Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China

•Clinopyroxene (Cpx) macrocrysts are recognized in the Xiaohaizi dyke, Tarim LIP.•Cpx macrocrysts are not in textural and chemical equilibrium with the host dyke.•Cpx macrocrysts are antecrysts that crystallized from progenitor magmas.•Sr–Nd isotopes are distinct between Cpx macrocrysts and Cpx in the Xiaohaizi wehrlites.•AFC process played an important role in the generation of the Xiaohaizi intrusion.

Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80–89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2 (0.26–1.09 wt.%), Al2O3 (1.15–3.10 wt.%) and Na2O (0.16–0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenocrysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The 87Sr/86Sri (0.7035–0.7037) and εNdi (4.5–4.8) of the clinopyroxene macrocrysts with high Mg# (80–89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the wehrlites (Mg# = 75–84, 87Sr/86Sri = 0.7038–0.7041, εNdi = 1.0–1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts.