Petrogenesis and metallogenesis of the Wajilitag and Puchang Fe-Ti oxide-rich intrusive complexes, northwestern Tarim Large Igneous Province

Abstract

The Wajilitag and Puchang intrusive complexes of the Tarim large igneous province (TLIP) are associated with significant resources of Fe–Ti oxide ores. These two mafic–ultramafic intrusions show differences in lithology and mineral chemistry. Clinopyroxenite and melagabbro are the dominant rock types in the Wajilitag complex, whereas the Puchang complex is generally gabbroic and anorthositic in composition with minor plagioclase–bearing clinopyroxenites in the marginal zone. Disseminated Fe–Ti oxide ores are found in the Wajilitag complex and closely associated with clinopyroxenites, whereas the Puchang complex hosts massive to disseminated Fe–Ti oxide ores mainly within its gabbroic rocks. The Wajilitag intrusive rocks are characterized by a restricted range of initial 87Sr/86Sr ratios (0.7038–0.7048) and positive εNd(t) (+0.04 − +3.01), indicating insignificant involvement of continental crustal contamination. The slightly higher initial 87Sr/86Sr ratios (0.7039–0.7059) and lower εNd(t) values (−1.05 − +2.35) of the Puchang intrusive rocks also suggest that the isotopic characteristics was controlled primarily by their mantle source, rather than by crustal contamination. Both complexes have Sr–Nd isotopic compositions close the neighboring kimberlitic rocks and their hosted mantle xenoliths in the TLIP. This indicates that the ferrobasaltic parental magmas were most probably originated from partial melting of a metasomatized subcontinental lithospheric mantle, modified recently with subduction–related materials by the impingement of the ascending mantle plume. The recycled subduction–related materials preserved in the lithospheric mantle could play a key role in the formation of the parental Fe–rich magmas in the context of an overall LIP system. The distinct variations in mineral assemblage for each complex and modeled results indicated that the Wajilitag and Puchang complexes experienced different crystallization path. Fe–Ti oxides in Wajilitag joined the liquidus earlier in the crystallization sequence, especially relative to the crystallization of plagioclase. This is attributed to the relatively high TFeO, TiO2 and initial H2O contents of the parental magma. In combination with definitive field and petrological evidence, the enrichment of highly incompatible elements (e.g., Zr, Hf, Nb and Ta) and the depletion of rare earth elements in the Fe–Ti oxide ores is consistent with the plausible interpretation that the ores could be formed by fractional crystallization and crystal accumulation of the Fe–Ti oxide crystals from the ferrobasaltic parental magmas. A considerable amount of the Fe–Ti oxides in the Puchang has transported and sunk from higher up in the chamber to the underlying unconsolidated silicate crystal pile. The highly dense Fe–Ti oxide crystal slurries further tended to effective accumulate Fe–Ti oxides to form high–grade Fe–Ti oxide ore bodies, and subsequent rapid collapse and intrusive into lower lithologies within the complex under a H2O–rich environment during the late–stage of magmatic differentiation. The development of massive Fe–Ti oxide ores in the case of the Puchang, could plausibly result from a combination of the protracted differentiation history of a Fe highly enriched parental magma and the later addition of external H2O from the country rocks (e.g., carbonates) to the slowly cooling magma chamber.