西天山造山带的古生代构造演化与地壳增生来自花岗岩的证据

1、西天山造山带的古生代构造演化与地壳增生：来自花岗岩的证据龙灵利1, 高俊1, 钱青1, Reiner Klemd2 1中国科学院地质与地球物理研究所，北京1000292Mineralogisches Institut, Universität Würzburg, Am Hubland 97074, Würzburg, Germany中亚造山带是不同于俯冲型和碰撞型的造山带，是全球显生宙大陆地壳增生最显著的地区，为造山带前沿热点研究对象。西天山造山带位于中亚造山带的西南缘，其对认识中亚造山过程和大陆地壳增生具有重要意义。西天山造山带古生代经历了复杂的构造演化过程，其

2、间发育的花岗岩记录了该过程的重要信息。同为中亚增生造山带的重要组成部分，相对于周边东准噶尔、西准噶尔、阿尔泰山及阿拉套山地区，西天山地区显生宙地壳增生的信息和证据较少。本次研究新获得西天山花岗岩25个高精度SHRIMP和LA-ICPMS锆石U-Pb年龄数据。花岗岩的形成分为两个阶段：896Ma的花岗片麻岩反映了前寒武纪基底岩系的形成时间，470Ma247Ma间的花岗岩记录了西天山古生代造山过程。西天山花岗岩由中基性岩、中性岩、中酸性岩到酸性岩组成，以中酸性和酸性岩为主。岩石类型主要为花岗岩、花岗闪长岩、石英正长岩及二长闪长岩。不同构造单元发育的不同类型花岗岩：伊犁板块北缘花岗岩主要形成于晚古生

3、代（413Ma281Ma），413Ma297Ma的花岗岩具有类似于大陆弧花岗岩的特征，281Ma镁质钙碱性偏铝质闪长岩为后碰撞造山阶段岩浆活动的产物；伊犁板块南缘发育早古生代470Ma加厚下地壳部分熔融形成的埃达克质闪长岩以及430Ma后碰撞碱长花岗岩，晚古生代348Ma火山弧花岗岩和三叠纪后碰撞花岗岩；中天山花岗岩形成于479Ma 247Ma，主要集中在433Ma321Ma间。479Ma321Ma的花岗岩大多为镁质钙性-碱性岩，少数为铁质钙碱性-碱性岩，它们表现为大陆弧花岗岩的特征，少量形成于276Ma247Ma间的后碰撞花岗岩一定程度继承了先存岩浆弧的特征；南天山（塔里木板块北缘）花岗岩形

4、成于420Ma411Ma和285Ma左右两个阶段，420Ma411Ma间的镁质钙性-碱性岩花岗岩，可能为拉伸环境下的产物，285Ma花岗岩主要为铁质钙碱性-碱钙性岩，SiO2 含量高、全碱含量高、表现明显的Eu，Ba，Sr，P，Ti的负异常，具有类似于A型花岗岩的地球化学特征，它们形成于后碰撞造山过程中的拉伸环境。初步建立西天山造山带古生代构造演化模型：早寒武纪，沿那拉提北缘断裂帖尔斯克依古洋形成，它将伊犁板块和中天山板块分隔开来，当时中天山板块和塔里木板块连在一起。帖尔斯克依古洋向两侧伊犁板块和中天山板块之下发生双向俯冲，晚奥陶纪早期，帖尔斯克依古洋闭合，伊犁板块和中天山板块碰撞缝合。同时，

5、中天山-塔里木板块开始伸展拉张，古南天山洋逐渐发育形成。早志留纪，古南天山洋开始向伊犁-中天山板块之下俯冲，形成火山弧型花岗岩。志留纪中期，古南天山洋达到相当规模，这一时期洋壳俯冲活动强烈，在中天山形成大量的花岗岩。石炭纪末期，古南天山洋消失，塔里木板块与伊犁-中天山板块碰撞。晚寒武纪，准噶尔洋在中天山北缘出现，奥陶纪开始向伊犁板块之下俯冲，在伊犁板块北缘发育岩浆弧。晚石炭纪，古准噶尔洋闭合消失。石炭纪末塔里木、伊犁-中天山和准噶尔板块拼合在一起。二叠纪开始西天山地区处于后碰撞造山阶段，该阶段的岩浆活动可能一直持续到早三叠纪。西天山花岗岩87Sr/86Sr初始值介于0.7032260.7163

6、43之间，Nd（t）值介于-6.502.03之间。Sr、Nd同位素特征综合显示西天山造山带花岗岩形成的源区并不单一，是壳源和幔源岩浆混合而成。古生代时期，西天山地区大陆地壳沿活动大陆边缘发生了显著的侧向增生，而后碰撞幔源物质添加导致的垂向生长并不很显著。Paleozoic tectonic evolution and continental growth of the West Tianshan Orogen: evidence from granitoids Lingli Long1, Jun Gao1, Qing Qian1, Reiner Klemd2 1Key Laboratory o

7、f Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029, China 2Mineralogisches Institut, Universität Würzburg, Am Hubland 97074, Würzburg, GermanyThe Central Asian Orogen Belt (CAOB), which is different from the subductional

8、orogen and the collisional orogen, is known as the most important site of crustal growth in the Phanerozoic, and it has been a hot spot for studying the orogenic belts. The Chinese West Tianshan Orogen is occupying the west-southern part of the CAOB and is of great importances for geologists to unde

9、rstand the orogenic processes and the continental growth in the Central Asia.The West Tianshan Orogen had undergone complex tectonic evolutional processes in Paleozoic times and large volumes granitic rocks have recorded important information about these processes. Litter is known about Phanerozoic

10、continental growth in the Western Tianshan area so far, compared with the other areas of the CAOB, such as eastern Junggar, western Junggar, Altai and Alakol. 25 precise SHRIMP U-Pb zircon and LA-ICPMS U-Pb zircon ages have been obtained in this study. The granitic rocks in western Tianshan had been

11、 formed during two periods: the granitic gneiss with an age of 896Ma, possibly representing the forming age of the Precambrian basement; the granitic rocks with ages varying from 470Ma to 247Ma, recording the Paleozoic orogenic process of western Tianshan.The granitoids in western Tianshan are compo

12、sed of intermediate-basic rocks, intermediate rocks, intermediate-acid rocks and acid rocks, mainly intermediate-acid rocks and acid rocks. They are mostly granite, granodiorite, quartz syenite and monzodiorite. Different types of granitic rocks are exposed in different tectonic units. The granitoid

13、s on the northern margin of the Yili Plate mainly formed in late Paleozoic (413Ma 281Ma), those with ages varying from 413Ma to 297Ma show continental arc affinities and the magnesian calc-alkalic metaluminous diorite of 281Ma display the geochemical characteristics similar to those of granites form

14、ed during the post-orogenic period. The granitiods on the southern margin of the Yili Plate include the adakite diorite of 470Ma which was formd by partial melting of thickened lower crust, the post-collisional alkali-feldspar granite of 430Ma, the volcanic arc granite of 348Ma and the Triassic post

15、-collisional granite. The granitoids in the Central Tianshan Plate formed in 479Ma 247Ma, mainly in 433Ma 321Ma. The granitic rocks with ages of 479Ma 321Ma are magnesian calc-alkalic to alkalic rocks with continental arc affinities. A few post-collisional granitoids of 276Ma 247Ma may have inherite

16、d the geochemical characteristics of pre-existing arc magma. The granitic rocks in Southern Tianshan (northern margin of the Tarim plate) formed two stages, 420Ma 411Ma and ca. 285Ma. The magnesian calcic to alkalic granites of 420Ma 411Ma may formed during the extension process of the continental m

17、argin. The granite of 285Ma includes mostly ferroan calc-alkalic to alkali-calcic rocks with high SiO2 and high alkaline contents, and obviously negative anomaly of Eu, Ba, Sr, P, Ti, similar to the geochemical characteristics of the A-type granite which is formed during post-collisional extension.A

18、 model for Paleozoic tectonic evolution of the West Tianshan Orogen has been proposed here on the basis of the new results obtained in this study and the previous published data. In Early Cambrian, the Terskey Ocean occurred along the North Nalati fault (NNF), and it separated the Yili plate from th

19、e Central Tianshan plate which was probably connected with the Tarim plate. The Terskey Ocean probably subducted towards south under the Central Tianshan plate and towards north under the Yili plate simultaneously. In the early stage of Late Ordovician, the Terskey Ocean had been closed, and the Yil

20、i and Central Tianshan plates collided. Meanwhile, extension happened within the joint Central Tianshan and Tarim plates gradually and the Paleo-South Tianshan Ocean had been formed. In Early Silurian, the Paleo-South Tianshan Ocean began to subduct beneath the composite Yili-Central Tianshan plate,

21、 which was intruded by volcanic arc granitoids. In Middle Silurian, the Paleo-South Tianshan Ocean, which had reached a certain width, was subducting strongly. And this subduction may have produced voluminous granitoids in the Central Tianshan plate. In the latest stage of Carboniferous, the Paleo-S

22、outh Tianshan ocean closed, and the Yili-Central Tianshan plate and Tarim plate collided. In Late Cambrian, Paleo-Junggar Ocean occurred to north of the Yili plate; and started to subduct towards south under the Yili plate in Ordovician. This subduction may have produced a magma arc on the northern margin of the Yili plate. In Late Carboniferous, the Paleo-Junggar Ocean ha