Etude pétrologique, géochimique et structurale de la zone de transition dunitique dans l’ophiolite d’Oman : Identification des processus pétrogénétiques à l’interface manteau/croûte

Abstract : The origin of the dunitic transition zone (DTZ) between the Earth mantle and the oceanic crust is poorly constrained. This work was dedicated to the petrological, geochemical and structural study of 20 cross-sections (600 samples) sampled in the Sumail massif (Oman ophiolite). The development of an analytical procedure allowed the determination of trace element compositions in dunites. The DTZ is made of pure dunites (olivine and chromite) and of impregnated dunites, containing a variable amount of interstitial minerals that crystallized from a percolating melt and including, in addition to those that clearly crystallized from a MORB (clinopyroxene and plagioclase), orthopyroxene, amphibole, garnet, and diopsides, witnesses of a hybridization process between the MORB and hydrous fluids. The high Mg# ratio and TiO2 content in orthopyroxene and amphibole as well as the clinopyroxene composition, intermediate between igneous clinopyroxene and hydrothermal diopside, allowed constraining the composition of the hybrid parent melt that would result from the mixing between a tholeiitic melt and supercritical water saturated with silica or a trondhjemitic melt issued from the hydrous incongruent melting of mantle orthopyroxene. Pure dunites have recorded a strong enrichment in incompatible trace elements (REE, Th, U, HFSE), probably early during their genesis following chemical reequilibration with enriched melts that can correspond to the hybrid one. The structural study of the DTZ in the Sumail massif demonstrates the influence of synmagmatic faulting on the DTZ structuration, having enhanced the melt and fluids percolation, responsible of the dunitization process, as well as the introduction of hydrothermal fluids. The hydrous characteristic of the reacting melt is interpreted as a critical condition for formation of dunites.
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Submitted on : Thursday, August 8, 2019 - 7:20:09 AM
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Mathieu Rospabé. Etude pétrologique, géochimique et structurale de la zone de transition dunitique dans l’ophiolite d’Oman : Identification des processus pétrogénétiques à l’interface manteau/croûte. Géochimie. Université Toulouse III Paul Sabatier (UT3 Paul Sabatier), 2018. Français. ⟨tel-02264959⟩

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