Using microstructural, foliation intersection axes (FIAs), thermodynamic modeling (McNCKFMASH) and chemical relationships, this research investigates in detail the metamorphic and deformation processes and the progression in the distribution of various index minerals phases (garnet, staurolite, andalusite and cordierite) in the Big Thompson Canyon region, Colorado Rocky Mountains, USA. This region forms part of a Proterozoic orogenic belt in the southwestern USA and provides a classic example of a large high-T-low-P terrane.
A progression of FIAs (foliation intersection/inflection axes preserved within porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods of staurolite growth and two growth phases each of cordierite and andalusite; these FIA based isograds migrated ~2.5 kms across the orogen. This progression of mineral development occurred about FIAs trending successively NE-SW, E-W, SE-NW and NNE-SSW. Granitoids were emplaced during both periods of orogenesis in the surrounding region but have no direct relationship to the isograds. Isograd migration took place away from a heat source to the WNW and combined with the lack of relationship to pluton boundaries to the north and south suggests that the latter rocks were not the heat source for metamorphism but rather product of it. A final period of andalusite, cordierite and fibrolitic sillimanite grew over the matrix foliation and consequently, no FIA was determined for it; the isograds for this last period of mineral growth lie sub-parallel to those mapped by previous workers. A strong correlation between the distribution of FIA trends and the axial trace of all folds present in the area suggests that pockets of low strain occur due to deformation partitioning, in spite of, or perhaps because of, the 4 successive changes in bulk shortening direction. These pockets preserve folds in the orientation in which they formed from subsequent rotation or destruction. They provide remarkable confirmation of the veracity of the FIA data and indeed reveal that such data can be used to determine successions of fold development from regional maps at which scale many overprinting criteria cannot be applied. FlAs (foliation intersection/inflection axes preserved within porphyroblasts) in garnet, staurolite, andalusite and cordierite from PreCambrian rocks in the northern Colorado Rocky Mountains reveal 4 periods of growth about axes trending NE-SW, E-W, SE-NW and NNESSW during an overall prograde path. The growth of garnet was always followed by staurolite for each of the 4 FIA sets with andalusite and cordierite following staurolite during development of the last 2 sets. Thermodynamic modelling in the MnNCKFMASH system reveals that well documented episodic growth occurred over a similar bulk compositional range and PT path for each FIA in the succession. The intersection of Ca, Mn, and Fe isopleths in garnet cores containing FIA set 1 indicate that growth of this mineral phase began at 540-550°C and 3.8-4.0 kbars. Similarly, the intersection of Ca, Mn, and Fe isopleths in garnet cores containing FIA sets 2 and set 3 indicate that these rocks never got above 4 kbars throughout the Colorado orogeny. Indeed, they remained at approximately the same depth with the onset of the much younger Berthoud orogeny, when the pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FIA set 4. A slightly clockwise P-T path occurred for both orogenies. In-situ dating of monazite grains preserved as inclusions within foliations defining FlAs (foliation inflection/intersection axes preserved within porphyroblasts) contained within garnet, staurolite, andalusite and cordierite porphyroblasts provides a chronology of ages that matches the FIA succession for the Big Thompson region of the northern Colorado Rocky Mountains. FIA sets I, 2 and 3 trending NE-SW, E-W and SE-NW formed at 1760.5±9.7, 1719.7±6.4 and 1674±1l Ma respectively. For 3 samples where garnet first grew during just one of each of these FlAs, the intersection of Ca, Mg, and Fe isopleths in their cores indicate that these rocks never got above 4 kbars throughout the Colorado orogeny. Furthermore, they remained around approximately the same depth for ~250 million years to the onset of the younger Belthoud orogeny at 1415±16 Ma when the pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FlA set 4 trending NNE-SSW. No porphyroblast growth occurred during the intervening ~250 million years of quiescence, even though the PT did not change over this period. This confirms microstructural evidence gathered over the past 25 years that crenulation deformation at the scale of a porphyroblast is required for reactions to re-initiate and enable further growth. Microstructural measurements of FIAs (foliation intersection/inflection axes preserved in porphyroblasts) in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado frontal range and 5 in Palaeozoic Western Maine. Monazite inclusions in staurolite have an absolute age of 1 765±23 Ma (FIA I), 1718.8±6.9 Ma (FIA 2), and 1674±16 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8.8 Ma (FIA 3), 372.1±5.5 Ma (FIA 4), and 352.7±4.2 Ma (FIA 5) in Maine, confirming the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Thermodynamic modeling in the NCMnKFMASH system reveals that this episodic growth of staurolite occurred over a similar bulk compositional range and PT path. Multiple phases of growth by one reaction in the same and adjacent rocks in both regions strongly suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that each stage of growth in both areas 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.
Coinvestigators: None Related JCU Research Themes: Industries and Economies in the Tropics
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