Early Paleozoic Bimodal Magmatism Documents the Transition from Collisional to Extensional Tectonics within the Central Himalaya

Abstract

The tectonic setting of the Himalaya during the Early Paleozoic has been a subject of enduring debate within the scientific community. Newly discovered bimodal intrusive rocks from comprehensive field geological investigation in the central mountain range were subjected to petrology, zircon U-Pb geochronology, zircon Hf isotopes and whole-rock geochemistry analyses. The Palie bimodal intrusive rocks, comprising amphibolite and granitic gneiss, were formed at ∼489 Ma. The amphibolite exhibits geochemical characteristics consistent with N-MORB, while the granitic gneiss is classified as high potassium calc-alkaline peraluminous S-type granite. Both igneous rocks exhibit negative zircon ε_Hf(t) values and display ancient T_DM^C ages. In conjunction with regional geological survey findings, it can be inferred that the formation of the Palie bimodal intrusive rocks occurred within a post-collision extensional tectonic setting. The amphibolite genesis involved partial melting of an enriched lithospheric mantle with some crustal assimilation, whereas the origin of the granitic gneiss can be attributed to partial melting of pre-existing felsic crust. Our data indicate that during the Early Paleozoic, the Himalaya underwent a transition from a pan-African collisional setting to post-collisional extensional tectonics.

Keywords

igneous rocks / tectonics / bimodal magmatism / post-collision extension / Ordovician / Palie

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2025 Geological Society of China