LOCAL DISEQUILIBRIUM OF PLAGIOCLASE IN HIGH-TEMPERATURE SHEAR ZONES OF THE IVREA ZONE, ITALY

Microstructural and chemical analysis of plagioclase in 20 superficial ly similar amphibolite facies ductile shear zones in metagabbors and a mphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditi ons lower than those of the wall rocks, occurs in the following four d ifferent microstructural situations with different chemical compositio ns: (i) relatively undeformed porphyroclasts, (ii) dynamically recryst allized 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 difference s in the An and Ba contents are caused by partial chemical equilibrati on of plagioclase in the shear zones during and partly after deformati on. Changes in An and Ba contents were caused by fluid-assisted grain- boundary migration recrystallization, as well as by solid-state diffus ion, while fluid activity was high. The relation between the compositi on 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 inter pretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystal lization and that are not adjacent to porphyroclasts reflect P-T condi tions during the dominant shear-zone deformation.