A TEST OF COMPETING HYPOTHESES ON THE DEFORMATIONAL HISTORY OF ~1.7 GA GRANITIC DIKES IN THE COALBANK PASS AREA, WESTERN NEEDLE MOUNTAINS, COLORADO

Paleoproterozoic gneisses in the western Needle Mountains near Coalbank Pass host a suite of ~1.7 Ga S-type peraluminous pegmatitic to aplitic granite dikes. Previous studies on these dikes have constrained their age and provided cursory descriptions of the rocks, but the timing of emplacement relative to Yavapai deformation in the Needle Mountains is a point of contention. Previous proposed hypotheses are that the dikes were either emplaced during or subsequent to regional deformation at ~1.7 Ga. Resolution of this problem is important for further understanding nature of deformation at ~1.7 Ga in southwestern Colorado and implications for the crustal evolution of Laurentia during the Yavapai orogeny.

Recent structural studies have concluded that the ~1715 Ma Ga Tenmile Granite in the western Needle Mountains was emplaced under regional N-S shortening with subhorizontal E-W extension. If this proposed regional strain pattern is valid then it could be reflected in the trends and fabrics of granitic dikes in the Coalbank Pass area if they were emplaced under these conditions. Preliminary conclusions of our detailed field and structural investigation of the dike complex establish that the dikes are mostly undeformed with a small fraction preserving ductile strain fabrics. At several locations, the dikes contain prominent E-W striking mylonitic fabrics with subhorizontal lineations consistent with ~E-W extension. Stereonet plots of dike orientations include a very crude conjugate pattern consistent with N-S shortening at the time of emplacement. Microstructures in some dike rocks preserve incipient brittle fabrics as exemplified by fractured and rotated feldspar and bent mica, but no obvious ductile fabrics.

We suggest that the ~1706 Ma granite dikes in the Coalbank Pass area record the waning stage of N-S shortening related to Yavapai deformation during the growth and development of the southern margin of Laurentia. Emplacement of the post-tectonic 1700 to 1690 Ma Bakers Bridge Granite marked the termination of Yavapai deformation. Intrusive complexes in the Needle Mountains thus record a continuum of deformation from syn- to post-tectonic over an extended period from 1730 Ma to 1690 Ma.