小兴安岭东南部晨明地区早古生代花岗岩锆石U-Pb年龄及岩石地球化学特征

Zircon U-Pb age and geochemical characteristics of the granites in Chenming area,southeastern Xiao Hinggan mountains

晨明岩体位于小兴安岭东南段,其岩石组合为花岗闪长岩和二长花岗岩。所测定样品中的锆石均具有振荡生长环带,结合Th/U比值,表明这些锆石为岩浆成因,LA-ICP MS U-Pb测年结果显示,其形成时代分别为432.8±1.2 Ma和431.9±1.7 Ma,即早志留世。晨明岩体的地球化学特征显示,其属于高钾钙碱性系列,ACNK值为1.16～1.40,属于过铝质花岗岩;微量元素表现出富集大离子亲石元素(Rb、Ba、U和Th)和轻稀土元素,亏损高场强元素(Nb、Ta、Ti)和P、Sr以及重稀土元素。上述地球化学特征暗示其主要来源于下地壳物质的部分熔融,样品的过铝质特征暗示其形成于陆壳加厚环境。结合前人研究成果,认为晨明岩体形成于松嫩地块和佳木斯地块的碰撞拼合环境,即松嫩地块和佳木斯地块可能在早志留世碰撞拼合。

Chenming pluton is located in the southeastern Xiao Hinggan mountains,composed of monzonitic granite and granodiorite.Combined with the Th/U ratios of zircons in all samples which possess oscillatory growth ring,it suggests that these zircons are magmatic in origin.LA-ICP MS U-Pb dating results show that the forming age is 432.8±1.2 and 431.9±1.7 Ma,which indicates the Early Silurian.According to its geochemical characteristics,the Chenming pluton belongs to high potassium calc-alkaline series in category.Its ACNK value is ranging between 1.16-1.40,which indicates to be peraluminous granite in nature.The trace elements are enriched by LILE（Rb,Ba,U and Th） and LREE,with loss of HFSE（Nb,Ta,Ti）,P,Sr and HREE,which implies that the Chenming pluton originated mainly from the partial melting of lower crust material,and its peraluminous characteristics suggested its forming in the environment of continental crust thickening.Combined with the results of previous researches,the Chenming pluton formed in the collision backgroud of Songnen massif with Jiamusi massif,that means the collision of Songnen massif and Jiamusi massif may occur in the Early Silurian.