Late Triassic Collision of Jinshajiang Suture Belt: Geochronological, Geochemical and Hf Isotope Evidences from Quartz Monzonite in Gonjo Area
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摘要: 金沙江结合带是三江特提斯构造域重要的结合带之一，是研究金沙江古特提斯洋陆俯冲-碰撞演化过程的重要窗口.然而，关于金沙江古特提斯碰撞闭合的准确时限争议颇多.选择位于藏东地区金沙江结合带西侧的贡觉花岗岩体为研究对象，对其中大规模出露的石英二长岩进行了年代学、地球化学和Hf同位素分析.LA-ICP-MS锆石U-Pb测年结果显示石英二长岩形成于231±1 Ma，代表了金沙江结合带晚三叠世岩浆活动事件.锆石Hf同位素分析获得石英二长岩锆石εHf（t）为-8.3~-5.5，二阶段模式年龄TDMC为1 611~1 788 Ma.岩石地球化学特征表明，石英二长岩为钾玄岩-高钾钙碱性系列的I型花岗岩，富集K、Th、Rb等大离子亲石元素，亏损Nb、Ta、Zr、Hf等高场强元素.此外，地球化学特征显示石英二长岩形成于碰撞环境，贡觉地区在晚三叠世早期（~231 Ma）处于碰撞挤压向后碰撞伸展环境的转换阶段，石英二长岩为下地壳中基性变质火成岩部分熔融的产物.结合前人研究，认为金沙江古特提斯洋是由南向北逐渐闭合的，区域地质背景的差异性和古特提斯洋斜向碰撞的复杂性是导致金沙江结合带不同地区碰撞闭合时限不一致的主要原因.Abstract: As one of the important suture belts in the Sanjiang Tethyan tectonic domain, the Jinshajiang suture belt has long been the focus of study on the process of Paleo-Tethys ocean-continent subduction and collision. However, controversies remain about the exact time for the collision and closure of the Jinshajiang Paleo-Tethys. In this paper, the Gonjo granite body was selected as the research object, which is located on the west side of the Jinshajiang suture belt in the eastern Tibet. Based on the chronological, geochemical and Hf isotope analyses of the large-scale exposed quartz monzonite, the geochemical characteristics and tectonic setting of the Gonjo granite body are revealed. LA-ICP-MS zircon U-Pb dating results yield a weighted mean 206Pb/238U age of 231±1 Ma for the sample, indicating that quartz monzonite was emplaced in Late Triassic. Zircon from the quartz monzonite exhibits a narrow range of initial Hf isotope ratios (εHf(t)=-8.3 to -5.5), with corresponding TDMC of 1 611-1 788 Ma.Geochemical studies show that the quartz monzonite is typical I-type granite, belonging to shoshonite and high K calc-alkali series, enriched in Rb, Th, K, and LILEs, but depleted in Nb, Ta, Zr, Hf and HFSEs. In addition, geochemical features show that the quartz monzonite was formed in the geological background of continent collision, suggesting that the Gonjo area was in a transition phase of syn-collision compressional to post-collisional extension stage in the Early Triassic (~231 Ma) and the quartz monzonite was a product of partial melting of lower crustal intermediate-basic igneous rocks. Combined with previous studies, it can be proposed that the Paleo-Tethys ocean of Jinshajiang suture was gradually closed from south to north, and the differences of regional geological background and the complexity of the Paleo-Tethys ocean oblique collision are the main reasons for the different collision-closure time of different regions of the Jinshajiang suture belt.
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Key words:
- Jinshajiang suture belt /
- quartz monzonite /
- zircon U-Pb dating /
- Hf isotope /
- Late Triassic /
- Paleo-Tethys
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三江特提斯位于全球特提斯构造域东段，经历了从特提斯大洋岩石圈向大陆岩石圈转换及其大陆边缘多岛弧-盆系的构造演化(邓军等, 2016; Wang et al., 2017)，完整地记录了冈瓦纳与潘吉亚大陆裂解-增生和欧亚大陆聚合，以及新生代印度-欧亚陆陆碰撞造山的地质历史(潘桂棠等, 2002; Zheng et al., 2013).伴随特提斯洋的俯冲-碰撞，在三江地区形成了甘孜-理塘、金沙江-哀牢山、龙木错-双湖-昌宁-孟连、班公湖-怒江等数条结合带及其间的中咱-中甸地块、昌都-思茅地块、保山-镇康地块等微陆块，表现出反“S”型扭转的大地构造格局(尹福光等, 2006; 王保弟等, 2018)（图 1a）.
图 1 三江地区大地构造单元划分简图（a）和贡觉地区金沙江结合带地质简图（b）图a据Zi et al. (2012)修改；图b据Tang et al. (2020)修改Fig. 1. Tectonic unit division of the Sanjiang areas (a) and sketch map of the Jinshajiang suture belt in the Gonjo area (b)长期以来，金沙江结合带作为古特提斯一条重要的结合带，其形成与演化备受关注.许多学者对金沙江结合带的地层、岩浆岩、蛇绿混杂岩和构造演化等方面进行了大量研究(Jian et al., 2009; Zi et al., 2012)，反映其经历了裂（陷）谷盆地（D）、洋盆扩张（C1-P1）、洋壳西向俯冲消减（P1-P2）和碰撞闭合（T1-T3？）等演化阶段(范蔚茗等, 2009; 王冬兵等, 2012; Zheng et al., 2013).然而，关于金沙江古特提斯碰撞闭合的准确时限争议颇多，如莫宣学和潘桂棠(2006)根据弧岩浆岩和碰撞型岩浆岩的时空分布认为，金沙江-哀牢山洋板块于早二叠世开始向西俯冲，大陆碰撞发生于早三叠世至晚三叠世（T1-T3）；高睿等(2010)通过对羊拉花岗闪长岩（~229 Ma）和贡卡花岗闪长岩（~232 Ma）的研究，提出金沙江弧-陆碰撞于晚二叠世末期开始、中三叠世早期结束；Metcalfe (2013)和Deng et al. (2014)综合了沉积古地理与古地磁、蛇绿岩套及俯冲型和碰撞型岩浆岩的地质与地球化学资料，认为金沙江洋开启于中泥盆世，自早二叠世开始向西俯冲于兰坪-思茅地块之下，最终闭合于早三叠世早期.
花岗岩类侵入体作为洋壳俯冲消减-碰撞闭合的客观记录，其对于研究相关结合带的构造-岩浆-成矿过程十分重要.前人对金沙江结合带岩浆活动的研究主要集中于四川巴塘、云南德钦地区及其以南一线(Hou et al., 2003; Zhu et al., 2011; Zhang et al., 2014)，对于巴塘地区以北，尤其出露于藏东地区与金沙江结合带密切相关的花岗岩体却鲜有报道，从而制约了对金沙江古特提斯洋碰撞闭合的全面认识.本文选择藏东贡觉地区金沙江结合带西侧大规模出露的花岗岩体（图 1b），开展锆石U-Pb年代学和Hf同位素、全岩地球化学研究，以限定该套花岗岩的岩浆活动时限，并探讨其构造环境、岩石成因及岩浆源区等，为深入认识金沙江古特提斯洋的构造演化提供新的证据.
1. 研究区地质背景
金沙江结合带作为三江特提斯东缘一条重要的古特提斯结合带，主要沿金沙江流域展布，西起青海玉树，经西藏贡觉、四川巴塘和云南德钦等地区向东南延伸，可与哀牢山结合带相接，宽60余千米，全长达千余千米.结合带东部以金沙江断裂带为界，与裂离于扬子大陆西缘的中咱微陆块相接；结合带西部增生于昌都微陆块东缘(王立全等, 1999; 许志琴等, 2013).该带主要为半深海-深海相泥灰岩、硅质条带灰岩、砂泥岩、放射虫硅质岩、洋脊型基性火山岩、凝灰岩等组成的火山-复理石沉积建造(王保弟等, 2018).
研究区行政区划上隶属西藏自治区昌都市贡觉县，地理上位于青藏高原东缘的金沙江西岸，大地构造位置处于中咱地块与昌都-思茅地块之间的金沙江结合带北（中）段（图 1b）.该段的金沙江结合带内主要由一系列规模不等、岩性不一和变形程度不同的构造岩块及强变形的基质组成，各岩块之间由诸多近平行于结合带走向的韧性-脆韧性断层呈网结状连接，表现出局部有序、整体无序的构造混杂岩特征（图 1b）.构造岩块按岩性组合可分为超基性岩-基性岩、大理岩、大理岩+变质基性岩、变质硅-灰-泥质岩及（退变）榴辉岩等.金沙江结合带内的基质主要为含石榴斜长二云片岩、斜长黑云片岩、含红柱黑云斜长片岩、含石榴二云石英片岩及少量黑云斜长片麻岩等(Tang et al., 2020).
本文的研究对象贡觉花岗岩体主体位于金沙江结合带西侧，出露面积较大，约200 km2，由岩基、岩株和一系列小岩枝组成，呈NNW-SSE向展布，自北侧的克日乡冲录村向南至罗麦乡及沙东乡西侧连续分布；此外还有少部分岩体以NW-SE向出露于金沙江结合带内部，自克日乡登巴村向SE一直延伸至金沙江东岸.
2. 岩石学特征
详细的野外地质调查揭示贡觉花岗岩体的岩性包括石英二长岩、二长花岗岩和少量正长花岗岩等，其中以大规模出露的石英二长岩为主，其余岩体出露分散、面积较小（图 1b）.该套花岗岩体东侧为金沙江蛇绿混杂岩（主要由榴辉岩岩块、蚀变超基性岩块、大理岩岩块、云母石英片岩和变质火山岩等组成），西侧主要为前寒武系宁多岩群变质岩系（主要包括云母片岩、斜长角闪岩等）（图 2），宁多岩群以西主要发育三叠纪洞卡组火山岩及其上的甲丕拉组、波里拉组、阿堵拉组和夺盖拉组碎屑岩/碳酸盐岩等，与花岗岩体无直接接触.石英二长岩与两侧围岩均为侵入接触关系，接触面多呈外倾的凹凸不平的波状曲面或弧形曲面（图 3b）.岩体内原生构造不发育，以次生节理为主.
图 2 金沙江结合带西侧石英二长岩地质剖面图剖面位置见图 1bFig. 2. Geologic profile of quartz monzonite on the west side of Jinshajiang suture belt
图 3 贡觉石英二长岩体野外宏观及显微镜下特征a.石英二长岩中发育暗色包体；b.石英二长岩侵入于条带状大理岩中；c、d.石英二长岩镜下矿物组合特征. Am.角闪石; Pl.斜长石; Mc.微斜长石; Qz.石英Fig. 3. Field outcrops and microscopic characteristics of quartz monzonite石英二长岩总体风化程度较弱，新鲜面为灰-灰白色，块状构造，中细粒花岗结构，矿物粒径为2~5 mm.矿物组成为石英（10%~15%）、斜长石（35%~40%）、钾长石（30%~35%）、普通角闪石（8%~15%）和黑云母（3%~5%）等（图 3c）.斜长石为灰白色，呈半自形板状，发育聚片双晶、卡钠复合双晶，发育环带结构；钾长石为浅灰色，呈他形粒状、板状，具卡氏双晶和格子双晶，属微斜长石（图 3d）；石英呈灰白色他形粒状，黑云母为黑褐色鳞片状，角闪石为黑褐色短柱状.
3. 分析测试方法
本次选取贡觉花岗岩体的主要岩相石英二长岩为研究对象.样品采自克日乡至罗麦乡一带公路边，采样点的坐标为98°47′16″~98°47′58″E、30°59′19″~30°56′22″N.锆石分选在河北区域地质矿产调查研究所实验室完成，原岩样品经人工粉碎、人工淘洗后去除轻矿物部分，将得到的重砂部分经电磁选后得到含有少量杂质的锆石样品，最后在双目镜下挑选出锆石晶体.选择晶型较好、无裂隙的锆石颗粒粘贴在环氧树脂表面制成锆石样品靶，打磨样品靶，使锆石的中心部位暴露出来，然后进行抛光.对锆石进行反射光、透射光显微照相和阴极发光（CL）图像分析，图像拍摄在武汉上谱分析科技有限责任公司完成.仪器为高真空扫描电子显微镜（JSM-IT100），配备有GATAN MINICL系统.工作电场电压为10.0~13.0 kV，钨灯丝电流为80~85 µA.最后根据反射光、透射光及锆石CL图像选择代表性的锆石颗粒和区域进行U-Pb测年.
锆石U-Pb同位素定年和微量元素含量在武汉上谱分析科技有限责任公司利用LA-ICP-MS同时分析完成.详细的仪器参数和分析流程见Zong et al.(2017).GeoLasPro激光剥蚀系统由COMPexPro 102ArF193nm准分子激光器和MicroLas光学系统组成，ICP-MS型号为Agilent 7700e.激光剥蚀过程中采用氦气作载气、氩气为补偿气以调节灵敏度，二者在进入ICP之前通过一个T型接头混合，激光剥蚀系统配置有信号平滑装置(Hu et al., 2015).本次分析的激光束斑直径和频率分别为32 µm和5 Hz.U-Pb同位素定年和微量元素含量处理中采用锆石标准91500和玻璃标准物质NIST610作外标分别进行同位素和微量元素分馏校正.每个时间分辨分析数据包括20~30 s空白信号和50 s样品信号.对分析数据的离线处理（包括对样品和空白信号的选择、仪器灵敏度漂移校正、元素含量及U-Pb同位素比值和年龄计算）采用软件ICPMSDataCal (Liu et al., 2008, 2010)完成.锆石样品的U-Pb年龄谐和图绘制和年龄加权平均计算采用Isoplot/Ex_ver3(Ludwig, 2003)完成.分析结果见附表1.
锆石Lu-Hf同位素测试在中国地质科学院国家地质实验测试中心完成，采用美国ASI (Applied Spectra Inc.)J-100飞秒激光和Neptune MC-ICP-MS(Thermo Finnigan)联用技术，采用5 Hz的激光频率，剥蚀线长度为40 μm，能量密度为2.0 J/cm2，具体分析参数见Zhou et al.(2018).由于锆石中176Lu/177Hf值非常低（通常小于0.002），176Lu对176Hf同质异位素干扰可以忽略，根据Yb自然丰度以及测试得到的173Yb/172Yb值以扣除176Yb对176Hf的干扰.εHf计算采用176Lu衰变常数1.865×10-11 a-1 (Scherer et al., 2001)，球粒陨石现今值176Hf/177Hf=0.282 772和176Lu/177Hf=0. 033 2 (Blichert-Toft and Albarède, 1997)；单阶段Hf模式年龄(TDM)计算采用现今亏损地幔值176Hf/177Hf=0.283 250和176Lu/177Hf=0.038 4 (Griffin et al., 2000），平均大陆地壳的176Lu/177Hf值采用0. 015 0 (Griffin et al., 2002).Hf同位素分析数据与计算结果列于附表2.
全岩主量元素测试在自然资源部西南矿产资源监督检测中心完成，采用XRF(Rigaku RIX 2100型)玻璃熔饼法完成，分析精度优于4%；全岩微量元素含量在武汉上谱分析科技有限责任公司利用Agilent 7700e ICP-MS分析完成.用于ICP-MS分析的样品处理如下：（1）将200目样品置于105 ℃烘箱中烘干12 h；（2）准确称取粉末样品50 mg置于Teflon溶样弹中；（3）先后依次缓慢加入1 mL高纯HNO3和1 mL高纯HF；（4）将Teflon溶样弹放入钢套，拧紧后置于190 ℃烘箱中加热24 h以上；（5）待溶样弹冷却，开盖后置于140 ℃电热板上蒸干，然后加入1 mL HNO3并再次蒸干；（6）加入1 mL高纯HNO3、1 mL MQ水和1mL内标In，再次将Teflon溶样弹放入钢套，拧紧后置于190 ℃烘箱中加热12 h以上；（7）将溶液转入聚乙烯塑料瓶中，并用2% HNO3稀释至100 g以备ICP-MS测试.岩石地球化学数据列于附表3.
4. 实验数据及分析结果
4.1 CL图像特征及锆石年代学
贡觉石英二长岩样品（D0004）的锆石颗粒自形程度良好，多呈短柱状，长150~300 μm，长宽比为2:1~3:1，阴极发光显示出良好的振荡环带或宽缓的生长环带，具有岩浆锆石形态（图 4）.锆石中U含量的变化范围为473×10-6~1 284×10-6，Th含量的变化范围为179×10-6~924×10-6，相应测点的Th/U值变化于0.41~0.92之间（表1，均大于0.4)，与典型的岩浆锆石特征一致(Hermann et al., 2001).样品的24个分析点均位于U-Pb谐和线上及其附近，206Pb/238U年龄分布于228~232 Ma，加权平均年龄为231±1 Ma（MSWD=0.2，n=24）（图 5），代表了贡觉石英二长岩的侵位年龄.
图 4 石英二长岩样品(D0004)锆石CL图像Fig. 4. CL images of zircons from quartz monzonite sample (D0004)
图 5 石英二长岩样品(D0004)锆石U-Pb谐和图Fig. 5. U-Pb concordia diagram of zircons from quartz monzonite sample (D0004)4.2 Hf同位素特征
在锆石U-Pb定年的基础上，本文利用MC-ICP-MS方法对样品D0004中15颗锆石进行了Lu-Hf同位素线剥蚀分析（图 4）.15颗锆石的176Yb/177Hf和176Lu/177Hf范围分别为0.022 235~0.046 529和0.000 671~0.001 335.176Lu/177Hf值非常接近或小于0.002，表明锆石在形成后没有明显的放射成因Hf的增加，测定的176Hf/177Hf值能够代表源区的Hf同位素组成(吴福元等, 2007a).176Hf/177Hf值为0.282 399~0.282 479，以各单颗粒锆石结晶年龄计算出锆石(176Hf/177Hf)i值为0.282 396~0.282 474，对应的εHf(t)为-8.3~ -5.5，变化范围较小；计算得到锆石二阶段Hf模式年龄TDMC为1 611~1 788 Ma，平均为1 695 Ma.
4.3 岩石地球化学特征
4.3.1 主量元素特征
贡觉石英二长岩样品的SiO2含量为62.98%~65.34%（附表3），TiO2含量为0.46%~0.60%，MgO含量为1.36%~1.86%，Al2O3含量为15.41%~16.44%，碱含量K2O+Na2O为6.46%~8.47%.在SiO2-(Na2O+K2O)岩石分类图中（图 6），样品多数落入石英二长岩区域，少数落入靠近石英二长岩的花岗闪长岩区域（实际对应石英二长岩更符合野外及薄片鉴定特征）.在SiO2-K2O图上，4个样品落入钾玄岩系列区域，2个样品落入高钾钙碱性区域（图 7a）.在A/NK-A/CNK关系图中，A/CNK值均 < 1，表现为准铝质特征（图 7b）.
图 6 贡觉地区花岗岩体SiO2-(Na2O+K2O)分类图解1.橄榄辉长岩；2a.碱性辉长岩；2b.亚碱性辉长岩；3.辉长闪长岩；4.闪长岩；5.花岗闪长岩；6.花岗岩；7.硅英岩；8.二长辉长岩；9.二长闪长岩；10.二长岩；11.石英二长岩；12.正长岩；13.副长石辉长岩；14.副长石二长闪长岩；15.副长石二长正长岩；16.副正长岩；17.副长深成岩；18.霓方钠岩/磷霞岩/粗白榴岩Fig. 6. SiO2-(Na2O+K2O) diagram for the Gonjo granites
图 7 贡觉石英二长岩SiO2-K2O图解(a)和A/CNK-A/NK图解（b）Fig. 7. Diagrams of SiO2-K2O (a) and A/CNK-A/NK (b) for quartz monzonite4.3.2 稀土和微量元素地球化学特征
石英二长岩样品的稀土元素总量∑REE为170.25×10-6 ~291.65×10-6，变化范围较小（附表3）.LREE/HREE为10.17~15.06，属轻稀土富集型.(La/Yb)N=11.98~20.84，轻稀土分馏明显，重稀土的Ho至Lu有逐渐变平趋势，(Ho/Lu)N=1.02~1.10.Eu/Eu*为0.65~0.71，显示较弱的负异常，表明岩浆源区有不同数量的斜长石残留.图 8a显示，不同样品的稀土元素配分曲线近乎一致，表现为典型的右倾型，富集轻稀土，Eu具有一定的亏损.
图 8 石英二长岩稀土元素配分模式（a）和微量元素蛛网图（b）球粒陨石、原始地幔数据引自Sun and McDonough（1989）Fig. 8. Chondrite-normalized REE pattern (a) and primitive mantle-normalized trace element spider diagram (b) for quartz monzonite样品的Ni含量为5.65×10-6~8.29×10-6，Cr含量为8.22×10-6~12.1×10-6.在原始地幔标准化微量元素蛛网图中（图 8b），6个样品的曲线相似，均富集Rb、Th、K等大离子亲石元素，亏损Nb、Ta、Zr、Hf等高场强元素，明显亏损Ti、P等元素.
5. 讨论
5.1 岩石成因
镜下薄片鉴定显示，贡觉石英二长岩发育黑云母和角闪石，未见白云母，在岩相学上表现出I型花岗岩的特征.全岩主量元素分析显示，石英二长岩具有高SiO2（62.98%~65.34%）、高Al2O3（15.41%~16.44%）的特征，轻重稀土分馏明显，富集K、Th、U、Rb等大离子亲石元素，亏损Nb、Ta、Zr、Hf等高场强元素.实验研究表明，在准铝质到弱过铝质的I型花岗岩浆中，磷灰石的溶解度很低，磷灰石容易达到过饱和而结晶分离；而在强过铝质S型花岗岩浆中，磷灰石溶解度变化趋势与此相反，常具有高的P2O5含量(Wolf and London, 1994; 王保弟等, 2011a).该特点常被用于I型和S型花岗岩的区分(Chappell, 1999).样品为准铝质岩石（A/CNK < 1），在ACF图解上（图 9）落入I型花岗岩区域，同时样品具有低的P2O5含量（均小于0.26%），上述特征均反映出贡觉石英二长岩为典型的I型花岗岩.
图 9 石英二长岩ACF图解据Nakada and Takahashi (1979). A=molar Al2O3+Fe2O3-(Na2O+K2O), C=molar CaO, F=molar FeO+MgO+MnO；A+C+F=100Fig. 9. ACF classification diagram for the quartz monzonite前人研究认为，I型花岗岩主要为壳内中基性变质火成岩部分熔融的产物(Chappell and Stephens, 1988; Kemp et al., 2007; Collins and Richards, 2008; 朱弟成等, 2009).图 8a显示石英二长岩具有平坦的HREE分配模式，并且Y/Yb值（8.56~9.29）接近10，表明角闪石为主要的残留相矿物，岩浆演化过程中斜长石的分离结晶造成了负铕异常的出现，部分熔融源区可能来源于深部.在岩石成因判别图解中（图 10），样品主要落入变安山岩熔体-角闪岩熔体区域，表明石英二长岩为下地壳中基性变质火成岩部分熔融的产物(Stern and Kilian, 1996; Altherr and Siebel, 2002; Kaygusuz et al., 2008).
图 10 贡觉花岗岩体的成因判别图解Fig. 10. Discrimination diagrams of petrogenesis for the Gonjo granites5.2 岩浆源区
锆石Hf同位素组成很少受到后期地质事件的影响，是探讨岩浆源区属性的重要工具(Griffin et al., 2002).锆石U-Pb测年和Hf同位素分析表明，贡觉石英二长岩中无继承或捕获锆石，Hf同位素具有均一、负的εHf(t)值（-8.3 ~ -5.5）.无继承或捕获锆石以及均一的Hf同位素组成表明锆石结晶于均一、无混染的岩浆源区.Hf同位素研究表明，εHf(t) < 0的岩石为古老地壳部分熔融形成(Vervoort et al., 2000).根据石英二长岩εHf(t) < 0，并且在εHf(t)-t图解上（图 11）样品点均分布在亏损地幔线及球粒陨石演化线之下，笔者推断其为古老地壳部分熔融的产物，TDMC为1.61~1.78 Ga的物源区应为主要物源区.
图 11 贡觉石英二长岩εHf(t)-t关系Fig. 11. εHf(t)-t plot for zircons from quartz monzonite in Gonjo5.3 构造环境
藏东贡觉地区发育的花岗岩体主要位于金沙江结合带西侧，呈近SN向的带状和长条状展布，其岩浆活动在时空分布上明显受控于金沙江古特提斯洋向西俯冲-碰撞制约.侵入于金沙江蛇绿混杂岩中的花岗岩主要为石英二长岩，发育角闪石和黑云母，铝饱和指数不高，A/CNK < 1，符合碰撞环境形成的花岗岩特征(Nironen et al., 2000; 莫宣学等, 2001; 张旗等, 2008; 张万平等, 2011).在Rb-(Y+Nb)和Ta-Yb构造环境判别图解中（图 12），样品落入同碰撞-(后碰撞)区域，进一步证明了石英二长岩形成于碰撞环境，显示为大陆碰撞的地球动力学背景，属金沙江古特提斯碰撞造山作用的产物.
图 12 石英二长岩构造环境判别图解据Pearce et al. (1984). Syn-CLOG.同碰撞花岗岩；Post-CLOG.后碰撞花岗岩；VAG.火山弧花岗岩；WPG.板内花岗岩；ORG.洋中脊花岗岩Fig. 12. Discrimination diagrams of tectonic setting for the quartz monzonite前人研究认为高钾钙碱性花岗质岩石可出现在各种构造环境中，往往指示一种构造体制的转换而不是特定的构造环境，比如主碰撞期后的张弛阶段(Barbarin, 1999)，而后碰撞岩浆活动多以高钾钙碱性的S型花岗质岩浆侵位为特点(侯增谦等, 2001).贡觉石英二长岩4个样品落入钾玄岩系列区域，2个落入高钾钙碱性系列区域（图 7a），A/CNK < 1为准铝质特征，明显区别于高钾钙碱性系列的过铝质S型花岗岩（图 7b）.然而，挤压背景下形成的花岗岩非常有限(吴福元等, 2007b)，由挤压作用向伸展作用转换的背景则有利于壳幔相互作用以及岩浆活动(邓军等, 2013; 李龚健等, 2013)，野外地质调查也显示贡觉花岗岩体原生构造不发育；表明岩浆上侵过程中遭受的挤压作用较小，暗示其很可能形成于伸展环境.
结合石英二长岩地球化学特征、锆石U-Pb测年和Hf同位素分析，本文认为晚三叠世藏东贡觉地区为大陆碰撞的地球动力学背景，处于由碰撞挤压作用向后碰撞伸展作用转换的阶段.金沙江古特提斯洋向西俯冲导致地壳不断增厚，增厚到一定程度后开始伸展减压，从而诱发下地壳变质中基性火成岩发生部分熔融，由此生成的大规模I型花岗岩浆逐渐上升，最终侵位于贡觉地区金沙江结合带西侧形成石英二长岩岩体.
5.4 对金沙江结合带碰撞作用的指示
前人基于古生物地层学、同位素年代学、岩石地球化学等研究对金沙江-哀牢山古特提斯演化过程进行了限定，即裂(陷)谷盆地(D)、洋盆扩张(C1-P1)、洋壳俯冲消减(P1-P2)和弧-陆碰撞造山(T1-T3？)四个主要演化阶段(王立全等, 1999; 孙晓猛和简平, 2004; Jian et al., 2009; 贺娟等，2018).其中关于弧-陆碰撞造山的精确时代争议最大，相关认识主要基于金沙江结合带南段的江达-德钦-维西火山岩浆弧年代学和岩石学等方面的研究.高睿等(2010)对云南德钦羊拉地区花岗闪长岩进行了研究，获得232~230 Ma的岩浆侵位年龄，认为其产于后碰撞环境并具有类似O型埃达克岩的特征，从而提出金沙江结合带弧-陆碰撞阶段可能从晚二叠世末期开始、中三叠世早期结束.王保弟等(2011b)报道了德钦人支雪山组双峰式火山岩套中流纹岩LA-ICP-MS锆石U-Pb年龄为249~247 Ma，认为金沙江结合带在早三叠世已经进入了后碰撞伸展时期；王新雨等(2017)对四川巴塘地区金沙江结合带内的花岗闪长岩开展的研究表明其形成于235~230 Ma，属后碰撞构造环境形成的I型花岗岩，同时具有一定的S型花岗岩特征，反映金沙江洋在晚三叠世之前完成了闭合；陈文等(2005)在金沙江结合带北端的青海玉树地区哈秀岛弧型石英闪长岩体中获得了216.4 Ma角闪石Ar-Ar年龄，认为金沙江结合带北段玉树地区在晚三叠世仍处于洋壳俯冲阶段，碰撞作用还未开始.这些结果反映出金沙江结合带在其不同区段的碰撞闭合时限以及花岗岩性质等方面具有较大的差异.
在板块构造中，S型花岗岩侵入体，特别是花岗岩基（granitic batholith）的出现，通常是大洋关闭和陆-陆碰撞使地壳加厚从而部分熔融形成花岗质岩浆上侵聚集和冷凝的结果(Pearce et al., 1984; 侯增谦等, 2001).在三江地区，临沧花岗岩和鲁甸花岗岩等岩基均是大洋关闭和陆-陆碰撞的结果(莫宣学等, 1993).本文在结合带西侧获得的贡觉石英二长岩的时代为231 Ma，属金沙江古特提斯碰撞造山背景下构造-岩浆响应的产物，结合其具有典型的I型花岗岩地球化学特征认为此时研究区处于由弧-陆碰撞作用向后碰撞伸展转换的时期，尚未进入大规模后碰撞阶段，这也与王新雨等(2017)对巴塘地区晚三叠世（235~230 Ma）花岗闪长岩形成的构造背景等认识一致.
最近，Tang et al. (2020)在贡觉地区金沙江蛇绿构造混杂岩内新发现榴辉岩透镜体，基于岩石地球化学特征和变质温压计的估算认为该榴辉岩是金沙江古特斯洋壳物质俯冲到岩石圈深部经高压/超高压变质作用形成，并获得了较为精确的榴辉岩相变质作用时代（245~240 Ma），据此认为金沙江古特提斯洋-陆俯冲作用在中三叠世（T2）已经结束.金沙江古特提斯洋向西俯冲及持续碰撞导致地壳不断增厚，在晚三叠世~231 Ma，加厚下地壳中基性变质火成岩部分熔融上侵形成了大规模的贡觉石英二长岩体，随后该地区进入后碰撞伸展时期，直至区域上晚三叠世磨拉石建造（甲丕拉组）不整合覆盖于结合带内不同层位的岩石聚合体之上(汪啸风等, 1999)，标志着藏东贡觉地区金沙江古特提斯构造演化的结束.
古大洋的关闭多具有“剪刀式”的斜向碰撞(Hennig et al., 2009)，在我国秦岭、广西等地存在这样的斜向碰撞(张复新等, 2004; 汪劲草等, 2017)，研究资料也显示金沙江结合带具有由南向北斜向闭合的特征(江元生等, 2002; 葛孟春等, 2002).杨天南等(2019)也提出古特提斯主洋盆的闭合具有穿时性，并进行了较系统的阐述和总结.曾普胜等(2018)系统测得羊拉地区南部的加仁岩体中通吉格花岗闪长岩，中部的路农、里农、江边花岗闪长岩，北部的贝吾花岗闪长岩的侵位年龄分别为246.1±3.5 Ma、238.1±5.3 Ma、239.0±5.7 Ma、227.9±5.1 Ma和213.6±6.9 Ma，从而认为金沙江古特提斯洋是由南向北逐渐闭合的，持续的时限可达33 Ma，且上述岩浆作用由南向北的穿时性侵位标志着金沙江古特提斯洋以斜向俯冲-碰撞的方式闭合.因此，结合区域研究资料，本文认为区域地质背景的差异性和古特提斯斜向碰撞的复杂性，可能是导致金沙江结合带不同地区碰撞闭合时限差异性的主要原因.
6. 结论
（1）锆石U-Pb同位素定年结果表明藏东地区贡觉石英二长岩的侵位年龄为231±1.0 Ma，代表了金沙江结合带晚三叠世印支期岩浆活动事件.
（2）贡觉石英二长岩为钾玄岩-高钾钙碱性系列的I型花岗岩，富集K、Th、U、Rb等大离子亲石元素，亏损Nb、Ta、Zr、Hf等高场强元素.Hf同位素特征表明其为壳源成因，为加厚下地壳中基性变质火成岩部分熔融的产物.
（3）藏东贡觉地区金沙江结合带在晚三叠世（~231 Ma）处于由碰撞挤压向后碰撞伸展转换的构造环境.结合前人研究，本文认为金沙江古特提斯洋是由南向北逐渐闭合的，区域地质背景的差异性和古特提斯洋斜向碰撞的复杂性是导致金沙江结合带不同地区碰撞闭合时限不一致的主要原因.
附表见本刊官网（http://www.earth-science.net）.
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图 1 三江地区大地构造单元划分简图（a）和贡觉地区金沙江结合带地质简图（b）
图a据Zi et al. (2012)修改；图b据Tang et al. (2020)修改
Fig. 1. Tectonic unit division of the Sanjiang areas (a) and sketch map of the Jinshajiang suture belt in the Gonjo area (b)
图 2 金沙江结合带西侧石英二长岩地质剖面图
剖面位置见图 1b
Fig. 2. Geologic profile of quartz monzonite on the west side of Jinshajiang suture belt
图 3 贡觉石英二长岩体野外宏观及显微镜下特征
a.石英二长岩中发育暗色包体；b.石英二长岩侵入于条带状大理岩中；c、d.石英二长岩镜下矿物组合特征. Am.角闪石; Pl.斜长石; Mc.微斜长石; Qz.石英
Fig. 3. Field outcrops and microscopic characteristics of quartz monzonite
图 4 石英二长岩样品(D0004)锆石CL图像
Fig. 4. CL images of zircons from quartz monzonite sample (D0004)
图 5 石英二长岩样品(D0004)锆石U-Pb谐和图
Fig. 5. U-Pb concordia diagram of zircons from quartz monzonite sample (D0004)
图 6 贡觉地区花岗岩体SiO2-(Na2O+K2O)分类图解
1.橄榄辉长岩；2a.碱性辉长岩；2b.亚碱性辉长岩；3.辉长闪长岩；4.闪长岩；5.花岗闪长岩；6.花岗岩；7.硅英岩；8.二长辉长岩；9.二长闪长岩；10.二长岩；11.石英二长岩；12.正长岩；13.副长石辉长岩；14.副长石二长闪长岩；15.副长石二长正长岩；16.副正长岩；17.副长深成岩；18.霓方钠岩/磷霞岩/粗白榴岩
Fig. 6. SiO2-(Na2O+K2O) diagram for the Gonjo granites
图 7 贡觉石英二长岩SiO2-K2O图解(a)和A/CNK-A/NK图解（b）
图a据Peccerillo and Taylor（1976）；图b据Rickwood（1989）
Fig. 7. Diagrams of SiO2-K2O (a) and A/CNK-A/NK (b) for quartz monzonite
图 8 石英二长岩稀土元素配分模式（a）和微量元素蛛网图（b）
球粒陨石、原始地幔数据引自Sun and McDonough（1989）
Fig. 8. Chondrite-normalized REE pattern (a) and primitive mantle-normalized trace element spider diagram (b) for quartz monzonite
图 9 石英二长岩ACF图解
据Nakada and Takahashi (1979). A=molar Al2O3+Fe2O3-(Na2O+K2O), C=molar CaO, F=molar FeO+MgO+MnO；A+C+F=100
Fig. 9. ACF classification diagram for the quartz monzonite
图 10 贡觉花岗岩体的成因判别图解
图a据Stern and Kilian（1996）；图b~d据Kaygusuz et al.（2008）和Altherr and Siebel（2002）
Fig. 10. Discrimination diagrams of petrogenesis for the Gonjo granites
图 11 贡觉石英二长岩εHf(t)-t关系
Fig. 11. εHf(t)-t plot for zircons from quartz monzonite in Gonjo
图 12 石英二长岩构造环境判别图解
据Pearce et al. (1984). Syn-CLOG.同碰撞花岗岩；Post-CLOG.后碰撞花岗岩；VAG.火山弧花岗岩；WPG.板内花岗岩；ORG.洋中脊花岗岩
Fig. 12. Discrimination diagrams of tectonic setting for the quartz monzonite
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