CO2-RICH FLUID INCLUSIONS IN THE WHITESTONE ANORTHOSITE - IMPLICATIONS FOR THE RETROGRADE HISTORY OF THE PARRY SOUND SHEAR ZONE, GRENVILLE PROVINCE, CANADA
Academic Article
Overview
Research
Identity
Additional Document Info
Other
View All
Overview
abstract
The Whitestone Anorthosite (WSA), located in southern Ontario, underwent granulite facies metamorphism during the Grenville orogeny at 1.16 Ga. During the waning stages of metamorphism fluids infiltrated the outer portions of the anorthosite and promoted the formation of an envelope comprised of upper amphibolite facies mineral assemblages. Also, this envelope corresponds to portions of the anorthosite that underwent deformation related to movement along a highgrade ductile shear zone. Samples from this portion of the anorthosite (the margin) contain CO2rich inclusions in plagioclase porphyroclasts (relict igneous phenocrysts), matrix plagioclase and garnet. These inclusions have features which normally are interpreted as indicating that they are texturally primary, but they have relatively low CO2 densities (0.610.95 g cm3). Plagioclase from the anorthosite interior contains texturally secondary inclusions with relatively high CO2 densities (generally from 0.99 to 1.10 g cm3). The high CO2 densities suggest that the inclusions in the plagioclase of the anorthosite core formed prior to inclusions in porphyroclast minerals of the outer portions of the anorthosite, an interpretation that is apparently inconsistent with inclusion textures. This apparent paradox indicates that most fluid inclusions from the anorthosite margin were formed during, or were modified by, the dynamic recrystallization that affected this portion of the WSA. In either case, late formation or modification, the texturally primary fluid inclusions do not contain pristine samples of the peak metamorphic fluid. Furthermore, because shearrelated deformation is apparently associated with entrapment of the lowest fluid densities, some strain localization persisted to relatively low temperatures (e.g. less than approximately 500 C). These results constrain a part of the retrograde PT path for this portion of the Grenville Orogen to temperatures of approximately 400500 C at pressures of approximately 12 kbar.
