Pegmatite dyke emplacement and the state of stress during cratonization – An example from the Dharwar Craton (South India)

Abstract

In this study we measured orientations of 797 pegmatite dykes intruding the ∼2.5 Ga Koppal Pluton (Dharwar Craton, South India) and use the data to evaluate the state of stress prevalent during dyke emplacement. We infer that the dykes were emplaced synchronously with late-D3 regional deformation that was controlled by NW-SE directed shortening. Dykes within the Koppal Pluton (KP) have a restricted range of orientations, which is indicative of fluid pressure (Pf) < σ2. Paleostress analysis using dykes yields orientation of σ2 to be steepest (with respect to other principal stress axes), while σ1 is shallow, thus implying oblique-slip during dyke emplacement, but with a strong strike-slip component. Maximum orientation of dykes intruding KP is NNW-SSE, which is oblique to the NNE-SSW oriented magnetic fabric that is known to have developed in the KP during earlier stages of D3. Statistical analysis of dyke thickness data from KP implies accumulation of melt in multiple pockets was important during the pegmatite emplacement process during late-D3 deformation. A kinematic model involving this aspect as well as deformation partitioning during late-D3 along anisotropy developed during early-D3 (magnetic fabric) is envisaged. This explains emplacement of pegmatite dykes having a NNW-SSE strike. Cratonization of Dharwar Craton is known to have occurred at ca 2.5 Ga, which is also the established age of KP. Therefore, the present work on pegmatite dykes provides information about the processes and paleostress prevalent during this cratonization.