The Mavuradonha Layered Complex: Neoproterozoic ... - ArchiMeD

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Abstract An understanding of the geological evolution of the Zambezi belt is important for the geodynamic evolution of the Neoproterozoic network of orogenic belts in West- Gondwana. A detailed petrological, geochronological, geochemical and isotopic characterisation of the Mavuradonha Layered Complex in northern Zimbabwe has been carried out in order to contribute to a better understanding of the geological evolution of the NE-part of the Zambezi belt. The Mavuradonha Layered Complex represents a lower crustal complex which was generated during an early Pan-African extensional event in the northeastern Zambezi belt. Emplacement of the complex was dated at 862 ± 4 Ma using SHRIMP and vapour transfer methods on zircons. This magmatic emplacement age is also supported by a Sm-Nd whole rock errorchron age of 899 ± 46 Ma. Additional age information was obtained on zircons of the Ocellar Gneiss, which structurally underlies the Mavuradonha Layered Complex. These zircons reveal a Kibaran protolith age of 1022 ± 7 Ma. Within the Mavuradonha Layered Complex a multiphase magmatic differentiation is recorded in macro rhythmic units and small-scale layering. This multiphase differentiation led to the formation of magmatic sequences comprising pyroxenites, gabbro/norites, leuco-gabbros within the layered gabbro series. Additionally, large amounts of ferro-gabbros are associated with anorthosites in the lower meta-anorthosite suite and the upper meta-anorthosite suite. It is also shown that there is evidence in the isotopic record for substantially depleted mantle values in the NE-Zambezi belt during the Neoproterozoic. Crustal contamination of the subalkaline, tholeiitic parental magma is indicated by variable εNd values between + 0.3 and + 6.6, as well as abundant xenoliths and inherited zircons in the layered gabbro series of the Mavuradonha Layered Complex. The Mavuradonha Layered Complex was overprinted by granulite-facies metamorphism with peak-conditions of 13 ± 2 kbar at 840 ± 30 °C during the Pan-African orogeny at about 554 ± 13 Ma. This age of metamorphism was estimated from metamorphic overgrowth on zircon using the SHRIMP method. After the granulite-facies peak, the complex was retrogressed under amphibolite-facies conditions at 11 ± 2 kbar and 680 ± 20 °C, and, locally further retrogression under lower amphibolite- to greenschistfacies conditions took place. The cooling event was accompanied by infiltration of highly enriched saline fluids as indicated by the abundance of late, chlorine-rich scapolites, amphiboles and corona-textures in the metagabbros. Retrogression occurred at about 546 ± 9 Ma and is constrained by Sm-Nd garnet-whole rock ages. Further cooling to greenschist-facies conditions occurred within the following 50 Ma as constrained by U-Pb III
utile ages ranging from 526 ± 7 to 501 ± 6 Ma. Taking into account the PT data, a PT path is suggested that resulted from crustal thickening related to plate convergence. The parental magma of the metagabbroic bodies that occur as inliers in the Mt. Darwin area also have a subalkaline, continental tholeiitic character. These inlier metagabbros have a geochemical composition that is different from the Mavuradonha Layered Complex metagabbros. They are slightly enriched in incompatible and rare earth elements. The isotope characteristics of these metagabbros also imply a crustal setting for the inlier metagabbros, but probably with higher degrees of contamination by continental crust (εNd ranging from + 2.4 to – 3.5) than is recognised for the Mavuradonha Layered Complex. The data presented in this study suggest that the evolution of the Zambezi belt in NE-Zimbabwe began as a failed rift or an intracratonic basin. The results of the geochronological studies imply no genetic link between intrusion of the mafic magmas and high-temperature granulite-facies metamorphism. The age data show that granulite-facies regional metamorphism occurred at least 300 Ma after the emplacement of the mafic rocks. Therefore, emplacement of the Mavuradonha Layered Complex and the inlier gabbros represents an episode of intense plutonic activity during an early stage of rifting. The basin or failed rift was closed during the late Pan-African orogeny. IV
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: This project was part of the Mainz-
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 and 22: 1 Introduction 7 Zambezi Belt Mount
Page 23 and 24: 1 Introduction 9 metamorphosed unde
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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Plate III 49 Plate III-8: Corona te
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Pl III-4 Am III-5 III-6 Am Pl Am Op
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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-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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