MINERAL COMPOSITIONS FROM VOLCANIC ASH BEDS OF THE LATE JURASSIC BRUSHY BASIN MEMBER OF THE MORRISON FORMATION, CENTRAL UTAH

The Morrison Formation contains many volcanic ash beds thought to have been erupted from a Late Jurassic volcanic arc along the western coast of North America. The ash is preserved in the Brushy Basin Member, which was deposited in a back bulge basin during the Nevadan Orogeny. The purpose of this study is to characterize phenocrysts in ash beds from a section of the Brushy Basin Member at Little Cedar Mountain, Utah in order to better understand the magma composition and infer the tectonic setting of the magmatism. Even though the glass in the ash beds (n=31) has altered to clay and zeolites, phenocrysts are preserved, including: quartz, sanidine, plagioclase, biotite, apatite, titanite, zircon, and in one bed, hornblende. Plagioclase is present in 84% of the beds and ranges from An₁₇ to An_44. Quartz and sanidine are both present in 29% of the ash beds, indicating a rhyolitic composition, the rest have dacitic mineral assemblages. Sanidine ranges from Or₃₇ to Or₆₇. Two-feldspar geothermometry (Elkins and Grove, 1990) indicates an average temperature of 765 ^oC, typical of rhyolites. Biotite phenocrysts are present in 61% of the ash beds and yield an average temperature of 768 ^oC (Henry et al. 2005). The Fe/ (Fe+Mg) ratios of biotite range from 0.3 to 0.6. The biotites which have Fe/ (Fe+Mg) higher than 0.5 are not typical subduction zone rhyolites and may have an anorogenic (A-type) character. Titanite occurs in 52% of the ash beds and is limited to ash beds with low Fe/ (Fe+Mg), indicating an oxidized magma source typical of subduction zones. Most of the titanite phenocrysts have Fe/Al ratios greater than one, typical of volcanic rocks. Apatite is present in 48% of the ash beds. All of these apatites are fluorapatites and have only minor Si substitution for P. Hornblende was present in only one ash bed, LCM-32. The average Fe/ (Fe+Mg) ratio of this hornblende was 0.4 indicating that it crystallized in an intermediate composition magma, a conclusion supported by the mineral assemblage which consists of hornblende, plagioclase, biotite, titanite and zircon. The mineral chemistry of these Late Jurassic ash beds confirms that they erupted during a transitional time in North America where subduction was changing to transform movement along the western plate boundary.