大兴安岭北段免渡河地区晚石炭世二长花岗岩成因:锆石U-Pb年代学、Hf同位素和地球化学证据

The Petrogenesis of Late Carboniferous Monzogranite in Mianduhe Area, Northern Great Xing’an Range: Evidence from Zircon U-Pb Ages, Hf Isotopic and Geochemical Features

为了确定大兴安岭北段晚石炭世二长花岗岩的成因及其构造背景,对免渡河地区二长花岗岩样品进行了锆石U‐Pb年代学、Hf同位素及地球化学分析测试.锆石U‐Pb测年结果显示,二长花岗岩形成于307~308 Ma,为晚石炭世岩浆活动的产物.主量元素特征表明,二长花岗岩具有富碱(全碱=7.28%~9.08%)、高钾(K_(2)O=3.45%~5.54%)和弱过铝质(A/CNK=1.02~1.19)特征,属高钾钙碱性I型花岗岩.微量元素特征表明,二长花岗岩具有中等的负铕异常(δEu=0.29~0.77),明显富集大离子亲石元素(Rb、K、Th、U)和轻稀土元素,显著亏损高场强元素(Nb、Ta、Ti)和P元素.此外Hf同位素特征显示,ε_(Hf)(t)值为介于+7.7~+12.5,t_(DM2)年龄介于501~764 Ma,表明岩浆来源于新元古代晚期下地壳新增生的角闪岩相物质部分熔融形成,并经历了显著的结晶分离过程.结合年代学、地球化学与区域地质特征,认为免渡河地区二长花岗岩是额尔古纳−兴安地块与松嫩地块碰撞拼贴后的后碰撞阶段的产物.

To determine the petrogenesis and tectonic setting of the Late Carboniferous monzogranite in the northern Great Xing’an Range,(Method) the paper analyzes the zircon U-Pb geochronology,Hf isotope and geochemistry of the monzogranite samples from Mianduhe area. The zircon U-Pb dating results show that the monzongranite was formed at 307-308 Ma, belonging to theLate Carboniferous. The characteristics of major elements reveal thatthe granite is characterized by alkali-rich(total alkali=7.28%-9.08%), high potassium(K_(2)O=3.45%-5.54%) and weak peraluminous(A/CNK=1.02-1.19), suggesting that it can be classified as high potassium calc-alkaline I-type granite.The characteristics of trace elements reveal thatthe granite has medium-negative Eu anomaly(δEu=0.29-0.77), obviously enriched LILES(Rb,K,Th and U)and LREE,significantly depleted HFSEs(Ti,Ta and Nb) and P. In addition, the Hf isotope characteristics show that the ε_(Hf)(t) values range from +7.73 to +12.46, with the t_(DM2)age of 501-764 Ma,indicating that their parent magma was formed by partial melting of the Neoproterozoic juvenile lower crust under the amphibolite facies, and then experienced significant fraction crystallization. Combined with previous study, it is suggested that the Mianduhe monzogranite was responded for the collage of the Erguna-Xing’an block and the Songnen block in an post-collision setting during the Late Carboniferous.