Ages of Rhyolitic Magmatism in the Salton Sea Geothermal Field Determined by Ion Microprobe Dating of Zircon

Abstract

U-Pb and U-Th zircon ages for igneous rocks from the Salton Sea geothermal field (SSGF) help constrain the timing of magmatic and volcanic activity in this part of the Salton Trough. Two types of samples were age-dated: (1) drill cuttings from three geothermal wells that penetrated deeply-concealed extrusive and intrusive rhyolites in the current depth range 1.6 -2.7 km; and (2) flow/dome rocks and xenoliths from the Salton Buttes—four Quaternary rhyolitic volcanic centers exposed along the western margin of the geothermal field. The subsurface rhyolites yield U-Pb zircon ages between 417 ± 6 ka and 476 ± 23 ka (1?). Average zircon crystallization ages are indistinguishable between the buried intrusive and extrusive rhyolites, and overlap (within the range of uncertainty) in two of the extrusive-rhyolite intercepts separated by ~300 m of intervening calcareous-siliciclastic strata. Zircons from the Salton Buttes rhyolites are much younger, yielding U-Th ages between 10 ± 1 ka and 18 ± 2 ka. The older ages closely overlap with those of granitic and finely-crystalline rhyolitic xenoliths (21 ± 2 ka and 18 ± 4 ka, respectively) in the exposed volcanics. Basaltic xenoliths in these volcanics host zircons, in remelt pockets, that yield ages between 30 ± 13 ka and 9 ± 7 ka. Subsurface-rhyolite zircon ages indicate that the inception of magmatism in this part of the U.S. Salton Trough was at least several 105 years earlier than indicated by the surface volcanic rocks and their xenoliths. Minimum sedimentation rates estimated from U-Pb zircon crystallization ages for the now buried but unambiguously extrusive rhyolites indicate that the rate of subsidence and sedimentation in the central portion of the SSGF—up to 4 mm/yr—was nearly twice as rapid as previously reported (2.4 mm/yr) for the northern part of the field. The ~30 - 9 ka age for the xenoliths of plutonic rock in the surface volcanics underscores the fact that heating in the modern geothermal system was initiated in the geologically recent past.