MAGNETIC ANOMALY BASED INTERPRETATION OF THE MAGMA FEEDERS TO THE TRACHYTE MESA LACCOLITH, HENRY MTS., UTAH

Magnetic anomaly data was collected within a grid of traverses adjacent to the Trachyte Mesa Laccolith in order to identify the magma feeding system to the laccolith. The area is covered with alluvium and located between a large intrusive center (Mt. Hillers) and a distant (5km) satellite laccolith (Trachyte Mesa). Across the alluvium SW of the exposed Trachyte Mesa laccolith, magnetic intensity data was collected from 5 NW trending and 2 NE trending traverses using the gradiometer mode and 3 NW trending traverses using the regular mode. More gradiometer data was taken because analysis of the data shows that the feeders are approximately less than 50 feet deep which is relatively shallow and allows for higher resolution. The 5 NW traverses reveal 3-4 anomalies, all of which appear to converge as the exposed intrusion is approached. The 2 NE traverses also have anomalies that correspond with the 5 NW traverses. The pattern of the anomalies across the grid is complex, although, by connecting each anomaly from one traverse to the next, an irregular anastomosing network of fingers is apparent. Additional support for this interpretation is located roughly .9 miles south of the Mesa where an irregular tube-like shaped intrusion is partially exposed and is situated between the two main intrusions (Hillers, TML). This small intrusion diverges and converges along its length and is believed to be very similar to the feeders imaged adjacent to the TML to the north and supports the anomaly interpretation of an anastomosing network as a feeding system. These data are not consistent with previous models of either a dike or sill-fed feeding system for the satellite laccoliths in the Henry Mts.