PAISANO VOLCANO REVISITED: A SECOND PASS AT PAISANO

The Paisano Volcano (~36 Ma), arguably the best exposed central volcano of the Trans-Pecos volcanic field, produced an eruptive sequence peralkaline rhyolite – quartz trachyte – trachyte, forming a shield lava complex 30 x 20 km in the southern Davis Mountains. Initial eruption of silicic domes and associated pyroclastic deposits were followed by two shield-forming units: the lower shield is comprised of a local ignimbrite and overlying extensive sparsely porphyritic quartz trachyte lavas; the upper shield unit contains ignimbrites and strongly porphyritic lavas, some of which are strongly glomeroporphyritic. A five km diameter caldera formed during the eruption of the upper shield unit. The volcano is underlain and overlain by mafic lava units dominated by mugearite and hawaiite. A later episode of intrusive activity produced plugs and laccoliths of peralkaline trachyte to rhyolite and nepheline trachyte. A mantle-normalized elemental plot shows extreme depletion in Ba, Sr, P, Eu and Ti, suggestive of extensive crystal fractionation of observed phenocrysts (pc, af, cpx, mt, ap). Fractionation modelling indicates that quartz trachyte can be produced by about 0.56 weight percent crystallization of trachyte, and that peralkaline rhyolite of the volcano can be produced by ~0.50 weight percent crystallization of quartz trachyte. Zr was buffered in silicic rocks by zircon fractionation. REE plots show LREE enrichment and flat HREE for the series, with well-developed negative Eu anomalies. Some peralkaline rhyolite shows depletion in LREE relative to quartz trachyte, possibly the result of fractionation of a LREE phase such as chevkinite. A strongly glomeroporphyritic trachyte lava and an ejected syenite block have textures and chemistry suggestive of magma mixing between mafic and silicic magmas. The coarse gabbroic textures of glomeroporphyritic clots suggest that these mixed rocks represent silicic magma incompletely separated from mafic plutons, rather than simple mixing of magmatic end members. Extensive silicic lavas of the lower shield unit show chemical and isotopic similarities to older (~37 Ma) flood rhyolite units and provide a model for their generation.