Metamorphic petrology and zircon geochronology of high‐grade rocks from the central Mozambique Belt of Tanzania: crustal recycling of Archean and Palaeoproterozoic material during the Pan‐African orogeny

Abstract

New data on the metamorphic petrology and zircon geochronology of high-grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan-African event. The metamorphic rocks are characterized by a clockwise P-T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite ⁄ sillimanite-bearing metapelites. The meta-granitoid rocks are of calc-alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet-clinopyroxene-plagioclase-quartz-biotite-amphibole ± K-feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet-biotite-plagioclase ± K-feldspar ± kyanite ⁄ sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12-13 kbar and 750-800°C. Pressures of 5-8 kbar and temperatures of 550-700°C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P-T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970-2500 Ma) and Palaeoproterozoic (c. 2124-1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan-African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan-African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

“…Similar conditions of 0.9-1.2 GPa and 750-880°C in the equivalent Vohibory Domain of Madagascar (Nicollet, 1986(Nicollet, , 1990Jöns and Schenk, 2008) are dated at 0.65-0.61 Ga with a younger overprint at 0.58-0.52 Ga (de Wit et al, 2001;Jöns and Schenk, 2008;Collins et al, 2012;Boger et al, 2015). In contrast, comparable peak conditions of 1.0-1.4 GPa and 700-800°C in the reworked Western Granulites (Sommer et al, 2003;Jöns and Schenk, 2004) reflect the younger event at 0.58-0.55 Ga (Cutten et al, 2006;Tenczer et al, 2013). Another difference is that although both Eastern and Western Granulites have a P-T path with substantial prograde and retrograde P change, the former is anticlockwise and the latter clockwise (Appel et al, 1998;Jöns and Schenk, 2004;Tenczer et al, 2011) consistent with suggestions they are distinct thrust nappes with different histories (Fritz et al, 2009.…”

Formation of giant spicules in the deep-sea hexactinellid Monorhaphis chuni (Schulze 1904): electron-microscopic and biochemical studies

“…Thin sections (150 μm) of giant basal spicules were prepared and subjected to electron microprobe analyses as described by Sommer et al (2003). Mineral analyses were performed by means of a JOEL 8900RL Superprobe device attached to a MICROSPEC wavelength dispersive system.…”

Pan-African granulites of central Dronning Maud Land and Mozambique: A comparison within the East-African-Antarctic orogen

“…The peak granulite-facies metamorphism is documented followed by an isothermal or near-isothermal decompression. Similar decompression is documented in Tanzania by resorption and corona growth on garnet, by migmatisation of gneisses and by the formation of cordierite-bearing mineral assemblages (Sommer et al, 2003). The eclogite facies conditions reported in Malawi are strongly overprinted by granulitefacies assemblages, showing an isothermal decompression (Ring et al, 2002).…”

“…Late Neoproterozoic/Early Paleozoic high-grade metamorphism is documented over a large region in Mozambique, Tanzania (Sommer et al, 2003), Malawi (Ring et al, 2002) and Madagascar (Markl et al, 2000). Granulite facies rocks are widespread in northern Mozambique (Norconsult Consortium, 2007).…”