勉略带三岔子蛇绿岩的变质特征及构造意义

Tectonic lmplication and Metamorphism of Sanchazi Metagabbro from Mian Lue Suture Zone of Qinling Orogenic Belt

勉略带是近年来秦岭造山带研究中揭示出来的代表晚古生代小洋盆的缝合带 ,组成复杂 ,它们经历了不同的变质、变形改造 ,构成了不同构造背景、不同构造层次、不同物质组成、具不同 PTt轨迹的蛇绿构造混杂带。文中 PTt轨迹揭示 ,秦岭造山带勉略带三岔子蛇绿岩中的变质辉长岩是由上地幔先处于伸展背景下等温快速降压、后又经历缓慢降压快速降温上升侵位的。变形前的早期变质可能发生在 60～ 70 km处的上地幔 ,然后变质辉长岩快速“底辟”构造侵位 ,上升至约4 0 km深处 ,可能与俯冲的冷洋壳并置 ,发生快速降温退变反应 ;因俯冲作用的开始 ,抑制了洋盆快速扩张 ,变质辉长岩在减速伸展环境下继续抬升减压 ;后期又由于仰冲推覆 ,最终出露地表。此外 ,变质作用 PTt轨迹还反映勉略小洋盆洋壳厚度与标准大洋型洋壳厚度差异较大。

In recent years, Mian Lue suture zone is revealed to represent a small ocean during the late Paleozoic existed in the Qinling orogen. This ophiolitic tectonic melange is composed of some complex slabs from different tectonic settings and structural levels, which underwent different metamorphism with different PTt paths and reworking. The PTt path study in this paper reveals that Sanchazi metagabbro from Mian Lue Suture zone of Qinling orogenic belt underwent two metamorphic and tectonic stages. The first is isothermal and fast decompressional metamorphism and uplifting of relict gabbro in the upper mantle under the extension setting. The subsequent stage is rapid cooling and slowly decompressional metamorphism. The early stage of metamorphism happened before deformation in the upper mantle at about 60 70km. Then metagabbro emplaced rapidly at about 40 km by diapirism and perhaps amalgamated with cool subducted oceanic crust. After that, rapid cooling retrograde metamorphism occurred. Early rapid ocean spreading was constrained because subduction appeared. Therefore, metagabbro continued to uplift under the slow extension. At last, it obducted and thrusted to outcrop on the surface. In addition, the metamorphic PTt path reflects that the thickness of oceanic crust of the Mian Lue small ocean is thinner than that of the standard oceanic crust.