Geochemistry and U-Pb zircon dating of the high-K calc-alkaline basaltic andesitic lavas from the Buanji Group, south-western Tanzania

SHRIMP zircon U-Pb and Sm-Nd isotopic data together with major and trace elements data are presented for the basalt andesitic lavas from the Buanji Group, south-western Tanzania in order to establish their emplacement age, ascertain their mantle sources and processes responsible for their generation. Zircon U-Pb data shows that the Buanji Group lavas were emplaced at 1674+/-15 Ma and do not therefore belong to the Neoproterozoic Bukoban Supergroup. Having erupted on top of sedimentary rocks, the 1674+/-15 Ma age of the volcanic rocks provides the younger limit to the age of deposition of underlying un-dated sediments. The Bunaji Group lavas are amygdaloidal and contain euhedral phenocrysts of plagioclase + orthopyroxene + clinopyroxene as well as subhedral quartz and opaque minerals mainly magnetite with quartz filling in the interstices. As a suite, they are compositionally uniform and are classified as high-K calc-alkaline basaltic andesites. The samples display coherent and fractionated REE patterns with La/Sm-CN and La/Yb-CN ratios of 2.48-2.72 and 4.03-5.92, respectively; and are characterized by negative Eu anomalies (Eu/Eu* = 0.76-0.84). They are depleted in Nb, Ta, Ti and Sr but enriched in the most incompatible elements Rb, Ba, Th, U, K, and Pb leading to high ratios of Ba/Ta (531-1358) and Ba/Nb (35-91, with one outlier). They exhibit Hf/Yb ratios = 1.31-1.52, sub-chondritic ratios of Nb/Ta (13-18) and Zr/Hf (32-38), characteristics of shallow melting of mantle derived magmas. The samples display Th/Nb ratios of 0.48-0.95 and have epsilon Nd (1.67 Ga) values of +0.22 to +2.34 which are much lower than the corresponding epsilon Nd (1.67 Ga) mantle value of +6.55; characteristics indicative of contamination by the older crust. The geochemical features of the Buanji Group high-K calc-alkaline basaltic andesites are interpreted in terms of derivation of these rocks by partial melting of the a spinel lherzolite mantle wedge that has been metasomatized by the subduction related fluids in a late Paleoproterozoic continental convergent margin. The resultant magmas were subsequently contaminated by the felsic crust prior to their emplacement. (C) 2013 Elsevier Ltd. All rights reserved.