Newly discovered youngest UHT metamorphism on Earth,Western Sulawesi,Indonesia

Ultrahigh-temperature(UHT)metamorphism represents an extreme crustal thermal event with peak conditions exceeding 900C at 7–13 kbar.In the modern-style plate tectonic system,records of the UHT metamorphism are relatively rare due to the secular cooling of Earth.In the Palu region of Western Sulawesi,we newly discovered a series of HT-UHT metamorphic rocks including amphibolite,granulite,eclogites and gneiss.Of them,two granulite samples(18CS14-2,18CS14-4)with high garnet content(>50 mol%)are chosen for petrographic observation,phase equilibrium modelling,and zircon U-Pb dating.These rocks are characterized by a relic M1 assemblage of Grt+Ky+Bt+Rt and a M2 assemblage of Grt+Sil+Pl+Spl+Crd±Qtz+Ilm+melt.Phase equilibrium modelling based on effective bulk compositions yields UHT conditions of 7.2–8.5 kbar/940–1080C(18CS14-2)and 7.0–7.3 kbar/1000–1040C(18CS14-4).U-Pb analysis reveals two generations of metamorphic zircon with evolving REE content that is intimately related to garnet growth and decomposition.Zircon age of 36–5.3 Ma is ascribed to syn-to post-M1 metamorphism,whereas the young zircon age of 5.1–3.8 Ma is linked to syn-and post-M2 stage.The UHT metamorphism was probably the consequence of the upwelling of asthenospheric mantle triggered by post-collisional delamination of lithosphere in the MiocenePliocene(ca.5 Ma).It could represent the youngest known UHT metamorphism on Earth.