哀牢山深变质带内两类晚渐新世花岗岩成因及其构造指示

郭小飞，王岳军，刘汇川 ，张玉芝

(1.中国科学院广州地球化学研究所同位素地球化学国家重点实验室, 广东 广州 510640;2.中山大学地球科学与工程学院, 广东 广州 510275;3.中国科学院大学, 北京 100049;4.广东省地球动力作用与地质灾害重点实验室, 广东 广州 510275 )

哀牢山深变质带内两类晚渐新世花岗岩成因及其构造指示

郭小飞1, 2, 3，王岳军2, 4，刘汇川2, 4，张玉芝2, 4

(1.中国科学院广州地球化学研究所同位素地球化学国家重点实验室, 广东 广州 510640;2.中山大学地球科学与工程学院, 广东 广州 510275;3.中国科学院大学, 北京 100049;4.广东省地球动力作用与地质灾害重点实验室, 广东 广州 510275 )

红河-哀牢山构造带发育了广泛的、与印度欧亚板块碰撞造山作用紧密相关的新生代构造-岩浆作用，出露于云南哀牢山变质带中段的两个花岗岩样品锆石U-Pb定年结果分别为26.2±0.5 Ma和26.8±0.5 Ma，说明哀牢山变质带并不全是扬子板块结晶基底的下元古界，也存在新生代岩浆岩。花岗岩锆石εHf(t)分别为+0.3～+6.9和-9.8～-0.7，平均为+3.4和-4.3，其二阶段模式年龄TDM2分别为526～853 Ma和903～1 355 Ma。低εHf(t)花岗岩源岩可能是元古宙哀牢山群变质杂砂岩，高εHf(t)花岗岩源自软流圈地幔热源加热诱发的新生下地壳部分熔融。结合前人研究成果，认为晚渐新世红河剪切带剪切作用切穿了岩石圈地幔；渐新世印支地块两侧的高黎贡山和哀牢山走滑剪切系统肢解了东南亚不同的块体，为印支地块向东南方向的挤出创造了条件。

哀牢山构造带; 花岗质岩石; 锆石年龄; Hf同位素; 走滑剪切; 挤出效应

我国滇西三江造山带属于东特提斯洋构造域的重要组成部分，经历了复杂的原、古、新三期特提斯洋演化过程[1-6]，是由一系列从冈瓦纳大陆北缘分离出来的外来陆块组成的多岛洋俯冲-碰撞造山带[7-11]。该构造带在喜山期印度板块与欧亚板块俯冲碰撞作用下于始新世-中新世发育了多条大规模走滑剪切带[12-14]，并随着新生代以来陆内造山带的构造应力调整，形成了滇西地区复杂的地质构造现象。

哀牢山构造带位于三江造山带东南部，带内广泛发育了新元古代-晚古生代-中生代和新生代构造-岩浆作用[15-20]。红河剪切带新生代剪切作用强烈改造了哀牢山地区的构造-岩浆特征，形成了哀牢山深变质岩带(即哀牢山群)。哀牢山群一直被认为形成于元古代，是扬子地台结晶基底的一部分，但是Liu et al.[19]在哀牢山群内识别出了晚三叠世花岗岩，进而提出哀牢山群可能是包含多期次岩浆岩和变质岩的混杂岩带，那么哀牢山群是否也包含新生代红河剪切作用诱发的岩浆岩呢？此外，当前关于红河剪切带研究另一个重要争论在于该剪切作用是否切穿了岩石圈地幔，因为该问题不仅关乎沿剪切带发育的三江地区富碱斑岩Cu-Mo-Au多金属矿产的成因，还对印支地块东南向的挤出作用和南海的打开具有重要的指示意义。前人对哀牢山红河剪切带开展了运动学和构造变形的分析，也对剪切带伴生的变质岩开展了变质、变形时代的研究[21-23]。但是对其中新生代岩浆岩研究尚存争论，而新生代岩浆岩，尤其是记录下地壳甚至深部地幔活动的岩浆岩报道较少，本文对云南省红河州甲寅地区出露的两类花岗质岩石开展了LA-ICP-MS锆石U-Pb年代学和Lu-Hf同位素的分析，以期为红河左旋走滑剪切作用在渐新世是否切穿岩石圈地幔，以及印支地块向东南方向的挤出过程提供资料。

1 地质背景和样品特征

哀牢山-红河变质带夹持于东西两侧的红河断裂和哀牢山断裂之间，是分隔印支地块和扬子地块的一条重要地质界线(图1)[26-27]。变质带主体呈北西-南东向延伸，宽度在20～30 km，长度超过500 km。变质带内遭受变质变形作用的深变质岩系从北西到南东依次包括点苍山群、哀牢山群、瑶山群和越北的大象山群，主要由斜长角闪岩、片麻岩、大理岩等及后期侵位的岩浆岩组成[7]。前人研究中普遍把哀牢山群归属于下元古界，认为其属于扬子基底的一部分或者与扬子基底具有相似性[17, 28-30]，然而随着锆石U-Pb定年等高精度同位素年代学技术的发展和应用，许多原来认为是元古代的花岗岩侵入体，如今被证明属中生代或新生代的变质-变形花岗岩[16, 19, 31-33]。研究区哀牢山变质带主体由哀牢山群和部分瑶山群变质岩系组成，为一套混合岩化强烈的中深变质岩，沿哀牢山山脉呈NW-SE向狭窄带状展布，两侧分别被哀牢山断裂和红河断裂所限，北延至南涧县密滴附近，被红河断裂所切，使哀牢山断裂尖灭，南延入越南范士版带[34-35]。变质带主体由东西两个变质带构成，即东部的高级变质岩和西部的低级变质岩。高级变质岩带由角闪岩-绿片岩相的片麻岩、角闪岩、大理岩和花岗岩组成，带内因受剪切带活动影响岩石已强烈糜棱岩化。低级变质带由低绿片岩相片岩、千枚岩和板岩等浅变质作用的早古生代碎屑岩组成。两者被一逆冲断裂分隔开，断裂大体沿山脉的主脊延伸[36]。大部分片麻岩与斜长角闪岩片理化良好且平行于剪切带方向，运动学标志指示左旋剪切。由于变质变形作用的叠加改造，加之混合岩化强烈，原岩面貌和层理特征基本消失，岩性复杂。

本文所研究的2个花岗质岩石样品(10HH-99B、10HH-105B)位于哀牢山红河剪切带中段滇西红河甲寅地区。10HH-99B采样坐标(N 23° 14′ 18.9″，E 102° 24′ 33.2″)，岩石类型为糜棱状变形花岗岩，样品呈灰色，糜棱状结构，块状构造。岩石内部的长英质矿物被压扁拉长形成线理，面理产状与区域性走滑面理产状一致(图2a)。由斜长石、钾长石、石英和少量黑云母、磁铁矿、榍石和锆石等矿物组成。石英因动态重结晶而发生晶体颗粒细粒化，斜长石表现出明显的环带结构，黑云母呈细小的鳞片状分布在长石和石英的颗粒边界(图2b)。10HH-105B采样坐标(N 23° 18′ 34.6″，E 102° 24′ 58.2″)，岩石类型为混合岩化浅色花岗岩脉，其基质为花岗片麻岩。样品所在的长英质脉体与岩石片麻理平行，脉体未发生明显变形，产出方向与剪切带构造面理方向一致。在野外可见长英质脉体被发育于更晚阶段的花岗质脉体侵入，二者呈现显著截切关系(图2c)。浅色花岗岩脉呈白色，块状构造，主要矿物为斜长石、钾长石、石英和少量黑云母、磁铁矿、榍石和锆石等(图2d)，可见长英质矿物的波状消光。

图1 (a) 哀牢山红河剪切带构造纲要图 (据[24-25]修改)；(b) 甲寅地区地质简图Fig.1 (a) Tectonic outline map of Ailaoshan-Red River shear zone and (b) geological map of the Jiayin area

图2 哀牢山红河剪切带花岗质岩石10HH-99B和10HH-105B野外照片(a和c)及正交偏光(10×4)显微照片(b和d，Qz-石英, Pl-斜长石)Fig.2 Field photos (a and c) and microphotographs (b and d) for granitic rocks of 10HH-99B and 10HH-105B from Ailaoshan-Red River shear zone

2 分析方法

锆石通过重选和磁选技术从新鲜的样品中分选出来，用双目显微镜挑选出无裂隙、无包体、透明干净的锆石颗粒，在玻璃板上用环氧树脂固定、抛光，然后进行反射光和透射光照相，并进行阴极发光(CL)图像分析以检查锆石内部结构。锆石U-Pb年龄及Lu-Hf同位素分析在中国科学院地质与地球物理研究所岩石圈演化国家重点实验室的Neptune型多接收电感耦合等离子体质谱仪(MC-ICP-MS)、Agilent7500a型四级杆电感耦合等离子体质谱仪(Q-ICPMS)和193 nm的ArF准分子激光系统上进行原位测定完成。详细的分析流程和原理参见[37-39]。数据处理采用Ludwig 2001 SQUID 1.02及ISOPLOT程序[40]。锆石U-Pb同位素比值计算采用标准样品91500作外标进行校正，分馏校正及计算结果采用ICPMSDataCal (8.4)[41]。

3 分析结果

3.1 锆石U-Pb年代学

从云南红河甲寅地区2个花岗岩样品中挑选出的锆石CL图像如图3所示，2个样品的LA-ICP-MS锆石U-Pb分析结果见表1和图4。

10HH-99B样品锆石呈自形或半自形长柱状，长度50～120 μm。依据锆石CL图像可分出明暗两种锆石，大部分为具有明显岩浆震荡环带的暗色锆石，亮色锆石则呈弱分带或者无分带特点(图3a)。此外，部分锆石具有亮色核和暗色边的核边结构，为具有老核新壳的岩浆复合型锆石。此类锆石的新壳具有岩浆锆石所具有的特点，其新壳年龄反映了岩体的结晶时间，老核的年龄为深部地质体提供的信息[16, 42]。对糜棱状变形花岗岩(10HH-99B)中的18个锆石颗粒进行了分析，所选测点晶形较好，无裂纹和包裹体。其中12个测点为暗色锆石颗粒，具有较高的U含量。这些锆石的Th/U比值在0.11～0.20变化，它们的206Pb/238U加权平均年龄值为26.2 ± 0.5 Ma (MSWD = 0.94; 图4a)。另外有2个测点18和21为亮色锆石，Th/U比值分别为0.60和0.19，对应表面年龄为40.1±2.8 Ma和40.0±2.1 Ma，可能反映了～ 40 Ma的变质事件；剩余的5个测点也为亮色锆石，Th/U比值变化于0.20～1.54，表面年龄在190～699 Ma之间，代表继承锆石年龄。我们认为，12个测点的加权平均年龄值26.2 ± 0.5 Ma代表糜棱状变形花岗岩的岩浆结晶年龄。

图3 哀牢山红河剪切带花岗质岩石 (a) 10HH-99B和 (b) 10HH-105B代表性锆石CL图像Fig.3 The cathodoluminescence (CL) images of representative zircons from the granitic rocks of (a) 10HH-99B and (b) 10HH-105B from Ailaoshan-Red River shear zone

样品分析点Th/U同位素质量分数比207Pb/235U±1σ206Pb/238U±1σ加权平均年龄/Ma207Pb/235U±1σ206Pb/238U±1σ10HH⁃99B020140026500012000430000126612274090401800275000130003900001276132530805015002490001200041000012501226309060201063300602011470005573552966999315070120028200014000410000128214265090801100280000180004100002281182641009020002460001200040000012471225709110180025000013000390000125113249081201800286000170004200001286172711013154020600011700299000101902991900651405802393001900037000015217915523449415033031160022200474000192754172298711616047026470023100406000172384186256810618060004110006500062000044096340128210190045600033000620000345332400212201400297000160004200001297162710923016003040001700040000013041725709240140026200015000430000226315275102501500310000230004000001310232540910HH⁃105B010110030800032000410000230832263120201400415000720004600004413702962803014003110003300041000023113226411050140030600037000400000230637261120701900371000510004400002370502811408011002510003300038000022523324312090150029400028000400000229528258111001500273000300004300002274302751511028003700003900040000023693825512120120027400031000420000227431272111301000353000400004200002352402691214017003440004500042000023434427112150210036600046000450000236545290121604000297000340004200002297332681217044024010011100351000142185912221871801600374000440004300003373432771819011003350002700042000023352726812200170036200040000390000236239252112101600285000260004100002285262641222014003610004200045000033604129017230190037400038000440000237337282112404600349000820004100003348812652125017003320003100044000023323128413

10HH-105B样品锆石晶面整洁光滑，大部分呈自形的长柱状，少量存在继承性锆石核，直径在50～220 μm之间，长短轴之比一般大于2。在CL图像上大部分锆石显示出典型的震荡生长环带，表明其为典型的岩浆锆石(图3b)。对浅色脉体的23个锆石颗粒进行了分析，所选测点晶形较好，同时避开裂纹和包裹体。测点17的Th/U比值为0.44，并给出了222.1±8.7 Ma的206Pb/238U年龄值。该测点位于锆石核部，可能代表前期热事件中残留的锆石。剩余22个测点Th/U比值在0.11～0.46，投点均落在谐和线上或其附近，它们的206Pb/238U加权平均年龄值为26.8 ± 0.5 Ma (MSWD = 0.94; 图4b)。这一年龄代表混合岩化浅色花岗岩脉的岩浆结晶年龄。

图4 哀牢山红河剪切带花岗质岩石 (a) 10HH-99B和 (b) 10HH-105B LA-ICP-MS锆石U-Pb定年结果Fig.4 Zircon LA-ICP-MS U-Pb isotopic data for granitic rocks of (a) 10HH-99B and (b) 10HH-105B from Ailaoshan-Red River shear zone

3.2 锆石Lu-Hf 同位素分析

两个样品的LA-ICP-MS锆石Lu-Hf同位素组成见表2。

变形花岗岩10HH-99B：进行了17颗锆石的原位Lu-Hf同位素分析，按t= 26.2 Ma的形成年龄，对其中12颗锆石计算出的εHf(t)为 +0.3～+6.9，平均+3.4，其二阶模式年龄TDM2介于526～853 Ma之间。其中10HH-99B-06继承锆石给出最老206Pb/238U表面年龄699.9 ± 31.5 Ma，它的εHf(t)和TDM2值分别为-2.99和1 575 Ma。剩余4颗继承锆石206Pb/238U表面年龄在190～298 Ma变化，εHf(t)介于-9.25～-2.81，TDM2介于1.23～1.58 Ga，如图5a和图6所示。

图5 哀牢山红河剪切带渐新世花岗质岩石 (a) 10HH-99B 和 (b) 10HH-105B 锆石Hf同位素组成Fig.5 Zircon Hf isotopic compositions for granitic rocks of (a) 10HH-99B and (b) 10HH-105B from Ailaoshan-Red River shear zone

样品分析点年龄/Ma176Yb/177Hf176Lu/177Hf176Hf/177Hf2σ176Hf/177HfiεHf(0)εHf(t)2σTDM1TDM2fLu/Hf10HH⁃99B0226201872160006449028283900000210282836242807686728-0810426201457830004988028286400000250282861323709616683-0850526201339180004687028285500000260282853293409625698-08606699900526560001789028227500000320282251-176-301114101575-0950726201415940004933028285800000310282856313511624692-0850826201144970004090028280800000280282806131810686780-0880926201533380005192028276700000270282764-020309774853-0841126202238680007677028287400000300282870364011653668-0771226201214120004263028284000000260282837242909641725-08713190000923420002955028244000000290282429-117-801012121397-09114234400666240002357028250700000340282496-94-461210951269-09315298700511670001747028251700000300282507-90-281110621233-09516256800136150000498028235400000280282351-148-931012531516-0982226201511090005289028295300000290282950646910479526-0842326201718270005892028289600000310282893444911580627-0822426202122830007280028289300000310282889434711612633-0782526201676760005872028282400000320282821182311698754-08210HH-105B0126800163000000627028249800000370282498-97-911310571319-0980226800107560000394028263400000360282633-49-43138631082-0990326800187280000683028258500000380282585-66-60139361167-0980526800269900000977028259300000380282592-63-58139341154-0970726800204390000738028273300000380282733-14-0813731908-0980826800169340000647028256700000310282566-73-67119621199-0980926800241380000870028266300000370282662-39-33138331032-0971026800288800000997028256700000350282566-73-67129701199-0971126800215090000778028273600000380282736-13-0713727903-0981226800210660000775028263100000330282630-50-44128751088-0981326800166490000632028247700000300282477-104-981110861355-0981426800199740000724028268800000360282687-30-2412794988-0981526800243160000873028269100000310282690-29-2311793983-0971626800302110001061028271200000360282711-21-1612768946-09717222101017860003277028252600000280282512-87-431010941244-0901826800202980000727028268900000320282688-29-2411793986-0981926800189570000711028256500000260282564-73-68099661203-0982026800197340000705028268800000320282688-30-2411794988-0982126800274640000980028265400000270282654-42-36108471046-0972226800169740000624028257500000270282574-70-64109501186-0982326800222280000798028266000000290282660-39-34108341036-0982426800202460000728028273100000280282730-15-0910734912-0982526800259700000944028262300000330282623-53-47128901101-097

图6 哀牢山红河剪切带渐新世花岗质岩石锆石εHf(t)-t图解Fig.6 εHf(t) vs.t diagrams for granitic rocks of 10HH-99B and 10HH-105B from Ailaoshan-Red River shear zone

混合岩脉体10HH-105B：进行了23颗锆石的原位Lu-Hf同位素分析，其中17为继承锆石，εHf(t)为-4.3、TDM2为1 244 Ma。其余22个分析点176Hf/177Hf = 0.282 477～0.282 736。按t=26.8 Ma的形成年龄，εHf(t)=-9.8～-0.7，平均-4.3，二阶模式年龄TDM2介于903～1 355 Ma，如图5b和图6。

4 讨 论

4.1 成岩过程

近年来，在金沙江-哀牢山构造带识别出许多新生代岩浆岩，为研究壳幔相互作用提供了丰富的地球化学信息[15, 22, 31]。这些新生代岩浆岩以花岗质岩石为主，普遍具有富钾的特征，其锆石Hf二阶段模式年龄集中在1078～1448 Ma，与哀牢山构造带内的前寒武纪变质岩系的Nd同位素模式年龄接近[7, 30, 43-44]，被认为是中元古代地壳物质重熔再造形成的，幔源物质贡献较少[15, 31, 45]。我们对哀牢山构造带红河甲寅地区花岗质岩石Lu-Hf同位素组成的测试结果表明，样品10HH-99B初始εHf值变化在+0.3～+6.9之间，对应的二阶段Hf模式年龄落在526～853 Ma之间，明显大于锆石的206Pb/238U表面年龄，与Liu et al.[19]提出的哀牢山新生基性下地壳模式年龄一致，表明该花岗岩与滑石板高εNd-εHf花岗岩具有相似的源区，主要源于526～853 Ma新生地壳的重熔。另一件样品(10HH-105B)初始εHf值变化在-9.8～-0.7之间，对应的Hf模式年龄落在903～1 355 Ma之间，远大于其结晶年龄。锆石具负εHf(t)值和古老的Hf两阶段模式年龄，揭示其源区主要为古老陆壳物质，可能来源于哀牢山高级变质带中新元古代地壳物质的部分熔融[21, 46]。因此，哀牢山构造带存在前寒武纪结晶基底，说明代表该区基底的哀牢山群形成时代包括中新元古代，综合显生宙时期各类岩石，可知哀牢山高级变质带是一个杂岩。结合区域构造演化，我们认为该花岗岩形成可能与印欧板块后碰撞背景下古老地壳熔融相关，是造山带在渐新世时期剪切过程因伸展松弛而发生减压熔融的产物。Lin et al.[45]在点苍山和哀牢山也识别出了晚渐新世低εHf(t)片麻岩(图8)，其成因应与本文低εHf(t)花岗岩相似。综合可知，低εHf(t)的花岗岩源岩可能是元古宙哀牢山群变质杂砂岩，高εHf(t)的花岗岩源自软流圈地幔热源加热新生下地壳的部分熔融[19, 47]。

Rapp & Watson[48]的实验表明变质玄武岩熔融的最低温度为1 000 ℃，地壳加厚和中下地壳的放射性元素生热显然无法产生这么高的温度，那么基性下地壳熔融的热源来自哪里?哀牢山构造带是特提斯-喜马拉雅构造域的重要地区，在青藏高原东南缘新生代以来的构造演化讨论中，围绕哀牢山构造带对印欧碰撞的响应机制一直存在诸多争议。目前，主要争论的焦点在于三江地区广泛分布的大型走滑断层在大陆挤压过程中扮演着怎样的角色[49]。一种观点认为，印度板块本质上是刚性的岩石圈块体，其变形主要集中在板块的边缘，红河断裂带是大陆块体侧向逃逸的东部边界。因此，走滑断层切割深达岩石圈地幔[12, 14, 50-52]。另一种观点认为挤压加厚的陆壳是一种薄的粘性席体，其内部变形是均匀的，主体上是非旋转岩石圈缩短，故而走滑断层纯粹是在地壳尺度[53-57]。此外需要注意的是，华南的大部分研究地区普遍受陆内深断裂的控制，许多深断裂继承了古俯冲带、古拼接带等板块边界构造，在后来发生的部分熔融事件中，新生岩浆继承了早先形成的与俯冲和碰撞有关的含有较多幔源物质的特征，在地球化学方面显示幔源组分参与特征[58]。Zhang和Schärer[15]认为在哀牢山-红河左旋走滑剪切过程中存在热异常。因此，处于后碰撞环境的研究区，最有可能的深部热源就是上地幔尺度上的深大断裂。此外，哀牢山富碱侵入岩不管是由古洋壳板块加岩石圈地幔部分熔融形成[59-61]，还是印欧板块俯冲过程中板片断离导致软流圈物质上涌诱发加厚大陆下地壳部分熔融形成[62-63]，均是处在强烈的区域性构造应力转换阶段[64]，可能深切至上地幔的大型走滑断裂系统是富碱斑岩岩浆的重要运移通道，也是地壳部分熔融的重要热源之一。我们认为，两类花岗岩的形成过程如下：在渐新世后碰撞构造环境中，地壳拉张和深大断裂活动导致地幔上涌，高εHf(t)花岗岩由被底侵的玄武质下地壳物质部分熔融形成，其热源可能是底侵的幔源镁铁质岩浆。受碰撞作用的影响，因地壳加厚及阶段性剪切作用，导致地壳重熔产生低εHf(t)花岗岩，岩石部分熔融的热源来自于块体的摩擦运动。

4.2 两类晚渐新世花岗岩的构造指示

近年来，哀牢山红河剪切带众多高封闭温度同位素体系的年代学，如独居石、锆石U-Th-Pb定年的研究，显示其经历了一个复杂的热历史(图7)。该区域发表的大部分花岗质岩石U-Pb年龄集中在35～21 Ma，最老年龄在35～38 Ma[15, 22-23, 65-67]。同时，研究区同剪切矿物的40Ar/39Ar定年结果给出了32～22 Ma的年龄范围[15, 68-69]。结合青藏高原东南、滇西地区以及越南地区广泛存在的高钾质岩浆岩，Liang et al.[70]认为初始左旋走滑运动发生在35～36 Ma。基于变质岩、热年代学数据不少学者提出初始左旋剪切至少在35 Ma[15,71-72]。我们对云南红河甲寅地区两类花岗质岩石研究结果显示，在晚渐新世存在一期岩浆岩的作用，并且同时软流圈地幔的上侵说明此时哀牢山局部地区可能转化为伸展背景，也就是说红河剪切带剪切作用在晚渐新世由压剪转为扭剪作用。这一结论也为我们在深变质带内识别的26 Ma变基性岩所证明(课题组待刊数据)。

图7 哀牢山红河剪切带新生代岩石锆石U-Pb年龄谱系图数据来源于文献[22-23, 31, 45, 73-77]以及本研究Fig.7 Zircon U-Pb age histograms for the Cenozoic rocks from the Ailaoshan-Red River shear zone Age data refer to the following sources:[22-23, 31, 45, 73-77] and this study

结合邻近的大地构造格局来看(图8)，哀牢山红河剪切带位于印支和扬子板块交界处，高黎贡山右旋走滑断裂带位于腾冲地块和保山地块间，是思茅-印支地块的西边界断层。这些韧性剪切带可能与红河剪切带一致，都是切穿至岩石圈地幔深度的断裂带。Wang et al.[78]认为右旋的高黎贡山剪切带和左旋的崇山剪切带始于～ 32 Ma，终止于约 15～17 Ma。同样不同学者对糜棱岩、片麻岩、花岗质脉中不同矿物进行同位素定年的结果表明，高黎贡山峰期变质时间介于38～22 Ma之间[79-81]。哀牢山红河剪切带剪切作用可能与高黎贡山右旋走滑剪切带和崇山左旋走滑剪切带具一致延续时限，它们都是新生代以来印欧板块碰撞造山事件在青藏高原东南缘的响应。高黎贡山剪切带和哀牢山红河剪切带肢解了至少两大块体(兰坪思茅地体和掸泰地块)(图8)，而不是由Tapponnier et al.[12]提出的东南缘各陆块为一个单一的刚性块体[82-83]。

图8 三江地区渐新世时期印支地块沿剪切带东南向挤出被肢解成不同的块体(改自[84])① 甘孜理塘缝合带; ② 金山江缝合带; ③ 龙木错-双湖缝合带; ④ 班公湖怒江缝合带; ⑤ 哀牢山缝合带; ⑥ 昌宁-孟连缝合带; ⑧ 密支那缝合带.GLSZ=高黎贡山剪切带; CSZ=崇山剪切带; ARSZ=哀牢山-红河剪切带; SGSZ=实皆剪切带; JLF=嘉黎断裂Fig.8 Distribution of the late Oligocene shear zones in Southeast Asia (revised from [84]) ① Garzê-Litang suture; ② Jinshajiang suture; ③ Longmu Tso-Shuanghu suture; ④ Banggong-Nujiang suture; ⑤ Ailaoshan suture; ⑥ Changning-Menglian suture; ⑦ Myitkyina suture GLSZ=Gaoligongshan shear zone; CSZ=Chongshan shear zone; ARSZ=Ailaoshan-Red River shear zone; JLF=Jiali fault; SGSZ=Saging shear zone

5 结 论

1) 云南红河甲寅地区识别出两类晚渐新世(～26 Ma)花岗质岩石，其锆石εHf(t)平均值分别为+3.4和-4.3，二阶段模式年龄分别为526～853 Ma和903～1 355 Ma。前者源自新生基性下地壳的部分熔融，后者源岩为元古宙哀牢山群变质杂砂岩。

2) 甲寅两类晚渐新世花岗岩的发现说明哀牢山群并不全是扬子板块的元古代结晶基底，也包含有新生代岩浆岩。

3) 甲寅两类花岗岩的发现还说明红河剪切带在晚渐新世切穿了岩石圈地幔，为我们认识东南亚陆块属性及南海打开提供了资料。

致谢:野外样品采集以及室内岩石分析工作得到蔡永丰、张爱梅等的帮助，编辑部老师和评审专家的意见，对提高论文质量起了重要作用，在此一并致以诚挚谢意。

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PetrogenesisoftwotypesoflateOligocenegranitesinAilaoshantectoniczoneandtheirtectonicimplications

GUOXiaofei1, 2, 3,WANGYuejun2, 4,LIUHuichuan2, 4,ZHANGYuzhi2, 4

(1.State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640, China;2. School of Earth Sciences and Engineering, Guangzhou 510275, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China;4. Guangdong Provincial Key Lab of Geodynamics and Geohazards, Sun Yat-sen University,Guangzhou 510275, China)

The widespread Cenozoic magmatism in the Red River-Ailaoshan tectonic zone is closely related to the Indian-Eurasian plate collision orogeny. Zircon U-Pb dating results of two granitic samples exposed in the middle part of the Ailaoshan metamorphic belt are 26.2 ± 0.5 Ma and 26.8 ± 0.5 Ma, respectively. This indicates that the Ailaoshan metamorphic zone is not entirely attributed to the Proterozoic crystalline basement of the Yangtze block, while it also contains Cenozoic magmatic rocks. Zircon εHf(t) values of the samples range from +0.3 to +6.9 and from -9.8 to -0.7 with mean values of +3.4 and -4.3, respectively and their corresponding two-stage Hf model ages (TDM2) range from 526 to 853 Ma and from 903 to 1 355 Ma. The high εHf(t) granites were derived from partial melting of juvenile crust induced by the asthenospheric mantle heat source, while low εHf(t) granites from Proterozoic metamorphic sandstones of the Ailaoshan group. In combination with previous research results, we suggest that the Red River fault has cut across the lithosphere mantle. In SE Asia, the Gaoligongshan and Ailaoshan strike-slip shear zones on both sides of Indosinian block dismembered different blocks and created conditions for their Oligocene extrusion to the southeast.

Ailaoshan tectonic zone; granitic rocks; zircon ages; Hf isotope; strike slip shear; extrusion effect

10.13471/j.cnki.acta.snus.2017.06.001

2016-10-20

国家重点研发计划项目(2016YFC0600303)；国家重点基础研究发展计划(973)项目(2014CB440901)；国家自然科学基金联合基金(U170160005)；中山大学高校基本业务费项目

郭小飞(1990年生)，男；研究方向构造地质学;E-mail:niubidrbsr@126.com

刘汇川(1986年生)，男；研究方向构造地质学;E-mail:liuhuichuan1986@126.com

P588.121

A

0529-6579(2017)06-0001-14