FIELD-BASED CONSIDERATIONS ON THE NATURE OF THE HP “MÉLANGE” OF NEW CALEDONIA

The eclogitic Pouébo unit of New Caledonia is considered a good example of HP mélange formed by intense deformation along the plates interface. Serpentinites within the mélange structure are alternatively interpreted as lower- or upper plate material variably mixed with subducting mafic and sedimentary rocks.

We provide new field observations that clarify the geological meaning of the Pouébo unit. The unit comprises a spatially discontinuous lithostratigraphy that includes variably serpentinized ultramafics, metagabbro, metabasalt and metasediments including (radiolarian) metachert, siliceous metatuff and micaschist. Ultramafics commonly preserve primary peridotite textures. Metavolcanics and metasediments are primarily associated and form repeated intercalations ranging in thickness from a few cm to several meters. Thicker metabasalt (>10m) bodies commonly show pillowed structure, while thinner (ca. 1-2m) layers intercalated within siliceous metasediments commonly show hyaloclastite textures (eruptive tectono-sedimentary structures) or in-situ cooling fracturing typical of oceanic lavas. Mafic polygenic breccias, also including minor clasts of radiolarian metachert, are equally found and locally show rounded shapes. This lithological association is variably deformed, from the rather pristine sequence to intensely disrupted structures. The most intense deformation is localized in the structurally upper part of the units, close to the contact with the overlying blueschist-facies metasediment-rich accretionary complex (i.e. the Diahot terrane). In this zone, all terms of the primary lithostratigraphic sequence are found as blocks of variable size (cm to tens of m) embedded in a metasomatized ultramafic/sedimentary matrix. Blocks commonly show intense deformation, and in many cases likely formed by intense folding/boudinage. No "exotic" blocks were found.

We propose that the Pouébo unit represents a coherent segment of Pacific lithosphere that only locally underwent intense tectonic disruption, probably favored by primary lithological intercalations and rheological contrasts along localized HP shear zones. Consequently, this terrane should not be considered as an HP mélange structures formed by intense mixing of lower- and upper plate material along the plates boundary.