PRESSURE-TEMPERATURE PATHS FROM FIAs, P-T PSEUDOSECTIONS AND ZONED GARNETS: SIGNIFICANCE AND POTENTIAL FOR ~1700 AND ~1400 MA DEFORMATION AND METAMORPHISM IN THE BIG THOMPSON REGION OF COLORADO ROCKIES, USA

A progression of FIAs (foliation intersection/inflection axes preserved within porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods of garnet and staurolite growth and two growth phases each of cordierite and andalusite. These minerals have grown in an overall prograde path, where growth of the garnet porphyroblast was always followed by the formation of staurolite, andalusite and cordierite phases in each FIA set. Inclusions of earlier minerals within the younger phases have supported the porphroblastic mineral sequence obtained through FIAs. Thermodynamic modelling in the MnNCKFMASH system reveals that this episodic growth occurred over a similar bulk compositional range and PT path. Multiple phases of growth by one reaction in these rocks strongly suggests that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by these porphyroblasts indicate that each stage of growth occurred during deformation and indicate that the local partitioning of deformation at the scale of a porphyroblast was the controlling factor on whether or not the reaction took place. In-situ dating of monazite grains preserved within porphyroblast from each FIA set has revealed that the first period of tectonism, occurred around 1759.7±9.8 Ma, recorded within the porphyroblasts of FIA set 1, where garnet nucleated at 540-550°C and 3.8-4.0 kbars. The intersection of Ca, Mn, and Fe isopleths in garnet cores for 3 samples, containing FIA set 1, set 2 (1759.7±9.8, 1721.0±6.4 Ma) and set 3 (1674±11 Ma), trending NE-SW, E-W and SE-NW respectively, indicate that these rocks never got above 4kbars throughout the Colorado Orogeny. They remained around the same depth until the onset of younger orogeny at 1415±16 Ma, when the pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FIA set 4 and trending NNE-SSW. A slightly clockwise P-T path occurred for both orogenies.