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Publication authorized by the Director, U. S. Geological Survey.

Abstract

Two Tertiary volcanic suites, predominantly rhyolitic ash flows and air-fall tuffs, near Beatty, Nevada, are believed to have been derived from two calderas. Subsidence of the calderas probably followed the catastrophic eruption of the ash flows, which drained the underlying magma chambers and caused them to collapse. Subsidiary subsidence of an area extending outward from the larger caldera may have resulted from later eruption of tuffs and flows. Late intrusions and flows of rhyolite and latite occurred along the margins of the calderas and help to delineate them. Paleozoic and Precambrian rocks of the pre-Tertiary basement were pushed up into the ash flows and tuffs in a small area within the larger caldera by re-entry of magma into the underlying chamber.

The volcanic rocks range in composition from basalt and basanite to latite and rhyolite; rhyolitic types predominate. Chemical analyses of 26 rocks are given and include quantitative spectrographic analyses of minor elements. The major oxides have been plotted against the differentiation index of Thornton and Tuttle (1960) invariation diagrams, and most of the trends are normal. The position of the norms of the most differentiated rhyolitic rocks in the ternary diagram quartz-orthoclase-albite, when compared with experimental data of Tuttle and Bowen (1958), indicates that the depth of the magma chamber may have been 7–9 miles.

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