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Abstract
Compositional and $\rm\sp{40}Ar/\sp{39}Ar$ ages in three tephra sequences from the southwestern Nevada record the chemical history and timing of eruption from the parent magmas producing these volcanoclastic sequences. Compositional changes within two of the tephra sequences, the Post-Grouse Canyon and the Pre-Rainier Mesa tephra sequences, correspond with changes in $\rm\sp{40}Ar/\sp{39}Ar$ ages.
The Post-Grouse Canyon tephra sequence is divided into three main corresponding age and compositional groups. The compositional groups are not easily related by crystal fractionation or magma mixing and are interpreted to represent distinct magma batches derived from separate sources. A highly compositionally variable layer occurring within the middle age group, is interpreted to reflect the eruption of an additional distinct magma type. This pattern of emplacement and eruption of unrelated magmas recorded in the Post-Grouse Canyon tephra sequence mimics the inferred processes operating in large-volume magmatic systems in this region.
The Pre-Rainier Mesa tephra sequence is divided into two discrete age and compositional groups. The upper sequence is the age and chemical equivalent of the high-silica, low Th/Nb portion of the overlying Rainier Mesa Tuff (11.6 Ma). The sequence is the age equivalent of the underlying Tiva Canyon Tuff (12.7 Ma), however is compositionally unlike the Tiva Canyon Tuff. The lower Pre-Rainier Mesa tephra sequence is consistent with at least two magma mixing combinations. Both combinations involve mixing of magma compositionally similar to the low Th/Nb, high silica group from the Rainier Mesa Tuff. The inferred presence of Rainier Mesa-like magma at 12.7 Ma suggests that more than one magma pulse was emplaced from the source rocks that produced the Rainier Mesa magma. The emplacement of multiple magma pulses is in contrast to the inferred single pulse emplacement style for the large-volume magmatic systems in the region.
The third tephra sequence, the Pre-Ammonia Tanks tephra sequence, does not exhibit a compositional change with respect to stratigraphic position. In general, the entire compositional range (70-78% SiO$\sb2$) is present throughout the sequence. This compositional variation is consistent with the overlying Ammonia Tanks ash-flow sheet. Ages for the Pre-Ammonia Tanks tephra sequence are not well constrained and may be partially reset.
