三江造山带兰坪盆地东缘火山岩锆石U-Pb年代学、Hf同位素组成

青藏高原东南缘"三江"复合造山带南、北两段发育若干火山-岩浆岩带,随着火山岩年代学、地球化学数据的积累,其时空关系渐趋清晰。但由于中-新生界沉积覆盖,这些火山岩带向南常终止于兰坪盆地附近,致使火山岩时空构架不明。兰坪盆地东部弥沙地区出露盆地基底岩石,由两个构造层组成,上部为碎屑岩及生物碎屑灰岩,下部出现大量火山碎屑物质。含火山岩段出露厚约150m,可分为三个喷发-沉积韵律,由流纹质熔结凝灰岩、晶屑岩屑熔结凝灰岩组成的火山碎屑岩与紫红、灰黑色泥岩、大理岩组成的沉积岩相间组成。火山岩锆石LA-ICP-MS U-Pb数据表明,岩浆活动始于或早于258Ma,一直持续到248Ma。锆石原位Lu-Hf分析结果显示,早期岩浆包含较多幔源物质,而晚期岩浆则几乎完全由古老地壳局部熔融形成。本文提供的新数据表明,兰坪盆地东缘火山岩与三江造山带其它地区古特提斯洋俯冲阶段火山岩具有一致的年代学组成、相同的Hf同位素变化规律,可能属于发育在扬子板块西缘之上江达-维西-云县弧火山岩带。

U-Pb geochronology, Hf isotopes of zircons from the volcanic rocks along the eastern margin of Lanping basin, Sanjiang orogenic belt.

Several volcanic belts developed in the Sanjiang orogenic belt, southeast of Tibetan Plateau. Recently published new geochronological and geochemical data gradually revealed the spatial and temporal relationship among these belts. However, these belts are commonly covered by Mesozoic to Cenozoic sediments of the Lanping basin, which have led to some erroneous tectonic interpretations. Basement rocks expose very well in Misha regions, eastern margin of the Lanping basin. The basement consists of two tectono-stratigraphic units:an upper one consisting of Late Triassic fine-grained clastic and bioclastic and a lower volcanic one. The lower one has a thickness of ca. 150m, which is made of three volcanics-sediments sequences. Each sequence is composed of ignimbrite and crystal-lithic tuff in lower and reddish or dark shale and limestone in upper. LA-ICP-MS U-Pb results of zircon from the volcanic rocks demonstrated that the volcanism started at least since 258Ma, and continued to 248Ma. In-situ zircon Lu-Hf isotopic data revealed a significant contribution of mantle-derived material to the earlier magmatic rocks, while the latter stage magmatic rocks seem to have completely resulted from partial melting of Precambrian crustal rocks. These new data indicate that the volcanic rocks along the eastern margin of the Lanping basin have the same geochronological framework and identical isotopic variation trend as other Paleo-Tethyan volcanics in the Sanjiang belt, suggesting they all form an entire continental arc belt developed along the western margin of Cathaysian affinity block(s).