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Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some "immobile" trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction. [ABSTRACT FROM AUTHOR]

Aim: Although New Caledonia (NC) is now considered an oceanic island that emerged ca. 60 Ma, a few terrestrial clades are significantly older, raising the question of the origin of these groups. Classically, old lineages on more recent islands are hypothesized to originate through a process of hopping on now‐vanished islands (i.e., island‐hopping hypothesis) or other territories. We aim to test this hypothesis by studying a group of cockroaches with several lineages found in NC. Location: New Caledonia, New Zealand, Australia. Taxon: Insects: Blattodea. Methods: We generated a dated phylogeny for blattid cockroaches (Blattidae and Tryonicidae) using Bayesian inference along with fossil calibrations. We reviewed studies on the palaeogeography of the Southwest Pacific region, including hypotheses about the existence of yet‐to‐be‐discovered past islands, and constructed biogeographical tests accordingly. We computed ancestral area estimation under different models in BioGeoBEARS (DEC, BAYAREALIKE, DIVALIKE, with or without +J) to test the role of an island‐hopping hypothesis in the establishment of NC blattid fauna. Results: We find divergence times older than 60 Ma for two NC clades. We show that these 'old' endemic lineages can partially be explained by indirect dispersal from Australia or New Zealand through now disappeared islands. Alternative hypotheses suggest multiple independent colonizations of NC from Antarctica or Australia. Main Conclusions: Our results indicate that island‐hopping may explain the presence of old groups in NC. The island‐hopping hypothesis is nonetheless only supported for a period‐area from which geological evidence is ambiguous. Our work highlights both the fruitful interactions between geology and biogeography and the underlying difficulties. The multiple colonization events inferred for NC provide additional insights into the composite nature of NC biota. [ABSTRACT FROM AUTHOR]

To unveil how forearc lithosphere cools and re‐equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto‐arc section generated during subduction infancy. A large data set, including more than 80 samples (peridotites and mafic‐ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE‐Y) and fast diffusing (TCa‐in‐Opx, TBKN, TCa‐in‐Ol, TOl‐Sp) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub‐oceanic mantle. Cooling rates deduced from TREE‐Y and TBKN yield values of ≈10−3°C/y, similar to those obtained with TCa‐in‐Ol. These features are consistent with a post‐melting history of emplacement, possibly along a transform fault, and thermal re‐equilibration via conduction. Cpx‐free harzburgites register a high‐T evolution, followed by quenching and obduction. The relatively high TCa‐in‐Ol, TOl‐Sp and cooling rates computed from TCa‐in‐Ol (≈10−3°C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe. Temperature profiles across the crust‐mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non‐cogenetic magma batches in the forearc lithosphere, followed by thermal re‐equilibration at rates of ≈10−4–10−3°C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic setting. Plain Language Summary: The New Caledonia ophiolite represents a rare example of a complete forearc section, extending from the lower crust to the mantle, originated at the beginning of the Eocene subduction. Our study aims to shed light on the thermal evolution of this ophiolite, which constitutes one of the largest ophiolite complexes worldwide, gaining‐at the same time ‐ insights into the mechanisms that govern cooling of forearc lithosphere. For this purpose, different geothermometers were applied to mantle peridotite and mafic‐ultramafic intrusive samples. Geothermometers are equations based on temperature‐dependent major element and/or trace element exchange reactions among mineral phases, which allow calculating equilibrium temperatures starting from mineral chemical compositions. Equilibrium temperatures obtained with various thermometers can be linked to the cooling evolution and heat extraction rates of the studied samples. With this study, we were able to track the post‐melting evolution and re‐equilibration at lower temperature conditions for all the investigated rock types. A comprehensive investigation of the Massif du Sud sequence, from the lower crust to the upper mantle, allowed us to build thermal profiles across the forearc lithosphere. Overall, our results contribute to unravel the cooling mechanisms of the crust‐mantle sequences in this poorly explored tectonic setting. Key Points: A comprehensive geothermometric investigation of the New Caledonia ophiolite has been carried outThe computed temperatures and cooling rates (≈10−4–10−3°C/y) contrast with previous results on ophiolitic and forearc peridotitesThe thermal conditions recorded by forearc sequences can be linked to specific areal processes and previous lithosphere evolution [ABSTRACT FROM AUTHOR]

The island of New Caledonia is the second largest rock exposure of the continent Zealandia. The New Caledonian basement rocks have been interpreted as representing a late Paleozoic to Mesozoic intra-oceanic arc system that was possibly correlative to contemporaneous terranes in eastern Australia and New Zealand. In order to understand tectonic relationships between the basement rocks of New Caledonia and other eastern Gondwanan terranes, we obtained >2200 new U-Pb ages of detrital zircon grains from New Caledonia. Our new results, combined with a synthesis of previously published geochronological data, show abundant pre-Mesozoic zircon ages, but an absence of Early Permian to Middle Triassic ages characteristic of eastern Gondwana magmatism. The results thus suggest that the detritus of the New Caledonian basement was derived from a local Paleozoic continental fragment that was rifted from the margin of Gondwana, most likely in the Early Permian. The results imply that dispersal of the Gondwanan margins started earlier than the Late Cretaceous opening of the Tasman and Coral seas, consistent with the Mesozoic endemism of both New Caledonia and New Zealand. [ABSTRACT FROM AUTHOR]

Abstract: The Poya Terrane of New Caledonia is a composite lithotectonic unit made of (i) Campanian‐Paleocene enriched mid‐ocean ridge basalt (E‐MORB) and BABB‐type (back‐arc basin basalt) basalts and abyssal argillite (Poya Terrane basalts) and (ii) Coniacian‐Santonian sandstone, turbidites, and abyssal argillite (Kone Facies) intruded by early Eocene E‐MORB sills. Remapping reveals that the Kone Facies is more extensive than previously thought. Field data, petrography, and U‐Pb geochronology of detrital zircons show that Kone Facies sediments have the same provenance as coeval autochthonous sediments (Formation a Charbon), albeit with more abundant contemporaneous zircons. They accumulated on the eastern continental slope of the Norfolk Ridge and eventually mixed with abyssal argillite. Temporally, sill emplacement is related to subduction inception at circa 56 Ma, thus suggesting a possible genetic link. We postulate that either (i) E‐MORB intrusion was related to oblique extension and thinning of the down going plate or, alternatively, (ii) the “enriched” (off axis?) partial melt zone of the ancient ridge swept the lower plate continentward, generating E‐MORB dikes in the upper marginal basin crust, and sills in passive margin sediments before it became extinct. Thereafter, sliced marginal basin upper crust, passive margin sediments, and associated dolerite sills were obliquely accreted to the fore‐arc region, and in the NE part of the terrane, subducted and recrystallized into the blueschist facies. The Poya Terrane was eventually thrust onto the Norfolk Ridge when the latter reached the subduction zone and debris from the thrust sheet fed middle to late Eocene syntectonic basins. At the same time, mafic portions of the Poya Terrane were subducted at depth where they recrystallized into the eclogite facies, mixed with serpentinite to form the Pouebo mélange, and, finally, were exhumed in the fore‐arc region. Finally, late Oligocene faulting and hydrothermal events overprinted the NE part of the terrane in probable connection with postobduction granitoid emplacement. [ABSTRACT FROM AUTHOR]

• Dating magmatic zircons from the world-class podiform Cr ore deposit of Tiébaghi (New Caledonia) yields an Eocene U-Pb age (48 ± 1 Ma) • Amphibole in the chromitite displays a similar 40Ar/39Ar age of 46 ± 2 Ma. • Chromite ore-forming processes occurred around the end of forearc magmatic event (55–47 Ma) • Eocene magmatic zircons display depleted mantle isotopic signatures (εHf and δ18O) • Trace elements contents of zircons suggest crystallization from silica-rich differentiated residual magma of depleted mantle origin. • Magma modelling from REE compositions of zircons suggests forearc basalt as a possible candidate for the melt that reacted with peridotite to form the chromite ore. Dominantly Mesozoic to Proterozoic xenocrystic zircons extracted from Tiébaghi chromitite coexist with a well-defined population (30%) of Eocene magmatic grains (U-Pb: 48.2 ± 1.1 Ma). Since Eocene zircons are absent in the peridotite host rock and are accompanied by rare rutiles with the same age range, it is suggested that both are derived from the circulating melt from which the chromite ore was generated. This hypothesis is strengthened by a similar 40Ar/39Ar age of 46 ± 2 Ma on amphibole that is closely associated with chromitite. These data confirm the Eocene age and supra-subduction setting of the Tiébaghi podiform chromitite, which thus was formed by interaction of peridotite and mafic melt in the final stage of the forearc magmatic event that occurred between 55 Ma and 47 Ma. Modelling the melt associated with chromite formation using zircon trace-elements and REE-zircon partitioning coefficients suggests forearc basalt as a likely candidate and the felsic by-product of peridotite-melt interaction a source for early Eocene magmatic zircons. [ABSTRACT FROM AUTHOR]

A lower Miocene coarse conglomerate that crops out in the Népoui Peninsula does not represent the base of the marine transgression that followed obduction in New Caledonia. Instead, the conglomeratic alluvial fan that contains peridotite cobbles and reworked weathering products records a short-lived episode of terrestrial erosion intercalated between two intervals of subsidence marked by marine carbonate deposition. Considering the Miocene sea level evolution reported in the literature, it is concluded that neither lower Miocene transgression nor erosion were driven by sea level variation. In contrast, a southeastward propagating slab tear that initiated at the latitude of the high pressure/low temperature metamorphic complex of northern New Caledonia likely generated east to west tilting of New Caledonia, subsidence along the West Coast and hence fringing reef development together with moderate erosion of older regolith. Coincidence between conglomerate deposition and hence prominent erosion that closely followed emplacement of postobduction granitoids influenced by a slab window suggests a genetic link. Therefore, it is concluded that short-lived lower Miocene erosion was due to slab breakoff and subsequent uplift that occurred at ~ 22 Ma. Lower Miocene erosion profoundly dissected the Peridotite Nappe and in the northern half of New Caledonia only left isolated klippes along the West Coast. [ABSTRACT FROM AUTHOR]

Abstract: The Late Cretaceous clastic coastal sediments of New Caledonia are contemporaneous with the latest stages of the eastern Australian marginal rifting. As such, they record the erosion of basement terranes located on uplifted and tilted blocks and a contemporaneous volcanic activity. Detrital zircon populations contain two major components, the younger of which is Early Cretaceous, and the older Early Paleozoic and Precambrian. Following recent advances in the knowledge of detrital zircon content of basement terranes, and at variance with previous interpretations, that hypothesised a possible direct Australian provenance for Precambrian zircons, the detrital zircon record of these syn-rift sediments allows a local recycled provenance to be established. In consequence, this new evidence confirms that New Caledonia was already isolated from Australia as early as Coniacian time (ca. 89–85Ma) a fact consistent with the development of faunal and floral endemism at that period. The prominent abundance of Early Cretaceous detrital zircons also establishes the importance of a previously unrecorded Early Cretaceous magmatism in the area. [Copyright &y& Elsevier]

New, high-quality multichannel seismic reflection data from the western New Caledonia offshore domain allow for the first time the direct, continuous connection of seismic reflectors between the Deep Sea Drilling Project 208 drill hole on the Lord Howe Rise and the New Caledonia Basin. A novel seismic interpretation is hence proposed for the northern New Caledonia Basin stratigraphy, which places the Eocene/Oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre-Oligocene sequences. A causal link is proposed between the obduction of the South Loyalty Basin over New Caledonia (NC) and the tectonic history of the northern New Caledonia Basin. Here it is suggested that as the South Loyalty Basin was being obducted during early Oligocene times, the NC Basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation, which resulted in (1) the uplift of the northern Fairway Ridge and (2) the sinking of the western flank of New Caledonia. This event also had repercussions farther west with the incipient subsidence of the Lord Howe Rise. [ABSTRACT FROM AUTHOR]

Supergene nickel deposits of New Caledonia that have been formed in the Neogene by weathering of obducted ultramafic rocks are controlled by fracture development. The relationship of tropical weathering and tectonic structures, faults and tension gashes, have been investigated in order to determine whether fractures play a passive role only, as previously thought; or alternatively, if brittle tectonics was acting together with alteration. Observation of time-relationship, textures, and mineralogy of various fracture fills and fault gouges shows that active faulting has played a prominent role not only in facilitating drainage and providing room for synkinematic crystallization of supergene nickel silicate, but also in mobilizing already formed sparse nickel ore, producing the very high grade ore nicknamed “green gold”. [ABSTRACT FROM AUTHOR]

In southern New Caledonia, Late Oligocene granodiorite and adamellite are intruded into an ultramafic allochthon emplaced in the Late Eocene period. Previous studies of these granitoids proposed an origin associated with the melting of the underlying continental crust, but our new data show that these high-K to medium-K calc-alkaline granitoids display the geochemical and isotopic features of volcanic arc magmas uncontaminated by crust-derived melts. These magmas were probably generated in a post-Eocene and pre-Miocene subduction, the geophysical traces of which have been detected along the western coast of New Caledonia. Sr, Nd and Pb isotopic ratios indicate derivation from an almost isotopically homogeneous mantle wedge, but in contrast, some variation in trace element ratios uncorrelated to differentiation is indicative of source heterogeneity. Prominent heavy rare earth element (HREE) depletion of some of the younger granitoids may be the result of an equilibrium achieved with garnet-bearing subcrustal material (granulite) found as xenoliths, while a relative Nb, Ta and Hf enrichment, irrespective of crystal fractionation, may be related to either a modest contamination by previously underplated mafic material, heterogeneous hydration of the mantle wedge, or mixing with uplifted Nb-rich mantle. Post-obduction slab break-off can be proposed to have played a role in sublithospheric mantle mixing and the subsequent heterogeneity. The Late Oligocene subduction described here may be tentatively extended southward into northern New Zealand allochthons. [ABSTRACT FROM AUTHOR]

The New Caledonia ophiolite (Peridotite Nappe) is dominated by mantle lithologies, composed of forearc-related refractory harzburgites and minor lherzolites in both the spinel and plagioclase facies. In this study, a comprehensive geochemical data set (major, trace elements and Sr–Nd isotopes) is used to constrain the mantle evolution of the lherzolites and their relationships with the basalts from the Poya Terrane, which tectonically underlies the mantle rocks. The majority of the lherzolites are low-strain porphyroclastic tectonites. They likely record an asthenospheric origin followed by re-equilibration at lithospheric conditions, as supported by geothermometric estimates (T = 1100–940 °C and 920–890 °C for porphyroclastic and neoblastic spinel-facies assemblages, respectively). Olivine composition (Fo = 88.5–90.0 mol%), spinel Cr# ([molar 100 • Cr/(Cr + Al)] = 13–17) and relatively high amounts (7–8 vol%) of Al 2 O 3 - and Na 2 O-rich clinopyroxene (up to 0.5 and 6.5 wt.%, respectively) indicate a moderately depleted geochemical signature for the spinel lherzolites. Bulk rock and clinopyroxene rare earth elements (REE) patterns display a typical abyssal-type signature, i.e. steeply plunging LREE accompanied by nearly flat HREE to MREE. Clinopyroxene REE compositions of the spinel lherzolites may be reproduced by small amounts of fractional melting of a garnet lherzolite precursor (~ 4%), followed by 4%–5% melting in the spinel peridotite field. The plagioclase lherzolites show melt impregnation microstructures, Cr- and Ti-rich spinels and incompatible trace element enrichments (REE, Ti, Y, and Zr) in bulk rocks and clinopyroxenes. Impregnation modelling for these elements suggests that the plagioclase lherzolites originated from residual spinel lherzolites by entrapment of highly depleted (non-aggregated) MORB melt fractions in the shallow oceanic lithosphere. Nd isotope compositions of the investigated peridotites are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion event. This evolution was most likely accomplished in a spreading ridge. However, geochemical trace element modelling and Nd isotopes do not support a genetic mantle–crust link between the lherzolites and enriched-MOR-type basalts from the Poya Terrane. [ABSTRACT FROM AUTHOR]

Abstract: The ichnospecies Asthenopodichnium xylobiontum comprises U-shaped traces in wood, usually attributed to the activity of mayfly nymphs (Insecta, Ephemeroptera), which indicate the presence of freshwater palaeoenvironments. The record of a new ichnospecies in wood, Asthenopodichnium lignorum, in the Miocene of New Caledonia, strongly indicates that it may correspond to other trace makers because the potential producing Ephemeroptera are absent around these Miocene oceanic islands and in a wide area comprising New Zealand and Australia. We present herein the evidence for wood rotting fungi as potential trace maker of the new ichnospecies that has been previously recorded from Argentina, Egypt, Czech Republic, USA, and Lithuania. The new ichnospecies A. lignorum, in contrast with A. xylobiontum, is an indicator of terrestrial, humid to subhumid, palaeonvironments. [Copyright &y& Elsevier]

Abstract: The origin of the New Caledonia ophiolite (South West Pacific), one of the largest in the world, is controversial. This nappe of ultramafic rocks (300km long, 50km wide and 2km thick) is thrust upon a smaller nappe (Poya terrane) composed of basalts from mid-ocean ridges (MORB), back arc basins (BABB) and ocean islands (OIB). This nappe was tectonically accreted from the subducting plate prior and during the obduction of the ultramafic nappe. The bulk of the ophiolite is composed of highly depleted harzburgites (±dunites) with characteristic U-shaped bulk-rock rare-earth element (REE) patterns that are attributed to their formation in a forearc environment. In contrast, the origin of spoon-shaped REE patterns of lherzolites in the northernmost klippes was unclear. Our new major element and REE data on whole rocks, spinel and clinopyroxene establish the abyssal affinity of these lherzolites. Significant LREE enrichment in the lherzolites is best explained by partial melting in a spreading ridge, followed by near in-situ refertilization from deeper mantle melts. Using equilibrium melting equations, we show that melts extracted from these lherzolites are compositionally similar to the MORB of the Poya terrane. This is used to infer that the ultramafic nappe and the mafic Poya terrane represent oceanic lithosphere of a single marginal basin that formed during the late Cretaceous. In contrast, our spinel data highlights the strong forearc affinities of the most depleted harzburgites whose compositions are best modeled by hydrous melting of a source that had previously experienced depletion in a spreading ridge. The New Caledonian boninites probably formed during this second stage of partial melting. The two melting events in the New Caledonia ophiolite record the rapid transition from oceanic accretion to convergence in the South Loyalty Basin during the Late Paleocene, with initiation of a new subduction zone at or near the ridge axis. [Copyright &y& Elsevier]