大兴安岭东北部侏罗纪花岗质岩石的锆石U-Pb年龄、地球化学特征及成因

Zircon U-Pb ages,geochemistry and its petrogenesis of Jurassic granites in northeastern part of the Da Hinggan-Mts

大兴安岭东北部花岗岩锆石的LA-ICPMS U-Pb年龄测定结果表明,前人划分的元古代、早古生代和晚古生代的花岗岩有相当一部分是侏罗纪花岗岩。根据时间先后关系,可将本区侏罗纪花岗岩的侵位顺序划分为早、晚两期,其锆石U-Pb年龄分别为188～190Ma和171～181Ma,这些侏罗纪花岗岩的年龄数据与东北其它地区侏罗纪花岗岩完全可以对比。在地球化学特征上可以将这些花岗质岩石划分为低Sr高Yb型和高Sr低Yb型两类,它们有相同或相似的源岩组成而起源的深度不同,早期侵入的花岗岩为低Sr高Yb型,起源于压力较低的中地壳,而晚期侵入的花岗岩为高Sr低Yb型,类似于"C"型埃达克岩,起源于压力较高的下地壳。锆石Hf同位素成分特征表明,侏罗纪花岗岩的源区物质主要为新元古代和显生宙期间增生的地壳物质及两期增生地壳物质的混合物。本区侏罗纪花岗岩以花岗闪长岩-二长花岗岩为主,为准铝质或弱过铝质、高钾钙碱性系列的Ⅰ型花岗岩,具有类似于活动大陆边缘花岗岩的岩石组合特征,属于呈北北东向带状展布的中国东北地区侏罗纪花岗岩带的一部分,其形成与古太平洋板块的俯冲作用有关。

Abstract The U-Pb dating results determined by LA-ICPMS show that quite a large of Jurassic granites were still distributed inside the Proterozoic , Early and Late Paleozoic granite, which were recognized and categorized by predecessors. The intrusion sequence of Jurassic granite in the studied area could be classified into two stages, i.e. early and late stages on the basis of event occurring time, whose U-Pb dating in zircon are 188 ~ 190Ma and 171~181Ma, respectively. The dating data of Jurassic granite are fully comparable to those of Jurassic granite existing in the other part of Northeast China. Geochemically, these granitics could be distinguished by two types-low-Sr and high-Yb granitics and high-Sr and low-Yb granitics, they possess the same or similar source rock compositions but originated at different depth. The early intruded granitics, characterized by the low-Sr and high Yb, originated at the low pressure in the mid crust, however, the late intruded ones, characterized by high Sr and Law Yb, similarly to C-type adakites and it originated at the relatively high pressure in lower part of the crust. Characteristics of zircon Hf isotope indicated that the source materials of Jurassic granitics came from newly accreted crustal materials and their mixture in the Neoproterozoic and phanerozoic. The Jurassic granitics in the studied area are dominantly composed of granodiorite and adamellite, which belong to aluminum saturated or oversaturated, high potassium calc-alkaline series-the granite in type I, with similar feature of the granite associations at the active continental margin. It is a part of stripped or belted Jurassic granitics in Northeast China distributed alone NNE direction, its origin might be related to the subduction of paleo-Pacific tectonic plate.