新疆乌伦古河碱性花岗岩的地球化学及其构造意义

GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF ALKALI GRANITES ALONG ULUNGUR RIVER, XINJIANG

在新疆北部准噶尔板块与阿尔泰造山带的缝合带即阿尔曼泰－扎河坝蛇绿混杂岩带附近，沿乌伦古河南岸分布一条碱性花岗岩带。它们形成的时代为２９２—３０９Ｍａ，是阿尔泰地区海西期继同碰撞ｓ型花岗岩类、碰撞后抬升Ⅰ型花岗岩类之后的最后一次岩浆活动的产物。这些碱性花岗岩以出现霓石、钠铁闪石、高硅、高碱、低钙、低镁、富集高场强元素为特征，属于典型的Ａ型花岗岩。碱性花岗岩是海西期岩浆旋回的最后产物，活动时间很短暂，在空间上与蛇绿岩带伴生，为后造山Ａ型碱性花岗岩（ＰＡ型），是阿尔泰海西期造山运动结束的重要标志。

In North Xinjiang, there is an alkali granite zone extended in NW-SE direction along the southern bank of Ulungur River and parallel to the suture zone, i.e.Aermantai-Zhaheba Ophiolitic Melange Zone, between Junggar Plate and Altay Orogenic Belt. The Rb-Sr isochron ages of these granites are in range of 292-309Ma, showing that they are generated by the latest Hercynian magmatism after the syncollisional S-type and postcollisional uplift I-type granitoids in Altay region. The lkali granite are mineralogically characterized by the occurence of aegirine and arfvedsonite and chemically by high silicon and alkali, low calcium and magnesium, and enrichment of HFS elements, being typical A-type granites. The alkali granites were formed in the late stage of the Hercynian cafe-alkaline magmatism cycle in a very short period.They are in line with postorogenic A-type alkali granites, marking the end of the Hercynian Orogeny in Altay region and implying that the tectonic regime was changed from compression to extension.