Deformation mechanisms and phase distribution in mafic high-temperature mylonites from the Jotun Nappe, southern Norway

Abstract

Shear zones and mylonites in anorthositic to gabbroic rocks from the Jotun Nappe in southern Norway have been studied as an example of high-temperature deformation in polyphase rocks. The deformation has taken place at approximately 700°C and maximum confining pressures of 900 MPa. The constituent phases amphibole, pyroxene, and plagioclase have recrystallized synkinematically, reducing the grain size of the rock to about 20 to 70 μm. Two different microstructures develop, depending on the recrystallization mechanism. Dynamic recrystallization of plagioclase, hornblende and pyroxenes produces monophase layers, which extend from porphyroclasts. Heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase. These phase mixtures extend from hornblende or clinopyroxene porphyroclasts. The product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende, pyroxene and plagioclase layers, which alternate with abundant layers of phase mixtures. Different deformation mechanisms operate in the different types of microstructures. Dislocation substructures, dynamic recrystallization microstructures and a crystallographic preferred orientation (measured by Ji and Mainprice (1988)on samples of the same shear zone) indicate crystal plastic deformation in monophase plagioclase layers. Dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation. The phase distribution of plagioclase and hornblende, grain shapes, and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow (grain boundary sliding, probably accommodated by diffusional mass transfer). In the plagioclase–hornblende mixtures, the phases show an anticlustered, non-random distribution. Grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts. The reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries.