Isograds of kyanite, chloritoid, corundum, and high-alumina equilibria in greenschist metamorphism (Patom highlands)

Abstract

In the greenschist facies of the Patom highlands, three grades of equilibrium, based on the parageneses of high-alumina rocks, have been recognized within the biotite subfacies. Under conditions of the lowest temperature pyrophyllite-chlorite grade, chloritoid is still not stable, but instead, we have pyrophyllite with a chlorite with any iron index. The boundary with the next pyrophyllitechlorite-chloritoid grade has been plotted on the appearance of early, almost purely iron chloritoids; with increase in temperature, their magnesium content increases. The breakdown of diaspore and the formation of corundum take place in the upper portions of this grade. Dehydration of pyrophyllite and crystallization of kyanite denotes the transition to the third, kyanite-chloritoid grade of biotite subfacies where the iron index of chloritoid reaches 70%. Under conditions of the almandine-chlorite-chloritoid subfacies, in the critical paragenesis, kyanite + chloritoid + chlorite, the iron index of the chloritoid is 73%...