New Mexico Geological Society Annual Spring Meeting — Abstracts

Melted xenoliths are present at Vulcan cone, highest ofthe middle Pleistocene Albuquerque volcanoes. The inclusions at vulcan cone are light colored, are morethan 90% glass and, in many cases, are extremely vesicular. Petrographic examinations show that the inclusions are about 15-30% rounded quartz grains and glass (locally hydrated to unidentified zeolites) and vesicles.

XRF analyses indicate a SiO₂ content of 84%. This composition is unreasonable for an igneous protolith but, along with. the other oxides, is consistent with· an arkosic sandstone (70% quartz, 13% K-feldspar, 17% plagioclase [An28]).

The basalt is not quenched where it is in contact with thee:xotic melt leading to the conclusion that the magma had reached thermal equilibrium. In places the melts have thoroughly mingled and a hybrid liquid appears to have formed. We have found no reference to similar extremely melted sedimentary xenoliths, suggesting unusual conditions for their formation.

Our hypothesis is that the extreme melting was a consequence of wet-melting at sufficient depth that the pore water was not expelled but participated in the melting reactions and was incorporated into the newly formed melt. The extreme vesicularity of the ejecta affirms a high dissolved volatile content for the melted xenoliths. Mingling of the two melts also suggests similar viscosity at some point in their combined history, despite striking contrast in silica content. This might also be explained by a high water content for the felsic melt. To test this idea we are currently calculating the viscosity ofthe two melts for different water contents. These water contents will be compared to experimental haplogranite water solubility data to estimate a depth ofmelting. This depth will be compared to the inferred stratigraphic section beneath the volcanoes for consideration of potential protolith horizons.