Garnet clinopyroxenite and garnet websterite layers occur locally within mantle peridotite bodies from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These ophiolites were derived from an ocean–continent transition similar to the present-day western Iberian margin. The garnet clinopyroxenites are mafic rocks with a primary mineral assemblage of pyrope-rich garnet + sodic Al-augite (Na₂O ∼2·5 wt %, Al₂O₃ ∼12·5 wt %), with accessory graphite, Fe–Ni sulphides and rutile. Decompression caused Na-rich plagioclase (An_50–45) exsolution in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene + plagioclase (An_85–72) + Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine + plagioclase-bearing assemblage, locally under syn-kinematic conditions, at the expense of two-pyroxenes + Al-spinel. Mg-rich garnet has been also found in the websterite layers, which are commonly characterized by the occurrence of symplectites made of orthopyroxene + Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu–Hf and Sm–Nd mineral isochrons (220 ± 13 Ma and 186.0 ± 1·8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T ∼1100°C and P ∼2·8 GPa. The early decompression was associated with moderate cooling, corresponding to T ∼950°, and development of a spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase–olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution occurred under a brittle regime and led to the superposition of hornblende to serpentine veining stages. The garnet pyroxenite-bearing mantle from the External Liguride ophiolites represents a rare tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. The exhumation was probably accomplished through a two-step process that started during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, probably including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence became involved in an extensional detachment process that resulted in sea-floor denudation.

Convergence and subduction started in the Late Paleocene, to the east of New Caledonia in the South Loyalty Basin/Loyalty Basin, leading to the formation of the Subduction–Obduction Complex of Grande Terre. Convergence during the Eocene consumed the oceanic South Loyalty Basin and the northeasternmost margin of Zealandia (the Norfolk Ridge). The attempted subduction of the Norfolk Ridge eventually led to the end-Eocene obduction. Intra-oceanic subduction started in the South Loyalty Basin, as indicated by high-temperature amphibolite (56 Ma), boninite and adakite series dykes (55–50 Ma) and changes in the sedimentation regime (55 Ma). The South Loyalty Basin and its margin were dragged to a maximum depth of 70 km, forming the high-pressure–low-temperature Pouébo Terrane and the Diahot–Panié Metamorphic Complex, before being exhumed at 38–34 Ma. The obduction complex was formed by the stacking from NE to SW of several allochthonous units over autochthonous Zealandia, including the Montagnes Blanches Nappe (Norfolk Ridge crust), the Poya Terrane (the crust of the South Loyalty Basin) and the Peridotite Nappe (the mantle lithosphere of the Loyalty Basin). A model of continental subduction accepted by most researchers is proposed and discussed. Offshore continuations and comparable units in Papua New Guinea and New Zealand are presented.

The last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid‐present conditions were studied in micaschist recovered from the cuttings of the Pontremoli1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th‐silicate, allanite and epidote. The chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid‐mediated coupled dissolution–reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P–T evolution. The partial dissolution and replacement by rare earth element‐accessory minerals (apatite–allanite–epidote) occurred during a fluid‐present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P–T path or two clockwise P–T loops can fit the above P–T constraints. The former path may be related to a context of late Variscan strike‐slip‐dominated exhumation with minor Tertiary (Alpine‐related) reworking and fluid infiltration, while the latter requires an Oligocene–Miocene fluid‐present tectono‐metamorphic overprint on the Variscan paragenesis.