ISSN:
1525-1314
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
Sm–Nd garnet-whole rock geochronology, phase equilibria, and thermobarometry results from Garnet Ledge, south-eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores 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 minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single-stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim-matrix data from the two samples constrain garnet growth duration to ≤5.5 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 and older than the 88.4 ± 2.5 Ma rim-matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet-in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet-biotite-sillimanite-plagioclase-quartz calculated by THERMOCALC in average P–T mode, and exchange thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure-temperature-time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1046/j.0263-4929.2001.00337.x
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