The Mono Craters-Mono Lake islands volcanic complex consists of Late Pleistocene to Recent, basalt through high-silica rhyolite lavas. The lavas, exclusive 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 rhyolites due to fractionation of sanidine. &egr;Nd 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: (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. ¿1990 American Geophysical Union