西藏中北部双湖地区蓝片岩带及其构造涵义  

The Blueschist Belt in the Shuanghu Region, Central-Northern Tibet and Its Tectonic Implications

作　　者：鲍佩声[1] 肖序常[1] 王军[1] 李才[2] 胡克[2] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]长春科技大学地球科学院,130061

出　　处：《地质学报》1999年第4期302-314,T001,共14页Acta Geologica Sinica

基　　金：地质矿产部"八五"重点基础研究项目;地质矿产部"九五"重点基础研究项目(编号850220和950110)的部分成果

摘　　要：西藏中北部的蓝片岩呈岩块断续出露长达300 km.东起西亚尔岗双湖及其以西的恰格勒拉,经纳若、角木日、冈塘错、果干加年山、玛依岗日,西至冈玛错一带。除恰格勒拉及果干加年山的蓝片岩早年有简要报导外,其余均为李才等近年的新发现。蓝片岩的原岩主要为冈瓦纳大陆北缘晚石炭世-早二叠世裂谷型(裂陷槽)沉积-火山岩系,包括基性火山岩(或火山碎屑岩)、泥质砂岩、泥质灰岩等,尚有少量基性煌斑岩。高压低温变质矿物主要为青铝闪石,次为镁钠闪石,少量蓝透闪石。缺失典型蓝片岩相的硬玉和硬柱石。根据钠质角闪石类矿物成分所估算的变质压力为0.67～0.75 GPa,变质温度为410～460 C,视为蓝片岩与绿片岩之间的过渡相更为确切。 蓝片岩典型的矿物组合为:(1)绿泥石+绿帘石+青铝闪石+钠长石;(2)绢云母+青铝闪石(±镁钠闪石)-蓝透闪石-斜长石+石英;(3)方解石+青铝闪石-绿帘石:(4)绿泥石-透闪石+蓝透闪石(±单斜辉石残晶)。 蓝闪石的^(46)Ar-^(48)Ar年龄值为222.5±3.7 Ma,故其变质年龄为晚古生代-早中生代。高压变质带及其伴随的“蛇绿混杂岩”很可能是冈瓦纳古陆北缘边缘海内裂陷槽或“初始有限洋盆”从离解到汇聚消亡的地质记录,龙木错-冈玛错-双湖-丁青-澜沧江一带作为冈瓦纳古陆与欧亚古陆分界地带.基本是可取的。The blueschist blocks extend discontinuously over 300 km in central-northern Tibet from the Qiagelela town, 13 km west of Shuanghu, Xiyaergang, in the east Tibet, through Nare, Gacuo, Jiaomuri, Gangtangcuo, Guoganjianian Mt. , Mayigangri to Gangmacuo-Longmucuo in the west. The blueschists are newly-discovered by Li Cai et al. in recenr years, except those in Qiagelela and Guogan-jianian Mt. There are also some associated 'Ophiolite melanges'.The protoliths of the blueschist are mainly of the Late Carboniferous rifted sedimentary-volcanic rock series at the northern margin of the Gondwana land, including basic volcanic rocks (or pyroclastic rocks), argillaceous sandstone, argillaceous limestone etc. , and minor basic lampro-phy.The high pressure-low temperature minerals mainly consist of crossite, secondly of magne-sio-riebeckite, with minor winchite. The estimated metamorphic pressure of the blueschist varies from 0. 67-0. 75 GPa and the metamorphic temperature from 410-460c corresponding to the transitional facies between blueschist and green schist.The typical mineral assemblages are as follows; (1) epidote+chlorite+crossite+albite; (2) sericite+chlorite+magnesio-riebeckite (±winchite) + feldspar + quartz ; (3) calcite+crossite + epidote; and (4) chlorite + actinolite+winchite (iclinopyroxene relict).The 40Ar/39Ar isotope age of crossite is 222. 5± 3. 7 Ma, revealing its metamorphic age as between the Late Paleozoic and the Early Mesozoic.The high-pressure metamorphic zone, with its associated 'ophiolite melange', may very well be a geologic record from its divergence to convergence and consumption of a rift trough or 'initial limited oceanic basin' within the marginal sea at the northern margin of the Gondwana land. It is reasonable to take the Longmucuo-Gangmacuo-Shuanghu-Dengqen Lancang River belt as a boundary between the Gondwana land and the Eurasian land.

关 键 词：蓝片岩 构造涵义 双湖地区 藏北 西藏 

分 类 号：P588.344[天文地球—岩石学]