MICROSTRUCTURAL FEATURES OF HIGH-TEMPERATURE SHEAR ZONES IN GABBROS OF THE NORTHERN APENNINE OPHIOLITES

This paper presents preliminary data on the microstructural evolution of upper amphibolite facies mylonitic metagabbros from the Bracco Ophiolite Complex, Northern Apennines (Italy). High temperature deformation, possibly related to the initial stages of breakup and oceanization in the Ligurian Tethys, led to the development of three different types of fabrics in the coarse grained Mg-gabbros, the most widespread rock-type in this area. With progressive deformations, coarse-grained porphyroclastics mylonites (fabric type 1), banded (fabric type 2) and 'flinty' ultramylonites (fabric type 3) were formed. In plagioclase and clinopyroxene, the two main minerals in the studied rocks, optical microstructures and scanning electron microscopy analyses indicate crystal plasticity as the main deformation mechanism. A possible change from 'rotation'-recrystallization regime, through grain boundary migration to diffusion-assisted high temperature grain boundary sliding, is suggested for the plagioclase grains in high temperature shear zones.