USING MINERAL CHEMISTRY TO UNDERSTAND THE ASCENT OF MAGMA THROUGH THE COLORADO PLATEAU - A STUDY OF THE HENRY MOUNTAINS LACCOLITH

Laccoliths are large, lens-shaped intrusions of magma into sedimentary rock layers of the shallow crust. They are well exposed in southern Utah due to uplift and erosion since the Oligocene Epoch. Early studies of these intrusions focused on general rock descriptions and morphology (G.K. Gilbert, 1877). Recent studies have focused on the physical properties of the magma as it intrudes and deforms the strata of the shallow crust (e.g. Wilson et al. 2016), leaving an opportunity to use rock and mineral chemistry to better understand the deep crust. The 30-23 Ma laccolith intrusions of the Henry Mountains represent the last stages of magma traveling through the thick crust of the Colorado Plateau. During the ascent, the magma composition changed through multiple interactions with the surrounding crust. Those physical and chemical signatures are recorded in the mineral assemblage of the intrusions. We have collected samples from several intrusions of the Mt. Hillers Complex at the southern end of the Henry Mountains. The complex consists of a plagioclase-hornblende porphyry with amphibolite xenoliths. Data from the xenoliths is shedding light on the composition of the base of the crust beneath the Colorado Plateau while plagioclase and amphibole from the host magma is recording signatures from the mid and upper crust. With mineral compositions collected via electron microprobe, we have calculated the depths at which hornblende crystallized (Ridolfi et al., 2012). We calculated hornblende P/T conditions ranging from 1.8-7.2 kbar with an average of 4.95 kbar signifying a mid-crustal staging area for the magma prior to shallow emplacement of the laccoliths. We also calculated P/T conditions for amphibolite xenoliths found in the samples. These xenoliths contain hornblende that crystallized between 12-17 kbar, or the base of the Colorado Plateau during arc magmatism. We plan to use these data with plagioclase data to better reconstruct the geometry of the crust beneath the Colorado Plateau.