晓天—磨子潭断裂糜棱岩化过程的温压环境及其指示意义

THE p-t CONDITION AND ITS INDICATION OF MYLONITIZATION IN THE XIAOTIAN-MOZITAN FAULT ZONE

晓天—磨子潭断裂带是大别造山带北部一条重要的断裂。探针分析表明，断裂带内糜棱岩中基质白云母每单位分子平均硅原子数大于3．3p．f．u.，为典型的高压多硅白云母。利用Massonne多硅白云母压力计获得基质多硅白云母指示的平均压力值为1．0GPa，由此计算断裂带变形深度为35km左右。矿物变形记录的糜棱岩变形温度在600-700℃之间，对应的地温梯度为16．0～22．5℃／km。认为晓天--磨子潭断裂记录了高压-超高压岩石下地壳的折返事件，为同造山折返边界。糜棱岩中残斑白云母的探针分析结果与基质白云母相似，图解计算的压力与基质白云母相当，指示了变形的压力环境为残斑白云母生长的压力环境的极小值，推测糜棱岩原岩很可能经历了糜棱岩化之前的更高压力环境的变质作用。

The Xiaotian-Mozitan fault zone （XMF） is a very important fault in the northern part of the Dabie Moun- tain. The compositional analysis of muscovite in mylonites of XMF was carried out by means of an electronic microprobe. The Si atom of the matrix muscovite is more than 3.3 （p. f. u） on average, which shows these muscovites are classic phengites. And the data show that the growing pressure of these phengites was 1.0 GPa on average as determined by Massonne＇s geobarometer and these phengites were formed at the depth of about 35 km These results showed that the deformation temperature of mylonites is 600--700℃ as revealed by the deformation characteristics of quartz and feldspar and the geothermal gradient is 16. 0 -- 22. 5 ℃/km. All of these suggest that the XMF recorded the information about HP-UHP rocks exhumation as a synorogenic exhumation boundary. Additionally, the por-phyroclastic muscovite is similar to the matrix muscovite in Si atom, which indicates that 1.0 GPa is the minimum pressure during the formation of porphyroclastic muscovite, suggesting the protolith of the mylonites had possibUy experienced higher-degree metamorphism.