USING PRESERVED SECULAR VARIATION TO CONSTRAIN EMPLACEMENT DURATION IN LARGE SCALE SILLS, COPPER RIDGE INTRUSION, MT ELLEN, HENRY MOUNTAINS, UT

The Copper Ridge intrusion is a tongue-shaped igneous body that extends to the southeast out of the Mt. Ellen intrusive center in the Henry Mountains, UT. This andesite porphyry intrusion consists of two major sills 100-200 m thick separated by a horizontal package of country rock. In this study we examined the remanent magnetic signal preserved in these sills. The vertical thickness of each sill was sampled at an approximately evenly spaced interval with a field drill. Samples were subjected to alternating field demagnetization and the remanent magnetism measured using a spinner magnetometer.

The overall remanent magnetism from the upper sill was scattered and difficult to interpret. Of ten sites sampled of the upper sill, eight yielded internally consistent results. Six indicated a reverse polarity and the remaining two showed a normal polarity. The normal sites, however, were not antipodal to the reverse polarity sites. Of the six sites with a reverse polarity, two exhibited remanent magnetizations inconsistent with the remaining four. A mean vector calculated from these four sites yielded an orientation consistent with the expected direction for emplacement ~25 Ma. The demagnetization patterns (i.e. one vs. two vs. three component systems) collected from the upper sill tended to be consistent at a single site, but varied greatly from site-to-site. The variability of magnetic properties suggests a variable magnetic mineralogy, which is consistent with the hypothesis that this sill formed as the result of multiple pulses of magma.

Paleomagnetic results from the lower sill were far more consistent. Of eleven sites sampled, nine provided internally consistent results, all of which recorded a reverse polarity. Calculation of a virtual geographic pole (VGP) for each of these nine sites yielded estimates of the position of the magnetic pole during emplacement. Summing the difference in position between each VGP suggests that the magnetic pole experienced a total of 139° of secular variation during emplacement. Assuming Holocene rates of secular variation are a good proxy for the Oligocene, the data suggests the middle sill of the Copper Ridge intrusion was emplaced through multiple magma pulses over the period of 90 to 1500 years.