Euhedral muscovites from the granites of the Leinster batholith, Ireland, have zone textures that clearly demonstrate their magmatic origin. These textures include corrosion, synneusis, development of transient faces, zone thickening, face widening, and rational and irrational growth in response to inclusions. Inclusions of biotite, zircon, and sillimanite are zonally controlled, whereas those of quartz, apatite, and monazite are not. Muscovite nucleation over a wide span of magmatic evolution from a strongly peraluminous melt under conditions of enhanced muscovite stability is implied. These micas provide and validate criteria that distinguish primary from hydrothermal muscovites. The observations here significantly alter previous models for the evolution of the Leinster granites and provide a potentially powerful means of discriminating primary from hydrothermal muscovites in other two-mica granites.

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