Ultra - high temperature granulite - facies metamorphic rocks from the Mozambique belt of SW Tanzania

Sommer, Holger and Kröner, A. (2013) Ultra - high temperature granulite - facies metamorphic rocks from the Mozambique belt of SW Tanzania. Lithos, 170/17 . pp. 117-143. ISSN 0024-4937

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Abstract

The metamorphic rocks in the Neoproterozoic (Pan-African) Mozambique belt of southwestern Tanzania, around the town of Songea, can be subdivided into one- and two pyroxene bearing charnockitic gneisses, migmatitic granitoid gneisses and amphibolite-facies metapelites. Lower-grade amphibolite-facies rocks are rare and can be classi!ed as sillimanite- and/or garnet-bearing metapelites. Most of the studied charnockitic gneisses show excellent corona textureswith large orthopyroxene grains rimmed by clinopyroxene, followed by quartz and well developed garnet rims due to the reaction Opx + Pl = Grt + Cpx + Qtz that formed during isobaric cooling. These and other charnockitic gneisses show symplectites of orthopyroxene and An-rich plagioclase that resulted from the breakdown of garnet during isothermal decompression due to the reaction Grt + Cpx + Qtz = Opx + Pl. Geothermobarometric calculations yield up to ~1050 °C and up to ~12 kbar for peak metamorphic conditions. These are higher temperature and slightly lower pressure conditions than reported for other granulite-facies terrains in the Mozambique belt of Tanzania. Single zircon Pb–Pb evaporation and U-Pb SHRIMPages for magmatic zircons extracted fromtwo charnockitic and two granitic gneisses cluster in two groups, one at ~750 Ma and one at ~1150 Ma with the older re"ecting the time of emplacement of the igneous precursors, and the younger approximating the time of charnockitization. These protolith ages are similar to those farther east in theMasasi area of southern Tanzania, aswell as in northern Mozambique and in southern Malawi, and suggest that the Mozambique belt consists of chronologically heterogeneous assemblages whose pre-metamorphic tectonic setting remains obscure.

Item Type:	Journal Article
Subjects:	Q Science > QE Geology
Divisions:	Faculty of Science, Technology and Environment (FSTE) > School of Geography, Earth Science and Environment
Depositing User:	Holger Sommer
Date Deposited:	15 Sep 2014 23:58
Last Modified:	06 Jul 2016 23:54
URI:	http://repository.usp.ac.fj/id/eprint/7624

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