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

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、变质岩的年龄较为一致,指示这期年轻的变质事件与岩石圈尺度大规模剪切运动有关。此外,两类变沉积岩中6颗继承性碎屑锆石的年龄分布范围为528~783Ma,这些锆石具有锥形的锆石形态,清晰的振荡环带,表面发育蚀痕和凹坑,较高的Th/U比(>0.1),陡倾的HREE配分曲线,表明这些锆石为经过剥蚀-搬运-沉积的岩浆锆石,具有继承性碎屑锆石的特征,说明哀牢山杂岩变沉积岩中至少应包含新元古代和早古生代的沉积物源,指示研究区哀牢山杂岩带部分岩石并不属于真正意义上的扬子结晶基底。

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 biotite monzogneiss had experienced peritectic reaction and most zircons show rounded or ovoid external morphology. Overgrown rims contain inclusion assemblage of sillimanite ＋ K-feldspar ＋ quartz ＋ biotite ＋ monazite,with low Th/U ratios（ 0. 01 ~ 0. 1）,flat REE pattern（（ Lu/Gd）N= 0. 45 ~7. 59） and moderate negative Eu anomalies,all of which demonstrate that the metamorphic zircons were grown in a background coexisting with garnet and feldspar and,the anatexis melting possibly corresponded to the regional ductile strike-slip shear in Oligocene. Besides,six inherited zircons from the two samples have ages from 528 Ma to 783 Ma,high Th/U ratios（ 0. 1）,steep HREE pattern,exhibit oscillatory zoning in the CL images and prismatic shapes with etching and pitting surface. All these features are corresponding to magmatic zircon underwenting an erosion-transportation-deposition process. As a consequence,the meta-sedimentary rocks in the ALSC should contain the Neoproterozoic and Early Paleozoic source,indicating that the ALSC in studying area do not belong to the basement of the Yangtze plate.