Pressure–Temperature Evolution of a Late Palaeozoic Paired Metamorphic Belt in North–Central Chile (34–35300S)

Eastern Series, constitute coeval parts of a Late Palaeozoic paired metamorphic belt dominated by siliciclastic metasediments. The Western Series also contains rocks from the upper oceanic crust and represents an accretionary prism. Omnipresent high-pressure conditions are reflected by Na–Ca-amphibole and phengite in green-schists. Peak PT conditions of 70–93 kbar, 380–420C point to a metamorphic gradient of 11–16 C/km. Three unique occurrences of blueschist yield deviating conditions of 95–107 kbar, 350– 385C and are interpreted as relics from the lowermost part of the basal accretion zone preserving the original gradient of 9–11C/km along the subducting slab. Pervasive ductile deformation related to basal accretion occurred near peak PT conditions. Deformation and PT evolution of the metapsammopelitic rocks is similar to that of the metabasites. However, a rare garnet mica-schist yields peak PT conditions of 96–147 kbar, 390–440C reflecting a retrograd