大兴安岭南段罕苏木地区二长花岗岩锆石U-Pb年龄、Hf同位素特征及其伸展构造作用

Zircon U-Pb ages,Hf isotope and extensional tectonics of monzogranite in the Hansumu area of southern Great Khingan

大兴安岭南段罕苏木地区出露大面积的二长花岗岩,为了正确认识该岩体的形成时代及其伸展构造作用,本文采集相关样品,对罕苏木地区出露的二长花岗岩体开展了岩相学、LA-MC-ICP-MS锆石年代学和Hf同位素分析研究。研究结果表明:罕苏木地区二长花岗岩岩性为微细粒斑状含角闪黑云二长花岗岩和细粒斑状含黑云二长花岗岩,具有斑状和似斑状结构,块状构造。二长花岗岩中的锆石为岩浆成因,测得^(206)Pb/^(238)U年龄的加权平均值为(136±1)Ma(MSWD=1.4),属于早白垩世晚期岩浆活动的产物。这一结果与研究区及周边,甚至是与区域上中国东北地区114~145 Ma岩浆活动相吻合,都属于同一岩浆活动阶段的产物。罕苏木地区二长花岗岩锆石的ε_(Hf)(t)值均为正值,变化范围为7.1~14.4,并且具有较年轻的二阶段模式年龄,T_(DM2)为324~959 Ma。年轻的Hf同位素模式年龄,暗示在新元古代—晚古生代曾发生一次重要的地壳增生事件。结合区域地质,表明研究区二长花岗岩的岩浆可能是来源于从亏损地幔中新增生的年轻地壳发生部分熔融的产物,在侵位过程中受到了地壳或岩石圈地幔的混染,可能形成在造山后岩石圈伸展环境下,与古太平洋板块向欧亚大陆俯冲有关。

Large area of monzogranite is outcropped in the Hansumu area of southern Great Khingan.In order to recognize the formation age of the rock and its tectonic significance,samples were collected from the monzogranite pluton to study its petrography,zircon LA-MC-ICP-MS age and Hf isotope.The results show that the pluton is composed of fine-grained porphyritic amphibolit-biotite monzogranite and fine-grained porphyritic biotite monzogranite with porphyritic and porphyritic structures and massive structures.The zircons from the monzogranite is of magmatic origin,and yields ^(206)Pb/^(238)U age of 136±1 Ma(MSWD=1.4),which suggests that the pluton was formed in the late Early Cretaceous.This result is strongly consistent with the study area and its surrounding areas,and even with the 114-145 Ma magmatic activities in the northeastern of China,indicating the products of the same magmatic activity stage.The monzogranite in the Hansumu area has positiveε_(Hf)(t)values of 7.1 to 14.4 and young Hf two-stage model ages,and T_(DM2) is 324 to 959 Ma.Young Hf isotope model ages imply that this area could have experienced an important crustal accretion event during the Neoproterozoic-Late Proterozoic.Combined with regional geology,it is suggested that the monzogranite was likely derived from the partial melting of young mantle,and possibly was formed in a lithosphere extensional tectonics environment related to the subduction of the Paleo-Asian plate to Eurasia Plate.