LATE JURASSIC VOLCANO-SEDIMENTARY INTERACTIONS AND IN THE HODGE VOLCANIC SERIES, IRON MOUNTAIN, CENTRAL MOJAVE DESERT, CALIFORNIA

This study examines the interbedded volcanic and metasedimentary rocks of the Hodge Series in the Iron Mountain area in the Central Mojave Desert to evaluate the relationship of volcanism in the Late Jurassic to local sedimentation in the basin, and more broadly in the disarticulated section of the Jurassic belt of the Mojave. The Hodge sequence, initially mapped by Dibblee (1967) and later detailed by Boettcher and Walker (1993), encompasses approximately 3 km of exposed metasedimentary and volcanic strata predominantly composed of andesitic porphyries to rhyolite tuffs. Metamorphic mineral assemblages of biotite + quartz + muscovite + clinozoisite + plagioclase ± sillimanite in aluminous sections is typical, suggesting that metamorphic temperature conditions of the region reached the amphibolite facies. Field examination of the sequence was conducted to establish the relationship of volcanic and sedimentary interactions. Instances where volcanics are interbedded with metasedimentary rocks at scales of <1 m thick were observed in the field, suggesting regular volcanic eruptions concurrent with sediment deposition in the basin. New U-Pb zircon geochronology obtained a weighted mean age of 147.9±1.6 Ma (n = 12 grains) on Boettcher and Walker’s post-tectonic “Granite of Wild Crossing” which is interpreted to intrude the sequence. Volcanics within the sedimentary sequence range in age from 146.1 to 149.4 Ma, within uncertainty of the age of the post-tectonic granite; sediments interbedded with volcanic rocks contain zircon ranging in age from 150 to 230 Ma with the majority being ca. 164 Ma. These findings portray a vigorous pace of volcanism and sedimentation in the Late Jurassic. The preponderance of Late Jurassic ages also suggests a more likely correlation of the Hodge Volcanic Series with the upper Sidewinder Volcanic Series to the southwest, underscoring a dynamic interplay of the Late Jurassic extension, volcanism, and sedimentation of the Mojave Desert region.