岩石学报  2018, Vol. 34 Issue (3): 513-546   PDF    
藏南冈底斯带中段始新世岩浆作用的厘定及其大地构造意义
孟元库1,2,3 , 许志琴3,4 , 高存山5 , 徐扬2 , 李日辉2     
1. 山东科技大学地球科学与工程学院, 青岛 266590;
2. 中国地质调查局青岛海洋地质研究所, 青岛 266071;
3. 中国地质科学院地质研究所, 北京 100037;
4. 南京大学地球科学与工程学院, 南京 210023;
5. 山东省第七地质矿产勘查院, 临沂 276006
摘要:本文对藏南冈底斯带中段的花岗岩类和角闪辉长岩进行了锆石U-Pb年代学和全岩地球化学分析,据此阐明了岩体的形成机制与演化过程,并探讨了成岩时的大地构造背景。分析结果显示,研究区内花岗岩类和角闪辉长岩体的LA-ICP-MS锆石U-Pb定年结果为41~55Ma,为始新世早-中期岩浆活动的产物,代表了区内岩体的成岩年龄。在地球化学组成上,花岗岩类属于钙碱性到高钾钙碱性系列,均富集轻稀土(LREE)和大离子亲石元素(LILE)(Rb、Ba和K),强烈亏损Nb、Ta、P等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。此外,花岗岩类的铝饱和指数(A/CNK)小于1.1,属于准铝质到弱过铝质的I型花岗岩。角闪辉长岩为石榴橄榄岩部分熔融的产物,并在后期侵位的过程中遭受到了壳源物质的混染。综合分析表明,研究区内的岩体形成于初始碰撞向主碰撞的转化阶段。始新世早期(~50Ma)新特提斯洋板片的断离引起软流圈物质上涌,导致岩石圈地幔发生部分熔融形成基性岩浆,随后基性岩浆底侵至下地壳并诱发下地壳发生部分熔融形成花岗岩质岩浆,最后经过岩浆混合作用形成始新世早-中期冈底斯地区的花岗岩类。
关键词: 冈底斯     花岗岩类     地球化学     始新世     锆石U-Pb定年    
The identification of the Eocene magmatism and tectonic significance in the middle Gangdese magmatic belt, southern Tibet
MENG YuanKu1,2,3, XU ZhiQin3,4, GAO CunShan5, XU Yang2, LI RiHui2     
1. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China;
2. Qingdao Institute of Marine Geology, China Geological Survey, Qingdao 266071, China;
3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
4. School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;
5. The 7th Institute of Geology and Mineral Exploration of Shandong Province, Linyi 276006, China
Abstract: We conducted systematically zircon LA-ICP-MS U-Pb dating and the whole-rock geochemical analyses for the granitic plutons and hornblende gabbro pluton in the middle Gangdese magmatic belt, southern Tibet. Based on studied results, we elucidate their petrogenesis, evolutionary geodynamic processes and tectonic setting. The zircon U-Pb analyses demonstrate that crystallization and emplacement ages of the plutons range from 41Ma to 55Ma, belonging to Early-Middle Eocene magmatic activities. Geochemically, the granitic samples lie in calc-alkaline and high K calc-alkaline fields, integrated with enrichments of LREE and LILE (Rb, Ba and K) and depletions of Nb, Ta and P (HFSE), suggesting arc-related geochemical affinities. Moreover, the Aluminium saturation index (A/CNK) of the granitic samples show the metaluminous and weak peraluminous features characterized by I-type granitic rocks. Hornblende gabbro was the product of partial melting of garnet peridotite and was contaminated by crustal material during ascending. Based on petrology, geochemistry, geochronology and regional setting, we address that the granitoids and related igneous rocks were formed in a transitional tectonic setting from initional collision to syn-collisional setting. In the early stage of Eocene, the breakoff of Neo-Tethys oceanic slab triggered asthenosphere upwelling, resulting in partial melting of lithospheric mantle forming basic magmas. Subsequently, the basic magmas were emplaced to lower crust and resulted in partial melting of lower crustal materials forming granitic magma through magma mixing in the Gangdese belt.
Key words: Gangdese     Granitoids     Geochemistry     Eocene     Zircon U-Pb dating    

青藏高原号称“世界屋脊”,是研究大陆动力学的理想场所(Yin and Harrison, 2000; 莫宣学等, 2009; 许志琴等, 2011; Zhang et al., 2017)。位于青藏高原腹地的拉萨地体记录了多期次碰撞造山和特提斯洋演化的关键信息(许志琴等, 2006)。分布于拉萨地体南缘的冈底斯岩浆带(图 1a),常被看作是新特提斯洋盆向北消减、闭合以及印度-亚洲陆陆碰撞的产物,保存有大量“构造-岩浆-变质-成矿”的时空和历史演化记录(Chung et al., 2003; Dong et al., 2005; 莫宣学等, 2005, 2009; 潘桂棠等, 2006; 侯增谦等, 2008; Ji et al., 2009a, b; Mo et al., 2009; 徐旺春, 2010; Zhu et al., 2011, 2015, 2017; Zhang et al., 2013, 2017; Tang et al., 2015; Wang et al., 2015a; Sun et al., 2017)。

图 1 青藏高原大地构造简图(a, 据莫宣学等, 2009)和冈底斯带中段地质简图(b, 据西藏自治区地质调查院, 2002, 2006修改)) Fig. 1 The tectonic sketch of the Tibetan Plateau (a, after Mo et al., 2009) and simplified map of the middle Gangdese batholith, southern Tibet (b)

西藏自治区地质调查院. 2002. 1:25, 0000日喀则市幅地质图

西藏自治区地质调查院. 2006. 1:25, 0000拉萨市幅地质图

冈底斯岩浆带的研究始于20世纪60年代,研究内容主要集中于地质年代学、矿物岩石学、地球化学以及同位素地质学,并且取得了重要的突破和进展,这为进一步了解青藏高原的形成和演化奠定了坚实的基础(金成伟和周云生, 1978; 涂光炽等, 1981; 张玉泉等, 1981; 桂训唐等, 1982; Maluski et al., 1982; Schörer et al., 1984; 许荣华和金成伟, 1984; Harris et al., 1985; Chang et al., 1986; Debon et al., 1986; Chung et al., 2003, 2005; Hou et al., 2004; Mo et al., 2005, 2007, 2008; 潘桂棠等, 2006; 赵志丹等, 2006; 侯增谦等, 2008; Wen et al., 2008; Zhu et al., 2008, 2011, 2015, 2017; Ji et al., 2009a, b; 徐旺春, 2010; Ma et al., 2015; 邱检生等, 2015; Wang et al., 2015a, b, 2016; Meng et al., 2016a, b)。尽管前人对冈底斯带进行了深入的研究,但在一些关键问题上仍然存在着广泛的争议。古新世-始新世的岩石成因目前仍然存在着激烈的争论。早期的学者认为该时期的花岗岩类是新特提斯洋板片持续俯冲的产物,具有岛弧花岗岩的地球化学特征(Schörer et al., 1984; Searle et al., 1987)。后来部分学者通过对林子宗火山岩的系统研究后认为该时期的岩浆具有同碰撞的成因机制,认为印度-亚洲大陆的碰撞,巨大的阻力使得新特提斯洋板片的俯冲速率减慢,有充分的时间与上覆热的岛弧岩石圈平衡并达到一定的温压条件后,使其发生部分熔融(莫宣学等, 2003, 2005; Mo et al., 2008; 徐旺春, 2010)。而另一种普遍观点却认为,该时期的岩浆活动和新特提斯洋板片的回转(rollback)、断离(breakoff)等具有密切的关系(Wen et al., 2008; Ji et al., 2009a, 2012, 2016; Wang et al., 2015a; Zhu et al., 2015; 王青, 2016; Ma et al., 2017a)。对于始新世花岗岩类的起源,部分学者认为其主要源于新生地壳的部分熔融(丛源等, 2012; 黄勇等, 2015; 孟元库等, 2015),而Guan et al. (2012)对同时期的花岗质岩石和闪长质包体进行研究后认为,花岗质岩石和闪长质包体分别来自加厚下地壳和富集岩石圈地幔部分熔融的产物。印度-亚洲碰撞的时限:尽管大多数学者认为印-亚板块碰撞的时间为始新世早期(~55Ma)(Ji et al., 2009a, 2016; 许志琴等, 2011; Donaldson et al., 2013; Zhu et al., 2015; Hu et al., 2015, 2016; 王青, 2016),但也有少数学者认为这一事件发生在~45Ma(Wang, 2017),甚至晚到34Ma左右(Aitchison et al., 2007)。此外,藏南冈底斯地区地壳增厚的时间:构造地质学证据显示拉萨地体地壳增厚的时间开始于晚白垩世初期(~100Ma)(Murphy et al., 1997; Kapp et al., 2005),而来自于冈底斯带火成岩的地球化学和岩石学证据显示地壳增厚开始于古新世末期-始新世初期(Mo et al., 2007; Guang et al., 2012; Ji et al., 2012; Jiang et al., 2014)。综上所述,冈底斯带始新世岩浆作用的厘定对理解新特提斯洋板片演化的方式、藏南冈底斯地区地壳增厚的时间和限定印-亚大陆碰撞的时限具有重要的科学意义。

针对以上问题,本文在前人工作的基础之上,以花岗岩类为研究对象,对冈底斯带中段研究程度较弱和高海拔地区进行了样品采集。通过对采集的岩体进行锆石U-Pb年龄和岩石地球化学分析,精确厘定了各个岩体的成岩时代和岩石成因类型、归属,据此反演成岩时的大地构造背景,探讨了相关的壳-幔作用过程。

1 区域地质背景

青藏高原从南向北主要由喜马拉雅、拉萨、羌塘、松潘甘孜和祁连-昆仑地体组成,这些地体和特提斯洋的演化具有密切的关系(李才, 1987; Yin and Harrison, 2000; 潘桂堂等, 2006; 许志琴等, 2011; Zhang et al., 2017)。特提斯洋的开启、俯冲、闭合伴随着地体间的裂解和碰撞。经过多期次的俯冲、碰撞最终形成了现今的高原地貌。拉萨地体位于雅鲁藏布江缝合带和班公湖-怒江缝合带之间(图 1a),是欧亚板块和印度板块发生拼贴、碰撞的主要场所,记录了强烈的构造变形和岩浆活动(Ji et al., 2009a, b; 莫宣学等, 2009; 许志琴等, 2011; Zhu et al., 2011, 2015; 孟元库等, 2016c, d)。冈底斯岩浆带又称南拉萨地体,北侧以狮泉河-隆格尔-措麦断裂为界,南侧紧邻雅鲁藏布江缝合带,主要出露由岩基和大岩株构成大规模的花岗岩带及同碰撞中酸性火山岩带(林子宗火山岩系),两者共同占据冈底斯岩浆带总面积的60%以上(莫宣学等, 2003, 2009; Ji et al., 2009b; 徐旺春, 2010; Zhu et al., 2011; Wang et al., 2015a)。

冈底斯岩浆带中段及其邻区主要出露岩性以花岗岩类和林子宗火山岩为主,其次分布有少量侏罗纪-白垩纪的沉积岩系(图 1b)。冈底斯带中段的花岗岩类主要岩性包括闪长岩、花岗闪长岩、石英闪长岩、石英二长岩、正长岩、二云母花岗岩和二长花岗岩以及少量基性杂岩体(董国臣等, 2008; Ji et al., 2009b; 莫宣学等, 2009; 邱检生等, 2015)。前人研究表明,在冈底斯带中段南木林卡孜乡、大竹卡以及曲水其奴村等分布有晚三叠世(212~202Ma)的花岗岩和角闪辉长岩,它们多以岩株、岩瘤或者岩滴的形式产出在古新世-始新世花岗岩类中。目前,这些岩体是冈底斯带中段年龄最老的的侵入体,它们的形成和新特提斯洋早期向拉萨地体的北向俯冲有关(Chu et al., 2006; Ji et al., 2009a; Meng et al., 2016a; Ma et al., 2017b)。随后,在冈底斯带中段南缘还识别出了早-中侏罗世的花岗岩类和部分基性杂岩体,花岗岩类主要由闪长岩、花岗闪长岩和花岗岩组成,基性杂岩体主要以角闪辉长岩为主,其次还分布有少量辉长闪长岩、角闪石岩等。早-中侏罗世的岩石成因也归结为新特提斯洋板片向拉萨地体的北向俯冲,其中辉长岩类主要是俯冲板片流体交代上覆地幔楔部分熔融的产物,花岗岩类主要是新生地壳的部分熔融(张宏飞等, 2007; Ji et al., 2009b; 邱检生等, 2015; Meng et al., 2016a, b; Ma et al., 2017b)。白垩纪花岗岩体在冈底斯带中段地区分布较为零星,主要分布在冈底斯带东段波密、察隅等地(莫宣学等, 2009)。古新世-始新世是冈底斯带中段岩浆活动最为剧烈的时期(莫宣学等, 2003, 2009; Zhu et al., 2015; Wang et al., 2015a, b),该时期的花岗岩类构成了冈底斯带的主体。曲水岩基是古新世-始新世时冈底斯带中段的代表性复式岩体,岩性以花岗闪长岩、石英闪长岩、二长花岗岩以及正长花岗岩为主。SHRIMP锆石U-Pb定年显示,曲水岩基的主体形成于47~53Ma,是印度-亚洲碰撞的产物(Ji et al., 2009b; 莫宣学等, 2009; 徐旺春, 2010)。此外,曲水岩基的花岗岩类中普遍发育有镁铁质微粒包体(mafic microgranular enclaves, MME),Mo et al. (2005)Dong et al. (2005)对其进行了地质年代学和地球化学研究,结果表明基性岩浆底侵与花岗岩的成因具有密切的关系。除了广泛分布的古新世-始新世花岗岩体外,在冈底斯带中段谢通门-尼木-曲水-墨竹工卡一带还分布有规模较小的花岗斑岩体,他们多以岩株或岩脉的形式产出,成岩时代主要集中10~20Ma(侯增谦等, 2008; Wang et al., 2015a)。现已发现的大规模铜多金属矿产资源和这些斑岩体具有密切的成因联系(Wang et al., 2015a)。

除了花岗岩之外,在冈底斯带中段的中-北部地区分布有大面积的林子宗火山岩。林子宗火山岩从下到上可以分为三个组:典中组(2400m)、年波组(700m)和帕那组(2200m)(莫宣学等, 2003; Zhou et al., 2004)。通过对林周盆地林子宗火山岩的典型剖面同位素测年(40Ar-39Ar测年),获得了三个火山岩组的形成时代为65~41Ma,其中典中组为65~60Ma,年波组为60~50Ma,帕那组为50~41Ma(莫宣学等, 2003, 2009; Zhou et al., 2004)。对于林子宗火山岩的成因,莫宣学等(2003, 2009)认为林子宗火山岩是俯冲到碰撞转换的产物。

2 采样位置及样品特征分析

本文所采集的样品主要位于冈底斯带中段南缘,冈底斯逆冲断裂(Gangdese thrust, GT)以北,紧邻雅鲁藏布江缝合带(图 1b)。研究区主要出露花岗岩类,此外还发育有少量的中新生代沉积地层。从研究区西侧至东侧,可以划分为五个采样区域,分别为仁庆则、东嘎乡北、大竹卡北、尼木北和曲水北,野外GPS采样点见表 1

表 1 藏南冈底斯带中段野外采样GPS坐标点 Table 1 The GPS data of sampling locations in the middle Gangdese belt, southern Tibet
2.1 仁庆则地区

采样岩体邻近仁庆则乡(图 1b),野外露头良好(图 2a),为典型的正长花岗岩,似斑状结构(图 2b),主要由石英(20%~25%),正长石(条纹长石)(55%~60%)和斜长石(~10%)组成,还含有少黑云母(5%~10%)以及副矿物榍石、锆石、磷灰石、磁铁矿等(~5%)(图 2c, d)。镜下矿物特征主要为:黑云母呈浅黄绿色到暗绿色,属于富铁黑云母;石英为他形粒状,可见动态重结晶形成的亚颗粒(图 2c, d);钾长石为半自形-自形,发育明显的卡式双晶,可见变形纹以及后期构造运动形成的剪裂隙;斜长石为半自形-自形,可见弱的聚片双晶(图 2c, d)。野外观测显示,该岩体遭受过轻微的韧性剪切,为典型的糜棱岩化花岗岩。除了部分矿物的机械破碎和部分长石有高岭土化外,该岩体整体较为新鲜,其余矿物没有发生明显的蚀变(图 2)。

图 2 仁庆则地区岩体野外露头及镜下显微照片 Qtz-石英;Pl-斜长石;Kfs-钾长石;Bt-黑云母;Sph-榍石 Fig. 2 Field photos and typical micrograph photos of granites in Renqingze area Qtz-quartz; Pl-plagioclase; Kfs-K-feldspar; Bt-biotite; Sph-sphene
2.2 东嘎乡地区

采样岩体位于东嘎乡北侧(图 1b),岩体野外露头较好(图 3a, b),为典型的块状构造,中-粗粒结构(图 3c)。主要岩性以正长花岗岩和黑云母花岗岩为主,野外岩性呈渐变过渡关系。正长花岗岩主要矿物由石英(~20%),斜长石(~10%),钾长石(~60%)和黑云母(5%~10%)组成,副矿物包括锆石、磷灰石、榍石等(0~5%)(图 3e)。黑云母花岗岩,似斑状结构(图 3f),主要由石英(~25%),钾长石(~50%),黑云母(~15%),斜长石(~5%)组成;此外,含有少量副矿物和新生矿物(5%~10%);钾长石可见明显的卡式双晶和格子双晶(微斜长石)。镜下特征显示,长石多已粘土化和绢云母化,表面较为浑浊(图 3f)。

图 3 东嘎乡北花岗岩体野外露头及镜下显微照片 Fig. 3 Field photos and typical micrograph photos of granites in northern Dongga area
2.3 大竹卡地区

采样岩体位于大竹卡北侧,主要由两个相邻的小型岩株组成(图 1b)。位于最北侧的小型岩株野外露头良好,块状构造,中粒结构(图 4a, b),岩性为典型的闪长岩,其中斜长石含量为~55%,钾长石为~15%,石英~10%,黑云母10%~15%,角闪石5%~10%。其中,斜长石可见明显的聚片双晶,钾长石可见卡式双晶,此外,黑云母外围可见角闪石的反应边,表明了部分黑云母由角闪石蜕变而来(图 4d)。另外一个小岩体为典型的角闪辉长岩,野外为块状构造(图 4c),中细粒结构(图 4e),主要由角闪石(~45%),斜长石(~45%),石英(~5%)以及其他副矿物组成(~5%)。角闪石镜下为半自形晶体,可见典型的灰绿色到墨绿色干涉色;斜长石镜下以半自形晶为主,石英为他形、粒状(图 4e)。

图 4 大竹卡地区采样岩体野外露头及样品显微照片Hbl-角闪石 Fig. 4 Field photos and typical micrograph photos of granitoids in Dazhuka area Hbl-hornblende
2.4 尼木地区

采样岩体位于尼木县西北15km(图 1b),交通较为便利。该岩体出露规模中等,露头较好(图 5a),块状构造,中-粗粒结构,岩性为黑云母花岗岩。主要组成矿物为斜长石(~45%),钾长石(~15%),石英(~25%),黑云母(~10%)以及少量副矿物和新生矿物(~5%)(图 5b-c)。长石颗粒以自形到半自形为主,石英颗粒为他形粒状。单偏光下,石英颗粒表面较为干净,而长石表面较为浑浊(图 5b, c)。此外,斜长石可见明显的聚片双晶,钾长石可见卡式双晶,黑云母可见典型的棕褐色干涉色(图 5c),其次,黑云母的边部可见角闪石的反应边,表明了黑云母很可能由角闪石蜕变而来(图 5b)。

图 5 尼木北地区采样岩体野外露头及显微照片 Fig. 5 Field photo and typical micrograph photos of granite in northern Nyemo area
2.5 曲水地区

采样岩体位于曲水县城西北20km(图 1b)。前人的研究主要集中在曲水岩基的南缘(Ji et al., 2009a, b; 莫宣学等, 2009; 徐旺春, 2010; Wang et al., 2015a; Ma et al., 2016),而曲水岩基中北部岩体(即本文中的采样岩体)由于自然条件和交通等原因的限制,研究程度相对较低。1:25万区域地质调查时,曲水岩基北部岩体多以K-Ar法进行定年,所获得的年代多为晚白垩世,本文首次通过精确的锆石U-Pb法对其成岩时代进行了厘定(始新世)。采样岩体为典型的块状构造,中-粗粒结构,岩性为黑云母二长花岗岩(图 6a, b)。黑云母二长花岗岩主要由斜长石(~35%),钾长石(~25%),石英(25%~30%),黑云母(10%~15%)组成。斜长石可见明显的聚片双晶,钾长石可见明显的格子双晶和卡式双晶(图 6c-f)。野外及镜下显示,该岩体风化较弱,整体较为新鲜。

图 6 曲水北部地区采样岩体野外露头及显微照片 Fig. 6 Field photos and typical micrograph photos of granite in northern Quxu area
3 分析方法 3.1 锆石U-Pb同位素测试

本论文锆石的分选在河北廊坊科大岩石矿物分选技术服务有限公司和河北省地质测绘院岩矿中心挑选完成。具体步骤如下:首先将所测试的样品物理粉碎,按照重力和磁选的方法进行初步筛选,然后再在双目镜下进一步挑选(挑纯),选出晶形较好、透明度和色泽度高的锆石,并且确保已选的锆石完整,没有微小裂隙或破裂。然后,把已经挑选好的锆石粘在环氧树脂上,经抛光后进行透射光、反射光和阴极发光扫描电镜照相(Cathode-luminescence images)。阴极发光照相在中国地质科学院地质研究所国土资源部大陆构造与动力学重点实验室高分辨扫描电镜-阴极发光实验室(SEM-EDS-CL)完成。最后根据CL图像选择环带较为发育和自形程度高的锆石进行LA-ICP-MS U-Pb定年。

锆石U-Pb定年在南京大学内生金属矿床成矿机制研究国家重点实验室完成。所测试的仪器型号为New Wave 213nm激光剥蚀系统和Agilent 7500s电感耦合等离子质谱仪(ICP-MS),激光束斑直径为32μm,频率为5Hz。实验中,剥蚀物质的载气为He气,气体流速为270mL/min,工作电压为27.1kV,剥蚀激光的能量为29J/am2。U-Pb分馏根据澳大利亚锆石标样GEMOC Gj-1 (207Pb/206Pb=608.5Ma) (Jackson et al., 2004)进行校正,并采用锆石标准样品Mud Tank (740Ma)(Black and Gulson, 1978)作为内标来控制和验证分析精度。每隔8个测点用2个GJ标样校正,另外测试一个MT标样,更加具体的实验操作方法见Griffin et al. (2004)Jackson et al. (2004)。实验结束后,采用Glitter软件进行样品的同位素比值和U-Pb表面年龄数据处理,通常由于204Pb信号太弱以及剥蚀物质中载气204Hg的干扰,该方法不能直接精确地测定样品的年龄。因此,通常利用Com Pb Corr#3_15G方法对所测的数据进行了普通铅矫正(Andersen, 2002)。锆石谐和年龄和加权平均年龄图采用Isoplot 4.11宏加载在Excel 2010中完成(Ludwig, 2003)。

3.2 全岩粉末地球化学测试

全岩粉末地球化学样品的制备是先将所测样品物理粉碎,然后清洗干净,选择新鲜、未风化的样品,然后研磨至200目以上。主量元素和微量元素成分分析在广州澳实矿物实验中心完成,其中主量元素采用熔X-射线荧光谱法(XRF)测定,并利用等离子光谱和化学法进行互相检测。稀土和微量元素采用等离子质谱仪(PerkinElmer)完成。所有分析精度均优于5%。具体步骤及操作流程见Qian et al. (2014)

4 分析结果 4.1 锆石U-Pb定年结果

研究区内所有样品的锆石LA-ICP-MS U-Pb测试结果见表 2,代表性锆石的CL图像见图 7,红色圆圈代表锆石LA-ICP-MS U-Pb测试区域。

表 2 藏南冈底斯带中段始新世岩体锆石LA-ICP-MS U-Pb测年结果 Table 2 Zircon U-Pb dating results of the Eocene plutons in the middle Gangdese, southern Tibet

图 7 研究区采样岩体代表性锆石阴极发光CL图像 Fig. 7 CL images of representative zircons of the studied plutons in the study region
4.1.1 仁庆则地区采样岩体锆石U-Pb定年结果

阴极发光CL图像显示,锆石颗粒大小相对均匀,半自形到自形,短柱状到长柱状,大小从80μm到200μm不等,长短轴之比为1:1到3:1,部分锆石内部发育有暗色包体。CL阴极发光图中锆石可见明显的岩浆韵律环带(图 7a),并且所测锆石的Th/U比值均大于0.4(表 2),反映所有测试的锆石均为岩浆成因锆石(Hoskin and Schaltegger, 2003; Wu and Zheng, 2004)。在207Pb/235U-206Pb/238U谐和图上,测点均落在谐和曲线上或者谐和曲线附近(图 8a),表明锆石在形成后没有明显的普通铅丢失。对206Pb/238U年龄进行加权平均计算,该岩体(样品Mk441)的形成年龄为40.6±0.4Ma(95%置信度,MSWD=0.56),结果精确度较高,可以准确的反映采样岩体的成岩年龄。

图 8 冈底斯带中段仁庆则及东嘎乡地区采样岩体锆石U-Pb年龄谐和图 Fig. 8 Zircon U-Pb concordia diagrams of granitic plutons from Renqingze and Dongga areas in the middle Gangdese belt
4.1.2 东嘎地区采样岩体锆石U-Pb定年结果

东嘎地区采样岩体的锆石CL图像显示,大多数被测锆石为透明-半透明状,锆石颗粒大小相对均匀,半自形到自形,短柱状到长柱状,颗粒大小从150μm到50μm不等,长短轴之比为1:1到3:1,部分锆石内部可见暗色包体,少量锆石发生蜕晶化(图 7b)。CL阴极发光图像中锆石可见明显的岩浆韵律环带(图 7b),结合锆石形态学和Th/U比值(>0.4)(表 2),所测试的锆石均为岩浆结晶锆石(Hoskin and Schaltegger, 2003; Wu and Zheng, 2004)。在207Pb/235U-206Pb/238U谐和图上,5件样品均落在谐和曲线上或者谐和曲线附近(图 8b-f),表明锆石在形成后没有明显的普通铅丢失。对五组样品的206Pb/238U年龄进行加权平均计算,获得的年龄分别为:样品Mk961为46.9±0.6Ma(95%置信度,MSWD=0.65);样品Mk941为46.6±0.6Ma(95%置信度,MSWD=0.35);样品Mk934为49.9±0.9Ma(95%置信度,MSWD=1.14);样品Mk922为47.6±0.9Ma(95%置信度,MSWD=2.0);样品Mk321为46.9±0.6Ma(95%置信度,MSWD=0.86)。5件样品的206Pb/238U年龄在误差范围内相一致,代表了东嘎地区采样岩体的成岩时代。

4.1.3 大竹卡地区采样岩体锆石U-Pb定年结果

大竹卡地区采样岩体包括闪长岩和角闪辉长岩。CL图像显示,闪长岩(Mk132)的锆石颗粒为自形-半自形,锆石具有明显的岩浆韵律环带(图 7c),大小从60μm到180μm,长宽比为1:1到3:1。角闪辉长岩的锆石颗粒多为半自形(Mk252),可见较弱的岩浆韵律环带,颗粒相对较小,大小从40μm到100μm不等,长宽比为1:1到2:1(图 7c)。2个岩体的锆石均具有较高的Th/U比值(>0.4)(表 2),为典型的岩浆成因锆石(Hoskin and Schaltegger, 2003; Wu and Zheng, 2004)。在207Pb/235U-206Pb/238U谐和图上,所有测点均位于谐和曲线上或者谐和线附近(图 9a, b),表明了锆石没有明显的普通铅丢失。2件样品的锆石206Pb/238U加权平均年龄代表了闪长岩和角闪辉长岩的成岩年龄(95%置信度,Mk132=48.8±0.8Ma,MSWD=0.28;95%置信度,Mk252=48.2±0.6Ma,MSWD=1.19)。

图 9 冈底斯带中段大竹卡、尼木和曲水地区采样岩体锆石U-Pb年龄谐和图 Fig. 9 Zircon U-Pb concordia diagrams of Dazhuka, Nyemo and Quxu granitic plutons in the middle Gangdese belt
4.1.4 尼木地区采样岩体锆石U-Pb定年结果

采样岩体位于尼木县西北方向,为典型的中-粗粒黑云母花岗岩。CL图像显示,锆石以自形晶为主,可见明显的环带和扇状结构,大小为70μm到150μm,长宽比为1:1到2:1(图 7d)。Th/U比值高(Th/U>0.4)(表 2),反映了所有测试锆石均为岩浆成因锆石(Hoskin and Schaltegger, 2003; Wu and Zheng, 2004)。在207Pb/235U-206Pb/238U谐和图上,测试点均位于谐和线上或者谐和线附近,表明没有遭受明显的后期热事件的影响和普通铅丢失(图 9c)。对所获得结果进行加权平均年龄计算206Pb/238U=55.1±0.7Ma(95%置信度,MSWD=1.03),该年龄代表了尼木地区采样岩体的成岩年龄。

4.1.5 曲水地区采样岩体锆石U-Pb定年结果

CL图像显示,所测试的3件样品锆石均为半透明状,半自形到自形,长度从75μm到300μm,长宽比为1:1到4:1,局部可见暗色包裹体,CL图中锆石可见岩浆韵律环带(图 7e)。高的Th/U比值(>0.4)(表 2)反映所有测试的锆石均为岩浆结晶锆石(Hoskin and Schaltegger, 2003; Wu and Zheng, 2004)。在207Pb/235U-206Pb/238U谐和图上,测试点均位于谐和线上或者谐和线附近,表明没有明显的普通铅丢失(图 9d-f)。对3件样品进行加权平均年龄计算,样品M521为46.9±0.7Ma(95%置信度,MSWD=0.21);样品M531为47.8±0.6Ma(95%置信度,MSWD=0.9);样品M541为47.1±0.4Ma(95%置信度,MSWD=0.86)。3件样品的年龄在误差范围内一致,代表了曲水地区采样岩体的成岩年龄。

4.2 始新世岩体地球化学分析结果

野外共采集岩石地球化学样品30件,分析结果见表 3

表 3 藏南冈底斯带中段始新世岩体地球化学测试结果(主量元素:wt%;稀土和微量元素:×10-6) Table 3 Geochemical data of the Eocene plutons in the middle Gangdese batholith, southern Tibet (major elements: wt%; trace elements: ×10-6)
4.2.1 主量元素

除1个基性岩体外,本区花岗岩类的SiO2含量为55.77%~74.77%,Al2O3含量相对较低,介于13.22%~17.12%,全碱含量(Na2O+K2O)变化较大,介于6.29%到9.23%。此外,部分样品相对富钠,Na2O/K2O比值为0.58~2.72。所测样品的里特曼指数(σ)介于1.82~3.12之间,为典型的钙碱性系列; 极个别样品的σ值1.41,为钙性系列。在莱特碱度和SiO2的协变图解上(图 10a),绝大多数样品投在钙碱性区域。在TAS图解上(图 10b),所有样品均为亚碱性系列,岩性以花岗闪长岩和花岗岩为主,少部分样品落在辉长岩到闪长岩范围。在SiO2-K2O图解上,样品属于典型的钙碱性到高钾钙碱性系列(图 10c)。图 10d中显示,铝饱和指数A/CNK值为0.69~1.11,属准铝质到弱过铝质(样品Mk9-4-2的A/CNK=1.11为过铝质)。

图 10 冈底斯带中段始新世岩体地球化学属性判别图解 (a) SiO2-A.R.图解(Wright, 1969);(b) SiO2-Na2O+K2O图解(Irvine and Baragar, 1971; Middlemost, 1994);(c) SiO2-K2O图解(Peccerillo and Taylor, 1976);(d) A/CNK-A/NK图解(Maniar and Piccoli, 1989虚线代表I-S型花岗岩的分界线,据Chappell and White, 1992) Fig. 10 Geochemical characteristic diagrams for the Eocene plutons in the middle Gangdese belt (a) SiO2 vs. A.R diagram (Wright, 1969); (b) Diagrams of SiO2 vs. Na2O+K2O (Middlemost, 1994; Irvine and Baragar, 1971); (c) SiO2 vs. K2O diagram (Peccerillo and Taylor, 1976); (d) A/CNK vs. A/NK (Maniar and Piccoli, 1989; dotted line for I-S type granites discrimination boundary, after Chappell and White, 1992)
4.2.2 微量及稀土元素

研究区内所测样品的稀土总量变化较大,∑REE介于12.15×10-6~665.6×10-6,其中辉长岩体∑REE相对较小(表 3);轻、重稀土元素之间的分馏非常明显,LREE/HREE的平均比值为19.0,(La/Yb)N=4.21~78.9。LREE相对富集,HREE相对亏损,轻稀土的内部的分异也非常明显,(La/Sm)N=1.90~20.3,平均比值为6.19。在球粒陨石标准化稀土元素配分图上(图 11a),所测试的30件样品均为典型的右倾型,铕Eu异常的变化范围较大(δEu=0.63~2.19,δEu平均值为1.01)。采自大竹卡地区角闪辉长岩样品铕Eu的正异常很可能和源区角闪石的残留有关,曲水地区的样品也表现出一定铕Eu的正异常,说明了源区可能存在斜长石的堆晶作用。而其他区域采集的样品具有一定程度铕Eu的负异常,表明了源区存在斜长石的分离结晶。在原始地幔标准化微量元素蛛网图解上(图 11b),大部分样品强烈富集Rb、Ba、Th等大离子亲石元素(LILE)以及La、Ce等轻稀土元素,而强烈亏损Nb、Ta、P等高场强元素(HFSE),表现出弧型或者壳源岩浆岩的地球化学属性。部分样品Sr的负异常可能和源区斜长石的结晶分离有关,而大多数样品P的负异常主要受控于磷灰石等矿物的制约。另外,所有花岗岩样品的Sr/Y的比值均大于1(13.9~64.4),Rb/Sr比值均小于1(0.12~0.68),而角闪辉长岩具有相对更小的Rb/Sr比值(0.01)。

图 11 冈底斯带始新世岩体球粒陨石标准化稀土元素配分图(a, 标准化值据Boynton, 1984)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) Fig. 11 Chondrite-normalized rare earth element distribution patterns (a, normalizing values after Boynton, 1984) and primitive mantle-normalized trace element spider diagram (b, normalizing values after Sun and McDonough, 1989) of the Eocene plutons in the Gangdese belt

此外,图 11b显示,花岗岩类在蛛网图上呈现出明显的Ba亏损谷,而角闪辉长岩却表现为Ba正异常,暗示了它们并非同源岩浆分异演化的产物。图 11显示,角闪辉长岩和花岗岩类具有相似的稀土元素配分曲线和微量元素蛛网图解,均富集轻稀土,亏损Nb、Ta等高场强元素,具有弧型岩浆岩的地球化学特征。

5 讨论 5.1 年代学格架

目前冈底斯带中段最老的花岗岩体可以追溯到晚三叠世~212Ma(Chu et al., 2006; Ji et al., 2009a; Ma et al., 2017b)。根据岩浆活动的规律和特征,冈底斯带的岩浆活动可以分为212~152Ma、109~80Ma、65~41Ma和33~13Ma四个阶段(Ji et al., 2009a; 徐旺春, 2010; Ma et al., 2017b)(图 12a)。其中65~41Ma是冈底斯带岩浆活动最为剧烈的时期,该时期的花岗岩带构成了冈底斯岩基的主体,并且在50Ma左右岩浆活动达到了顶峰(Wen et al., 2008; Ji et al., 2009a; Zhu et al., 2011)。本文测年样品共计12件,均分布在冈底斯带中段(图 1b),年龄范围为55.1~40.6Ma,处于冈底斯岩浆活动的第三阶段(65~41Ma),统一为始新世早-中期岩浆作用的产物。本文所测的锆石均为棱柱状,透明-半透明状,具有高的Th/U比值(>0.4),为典型的岩浆结晶锆石,锆石U-Pb年龄横跨始新世早期(55.1Ma)-中期(40.6Ma),主要年龄峰期集中在47Ma左右(图 12b),与冈底斯带岩浆活动的峰期相一致。冈底斯带该时期的岩浆活动可能是印度-亚洲板块初始碰撞的产物(许志琴等, 2011; Zhu et al., 2015; Wang, 2017)。

图 12 冈底斯带花岗岩类锆石U-Pb年龄分布格架图(部分年龄据Ji et al., 2009b; 徐旺春, 2010; Wang et al., 2015a, b; 王睿强等, 2016; Ma et al., 2017a, b修改) Fig. 12 Zircon U-Pb geochronological framework for the granitoid rocks in the Gangedese belt (some age data after Ji et al., 2009b; Xu, 2010; Wang et al., 2015a, b; Wang et al., 2016; Ma et al., 2017a, b)

另外,研究结果显示分布在曲水北部和东嘎乡北部地区的花岗岩体早先被认为是晚白垩世的侵入体,锆石U-Pb测年显示,这些岩体的成岩年龄均为始新世。本次测年结果进一步丰富了冈底斯带中段地区的年代学格架。

5.2 花岗岩成因及岩浆源区

研究区内绝大多数花岗岩体的铝饱和指数A/CNK值小于1.1(图 10d),为典型的准铝质到弱过铝质,具有I型花岗岩的地球化学特征。在矿物组成上,样品主要由斜长石、钾长石、角闪石以及少量黑云母组成,缺乏白云母、堇青石等过铝质矿物,区别于S型花岗岩。在10000Ga/Zr判别图解上,所测样品位于I、S或者M区域中,不同于传统的A型花岗岩(图 13a)。在SiO2-P2O5图解上,花岗岩类样品的P2O5含量随着SiO2含量的升高的而降低,与I型花岗岩的演化趋势线相一致(图 13b)。此外,王德滋和刘昌实(1993)研究认为Rb元素随着壳-幔的分离和陆壳的演化倾向于成熟的地壳中,而Sr元素却倾向于成熟度低、演化不充分的地壳中,因而Rb/Sr比值可以灵敏地记录花岗质岩浆源区的属性。一般Rb/Sr比值>0.9时具有S型花岗岩的特征,<0.9时则为I型花岗岩。研究区内所有花岗岩体的Rb/Sr比值介于0.12~0.68,平均值为0.35,远小于0.9,具有I型花岗岩的地化特征。以上矿物岩石学和地球化学特征均表明了研究区内的花岗岩体为典型的I型花岗岩。通常花岗岩类的Rb/Sr比值大于5表明部分熔融反应和白云母的脱水熔融有关,而Rb/Sr比值小于5与黑云母的脱水熔融有关(Visonà and Lombardo, 2002)。研究区内花岗岩类的Rb/Sr比值介于0.12~0.68,平均值为0.35,均小于5,指示了花岗岩类源区与黑云母的脱水部分熔融有关。

图 13 花岗质岩石10000Ga/Al-Zr判别图解(a, 据Whalen and Chappell, 1988)和SiO2-P2O5判别图解(b, 据Chappell and White, 1992) Fig. 13 10000Ga/Al vs. Zr (a, after Whalen and Chappell, 1988) and SiO2 vs. P2O5 (b, after Chappell and White, 1992) discrimination diagrams for granitoid rocks

在Nb-Nb/Th图解中(图 14a),大多数样品靠近大陆地壳,而远离原始地幔和MORB区域,与弧火山岩具有相一致的地球化学属性;在Nb/Y-Th/Y图解中(图 14b),绝大多数样品远离上地壳,落在Th/Nb=1和Th/Nb=10趋势线之间,接近中下地壳的平均组分。另外,样品的La/Nb比值介于2.59~11.4,平均值为4.49,均远大于1.0而不同于地幔来源的岩浆(DePaolo and Daley, 2000);样品的Rb/Sr比值介于0.12~0.68,平均值为0.35,也接近大陆地壳的平均值(Taylor and McLennan, 1985; Gao et al., 1998)。此外,样品低的MgO和Cr含量(表 3)也暗示了区内花岗岩类来自于壳源物质的部分熔融。

图 14 冈底斯中段始新世花岗质岩石Nb-Nb/Th (a)和Nb/Y-Th/Y (b)判别图解(据Boztu ğ et al., 2007) 数据来源:原始地幔(Hofmann, 1988); 大陆地壳和弧火山岩以及MORB-OIB(Schmidberger and Hegner, 1999) Fig. 14 Nb vs. Nb/Th (a) and Nb/Y vs. Th/Y (b) discrimination diagramsin the middle Gangdese belt (after Boztu ğ et al., 2007) Data sources: Primitive mantle (Hofmann, 1988); continental crust, MORB, OIB and arc volcanic rock (Schmidberger and Hegner, 1999)

在稀土元素配分图和微量元素蛛网图中,区内的花岗岩体均富集轻稀土和大离子亲石元素Rb、Ba、K等,亏损重稀土和Nb、Ta和P等高场强元素(图 11),具有弧型或者壳源岩浆岩的地球化学特征(Kelemen et al., 1990)。此外,样品的重稀土分布较为平坦,(Ho/Yb)N=0.67~1.14,平均数值为0.97,HoN和YbN大体相当,暗示了源区可能有角闪石的存在。另外,所有花岗岩类样品具有低的Y和HREE说明其源区残留相中可能含有石榴子石。其次,花岗岩体的Sr和Yb(以及Sr/Yb和Sr/Y)含量的高低能很好的反映岩浆源区的深度(张旗等, 2006)。测试结果显示(表 3),区内的花岗岩体均具有相对较低的Sr、Yb、Y含量,暗示了花岗岩体形成于中高压环境(0.8~1.5GPa),温-压上相当于麻粒岩相条件,残留相由石榴子石、辉石以及斜长石组成(张旗等, 2006)。在图 15中,所有被测花岗岩样品均落入石英榴辉岩、含石英角闪岩和角闪岩区,也暗示了研究区的花岗岩类起源于中高压环境的基性下地壳。

图 15 冈底斯带中段花岗岩类的(La/Yb)N-YbN图解(据江博明和张宗清, 1985; 朱凯等, 2016) 虚线代表玄武质成分不同源区的熔融趋势.CFB-大陆溢流玄武岩;UM-上地幔 Fig. 15 (La/Yb)N vs. YbN discrimination diagram of granitoid rocks in the middle Gangdese belt (after Jahn and Zhang, 1985; Zhu et al., 2016) Dotted line for melting curves from different basaltic sources. CFB-continental flood basalt; UM-upper mantle

实验岩石学证明在相对较宽的温度和压力条件下,不同组分的陆壳发生部分熔融可以产生不同类型的花岗质岩浆(Winther, 1996; Skjerlie and Patiño Douce, 2002)。一般地壳中基性岩类的部分熔融形成化学成分偏基性的准铝质-弱过铝质的I型花岗岩(Wolf and Wyllie, 1994; Sisson et al., 2005),而地壳中沉积岩系的部分熔融往往形成成分偏酸性的过铝质花岗岩类(Johannes and Holtz, 1996)。在前人研究的基础之上,结合岩石矿物学特征和有效的判别图解,研究区内的花岗岩体很可能来自基性下地壳的部分熔融。

上述论证表明研究区内的岩体为典型的I型花岗岩。I型花岗岩是未经风化的火成岩部分熔融的产物(Chappell and White, 1974),但是目前普遍的观点认为I型花岗岩的形成过程中有幔源物质的加入,是壳源岩浆和幔源岩浆不等比例混合的产物(Griffin et al., 2002; Kemp et al., 2007)。在反映岩浆演化的MgO-FeOT(图 16a)和Rb/Sr-Ti/Zr(图 16b)图解中,所有样品呈现出远离结晶分异曲线,而趋近于岩浆混合曲线,表明在花岗质岩浆形成的过程中曾经发生过岩浆混合作用(Castro et al., 1991)。此外,在La-La/Sm(图 17a)和Th-Th/Nd(图 17b)图解中,所有始新世早-中期的样品均表现出正相关性且沿一条倾斜的直线分布,表明了研究区内的花岗岩体在形成的过程中经历了部分熔融和岩浆混染作用(Schiano et al., 2010)。与以上岩石近乎同时出现的岩浆混合作用在冈底斯带南缘已经得到了充分的证明(Dong et al., 2005; Mo et al., 2005, 2007, 2008, 2009; 董铭淳等, 2015; 邱检生等, 2015; Ma et al., 2017a)。

图 16 藏南冈底斯中段始新世样品MgO-FeOT (a, 据Zorpi et al., 1991)和Rb/Sr-Ti/Zr (b, 据de Hollanda et al., 2003)图解 Fig. 16 MgO vs. FeOT (a, after Zorpi et al., 1991) and Rb/Sr vs. Ti/Zr (b, after de Hollanda et al., 2003) diagrams of Eocene rocks in the middle Gangdese, southern Tibet

图 17 藏南冈底斯中段始新世样品La-La/Sm (a)和Th-Th/Nd (b)图解(据Schiano et al., 2010) PM-部分熔融;AFC-同化重结晶作用;MM-岩浆混合 Fig. 17 La vs. La/Sm (a) and Th vs. Th/Nd (b) diagrams of Eocene rocks in the middle Gangdese, southern Tibet (after Schiano et al., 2010) PM-partila melting; AFC-assimilation fractional crystallization; MM-magmas mixing

冈底斯带早-中始新世(53~40Ma)的花岗岩中普遍发育有MME包体(Dong et al., 2005; 董国臣等, 2008; 邱检生等, 2015; Ma et al., 2017a)。Dong et al. (2005)Mo et al. (2005)对MME包体和围岩进行研究后认为,基性岩浆底侵和岩浆混合作用与始新世花岗岩的成因具有密切的关系。在花岗质岩浆形成的过程中,底侵的基性岩浆扮演了重要的角色,一方面为花岗质岩石的源区发生部分熔融提供了热源,另一方面通过岩浆混合作用改变了花岗质岩石的成分(Foley and Wheller, 1990; Sajona et al., 1996; 莫宣学等, 2009; Ma et al., 2017b),这一过程也得到了实验岩石学的支持。一般情况下,1g玄武质岩浆的温度从1200℃降到775℃时,释放的能量可以产生3.5g的花岗质熔体(Wiebe et al., 2004)。研究结果显示,除个别样品之外,大部分样品普遍具有较高的Mg#值(>40)(表 3),这与有幔源基性物质加入时形成的岩石Mg#值相一致(Rapp and Watson, 1995)。在冈底斯带中段南缘的谢通门-曲水地区分布有20余个大小不等的基性杂岩体,它们多呈岩株、岩瘤或者岩滴状分布(Dong et al., 2005; Mo et al., 2005; 董国臣等, 2008; 邱检生等, 2015)。在岩石成因上,它们和花岗岩之间具有密切的关系(Dong et al., 2005; Mo et al., 2005; 莫宣学等, 2009; 邱检生等, 2015)。在野外,MME包体相对比较发育的花岗岩体主要分布在冈底斯岩基的南缘,紧邻基性岩体,而区内个别低Mg#值(<40)的样品分布相对靠北,远离基性岩体。

因此,研究区内花岗岩体的形成可能和幔源基性岩浆的底侵有关。基性岩浆的底侵带来了大量的热和流体/熔体,导致了研究区内基性下地壳的部分熔融,在部分熔融的过程中,基性岩浆也参与了花岗质岩浆的演化(混合作用),最终形成了始新世早-中期的花岗岩类。此外,从分离结晶模拟构筑的微量元素判别图中可以看出(图 18),研究区内的花岗岩体在成岩过程中存在着角闪石和斜长石等矿物的分离结晶。

图 18 冈底斯带中段采样岩体Sr-Ba (a)和Sr-Rb/Sr (b)关系图及其分离结晶趋势(矿物分配系数据Rollinson, 1993) Pl-斜长石;Kfs-钾长石;Bt-黑云母;Amp-角闪石 Fig. 18 Sr vs. Ba (a) and Sr vs. Rb/Sr (b) diagrams showing the fractional crystallization trends of the sampled plutons in the gangdese belt (after Rollinson, 1993) Pl-plagioclase; Kfs-K-feldspar; Bt-biotite; Amp-amphibolite
5.3 角闪辉长岩成因

角闪辉长岩野外呈岩滴状产出,共采集2件地球化学样品和1件年龄样品。地球化学分析显示,2件样品具有较为一致的SiO2含量(48.48%和49.01%)。锆石CL图像显示所测的锆石为半自形-自形,粒度较小,呈现出简单的宽条状结构,Th/U比值大于0.4,属于典型的岩浆结晶锆石。锆石U-Pb定年结果显示角闪辉长岩的成岩年龄为48.2Ma,和研究区内花岗岩类的成岩年龄相近。2件样品的Mg#分别为53和52,接近原生玄武质岩浆的Mg#值范围。图La/Yb-Sm/Yb显示(图 19a),角闪辉长岩的源区可能是~20%石榴橄榄岩部分熔融的产物。此外,相对平坦的HREE和低的Y值,反映了岩浆源区可能存在石榴子石残留相,减压部分熔融深度小于50km(Atherton and Ghani, 2002),并且在后期岩浆就位的过程中遭受到了壳源物质的混染(图 19b)。

图 19 大竹卡角闪辉长岩La/Yb-Sm/Yb判别图解(a, 据Johnson et al., 1990)和(Th/Nb)N-Nb/La图解(b, 据Kieffer et al., 2004; 标准化数值据Sun and McDonough, 1989) Fig. 19 Plot of La/Yb vs. Sm/Yb (a, after Johnson et al., 1990) and plot of (Th/Nb)N vs. Nb/La (b, after Kieffer et al., 2004; normalized values from Sun and McDonough, 1989) for hornblende gabbro samples from Dazhuka area
5.4 大地构造意义

印度-亚洲板块的碰撞是地球上显生宙以来最为壮观的地质事件之一,两板块的碰撞导致了青藏高原整体的隆升,引起了气候和环境的变化。然而,关于碰撞的时限至今仍然存在着广泛的争议,众说纷纭,碰撞时限从70Ma到34Ma均得到不同学者的支持(Garzanti et al., 1987; Yin and Harrison, 2000; Aitchison et al., 2002, 2007; Mo et al., 2007; Najman et al., 2010; 许志琴等, 2011; Bouilhol et al., 2013; Donaldson et al., 2013; DeCelles et al., 2014; Lippert et al., 2014; Hu et al., 2015, 2016; Yang et al., 2015; Zhu et al., 2015; Ding et al., 2016; Wang, 2017)。通过梳理前人的研究成果(图 20),大多数学者支持印度-亚洲板块的碰撞应该在古新世末期到始新世初期完成(60~55Ma)。因此,始新世岩浆作用对限定印度-亚洲板块初始碰撞时间和对藏南冈底斯地区地壳增厚机制的理解具有重要意义。

图 20 印度-亚洲板块碰撞时限综述 Fig. 20 Over-review of Indian-Asian plates collision time

花岗岩类的形成和大地构造环境具有密切的关系(Pitcher, 1979; Barbarin, 1999; Winter, 2001; 王涛等, 2017),有效的判别图解为了解花岗岩的成岩环境提供了可能。通常Pearce et al. (1984)Pearce and Peat (1995)判别图解在区分火山弧花岗岩和同碰撞花岗岩方面具有较大的优势。Ji et al. (2009b)对冈底斯带花岗岩类进行投图后发现几乎所有的样品全部落入火山弧花岗岩,因此Pearce图解无法区分中生代火山弧花岗岩和新生代碰撞及后碰撞花岗岩。为此,张旗等(2008)认为Sr-Yb判别图解能较为成功地区别造山前、造山和造山后的花岗岩类。在Sr-Yb图解上(图 21a),几乎所有样品均落入了Ⅰ、Ⅱ和Ⅳ区域,说明了此时地壳具有一定的厚度,应处于碰撞阶段或造山阶段。在Zr-Y图解上(图 21b),区内的花岗质样品均落入板内构造环境,也暗示了始新世早期印度-亚洲板块已经发生碰撞,藏南地区处于碰撞环境。另外,Hu et al. (2016)强调,印度大陆最北缘碎屑岩物源变化(亚洲碎屑物质最早到达印度北缘)的时间和缝合带两侧沉积盆地性质变化(周缘前陆盆地启动)的时间最接近初始碰撞时间。而其它的方法(岩浆、构造、变质以及古地磁等)虽然也能对初始碰撞时间提供约束,但由于精度和约束条件的限制,不如地层记录准确,给出的“碰撞时间”滞后于真正的初始碰撞时间,其误差可达5~40Ma。在方法学认识的基础之上,再结合缝合带两侧的地层记录,Hu et al. (2016)将印度-亚洲大陆初始碰撞时间精确限定为古新世中晚期(59±1Ma),且碰撞作用沿走向无明显的穿时性。综上所述,本文认为印度-亚洲板块碰撞的初始时间不晚于55Ma。

图 21 藏南冈底斯中段花岗岩类的Sr-Yb(a, 据张旗等, 2008)及Zr-Y相关构造背景判别图解(b, 据Müller and Groves, 2001) Fig. 21 Sr vs. Yb (a, after Zhang et al., 2008) and Zr-Y discrimination (b, after Müller and Groves, 2001) diagrams of granitoid rocks in the Gangdese, southern Tibet

古新世-始新世是冈底斯地区岩浆大爆发的时期,该时期的花岗岩类和林子宗火山岩构成了冈底斯岩浆带的主体(莫宣学等, 2003, 2005, 2009; Mo et al., 2008; Zhu et al., 2011; Ma et al., 2017b)。本论文所采集的样品锆石U-Pb年龄为41~55Ma,主要形成于早-中始新世。所有样品均显示出俯冲带型岩浆岩的地球化学特征(图 11)。然而,Ji et al. (2009b)认为只要岩浆源区熔融的过程中残留有金红石、钛铁矿等富集高场强元素(Nb、Ta、Ti等)的矿物,就会使得岩浆的源区亏损高场强元素,显示出俯冲带型岩浆岩的特征。在前人研究的基础上,通过对研究区内的数据进行梳理,发现在~50Ma时,Th/Y、La/Yb和Sr/Y的比值随着时间显著增加(图 22)。通常情况下,地壳与地幔相比而言含,Th的含量相对较高,但是它们二者却有相似的Y含量(Rudnick and Gao, 2003)。Th/Y比值的升高暗示了再造地壳的增加,而具有弧岩浆的La/Yb比值的升高通常和地壳的加厚有关(Haschke and Günther, 2003)。高的La/Yb比值暗示了重稀土的亏损和岩浆源区石榴子石以及角闪石的残留有关(Kay et al., 1994)。通常情况下,稀土的分馏和角闪石在源区的残留以及低压下角闪石的分异有关,在源区没有石榴子石时,角闪石单独存在时将会引起稀土配分曲线呈现出一个向上的“凹”型模式,即表现为中稀土(MREE)的亏损,显示出较高的La/Yb比值(Kay et al., 1989)。高的La/Yb比值和强烈分馏的稀土配分模式(图 11a)暗示了花岗质样品的源区存在石榴子石和角闪石。一般而言,石榴子石能稳定的存在于地壳厚度大于40km,压力大于1.2GPa的镁铁质岩浆中(Rapp and Watson, 1995)。因此,研究区的样品暗示了冈底斯中段地区在始新世早-中期时地壳厚度大于40km,这也和Zhu et al. (2017)近期取得的认识相一致。此外,王青(2016)首次在冈底斯岩基中报道了43Ma的中等程度分异和高分异的花岗岩,并结合它们的HREEs和Y含量,认为冈底斯地区地壳在43Ma以前已经增厚了。在Sr/Yb图解中,~50Ma之后Sr/Yb比值的不断升高也暗示了地壳处于一个持续加厚的阶段。来自冈底斯岩浆带其他的证据显示,到了晚始新世时,冈底斯地区的地壳厚度已经达到50~55km(Ji et al., 2012; Zhu et al., 2017)。以上研究表明,从始新世早期开始到始新世中期,冈底斯地区的地壳厚度处于一个持续加厚的过程。

图 22 藏南冈底斯带中段研究区始新世岩体Th/Y比值(a)、La/Yb比值(b)和Sr/Y比值(c)与年龄协变关系图解(文献数据来自Ji et al., 2012; Wang et al., 2015b) Fig. 22 Th/Y vs. ages (a), La/Yb vs. ages (b) and Sr/Y vs. ages (c) diagrams of Eocene plutons in the middle Gangdese belt, southern Tibet (Reference data from Ji et al., 2012; Wang et al., 2015b)

冈底斯地区该时期地壳增厚原因可能包括三个因素:1)印度板块的下插;2)地壳缩短;3)岩浆底侵(Gill, 1981; Sheffels, 1990; Chung et al., 2009; Ji et al., 2012; Wang et al., 2015b)。地壳加厚通常被认为是一个与构造相关的过程。然而,Mo et al. (2007)Chung et al. (2009)研究认为岩浆作用在地壳加厚方面同样扮演有重要的作用。例如在Andes地区,玄武质岩浆的底侵(underplating)是地壳加厚的主要的因素(Atherton and Petford, 1993; Haschke et al., 2002)。前人研究表明,晚白垩世时(90~80Ma)冈底斯地区经历了地壳的强烈缩短和加厚(Kapp et al., 2007),形成了晚白垩世埃达克岩(管琪等, 2010; 孟繁一等, 2010)。然而,晚白垩世末时(~69Ma)增厚的岩石圈地幔发生拆沉(Kapp et al., 2007; Ji et al., 2012),进一步加速了新特提斯洋板片的回转导致了软流圈物质的上涌,诱发了古新世时冈底斯地区强烈的岩浆活动,此时藏南冈底斯地区地壳的平均厚度为35km,恢复到了正常的地壳厚度(Ji et al., 2012)。随后,印度-亚洲板块的碰撞,巨大的阻力使得新特提斯洋板片的俯冲角度变陡(董国臣等, 2011),并在其结合部位应力不断积累,形成窄的裂谷,最终导致板片撕裂并发生断离(break off)(~50Ma)(王青, 2016)。这也得到了地球物理和数值模拟方面的支持(李忠海和许志琴, 2015; Liang et al., 2016)。

冈底斯岩基中的MME包体和林子宗火山岩中的基性岩墙群的形成时代介于53~47Ma(Dong et al., 2005; Mo et al., 2005),与围岩花岗质岩石以及林子宗火山岩形成的时代相一致,均被认为和新特提斯洋板片的断离有关(Wen et al., 2008; Chung et al., 2009; Ji et al., 2009a, b; Lee et al., 2009)。板片的断离导致软流圈地幔减压熔融上涌,形成了始新世早期(55~47Ma)的玄武质岩浆。随后上涌的玄武质岩浆底侵到冈底斯地区地壳下部,直接导致了冈底斯地区地壳厚度的垂向增厚。此外,底侵的玄武质岩浆促使了冈底斯地区新生下地壳的部分熔融,这一新生地壳的部分熔融与底侵的玄武质岩浆经过岩浆混合形成了始新世早期的花岗岩类。冈底斯岩基中的MME包体是持续底侵的基性岩浆侵入到部分结晶的花岗质岩浆时,由于两者存在明显的黏度差,使得彼此不能完全混合,主要以机械混合(mingling)为主,形成了囊状的基性体——MME包体(喻思斌等, 2016)。始新世中期(45~40Ma)的岩浆作用是新特提斯洋板片断离后的岩浆响应(王青, 2016)。

Replumaz et al. (2010)van Hinsbergen et al. (2012)研究认为印度克拉通和亚洲板块前缘的硬碰撞(hard collision)大约发生在35Ma,后碰撞主要发生在渐新世和中新世。此外,Chu et al. (2011)选择亏损地幔和印度地壳作为二元混合的端元组分,对冈底斯岩带始新世的花岗质岩石进行计算,结果表明该时期的岩浆主要以拉萨地体新生地壳组分为主,印度陆壳物质的贡献不足5%。其次,冈底斯带始新世早-中期的花岗岩类Hf同位素组成较为亏损,明显不同于Hf同位素富集的印度陆壳(Ji et al., 2009a; 徐旺春, 2010; 王睿强等, 2016)。基于前人研究的基础上(Chung et al., 2009; Mo et al., 2007; Replumaz et al., 2010; van Hinsbergen et al., 2012; Wang et al., 2015b; 王青, 2016; Ma et al., 2017a),我们认为始新世早-中期地壳的加厚可能和岩石圈缩短及印度陆壳的下插没有直接的关系,主要和新特提斯洋板片断离导致软流圈物质上涌引起岩石地幔部分熔融形成的基性岩浆的底侵有关。综上所述,冈底斯带始新世早-中期强烈的岩浆作用和地壳增厚是新特提斯洋板片断离引起软流圈物质上涌的结果。

6 结论

(1)研究区采样岩体的锆石LA-ICP-MS U-Pb年龄为55~41Ma,属于始新世早-中期阶段岩浆活动的产物。这些年龄代表了研究区内采样岩体的成岩年龄。

(2) 在地球化学属性上,区内的花岗质样品属于准铝质到弱过铝质、钙碱性到高钾钙碱性的I型花岗岩。所有样品均富集轻稀土,相对亏损重稀土,富集大离子亲石元素,亏损Nb、Ta和P等高场强元素,具有弧型岩浆的地球化学属性。

(3) 研究区花岗岩类起源于拉萨地体新生基性下地壳的部分熔融;角闪辉长岩源于石榴橄榄岩的部分熔融,并在后期侵位的过程中遭受到了壳源物质的混染。

(4) 始新世岩浆大爆发很可能和新特提斯洋板片的断离有关。板片的断离引起了软流圈物质上涌,诱发岩石圈地幔发生部分熔融形成大规模基性岩浆,而基性岩浆的底侵是藏南地区始新世早期(~50Ma)地壳加厚的主要因素。

致谢 两位评审专家对论文进行了认真地审阅,从论文结构、立意和内容等方面提出了中肯的修改意见,使论文的质量得到了质的提升,让笔者受益匪浅,在此对两位专家表示衷心的感谢。在藏南野外地质调查中,得到了孙敬博助理研究员、陈希杰副研究员、马绪宣博士和司机赤来曲扎的协助。锆石LA-ICP-MS U-Pb定年方面,得到了南京大学内生金属矿床成矿机制研究国家重点实验室武兵高级工程师和姜鼎盛硕士研究生的大力帮助。
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