Cooling Before Super-Eruption: No Evidence of Rejuvenation in a Crystal-Rich Dacite Magma Body, Southern Great Basin Ignimbrite Province, Utah and Nevada

Abstract:

The model of rejuvenation of a near-solidus crystal mush to produce large volumes of crystal-rich magma is tested here by analyzing the mineralogical, chemical, modal, and physical characteristics of the 31.1 Ma super-eruptive (2000 km³) Cottonwood Wash Tuff. It is the oldest in a series of three so-called “monotonous intermediate” ignimbrites from the Indian Peak-Caliente volcanic field in southern Utah and Nevada. A crystal-rich (~50% Pl + Qz + Hbl + Bt + Mag + Ilm + Cpx + Zrn + Ap + Po) dacite (62 – 69 wt% SiO₂), the Cottonwood Wash Tuff is similar in age, volume, mineralogy, crystallinity, and elemental composition to the 28.0 Ma, ~5000 km³ Fish Canyon Tuff (~45% Pl + Kfs + Qz + Hbl + Bt + Ttn + Mag + Ilm + Ap + Zrn + Po, 66 – 68 wt% SiO₂), used as the basis of the rejuvenation model. The Cottonwood Wash magma chamber was compositionally varied as shown by mineral and juvenile clast compositions. Whole-rock compositional variations are likely due to the variation of mineral proportions induced by shear in the magma chamber. Mineral compositions and experimental phase relationships show the pre-eruption magma crystallized at 800°C, 2.5 kb under water-undersaturated but oxidized conditions (delta QFM = 2.1). The majority of plagioclase and amphibole grains exhibit small-scale oscillatory zonation; where systematic compositional zonation exists, normal and reverse zonation are equally present. Cathodoluminescence of quartz reveals typically normally zoned phenocrysts with late resorption, considered to be the result of eruptive decompression. Many of the characteristics used to identify the rejuvenation of a near-solidus mush for the Fish Canyon Tuff are not present in the Cottonwood Wash Tuff [i.e., reversely zoned hornblende or plagioclase, partially remelted mineral aggregates, evidence of fluid saturation, resorption textures not related to decompression, rapakivi mantles, and hybrid andesite inclusions.] The Cottonwood Wash magma system did not undergo rejuvenation from a near-solidus state. Instead, the magma was apparently cooling and crystallizing just prior to eruption.