Remnants of oceanic lower crust in the subcontinental lithospheric mantle: Trace element and Sr–Nd–O isotope evidence from aluminous garnet pyroxenite xenoliths from Jiaohe, Northeast China

Pyroxenite xenoliths entrained in alkali basalts are usually interpreted to be high pressure cumulates from mantle-derived melts. However, those from Jiaohe, Northeast China may have originated from subducted oceanic crust. Their high bulk Al2O3 contents (12.2–17.4%) and Mg# (up to 0.88) resemble those of olivine-bearing cumulative gabbros. A crustal origin for the Jiaohe pyroxenites from plagioclase-bearing protoliths is further supported by positive Eu and Sr anomalies in both whole rock and in garnet and clinopyroxene analyses, low HREE abundances in garnet and low δ18O (4.9–5.1‰) relative to typical mantle values (5.5‰). Compared with average values for the corresponding mantle minerals, the δ18O values of the Jiaohe clinopyroxene (4.5–5.4‰) are lower, the δ18O values of garnet (5.1–5.6‰) are similar, whereas those of spinel are higher (5.2‰). In particular, there are disequilibrium O isotopic fractionations between garnet/spinel and clinopyroxene, resembling those of plagioclase/clinopyroxene pairs in seawater-altered oceanic crust. This is attributed to spinel and garnet inheriting their O isotope characteristics from plagioclase prior to transformation to these minerals during subduction metamorphism of altered oceanic gabbros, without isotopic re-equilibration at mantle conditions. Identification of remnants of subducted oceanic crust in the subcontinental lithospheric mantle is important to understanding mantle heterogeneity and genesis of intraplate magmatism.