Lawsonite equilibria are predicted to occur over a broad P-T spectrum developed during subduction, yet lawsonite-bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism (4-23 kbar, 400-750°C) using the system Na 2 O-CaO-K 2 OFeO-MgO-Al 2 O 3 -SiO 2 -H 2 O and the software THERMOCALC THERMOCALC, unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages because of water under-saturation with non-occurrence of recrystallization. If formed, lawsonite-bearing assemblages are observed to be highly unstable; their preservation requires that exhumation be accompanied by substantial cooling. The amount of structurally bound H 2 O in minerals plays a critical role in the formation and preservation of mineral assemblages, controlling key changes in rocks undergoing subduction.

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The Poue´bo 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°C 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.

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Low-P granulite facies metapelitic migmatites in the Wuluma Hills, Strangways Metamorphic Complex, Arunta Block, preserve evidence of polyphase deformation and migmatite formation which is of the same age of the c. 1730 Ma Wuluma granite. Mineral equilibria modelling of garnet-orthoproxenecordierite-bearing assemblages using THERMOCALC THERMOCALC is consistent with peak S3 conditions of 6.0-6.5 kbar and 850-900°C. The growth of orthopyroxene and garnet was primarily controlled by biotite breakdown during partial melting reactions. Whereas orthopyroxene in the cordierite-biotite mesosome shows enrichment of heavy-REE (HREE) relative to medium-REE (MREE), orthopyroxene in adjacent garnet-bearing leucosome shows depletion of HREE relative to MREE. There is no appreciable difference in major element contents of minerals common to both the mesosome and leucosome. The REE variations can be satisfactorily explained by decoupling of major element and REE partitioning, in the context of appropriate phase-equilibria modelling of a prograde path at 6 kbar. Sparse garnet nucleii formed at 760°C, along with concentrated leucosome development and preferentially partitioned HREE. Further heating to 800°C at constant or subtly increasing pressure conditions additionally stabilized orthopyroxene and decreased the garnet mode. Orthopyroxene in the leucosome inherited an REE pattern consequent to the partial consumption of garnet, it being distinct from the REE pattern in mesosome orthoproxene that was mostly controlled by biotite breakdown. Such withinsample variability in the enrichment of heavy REE indicates that caution needs to be exercised in the application of common elemental partitioning coefficients in spatially complex metamorphic rocks.