REINTERPRETATION OF SLEEPING LION RHYOLITE AS RHEOMORPHIC TUFF

Sleeping Lion Rhyolite (~50 km³) of the Davis Mountains Volcanic Field in southwestern Texas has been interpreted by some workers, including this author, as an extensive silicic lava flow based upon its thick and widespread upper and lower breccia and flow features such as ramp structures. The unit is suspected to have been erupted from vents in the western Davis Mountains and to have flowed into and filled a paleocanyon system in the southeastern Davis Mountains. Despite its lava features, doubt remains as to its origin, as some vitrophyre contain fiamme and glass shards and, in at least one locality, granophyre contains foreign lithic inclusions. It also strongly resembles Moore Tuff (~15 km³), which filled in the small (4 x 3.5 km) Muerto Caldera in the western Davis Mountains. Sleeping Lion Rhyolite and Moore Tuff are both composed of low silica rhyolite, marginally peralkaline, with abundant alkali feldspar phenocrysts, which are, in many samples, chatoyant. Ar-Ar ages of the two units are, within analytical error, identical (~36.5 Ma). Major and trace element contents of the two units are broadly similar. Ratio plots of incompatible trace element ratios for Davis Mountains units may be used to “fingerprint” them; ratio plots for Sleeping Lion Rhyolite and Moore Tuff are virtually identical. Sr and Ba contents, and, to a lesser extent, Y, however, are lower in Moore Tuff, whereas Rb and Th contents are higher (~20%), as is Zr (~10%). If the correlation between the two units is correct, the outflow unit (Sleeping Lion Rhyolite) is less differentiated than the intracaldera Moore Tuff, a pattern similar to that of the younger Wild Cherry Tuff of the Paradise Mountain caldera in the Davis Mountains.