Abstract

The Grouse Canyon Member of the upper Miocene Belted Range Tuff, southern Nevada, is an areally extensive ash-flow sheet of comenditic composition. Most rocks of the unit are densely welded and exhibit primary groundmass crystallization. Aphyric nonhydrated glassy tuff from the basal glassy zone of the sheet contains 5.35 to 5.40 weight percent Na2O. Aphyric to phenocryst-poor granophyrically crystalized and devitrified tuff from the lower part of the ash-flow sheet, which otherwise is chemically very similar to the glassy tuff, contains 4.1 to 5.1, median 4.75, weight percent Na2O.

The crystallized tuff undoubtedly lost sodium during or shortly after crystallization and cooling. The loss is a direct result of the peralkaline character of the parent melt: not enough Al and Fe was available to combine with all the Na, K, and Ca present in the glass to form feldspar, pyroxene, and amphibole. Crystallized comendites, pantellerites, and “alkali granites” almost invariably have significantly lower Na2O contents than do comendite and pantellerite glasses that are otherwise chemically comparable. Loss of sodium appears to be a general feature of the final stages of crystallization of moderately and highly peralkalinc silicic melts.

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