大兴安岭查巴奇地区中生代侵入岩岩石地球化学特征及构造背景

The petrogeochemical characteristics and tectonic setting of Mesozoic intrusive rocks in Chabaqi area of the Da Hinggan Mountains

大兴安岭查巴奇地区中生代侵入岩位于嫩江断裂带西侧,其东侧为隆起带与断陷盆地连结处,该区构造研究可为大兴安岭的隆升造山研究提供信息。研究区花岗质岩SHRIMP锆石U-Pb年龄测试结果表明,这些侵入岩主要形成于早侏罗世和晚侏罗世,重新修正了前人把研究区二长花岗岩划归晚二叠世的观点。岩石化学特征表明,研究区侵入岩均属于钙碱性岩,具有属于高钾、硅饱和或过饱和岩石;依据系列判别图解显示研究区不同时期的花岗质岩石均为壳幔混熔的I型花岗岩,均形成于长期稳定环境下,两期花岗岩形成环境又存在差异,早期石英和英云闪长岩为挤压型,形成压力较高的深源,晚期二长花岗岩为挤压-引张过渡型,形成于压力相对较小的浅源,形成过程中可能存在花岗岩时空上的演化不均一性,该区中生代侵入岩的形成与地壳厚度的关系可作为大兴安岭中生代地壳乃至岩石圈的减薄及伸展造山过程的辅证。

The Mesozoic intrusive rocks in Chabaqi area are located in the west of Nenjiang fault zone, and the east side of the study area is the joint between apophysis and graben basin. Hence the study of the structure of the study area can provide the information for the orogenic uplift of the Da Hinggan Mountains. The result of SHRIMP U-Pb zircon dating shows that the intrusive rocks were mainly formed in the early Jurassic and late Jurassic period, and the petrogeochemical characteristics show that the intrusive rocks are high-potassium and Si- saturated rocks, belonging to calc-alkaline series. Discrimination diagrams show that the intrusive rocks belong to I-type granite of crust-mantle miscibility,and they were formed in a prolonged stable environment but under different formation conditions. The early Jurassic quartz diorite and tonalite have the characteristics of extrusion type, so that they should be formed in a relatively high pressure environment, whereas the late Jurassic monzonitic granites belong to the transition type of extension-extrusion, and hence they should be formed in a high differentiation environment of relative low pressure. The spatial and temporal granite evolution heterogeneity was probably existent in the formation process. Therefore, the relationship between the formation of Jurassic granites and the thickness of the earth＇s crust can serve as the supporting evidence for the thinning of the earth＇s crust and lithosphere as well as the stretching orogenic process of the Da Hinggan Mountains.