CRENULATION CLEAVAGE DEVELOPMENT AND ITS RHEOLOGICAL IMPLICATIONS

The metapelitic rocks that make up the Mooselookmeguntic pluton aureole in western Maine are ideal for studying crenulation cleavage development. In this area all stages of crenulation cleavage development are present, from an insipient micro-folding approximately 3 km away from the pluton to a pervasive new foliation, adjacent and parallel to the pluton margin. The preservation of such a spatial gradient in crenulation cleavage development is rare, and is in this case partly related to the fact that the pluton (~370 Ma) postdates the regional Devonian-aged deformation by ~30 m.y.

The characteristic mineralogical differentiation that occurs during crenulation cleavage development is thought to be the result of dissolution of quartz and feldspar to form phyllosilicate-rich domains, and precipitation of the dissolved material in quartz- and feldspar-rich domains. The occurrence and evolution of this, partly deformational and partly metamorphic, process in the Mooselookmeguntic pluton aureole is revealed by crystallographic orientation data obtained by electron backscatter diffraction methods, and by high resolution compositional maps obtained by wavelength dispersive spectrometry. The stress and strain controls on mass transfer that lead to the differentiation are being explored using 2D and 3D numerical experiments that account for elastic interaction of individual minerals. Mineralogical differentiation leads to marked changes in the orientation and degree of mechanical anisotropy. Incorporation of texturally induced anisotropies into 3D numerical models shows that these have a profound effect on crustal-scale rheological evolution and strain localization.