满洲里南部中生代花岗岩的锆石U--Pb年龄及Hf同位素特征

满洲里南部地区花岗岩主要由碱长花岗岩、正长花岗岩、二长花岗岩及花岗斑岩组成。采用LA--ICP--MS技术,对满洲里南部花岗岩进行的锆石U--Pb年龄测定表明,该区中生代花岗岩浆活动分为3期:中—晚三叠世(208~239 Ma)、早侏罗世(179~185 Ma)和晚侏罗世—早白垩世(137~151 Ma),与整个大兴安岭中生代花岗岩的年代学格架基本一致,与东部的张广才岭—小兴安岭地区中生代岩浆活动时代也可以对比。锆石LA--MC--ICP--MS Hf同位素研究显示,本区中生代花岗岩的锆石εHf(t)多数为+0.7~+9.5,二阶段模式年龄为0.6~1.2 Ga,表明花岗岩浆主要源于中—新元古代增生的地壳物质。结合额尔古纳地块其他花岗岩的锆石Hf同位素资料,认为额尔古纳地块在中—新元古代时期曾发生一次重要的地壳增生事件,与兴安地块的地壳增生时间为新元古代—显生宙的特点不同。

Zircon U-Pb ages and Hf isotopic characteristics of Mesozoic granitoids from southern Manzhouli,Inner Mongolia

The granites in southern Manzhouli region of Inner Mongolia are composed mainly of alkaline-feldspar,orthoclase and adamellite granite.We present new zircon U-Pb dating of granites using ablation LA-ICP-MS technique.The result shows that three stages of granites in the southern Manzhouli region：Middle to Late Triassic （208 ～239 Ma）,Early Jurassic （179 ～ 185 Ma）,Late Jurassic to Early Cretaceous （137 ～151 Ma）,which are consistent with all of the Da Hinggan Mountain,and this geochronological framework is also comparable to that in the Zhangguangcai-Lessor Hinggan Mountain in the East.Zircon Hf analyses conducted by LA-MC-ICP-MS show that these granites have εHf（t） values of ＋ 0.7 ～ ＋ 9.5 with Hf modal ages of 0.6 ～ 1.2 Ga,indicating that the crust in the Ergun block was extracted from mantle during Meso-Neoproterozoic,which is different from the Xing＇an block to the south,where the crustal accretion was aged in the Neoproterozoic-Phanerozoic.