Permian A-type granites in Tarim and western part of Central Asian Orogenic Belt (CAOB): Genetically related to a common Permian mantle plume?

Abstract

Zircon U–Pb ages, whole-rock geochemical and zircon Hf isotope data are reported for the Permian A-type quartz syenites–granites in western Tarim Block, Northwest China. Zircon U–Pb dating indicates these alkali plutons were emplaced at 270–277Ma. Elemental geochemistry shows their typical A-type signatures, i.e., high total alkali, HFSE, REE contents and Ga/Al, FeOtot/(FeOtot+MgO) and Nb/Y ratios. Isotopically, their epsilon Nd values vary from −2.6 to 2.0. Based on zircon Hf isotope compositions, they could be divided into two sub-groups: Group I exhibits high positive εHf(t) ranging from 3 to 11 (mostly within the range of 5 to 9), which is highly comparable with the syenites in Bachu area located just south of these plutons. Group II has εHf(t) ranging from −3 to 2. The second group exhibits comparable Nd–Hf isotope compositions with the spatially and temporally related Piqiang ultramafic–mafic complex. Integrating geological, geochemical and Nd–Hf isotope data, we suggest that the A-type syenite–granites were formed via intensive crystal fractionation from a common plume-derived parental mafic magma, coupled with variable extent of crustal contamination. Moreover, the extent of the crustal contamination increase from inner Tarim towards South Tianshan Collisional Belt.A comprehensive synthesis of the coeval A-type granites in the western part of the Central Asian Orogenic Belt (CAOB) shows that they exhibit distinct geochemical features, e.g., large range of εNd(t) and (87Sr/86Sr)i values, high Y/Nb, Yb/Ta and low Nb/La ratios. All these features argue that they were derived from juvenile mafic crust with variable addition of the mafic materials from mantle sources and some old crustal materials or mixing of mantle-derived mafic magma with Precambrian crustal components. We suggest that both the Permian A-type granites from Tarim and western section of CAOB were genetically linked to the Permian Tarim mantle plume, which is concurrent with the two mantle domains for the Permian Tarim Large Igneous Province (LIP).