ANISOTROPY OF MAGNETIC SUSCEPTIBILITY ANALYSIS OF COLORADO FRONT RANGE CLASTIC DIKES

Abstract

Clastic dikes within the Pikes Peak Granite on the eastern flank of the Colorado Front Range have been a point of interest since their first written observation in 1893. They are massive, quartzarenites ranging from centimeters in width to tens of meters and have no internal depositional stratigraphy. They trend N-S along the Ute Pass Fault Zone and other Front Range faults and lack a nearby source rock – although the local Cambrian Sawatch Sandstone has been suggested as being lithologically comparable. Geochronological dating has constrained timing of dike emplacement to the Proterozoic although studies have not agreed on an emplacement mechanism for the dikes. There are two primary hypotheses for method of emplacement: (1) glacially mediated deposition and (2) liquefaction and remobilization of sandstone. This study focuses on characterizing magnetic minerals through rock magnetic analysis, paleomagnetic analysis, and microscopy to understand possible emplacement mechanisms of the dikes and uses paleomagnetic dating to confirm age constraints of the dikes. Scanning Electron Microscopy (SEM) and transmitted light microscopy were conducted to confirm presence of iron oxides, hysteresis and Curie Temperature analysis were used to confirm the presence of hematite as the primary magnetic carrier, and Anisotropy of Magnetic Susceptibility (AMS) analysis to discern the presence of magnetic fabric alignment. AMS analysis was run on 176 core specimens from eight sites with results indicating primarily oblate magnetic susceptibility ellipsoid shapes, a northwesterly-oriented foliation trend, and magnetic fabrics interpreted to represent sedimentary and intermediate fabrics with minor deformation signals – contrary to expectation that significant tectonic overprinting would be observed due to multiple orogenic episodes. Although a good portion of the samples display AMS, there is another portion which preserve remanent magnetizations. Previous study has shown three characteristic remanent magnetizations observed in the clastic dikes with a characteristic remanent magnetization overlapping other Precambrian pole positions, (virtual geomagnetic pole = -13.9°N, 153.5°E°, dp=18.9, dm=33.5), and two additional component remanent magnetizations interpreted as chemical remanent magnetizations (CRMs) acquired during the late Paleozoic and end Mesozoic. All CRMs were interpreted to reside in hematite based on unblocking temperatures above 580 °C after stepwise thermal and AF demagnetization. The age of the clastic dikes along with the oblate, primary sedimentary AMS magnetic fabric suggesting an emplacement mechanism in an over-pressurized, fluid-rich system, where deposition is neither catastrophic nor passive.