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INTERSECTING ISOGRADS IN REGIONALLY METAMORPHOSED SILICEOUS DOLOMITIC MARBLE OF THE SHARBOT LAKE DOMAIN, GRENVILLE PROVINCE, SOUTHEASTERN ONTARIO

Jo-Anne S. Goodwin-Bell

Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada

E-mail address: jsgoodwinbell{at}gmail.com

The aim of this paper is to describe the mineralogy and petrology of siliceous, dolomitic marbles in the Sharbot Lake domain, as exposed in the Almonte – Carleton Place area of Ontario. Five isograds based on specific metamorphic reactions were mapped in the area. The delineation of the isograds is based on detailed mapping, petrographic analysis of assemblages of coexisting minerals, and a polybaric T–X(CO₂) diagram, calculated using thermobarometric data from associated gneissic rocks. The T–X section is based on a field gradient of ~47°C/km. In order of increasing metamorphic grade, the isograds are:

Mineral assemblages above and below each isograd are described, and relevant examples are shown. These isograds intersect in a manner that can be explained in terms of a fluid gradient in a mixed-volatile fluid (an example of fluid infiltration). Evidence for internal buffering of the mixed-volatile fluid is also presented. The locations of two isobaric invariant points in the study area have been identified by the intersection of the isograds and by determining the bracketing assemblages from above and below the invariant point. Data from this study suggest that the Sharbot Lake domain marble and associated rock in the area underwent regional metamorphism consistent with the greenschist- to amphibolite-facies conditions.

Keywords: marble, field gradient, isograd, T, X(CO₂) diagram, fluid composition, fluid gradient, internal buffering, greenschist to amphibolite facies, Sharbot Lake domain, Grenville Province, Ontario.