Chemical variation in micas from the Cairngorm pluton, Scotland

Electron microprobe analyses of micas from the Cairngorm pluton in the Eastern Grampian Highlands of Scotland show extensive compositional variation in biotite, despite a lack of chemical variation in the host granite. Biotite has high Fe/(Fe + Mg) (0.6--0.85) and A1 v ' (0.6-2.1 a.f.u.), and enrichment trends in these two parameters are attributable to the AI-Tschermak and dioctahedral-trioctahedral substitutions, the latter becoming dominant with increasing A1 vi content. Ti content is low (0.2-0.4 a.f.u.), and is largely controlled by a Tschermak-type substitution. Biotite is also unusually rich in Mn (up to 2.57 wt. % MnO), which increases with both A1 vi and Fe/(Fe+Mg). F contents generally range between 0.55 and 2,05 wt. % All compositional variation in biotite can be attributed to the extensive development of a fluid phase during the late-magmatic and subsolidus evolution of the pluton, The presence of an abundant fluid phase has resulted in the alteration of biotite to muscovite, which has occurred in response to de-stabilization f the biotite as octahedral R2+ cations are lost in favour of A1. Extreme build-up of this fluid phase has resulted in the crystallization of muscovite as a late, interstitial primary phase. Both primary and replacive muscovite have Fe/(Fe+Mg)> 0.50, 15-36 tool. % celadonite and <1 tool. %paragonite