The Geochemistry of the Mono Craters‐Mono Lake Islands Volcanic Complex, eastern California

The Mono Craters-Mono Lake islands volcanic complex consists of Late Pleistocene to Recent, basalt through high-silica rhyolite lavas. The lavas, excluive of high-silica rhyolites, are divided into high-and low-Ba series. Rocks of the high-Ba series include basalts, dacites, and rhyolites with SiO2 as high as 72 wt. %, all found in the Mono Basin. “Fp” (finely porphyritic) rhyolites of the Inyo chain extend this series to 74 wt. % SiO2. Basaltic rocks of the high-Ba series are depleted in Nb relative to large ion lithophile elements, and have trace-element signatures similar to subduction-related lavas. Ba concentrations increase from basalt to dacite, where they reach a maximum of about 1500 ppm, and then fall in the ryolites due to fractionation of sanidine. eNd values of the high-Ba series range from +1.6 to −2.0, and they show no regular variation with silica contents. This series probably represents numerous batches of magmas from an isotopically heterogeneous source. The high-Ba series is believed to contain a relatively large mantle component, although various magma batches evolved separately by crystal fractionation, assimilation, and mixing. The low-Ba series is volumetrically minor and is composed of basaltic and andesitic inclusions and one dacite dome in the Mono Craters. The low-Ba dacite (300 ppm Ba) appears to be derived from a crustal source that contained residual K-spar. The andesitic inclusions are hybrid and also probably contain a significant crustal component. The high-silica rhyolites of the Mono Craters, although monotonous in major-element chemistry, can be divided into the following three groups based on phenocryst mineralogy and trace-element chemistry: (1) biotite-bearing, (2) orthopyroxene-bearing, and (3) fayalite-bearing plus crystal-poor lavas. The biotite-bearing domes are the oldest and are not directly related to the other high-silica rhyolites. The orthopyroxene-bearing domes have the highest concentrations of light rare-earth elements and Zr and are the least evolved high-silica rhyolites of the chain. The fayalite-bearing and crystal-poor rhyolites are chemically homogeneous, and the trace-element data are consistent with their derivation from the orthopyroxene-bearing domes by fractionation of a feldspar-dominated, allanite-bearing assemblage. It is unlikely that the high-silica rhyolites are related to magmas similar to the low-Ba dacite. For example, the dacites and rhyolites contain zircon; thus Zr concentrations should fall and Nb should rise during fractionation from dacite to rhyolite. The high-silica rhyolites have similar concentrations of Zr and Nb as the low-Ba dacite, indicating that the former are not related to the latter. The geochemical data are qualitatively consistent with the high-silica rhyolites forming by fractionation of high-Ba rhyolites similar to the “fp” lavas of the Inyo chain. If this model is correct, then the high-silica rhyolites of the Mono Craters probably contain a large mantle component, and several cubic kilometers of new crust with an important cumulate component were added to the basement of the Eastern Sierra Nevada in Quaternary time.