ORIGINS OF PLEISTOCENE RHYOLITE AT AUGUSTINE VOLCANO: CLUES FROM A HIGH-P DACITE

Relatively small explosions (VEI 2-4) and eruptive products of intermediate silica content have dominated activity at Augustine Volcano, Alaska, in recent Holocene time. In stark contrast, there exists both Pleistocene basalt and rhyolite (ca. 25 ka), the latter of which comprises a large deposit of pumice fall material at least 30 m thick in some locations. Trace element patterns and zircons of identical ages link the rhyolite to dioritic inclusions erupted within andesitic material in 2006. Coombs and Vazquez (2014) postulate that the two were cogenetic, with their origins both in a crystal-rich dacite. Crystal-poor rhyolitic interstitial melt was extracted from the mush to erupt as the rhyolite pumice fall, leaving behind the crystalline restite of dioritic composition to be later remobilized and entrained in the 2006 andesite. This un-mixing of dacite into rhyolite and diorite is clearly supported by some major element Harker diagrams for Augustine eruptive products, particularly P₂O₅. Also plotting along the un-mixing trend is a high-P dacite, which lies just above the rhyolite stratigraphically (Wallace and Coombs, 2013).

Initial investigation of the rhyolite’s mineralogy and volatile content (Nadeau et al., 2015) has already yielded insight into the magma chamber conditions at the time of the rhyolite’s eruption, but questions remain regarding magmatic processes that first segregated the rhyolite from the dacite, and in turn, into the three distinct rhyolitic lithologies recognized at Augustine.

Here we report results of preliminary comparisons of samples of the high-P dacite, likely genetically linked to the rhyolite, to the rhyolite itself. We also investigate the possibility of a coeval basaltic melt interacting with the dacite and/or the rhyolite, in terms of chemical or thermal input. We present results contrasting mineralogy, mineral zonation patterns, melt inclusion chemistry, and volatile content as a means to better elucidate the origins of the voluminous rhyolite at Augustine.

Coombs, M.L., Vazquez, J.A., 2014. Geochem. Geophys. Geosyst., 15 (12), 4846–4865.

Nadeau, P.A., et al., 2015. J. Volcanol. Geotherm. Res., 304, 304–323.

Wallace, K., Coombs, M.L., 2013. Abstract V23C-2839 presented at AGU Fall Meeting, San Francisco, Calif, 9-13 Dec.