Magmatic Garnets in the Cairngorm Granite, Scotland

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Abstract
Small, euhedral Mn-rich garnets (32–52 mol. % spessartine) from the Cairngorm granite, Eastern Grampian Highlands, Scotland, are considered to be of magmatic origin and have not been derived from the assimilation of metasedimentary material, despite their occurrence largely at the margins of the pluton. Similar garnets also occur in a late cross-cutting aplite sheet. The garnets in the granite crystallized early in the sequence and are thought to have formed in response to the ponding of Mn-rich fluids against the wall of the pluton. This Mn enrichment of the fluid phase continued throughout the evolution of the pluton, resulting in Mn-rich biotites and opaque oxides and the localized crystallization of Mn-rich garnets in aplite. Garnet contains up to 1.67 wt. % Y, but has not played a major role in the geochemical evolution of the Cairngorm granite, which has high SiO2 (72–77%) and is enriched in Y and HREE. Chemical analyses of garnets, biotites and rocks are given.