Zircon U-Pb and Hf Isotopic Composition of Permian Felsic Tuffs in Southeastern Margin of Lhasa, Tibet
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摘要: 西藏东南缘记录的唐加-松多古特提斯增生杂岩带对认识古特提斯洋晚古生代的构造演化提供了新的证据.针对该杂岩带中新发现的长英质凝灰岩开展了全岩主、微量元素,锆石LA-ICP-MS U-Pb定年和锆石Hf同位素组成的研究.结果显示冲尼凝灰岩喷发于278~275 Ma,具有较高的SiO2含量(63.47%~72.65%)、Al2O3含量(14.53%~21.31%),较低的K2O含量(1.30%~2.51%)和TiO2含量(0.50%~1.17%),MgO含量较低,介于0.92%~2.00%,Mg#范围在19.9~34.2(均低于40).富集大离子亲石元素(LILE)、亏损高场强元素(HFSE).锆石具有较高的εHf(t)值(+10.2~+14.4)和相对年轻的地壳模式年龄TDMc=351~621 Ma,认为冲尼凝灰岩是唐加-松多古特提斯洋向北俯冲背景下的新生地壳部分熔融的产物,洋盆俯冲消减的开始时代不晚于早二叠世,并且在早二叠世拉萨地体东南缘存在新生地壳生长事件.Abstract: The Tangjia-Sumdo Paleo-Tethys accretionary complex belt, recorded in the southeastern margin of Lhasa, provides new evidence for the understanding of the Late Paleozoic tectonic evolution of the Paleo-Tethys Ocean. In this paper, it reports new data for the felsic tuff in this accretionary complex belt, including petrology, major and trace element compositions, zircon U-Pb age, and in-situ Hf isotopic compositions. U-Pb zircon dating indicates that the timing of eruption of the Chongni tuffs was ca. 278-275 Ma. They are characterized by relatively high SiO2 (63.47%-72.65%), and high Al2O3 (14.53%-21.31%), relatively low K2O (1.30%-2.51%), TiO2 (0.50%-1.17%), MgO (0.92%-2.00%), and Mg# (19.9-34.2). These tuffs exhibit LILE enrichment and HFSE depletetion. The zircons participating in the weighted age calculation have positive εHf(t) values of +10.2 to +14.4 and relatively young zircon Hf crustal model ages (TDMc=351-621 Ma). It agrees that Chongni tuffs were derived from the partial melting of the juvenile crust under the background of the northward subduction of the Tangjia-Sumdo Paleo-Tethys Ocean slab, and the beginning time of the ocean slab subduction is not later than Early Permian, before which there were the events that crust grew in the southeastern margin of Lhasa.
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Key words:
- Early Permian /
- Lhasa terrane /
- Hf isotope /
- U-Pb age /
- Tangjia-Sumdo Paleo-Tethys /
- petrology
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位于冈瓦纳大陆与劳亚大陆之间的古特提斯洋(Zhu et al., 2013),其形成与演化改变了晚古生代全球尺度的海陆格局,青藏高原内部因记录了古特提斯洋盆演化有关的沉积盆地、构造‒岩浆作用而备受地学界关注(Cawood et al., 2007;Zhu et al., 2011a;Zhai et al., 2016).龙木措‒双湖‒澜沧江缝合带作为青藏高原发育重要的晚古生代缝合带,在过去几十年间,报道了该带保存了原、古特提斯洋洋壳残余、俯冲消减的各类岩石,代表了青藏高原古特提斯大洋发育的主洋盆,其发育演化时代为晚寒武世‒晚三叠世(李才,1987;李才等,2008;吴彦旺,2013;Zhai et al., 2016;Fan et al., 2017).但自杨经绥等(2006)在松多地区发现榴辉岩后,改变了原有的古特提斯演化格局,将古特提斯洋的构造域从羌塘延伸到拉萨地体(Liu et al., 2020),使其近年成为国内外学者研究的热点(李化启等,2008;Yang et al., 2009;Zhang and Tang, 2009;董昕等,2010;董昕和张泽明,2013;吴兴源等,2013;Weller et al., 2016;郎兴海等,2017;Wang et al., 2019).尽管松多地区的超高压‒高压变质岩有着较为深入探讨(Li et al., 2009;Liu et al., 2009, 2020;Yang et al., 2009;Cheng et al., 2012, 2015;Weller et al., 2016;Wang et al., 2019),但与大洋演化直接相关的蛇绿混杂岩及增生杂岩研究较为薄弱.此外,整个拉萨地体二叠纪报道大部分基性火山岩是利用地层对比与化石来确定时代(耿全如等,2007;王立全等,2008;Zhu et al., 2010),有精确年龄约束的二叠纪岩浆岩仅在鸭洼地区与唐加‒松多‒布久等地有报道(Zhu et al., 2009;李奋其等,2012;牛志祥,2019;Wang et al., 2019, 2021;李楠等,2020;于云鹏,2020),这使得不同学者对拉萨地体二叠纪时期的构造背景还存在不同的认识.新一轮的1∶5万区域地质调查资料确立了位于拉萨地体东南缘的唐加‒松多古特提斯增生杂岩带,该带保存了相对完整的与古特提斯洋演化以及洋陆转换相关的混杂岩(李光明等,2020;解超明等,2020),新识别出了晚古生代蛇绿混杂岩(王斌等,2017)、中二叠世洋岛残片(Wang et al., 2019)、中二叠世岛弧岩浆岩(李楠等,2020;于云鹏,2020;Wang et al., 2021)以及晚三叠世‒早侏罗世弧火山岩(李光明等,2020)等关键地质体.这为研究古特提斯洋的演化提供了有利的约束,同时也对拉萨地体二叠纪的演化具有重要的指导意义.最近,笔者在唐加乡地区冲尼村附近首次发现一处早二叠世长英质凝灰岩的露头,本文对其锆石U-Pb年龄、全岩主微量地球化学以及锆石Hf同位素组成开展研究,并且在综合区域资料对比基础上,探讨唐加‒松多古特提斯早二叠世的俯冲消减作用以及与之相关的地壳生长.
1. 地质背景与岩石特征
青藏高原自北向南由一系列构造块体拼合而成,主要以东昆仑缝合带、金沙江缝合带、龙木措‒双湖‒澜沧江缝合带、班公湖‒怒江缝合带和雅鲁藏布缝合带为界划分成松潘‒甘孜地体、羌北‒昌都地体、羌南‒保山地体、拉萨地体和喜马拉雅地体(图 1a)(Yin and Harrison, 2000;Xu et al., 2015).其中,拉萨地体夹持于班公湖‒怒江缝合带和雅鲁藏布缝合带之间,以往的研究以狮泉河‒拉果错‒永珠‒纳木错‒嘉黎蛇绿混杂岩带和洛巴堆‒米拉山断裂带为界,由北向南分为北部拉萨地体、中部拉萨地体和南部拉萨地体(Zhu et al., 2008).南拉萨地体主要以新生地壳为主,出露了白垩纪‒第三纪冈底斯岩基和古近系林子宗火山岩,三叠纪‒白垩纪火山‒沉积地层主要分布在其东部;中拉萨地体代表一个微大陆与前寒武纪结晶基底岩石,覆盖了寒武系‒二叠系沉积岩和上侏罗统‒下白垩统火山岩;北拉萨地体整体也主要是新生地壳而非古老的改造地壳,中‒上三叠统沉积岩被中侏罗统碎屑岩不整合覆盖,主要分布在其东部,而下白垩统地层沿着其走向广泛发育(Zhu et al., 2008, 2010).
图 1 青藏高原地质简图以及唐加‒松多地区地质简图a.青藏高原构造单元划分简图(李才等,2008;Zhang et al., 2014;Xu et al., 2015);b.唐加‒松多地区地质简图.数据来源:陈松永等(2008);Yang et al.(2009);曾令森等(2009);Cheng et al.(2012, 2015);Weller et al.(2016);Wang et al.(2019, 2021);李楠等(2020);于云鹏(2020)Fig. 1. Tangjia-Sumdo geological map in the Gangdese, Tibet基于松多榴辉岩的发现,表明拉萨地体内部发育了一条唐加‒松多古特提增生杂岩带(杨经绥等,2006;李光明等,2020),该带东缘与洛巴堆‒米拉山断裂带近乎重叠展布,而后有学者以这条晚古生代杂岩带为界将拉萨地体划分为南、北地体(Zhang et al., 2014).本文研究区位于拉萨市东北墨竹工卡县唐加乡一带,出露的地质单元主要包括石炭‒二叠系松多岩组(C-Ps)、上三叠统‒下侏罗统火山‒沉积地层雄来组(T3J1x)和渐新统日贡拉组(E3r)以及俯冲增生杂岩和中‒新生代岩浆岩(图 1b).其中松多岩组为一套局部有序、宏观无序的地质体,其岩石组合主要为变质石英砂岩、白云母石英片岩,局部夹薄层中基性火山岩;雄来组为角度不整合于石炭‒二叠系松多岩组之上的碎屑岩与火山岩组合.
本文研究的长英质凝灰岩出露于唐加乡冲尼村以西,岩石整体呈灰绿色、灰白色,具凝灰结构,块状构造(图 2a),岩石致密坚硬,可见厚度约为15 m(图 2b),整体呈透镜体状与雄来组浅变质灰岩围岩断层接触.显微岩石学观察显示冲尼凝灰岩主要由晶屑、火山灰胶结物组成,具有典型的火山碎屑结构,玻屑与岩屑均不发育.晶屑含量整体约为15%~30%,大小在0.05~2 mm,成分主要为石英、斜长石,以及少量黑云母,故定名为长英质凝灰岩;晶屑多呈港湾状、棱角‒次棱角状,未见来自沉积岩或岩浆岩的岩屑,推测冲尼凝灰岩未经历长距离的搬运,为火山喷发后原地堆积形成;胶结物为火山灰,主要由粒径 < 0.05 mm的长英质矿物组成,见部分铁质矿物析出,呈他形粒状分布于胶结物之中(图 2c,2d).
2. 分析方法
本次野外采样工作尽量选择新鲜样品,经过室内洗净并晾干后备用.样品的无污染碎样在河北廊坊市(宇能)宇恒矿岩技术服务有限公司完成,主量元素和微量元素的测试在武汉上谱分析科技有限公司实验室完成,其中主量元素用XRF法测定,微量元素采用等离子质谱仪ICP-MS(Agilent 7700e)测定,分析精度优于5%~10%.
锆石分选在河北廊坊市(宇能)宇恒矿岩技术服务有限公司完成,首先对样品进行破碎、淘洗、电磁和重液分选,而后在双目镜下挑选出晶形符合条件的锆石颗粒.在北京锆年领航科技有限公司实验室进行了锆石的制靶、透射光和反射光显微照相、阴极发光(CL)显微照相以及锆石微量元素和LA-ICP-MS微区原位U-Pb同位素、Hf同位素分析.锆石U-Pb同位素采用的分析仪器为激光剥蚀‒电感耦合等离子体质谱仪(LA-ICP-MS,Agilent 7900型),激光剥蚀平台为Resolution SE型,利用193 nm深紫外激光进行剥蚀,剥蚀直径采用30 µm,能量密度为2 J/cm2,频率为5 Hz,以锆石91500作为校正标样,GJ-1作为检测标样,锆石微量元素测定以NIST 610作为外标,91Zr作为内标计算含量,数据采用Iolite程序处理(Paton et al., 2010).
锆石Hf同位素测试在锆石完成定年后进行,所有的Hf同位素分析点均位于相同锆石U-Pb测试点同一位置.利用激光剥蚀多接收器电感耦合等离子体质谱仪,系统为NWR213nm固体激光器,仪器运行条件及分析方法详见文献(Hu et al., 2012).激光剥蚀物质以He作为剥蚀物质载气,剥蚀直径采用20 µm×40 µm,能量密度为5 J/cm2,频率为8 Hz,测定时使用锆石国际标样Plešovice作为参考物质.分析过程中锆石标样Plešovice的176Hf/177Hf测试加权平均值为0.282 479 3 ± 0.000 004(2σ,n=15),与文献中标样的测试值在误差范围内完全一致(Sláma et al., 2008).
3. 分析结果
3.1 锆石U-Pb年龄与微量元素
本文对冲尼地区的两件凝灰岩(D0021-N1、D0021-N6)(图 1b)进行了锆石U-Pb定年与稀土元素测定,分析结果分别列于附表1、附表2中.样品D0021-N1、D0021-N6分别各获取了25个锆石年龄测点,其稀土元素含量普遍偏高(D0021-N1:ΣREE=728~7 030 μg/g;D0021-N6:ΣREE = 587~2 674 μg/g).在球粒陨石标准化配分图中,轻、重稀土分馏明显,具有Ce的正异常和Eu的负异常(图 3b,3d),呈典型的火成岩锆石稀土元素配分模式;此外,锆石颗粒无色透明呈半自形晶,在CL图像中呈现规则的韵律环带,颗粒粒度较小(粒径一般 < 100 µm)(图 4),且具有较高的Th/U比值(0.29~1.57),显示典型的岩浆成因锆石特征(Hoskin and Black, 2000).剔除不谐和的年龄后,D0021-N1锆石的206Pb/238U加权平均年龄为278.0±3.5 Ma(2σ,MSWD=3.1,n= 8)(图 3a,3b);D0021-N6锆石的206Pb/238U加权平均年龄为275.6±0.9 Ma(2σ,MSWD=0.54,n=12)(图 3c,3d),两件凝灰岩样品的锆石U-Pb年龄在误差范围内相近,说明冲尼凝灰岩喷发于早二叠世.除此之外两件凝灰岩中均存在年龄值较老的锆石:D0021-N1中存在302~289 Ma、326~320 Ma、372~363 Ma的锆石年龄,D0021-N6中具有302~291 Ma、318~313 Ma、460 Ma的锆石年龄,这些锆石没有发现明显的磨圆,部分锆石为次棱角状、边部具熔蚀边或存在吸收现象(例如测点:D0021-N1-11、D0021-N1-12、D0021-N1-25,D0021-N6-6、D0021-N6-11、D0021-N6-12、D0021-N6-14、D0021-N6-15等)(图 4),锆石的形态特征也显示出未经历过长距离的搬运.其次,岩石显微特征呈现以晶屑为主、未见岩屑的特点,而晶屑一般是由早期结晶的斑晶崩碎或火山通道的围岩岩石炸碎后形成.合理地推测冲尼凝灰岩的中酸性母质岩浆熔体在上升的过程中捕获了早期岩浆作用形成的锆石,岩浆喷发后随着火山灰就近堆积在火山口附近.
3.2 锆石Hf同位素
针对冲尼村的两件凝灰岩,共计39颗锆石进行了原位Hf同位素的分析(附表3).176Hf/177Hf初始比值和εHf(t)值根据同一锆石U-Pb年龄值计算,整体176Yb/177Hf和176Lu/177Hf比值范围分别为0.028 007~0.336 858和0.000 789~0.009 060、176Hf/177Hf初始比值范围为0.282 517~0.283 137、εHf(t)值变化于-8.7~+15.0;其中参与锆石加权年龄(年龄范围为270~285 Ma)计算的测点的εHf(t)值为+10.2~+14.4,具有相对年轻的地壳模式年龄TDMc =351~621 Ma,捕获锆石(290~460 Ma)的εHf(t)值为-8.7~+15.0,整体地壳模式年龄TDMc=324~1 894 Ma.以上均为排除其中测点D0021-N1-18的结果(模式年龄小于结晶年龄).
3.3 岩石地球化学特征
冲尼早二叠世凝灰岩的全岩主微量元素测试结果见附表4.样品主量元素中SiO2含量变化于63.47%~72.65%,TiO2含量为0.50%~1.17%,有较高的Al2O3含量(14.53%~21.31%),有较高铝饱和指数(A/CNK=1.07~4.72,平均值为1.81),属过铝质岩石.MgO含量较低,介于0.92%~2.00%,Mg#范围在19.9~34.2(均低于40),使用针对蚀变火山岩分类有效的图解(Zr/TiO2-Nb/Y、Th-Co图解)进行判别,样品落在安山岩和安山岩或玄武岩内(图 5a),属于钙碱性系列(图 5b).
图 5 冲尼凝灰岩岩石判别图a. Zr/TiO2-Nb/Y图解,据Winchester and Floyd(1977);b. Th-Co图解,据Hastie et al.(2007).数据来源:大陆平均弧安山岩(Kelemen et al., 2007);中二叠世深成岩(李楠等,2020;于云鹏,2020);埃达克质花岗岩(Wang et al., 2021);皮康花岗岩(Zhu et al., 2009)Fig. 5. Classification diagrams of Chongni tuffs冲尼凝灰岩在原始地幔标准化微量元素蛛网图(图 6a)上明显富集Rb、Ba、Th、U、K等大离子亲石元素(LILEs),并且强烈亏损Nb、Ta、Ti等高场强元素(HFSEs).其次在球粒陨石稀土元素配分图(图 6b)上轻、重稀土元素分馏作用较弱((LaN/YbN)=2.04~4.95),具有不同程度的轻微正和负Eu异常(δEu=0.77~1.23),表明斜长石分离结晶作用或源区斜长石的残留不明显.
图 6 原始地幔标准化微量元素蛛网图及球粒陨石标准化稀土配分曲线图标准化数据引自Sun and McDonough(1989).数据引用参看图 5Fig. 6. Primitive mantle-normalized trace element spidergram and chondrite-normalized REE pattern4. 讨论
4.1 岩浆源区与构造背景
锆石Hf同位素可以很好地反映岩浆源区特征(Griffin et al., 2000),冲尼凝灰岩具有正εHf(t)值(+10.2~+14.4),与研究区时代相近的深成岩的锆石Hf同位素组成具有很高的相似度(εHf(t)= +12.2~+12.8,于云鹏,2020;εHf(t)=+13.8~ +15.7,Wang et al., 2021),不同于早二叠世布久中基性岩浆岩(εHf(t)=-2.7~+0.1)(牛志祥,2019)和中二叠世皮康花岗岩(εHf(t)=-4.5~+1.9)(Zhu et al., 2009).对于正的εHf(t)值,目前存在3种不同的解释:(1)变质作用的影响.锆石受变质作用形成残留锆石、重结晶锆石和新生的变晶锆石(吴福元等,2007),其中新生的变晶锆石周围的流体/矿物对其Hf同位素组成有重要的影响(吴福元等,2007).当变质锆石结晶时源区存在石榴石,会导致锆石176Hf/177Hf比值明显升高,出现相对较高的εHf(t)值(Zheng et al., 2005;Wu et al., 2006),从野外和室内镜下观察并未发现凝灰岩存在明显的变质,且锆石都具较高的Th/U比值、火成岩稀土配分特征和韵律环带,可以排除变质产生的影响.(2)亏损地幔来源的岩浆直接结晶分异.部分学者认为地幔玄武质岩浆可以直接结晶分异出少量的花岗质岩浆(Grove and Donnelly-Nolan,1986;Bacon and Druitt, 1988),事实上最多形成相当于高镁安山岩成分的岩浆(Hofmann,1988),冲尼凝灰岩具有较高的SiO2含量(63.47%~72.65%),这只有在地幔极低部分熔融的条件下形成,产生的少量熔体难以聚集上涌并出现岩浆喷发,故不太可能是亏损地幔来源.(3)新生(juvenile)地壳的部分熔融.Hf同位素常用来区别以陆壳再循环为主或以新生地壳生长为主的造山过程(Collins et al., 2011),俯冲作用会逐渐移除掉古老的下地壳,地幔楔派生的新生地壳会持续增加,与俯冲相关的岩浆活动也会呈现出εHf(t)值越来越大的特征(Collins et al., 2011),冲尼凝灰岩具有较正εHf(t)值,明显不同于古老地壳的特征(即较负εHf(t)值).其较为年轻的地壳模式年龄(TDMc=351~621 Ma),表明具有相对年轻的岩浆来源,加之相对低的Mg#值(19.9~34.2)也反映壳源特征,故笔者认为冲尼凝灰岩应来自于新生地壳的部分熔融.
岩浆岩锆石中微量元素(如Y、Th、U、Nb、Ta)能很好地反映其岩石类型和结晶环境(Heaman et al., 1990;Belousova et al., 2002),常被用于判别其形成的大地构造背景(Belousova et al., 2002;Yang et al., 2012).通过统计大量的岩浆岩锆石微量元素特征,利用Y-Yb/Sm与Y-U图解区分出几种不同类型的锆石微量元素特征组合(Belousova et al., 2002),本文样品落入花岗岩类与镁铁质岩重叠区内(图 7a,7b).其次锆石总稀土元素含量都偏高(D0021-N1:ΣREE = 728~7 030 μg/g;D0021-N6:ΣREE = 587~2 674 μg/g),与Belousova et al.(2002)报道的花岗岩中锆石微量元素特征相当,表明其更可能为花岗岩类锆石.在此基础上,利用锆石中微量元素的Th、U、Nb、Hf来约束其构造背景(Yang et al., 2012),样品均落入岩浆弧区域(图 7c,7d).此外,冲尼凝灰岩在原始地幔标准化微量元素蛛网图(图 6a)和球粒陨石稀土元素配分图(图 6b)上具有明显的岛弧特征,如强烈亏损Nb、Ta、Ti,与典型的大陆平均原始弧安山岩特征相似(Kelemen et al., 2007),但在大陆岛弧、活动大陆边缘以及同碰撞环境都能显示这种特征.根据松多地区榴辉岩273~260 Ma的峰期变质年龄,表明该时段洋盆发生一期深俯冲作用(陈松永等,2008;Yang et al., 2009;Cheng et al., 2012;Weller et al., 2016).冲尼凝灰岩喷发于278~275 Ma与松多榴辉岩进变质年龄相仿,且两者处于同一构造带内相距约50 km,可以排除同碰撞的环境;而后出现与岛弧相关的镁铁质岩浆活动(270 Ma,李楠等,2020)、埃达克质花岗岩(269~266 Ma,Wang et al., 2021)以及闪长质岩浆活动(262 Ma,于云鹏,2020),表明早‒中二叠世唐加‒松多地区处于洋壳俯冲消减的大地构造背景.综上笔者认为冲尼凝灰岩形成于古特提斯洋俯冲背景下新生地壳部分熔融的大陆岛弧环境.
图 7 冲尼凝灰岩锆石微量元素特征判别图a,b.据Belousova et al.(2002);c,d.据Yang et al.(2012)Fig. 7. Zircon trace element plots of Chongni tuffs4.2 唐加‒松多古特提斯洋俯冲时限与地壳生长
前人研究表明,唐加‒松多古特提斯杂岩带中发现原岩为洋壳玄武岩的榴辉岩(306 Ma,全岩Sm⁃Nd,Li et al., 2009;290 Ma,锆石U⁃Pb,Cheng et al., 2012),以及洋岛玄武岩(306 Ma,陈松永,2008),说明洋盆在晚石炭世‒早二叠世早期就已经打开.冲尼凝灰岩喷发年龄为278~ 275 Ma以及松多榴辉岩存在273 Ma的进变质年龄(Weller et al., 2016),表明唐加‒松多古特提斯洋俯冲不会晚于早二叠世.其次,上述冲尼凝灰岩形成于古特提斯洋俯冲背景下新生地壳部分熔融的岛弧环境,样品中捕获的锆石年龄在290~ 460 Ma的测点εHf(t)值为-8.7~+15.0,其地壳模式年龄TDMc = 324~1 894 Ma,整体跨度较大,合理的解释是新生地壳主要是来源于板块汇聚边缘,洋壳的俯冲导致地幔楔的部分熔融,从而引发弧岩浆作用形成新生地壳;进一步的俯冲形成流体向上运移或岩浆底侵使得新生地壳形成后便很快又部分熔融,形成长英质熔体,而俯冲作用引发的新生地壳部分熔融带来的时空变化差异,使得唐加‒松多杂岩带中与弧岩浆有关的锆石整体呈现出εHf(t)值随着锆石结晶年龄变年轻而变正的趋势(图 8),这表明在早二叠世拉萨地体东南缘就已经开始发育新生地壳,出现地壳生长事件.
再者,温木朗洋岛残片(268~260 Ma)表明中二叠世唐加‒松多古特提斯洋还存在洋盆(Wang et al., 2019).松多榴辉岩识别出原岩为N-MORB型玄武质岩浆,全岩Sm-Nd等时线年龄为239± 3.5 Ma,榴辉岩作为俯冲折返的产物,暗示着中三叠世晚期还存在着洋壳俯冲事件(曾令森等,2009).白朗榴辉岩研究表明其原岩为OIB型玄武质岩浆,并提出唐加‒松多古特提洋不会早于230 Ma关闭(Cheng et al., 2015).综上表明唐加‒松多古特提斯洋至少从晚石炭世一直演化到晚三叠世.此外,唐加‒松多增生杂岩带北侧发育以古老地壳为基底的晚古生代火山‒沉积地层(王立全等,2008),而南侧南拉萨地体主要是以新生地壳为主(Zhu et al., 2011b),缺乏前寒武纪基底.从石炭‒二叠纪到早中生代的弧岩浆岩显示不断向南迁移的趋势,并且带内发育倾向北的多期次透入性面理构造(李光明等,2020),故笔者认为唐加‒松多古特提斯洋俯冲极性为向北俯冲.
最近发现唐加‒松多增生杂岩带内发育了不整合于晚古生代混杂岩之上的晚三叠世‒早侏罗世沉积‒火山盆地(雄来组),表明该地区存在晚三叠世的造山作用,但不能简单地认为是古特提斯洋闭合造山.因为雄来组出露大量具弧火山岩特征的中基性火山岩夹层(以安山岩为主),正如前文提到,此类性质的岩石在大陆岛弧、活动大陆边缘以及同碰撞环境都能产出,这也可能代表了唐加‒松多古特提斯洋于晚三叠世‒早侏罗世还在俯冲,并且俯冲增生同样也会出现区域性角度不整合(肖文交等,2019).因此,唐加‒松多古特提斯洋的闭合时限还需进一步研究.
5. 结论
(1)拉萨地体东南缘冲尼凝灰岩的锆石U-Pb测年结果显示,其结晶年龄分别为278.0±3.5 Ma和275.6±0.9 Ma,形成时代为早二叠世.
(2)冲尼长英质凝灰岩具有正的εHf(t)值、明显亏损Nb、Ta等高场强元素,表明其形成于唐加‒松多古特提斯洋北向俯冲背景下的新生地壳部分熔融的岛弧环境.
(3)拉萨地体东南缘在早二叠世就已经开始发育新生地壳,存在地壳增长事件.
附表见本刊官网(http://www.earth-science.net).
致谢: 感谢成都理工大学李博博士、李楠硕士、周豫硕士在野外工作中提供的帮助,两位匿名审稿专家也提供了许多建设性的意见,极大地改进了文章内容,在此谨致谢忱! -
图 1 青藏高原地质简图以及唐加‒松多地区地质简图
a.青藏高原构造单元划分简图(李才等,2008;Zhang et al., 2014;Xu et al., 2015);b.唐加‒松多地区地质简图.数据来源:陈松永等(2008);Yang et al.(2009);曾令森等(2009);Cheng et al.(2012, 2015);Weller et al.(2016);Wang et al.(2019, 2021);李楠等(2020);于云鹏(2020)
Fig. 1. Tangjia-Sumdo geological map in the Gangdese, Tibet
图 5 冲尼凝灰岩岩石判别图
a. Zr/TiO2-Nb/Y图解,据Winchester and Floyd(1977);b. Th-Co图解,据Hastie et al.(2007).数据来源:大陆平均弧安山岩(Kelemen et al., 2007);中二叠世深成岩(李楠等,2020;于云鹏,2020);埃达克质花岗岩(Wang et al., 2021);皮康花岗岩(Zhu et al., 2009)
Fig. 5. Classification diagrams of Chongni tuffs
图 6 原始地幔标准化微量元素蛛网图及球粒陨石标准化稀土配分曲线图
标准化数据引自Sun and McDonough(1989).数据引用参看图 5
Fig. 6. Primitive mantle-normalized trace element spidergram and chondrite-normalized REE pattern
图 7 冲尼凝灰岩锆石微量元素特征判别图
a,b.据Belousova et al.(2002);c,d.据Yang et al.(2012)
Fig. 7. Zircon trace element plots of Chongni tuffs
图 8 冲尼凝灰岩εHf(t)‒锆石年龄图
数据来源于Zhu et al.(2009);牛志祥(2019);于云鹏(2020);Wang et al.(2021)
Fig. 8. εHf(t) vs. age diagram of Chongni tuffs
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