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.

Accessory mineral U–Th–Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

“…Manley and Bacon (2000) used the Al-in-hornblende barometer to show that mineral growth within the *0.6 to 0.3 Ma rhyolites imply that the depth to the top of the silicic magma body was *10 km (*270 MPa) and that by *0.04 Ma it had risen to *5.4 km (140 MPa). Following Bacon et al (1981), Mordick and Glazner (2006) concluded that Pleistocene basalts likely provided heat needed to generate Pleistocene rhyolite and may have been trapped beneath the Pleistocene rhyolite magma body, a possible explanation for why the Pleistocene basalts records greater depths than the Pliocene basalts.…”

“…The dome names (numbers), their field relationships, and their major and trace element compositions used here to develop models for the petrogenetic and eruption history of the magma system come from Duffield et al (1980) and Bacon et al (1981). In general, Coso rhyolites are crystal-poor with the two oldest domes (38 and 28) and a third dome (5) located at the north end of the field being the exceptions.…”

“…Bacon et al (1981) assigned the 38 Quaternary rhyolite extrusions to seven groups, each of which consisted of chemically similar units that had similar apparent eruption ages. Although the precision and accuracy of 40 Ar/ 39 Ar dating techniques are typically superior to those of dates obtained by the K-Ar method, which may be biased by excess 40 Ar contamination (e.g., Renne et al 1997) or incomplete extraction of argon from sanidine (e.g., McDowell 1983), the K-Ar results are adequate to define the general history of the Coso volcanic field.…”

“…magma centers and to improve our understanding of the time scales of silicic magma processes. Further dating of Coso rhyolites provides an opportunity to explore and refine ideas on how large shallow silicic magma systems develop and how quickly their magmatic and eruptive behavior evolves (e.g., Bacon et al 1981).…”

Chemical Signatures of and Precursors to Fractures Using Fluid Inclusion Stratigraphy

“…The Pleistocene volcanic rocks consist of 38 separate domes and flows of high-silica rhyolite, and most of them are quite young; younger than 300,000 years. Bacon et al (1981) inferred from the rhyolite magma that there was a chemically stratified siliceous reservoir at depth. Most of the first production of the geothermal system was near Dome 53, which is near the Devil's Kitchen fumarolic area.…”