Geochemical similarities between volcanic units at Yucca Mountain and Pahute Mesa: evidence for a common magmatic origin for volcanic sequences that flank the Timber Mountain Caldera

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Chemical compositions have been determined for sanidine, plagioclase, biotite, and hornblende phenocrysts by electron microprobe for a comprehensive set of samples of Crater Flat Tuff and tuffs of Calico Hills. Most of these samples were obtained from drill holes at Yucca Mountain. Samples of tuffs and lavas of Area 20, obtained from locations at Pahute Mesa, have similarly been subjected to microprobe analysis. Complete modal petrography has been determined for all samples. Biotite and hornblende in the samples from both Yucca Mountain and Pahute Mesa have Fe-rich compositions that contract strikingly with Fe-poor compositions in the overlying Paintbrush Tuff and the underlying Lithic Ridge Tuff at Yucca Mountain. Each unit from Yucca Mountain has distinctive compositions for both sanidine and plagioclase that very closely match compositions for a corresponding unit identified within the lower, middle and upper portions of the Area 20 tuffs and lavas from Pahute Mesa. Each of these paired units probably originated from a common parental magma and was eruptd contemporaneously or nearly so. Each pair of units with matching phenocryst chemistries has a similar, but not identical set of petrographic characteristics. The petrographic differences, as well as small differences in phenocryst chemistry, result from a sonal distribution of phenocrysts within the parent magma chamber and eruption through earlier units that differ markedly between Yucca Mountain and Pahute Mesa.