岩石矿物学杂志2012，Vol.31Issue(2)：119-132,14.
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
The continuation of the subduction of Paleo-Tethys Ocean within Lhasa block in Early-Middle Triassic: evidence from volcanic rocks and HP metamorphic rocks
摘要
Abstract
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 ofalmandine 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.关键词
火山岩/变斑晶/锆石U-Pb年龄/岛弧/俯冲Key words
volcanic rocks/ metacrystal/ zircon U-Pb ages/ island arc/ subduction分类
天文与地球科学引用本文复制引用
李奋其,刘伟,王保弟,张士贞..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[J].岩石矿物学杂志,2012,31(2):119-132,14.基金项目
中国地质调查局国土资源大调查项目(1212011086039) （1212011086039）
