Abstract

Devonian submarine island-arc deposits of the Sierra Buttes Formation in the northern Sierra Nevada are penetrated by numerous penecontemporaneous hypabyssal intrusions, but related extrusive lavas are absent. Significant parts of the arc succession are occupied by andesitic to rhyolitic intrusive complexes that record the quenching and disruption of large volumes of magma injected into wet sediments at shallow levels beneath the sea floor. Extensive masses of intrusive hydroclastic breccia are characterized by in situ fragmentation textures and developed by progressive brittle disintegration of quenched magma. Pillows dispersed within the breccia formed as a series of interconnected tubes that supplied magma to the growing intrusive masses. Beds of redeposited hydroclastic breccia indicate that the intrusions locally reached the sea floor to undergo slumping and debris flow. Large-scale steam explosions did not occur during magma/wet-sediment interaction, probably due at least in part to the lithostatic and hydrostatic pressure acting on the intrusions. Low-viscosity rhyolitic magma developed intrusive pillows and in situ hydroclastic breccia in the same fashion as quenched andesitic magma. This fluid behavior was favored by retention of volatiles in the rhyolitic magma under relatively deep marine conditions.

Quenched and disrupted hypabyssal complexes of the type described herein may be common elements of the submarine parts of island arcs and other volcanic environments where uprise of magma through thick sequences of wet sediment is impeded due to buoyancy constraints and rapid chilling.

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