内蒙古塔尔气地区晚古生代花岗岩的锆石U-Pb年龄及Hf同位素特征

Zircon U-Pb ages and Hf isotopes of Late Paleozoic granites in Taerqi area,Inner Mongolia

内蒙古塔尔气地区位于大兴安岭中段兴安地块上,该区出露大面积晚古生代花岗岩。其锆石的LA-ICP-MS U-Pb测年结果显示,正长花岗岩形成于335±5 Ma,二长花岗岩形成于313±3 Ma,花岗闪长岩形成于320±1 Ma,表明塔尔气地区花岗岩为晚古生代多期花岗质岩浆活动的产物。花岗质岩浆的就位与古生代时期古亚洲洋闭合中洋壳俯冲作用有重要关系。花岗闪长岩的176Hf/177Hf为0.282 833～0.282 951,εHf(t)为+1.1～+5.6,TDM2为525～752 Ma,暗示花岗岩的源岩为新元古代—早古生代时期亏损地幔来源的基性下地壳物质。结合目前已发表的花岗岩Sr--Nd和锆石Hf同位素资料,认为兴安地块可能从中元古代就开始地壳增生,峰期集中在新元古代—早古生代。

Taerqi area is located at Xing＇an Block in central Great Xing＇an Range, where exposed widespread Late Paleozoic granites. Ages dated by zircon LA-ICP-MS U-Pb are 313 ＋3 Ma, 335 ＋5 Ma and 320 ＋ 1 Ma for monzogranite, syenogranite and granodiorite, respectively, indicating the Taerqi granites experienced multi-stage magamatism in the l_ate Paleozoic. There is a genetics of the emplacement of granitic magma was related closely to the oceanic crust subduction during the closure of the Paleo-Asia Ocean in Paleozoic. 176Hf/177Hf ratios of zircons from the granodiorite range from 0. 282 833 to 0. 282 951, corresponding to εHf （t） values from ＋ 1.1 to ＋5.6 and TDM2 from 525 to 752 Ma. Hf isotopic characteristics imply that these granites originated from the mafic lower crust formed during Neoproterozoic-Early Paleozoic with depleted mantle affinity. Combined with the Sr-Nd and zircon Hf isotopic data published, it is concluded that the crustal accreation event in Xing＇an block mainly took place at Neoproterozic-Phanerozoic, probably started at the Mid-Proterozoic.