西天山造山帶的古生代構(gòu)造演化與地殼增生來自花崗巖的證據(jù)

1、西天山造山帶的古生代構(gòu)造演化與地殼增生：來自花崗巖的證據(jù)龍靈利1, 高俊1, 錢青1, Reiner Klemd2 1中國科學(xué)院地質(zhì)與地球物理研究所，北京1000292Mineralogisches Institut, Universität Würzburg, Am Hubland 97074, Würzburg, Germany中亞造山帶是不同于俯沖型和碰撞型的造山帶，是全球顯生宙大陸地殼增生最顯著的地區(qū)，為造山帶前沿?zé)狳c(diǎn)研究對象。西天山造山帶位于中亞造山帶的西南緣，其對認(rèn)識中亞造山過程和大陸地殼增生具有重要意義。西天山造山帶古生代經(jīng)歷了復(fù)雜的構(gòu)造演化過程，其

2、間發(fā)育的花崗巖記錄了該過程的重要信息。同為中亞增生造山帶的重要組成部分，相對于周邊東準(zhǔn)噶爾、西準(zhǔn)噶爾、阿爾泰山及阿拉套山地區(qū)，西天山地區(qū)顯生宙地殼增生的信息和證據(jù)較少。本次研究新獲得西天山花崗巖25個高精度SHRIMP和LA-ICPMS鋯石U-Pb年齡數(shù)據(jù)。花崗巖的形成分為兩個階段：896Ma的花崗片麻巖反映了前寒武紀(jì)基底巖系的形成時(shí)間，470Ma247Ma間的花崗巖記錄了西天山古生代造山過程。西天山花崗巖由中基性巖、中性巖、中酸性巖到酸性巖組成，以中酸性和酸性巖為主。巖石類型主要為花崗巖、花崗閃長巖、石英正長巖及二長閃長巖。不同構(gòu)造單元發(fā)育的不同類型花崗巖：伊犁板塊北緣花崗巖主要形成于晚古生

3、代（413Ma281Ma），413Ma297Ma的花崗巖具有類似于大陸弧花崗巖的特征，281Ma鎂質(zhì)鈣堿性偏鋁質(zhì)閃長巖為后碰撞造山階段巖漿活動的產(chǎn)物；伊犁板塊南緣發(fā)育早古生代470Ma加厚下地殼部分熔融形成的埃達(dá)克質(zhì)閃長巖以及430Ma后碰撞堿長花崗巖，晚古生代348Ma火山弧花崗巖和三疊紀(jì)后碰撞花崗巖；中天山花崗巖形成于479Ma 247Ma，主要集中在433Ma321Ma間。479Ma321Ma的花崗巖大多為鎂質(zhì)鈣性-堿性巖，少數(shù)為鐵質(zhì)鈣堿性-堿性巖，它們表現(xiàn)為大陸弧花崗巖的特征，少量形成于276Ma247Ma間的后碰撞花崗巖一定程度繼承了先存巖漿弧的特征；南天山（塔里木板塊北緣）花崗巖形

4、成于420Ma411Ma和285Ma左右兩個階段，420Ma411Ma間的鎂質(zhì)鈣性-堿性巖花崗巖，可能為拉伸環(huán)境下的產(chǎn)物，285Ma花崗巖主要為鐵質(zhì)鈣堿性-堿鈣性巖，SiO2 含量高、全堿含量高、表現(xiàn)明顯的Eu，Ba，Sr，P，Ti的負(fù)異常，具有類似于A型花崗巖的地球化學(xué)特征，它們形成于后碰撞造山過程中的拉伸環(huán)境。初步建立西天山造山帶古生代構(gòu)造演化模型：早寒武紀(jì)，沿那拉提北緣斷裂帖爾斯克依古洋形成，它將伊犁板塊和中天山板塊分隔開來，當(dāng)時(shí)中天山板塊和塔里木板塊連在一起。帖爾斯克依古洋向兩側(cè)伊犁板塊和中天山板塊之下發(fā)生雙向俯沖，晚奧陶紀(jì)早期，帖爾斯克依古洋閉合，伊犁板塊和中天山板塊碰撞縫合。同時(shí)，

5、中天山-塔里木板塊開始伸展拉張，古南天山洋逐漸發(fā)育形成。早志留紀(jì)，古南天山洋開始向伊犁-中天山板塊之下俯沖，形成火山弧型花崗巖。志留紀(jì)中期，古南天山洋達(dá)到相當(dāng)規(guī)模，這一時(shí)期洋殼俯沖活動強(qiáng)烈，在中天山形成大量的花崗巖。石炭紀(jì)末期，古南天山洋消失，塔里木板塊與伊犁-中天山板塊碰撞。晚寒武紀(jì)，準(zhǔn)噶爾洋在中天山北緣出現(xiàn)，奧陶紀(jì)開始向伊犁板塊之下俯沖，在伊犁板塊北緣發(fā)育巖漿弧。晚石炭紀(jì)，古準(zhǔn)噶爾洋閉合消失。石炭紀(jì)末塔里木、伊犁-中天山和準(zhǔn)噶爾板塊拼合在一起。二疊紀(jì)開始西天山地區(qū)處于后碰撞造山階段，該階段的巖漿活動可能一直持續(xù)到早三疊紀(jì)。西天山花崗巖87Sr/86Sr初始值介于0.7032260.7163

6、43之間，Nd（t）值介于-6.502.03之間。Sr、Nd同位素特征綜合顯示西天山造山帶花崗巖形成的源區(qū)并不單一，是殼源和幔源巖漿混合而成。古生代時(shí)期，西天山地區(qū)大陸地殼沿活動大陸邊緣發(fā)生了顯著的側(cè)向增生，而后碰撞幔源物質(zhì)添加導(dǎo)致的垂向生長并不很顯著。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