Electron microprobe in situ monazite dating of the Slide Lake shear zone

The Colorado mineral belt (CMB) shear zone system records Proterozoic deformation episodes documenting the progressive tectonic accretion of Laurentia. Regional fold formation (D₁ & D₂) is coeval with the ca. 1700 – 1600 Ma Yavapai and Mazatzal orogenies, while Mesoproterozoic sub-vertical shear zones (D₃ & D₄) are a response to 1450 – 1300 Ma intracontinental deformation. The Slide Lake shear zone (SLSZ) is a sub-horizontal shear zone located in the Northern Sawatch Range, central Colorado. Aside from its orientation, the SLSZ records similar deformational structures to other shear zones in the CMB, but there is no timing constraint to test this relationship. Monazite is used as a geochronometer for poly-deformed metamorphic assemblages that record phases of growth through compositional zoning related to deformation and fluid-alteration. This thesis implements microstructural observations, electron microprobe in-situ monazite U-Th-Pb dating, and trace element analysis to place a timing constraint on SLSZ activity. Monazite dates from this research dominantly range from 1716 – 1580 Ma and 1538 – 1413 Ma, the latter of which roughly coincides with dates from the CMB shear zones. Monazite domain-age relationships in some grains suggest that U-Th-Pb monazite system was incompletely reset, commonly leading to younger inner-cores and older outer-cores. Core domains from HS09-42a record older dates (avg: 1700 ± 6.9 Ma) than is expected from a mylonitic sample, which may imply that monazite grains from Paleoproterozoic deformation (D₁ & D₂) were not reset by fluid alteration during shear zone activity (D₃ & D₄). The ranges of monazite sample dates shows a progression of younger average ages in samples with more advanced grain-size reduction (HS09-36: 1640 ± 23 Ma → HS09-35: 1502 ± 39 Ma → SL08-07: 1421 ± 7.6 Ma), paralleling the structural evolution of the CMB shear zones. Low-Y domains from HS09-35 (1423 and 1420 Ma) and SL08-07 (avg: 1421 ± 7.6 Ma) support fluid-alteration in the shear zone at ⁓1421 Ma, possibly corresponding with Mesoproterozoic deformation (D₃ & D₄) in the CMB. Monazite dates from mylonitic samples in the SLSZ are coeval with ages of the neighboring Homestake shear zone (HSZ), suggesting that the shear zones were active at similar times.