Discontinuous grain growth in recrystallised vein quartz — implications for grain boundary structure, grain boundary mobility, crystallographic preferred orientation, and stress history

Grain growth is a process driven by the reduction of interfacial free energy, leading to an increase in grain size and a reduction of the number of grains in a given volume of a single phase polycrystalline material. Discontinuous grain growth (DCGG), as opposed to normal grain growth (NGG) involves just a few grains (blasts) growing at the expense of a matrix, that itself undergoes much slower modification in grain size, hence producing a bimodal grain size distribution as a transient state prior to impingement of the blasts. Such a transient state has been found in a quartz layer in metamorphic rocks of the Sesia Zone, Western Alps. There, large strain-free quartz grains occur that reveal concave outward grain boundaries against a matrix of small grains with a pronounced crystallographic and a weak dimensional preferred orientation. The misorientation of the blasts with respect to the matrix grains systematically exceeds 30°. This finding suggests that the mobility of high-angle grain boundaries in a natural quartzite depends on the degree of their misorientation (among other factors), as known for metals and alloys. The width of the grain boundaries must be sufficiently small to allow for this effect. DCGG is promoted by a pronounced crystallographic preferred orientation (CPO), that leaves only a few grain boundaries with sufficient misorientation to be mobile. In contrast to NGG, DCGG wipes out any pre-existing CPO and information on activated glide systems or kinematics during the preceding stage of flow is lost after DCGG has gone to completion. A complementary new CPO could even develop, that could be difficult to interpret. With respect to tectonic history, interfacial free energy driven grain growth reflects a period of low stress annealing and thus constitutes a first order signal.