MAGMA MIXING AND MAGMA CHAMBER INVERSION RECORDED IN HIGH-SILICA ROCKS OF THE BISHOP TUFF, EASTERN CALIFORNIA

We present electron microprobe analyses for 11 samples of Bishop Tuff that show evidence for magma mixing and magma chamber inversion during eruption. The Bishop Tuff, a high-silica pyroclastic deposit, formed when the Long Valley Caldera erupted 760,000 years ago in Eastern California. Samples were collected on a 126 m vertical transect in the Owens River Gorge to test the hypotheses (1) that phenocrysts in the tuff show evidence for magma mixing and (2) that stratigraphic variations in phenocryst/glass composition reflect magma chamber inversion during eruption.

Microprobe transects across feldspar and biotite phenocrysts show evidence that magma mixing was an important process in Bishop Tuff formation. Feldspar grains show K-rich cores with Na, Ca, and Ba increasing toward the edges of the grains; biotite phenocrysts show K-rich cores with F and Mg increasing on the rims. These variations are evidence for magma mixing of an initial potassium-rich high silica rhyolite with a more Mg- and plagioclase-rich low-silica rhyolite. Plotted on a vertical transect, average concentrations of elements in phenocrysts and glass show little variation from 0 to 65 meters of Tuff above the bottom of Owens River Gorge, followed by large variations from 65 to 126 meters. We interpret the first trend to represent the crystallization of an initial high-silica rhyolite. The distinct change above 65 meters represents mixing of high- and low-silica rhyolites and/or tapping of different parts of the magma chamber.

This study provides evidence that the Bishop Tuff formed through complex modification processes. Chemical variations across phenocrysts are inconsistent with fractional crystallization; instead, mixing between high- and low-silica rhyolites must be invoked. Stratigraphic chemical variations indicate complex mixing and magma chamber inversion during eruption of the tuff, tapping a high-silica rhyolite first, followed by extraction of deeper less silica-rich melts that may have mixed in varying proportions with the original high silica rhyolite as they erupted. This study provides a better understanding of magma chamber processes and indicates that magma mixing prior to or during eruption and magma chamber inversion during eruption may play key roles in large-scale pyroclastic eruptions.