西南天山晚古生代后碰撞岩浆作用:以阔克萨彦岭地区巴雷公花岗岩为例

Late Paleozoic post-collisional magmatism in the Southwestern Tianshan orogenic belt,take the Baleigong pluton in the Kokshal region as an example.

西南天山阔克萨彦岭地区巴雷公钾长花岗岩出露于蛇绿混杂岩南侧。地球化学特征显示，该岩石富碱（K2O＋Na2O为8.25～8.72％〉8％），富钾（K2O／Na2O为1.34～1.56），准铝质（A／CNK为0.94～1.05），为高钾钙碱性-钾玄岩系列；岩石富集大离子亲石元素和轻稀土，亏损Sr、P等大离子亲石元素及Nb、Ti等高场强元素，具有中等的负铕异常（δEu=0.49～0.59），为向A型花岗岩过渡的后碰撞高钾花岗岩特征；岩石的Nd／Th（1.64～3.19）、Th／U（5.95～7.11）、Nb／Ta（7.26～9.17）和高K2O／Na2O比值、低Sr／Ba比值特征表明，巴雷公花岗岩来源于中下地壳物质的部分熔融，残留相为斜长角闪岩。阴极发光图像显示，该岩石中锆石多呈完好的自形晶，具有岩浆锆石特有的韵律环带结构，LA-ICP-MS微区原位U-Pb定年结果表明该花岗岩的结晶侵位年龄为273±2Ma。综合南天山已有研究成果，推测南天山造山带后碰撞花岗质岩浆活动主要发生在282～259Ma之间，具有从高钾钙碱系列（282～266Ma）向碱性系列（266～259Ma）演化的特征，暗示了一个后碰撞阶段的造山带垮塌、陆壳连续伸展减薄的过程。巴雷公高钾花岗岩应为南天山造山带碰撞造山峰期变质后垮塌过程中的第一个阶段或碰撞造山到后造山的一个转折阶段的中下地壳熔融的产物，指示南天山西段古洋盆在中二叠世以前已经闭合，西南天山已进入后碰撞演化阶段，代表了古亚洲洋南部的闭合和中亚南部增生造山作用的结束。这一认识为深入探讨中亚后碰撞岩浆作用的时限和机制提供了新的约束资料。

Abstract Southern Tianshan is one of the regions with most complex structural evolution history in the Tianshan Orogenic belt. Compared with other regions in the northern Xinjiang, studies on problems of continental dynamics of the post-collision procedure and the magma genesis are relatively weak. Baleigong pluton, which is located in the southwestern section of the Southern Tianshan, is carefully studied in this study. Based on geochemistry and geochronology, the petrogenesis, characteristics of the magma source, as well as the geodynamic are discussed, in order to provide some constraints for the further studies on the evolutionary patterns of the ocean-crust system in the region such as closure of the oceanic crust and timing of the collisional orogeny. The Baleigong pluton located in the Southwestern Kokshal area, the western segment of the Southwestern Tianshan; and emplaced into the south of the Baleigong ophiolitic m lange. Lithologically, the pluton consists mainly of biotite moyites. Geochemically, the rocks are high-K calc-alkaline to shoshonitic series, and rich in alkali （ K2O ＋ Na2O为8.25% ～ 8.72% ） and K （ K2O/Na2O 为 1.34 ～ 1.56） , with the A/CNK values between 0.94 and 1.05, which fall into the range of metaluminous to slightly peraluminous rocks. The rocks are also characterized by enrichment of LREE and LILE （Cs, Rb, Th and Ba） and depletion of Sr, P, Nb and Ti, with moderate negative Eu anomaly （δEu为0.49～0.59）, showing the transitional features between high-K calc-alkaline and A-typo granites. The elemental ratios of the Baleigong pluton, such as Nd/Th （1.64～3.19） , Th/U （5.95～7.11 ） , Nb/Ta （7.26～9.17, combined with the high K2O/Na2O and low Sr/Ba ratios, indicate the partial melting of middle to lower crust with intermediate magmatic rocks as protoliths, with plagioclase amphibolite as the restite. In situ zircon U-Pb LA-ICP-MS dating on the magmatic genetic zircons yielded an age of the 273±2Ma, representing the emplacement age of the pluton. Combined with the previous studies, it is concluded that the Late Paleozoic post-collisional granitic magmatism in the South Tianshan took place in the period between 282 and 259Ma and obviously later than that of other areas in the Northern Xinjiang. Together, these studies also indicate that the post-collisional granitic magmas evolved from high-K calc-alkaline series （282～266Ma） to alkaline series （266～259Ma） successively, which suggest the process of orogenic collapse and continuously extensional tinning of the continental crust during the post-collisional stage. The Baleigong high-K granite was probably formed during the first stage of the collapse after the collisional orogeny or during the transitional stage between collision and post-collision, by partial melting of the middle to lower crust materials, The post-collisional magmatism in the South Tianshan indicates that before the Middle Permian, the Paleozoic South Tianshan Ocean has been closed and the South Tianshan was in a post-collisional setting, which probably represents the last closure of southern part of the Paleo-Asia oceanic crust, as well as the ending of the accretionary orogeny in the southern section of the Central Asia. These conclusions provide some new constraints to the further study on the timing and potrogenesis of the post-collisional magmatism in the Central Asia.