Contact metamorphic P-T-t paths from Sm-Nd garnet ages, phase equilibria modelling and thermobarometry: Garnet Ledge, south-eastern Alaska, USA

Sm-Nd garnet-whole rock geochronology, phase equilibria, and thermobarometr y results from Garnet Ledge, south-eastern Alaska, provide the first precis ely constrained P-T-t path for garnet zone contact metamorphism. Garnet cor es from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 +/- 3.6 Ma. Garnet rims and matrix m inerals from the same samples yield a five point isochron age for final gar net growth of 89 +/- 1 Ma. Six size fractions of zircon from the adjacent p luton yield a concordant U-Pb age of 91.6 +/- 0.5 Ma. The garnet core and r im, and zircon ages are compatible with single-stage garnet growth during a nd/or after pluton emplacement. All garnet core-whole rock and garnet rim-m atrix data from the two samples constrain garnet growth duration to less th an or equal to5.5 my. A garnet mid-point and the associated matrix from one of the two garnet crystals yield an age of 90.0 +/- 1.0 Ma. This mid-point result is logically younger than the 90.7 +/- 5.6 Ma core-whole rock age a nd older than the 88.4 +/- 2.5 Ma rim-matrix age for this sample. A MnNaCaK FMASH phase diagram (P-T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 +/- 20 degreesC and 5 +/- 1 kbar. This exceeds the temperature of the garnet-in reaction by c. 50 degreesC and is compatible with overstepping of the garnet growth re action during contact metamorphism. Intersection of three reactions involvi ng garnet-biotite-sillimanite-plagioclase-quartz calculated by THERMOCALC i n average PT mode, and exchange thermobarometry were used to estimate peak metamorphic conditions or 678 +/- 58 degreesC at 6.1 +/- 0.9 kbar and 685 /- 50 degreesC at 6.3 +/- 1 kbar, respectively. Integration of pressure. te mperature, and age estimates yields a pressure-temperature-time path compat ible with near isobaric garnet growth over an interval of c. 70 degreesC an d c. 2.3 my.