Geochemical characteristics of the intra-oceanic arc type metabasic-volcanics in Chencai accretion complex of Zhejiang Province and their geological significance

This paper reports the discovery of a special kind of metabasic-volcanics combination in Chenccai accretion complex of Zhejiang Province. It is mainly composed of amphibolite and hornblendite. Geochemical characteristics of the rocks are low Ti and high Mg. Their protoliths are tholeiite and calcalkali basalts. The rocks have low values of REE, and the average REE is 25.46×10^-6. The ratios of LREE to HREE and La_N toYb_N are lower, and the average of the ratios are respectively 1.87 and 1.30. The average of δ (Eu)is 0.97. The REE distribution model is similar to N-MORB and T-MORB. Trace elements show enrichment of incompatible elements Cr, Ni and LILE such as K, Rb, Ba and U and depleted in HFSE such as Nb, Zr, P and Ti. The geochemical characteristics are very similar to those of boninites. The ratios and discrimination diagrams of trace elements indicate that the metabasic-volcanics were formed in an intra oceanic arc. The discovery of the intra oceanic arc type metabasic- volcanics shows that Chencai Group resulted most probably from the subducted accretionary complex which contained ancient fragments of oceanic crust from the middle Neoproterozoic to early Paleozoic rather than the Cathaysia Block basement.