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M. Sc. Thesis

Contamination of Plutons by Manganiferous Country Rock in the Governor Lake Area, North-Central Meguma Terrane, Nova Scotia

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The Governor Lake area, north-central Meguma terrane, consists mainly of ca. 373 Ma granitoid rocks with surrounding metasedimentary rocks. Garnet is abundant in contact-metamorphosed Beaverbank Foramtion and its enclaves in the granitoid rocks, as well as in granitic plutons, where it is present as crystals up to 3 cm in diameter. From garnetiferous samples, it was noticed that garnet is present in country rock, coherent and variably disaggregated xenoliths, and granitic samples. Based on petrographic examination and electron microprobe analyses, four different types and six subtypes of garnet have been identified. Type 1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps19-74) grains that are concentrated in irregular to planar aggregates. Crystals range from homogeneous (Type 1A) to zoned towards Mn-enriched rims (Type 1B). Type 2 garnet in the country rocks forms small, spessartine-rich (Sps22-71) grains that are zoned from Mn-rich cores to Mn-poor rims. Type 3 garnet contains abundant metamorphic inclusions (e.g., sillimanite) and commonly has distinct inclusion-rich cores with inclusion-poor, euhedral rims. Type 3A has Mn-rich cores (Sps21-52) while Type 3B has Mn-rich rims (Sps15-21). Type 4 garnet grains are large, subhedral to euhedral crystals with abundant inclusions of apatite, ilmenite, and plagioclase interpretared to be of igneous origin. Type 4A is spessartine-poor (Sps4-11) and weakly zoned, with local Mn-enrichment in rims adjacent to retrograde biotite. Zoning in Type 4B garnet suggest that Mn-poor cores (Sps8-16) were partially resorbed and overgrown by Mn-rich rims (Sps11-26). Based on the combination of textural and compositional data, Types 1A, 1B, 2, and 3B are interpreted to be metamorphic/xenocrystic, Type 3A to have xenocrystic cores overgrown by magmatic rims, and Types 4A and 4B to be magmatic. Whole-rock geochemistry shows slight MnO- and Fe2O3T-enrichment in samples containing garnet and/or xenolithic material. The presence of xenocrystic garnet in granitic samples and supporting geochemistry suggest that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous country rocks, probably by incorporation and assimilation of xenolithic material derived from the Beaverbank Formation. Garnet in these plutons is similar to garnet elsewhere in the Meguma terrane that has been attributed to contamination as well as magmatic garnet in plutons around the world that have been attributed to Mn-rich magma.

Pages: 312
Supervisors: Rebecca Jamieson,