Thesis (Ph. D.)--University of Oklahoma, 1998. / Includes bibliographical references (leaves 156-169).

Metamorphic rocks constitute a vast volumetric proportion of the Earth’s continental lithosphere and are invaluable recorders of the mechanisms and rates of deformation and metamorphism that occur at the micro-, meso- and macro-scale. As such, they have the potential to provide detailed insight into important tectonic processes such as the subductive transport of material into, and back from, mantle depths and also folding, faulting and thickening of crust that occurs during collisional orogeny. The Himalayan-Karakoram-Tibetan orogen is the youngest and most prominent example of a continent-continent collisional mountain belt on Earth today and is a product of the on-going convergence of the Indian and Asian plates that initiated in the Early Eocene. Thus, it provides an exceptional natural laboratory for the investigation of such processes. Recent advances in the computational ability to replicate natural mineral assemblages through a variety of metamorphic modelling techniques have led to improvements in the amount (and quality) of petrographic data that may be obtained from a typical metamorphic rock. In this study, phase equilibria modelling (pseudosection construction) using THERMOCALC, amongst other techniques, has been integrated with in-situ U–Pb and Th–Pb geochronology of accessory monazite in order to constrain the tectonothermal evolution of four regions intimately associated with the Himalayan-Karakoram-Tibetan orogen. These regions comprise the Karakoram metamorphic complex (north Pakistan), the Tso Morari massif (north-west India), the eastern Himalayan syntaxis (south-east Tibet) and the Day Nui Con Voi metamorphic core complex of the Red River shear zone (North Vietnam). Each case study documents previously unreported metamorphic, magmatic or deformational events that are associated with the India-Asia collision. These data have allowed original interpretations to be made regarding the tectonic evolution of each individual region as well as the large-scale evolution of the Himalayan-Karakoram-Tibetan orogenic system as a whole.

Mémoire (M.Sc.A.))--Université du Québec à Chicoutimi, 1989. / Document électronique également accessible en format PDF. CaQCU

Thèse (D.R.M.)--Université du Québec à Chicoutimi, 1997. / Document électronique également accessible en format PDF. CaQCU

Thèse (D.R.M.)--Université du Québec à Chicoutimi, 1997. / Document électronique également accessible en format PDF. CaQCU

In recent decades there has been an increased interest in the carbon content of Earth’s geochemical reservoirs due to the impact of atmospheric carbon on the habitability of our planet. Earth’s interior likely hosts a greater mass of carbon than that of the oceans, atmosphere and crust combined, which has buffered the carbon content of the atmosphere over geological time. Yet only a few direct measurements of carbon from the upper mantle, and none from the lower mantle, have been made. Undegassed basalts erupted at mid-ocean ridges have previously been used to estimate the carbon content of the upper mantle. However, due to the low solubility of carbon within silicate melt, these undegassed basalt suites are rare. The majority of basalts have lost their mantle carbon information en route to eruption through the crust. Various crustal processes act to modify the geochemistry of melts before eruption, therefore it is important to be able to characterise the effect of these processes to better interpret the volatile signals preserved in erupted products. Pressure, and therefore depth, is a key parameter controlling volatile solubility and can be estimated using a variety of igneous barometers. This thesis presents results from crys- tallisation experiments conducted on basaltic glass from the Miðfell eruption, Iceland. The experiments provide new data that has been used to test a variety of barometers and crystalli- sation models used by igneous petrologists, and could aid future barometer recalibration. A key part of this work was the development of an experimental method for stabilising 5 kbar conditions in a piston cylinder apparatus. The experiments have shown that clinopyroxene- liquid barometry is more reliable than multi-reaction barometry. However, knowledge of equilibrium clinopyroxene compositions is crucial for accurately determining pressure using the clinopyroxene-liquid barometer. More experiments conducted at mid-crustal pressures are required for a full recalibration of these barometers. The results of testing igneous barometers and crystallisation models have been applied to two suites of olivine-hosted melt inclusions from the Kistufell and Miðfell eruptions to help determine the melt evolution history of these basalts. These eruptions were targeted due to previously measured noble gas isotopic ratios that suggest a primordial mantle component present in their melting regions, and therefore evoking the possibility that they could hold information about deep mantle carbon. Barometry suggests that Miðfell phases equilibrated, and therefore crystallised, at mid-crustal pressures (5–7 kbar), which could allow for the entrapment of undegassed melt inclusions within olivine. The two melt inclusion suites were found to differ in trace element variability, with the observation that the more trace element enriched eruption, Kistufell, had lower relative trace element variability than the more depleted eruption, Miðfell. Several processes, both in the crust and the mantle, are likely responsible for the level of trace element enrichment and variability, including extent of mantle melting, source heterogeneity, and melt transport. The depleted nature of the Miðfell melt inclusions has allowed them to preserve some of the highest CO$_2$/Ba and CO$_2$/Nb ratios ever recorded in basaltic glass, with ratios over five times greater than undegassed mid-ocean ridge basalt values. This carbon enrichment is not due to any crustal melt modification process, but rather pertaining to lower mantle carbon-rich lithologies that have been tapped by the Icelandic mantle plume. The carbon reservoir beneath Miðfell is estimated to contain 744 $\pm$ 188 ppm carbon, 15 times greater than the depleted upper mantle. This value matches estimates of bulk mantle carbon from planetary mass balance calculations and provides evidence for carbon-rich domains within the Earth.

L’ophiolite de Nouvelle Calédonie présente une des plus grandes sections de manteau océanique obductées au monde, offrant une perspective unique pour l’étude des processus du manteau supérieur. Les roches du manteau appartiennent à une séquence ophiolitique “atypique”, dominée par des harzburgites réfractaires avec quelques lherzolites à spinelle et à plagioclase. À l'exception de quelques cumulats mafiques-ultramafiques, la partie crustale de l'ophiolite est totalement absente. Cette ophiolite a été étudiée depuis plusieurs décennies, toutefois sa nature ultra-appauvrie a rendu très difficile une caractérisation géochimique détaillée. La littérature scientifique ne regroupe que quelques données sur les éléments en trace et les données isotopiques sont totalement inexistantes. Dans ce travail de thèse, une étude géochimique exhaustive (éléments majeurs, en trace et isotopes Sr-Nd-Pb) des péridotites et des roches mafiques associées à l’ophiolite a été réalisée. Les péridotites sont des tectonites avec des textures porphyroclastiques. Les lherzolites à spinelle ont 7-8 vol.% de clinopyroxène riche en Na2O et Al2O3 (jusqu’à Na2O 0.5 wt.%; 6.5 wt.% Al2O3), teneur en Fo de l’olivine de 88.5 à 90.0 mol.%, bas valeurs du Cr# du spinelle (13-17), attestant la nature fertiles de ces roches. A l’inverse les harzburgites costituent des roches très réfractaires : ils ne contiennent pas de clinopyroxène primaire et les teneurs en Fo de l’olivine (90.9-92.9 mol.%), le Mg# de l’orthopyroxène et le Cr# du spinelle (39-71) sont élevés. Les spectres de concentrations en REE présentent des caractéristiques typiques de formation dans un environnement abyssal pour les lherzolites à spinelle, alors que les harzburgites ont des spectres en U typiques d’environnement d’avant-arc. Les compositions en REE des lherzolites à spinelle sont compatibles avec un bas degré de fusion fractionnée (8-9%) d'une source DMM, commençant dans le domaine de stabilité du grenat. Au contraire les concentrations en REE des harzburgites indiquent un haut degré de fusion d’une source DMM, en accord avec une fusion hydratée en environnement d’avant-arc. Les lherzolites à plagioclase présentent des microtextures d’imprégnation, des spinelles riches en Cr2O3 et TiO2 et un enrichissement progressif en REE, Ti, Y, Zr. Les modèles des éléments en trace indiquent que les lherzolites à plagioclase résultent des lherzolites à spinelle par séquestration des liquides MORB très appauvris au sein de la lithosphère océanique.Les roches intrusives sont des gabbronorites à olivine avec des compositions très appauvries (87.3≤Fo ol≤88.9 mol.%, 87.7≤Mg# Cpx≤92.2, An Pl=90-96 mol.%). Le haut Mg#, le bas teneur de TiO2 des pyroxènes, la composition en anorthite du plagioclase et le modèle des éléments en trace montrent que les magmas parents des gabbronorites sont des magmas primitifs, très appauvries, formés dans un environnement de subduction. / The New Caledonia ophiolite hosts one of the largest obducted mantle section in the world, hence providing a unique insight for the study of upper mantle processes. These mantle rocks belong to an “atypical” ophiolitic sequence, which is dominated by refractory harzburgites but it also includes minor spinel and plagioclase lherzolites. Upper crust is notably absent in the ophiolite, with the exception of some mafic-ultramafic cumulates cropping out in the southern part of the island. Although the New Caledonia ophiolite has been under investigation for decades, its ultra-depleted nature has made its characterization an analytical challenge, so that few trace element data are available, while isotopic data are completely missing. In this thesis a comprehensive geochemical study (major, trace element and Sr-Nd-Pb isotopes) of the peridotites and the associated intrusive mafic rocks from the New Caledonia ophiolite has been carried out. The peridotites are low-strain tectonites showing porphyroclastic textures. Spinel lherzolites are undepleted lithotypes, as attested by the presence of 7-8 vol.% of Na2O and Al2O3-rich clinopyroxene (up to 0.5 wt.% Na2O; 6.5 wt.% Al2O3), Fo content of olivine (88.5-90.0 mol.%) and low Cr# of spinel (13-17). Conversely, harzburgites display a refractory nature, proven by the remarkable absence of primary clinopyroxene, very high Fo content in olivine (90.9-92.9 mol.%), high Mg# in orthopyroxene (89.8-94.2) and Cr# in spinel (39-71). REE contents show abyssal-type patterns for spinel lherzolites, while harzburgites display U-shaped patterns, typical of fore-arc settings.Spinel lherzolites REE compositions are consistent with relatively low degree (8-9%) of fractional melting of a DMM source, starting in the garnet stability field. Conversely, REE models for harzburgites indicate high melting degrees (20-25%) of a DMM mantle source under spinel faies conditions, consistent with hydrous melting in forearc setting. Plagioclase lherzolites exhibit melt impregnation microtextures, Cr- and TiO2-enriched spinels and REE, Ti, Y, Zr progressive increase with respect to spinel lherzolites. Impregnation models indicate that plagioclase lherzolites may derive from spinel lherzolites by entrapment of highly depleted MORB melts in the shallow oceanic lithosphere. Mafic intrusives are olivine gabbronorites with a very refractory composition, as attested by high Fo content of olivine (87.3-88.9 mol.%), very high Mg# of clinopyroxene (87.7-92.2) and extreme anorthitic content of plagioclase (An = 90-96 mol.%). The high Mg#, low TiO2 concentrations in pyroxenes and the anorthitic composition of plagioclase point out an origin from ultra-depleted primitive magmas in a convergent setting. Geochemical trace element models show that the parental melts of gabbronorites are primitive magmas with striking depleted compositions, bearing only in part similarities with the primitive boninitic melts of Bonin Islands. The first Sr, Nd and Pb isotope data obtained for the New Caledonia ophiolite highlight the presence of DM mantle source variably modified by different processes.Nd-Sr-Pb isotopic ratios for the lherzolites (+6.98≤ƐNdi≤+10.97) indicate a DM source that suffered low-temperature hydrothermal reactions. Harzburgites are characterized by a wide variation of Sr, Nd and Pb isotopic values, extending from DM-type to EMII compositions (-0.82≤ƐNdi≤+17.55), suggesting that harzburgite source was strongly affected by subduction-related processes. Conversely, combined trace element and Sr-Nd-Pb isotopic data for gabbronorites indicate a derivation from a source with composition similar to Indian-type mantle, but affected by fluid input in subduction environment.

The Tibetan plateau is a vast, elevated region located in central Asia, which is underlain by the thickest crust known on Earth (up to 90 km). An outstanding question of importance to many fields within geology is how and why did the Tibetan plateau form? Models attribute the growth of the plateau to a consequence of the ongoing India-Asia continental collision, but differ in the details of how the crustal thickening was accommodated: was it by underplating of Indian lower crust or by homogeneous shortening? High-grade metamorphic rocks sampled from the region potentially hold the key to answering this question, as they contain a record of past tectonic events that can discriminate between the various proposed models. This record can be decoded by integrating field, thermobarometric and geochronological techniques, to elucidate a detailed thermotectonic understanding of a region. This methodology was applied to three case studies, each of which targeted rare tectonic windows into the mid-crust of the plateau. These regions comprise Danba in eastern Tibet, Basong Tso in south-eastern Tibet and the Western Nyainqentanglha in southern Tibet. Each case study documents previously unreported metamorphic events that have allowed original interpretations to be made regarding tectonic evolution: in Danba, all metamorphism is shown to be early Jurassic; in Basong Tso, two metamorphic belts are documented that reveal a late Triassic--early Jurassic orogenic event; and in the Western Nyainqengtanglha, Cretaceous--Neogene magmatism is shown to overprint late Triassic metamorphism. Integration of the results has enabled commentary on the large scale evolution of the Tibetan plateau from the Permian until the present day, and even hinted at its future. The results indicate that the closure of the Paleotethys played an important role in the construction of the Tibetan plateau, and suggest that homogeneous crustal thickening is not a viable model for the documented exposure levels.