My presentation of these results and interpretations in a talk at the GSASouth Central-Rocky Mountain combined section meeting was awarded the prize for Best Student Paper at that meeting.
KOHL, J.A., J.W. GOODGE, and V.L. HANSEN, Dept. of Geological Sciences, Southern Methodist University, Dallas, TX 75275
The NE-trending Idaho Springs-Ralston shear zone (ISRSZ) in the east-central Front Ranges of Colorado is >= 4 km wide and deforms Proterozoic igneous and metamorphic rocks of the Colorado Province, including granodioritic plutons that may be part of the ca. 1.7 Ga Boulder Creek Batholith. L-S tectonites of the ISRSZ are characterized by a moderately to steeply SE-dipping foliation with two distinct elongation lineations. L1, a prominent penetrative dip-slip lineation characterized by rodded quartz and elongated micas, amphiboles, and feldspars, plunges SSE to SW. L1 appears to be overprinted locally by a weaker, less penetrative NE-SW trending strike-slip ineation (L2). L2 occurs within a narrow sub-domain in the central part of the shear zone.
Well-preserved macroscopic and microscopic kinematic indicators associated with both L1 and L2 include S-C fabrics, winged inclusions, quartz grain-shape preferred orientations, myrmekite, mica mats, and quartz c-axis fabrics. Microstructures associated with L1 record top-NE to top-NW (sinistral-reverse) shear. L2 microstructures consistently record dextral strike-slip shear.
We interpret these ductile L-S fabrics as the result of two distinct deformations on the basis of their different kinematics and the apparent cross-cutting relations; however, it is possible that they formed at the same time as a result of strain partitioning. We cannot differentiate the crustal levels of L1 and L2 deformations by mineral assemblages. L1 and L2 microstructures both show fibrolitic Sil aligned in anastomosing sheaves with S-C geometry, delicate oriented Sil needles passing through quartz grains that show ductile strain, and bundles of Sil showing discrete cross-fractures and boudinage, together indicating that Sil growth occurred prior to and/or during deformation. The absence of annealed quartz and a lack of randomly oriented Sil overprinting in either L1 or L2 tectonite fabrics indicates that cooling accompanied the late stages of both deformations. Structural relations therefore indicate that Proterozoic (?) displacement on the ISRSZ involved contraction, perhaps with a late strike-slip component, followed by cooling in the absence of static reheating.
My presentation of these results and interpretations in a talk at the GSASouth Central-Rocky Mountain combined section meeting was awarded the prize for Best Student Paper at that meeting.