The rheology and strength of the lower crust play a key role in lithosphere dynamics, influencing the orogenic cycle and how plate tectonics work. Despite their geological importance, the processes that cause weakening of the lower crust and strain localization are still poorly understood. Through microstructural analysis of naturally deformed samples, this PhD aims to investigate how weakening and strain localization occurs in the mafic continental lower crust. Mafic granulites are analysed from two unrelated continental lower crustal shear zones which share comparable mineralogical assemblages and high-grade deformation conditions (T > 700 °C and P > 6 Kbar): the Seiland Igneous Province in northern Norway (case-study 1) and the Finero mafic complex in the Italian Southern Alps (case-study 2). Case-study 1 investigates a metagabbroic dyke embedded in a lower crustal metasedimentary shear zone undergoing partial melting. Shearing of the dyke was accompanied by infiltration of felsic melt from the adjacent partially molten metapelites. Findings of case-study 1 show that weakening of dry and strong mafic rocks can result from melt infiltration from nearby partially molten metasediments. The infiltrated melt triggers melt-rock reactions and nucleation of a fine-grained (< 10 µm average grain size) polyphase matrix. This fine-grained mixture deforms by diffusion creep, causing significant rheological weakening. Case-study 2 investigates a lower crustal shear zone in a compositionally-layered mafic complex made of amphibole-rich and amphibole-poor metagabbros. Findings of case-study 2 show that during prograde metamorphism (T > 800 °C), the presence of amphibole undergoing dehydration melting reactions is key to weakening and strain localization. Dehydration of amphibole generates fine-grained symplectic intergrowths of pyroxene + plagioclase. These reaction products form an interconnected network of fine-grained (< 20 µm average grain size) polyphase material that deforms by diffusion creep, causing strain partitioning and localization in amphibole-rich layers. Those layers without amphibole fail to produce an interconnected network of fine grained material. In this layers, plagioclase deforms by dislocation creep, and pyroxene by microfracturing and neocrystallization. Overall, this PhD research highlights that weakening and strain localization in the mafic lower crust is governed by high-T mineral and chemical reactions that drastically reduce grain size and trigger diffusion creep.

The oldest dated rocks from the Acasta gneisses of the western Slave Province, Canada present an igneous age of ~4030 Ma. Following this the detrital zircons from the Jack Hills, Narryer Gneiss Terrane, Yilgarn Craton, Western Australia are identified as 4404 ±8 Ma. These discoveries suggest that crustal formation started as early as the Priscian Eon. Hitherto the Earth has gone through a series of interactions involving the atmosphere, hydrosphere, crust, mantle and core. However, only limited remnants of these early processes remain on the accessible crust due to extensive crustal reworking. The Southern Granulite Terrane (SGT) in the southern part of India represents the most extensive exposure of lower crustal granulite terranes in the world. This study mainly focuses on the characteristics of Archean (~2500 Ma) tectonics and nature of subsequent crustal growth, which led to the formation of Archean Nilgiri Block. Detailed fieldwork in this terrane and subsequent petrographic analysis revealed charnockites, hornblende-biotite gneiss, metagabbro/mafic granulite, websterite, amphibolite, Grt-Ky metasediment, metatuff and banded iron formation as the main rock types in this terrane. Field and petrographic results show a regional trend with garnet-orthopyroxene-biotite-quartz-plagioclase-K- feldspar bearing charnockites in the southern part which gets subsequently enriched in clinopyroxene that forms garnet-absent two pyroxene granulites consisting of orthopyroxene-clinopyroxene-quartz-plagioclase-K-feldspar towards the central part. Further north, metagabbro/mafic granulite is enriched in garnet-clinopyroxene-plagioclase assemblage. Websterite, amphibolite, metasediment, metatuff and banded iron formation are stacked and closely associated within this mafic belt in the north. The metagabbro represents peak P-T conditions of ~850°C and ~14kbar compared to the charnockites, which recorded a peak P-T of ~850°C and 9-10kbar. Petrographic results of oxide minerals show that the southern charnockitic part is abundant in rutile-ilmenite association represent reduced conditions compared to the oxidized magnetite-hematite-ilmenite associations in the mafic rocks. This oxidation trend is followed by pyrrhotite-chalcopyrite enriched southern charnockitic region that transforms to pyrite rich northern mafic belt. Ilmenite¬titanite association with no sulphides characterizes the hornblende-biotite gneiss in the entire Nilgiri Block. The geochemical variations of major, trace and rare earth elements show that the granulite-amphibolite grade felsic rocks evolved in an arc magmatic process leaving behind mafic magma, which later intruded into these rocks, in a subduction related arc magmatic process. The U-Pb LA-ICPMS and SHRIMP dating of charnockite, hornblende-biotite gneiss and met gabbros shows ca. 2550 Ma formation age and ca. 2450 Ma metamorphism in this terrane.

The origin of silicic magma in basalt-dominated oceanic settings is fundamental to our understanding of magmatic processes and formation of the earliest continental crust. Particularly significant is magma-crust interaction that can modify the composition of magma and the dynamics of volcanism. This thesis investigates silicic magma genesis on different scales in two ocean island settings. First, volcanic products from a series of voluminous Neogene silicic centres in northeast Iceland are investigated using rock and mineral geochemistry, U-Pb geochronology, and oxygen isotope analysis. Second, interfacial processes of magma-crust interaction are investigated using geochemistry and 3D X-ray computed microtomography on crustal xenoliths from the 2011-12 El Hierro eruption, Canary Islands. The results from northeast Iceland constrain a rapid outburst of silicic magmatism driven by a flare of the Iceland plume and/or by formation of a new rift zone, causing large volume injection of basaltic magma into hydrated basaltic crust. This promoted crustal recycling by partial melting of the hydrothermally altered Icelandic crust, thereby producing mixed-origin silicic melt pockets that reflect the heterogeneous nature of the crustal protolith with respect to oxygen isotopes. In particular, a previously unrecognised high-δ18O end-member on Iceland was documented, which implies potentially complex multi-component assimilation histories for magmas ascending through the Icelandic crust. Common geochemical traits between Icelandic and Hadean zircon populations strengthen the concept of Iceland as an analogue for early Earth, implying that crustal recycling in emergent rifts was pivotal in generating Earth’s earliest continental silicic crust. Crustal xenoliths from the El Hierro 2011-2012 eruption underline the role of partial melting and assimilation of pre-island sedimentary layers in the early shield-building phase of ocean islands. This phenomenon may contribute to the formation of evolved magmas, and importantly, the release of volatiles from the xenoliths may be sufficient to increase the volatile load of the magma and temporarily alter the character and intensity of an eruption. This thesis sheds new light on the generation of silicic magma in basalt-dominated oceanic settings and emphasises the relevance of magma-crust interaction for magma evolution, silicic crust formation, and eruption style from early Earth to present.

Le travail de thèse reporté dans ce manuscrit se focalise sur la reconnaissance ainsi que l'expression des processus de différenciation crustale à l'Archéen. Cet éon représente à lui tout seul 1/3 des temps géologiques, et se caractérise notamment par des lithologies diagnostiques, ainsi que des contextes géodynamiques complexes. La majorité des études sont portées sur l’investigation des phases alumineuses qui permettent de contraindre précisément les événements métamorphiques au-cours d’une géodynamique d’épaississement crustal. Toutefois, ces phases alumineuses représentent rarement plus de 10% des terrains archéens, alors qu’ils sont faits à plus de 75% de gneiss orthodérivés. L'étude qui suit est une caractérisation du complexe de gneiss gris composite de la marge Nord du craton archéen du Kaapvaal en Afrique du Sud. Les résultats produits durant cette investigation ont amené à plusieurs conclusions importantes au regard de la géodynamique archéenne. L'étude isotopique U-Pb/Lu-Hf sur zircon couplée à des analyses pétro-métamorphiques montre que la construction d’un complexe de gneiss gris composite correspond à une géodynamique prolongée dans le temps, accomplie au-travers de processus de différenciations crustales internes à la Zone accrétée, 1.e. le bloc crustal évolue en système thermodynamiquement fermé. Les complexes de gneiss gris ne sont que modérément étudiés toutefois, les informations contenues dans ces lithologies apparaissent complémentaires avec celles obtenues par les études métamorphiques sur les lithologies alumineuses. Il est donc nécessaire d'approfondir ce type d’investigations afin de mieux contraindre les modèles géodynamiques archéens / The PhD work presented in this manuscript focuses on the recognition and the manifestation of Archean crustal differentiation processes. The Archean eon which represents 1/3 of the geological record is featured by both lithologies unrecognized in younger eons and cryptic geodynamics. Most of investigations concentrate on the characterisation of aluminium-rich lithologies that allow an accurate determination of the pressure-temperature evolution underwent by crustal materials during crustal thickening geodynamics. However, aluminium-rich lithologies - mainly represented by metasediments - account for only 10% on average of Archean terranes whereas orthoderived gneisses - which also testify for crustal differentiation processes - form around 75% of these terranes. The following contribution depicts an Archean composite grey gneiss complex located at the northern edge of the Kaapvaal craton is South Africa. Results carried out during this PhD study have major consequences on Archean geodynamics. The zircon U-Pb/Lu-Hf isotope Investigation coupled with strong petro-metamorphic observations show that composite grey gneiss complexes may be built over a protracted time span, achieved through self-refinement of crustal materials, i.e. the crustal block evolved in a thermodynamically closed system. Grey gneiss compiexes are only moderately investigated even though information enclosed in these lithologies is complementary with those from aluminium-rich rocks. Therefore, deeper investigations of these geological objects must be a central scope in order to improve the knowledge of the Archean eon and appears necessary for the building of even more realistic geotectonic models

La lithosphère continentale est physiquement et chimiquement segmentée. La cartographie des isotopes radiogéniques de roches plutoniques acides, représentatives de la croûte continentale, et de laves basiques, représentatives du manteau, possède des similarités avec la cartographie sismique de la lithosphère sous-jacente. Ces similitudes permettent d’interpréter les observations sismiques en étudiant leurs caractéristiques chimiques et leur âge. Les isotopes du plomb permettent de dater et d’identifier l’empilement de segments crustaux qui forment la croûte. L’écart des âges modèles du plomb avec d’autres systèmes identifie le recyclage crustal et le réchauffement de la croûte au dessus de la température du système plomb-plomb. Le système plomb-plomb donne également accès au sous-étudié rapport Th/U qui contraint la profondeur de la source des roches continentales. Certains échantillons de l’ouest U.S.A. proviennent de la croute inférieure, et se sont formés par l’extension crustale ou par un flux de matériel au sein de la croûte. Les isotopes du néodyme et de l’hafnium marquent la fusion du manteau lithosphérique enrichis sous le Colorado Plateau, une région où est observée le détachement du manteau lithosphérique sub-continental. Ce manteau fond par décompression adiabatique, par extension localisée ou remontée asthénosphérique engendrée par la convection locale. Au final, l’association des systèmes isotopiques du plomb, du néodyme, et de l’hafnium avec la sismologie est une approche puissante pour étudier la formation et la déformation de la lithosphère continentale. / Continental lithosphere is physically and chemically segmented. The mapping at a continent size scale of radiogenic isotopes from plutonic acid rocks, sampling the continental crust, and from mafic lavas, sampling the mantle, has similarities with the seismic mapping of the underlying lithosphere. These similarities allow to interpret the seismic observations by studying their chemical characteristics and age. Lead isotopes are used to date and identify the stacking of crustal segments that form the crust. The deviation of Lead model ages with other system is used to identify crustal recycling and the heating of the crust above the Lead-Lead system closing temperature. The Lead-Lead system also give access to the under-studied Th/U ratio that constrains the depth of the continental rock sources. Somes samples from the Western U.S. are coming from the lower crust, formed by crustal extension or crustal flowing within the crust. Neodymium and Hafnium isotopes identify the fusion of an enriched lithospheric mantle under the Colorado Plateau, a place where lithospheric delamination has been observed. This mantle melts by adiabatic decompression due to localized expansion or asthenospheric upwelling caused by secondary convection. At the end, the merging of Lead, Neodymium, and Hafnium isotopic systems with seismology is a powerful tool to study the formation and deformation of the continental lithosphere.

Thesis: Ph. D. in Geochemistry, Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016. / "February 2016." Cataloged from PDF version of thesis. / Includes bibliographical references. / Fractional crystallization of mantle-derived basaltic melts is a critical process in producing a compositionally stratified continental crust characterized by a silicic upper crust and a mafic lower crust. This thesis explores outstanding questions associated with fractional crystallization through detailed field, petrological, and geochemical studies of the Dariv Igneous Complex in Western Mongolia. The Dariv Igneous Complex records the crystallization of a high-K primitive arc melt at shallow crustal levels, preserving both biotite-bearing ultramafic and mafic cumulates, as well as liquid-like evolved plutonics, such as (quartz-)monzonites. Chapter 2 presents comprehensive field and petrographic descriptions of the complex and establishes the petrogenetic groundwork to understand the conditions under which it formed. Results of this study indicate that the observed lithologies formed through the fractional crystallization of a high-K hydrous basalt, typical of alkali-rich basalts found in subduction zone settings, at 0.2-0.5 GPa and elevated oxygen fugacities. Chapter 3 presents a quantitatively modeled liquid line of descent (LLD) for the complex based on whole rock geochemical analyses, which is able to explain the trends observed in the monzonitic plutonic series observed in continental arcs. The oxygen isotope trajectory of fractionally crystallizing melts is rigorously constrained through modeling and mineral analyses in Chapter 4. This study indicates that large (1 to 1.8%o) increases in [delta]18O as a melt evolves from basaltic to granitic in composition due to the fractionation of low [delta]18O minerals. As such, the majority of [delta]180 values of upper crustal silicic plutonics can be explained through fractional crystallization of primitive arc basalts alone without needing to invoke assimilation of high [delta]18O crustal material. Finally, Chapter 5 explores the timescales associated with fractional crystallization through high precision U-Pb geochronology of zircon from the Dariv Igneous Complex. Evolution from a basaltic melt to a silica-rich monzonitic melt in the Dariv Igneous Complex occurred in <300 ka. If rates of fractional crystallization are primarily a function of cooling, this study provides an end-member constraint for fractional crystallization of a basaltic melt at relatively cool, shallow crustal levels. Together, these studies advance our understanding of the compositional, isotopic, and temporal variations associated with the formation of the continental crust. / by Claire Elizabeth Bucholz. / Ph. D. in Geochemistry