Structure and Tectonics of the Ampanihy Group in the Vicinity of the Ankafotia and Saririaky Anorthosite Bodies, Southwestern Madagascar
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STRUCTURE AND TECTONICS OF THE AMPANIHY GROUP IN THE VICINITY OF THE ANKAFOTIA AND SARIRIAKY ANORTHOSITE BODIES, SOUTHWESTERN MADAGASCAR by LEON GABRIEL RANDRIANASOLO THESIS Submitted in fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology in the FACULTY OF SCIENCE at the RAND AFRIKAANS UNIVERSITY Promotors: Prof. ASHWAL L. D. (R.A.U) Prof. DE WIT M. J. (U.C.T) August 1996 ACKNOWLEDGEMENTS I thank Prof. L.D. Ashwal and Prof de Wit M. J. for supervising this project and providing encouragement, support, advice and constructive discussions. Financial support for this study from the Foundation for Research and Development (FRD) and from the Rand Afrikaans University is gratefully acknowledged. I also wish to express my appreciation to Prof Roger Rambeloson of University of Antananarivo (Departement des Sciences de La Terre) for assistance and support. My thanks to all the other students and members of staff of the Department of Geology at R.A.U. for constructive discussions, especially Profs. Chris Roering, Dirk van Reenen, Dr. R. M. Cox, Mr. K. Mogathla, Mr. V. Morel, Mr. L L. Raoelison, Mr. B. Muller, Mr. H. Dirr and M. Legrange. I thank Ms. Nellie Day for technical assistance during the microprobe time. I thank Mr. Hennie Jonker for his technical support. I also acknowledge the Lunar and Planetary Institute of Houston, Texas, U. S. A. for providing satellite imagery. I wish to thank the Consulate of R. S. A. and the 'Compagnie Air Madagascar' of Antananarivo for transport facilities. My thanks to my compatriots I. L. Raoelison and N. Rakotosolofo who made the time spent during the duration of this study pleasant. I also owe a great deal to my wife and my children for their unfailing support. Finally, all thanks to God who made it possible for me, through his everlasting love. ABSTRACT The Ampanihy Group is part of the Pan-African terrains in southwestern Madagascar, bounded to the north by the Bongolava- Ranotsara shear zone (BRSZ). Lithotectonically, it is separated from the Vohibory Group to the west by the Ampanihy shear zone (ASZ), and from the Ampandrandava Group to the east by the Vorokafotra shear zone (VSZ). The Ampanihy Group comprises a variety of rock types that have been metamorphosed at granulite facies. The most common rocks are graphite schists, leptynites, marbles, gneissic amphibolites-pyroxenites, quartzites and granitoid gneisses. Aside from these rocks types, the Ampanihy Group also contains four anorthosite bodies ranging in areal extent from 25 km 2 to 100 km2; two of them (Ankafotia and Saririaky) are located within the Ampanihy shear zone. Structures of three episodes of deformation have been recognized (D2-D4), Di having been destroyed and overprinted by D3. D2 produced upright to steep E-overturned folds, but most of the structures related to D2 have also been obliterated. Remnants of D2 structures are prominently visible as fold closures to the north and south of the two anorthosite bodies, and in the eastern part of the area. Petrographic observations reveal grt+kf+sil+qtz+pl assemblages in metapelitic rocks (sillimanite- graphite gneiss), indicating that D2 was accompanied by medium-grade metamorphism (upper amphibolite fides) with estimated P-T conditions of 500° C - 680° C and 5.5 kbar - 7.1 kbar. Combined evidence from field and laboratory observations on asymmetric tight-to isoclinal folds, flattened boudins, flattened feldspathic porphy' roblasts with symmetric pressure shadows, and ribbon-quartz and K-feldspars suggests that D3 was an intense deformational flattening event caused by east-west shortening stresses. D3 was accompanied by high-grade metamorphism (T= 710° - 850° C; P= 7 - 9 kbar). The present structural pattern of the ASZ reflects this D3 deformation. L3 lineations are moderate to steep (40°-80°) plunging either to north or south. They are mostly intersection lineations produced by D2/D3 interference. S3 foliations are steep to subvertical (60 0-871, striking between N 25° W and N 25° E, mostly resulting from S2 transposition into S3. D3 effects have resulted in sheath-like geometry of the two anorthosite bodies Ankafotia and Saririaky. This structural pattern is supported by the presence of fold closures at each end of the bodies bounding them and facing their openings to them. To the north and south of the bodies, lineations in the country rocks are respectively north and south-trending. Within the eastern and western margins of the anorthosites, L3 stretching lineations are subvertical. The subcircular shape of the Saririaky body suggests that it is tubular in the third dimension. D3 was subsquently followed by an uplift event (D4), accompanied by basic rock emplacement (gabbros?) and poorly-developed medium-to high retrograde metamorphism (M4) (granulite-amphibolite facies transition), during near-isothermal decompression (ITD). The basic rocks are massive, garnet amphibolites and pyroxenites without foliation structures, attesting to their post-D3 generation. D4, the last recognized deformation undergone by the area, is manifested by weak brittle deformation structures and prominent grain size reduction and cataclastic textures of the amphibole-pyroxene gneisses that cover the greater part of the area. The east-west compressional flow generating the ASZ could have been the result of continent-continent collision during Gondwana assembly, and may represent a form of escape tectonics generated by plate- interior adjustment (650-640 Ma) in eastern Gondwana following such a collisional event. The tectono-metamorphic evolution of the area, having started by folding, followed by flattening accompanied by granulite facies metamorphism and subsequent uplift, is consistent with such a collisional environment. 1 CONTENTS page Chapter 1: Introduction 1 1.1. Madagascar in a Gondwana context 1 1.2. Geological overview of Madagascar 4 1.2.1. Lithology and Stratigraphy 4 1.2.2. Precambrian structural features 10 1.3. Geographical setting of the study areas 13 1.4. Geological setting and structural framework of the study area 15 1.4.1. Geology of the Graphite System 15 1.4.2. Geology of the Ankafotia and other anorthosite bodies in the region 20 1.5. Recent investigations 22 1.5.1. General structure of the Ampanihy shear zone 22 Chapter 2: Objectives of the study 26 Chapter 3: Methods of study 29 3.1. Mapping 29 3.2. Sampling 29 3.3. Petrography 30 3.3.1. Pyroxene gneiss 31 3.3.2. Graphite schists 31 3.3.3. Amphibolite gneiss 31 3.3.4. Leptynites 33 3.3.5. Sillimanite gneiss 33 11 page 3.3.6. Garnet amphibolite 33 3.3.7. Quartzite 33 3.3.8. Pyroxenite 35 3.3.9. Marble 35 3.3.10. Leuconorite 35 3.3.11. Pegmatite 35 3.3.12. Mylonites 37 Chapter 4: Field occurrences of major lithologies 38 4.1. Pyroxene gneiss 38 4.2. Graphite schist 38 4.3. Amphibolite gneiss 39 4.4. Leptynites 39 4.5. Sillimanite gneiss 39 4.6. Garnet amphibolite 39 4.7. Quartzite 40 4.8. Pyroxenite 40 4.9. Marble 40 4.10. Leuconorite 40 4.11. Pegmatite 41 4.12. Mylonites 41 Chapter 5: Structural Geology 42 5.1. Meso-to micro-scale features 43 5.1.1. Foliations 43 5.1.2. Lineations and minor folds 45 5.1.3. Folding 50 111 page 5.1.4. Microstructures 50 5.2. Age of deformation 55 5.2.1. D1 deformation 55 5.2.2. D2 deformation 55 5.2.3. D3 deformation 57 5.2.3. D4 deformation 57 5.3. Ankafotia area 57 5.3.1. Outline of the geological structure 57 5.3.2. Structural description and interpretation 58 5.3.2.1. Domain 1 58 5.3.2.2. Domain 2 58 5.3.2.3. Domain 3 58 5.3.2.4. Domain 4 62 Interpretation 64 5.3.2.5. Domain 5 66 5.3.2.6. Domain 6 67 5.3.2.7. Domain 7 72 5.1.2.8. Domain 8 72 Interpretation 72 5.3.3. Depth of emplacement and thickness of the Ankafotia anorthosite body 77 5.3.4. Strain effects on the anorthosite and the country rocks and a model for their structural evolution 79 5.3.4.1. Foliation and lineation patterns 82 5.3.4.2. Sheath folding 84 5.3.5. Relation between deformation and metamorphism 84 5.3.5.1. Di deformation 86 iv page 5.3.5.2. D2 deformation 86 5.3.5.3. D3 deformation 86 5.3.5.4. D4 deformation 92 5.4. Saririaky area 95 5.4.1. Outline of the geological structure 95 5.4.1.1. Domain 1 95 5.4.1.2. Domain 2 95 5.4.1.3. Domain 3 99 5.4.1.4. Domain 4 99 5.4.1.5. Domain 5 101 5.4.2. Di deformation 101 5.4.3. D2 deformation 101 5.4.4. D3 deformation 103 Interpretation 103 Model 1 108 Model 2 118 5.4.5. D4 deformation 120 5.5. Structural interpretation 123 Chapter 6: Metamorphism 132 6.1. Prograde metamorphism 132 6.2. Textural features of regressive metamorphism 132 6.3. Retrograde P-T path and tectonic relationships 136 6.4. Relation between metamorphism and deformation 140 Chapter 7: Tectonic interpretations and Conclusion 143 References 164 V page Appendix 173 vi List of Figures page Fig. 1: Gondwanaland and components 2 Fig. 2: Correlations between Dharwar craton and Madagascar 5 Fig. 3: Geologic map of the Precambrian units in Madagascar 6 Fig. 4: Map showing the metamorphic zones and facies of the Precambrian of Madagascar 8 Fig. 5: Map showing the Precambrian main structures of Madagascar 11 Fig. 6: Location of the study area 14 Fig.