岩石学报  2018, Vol. 34 Issue (5): 1347-1358   PDF    
哀牢山-红河剪切带中段多阶段新生代花岗岩脉:同位素年代学及对于剪切应变型式转变时间的约束
陈玮1 , 刘俊来1,2 , 樊文魁1 , 冯嘉1 , 闫佳鑫1 , 陶海南1     
1. 中国地质大学(北京)地球科学与资源学院, 北京 100083;
2. 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083
摘要:哀牢山-红河剪切带是新生代印度板块与欧亚板块碰撞过程中发育的大规模走滑型剪切带,其发育对于碰撞过程中印支地块的南东向逃逸以及藏东南地区构造格局的形成具有重要的贡献。与剪切带演化相关,伴随发育多阶段花岗岩脉就位,它们为限定剪切变形时限、阐明剪切作用属性提供了重要证据。本文在野外观察基础上,应用显微构造和EBSD石英c-轴组构分析查明花岗岩脉的构造特点与应变型式,同时采用锆石LA-ICP-MS测年方法获得岩脉侵位与结晶年龄。年龄分析结果表明,岩脉年龄分别为27.09±0.48Ma、25.17±0.23Ma和25.16±0.50Ma,其中年龄为27.09±0.48Ma的花岗岩脉具有糜棱岩化现象,其变形特征体现为中温变形后叠加低温变形,且剪切变形形式由一般剪切转换为简单剪切;年龄为25.17±0.23Ma的花岗岩脉表现出同剪切晚期构造特征,且具有较低温度简单剪切变形特点;25.16±0.50Ma的切穿糜棱叶理,矿物未见变形,可能代表剪切期后岩脉。结合区域构造,推测剪切方式由纯剪为主的剪切向由单剪为主的剪切转换发生在27Ma和25Ma之间,哀牢山-红河剪切带中段在约25Ma走滑运动结束。
关键词: 哀牢山-红河剪切带     花岗质岩脉     变形显微构造     石英c轴组构     应变型式转换    
Multiple stages of granitic dykes along the Ailao Shan-Red River shear zone: Constraints on timing of switch of shear strain types.
CHEN Wei1, LIU JunLai1,2, FAN WenKui1, FENG Jia1, YAN JiaXin1, DAO HaiNam1     
1. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;
2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
Abstract: The Ailao Shan-Red River shear zone is a large scale strike slip fault resulted from collision between the Indian and Euro-Asia plates in Cenozoic. The formation and evolution contributed to the generation of the present structural framework and the southeastward extrusion of the Indochina block during the collision. Multiple stages of granitic dykes were emplaced in association with shearing along the shear zone. They provide evidence constraining the timing and nature of shearing. Based on field observations, microstructural and EBSD quartz c-axis analysis were conducted to clarify structural properties and strain patterns of the granitic intrusions. LA-ICP-MS zircon U-Pb dating results of the granitic emplacement and crystallization suggest that the three dated dykes were emplaced at 27.09±0.48Ma, 25.17±0.23Ma and 25.16±0.50Ma, respectively. The dyke with age of 27.09±0.48Ma possesses evidence for intensive low temperature mylonitization superimposed on an earlier middle temperature deformation. Fabric and microstructural analysis reveals an obvious change from pure-shear dominated to simple shear-dominated strains. The dyke with an age of 25.17±0.23Ma exhibits structures and fabrics characteristics of late syn-shearing dykes and of simple shearing at low temperatures. The dyke with age of 25.16±0.50Ma is a post-shearing dyke that cuts across the mylonitic foliations of the host rocks and show very weak mineral deformation. Combining with regional tectonic analysis, it is expected that shearing along the Ailao Shan-Red River shear zone transformed from pure shear to simple shear between 27Ma and 25Ma, and the cessation of strike-slip shearing along the middle segment of the Ailao Shan-Red River shear zone occurred at ca. 25Ma.
Key words: Ailao Shan-Red River shear zone     Granitic dyke     Deformation microstructures     Quartz c-axis fabrics     Strain pattern variation    

哀牢山-红河剪切带(图 1)是青藏高原东南缘的一条重要线性构造带,其发育调节了印度板块和欧亚板块碰撞引发的印支地块大规模南东向逃逸(Tapponnier et al., 1982, 1986; England and Houseman, 1986; Searle, 2006; Searle et al., 2010; 刘俊来等, 2006; Chen et al., 2015)。剪切带起始与终止时间、剪切过程中岩浆活动,变质事件与成矿作用、区域变形过程中变形的阶段划分及控制因素、不同变形阶段岩石矿物流变特征与变形机制等,是多年来的研究热点(Houseman and England, 1986, 1993; England and Molnar, 1990, 1997; Chung et al., 1997; 杨振宇等, 2001; Gilley et al., 2003; Searle, 2006; 刘俊来等, 2006; Yeh et al., 2008; Searle et al., 2010; Deng et al., 2014, 2017; Chen et al., 2015; 史鹏亮等, 2015; Deng and Wang, 2016; Zhang et al., 2017)。

图 1 哀牢山剪切带及其邻区大地构造简图 (据Leloup et al., 1995; 刘俊来等, 2006) Fig. 1 Skecth of tectonic map of Ailao Shan-Red River shear zone and adjacent area (modified after Leloup et al., 1995; Liu et al., 2006)

沿着哀牢山-红河剪切带存在大量不同时期的岩浆活动,包括新元古代岩浆活动(Cai et al., 2014; 戚学祥等, 2010; Qi et al., 2014, 2016; Chen et al., 2017a)、二叠纪-三叠纪花岗岩(Jian et al., 2009a, b; Fan et al., 2010; 刘俊来等, 2006; 刘翠等, 2011; 王舫等, 2011; Lai et al., 2014a, b; Liu et al., 2014; 冀磊等, 2017; Wu et al., 2017a)、以及大量新生代花岗岩(Schärer et al., 1990, 1994; Gilley et al., 2003; Sassier et al., 2009; Cao et al., 2011a, b; Tang et al., 2013; Liu et al., 2015b; Chen et al., 2017b)。它们多数遭受了不同程度的糜棱岩化作用的改造(Cao et al., 2011a, b; Tang et al., 2013; Liu et al., 2015b; Chen et al., 2016)。综合具有不同变形特点的花岗质岩石结晶年龄、岩石宏观、微观变形特点与组构特征、以及大量热年代学研究结果(见图 2a),目前基本限定了哀牢山-红河剪切带剪切变形时间在约31~21Ma(Leloup and Kienast, 1993; Leloup et al., 1995, 2001; Nam et al., 1998; Jolivet et al., 2001; Yeh et al., 2008; Anczkiewicz et al., 2007; Searle et al., 2010; Cao et al., 2011a; Liu et al., 2012; Tang et al., 2013; Chen et al., 2017b; Wu et al., 2017b)。然而,最近的详细研究揭示该剪切带剪切作用具有显著的阶段性,即早阶段的纯剪剪切为主的一般剪切变形和晚阶段以单剪剪切变形为主的一般剪切变形(Liu et al., 2012; Wu et al., 2017b),但是, 迄今对于变形体制转换时间仍没有明确的限定。

图 2 哀牢山杂岩构造简图(a,据Liu et al., 2012, 2015b)和元江-墨江地质简图(b,据云南地质矿产勘查开发局区域地质调查所,1996修编) Fig. 2 Structural outline of the Ailao Shan complex (a, after Liu et al., 2012, 2015b) and sketch geological map of Yuanjiang-Mojiang (b)

① 云南地质矿产勘查开发局区域地质调查所. 1996. 1/50000红光农场幅地质图

通过研究不同变形阶段侵位与剪切变形相关联花岗岩脉的侵位时代、变形特点的综合分析是解决上述问题的有效途径之一。本文选取哀牢山-红河剪切带中段元江-墨江剖面中具有不同变形特点的新生代花岗质岩脉为研究对象,基于宏观构造、显微构造与石英c轴组构分析,结合应用岩脉锆石U-Pb定年以期对哀牢山-红河剪切带剪切应变型式的转变提供约束,进而阐明不同阶段剪切变形的区域构造意义。

1 区域地质背景与哀牢山中段构造特点

哀牢山-红河剪切带位于青藏高原东南缘,长度大于1000km,宽度达20~30km,向南延伸进入南中国海。沿哀牢山-红河剪切带出露四个变质杂岩体,自北向南依次是雪龙山杂岩、点苍山杂岩、哀牢山杂岩和大象山杂岩(中国境内称瑶山杂岩),它们记载了哀牢山剪切带变形演化的历史。

位于哀牢山-红河剪切带中段的哀牢山杂岩北起云南南涧,其北段走向约290°,在元江咪哩附近发生转向为310°,之后在大平再次转向约290°,并向南经金平延伸进入越南境内范式坂地区。杂岩体平面上呈北西段狭窄,南东段开阔的楔形形态(刘俊来等, 2006),由深变质岩带和浅变质岩带两部分组成。深变质岩带位于东侧,与扬子地块沉积岩系之间以红河断裂分隔;浅变质岩带位于西侧,与毗邻的印支地块间发育九甲-安定断裂(Leloup et al., 1995; 刘俊来等, 2006; Liu et al., 2012)。在哀牢山杂岩体内,深变质岩带与浅变质岩带之间存在明显不连续性,该不连续面曾命名为哀牢山断裂,其两侧具有相似变形特点的岩石具有强烈的变质程度差异。

位于哀牢山杂岩带中段的元江-墨江剖面(图 2b)中,深变质岩带东起元江县红光农场,西止于咪哩-热海附近。深变质带岩石统称哀牢山岩群,自西向东依次划分为小羊街岩组、清水河岩组和阿龙岩组三部分,各岩组之间接触关系为断层接触。总体上看,变质岩系自西向东变质程度逐渐加强,由低角闪岩相逐渐过渡到高角闪岩相(Zhang and Schärer, 1999; 刘翠等, 2011; Liu et al., 2015a),局部零星出现麻粒岩相变质岩(戚学祥等, 2012; Liu et al., 2013, 2015a)。变质程度较高岩石可见部分熔融作用和混合岩化现象。区域变质岩岩石类型包括变质富铝质岩石、镁铁质岩石、变质碳酸盐岩、钙硅酸盐岩等,岩性包括麻粒岩、变粒岩、片麻岩、片岩、大理岩、斜长角闪岩等,特征变质矿物包括石榴石、矽线石、黑云母、斜长石、透辉石、透闪石、橄榄石、白云母等。在深变质杂岩中,由于新生代强烈的剪切变质作用,构造叠加和变质叠加现象明显(如叶理(Sn)被后期糜棱叶理(Sn+1)置换现象),致使深变质带内岩石基本层序消失或难以识别。变质叠加作用主要为退变质作用,一般认为由杂岩体剥露抬升引起(Searle et al., 2010; 戚学祥等, 2012; 冀磊等, 2017)。对特征变质矿物进行变质温压分析认为,哀牢山地区变质峰期温压条件为640~780℃、3.0~8.1kbar(Leloup and Kienast, 1993; Gilley et al., 2003);局部可达850~920℃、9.2~10.4kbar(戚学祥等, 2012),后期退变质条件为500~625℃、3.5~5.2kbar(Leloup and Kienast, 1993; Gilley et al., 2003; 戚学祥等, 2012)。

糜棱岩化作用在哀牢山杂岩带内发育广泛,在杂岩体内有多条糜棱岩带,糜棱岩化发生时间在古-中新世(刘俊来等, 2006; Liu et al., 2010a, 2011)。由于糜棱岩化作用,形成一系列L,L-S构造岩(Cao et al., 2011a, b, 2012; Liu et al., 2012, 2013)。其中的糜棱岩化花岗质岩石原岩时代包括二叠纪-三叠纪和新生代(Liu et al., 2011, 2012; Cao et al., 2012)。前者代表了古特提斯洋盆的闭合(Jian et al., 2009a, b; Fan et al., 2010; 刘翠等, 2011; 王舫等, 2011; Lai et al., 2014a, b; Liu et al., 2014; Wu et al., 2017a冀磊等, 2017)。由于始新世-渐新世走滑剪切作用改造,不同时期的花岗岩脉出现倾向NE-SW向,倾角中等或较陡(约45°~60°)糜棱叶理和倾伏向SE-NW,倾伏角平缓的线理。岩石中常可见左行指向标志(Tapponnier et al., 1990; Leloup et al., 1995; Liu et al., 2012)。由于新生代花岗岩脉就位之后遭受的剪切改造程度不一致,其就位时间不一,糜棱岩化程度不同。根据花岗岩脉与剪切作用时间关系,可以将花岗岩脉分为剪切前、同剪切和剪切后花岗岩脉。其中,同剪切花岗岩脉又可分为同剪切早期与同剪切晚期花岗岩脉(Searle, 2006; Cao et al., 2011a; 陈小宇等, 2017b)。其中,同剪切淡色花岗岩以及哀牢山杂岩带和浅变质岩带中均有出现的透镜状含角闪石碱性岩脉(Zhang and Schärer, 1999)可以用来限定哀牢山-红河剪切带剪切变形作用的时限。经历糜棱岩化改造的岩脉遭受变形作用后,形成透镜体或褶皱。同剪切早期脉体构成的褶皱两翼平行(图 3a),而同剪切晚期脉体的褶皱往往呈不对称状(Liu et al., 2012)。同时,剪切变形的岩脉常保留有SCC’组构(图 3b)、旋转碎斑系(图 3c)、云母鱼等剪切指向标志,显示左行走滑特点,糜棱岩化作用常导致矿物细粒化现象出现(图 3d)(Searle, 2006; Searle et al., 2010; Tang et al., 2013)。剪切作用之后侵入的花岗岩脉由于没有经历糜棱岩化作用,因此常切穿糜棱叶理,矿物也没有糜棱岩化改造后的细粒化现象(图 3e, f)。

图 3 元江-墨江剖面岩石变形构造与显微构造 (a)同剪切早期花岗岩脉褶皱;(b)S-C-C’组构;(c)长石残斑;(d)糜棱岩化过程中矿物细粒化,且同时发生矿物定向;(e、f)剪切期后花岗岩脉宏观、微观特点 Fig. 3 Structural geology along Yuanjiang-Mojiang section (a) folding of early-shearing dykes; (b) S-C-C' fabric; (c) porphyroclastic plagioclase; (d) grain size reduction of mineral grains by milonization and orientation of the grains; (e-f) post-shearing granitic intrusion in macroscale and mesoscale
2 剪切变形岩脉的宏观与微观构造特点

在元江-墨江剖面中,脉状印支期花岗岩与新生代花岗岩广泛发育,前者表现为糜棱岩化作用强烈,形成发育的糜棱叶理和拉伸线理。新生代花岗岩脉常常穿切前者,并表现出不一致的变形特点(图 4a, b)。

图 4 样品宏观产状与显微构造特点 (a、b)样品野外露头情况;(c、d)同剪切花岗岩脉变形特征;(e、f)剪切晚期花岗岩脉特征 Fig. 4 Mesoscopic and microscopic structures of the granitic dykes (a, b) photos of samples in field; (c, d) characteristics of syn-shearing granitic dykes; (e, f) characteristics of late-shearing granitic dykes

岩脉ALYJ1615-4-1具有同剪切早期花岗岩脉的宏观和微观特点(图 4b-d)。岩脉沿着围岩的叶理方向延伸,且具有显著的矿物定向排列特征形成叶理,叶理与拉伸线理产状与哀牢山-红河剪切带区域糜棱叶理、拉伸线理一致。显微观察可见黑云母具有一定定向性排列特征。钾长石可见卡式双晶,周边发育有蠕英构造。斜长石呈半自形-他形,可见机械双晶与变形纹现象,双晶纹宽度不一,且在一些颗粒内发生尖灭。石英颗粒呈他形粒状,偶尔可见波状消光现象,属于同剪切花岗岩脉特征。

岩脉ALYJ1615-3为同剪切晚期就位花岗岩脉。宏观露头体现出遭受剪切作用形成褶皱,脉壁宽度较为稳定,形态略有弯曲,并且在侧向上分散为两枝(图 4a)。围岩为强烈糜棱岩化并具有透入性叶理和线理的花岗质糜棱岩,岩脉切穿其糜棱叶理。同时,花岗岩脉末端呈现出肿缩状,表现出微弱变形改造特点。显微观察发现,岩石以保留较明显的花岗结构和构造特点为主。组成矿物多保持原始结晶状态,比如长石呈自形-半自形板柱状,边界较为平直,钾长石可见蠕英构造。石英呈他形粒状,可见波状消光与膨凸重结晶现象,边界呈现出锯齿状、港湾状特点,属于结晶后变形改造的证据(图 4e)。

岩脉ALYJ1615-5为同剪切晚期淡色石榴石白云母花岗岩。露头观察可以发现,岩脉与围岩界线平直,该岩脉切穿前期所有岩脉(图 4a)及这些岩脉中的糜棱叶理构造。矿物颗粒较细,具有花岗结构和块状构造特征。显微观察揭示出,长石粒径较大者呈半自形板柱状,较小者呈他形粒状,钾长石可见蠕英构造。斜长石颗粒部分可见少量机械双晶与变形纹,石英呈他形粒状,可见微弱波状消光现象,石榴石晶形较完整,说明该岩脉也遭受了一定程度的变形作用,但变形程度很弱,代表了该岩脉就位时间在变形晚期到变形结束(图 4f)。

3 技术方法 3.1 LA-ICP-MS锆石测年技术

将岩石样品(>5kg)粉碎后,采用重液分选和磁分选技术获得单矿物锆石颗粒样品。锆石分选后,使用双目镜挑选锆石颗粒(约200粒),粘贴在双面胶上,加注环氧树脂固化制作成靶。固化后的锆石靶打磨抛光,磨去约一半厚度,使锆石中心暴露。打磨后的靶在光学显微镜下拍摄透、反射图像,再通过阴极发光技术观察锆石振荡环带,确定适合分析的锆石颗粒。

LA-ICP-MS锆石U-Pb年代学测试在吉林大学东北亚矿产资源评价国土资源部重点实验室完成。激光剥蚀使用德国相干公司(Coherent)COMPExPro型ArF准分子激光器,质谱仪为美国安捷伦公司7500A型四极杆等离子质谱。激光条件为:激光束斑直径32μm,激光能量密度10J/cm2,剥蚀频率8Hz。剥蚀样品前首先采集30s的空白,随后进行30s的样品剥蚀,剥蚀完成后进行2min的样品池冲洗。载气使用高纯度He气,气流量为600mL/min;辅助气为Ar气,气流量为1.15L/min。对于不用同位素的采集时间,204Pb、206Pb、207Pb和208Pb为20ms,232Th、238U为15ms,49Ti为20ms,其余元素为6ms。使用标准锆石91500(1062Ma)作为外标进行同位素比值校正,标准锆石PLE/GJ-1/Qing Hu为监控盲样。元素含量以国际标样NIST610为外标,29Si为内标元素进行计算,NIST612和NIST614为监控盲样。测试分析后的数据使用ICPMSDataCal 10.2软件进行同位素比值及元素含量的计算,具体操作见(Liu et al., 2008, 2010b)。谐和年龄及图像使用Isoplot/Ex(3.0)给出(Ludwig, 2003)。普通铅校正使用Andersen (2002)给出的程序计算。分析数据及锆石U-Pb谐和图给出误差为1σ,表示95%的置信度。

3.2 石英c轴结晶学组构EBSD分析技术

石英c轴结晶学组构研究在中国地质大学(北京)地质过程与矿产资源国家重点实验室完成。当扫描电子显微镜入射电子束撞击分析样品满足布拉格衍射条件时,会产生菊池花样。通过分析菊池花样即可对待分析晶体进行研究(曹淑云和刘俊来, 2006)。定向光薄片使用BUEHLER MASTERMET非晶质硅酸胶体抛光液,在BUEHLER Alpha & Beta磨抛机中抛光2h。实验操作分析由Hitachi S-3400N Ⅱ扫描电子显微镜附加的HKL Nordlys电子背散射探头套件(EBSD)和HKL CHANNEL5软件获取、处理数据,工作电压15kV,工作距离18.4mm。标本准备及实验操作具体可见(刘俊来等, 2008)。

4 分析结果 4.1 锆石U-Pb测年结果

样品ALYJ1615-4-1的锆石自形程度较好,长宽比3: 1~4: 1,锆石振荡环带清晰,CL图像较亮。选取测试部分为锆石边部。每颗锆石测试1个数据点,测试点共计28个。去除偏离协和线较远、协和度低、或者有混合年龄可能的数据,共有12个数据点在谐和线上或附近(图 5a),这12颗锆石Th含量在568×10-6~3660×10-6之间,U含量在997×10-6~7135×10-6之间,Th/U值在0.47~0.84之间(表 1)。这12颗锆石的206Pb/238U年龄在26.0~28.6Ma之间,加权平均年龄为27.09±0.48Ma(MSWD=2.8),代表花岗岩脉结晶年龄。

图 5 锆石U-Pb年龄协和图和锆石阴极发光图像 (a)样品ALYJ1615-4-1;(b)样品ALYJ1615-3;(c)样品ALYJ1615-5 Fig. 5 CL images of zircons and LA-ICP-MS U-Pb concordia diagram

表 1 哀牢山杂岩中段花岗质岩脉锆石LA-ICP-MS U-Pb结果分析 Table 1 LA-ICP-MS zircon U-Pb isotopic data of the granitic dykes from the middle segment of the Ailaoshan Complex

样品ALYJ1615-3中的锆石呈自形-半自形,长宽比4: 1~2: 1,CL图像下环带较暗。选取测试部分为锆石边部,每颗锆石测试1个点,测试点共计28个。去除偏离协和线较远、协和度低、或者有混合年龄可能的数据,共有9个数据点落在谐和线上或附近(图 5b)。这9颗锆石Th含量在536×10-6~733×10-6之间,U含量在1807×10-6~2370×10-6之间,Th/U值在0.27~0.40之间。这9颗锆石206Pb/238U年龄在24.2~25.9Ma之间,加权平均年龄为25.17±0.23Ma(MSWD=1.3)代表花岗岩脉结晶年龄。

样品ALYJ1615-5的锆石自形程度较好,长宽比3: 1~5: 1,锆石振荡环带清晰,CL图像较亮。选取测试部分为锆石边部。每颗锆石测试1个点,测试点共计28个。去除偏离协和线较远、协和度低、或者有混合年龄可能的数据,共有15个数据点在谐和线上或附近(图 5c)。15个数据点中,4个数据反映的年龄为二叠纪-三叠纪,推测为继承锆石,剩下11个数据点反映新生代年龄。这11颗锆石Th含量在353×10-6~3167×10-6之间,U含量在1588×10-6~7675×10-6之间,Th/U值在0.06~0.48之间。这11颗锆石的206Pb/238U年龄在24.4~25.9Ma之间,加权平均谐和年龄为25.16±0.50Ma(MSWD=0.26),代表同剪切晚期岩浆结晶年龄。

4.2 EBSD石英c-轴组构

样品ALYJ1615-4-1的石英c-轴EBSD极图比较复杂(图 6a),其中以Y轴附近的一组较弱的不对称极密和一组Z轴附近较强的极密为主,并发育介于基圆和Y轴之间的极密组合以及一些有其他方向的复杂微弱极密组合。Y轴极密反映了中温变形的柱面<a>滑移,Z轴极密为低温底面<a>滑移的结果,介于基圆和Y轴之间的极密组合则反映了中-低温条件下菱面<a>滑移的重要性。分散的微弱极密可能是递进剪切变形过程的复杂性所致。

图 6 三个花岗岩脉样品EBSD石英c轴组构分析结果 Fig. 6 EBSD quartz c-axis fabrics for the three deformed granitic dyke samples

样品ALYJ1615-3石英c轴组构极密图以Z轴与X轴间的四个极密最为显著,其中Ⅱ-Ⅳ象限极密较强(图 6b),其Mad值可以达到大于11,而Ⅰ-Ⅲ象限极密微弱,Mad值仅仅介于3和4之间。四组极密的发育,反映了低温底面<a>滑移的重要性,而组构的对称性与不对称性,反映了纯剪应变与单剪应变的递进叠加关系。

样品ALYJ1615-5中石英c-轴组构型式简单,为一组Z轴极密为主,且极密有向着X轴方向延伸的趋势(图 6c)。上述组构特点反映了哀牢山-红河剪切带经历了低温递进单剪变形作用过程。

5 讨论 5.1 新生代剪切活动的阶段性

前述宏观构造描述中可以显见,哀牢山剪切带中的花岗岩脉具有显著不同的变形特点,同时组构分析也揭示出不同花岗岩脉石英c-轴组构的不一致性。这些差异为阐明变形的阶段性及应变类型提供依据。

岩脉ALYJ1615-4-1宏观以发育的糜棱叶理为特点,同时有拉伸线理存在。岩脉褶皱紧闭、同斜倒转,并以其轴面平行于糜棱叶理为特征。显微观察可见黑云母定向排列形成叶理,而石英常呈他形。石英c-轴组构分析显示出多种组构的叠加特点:其中的Y极密显示出柱面-a滑移系的启动,系中温变形作用的结果,纯剪剪切变形在这组极密发育过程中具有重要的意义。Z轴附近不对称极密的出现是由底面-a滑移所产生的组构型式,单剪剪切变形是其主要诱因。单剪剪切变形同样改造了早期的Y轴极密,使之变得具有不对称性。介于基圆和Y轴之间的极密组合的出现,充分说明随着变形温度环境的改变递进变形的持续性过程。同时,一些其他方向次极密的发育,与递进剪切变形的叠加和其他滑移系的部分作用有着密切的联系。结合宏观和显微构造分析可以认为,岩脉ALYJ1615-4-1经历了从中高温到低温递进剪切变形作用的改造,早期阶段较高温条件下的纯剪剪切变形构造(组构)叠加了晚期阶段的低温单剪剪切变形构造(组构)。

岩脉ALYJ1615-3宏观上切穿围岩糜棱叶理,同时遭受微弱的糜棱岩化作用改造并发生弯曲形成不对称褶皱构造,是剪切作用晚期阶段就位岩脉的典型特点。显微观察发现石英波状消光与膨凸重结晶现象发育,长石偶尔可观察到变形纹现象说明其经历了低温变形作用。石英c-轴组构X-Z轴间四个象限同时存在的极密表明底面-<a>滑移占主导,低温变形特点与膨凸重结晶作用的发育一致。石英c-轴组构型式中四组极密的共存表明纯剪变形仍然起着一定的作用,但极密显著的不对称性表明单剪变形是其主导的应变型式。显见,岩脉ALYJ1615-3代表了剪切作用晚期阶段花岗岩浆活动性,而且在较低温条件下经历了以简单剪切变形为主的一般剪切变形作用的改造。

岩脉ALYJ1615-5宏观上切穿了围岩糜棱叶理,其就位晚于糜棱叶理形成,并可以用以限定局部地区糜棱演化作用(韧性剪切变形作用)的结束时间。显微观察揭示出岩石中的长石和石英颗粒同样具有一定的单向定向性,可能是应力控制下的生长作用的结果。组构分析显示,石英c-轴组构主要为一组不对称的Z轴极密,并有从Z轴向X轴方向变化的趋势。这种组构型式显然是岩脉就位后经历低温递进剪切变形改造的结果。从简单的石英c-轴组构特点可以判断,单剪剪切变形是其主要的动力学机制。

上述研究成果与早期对于哀牢山剪切带关于变形条件及变形阶段性的研究成果一致。早期研究认为,哀牢山-红河剪切带糜棱岩化现象形成于杂岩体变质峰期高角闪岩相条件下(Tapponnier et al., 1990; Leloup and Kienast, 1993; Leloup et al., 1995, 2001)。然而,来自哀牢山-红河剪切带南段出露的大象山杂岩研究中,一系列研究发现糜棱岩化剪切作用不发生在峰期变质角闪岩相条件下,而是发生在后期叠加的绿片岩相条件下(Nam et al., 1998; Jolivet et al., 2001; Yeh et al., 2008)。同时,还有学者认为高角闪岩相变形之后由于递进变形作用叠加了后期绿片岩相变形作用(Anczkiewicz et al., 2007; Chen et al., 2016)。在哀牢山杂岩、点苍山杂岩研究过程中,也发现了低温剪切作用叠加在高温剪切作用之上的现象(Searle et al., 2010; Cao et al., 2011a; Tang et al., 2013; Wu et al., 2017b)。Liu et al. (2012)认为沿哀牢山-红河剪切带出露的三个杂岩体中(大象山杂岩、哀牢山杂岩和点苍山杂岩)普遍存在两期构造变形事件。第一阶段变形(D1)为高温纯剪变形,发育层内褶皱,并且伴随着透入性较强的叶理、对称的石香肠构造与透镜体,褶皱两翼对称,黑云母经常定向形成线理。石英c轴组构分析得到的极密图显示高温极密特征,且有代表纯剪变形的斜方对称特点;第二期阶段(D2)为低温单剪变形,发育大量不对称构造,是糜棱岩化形成的主要阶段。该阶段岩石组构以L型为主,糜棱岩化过程中形成运动指向标志,如残斑、S-C组构等。显微镜下石英膨凸重结晶、单晶条带、波状消光等现象也说明糜棱岩化作用发生在该阶段。石英c-轴EBSD组构分析也发现石英c-轴极密代表的温度较低,且存在单斜对称特点,表明较低温简单剪切作用。Wu et al. (2017b)则在哀牢山杂岩北段和平-水塘剖面应用剪切作用运动学涡度分析进一步验证了两阶段剪切变形的存在及其递进叠加特点,含有更多纯剪的一般剪切叠加了后期简单剪切(Cao et al., 2011a; Liu et al., 2012; Tang et al., 2013; Wu et al., 2017b)。

5.2 剪切变形转换时限的限定及其区域构造意义

从上述宏观、微观与组构分析结果可以看出,花岗岩浆活动伴随着哀牢山-红河剪切带剪切变形作用过程发生。一方面,在三阶段花岗岩脉就位过程中剪切带经历了从较高温到较低温的变形环境的变迁,伴随着剪切变形岩石的剥露(Chen et al., 2017a)。另一方面,在此过程中剪切变形发生了从以纯剪剪切变形为主的应变向以单剪剪切变形为主的应变的转变。该转变是一个递进的、渐变的过程,在剪切早期、剪切晚期和剪切后就位的岩脉中保留了相应的宏观、微观构造特点和石英c-轴组构型式。

对于花岗岩脉中岩浆结晶锆石的U-Pb定年分别给出了27Ma (样品ALYJ1615-4-1)和25Ma(样品ALYJ1615-3和ALYJ1615-5)的结果,可以认为上述应变型式的转变发生在约27~25Ma之间。同时,上述测年结果对于哀牢山中段韧性剪切变形结束的时间(即大约25Ma)给出了一定的约束。

研究表明,在不同大地构造环境(伸展或收缩)下,纯剪作用往往发育于早期阶段,而随着造山带演化,变形作用逐渐为单剪为主的剪切变形所替代。例如,Wallis(1995)根据Sannbagawa造山带型韧性剪切带进行涡度分析,发现在造山阶段末期发生了由含更多纯剪成分的一般剪切转换为简单剪切。Bailey and Eyster(2003)根据美国亚利桑那Pinaleno变质核杂岩分析认为单剪成分也越来越大,二者均代表了上覆地壳递进减薄过程中,垂向压缩作用减小,侧向流动速度增加。高喜马拉雅穹隆杂岩的剪切变形运动学涡度学分析则发现纯剪成分增加,代表了地壳加厚过程中,垂向压缩作用增大,侧向流动速度降低(Law et al., 2004; 张波等, 2005)。结合点苍山(Cao et al., 2011a; Tang et al., 2013)和哀牢山地区糜棱岩研究结果(Liu et al., 2011, 2012; Wu et al., 2017b; 本文),可以认为在大约27~25Ma时期,哀牢山-红河剪切带剪切发生了由纯剪为主的一般剪切向单剪为主的一般剪切的转变。此间垂向应变减弱,侧向流动速率增加,代表了印支地块在印度-欧亚板块汇聚作用过程中,发生了从隆升为主向侧向逃逸的转变。

6 结论

(1) 对于哀牢山-红河剪切带中段花岗岩脉的宏观、微观和组构分析识别出三阶段具有不同变形特点的花岗岩脉。包括具有中温到低温递进叠加变形特点的同剪切早期岩脉;低温变形为主的同剪切晚期岩脉和剪切期后岩脉。三组岩脉分别遭受了以纯剪剪切变形为主的一般剪切应变、单剪剪切变形为主的一般剪切应变和单剪剪切应变。

(2) LA-ICP-MS锆石U-Pb测年获得了三种花岗岩脉结晶年龄分别为25.17±0.23Ma、27.09±0.48Ma和25.16±0.50Ma。结合岩石宏观露头与显微镜下观察、EBSD组构分析,推测哀牢山-红河剪切带发育过程中在大约27~25Ma之间发生了剪切应变型式的转变,伴随着印支地块的南东向逃逸。同时, 哀牢山-红河剪切带中段韧性变形停止时间在约25Ma。

致谢 感谢吉林大学郑培玺老师在锆石U-Pb测年方面提供的帮助,以及美国印第安纳大学Robert Peter Wintsch教授对锆石U-Pb定年结果提出的建议。同时感谢两位匿名审稿人对本文提出的修改意见!
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