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The structural evolution of the Ox Mountains Granodiorite (478±12Ma) during and after its emplacement is described. This pluton has been emplaced within and synchronously with, a major transpressional shear zone which is expressed as a 11km wide belt of strongly deformed NE-SW striking, steeply dipping metasediments. The steep shear zone cleavage is intensified to a mylonitic fabric in a braided system of high strain zones formed through out the Ox Mountains Inlier. Pervasive ductile, sinistral deformation in the shear zone is interupted by the emplacement of the pluton and by the synchronous development in the country rocks of series of brittle thrust structures, which produced a displacement upwards and towards the centre of the shear zone. These thrusts are intimately associated with the emplacement of moderately inclined granodiorite sheets belonging to the main intrusive phase indicating a component of vertical extension in the country rocks at this time. The CMC is a heterogeneous, four component pluton internally composed of a series of large sheets or dykes. Minor muscovite granite sheets emplaced along the northern contacts, preceeded the main intrusive sheets of Group 1 and Group 2 granodiorite with associated diorites. Sheets of tonalites and minor components completed the emplacement history. A prolonged history of sinistral transpressional shearing has deformed the pluton. Discrete sinistral shears indicate an early localization event is overprinted by a main ductile penetrative fabric which cross-cuts ail internal contacts. This fouation is deformed by extensive sinistral S-C fabrics. Later deformation becomes increasingly partitioned into late sinistral and dextral shear zones which are locally mylonitic. Microstructural evidence suggests that the main foliation was formed under lower amphibolite facies and deformed by a steady state flow process. The S-C fabric and late shear zone formation, best developed in the granodiorites and granites, may have been initiated by a switch in the predominant alkalifeldspar deformation mechanism from crystal plastic to a diifusive mass transfer process. This may be a retrogessive effect, and the product is a grain size reduction which may lead to ultramylonite production. The emplacement model for the Ox Mountains Granodiorite is constrained by the original geometry of the dykes or sheets. These data rule out emplacement of the OMG in a releasing bend or pull-apart structure. Strain data does not allow a forceful mechanism and a permissive emplacement model is preferred, in which vertical extension during the intrusive episode created an area of dilation in which dyking occurred. This was caused by oblique movements on two upwardly converging high strain zones outside the pluton. Two satellite plutons, the Lough Talt Adamelhte and the Easkey Lough Adamellite were emplaced in extensional cavities created by reactivated sinistral movements on one of the high strain zones at a much later date. (c400Ma).Transpressional shear zones may initiate or enhance melting in the lower crust and mantle lithosphere where thermal perturbation has occurred. The shear zones may provide conduits for the melts and emplacement sites, especially where high transpressional strains are accommodated by vertical uplift.

The Central Asian Orogenic Belt (CAOB), the largest accretionary collage on the Earth, has a complicated and prolonged accretionary history which remains being highly debated. High-grade terranes were previously interpreted as Precambrian micro-continents which played a very important role during the evolution of the CAOB. However, some of their presumed old ages are challenged by recent high-resolution dating results which raise questions on their Precambrian origins. The Chinese Altai and Tseel Terrane in the SW CAOB, two typical high-grade terranes occupy vital structural positions, feature various lithological elements and exhibit complicated deformation-metamorphism patterns, making them key areas in the reconstructing of the evolution of central Asia. However, their origins are not firmly constrained. Paragneisses were considered as Precambrian basements, but yielded detrital zircon ages predominantly between 440 and 580 Ma. The associated granitic gneisses and amphibolite gave crystallization ages at 420-463 Ma. Geochemical and zircon Hf isotopic data of paragneisses support that their protoliths may represent significant erosion products of arc rocks that were developed in a subduction environment. This feature is similar with that of the associated low-grade volcanogenic schists which probably represent immature sediments in an active margin. Detrital zircons from the paragneisses and schists show similar age patterns, supporting derivation from similar provenance. Accordingly, our data reveal that these high-grade terranes do not represent Precambrian microcontinents. Moreover, the U-Pb age pattern for the detrital zircons, and some xenocrystic zircons from the associated granitoids, is comparable with the age patterns of the micro-continents and arc terranes in western Mongolia. The predominant zircon population of 440-580 Ma matches the widely distributed granitoids within the Neoproterozoic-early Paleozoic terranes in western Mongolia, while the minor Precambrian ages (>540 Ma) resemble those old rocks preserved in the Tuva-Mongolian (TM) block and its adjacent Neoproterozoic arc terranes. These features suggest that detrital and xenocrystic zircons more likely represent the detritus recycled from western Mongolia. Accordingly, the crustal growth of the SW CAOB in the early Paleozoic could be outlined by secular amalgamation of magmatic arcs around a Precambrian micro-continent. In addition, the TM-derived Precambrian zircons are further used to trace the origin of the TM block, which favors that the TM block was possibly rifted from the Indian block in the Neoproterozoic. Further efforts have been made to decipher the controversial tectono-metamorphic history. In the Chinese Altai, U-Pb dating on the metamorphic zircon portions yielded consistent ages of ~390 Ma. Temperature estimations using mineral-pair as well as Ti-in-zircon thermometers revealed high-temperature conditions up to ~720℃. Detailed investigations on the metamorphic rocks in the Tseel area revealed that middle-pressure metamorphic fabrics developed under progressive NNE-SSW convergent setting, possibly at 385-374 Ma. A later low-pressure/high-temperature metamorphic sequence developed during decompression, associated with high-level anatexis at 374-363 Ma. Collectively, our data support that the final amalgamation of North Mongolian Domain on its southern margin occurred at Middle-Late Devonian, and might be immediately followed by the subduction of an active oceanic ridge. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

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