A Permian Layered Intrusive Complex in the Western Tarim Block, Northwestern China: Product of a Ca. 275‐Ma Mantle Plume?

Zircon laser ablation inductively coupled plasma mass spectrometry U‐Pb age and geochemical and Sr‐Nd‐Hf isotopic data are reported for the Bachu layered intrusive complex (BLIC) in the western Tarim Block and are used to assess the possible presence of a Permian large igneous province (LIP) in the region. The BLIC intrudes the Silurian‐Devonian sedimentary rocks, and our U‐Pb zircon dating gives a crystallization age of $$274\pm 2$$ Ma. Rock types of the BLIC include pyroxenite, diorite, syenite, and quartz syenite, with a wide range of SiO2 contents (38.6%–68.7%) and variably high alkalinity ($$\mathrm{Na}\,_{2}\mathrm{O}\,+\mathrm{K}\,_{2}\mathrm{O}\,=1.5\% {\mbox{--}} 12.0\% $$ , $$\mathrm{K}\,_{2}\mathrm{O}\,/ \mathrm{Na}\,_{2}\mathrm{O}\,=0.23{\mbox{--}} 0.9$$ ). They are enriched in Rb, Ba, Th, Nb, Ta, Zr, Hf, and light rare earth elements. Isotopically, they are characterized by positive whole‐rock εNd(t) values (0.25–2.8, mostly above 2.0) and zircon εHf(t) values (5.8–8.9) and low initial 87Sr/86Sr ratios (0.7035–0.7045). These features suggest that the BLIC was likely formed by crystal cumulation and fractionation (with negligible crustal contamination) of alkali basalts derived from an ocean island basalt–like mantle source (i.e., the asthenospheric mantle) in an extensional regime. We suggest that these mid‐Permian igneous rocks, in combination with the voluminous coeval basalts and intrusive rocks covering an total area of ca. 250,000 km2 in the Tarim Block and surrounding regions, constitute an LIP (the “Bachu LIP”) and that the BLIC could be the residue of a feeder for this LIP.