MICROSTRUCTURAL CHARACTERIZATION OF MONAZITE GRAINS FROM THE SLIDE LAKE SHEAR ZONE, CO

The Slide Lake shear zone (SLSZ) is one of several north-east striking ductile shear zones within the Colorado mineral belt that were generated during the Proterozoic continental assembly of Laurentia. The sub-horizontal mylonite and ultramylonite zones of the SLSZ are located within Paleoproterozoic rocks, recording both southeast-side-up and southeast-side-down kinematics, with a minor dextral component (Lee et al., 2012). Several generations of deformation fabrics are recognized in the neighboring Homestake shear zone, from folded high-temperature penetrative foliations and layering, to localized sub-vertical, greenschist facies mylonite zones (Shaw et al., 2001).The eventual goal of this project is to implement the in-situ monazite dating technique on samples from the SLSZ. Preliminary results are the focus of this presentation. Microstructural and SEM analyses were used to locate monazite grains and relate them to the larger structural and petrological context. The monazite grains range from 5 to 100 µm, and are usually anhedral and elongated parallel to the mylonitic foliation. Euhedral grains are very rare, and likely represent secondary growth after shearing ceased. Grains with tail complexes are believed to have acted as rigid grains during ductile shearing, which may indicate grow during separate phases of deformation. Future electron microprobe mapping of selected grains followed by U-Th-Pb dating will attempt to constrain the timing of movement along the SLSZ.