西藏羌塘中部才多茶卡蓝闪石^(40)Ar/^(39)Ar年代学及地质意义

^(40)Ar/^(39)Ar Isotopic Dating of the Glaucophane in Caiduo Caka,Central Qiangtang Area,Northern Tibet,China and Its Geological Significance

本文报道了藏北羌塘中部才多茶卡地区晚三叠世蓝闪石片岩及蓝闪石矿物的新发现及地质意义。通过对才多茶卡构造混杂岩地质剖面的实测,在剖面第6层灰绿色变基性火山岩(绿片岩)中新发现以产蓝闪石类矿物为特征的中—高压变质岩系——蓝闪石片岩。对蓝闪石片岩的岩石学和矿物学特征进行了详细室内研究。从蓝闪石片岩中挑选出蓝闪石单矿物进行了40Ar/39Ar同位素年龄测定,结果表明,蓝闪石单矿物的坪年龄值为209±4Ma,等时线年龄值为216±4Ma,坪年龄与等时线年龄基本一致,属于晚三叠世诺利期,这与区域上羌塘中部发育的红脊山—双湖蓝闪石片岩的中—高压变质事件相吻合。由于蓝闪石类矿物的产出具有特殊的构造机制和地质意义,因此双湖以东的才多茶卡地区蓝闪石片岩的新发现及蓝闪石矿物的40Ar/39Ar同位素年代学研究进一步补充和丰富了青藏高原腹地羌塘中部龙木错-双湖构造混杂岩带的物质组成和同位素年代学信息,同时也为解决羌塘地区"龙木错-双湖构造混杂岩带是否向东延伸"等重大基础性地质问题,提供了新的重要资料。研究资料表明,羌塘中部自西向东从冈玛错、红脊山、绒马,经嘎尔错到才多茶卡地区,沿构造线方向断续分布的长达600km的含蓝闪石片岩的中—高压变质岩系,都是龙木错-双湖构造混杂岩带印支期构造活动事件的产物。

The paper reports a new discovery of the late Triassic blueschists and glaucophanes in Caiduo Caka,Shuanghu area,central Qiangtang,northern Tibet,China and its geological significance. Based on the field survey and measurement of the tectonic m langes geological section in the Caiduo Caka,Shuanghu region,the middle-high pressure metamorphic rocks （glaucophanes-bearing blueschists） were found to occur in the grey-green metamorphic basic volcanic rocks（green schist） along the sixth bed of this section. The petrology of the blueschists,and the mineralogy and petrochemistry of the glaucophanes have been researched in detail. Glaucophanes minerals were collected from the blueschists for ^40Ar/^39Ar isotopic dating. The testing yielded a plateau age of 209±4 Ma and an isochronal age of 216±4 Ma,both of which are basically consistent with each other,suggesting that they formed at the Norian stage of the late Triassic. The ages couple with regional middle-high pressure metamorphic event of the Gangmar-Hongjishang-Shuanghu blueschists belts in central Qiangtang. Due to the particular tectonic settings and geological significance of glaucophanes and blueschists occurrence,the discovery of the Caiduo Caka blueschists in eastern Shuanghu area and the ^40Ar/^39Ar isotopic chronological research have not only provided new important data to the Shuanghu tectonic m lange composition and the glaucophanes isotopic chronology,and but also solved the key geological problem about the ＂eastward extension of the Longmucuo-Shuanghu tectonic m lange zone in central Qiangtang＂. Research also indicates that the middle-high pressure blueschist metamorphic rocks zone extending discontinuously over 600 km from Gangmacuo-Hongjishan-Rongma-Gaercuo,sequentially eastward to Caidaocaka in central Qiangtang northern Tibet are bound up with the deep subduction orogenesis of the Longmucuo-Shuanghu tectonic m lange zone during the late Triassic.