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SUBRAMANIAM - i - CONTENTS PREFACE..........................................................viii ABSTRACT........................................................ xi CHAPTER I - LOCATION AND HISTORICAL REVIEW ...................1 Introduction, location................................. 1 Historical review . . . . . . . . . . . . .................... 2 Present Study . ........................................... 8 CHAPTER II - REGIONAL GEOLOGY AND THE SETTING OF THE COMPLEX . . . . 10 Archaean Geology of Peninsular India. ............... 10 Geology of the Anorthosite-eclogite gabbro complex . . . . . . 18 Structure of the complex. ............. 22 CHAPTER III - MINERALOGY......................................... 24 Introduction, methods of separation, and determination of optical properties . . . . . . . . . . . . . . . . . . . . . 24 Explanation of tabular statement of analyses of minerals. ... 51 Garnets............................... 55 Plagioclaee Feldspars . . . . . . . . . . . . . . . . . . . . . 42 Clinozoisite. . ................................ 57 ' Cordierite . . . . . . . . . . . . .............. . . . . . . 65 Anthophyllite . ............................ 70 r Clino-amphiboles. 75 • T, Chromite.................................................... 85 ^ Pyroxenes . . . . . . . . . . . . . . . . . . . . . . 89 r Scapolite . . . . . . .......... . . . . . . . . . . . . . . . 91 I ' Corundum, sillimanite, rutile, sphene, etc. 92 a "I Summary..................................................... 9^ c/ - ii - CHAPTER IV - PETROGRAPHY.......................................... 95 Anorthooite series-* general description* 95 Description of principal types ............. 102 Chromitites . ............ ............................. 108 Cordierite-corundum-anthophyllite-amorthite-bytownite rock. . . 109 Eclogite gabbro series-general description. 110 Description of principal types......... ............... . H5 Retrograde characters of eclogitee..........................117 Pyroxenite - Websterite 119 Amphibolite.............................................. I20 CHAPTER V - CHEMICAL GEOLOGY AND .................... 125 Anorthosite series • • • • • • • • • • • • • • • • • • • • • * 125 Epidote-plagioclase equilibrium • * . • • • • • • • • • • • • • 127 Metamorphic facies.................... ................... I2® Cordierite-corundum-anthophyllite-anorthite-bytownite rocks . . 129 Problem of the eclogites. 150 Eclogite rocks of the Sittampundi complex.................. 155 (FeO+MnO)tMgO,CaO, diagram................................. 15^ Origin of eclogites ............... . . • • • • • • • • • • • 157 Metamorphism of the complex as a whole . . * • . « • • • • • • 159 Origin of chromite 1^1 Origin of corundum................................... . 1^5 Origin of banding in the complex..........................1^5 Preferred orientation of minerals . . • * • • • • • • • • • • • 1^7 CHAPTER VI - FETROGENESIS..................................... 1*9 Comparison with stratiform sheets and evidence for igneous origin of the complex . * • • * ........... . • • • • 149 Summary......... .................................... BIBLIOGRAPHY.................................................. - ill - TABLES Par, I. Chemical composition and physical and optical properties of some minerals from the Sittampundi complex 52A II. Composition and physical properties of Garnet C.12 54 III. Physical properties of Garnet end members 56 Refractive indices of some garnets with estimated composition and the compositions of associated plagioclase and orthopyroxene IV. Composition and physical properties of Garnet C.41 57 V. Composition and physical properties of Garnet C.60 58 VI. Chemical composition and physical properties of eclogite garnets 59 VII. Chemical composition of anorthite from Sittampundi 44 Partial spectroscopic analyses of rome anorthitic feldspars VIII. Chemical composition and physical properties of some anorthites 4^ IX. Anorthite content of plagioclaees in Sittampundi rocks by various methods 50 X. Twinning laws of Plagioclase Feldspars in the Sittampundi Rocks 55 XI. Analyses of clinozoisite from Anorthite—corundum rocks of Salem 58 XII. Composition of clinozoisite C.59 59 XIII. Molecular formula and physical properties of clinozoisite C.59 60 XIV. Chemical composition and physical properties of some clinozoisites 6l XV. Composition of Cordlerite C.71 64 XVI. Composition and physical properties of someo agnesian cordierites 65 XVII. Composition and optical properties of Cordierite C.71 66 XVIII. Composition of Anthophyilite C.58 71 XIX. Composition of amphibole 0.54 74 XX. Composition of amphibole C.70 75 XXI. Composition of amphibole from rock C.55 (calculation) 76 XXII. Composition of amphibole C.55 77 XXIII. Structural account of the analyzed amphiboles, after Sundius, indicating affinities to the Edenlte and Fargaaite type 78 % - iv - XXIV. Optical and physical properties of analyzed amphiboles 79 XXV. Optical properties of some amphiboles 80 XXVI. Composition of Chromite from Chromitite C.101 85 XXVII. Calculation of atoms per unit cell of chromite C.101 86 XXVIII. Chemical composition of some chromites from stratiform sheets 87 XXIX. Approximate modes of rocks from the Sittampundi complex(weight^) 96 XXX. Chemical composition with normative and modal mineralogical composition of some rocks from the Sittampundi complex 120 XXXI. Analyses of anorthosites from South Africa, Sweden, and Sittampundi Complex 125 XXXII. Chemical composition of Eclogite gabbro C.4l, together with comparable analyses 124 XXXIII. Molecular ratios (FeO+MnO), C&O, and MgO of Qabbroic and eclogitic rocks, together with those of the garnets in them in so far as available, recalculated to 100$6 155 XXXIV. Comparative vertical sequence in some gravity stratified sheet complexes 150 * ILUJSTRATICNS Plate Between Pages I. 5-4 i)View of ancient corundum workings on anorthosite band, south of Sittampundi. Note barren nature of country. ii)Low hill of anorthosite north of Kattipalaiyam,towards the N.E. end of complex. II. 18-19 i)Steeply dipping anorthosite near Pamandapalaiyam. ii;Anorthosite outcrop east of Kattupaliyam,note lower dip. III. 18-19 i)Foliated amphibolite on northern margin of complex,south of Konaaamudram. ii)Typical exposure of weathered anorthosite south of Sittam­ pundi. IV. 20-21 i)Typical exposure of anorthosite with mafic bands on high ground south of Sittampundi. ii)0utcrop of eclogite-gabbro near Gangarapalaiyam, note visible foliation. V. 20-21 i)Anorthosite with mafic layers, near Molasi, western end of complex. ii)Large band of hornblende rock in anorthosite, outcrop near Sanarpalaiyam. VI. 20-21 i)Banded anorthosite near Syakkadupudur ii; Mafic bands in anorthosite, outcrop near Mannadipalaiyam. VII. 21-22 i)P«eudo-Boudinage structure of mafic bands in anorthosite, outcrop north of Kattipalaiyam ii) Seam of chromitite in old corundum quarry at Karungalpatti VIII. 21-22 i)Jointing in chromitite at Karungalpatti quarry. Xi)Jointing in chromitite at Karungalpatti quarry. IX. 21-22 i)Jointing in anorthosite, old corundum quarry south of Sittampundi. ii)Jointing in anorthosite outcrop at Kottakkaradu east of Karungalpatti. X. 22-25 Diagramstic interpretation of the form and structure of Complex*. Vi - XI. 54-55 i )Bhotomicrograph, showing garnet replacing anorthite. X 10 Ii)Photomicrograph, showing garnets with dactylitic rims, in eclogite-gabbro. X 55 XII. 54-55 Micrograph showing garnet selectively replacing anorthite. XIII. 41-42 Micrograph of garnet in eclogite gabbro, with plagioclase hornblende symplectite. XIV. 41-42 Micrograph of garnet crystal in eclogite gabbro XV. 48-49 Curve correlating the indices of refration and 'An1 content for calcic plagioclase indicating position of Sittampundi pl&giocl&ses. XVI. 51-52 Fnotomicrograph of anorthite clinozoisite rock showing a crystal of anorthite displaying complex albite-Ala twinning. X 22 XVII. 61-62 Micrograph showing replacement of anorthite cy clinozoisite XVIII. 61-62 Micrograph showing well developed crystal of clinozoisite in anorthite gneiss. XIX. 66-67 i)Fhotomicrograph of zoned crystal of clinozoisite. X 50• ii)Photomicrograph of cordierite showing twinning. X 100. XX. 66-67 Sketch of exposure of footwall of old dorundum quarry south of Sittampundi, showing paragenetic relationship of corundum, clinozoisite, garnet, and anorthite. XXI. 71-72 i)Fhotomicrograph showing bladed anthophyllite in anorthosite.X9 ii)Photomicrograph showing bladed anthophyllite in chromitite.X10 XXII. 71-72 Micrograph showing anthophyllite in anorthosite. XXXII. 82-65 Curves expressing the optical properties of hornblendes from Sittampundi rocks, in relation to their chemical composition. XXIV. 87-68 i)Faotomicrograph of polished section of chromite with rutile. X 180. ii)Photomicrograph of polished section of chromite with pAft.tea of unknown mineral, note their geometric orientation. X 400 - vii - XXV. 38-89 Triangular diagram after Stevens, showing position of Sittampundi chromite. XXVI. 90-91 i)Photomicrograph-8keletal crystals of diopside in anorthosite. X.22. ii)Photomicrograph-Ilmenite, rutile and aphene in snorthoBite, X.7. xxvii. 90-91 2-a.crograph of skeletal crystals of diopside in anorthosite showing p&ramorphic alteration to edenite. XXVIII. 91-92 Micrograph of corundum replacing anorthite. xxix. 96-9 7 i )Porphyroblast of corundum in anorthosite. ii)Porphyroblast of corundum in anorthosite. XXX. 105-104 i)Coarse anorthosite with anthophyllite ii )Chromitite layer in hand specimen of anorthosite. XXXI. 105-106 i)Photomicrograph of anorthosite displaying mosaic texture. X 10 ii)Fhotomicrograph of anorthosite showing preferred orientation. X7 XXXII. 105-106 i ^Photomicrograph of eclogite gabbro. X 9 • ii )Photomicrograph of bytownite gneiss. X 7» XXXIII. 114-115 Micrograph of eclogite XXXIV. 114-115 Micrographs of garnet coronas in eclogite XXXV. 114-115 i)Photomicrograph of eclogite gabbro. X 10 ii)Photomicrograph of bronzitic pyropite. X 10 XXXVI. 128-129 A C F Diagram for Amphibolite facies. XXXVII. 129-150 A C F Diagram for Sanidinite facies XXXVIII. 155-154 A C F Diagram for Eclogite facies. XXXIX. 155-154 (FeO+MnO), MgO, OaO diagram. XL. In pocket. Petrological map of Sittampundi complex. - viii - PREFACE During my official duties with the Geological Survey of India, I had opportunities to make a fairly detailed field study of a group of basic rocks from the Archaean complex of Peninsular India. The results presented in this dissertation are the outcome of laboratory studies of these rocks, carried out in the Department c-f Geology, Princeton University, for the last tvro years. I am deeply grateful to the Graduate School, Princeton University, for the award of Scholarship and Fellowship for the academic years 19^9~50f and 1950-51* covering the cost of tuition, and the Department of Geology for generous grants, covering the cost of 18 chemical analyses of rocks and minerals, 60 thin sections, and the illustrations in this paper. I am under a deep debt of gratitude to ProfeesorsA. F. Buddington and H. H. Hess, under whose general supervision this investigation was carried out. Their stimulating discusssions of the problem and helpful criticisms of the manuscript have greatly contributed to this study. The mineralogical and petrographic studies were carried out principally under the generous guidance of Professor Hess, and I am indebted to him for initiating me in several optical methods with which I was unfamiliar. To Professor E. Sampson, my thanks are due for useful discussions on the chromite problem and for introducing me to the study of polished sections. I am specially obliged to him for the photomicrographs in Plate XXIV, and for polishing some of my chromites for microscopic study. Ify thanks are also due to Dr. J. C. Maxwell for discussing some aspects of my problem and for various suggestions, much appreciated by me. To the other members of the faculty I am most grateful for the encouragement I have received