Structural studies of Quartzo-Feldspathic veins from parts of North-Odisha Singhbhum Craton: Special reference to Paleo-stress Analysis

Abstract

Singhbhum Craton in Peninsular India is one of the oldest Archean Craton that preserves the history and signature of the early continental crust. It preserves the records of several episodes of volcanism, sedimentation, and metallogenic processes from Palaeoarchean to the Mesoproterozoic. It is traversed by Singhbhum Shear Zone (SSZ). The shear zone separates a northern terrain of more highly metamorphosed rocks and a southern terrain of relatively less metamorphosed rocks. Anandpur area is a part of southern Singhbhum craton which is mostly dominated by Singhbhum Granite (SG) and Older Metamorphic Tonalite Gneiss (OMTG). These are present as enclaves or pockets within singhbhum granites. Granitic bodies are intruded by quartz and pegmatite veins. Pegmatite is intruded in the country rock in numerous places showing possibility of hydrothermal alteration in country rock due to pegmatite intrusion. The emplacement of quartz and pegmatite veins into the fracture plane is controlled by both pore fluid pressure (Pf) and normal stress (σn). By plotting the orientations of veins and fractures which are collected from the field in stereonet and Mohr plotter, pore fluid pressure and tectonic stress ratio are measured. Total 235 quartz veins and 65 fracture planes orientations are measured, and the most noticeable direction is NE SW. Although veins are cross cutting each other, it’s proven from the field photos that they are synchronous. In the stereoplot poles to the veins displays a girdle distribution pattern with an elliptical gap region in the middle of it and the plane striking N30°E represents σ1-σ2 plane. Further, 𝜃2 and 𝜃3 value is 17° and 34° respectively, and the trend and plunge amount of σ1, σ2 and σ3 is 075° / 81.5°, 210° / 06° and 300° / 06° respectively, which represents maximum expansion in NW-SE. The stress ratio (𝜙) and the driving pressure ratio (R’) are calculated as 0.72 and 0.91 respectively, high value of driving ratio represents broad range of fractures to dilate and the high φ value indicating uniaxial extension. 3D Mohr plot shows the variation of fluid pressure in between σ1 and σ2.