represent the extreme alteration product of the precursor rocks and the original mineralogy has been destroyed. The similarity between monomineralic types and the central portions of some heterogeneous dykes (see Sub-Section 4.S~supports this explanation.

ii) The monomineralic rodingites were derived from an originally plagioclase-rich rock.

Apart from diopside and plagioclase, whether or not the precursor rock contained any other major constituents is not known. Primary minerals such as olivine, orthopyroxene and amphibole might be expected to be associated with diopside and plagioclase. However, in Group 1 rodingites there is no trace of any of those minerals. Also the low A1 2o3 contents of diopsides from Group 1 rodingites (see Table 20 , p 167) suggest that the original rock was olivine-poor (LeBas, 1962}. 222

Ashley (1973) reported that rodingitic rocks from

the southern part of the Coolac Serpentine Belt were

derived from a number of rock types including wehrlite, hornblende gabbro, amphibolite and diorite. The

Mt. Lightning Group 1 rodingites could not have been

derived from hornblende gabb::::-o, amphiboli te or diori te s1.1.ff ~c.i.ent since none of these rocks contains clinopyroxene. A A wehrlite precursor is also unlikely because of the olivine-poor nature of the pristine rock (see above).

Therefore it may be concluded that a gabbro (or

an eucritic gabbro) or its finer grained equivalent is

the most likely parent rock for Group 1 rodingites. It

is also possible that progressive alteration of a coarse

grained precursor rock gives rise to a finer grained prod11ct. Thus the fine-grained varieties of rodingites may have been derived from an originally gabbroic rather

than a basaltic parent.

Recently, Menzies and Allen (1974) have reported

occurrences of anorthositic veins in the harzburgitic member of the Othris ophiolite. These authors have

also noted partly to completely rodingitized gabbroic

(clinopyroxene and plagioclase) pockets within lherzolite

masses at Othris. It seems possible that a similar

situation exists in the Coolac Serpentine Belt. Partially

rodingitized lenses are common in the northern part of

the belt at Mooney Mooney Range (Franklin, 1975) while 223

at Mt. Lightning the Group 1 rodingites represent the

completely meta~:oma ti zed clinopyroxene-plagioclase rocks.

Although no anorthositic veins or lenses have yet been

discovered from the Coolac Serpentine Belt, it is

possible that some of the monomineralic variants of

Group 1 rodingites are altered anorthositic bodies.

The present writer believes that the gabbroic

(and perhaps anorthositic) precursors of Group 1 rodingites

represent pockets of magma that was produced by a small

degree of partial melting of upper mantle peridotite.

These magma pockets are considered to have been trapped within the mantle peridotite and probably intruded the

host peridotite and the chromite pods at a later stage.

The slickensided surfaces that are occasionally observed

on the outer margins of these rodingites indicate movement

of these bodies within the host rock.

A small amount of partial melting of upper mantle

peridotite can produce a melt of plagioclase and clino

pyroxene composition (Boudier and Nicolas, 1972). These

authors suggested that feldspathic lenses within the

Lanzo lherzolite were produced by less than 4% of mainly

feldspathic melting in the original lherzolite, while

a 5% partial melting would result in a melt of clino

pyroxene and plagioclase (or gabbroic) composition. The

relatively small volume of Group 1 rodingites as compared

with the large volume of ultramafic rocks in the 224

Mt. Lightning area is in accordance with the theory of

a small volume of partial melting ( { 5%) of upper mantle

material.

The restricted occurrence of Group 1 rodingites

within only ultramafic rocks suggest that the partial

melting-segregation process of primary gabbroic melt was

confined to the upper mantle. The absence of Group 1

rodingites in the overlying spilitic rocks, which are

believed to represent oceanic crust (see Chapter 3 ),

also supports the claim that partial melting-segregation

process occurred beneath the ocea.nic crust.

4 .821 Rodingitization and Serpentinization:

Metasomatic transformation of gabbroic rocks to

Group 1 rodingites will undoubtedly involve chemical

changes. For instance, some silica will have to be

removed from a gabbroic rock whilst water needs to be

introduced in order to obtain a composition similar to

that of Group 1 rodingites. However, it is not possiblEj

to determine the net gain or loss of any individual oxide

without knowing the chemical composition of the precursor

rock. Therefore, no such attempt has been made by the

present writer. Nevertheless, it is felt that some

important chemical changes during rodingitization, that

have been proposed by various authors, should be discussed

briefly. 225

Coleman (1967) remarked that desilication and calcium enrichment are universal in rodingitic rocks.

The present author accepts that desilication is necessary for the derivation of rodingites from gabbroic parents.

However, whether lime must be introduced is questionable.

This will undoubtedly depend on the lime content of the precursor rock. A gabbro consisting of essential diopside and basic plagioclase (bytownite) can contain approximately 22 wt.% of Cao assuming a 1:1 ratio of diopside and bytownite. It should be noted that th1s value of lime is very similar to that of the average

Group 1 rodingite (22.85 wt.%, see Table 26, p 190).

Thus from a starting material containing 50% diopside and 50% bytownite, very little or no lime needs be introduced to form Group 1 rodingites.

The concept that calcium for rodingitization at

Mt. Lightning was released from the host ultramafic rocks is also questionable for the following reasons:

i) The host rocks are predominantly harzburgitic.

They contain small amounts cf clinopyroxene

which resists serpentinization to a greater

extent than olivine and orthopyroxene and which

com.~only remains largely unaltered even at

advanced stages of serpentinization. Also,

clinopyroxene lamellae in orthopyroxene grains

are usually extremely well preserved even though 226

the host orthopyroxene grains have been

serpen tinized.

ii} Chemical analyses of serpentinites collected

from the immediate vicinities of Group 1

rodingites do not show unusual Cao values (see

Section4.7}. It is believed by the present

writer that the lime content of serpentinite

from Mt. Lightning is mainly dependent on the

clinopyroxene content of the primary ultramafic

rock rather than on the serpentinization process.

In other words a low Cao value does not necessa

rily indicate a high degree of serpentinization

but may be attributed to a low clinopyroxene

content of the primary peridotite.

The concept that rodingitization and serpentiniza tion are complementary processes appears to be an over simplification. A number of authors, including Bilgrami and Howie (1959), Coleman (1967) and Honnorez and Kirst

(1975), have ascribed rodingitization to lime released

from clinopyroxenes of host rocks during serpentinization despite their recognition that clinopyroxene is preserved

as a relict mineral in rodingite. The present writer

challenges this view. It would be expected that in a given environment of serpentinization clinopyroxene in

rodingite precursor rocks and in host ultramafics would behave in a similar manner. Not only does the view of 227

the cited authors conflict with this expectation but it implies that lime removed from host rocks is preferenti ally incorporated in rocks which contain relatively abundant intrinsic lime, thus introducing additional and unexplained complications.

Also, assuming that lime, released during the serpentinization of associated ultramafic rocks, enters the rodingitic precursors, it is reasonable to expect that margins of ro

Mt. Lightning Group 1 rodingite increases from the margins toward the centres {see Sub-Section 4.711).

Therefore, it is concluded that lime needed to form Group 1 rodingites was available from the precursor rock (or magma) itself. l\.d.illittedly, the model of an initial gabbroic composition with 50% diopside and 50% bytownite, as has been suggested above, is an over simplification. Nevertheless, it shows that added lime may not be necessary to produce a rodingite. Such a hypothetical gabbro can contain approximately 17 wt.%

Al2o3 and thus coul

Al2o3 , see Table 26 , p 190 ) •

Of the other major oxides that would need to be 228

added or removed from the hypothetical gabbro MgO (and also H2o) seems to have been introduced from an extraneous source. The magnesia content of the average Group 1 rodingites (13.33 wt.%) is much higher than that expected (approx. 9 wt.%) from such an assumed gabbroic composition. It is possible that both MgO and H2o may have been added during reaction with sea water. Recent experimental investigation by Hajash (1975) indicates that during reactions between basaltic rocks and sea water at temperatures of 200° to soo 0 c, the compositions of both sea water and basalts would change. One of the most important changes, according to Hajash, is the introduction of MgO from the sea water into basalts.

Group 1 rodingites are believed to have formed under high activities of H2o and Cao and low activities of sio2 and co2 . The commonly high water content of

Group 1 rodingites indicates that H2o-activity was generally high, although it was probably variable since the monomineralic garnet rocks contain mainly anhydrous grossularite. The abundance of grossularite and/or vesuvianite points to a high CaO activity and a low Sio2 activity. Reported occurr.ences of lime garnet and vesuvianite in nepheline syenites and ijolites suggest that undersaturation of silica is an important factor for the formation of these two minerals (see Honnorez and

Kirst, 1975). The absence of carbonates in Group 1 rodingites accords with a low co2-activity during rodingitization. 229

Whether or not serpentinization and rodingiti zation of Mt. Lightning rocks took place more or less at the same tim9 is difficult to prove. In fact both processes might have taken place at various times and at various depths without being concurrent. Nevertheless, the world-wide association of rodingites and serpentinized ultramafic rocks may not be accidental. Both rodingiti zation and serpentinization require a high H2o-activity and a low Sio2-activity. The occurrence of rodingites in association with the highly serpentinized ul trainafic rocks in the area studied and the presence of relict clinopyroxene both in rodingites and in serpentinites

(see above) suggest that rodingitization and serpentini zation took place under similar physico-chemical conditions. 230

4.83 Origin of Group 2 Rodingites from Mt. Lightning:

The characteristic features of Group 2 rodingites are summarised below:

i) They occur along the contact between serpentinite

and variolite spilites.

ii) They generally show a thin tremolite-actinolite

rich border zone near the contact'with serpentinite.

Monomineralic chlorite rock is occasionally present

between variolitic spilites and rodingites.

iii) They are predominantly monomineralic rocks, the

mineralogy being dominated by zoisite or pr.ehnite.

Monomineralic garnet rock, although present, is rar.e.

iv) Their bulk chemical composition is characterized

by very high Al2o 3 and Cao, and by low Sio2 , MgO,

co2 , total iron oxides alkalies, Cr and Ni.

The nature of the parent reacting rocks for

Group 2 rodingites is less problematic than that for

Group 1 types because of their (Group 2 rodingites) close association or gradational relationship with variolitic spilites in the area. The idea that excess lime for this rodingitization was derived from the associated ultramafic rocks is largely discounted by the writer for the reasons 231

given in the previous sub-section (4.82) although the hydrous environment that promoted serpentinization might also have favoured rodingitization.

It is believed by the present writer that albite in the spilitic rocks was derived from a more calcic plagioclase (see Sub-Sections 3. 41 and 3. 7 ) • This albitization process would release both Ca and Al from the parental material of spilites. Vallance (1969b) suggested that removed Al would be precipitated close by mostly in the form of silicates such as chlorite, feldspar, hydromica and epidotes, and a low co2-activity might favour the formation of calcium aluminium silicates.

The abundance of calcium aluminium sillcate phases, such as zoisite and prehnite, and, less commonly, grossu larite, in Group 2 rodingites may be explained in a similar way. The absence of carbonates in Group 2 rodingites confirms that Co2-activity was low during rodingitization.

Wheth€!r zoisite or prehni te becomes the dominant constituent in these rodingites would depend on the activity of silica assuming constant pressure and temper ature conditions and constant activities of aluminium and magnesium (Coleman, 1967). At lower Sio2-activity zoisite will form instead of prehnite. This may explain the formation of mainly two monomineralic variants of 232

Group 2 rodingites namely the zoisite rock and prehnite rock. However, Coleman's claim that these two minerals usually do not coexist in rodingitic assemblages is not true for the Mt. Lightning examples. As mentioned before in Section 4.6 prehni te is ccmmonly found in the inter stitial spaces between zoisite euhedra in zoisite rocks.

Also veins of zoisite in prehnite rocks and vice versa are common in Group 2 rodingites. It is possible that during the formation of these rodingites the activity of Sio2 was variable.

The monomineralic garnet rock probably formed at a higher calcium-activity and a.lower H2o-activity than that required for the precipitation of either zoisite or prehni te. Composition determination has shown (see Sub-

Section 4. 624 ) that garnets from Group 2 rodingi tes are largely anhydrous. Therefore, crystallization of garnet in these rocks might have taken place at a lower H2o-acti vity •.

As mentioned before, Group 2 rodingites are similar to some of the "reaction zone" rocks described by Coleman (1967). Coleman envisaged that rodingitic rocks resulted from rnetasomatic interchange of elements between serpentine and country rocks. The formation of chlorite along the outer margins of rodingites according to Coleman is due to an increase in both silica and alumina while tremolite is believed to have formed due to reaction 233

between chlorite and serpentine if the silica and calcium activities are increased. Coleman (1967, p 12} also remarked "the serpentinites associated with the rodingites commonly show increases in both calcium and aluminium, ...... II

It is difficult to be certain that tremolite actinolite and chlorite rocks associated with Group 2 rodingites at Mt. Lightning formed due to reaction with host ultramafic rocks. There is definitely no indication of calcium and aluminium enrichment in the adjacent serpentini tes (see Sub-Section 4. 721 ) . Also the boundary between serpentinite and the tremolite-actinolite rim of Group 2 rodingites is sharp. It is possible that due to post-rodingitization movement serpentinites which show reaction effects are no longer in direct contact with Group 2 rodingites. However, tremolite-actinoli·te rims may also originate by reaction between seawater and basaltic rocks as suggested by Hajash (1975).

Hajash has recently demonstrated from experimental work that reaction between seawater and basaltic rocks at temperatures between 200° - soo 0 c can produce assemblages containing among other minerals tremolite- actinolite and prehnite. He concluded " ••.....••...• the experimentally produced assemblages are impressively similar to those found .•...•.•...... in metamorphic rocks from the oceanic crust, and in metamorphosed pillow 234

basalts from ophiolitic complexes. Such reactions between seawater and mafic rocks of the Honeysuckle Beds might have taken place in the oceanic crust (see Section 3.9 ) before its emplacement to the present position. The origin of tremolite-actinolite rocks and, in fact, that of Group 2 rodingites could also be explained in a similar manner. 235

4,84 Estimation of Temperature and Pressure Conditions:

Estimations of physical conditions during the forma tion of rodingitic rocks are difficult to infer mainly because the stability fields of commonly occurring calc-silicate minerals are not well established. On the basis of the stability of serpentine minerals, the composition of garnet and the presence of xonotlite, Coleman(l967) suggested a maximum ten:tperature of soo0 c and a minimum of 2S0°c for the formation of rodingitic rocks of the West Pacific Coast area.

Xonotlite is absent in Mt.Lightning rocks. The use of garnet compositions as a geothermometer is questionable

(see discussion in Honnorez and Kirst,1975,p 249, and Zabinski1 1964). An attempt has,therefore, been made to estimate the temperature of formal:ion of Mt.Lightning rodingites from the prehnite stability field and from the nature of associated serpentine minerals.

Experimental work by Liou(l971) shows that at about 400°c prehnite breaks down to zoisite + grossular +quartz+ H2o between 3 and 5 kbars. The dominant lizardite-chrysotile assemblage suggests a possible temperature range of 100°c to 300°c (see also section 2.6). Thus a temperature range of 100°c to 4oo0 c is plausible for the formation of Mt.Lightning rodingi tes.

The absence of high pressure index minerals such as lawsonite and jadeite, and that of blueschist facies metamor phism tend to suggest that the pressure during rodingitization was less than 6 kbars (see also section 2.6). 236

C H A P T E R 5

T R O N D H J E M I T E S

A N D

A L B I T I T E S

5.1 INTRODUCTION:

Leucocratic feldspathic rocks mainly occur as

enclosures within the mafic-ultramafic complex in the vicinity of Mt. Lightning. They outcrop essentially

along or near the western margin of the untramafics at

the junction of sheared serpentinite and spilites, except

for two dyke-like bodies which occur at the eastern

junction between the Young Granodiorite and peridotite.

Similar leucocratic rocks have been reported

from other alpine-type intrusive complexes or ophiolitic

assemblages by various authors including Thay1=r (1957), 237

Thayer and Himmelberg (1968), Aumento (1969), Aumento et al

(1971), Davies (1971), Moores and Vine (1971), Coleman and

Peterman (1975) and Ishizaka and Yanagi (1975). These rocks have been variably described as trondhjemite, quartz diorite, albite granite, tonalite, plagiogranite and albitite. Apparently similar sodium-rich acid igneous rocks have also been recorded in Precambrian shield areas all over the world (see Glikson and Sheraton, 1972).

Recently, Coleman and Peterman (1975) introduced the term "oceanic plagiogranite" as a general descriptive and collective term for various leucocratic rocks from ophiolitic assemblages. Following these authors,

Ishizaka and Yanagi (1975) have also used the term "oceanic plagiogranites" for trondhjemitic and quartz dioritic rocks from the Nagato tectonic zone in Japan. However, the prefix "oceanic" has a genetic implication undesirable in a descriptive term. Furthermore, some c.f t'.1ese leucocratic rocks (eg, some albitites and some diorites) do not contain modal quartz, so that the term "plagiogra nite" is not appropriate. Consequently, the present writer has retained such terms as albitite and trondhjemite for descriptive purposes. 2,38

5. 2 GENERAL STATEMENT:

Trondhjemi tes and albi ti tes :ire thE? two main varieties of acid feldspathic rocks encountered at

Mt. Lightning. Definitions of these terms and their petrographic descriptions are discussed in the following section (Section 5.3). These light coloured rocks constitute less than 1% of the area mapped by the author.

Although not common in the area, these rocks merit special attention for two reasons: i) they are the only acid feldspathic rocks in an otherwise almost exclusively mafic-ultramafic assemblage and, ii) the problem of their genetic affinities in other ophiolite assemblages has attracted considerable attention.

At Mt. Lightning it is difficult to determine whether or not these leucocratic feldspathic rocks are completely enclosed within the ultramafics. Structural relationships between either mafic or ultramafic rocks and acid feldspathic rocks are difficult to establish.

For instance, in the vicinity of Haystack Creek, south of the Murrumbidgee River, trondhjemites and albitites are closely associated with rodingites, variolitic spilites and serpentinites but well-defined contacts are lacking.

Further south, near the top of Haystack Creek, two small sub-rounded lens-like exposures of trondhjemite appear to be completely enclosed within serpentinites, while at the top of the ridge west of Mt. Lightning peak a large dyke- 239

like body of albitite seems to have intruded spilite. To

the north of the Murrumbidgee River small pockets of

trondhjemite occur sporadically along the intertonguing boundary between mafic and ultramafic rocks on the west

of the belt. Two large dyke-like trondhjemitic bodies,

one to the north and the other to the S()uth of the

Murrumbidgee River, occur along the contact between the

ultramafic mass and Young Granodiorite to the east.

The principal conclusions concerning the field

relationships of the acid feldspathic rocks at Mt. Light ning are: i) they tend to occur near both eastern and western margins of the ultramafic mass and, ii) they

are associated with highly altered ultrarnafics and not with slightly serpentinized peridotite. A similar

relationship of acid rock mass to highly serpentinized

alpine-type peridotites from Venezuela was reported by

MacKenzie (1960).

Leucocratic feldspathic rocks generally form

small sub-rounded outcrops with di~~eters less than 2 m.

Larger masses include a dyke-like body of albitite (about

8 m long and 5 m wide) and a trondhjemitic mass (about

5 rn long and 2 rn wide) occurring to the east of Mt. Light

ning near the contact with Young Granodiorite.

In hand specimen these rocks vary from almost

pure white to greyish white in colour, the amount of mafic 240

constituents being extremely small. Weathered surfaces show a yellowish brown coating of about 1 mm in thickness.

In some coarse-grained examples plagioclase twinning is distinct in the hand specimen but, some fine-grained varieties resemble chert.

Joints are common in the larger outcrops. Sub parallel joint planes often give the outcrop a bedded or foliated appearance (Fig. 57, p 241). The strike of the major set of joint planes is usually parallel to that of the ultramafic belt (ie, NNW - SSE). Occasional outcrops of medium- to fine-grained varieties show gneissosity or banding resulting from alternating streaks or ban~s of mafic minerals (chlorite) and light coloured minerals

(quartz and plagioclase). The darker layers (up to 1 mm in thickness) are much thinner than the white layers (2 mm to 4 mm in thickness). Lam;r1ahon Fig.57. An outcrop of albitite showing (pseudo-bedding). I\ 241 242

5.3 PETROGRAPHY:

Leucocratic rocks at Mt. Lightning consist predominantly of plagioclase (An2 to An 15 } and quartz. Other minerals which are present in minor amounts (< 15% by volume} include chlorite, epidote, sericite, sphene and magnetite.

Ferromagnesian minerals in these rocks do not exceed 15% by volume. Potash feldspar is typically absent.

Therefore, these leucocratic rocks may be called "trondhjemites" (Streckeisen et al, 1973). According to Joplin

(1971, p 20} a trondhjemite consists essentially of oligoclase or andesine, quartz and biotite; orthoclase and hornblende are rare or absent. The type of plagioclase in Mt. Lightning leucocratic rocks however, is not necessarily oligoclase. It ranges from oligoclase to albite. However, as will be discussed later, it seems possible, that these rocks have been affected by albitization. Therefore, trondhjemite is still the most a.ppropriate descriptive term for these rocks.

The term "albitite" has been used by the author to describe rocks containing more than 90% (by volume} albite.

Modal analyses of nine samples, which have also been chemically analysed, are given in Table 32, p 244. It 243

is clearly seen from the modes, that there is a consider able range in the proportions of plagioclase and quartz.

Plagioclase content varies from 60% to 92% (by volume) while quartz shows a range from 3% to 31% {by volume).

Coleman and Peterman {1975) have also reported a wide range in the quartz/feldspar ratio in leucocratic rocks from ophiolitic assemblages.

Despite careful search, K-feldspar has not been found in either albitite or in trondhjemite at Mt. Light ning. A few plagioclase grains showing simple twinning resemble a K-feldspar, but application af the selective staining technique of Bailey and Stevens {1960) on three thin sections confirmed that discrete grains of K-feldspar are absent in these rocks.

Scarcity of K-feldspar in leucocratic rocks from the oceanic crustal-sub-crustal association has been commented upon by var~ous authors. For example, Aumento

(1969) reported that only one of numerous dredged diorite samples from the Mid-Atlantic Ridge at 45°N contains distinct grains of K-feldspar. Moores and Vine (1971) have not mentioned the presence of K-feldspar in quartz dioritic rocks from the Troodos massifs, Cyprus. In their discussion on oceanic plagiogranites, Coleman and

Peterman (1975) suggested that ophiolitic leucocratic rocks rarely contain potassium feldspar in contrast to granophyres associated with continental mafic intrusions. TABLE 3 2-

MODAL A.NALYSES OF TRONDHJEMITES AND ALBITITES FROM Mr. LIGHTNING AREA

(arranged in order of increasing plagioclase content)

1 2 3 4 5 6 7 8 9

Quartz 31 21 22 24 18 17 10 5 3

Plagioclase 60 65 66 66 70 72 78 90 92

Chlorite 5 8 6 5 4 6 3 1

Scrici te 1 2 1 3 1 2 2 4

Epidote 2 1 2 4 2 3 1 1

Sphene 1 2 1 3 1 1 3 1

Magnetite l 2 1 1 1 1

Samples as in Table 33 , p 2.S6 . 245

In fact potassium deficiency in oceanic leucocratic rocks has been utilized by these authors to distinguish them from continental occurrences.

Modal analyses of Mt. Lightning trondhjemites and albitites, when plotted on a quartz-plagioclase-alkali feldspar diagram (after Streckeisen, 1973) show these rocks to lie on the quartz-plagioclase join (see Fig. 58, p 246).

Albitites and two trondhjemites fall in the diorite field of Streckeisen. However, they are not diorites because their content of mafic constituents is always less than

10% by volume (see Table, 32 , p 244 ) •

Plagioclase grains in Mt. Lightning trondhjemites are commonly twinned and zoned. Composition determinations in strongly zoned sections indicate a decrease in An cont8nt from the core towards the margin of a grain, an observation in accordance with those recorded from similar rock types elsewhere (Aumento, 1969; Moores and Vine, 1971).

The An content of plagioclase grains in Mt. Lightning trondhjemites varies from 8 to 15 (mol.%) in the core, while the rim shows a range from 4 to 10 mol.% An. The method used to determine these compositions is the same as that described in Section 3.4,p 96.

Zoned plagioclase grains are much less common in albitites than in trondhjemites. HowEiver, where zoning is observed, the An content shows a similar trend to that Fig. 58. Modal analyses of trondhjemites and albitites from Mt.Lightning plotted on a Quartz-Alkali feldspar-Plagoclase diagram (classification after Streckeisen et al,1973).

Fig.59. Plots of normative feldspars of trondhjemites and albitites from Mt.Lightning. The dotted· lines separate various rock types on the basis of their normative feldspar ratios (after O'Connor, 1965). 246

Quartz

Granite

Alk- felds. syenite syenite Monzonite Monzo diorite

-··

I I I I / C, . q, I~ .:,<,,; I o~ ~ ·s ~-,; I t> ~ 1 ~o - I~-- I ~ I -- I ...~ -- ...... '- ...... -t-: (~ /-- -- • • I ---- • Tro'ldhjemite• • / I Granite ------__ _ Ab 247

described above, although the range (An3 to An10) is much more restricted in albitites. Unzoned plagioclase sections vary in composition from An 3 to An6 indicating a slightly more sodic nature than the zoned sections.

Refractive index determinations on four unzoned plagioclase sections (p = 1.534 to 1.537) also suggest a similar low An content.

Coleman and Peterman (1975) remarked that the most com.~only identified ferromagnesian mineral in leucocratic rocks from ophiolitic assemblages is either hornblende or pyroxene. In Mt. Lightning leucocratic rocks, however, chlorite is the predominant mafic mineral and occurs as elongated and often shredded flakes. It is also occasionally found to be enclosed poikilitically in large albite grains. Whether any hornblende or pyroxene was origi~ally present, and was later transformed to chlorite, is not known. However, such features as pleochroic haloes and mottled extinction, occasionally observed in chlorite from

Mt. Lightning samples, may indicate pseudomorphous transformation of original biotite rather than hornblende or pyroxene.

Pleochroism is usually weak in these chlorites suggesting a low iron content (Deer et al, 1967, p 237).

The absorption formula may be written as oc =(3 > Y , where ocand ~ are pale yellow to colourless, and~ is yellowish green. Anomalous blue interference colour is 248

characteristic, the 2V is usually low (10° to 25°) and the optic sign is always positive.

Sericite is the most common alteration product in trondhjemites and albitites at Mt. Lightning. Usually it occurs as aggregates of shreds in the groundmass.

Sericite is particularly common in saussuritized plagioclase. The term sericite is used broadly here for fine-grained non-pleochroic white mica (Deer et al, 1967, p 20 2) • Whether this sericite is similar in composition to muscovite or paragonite has not been determined.

However, considering the generally low K2o content of the host rocks, and the abundance of sodic plagioclase, paragonite seems the likely species in these rocks.

The other accessory minerals (epidote, sphene and magnetite) occur as disseminated interstitial grains.

In some thin sections epidote is also found to be poikilitically enclosed in plagioclase grains. The presence of such epidote inclusions may indicate a secondary origin for the host sodic plagioclase. Pale yellow colour in plane polarized light and typically high birefringence indicate a high pistacite content for epidotes in these rocks. 249

5. 4 MICROTEXTURE:

Trondhjemites and albitites at Mt. Lightning are extremely variable in microtexture. Both igneous and metamorphic textures are apparent. For example, some specimens show a true granitic texture (hypidiomorphic to allotriomorphic granular) while cataclastic and gneissic textures are characteristic in others.

One notable textural characteristic of these rocks is that the quartz-rich varieties show a more pronounced cataclastic texture than the quartz-poor types.

Gilluly (1933) observed that the more siliceous varieties of albite granites from Sparta, Oregon, are notably cataclastic. Indeed, he recognized a direct correlation between brecciation and high quartz content. A similar correlation between the quartz content and the degree of cataclasis is evident in the Mt. Lightning occurrences.

5.41 Granitic Texture:

Hypidiomorphic to allotriomorphic granular texture

is best shown by albitites and the quartz-poor varieties of

trondhjemites. The grain size is usually medium to

coarse (average rv 2 mm). Some medium-grained samples

resemble aplite in thin sections. Gradations between

the coarse-grained and the medium-grained varieties are

common in a single thin section. 250

The effects of deformation such as undulatory extinction can be seen in plagioclase and quartz grains.

Plagioclase grains also show bent twin lamellae (Fig.60 p 251) and rarely, microfaulting. Features such as banding or gneissic structure, mylonitization and breccia tion are absent in these rocks.

5.42 Gneissic or banded texture:

This texture, characterized by alternating mafic

(chlorite) and leucocratic (quartz and plagioclase) bands, is restricted to trondhjemites and is believed to indicate penetrative deformation (Fig. 61, p 251). All constituents are elongated parallel to the direction of foliation and this mineral alignment becomes more prominent with an increase in chlorite content.

Quartz is usually lobate and is moderately to strongly sutured and strongly undulatory. Plagioclase is bent, locally fractured and faulted. Chlorite is commonly stretched and sometimes shredded. With an increase in deformation these gneissic varieties grade into mylonitic types.

5.43 Mylonitic texture:

Mylonitic varieties are characterized by extreme granulation. Banding is usually present but is less Fig.60. Photomicrograph showing hypidiomorphic to allotriomorphic granular texture in a trondhjemite. Specimen No: 5/24. Crossed polars. Frame length: 1.5mm.

Fig.61. Photomicrograph of a gneissic trondhjemite showing alternating light coloured (consisting of quartz and albite) and dark coloured bands (consisting of chlorite). Specimen No: 2/39. Ordinary light. Frame length: 1mm. 251 252

prominent than in gneissic varieties. A fine-grained matrix of quartz and plagioclase surrounds porphyroclasts of plagioclase and, less frequently, of quartz

Occasionally porphyroclasts are deformed into lenticles producing "augen" texture. Chlorite in these rocks is invariably shredded.

5.44 Microbrecciated texture:

Banded Mylonitic varieties are associated with massive microbrecciation types. The latter apparently

,rl.,I..C1'0- Were derived from the former where local intense breccia- ~ tion had erased the earlier linear structure of the mylonites. One noteworthy feature of microbreccias is the abundant microfaulting in plagioclase grains (Figs. 62&63, p 253 ) •

5.45 Other textures:

Porphyroclastic and poikiloblastic textures are relatively uncommon in Mt. Lightning leucocratic rocks.

The porphyroclasts of plagioclase and/or quartz occur in a finer-grained groundmass of similar composition.

Rarely, snall chlorite grains are found poikili

tically enclosed in a large grain of plagioclase (Fig. 64

.p 253 ). Fine-grained plagioclase (possibly of an early

generation) and quartz accompany chlorite in some sections Fig.62. Photomicrograph of a trondhjemite showing micro brecciated texture formed by subangular grains of quartz (clear) and plagioclase (cloudy grains) in a fine-grained matrix consisting of quartz, albite and minor chlorite. Specimen No: 5/13. Crossed polars. Frame length: l.Snun.

Fig.63. Photomicrograph of a trondhjemite breccia showing a large microfaulted grain of albite. Specimen No:4/30 •. Crossed polars. Frame length: 1.5mm.

Fig.64. Photomicrograph showing small grains of chlorite (isotropic) poikilitically enclosed in albite (greyish white) in a trondhjemite. Specimen No: 5/14. Crossed polars. Frame length: 0.7mm. 253 254

of plagioclase (cf. granoblastic texture of Gilluly, 1933).

Whether or not the host plagioclase is a product of recrystallization is difficult to determine. Both the host.. and the included plagioclase grains show undulatory extinction, although they do not extinguish simultaneously.

Myrmeketic and granophyric textures have been reported from similar leucocratic rocks elsewhere (Gilluly,

1933: Coleman and Peterman, 1975). However, these textural varieties are absent in Mt. Lightning rocks.

From the above discussion, it is clear that leucocratic rocks at Mt. Lightning underwent more than one period of deformation. As is to be expected, there are samples which show combinations of textural varieties. At least four stages of deformation have affected these rocks:

(1) post crystalline deformation producing undulatory extinctions in quartz and feldspar, (2) penetrative deformation resulting in a strong mineral alignment,

(3) later penetrative deformation producing mylonitization and (4) further deformation causing directionless microbre cciation.

Thin section examination of textures has revealed a complete gradation between quartz-rich varieties of trondhjemites and quartz-poor albitic rocks, a fact difficult to establish from field evidences due to sporadic outcrops in the area. 255

5.5 WHOLE ROCK CHEMISTRY:

Nine whole rock analyses of leucocratic rocks

(including seven trondhjemites and two albitites) from

Mt. Lightning are given in Table 33, p 256. It is apparent that these rocks are characterized by high silica and soda, typically low potash and low total iron, magnesia and lime contents.

Approximate ranges of variation of major consti tuents (in weight per cent) are as follows:

Silica 9, alumina 6, soda 6, ferric iron 1.5,

magnesia 1.5, lime 1.5 and ferrons iron 1.

Other oxides (titanium, manganese, potassium and phospho rous) show variations over a range of less than 1% by weight.

Plots of normative An - Ab - Or for Mt. Lightning trondhjemites and albitites are shown in Fig. 59, p 246.

This diagram shows that these rocks fall in the trondhje mite field of O'Connor (1965). Normative contents of An,

Ab and Or for oceanic leucocratic rocks and those for continental trondhjemite (data from Coleman and Peterman,

1975) are also plotted on the same diagram for comparison.

Coleman and Peterman have remarked from their study of oceanic leucocratic rocks that the normative orthoclase TABLE 33

1. Trondhjemite. Specimen No: 4/13, GR 255752 Tumorrama 1:50,000 Sheet. 2. Trondhjemite. Specimen No: 2/39, GR 249762 Jugiong 1:50,000 Sheet. 3. Trondhjemite. Specimen No: 4/308, GR 252752 Turnorrama 1:50,000 Sheet. 4. Trondhjemite. Specimen No: 2/38, GR 248766 Jugiong 1:50,000 Sheet. 5. Trondhjemite. Specimen No: 4/30A, GR 256762 Jugiong 1:50,000 Sheet. 6. Trondhjemite. Specimen No: 5/24, GR 236776 Coolac 1:50,000 Sheet. 7. Trondhjemite. Specimen No: 5/14, GR 238773 Coolac 1:50,000 Sheet. 8. Albitite. Specimen No: 2/11, GR 246756 Gundagai 1:50,000 Sheet. 9. Albitite. Specimen No: 3/9, GR 237758 Gundagai 1:50,000 Sheet. TAilLE 3 3

CHEMICAL ANALYSES AND NORMS OF TRONDHJEMITES AND ALBITITES FROM MT. LIGHTNING AREA

Major Elements (in weight per cent)

1 2 3 4 5 6 7 8 9

Si02 75 .11 73.14 73.82 72. 02 72 .41 69.62 67.24 67.07 66.24 Ti02 0.34 0.42 0.31 0.65 0.26 0.28 0.52 0.16 0.38 A1 203 13.84 14.01 12.64 14.82 13.27 14.62 16.40 18.31 17 .63 Fe2o3 0.48 0.82 1. 75 1.62 1.68 1.94 1. 82 0.41 0.62 Fe0 1.04 0.62 0.82 1.21 0.74 1.24 0.91 0.20 0.18 Mn0 0.16 0.04 0.09 0.14 0.18 0.10 0.10 0.03 0.09 Mg0 0.48 0.66 0.68 1.06 0.92 1.96 1.82 1.20 0.82 Cao 1.07 1. 81 2.42 1.89 2.33 1.92 1.64 1.59 1.02 Na2o 3.84 5.82 4.73 4.74 5.24 5.46 7.01 9.50 9.89 I K20 0.79 0.68 0.41 0.38 0.34 0.40 0.22 0.05 0.12 P205 0.11 0.04 0.09 0.12 0.12 0.03 0.07 0.04 0.10 H2o+ 1.21 1.42 1.68 1.62 1. 84 1.62 1.82 0. 71 1.34 H2o- 0.69 0.74 0.74 0.63 0.68 0.40 1.04 0.34 0.61 co2 0.04 0.07 0.06 0.06 0.04 0.03 0.04 0.05 0.04

Total 99.20 100.29 100.24 100.96 100.05 99.62 100.65 99.66 99.08

Na20/K2o 4.86 8.56 11.54 12.47 15.41 13.65 31.86 190.00 82.42

Norms (CIPW)

Qz 47.18 32.15 41.12 37.27 36.88 31.45 19.19 6.74 6.03 Or 4.67 4.02 2.42 2.25 2.01 2.36 1.29 0.30 0.71 Ab 32.48 49.22 40.00 40.00 44.32 46.18 59.29 80.35 83.65 An 4.34 8.27 10.52 8.21 10.17 8.54 7.44 7.19 4.24 C 4.08 1.67 2.19 3.44 2.25 3.68 3.97 - - Di ------0.10 - Hy 1.20 1.64 1.69 0.88 2.29 5.28 4.53 2.94 2. 71 Mt 1.08 0.91 2.03 2.35 2.21 2.81 1. 75 0.28 - Hm 0.30 0.19 0.35 - 0.15 - 0.61 0.22 0.62 11 0.65 0.80 0.59 1.23 0.49 0.53 0.99 0.30 0.57 Ap 0.25 0.09 0.21 0.28 0.28 0.07 0 .16 0.09 0.23 Cc 0.09 0.16 0.14 0.14 0.09 0.07 0.09 0.11 0.09 H2o 1.90 2.16 2.42 2 .25 · 2.52 2.02 2.86 1.04 1.95

Analyses by the author. 257

content is usually less than 4 mol.%. Most of the

analysed Mt. Lightning samples also have less than 4 mol.% normative orthoclase but, two samples (Nos. 2/39 and

4/13) have 4.02 and 4.67 mol. percentages respectively.

However, compared with oceanic plagiogranites of Coleman and Peterman, Mt. Lightning rocks show considerably less variation in normative An. In fact, as shown in Fig. 59, p 246, they lie close to the Ab apex.

Fig. 59 also demonstrates that Mt. Lightning

leucocratic rocks have a higher Ab/Or ratio than that of continental trondhjemite. Absence of modal K-feldspar in Mt. Lightning rocks certainly accounts for that.

Table 33 , p 256 , clearly indicates that

Mt. Lightning trondhjemites and albitites are character

ized by high Na 2o/K2o ra tic (> 4. 8) , which is stated to be the distinctive feature of acid to intermediate plutonic rocks associated with alpine-type ultramafics (Thayer, 1967).

A high Na 20/K2o ratio for leucocratic plutonic rocks from similar ultramafic-mafic environment elsewhere has been reported by various authors including Aumento (1969),

Aumento et al (1971) and Coleman and Peterman (1975).

Coleman and Peterman (1975) have distinguished

oceanic leucocratic rocks from continental trondhjemites

utilizing a Sio2 versus K2o diagram. However, when plotted on the same diagram (Fig. 65, p 258) Mt. Lightning 258

Continental trondhjemite- "- ..-'~ (/ \ 10 ' ', . t ,I,..---.._ ...,...,...... , '.- I I "l,-...."- / / Oceanic pla~iogranit~' '\ { ) \ / 0-1 '-, ____ .,,,,, ...... --- . ., .....

om~-.....J...-__,, __...__...... L. __,1_ _ _,__-J-_ _J 40 50 60 70 80 Wt.% 5102

Fig.65. Semilog plot of Si02 against K2o in trondhjemites and albitites from Mt.Lightning. Defined fields after Coleman and Peterrnan,1975). 259

leucocratic rocks lie in both continental and oceanic fields. Thus, the use of K2o vs Sio2 plots to differen- tiate continental trondhjemites from those of oceanic areas is disputable.

The average major element composition of Mt.

Lightning leucocratic rocks is given in Table 34, p 260.

Chemical analysis of a leucocratic variety of diorite dredged from the Mid-Atlantic Ridge at 45°N (Aumento,

1969), the average composition of three plagiogranite samples (data from Coleman and Peterma.n, 1975) and the average granite composition (Nockolds, 1954) are also listed in Table34 for comparison.

There is a distinct similarity in major element composition between the average Mt. Lightning leucocratic rock and Aumento's leuco-diorite (Table 34, p 260). The

FeO*/MgO ratio (total iron as FeO) of the average

Mt. Lightning sample ( 1.76) is closely comparable with that of Aumento's leuco-diorite (FeO*/MgO = 2.05), but is considerably lower than those of Coleman and Peterman's average plagiogranite and Nockold's average granite

(FeO*/MgO = 3.41 and 5.08 respectively). Coleman and Peterman's average plagiogranite is also distinctly rich in lime compared with the others.

As part of the evidences for uniqueness of oceanic leucocratic rocks, Coleman and Peterman (1975) remarked i6o

TABLE 34

AVERAGE CHEMICAL COMPOSITION OF MT. LIGHTNING LEUCOCRATIC ROCKS COMPARED WITH THOSE OF SIMILAR ROCK TYPES FROM ELSE- WHERE AND WITH THAT OF GRANITE (in weight per cent) 1 2 3 4

Si02 70.74 72.47 66.66 72.08 Ti02 0.37 0.33 o. 72 0.37 Al 203 15.06 14. 77 14.00 13.86 Fe2o3 1.30 1.85 3.36 0.86 FeO 0. 71 1.19 2.46 1.87 MnO 0.10 0.08 0.05 0.06 MgO 1.07 1.39 1.61 0.52 Cao 1. 74 1.48 4.93 1.33 Na2o 6.25 5.55 2.73 3.08

K20 0.38 0.24 0.34 5.46

P205 0.08 0.06 0.05 o. 18

H2o+ 1.47 0.90 1.55 0.53 H2o ... 0.65 0.68

Co2 0.05 < 0.01 < 0.05

FeO*(total 1. 76 2.05 3.41 5.08 Fe as FeO/ MgO

1. Average Mt. Lightning Leucocratic rock (the average of Anal. 1 to 9, Table 33, p 2.S-6).

2. Leuco-diorite from the Mid-Atlantic Ridge at 45°N (Aumento, 1969).

3. Average plagiogranite from the Troodos Complex, Cyprus (the average of three analyses given in Table I in Coleman and Peterman, 1975, specimens Cy-55C, Cy-52 and Cy-SSA).

4. Average granite (Nockolds, 1954). 261

on their low Rb and Sr contents. Available data, although limited, on Rb contents of leucocratic rocks dredged from oceanic areas and of those from alpine-type ultramafic associations support the above authors' opinion.

Mt. Lightning trondhjemites and albitites are no exceptions

(the Rb values range from 0 to 6 ppm, Table 35 , p 262 ) •

However, as regards Sr content, the data from

Mt. Lightning rocks (Table 35, p 262) show a wide variation ranging from 225 to 760 ppm. Compared with other similar geochemical characteristics (low potash content and high Na 20/K20 ratio) amongst leucocratic feldspathic rocks from oceanic environments, the wide variation in Sr content of Mt. Lightning samples is notable.

Recently, Ishizaka and Yanagi (1975) have also reported a similar variation in Sr content (ranging from 218 to 892 ppm) in trondhjemitic rocks associated with serpentinites from South-west Japan. These authors state that Sr contents are apparently controlled by those in plagioclase.

The Ba content of Mt. Lightning leucocratic rocks is also quite variable, ranging from 79 to 225 ppm.

Coleman and Peterman (1975)r however, have reported very low Ba concentrations ( < 20 ppm) in oceanic plagiogranites from Troodos, Cyprus. The only analysis of a leuco diorite sample from the Mid-Atlantic Ridge (Aumento, 1969), on the other hand, gives a value of 200 ppm Ba. TABLE 3S-

TRACE ELEMENT CONTENTS OF TRONDHJEMITES AND ALBITITES FROM MT. LIGHTNING AREA

(in ppm)

1 2 3 4 5 6 7 8 9

V 2 8 10 * 25 * 27 19 16 Cr 3 18 2 6 22 15 2 10 5

Ni 64 33 25 56 45 50 58 70 60

Rb * 2 * * 4 6 6 2 4

Sr 290 536 225 372 728 300 475 760 640 y 12 6 25 10 16 10 20 18 22

Zr 180 145 51 86 49 75 105 119 98

Ba 79 160 180 122 108 150 205 146 225

Analyses by the author. &by Dr. B. Chappell, A.N.U. 263

Mt. Lightning trondhjemites and albitites contain considerable Ni (ranging from 25 to 70 ppm) compared with average granite (0.5 ppm, Taylor, 1965). The Ni enrichment, however, is not unexpected in a mafic-ultramafic environ ment. The er content of Mt. Lightning leucocratic rocks

(2 to 22 ppm Cr) is not consistently high compared with that of •raylor·-1 s average granite {4 ppm Cr). 264

5.6 GENESIS:

Thayer (1967) commented on the close relationship between feldspathic and mafic-ultramafic members of alpine- type gabbro-peridotite complexes. However, the genetic relationship between these members is uncertain according to Thayer.

Aumento {1969) was of the opinion that diorities, which occur on the faulted scarps of two seamounts at Mid

Atlantic Ridge, are directly related to the associated mafic and ultramafic rocks. He concluded " ..... magmatic differentiation has progressed further than is generally

thought possible in mid-oceanic ridge environment". In a later report on the geology of Mid-Atlantic Ridge at

45°N, Aumento et al (1971) suggested that the trondhjemites or albite granites are products of residual magmatic liquids which intruded at a late stage into serpentinite bodies.

Coleman and Peterman (1975) favour the idea that

oceanic plagiogranites result from differentiation during

the development of oceanic crust. The similarity of

87sr;86sr ratios of mafic and silicic rocks of the

ophiolite suite at Troodos is believed by these authors

to be consistent with the idea of low-pressure differen

tiation of sub-alkaline basaltic magma to produce K-poor

leucocratic rocks. 265

The world-wide occurrence of oversaturated leucocratic rocks in alpine-type or ophiolitic assembla ges is perhaps the most conclusive evidence in favour of their genetic relationshipsto the mafic and/or ultramafic associates. In fact, Coleman and Peterman (1975) did suggest that oceanic plagiogranites should be considered as a part of the ophiolite association.

The position of these dioritic rocks in the ophiolite sequence is difficult to determine. Commonly they occur in close association with the gabbroic parts of ophiolites (Thayer, 1967; Aumento, 1969; Aumento et al 1971; Moores and Vine, 1971). However, as has been discussed previously in Section 5.2 (p 239 ), these leucocratic rocks at·Mt. Lightning appear to have intruded both spilites as well as serpentinized ultramafics and occur close to the margin of the ultramafic mass.

Unaltered gabbros are absent in the area.

Three possibilities may be considered to explain the origin of trondhjemites at Mt. Lightning:

(1) Partial melting of greywackes, producing

alkali-rich acidic liquids;

(2} Partial melting of amphibolite or eclogite

at mantle depths producing acidic liquids;

(3} Fractionation of abyssal tholeiitic magma

giving rise to silica-rich melts. 266

Greywackes are common in the western part of

the area and form part of the Honeysuckle beds. Partial melting of these greywackes may produce an early sodic magma (see Glikson and Sheraton, 1972) or a quartz diorite magma (see Arth and Hanson, 1972). Although greywackes

from the Mt. Lightning area have not been analysed, petrographic investigation reveals that they contain K feldspar (see Sectionl.4, p 18) and hence, will have a higher potash than trondhjemites which lack K-feldspar.

Arth and Hanson (1972) have remarked that partial melting of greywacke must give a more potash-rich melt than the parent greywacke. The extremely low K2o content of Mt. Lightning trondhjemites precludes the possible genetic relationship between greywackes and trondhjemites at

Mt. Lightning. Moreover, the relatively high Ni content of these trondhjemites as compared with that of the avsrage granite of Taylor (see Section 5. 5 , p 263 ) supports their genetic relationship with basic or ultrabasic rocks.

High pressure experiments (Green and Ringwood,

1968) have revealed that acidic and intermediate magmas may be produced by partial melting of amphibolite or eclogite of basaltic or gabbroic composition at mantle depths. The theory has also been exploited by Arth and

Hanson (1972) to explain the origin of quartz diorites and trondhjemites from Minnesota, Ontario and California

in the United States. These authors have envisaged

transformation of basalt or gabbro to amphibolite or 267

eclogite along a subduction zone with subsequent partial melting at mantle depths. However, although liquids arising from melting of hydrated eclogite or amphibclite are typically sodic, high pressure experiments (Green and Ringwood, 1968) also reveal that fractional separation of plagioclase under crustal pressure conditions could lead to potash enrichment. The lack of equivalent potash-rich members in the Mt. Lightning area or in the Coolac

Serpentine belt (Ashley, 1973; Fr~nklin, 1975) makes the origin of these leucocratic rocks through partial melting of eclogite or amphibolite doubtful.

The origin of gabbroic and associated silica-rich rocks from mid-oceanic ridge and ophiolitic assemblages by crystallization and differentiation of tholeiitic magma has been commented upon by a number of authors including

Thayer (1963, 1967, 1969), Thayer and Himmelberg (1968),

Moores (1969), Aumento (1969) ,Miyashiro et al (1970),

Davies (1971), Coleman (1971a) and Coleman and Peterman

(1975). Although gabbros and diorites do not occur in the Mt. Lightning area, they have been described from elsewhere in the Coolac serpentine belt by Golding (1966,

1969), Ashley (1971, 1973) and Franklin (1975). The genetic relationship between gabbros and trondhjemites from the Coolac rocks has been noted by both Ashley (1973) and Franklin (1975).

High pressure experiments have shown that 268

tholeiitic magma may be produced by partial melting of

"pyrolite" in the upper mantle. (Green and Ringwood,

196 ) . Differentiation of such a tholeiitic magma will result in a silica and alkali-rich rei::t magma from which dioritic rocks may crystallize. The presence of cumulate ultrabasic (wehrlites, pyroxenites and minor dunites) and basic rocks in the Coolac serpentine belt and their close association with dioritic rocks (Franklin,

1975) indicate that crystallization and differentiation may have been responsible for the origin of such an association.

Ashley (1973) favoured the idea that trondhjemites from the southern part of Coolac belt were formed directly from a residual Na 2o and Sio2-rich melt (produced by differentiation of an abyssal tholeiitic magma). Thus the albitic plagioclase in these rocks is presumably primary in origin according to Ashlay. However, the presence of secondary epidote in feldspar hosts (see

Section 5. 3 , p 248 ) tends to suggest that the sodic plagioclase in the leucocratic rocks at Mt. Lightning was derived from an originally more Ca-rich plagioclase and is thus secondary in origin.

It is believed by the present author that trondhjemites and albitites from Mt. Lightning were derived by alteration of a diorite (or a quartz diorite) precursor. Although no fresh diorite has been found in the vicinity of Mt. Lightning, Franklin's (1975) work 269

shows that there is an actual gradation between diorites and trondhjemi tes (and albi ti tes) in th

Complex of the Coolac serpentine belt.

Albitites in the Mt. Lightning area are believed to be the extreme alteration produce of dioritic parental material through an intermediate transitional state of trondhjemites. The complete gradation between trondhjemites and albitites, as evidenced by textures (see

Section 5. 4, p. 254 ) , may be significant in this respect.

Moveover, the occurrence of almost pure albite in

Mt. Lightning albitites makes an igneous origin for these albitites doubtful (Gilluly, 1933). It is possible that these albite crystals are pseudomorphs after an originally more calcic feldspar. 270

CHAPTER 6

C H R O M I T I T E S

6.1 INTRODUCTION:

One of the distinctive features of Mt. Lightning

is the number and diversity of the chromitite pods

(chromite deposits) that occur within its confines. About

40% of the chromite pods known from the Coolac ultramafic belt as a whole occur at Mt. Lightning and these include

some of the largest pods (Fig. 66, p 271). Moreover

the Mt. Lightning chromitites exhibit a textural and mineralogical diversity, and a range of chromite composi

tions (between pods), that closely approach the diversity

and compositional range recorded for chromitite pods from

the belt as a whole (Golding, 1_)t?rsonal communication) •

Chromite has been mined or prospected at more

than 20 sites between the Murrumbidgee River and the Fig.66. Variation in chemical composition of chrome spinels from 38 pods along the Coolac Ultramafic Belt (after Golding and Johnson,1971). H = harzburgite S = serpentinite W = wehrlite and clinopyroxenite G = gabbro

V = basic volcanics 271

I I I KZSZSZ9

V .- .- ----====------c. MT. --- LIHTNING ---- r.::e-- r,, • t,,; ,,, , ttSZSZSI " ..::. ·::::::----- &5¥1¥& = •· z I I I I I I I I I ,-j LZS2S2S.i ·- ---~-___ ---...... _ f ?-e;;; •• t f K?SZY s ------

KANGAROO MINES ----

G---• w------411'4 G----• -----

0 10 20 30 '40 so 60 70 60 qo ,oo WE IGHT PER CENT 0 2 E3 czs:n FeO MgO 272

Adjungbilly Creek. Some former pits and shafts have been

filled-in but abundant material for sampling remains at

3 or 4 sites. Most of the pods are clustered at Quilters

East Mines, Quilters West Mines, Quilters Central (or South)

Mine on the northern slopes of Mt. Lightning and at

Mt. Mary Mines on the southern slopes (Fig. 67, p 273).

Apart from the possible economic importance of

the Mt. Lightning chromitites as a source of chromite, the

chromitites merit study as members of the mafic-ultramafic

rock association at Mt. Lightning. The chromit.ites might be' regarded as the end member of a spectrum of rock types which ranges from chromitite through dunitic, peridotitic,

gabbroic and basaltic members, or their metasomatic

derivatives, to trondhjemites and albitites. In addition

to the petrogenetic problems relating to this association

as a whole, and the problem of the inter-relationships of

the members, the chromitites present a number of specific

problems that set them apart from the other members.

(Thayer, 1964, 1969, 1970; Irvine 1967; Golding and

Johnson, 1971; Dickey 1975; Golding 1975).

In view of the considerable data on the Mt.

Lightning chromitites that are already available (Golding,

1966, 1975; Golding and Bayliss, 1968; Golding and

Johnson, 1971} the specific contribution by the present

writer to the petrological investigation of these rocks

was a matter for deliberation. ·Golding (personal

communication) drew attention to the following relevant 273

+ + + + +~~ + + + + + + + + + + + + + + + s-. + + + + + + cl't>. + ~~ + +

WELCH'S MT. ,, ..... •• ,

Fig.67. Sketch map showing locatuions of analysed chromite samples from Mt.Lightning- Welch's Mt. area. 274

circumstances:

(i) The :-.ilable chemical analyses of the chrumi te

cons: i:uent of the Mt. Lightning chromitites

indicate a range of compositions from > 60%

and > 30% A1 2o3 , in wt per cent (Fig. 66, p 271 ).

(ii) This compositional range has seldom been

documented for chromite pods from similarly

restricted areas within comparable ultramafic

belts elsewhere, was the subject of specific

comment by Thayer (1969, p 145; 1970, p 386)

and is of considerable petrogenetic interest

(Irvine, 1967; Dickey, 1975).

(iii) The available chemical analyses of the

Mt. Lightning chromites however, were made mostly

on chromite sub-samples from which associated

silicates, secondary chromite and magnetite

had been incompletely removed.

(iv) It was therefore desirable to re-determine the

major element composition of the primary (unaltered),

pure chromite constituent from a selection of

Mt. Lightning chromitites by means of the

electron microprobe analyser. 275

The writer, influenced by these considerations, has therefore focussed attention on the chromite composi tions. Eleven new analyses of chromites, 10 from Mt. Lightning and one from Welch's Mt., have been made using the electron microprobe technique. The Welch's Mt. sample was analysed because of its relative proximity to Mt. Lightning and because of its previously reported high er-content. 276

6.2 GENERAL FEATURES OF THE MT. LIGHTNING

CHROMITITE PODS:

6.21 Distribution of Pods:

On the northern fall of Mt. Lightning, chromitite pods occur at intervals over much of the width of the ultramafic belt. The largest quarries and underground workings are those of Quilter's East Mines in the blocky harzburgite (e~stern) sector of the belt. The Quilter's

West workings occur in sheared serpentinites between the

Haystack Creek variolitic spilites and the western margin of the belt. The pod at Quilter's Central Mine lies within blocky and sheared harzburgite relatively close to the junction of the eastern and western sectors of the belt. The distribution of the pods with respect to the ultramafic lithologies and hence, to possible stratigraphic divisi.ons within the ultramafic member of the association, therefore seems to be random.

6.22 Shape, Dimension and Orientation of Pods:

As inferred from numerous excavations, most pods were lenticular (podiform), aligned parallel to the strike of the belt, ar.d were 5 - 50 m long, 1 - 5 m wide and

5 - 10 m deep. Quilter's Central pod may have been torpedo-shaped prior to erosion. A cross-sectional segment of this pod is exposed in an upper quarry and 277

lower slope. The quarry face, 8 - 10 m wide and 2 m deep,displays considerable chromite in situ, but post er:t:?laceme:·Tt ~lock faulting has resulted in a highly irregular tihape for the lower part of this pod, as revealed in the adit and mine. Not all the Mt. Light ning chromitite occurrences are lenticular. Some are irregularly shaped masses of chromite-serpentinite breccia, others are of chromite-rodingite breccia and a few are simply angular blocks of chromitite occurring in serpentinized harzburgite or dunitic serpentinite. At Quilter's West closely spaced pods that occur in parallel may represent faulted and off-set fragments of a pre-existing larger pod.

6 .23 Internal Structure, Textures and Contacts of Pods:

At QuiltBr's Central, massive chromitite {50% -

80% chroni h~) ,:;ha::!'.'.:1.c·c-=rizes the central and western side of the pod. Such chromitite is coarse-grained, and consists of nodular or faceted chromite textural units about 5 mm in diameter. This massive chromitite sharply abuts blocky harzburgite on the west. Towards the eastern margin of the pod fine-grained chromitites appear {see Figs. 68 and 69, p 278) and the contact is marked by fine-grained disseminated chromite in dunitic serpent inite which merges with sheared serpentinized harzburgite. Chromitite from Quilter's West mines is either fine grained and massive (70% - 95% chromite) with a polygonal granular texture as revealed under the microscope, or is Fig.68. Sawn surface of a specimen from Quilter's Central mine showing a sudden change in grain size of chromite that may indicate layering. Black: chromite, grey: chlorite. Frame length: 4.8mm.

Fig.69. Sawn surface of a specimen from Quilter's Central mine illustrates nodular, disaggre gated nodular and (lower centre) re-aggregated chromite. Black:chromite, grey:chlorite. Frame length: 4.8mm. 278 279

brecciated so that prior textures have been obliterated.

Small isolated masses of chromitite flanking the main working at Quilte:r:'s East include lineated nodular chromitite (Fig. 70, p 280) and gneissic chromitite

(Fig. 71, p 280).

6.24 Rodingite-Chromitite Relationships:

All the larger excavations at Mt. Lightning reveal some rodingite-chromitite breccia. The rodingite is of Group 1 type. Narrow dykes (5 - 10 cm wide) and narrower veins of rodingite also transect chromitite and

"hair cracks" in chromitite commonly contain rodingitic minerals. To some extent excavations in harzburgite or serpentinite, that is, free of chromitite, would probably reveal rodingite almost anywhere at Mt. Lightning. There is little doubt, however, that the ease of fracturing of brittle and massive chromitite favoured the preferential entry of mafic magmas that consolidated to the gabbroic and doleritic precursors of the Group 1 rodingites.

However, it should also be noted that several of the most impressive single dykes or dyke-like masses of Group 1 rodingite appear to be unassociated with and unrelated to chromi ti tes. Fig.70. Hand specimen of a lineated, nodular chromitite in which the chromite nodules are associated with bronzite grains. The chromite nodules (black) contain inclusions of bronzite and other silicates (white) and are separeted by grey and white bronzite grains. Specimen No:284~ Frame length: 8.2cm.

Fig.71. Hand specimen of an olivine-chromitite 'pencil gneiss'. The specimen has been sawn in two directions to show the lineation. A vein of serpentine intrudes the specimen and indicates that veining post-dated the lineation, Black: chromite, grey and white: serpentine. Specimen No:246. Frame length:lOcm. 280 281

6.3 MINERALOGY OF CHROMITITES:

The mineralogy of the chromitites is extremely complex in detail. Very broadly, er-rich chromite appears to have been initially associated with predominant olivine and Al-rich chromite apparently occurred with olivine and clinopyroxene or with clinopyroxene only. At Mt. Lightning er-rich chromite with minor secondary magnetite rims is commonly associated with lizardite-chrysotile serpentinite together with rodingitic minerals. Al-rich chromite accompanied by ferrit chromite is commonly associated with chlorite some of which contains relict clinopyroxene.

The clinopyroxene is partly altered to chlorite that is peppered with minute grossularitic garnets. Interaction between rodingitic minerals and chromite at Mt. Lightning has resulted in chrome chlorite, chrome grossularite and rarely chrome vesuvianite. Chalcedony, opaline silica, and carbonates also occur in veinlets in Mt. Lightning chromi ti tes. A chromi te-bronzi te association (Fig. 70 , p 280) is rare and restricted to a single block from

Mt. Lightning. Other constituents of the chromitites including awaruite and metallic minerals have been observed in serpentine minerals within chromitites (Golding, 1966). 282

6.4 CHEMISTRY OF CHROMITES:

6.41 Location and Brief Description of Analysed

Samples:

Specimen No. 199 (From Quilter's South Mine

- Lower Workings)

The hand specimen displays euhedral to sub-hedral

chromite grains ( rv 5 mm in diameter) and resembles an

orthocumulate but may be a disaggregated, annealed

(polygonal granular) accumulate (Golding, personal

communication). The polished section shows chromite with an orthogonal network of silicate-filled fissures.

There is about 5% of higher-reflecting secondary chromite, which forms rims to the chlorite-filled fissures.

Specimen No. 179 (From Quilter's South Mine

- Upper Workings)

The hand specimen consists of greyish black

chromite with a few per cent of grey chlorite that includes

traces of clinopyroxene and grossularite. The polished

section is similar to Specimen Number 199.

Specimen No. 317 (From Mt. Mary Mines)

The hand specimen is of nearly pure chromite that

has a vitreous lustre and shows red internal reflections 283

on thin edges. The polished section reveals faint

sinuous grain boundaries in the chromite (Fig. 72 , p 284 ) which is essentially unaltered. Small amounts of colour-

less diopside, amphibole and chlorite are present.

Specimen No. 155 (From Quilter's West Mines)

The hand specimen contains about 70% dull greyish black chromite. It is brecciated and veined with chlorite, green chrome-grossularite, pink vesuvianite or mixtures of these minerals. The polished specimen is complexly veined and altered (see Fig. 50 in Golding, 1966).

Specimen No. 357 (From Mt. Mary Mines)

The hand specimen shows 90% of brecciated chromite and 10% of chlorite. The polished section reveals that about 10% of the chromite is altered at

junctions with chlorite.

Specimen No. 274 (From Quilter's East Mines)

The hand specimen may be described as a chromite

serpentine breccia. The polished section shows fractured

chromite with serpentine minerals, chlorite and chalcedony

occupying fissures which are bordered by altered chromite. Fig.72. Photomicrograph of a polished section showing chromite grain shape and boundaries. The grain boundaries are barely visible but havebeen accentuated with ink. Minor pyroxene, amphibole and chlorite (dark grey) accompany the chromite. Specimen No: 317. Frame length: 2.1mm.

Fig.73. Photomicrograph of a polished section showing chromite grain shape with~a polygonal-granular aggregate of chromite (white) associated with small amounts of silicates (black and dark grey). Specimen No: 291. Frame length:3.Smm. 284

I . / ' \ ... '\ ~\

~ r--~:} \ . .... /

\ ~ ,1 ' J . \ ' \ I • , / -- \ --r/;

~ ~ - ~ , ' '. 1 ~. :. ,. ., - .j

J I ' I

.,. , . . ' ' ..._.: ., ' ., . -, I {'',...... - r-· . _,, ... ., lI I • (; • • . • I • q J 285

Specimen No. 291 (From Quilter's East Mines)

The hand specimen shows about 85% of dull greyish black chromite associated with grey chlorite. The polished section reveals a polygonal-granular texture with chromite grains up to 2 mm in diameter (Fig. 73 , p 284).

Specimen No. 284 (From Quilter··' s East Mines)

The hand specimen exhibits lineated, ellipsoidal, nodules of chromite associated with grains of bronzite

(Fig. 70, p 280 ). The chromite nodules are polycrysta- lline and contain silicate inclusions. The chromite is unaltered.

Specimen No. 113 (From Quilter's West Mines)

The hand specimen is similar to specimen number 155. The polished section shows about 70% of chromite in polygonal-granular aggregates of grains (...... , 1 mm wide). The chromite is altered to porous ferrit-chromite along selvedges to veinlcts.

Specimen No. 246 (From Quilter's East Mines)

In hand specimen this sample is strongly lineated and may be termed a "serpentinized olivine-chromite pencil gneiss" (Golding, personal communication). The chromite 286

occurs in "strings" of small polygonal grains. The

. "strings" are separated by "pencils" of serpentine

minerals. In polished section the chromite is homogeneous

apart from thin magnetite rims to some grains.

Specimen No. 402 (From Welch's Mine}

The hand specimen contains about 98% of greyish

black chromite. The polished section reveals that the

chromite is brecciated and contains a few per cent of

chlorite and chrome-grossularite. About 5% of the

chromite is altered.

6.42 Analytical Results:

Electron microprobe analyses were carried out on

sub-samples of the 11 specimens collected by Dr. H. G.

Golding. For each sample analyses were made at two spots

about 100 microns apart. It was found that oxide values

for each pair of analyses do not differ by more than 0.05 wt%.

Averages of two analyses carried out per specimen are

given in Table 37 , p 287 , where analyses are arranged in

order of increasing cr2o3 values.

In the electron microprobe analyses carried out

by the author total iron was recorded as FeO. Assuming

spinel stoichiometry, these total iron oxide values were

distributed to FeO and Fe2o3 using the method of Bateman

?"' ?"'

IQ IQ

~ ~

7 7

.3 .3

tr tr

11 11

tr tr

8.4 8.4

0.11 0.11

7.79 7.79

7.43 7.43

402 402

31. 31.

68.3 68.3

77 77

14.3 14.3

14.05 14.05

11.62 11.62

60.04 60.04

101.04 101.04

tr tr

10 10

0.10 0.10

0.12 0.12

9.4 9.4

113 113

7.84 7.84

9.84 9.84

9.91 9.91

26.5 26.5

73.5 73.5

18.6 18.6

15.43 15.43

57.01 57.01

100.25 100.25

tr tr

9 9

7.8 7.8

0.08 0.08

246 246

6.41 6.41

69.3 69.3

73.0 73.0 72.0

19.2 19.2

30.7 30.7

10.06 10.06

14.36 14.36

11.34 11.34

99.25 99.25

57.00 57.00

7 7

77 77

tr tr

8 8 tr tr

author. author.

9.5 9.5

7.99 7.99

0.05 0.05

274 274

0.09 0.09

9. 9.

71. 71.

26.0 26.0

74.0 74.0

18.8 18.8

10.00 10.00

15.58 15.58

56.91 56.91

100.39 100.39

the the

by by

7 7

tr tr

6.2 6.2

0.07 0.07

291 291

9.58 9.58

5.22 5.22

74.8 74.8

27.7 27.7

66.1 66.1

15.89 15.89

14.91 14.91

52.96 52.96

98.70 98.70

Analyses Analyses

) )

CHROME-SPINELS CHROME-SPINELS

284 284

7.90 7.90

0.05 0.05

9.05 9.05

20.6 20.6 25.2

79.4 79.4

26.7 26.7

50.90 50.90 10.7 10.7

62.6 62.6

17.13 17.13

14.60 14.60

99.82 99.82

OF OF

37 37

CHROMITITES CHROMITITES

5 5 6

357 357

8.4 8.4

0.11 0.11 0.11

7.23 7.23

0.11 0.11 0.08 0.07

29.4 29.4

70.6 70.6

61.9 61.9

11.29 11.29

16.15 16.15

50.09 50.09

15.16 15.16

100.23 100.23

Al~ALYSES Al~ALYSES

TABLE TABLE

. . h

LIGHTNING LIGHTNING

text. text.

.11 .11

tr tr tr tr tr

4 4

C C

155 155

0.05 0.05 0.09

0.06 0.06

0.12 0.12

9.51 9.51

79.0 79.0

11.2 11.2

21.0 21.0

59.2 59.2

48.05 48.05

29.6 29.6 29.8

16 16

17.01 17.01

98.96 98.96

MT. MT.

see see

MICROPROBE MICROPROBE

FROM FROM

data data

tr tr 3 3

tr tr

317 317

8.6 8.6

7.67 7.67

0.07 0.07

0.10 0.10

8.51 8.51 8.05

78.5 78.5

49.6 49.6

21.5 21.5

99.03 99.03

17.51 17.51

23.58 23.58

41.59 41.59 41.8 41.8

ELECTRON ELECTRON

sample sample

72 72

2 2

tr tr

199 199

8.2 8.2

0.07 0.07

0.11 0.11

7.43 7.43

8.97 8.97

For For

78.1 78.1

35.52 35.52

21.9 21.9

29. 29.

40.8 40.8

51.0 51.0

18.01 18.01

99.92 99.92

1 1

tr tr

179 179

0.07 0.07

7.4 7.4

0.08 0.08

0.12 0.12

6.92 6.92

39.8 39.8

74.9 74.9

31.15 31.15 35.17 35.17

52.8 52.8 25.1 25.1

10.47 10.47

17.51 17.51

101.49 101.49

%) %)

3 3

2 2

(mol. (mol.

Number Number

FeO FeO

RO RO

Specimen Specimen

R203 R203

V203 V203

MgO MgO

Total Total Al

MnO MnO

MgO MgO

NiO NiO Ti0 288

(1945). The RO and R2o 3 values were then recalculated to 100% (Table 37, p 287).

A considerable variation in the composition of chromites is revealed by th~se analyses. The approximate ranges of oxide values are as follows (given in wt%): cr2o 3 - 25, A1 2o 3 - 23.5, Fe2o 3 - 4.5, FeO - 4 and

MgO - 4. The other oxides (v2o 3 , MnO, Tio2 and NiO) vary over a range of less than 1 wt%.

The range and spread of values for cr2o 3 and

A1 2o 3 substantially confirm those of the earlier analyses

{Golding, 1966) in which cr2o 3 varied from 35.7 to 61.2 wt%

{or by 25.5 wt%} and A1 2o 3 from 8.8 to 32.8 wt% {or by 24 wt%) for chromites from the same mine deposits. Most new individual values of these oxides, however, are low2r than former values by 0.1 to 3.0 wt%.

The ranges of Fe2o3 , FeO and MgO have contracted from approximately 7, 7 and 6 wt% {in earlier analyses by Golding, 1966} to 4.5, 4 and 4 wt% respectively in these new analyses. Most new individual values of Fe2o 3 and MgO are higher whilst those of FeO are lower, by 1 to

3 wt%, than the old values.

On a Thayer-type diagram (Fig. 74 , p 290 } R2o3 molecular values are plotted and compared with previously obtained values (Golding, 1966) for samples from the same 289

deposits. The general correspondence between the new and the old analyses is well defined although the higher Fe2o3 and lower cr2o3 and Al2o3 values in the new analyses of several samples are evident.

The minor element content of the chromitites is of interest. v2o3 and NiO were determined in only four of Golding's analyses of Mt. Lightning chromitites where both v2o3 and NiO were reported to be 0.1 wt% in one sample and "nil" in three samples. Ti02 and MnO values were reported in Golding's analyses as 0.06 to 0.13 wt% and

0.10 to 0.34 wt% respectively. Because of the presence of impurities in Gelding's material the location of these constituents in the chromites was in doubt. The data obtained by the writer show:

(i) v2o3 ~ 0.05 wt% in all but the most er-rich chromites (Analyses 9, 10 & 11 in Table 37,

p 287) where only trace amounts of v2o3 are obtained.

(ii) NiO ~ 0.09 wt% in all but one chromite

(Analysis 7, Table 37 , p 287 ) •

(iii} Tio2 value ranges from 0.07 to 0.11 wt% in 8 sub-samples but was detected in only trace

amounts in the others.

(iv) MnO was not detected above trace amounts. N U) 0

2 :\ (0

,10

---.

cent

deposits.

80 20

per

represent

single analyses.

Mt.Lightning

mole

dots old

the

from

(in

70 30

2 from

Fe circled

represent

chromites

and

3 samples of

eleven o

The

triangles

chromite

the

in area.

compositions

in )

Mt.

join R

analyses,

50 50

total

Variation new Welch's Lines

Fig.74.

A.l293~ 291

6.5 DISCUSSION:

6.51 Conclusions Based on the Author's

Investigations:

1. There is no correlation between the E - W location in the belt (or the "stratigraphic level") of the pods and their size, mineralogy, texture, or chromite compositional type. In particular, er-rich chromites occur in pods: on the east (at Quilter's East) and on the west (at Quilter's West) of the belt. Al-rich chromites characterize the occurrence at Quilter's Central but, a rather Al-rich type also occurs in the most easterly pod

(East Ridge). The distribution of the pods on the basis of chemical compositions of chromites thus seems to be random within harzburgite and serpentinite in the Mt.

Lightning area. However, it has been noted (Golding,

1966) that whereas er-rich chromite occurs in pods throughout the Coolac belt as a whole, such pods are interspersed with pods containing Al-rich chromite principally in the north (Mt. Lightning and the Mooney

Mooney Range) where mafic rock members of the association are most prominent.

2. Chemical analyses by the writer accord with the tendency previously recorded (Golding, 1966) for the larger deposits to be mainly er-rich (52 - 62 wt% cr2o3 ) with a lesser number containing Al-rich chromites (34 - 292

46 wt% Al2o3 ) but some small pods contain chromites of transitional composition (48 - 52 wt% cr2o3 ).

3. There is a tendency for MgO to increase with Al2o3, although Anal. 5 and 6 (Table 37, p 287) depart from this trend.

4. The chromites predate magmas that consoli dated to gabbroic, doleritic and basaltic rocks (subse quently rodingitized) which penetrate chromitite as narrow dykes, veins and veinlets. One sample from Quilter's

East (Specimen No. 246) and awaruite-rich serpentinite veins in chromitite at Quilter's Central suggest that dunitic magma may also have been available after chromite consolidation. The similarity of many of these rodingitic vein-rocks to the dyke-like masses of Gp. 1 ·rodingite that lie within the harzburgite and the form of the latter as dykes rather than as schlieren, suggest that the rodingitic precursors post-dated both chromitite and harzburgite.

These relationships suggest that, in general, as noted by

Golding (1966, 1969), the harzburgite was essentially solid prior to the existence of magmas which gave rise to chromitite and dunitic and gabbroic veins.

6.52 Aspects of Genesis of the Podiform

Chromitites:

Most recent observers of podiform chromitites 293

favour their formation in the upper mantle at depths of

10 - 25 j,:rn be low marginal seas or island arc sites. The chromite is usually regarded as a precipitate from magmas that are stated to be mafic, tholeiitic or picritic, although some authors leave the magma composition unstated.

Thayer (1969, 1970) considered that podiform chromitites, together with their harzburgitic and dunitic host rocks, and associated gabbroic robks, originated in large sub-crustal layered "supercomplexes". He envisaged that the chromitite layers were broken up and re-emplaced together with the harzburgitic, dunitic and gabbroic mushes to crustal levels. Golding and Johnson (1971) also envisaged the possibility of such an origin for the chromitite pods of the Coolac belt but, suggested that the observed rock assc~blages also included significant amounts of depleted mantle peridotite which formed the environment for the postulated layered complex. Peters and Kramers (1974) believed that podiform chromititesin the North Oman ophiolites precipitated from magma at a specific level in pre-existing depleted mantle peridotite within which the magmas had ascended. This level was considered to correspond to the transition between the stability fields of spinel- and plagioclase-lherzolite.

Dickey (1975) and Golding (1975) regarded podiform chromititesas polygenetic. Except where former pods have been fragmented, individual pods were considered 294

to have formed in individual magma pockets or small magma chambers. Pods therefore formed at somewhat different times, different levels and different conditions within host rocks that were regarded by both authors as essentially depleted mantle peridotite. According to Dickey (1975)

Cr/Al ratios would be expected to decrease with temperature

(in the absence of plagioclase) but the major influence in chromite composition is probably er-depletion and Al enrichment in fractionating magmas.

Pods containing Al-rich chromite in the Coolac belt may have formed at a higher level than those containing er-rich chromite. It is possible that the former are lowermost, earliest members of "transitional" diopside-rich cumulates that, in the north of the Mooney Mooney Range, overlie the harzburgite (Franklin, 1975). Such

"transitional" cumulates may formerly have existed more or less continuously above the harzburgite. Pods containing Al-rich chromite may have descended into the subjacent harzburgite and become mixed with pods containing er-rich ch~0~ite (Golding, 1966, 1975).

A bimodal distribution of pods on the basis of chromite composition was indicated for the Coolac occurrences by Golding (1966). Most pods contain er-rich chromite (52 - 62 wt% cr2o3 ). A lesser number of pods contain Al-rich chromite (34 - 46 wt% cr2o3 ). Very few pods contain chromite with 48 - 50 wt% cr2o3 and these are 295

very small. Subsequently, (Thayer, 1970; Golding and

Johnson, 1971) this bimodal tendency was recognized as a general feature of the class of podiform chromite. The reason for the gap in chromite composition is not yet clear (see also Dickey, 1975).

l~l though textures of the Mt. Lightning chromi ti tes can be explained broadly as modified settled textures, the problem of origin of chromite nodules is unresolved.

Chromite of most pods presumably precipitated in the magma in which the chromite settled but the possibility of other origins is not excluded. For example, during the diapiric ascent of a peridotitic mush, partial melting of silicates might produce sufficient volumes of liquid to permit pre existing residual accessory or disseminated chromite to become concentrated at the base of the diapir. Podiform chromi ti tes are therefore regarded by the present author as types of cumulate sensu lato. Some are probably "normal" cumulates in which euhedral chromite crystals precipitated and settled in a given magma. Some are modified cumulates in which the settled aggregate developed a polygonal granular texture as a result of grain boundary re-crysta llization (static annealing). Nodular chromitites may also have formed by settling and concentration of nodules but the origin of the nodules is not clear. Finally some chromitites may contain settled units that are older than the magma in which they accumulated. 296

In conclusion, it may be said that the compositional and textural diversity of the chromitites at Mt. Lightning and their association with a variety of minor rock ~ypes of magmatic origin (dunites, rodingites, trondhjemites and albitites) are features that support the view that the

Mt. Lightning chromitites are polygenetic, represent magmatic cumulates that formed in a number of discrete magma pockets scattered within, and perhaps partly above, depleted mantle peridotite. 297

C H A P T E R 7

D I S C U S S I O N

AND

C O N C L U S I O N

7.1 OPHIOLITES AND THE MT. LIGHTNING ROCKS:

The name "ophiolite" was introduced by Steinmann

{1905, 1927). According to Church's {1972) translation of Steinmann's {1927) description, the definition reads as follows:

"As ophiolite one should designate only the

consanguineous association of chiefly ultrabasic

rocks, the most important component of which

consists of peridotite {serpentinite) the more

subordinate of gabbro, diabase, spilite or norite

and related rocks".

A pa.rticula1.· association of red chert, pillow lava and serpentinite is often c.esignated as "Steinmann's

Trinity". It should be emphasized that synonymous terms 298

such as 'ophiolite suite', 'ophiolite assemblage' or simply 'ophiolite',refer to a specific assemblage of rocks and not to a single rock type. The rock types of an ophiolite suite are, as pointed out by Gass et al (1975), sometimes chaotically mixed together in which case the

Recently in the Penrose Ophiolite Symposium

(1972) a model profile of an ophiolite sequence, similar to that described by Steinmann (see Church, 1972), was adopted (see Table 38, p 300).

In this thesis the term 'ophioli te' has been used in a purely descriptive sense with no genetic implications as has also been suggested by Gass et al

(1975).

The association of ul tramafic, ma fie .J.nd sedi- men tary rocks in the Coolac-Tumut region including the

Mt. Lightning area seems to be similar to a typical ophiolite assemblage. Almost every stratigraphic member of an ophiolite sequence is represented in this region although at places one or more of the members may be missing.

A feature of the Mt. Lightning rock assemblage is that only the lowermost (basal peridotites with tectonic fabric) and the upper members (sediments and spilitic 299

rocks) are clearly represented. Minor bodies such as podiform chromites and sodic felsic intrusives (trondh- emites and albitites) are also well exposed. However, the gabbroic complex containing cumulus peridotites, pyroxenites and gabbros, and the mafic sheeted dyke complex are absent in the vicinity of Mt. Lightning.

Excellent examples of diopside-rich cumulates(including wehrlite,clinopyroxenite and gabbro) belonging to the gabbroic complex of an idealized ophiolite sequence occur in the North Mooney area near Coolac (Franklin,

1975). Such rocks may have overlain the basal peridotites everywhere prior to its emplacement in the Coolac

Tumut region but have been tectonically removed in places.

Perhaps the best exposed ophiolite sequence in south-eastern Australia occurs about 10 km north of

Mt. Lightning in the North Mooney area (see Table 38 , p 300).

The only missing member of an idealized ophiolite sequence is the sheeted dyke complex (Franklin, 1975). As yet a sheeted dyke complex has not been recognized in the vicinity of the Coolac Serpentine Belt. Sheeted dyke complexes are also absent in many other ophiolites, eg, the Papuan ophiolite. It should be pointed out that an idealized ophiolite sequence is rarely encountered and a whole section of the sequence may be missing (see Gass et al, 1975; Coleman and Irwin, 1975). Therefore even

though sheeted dykes are absent, the ultramafic-mafic

sedirnentary association in the Coolac-Tumut region may 300

still be regarded as an ophiolite, perhaps an incomplete or a dismembered ophiolite.

TABLE 38

~mparison of ultramafic-mafic-sedimentary complex of Mt.Lightning rith an ideal ophiolite sequence and with rocks of the North Mooney complex.

Ophiolite sequence North Mooney complex Mt.Lightning (Penrose Conference, (Franklin,1975) 197 2)

Sedimentary section- 'Sedimentary section- Sedimentary section ribbon cherts, thin tuffs, sandstones cherts, quatzites shale and minor 1 ime and silts tones. greywackes and tuffs. stones.

Ma f i c v o 1 can i c Metabasalts and Spilitic rocks. complex commonly meta-andesites. pillowed. i Mafic sheeted dyke Absent Absent complex.

Gabbro complex- Wehrlite, pyroxe- Absent peridotites and pyr- nites, dunites, oxenites with gabbro and diorites. cumulus fabric.

Ultramaf ic complex Harzburgites and Harzburgites and ~rzburgite, dunite dunites with minor lherzolites and lherzolite with tectonic fabric. with tectonic fabric. tectonic fabric. 301

7. 2 MT. LIGHTNING ROCKS AS PART OF A LAYERED

OCEANIC CRUST-UPPER MANTLE SEQUENCE:

The similarity between the ophiolite sequence and layered models for the oceanic crust and upper mantle strongly suggests a genetic relationship. For instance,

Gass et al (1975) commented "there are few who would question the oceanic setting for the genesis of ophiolite rocks", while Dewey (1976) remarked" ...... • there has been a great revival of interest in Alpine-type mafic/ ultramafic complexes mainly as a consequence of the hypothesis that ophiolite complexes represent slices of oceanic crust and mantle •.....•...• ".

It seems likely to the writer that the .Mt.

Lightning mafic-ultramafic rocks represent a slice of oceanic crust and upper mantle. This view is based on the textural similarity between Mt. Lightning peridotites and mantle-derived xenoliths (see Sub-Sections 2.31 and

2.57), the chemical similarity between spilitic rocks in the area and oceanic basalts (see Sections 3.6 and

3.9 ) and the typically K2O-poor nature of mafic and felsic rocks, characteristic of oceanic rocks. The local

(ophiolitic) and region3l tectonic settings of Mt. Light ning rocks (see Section 7.3) are also compatible with an oceanic origin for these rocks.

Assuming the incomplete or dismembered ophiolite sequence exposed in the vicinity of Mt. Lightning to be 302

representative of oceanic crust and upper mantle, a correlation between Mt. Lightning ultramafic-mafic sedimentary association and sub-oceanic layers may be attempted. The bulk of the information on the oceanic lithosphere comes from geophysical and deep sea drilling data (Raitt, 1963; Raitt et al 1969; Cann, 1968, 1970b;

Miyashiro 1969, 1970; Melson and Thompson, 1970; Barret and Aumento, 1970; Christensen, 1970, 1972; Bonatti et al 1971; Aumento, 1972; Vine and Moores, 1972 and

Christensan and Salisbury, l975). Below is an attempted correlation between .Mt. Lightning rocks and the sub-oceanic layers, drawn mainly from the work of Dewey and Bird (1971),

Coleman (1971b) and Aumento (1972).

Layer 1: consists mainly of chert, pelagic limestone,

foraminiferal ooze and compacted sediments.

At Mt. Lightning this layer is represented

by the sedimentary portions of the Honeysuckle Beds

(chert, phyllite, quartzite and greywackes- see

Sub-Section 1.4 ).

Layer 2: composed mainly of extrusive basalts and

spilites with minor intrusions.

At Mt. Lig-htning layer 2 is represented by

the spilitic rocks of the Honeysuckle Beds.

Layer 3: contains mainly gabbroic rocks (gabbro, wehrlite,

pyroxenite) with minor diorites, trondhjemites and

albitites. 303

At Mt. Lightning layer 3 is not clearly

represented apart from the minor rock bodies of

trondhjemites and albitites •

• • • • • • • • • . • • . • • • MOHOROVICIC DISCONTINUITY • •••••••••••• ; •••

Layer 4: consists of upper mantle rocks - harzburgites,

dunites and lherzolites.

:At Mt. Lightning this layer is represented

by the ultramafic rocks. 304

7.3 TECTONIC EVOLUTION OF THE AREA STUDIED:

The Coolac Serpentine Belt lies in the Cowra

Trough which in turn is a part of the Lachlan Fold Belt.

In recent years a number of authors including Packham

(1960, 1969, 1973), Packha~ and Falvey (1971), Packham and

Leitch (1974), Oversby (1971, 1972) and Scheibner (1972a and b, 1973, 1974, 1976) have discussed the tectonic development of south-eastern Australia in terms of plate. tectonic theories. In view of the fact that the origin and emplacement of ophiolites is currently under debate

(see Church, 1972; Coleman and Irwin, 1974; Gass et al,

1975; Dewey, 1976), it is felt that a summary of some of the hypotheses proposed to explain the geological history and setting of the Cowra Trough, be undertaken.

Perhaps the oldest tectonic event in the

Palaeozoic history of the southern and central parts of

Lachlan Fold Belt was the formation of the Girilarnbone

Flysch Wedge between a separated frontal part of the

Proterozoic Australian craton and an oceanic trench

(Scheibner 1972a, 1973). The Girilambone Beds include low-grade metamorphosed deep water sediments, basalts and rare serpentinites and are believed by Scheibner to be of Cambrian age since they are overlain by Upper

Ordovician rocks. The Jindalee Beds which occur to the west of Coolac may represent the southern extension of the

Girilambone Beds. Packham (1973), however, suggested 305

that due to likely widespread Upper Cambrian orogenesis little could be said about the Cambrian history of the sc-uth-west Pacific region.

Scheibner (1972a and b) envisaged an outward retrograde motion of the oceanic trench in Middle

Ordovici2.n t..i.me. This resulted in a separation of the

Girilarrbone Beds thus forming a marginal sea - the Wagga

Marginal Basin. The separated part of the Girilarnbone

Flysch Wedge formed the Parkes Shelf to the east of the

Wa.gga Marginal Basin which was bounded to the west by the

Australian craton. In the Middle-Upper Ordovician calc- alkaline volcanism took place east of the Parkes Shelf forming the Molong Volcanic Rise to the north and the

Yass-Canberra Rise to the south, while a new flysch wedge accumulated in the Monaro Slope and Basin to the east of the Nolong-Yass-Canberra Rise. During the Late Ordovician. the main part of the Molong-Yass-Canherra Rise split away from the western part under tension and a marginal sea

(the Initial Cowra Trough} developed (Fig. 75.. , p 306).

The formati0n of an Ordovician marginal sea to the west of the volcanic arc ( Molong-Yass-Canberra High) was also proposed by Oversby {1971, 72) and Packham (1973). These authors suggested that sedimentation in this marginal sea took place mainly in the Ordovician.

Scheibner {1972a, 1974, 1976) is of the opinion that strong extension in the Early to Middle Silurian 306

100 0 IOO Kilomet•re

Fig.75. Palinspatic map of New South Wales showing the initial development of the Cowra Trough during the Late Ordovician time (after Scheibner, 1976). 307

widened the already formed Cowra Trough, where oceanic crust developed. The Early to Middle Silurian Honeysuckle

Beds in the area studied could represent such an oceanic crust.

Calc--alkali volcanism and associated orogenic plutonism {Bamilton, 1969) took place on the western edge of the Molong-Yass-Canberra Rise during Early to Middle

Silurian. The volcanic rocks are represented by the

Blowering Beds and DC)ta'o Volcanics and are mainly rhyodacite, dacite and less commonly andesite. The plutonic rocks are represented by the Young Granodiorite. Ashley {1973) and Franklin (1975) have shown from chemical and field evidence that these volcanic and plutonic rocks are related.

During Late Silurian-Early Devonian time a relative eastward movement of the Australian Plate might have been responsible for the closure of the Cowra Trough, such closure being accompanied by subduction of the oceanic crust of the Cowra Trough both to the east under the Molong

Yass-Canberra Rise and to the west under the Parkes High

{Scheibner, 1972a, 1976; Ashley, 1973; Franklin, 1975).

Coleman {1971b) suggested that during convergence

of oceanic and continental crusts slabs of oceanic crust

and underlying upper mantle might be emplaced over

continental crust by what he called an 'abduction' process. 308

A similar abduction of the oceanic lithosphere of the Cowra

Trough is believed to have taken place (Scheibner, 1972a and b, 1976; Ashley, 1973; Franklin, 1975). Scheibner believes that the Coolac Serpentine Belt with the adjacent western region is an upthrust or abducted overturned

Silurian subduction zone. However, it should be mentioned that abduction of ophiolites during convergence of continen tal and oceanic plates is not necessarily preceded by subduction(Coleman and Irwin, 197.4).

Initially the abduction of oceanic crust-upper mantle over the Molong-Yass-Canberra Rise to the east may have been at a low angle (Coleman, 1971b; Davies, 1971) but with increasing compression the oceanic lithosphere may have assumed an overturned position. The upside down facing of ultramafic-mafic-sedimentary association in the Coolac-Tumut region may be explained by such a process.

Watts (1971) has suggested from gravity data that the Young Granodiorite of the Molong-Yass-Canberra

Rise had been thrust back over the Coolac Serpentine Belt.

The tectonic contact between the ultramafic rocks of the

Coolac Belt and the Young Granodiorite is tentatively ascribed to this reversed thrusting process during the final stages of closure of the Cowra Trough.

In Early to Middle Devonian time the rocks of the 309

Coolac Serpentine Belt and the adjoining areas were

intruded by granitic plutons which include the Bogong

Granite to the south near Tumut (see Ashley, 1973;

Ashley and Basden, 1973). After Middle Devonian time

the area surrounding the Coolac Serpentine Belt became a stable cratonic area. Apart from the Upper Devonian

Hervey Group near Cootamundra and the Hatchery Creek conglomerate near Wee Jasper (Roberts et al, 1971;

Webby, 1972) there is virtually no evidence of further sedimentation in the Cowra Trough, while the alkali basalts of possible Oligocene-Miocene age, that crop out near

Red Hill north-east of Tumut, seem to be the only evidence of igneous activity since Devonian.

7.31 Marginal Basins and Emplacement of Ophiolites:

It is clear from the above summary that there

is a general agreement regarding the formation of ultramafic-mafic rocks of the Coolac Serpentine Belt in

a marginal sea. In fact the association of Coolac ophiolite with rocks of volcanic arc affinities (the

Durre volcanics to the north, the Blowering Beds to the west and the Young Granodiorite to the east) suggests

a rear-arc basin site for this generation (see Dewey,1976).

Marginal seas are semi-isolated small ocean

basins lying behind island arcs, which separate them from

the main oceans, and are marginal to continents as well as 310

to major oceans (Karig, 1970; Scheibner, 1976). In recent years, the tectonic significance of ophiolites has grown considerably as they are believed to mark sites of marginal seas (Karig, 1971; Bird et al, 1971; Dewey,

1976). A ~umber of hypotheses have been proposed regarding the development of oceanic crust in marginal seas. These theories include:

1) Rifting of a migrating volcanic arc causing the generation of marginal basin(s) (Karig, 1970; Packham and Falvey, 1971). Packham and Falvey suggested an olivine basalt composition for the oceanic crust developed in such marginal basins.

2) Diapiric "pull-apart" mechanism causing the formation of a marginal sea (Karig, 1971). Karig suggested an island arc tholeiitic composition for the oceanic crust of such marginal seas.

3) Fast spreading of a mid-oceanic ridge resulting in the formation of small ocean basins (Scheibner, 1972b).

An abyssal tholeiitic composition for the oceanic crust is proposed by Scheibner.

4) Retrograde motion of a sinking slab of the Benioff zone causing tension in the adjoining continental plate.

Diffused oceanic crust of abyssal tholeiitic composition is believed to be formed without the development of a 311

mid-oceanic ridge (Elsasser, 1971; Dewey and Bird, 1971;

Scheibner, 1972; Sclater et al, 1972).

It is not certain which of the above four hypotheses might be applicable to explain the generation of oceanic cru3t in the Cowra Trough. The difference between tholeiitic rocks formed in island arcs and those from mid-oceanic ridges is currently under debate (see

Miyashiro, 1975a and b, Hynes, 1975). Also, due to the fact that the mafic rocks of ophiolites are generally altered to low-grade metamorphic rocks, it is difficult to be certain about their parentage. Nevertheless, the spilitic rocks of Honeysuckle Beds in the Mt. Lightning area may have been derived from abyssal tholeiites (see

Section 3.9 ) ; therefore, hypotheses 3 and 4 may be applicable to the origin of Honeysuckle Bed spilites in the Cowra Trough.

The emplacement of ophiolites is a difficult problem and is not clearly understood (see Gass et al, 1975).

Various hypotheses have been advanced to explain the

occurrence of dense oceanic lithosphere above lighter

continental crust (see Coleman and Irwin, 1974, for a

discussion). But as Coleman and Irwin remarked" ..•.•••

none of the arguments satisfactorily explains the internal

petrological evolution of many ophioli tes .••.•• ". For the

Coola.c ophiolite, it may be concluded that Scheibner's

(1972a and b, 1973, 1974, 1976) concept appears to be

compatible with the regional setting of the Coolac

Serpentine Belt and associated areas. 312

7.4 CONCLUSIONS:

The Mt. Lightning area is characterized by an ultramafic-m-3.fic rock association. The predominant ultramafic rock is variably serpentinized alpine-type harzburgite, which contains typically Mg-rich olivine and enstatite with subsidiary diopside and chrome-spinel.

With increasing diopside content (> 5% by volume), the harzburgite grades locally into lherzolite. Dunitic variants are rare and generally occur associated with podiform chromite deposits. Serpentinization increases from the east toward the west in the ultramafic mass and ranges from partial to complete, lizardite-chrysotile being the predominant serpentine minerals.

Two broad types of microtextures have been observed in Mt. Lightning ultramafic rocks - a high temper ature ty?e characterized by deformation and recrystalli zation of primary minerals, and a low temperature cataclas tic type associated with serpentinization. The deformation and recrystallization textures are similar to those reported in mantle-derived ultramafic xenoliths.

The chemical composition of the primary minerals and the microtextures accord with an upper mantle origin for the Mt. Lightning ultramafic rocks.

Spilites are the most abundant mafic rocks in 313

the area and can be divided into variolitic and massive types, the former being subordinate to the latter.

Variolitic spilites occur only near the junction with u.ltramafic rocks except for one large body of variolite that is enclosed within the ultramafic mass. Massive spilites are abundant in the western sector of the study area.

The spilites are characterized by a greenschist· faci.es mineralogy and are considered to be secondary in origin. Chemical composition of relict clinopyroxenes indicate a tholeiitic parentage.

Minor bodies of rodingites, acid feldspathic rocks (trondhjemites and albitites) and chromitites are common in the area. The rodingites can be divided into two broad groups. Group 1 rodingites, which are comple~ely enclosed within the ultramafic rocks forming dyke-like masses, contain grossula.rite, vesuvianite, chlorite and relict diopside. They are characterized by high lime, Cr and Ni contents, and by high Mg/Fe and

Cr/V ratios. It is believed by the present writer that

Group 1 rodingites were derived from gabbroic parents.

Group 2 rodingites occur as selvedges around variolitic spilites near the contacts with ultramafic

rocks. They contain zoisite, prehnite, grossularite, 314

chlorite and sphene and are characterized by high lime, high alumina, low Cr and Ni, and by low Mg/Fe and Cr/V ratios.

The widely accepted view that rodingites owe their high lime content to associated ultramafic rocks is disputed by the present writer for Mt. Lightning rodingites.

It is argued that sufficient amounts of lime might have been available in the precursor gabbroic rocks and in the variolitic spilites to form the Group 1 and Group 2 rodingites respectively. However, episodes of rodingitization and serpentinization may have proceeded concurrently.

The trondhjemites and albitites occur as small bodies enclosed within both ultramafic and mafic rocks.

They are characterized by low K2o content and high Na2o/ A gradational relationship between trondh jemites and albitites could not be established from field study. However, such a relationship is indicated by textural evidence. Textural observations also suggest that the sodic plagioclase in these rocks is secondary in origin being derived from relatively more calcic plagioclase. It is concluded that these acid feldspathic rocks represent altered diorites.

Mt. Lightning chromitites are characterized by textural and chemical diversities. Both er-rich and Al- rich chromites are to be found. Their association with 315

a variety of minor rock types, eg, Group 1 rodingites,

dunites and acid feldspathic rocks, and their compositional

and textural diversities suggest that Mt. Lightning

chromites are polygenetic being derived from a number of

discrete magma pockets.

The derivation of principal and minor rock types

at Mt. Lightning from partial melting of upper mantle

seems plausible. The regional geological setting of the

Coolac Serpentine Belt and the flanking rocks to the west

is compatible with an oceanic origin for the ultramafic mafic- sedimentary association at Mt. Lightning. 316

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.APPENDI.X

Mineralogical Studies:

Standard transmitted and reflected light (for chromite ores) methods were used for most mineralogical investigations. A 4-axis Leitz Universal Stage was used to determine compositions of plagioclase and optic axial angles (2V) of various minerals. For refractive index measurements Series Ml Cargille R.I. fluids were used and calibrated with an "Abbey micro-refractometer". The refractive indices measured by the author are believed to be accurate to+ 0.005.

X-ray diffraction work was carried out on a Philips X-ray diffraction unit. CoK~ radiation and Fe filter were used for all samples. Sio2 or CaF2 was used as internal standard in most samples. The d-spacings calculated from the X-ray diffraction traces were compared with the A.S.T.M. Index and the 1972 Powder Diffraction File and the calculated X-ray powder patterns for silicate minerals (after Borg and Smith, 1969). For unit cell determinations on garnets Taylor, Sinclair, Nelson and Riley's extrapolation was used to correct for absorption and divergent radiations. •• 11

Whole Rock Chemical Analyses:

Hand specimens selected for chemical analyses were crusted in a Terna tungsten carbide vibrating grinding milL to -200 mesh size. Both X-ray fluorescence spectrometry and wet chemical techniques were used to determine the major oxide values. Si02, Tio2, Al2o3, Fe2o3 (total Fe as Fe2o 3), MnO, MgO, Cao, K2o and P2o5 were determined at Macquarie University by X-ray fluorescence spectrometry using a Siemen's S.R.S. spectrometer with a 4 Kw generator and standard counting rack.

Samples for major element analyses were prepared following the lithium borate glass disc method of Norrish and Hutton (1964). The U.S. Geological Survey rock standards GSP-1, AGV-1, BCR-1 and PCC-1 were used. In Table A-1, oxide values of the abovementioned standards obtained in the present study are compared with the recommended values.

FeO was determined following the method of Riley (1955) and Na was determined by flame photometer using artificial standards. Li2so4 was used to suppress inter- element interference.

Trace element analyses were carried out at the· Australian National University by X-ray fluorescence spectrometry using a Philips XRF unit. The detection limits TABLE Al

MAJOR ELEMENT OXIDE ANALYSES OF U.S. GEOLOGICAL SURVEY ROCK STANDARDS

(in weight per cent)

GSP - 1 AGV - 1 , This study Flanagan This study Flanagan (1969) (1969)

Si02 67.75 67 .27 58.92 59.00

Ti02 0.66 0.66 1.09 1.05 Al 2o3 15.12 15.31 17.07 17.14 Fe2o3 4.28 4.17 6.80 6.92 MnO 0.05 0.05 0.10 0.10 MgO 1.05 0.98 1.58 1.50 CaO 1.98 2.07 5.02 4.91 K20 5.40 5.51 2,. 77 2.86 P205 0.28 0.28 0.50 0.49

B.C.R - 1 PCC -.1 This study Flanagan This study Flanagan (1969) (1969

Si02 54.08 54.13 42.06 41.91 Ti02 2.25 2.26 0.00 0.01 A1 2o3 13.42 13.67 0.62 0.76 Fe2o3 13.05 12.64 8.16 7.93 MnO 0.18 0.19 0.12 0.12 MgO 3.52 3.49 43.40 43.32 CaO 7.01 6.92 0.49 0.42 K20 1. 72 1.69 0.01 0.005 P205 0.37 0.35 0.00 0.005 TABLE A.2

Detection limits of various trace elements analysed in the present study

Element X-ray tube Detection limit* (in ppm)

Ba w 2

Rb Mo 1

Sr Mo 1

Zr w 2

Nb w 2

y Mo 0.5

V w 2

Cr w 1

Ni Au 1

Cu Au 1

* Obtained from R. Freeman, A.N.U. (personal communication). ... il1.

of the elements analysed are given in Table A-2 (R. Freeman, pers. comm.).

Electron Microprobe Analysis:

Quantitative electron microprobe a_nalysis was carried out using an Applied Research Laboratories EMX mark II Electron microprobe X-ray analyser.

Three fully focussing spectrometer~ utilising Lithium fluoride, Ammonium dihydrogen phosphate and Rubidium acid pthalate crystals cover the normal character istic wavelength range of silicates and oxides.

An accelerating voltage of 25 KV produces a specimen current of 0.01~ amps (on brass) and typically a spot size of 1 micron.

X-rays were electronically processed through sealed (LiF and ADP) and flow (RAP) proportional counters, pulse height analysers, linear ratemeters and finally displayed through scaler times collected under fixed beam current conditions.

The data was transferred on to punch cards and processed through on IBM 360/50 computer. The data reduction programme applied corrections for dead time, background, absorption, fluorescence and atomic number effects, • 1V

The detection limits of various oxides analysed were calculated using the following equation:-

Lower limit of detection=~-m.v~ fCb.

where m = number of counts/sec/unit of ;

concentration for the element in the specimen ..

~ = background counts T = measuring time.

The calculated detection limits are given below (in weight per cent):-

SiO2 .. 0.04 cao . . 0.03

TiO2 . . 0.04 Cr2o 3 .. o.os

Al2o 3 .. o.os V203 . . 0.07 FeO . . 0.04 P2O5 .. 0.06

MnO . . 0.04 Na2o .. 0.07

MgO . . o.os NiO • • o.os

References:

Borg, I.Y., and Smith, D.K., 1969. Calculated X-ray powder patterns for silicate minerals. Geol. Soc. Am., Mem., 122. V

Flanagan, F.J., 1969, U.S. Geological Survey Standards II1 First compilation of data for the new u.s.G.S. rocks. Geochim. Cosmochim. Acta, 33, pp 81-120.

Norrish, K., and Chappell, B.W., 1967. X-ray fluorescence spectrography, in Physical Methods of Determina tive Mineralogy (Zussman, J.Ed.), Academic Press, London., pp 161-214.

Norrish, K., and Hutton, J.T., 1964. Preparation of samples for analysis by X-ray fluorescent SP.ectrography, Divl. Rep. Div. Soils CSIRO 3/64.

Sweatman, T.R., and Long, J.V.P., 1969. Quantitative electron probe microanalysis of rock forming minerals. J. Petrol., 10, pp 332-379.