Refertilization of lithospheric mantle beneath the North China Craton in Mesozoic: Evidence from in situ Sr isotopes of Fuxin peridotite

Abstract

The Mesozoic was a critical period for the evolution of the North China Craton (NCC). We present the petrological observations and in situ mineral major and trace elements and Sr isotopic data for spinel harzburgite and lherzolite xenoliths in Late Cretaceous Fuxin basalts from the northern margin of the NCC. These data are used to investigate the nature and evolution of the subcontinental lithospheric mantle (SCLM) beneath the NCC during the Mesozoic. The peridotite xenoliths commonly have porphyroclastic and granoblastic textures, and can be divided into high-Mg# and low-Mg# peridotites. Compared with the low-Mg# peridotites, the high-Mg# peridotites have higher olivine Fo (>91) and spinel Cr# values (>35), and their clinopyroxenes are light REE-enriched with uniform 87Sr/86Sr ratios (ca. 0.7042). These observations suggest that the high-Mg# peridotites could be relics of old SCLM and were metasomatized by melts derived from subducted Paleo-Asian oceanic slab. The low-Mg# peridotites are characterized by minerals with sieve textures and melt pockets. The olivine Fo of the low-Mg# peridotites are low and variable (81.2–90.4), and decrease from core to rim in some crystals. Thus, the low-Mg# peridotites underwent extensive peridotite–melt interactions. Their clinopyroxenes are enriched in LREE and show large variations in the 87Sr/86Sr ratios (0.7039–0.7052). Moreover, 87Sr/86Sr ratios positively correlate with MgO and negatively correlate with Al2O3, Eu and Dy contents. These observations suggest that the low-Mg# peridotites experienced multiple stages of mantle metasomatism, involving high-87Sr/86Sr melts derived from the subducted Paleo-Asian oceanic crust in early stage and low-87Sr/86Sr melts derived from more recent asthenospheric upwelling. Therefore, the low-Mg# peridotites are representative of SCLM fragments that were strongly modified beneath the Fuxin region during the Late Cretaceous. Combined with previous studies, our findings suggest that refertilization of the SCLM by asthenospheric melts beneath the northern margin of the NCC commenced before eruption of the Fuxin basalts (ca. 100 Ma), which is much earlier than previously thought. This process resulted in a highly heterogeneous SCLM. In addition, the Tan–Lu Fault contributed substantially to the evolution of the SCLM beneath the NCC during the Mesozoic.