FABRIC ANALYSIS, MAGNETIC SUSCEPTIBILITY, AND CHEMICAL ANALYSIS OF A CORE INTO THE TRACHYTE MESA LACCOLITH IN THE HENRY MOUNTAINS, UTAH

The Trachyte Mesa laccolith, located in Utah’s Henry Mountains, is a magmatic intrusion undisturbed by tectonic processes within the Colorado Plateau. Several models have been proposed for the emplacement of this intrusion, including a single event pulse resulting in one large magmatic structure, several smaller magma pulses occurring over time, or a continuous flow resulting in sheet-like magmatic structures. Using fabric analysis, anisotropy of magnetic susceptibility (AMS), and chemical analysis of samples taken from a 1.3 m deep core drilled into the top of the Trachyte Mesa laccolith, we developed two kinematic models of magmatic intrusion.

From the core, twenty polished thin sections were made. Fabric analysis was obtained using two image analysis programs: autocorrelation and intercepts. This process determined the orientation of hornblende and other oxide crystals in the magma. These orientations are compared to the magnetic foliation to determine flow models and magnetic signal origination. AMS data indicated an abrupt increase in susceptibility halfway down the core.

The first model includes the intrusion of a large sheet that cooled faster on its exterior than on its interior, causing an element of vertical strain on the faster flowing interior minerals in contact with the cooler outer edges. A lower AMS value indicates the exterior of the sheet cooled faster developing fewer exsolved magnetites, a higher AMS value occurs in the interior of the sheet where slower cooling allowed for more exsolution of magnetite.

The second model includes the multiple sheet hypothesis. The point at which high and low AMS values intersect indicates a contact between two magma sheets. Image analysis shows a change in angle of foliation at this contact. Observations from thin sections allowed for qualitative analysis of fractures. High levels of fractures filled with late stage oxides indicate fluid infiltration in the upper sheet which may have been caused by the intrusion of the lower sheet.