Richard M. Chamberlin
Abstract-- The Socorro caldera, source of the 31.9 Ma Hells Mesa Tuff, erupted 1200 km3 of rhyolitic crystal mush (69–75 wt.% SiO2, 30–55 vol.% phenocrysts) from a collapse structure 24 km in diameter. The primary collapse-phase of eruption is distinguished by 2 km of mostly xenolith-rich, lower Hells Mesa Tuff exposed in the core of the caldera. Waning-stage volcanism, immediately after collapse, was limited to about 20–30 small volume eruptions (total volume 10–50 km3) of variably welded, phenocryst-rich ignimbrite that contain many thin tongues of lithic lag breccias. Breccia clasts are mineralogically and compositionally similar to the enclosing crudely bedded tuffs; the clasts are termed comagmatic lithics. In the northeast sector, densely welded ignimbrites of the upper Hells Mesa Tuff contain angular to moderately flattened lag-breccia clasts of crystal-rich spherulitic rhyolite. Spherulitic clasts are interpreted as mostly degassed crystal-rich magma that rapidly crystallized along dike margins and temporarily clogged shallow fissure vents. Slowly rebuilding volatile pressures periodically cleared the clogged vents by explosive eruptions and small volume pyroclastic flows. The latter are commonly capped by thin, fine-grained, ash-fall deposits that signify brief periods of repose. Crudely bedded comagmatic lag breccias in the southwest sector contain abundant angular fragments of xenolith-poor, lower caldera-facies Hells Mesa Tuff, apparently derived from unstable vent walls during small explosive pyroclastic eruptions. Conformable contact relationships imply that resurgent doming was not associated with these small volume waning-stage ignimbrites.
The final stage of eruptions from the Hells Mesa magma chamber (< 0.3 km3) is represented by a small, coarsely porphyritic, ring-fracture lava dome and derivative tuff breccias that were emplaced in the southeast sector of the caldera. The inner cauldron block was locally uplifted approximately 70 m prior to emplacement of the crystal-rich rhyolite lava dome (~52 vol.% phenocrysts, ~73 wt.% SiO2). Coarse-grained sanidine phenocrysts in the lava and coeval tuff breccias are about 6–9 mm long and commonly exhibit oscillatory zoning (Ba-rich zones). Crude flow banding, microfaulted phenocrysts and lack of vesicular zones in the lava dome suggest that it was extremely viscous and volatile poor at the time of extrusion. Precise 40Ar/39Ar sanidine ages of the Hells Mesa ignimbrite facies and the comagmatic lava dome (n=4), are analytically indistinguishable and tightly clustered between 31.85 and 31.94 Ma, with an average error of ± 0.16 Ma. Age data, stratigraphic indications of local uplift and a cooling break, plus greater phenocryst sizes in the dome rocks, taken together, suggest that the coarsely porphyritic lava dome is about 103-104 years younger than caldera collapse. Maximum phenocryst size and total phenocryst content are correlative, both generally increase upwards within the Hells Mesa eruptive sequence. The coarsely porphyritic lava dome closely approaches a well-known crystallinity-flow barrier at about 55% phenocrysts. These observations provide evidence that the Hells Mesa magma body was: 1) largely exhausted of volatiles shortly after caldera collapse; and 2) became immobile through progressive growth of interlocking crystals shortly after emplacement of the crystal-rich lava dome. Post-collapse activity of the Socorro caldera was anomalously brief in comparison to most large ignimbrite calderas. (Reprints of this article are available on request.)