PRELIMINARY PALEOMAGNETIC RESULTS OF THE MID-TERTIARY CRIPPLE CREEK DIATREME COMPLEX, FRONT RANGE OF COLORADO

The Cripple Creek diatreme complex located near Cripple Creek, Colorado, consists of breccia (the primary rock type in the district) that is intruded by lamprophyre dikes, aphanitic phonolite dikes, porphyritic phonolite, and syenite intrusions. The igneous rocks presently exposed at the surface were emplaced less that a few kilometers below the paleosurface, between about 32.5 and 28.7 Myr, on the basis of previously reported high-precision Ar/Ar age determinations. The complex is host to substantial gold mineralization and is presently being mined in an open-pit fashion, allowing for outstanding, fresh, three-dimensional exposures. Laramide contractional deformation (about 70 to about 45 Myr) resulted in considerable topography throughout the Front Range of Colorado and exposed a complex assemblage of Precambrian crystalline rocks. Following contractional deformation the dominant mode of deformation has been extension. The primary extensional feature, the Rio Grande rift, which formed later than 30 Myr, extends from southern New Mexico to north-central Colorado. Evaluation of Laramide and younger deformation in Colorado is difficult because of the dominance of Precambrian rocks with complex structural histories. Parts of the Front Range expose Laramie to mid-Tertiary intrusive igneous rocks, and structures that deform these rocks clearly have a component of deformation that post-dates their intrusion. Detailed examination of the structures affecting the igneous rocks of the complex will shed light on the stresses affecting this area since Laramide contractional deformation. Samples from Cripple Creek have been subjected to progressive alternating field and thermal demagnetization. Our preliminary results show that samples collected are capable of carrying geologically stable magnetizations and reveal excellent demagnetization behavior. Fully completed demagnetization data yield well-defined trajectories; for example, the site mean calculated from 8 samples from site cc40 is Decl.=132.2°, Incl.=-59.5°, a95=5.22, and paleopoles of latitude=-53.5° and longitude=3°.