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Genesis and Characteristics of Mantle Sources of Cenozoic Jiande Basalts from Zhejiang Province
- HE Ye,CHEN Lihui,ZENG Gang,WANG Xiaojun
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2020, 26(3):
241-254.
DOI: 10.16108/j.issn1006-7493.2019043
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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 Fe2O3T (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.