Source of highly potassic basalts in northeast China: Evidence from Re-Os, Sr-Nd-Hf isotopes and PGE geochemistry

- Unradiogenic to only moderately radiogenic Os isotopic compositions
- Only slight PPGE (Pt and Pd) enrichments relative to IPGE (Ir, Os and Ru)
- Minor lower crustal contamination reflected by Os isotopes and Pd/Ir ratios
- The old lower crust in WEK area has foundered together with SCLM.
- The newly-formed SCLM has been metasomatized by delaminated old lower crust.

The origin of the very young (0.5 Ma to 1721 AD) Wudalianchi-Erkeshan (WDLC and EKS) highly potassic basalts in northeast (NE) China has been the subject of considerable debate. In this study, we present new major-, trace- and platinum group element (PGE) data together with Re-Os, Lu-Hf, and Sr-Nd isotopic analyses for these potassic basalts in order to further constrain their source. It has been shown that the WDLC and EKS basalts have unradiogenic to only moderately radiogenic osmium isotope ratios with 187Os/188Os ranging from 0.1187 to ~ 0.17, and only slight PPGE (Pt and Pd) enrichments relative to IPGE (Os, Ir and Ru). The positive correlations between 187Os/188Os and 1/Os suggests that these basalts have probably experienced about 2-8% of lower continental crust addition during magma ascent. On the other hand, the incorporation of sub-continental lithospheric mantle (SCLM)-derived, primary sulphides and/or PGE micro-alloys during magma ascent seems likely in some basalts, which preserve particularly unradiogenic Os compositions. The complex interplay of signatures derived from crustal and SCLM contamination means that the Os isotope systematics of the basalts do not unequivocally fingerprint the source of the WDLC and EKS basalts.The strong enrichments of light rare earth elements (LREE) and large ion lithophile elements (LILE), high-K, EM1-like Sr-Nd-Hf isotopic characteristics, and particularly strong fractionation of the heavy REE ((Sm/Yb)N = 9.7 ± 0.6), suggest that the WDLC and EKS basalts mainly originated from phlogopite-bearing garnet-peridotite in the SCLM. Combined with low Ce/Pb ratios (~ 9.4-13.5), the most likely source of WDLC and EKS basalts is SCLM that has been metasomatized by delaminated old, lower continental crust. This is different from previous hypotheses that implicate a dominantly asthenospheric source with a contribution from delaminated ancient SCLM or subducted oceanic crust with sediments, or a SCLM source that has been metasomatized by melts derived from deep asthenosphere during the Proterozoic.