HR: 1340h
AN: V13A-0515    [Abstracts]
TI: Rates of Metamorphic and Tectonic Processes Derived From Garnet Chemistry and Phase Diagrams
AU: * Stowell, H H
EM: hstowell@geo.ua.edu
AF: Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, AL 35487-0338 United States
AB: Sm and Nd isotope data for the interior parts of a garnet crystal (core) and the whole rock can be used to calculate the timing of initial growth. Isotope data for the external parts of a garnet crystal (rim) and the rock matrix can be used to calculate the timing of final growth. Similarly, major element chemistry can be used to calculate pressures (P) and temperatures (T) of metamorphism for initial and final garnet growth. Combination of these data allows estimation of garnet growth, heating, and loading rates. Sm-Nd isochrons were constructed for garnet core and whole rock, and for garnet rim and rock matrix (whole rock minus garnet). For core and rim ages that are indistinguishable, uncertainties for a single isochron constrain the duration of garnet growth. P and T were estimated from the intersection of garnet chemical isopleths on P-T phase diagrams for specific rocks (pseudosections), and garnet rim thermobarometry and/or pseudosection peak mineral stability fields. Results from Cretaceous contact and regional metamorphic environments provide preliminary data for comparison. Schist from Garnet Ledge, AK. 20 mm garnet crystals in pelite less than 1 km from a diorite pluton grew during contact metamorphism over ca. 0.5 m.y. during a 70°C increase in T and 0.6 kbar increase in P. Growth, heating, and loading rates are 20 mm/m.y., 140°C/m.y., 1.2 kbar/m.y., respectively. Chiwaukum Schist from near the Mount Stuart batholith, WA. 14 mm garnet crystals in pelite less than 2 km from the Mt. Stuart batholith grew during regional metamorphism over <1.4 m.y. during a 75°C increase in T and 1.2 kbar increase in P. Growth, heating, and loading rates are 5 mm/m.y., 54°C/m.y., 0.9 kbar/m.y., respectively. 5 mm garnet crystals in pelite adjacent to orthogneiss grew during contact metamorphism over <1.8 m.y. while T and P increased to peak conditions of 630°C and 6.8 kbar. Pembroke granulite from Fiordland, New Zealand. 16 mm peritectic garnet crystals in lower-crustal mafic migmatite grew during partial melting over <1.3 m.y. at about 14 kbar and presumably during little or no change in T. Garnet growth durations range from ca. 0.5 to 6.6 m.y. Growth and heating rates for the contact metamorphic garnet are >double those for regional metamorphism; however, additional data are needed to confirm general differences. In addition, uncertainties in the above ages and rates are generally similar to the absolute differences between core and rim ages, and the derived rates. Therefore, the precision on isotopic ages clearly needs to be improved in order to better constrain rates of metamorphic processes.
DE: 1100 GEOCHRONOLOGY
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005