Petrogenetic modeling of three mafic–ultramafic layered intrusions in the Emeishan large igneous province, SW China, based on isotopic and bulk chemical constraints

Abstract

Mafic–ultramafic layered intrusions align along a N–S trending fault zone in the central part of the late Permian Emeishan large igneous province, SW China. We present major and trace element and clinopyroxene Sr, Nd, Pb and O isotopic compositions of three representative mafic–ultramafic layered intrusions, the Panzhihua gabbro intrusion that hosts V–Ti–Fe ores, the Limahe peridotite intrusion that hosts Cu–Ni ores, and the Xinjie peridotite and pyroxenite intrusion that hosts both V–Ti–Fe and Cu–Ni–PGE ores. The Panzhihua intrusion is more evolved, and rich in Fe and Ti, whereas the Limahe ore-bearing rocks are rich in Mg, and the unmineralized gabbro-diorites are rich in Ca and Al. Both the petrology and geochemistry of the Xinjie intrusion are transitional between those of the two other intrusions. Petrography and geochemistry indicate that there is a large amount of cumulus magnetite and ilmenite in the Panzhihua and Xinjie intrusions. This has changed the ratios of elements normally considered incompatible in basaltic systems (e.g., La/Zr and La/Nb), because high field strength elements (Zr, Hf, Nb and Ta) show significant partitioning into the magnetite structure [Klemme, S., Günthe, D., Hametner, K., Prowatke, S., Zack, T., 2006. The partitioning of trace elements between ilmenite, ulvospinel, armalcolite and silicate melts with implications for the early differentiation of the moon. Chemical Geology 234, 251–263]. The parental magma of the Panzhihua intrusion is estimated to be ferropicrite. Thus, in combination with their low δ18O values (<6‰), we infer that the generation of the Panzhihua intrusion could be ascribed to ascending plume-derived magma contaminated by a Fe- and Ti-rich lithospheric mantle source. In contrast, negative εNd(t) values (−0.6 to −4.13), high δ18O values (>6‰), radiogenic Pb isotopic compositions, and high La/Nb and La/Ta ratios imply that the Limahe intrusion was derived from mantle-plume magmas contaminated by granitic crustal materials. The δ18O values (4.8–8.0‰) and Pb isotope ratios suggest that the Xinjie intrusion may result from plume-derived magmas variably contaminated by basic crustal materials in situ. The formation of the different types of the magmatic-ore deposits within the Emeishan LIP may be mainly attributed to their different sources.