High-resolution garnet chronometry and the rates of metamorphic processes

Details High-resolution garnet chronometry and the rates of metamorphic processes High-resolution garnet chronometry and the rates of metamorphic processes

Dec 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.107..649b&link_type=abstract

Earth and Planetary Science Letters, Volume 107, Issue 3-4, p. 649-671.

Computer Science

34

Scientific paper

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°C and 5.2 +/- 0.5 kbar under conditions of low aH2O, and yield indistinguishable 147Sm-143Nd and 238U-206Pb 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°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°C and 7.0 +/- 1.0 kbar. The inclusion-free garnet rims in this assemblage record indistinguishable 147Sm-143Nd and 238U-206Pb 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°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°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.

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