BIOTITE AND TITANITE COMPOSITIONS FROM ALTERED ASH BEDS OF THE BRUSHY BASIN MEMBER OF THE MORRISON FORMATION: FURTHER EVIDENCE FOR TRANSTENSION IN THE CONTINENTAL MARGIN ARC OF THE LATE JURASSIC, WESTERN NORTH AMERICA

The Brushy Basin Member of the Morrison Formation contains numerous volcanic ash beds thought to have erupted from a Late Jurassic volcanic arc along the west coast of North America. These sediments were deposited in a back bulge basin during the Nevadan orogeny. Phenocrysts in 31 ash beds from a section near Huntington, Utah were studied to better understand magma compositions and infer the tectonic setting of the magmatism. Glass has altered to clay and zeolites, but phenocrysts are preserved, including: quartz, sanidine, plagioclase, biotite, apatite, titanite, zircon, and, in one bed, hornblende. Biotite was found in 19 of the samples and is by far the most abundant mafic phase. Compositions of biotites in terms of Fe/(Fe + Mg) and Al generally fall in the area of calc-alkaline igneous rocks. However, some samples trend into the A-type granite field. This is different than in Middle Jurassic ash beds in southern Utah, which overlap almost perfectly with the calc-alkaline Fish Canyon Tuff. Plotted as log(X_Mg/X_Fe) vs. log(X_F/X_OH), most of the biotites fall into or to the right of the highly oxidized fields, but scatter across the moderately contaminated (I-MC) to strongly contaminated (I-SC) fields. They are more strongly contaminated (i.e. higher F/OH ratios), on average, than biotites from Middle Jurassic ash beds in the Temple Cap and Carmel Fms., but not quite as enriched in F as the biotites in the Fish Canyon Tuff. However, a significant number of the Late Jurassic biotites scatter into the strongly contaminated reduced field (I-SCR), unlike the Middle Jurassic. Temperatures from biotites (Henry et al., 2005) ranged from 730-800°C, with an average of 768°C and are comparable to two-feldspar temperatures (703-844°C) with an average of 752°C. Many of these Late Jurassic ash beds have titanite. Biotites, from ash beds that contain titanite, are compositionally distinctive and typically have low Fe/(Fe + Mg) and high log(X_Mg/X_Fe). They also have higher log(X_F/X_OH), similar to the titanite-bearing Fish Canyon Tuff. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America where subduction was changing to transtensional movement along the western plate boundary.