HIGH-RESOLUTION GARNET CHRONOMETRY AND THE RATES OF METAMORPHIC PROCESSES

Garnets in an amphibolite-facies metasediment from Sulitjelma, North Norway yield precise and concordant Sm-Nd, U-Pb and Rb-Sr ages that relate directly to the pressure (P) and temperature (T) conditions of mineral growth. Differential mineral reaction between graphitic and non-graphitic layers within this sample preserves a record of the P-T and time (t) history experienced during Barrovian regional metamorphism. Garnets in graphitic layers grew during prograde metamorphism at 462 +/- 16-degrees-C and 5.2 +/- 0.5 kbar under conditions of low aH2O, and yield indistinguishable Sm-147-Nd-143 and U-238-Pb-206 ages of 434.1 +/- 1.2 Ma and 433.9 +/- 1.0 Ma, respectively. In contrast, garnet growth in adjacent graphite-free layers did not occur until P-T conditions of 540 +/- 18-degrees-C and 8.0 +/- 1.0 kbar were attained, with continued growth in response to minor heating and decompression with final matrix equilibration at 544 +/- 16-degrees-C and 7.0 +/- 1.0 kbar. The inclusion-free garnet rims in this assemblage record indistinguishable Sm-147-Nd-143 and U-238-Pb-206 ages of 424.6 +/- 1.2 Ma and 423.4 +/- 1.7 Ma, respectively. These results provide precise estimates for average heating and burial rates during prograde metamorphism of 8.6(-4.4)+7.5-degrees-C Ma-1 and 0.8(-0.5)+0.9 km Ma-1, respectively. Rb and Sr exchange between coexisting silicates in the graphite-free assemblage continued for some 37 Ma after the "peak" of metamorphism, and require an average cooling rate of about 4.0-degrees-C Ma-1 during uplift. These results illustrate a clear relationship between reaction history and the timing of mineral growth and provide definitive constraints on the rates of thermal and tectonic processes accompanying regional metamorphism.