辽宁省本溪地区南孤山子岩体锆石U-Pb年代学及其构造意义

ZIRCON U-Pb DATING OF THE NANGUSHANZI INTRUSIVE ROCK IN BENXI COUNTY, LIAONING PROVINCE: Tectonic implication

南孤山子岩体位于辽宁省本溪县东南部,其岩性为中细粒碱长花岗岩.锆石U-Pb同位素年龄测试显示,该岩体形成于120-130 Ma的早白垩世,而不是前人认为的侏罗世.岩体的主量元素特征表明了它是高钾钙碱性系列.球粒陨石标准化稀土元素分配模式显示LREE相对富集,HREE相对亏损,具有较强亏损的负Eu异常.微量元素显示富集大离子亲石元素（LILE）Rb、Th、K,贫高场强元素（HFSE）Ta、P、Ti.碱长花岗岩微量元素组成表明本区的花岗岩岩浆形成压力小于0.8-1.0 GPa,源区物质存在于30-40 km的地壳深处,属于减薄型地壳,此时辽东陆块处于陆内伸展环境.上述研究充分地显示,我国东部早白垩世岩浆活动发育在伸展构造环境,并与岩石圈减薄存在密切的联系.

The Nangushanzi intrusive rock, located in the southeastern Benxi County of Liaoning Province, is lithologicallymedium-fine grained alkali-feldspar granite. The zircon U-Pb isotopic dating result shows that the rock was formed in theEarly Cretaceous Epoch of 120 - 130 Ma, but not the Jurassic Epoch as previously thought. The major element featuresindicate that the rock belongs to high-K calc-alkaline series. The chondrite-normalized REE patterns show enrichment ofLREEs, depletion of HREEs, and strongly negative Eu anomalies. The trace elements are characterized by enriched large-ion lithophile elements （LILE, such as Rb, Th and K） and depleted high field strength elements （HFSE, such as Ta, P andTi）. The trace element compositions of alkali-feldspar granite suggest that the magma pressure be less than 0.8 - 1.0 GPa,derived from the depth of 30 - 40 km in the crust, belonging to thinning crust, when the Liaodong massif is underintracontinental extension environment. The study fully proves that the Early Cretaceous magmatism in eastern Chinadeveloped in an extensional tectonic environment which was induced by thinning lithosphere.