Subducted slab-plume interaction traced by magnesium isotopes in the northern margin of the Tarim Large Igneous Province

Abstract

Incorporation of subducted slabs may account for the geochemical and isotopic variations of large igneous provinces (LIPs). However, the mechanism and process by which subducted slabs are involved into magmas is still highly debated. Here, we report a set of high resolution Mg isotopes for a suite of alkaline and Fe-rich rocks (including basalts, mafic–ultramafic layered intrusions, diabase dykes and mantle xenoliths in the kimberlitic rocks) from Tarim Large Igneous Province (TLIP). We observed that δ26Mg values of basalts range from −0.29 to −0.45‰, −0.31 to −0.42‰ for mafic–ultramafic layered intrusions, −0.28 to −0.31‰ for diabase dykes and −0.29 to −0.44‰ for pyroxenite xenoliths from the kimberlitic rocks, typically lighter than the normal mantle source (−0.25‰±0.04, 2 SD). After carefully precluding other possibilities, we propose that the light Mg isotopic compositions and high FeO contents should be ascribed to the involvement of recycled sedimentary carbonate rocks and pyroxenite/eclogite. Moreover, from basalts, through layered intrusions to diabase dykes, (87Sr/86Sr)i values and δ18OV-SMOW declined, whereas ε(Nd)t and δ26Mg values increased with progressive partial melting of mantle, indicating that components of carbonate rock and pyroxenite/eclogite in the mantle sources were waning over time. In combination with the previous reported Mg isotopes for carbonatite, nephelinite and kimberlitic rocks in TLIP, two distinct mantle domains are recognized for this province: 1) a lithospheric mantle source for basalts and mafic–ultramafic layered intrusions which were modified by calcite/dolomite and eclogite-derived high-Si melts, as evidenced by enriched Sr–Nd–O and light Mg isotopic compositions; 2) a plume source for carbonatite, nephelinite and kimberlitic rocks which were related to magnesite or periclase/perovskite involvement as reflected by depleted Sr–Nd–O and extremely light Mg isotopes. Ultimately, our study suggests that subducted slabs could make important contributions to LIP generation, and establishes a potential linkage between plate tectonics and mantle plume.