Isotope and trace element studies of the Xingdi II mafic–ultramafic complex in the northern rim of the Tarim Craton: Evidence for emplacement in a Neoproterozoic subduction zone

Abstract

The Xingdi II mafic–ultramafic complex is located in the northern rim of the Tarim Craton, Northern Xinjiang, NW China. This complex is mainly composed of gabbro, gabbronorite, websterite and lherzolite, plus minor granodiorites and granites. The geodynamic setting of this complex and other Neoproterozoic mafic–ultramafic complexes in the region is debated, with opinions varying from mantle plume to a continental arc. A new zircon U–Pb age from this study reveals >7-myr difference for two gabbroic intrusive phases in the Xingdi II mafic–ultramafic complex and up to a 21-myr difference between this complex and the nearby mafic–ultramafic complexes that occur <12km from this complex. The age changes for the Neoproterozoic (730–820Ma) mafic–ultramafic complexes in the area occur in different directions and hence a mantle plume origin for the complexes is unlikely. The Xingdi II mafic–ultramafic intrusive rocks are characterized by moderate light-REE enrichments and pronounced negative Nb anomaly, which are common for mafic–ultramafic rocks in convergent tectonic settings, and negative εNd(t) values (−9.2 to −5.1) and elevated initial 87Sr/86Sr ratios (0.7061 to 0.7086), which indicate enriched source mantle plus crustal contamination. The εHf(t) values of zircon crystals from the Xingdi II mafic–ultramafic complex are between −2.8 and −7.0. The trace element and isotope data together indicate that this complex formed from multiple pulses of contaminated subduction-related basaltic magma that were emplaced separately with a time gap of at least 7myr. The results from this study support the viewpoint that in the Neoproterozoic the northern rim of the Tarim Craton was an active continental margin that belongs to the Circum-Rodinia Subduction System.