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  中国地质 2021, Vol. 48 Issue (5): 1623-1638  
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吴昊, 翟庆国, 胡培远, 唐跃, 朱志才, 王伟, 谢超明, 强巴扎西. 2021. 西藏班戈北部早白垩世火山岩:班公湖—怒江洋闭合的岩浆记录[J]. 中国地质, 48(5): 1623-1638.  
Wu Hao, Zhai Qingguo, Hu Peiyuan, Tang Yue, Zhu Zhicai, Wang Wei, Xie Chaoming, Qiangba Zhaxi. 2021. Early Cretaceous volcanic rocks in northern Baingoin, Tibet: Magmatic record of the closure of the Bangong-Nujiang Ocean[J]. Geology in China, 48(5): 1623-1638. (in Chinese with English abstract).  

西藏班戈北部早白垩世火山岩:班公湖—怒江洋闭合的岩浆记录
吴昊1, 翟庆国1, 胡培远1, 唐跃1, 朱志才1, 王伟1, 谢超明2, 强巴扎西3    
1. 自然资源部深地动力学重点实验室, 中国地质科学院地质研究所, 北京 100037;
2. 吉林大学地球科学学院, 吉林 长春 130061;
3. 西藏地勘局区域地质调查大队, 西藏 拉萨 851400
摘要:对班戈县北部马前乡地区的早白垩世安山岩和英安岩进行了详细的地质填图及岩石学、年代学、地球化学和Hf同位素研究。锆石U-Pb定年获得安山岩年龄分别为(108.0±1.5)Ma和(113.6±0.9)Ma;英安岩年龄为(106.7±1.9)Ma和(113.6±0.8)Ma。安山岩富集Th和U,亏损Nb、Ta和Ti,具有变化范围较大的Mg#值(25~63),锆石εHft)值(-8.6~+1.5)以负值为主,应当为幔源镁铁质熔体与壳源熔体的混合产物。英安岩具有与安山岩类似的微量元素成分特征及负的锆石εHft)值(-12.3~-8.1),应当是地壳部分熔融的产物。结合前人研究成果认为,这些早白垩世岩浆岩是约110 Ma沿班公湖-怒江缝合带岩浆大爆发的产物,可能与班公湖-怒江洋闭合之后的拉萨与羌塘地块陆-陆碰撞有关。
关键词早白垩世火山岩    地球化学    岩石成因    岩浆混合    地质调查工程    班公湖—怒江缝合带        
中图分类号:P588.142;P588.144            文献标志码:A             文章编号:1000-3657(2021)05-1623-16
Early Cretaceous volcanic rocks in northern Baingoin, Tibet: Magmatic record of the closure of the Bangong-Nujiang Ocean
WU Hao1, ZHAI Qingguo1, HU Peiyuan1, TANG Yue1, ZHU Zhicai1, WANG Wei1, XIE Chaoming2, QIANGBA Zhaxi3    
1. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. College of Earth Science, Jilin University, Changchun 130061, Jilin, China;
3. Regional Geological Survey Party, Tibet Bureau of Geology and Mineral Exploration and Development, Lhasa 851400, Tibet, China
Abstract: The petrological, zircon U-Pb dating, whole-rock geochemical, and zircon Hf isotopic data of the Early Cretaceous andesites and dacites in the Maqianxiang area of Baingoin County, Tibet, are reported. The zircon U-Pb ages of andesite are (108.0±1.5)Ma and (113.6±0.9) Ma, and those of dacite are (106.7±1.9) Ma and (113.6±0.8) Ma. The andesites are enriched in Th and U and depleted in Nb, Ta, and Ti, have variable Mg# values (25-63), and show mainly negative zircon εHf(t) values (-8.6 to +1.5). They are probably generated by mixing of mantle- and crust-derived melts. Dacite shares similar trace element features with the coeval andesite, and has negative zircon εHf(t) values (-12.3 to -8.1). It is interpreted as a product of partially melting crust. The andesite and dacite are interpreted as a product of the ca.110 Ma magmatism along the Bangong-Nujiang suture zone, and may be related to the continent-continent collision process after the closure of the Bangong-Nujiang Ocean.
Key words: Early Cretaceous volcanics    geochemistry    petrogenesis    magma mixing    geological survey engineering    Bangong-Nujiang suture zone    Tibet    

1 引言

青藏高原记录了特提斯洋以及印度—亚洲大陆的聚合、碰撞造山的地质演化过程,是当今地球科学研究的热点和前沿(侯增谦等, 2004, 2006许志琴等, 2006, 2016杨经绥等,2004李海兵等,2006赵慧等,2015)。班公湖—怒江缝合带横亘于青藏高原中部,记录了班公湖—怒江中特提斯洋的俯冲消减以及拉萨地块和南羌塘地块的碰撞过程,是研究中特提斯洋从洋-陆俯冲到陆-陆碰撞演化历史的关键。

截至目前,班公湖—怒江缝合带的研究依然存在较多争议,很多关键科学问题尚未解决,尤其是班公湖—怒江中特提斯洋的闭合时间(Girardeau et al., 1984Kapp et al., 2007Hu et al., 2017)。当前主要分为两种观点,一种观点认为,班公湖—怒江洋盆一直持续到晚白垩世才发生闭合(朱弟成等,2006Zhang et al., 2004, 2012;Fan et al., 2014Wu et al., 2015Liu et al., 2018),主要证据为早白垩世晚期(120~108 Ma)的“洋岛型”岩石组合(包括OIB型玄武岩和伴生的灰岩)(Zhu et al., 2006Liu et al., 2014Zhang et al., 2014Fan et al., 2015)。另一种观点认为,班公湖—怒江特提斯洋在早白垩世之前就已经闭合并进入陆-陆碰撞阶段(Xu et al., 1985; Guynn et al., 2006Kapp et al., 2007Leier et al., 2007;Chen et al., 2015, 2017a),主要依据古地磁(Matte et al., 1996Lippert et al., 2014Zhu et al., 2015)和沉积学(Kapp et al., 2007)方面的证据。

近年来的研究显示,在班公湖—怒江缝合带内部及两侧出露有大量早白垩世中酸性火山岩和花岗岩(Zhu et al., 2009),它们可以为研究班公湖—怒江特提斯洋的演化历史提供重要约束。本文报道了新近在班公湖—怒江缝合带中段班戈县马前乡地区的早白垩世去申拉组安山岩和英安岩。在野外填图的基础上,对这些岩石开展了系统的岩石学、地球化学和锆石U-Pb年代学研究工作。结合班公湖—怒江缝合带内已有同期岩浆岩的资料,本文讨论了马前乡地区早白垩世火山岩的岩浆源区和岩石成因,及其对班公湖—怒江中特提斯洋的闭合时间的约束。

2 地质概况

青藏高原是研究大陆动力学和板块构造理论的天然实验室。前人研究表明,青藏高原由一系列的陆块和缝合带组成。这些陆块包括昆仑、北羌塘、南羌塘、拉萨和喜马拉雅地块,这些地块被金沙江缝合带、龙木错—双湖—澜沧江缝合带、班公湖—怒江缝合带和雅鲁藏布江缝合带分割(图 1a)。

图 1 青藏高原构造划分简图(a, 据Hu et al., 2017)和西藏班戈县马前乡地区地质简图(b) BNSZ—班公湖—怒江缝合带;LSSZ—龙木错—双湖缝合带;IYZSZ—雅鲁藏布江缝合带;JSSZ—金沙江缝合带;1—第四系;2—古近系牛堡组;3—白垩系去申拉组安山岩;4—白垩系去申拉组英安岩;5—白垩系去申拉组砂岩;6—侏罗系接奴群;7—三叠系确哈拉群;8—三叠纪花岗岩;9—蛇绿岩;10—断层;11—不整合接触;12—年龄采样点;13—产状 Fig. 1 Tectonic framework of the Tibetan Plateau (a, after Hu et al., 2017) and simplified geological map (b) of Maqian Town, Baingoin County BNSZ-Bangong Co-Nujiang suture zone; LSSZ-Longmu Co-Shuanghu suture zone; IYZSZ-Yarlung Zangbo suture zone; JSSZ-Jinshajiang suture zone; 1-Quaternary; 2-The Paleogene Niubao Formation; 3-The Cretaceous andesite of Qushenla Formation; 4-The Cretaceous dacite of Qushenla Formation; 5-The Cretaceous sandstone of Qushenla Formation; 6-The Jurassic Jienu Group; 7-The Triassic Quehala Group; 8-The Triassic granite; 9-Ophiolite; 10-Fault; 11-Unconformable contact; 12-Sampling site; 13-Occurrence

研究区位于班公湖—怒江缝合带中段,行政规划属于班戈县马前乡(图 1a),大地构造上处在拉萨地块北缘火山岩浆弧带中。研究区出露的地层主要为三叠系、侏罗系、白垩系和古近系(图 1b)。三叠系主要为上三叠统确哈拉群(T3q),岩性以细砂岩和含砾粗砂岩为主,二者呈互层状产出,顶部发育薄层灰岩。侏罗系主要为中上侏罗统接奴群(J2-3jn),以变质粉砂岩、变质砂岩和灰黑色板岩为主,与下白垩统去申拉组(K1q)以断层相接触。白垩系去申拉组不整合覆盖于确哈拉群之上,岩性主要为安山岩、英安岩、砂岩和少量火山碎屑岩,其中安山岩与砂岩多呈互层状产出。古近系主要是牛堡组(E1-2n)红层,岩性以中—厚层状砂岩和砾岩为主。

3 岩石学特征

本文采集的火山岩样品来自于去申拉组,岩石类型包括安山岩和英安岩,采样位置见图 1b,样品包括4件同位素测年样品和28件岩石和地球化学样品,其中典型样品的岩石学特征如下:

安山岩:呈灰色、深灰色,斑状结构,块状构造(图 2f)。斑晶总含量约占20%,主要为斜长石(约60%)、角闪石(约25%)、辉石(约10%)和少量黑云母(约5%)。斜长石呈板柱状,长度为50~2000 μm,发育聚片双晶(图 2h),部分发生了蚀变作用而形成绢云母。辉石呈板状,长度为100~2000 μm,斜消光,消光角为40°,部分发生了绿泥石化和绿帘石化。角闪石多呈长柱状和粒状,部分蚀变。黑云母呈片状(0.2~0.7 mm),一组极完全解理,平行消光,有的具暗化边结构,部分蚀变。基质主要为斜长石微晶、暗色矿物。

图 2 西藏班戈县马前乡火山岩野外露头及显微照片 a—安山岩远景;b—安山岩夹火山角砾岩;c, d, e—火山岩与砂岩界限;f—安山岩近景;g—英安岩近景;h—安山岩镜下照片;i—英安岩镜下照片;Pl—斜长石;Px—辉石;Ser—绢云母;Q—石英 Fig. 2 Field photographs and microphotographs of the volcanic rocks in Maqian Town of Baingoin County, Tibet a-Distant view of andesites; b-Andesites intermingled with volcanic breccia; c, d, e-The boundary between volcanic rock and sandstone; f-Tight shot of andesites; g-Tight shot of dacites; h-Microphotographs of andesites; i-Microphotographs of dacites; Pl-Plagioclase; Px-Pyroxene; SerSericite; Q-Quartz

英安岩:灰绿色,具斑状结构,块状构造(图 2g)。斑晶占10%~15%,由斜长石和少量石英组成。斜长石为自形—半自形板柱状,石英斑晶呈现珍珠光泽,基质为隐晶质,多为微晶或针状长石类矿物和暗色矿物。

4 分析方法

锆石U-Pb测年样品中锆石的分选在河北省区域地质调查队实验室完成,采用常规的重液和磁选方法进行分选,最后在双目显微镜下挑纯。样品靶的制备在中国地质科学院地质研究所完成,制成的样品靶直径为25 mm。锆石的阴极荧光图像分析在中国地质科学院地质研究所的阴极荧光分析系统(HITACH S-3000N型场发射环境扫描电镜和Gatan公司Chroma阴极荧光谱仪)上完成。样品的锆石U-Th-Pb分析在北京科荟测试技术有限公司完成,分析仪器为美国ESI公司生产的NWR 193nm激光剥蚀进样系统和德国AnlyitikJena公司生产的PQMS Elite型四级杆等离子体质谱仪联合构成的激光等离子体质谱仪(LA-ICP-MS)。本次分析193 nm激光器工作频率为10 Hz;测试点束斑直径为25 μm,剥蚀采样时间为45 s,具体分析流程见相关文献(侯可军等,2009)。锆石GJ-1(Jackson et al., 2004)作为外部标准来校正分析过程中的同位素分馏;NIST610作为外部标准来获得分析点的Th和U的含量。锆石U-Pb年龄用ICPMSDataCal数据处理软件(Liu et al., 2010)计算获得,加权平均年龄的计算和谐和图的绘制采用ISOPLOT 3.0程序(Ludwig et al., 2003)。锆石Hf同位素分析也在北京科荟测试技术有限公司完成,在Neptune Plus多接收电感耦合等离子质谱仪(MC-ICPMS)和NWR 213 nm激光取样系统上进行,分析中,标准锆石176Hf/177Hf比值范围为0.282496 ± 9(2σn= 276),仪器的运行条件及详细的分析过程参见相关文献(Wu et al., 2006)。采用单点剥蚀模式,斑束固定为44 μm。全岩主量、微量和稀土元素的分析在国家地质实验测试中心。主量元素采用X-射线荧光光谱仪(PW4400)分析。微量元素和稀土元素的分析仪器为X-series等离子质谱仪,实验室分析详细方法见相关文献(Hu et al., 2018)。

5 分析结果 5.1 锆石U-Pb年龄

西藏班戈北部马前乡地区岩浆岩的4件样品的锆石定年结果见表 1

表 1 西藏班戈县马前乡火山岩锆石U-Th-Pb同位素数据 Table 1 LA-ICP-MS zircon U-Th-Pb data of the volcanic rocks in Maqian Town of Baingoin County, Tibet

安山岩样品16T337和18T097中的锆石呈短柱状,自形—半自形,粒径范围在100~150 μm,具有明显的岩浆振荡环带(图 3ab)。锆石Th/U值分别介于0.74~2.41和0.40~1.03,表明这些锆石属于岩浆成因锆石。在U-Pb年龄谐和图上,分析点均落在曲线上及附近,测点的206Pb/238U年龄范围分别为106~109 Ma和110~114 Ma,年龄加权平均值分别为(108.0 ± 1.5)Ma(MSWD = 0.072)和(113.6 ± 0.9)Ma(MSWD = 0.20),代表了安山岩的岩浆结晶年龄。

图 3 西藏班戈县马前乡安山岩(a,b)和英安岩(c,d)的LA-ICP-MS锆石U-Pb谐和图(比例尺代表100 μm) Fig. 3 LA-ICP-MS zircon U-Pb concordant diagrams of the andesite (a, b) and dacite (c, d) in Maqian Town of Baingoin County, Tibet (The scale bar on the CL images representing 100 μm)

英安岩样品16T348和18T303中的锆石多呈长柱状,自形,粒度较安山岩小,粒径范围在50~100 μm,锆石有明显的岩浆振荡环带(图 3c)。锆石Th/U比值分别在0.60~1.31和1.03~1.98(>0.1),显示出典型岩浆锆石的特征,测点的206Pb/238U年龄变化范围分别为(105~109)Ma和113~114 Ma,加权平均值分别为(106.7 ± 1.9)Ma(MSWD = 0.23)和(113.6 ± 0.8)Ma(MSWD = 0.042),代表了英安岩的岩浆结晶时代。

5.2 全岩地球化学

28件样品主全岩主量、微量和稀土元素分析结果见表 2。如前文所述,样品发生了不同程度的后期蚀变作用,显示出较高、且变化范围较大的烧失量(1.31%~5.90%),因此,Ba、K、Na、Pb、Rb、Sr、U等活动性元素不宜用来探讨岩石成因及其形成的大地构造环境。通常认为,高场强元素(如Nb、Ta、Zr、Hf等)、相容元素(如Cr、Ni)和稀土元素受蚀变作用的影响较小,本文主要根据这些元素的组成特征,讨论火山岩的类型和成因等(Winchester and Floyd, 1977; Hastie et al., 2007)。

表 2 西藏班戈县马前乡火山岩主量(%)和微量元素(10-6)成分分析结果 Table 2 Major (%) and trace (10-6) elements data of the volcanic rocks in the Maqianxiang area, Baingoin County, Tibet

安山岩样品主要的氧化物质量分数及比值如下:SiO2为52.77%~63.40%,Al2O3为14.86%~18.73%,MgO为0.63%~5.27%,K2O为0.73%~2.53%,K2O/Na2O为0.18~0.69,TiO2为0.76%~0.98%。其中,部分样品的MgO含量较高(表 2),与高镁安山岩类似。在Zr/TiO2*0.0001-SiO2图解上(图 4a),样品投点主要落在安山岩的区域内。在Co-Th图解中(图 4b),样品点位于高钾钙碱性岩石区域。稀土元素含量较高(ΣREE=96×10-6~149×10-6),球粒陨石标准化稀土元素配分曲线(图 5b)呈轻稀土元素富集、重稀土元素亏损的右倾模式((La/Yb)N=4.78~8.08),并且具有弱的Eu负异常(Eu/Eu*=0.72~0.85)。在原始地幔标准化微量元素蛛网图(图 5a)上,样品具有明显富集Th和U,亏损Nb、Ta和Ti的特征,这与典型岛弧火山岩类似。

图 4 西藏班戈县马前乡火山岩的Zr/TiO2*0.0001-SiO2(a)和Co-Th图解(b) Fig. 4 Zr/TiO2*0.0001-SiO2(a) and Co-Th diagrams (b) of the volcanic rocks in Maqian Town of Baingoin County, Tibet
图 5 西藏班戈县马前乡安山岩(a,b)和英安岩(c,d)的微量元素原始地幔标准化蛛网图和稀土元素球粒陨石标准化配分曲线标准化数据和上地壳数据引自参考文献(据Sun and McDonough, 1989 Fig. 5 Primitive-mantle-normalized trace elements spidergrams and Chondrite-normalized rare-earth element patterns of the andesite (a, b) and dacites (c, d) in Maqian Town of Baingoin County, Tibet (Normalizing and upper crust data from reference, after Sun and McDonough, 1989)

英安岩样品主要的氧化物质量分数如下:Al2O3为14.40%~16.05%,MgO为0.52%~1.93%,K2O为1.47%~44.42%,TiO2为0.52%~0.73%。与安山岩样品相比,英安岩样品具有较高的SiO2含量,为62.03%~67.86%。在SiO2-Zr/TiO2*0.0001图解中(图 4a),样品投点主要落在流纹英安岩/英安岩的区域内。在Co-Th图解中(图 4b),样品点全部位于高钾钙碱性系列岩石的范围内。稀土元素球粒陨石标准化配分曲线(图 5d)呈轻稀土元素富集、重稀土元素亏损的右倾模式,具有弱的Eu负异常(Eu/Eu*=0.42~0.78)。英安岩与安山岩样品呈现出类似的微量元素成分特征(图 5c),均富集Th和U,亏损Nb、Ta和Ti。

5.3 锆石Hf同位素

本项研究对所有测年锆石进行Hf同位素分析,共获得了43个锆石的测试数据,分析点均在锆石U-Pb定年的原位或相邻部位进行,数据结果见表 3。安山岩样品中锆石的初始176Hf/177Hf比值为0.282545~0.282658,锆石εHf(t)值(-8.6~+1.5)以负值为主(图 6),对应的二阶段模式年龄(TDMC)为1069~1712 Ma。英安岩样品中锆石的初始176Hf/177Hf比值为0.282356~0.282474,对应的锆石εHf(t)值在-12.3~-8.1(图 6),二阶段模式年龄(TDMC)为1748~1952 Ma。

表 3 西藏班戈县马前火山岩的锆石Hf同位素分析结果 Table 3 Zircon Hf isotopic data of the volcanic rocks in the Maqianxiang area, Baingoin County, Tibet
图 6 西藏班戈县马前乡火山岩的εHf(t)-U-Pb年龄图解 Fig. 6 Plots of εHf(t) vs. U-Pb ages of the volcanic rocks in Maqian Town of Baingoin County, Tibet
6 讨论 6.1 去申拉组火山岩的形成时代

去申拉组火山岩断续分布于班公湖—怒江缝合带中,是探索该缝合带构造演化过程的重要载体。陈玉禄(2002)最早在班戈—切里错地区对去申拉组进行了岩石学的论述,同时获126 Ma的全岩Rb-Sr同位素年龄值,从而初步证实去申拉组火山岩时代为早白垩世。吴浩等(2013)对塔色普勒地区的去申拉组火山岩进行了地球化学和年代学研究,认为其形成的地球动力学背景是早白垩世晚期班公湖—怒江洋壳南向俯冲过程中发生的板片断离,并指示该地区班公湖—怒江洋的陆-陆碰撞时间在107 Ma左右。朱弟成等(2006)对该期火山活动进行了较为详细的地球化学、年代学及同位素的研究,提出拉萨地块中北部地区在140~110 Ma可能一直处于与羌塘碰撞有关的同碰撞背景。

尽管前人已经获得了一些去申拉组火山岩的年代学资料,但是主要集中于班公湖—怒江缝合带西段,中—东段的相关年代学资料仍然较少。本次研究样品采自班戈北部马前乡地区,位于班公湖—怒江缝合带中段。安山岩和英安岩样品中锆石具有明显的岩浆成因振荡环带,显示出明显岩浆锆石的特征。获得的锆石U-Pb年龄为105~114 Ma,代表了火山岩的岩浆结晶时代,即班戈北部马前乡地区去申拉组火山岩的形成时代为早白垩世,从而为去申拉组火山岩的准确时代提供了新的约束。

6.2 岩石成因与源区性质

对于长英质火山岩的成因,通常有2种解释:(1)起源于幔源基性岩浆经历广泛的结晶分异和同化混染作用(Bacon and Oruitt, 1988; Ingle et al., 2002);(2)来自幔源基性岩浆的热量促使地壳物质部分熔融的产物(Roberts and Clemens, 1993Tepper et al., 1993)。第一种成因解释要求有大量的中—基性岩浆岩,这与研究区内并没有发现大规模同时代基性岩相悖。虽然本项研究发现了较多的与英安岩(εHf(t)=-12.3~-8.1)同时代的安山岩(εHf(t)=-8.6~+1.5),但是它们具有明显不同的Hf同位素成分(图 6),显示出其岩浆源区的差异。英安岩具有与上地壳类似的稀土和微量元素组成,由此推测它们应当是地壳物质部分熔融的产物。考虑到它们具有较古老的锆石二阶段Hf模式年龄(TDMC=1748~1952 Ma),初步推测它们是古元古代地壳物质熔融的产物。

对于中性火山岩的成因,一般有以下几种解释:(1)幔源玄武质熔体的结晶分异作用(Sisson and Grove, 1993Müntener et al., 2001);(2)被俯冲流体或熔体交代过的地幔楔橄榄岩的部分熔融(Tatsumi,1982Mccarron and Smellie, 1998);(3)镁铁质中、下地壳岩石部分熔融(Smith and Leeman, 1987);(4)基性岩浆与酸性岩浆的岩浆混合(Heiken and Eichelberger, 1980)。本文倾向于最后一种解释,理由如下:首先,安山岩样品的锆石εHf(t) 值(-8.6~+1.5)以负值为主,与幔源岩浆(正的锆石εHf(t)值)不一致,因而不应该是简单的幔源基性熔体结晶分异作用或者地幔楔橄榄岩部分熔融的产物,其成岩过程应当有地壳物质的参与;其次,安山岩样品具有变化范围较大的Mg#值(25~63),而中、下地壳岩石部分熔融形成的岩石通常只具有较低的(< 40)的Mg#值(Smith and Leeman, 1987);再次,在TFeO-MgO和La-La/Sm图解(图 7)上,安山岩显示出明显的岩浆混合趋势。本文报道的英安岩可能代表了岩浆混合中的酸性端元,而Zhu et al.(2009)在拉萨地块北部报道的同时代幔源玄武安山岩则可能代表了基性端元,安山岩可能是这两类岩浆混合的产物。

图 7 西藏班戈县马前乡火山岩的MgO-TFeO图解(据Zorpi et al., 1989)(a)和La-La/Sm图解(据Schiano et al., 2010)(b) Fig. 7 MgO-TFeO (a) and La-La/Sm diagrams (b) (after Zorpi et al., 1989) and La-La/Sm diagrams (after Schiano et al., 2010) of the volcanic rocks in Maqian Town of Baingoin County, Tibet
6.3 构造意义

班公湖—怒江中特提斯洋的闭合时间一直缺乏准确的约束。部分学者把沿班公湖—怒江缝合带展布的早白垩世晚期(120~110 Ma)OIB型基性岩及其伴生的层状灰岩解释为洋岛岩石组合,进而由此推测此时班公湖—怒江缝合带仍未完全关闭(Zhu et al., 2006Liu et al., 2014Zhang et al., 2014Fan et al., 2015)。然而,新近对班公湖—怒江缝合带上120~110 Ma基性岩进行的系统地球化学研究表明,这一期基性岩浆事件具有混合岩浆源区,分别为浅层的MORB型源区和深层的OIB型源区,与典型的洋岛环境具有一定差别(Zhu et al., 2015)。

通常认为,大洋板片俯冲环境下形成的岩浆岩主要为钙碱性系列岩石,而在随后的陆-陆碰撞阶段主要形成高钾钙碱性岩石(LeBas et al., 1986; Zhu et al., 2011Hu et al., 2018, 2019)。已有研究资料表明,沿班公湖—怒江缝合带及其两侧出露大量113 Ma左右的岩浆岩(Zhu et al., 2011)。本次研究在马前乡地区识别出的早白垩世安山岩和英安岩的形成时代为107~114 Ma,与这些岩浆岩的时代基本一致。如前文所述,马前乡地区的早白垩世火山岩主要为高钾钙碱性系列岩石,它们与拉萨地块中北部的近同时代岩浆岩一致,其形成均与拉萨与南羌塘地块的陆-陆碰撞相关。因此,我们认为这些岩浆岩的大规模产出可以推测班公湖—怒江缝合带在早白垩世就已经闭合。这一推断也得到了古地测资料的支持。在110~50 Ma之间,拉萨地块南缘的古纬度为~20 ± 4°N(Lippert et al., 2014)。考虑到此时拉萨地块应当具有600 km左右的宽度,拉萨地块北缘的古纬度可能为~26 ± 4°N,与此时南羌塘地块南缘的古纬度(29.3 ± 5.7°N)误差范围内部分重合(Chen et al., 2014),从而指示此时陆-陆碰撞可能已经发生。班公湖—怒江缝合带内120~108 Ma玄武岩的地球化学数据指向混合地幔源区,并且这些玄武岩可能来源于拉萨—羌塘碰撞后软流圈物质的减压熔融(Zhu et al., 2016)。构造填图和碎屑锆石研究也表明,南羌塘地块在晚侏罗世—早白垩世发生了非常明显的地壳缩短,其缩短程度甚至可以与新生代的印度—欧亚大陆碰撞相对比(Raterman et al., 2014)。班公湖—怒江缝合带中段尼玛县的沉积学研究表明:在125~118 Ma该地区沉积环境就已由海相转化为陆相,这与拉萨与南羌塘地块陆-陆碰撞事件相关(Kapp et al., 2007)。

7 结论

(1)班公湖—怒江缝合带中段马前乡地区的安山岩和英安岩的形成时代为105~114 Ma,揭示了去申拉组火山岩的形成时代为早白垩世。

(2)安山岩富集Th和U,亏损Nb、Ta和Ti,具有变化范围较大的Mg#值(25~63),锆石εHf(t) 值(-8.6~+1.5)也以负值为主,应当为幔源镁铁质熔体与壳源熔体的混合产物。英安岩具有与安山岩类似的微量元素成分特征,以及负的锆石εHf(t)值(-12.3~-8.1),应当是地壳部分熔融的产物。

(3)马前乡地区早白垩世火山岩支持班公湖—怒江中特提斯洋的闭合时间为早白垩世,此时拉萨与南羌塘地块已经发生了陆-陆碰撞。

致谢: 锆石LA-ICP-MS U-Pb定年和Hf同位素分析得到了中国地质科学院矿产资源研究所侯可军副研究员的帮助,在此致以衷心的感谢。

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