2020, Vol. 36
2. 北京大学地球与空间科学学院, 北京 100871
2. School of Earth and Space Sciences, Peking University, Beijing 100871, China
喜马拉雅造山带的崛起是印度-欧亚大陆碰撞造山作用的结果，并伴随着广泛的高级变质作用和地壳深熔作用(Allégre et al., 1984; Hodges, 2000; Yin and Harrison, 2000;吴福元等, 2015; Zeng et al., 2011, 2015; Gao et al., 2017; Wang et al., 2015, 2017a;曾令森和高利娥, 2017; Zheng and Wu, 2018)。在新生代碰撞之前，印度大陆是冈瓦纳大陆的一部分，喜马拉雅地块位于东冈瓦纳大陆的北缘。了解印度大陆北缘的物质和构造属性有助于甄别哪些特征是欧亚大陆与印度大陆碰撞的产物，哪些是在碰撞之前就存在的继承性构造(Beaumont et al., 2001; Zeng et al., 2009; DeCelles et al., 2004; Gehrels et al., 2006a, 2011; Zhang et al., 2012; Palin et al., 2018; van Hinsbergen et al., 2019; Gao et al., 2019)。了解碰撞前喜马拉雅地块的物质组成、结构和来源，有助于解译新生代变质作用的性质和地壳深熔作用的源区组成，有助于冈瓦纳古大陆的重建(DeCelles et al., 2000; Miller et al., 2001; Gehrels et al., 2006b, 2011;许志琴等, 2005; Cawood et al., 2007; Li et al., 2010; Zhang et al., 2012; Wang et al., 2017; Martin, 2017; Palin et al., 2018; Gao et al., 2019)。
现今的喜马拉雅造山带(或喜马拉雅地块)由北向南主要分为特提斯喜马拉雅沉积岩系(Tethyan Himalayan Sequence, THS)、高喜马拉雅结晶岩系(Greater Himalayan Crystalline Sequence, GHCS)、低喜马拉雅岩系(Lesser Himalayan Sequence, LHS)和次喜马拉雅岩系(Sub-Himalayan Sequence, SHS)等四个构造单元，它们之间分别被藏南拆离系(South Tibetan Detachment System, STDS)、主中央逆冲断层(Main Central Thrust, MCT)和主边界逆冲断层(Main boundary Thrust, MBT)所分隔(图 1)。喜马拉雅地块包含大量原岩年龄为晚元古代到早古生代的花岗质片麻岩(Collins and Pisarevsky, 2005;许志琴等, 2005; Gehrels et al., 2006a, 2011; Cawood et al., 2007; Zeng et al., 2011, 2012; Wang et al., 2013b; Mottram et al., 2014; Gao and Zeng, 2014;高利娥等, 2015; Liu et al., 2016;丁慧霞等, 2017; Martin, 2017; Gao et al., 2019; Yoshida et al., 2019)，表明了喜马拉雅地体在碰撞前经历过较广泛的晚元古代到早古生代构造岩浆事件。除此之外，在印度北部、喜马拉雅造山带和拉萨地块内都发育寒武系与奥陶系角度不整合接触(DeCelles et al., 2000; Myrow et al., 2003, 2010, 2016, 2018; Liu et al., 2019; Gehrels et al., 2006b, 2011; Long et al., 2011; McQuarrie et al., 2008, 2013; Hughes et al., 2019)。早古生代岩浆作用记录和构造角度不整合都指示冈瓦纳大陆北缘在早古生代发生过一期重要的构造事件。不同学者们提出了不同的模型，来解释印度大陆北缘与相邻地块的构造演化关系，归结起来主要有两类模型：第一类模型是安第斯型俯冲模型(Gehrels et al., 2006b, 2011; Cawood et al., 2007; Palin et al., 2018)，认为印度大陆北缘在早古生代由被动陆缘转变为主动陆缘，原特提斯洋向南俯冲，发生增生造山作用。奥陶纪之后又转为被动陆缘，形成寒武系-奥陶系角度不整合。另一些学者虽然提出了稍微不同的模型(Zhu et al., 2012; Wang et al., 2016; Martin, 2017; Yoshida et al., 2019)，但都是在该模型的基础之上提出的，主要差异在邻区地块的空间方位和构造背景。第二类模型是被动陆缘演化模型(Brookfield, 1993; Mottram et al., 2014; Liu et al., 2019, 2020)，认为从晚元古代末期到早古生代，印度大陆北缘没有经历过原特提斯洋的俯冲，一直处于被动陆缘状态，认为在该时期的岩浆-构造事件与裂谷闭合或者岩石圈减薄相关。为了探究晚元古代和早古生代岩浆-构造事件成因，本次研究采集了喜马拉雅造山带东部错那县麻玛沟地区的花岗片麻岩和淡色花岗岩，开展了锆石SHRIMP U-Pb定年和主、微量元素测试，结合前人数据来探讨晚元古代到早古生代印度大陆北缘构造演化过程。
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图 1 喜马拉雅造山带地质简图(据Guillot et al., 2008; Zeng et al., 2011; Zhang et al., 2017修改) 图中年龄数据来源于表 3 Fig. 1 Simplified geological map of Himalayan orogen (modified after Guillot et al., 2008; Zeng et al., 2011; Zhang et al., 2017) Age data in Fig. 1 from Table 3 |
喜马拉雅造山带的四个构造单元中，THS和LHS为浅变质或未变质的大陆边缘相沉积岩系(Myrow et al., 2003, 2016; Gehrels et al., 2011; Hughes et al., 2019)。其中THS整体上以海相沉积为主，并夹有少量陆相碎屑沉积，并且寒武系-奥陶系角度不整合普遍存在(周志广等, 2004; Myrow et al., 2018)，典型剖面有Zanskar、Spiti和康马地区等。而LHS中陆相和过渡相沉积占大部分，少部分为浅海碳酸盐相，也存在寒武系-奥陶系不整合，如Ambar和Swabi地区(Myrow et al., 2018)。在巴基斯坦和苏丹局部地区的寒武系与晚于奥陶系的更年轻地层不整合接触(Long et al., 2011; Myrow et al., 2016)。SHS为喜马拉雅造山带的前陆盆地，除了上部少量新生代沉积外，下部地层与LHS相似(Thakur et al., 2007)，但是除了巴基斯坦的Salt Range地区，沿造山带走向的其他地区都还没有发现有寒武系地层出露，而Salt Range和印度克拉通北部的Rajasthan地区都存在寒武系-二叠系不整合接触(Hughes et al., 2019)。GHCS的变质程度普遍达到了角闪岩相以上，阻碍了原生层序的划分，在该构造单元的来源上的认识存在较大分歧(DeCelles et al., 2000;张泽明等, 2008a, b ; Myrow et al., 2003, 2010, 2016, 2018; Hughes et al., 2019)。GHCS的划分方法有多种(Wang et al., 2013a)，本文参照更为简便的Le Fort (1975)的划分方法，即GHCS按照变质程度和矿物共生组合等特征由下到上划分成三组，组-1为含蓝晶石-石榴石变泥质岩、混合岩和石英岩；组-2主要为含石榴子石-透辉石大理岩和钙硅酸盐类；组-3为含夕线石变长英质岩，夹杂少量钙硅酸盐类，并且含有大量正片麻岩。
本次研究的采集的样品为STDS以南的GHCS组-3花岗片麻岩类和淡色花岗岩，采样地点位于错那县南部的麻玛沟地区(图 2)；STDS以北为错那裂谷带，广泛分布近东西走向的辉绿岩脉(Wang et al., 2018)和规模不等的淡色花岗岩体(高利娥等, 2012;王晓先等, 2016;石卿尚等, 2017)。MM15系列样品为含石榴子石花岗片麻岩，MMG-LG和T0713为淡色花岗岩。花岗片麻岩(MM15)的主要矿物为石英、钾长石、斜长石、黑云母、石榴子石和少量白云母(图 3a, b)，其中黑云母定向，斜长石、钾长石和石榴子石呈半定向结构，在野外露头上显示出片麻状构造特征。其中黑云母半自形-他形结构，多呈长柱状或条状，部分形态不规则的颗粒生长在斜长石周围；斜长石和钾长石为半自形-他形的板状结构，且钾长石颗粒相对较大；石榴子石他形粒状结构，裂隙发育。由于该岩体与围岩共同经历了高级变质作用事件，因此二者的边界难以准确界定。
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图 2 西藏南部错那地区地质简图(据丁慧霞等, 2017修改) Fig. 2 Simplified geological map of the Cona area, South Tibet (modified after Ding et al., 2017) |
淡色花岗岩(MMG-LG和T0713)产状为脉体，斜切围岩的片理或片麻理。2条脉体除了钾长石颗粒大小不同外，其他矿物组合、结构和含量均相同，都为含石榴子石淡色花岗岩(图 3c, d)。主要矿物为石英、钾长石、斜长石、石榴子石和少量云母，石榴子石大部分为他形粒状结构，其核部包裹钾长石、石英等矿物，所有矿物没有明显的定向，表明这些脉体没有受到明显的构造变形的影响。样品MMG-LG中的钾长石大小相对不均一，为0.01~1mm之间；样品T0713中的钾长石颗粒大小相对均一，大部分颗粒位于0.1~0.5mm之间。从岩相学上，花岗片麻岩和淡色花岗岩明显不同，前者黑云母含量较多，石榴子石含量较少，而淡色花岗岩反之。
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图 3 麻玛沟花岗片麻岩(a、b)和淡色花岗岩(c、d)显微照片 (a、b)为样品MM15；(c)为样品MMG-LG；(d)为样品T0713. Gt-石榴子石；Kf-钾长石；Pl-斜长石；Bi-黑云母；Q-石英 Fig. 3 Microphotographs of the granitic gneiss (a, b) and the leucogranite (c, d) from the Mama Valley (a, b) Sample MM15; (c) Sample MMG-LG; (d) Sample T0713. Gt-garnet; Kf-K-feldspar; Pl-plagioclase; Bi-biotite; Q-quartz |
样品的全岩主微量元素测试在自然资源部国家地质实验测试中心进行，其中主量元素通过X射线荧光法(XRF)进行测定，仪器型号为Rigaku-3080，分析精度优于5%；微量元素利用等离子质谱仪(ICP-MS)进行分析，仪器型号为Finigan ELEMENT 2 ICP-MS，其分析精度可保持在5%~10%。
2.2 锆石SHRIMP U-Pb年龄锆石同位素年龄测试和锆石靶的制作在北京离子探针中心进行，采用高分辨率二次离子探针质谱仪(SHRIMPⅡ)进行测定，离子源为氧源，粒子流强度为4nA，束斑直径约为25μm×15μm，仪器质量分辨率为5000。锆石标样采用M257(U：846×10-6)和TEM(417Ma)双标样标定，分别标定U含量和年龄。样品测定时，每测定5个样品点进行一次标样测定，每个测试点之间最短为180秒，用以测定后通道内的清洗。锆石靶的制作流程可参见宋彪等(2002)。
3 分析结果 3.1 主量元素特征两种岩石中，淡色花岗岩(MM15)的SiO2(73.94%~75.08%)、Na2O(3.41%~4.12%)、K2O(4.02%~5.55%)含量最高，TiO2(0.01%~0.22%)、FeO+Fe2O3(0.44%~1.59%)、MgO(0.11%~0.35%)、CaO(0.53%~1.31%)含量最低，并具有较高的Al2O3(13.44%~14.51%)，K2O/Na2O(1.14~1.44)和A/CNK(1.08~1.13)比值都较高，表明该岩石为富钾过铝质花岗岩(表 1、图 4)。
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表 1 麻玛沟花岗岩和变泥质岩主量(wt%)及微量(×10-6)元素数据 Table 1 Major (wt%) and trace (×10-6) element data of granite and metapelite from Mama Valley |
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图 4 麻玛沟花岗片麻岩和淡色花岗岩主量元素关系图解 Fig. 4 Major element diagrams for the granitic gneisses and the leucogranites from the Mama Valley |
花岗片麻岩(T0713、MMG-LG)，SiO2含量在72.44%~74.60%之间，具有较高的Al2O3(12.63%~13.48%)、Na2O(2.58%~2.85%)、K2O(4.02%~4.73%)和较低的TiO2(0.27%~0.30%)、FeO+Fe2O3(2.90%~3.18%)、MgO(0.56%~0.63%)、CaO(1.42%~1.67%)含量，其K2O/Na2O比值很高，位于1.50~1.83之间，A/CNK值在1.09~1.11之间，也属于富钾过铝质花岗岩(表 1、图 4)。
3.2 微量元素特征花岗片麻岩的球粒陨石标准化稀土元素曲线右倾(图 5a)，其富集程度低于A型花岗岩的平均值，且具有较低的(La/Yb)N(4.30~5.89)比值，较大的Eu负异常(Eu/Eu*=0.28~0.36)；从微量元素(表 1、图 5b、图 6)中可看出，这些样品富集Cs(6.33×10-6~7.01×10-6)、Rb(229×10-6~264×10-6)、Th(25.9×10-6~31.8×10-6)、U(5.6×10-6~7.61×10-6)、Pb(35.1×10-6~47.6×10-6)、Nb(11.4×10-6~12.6×10-6)、Ta(1.33×10-6~1.64×10-6)，亏损Sr(56.7×10-6~62.8×10-6)。
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图 5 麻玛沟花岗片麻岩和淡色花岗岩的球粒陨石标准化稀土元素配分图(a, 标准化值据Anders and Grevesse, 1989)和原始地幔标准化微量元素蛛网图(b, 标准化值据Sun and McDonough, 1989) A型花岗岩数据引自Wu et al. (2002)，图 6同 Fig. 5 Chondrite-normalized REE patterns (a, normalization values after Anders and Grevesse, 1989) and primitive mantle-normalized trace element spider diagrams (b, normalization values after Sun and McDonough, 1989) for the granitic gneisses and the leucogranites from the Mama Valley Data of A-type granite from Wu et al. (2002), also in Fig. 6 |
淡色花岗岩的稀土元素含量变化较大(图 5a)，其REE标准化曲线形态各异，富集程度不及花岗片麻岩和A型花岗岩。MMG-LG呈现出右倾趋势((La/Yb)N=12.79)，且具有较明显的Eu负异常(Eu/Eu*=0.63)，相对富集轻稀土元素(LREE)，亏损重稀土元素(HREE)。T0713-15-LG的REE标准化曲线为近水平((La/Yb)N=1.02)，较明显的Eu负异常(Eu/Eu*=0.52)。T0713的REE标准化曲线总体右倾((La/Yb)N=2.46~4.69)，具微弱的Eu正异常(Eu/Eu*=1.17~1.18)和Ce负异常(Ce/Ce*=0.87~0.88)，LREE和HREE相对富集，而中稀土(MREE)相对亏损。淡色花岗岩的微量元素(图 6)含量总体上也富集Cs(1.38×10-6~15.5×10-6)、Rb(150×10-6~353×10-6)、U(1.29×10-6~16×10-6)、Pb(31×10-6~87.4×10-6)，但相对亏损Th(0.15×10-6~16.2×10-6)、Nb(1.57×10-6~12.8×10-6)、Ta(0.07×10-6~2.22×10-6)、Sr(9.75×10-6~96.3×10-6)。
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图 6 麻玛沟花岗片麻岩和淡色花岗岩的Rb-Sr (a)、Ba-Sr (b)、Nb-Ta (c)和Th-U (d)图解 Fig. 6 Rb vs. Sr (a), Ba vs. Sr (b), Nb vs. Ta (c) and Th vs. U (d) diagrams for the granitic gneisses and the leucogranites from the Mama Valley |
花岗片麻岩样品的锆石颗粒(图 7a)为长柱状，其长轴/短轴的值在2~7之间，平均值约为3.5，自形-半自形结构，锆石CL图像显示内部呈韵律环带结构，显示岩浆锆石的特征，部分锆石的边部颜色比核部更深，说明到锆石结晶后期U、Th含量增加。部分颗粒边部均匀无韵律环带，可能代表了后期变质作用的结果。从结构来看，锆石内部的韵律环带部分记录了该岩石的结晶年龄，而边部分可能是记录了该岩体后期变质作用的年龄，但边部太窄，小于SHRIMP束斑大小，无法获取有地质意义的年龄。对该样品，选择了20颗锆石，进行了27点测试(表 2)，测试结果显示这些锆石的U(208×10-6~11945×10-6)和Th(49×10-6~506×10-6)的含量变化较大，具有高U效应，Th/U比值在0.04~0.61之间，206Pb/238U年龄除了点6.1(937Ma)和17.2(279.4Ma)外，集中在481.8~541.5Ma之间(图 8a)。点6.1的锆石为继承锆石，其Th/U比值为0.18，锆石核为新元古代的岩浆锆石，点17.2可能是锆石的环带部分与反应边的混合年龄，因此其年龄远小于平均年龄，剔除这2个点后的加权平均年龄为500.7±4.5Ma(N=25，MSWD=1.9)。所有的点均位于或近似位于谐和线上，没有明显的Pb丢失，为谐和年龄，代表该岩体的岩浆结晶年龄。
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图 7 麻玛沟花岗片麻岩和淡色花岗岩样品的锆石阴极发光图像 圆圈代表激光剥蚀位置，数字代表年龄(Ma)；括号内数字代表晶面符号 Fig. 7 Cathodoluminescence (CL) image of the zircon grains for granitic gneiss and leucogranite from Mama Valley The circles stand for the ablated locations by laser, the numbers stand for age (Ma); the numbers in brackets are symbol of crystal face |
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表 2 麻玛沟花岗片麻岩和淡色花岗岩锆石SHRIMP U-Pb年龄 Table 2 SHRIMP zircon U-Pb data for granitic gneiss and leucogranite from Mama Valley |
淡色花岗岩样品的锆石颗粒形态与花岗片麻岩类似(图 7b)，均为长柱状，长轴/短轴的值为1~5.5之间，平均值约为3.0，半自形-他形结构，CL图像显示其内部结构也普遍具有韵律环带结构，且边部的环带变暗。但与花岗片麻岩的锆石不同的是，该样品的部分锆石颗粒发育核-幔-边结构或核-边结构。其中核部为具有环带结构的岩浆锆石，为锆石的主体部分，U(322×10-6~4061×10-6)和Th(32×10-6~1257×10-6)含量变化较大(表 2)，Th/U比值在0.01~0.66之间，206Pb/238U年龄除了点1.2(427.5Ma)和13.1(238.1Ma)离群外，集中在488.5~512.6Ma之间(图 8c, d)。点1.2和13.1的年龄均小于平均年龄，推测为核部与慢部(或边部)的混合年龄，将两点的年龄剔除后，核部的加权平均年龄为498.6±3.4Ma(N=16，MSWD=0.88)，所有的点都位于或近似位于谐和线上，为谐和年龄，无明显的Pb丢失。CL图像显示锆石的幔部无环带，呈现出较均匀的深灰色调，U(2539×10-6~4194×10-6)和Th(10×10-6~89×10-6)含量变化相对较小，无明显高U效应，Th/U比值很低，在0.001~0.03之间，但绝大多数的值为0.01，这是淡色花岗岩中结晶锆石的通性(曾令森等, 2019)，206Pb/238U年龄集中在15.7~25.1Ma之间，但是年龄数据比较分散(图 8b)，给出的加权年龄无太大意义，需要结合相邻淡色花岗岩的年龄数据进行讨论(见下文讨论部分)。该样品锆石的边部无环带或不明显的环带，总体上边部显示灰色的亮边，与暗灰色的幔部区别开来，边部厚度很小，个别锆石颗粒出现了边部而幔部不发育，还有极少的锆石出现了幔部而边部不发育，而核部几乎出现在每颗锆石中，且占锆石颗粒的主要部分，控制着锆石再生长过程中的大致的几何形状，有些锆石可见核部被幔部侵蚀成港湾状(例如图 7b的前两颗锆石)。因此我们判断核部为继承锆石，其年龄代表淡色花岗岩形成之前的岩浆作用事件；幔部和边部为淡色花岗岩岩浆结晶作用的产物，其中幔部为结晶峰期时形成，而边部为结晶末期形成。
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图 8 麻玛沟花岗片麻岩(a)和淡色花岗岩(b-d)的锆石U-Pb年龄谐和图 Fig. 8 Zircon U-Pb Concordia diagram for the granite gneiss (a) and the leucogranite (b-d) from Mama Valley |
利用CL图像，我们可以观察锆石晶面中锥面的发育情况(Pupin, 1980; Vavra, 1990, 1993; Li et al., 2014)，如果锆石颗粒发育(101)锥面，则锆石尖端角度较大，呈钝角或近直角形态，如果(211)锥面发育，则锆石尖端呈锐角。由于CL图像为二维图像，锆石的柱面类型不容易区分，但是可以观察其长轴/短轴的比值。通过对比，花岗片麻岩(MM15)和淡色花岗岩(MMG-LG)样品中的锆石形态相近(图 7)，主要表现为：长轴/短轴比值相近，其平均值在3~3.5之间；锆石晶面发育相近，两种锆石均发育(101)锥面，(211)锥面发育较差。Pupin (1980)利用花岗岩中的锆石形态对其进行成因分类，主要分成壳源的过铝质淡色花岗岩、二长花岗岩、花岗闪长岩，壳幔混合型的亚碱性、钙碱性花岗岩，和幔源的碱性、拉斑玄武系列的花岗岩。样品中的锆石形态特征符合壳源成分的过铝质花岗岩，与岩石的化学成分分类相吻合，表明花岗片麻岩从岩浆作用开始到现在，虽然经历过较强烈的变质作用，但其全岩主量元素成分变化不大。而淡色花岗岩中的锆石核部占有很大比重，控制了增生后锆石的整体形态，因此花岗片麻岩中的锆石与淡色花岗岩中的锆石核部形态相近。
麻玛沟的花岗片麻岩加权平均年龄为500.7Ma(图 8a)；淡色花岗岩继承锆石的加权平均年龄为498.6Ma(图 8c, d)。这些年龄与前人报道的印度大陆北缘的早古生代岩浆事件相吻合(Cawood et al., 2007; Zhu et al., 2012; Gao et al., 2019)，为同一期构造岩浆事件的产物。从锆石的形态学分类可看出麻玛沟地区花岗片麻岩和淡色花岗岩都属于造山型的过铝质花岗岩，而非岩石圈减薄过程中产生的钙碱性或碱性火成岩。
4.1.2 地球化学分析虽然目前精准判别花岗岩的构造环境仍有困难(Pitcher, 1997;吴福元等, 2007)，但不同构造环境下的花岗岩总体上还是具有明显的差异性(吴福元等, 2007;舒良树和王博, 2019)。本文的两种花岗岩都属于过铝质花岗岩，部分非造山和造山后花岗岩也会出现过铝质特征(Pitcher, 1997; Wu et al., 2002;吴福元等, 2007)，但其微量元素特征能够较好地区分过铝质花岗岩是否是造山型过铝质花岗岩。例如非造山的A型花岗岩比造山型过铝质花岗岩的U、Th、Sr和Ba含量要低，具有更高的Zr、Hf和Nb/Ta比值，(图 5b、图 6；Wu et al., 2002)，通常具有非常显著的Eu负异常和较高(La/Yb)N比值，与麻玛沟花岗片麻岩的稀土配分曲线差异较大(图 5a)。进一步地，在新元古代早期(约800Ma)也发生过大规模的岩浆作用时间，普遍认为是Rodinia超大陆裂解的产物(Singh et al., 2002; Ding and Zhang, 2016; Wang et al., 2016, 2017b;王一伟等, 2016; Spencer et al., 2019;董昕和田作林, 2019)，这些新元古代岩浆岩的锆石Th/U比值明显高于本文早古生代花岗岩的数据(图 9)，说明二者的形成环境不同。上述分析表明，早古生代过铝质花岗岩可能是造山型过铝质花岗岩，不是拉张环境下的非造山特征，支持前文所述的安第斯型俯冲模型。
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图 9 麻玛沟花岗片麻岩和淡色花岗岩锆石U-Th/U图解 800Ma的数据来源于Wang et al. (2017)，其他数据来源于表 2 Fig. 9 Diagram of U vs. Th/U for zircon from granitic gneiss and leucogranite from Mama Valley Except for the data of 800Ma after Wang et al. (2017), others are from Table 2 |
喜马拉雅造山带保存着大量的晚元古代-早古生代岩浆作用的纪录(图 1、表 3)，分布于THS、GHCS和LHS等三个构造单元，从东构造节的南迦巴瓦地区到西部的巴基斯坦地区均有出露。如前所述，印度大陆北缘早古生代岩浆作用可能是原特提斯洋俯冲作用的结果，在俯冲作用发生之前，印度大陆与原特提斯洋是被动陆缘的连接模式(Cawood et al., 2007; Zhu et al., 2012; Gao et al., 2019)。因此，在新元古代到早古生代期间，其板块边缘必定发生了从离散状态到会聚状态的构造转换，从而导致原特提斯洋开始向印度大陆俯冲，那么，相对于印度大陆，原特提斯洋初始俯冲是正向俯冲，还是从一侧俯冲后逐渐扩展到另一侧？针对该问题，笔者整理了喜马拉雅造山带晚元古代-早古生代花岗岩年龄数据(表 3)，发现从喜马拉雅造山带西部到东部(图 10)，除了Kaplas地区的花岗岩外，其他花岗岩的年龄均匀变大的趋势，如图 10中黄色箭头线所示，从约470Ma增大到510Ma左右，间隔约40Myr，预示着东部的岩浆活动要早于西部，原特提斯洋初始俯冲从东部开始，并逐渐向西部扩展。印度大陆北缘在经历岩浆作用的同时必然也经历了强烈的变质作用事件，GHCS内的变质岩可能会残留早古生代变质作用的痕迹。
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表 3 喜马拉雅造山带的花岗岩锆石U-Pb年龄数据 Table 3 U-Pb age data of granite from Himalayan orogen |
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图 10 喜马拉雅造山带花岗岩锆石U-Pb年龄分布图 数据来源于表 3 Fig. 10 Zircon U-Pb age of granite from the Himalayan orogen The data is from Table 3 |
喜马拉雅造山带中新世淡色花岗岩分布广泛，在THS和GHCS构造单元内均有出露，构成喜马拉雅造山带新生代花岗岩的主体部分(吴福元等, 2015;曾令森和高利娥, 2017)，其形成机制主要有两种(Inger and Harris, 1992; Gao et al., 2014;曾令森和高利娥, 2017)：
(1) 白云母脱水熔融：22Ms+7Pl+8Qz=25Melt+5Kf+5Sil+2Bt
(2) 水致白云母熔融：9Ms+15Pl+7Qz+xH2O=31Melt
其中Ms为白云母，Pl为斜长石，Qz为石英，Melt为熔体，Kf为钾长石，Sil为夕线石，Bt为黑云母。
如上文所述，麻玛沟淡色花岗岩的结晶年龄在15.7~25.1Ma之间，为了探讨其形成机制，本文整理了邻近的三个淡色花岗岩体(亚马荣淡色花岗、错那洞淡色花岗岩和雅拉香波淡色花岗岩)年龄(图 11)和Ba-Rb/Sr关系数据(图 12)。这四套淡色花岗岩近似位于同一经度，最南端为麻玛沟淡色花岗岩，最北端为雅拉香波淡色花岗岩，其中麻玛沟和亚马荣为高喜马拉雅淡色花岗岩，而错那洞和雅拉香波为THS单元内与穹窿构造相伴生的淡色花岗岩。
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图 11 麻玛沟、亚马荣、错那洞和雅拉香波的中新世淡色花岗岩的锆石U-Pb加权年龄分布 数据来源：亚马荣据Aikman et al. (2008)和石卿尚等(2017)；错那洞据Fu et al. (2018)；雅拉香波据曾令森等(2019) Fig. 11 Zircon U-Pb weighted average ages for the Miocene leucogranite from the Mama Valley, Yamarong, Conadong and Yardoi Data source: the Yamarong from Aikman et al. (2008) and Shi et al. (2017); the Conadong from Fu et al. (2018); the Yardoi from Zeng et al. (2019) |
根据整理的年龄数据(图 11)可知，该地区的淡色花岗岩至少可分为5个期次，分别为1(~23Ma)、2(~20.5Ma)、3(~19Ma)、4(~17Ma)和5(~14Ma)，其中第一期在错那洞和雅拉香波淡色花岗岩中不发育，说明中新世淡色花岗岩岩浆可能在GHCS地区先形成，随后才在THS地区出现，但是热源并不是从南向北逐渐扩展，因为后四个期次大部分中新世淡色花岗岩中都能见到(曾令森和高利娥, 2017; Gao et al., 2017)，因此至今大部分学者认为GHCS和THS地区的中新世淡色花岗岩的成因机制相似(吴福元等, 2015;石卿尚等, 2017;王晓先等, 2016)。Ba-Rb/Sr系统关系(图 12)显示四个区域的淡色花岗岩为主要为白云母脱水熔融的产物(Inger and Harris, 1993; Patiño Douce and Harris, 1998;黄春梅等, 2018)，但不排除部分淡色花岗岩为水致白云母熔融成因(Le Fort, 1975; Gao and Zeng, 2014; Gao et al., 2017)。麻玛沟、亚马荣、错那洞和雅拉香波位于南北走向的错那裂谷带中，其成因机制主要为白云母脱水熔融，在邻近区域也发现了水致白云母熔融成因的18Ma左右的淡色花岗岩体(高利娥等, 待发表数据)，因此喜马拉雅中新世淡色花岗岩可能是两种形成机制共同作用的结果，并导致其岩浆活动至少有五个期次。
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图 12 麻玛沟、亚马荣、错那洞和雅拉香波的中新世淡色花岗岩的Ba-Rb/Sr图解 数据来源：亚马荣据石卿尚等(2017)；错那洞据Xie et al. (2018)和黄春梅等(2018)；雅拉香波据曾令森等(2019) Fig. 12 Ba vs. Rb/Sr diagram for the Miocene leucogranite from the Mama Valley, Yamarong, Conadong and Yardoi Data source: the Yamarong from Shi et al. (2017); the Conadong from Xie et al. (2018) and Huang et al. (2018); the Yardoi from Zeng et al. (2019) |
大量野外和实验数据表明，喜马拉雅造山带绝大多数淡色花岗岩的源岩为变沉积岩。与沉积岩中碎屑锆石普遍具有分散的年龄特征不同，麻玛沟淡色花岗岩的继承锆石年龄非常集中(图 8c, d)，更像岩浆岩中的锆石年龄特征。同时，麻玛沟淡色花岗岩的锆石与本文报道的花岗片麻岩的锆石形态特征相近(图 7)，因此我们推测麻玛沟淡色花岗岩的源岩可能是晚元古代-早古生代花岗质片麻岩。
与多数变泥质岩相比，晚元古代-早古生代花岗岩的原岩白云母含量明显较低，即使发生白云母脱水部分熔融作用，形成的岩浆规模有限。与喜马拉雅造山带其他地区的中新世淡色花岗岩相比，麻玛沟淡色花岗岩的(La/Yb)N比值较低，具更显著的Eu负异常(图 5a)，但K2O/Na2O比值和K2O等主量元素含量相近(图 4)。如果麻玛沟淡色花岗岩的源区为晚元古代-早古生代花岗质片麻岩白云母脱水熔融的产物，那么这些脉状产出的淡色花岗岩可能代表花岗质片麻岩低度部分熔融的产物。
5 结论本文通过对错那县麻玛沟的花岗片麻岩和淡色花岗岩进行锆石SHRIMP U-Pb年龄和全岩成分的研究，并结合前人的数据，有如下认识：
(1) 本次研究得出麻玛沟花岗片麻岩和淡色花岗岩中继承锆石的结晶年龄分别为500.7±4.5Ma和498.6±3.4Ma，属于早古生代岩浆作用的产物。
(2) 通过岩石主量元素和锆石形态可知，麻玛沟花岗片麻岩和淡色花岗岩均为过铝质花岗岩，并相对富集Cs、Rb、U、Pb，亏损Zr、Hf、Nb/Ta等与非造山型或造山后花岗岩区别开来，属于造山型花岗岩，由此推知在早古生代期间印度大陆北缘为俯冲汇聚型大陆边缘。
(3) 结合前人数据推知，从晚元古代末期开始，印度大陆北缘从被动陆缘转变为主动陆缘，可能从东部开始俯冲，并逐渐向西部扩展，形成自东向西的俯冲模式。
(4) 麻玛沟淡色花岗岩可能为晚元古代-早古生代花岗岩低程度部分熔融的产物。结合前人数据，推测喜马拉雅中新世淡色花岗岩可能为白云母脱水熔融和水致白云母熔融共同作用的结果，并导致其岩浆活动至少有五个期次。
致谢 感谢中国地质科学院地质研究所董昕副研究员和中国科学院地质与地球物理研究所王佳敏副研究员审阅稿件，并提出诸多建设性意见和建议。
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