辽东岫岩王家堡子地区二长花岗岩U-Pb年代学、地球化学特征及其地质意义

U-Pb Geochronology, Geochemistry and Geological Significance of Early Cretaceous Monzonitic in Wangjiabaozi Area of Xiuyan, Liaodong Peninsula

辽东半岛岫岩王家堡子地区位于胶—辽—吉造山/活动带中段。本文对该区内的二长花岗岩(龙潭沟岩体与罗圈沟岩体)进行了U-Pb年代学与地球化学特征研究,并探讨了其地质意义。结果表明:龙潭沟岩体与罗圈沟岩体侵位年龄分别为(122.37±0.30) Ma和(124.37±0.91) Ma,年代为早白垩世;二岩体具高SiO2质量分数,低Mg、Co、Cr、Ni质量分数,富集LREE和LILE,亏损HFSE,说明二者为地壳物质熔融的产物;高Al2O3、K2O质量分数,低Na2O、MnO和CaO质量分数,高Sr质量分数,低Y、Yb质量分数,富集LILE及LREE,Eu呈负异常,亏损HREE等地球化学特征,也表明二者具有埃达克岩与高钾钙碱性I型花岗岩的性质。龙潭沟岩体岩浆来源于洋壳俯冲导致的加厚陆壳底部基性岩部分熔融,而罗圈沟岩体岩浆则是俯冲洋壳导致地幔物质上涌,使得玄武质下地壳发生部分熔融的结果,并且岩浆受到了地幔橄榄岩的混染。认为龙潭沟岩体与罗圈沟岩体均为碰撞环境中的产物,辽东岫岩王家堡子地区在124~122 Ma处于由古太平洋板块俯冲引起的挤压环境中,构造挤压向拉伸环境的转换应在122~110 Ma之间完成,岩石圈的减薄与破坏也随之开始。

According to the U-Pb geochronology and geochemical characteristics of the monzonitic granite of Longtangou pluton and Luoquangou pluton in Wangjiabaozi area of Xiuyan, Liaodong Peninsula in the middle of Jiao-Liao-Ji orogenic active zone, we carried out a research. The results show that the emplacement ages of Longtangou pluton and Luoquangou pluton are(122.37±0.30) Ma and(124.37±0.91) Ma respectively, both in Early Cretaceous. According to the mass fraction of high SiO2, low Mg, Co, Cr, and Ni, enrichment of LREE and LILE, and loss of HFSE,Longtangou and Luoquangou plutons are the products of crustal material melting. According to the mass fraction of high Al2O3 and K2O, low Na2O, MnO, and CaO, high Sr, low Y and Yb, enrichment of LILE and LREE, negative anomalies of Eu, and depletion of HREE, the monzonitic granite possesses the properties of adakite and high potassium calc-alkaline type I granite. The magma of Longtangou pluton was resulted from the partial melting of the base rock at the bottom of the thickened continental crust caused by the subduction of the oceanic crust, while the magma of Luoquangou pluton was the result of the upwelling of mantle material caused by the subduction of the oceanic crust, which led to the partially melting of the lower basalt and mixing with mantle peridotite. The research shows that both Longtangou and Luoquangou plutons are the products of collision environment caused by the subduction of the ancient Pacific plate at the age of 124-122 Ma. The transition from tectonic extrusion to tensile environment should be completed between 122-110 Ma. The lithospheric thinning and destruction began with the transformation.