Mafic-ultramafic intrusion formed by multi-stage evolution of hydrous basaltic melts

Abstract

AbstractThe magmatic processes beneath the active continental margins are very complicated and affect structures and compositions of the arc roots. Neoproterozoic igneous rocks are widely distributed around the margins of the Tarim Block in NW China. The Xingdier mafic-ultramafic intrusion is a composite body, located at the northern margin of the Tarim Block, and consists of gabbro, pyroxenite, and peridotite units. The gabbro unit has a secondary ion mass spectrometry zircon U-Pb age of 727 ± 5 Ma. Rocks from the Xingdier intrusion have a large range of MgO (12.9–32.8 wt%) and SiO2 (43.0–57.9 wt%), and low K2O + Na2O (0.11–2.25 wt%) contents. They have right inclined chondrite-normalized rare earth element patterns with (La/Yb)N ratios of 2.2–8.6. Their primitive mantle normalized trace element patterns show arc-affinity geochemical features characterized by enrichment in Rb, Ba, Th, U, and Pb and depletion in Nb, Ta, and Ti. They have variable initial 87Sr/86Sr ratios (0.7063–0.7093), εNd(t) values (−2.9 to −7.8), 206Pb/204Pb (17.08–17.80), 207Pb/204Pb (15.42–15.49), and 208Pb/204Pb ratios (37.48–38.05), forming an evolution trend from the peridotite unit to the gabbro and pyroxenite units. Clinopyroxene in the three units is chemically similar to those formed in hydrous magmas. The spinel inclusions in olivine from the peridotite unit show unmixing texture and have high Al contents and oxygen fugacity of ~FMQ+1. Therefore, the parental magma was probably derived from a lithospheric mantle enriched by slab-derived fluids. Rocks from the gabbro and peridotite units are proposed to have been derived from olivine-normative melts, whereas rocks from the pyroxenite unit are cumulates from the quartz-normative melts. Such contrasting parental magmas resulted from variable degrees of crustal contamination and fractional crystallization in the arc root.