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Local disequilibrium of plagioclase in high‐temperature shear zones of the Ivrea Zone, Italy
Author(s) -
ALTENBERGER U.
Publication year - 1995
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1995.tb00242.x
Subject(s) - plagioclase , shear zone , geology , mylonite , shear (geology) , dynamic recrystallization , geochemistry , mineralogy , recrystallization (geology) , petrology , microstructure , materials science , composite material , seismology , tectonics , hot working , paleontology , quartz
Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P‐T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross‐cutting the porphyroclasts and (iv) fine‐grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid‐assisted grain‐boundary migration recrystallization, as well as by solid‐state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history. The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain‐boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P‐T conditions during the dominant shear‐zone deformation.