拉萨地块内部古特提斯洋早中三叠世仍在俯冲——来自火山岩和高压变质岩的证据

重点对拉萨地块查曲浦组火山岩地球化学特征和工布江达地区岔萨岗岩组变质基性火山岩矿物特征、时代进行了研究。查曲浦组火山岩以中基性岩为主,具有从基性向酸性演化的特征,富集大离子亲石元素K、放射性生热元素U和Th,相对亏损高场强元素Nb、Ta和Ti,轻稀土元素明显富集,具有类似于俯冲带火山岩的地球化学特征。岔萨岗岩组变质基性火山岩发育石榴石、角闪石等变斑晶,前者以铁铝榴石和钙铝榴石为主,后者包括镁质普通角闪石、铁质普通角闪石、韭闪石和铁阳起石等,它们是在高压、相对低温环境下形成的。变质基性火山岩中的锆石为具有新壳和老核的复合型锆石,新壳不具振荡环带结构,Th/U比值小于0.1,系后期构造-热事件形成;SHRIMP定年结果显示,新壳的U-Pb年龄分为2组:8个测点加权平均年龄为237.7±7.1 Ma(MSWD=1.3),与早中三叠世造弧运动对应;6个测点加权平均年龄为273.4±5.5 Ma(MSWD=0.15),与早中二叠世造弧运动对应;4个测点年龄值在304~346 Ma之间,为混合年龄,反映变质基性岩原岩时代至少为石炭纪。在以上基础上,结合前人资料,认为拉萨地块古特提斯洋在早中三叠世时期还在俯冲,于晚三叠世消亡。

The continuation of the subduction of Paleo-Tethys Ocean within Lhasa block in Early-Middle Triassic: evidence from volcanic rocks and HP metamorphic rocks

The Sumdo eclogite belt is regarded as remains of the Paleo-Tethys Ocean basin in Lhasa block.This paper deals with the problem whether it still subducted in Early-Middle Triassic or not.Early-Middle Triassic Chaqupu Formation and Chasagang Formation serving as the wall rock of eclogite were selected for the study.The authors not only emphatically studied geochemical features of Early-Middle Triassic volcanic rocks lying in Chaqupu area but also investigated mineralogical characteristics and age of meta-basic volcanic rocks of Chasagang Formation in Gongbujiangda area.The Early-Middle Triassic volcanic rocks consist of basic-intermediate rocks with the evolution features from basic to acid,and show intense enrichment of K in LILE and radioactive elements Th and U,relative depletion of Nb,Ta and Ti in HFSE,and enrichment of LREE,similar to geochemical features of volcanic rocks produced in subduction zones.The meta-basic volcanic rocks of Chasagang Formation not only have hornblende metacrystal but also garnet metacrystal;the former consists of almandine and grossular,whereas the latter mainly consists of magnesian hornblende,ferruginous hornblende and ferruginous actinolite,both formed under high-p and relatively low-T conditions.All of the zircons from meta-basic volcanic rocks are metamorphic compound ones composed of old core and young shells.And the young shells have no zoned structure and were formed probably by late thermo-tectonic event.The SHRIMP technique was applied to determine U-Pb age of the metabasic volcanic rocks.The results show that the young shells can be divided into two groups by their ages,one group yielded 237.7±7.1 Ma(MSWD=1.3)which is consistent with the island arc produced(IAP) in Early-Middle Triassic,the other yielded 273.4±5.5 Ma(MSWD=0.15)which is consistent with the IAP of Early-Middle Permian.The mixtures of old core and young shell of 4 zircon measuring points yielded the ages from 304 to 346 Ma,suggesting that the protolith of meta-basic volcanic rocks were formed at least in Carboniferous.Based on these results in combination with the conformity between Early-Middle Triassic Chaqupu Formation and Carboniferous-Permian island-arc volcanic rocks,the unconformity between Middle Jurassic and Lower-Middle Triassic in Lhasa area as well as between Lower Jurassic and Upper Triassic in Coqin area,and the existence of Late-Triassic collisonal granite in Nanmulin area,the authors have reached the conclusion that the Paleo-Tethys Ocean between the southern part and the northern part of Lhasa block,like Longmu Co-Shuanghu and Xijinwulan-Jinshajiang Paleo-Tethys Ocean,subducted in Early-Middle Triassic and disappeared in Late Triassic.