Reconnaissance SHRIMP U-Pb zircon geochronology of the Tanzania Craton: Evidence for Neoarchean granitoid-greenstone belts in the Central Tanzania Region and the Southern East African Orogen

摘要

Reconnaissance U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon dating of gneisses, granitoids and greenstones from well-documented study areas within the Tanzania Craton indicates that: (1) ~2815-2691 Ma greenschist-amphibolite facies greenstones and associated granitoids are confined within extensive >3600 Ma granitoid-gneisses in the Central Tanzania Region; (2) greenschist-amphibolite facies greenstone rocks from the Singida-Mayamaya Terrane in the south-eastern Lake Nyanza Superterrane, Lake Victoria Region are older than 2681 Ma; (3) greenschist to lower-granulite facies granitoid-greenstone belts from the Kilindi-Handeni Superterrane, within the largely Neoproterozoic Southern East African Orogen are older than 2670 Ma; and (4) the granitoid-greenstone belts within the Dodoma Basement Superterrane, Central Tanzania Region and Kilindi-Handeni Superterrane, Southern East African Orogen are broadly coeval with ~2823-2671 Ma granitoid-greenstone belts in the Lake Nyanza Superterrane, in the Lake Victoria Region.The basement to juvenile greenstone rocks in the Central Tanzania Region includes E-W-trending orthogneisses. These comprise largely >3140 Ma diorite to granodiorite gneisses with rafts and/or tectonic enclaves of supracrustal rocks, including ~3600 Ma fuchsitic sericite quartzite, which forms part of the ~25. km by 5. km Simba-Nguru Hills in the Undewa-Ilangali Terrane. This quartzite contains detrital 4013-3600 Ma zircons that define ancestral cycles of protracted magmatism in their as yet undetected source terranes.In addition to the ~2815-2670 Ma granitoids and greenstones in the >3000 Ma gneisses and granitoids within the widely accepted marginal zone of the Tanzania Craton, the Lake Nyanza Superterrane extends east into the Kilindi-Handeni Superterrane, in the largely Neoproterozoic Southern East African Orogen. In this Superterrane, Neoarchean igneous and sedimentary rocks in the Mkurumu-Magamba Terrane record ~620-603 Ma amphibolite-granulite facies metamorphism, ~585-575 Ma partial-melting, and emplacement of enderbitic-charnockitic granitoids. They also record a short-lived, but significant, 570-560 Ma period of exhumation and emplacement of high-grade metamorphic rocks on to basement rocks of the proto-Archean craton within the Central Tectonic Zone in the Southern East African Orogen.