Tephra sequences as indicators of magma evolution: 40Ar/ 39Ar ages and geochemistry of tephra sequences in the southwest Nevada volcanic field

Abstract

Changes in rock chemistry with 40Ar/ 39Ar ages in tephra layers record the temporal and magmatic history of two volcanic systems in southwestern Nevada. Tephra layers from the Post-Grouse Canyon tephra sequence record three distinct groups. These groups are chemically distinct and have 40Ar/ 39Ar ages of 13.52±0.06, 13.31±0.18, and 12.95±0.10 Ma. The age groups correspond to three distinct chemical groups based on trace element distributions. These chemical groups cannot be related by any reasonable fractional crystallization or magma mixing model and are interpreted as distinct magma batches. The Pre-Rainier Mesa tephra sequence records two 40Ar/ 39Ar ages (12.79±0.10 and 11.84±0.18 Ma). The upper portion of this sequence is equivalent in age and chemistry to part of the overlying Rainier Mesa ash-flow sheet. The lower portion of the sequence is equivalent in age to the underlying Tiva Canyon ash-flow sheet but is chemically distinct from this sheet. The formation of this chemical group is consistent with mixing of low silica Tiva Canyon and high silica, low Th, Rainier Mesa magma. Post-Grouse Canyon magmas were most likely emplaced as a series of small, unrelated magma bodies, which allowed them to evolve independently. The mixed Pre-Rainier Mesa magma was produced by infilling of the Tiva Canyon magma chamber with Rainier Mesa-like magma after eruption of the Tiva Canyon ash-flow sheet at approximately 12.8 Ma. The upper portion of the Pre-Rainier Mesa tephra sequence represents eruption of Rainier Mesa magma less than 0.3 My. before that of the voluminous (1200 km 3) Rainier Mesa ash-flow sheet at approximately 11.71 Ma.