滇西古特提斯俯冲-碰撞过程的岩浆作用记录

Triassic magmatism in the southern Lancangjiang zone,southwestern China and its constraints on the tectonic evolution of Paleo-Tethys

三叠纪岩浆作用广泛发育于澜沧江南带,锆石U-Pb年代学结果及岩石组合特征显示:(1)该区残留有少量的早三叠世火山岩(约248Ma),主要为一套薄层的以安山岩为主的火山岩;(2)中三叠世以大规模的酸性岩浆作用为主,部分地段的中部夹有少量的玄武岩,其中晚期的流纹岩与该区出露巨大的临沧花岗岩基主体具有一致的形成时代(约230Ma);(3)晚三叠世则以基性火山岩为主,夹少量流纹岩为特征的"双峰式"火山岩组合。元素地球化学特征表明,早三叠世火山岩具有典型的岛弧火山岩的特征,暗示其很可能形成于俯冲的构造环境;中三叠世晚期的流纹岩与临沧花岗岩基主体具有非常相似的地球化学特征,结合其流纹岩具有A型花岗岩属性,表明该区中三叠世火成岩形成于碰撞后的构造背景;而晚三叠世基性火山岩虽然在地球化学特征上显示出与岛弧火山岩相似的高铝和Nb-Ta的亏损,但其中部分火山岩具有高镁和板内玄武岩的微量元素比值特征,说明这些火山岩可能形成于碰撞后裂谷(post-collisional rifting)环境。结合区域上前人的研究资料,我们认为西南三江地区古特提斯主洋盆的俯冲闭合可能直到早三叠世晚期才完成,其后的陆-陆/弧碰撞作用可能发生在中三叠世早期,中三叠世中晚期该区进入碰撞后演化阶段,晚三叠世开始处于碰撞后的裂谷阶段,古特提斯在该区的演化最终结束于三叠纪末。

Triassic magmatism widely developed in the southern Lancangjiang zone,southwestern China.The study of their precise U-Pb geochronology and characteristics of rock associations shows that（1）small amount of Early-Triassic（-248 Ma）volcanic rocks were preserved and cropped out in the area,which consist mainly of thin bed of andesitic rocks intercalated with minor basaltic andesites;that（2）the Middle-Triassic magmatism was characterized by extensive felsic igneous rocks with minor basalts interbedded in the middle of volcanic sections in part of areas,and the late Middle-Triassic rhyolites and main body of Lincang granitic batholith have the same extrusive/intrusive time with the age of ~230 Ma;that（3）the Late-Triassic volcanic sequences are principally composed of basalts with minor interlayered rhyolites,exhibiting typical characteristics of bimodal volcanics.In addition,the features of elemental geochemistry of all samples suggest that（1）the Early-Triassic andesitic rocks,characterized by characteristic natures of island arc volcanics,likely erupted in the subduction setting;that（2）the generation of Late-Triassic rhyolites with an affinity of A-like granite and synchronously main body granites in the Lincang batholith displaying similar geochemical features,is closely related with the post-collision or late-collision setting;that（3）although the Late-Triassic basic rocks partly show island arc volcanics such as high-Al contents and Nb-Ta depletion,some of them possessing high-Mg contents and mostly having trace-element ratios similar to intraplate basalt,which indicates that they erupted in the post-collision rift environment.Integrated with previous data in the study area,we conclude that the subduction and closing of main Paleo-Tethys Ocean（Changning-Menglian Ocean）in the Sanjiang area of southwestern China probably had completed until the late Early-Triassic,and the succeeding continent-continent/arc collision and post-collision rifting developed in the Middle-Triassic and Late-Triassic,respectively,and finally the evolution of Paleo-Tethys finished in the end of the Late-Triassic.