Petrogenesis of the early Permian Yejili mafic–ultramafic intrusion in the Alxa Block, western margin of the North China Craton

Abstract

The ~287 Ma Yejili mafic–ultramafic intrusion in the southern Alxa Block mainly consists of lherzolite, wehrlite, and gabbro. Rocks from the lherzolite and wehrlite have olivine with Fo values ranging from 87.4 to 88.6 mol.% and from 81.6 to 87.4 mol.%, respectively, indicating crystallization from a moderately evolved magma, that is a mixing of 5% to 7% upper crust contaminant with the primitive magma modelling by Sr–Nd isotopic compositions. Samples from the Yejili intrusion generally have low and variable PGE (0.2 to 4.7 ppb) and display trough‐like primitive mantle‐normalized chalcophile element patterns with relatively high Cu/Pd ratios (40,967 to 412,727) and positive correlation between Y/Pd and Y/Pt ratios, indicating derivation possibly from sulphide‐saturated magmas. However, the small degree of crustal contamination and high Ni concentrations of olivine from lherzolite and wehrlite suggest possible small‐scale sulphide segregation for the Yejili intrusion. In addition, the positive correlation between Fo and Ni concentrations of olivine from the wehrlite suggests the sulphide segregation may have occurred in a deep‐seated staging magma chamber and the sulphide did not enter into the shallow magma chamber because of the high densities. On the other hand, the Yejili intrusion has εNd(t) values (+1.1 to +2.6) much lower than the corresponding depleted mantle and the initial 206Pb/204Pb ratios ranging from 17.88 to 18.11, indicating a weakly depleted source. In addition, the slight enrichment of LREE relative to HREE and positive LILE and negative HFSE (e.g., Nb, Ta, Zr, and Hf) anomalies in primitive mantle‐normalized trace element patterns of rocks from the Yejili intrusion differ from MORB and OIB from the convecting mantle, suggesting the weakly depleted source may have resulted from the metasomatism of the subcontinental lithospheric mantle by slab‐derived fluid/melt. Therefore, the arc‐like geochemical features and low Ca concentration of olivine from lherzolite and wehrlite for the Yejili intrusion indicate a subduction‐related origin, rather than an extensional environment. We thus suggest that the early Permian magmatism in the periphery of the Alxa Block was associated with southward subduction of the Paleo‐Asian Ocean.