Low-grade blueschists in the central Yarlung Zangbo Suture Zone, southern Tibet: Petrogenesis and tectonic implications for the Neo-Tethyan Ocean evolution

The Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet preserves critical geological records of the Neo-Tethyan Ocean, from oceanic crust formation to final subduction and continental collision. This study presents new petrological, mineral chemical, geochronological, and thermodynamic modelling data from two isolated occurrences of blueschists in the Sangsang and Kadui areas of the central YZSZ, with the aim of constraining their pressure–temperature–time (P–T–t) evolution, protolith nature, and tectonic significance. The Sangsang blueschists record a peak mineral assemblage of magnesioriebeckite, aegirine-augite, phengite, epidote, chlorite, albite, and sphene, whereas the Kadui blueschists lack phengite and contain a peak assemblage of riebeckite, aegirine-augite, epidote, chlorite, albite, and sphene. Phase equilibrium modelling constrains peak metamorphic conditions to 5.9–7.5 kbar at 314–368 °C for Sangsang and 5.5–7.1 kbar at 325–380 °C for Kadui, indicating high geothermal gradients and shallow subduction. Zircon U–Pb dating yields protolith ages of 150–148 Ma for Sangsang and 192–188 Ma for Kadui, the latter further supported by apatite U–Pb results. Published 40Ar/39Ar ages of sodium amphiboles from both localities indicate blueschist-facies metamorphism during the Paleocene (65–59 Ma), synchronous with the onset of the India–Asia collision. Field relationships show that the blueschists occur as layered or lenticular bodies intercalated with metachert and marble within sedimentary-matrix mélanges situated south of the YZSZ ophiolite belt. These blueschists exhibit distinctly higher thermal gradients and lower peak pressures compared to the high-P/low-T metamorphic rocks in the Indus Suture Zone and the Nagaland Ophiolite Complex. These characteristics suggest that the Sangsang and Kadui blueschists represent upper oceanic crustal fragments, likely derived from seamounts, that were offscraped and metamorphosed within the basal part of an accretionary wedge rather than descending into the subduction channel. By integrating new metamorphic, geochemical, and geochronological constraints with existing regional data, this study reconstructs a refined tectonic evolution model of the Neo-Tethyan Ocean. It outlines key transitions from oceanic crust generation to shallow subduction, accretion, and eventual continental collision along the southern margin of the Lhasa Block, providing new insights into the geodynamic evolution of the Tibetan Plateau.