关键词: 碳酸盐组分, 强碱性玄武岩, 中国东南部, 低程度部分熔融, 地幔岩性

Abstract: The distribution of Cenozoic continental basalts from Zhejiang Province, southeast China are controlled by three northeast-southwest trending faults. According to their outcropping locations and eruption ages, these basalts are divided into three groups: western part, central part and eastern part basalts in Zhejiang province. Here, we carried out a geochemical study on Cenozoic Jiande basalts, the oldest basalts (~40 Ma) cropping out at Jiangshan-Shaoxing fault in the west of Zhejiang, to further explore the nature of mantle source beneath Zhejiang province. These volcanic rocks are mainly alkaline (sodic) basanites with low contents of SiO₂  (41.3-42.3 wt%) and Al₂O₃ (9.70-12.6 wt%), high contents of MgO (8.90-15.6 wt%), CaO (8.92-12.1 wt%), TiO₂ (2.78-3.18 wt%) and Fe₂O₃^T (14.1-16.2 wt%). They are characterized by negative K, Zr, Hf, and Ti anomalies (Hf/Hf*=0.74-0.77, Ti/Ti*=0.70-0.74), and high Zr/Hf (48.5-50.1), Ca/Al (1.02-1.16) ratios, which suggest the contribution of recycled carbonated materials in the mantle source. They show depleted Sr, Nd, Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.7032-0.7034, εNd=5.85-5.95, εHf=7.78-8.56) and have ²⁰⁶Pb/²⁰⁴Pb ratios of 18.491-18.554, ²⁰⁷Pb/²⁰⁴Pb ratios of 15.488-15.518 and ²⁰⁸Pb/²⁰⁴Pb ratios of 38.387-38.523. Compared to weakly alkaline basalts in central and eastern part of Zhejiang, the strongly alkaline basalts in western part of Zhejiang show higher contents of trace elements and more pronounced negative Zr, Hf, and Ti anomalies, which suggest that the strongly alkaline basalts in western part of Zhejiang are generated by low-degree partial melting of a carbonated mantle source. The good correlations of Ti/Ti* with Ba/Th and K/La of Zhejiang Cenozoic basalts suggest a mixing process of two components. Carbonated melts derived from partial melting of carbonate-bearing oceanic crust reacted with the depleted asthenosphere mantle, generating the carbonated mantle source with low Ba/Th, K/La, and Ti/Ti* ratios (reflected in basalts in western part of Zhejiang). Another enriched component is the melting residue of recycled oceanic crust with high Ba/Th, K/La and Ti/Ti* ratios (reflected in basalts in eastern part of Zhejiang). Therefore, the mantle source of Cenozoic Zhejiang basalts, which is mainly controlled by two components with genetic relationship, is heterogeneous.

Key words: recycled carbonated component, strongly alkaline basalts, southeast China, low-degree partial melting, mantle source lithology