Pleistocene high‐silica rhyolites of the Coso Volcanic Field, Inyo County, California

Abstract

The high‐silica rhyolite domes and lava flows of the bimodal Pleistocene part of the Coso volcanic field provide an example of the early stages of evolution of a silicic magmatic system of substantial size and longevity. The rhyolites are sparsely porphyritic to virtually aphyric, containing qz + pl + san + bi + hb + mt ± allanite ± opx ± cpx ± fa ± il ± ap ± zircon phenocrysts. Major and trace element compositions of all 38 rhyolite extrusions are consistent with derivation from somewhat less silicic parental material by liquid state differentiation processes in compositionally and thermally zoned magmatic systems. Seven chemically homogeneous eruptive groups emplaced approximately 1.0, 0.6, 0.24, 0.17, 0.16(?), 0.09, and 0.06 m.y. ago can be distinguished on the basis of trace element and K‐Ar data. The oldest two groups are volumetrically minor and geochemically distinct from the younger groups, all five of which appear to have evolved from the same magmatic system. Erupted volume‐time relations suggest that small amounts of magma were bled from the top of a silicic reservoir at a nearly constant long‐term rate over the last 0.24 m.y. The interval of repose between eruptions appears to be proportional to the volume of the preceding eruptive group. This relationship suggests that eruptions take place when some parameter which increases at a constant rate reaches a critical value; this parameter may be extensional strain accumulated in roof rocks. Extension of the lithosphere favors intrusion of basalt into the crust, attendant partial melting, and maintenance of a long‐lived silicic magmatic system. Consideration of the mass of magma that must have been intruded into the crust in order to explain the anomalously high heat flow near the center of the rhyolite field and comparison of age, volume, mineralogic, and compositional characteristics of the rhyolites with those of caldera‐forming systems suggest that the Coso silicic system may contain a few hundred cubic kilometers of magma. The Coso magmatic system may eventually have the potential for producing voluminous pyroclastic eruptions if the safety valve provided by rapid crustal extension becomes inadequate to (1) defuse the system through episodic removal of volatile‐rich magma from its top and (2) prohibit migration of the reservoir to a shallow crustal level.