Slab-derived origin of tremolite–antigorite veins in a supra-subduction ophiolite: the Peridotite Nappe (New Caledonia) as a case study

Abstract

Hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks (reservoirs involved, fluid/rock ratios, temperature of interaction). Highly depleted harzburgites and dunites of the Peridotite Nappe of New Caledonia are crosscut by fractures, which have been pervasively serpentinized, producing lizardite, brucite, magnetite and minor chrysotile in a near-static environment, probably by sea water circulating in cooling joints. This event is generally referred to as "primary serpentinization". In a next step, already serpentinized joints were reopened to produce tension and shear cracks sealed by highertemperature synkinematic fibrous minerals. Locally, tremolite-bearing veins and pockets, which do not display evidence for void infill, were generated by metasomatic replacement of the wall rock peridotite. Most veins only contain fibrous antigorite but some display tremolite-antigorite intergrowths or even pure tremolite. The latter rocks yield high contents of Ca and incompatible elements, which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source. Whole-rock geochemical and isotopic features (87 Sr/ 86 Sr,  18 O and H) suggest that antigorite veins, which bear highly radiogenic Sr isotope signatures, were strongly influenced by fluids emitted by the subducted slab and associated sediments. In contrast, the geochemical and isotopic signatures of tremolite-bearing rocks suggest a genetic link with Early Eocene supra-subduction dykes and fluids that leached them. Calcium, strontium and REE-bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation, chromite alteration and high Cr contents in tremolite. The bulk of syn-tectonic fluid-rock interaction associated with shear and tensile fracture development probably occurred at the onset of Eocene intra-oceanic subduction, when the buoyant lower plate (the South Loyalty Basin) obliquely forced its way beneath the nascent Loyalty fore-arc. Mobile elements extracted from supra-subduction dykes temporarily enriched the circulating fluids and generated tremolite, antigorite, Mg-chlorite and magnetite instead of antigorite and magnetite solely. Tremolite crystallization probably ceased due to the exhaustion of Ca-rich fluids in a globally cooling fore-arc environment.

Eocene pre- and syn-obduction tectonics in New Caledonia (Southwest Pacific), a case for oblique subduction, transcurrent tectonics and oroclinal bending; structural and paleomagnetic evidence

“…Therefore, the relative chronology above records: 1) a net increase in temperature after primary serpentinization due to the ingress of slab-derived fluids J o u r n a l P r e -p r o o f during the formation of tremolite-antigorite bearing veins, 2) a gentle temperature decrease at the end of tremolite crystallization, and 3) a final cooling down to supergene conditions (ca. 50°C-35°C) (Cluzel et al, 2019). The predominance of strike-slip or oblique slip displacements denotes transtensional conditions in the upper plate consistent with oblique subduction (Chemenda et al, 2000;Gurnis et al, 2004;Gutscher et al, 1998Gutscher et al, , 2000.…”

“…Because antigorite and tremolite form at higher temperature than lizardite and chrysotile, the former record syntectonic high-temperature and Ca-rich fluid input. A connection with fluids emitted by the subduction zone, or having leached lower Eocene supra-subduction dykes is suggested by their strontium and oxygen isotope signatures (Cluzel et al, 2019) and thus places them within or by the end of the 55-50 Ma interval.…”

Serpentinites of Different Tectonic Origin in an Exhumed Subduction Complex (New Caledonia, SW Pacific)

“…2020)). Weathered samples and samples for which antigorite was not specified in Cluzel et al (2020) were excluded. Upper mantle values from Eiler (2001), Kyser and O'Neil (1984), and Mattey et al (1994).…”

“…Oxidizing conditions during decompression of the HP/LT complex (but preceding the juxtaposition of the Pouébo Terrane and Diahot-Panié Complex) are predicted by thermodynamic modeling of mafic blueschist and eclogite (Carson et al, 2000;Clarke et al, 1997). In the overlying ophiolite, Cluzel et al (2020) posits the formation of Cr-rich magnetite around chromite grains in slab-derived tremolite veins requires oxidizing fluids. They report this result is consistent with their observation of leaching of Eu from slab-related felsic dikes believed to be the source of fluids percolating into the overlying mantle (the same source we favor for partially serpentinizing the mantle protolith for Group II).…”

“…(a) Hydrogen (δD) and oxygen (δ 18 O) stable isotope compositions of New Caledonia high-pressure/low-temperature (HP/LT) serpentinites compared with existing data for serpentine in the overlying Peridotite Nappe (ophiolite groups afterCluzel et al (2020) andUlrich et al (2020)). Weathered samples and samples for which antigorite was not specified inCluzel et al (2020) were excluded. Upper mantle values fromEiler (2001),Kyser and O'Neil (1984), andMattey et al (1994).…”