http://www.sciencedirect.com/science/article/pii/S1674987114000449 |
| |
| Authors: | Oleg GSafonov Leonid YAranovich |
| |
| Institution: | 1. Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow Region, Russia ; Department of Petrology, Moscow State University, Moscow, Russia ;Department of Geology, University of Johannesburg, Johannesburg, South Africa
2. Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia ;Department of Geology, University of Johannesburg, Johannesburg, South Africa |
| |
| Abstract: | We review petrologic observations of reaction textures from high-grade rocks that suggest the passage of fluids with variable alkali activities.Development of these reaction textures is accompanied by regular compositional variations in plagioclase,pyroxenes,biotite,amphibole and garnet.The textures are interpreted in terms of exchange and net-transfer reactions controlled by the K and Na activities in the fluids.On the regional scale,these reactions operate in granitized,charnockitized.syenitized etc.shear zones within high-grade complexes.Thermodynamic calculations in simple chemical systems show that changes in mineral assemblages,including the transition from the hydrous to the anhydrous ones.may occur at constant pressure and temperature due only to variations in the H_2O and the alkali activities.A simple procedure for estimating the activity of the two major alkali oxides,K_2O and Na_2O,is implemented in the TWQ software Examples of calculations are presented for well-documented dehydration zones from South Africa.southern India.and Sri Lanka.The calculations have revealed two end-member regimes of alkalis during specific metamorphic processes:rock buffered,which is characteristic for the precursor rocks containing two feldspars,and fluid-buffered for the precursor rocks without K-feldspar.The observed reaction textures and the results of thermodynamic modeling are compared with the results of available experimental studies on the interaction of the alkali chloride and carbonate-bearing fluids with metamorphic rocks at mid-crustal conditions.The experiments show the complex effect of alkali activities in the fluid phase on the mineral assemblages.Both thermodynamic calculations and experiments closely reproduce paragenetic relations theoretically predicted by D.S.Korzhinskii in the1940 s. |
| |
| Keywords: | High-grade metamorphism Alkali activity Fluids Reaction textures Fluid-mineral reactions Thermodynamic modeling |
| 本文献已被 CNKI 维普 万方数据 ScienceDirect 等数据库收录！ |
| 点击此处可从《地学前缘(英文版)》浏览原始摘要信息 |
| 点击此处可从《地学前缘(英文版)》下载免费的PDF全文 |
|
