Metamorphic and Structural Evolution and Provenance of the Blue Ridge Area, Fremont County, Colorado.

摘要

New field studies combined with U-Pb zircon/apatite geochronology, thermodynamic equilibrium modeling, whole-rock/rare earth element geochemistry, heavy mineral comparison, and petrographic analysis yield new insights into the age and timing of deposition, deformation, metamorphism, and the provenance and boundary relationships of exposed Precambrian metasedimentary and associated granitic and pegmatitic rock in the Blue Ridge area, Fremont County, Colorado. The Blue Ridge area is comprised of alternating layers of quartzite, schist, and quartzitic-gneiss, and is in contact with ~ 1.7 Ga granitic basement at both its northern and southern boundary, with ~1.4 Ga concordant and discordant pegmatite dikes exposed throughout. Previous research has used the boundary relationships and U-Pb crystallization and detrital zircon ages to propose two hypotheses: 1) The northern boundary is depositional in nature and represents original sedimentation of quartz-rich sediment sourced from the adjacent granitic basement onto a weathered granitic surface after a period of very efficient and rapid weathering at 1.7 Ga due to altered ocean chemistry (Jones et al. 2009, Medaris et al. 2003, Cox et al. 2002); and; 2) The major deformational and tectonic events that are responsible for the present configuration at Blue Ridge occurred during emplacement of granitic basement at 1.7 Ga (Mai 2002) and/or during inboard deformation associated with the ca. 1.66 -- 1.60 Ga Mazatzal orogeny (Jones et al. 2009). My field observations coupled with geochronologic and geochemical data acquired from previously undocumented structural marker units infer that an episode of deformation occurred at ~1.4 Ga during pegmatite emplacement, and that an additional pulse of igneous activity occurred at ~1.1 Ga associated with emplacement of the Pike's Peak batholith. The newly identified structural marker units include syndeformational pegmatite dikes located at both the northern and southern boundary, an amphibole-bearing leucosome infiltrating quartzite and augen-gneiss along the southern boundary, and the identification of granodiorite and an associated cross-cutting diabase dike that appear to cross-cut existing metasedimentary units.;The presence of attenuated and ductiley deformed pegmatite dikes found at both boundaries, combined with published U-Pb zircon ages ca. 1436 -- 1431 Ma from pegmatites at Blue Ridge (Jones et al., 2009), show that movement and deformation occurred at this time. U-Pb geochronologic data from the amphibole-bearing leucosome define discordia arrays with lower intercept ages of 1097 +/- 93 Ma and 1109 +/- 43 Ma for zircon and 1185 +/- 39 Ma for apatite. The granodiorite yielded little to no zircon, but had a U-Pb apatite lower intercept age of 1016 +/- 65 Ma. Sm/Nd isotopic data of the diabase dike revealed a depleted mantle model age of 1.2 Ga. Field observations coupled with geochemical and petrographic analyses from the northern boundary show that the contact of granite with metasedimentary rocks was initially depositional in nature, but subsequently deformed as a shear zone ca. 1.4 Ga. Rare earth element (REE) patterns from the northern granite and adjacent metasedimentary rocks are very similar, leading to the interpretation of the granite being the source for the metasediments. Heavy mineral comparisons across the northern boundary are less clear, but apatite/zircon ratios within the granite and basal quartz pebble conglomerate (QPC) of the metasedimentary package display moderate similarities. Additionally, new pressure-temperature (P-T) thermodynamic modeling based on schists and quartz-schists combined with garnet-biotite thermometry was used to quantify that amphibolite-grade metamorphism reached a maximum pressure of 5.2 kb and a maximum temperature of 635 °C.