266 Pages
English

The Mavuradonha layered complex [Elektronische Ressource] : neoproterozoic emplacement and Pan-African granulite facies metamorphism in the Zambezi Allochthonous Terrane of the Mt. Darwin Area, Zambezi belt, NE-Zimbabwe / Mario Müller

-

Gain access to the library to view online
Learn more

Description

The Mavuradonha Layered Complex: Neoproterozoic emplacement and Pan-African granulite-facies metamorphism in the Zambezi Allochthonous Terrane of the Mt. Darwin Area, Zambezi belt, NE-Zimbabwe Dissertation zur Erlangung des Grades „Doktor der Naturwissenschaften“ am Fachbereich für Geowissenschaften der Johannes Gutenberg-Universität in Mainz Mario Müller geboren in Speyer am Rhein Mainz 2004 Tag der mündlichen Prüfung: 07.05.2004 All views and results presented in this thesis are those of the author, unless stated otherwise. Ich versichere, dass ich die vorliegende Arbeit selbständig und nur unter Verwendung der angegebenen Quellen und Hilfsmittel verfasst habe. Mühlhausen, den 30. Januar 2004 And I think to myself, what a wonderful world ....L. ArmstrongTable of contents Acknowledgements .......................................................................................................... I Abstract .........................................................................................................................III ZusammenfassungV 1 Introduction .......................................................................................................1 1.1 Geological evolution of Gondwana ............................................................................... 1 1.2 The Zambezi belt ...

Subjects

Informations

Published by
Published 01 January 2004
Reads 29
Language English
Document size 24 MB




The Mavuradonha Layered Complex:
Neoproterozoic emplacement and
Pan-African granulite-facies metamorphism in the
Zambezi Allochthonous Terrane of the Mt. Darwin Area,
Zambezi belt, NE-Zimbabwe








Dissertation
zur Erlangung des Grades
„Doktor der Naturwissenschaften“
am Fachbereich für Geowissenschaften
der Johannes Gutenberg-Universität in Mainz






Mario Müller
geboren in Speyer am Rhein

Mainz 2004
























Tag der mündlichen Prüfung: 07.05.2004























All views and results presented in this thesis are those of the author, unless stated
otherwise.


Ich versichere, dass ich die vorliegende Arbeit selbständig und nur unter Verwendung der
angegebenen Quellen und Hilfsmittel verfasst habe.



Mühlhausen, den 30. Januar 2004 And I think to myself, what a wonderful world ....
L. ArmstrongTable of contents

Acknowledgements .......................................................................................................... I
Abstract .........................................................................................................................III
ZusammenfassungV
1 Introduction .......................................................................................................1
1.1 Geological evolution of Gondwana ............................................................................... 1
1.2 The Zambezi belt ........................................................................................................... 2
1.2.1 General geology............................................................................................................. 2
1.2.2 The Zambezi belt in NE-Zimbabwe .............................................................................. 6
1.3 Previous work ................................................................................................................ 9
1.4 Aim of this thesis ......................................................................................................... 10
2 Geology of the study areas12
2.1 Geology of the Mavuradonha Mountain range............................................................ 12
2.2 of the inlier.................................................................................................... 25
2.2.1 Nyamhanda Inlier ........................................................................................................ 25
2.2.2 Chimwaya Hill Inlier ................................................................................................... 27
3 Analytical methods..........................................................................................29
4 Petrology and metamorphic evolution ..........................................................36
4.1 Petrography - mineral assemblages and textures ......................................................... 36
4.1.1 Mavuradonha Layered Complex ................................................................................. 36
4.1.1.1 Serpentinite.................................................................................................................. 37
4.1.1.2 Metapyroxenites .......................................................................................................... 37
4.1.1.3 Amphibolites................................................................................................................ 38
4.1.1.3.1 Garnet-bearing amphibolites........................................................................................ 38
4.1.1.3.2 Garnet-free amphibolites ............................................................................................. 38
4.1.1.4 Fine-grained metagabbros............................................................................................ 39
4.1.1.5 Coarse-grained metagabbros 39
4.1.1.6 Garnet-bearing metagabbros 40
4.1.1.7 Leuco-metagabbros and leuco-amphibolites ............................................................... 40
4.1.1.8 Meta-anorthosites ........................................................................................................ 41
4.1.1.9 Pegmatites.................................................................................................................... 41
4.1.2 Nyamhanda Inlier and Chimwaya Hill Inlier 45
4.1.2.1 Amphibolites................................................................................................................ 45
4.1.2.1.1 Garnet-bearing amphibolites........................................................................................ 45
4.1.2.1.2 Garnet-free amphibolites ............................................................................................. 46
4.1.2.2 Metagabbros 46
4.1.2.2.1 Chimwaya Hill Inlier ................................................................................................... 46
4.1.2.2.2 Nyamhanda Inlier ........................................................................................................ 47 4.1.3 Ocellar Gneiss.............................................................................................................. 53
4.2 Mineral chemistry of samples from the Mavuradonha Layered Complex .................. 53
4.2.1 Garnet ..........................................................................................................................54
4.2.2 Pyroxene ...................................................................................................................... 60
4.2.2.1 Clinopyroxene 60
4.2.2.2 Orthopyroxene ............................................................................................................. 62
4.2.2.3 Pigeonite 62
4.2.3 Plagioclase ................................................................................................................... 63
4.2.4 Amphibole 64
4.2.5 Scapolite 65
4.3 PT evolution of the Mavuradonha Layered Complex.................................................. 65
4.3.1 Granulite-facies PT conditions .................................................................................... 65
4.3.2 Retrograde PT evolution.............................................................................................. 71
4.3.3 PT conditions calculated using TWQ .......................................................................... 72
4.4 Summary and interpretation of textures and PT-evolution of the Mavuradonha
Layered Complex......................................................................................................... 75
5 Geochemistry ...................................................................................................78
5.1 Classification of the metagabbros................................................................................ 80
5.2 Major and trace element variations.............................................................................. 81
5.2.1 Mavuradonha Layered Complex ................................................................................. 81
5.2.2 Nyamhanda Inlier and Chimwaya Hill Inlier............................................................... 86
5.3 TiO , LIL and HFS element correlations..................................................................... 90 2
5.4 Summary and interpretation......................................................................................... 90
5.4.1 Mavuradonha Layered Complex 90
5.4.2 Nyamhanda Inlier and Chimwaya Hill Inlier 93
6 Geochronology and zircon-cathodoluminescence ........................................97
6.1 Cathodoluminescence imaging .................................................................................... 97
6.2 Zircon U-Pb and Pb-Pb geochronology....................................................................... 99
6.2.1 Mavuradonha Layered Complex ................................................................................. 99
6.2.1.1 Metagabbro sample ZZB 123 ...................................................................................... 99
6.2.1.2 Ferro-metagabbro sample ZZB 166........................................................................... 106
6.2.1.3 Summary of age interpretation for zircons from the Mavuradonha Layered
Complex..................................................................................................................... 107
6.2.2 Pegmatites within the Mavuradonha Layered Complex............................................ 109
6.2.2.1 ZZB 60: pegmatite, northern Mavuradonha Mountains 109
6.2.2.2 ZZB 128: pegmatite, south of the Mavuradonha Mts................................................ 109
6.2.2.3 Interpretation.............................................................................................................. 112
6.2.3 Ocellar Gneiss............................................................................................................ 112
6.2.3.1 ZIM 30 (Fig. 6-9 a & b; Fig. 6-10 a) ......................................................................... 112
6.2.3.2 ZIM 56 (Fig. 6-9 c & d; Fig. 6-10 b) 113
6.2.3.3 ZIM 55 (Fig. 6-9 e & f; Fig. 6-10 c) .......................................................................... 116 6.2.3.4 Summary and interpretation of the Ocellar Gneiss.................................................... 117
6.3 Rutile U-Pb geochronology ....................................................................................... 117
6.4 Sm-Nd garnet - whole-rock geochronology .............................................................. 118
6.4.1 Garnet - whole-rock age determinations.................................................................... 119
6.5 Summary.................................................................................................................... 120
7 Sm-Nd, Rb-Sr and Pb-Pb isotopic systematics...........................................122
7.1 Sm-Nd isotopic systematics 124
7.1.1 Mavuradonha Layered Complex ............................................................................... 124
7.1.2 Nyamhanda Inlier and Chimwaya Hill Inlier............................................................. 128
7.2 Rb-Sr isotopic system ................................................................................................ 130
7.2.1 Mavuradonha Layered Complex 130
7.2.2 Nyamhanda Inlier and Chimwaya Hill Inlier 132
7.3 Pb isotopic systematics of feldspars .......................................................................... 133
7.3.1 Mavuradonha Layered Complex 133
7.3.2 Nyamhanda Inlier and Chimwaya Hill Inlier............................................................. 135
7.4 Summary and geotectonic implications..................................................................... 137
7.4.1 .................................................................................................................... 137
7.4.2 Geotectonic implications ........................................................................................... 138
8 Petrogenetic and geotectonic implications ..................................................141
8.1 Mavuradonha Layered Complex ............................................................................... 141
8.1.1 Early Neoproterozoic magmatic evolution ................................................................ 141
8.1.1.1 Crystallisation age and implications for the parental magma.................................... 141
8.1.1.2 Geochemical evolution during crystallisation of the complex................................... 142
8.1.1.2.1 Aspects of layering in the Mavuradonha Layered Complex...................................... 144
8.1.1.3 Isotopic evolution and contamination of the complex............................................... 147
8.1.2 Implications for the tectono-metamorphic evolution of the Mavuradonha
Layered Complex....................................................................................................... 152
8.2 Nyamhanda Inlier and Chimwaya Hill Inlier............................................................. 154
9 Interpretation in the regional context .........................................................157
9.1 Implications for the evolution of the Zambezi Allochthonous Terrain ..................... 157
9.2 Implications for the Pan-African orogenic belt system ............................................. 162
References ....................................................................................................................164
Appendix ......................................................................................................................180
Curriculum Vitae.........................................................................................................181
I

Acknowledgements

First of all, I would like to thank my supervisor for his guidance during my study. It
was a pleasure to work with him, and with the freedom he gave me it was possible to
approach the Pan-African geology and zircon geochronology. Thanks for his enthusiasm
and patience during the last six years.
Special thanks are due to my field assistants who accompanied me during the field
trips. I will never forget the great time with them in the field, and I am reminded of them
many times, because sometimes I would have been lost without their assistance in the
Chiswiti Communal Land. Cordial thanks to all friends I met in the Chiswiti Communal
Land, particularly the staff at Pachanza School, who arranged the best accommodation
possible in the area I ever had. Special thanks to all my friends at the Chividze General
Dealer for all cold cokes and beers after a day in work. It was a great honour for me to
meet all these friends.
Thanks to the staff at the Department of Geology, University of Zimbabwe, Harare,
for their support during field work. Especially for providing field equipment, their
assistance in the organization, as well as the sawing and crushing of samples.
I particularly thank the staff at the MPI for the introduction to isotope geochemistry
and geochronology, for their support, useful discussions and critical comments during
laboratory work at the MPI, during the writing of this thesis and reviews of the manuscript.
In addition, I would like to thank the “MPI lab group” for being great lab
colleagues and their never ending support. Thanks to all colleagues and friends for helpful
discussions.
In addition I thank the staff at the Geosciences Department of Johannes Gutenberg-
University for support and help at the microprobe, the XRF, in the geochemistry and rock-
crushing lab and at mineral separation. I am very grateful to the secretaries for their help
with administrative questions.

Last but not least I thank my family for their patience and everything they did for
me during the last 34 years. Very special thanks to my wife Angelika and daughters Ann-
Cathrin and Helen for their support, patience and understanding for missing family-live.
They helped and encouraged me to study African geology and to finish the presented
study. II

This project was part of the Mainz-Harare collaborative project “The amalgamation
of Gondwanaland in southern Africa”, founded by the Volkswagen Foundation, and the
Graduiertenkolleg „Stoffbestand und Entwicklung von Kruste und Mantel“ at the
Geosciences Department of Johannes Gutenberg-University and the Max-Planck-Insitut für
Chemie in Mainz, funded by grant GK392/1 of the German Science Foundation. III

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 ε Nd
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 greenschist-
facies 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