The Mavuradonha Layered Complex: Neoproterozoic ... - ArchiMeD

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1 Introduction 8 The Rushinga Metamorphic Suite is interpreted as a sequence of shallow water sediments representing sedimentation along the margin of the Zimbabwe Craton. This strongly deformed sequence shows different metamorphic grades ranging from amphibolite-facies to granulite-facies. The Chimanda Metamorphic Suite consists of reworked basement and supracrustal rocks. The Zambezi Allochthonous Terrain is the structurally highest unit in the north of the Zambezi belt. This terrain is bordered in the north by the Zambezi Escarpment and to the south by the Masoso thrust of the Rushinga Metamorphic Suite. The Zambezi Allochthonous Terrain is interpreted as a thrust pile composed of felsic and mafic intrusive rocks of the Masoso Metamorphic Suite and Mavuradonha Metamorphic Suite which were thrust onto the Marginal Gneiss Terrain. The Zambezi Allochthonous Terrain contains rocks of the highest grade of metamorphism that were found in this part of the Zambezi belt and consists of garnet- and orthopyroxene-bearing granulites. The Masoso Metamorphic Suite forms the basal unit of the Zambezi Allochthonous Terrain and consists of a bimodal suite of leucomigmatites and striped mafic gneisses and metaplutonic rocks of amphibolite-facies grade. Locally, within mafic horizons garnetbearing granulites are observed. Some of the Masoso rocks show geochemical affinities to the Basal Rushinga Intrusive Complex. The Mavuradonha Metamorphic Suite is the structurally highest unit in the northeastern part of Zambezi belt and was thrust onto the Masoso Metamorphic Suite along the Mavuradonha thrust. The Mavuradonha Metamorphic Suite was overprinted under granulite-grade peak metamorphic conditions, indicating the highest metamorphic conditions in this part of the Zambezi belt. At the base it is composed of the Nyamasoto gneiss (Barton et al., 1991) and the Ocellar gneiss (Bache et al., 1983). Granulite-facies metagabbros, which are retrogressed under amphibolite-facies conditions, rest structurally on top of the gneisses. Gabbroic to anorthositic garnet- and clinopyroxene-bearing granulites are exposed next to the Zambezi Escarpment. The base of the Mavuradonha Metamorphic Suite exhibits large bodies of garnet-free amphibolites and garnet-bearing amphibolites that locally preserve gabbroic textures. Three major deformation events were observed in the Zambezi belt of NE Zimbabwe, which are referred to as the DZM 1 to DZM 3 (Deformation episodes Zambezi-Mozambique) by Barton et al. (1991; 1993). DZM 1 refers to the planar compositional layering and is presumed to be the result of deep crustal extension of thickened crust due to underplating. This event is restricted to the Masoso Metamorphic Suite and the Mavuradonha Metamorphic Suite. Emplacement of allochthonous terranes of the Zambezi Allochthonous Terrain onto the craton is correlated with the DZM 2 episode. During this event, the Marginal Gneiss Terrain was
1 Introduction 9 metamorphosed under high-pressure granulite-facies conditions of ~800 °C and 12 kbar (Treloar et al., 1990; Carney et al., 1991). This event was accompanied by exhumation of deep crustal granulites. DZM 3 indicates infolding and north-directed overthrusting which is related to compressional tectonics and the closure of a basin. These deformational episodes have recently been revised by Dirks et al. (1998). The authors divided the deformational episodes into three groups. The D1 event proposed by Dirks et al. (1998) represents structures of Archaean age which are restricted to the Archaean basement. D2 deformation is supposed to be Pan-African structures indicating E- to NNE-directed shearing that resulted in the formation of isoclinal, recumbent and upright fold geometries that affected the entire Zambezi belt. Structures related to D2 are inferred to be coeval with the emplacement of the Basal Rushinga Intrusive Complex, and mafic dykes which are suggested to be related to extensional processes. D3 structures are restricted to shear bands that truncate the D2 fabric. 1.3 Previous work The first mapping was undertaken by Leitner & Phaup (1974) who named the units in the Mt. Darwin area and subdivided the Rushinga Group into three units: the lower, the middle and the Upper Rushinga Group. They named the metagabbros west of the village of Dotito the Nyamhanda Inlier and interpreted these rocks as complexly infolded with the underlying gneisses. The upper Rushinga Group was re-named by Bache et al. (1983) who also mapped the Mavuradonha Mountains and several other gabbroic bodies in the Mt. Darwin and Centenary areas. They interpreted the metagabbros in the Mavuradonha Mountains as a thick sill which intruded into the underlying amphibolites. Other metagabbros were interpreted as outliers which were thrust onto the felsic gneisses. More recently, Barton et al. (1991) mapped the Rushinga area and neighbouring terrains and interpreted the metagabbro-leuco-metagabbro-amphibolite-association in the Mavuradonha Mountains as highly fractionated rocks. Published radiometric whole-rock Rb/Sr age data fall into two groups: the first group with ages of 830 ± 30 Ma reflects the metamorphic and plutonic climax. The second group shows ages between ~660 and ~470 Ma and was interpreted as cooling ages. Treloar et al. (1990) and Carney et al. (1991) reported metamorphic PT conditions of ~800 °C and 12 kbar for the high-grade regional metamorphism corresponding to DZM 1, and 625 to 700 °C and 7 ± 2 kbar for metamorphism related to the DZM 2 in the neighbouring Rushinga area. Collectively, these data suggest an anticlockwise PT path. Makanza (1993) and Muzuwa (1993) mapped the area next to the Zambezi Escarpment north of the main ridge of the Mavuradonha Mountains and interpreted the metagabbro-anorthosite-amphibolite association as layered sequences that were metamorphosed under amphibolite- to granulite-facies conditions.
Page 1 and 2: The Mavuradonha Layered Complex: Ne
Page 3 and 4: All views and results presented in
Page 5 and 6: Table of contents Acknowledgements
Page 7 and 8: 6.2.3.4 Summary and interpretation
Page 9 and 10: This project was part of the Mainz-
Page 11 and 12: utile ages ranging from 526 ± 7 to
Page 13 and 14: Alter von 899 ± 46 Ma untermauert.
Page 15 and 16: 1 Introduction 1 1 Introduction 1.1
Page 17 and 18: 1 Introduction 3 1993). In the West
Page 19 and 20: 1 Introduction 5 Moreover, the stru
Page 21: 1 Introduction 7 Zambezi Belt Mount
Page 25 and 26: 1 Introduction 11 on minerals with
Page 27 and 28: 2 Geology of the study areas 13 Zam
Page 29 and 30: 2 Geology of the study areas 15 The
Page 31 and 32: Plate I Plate I-1: View at the land
Page 33 and 34: Plate I Plate I-3: Layering in the
Page 35 and 36: Plate I Plate I-5: Compositional la
Page 37 and 38: Plate I Plate I-7: Shearzone within
Page 39 and 40: 2 Geology of the study areas 25 def
Page 41 and 42: 2 Geology of the study areas 27 Two
Page 43 and 44: 3 Analytical methods 29 3 Analytica
Page 45 and 46: 3 Analytical methods 31 Mineral sep
Page 47 and 48: 3 Analytical methods 33 U-Pb rutile
Page 49 and 50: 3 Analytical methods 35 microscopic
Page 51 and 52: 4 Petrology and metamorphic evoluti
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4 Petrology and metamorphic evoluti
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5 Geochemistry 79 a) Gabbro / Norit
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5 Geochemistry 81 Chimwaya Hill Inl
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31 28 25 22 19 16 13 10 2.5 1.6 1.4
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5 Geochemistry 85 25 20 15 10 5 Ga
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5 Geochemistry 87 The trace element
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5 Geochemistry 89 1000 900 800 700
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5 Geochemistry 91 600 500 400 300 2
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5 Geochemistry 93 proposed for gabb
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5 Geochemistry 95 1000 900 800 700
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6 Geochronology and zircon-cathodol
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6 Geochronology and zircon cathodol
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6 Geochronology and zircon cathodol
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7 Sm-Nd, Rb-Sr and Pb-Pb isotopic s
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8 Petrogenetic and geotectonic impl
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9 Interpretation in the regional co
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References 164 References ARANOVICH
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References 166 COMPSTON, W., WILLIA
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References 168 GRAHAM, C.M. and POW
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References 170 JOHN, T., SCHENK, V.
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References 172 KROGH RAVNA, E. (200
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References 174 MEZGER, K., HANSON,
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References 176 POLLER, U. (1999): A
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References 178 TEERTSTRA, D.K. and
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Appendix 180 Appendix A-I Geology o
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Appendix A-I. Geology of the study
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Appendix A-I. Geology of the study
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Appendix A-II. Analytical methods v
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Appendix A-II. Analytical methods v
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Appendix A-III. Petrology and metam
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Appendix A-III. Petrography and met
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Appendix A-IV. Geochemistry xlviii
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Appendix A-IV. Geochemistry l Table
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Appendix A-IV. Geochemistry lii Tab
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Appendix A-IV. Geochemistry liv Tab
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Appendix A-IV. Geochemistry lviii T
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Appendix A-V. Geochronology and cat
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Appendix A-VI. Sm-Nd, Rb-Sr and Pb-
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Appendix A-VI. Sm-Nd, Rb-Sr and Pb-
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Curriculum vitae 181 Curriculum Vit
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