吐格尔明背斜核部花岗岩的年代学、地球化学与构造环境及其对塔里木地块北缘古生代伸展聚敛旋回的揭示

摘要

塔里木盆地北缘库车坳陷东部地区吐格尔明背斜核部花岗岩发育,该套花岗岩侵位于以云母石英片岩为主的元古界基底岩系中,上覆的三叠系以角度不整合与其接触.本区花岗岩的研究对揭示库车地区的基底性质与构造演化历史具有重要意义.对吐格尔明花岗岩锆石的U-Pb年代学和岩石地球化学的研究表明,吐格尔明背斜核部的花岗岩锆石为典型岩浆成因锆石,其锆石SHRIMP U-Pb定年结果分别为626.4±5.2Ma～643.3±4.0Ma,表明它们的形成时代为新元古代晚期埃迪卡拉纪早期.该套花岗岩的SiO2(67.95%～78.59%)和Al2O3(11.81%～16.21%)含量均很高,A/CNK在1.2～1.6,为硅和铝过饱和类型,属典型的过铝质花岗岩;稀土元素总量为106×10-6～207×10-6,稀土元素配分曲线呈右倾型,具有明显的负Eu异常;高场强元素(Ta、Nb、Ti等)具有明显的负异常,大离子亲石元素(Rb、Ba、Sr等)具有明显的正异常.吐格尔明花岗岩是由地壳物质在底侵热源或构造动力作用下引起的局部熔融形成的,属典型的壳源成因类型,形成于挤压环境向伸展环境转化阶段,推测塔里木北缘在震旦纪已开始伸展,代表了古亚洲洋早期的伸展构造环境.实验测得花岗岩钾长石40Ar/39Ar 年龄平均为291.4～255.8Ma,代表了花岗岩的隆升剥蚀年龄,反映出吐格尔明花岗岩在二叠纪受到挤压发生剥露的过程.因而从年代学框架上更好地约束了塔里木地块北缘在古生代由伸展裂解到聚敛隆升的构造旋回.%Granites occur at the core of Tugerming anticline of eastern Kuqa depression at the northern margin of Tarim Basin. It intrudes in Proterozoic mica-quartz schist, and is overlain unconformably by Upper Triassic series. The study on granites in the area is a key to address the basement character and tectonic processes in Kuqa depression. The research on zircon U-Pb dating and rock geochemistry of granites in Tugerming anticline shows that the zircon of granites is typical of magmatic genesis. Its SHRIMP U-Pb dating centers around 626.4 ±5. 2Ma～643.3 ±4.0Ma,indicating a formation age of Early Ediacaran period. The granite has a high content of SiO2(67. 95％ ～78.59％) and Al2O2(11.81％ ～ 16. 21％ ) ,A/CNK around 1.2～1.6. It is over-saturated of silicon and aluminium, and is the typical type of over-aluminous granite. The average value of ∑REE is 106 × 10-6 ～ 207 x 10 -6. The chondrite normalized REE pattern curve is right-sloping type, and the rocks possess marked Eu anomalies. The primitive mantle standardized trace elements are characterized that HFSE(Ta, Nb, Ti, etc. )has remarkable negative anomalies, while LILE(Rb, Ba, Sr, etc. )has remarkable positive anomalies. The granites at Tugerming anticline is partially melted resulted from the bottom-intruded thermal origin or tectonic movement processes, and is predominantly originated from crust It is produced in the transitional period from compression to extension. It is inferred the northern margin of Tarim continental block has extended since Sinian, referring to the extensional setting of earlier Paleo-Asia Ocean. The 40Ar/39Ar dating of the potassic-feldspar in granite gives an age of 291.4 ～255. 8Ma, indicating the uplifting and eroding age of the granites, and corresponding to the period of compression and thus elevation during Permian Period. It constraints better from chronological framework on the tectonic cycle from extension to compression during Paleozoic at the northern margin of Tarim continental block.