拉萨地块松多高压变质带不同类型榴辉岩的变质演化过程及其限定方法探讨

Metamorphic processes of various eclogites from Sumdo high pressure metamorphic belt of Lhasa block and the discussion of its calculation methods

拉萨地块中东部松多高压变质带是揭示拉萨地体形成与演化过程的重要研究对象。松多变质带记录了古特提斯洋的俯冲和闭合过程。前人对松多地区出露的榴辉岩及围岩开展了大量的岩石学工作,但变质峰期温压条件没有得到很好的限定,温压分布范围较广,且变质演化过程仍存争议。笔者等总结了松多变质带不同地区榴辉岩的岩石学和矿物学特征,汇总了不同计算方法得到的温压条件。通过对比发现,松多高压变质带内榴辉岩的峰期温压条件处于465~880℃,2.5~3.9 GPa的范围,其宽泛的峰期温压条件是由于不同计算方法和折返机制造成的。与传统矿物对温压计相比,变质相平衡模拟方法更适合低温榴辉岩的峰期温压条件及变质过程的限定。

The Sumdo high pressure metamorphic belt in the central and eastern part of Lhasa block is an important place for revealing the formation and evolution of Lhasa block.The Sumdo metamorphic belt records processes for the subduction and closure of the Paleo-Tethys Ocean.Numerous studies on eclogites and the country rocks have been carried out in the Sumdo metamorphic belt,but the peak P—T conditions are not well constrained and the metamorphic evolution is still controversial.In this contribution,we summarize the petrological characteristics of eclogites in different areas in Sumdo and the peak P—T conditions on the(U)HPM using different methods.The broad peak P—T conditions of the Sumdo high pressure metamorphic belt in the range of 465~880℃and 2.5~3.9 GPa can be explained by different calculation methods and exhumation mechanisms.Our study shows that the phase equilibrium calculation is more suitable to constrain the metamorphic process of the low-temperature eclogites compared with conventional geothermobarometry.