岩石学报  2015, Vol. 31 Issue (12): 3595-3608   PDF    
秦岭-大别-苏鲁印支造山带连接枢纽的形成时代——来自宁陕断裂带同构造花岗岩锆石U-Pb年代学的限定
李源1, 许志琴1, 裴先治2, 张健3, 赵佳楠4, 张岚4, 武勇4     
1. 大陆构造与动力学国家重点实验室, 中国地质科学院地质研究所, 北京 100037;
2. 长安大学地球科学与资源学院, 西安 710054;
3. 天津地质矿产研究所, 天津 300170;
4. 中国地质大学地球科学与资源学院, 北京 100083
摘要: 最新的研究表明,南秦岭勉略缝合带可以经宁陕左行走滑断裂带与大别苏鲁的高压/超高压变质带相连。对于这个模型,两带间的"连接枢纽"—宁陕走滑断层的活动时间是关键问题之一。研究显示宁陕断裂带是南秦岭中的一条走向近E-W的走滑剪切带,早期为左行韧性剪切变形,晚期叠加了左行脆性剪切变形。对带内千糜岩化石英片岩中的两期同构造花岗岩脉的构造地质学、岩石学和锆石U-Pb和Lu-Hf同位素研究,获得早期面理化细粒花岗岩的年龄为214.4±1.1Ma(MSWD=1.3),εHf(t)主要集中在-8.58~-0.29之间,tDM2=2.45~1.62Ga;晚期钾长花岗岩脉的年龄212.8±1.6Ma(MSWD=2.1),εHf(t)=-5.79~2.07,tDM2=2.53~1.49Ga。同位素数据表明两期花岗岩脉具有相同的岩浆源区,是古老地壳物质的再循环;晚期钾长花岗岩脉是早期花岗岩演化的产物。两期同构造花岗岩脉年龄的确定,表明宁陕左行走滑断层至少从晚三叠世中期之前就已经开始活动,而不是前人认为的早-中侏罗世或晚三叠末。尤其是宁陕左行走滑断裂带与勉略缝合带具有相同的左行韧性走滑叠加晚期脆性走滑的构造样式和活动时间,表明二者的形成可能都与古特提斯洋的斜向俯冲或者扬子板块的顺时针旋转有关。本研究成果为南秦岭的"古特提斯洋缝合带"——勉略缝合带向东经宁陕断裂带与大陆俯冲和深俯冲形成的耀岭河-桐柏-大别-苏鲁高压/超高压变质带相接提供了关键的年代学证据。
关键词: 秦岭造山带     古特提斯洋缝合带     勉略缝合带     宁陕左行走滑剪切带     同构造花岗岩    
The probability of the Mianlue suture zone, South Qinling extends to Dabie-Sulu UHP belt, East Qinling: Constraint from the activity time of Ningshan shear zone
LI Yuan1, XU ZhiQin1, PEI XianZhi2, ZHANG Jian3, ZHAO JiaNan4, ZHANG Lan4, WU Yong4    
1. State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. College of Earth Sciences and Resources, Chang'an University, Xi'an 710054, China;
3. Tianjin Institute of Geology and Mineral Resources, CGS, Tianjian 300170, China;
4. School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
Abstract: A new study suggests that the Mianlue suture zone is connected with Dabie-Sulu UHP belt by Ningshan left-lateral shear zone. Thus, the activity time of Ningshan shear zone is the key issue in this model. Structure studies revealed that Ningshan fualt is a large scale WE-trending strike-slip fault characterized by the early left-lateral ductile defomation and overlapped by a late brittle deformation in the southern Qinling Orogen. The zircon LA-MC-ICP-MS U-Pb and Lu-Hf on the syn-tectonic, foliated fine-grain granite yield an age of 214.4±1.1Ma (MSWD=1.3), εHf(t)=-8.58~-0.29, and on the syn-tectonic coarse-grain K-feldspar ganite yield an age of 212.8±1.6Ma (MSWD=2.1), εHf(t)=-5.79~2.07. The geochronologyical results defined that the deformation time of Ningshan fault is at least before Late Triassic, similar with the left-lateral shear deformation time in the Mianlue suture zone. The new age of the Ningshan fault activity time confirmed the suggestion of the Paleo-Tethys Mianlue suture zone extends to the Qinling-Dabie-Sulu UHP orogenic belt.
Key words: Qinling Orogenic Belt     Mianlue Suture Zone     Ningshan left-lateral strike-slip fault     Syn-tectonic granite    
1 引言

位于中国中央造山带中东部的秦岭-大别-苏鲁印支造山带是北中国板块与扬子陆块直接碰撞的产物(许志琴等,1986; Li et al., 1993; 张国伟等, 19952001)。李春昱等(19781980)早在40年前提出秦岭造山带中存在印支期勉略蛇绿岩带,并将其向西延伸与东昆仑的阿尼玛卿缝合带相接。之后,更多的证据表明勉略缝合带总体形成于晚古生代至中生代初(约300~200Ma):(1)勉略区段蛇绿岩中硅质岩层内放射虫动物群化石年代为晚泥盆世-早石炭世(冯庆来等,1996; 王宗起等,2009);(2)勉略蛇绿混杂岩变基性火山岩Sm-Nd同位素年龄集中在约283~219Ma(Li et al., 1996)。与基性火山岩共生的斜长花岗岩的锆石U-Pb年龄为300±61Ma(Li et al., 2004);(3)沿勉略带内及北侧发育一系列220~205Ma的俯冲碰撞型花岗岩,是勉略带向北俯冲的重要证据(孙卫东等,2000);(4)勉略带中具有岛弧性质的基性麻粒岩(李三忠等,2000),原岩时代为477Ma,角闪岩相变质时代214Ma,代表了洋壳俯冲折返的时代(梁莎等,2013);(5)西秦岭三叠系沉积岩碎屑锆石U-Pb年龄谱显示中央造山带中存在一条由古特提斯洋向北俯冲形成的三叠纪增生岩浆弧(闫臻等,2012; Yan et al., 2014)。近年来,对勉略蛇绿混杂岩带中的镁铁质岩块和花岗岩的研究,获得了一些新元古代的同位素年代学数据(张宗清等,2005; 闫全人等,2007; 李瑞保等,2009)。对这些数据,一种解释认为南秦岭不存在印支期缝合带,勉略蛇绿岩混杂岩是新元古代缝合带(夏林圻等,1996; 张传林等,2000; 李亚林等,2001;冯益民等,2004; 闫全人等,2007)。另一种观点认为印支期勉略缝合带混入了古老的物质;或者勉略缝合带经历了新元古代和印支期多期构造事件叠加(张国伟等,2004; 张宗清等,2005; 李瑞保等,2009; 许志琴等,2015)。我们认为在蛇绿混杂岩中包含古老的物质是正常的,例如在中央造山带西段的阿尼玛卿蛇绿混杂岩中,不但包含早古生代辉长岩和闪长岩(边千韬等, 19992001; 李王晔等,2007; 刘战庆等,2011),还包含了石炭纪辉长岩和玄武岩(杨经绥等,2004)。而阿尼玛卿蛇绿岩最终形成时代为三叠纪(姜春发等,2009; 许志琴等,1996; Yang et al., 1996)。

中国大陆中部构造格架另一个关键的问题:勉略缝合带如何向东延伸,一直也是秦岭造山带中印支期造山作用主要的争论之一。其中大部分突破性研究出现在20年前,张国伟等(1995200120032004)将秦岭造山带中的印支期勉略缝合带的构造属性确定为向西连接昆仑,向东经巴山弧形带延伸至大别南缘的,中国大陆最后拼接的主要结合带之一。最近十年,为数不多的研究者提出了不同的意见。有研究者认为勉略蛇绿岩带并不能沿大巴山弧形断裂带延伸(胡健民等,2008),而是被宁陕断裂切断,由勉县向东并左行位移到西峡一带与商丹缝合带拼合在一起(Meng et al., 2005)。杨经绥等(2002a; Yang et al., 2003b)提出南大别-苏鲁印支期超高压变质带可以与南秦岭勉略蛇绿岩带相连,代表秦岭造山带中发生的一次印支期的板块裂解和俯冲碰撞作用,但是研究者并没有指出两个造山带如何连接以及连接带的位置。

许志琴等(2015)基于对秦岭、桐柏地区的野外地质调查和前人的研究成果,提出勉略缝合带向 东经洋县-宁陕-湘河左行走滑剪切带、南秦岭北部的耀岭河高压变质带(Wu and Zheng, 2013)向东延至桐柏高压变质带、红安-大别高压/超高压变质带,最终抵达苏鲁高压/超高压变质带的新连接方式。对于该模式,连接秦岭-大别-苏鲁造山带与勉略缝合带的枢纽—宁陕走滑断裂的活动时代就成为一个关键的问题。本文通过对宁陕左行走滑断裂内同构造花岗岩脉的研究,配合LA-MC-ICP-MS锆石U-Pb和Lu-Hf同位素年代学,确定同构造花岗岩脉形成于晚三叠世,说明宁陕走滑断裂带至少在此之前就已经开始活动,与勉略带缝合带内中-晚三叠世左行走滑韧性剪切带一起构成了一条区域性走滑断层,将勉略古特提斯洋缝合带与秦岭-大别-苏鲁陆陆碰撞造山带连接在一起。

2 区域地质背景 2.1 秦岭印支期造山带

秦岭造山带位于中央造山带的核部,处于北中国板块和扬子板块的结合部,是由北部的早古生代造山带和南部的印支造山带拼合而成的复合造山系(Mattauer et al., 1985; 许志琴等,1988; 张国伟等, 19952001)(图 1)。印支期造山带地质记录主要保存在秦岭造山带南缘的勉略缝合带和大别-苏鲁高压/超高压变质带中(图 2)。

图 1 中央造山带构造格架示意图(据Yang et al., 2003a) N.COB-北中央早古生代造山带;S.COB-南中央中生代造山带 Fig. 1 Simplified geological map of the Central Orogenic belt,showing various continents blocks,suture zones and tectonic feature(after Yang et al., 2003a) N.COB-Northern Central Orogenic belt; S.COB-Southern Central Orogenic belt

图 2 秦岭-桐柏-大别-苏鲁造山带中的古特提斯构造格架图(据许志琴等,2015) Ⅰ-北中国板块;Ⅱ-北秦岭-北桐柏早古生代造山带;Ⅲ+Ⅳ-南秦岭-三叠纪造山带;Ⅲ1-南秦岭-大别古特提斯活动陆缘(D-T);Ⅲ2-白水江古特提斯增生混杂带;Ⅲ3a-勉略古特提斯蛇绿岩混杂带;Ⅲ3b-宁陕-湘河左行走滑剪切带;Ⅲ3c-耀岭河高压变质带;Ⅲ3d-桐柏-大别高压变质带(HP);Ⅲ3e-大别超高压变质带(UHP);Ⅳ-南秦岭-南桐柏被动陆缘;Ⅳ1-南秦岭盖层(Z-T、Pz1);Ⅳ2-南秦岭基底(Pt3);Ⅳ3-桐柏-罗田中生代杂岩(TB、LT);Ⅳ4-大巴山中生代前陆逆冲带;Ⅴ-扬子陆块(YZB);SDSZ-商丹缝合带;NXF-宁陕-湘河左行走滑断裂;TLF-郯庐断裂 Fig. 2 Paleo-Tethys tectonic framework of the Qinling-Tongbai-Dabie-Sulu Orogen(after Xu et al., 2015) Ⅰ-North China Block; Ⅱ-North Qiling Early Paleozoic Orogen; Ⅲ+Ⅳ-South Qinling Triassic Orogen; Ⅲ1-South Qiling-Dabie Paleo-Tethys active continental margin; Ⅲ2-Baishujiang Paleo-Tethys accretional belt; Ⅲ3a-Mianlue Paleo-Tethys ophiolitic mélange zone; Ⅲ3b-Ningshan left-lateral strike-slip shear zone; Ⅲ3c-Yaolinghe HP metamorphic belt; Ⅲ3d-Tongbai-Dabie HP metamorhphic belt; Ⅲ3e-Dabie UHP metamorphic belt; Ⅳ-South Qinling-South Tongbai passive continental margin; Ⅳ1-South Qinling covers(Z-T); Ⅳ2-South Qiling basement; Ⅳ3-Tongbai-Luotian Mesozoic complex; Ⅳ4-Dabaishan forel and thrust belt; Ⅴ-Yangtze Block; SDSZ-Shangdan suture zone; NXF-Ningshan-Xianhe shear zone; TLF-Tanlu fault

勉略缝合带位于秦岭造山带南缘,是由岛弧火山岩、洋脊蛇绿岩残片和混杂堆积组成的蛇绿构造混杂岩带(张国伟等, 1995200120032004; 许继锋和韩吟文,1996; 赖绍聪等, 19982003; Dong et al., 19992011; Zhang et al., 2004)。中、晚三叠世勉略洋的俯冲和华南与华北板块的碰撞在带内形成了多级次南北向逆冲推覆构造变形,碰撞后的陆内变形形成了近东西向的左行走滑韧性和脆性剪切变形(李三忠等,2002; Zhang et al., 2004; Li et al., 2007)。

勉略缝合带的时代一直存在争议,一种观点认为勉略蛇绿岩代表的洋盆的形成时代始于泥盆纪中期,至早-中三叠闭合(张国伟等, 19952001; Li et al., 1996; 冯庆来等,1996; 孙卫东等,2000; 赖绍聪等,2003; Dong et al., 2011),与南秦岭晚古生代-三叠纪增生楔(王宗起等,2009; 王涛等,2011)一起构成了西秦岭古特提斯洋的板块边界(许志琴等,2015)。证据主要包括:(1)勉略蛇绿岩硅质岩中放射虫化石年代为晚泥盆世-早石炭世(冯庆来等,1996; 王宗起等,2009);(2)勉略蛇绿混杂岩中变基性火山岩Sm-Nd同位素年龄集中在约283~219Ma(Li et al., 1996);斜长花岗岩的锆石年龄为300±81Ma(Li et al., 2004);(3)勉略带内及北侧发育220~205Ma的俯冲碰撞型花岗岩,是勉略带向北俯冲的重要证据(孙卫东等,2000);(4)勉略带中具有岛弧性质的基性麻粒岩(李三忠等,2000)的退变质时代214±11Ma代表了洋壳俯冲折返的时代(梁莎等,2013)。近年来,对勉略蛇绿混杂岩带中镁铁质岩块和花岗岩的研究,还获得了一些新元古代的同位素年代学数据(张宗清等,2005; 闫全人等,2007; 李瑞保等,2009),表明勉略带中不仅存在晚古生代洋盆,而且还可能存在更古老的洋盆物质记录。

中央造山带中另一印支期壮观事件是秦岭大别造山带东端的大别-苏鲁造山带高压/超高压变质带,由华北和扬子板块陆陆碰撞形成(Wang et al., 1989; Li et al., 1993; Zhang and Liou, 1998; 杨经绥等,2002b)。最近的研究表明,南秦岭以东的桐柏高压变质带高压榴辉岩峰期变质年龄为255~256Ma,折返冷却年龄为238~217Ma(Liu et al., 20082010)。红安高压/超高压变质带超高压峰期变质年龄在239~223Ma(Wu et al., 2008; Zhou et al., 2011),折返冷却年龄在216~212Ma(Liu et al., 2004b; Wu et al., 2011; Li et al., 2011)。位于商丹缝合带以南赵川地区的陆内深层滑脱构造形成的蓝片岩钠闪石和多硅白云母Ar-Ar年龄分别为232±5Ma和216±7Ma(Mattauer et al., 1985; 许志琴等,1988)。高压/超高压变质带形成-折返时间上的一致性和空间上的关联性,表明大别-苏鲁地区的超高压变质带可以向东一直延伸到耀岭河地区的高压变质带(Wu and Zheng, 2013; 许志琴等,2015)。

2.2 洋县-宁陕左行走滑断裂

洋县-宁陕断裂是发育在南秦岭构造带内部的一条近东西向断裂,宽约2~2.5km,向西与青川-阳平关断裂相连,向东与山阳-凤县断裂相接,再向东在西峡一带并入商丹断裂带,将南秦岭构造带划分为北西构造带和南东构造带两个不同构造的单元(胡健民等,2008)。带内发育大量以左行走滑韧性剪切变形,平行于断裂带的A型褶皱和近水平的拉伸线理(图 3a,b)(胡健民等,2008; 陈虹等,2014; 许志琴等,2015)。Meng et al.(2005)认为宁陕断裂于晚三叠世末就开始活动。胡健民等(2008)获得带内韧性变形形成的黑云母和白云母的40 Ar-39 Ar年龄在169~162Ma,认为属于秦岭造山带碰撞后陆内变形阶段产物。陈虹等(2014)获得断裂带内同构造花岗岩脉锆石SHRIMP U-Pb年龄186±4.0Ma。洋县-宁陕断裂晚期叠加了时代约为早白垩世的脆性变形(李建华等,2010)。宁陕左行走滑断层的形成被认为是印支期华北和扬子碰撞后陆内变形阶段的产物,与中晚侏罗世-早白垩世期间扬子地块的顺时针旋转相关(Meng et al., 2005; 胡健民等,2008)。

3 宁陕断裂同构造花岗岩和伟晶岩脉的地质特征

本文的研究区位于宁陕县城西汤平金水河,由志留-泥盆纪千糜岩化的石英片岩组成,变形强烈,面理SE向,近直立,拉伸线理近水平(图 3b)。剪切带内地层内的拖拽应变显示出明显的高温左行走滑特征(图 3a)。薄片下带状石英由亚颗粒旋转而产生的小的石英动态重结晶而成(图 4a,d),石英重结晶颗粒边界发生了明显的迁移,显示出中-高温的显微构造特征。石英的波状消光则显示出岩石低温变形特征。

图 3 宁陕左行走滑剪切带中的变形特征 (a)宁陕剪切带中同构造钾长花岗岩脉的左行走滑运动指向;(b)宁陕剪切带中E-W向直立面理和水平拉伸线理;(c)宁陕剪切带中同构造面理化花岗岩;(d)面理化细粒花岗岩的E-W流动线理 Fig. 3 Field photographs of the Ningshan lefe-lateral strike-slip shear zone (a)syn-tectonic granitic veins show the left-lateral shear sense;(b)the sub-vertical E-W foliation and horizontal lineation in the Ningshan shear zone;(c)syn-tectonic foliated,fine-grain granite;(d)the foliated fine-grain granite show the E-W lineation

带中可见两期粒度不同的花岗脉。早期的细粒花岗岩岩脉(XSQ2.8.1)(图 3c),最宽处近2m,岩体内面理和流动线理与围岩面理、拉伸线理产状一致(图 3d),但变形程度明显弱于围岩云母石英片岩。薄片下细粒花岗岩岩脉主要由细粒的长石、石英和黑云母组成(图 4b),石英颗粒具有波状消光,亚颗粒化,定向排列构成了花岗岩的面理和流动线理(图 4e)。长石内部塑性变形不明显,近平行面理定向排列,无明显动态重结晶。晚期粗粒钾长花岗岩(XSQ2.8.2)主要由斜长石、钾长石和石英组成(图 4c),产在细粒花岗岩边部,或者顺石英片岩层侵入,局部可见穿插早期细粒花岗岩现象,脉体产状同样受区域性面理的控制。以上特征表明两期花岗岩均为走滑带内的同构造花岗岩。

图 4 宁陕左行走滑剪切带中的岩石学和显微构造特征 (a)石英片岩显微照片,箭头指示拉伸线理方向;(b)早期面理化细粒花岗岩同构造花岗岩显微照片;(c)晚期钾长花岗岩脉显微照片;(d)石英片岩带状石英(Q)颗粒,有亚颗粒旋转形成的小石英动态重结晶组成;(e)面理化细粒花岗岩不规则的石英颗粒,通过动态重结晶颗粒边界迁移形成;(f)钾长花岗岩石英的波状消光.Ms-白云母;Qz-石英;Pl-斜长石;Kf-钾长石 Fig. 4 Micrograph of the syn-tectonic granite and microstructure of the ductile (a)micrograph of quartz schist,the arrowhead indicating the direction of lineation;(b)micrograph of syn-tectonic fine-grain granite;(c)coarse-grain K-feldspar granite;(d)ribboned quartz grains composed of dynamically recrystallized granis formed by subgrain rotation;(e)dynamiclly recrystllized quarzt grains with irregular boundaries formed in response to grain boundary migration;(f)wavy extinction quartz in the coarse-grain K-feldspar granite. Ms-muscovite;Qz-quartz;Pl-plagioclase;Kf-K-feldspar
4 两期花岗岩锆石U-Pb和Lu-Hf同位素年代学

选择早期细粒花岗岩样品XSQ2.8.1和晚期钾长花岗岩脉样品XSQ2.8.2进行了锆石U-Pb和Lu-Hf同位素年代学测试。测试在天津地质矿产研究所完成,所用仪器为Neptune多接收等离子质谱和Newwave UP193紫外激光剥蚀系统(LA-MC-ICP-MS),U-Pb同位素测试方法及流程见李怀坤等(2010),Lu-Hf同位素测试方法及流程见耿建珍等(2011)。测试结果见表 1表 2

表 1 宁陕左行走滑剪切带中同构造花岗岩(XSQ2.8.1、XSQ2.8.2)锆石U-Pb同位素分析结果 Table 1 LA-MC-ICP-MS U-Pb isotopic data of zircon from the syn-tectonic granite(XSQ2.8.1,XSQ2.8.2)in the Ningshan left-lateral strike-slip fault

表 2 宁陕左行走滑剪切带中同构造花岗岩(XSQ2.8.1、XSQ2.8.2)锆石Lu-Hf同位素分析结果 Table 2 LA-MC-ICP-MS Lu-Hf isotopic data of zircon from the syn-tectonic granite(XSQ2.8.1,XSQ2.8.2)in the Ningshan left-lateral strike-slip fault
4.1 锆石U-Pb同位素

细粒花岗岩样品XSQ2.8.1的锆石镜下多为无色透明,短柱状、自形-半自形晶,少量为外形不规则的他形锆石。锆石粒径变化在100~200μm之间。大多数锆石表面光洁且内部较纯净,几乎没有包裹体和裂纹。阴极发光图像(CL)显示,多数锆石具有原生的岩浆振荡环带,少数具有核边构造锆石边部也显示出明显的环带构造(图 5a)。上述特征是岩浆成因锆石的典型特征(Vavra et al., 1999)。XSQ2.8.1样品共测试32点。分布在谐和线之上的27个点,Th/U=0.14~1.1,获得206Pb/238U加权平均年龄214.4±1.1Ma(MSWD=1.3)(图 5b),代表花岗岩结晶年龄。还获得5核部年龄206Pb/238U年龄分别为786Ma、607Ma、561Ma、441Ma和419Ma,代表花岗质岩浆捕获的古老锆石或者是花岗岩原岩残留锆石的年龄。

图 5 宁陕左行走滑剪切带中同构造花岗岩锆石阴极发光图像(a、c)和锆石UPb谐和年龄图(b、d) Fig. 5 CL images of zircon (a,c) and LA-MC-ICP-MS U-Pbzircon concordia diagrams (b,d) for the syn-tectonic granite in the Ningshan left-lateral strike-slip shear zone

钾长花岗岩脉样品XSQ2.8.2锆石粒径变化在100~200μm之间,自形-半自形、少数呈碎片状他形,锆石表面光洁且内部较纯净,阴极发光图像(CL)显示出细密的岩浆振荡环带,个别锆石CL呈现出斑杂状,显示曾受到后期变质流体的蚀变和改造作用的影响(图 5c)。样品共测试32点,7个不在谐和线上测点显示出明显的Pb丢失。25个测点落在谐和线上,Th/U=0.5~1.49,获得206Pb/238U加权平均年龄212.8±1.6Ma(MSWD=2.1)(图 5d),代表钾长花岗岩脉的结晶年龄。还获得4个锆石206Pb/238U年龄分别为433Ma、402Ma、419Ma和413Ma,显示出明显的Pb丢失,可能代表花岗质岩浆捕获的古老锆石。

4.2 锆石Lu-Hf同位素

分别对样品XSQ2.8.1和XSQ2.8.2锆石进行了Lu-Hf 同位素分析。Lu-Hf测点紧邻锆石U-Pb年龄分析点(图 5a,c)。两个样品分析点的176Lu/177Hf比值均很低,在0.0004~0.0018之间,表明这些锆石在形成后基本没有放射性成因Hf的积累,所测定的176Hf/177Hf比值代表其形成时的Hf同位素组成。 XSQ2.8.1样品共测试20点,176Hf/177Hf比值为0.281871~0.282766,对应的εHf(t)在-19.06~4.52之间,二阶段模式年龄tDM2集中在2.45~1.27Ga之间(图 6a,b),表明该岩石主要来自古老地壳岩石的部分熔融。XSQ2.8.2样品共测试15点,176Hf/177Hf比值为0.282361~0.282702,对应的εHf(t)在-5.79~2.07,二阶段模式年龄tDM2集中在2.53~1.49Ga之间(图 6a,b)。锆石Lu-Hf同位素数据表明两期花岗岩脉具有相同的源区,主要来自古老地壳岩石的部分熔融,晚期钾长花岗岩脉是早期花岗岩浆演化的产物。

图 6 宁陕左行走滑剪切带中同构造花岗岩锆石U-Pb年龄-εHf(t)图解(a)和Hf二阶段模式年龄(tDM2)图解(b) Fig. 6 Diagram of U-Pb age vs. εHf(t)(a) and tDM2 spectra(b)of zircons from the syn-tectonic granite in the Ningshan left-lateral strike-slip shear zone
5 讨论与结论 5.1 宁陕左行走滑断层的活动时间

由于白云母和黑云母的40Ar-39Ar体系的封闭温度较低,因此宁陕断裂带内的云母40Ar-39Ar年龄169~162Ma只代表了断裂带中变形岩石的冷却年龄(胡健民等,2008)。前文研究表明,汤平金水河千糜岩化的石英片岩中的两期花岗岩均为走滑带内的同构造花岗岩,可以确定的是宁陕左行走滑的活动时代至少不会晚于花岗岩锆石U-Pb时代。利用同构造花岗岩锆石U-Pb年代学来制约宁陕左行走滑的活动时代相对于变形矿物的40Ar-39Ar年龄更为准确。陈虹等(2014)曾获得断裂带内宁陕县渔湾镇含石榴石黑云母片岩中同构造花岗岩脉锆石SHRIMP U-Pb年龄186±4.0Ma。从区域构造和围岩的特征可以看出该地区变质程度较低,并没有受剪切作用的同构造混合岩化作用形成,可以排除该岩脉是同构造淡色熔融脉体。从野外产状可以看出(见陈虹等,2014中的图 11),该花岗岩脉与围岩片理成小角度侵入接触,岩体形状与区域性面理并不一致,虽然脉体边界处可见围岩片理发生牵引褶皱,但是花岗岩内部无变形。因此,该岩脉可能是走滑断裂同构造变形较晚期形成的花岗岩脉,并受到了更晚期变形作用的影响。

本文在位于宁陕县城西汤平金水河千糜岩化的石英片岩中发现两期产状、粒度、以及岩石学特征不同的花岗岩脉体。早期的细粒花岗岩岩脉产状、岩体内面理和流动线理与围岩石英片岩的面理、拉伸线理产状一致(图 3a),产状完全受到剪切带的控制。薄片中较大的带状石英由亚颗粒旋转而产生的小的石英动态重结晶而成,重结晶的石英颗粒边界发生了明显的迁移(图 4e),这些结构是中-高温(400~700℃)条件下典型的变形结构。另外,石英的波状消光表明花岗岩脉还受到了后期的低温变形(<300℃)(图 4e)。从显微构造看,该花岗岩脉与围岩石英片岩具有一致的剪切应变(图 4b,e),表明其受到了从高温(>400℃)到低温(<300℃)的连续的剪切变形。上述特征表明面理化细粒花岗岩具有典型的剪切带同构造花岗岩的特征(Tapponnier et al., 1990; Huton and Reavy, 1992),其形成年龄214.4±1.1Ma可以代表剪切带的活动时间。晚期粗粒钾长花岗岩产在早期细粒面理化花岗岩边部,或者顺石英片岩层(S1)侵入,剖面上可以看到穿切早期面理化花岗岩的现象,其产状也受到围岩石英片岩的控制。虽然野外宏观上没有观察到粗粒钾长花岗岩的变形特征,但是薄片中同样发育石英动态重结晶亚颗粒和波状消光现象(图 4f)。上述特征表明该岩脉是早期同构造花岗岩浆演化的结果,两期花岗岩脉锆石具有相同的Hf同位素变化范围的也支持同样的结论。因此,其形成年龄212.8±1.6Ma同样可以限定剪切带活动的时间。

结合前人获得的同位素年代学数据来看,宁陕左行走滑剪切带可能经历了较长时间的生长过程,至少在晚三叠世早期就已经开始活动,并一直持续到了早-中侏罗世。而不是前人认为的宁陕左行走滑剪切自早-中侏罗世或晚三叠末才开始活动(Meng et al., 2005; 胡健民等,2011; 陈虹等,2014)。

5.2 勉略缝合带与大别-苏鲁高压-超高压变质带的连接

中央造山带的印支造山带是中国大陆最后拼接的主要结合带之一。其东端发育大别-苏鲁造山带高压/超高压变质带,是北中国板块和扬子板块陆陆碰撞的产物(Wang et al., 1989; Li et al., 1993; Zhang and Liou, 1998; 杨经绥等,2002b),两个板块的碰撞在带内形成了强烈的左行走滑剪切变形(Ratschbacher et al., 2003)。对含柯世英锆石的U-Pb定年结果显示超高压变质发生在240~225Ma(Yang et al., 2003a; Liu et al., 2004a2006)。超高压岩片折返时间在225~205Ma(Liu et al., 2006; Zheng et al., 2006)。最新的研究表明,桐柏-红安高压变质带的的变质年龄在中三叠世,折返年龄在中-晚三叠世(Liu et al., 2004b200820102013; Wu et al., 20082011; Zhou et al., 2011; Li et al., 2011),表明大别-苏鲁地区的超高压变质带向西可以经桐柏-红安高压变质带与耀岭河地区高压变质带连接(Wu and Zheng, 2013; 许志琴等,2015)。

中央造山带中段的印支期勉略缝合带是北中国板块与南中国板块最后的拼接的结合带(张国伟等, 199520012003; Zhang et al., 2004)。勉略古特提斯洋盆俯冲形成的岛弧在242~220Ma(Li et al., 19962004)。闫臻等(2012)和Yan et al.(2014)对西秦岭三叠系沉积岩碎屑锆石的研究,表明秦岭在三叠纪就已经存在一条由古特提斯洋向北俯冲形成的增生岩浆弧。Zhang et al.(2002)曾获得勉略带内基性麻粒岩黑云母40Ar-39Ar年龄199.7±1.7Ma,全岩Sm-Nd等时线年龄206±55Ma;梁莎等(2013)获得安子山麻粒岩角闪岩相退变质年龄214Ma。勉略缝合带北部发育的花岗岩年龄则表明北中国板块的从碰撞到碰撞后伸展在245~200Ma(孙卫东等,2000; 胡健民等,2004; 金维浚等,2005; 张成立等, 20052008)。勉略缝合带和大别-苏鲁造山带高压/超高压变质带形成时间上的一致性表明其可能是一条带(杨经绥等,2002a; Yang et al., 2003b)。

张国伟等(1995)李三忠等(2002)根据地层组合的相似性将勉略缝合带向东南与城口-房县-大巴山弧形断裂相连,直至大别南缘。最新的研究表明城口-房县-大巴山的构造样式与勉略缝合带并不相同(胡健民等,2008)。从空间的构造样式上看,勉略带缝合带早期变形为俯冲-碰撞形成的多级次南北向逆冲推覆构造的样式,晚期为近陆陆碰撞造山形成的东西向的左行走滑韧性和脆性剪切带,由古特提斯洋和扬子板块的斜向俯冲形成(李三忠等,2002; Zhang et al., 2004; Li et al., 2007)。陈虹等(2010)获得勉略缝合带左行韧性剪切带中白云母40Ar-39Ar年龄223±2Ma,表明陆陆碰撞斜向引发的走滑变形至少在此之前已经开始。而宁陕左行走滑断裂的确认(Meng et al., 2005; 胡健民等,2011; 许志琴等,2015),表明勉略缝合带与宁陕断裂带具有相同的左行韧性走滑的构造样式。本文获得宁陕剪切带中同构造花岗岩214.4±1.1Ma,表明宁陕断裂左行走滑与勉略缝合带左行走滑活动期也一致,二者可能都与古特提斯洋的斜向俯冲(李三忠等,2001; Zhang et al., 2004; Li et al., 2007)有关,或者与扬子板块的顺时针旋转有关(Zhao and Coe, 1987; Meng et al., 2005)。

相同的构造样式和活动时间表明勉略缝合带带内的左行走滑可以向东可以与宁陕左行走滑拼合,使南秦岭的“古特提斯洋缝合带”——勉略缝合带向西与东昆仑的阿尼玛卿缝合带相连,向东经宁陕断裂带与大陆俯冲和深俯冲形成“耀岭河-桐柏-大别-苏鲁”高压-超高压变质带连接,构成了横穿中国大陆的古特提斯缝合带(许志琴等,2015)。

致谢 天津地质矿产研究所同位素实验室周红英教授级高级工程师、耿建珍高级工程师在LA-MC-ICP-MS实验过程中给予了指导,保证了数据的精确;张泽明研究员和另外一名匿名审稿人对本文提出了宝贵的修改意见;在此深表谢意!

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