北山造山带南缘柳园地区新元古代花岗岩的地球化学特征及其地质意义

Geochemistry of Neoproterozoic granite in Liuyuan area of southern Beishan orogenic belt and its geological significance

为深入认识中亚造山带南缘早前寒武纪的地质演化,对中亚造山带南缘柳园地区新元古代花岗质片麻岩的主、微量元素和锆石U-Pb年龄、微量元素、Hf同位素进行了分析。花岗质片麻岩具有高钾的特点(K2O=5.31%-7.02%),为高钾钙碱性系列。富集轻稀土元素(LREE)和大离子亲石元素(LILE),Nb、Ta、Ti和Sr显著亏损,Pb表现出正异常,具有后碰撞花岗岩的岩石地球化学特征。LA-ICP-MS锆石U-Pb年龄和Hf同位素分析结果表明,柳园地区新元古代花岗岩形成于902Ma±5Ma(2σ),亏损地幔的两阶段模式年龄约为1.6Ga,结合约1.6Ga继承锆石的出现,认为这些花岗岩是古老地壳物质熔融的产物。另外,花岗岩具有较高的εHf(t)值(-2.0-1.0),表明母岩浆含有新生幔源物质的加入。因此,柳园地区新元古代花岗岩可能具有古老地壳物质熔融再与新生幔源物质混合的成因,认为北山柳园地区新元古代花岗岩的形成是Rodinia超大陆汇聚的响应。

In order to constrain the early Precambrian geological evolution of the southern Central Asian Orogenic Belt （CAOB）, the authors studied geochemistry, zircon U-Pb ages, trace elements and Hf isotope of the Neoproterozic granitic gneisses in Liuyuan area of the southern Beishan orogenic belt, which is considered to be the southernmost region of the CAOB. These granitic gneisses belong to high-K calc-alkaline series with K2O content of 5.31%-7.02% and are characterized by enrichment of light rare earth elements （LREE） and large ion lithophile elements （LILE）, depletion of Nb, Ta, Ti and Sr, and existence of positive Pb anomalies, similar to characteristics of post-collision granites. In-situ zircon U-Pb dating and trace element analysis using LA-ICP-MS reveal that 902Ma±5Ma（2σ） represents the formation age of the Liuyuan Neoproterozoic granite. The presence of -1.6Ga inherited zircon core indicates the adding of ancient crustal materials. On the other hand, these granites have relatively high εHf（t） values （-2.0-1.0）, indi- cating that the parental magma contained juvenile mantle-derived materials. Therefore, Liuyuan Neoproterozoic granites might have been derived from the partial melting of ancient crust and juvenile mantle materials. In combination with previous studies, the authors hold that Liuyuan Neoproterozoic granite could be considered to be a result of the assembly of Rodinia supercontinent.