Submagmatic microfractures in granites

Microstructures in Hercynian granite plutons of the Pyrenees indicate fracturing of plagioclase crystals in the presence of melt. This is represented mainly by quartz and/or feldspar that fills fractures. Electron-microprobe analyses demonstrate that, in compositional agreement with the rims of the zoned plagioclase, the feldspar infillings are more sodic with decreasing melt fraction. Fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic, i.e., when the melt was below the critical fraction for magmatic flow. Material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures, because during cooling the volume fraction of melt remains low for a large temperature range. A strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas.