Zircon U–Pb ages and Hf–O isotopic signatures of the Wajilitag and Puchang Fe–Ti oxide–bearing intrusive complexes: Constraints on their source characteristics and temporal–spatial evolution of the Tarim large igneous province

Abstract

The Wajilitag and Puchang igneous complexes host two known economic Fe–Ti oxide deposits in the recently recognized Tarim large igneous province (TLIP). The Wajilitag complex comprises clinopyroxenite and melagabbro, whereas the Puchang complex is generally gabbroic and anorthositic in lithology apart from minor plagioclase-bearing clinopyroxenites in the marginal contact zone. The Fe–Ti oxide ores are disseminated throughout the Wajilitag complex and principally restricted to the ultramafic unit, whereas the Puchang complex contains massive to disseminated Fe–Ti oxide ores mainly hosted within the gabbroic rocks. Both secondary ion mass spectroscopy and laser ablation-inductively coupled plasma-mass spectrometry U–Pb dating of zircon grains from the Wajilitag and Puchang complexes yield U–Pb zircon ages of ca. 283Ma and ca. 275Ma, respectively, clearly indicating that there were two independent episodes of the magmatic events related to Fe–Ti oxide mineralization in the TLIP. The new zircon U–Pb ages of intrusive rocks studied here, coupled with available geochronological data from elsewhere in the TLIP, show a long duration of magmatism (up to 30Myr), although the precise age of the TLIP remains to be determined. The two complexes are late-stage events that notably postdate most, if not all, the basaltic lava flows. Furthermore, the occurrence of the earliest manifestation (e.g., ca. 300Ma kimberlitic rocks) of a proposed mantle plume in the Bachu area and the potential temporal migration of the late-stage magmatism from the Bachu and Keping areas to the edges of the Tarim Craton, indicate a possible plume centre near the Bachu–Keping district. The εHf(t) values of zircons from each complex show a range of several εHf(t) units (Wajilitag: +2.7 to +9.2, Puchang: −5.2 to +2.6), probably suggesting late-stage crustal contamination in magma chambers at the time of zircon saturation. Unlike the Lu–Hf isotopic system, the zircons may preserve the original O isotope signature of their mantle sources. The increase of O isotopic composition from the Wajilitag complex (δ18O=5.2–5.9‰) to the Puchang complex (δ18O=5.6–7.1‰), indicates a relatively high proportion of recycled subduction-related materials (e.g., eclogite and garnet pyroxenite) incorporated into the subcontinental lithospheric mantle source for the Puchang intrusive rocks. Partial melting of the refertilized subcontinental lithospheric mantle with the involvement of garnet-bearing mafic components can be of great importance for the formation of parental Fe-rich magmas and ultimately Fe–Ti oxide deposits. This observation is consistent with the occurrence of some mineralized LIPs (e.g., Emeishan) in formerly active convergent plate margins of ancient cratonic blocks, contributing to a global understanding valuable to exploration efforts.