Fifty Shades of Grey: Zircon Perspectives on the Timing and Chemistry of Magma Interactions Reflected in the Bishop Tuff

The ~0.76 Ma Bishop Tuff eruption in eastern California is an archetypal example of the products of a compositionally stratified magma chamber. Key issues regarding the melt-dominant magma body that have arisen from recent studies include the role of horizontal variability versus vertical stratification, and the nature and timing of a late-stage ingress of compositionally contrasting melt. The latter is reflected in, for example, CL-bright rims on quartz, and Sr- and Ba-rich rims on sanidines, both of which are reported in samples from ignimbrite erupted from vents along the northern caldera rim. Various studies have placed contrasting temporal constraints on assembly and evacuation of the Bishop Tuff magma body and the processes occurring within it from one or two mineral phases. Although the overall crystal specific record is more complex, it has the potential to distinguish between mixing, in-situ crystallisation and other magmatic processes, and resolve the associated timings of these events. Zircon has been widely utilised in the Bishop Tuff and other silicic systems because of its unique records of information about the ages and P-T-X environments of growth. Here we present SHRIMP-RG ion probe data from Bishop Tuff zircons, collected from samples spanning the full eruptive sequence. A combination of age data, textural analysis and trace element analysis is used to yield insights into magma chamber development and the pre-eruptive state of the melt-dominant magma body, as evidenced in material erupted from different vent areas around what became the Long Valley caldera. U-Pb dating of 307 spots from all zones in crystals from all samples yields a strongly unimodal age spectrum, with a mean age of 794+/-3 ka (10 spots rejected, MSWD = 2.0). CL images of the dated zircon suites show the appearance of bright-rimmed zircons in later-erupted ignimbrite packages (accompanied generally by bright-rimmed sanidine and quartz crystals), but with diversity in the proportions of the bright-rimmed zircons and the thicknesses of the rims. Diverse zircon CL patterns between the Ig2N and Ig2NW units implies that zircon growth regimes varied along the northern caldera rim, and indicate the presence of geographic variation in crystals across the Bishop Tuff magma body. Trace element analyses of the 'standard' Bishop non-rimmed sector-zoned zircons show strong variations in contents of elements including U, Th, P and Ti between dark and light sectors in growth zones of the same age (as indicated by subordinate oscillatory zoning). This study highlights the importance of examining samples which span the entire eruptive sequence, and illustrates lateral variations in the magma body which add complexity to the pre-existing 'early' versus 'late' contrasts emphasized by many workers.