下扬子地区姚村A型花岗岩年代学、地球化学特征及岩石成因

Geochronology,geochemistry and petrogenesis of early Cretaceous Yaocun A-type granite in the Lower Yangtze region

姚村岩体位于下扬子江南造山带东北端,主要由中心相的中粗粒正长花岗岩和边缘相的细粒似斑状正长花岗岩组成。本文对该岩体进行了详细的锆石U-Pb年代学、主量元素、微量元素以及Nd-Hf同位素研究。LA-ICPMS锆石U-Pb定年表明姚村岩体的形成年龄为(127.6±1.4)Ma,为燕山晚期岩浆活动的产物。岩石地球化学研究表明姚村岩体属于A型花岗岩,具高硅、富铁,锆饱和温度高、轻重稀土分馏明显、富集Rb、Th、U、K、Pb等元素而亏损Ba、Nb、Sr和Ti等元素、铕负异常显著(Eu/Eu*=0.22~0.46)的特点。姚村岩体的全岩εNd(t)值与锆石εHf(t)值分别变化于-6.2~-5.7和-13.9~-5.0,两阶段Nd和Hf同位素模式年龄分别为T_(DM2)(Nd)=1439~1532 Ma和T_(DM2)(Hf)=1508~2062 Ma,Nd同位素的模式年龄重叠于Hf同位素模式年龄。结合区域地质研究成果认为,姚村岩体可能于早白垩世古太平洋板块俯冲作用之后伸展-拉伸环境下,由中元古代地壳重熔而成。

The Yaocun pluton in Southern Anhui Province,lithologically consisting mainly of medium-to coarse-grained syenogranite in its centre and fine-grained porphyritic syenogranite on its margin,is outcropped around the northeastern Jiangnan orogeny of the Lower Yangtze region.This paper reports detailed study results of its the LA-ICP-MS zircon U-Pb dating,major elements,trace elements,whole-rock Nd isotopic compositions and zircon Hf isotopic compositions.LA-ICP-MS zircon U-Pb ages show that the Yaocun pluton,as the product of magmatic activity in the late Yanshanian period,was emplaced at 127.6±1.4Ma.The studies of petrography and geochemistry of this rock indicate that it is A-type granite,and is characterized by rich silica,high iron,high zircon saturation temperatures,enrichment of Rb,Th,U,K and Pb,depletion of Ba,Nb,Sr and Ti,and significant negative Eu anomalies(Eu/Eu*=0.22-0.46).ItsεNd(t)andεHf(t)values range from-6.2 to-5.7 and from-13.9 to-5.0 respectively,and the calculated two-stage model ages(TDM2)of Nd and Hf isotopes from 1439 Ma to 1532 Ma and from 1508 Ma to 2062 Ma respectively.Combined with the results of regional geological research,it is suggested that the Yaocun pluton might be formed by the Mesoproterozoic crust remelting under extension-tension environment after the subduction of the Paleo-Pacific plate during the Early Cretaceous.