Protracted magmatism and magnetization around the McClure Mountain alkaline igneous complex

Abstract

AbstractThe McClure Mountain–Iron Mountain igneous complex is an alkalic intrusive center in the northern Wet Mountains of southern Colorado. It was emplaced in early Cambrian time into gneissic/granitic 1.75–1.45 Ga Proterozoic host rocks. Numerous dikes are associated with the complex, primarily along the western side. Although the main intrusive nepheline-syenite body is well dated, the ages of the surrounding dikes are poorly known. Crosscutting relationships and poorly defined K-Ar dates suggest that the dikes are younger than the main intrusion. Paleomagnetic samples were collected from dikes associated with the McClure Mountain igneous complex. Geochronologic samples were also collected from two dikes sampled for their paleomagnetism. We obtained U-Pb zircon ages of 526 ± 8 Ma for a lamprophyric extracomplex dike and 483 ± 2 Ma for a trachytic extracomplex dike. These ages suggest either multistage or protracted dike intrusion around the ca. 524 Ma McClure Mountain complex. Our paleomagnetic data are consistent with previously published results. Dikes of the complex primarily exhibit southeast and shallow paleomagnetic directions, with variable declinations. Results from several baked contact tests indicate that the magnetizations are secondary. A steeply inclined magnetization is pervasive and was acquired over a protracted interval from late Laramide time to the present day.