CLASTIC DIKES WITHIN THE PIKES PEAK GRANITE OF THE FRONT RANGE, COLORADO: USING PALEOMAGNETISM TO CONSTRAIN AGE OF EMPLACEMENT

Clastic dikes of unknown age and origin occur within the Pikes Peak Granite along nearly 75km of the Front Range of Colorado. These dikes are spatially associated with thrust faults along the eastern margins of the range, and vary in size from a few centimeters across to over one hundred meters wide. The study area focuses on clastic dikes which are parallel to the predominately northern striking Ute Pass Fault near Woodland Park, Colorado, as well as dikes that are parallel to the northwest striking Pine Gulch Fault near Pine, Colorado.

Paleomagnetism is a useful tool for age-dating sediments whose age cannot be stratigraphically determined, as in the case of these clastic dikes. Paleomagnetic sampling and subsequent demagnetization of fifteen clastic dikes along the Front Range indicates the presence of a complex magnetization. An easterly and moderate down component residing in hematite was resolved from six dikes, with corresponding pole positions which fall on or near the poorly defined Neoproterozic-early Paleozoic portion of the apparent polar wander path of North America. A mean of means for the six dikes(D = 94.9°, I = 26.3°); this yields a pole position of 5.1°S and 151°E; dp= 14.7, dm=27.1. This mean pole plots near poles for Cambrian granites. This magnetization is interpreted as early in origin and was probably acquired during or soon after emplacement of the dikes. The paleomagnetic data place an upper limit on the age of the dikes; the dikes can be no younger than early Paleozoic. A southeast and shallow magnetization, also residing in hematite, was found in three dikes and yields a grouping of poles of late Paleozoic age. This is interpreted as a chemical remanent remagnetization (CRM), possibly associated with fluids activated by the Ancestral Rockies uplift. Other dikes yield scattered or streaked distributions.