Petrographic and Petrochemical Convergence of Temporally Distinct Granitic Gneisses: A Case Study from Swat and Chakdara, NW Himalayas

Abstract

This study presents a comparative petrographic, modal mineralogical, and whole-rock major element petrochemical investigation of the Swat and Chakdara granitic gneisses, which represent important deformed granitoid bodies exposed south of the Main Mantle Thrust in the northwestern Himalaya, northern Pakistan. The Late Cambrian-Early Ordovician Swat granitic gneisses exhibit fine-grained to mega-porphyritic textures, with well-developed augen-flaser fabrics defined by quartz, K-feldspar, plagioclase, muscovite, and biotite. In contrast, the Permian Chakdara orthogneisses, with a reported U-Pb zircon age of ca. 278 ± 4 Ma, display a more homogeneous, medium-grained, and well-foliated fabric. Petrographic and mineralogical data classify the Swat samples mainly as granitic to alkali-feldspar granite, whereas the Chakdara samples plot predominantly in the alkali-feldspar granite field. Whole rock major element petrochemical data indicate that both suites are generally magnesian, high-K calc-alkaline to alkalic-acidic, and peraluminous, consistent with an S-type affinity supported by abundant mica and the absence of primary hornblende. Despite these mineralogical and petrochemical similarities, the significant age gap between the Cambro-Ordovician Swat basement gneisses and the Permian Chakdara orthogneisses precludes a direct cogenetic relationship. Instead, their similarities are interpreted to reflect comparable crustal melting processes during separate tectono-magmatic episodes along the evolving northern margin of the indian Plate, followed by later Himalayan deformation overprinting.