西天山造山帶的古生代構(gòu)造演化與地殼增生來(lái)自花崗

1、西天山造山帶的古生代構(gòu)造演 化與地殼增生：來(lái)自花崗西天山造山帶的古生代構(gòu)造演化與地殼增生： 來(lái)自花崗巖的證據(jù)龍靈利1,高俊1,錢青1, Reiner Klemd 21中國(guó)科學(xué)院地質(zhì)與地球物理研究所，北京 1000292Mineralogisches Institut, Universit? t W urzburg, Am Hubla nd 97074, W uzburg, Germa ny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(duì)象。西天山造山帶位于中亞造山帶的西 南緣，其對(duì)認(rèn)識(shí)中亞造山過(guò)程和大陸地殼增生具有重要意義。西天山造山帶

2、古生代經(jīng)歷了復(fù)雜的構(gòu)造演化過(guò)程， 其間發(fā)育的花崗巖記錄了 該過(guò)程的重要信息。同為中亞增生造山帶的重要組成部分，相對(duì)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(gè)高精度SHRIMP和LA-ICPMS鋯石 U-Pb年齡數(shù)據(jù)?；◢弾r的形成分為兩個(gè)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(guò)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(zhǎng)巖、石英正長(zhǎng)巖及二長(zhǎng)閃長(zhǎng)巖。不 同構(gòu)造單元發(fā)

3、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(zhǎng)巖為后碰撞造山階段巖漿活動(dòng)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(zhǎng)巖以及 430Ma后碰撞堿長(zhǎng)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

4、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)花崗巖形成于 420Ma411Ma和285Ma左右兩個(gè)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(guò)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(kāi)來(lái)，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

5、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)，中天山-塔里木板塊開(kāi)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(kāi)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(dòng)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(kāi)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(kāi)始西天山地區(qū)處于后碰撞造山階 段，該

6、階段的巖漿活動(dòng)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0.7032260.716343之間，馭d (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(dòng)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(zhǎng)并 不很顯著。Paleozoic tectonic evolution and continentai growth of the WestTian sha n Oroge n: evide nee from gran itoids11i2Li

7、n gli Long , Jun Gao , Qi ng Qia n , Reiner Klemd1Key Laboratory of Min eral Resources, I nstitute of Geology and Geophysics, Chin ese Academy of Scie nces, PO Box 9825, Beiji ng 100029, Chi na 2Mineralogisches Institut, Universit? t W trzburg, Am Hubla nd 97074, W rzburg, Germa nyThe Central Asian

8、Orogen Belt (CAOB), which is different from the subductional orogen and the collisional orogen, is known as the most important site of crustal growth in the Pha nerozoic, and it has bee n a hot spot' for study ing the orogenic belts. The Chinese West Tianshan Orogen is occupying the west-souther

9、n part of the CAOB and is of great importances for geologists to understand 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 importa nt

10、 in formati on about these processes. Litter is known about Pha nerozoic con ti nen tai growth in the Western Tian sha n 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

11、 been obtained in this study. The granitic rocks in western Tianshan had been 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 orog

12、enic process of wester n Tian sha n.The granitoids in western Tianshan are composed of intermediate-basic rocks, in termediate rocks, in termediate-acid rocks and acid rocks, main ly intermediate-acid rocks and acid rocks. They are mostly granite, granodiorite, quartz sye nite and mon zodiorite. Dif

13、fere nt types of gran itic rocks are exposed in differe nt tect on ic un its. The gran itoids on the n orther n margin of the Yili Plate mainly formed in late Paleozoic (413Ma 281Ma), those with ages vary ing from 413Ma to 297Ma show con ti nen tal arc affin ities and the magn esia n calc-alkalicmet

14、alumi nous diorite of 281Ma display the geochemical characteristics similar to those of granites formed during the post-orogenic period. The granitiods on the souther n margin of the Yili Plate in clude the adakite diorite of 470Ma which was formd by partialmelting of thickened lower crust, the post

15、-collisionalalkali-feldspar granite of 430Ma, the volcanic arc granite of 348Ma and the Triassic post-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 rock

16、s with continental arc affinities. A few post-collisional granitoids of 276Ma 247Ma may have inherited the geochemical characteristics of pre-existing arc magma The gran itic rocks in Souther n Tian sha n (n orther n margin of the Tarim plate) formed two stages, 420Ma 411Ma and ca. 285Ma. The magnes

17、ian calcic to alkalic granites of 420Ma 411Ma may formed during the extension process of the continental margin. The granite of 285Ma includes mostly ferroan calc-alkalic to alkali-calcic rocks with high SiO 2 and high alkali ne conten ts, and obviously negative anomaly of Eu, Ba, Sr, P, Ti, similar

18、 to the geochemical characteristics of the A-type granite which is formed during post-collisional exte nsion.A model for Paleozoic tecto nic evoluti on of theWest Tian sha n Oroge n hasbee n proposed here on the basis of the new results obta ined in this study and the previous published data. In Ear

19、ly Cambria n, the Terskey Ocea n occurred along the North Nalati fault (NNF), and it separated the Yili plate from the CentralTian sha n plate which was probably conn ected with the Tarim plate. The Terskey Ocea n probably subducted towards south un der the Cen tral Tian sha n plate and towards nort

20、h under the Yili plate simultaneously. In the early stage of Late Ordovician, the Terskey Ocean had been closed, and the Yili and Central Tian sha n plates collided. Mean while, exte nsion happe ned withi n the joint Central Tianshan and Tarim plates gradually and the Paleo-South Tianshan Ocean had

21、been formed. In Early Silurian, the Paleo-South Tianshan Ocean bega n to subduct ben eath the composite Yili-Ce ntral Tian sha n plate, which was intrudedby volcanic arc granitoids. In Middle Silurian, the Paleo-SouthTianshan Ocean, which had reached a certain width, was subducting strongly. And thi

22、s subduct ion may have produced voluminous gran itoids in the Cen tral Tianshan plate. In the latest stage of Carboniferous, the Paleo-South 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 subducti on may have produced a magma arc on the n orther n margi n of the Yili plate. In Late Carboniferous, the Paleo-Junggar Ocean