T89 Magmas Assemble! Petrologic, Geochemical, Chronologic, and Geophysical Insights into the Architecture and Timescales of Magmatic Systems

Monday, 5 November 2018
8:00 AM - 12:00 PM
Room 240-241

INSIGHTS INTO THE MAGMATIC ASSEMBLY OF A VOLUMINOUS, LOW δ18O, AND STRONGLY TRACE ELEMENT ZONED HIGH-SILICA RHYOLITE BODY: THE DEVINE CANYON TUFF, OREGON

Paper number: T89-11
Program: Technical Sessions
Time: 10:30 AM – 10:45 AM

Authors

Abstract

The Devine Canyon Tuff is a crystal-rich (3 to 22%), large-volume (300 km3), high-silica rhyolitic ignimbrite that erupted ~9.7 Ma, from a presumed source in the Harney Basin of southeastern Oregon. The tuff erupted as a simple cooling unit after an initial Plinian phase with bulk rock and pumice analyses indicating that all of the tuff (>99%), except for the sparse dacite and dacite/rhyolite mixed pumices, are strongly trace-element zoned. Individual rhyolitic pumices extend the compositional range of bulk tuff samples [ppm]: e.g. Zr 611-1600, Nb 46-112, Rb 52-180, Y 75-204. All pumices analyzed fall into compositional clusters with or without gaps. Quartz and feldspar phenocrysts from the most evolved rhyolite to the least evolved dacite pumices have variable and low δ18O values. The maximum range of calculated δ18O melt values are 1.5‰ from 4.28‰ to 5.76‰. Quantitative crystallinity data on pumices (and fallout) from the rhyolite magmas reveal a span of 3 to 19% with the dacite pumice having a crystallinity of 22%. However, size distribution of phenocrysts among the rhyolites change progressively, with narrow and overlapping feldspar and pyroxene compositions across all rhyolites (sanidine with Ab: 54-59, Or: 46-41; cpx with En: 57-58 Wo: 41-41.5) and slightly wider range of minerals from dacite.

Clustered mineral compositions, banded pumices composed of different rhyolites, and low δ18O values strongly suggest different rhyolites erupted from a contiguous and shallow reservoir rather than separated reservoirs. The addition of hydrothermally altered material is evident from the low δ18O values and we suggest the addition of this material occurred prior to the trace element zonation of the highly fractionated rhyolites (Eu/Eu* 0.08-0.16). Intermediate rhyolites cannot be solely generated through the mixing of the end-member rhyolite melts but require, as minimum, some subsequent modification. Mixing of a more mafic magma appears to have generated the dacitic magma and our data do not suggest it to be a remobilized crystal mush. All this suggests magmatic evolution occurring within melt rich rhyolite magma(s) and nearby evolution of intermediate magmas rather than punctuated extraction of melt from intermediate crystal mush.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-323454

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