Origin of a silica-oversaturated quartz trachyte-rhyolite suite through combined crustal melting, magma mixing, and fractional crystallization; the Leyva Canyon Volcano, Trans-Pecos magmatic province, Texas.

Abstract

The Leyva Canyon Member of the Rawls Formation is a sequence of Oligocene 27.3-27.1 Ma 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 approximately 40% crustal input, followed by approximately 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-Al sub 2 O sub 3 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.

28 samples from this reference