大兴安岭北部永庆林场-十八站花岗岩锆石U-Pb年龄、Hf同位素特征

Zircon U-Pb Ages, Hf isotopic Characteristics of the Yongqinglinchang-Shibazhan Granites in the Northern Da Hinggan Mountains, Northeastern China

永庆林场-十八站花岗岩体位于大兴安岭东北部的额尔古纳地块,主要由花岗闪长岩组成,二长花岗岩和石英闪长岩在岩体中零星出露。岩体中锆石呈自形晶,发育振荡生长环带,显示高Th/U比值(0.23～1.35),表明锆石岩浆成因。锆石的LA-ICP-MS U-Pb定年结果为443.5～447.5 Ma,属于晚奥陶世岩浆活动的产物,而非前人所划分为的新元古代。锆石的Hf同位素研究显示,2件花岗岩样品的εHf(t)值分别为-1.1～+2.4和-0.4～+3.6,二阶段模式年龄为1.2～1.5 Ga。结合额尔古纳地块已有的早古生代和中生代花岗岩锆石Hf同位素资料,笔者认为额尔古纳地块不同时代的花岗岩具有相似的模式年龄,其地壳增生的时间主要发生在中-新元古代。目前已有研究表明,兴安地块地壳增生发生在新元古代-显生宙,暗示它们具有不同的地壳演化过程。

The Yongqinglinchang-Shibazhan granites are located in the Eerguna block in the northeastern Da Hinggan Mountains, and mainly consist of granodiorite, with minor adamellite and quartz diorite. The zircon grains separated from the granodiorite sample in the Yongqinglinchang-Shibazhan granites show generally euhe-dral form with fine oscillatory zoning on their CL images, and high Th/U ratios（0.23-1.35）, which exhibit the fea- tures of magmatic zircons. LA-ICP-MS zircon U-Pb dating reveals that the Yongqinglinchang-Shibazhan granites emplaced at 443.5-447.5 Ma, belonging to the products of upper Ordovician magmatism, not to the Neoprotero-zoic recognized and categorized by predecessors. Zircon Hf isotope characteristics indicate that two granotoid samples have zircon sill（t） values ranging from-1.1 to ＋2.4 and -0.4 to ＋3.6, with model ages from 1.2 Ga to 1.5 Ga. Combined with previous zircon Hf isotope data obtained from Early Paleozoic and Mesozoic granites in the Eerguna block, the authors hold that the granites have the similar model ages although they were formed in differ- ent geological ages. Meanwhile it indicates that the crustal accretion event of Eerguna block occurred mainly in Middle and Neo-Proterozoic period. Previous studies indicate that the Xing＇ an block accreted in Neoproterozo-ic-Phanerozoic. It suggests that they have the different tectonic natures and evolvement history.