Structure and Tectonics of the Ampanihy Group in the Vicinity of the Ankafotia and Saririaky Anorthosite Bodies, Southwestern Madagascar

STRUCTURE AND TECTONICS OF THE AMPANIHY GROUP IN THE VICINITY OF THE ANKAFOTIA AND SARIRIAKY ANORTHOSITE BODIES, SOUTHWESTERN MADAGASCAR

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. 7: Map showing the study area surroundings 16 Fig. 8: Geographical distribution of the Graphite System 17 Fig. 9: Map showing the Precambrian geology of southern Madagascar 19 Fig. 10: Schematic maps of the four anorthosite bodies in SW Madagascar 21 Fig. 11: Location of the Ampanihy and Vorokafotra shear zones in SW Madagascar 24 Fig. 12: Satellite image of the two anorthosite massifs in the Ampanihy shear zone 25 Fig. 13: Sketches showing the two models: boudin and sheath fold 27 Fig. 14: Pyroxene gneiss photomicrograph 32 Fig. 15: Graphite gneiss photomicrograph 32 Fig. 16: Sillimanite relicts photomicrogaph 34 Fig. 17: Retrogression structure (corona texture) in garnet 34 Fig. 18: Photomicrograph of marble showing serpentinized forsterite and deformation twins in calcite 36 vii

page Fig. 19: Banded garnet gneiss interlayered with amphibolite 44 Fig. 20: W-dipping flattened marble outcrop 44 Fig. 21a: Moderately N-plunging intersection lineations (quartz and garnet) 46 Fig. 21b: Diagrammatic sketch showing planar and linear fabrics 46 Fig. 22: Moderately S-plunging elongation lineations (garnet) 47 Fig. 23: Moderately N-plunging intersection lineations (garnet and amphibole) 47 Fig. 24a: Shallowly S-plunging intersection lineations (quartz) 48 Fig. 24b: Diagrammatic sketch showing planar and linear fabrics 48 Fig. 25: Garnet amphibolite flattened boudins 49 Fig. 26: Tight folded layered garnet gneiss and garnet amphibolite 51 Fig. 27: Similar tight-folded layered garnet gneiss and garnet amphibolite 51 Fig. 28: Asymmetric isoclinal folded garnet gneiss 52 Fig. 29: Minor folds in garnet gneiss 52 Fig. 30: Crossed foliations on mylonite 53 Fig. 31: Deformation associated to D4: cataclastic structure 53 Fig. 32: Mortar structure in mylonite 54 Fig. 33: K-feldspar porphyroblast deflecting ribbon-like quartz and feldspar (S3) 54 Fig. 34: Prominent ribbon-like quartz and K-feldspar in mylonite 56 Fig. 35: Perthites in feldspathic matrix and undulose extinction of quartz in leptynite 56 Fig. 36: Structural map of Ankafotia area surroundings 59 Fig. 37: Location of structural domains 1 and 2 of Ankafotia area with stereograms 60 viii

page Fig. 38: Location of structural domains 3 and 4 of Ankafotia area with stereograms 61 Fig. 39: Symmetric K-feldspathic porphyroblast in garnet leptynite outcrop 63 Fig. 40: Pre-tectonic garnet with pressure shadow 65 Fig. 41: Post-tectonic garnet 65 Fig. 42: Location of structural domains 5 and 6 of Ankafotia area with stereograms 67 Fig. 43: Schematic structural map of domain 5 (Ankafotia area) 68 Fig. 44: Folded layered graphite schist in the southern closure (Ankafotia area) 69 Fig. 45: Schematic geological map of domain 6 (Ankafotia area) 70 Fig. 46: Parasitic fold in graphite gneiss in the northern closure (Ankafotia area) 71 Fig. 47: Location of structural domains 7 and 8 of Ankafotia area with stereograms 73 Fig. 48: Structural map of domain 7 (Ankafotia area) 74 Fig. 49: Structural map of domain 8 (Ankafotia area) 75 Fig. 50: Folds exposed in Ankafotia eastern margin 76 Fig. 51: Fold closure in Ankafotia eastern margin 76 Fig. 52: Stereograms showing lineation and foliation attitudes for entire envelope around the Ankafotia anorthosite body 78 Fig. 53: Kinematic evolution of the ASZ structures in the vicinity of the Ankafotia massif 80 Fig. 54: Diagrammatic representation of lineation orientations on the Ankafotia massif and the country rocks 83 ix

page Fig. 55: Diagrammatic representation of the main structural features of the Ankafotia area 85 Fig. 56: Stereograms for D3 deformation from domains 1 and 2 (Ankafotia area) 87 Fig. 57: Stereograms for D3 deformation from domains 3 and 4 (Ankafotia area) 89 Fig. 58: Stereograms for D3 deformation from domains 5 and 6 (Ankafotia area) 90 Fig. 59: Stereograms for D3 deformation from domains 7 and 8 (Ankafotia area) 91 Fig. 60: Sillimanite grain folded by D3 93 Fig. 61: Cataclastic and protomylonitic structures produced by D4 93 Fig. 62: Deformation associated to D4: broken and crushed

garnet in pyroxene gneiss 94 Fig. 63: Deformation associated to D4: fractured and bent plagioclase in garnet amphibolite 94 Fig. 64: Structural photogeological map of the Saririaky body surroundings 96 Fig. 65: Location of structural domain 1 of Saririaky area with stereograms 97 Fig. 66: Location of structural domains 2 and 3 of Saririaky area with stereograms 98 Fig. 67: Mylonitized gneiss outcrop in Saririaky western margin 100 Fig. 68: Flattened boudins in Saririaky western margin 100 Fig. 69: Location of structural domains 4 and 5 of Saririaky area with stereograms 102 Fig: 70: Transposition structure initiation on gneiss outcrop in Saririaky eastern margin 104

page Fig. 71: Diagram showing tubular fold parameters 105 Fig. 72: Schematic diagram illustrating the two alternative rotation models 107 Fig. 73a: Obliquity between two mylonitized rocks in Saririaky western margin 109 Fig. 73b: Figure 73a locality 109 Fig. 74: Diagram representing the structural features indicative of the anticlockwise rotation of the Saririaky body 110 Fig. 75: S-folded graphite schist in Saririaky southern margin 111 Fig. 76: Diagram showing the cause of the deflection of the foliation north of domains 1 and 2 of the Saririaky anorthosite body 113 Fig. 77: Satellite image illustrating the effects of the rotation on the country rocks north of the Saririaky anorthosite body 114 Fig. 78: Schematic diagram showing the possible pattern of lineations and foliations around the Saririaky body prior to rotation and tilting 115 Fig. 79: Schematic diagram displaying the structural features indicative of the Saririaky anorthosite body tilting 116 Fig. 80: a-8 feldspathic porphyroblast on gneiss outcrop in Saririaky southeastern margin 117 Fig. 81: Flattened quartzo-feldspathic veins and feldspathic porphyroblasts on gneiss outcrop in Saririaky southeastern margin 117 Fig. 82: Schematic diagram illustrating possible regional trend prior to D3 and after D3 119 xi

page Fig. 83: Stereograms for D3 deformation from domains 1 and 2 of Saririaky area 121 Fig. 84: Stereograms for D3 deformation from domains 3, 4 and 5 of Saririaky area 122 Fig. 85: Stereograms of foliation and lineation attitudes for the envelope area around the Saririaky body 124 Fig. 86: Isoclinal folded feldpathic vein (intrafolial fold) on garnet gneiss outcrop in Ankafotia western margin 125 Fig. 87: Sketch illustrating the axial traces of the two anorthosite bodies 127 Fig. 88: Relation between mineral formation and deformation 129 Fig. 89: Isoclinal layered garnet gneiss with steep fold axis plunge in Ankafotia western margin 130 Fig. 90: Flattened quartzo-fedspathic veins producing small-scale folds on garnet gneiss outcrop in Ankafotia western margin 130 Fig. 91a: ACF-A1CF projection showing mineral assemblage of metapelitic rocks in the amphibolite facies 133 Fig. 91b: ACF projection showing mineral assemblage of metabasic rocks in the amphibolite facies 133 Fig. 92a: ACF-AIKF projection showing mineral assemblage of metapelitic rocks in the granulite facies 134 Fig. 92b: ACF projection illustrating mineral assemblage of mafic rocks in the granulite facies 134 Fig. 93: Illustration showing zoning patterns in amphibole 137 Fig. 94: Illustration showing zoning patterns in garnet 138 Fig. 95: Diagram showing possible decompressional (ITD) path for granulite terranes 139

xii

page Fig. 96: Schematic diagram illustrating the kinematic evolution of the Ampanihy Group during D2 and D3 143 Fig. 97: Schematic profiles through the northern (Ankafotia) and southern (Saririaky) anorthosite bodies 145 Fig. 98: Illustration showing the Precambrian structure of Eastern Africa, Madagascar, southern India and Sri Lanka 147 Fig. 99: Map showing Figure localities in Ankafotia area 152

Fig. 100: Map showing Figure localities in Saririaky area 153 List of tables

page Table 1: Subdivisions of Precambrian of Madagascar 9 Table 2: Metamorphic facies of southern Madagascar 9 Table 3: Graphite System: Groups and lithologies 18 Table 4: Recent geochronological and thermobarometric data from south Madagascar 23 Table 5: Microprobe analysis of garnet 154

Table 6: Microprobe analysis of pyroxenes 157 Table 7: Microprobe analysis of amphibole 160 Table 8: Microprobe analysis of plagioclase 162 Table 9: Summary of geological history of the rocks within the map areas 142 Table 10: Usagaran Complex and Ampanihy Group

correlations 151

(Tables 5, 6, 7 and 8 are at the end of the text) x i v

List of maps

Map 1: Geological map of Ankafotia massif envelope Map 2: Geological map of Saririaky massif envelope Map 3: Map showing structural data between the two anorthosite bodies 1

Chapter 1: INTRODUCTION

1.1. MADAGASCAR IN A GONDWANA CONTEXT Madagascar is the world's fourth largest island with an area of 595 000 km2. It is located on the eastern side of the African continent and separated from it by the Mozambique Channel, which is about 400 km wide. Geologically, the eastern two-thirds of the island is made up of Precambrian rocks, whereas the western third is characterized by sedimentary basins of Devonian to Recent ages. Madagascar occupied a position in the interior of the Gondwana supercontinent prior to Gondwana break-up (Fig. 1). The Gondwana supercontinent was formed at the end of a supercontinental cycle (Worsley et al., 1984; Nance et al., 1986); following rifting and break-up of an older supercontinent. This Gondwana cycle was followed by a phase of assembly during which the cratons rearranged, collided and sutured to form a new supercontinent (Hartnady, 1991) termed Gondwana (de Wit et al., 1988). The older supercontinent, termed Rodinia, that produced the continental plates constituting the components of Gondwanaland, was assembled in the global orogeny that produced the mobile belts of 1300-1000 Ma age (Unrug, 1992). The component cratons of Rodinia, containing rocks older than 1700 Ma, include the Gondwana cratons of Uweinat-Nile, West Africa, Amazonia, Arequipa, Rio de la Plata, Kalahari, Sao Francisco-Congo, Dharwar, Bundelkhand, Western Australia, Kimberley, Arunta, Gawler and East Antarctica (Fig. 1) (Unrug, 1992). The fragmentation of Rodinia was followed by the amalgamation of the Gondwana supercontinent in the Neoproterozoic tectogenetic cycle culminating in widespread orogeny between 820-540 Ma. Madagascar occupies a key position between east and 2

West Uweinar Africa Nile Madagascar •AF

- Dharwar Sao Francisco 4 Bundelkhand IC/ Amazonian SFC ANC W. Australia Congo WAS res Kimberley cast R ARC PG Antarczic 'SAN Arequipa • Gawler Arunta Rio de la Plata — Goias Kalahari

Archaean - tarty Prcterazcic

Figure 1: Schematic illustration showing Gondwana, its principal components, and the location of Madagascar. (After Unrug, 1992). 3 west Gondwana. It lies at the eastern edge of the ANEKT/Mozambique orogen [ANEKT refers to the northward extension of the Mozambique Belt into the Arabian-Nubian shield, Ethiopia, Kenya and Tanzania (LInrug, 1992, 1993)]. The ANEKT/Mozambique orogenic belt forms the suture between east and west Gondwanaland. The Mozambique Belt is a complex assemblage of Proterozoic terranes of different ages sutured during Pan- African events between 550 and 950 Ma ago. The wider Pan-African belt of ANEKT includes the Arabian-Nubian shield, the Mozambique Belt of E- Africa, Southern Madagascar, Southern India and Lutzow-Holm Bay area of East Antarctica. The entire zone of Pan-African activity has undergone amphibolite to granulite facies metamorphism during the Pan-African (Pinna et al, 1993). The term "Pan-African" was introduced by Kennedy (1964) to describe a continent-wide thermotectonic event that affected large parts of Africa, including the Mozambique Belt of Holmes (Holmes, 1951) at about 500 Ma ago. It was an Africa-wide tectonothermal event that led to the structural differentiation of the African continental crust into cratons and circumstructural or orogenic areas at about 500 Ma (Kroner, 1979). The Pan- African was preceded by "geosynclinal" sedimentation in some parts (e.g. Katangan in Zambia and Southwest Zaire). In other areas, like the Mozambique Belt (Holmes, 1951), Kennedy believed the structural differentiation only involved reactivation of pre-existing basement rocks. In either case, Kennedy envisaged the Pan-African as a time interval during which unspecified regional tectonic and thermal events affected large parts of Africa. The effect of Pan-African orogeny can be traced into other Gondwana fragments (Muhongo, 1989; Kroner, 1991). The distribution of the Pan-African mobile belts in the Gonwana has recently been described by Kroner (1991), Pinna et al. (1993) and Kriegsman (1995). 4 Geologic similarities, tectonic correlations, magnetic anomalies, magnetic satellite (MAGSAT) and other geophysical anomalies have been used to better constrain the precise links between Madagascar, Africa, India, Sri Lanka and Antarctica (Negi et al., 1986). The results of these studies reveal that the anorthosite massifs located in the charnockite gneissic belt in the eastern and southern region of the Indian shield may extend into the southern part of Madagascar (Agrawal et al., 1992), where they are represented by the Ankafotia, Saririaky, Manambahy and Volovolo bodies, which in turn may also have counterparts in NE Tanzania and SW Kenya (de Wit et al., 1995). Moreover, some Archean areas on the eastern coast of Madagascar could correlate with the Dharwar craton of India (Katz and Premoli, 1979) and the Bongolava-Ranotsara lineament of southern Madagascar may correlate with the Moyar-Bhavani, Achankovil or the Palghat-Cauvery shear zones of southern India (de Wit et al., 1995; Ghosh et al., 1996). Finally, the escarpment structure with Cretaceous volcanics and dykes along the east coast of Madagacar (Fig. 2) is similar to that associated with the Deccan Plateau basalts of the India. The first significant break-up between Madagascar and India appears to have occured at about 140 Ma (Besse and Courtillot, 1988). Rifting and spreading between Africa and Madagascar ended at about 123 Ma. At about 80 Ma, rifting took place along the eastern margin of Madagascar, giving rise to the separation from the west coast of India (Agrawal et al., 1992).

1.2. GEOLOGICAL OVERVIEW OF MADAGASCAR

1.2.1. Lithology and Stratirsay The Malagasy Precambrian (Fig. 3) has been divided by Besairie (1954) into three distinct Systems, which are inhomogeneously distributed throughout the island. 5

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ZU8CPCsa vOLC AMC S Sr, MARY (IL ANO GROUP vOLC.INIC 5 MASSIVE AMCIRTMOSITE• 0CCVRRENCES STRUCTURAL TREM01 upPER CRETICEOU3 VOLCANIC3 DeccIn TRAPS ronaLinC onfiSSE3 NICOLE PRoTEROZOIC SEOtmEnraRt gaSin SCHIS r emus wirnif.. nuctlii

Figure 2: Illustration representing some of the inferred geologic,

tectonic and volcanic correlation between the Dharwar craton

(South Indian shield) and Madagascar. (After Agrawal et al.,

1 9 9 2 ) . Figure 3: Generalized geological map of the Precambrian units of

Madagascar as defined by Besairie (1971) and modified by Boast and Nairn (1982). 7 Based on geochronological and lithological evidence, it was proposed that the NW-SE trending sinistral Bongolava-Ranotsara shear zone (BRSZ) (Hottin, 1970; Jourde, 1971) subdivides the Precambrian basement into central-northern and southern sectors, each of which can be further subdivided into six tectonic belts and three zones, respectively (Windley et al., 1994). The central-northern sector consists of a high grade zone dominated by rocks of granulite and amphibolite facies, migmatites, conformable granite sheets and granite plutons (Nicollet, 1990). On either side of the high grade zone occur tectonic zones with small areas containing rocks that preserve greenschist facies assemblages. The central northern sector can be subdivided into two series (Hottin, 1976; Vachette, 1979). The Antongilian series, older than 3000 Ma (Vachette, 1979), is characterized by granitoid migmatites and gneisses, mostly confined to the eastern part of the island. The Andriamena series, on the other hand, is composed of amphibolites, mica schists and ultrabasites and might be about 2600 Ma in age (Vachette, 1979). These ages, however, are based on old Rb-Sr data, and must be corroborated by modern geochronology. The southern sector, considered by some to be Late to Mid- Proterozoic in age (Hottin, 1976; Vachette, 1979), consists of three Systems (Besairie, 1967; Nicollet, 1983). From the west to east, these are the Vohibory, Graphite and Androyan Systems. The Androyan System is located in the eastern part of southern Madagascar. Stratigraphically, it consists from the base to the top, of the Fort-Dauphin, Tranomaro and Ampandrandava Groups (Table 1). In this System, intermediate-pressure granulite facies conditions prevail (Nicollet, 1990) (Table 2) (Fig. 4). \.(7 rf-s-C1,

ZCS t-

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Figure 4: Map of the metamorphic zones and facies of the Precambrian

of Madagascar, modified by Windley et al., 1994 (After Hottin,

1 9 7 6). 9

Table 1: Subdivisions of Precambrian of Madagascar (oldest at base). After Besairie, 1954.

GROUPS LITHOLOGIES

VOHIBORY SYSTEM AMBOROMPOTSY Orthoamphibolites, MAEVATANANA para-amphibolites, BEFORONA marbles, chlorite schists, V OHIBORY talc schists, pyroxenites.

GRAPHITE SYSTEM MANAMPMSY Graphite schists, ANDRIBA garnet leptynites, AMBATOLAMPY marbles, quartzites, AMPANIHY gneisses, migmatites, mica schists.

ANDROYAN SYSTEM AMPANDRANDAVA Garnet-cordierite leptynites, TRANOMARO pyroxenites, marbles, FORT-DAUPHIN quartzites, gneisses.

Table 2: Metamorphic facies of southern Madagascar

VOHIBORY GROUP AMPANIHY GROUP ANDROYAN SYSTEM

AMPHIBOLITE FACIES GRANULITE FACIES GRANULITE FACIES

9 - 11.5 kbar 7 - 9 kbar 5 kbar 750°C - 800°C 710°C - 890°C 800°C (Nicollet, 1990) (Nicollet, 1990) (Nicollet, 1990)

The Graphite System crops out between the Androyan and Vohibory Systems and from the base to the top, it consists of four groups: Ampanihy, Ambatolampy, Andriba and Manampotsy, with various lithologies (Table 1). Nicollet (1990) stated that the Ampanihy Group preserves conditions of high-pressure granulite facies (Table 2). 10 The Vohibory System is the westernmost of the three Systems of the southern sector. It comprises from the base to the top, four groups which are: the Vohibory, Beforona, Maevatanana and Amborompotsy Groups mainly characterized by metabasites (Table 1). The Vohibory Group is characterized by metamorphic conditions near the amphibolite-granulite facies transition (Nicollet, 1990). On both sides of the BRSZ are located the Itremo (SQC) and Amborompotsy-Ikalamavony Groups. The Itremo Group, affected by greenschist facies and amphibolite metamorphism (700-800 Ma, Cox et al., 1995) in the east and west-centre, respectively, consists of quartzites, metapelites, marbles, all spatially associated with igneous rocks (amphibolites, gabbros, granites) (Cox et al., 1996) (Fig. 4). The SQC is the largest and best preserved Precambrian sedimentary group in Madagascar (Moine, 1974), U-Pb SHRIMP studies of detrital zircons indicate it is of Mid- Late Proterozoic in age, between 700-800 Ma and 1800 Ma (Cox et at, 1996). The Amborompotsy-Ikalamavony group was subjected to amphibolite facies metamorphism (Moine, 1974) and consists of gneisses, mica schists and marbles.

1.2.2. Precambrian structural features Structurally, the Precambrian crystalline basement located to the north of BRSZ is characterized by ten N-S trending lithotectonic units or belts (Fig. 5) (Hottin, 1976). These structures present different trends, which are NNE south of Antananarivo, from N-S to NNW at the latitude of Maroantsetra, and NW in the north. Windley et al. (1994) stated that the centre of Madagascar is traversed by a 100 km wide N-S trending sinistral shear zone (of probable Pan-African age) that transects Proterozoic and Archean basement. This sinistral shear zone is defined by the prominent regional drag on the western side. 11.

46° so 12°

Amsoranana

140

Maiunga 9 160 rrCS Maroantsetra

Tamatave

Mananarivo

20°

1. Bongolava belt.

2° .2. Maevatanana belt Ardriba-Arnbatolampy-Tolongoina ben Andriamena belt . Mampikony-Anjaty-Vondrozo bert. Tuliar Ampasary-Eletorona-Alaotra-Andipna t Betody-Brickaville-Ankaizina ben . Vavatenina-Sanantaha-Sambirano belt 9. Ivontaka-Antongil belt BRSZ \ 10. Bemarivo-Ampanelena-Mananjeoa oak Fort-Dauphin

0 60km BRSZ: Bongolava-Ranotsara shear zone ASZ: Ampanihy shear zone VSZ: Vorokafotra shear zone

Figure 5: Generalized map showing the main Precambrian structures

of Madagascar (Modified from Hottin, 1976). 12 A striking structural feature of the Malagasy Precambrian basement is the sinistral Bongolava-Ranotsara shear zone (BRSZ) (Fig. 5), which trends NW-SE. Its continuation in the Mozambique belt of Tanzania might be represented by the Surma shear zone, and in southern India by the Achankovil shear zone (Windley et al., 1994). According to Ghosh et al., 1996, the BRSZ might be linked up with either the Achankovil or the Palghat-Cauvery shear zones of southern India. This precise linking of the shear zones has not yet been solved, and is part of ongoing geochronology work, both in Madagascar and in India (Ghosh et al., 1996). In central Madagascar, the Itremo Group is characterized by a series of recumbent folds and thrusts separated by shear zones. The deformation was synchronous with the metamorphism for the eastern part of the belt and post-dated it for the western part (Cox et al., 1995). South of BRSZ, two prominent shear zones have been recently distinguished. These are the Ampanihy shear zone (ASZ), which separates the Vohibory from the Ampanihy Group, and the Vorokafotra shear zone (VSZ) which separates the latter from the Ampandrandava Group. They generally trend N-S (Fig. 5). The Ampanihy Group includes four anorthosite bodies; the structure and emplacement of two of them (the Ankafotia and Saririaky massifs, located in the ASZ) have been interpreted as either: (i) megaboudins resulting from simple shear, and (ii) sheath-folds, folded by pure shear flattening (Kilmer and Duncan, 1990; Rolin, 1991; de Wit et al., 1993). The present study is based, in part, on providing a critical evaluation of these different hypotheses. Based on lithostratigraphic and structural evidence, Windley et al. (1994), divided the southern sector of Madagascar into six different tectonic belts separated from each other by ductile shear zones. These belts are from west to east: the Vohibory, Ampanihy, Bekily, Betroka, Tranomaro and Fort-Dauphin-Anosy belts. 13 All statements mentioned above concerning Malagasy Precambrian geology are still vague. For example, the map of metamorphic zones and facies established by Bazot et al. (1971) and Hottin (1976), did not take into account retrogression or possible successive metamorphic overprints to which the Precambrian basement has been subjected. Age determinations gist for Madagascar, in contrast with the situation in Mozambique, Kenya and Tanzania, but many of these are Rb-Sr ages and which record "reconnaissance ages" only (Vachette, 1979). Precise geochronological data on the Precambrian are sparse and most of the published age determinations were summarized and critically evaluated by Cahen et al., (1984). Moreover, the influence of the Pan-African tectonism and granulite metamorphism is very widespread throughout the Malagasy Precambrian basement. Hence, primary ages of the rocks and earlier metamorphisms have probably been reset. Moreover, the lack of sufficient structural studies is a handicap in the interpretation of the numerous ages obtained. However, recent investigations have been undertaken concerning Precambrian lithology and petrology (Ashwal et al., 1993; Cox et al., 1994), structures and tectonics (de Wit et al., 1993; Windley et al., 1994; this study), and geochronology (Paquette et al., 1994) to unravel the geologic and tectonic history of Madagascar.

1.3. GEOGRAPHICAL SETTING OF THE STUDY AREA The study area is located in southwestern Madagascar (Fig. 6), between approximately 23°49 S and 45°06 E and 24°22 S and 44°47 E. The region is of middle high relief (highest point = 520 m) composed of subparallel ridges which embody the two bodies. It is a semi-desert region with scattered vegetation dominated by thorny bushes. Antanosy and Mahafaly are the local people with subsistence farming as their main occupation. 14

440 460 480

STUDY AREA

Fort.Dauphin

), Tulear

STUDY AREA

I Anorihosite body

Fort-Dauphin

50krn

Figure 6: Map of the southern Madagascar showing the location of the study area, and the locations of the Ankafotia and Saririaky anorthosites that bound the study area to the north and south, respectively. 15 The main drainage in the northern area (Ankafotia area) is through the Vatanalialy and Beamalo Rivers, which flow northwards along the western side of the Ankafotia massif. The Kafotia River crosses the massif from the southeast to the northwest to join the Vatanalialy River. Small roads and tracks forking from the main Bekily-Fotadrevo road penetrate east-west into the area (Fig. 7). The southern area (Saririaky area) is drained only by the Manakaravavy River and its tributaries, which cross the anorthosite body flowing westwards. Smaller roads and tracks across the area link the main villages Ampanihy, Maniry, Bokonaky, Ankilimivory and Bekily located in and around the massif, facilitating access to the area (Fig. 7).

1.4. GEOLOGICAL SETTING AND STRUCTURAL FRAMEWORK OF THE STUDY AREA 1.4.1. Geology of the Graphite System Rocks of the Graphite System (Fig. 8) are affected by granulite facies metamorphism (Nicollet, 1988) and are characterized by the presence of graphite in all its units. The System has been subdivided as shown in Table 3 (Besairie, 1954; Hottin, 1976) as follows : 23°40

th•

kftircyllv 230 I '1 1•JF.V11.."4 ko3 so

11 1- Pgai.

24°C0 it Study area Fwasr.-3,

24°10

,44. .] 4e;<# e2 .G°99939° • f rg?

eet •

44 40, 450oa 45°10

/ Road Anorthosite body 0 5 KM Vohibory Series 41 Big village Atnpanihy Series

Recent formations Afripandrandava Series

Figure 7: Geologic map showing the study area in its regional

surroundings. Modified after BoUlanger et al., 1958. 17

0 60 km

Figure 8: Map showing the geographical distribution of the Graphite

System. (After Besairie, 1973). 18

Table 3: Graphite System: Groups and lithologies

GROUPS LITHOLOGIES

AMPANIHY (southwest) * Pyroxene gneisses, garnet leptynites, marbles, quartzites, migmatites, * In study area graphite gneisses, amphibolites.

AMBATOLAMPY (centre) Graphite-sillimanite gneisses, migmatites, rare marbles, mica schists, quartzites.

MANAMPOTSY (east) Garnet-sillimanite gneisses, leptynites, quartzites, marbles, migmatites.

ANDRIBA (northwest) Sillimanite-garnet gneisses, quartzites, migmatites.

South of the BRSZ, the Graphite System is represented by the Ampanihy Group, which is composed of gneisses, leptynites and marbles, and includes four anorthosite bodies. To the west and east, the Ampanihy Group is bounded respectively by the Vohibory and Ampandrandava (Benato-Horombe) Groups (Fig. 7 and Fig. 9). The Graphite System in the study area is mostly isoclinally folded and overturned to the east. Fold axis trends range from N-S to N 25° E, but local deviations occur. Dips of foliation planes generally vary between 45° W to subvertical. A shear-zone of NNE-SSW trend, termed the Ampanihy shear-zone (ASZ), is located between the Vohibory and Ampanihy Groups. Four anorthosite bodies have been recognized within the Graphite System: 19

--so

PRECAMBRIAN GEOLOGY OF SOUTHERN MADAGASCAR

.21)

Recent formations

Vohibory GIOUD

-240

Ampanihy Group

\ \ - \ \ Ampandrandava Grouc ti i I • I.! .Tranomaro Group

J Fort Dauchin Grouts Fort-Dauphin Archean and Katarcnean

Anorthosite-body

BRSZ: Bongolava-Ranotsara shear zone 0 501tm

Figure 9: Generalized map showing the Precambrian geology of the

southern sector of Madagascar. (After Hottin, 1976). 20 Ankafotia, Saririaky, Manambahy, Volovolo, ranging in size from 25 to 100 km2

1.4.2. Geology of the Ankafotia and other anorthosite bodies in the region The term "anorthosite body" is preferred over "anorthosite massif" in the text, because the term "body" implies a single intrusive unit, which appears to be the case for the Ankafotia and Saririaky anorthosites. The term "massif" implies a larger, composite plutonic complex, with several individual components. The four anorthosite bodies mentioned above occur in the Ampanihy Group (Fig. 5, 6, 7 and 10). The Ankafotia and the Saririaky bodies are located in the western part of the Group, while the Manambahy and Volovolo bodies occur in the eastern part (Fig. 6 and 7). Stratigraphically, the four bodies occur in the same unit of the Graphite System, i.e. the Ampanihy Group (Fig. 9). Ankafotia is the northernmost of anorthosite bodies in the area. It has an almond-shape of 16 km in length and 6 km in width (Fig. 10 A), and consists of an anorthositic core surrounded by leuconorites. Secondary rock types are pyroxene-garnet anorthosite and gabbronorite. Outcrops of anorthosite consist mostly of plagioclase crystals reaching sizes up to 10 cm, and with garnet and pyroxene up to 2 cm (Morel, 1996). The Saririaky anorthosite occurs 40 km to the south of the Ankafotia body. It is a subcircular body, 11 km in diameter (Fig. 10 B), consisting of a coarse-grained anorthositic core passing to the south into a leuconoritic gneiss that dips moderately to the north. Mapping as part of this study carried out in the envelope of the massif shows different lithologies: leptynites, graphite schists, marbles, quartzites, garnet gneisses, all of which dip steeply to the west and wrap around the anorthosite. 21

ANKAFOTIA MASSIF SARISIAKY MASSIF

A B

• • r J " -3r=, •

LEGEND

C0ost1ve ma rms. le IMCcrner os,ie E._71.0scncri re i C.I. 10 - is t! I Lncmait, i C.I. 20 - 5krn 40-4Ct react '101.00103a -=:- Crew, Pe oecnol recxs Anorthosite BOULANGER, 1959 Leuconorite MOREL, 1996 ;ionrses

MANAMBAHY MASSIF VOLOVOLO MASSIF C D

•

- arVIC-1,1-Thca÷

3 krn ••• ------• Anorthosite BOULANGER, 1959 yr.d: 7.erz Leuconorite Anorthosite 0 ;i0, BOULANGER, 1959

Figure 10: Schematic maps showing rock types constituting the four anorthosite bodies of southwestern Madagascar. (After Boulanger, 1959 and Morel, 1996). 22 The Manambahy anorthosite body has an elongated shape, 9 km in length and 3 km in width (Fig. 10 C). The anorthosite and leuconorite rocks are generally paler in colour than the other bodies. The margin is almost completely unexposed (Boulanger, 1959). The Volovolo anorthosite body has a figure-eight shaped outcrop pattern (Fig. 10 D). Its northern part has a globular shape 5 km in diameter, consisting of leuconoritic rocks surrounded by a thick anorthosite margin. The southern part has a subcircular form, 4 km in diameter. Its almond- shape core is anorthositic while the envelope is leuconoritic (Boulanger, 1959). The leuconoritic rocks are characterized by increasing abundance of pyroxene (hypersthene, diopside), garnet and ilmenite, indicating a gradational relationship between the anorthosite massifs and their gneissic envelopes.

1.5. RECENT INVESTIGATIONS Renewed interest in the Malagasy Precambian has yielded some recent new geochronological and geological data from the study area and surroundings (Table 4). Two major ductile shear zones trending NNE-SSW, the Ampanihy and Vorokafotra shear zones (Rolin, 1991), occur in the area. The Arnpanihy shear zone occurs to the west within the Ampanihy Group. The Vorokafotra shear zone, to the east separates Ampandrandava Group from the Fort-Dauphin Group (Fig. 11).

1.5.1. General structure of the Ampanihy shear zone The Ampanihy shear zone (Rolin, 1991) is a large ductile shear zone 10 to 25 km wide, and about 250 km along strike, developed mainly in granulite facies rocks of the Ampanihy Group. It separates the Vohibory and 23 Androyan sequences into two tectonometamorphic terranes (de Wit et al, 1993) and includes the Ankafotia and Saririaky anorthosite massifs (Fig. 11), which are separated from each other by about 40 km (Fig. 12). On either side of the shear zone, the Vohibory and Ampandrandava terranes display large recumbent folds. Table 4: Recent geochronological and thermobarometric data from southern Madagascar

VOHIBORY GROUP AMPANIHY GROUP ANDROYAN SYSTEM

AGES (Ma) (Method)

520 Late Pan-Afr. activity (4) (U-Pb/zircon; Sm-Nd)

550- 600 Granulite facies (U-Pb/zircon) metamorphism (3)

560-580 Tranomaro Group (4) (U-Pb zircon; Fort-Dauphin leptynite (4) Sm-Nd) Anosyan chamocicite (4) Vohimena granite (4)

640 Garnet-biotite schist (U-Pb/monazite) (3)

645 Garnet psammite (3) (U-Pb/monazite)

828 Opx amphibolite (3) (U-Pb/monazite)

900- 1000 Ankafotia and (Sm-Nd) Saririaky crystallization (3)

952 Cordierite gneiss (3) (U-Pb/monazite)

METAMORPH. 750 - 800°C 710 - 890°C 850°C CONDITIONS 9 -11.5 kbar (1) 7 - 9 kbar (1) 5 kbar (1) (2)

Nicollet, 1990 (3) Ashwal et al., 1996 Nedelec, 1992 (4) Paquette et al., 1994 24 • o f

ASZ: amoanthy shear zone

VSZ : vorokalotra snear zone

Ankafotia massif Saririaky massif

ASZ vsz

4,1 I moaniny 1 1'6' ti / Li"lire 41 Ill

Ins Is 1/1/ 1 7 7

J — • '31 ,0 -

r II lir. S1 aiitit eihr nrAiMAC AN? 41!::.".

, LANDSAT THEMATIC MAPPER (SCENES 160 - 76/77)

SATELLITE IMAGE INTERPRETATION OF THE STRUCTURAL ELEMENTS OF THE PRECAMBRIAN BEDROCK, SW MADAGASCAR. (De Wit, 1992) 50 Km 0

Figure 11: Illustration showing the location of the shear zones in SW

Madagascar. (After de Wit, 1992; unpublished).

0 14 km

Figure 12: Satellite image showing the two anorthosite bodies in the Ampanihy shear zone. (Landsat thematic mapper, scenes 160-76/77). 26

Chapter 2: OBJECTIVES OF THE PRESENT STUDY

Two hypotheses have been proposed concerning the structural evolution of the Ankafotia and Saririaky bodies (Fig. 13). The first considers them as megaboudins, resulting from simple shear deformation, and the second considers them as sheath-folds, resulting from pure shear flattening. Based on satellite imagery and subhorizontal lineations recorded in the field (Rolin, 1991), Kilmer and Duncan (1991), Nicollet (1990), Windley et al. (1994), proposed that the two anorthosite bodies show structural characteristics of boudins in a ductile matrix, and have been pulled apart horizontally in a dextral shear zone by about 40 km in the Ampanihy Group. Based on remote sensing analysis and field data, Martelat et al. (1995) proposed sinistral shear zone. The theory stated that an anorthosite sill with its host rocks were subjected to a shear stress ai within the sinistral

Ampanihy shear zone. The effect of the stress was simple shear deformation in the host rocks, with contemporaneous boudinaging of the anorthosite due to its higher competence in comparison with the host rocks (graphite-bearing rocks and graphite schists). However, recent detailed structural mapping (Morel, 1996 and this study) has. demonstrated the existence of fold closures situated respectively in the north and south of the Ankafotia massif, which indicates that the host rocks wrap around the anorthosite. This is inconsistent with a simple shear and boudin hypothesis. Alternatively, de Wit et al (1993) interpreted the anorthosite bodies as eastward verging Di. / D2 sheath folds that were rotated into the vertical during intense flattening (i.e. pure shear deformation) (Fig. 13). Morel et al. (1995), supported an eastward thrusting of sheets of metasediments underlain by anorthosite across an earlier crustal-scale L/

(A): BOUDINS host rock not folded round Anorthosite

-present day surface

expected stretching lineations in all directions

, (B): SHEATH FOLDS

present day surface

expected stretching lineations (unimodal)

Figure 13: Sketch illustrating the two models for the structural evolution of the Ampanihy shear zone, modified after Nicollet (1990), Bolin (1991), Kilmer and Duncan (1991), Windley et a I. ( 1 9 9 4 ), modified after de Wit et al. (1993) and Morel ( 1 9 9 6 ) . 28 ramp. The thrust stack underwent shortening, with flattening concentrated across the rotating ramp. The aim of the present study is to try to distinguish between the different models mentioned above, and to define and establish the structural and tectonic style of the area, firstly by detailing structures and microstructures in the rocks, and secondly by compiling a structural and tectonic map of the area from the foliation and lineation orientations recorded in the field. Thus, this thesis was conceived as a study of the structural and tectonic features of the Ampanihy Group predominantly in the vicinity of the Ankafotia and Saririaky anorthosite bodies. The study forms a part of the Gondwana Reconstruction Project that was initiated in the second half of 1980s to investigate the dynamic history of the Gondwana. The study is, therefore, a contribution towards the understanding the history of the Mozambique Belt of East Africa and an update of the Precambrian geology of Madagascar. 29 Chapter 3: METHODS OF STUDY

In order to realize the objectives of the work, the thesis first involved the preparation of a geologic map presenting lithologies and structural data. To facilitate the description, the study area has been subdivided into three sectors: the Ankafotia area to the north, the middle and the Saririaky area to the south (Fig. 6). The field work was carried out during two periods. The first was in 1993 for two months (May-June) and essentially focussed on the northern area (Ankafotia area). The second period lasted six weeks (July-August 1995) and was spent investigating all the three areas but concentrating on the southern one (Saririaky). Detailed mapping has been done for the northern and southern areas. Reconnaissance only has been done for the sector between them.

3.1. MAPPING Detailed mapping of the country rocks surrounding the two anorthosite bodies was carried out using aerial photographs on scale of 1:40,000 and topographic maps (Ianapera sheet, H 59; Ampanihy sheet, G 61) at 1:100,000, enlarged to a scale of 1:12,500. Orientations of foliations and lineations were recorded, allowing the compilation of lithostratigraphic and structural maps of the areas on a scale of 1:12,500 (see maps, appendix). The stratigraphic map distinguishes all rock types exposed in the area and shows their structural features.

3.2. SAMPLING Samples from different units were collected from locations distributed throughout the study area. They were studied petrographically, 30 focusing on metamorphism and microtextures produced by the deformational events.

3.3. PETROGRAPHY The study area is characterized by varieties of rock types. However, petrographic observations using thin sections show that common features exist between them. Most of the lithologies, including leptynites, gneisses, graphite schists, and quartzites are mylonitized. They display ribbon-like quartz and feldspars as a distinct and common feature. Quartz and feldspar usually show undulose extinction, and amphibole (hornblende) and pyroxene (hypersthene) respectively, are pleochroic from dark-green, pale- brown to brown and from pale-green to green and pink in plane polarized light. Perthites are visible in most of the leptynite thin-sections. The Albite and Carlsbad twinning laws are preserved in all plagioclases. Garnet is generally anhedral, porphyroclastic, and has abundant quartz, feldspar and/or pyroxene as inclusions. Most of the rocks in the region exhibit granoblastic polygonal textures, indicative of late post-tectonic recrystallisation.

Definition of gneiss: Gneiss is a medium-to coarse grained metamorphic rock with compositional layering (foliation) termed gneissose banding or gneissosity as a characteristic feature. The compositional layering is similar to bedding but of metamorphic or deformational origin. In the petrological descriptions, the grain size definitions referred to in the text are as follows (after Kretz, 1994): 1/8 - 1 mm = fine-grained 1 - 4 mm = medium-grained 4 - 16 mm = coarse-grained 32

Figure 14: Photomicrograph of pyroxene gneiss showing plagioclase (p1), hypersthene (opx), quartz (qtz), K-feldspar (kf) and garnet (grt). Crossed Nicols. Long dimension of photo is 1 cm. Sample R 22. Locality 1, I: 24°33.64'; L: 44°54.90'.(Fig. 99).

Qer O4 1 ". r.- • z ---- •

Figure 15: Photomicrograph of graphite gneiss showing graphite flakes (black) curved by D3. Polarized plane light. Long dimension of photo is 1 cm. Sample G 20, cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Locality 2, I: 23°56.47'; L: 45°08.90'. (Fig. 99). 34

Figure 16: Photomicrograph of sillimanite gneiss showing sillimanite (high birefringence) relicts (sil) surrounded by garnet grains. Sample G 428. Crossed Nicols. Long dimension of photo is 1 cm. Locality 3, 1:23°52.32'; L: 45°10.80'. (Fig. 99).

Figure 17: Photomicrograph of garnet amphibolite showing retrogression structure (corona texture) consisting of garnet core (grt) surrounded by plagioclase and hypersthene intergrowth. Sample G 163. Crossed Nicols. Long dimension of photo is 0.70 cm. Locality 4, I: 23°52.37'; L: 45°06.66'. (Fig. 99). 35 3.3.7. Quartzite These leucocratic rocks are fine to medium-grained (0.5-1 mm), foliated, and consist generally of quartz (70-85%), K-feldspar (5-15%), and garnet (10-15%). Garnets are very conspicuous and usually define the foliation (e.g., sample G 25). Some of the quartzite outcrops in Saririaky are garnet-free and are, therefore, poorly foliated.

3.3.8. Pyroxenite This rock type is massive and melanocratic, having a granoblastic texture, and general grain size ranging between 1 mm and 2 mm. It is predominantly composed of clinopyroxene (70-80%), plagioclase (3%) with compositions between Ann and An14, amphibole (5%), garnet (10%), biotite (3%), and minor quartz and Fe-Ti oxides (1%) (e.g., sample G 511).

3.3.9. Ma These rocks are generally massive, granoblastic, medium to coarse- grained (2-10 mm), and poorly foliated. They comprise dolomite (40-50%), calcite (20-30%), serpentinized forsterite (10-15%), pyroxene (7%), minor garnet (1%) and occasional graphite (e.g., sample G 1) (Fig. 18). Calcite and dolomite commonly show oblique twinning. In some of the outcrops, mafic minerals are absent.

3.3.10. Leuconorite This rock type is granoblastic, inequigranular, mesocratic, foliated and coarse-grained (4-25 mm). Commonly, pyroxene and garnet are present as porphyroblasts in the feldspathic matrix. Leuconorites contain plagioclase (70-80%) with compositions ranging from Anzio to Ana), hypersthene (15-

20%), garnet (10-15%) and ilmenite (3%). 36

Figure 18: Photomicrograph of marble sample GC 1, showing serpentinized forsterite (fo) and deformation twins in calcite (cal). Crossed Nicols. Long dimension of photo is 1.5 cm. Locality 5, 1:45°07.19'; L: 23°56.60'. (Fig. 99). 37 3.3.11. Pegmatite Commonly, this rock type is leucocratic, granoblastic with general grain size over than 10 mm. It consists of K-feldspar (50%), quartz (30%), symplectites (15%) and minor plagioclase (3%) (e.g., sample G 462).

3.3.12. Mylonites In the Ankafotia area, two varieties of mylonite have been identified in the field. Commonly, their matrix is fine-grained (0.2-1 mm) and includes K-feldspar porphyroblasts ranging in size between 2 mm and 20 mm. The first is a leucocratic type containing quartz (40-50%) and K-feldspar (30-40%) with subordinate plagioclase (1%) (e.g., sample G 148). The second is a mesocratic type consisting of quartz (35-40%), biotite (20-25%), plagioclase (An5-7) (7%), graphite (10-15%), garnet (5%) and pyroxene (5%)

(e.g., sample G 341). . 38

Chapter 4: HELD OCCURRENCES OF MAJOR LITHOLOGIES

The dominant lithologies of the Ankafotia and Saririaky envelopes include pyroxene-bearing quartzo-feldspathic gneisses, representing 60% of the entire area, with numerous intercalations of leptynites, amphibolites, graphite schists, and leuconorites. Commonly, these units are structurally conformable to each other, and repetitively interlayered.

4.1. Pyroxene gneiss These rocks strike from N 15° W to N 25° E throughout the area (Map 1 and Map 2, app.) and dip between from 60° W to subvertical. The rocks contain considerable modal amphibole in the northwestern part of the Ankafotia area. The other rocks (described below) are interlayered within this pyroxene gneiss on a metre scale.

4.2. Graphite schist Graphite schists are very prominent in the northern and southern parts of the Ankafotia anorthosite body (Map 1, app.). Along the southern margin of the anorthosite, they define a fold closure and strike E-W at the hinge. Along the eastern and western sides of the anorthosite, these rocks dip steeply (70°) to the west. Along the northern margin, graphite schists around the fold closure are not well exposed. Graphite schists are very prominent exposures in the area and can be up to 80 m in thickness. Other extensive layers occur in the eastern and northeastern margins of the anorthosite body, varying between 2 m and 40 m in width. In the Saririaky area, this rock type crops out mostly along the western edge of the body, 39 with thickness ranging between 5 m and 25 m (Map 2, app.). Graphite constitutes up 80% of some layers.

4.3. Amphibolite gneiss Layers of amphibolite gneiss are from 1 to 5 m wide, regularly alternating with other rock types. They are prominent in the eastern and western parts of Ankafotia area (Map 1, app.) but poorly visible in Saririaky area (Map 2, app.). Some of the exposures show this rock type extremely folded. Commonly, they dip toward the west, steeply to subvertical.

4.4. Leptynites Prominent outcrops of leptynite are visible all around the anorthosite bodies (Map 1 and Map 2, app.); the orientations of their foliations are parallel to that of the other rock types. Layers range from 2 m to 25 m in width. This rock type makes up more than 15% of the total country rock sequence.

4.5. Sillimanite gneiss This rock type is well exposed along the eastern margin of the Ankafotia anorthosite massif (Map 1, app.), and possesses a prominent reddish colour due to iron oxide staining. In thickness, it ranges up to 10 m in width. Dips and strikes of foliation of this rock are conformable with the other layers.

4.6. Garnet amphibolite Outcrops of this rock type are uncommon. Layers up to 2 m wide are structurally conformable to the other rocks. They are mostly confined to the western sector of the Ankafotia area. 40

4.7. Quartzite In the Ankafotia area, prominent exposures of quartzite ranging from 2 to 8 m in thickness are located within the southern fold closure, dipping steeply to the south and bounding the graphite schist to the north (Map 1, app.). In the Saririaky area, quartzites are mostly confined to the northern margin of the anorthosite body (Map 2, app.) and generally dip steeply to the west Quartzites are up to 20 m in width.

4.8. Pyroxenite Garnet pyroxenite outcrops are uncommon. The only visible outcrop of this rock type is located southeast of Ankafotia body where it reaches up to 3 m in width.

4.9. Marble Marbles are exposed as prominent ridges around the Ankafotia anorthosite body, especially in the east, west and north (Map 1, app.). They are up to 50 m wide and conformable with the regional trend. They display spectacular minor folds defined by thin layers rich in forsterite. Other exposures show them wrapping around amphibolite and gneiss boudins. In the Saririaky area, they make up prominent ridges located in the northeastern sector and are up to 5 m thick in the southern sector (Map 2, app.).

4.10. Leuconorite This is the unit immediately adjacent to the Ankafotia anorthosite body and displays a mineralogical composition varying between anorthosite and leuconorite or norite, i.e. the modal orthopyroxene and garnet increase 41 towards the margin of the anorthosites. Leuconorite exposures are widely distributed around both the Ankafotia and Saririaky bodies and range in width from 2m to 20 m. Their orientations are conformable to the country rocks.

4.11. Pegmatite The only prominent pegmatite occurences of the area are located along the eastern and western sides of the Ankafotia anorthosite body (Map 1, app.), striking N-S and NE-SW, i.e. more or less conformable to the other units. Their thicknesses range between 3 m and 10 m.

4.12. Mylonites This rock type is very prominent in the northwestern part of the Ankafotia massif (Map. 1, app.), ranging from 0.2 m to 1.5 m in width and structurally conformable in dip and strike to the other rocks. In the Saririaky area, mylonites are very prominent along the western margin of the anorthosite body, as layers varying between 2 m and 30 m in thickness (Map 2, app.), dipping steeply to the west. 42 Chapter 5: STRUCTURAL GEOLOGY

The Ankafotia and Saririaky envelopes consist of extremely deformed rocks and to constrain the structural feature of these areas, they have been subdivided into 8 and 5 structural domains, respectively. In structural analysis, it is particularly important to subdivide the area into homogenous domains with respect to the attitudes of the linear elements (Whitten, 1969). Definitions Cleavage: All types of mesoscopically recognisable layering of metamorphic origin, such as axial planar surfaces are denoted Si to Sri, where the subscript denotes the relative order of development. Layering: Layering is the characteristic feature of gneissic rocks. It is a survival of primary structure of sedimentary or igneous origin or may be of secondary origin and the result of either intense deformation or solid state metamorphic differentiation. Lineation: These are linear structures that are penetrative at the scale of hand specimens or in small exposures. They comprise S-surface intersections (intersection lineations), or preferred orientation of elongate mineral grains (mineral stretching lineations). Larger linear structures include fold axes of megascopic folds. The designation Li to L n is applied to lineations, and Fn to fold axes, with the subscript denoting the relative order of development. A summary of this terminology is as follows:

Deformation phase D2 D3 D4 Foliation and axial foliation S2 S3 Fold axis F2 F3 Associated L-lineation L2 L3 Associated metamorphic event M2 M3 M4 43

All structural data shown on stereographic projections have been plotted on the lower hemisphere of a Schmidt equal area net. H-diagrams have been used rather than I3-diagrams, which are limited in their application. The advantage of the IT-diagram method is the simplicity with which large amounts of data may be incorporated into the analysis. The method is also more accurate to obtain statistical criteria such as best fit great circles, and to locate centres of maximum concentration. Refolding of earlier axial planes, foliations and lineations, and porphyroblast-foliation relationships (c.f. Spry, 1969), are the criteria used to determine the succession of various deformation phases.

5.1. MESO-TO MICRO-SCALE FEATURES

5.1.1. Foliations Most of the outcrops, with the exception of marble, garnet pyroxenite and garnet 'amphibolite exhibit very prominent foliation planes (S- tectonites) due to the intense oblate deformation undergOne by the rocks. The foliations are usually defined by the parallel alignment of alternating mafic (garnet or amphibole) and felsic (quartz and K-feldspar) layers up to 5 cm in width (Fig. 19). Their orientations are different in each domain. Foliations and layering on both sides of the anorthosite bodies (Map 1 and Map 2, app.) have approximately the same orientations, i.e. they are W- dipping steep to subvertical (Fig. 20), and striking from N 25° W to N 25° E. The foliations of rocks in the northern and southern parts of the areas display mostly the same attitudes, with exception of the southern Saririaky body where foliations mostly strike E-W and dip northwards (Map 2, app.). 44

Figure 19: Photograph of banded garnet gneiss interlayered with amphibolite. Individual layers (foliae) are up to 5 cm in thickness. Outcrop 1.8 km east of Seta village. Field of view is approximately 4 m. Locality 6, I: 24°06.32'; L: 45°02.27'. (Map 3 , app.).

Figure 20: Photograph of W-dipping flattened marbles. Outcrop 2 km east of Seta village. Field of view is approximately 10 m. Locality 7, I: 24°06.72'; L: 45°02.58'. (Map 3, app.). 45 The rocks in both areas are moderately to steeply west-dipping all around the anorthosite bodies. Commonly, strikes range from N 25° W to N 25° E wrapping around the bodies. Dips to the east are present, but are not abundant. It is clear that under the conditions of extreme rock deformation, most of previous structures were destroyed or so modified that their original nature cannot be ascertained.

5.1.2. Lineations and minor fold axes Two types of lineations have been distinguished on foliation planes: intersection and mineral elongation (stretching) lineations. Intersection lineations (Fig. 21a, 21b) representing about 97% of all observed lineations, are chiefly defined by quartz, amphibole or pyroxene, whereas mineral elongation (stretching) lineations (Fig. 22) are commonly defined by garnet, and are very conspicuous within leptynites. Trends of the intersection lineations are different for each domain. Their plunges vary from shallow to moderate, trending northwards or southwards (Map 1 and Map 2, app.) depending on the domain (Fig. 23). As a first approximation, fold axes and other linear features are parallel to one another wherever they can be observed together in the field. Where the.axial plane can be observed, the linear fabrics are subparallel to the fold axis within this plane. This relation suggests that the linear fabrics mostly represent the intersection of two planes (fold axial planes and bedding-foliations) that have been flattened (Fig. 24a, 24b). Boudinage is a prominent feature along the western margin of the Saririaky body (Fig. 25). The boudins generally consist of garnet amphibolites within gneisses, and are always parallel to the local fold axes, showing long, rod-like structures. Their plunges vary from moderate to steep, conformable with that of the adjacent fold axes. 46

Figure 21a: Photograph of intersection lineation (accentuated by quartz and garnet) moderately plunging to the north. Outcrop 2.1 km southeast of Vohibe village (North of Saririaky). Field of view is approximately 3 m. Locality 8, I: 24°24.79'; L: 44°56.99'. (Fig. 100).

Fold axis axial planar foliation

edding-plane

---- .---- ------intersection lineation -..-■ __-- ....------.------.- .------— ----

Figure 21b: Diagrammatic sketch showing planar and linear fabrics on the flattened leptynite outcrop in Fig. 21a. 47

Figure 22: Photograph of elongation mineral (garnet) lineation, plunging moderately to the south. Garnet leptynite. Outcrop 0.8 km east of Ankafotia body. Pen 14.5 cm, is aligned N-S. Locality 9, I: 23°53.48'; L: 45°11.25'. (Fig. 99).

Figure 23: Photograph showing intersection lineation (accentuated by quartz and amphibole) moderately plunging to the north. Outcrop along the Ankafotia western margin. Field of view is approximately• 1.5 m. Locality 10, I: 23°06.64'; L: 45°06.64'. (Fig. 99).

- 4 -axoan74-i r . -- ';•0:• -,--.? n 4sti'L- ' ,ar! - ffie . -.1-44.7.,...... - ent,-'7,*-•;..r,,... ! .;:t ,i3",,,, • •r o- ; ..-,..trif ".2..1.-,xi .. 1 7' .1t--...11Tiaia•• ...Is. 1 : ::-I''''' .--Te>;,:freld• ^ - ,..-7.,-"‘- -.5424).3.: -"a': .17.4.1.....e_:se ;,?.41,D--.-,,i- f, ./..---:•:.--:cii-st."-.!?..:71::- •.:- Jofeta:-/ -0. tf...-tyr4- - r.."-Td•:,-;-2", 447eff4s...114;;P:kPrt ,:st.f,- Clfikr. • 4 -'4! rte..c ,b-t:"- ;-7,.,-ertt... • ,44...e rst...? 4 ;1..2 . 47.4.if,,,;e

-it • ...7:. 0.,.;:if - • , -

r'rc.ttsq,F;x.- ,.n• trAt''...;t71.2;.-..... r I ....4tri er "cra,-‘c,:, i., ,,,•- it ,47::::4-I1S.;:frtif ”. • • • 'T"-- Je . .-Yete.:44;111C-4.04.`• 0.7 -,4:14-0-0 . -0 SS - :r .• r--P:7.-..':e1.1:.9% -- C”-z. c 4-•;• .1. • %fir /IP .„,r•-...22, ',,,r' ?•••• •&el's,: -.4111n1....,..,- A".0 4}:la net. --:?-3,1:50.3,-1 ,111frrti t•••••{3?7,17/ • #11tio: "71...Mo,T ,4 i:fee-Nr4,1 I ....1.14,„ : rfr..H., • dittrelPb yoitott\s;* TZ- - •

...tekiler r

Figure 24a: Photograph of intersection lineation (quartz) plunging shallowly to the south. Ouartzo-feldspathic rock. Outcrop 1.8 km east of Beseva village. Field of view is approximately 2 m. Locality 11, I: 24°07.60'; L: 45°02.90'. (Map 3, app.).

Fold axis

axial planar foliation

bedding-plane

....c.:>- intersection lineation

_1-

Figure 24b: Diagrammatic sketch showing planar and linear fabrics on the flattened quartzo-feldspathic rock outcrop in Fig. 24a. 49

Figure 25: Photograph of elongated boudins of garnet amphibolite within garnet gneiss. Outcrop in the bed of a river 0.9 km southeast of Ankilimihamy village (west of Saririaky). Pen 14.5 cm, is aligned N-S. Locality 12, I: 24°27.63'; L: 44°51.25'. (Fig. 100). 50 5.1.3. Folding Tight to isoclinal folds characterize the areas. Folding occurs in all lithologies: graphite schists, interlayered layers such as garnet leptynite, gneissic amphibolite and garnet gneisses (Fig. 26). Fold axes mainly plunge moderately to steeply north and south. Commonly, they are asymmetric and similar, due to the marked attenuation of a limb, generally the western limbs (Fig. 27). Limbs are generally W-dipping, and steep to subvertical. The shape and style of these folds suggest that they are flexural folds formed in ductile environment at high metamorphic grade (flexural flow) (Fig. 28). They are minor (Fig. 29) to large scale folds. The largest ones are mainly located at the ends of the anorthosite bodies "Map 1 and Map 2, app.) and their openings commonly face towards t, '..ody. In certain places, the harmony in the shape of these folds was disrupted by the presence of cm- scale mylonite intercalations, which have slip planes between them.

5.1.4. Microstructures Thin sections of rock samples collected from the area reveal the extreme intensity of the deformation they have undergone. Evidence of deformation effects within the rocks is manifested by the development of preferred orientation in most of the rocks. This is mainly defined by the alignment of more or less flattened quartz and feldspar (felsic minerals), intercalated with alignments of amphibole, pyroxene or garnet (mafic minerals) giving rise to gneissose foliation. Different orientations indicating more than one foliation are commonly conspicuous within these thin sections (Fig. 30). Cataclastic structure is well visible in the pyroxene gneisses, which cover the largest part of the area (Fig. 31). This rock type is relatively fine-grained and weakly foliated. Other characteristics are porphyroclastic and mortar structures indicative of mylonitized rocks (Fig. 32), which commonly consist of large porphyroclasts of garnet or feldspar 51

Figure 26: Photograph of tight fold in layered garnet gneiss and garnet amphibolite. Outcrop in western margin (Ankafotia area). Pencil 12.5 cm, is aligned N-S. Locality 13, I: 23°50.75'; L: 45°07.66'. (Fig. 99).

Figure 27: Photograph depicting layered garnet gneiss folded into similar asymmetric folds. Note the attenuated limb indicated by the pen. Outcrop along Saririaky body northwestern margin. Pen 14.5 cm, is aligned N-S. Locality 14, I: 24°25.62'; L: 44°53.94'. (Fig. 100). 52

- , )17--

a ( A

I t ‘"- ct ' .

....."•••"' • :'•-.1--Dr--7 .1". -• ---- . e •

Matt .1

Figure 28: Photograph of asymmetric isoclinal fold in garnet gneiss. Outcrop 0.2 km east of Ankafotia body. Pen 14.5 cm, is aligned N-S. Locality 15, I: 23°56.25'; L: 45°09.12'. (Fig. 99).

Figure 29: Photograph depicting minor folds in garnet gneiss. Outcrop 2 km east of Vohidrakitse village (south of Saririaky massif). Pen 14.5 cm, is aligned N-S. Locality 16, I: 24°33.75'; L: 44°54.73'. (Fig. 100). 53

Figure 30: Photomicrograph showing cross-cutting foliations

emphasized by ribbon-like quartz and K-feldspar. Mylonite, sample G 57. Sample is cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Crossed Nicols. Long dimension of photo is 2 cm. Locality 17, I: 23°58.14'; L: 45°08.11'. (Fig. 99).

Figure 31: Photomicrograph showing deformation associated to 04:

broken and crushed grains giving rise to cataclastic structure. Pyroxene gneiss, sample G 38. Long dimension of photo is 2 cm. Locality 18, 1:23°57.28'; L: 45°08.80'. (Fig. 99). 54

Figure 32: Photomicrograph of K-feldspar (kf) porphyroblast (M2) deflecting ribbon quartz and feldspar (S3), surrounded by recrystallized grains. Mortar structure in mylonite, sample G 113. Sample is cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Crossed Nicols. Long dimension of photo is 2 cm. Locality 19, I: 23°50.32'; L: 45°08.09'. (Fig. 99).

Figure 33: Photomicrograph of K-feldspar (kf) porphyroblast (M2) deflecting ribbon-like quartz and feldspar (S3). Crossed Nicols. Sample G 106, cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Long dimension of photo is 2 cm. Locality 20, I: 23°54.72'; L: 45°06.65'. (Fig. 99). 55 surrounded by a fine aggregate of recrystallized grains mainly composed of quartz and feldspar (Fig. 33). Other striking features include ribbon-like quartz and feldspar that are well developed in these rocks (Fig. 34). Deformation twins in calcite are commonly visible in marbles (Fig. 18), and in plagioclase in the foliated rocks. Lastly, undulose extinction is commonly exhibited by quartz and feldspar in gneisses and leptynites (Fig. 35).

In conclusion, most of structural features indicative of intense deformation are prominently visible in thin sections from rocks of the area. Preferred orientations and mylonitic structures are the most conspicuous features. Cross-cutting foliations are prominent in some of the thin sections. Recrystallization and grain size reduction is prominent, giving rise in many outcrops to mylonitic structures. The process is best preserved in structures of quartz and feldspar. Finally, quartz and feldspar define very striking ribbon structures.

5.2. AGE OF DEFORMATION

5.2.1. Di deformation No remnants of Di structures have been distinguished in the field. Due to the intense deformations that postdated this event, these have been obliterated and overprinted in certain parts of the region.

5.2.2. 122 deformation. From the dominantly similar tight to isoclinal fold style that has been produced by D3, observed in the field, D2 deformation could have been the buckling event that produced upright to E-overturned folds. As the transposition of D2 structures into D3 was not thoroughly penetrative 56

Figure 34: Photomicrograph of prominent ribbon-like quartz (qtz) and K-feldspar (kf) in mylonite, forming S3 foliation. Crossed Nicols. Sample G 311, cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Long dimension of photo is 1.5 cm. Locality 21, I: 23°54.25'; L: 45°06.37'. (Fig. 99).

Figure 35: Photomicrograph showing perthites (kf) in feldspathic matrix and undulose extinction of quartz (qtz). Sample G 136. Leptynite. Long dimension of photo is 1.5 cm. Locality 22, I: 23°54.31'; L: 45°07.64'. (Fig. 99). 57 throughout the area, many of the folds still retain S2 foliations and remnant F2 fold closures.

5.2.3. D3 deformations. Most of the structures found throughout the area are of D3 age. D3 may have been the result of compressional stresses at the same orientation as that which produced D2. The transition between both events is unclear.

5.2.4. D4 deformation D4 was a brittle deformation mostly visible in the pyroxene gneiss.

The cataclastic structure and the finer grain size of this lithology are attributed to D4. The growth or recrystallisation of a garnet, post-tectonic to D3, may have taken place during or after D4.

5.3. ANKAFOTIA AREA

5.3.1. OUTLINE OF THE GEOLOGICAL STRUCTURE On regional scale, the dominant structure of the area is an anorthosite body enveloped by different lithologies, with foliations striking between N 25° W and N 25° E and dipping steeply to the west. This setting is in turn enclosed in a north-south trending shear zone (Ampanihy shear zone) (Fig. 11), of about 25 km wide and 250 km long. Most of the lithologies described in Section 3.3 are visible in this area. On either side of the massif, they are concordant in dip and strike except for the graphite schists which close in a V-form at the northern and southern ends of the body. As mentioned above, the area is subdivided into 8 structural domains. Each domain will be described in terms of its dominant structural features and their orientations. 58 5.3.2. Structural description and interpretation. Figure 36 depicts the structural pattern of the surroundings of the Ankafotia body. It is dominated by straight and subparallel layers, deformed into tight to isoclinal folds with axes subparallel to the general trend (S3) of the area. It also shows the consistence of the axial trace of the anorthosite body with S3.

5.3.2.1. Domain 1 Domain 1, located to the northern end of the anorthosite body (Fig. 37) is composed of gneisses, marbles and graphite schists. The rocks are W- dipping and display N-plunging lineations of about 50°. The layers present two strike directions which vary between N 30° W and 25° E. These two directions describe the "V-form",- of the fold closure around the northern margin of the body (Map 1, app.).

5.3.2.2. Domain 2 Domain 2 (Fig. 37) includes rocks flanking the eastern margin of the anorthosite body. As mentioned above, rocks of this sector dip steeply westwards between 70° and 85°, and lineations plunge moderately 40° toward the south. The rocks of this domain consist of marbles, gneisses, amphibolites, graphite schists, leuconorites and leptynites, each with thickness less than 50 m.

5.3.2.3. Domain 3 The rocks at the southern end of the body (Fig. 38) consist of gneisses, leptynites, marbles, amphibolites and graphite schists mainly dipping to the west at about 70° to 90°. Lineations within these rocks are subvertical or plunge steeply toward the south. This domain represents an another V- shaped closure wrapping around the body (Map 1, app.).

f " li"\■ ..A\ \\\\ D3 axial trace (not rotated unlike In , 1 ∎ 1,‘ \ Saririaky massif, 1 ‘ V\\\\\ \ \ parallel to Ampanihy k ( 0, \k shear zone )

■ ,..ykl \\. . \

k1111„‘V ,

11\\(4\EISC)‘)\ \\":

I/'1 0\ \Zt3 \\‘ \ l W\ ok l \\\ \ 1 \\

\\) \

ail \ \ \ I \‘ \ ) 1. 21 ‘ I Ili

C%-- / / I/ 1//) %/ // I 7/ 0/ / g / ///„ .„ Figure 36: Structural photogeological map of the Ankafotia massif surroundings (reduced at a scale of 1:40,000 within the major 0 21cm N-NE trending Ampanihy shear zone. Tectonic fabrics show a transposition of S2 (preserved in D3 fold hinges) by S3 (the 60

Figure 37: Location of structural domains 1 and 2 with stereograms showing foliations (a) and intersection lineations (b) for 2 subdomains surrounding the Ankafotia anorthosite body.

a N: 76 GC: 38° 14°E I': 308° 76°

T: 8.0° 13: 50.6° cc.: 10.1°

a N: 140 GC: 56° 27°S P: 326° 63° •

stereoplot of poles to foliation GC: great circle of poles to foliation GC: 38° 14°E Az= 38° D= 14° D. D.= E T: 200.3° P: 40.1° is pole to great circle = fold axis (F3) c.c.: 7.6° P: 308° 76° T= 308° p= 76° N: number of poles b: stereoplot of intersection lineations a mean value of lineations T: trend P: plunge 0 c.c.: 95% confidence cone 61

Figure 38: Location of structural domains 3 and 4 with stereograms showing foliations (a) and intersection lineations (b) for 2 subdomains surrounding the Ankafotia anorthosite body.

N: 74 CC: 82° 13 °S I': 352° 77°

T: 27.8° I': 76.2° c.c.: 14.3° (not accurate)

N: 74 a GC: 60° 20°5 1:1: 330° 70° a: stereoplot of poles to foliation GC: great circle of poles to foliation GC: 82° 13°S Az= 82° D= 13° D. D.=S pole to great circle = fold axis (F3) P: 352° 77° T= 352' p= 77° N: number of poles b: stereoplot of intersection lineations mean value of lineations T: trend F.: plunge 0 c.c.: 95% confidence cone

T: 225.2° P: 86.2° cc.: 8.4° 62

5.3.2.4. Domain 4 This domain (Fig. 38) flanks the western side of the anorthosite body. Rocks of this sector are composed of mylonites, amphibolites, gneisses, marbles and graphite schists. They dip steeply westwards, and lineations should present two maxima (Fig. 38): the first, N-plunging moderate to steep at about 50° and the second, S-plunging at about 45° as mean value. In the field, the rocks appear as subparallel layers more or less conformable to each other in dip and strike over long distances and commonly repetitive across strike. Generally, foliations are well developed, giving rise to conspicuous S-tectonites, defined by elongate and flattened grains of quartz, or by the parallel alignment of other minerals like garnet, amphibole, feldspar. The alignments give rise to pale and dark layering from 2 mm to 5 mm wide. Lineations on foliation planes are locally well developed, and are defined by amphibole, quartz or garnet, depending on lithology. S-L tectonites are not very common. These lineations predominantly represent flattened D2/D3 intersection (de Wit et al., 1993).

Mylonites are present throughout the area. They are very prominent in the western part of the area (domain 4) (Map 1, app.) and are structurally conformable with the other rocks. The mylonites reach up to 20 m in thickness, and contain flattened lens-shaped crystals of K-felsdpar (augen) up to 20 cm in size with long axes subparallel to the foliation (Fig. 39). Similar porphyroclasts are also present in graphite schists. In some outcrops, they give rise to a succession of small boudins up to 2 m long, and parallel to the foliation. Quartz veins parallel to the foliation are exposed mainly within outcrops of graphite schists. 63

Figure 39: Photograph of symmetric porphyroclast of K-feldspar in garnet leptynite. Saririaky eastern margin. Pen 14.5 cm, is aligned N-S. Locality 23. I: 24°28.72'; L: 44°57.99' (Fig. 100). 64 Microscopically, the mylonite rocks reveal two types of textures. The first is a cataclastic texture (Fig. 31) mainly present within the pyroxene gneiss. The second is a preferentially oriented texture visible in the remaining rock types (gneissic amphibolites, graphite schists, leptynites) due to deformation mechanisms such as crystal-plastic deformation and dynamic recrystallization involving quartz grains. Feldpar, amphibole, pyroxene and graphite of some rocks are also commonly aligned in planar or linear fabrics. Most of the rocks have mylonitic textures in thin sections. S3 (the dominant foliation) is well developed because of prominent parallel alignments of ribbon-like quartz and feldspar in most of rocks. Very common also are garnet porphyroclasts with sieve texture (Fig. 17). These porphyroclasts deflect (pre-tectonic) (Fig. 40) or overprint (post-tectonic) (Fig. 41) the S3 foliation and associated ribbon textures. Pre-tectonic garnets have symmetric pressure shadows mostly composed of quartz grains. Some samples exhibit obliquity between two foliations (52/53) ranging from 20° to approximately 25 0 (Fig. 30). Uncommonly, oxides and dark minerals make up alignments subparallel or oblique to the foliation.

Interpretation The structures and microstructures mentioned above indicate that the area has undergone an intense deformation. They were formed in response to a 02 / D3 deformation due to an unevenly distributed subhorizontal west-east compressional stresses (de Wit, et al. 1993). The eastwards directed stresses were of greater intensity; evidence for this includes the steeply W-dipping (85°) orientation of the rocks on either side of the anorthosite body, giving rise to tight to isoclinal easterly verging, overturned folds, with axial planes generally trending either to north- south. The intense deformation of the area is also emphasized by the 65

Figure 40: Photomicrograph of pre-tectonic garnet (grt) with pressure shadow, deflecting S3 foliation made up of ribbon-like quartz and feldspar. Garnet amphibolite, sample R 51 cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Crossed Nicols. Long dimension of photo is 1 cm. Locality 24, I: 24°28.60'; L: 44°50.24'. (Fig. 100).

Figure 41: Photomicrograph of post-tectonic garnet (grt) (black) overprinting S3 foliation (horizontal). Garnet leptynite, sample G 105 cut perpendicular to foliation (XY) and parallel to intersection lineation (X). Crossed Nicols. Long dimension of photo is 1 cm. Locality 25, I: 23°54.68'; L: 45°06.40'. (Fig.9 9). 66 structural features, isoclinal and flattened, of mesoscopic folds mainly exposed in the sub-domains 5, 6, 7 and 8 (Fig. 42 and 47).

5.3.2.5. Domain 5 This domain displays a fold closure (Fig. 42) composed of graphite schists interlayered with mylonitic rocks near the southern tip of the anorthosite body. The fold opens toward the north and consists of garnet quartzite exposed near the hinge and graphite schists dipping moderately to steeply (30-70°) to the south (Fig. 43). Fold axes trend southwards and plunge on average about 54°. The presence of mylonitic rocks within the graphite schist is evidence of the deformation undergone by the domain. The style of the fold (see Section 5.3.5) and the symmetric structures of the mylonite (porpyroclasts with symmetric pressure shadows) interlayered within the fold indicate that it was flattened by pure shear. The mylonitization of the layer between the graphite schist and the quartzite may have occurred during D2. The graphite schist and quartzite were subsequently folded by D3 and the mylonitized layer is folded around and confined to the hinge of S2 folded garnet quartzite. Minor folds have been identified, displaying fold axes plunging moderately southwards (Fig. 44).

5.3.2.6. Domain 6 This domain comprises the northernmost fold closure in the area (Fig. 42). The fold is entirely composed of graphite schists (S2) with a southwards opening around the anorthosite body. Limbs dip steeply westwards, and the fold axis, as documented by intersection lineations and other fold axes recorded in the domain trends about 12° E and plunges 43°- 62° to the north (Fig. 45). Minor folds are visible in the hinges (Fig. 46). 67

Figure 42: Location of structural domains 5 and 6 with stereograms showing foliations (a) and intersection lineations (b) for 2 subdomains surrounding the Ankafotia anorthosite body.

T: 12.2° 13: 57.1° ■ c.c.: 21.8° (not accurate/

N: 8 GC: 65° 25 °S P: 335° 65° N

1----AIVO RTHOSITE

a: stereoplot of poles to foliation GC great circle of poles to foliation GC: 329° 10°E Az= 329° D= 10° a D. D.=E N: 20 pole to great circle = fold axis (F3) GC: 329° 10°E P: 239° 80° P: 239° 80° T= 239° p= 80° N: number of poles hi„stes2pjoisfintensitualimationa mean value of lineations T: trend F.: plunge 0 cc: 95% confidence cone

T: 199.0° 13: 53.9° 68

_ r / ./ \ -47'1 i / P. _..<1..1? •// / / 1 80' '''• •••- •*7•.:.1;;•:..:• '.;AV I \

\1 \\ \\\* \ C‘•3• i.7.7../: .7/// / //„ \ \ I Ai 71 / / 7,e

\ aak 1104/1/ 47 // \ 11 \ YM// /// '7/ / 1 .1/ / / /' , \ \ 11/ /, // 1\ \ I P / i

it I 1 1 1 45 ( ) /46 33k / i Fig. 44 178 /I

H 1 1 1 / 38 8 / Strike and dip al bedding ' 73/, • Stnke ana dip oi foliation

$2 1/ / Trend and plunge of intersection lineation 30 7 i r / / 367' Trend ana plunge al fold axis .1 / Minor laid / I • Mylonite

Graphite scnist

7r.7.. Garner quartzite

0 120 m

Figure 43: Geological map of domain 5 representing the orientations

of the different structural features. 69

Figure 44:. Photograph of folded, layered graphite schist. Outcrop in the fold closure around the southern margin of the Ankafotia body. Pen 14.5 cm, is oriented N-S. Locality 26, I: 24°00.25'; L: 45°07.90'. (Fig. 99). 70

Fig. 46 6d/ / 7Th

is2/// -fik /

70 ,, Strike and dip of bedding

43 Trend and plunge of fold axis Graphite schist

0 15m

Figure 45: Schematic geological map of domain 6 depicting D3 fold and the orientation of associated structural elements. 71

Figure 46: Photograph of parasitic fold in graphite gneiss. Outcrop at the northern closure domain 6 (Ankafotia area). Pen 14.5 cm, is oriented N-S. Locality 27, I: 23°49.09'; L: 45°09.41'. (Fig. 99). 72 5.3.2.7. Domain 7 Domain 7 is located along the western margin of the body (Fig. 47). Here tight to isoclinal similar folds of mesoscopic size, not exceeding 2 m in wavelength are common. Mylonites both folded around, and axial planar to the folds are very prominent in this sector. They are conformable to the general trend of the folds, i.e. N 20° W to 20° E. The eastern limbs of these folds are short and flattened. Fold axis plunges range between 60° and 80° to the north and south (Fig. 48). In this domain, remnants of D2 structures are observable since they represent prominent fold closures resulting from D3. The main folding in the area is of D3 age.

5.3.2.8. Domain 8 The structural orientation (NE-SW) of this domain (Fig. 47) is different compared with the regional trend of the area (Fig. 49 and 50). Fold axes and intersection lineations trend between 20° and 60° (Fig. 51) to southwest and northeast, for the northern and southern sectors respectively (Fig. 49). Dips are mostly to NW and range from 60° to 80°. In places, the structures appear 'cut-off' by the anorthosite. Unfortunately, the outcrop is insufficient for a complete analysis of this area.

Interpretation The structures in domain 8 suggest that the deformation, penetrative and evident throughout the area, was of much lower intensity in this domain. Layers in this domain are not completely rotated by D3. The continuity of the dominant foliation S3 further away to the east and subparallel to the edge of the body (Fig. 49) suggests that there was a strain partitioning in the domain during D3. Thus, the transposition of S2 into S3 was not effective. The folding of the anorthosite body during D2 has

resulted in the discordance (Fig. 49) between the body and the country rocks 73

11 J

N: 9 GC: 314 ° 16°E P: 224° 74°

T: 290° P: 88.1° cc.: 18.9° (not accurate)

N: 15 GC: 182° 37°E P: 272° 53° a: stereoplot of poles to foliation GC: great circle of poles to foliation GC: 314° 16°E Az= 314° D= 16° D. D.=E ti pole to great circle = fold axis (F3) P: 224° 74° T= 224° p= 74° N: number of poles 12Stplot of intersection lineations ■ mean value of lineations T: trend P: plunge CD c.c.: 95% confidence cone T: 259.8° P: 64.4° cc: 16.2° (not accurate)

Figure 47: Location of structural domains 7 and 8 with stereograms showing foliations (a) and intersection lineations (b) for 2

snhriomains surroundinn the Ankafntia anorthosite body. 74

71.1 if

1 S2 D2 fold closure

Mylonites • 707' Strace and dip of bedding

637 Trend and plunge of fold axle

0 3.5m

D2 fold closure

ras

Figure 48: Structural map of domain 7 showing style of D3 folding. Note the presence of several D2 fold closures.

43/

24,7

/ 1 / Ss 1/

I /1 Fig. 51 •

/ if I // 1/ / / 1 Vt .5)■ a S2 / S3/

Fig. 50

/ I / / /

/ / 1 / / 1 / IStrite and dip or bedding

, / Trend and plunge of intersect,: I 25 II nation •••••"" Trend and plunge of lo id axis 24 11 / / ...... 1„,..KBendodwinn7conzact / / • / / Foliation

//// 4 Graphite scnist 0 250

Figure 49: Structural map of domain 8. 76

Figure 50:: Photograph showing folded garnet leptynite exposed 2.5 km south of Ampamata village (Eastern margin of Ankafotia body). Note their oblique trend relative to the massif direction (upper right hand corner of the photograph). Field of view is approximately 30 m. Locality 28, I: 23°55.83'; L: 45°09.26'. (Fig. 99).

soh ` e n

Jo- • r

:, 7.C.• p • t•r- -71' 1 to- !-*

Figure 51: Photograph of folded garnet leptynite, fold closure 2 km south of Ampamata village (Eastern margin of Ankafotia body) Note the L3 intersection lineation steeply plunging to the southwest. Field of view is approximately 3 m. Locality 29, I: 23°55.77'; L: 45°09.38'. (Fig. 99). 77 (S2). Effectively, field observations suggest that some of limbs and hinges of these folds are obliquely cut by the edge of the body (indicative intrusive relation). As mentioned above, the country rocks have wrapped around the anorthosite body.

A stereoplot of all foliations of the map area (Fig. 52a), displays a concentration of poles predominantly confirming the steeply W-dipping of the rocks on each side of the body. A stereoplot (Fig. 52b) depicts three main attitudes of the lineations. The concentration in the centre represents the few lineations mostly confined to the southern part of the body (Map 1, app.). They plunge steeply to subvertical (70°-88°). The two other maxima, respectively, represent the remaining lineations that trend north or south with plunges varying between 30° and 50°. It is thus confirmed that intersection lineations are not horizontal but moderate to very steep, plunging NE and SW.

5.3.3. Depth of igneous emplacement and thickness of the Ankafotia anorthosite body Many anorthosite bodies were emplaced as elongate sills or subconcordant sheets at relatively shallow levels (Ashwal, 1993), e.g. the Archean Fiskenaesset Complex of west Greenland and the Messina Anorthosite in the Limpopo mobile belt of South Africa, both of which occur as sheet-like layered intrusions. As emplacement depths, it was suggested that they commonly occur at intermediate-to-shallow level in the earth crust, e.g. the Proterozoic massif-type Nain Anorthosite of Labrador was emplaced at 6-14 km of depth (Ashwal, 1993) and the original thickness of Laramie Anorthosite between 10 and 15 km. From geophysical investigations, it was concluded that the Marcy massif of Adirondacks was a tabular body, 3-4.5 km in thickness (Ashwal, 1993), the Morin anorthosite 78

Figure 52: Stereograms showing lineation and foliation attitudes for entire envelope around the Ankafotia anorthosite body.

; • 1:153° / P: 39.4° c.c.= 6.0° 1, \:.,N . --,' • % , 1 ♦ 4 ■ .--7 • '• 1 ---------...._ ....""--1 4 ,----.....______P: S8.7° /,' I c.c.= 4.5 / 4 7:- ! ------:.. 4 -7 ■

, I 7: 201.4° b. P: 32.4° c.c.= 5.3°

•

a N= 160 N: 409 C CC: 17° 12°E P: 287° 78°

a: stereoplot of poles to foliation GC: great circle of poles to foliation GC: 17° 12°E Az= 17° D. 12° D. D.=E a pole to great circle = fold axis (F3) P: 287° 78° T= 287° p= 780 N: number of poles b and c: stereoplots of intersection lineations a mean value of lineations T: trend I': plunge 0 c.c.: 95% confidence cone 79 (Quebec) 2-4 km in thickness (Ashwal, 1993), and 5 to 6 km was suggested for the Laramie anorthosite massif (Ashwal, 1993). For the Ankafotia and Saririaky anorthosite bodies, Nicollet (1990), proposed an emplacement depth between 15 and 30 km. Due to subsequent deformation as described above, the original shape and thickness are still unknown. Thus, in view of emplacement depths and shapes of other anorthosite massifs in the world, intermediate-to-shallow crustal level is also suggested for the emplacement of the Ankafotia and Saririaky anorthosite bodies, and sheet-like or tabular for their shape. Their thickness is still unconstrained.

5.3.4. Strain effects on the anorthosite body and the country rocks, and model for their structural evolution The deformations described in previous sections did not spare the anorthosite body, which, prior to the onset of the compression, could have been a subhorizontal sheet intruded perhaps along basement-cover interface (Fig. 53). The sheet-like form of such an anorthosite sill implies a lack of tectonic compressive stress during emplacement (Windley, 1977). Fold-forming forces are usually related to horizontal tectonic movements. For this reason, simple compression in a geological context is virtually synonymous with horizontal compression (Jaroszewski, 1984). Therefore, the effect of the compression on a sub-horizontal basement, including a sheet-like anorthosite, is the formation of folds (buckling), whose axes are perpendicular to the compression axis (Fig. 53b). The results of such compression might be upright folds whose axes might variably be plunging and the strikes of layers, may be not everywhere perpendicular to the compression axis (Jaroszweski, 1984). The deflection of the foliations around the anorthosite body may have been due to the higher competence of the anorthosite body resisting the flattening effect. The eastwards bU

Figure 53: Model representing. the possible kinematic evolution (schematic) of the Ampanihy shear zone structures in the vicinity of the Ankafotia body.

tolia don boudin irtsion fissure parasitic told 17/4!,<_1;" anorchosice body -If I xenolith (gCiphite schist) I --lir I graphite schist. LI S.

This diagram shows the onset of the buckling initiated by an east-directed compressional stresses, giving way to upright to E- overturned open folds. The anorthosite is Interlayered within the basement units. During its ascent, the anorthoslte incorporated some rocks of the basement (graphite schists xenoliths). On the limbs which are at an angle of over 45° to the axial plane, progressive compression produced parasitic folds (de Sitter, 1958). The sheet-like anorthosite, due to its higher competence and lower ductility in comparison to the country rocks, follows progressive buckling. This schematic illustration summarizes the effects of the compression on the different layers during folding. Layers with moderate competence and ductility, form parasitic folds on the flanks and in the hinges of the folds. For the anorthosite body, tension fissures and fractures appear in the outer arc due to its higher competence and lower ductility. This schematic illustration summarizes the onset of the flattening (pure shear deformation) and its effect on the fold. Progressive compression on the flanks of the upright fold change its shape Into tight and Isoclinal fold, revealing the onset of the flattening. Thin myionitic layers representing small-scale shear zones produced by inhomogeneous strain participation separate parasitic folds. Regionally, the flattening generates an expansion of the hinge, owing to the shortening of the flanks (limbs). Around the anorthosite body, in the country rocks, lineations are reoriented to north and south plunging moderately (40°-60°) whilst In the body, lineations (stretching) become vertical. At this stage, the body acquires the sheath-like geometry. Graphite schists enclosed as large septums in the anorthosite body during its folding have been squeezed and expanded in fractures within the body (case of the large graphite schist layer in the southern part of the body). 82 vergence of the folding is indicative of tectonic transport generally eastwards. It is worthy to note that the graphite bearing layers may have played an effective role in the development of any deformation (buckling and flattening) by lubricating the friction within and between the layers. Figure 53 depicts such a possible tectonic evolution following the initial intrusion of the anorthosite as sub-horizontal sill.

5.3.4.1. Foliation and lineation patterns Compressional effects may have produced two types of deformation within the anorthosite body: the first in response to shortening, mostly effective along the eastern and western edges of the body, and the second in response to extension, mainly confined to the northern and southern ends of the body (Guglielmo, 1994). Such a model would explain the formation of foliations parallel to the sides of the massif, indicative of intense flattening. The compression would also have given rise to constrictional strains, its effects are evident by the presence of mainly moderate to steep dipping lineations (Fig. 53c). Within the northern sector of the area, lineations plunge approximately northwards, whilst along the southern half of the body, lineations plunge southwards. The progressive flattening could have been more severe along the N-S trending of the anorthosite body inducing bending of its axis and hinge giving rise effectively to a sheath-like geometry (Fig. 53d and 53c). Evidence of the constrictional strains within the anorthosite include: subvertical stretching lineations visible in much of some outcrops of the anorthosite body (Morel, 1996), and similar opposite trends of the lineation plunges as seen in the envelope of the body described here. Figure 54 is a schematic diagram depicting the geometric relationships of the linear structural elements. 83

Figure 54 : Diagrammatic representation displaying the lineation orientation within and around the Ankafotia anorthosite body.

---- Stretching lineation (from Morel, 1995)

r Intersection lineation 84 5.3.4.2. Sheath folding With the structural features mentioned above, the Ankafotia body is interpreted as a sheath fold whose final shape formed during D3 flattening deformation (pure shear). It is an overturned sheath fold included within folded country rocks, with limbs dipping at about 80° to the west and an axial plane striking N 17° E. Lineations in the northern and southern ends plunge respectively about 50° northwards and 40° southwards, emphasizing its sheath-like fold geometry. The anorthosite body is bounded to the north and south by V-form fold closures whose openings both face toward the body (Fig. 55). The presence of these fold closures composed of graphite schists and marbles, which are part of the country rocks wrapping around the anorthosite, disagrees with the megaboudinage hypothesis supported by Rolin (1991) and Kilmer and Duncan (1991). According to Whitten (1966), boudinage is characterized by the attenuation of a more competent layer or unit and concomitant less competent adjacent units into the "neck" zones resulting from the attenuation. Continued attenuation commonly results in separation of the competent rock into isolated cylinder-like units, each of which was referred to as a boudin. In other words, the adjacent layers or units must be continuous between boudins. In the Ankafotia case, the adjacent layers (graphite schist) are folded, facing their openings towards it, i.e. bounding it to the north and south (Fig. 55). Effectively, the graphite schist and marble-leptynite unit (see Map 1, app.) wrap around the anorthosite body.

5.3.5. Association of deformation to metamorphism The study area is part of the Ampanihy Group, which has been subjected to granulite facies metamorphism at about 560-545 Ma (U- Pb/monazite) (de Wit et al., 1993). This metamorphic event has overprinted 85

Ariorthosite massif

V-form fold closure

Steep to subvertical country rocks

Figure 55; Diagrammatic representation showing the main structural features of the area surrounding the Ankafotia anorthosite body. 86 the structural pattern described above which took place about 640-650 Ma ago (U-Pb/monazite) (de Wit et al., 1993). Two episodes of deformation D3 and D4, the later ones, have been recognized in the field; D2 structures are only sporadically preserved (i.e. see Fig. 48) and Di structures can only be inferred from more regional observation (de Wit, 1996, personal communication).

5.3.5.1. Di deformation In the field, Di structures are not observable due to the extreme intensity of the subsequent tectonic phases D2 and D3, and has been completely obliterated and overprinted.

5.3.5.2. D2 deformation In the Ankafotia area, remnants of D2 structures are represented by fold closures in domains 5, 6, 7, and 8. In these domains, S2 foliations are very prominent, forming limbs of the main folds which are of D3 age (Fig. 49). Their orientations are given in the domain description (see Section 5.3.2 ).

5.3.5.3. D3 deformation D3 structures were formed in response to the intense east-directed flattening that has generated the present Ampanihy shear zone. Figures 56 to 59 illustrate D3 deformations from each domain of the Ankafotia area. In domain 5 (Fig. 42 and 43), S3 is axial planar to the large-scale fold composed of graphite schist. Its limbs strike from N 15° W to N 45° E and tend to parallel the general strike of S3. In the garnet quartzite, to the north, S2 is still preserved; it is defined by garnet alignment, which ranges in strike between N 70° W and N 40° E at the limbs, and between N 130° E and N 90° 87

Figure 56: Structural data for D3 deformation from domain 1 in the Ankafotia area Lsee location Fig. 37) The stereoplot shows the northwards orientation of L3 with moderate plunges. S2 dip steeply to the west. L3 and F3 are slightly different in orientation. 76 poles to foliation plane S2 L3

GC: 38°14°E ■ • P : 308°76°

T : 8.8° AP: 3°70°W P : 50.6° c.c.=10.1°

Structural data for D3 deformation AP from domain 2 (see location Fig. 37) The stereogram depicts prominent southwards orientation of L3 plunging moderate to steep. S2 dip moderately to steeply to the west. L3 and. F3 display opposite b orientations. N= 140 c I. = 2.0°4/1% area 140 poles to foliation plane S3 lineations L3. GC: 56°27°S P: 326°63°

T: 200.3° AP: 18°62°W P: 40.1° c.c.=7.6° GC: Great circle (Az, D, D.D) ■ P: Pole to great circle (T, P) a Mean value of lineation T: Trend P: Plunge 0 c.c.: 95% confidence cone AP: Axial plane (Az,D, D.D) 88 E at the hinges. Dips are southwards between 60° and 80°. The fold axis (F3) of this large-scale fold trends SW and plunges at about 80° (Fig. 58a). In domain 6, D3 has refolded S2, outlining another large-scale fold closure (Fig. 42 and 45). S2 strikes range N 10° W to N 55° E and dips are from 65° W to subvertical. F3 is NW-trending with a plunge of about 65°

(Fig. 58b). The two large scale folds of domains 5 and 6 are of class 3 (Ramsay and Huber, 1987), i.e. their hinges are thicker than the limbs. However, the fold of domain 5 is more flattened and may be considered as of flattened class 3 (Ramsay and Huber, 1987). Along the western side of the massif, in domain 7 (Fig. 47 and 48), S2 has been flattened by D3 into asymmetric tight to isoclinal folds with western attenuated limbs. S3 is axial planar to these folds and F3 plunges about 70° towards 230° (Fig. 59a). To the east, in domain 8 (Fig. 47 and 49), S2 has been rotated by D3 into a SW-NE orientation. This is different from the regional structure (Fig. 59b). In the northern part of the domain, S2 strikes SW and dips NW, whereas in the southern part, strikes and dips are NW-SE and SW, respectively. These folds are tight to isoclinal and slightly refolded (Fig. 49). S3, which is the regional and dominant foliation in the area, is ,W- dipping steep to subvertical and strikes between N 20° W and N 20° E all around the anorthosite body. Lineations L3 produced by D3 (Map 1, app.), present opposite trends (north and south) all around the body. This pattern is consistent with a sheath fold geometry of the Ankafotia body (Fig. 52b, 53e, 54 and 55). Stereoplots of each domain depict that most of L3 lineations trend N and S, moderately to steeply, indicating the sheath-fold geometry of the body. F3 has refolded S2 into asymmetric tight to isoclinal fold whose axis F3 is concordant with L3 in terms of trend and plunge. However, the flattening may have differently reoriented F3 and L3. 89

Figure 57: Structural data for D3 deformation from domain 3 in the Ankafotia area Lsee location Fig. 381 L3 are concentrated in the centre part of the stereogram. Their orientations are north and south and plunges are steep to subvertical. S2 are confined to the eastern part of the stereogram, indicating that they a are W-dipping steep to subvertical. N = 74 C.I. = 2.0%/1% area L3 and F3 are consistent in GC: 42°20°S concentration. P:312°70° 74 poles to foliation plane S2 7:225.2° AP: 2°66°W lineations L3. P: 86.2° c.c=8.4°

Structural data for D3 deformation from domain 4 (see location Fig. 38) In this diagram, L3 concentration is oriented N-S, and they plunge moderately to steeply to the north and south. S2 are steep to subvertical, dipping to the west. L3 and F3 are prominently b consistent in concentration.

N = 74 C.I. = 2.0%/1% area 74 poles to foliation plane S3

GC: 82°13°S Lineation L3 P: 352°77°

T: 27.8° AP: 14°78°W P: 76.2° c.c.=14.3° (not accurate) GC: Great circle (Az, D, D.D) :• P: Pole to great circle (1, P) Mean value of lineation T: Trend P: Plunge 0 c.c.: 95% confidence cone AP: Axial plane (Az,D, D.D) 90

Figure 58: Structural data for D1 deformation from domain 5 in the Antifotia area tsee location Fig. 42). This diagram shows the moderately southwards plunges of L3 in this domain. S2 are W-dipping, steep to subvertical. Consistence in concentration between L3 and F3 is displayed. 20 poles to foilation plane S2 * Lineation L3 GC: 329°10°E ■ P; 239°80° T: 199° AP: 1°70°W ❑ P: 53.9° c.c.=6.7°

Structural •ata• for D3 deformation from domain 6 (see location Fig. 42) In this domain, L3 moderately plunge northwards whereas S2 dip moderately to steeply to the northwest and west. F3 and L3 are slightly consistent in concentration. b 7 poles •to foliation plane S2 2.0%/1% area • Lineation L3

GC: 65°25°S P: 335°65°

T: 12.2° AP: 24°58°W P:57.7° c.c.=21°8 (not accurate) GC: Great circle (Az, D, D.D) P: Pole to great circle (T, P) Mean value of lineation T: Trend P: Plunge oc.c.: 95% confidence cone

AP: Axial plane (Az,D, D.D) 91

Figure 59: Structural data for D3 deformation from domain 7 in the Ankafofia area Lsee location Fig. 47)_ In this domain, L3 are north and south-plunging, steep to subvertical. S2 foliation planes are west-dipping, steep to subvertical. L3 and F3 are consistent in concentration. 9 poles to foliation plane S2 Lineation L3 GC: 314°16°E • P:224°74° T: 290° AP: 6°70°W 17 P: 88.1° c.c.=18.4° (not accurate)

Structural data for D3 deformation from domain 8 (see location Fig. 47) The diagram displays the prominent westwards orientation of L3 which plunge moderately. S2 dip moderately to steeply to the northwest and moderately to the southwest. Consistence in concentration are prominent b between L3 and F3. N = 15 C.I. - 2.0%/1% area 15 poles to foliation plane S2 GC: 182°37°E P: 272°53° Lineation L3

T: 259.8° AP: 290°62°S P: 54.4° c.c.=16.2° (not accurate)

GC: Great circle (Az, b, DAD) P: Pole to great circle (T, P) Mean value of lineation T: Trend P: Plunge Q c.c.: 95% confidence cone AP. Axial Mane (Az D. D:D1 92 Observations of S3 on outcrops and in thin-sections from different rocks reveal that opx, cpx, grt and pl were syntectonic with respect to D3. They grew or recrystallized during M3 and shared the preferred orientation mainly imposed by the transposition of S2 into S3 during D3. Observations of some thin sections, however, also indicate significant pre-tectonic (with respect to S3) growth of minerals such as garnet (Fig. 40) K-feldspar (Fig. 31) and sillimanite (Fig. 16), as documented by the deflection of ribbon-like quartz and K-feldspar forming S3 around these minerals. Commonly, garnet and feldspar display symmetric pressure shadows. Sillimanite may be folded or curved parallel to S3, indicating that it also grew or prior to the onset of D3 (Fig. 60).

5.3.5.4. D4 deformation D4 is the last recognisable phase of deformation within the map area (de Wit et al., 1993). It has weakly affected the area by giving rise to the cataclastic and protomylonitic structures of the pyroxene gneiss. D4 has partly destroyed the ribbon-like structure acquired by the gneiss during D3,

by microfracturing and breaking of quartz and feldspar (Fig. 61). Garnet clusters were produced by crushing of pre-existing porphyroclasts (Fig. 62). In massive, unfoliated garnet-pyroxenite and garnet-amphibolite samples, plagioclase grains are tapered, fractured and slightly bent (Fig. 63).

The conclusion based on a combination of the above criteria is that the granulite facies metamorphism as indicated by the mineral paragenesis (cpx+opx+grt+pl) was prevalent during D3. It is worthy to note at this point that at D3 and 04, basic igneous rocks (gabbros?) must have been intruded. These rocks do not have D3 structures, but have been metamorphosed into massive garnet amphibolites and pyroxenites by a second granulite or amphibolite facies metamorphism event M4, which 93

Figure 60: Photomicrograph of sillimanite gneiss, showing sillimanite grain (sil) folded by F3, indicative of amphibolite facies metamorphism during M2. Sample G 428. Long dimension of photo is 1 cm. Locality 30, I: 23°52.32; L: 45°10.80'. (Fig. 99).

Figure 61: Photomicrograph of pyroxene gneiss, sample G 410, showing cataclastic and protomylonitic structures produced during D4 deformation. Crossed Nicols. Long dimension of photo is 2 cm. Locality 31, I: 23°50.75'; L: 45°10.50'. (Fig. 99). 94

Figure 62: Photomicrograph of garnet gneiss, sample G 25, showing late cataclastic deformation associated with D4: broken and crushed garnet (grt) producing grain clusters. Crosed Nicols. Long dimension of photo is 2. cm. Locality 32, I: 23°56.34; L: 45°09.40'. (Fig. 99).

Figure 63: Photomicrograph of garnet amphibolite, showing late deformation associated with D4: fractured and bent plagioclase (p1) in the middle of the photo. Crossed Nicols. Long dimension of photo is 1 cm. Sample G 99. Locality 33, I: 23°55.65'; L: 45°06.62'. (Fig. 99). 95 probably accompanied D4. The metamorphic conditions and history of these rocks will be further discussed in Chapter 6.

5.4. SARIRIAKY AREA Figure 64 represents the structural pattern in the surroundings of the Saririaky anorthosite body. Its characteristic feature is the prominent asymmetry existing between the northern and southern ends of the body, emphasized by the axial trace passing through it. This suggests that the body was rotated during D3 flattening. For the Ankafotia body, the axial trace is slightly straight (Fig. 36), subparallel to regional trend S3 of the shear zone.

5.4.1. Outline of the geological structure As mentioned previously, the Saririaky area is sudivided into 5 structurally homogeneous domains (Fig. 65, 66 and 69).

5.4.1.1. Domain 1 This domain comprises the northernmost part of the area (Fig. 65). It consists of biotite gneiss, pyroxene gneiss, garnet leptynite, marbles, graphite schist, mylonites, amphibolite gneiss and quartzites. Generally, the foliations strike from N 20° E to N 30° E and dip to the west from steep to subvertical. Intersection lineations, mostly accentuated by garnet and quartz, plunge shallowly to moderately north or south-plunging (16°-51°).

5.4.1.2. Domain 2 Domain 2 represents the western side (Fig. 66) of the envelope around the anorthosite body. It is characterized by mylonites, pyroxene gneiss, biotite gneiss, garnet leptynite, graphite schist, and amphibolite gneiss. The general strike of these rocks is to the northeast and dips are mainly steep to the west. Lineation plunges are moderate, with a mean 96

D3 axial trace (rotated)

/,/ I/ 1,1/ //I/ a 1 • 1/1/, I /

, ' /

IJ \

0 2 krn

Figure 64: Structural photogeological map of the Saririaky body surroundings (reduced from a scale of 1:40,000) with a major N-NE trending. Straight lines indicate S3 foliations.

• . . _ _ --•- innafinne 97

N: 35 GC: 303° 36°N P 213° 54°

T: 179.7° b P: 50.3° cc.: 24° (not accurate)

Figure 65: Location of structural domain 1 and stereograms showing foliation (a) and intersection lineation (b) attitudes.

A: stereoplot of poles to foliation GC: great circle of poles to foliation GC: 303° 36°N Az= 303° D= 36° D. D.= N • pole to great circle = fold axis (F3) P: 213° 54° T= 213° p= 540 N: number of poles b : stereoplots of lineations ■ mean value of lineations T: trend P: plunge 0 c.c.: 95% confidence cone 98

Figure 66: Location of structural domains and stereograms showing foliation (a) and intersection lineation (b) attitudes for domains 2 and 3.

b T: 52.5° P: 33.9° cc.: 12°

N: 2S GC: 119° 33°S P: 29° sr

N: 31 b GC: 144° 44°W T: 63° P: 54° 46° a P: 42.2° cc.: 29°.0 (not accurate) stereoplot of poles to foliation GC: great circle of poles to foliation GC: 144° 44°W Az= 144° D= 44° D. D.=W ■ pole to great circle = fold axis (F3) P: 54° 46° T= 54° p= 46° N: number of poles b stereoplots of intersection lineations to mean value of lineations T: trend P: plunge

() cc.: 95% confidence cone 99 value of about 34° NE. Mylonites are very prominent in this area, reaching thicknesses of up to 30 m (Fig. 67). Some of the exposures in this area preserve mesoscopic similar folds with wavelengths of 20 cm to 80 cm. The western limbs are generally flattened (Fig. 27). Fold axes trend N 29° E and plunge about 57° N. Boudins are common in this domain, and generally consist of amphibolite enveloped by a gneissic matrix. Individual boudins, measured from neck to neck, range from 20 cm to 1 m (Fig. 68). The northern part of this domain (Map 2, app.) reveals units (graphite schists and quartzite) obliquely cut by the edge of the anorthosite body (discordance). (This structure will be further discussed in Model 1 and Section 5.5).

5.4.1.3. Domain 3 This domain is located in the southern part of the anorthosite body (Fig. 66). It is made up of amphibolite gneiss, graphite schist, marble, garnet leptynite and garnet-biotite gneiss. These rocks strike between N 25° E and N 100° E. Dips are mostly to the northeast and southeast between 65° and 80°. Lineations are not common in this domain. However, two sets of folds have been distinguished: the first contains fold axes trending NE, and plunging about 10° N, whereas the second is E-trending with a mean plunge of about 45° E. The units in this area make up a large open fold (Vohidrakitse fold) with a fold axis trending about N 56° E, and plunging about 54° N (Map 2, app.).

5.4.1.4. Domain 4 This domain is also located directly south of the anorthosite body (Fig. 69). It is composed of amphibolite gneiss, biotite gneiss, garnet leptynite 100

Figure 67: Photograph of mylonitized gneiss. Outcrop in the bed of Andranozirioky River (Saririaky western margin). Field of view is approximately 15 m.. Locality 34, I: 24°28.60'; L: 44°50.24'. (Fig. 100).

Figure 68: Photograph showing flattened boudins of garnet amphibolite gneiss. Outcrop in river bed 1.4 km south of Ankilimihamy village (west of Saririaky). Field of view is approximately 3 m. Locality 35, I: 24°27.69'; L: 44°51.28'. (Fig. 100). 101 and quartzite, which strike between N 10° E and N 60° E and dip southeastwards from about 50 0 to subvertical. Lineations trend northwards and plunges are from moderate to steep, with a mean value of about 67 0 N. This domain covers the region of a refolded 02 fold. The axis of the D3 fold trends northeast and plunges about 62° NE (Map 2, app.).

5.4.1.5. Domain 5 This domain represents the eastern part of the study area (Fig. 69). Units consist of garnet leptynites, biotite gneiss, marbles, quartzites, amphibolite gneiss and quartzo-feldspathic rock, displaying strikes ranging from N 10° W to N 25° E. Dips are commonly to the west, steep to subvertical (65°-88°). Lineations are mostly shallow to moderate S-plunging (16°-55°). Small-scale folds with wavelengths of about 10 cm are visible in some outcrops of gneiss. They are similar, tight and flattened folds displaying axes trending at about N 10° and plunging from 27° to 40° S. Worthy of note are that many of their western limbs are flattened. Other remarkable features are late transposition structures found on some of gneiss outcrops (Fig. 70).

5.4.2. Di deformation Di is not observable throughout the study area.

5.4.3. D2 deformation. Remnants of D2 structures in this area are prominent in the southern domains (Fig. 66 and 69; Map 2, app.). S2 is represented by limbs of the Vohidrakitse D3 open fold and adjacent to this fold, F2 has been refolded by D3. Attitudes of these D2 structures are described in the previous sub-sections. 1UL

Figure 69: Location of structural domains and stereograms showing foliation (a) and intersection lineation (b) attitudes for domains 4 and 5.

T: 172° P: 42.1° c.c.: 17.8° (not accurate)

N: 43 GC: 300° 25°N P: 210P 65'

N: 10 GC: 163° 23°W I": 73° 62°

a: stereoplot of poles to foliation GC: great circle of poles to foliation GC: 163° 28°W Az= 163° 13= 28° D. D.=W pole to great circle = fold axis (F9 T: 19.5° P: 73° 62° T= 73° P: 66.9° p= 62° cc.: 27.8° (not accurate) N: number of poles b : stereoplots of intersection lineations mean value of lineations T: trend P: plunge 0 c.c.: 95% confidence cone 103

5.4.4. D3 deformation Most of the structures found throughout the area were produced during D3, and their onset might be contemporaneous with D3 in the Ankafotia area. However, D3 effects were not similar for the two bodies. The spectacular difference in the shapes of the anorthosite bodies suggests that D3 was more intense or more direct around the Ankafotia body. Two alternative models (see models 1 and 2 p. 108 and 118) are offered to explain the origin of the subcircular form of the Saririaky body that may be a tubular fold (or a very tight sheath fold) with the following parameters (Skjernaa, 1989): w < 20°, x:y >1, and z:y between 1:1 and 1:10 (see Fig. 71 for parameter definitions), mostly by its long and short axes. The subsequent movement undergone by the anorthosite body during D3 after the sheath folding process was an eastwards to southeastwards tilting (see further discussion in models 1 and 2 p. 108 and 118). This last movement has imposed the present structural features observed all around the anorthosite body. The flattening has continued after the cessation of the tilting.

Interpretation Because the Saririaky area is also part of the Ampanihy shear zone, mapping carried out around the anorthosite body (Map 2, app.) has revealed the shear zone characteristics of the area. Within the structural domains identified, layering is subparallel and at near constant thickness along lengthy strike sections (Map 2, app.). Axial planes of isoclinal and tight folds strike conformably to the general structure of the area. Mylonites are very prominent in the western part of the area (Fig. 67), indicative of ductile processes that allowed the area to accomodate the imposed strain (White et al., 1980). 104

Figure 70: Photograph showing initiation of transposition structure. Garnet-biotite gneiss. Outcrop east of Saririaky massif. Field of view is approximately 1 m. el Locality 36, I: 24°28.36'; L: 44°57.80'. (Fig. 100).

' 1 ff 4a.--; .. ...- t•t•- ' r1SGone,..--e.. -..: .Afj.:..- . ^agi 7; . ‘t e re.nt . 1 S" At 14 :04311* l' ”;- -- 74*r 105

(b)

2 3 Figure 71: (a) Geometrical position of the x. y and axes and the angle a) of a tubular fold. (b) .ry sections through non-cylindrical folds. a, b. c and d have w < 20° measured at section 1, e, f and g have ca < 90° measured at section 2 and h has ro > 90° at section 3. a. b and c have x:y ratios >1, d. c. f have x:y ratios <1 but >1/ 1 . g and h have x:y ratios <1/4. a, b, c are tubular folds, d, e and f are sheath folds, g and h are non-cylindrical folds. (After Skjernaa, 1989) 106 The dominant W-dip (70°-87°) of the units on both sides of the anorthosite body, and flattened western limbs of tight or isoclinal folds, suggest that the intensity rate of the compression from the west was higher during the deformation. As the two anorthosite bodies are contained in the same lithotectonic unit, the Ampanihy Group, it is suggested that they were originally emplaced at similar crustal levels prior to deformation. The difference in shape between the two anorthosite bodies seems very significant and may be interpreted as due to variations in the flattening deformation undergone by these bodies. The lensoid-form of the Ankafotia body suggests that it was more intensely flattened than the Saririaky body. The E-W compressive stresses were probably similar to that around the Ankafotia body, but around the Saririaky body, an additional (sinistral) rotation has probably occurred to cause the asymmetry. This could have been produced by a reorientation of al during the deformation history, or because of originally different orientation of D2 structures between both areas. Mapping around the Saririaky body reveals a prominent asymmetry between its northern and southern extremities (closures) (Map 2, app.); the southern closure appears "displaced" eastwards with respect to a line passing through the northern closure of the body, subparallel to the general orientation of the Ampanihy shear zone.

Two alternative models can explain these observations. The first involves a small rotation of the body and its immediate margin during deformation. The second implies that the D2 structural trends were orientated between N 45° W and N 60° W or NW-SE, i.e. at a different configuration from that around the Ankafotia body (see Fig. 72). 107

D3 anorthosite

Figure 72: Schematic diagram showing the two models: Model 1: the internal rotation of the body during the flattening (pure shear) was accompanied by sinistral shear along the Ampanihy shear zone, Model 2: the envelope only has rotated during D3 (flattening) without necessary simple shear along the Ampanihy shear zone. Note D2 orientations prior to the onset of D3. 108 a. Model 1 In domain 2, two converging mylonites have been distinguished (Fig. 73); the first is a mylonitized interlayered biotite gneiss and amphibolite striking N 25° E, and conforming to the regional trend, and dipping at about 85° W. The second is a mylonitized garnet amphibolite striking N 50° E and dipping from 70° W to vertical. In domain 3 (Fig. 66), folds trend to the northeast from N 40° E to N 60° E, plunge between 7° and 60° NE. The axis of the Vohidrakitse large-scale open fold (Fig. 74) trends to the NE and plunges at about 56° N, and in the eastern part of the area, in domain 5 (Fig. 69 and 74) there are drag-folds whose axes plunge towards south. Observations of satellite imagery, aerial photos, field mapping carried out for this study, and the structures described above reveal that the Saririaky body and its immediate margin may have been rotated during deformation. The rotation was anticlockwise or sinistral, as indicated by the shifting towards east and west, of the southern and northern closures, respectively. The rotation of the body was due to the friction produced by the ductility contrast between the country rocks and the body. The margin' appears to have reacted in a ductile manner. The presence of graphite within the different units may have lubricated the movement, perhaps facilitating the rotation that has resulted in the prominent structures described above in the country rocks. For example, the outcrop in domain 2 appears to represent a well developed late (sinistral) shear zone, probably produced by the rotation of the anorthosite. This structure in domain 3 is also suggestive of the rotation of the body, because they are interpreted as drag-folds produced by the differential rotation between the body and its host rock environment due to their ductility contrast (Fig. 75). The orientation of the Vohidrakitse fold is also probably the result of the differential rotation between the margin and the body. 109

Figure 73:a Photograph showing the obliquity between two mylonitized rocks: amphibolite interlayered with garnet gneiss (upper right part of the photo) and amphibolite. Outcrop in the bed of the Andranozirioky River (Saririaky western margin). Pen 14.5 cm, is oriented N-S. Locality 36, I: 24°28.60'; L: 44°50.24'. (Fig. 100).

/ Fig. 73-e-; 77 7 / 7 _.....-- // / 7'''.-----C--- I / i // i dextral shear zone // i \ h / / ' I \ / ) /

/ / / 7

Figure 73b: Figure 73a was taken at the northern contact

Fig. 74) between the regional foliation and the foliation produced by the anorthosite rotation. Disregarding the rotation of the body, structurally, it may be taken as a sinistral classic shear zone. The structure in the southern contact (•) (in the same margin) would display a dextral classic shear zone, real evidence of the anticlockwise rotation of the anorthosite 110

Figure 74 : Schematic diagram showing the structural features indicative of the anticlockwise (sinistral) rotation (arrows) of the body and the immediate margin: the southern D3 fold closure (Vohidrakitse fold) (1) was prominently rotated to the east, the large-scale fold along the eastern side of the body (2) has its fold axis plunging steeply to the south, and immediately south of the body, drag-folds (3) (Fig. 75) are observed on the northern limb of the large open fold (1) (see Map 2, app. and Fig. 100) whose axial orientations conform to the trend of the latter.

Enlargement of Vohidrakitse fold (1) 111

Figure 75: Photograph of S-folded graphite schist. Outcrop 0.9 km east of Tsiatsimo village (south of Saririaky). Pen 14.5 cm, is oriented N-S. Locality 37, I: 24°32.76'; L: 44°53.85'. (Fig. 100). 112 North of domain 1 (Fig. 76 and 77), anticlockwise rotation of the body has produced a westward deflection of foliations on the country rocks. The subcircular-form of the anorthosite body suggests that the flattening was not as intense as around the Ankafotia body. In the northern sector of domain 2 (Fig. 66; Map 2, app.) graphite schist, quartzite and quartzo-feldspathic rock layers have been cross-cut by the body. This structure suggests that the country rocks have been stacked against the body before its rotation (Fig. 78). Subsequently, the sinistral rotation of the body has slightly deflected the layers (Fig. 76). The rotation movement of the body appears to have been accompanied by its wholescale slight tilting to the east or southeast. Evidence of this tilting includes the opposite trending of the lineations on either side of the body (Fig. 79). Evidence of this tilting is also documented by the lineation attitudes along the southeastern margin of the body. Effectively, in this sector, the converging trends (to the north and south) of the lineations along the margin are indicative of downwards movement of the body to this side. Flattening probably continued after the cessation of the rotation and tilting. Further evidence for rotation and tilting during the flattening in this area comes from a a-5 K-feldspar-blast (Fig. 80) and boudinaged and flattened quartzo-feldspathic veins including feldspathic porphyroblasts (Fig. 81), found on a subvertical bank of the Andranomaleotsy River, S-E Ankilifolo village (Map 2, app.). This structure can be interpreted as involving sinistral rotation during flattening (c.f. Passchier and Simpson, 1986). According to Simpson and De Paor (1993), natural examples of such porphyroclasts imply either that: "(a) the temperature rose during a constant rate deformation; (b) the shear zone remained at the same temperature during a decreasing strain rate deformation; or (c) the shear zone maintained constant temperature and strain rate, but the blast itself 113

Figure 76: Diagram showing the progressive stage having resulted in the deflection of the foliation north of domains 1 and 2. The internal rotation of the body (a) has resulted in the deflection of S3 (b) producing the cross-cutting contact between the body and the host-rocks (graphite schist, quartzites,. • .) (Map 2. app.). 114

0 9 icn

Figure 77: Satellite image of the southern area: in' square, rotation of the body has moved apart the foliation of the country rocks located to the north of the body. The image shows a clear asymmetry between the anorthosite body and the closures at its ends (Map 2, app.) 115

Anorthosite massif

---.. intersection lineation

stretching lineation

Figure 78: This schematic diagram illustrates the possible pattern of lineations and foliations around the body prior to the diffefential rotation and tilting (sheath folding phase). Subvertical stretching lineations occur in the anorthosite body. Inteisection lineations with opposite trends (N and S) on the subvertical to W-dipping foliation planes in the surrounding host rock envelope are subparallel to symmetric fold closures . with respect to the axis of the body. 116

VF: Vorokafotra fold. ---- lineation

Figure 79: Schematic diagram showing the structural features indicative of the Saririaky anorthosite body tilting downwards to the east. The tilting of the body has reoriented the intersection lineations on the foliation planes along the western margin of the body, lineations are NE-trending and moderately plunging to the north whereas along the eastern margin, they are S-trending with moderate plunge. At the southern end of the body, lineations are again NE-trending with moderate plunge. 11/ E Figure 80: Photograph showing a a--8 type of feldspathic porphyroclast on a subvertical bank of Andranomaleotsy River, 1 km southeast Ankilifolo village. Southeastern Saririaky margin. Arrow is 7cm, indicating the top. View looking southwest. Shear sense is sinistral. Locality 38, I: 24°32.02'; L:. 44°57.35'. (Fig. 100).

Shear sense

Figure 81: Photograph of boudinaged and flattened quartzo- feldspathic veins including feldspathic porphyroblasts. Subvertical bank of Andranomaleotsy River, 1 km southeast of Ankilifolo village. Southeastern Saririaky margin. Arrow is 10 cm, indicating the top. View looking southwest. Locality 39, I: 24°32.02'; L: 44°57.35'. (Fig. 100). 118 was initially elliptical in cross-section, and therefore changed its rotation rate during deformation". It is suggested that for the study area, explanation (c) is the most likely; in such a model, the area was subjected to rotation at constant temperature (granulite facies conditions). Tilting progressively decreased the rotation rate of the body whilst overall flattening continued.

b. Modell A second possible model to explain the observations is that the general trend of S2 may have been at a large angle to the principal flattening stress field. An original orientation between N 45 0 W and N 600 W, for example, might have been the regional trend of S2 around the body prior to the flattening (Fig. 72 and 82). These strikes are still visible in the southwestern margin of the body (Map 2) in the domain 3. The effect of eastward-directed compressional flow, oblique with respect to the S2 strike was that the dominant part of the stress did not reach the body, i.e. the country rocks have more or less rotated around the body. In this way, the anorthosite body might have preserved its shape or form acquired, in great part, during D2. Other evidence of the NW-SE structural pattern of D2 prior to D3 is the style of the folds located in the northeastern side of the body (Fig. 64 and 74). Effectively, this area contains an interference of two fold generations. Earlier D2 folds have an axial plane striking roughly E-W, whilst the second phase folds (F3) trend N-NW. The effects of the compression on layers with pre-D3-orientation may have produced folds with axial planes normal or oblique to its general direction. The field evidence cannot distinguish between these two models. The first involves internal rotation of the body during flattening (pure shear) accompanied by sinistral simple shear along the Ampanihy shear zone (Fig. 72a). The second argues for an external rotation of the envelope during flattening (pure shear) (Fig. 72b). 119

t. I • t ! if ! i i. i ! " \• I it • •

i 1 1 il ' ! i a i ! i '.\. I 7 1 /• i i 1I 1 • i . . / I. \ I I / i I il I ) S.2 1 / j i I .1 I ! / • I ri 1 /. / ,/ii j i' 1 I I•I / . / /I .j /• • / • I' • • / ' / / ' • 1 /11 / / i • j ./ / / / / / / / / / i/.7 / / ./ /. // /' ;/ / 1 I ii•• / • / • 'i // i i • . / •/ / • /• i / / ' // • I / / i / / ' i • . 1 I / / / / / / ' ./1 . • / / / ,/' / • !Oil I / / // / / / ' A • • • • • , ./ /III ( 1. ./ 1 / i i I- Raririakv ; / I . // ' •

Figure 82: Schematic diagram showing possible regional trend (a) prior to D3 and (b) after D3. This model suggests that rotation of the host rocks has caused the sinistral rotational structures observed. 120 The two models do not disagree with the flattening or pure shear deformation stated previously for the study area. Figures 83 and 84 display structural data for D3 deformation from each domain of the Saririaky area. The stereoplot of each domain shows that D3 has refolded S2 into asymmetric tight to isoclinal folds. Consistence in concentration between F3 and L3, i.e. they more or less display the same orientation in terms of plunges and trends, suggests that they are probably of the same D3 generation. S3, the planar fabric produced by D3 is defined by mm-scale banding, formed by variable amounts of garnet, biotite, graphite and quartz-K- feldspar. It was produced by intense flattening leading to the transposition of 52 into parallelism with S3. S3 is very prominent (S-tectonites) in domains 1 (Fig. 65), 2 (Fig. 66) and 5 (Fig. 69), and is generally subparallel, striking NE-SW, dipping steep to subvertical and axial planar to similar, tight and isoclinal folds in these domains. Obviously, the orientation is concordant with the general trend of D3. In domains 3 (Fig. 66) and 4 (Fig. 69), S3 is axial planar to the large scale folds of these domains (Fig. 74).

Concerning the superposed fold with subperpendicular axial planes located north of domain 5 (Fig. 69 and 74), it was synchronously formed during D3. The tilting process produced during D3 has emphasized the asymmetry between the two northern and southern closures and the body. Effectively, the downwards tilting of the body to the east and southeast has carried along, to these directions, all the structures folds, lineations) in this side. Evidence of this process is the orientation of Vohidrakitse fold axis that plunges NE at about 50°.

5.4.5. D4 deformation The effects of D4 are the same as in Ankafotia area, i.e. cataclastic structure and prominent grain size reduction for the pyroxene gneiss.

121

Figure 83: Structural data for D3 deformation from domain 1 in the SaririakV area (..see location Fig. 65) The stereoplot shows that L3 are I prominently oriented southwards with moderate plunges, in this domain. S3 are west-dipping, steep to subvertical. F3 is concordant in orientation with L3. 35 poles to foliation plane S3 " Lineation L3 GC: 303°36°N P: 213°54°

T: 179.7° AP:16°70°W P: 50.3° c.c.=24° (not accurate)

Structural data for D3 deformation from domain 2 (see location Fig. 66) L3 are oriented to NE and plunge moderately. S3, in this domain, dip steeply to the northwest. Consistence in concentration between L3 and F3 is shown. 28 poles to foliation plane S3 * Lineation L3 N o 28 2.0°M1% area

GC: 119°33°S P: 29°97°

T: 52.5° AP: 52°69°W P: 33.9° c.c.=12° (not accurate) GC: Great circle (Az, D, D.D) P: Pole to great circle (T, P) Mean value of lineation T: Trend P: Plunge 0 c.c.: 95% confidence cone 122

Figure 84: Structural data for D3 deformatior from domain 3 (see location

Fig. 66) The stereoplot shows northeastwards and eastwards orientations of L3 in the domain. Their plunges are shallow to steep a S2 dip moderately to steeply to the C.I. = 2.0%/1% area southeast and northeast. F3 and L3

❑ are concordant in orientation. GC: 144°44°W T: 63° P:54°46° P: 42.2° 31 poles to foliation plane S3 c.c.=29° (not accurate) AP: 64°80°N Lineation L3

Structural data for D3 deformation from domain 4 (see location Fig. 69) L3 in this domain trend NE with moderate to steep plunges. S2 dip

moderately to steeply to the southeast. F3 and L3 are consistent in concentration. b N= 10 C.I.. 2.0%/1% area 10 poles to foliation plane S3 Lineation L3 GC: 344°24°W ❑ T:23° P: 74°65° P: 42.1° AP: 40°66°E c.c=17.8° (not accurate)

Structural data for D3 deformation from domain 5 (see location Fig. 69) In this domain, L3 are prominently southwards. Their plunges are moderate. S3 dips are steep to subvertical to the west. 23 poles to foliation plane S3 Lineation L3

GC: Great circle (Az. D, D.D) N = 44 C.I. = 2.0%/1% area P: Pole to great circle (T, P) Mean value of lineation T: 169.5° ■ GC: 120°28°N T: Trend P:210°62° P: 66.9° P: Plunge AP: 8°78°W c.c.=27.8° (not accurate) 0 c.c.: 95% confidence cone 123

In conclusion, D3 has imposed a shear zone geometry to the

Ampanihy Group. All structural features of the area, from micro-to meso- scale, described above suggest that the shearing is of pure shear style (flattening deformation). The Saririaky body and its immediate margin only display a sinistral simple shear but this fact does not disturb the structural pattern of the Group. Regionally, foliations are subparallel with the eastern and western boundaries of the shear zone. Axial traces of isoclinal and tight folds follow the S3 regional trend, i.e. most of the structures have been

rotated into parallelism. In the micro-scale structures, pressure-shadows at the ends of garnets and K-feldspar porphyroblasts (augen) are symmetric, i.e. subparallel to the host-rock foliations. D3 resulted in the sheath-fold

geometry of both anorthosite bodies (Fig. 52b and 85b), but as described above, the process was produced by a pure flattening (east-west shortening) mainly for the northern body. No evidence of shear sense criteria was identified either in the field or in the laboratory to conclude that the ductile shearing was simple. All observations are consistent with a pure shear deformation.

5.5. STRUCTURAL INTERPRETATION S3 is the dominant foliation developed by D3, by pure shear

deformation (flattening). High-grade metamorphism (granulite facies) occurred contemporaneously with D3, producing opx+cpx+hbl+pl+grt as a key mineral assemblage mainly present in leptynites and gneisses. D3 was not penetrative across the entire area. Figures 36 and 64 display the structures in the study areas around the bodies. In these figures, S2 foliations are very prominently preserved in D3 fold hinges along the eastern side of the bodies. S2 surfaces have been deformed into similar tight to isoclinal D3 folds at all scales (Fig. 86). Farther to the east (Fig. 36), folds 124

Figure 85: Stereograms showing foliation and intersection lineation attitudes for the envelope area around the Saririaky body.

T: 47.3° I": 21.4° c.c.= 7.5°

T: 25.? P: 49.7° a c.c.= 7.4° N: 121 GC: 113° 18°S P: 23° 72°

T: 184.1° P: 37.8° cc... 7.8° b

•

a: stereoplot of poles to foliation GC: great circle of poles to foliation GC: 113° 18°S Az= 113° D= 18° D. D.=S gl pole to great circle = fold axis (F3 P: 23° 72° T= 23° p= 72° N: number of poles

sl mean value of lineations T: trend P: plunge c.c.: 95% confidence cone 125

Figure 86: Photograph of isoclinally folded feldspathic vein (intrafolial fold) in garnet gneiss layer. S3 is axial planar to the fold. Ankafotia western margin. Pen 14.5 cm, is oriented N-S. Locality 40, I: 23°51.87'; L: 45°06.90'. (Fig. 99). 126 are S-form, preserving their previous styles; they originally were D2 drag- folds but have been flattened by D3. To the west of the bodies, folds are uncommon. Here, transposition of S2 into S3 appears to have been more effective and S3 is more prominent in terms of continuity and density. Immediately north and south of the body axis extension (Fig. 36), folds are similar, and tight to isoclinal. The axial traces of the isoclinal folds to the north and to the south of the Ankafotia body suggest that the pure shear deformation undergone by the region dominated in this area, without significant simple shear along the Ampanihy shear zone. In contrast, the structures around the Saririaky massif (Fig. 64) reveal a distinct asymmetry along the axial traces north and south of the body. This suggests significant rotation, either of the body or of the envelope rocks during the flattening (Fig. 87). The cross-cutting relationships between the body and the country rocks in domains 8 and 2 of Ankafotia and Saririaky areas, respectively, are not indicative of the intrusive emplacement of the bodies. They may be interpreted as the layers were not sufficiently rotated to follow D3 trend. This structure reflects also an oblique contact between them before D2, i.e. there was a slight discordance between the anorthosite-sill and the basement-cover interfaces.

A determination of the complete sequence of deformation events through which the rocks in the map areas have undergone is not possible due to the extreme intensity of D3 deformation, which has mostly overprinted Di and D2. Thus, two phases of deformation (D3 and D4) and three metamorphic events have been most readily recognized. However, D2 can be defined and identified according to the observations made in the field (S2 foliations in the hinges of large-scale D3 folds) and in the laboratory (pre-tectonic garnet with respect to S3). Most of hinges of the large-scale 127

Axis

symmetrical Ankafotia

Axis rotation

asymmetrical Sarirlaky

Figure 87: Sketch illustrating the axial traces of the two anorthosite bodies. Displacement of the axial trace in Saririaky body indicates a measure of simple shear (sinistral). 128 folds are composed of S2 and have retained evidence of pre-tectonic

porphyroblasts (garnet, sillimanite, K-feldspar). It is suggested that at the time when compressional deformation began, the region was composed of subhorizontal layers with interbedded sheet-like or tabular anorthosite separately located in the north (Ankafotia) and in the south (Saririaky). The cause of deformation was due to E-W directed compressional stresses, and with greater strain concentration along the western margin of the body, which is more flattened than the eastern margin. The effects of the compression on the subhorizontal layers were first the initiation of folding (buckling) followed by formation of upright to E-overturned buckle folds. Aided by flexural flow at amphibolite-grade metamorphism, the buckling produced asymmetric similar folds steeply overturned to the east. The mineral assemblage (M2) during this event D2

could have been cpx+grt+pl+hbl+sil+bt as preserved in some gneisses and graphite schists (Fig. 88). The deformation could have resulted from a local collision or the impact of a continental collision occurring in the Mozambique Belt. S3 is the prominent and penetrative D3 structure in the region. The D3 event took place when the progressive compression gave rise to asymmetric tight to isoclinal folds (Fig. 89) in the surroundings of the anorthosite bodies. During this event, flattening occurred in the area as a result of the compressional deformation. The flattening was not sufficiently penetrative due to stress and strain partitioning. The result is evident in the elongate shape of the Ankafotia body and subcircular shape of the Saririaky body. The present shapes of the bodies could be attributed to variations in the local orientation of D2 structures. In the northern area, the general trend of D2 approximately conformed to the present orientation of the

Ankafotia massif, i.e. between N-S and N 15° E. The effects of the high flattening strains were well-developed subvertical S-tectonites (S3) within 129

Deformation phases

Dl D2 D3 D4

Biotite ------

Plagioclase ------

Clinopyroxene --- ------

Garnet ------

Hornblende — —

Hypersthene

Sillimanite

Amphibolite Granulite Granulite- fades (M2) facies (M3) amphibolite . facies transition ? (M4)

Figure 88z Relation between mineral formation and deformation. This

diagram shows that D3 deformation took place during a granulite

metamorphism event. D3 and M3 have developed between an

amphibolite facies metamorphism M2 (with pre-D3 sillimanite)

and a granulite-amphibolite facies transition (M4) (with post-D3

garnet). 130

? • 047? ;tit f -t* -1W-al:r 110., f, 2 I, • --: /a z. -••• 13.. a • -•-••`••": 7 :- ti -. • j..C.g.t.

sat.- e -.• •ti . 4111 '

Figure 89: Photograph of isoclinally folded layered garnet gneiss. Note the steep plunge to the south of the fold axis. Outcrop in Ankafotia western margin. Pen 14.5 cm, is oriented N-S. Locality 41, I: 23°50.59'; L: 45°07.90'. (Fig. 99).

`•

-et.*

/ -••••••.4 ,e• • 'te' '6..• • ‘• •

, ',.C•• • ••

Figure 90: Photograph showing flattened quartzo-feldspathic veins producing small scale folding in garnet gneiss. Outcrop in Ankafotia western margin. Pen 14.5 cm, is oriented N-S. Locality 42, I: 23°51.82'; L: 45°06.95'. (Fig. 99). 132 Chapter 6: METAMORPHISM

6.1. PROGRADE METAMORPHISM Textural evidence observed in thin sections in which S3 wraps around K-feldspar (Fig. 32), sillimanite (Fig. 16), garnet porphyroblasts (Fig. 40), and biotite (sample L 536), suggests that upper amphibolite facies metamorphism occurred prior to D3. The paragenesis crystallized prior to D3 (=M2) is presumed to be sil+kf+bio+grt+pl+qtz (Winkler, 1979) (Fig. 91a). The P-T conditions of this pelitic assemblage reached a maximum temperature between 500°C and 680°C (Bucher and Frey, 1994) and a maximum pressure between 5.5 and 7.1 kbar (Carmichael, 1978), since 500°C represents the first appearence of the garnet. In metabasic rocks, the mineral assemblage is hbl+pl+grt+cpx±qtz (Fig. 91b). The cpx marks the beginning of upper amphibolite facies and a representative temperature is about 650°C (Bucher and Frey, 1994). Other thin sections (sample R 140) reveal high-grade metamorphic mineral assemblages. In metabasic rocks (amphibolite gneiss), the mineral assemblage is: opx+cp+pl+grt+hbaqtz and in metapelitic and quartzo- feldspathic rocks (sample), the observed mineral assemblage is kf+pl+opx+cpx+grt+qtz (Fig. 92a). These are indicative of granulite facies metamorphism. Nicollet (1990) stated that the Ampanihy Group is of high pressure metamorphic grade ranging between 7 and 9 kbar, and with temperatures between 710°C and 890°C.

6.2. TEXTURAL FEATURES OF RETROGRESSIVE METAMORPHISM Observations of thin sections from rocks of the area however, indicate a granulite-amphibolite facies transition zone. Effectively, a high proportion of amphibole (hornblende; mostly brown in colour) in most of 133

Figure 91a: Amphibolite facies: ACF-A'KF projection showing mineral

assemblage in metapelitic rocks (after Winkler, 1979).

•

Figure 91b: Amphibolite facies: ACF projection showing mineral assemblage in metabasic rocks (after Winkler, 1979). 134

Figure 92a: Granulite facies: ACF-ICKF projection illustrating mineral assemblage in metapelitic rocks (After Winkler, 1 9 7 9 ) .

Figure 92b: Granulite facies: ACF projection illustrating mineral assemblage in mafic rocks (After Winkler, 1979). 135 the rocks suggests retrograde amphibolite facies (hydrous facies). In addition, partial or complete pseudomorphing, corona textures and rims are also visible in thin sections from rocks of the area. These textural features are indicative of incomplete re-equilibration of granulite facies assemblages at amphibolite facies conditions. Partial or complete pseudomorphing are visible as chloritization of garnet and as amphibole replacement of pyroxene. Corona textures in some rocks (amphibolites) show garnet porphyroclasts rimmed by symplectitic intergrowths of opx and pl. Opx-pl symplectites are formed at the expense of garnet in both qtz-present and qtz- absent rock types, produced by the reactions: grt + qtz = opx + pl and grt + cpx + qtz = opx + pl (Harley, 1989). The latter shows the instability of grt±cpx+qtz, indicative of new metamorphic conditions of temperatures and pressions. Zoning as measured by electron microprobe on certain minerals including garnet, orthopyroxene, clinopyroxene, amphibole and plagioclase (Tables 5, 6, 7 and 8, app.) also emphasize the retrogressive metamorphism affecting the area. Traverses from rim to rim have been made for each phase, isolated and in contact (for example garnet in contact with orthopyroxene). The characteristic chemical profiles of garnets that have undergone homogenization and retrograde zoning include a core to rim increase in Mn, a decrease in Mg, and a Ca flat profile. This contrasts with growth zoning, which is usually characterized by bell-shaped Mn profiles and core to rim increase in Mg (Tucillo et al., 1990). According to Loomis (1975), compositional zoning of garnet preserves evidence of the equilibration history of the sample and can be interpreted in terms of a growth-fractionation, diffusion-exchange or diffusion-reaction models. Changes in the metamorphic conditions (P and T) lead to the re- equilibration of the granulite facies rocks into amphibolite facies conditions 136 and diffusion-exchange between garnet and other surrounding phases is favored by the same factors that favor retrogression: time, temperature, and volatiles. Diffusional retrograde effects, thus, result in chemical variations of Fe, Mg, Mn and Ca from core to rim of the minerals (Fig. 93). Grant and Weiblen (1971), also stated that rims of high-grade garnets are formed as a result of diffusive exchange with matrix minerals during cooling. Profiles in Figure 94, from garnet, depict an increase in Mn from core to rim and a slight decrease in Mg; Fe and Ca display opposite profile patterns. These profile patterns are consistent with retrograde zoning. The amphibolitization of the granulites can be attributed to fluid infiltrations that can introduce or remove major or minor elements (St-Onge et al., 1995).

6.3. RETROGRADE P-T PATH AND TECTONIC RELATIONSHIPS The core garnet compositions record information concerning the metamorphic conditions at peak metamorphic temperatures, and the garnet rims record information during cooling (Bohlen, 1987). Barker (1990) suggested that coronas and rims are indicative of disequilibrium betWeen the porphyroblasts and the matrix due to shearing or in association with uplift and cooling. This argument coincides with the study area setting in the fact that fluid must have passed along the shear zone and fluid infiltration may have utilized network fractures as principal fluid pathways. Harley (1989) noted that opx-pl symplectites around embayed garnet grains are characteristic of mafic granulites that have undergone near- isothermal decompression (ITD) paths, i.e. the maximum pressures the rocks experience occur long before the thermal maximum (i.e. a clockwise P-T-time progression as viewed on a P-T diagram) (Fig. 95). Most of the ITD paths are found in granulites metamorphosed in the range 6 - 9 kbar and 137

rim core rim 1.86

1.84

1.82 Ca 1.8

1.78

1.76 1.5 mm 1.74

rim core rim 0.04

0.035 0.03

0.025 0.02

0.015

0.01 0.005

0 1.5 mm

Figure 93: Illustration showing zoning patterns (Ca, Mn. Mg, Fe atoms per formula unit) in amphibole from amphibolite (sample G 85). 138

rim core rim 20

19.5

18.5

17.5

3 — rim core rim

2.5 —

2 — Mn 1.5 —

0.5 2.5 mm 0

55 rim core rim

50 Fe - 45

35 Mg 30

25 2.5 mm 20

Figure 94: Illustration showing zoning patterns (atomic percent Fe, Mg, Ca and Mn) in garnet (Sample G 85). Figure 95: T-time path. conditions thatwouldberecordedbyrockspassingalongtheP- 1989). Boxenclosestherelativelyrestrictedrangeofpeak time path(clockwisepath)forgranuliteterrains(Harley, Bohlen (1987).Filledcircleindicatesthelikely'peak'P-T metamorphic conditionsbyNewtonandPerkins(1982) Diagramshowingpossibledecompressional(ITD)P-T-

Pressure ( k bar) Temperature (92) 139 140 700 - 850°C, and tectonic thickening is the principal environments for the genesis of ITD-type granulites (Harley, 1989). Moreover, Passchier et al. (1990) argued that ITD-path also is indicated as typical of uplift and erosion of continental crust that was thickened by collision.

6.4.RELATIONSHIP BETWEEN METAMORPHISM AND DEFORMATION The tectono-metamorphic evolution of the Ampanihy Group has culminated in the development of granulite facies metamorphism throughout the region. This phase was preceded by an amphibolite facies metamorphism M2 coeval with D2, with the poorly preserved mineral assemblage of grt+cpx+hbl+pl+sil. The M2 assemblage suggests temperatures between 500°C and 680°C and pressures between 5.5 and 7.1 kbar (Carmichael, 1978). The M2 event was followed by granulite facies metamorphism (M3), a separate metamorphic event, during deformation (D3) represented -by folding (tight to isoclinal folds) and flattening (pure shear deformation), which was responsible for the present structural pattern of the shear zone. M3 mineral assemblages consist of grt+opx+cpx+pl+qtz, and a conspicuous S3 foliation that deflects around D2 garnet, K-feldspar or sillimanite porphyroblasts. The maximum temperature during M3 is estimated to be between 710°C and 890°C and maximum pressure between 7 kbar and 9 kbar (Nicollet, 1988). The peak metamorphic event M3 was followed by a retrograde metamorphism (M4), of granulite-amphibolite transition facies condition. M4 is poorly developed in the field and is difficult to separate from D3. M4 occurred during the uplift event subsequent to D3. The retrograde metamorphism is recorded by partial to complete pseudomorphing of garnet, and by amphibole pseudomorph of pyroxene, and by corona reaction 141 rims. The latter consists of garnet cores surrounded by opx and pl intergrowths (symplectites) (Fig. 17), indicative of medium-to high retrograde metamorphism during isothermal decompression (ITD) (c.f. Harley, 1989). The growth and recrystallization of the post-D3 garnet throughout the region may also be attributed to this event. During the uplift event also occurred emplacement of basic rocks; these rocks were subsequently metamorphosed into massive garnet amphibolites and pyroxenites during M4 (Table 9). 142 t) lif ) Up Ma ( 50 ing 4 hear 580- (

• Post-s

de a r lite

) s

h-g

anu ie ) r ing fac (g Hig Ma ar he 640 n-s 650- Sy (

ing r hea -s Pre

Destroyed, 5 overprinted

del to o cn 0 ..0 d m 0 C an

n evt n pC) io

t 7 C -N5, me Y tn 4-1 41 rma 1..

iron i-i <12 fo

v e" f o En

v, cu s.— ad cs) >-, ..c c— C, M. E -tt 143

Chapter 7: TECTONIC INTERPRETATIONS AND CONCLUSION

As noted previously, during E-W compressional stresses, flattening strains (pure shear) during D3 produced the Ampanihy shear zone. It is

possible that this may have occurred during the amalgamation of the Gondwana supercontinent. Collision at active plate margins is commonly accompanied by an episode of escape tectonics (Burke and Sengor, 1986). Escape tectonic structures may be far within the plate interiors where major strike-slip faulting, short-lived sedimentary basin formation and deformation in thrust and fold belts may occur. Large systems of late shear zones hundreds of kilometres wide and thousands of kilometres long are present in all Neoproterozoic orogens of Gondwana (Unrug, 1990). Individual mylonite belts are up to several km wide. Some of these shear zones are in structural continuity with transpressional fold belts and formed simultaneously with the fold belts under amphibolite facies conditions (Pinna, 1993). Deformation in the Neoproterozoic mobile belts of Gondwana apparently was completed by 550 Ma (Unrug, 1992). Collision and deformation events occurred in the 820-540 Ma interval during the assembly of Gondwana supercontinent (Unrug, 1992). The Mozambique Belt is interpreted as having formed during Tibetan style collision of continental plates (Chewaka and de Wit, 1981; Burke and Sengor, 1986). The data presented in this thesis suggest that the Ampanihy shear zone (ASZ), might also have been generated within this Mozambique Belt continent-continent collisional environment. The Ampanihy shear zone may be interpreted as result of crustal shortening conditions having progressed from west to east across a major collision zone (Fig. 96). 144

Ampanihy terrane

Ampanihy shear zone Anorthosite

Ampanihy terrane

Figure 96: Schematic profiles illustrating the kinematic evolution of the Ampanihy Group during D2 and D3 events. 145 Locally, the flattening could be interpreted as the effect of eastward displacement of the Vohibory terrane against the Ampandrandava terrane (Fig. 97). The motion of the Vohibory terrane had started after the Ampanihy terrane had overthrust the Ampandrandava terrane along a crustal scale ramp. During its eastward displacement, the terrane compressed Ampanihy terrane against the Ampandrandava terrane lateral ramp. In this way, the Ampanihy terrane had been "sandwiched" between the Vohibory terrane front and the Ampandrandava terrane ramp. Compression was approximately E-W, imposing a pure shear deformation to the Ampanihy terrane. Geochronology of the Ampanihy shear zone indicates its formation (and hence D3) by about 650 and 640 Ma (de Wit et al., 1993), therefore, significant plate convergence in the Mozambique Belt must have occurred during and prior to this time. D3 was subsquently followed by uplift accompanied by medium-to high retrograde metamorphism M4 (near-isothermal decompression).

The structural pattern of the Ampanihy shear zone, i.e. its N-S to NNE-trend and steep W-to subvertical dip suggests that these characteristics are consistent with the main Pan-African structures (Fig. 98). Effectively, N to NNE-trending transpressive shear zones and ductile belts are the main Pan-African structures, and compression ranges from NNW to NNE (Pinna, 1993). It is noteworthy that according to de Wit et al., (1993), the Ampanihy Group probably represents mid-late Proterozoic sediments thrust over an early-Proterozoic-Archean basement complex to the east (Ampandrandava Group), whilst the Vohibory terrane was in turn thrust over eastward across the late Proterozoic cover. This structural feature represents crustal thickening and uplift, indicative of a collisional environment (c.f. Bohlen. 1991; Ashwal et al., 1992). Other evidence is the granulite facies metamorphism in the region. It seems, therefore, that the 146

ASZ

Ankafotia massif

7 a

ASZ

Saririaky massif

V: Vohibory terrane APH: Ampanihy terrane APD: Ampandrandava terrane ASZ: Ampanihy shear zone

Figure 97: Schematic profiles through the northern (Ankafotia) (a) and southern (Saririaky) (b) anorthosite bodies. 147

A `•\ 1 1

\II I. \ I :

It 11\ ‘,..—, I It. 0\

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SOC Group

Serpentinite / Ophiolite

J Trend .0 /"5-• Structural

(D Granulite Klippe

Shear Zone .0 A Achankovil 0 300km S B Buur Massif O

Figure 98: Illustration showing the main Precambrian structure of Eastern Africa, Madagascar, southern India and Sri Lanka (After Windley et al., 1994). 148 most appropriate tectonic setting for the study area is a continent-continent collisional environment. The Ampanihy shear zone and the two anorthosite bodies, Ankafotia and Saririaky within it, have been the subject of different hypotheses related to their structural evolution. Kilmer and Duncan (1991) proposed a model of megaboudinaging for the two bodies in a ductile simple shear zone. Rolin (1991) supported a dextral shear sense for the Ampanihy shear zone, with formation of subhorizontal lineations; this was also the preferred interpretation of Windley et al., (1994). Martelat et al. (1995) however, argued for a sinistral shear sense whilst de Wit et al. (1993), emphasized the pure shear (flattening) origin for the shear zone. Morel (1996), on the basis of detailed geology of the Ankafotia body, determined that subvertical lineations predominated within the anorthosite body and emphasized a sheath-like geometry for the body. This study has substantiated that the structural characteristics of the Ampanihy shear zone are consistent with pure shear deformation (flattening), result of a E-W compressional stresses approximately. During this shear zone forming event, termed (D2-D3) the two anorthosite bodies

acquired sheath folding characteristics with steep-moderate lineations. This structural evolution is substantiated by the moderate to steep intersection lineation pattern all around the bodies. Lineations, mainly composed of intersection lineations, are either north or south-trending with plunges ranging from moderately to steeply (40 0-881 (Table 10). 149

Table 10: Plunges of lineations

Domain Ankafotia area Saririaky area

1 50° 50° 2 40° 340 3 86° 42° 4 76° 67° 5 54° 40° 6 58° 7 88° 8 54°

Mean value of From Fig. 52b: From Fig. 85b: concentration 39° 21° 89° 50° 32° 38°

On the other hand, the anorthosite outcrops of the Ankafotia body display subvertical stretching lineations (Morel, 1996). Other evidence for a sheath- like geometry of the body includes the presence of opposing fold closures at the ends of the bodies and the apparent fold 'wrapping' of the envelope lithologies entirely around both bodies. D3 was accompanied by granulite facies metamorphism (M3), with a maximum temperature between 710°C and 890°C and a maximum pressure between 7 and 9 kbar. (M3) was subsequently followed by high-to medium retrograde metamorphism (M4), of isothermal decompression (ITD) type, having grown or recrystallized the garnet visible in most of the units of the region. Finally, the region has been subject to brittle deformation (D4) resulting in the grain size reduction and cataclastic structure of the pyroxene gneiss. 150 All the structural features recorded above are compatible with a model of a continent-continent collisional event during Gondwana assembly. The pure shear deformation that generated the Ampanihy shear zone could have been a post-collisional response following an eastward- stacking or shortening of the Ampanihy Group across the Ampandrandava lateral ramp by the Vohibory Group. According to de Wit (1993), the deformation took place between 650-640 Ma (U/Pb monazite). In a regional Gondwana context, the Ampanihy Group can be linked with the Usagaran Complex of northeastern Tanzania (Table 11). Effectively, both areas have been subject to amphibolite and granulite facies metamorphisms that produced amphibolites, gneisses, marbles and graphite gneisses. Anorthosite occurrences are prominent in both areas. Structural patterns display similarities both in fold vergences, trends, styles (Table 11) and inferred tectonic transport direction. Having constrained the tectonic transport (eastward) in the southwestern part of Madagascar (in this study), it is now important to also constrain other tectonic elements in Madagascar so that the dynamics of the Mozambique Belt might be better understood. For example, the vergence of the tectonic transport north of the Bongolava-Ranotsara shear zone, and the BRSZ equivalent in northeastern Africa is unknown. Constraining these parameters will help to unravel the tectonic history of adjoining regions in Africa and India. The tectonic vergence direction (facing direction of thrusts, overfolds and other asymmetric structures) is of fundamental significance in understanding the way in which a crustal terrane has been deformed (Park, 1988). 151

Table 11: Usagaran Complex and Ampartihy Group correlations

USAGARAN COMPLEX AMPANIHY GROUP

Location Eastern Tanzania Southern Madagascar

Metamorphism Upper amphibolite- Upper amphibolite- granulite facies (Maboko, granulite facies 1995)

Lithology Granulites, amphibolites, Leptynites, gneisses, graphite gneisses, marbles, graphite schists, scattered mafic and marbles, quartzites, ultramafic bodies (Malisa amphibolites and Muhongo, 1990)

Occurrences Two anorthosite bodies Four anorthosite bodies

Mineral assemblage Hbl-opx-cpx-grt-pl-scp-qtz Hbl-opx-cpx-grt-pl-kf-qtz

Trends N to NNE (Pinna, 1993) N to NNE

Vergences Westward and eastward; Eastward NNW (Pinna, 1993)

Main structures -Transpressive shear zones -Compressive shear zone -Ductile belts (Pinna, 1994) (de Wit et al., 1993) -Recumbent folds (Pinna, -Isoclinal folds 1993) -Thrusting (de Wit et al., -Thrusting (Muhongo, 1991) 1993)

Geochronology -652±10 (U-Pb /zircon): -640-650 (U-Pb /monazite): (Ma) granulite facies (Coolen et flattening (de Wit et al., al., 1982) 1993) -695±4 (U-Pb /zircon): -ca. 1 Ga (Sm-Nd): anorthosites (Muhongo and anorthosite emplacement Lenoir, 1994) (Ashwal et al., 1994) • -618±16 (Sm-Nd): granulite -550-650 (U-Pb/zircon): facies (Maboko and granulite facies (Ashwal et Nakamura, 1995) al., 1994) 23049 4

IN 23050

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23055

23056 Fenoanevo •28

15'• • 32. • 2

23057

23058

23059

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_ 24000

0 2 km

45010 45011 45006

Figure 99: Map showing figure localities in Ankafotia area. 153

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