Abstract

Radiometrically dated hypabyssal tabular intrusive masses in the northern part of the Front Range, Colorado, were emplaced about 63 m.y. ago. Comparison of the remanent magnetic directions of these igneous bodies with their unrelated magnetic reference directions indicates that the intrusive rocks and the enclosing Paleozoic and Mesozoic sedimentary rocks have undergone variable amounts of rotation, depending on their geographic location. The host beds of the Ralston intrusive near Golden were rotated to their present attitude after the emplacement of the intrusive, whereas the enclosing sedimentary rocks at three sites near Boulder were rotated in part prior to and in part after intrusion. The host beds of the sill near Left Hand Canyon were probably rotated prior to intrusion. It is likely that rotations are due to the vertical and (or) eastward translation of the Precambrian crystalline rocks adjacent to the monocline.

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