Geochemical evidence of plume sources for high-MgO lavas in the WK Orogenic Belt

Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian-Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6-33.8 wt.%) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2±0.9 wt.%. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg-Fe equilibria, Al-in-olivine and olivine-spinel thermometry and oxybarometer yield a mantle potential temperature of 1522-1556 °C and high oxygen fugacity of FMQ+0.93. The H2O contents in the picrite flows are estimated as 3.67±1.0 wt.%, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd-O-Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.