GARNETIFEROUS CORONITE IN ARCHEAN GABBRO FROM THE YENISEY RIDGE

Abstract

Outcrops on the left bank of the Yenisey River expose banded gabbro, gabbro-norite and norite in the western part of this pluton near its floor. In isolated places these rocks were converted by high-pressure metamorphism to coronite. Calculation made using real mineral compositions show that the coronite was formed chiefly from pyroxenes and plagioclase, aluminum being derived preferentially from plagioclase and magnesium, iron and manganese from pyroxenes. An excess of silicon led to the growth of quartz. Biotite participating in the reaction was broken down into orthoclase and ilmenite. Material balance during the reaction was reached for all rock-forming components except sodium and water. Although the basicity of plagioclase decreased slightly during metamorphism, sodium was undoubtedly removed from the system for the most part. If sodium was not disseminated into the surrounding plagioclase grains, the reaction which formed the coronite can most likely be regarded as the source of an alkaline fluid, migrating from deep crustal levels in regional metamorphism.