The metamorphic history of rocks buried, accreted and exhumed in an accretionary prism: an example from the Otago Schist, New Zealand

摘要: Although subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these poorly understood. Subgreenschist transitions have been invoked as critical controls on mechanical behaviour of within crustal seismogenic zone, calling for further study very low-grade assemblages. In this a multi-technique thermobarometric Chrystalls Beach Complex melange, which is located Otago Schist accretion-collision assemblage South Island New Zealand, presented. The comprises highly sheared trench-fill sedimentary and scattered pillow basalts, inferred to formed during Jurassic subduction under paleo-Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions range 400–550 MPa 250–300 °C, supported by chlorite thermometry. Relatively high pressures burial accretion foliation development from phengite content b0 spacing analyses white mica. Rare lawsonite occurs post-foliation vein, illite ‘crystallinity’ measurements thermal overprint exhumation. These estimates their relative chronology that developed along clockwise path. Based variability different techniques, largely overprinted exhumation at similar or higher-T than experienced prograde Observed subduction-related therefore likely lower-P subduction, higher-P compositions re-equilibrate path shape paths higher grade parts Schist, other exhumed accretionary prisms around world, probably common buried, accreted prisms.