东昆仑丘吉东沟地区泥盆纪花岗岩的成因机制及其对原特提斯造山作用的启示

PETROGENESIS OF DEVONIAN GRANITES FROM THE QIUJIGONGGOU AREA IN EAST KUNLUN AND ITS IMPLICATIONS FOR PROTO-TETHYS O-ROGENY

东昆仑造山带发育巨量显生宙花岗岩,记录了特提斯造山作用的岩浆-构造演化过程。本次研究以东昆仑丘吉东沟地区泥盆纪花岗岩体为研究对象,开展系统的岩石学、年代学和地球化学研究,厘定其成因类型,揭示岩石成因及其对原特提斯造山作用的启示。丘吉东沟花岗岩体由斑状花岗闪长岩组成,岩体内发育暗色微粒包体。LA-ICP-MS锆石U-Pb年代学分析显示,斑状花岗闪长岩形成于382.7±3.6 Ma,判定其为中泥盆世岩浆活动的产物,岩石具有富硅(w(SiO2)=66.80%~73.15%)、富碱(w(K2O+Na2O)=7.07%~8.50%)和低铝(A/CNK=0.88~0.97)的特征,属于准铝质高钾钙碱性岩石。岩石的w(MgO)和w(FeO)分别为0.74%~1.77%和2.13%~4.98%,具有变化范围较大的Mg#值(21~50)。岩石富集Zr, Nb, Ce和Y等高场强元素(Zr+Nb+Ce+Y=330~540×10-6),亏损Ba, Sr, P,Ti和Eu等元素。岩石的稀土分配模式呈右倾的"海鸥式"特征,具有显著的负Eu异常(δEu=0.12~0.35)和较高的10 000*Ga/Al比值(2.65~3.72),属于典型的A型花岗岩。构造判别图解指示岩石具有A2型花岗岩特征,形成于后碰撞伸展构造环境。综合分析表明,丘吉东沟泥盆纪A型花岗岩起源于低压高温条件下长英质地壳的部分熔融,具有一定的岩浆混合印记,其成因背景与东昆仑原特提斯域碰撞后伸展作用相关。

Large amount of Phanerozoic granites are developed in the East Kunlun orogenic belt, which records the magmatic-tectonic evolution process of the Tethyan orogeny and the dynamic process of oceanic crust subduction and terrane collision. In this paper, the Devonian granitoids in the Qiujidonggou area of East Kunlun are studied systematically in accordance with petrology, chronology and geochemistry to determine their genetic types and reveal the petrogenesis and its implications for the proto-Tethys orogeny. Qiujidonggou granitic body is composed of porphyritic granodiorite with dark microgranular enclaves in the granitic body. LA-ICP-MS zircon U-Pb geochronological analysis shows that the porphyritic granodiorite was formed at 382.7±3.6 Ma, which indicates that it is the product of magmatic activity in the Middle Devonian. Geochemical analysis shows that the rocks have some typical features, which are rich in silicon w(SiO2)=66.80%-73.15% and alkali w(K2O+Na2O)=7.07%-8.50% and poor in aluminum(A/CNK=0.88-0.97), belonging to quasi-aluminous high potassium alkali-calcium rock. The w(MgO) and w(FeO) contents of rocks respectively range from 0.74% to 1.77% and 2.13% to 4.98% with a wide range of Mg# values(21-50). The rocks are enriched in Zr, Nb, Ce and Y(Zr+Nb+Ce+Y=330-540×10-6 and depleted in Ba, Sr, P, Ti and Eu. REE distribution of the rocks shows right-dip "seagul-like" pattern, with significant negative Eu anomaly(δEu=0.12-0.35) and high 10 000*Ga/Al ratio(2.65-3.72), belonging to typical A-type granites. Tectonic discrimination diagram indicates that the rock has the characteristics of A2-type granite and is formed in the post-collision extensional tectonic environment. Through comprehensive analysis, it is considered that the Qiujidonggou Devonian A-type granite is originated from the partial melting of felsic crust under the conditions of low pressure and high temperature with a certain magmatic mixed mark. Its genetic background is related to the extension of the East Kunlun proto-Tethys domain after the collision.