Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep

In the Gran Paradiso metagranodiorite (Western Alps) small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture. Polycrystalline quartz aggregates in the mylonite deform by dislocation creep developing a crystallographic preferred orientation (CPO) and a monoclinic surface orientation distribution function (ODF). The polymineralic matrix of the mylonite and the ultramylonite deform by diffusion creep. In the ultramylonite the quartz CPO is randomized and the surface ODF becomes orthorhombic. The transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates, concomitant with the precipitation of K-feldspar (±biotite) between quartz grains.In quartz, reduction from the dynamically recrystallized grain size in the aggregates (110 μm) to the size of the dispersed grains in the ultramylonite (25 μm) occurs through the following processes: K-feldspar precipitates at opening sites along grain boundaries (strain incompatibility) pinning the grain size in quartz aggregates. Coalescence of K-feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates. Solution precipitation reduces the size of the dispersed grains to less than subgrain size (∼45 μm).

► Quartz aggregates in the mylonite deform by dislocation creep. ► Polymineralic fine grained matrix deforms by diffusion creep. ► The ultramylonite (qtz–kfs–plg–bio) deforms by diffusion creep. ► Disintegration of quartz aggregates by nucleation and growth of kfs and bt. ► Quartz grain size decrease by combination of pinning and dissolution.