ASSESSMENT OF CRUSTAL CONTAMINATION IN VOLCANIC ROCKS LOCATED WITHIN THE COLORADO PLATEAU OF CENTRAL UTAH

Tertiary volcanic rocks are a common part of the Colorado plateau in Central Utah. These volcanic rocks have erupted through continental crust that thickens from the edge of the Colorado plateau towards its interior. Consequently, volcanic rocks should see increasing interaction with continental crust and changes in their geochemistry from the edge of the Colorado plateau towards the interior. To test this hypothesis we collected samples along a west to east transect of the Colorado plateau in Central Utah. From west to east this transect includes the Tushar Mountains, Monroe Mountain, Fish Lake Plateau, and Thousand Lake Mountain. Many of these rocks have never been studied and their geochemistry is unknown. Consequently, this study will provide new data for the geologic community. Samples were sectioned and examined with a polarized light microscope to determine phenocryst assemblages and volcanic textures. The Michelle Levy was used to determine the plagioclase compositions in samples where it was possible. The data collected shows that the Tushar Mountains are dominated by felsic material with some subordinate mafic material from the most recent eruptions. To the east, the mountain ranges are dominated by more mafic compositions, and farthest to the east we once again encountered a reoccurrence in felsic material. Also in this area we encountered strong evidence for magma mixing. This evidence includes resorbed grains, sieve textures, and reaction rings around grains. This magma mixing was most likely due to the mafic magmas interacting with the thickened continental crust of the Colorado Plateau. Selected samples will be analyzed using X-ray fluorescence techniques. From this, major element and trace element data will be collected. XRF data should provide more detailed information about the sources of magmas as well as possibly provide more extensive evidence for crustal contamination in these areas.