WERE LATE JURASSIC PLUTONS OF THE MOJAVE DESERT REGION IN SOUTHERN CALIFORNIA ERUPTIVE SOURCES FOR SOME ASH BEDS IN THE MORRISON FORMATION , UTAH?

The Late Jurassic Morrison Formation of the western United States contains many beds of fallout tuff. The eruptive sources for the ashes are not known. A few plutons in southern California (Rattlesnake Creek, White Tank, Crystal Creek, John Bull Flat, and Juniper Flats) are of the right age (about 150 Ma according Barth et al., in review) and the right Pb-isotopic composition to be possible eruptive sources. To test this correlation, compositions of apatite, biotite, plagioclase, alkali feldspar, and titanite were analyzed from these granitic rocks and ash beds by electron microprobe. We analyzed phenocrysts from 33 ash beds from Little Cedar Mountain near Huntington, Utah, and 5 samples from the plutons listed above.

A difficulty with this approach is that plutonic minerals re-equilibrate at subsolidus temperatures. Alkali feldspar and biotite become K-rich and Na-poor, whereas plagioclase loses K. Apatite becomes Cl-poor and F-rich. Biotite is Cl-poor in the plutonic rocks. Biotite geothermometry (Henry et al., 2005) shows that the biotites in the intrusive rocks continued to equilibrate to low temperatures (530-730°C), while biotite in the ash beds yield temperatures more typical of magmatic rocks (715 to 800°C). This suggests that this geothermometer may be used to estimate eruption temperatures for volcanic rocks.

Nonetheless, some elements in these plutonic minerals (Fe, Al, Mn, Y, and LREE) may be useful for identifying eruptive equivalents. For example, the alkaline plutonic rocks have apatite with anomalously high LREE and Mn, biotite with high Mn, and titanite with high Mn, Y, and LREE. Apatite, biotite and titanite in some of the ash beds have similar anomalous concentrations of these elements. This is consistent with our hypothesis that these plutons may have been eruptive sources for some of the volcanic ash beds in the Morrison Formation.