THERMAL REGIME AND TECTONO-STRATIGRAPHY ACROSS THE BLUESCHIST-ECLOGITE TRANSITION: INSIGHTS FROM VARIOUS HP BELTS OF TETHYAN AND PACIFIC TYPE

The blueschist-eclogite transition of fossil and present day subduction zones deserves broad interest in the Earth Sciences because of its central role in subduction dynamics. In fold-thrust belts, blueschist and eclogite-facies rocks commonly occur in different terrnanes separated by tectonic contacts, and lack pristine continuity. We compared the nature of the blueschist-eclogite transition in different orogenic belts including both Tethyan-type (Alpine Corsica, the Western Alps and Oman) and Pacific-type (New Caledonia). Our first aim was to compare the thermal regime of these HP belts by means of the same petrological techniques, namely Raman Spectroscopy of Carbonaceous Material (RSCM) and PT pseudosections. All these belts show comparable Pressure-Temperature (PT) estimates and PT path, and testify a rather equivalent thermal regime independently from the above-mentioned initial setups. Blueschist-facies terranes are metasediment-rich and show a constant metamorphic gradient increasing from ca. 300 °C and 1 GPa to ca.480-500 °C and 1.8-2.0 GPa toward the blueschits-eclogite boundary, and preserve progressive metamorphic mineral isograds (e.g. carpholite-, lawsonite-, Na-amphibole-, chloritoid-, garnet-in). Eclogite-facies terranes are otherwise metaophiolite-rich (or continental basement-rich in the case of Oman), and show a narrower PT estimates increase from ca. 490-500 to ca. 550 °C and rather constant P estimates at and 2.3-2.5 GPa. On the field, the blueschist-eclogite transition occurs at ca. 480-500 °C, and corresponds to a sharp P gap in the range of ca. 0.4-0.6 GPa. Importantly, no significant Tmax gap is observed across the blueschist-eclogite boundary.

Tectonostratigraphic data indicate that the two metamorphic terranes originated in different paleogeographic contexts, structure of which possibly controls their tectonometamorphic evolution during subduction and mountain building.

We conclude that the blueschist-eclogite transition shows precise metamorphic and tettonostratigraphic patterns in HP belts originated in both Tethyan and Pacific subduction zones, and that structural inheritance plays a crucial role in selective accretion of subducting material and mountain building.