PETROLOGY OF ULTRAMAFIC AND RELATED ROCKS FROM MT. LIGHTNING, NEAR GUNDAGAI, NEW SOUTH WALES, AUSTRALIA. By A. S. Ray, M.Sc. A THESIS SUBMITTED TO THE UNIVERSITY OF NEW SOUTH WALES FOR THE DEGREE OF DOCTOR OF PHILOSOPHY, 1977. ABSTRACT A rock assemblage in the Mt. Lightning area of the Coolac Serpentine Belt in south-eastern New South Wales has been mapped and studied by means of petrographic and chemical techniques. The principal lithologies include massive to locally foliated, partially serpentinized, olivine-rich harzburgites, with deformed and recrystallized textures, containing consistently Mg-rich olivine {Fo89 _92} and enstatite {En87_90 } with subordinate diopside and chrome-spine!. These rocks are associated with blocky and sheared lizardite- chrysotile serpentinites. Massive and variolitic varieties of low-potash spilites containing tholeiitic pyroxenes as inferred from chemical compositions, occur within and flanking the serpentinites. Smaller rock masses enclosed by, or flanking the ultramafic rocks, include low-iron rodingites of two genetic groups, a series of low-potash feldspathic rocks ranging from quartz-rich trondhjemites to quartz-poor albitites, and both er-rich and Al-rich chromitites. Group 1 rodingites containing grossularite, vesuvia­ nite, chlorite and relict diopside, and characterized by high Cr {> 200 ppm} and Ni {> 150 ppm} contents, and high Cr/V ( > 2) and Mg/Fe (> 2) ratios, are considered to be metasomatized mafic rocks. Group 2 rodingites are reaction zone products that formed at contacts of spilite and ultra- mafic rocks. They contain zoisite, prehnite, chlorite and grossularite but lack vesuvianite and relict pyroxene, have low er ( <. 40 ppm) and Ni ( <. 50 ppm) contents, and low Cr/V ( < 1) and Mg/Fe (<1.5') ratios. The trondhjemitic rocks are devoid of k-feldspar and are typically K2o-poor ( <. 0. 8 wt%) ; they are considered to be alteration products of dioritic parents. The ultramafic-mafic-sedimentary rock assemblage at Mt. Lightning accords with that of an ophiolite in which the harzburgite represents the basal member. The mineral chemistry and textures of the harzburgite also accord with a status as upper mantle residue. It is therefore possible that the Mt. Lightning rock assemblage represents a fragment of Lower Palaeozoic oceanic lithosphere with part of the crustal section missing. A partial melting model of upper mantle is tentatively proposed for the generation of the principal and minor rock types at Mt. Lightning. The ultramafic rocks are considered to be refractory residues from partial melting processes. At lease two stages of partial melting might have taken place - a low degree (~ 5% by volume) of partial melting producing the gabbroic parent (s) of Group 1 rodingites and a relatively high amount (~ 20% by volume) of melting giving rise to the tholeiitic parent of the spilites. Differentiation of the latter tholeiitic magma is believed to have resulted in dioritic rocks from which the trondh­ jemites and albitites were derived. ACKNOWLEDGEMENTS I have received much assistance from a large number of people. To one and all I express my thanks, However, I would like to pay special tribute to the following people who assisted me greatly:- • Dr. H.G. Golding who acted as my Supervisor. I am most indebted to him for all his personal help, encouragement and constant guidance during the course of my present study. The numerous discussions that I had with him on . this topic have all provided me with an invaluable help. With all confidence I can say that without his supervision this work could never have been completed. • Dr. B.J. Hensen, Dr. P.C. Rickwood and Dr. M.B. Katz with whom I had helpful discu­ ssions from time to time. • Prof. T.G. Vallance of the University of Sydney with whom I had valuable discussions on spilitic rocks. • Dr. B.J. Franklin who provided the facili­ ties to take microphotographs at the Institute of Technology. • Dr. B. Chappell and Mr. R. Freeman of the Australian National University, Dr. R. Flood and Mr. G. Pooley of the Macquarie Univer- sity for X-ray fluorescence analys~s, and from the University of New South Wales: Mr. F. Scott for electron microprobe analyses, Ms G. Chorley and Mr. M. Walker for wet chemical analyses, Mr. G. Small for assistance with photography and Ms M. Clark for typing the tables. My list of acknowledgements would not be complete without my payins tribute to the late Professor J.J. Frankel who provided me with the initial inspira­ tion to commence this thesis. CONTENTS CHAPTER 1. INTRODUCTION Page No. 1.1 PREAMBLE 1 1.2 LOCATION, PHYSIOGRAPHY AND MAPPING 1.21 Location and Physiography 4 1.22 Mapping 7 1.3 PREVIOUS REFERENCES 8 1.4 REGIONAL GEOLOGICAL SETTING 1.41 Introduction 11 1.42 The Coolac Serpentine Belt 11 1.43 Honeysuckle Beds 17 1.44 Young Granodiorite 19 CHAPTER 2. ULTRAMAFIC ROCKS 2.1 INTRODUCTION 23 2.2 GENERAL STATEMENT 25 2.3 PETROGRAPHY AND TEXTURE 29 2.31 Microtexture 29 2.32 Mineralogy and mineral chemistry 42 2.321 Olivine 42 2.322 Orthopyroxene 44 2.323 Clinopyroxene 48 2.324 Chrome-spinel 51 2.325 Serpentine-group 53 2.326 Other secondary minerals 59 2.4 WHOLE ROCK CHEMISTRY OF MT. LIGHTNING PERIDOTITES 60 Page No. 2.5 PETROGENESIS OF PRIMARY ULTRAMAFIC ROCKS 64 2.51 Temperature-pressure estimation based.on CaSiOrl and Al2o3 contents of clinopyroxe e . ' 64 2.52 Temperature estimation based on cation distribution in co-existing· orthopyroxene and clinopyroxene p6 2.53 Temperature estimation based on cation distribution in co-existing olivine and ·orthopyroxene 67 2.54 Summary 69 2.55 Classification of Mt. Lightning ultramafic rocks 71 2.56 Origin of Alpine-type ul tramafic . rocks 73 2.57 Origin of Mt. Lightning ultramafic rocks 80 2.6 SERPENTINIZATION OF COOLAC ULTRAMAFIC ROCKS 82 CHAPTER 3. SPILITES 3.1 INTRODUCTION 88 3.2 GENERAL STATEMENT 90 3.3 PETROGRAPHY 94 3.4 MINERALOGY 3 .41£. fAlbi te 95 3. 42 r•" 0 hroup Minerals 97 3.43,. Chlorite 100 3.44 Pyroxene 102 3.45 Other minerals 103 3.5 TEXTURE 106 3.6 WHOLE ROCK CHEMISTRY 110 3.7 MT. LIGHTNING SPILITES AS SECONDARY ROCKS 124 Page No. 3.8 DETERMINATION OF SPILITIC PARENTAGE 3.81 Using major element chemistry 127 3.82 Using trace element data 131 3.83 Using clinopyroxene composition 134 3.9 ORIGIN OF MT. LIGHTNING SPILITES 138 CHAPTER 4. RODINGITES 4.1 INTRODUCTION 141 4.2 NOMENCLATURE 142 4.3 CLASSIFICATION OF RODINGITES 143 4.4 MODE OF OCCURRENCE 4.41 Group 1 rodingites 145 4.42 Group 2 rodingites 146 4.5 PETROGRAPHY AND TEXTURE 4. 51 Group 1 rodingi tes 149 4.52 Group 2 rodingites 152 4.6 MINERALOGY 4.61 Group 1 rodingites 4.611 Garnet 156 4.612 Vesuvianite 158 4.613 Clinopyroxene 164 4.614 Chlorite 168 4.615 Other minerals 169 4.62 Group 2 rodingites 4.621 Zoisite 173 4.622 Prehnite 179 4.623 Chlorite 182 4. 624 G\rnet 185 4. 625 SpAene 186 4.626 Tremolite-actinolite 186 Page No. 4.7 CHEMISTRY OF RODINGITES 4.71 Group 1 rodingites 187 4.711 Variation of composition within Group 1 rodingite bodies at Mt. Lightning 197 4.72 Group 2 rodingites 200 4.721 Variation of composition across Group 2 rodingite bodies 206 4.8 GENESIS OF RODINGITE 210 4.81 Summary of various hypothesis for the origin of rodingites 211 4.82 Origin of Group 1 rodingites from Mt. Lightning 216 4.821 Rodingitization and Serpentinization 224 4.83 Origin of Group 2 rodingites from Mt. Lightning 230 4.84 Temperature-pressure estimation 234 CHAPTER 5. TRONDHJEMITES AND ALBITITES 5.1 INTRODUCTION 236 5.2 GENERAL STATEMENT 238 5.3 PETROGRAPHY 242 5.4 MICROTEXTURE 5.41 Granitic texture 249 5.42 Gneissic or Banded texture 250 5.43 Mylonitic texture 250 5.44 Microbrecciated texture 252 5.45 Other textures 252 5.5 WHOLE ROCK CHEMISTRY 255 5.6 GENESIS 264 CHAPTER 6. CHROMITITES Page No. 6.1 INTRODUCTION 270 6.2 GENERAL FEATURES OF THE MT. LIGHTNING CHROMITE PODS 276 6.21 Distribution of Pods 276 6.22 Shape, dimension and orientation J of Pods 276 6.23 Internal structure, textures and contacts of Pods 277 6.24 Rodingite-chromitite relationships 279 6.3 MINERALOGY OF CHROMITITES 281 6.4 CHEMISTRY OF CHROMITES 282 6.41 Location and brief description of analysed samples 282 6.42 Analytical results 286 6.5 DISCUSSION 6.51 Conclusions based on the author's investigations 291 6.52 Aspects of the genesis of the Podiform chromitites 292 CHAPTER 7. DISCUSSION 7.1 OPHIOLITES AND THE MT. LIGHTNING ROCKS 297 7.2 MT. LIGHTNING ROCKS AS PART OF A LAYERED OCEANIC CRUST-UPPER MANTLE SEQUENCE 301 7.3 TECTONIC EVOLUTION OF THE AREA STUDIED 304 7.31 Marginal Basins and emplacement of ophiolites 309 7-4 CONCLUSIONS 312 BIBLIOGRAPHY 316 APPENDIX 1 C H A P T E R 1 I N T R O D U C T I O N 1.1 PREAMBLE: Ultramafic rocks that occupy extens~ve fault­ bounded belts have received increasing attention in recent years largely as a consequence of plate tectonic concepts and the hypothesis that such rock masses are solid-emplaced, on-land fragments of pre-existing oceanic sub-crust, but broad generalizations concerning such ultramafic belts have tended to outstrip knowledge of their detailed constitution. Initial investigations along some belts have revealed a high degree of internal lithologic diversity that precludes a rapid appraisal of their detailed petrology. For such occurrences a number 2 of restricted or "type" areas may be selected for detailed study, each study contributing to the overall picture.