Melt Inclusion Insights Into Coeval, Late-Erupted Portions of the Bishop Tuff

A remaining area of uncertainty in the Bishop Tuff is tying together the late stages of ignimbrite activity from vents on the northern and eastern margins of Long Valley caldera with their coeval fall deposits (F9). To this end, we present new data on quartz-hosted melt inclusion compositions from stratigraphically controlled samples from the thickest known (2.1 m) section of F9 at a location c. 40 km SE of Long Valley caldera where the fall deposit is uninterrupted by thick ignimbrite deposition. We analyzed major and trace elements in quartz-hosted melt inclusions from four layers within F9 using electron microprobe and LA-ICPMS. We use these data along with those of Wallace et al. (1999; J Geophys Res 104:20097) and Chamberlain et al. (2015; J Petrol 56:605) to investigate the diversity of melt compositions represented in this sparsely documented portion of the eruption. Major and trace element data for the F9 melt inclusions include compositions that span both those of the east-side, later erupted, crystal-poorer ignimbrite (Ig2E) and north-side, later erupted, crystal-richer ignimbrite (Ig2NW, Ig2N). Trace element concentrations for the F9 inclusions are highly variable (e.g. 118-233 ppm Rb; 316-662 ppm Ti; 0.4-8.9 ppm Sr). Least evolved compositions of melt inclusions in F9 quartz crystals are closely similar to those of melt inclusions and pumice glass from north-side Bishop Tuff Ig 2 lobes. Finding material representative of the northern ignimbrite lobes in these fall deposit samples implies that the north and east Ig2 lobes were coevally erupted, as suggested by previous stratigraphic correlations. Initial cathodoluminescence examination of the host quartz crystals from F9 shows the presence of both homogenous and bright-rimmed grains. However, some of the least evolved melt inclusions, with compositions similar to those of melt inclusions from the northern ignimbrite lobes that are trapped in bright-CL quartz rims, come from grains lacking bright rims. These observations, coupled with the continuously variable range in melt inclusion compositions (also in Wallace et al. 1999; Roberge et al. 2013; Contrib Mineral Petrol 165:237) reinforce the view that there was compositional zonation in the deeper part of the Bishop magma body, reflecting prolonged interaction with less-evolved input magma(s).