Development of P‐T prograde and P‐retrograde, T‐prograde isogradic surfaces during blueschist to eclogite regional deformation/metamorphism in New Caledonia, as indicated by progressively developed porphyroblast microstructures

Abstract

Abstract The mid‐Tertiary blueschists, eclogites and eclogitic gneisses of northern New Caledonia are the products of four phases of regional metamorphism and deformation (D 1 –D 4 ). Omphacite, lawsonite and Mn‐rich garnet isogradic surfaces were developed during the second deformation (D 2 ) under prograde pressure and temperature conditions. Subsequent deformations (D 3 –D 4 ) folded these D 2 isogradic surfaces. However, within the P‐retrograde, T‐prograde metamorphic environment of the D 4 phase, omphacite altered to albite and chlorite; as a result, a late‐stage sub‐horizontal isogradic surface developed for omphacite‐out where this mineral preserved as relics within syn‐D 4 albite porphyroblasts. Other minerals that crystallized for the first time (epidote) or had rim additions (almandine phengite) during D 4 , also form nearly horizontal isogradic surfaces. Porphyroblastic garnet and albite contain inclusion trails, which allow their microstructural development and crystallization of the matrix to be traced from D 2 to D 4 . Late syn‐D 4 the temperature increased markedly in association with an extensive exothermic decarbonation, even though the rocks were in a state of pressure retrogression. This caused considerable neocrystallization, recrystallization and growth of mattix and porphyroblasts such that, although S 2 foliation crenulated by D 3 and D 4 is readily observable, almost all signs of stored strain due to D 3 and D 4 have been removed, and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization, post‐dating deformation.