The Origin of Quartz Phenocrysts in the Bishop Tuff Rhyolitic Magma Based on Trace Tlements in Melt Inclusions and Pumice Matrix Glass

The origin of crystals in silicic volcanic rocks has been challenged by textural and isotopic studies of phenocrysts and melt inclusions in some silicic ignimbrites. These studies suggest that quartz and feldspar are `xenocrysts' derived either from re-melting of previously solidified magma bodies or disaggregation from crystal-rich rinds along the margins of the magma body. To investigate the origin of quartz in the Bishop Tuff, we have analyzed trace elements in the matrix glass of pumice clasts and in quartz-hosted melt inclusions. Our results suggest contrasting origins for quartz in different parts of the Bishop Tuff. In all samples from the early part of the eruption (F1-F7, terminology of Wilson and Hildreth, 1997) and tephra fall samples from the middle part (F9), trace element compositions of matrix glasses are similar to but slightly more evolved than quartz-hosted melt inclusions. This indicates a cogenetic relationship between quartz crystals and their surrounding matrix glass, consistent with in situ crystallization. The range of incompatible element concentrations in melt inclusions and matrix glass from single pumice clasts requires 16-20 wt.% in situ crystallization. This is greater than the actual crystal content of the pumices (<15% crystals). Interestingly, pyroclastic flow samples from the middle part of the eruption (Ig2Ea, Ig2Eb) show contrasting trends: in some clasts the matrix is more evolved than the inclusions whereas in other clasts the matrix is less evolved than the inclusions. In the late Bishop Tuff (Ig2Na, Ig2NWa) all crystal-rich samples have matrix glasses that are significantly less evolved than quartz-hosted inclusions. Trace element abundances indicate that most of the quartz in the late Bishop Tuff crystallized from more differentiated rhyolitic magma similar in composition to early erupted Bishop Tuff. We used cathodoluminescence images of melt inclusion-bearing quartz crystals to correlate growth in the host crystal with compositional data from melt inclusions to assess the chronological evolution of melt inclusion compositions. All quartz crystals show a variety of oscillatory zoning textures. Melt inclusions in samples from the early part of the eruption and tephra fall samples from the middle part are less evolved in the core of the crystal and are more evolved closer to the crystal rim, consistent with in situ crystallization of quartz. However, quartz phenocrysts from late-erupted samples have bright luminescent rims that are not observed in early and middle erupted samples (Peppard et al., 2001, Am. Min.). Highly differentiated melt inclusions similar to those in early samples occur within the dark cores whereas less differentiated inclusions are associated with the bright rims. Our results for late erupted Bishop Tuff are compatible with either crystal settling of quartz from shallower levels within the magma body or late-stage mixing of a new batch of less differentiated rhyolitic magma in the deeper levels of the Bishop magma chamber.