中天山科克苏河地区隆升剥蚀历史
——来自(U-Th)/He年龄的制约
2016-09-29喻顺陈文张斌孙敬博李超袁霞沈泽杨莉马勋
喻顺,陈文*,张斌,孙敬博,李超,袁霞,沈泽,杨莉,马勋
1 中国地质科学院地质研究所同位素热年代学实验室,北京 100037 2 中国地质科学院Re-Os同位素地球化学重点实验室,北京 100037 3 中原油田分公司采油五厂,河南 濮阳 457001
中天山科克苏河地区隆升剥蚀历史
——来自(U-Th)/He年龄的制约
喻顺1,陈文1*,张斌1,孙敬博1,李超2,袁霞1,沈泽1,杨莉1,马勋3
1 中国地质科学院地质研究所同位素热年代学实验室,北京100037 2 中国地质科学院Re-Os同位素地球化学重点实验室,北京100037 3 中原油田分公司采油五厂,河南 濮阳457001
天山是中亚造山带重要组成部分,其中-新生代构造热演化及隆升剥露史研究是认识中亚造山带构造变形过程与机制的关键.本文应用磷灰石(U-Th)/He技术重建中天山南缘科克苏河地区中-新生代构造热演化及隆升剥蚀过程.磷灰石(U-Th)/He数据综合解释及热演化史模拟表明该地区至少存在晚白垩世、早中新世、晚中新世3期快速隆升剥蚀事件,起始时间分别为~90 Ma、~13 Ma及~5 Ma,且这3期隆升剥蚀事件在整个天山地区具有广泛的可对比性.相对于磷灰石裂变径迹,磷灰石 (U-Th)/He年龄记录了中天山南缘地质演化史中更新和更近的热信息,即中天山在晚中新世(~5 Ma)快速隆升剥蚀,其剥蚀速率为~0.47 mm·a-1,剥蚀厚度为~2300 m.总体上,中天山科克苏地区隆升剥蚀起始时间从天山造山带向昭苏盆地(由南向北)逐渐变老,表明了中天山南缘隆升剥蚀存在不均一性,并发生了多期揭顶剥蚀事件.
中天山;科克苏;隆升剥蚀;(U-Th)/He;低温热年代学
1 引言
天山造山带东西绵延约2500 km,是中亚造山带重要的组成部分(图1a).天山造山带形成于古生代,在中-新生代经历了多次构造叠加和改造过程.现今天山造山带是受新生代印度—欧亚大陆板块碰撞作用的影响再次褶皱隆升形成(Avouac et al.,1993;Tapponnier and Molnar,1979).中-新生代以来天山复杂的构造演化过程导致了天山地区存在多阶段、多期次的隆升剥蚀事件.前人研究天山中-新生代隆升剥蚀历史,取得了一系列重要的成果:① 根据不整合面的发育推测天山经历了三叠纪-晚侏罗世夷平、晚侏罗世-早白垩世隆升及晚白垩世夷平3个阶段(张良臣和吴乃元,1985);② 裂变径迹技术研究表明天山在早白垩世、晚白垩世期间都发生了明显的快速隆升(杨庚和钱祥麟,1995;杨树锋等,2003;杜治利和王清晨,2007;王彦斌等,2001;贾承造等,2003;马前等,2006;陈正乐等,2006),而新生代天山的快速隆升起始时间有始新世(杜治利和王清晨,2007;Wang et al.,2009)、渐新世(Hendrix et al.,1994;Sobel and Dumitru,1997;Dumitru et al.,2001;De Grave et al.,2013)及中新世(11~17 Ma)( Bullen et al.,2001)等3种观点;③ 根据天山周缘盆地的沉积速率、磁性地层学及构造特征等反演了天山山脉的隆升时间(Windley et al.,1990;Yin et al.,1998 ;Sun et al.,2004,2009;Charreau et al.,2006;Huang et al.,2006);④ 根据山前沉积盆地沉积特征、沉降中心变化、沉积物成分及重矿物含量特征等揭示天山隆升剥蚀特征(李忠等,2003;李双建等,2007);⑤利用GPS对天山大地测量推断天山中新世快速抬升(Abdrakhmatov et al.,1996).上述成果促进了我们对天山造山带中-新生代剥蚀历史的认识,同时也表明天山中-新生代隆升剥蚀历史仍存在争议,这可能是由于研究方法的不同引起的,也可能是隆升剥蚀的时空分布差异性引起的.以往的研究主要是根据天山山前盆地资料反演天山隆升剥蚀史,而利用热年代学技术研究中天山南缘岩体隆升剥蚀主要集中在境外中天山(吉尔吉斯段)(Glorie et al.,2010;De Grave et al.,2011,2013).另外,天山地区晚中新世快速隆升剥蚀已被山前盆地内磁性地层、岩石磁及生长地层等多种证据证实(Huang et al.,2006;Sun et al.,2004,2009;Zhang et al.,2014;Yu et al.,2014;喻顺等,2014),然而在天山造山带少有该期隆升剥蚀事件的年代学证据.
近年来,国际上新发展的磷灰石(U-Th)/He定年技术具有对低温条件的敏感性(40~85 ℃)和较低的封闭温度(Wolf et al.,1998),能有效记录地质体经历较低温度范围的时代和温度信息,即地质演化史中最新和最近的信息(地表下1~3 km 深度),已成为研究造山带抬升剥露作用的有效手段(Ehlers and Farley,2003;Reiners et al.,2002).本文利用磷灰石(U-Th)/He技术恢复中天山南缘科克苏河地区中-新生代构造热演化史,揭示该区中-新生代隆升剥蚀过程,为天山隆升剥蚀事件研究提供年代学证据,同时结合前人资料探讨天山造山带隆升剥蚀特征,这有助于研究天山地区构造演化机制、过程及建立构造热演化事件对比框架,对于深入认识中亚造山带构造变形过程与机理具有重要意义.
2 地质概况
在古生代晚泥盆世-早石炭世、晚石炭世-早二叠世等期间,多个地块之间碰撞、增生融合逐步形成了天山造山带(Windley et al.,1990;Avouac et al.,1993;Yin et al.,1998;Gao et al.,1998;Xiao et al.,2004).晚二叠世天山抬升作用加速,在天山地区广泛发育陆相磨拉石(Gao et al.,1998;Chen et al.,1999),中生代早期(三叠纪-晚侏罗世)天山在地貌上处于准平原化状态,晚侏罗世-早白垩世进入隆升作用阶段,中生代末期(晚白垩世)再次进入区域剥蚀夷平状态(张良臣和吴乃元,1985;Allen et al.,1993;Bullen et al.,2001;Jolivet et al.,2010).新生代以来,印度—欧亚大陆板块碰撞作用的远程效应使天山再次活跃,天山遭受了强烈的挤压和隆升变形作用而形成陆内造山带(Molnar and Tapponnier,1975;Allen et al.,1993;Hendrix et al.,1994),并在其两侧发育了与陆内造山带相关的再生前陆盆地(Lu et al.,1994).
图1 天山地区地质简图(a)中亚造山带构造简图;(b)天山造山带地貌图,图中断裂据Klemd等(2015)修改;(c)中天山南缘科克苏地区地质简图(据Wang等(2010)修改).Fig.1 Geological and structural sketch map of southwestern Chinese Tianshan(a)Structural sketch map of the Central Asian Orogenic Belt;(b)Present geomorphological map of Tianshan orogeny;(c)Geological map of Kekesu area in the south of Center Tianshan.
天山造山带发育多条不同时代的板块缝合线,其中北天山缝合带和中天山南缘缝合带(图1b)(Windley et al.,1990;Allen et al.,1993;Gao et al.,1998)将天山造山带自北向南划分为北天山、中天山和南天山.中天山南缘缝合带以北的伊犁—中天山板块南缘出露元古代变质岩系,主要分布在那拉提北缘断裂与中天山南缘缝合带之间.高压变质岩带宽约20 km,主要由绿片岩、蓝片岩、榴辉岩和少量大理岩(Gao et al.,1995,1998)组成(图1c).由于南天山洋向北俯冲,伊犁-中天山板块南缘发育了早古生代晚期至晚古生代的岛弧型火山岩、花岗岩,这些岛弧型岩浆岩及高压变质岩的形成时代主要是早志留世-早石炭世(Gao et al.,1995,1998;Klemd et al.,2005;Zhu et al.,2005;朱志新等,2006).在那拉提北缘断裂与那拉提南缘断裂(或中天山南缘断裂)之间,高压变质带的北侧发育一条宽近8 km的大型韧性剪切带,其中心部位遭后期花岗岩侵入、破坏(高俊等,1995;Wang et al.,2007;Xia et al.,2014).
本文的研究区为新疆特克斯县南科克苏河谷剖面,该剖面位于伊犁-中天山地块南缘至哈尔克山北缘(图2),跨越多个不同的岩石构造单元,是研究天山构造演化的重要窗口.科克苏河剖面从特克斯县向南到韧性剪切带广泛出露石炭系大哈拉军山组和阿克沙克组火山岩及陆源碎屑岩.火山岩主要由玄武岩、玄武安山岩、安山岩、粗面岩、流纹岩、凝灰岩等组成(Wang et al.,2007),并伴有大量同时代侵入岩(王博等,2007).侵入岩主要为辉长岩、花岗闪长岩、英云闪长岩、钾长石花岗岩和花岗质岩脉.沉积岩则主要为凝灰岩、砂砾岩、薄层灰岩和泥质岩.近年来,很多高精度同位素年代学研究成果陆续发表,其中大哈拉军山组辉长玢岩中辉石39Ar-40Ar年龄为326.85±15 Ma(刘友梅等,1994),火山岩锆石SHRIMP U-Pb年龄分布在354±5 Ma~313±4 Ma(Zhu et al.,2005).角闪花岗岩和钾长花岗岩锆石U-Pb年龄分布在352±6 Ma~338±8 Ma(王博等,2007;Gao et al.,2009),花岗闪长岩锆石U-Pb年龄为313±4 Ma,黑云母坪39Ar-40Ar年龄为263.4±0.6 Ma(王博等,2007),被认为可能与岩体经历的后期热事件改造有关;闪长岩锆石U-Pb年龄为398~433 Ma(Gao et al.,2009).那拉提二长花岗岩的锆石SHRIMP U-Pb 年龄为436 Ma和370~366 Ma(朱志新等,2006).
3 样品和实验
3.1样品采集及制备
科克苏一带发育大量的海西期花岗岩,部分岩体侵入到高压变质杂岩带中.在高压变质杂岩带北侧,二叠纪发生过大规模的区域韧性走滑事件.剖面南部那拉提断裂区正长岩侵入中-新元古代黑云石英片岩、石榴石斜长麻粒岩、片麻岩、斜长角闪岩及古生代绿片岩(图2,图3).剖面北部出露一个复合闪长岩体,主要由闪长岩、角闪花岗岩及二长闪长岩组成,该岩体侵入中-新元古代变质岩中,同时该岩体被黑云母花岗岩侵入.样品TS1370及TS1371位于那拉提韧性剪切带中(图2),构造上位于那拉提北缘断裂与中天山南缘缝合带之间,TS1370为黑云母石英片岩,分布在前寒武纪地层中;TS1371为闪长岩,矿物颗粒较粗,该岩体锆石U-Pb年龄为398±1 Ma(Gao et al.,2009).TS1372黑云母花岗岩与TS1373角闪黑云母花岗岩位于那拉提北缘断裂的北部,黑云母花岗岩锆石U-Pb年龄为352±6 Ma(Gao et al.,2009);TS1373角闪黑云母花岗岩锆石 U-Pb年龄为349±6 Ma(Gao et al.,2009).样品TS1374石英正长岩位于科克苏河剖面最北端,即昭苏盆地南缘断裂南侧,这些岩体及地层自中-新生代以来发生了强烈的抬升剥蚀(Glorie et al.,2010).
图2 天山科克苏河地区地质图(改自Gao等(2009)及Wang等(2010))(图中锆石U-Pb年龄引自Gao 等(2009))Fig.2 Geological and structural sketch map of Kekesu section in Chinese Tianshan (modified from Gao et al. (2009)and Wang et al.(2010))
图3 新疆天山科克苏河剖面示意图(剖面位置见图2,图中(U-Th)/He年龄见表1)Fig.3 Schematic regional cross section along the Kekesu River (location shown in figure 2)
磷灰石单矿物的分选及制备采用Donelick等(2005)描述的程序.样品TS1370磷灰石晶体形态较好,晶体直径主要分布在60~100 μm;TS1371磷灰石直径以大于70 μm为主,矿物表面较为粗糙,晶体形态较完整.TS1372样品磷灰石颗粒形态较差,矿物直径较小.TS1373磷灰石形态较好、表面光滑、透明均一、无包裹体,然而该样品磷灰石直径较小(主要分布在40~60 μm),仅少量磷灰石直径大于80 μm.TS1374磷灰石晶体形态较好,磷灰石直径多数大于70 μm、透明均一、无包裹体.磷灰石(U-Th)/He实验显微镜下矿物挑选原则为:挑选晶形较好(自形程度高)、不含包裹体(尽可能少)、无裂缝、干净透明,且直径大于70 μm的磷灰石颗粒(减少Ft校正误差及增加单颗粒He含量),并在显微镜下测量矿物颗粒的尺寸,用于计算α校正因子(Ft)(Farley et al.,1996).上述每个样品在显微镜下精选5~10粒磷灰石,测量磷灰石颗粒的大小并照相,再进行综合对比分析,最终挑选出3~5粒磷灰石用于(U-Th)/He测试(图4).
3.2(U-Th)/He年龄测试
磷灰石(U-Th)/He年龄测试在中国地质科学院地质研究所同位素热年代学实验室进行.磷灰石(U-Th)/He年龄是根据矿物中U、Th及He含量计算获得,其实验流程如下.
利用Alphachron II四极杆质谱系统完成磷灰石He测量.将已完成尺寸测量磷灰石颗粒放入Pt包中,加载到Alphachron II仪器激光室.利用970 nm半导体二极管激光加热两次,加热温度约为900~1000 ℃,持续5 min,完全提取磷灰石4He.之后,将4He与同位素稀释剂3He充分混合,利用Pfeiffer Prisma四极杆质谱测量He同位素比值;根据标准罐气体4He校正同位素稀释剂3He量;依据同位素稀释剂3He量计算磷灰石4He量,总体上He的测量精确度小于1%.
磷灰石U、Th含量测定是根据同位素稀释电感耦合等离子体质谱法(ID ICP MS).在测完He的磷灰石样品中加入25 μL浓度为50% HNO3(体积比,大约7 mol/L)的稀释剂溶液(包含约15ppb235U 和5 ppb230Th),然后置于超声波清洗槽15 min,随后样品在酸性稀释剂溶液中溶解12 h.向25 μL标准溶液(25ppb U和25 ppb Th)中加入25 μL稀释剂溶液.在标准溶液和样品溶液中各加入250 μL 蒸馏水.利用Thermofisher-X2(中国科学院地质与地球物理研究所)电感耦合等离子体质谱仪(ICPMS)测量化学处理后溶液的U、Th同位素比值.U、Th同位素比值测量精确度(相对标准偏差)小于2%.样品中U、Th同位素含量依据标准溶液U、Th含量计算获得.磷灰石(U-Th)/He实验测量精确度依据磷灰石标准样品年龄的测量结果确定,中国地质科学院地质研究所同位素热年代学实验室测试21粒Durango磷灰石(U-Th)/He年龄为31.5±1.4(1σ)Ma(参考年龄为31.02±1.01 Ma(McDowell et al.,2005)),磷灰石(U-Th)/He年龄测试精确度(相对标准偏差)小于2%.
图4 科克苏河磷灰石镜下测量照片Fig.4 Images of apatite grains analyzed in this study
4 构造热演化史及地质意义
4.1(U-Th)/He测年结果判读
科克苏河剖面样品磷灰石(U-Th)/He年龄测试分析结果见表1.(U-Th)/He未校正年龄分布在2~160 Ma,经过Ft校正的年龄分布在2.7~226 Ma.本次采集样品(TS1371-TS1374)的岩性为闪长岩、花岗岩及石英正长岩,这些岩石在该地区广泛发育,前人研究表明它们形成于二叠纪及以前(王博等,2007;Gao et al.,2009;Wang et al.,2010),且测试的磷灰石(U-Th)/He年龄远小于岩体的形成年龄,表明磷灰石(U-Th)/He年龄为岩体中-新生代冷却年龄;另外一个样品TS1370岩性为新元古代片岩,其磷灰石(U-Th)/He年龄也代表了该样品中-新生代冷却年龄.这些样品冷却年龄与该地区后期发生的构造事件有关,记录了样品抬升或逆冲引起的剥蚀信息.
科克苏河剖面部分样品磷灰石 (U-Th)/He年龄较为分散,如样品TS1373和TS1374.磷灰石(U-Th)/He年龄分散原因为:① 未识别的富含U、Th的包裹体(Farley,2002;Lippolt et al.,1994)的影响.通过在显微镜下精细地挑选磷灰石,可以将此种影响因素减小到最低.另外,在测量He时第二次加热去气结果也可以作为一个辅助参数,如第二次加热释放的He量与热空白接近,表明磷灰石含有包裹体可能性小,本次实验中第二次所测量的磷灰石He量与热空白较为接近,表明了矿物包裹体可能性较小.② 磷灰石颗粒大小变化导致磷灰石He封闭温度的差异(Farley,2000).Wolf 等 (1998)建立磷灰石热扩散模型,表明了如果磷灰石长时间停留在He部分保留区,磷灰石颗粒半径的差异可引起(U-Th)/He年龄分散,本次测量的样品磷灰石颗粒半径与(U-Th)/He年龄关系见图5,TS1374磷灰石(U-Th)/He年龄与颗粒半径正相关,表明其年龄分散可能与半径有关.③ U和Th的植入效应(Farley,2002;Spiegel et al.,2009).当磷灰石颗粒内U和Th含量远低于其周围岩层U和Th含量,特别是在磷灰石颗粒内eU浓度(eU=U+0.235Th)小于5 ppm时,U和Th的植入效应影响明显(Spiegel et al.,2009),科克苏河地区样品仅TS1370-3磷灰石eU浓度较小(表1),仅在7.5 ppm左右,然而样品TS1370各颗粒磷灰石(U-Th)/He年龄较集中,表明了年龄受此影响因素不明显.④ 增强He保留效应(辐射损伤)(Fitzgerald et al.,2006;Flowers et al.,2009;Gautheron et al.,2009;Green and Duddy,2006;Shuster et al.,2006;Shuster and Farley,2009)导致(U-Th)/He年龄偏大.Shuster 等 (2006)和Flowers 等(2007)研究成果表明,如磷灰石(U-Th)/He年龄与eU浓度和He浓度呈正相关,说明它们受辐射损伤影响明显.图5表明科克苏河地区样品仅TS1374磷灰石(U-Th)/He年龄与eU浓度正相关,表明TS1374年龄的分散可能与增强He保留效应有关.另外,样品TS1372-1磷灰石(U-Th)/He年龄与其他颗粒年龄差异较大,分析原因可能是该磷灰石颗粒直径53 μm(图4),(U-Th)/He年龄校正系数Ft(0.43)较小,影响了测年结果的准确性.因此,该颗粒不参与样品的平均(U-Th)/He年龄计算.图2显示了自南向北样品磷灰石(U-Th)/He平均年龄总体上增大的特征.
表1 科克苏河磷灰石(U-Th)/He年龄测试结果
注:rad 颗粒等效半径;Unc.age 未校正年龄;Cor.Age 校正年龄;eU=U+0.235Th(Shuster et al.,2006);Ft为α粒子射出效应的校正参数(Farley et al.,1996);* 表示该磷灰石颗粒半径较小(见图4),Ft校正系数较大,未参与平均年龄计算;黑体数据为样品单颗粒年龄的平均.
图5 科克苏河磷灰石半径和eU与(U-Th)/He年龄关系图Fig.5 Apatite (U-Th)/He ages versus effective U concentration (eU=U+0.235Th)and radius for the analyzed samples in Kekesu section
4.2构造热演化史模拟原理
低温热演化历史可以利用数值模拟软件进行模拟,如QTQt(Gallagher,2012)软件等.QTQt 软件模拟原理是以Bayesian(贝叶斯)统计方法为基础,采用Markov Chain Monte Carlo(马尔可夫链蒙特卡尔)理论模拟计算,获取样品一定范围内可能的热演化模型,作为样品经历热演化的时间和温度先验信息,利用Bayesian方法获取适合数据的简单热史模型,优选概率值较大的热史模型,定量化可接受模型的概率分布(后验概率分布模型).上述软件模拟时也考虑了颗粒尺寸、辐射损伤及冷却速率等对热史的影响.Flowers等(2007,2009)认为磷灰石(U-Th)/He年龄与eU浓度和He浓度的相关性对样品所经历的热演化史敏感,当样品经历一样的热史,如磷灰石(U-Th)/He年龄与eU浓度呈非线性正相关,认为磷灰石(U-Th)/He体系受辐射损伤退火影响明显,并建立了磷灰石(U-Th)/He辐射损伤退火模型(RDAAM模型)处理这类样品.图5显示样品TS1374磷灰石(U-Th)/He年龄与eU浓度和He浓度的呈正相关,在运用QTQt软件对研究区(U-Th)/He数据模拟时,样品TS1374选择辐射损伤退火模型进行定量模拟.
对于用于模拟的同一样品,所有的磷灰石颗粒都经历了相同的地质演化过程,其(U-Th)/He数据是样品所经历的热演化过程反映,模拟时根据基础地质演化史输入温度-年龄等约束条件.将已知的现今地表温度(15±5 ℃)输入软件作为边界条件.科克苏地区发育大量的海西期花岗岩,部分岩体侵入到高压变质杂岩体中.在高压变质带的北侧,二叠纪发生了大型的区域韧性走滑事件,北侧花岗闪长岩岩体黑云母39Ar-40Ar坪年龄为263.4±0.6 Ma(王博等,2007),样品TS1371(闪长岩)黑云母39Ar-40Ar坪年龄为294.8±1.6 Ma(另文发表),这些年龄可能与岩体经历了此期构造热事件有关.因此,根据以上地质条件假设样品的模拟起始时间为260±20 Ma,起始温度为350±50 ℃.图6显示了距今最近且最相关的热演化史.热模拟模型对样品早期的热演化路径约束较差,但对距今最近的冷却和剥蚀路径给予了较好的约束,即使这些热模拟模型不能给出精确的剥蚀路径,但是它们能提供样品随时间-温度演化的窗口,显示样品开始冷却及通过磷灰石He部分保留区的时间,因而为样品经历低温阶段冷却历史提供了有力的信息.
4.3构造热演化史
样品TS1370和TS1371位于那拉提韧性剪切带中,其中样品TS1370为前寒武纪黑云母石英片岩;TS1371闪长岩锆石U-Pb年龄为398±1 Ma(Gao et al.,2009).TS1370磷灰石(U-Th)/He年龄分布在4.5~8.5 Ma,平均年龄为6.0 Ma;TS1371磷灰石(U-Th)/He年龄分布在2.7~4.4 Ma,平均年龄为3.5 Ma,因此,将剪切带内上述两个样品 (U-Th)/He年龄解释为冷却年龄,它们记录了岩体经过磷灰石He部分保留区间的时间.热模拟(图6)结果显示韧性剪切带内样品在新生代存在一期快速冷却事件,快速冷却的起始时间为~5 Ma,即样品~5 Ma开始快速冷却通过磷灰石He部分保留区,表明了该地区新生代快速隆升剥蚀事件起始时间为~5 Ma.假设地温梯度为30 ℃/km(冯昌格等,2009;王良书等,2003),地表温度为15 ℃,磷灰石(U-Th)/He封闭温度为85 ℃(喻顺等,2014;Qiu et al.,2012),计算新生代晚中新世该地区隆升剥蚀厚度为~2300 m,剥蚀速率为~0.47 mm·a-1.
样品TS1372为黑云母花岗岩,锆石U-Pb年龄为352±6 Ma(Gao et al.,2009),该样品测试了3粒磷灰石,其(U-Th)/He年龄分别为138.6 Ma、98.6 Ma及87.7 Ma,其中年龄为138.6 Ma的磷灰石颗粒直径较小,未用于热史模拟,另外两粒磷灰石(U-Th)/He年龄较年轻,记录了该区黑云母花岗岩热演化信息,这两粒磷灰石(U-Th)/He数据模拟结果见图6.模拟结果揭示了该样品经历了3阶段的冷却,第一阶段起始于温度大于120 ℃,接着从90 Ma到85 Ma迅速的冷却通过磷灰石He半保留区,最后缓慢抬升冷却至地表.
图6 科克苏地区样品QTQt软件热史模拟图左:样品热演化史模拟(频率分布图);右:样品期望热史模拟.Max.Like Model 最大似然模型;Max.Post.Model 最大后验模型;Expected Model 预期模型;Max.Mode Model 最大模态模型(模型详细解释见Gallagher(2012)).Fig.6 Time-temperature paths of AHe ages modeled using QTQt softwareLeft:Thermal history modeling of the samples;right:Thermal history of the Expected model.
样品TS1373角闪黑云母花岗岩锆石U-Pb年龄为349±6 Ma(Gao et al.,2009).该样品测试了5粒磷灰石,其磷灰石(U-Th)/He年龄较为分散,分别为~48 Ma,35.8 Ma,70.7 Ma,59.7 Ma 和28.3 Ma,平均年龄为48.5 Ma.热史模拟结果显示该样品经历了2个阶段的冷却史,第一阶段为大于13 Ma,样品缓慢冷却通过磷灰石He半保留区,之后样品快速抬升剥蚀至地表.由图6可见该样品长期停留于磷灰石He半保留区(40~85 ℃)(Wolf et al.,1998),这可能是引起磷灰石(U-Th)/He年龄分散的原因之一.
TS1374石英正长岩锆石(U-Th)/He年龄约为320 Ma(另文发表).该区花岗闪长岩岩体黑云母坪39Ar-40Ar年龄为263.4±0.6 Ma(王博等,2007),4粒磷灰石(U-Th)/He年龄分别为226.7 Ma、191.3 Ma、176.8 Ma及207.8 Ma(平均年龄为201 Ma),小于该区锆石(U-Th)/He年龄及黑云母39Ar-40Ar年龄.热史模拟结果见图6,热史可分为两个阶段,即~220 Ma之前的快速冷却阶段和~220 Ma之后的缓慢冷却阶段,表明了该岩体自~220 Ma以来处于缓慢隆升剥蚀阶段,这可能与该岩体处于盆地边缘相对较低的部位(低海拔)及受后期构造运动事件影响较小有关.
5 讨论
天山造山带经历了古生代、中生代及新生代多旋回、复杂的构造和岩浆作用叠加、改造过程.磷灰石(U-Th)/He年龄数据热演化史模拟表明该区至少存在3期快速隆升剥蚀事件,分别起始于90 Ma、13 Ma及5 Ma,这3期隆升剥蚀事件在区域上具有广泛的可对比性.中天山拉尔墩黑云母花岗岩磷灰石裂变径迹年龄89 Ma(王彦斌等,2001)记录了中天山晚白垩世隆升剥蚀事件,库车盆地花岗岩裂变径迹分析表明该地区晚白垩世隆升剥蚀事件发生在89 Ma前(贾承造等,2003).De Grave等(2013)及Glorie等(2010)利用磷灰石裂变径迹及(U-Th)/He技术研究吉尔吉斯段天山岩体获得了110~90 Ma冷却年龄,天山那拉提山脉东部也获得相似磷灰石裂变径迹及(U-Th)/He年龄(Jolivet et al.,2010;Dumitru et al.,2001).库车盆地北缘碎屑岩磷灰石裂变径迹研究表明天山存在~90 Ma晚白垩世快速隆升剥蚀事件(Hendrix et al.,1994;罗梦等,2014;杜治利和王清晨,2007).柴窝堡盆地、库车盆地及巴伦台裂变径迹数据表明天山在~96 Ma发生了较快的抬升剥蚀(杜治利和王清晨,2007).吉尔吉斯境内的西天山磷灰石裂变径迹研究表明该地区早白垩世-古近纪经历过一次快速抬升作用(Dobrestsov et al.,1996).陈正乐等(2006)根据西天山察汗乌苏山石炭纪火山岩的裂变径迹数据分析揭示该地区存在110~80 Ma的快速隆升剥蚀事件.吐鲁番—哈密盆地南部造山带的磷灰石裂变径迹年龄数据表明构造抬升发生在 88~97 Ma(郭召杰等,2002).这些研究成果与科克苏河磷灰石(U-Th)/He记录的年龄一致.另一方面天山山脉中普遍缺乏了白垩系的沉积,这与晚白垩世区域性隆升剥蚀有关,如库车坳陷乃至整个塔里木盆地东部地区缺失上白垩统,古近系直接覆盖在下白垩统之上;柴窝堡盆地普遍缺失上白垩统(曹守连和何登发,1997);吐哈盆地上、下白垩统之间的角度不整合,也揭示在白垩纪时期发生过大面积的隆升剥露作用.上述研究表明整个天山山脉在晚白垩世存在一期强烈的抬升剥蚀事件,该期构造事件可能是青藏高原地区Kohistan-Dras岛弧与拉萨地体碰撞远距离效应的结果.
QTQt软件及磷灰石(U-Th)/He数据模拟揭示了科克苏河角闪黑云母花岗岩快速抬升剥蚀发生在~13 Ma.在天山其他地区,关于~13~10 Ma的快速隆升剥蚀时间已有相关报道.磷灰石裂变径迹记录了西天山山前带快速逆冲抬升剥蚀的时间为13.6±2.2(1σ)(Sobel and Dumitru,1997);根据磁性地层及地质年代学数据研究表明吉尔吉斯西段的天山快速隆起开始于~11 Ma(Bullen et al.,2001;Sobel et al.,2006).另外,根据磁性地层和岩石磁学研究奎屯河沉积剖面表明北天山在~10 Ma快速抬升(Charreau et al.,2005).Abdrakhmatov等(1996)和 Reigber 等(2001)通过GPS测量现今地壳缩短率推算天山新生代重新活动起始于~10 Ma.Charreau等(2006)通过对克拉苏—依奇克里克构造带南部牙哈剖面详细调查,认为中新世沉积地层沉积速率快速增大,指示了天山地区抬升剥蚀时间为~11 Ma.刘志宏等(2000)利用生长断层相关褶皱理论模型研究克拉苏背斜,认为变形时间起始于新近系康村组(13.5~5.9 Ma)沉积期,即约为13.5 Ma.库车盆地构造地质调查、地震地层分析及古地磁定年研究表明天山在~13 Ma开始加速变形(Zhang et al.,2014).Yu等(2014)根据磷灰石(U-Th)/He数据模拟揭示库车盆地克拉苏—依奇克里克构造带北缘边界断层上盘快速抬升剥蚀发生在~10 Ma.这些研究都表明在中新世(~13~10 Ma)天山地区(包括南天山、中天山及北天山)整体抬升,且抬升范围和强度有增大的趋势,同时这次抬升事件导致了科克苏河地区快速隆升剥蚀事件.
磷灰石(U-Th)/He年龄及热史模拟研究表明科克苏河韧性剪切带晚中新世快速隆升剥蚀的起始时间为~5 Ma,这一重要的隆升剥蚀事件在天山南北缘有地层学、古地磁学及热年代学记录.磷灰石(U-Th)/He数据研究库车盆地坎亚肯背斜及吐孜洛克背斜表明~5~6 Ma存在一期快速隆升剥蚀事件(喻顺等,2014;Yu et al.,2014);利用塔北隆起磷灰石(U-Th)/He数据模拟反演表明其物源区天山快速隆升剥蚀发生在~8~5 Ma(Qiu et al.,2012).库车盆地克拉苏—依奇克里克构造带一些钻井(如吐孜2,克拉204)缺乏了库车组及其上覆地层,这表明该构造带隆升剥蚀起始时间约为5.9 Ma.Huang等(2006)及Zhang等(2014)研究库车盆地磁性地层、岩石磁及平衡剖面表明盆地沉积速率在~7~6.5 Ma显著增加,并认为南天山地区在~7~6.5 Ma快速隆升剥蚀.天山南北缘山前盆地古地磁学、生长地层及岩层几何形态等相结合研究显示天山在7~6 Ma快速隆升剥蚀(Sun et al.,2004,2009;Sun and Zhang,2009).
然而,这期隆升剥蚀事件并没有被天山地区磷灰石裂变径迹数据记录(Dumitru et al.,2001;Hendrix et al.,1994;Sobel and Dumitru,1997;杜治利等,2007;罗梦等,2012;杨庚和钱祥麟,1995;贾承造等,2003;马前等,2006),其原因可能是相对于天山新生代晚期的快速抬升,磷灰石裂变径迹退火温度显得“过高”,当构造抬升至较浅位置后,磷灰石裂变径迹已不能记录“更低温度”的抬升历史(图7a),地表样品的磷灰石裂变径迹未能记录该区域中新世隆升剥蚀事件;对沉积盆地样品而言,相对于磷灰石裂变径迹退火温度(60~120 ℃)(Green,1988),磷灰石(U-Th)/He体系具有更低敏感温度区间(40~85 ℃),当沉积地层埋藏深度小于裂变径迹退火深度而大于磷灰石(U-Th)/He封闭温度(深度)时,磷灰石(U-Th)/He体系可记录快速隆升剥蚀事件(图 7b),这也是天山山前盆地大量裂变径迹数据只记录了物源区侏罗纪-白垩纪及渐新世等隆升剥蚀事件,而磷灰石(U-Th)/He年龄却能记录盆地原位晚中新世隆升剥蚀事件的原因(Yu et al.,2014).
图7 磷灰石裂变径迹及(U-Th)/He对晚新生代热演化史的约束示意图AHePRZ指磷灰石He部分保留区,APAZ表示磷灰石裂变径迹部分退火带.Fig.7 Sketch map of thermal history during late Cenozoic based on AFT and AHe data
总体上,科克苏河地区磷灰石(U-Th)/He年龄被解释为冷却年龄,QTQt软件模拟表明中天山南缘科克苏河地区自造山带向盆地方向隆升剥蚀起始时间变老,即韧性剪切带(靠近变质带区)在~5 Ma快速隆升剥蚀,向北扩展存在~13 Ma及~90 Ma大范围隆升剥蚀事件,而在昭苏盆地边缘区中-新生代缓慢抬升剥蚀,这不仅体现了科克苏地区隆升剥蚀的不均一性,也表明在该地区存在多期揭顶剥蚀事件.
6 结论
(1)中天山南缘科克苏河地区磷灰石(U-Th)/He年龄有效地记录了该地区中-新生代抬升剥蚀时间及温度演化信息.磷灰石 (U-Th)/He数据综合解释及热演化史模拟表明晚白垩世以来中天山南缘至少存在3期快速隆升剥蚀事件,起始时间分别为~90 Ma、~13 Ma及~5 Ma,且这3期隆升剥蚀事件在整个天山区域上具有广泛的可对比性,如中天山南缘在晚白垩世及中新世发生了强烈的快速隆升剥蚀,在南天山及北天山地区均有相对应的热年代学及地层学证据,这表明了天山地区存在多阶段整体抬升剥蚀.
(2)相对于磷灰石裂变径迹数据,磷灰石 (U-Th)/He记录了中天山南缘地质演化中最新和最近的信息,即中天山南缘在晚中新世快速隆升剥蚀起始时间为~5 Ma,隆升剥蚀厚度为~2300 m,剥蚀速率为~0.47 mm·a-1,这期隆升剥蚀事件得到了天山两侧沉积盆地古地磁学、生长地层及地层不整合等研究成果的证实.
(3)中天山南缘隆升剥蚀起始时间从天山造山带向昭苏盆地(由南向北)逐渐变老,体现了科克苏地区隆升剥蚀具有强烈的不均一性,也表明在该地区存在多期揭顶剥蚀事件.
致谢磷灰石单矿物分选工作在河北省区域地质矿产调查所实验室完成,磷灰石化学处理工作得到了张巧大研究员的帮助,U、Th元素质谱分析测试得到了中国科学院地质与地球物理研究所王非研究员、吴林博士、张炜斌等人的帮助,苏榕、徐志华等参与了野外地质调查工作,在此一并表示衷心的感谢!
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(本文编辑胡素芳)
Mesozoic and Cenozoic uplift and exhumation history of the Kekesu section in the Center Tianshan:constrained from (U-Th)/He thermochronometry
YU Shun1,CHEN Wen1*,ZHANG Bin1,SUN Jing-Bo1,LI Chao2,YUAN Xia1,SHEN Ze1,YANG Li1,MA Xun3
1 Laboratory of Isotope Thermochronology,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China 2 Key Laboratory of Re-Os Isotope Geochemistry,Chinese Academy of Geological Sciences,Beijing 100037,China 3 NO.5 Oil Production Plant,Zhongyuan Oil Field Company,SINOPEC,Puyang Henan 457001,China
The Tianshan is the main part of the Central Asian Orogenic Belt.Knowledge of Mesozoic-Cenozoic tectonothermal evolution and exhumational history is critical to better understand the deformed process and mechanism of the Central Asian Orogenic Belt.Apatite (U-Th)/He thermochronometry was applied to compile a low temperature,multi-stage thermal history of tectonic uplift and exhumation of the Kekesu section in the South of Center Tianshan.New apatite (U-Th)/He ages and modeling indicated that the Center Tianshan experienced at least three-stage events of uplift and exhumation during Mesozoic and Cenozoic,which initiated at ~90 Ma,~13 Ma and ~5 Ma,respectively.The three-stage events of uplift and exhumation were also discovered in the South and North Tianshan.Compared with apatite fission track ages,apatite (U-Th)/He ages recorded the younger event (~5 Ma)of uplift and exhumation since the Miocene,which means in the last ~5 Ma,~2300 m of overburden was removed during Cenozoic folding and thrusting with a corresponding denudation rate of ~0.47 mm·a-1.It was suggested the initial timing of uplift and exhumation gradually became younger from the South to the North by QTQt modeling of the AHe ages in the Kekesu section during the Mesozoic-Cenozoic,which is more likely that there has been differential uplift/exhumation and multistage unroofing in the South of Center Tianshan.
Center Tianshan;Kekesu;Uplift and exhumation;(U-Th)/He ages;Low-temperature thermochronology
喻顺,陈文,张斌等.2016.中天山科克苏河地区隆升剥蚀历史——来自(U-Th)/He年龄的制约.地球物理学报,59(8):2922-2936,
10.6038/cjg20160817.
Yu S,Chen W,Zhang B,et al.2016.Mesozoic and Cenozoic uplift and exhumation history of the Kekesu section in the Center Tianshan:constrained from (U-Th)/He thermochronometry.Chinese J.Geophys.(in Chinese),59(8):2922-2936,doi:10.6038/cjg20160817.
国家自然科学基金(41503058,41473053,41503057),公益性行业专项经费(201511064-2),地质矿产调查评价项目(12120113015600,DD20160123-02)和中国地质科学院基本科研业务费项目(J1625)资助.
喻顺,男,1982年生,博士,主要从事同位素热年代学研究.E-mail:yushun0722@163.com
陈文,男,1962年生,博士,研究员,博士生导师,从事同位素地质年代学研究.E-mail:chenwenf@vip.sina.com
10.6038/cjg20160817
P314,P597
2016-01-17,2016-04-01收修定稿