Fluid assisted rejuvenation of precursor brittle fractures as the habitats of ductile shear zones: An example from the ~2.6 Ga Bundelkhand Granitoid of north-central India

Abstract

Ductile shear zones frequently nucleate on precursor fractures, and usually imitate their initial geometries. The fractures may either predate the ductile shearing episode or form at an early stage of the ductile shearing. In either case, fluids play a pivotal role in the transformation from brittle to ductile mechanisms. However, the exact mechanism(s) triggering the transition from brittle fracturing to ductile shearing remains elusive. The granitoid rocks of the Bundelkhand Craton preserve evidences of at least five growth stages of ductile shear zone, nucleated on precursor fractures. Field and microstructural investigations suggest that these ductile shear zones are always associated with epidote (~70–80% epidote + ~30-20% quartz ± ~5% chlorite) veins. Here, we argue that a fluid, now represented by epidote vein, originated from the breakdown of mafic minerals in the host granitoid, essentially percolated in the fractures that promoted cataclasis at first. The finer grain size resulted from cataclasis coupled with the fluid had weakened the host rock along fractures which eventually nucleated the ductile shear zone. Protracted supply of the fluid and syn-kinematic redistribution by the fluid phase widened the shear zone.