The record of Middle Jurassic volcanism in the Carmel and Temple Cap Formations of southwestern Utah

Altered volcanic ash beds in the Middle Jurassic Temple Cap and Carmel Form ations in southwestern Utah record a pulse of active are-related volcanism between 166 and 171 Ma. A second pulse between 148 and 155 Ma has previousl y been documented in the Upper Jurassic Morrison Formation. Volcanic and vo lcaniclastic rocks of these same ages have also been identified closer to o r within the are in California in the Inyo Mountains, the Cow hole Mountain s, the Palen Mountains, and the central Mojave Desert. The upper part of th e volcaniclastic Mount Wrightson Formation and the strata of Cobre Ridge in southern Arizona are ca, 170 Ma in age and appear to be time correlative w ith the Middle Jurassic formations in southwestern Utah. The altered ash beds found in the Temple Cap and Carmel Formations typicall y contain phenocrysts of sanidine, quartz, biotite, apatite, zircon, and ti tanite. Plagioclase was likely present originally in all of the ashes, but was removed by alteration and is now found only in the Temple Cap red beds. Quartz and sanidine are absent in two crystal-poor ash beds that contain t wo pyroxenes, hornblende, and biotite. Although major and trace element con centrations in the ash beds have been substantially modified, compositions of relict phenocrysts reveal that the magmas were calc-alkaline rhyolites t o andesites. Two-pyroxene, two-feld- spar, biotite, and biotite-apatite the rmometers suggest that crystallization occurred at temperatures ranging fro m 740 to 910 degreesC. Hornblende geobarometry yields pressures of 1-2 kilo bars for the two ash beds that contain the appropriate buffer assemblage. T he mafic silicates all have moderately high Mg/Fe ratios. This fact, combin ed with the presence of hornblende, biotite, and titanite, suggests that th e phenocrysts crystallized at high oxygen fugacities similar to those of th e granites of the batholiths of California. The ash probably erupted from a low-lying are cut by strike-slip faults in what is now southern California and western Nevada. Major Jurassic unconformities occur near or within the ash-bearing formatio ns in southwestern Utah. Laser-fusion single-crystal Ar-40/Ar-39 measuremen ts have defined the ages of the unconformities and the associated volcanism , The age of the J-1 unconformity, found at the base of the Temple Cap Form ation in southwestern Utah, is older than ca, 170.5 Ma. The 5-2 unconformit y, which lies between the Temple Cap and Carmel Formations, formed between ca, 169 and 168 Ma. The origin of these unconformities is still unclear, bu t may be related to the Middle Jurassic pulse of magmatism and the oblique plate convergence along the western margin of North America. The age range of ash beds in the Carmel Formation between 166.3 and 168.0 +/- similar to0 .5 Ma is consistent with a Bajocian-Bathonian boundary of ca. 166 Ma.