敦煌地块南部古生代花岗岩地球化学、锆石U-Pb定年及Hf同位素特征研究

Geochemistry,zircon U-Pb geochronology and Hf isotopic characteristics for Granites in southern Dunhuang block

敦煌地块出露有大量古生代花岗岩，对这些花岗岩的研究可以更深入的了解敦煌地块的构造演化及其地球动力学意义。本文对敦煌地块南部的安盆沟复式花岗岩体和小草湖花岗岩体进行了岩相学、地球化学、锆石U-Pb年代学和Hf同位素的研究。研究结果表明：（1）安盆沟岩体岩石组合为正长花岗岩．花岗岩，其中正长花岗岩侵位于早古生代（431Ma），花岗岩侵位于泥盆纪（～360Ma）和石炭纪（～340Ma）；小草湖岩体主要为似斑状花岗岩，侵位时代为石炭纪（340Ma）。（2）安盆沟早古生代正长花岗岩的sHf（t）为-11．7到-6．3，tDM2为1．8—2．2Ga，而晚古生代花岗岩的sHf（t）为-12．3到-5．5，tDM2为1．7～2．1Ga，二者Hf同位素特征相似，推测源岩均为古元古代-中元古代时期的含泥质成分的（变）砂质岩。小草湖似斑状花岗岩为加厚下地壳部分熔融成因的埃达克质岩石，εHf（t）变化于-16．7至-4．9，tDM2变化于1．8～2．4Ga之间，源岩为古元古代古老地壳物质。两个岩体的部分源岩可能与Columbia超大陆的汇聚和裂解有关。（3）安盆沟正长花岗岩、花岗岩和小草湖似斑状花岗岩的残留相分别为麻粒岩、角闪岩和麻粒岩-角闪榴辉岩，反映了形成深度和变质级别的不同。（4）敦煌地块内的早古生代造山活动结束时间明显晚于中央造山带，并且早古生代造山活动以及早-晚古生代花岗岩与北侧的天山-北山造山带存在时空上的对应关系，敦煌地块可能在古生代期间卷入了中亚造山带的造山活动中。

Numerous Paleozoic granitoids emplaced in Dunhuang block. Aiming to better understand the tectonic evolution of Dunhuang block and the geodynamic significance, in this paper, Anpengou composite granites and Xiaocaohu granite in southern Dunhuang block were chosen to study by means of petrography, geochemistry, zircon U-Pb chronology and Hf isotopic analyses. The results show that ： （ 1 ） the two granites had different rock assemblage types. Anpengou composite granites were mainly composed of Early Paleozoic syenogranite emplaced during the Silurian period （431Ma） and granite emplaced during the Devonian period （ - 360Ma） and the Carboniferous period （ - 340Ma）, while Xiaocaohu porphyritic granite was emplaced during the Carboniferous period （ - 340Ma ） . （2） Both Anpengou syenogranite and granite showed S-type granite features and similar Hf isotopic characteristics, with 8nf（t） ranging from - 11.7 to -6. 3 and - 12. 3 to -5.5, and two-stage model age was varied in 1.8 -2. 2Ga and 1.7 - 2. 1Ga, respectively. Both source rocks of the Early Paleozoic syenogranite and the Late Paleozoic granite derived from meta- greywacke with part of metapelitic materials of Paleoproterozoic to Mesoproterozoic. Xiaocaohu porphyritic granite showed features of adakitic rocks and was generated by partial melting of thickened crust, with 8Hf （t） ranging from - 16.7 to -4.9 and two-stage model age was varied in 1.8 - 2.4Ga, indicating that the source rocks of Xiaocaohu porphyritic granite derived from the Paleoproterozoic crustal materials. In addition, some of the source rocks of the two granites may be related to the assemblage and break-up of Columbia supercontinent. （3） The residual rocks of Anpengou syneogranite, Anpengou granite and Xiaocaohu porphyritic granite were granulite, amphibolite and granulite-amphibole eclogite, which reflected the differences in the formation depth and grade of metamorphism. （4） The Early Paleozoic orogenic movement of Dunhuang block ended much later than the Central Orogenic Belt, but exhibited closely temporal-spatial coupling retionship with the Tianshan-Beishan orogenic belt in timing limitation and the distribution of the Paleozoic granites. Therefore, we proposed that the Dunhuang block was involved in the orogenic event of the Central Asian Orogenic Belt in both Early Paleozoic and Late Paleozoic.