New Mexico Geological Society Annual Spring Meeting — Abstracts

Quartz xenocrysts with granular, green halos are common in the rhyolitic EI Cajete pumice, Jemez Mountains, North-Central New Mexico. Compositional and textural analysis of augite in the reaction rims suggests that the quartz and augite coronas resulted from multiple mafic-felsic mingling events.

SEM examination using backscattered electron imaging revealed sparse-tomoderate amounts of augite oriented radially with respect to the fractured and 'crackeled' quartz grains. Quartz grains were embedded in a coarsely vesicular pumice halo about 1mm wide which was in turn surrounded by finely vesicular pumice. Petrographic study showed that the quartz grains were rounded and embayed and occasionally polycrystalline. Augite coronas were discontinuous and separated from the quartz by a glassy interval but always occurred within the coarsely vesicular halo. Individual augite grains in the corona were generally euhedral; however, single crystals and patches of finely granular augite occurred in the groundmass. Two varieties of feldspar phenocryst were present; wormy and fritted sadic plagioclase, some with clear exterior rims, and largely pristine calcic plagioclase. Some feldspar phenocrysts had inclusions of brown glass. Groundmass minerals were pyroxene, magnesian olivine, Fe-Ti oxides, and euhedral amphibole, sometimes jacketing biotite.

Based on our results, we concluded that EI Cajete quartz was xenocrystic in a more mafic magma and augite coronas grew in response to diffusion-limited reactions in dissolution boundary layers at crystal margins. The mafic magma provided the thermal energy necessary to induce partial melting of a granodiorite (reported by Gardner et al.1995, to be the parent of the EI Cajete) and, as limited mixing occurred, quartz xenocrysts and coronas were assimilated into a new, more silicic, melt. During eruption of the EI Cajete pumice the quartz xenocrysts acted as nucleation sites for vesiculation and thus the coarsely vesicular halo formed. Continued rapid vesiculation during eruption probably dispersed many of the augite crystals into the groundmass.