GEOCHEMISTRY OF MAFIC-ULTRAMAFIC ROCKS AROUND GOGUNDA, DISTRICT UDAIPUR, RAJASTHAN

SUBMITTED IN PARTIAL FULFILMENT FOR THE DEGREE OF Muittv of ^{)UogDpfip in

BY ZIAD SALEM HUSSAIN ABU-HAMATTEH

DEPARTMENT OF GEOLOGY ALIGARH MUSLIM UNIVERSITY ALIGARH (INDIA) 1992 [ITAZAI) t;

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(D&Dmm&D TO mr mcymeR In the Name of Allah the Compassion, the Merciful, Praise be to Allah, Lord of the Universe, and peace and prayers be upon His Final Prophet and Messenger.

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In the Name of Allah, Most Gracious, Most Merciful

Proclaim! (or Read!) In the name of thy Lord and Cberisber Who created* Created man, out of A (mere) clot of congeated blood* Proclaim! And thy Lord Is Most Bountiful* He Who taught (The use of) the Pen* Taught man that Which be knew not*

Surah 96 Al' ALAQ. CERTIFICATE

This is to certify that the dissertation entitled

"Geochemistry of Mafic-Ultramafic rocks around Gogunda,

District Udaipur, Rajasthan" is the record of bonafide

research carried out by Mr. Ziad Salem Hussain Abu-Hamatteh

under our joint supervision. This work is an original

contribution to the existing knowledge of the subject.

We allow Mr. Abu-Hamatteh to submit the work for the

award of Master of Philosophy degree of the Aligarh Muslim

University, Aligarh.

U2^^^-^ Dr. Talcit Ahmad Dr. Mahshar Raza Scientist-C Reader Wadia Institute of Deptt. of Geology Himalayan Geology, A.M.U., Aligarh. Dehradun. CONTENTS

Acknowledgement i List of Tables iv List of Figures v List of Plates ix

INTRODUCTION 1

CHAPTER-I

GEOLOGICAL SETTING 7

Geneiral Geology of Aravalli Mountain Belt 7 Distribution of Mafic-Ultramafic Rocks in Aravalli Mountain Belt 9 Geology of Udaipur-Jharol Basin 10 Field Occurrence of Mafic-Ultramafic Rocks of Jharol Group 13 Age and Correlation 16

CHAPTER- II

PETE^OGRAPHY 18

CHAPTER- III

GEOCHEMISTRY 2 3

Analytical Procedure 23 Accuracy and Precision 27 Results 29 Major Elements Distribution 30 General Statement 30 Jharol Ultramafics 31 Gopir Volcanics 34 Bagdunda Volcanics 36 Trace Elements Distribution 37 General Statement 37 Jharol Ultramafics 38 Gopir Volcanics 39 Bagdunda Volcanics 41 Rare - Earth Elements 43 Gopir Volcanics 43 Bagdunda Volcanics 44 Effect of Alteration 45 Major Elements Variability 48 Trace Elements Variability 52

CHAPTER- IV MAGMA TYPE CLASSIFICATION 54 General Statement 54 Magma Series Classification 56 Major Element Relationship 57 Immobile Minor and Trace Elements Criteria 58 Tectonic Setting Classification 59 Immobile Minor and Trace Elements diagrams 62 Geochemical Patterns 63

CHAPTER-V PETROGENETIC CONSIDERATIONS AND SOURCE GEOCHEMISTRY 67 Petrogenetic Considerations 67 Source Modelling 75

CHAPTER- VI TECTONIC IMPLICATIONS 80 CHAPTER- VII

SUMMARY AND CONCLUSION 89

TABLES 1 to 11 97

REFERENCES 117 (i)

ACKNOWLEDGEMENT

This work has been suggested and supervised by Dr.

Mahsher Raza, Deptt. of Geology, A.M.U., Aligarh and Dr.

Talat Ahmad, Wadia Institute of Himalayan Geology, Dehradun.

I would like to express my deep and sincere gratitude to

them for their valuable guidance, encouragement and

cooperation in all stages of this work.

I am also grateful to Prof. S.N. Bhalla, Chairman,

Deptt. of Geology, A.M.U. Aligarh and Dr. V.C. Thakur,

Director, W.I.H.G., Dehradun for providing all the necessary

facilities for the completion of this work.

My sincere thanks are due to Dr. P.P. Khanna, Dr.

P.K. Mukherjee, and Mr. Saini, Scientists,W.I.E.G., Dehradun

for their help in the analytical work.

The help received from Dr. S.M. Zainuddin, Dr.

K.K. Ghauri and Dr. S. Farooq, Deptt. of Geology, A.M.U. ,

Aligarh and Dr. M.S. Rathi and Dr. R. Islam, W.I.H.G.,

Dehradun, in the various stages of this work is thankfully

acknowledged.

I am further grateful to Dr. N.K. Chauhan and my

friend Mr. Sabah A. Moh'd, Deptt. of Geology, Sukadia (ii)

University, Udaipur, for their help during my field work.

I would also like to extend my sincere thanks to

my collegues Dr. S.H. Alvi, Dr. M.S. Khan and Mr. Safdare

Azam for their critical views and constructive suggestions.

Mr. Salimuddin Ahmad, Mr. Zakir Hussain, Mr. Feroz

Javid and Mr. Habbib Ahmad, Deptt. of Geology, A,M.U.,

Aligarh and Mr. Saeed Ahmad and Dr. B.K. Choudhary,

W.I.H.G., Dehradun and Mt .Kazem Rangzan, R. S . A.C.R.E .G. ,

A.M.U., Aligarh are acknowledged for their cooperation and help during the course of this work.

I am grateful to Ms. S.Kombe for her help and

encouragement throughout this work.

I would like to extend my gratitude to my brother

Mr. Naif Salem for financing my research studies.

I am very much thankful to Mr. Ahmad Hussein and everyone who lent his assistance in the completion of this work. (iii)

And last, but not least, I am grateful to My Mother and members of my family for their moral and financial support.

(Ziad S.H. Abu-Hamatteh) (iv)

LIST O F TABLES

Table 1, General geological succession of Aravalli Supergroup in the type area. Table 2. Precision, coefficient of variation and detection limits for various oxides and elements.

Table 3, The correlation coefficient values between the various major oxides of Gopir volcanics.

Table 4, The correlation coefficient values between the various major oxides of Bagdunda volcanics. Table 5. The correlation coefficient values between the various major oxides of Jharol ultramafics.

Table 6, Major element abundances (in wt%) and trace element concentrations (in ppm) of Gopir and Bagdunda volcanics.

Table 7, Major element abundances (in wt%) and trace element concentrations (in ppm) of Jharol ultramafics. Table 8, Range of variation and average chemical compositions (major oxides and trace elements) of Gopir volcanics, Bagdunda volcanics and Jharol ultramafics. Table 9. Element ratios in Gopir and Bagdunda volcanics. Table 10, Element ratios in Jharol ultramafics. Table 11, Comparison of the average chemical composition of Gopir and Bagdunda volcanics with the other volcanic rocks. (v)

LIST O F FIGURES

Figure 1. (A) Map of India.

(B) Geological map of Aravalli mountain belt, showing distribution of Precambrian rocks.

(C) Simplified geological map of Jharol basin, showing distribution of various lithological units.

Figure 2. (A) Feo^, MgO, and TiO-- versu^^^^^u^s variouv^4.^v>^^s^ major oxides and trace elements variation plots of Jharol ultramafics. (B) Ni and Cr versus various major oxides and trace elements variation plots of Jharol ultramafics.

Figure 3. Chondrite normalized REE patterns of Gopir and Bagdunda volcanics. Normalizing values are those of Sun and McDonough (1989).

Figure 4. Zr versus TiO-, PpOc and Al„0^ of Gopir and Bagdunda volcanics variation diagram, showing distribution of these elements relative to Zr.

Figure 5, Zr versus Y covariation diagram, showing the relative enrichment of Zr over Y in Gopir and Bagdunda volcanics.

Figure 6. (A) Zr versus Ce covariation diagram, showing the less mobile nature of Gopir and Bagdunda volcanics.

(B) Zr versus Nb covariation diagram, showing the enrichment of Nb in Gopir and Bagdunda volcanics. (vi)

Figure 7. CaO/Al-O^-MgO/lO-SiO^/lOO ternary variation diagram (after Schweitzer and Kroner 1985), showing unaltered nature of Gopir and Bagdunda volcanics.

Figure 8, CaO/TiO„ versus MgO/TiO„ binary diagram for Gopir and Bagdunda volcanics, illustrating the sympathetic relationship an indication for their primary magmatic character.

Figure 9. Mg-number versus TiO» binary diagram (after Dungan and Rhodes 1978) for Gopir and Bagdunda volcanics, showing relative olivine fractionation in these volcanics.

Figure 10. MgO/CaO versus principal magmatic component i.e., TiO- variation diagram of Gopir and Bagdunda volcanics, showing the fractionation of FOn^ and Cpx.

Figure 11, TiO- versus CaO/Al„0^ variation diagram (after Dungan and Rhodes 1978), showing relative olivine and plagioclase fractionation in Gopir and Bagdunda volcanics.

Figure 12, FeO /MgO versus CaO/Al„0,, diagram for Gopir and Bagdunda volcanics, showing olivine and clinopyroxene fractionation.

Figure 13. TiO„ versus Al-0,, variation diagram, showing identical trends of variation in Gopir and Bagdunda volcanics.

Figure 14, Zr versus Zr/Y variation diagram of Gopir and Bagdunda volcanics (after Pearce and Norry 1979).

Figure 15, Nd versus Ce plot • for Gopir and Bagdunda (vii)

volcanics with primitive mantle and chondritic ratio, showing the least effect of contamination and possible extent of melting. Figure 16. A (Na20+K20)-F (FeO^)-M (MgO), ternary variation diagram for Gopir and Bagdunda volcanics, showing their tholeiite affinity. Figure 17. Jensen's cation ternary plot for Gopir and Bagdunda volcanics, showing compositional variation of these volcanics from high Mg-tholeiite to basaltic komatiite affinity.

Figure 18. Y (Y+Zr)-T (TiO-)- Cr variation diagram, showing the komatiite-tholeiite affinity of Gopir and Bagdunda volcanics. Figure 19. Nb/Y versus Zr/TiO„ variation diagram (after Winchester and Floyd 1977) for Gopir and Bagdunda volcanics, showing their sub-alkaline and basaltic andesite nature. Figure 20, TiOp versus PoO^ variation diagram of Gopir and Bagdunda volcanics, showing their oceanic ridge basalt affinity. Figure 21.(A) Zr versus TiO_ covariation diagram, illustrating MORE affinity of Gopir and Bagdunda volcanics. (B) Y versus Cr covariation diagram, illustrating MORE affinity of Gopir and Bagdunda volcanics. Figtire 22. Ti/Y versus Zr/Y diagram of Gopir and Bagdunda volcanics, indicating their plate margin nature. Figure 23. Ti/100-Zr-Yx3, ternary tectomagmatic discrimination diagram of Gopir and Bagdunda (viii)

volcanics, showing their MORE and lAE affinity. Figure 24.(A) MORE normalized multi element patterns of Gopir and Bagdunda volcanics. (B) MORE normalized multi element patterns of basalts from various tectonic settings. Figure 25.(A) Primordial mantle normalized incompatible element patterns of Gopir and Bagdunda volcanics. (B) Primordial mantle normalized incompatible element patterns of basalts from various tectonic settings. Figure 26.(A) Nb-normalized geochemical patterns of Gopir and Bagdunda volcanics. (B) Nb-normalized geochemical patterns of basalts from various tectonic settings. Figure 27.(A) Primitive mantle (PM) normalized ratio spidergram for average Gopir and Bagdunda volcanics. (B) Primitive mantle (PM) normalized ratio spidergram for average komatiitic basalt and tholeiites of basal Aravalli volcanics. Figure 28. Chondrite-normalized patterns, for the calculated source for sample No. G5, calculated REE patterns for 3%, 8%, 10%, 12% and 20% melting of the source and REE patterns for Gopir volcanics, for comparison. (ix)

LIST O F PLATES

Plate 1, The intercalation of Jharol ultramafics with quartzite near Jhameshwarjee temple.

Plate 2, Ill, preserved pillow structure near Challi village.

Plate 3, Jharol "altramafics sho-wing the xeinoval of soiae phases giving rise to pitted appearance, indicating their altered nature. Jfntroliuctton INTRODUCTION

Basic magmatism has played an important role in the evolution of the crust since the earliest times and its products, the basic igneous rocks, are the most wide-spread rocks on the earth at present and have been equally important throughout geological periods (Condie 1985). Geochemical data and experimental studies have pointed out that the primary magmas of basic or ultra-basic composition are produced by varying degrees of partial melting of more fusible constituents of the upper mantle and/or subducting lithosphere. These melts rise to the surface and solidify, adding, from time to time, new rocks to the crust. During their transportation and storage in high level magma chambers, their composition is modified by subsequent differentiation processes, including simple fractional crystallization, assimilation fractional crystallization (AFC), magma mixing, crustal contamination, etc. and any combination of these and other processes. The effects of all these processes are generally reflected in the chemical composition of the finally produced rocks. Therefore, the geochemical attributes of the volcanic suites provide insight into the history and composition of source regions, conditions of melting, extent of melting and modification of primary magmas by processes operating between generation of the magma and the emplacement of the end product. The geochemical characteristics of basic volcanic sequences, coupled with geochronological and stratigraphic controls, can provide means of evaluating geochemical and thermal evolution of mantle through time, possible mantle heterogeneities and petrogenetic processes. Because these factors vary from one tectonic setting to another, the basic volcanic rocks with distinct chemical characteristics are found associated with specific tectonic environments in the plate tectonic framework (Pearce and Cann 1973; Wood 1980).

Though a close relationship between volcanism and tectonism is known since a long period of time, the concept of plate tectonic has led to the classification of volcanic rocks on the basis of their occurrence in different tectonic, settings. In this regard, the mafic volcanic rocks have a great significance in the sense that they have distinct chemical signatures of their eruptive environment, namely, oceanic, continental or transitional (Pearce and Cann 1973; Floyd and Winchester 1975; Saunders and Tarney 1979). Such a Chemical discrimination is possible even for altered and metamorphosed volcanic rocks (Cann 1970; Pearce and Cann 1973; Field and Elliot 1974; Floyd and Winchester 1978; Pharaoh and Pearce. 1984). These rocks, which may potentially provide information about the earth's mantle and crust subsequent to ca. 3.8 b.y., are well preserved in geological records representing various episodes of the earth's history. The Precambrian volcanics, which characteristically constitute major components of Archaean greenstone belts and Proterozoic mobile belts can provide useful information on the evolution of the crust and the mantle during the early period of the earth's history.

In India, the Aravalli-Delhi belt which borders the north western margin of Indian shield, preserves one of the best developed Proterozoic sequence of the Indian subcontinent (figure-1). Though considerable work has been done on its geological framework, structure, metamorphism, geochronology and sedimentation (Heron 1953; Chaudhary et al. 1984; Mohanty and Naha 1986; Roy 1988; Sharma 1988;

Singh 1988), the origin and evolutionary history of this great belt, even today, is not fully understood and reconstruction of sequences of geological events is still an enigma.

In this region, along with different types of metasediments, there exists numerous volcanic sequences, which represent various Proterozoic episodes of Aravalli history. Except for the basal Aravalli volcanics of

Delwara-Nathdwara area (Ahmad and Rajamani 1991), these volcanic sequences have not received due attention although they provide an excellent opportunity to investigate the petrology and geochemistry of rocks of this age. Such V3l30'*;

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Figure 1- (Inset) (A) Map of India, (B) Geological map of Aravalli mountain belt, showing distribution of Precambrian rocks (after Deb and Sarkar 1990), (c) Simplified geological map of Jharol basin, showing the occurrence of volcanic rocks (after Roy et al. 1988). investigations may prove fruitful to solve the problems related to Proterozoic magma genesis and eruptive environment in general and evolutionary history of Aravalli belt in particular (Ahmad and Rajamani 1991; Ahmad and

Tarney 1991).

The present study is an attempt to visualize the chemical characteristics of the lower Proterozoic volcanic rocks of Jharol belt, occurring west of Udaipur city in

south western Rajasthan. The metasedimentary rocks of Jharol

Group are considered to be the deep water facies of Aravalli

Supergroup, which are well developed around Udaipur.

Although detailed information on geology and stratigraphic positions of these rocks are available in literature (Gupta et al. 1980; Roy and Paliwal 1981; Roy et al. 1988), the details of their geochemical characterisations are not available so far. In this area the basic volcanic rocks occur at the base and showing uncomformable relationship with Archaean gneisses is seen around Bagdunda and has been referred to as Bagdunda amphibolites (Sharma et al. 1988).

The mafic rocks occurring within the deep water metasediments of Jharol Group, as a linear belt between

Modri and Rakhabdev, are generally referred to as ultramafic intrusions. Because of complex structure and deformation, the relationship between these mafic rocks and associated sediments has not been clearly defined. The present study however, suggests that the linear belt between Modri and Gopir (figure-1) consists of metamorphosed (up to green schist facies) basic flows with a subordinate amount of ultramafics which are also seen to be of eruptive nature. However, the presence of some intrusive ultramafic bodies can not be ruled - out. Although the relationship of the metasediments with the ultramafic and mafic bodies is not clear, some workers have considered these mafic rocks as part of an ophiolitic sequence (Sen 1981; Brahmdatta et al. 1986; Sinha-Roy 1984), while other workers (Roy et al. 1988) consider their emplacement in a rifted basin contemporaneous with sedimentation.

The present study is carried out to provide the detailed geochemical data on- mafic and ultramafic rocks of Jharol belt and utilizes them to draw petrological and tectonic conclusions. The purpose of this study is to identify their nature of magma type or types, their genetic relationship, nature of their source region and the environment of eruption and its significance in interpretation of regional tectonics. For this purpose, analyses of the major and trace elements were performed on critically selected twenty two least altered samples, of basal Jharol volcanics (Bagdunda volcanics) and raafic-ultramafic rocks occurring as a linear belt between Modri and Gopir areas (referred to as Gopir volcanics). The chemical data along with petrography and field charcicteristics are used for petrogenetic and tectonic interpretations.

The work is presented in seven chapters: Chapter-I deals with geological setting of the area along with the age and field characteristics of Jharol volcanics. In chapter-II, the Petrography of Jharol volcanics is discussed and an attempt is made to understand the mineralogical and textural features of these rocks. Chapter-Ill, deals with geochemistry of the mafic and ultramafic rocks of Jharol volcanics, covering analytical methods, major/trace and REE distribution, elements variability and effect of alteration. In chapter-IV, the geochemical classification and magma type is discussed, in terms of magma series and tectonic setting. In chapter-V, major, trace and rare earth elements are used to elucidate the genesis, and source chemistry of the studied volcanic rocks. In chapter-VI, the geochemical data along with geological evidences are used to elucidate the environment of these volcanics and associated tectonic events in the early history of Jharol sequence (Tectonic implications). A summary of the work and conclusions are presented in Chapter-VII. Chapter I

(Seologital ^ettins CHAPTER ONE

GEOLOGICAL SETTING

General Geology of Aravalli Mountain Belt :

The Aravalli craton which occupies the north western part of the Indian shield is bounded by the main' boundary thrust (MBT) of Himalayan foot hills in the north, the Cambay Basin in the south-west and the Son-Narmada lineament in the south and south-east. The western boundary of this craton. is covered by recent deserts and could well extended up to Pakistan Himalayas (Naqvi and Rogers 1987). The rocks comprising this craton range in age from 3.5 to 0.75 b.y. (Deb and Sarkar 1990), indicating the continuity of geological processes in this area for a long geological period.

The western margin of this craton is marked by the presence of Aravalli mountain range, which runs for about 700 Km between Delhi and Ahmedabad, with a general strike of north east - south west. These mountains are made up principally of the following three geological units (figure-1):

The Banded Gneissic Complex (B.G.C.) (3500-2500 Ma.) ;

It is a heterogeneous rocks assemblage consisting 8 of granitic gneisses, amphibolite and metasediments. There is a general agreement that this unit has served as the basement of younger Proterozoic rocks. The B.G.C. comprises of two tectonic blocks (Sen 1980) i.e. the northern block and the southern block. The rocks of the nothern block have been metamorphosed under P-T conditions of the amphibolite to granulite facies, while the rocks of southern block are metmtiorphosed up to the grade of amphibolite facies.

Sm-Nd isochron data suggest the B.G.C. contains rocks as old as 3.5 Ga'.(MacDougall et al. 1983).

The Aravalli Supergroup ; (2500-2000 Ma.) ;

The Aravalli Supergroup is the oldest Proterozoic cover unit. The rocks of this group are best developed around Udaipur, where they are represented by two contrasting sedimentary facies associations, distributed along two roughly north-south running belts (figure-1), comprising the Udaipur-Jharol basin. The eastern belt represents a shelf sequence in which carbonates from an important and dominant component (Aravalli belt), whereas the western belt, which comprises carbonate free sequence of mica schist with some bands of quartzite, has been interpreted as deep sea facies (Jharol belt) (Roy and

Paliwal 1981) . The Delhi Supergroup (2000-1200 Ma.) ;

This is the younger unit, made up of various types of metasediments with basic volcanics of different ages.

Distribution of Mafic - Dltramafic rocks in Aravalli Mountain Belt :

In Aravalli mountain belt, mafic and ultramafic

rocks occur at different stratigraphic levels. In this

region the oldest mafic magmatism is represented by 3.5 b.y.

old amphibolites within the B.G.C. (MacDougall et al. 1983;

Srivastava 1986). The next phase of mafic magmatism is

represented by komatiitic and tholeiitic lava flows

occurring at the base of Aravalli Supergroup (Ahmad and

Rajamani 1991). The mafic and ultramafic rocks of Jharol

Group may be considered as the next younger phase of mafic

magmatism in Aravalli region.

The next younger phase is represented by volcanic

rocks which are associated with metasediments of Delhi

Supergroup. Two main volcanic episodes are recognised, one

at the base of Alwars and the other within the Ajabgarh. The

first episode is best developed in Byana and Tehla areas of

the north-eastern Rajasthan. In the Ajabgarh group of rocks,

metavolcanics and metasedimentary rocks occur intermittently

in a linear belt approximately 450 Km long and up to 30 Km 10 wide from Khetri in the north to Deri and Ambaji in the south. These rocks have been considered as older than 1600 m.y. (Srivastava 1986).

Geology of Udaipur - Jharol Basin :

In the type area of Udaipur, the rocks of the lower Proterozoic Aravalli Supergroup constitute an inverted

"V" shaped linear belt which narrows towards the north. The rocks in this area consist of two contrasting facies, i.e., a carbonate - bearing shelf facies of Udaipur basin and a carbonate free deep water facies of Jharol basin. In view of the fact that these two types of lithofacies represent two segments of a single basin, they are discussed herein together under the same heading. Although generalized geological framework of this basin has been given by many workers (Heron 1953; Raja Rao et al. 1971; Gupta et al.

1980), The detailed stratigraphic evolution of the basin has been worked out by Roy et al. (1988) who established the geological succession as given in table-1.

The shelf sequence of the Aravalli belt is again divided into two groups, i.e., lower and upper. The lower group starts with conglomerates, quartzite and basic volcanics referred to as Delwara Formation. This Formation is overlain by a sequence of rocks comprising phyllites 11

(locally carbonaceous) dolomites, quartzite and stromatolitic phosphorites (Jhamarkotra Formation). The upper group contains graywacke-slate-phyllite, rhythmites, lithic-arenites at the bottom, overlain by quartzite, dolomite, and silty arenite in the middle and slate phyllite at the top. The rocks of the Aravalli belt thin out towards the north and occur as irregular smears between the B.G.C. and Delhi belt rocks. The grade of metamorphism does not exceed green schist facies, except in rare instances (Naha et al. 1968). No felsic magmatism is reported in the

Aravalli Supergroup.

The deep water facies rocks of Aravalli Supergroup form a north-south trending linear belt, west of Udaipur which extends from south of Rakhabdev (24°5' : 73°42') to

Gogunda in the north (24°45'' : 73°34') (figure-1). The rocks of Jharol Group include phyllite, mica schist, sheared amphibolites, thin persistent beds of quartzite and numerous ultramafics. This association of rocks is believed to have been deposited in the deeper part of Aravalli basin (Roy and

Paliwal 1981; Sharma and Roy 1986).

The mafic and ultramafic rocks in this sequence are found at two different stratigraphic levels. The sequence starts with a basal volcanic-quartzite unit which occurs in direct contact with an exposed gneissic basement 12 within the Jharol belt in Bagdunda area (Sharma et al.

1988). Another unit of mafic rocks occur within the Jharol

Group at a higher stratigraphic level and forms a linear belt between Modri in the north to Damana in the south. Due to complex structure of the Jharol belt, the relationship between these mafic-ultramafic rocks and the associated metasediments is not clear. Although the relationship between various units is not very much clear, the lithostratigraphic sequence of Jharol Group may be summarized as follows:

Basal quartzite, which is exposed near Bagdunda village, forms a complete oval shaped outcrop (figure-1).

These basal quartzites are followed by the amphibolites.

Amphibolites are also intercalated with thin bands of quartzite. They are generally of light to dark-green colour.

Amphibolites are followed by garnet mica schist. They are exposed right from Bagdunda village in the south to Phutiya village in the extreme north. The mica-schist forms the major unit in that area. Isolated ultramafics (talc-chlorite schist) are observed within the garnet mica schist north east of Bagdunda village.

Garnet mica schist is comformably followed by younger quartzite. These quartzite bands are exposed on the west of Bagdunda, which continue in the north for about four 13 kilometers, whereas the quartzite bands exposed north west of Bagdunda continue right from the north up to

Majawaton-Ka-Gurh, where it appears again in the west of

Phutiya and continues in the north.

The garnet mica schist is followed by mafic-ultramafic rocks, with thin bands of intercalated quartzite. They are exposed between Modri and Gopir, forming a linear belt of metamorphosed basic rocks and serpentinized ultramafics. They are sometimes associated with chert bands in the northern part of the belt.

Field Occurrence of Mafic-Ultramafic Rocks of Jharol Group :

Throughout the belt, the ultramafics are associated with mafic rocks. Although the mafic rocks of

Jharol Group have been described as ultramafic intrusions by various workers, the present study suggests that the linear belt between Modri in the north and Gopir in the south consists of both mafic and ultramafic rocks. Even though the deformation and metamorphism (up to green schist facies) have obliterated their primary structure, there are some features still preserved, which suggest that some of the mafic-ultramafic rocks were erupted contemporaneously with the sedimentation of Jharol Group. Such presumption can be evident from the following features. Near Jhameshwarjee 14 temple, the ultramafics are intercalated with quartzite bands (plate-1). Moreover, ill preserved possible pillow

structure (plate-2) and vesicles are found near Challi area in Playal Ghati mines, 20 Km from Gogunda, on the road side connecting Gogunda with Jharol. Furthermore, the vesicular basalt is found 1 Km north west of Jhameshwarjee temple, give a pitted appearance, which may reflect the removal of the fillings from the vesicules (plate-3).

The mafic-ultramafic rocks of Jharol Group are represented by serpentinite and talc-chlorite schist. They are generally deformed and cofolded with the metasediments.

These mafic-ultramafic rocks along with phyllite and carbonates, have been considered as a possible ophiolitic

sequence (Sinha-Roy 1988). These rocks are found enclosed in the upper part of Jharol Group. According to Middlemiss

(1921), the talc-serpentine rocks are metamorphosed ultramafic intrusions of peridotite or pyroxene. These rocks are characterised by green colour and are composed of talc,

serpentine and chlorite in varying proportions.

Actinolite-tremolite, calcite, dolomite and magnesite are also present.

In the northern part of the belt, the mafic-ultramafic unit of Jharol Group contains thin bands of garnet mica schist in the south of Majam village and its Plate 1,

Plate 1. The intercalation of Jharol ultramafics with quartzite near Jhameshwarjee temple. Plate 2.

Plate 2- 111 preserved pillow structure near Challi village. Plate 3.

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MSr-

Plate 3. Jharol ultramafics , showing the removal of some phases giving rise to pitted appearance, indicating their altered nature. 15

outcrop width increases in the north, where maximum outcrop width is observed in the south of Jogryon-Ka-Gurha and narrows again near Jhameshwarjee temple. At some places, the thin bands of quartzite are found intercalated with the ultramafics (plate-l), which may reflect their extrusive nature. At other places, the Jharol ultramafics occur as isolated outcrops in garnet mica schist north west

Badundia. Several mining pits for talc, amphibolite, asbestos and calcite are dug along the strike. In general, they are deformed, altered and metamorphosed up to green schist facies. In hand spacemen , the grain size ranges from fine to medium and in some localities, the crystals of actinolite - tremolite are large enough to be measured by scale. The ultramafics exposed near Challi area are hard, talcish and dark in colour, with very fine grain size.

Further in the south, six kilometers from Jharol towards

Damana, in Gopir area, the ultramafic rocks are found associated with mafic unit. They are hard and dark in colour.

The coarse grained amphibolite units, probably derived from gabbroic protolith, are readily distinguished from fine grained amphibolites, for which a volcanic origin is inferred. The mafic rocks (amphibolites), which are found within the domal structure near Bagdunda village, are massive, and hard. They are deformed, dark in colour, where 16 almost the whole of the Bagdunda village lies on the amphibolites.

Age and Correlation :

The Sm/Nd isotops studies on the amphibolites and biotite gneisses, collected from Banded Gneissic Complex, east of Udaipur, gave an isochron age 3500 Ma. (Roy et al. 1984). While the late phase granitic intrusion (the Untala granite and the Gingle granite) in the Banded Gneissic Complex, show Rb/Sr isochron age 2950 + 150 Ma. (Chaudhry et al. 1984). Berach granite has been proposed to be the youngest rock to the pre-Aravalli history having a model age 2500 Ma. (Crawford 1970). The Early Proterozoic age of Aravalli rocks is based on 2000 Ma. Rb/Sr isochron age of Dharwal granite, which is believed to have been emplaced during the earliest phase of deformation of Aravalli rocks. (Naha et al. 1968) .

The basal volcanics (Delwara Formation) of the Aravalli Supergroup are directly overlain by Debri quartzites. Crawford (1970) and Sarkar (1980), determined the ages of Dabari quartzite as 2000-2500 Ma. and 1600-2000 Ma. respectively. Gopalan et al. (19 90) carried out the Sm/Nd isotops of tonalitic to granodioritic gneisses from the basement gneiss of the Aravalli region and gave an 17 isochron age of 3.3 Ga. for the rocks in Rajasthan, north western India. They suggested that the mafic volcanism may have been associated with the formation of the granite.

Recent Nd model ages indicate that the basal Aravalli volcanics may not be older than 2.6 Ga. (MacDougall et al. 1984). The basal stratigraphy of the Jharol belt in the Bagdunda area (Sharma et al. 1988) has a striking similarity with the basal formations of main Aravalli belt in the type area of Udaipur. Therefore, the volcanic rocks occurring around Bagdunda are considered as /-^^ 2500 m.y. old and the mafic - ultramafic unit of Jharol Group is considered as younger than the basal unit. Chapter II

^ttroaraplJP CHAPTER TWO

PETROGRAPHY

Petrography has been defined by many workers in different ways with different objectives. The simplest is that, petrography is the systematic description of rocks in hand specimen and thin sections (Whitten and Brooks 1972). A generalized knowledge of the lithological aspects of the rock is essential for understanding the geology of an area, therefore, it has been found necessary to work out the petrography of mafic-ultramafic rocks of Jharol volcanics present in the area.

After a careful examination of all the samples in hand specimen, about sixty thin sections of selected samples have been prepared and studied from mineralogical and textural points of view.

The field characteristics of Jharol mafic-ultramafic rocks have been discussed in the preceding chapter. Petrographically, the Jharol volcanics comprising ultramafics and mafics show common mineral assemblages and textural relations, like many other altered corresponding igneous rocks. The Jharol volcanics have suffered multi-phases of deformations and regional metamorphism up to 19

the grade of green schist facies, thus resulting in general obliteration of pristine mineralogy and textures- However, the extrusive nature of these volcanics can be sometimes recognised in the field on the basis of certain characteristics, such as, the intercalation of ultramafics with the quartzite bands (plate-1), as well as the presence of ill preserved pillow structures (plate-2) and deformed vesicles.

In this chapter, the petrographical and mineralogical features of the mafic-ultramafics of Aravalli

Supergroup are discussed as they are observed in the microscope.

MINERALOGY :

Majority of the rock samples from Jharol mafic-ultramafic rocks are composed of secondary minerals, such as, serpentine, hornblende, actinolite-tremolite, chlorite, calcite, sphene, zoisite, palagonite, talc, plagioclase and quartz- The ultramafics consist mainly of serpentinite which may be present as an alteration product of pyroxenes and olivines. Magnetite is a common accessory mineral. On the other hand, the mafic unit contains more than 50 per cent hornblende. However, Bagdunda mafics have a fresher appearance than those of Gopir, with more pronounced mineralogy. 20

Serpentine is the major constituent mineral of

Jharol ultramafics. Antigorite is the most common variety of

serpentine in these rocks. It occurs in the form of fine

sheafs and plates often in a criss-cross mesh work. The antigorite crystals are colourless in plain polarized light, possess straight extinction and one set of cleavage.

Actinolite and hornblende are the main varieties of amphiboles. These minerals occur in the form of phenocryst as well as in the ground mass. Actinolite

crystals are light green in plain polarized light, with prismatic shape and show faint pleochroism. They show high

second order polarization colours and possess an- extinction angle range from 0°-8°. The crystals of hornblende, are prismatic in shape, having green colour with marked pleochroism in the shades of green, bluish green, deep green, brownish green to green. The extinction angle is about 24°-27°. The crystals show second order interference colour and bear two sets of cleavage- Inclusions in the hornblende are generally of iron oxides and quartz.

In some of the rock samples, zoisite (epidotes) is the major constituent mineral. The prismatic crystals of

zoisite possess straight extinction angle in any section, with perfect one set of cleavage. They are colourless in plain polarized light, showing high relief and deep sea blue interference colours under cross-nicols. They also 21

occur as columnar aggregates.

Chlorite is another important constituent mineral of Jharol volcanics. The flakes of chlorite show random orientation. They are of low interference colour and show straight extinction angle. In more altered rocks, chlorites are also often found. They occur as fibro shaped and possess very low relief, showing second order interference colour.

Crystals of calcite have perfect three sets of cleavage. Some crystals show deformation effect, as the outlines of the crystals appear to be curved. . They show first order of interference colour. The calcite crystals are generally found in association with antigorite crystals in most of the thin sections.

In fresh looking samples, the laths of plagioclase

show lamellar twinning, having subhedral to anhedral crystal

shapes. They bear the composition of oligoclase. Although they are in a very small amount, they are quite distinguishable in the thin sections. Plagioclase crystals

seem to be the relicts which have escaped alteration and metamorphism.

The crystals of palagonite are green in colour in the plain polarized light, but completely isotropic under 22 cross-nicols. They show no pleochroism , they are massive in form with low birefringence.

Sphene is found in some thin sections. They are colourless, having lozenge shape with high relief and two sets of cleavage.

Quartz is found in minor amounts in some of the thin sections, mainly in those of Bagdunda mafics.

Opaques occur as enhedral crystals, or irregular masses or fine dust, disseminated throughout the rock samples. They are mainly magnetite and ilmenite. Chapter III

(Seotl^emtsitrp CEAPTER THREE

GEOCHEMISTRY

Analytical Procedure :

Sample Processing ;

The first objective in sample processing is to reduce the sample size without contaminating it, in such a manner that the sample should represent the whole rock. The samples collected from the field were initially reduced to

1/2 cm size in laboratory, using hammer and then crushed by steel mortar. After crushing each sample, the mortar was cleaned with acetone and dried. The crushed samples were powdered to -200 mesh size in an agate mill. Major, trace and rare earth elements were analysed at Wadia Institute of

Himalayan Geology, Dehradun.

Analytical techniques ;

X-ray Fluorescence Spectrometery (XRF) and

Inductively Couple Plasma Atomic Emission Spectoscopy

(ICP-AES), were used in performing the analyses of the ultramafic and mafic samples. For the mafic samples, the major oxides SiO^, ^120^, MgO, CaO, Na20, K 0, TiO- and the 24

trace elements Cu, Zr, Ga, Th, Pb, Ni, Rb, Zn, Sr, were determined by XRF. The trace elements Ba, Cr, Co, V, Y, Nb and Sc, and MnO, were analysed by ICP. For ultramafic samples the analysis of SiO« was performed on XRF. The major elements Al-O^/ FeO , MgO, CaO, TiO- and MnO and the trace elements Co, Ni, Cr, and V, were analysed by ICP. The rare earth elements (REE), were separated from other major and trace elements using chromatographic columns following the methods of Walsh et al. (1981). The separated REE were analysed using ICP-AES, at W.I.H.G., Dehradun.

Pellets preparation :

To analyse the samples by XRF, the pressed powder pellets were prepared. Three grams of sample was weighed, and transfered to an agate mortar, where two to three drops of Polyvinyl Alcohol was added to it, and mixed thoroughly for two to three minutes. Thereafter, the sample was pressed using compression cylinder where two spoons of boric acid crystals had been added as a base and around 15 K.b. pressure was applied.

B-Solution :

B-solutions are used to carry out analyses by ICP.

To prepare B-solution, flat bottomed teflon beakers are just 25

rinsed with HCl (1:1) and double distilled water (ddw), then two to three drops of ddw is added. One gram of sample powder is transfered into the beaker and then, 15 ml of HF and 5 ml of HCIO. were added before heating it on a hot plate (below 180°c) until no liquid remained, but just a damp sludge (i.e., incipient dryness). Five ml of HCIO, and a little d.d. water were again added until the sample was completely dissolved. The solution was dried and the residue was dissolved in 20 ml of 25 per cent HCl by heating the

solution for 30 minutes at 50-60°c. Later, the solution was transfered to 100 ml glass beaker. The sample was cooled and the volume was made up to 100 ml using a volumetric flask.

Later, the solution was split into two fractions of 50 ml. each for REE and major and trace elements analyses.

REE separation ;

To separate the REE from B-solution, three types of normalities of HCl were prepared, they are as following:

(1) 1 N. (218.9 ml. HCl).

(2) 1.7 N. (372.13 ml. HCl).

(3) 4 N. (875.6 ml. HCl).

The above normalities of HCl were placed in their respective bottles whereby the volume was brought up, for each of them, to 2500 ml. (2.5 Lts.) by adding d.d. water (or as required 26 for different normalities, but with, the same ratio as above), the rasin in each glass column was stirred with sterilized glass rod, for the purpose of purification and rejuvination of the rasin in each glass column. 250 ml. of 4 N. respectively were added, the resultant solutions were discarded.

In six of the eight columns 50 ml. of samples were added, blank solution was added into the seventh and standard to the eighth column, here too the resultant solutions were discarded. Thereafter, 450 ml. of 1.7 N. was added in each of the eight columns and the • resultant solutions were again discarded. Finally, 600 ml. of 1 N. was added to each of the eight columns and the resultant solutions were carefully collected into 1000 ml. beakers, sterilized with nitric acid overnight. The resultant solutions were heated on a hot plate to complete/absolute dryness, then the volume was made by adding 10 ml. of 10 per cent nitric acid to each beaker and stirred, thereby they were transfered into their respective storage 10 ml. flasks ready for the analysis of REE.

Loss on Ignition (LOI) determination ;

The loss on ignition was determined by using silica crucibles, which had been heated in an oven at 1000°c 27 for two hours. The silica crucible was then cooled and weighed, as W, . Five grams of sample was added to the crucible and again weighed as W- (W_-W, =weight of the sample). The crucible containing the sample was then heated at the same temperature for two hours, thereafter the crucible was cooled and re-weighed, as W^. Thereafter, the percentage of LOI was calculated.

Accuracy and Precision :

Accuracy of a determination can simply be defined as the concordance between it and the true or most probable values. The error will be the numerical difference between the two. In other words, accuracy of geochemical analysis completely rely on the standards used during the chemical analysis. The standards must satisfy at least the following features. Firstly, the standards should have a matrix similar to that of the samples. Secondly, they should incorporate a range of values obtained for the samples. Lastly, the values obtained for the different elemental concentration of the standards should well agree with the different instrumental techniques.

In the present study, we have used international standards BRl, BEN, BHVO-1, BIR-1, AGV-1, W-2, MRG-1, UBN, PCC-1, GSR3, GSR2, JBla and JPl standards for XRF analysis. 28

For ICP the standards JP-1, SP-13, PCC-1, BHVO-1, W-2, and JB-1 were used.

The precision of the analysis is governed by various factors such as, weighing instrumental precision, dilutions, sample homogeneity, etc. The precision is obtained by replicate determination of a particular constituent in different aliquots of the same sample. As the difference between the individual values decreases^ the precision becomes high. The precision, coefficient of variation and detection limits for various oxides and elements are presented in table-2.

Specification of the Instruments used ;

Energy Dispersive X-Ray Fluorescence Spectrometer (EDXRF) model EDAXEXAM six with Philips PV 9100 was used for pressed powder pellets. The operating conditions for major oxides determinations were Anode-Ag-X-Ray tube, operating voltage - 12 KV, path vacuum and filter nil. While for trace elements Anode-Ag-X-Ray tube, voltage was 40 KV and Ag filter was used to reduce the back ground in spectrum.

The ICP instrument used was a Jobin Yvon model JY 70 Plus ICP, with a 1-m focal length Czerny-Turner, holographic grating, 36 00 grooves/mm. and a Meinhard 29

concentric glass nebulizer "type C". The RF generator was

40.68 MHz single-phase unit, with power stable to better than 0.01 per cent. The gas flow-rates (l./min.) were: outer

(coolant) 12; intermediate (auxiliary) 0; central

(carrier + sheath) 0.55 (Rathi et al. 1991), where BHVO-1 standard was used.

Results :

In the present study, twenty two samples of mafic - ultramafic rocks of Jharol belt around Gogunda area are analysed for major, minor and trace elements including REE. After the chemical analyses of the rocks, the data were fed to the computer for calculation of element ratios (tables - 9 and 10), correlation coefficient between the various elements (table - 3,4 and 5) and other parameters of petrogenetic significance. The data are presented in different tables. The data of Gopir and

Bagdunda mafics (major elements, trace elements and REE) are presented in table - 6 and those of Jharol ultramafics

(major and trace elements) are presented in table - 7. 30

Major Elements Distribution :

General statement :

Among the many petrochemical Indices, Si, Al, Fe,

Mg, Ca, Na, K, Ti, P and Mn are commonly included in major elements analyses. In igneous suites, these elements show systematic variation. Thus, they play an important role in determining the composition and classification of various rock types. Among these elements Si, Al and Ca are essential structural constituents (e.s.Cs) in basaltic mineralogy because they almost fully occupy sites in olivine, pyroxene, plagioclase and garnet. Therefore, the distribution of these elements between solid and melt is independent of both the abundance of the element in the system and proportion of phase present (Langmuir and Hanson 1980). Mg and Fe are intermediate elements in basaltic rocks as they remain in the solid solution and do not completely occupy any one site in any mineral in the melt system but are abundant enough to be stoichiometrically important. The abundance of these elements appear to be a function of the present composition as well as the extent of melting and proportion of minerals present in the system (Langmuir and Hanson 1980).

Na, K, Ti, P and Mn concentrations in basaltic rocks are generally low and there are no stoichiometric 31

constraints on their abundances in any of the phases in the system. Unless a mineral contains these elements as the major component in a mineral-melt system they may be treated as trace elements (Langmuir and Hanson 1980).

The compositional variation is a consequence of crystal-liquid fractionation processes, either partial melting or fractional crystallization. The relationships between the major elements have been utilized variously to distinguish the evolutionary trends of the magmas (Kuno

1960, 1968; Murata 1960; Poldervaart 1964; Miyashiro 1974,

1975), their tectonic settings (Pearce et al. 1977; Rogers

1982; Kullen 1983), crustal thickness (Condie and Potts

1969; Naqvi and Hussain 1973 a,b; Dickinson 1975; Pearce et al. 1975) and the degree of partial melting as well as physico-chemical conditions during partial melting (Hanson and Langmuir 1978; Sun et al. 1979; Lungmuir and Hanson

1980; Francis et al. 1983).

Jharol ultramafics :

The range of variation and mean values of various oxides in Bagdunda volcanics, Gopir volcanics and Jharol ultramafics are compared in table-8. However, the important chemical characteristics are stated as follows. The ultramafic samples of Jharol Group are characterized by 32

high concentration of ferromagnesium elements. For example their MgO content varies from 36.6 to 39.5 per cent with an average of 38.3 per cent (table-8). On the other hand, the associated basic rocks (Gopir volcanics) contain 11.9 to

13.9 per cent MgO. In Jharol ultramafics the SiO-, which plays an important role in the determination of various rock types due to its highly variable nature, range from 39.3 to

41.6 per cent. However, the average of all the samples appear as 40.4 per cent.

The Al„0-5 concentration shows a variation from 1.6 per cent to 3.6 per cent and averages at 2.1 per cent. FeO

(total iron) show a large range of variation, from 3.01 per cent to 9.50 per cent with an average value of 6.13 per cent. The CaO content varies from 0.01 per cent to 0.53 per cent and averages at 0.12 per cent. The MnO ranging from

0.01 per cent to 0.08 per cent with 0.06 as an average value. The TiO„ content, which sometimes provides createst discrimination power among the major oxides (Chayes 1964), shows variation from 0.01 per cent to 0.09 per cent and averages at 0.06 per cent. The concentration of P^O-, Na„0 and KpO are below detection limits in these rocks.

The Mg number (100 MgO/MgO+FeO) ranges between

86.51 to 95.63, with an average value of 91.32. L.I.O. ranging between 9.0 2 per cent and 14.70 per cent vith an 33

average of 12.71 per cent.

In figure-2 a and b, various oxides are plotted

against less mobile or immobile elements such as TiO-, Ni,

MgO, Cr, and FeO . The scatter of plots may be due to post

igneous alteration and mobilization.

Since the discovery of komatiite and the related

rocks from South Africa (Viljon and Viljon 1969), the high

Mg rocks from Precambrian sequences have attracted a

considerabele attention. In recent years the ultramafic and

mafic rocks, from various Proterozoic supracrustal sequences

of the world have been classified as komatiite-tholeiite

associations (Baragar and Scoats 1987; Arndt et al. 1987;

Crow and Condie 1990; Raza and Khan 1990; Ahmad and Rajamani

1988, 1991). In general the komatiites are characterised by

high CaO/Al^O^ (0.8 to 2.0) and MgO/Al202 (> 0.6) ratios,

low TiO- (< 0.9) and low alkali content (< 0.9), along with

their high MgO contents (> 18.00) (Arndt and Nesbitt 1982),

Their important characteristic is the presence of quench

texture or spinifex texture. This characteristic has been

considered as an essential feature of rocks to be called as

komatiite. Jharol ultramafics do not record any presence of

spinifex texture or a like texture. Also their CaO/Al^O^

ratio is too low (avg. 0.07), and MgO/Al„0^ ratio is too

great (avg. 19.88). These features of Jharol ultramafics 5000 • • I •0.53 4500 - •0.35 3000 4000 - • 0.16 • 2800 - 3500 E 01* • Jharol Ultra-mali'cs 2600 Q.3000 • _ tx - • _ •" 0.12 - 2400 - • u 2500 " • 0 « '-' 0.10 2200 • 2000 • 008 - • • 2000 _ • 1500 « 007 • 1800 - • • 1000 0.05. 1600 • 500 • 0.03 _* ^ 9 • 0,09 - • • •035 0" 0.01 • • B - 0 16 - • 0.06 • • • 7 • 014 - - 3 - 0 0.04 • • • • • c e - 012 -. ;•» • ^ 002 • • • 5 - • • 0 0.10 ^ •' 1 • 0 lo" I0.35 u 0.16 4 - 0.08 • 0 • • • , • O.U • 3 - • 0.06 - 4500 - • 42 - 0.12 2 0.04 - • 4000 - - S " * 1 • • in » • » • aio - 3 0.02 - • 3500 . 40 • • £ • 0.06 • 2 - 0.00 ^ 3000 39 • • • - • 0.06 - • 0.08 - • •• u 2500 . « 9 ^0.06 0.04 • 0 - • • • 2000 '• • 8 • • 2 0.04 • • • 1500 0.02 • - • - • • _. 7 • • • 0.14 _ • 0.02 - 1000 - "b 6 • a/ 1 0.12 0.00 500 u. 150 - - 5 • • 130 - 010 - SO 150 4 110 0 08 6 50 • 130 • • • - • • • 3 • a ^ e 90 0 06 ^ 40 . • • a 110 1 • 2 •• •• • 0 1 70 - 0,04 * 30 •^ 90 . • • • 1000 • • • • • • 002 20 70 - 2800 ~ 50 - * 0 • 150 50 • • t 2C0U • a 6 "° 60 • i 2400 • a • 42 42 ^m & 50 • • 2200 t tx0 MO • • • • • a. 1 • 41 - • • • 41 * •• •^ 90 • • • ' 40 • 2000 • . • • ••. - • • * 40 40 70 - 30 . • 18 00 • • 19 1 ' ,• , ,• , , 39 .4 1 1 'i 1 1 50 1 1 1 1 1 1*1 \ 1 20 1 1 1 t 1 1600 1 1*1 1 t 1 5 6 7 S 9 10 33 34 35 36 37 38 39 40 41 3 3 34 35 36 37 38 39 40 41 42 0 .03 •04 .06 .06 .10 .12 .U 0 -02 .04 .06 Oe 10 .12 .14 F.o'v. MgO V. MgO V. TIG?'/. TiO?V, Figure 2-A. FeO^, MgO and TiO„ versus various oxides and elements in Jharol ultramafics^showing a sea tter of plots, may be due to post- crystallization alteration and mobilization. Jharol Ullra-mallci 0.08

>-• 0.06 o • f 9 I 0.04 0 • 'CSS a •03 5 8 0.02 u • 10.53 7 • 000 0-17 6 • • :o.3s 0.15 - • • • • 5 o 2 0.13 - I - 1 O.ll - • 3 0 0.09 - • 0.16 • 2 O.K 60 0.07 - • • • 4500 - 0 12 50 0.05 - • 4000 - U O.'O 40 0.03 • O • 3500 30k 0 01 • '-' 0.08 - 3000 - 0.06 JO I • • 2500 • 0.04 3000 2000 • 0.02 2600

15 00 « 0 2600 • 1000 - 008 g2400 • 500 -• 0.06 Q.2200 o 2000 ISO I 0.04 • 130 - 0.02 1800 110 0 1600 • 90 42 1400

70 - 'q 41 1200 • • in 50 tooo 40 60 - 130 « 39 50 - •• . E no • • 3 a 40 - • • n « o 90 • 70 30 - • •

20 1 1 1 1.. L . 50l I " ' I I ' ' I L. 1600 2000 2400 2 BOO 1600 2000 2400 2800 5 00 1500 2500 3500 4500 500 1500 2500 3500 4500 N i p p m Nt ppm Cr ppm Cr ppm Figure 2-B. Ni and Cr versus major oxides and elements in Jharol ultramafics, showing a scatter of plots. 34

preclude to classify them as komatiites.

Due to the highly altered nature (plate-3) and

lack of trace elements and REE data, the Jharol ultramafics will not be considered for farther petrogenetic discussions.

These rocks were not analysed for incompatible elements such as HFSE and REE because the concentration of these elements are likely to be very low, and this is evidenced from the analyses of K-O, Na^O, and PpOc which are found to be below detection limits. Therefore , the mafic units of Jharol volcanics will be subjected for further detailed studies.

Jharol mafics (Gopir volcanics) :

The major elements composition of mafic samples of

Jharol Group are given in table-6. These samples were

collected from Gopir area (figure-1), therefore, they are discussed here as Gopir volcanics. The SiO^ content in Gopir volcanics ranges from 38.2 per cent to 45.1 per cent, which may be considered as a wide variation. The lowest content of

SiO„ is found in samples G3 and G4. These two samples also

contain high CaO concentrations (17.75 and 17.21 per cent respectively). The low SiO^ and abnormally high CaO contents

in these samples may be due to the removal of SiO„ and addition of CaO during post ingeous processes, most probably

from adjacent calcareous sediments. Comparatively, the 35

Gopir volcanics contain higher content of SiO„ (avg. 43.8 per cent, except samples G3 and G4), than the associated ultramafics. The TiO„ content in these rocks ranges from

1.70 per cent to 2.10 per cent with an average of 1.78 per cent, which is much higher compared to that of associated

Jharol ultramafics (avg. 0.06 per cent). -^lo^S ^^^^ ^ restricted range of variation (11.4 per cent to 15.21 per cent), and appears to be very high (avg. 13.8 per cent) than those of associated ultramafics (range 1.00 to 3.6 per cent, avg. 2.1 per cent). FeO (total iron) is also very high in the mafic rocks (avg. 12.35) than in the ultramafics (avg.

6.13 per cent). The MgO is ranging between 11.9 per cent to

13.9 per cent, with an average of 12.7 per cent. However, in ultramafic rocks the MgO varies from 36.3 to 41.5 per cent, and averages at 38.3 percent. CaO content ranges from 12.39 to 17.75 per cent, with an average value of 14,55 per cent.

The CaO content in these rocks is much higher than in the

Jharol ultramafic (range 0.10 to 0.53 per cent, avg. 0.12 per cent). The ^2^5 content in Gopir volcanics ranges from

0.11 per cent to 9.23 per cent, with an average of 0.18 per cent. In the associated ultramafics the PpOc content is below detection limit in all the samples. ^SL^O content in these volcanics ranges from 0.68 per cent to 2.09 per cent, with the average of 1.48 per cent. K^O content ranging frcra

0.66 per cent to 0.96 per cent with an average of 0,82 per cent. In the associated ultramafics, both Na„0 and K-O are 36 not detectable. MnO content in Gopir volcanics varies from 0.17 per cent to 0.34 per cent, with an average value of 0.27 per cent.

The loss on ignition has been calculated for the samples at 1000°c, which is found to have an average of 2.16 per cent. The Mg number (100 MgO/MgO+FeO) show a range of variation from 60 to 64, and are much lower than those of associated ultramafics (86 to 91).

Bagdunda volcanics :

The major elements composition of four mafic samples of Bagdunda volcanics are given in table-6. These samples were collected from Bagdunda area. These volcanic rocks show higher contents of SiO- (range 49.5 to 52.2 per cent, avg. 51.0 per cent), and lower contents of MgO (8.2 to 8.9 per cent, avg. 8.55 per cent) and CaO (9.79 to 12.75 per cent, avg. 11.45 per cent) than Gopir volcanics. The TiO- content (avg. 1.34 per cent), is almost similar to those of Gopir volcanics, but much higher than that of Jharol ultramafics (avg. 0.06 per cent). Bagdunda volcanics also contain lower FeO contents (avg. 10.94) than Gopir (avg. 12.35) volcanics.. P^^B contents are almost similar in Bagdunda and Gopir volcanics which averages at 0.18 per cent. The Mg number in these volcanics ranges from 54 to 59 37 and are considerably lower than those of Gopir volcanics (60 to 64), Thus, suggesting their more primitive or less differentiated nature.

Trace Elements Distribution :

General statement :

Trace elements have distinct advantages over the major elements particularly in modelling the igneous processes. This has become a very important tool with the ever improving data on their partitioning between the mineral and melts (eg. Neumann et al. 1954; Cast 1968; Greenland 1970; Paster et al. 1974; Allegre et al. 1977; Hanson 1980; Carr and Fardy 1984). The trace element concentration of a melt is dependent on the trace element concentration of the source region, the type and extent of melting, the residual phase remaining at the time of removal of the melt, any differentiation prior to solidification, melts or fluids (Hanson 1980).

Trace element abundances in Archaean volcanics has been used by many workers to deduce the genesis of the magma and physico-chemical environment in the early period of the geological past (eg. Hart et al. 1970; White et al. 1971; Jahn et al. 1974). 38

Till date, no analytical data is available on the trace elements composition of mafic rocks of Jharol area.

The present study provides, for the first time, the geochemical data of advance nature on these rocks. Due to extremely low concentration (below detection limits) of some incompatible elements (eg. Zr, Nb, Y) in ultramafics, they are analysed only for transitional elements i.e., Ni, Cr, Co and V.

Jharol ultramafics :

The analytical results are presented in table-7, while range of variation within the trace elements are presented in table-8.

Nickel is considered to be a very sensitive element for olivine fractional crystallization in the early stages of differentiation. The nickel concentration in

Jharol ultramafics show a wide range of variation, ranging from 1159 ppm to 2344 ppm with an average of 2027 ppm. On the other hand, the associated basic rocks contain 157 pen to 245 ppm nickel with an average of 194 ppm.

Cobalt concentration ranges from 56 ppm to 141 ppn

With an average of 99 ppm, whereas in Gopir basics it varies from 33 ppm to 71 ppm with an average of 53 ppm. Chromi-uri 39

which has also been considered as a sensitive element to fractional crystallization, shows a wide variation ranging from 650 ppm to 4742 ppm with an average of 2575 ppm. In

Gopir volcanics, it shows a variation from 169 ppm to 590 ppm with an average of 344 ppm. Vanadium like chromium, differs from other transitional elements, in having three common" valence states underterrestrial conditions that exhibit strongly contrasting geochemical behaviour. Vanadium in Jharol ultramafics, ranges from 28 ppm to 78 ppm with an average of 50 ppm. On the other hand, the associated basic rocks are relatively enriched in V, showing a range of variation from 172 ppm. to 326 ppm with an average value of

229 ppm.

Gopir volcanics :

The trace elements compostion of the mafic unit of

Jharol volcanics are presented in table-6, and compared with other rocks of Jharol belt in table-8. As discussed in previous paragraphs the Gopir volcanics contain comparatively lower concentration of transitional elements

Ni, Cr, Co and V than the associated ultramafics. Compared to Bagdunda volcanics the concentrations of these elements are well matched.

Elements having small ionic radii and low 40

radius/charge ratio are called high field strength elements

(HFSE). They tend to be strongly incompatible, having very small bulk partition coefficient in most situations and are considered immobile during low temperature alteration

(Pearce and Norry 1979; Saunders et al. 1980; Shervais

1982). This property together with their systematic variation in fresh lavas, make them sensitive indicators of their source regions (Erlank and Kable 1976; Pearce and

Norry 1979; Sun et al. 1979; Pearce 1982). Ti, Zr, Hf, Nb and Y are the elements which are included in the group of high field strength elements. The abundance of these elements in basaltic magma show a general increase from island arc through ocean floor to within plate magm.a types

(Pearce and Cann 1973; Pearce and Gale 1977; Pearce 1982,

1983).

Zirconium contents of Gopir volcanics vary from

144 ppm to 244 ppm with an average of 199 ppm (table-8). The average Zr content of Gopir volcanics, however, resembles with those of continental rift tholeiite (200 ppm), and

Archaean andesite (190 ppm) (table-11). In Eagdunda volcanics Zr content varies from 71 ppm to 170 ppm with an average of 104 ppm. Yittrium, show a variation between 38 ppm to 69 ppm with an average value of 53 ppm, while in

Bagdunda volcanics it varies from 25 ppm to 33 ppm v/ith 29 ppm as an average value. Niobium (Nb) content in these 41

mafics varies from 14 ppm to 23 ppm with an average of 19 ppm. On the other hand, in Bagdunda volcanics it is ranging from 7 ppm to 15 ppm with lower average value as 11 ppm

(table-8). Compared to Bagdunda volcanics, the Gopir volcanics appear to be more enriched in HFSE

(tables- 6 and 8).

Bagdunda volcanics :

The trace elements composition of Bagdunda volcanics are presented in table-6, and compared with Gopir volcanics and Jharol ultramafics in table-8.

Rubidium in basic rocks appears to be present in detectable amount only in feldspars and felspathoids

(Prinz 1967). However, the presence of Rb in pyroxenes, analcite and orthoclase has been also observed (Wager and

Mitchell 1951; Wilkinson 1959). In these mafics Rb content varies from 3 ppm to 4 ppm with an average of 4 ppm, which is lower than that of Gopir volcanics as it varies from 8 ppm to 11 ppm with an average of 10 ppm. Strontium in basaltic rocks is believed to be present in Ca-rich minerals like pyroxene, plagioclase apatite and in high temperature

K-bearing minerals such as feldspars and felspathoids

(Nockolds and Allen 1953; Butler and Skiba 1962; Heir 1962).

However, it has also been observed that clinopyroxene, 42 though have high Ca content, is depleted in Sr than in coexisting plagioclase (Wager and Mitchell 1951; Cronwall and Rose 1957; Wilkinson 1959). Strontium in these volcanics varies from 108 ppm to 513 ppm with an average of 225 ppm. On the other hand, it varies from 359 ppm to 966 ppm with an average value of 680 ppm in Gopir volcanics. It has been suggested that, the concentration of Bariiim (Ba) and Strontium (Sr) depends upon the temperature of crystallization. In Bagdunda volcanics, the barium content ranges from 26 ppm to 59 ppm with an average of 43 ppm, whereas in Gopir volcanics it varies from 8 ppm to 33 ppm with an average of 22 ppm.

Zn, replaces Fe+ 2 and Mg diadochically in the mineral structures. Early magmatic oxides are weakly zinciferous as Zn enriches into the structures of magnetite and ilmenite, because during magmatic differentiation it remains largely in residual melt throughout the main stage of crystallization (Rankama and Sahama 1950). Zn content of Bagdunda volcanics show a range of variation ranging from 117 ppm to 143 ppm with an average of 132 ppm. In Gopir volcanics, it averages at 121 ppm and ranges between 62 ppm and 197 ppm. 43

Rare - Earth Elements :

The rare-earth elements (REE), has been proved very useful particularly in petrogenetic studies of basaltic rocks because their geochemical behaviour changes gradually, from the lightest lanthanum (La) to the heaviest lutetium

(Lu). Thus a given REE has geochemical characters very similar to those of its nearest atomic neighbour but, differing systematically from those of REE with greater or smaller atomic numbers (Hanson 1980). They are therefore, believed to be fractionated by magmatic processes and their relative abundances appear to record reliably the effect of primary differentiation processes in igneous geochemistry.

REE in general, have been inferred to indicate the primary nature of the magma chemistry (Sun and Nesbitt 1978; Jahn and Sun 1979).

REE data on these rocks is not available in literature. In the present study four samples of Gopir volcanics and two samples of Bagdunda volcanics have been analysed for 12 REE, viz: La, Ce, Nd, Sm, Eu, Gd, Tb, Dy,

Ho, Er, Yb, and Lu. The data is presented in table-6.

Gopir volcanics :

In general the total REE of Gopir volcanics 44

ranges from 55.67 ppm to 70.48 ppm. However, sample G3 is highly enriched in REE with total REE content of 219.19 ppm.

Chondrite normalized REE patterns of Gopir volcanics change from nearly flat with (La/Yb) =1.29 to LREE enriched with

(La/Yh)jj=8.63 (N = chondrite - normalized). Among all the samples showing flat pattern sample G7 shows strongest positive Eu anomaly. All but, one sample of these volcanics show flat to slightly enriched REE pattern (about 30-40 x chondrite La; and 20-25 x chondrite Yb) (figure-3). The exception is sample G3 which shows LREE enriched pattern

(about 320 x chondrite La; and 37 x chondrite Yb). It is not clear whether the negative Eu anomaly of G3 is a primary feature or resulted from alteration processes. The REE abundance of sample G3 suggest that it might have been derived from a different source. This possibility will be further discussed in chapter V. Another feature of this sample (G3) is that, HREE abundances are less variable than the concentration of LREE (Tb/Yb= 0.308; and La/Sm= 5.719).

Bagdunda volcanics :

The total REE of Bagdunda volcanics ranges from

33.42 ppm to 67.98 ppm. Chondrite normalized pattern of

Bagdunda volcanics (figure-3), changes from LREE enriched in sample BA3 (about 35 x chondrite La; and 16 x chondrite

Yb) to LREE depleted in sample BA5 (about 11 x chondrite La; 1000 r

G3 G5 1^ Gopir 200 Ge Volcanics G7 ^IBagdunda * BAsj Volcanics 100

50

i 20 o:

10

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 3. Chondrite normalized REE abundance in Gopir and Bagdunda volcanics. Normalizing values after Sun and McDonough (1989). 45

and 16 x chondrite Yb). Such depletion of LREE may be due to alteration (Ludden and Thompson 1979), or it may be due to the influence of asthenospheric (N-type MORE source) which gave rise to such depleted pattern. This can be clarified only with more samples from this area being analysed for their REE concentrations. In these rocks the (La/Yh)^ ratio vary from 0.68 to 2.78. The relative range of abundance of individual REE of Bagdunda volcanics are compared to Gopir volcanics and presented in table-8.

Effect of iQteration :

Most of the ancient volcanic rocks have undergone some degree of alteration, so that the original nature may be obscured. The most common types of alteration include albitization, epidotization, silicification, carbonation and chloritization. Mineral assemblages formed by such types of alteration are similar to low grade metamorphic assemblages

(Jolly and Smith 1972; Condie et al. 1977; Strong et al.

1979). In some instances, these alterations occur purely as response to progressive low grade regional metamorphism, while in other cases secondary minerals such as carbonate, quartz and sericite may replace low grade assemblages and show cross cut relationship with the foliation, indicating post metamorphic alteration (Condie et al. 1977;

Condie 1982 a). 46

In interpretations based on chemical characteristics of ancient rocks, the most critical question arises as, what was their original composition and to what extent their chemical constituents have been affected by the alteration processes. Numerous studies document the mobility of various elements during alteration processes (eg. Lisitsyana 1968; Cann 1969; Babcock 1973; Hart et al. 1974; Scott and Hajash 1976; Condie et al. 1977; Beswich and Souice 1978). Although there is a general agreement, that major elements such as Na, K, Ca, and Si are appreciably mobile during alteration, there are conflicting results of mobilities of many other elements (Jolly and Smith 1972; Condie et al. 1977, Humphris and Thompson 1978). The major elements such as Al^O,., MgO, FeO, TiO-, P^'^B ^^'^ ^^^ ^""^^ considered either less mobile or immobile and thus, have been used variously in petrogenetic interpretations (eg. Pearce 1970; Miyashiro 1974, 1975; Pearce et al. 1977; Mullen 1983). It is, therefore, appropriate to determine the extent and effect of post crystallization processes (alteration and metamorphism) on the rock chemistry, before using the data for petrogenetic interpretations.

The Jharol volcanics have been affected by low grade regional metamorphism, as well as, intensive deformation, therefore, major compositional changes are likely. In the present study various diagrams and chemical 47 criteria are used to investigate the possible extent and effect of alteration on the chemical composition of Gopir and Bagdunda volcanics of Jharol Group. In order to determine the nature and extent of mobility of various elements and to assess the effect of secondary processes on rock chemistry, the major and trace elements which are known to be mobile during these processes are plotted against Zr (figures- 4,5 and 6) which is known to be relatively immobile (Arndt and Jenner 1986). Despite some scatter only TiO„, PnOc and AlpO-. show the trends consistent with an evolving magma, therefore, indicating their primary concentrations (figure-4).

Schweitzer and Kroner (1985), have used CaO/Al„O-,-MgO/10-SiO2/100 ternary diagram to recognize the unaltered volcanic rocks. In this diagram (figure-7), all the analysed samples of Gopir and Bagdunda volcanics plot in the field of unaltered basalt. The Gopir volcanic rocks shov/ some inclination towards the Mg-apex. This probably reflect their originally high MgO content. It has been pointed out (eg. Pearce and Cann 1973; Pearce et al. 1975; Winchester and Floyd 1976, 1977; Mullen 1983), that Ti and a number of other elements are not mobile during various alteration processes, including low grade metamorphism. Thus in order to check the validity of Mgo and CaO content of Gopir and Bagdunda volcanics, their MgO/TiOp and CaO/Tio„ratios are • Gopir Volcanics o Bogdunda Volcanics 15

a

en O 5"'3

12

11

Figure 4. Zr versus Ti02f ^2°5 and AI2O3 of Gopir and Bagdunda volcanics, illustrating the less mobile nature of these major oxides. Zr is assumed here to be least mobile element. 100 • Gopir Volcanics o Bagdunda Pm Volcanics 60

150 200 250 Zr Figure 5. Zr-Y covariation diagram of Gopir and Bagdunda volcanics, showing the relative enrichment of Zr over Y. 160.8 t

• Gopir Volcanics o Bagdunda Volcanics

20

£ Q. o. 10 (b O © 0

30 e Enriched Q. a20

10

0 50 100 150 200 250 Zr ppm

Figure 6. (A) Zr versus Ce and (B) Zr versus Nb diagrams of Gopir and Bagdunda volcanics, showing the less mobile nature of these elements and illustrating the sensitivity of Zr/Nb ratio to source composition (PM ratio after Sun and McDonough 1989). CaO/Al203

MgO/10

Figure 7. CaO/Al2O2-MgO/10-SiO2/100 ternary variation diagram (after Schweitzer and Kroner 1985), of Gopir and Bagdunda volcanics, where all of the samples plot in the field of unaltered rocks/ although these major elements individually are demonstrated to be mobile in earlier plots. This diagram indicates either that the ratio were less affected by post-crystallization processes or put severe limitation on the suitability of this diagram. 48 plotted against each other (figure-8), the sympathetic relationships between these two ratios do negate any serious impact of secondary alteration on these two elements.

Although some major elements of Gopir and Bagdunda volcanics appear to reflect their primary concentrations, some of them are unlikely to reflect abundances at eruption. Therefore, no reliance is placed on these elements in the interpretations of petrogenesis and tectonic setting. Our interpretations rely in preference on the minor, trace and rare earth elements which are considered less mobile. The chemical evidence adduced in this study from major elements are confirmed by immobile elements before reaching any conclusion.

Major Elements Variability :

The compositional variation in an igneous suite is a consequence of crystal liquid fractionation processes, related either to partial melting or fractional crystallization. The chemical differences and trends shown by volcanic suite can reasonably be presented in a simple way by using variation diagrams. Such diagrams are considered to be a useful way of synthesising a large volume of chemical data, which is difficult to compare in table form. These diagrams have proved very useful in • Gopir Volcanics o Bagdunda Volcanr'cs

12

CM 10 5 8 en 2 6

- o 2L 8 10 12 CaO/Ti02

Pigiire 8. MgO/TiOp versus CaO/TiO„ binary diagram for Gopir and Bagdunda volcanics, illustrating the sympathetic relationship, as an indication for their primary magma tic character, in terms of major element ratios. 49 derivation of models to explain the petrogenesis of a particular igneous assemblage. In the present section, major element oxides of Gopir and Bagdunda volcanics are plotted against different parameters to see the chemical differences within and between these suites. These differences will be a key to the evolution of these rocks.

Variation in basalt chemistry are generally examined with respect to Mg number (100 MgO/MgO+FeO) . It is evident from table-6, that Gopir samples have higher Mg number than that of Bagdunda volcanics which indicate that Gopir volcanics are less fractionated compared to Bagdunda volcanics. Their less fractionated nature is also reflected by transitional elements such as Ni, Co and Cr. In ultramafic rocks of Jharol Group, the Mg numbers are more than 85, indicating their rather primitive character, with very low extent of fractionation.

According to Hanson and Langmuir (1978), Mg-number is sensitive to olivine fractionation and since TiO_ acts as relatively magmaphile element (olivine or plagioclase Kd=0.01) (Dungan and Rhodes 1978), the relationship between these two would clearly indicate the olivine fractionation. In figure-9 the Mg-number and TiO- contents of Jharol volcanics are plotted to examine the extent of fractionation in these rocks. It appears that the samples from Gopir Olivine, Fractionation Path From Primitive^ Parental Composition

2 - • Gopir Volcanics o Bagdunda Volcanics

0 30 AO 50 60 70 80 Mg- Number Figure 9, Mg-number versus Ti02 binarbinaryy diagram (after Dungan and Rhodes 1975) for Gopir and Bagdunda volcanics, indicating olivine fractionation in these volcanics. 50 volcanics have undergone less than 12 per cent olivine fractionation, where in Bagdunda volcanics it is greater than 12 per cent.

Baragar and Scoates (1987), while studying the komatiite - tholeiite association of Circum - Superior belt used the MgO/CaO ratio. as a measure of olivine fractionation. During the course of fractionation within mafic and ultramafic, range of compositions of CaO increases with separation of olivine and decreases with the fractionation of clinopyroxene. They interpreted the MgO/CaO of about one as the point at which the clinopyroxene appears on the liguidus. Variation diagrams of Gopir and Bagdunda volcanics are shown in figure-10, where TiO„ content is plotted against MgO/CaO ratios. The arrows shown in these plots are after Baragar and Scoates (1987), and show direction of compositional changes in the magmas which affected by withdrawal of the phases indicated. The plots in figure-10 demonstrate , that, the variation in composition of Gopir and Bagdunda volcanics may be the result of olivine and possibly pyroxene fractionation.

CaO/Al„0^ ratio has been considered as one of the most important characteristics of the basic volcanic rocks. Since in the primary melts it reflects the ratio of their mantle source (Cawthorn and Strong 1974; • Gopir Volcanlcs o Bagdunda Volcanics O

2.0

1.5

rsi O 1.0

0.5

0.0 _L 0.6 0.7 0.8 0.9 I.O MgO/CaO Figure 10. MgO/CaO versus TiO_ variation diagram (after Baragar and Scoates 1987) of Gopir and Bagdunda volcanics, MgO/CaO is used as measure of fractionation. Arrows in the diagram indicate the direction of compositional changes in the magma with the fractionation of phases indicated. 51

Perfit et al. 1980). In Gopir and Bagdunda volcanics

CaO/Al^O^ ratio varies from 0.89 to 1.18 and 0.82 to 1.04 with the averages 1.05 and 0.95 respectively (table-9).

Figure-11, illustrates the relationship between CaO/Al^O^ ratio and TiO„ contents of Gopir and Bagdunda volcanics.

This figure also suggests the fractionation of olivine in both the volcanics because CaO/Al„0^ ratio remains constant with increasing TiO„ (Dungan and Rhodes 1978).

Bell et al. (1985), noted that decreasing trend of

CaO/Al„0-. ratio with FeO /MgO ratio is an indication of

clinopyroxene fractionation- It is evident from figure-12 that in both Gopir and Bagdunda volcanics clinopyroxene has a dominant role during the magma evolution, although it seems more pronounced in Gopir volcanics.

Al_0^/TiO„ ratio of the basaltic rocks has been

variously interpreted (eg. Sun and Nesbitt 1978; Nesbitt et al. 1979; Sun et al. 1979; Hickey and Frey 1982).

Al„0-s/TiO_ ratio of Gopir and Bagdunda volcanics are plotted

against their TiO^ contents in figure-13. AlpO-,/TiO- ratios

of these volcanics plot in the field of mid-oceanic ridge basalt (MORE), and beyond towards higher TiO- contents,

indicating variably enriched source characteristics and/or variable extents of partial melting of mantle sources. o • Gopir Volcanics £ o Bagdunda Volcanics Or GC

a u o

en o O Olivine Removal CM

o o u

1.^

1.2 -

1.0 - O 0

0.8 - 0

O.fi 1 1 0-8 1.2 1.6 2.0 2.A T1O2V.

Figure 11. Ti02 versus CaO/Al-O^ variation diagram (after Dungan and Rhodes 1978). showing relative olivine and plagioclase fractionation in Gopir and Bagdunda volcanics. • Gopir Volcanics 2.5 o Bagdunda Volcanics

n 1.5 O »-0l.0px CN < O a o Island Arcs 0.5

0 JL 0.5 1.5 2.5 FeO^ /MgO

Figure 12. FeO /MgO versus CaO/Al20^ diagram (after Bell et al. 1985) for Gopir and Bagdunda volcanics. Arrows indicate the mineral fractionation trends. Data plot indicate olivine and clinopyroxene fractionation. • Gopir Volcani'cs o Bagdunda Volcanlcs

AO CN O 30 *-^ m O CNl 20 P < o 10 • • •• • o 1 1 t 1 0.5 1.5 Ti02 7o

Figure 13. TiO- versus Al-O^/TiO- variation diagram (after Sun et al. 1979), showing identical trend of variation for Gopir and Bagdunda volcanics and relatibely large variation in TiO-• 52

Trace Elements Variability :

Trace elements are particularly useful in petrogenetic studies of igneous rocks (Taylor 1965; Cast 1968; Weill et al. 1974; Hanson 1980; Carr and Fardy 1984), because they have several distinct advantages over major elements. The trace elements are seemingly partitioned more strongly than major elements into either crystalline or liquid phase making them more sensitive indicators of both degree and mechanism of differentiation. The concentration of some major elements in volcanic rocks often drastically modified by secondary processes, whereas the trace elements, notably the high field strength elements (HFSE), transitional elements and rare earth elements (REE) remain immobile. In view of variable state of preservation of Proterozoic volcanics, the trace elements are likely to prove more useful, particularly in the assessment of their petrogenesis.

The Zr/Nb ratio has been found a powerful discriminant between enriched mantle source and depleted mantle source (Erlank and Kable 1976; Le Roex et al. 1983). The colinear relationship between Ce and Zr (figure-6 a), suggest that the Ce concentration have not been affected by alteration processes. The relationship between Zr and Nb 53

(figure-6 b), indicates the relative enrichment of Nb compared to Zr, indicating both, less mobile nature of Nb and enriched source characteristics.

The Zr/Y ratios in these volcanics, vary from 3.5 to 4.8 and 2.4 to 5.7 for Gopir and Bagdunda volcanics respectively (table-9), with an average of 3.84 for Gopir volcanics and 3.58 for Bagdunda volcanics. These values are higher than chondritic ratio (2.5) as reported earlier by Nesbitt and Sun (1976). On a Zr/Y versus Zr plot (figure-14) , Bagdunda volcanics show a systematic increase of Zr/Y ratio with similar increasing in Zr content. However, the Gopir volcanics do not show any relationship.

The Ce and Nd contents of Gopir and Bagdunda volcanics are plotted in figure-15, where the Ce and Nd concentration for Primordial Mantle is also shown (Taylor and McLennan 1981). The plots of both the volcanics fall on the same line which intersect the origin. Because of highly enriched nature of sample G3, it plots well outside the diagram but, on the same trend.

In figure-5 Zr content of both Gopir and Bagdunda volcanics are plotted against Y. In Zr versus Y diagram, the plot of data below PM line suggest that these rocks are enriched in Zr. relative to Y. • GopiV Volcanics 5 [_ o Bagdunda Volcanics o

£ 5 a a

N

O O

0 100 150 200 250 Zr ppm

Figure 14. Zr versus Zr/Y variation diagram (after Pearce and Norry 1979) of Gopir and Bagdunda volcanics. 160.8 .71.17 • Gopir Volcanics o.r/o 100 LOWER EXTENTS o Bagdunda Volcanics jf^ \ OF MELTING

80

E Q. a 60

U

AO

20 107o^/^ MANTLE yy^ .1 >^' HIGHER EXTENTS \ ^O ^^^ MELTING 0 0 10 20 30 40 50 Nd (ppm )

Figure 15. Plots of Ce versus Nd for Gopir and Bagdunda volcanics, showing the least effect of contamination and reflecting their magmatic characteristics (after Horan et al. 1987). Chapter IV inagma tE^ppe Clas(£(ifttation CHAPTER FOUR

GEOCHEMICAL CLASSIFICATION

General Statement :

In the past, there was considerable ambiguity in the concept of igneous rocks series because of the intrinsic and more casual characteristics of the series of rocks.

Subsequently, the characteristics of various volcanic rocks from different tectonic settings were determined and as a result a close relationship between tectonic environment and chemical composition of volcanic rocks has been suggested by many workers (eg, Kuno 1959, 1960, 1966, 1968; Chayes 1964;

MacDonald and Katasura 1964; Engel et al. 1965; Dickinson and Hatherton 1967; Dickinson 1968; Cast 1968; Jakes and

White 1972).

In the meantime the plate tectonic concept (Le

Pichon 1968; Dewey and Horsfield 1970; Dickinson 1971} caused a revolution in the geological thinking and as a result three principal magma series have been recognized, each composed of a group of closely related magma types that are emplaced in or on the earth crust. These are tholeiite, calc-alkaline and alkali series. Tholeiite and alkaline series occur in almost all types of tectonic environments 55 whereas, calc-alkaline series occur, characteristically in island arcs and continental margins. At present there are two approaches to the problem of the classification of volcanic rocks. They are either considered to belong to a specific rock association or they are regarded as having evolved in a specific tectonic and/or thermal environment. These different methods of classification do not always complement one another (Middlemost 1985).

The volcanic rocks may be sub-divided into two major magma series, alkalic and sub-alkalic. Such a division was employed and elaborated by many workers (Barker 1909; MacDonald and Katasura 1964; MacDonald 1968; Irvine and Baragar 1971; Miyashiro 1978). Each of these magma series contains rocks ranging from basic to acidic in composition. Broadly speaking, the compositional range of volcanic rocks may be regarded as a consequence of two dominating fundamental processes namely, partial melting and fractional crystallization. In general, the sub-alkalic magma series can be sub-divided into a calc-alkaline series and a low-K tholeiitic series. The sub-alkalic basalts are the most common type of volcanic rocks found within both continental and ocean environments. Low-K, sub-alkalic basalts, or tholeiitic basalts are the dominant magma type in many within continent flood basalt associations. These basalts are generally depleted in K and related elements. The MORE 56

like basalts are also found in the back arc basins associated with subduction related volcanic arc in terms of their major elements. The calc-alkaline rocks are totally restricted in their occurrence to subduction related tectonic settings of present time. In general, the alkali basalts and their differentiations are commonly found in interplate tectonic settings such as oceanic island and intra continental plate rifts.

Because one of the objectives of this work is to utilize the geochemical criteria to classify the magma type and to investigate the eruptional environments of Gopir and

Bagdunda volcanics, it is necessary to identify the magma type both in terms of rock association as well as tectonic setting. In the present chapter, major elements, trace elements and REE concentrations of these volcanics are used, and with the help of various discriminant diagrams and other parameters, an attempt is made to achieve this objective.

Magma Series Classification :

In the geochemical studies of basic igneous rocks, the most commonly used method to classify magma type has been the Na^O + K„0 versus SiO„ variation diagram (MacDonald and Katasura 1964; Irvine and Baragar 1971; Cox et al. 1979).

However, this diagram may prove to be of little use for 57

Jharol volcanics because of secondary changes in their Na^O

and K„0 contents. In view of this, the following discussion

of magma classification is based on major and trace elements which are generally considered as less mobile.

Major element: relationships ;

For many years, AFM (A= Na_0+K„0, F= total iron,

M= MgO, all in weight per cent) diagram was widely used for

classification purposes, particularly to differentiate

tholeiite and calc-alkaline series. In this diagram

(figure-16) the data from both Gopir and Bagdunda volcanics

plot above the demarcating line, thus, suggesting their

tholeiitic nature.

In recent years, the Jensen cation plot has been widely used to identify chemically the komatiite, tholeiite

and calc-alkaline suites. The plot was developed by Jensen

(1976), who used the concentration of Al-O^, FeO +Ti02 and

MgO in cation per cents. The analyses of Gopir and Bagdunda

volcanics are plotted in figure-17 in a manner proposed by

Jensen (1976). It is significant to note that both Gopir and

Bagdunda volcanics are classified as high Mg rocks. The

basaltic komatiite affinity is pronounced in Gopir samples. Tholeiile

• Gopir Volcanics o Bagdunda Volcanics

.^JL 50 M Figure 15. A (Na20 + K-O) -F (FeO )-M(MgO), ternary variation diagram of Gopir and Bagdunda volcanics, showing their tholeiite affinity. Compositional variation line after Irvine and Baragar (1971). Fe+Ti

• Gopir Volcanics o Bagdunda Volcanics

50y .50 <5, '^/6' ^^^ '^ High Fe ^z,- Tholeiite "t.

^ o o High Mg •• ^Tholeiite A^^ ^V^^ \^ C<^ Peridolite Komatiite JiZ_ Al 50 Mg

Figure 17- Jensen's (1976) cation ternary plots, showing compositional variation of Gopir and Bagdunda volcanics from high Mg-tholeiite to basaltic komatiite affinity. 58

Immobile minor and trace elements criteria :

Classification of metamorphosed volcanic rock suites based on major elements may prove doubtful and unreliable due to the mobile nature of many of these elements, especially the alkalies. In various variation diagrams as discussed above the Gopir and Bagdunda volcanics appear to be very similar to tholeiite series. This needs firther confirmation in view of the possibility of the major elements being mobile during the post igneous solidification. In order to classify the magma types more objectively the methods based on those minor and trace elements (Ti, Y, Zr, Nb, Cr and P) which are generally considered to be relatively less mobile during alteration and metamorphic processes (Cann 1970; Pearce and Cann 1973;

Field and Elliot 1974; Winchester and Floyd 1976; Floyd and

Winchester 1978), are used.

Davies et al. (1979), devised a ternary diagram by using Y, Zr, TiO„ and Cr which are considered immobile during secondary processes. They proposed that the plot of

Y+Zr-lOOxTiO^-Cr (YTC) is analogous to AFM diagram because Y and Zr are likely to get enriched progressively during fractional crystallization like Na-O and K„0; TiO^ behaves like Fe^O^ being enriched in tholeiitic series and decrease systematically in calc-alkaline suites; whereas Cr closely 59

follows MgO. Thus this diagram is considered to show very closely the original trends and thus, it is more reliable in the study of altered volcanic rocks. The plots of Gopir and

Bagdunda volcanics lie closo to the tholciitic trend, and extend well into the field of magnesian suites (figure-18), showing a komatiite-tholeiite affinity. Moreover, it also confirms the interpreted tholeiite trend in the AFM diagram as discussed earlier.

The Nb/Y versus Zr/TiO„ diagram of Winchester and

Floyd (1977), based on least mobile incompatible trace elements, indicates a compositional variation of Jharol volcanics from sub-alkaline basalt to basaltic andesite

(figure-19). The range of Nb/Y ratio (0.22-0.55) (table-9), is typical of sub-alkaline magmatic series which exhibits

Nb/Y <1 (Pearce and Gale 1977).

Tectonic Setting Classification :

During the last two decades or so a close relationship between tectonic environment and the composition of volcanic rocks has been observed by many workers (Pearce et al. 1975, 1977; Pearce and Cann 1973;

Pearce 1975, 1982; Mullen 1983; Meschede 1986). This has led to the development of tectonomagmatic discrimination techniques, which have been widely used to elucidate the 1102X100

• Gopir Volcanics o BagdundaVolcanics

50> .50 ^A o/<= K

'Vo C tPo. Q/c. Alk a// He 7- '•e ^c/.

Zr+Y 50 Cr

Figure 18. Y+Zr-TiO--Cr variation diagram (after Davies et al. 1979), showing the komatiite-tholeiite affinity of Gopir and Bagdunda volcanics. •O'.i / JZ 60

tectonic setting of ancient volcanic suites (Rogers et al.

1974; Pearce 1975; Bhat et al. 1981; Raza 1981; Bhartacharya et al. 1988). However, the geochemical characterization of

Proterozoic volcanics to decipher tectonic setting faces two major problems. The first and foremost, is the problem of alteration and metamorphism. The Jharol volcanics have suffered regional metamorphism up to the grade of green schist facies, as a result of which concentration of LIL incompatible elements such as K, Rb, Ba etc, are unlikely to reflect abundance at eruption. Therefore, no reliance is placed on these elements in the interpretation of tectonic

setting of Jharol volcanics. The second problem is the question of validity of uniformaterian approach for rocks older than 600 m.y. In most of the cases the interpretation of the tectonic environment is based to larger degree on classification and discrimination diagrams with the field using chemical compositions of Mesozoic and younger volcanic

suites. The workers like Pharaoh and Pearce (1984), who have raised this problem and discussed it extensively, have

strongly recommended the application of discrimination techniques to Proterozoic volcanic assemblages. Watters and

Pearce (1987), have also advocated the use of these methods for tectonic classification of Precambrian volcanic

sequences. They observed that there appears reasonably good agreement between tectonic interpretation based on geological evidence and geochemical criteria. Such 61 observations make a strong case for validity of the application of chemical criteria to Proterozoic volcanic assemblages. Because the use of multiple discriminant diagrams is essential for more accurate results (Saunders et al. 1980), a series of diagrams based on both major and trace elements are used here to reach some meaningful conclusions.

In the PpOj- versus TiO- diagram proposed by Ridley et al. (1974), all the plots of both Gopir and Bagdunda volcanics fall within the field of ocean ridge basalts (figure-20). This diagram appears more useful as the minor elements P^Oc and TiO_ have been considered immobile or less mobile during secondary processes (Pearce and Cann 1973; Winchester and Floyd 1977; Mullen 1983).

Although the above method, principally based on major and minor elements, indicate ocean floor tholeiite affinity of Jharol volcanics, it does not confirm this interpretation. There is no discrimination field for continental setting. Thus, although a rock actually belonging to continental setting has to plot somewhere in these diagrams, it does not mean they are of oceanic affinity. Therefore, the confirmation of oceanic basalts affinity, if it is true for Jharol volcanics, is possible by using the diagrams which are based on immobile or less mobile • Gopir Volcanics o Bagdunda Volcanics 0.8

0.6

O O.A CM Q.

/ 0.2 - /o x^ / ^^ m /

0 0 T;O2 •/. Figure 20. TiO_ versus P2O5 variation diagram (after Ridley et al. 1974) of Gopir and Bagdunda volcanicsv showing their oceanic ridge basalt affinity. 62 minor and trace elements, as well as have discrimination, criteria between oceanic and continental magmatism.

Immobile minor and trace elements diagrams :

The TiO_-Zr covariation diagram (Pearce 1982; Pharaoh and Pearce 1984), is not useful in discrimination completely between the island arc basalt and MORE, but effectively distinguishes between island arc basalt and within plate basalts (Pearce and Cann 1973). The Jharol volcanics of both Gopir and Bagdunda areas cluster in the field of MORE and WPB (figure-21 a). Likewise in a Y-Cr plot (Pearce and Cann 1973), Gopir volcanics and Eagdunda volcanics fall in the MORE field (figure-21 b) , which overlaps the field of within plate basalts.

In Zr/Y - Ti/Y variation diagram (Pearce and Gale 1977) samples of both the Gopir volcanics and the Bagdunda volcanics plot within the field of plate margin basalts (figure-22).

In Ti-Zr-Y diagram (Pearce and Cann 1973), the plots of Jharol volcanics fall in field three (3), which represents an overlap of mid oceanic ridge basalt and island arc basalt (figure-23). 5 r ®

/ CM O / 0.5 i

oi 100 1000 Zr ppm

1000 r • Gopir Volcanics ® o Bagdunda Volcanics

300 E S 200

150

100

VAB

0 10 20 30 40 50 100 Y ppm

Figure 21. (A) Zr versus T^^O^ and (B) Y versus Cr covariation diagrams (after Pharaoh and Pearce 1984; Pearce 1982), illustrating MORE affinity of Gopir and Bagdunda volcanics. 30 • Gopir Volcanlcs o Bagdunda Volcanics

WITHIN PLATE 20 I PLATE MARGIN BASALTS BASALTS

>

L. N

10

00 0 100 200 300 ^00 500 600 Tl/Y Figure 22. Ti/Y versus Zr/Y diagram (after Pearce and Gale 1977) of Gopir and Bagdunda volcanics, indicating their plate margin nature. T;/IOO

1-WITHIN PLATE BASALT 2 ISLAND ARC THOLEIITES MORB-MID-OCEANIC RIDGE- lAB-ISLAND ARC BASALT CALC.-ALKALINE BASALT

• Gopir Volcanics o Bagdunda Volcanics

Y_ Zr 50 YX3

Figure 23. Ti/10 0-Zr-Yx3, ternary tectomagmatic discrimination diagram of Gopir and Bagdunda volcanics,showing their MORB and lAB affinity (after Pearce and Cann 1973). 63

In Nb-Zr/4-Y diagram (Meschede 1986), the plots of Jharol volcanics are scattered and do not define a particular environment (diagram is not shown).

Geochemical Patterns :

MORE or Primordial mantle normalized geochemical patterns provide an effective means of comparing ancient with modern lavas (Pharaoh and Pearce 1984; Holm 1985). The trace element diagrams are particularly useful measures in which multi-element variation can be observed simultaneously to distinguish the tectonic setting and source characteristics of basic volcanics. These patterns reveal variation in incompatible elements behaviour providing a geochemical finger-print of plate tectonic association.

MORE normalized incompatible element abundances of Gopir and Bagdunda volcanics are plotted in figure-24 a. The elements used in the patterns behave incompatibly during most of the partial melting and crystallization events. The normalized values of different elements are from Sun and McDonough (1989). The elements are arranged in order of decreasing incompatability from left to right. The elements Sr to Ba are classed as mobile and plot at the left of the patterns whereas elements Ta to Yb are generaly immobile and plot at the right. Figure 24, (A) MORB-normalized multi element patterns of Gopir and Bagdunda volcanics. Normalizing values after Sun and McDonough (1989). (B) Compared with various tectonic settings i.e., basal Aravalli volcanics ( A , Raza and Khan 1990), within plate basalt ( ^ , Sanke river, Thompson et al. 1983), initial rift tholeiites ( X , Holm 1985), back arc basin basalt ( • , Saunders and Tarney 1984) and island arc basalts (o , South Sandwich island arc, Luff 1982). Normalizing values after Pearce (1982). 64

In this diagram the patterns of all the samples (except G3) of Gopir and Bagdunda volcanics are almost parallel exhibiting progressive enrichment of most of the elements from right to left relative to N-type MORE. Contrary to result obtained from various discrimination diagrams discussed earlier, this feature indicates their close affinity with tholeiitic within plate basalts. Calc-alkaline basalts also exhibit a subduction related enrichment in LILE and REE, but this may not be true for Gopir and Bagdunda volcanics, because geochemically they are identified as tholeiite basalt (figures-16,17 and 18). The tholeiitic volcanic arc basalts generally show a depletion of Nb, Ti, Y, Zr, Hf, P and Sm relative to N-MORB, but with low or little La, Ce, enrichment (Pearce 1983). MORE normalized geochemical patterns from various tectonic settings are also given in figure-24 b, for the purpose of comparison. Among all the patterns shown in this figure the patterns of Gopir and Bagdunda volcanics are matched more closely with initial rifting tholeiites (IRT) which erupted at the time of abortive continental break up (Holm 1985).

A within plate continental signature and close affinity with initial rifting tholeiites is also observed in primordial mantle-normalized diagram (figure-25 a,b) . In this diagram also the patterns of all the samples of Gopir and Bagdunda volcanics show parallel patterns. The only 100

ut _j I— 20 2 < 2

Q 10 IX o cc CL *^ u o Q:

Rb Ba K Nb La Ce Sr Nd Zr Lu DC Ni

Figure 25. (A) Primordial mantle normalized incompatible element patterns of Gopir and Bagdunda volcanics. Normalizing values after Sun and McDonough (1989). (B) Compared with basal Aravalli ( • , Raza and Khan 1990), continental back arc basalts (X , Holm 1985), oceanic back arc basalts (A , Holm 1985), initial rift tholeiite (O , Holm 1985), continental tholeiites {a , Holm 1985) and contaminated basaltic rocks (* , Holm 1985). 2-Jormalizing values after Wood et al. (1979). 65 exception is sample G3, which show a pattern with some pronounced anamolies.

In order to further examine the tectonic setting of Gopir and Bagdunda volcanics, variation in seven elements Nb, La, Ce, P, Zr, Ti and Y have been evaluated in a manner following Myers et al. (1987). These elements have been used for the reason that they are all relatively immobile during alteration and that their concentrations in basic magmas are not affected by assimilation of crustal materials. The data are plotted in figure-26 a. This figure also demonstrates a strong affinity of Gopir and Bagdunda volcanics with continental basalts as their patterns are closely similar to those of within plate basalt and initial rift tholeiites (figure-26 b). Here again the pattern of sample G3 is not similar to those of other samples, and shows an irregular pattern. The obvious conclusion from multi-element geochemical patterns is very likely to be the correct one because this method involves all the iiranobile-incompatible elements simultaneously to distinguish the tectonic setting. The apparent MORB affinity of these volcanics in various discrimination diagrams as discussed in previous paragraphs (figure-13,20,21 a,b and 23), seems to be incorrect. It has been observed that some suites of continental tholeiite may Gopir Volcanics

^^^31 Bagdunda BAs) Volcanics

Figrire 26. (A) Nb-normalized geochemical patterns of Gopir and Bagdunda volcanics. Normalizing values after Sun and McDonough (1989). (B) Compared with basal Aravalli volcanics (• , Raza and Khan 1990), island arc tholeiites ( o , Luff 1982), back arc basin ( A , Saunders and Tarney 1984), within plate basalts (X , Thompson et al. 1983), and initial rift tholeiites (^ , Holm 1985). Normalizing values after Wood et al. (1979). 66

plot in the MORE field in various variation diagrams and may contain high concentration of Nb (Dupuy and Dostal 1984;

Papezik and Hodych 1980; Greenough et al. 1985; Smith and

Holm 1987). Chapter V

^etrogenetit Consiiberationiot ^nb Source (Seotfiemtsitrp CHAPTER FIVE

PETROGENETIC CONSIDERATION AND SOURCE GEOCHEMISTRY

Petrogenetic Consideration :

In the preceding chapters, the geochemical characteristics of Jharol volcanics in terms of major, trace and rare earth elements, have been discussed in detail. The behaviour and relationship of various elements have brought certain significant and interesting features concerning their origin. In the present chapter, attempt has been made to elucidate such features within the limited scope of this work.

The petrogenetic studies of igneous rocks involve characterization of source region of the magma, conditions at the site of melting and the extent of modification in the primary melt, subsequent to its generation, during transport and storage in the magma chambers or in conduits.

The high Mg-number rocks can be considered as more primitive. The rocks with low Mg-number can be related with those of high Mg-numbers by fractional crystalliza~ion or progressive partial melting of a mantle source (Nesbitt and 68

Sun 1976). The Mg-numbers and MgO contents of Gopir volcanics (59.12 to 66.91; 13.9 to 11.9 per cent) and Bagdunda volcanics (54.11 to 58.70; 8.2 to 8.9 per cent), are lower than those predicted for primary basalt melts, suggesting that they have undergone magmatic differentiation. However, the relatively lower Mg-numbers and MgO contents of Bagdunda volcanics, compared to those of Gopir volcanics, suggest that the former are more differentiated than the latter. The relationships of Mg-number versus TiO^, MgO/CaO versus TiO-, TiO- versus CaO/Al202 and FeO^/MgO versus CaO/Al202 (figures-9,10,11 and 12), indicate possible fractionation of olivine and clinopyroxene in both Gopir and Bagdunda volcanics. Moreover, the relationship between Zr, Ti and Y has been used to interpret the fractional crystallization of basic magma (Pearce and Norry 1979). The plots of TiO- and Y versus Zr contents of Gopir and Bagdunda volcanics (figures- 4 and 5) also indicate a combination of crystallization phase of olivine-plagioclase-clinopyroxene (Pearce and Norry 1979). The Mg-numbers versus TiOj diagram (figure-9) also suggests that the Gopir volcanics have undergone less amount of olivine fractionation (<^ 12 per cent) than the Bagdunda volcanics (^ 12 per cent).

Slight positive Eu anomaly (figure-3) in almost all the samples negate any plagioclase fractionation, except 69

in sample G3 which shows negative Eu anomaly (figure-3).

This may be due to either plagioclase fractionation or post crystallization alterations.

In Ti-Zr diagram (figure-21 a), these volcanics plot in the field of MORE, indicating that no titanium bearing phase has been fractionated. Moreover, the fractionation of Ti bearing phase will drastically lower Cr and Ni abundance (Kds of Ni and Cr for magnetite are 29 and

153 respectively) (Dostal et al. 1983). In these rocks the samples with low TiO^ contain higher concentration of Cr and

Ni.

In Al^O^/TiO^ versus TiO„ diagram, the Bagdunda and Gopir volcanics plot throughout the field of MORE, showing an identical trend (figure-13). It is believed that at lower degree of partial melting, Ti behaves as an incompatible element, becoming more enriched in the melt relative to Al. Thus the TiO„ content decreases and

AlpO-/TiO„ ratio increases with the increasing degree of partial melting (Sun et al. 1979). The TiO- content of about

1.5 per cent with Al202/Ti02 ratio about 15, represent about

15 per cent partial melting (Sun et al. 1979). Eagdunda and

Gopir volcanics with MORE like Al20^/Ti02 ratios and TiO„ abundance may represent a variable degree (10-20 per cent) of partial melting of similar mantle source. However, their 70

derivation from a mantle source like that for MORE is not

evident from incompatible element abundances and ratios

(figure-24 and 27).

TiO^/P^Oi- ratio of the magma is dependent on the percentage of water in the original melt and the amount of

fractionation of silicate phases and depth. If OH is present in the mantle, then with the increasing depth of

partial melting, the concentration of both elements Ti and P will increase in the melt (Chazen and Vogel'1974). The lack

of collinearity between TiO„ and PoO,- (figure-20) in these

rocks would imply that the melt of Gopir and Bagdunda

volcanics were produced from a heterogeneous source or they

had a variable water content.

The plots of Ti, Y and Nb (figures-4,5 and 6b)

clearly depict the enriched source of both Bagdunda and

Gopir volcanics, because these rocks have higher Zr/Y, Zr/Ti

and Nb/Zr ratios relative to primitive mantle ( PM; Sun and

McDonough 1989). This is because Nb is more incompatible

than Zr and Zr is more incompatible relative to Ti which in

turn is more incompatible than Y and Yb during melting of mantle sources (Sun 1980). However, it is possible to

generate enriched melt (with higher Zr/Y, Zr/Ti ratios) by

very low extent of melting of primitive or even depleted mantle sources (Fitton and Dunlop 1985; X KOMATIITIC BASALT

9 8 0.01 7 IiIiIiILIiIii£lrliLrXX IL 6 Th Nb Ce Zr Y Yb Nb Ce Y Yb Rb Ba Nb Ce 5 • Average. 65,05 and. G7 — Gopir Volcanics U o Average. BA3 and BA5 — Bogdundo Volcanics

Ti Ti T! Ti Ti Zr Zr Zr Zr Y Y Y Y Nb Ce Zr Y Yb Nb Ce Y Yb Rb Ba Nb Ce Figure 27. (A) Normalized incompatible trace element ratio patterns (spidergram), indicating the enrichment of Gopir and Bagdunda volcanics. (B) Compared with komatiitic basalt and tholeiites of basal Aravalli (Ahmad and Tarney 1992). PM. values after Sun and McDonough (1989). 71

Ahmad and Tarney 1992).

Figure-3 compares the REE patterns of individual samples of Gopir and Bagdunda volcanics. The REE patterns of three samples of the Gopir volcanics are remarkably similar, only sample G3 exhibits a highly enriched LREE pattern. If Mg number is considered to be a measure of fractional crystallization, the Gopir volcanics are expected to show enrichment in REE with decreasing of Mg numbers. This is because the fractionation of mafic phases in magma chamber or enroute to the surface result in increase of incompatible element concentrations relative to those of high Mg number primary magmas. However, such a relationship between Mg numbers and REE concentration is not significantly observed in Gopir volcanics. Although, there is a large difference in the Mg numbers (59-65), the difference in degree of REE enrichment is not significant. The parallel REE patterns of samples G5, G6 and G7 and the weak relationship between their Mg-number and REE contents suggest that, these samples could be related to fractionation of pyroxene and separation of sulphide phase from parental melts derived from different extents of melting of uniformally enriched source or sources as suggested for the Abor volcanics (Bhat and Ahmad 1990). The remaining possibility is that they have been derived from the melts generated by different degrees of melting of mantle sources with variable chemistry or in other words, a 72

heterogeneous mantle with each magma phase undergoing fractionation independently. Sample G3 is highly enriched in

LREE, while its Mg number is the highest among all the samples of Gopir volcanics (66.43). Two possible interpretations can be suggested for such a behaviour.

Firstly, this could be reflecting the heterogeneity of the source or, in other words, this sample has been derived from a source quite different from that of other samples of Gopir volcan,ics. Secondly, such enrichment in LREE may be a result of secondary processes, such as addition of material from adjacent calcareous rocks.

The consideration of incompatible elements and element ratio emphasizes the enriched character of Gopir and

Bagdunda volcanics. The enriched character of these rocks is also evident from multi-element patterns (spidergrams)

(figures-24,25 and 26), which strongly suggest their continental basalt affinity.

In recent years, the origin of the distinctive features of modern continental basalt have been interpreted as the product of two quite different processes, namely (a) crustal contamination of magma derived from a depleted mantle source(Thompson et al. 1983; Dupuy and Dostal 1984;

Fodor and Vetter 1984; Arndt and Jenner 1986), and

(b) derivation from mantle with enriched characteristics. 73

Therefore, the character of Bagdunda and Gopir volcanics may be ascribed to either crustal contamination or derivation from incompatible elements enriched lithospheric source (s) (Weaver and Tarney 1983). A good case can be made, that the chemical features of Jharol volcanics have not resulted from crustal contamination.

The most characteristic feature of the multi-element patterns of both Gopir and Bagdunda volcanics is the positive Nb anomaly. Its presence precludes explaining the enrichment of these rocks in HFS elements by crustal contamination. Since all likely contaminants have large negative Nb anomalies (Weaver and Tarney 1983; Pearce 1983), therefore, the Nb anomaly is interpreted as representative of the composition of the source material. The uncontaminated nature of these rocks is also evidenr from primordial mantle normalized multi-element patterns (figure-25 a), where they show rather flat patterns as compared to contaminated rocks, which have characteristic feature of having steep slope of patterns, and size of anomalies is more pronounced notably in segment La-Ti (Holn 1985). Only sample G3 exhibits this characteristic and matches well with the pattern of contaminated basaltic rocks (figure-25 b).

Extensive geochemical studies of ocean ridge and 74 ocean island have shown that Zr/Nb ratio is a powerful discriminant between enriched source and depleted source (Erlank and Kable 1976; Le Roex et al. 1983). The plot of Gopir and Bagdunda volcanics in Zr versus Nb diagram (figure-6 b) , suggest that they have been derived from an enriched source.

To further assess this possibility in a much broader framework, we have used a multi-element approach using incompatible trace element ratios in the form of spidergrams (figure-27). To construct this spidergram, the immobile incompatible trace ratios are normalized by similar ratio of primitive mantle (Sun and McDonough 1989). In this manner, the various incompatible element ratios are compared simultaneously to standard (PM) ratio (Thompson et al. 1984; Myers and Breitkopf 1989). By such double normalization the effect of fractionation on the melt composition is lainimized (Ahmad and Tarney 1992), so that the elemental ratio of the samples reflect the mantle source ratio (Myers et al. 1987; Myers and Breitkoph 1989). In figure-27 the normalized ratio (average) of Gopir and Bagdunda volcanics are plotted, using the logic of relative incompatibility between the two elements used in ratios. In this diagram the incompatible elements ratio patterns of both the Bagdunda and Gopir volcanics are parallel to each other, showing their enriched nature relative to primordial mantle. Therefore, it may be 75

suggested that the enriched nature of these volcanic rocks is a source characteristic and not the result of crustal contamination. This interpretation is strongly supported by Ce-Nd relationship (figure-15), where the plots of both Gopir and Bagdunda volcanics fall along a line that intersects the origin. It has been suggested (Horan et al. 1987; Bhat and Ahmad 1990), that any contamination or assimilation with fractionation or mixing of primary melt with a melt derived by a low degree of melting of a source with similar Ce-Nd ratio cannot result in the observed relationship. In each case, the regression line would pass through Nd axis.

Source Modelling :

The geochemical characteristics of Gopir and Bagdunda volcanics, as discussed in the preceding paragraphs, overwhelmingly suggest that they have been derived either from different extents of melting of a uniformally enriched source followed by fractionation of pyroxene and sulphide phase or from the melts generated by different extent of melting of mantle sources with variable chemistry with each magma phase suffered fractionation (Bhat and Ahmad 19 90). It has been observed that chemical heterogenity, observed in both oceanic and continental basalts, may result from successive depletion and enrichment 76

events which have affected their source region (Hanson 1977;

Frey et al. 1978).

To evalute this possibility, we have calculated the REE composition of the source for sample G5 which appears to be the most primitive sample among Gopir volcanics (Mg No. 65). Its "major and trace elements composition appear to be in agreement with its low REE abundances. The calculation of the source was done by assumption based on the Ti, Y and La contents of the sample

G5 and Ce-Nd (figure-15), where it falls in the range of the chondrite ratio, that is represented by a line which intersects the origin. By using the equation C /C =1/F, when J-j O D=0 (Hanson 1980), (where D-bulk distribution coefficient;

C^=concentration of the element in the melt; C =initial L o concentration of the element in the source; F= fraction of melt), we calculated that sample G5 represents about 12 per cent partial melting. A similar extent of partial melting can be inferred based on its TiO_ content (1.49 per cent) if

1.5 per cent represent about 15 per cent melting (Sun et al. 1979). The REE patterns for 3,8,10 and 20 per cents melting of the modelled source were calculated using the batch melting equation and Kds value of Hanson (1980), and assuming a Lherzolite mantle source consisting originally of

55 per cent olivine, 25 per cent orthopyroxene, and 20 per cent clinopyroxene, leaving a residual mineralogy of 60 77 per cent olivine, 25 per cent orthopyroxene'^rsd^dtS^^^^l: cent clinopyroxene at 12 per cent partial melting of the model source. The assumed melting proportion of 20 per cent olivine, 25 per cent orthpyroxene and 55 per cent clinopyroxene (after Hanson 1980), was:;5i^^^^o^24^Nwlation-

The REE patterns for 20 pe«r cent, 12 per ceat/J 10 per cent, 8 per cent and 3 per cent meil.^fcfgvP#fyt.^e^odel source are shown in figure-28. It is evident that the calculated patterns for 8 per cent and 10 per cent melting match very well with those samples G6 and G7 (figure-28). The REE abundances of G3 do not match with melts derived from the calculated source and suggest that sample G3 cannot be produced even by the lowest degree (3 per cent) of melting of this source. Probably as mentioned in previous paragraphs, sample G3 has been derived from a different source or a more enriched source or it has been contaminated by some calcareous sediments resulting in significant LREE enrichment. Note that G3 has high CaO content (17.75 per cent) and low SiO^ content (38.5 per cent), which could reflect the addition of CaO and removal of Si02.

At 20 per cent melting Ce/Sm ratio is 0.985 (figure-28), which shows that with further increase in the extent of melting of the modelled source, more depleted patterns will be generated, which makes the REE analysis for AOO

G5 Gopir a Gs Volcanics X G7

100

cc Q z o X u it: -107o u 12 7o o cc 207o Ce/5m=0.985

10

Calculated Source for Sample No. G5

/

Ce Nd Sm Eu Gd Dy Er Yb Figure 28,. Chondrite-normalized REE pattern of the calculated source for sample G5, assuming it represents 12 per cent partial melting. Calculated REE patterns for 3,8 and 10 per cent melting of the source and REE patterns for samples G3,G5,G6 and G7, for comparison. The mantle mineralogy and melting proportions are after Hanson (1980). 78 ultramafic rocks a very difficult task, because they are generated at very high degrees of melting giving rise to very low REE and other incompatible element concentrations.

Detailed studies on mantle-derived xenoliths have indicated that the lithospheric mantle may be variably metasomatized or veined (Frey and Green 1974). Tarney et al. (1980), have shown that it is possible to generate a wide spectrum of mantle sources, ranging from LREE depleted to strongly LREE enriched, where the composition of the final liquid depends on the composition of the host mantle, the composition and percentage of veins and the percentage of melting of the source. Richard et al. (1985), have suggested that the process of metasomatism may be a periodic or quasicontinuous process which replenishes the LREE and other highly incompatible elements in the lithosphere as well as replacing major minerals, such as diopside, which are depleted during melting. Re-enrichment of a depleted mantle may occur when under saturated fluids, enriched in highly incompatible elements, traverse through the weak zones forming veined sources (Frey and Green 1974). Studies on basaltic rocks related to lithospheric rifting, xenoliths, and volcanic gases indicate that the chemical features of these rocks are controlled by source metasomatism that immediately precedes incipient partial melting, lithospheric rifting and lava eruption (Lloyd and Baily 1975; 79

Frey et al. 1978, 1980; Norry et al. 1980; Schilling et al. 1980; Wass and Rogers 1980; Wass et al. 1980; Baily 1980, 1982, 1983).

The geochemical characteristics of Jharol volcanics appear to have been developed by similar phenomenon. Similar multi-element patterns of incompatible trace elements and REE for Gopir volcanics suggest enrichment of the source after the eruption of Bagdunda volcanics. Chapter VI

%ttlmit f mplitattonis CHAPTER SIX

TECTONIC IMPLICATIONS

The geochemical studies have proved to be very useful in deducing the tectonic environment of magmatic eruptions. There have been several successful attempts to relate the chemical compositions of basic volcanics to the natures of the crust and environment of magmatism. A direct correlation has been demonstrated between the chemical compositions of the basic rocks and the nature of crust on which they erupted (Jakes and White 1972; Mohr 1972 etc.).

Such relations have been widely used to recognize the nature and evolution of the earth's crust in different geological periods (Shaw 1972; Naqvi and Hussain 1973; Naqvi et al.

1974; Rogers et al. 1974).

Since the development of plate tectonic theory, attempts have been made to interpret the folded mountain belts in terms of large scale horizontal plate tectonics.

However, there is disagreement among the geoscientists as to whether or not the Phanerozoic type of plate tectonic processes existed during the Proterozoic period. One group which believes in uniformitarianism, maintains that since the formation of rigid crust in Archaean, the presently operating global mechanism is continued (eg. Windley 1977, 81

1981; McWilliams 1981). The other group does not agree with the existence of plate tectonic processes in Precambrian, because of significant changes in physical conditions in the lithosphere and underlying mantle with geological time

(Ramburg 1967; Stephanson and Johnson 1976; Watson 1976;

Sutton 1977; Schwerdtner et al. 1979 etc.). The third group believed that the plate tectonics did not strictly follow uniformitarian principles and therefore, it was initiated and operated in different ways as compared to the present. A number of workers (Condie 1973; Katz 1974; Kroner 1981), have invoked the plate tectonic concept for the Precambrian orogenies.

In recent years, with the addition of new and more reliable data (Hoffman 1980; Naqvi 1985; Kerr 1986; Watters and Pearce 1987; Naqvi et al. 1988; Condie 1989), the extension of plate tectonics into Precambrian has gained considerable ground. A growing number of evidences seem to indicate that these processes were broadly similar to the

Proterozoic ones, although there might have been differences in the scale and geometry of the response between

Proterozoic and Phanerozoic. If such comparison of

Proterozoic and Phanerozoic settings are valid, the volcanic rocks of Proterozoic sequences can be expected to throw light on the tectonic settings at the time of their eruption. The basic volcanic rocks are found to be most 82

useful for realising this objective. This is because of the

fact that, the geochemical signatures of basic volcanics of younger ages are found to reasonably define their tectonic

settings. Due to this fact the geochemical composition of

basic volcanics have been used successfully, to model the

tectonic evolution of Archaean and Proterozoic terrains.

The evolutionary history of Aravalli has been in

debate for a long time. Various models, involving Wilson

cycle (Sinha-Roy 1988, 1990) to resurgent rifting (Roy 1988,

1990; Bhattacharya 1990) have been proposed.

Jharol belt with its deep water sediments and

abundant mafic rocks occur as a wedge shaped area between

shallow water Aravalli rocks of Udaipur basin in the east

and south Delhi fold belt in the west. Geochemical studies

on basic volcanics of Udaipur basin (Ahmad and Tarney 1992;

Ahmad and Rajamani 1988, 1991; Raza and Khan 1990) have

shown that they have erupted in an initial rifting

environment. On the other hand, the geochemical data on the

basic volcanic rocks of south Delhi fold belt indicate, that

there are two independent rock suites, one with element

abundances and ratios resembling MORE and another with

relatively high A1_0^ and low TiO„ and high LILE coupled

with low HFSE and REE resembling oceanic arc basalts (Volpe

and Macdougall 1990). These informations indicate that 83

somewhat similar tectonic processes, with regard to modern

Phanerozoic plate tectonics, were operating in this region during Proterozoic period.

In one of the models mafic rocks of Jharol belt have been considered as a part of obducted oceanic crust emplaced during the closure of the basin (Sychathavong and

Desai 1977; Sen 1981; Sinha-Roy 1988). However, the field evidence suggests that these rocks are predominantly extrusive in nature, erupted contemporaneously with sedimentation. Although, the region has had a complex history of structural deformation their extrusive nature is illustrated by the presence of ill preserved vesicles, pillows (also Deb and Sarkar 1990) and thin beds of intercalated quartzite.

It is known that, geochemistry can be a very powerful tool applied to the petrogenesis of mafic rocks. because different characteristics of geochemical nature of the mafic magmatism and thereby, their mantle sources are likely to represent different tectonic settings. Thus, the geochemical and petrogenetic informations regarding the source (s), its oxidation state and melting conditions may provide useful clues to interpret the tectonic settings prevailing during the magmatism. 84

Although discrimination diagrams have been widely used to decipher the tectonic setting of ancient mafic volcanics, many studies have shown that such diagrams are sometimes ambiguous even for modern basalts (Holm 1985; Myers et al. 1987; Myers and Breitkopf 1989). Therefore, these diagrams need extreme care to decipher the tectonic environment of the Proterozoic sequences. In recent years the multi-element spidergrams have been used to determine the source characteristics of mafic volcanic rocks. In this regard the MORB or primordial mantle normalized multi-element diagram (Pearce 1982; Holm 1985) and also the ratio plots (Myers et al. 1987) are particularly very useful. They are also found useful in comparing the ancient and modern lavas (Ahmad and Tarney 1991) as the pattern displayed by the incompatible trace and rare earth elements are likely to reveal no-differentiation related to compositional variation in tectonoraagmatic element behaviour providing a geochemical finger-print of plate tectonic association. Furthermore, these elements remain largely immobile in response to alteration.

The plots of Jharol volcanics on different discrimination diagrams based on major and minor elements as well as, immobile trace elements suggest their close affinity with mid-ocean ridge basalts (MORB). However, MORB normalized multi-element patterns (figure-24) show that the 85

Jharol mafic rocks are highly enriched with respect to typical MORB. The MORE normalized, primordial mantle normalized and primordial mantle/Nb normalized multi-element ratio patterns (figures-24,25 and 26), suggest that both Bagdunda and Gopir volcanics have been derived from an enriched source and resemble more closely with continental basalts. Although, the Bagdunda volcanics are intercalated with quartzite bands and unconformably overlie the B.G.C. basement, the younger lavas occur within a thick column of deep water metasediments, mainly mica schist. However, they are locally intercalated with quartzite bands, which may indicate local fluctuation in the water level, at the time of the volcanic eruptions. In the recent island arcs and marginal continental settings, marine sediments are frequently intercalated with the volcanic products (Thorpe 1982). However, Jharol volcanic rocks, having intercalated with deep water facies sediments and possibly situated near an Archaean continental mass (Roy 1988), do not show any subduction zone signature in their chemistry. Thus, no evidence support the oceanic or subduction related origin for these volcanics. Hence these characteristics are best explained by an intermediate situation in which extension of sub-continental lithosphere produced highly attenuated continental crust and marginal sea which could not fully develop to ocean stage. The basic volcanics erupted within the rift thus, show some of their chemical characteristics 86 closely similar to those of rocks which erupt along continental margins and initial rifting of continents (Initial Rift volcanism-IRV: Holm 1985).

The oldest unit of the Jharol basin comprises of the Bagdunda volcanics and intercalated quartzite that overlie the Archaean gneissis. Geochemistry of these volcanics, as discussed in the previous chapters, suggests that they have continental basalt chemistry and erupted in a within plate continental setting. These lavas are intercalated with clean washed quartzite, indicating long transport on the continent and their deposition along the continental margins. Therefore, it is evident that the lava were introduced into a stable sedimentary environment of shallow marine or fluviatile and continentally derived sediments.

Gopir volcanics with relatively high MgO and less fractionated nature, occur higher up in the stratigraphic column. Element abundances, ratios and other geochemical characteristics of these rocks suggest that they have also been derived from a sub-continental lithospheric source, or a source that had influence of both sub-continental lithosphere and asthenosphere in variable proportions.

These characteristics can be interpreted in terms 87 of a model in which the chemistry of magma reaching the surface are constrained by the sedimentary processes operating on the crust. As discussed above, the geochemistry of Bagdunda volcanics and their occurrence with clean washed quartzite are the features which can be best explained by extensional rifting which produced highly attenuated continental crust. The absence of evidence for any crustal contamination support their ascent through a thin continental crust which was undergoing extension. It seems likely that, this thin crust diminished the velocity of rising magma, but could not force the magma to stop and pool into large reservoirs. Thus, the magma reached the surface through conduits/fractures etc. without forming any large reservoirs. The magma evolved chemically by process of crystal fractionation during its upward journey through these conduits. Since the magma did not pool in large reservoirs, the effect of wall rock contamination was minimum.

With continued rifting, the next phase of volcanic activity occurred in a later stage of Jharol sedimentation. During this period, the crust had become very thin and the pulses of primitive magma, rising from the mantle, were much less hindered and reached the surface through a fracture controlled conduit system (Francis et al. 1983). The high Mg basalts and picritic lavas reached the surface 88

contemporaneously with sedimentation. At this stage, with progressive rifting, a crust very similar to marginal sea developed, but could not mature into a true oceanic crust. It appears that, the rift developed in the Archaean continent (BGC) and its sub-continental lithosphere, which produced melts with enriched characteristics (figure-27). Under such extensional regime the continiental crust becomes thin, thus, the crustal contamination becomes less significant (figure-15). Chapter VII

^ummarp ^nb Contlu£(ion CHAPTER SEVEN

SUMMARY AND CONCLUSION

The Aravalli mountain belt of the north western part of the Indian shield, preserves well developed rock sequences of Proterozoic age, ranging from 2500 m.y. to 500 m.y. In this region, the thick sequences of metasedimentary rocks contain numerous mafic and ultramafic rocks of extrusive and intrusive nature. Although considerable work has been done on various aspects of geology of the Aravalli region, geochemical studies on basic and ultrabasic rocks have been given less attention.

In Udaipur area of Rajasthan, the lower Proterozoic supracrustal sequence is represented by the Aravalli Supergroup, consisting of two contrasting facies i.e., a carbonate free deep water facies of Jharol basin and a carbonate bearing shelf facies of Udaipur basin. The deep water facies rocks form a north-south trending linear belt, west of Udaipur which extends from south of Rakabdev to Gogunda in the north The rocks of Jharol Group include phyllite, mica schist, sheard amphibolites, thin persistent bands of quartzite and numerous mafic and ultramafic rocks. 90

The mafic rocks in Jharol Group are found at two different stratigraphic levels. The older volcanic rocks occur at the base of the sequences, where they constitute a basal volcanic-quartzite unit. This unit occurs in direct contact with an exposed gneissic basement (B.G.C.) within the Jharol belt in Bagdunda area (referred to as Bagdunda volcanics). Another unit of mafic rocks is found associated with ultramafic rocks occurring at a slightly higher stratigraphic level and forms a linear belt between Modri in the north (5 Km north of Gogunda) to Gopir in the south. These rocks have been described earlier as intrusive ultramafics. The present study suggests the widespread occurrence of mafic volcanic rocks occurring with ultramafics. The mafic and ultramafic rocks of Jharol Group are the subject of the present study.

Petrographic examination and determination of major, trace and rare earth elements were carried out on the selected samples of the mafic and ultramafic rocks of Jharol Group occurring around Bagdunda (Bagdunda volcanics) and Gopir (Gopir volcanics). Although major and some trace element analyses of ultramafic unit are presented, they are excluded from detailed petrogenetic studies due to their highly altered nature. Only the mafic units are subjected to detailed studies The data has been utilized to discuss their petrogenesis, tectonic environment and source 91 characteristics.

Microscopic examination of the volcanic rocks of Jharol belt display almost uniform mineral assemblage and textural relations, like many other altered basalts. They are generally composed of secondary minerals, such as amphiboles, chlorite, zoisite, calcite, antigorite quartz etc., where the primary minerals have been obliterated. However, the Bagdunda volcanics are of more pronounced mineralogy compared to Gopir volcanics.

To examine the degree of metamorphic alteration and the extent of alteration in the whole rock chemistry of Gopir and Bagdunda volcanics, the post eruption changes have been determined by using various diagrams and chemical criteria. The chemical data shows that the impact of alteration is mostly on SiO^, CaO, K^O, Na-O and MnO. The elements Al^O, FeO , MgO TiO„ and PpOj- do not show any serious effect of altration and change in their magmatic concentrations. However, the chemical evidences adduced in this study, rely preferably on minor and trace elements, including REE, which are considered to be immobile or less mobile during secondary processes.

In AFM diagram, the Gopir and Bagdunda volcanics show an iron enrichment trend, suggesting their tholeiitic 92 affinity. However, in Jensen's plot these volcanic rocks display a compositional variation from high Mg tholeiite to basaltic komatiite, which is also observed in Y+Zr-TiO»-Cr variation diagram. Other variation diagrams, based on major and immobile trace elements and element ratios, suggest that the Gopir and Bagdunda volcanics are tholeiitic basalts.

The chondrite normalized REE patterns of all the samples (except sample G3) of Gopir volcanics are almost parallel, showing a slight enrichment in LREE with moderate to strong positive Eu anomaly. Sample G3 is highly enriched in LREE with negative Eu anomaly which may be a primary characteristic or a feature related to alteration processes. Though the similarity in the REE patterns is maintained over a large range of Mg numbers (54 to 66), the relationship between REE enrichment with decreasing Mg number is not significantly observed in Gopir volcanics.

In Bagdunda volcanics, one sample (BA3) is LREE enriched, while another sample (BA5) is showing a depleted pattern which is likely to be due to alteration or may be due to influence of the asthenospheric source. Such possibility can be further clarified only with more samples from this area being analysed for their REE concentration.

In terms of some major element and trace element 93 relationships, the evolution of both Gopir and Bagdunda volcanics seems to be compatible with relatively simple mechanism of fractional crystallization. The relationships of Mg number versus TiO^/ MgO/CaO versus TiO^, TiOj versus CaO/Al-O^ and FeO /MgO versus CaO/Al^O^, suggest the fractionation of olivine and clinopyroxene. TiO_ versus Y and TiO- versus Zr diagrams also indicate a combination of crystallization phase of olivine-plagioclase-clinopyroxene. However, a slight positive Eu anomaly in almost all the samples negate any plagioclase fractionation, except in sample G3.

The Mg numbers versus TiO„ diagram, suggests that the Gopir volcanics have undergone olivine fractionation £ 12 per cent, white Bagdunda volcanics have undergone olivine fractionation >_ 12 per cent.

Geochemical data particularly incompatible trace elements and REE abundances, emphasize an enriched character of these volcanics. The chemical features overwhelmingly indicate that, the compositional characteristics have been inherited from their mantle source, with a very minimized effect of crustal contamination. The rocks are considered to have been derived from an enriched source (metasomatized) of sub-continental mantle source. 94

On the discrimination diagrams, based on major elements (Al-O^f MgO and FeO ), minor elements (TiO- and P_0_) and trace elements (Zr, Y and Cr), these volcanics fall in the field of MORE. However, the multi element geochemical patterns involving the immobile-incompatible elements indicate, a progressive enrichment of more incompatible elements. This feature, indicates their close affinity with continental tholeiites. The geochemical patterns match more closely with initial rifting thoeiites, which erupt at the time of the continental break up.

To evalute the possibility of Gopir and Bagdunda volcanics as being derived either from different extents of melting of a uniformally enriched source followed by fractionation of clinopyroxene and sulphide phase or from the melts generated by different extents of melting of mantle source with variable chemistry, we have calculated the REE composition of the source (Lherzolite mantle source) for sample G5 (Mg No. 65), which appears to be the most primitive sample among Gopir volcanics. The calculations of the source were done by assumption based on its TiO_, Y and La contents and Ce-Nd relationship, using Hanson's (1980) equation C-./C = 1/F. The results show that, sample G5 1J O represents about 12 per cent partial melting.

Trace elements, including REE based petrogenetic 95 modelling, indicates that (a) the source had about 3 x chondritic REE abundances with nearly a flat to slightly depleted pattern and (b) that these rocks (Gopir volcanics) were generated by about 8 to 12 per cent partial melting of the modelled source followed by some olivine and clinopyroxene fractionation.

The geochemical characteristics and the geological occurrence of Gopir and Bagdunda volcanics can be interpreted in terms of a model in which the chemistry of magma reaching the surface are constrained by the sedimentary processes operating on the crust. The geochemistry of Bagdunda volcanics and their occurrence with clean washed quartzite are the features which can be best explained by extensional rifting which produced highly attenuated continental crust. The absence of evidence for any crustal contamination support their ascent through a thin continental crust which was undergoing extension. It seems likely that, this thin crust diminished the velocity of rising magma but could not force the magma tc stop and pool into a large reservoir. Thus, the magma reached the surface through conduits/fractures etc. without forming any large reservoirs. The magma evolved chemically by a process of crystal fractionation during its upward journey through these conduits. 96

With continued rifting, the next phase of volcanic activity occurred in a later stage of Jharol sedimentation. During this period, the crust had become more thin and the pulses of primitive magma, rising from the mantle were much less hindered and reached the surface through a fracture controlled conduit system. At this stage, with progressive rifting, a crust very similar to marginal sea developed, but could not mature into a true oceanic crust. It appears that, the rift developed in the Archaean continent (BGC) and its sub-continental lithosphere produced melts with enriched characteristics. ZMti I to It 97

TABLE - 1

Stratigraphic succession of the Aravalli Supergroup in the Type Area (after Roy et al. 1988).

Formation Shelf sequence Llthology Deep water Type Area sequence

Ultramafic intrusive of Kherwar - Dungarpur. Tidi Tidi, Slaty Phyllite with dolomitic Mica schists Formation Zawar and quartzitic interbeds phyllite and thin bands of quartzite

Machhlamagra Udaipur Quartzite and quartz Formation Phyllite (local conglomerate).

Zawar Zawar Dolomite, carbonaceous Formation phyllite and quartzite, with pyrite, galena, sphalerite and silver ore bodies.

Udaipur Udaipur Greywacke-phyllite, lithic Formation arenite, local diamictic conglomerate.

Debari Debari Smelter Conglomerate-arkose-quartzite Formation region and minor bodies of phyllite.

Sishmagra Sarara Congolomerate and Dantala Inlier quartzite. Formation

UNCONFORMITY Jhamarkotra Jhamarkotra Dolomite, quartzite phyllite, Formation carbonaceour phyllite, stromato- litic phosphorite (local), with copper and uranium ore bodies.

Delwara Delwara Metavolcanics with interbeds of Formation quartzite, minor phyllites and dolomite, local conglomerate. White mica at the base. FIRST ORDER UNCONFORMITY

Banded Gneissic Complex (BGC) 98

TABLE - 2 Precision, coefficient of variation and detection limits for various oxides and elements.

BHVO-1 (USGS), CRPG - BR. Oxides Detection CV% Coefficient Elements Precision RSD% limits of variation relative RSD% precision standard devi­ ations.

Si02 0.06 0.25 Ni 01

AI2O3 0.12 1.03 Cu 11

0.11 0.21 Zn 3.9 ^^2°3 MgO 0.39 2.07 Ga 2.9

CaO 0.16 0.20 Pb 37

Na^O 1.80 4.70 Th 13

K^O 0.18 1.41 Rb 1.7

0.57 2.44 U 6.9 ^2°5 TiO^ 0.014 0.60 Sr 0.5

MnO 0.009 1.42 Y 0.11

Zr 01

Nb 0.3 98

TABLE - 2 Precision, coefficient of variation and detection limits for various oxides and elements.

BHVO-1 (USGS). CRPG - BR. Oxides Detection CV% Coefficient Elements Precision RSD% limits of variation relative RSD% precision standard devi­ ations.

Si02 0.06 0.25 Ni 01

AI2O3 0.12 1.03 Cu 11

Fe^O^ 0.11 0.21 Zn 3.9

MgO 0.39 2.07 Ga 2.9

CaO 0.16 0.20 Pb 37

Na^O 1.80 4.70 Th 13

K2O 0.18 1.41 Rb 1.7

0.57 2.44 U 6.9 ^2^5 TiO^ 0.014 0.60 Sr 0.5

MnO 0.009 1.42 Y 0.11

Zr 01

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TABLE - 5

The correlation coefficient values between the various major oxides of Jharol ultramafics.

Oxides SiO_ A1_0_ TiO. FeO^ MgO CaO MaO 2 2 3 2

Si02 1 0.4314 -0.3025 .-0.0333 -0.0242 0.2430 0.8151

AI2O3 1 0.4097 0.1496 0.2856 -0.2466 0.1603

Ti02 1 0.5416 0.0518 -0.2517 -0.3989

FeO^ 1 -0.1834 0.0646 -0.0617

MgO 1 0.0328 -0.3366

CaO 1 0.3197

MaO 1 102

TABLE - 6

Major element abundances (in wt%) and Trace element concentrations (in ppm) of Gopir and Bagdunda volcanics.

Sample G5 G6 G7 BA2 BA3 M4 BA5 No. G3 G4

Si02 38.5 38.2 45.1 44.0 42.3 52.2 49.5 50.5 51.8

AI2O3 15.0 15.2 11.4 13.4 13.9 12.2 11.7 12.3 11.8

TiO^ 1.81 1.78 1.49 2.10 1.70 0.95 2.25 1.16 1.00

FeO^ 10.95 11.57 12.78 12.51 13.93 10.39 12.51 10.80 10.05

MgO 12.9 13.9 12.6 11.9 12.0 8.8 8.9 8.2 8.3

CaO 17.75 17.21 12.75 12.67 12.39 9.97 11.18 12.75 11.89

Na^O 0.68 1.33 1.39 1.92 2.09 1.57 2.46 3.18 1.99

K2O - - 0.66 0.85 0.96 0.90 0.40 0.25 0.26

MaO 0.17 0.17 0.33 0.33 0.34 0.17 0.19 0.20 0.18

0.11 0.23 0.17 0.18 0.23 0.18 0.24 0.15 0.14 ^2°5 L.O.I. 3.10 3.00 1.66 1.51 1.52 2.31 1.79 2.01 1.98

Total 100.97 102.59 99.35 101.37 101.36 99.64 101.12 101.50 99.93

Trace elements In ppm.

V 172 180 228 237 326 272 261 351 304

Cr 173 169 359 590 431 418 291 284 325

Ni 157 163 171 245 233 169 171 222 160

Zn 62 94 69 181 197 143 124 117 143

Ga 23 19 24 29 28 24 22 25 19

Rb — — 8 10 11 4 — 3 -

Table No. 6 Contd. 103

Scunple No. G3 G4 G5 G6 G7 BA2 BA3 M4 BA5

Sr 966 856 359 616 603 164 513 108 115

Y 69 63 38 42 51 25 30 33 29

Zr 244 221 144 203 185 96 170 80 71

Nb 15 14 21 22 23 7 15 12 9

Ba 10 8 28 31 33 59 32 26 54

Co 33 35 65 63 71 44 56 51 52

Th 18 16 1 - - 5 6 - 4

Pb 17 36 38 53 50 20 - 16 5

Sc 30 32 52 71 60 49 41 57 52

Cu 204 193 313 299 283 120 144 219 73

La 76.06 * 6.56 8.64 7.87 * 8.05 * 2.68

Ce 160.8 * 13.01 18.80 17.57 * 19.72 * 4.92

Nd 70.17 * 11.24 14.69 12.02 * 17.23 * 6.51

Sm 13.30 * 3.03 3.99 3.40 * 4.44 * 1.86

Eu 2.83 * 1.32 1.64 1.85 * 1.82 * 0.83

Gd 11.47 * 4.32 5.22 5.11 * 5.60 * 3.79

Tb 1.96 * 0.98 0.79 0.78 * 0.60 * 0.40

Dy 11.72 * 5.98 6.82 6.57 * 5.09 * 4.52

Ho 2.15 * 1.36 1.35 1.41 * 0.86 * 0.79

Er 6.17 * 3.81 3.99 4.22 * 2.21 * 2.87

Table Ko. 6 Coatd. 104

Sample G3 G4 G5 G6 G7 BA2 BA3 M4 BA5 No.

Yb 6.37 * 3.54 4.03 4.37 * 2.08 * 2.84

Lu 0.91 * 0.52 0.52 0.57 * 0.28 * 0.41

REE 219.19 * 55.67 70.48 65.74 * 67.98 * 32.42

: Below detec.tldn. limit,

* : Not analysed for REE.

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