Abstract
The Leyva Canyon Member of the Rawls Formation is a sequence of Oligocene (27.3–27.1Ma) silicic lava, tuff, and volcaniclastic rock that comprise a trachytic shield volcano in the central Bofecillos Mountains of Big Bend Ranch State Park, Texas. This silicic unit developed within a volcanic field that was otherwise dominated by silica-undersaturated, mafic to intermediate lava. Quartz trachyte to low-silica rhyolite of the Leyva Canyon volcano appears to be the result of magma mixing between mantle-derived, alkalic mafic magmas and peraluminous crustal melt (∼40% crustal input), followed by ∼65% fractional crystallization. The parental mafic component was probably similar to silica-undersaturated, mafic lavas of the Rawls Formation. Peraluminous, A-type high-silica rhyolite represents the earliest-erupted lavas of the Leyva Canyon volcano and is unrelated to quartz trachyte and low-silica rhyolite via fractional crystallization. The high-silica rhyolite provides evidence for an episode of crustal melting beneath the Leyva Canyon volcano.Data from the Rawls Formation suggest that silica-undersaturated, alkalic (ne-normative) mafic magmas may evolve via fractional crystallization alone to silica-undersaturated silicic compositions (ne-trachyte), but require magma mixing with crustal-derived silicic magmas coupled with fractional crystallization to produce silica-oversaturated magmas (quartz trachyte–rhyolite). Mixing with a high-Al2O3 crustal component results in silica-oversaturated, mildly peraluminous and metaluminous to mildly peralkalic felsic rocks, rather than the strongly peralkalic rocks more commonly associated with alkalic mafic magmas.
Published Version
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