南秦岭白水江群变基性火山岩块体的形成时代及其地球化学特征

The formation age and geochemical characteristics of the metavolcanic rock blocks of the Baishuijiang Group in South Qinling

在南秦岭白水江群大堡组中出露的变基性火山岩块体,高精度锆石SHRIMPⅡU-Pb定年测得年龄值为778Ma、771Ma和667Ma。岩石地球化学分析结果表明具有较高含量的TiO_2和P_2O_5,属于亚碱性玄武岩和碱性玄武岩。轻稀土富集、无δEu异常,富集大离子亲石元素,无明显的Nb-Ta-Zr-Hf等高场强元素异常,以及具有较低的正ε_(Nd)(t)(1.13～4.61)值,其特征与来自于富集地幔源区的洋岛玄武岩(OIB)相似,结合在白水江群中发现的早古生代洋岛型玄武岩,揭示了在新元古代和早古生代期间南秦岭地区存在着古洋盆。因此,综合在白水江群基质和不同岩块中获得的最新研究成果,以及勉-略混杂带中的资料,表明南秦岭增生杂岩最终形成于晚古生代。

The zircon SHRIMPⅡU-Pb dating indicates the metavolcanic rock blocks from the Dabao Formation of Baishuijiang Group, in South Qinling orogenic belt, were emplaced at 667~778Ma. And the blocks are belonging to sub-alkaline and alkaline tholeiite basalts, which have high concentration of TiO2 and P2O5, and showed enrichment in light REE and large ion lithophile elements, without δEu （δEu=1.00~1.17） and Nb-Ta-Zr-Hf abnormally, a low positive εNd（t） （1.13~4.61） value. Their geochemical characters and Sr-Nd istopic composition are similar to those of oceanic island basalt （OIB） that derived from enriched mantle. The new found OIB together with early found Paleozoic oceanic island basalts revealed there was an ancient ocean during the Neoproterozoic and Early Paleozoic in South Qinling. Therefore,the current research results of the matrix and blocks in the Baishuijiang Group and Mian-Lue mélange indicate the South Qinling accretionary complex eventually formed in Late Paleozoic.