岩石学报  2017, Vol. 33 Issue (12): 3710-3728   PDF    
喜马拉雅造山带纳木那尼穹隆古元古代岩石单元深熔作用
胡古月1 , 曾令森2 , 高利娥2 , 陈翰3 , 刘秋平4 , 郭英帅3     
1. 中国地质科学院矿产资源研究所, 国土资源部成矿作用与资源评价重点实验室, 北京 100037;
2. 中国地质科学院地质研究所, 北京 100037;
3. 成都理工大学地球科学学院, 成都 610059;
4. 中国地质大学地球科学与资源学院, 北京 100083
摘要:纳木那尼穹窿位于特提斯喜马拉雅带西段,属变质杂岩体,由黑云母片麻岩、花岗片麻岩、糜棱岩、混合岩、变杂砂岩、角闪岩、大理岩及后期侵位的电气石花岗岩和二云母花岗岩组成。本次研究对穹隆核部出露的混合岩、花岗片麻岩、电气石花岗岩及边缘出露的二云母花岗岩和黑云母片麻岩进行了岩相学、锆石U-Pb定年及地球化学研究,结果表明:(1)混合岩(T0768-4A-4C)锆石206Pb/238U谐和图上交点年龄为1873±28Ma,207Pb/206Pb加权平均年龄为1877±21Ma。混合岩Sr同位素比值(1.25018~1.44452)和εNdt)值(-28.8~-28.5)指示其具其有低喜马拉雅岩石单元的地球化学属性;(2)花岗片麻岩锆石核部206Pb/238U谐和图上交点年龄为1878±9Ma,下交点年龄为10.9±0.5Ma。个别震荡环带边记录有13.1±0.3Ma的年龄数据,表明古元古代花岗片麻岩可能经历了~10Ma左右的熔融事件;(3)侵位于古元古代混合岩和花岗片麻岩之中的电气石花岗岩(T0768-LG)具有与深熔事件相一致的年龄,其206Pb/238U谐和年龄为9.0±0.2Ma;(4)穹隆核部电气石花岗岩εNdt)值集中在(-18.9~-16.1),显著低于穹隆边缘的二云母花岗岩(εNdt)=-14.4~-10.3),指示电气石花岗岩部分熔融源区有更多成熟地壳物质的加入;(5)个别电气石花岗岩εNdt)值为-12.6,可能是岩浆上升过程中受到变泥质岩的混染所致。本次在纳木那尼穹隆的研究结果支持19~13Ma左右喜马拉雅造山带发生构造转换的模型(Zhang et al.,2012),并表明这种构造转化可能进一步引发了淡色花岗岩部分熔融源区的变化。南北伸展阶段为深度相对较浅的高喜马拉雅变泥质岩和杂砂岩等发生部分熔融,形成穹隆边缘的二云母花岗岩(~16Ma);进入东西向伸展阶段后,深熔作用导致深部主中央逆冲断层(MCT)附近的古元古代岩石单元和变泥质岩混合源区发生部分熔融,岩浆沿着南北向断裂带上升,形成电气石花岗岩体(~9Ma)。
关键词: 喜马拉雅造山带     纳木那尼穹隆     古元古代岩石单元     深熔作用     晚中新世    
Anatexis of Paleoproterozoic rock units in the Gurla Mandhata dome, Himalayan orogen
HU GuYue1, ZENG LingSen2, GAO LiE2, CHEN Han3, LIU QiuPing4, GUO YingShuai3     
1. MRL Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resource, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. College of Earth Science, Chengdu University of Technology, Chengdu 610059, China;
4. School of Earth Science and Resources, China University of Geoscience, Beijing 100083, China
Abstract: The Gurla Mandhata dome is located in the western Tethyan Himalaya. It consists of biotite gneiss, granitic gneiss, mylonite, migmatite, meta-sandstone, amphibolite and marble intruded by tourmaline granite and two-mica granite. An integrated petrographic, zircon U-Pb dating and geochemical study has been carried out on the migmatite, granitic gneiss, tourmaline granite and two-mica granite in Gurla Mandhata dome. The data results show that:(1) the upper intercept 206Pb/238U age of the migmatite (T0768-4A-4C) is 1873±28Ma, with weighted 207Pb/206Pb age at 1877±21Ma. Meanwhile, Sr (87Sr/86Sr(i)=1.25018~1.44452) and Nd (εNd(t)=-28.8~-28.5) isotopic compositions of the migmatite indicate that it has the same geochemical properties as those of the Lower Himalayan Sequence. (2) The upper and lower intercept 206Pb/238U ages of the granitic gneiss (TG-LE-11) are 1878±9Ma and 10.9±0.5Ma, respectively. A 13.1±0.3Ma magmatic crystallized zircon age was obtained in the granitic gneiss, indicated an anatexis event happened in the Mandhata Massif around 10Ma; (3) the concordant age of tourmaline granites that intruded into the Paleoproterozoic migmatite is 9.0±0.2Ma, in accordance with the anatexis age of the granitic gniess; (4) εNd(t) values (-18.9~-16.1) of tourmaline granite are lower than that of the two-mica granite (εNd(t)=-14.4~-10.3), indicating the contribution of mature crust materials in the source melting region of the tourmaline granitic pluton; (5) the relative lower εNd(t) values (-12.6) of the individual tourmaline granite may be resulted from partly dissemination of the magma druing the rising stage. This study supports the model proposed by Zhang et al. (2012) that the tectonic regime of Himalayan orogen was in a period of transition and adjustment from N-S extension to E-W extension during 19~13Ma, and further induced the transition of the melting source of the Himalayan leucogranite. During the period of the N-S extension stage, melting of the meta-pelite and meta-sandstone formed the two-mica granite (~16Ma); while during the stage of the E-W extension happened along the southern Tibetan rift system, the mixing source of metapelites and Paleoproterozoic crust materials adjacent to the Main Central Thrust (MCT) began to melt, giving rise to the drawing up of the magmas along the S-N normal fault and finally forming the tourmaline-bearing leucogranitic plutons.
Key words: Himalayan orogen     Gurla Mandhata dome     Paleoproterozoic crust materials     Anatexis     Late-Miocene    

地壳物质部分熔融作用,即地壳深熔作用,是造山带构造演化中的重要地质作用(Hollister, 1993; Brown, 2013)。在喜马拉雅造山带,地壳深熔作用与喜马拉雅造山带的构造演化密切相关(Burg and Chen, 1984; Harris and Inger, 1992; Yin, 2000; Lee et al., 2004; Zhang et al., 2004; Zhang et al., 2012; 吴福元等,2015; Gao et al., 2016; 曾令森和高利娥,2017),与喜马拉雅高级变质结晶岩系的快速折返(吴福元等,2015; Liu et al., 2014; Gao et al., 2017)、藏南拆离系(Aoya et al., 2005; Lee and Whitehouse, 2007; Zeng et al., 2009; Wang et al., 2013)和南北向伸展(Gao and Zeng, 2014Liu et al., 2014; 胡古月等,2016)的活动密切相关。新生代淡色花岗岩主体以大型岩体、透镜体或岩脉的形式分布于北喜马拉雅片麻岩穹窿和高喜马拉雅结晶岩系中,少部分以岩脉的形式侵入至低喜马拉雅岩系(Harrison et al., 1997; Visona and Lombardo, 2002; Zhang et al., 2004; 吴福元等, 2015)。早期研究认为这些花岗岩主要形成于28~10Ma (Harris and Massey, 1994; Harris et al., 1995; Guillot and Le Fort, 1995; Harrison et al., 1997, 1999a, b; Simpson et al., 2000; Zhang et al., 2004),是在高喜马拉雅高级岩片快速折返过程中高级变泥质岩白云母脱水部分熔融的产物(Le Fort, 1981; Harrison et al., 1997; Patiño Douce and Harris, 1998)。但最近的研究揭示了在印度-欧亚大陆陆陆碰撞造山作用期间,喜马拉雅造山带经历了复杂多样的深熔作用,如角闪岩部分熔融作用(Zeng et al., 2009, 2011, 2015; 高利娥等,2009; Liu et al., 2014)和变泥质岩水致白云母部分熔融作用(Prince et al., 2001; Zhang et al., 2004; King et al., 2010; Guo and Zeng, 2014; 曾令森和高利娥,2017)。

除了新生代淡色花岗岩外,喜马拉雅造山带广泛发育不同时代和不同地球化学性质的花岗质片麻岩,其原岩时代包括:古元古代(1850~1800Ma) (Le Fort, 1981; Ahmad et al., 2000),新元古代(850~800Ma) (DiPietro and Isachsen, 2001; 许志琴等,2005; Richards et al., 2006; Chu et al., 2011; Ahmad et al., 2013; Wang et al., 2017)和早古生代(520~480Ma)(Schärer et al., 1986; Miller et al., 2001; 许志琴等,2005; Gehrels et al., 2006a, b; Cawood et al., 2007; Quigley et al., 2008; 张泽明等,2008高利娥等, 2012, 2015; Wang et al., 2013; 王誉桦等,2014)。许多花岗片麻岩经历了广泛的混合岩化作用,发育含石榴子石的淡色体(Pognante et al., 1990),同时还记录了20~18Ma的变质和部分熔融年龄(许志琴等, 2005),表明花岗质片麻岩也可能发生深熔作用,形成花岗质岩浆。但是,花岗质片麻岩在淡色花岗岩形成中的作用并不清楚。在日玛那穹窿和麻珈穹窿中,中新世淡色花岗岩具有特殊的Sr-Nd同位素系统特征(Zhang et al., 2004; 于俊杰等, 2011; 胡古月等, 2016)。日玛那穹隆其周缘地区发育Sr-Nd同位素特征与低喜马拉雅岩石单元相似的原岩时代为古元古代的正变质岩和新生代淡色花岗岩(Goscombe et al., 2006),预示着低喜马拉雅结晶岩石也可能是喜马拉雅淡色花岗岩的源区之一(吴福元等,2015; 胡古月等,2016)。自中新世以来,日玛那穹隆-麻迦穹隆经历了长期的部分熔融作用,从22~21Ma开始发生部分熔融,在16Ma进一步深熔,沿STDS上升构成定结淡色花岗岩(于俊杰等,2011);随后,在12~10Ma之间,MCT附近的古元古代岩石单元再次发生部分熔融作用,延南北向正断层上升,构成侵位至北喜马拉雅特提斯沉积岩系之中的麻迦淡色花岗岩(胡古月等,2016)。与多数淡色花岗岩相比,麻迦淡色花岗岩具有异常低的Nd同位素组成(Zhang et al., 2004; 胡古月等,2016),表明此类淡色花岗岩的源区可能与原岩时代为古元古代的正变质岩相关。此种深熔机制是否在喜马拉雅造山带具有普遍性,还需进一步开展细致的岩石学和地球化学工作加以验证。

纳木那尼穹隆与日玛那穹隆的构造背景类似,发育在南北向伸展断层下盘,穹隆内的淡色花岗岩形成后仍有被南北向正断层切割的现象(Edwards and Harrison, 1997),在成因上被认为是与南北向正断层的活动相关(Murphy, 2007)。本文重点对纳木那尼穹隆出露的高级变质岩和淡色花岗岩开展元素地球化学、锆石U-Pb定年和Sr-Nd同位素工作,试图探讨喜马拉雅造山带发生构造转换后(< 13Ma),深部MCT附近的古元古代岩石单元在东西向伸展减压构造背景下,发生深熔作用而沿南北向正断层上升,形成具有异常Sr-Nd同位素特征淡色花岗岩体的可能性。

1 地质背景与采样 1.1 喜马拉雅造山带基本构造单元

喜马拉雅造山带呈EW向弧形展布,长约2500km,宽约300~500km,北侧以雅鲁藏布江缝合带(YZS)与拉萨地体南部的冈底斯活动陆缘增生带为邻,南侧以喜马拉雅主前缘逆冲断层(MFT)与印度克拉通为界(图 1)。自南向北依次划分为4个基本构造单元:西瓦里克群(SG)、低喜马拉雅岩系(LHS)、高喜马拉雅结晶岩系(HCS)和特提斯喜马拉雅带(THS) (图 1)。

图 1 喜马拉雅造山带古元古代岩石单元出露的纳木那尼和日玛那穹隆分布简图(据Zhang and Guo, 2007; Guo and Wilson, 2012修改) Fig. 1 Simplified geologic map showing the distribution of Gurla Mandhata dome and Ama Drime dome in Himalayan orogen (modified after Zhang and Guo, 2007; Guo and Wilson, 2012)

西瓦里克组(SG)属喜马拉雅山的前陆沉积,由中新世到更新世的磨拉石组成,其物源为来自喜马拉雅山的剥蚀物质。这些磨拉石沉积物内部褶皱和叠瓦状构造发育,逆冲到第四系的河流冲积层之上,表明喜马拉雅山依然是一个正在活动的造山带。

低喜马拉雅岩系(LHS)主要由形成在印度大陆被动陆缘的早元古代至早寒武纪的浅变质沉积岩和片麻岩组成,其中眼球状花岗质正片麻岩的锆石U-Pb和Sm-Nd年龄均为1850Ma左右(Robinson et al., 2001及其中参考文献),变沉积岩碎屑锆石的年龄为2.6~1.8Ga之间(DeCelles et al., 2000; Richard et al., 2005; Robinson et al., 2006)。

高喜马拉雅结晶岩系(HCS)是一个约30km厚的中、高级变质岩系,主要由800Ma之后形成的变质沉积岩组成,发育早奥陶纪和新生代花岗岩,普遍经历了巴罗型区域变质作用(Parrish and Hodges, 1996; Robinson et al., 2001, 2006; Richard et al., 2005)。一般认为,高喜马拉雅结晶岩系的变质沉积岩为上覆的特提斯喜马拉雅底部沉积岩系变质作用的产物。高喜马拉雅结晶岩系沿向北倾斜的主中央逆冲断裂带(MCT)逆冲到低喜马拉雅之上。但是,高级变质的高喜马拉雅结晶岩系主要由新元古代-寒武纪变质岩组成,而低级变质的低喜马拉雅岩系主要由古-中元古代变沉积岩组成,作为两者之间分界线的新生代断层(MCT)的具体位置一直存在争议,尤其体现在对MCT附近的古元古代Ulleri组糜棱岩的层位划分问题上(Goscombe et al., 2006; Searle et al., 2008)。

特提斯喜马拉雅带(THS)是一个约100km宽的复式向斜带,主要由低级变质或未变质的沉积岩组成,保存了几乎完整的从奥陶纪至始新世的海相沉积地层(Garzanti et al., 1986; Brookfield, 1993),主体沉积物源来自印度大陆的北部边缘(Guillot et al., 2008; Yin, 2006; Hodges, 2000),也有少部分可能来自拉萨地体(Li et al., 2010)。特提斯沉积带内发育一系列的片麻岩穹窿(Zeng et al., 2011),被称为北喜马拉雅片麻岩穹窿带(Zhang and Guo, 2007)。这些变质穹窿在构造上位于特提斯喜马拉雅之下,由新元古代至寒武纪的变质沉积岩和片麻岩组成。这些片麻岩穹窿被始新世至中新世的花岗岩(44~7Ma)侵位(Zeng et al., 2011, 2015; Hou et al., 2012; Zhang et al., 2012; Liu et al., 2014)。

1.2 南北向裂谷

南北向裂谷(NSTR)南段岩墙最老时代为18Ma (Williams et al., 2001),拉萨地体以北的岩墙年龄为47~38Ma (Wang et al., 2010)。一般认为,喜马拉雅地区在19~13Ma发生构造转换,由南北向挤压转化为东西向的伸展(Garzione et al., 2003; Murphy and Copeland, 2005; Thiede et al., 2006; Zhang et al., 2012)。对于裂谷的成因机制,普遍观点认为是青藏高原隆升至其最大高度后垮塌的标志(Colemann and Hodges, 1995),但也存在有南北向挤压(Zhang and Guo, 2007)、侧向挤出(Armijo et al., 1989)或者是岩石圈地幔和地壳的空间异质性(Blisniuk et al., 2001)等诸多观点。青藏高原自晚中新世开始经历广泛的东西向伸展(Yin, 2000, 2006; Zhang and Guo, 2007),所形成的南北向裂谷成为藏南及北喜马拉雅地区最为明显的构造现象(梁维等,2015),可北延至羌塘地块(Yin et al., 1999; Wang et al., 2010),比较著名的有亚东-谷露裂谷、尼玛-定日裂谷和申扎-定结裂谷等。

1.3 纳木那尼穹隆及采样

纳木那尼穹隆位于喜马拉雅造山带片麻岩穹隆带西段(图 1)、雅鲁藏布缝合带南侧(图 2),由一系列西倾的低角度(22°~45°)正断层构成(Murphy et al., 2002),平面上东西跨度约100km,南北跨度约40km (Murphy and Copeland, 2005)。云母Ar-Ar年代学工作表明穹隆西侧出露的片麻岩从9Ma和400℃开始冷却,即推测9Ma以来该地区至少隆升20km以上(Murphy et al., 2002Murphy, 2007)。

图 2 喜马拉雅造山带纳木那尼穹隆地质简图(据河北省地质调查院,2005Pullen et al., 2011修改) Fig. 2 Simplified geological map of Gurla Mandhata dome, Himalayan orogeny (modified after Pullen et al., 2011)

① 河北省地质调查院.2005. 1:25万普兰幅区域地质图

纳木那尼穹隆低角度正断层上盘岩石单元主要是蛇绿岩套、特提斯及新生代沉积岩(图 2),下盘为穹隆内的变质岩和淡色花岗岩。纳木那尼西南坡(Murphy et al., 2002)和小纳木那尼(Pullen et al., 2011)地质与地球化学工作表明,纳木那尼主要由“泛非期”片麻岩(图 3b)及其深熔形成的各类淡色花岗岩构成,但穹隆内部个别混合岩及出熔浅色体的Sr-Nd同位素地球化学特征显示其部分熔融源区有低喜马拉雅的贡献(Murphy, 2007)。穹隆边部普遍发生强韧性剪切糜棱岩化作用(图 3a, d),岩性上可分为糜棱片岩和长英质糜棱片麻岩两类(Murphy et al., 2002)。花岗质糜棱岩SHRIMP锆石定年获得谐和线上交点年龄为1863.8±7.5Ma(戚学祥等,2006)。在穹隆北坡位置,侵位于黑云母片麻岩之中的二云母花岗岩在穹隆隆升过程中发生同变形作用,矿物发生明显的定向排列(图 3h),SHRIMP锆石U-Pb年龄为16.1±0.3Ma(王英超等,2005)。穹隆核部则相对变形较弱,由混合岩、花岗片麻岩、变杂砂岩、角闪岩和少量大理岩等构成(河北省地质调查院,2005)。穹隆核部出露花岗岩主要为电气石花岗岩,其主矿物成分与边部二云母花岗岩的相近,为石英、斜长石和白云母,两者的区别仅表现为副矿物电气石含量的不同(图 3h, i)。电气石淡色花岗岩则侵位于各类型高级变质岩,并有穿切二云母花岗岩的现象。穹隆核部随处可见古元古代糜棱岩和出熔于花岗片麻岩中的含电气石浅色体(图 3c, d)。

图 3 纳木那尼穹隆岩石的野外露头(a-c)、手标本(d)和镜下照片(e-i) (a)纳木那尼穹隆边缘出露的古元古代混合岩和电气石花岗岩体(T0768系列);(b)二云母花岗岩岩体侵入黑云母片麻岩;含电气石浅色体常见于角闪混合岩(c)和糜棱岩(d);(e)变杂砂岩(T0768-GN)镜下照片,主要为定向分布的石英和白云母;(f)含角闪石混合岩(T0768-4A)镜下照片,内含少量石榴子石;(g)纳木那尼北坡采集的花岗片麻岩(TG-LE-11);(h)受南北向正断层作用,二云母花岗岩(T0771-LG)镜下见石英晶格扭曲而发生的波状消光,属塑性变形的产物;(i)纳木那尼电气石花岗岩(T0768-LG)镜下照片. Bt-黑云母;Ga-石榴子石;Ms-白云母;Pl-斜长石;Qtz-石英;Tou-电气石 Fig. 3 Photographs of outcrops (a-c), hand specimen (d) and micro-photographs (e-i) of Gurla Mandhata dome (a) the Paleoproterozoic migmatite and tourmaline granite pluton outcropped at the margin of Gurla Mandhata dome; (b) biotite gneiss intruded by two-mica granite; toumaline-bearing leocosome which can be observed at the amphibolite migmatite (c) and mylonite (d); (e) micro-photograph of meta-sandstone (T0768-GN), composed of quartz and muscovite; (f) micro-photograph of amphobolite-bearing migmatite (T0768-4A), with a few garnets therein; (g) granitic gneiss (TG-LE-11) sampled from the northern slope of Gurla Mandhata dome; (h) the wave extinction of quartz indicating a plastic deformation of the two-mica granite (T0771-LG) as effected by NSTR; and (i) micro-photograph of tourmaline granite (T0768-LG). Bt-biotite; Ga-garnet; Ms-muscovite; Pl-plagioclase; Qtz-quartz; Tou-tourmaline

本次研究沿着纳木那尼穹隆西坡和北坡的山脊露头进行了野外考察和采样(图 2)。在纳木那尼穹隆北坡位置采集含花岗片麻岩样品TG-LE-11,岩石主体由长英质矿物构成,少量黑云母发生定向排列(图 3g)。在穹隆北坡的T0771位置见有二云母花岗岩侵入黑云母片麻岩野外露头(图 3b),采集侵位于黑云母片麻岩之中的二云母花岗岩(T0771-LG系列) (图 3h)。西坡T0768位置发现含电气石花岗岩侵入糜棱岩和条带状混合岩的野外露头(图 3a),采集电气石花岗岩(T0768-LG系列)、含角闪石条带状混合岩(T0768-4A,T0768-4B,T0768-4C)和眼球状变杂砂岩(T0768-GN)(图 3e)。其中,电气石花岗岩内电气石颗粒最大者可达1cm,并可见矿物发生于糜棱岩同方向的剪切变形作用,大颗粒单矿物普遍发生碎裂(图 3i);条带状混合岩由长英质矿物组成浅色条带,角闪石和少量石榴子石组成暗色条带(图 3f)。

2 分析方法 2.1 元素地球化学分析

为确定喜马拉雅造山带纳木那尼穹隆电气石花岗岩(T0768-LG系列)、条带状混合岩(T0768-4系列)及二云母花岗岩(T0771-LG系列)的元素地球化学特征,分析了它们的全岩主量和微量元素组成。主量及微量元素的测试在国家地质实验测试中心完成。其中,主量元素通过XRF(X荧光光谱仪3080E)方法测试,分析精度为5%;微量元素和稀土元素(REE)通过等离子质谱仪分析,含量大于10×10-6的元素的测试精度为5%,而小于10×10-6的元素精度为10%,个别在样品中含量低的元素,测试误差大于10%。分析结果列在表 1中。

表 1 喜马拉雅造山带纳木那尼穹隆淡色花岗岩和混合岩主量元素(wt%)和微量元素(×10-6)地球化学特征 Table 1 Major (wt%) and trace (×10-6)element geochemical characteristics of leucogranite and migmatite in Gurla Mandhata dome, Himalayan orogen
2.2 LA-MC-ICP-MS锆石U-Pb定年

对纳木那尼穹隆出露的花岗片麻岩(TG-LE-11),混合岩(T0768-4A,4C)、电气石花岗岩(T0768-LG)和二云母花岗岩(T0771-LG)进行了破碎和锆石挑选。阴极发光照相(CL)在配备阴极荧光探头的JSM6510型扫描电镜上完成。测试分析在中国地质科学院地质研究所大陆构造与动力学实验室完成。锆石U-Pb定年工作所用的MC-ICP-MS为美国Thermo Fisher公司最新一代Neptune Plus型多接收等离子体质谱仪。采用的激光剥蚀系统为美国Coherent公司生产的GeoLasPro 193nm。激光剥蚀以氦气作为剥蚀物质的载气,激光剥蚀束斑直径为30μm,激光能量密度为10J/cm2,频率为8Hz。锆石中的U、Pb在8000℃以上的高温等离子体中发生离子化,利用动态变焦扩大色散可以同时接收质量数相差很大的U-Pb同位素,从而进行锆石微区U-Pb同位素原位同时测定。每个分析点的气体背景采集时间为4s,信号采集时间为23s。数据分析前用国际上通用的锆石标样91500作为参考物质进行仪器的最佳化,使仪器达到最大的灵敏度、最小的氧化物产率(ThO+/Th+ < 2%)和最低的背景值。选用GJ-1作为辅助标样对数据的准确性进行验证。ICP-MS数据采集选用一个质量峰采集一点的跳峰方式。每测定5~10个样品点,测定一组标样(1个标样91500点和1个GJ-1点)。采用中国地质大学刘勇胜教授研发的ICPMSDataCal程序(Liu et al., 2010)和Ludwig KR的Isoplot程序(Ludwig,2003)进行数据处理;年龄计算以标准锆石91500为外标进行同位素比值分馏。测试结果见表 2

表 2 喜马拉雅造山带纳木那尼穹隆混合岩(T0768-4A-4C)、花岗片麻岩(TG-LE-11)和电气石花岗岩(T0768-LG)的LA-MC-ICP-MS锆石U-Pb定年数据 Table 2 Zircon LA-MC-ICP-MS U-Pb isotopic data of the migmatite (T0768-4A-4C), granitic gneiss (TG-LE-11) and tourmaline granites (T0768-LG) in Gurla Mandhata dome, Himalayan orogen
2.3 Sr和Nd同位素测试

喜马拉雅造山带纳木那尼穹隆的电气石花岗岩(T0768-LG系列)、二云母花岗岩(T0771-LG系列)和混合岩(T0768-4系列)的全岩Sr-Nd同位素测试在中国科学技术大学壳幔物质与环境重点实验室进行。使用Finnigan MAT-262质谱仪获得了Sr同位素组成及Rb、Sr、Sm和Nd的浓度。利用Nu Plasam HR MC-ICP-MS多接收等离子质谱仪进行Nd同位素分析。Sr-Nd同位素结果分别按146Nd/144Nd=0.7219和86Sr/88Sr=0.1194标准化,进行质量分馏校正。在分析样品期间,Sr同位素测试标准物质为NBS987,测试值为0.710247±12(2σ)。Nd同位素标准物质为JMC Nd,测试值为0.511127±12(2σ)。Sr-Nd同位素测试精度分别为±0.000010(n=20)和±0.000011(n=20)。Sr-Nd同位素的初始值参考LA-MC-ICP-MS锆石U-Pb同位素定年数据(见表 2)和王英超等(2005),分析结果见表 3

表 3 喜马拉雅造山带纳木那尼穹隆淡色花岗岩和混合岩Sr-Nd同位素地球化学特征 Table 3 Sr-Nd data for the migmatite and leucogranite in Gurla Mandhata dome of Himalayan orogen
3 数据及结果 3.1 全岩元素地球化学特征

表 1图 4可见,纳木那尼穹隆西坡出露的电气石花岗岩主量元素特征整体类似,但存在差异。电气石花岗岩可分为两类,其中T0768-LG和T0768-LG4的SiO2(70.0%)含量偏低,但FeOT(2.34%~2.95%)和TiO2(0.10%~0.47%)含量相对较高,这与岩石中出现少量黑云母有关。同时,T0768-LG和T0768-LG4的K2O/Na2O (1.27~1.45)比值相对较高,表明其部分熔融源区的泥质组份相对其他T0768-LG系列淡色花岗岩可能更高。其他T0768-LG系列淡色花岗岩与二云母花岗岩主量元素地球化学特征类似,均表现为SiO2含量(73.8%~76.0%和71.8%~74.3%)和Al2O3含量(13.9%~14.8%和14.6%~15.5%)较高,FeO (0.45%~0.68%和0.23%~0.77%)、MnO (0.02%~0.05%和0.01%~0.02%)、MgO (0.17%~0.30%和0.11%~0.30%)和TiO2(0.05%~0.07%和0.02%~0.08%)含量较低。两者的CaO含量均较低(0.57%~0.90%和0.62%~0.91%),未显示水致白云母部分熔融(Gao and Zeng, 2014; Gao et al., 2016b)的特征。

图 4 纳木那尼电气石花岗岩(T0768-LG, T0768-LG4)、电气石花岗岩(T0768-LG系列其他样品)、二云母花岗岩(T0771-LG系列)和混合岩(T0768-4系列)的Al2O3(a)、FeOT(b)、CaO (c)和K2O/Na2O比值(d)与SiO2的关系图解 Fig. 4 Selected major oxides of Al2O3(a), FeOT(b), CaO (c) and K2O/Na2O ratio (d) plotted against SiO2 for the tourmaline granite (T0768-LG, T0768-LG4), tourmaline granite (other samples of T0768-LG series), two-mica granite (T0771-LG series) and migmatite (T0768-4 series) in Gurla Mandhata dome

表 1中,纳木那尼穹隆采集的3件混合岩样品显示高SiO2(73.6%~74.0%)、Al2O3(12.9%~13.1%)、K2O (5.41%~5.76%)、A/CNK(1.01~1.17)、K2O/Na2O(0.81~ 2.17)和低MgO (0.17%~0.20%)和TiO2(0.20%)。在主量元素特征上,与变泥质岩(Harris and Inger, 1992)和日玛那糜棱岩(胡古月等,2016)类似。

在微量元素方面,纳木那尼穹隆出露的电气石花岗岩、二云母花岗岩与喜马拉雅造山带白云母脱水部分熔融成因淡色花岗岩表现出一致的特征。整体显示高度演化的S型花岗岩富集大离子亲石性元素和亏损高场强元素的微量元素地球化学特性(图 5)。其中,T0768-LG4以较高的TZr(850℃)、∑REE (225×10-6)和Ba (597×10-6)区别于其他T0768-LG系列样品。电气石花岗岩和二云母花岗岩具有较低的Zr/Hf比值(14.6~31.5)和Nb/Ta比值(1.73~9.74),显示为高度演化花岗岩的特征(Irber, 1999)。同时,两种花岗岩的Sr含量为13.4×10-6~96.3×10-6,Rb/Sr比值为3.7~41.8,均符合白云母脱水部分熔融的地球化学特征(曾令森和高利娥,2017)。但是,相对于二云母花岗岩和混合岩,电气石花岗岩更富集大离子亲石性元素(如Cs, Tl, Rb)。

图 5 纳木那尼电气石花岗岩(T0768-LG4)、电气石花岗岩(T0768-LG系列其他样品)、二云母花岗岩(T0771-LG系列)及混合岩(T0768-4系列)原始地幔标准化微量元素蛛网图(标准化值据McDonough and Sun, 1995) Fig. 5 Primitive mantle-normalized trace element distribution patterns for the tourmaline granite (T0768-LG4), tourmaline granite (other samples of the T0768-LG series), two-mica granite (the T0771-LG series) and migmatite (the T0768-4 series) of the Gurla Mandhata dome (normalization values after McDonough and Sun, 1995)

在稀土元素方面,电气石花岗岩∑REE值(31.4×10-6~225×10-6)相对二云母花岗岩(16.5×10-6~29.0×10-6)富集,而相对混合岩(752×10-6~785×10-6)和变泥质岩(Harris and Inger, 1992)亏损(图 6)。相对于日玛那糜棱岩和高喜马拉雅变泥质岩,纳木那尼穹隆出露的混合岩显示陡“右倾”的稀土元素配分形式(图 6)。

图 6 纳木那尼电气石花岗岩(T0768-LG3)、电气石花岗岩(T0768-LG4)、电气石花岗岩(T0768-LG系列其他样品)、二云母花岗岩(T0771-LG系列)和混合岩(T0768-4系列)的球粒陨石标准化稀土元素配分图(标准化值据McDonough and Sun, 1995) 日玛那糜棱岩数据胡古月等(2016)、变泥质岩数据于俊杰等(2011) Fig. 6 Chondrite-normalized rare earth elements distribution pattern for the tourmaline granite (T0768-LG3), tourmaline granite (T0768-LG4), tourmaline granite (other samples of the T0768-LG series), two-mica granite (the T0771-LG series) and migmatite (the T0768-4 series) of Gurla Mandhata dome (normalization values after McDonough and Sun, 1995) Ama Drime mylonite data after Hu et al. (2016), and the metapelite data after Yu et al. (2011)
3.2 LA-MC-ICP-MS锆石U-Pb年龄 3.2.1 混合岩(T0768-4A-4C)和花岗片麻岩(TG-LE-11)定年结果

在喜马拉雅造山带西段的纳木纳尼穹窿(Murphy, 2007)和中段的日玛那穹窿(Goscombe et al., 2006; Searle et al., 2008)均发现高级变质岩的Sr-Nd同位素特征上类似于MCT附近Ulleri组的岩石单元。因此,为确定纳木那尼穹窿内出露的混合岩(T0768-4A-4C)和花岗片麻岩(TG-LE-11)的原岩时代,本次研究进行了LA-MC-ICP-MS锆石U-Pb定年工作。混合岩和花岗片麻岩中锆石纵剖面长轴约为200~250μm,短轴约为80~100μm,整体呈柱状,显示震荡环带结构(图 7)。本次研究对纳木那尼穹窿出露的混合岩(T0768-4A-4C)锆石进行了25个点位的测试。其中6个测试点(点位:01, 06, 08, 09, 11, 22)的数据结果谐和度小于90%、1个测试点(T0768-4A-4C-18)误差太大,不做统计;另外18个岩浆锆石尽管其U (72×10-6~1602×10-6)、Th (23×10-6~537×10-6)测试结果显示较大的变化范围,但在206Pb/238U-207Pb/235U图上,均集中投点在一条直线上,上交点年龄为1873±28Ma (MSWD=2.6) (图 8a)。锆石207Pb/206Pb年龄介于1814±4Ma~1944±3Ma之间,加权平均值为1877±21Ma (MSWD=0.21) (图 8b)。这18个岩浆锆石边的LA-MC-ICP-MS测试结果中Th/U比值为0.31~0.95(表 2),锆石CL图像上明显的震荡环带也表明获得的是岩浆年龄。考虑到MSWD值要求大于1,且谐和图上数据在同一条直线上,本研究认为混合岩的原岩形成时代为1873±28Ma。

图 7 纳木那尼穹窿混合岩(T0768-4A-4C)、花岗片麻岩(TG-LE-11)和电气石花岗岩(T0768-LG)的锆石阴极发光图像(CL)和LA-MC-ICP-MS锆石U-Pb定年代表性结果 T0768-4A-4C和TG-LE-11为207Pb/206Pb年龄,T0768-LG和TG-LE-11-17为206Pb/238U年龄 Fig. 7 Cathodoluminescence (CL) images showing the textures and spots for LA-MC-ICP-MS U-Pb analysis on zircon grains from the layered migmatite (T0768-4A-4C), the granitic gneiss (TG-LE-11) and the Tou-granite (T0768-LG) of the Gurla Mandhata dome T0768-4A-4C and TG-LE-11 are ages of 207Pb/206Pb; T0768-LG and TG-LE-11-17 are ages of 206Pb/238U

图 8 纳木那尼穹窿混合岩(T0768-4A-4C)、花岗片麻岩(TG-LE-11)和电气石花岗岩(T0768-LG)的LA-MC-ICP-MS锆石U-Pb定年谐和图和年龄分布图 Fig. 8 U-Pb concordia and age distribution diagrams for LA-MC-ICP-MS zircon U-Pb analyses of the migmatite (T0768-4A-4C), the granitic gneiss (TG-LE-11) and the Tou-granite (T0768-LG) from Gurla Mandhata dome

对纳木那尼穹窿出露的花岗片麻岩(TG-LE-11)锆石进行了25个点位的测试。其中1个测试点(TG-LE-11-08)年龄偏大(2221±3Ma)不作统计。另外24个岩浆锆石在206Pb/238U-207Pb/235U图上,均集中在一条直线(图 8c),上交点年龄为1878±9Ma,下交点年龄为10.9±0.5Ma (MSWD=1.08)。207Pb/206Pb年龄介于1861±3Ma~1902±4Ma之间,加权平均值为1877±9Ma (MSWD=28)。24个岩浆锆石边的LA-MC-ICP-MS测试结果中Th/U比值为0.40~0.77(表 2),锆石CL图像上明显的震荡环带也表明获得的是岩浆年龄,判断原岩(岩浆岩)形成时代为1878±9Ma。另外,花岗片麻岩(TG-LE-11)的锆石测试点TG-LE-11-07位于震荡环带边,年龄为13.1±0.3Ma,因此,认为下交点(10.9±0.5Ma)年龄可能代表了花岗片麻岩的一次深熔作用事件。

因此,LA-MC-ICP-MS锆石U-Pb定年结果表明,纳木那尼混合岩的原岩类型和时代与日玛那穹窿东侧正断层上盘出露的眼球状正片麻岩一致,与MCT附近的Ulleri组古元古代正片麻岩(Robinson et al., 2001)可能属同一岩石单元在不同位置的出露。纳木那尼的古元古代岩石单元可能在10Ma左右经历了一次深熔作用过程,形成了新生岩浆锆石边(TG-LE-11-07)。

3.2.2 电气石花岗岩(T0768-LG)定年结果

为更好地限制纳木那尼穹隆出露的电气石花岗岩(T0768-LG)的岩浆结晶年龄,本次研究对其进行了LA-MC-ICP-MS锆石U-Pb定年工作。在淡色花岗岩锆石阴极发光图像上,电气石花岗岩的大部分锆石颗粒为自形-半自形、长柱状,棱角清晰,粒度在100~250μm之间,长宽比一般为2:1~3:1。部分锆石核部为残留变质或岩浆锆石核,锆石边部具有韵律环带结构,为典型的岩浆锆石(图 7)。

对电气石花岗岩样品(T0768-LG)共选取了25个测点进行LA-MC-ICP-MS锆石U-Pb测年,其中17个测点位于锆石边缘环带,8个测点位于锆石残留核部。边部3个测试点(10, 19, 25)数据结果谐和度小于90%,不作统计。其余锆石边部统计的14个测试点U和Th含量分别在6091×10-6~33410×10-6和84.4×10-6~1119×10-6之间,Th/U值在0.01~0.05。虽然后者数值较正常岩浆锆石偏低,但其具有明显的岩浆震荡环带边,说明其为岩浆成因。测试结果表明:具有韵律生长环带的边部锆石206Pb/238U年龄在8.4~9.5Ma之间,谐和年龄为9.0±0.2Ma (MSWD=3.1)。在这些锆石中,206Pb/238U年龄与U含量之间不存在负相关关系,表明这些锆石的U含量虽然较高,但它们的年龄未受U含量的影响(表 2)。另外,核部锆石U和Th含量分别在268×10-6~6949×10-6和39×10-6~533×10-6之间,Th/U值在0.05~1.77之间(表 2),207Pb/206Pb年龄在347~1748Ma之间,表明这些锆石核部继承了少量古元古代残留锆石,这一特征与麻迦淡色花岗岩的锆石继承核(胡古月等,2016)具有可对比性。

另外,王英超等(2005)曾对纳木那尼穹隆北坡出露的二云母花岗岩进行了SHRIMP锆石定年,获得206Pb/238U谐和年龄为16±0.5Ma (MSWD=1.18)。因此,本次研究采集的二云母花岗岩样品为同一地区,将16Ma作为二云母花岗岩(T0770-LG系列)的岩浆结晶年龄,并作为Sr-Nd同位素计算依据。

3.3 Sr-Nd同位素地球化学特征

混合岩(T0768-4A-4C)样品的Rb含量(215×10-6~230×10-6)较高,Sr含量(19.4×10-6~22.6×10-6)较低,Rb/Sr比值(31.32~34.55)极高。Sm (18.5×10-6~19.1×10-6)、Nd (118×10-6~127×10-6)及Sm/Nd (0.0909~0.0948)比值的变化范围较小。混合岩的初始Sr同位素比值(1.250184~1.444517)高于高喜马拉雅变泥质岩,而接近低喜马拉雅岩石单元和Ulliri组,同时低的εNd(t)值(-28.8~-28.5)表明其属古老地壳源区变质成因。

电气石花岗岩(T0768-LG系列)样品中T0768-LG4显示高Sr (68.9×10-6)、Nd (31.7×10-6)和Sm(7.39×10-6)含量,并以相对较高的εNd(t)值(-12.6)区别于其他的电气石花岗岩。电气石花岗岩(T0768-LG系列其他样品)的Rb含量(236×10-6~572×10-6)较高,Sr含量(13.4×10-6~74.0×10-6)较低,Rb/Sr比值(11.65~121.8)较高。Sm (0.93×10-6~2.71×10-6)、Nd (2.85×10-6~9.32×10-6)及Sm/Nd (0.1491~0.2218)比值的变化范围相对较大。淡色花岗岩的初始Sr同位素比值(0.763205~0.793346)高于高喜马拉雅花岗岩和雅拉香波花岗岩,但仍在喜马拉雅基底范围内,同时低的εNd(t)值(-18.9~-16.1)表明其属于古老地壳源区重熔而来的花岗岩,少量的高εNd(t)值(-12.1)可能是岩浆上升过程中遭受高喜马拉雅变泥质岩(图 9a)的混染。

图 9 喜马拉雅造山带部分岩石单元(a)和纳木那尼穹隆的混合岩和花岗岩以及高喜马拉雅和特提斯喜马拉雅花岗岩(b)Sr-Nd同位素组成 数据来源:特提斯喜马拉雅角闪岩(Zeng et al., 2011);高喜马拉雅和特提斯喜马拉雅变泥质岩(Inger and Harris, 1993; 杨晓松和金振明, 2001; Zeng et al., 2009, 2011, 2012);低喜马拉雅岩系(Robinson et al., 2001; King et al., 2010);雅拉香波始新世花岗岩(Zeng et al., 2009胡古月等, 2011);高喜马拉雅花岗岩(Deniel et al., 1987; 杨晓松和金振明, 2001; Zhang et al., 2004; Liao et al., 2008);纳木那尼混合岩和片麻岩(Murphy, 2007; 本文);纳木那尼电气石花岗岩和二云母花岗岩(本文) Fig. 9 Sr-Nd isotopic compositions of some rock units in the Himalayan orogen (a) and the migmatite and leucogranite in the Gurla Mandhata dome, and leucogranites in the High Himalaya and the Tethyan Himalaya (b) Data source: Amphibolite of the Tethyan Himalaya (Zeng et al., 2011); Metapelites of the High Himalaya and the Tethyan Himalaya (Inger and Harris, 1993; Yang and Jin, 2001; Zeng et al., 2009, 2011, 2012); the rock units in the Lower Himalaya (Robinson et al., 2001; King et al., 2010); the Eocene Yardoi granite (Zeng et al., 2009; Hu et al., 2011); the granites of the High Himalaya (Deniel et al., 1987; Yang and Jin, 2001; Zhang et al., 2004; Liao et al., 2008); the migmatite and gneiss of the Gurla Mandhata dome (Murphy, 2007; this paper); and the tourmaline granite and the two-mica granite of the Gurla Mandhata dome (this paper)

二云母花岗岩(T0771-LG)样品的Rb含量(243×10-6~334×10-6)较高,Sr含量(64.0×10-6~96.3×10-6)较低,Rb/Sr比值(8.95~12.5)相对较低。Sm (0.48×10-6~0.93×10-6)、Nd (1.48×10-6~3.26×10-6)及Sm/Nd (0.1725~0.2344)比值的变化范围相对较大。淡色花岗岩的εNd(t)值(-14.4~-10.3)与高喜马拉雅花岗岩和变泥质岩一致,但初始Sr同位素比值(0.727926~0.733050)相对偏低,表明其可能来自变泥质岩和变杂砂岩混合源区的深熔。

4 讨论 4.1 纳木那尼晚中新世花岗岩部分熔融源区的厘定

纳木那尼穹隆出露的淡色花岗岩岩相上可分为二云母花岗岩和电气石花岗岩两大类。喜马拉雅造山带大部分淡色花岗岩Rb含量高、Sr含量低、Rb/Sr比值较高(Rb/Sr > 1.0),一般被认为是变泥质岩部分熔融的产物(Harris and Massey, 1994),或者是有少量新元古代-早古生代正片麻岩参与深熔作用(于俊杰等,2011)。Rb主要含于云母类矿物中,而Sr主要含于长石矿物中,在饱水部分熔融作用过程中,长石熔融的量远多于云母的量,因而饱水部分不会使熔体中Rb/Sr比值提高(Harris and Inger, 1992; Harrison et al., 1999b; Zeng et al., 2005a)。纳木那尼穹隆出露的二云母花岗岩和电气石花岗岩Rb/Sr大于8.94,显示为云母类矿物脱水部分熔融特征(King et al., 2010)。但是,纳木那尼穹隆的电气石花岗岩相对二云母花岗岩更富集大离子亲石性元素和稀土元素(图 5图 6)。随着花岗质岩浆的演化,大离子亲石性元素不断富集,而REE值不断减低(赵振华,2016)。假如电气石花岗岩是二云母花岗岩的演化晚期阶段,则应显示相对富集大离子亲石性元素和亏损REE,反之亦然。因此,元素地球化学特征表明电气石花岗岩与二云母花岗岩的部分熔融源区尽管都富含云母类矿物,但二者可能有所区别。

Sr-Nd同位素对于判断S型花岗岩的部分熔融源区具有决定性意义。由图 9a可见,高喜马拉雅带、北喜马拉雅带和特提斯带岩石单元的Nd同位素组成较高(εNd(t) > -15.0)(Zeng et al., 2009, 2011; 胡古月等,2011高利娥等,2013Gao and Zeng, 2014)。由古-中元古代变质碎屑岩和片麻岩组成(Brookfield, 1993; Miller et al., 2000; Goscombe et al., 2006; Chambers et al., 2008)的低喜马拉雅结晶岩系εNd(t)值(-29~-18)相对较低。在地壳岩石部分熔融过程中,尤其是较低度部分熔融情况下,熔体的Sr-Nd同位素可能出现同位素不平衡的现象(Knesel and Davison, 2002; Zeng et al., 2005b),但对于规模较大的淡色花岗岩体,它们是部分熔融体累积汇集的结果,其Sr和Nd同位素组成应是部分熔融反应和源区的共同反映,接近于源区的同位素特征(Knesel and Davison, 2002; Zeng et al., 2005a)。从图 9b表 3中可见,二云母花岗岩εNd(t)值(-14.5~-10.3)与纳木那尼穹隆内出露的泛非期黑云母片麻岩(Pullen et al., 2011) Sr-Nd同位素组成(Murphy, 2007)相近,但稍偏低。因此,二云母花岗岩部分熔融源区主体为高喜马拉雅变泥质岩,但较低的初始Sr同位素比值(0.731054~0.735594)指示部分熔融源区可能有大量的变杂砂岩参与,这也与纳木那尼穹隆出露有较多变杂砂岩(图 3e)相一致。

相对而言,电气石花岗岩(T0768-LG系列)的地球化学成分更复杂。样品T0768-LG4以较高的εNd(t)值(-12.6)和∑REE值(225×10-6)区别于其他电气石花岗岩,而更接近于高喜马拉雅变泥质岩的特征(图 5图 6)。纳木那尼穹隆其他的电气石花岗岩以极低的εNd(t)值(-18.9~-12.6)明显不同于喜马拉雅造山带的其他淡色花岗岩(图 9b),说明其源区应具有特殊性。由图 9b可见,纳木那尼穹隆的电气石花岗岩的初始Sr和Nd同位素组成明显低于高喜马拉雅和特提斯喜马拉雅的泛非期变泥质岩和花岗片麻岩,因此,变泥质岩和花岗片麻岩不太可能是纳木那尼电气石花岗岩的直接源区。异常低的Nd同位素组成(εNd(t) < -18.0),要求其源区为比高喜马拉雅变泥质岩更古老的地壳物质。纳木那尼穹窿出露的古元古代混合岩与高喜马拉雅变泥质岩微量和稀土元素地球化学特征十分接近(图 5图 6)。因此,纳木那尼电气石花岗岩的原岩是属于变泥质岩和古元古代正片麻岩的其中一种,亦或是二者的混合。在表 2中可见,电气石花岗岩的锆石核部207Pb/206Pb年龄分布在476~2719Ma,大量泛非期继承核的存在表明部分熔融源区为古元古代岩石单元与泛非期变泥质岩的混合源区。另外,此种古元古代岩石单元在野外和手标本上以出熔含电气石浅色体(图 3c, d)而区别于穹隆之中黑云母片麻岩。古元古代花岗片麻岩206Pb/238U谐和图中下交点年龄为10.9±0.5Ma,锆石单点(TG-LE-11-17)也记录有13.1±0.3Ma的岩浆年龄,说明古元古代岩石单元可能在晚中新世(~10Ma)发生深熔作用,并形成含电气石淡色花岗岩岩体。少数高εNd(t)值(-12.6)样品(T0768-LG4)的出现可能是由于岩浆上升就位过程中混染了高喜马拉雅和北喜马拉雅穹隆主体泛非期变泥质岩(εNd(t)值为-16.9~-11.2(数据参考Inger and Harris, 1993; 杨晓松和金振明, 2001; Zeng et al., 2009, 2011, 2012; Pullen et al., 2011)的结果。

4.2 MCT附近古元古代岩石单元参与部分熔融的构造机制

低喜马拉雅岩系(LHS)为印度大陆被动陆缘的早元古代至早寒武纪的浅变质沉积岩,其碎屑锆石的年龄为2.6~1.8 Ga之间(DeCelles et al., 2000; Richard et al., 2005; Robinson et al., 2006),是喜马拉雅造山带出露的主要的古元古代岩石单元。低喜马拉雅岩系发生绿片岩相变质作用,其上部为主要由800Ma之后形成的中、高级变质岩组成的高喜马拉雅结晶岩系(HCS) (Robinson et al., 2001, 2006; Richard et al., 2005),两者构成著名的喜马拉雅倒转变质带。由图 8可见,本次研究采集自纳木那尼的混合岩和花岗片麻岩样品(T0768-4A-4C和TG-LE-11)锆石核部显示明显的岩浆震荡环带,谐和图上交点年龄分别为1873±28Ma和1878±9Ma。因此,本次研究采集的混合岩和花岗片麻岩具备低喜马拉雅岩石单元的地球化学属性(Goscombe et al., 2006; Liao et al., 2008; 胡古月等,2016)和高喜马拉雅岩石单元的构造属性(Searle et al., 2008)。实际上,此种锆石U-Pb和全岩Sm-Nd年龄为1850Ma左右的眼球状花岗质正片麻岩在喜马拉雅造山带MCT的附近常有出露(Upreti, 1999; Robinson et al., 2001; Imayama and Arita, 2008),并被称为Ulleri组眼球状片麻岩。高喜马拉雅结晶岩系沿北倾的主中央逆冲断裂带(MCT)逆冲到低喜马拉雅之上。在新生代MCT长期活动过程中,剪切面主要在新元古代-寒武纪地层之中发育,但下盘的某些古元古代变沉积岩亦可参与其中。少量参与剪切动力学变质过程的低喜马拉雅岩石单元在构造属性与高喜马拉雅结晶岩系一致,但继承低喜马拉雅岩系的地球化学属性。喜马拉雅造山带出露的Ulliri组眼球状花岗片麻岩(Miller et al., 2000; Robinson et al., 2001)可能是MCT长期活动过程中从中地壳层次带来的古元古代构造岩片。此种构造岩片可能参与了喜马拉雅淡色花岗岩部分熔融过程,形成的熔体就位于纳木那尼穹隆。

由上述讨论可知,二云母花岗岩部分熔融源区为高喜马拉雅变泥质岩和变杂砂岩,岩浆结晶时代为16Ma (王英超等,2005);电气石花岗岩为古元古代岩石单元和泛非期变泥质岩组成的混合源区,岩浆结晶时代为9.0±0.2Ma。因此,纳木那尼地区在16~9Ma的时间范围内发生了淡色花岗岩部分熔融源区的变化。据Zhang et al. (2012)所提出的模式,中新世中期(19~13Ma)喜马拉雅造山带发生了由南北向伸展向南北向挤压的构造体制转换,这一构造体制转换最终完成的时间为13Ma。纳木那尼穹窿位于南北向裂谷带的南北向裂谷内(Murphy et al., 2002),中地壳层次MCT附近的、地球化学特征上接近低喜马拉雅的岩石单元,有可能在东西向伸展引发的深大断裂减压作用下,与变泥质岩一同发生部分熔融,形成Sr和Nd同位素组成上更亲近于低喜马拉雅岩系的晚中新世(< 13Ma)喜马拉雅淡色花岗岩。

另外,胡古月等(2016)曾对麻迦淡色花岗岩和日玛那糜棱岩进行了Sr-Nd同位素和SHRIMP锆石U-Pb定年工作,结果表明花岗岩(~10Ma)表现出高Sr同位素初始比值(0.7621~0.8846)和低εNd(t)值(-20.2~-13.0),糜棱岩显示极低的εNd(t)值(-21.0~-19.6),认为麻迦淡色淡色花岗岩可能代表着变泥质岩和古元古代岩石单元的混合源区部分熔融的结果。纳木那尼穹隆与日玛那穹隆分别位于高喜马拉雅带的西段和中段,相距达800余千米。因此,岩浆结晶年龄 < 13Ma、发育在喜马拉雅造山带南北向正断层下盘的淡色花岗岩体的部分熔融源区可能与传统的变泥质岩和角闪岩区稍有区别。来自中地壳层次MCT附近的古元古代构造岩片可能会参与东西向伸展背景下的减压部分熔融过程,形成εNd(t)值较低的淡色花岗岩体。

5 结论

(1) 纳木那尼出露的古元古代岩石单元包括糜棱岩、混合岩和花岗片麻岩,年龄分布在1863~1877Ma,可能是MCT长期活动过程中混入高喜马拉雅结晶基底的亲低喜马拉雅岩系的构造岩片;

(2) 纳木那尼穹隆出露的淡色花岗岩可分为两期:早期(16Ma)二云母花岗岩来自于泛非期变泥质岩和杂砂岩的深熔作用;晚期(9.0Ma)电气石花岗岩来自于古元古代岩石单元和泛非期混合源区的深熔作用。部分熔融源区的改变与约13Ma喜马拉雅地区发生构造形式转换这一模型相一致。

致谢 感谢中国科学技术大学地球和空间科学学院陈伊翔副教授和中国科学院青藏高原研究所李金祥副研究员在审稿过程中提出的修改意见和建议。
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