DEFORMATION OF GRANITOIDS AT LOW METAMORPHIC GRADE .2. GRANULAR FLOW IN ALBITE-RICH MYLONITES

The high strain deformation at low metamorphic grades of three investi gated granitoids is dominated by the granular flow of albite-rich poly phase aggregates. These aggregates formed by retrograde breakdown reac tions from intermediate plagioclase and K-feldspar. The predominant de formation mechanism changes in granitoids as the grain size is reduced : coarse-grained (low strain) examples are deformed by a combination o f intracrystalline plasticity (quartz) and fracturing (feldspar). In t he mylonites, intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non-cataclastic gran ular flow of polyphase aggregates, consisting largely of albite and qu artz. Deformation appears to be stable, probably because grain growth in albite-quartz mixtures is inhibited. These fine-grained aggregates are mechanically weaker than pure quartz aggregates. Thus, the change in deformation mechanism, mainly due to feldspar breakdown reactions, appears to be important for the localization of high shear strain defo rmation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar. The rheological behaviour of albite-dom inated mineral aggregates may have two consequences for middle to uppe r crustal deformation of modally feldspar-dominated lithologies: (1) f eldspar, in the presence of aqueous fluids, can be an important minera l controlling the rheology at low metamorphic grades, that is, below a mphibolite facies P-T conditions; (2) the occurrence of granular flow of fine-grained polyphase aggregates in low-grade granitoids is probab ly common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity o f single minerals such as quartz.