Geochemistry and U-Pb zircon geochronology of S-type granites in the Karagwe Ankolean Belt, northwestern Tanzania: Implications for rare-metals mineralization

The Mesoproterozoic Karagwe Ankolean Belt in the Central Eastern Africa region hosts suites of granites that intruded supracrustal rocks and are associated with Sn-W mineralization. In northwestern Tanzania, the granites are S-type, peraluminous and calc-alkalic to alkalic-calcic in composition. Geochemical analyses of the granites reveal the following concentrations: SiO2 (71-77 wt%), Al2O3 (11-15 wt%), K2O (3-7 wt%), MgO (0.06-0.86 wt %), TiO2 (0.07-0.36 wt%) and Fe2O3T (0.71-2.40 wt%). The SiO2 contents correlate negatively with MgO, CaO and K2O and positively correlate with Nb, Ta, Zr, Sr, Ba, Hf, Th and the REE (Eu, Ce and Sm). Primitive mantle normalized spider plots show elevated LILE against depletions in Ba, Nb, Ti and La, suggesting inherited arcmagmatic signatures from the protolith. Chondrite-normalized REE spider plots show elevated LREE and relatively flat MREE and HREE as indicated by (La/Yb)cn = 2-9 and negative Eu* anomalies (0.05-0.24). Modelling of nearby Uha group sedimentary rocks partial melts, supports fractional crystallization and upper crustal contamination for generation of the KAB granites. Trace elemental ratios of Y/Nb, Nb/Ta, Zr/Hf and P205 suggest the granites are weakly mineralized to barren in Sn-W. The U-Pb zircon ages point to two successive and overlapping periods: from 1418 +/- 15 Ma to 1395 +/- 6 Ma (mean = 1406 Ma) and from 1385 +/- 8 Ma to 1376 +/- 11 Ma (mean = 1381 Ma). Inheritance U-Pb dates of xenocryst zircons, comprise early to late Paleoproterozoic ages ranging from 2376 +/- 30 Ma, 2000 +/- 27 Ma to 1901 +/- 39 Ma, and from 1795 +/- 7 Ma to 1738 +/- 7 Ma, implying the participation of the Paleoproterozoic sedimentary rocks in the petrogenesis of the granites. A zircon rim overgrowth age of 1015 +/- 26 Ma is interpreted to date a metamorphic/anatexis event that coincided with the c.1000 Ma amalgamation of the supercontinent Rodinia and the emplacement of Sn-W enriched granites. The ages of S-type granite magmatism (1418-1376 Ma) obtained from this study are restricted to the Mesoproterozoic within the widespread bimodal magmatism event c.1375 Ma.