Volker Schenk

Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania

U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth9s history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

U-Pb dating of metamorphic minerals: Pan-African metamorphism and prolonged slow cooling of high pressure granulites in Tanzania, East Africa

Abstract U–Pb monazite and zircon ages reveal that the high pressure granulites from eastern Tanzania were metamorphosed during a Pan-African tectonothermal episode. These mineral ages range from 610 to 655 Ma and indicate that peak metamorphic conditions were diachronous in the different granulite domains. U–Pb titanite and rutile ages define integrated cooling rates of 2–5°C/Ma for all investigated granulite areas, and suggest a common process for the post-metamorphic histories of the different granulite areas. Prolonged slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thickening during heating did not produce isostatic instability and fast erosion-driven or tectonic exhumation. The thermal history determined in this study is not consistent with the collision of East- and West-Gondwana as the cause of granulite facies metamorphism. Palaeomagnetic data have shown that this collision did not occur until 550 Ma, when the Pan-African granulites in Tanzania had already cooled below 500°C. The high pressure granulites of eastern Tanzania are thus interpreted as having attained their metamorphic peak prior to the final amalgamation of Gondwana, probably in an active continental margin setting.

Timing and PT evolution of whiteschist metamorphism in the Lufilian Arc-Zambezi Belt orogen (Zambia): Implications for the assembly of Gondwana

One of the world’s most extensive occurrences of whiteschists (talc‐kyanite schists) is located in south central Africa with several exposures along a ca. 700 km northwest‐southeast striking zone. The metamorphic evolution and age relations of whiteschists and associated rocks from four localities, three in the Lufilian Arc and one in the Zambezi Belt, were investigated. In the Lufilian Arc whiteschists, associated garnet‐amphibolites and biotite‐kyanite‐garnet gneisses record peak metamorphic conditions of about \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context: implications for the amalgamation of the Gondwana supercontinent

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From Closure of the Mozambique Ocean to Gondwana Breakup : New Evidence from Geochronological Data of the Vohibory Terrane, Southwest Madagascar

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Fluid inclusions in granulite-facies metapelites of the Hercynian ancient lower crust of the Serre, Calabria, Southern Italy

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