点苍山-哀牢山杂岩带多期变质作用:嘎洒地区变沉积岩锆石微量元素与U-Pb年代学制约  

Ployphase metamorphism in Diancang Shan-Ailao Shan complex zone: Constraints from U-Pb dating and trace elements of zircons in metasedimenary rocks,Gasa area

作　　者：冀磊[1] 刘福来[1] 王舫[1] 蔡佳[1] 王伟[1] 田忠华[1] 刘利双 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2017年第2期605-621,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41372069);国家重点研发计划(2016YFC0600310)联合资助

摘　　要：点苍山-哀牢山杂岩带位于青藏高原东南缘,为云南三江地区一条重要的造山带,由扬子板块和印支板块于晚二叠世-中晚三叠世碰撞拼合而成。杂岩带主要由各类副片麻岩、片岩、石英岩、大理岩和斜长角闪岩构成,岩石发育强烈糜棱岩化和深熔作用。本文选取哀牢山北段新平嘎洒地区变沉积岩为研究对象,通过对变沉积岩锆石的阴极发光图像、微量元素、矿物包裹体组合、表面形态和U-Pb年龄的综合研究,揭示出嘎洒地区哀牢山杂岩经历了两期变质事件:其中,含石榴子石斜长二云母片岩中30颗变质锆石获得了较为一致的206Pb/238U年龄215±6Ma^227±5Ma,加权平均年龄为222.3±1.2Ma(n=30,MSWD=0.27),这些锆石具有浑圆状或椭圆状形态、较为均匀的阴极发光图像、平坦的HREE配分模式((Lu/Gd)N=0.73~4.08)和弱的负Eu异常,这些特征与典型的高级变质岩中变质锆石相似,而锆石的Th/U比值较为分散为0.06~0.84,平均值为0.45,可能与变质过程中富Th矿物独居石分解有关。变质年龄与杂岩带中点苍山和元阳地区变质岩中、晚三叠世变质年龄极为吻合,指示这期变质事件与中-晚三叠世古特提斯洋闭合-造山有关,标志着点苍山-哀牢山杂岩带为三江地区一条重要的古缝合线。此外,嘎洒地区夕线石榴黑云二长片麻岩的岩相学特征显示,岩石经历了石榴子石的转熔作用,除两颗锆石年龄为35.4Ma外,28颗锆石(增生边)给出了误差范围内较为一致的206Pb/238U年龄(27.3±0.5Ma^31.9±0.5Ma),加权平均年龄为29.4±0.53Ma(n=28,MSWD=2.0)。这些锆石的增生边中的矿物包裹体组合为夕线石+钾长石+黑云母+石英+独居石,且具有较低的Th/U比值(0.01~0.1),平坦的重稀土(HREE)配分模式((Lu/Gd)N=0.45~7.59)和中等程度的负Eu异常,这些特征指示该类锆石为典型的变质锆石。变质年龄与新生代红河-哀牢山剪切带内大量发现的同剪切岩浆岩、变质�The Diancang Shan-Ailao Shan metamorphic complex belt is located on the southeastern margin of Tibetan Plateau,which formed by the Late Permian to Middle-Late Triassic collision of the Yangtze plate and Indochina plate. The complex belt mainly comprises different kind of paragneisses,schists,quartzites,marbles,amphibolites,which have experienced extensive mylonitization and anatectic melting. In this paper a comprehensive study of zircon cathodoluminescence（ CL） image,trace element,mineral inclusion,external morphology and LA-ICP-MS U-Pb dating on meta-sedimentary rocks from the Gasa area was accomplished. Two different metamorphic events are revealed by garnet-bearing plagioclase two-mica schist and sillimanite-garnet biotite monzogneiss from the Ailao Shan Complex（ ALSC）. Thirty metamorphic zircons from the garnet-bearing plagioclase two-mica schist yield consistent ages ranging from 215 ± 6Ma to 227 ± 5Ma and give a weighted mean206Pb/238 U age of 222. 3 ± 1. 2Ma（ n = 30,MSWD = 0. 27）,In addition,most of these zircons show rounded or ovoid external morphology,flat HREE pattern（（ Lu/Gd）N= 0. 73 ~ 4. 08） and weak negative Eu anomaly,similar to metamorphic zircon from typical high grade metamorphic rocks. These zircons gives a large range of Th/U ratios（ 0. 06 ~ 0. 84,mean = 0. 45）,which maybe associated with the decompositon of Th-enriched mineral（ such as monazite）. The nearly identical metamorphic ages with those from the Diancang Shan and Yuanyang areas indicate a metamorphic event associated with the closure of Paleo-Tethys in Middle-Late Triassic and the Diancang Shan-Ailao Shan complex belt is one of essential sutures zones in the Sanjiang area,southeast of Tibetan Plateau. Moreover,twenty-eight metamorphic zircons（ rims） in the sillimanitegarnet biotite monzogneiss yield concordant ages ranging from 27. 3 ± 0. 5Ma to 31. 9 ± 0. 5Ma,and give a weighted mean U-Pb age of29. 4 ± 0. 53Ma（ n = 28,MSWD = 2. 0）. Petrographic study indicated that the sillimanite-garnet bio

关 键 词：哀牢山杂岩带 多期变质作用 U-PB定年 锆石微量 嘎洒地区 

分 类 号：P588.34[天文地球—岩石学] P597.3[天文地球—地质学]