A metasomatized mantle wedge origin for low-δ 18O olivine in late Cretaceous Junan and Qingdao basalts in the Sulu orogen

The North China Block (NCB) experienced extensive lithospheric thinning and subsequent mantle accretion in the Mesozoic to Cenozoic, but their mechanism remains controversial. This paper reports in situ chemical and oxygen isotope analyses on olivine xenocrysts and porphyroclastic olivines in mantle xenoliths hosted in the late Cretaceous basalts from Junan and Qingdao, the Sulu orogen. The results show that all these olivines have low delta O-18 values relative to normal mantle peridotite. The olivine xenocrysts and porphyroclastic olivines from Junan have Fo=87.5-89.8, delta O-18=4.1aEuro degrees aEuro5.2aEuro degrees with an average of 4.8aEuro degrees; the porphyroclastic olivines from Qingdao also have Fo=89.0-89.9, delta O-18=4.1aEuro degrees aEuro5.2aEuro degrees with an average of 4.8aEuro degrees. These peridotite xenoliths have petrological and geochemical affinities similar to newly accreted MORB-type mantle, we hence consider such low-delta O-18 features in the olivines to inherit from a mantle wedge that was metasomatized by melts derived from the subducted oceanic crust, which had experienced high-temperature hydrothermal alteration to acquire the low-delta O-18 signatures. Combined the existence of Cenozoic low-delta O-18 basalts and garnet pyroxenite xenoliths (relicts of recycled oceanic crust) hosted in Cenozoic basalts in the NCB, the subducted oceanic crust likely played an important role in the lithospheric evolution of the NCB during the Mesozoic to Cenozoic.