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 rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low-grade metamorphic assemblages. In this study a multi-technique thermobarometric study of the Chrystalls Beach Complex mélange, which is located within the Otago Schist accretion-collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench-fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo-Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P-T conditions in the range 400-550 MPa and 250-300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b_0 spacing analyses of white mica. Rare lawsonite occurs in a post-foliation vein, and illite 'crystallinity' measurements indicate a thermal overprint during exhumation. These P-T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P-T path. Based on variability in P-T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher-T than experienced along the prograde path. Observed subduction-related subgreenschist assemblages are therefore likely to indicate lower-P than experienced during subduction, as higher-P mineral compositions re-equilibrate during exhumation. The P-T path inferred in this study is similar in shape to P-T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.