Petrogenesis and tectonic implications of Paleozoic S-type granitic gneisses of Nanga Parbat Syntaxis, North Pakistan

Abstract

This study integrates in-situ zircon U-Pb and Hf isotope data, and whole-rock geochemical analyses, along with field and petrographic features of granitic gneisses exposed in the Nanga Parbat Syntaxis. The aim is to constrain the petrogenesis and tectonic evolution of these rocks. Field and petrographic observations distinguish the studied rocks into two varieties: leucogranitic gneisses referred as type 1 and augen gneisses referred as type 2. Zircon U-Pb dating of type 1 yields a weighted mean age of ∼460 Ma while type 2 yields an age of ∼480 Ma, indicating that both types were emplaced during the early Paleozoic. Geochemically both types are strongly peraluminous, high-K calc-alkaline, and show high normative corundum, indicating an S-type granite magma source derived from the anataxis of heterogeneous supracrustal metasedimentary rocks. The highly negative εHf(t) values (−29.5 ∼ −8.8) further confirm types 1 and 2 rocks were derived from crustal material without a significant contribution from the mantle. Type 1 rocks are characterized by the presence of tourmaline, high Al₂O₃/TiO₂ (67.3–84.2)_, and low CaO/Na₂O (0.25–0.37) ratios, suggesting their formation through the partial melting of boron-rich metapelites under moderate temperatures (avg. 763 °C, based on Ti-in-zircon thermometry). In contrast, type 2 rocks lack tourmaline, show higher CaO/Na₂O ratios (0.60–0.70), lower Al₂O₃/ TiO₂ ratios (19.3–23.9), and are likely formed from hydration-driven partial melting of metagraywacke at an average temperature of ∼704 °C. The geochemical and isotopic data in this paper, and their comparison with coeval rocks from the Himalaya and other regions, suggest that magma of the studied Paleozoic granites originated from heterogenous metasedimentary sources, likely deposited in a back-arc basin followed by the subduction of the Proto-Tethyan Oceanic lithosphere, followed by slab roll-back and break-off. Such a tectonic setting possibly triggered mafic magmatic underplating and induced partial melting of the back-arc assemblages. Both rock types contain occasionally inherited zircon cores that yield ages of ∼2065 to ∼773 Ma, indicating remnants of earlier crustal growth events. Additionally, a few zircon grains with ages of ca. 53 and 25 Ma could be linked with the Cenozoic India-Asia collision-related magmatism, metamorphism, and post-collision anataxis.