Abstract:

The Cenozoic basalts in Fujian are widely developed and contain abundant mantle xenoliths, whose mineralogical characteristic provide insights into the properties of lithospheric mantle. This study conducted electron microprobes, clinopyroxene LA-ICP-MS trace element, and in-situ Sr isotopes analyses on the minerals in Cenozoic basalts and peridotite xenoliths from Xiamen along the Fujian coast, as well as Mingxi and Minqing in the inland region. The results indicate that the clinopyroxenes in both basalt and peridotite xenoliths are predominantly diopside. The peridotite xenoliths are divided into two groups based on mineralogical characteristics: Group A (Mg# > 90), representing an ancient, refractory lithospheric mantle, and Group B (Mg# < 90), representing a younger, modified enriched lithospheric mantle. Group A clinopyroxene exhibit slight negative anomalies in high-field strength elements Nb and Ti, and strong negative anomalies in Hf, Zr, and Ta. In Group B peridotite, clinopyroxene shows differences, with slightly negative anomaly in high high-field strength elements Zr and Hf and a pronounced negative anomaly in the large ion lithophile element Ba. The clinopyroxene in both the phenocrysts and matrix of the basalt exhibits LREE enrichment and HREE depletion, with the rims showing greater depletions in high-field strength elements Th and Nb compared to the cores, along with enrichments in large ion lithophile elements Sr and Eu. Most clinopyroxenes in both Group A and Group B xenoliths have experienced silicate melt metasomatism, while a few Group A clinopyroxene suggest a mix between silicate and carbonate melt metasomatism. In-situ Sr isotope ratios for clinopyroxene range from 0.702 57 to 0.703 67, with most rims showing higher 87Sr/86Sr ratios than the cores, like due to interaction with materials of higher 87Sr/86Sr ratios. The Fujian peridotite probably represents a newly modified lithospheric mantle of the Pacific plate subduction asthenosphere mantle, following decompression ascent.

Key words: Cenozoic basalts, peridotite xenoliths, mineralogy, in-situ Sr isotope, lithospheric mantle