MAGMA FINGERS FEEDING THE EMPLACEMENT OF THE TRACHYTE MESA SILL/LACCOLITH, HENRY MTS., UTAH, BASED ON AMS, MAGNETIC, AND GRAVITY DATA

The emplacement of the Oligo-Miocene Trachyte Mesa sill/laccolith in the Henry Mts., Utah is modeled as occurring in three stages based on detailed geophysical surveys. The andesite porphyry intrusion is transitional in shape between a sill and a laccolith: the intrusion is 1.7 km long, 0.6 km wide, and only 20 m thick, it is composed of several sheets, the sides of the intrusion are bulbous, and locally the margins exhibit prongs or individual flattened "finger-like" shapes extruding into the country rocks.

Samples from 103 AMS stations have been collected on the top of the intrusion, which is exposed at its top contact. The NE elongated intrusion exhibits a NE trending centralized zone of Kmax lineations which also trend NE. Lineations radiate outward (NW & SE) from this central elongated zone.

Magnetic intensity was collected from 3 NW trending Magnetometer traverses across the alluvium SW of the intrusion (towards one of the main intrusive centers in the Henry Mts.). Traverses reveal 3 broad magnetic highs that become weaker in intensity but broader in width with distance from the intrusion. Connecting the magnetic highs between the traverses reveal three linear zones which trend NE, but which are not parallel to each other. These linear zones of high magnetic intensity intersect one another at the SE margin of the laccolith. Data from one gravimeter traverse illustrates that gravity lows correlate with the magnetic highs.

We interpret the data to suggest that the laccolith was fed by three feeder sills which are shaped like flattened fingers and which may or may not be connected. These three NE trending magma "fingers" originated at an intrusive center in the SE (Mt. Hillers) and were transporting magma NE. We suggest three stages of magma transport/emplacement: 1) Production of a magma chamber as a result of the intersection of these three magma feeder "fingers", 2) Production of a NE elongated central magma body as a result of the NE direction of magma transport, and 3) Outward flow (NW and SE) from the central NE trending zone resulting in the radial pattern of AMS lineations.