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L'origine de la zone de transition dunitique (DTZ) à l'interface manteau-croûte est mal connue, ainsi que les processus physico-chimiques impliqués dans sa genèse. Pour aborder cette question, ce travail a porté sur l'étude pétrologique, géochimique et structurale de 20 coupes (600 échantillons) levées dans la DTZ du massif de Sumail (ophiolite d'Oman), épaisse de plus de 400 mètres à l'aplomb d'un paléo-diapir mantellique. Au-delà des données in situ sur minéraux (microsonde, LA-ICP-MS) et des compositions en éléments majeurs des roches totales, le développement d'une procédure analytique a permis l'acquisition des compositions en éléments en traces des dunites dont les teneurs sont de l'ordre du ng.g-1. La DTZ est faite de dunites pures (olivine et chromite) et de dunites imprégnées, contenant une quantité variable de minéraux interstitiels ayant cristallisé à partir d'un magma percolant. Ces faciès renferment des minéraux d'une variété insoupçonnée incluant, en plus de ceux clairement issus d'un MORB (clinopyroxène et plagioclase), de l'orthopyroxène, amphibole, grenat, et des diopsides témoignant d'un processus d'hybridation entre le MORB et des fluides hydratés. Les forts rapports Mg# et teneurs en TiO2 des orthopyroxènes et amphiboles ainsi que la composition des clinopyroxènes, intermédiaire entre clinopyroxènes magmatiques et diopsides hydrothermaux, a permis de contraindre la composition du magma hybride qui résulterait du mélange entre un magma d'affinité tholéiitique et un fluide supercritique riche en silice, voire trondhjémitique issu de la fusion incongruente hydratée des orthopyroxènes mantelliques, similaire au produit de fusion hydratée des roches environnantes (péridotites serpentinisées, troctolites, gabbros). Ces minéraux sont observés en position interstitielle et en inclusion dans les chromites, témoignant de leur origine précoce et du fait que les magmas hybrides ont participé à la formation de la DTZ. La combinaison des interprétations des données in situ et des données roches totales a permis la déconvolution du message polyphasé enregistré par les dunites : la signature du protolithe, celles de la dunitisation et du rééquilibrage de la matrice d'olivine avec un MORB percolant (métasomatisme cryptique), la signature de refertilisation par la cristallisation des minéraux interstitiels (métasomatisme modal), ainsi que les effets de la serpentinisation. Il apparaît que les dunites pures, caractérisées par un spectres de terres rares en forme de U ou de V, semblent avoir acquis cette signature très précocement, probablement lors de la phase initiale de leur genèse sous l'effet de rééquilibrages avec des liquides très riches en éléments incompatibles (REE, Th, U, HFSE) et pouvant correspondre au magma hybride. L'étude structurale de la DTZ dans le massif de Sumail a montré l'influence de la tectonique synmagmatique sur la structuration de la DTZ, se traduisant par l'alternance d'horizons imprégnés ou non ainsi que par l'évolution verticale sur plusieurs dizaines de mètres des compositions chimiques à l'approche des zones de failles. On l'observe notamment pour les teneurs en éléments immobiles dans les fluides tels que le Ti, les REE ou le Th. La DTZ semble s'être développée dans un environnement transtensif dont les deux systèmes de failles principaux N130 et N165-180 ont accommodé la percolation des magmas et fluides responsables de la dunitisation ainsi que l'introduction des fluides hydrothermaux pouvant conditionner les échanges globaux avec les enveloppes externes.La comparaison avec les DTZ d'autres massifs en Oman ou à Trinity (Californie), ayant évolué dans un contexte magmatique différent, montre également l'importance des failles synmagmatiques dans la structuration de la DTZ. Les liquides qui ont percolé dans ces DTZ apparaissent systématiquement sous-saturés en Al et saturés en H2O, amenant à interpréter le caractère hydraté comme une condition critique pour la genèse des dunites. / The origin of the dunitic transition zone (DTZ) between the mantle and the crust is still largely unknown, as well as the physical and chemical processes involved in its genesis. To address this topic, this thesis focused on the petrological, geochemical and structural study of 20 cross-sections (600 samples) collected along the DTZ from the Sumail massif, Oman ophiolite, 400 meters thick and located above a former paleo-mantle diapir. In addition to mineral compositions acquired using in situ methods (microprobe, LA-ICP-MS) and to whole rock major elements, the development of an analytical procedure permitted to determine trace element contents in dunites that display low concentrations (regularly about one ng.g-1). The DTZ is made of pure dunites (olivine and minor chromites), and of impregnated ones, containing a variable amount of interstitial minerals that crystallized from a percolating melt. These latter rocks contain an unexpected mineralogical variety with, in addition to clinopyroxene and plagioclase showing a MORB affinity, the presence of orthopyroxene, amphibole, garnet and diopsides that highlights a hybridization process between the MORB and hydrated fluids. The high Mg# ratio and TiO2 content in orthopyroxene and amphibole together with the clinopyroxene composition, intermediate between igneous clinopyroxene and pure hydrothermal diopside, allow deciphering the nature of the parent melt as the result of the mixing between tholeiitic melt and a supercritical water enriched in silica, or trondhjemitic fluid issued from the hydrated incongruent melting of mantle orthopyroxene, similar to melts produced by the hydrated melting of country rocks (serpentinized peridotites, troctolites, gabbros). All these minerals are observed both in interstitial position and as inclusions in chromite, showing that they crystallized early and that hybrid melts participated to the genesis of the DTZ. The comparison between mineral and whole rock compositions permitted to highlight the different processes that led to the observed chemical signatures of dunites: the protolithe signature, the dunitization process, chemical reequilibration between the olivine matrix and the percolating MORB, refertilization following the crystallization of interstitial minerals, as well as the effects of later serpentinization. Pure dunites, characterized by U or V-shaped REE patterns, seem to have acquired early the LREE-enriched signature that probably results from the reequilibration with silica- and incompatible trace elements-rich fluids (REE, Th, U, HFSE) generated through the harzburgite orthopyroxenes incongruent melting and probably reflecting the hybrid melt that crystallized interstitial hydrous minerals. The structural study of the DTZ in Sumail highlights the effect of synmagmatic faults on the DTZ development, resulting in the alternation between pure and impregnated horizons as well as in the vertical chemical structuration with compositions evolving on few tens of meters until fault zones. This is particularly true for chemical species expected as immobile during weathering as Ti, REE or Th. The DTZ seems to have been developed in a transtensional environment structured by two main faults systems, oriented N130 and N165-180. These faults spatially constrained both the melt flow, thus the dunitization, and the introduction of hydrothermal fluids probably oceanic in origin. This meeting zone between igneous and hydrothermal fluids can strongly influence the chemical exchanges and distribution between the deep lithosphere and the surface. The comparison between the Sumail DTZ and other ones from Oman or Trinity (California) ophiolites, which evolved in a different magmatic setting, shows the systematic role of synmagmatic faults. Melts that percolated these other DTZ were under-saturated in Al and saturated in water, allowing to interpret the hydrated component as an essential condition for dunites genesis at the mantle-crust transition.

The thesis presents the application of the magnetotelluric (MT) sounding method to image Earth’s crust in Oman and South Australia. The aim of these MT surveys is to provide constraints on the geological interpretation of emplacement scenarios and the tectonic evolution of the geological domain. The thesis concentrates on the methodological aspects of the MT technique, e.g. the data analysis and modelling of electromagnetic fields. The phase tensor approach by Caldwell et al. (2004) is applied to the data and provides insights into the dimensionality of the MT data in even complex and electrically distorted terranes. Modelling and inversion of the MT data is performed with various 2-D and 3-D codes to show how the interpretation of the data can benefit from multiple modelling approaches. Data collected in a 2-D survey across the Oman ophiolite mountains show complex behaviour and 2-D inversion and 3-D forward modelling resolve ambiguities in the emplacement scenario of the Oman ophiolite. It is believed that initial underthrusting of the Jurassic-Cretaceous oceanic lithosphere was followed by exhumation. Further oceanic thrusting subsequently led to rising of lower-plate eclogites and eventually gravitational collapse of the ophiolite onto the margin (Gray et al., 2000). The 3-D inversion code by (Siripunvaraporn et al., 2005a) was expanded to incorporate static shift corrections and inversion model misfits have therefore improved significantly compared to inversion models without static shift correction. 2-D and 3-D surveys across the South Australian Gawler Craton reveal deep crustal conductors which are connected to near surface mineralisation systems of the IOCG Olympic Dam deposit in the north-eastern part of the craton and the Au-dominated central Gawler Craton provinces. / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008