PERALKALINE RHYOLITE AND TRACHYTE IN BIG BEND NATIONAL PARK, TEXAS

In their pioneering work in Big Bend National Park (BBNP), Maxwell et al. (1967) assigned outcrops of “riebeckite rhyolite” in the Chisos Mountains and Castolon Graben to the Oligocene South Rim Formation (SRF). Subsequent detailed studies of the petrology and geochemistry of the SRF were conducted primarily by Daniel Barker and his academic descendants which have revealed a great deal of complexity in the geologic history, volcanology and petrogenesis of these rocks which are now divided into the older (32 Ma) SRF as well as the younger (28 Ma) Burro Mesa Formation (BUM). Although all are broadly similar, consisting of metaluminous to peralkaline trachyte and rhyolite (comendite) ultimately derived from transitional basalt similar to the 34-28 Ma Bee Mountain Basalt Formation (BMB), there are significant differences with respect to their geochemistry and origin. The SRF crops out entirely within the Chisos Mountains, where it erupted from the small (~6 km diameter) Pine Canyon caldera as well as from several extra-caldera lava domes. The SRF is divided into the Pine Canyon Rhyolite (PCR), Boot Rock (BRM), and Emory Peak Rhyolite (EPR) members, which comprise two unrelated series. The BRM-PCR series was derived via fractional crystallization of BMB coupled with assimilation of shale to produce metalmunious trachyte, followed by fractional crystallization that resulted in comenditic trachyte and comendite. The EPR, which consists of a distinctly different high-silica comendite, was most likely ultimately derived from ~1-5% flux-induced partial melting of lower crustal mafic granulite. The Burro Mesa Rhyolite Formtion (BUM) crops out entirely within the Castolon Graben, where it erupted from several different volcanic centers that comprise a dome field similar to Olkaria, Kenya. Unlike the SRF, which is primarily the result of open-system processes, the comendite lavas and ignimbrites of the younger Burro Mesa Rhyolite Formation (BUM) can be modeled as the result of closed-system processes—fractional crystallization of BMB-like basalt which evolved to a mugearite, metaluminous trachyte, comenditic trachyte, and comendite. These lavas and tuffs show that peralkaline magmas can form via a wide variety of petrogenetic processes, but they are all ultimately derived from alkali basalt.