CONSTRAINTS ON RATE OF IGNEOUS EMPLACEMENT USING PALEOMAGNETIC SECULAR VARIATION, MAIDEN CREEK SILL, HENRY MOUNTAINS, UT

The rate of emplacement of small plutons is commonly difficult to constrain because the timescale of emplacement is within the uncertainty of absolute age dating methods. Paleomagnetic analysis of incrementally constructed intrusions may offer an alternate method for constraining rate of emplacement. Within a sheeted intrusion, cooling of a sheet following emplacement captures the position of the magnetic pole at that time. Variation of the position of the pole (secular variation) recorded by adjacent sheets provides information about the time elapsed between emplacement of one sheet and the next. In this study we examine paleomagnetic data collected from the Maiden Creek Sill in the Henry Mountains of Utah. The Oligocene andesite porphyries of the Henrys were emplaced into the nearly flat lying stratigraphy of the Colorado Plateau. The Maiden Creek Sill is a nearly horizontal, small scale (0.05 km3), satellite intrusion on the flank of the much larger Mt. Hillers intrusive complex. Bulbous terminations, solid state shear zones, and intercalated country rock all suggest the sill consists of two sheets each of which is approximately ten meters thick. If this is the case then the paleomagnetic remanence vector orientation preserved in each sheet should be internally consistent, with the exception of any contact metamorphic effects associated with emplacement of the second sheet. Paleomagnetic data collected at a one meter vertical interval throughout both sheets, however, displays much greater variation. Analysis of multiple cores from a single sample site show remarkable uniformity (a95 <4˚). Samples one meter above or below, however, are different. These data suggest that each of the two large sheets identified in the field are in fact composed of approximately ten one-meter-thick sheets, each of which intruded at a different time and captured a different position of the magnetic pole. Assuming that the rate of secular variation in the Oligocene coincides with that in the Holocene we estimate that the Maiden Creek Sill took between 90 and 1600 years to be emplaced, with a median of 280 years. Analysis of the secular variation recorded by sheeted intrusions may provide a way to examine rates of emplacement on the 100 to 1000 year timescale, a much shorter timescale than can be examined using absolute age dating methods.