Abstract

Major and trace elements and oxygen-isotope compositions are evaluated to delineate the sequence of crystallization in the lithologically and chemically zoned Tin Mountain pegmatite. The pegmatite consists of four stacked tabular zones surrounded by an enclosing wall zone. Each zone is lithologically, compositionally and texturally distinct, although intrazone contacts are gradational. Oxygen-isotope geothermometry, rare-earth-element patterns, low concentrations of Rb, Cs, and Li, and high concentrations of Ba and Sr in the wall zone indicate that it crystallized first. Cooling trends indicated by two-feldspar-geothermometry, the presence of positive Eu anomalies in feldspars, and low concentrations of Rb and Cs in the uppermost inner zone suggest that it crystallized next. The three remaining lower zones then crystallized simultaneously. It is suggested that an aqueous-fluid film connected all crystallization fronts in the body. This aqueous fluid vertically redistributed Si, Al, K, Na, and possibly Rb and Cs in such a manner that the melt retained a ternary-minimum melt composition throughout crystallization.

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