CRENULATION CLEAVAGE FORMATION - EVOLVING DIFFUSION, DEFORMATION AND EQUILIBRATION MECHANISMS WITH INCREASING METAMORPHIC GRADE

An integrated microstructural and chemical study of crenulation cleava ges developed in chlorite-to garnet-zone schists from the Wenchuan-Mao wen shear zone, central China, has resulted in the recognition of the deformation and equilibration mechanisms operating during their format ion at a range of temperatures. Deformation during crenulation cleavag e formation at chlorite-to biotite-zone temperatures (similar to 520 d egrees C) is essentially a solution-deposition process, in which trans port occurred by a combination of localized advection and grain-bounda ry diffusion. Recrystallization and hence equilibration of muscovite i n discrete cleavages is recognized by distinct compositional differenc es between muscovite from different domains. Crenulation cleavages dev eloped in garnet-zone schist are characterized by homogeneous muscovit e compositions across domains and microstructures related to intracrys talline deformation, indicating an increasing importance of volume dif fusion and diffusive equilibration of mica at temperatures greater tha n similar to 520 degrees C. (C) 1997 Elsevier Science Ltd.