BUBBLE ACCUMULATION AND ORIGIN OF A VESICULAR MICROGRANITE

A 4 km section through the Miocene Aztec Wash pluton (southern Nevada) was exposed by extension-related tilting. The pluton is strongly zoned, with a lower portion composed of interlayered granitoids and mafic rocks, and an upper, more homogeneous, granitic zone. Close to the roof of the intrusion, distinctive, finer-grained rocks include an extremely felsic granite with abundant cavities up to several mm diameter.

Using x-ray tomography, we are quantitatively studying the size, shape and distribution of cavities present. Hand-specimens were imaged at the NDE X-ray Lab, Argonne National Laboratory. Image processing was performed using Blob3D and routines written by us in IDL. Resulting images are 3D maps of the linear attenuation coefficient, and allow easy distinction of voids and the quartzo-feldspathic matrix.

The largest cavities imaged so far are 4-5 mm in diameter. Crystals that surround the cavities are close to or smaller than 1 mm in diameter, and no phenocrysts are present, indicating that the cavities must have formed before significant crystallization. In this sense, they are true vesicles, and record the presence of bubbles in a largely molten, highly fractionated granitic liquid. Recognition of the vesicular nature of this microgranite has important petrologic consequences. First, a bubble-rich, low-density magma like this would be prone to erupting to the surface. High-silica rhyolites of similar age in volcanic successions in the vicinity of Aztec Wash are potentially erupted equivalents. Second, the abundance and size of the vesicles increase with stratigraphic height, with a range in porosity from about 1 vol. % up to 10 % or more. Extensive exsolution of volatiles would promote crystallization of phenocrysts, which is not observed; hence, we interpret the upward increase in vesicle porosity to reflect accumulation of bubbles towards the upper portions of the unit.

Plausibly the vesicular microgranite formed from fractionated liquids that collected beneath the roof. Large bubbles developed by a combination of growth and coalescence during ascent to this zone and, possibly, by accumulation of bubbles rising from the granitic mush column below. We speculate that at times during the evolution of the pluton, this layer may have become unstable leading to eruptions.