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

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 classified as sillimanite- and/or garnet-bearing metapelites. Most of the studied charnockitic gneisses show excellent corona textures with 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 ~12kbar 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 SHRIMP ages for magmatic zircons extracted from two charnockitic and two granitic gneisses cluster in two groups, one at ~750Ma and one at ~1150Ma with the older reflecting 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 the Masasi area of southern Tanzania, as well 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.