Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single-polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1°, I = −41.9° ( k: = 94, α₉₅ = 3.3°, N = 21 sites) and a paleomagnetic pole at 21.1°S, 221.1°E, (K = 89, A₉₅ = 3.4°). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47–1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5–1.4 Ga poles of the North American apparent polar wander path.