ARE TITANITE GRAINS IN MIDDLE AND LATE JURASSIC TUFFS OF THE CARMEL AND MORRISON FORMATIONS OF UTAH VOLCANIC IN ORIGIN OR DETRITAL GRAINS FROM NON-VOLCANIC SOURCES?

Titanite (sphene) is a common accessory phase in plutonic and metamorphic rocks, but has been reported to be uncommon in volcanic rocks. This does not appear to be true for altered Middle and Late Jurassic fallout tuffs preserved in the Temple Cap, Carmel, and Morrison formations in Utah where about 40% of the ash beds contain titanite. However, since these titanite grains only comprise a trace percentage of the total residual minerals recovered from any of the thin altered ash beds, their origin might be questioned. Were they erupted along with other recovered phenoclasts, or are they detrital in origin? In order to answer this question, we have accumulated a large number of geochemical analyses on major and trace elements from the literature, as well as from our own samples for comparison. Titanite from the altered tuffs is most similar to titanite from other volcanic rocks. This is apparent for REE, Y, Fe, and Mn concentrations, which are typically higher in volcanic titanite, and for Al and F, which are typically lower. They are also similar to plutonic titanite grains, particularly those from Jurassic plutons of southern California. However, titanite grains from the Jurassic tuffs and other volcanic rocks are typically not as strongly zoned as plutonic titanite and lack rim zones that in many plutonic titanites are low in REE and Y and higher in F. We interpret this stronger zoning to be the result of long crystallization and cooling histories of plutonic titanite. Titanite compositions from metamorphic and hydrothermal environments are quite different than titanite from the Jurassic tuffs with higher Al and F and lower REE, Fe, Y, and Mn. With only a few exceptions, the geochemical data support a volcanic origin for the titanite found in the altered tuff beds of the Carmel, Temple Cap, and Morrison formations.