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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 δ18O values relative to normal mantle peridotite. The olivine xenocrysts and porphyroclastic olivines from Junan have Fo=87.5–89.8, δ18O=4.1‰–5.2‰ with an average of 4.8‰; the porphyroclastic olivines from Qingdao also have Fo=89.0–89.9, δ18O=4.1‰–5.2‰ with an average of 4.8‰. These peridotite xenoliths have petrological and geochemical affinities similar to newly accreted MORB-type mantle, we hence consider such low- δ18O 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- δ18O signatures. Combined the existence of Cenozoic low- δ18O 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. [ABSTRACT FROM AUTHOR]

The Late Paleozoic layered or stratoid-layered basalts in western Guangxi have similar elemental and isotopic compositions to Emeishan high-Ti basalts. Whole-rock 40Ar/39Ar and SHRIMP zircon U-Pb dating were carried out for the representative basalt samples in three typical profiles in the area. Three basalts from the upper segment of Yangxu profile and lower segment of Yufeng and Min'an profiles yield the 40Ar/39Ar plateau ages of 253.6±0.4 Ma (20BS-71), 255.4±0.4 Ma (20BS-99) and 256.2±0.8 Ma (20BS-119), respectively. Twenty-three analyses on 23 zircons of the basalt from the upper segment of Yangxu profile give a weighted mean 206pb/238U age of 253.7±6.1 Ma with an MSWD = 2.8. These new and published geochronological data for Emeishan large igneous province (LIP) indicate that the Emeishan LIP was initiated at -260 Ma, voluminously erupted between 253 and 256 Ma, and possibly ended at ∼251–253 Ma. The age (251–260 Ma) is generally consistent with that of the associated environmental deterioration and mass extinction events at the end-Guadalupian and Permo-Triassic boundary. These precise geochronological data provide important constraints on the dominantly eruptive time of the Emeishan LIP and understanding of the distribution of Emeishan high-Ti basalts and its mantle plume dynamics. [ABSTRACT FROM AUTHOR]