Petrogenesis of tonalite-trondhjemite and potassic granite, Geita Greenstone Belt, Tanzania Craton: Implication for crustal evolution during Neoarchean plate tectonic initiation

Geochemical and zircon isotope data reveal a complex petrogenetic history of the Neoarchean granitoids of the Geita Greenstone Belt (GGB), northern Tanzania Craton. Geochemically, the granitoids are categorized into: (1) tonalite-low-silica trondhjemite; (2) high-silica trondhjemite; (3) transitional granitoids; and (4) potassic granite. Compared to tonalite-low-silica trondhjemite, the high-silica trondhjemite displays significant depletion in HREE and HFSE, and have an exceptional high Sr/Y and La N /Yb N ratios. Both categories display similar LREE and LILE patterns. Transitional granitoids comprise dykes and dyke-like bodies of low-silica trondhjemite and quartz monzonite. Characteristically, they have low La N /Yb N and Sr/Y ratios, and are strongly-enriched in LREE and LILE and less depleted in HREE and HFSE. Potassic granite is represented by small intrusions and dykes that exhibit low La N /Yb N and low to moderate Sr/Y ratios, and are moderately-enriched in LREE, strongly-enriched in LILE, and moderately-depleted in HREE and HFSE. The tonalite and low-silica trondhjemite generated (∼2699–2714 Ma) by partial melting of basaltic rock at amphibolite facies metamorphic condition. The high-silica trondhjemite evolved from fractionation of the tonalitic melt. Lithospheric delamination produced high-K mafic magma that fractionated into the transitional granitoids (∼2660–2683 Ma), and caused partial melting of the tonalite-trondhjemites to generate the potassic granite (∼2646 – 2685 Ma). Geochemical signature of the tonalite-trondhjemites and potassic granite, as indicated by the negative Nb-Ta-Ti anomalies, and strongly-fractionated REEs, supported with the structural style near and at the contact between the felsic gneisses and the greenstone belt infer generation of the gneisses at a convergent plate boundary. Partial melting at the base of an oceanic plateau or a flat subducting slab has been suggested to interpret the evolution of the northern Tanzania Craton (2700–2650 Ma). A metasomatized mantle was introduced by lower crustal delamination/break-off of the down-going slab.