西南三江德钦-维西地区中二叠-晚三叠世岩浆岩与古特提斯演化

Middle Permian-Late Triassic magmatism in the Deqen-Weixi area of the Sanjiang Orogenic Belt:Implications for Paleo-tethyan evolution

西南三江的德钦-维西地区出露有大量中二叠-晚三叠世岩浆岩,但是它们的形成与两个古特提斯分支洋(即金沙江洋和昌宁-孟连洋)的关系存在很大争议,制约了我们对该地区古特提斯构造演化的理解。本文对得荣县附近的玄武岩和流纹岩、兰坪县附近的安山岩以及德钦县附近的沉火山角砾岩进行了系统的野外地质调查、岩石学和地球化学研究,目的是查明它们的成因和形成环境,为深入探讨古特提斯构造演化提供有力的制约。LA-ICP-MS锆石U-Pb定年结果表明,得荣玄武岩(约236Ma)、兰坪安山岩(约233Ma)以及德钦沉火山角砾岩(约232Ma)均形成于晚三叠世,而得荣流纹岩则形成于中二叠世(约261Ma)。地球化学研究表明,得荣玄武岩多属于拉斑质玄武岩,具有变化的TiO_(2)和MgO含量,缺乏明显的轻重稀土元素分馏,以不同程度亏损Nb、Ta和Ti为特征,它们可能是大陆弧环境下尖晶石稳定域内"肥沃"岩石圈地幔较高程度部分熔融的产物,并经历了不同程度的结晶分异和地壳混染;兰坪安山岩具有较高的MgO含量和Mg#值,属于高镁安山岩,其微量元素组成类似于陆壳物质,应该形成于沉积物起源熔体与地幔楔岩石之间的相互作用;得荣流纹岩以高SiO_(2)和低TiO_(2)含量为特征,具有较高的锆饱和温度,其地球化学组成类似于典型的高分异A型花岗岩,起源于伸展环境下地幔物质上涌导致的地壳岩石部分熔融。结合相关区域地质资料,我们提出中二叠-中三叠世时期昌宁-孟连古特提斯洋向东(北)俯冲使得扬子地块与昌都-兰坪-思茅地块之间以弧后伸展(即金沙江弧后盆地)的方式打开,而晚三叠世时期昌宁-孟连洋闭合后的陆-陆碰撞(即保山地块和昌都-兰坪-思茅地块)则导致金沙江弧后盆地的闭合。

The Middle Permia-Late Triassic magmatic rocks are widespread in the Deqen-Weixi area of the Sanjiang Orogenic Belt.However,the relation between their petrogenesis and the two branches of the Paleo-Tethys Ocean remains the subject of intensive debate,which constrains our understanding of the Paleo-Tethyan evolution.Based on field investigation,the petrological study,zircon U-Pb isotopic dating,and major and trace element analyses are carried out upon the basalts and rhyolites near the Derong County,and andesites near the Lanping County and sedimentary volcanic breccia near the Deqen County to investigate their petrogenesis and tectonic settings,and further provide insights into the Paleo-tethyan evolution.The LA-ICP-MS zircon U-Pb analyses yield Late Triassic eruption ages for the basalts(~236 Ma),the andesites(~233 Ma)and the sedimentary volcanic breccia(~232 Ma),and a Middle Permian age of~261 Ma for the rhyolites.The Derong basalts are tholeiitic,with variable TiO_(2) and MgO contents,and low LREE/HREE ratios,and are characterized by varying degrees of negative Nb,Ta and Ti anomalies.They are inferred to be derived from high degree partial melting of a relatively fertile lithospheric mantle in the spinel stability field within a continental arc,with some fractional crystallization and crustal assimilation.The Lanping andesite has geochemical features of high-Mg andesites(high MgO content and Mg#value)with trace elements similar to continental crustal material and may have been generated by interaction between sedimentderived melts and a mantle wedge.The Derong rhyolite is characterized by high SiO_(2) and low TiO_(2) contents,and high Zr saturation temperatures,with geochemical features similar to fractionated A-type granites,and it was produced by partial melting of crustal material at high temperatures and low pressures,corresponding to an extensional setting with mantle upwelling.These data,in combination with previous studies on these Middle Permian-Late Triassic magmatic rocks and palaeogeographic reconstruction of Paleotethyan realm,allow us to discuss the tectonic evolution of the Paleo-Tethys Ocean described as belows:(1)During the Middle Permian-Middle Triassic,the eastward or northwestward subduction of the Changning-Menglian Paleo-Tethys Ocean(CMPTO)opened the Jinshajiang back-arc basin between the South China Block(SCB)and the Changdu-Lanping-Simao Block(CLSB);(2)During the Late Triassic,the continent-continent collision between the SCB and the CLSB following the closure of the CMPTO resulted in the westward or southwestward subduction of the Jinshajiang back-arc basin.