The origin and P-T evolution of peridotites and serpentinites of NE New Caledonia: prograde interaction between continental margin and the mantle wedge

摘要

The Pouebo and Diahot terranes of NE New Caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c. 40 Ma metamorphism. This Eocene high-P event has been linked with the SW-directed obduction of the New Caledonian Ophiolite, an extensive ultramafic nappe that dominates outcrop in the south of the island. In the north, ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with, but separated from, the eclogites and blueschists by foliated talc-chlorite-serpentine-carbonate-bearing rocks. The base of the largest and best-preserved ultramafic body at Yambe is marked by a distinctive (2 m thick) layer of high-P mylonite that preserves evidence for early blueschist facies conditions (S1) as inclusions in eclogite facies minerals. Textural evidence preserved in olivine-bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high-P terrane as serpentinite tectonites sourced from hydrated mantle, formerly in the hangingwall of the Eocene subduction zone. Serpentinite emplacement accompanied burial of the NE New Caledonian margin at T<500 degreesC and P<16 kbar. The ultramafic fragments were buried to depths of 50-60 km in the subduction zone, where olivine was stable and coarse-grained garnet-omphacite-rich assemblages developed in low strain domains within enclosing mylonites. Host metabasic and metasedimentary rocks from the structurally highest portions of the high-P belt have a prograde record identical to that of the ultramafic tectonites. The early emplacement and similar P-T history of host rocks and ultramafic masses suggest that NE New Caledonia preserves a fossil slab/mantle-wedge boundary reactivated during exhumation.