Metasomatized Granulites of the Mozambique Belt: Consequences for Lithospheric U, Th, REE Fertilization and Metallogenesis in the Ancient Gondwanaland Supercontinent.

摘要

The 1.0 Ga old Lurio belt extends for approximately 1 000 km from Nsanje (S Malawi) through NE Mozambique to the Indian ocean. Lower crustal levels are locally exposed along its southern tectonic front. In this article mineralogical and geochemical data for an andesinite-mafic-ultramafic suite from Nsanje were reported.The results indicate that this complex terrane equilibrated first at P /similar to/ 13 kbr and T /similar to/ 900 deg C and subsequently cooled under an eclogite-garnet granulite georem. During a later event (P /similar to/ 7-10 kbar, T /similar to/ 650 - 800 deg C) distinctive metasomatic mica, amphibole, scapolite, apatite, diopsidic pyroxene (MASAD)-bearing assemblages and pegmatoids were formed by CO/sub 2/, Cl, H/sub 2/O, S and F rich fluids with higher REE, U, Th and Zr concentrations than the high-grade precursors. MASAD and older high-pressure granulite parageneses underwent subsequent dehydratation and reequilibration under medium-pressure granulite facies conditions perhaps during the Lurio orogenic event. MASAD-like assemblages are relatively common within the late Proterozoic medium- and high-pressure granulite terranes of Central Gondwana, especially in the newly defined Lurio-Zambezi Eclogite Province. The data provide indicate that the metasomatizing, MASAD-forming fluids had crypto-carbonatitic affinities and were introduced into the crust from the upper mantle during protorifting episodes between ca. 1,1 and 0,5 Ga ago. The MASAD assemblages may therefore represent the crustal equivalent of the metasomatic and MARID suites discovered in mantle xenoliths. (Atomindex citation 19:092133)