Evolution of boudins under progressively decreasing pore pressure –A case study of pegmatites enclosed in marble deforming at high grade metamorphic conditions, Naxos, Greece

During ductile deformation of marbles under high grade metamorphic conditions on Naxos, Greece, pegmatites enclosed in the marbles were deformed in a brittle fashion forming blocky boudins with quartz crystallized in the interboudin zones. We studied the three-dimensional geometry of the boudins in the field and in two large samples. The first sample was serially sectioned to observe the structural and microstructural evolution; in the second sample we mapped the surface morphology of the deformed pegmatite. In profile, the boudins can be classified as symmetric torn type boudins, evolving towards asymmetric boudins with a later domino boudin component. In three dimensions, however, the morphology of the boudinaged pegma- tite is a simple set of normal faults with mode-I fractures in the fault tips and rotation of the fault blocks to accommodate the extension in the marble. Deformation history was constrained by petrology and microstructures in combi- nation with simple order of magnitude calculations of both the cooling and pore fluid pressure evolution. The dynamically recrystallized, coarse-grained calcite of the marble provides clear evidence that after the pegmatite intruded the marble and solidified, it was deformed at peak conditions of M2b metamorphism (� 670 °C and � 0.6 GPa). Such pegmatitic melts contain � 10 percent H2O which is released during crystalliza- tion. We infer that after crystallization of the pegmatite the pore fluid pressure in the pegmatite remained close to lithostatic due to the very low permeability of the surrounding marble, and the pegmatite was deformed at very low effective stress which led to brittle deformation of feldspar and mode-I fracturing of the pegmatite, forming torn type boudins. With time the pore fluid pressure slowly decreased, increasing the effective stress. Ongoing N-S extension thus resulted in slip along the quartz-filled interboudin zones and in block rotation, producing domino boudins.