辽宁大石桥早白垩世建一岩体年代学、地球化学及成因研究

GEOCHRONOLOGY, GEOCHEMISTRY AND ORIGIN OF THE EARLY CRETACEOUS JIANYI PLUTON IN DASHIQIAO, LIAONING PROVINCE

建一岩体岩性主要为花岗闪长岩.岩石中的锆石呈自形,发育岩浆振荡生长环带,具有较高的Th/U比值（〉0.1）,均暗示其为典型的岩浆成因锆石.锆石LA-ICP-MS U-Pb定年结果显示,所测锆石206Pb/238U年龄集中于138-145 Ma,其加权平均年龄为140 Ma,说明花岗闪长岩的形成时代为早白垩世.主量元素特征显示,该岩体为铝不饱和的I型花岗岩.球粒陨石标准化稀土元素分配模式显示轻稀土元素（LREE）相对富集,中稀土和重稀土元素（MREE和HREE）相对亏损,具有轻微的负Eu异常,暗示岩浆演化过程中角闪石分离结晶明显而斜长石分离结晶较弱.微量元素蛛网图显示富集大离子亲石元素（LILE）Rb、Th、U,贫高场强元素（HFSE）Nb、Ta,其Nb/Ta、La/Nb和Th/Nb比值介于地壳和地幔平均值之间,显示建一岩体岩浆具有壳幔混源的特点.结合建一岩体岩石地球化学特点和区域构造背景,认为该岩体的形成可能与古太平洋板块向欧亚大陆下俯冲引起的类似于弧后盆地的伸展环境有关.

The Jianyi pluton, located in Dashiqiao, Liaoning Province, Northeast China, is dominated by granodiorite. The zircons from the granodiorite show euhedral shape, oscillatory and sector-zoning, with high Th/U ratios（〉0.1）, suggesting a typical magmatic origin. LA-ICP-MS zircon U-Pb dating indicates that the ^206Pb/^238U ages of zircons range from 138 to145 Ma, yielding a weighted mean age of 140 Ma, which probably represents the formation age of the granodiorite, i.e.,Early Cretaceous. Major element characteristics show that the Jianyi pluton belongs to metaluminous I-type granite. The chondrite-normalized REE patterns suggest enrichment of LREEs, and depletion of MREEs and HREEs, with slightly negative Eu anomaly, indicating a strong fractional crystallization of amphibole and weak fractional crystallization of plagioclase. The trace element spider diagram shows that they are enriched in large ion lithophile elements（LILEs, such as Rb, Th and U）and depleted in high field strength elements（HFSEs, Nb and Ta）. The Nb/Ta, La/Nb and Th/Nb values are between the data of the crust and mantle, showing that the magma is derived from crust-mantle mixed source.Integration of our new data with recent geological studies concludes that the Jianyi pluton and the Early Cretaceous magmatism in the eastern North China craton was formed in an extensional setting similar to back-arc basin, which may be related to the subduction of the Paleo-Pacific oceanic plate.