Subduction and Continental Collision in the Lufilian Arc
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Subduction and continental collision in the Lufilian Arc-Zambesi Belt orogen: A petrological, geochemical, and geochronological study of eclogites and whiteschists (Zambia) Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Timm John Kiel 2001 Vorwort 1 Introduction and Summary 3 CHAPTER ONE 6 Evidence for a Neoproterozoic ocean in south central Africa from MORB-type geochemical signatures and P-T estimates of Zambian eclogites 1.1 Abstract 6 1.2 Introduction 7 1.3 Geological overview 7 1.4 Petrography and mineral chemistry 9 1.5 Thermobarometry and P-T evolution 9 1.6 Geochemistry 12 1.7 Geochronology 14 1.8 Conclusions 15 CHAPTER TWO 16 Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration 2.1 Abstract 16 2.2 Introduction 17 2.3 Geological setting 18 2.4 Petrology 20 2.4.1 Petrography 20 2.4.2 Mineral chemistry and growth history 24 2.4.3 P-T conditions and phase relations 29 2.5 Processes occurring during eclogitisation 31 2.5.1 Dissolution and precipitation mechanism 32 2.5.2 Formation of pseudomorphs 33 2.5.3 Vein formation 34 2.6 Fluid source 35 2.7 Discussion and conclusions 36 CHAPTER THREE 37 Timing and P-T evolution of whiteschist metamorphism in the Lufilian Arc-Zambesi Belt orogen (Zambia): implications to the Gondwana assembly 3.1 Abstract 37 3.2 Introduction 38 3.3 Regional geology 39 3.4 Sample localities 41 3.5 Petrography and mineral chemisty 43 3.5.1 Zambesi Belt: Whiteschists and associated rocks 43 3.5.2 Lufilian Arc: Whiteschists and associated rocks 49 3.6 Reaction history and P-T evolution 51 3.6.1 Zambesi Belt: Chowe River rocks 51 3.6.1.1 Prograde evolution 51 3.6.1.2 Peak-metamorphic conditions 51 3.6.1.3 Retrograde evolution 53 3.6.2 Lufilian Arc: Solwezi Dome rocks 53 3.6.2.1 Metamorphic evolution and P-T path 53 3.6.3 Discussion and comparison with previous studies 54 3.7 Geochronology 56 3.7.1 Sample description 56 3.7.2 Analytical procedures 56 3.7.3 Results of U-Pb and Rb-Sr geochronology 57 3.7.2 Comparison with previous studies and discussion 62 3.8 Conclusions 64 References 66 Vorwort 1 Vorwort Diese Arbeit besteht aus drei Kapiteln, denen eine allgemeine Einführung und Zusammenfassung vorangestellt ist. Die Kapitel sind so konzipiert, daß sie zwar getrennt voneinander als Artikel publiziert werden können, aber auch eine zusammenhängende Monographie darstellen. Die Arbeit entstand im Rahmen eines von der Deutschen Forschungs- gemeinschaft geförderten und von Volker Schenk initiierten Forschungsprojekts (Sche 265/10-1 und Sche 265/10-2). Volker Schenk möchte ich an dieser Stelle in besonderem Maße danken: für die Gelegenheit an diesem Projekt mitzuarbeiten und die Möglichkeit zu promovieren. Auch seinem großen Engagement, dem intensiven Einsatz und der fördernden Betreuung während des Geländeaufenthaltes in Zambia und der Bearbeitung der Proben hier in Kiel gilt mein herzlicher Dank. Von seiner Leidenschaft für die Petrologie habe ich mich gern anstecken lassen. Klaus Mezger hat mich während meines Gastaufenthaltes am Zentrallaboratorium für Geochronologie der Universität Münster hervorragend betreut und stets nach “Niederlagen” im Labor wieder aufgebaut. Vielen Dank für die kompetente Einführung in die Geochronologie, unzählige Denkanstöße, viele Lacher und nicht zuletzt die Gewährung von Unterschlupf. Für die gute Zusammenarbeit in der Arbeitsgruppe in Kiel bedanke ich mich bei Petra Herms, Peter Raase und Peter Appel, die mir stets hilfreich zur Seite standen. Besonders die Ratschläge zu Fragen der Dünnschliffmikroskopie haben mir immer wieder geholfen. Karsten Haase hat mir in vielen langen Diskussionen die Geochemie näher gebracht, unter anderem danke ich ihm dafür. Dieter Garbe- Schönberg danke ich herzlichst für die Durchführung der Messungen von Spurenelementen an der ICP-MS. Bei der Probenaufbereitung wurde ich hervorragend von Stefan Bredemeyer und Frau Astrid Weinkauf unterstützt. Andreas Fehler danke ich für die Herstellung vieler Gesteins-Dünnschliffe. Herr Dr. Dietrich Ackermand, Frau Barbara Mader und Peter Appel halfen mir tatkräftig bei der Durchführung der Mikrosondenanalysen. Der Münster-Crew sei erst einmal im Gesamten gedankt. Ihr wißt schon wofür. Bei Bart Willigers, Erik Scherer, Kilian Pollok und Thorsten Kleine möchte ich mich vor allem für die anregenden Diskussionen bedanken. Erik, danke für much more than the big Lu’s and Hf‘s. Ohne die vielen Hilfestellungen von Heidi Baier im Labor und an den Massenspektrometern wäre wohl kein Isotop von mir gemessen worden. Ein Dank geht auch an Andreas Kronz vom Institut für Geochemie der Universität Göttingen, der mir bei Messungen an der dortigen Mikrosonde geholfen hat. Vorwort 2 I greatfully acknowledge the support of the Geology Department of the University of Zambia and the Zambian Geological Survey. I am especially thankful for the strong support I had from Francis Tembo during the whole study. Pete and Spana Mosley and Bill Barclay, I thank you for the support and the nice time in Lusaka. Last but not least, thanks to Ben Mapani. Andrea, Klaus, Evi, Geli, Alex, Dirk, Imke, Lutz, Martin und Uwe – danke. Außerdem danke ich allen, die mir über die Jahre geholfen, mich zum Lachen gebracht oder einfach in Ruhe gelassen haben. Introduction and Summary 3 Introduction and Summary consumption occurs at convergent plate boundaries (subduction zones) where oceanic The study is focused on eclogites and other lithosphere descends beneath less dense potentially subduction related rocks from the continental lithosphere, or older (denser) late Proterozoic orogenic belts in south central beneath younger (lighter) oceanic lithosphere. Africa in order to get new insights into Orogens usually form by the closure of an subduction zone processes at Precambrian times ocean basin (e.g., Alps), or subduction of and, moreover, to clarify their geodynamic oceanic beneath continental lithosphere (e.g., relation to the Pan-African orogenic cycle. Andes). In rare cases, orogens may form by Eclogites are formed from rocks of basaltic compression of sedimentary basins in bulk-composition due to crustal thickening or continental rifts. Thus, ophiolites and eclogites subduction zone processes. Rocks transform to representing relics of oceanic basins can eclogites at great depth under low geothermal provide key information to distinguish between gradients, and thus, the occurrence of eclogites different former large-scale geodynamic indicates specific plate-tectonic processes. If processes. They provide geochemical eclogites are formed from oceanic crust, their information about the geotectonic setting in outcrops may mark a suture zone, which is the which they were generated, the time of their region where two blocks of lithosphere are formation, and can preserve a record of the welded together (e.g., Condie, 1989). Other pressure-temperature conditions within the potentially subduction related rocks besides subduction zone in which they were eclogites are whiteschists (talc-kyanite schists) metamorphosed. In addition, whiteschists (Schreyer, 1973). Whiteschists typically are usually contain monazite which is suitable for found in Alpine-type orogens, e.g., Western Alps, Trans-Himalaya (Sar e Sang - Afghanistan), high-precision U-Pb geochronology due to the and are usually interpreted as being of incorporation of uranium into the structure. metasomatic origin (e.g., Schreyer & Abraham, Since whiteschists and monazites are formed 1976). The stability field of the talc-kyanite by metamorphic processes, they provide, assemblage is restricted to temperatures between similar to eclogites, information about the c. 600 and 800 °C, and pressures from c. 6 kbar conditions and the time of their formation. up to conditions of ultra-high-pressure metamorphism (Chopin, 1984; Schreyer, 1988; The Proterozoic mobile belts of central Massonne, 1989). Whiteschists of all known southern Africa (e.g., Irumide Belt and Lufilian occurrences are formed during crustal Arc-Zambesi Belt) formed between the Congo thickening under low geothermal gradients. and Kalahari cratons during the evolution of Ocean basins form by seafloor spreading the supercontinents Rodinia and Gondwana processes following the rifting of continental (e.g., Unrug, 1996; Weil et al., 1998). Two lithosphere. In some cases, the closure of an contrasting models exist to explain the origin of ocean basin occurred repeatedly between the these mobile belts: one suggesting an same parts of continental lithosphere which intracratonic formation (Daly, 1986; Hanson et formed one continuous block prior to rifting. al., 1994), and the other involves the formation This process of ocean basin formation and of an ocean basin (Coward & Daly, 1984, closure, resulting in a continent-continent Porada & Berhorst, 2000). Central Zambia collision, is called a Wilson cycle. During the exhibits the largest occurrence of Precambrian closure of an ocean basin, oceanic crust eclogites in southern Africa (Vrana et al., Introduction and Summary 4 1975). Only two other eclogite localities exist main metamorphic processes that control the in this region, one in northern Zambia (Cosi et gabbro-to-eclogite transformation. al., 1992) and the other in northern Zimbabwe Furthermore, Zambia hosts one of the largest (Dirks and Sithole, 1999). Both could be occurrences of whiteschists in the world with related to the main central Zambian occurrence,