Evidence For Step-Function Zoning Of Magma And Eruptive Dynamics, Toquima Caldera Complex, Nevada

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Journal Article: Evidence For Step-Function Zoning Of Magma And Eruptive Dynamics, Toquima Caldera Complex, Nevada

Analyses of bulk tuff, fiamme groundmasses, and biotite of eruptive units from the Toquima caldera complex, provide insights on the nature of compositional zoning of magma and on the eruptive dynamics of a zoned chamber. Samples of bulk tuff consist of one to several petrographically and compositionally different types of fiamme. The diversity of fiamme is generally least in the first-erupted parts of units but increases with stratigraphic height as less silicic fiamme types join the assemblage. As a result, analyses of bulk tuff, although yielding systematic zoning trends, represent only a minimum range of magma compositions tapped because of mixing of fiamme groups. Microprobe analyses of fiamme groundmasses cluster into tight compositional groups corresponding to petrographically distinctive types of fiamme. Fiamme groups are separated by small, but significant compositional gaps (<4% SiO2) which are not evident in analyses of bulk tuff. Biotite compositional data mimic that of fiamme and cluster into discrete groups that also correlate with petrographically identifiable types of fiamme. The discontinuous compositional zoning of fiamme and biotite suggests that magma was zoned in a stepwise fashion, analogous to zoning patterns developed in experiments modelling boundary-layer and convective fractionation in high-level magma chambers. In such experiments, a series of compositionally distinct, internally convecting layers are produced whose boundaries are characterized by abrupt transitions in composition and density. Reversals in the upward mafic trend of units correspond to discontinuous horizons of tuffaceous sedimentary rocks. These compositional overlaps are interpreted to reflect eruption of residual silicic layers of magma that restratified below the vent during pauses in the eruptive cycle. Similarly, horizons of tuff containing several types of fiamme and multiple compositional populations of biotite are considered to form during periods of accelerated rates of eruption, perhaps occasioned by cauldron subsidence and vent widening, in which several compositional layers of magma are tapped concurrently. Careful analysis of juvenile constituents in ash-flow tuffs in conjunction with well-constrained field relations can provide meaningful information on compositional zoning of magma and dynamics of eruption.

David R. Boden

Published Journal 
Journal of Volcanology and Geothermal Research, 1989

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David R. Boden. 1989. Evidence For Step-Function Zoning Of Magma And Eruptive Dynamics, Toquima Caldera Complex, Nevada. Journal of Volcanology and Geothermal Research. (!) .