A PETROLOGIC ANALYSIS OF THE HENRY MOUNTAINS LACCOLITH

Laccoliths are a mass of igneous rock, typically lens-shaped, that intrude conformably between rock strata causing uplift in the shape of a dome. The formation of laccoliths has been challenging to comprehend. While many researchers have attempted to answer questions regarding the formation through wall-rock deformation, few have delved into the geochemistry of the intrusions themselves. We plan to use the geochemical characteristics to determine the age and architecture of the intrusion. The Henry Mountains have been the site of many structural, thermomechanical, and geomorphological studies and thus provide a great framework for our geochemical exploration. This location is ideal because the laccolith is well exposed and previous research has mapped out multiple intrusions based on spatial distribution. We so far have collected samples from previously defined intrusions of the Mt. Hillers intrusive complex along the southern end of the Henry Mountains. Our goals are to 1) use mineralogy, igneous textures, and bulk rock chemical compositions to confirm the number of intrusions that make up the Henry Mountains, 2) determine the age of the intrusion(s), and 3) use the geochronology to determine the timescale of the intrusion(s).

Field and petrographic observations of the intrusions reveal wide variations in mineralogy and compositions. The dominant phenocryst assemblage of the Mt. Hillers intrusive complex is plagioclase and amphibole with 10-70% matrix depending on the intrusion. Plagioclase crystals are subhedral and vary between 1-10 mm, also depending on the intrusion. Amphibole is not present in every intrusion, but makes up as much as 40% of the phenocryst assemblage when present in some intrusions. These amphiboles are subhedral, elongate, crystal that are up to 2 mm in length. Mid crustal (amphibole) and deep crustal (garnet and olivine) xenoliths are present in the intrusions of the Mt. Hillers complex. These heterogeneities suggest that the Mt. Hillers complex may be made of multiple intrusions. We expect bulk-rock and in-situ mineral chemistry on plagioclase and amphibole as well as zircon geochronology to corroborate these interpretations.