Mineral reactions participating in intragranular fracture propagation: implications for stress corrosion cracking

Two examples of mineral reactions accompanying intragranular fracturing of silicates are described from amphibolites at the Grenville front, Coniston, Ontario, and a granodiorite within the Miéville shear zone, Switzerland. At these localities coarse grained (> 1 mm) hornblende and biotite respectively have undergone initial deformation by intense transgranular fracturing. The ambient temperature of deformation is estimated at 400–500°C for the amphibolites, and 250–300°C for the granodiorite.Fracture intensity within hornblende increases with progressive deformation until fractures coalesce and grains lose cohesion. Fractures are occupied by hornblende in close optical continuity with the parent grain, and typically contain median sutures decorated by arrays of solid inclusions. Relative to the parent grain, hornblende in fractures is depleted in Ti, Al, plus K, and enriched in Mg. Given the preferential partitioning of Mg, Al^VI and Ti into the M2 site of calcic amphiboles, the decrease of Ti and Al^VI in the host-to-crack transition is consistent with the corresponding increase of the Mg/(Mg + Fe²⁺) ratio.Fractures within biotite are bounded by an envelope of paler brown biotite which corresponds to a decrease of Ti and increase of Fe + Mg relative to regions unaffected by cracking. Fractures are occupied by secondary ilmenite, low-Ti biotite and high-Ti muscovite. Ti and Al do not vary significantly as a function of Mg/(Fe + Mg) in the host-to-crack transition, as anticipated from the approximately equal partitioning of these two cations into the M1 and M2 octahedral cation sites. The direct relationship of the mineral reactions to the fractures is taken as evidence for the participation of the reactions in crack propagation. These features may thus represent examples of natural stress corrosion cracking.