岩石学报  2020, Vol. 36 Issue (9): 2631-2645, doi: 10.18654/1000-0569/2020.09.03   PDF    
冈底斯岩浆弧东端泥质岩的变质作用及构造意义
江媛媛1, 张泽明1,2, 丁慧霞1, 秦圣凯2, 李文坛1, 张成圆1, 康东艳1, 莫宣学1     
1. 中国地质大学地质过程与矿产资源国家重点实验室, 中国地质大学地球科学与资源学院, 北京 100083;
2. 中国地质科学院地质研究所, 北京 100037
摘要: 冈底斯岩浆弧东端林芝地区出露的中-高级变质岩来自岩浆弧的中、下地壳,是研究岩浆弧深部组成与形成演化的窗口。本文对林芝布久地区西部产出的泥质片岩进行了岩石学和锆石U-Pb年代学研究。研究表明,含夕线石石榴石云母片岩的峰期矿物组合为石榴石+黑云母+斜长石+白云母+夕线石+石英+金红石,经历了角闪岩相变质作用和部分熔融,峰期变质温度和压力条件为~7.4kbar和~715℃。片岩的进变质和部分熔融作用很可能开始于~70Ma,退变质和熔体结晶作用发生在61~48Ma。本文和现有研究成果表明,冈底斯岩浆弧东端变质岩的变质条件存在明显的空间变化,角闪岩相和麻粒岩相变质带变质岩分别代表岩浆弧的中地壳和下地壳组成。同时,岩浆弧的下地壳主要由变质的基性和长英质岩浆岩组成,含少量变质沉积岩,而中地壳主要由变质的花岗质岩浆岩和变质沉积岩组成。我们认为在晚中生代-早新生代,印度大陆与亚洲大陆的碰撞和俯冲的新特提斯洋岩石圈的断离,引起了岩浆弧地壳构造缩短加厚和幔源岩浆增生,进而导致上地壳的沉积岩被埋藏到中、下地壳,并经历长期持续的高温变质和部分熔融作用。本研究不仅揭示出冈底斯岩浆弧经历了晚中生代-早新生代的变质作用,也为岩浆弧的地壳组成与空间变化提供了重要信息。
关键词: 泥质岩    角闪岩相变质    晚中生代-早新生代    地壳组成    地壳加厚    冈底斯岩浆弧    
Metamorphism of pelites in the eastern Gangdese magmatic arc and its tectonic implications
JIANG YuanYuan1, ZHANG ZeMing1,2, DING HuiXia1, QIN ShengKai2, LI WenTan1, ZHANG ChengYuan1, KANG DongYan1, MO XuanXue1     
1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;
2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract: The Nyingchi area, eastern Gangdese magmatic arc, exposed middle-to high-grade metamorphic rocks, representing the middle and lower crustal components of the arc, and therefore, it provides a window into investigating the deep components, formation, and evolution of the arc. In this paper, we conduct a study of petrology and zircon U-Pb geochronology of pelitic schist from the west of the Bujiu, the Nyingchi area. The results show that the sillimanite-bearing garnet mica schist has the peak assemblage of garnet+biotite+plagioclase+muscovite+sillimanite+quartz+rutile. Phase equilibrium modeling and geothermobarometry reveal that the schists experienced amphibolite-facies metamorphism and partial melting under peak metamorphic conditions of ~7.4kbar and ~715℃. The prograde process and associated partial melting of these schists probably initiated at ~70Ma, and the retrograde process and melt crystallization occurred at 61~48Ma. Combined available results, we conclude that the metamorphic conditions of metamorphic rocks from the eastern Gangdese arc have obvious spatial variation, and amphibolite-facies and granulite-facies metamorphic belt correspond to the component of arc middle crust and lower crust, respectively. Moreover, the Gangdese arc lower crust is composed mostly of meta-mafic and felsic igneous rocks, including minor meta-sedimentary rocks, and meta-granitic igneous rocks and meta-sedimentary rocks are the main components of the middle crust. We suggest that the continental collision of Indo and Asia and the roll-back and breakoff of the subducted Neo-Tethyan slab during Late Mesozoic-Early Cenozoic, resulted in the shortening and thickening of the Gangdese arc crust and the accretion of voluminous mantle-derived magma, which in turn induced the sedimentary rocks from upper crust transported into the middle and lower crust and experienced long-lasting high-temperature metamorphism and partial melting. The present study does not only reveal that the eastern Gangdese arc experienced Late Mesozoic-Early Cenozoic metamorphism, but also provides a key constraint on the component and spatial variation of the arc crust.
Key words: Pelites    Amphibolite-facies    Late Mesozoic-Early Cenozoic    Crustal component    Crustal thickening    Gangdese magmatic arc    

大陆岩浆弧形成在上覆板片为大陆岩石圈的汇聚板块边界,是与板块俯冲有关的岩浆作用产物,是研究板块构造、壳-幔相互作用和大陆地壳生长的天然实验室(Ducea et al., 2015)。由于俯冲作用是全球性的持续过程,因此弧岩浆作用被认为是太古代以来大陆地壳生长的主要机制(Jagoutz and Schmidt, 2012; Jagoutz and Kelemen, 2015; Palin et al., 2016)。位于青藏高原南部的冈底斯岩浆弧形成在中生代新特提斯洋北向俯冲导致的安第斯型造山作用和新生代印度与欧亚大陆碰撞导致的喜马拉雅型造山过程中,是典型的复合型大陆岩浆弧(图 1; Yin and Harrison, 2000)。前人已经对冈底斯弧大面积出露的中-新生代岩浆岩进行了广泛的研究,在岩浆作用和构造演化上取得了大量进展(莫宣学等, 2003, 2005; Chung et al., 2005, 2009; 张宏飞等, 2007; Hou et al., 2004, 2012; Nomade et al., 2004; Chu et al., 2006; Kapp et al., 2007; Liao et al., 2007; Mo et al., 2007, 2008; Zhu et al., 2008, 2011, 2015, 2018; Ji et al., 2009; Zhao et al., 2009; Wu et al., 2010)。然而,目前对岩浆弧的深部地壳组成,以及地壳的生长和再造过程的研究还相对薄弱(Guo et al., 2011, 2012; Zhang et al., 2013, 2014, 2015)。

图 1 青藏高原和冈底斯岩浆弧(a)及冈底斯弧东端(b)地质简图(据Zhang et al., 2020修改) 已发表的岩石变质条件数据来自于:董昕等(2012)Zhang et al. (2013)Palin et al. (2014)康东艳等(2019)张成圆等(2020) Fig. 1 Sketch geological map of the Tibetan Plateau and Gangdese magmatic arc (a) and geological map of the eastern Gangdese arc (b) (modified after Zhang et al., 2020)

在冈底斯弧东南缘林芝地区,由于新近纪以来地壳的抬升和剥蚀作用,出露了一系列中-高级变质岩(图 1b)。这些变质岩的原岩主要为中、新生代弧岩浆岩,因此,代表了冈底斯弧的中-下地壳组成,是研究岩浆弧的深部组成和变质-深熔-岩浆作用的窗口(Burg et al., 1997; Searle et al., 2011; Zhang et al., 2010b, 2020; Palin et al., 2014)。最近的研究表明,这些变质岩经历了不同时代和不同程度的变质和部分熔融作用(Guo et al., 2012, 2013; Zhang et al., 2013, 2015; Palin et al., 2014; 康东艳等, 2019; 牛志祥等, 2019; 秦圣凯等, 2019)。但是,这些岩石变质条件与组成的空间变化还缺乏深入研究。本文对分布于林芝布久西部的泥质片岩,即含夕线石石榴石云母片岩,进行了岩石学和锆石年代学研究,限定了岩石的变质条件和时间,并结合现有研究成果,探讨了表壳岩在大陆碰撞过程中被埋藏到岩浆弧中下地壳的机制,以及岩浆弧地壳组成的空间变化,为冈底斯弧的晚中生代-新生代构造演化提供了重要限定。

1 地质背景

青藏高原从北到南由松潘-甘孜地体、北羌塘、南羌塘、拉萨地体和喜马拉雅带组成(图 1a; Yin and Harrison, 2000)。位于拉萨地体南部的冈底斯岩浆弧,主要是由大面积分布的白垩纪至第三纪冈底斯岩基和古近纪林子宗火山岩系组成,含少量的三叠纪至白垩纪侵入岩和火山-沉积岩(图 1a; Allégre et al., 1984; Coulon et al., 1986; Debon et al., 1986; Pan et al., 2004; Chung et al., 2005; Wen et al., 2008a, b; Zhu et al., 2015, 2018; Wang et al., 2016)。本文的研究区处于冈底斯岩浆弧东端,东喜马拉雅构造结的西侧。该研究区包括三个构造单元:喜马拉雅带、印度-雅鲁藏布江缝合带和东冈底斯弧(图 1; Yin, 2006)。研究区的喜马拉雅带包括特提斯-喜马拉雅序列和高喜马拉雅序列,印度-雅鲁藏布江缝合带代表了新特提斯洋壳的残余。研究区内的冈底斯弧岩石组成包括古生代-中生代沉积岩、变沉积岩。最近的研究表明林芝地区变质沉积岩的原岩沉积年龄为石炭纪(Guo et al., 2016)。这些岩石被大面积分布的侏罗纪-白垩纪花岗岩、晚白垩世辉长岩-花岗闪长岩(里龙岩基)和古新世-始新世花岗岩,以及少量的渐新世花岗岩侵入(图 1b)。越来越多的研究表明,这些岩浆岩和沉积岩在晚白垩世至始新世期间经历了多期角闪岩相到麻粒岩相变质和深熔作用(王金丽等, 2008; Dong et al., 2010; 董昕等, 2010, 2012; Zhang et al., 2010a, b, 2013, 2014, 2015; Guo et al., 2011, 2012, 2013; Xu et al., 2013; Palin et al., 2014; Ding and Zhang, 2018; 康东艳等, 2019; 牛志祥等, 2019; 秦圣凯等, 2019)。这些由不同时代原岩组成的中-高级变质岩被称为林芝杂岩(Zhang et al., 2013)。通过详细的地质填图,Zhang et al. (2020)发现林芝杂岩具有自东南向西北变质程度降低的趋势,可划分为麻粒岩相、角闪岩相和绿片岩相变质带(图 1b),分别代表冈底斯弧下地壳至中-上地壳组成。

本文所研究的样品采集于林芝布久地区以西约8km处,位于角闪岩相变质带(图 1b)。我们对3个泥质片岩(D680108、D680109、D680110)进行岩石学和锆石U-Pb定年,并对其中1个片岩样品(D680108)进行了详细的矿物化学分析、地质温压计计算和相平衡模拟研究。本文中所使用的矿物代号据Whitney and Evans (2010)

2 分析方法

本文的矿物主量元素成分分析在中国地质科学院地质研究所完成,实验中所用的仪器为JEOL JXA-8100电子探针(EPM)。实验分析所需的条件为15kV的电压,20nA的电流,并采用了直径为5μm的探针,选取10s的峰期和背景时间,ZAF校正,实验分析误差在2%以内。文中的矿物分子式均采用Ax软件计算(Holland; https://www.esc.cam.ac.uk/research/research-groups/research-projects/tim-hollands-software-pages/ax)。

锆石LA-ICPMS(激光剥蚀-电感耦合等离子质谱仪)U-Pb同位素定年和微量元素含量的测定,是在武汉上谱分析科技有限责任公司同时分析完成的。仪器的详细操作流程和分析方法见Zong et al. (2017)。本次分析采用24μm或32μm的激光束斑直径。所使用的系统为GeolasPro激光剥蚀系统。此外,分析的质谱仪型号为Agilent 7700e。在激光剥蚀过程中,利用氦气作载气,氩气为补偿气来调节灵敏度,它们通过一个T型的接头混合后,再进入ICP装置,激光剥蚀系统中配有信号平滑的装置(Hu et al., 2015b)。U-Pb同位素定年是采用锆石标准91500进行同位素分馏校正,而微量元素含量处理中使用玻璃标准物质NIST610作外标进行微量元素分馏校正。每个分析数据都包含了大约20~30s的空白背景信号,以及50s的数据信号。本文采用基于Excel软件的ICPMSDataCal软件(Liu et al., 2010),对信号进行选择、仪器灵敏度漂移校正、以及微量元素含量和U-Pb同位素比值数据的离线处理。我们使用Isoplot/Ex_ver3(Ludwig, 2003)来绘制锆石U-Pb年龄谐和图,以及计算锆石的加权平均年龄。锆石的U-Pb同位素定年分析误差小于2%,同时微量元素含量的分析误差小于5%。

3 岩相学与矿物化学

含夕线石石榴石云母片岩(D680108)具有斑状变晶结构,呈片状构造。主要的矿物组成为:石英(~35%)、黑云母(~20%)、斜长石(~20%)、石榴石(~10%)、白云母(~10%)及少量的夕线石(~2%)。不透明的副矿物为金红石和钛铁矿(图 2a, b)。石榴石为变斑晶,粒径达4~5mm,而其它的细粒矿物为变基质(图 2b)。变斑晶石榴石中含有黑云母、白云母和石英的包裹体,以及由黑云母+斜长石或黑云母+石英组成的两相包裹体(图 2b)。而石榴石的边部部分被斜长石、石英和黑云母的冠状体所取代(图 2b),这可能是退变质过程中,石榴石与熔体反应的结果。变基质中的金红石边部被钛铁矿替代(图 2a)。所研究的含夕线石石榴石云母片岩峰期矿物组合应为石榴石+黑云母+斜长石+白云母+夕线石+石英+金红石+熔体。另外2个夕线石云母片岩(D680109,D680110)具有粒状鳞片状变晶结构,片状构造。由石英、黑云母、夕线石、白云母、斜长石、钛铁矿和金红石组成。岩石中定向分布的云母和夕线石,以及拉长的石英颗粒构成了面理(图 2c, d)。

图 2 云母片岩显微照片 (a、b)含夕线石石榴石云母片岩(D680108),石榴石(Grt)为变斑晶,石英(Qz)、黑云母(Bt)、白云母(Ms)和斜长石(Pl)等细粒矿物为变基质.石榴石边部被Pl+Qz+Bt替代.穿过石榴石变斑晶的红色实线为电子探针成分剖面分析的位置;(c、d)夕线石云母片岩(D680109和D680110),主要由石英、黑云母、夕线石(Sil)、白云母和斜长石等矿物组成.文中其他矿物的缩写为:Kfs-钾长石;Ilm-钛铁矿;Rt-金红石;M-熔体 Fig. 2 Photomicrographs of the mica schists

本文对含夕线石石榴石云母片岩(D680108)中的矿物化学成分,进行了电子探针成分分析,代表性矿物的成分分析结果见表 1。详细的成分剖面分析显示,石榴石变斑晶成分较为均一,具有较高的铁铝榴石(Alm:0.68~0.71,摩尔分数),较低的镁铝榴石(Prp:0.12~0.16,摩尔分数)、锰铝榴石(Sps:0.09~0.11,摩尔分数)和钙铝榴石(Grs:0.05~0.08,摩尔分数)组分(图 3a表 1)。石榴石变斑晶中有多处表现为镁铝榴石组分降低和锰铝榴石组分升高的现象(图 3a),这些变化发生在含包裹体或发育裂隙的区域(图 2b),应该是退变质冷却过程中的成分扩散所致。黑云母成分较均匀,其Ti阳离子数在0.14~0.18(apfu)之间变化,Mg阳离子数为0.88~1.05(apfu)(表 1图 3b)。斜长石的钙长石组分(An=Ca/(Ca+K+Na))为0.31~0.39(表 1),属于中长石。其中基质斜长石具有较低的CaO含量,其An=0.31~0.35,而冠状体中的斜长石具有较高的CaO含量,An=0.35~0.39(图 3c)。

表 1 含夕线石石榴石云母片岩代表性矿物化学成分的分析结果(wt%) Table 1 Chemical compositions of representative minerals of the sillimanite-bearing garnet mica schist (wt%)

图 3 含夕线石石榴石云母片岩矿物化学成分变化图 (a)石榴石成分剖面;(b)黑云母;(c)斜长石 Fig. 3 Chemical compositions of minerals of the sillimanite-bearing garnet mica schist
4 变质作用P-T条件 4.1 地质温压计

我们采用石榴石-黑云母(GB)地质温度计(Holdaway, 2000)和石榴石-黑云母-斜长石-石英(GBPQ)地质压力计(Wu et al., 2004),对含夕线石石榴石云母片岩(D680108)的变质P-T条件进行了估算。利用具有最高XPrp的石榴石成分、具有最高Ti的黑云母成分,以及An最低的斜长石成分进行计算,获得的温压条件为7.3~7.5kbar和710~720℃,应代表片岩的峰期变质条件。

4.2 相平衡模拟

相平衡模拟应用PERPLE_X程序(Connolly, 2005;版本6.7.4),选择Holland and Powell (1998)的升级版数据库,以及下列矿物和熔体相活度-成分关系模型:黑云母(Tajčmanová et al., 2009),石榴石(White et al., 2007),堇青石和十字石(Holland and Powell, 1998),长石(Fuhrman and Lindsley, 1988),白云母(Coggon and Holland, 2002),钛铁矿(White et al., 2007),绿泥石和硬绿泥石(White et al., 2014)和熔体(White et al., 2001)。考虑到P2O5主要赋存于磷灰石中,且在所研究岩石中含量很低,因此被忽略。本文相平衡模拟选择接近变泥质岩石实际组成的MnO-Na2O-CaO-K2O-FeOtotal-MgO-Al2O3-SiO2-H2O-TiO2-O2(MnNCKFMASHTO)体系。全岩成分是基于薄片中的矿物含量和化学成分计算所得,其中SiO2=66.43%、TiO2=1.60%、Al2O3=14.43%、FeO=7.30%、MnO=0.53%、MgO=1.85%、CaO=2.37%、Na2O=2.30%、K2O=2.14%和H2O=1.01%。

图 4为相平衡模拟出的P-T视剖面图,在计算的3~9kbar和550~800℃范围内,石榴石、斜长石和黑云母稳定存在,白云母只在低温区域出现,温度小于ca. 630~750℃,钾长石在高温条件下稳定,体系的固相线位于675~685℃。模拟结果表明,所观察到的峰期矿物组合Grt+Bt+Pl+Kfs+Ms+Qz+Sil+Rt+M(melt)稳定在5~8kbar和690~750℃温压范围内(图 4)。基于温压计得出的温、压条件接近或位于峰期变质矿物组合稳定域内(图 4的黄色填充矩形),进一步表明岩石的峰期变质条件很可能是在~7.4kbar和~715℃。在峰期变质条件下的熔体含量约6vol%,表明岩石发生了低程度的部分熔融。

图 4 含夕线石石榴石云母片岩P-T视剖面图 图中的红色虚线为熔体的含量等值线,红色和绿色实线分别为该体系的固相线和钾长石的出现线.黄色填充矩形是地质温压计估算出的云母片岩峰期变质条件 Fig. 4 P-T pseudosection for the sillimanite-bearing garnet mica schist
5 锆石U-Pb年代学

我们对3个云母片岩进行了锆石U-Pb同位素定年和微量元素原位分析,分析的结果见表 2表 3。代表性锆石的阴极发光图像见图 5,可以看出片岩中的锆石特征相似,为半自形长柱状,粒度约为50~200μm。多数锆石颗粒发育核-边结构,由继承的碎屑核和增生边组成,继承核具有不同的形状和大小,发育震荡环带或不发育环带,边部发育微弱的震荡环带或弱的补丁状环带(图 5)。

表 2 云母片岩中锆石U-Pb定年结果 Table 2 Zircon U-Pb dating of the mica schists

表 3 云母片岩中锆石稀土元素含量(×10-6) Table 3 Rare earth element contents of zircons from the mica schists (×10-6)

图 5 云母片岩中锆石的阴极发光图像和分析点位置与相应年龄(Ma) Fig. 5 Cathodoluminescence (CL) images of zircons from the mica schists, showing the locations of analyzed spots and relevant ages (Ma)

锆石增生边U-Pb定年和微量元素分析结果表明,3个样品均给出了近谐和的206Pb/238U年龄,分别为54~61Ma、53~61Ma和48~58Ma(图 6a-c表 2)。这些增生边具有较低的Th/U值(0.058~0.396、0.005~0.012和0.008~0.018),较高的HREE含量(122×10-6~687×10-6、607×10-6~1161×10-6和958×10-6~1431×10-6表 3),轻、重稀土明显分馏的REE模式(图 6d-f),并具有明显负Eu异常(δEu=0.061~0.188、0.040~0.241和0.087~0.279;表 3)。

图 6 云母片岩中锆石增生边的U-Pb年龄谐和图(a-c)和球粒陨石标准化稀土元素配分模式图(d-f, 标准化值据Sun and McDonough, 1989) Fig. 6 U-Pb concordia diagrams (a-c) and chondrite-normalized REE patterns (d-f, normalization values after Sun and McDonough, 1989) of zircon overgrowth rims from the mica schists
6 讨论 6.1 冈底斯弧东端变质作用条件与地壳组成的空间变化

前人的研究表明冈底斯弧东端林芝至布久地区的变质岩经历了古新世至始新世变质作用和部分熔融,但变质条件明显不同(图 1b)。董昕等(2012)对林芝市区附近的副片麻岩和片岩的研究表明,岩石经历了4~5.5kbar和625~679℃的低角闪岩相变质作用。Palin et al. (2014)研究显示,林芝市区南部的泥质混合岩经历了低压麻粒岩相变质作用(温压条件为~6kbar和~750℃)。最近,张成圆等(2020)对林芝市区东南部约15km处的变沉积岩进行了变质P-T条件计算,表明其经历了~8.0kbar和~750℃的高角闪岩相变质作用。康东艳等(2019)揭示出布久地区的石榴石夕线石云母片岩经历了高压麻粒岩相变质作用,变质条件为~10.5kbar和~775℃。布久东南江河汇流处的变质岩经历了早新生代的麻粒岩相变质作用和部分熔融,P-T条件为9~11kbar和800~830℃(Zhang et al., 2013)。这些研究表明从北部的林芝市区到南部的布久和江河汇流处,林芝杂岩依次经历了早新生代低角闪岩相、高角闪岩相和麻粒岩相变质作用,即变质和部分熔融程度由北向南逐渐升高。

本文所研究的片岩样品位于布久向西约8km处,相平衡模拟和地质温压计结果表明,含夕线石石榴石云母片岩经历了角闪岩相变质作用和低程度的部分熔融,峰期变质条件为~7.4kbar和~715℃。所获得的峰期变质条件明显低于东部布久和江河汇流地区的麻粒岩相变质温、压条件。这表明林芝杂岩的变质程度从东向西是降低的,即从东部的麻粒岩相到西部的角闪岩相。现有研究表明,布久-江河汇流-米林-扎西-里龙地区的麻粒岩相变质岩变质压力大于10kbar,所以应代表冈底斯岩浆弧的下地壳组成;而本文样品位置,以及西北部地区的岩石角闪岩相变质岩变质压力在4~8kbar,应代表冈底斯弧的中-上地壳组成。基于最新发表的填图结果,我们发现冈底斯弧的下地壳主要组成是变质的基性和花岗闪长质岩浆岩,此外含有少量的变质花岗岩和变质沉积岩,而北部的冈底斯弧中上地壳主要由变质沉积岩、变质花岗岩组成,含少量的变质基性岩(图 1b)。这很可能表明岩浆弧的下地壳总体上具有中-基性成分,而上地壳具有中-酸性成分。郭京梁等(2019)通过分析拉萨地体东南部地球物理、岩石物性和岩浆岩地球化学资料,证明下地壳由中性岩石组成。此外最近的研究也表明,冈底斯弧下地壳平均成分相当于玄武安山质,SiO2含量平均为~54%(Guo et al., 2020)。

6.2 变质作用时间与持续过程

如上文所述,所研究云母片岩中的锆石多为半自形长柱状,锆石增生边显示出弱的补丁状环带或震荡环带特征(图 5)、低的Th/U值(表 2)、负Eu异常、轻重稀土明显分异(图 6d-f)。这很可能表明这些锆石的增生边形成在片岩的退变质与残余熔体结晶过程中。在含石榴石的变质岩中,重稀土元素主要赋存在石榴石中,石榴石分解会释放出HREE,从而导致同时生长的锆石具有较高的HREE含量(Rubatto, 2002; Rubatto et al., 2013)。所研究的锆石增生边具有高的HREE含量(表 3)和陡的HREE模式(图 6d-f),表明锆石生长的过程中,伴随着石榴石的分解。而岩相学观察显示,片岩在退变质过程中,部分石榴石边部被Bt+Pl+Qz组成的冠状体替代(图 2b)。因此所获得的锆石增生边年龄,即61~48Ma应代表片岩的退变质和熔体结晶时间。

Palin et al. (2014)通过独居石原位定年,在林芝市区附近的变泥质岩中获得了71~50Ma的变质年龄。张成圆等(2020)在对布久北部约15km的变质沉积岩中获得了72~51Ma的变质年龄。结合本文的定年结果,我们认为布久到林芝市区的变质岩经历了晚中生代至早新生代的长期变质作用,进变质作用很可能发生在70~61Ma,而退变质和熔体结晶很可能发生在61~48Ma,整个变质作用持续约20Myr。这与Zhang et al. (2013)在布久和江河汇流地区的混合岩化片岩中获得的~67Ma到~52Ma的变质和深熔作用年龄基本上是一致的。

6.3 冈底斯弧新生代的构造演化

岩浆弧的下地壳通常是由幔源岩浆的底垫增生形成,即主要由基性的辉长岩组成,而且会普遍经历角闪岩相至麻粒岩相变质和部分熔融(Depine et al., 2008; Ducea et al., 2015)。然而详细的地质填图表明,冈底斯弧下地壳含有相当多的长英质麻粒岩,少量的变质沉积岩,并具有中性岩的平均成分(SiO2含量平均为57.61%,Mg#(=Mg/(Mg+Fe))平均为0.49;Zhang et al., 2020)。本文认为岩浆弧上地壳的中酸性岩浆岩和沉积岩被埋藏到了岩浆弧的中下地壳,由此导致了岩浆弧中-下地壳组成的变化。正如Zhang et al. (2020)认为,表壳岩在碰撞过程中被构造运移到深部地壳导致了冈底斯弧中下地壳由镁铁质转变成安山质。在印度与亚洲碰撞过程中,大量的长英质岩石加入到下地壳可能使冈底斯弧下地壳由玄武质转变为安山质(Guo et al., 2019)。此外Chin et al. (2013)Chapman et al. (2014)的研究也表明,岩浆弧下地壳中的变沉积岩可以使下地壳的岩石组成和化学成分发生明显变化。

大量的研究表明,印度大陆与亚洲大陆的碰撞发生在约65~50Ma的早新生代(Searle et al., 1987; Mo et al., 2007, 2008; Najman et al., 2010; Liebke et al., 2010; Meng et al., 2012; Hu et al., 2015a, 2016; Zhu et al., 2015, 2017, 2018; Ding et al., 2016a, b)。同时在晚中生代-早新生代(ca. 70~50Ma),俯冲的新特提斯洋岩石圈发生回卷和断离(Chung et al., 2009; Zhu et al., 2017, 2018),导致了热的软流圈物质上涌,诱发了地幔、新生和古老地壳的部分熔融(Ji et al., 2012; 张泽明等, 2019)。强烈的幔源岩浆增生,使弧地壳加厚到50~58km(Zhu et al., 2017, 2018)。同时地壳的明显构造挤压缩短也是地壳加厚的重要机制(Mo et al., 2005, 2007, 2008; Chung et al., 2009)。如Zhu et al. (2015)把~50Ma的冈底斯岩基具有负的全岩Nd同位素组成和锆石Hf同位素组成这一特征,解释为印度地壳物质的参与。此外,特提斯喜马拉雅在55~45Ma发生挤压褶皱变形(Ratschbacher et al., 1994; Smit et al., 2014),以及其中的打拉组在48~45Ma发生中压变质作用,显示出印度上地壳被埋藏到冈底斯弧20~30km地壳深部(Ding et al., 2016a)。冈底斯弧东端分布有加厚下地壳部分熔融产生的早新生代埃达克质岩石,为岩浆弧具有一个厚的地壳提供了重要证据(Ji et al., 2012; Jiang et al., 2014; Ma et al., 2014)。因此,我们认为印度和亚洲大陆碰撞导致了岩浆弧地壳的强烈逆冲推覆和缩短加厚,俯冲的新特提斯洋岩石圈的回卷和断离导致了大体积幔源岩浆的底侵和地壳加厚。这些机制一起导致了岩浆弧上地壳的变质沉积岩和中-酸性岩浆岩被埋藏到中、下地壳,由此经历长期持续的角闪岩相至麻粒岩相变质和深熔作用。

7 结论

(1) 冈底斯弧东端布久地区西部的含夕线石石榴石云母片岩由石榴石+黑云母+斜长石+白云母+夕线石+石英+金红石组成,经历了角闪岩相变质作用和部分熔融,峰期变质条件为~7.4kbar和~715℃。

(2) 所研究片岩中锆石增生边给出了古新世(61~48Ma)的变质年龄。结合前人定年结果,我们认为云母片岩的变质作用很可能开始于~70Ma,持续了约20Myr。

(3) 本文研究区经历了角闪岩相变质作用,代表冈底斯岩浆弧的中地壳,表明岩浆弧的中地壳含有大体积的变质沉积岩。本研究进一步表明,冈底斯弧东端出露的变质岩代表不同的地壳层次,其变质程度和组成存在系统的空间变化。

(4) 本研究表明,在晚古生代-早新生代,大陆碰撞导致的地壳逆冲推覆和缩短加厚,以及幔源岩浆底侵导致的地壳加厚,是表壳岩被埋藏至岩浆弧中、下地壳并发生持续的高温变质和深熔作用的构造机制。

致谢      感谢郭亮副教授和杜瑾雪副教授对本文提出的宝贵修改意见!

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