Petrogenesis of the Fe–P–REE mineralized Shangzhuang ultramafic intrusion in the Lajishan tectonic belt, South Qilian Belt: Implications for mantle metasomatism and tectonic setting

Abstract

The Shangzhuang ultramafic intrusion in the Lajishan tectonic belt, South Qilian Belt, is mainly composed of serpentinite, biotite clinopyroxenite, apatite clinopyroxenite, and hornblendite. Zircon grains selected from biotite clinopyroxenite have distinctly oscillatory and sector zoning and yield a weighted average 206Pb/238U age of 492.6 ± 4.1 Ma, which can be considered as the crystallization age of the Shangzhuang intrusion. Initial 206Pb/204Pb and 87Sr/86Sr ratios of the intrusion suggest an enriched mantle I source origin. Rocks from the intrusion have Rb/Cs from 16 to 29 and Nb/U from 0.2 to 11.7, different from mid‐ocean ridge basalt and ocean island basalt from the convecting mantle, suggesting a subcontinental lithospheric mantle source. In comparison with Sr–Nd isotopes of the regional 456‐ to 441‐Ma mafic intrusions and basic volcanic rocks, a gradually decreasing trend of εNd(t) values from +0.90 via −0.86 to −2.57 from 493 to 441 Ma argued that the subcontinental lithospheric mantle source of the Shangzhuang intrusion was progressively enriched by mantle metasomatism in the Early Palaeozoic. The elevated (Tb/Yb)N ratios of the Shangzhuang intrusion and positive correlations between light rare earth elements (La and Nd) and P2O5 of metamorphosed basalts from the regional Hualong Group imply that the source region was an apatite‐bearing garnet‐facies peridotite as a result of modal metasomatism. In general, the Shangzhuang intrusion was formed due to the northward subduction of the Proto‐Tethys Ocean underneath the Qilian Block during late Cambrian. Together with the ~491‐Ma regional ophiolites, we suggest that the Early Palaeozoic magmatism in the Lajishan belt was formed in a subduction zone, rather than intracontinental rifting or mantle plume settings.