CONTINUOUS INJECTION OF PLUME COMPONENTS TO SUB-CONTINENTAL LITHOSPHERIC MANTLE: EVIDENCE FROM THE EARLY PERMIAN TARIM LARGE IGNEOUS PROVINCE (NW CHINA)

The Tarim continental flood basalts (CFBs) provide important clues to address some of the controversial issues relating to the Early Permian Tarim Large Igneous Province (TLIP) in northwestern China. Here we present results from in-situ LA–MC–ICPMS Lu–Hf isotopic analyses of Early Permian zircons from the Tarim CFBs in the Keping area of the western Tarim Basin. Our results reveal that the zircons from two basalt sub-groups (Groups 1a and 1b) have similar Lu–Hf isotopic compositions and exhibit a narrow range of ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.282422 and 0.282568. Their corresponding εHf(T) values (-6.8~-1.4) are generally lower than the whole-rock εHf(T) values of their host basalts (-2.3~+2.1), and distinct from other known intrusive rocks (+3.0~+7.1) and their zircons (+4.9~+8.8) in the TLIP. These zircons are interpreted to have crystallized in a concealed pluton shortly prior to the extrusion of basalts and preserve the initial Hf isotopic compositions from their original source region, which was probably the sub-continental lithospheric mantle (SCLM) beneath the Tarim Block. They have relatively higher εHf(T) values compared with the inferred Tarim SCLM (-8.7~-5.2) in the Early Permian, indicating minor addition of depleted mantle magmas during the partial melting of the latter. A comparison of the Hf isotopic characteristics between the zircons and their host basalts suggests a similar mantle source in the SCLM, but the latter was probably mixed with more depleted mantle components than the former. Furthermore, a coherently depleting trend on Sr–Nd–Hf isotopic compositions from the earlier erupted Groups 1b, 1a and 2 basalts, and then to later emplaced ultramafic–mafic intrusive rocks and syenitic rocks can be traced, correlating with a rising mantle plume beneath the Tarim Block and its successive interaction with the SCLM with continuous injection of depleted mantle components into the evolved magma source to produce the various igneous rock units of the TLIP.