New Mexico Geological Society Annual Spring Meeting — Abstracts

Compositionally zoned, high-silica rhyolite ash-flow tuffs are commonly interpreted as the inverted equivalents of large vertically zoned magma chambers in which silica, volatiles, and large or highly charged cations are strongly enriched at the top. In the uppermost high-silica
magma zone (77.5 to 75 wt. % SiO₂, volatile free), concentrations of Rb, Nb, Ta and U progressively decrease downward, whereas Ba and Sr antipathetically increase downward. As a working hypothesis, we suggest that the small volume (0.82 km³), high-silica rhyolite lavas
(~76% SiO₂) that form the No Agua Peaks, in north-central New Mexico, represent the discharge from a small (narrow) compositionally zoned magma chamber approximately 3.9 million years ago (published K-Ar age: 3.91 ± 0.27 Ma).

Published geologic maps, Rb, Sr and Ba concentration data from obsidian, and topographic data suggest the following eruptive history. (1) A Rb-rich, high-silica composite lava dome (interleaved pile of stubby flows) as much as 270 m high was slowly extruded over the primary conduit at "West Peak"; presumably this steep-sided dome was surrounded by a cogenetic ring or mounds of tuff breccia. (2) The composite dome then acted as a stopper and allowed vapor pressure to build in the magma column to a level approximately equivalent to 0.7% water at a nominal magma temperature of 800°C. (3) As vapor pressure exceeded litho static load, a slightly(?) less Rb-rich high-silica magma squeezed out from under the west flank of the dome to form a relatively thin (75 m), highly vesicular flattopped glassy flow that spread in a semi-circular geometry as much as 1.2 km from the lateral vent. (4) Following another period of quiescence and vapor pressure increase, subterranean swelling induced a radial fracture on the northeast flank of the composite dome; this relatively unconstrained radial vent allowed rapid degassing of the high-silica magma, possibly by small ash-flow events. (5) Finally, late phase low-Rb high-silica magma was then slowly extruded from the radial dike to produce three thick (120-150 m) lobate flows which pushed eastward through and around a mound of early tuff breccia, thereby forming a kipuka (opening).

Preferential erosion of the soft tuff breccias from the kipuka has since formed a large depression within the thick low-Rb flows. This topographic inversion has caused earlier workers to regard the more irregularly shaped low-Rb flows as steep-sided domes above
another conduit or conduits (two to four vents). Rapid chilling of the tops and steep toes of the viscous No Agua flows produced obsidian zones as much as 45 m thick. Chemical weathering (hydration) of highly fractured and vesicular obsidian zones by meteoric waters
has since formed the world's largest perlite deposit at No Agua. Ore distribution was largely predetermined by the eruptive history of the No Agua volcano. Thick flows cool more slowly than thin flows so they have thicker crystalline cores and relatively less obsidian. Thin flows
have thin cores and relatively more obsidian (and potential perlite) content than thick flows.