Volcanology and petrology of the Rattlesnake Ash-Flow Tuff, eastern Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kh04ds239

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  • The Rattlesnake Ash-Flow Tuff erupted 7.05±0.01 Ma from the western Harney Basin, southeastern Oregon. The location of the vent area is inferred based on vent-ward increases in size of pumices, in degree of welding, and in degree of post-emplacement crystallization. Today's outcrops cover 9000 km2 and estimated original outcrop coverage was ca. 35000 km2. Tuff thickness is uniform ranging mainly between 5 and 30 in with maxima up to 70 in. Estimated eruption volume is 280 km3 dense rock equivalent (DRE). Lithological variations include vitric non- to densely welded tuff, vapor-phase, devitrified, spherulitic, lithophysal, and rheomorphic tuff. Lithological zoning characteristics of the tuff change locally at nearly constant tuff thickness over distances of 1 to 3 km grading from incipiently welded tuff to highly zoned sections. Regional variations become apparent by integrating many sections from one area. A three-dimensional facies model is developed describing the local and regional facies variations. The Rattlesnake Tuff consists of high-silica rhyolite (HSR) erupted as pumices and glass shards. Dacite pumices make up less than 1% of the total volume and quenched basalt and basaltic andesite inclusions inside dacite pumices constitute << 0.1 volume %. HSR pumices cluster in 4 to 5 compositional groups which are discerned best by La, Eu, Ba, Hf(Zr), Ta(Nb). Major element variations are minor but consistent between groups with Si02 increasing and FeO*, MgO, Ti02, and CaO decreasing with differentiation. Modal mineralogy, mineral chemistry, and partition coefficients also change progressively. The diversity of HSR is likely the product of crystal fractionation processes. A model is proposed by which a stratified magma chamber is generated from the roof of the chamber downward by progressively more evolved HSR. Least evolved HSR is likely the product of dehydration melting of high-grade intermediate to mafic protoliths. High Ba/Rb ratios of 30 in the least evolved HSR limit the potential protoliths to lithologies with equal or higher Ba/Rb. Dacite pumices and mafic inclusions reveal the nature of the mafic root zone to the Rattlesnake Tuff magma chamber. Quenching and mingling textures with the host pumice indicate that basaltic andesite and basalt inclusions are cognate. Dacite formed at the interface between HSR and underlying enriched basaltic andesite and was generated by mixing of these two components. Basaltic andesite is fractionated and enriched in trace elements compared to regional primitive tholeiite (HAOT). Basaltic andesite has evolved from HAOT's mainly through fractionation and recharge while being stalled underneath a silicic cap.
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