Petrogenesis and geological significance of the Paleogene Magmatic Rocks in the Xima–Mengnongjie area, Tengchong Block, Yunnan province: evidence from petrology, geochemistry and zircon U-Pb geochronology

The Paleogene magmatic rocks in the Xima–Mengnongjie area of the Tengchong Block offer critical insights into the Cenozoic tectono-magmatic evolution of the eastern Tethyan Tectonic Domain. LA-ICP-MS zircon U–Pb geochronology identifies two emplacement phases at 64.7–62.4 Ma and 54.7–52.3 Ma, recording Early Paleocene and Early Eocene magmatic events with Ti-in-zircon crystallization temperatures of 661 °C–666 °C and 679 °C–703 °C, respectively. Geochemically, both the Early Paleocene and the Early Eocene magmatic rocks belong to acidic to intermediate-felsic, peraluminous to weakly metaluminous, exhibiting pronounced enrichment in LREE and large-ion lithophile elements (Rb, U, La, Ce, Zr and Hf) and conspicuous depletion in high-field-strength elements (Nb and Ti) together with Sr, Ba and HREE. The Early Paleocene granitoids exhibit S-type affinity, indicating magma sourced from meta-sedimentary and felsic crustal materials. The Early Eocene rocks consist of S-type granites and intermediate lithologies with crust-mantle mixing features. The Early Paleocene granitoids formed in a volcanic-arc (VAG) setting related to Neo-Tethyan oceanic subduction, whereas the Early Eocene rocks formed in syn-collisional to post-collisional (syn-COLG) settings. The age of ca. 55 Ma marks a critical tectonic transition from subduction to collision, accompanied by slab rollback and asthenospheric upwelling that provided extra heat for crustal melting and mantle melting during the India-Eurasia convergence.