Дисертації з теми "Island arc magmatism"

Field evidence from exposed plutonic rocks indicates that mafic-felsic magma interaction is an important process during the construction and evolution of magma chambers. The exhumed, ~140 Ma, Halfmoon Pluton of Stewart Island, New Zealand is characterized by a sequence of mingled mafic sheets and enclaves preserved within an intermediate-felsic host, and provides a unique opportunity to directly investigate the physico-chemical processes that operate within an arc setting. Interpretation of mingling structures and textures in the field, in combination with extensive petrographic, geochemical and geochronological data, allow for conclusions to be reached regarding the nature of the mafic-felsic magma interactions, and the physical, chemical and thermal processes responsible for the generation and evolution of the calc-alkaline magmas. Detailed documentation and interpretation of mafic-felsic magma mingling structures and textures reveal that the Halfmoon Pluton formed incrementally as the result of episodic replenishments of mafic magma emplaced onto the floor of an aggrading intermediate-felsic magma chamber. Physico-chemical processes identified include fractional crystallization and accumulation of a plagioclase – hornblende – apatite – zircon mineral assemblage, episodic replenishment by hot, wet basaltic magmas, magmatic flow and compaction. Early amphibole and apatite crystallization played an important role in the compositional diversity within the Halfmoon Pluton. Variations in the style of magma mingling preserved within the magmatic “stratigraphy” indicate that processes operating within the chamber varied in space and time. Variations in mineral zoning and composition within hornblende indicate that the Halfmoon Pluton crystallized within a magma in which melt composition fluctuated in response to repeated mafic magma replenishments, fractionation, crystal settling and convection. Mineral assemblages, chemical characteristics, isotopic data and geochronological evidence indicate that the amphibole-rich calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting, most probably an island-arc. Data are consistent with a model whereby ‘wet’ amphibole-rich basaltic magmas pond at the crust-mantle interface and episodically rise, inject and mingle with an overlying intermediate-felsic magma chamber that itself represents the fractionated product of the mantle melts.

The Kohistan arc, situated in the northwestern Himalaya of North Pakistan, is a Cretaceous intra-oceanic island arc which was erected during subduction of the Tethys Ocean consequent on the northward flight of India. Following accretion to the Asian margin, the arc behaved as an Andean-type continental arc prior to the collision of India with the Asian continent, which uplifted the arc and tilted it, thus providing a unique opportunity to study a complete succession of rocks from the very earliest stages of arc evolution. A combination of fieldwork, geochemical and radiogenic isotope analysis, and rare earth element modelling are used to determine the main magma sources in the mantle beneath the arc. The Kamila Amphibolites fall into two successions. The 'E-type' rocks have a MORB-type signature and were formed from 6% partial melting of a primitive, garnet¬bearing, mantle source. The 'D-type' Kamila Amphibolites have an arc signature and represent the earliest arc volcanic rocks. They originated from 15.5% partial melting of a primitive spinel-bearing, mantle source. The Jaglot Group (Gashu Confluence Volcanics and Peshmal Volcanics) and Western Volcanics of the Chalt Volcanic Group were generated by 7.5%, 13% and 2.5% partial melting, respectively, of a primitive, garnet¬bearing, mantle source. The Hunza Valley Volcanics of the ChaIt Volcanic Group, which contain boninites, have MORB-type chemistries and an arc signature. These rocks were generated from 15% melting of a depleted, spinel-bearing mantle source which may be the residuum from partial melting which produced the 'E-type' Kamila Amphibolites. That the Hunza Valley Volcanics were generated from this source is consistent with the 'E-type' Kamila Amphibolites fonning the basement and the Hunza Valley Volcanics occurring in the back-arc. Radiogenic Nd, Sr and Pb isotope analyses address an hypothesis that the magmas were generated from 'Dupal' -type mantle. The results indicate that this is not the case, but show that the isotopic chemistry of the rocks is the result of fluids from dehydration and melting of sediments carried on the downgoing ocean crust into the subduction zone affecting magma chemistry. A recent controversy concerns the polarity of subduction beneath the arc. This is partly' based on the presence of boninites, and the prevalent understanding that this rock-type occurs solely in the fore-arc. Evidence from this thesis suggests that the boninites of the Hunza Valley Volcanics were erupted into a back-arc setting, and arguments are made that boninites may also be erupted into the back-arc, and that the polarity of subduction was to the north. A model is presented for the erection of the Kohistan arc in which subduction was initiated by gravitational instability at an oceanic transform fault which connected two mid-ocean spreading ridge segments. The first arc magmas were generated by decompression melting beneath extending lithosphere during initial subsidence, and as this turned into subduction, magmas were generated at progressively greater depths in the spinel and garnet lherzolite facies, respectively. Lithospheric extension and rifting behind the volcanic front provided the setting for deompression melting to produce the back-arc Hunza Valley Volcanics, including high-Mg basalts and andesites. The arc signature in these rocks became reduced as the spreading centre developed and became progressively removed from the vicinity of the subduction zone.

The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs.

Dans le secteur de taxco-zihuatanejo (Mexique méridional) affleurent six séquences volcano-sédimentaires et/ou volcano-plutoniques d'arc datées du jurassique supérieur au crétacé inferieur qui se sont accrétées au craton nord-américain à la fin du crétacé inférieur. La séquence de taxco-taxco viejo comprend des andésites, des dacites et des rhyolites calco-alcalines inter stratifiées dans une sédimentation exclusivement détritique. Elle est affectée par un métamorphisme syncinématique de basse température (221-276c). Cette formation représente vraisemblablement le témoin d'un arc insulaire édifié sur un substratum continental. La séquence de teloloapan comprend des pillow lavas basiques surmontes en concordance par des dépôts volanoclastiques à lentilles de calcaires récifaux de l'aptien et des calcaires récifaux de aptien-albien. Elle est affectée par deux métamorphismes de bas degré: (i) hydrothermal océanique et (ii) syn-cinématique. Le volcanisme comprend surtout des basaltes et des andésites calco-alcalins avec de rares roches acides (andésites et rhyolites tholeiitiques). Comparées aux séries calco-alcalines d'arc intra-océanique les basaltes et andésites sont enrichis en hfse et lree. Les basaltes et andésites présentent des différences géochimiques et un nd compris entre 4,6-1,6. Cette séquence s'est développée dans un environnement d'arc insulaire intra-océanique. La séquence plutono-volcanique d'arcelia comprend un ensemble plutonique qui repose en klippe sur des basaltes en coussins recoupes par des filons basiques. La sédimentation est soit micritique au sein de la pile volcanique soit pelitique à radiolaires au sommet et datée de l'albien-cénomanien. Laves et filons sont affectés par un métamorphisme statique et hydrothermal prehnite-pumpellyite. Les roches basiques d'arcelia y compris les rhyolites montrent des affinités de tholeiite typique d'arc insulaire intra-océanique (nd compris entre +8 et +6). La séquence de huetamo représente le comblement d'un bassin fortement subsident qui se développe entre des îles volcaniques appartenant à un environnement d'arc insulaire. La séquence volcano-sédimentaire de zihuatanejo alocenomanienne est composée de pyroclastites et de laves calco-alcalines déjà différenciées, associées à des calcaires récifaux et/ou des couches rouges continentales. Le complexe de subduction de las ollas comprend des blocs de roches basiques et ultrabasiques enchâssés dans une matrice de serpentine ou de flysch. Ces blocs sont affectés par un métamorphisme hp-bt. Les roches basiques montrent des affinités de tholeiites d'arc, appauvries en terres rares légères. Leurs caractères géochimiques communs suggèrent qu'elles représentent des fragments dissociés d'une croûte supérieure d'un arc insulaire intra-océanique, formée aux tous premiers stades de l'activité de l'arc. Les affinites magmatiques des séries d'arc mésozoïques du Guerrero terrane sont très diversifiées à la fois d'une séquence à l'autre et à l'intérieur d'une même séquence. Cependant, deux ensembles peuvent être reconnus: (i) des tholeiites d'arc appauvries à légèrement enrichies en lree, composées exclusivement de basaltes et de leurs filons nourriciers et présentés a arcelia et las ollas. Quelle que soit la séquence, des cumulats ultrabasiques et basiques sont tectoniquement associés aux laves. Leur source mantellique appauvrie (nd compris entre +8 et +5. 5) est du type lherzolite a spinelles ; (ii) des séries calco-alcalines enrichies ou appauvries en hfs. Les roches basiques prédominent dans la série calco-alcaline enrichie en hfs (famille i), représentée par les basaltes et les andésites de l'aptien-albien de teloloapan et qui dérivent d'une source enrichie de type lherzolite à grenat. La série appauvrie en hfs (nd compris entre +9 et +7,5) est représentée par les andésites de zihuatanejo et les galets de l'aptien-albien de huetamo (famille ii) et qui dérive d'une source mantellique appauvrie, identique à celle des tholeiites mais avec des taux de fusion partielle moins élevés. Enfin la famille iii regroupe les laves de taxco et présente des caractères intermédiaires entre les familles i et ii. Ainsi, les séquences magmatiques orogéniques du Guerrero terrane reflètent la complexité de cet arc ou de ces arcs qui, néanmoins, ont fonctionné pratiquement en même temps

Les régions caraïbes et nordandines comportent, au crétacé, des séries magmatiques basiques, volcaniques ou ophiolitiques. L'étude petrologique analytique (majeurs, traces, terres rares, microsonde) de quelques séries du Costa Rica, de Colombie et d'Équateur, a permis leur identification magmatique et dynamique. La comparaison des laves à certaines séries volcaniques océaniques actuelles a conduit à une réinterprétation magmatique et géodynamique globale. Au Costa Rica, la péninsule de Santa Elena est formée d'une large nappe ophiolitique tholeiitique avec péridotites, cumulats gabbroiques et dolerites diverses (n-morb). Les iles Murcielago sont couvertes de ferrobasaltes t-morb. Santa Elena représente un témoin de croute océanique crétacée mis en place vers 70 ma et Murcielago un lambeau de plateau océanique soudé à l'Amérique centrale. La Colombie offre, au crétacé, et du nord au sud de la cordillère occidentale, un large éventail de formations océaniques: la série du Boqueron de Toyo, à volcanisme basaltique et intrusions diorito-tonalitiques (92 ma) témoigne du fonctionnement d'un arc insulaire immature. La série d'Altamira, a cumulats gabbroiques et basaltes primitifs illustre l'ouverture vers 80 ma d'un bassin en arrière de l'arc précédent. Le massif de Bolivar, correspond, avec ses cumulats tholeiitiques (i ou iia), a la croute océanique. La coupe de Buenaventura a Buga, avec ses nappes empilées riches en sédiments océaniques et en basaltes de type t-morb évoque des terrains constitués en plateau océanique et accrétés à la marge sud-américaine. En Équateur, le crétacé supérieur de la cordillère occidentale offre une situation analogue: des lambeaux de croute océanique sont dispersés le long d'une grande suture ophiolitique oblitérée par l'arc volcanique de Macuchi. La série de la Quebrada San Juan est l'équivalent de celle de Bolivar. Les basaltes (t-morb) du Grupo Pinon de la cote correspondent aussi à du matériel de plateau océanique accrété au bâti sud-américain

Volcanic arcs are the primary seat of subaerial volcanism and where continental crust is created. Since the advent of plate tectonics theory in the last half century, many of the processes that govern arc magmatism have been described in detail. However, understanding the development and eruption of upper crustal magma reservoirs remains a fundamental challenge. Here we develop and implement new geochemical approaches, and combine our results with those from other disciplines, to explore the location, formation, and eruption of upper crustal magma reservoirs, with the ultimate goal of linking these processes to the underlying process of plate tectonics. Our study area is the central-eastern Aleutian arc, one of the most volcanically active regions on earth, where significant along-strike variability in subduction parameters, magma compositions, and volcanic activity exists. To advance understanding of magma reservoirs, it is essential to hone our tools for gauging magma depth. Melt inclusion analysis is the first in the toolbox for petrologists, but recent studies have raised questions about the accuracy of this approach. Vapor bubbles commonly form in melt inclusions after entrapment. These bubbles may sequester a substantial portion of the total volatile contents of the melt inclusion, which is problematic because depth estimates are based on melt volatile contents. In Chapter 1, we explore vapor bubble growth in melt inclusions by describing the processes, and their timescales, that lead to bubble growth and developing new methods to retrieve accurate depth estimates from melt inclusions. Our new methods have situational strengths. In concert, they enable extraction of reliable depth information, unlocking the true potential of melt inclusions to measure depth. With an improved understanding of melt inclusions, we next investigate eruption run-up. During run-up, crustal-scale magmatic systems can be activated, providing a unique opportunity to peer into their structure. In Chapter 2, our goal is to study eruption run-up and determine how magmas are stored in the months, days, and hours leading to volcanic eruption. As a case study, we investigate the 1999 eruption of Shishaldin volcano, of interest because the run-up was months, an unusually long duration, and, despite 43 million cubic meters of tephra ejected in the eruption, no eruption-related deformation was detected in satellite imagery. We develop a new approach for studying run-up that combines diffusion modeling, which gives information on the timing of magmatic processes preceding eruption, with melt inclusion analysis, which gives depth information. Results are combined with those from shear-wave splitting analysis and other geophysical methods. We identify a shallow magmatic system that existed prior to the run-up to the 1999 eruption. A substantial fraction of the magma that was erupted was delivered to the shallow reservoir ~50 days prior to the eruption. More broadly, our results indicate that open-system volcanoes, such as Shishaldin, may commonly have long run-up durations. Our work on run-up demonstrates the strength of the forensic approach for studying magma reservoirs, but it leads us to question what can be understood in real time. One powerful approach for understanding the state and location of magma reservoirs in real time is the study of volcanic gas emissions. However, interpretation of gas data is a major challenge. To improve our ability to use gas data to understand plumbing systems, and to investigate the shallow magmatic plumbing system of an open-vent volcano, we perform a melt inclusion study of the degassing system at Cleveland in Chapter 3. We focus on Cleveland volcano, one of the most active volcanoes in the US. We develop an empirical degassing model based on melt inclusion data. We use the degassing model to interpret gas composition and flux measurements at Cleveland. Our results indicate gas emissions are generated in a shallow, convecting magmatic system, which is consistent with geophysical observations. After detailing plumbing systems at Shishaldin and Cleveland, we investigate global trends in magma storage depth in Chapter 4. Geophysically imaged magma storage depths are mostly ~0-20 km depths. The reason for the dramatic variability is not well known. We compile geophysical estimates of magma storage depth and compare these data to magmatic water contents. The initial water content of magma is thought to exert a key control over magma storage depth because as magmas ascent, they degas water. Concurrently, melt viscosity increases and crystallization may be induced. Both these processes promote slowing of magma ascent. We find a strong correlation between magma storage depth and magmatic water contents at the 24 volcanoes that have estimates for both. A global compilation of magma storage depths at 97 volcanoes has a distinct mode at 6 ±3 km, which coincides very closely with the average depth at which arc magmas become water saturated (6 ±3 km) based on maximum water content estimates from a compilation of 77 volcanoes. Melt inclusions from the eastern-central Aleutians do not show evidence of degassing or diffusive loss of water, indicating that water content is a strong control over magma storage depth. The trend exists globally, despite a large range in potential upper crustal controls (thickness, age, stress state, etc.). In Chapter 5, we move deeper in the arc to understand the underlying processes of subduction and arc magma genesis. Slab depth in the central-eastern Aleutians varies from a near global minimum of 65 km in west (near Seguam) to a more common value of 100 km in the east (near Shishaldin). The cause for this variability is not well known. The thermal structure of the wedge is thought to play a key role in determining where mantle melting occurs, and subduction parameters (slab age, dip, velocity, etc.) exert first order controls on wedge thermal structure. Therefore, subduction parameters are likely to some extent modulate slab depth. However, the mantle-slab coupling depth also has a key influence on the thermal structure of the wedge. Therefore, the coupling depth must also play a role in determining slab depth. A potential third factor is the extent of lateral melt migration in the wedge. In the central-eastern Aleutians, variations in slab depth are reflected in variations in major, trace, and volatile elements. Chemical trends are most consistent with a shallow slab-mantle coupling depth of 50 km throughout the corridor. Results from slab top thermometry suggest that significant lateral variation is unlikely. We speculate that the change in slab depth across the corridor is likely a result of the significant decrease in trench fill sediment thickness moving east.

This item is only available electronically.
Physical controls on fractional crystallisation in subduction settings around the globe are the focus of much discussion within academia. It is frequently observed that the typical magma to be erupted in arc settings is andesite from the differentiation of partially melted peridotite mantle wedge. It is less common in these settings to see more felsic end member magmas such as dacite or rhyolite being erupted in cataclysmic events. Discussion has arisen into what physical process or processes can drive a volcano, or set of volcanoes, within an arc to produce dacite or rhyolite magmas where fractional crystallisation is known to be the process of initial crystallisation. Research into these processes can be applied to Mount Rinjani (Lombok) within the Sunda Arc system, where the normally composite andesite volcano produced a cataclysmic eruption of dacite magma known as the Rinjani Pumice at 1300AD. Data is presented here in order to investigate processes within the sub-volcanic magma plumbing at Rinjani, primarily using volatiles from melt inclusions, coupled with petrology, pressure, temperature and water saturation estimates. It is theorised that ascent-driven crystallisation from a parent basalt drove fractional crystallisation to andesite composition beneath Rinjani volcano. Water undersaturated conditions induced ascent of the andesite magma to a shallow reservoir at ~3 kbar at temperatures ~1015 °C, with water saturation ~3.7 weight percent. Melt inclusion data from these same crystals reveals the interstitial liquid in the andesite magma to be of evolved dacite composition (~66 wt % SiO2) compared to the andesite (~50-54 wt % SiO2). Critical crystallinity is the physical process believed to have acted upon fractional crystallisation where the percentage of crystals forming exceeded the ability of the magma chamber to convect within this reservoir and therefore physical separation of the evolved dacite liquid from the andesite magma occurred. The buoyant dacitic liquid ascends to another shallow reservoir directly beneath Rinjani volcano and reaches its saturation pressure at 1.8 kbar at a temperature of ~900 °C, with water saturation increased to ~5 wt %. Sub-plinian cataclysmic eruption of the 1300AD Rinjani Pumice occurred due to pressure increase involved in saturation of the dacite, and syn-eruptive degassing aids in excavating the western flank of the Rinjani stratocone leaving a caldera. Subsequent volcanism occurs where the magma ascent path migrates west producing more cataclysmic eruptions increasing the volume of the edifice. Today, resurgent volcanism can be seen in the central eastern part of Segara Anak lake at Gunung Baru within the Rinjani caldera to produce basalt and andesite lavas. The application of physical mechanisms acting upon fractional crystallisation proposed here may be applied to other arc settings around the globe, where fractional crystallisation is found to be the primary driving force of felsic end member cataclysmic eruptions at what is typically an andesitic arc.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2009

This item is only available electronically.
The Arabian Nubian Shield (ANS) records multiple episodes of terrane amalgamation associated with the closure of the Mozambique Ocean and the amalgamation of Gondwana. Evidence for such amalgamation events are recorded by repeated ophiolite decorated sutures across the ANS. The Jebel Tays ophiolite is situated in the eastern portion of the ANS, within the Ad Dawadimi Basin and surrounded by the Abt Schist. This basin separates the Afif and Ar Rayn arc terranes and represents one of the last terrane amalgamation events affecting the exposed ANS. I identify mafic magmatism associated with Jebel Tays as a low titanium island arc tholeiite with forearc affinity (boninitic) in contrast to the island arc tholeiite magmatism of the nearby Halaban ophiolite. Furthermore, I place robust zircon ages on mafic magmatism of 678 ± 5.1 Ma for the gabbros of Jebel Tays and 674.4 ± 5.7 Ma for the Halaban ophiolite. Based on the stratigraphy and structure of Jebel Tays and the sub-greenschist nature of the thrust contacts I propose that Jebel Tays represents a forearc ophiolitic thrust sheet. Its obduction was associated with extensive serpentinite mud volcanism within a forearc environment associated with a west dipping subducting slab. From detrital zircons obtained from the Abt Formation I constrain deposition of the Ad Dawadimi basin sediments to ca 600 Ma, making them Ediacaran in age, a date that coincides with a new LA-ICP-MS monazite metamorphic age of 599.8 ± 5.8 Ma obtained from a trondhjemite that cross cuts the Jebel Tays body. This is in agreement with previously published 40Ar/39Ar ages which I interpret as metamorphism associated with final basin closure and the amalgamation of the Afif and Ar Rayn Terranes.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2009

The Arabian-Nubian shield reflects the complex interplay between juvenile oceanic and continental arc fragments accreted during the final stages of Gondwanian super continental assembly. To date, much of the geochronological and geochemical data from the Arabian Shield, Saudi Arabia, is absent or poorly constrained and extrapolated from neighbouring Middle Eastern and African countries. Little attention has been paid to the petrogenesis and tectonic significance of the plutonic rocks pursuant to lithospheric orogenesis. A total of 137 samples from 26 geological units were collected from the Midyan, Hijaz, Asir, Tathlith, Afif, Ad Dawadimi and Ha’il terranes with particular emphasis on accretionary suture zone and within plate setting relationships. Extensive data bases are constructed using zircon U-Pb geochronology and Hf isotopes to evaluate Gondwanian significance and whole rock major and trace element geochemistry, Nd, Sm, Sr isotopes and zircon geochemistry to determine their petrogenetic properties. These parameters provide new insight into changing mantle conditions beneath collisional sutures (Yanbu, Nabitah and Halaban) and within plate asthenospheric upwelling. 19 granitic units are subdivided into metaluminous, peraluminous and peralkaline groups that possess distinguished island arc (~950-730Ma), syncollisional (~<730-636Ma), post tectonic (~<636-600Ma) and anorogenic (<600Ma) U-Pb geochronology. These magmatic phases represent accretionary cycles initiating from the dismantlement of Rodinia, closure of the Mozambique Ocean and final Gondwana amalgamation. Evidence for final assembly is recorded at ~525Ma (Najd fault reactivation) which is now the youngest dated magmatism in the Arabian-Nubian Shield and warrants repositioning of the regional unconformity at ~542Ma. Emplacement of sampled Arabian Shield classic A-type post-tectonic and anorogenic granitoids falls into three categories: 1) Intrude sutures immediately following collision which contain extensive mafic cumulate fractionation and N-MORB affiliation. 2) Plate boundary juxtaposed suites without obvious mafic cumulates, but posses contaminated N-MORB geochemistry. 3) Within plate granitoids isolated from plate boundaries and also without obvious mafic cumulates, but with a distinctive enriched (OIB) like asthenospheric mantle source. All categories produce similar felsic endmembers, but contain isotopically distinct mantle source. These are differentiated using a newly developed geochemical scheme (contaminated and enriched mantle granitoids) that is successfully applied to regional Arabian-Nubian examples. The diachronous Nabitah Orogenic Belt symbolises collision and subduction between western oceanic and eastern continental terranes that was terminated by the appearance of category 1 post-tectonic granitoids. This long lived (~50Ma) granitic magmatism contains mingling textures, discrete crystallisation ages, distinguished zircon morphologies and isotopically less juvenile mafics that geochronologically and geochemically reflect magmatic pulsing from a contaminated lower crustal MASH zone. The transition from N-MORB like mafics to isotopically enriched granitoids (isotopically similar to category 3 suites) reflects subduction magmatism followed by slab tear and asthenospheric influx. Conversely, the appearance of category 3 anorogenic plutons is characterised by widespread, tightly constrained (<10Ma) magmatism that is geochemically enriched, economic and symbolic of lithospheric delamination and asthenospheric (OIB like) upwelling. Differences between category 1, 2 and 3 zircon geochemistry constrain further contaminated and enriched mantle source behaviour that produces similar felsic products from distinguished petrogenetic processes. In summary, the work presented in this thesis establishes clear distinctions between accretionary syncollisional suites and anorogenic suites, but more significantly, post-orogenic plutons confined to suture zones from those confined to within plate settings. This allows new petrogenetic insights into changing juvenile mantle beneath the Arabian Shield.
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2014