FIRST CYCLE DEPOSITION OF QUARTZITES ON GRANITIC BASEMENT AT BLUE RIDGE, COLORADO? EVIDENCE FROM HEAVY MINERAL COMPARISON AND U-PB GEOCHRONOLOGY

Previous studies have stated that a quartz pebble conglomerate (QPC) and overlying orthoquartzites found in the Blue Ridge area, Fremont County, Colorado form a conformable, depositional contact with the underlying granitic basement along the northern boundary of the exposure (e.g. Knepper 1972). Other studies on these rock units contend that the QPC is directly derived from the adjacent granitic basement, and represents first cycle deposition during a period of increased weathering rates in the Paleoproterozoic (Jones et al. 2009, Canfield and Teske 1996). The ongoing research for this project examines if the QPC and overlying orthoquartzites can be the result of a single cycle of weathering and deposition by studying the type, abundance and ages of heavy minerals. The QEMSCAN laboratory at the Colorado School of Mines offers a quantitative way to analyze heavy mineral modes from each sample and our research compares heavy mineral separates from several samples of metasediments and adjacent granitic basement from the Blue Ridge area to determine if these can be correlated. QEMSCAN analyses also ensure that no user bias is introduced in mineral identification (MID), which is a potential problem in optical microscopy MID. U-Pb geochronology on zircon and potentially rutile from the heavy mineral separates will be used to help determine the provenance, timing of sedimentation, and post metamorphic cooling in the Blue Ridge area. Rutile is an unlikely mineral in the granitic rocks, but it was reported by Mai (2002) as occurring in quartzite and pebble quartzite samples, implying either that the rutile is metamorphic, or that there was a metamorphic rock source eroded to provide detrital rutile. If our research reveals that the heavy minerals from the granitic basement and the QPC and orthoquartzites do not match then a new model of intrusion, deposition, and metamorphism will be needed for the Blue Ridge area.

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

Knepper, D., 1972, (M.S. Thesis) Univ. of KS, 53 p.

Mai, K., 2002, (M.S. Thesis) Univ. of OK, 210 p.