Field photographs of primary features and structural elements observed in basement assemblages in the southern Sawatch Range, Colorado. A, View down on preserved pillow structures in amphibolite. B, Cross-bedding in quartz-rich gneiss. C, Isoclinal fold (F1) in xenolith of interlayered quartz-rich gneiss and biotite schist within the Taylor River granite. Axial planar fabric (S1) is defined by fine-grained, aligned mica. D, Continuation of previous photo showing composite fabric (S0/S1) in metasedimentary rocks folded by upright, meter-scale F2 fold. Axial-planar fabric (S2) is defined in more schistose units by aligned mica.
Summary of structural data from basement assemblages and granitic rocks exposed in the southern Sawatch Range, Colo. Lower-hemisphere, equal-area stereonet diagrams represent poles to S0/S1 (A) and S2 (B) in basement assemblages and poles to foliation in the Henry Mountain granite (C) and Taylor River granite (D). Evidence for the interpretation of fabric elements in the Henry Mountain granite is discussed in the text. Data were plotted and average orientations were calculated using GEOrient 9.1 (Holcombe, 2003).
Hand sample photographs of Proterozoic granitic rocks exposed in the southern Sawatch Range, Colo. that were sampled for new U-Pb geochronology. The magnifying lens used for scale is approximately 2 cm wide. A, Henry Mountain granite. B, Monarch Pass granodiorite. C, Taylor River granite. D, Horsethief granite.
Field photographs of deformation features observed in granitic rocks in the southern Sawatch Range, Colo. A, Outcrop view of the contact between Henry Mountain granite (Xhmg) and Taylor River granite (Ytrg). A solid-state foliation defined by biotite cuts across the contact and is approximated by the dashed lines. B, Xenolith of quartz-rich gneiss in Taylor River granite contains composite S0/S1 foliation folded by upright F2 fold. S0/S1 and the S2 axial-planar cleavage in the xenolith are sharply truncated by the granite, but the granite contains a solid-state mica foliation (S3; approximated by dashed lines) that is subparallel to S2 in the xenolith. The fabrics observed in the xenolith are similar in orientation to those observed in outcrops outside of the pluton.
U-Pb concordia diagrams for samples from the southern Sawatch Range, Colorado. Probability of fit (POF) for linear regressions (l.i. = lower intercept) and mean square of weighted deviates (MSWD) for weighted mean ages are included in parentheses. Weighted mean 207Pb/206Pb ages were calculated following the methods of Ludwig (1998). See the text for a detailed discussion of age interpretations. All uncertainties are reported at the 2-sigma level.
U-Pb zircon age data for the Taylor River Granite. A, U-Pb concordia diagram for all zircon grains. B, Probability age-distribution diagram for all grains with histogram bins of 25 m.y. C, U-Pb concordia diagram for 7 youngest zircon grains with probability of fit (POF) for linear regression forced through the origin (0 ± 50 Ma). See the text for a detailed discussion of age determinations and interpretations.
Structural model (from Hetherington, 1991) for D2 and possibly D3 deformation in the western portion of the southern Sawatch Range, Colorado. Deformation was concentrated along and adjacent to the contact between metavolcanic rocks (Xas) of the Cochetopa succession and metasedimentary rocks (Xms) of the Black Canyon succession, possibly due to a thrust fault that ramps up and over the Henry Mountain granite (Xhmg) pluton (Hetherington, 1991).