Comparative Sr–Nd–Hf–Os–Pb isotope systematics of xenolithic peridotites from Yangyuan, North China Craton: Additional evidence for a Paleoproterozoic age

Abstract: Concentrations of trace elements in orthopyroxenes and clinopyroxenes, and the Sr, Nd, Hf and Pb isotopic compositions of clinopyroxenes from 11 well-characterized spinel peridotite xenoliths carried in the ~30Ma Yangyuan alkali basalts, North China Craton, reveal a complex history of chemical modification of the lithospheric mantle. The clinopyroxenes were separated from a suite of peridotites for which whole-rock Re–Os isotopic systematics have previously suggested a melt depletion event at ~1.8Ga, and were chosen to span a range of rare earth element (REE) patterns with (La/Yb) N =0.13 to 8.2. Present-day isotopic compositions are highly variable: 87Sr/86Sr=0.70229 to 0.70443, ε Nd =−0.6 to +24, 206Pb/204Pb=15.74 to 19.08, and ε Hf =+13.5 to +167. Partial melting models indicate that the Yangyuan clinopyroxenes have light REE (LREE) abundances that are too high to be pristine residues of fractional melting, reflecting subsequent enrichment of the LREE, e.g., via melt addition. Ancient melt addition accounts for the decoupling observed between Nd and Hf isotopic data for most samples, although a few samples may also have been modified by recent melt addition, shortly before the entrainment of the xenoliths into the host basalt. Strontium, Nd and Pb isotopic systems provide no useful age constraints for this suite, although some samples have Paleoproterozoic model ages. By contrast, Yangyuan clinopyroxenes yield a Lu–Hf correlation trend corresponding to an apparent age of 1.66±0.10Ga (2σ) that is within uncertainty of their whole-rock Os model age (1.8±0.2Ga). Given that the crust in this area originally formed during the Archean, these results suggest mantle lithosphere removal and replacement during the Paleoproterozoic. Our data demonstrate that, in some instances, the Lu–Hf isotope system may be used as a complement to the whole‐rock Re–Os isotope system in dating melt depletion of peridotites. [Copyright &y& Elsevier]