The Mahogany Mountain caldera is one of several volcanic centers in southeastern Oregon that erupted silicic peralkaline tuffs during the middle Miocene. The 15-by 20-km caldera structure coincides with a 25 mGal gravity low that is filled with an estimated 280 km3 of ash-flow and air-fall tuff. An unusually well-preserved central-vent complex is exposed in the resurgent dome of the caldera. Facies changes within the caldera indicate the majority of the tuffs erupted from the central vent. Near-vent facies include, surge deposits, lag breccias, and ash-flow and airfall tuffs containing abundant blocks. Major-and trace-element data indicate that both the intracaldera tuffs and the vent intrusion erupted from a bimodal magmatic system consisting of high-silica comendite, and zoned intermediate comenditic trachyte. The high-silica melt that formed the upper part of the magma chamber appears to have evolved as an open system with a small compositional gradient. Unusually low Nb, Zr, and Y values are characteristic of rocks that erupted from this part of the magma chamber. In contrast, an extreme range of Nb, Zr, and Y values is characteristic of rocks erupted from the underlying intermediate-silica melt. This melt evolved in a closed magmatic system that developed a steep compositional gradient. The central-vent eruption tapped both parts of the magma chamber simultaneously. Pyroclastic fragments that comprise the intracaldera tuffs and vent intrusion are a composite of both melts.
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