A comparative study of wollastonite skarn genesis in the Central Metasedimentary Belt, southeastern Ontario, Canada

Abstract

Potentially economic wollastonite-rich skarns are widespread in the Central Metasedimentary Belt of southeastern Ontario, Canada. Their genesis is evaluated through a comparative study of: the Platinova skarn (ca. 4 Mt grading 35% wollastonite), hosted by greenschist facies marble in the Elzevir terrane; the Olden skarn (ca. 2.8 Mt; 35%) in amphibolite facies marble of the Sharbot Lake terrane; and the St. Lawrence skarn (ca. 9 Mt; 41%) in granulite facies strata of the Frontenac terrane. The protolith for all of the wollastonite skarns was calcitic marble, locally containing impurities, but dolomitic marble may have been present at the St. Lawrence skarn. Each skarn is contiguous with an intrusive complex ranging from gabbro to granite in composition, and displaying megascopic fabrics indicative of magma commingling and mixing. The plutons are, however, of different ages and exhibit different geochemical characteristics. In each skarn, wollastonite is associated with varying proportions of diopside–hedenbergite and grossular–andradite. Skarn development is ascribed to the incursion of magmatogene, silica-rich, CO 2-poor ( X CO 2 between 0.1 and 0.3) fluids, rather than to regional metamorphism, and pressures varying between 200 MPa at Platinova and 350 MPa at Olden, at temperatures ca. of 500 to 650 °C. The Mountain Grove pluton has an U–Pb (zircon) age of 1153 ± 2 Ma. Hornblende and biotite from the same sample gave 40Ar– 39Ar plateau dates of 1058 ± 14 and 1047 ± 4 Ma, respectively, and the Olden skarn has been dated at 1074 ± 5 Ma ( 40Ar– 39Ar, phlogopite). These 40Ar– 39Ar dates are interpreted to record resetting by the contiguous 1070 Ma McLean pluton. U–Pb on titanite and 40Ar– 39Ar on amphibole separates from the St. Lawrence skarn yield 1147 ± ca. 8 and 1159 ± 6 Ma dates, respectively, similar to the U–Pb badelleyite age of the adjacent 1167 ± 2 Ma Leo Lake pluton. The age similarities between the skarns and the adjacent plutonic rocks are permissive evidence for a genetic link. Evidence of magma mixing and commingling in all the plutons contiguous with the wollastonite skarns may indicate that the concentration and egress of magmatogene brines at pressures locally attending 400 MPa was facilitated by thermal and compositional perturbation in the parental magma chambers.