IDENTIFICATION OF PYROXENE HORNFELS FACIES MAFIC ROCKS WITHIN THE METAMORPHIC AUREOLE OF THE SOUTH RANGE OF THE SUDBURY IGNEOUS COMPLEX, ON, CANADA

The 1.85 Ga. Sudbury Igneous Complex (SIC) is surrounded by a well-developed contact metamorphic aureole along its northern margin, but a corresponding aureole along the southern margin (South Range) has been more difficult to define, and is commonly reported to have been destroyed by younger low-grade regional metamorphism or by deformation. Although an anatexis isograd locally exists in granitoid rocks of the South Range, metamorphic isograds and facies boundaries have not been reported in the voluminous mafic rocks within the Elsie Mountain Formation, interpreted as a succession of pillowed and massive basalts, mafic and intermediate pyroclastic rocks, and associated quartz-rich metasediments. Recent mapping in the Little Stobie area indicates the local existence of relatively well preserved pyroxene hornfels facies assemblages that could allow mapping of the aureole over large distances within the voluminous mafic rocks of the South Range.

The dominant mafic lithology within the study area is a pyroxene hornfels with 0.2-0.5 cm poikiloblastic hornblende porphyroblasts. The matrix is dominated by plagioclase, granoblastic polygonal clino- and orthopyroxene, and minor amounts of magnetite. This mafic lithology can be mapped at least 800 m along strike parallel to the margin of the SIC, and the pyroxene-bearing hornfels assemblage is locally observed up to 400 m away from the SIC contact. Although initial petrographic analysis indicates the observed pyroxenes exhibits thermal metamorphic textures, it is uncertain at this stage if pyroxene formed by thermal recrystallization of pre-existing pyroxene or if it formed during hornblende-consuming dehydration reactions.

These observations suggest the South Range metamorphic aureole surrounding the SIC can be resolved despite complications due to post-metamorphic events, heterogeneities in the original footwall lithologies and the geometry of the SIC contact. Furthermore, the high temperature part of the aureole containing pyroxene hornfels facies assemblages in the South Range is thicker than in the North Range, consistent with the SIC Main Mass being thicker in the South Range. Defining the aureole geometry can provide constraints on the thermal evolution of the country rocks below the SIC and the degree of thermomechanical erosion.