US EPA

6901975 

Journal Article 

Deformation of amphibolites via dissolution-precipitation creep in the middle and lower crust 

Stokes, MR; Wintsch, RP; Southworth, CS; , 

2012 

Yes 

Journal of Metamorphic Geology
ISSN: 0263-4929
EISSN: 1525-1314 

WILEY-BLACKWELL 

HOBOKEN 

30 

7 

723-737 

English 

10.1111/j.1525-1314.2012.00989.x 

WOS:000307361000008 

http://doi.wiley.com/10.1111/j.1525-1314.2012.00989.x
Exit
 

Continuous compositional zoning in amphibole grains in strongly deformed and lineated amphibolites from the Eastern Blue Ridge, North Carolina indicates that most of the deformation was accommodated by dissolutionprecipitation creep. Amphibole in most samples shows moderate prograde and/or retrograde zoning parallel to the long-axis with compositions ranging between magnesiohornblende and tschermakite. In one sample, grains are zoned from actinolitic (Si = 7.9 p.f.u.) cores to tschermakitic (Si = 6.2 p.f.u) rims. Amphibole-plagioclase thermometry suggests prograde growth temperatures as low as 400 degrees C, but typically range from 650 to 730 degrees C and retrograde growth temperatures <700 degrees C. These estimates are corroborated quantitatively with amphibole-garnet-plagioclase thermobarometry and qualitatively with a positive correlation between TiO2 concentration in amphibole and calculated temperature. This growth zoning provides persuasive evidence that amphibole precipitation produced the fabric, but evidence for dissolution is less common. It is present, however in the form of truncations of complicated zoning patterns produced by healed fractures and overgrowths in low-temperature cores by high-temperature tschermakitic grains lacking similar internal structures. The preservation of this network of straight cracks filled with optically continuous amphibole also provides evidence against the operation of dislocation creep even to temperatures >700 degrees C because dislocation-creep would have deformed the fracture network. Thus, these amphibolites deformed by dissolutionprecipitation creep that produced a strong linear fabric under upper amphibolite facies, middle-to-lower crustal conditions. The significance of this discovery is that dissolutionprecipitation creep is activated at lower stresses than dislocation creep and that the strength of the lower crust, where amphibole is the dominant mineral is probably lower than that derived from experimental studies. 

amphibolite; Blue Ridge; dissolution-precipitation creep; metamorphic reaction mechanisms