Nucleation of ductile shear zones in a granodiorite under greenschist facies conditions, Neouvielle massif, Pyrenees, France

The problem of ductile shear zone nucleation under greenschist facies condi tions is approached from the example of small-scale shear zones developed i n the Neouvielle granodioritic pluton (Pyrenees, France) by means of field and microstructural observations and chemical analyses. These shear zones a re not related to pre-existing fractures and exhibit networks of numerous c onjugate, fairly parallel and regularly spaced centimetre-scale brittle-duc tile shear zones involving diffuse localization mechanisms. Although the mode of deformation depends on the minerals present, deformati on in the shear zones is basically controlled by hydration processes. Hydra tion and consequent fluid-controlled alteration and deformation are related to fluid migration towards developing cracks. All these cracks are extensi onal and formed on the scale of at most a few grains and frequently single grains. A comparison of chemical compositions of undeformed and sheared gra nodiorite shows that the shear zones can be interpreted as isochemical and isovolumetric systems. We propose a sequence of mechanisms by which the nucleation of a small-scal e brittle-ductile shear zones spreading out within a granodioritic rock may occur. In these mechanisms shear zone nucleation occurs independently of p re-existing fractures and results from the heterogeneous character of the p olymineralic rock. In the studied granodiorite the mineral heterogeneities favour focusing of locally derived fluids by processes involving grain-scal e hydraulic fracturing. Fluid focusing generates instabilities by local sof tening and subsequently shear zone nucleation. (C) 1999 Elsevier Science Lt d. All rights reserved.