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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
Author(s) -
BELL T. H.,
BROTHERS R. N.
Publication year - 1985
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1985.tb00305.x
Subject(s) - omphacite , geology , eclogite , metamorphism , blueschist , geochemistry , kyanite , lawsonite , albite , phengite , metamorphic rock , recrystallization (geology) , gneiss , glaucophane , schist , petrology , subduction , quartz , seismology , tectonics , paleontology
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.