伊犁地块南缘古生代花岗岩类的地质特征及构造意义

Geological Features and Tectonic Implications of the Paleozoic Granitoids in the Southern Yili Block

伊犁地块南缘元古宙特克斯群变质岩中侵入有花岗岩类,其形成时代和构造背景一直没有详细的研究。文章通过野外观察和室内分析,确定花岗质侵入体主要由弱面理化黑云母花岗岩、强面理化二云母花岗岩和未变形黑云母花岗岩组成。全岩地球化学和锆石U-Pb年代学分析显示,弱面理化黑云母花岗岩和强面理化二云母花岗岩属于过铝质钙碱性系列,其年龄分别为438 Ma和426 Ma;未变形黑云母花岗岩则属于典型的钙碱性系列,富集大离子亲石元素,相对亏损高场强元素,其年龄介于400~380 Ma之间。结合前人对区域地质的研究认识,笔者认为这些花岗岩类记录了两期不同构造背景的岩浆作用,指示研究区经历了两阶段构造—岩浆演化,即早古生代过铝质钙碱性弱面理化黑云母花岗岩与强面理化二云母花岗岩形成于哈萨克斯坦微大陆汇聚拼贴过程的后碰撞造山环境;中—晚古生代钙碱性未变形黑云母花岗岩则形成于准噶尔洋俯冲作用的活动大陆边缘。

Granitoids are well developed in the southern Yili Block intruding the Proterozoic metamorphic rocks of the Tekes Group.Their ages and tectonic settings are poorly constrained.Our field investigations indicate that the granitoids are composed of weakly foliated biotite granites,strongly foliated two mica granites and undeformed biotite granites.Whole-rock geochemical and zircon U-Pb geochronological analyses revealed that the weakly foliated biotite granites and strongly foliated two mica granites belong to peraluminous calc-alkaline series and formed during 438 Ma and 426 Ma,respectively.Whereas,the undeformed biotite granites emplaced during 400-380 Ma and have geochemical features similar to typical calc-alkaline series,showing enrichment in the large ion lithophile elements and depletion of the high field strength elements.Combined with regional geological data and previous results,we propose that the Paleozoic granitoids in the study area resulted from two episodes of magmatism related to different tectonic settings,namely,the Early Paleozoic peraluminous calc-alkaline,weakly foliated biotite granites and strongly foliated two mica granites were generated in the post-orogenic setting during the amalgamation of the Kazakhstan microcontinent,and the mid-late Paleozoic calc-alkaline,undeformed biotite granites were produced in an active margin related to subduction of the Junggar Ocean.