OIB‐Type Garnet Amphibolites From the Xuerongzangbu Area, Central Lhasa Terrane, Tibet: Implications for Songduo Paleo‐Tethys Ocean Evolution and Permian Mantle Activity Along North Gondwana

Abstract

Whether the Songduo metamorphic belt represents an in situ Paleo‐Tethyan suture within the Lhasa Terrane or far‐traveled allochthonous high‐pressure slices, and how Permian mantle activity along the North Gondwana margin relates to subsequent Tethyan evolution, remain central questions in Tibetan tectonics. We investigated retrogressed garnet amphibolites from the Xuerongzangbu area of the Central Lhasa Terrane using petrography, mineral and whole‐rock geochemistry, Sr–Nd isotopes, SHRIMP zircon U–Pb geochronology, ⁴⁰ Ar/ ³⁹ Ar thermochronology, and phase‐equilibrium modeling. The rocks record prograde burial, peak high‐pressure amphibolite‐facies metamorphism, and fluid‐assisted retrogression during exhumation. Modeling of a weakly retrogressed Type I amphibolite yields peak conditions of 15.4–16.0 kbar and 600–660°C, whereas more strongly retrogressed Type II amphibolites preserve relict kyanite, high‐Si amphibole, phengite, and rutile. The amphibolites' OIB‐type trace‐element patterns, initial ⁸⁷ Sr/ ⁸⁶ Sr ratios of 0.7050–0.7057, ε _Nd (t) values of −1.59 to +0.16, and Nd model ages of 0.97–1.08 Ga indicate derivation from an enriched intraplate mantle source with limited crustal contamination. Zircon U–Pb dating gives a protolith crystallization age of 271.0 ± 3.0 Ma, whereas hornblende and mica ⁴⁰ Ar/ ³⁹ Ar ages of 232.9–216.9 Ma record Late Triassic cooling and exhumation. Integrated with regional geological evidence, the protoliths are interpreted as seamount or ocean‐island basalts formed on the Songduo oceanic lithosphere. Together with regional Permian plume‐affinity mafic magmatism, these data suggest sustained mantle heating beneath North Gondwana and a possible temporal and geodynamic link between Songduo Ocean closure and the development of younger Tethyan branches.