滇西兰坪盆地西缘火山岩LA-ICP-MS锆石U-Pb年代学、地球化学特征及其大地构造意义

LA-ICP-MS zircon U-Pb geochronology and geochemistry of volcanic rocks on the western margin of Lanping Basin in western Yunnan and their tectonic implications

青藏高原东南缘"三江"复合造山成矿带南、北两段发育若干火成岩带,其间的时空关系并不清晰,原因在于位于构造带中段的兰坪盆地内火山岩缺乏年代学、地球化学制约。兰坪盆地西侧火山岩带由下至上由安山质晶屑凝灰岩、安山质集块-块状安山岩、玄武质熔岩组成,上部玄武质熔岩夹紫红色泥岩,并被紫红色泥岩覆盖。LAICP-MS锆石U-Pb测年结果表明,火山岩下部的晶屑凝灰岩形成时代为238 Ma,中段安山岩喷发时间为233 Ma,上段玄武质熔岩形成时代为229 Ma。这套火山岩属于钙碱性-高钾钙碱性系列,具有高Al2O3和低MnO、P2O5和TiO2的特征,具有轻稀土元素富集的右倾型配分模式,无明显Eu异常。安山岩相对富集La、U等大离子亲石元素,Nb、Ta等高场强元素明显亏损,具有俯冲带弧火山岩典型特征,而玄武质熔岩具有富集型洋中脊玄武岩地球化学特征。结合其野外产状(覆于石炭纪陆缘碎屑岩之上)、地球化学特点及其随时间变化规律,认为这套岩石形成于陆缘弧环境,而晚期岩浆活动可能与俯冲板片断离、拆沉过程有关。兰坪盆地西缘火山岩与造山带内其他同时代火成岩一起组成一条近连续的大型岩浆岩带,带内绝大部分岩浆岩具有弧岩浆岩构造属性,部分显示板内岩浆岩地球化学特征。这条岩浆岩带可与具同等规模的龙木错-双湖(-澜沧江)-昌宁-孟连缝合带对应,推测其为该缝合带所代表的古特提斯主洋盆向由西(南)向东、北(东)方向俯冲过程的产物。

A curved belt consisting of terranes or blocks and associated Late Paleozoic-Early Mesozoic sutures features the southeastern part of the Tibetan Plateau.It is the Sanjiang （Three-Rivers） multiple orogenic and oreforming belt.In spite of numerous studies,its Paleotethyan （Late Paleozoic to Early Mesozoic） tectonic evolution remains a topic of much controversy.Recently published geochronological and geochemical data derived from modern techniques have revealed several magmatic zones with variable tectonic affinity.However,the temporal and spatial distribution pattern of the magmatic rocks didn＇t appear,mainly due to the absence of high quality data from some regions.As the Lanping Basin is located in the middle portion of the Sanjiang orogenic belt,the time and tectonic affinity of the volcanic rocks in this basin play a significant role in revealing the connection between the magmatic zones in the southern and northern segments of the Sanjiang belt.This study focused on the volcanic rocks exposed on the western margin of the Lanping Basin where high-quality geochronological and geochemical data are rare.The volcanic rocks in western Lanping Basin consist of andesitic tuff in the lower segment,andesite in the middle segment,and basaltic lava at the top.This volcanic sequence is developed upon the Carboniferous terrigenous clastics.The basaltic lavas are interlayered with reddish mudstone.The content of mudstone increases upwards.Geochemical data demonstrate that these volcanic rocks are of calc-alkaline or highK calc-alkaline series,characterized by high Al2O3 and very low TiO2,MnO2,and P2O5 content.The andesitic samples exhibit arc-like chondrite-or primitive-mantle-normalized trace element pattern,whereas the basaltic lava displays enriched-middle-oceanic-ridge-basalt pattern.These features,combined with the field relationship,suggest that these volcanic rocks were formed in a continental arc setting.New LA-ICP-MS zircon U/Pb dating results indicate that the bottom crystal tuff was formed at 238 Ma,the andesite of the middle segment at 233 Ma,and the basaltic lava at the top of the volcanic sequence erupted at 229 Ma.These new data suggest that,at the end of the subduction,the breakup of the subducted slab might have taken place which led to the uprising of asthenosphere and induced partial melting of the enriched mantle wedge.The new data obtained by the authors have filled the spatial gap of geochronological data in the Sanjiang orogenic belt.As a result,a large scale （with a length 〉2 000 km） successive arc-like volcanic belt along the entire Sanjiang belt has become clear.Such a large arc-volcanic belt should be affiliated to a large-scale subduction zone （suture）.Synthesizing available geological,geochemical,and geochronological data,the authors hold that the eastward or northeastward subduction of the Paleotethys along the Longmu Co-Shuanghu（-Lancangjiang）-Changning-Menglian suture probably resulted in the formation of the large-scale volcanic belt.