浙江“陈蔡增生杂岩”中洋内弧型变基性火山岩的地球化学特征及其地质意义

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

文章报道了浙江"陈蔡增生杂岩"中新发现一类特殊的变质基性火山岩组合,主要由斜长角闪岩和角闪岩组成,岩石具有低Ti高Mg的地球化学特征,原岩为拉斑玄武岩和钙碱性玄武岩。岩石稀土总量较低,ΣREE平均为25.46×10^(-6),轻重稀土比值LREE/HREE及(La/Yb)_N比值较小,平均分别为1.87和1.30,δEu平均为0.97,稀土配分与N-MORB及T-MORB类似,而微量元素显示为富集相容元素Cr、Ni及大离子亲石元素(LILE)K、Rb、Ba、U,亏损高场强元素(HFSE)Nb、Zr、P、Ti。上述地球化学特征与玻安质岩石十分相似,微量元素比值及图解判别均指示该套变基性火山岩形成于大洋岛弧(洋内弧)环境。该洋内弧型变基性火山岩的发现表明原定陈蔡群极有可能为新元古代中期至早古生代含有古大洋地壳残片的俯冲增生杂岩,而不是华夏古陆块的基底。

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 andhigh 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 LaNto YbNare 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.