Petrology of the Bend pumice and Tumalo tuff, a Pleistocene Cascade eruption involving magma mixing Public Deposited

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  • The Bend pumice and Tumalo tuff are products of a plinian eruption which occurred sometime between 0.89 and 2.6 m.y. The Bend pumice is a poorly consolidated, air-fall vitric lapilli tuff, which overlies a zone of reworked tephra. Perlitic obsidian in the reworked zone probably represents the remains of a dome which filled the eruptive vent and is chemically related to the Bend pumice magma. Detailed grain size analysis of the air-fall part of the Bend pumice shows that the eruptive vent was located approximately 10-20 km west of Bend, Oregon. Grain size variations in vertical section are probably related to fluctuations in the diameter of the vent rather than interruptions in deposition of the Bend pumice. The Tumalo tuff is nonwelded to moderately welded ash-flow tuff which directly overlies the Bend pumice. Lack of discernable normal grading in the upper 50 cm of the Bend pumice indicates that the Tumalo tuff was emplaced before the Bend pumice was completely deposited and leads to the conclusion that the Tumalo tuff is the product of collapse of the Bend pumice eruption column. The Tumalo tuff was formed by one episode of flow and has a well developed basal 2a layer. Variations in the distal character of layer 2a are thought to represent complex flow conditions in the head of the Tumalo tuff ash flow. Mixed pumices also are found in proximal Tumalo tuff deposits. The Bend pumice and Tumalo tuff are peraluminous and rhyodacitic. Within analytical uncertainties, they have identical major, minor, and trace element abundances. Both contain fresh hornblende in the mineral assemblage Plg + Opx + Mgt + Zr + Ap. The hornblende appears to have been a liquidus phase and indicates that the rhyodacite evolved under high pressure, hydrous conditions. A hight La to Ce ratio and a strong negative Eu anomaly in the B-T rhyodacite further indicates that the Bend pumice and Tumalo tuff evolved under physical conditions quite distinct from other rhyodacites in the central Oregon High Cascades analysed by Hughes (1982). Mixed pumices in the Tumalo tuff represent the incomplete mixing between Bend-Tumalo rhyodacite and a dacitic magma. Trace element modeling fails to provide an unequivocally common path of crystal fractionation between these two magmas. The magmas can not be directly related through thermogravitational diffusion or assimilation. While mixed pumice formation is usually attributed to mixing of genetically related magmas, Tumalo tuff mixed pumices were produced through the mixing of genetically unrelated magmas.
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