Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh

This research examines the crystalline basement rocks encountered between depths of 496.8 and 524.3 m in drill hole GDH-62 and recovered as continuous core. The crystalline basement rocks in Dighipara, Dinajpur District, northwest Bangladesh, are dominantly felsic and crosscut by appinitic dykes in the lower part of the drill hole. The felsic rocks, mainly comprising granodiorite, are intersected near the base of the drill core by an appinitic dyke approximately 7.3 m thick. The felsic rocks are characterized by leucocratic- to mesocratic appearance and display hypidiomorphic, inequigranular textures with fine- to medium-grained interlocking fabric. The modal composition of these felsic suites includes plagioclase (33.4–49.2%), quartz (11.8–27.0%), biotite (6.6–27.2%), and K-feldspar (6.7–11.7%), accompanied by secondary minerals chlorite and epidote and accessory phases such as muscovite, zircon, hematite, and unidentified opaque minerals. From granodiorite to appinite, quartz and plagioclase show a general decreasing trend, whereas biotite increases. K-feldspar shows no consistent trend. In contrast, appinite exhibits mesocratic- to melanocratic appearance and is characterized by hypidiomorphic, inequigranular textures with fine- to medium-grained interlocking fabric. Appinite is dominated by hornblende (67–73%), accompanied by biotite, plagioclase, chlorite, hematite, and accessory opaque minerals, with calcite as micro-veins and veinlets as secondary minerals. Lensoidal quartz veins in granodiorite are interpreted to have formed by syntaxial growth and may indicate deformation under differential stress, whereas monomineralic calcite veins in appinite are formed by antitaxial growth, suggesting progressive stress-related deformation over time. The contact relationship suggests emplacement after consolidation of the felsic host rock, but the emplacement depth cannot be constrained by petrography alone. The contact is sharp, non-gradational, with no chilled margin observed.