DUCTILY DEFORMED PEGMATITES AND AN AMPHIBOLE-BEARING LEUCOSOME: CONSTRAINTS ON GEOLOGIC EVENTS AT PRECAMBRIAN BOUNDARIES USING STRUCTURAL MARKER UNITS, BLUE RIDGE, COLORADO

The Blue Ridge area, Fremont County, Colorado, is an exposure of alternating layers of Precambrian quartzite, schist, and quartzitic-gneiss in direct contact with granitic basement at its northern and southern boundary. Previous studies have used field relationships and U-Pb ages to propose two hypotheses: 1) the northern boundary is depositional, representing original sedimentation of quartz-rich sediment sourced from the granitic basement onto a weathered granitic surface; and that this is a type locale for a period of efficient weathering at 1.7 Ga due to altered ocean chemistry (Jones et al. 2009, Cox et al. 2002), and 2) the major deformational and tectonic events occurred shortly after emplacement of the granitic basement at 1.7 Ga (Mai 2002, Jones et al. 2009). Major periods of igneous and metamorphic activity occurred in this region at 1,781 ± 66 Ma and 1,450 ± 100 Ma (Vera & Van Schmus 1974) and these ages coincide with granitic basement and pegmatite emplacement at Blue Ridge, respectively. Based on field observations we propose that the northern contact is non-depositional and that major deformation and metamorphism occurred during pegmatite intrusion at ca. 1436 Ma (Jones et al. 2009). Field observations have introduced new structural markers in the form of ductily deformed pegmatite at both the northern and southern boundary and an undocumented amphibole-bearing leucosome also at the southern boundary. The leucosome contains randomly oriented amphiboles, infiltrates sheared basement and thus post-dates pegmatite intrusion. Whereas Jones et al. (2009) interpret the pegmatite to cross-cut the boundary and sheared basement, field evidence shows the pegmatite to be deformed within the shear zone. The deformed pegmatite at the northern boundary also infers that deformation was synkinematic with pegmatite intrusion. Analytical work currently underway involving monazite and zircon U-Pb geochronology and thermobarometry, micro-texture grain analysis and heavy mineral composition and abundance analysis will further test our hypothesis.

Cox, R., et al., 2002, Geology, v. 30, p. 323-326.

Jones, J.V., III., et al., 2009, Geol. Soc. Am. Bull., v. 121, p. 247-264

Mai, K., 2002, [M.S. Thesis]: Univ. of OK, 105 p.

Vera, R.H. & Van Schmus, W.R., 1974, Geol. Soc. Am. Bull., v. 85 p. 77-82