大别山榴辉岩富流体退变质阶段的白色云母

大别山榴辉岩的退变质过程可分为贫流体、弱流体和富流体3个阶段。贫流体阶段的主要作用是榴辉岩造岩矿物的重结晶、同质多象转变和固溶体出溶;弱流体阶段有少量流体参与,并且其压力、温度较高,形成的白色云母主要为多硅白云母;富流体阶段在大量流体参与下,形成大量的低压含水矿物,其中包括多种白色云母。富流体阶段早期的白色云母主要为多硅白云母,其Si原子数介于6．5pfu和7．2pfu之间;晚期白色云母成分复杂,包括钠云母、白云母和珍珠云母。黑云母是退变质作用最晚期形成的云母。退变质云母具环带结构,其内带为钠云母,中带为白云母和珍珠云母的交生体,外带为黑云母。

White micas formed at the fluid-rich retrograde stage of the Dabieshan eclogites

The retrograde metamorphism of eclogites in Dabieshan area can be divided into fluid_poor, fluid_bearing and fluid_rich stages. Recrystallization, polymorphous conversion and exsolution of rock_forming minerals of the eclogites seem to have been the main processes of the fluid_poor stage. The fluid_bearing stage proceeded at high temperatures and pressures, and is characterized by the participation of minor fluids. White mica formed at this stage is dominated by phengite. Due to the participation of large quantities of fluids, the fluid_rich stage is characterized by the formation of various low_pressure and hydrous minerals including a variety of white micas. White mica formed at earlier fluid_rich stage is dominantly phengite, with atom number of Si varying between 6.5 and 7.2 pfu. White mica of the late fluid_rich stage is markedly variable in composition, consisting of paragonite, muscovite and margarite. Biotite is the last kind of mica formed in the process of diaphthoresis. Also studied in this paper is a zoned structure of retrograde mica, which consists of an inner zone of paragonite, an intermediate zone of muscovite_margarite intergrowths and an outer zone of biotite.