The occurrence of paragonite in chloritoid schists from Stonehaven, Scotland

Abstract

SummaryParagonite has been found in two chloritoid-chlorite-mica schists from the Dalradian, north of Stonehaven. The principal bulk-chemical constraints on the occurrence of this mineral within the chloritoid zone appear to be the ratios of soda to potash and the Al2O3 contents of the rock. The composition of paragonite coexisting with muscovite indicates a crystallization temperature of 550 °C for rocks some way above the chloritoid isograd. Although this result is supported by other mineral equilibria, the muscovite compositions suggest a temperature of 690 °C which is too high because the mineral assemblage chloritoid-paragonite is unstable above about 600 °C By combining data on the breakdown of the chloritoid-paragonite assemblage with a mineral equilibrium for the assemblage garnet-plagioclase-muscovite-biotite, it has been deduced that rocks close to the staurolite isograd crystallized at about 7·5 kb and 600 °C.