南大别太湖地区榴辉岩峰期变质温压条件及所揭示的构造含义

The peak p-T conditions of eclogites from Taihu area in southern Dabie and its tectonic implications

通过岩石学、矿物成分和峰期变质温压条件的研究表明,南大别变质块体由超高压带、高压/超高压“混合”带和高压带3个变质单元组成,具有区域上的温压渐变趋势。应用5个 Crt-Cpx 温度计和 Grt-Cpx-Phe 压力计,对榴辉岩的峰期变质温压条件计算显示,自北向南,可分为4个温压域,温度压力逐渐降低：760～950℃,3.5～4.1 GPa→ 660～750℃,2.7～3.1 GPa→640～780℃,2.0～2.7 GPa→550～680℃,1.5～2.3 GPa,这表明南大别变质块体在俯冲过程中是一个连续的块体。然而,超高压带和高压带之间的压力差异以及不同变质带内相邻榴辉岩之间的压力差、无序分布特征及其与围岩的构造接触关系显示,南大别变质块体经历了强烈的缩短过程,现今榴辉岩的分布特征反映的是折返至地表的状态。

On the basis of petrology, mineral composition and peak p-T conditions of the area in southern Dabie terrain, the authors hold that southern Dabie Terrain is composed eclogites from Taihu of three metamorphic zones, namely, the ultrahigh pressure zone, the high/ultrahigh pressure ‘mixed＇ zone, and the high pressure zone, with a regional p-T gradient trend. Five Grt-Cpx thermometers and Grt-Cpx-Phe barometers were used to evaluate the peak p-T conditions of these eclogites. Four p-T regions have been identified, and their metamorphic temperature and pressure decrease gradually from north to south, being 760-950℃, 3.5-4.1 GPa; 660-750℃, 2.7-3.1 GPa; 640-780℃, 2.0-2.7 GPa; and 550-680℃, 1.5-2.3 GPa respectively. These features imply that southern Dabie Terrain was a coherent terrain during the subduction. However, the existence of the pressure gaps between the UHP zone and the liP zone or among adjacent eclogites in different metamorphic zones, the chaotic spatial distribution of eclogites and their relationship with country rocks all demonstrate that southern Dabie terrain was shortened or juxtaposed during the exhumation, and that the present distribution of eclogites represent the status of exhumed eclogites.