Early postcaldera rhyolite and structural resurgence at Long Valley Caldera, California

Abstract

After the 767-ka caldera-forming eruption of 650km3 of rhyolite magma as the Bishop Tuff, 90–100km3 of similar rhyolite erupted in the west-central part of Long Valley caldera in as many as 40 batches spread over the 110,000-year interval from ~750ka to ~640ka. Centrally, this Early Rhyolite (ER) is as thick as 622m, but it spread radially to cover much of the caldera floor, where half its area is now concealed by post-ER sediments and lavas. At least 75% of the ER is aphyric rhyolite tuff. Drillholes encountered ~22 (altered) ER lava flows intercalated in the pyroclastic pile, and another 11units of (largely fresh) ER lava are exposed on the caldera's resurgent dome and at Lookout Mountain. Exposed units have been distinguished, mapped, studied petrographically and chemically, and radioisotopically dated; each is described in detail. Their phenocryst contents range from 0 to 2.5wt%. All the phyric units have plagioclase, orthopyroxene, and ilmenite; most have biotite and rare tiny magnetite, and a few contain rare zircon. The compositional range of fresh obsidians is narrow—74.3–75.0% SiO2, 1.21–1.37% FeO*, and 5.12–5.26% K2O, but wider variations in Ti, Ba, Sr, and Zr permit distinction of individual units and eruptive groups. The limited chemical and petrographic variability shown by so many ER batches released episodically for ~110,000years suggests a thermally buffered and well-stirred reservoir.The ER central area, where ER eruptions had taken place, was uplifted ~400m to form a structural dome ~10km in diameter. Most of the inflation is attributable to 10 sills of ER that intrude the Bishop Tuff beneath the uplift, but other processes potentially contributing to resurgence are also considered. As shown by erratics of Mesozoic rocks ice-rafted from the Sierra Nevada and dropped on ER lavas, much of the ER had erupted early enough and at low enough elevation to be inundated by the intracaldera lake and was only later lifted by the resurgence that also raised clusters of the erratics hundreds of meters higher than any shoreline. Most of the uplift was over by ~570ka, but dome-crossing faults that exhibit normal throw of 10–30m cut lavas as young as 175–125ka.For most elements, chemical ranges of the ER lie within those of the zoned Bishop Tuff, which had erupted earlier from the same place. Only Ba, Zr, Hf, and Eu/Eu* extend to ranges outside those of the Bishop Tuff, nominally to less evolved compositions. Initial 87Sr/86Sr values of ER are likewise within the range of the Bishop Tuff, but ER ratios of 143Nd/144Nd and 206Pb/204Pb extend beyond those of the Bishop Tuff to values slightly more influenced by upper-crustal contributions. FeTi-oxide geothermometry yields 752°–844°C for ER, compared to 700°–820°C for the Bishop Tuff. ER fO2 values are 0.5–1.0 log units more reduced than those of the T–fO2 array of the Bishop Tuff. The postcaldera reduction may reflect reaction with graphite from the black lithics of Paleozoic graphitic metapelite so abundant in the Bishop Tuff. Much of the pumice emplaced during the later half of the Bishop Tuff eruption has 10–25wt% phenocrysts, dominantly quartz and sanidine, but the 100km3 of ER has only 0–2.5wt% and completely lacks quartz and sanidine. Postcaldera processes, including mixing, volatile ascent, and crystal resorption, as well as potential contaminants and magmatic inputs, are all considered.