沉积学报  2018, Vol. 36 Issue (3): 446−455

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石鑫, 侯明才, 黄虎, 胡小龙, 江文剑, 吴斌, 缪宗利, 郑斯赫
SHI Xin, HOU MingCai, HUANG Hu, HU XiaoLong, JIANG WenJian, WU Bin, MIAO ZongLi, ZHENG SiHe
伊犁盆地南缘中-下侏罗统物源分析及其对南天山造山带演化的启示
Provenance Analysis of the Middle-Lower Jurassic Clastic Rocks in the Southern Margin of the Yili Basin and Their Insight into the Evolution of Southern Tianshan Orogenic Belt
沉积学报, 2018, 36(3): 446-455
ACTA SEDIMENTOLOGICA SINCA, 2018, 36(3): 446-455
10.14027/j.issn.1000-0550.2018.115

文章历史

收稿日期:2018-02-13
收修改稿日期: 2018-03-16
伊犁盆地南缘中-下侏罗统物源分析及其对南天山造山带演化的启示
石鑫1,2, 侯明才1,2, 黄虎1,2, 胡小龙1,2, 江文剑3, 吴斌1,2, 缪宗利1,2, 郑斯赫1,2     
1. 油气藏地质及开发工程国家重点实验室(成都理工大学), 成都 610059;
2. 成都理工大学沉积地质研究院, 成都 610059;
3. 东华理工大学地球科学学院, 南昌 330013
摘要: 伊犁盆地南缘中-下侏罗统碎屑岩的物源特征,可为南天山造山带的演化提供重要证据。对其碎屑岩锆石U-Pb定年研究结果表明,伊犁盆地南缘坎乡下侏罗统八道湾组砂岩的碎屑锆石年龄集中在290~260 Ma,而下侏罗统三工河组的碎屑锆石年龄集中在350~290 Ma和460~390 Ma,中侏罗统西山窑组的碎屑锆石年龄集中在370~320 Ma和450~390 Ma。所有测试样品中前寒武纪的年龄记录非常少。这些特征表明,伊犁盆地南缘中生代碎屑沉积物主要来自于伊犁-中天山地块南部。测试样品中几乎不存在晚二叠世-中三叠世的碎屑锆石,与南天山造山带的岩浆岩记录一致,暗示在晚二叠世-中三叠世南天山地区并没有发生强烈的与碰撞或后碰撞相关的岩浆活动。该结果不支持塔里木克拉通与伊犁-中天山地块在晚二叠世-中三叠世碰撞的观点。结合高压-超高压变质岩的数据和地层记录,认为塔里木克拉通与伊犁-中天山地块的碰撞发生在晚石炭世。同时,样品中最年轻锆石的年龄数据从早侏罗世到中侏罗世逐渐增大,显示了揭顶沉积的特点。对伊犁盆地南部中生代的锆石年龄数据与同时代南天山地区的锆石年龄数据进行综合对比表明在早-中侏罗世发生构造沉积夷平的特征。
关键词物源     中下侏罗统     伊犁盆地南缘     南天山造山带     锆石U-Pb年代学    
Provenance Analysis of the Middle-Lower Jurassic Clastic Rocks in the Southern Margin of the Yili Basin and Their Insight into the Evolution of Southern Tianshan Orogenic Belt
SHI Xin1,2, HOU MingCai1,2, HUANG Hu1,2, HU XiaoLong1,2, JIANG WenJian3, WU Bin1,2, MIAO ZongLi1,2, ZHENG SiHe1,2     
1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China;
2. Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China;
3. School of Earth Science, East China University of Technology, Nanchang 330013, China
Foundation: National Key Basic Research Program of China(973 Program), No. 2015CB453001; Innovation Team of Sedimentary Geology of Chengdu University of Technology, No. KYTD201703; Cultivating Program of Middle-Aged Key Teachers of Chengdu University of Technology, No. KYGG201718
Abstract: The provenance features of the middle and lower Jurassic clastic rocks in southern margin of the Yili Basin can provide important information for insights into the evolution of southern Tianshan Orogenic Belt. Detrital zircon age of the Badaowan Formation sandstone of lower Jurassic in southern margin of the Yili Basin demonstrates a prominent age population at 290-260 Ma, whereas the Sangonghe Formation of lower Jurassic shows two prominent age populations at 350-290 Ma and 460-390 Ma, respectively. The Xishanyao Formation of middle Jurassic has two prominent detrital zircon age populations at 370-320 Ma and 450-390 Ma. All study samples have little zircon age record of Precambrian period. These features indicate that Mesozoic detrital sediments in southern margin of the Yili Basin predominantly might be derived from the south part of the Yili-Central Tianshan Block. The late Permian to middle Triassic zircons are nearly absent, consistenting with magmatic record in the South Tianshan Orogenic Belt and probably indicating no strong magmatic activities associated with collision or post-collision occurred in South Tianshan Block during late Permian-middle Triassic periods, which does not support the idea that the collision between the Tarim Craton and Yili-CTS Block occurred in the late Permian-middle Triassic periods. Combined with previously published data of tectonic exhumation of high to ultrahigh pressure metamorphic rocks and stratigraphic records, these data support the idea that the collision between the Tarim Craton and Yili-CTS Block occurred in the late Carboniferous period. The youngest detrital zircon ages of the studied samples are increasing from the middle-late Triassic to early Jurassic periods to middle Jurassic period, indicating a feature of unroofing and sedimentation. Integration of the provenance data of Mesozoic strata in the southern Yili Basin with the time equivalent of the South Tianshan Belt, provides an integrated record of the tectonic subsidence and planation in the early-middle Jurassic period.
Key words: provenance     middle-lower Jurassic     southern margin of the Yili Basin     South Tianshan Orogenic Belt     zircon U-Pb chronology    
0 引言

天山造山带位于中亚造山带南缘,由塔里木克拉通向北与中天山、哈萨克斯坦—伊犁、准噶尔构造地块拼合而成[1-4](图 1a)。伊犁盆地呈狭长的三角形夹于北天山和中天山之间(图 1b),而位于伊犁地块中间的伊犁盆地是重要的铀矿富集区,主要的含矿层位为侏罗纪早期的水西沟群砂岩。对伊犁盆地中生代侏罗纪沉积物物源的研究,可为矿产勘查提供重要信息。然而,前人有关该方面的研究较少。同时,前人的研究普遍认为南天山洋的最终关闭标志着古亚洲洋在中亚造山带西南部的结束[3, 5]。而对塔里木克拉通北部与伊犁陆块之间的碰撞时间的争论主要集中在晚泥盆世到中三叠世[1, 5-9]

图 1 西天山地区构造地质图[10-11] ①北天山断裂;②尼古拉耶夫线—北那拉提断裂;③阿特巴希—依尼尔切克—南那拉提断裂;④北塔里木断裂;⑤塔拉斯—费尔干纳断裂 Figure 1 Tectonic geological map of West Tianshan Block[10-11] ①North Tianshan Fault; ②Nikolaev Line-North Nalati Fault; ③Atbashi-Inylchek-South Nalati Fault; ④North Tarim Fault; ⑤Talas-Ferghana Fault

本文通过对伊犁盆地南部中下侏罗统碎屑岩的碎屑锆石U-Pb年龄的研究,揭示沉积物的来源,并通过这些年龄数据与同时代南天山造山带的年龄数据对比分析,能够更好地约束伊犁—中天山地块与南天山造山带碰撞时间,明确构造演化的过程。

1 地质背景和采样位置

天山造山带是一个拼合地块,自中国西北部新疆维吾尔自治区开始,横穿中国西部邻国吉尔吉斯斯坦和哈萨克斯坦南部,延伸至塔吉克斯坦及乌兹别克斯坦(图 1b),东西延伸至少2 500 km。而在中国境内,天山构造带被分为北天山地块、南天山地块以及夹持于中间的伊犁—中天山地块[10-12](图 1b)。伊犁—中天山地块是由伊犁地块和中天山地块在泥盆纪之前形成的统一地块[3],其古老基底是由中—新元古代的花岗片麻岩、斜长角闪岩、混合岩、石英岩、大理石以及各种片岩组成[13-14]。在前寒武纪基底岩石上覆盖有差异明显的古生代火山过渡层和中新生代沉积盖层[15]。南天山造山带夹于伊犁—中天山地块南部与塔里木克拉通北部之间,其北部边界为阿特巴希—依尼尔切克—南那拉提断裂,南部边界为塔里木北缘断裂[4]。该造山带主要由上奥陶统—上石炭统海相火山沉积岩、二叠系陆相碎屑岩和火山岩,以及三叠系—侏罗系陆相碎屑岩组成[4]

本文选取了伊犁盆地南部坎乡地区中—下侏罗统地层剖面进行研究(图 1b图 2)。该剖面侏罗系地层主要发育下侏罗统八道湾组、三工河组以及中侏罗统西山窑组。其中,侏罗系底部的八道湾组发育砾岩、含砾粗砂岩、砂岩、泥岩及煤层,由下到上发育两套正旋回沉积,旋回底部砾岩中可见块状层理(图 3b),与下伏三叠系小泉沟群呈平行不整合接触(图 3a),中上部的砂岩中可见交错层理和冲刷面[16-17](图 3c)。三工河组整合覆盖在八道湾组之上,属于三角洲平原亚相和三角洲前缘亚相[18-19],岩性以中—细砂岩、粉砂岩及泥岩为特征,平原河道砂体可见冲刷—充填构造和交错层理。西山窑组是伊犁盆地中生代煤层发育最好的地层[18],主要发育曲流河三角洲沉积,岩性由粗砂岩、中细砂岩、泥岩夹煤层组成,地层中可见发育于河道沉积的冲刷构造和交错层理。本次所取样品均来自于坎乡侏罗系剖面,地层包括下侏罗统八道湾组(J1b-2)、下侏罗统三工河组(J1s-5)以及中侏罗统西山窑组(J2x-3)。坎乡剖面中,砂岩以岩屑砂岩为主,岩屑含量约15%~65%,以硅质岩岩屑为主,流纹岩岩屑次之(图 3d),另外含有一些变质岩岩屑。碎屑颗粒中,可见大量具有溶蚀现象的单晶石英(图 3f)及一些长轴具有定向性的多晶石英;另外可见一些鳞片状绢云母(图 3e)和具有格子双晶的微斜长石(图 3g)。总体上,样品中碎屑颗粒分选和磨圆较差,具有来自中酸性火山岩、花岗岩或者低级变质岩的近源沉积特点[16]

图 2 伊犁盆地南缘侏罗系出露区及邻区构造地质图[12] Figure 2 Jurassic outcrop zone of southern margin of Yili Basin and tectonic geological map of adjacent blocks[12]
图 3 伊犁盆地南缘坎乡中—下侏罗统地层综合柱状图 a.八道湾组底部与下伏小泉沟不整合接触;b.砾岩,块状层理,八道湾组底部;c.砂岩,板状交错层和冲刷面,八道湾组;d.流纹岩岩屑,单偏光,八道湾组;e.绢云母,正交偏光,三工河组;f.具溶蚀现象的石英,正交偏光,西山窑组;g.微斜长石,正交偏光,八道湾组;Ser.绢云母;Lvf.流纹岩屑;Qm.单晶石英;Mc.微斜长石;Ca.钙质胶结 Figure 3 Comprehensive stratigraphic columns of middle-lower Jurassic strata at Kanxiang, southern margin of Yili Basin a. unconformity between the bottom of Badaowan Formation and underlying Xiaoquangou Formation; b. conglomerate, massive bedding, bottom of Badaowan Formation; c. sandstone, cross bedding and scouring surface, Badaowan Formation; d. rhyolite cuttings, plane-polarized light, Badaowan Formation; e. sericite, cross-polarized light, Sangonghe Formation; f. quartz with erosion phenomenon, cross-polarized light, Xishanyao Formation; g. microcline, cross-polarized light, Badaowan Formation; Ser. sericite; Lvf. rhyolite cuttings; Qm. single crystal quartz; Mc. micro plagioclase; Ca. calcareous cementation
2 分析方法

样品的破碎和锆石的挑选由河北省廊坊市国家重点实验室完成。锆石阴极发光图像拍摄在武汉上谱分析科技有限责任公司完成,锆石U-Pb同位素定年在武汉上谱分析科技有限责任公司利用LA-ICP-MS同时分析完成。所选仪器型号为Agilent 7700e,激光剥蚀系统配置有信号平滑装置[20]。本次分析的激光束斑为32 μm,锆石的U-Pb同位素定年采用锆石标准91500和玻璃标准物质NIST610作外标分别进行同位素和微量元素分馏校正。采用ICPMSDataCal[21-22]对锆石数据进行分析处理,通过IsoPlot软件对处理的锆石进行U-Pb直方图和频率曲线图的制作。数据处理过程中,为了获得更准确的锆石U-Pb年龄数据,减少Pb同位素分馏以及235U衰变成207Pb的影响,对于大于1 000 Ma的锆石年龄选取207Pb/206Pb年龄数据,年龄小于1 000 Ma的锆石,选择206Pb/238U年龄数据[23]

3 碎屑锆石U-Pb年龄

对本次三个样品共220个锆石测点获得的220个锆石数据进行分析,排除其中谐和度小于90%的数据。对剩下的203个锆石数据进行U-Pb年龄分析,获得年龄范围为3 121~244 Ma。从阴极发光图像中可见,大多数锆石呈自形/棱柱状,暗示离物源可能较近,锆石粒径为50~200 μm,长宽为1:1~3:1。大多数锆石阴极发光图像具有清晰的振荡环带(图 4)和较高的Th / U比值(绝大部分大于0.4),指示具有岩浆成因[23]

图 4 伊犁盆地南缘中—下侏罗世碎屑岩锆石阴极发光图像 Figure 4 Cathodoluminescence images for detrital zircons from the middle-lower Jurassic clastic rocks in the southern margin of Yili Basin

下侏罗统八道湾组的砂岩样品J1b-2所获得的71个碎屑锆石进行测试分析。其中有60个年龄数据谐和度超过90%,锆石年龄点都落在锆石U-Pb年龄谐和曲线上(图 5a)。最年轻的51颗锆石U-Pb年龄加权平均值为(273.1±2.2)Ma(MSWD=1.6)。谐和度大于90%的锆石,其年龄分布在367~261 Ma之间,其主要年龄组为290~260 Ma,次要年龄组为370~310 Ma,另有一颗锆石年龄为232 Ma (图 6a)。

图 5 伊犁盆地南缘中—下侏罗统碎屑锆石U-Pb测年谐和图 Figure 5 Concordia plot of middle-lower Jurassic detrital zircon U-Pb dating in southern margin of Yili Basin

下侏罗统三工河组的砂岩样品J1s-5所获得的75个碎屑锆石中,有1个锆石颗粒的谐和度低于90%,其余锆石年龄点都落在锆石U-Pb年龄谐和曲线上(图 5b)。最年轻的38颗锆石U-Pb年龄加权平均值为310.2±4.7 Ma(MSWD=6.2)。谐和锆石的年龄分布在3 121~244 Ma之间,主要年龄组为350~290 Ma和460~380 Ma,次要年龄组为1 000~900 Ma,分布较少的年龄为260~240 Ma、600 Ma、1 200 Ma、1 400 Ma、1 800 Ma、2 000 Ma和3 000 Ma(图 6b)。

图 6 研究区及邻区碎屑锆石年龄概率图和岩浆岩结晶年龄直方图 a, b, c.伊犁盆地南缘研究样品碎屑锆石年龄直方图和概率曲线图;d.南天山造山带早—中侏罗世碎屑锆石直方图和概率曲线图[10, 24];e~h.塔里木克拉通和天山地区岩浆岩结晶年龄直方图[10-11]及其参考文献) (直方图横坐标上小于600 Ma区域与大于600 Ma区域刻度不同) Figure 6 Probability curve map of detrital zircon age and histograms for compiled crystallization ages of magmatic rocks in study block and its adjacent blocks a, b, c. histograms and probability curves for detrital zircon ages of three samples in the southern Yili Block; d.histogram and probability curves of early-middle Jurassic detrital zircon age in south Tianshan orogenic belt[10, 24]; e~h. histogram of igneous rock crystallization age of Tarim Craton and Tianshan Block[10-11] (The X-axis scales are different for < 600 Ma and > 600 Ma portions)

中侏罗统西山窑组的砂岩样品J2x-3所获得的74个碎屑锆石进行测试分析,其中有70个谐和度大于90%,只有一颗锆石年龄点落在锆石U-Pb年龄谐和曲线下方(图 5c),最年轻的31颗锆石U-Pb年龄加权平均值为346.8±4.2 Ma(MSWD=2.7)。谐和锆石的年龄分布在1 729~301 Ma之间,主要年龄组为380~320 Ma和450~390 Ma,次要年龄组为310~300 Ma和480~410 Ma,另有一颗年龄为1 729 Ma(图 6c)。

4 碎屑锆石U-Pb年龄 4.1 沉积物源

通过收集伊犁—中天山地块、南天山造山带和塔里木克拉通已发表的岩浆岩结晶年龄[10-11],来对比区域岩浆活动和所获样品碎屑锆石的来源。在研究中,为了显示岩浆活动的规律,把伊犁—中天山地块进一步划为伊犁—中天山地块南部和伊犁—中天山地块北部。在塔里木克拉通,岩浆活动主要发生在300~270 Ma、430~390 Ma、460~450 Ma、1.1~0.6 Ga、1.9~1.7 Ga、2.6~2.3 Ga(图 6e)。南天山造山带与塔里木克拉通具有相似的岩浆活动期次,但缺少460~450 Ma和前寒武纪的岩浆岩记录(图 6ef)。与塔里木地块和南天山造山带相比,伊犁—中天山地块在380~310 Ma的岩浆岩记录比较丰富[24],并且年龄570~470 Ma和1.7~1.1 Ga的岩浆岩只出现在伊犁—中天山地块南部(图 6gh)。此外,由于北天山洋地块俯冲的间歇,在伊犁—中天山地块的北缘没有340~320 Ma岩浆岩记录[25-26],而该年龄在伊犁—中天山地块南部有丰富的记录(图 6gh)。

从中—下侏罗统的碎屑锆石年龄谱来看,样品中锆石年龄主要集中在460~260 Ma之间,前寒武纪的年龄分布较少。年龄在400~300 Ma的锆石在同时代的塔里木北部和南天山造山带的岩浆岩基本不存在,而在伊犁—中天山地块广泛存在[10-11] (图 6e~h)。在相同地层时代的南天山造山带中有大量的前寒武纪和二叠纪的碎屑锆石(图 6d),而在本次的研究样品中只有八道湾组有二叠纪的碎屑锆石,三工河组含有少量前寒武纪的碎屑锆石和两粒三叠纪的碎屑锆石,西山窑组没有二叠纪锆石出现(图 6abc)。这些特征表明南天山造山带和塔里木克拉通可能不是研究区碎屑物的主要来源。古生代的锆石形态多呈自形或菱形,有清晰的震荡环带和较高的Th/U比值,显示了近源沉积的特点。综上所述,研究样品中的碎屑锆石U-Pb年龄特征显示主要源区为伊犁—中天山地块南部。

同时,研究剖面侏罗系地层底部的八道湾组最年轻碎屑锆石的主要年龄组为290~260 Ma,主峰年龄为290 Ma。而八道湾组上覆的三工河组最年轻碎屑锆石的主要年龄组为350~290 Ma,其峰值年龄为295 Ma和310 Ma,大于八道湾组主峰年龄的。更重要的是三工河组上覆的西山窑组主要年龄组为380~320 Ma的最年轻碎屑锆石,其峰值年龄为330 Ma和355 Ma,大于八道湾组和三工河组最年轻碎屑锆石的主峰年龄。可以看出,由底部的八道湾组到三工河组再到西山窑组,样品中最年轻碎屑锆石年龄逐渐增大(图 6abc),暗示物源区具有揭顶的特征。

4.2 构造意义

在南天山造山带发现的巴雷公蛇绿岩年龄在450 Ma[27]、库勒湖蛇绿岩年龄425 Ma、418 Ma、330 Ma[28-30],这表明南天山洋在晚志留世到早石炭世一直没有关闭。然而,南天山洋的最终关闭时间以及塔里木克拉通地块和伊犁—中天山地块的碰撞时间一直存在以下几种争议:晚泥盆世—晚石炭世[31-32]、晚石炭世[33]、晚二叠世—中三叠世[6, 34-35]

有学者根据该地区的区域不整合特征,认为南天山造山带在晚泥盆世—早石炭世发生碰撞[36-37]。然而这与在北天山和南天山发现的早石炭世蛇绿岩是矛盾的[38-39]

部分学者通过研究蛇绿混杂岩中的放射虫化石的年龄和超高压变质岩的年龄认为碰撞主要发生在晚二叠世—中三叠世[40-41]。然而,Zhang et al. [41]发现的U-Pb年龄为233~226 Ma的年轻锆石,已经被认为是由于后期流体作用或者是锆石颗粒的重结晶[42]或者是衰变作用[43]导致的。蛇绿混杂岩中记录的晚二叠世放射虫样本[34]因为保存差,也一直广受质疑[8]。同时,对南天山造山带的榴辉岩进行放射性同位素测年发现,其变质峰值年龄为320~310 Ma[44]。此次研究中,侏罗系地层碎屑锆石样品的年龄谱显示,伊犁—中天山地块南缘基本没有晚二叠世和中生代的年龄记录。暗示在晚二叠世—中三叠世缺乏由同碰撞和后碰撞造山作用引起的岩浆活动。这些证据结合二叠纪的陆相沉积特征[8],并不能证明南天山造山带在晚二叠世—中三叠世存在碰撞活动。

在南天山造山带,年龄在295~285 Ma的早二叠世火山岩不整合覆盖在发生强烈褶皱的石炭纪早期和石炭纪晚期地层之上[45]。而且榴辉岩和蓝片岩的Rb/Sr和40Ar/39Ar的测年分析显示,超高压变质岩大约在330~300 Ma(主要发生在310 Ma左右)发生了快速的构造剥蚀[44]。更重要的是,在阿特巴希地区,石炭纪末期(304~299 Ma)磨拉石沉积建造中砾岩(轻微变形但未变质)不整合覆盖在高压变质岩之上[44]。同时,在伊犁盆地南缘特克斯地区上石炭统东图津河组也发现了榴辉岩砾石[11]。而砾岩中榴辉岩的发现,表明该区高压变质岩的暴露和剥蚀大约发生在300 Ma之前[11, 44]。这些地层特征和超高压变质岩的研究表明,塔里木克地块可能在晚石炭世与伊犁—中天山发生了碰撞。

在晚石炭世—早二叠世(大约320~270 Ma),南天山造山带和伊犁—中天山地块南部广泛发育大量的双峰式火山岩和S型、A型和高K花岗岩以及少量的富K正长石[33, 46]。来自于南天山造山带的碎屑岩的最年轻碎屑锆石峰值年龄从299~288 Ma略有不同,但是分布特征比较单一[47]。然而,在中—晚二叠世期间,随着来自伊犁—中天山地块南部和南天山造山带隆起的老火山沉积岩及基底剥蚀物质逐渐增多,使得年龄大于300 Ma的碎屑锆石也逐渐增多[11]。这表明,随着伊犁—中天山地块南部和南天山造山带隆起被剥蚀和夷平作用的发生,基底已经逐渐暴露[11, 47]。结合伊犁—中天山地块南部和南天山造山带发育的二叠纪正断层[45],表明后碰撞的伸展过程可能持续到晚二叠世。来自南天山造山带晚三叠世的年龄为260~220 Ma的碎屑锆石,与塔里木盆地阿图什喀若勒以北的辉绿岩年龄相对照,其源区可能为塔里木盆地西北缘[48],可能与塔里木盆地西北缘晚二叠世—晚三叠世岩浆活动有关[48]。在伊犁盆地南部侏罗系地层中,从下到上,最年轻锆石的峰值年龄逐渐增大。同时,在伊犁—中天山地块南部和南天山造山带的中下侏罗统地层中,广泛分布着煤和油页岩矿床[27, 47]。这些地层和年龄记录反映了早—中侏罗世随着天山山脉水系扩大带来的构造沉降和夷平现象。

5 结论

伊犁盆地南缘坎乡下侏罗统碎屑锆石年龄集中在370~260 Ma和450~390 Ma,缺少前寒武纪的年龄记录,表明碎屑沉积物主要来自于伊犁—中天山地块南部。样品中几乎不存在晚二叠世到中三叠世的碎屑锆石,这与南天山造山带的岩浆岩记录一致,该结果不支持塔里木克拉通与伊犁—中天山地块在晚二叠世—中三叠世发生碰撞的观点。结合该地区的地层特征,以及前人对该区高压变质岩的暴露和剥蚀年龄(约300 Ma之前)的研究,我们认为塔里木地块与伊犁—中天山地块可能在晚石炭世发生碰撞。碎屑锆石年龄特征表明,坎乡侏罗系地层由老到新,最年轻碎屑锆石峰值年龄逐渐增大,具有明显的揭顶现象,与早—中侏罗世发生的构造沉积夷平有关。

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