Long duration ∼600–500 Ma high-T metamorphism ended with early Ordovician rapid intermediate-T cooling and stabilization of Sri Lanka in central Gondwana

Abstract Continent formation and its stabilization are key factors for understanding tectonic processes and histories across geologic time. Sri Lanka consists of a Central Highland (HC) granulite/UHT terrane bounded by tectonic sutures and medium-to-high-grade magmatic arc terranes likely formed via Neoproterozoic double-sided subduction and collision associated with assembly of Gondwana. EMP Chemical Th-U-Pb dating of monazite within the eastern suture is dominated by 595–635 Ma dates, consistent with juxtaposition ca. 600 Ma as arc magmatism ended. Chemical analysis of metamorphic monazite dates from the eastern HC indicates prograde HT metamorphism (M1) at ca. 570 Ma during garnet growth (lower Y monazite) and retrograde HT metamorphism (M2) at ca. 560–550 Ma (higher Y monazite). These ages reflect orogenic thickening associated with arc collisions (M1) and retrograde metamorphism (M2) during deep-crustal exhumation of HC rocks. Regional long duration (>50–100 Ma) HT metamorphism, which continued until 520 Ma, and possibly to ca. 480 Ma, was followed shortly by rapid early Ordovician (480–490 Ma) lower to mid-crustal cooling based on near concordant 40 Ar- 39 Ar hornblende and biotite ages. Rapid cooling occurred concurrently with metasomatism (He et al. 2016), and region-wide exhumation during orogenic collapse documented in adjacent portions of Gondwana. The cessation of long-duration HT metamorphism linked to the onset of rapid Ordovician intermediate-temperature (>500–<300 °C) cooling and exhumation via orogenic collapse resulted in young stabilized continental crust. The Neoproterozoic-Early Palaeozoic metamorphic/thermal evolution of Sri Lanka (and correlated regions) within Gondwana attests to the timing and process of rapid stabilization of central Gondwanaland.