Differential partial melting process for temporal variations of Shandong basalts revealed by melt inclusions and their host olivines

The link between the geochemical variability of lavas and their source lithologies is very important in understanding the nature of mantle sources and melting processes. The Cenozoic basalts of Shandong province are ideal targets for addressing this issue. Previous studies have revealed two different types of basalts in Shandong province. The first type includes older, slightly alkalic basalts (SAB) with EMI oceanic island basalt (OIB)-like trace element and isotope compositions, whereas the second type includes younger, highly alkalic basalts (HAB) with HIMU OIB-like trace element compositions, but depleted isotope compositions. However, the source lithologies and temporal variations of these basalts are still debated. In this study, we present data from olivine-hosted melt inclusions and whole rock samples for the Cenozoic Yujiagou basalts of Shandong province in the North China Craton. The Yujiagou basalts are alkalic basalts with trace element compositions more like those of EMI OIB, suggesting that the Yujiagou basalts are SAB. In contrast, the olivine-hosted melt inclusions have more variable compositions, straddling the compositional field between the SAB and the HAB. In particular, the melt inclusions exhibit variable Pb isotope compositions ranging from the HAB to the SAB, arguing for the coexistence of HAB and SAB melts in the Yujiagou basalts. Compared with olivines crystallized from partial melts of peridotite, olivines from the Yujiagou basalts show lower Ca concentrations, but higher Ni abundances and Fe/Mn ratios. Combined with the low CaO contents of the bulk rocks and melt inclusions, this evidence suggests that the Yujiagou basalts represent the products of partial melting of pyroxenite. The mantle potential temperature and crystallizing temperatures of primary magmas calculated based on melt inclusions within high Fo olivines (Fo = similar to 90) are > 100 degrees C higher than those of the ambient mantle, suggesting a probable thermal anomaly in the mantle. Combined with experimental results, we deduce that the temporal change in Cenozoic basalt compositions in Shandong province is due to differential partial melting processes that could be attributable to a thermal anomaly. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.