gems, Geology, East Africa

The East African Orogen, a vast Neoproterozoic (Pan-African) orogenic belt extending from Arabia to Mozambique, exhibits both accretionary and collisional tectonic features. Its northern segment, the Arabian-Nubian Shield (ANS), comprises Neoproterozoic juvenile arc terranes accreted along ophiolite-decorated sutures, while the southern segment, the Mozambique Belt (MB), mainly reflects middle to lower crustal processes and remains poorly understood. New isotopic and zircon age data from Tanzania reveal that much of the high-grade MB consists of reworked late Archaean to Palaeoproterozoic granitoid gneisses, which may either represent an eastern extension of the Tanzania craton or separate terranes tectonically mixed with Neoproterozoic gneisses, akin to Madagascar. At many sites, >1800 Ma and 650–800 Ma gneisses occur in tight tectonic interlayers, suggesting that up to 70% of the MB is composed of pre-Neoproterozoic crust. Limited ∼1000–1100 Ma granitoids appear only in southern Tanzania, and their role in regional tectonics remains unclear. High-grade metamorphism in Tanzania’s MB occurred at 620–640 Ma—earlier than in Mozambique (∼615 Ma), Malawi (∼550–580 Ma), and Madagascar (∼550–560 Ma)—though the implications of these age variations are not yet known. Petrological evidence shows both clockwise and anticlockwise pressure–temperature paths, indicating diverse tectonic environments during metamorphism. Although the MB lacks clear arc-related structures like the ANS, similar processes of arc and microcontinent accretion likely occurred but have been obscured by intense ductile deformation and metamorphism. Recent identification of major shear zones and seismic imaging in Tanzania offer promising avenues for deciphering the deep crustal architecture of this complex orogen and the broader tectonics of East–West Gondwana amalgamation.