2020, Vol. 36
2. 云南省地质矿产勘查院, 昆明 650000;
3. 北京大学地球与空间科学学院, 北京 100871;
4. 中国地质调查局天津地质调查中心, 天津 300170;
5. 西藏自然科学博物馆, 拉萨 850000
2. Yunnan Institute of Geology & Mineral Resources Exploration, Kunming 650000, China;
3. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
4. Tianjin Geological Survey Center of China Geological Survey, Tianjin 300170, China;
5. Tibet Museum of Natural Science, Lhasa 850000, China
班公湖-怒江缝合带位于青藏高原中部拉萨-腾冲地块与羌塘-保山地块之间，西自班公湖，经班戈、东巧、丁青至东构造结，再向南沿高黎贡构造带延伸至缅甸境内(图 1a)。西段(班公湖-东构造结)构成拉萨地块与南羌塘地块的边界，走向近东西，长达2000km，断续分布的蛇绿混杂岩带(251~150Ma; Shi, 2007；Shi et al., 2008, 2012；Wang et al., 2008; 强巴扎西等, 2009；黄启帅等, 2012; Liu et al., 2014; 周涛等, 2014)、高压变质岩带(190~170Ma; Guynn et al., 2006, 2013；Zhang et al., 2010, 2012b, 2014；王保弟等，2015; Wang et al., 2016)及两侧侏罗纪-早白垩世科迪勒拉型岩浆岩带(170~150Ma，140~105Ma；朱弟成等, 2006；Kapp et al., 2007; 康志强等, 2008; Chiu et al., 2009; Ji et al., 2009; Zhu et al., 2009a, b, 2016;曲晓明等, 2009; 杜德道等, 2011; Sui et al., 2013; Li et al., 2014a, b; Liu et al., 2014; Chen et al., 2017; Fan et al., 2015)为厘定其中特提斯缝合带的属性提供了关键性标志。南段(高黎贡构造带)内形成于大陆边缘弧背景的早白垩世岩浆岩带(杨启军等，2006；Zhu et al., 2009b, 2015; 谢韬等，2010；戚学祥等，2011；杨启军和徐义刚, 2011; Xu et al., 2012；白宪洲等，2012；高永娟等，2012；李再会等，2012; Cao et al., 2014；Qi et al., 2015, 2019；Xie et al., 2016; Zhao et al., 2016)及增生杂岩带(刘本培等，2002；储著银等，2009；尹福光等，2012；Xie et al., 2016；Qi et al., 2019)为其作为怒江洋俯冲和腾冲地块与保山地块碰撞的缝合带及班怒带经东构造结向南延伸的部分提供了依据。近年来在高黎贡构造带东南缘龙陵-瑞丽一带相继发现断续分布与怒江洋俯冲/闭合有关的早白垩世中酸性火山岩(白宪洲等，2012；高永娟等，2012)，为高黎贡构造带作为班怒带南向延伸的大陆边缘弧提供了完整的岩浆岩序列。但是，该构造带向北是否还存在与侵入岩配套的同时代火山岩？至今尚未有明确的证据。为此，笔者在野外地质调研过程中，通过科研填图、镜下观察、锆石LA-ICP-MS U-Pb定年，查明高黎贡构造带西北缘原定为晚元古代梅家山群二道河组流纹岩(云南省地质矿产勘查开发局, 2011①)的时代实为早白垩世(图 1b)，并对其形成的构造背景及其与高黎贡造山带内同时代侵入岩的关系进行了探讨。
① 云南省地质矿产勘查开发局. 2011.云南省1/250000腾冲幅区域地质图(G47C003002)
1 地质背景高黎贡构造带位于青藏高原东构造结东南缘，北起察隅，呈南北向经贡山、泸水至龙陵转为南西向，经瑞丽至缅甸为Sagaing断裂带所截，全长约500km，宽10~20km，构成腾冲地块和保山(Sibumusu)地块的边界(图 1)。构造带由中部的高绿片岩相-角闪岩相(局部达中压麻粒岩相)(Song et al., 2010)的深变质变形带和两侧的低绿片岩相到绿片岩相浅变质带组成，其间分别以泸水-龙陵-瑞丽断裂带和龙川江断裂带构成其东、西边界(图 1b)。深变质带变质岩原岩主要为早古生代沉积岩(戚学祥等，2019)和古生代、中生代及新生代岩浆岩，并在新生代块体旋转挤出过程中叠加右行韧性剪切变形(Wang and Burchfiel, 1997；季建清等，2000；Wang et al., 2006；杨启军等，2006; Lin et al., 2009; Xu et al., 2012, 2015；Zhang et al., 2012a；高永娟等，2012；Eroǧlu et al., 2013; 邹光富等, 2013, 2014; Huang et al., 2015；Qi et al., 2015, 2019)，发生不同程度糜棱岩化，形成长英质、花岗质和角闪质糜棱岩及糜棱岩化角闪岩和大理岩。构造带东侧浅变质岩原岩由古生代半深海、浅海和滨海相沉积岩及中生代浅海到海陆交互相沉积岩组成，并在早二叠和中侏罗统地层中夹有海相火山岩(刘旭峰等，2019)，但缺失中、晚石炭和晚三叠-早侏罗统地层。在这套低绿片岩相变质岩中发育一套由蛇纹石化橄榄岩、玄武岩、硅质岩、碳酸盐岩岩块和含层状放射虫硅质岩的复理石沉积岩基质构成的增生杂岩带，分布于龙陵-瑞丽一带(图 1b)。构造带西侧浅变质岩为晚古生代浅海-滨海相和中生代浅海相和开阔台地相沉积岩经低绿片岩相变质形成的，以缺失中、晚泥盆统-石炭系、晚二叠-中三叠统和侏罗-白垩系地层而明显不同于东侧浅变质岩带。
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图 1 高黎贡构造带地质略图(据Qi et al., 2015修改) MBT-主边断裂带；YTS-雅鲁藏布缝合带；CL-中拉萨地块；NL-北拉萨地块；BNS-班公湖-怒江缝合带；LLRF-泸水-龙陵-瑞丽断裂带；LCJF-龙川江断裂带；DYJF-大盈江断裂带.白色圈为引用文献U-Pb年龄数据，来源于杨启军等, 2006; Cong et al., 2011a, b; 戚学祥等，2011；Xu et al., 2012; Qi et al., 2015, 2019; Xie et al., 2016 Fig. 1 Simplified geological map of the Gaoligong orogen (modified after Qi et al., 2015) MBT-Main boundary thrust; YTS-Yarlung zangbo suture; CL-Center Lhasa block; NL-North Lhasa block; BNS-Banggonghu-Nujiang suture; LLRF-Lushui-Longling-Ruili fault; LCJF-Longchuanjiang fault; DYJF-Dayingjiang fault.White circle-data of U-Pb ages from Yang et al., 2006; Cong et al., 2011a; Qi et al., 2011, 2015, 2019; Xie et al., 2016 |
泸水-龙陵-瑞丽断裂带走向与高黎贡构造带一致，由一系列近于平行的次级断裂组成。在龙陵以北呈近南北向，产状与糜棱面理一致，倾向东，倾角50°~60°，具有右行走滑剪切的特点，经龙陵呈弧型向西南方向延伸，进入龙陵-瑞丽段，由西部的龙潭-楠木桥断裂和东部的龙陵-瑞丽断裂等组成，宽2~5km，分别分隔混杂岩带于西北侧的高黎贡群变质岩和东侧的古生代、中侏罗统和上白垩统沉积岩。其中，龙潭-楠木桥断裂带总体倾向北西，倾角变化较大，多分布在50°~80°之间，糜棱岩化变形变质强烈，带内眼球状构造、糜棱面理、矿物拉伸线理普遍存在，展示其右行走滑的特点，形成时代为35~10Ma，是中新世晚期-上新世右行走滑剪切变形的产物(Lin et al., 2009; Zhang et al., 2012a；Eroǧlu et al., 2013; 王丹丹等，2013)。龙陵-瑞丽断裂呈舒缓波状，总体倾向W-NW，倾角60°~76°，是控制了龙陵-瑞丽增生杂岩带东部边界的伸展构造(图 1b)。
龙川江断裂带沿高黎贡构造带西缘分布，总体走向与构造带一致(图 1b)，断裂带宽3~5km，局部为第四纪火山-沉积物覆盖。沿带存在明显的断层三角面及20~50m宽的次棱角状构造角砾岩带和强片理化带，以及各种牵引褶皱、揉皱、劈理化带等，表现出明显的脆性变形特点。在断裂带北段新生代火山岩盆地相继分布，并展示出不规则菱形特征，揭示其在新生代时期以伸展、拉分变形为主，以及对该时期火山-沉积盆地的分布及火山岩浆喷发的控制作用。
构造带内岩浆活动频繁，主要经历了早古生代、中生代和新生代三期大规模岩浆活动。其中，中生代侵入岩主要为早白垩世闪长岩-花岗闪长岩-花岗岩，呈带状分布于构造带中，与围岩呈侵入接触关系。中生代火山岩为早白垩世英安岩和流纹岩，主要分布于高黎贡东南缘的龙陵-瑞丽和西北缘的曲石一带(图 1b)。龙陵-瑞丽一带的中酸性火山岩夹于弧前/弧间粉砂岩、泥质粉砂岩和泥岩中，与侏罗纪复理石沉积建造一起构成混杂岩的一部分，曲石一带中生代火山岩为项目组近年新厘定出来的，是本次研究的对象。
2 流纹岩岩石学特征滇西高黎贡西北缘早白垩世火山岩主要分布于曲石街一带，由高家寨和濮家寨两部分组成，总体呈近南北向展布，东部与高黎贡群变质岩呈断层接触，西部为第四纪沉积物覆盖(图 1b)。高家寨火山岩由西部的高家寨和东部的白马山两部分组成(图 2a)，岩性为流纹岩。其中，高家寨流纹岩东缘与高黎贡群浅变质砂岩、泥质粉砂岩之间为韧性剪切带所间隔，西部为新近纪芒棒组沉积岩覆盖，并为同时代花岗岩侵入。流纹岩出露长约5km，宽1~4km，面积约20km2，大部分都发生不同程度的糜棱岩化韧性变形(图 3a)。白马山流纹岩位于高家寨流纹岩东约4km，呈近南北向窄带状展布，长约25km，宽1~5km，出露面积约60km2，强糜棱岩化，与两侧高黎贡群变质岩的边界为向东倾斜的近南北向逆冲断层并叠加右行韧性走滑剪切变形的断裂带。濮家寨火山岩位于高家寨流纹岩正南8km，呈近南北向展布，长约14km，宽1~1.5km，出露面积约18km2，岩性为流纹岩，经历了不同程度的糜棱岩化韧性变形(图 3b)，与西侧晚古生代沉积岩和花岗岩间为右行韧性剪切变形叠加向东倾的正断层，与东部花岗片麻岩为右行韧性剪切带接触(图 2b)。
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图 2 高黎贡西北缘早白垩世流纹岩分布区域地质图(据云南省地质调查院, 2004修编①) Fig. 2 Geological map of the Early Cretaceous rhyolites in the northwestern Gaoligong orogen |
① 云南省地质调查院. 2004. 1/50000曲石街幅地质图(G47E017011)
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图 3 高黎贡西北缘早白垩世流纹岩岩石学特征 高家寨和濮家寨流纹岩野外产出照片(a、b)和镜下照片(c、d). Kf-钾长石；Pl-斜长石；Fsp-斜长石+钾长石；Qz-石英；Bi-黑云母；Zoi-黝帘石 Fig. 3 Petrological features of the Early Cretaceous volcanics in the westnorthern Gaoligong orogen Field occurrences (a, b) and photomicrographs (c, d) of the rhyolite in Gaojiazhai and Pujiazhai. Kf-K-feldspar; Pl-plagioclase; Fsp-plagioclase+K-feldspar; Qz-quartz; Bi-biolite; Zoi-zoisite |
高家寨流纹岩多发生韧性变形，主体为糜棱岩化流纹岩或流纹质糜棱岩。岩石多具有糜棱状构造、变余斑状结构。斑晶主要为斜长石、钾长石、石英和黑云母。其中，斜长石和钾长石都呈半自形-他形短柱状，粒度在0.2~0.8mm之间，含量约10%~15%，在后期糜棱岩化过程中边部出现细粒化，以及不同程度的钠黝帘石化、绢云母化和高岭土化蚀变(图 3c)。石英呈他形粒状，弱定向性，部分石英边部具有碎裂化，含量约7%~10%，黑云母呈不规则片状集合体定向分布于基质中或长石边部，含量3%~5%(图 3c)。基质为微晶石英、长石和隐晶质。
濮家寨流纹岩也经历了韧性变形，具有典型的糜棱状构造、变余斑状结构。斑晶主要为长石、石英和黑云母。其中，长石呈他形短柱状或粒状，部分在后期糜棱岩化过程中边部出现细粒化，使长石呈椭圆状，构成旋转碎斑(图 3d)。长石粒度在0.2~1.0mm之间，含量约10%~20%，并发生不同程度的钠黝帘石化、绢云母化和高岭土化蚀变(图 3d)。石英呈他形粒状，少量变形重结晶形成亚颗粒集合体，部分边部具有弱碎裂化，含量约为8%~10%，黑云母呈不规则片状集合体定向分布于基质中或长石边部，构成面理，含量3%~5%(图 3d)。基质为微晶石英、长石和隐晶质。
3 测试分析方法研究区内的样品采自高家寨和濮家寨两个区域(图 2)，在野外选择岩石出露面大、岩石新鲜、无脉体穿插、弱变形的露头采集流纹岩样品，对经薄片观察确定其无蚀变、无细脉体的流纹岩进行锆石分选和岩石地球化学分析，以保证锆石来源的单一性和地球化学成分的代表性。
锆石分选在河北省地质调查研究院完成。样品经常规的粉碎和重选，分选出纯度较高的锆石，然后在双目镜下经人工挑选出纯度在99%以上的锆石样品。在北京讯得丰科技发展有限公司用环氧树脂将锆石样品和标样固定成圆饼状，用不同型号砂纸和磨料将锆石磨去一半并抛光后，对抛光好的锆石进行阴极发光成像观察，查明锆石内部生长层的分布和结构，然后在中国地质大学(武汉)地质过程与矿产资源国家重点实验室用GeoLas 2005 ArF准分子激光剥蚀系统(LA)和Agilent7500a四级杆质谱(ICP-MS)进行锆石U-Pb同位素定年和锆石成分测试。其中，激光波长193nm，能量密度14J/cm2，频率8Hz，光斑直径24~32μm。锆石U-Pb年龄测定采用国际标准锆石91500作为外标校正方法，以29Si (锆石中SiO2的含量为32.18%)作为内标，测定锆石中U、Th和Pb的含量。每测定3~5个点后插入一次标样测定，以便及时校正。以合成硅酸岩玻璃NIST 610标示仪器的运行状态。采用ICPMSDataCal (V3.7)软件对同位素比值数据进行处理，详细的仪器操作条件和数据处理方法见(Liu et al., 2010)。ISOPLOT程序(Ludwig, 2001)进行锆石加权平均年龄计算及谐和图的绘制。
岩石地球化学分析样品在河北省地质调查研究院完成。样品经仔细挑选，确定无任何脉状体后，洗净、晒干、磨成200目粉末。岩石的常量元素、微量元素和稀土元素分析在国家地质测试实验中心完成。其中，常量元素依据GB/T14506.28-2010标准，采用X-射线荧光光谱仪(PW4400)进行测定，并用等离子光谱法进行验证，分析精度优于5%；微量元素和稀土元素依据GB/T 14506.30-2010标准，采用等离子质谱仪ICP-MS(Inductively Coupled Plasma Mass Spectrometry)(PE300D)方法进行测定，含量大于10×10-6的元素测试精度为5%，而小于10×10-6的元素测试精度为10%。
4 岩石地球化学岩石地球化学全分析结果表明濮家寨(15QDBW64-68)和高家寨(15QDBW70-74)流纹岩地球化学成分非常相近(表 1)，仅在常量元素方面存在微小的差别。
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表 1 高黎贡西北缘早白垩世流纹岩的常量元素(wt%)、稀土元素和微量元素(×10-6) Table 1 Compositions of major elements (wt%), rare earth and trace elements (×10-6) for the Early Cretaceous rhyolites in northwestern Gaoligong orogen |
高家寨流纹岩的SiO2含量(68.10%~70.49%)略低于濮家寨流纹岩(70.61%~73.43%)外，Al2O3、TiO2、Na2O和P2O5含量及Na2O/K2O比值都略高于濮家寨流纹岩。在TAS图解上，大部分样品都分布在流纹岩区，少量分布于英安岩区，且紧邻流纹岩和英安岩分界线(图 4a)，说明这套岩石以流纹岩为主，少量样品虽然落在英安岩区，但与其紧邻的流纹岩样品中SiO2含量仅相差1%~2%，在化学分析误差范围内，且镜下鉴定也难以区分，因此我们都将其归为流纹岩类。岩石A/NK集中于1.61~2.08之间，个别达2.53，A/CNK值绝大部分大于1.0，个别小于1.0(表 1)，其差异与样品中斜长石斑晶含量有关，反映岩石属于弱过铝质-过铝质性质。岩石里特曼指数(σ)介于0.7~1.93之间，在SiO2-(Na2O+K2O-CaO)和K2O-SiO2及SiO2-FeO/(FeO+MgO)图解上落于钙、钙碱性和高钾钙碱性及镁质岩区(Frost et al., 2001)，说明该区流纹岩属于镁质高钾钙碱性岩类(图 4b-d)。
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图 4 TAS火成岩分类图解(a)、SiO2-FeO/(FeO+MgO)图解(b, Frost et al., 2001)、SiO2-(Na2O+K2O-CaO)图解(c)和K2O-SiO2图解(d, Le Maitre, 1989) 图中花岗闪长岩和花岗岩数据来源于Qi et al. (2019)，图 8-图 10同 Fig. 4 TAS classification (a), SiO2 vs. FeO/(FeO+MgO) (b, Frost et al., 2001), SiO2 vs.(Na2O+K2O-CaO)(c) and K2O vs. SiO2 (d, Le Maitre, 1989) diagrams for the igneous rocks of the Gaoligong Orogen The shaded bands are the fields with boundary lines of Peccerillo and Taylor (1976).Data of granodiorite and granite is from Qi et al., 2019, also in Fig. 8-Fig. 10 |
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图 8 锆石εHf(t)-ages (a)和地壳模式年龄(tDMC)直方图(b) Fig. 8 Zircon εHf(t) versus ages diagram (a) and histogram of tDMC(b) Reference line representing chondritic Hf evolution (CHUR) is from Blichert-Toft and Albarède (1997) |
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图 9 高黎贡构造带早白垩世岩浆岩SiO2-Mg#图解 纯地壳部分融熔区：低钾玄武岩在8~16kbar and 1000~1050℃条件下脱水融熔实验(Rapp and Watson, 1995)；中-高钾玄武岩在7kbar and 825~950℃含1.7%~2.3%水条件下融熔实验(Sisson et al., 2005).安第斯南流纹岩带新近纪流纹岩区据López-Escobar et al., 1993；地幔AFC曲线据Depaolo，1981 Fig. 9 Mg# vs SiO2 diagram the Early Cretaceous plutonic rocks in the Gaoligong orogen Fields shown are as follows: pure crustal partial melts obtained in experimental studies by dehydration melting of low-K basaltic rocks at 8~16kbar and 1000~1050℃ (Rapp and Watson, 1995); pure crustal melts obtained in experimental studies by the moderately hydrous (1.7%~2.3% H2O) melting of medium-to high-K basaltic rocks at 7 kbar and 825~950℃ (Sisson et al., 2005); mantle melts (basalts); and Quaternary volcanic rocks from the Andean southern volcanic zone (López-Escobar et al., 1993). The curve for mantle AFC was calculated following Depaolo (1981) with a mass assimilation/fractionation ratio of r=2, reflecting a relatively hot mantle wedge, and 80% amphibole +20% clinopyroxene as the fractionating phases (Stern and Kilian, 1996) |
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图 10 高黎贡构造带早白垩世岩浆岩构造环境判别图解(a, 据Harris et al., 1986; b, 据Pearce et al., 1984) VAG-火山弧花岗岩；S-COLG-同碰撞花岗岩；WPG-板内花岗岩；ORG-洋脊花岗岩；A-ORG-异常洋脊花岗岩 Fig. 10 Discrimination diagrams for the tectonic settings of the Early Cretaceous magmatic rocks in the Gaoligong orogen (a, after Harris et al., 1986; b, after Pearce et al., 1984) VAG-volcanic arc granites; S-COLG-syn-collisional granites; WPG-within-plate granites; ORG-oceanic ridge granites; A-ORG-abnormal oceanic ridge granites |
高家寨和濮家寨两地流纹岩的稀土和微量元素特征几乎完全一致，其ΣREE变化于173×10-6~209×10-6，LREE/HREE在8.35×10-6~11.93之间，(La/Sm)N在3.92~5.80范围内，(Gd/Yb)N为1.47~1.75，δEu为0.49~0.60，展示轻稀土富集、分馏程度高、重稀土相对亏损、Eu中度亏损的特点。球粒陨石标准化稀土元素配分图表明，所有样品呈基本一致的向右倾斜、中间呈“V”字型的稀土配分模式(图 5a)。原始地幔标准化微量元素蛛网图(图 5b)呈现出向右倾斜的“M”型多峰谷模式，大离子亲石元素(LILE)Rb和K，放射性生热元素Th和U及亲岩浆元素Ce、La、Zr和Hf相对原始地幔强烈富集，高场强元素(HSFE)Nb、Ta、P、Ti及LILE中的Sr和Ba表现出明显的负异常。
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图 5 高黎贡西北缘早白垩世流纹岩球粒陨石标准化稀土元素配分模式(a)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. 5 Chondrite-normalized REE patterns (a) and primitive-mantle-normalized trace element spider diagrams (b) for the Early Cretaceous rhyolites in northwestern Gaoligong orogen (normalization values after Sun and McDonough, 1989) |
15QDBW-63和15QDBW-69样品锆石晶形特征非常相似，大部分呈自形的柱状，晶面整洁光滑，粒度在50×120μm~70×180μm之间，长宽比一般为2.5:1，少量为1.5:1。锆石阴极发光图像显示这2个样品中的大部分锆石都是韵律环带清晰的单一锆石，少量存在继承性锆石核和韵律环带清晰的新生锆石边(图 6a，b)。锆石的U、Th含量都较低，其中U含量大部分变化于111×10-6~571×10-6之间，Th含量为116×10-6~931×10-6，个别测试点仅为2×10-6(表 2)。Th/U比值大部分都在0.9以上，仅个别为0.5。Pb含量大部分在3×10-6~910×10-6之间，少量分布于11×10-6~20×10-6(表 2)。总体来看，2件样品的锆石阴极发光图像显示出十分清晰的生长韵律环带，Th/U比值都远大于0.1，表现出典型的岩浆锆石特点，但其Pb含量普遍偏低，反映其可能存在一定程度的Pb丢失。
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图 6 高黎贡西北缘早白垩世流纹岩中代表性锆石阴极发光图像 Fig. 6 Cathodoluminescence images of representing zircons from the Early Cretaceous rhyolites in northwestern Gaoligong orogen |
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表 2 高黎贡西北缘早白垩世流纹岩中锆石LA-ICP-MS U-Pb定年数据 Table 2 Zircon LA-ICP-MS U-Pb data of the Early Cretaceous rhyolites in northwestern Gaoligong orogen |
锆石LA-ICP-MS U-Pb定年结果表明，15QDBW-63样品锆石的206Pb/238U年龄数据中点13为142Ma和点20为523Ma，前者明显不同于其他锆石点年龄，故未参与加权平均年龄计算，后者测点位于锆石核部(图 6a)，反映其代表了继承性锆石年龄。除此以外，所有测点年龄都集中分布于120~130Ma。在谐和图上，只有3、6、7、16和19号测点位于协和线上，其他测点远离协和线，反映其存在明显的Pb丢失，未参与锆石206Pb/238U加权平均年龄计算，位于协和线上5个测点锆石206Pb/238U加权平均年龄为121±1.9 Ma(MSWD=0.6)(图 7a)，代表流纹岩中锆石结晶年龄。15QDBW-69样品锆石的206Pb/238U年龄除点20为继承性锆石年龄(452Ma)外，其他都集中分布于116~125 Ma区间(表 2)，在谐和图上，测点3、5、7、9、10、12、15分布于协和线上外，其他都远离协和线，可能与糜棱岩化变形变质引起Pb丢失或锆石均一化有关，故仅将集中于协和线上的7个测点锆石206Pb/238U进行了加权平均年龄计算，结果为122±2.2Ma(MSDW=1.2)(图 7b)，代表流纹岩中锆石结晶年龄。总体来看，计算加权平均年龄的测点值都选择锆石韵律环带清晰的边部，Th/U比值均大于0.9，反映锆石的结晶年龄，代表流纹岩喷出时代。
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图 7 高黎贡西北缘早白垩世流纹岩中锆石LA-ICP-MS U-Pb定年谐和图 图中红色曲线部分未参与加权平均年龄计算 Fig. 7 Zircon LA-ICP-MS U-Pb concordia diagrams from the Early Cretaceous rhyolites in northwestern Gaoligong orogen The red curves are excluded the weighted mean age calculation |
对2件样品(15QDBW-63和15QDBW-69)中与U-Pb定年测点相同点或结构相似的部位进行了锆石Lu-Hf同位素分析(表 3)。所有锆石的176Lu/177Hf均小于0.002，显示出176Hf的低放射成因。根据各自锆石U-Pb年龄值计算εHf(t)、tDM和tDMC值。
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表 3 高黎贡西北缘早白垩世流纹岩中锆石LA-ICP-MS Lu-Hf同位素数据 Table 3 Zircon LA-ICP-MS Lu-Hf isotopic data of the Early Cretaceous rhyolites in northwestern Gaoligong orogen |
样品15QDBW-63的锆石εHf(t)值除点7为-2.7±0.9，对应的地壳模式年龄为1193Ma外，其它测点的εHf(t)值变化于-9.6~-5.1之间，对应的地壳模型年龄(tDMC)为1026~1572Ma。样品15QDBW-69的锆石εHf(t)值除点5和15为-2.6±0.9和-4.1±0.7，对应的地壳模式年龄为1189Ma和1270Ma外，其它测点的εHf(t)值变化于-9.3~-5.7之间，对应的地壳模型年龄(tDMC)为1365~1562Ma。
6 讨论 6.1 岩浆来源高黎贡构造带内早白垩世侵入岩岩石组合为闪长岩、花岗闪长岩和花岗岩，锆石U-Pb年龄集中分布于118~134Ma(杨启军等，2006；Cong et al., 2011a, b；戚学祥等，2011；Xie et al., 2016；Qi et al. 2019)。其中，闪长岩的锆石εHf(t)值落在+1.2~+5.4的范围内，tDMC年龄为744~979Ma，为镁质、钙碱性，富含LILEs和LREEs，并具有强烈的Nb、Ta、P和Ti负异常以及高Mg#值，反映其岩浆主要来源于交代地幔楔部分熔融的产物(Qi et al. 2019)；花岗闪长岩的锆石εHf(t)值(-9.8~+2.9，总变化量约为13个εHf单位)和tDMC值(年龄为888~1564Ma)的变化范围大，在εHf(t)对U-Pb年龄图解中样品分布于CHUR线附近(图 8a)，以及花岗闪长岩中存在大量同时代暗色包体，钾长石(Kf)呈半自形-他形板状，并具有侵蚀边的斜长石(Pl)包裹体，角闪石存在辉石核，高TiO2、Na2O和MgO含量、高Mg#值和中-高钾钙碱性特征，揭示其岩浆是由幔源岩浆和古老壳源岩浆混合的产物(戚学祥等，2011；Qi et al. 2019)；花岗岩主要为以锆石εHf(t)值为-8.0~-1.1，平均值为-4.4，tDMC值为1101~1488 Ma，Ba和Sr或Rb和Y之间没有明显的相关性，具高钾钙碱性、镁质、准铝质特征，岩浆以壳源为主，混入少量幔源物质的I型花岗岩(Zhu et al., 2009b；戚学祥等，2011；Xie et al., 2016；Zhu et al., 2017；Qi et al. 2019)。同时存在少量以含石榴石、电气石、钙碱性、过铝质和Eu、Nb、Ta、Sr、P、Ti负异常、Mg#值低为特征，由纯壳源物质部分熔融形成的S型花岗岩(Xu et al. 2012)。
高家寨与濮家寨的流纹岩在斑晶组成和主量元素特征上虽然存在细小差别，但它们都分布在高黎贡构造带西北缘、相距不到10km，都属于高黎贡早白垩世岩浆岩带的一部分，形成的构造背景相同，其高钾钙碱性、准铝质-过铝质，尤其是稀土元素配分模式和微量元素蛛网图几乎完全一致，说明其岩浆来源相同。前人研究成果表明高钾钙碱性中、酸性岩浆岩的成因存在以下三种情况：1)幔源岩浆结晶分异作用(DePaolo, 1981; Be'eri-Shlevin et al., 2010; Weissman et al., 2013)；2)幔源岩浆侵位过程中诱发壳源物质部分融熔作用(Beard and Lofgren, 1989; Weissman et al., 2013)；3)壳源岩浆与少量幔源岩浆混合作用(DePaolo, 1981; Bergantz, 1989; Roberts and Clemens, 1993; Droop et al., 2003)。高黎贡构造带内至今未发现早白垩世基性岩，中性岩仅有少量出露(出露面积小于2%，Qi et al. 2019)，因此由幔源基性岩浆结晶分异形成大规模酸性岩浆岩的可能性可以排除。图 6显示研究区内流纹岩部分锆石存在核边结构，核部的年龄为早古生代，说明岩浆中有壳源物质存在。锆石εHf(t)值主要分布于-9.6~-2.6，总变化量存在7个εHf单位，在εHf(t)值对U-Pb年龄图解中呈线状分布于CHUR线下方，tDMC值介于1193~1572Ma之间(表 3、图 8)，流纹岩中存在黑云母斑晶、Mg#值高(43~58)，以及在SiO2-Mg#图解上样品落在纯壳源物质部分融熔区上方(图 9)，说明该区流纹岩岩浆除来源于壳源物质部分融熔的产物外，还有少量幔源组分的混入，即流纹岩岩浆为壳-幔混源岩浆。此外，流纹岩的主要矿物组成、高钾钙碱性、准铝质-过铝质、高Mg#值性质、稀土配分模式和微量元素蛛网图，以及锆石εHf(t)值和地壳模式年龄都与该区同时代、主要来源于壳、幔岩浆混合成因的花岗岩类(戚学祥等, 2011; Qi et al. 2019)一致(图 9)，而与龙陵-瑞丽一带(斑晶为钾长石、石英和少量白云母)、低钠、低εHf(t)值，完全由壳源物质部分融熔形成的流纹岩(另文发表)明显不同，进一步说明该区流纹岩是高黎贡早白垩世岩浆岩带的组成部分，是壳-幔物质混合的产物。
6.2 构造背景高黎贡构造带西北缘流纹岩带西以龙川江断裂带为界东与高黎贡群以断裂带相隔、或为早白垩世花岗岩侵位，在空间上与高黎贡早白垩世侵入岩融为一体，构成南至缅甸Mogok，北经藏东波密-察隅至拉萨地块北缘弧形展布的早白垩世岩浆岩带的一部分(图 1a)。带内中性、中酸性和酸性岩浆岩组合、镁质、中-高钾钙碱性、准铝质-过铝质及Nb、Ta负异常的地球化学特征，以及沿带断续分布的侏罗纪-早白垩世增生杂岩带说明该岩浆岩带的形成与班怒洋俯冲及拉萨-腾冲地块与南羌塘-保山地块拼合有关(刘本培等，2002；Zhu et al., 2009a；2017；戚学祥等，2011；Xu et al., 2012; 李再会等, 2012; 尹福光等，2012; Liao et al., 2015；Xie et al., 2016；Chen et al., 2017; Qi et al. 2019)。高黎贡西北缘早白垩世流纹岩与围岩多为断层接触，岩性单一(图 2)，但与其相对应的高黎贡东南缘早白垩世流纹岩带赋存于弧前/弧间海相沉积岩中，它们的岩石学、常量元素特征虽然存在一定差异，但其稀土配分模式、微量元素蛛网图、尤其是大离子亲石元素(Th、U、Ba、K和Rb)富集、高场强元素(Nb、Ta、P和Ti)和Eu负异常一致，以及在La/Yb-Th/Yb和Nb-Y构造环境判别图解上与同时代花岗岩一样，所有样品都落在大陆边缘弧或岛弧-同碰撞区(图 10)的特点，说明研究区内流纹岩形成的构造背景与侵入岩一致，都形成于大陆边缘弧环境。此外，如上节所述，研究区内流纹岩岩浆来源与同时代侵入岩一样具有壳幔混合的特征，其幔源组分来源于班怒洋壳俯冲诱发地幔楔物质部分熔融的基性岩浆(Zhu et al., 2009b, 2017；戚学祥等，2011；李再会等, 2012; 尹福光等，2012；Liao et al., 2015；Xie et al., 2016；Chen et al., 2017; Qi et al. 2019)，说明流纹岩的形成与洋壳俯冲有关。由此可见，高黎贡西北缘早白垩世流纹岩形成于活动大陆边缘，与班怒带南段怒江洋向腾冲地块下的俯冲活动有关。
7 结论高黎贡西北缘早白垩世流纹岩为镁质、弱过铝质-过铝质高钾钙碱性岩类，于121~122Ma喷发。岩石轻稀土富集、分馏程度高、重稀土相对亏损、分馏程度较低、Eu中度亏损及大离子亲石元素(LILE)Rb和K，放射性生热元素Th、U和亲岩浆元素Ce、La、Zr和Hf相对原始地幔强烈富集，高场强元素(HSFE)Nb、Ta、P、Ti负异常明显，以及εHf(t)值、tDMC值、稀土配分模式和微量元素蛛网图与该区同时代花岗岩几乎完全一致，说明其岩浆主要来源于壳源物质部分熔融的产物，并有少量幔源岩浆的混入，形成于怒江洋壳向腾冲地块俯冲过程中的活动大陆边缘环境。
致谢 本文在成文过程中与曾令森研究员和罗照华教授进行了有益的探讨; 孟繁聪研究员和蔡明海教授认真审阅了初稿并提出了中肯的修改意见；中国地质大学(武汉)地质过程与矿产资源国家重点实验室胡兆初教授协助完成锆石U-Pb定年和Hf同位素测试；在此一并表示衷心感谢！
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