Anisotropy of Magnetic Susceptibility of a Magma Flow Sheet in the Trachyte Mesa Laccolith of the Henry Mountains in South-Central Utah

The Trachyte Mesa Laccolith (TML) in the Henry Mountains of south-central Utah is thought to have been formed by the coalescing of numerous horizontal magma sheets that stemmed from the nearby igneous intrusion, Mount Hillers. To understand the flow behavior within a magma sheet, cores (ranging in length from 60 cm to 1.2 m) were cut through the top sheet of the laccolith with a construction grade drill. Individual samples were then cut every 5 cm down the core to obtain a downward gradient of foliation. These samples were cut parallel to lineation and were used for fabric analysis and the collection of Anisotropy of Magnetic Susceptibility (AMS) data. To determine average crystal percentage versus matrix in the sheet, thin sections were made from two samples from the top of the sheet. Each sample was divided into rows and columns and a microphotograph was taken for each section of the grid. The area calculation and crystal analysis of the microphotographs was done using NIH software. The software allowed each plagioclase and oxide crystal to be colored in and deleted from the total area which provided the crystal percentage of each photograph. An average percentage for each row of the thin sections was calculated and determined to be approximately 35 to 40%. In both samples, one row, several cm from the top of the sheet, deviates by 22% less than the average crystal percentage. The row lying directly underneath shows a 22% increase in crystallinity. These changes in crystal content are possibly due to grain dispersive forces that cause crystals to be displaced away from the contact shear zone. This is known as the Bagnold effect and helps to define the plug zone within the sheet where no shearing in occurring. Analysis of the AMS data revealed the bulk magnetic susceptibility (Km) to range from 2.71E-3 to 1.54E-2 SI with the average being 7.69E-3 SI. The Km values appear to be changing at approximately 40 cm down. At this same level the AMS foliation begins to curve from a northwesterly dip to a southeasterly dip in each of the core samples drilled. These data suggest there is a magma sheet contact at this depth. All evidence collected suggests the magma sheet was emplaced as a plug flow. This plug flow behavior is supported by the displacement of the crystal percentages, the curvature in the AMS foliation, and a change in the bulk magnetic susceptibility that corresponds to the change in foliation curvature.