TEXTURE AND COMPOSITION OF GARNET AS A GUIDE TO CONTAMINATION OF GRANITOID PLUTONS: AN EXAMPLE FROM THE GOVERNOR LAKE AREA, MEGUMA TERRANE, NOVA SCOTIA

摘要

The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps_(19–74)) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps_(22–71)) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions (e.g., sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps_(21–52)), whereas type-3B crystals have a Mn-rich rim (Sps_(15–21)). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions (e.g., apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps_(4–11)) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps_(8–16)) was partially resorbed and overgrown by a Mn-rich rim (Sps_(11–26)). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.