QUARTZITES AND GRANITIC BASEMENT: EXAMINING THE DEPOSITIONAL CONTACT HYPOTHESIS AT BLUE RIDGE, CO USING MICROSTRUCTURAL EVIDENCE

The project addresses the geological nature of the boundary between a metamorphosed sedimentary rock unit and adjacent granitic basement rocks in the Blue Ridge area near Canon City, Colorado. The outcrop, in particular a quartz pebble conglomerate (QPC) directly at the boundary, has been interpreted to support the hypothesis of a period of very fast and efficient weathering at ca. 1.7 Ga in the Paleoproterozoic, due to a different ocean chemical composition (Canfield and Teske, 1996). The interpretation that the contact between the units represents original deposition of pure quartz sediments on the weathered surface of a granite in a single sedimentary cycle (Jones et al. 2009) rather than representing a more structurally complex contact is crucial to this hypothesis. We are testing this hypothesis using petrographic and cathodoluminescence (CL) microscopy and macroscopic observations. The field evidence indicates the proposed model is unlikely e.g. because quartz grains "pebbles" in the QPC are larger than the quartz grains in the surrounding granites. Thin sections of granite and QPC have been compared for internal textures of the quartz grains. In the QPC muscovite trails are found to pseudomorph former grain boundaries within the quartz "pebbles". This suggests that the grains in the QPC are actually aggregates of multiple quartz grains providing further evidence that the QPC is not directly derived from weathering quartz grains from the granites. CL microscopy evidence may reveal further microtextures and inclusions and resolve whether the QPC has undergone a more complex history than a single cycle of erosion and deposition. Because this unit is one of the key units for depositional models to explain the rapid weathering in the Early Proterozoic (Jones et al., 2009), results indicating that the quartz pebbles are in fact aggregates of smaller grains that are not from first cycle sedimentation, a new depositional model and further geochronological data will be needed to explain the geological boundary at Blue Ridge. In general it needs to be established whether there is really evidence for a period of rapid weathering in the Early Proterozoic.

References

Canfield, D.E., Teske, A., 1996, Nature, v. 328 , p. 127-132.

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