This research is an exploration both of the Precambrian metamorphic geology of south-western Montana and the Wlaser ablation microprobe ^4OAt-³⁹At geochronological technique. Using the high spatial resolution of the UV laser microprobe it has been possible to produce systematic ^4OAt-³⁹Ar ages within individual mica grains. Further, the UV laser has been used to drill into biotite inclusions within garnet to look at the shielding effect of the garnet lattice, and to drill depth profiles into garnet to measure helium diffusion in laboratory experiments. The south-western Montana region of the Precambrian Wyoming Province was believed to have undergone at least three separate periods of metamorphism: M1- granulite facies in the Archaean, M2 - amphibolite facies in the Early Proterozoic, and M3 - greenschist to epidote-amphibolite facies in the Mid-Proterozoic. Because of the relatively low blocking temperature of the 40Ar- 39Ar geochronological system in mica (c.300-350 °C), it is readily reset by regional metamorphic events and previous K-Ar and 4OAr_38 Atgeochronology had found that the Early Proterozoic event dominated the ages obtained. UV laser 40Ar-39At dating of the matrix mica constrained the timing of cooling from the Early Proterozoic metamorphic event to between 1780 to 1740 Ma with a cooling rate between 1 and 8 °C/m.y. 4OAr_39Ar analyses of individual biotite inclusions in garnet also produced similar ages. However, Pb-Pb step leach dating of a small subset of garnet yielded ages between 1808 and 1765 Ma, demonstrating that the garnet did not grow during an Archaean event but, rather, during the Early Proterozoic metamorphic event. Thus, the shielding properties of garnet on biotite inclusions could not be easily assessed in these samples. However, where matrix biotite had been partially reset by the Cretaceous plutons, there was some evidence to suggest that a minority of the biotite inclusions in the same sample had been shielded from resetting. The influence of fractures, defects and other fast diffusion paths is believed to have prevented most of the inclusions from being shielded. No evidence was found to show that the rocks in south-western Montana were metamorphosed during the Archaean and it seems likely that the M1 and M2 events were not greatly separated in time and were both Early ProterozoiC in age. The high spatial resolution of the UV laser microprobe was used in order to date highly altered biotite within rocks that had undergone later (M3) greenschist metamorphism. Biotite was interlayered with chlorite, clinozoisite and prehnite but using the UV laser it was possible to separately analyse areas of unaltered biotite and areas of alteration within a single mineral and produce ages from both. It was thus possible to measure two ages from one sample: an unaltered age consistent with the timing of the Early Proterozoic metamorphism, and a younger age that could be linked to the c.1500-1360 Ma timing of Belt basin formation to the north and west of the region. This technique therefore was able to overcome the difficulties associated with producing meaningful ages from altered samples whilst constraining the timing of the M3 metamorphic event.

The offshore Karoo Supergroup (+2 km thick) of the Seychelles has been divided into five members based mainly on sandstone/mudstone development as revealed by wireline logs and lithological samples. The four lower members are characterized by thick sandstones with minor mudstones and the lowest member distinguished by the presence of occasional bands of lignite; the topmost member has thick mudstones as well as thick sandstones. Members 2 and 3, and 4 and 5 form two upward-fining megacycles in which members 2 and 4 have coarser-grained pebbly, sandstones while the sandstones of 3 and 5 are medium and fine-grained. The sediments are fluvial in origin, possibly braid plain except for the uppermost member which may have formed under a meandering system. Paleogeographic reconstruction invokes a source area in north-east Madagascar shedding debris eastwards towards the Seychelles which at that time (pre-mid-Jurassic) lay between Madagascar to the west, and India to the east. Burial curves are presented from considerations of sediment thickness and texture, structure, including the positions of unconformities and diagenesis as an aid in assessing oil potential. With possible source rocks in overlying marine sediments, hydrocarbon generation may have occurred in late Mesozoic and early Tertiary. Migration could possibly have taken place into fault-juxtaposed Karoo sandstones. For comparative purposes petrographic features of samples from the Elk Point Group Alberta, Canada were studied. This Devonian sequence comprises marginal and marine sediments including carbonates and evaporites, but the sandstones studied come from marginal deltaic deposits. Sandstones studied are remarkably similar in both original composition and diagenetic history. Primary mineralogy was determined by similar source areas - predominantly granitic with minor metamorphic and volcanic components. Despite being deposited in different sedimentary environments both groups show extensive diagenesis involving dissolution of feldspars and lithoclasts, precipitation of quartz, and a second phase of dissolution of feldspar, lithoclasts and quartz. Kaolinite is the dominating clay in the sandstones of Karoo Supergroup up to depths of 10,000 ft, while illite prevails over other clays in the sequence between 10,000-12,500 ft. Illite and chlorite are present in the Elk Point Group: kaolinite is absent. The last stage of cementation in Elk Point sandstones was anhydrite precipitation during Cretaceous times. Precipitation of authigenic cements ended with the formation of carbonates in Karoo sandstones. Cementation, especially by quartz appears to have considerably reduced porosity in many places in both the successions. In both the study areas early diagenesis was controlled by meteoric waters while burial and marine influence are responsible for later diagenetic episodes. The presence of illite and chlorite against kaolinite in the Elk Point Group is due to greater depths of burial than Karoo sandstones. Presence of evaporitic environments lead to the formation of anhydrite in the sandstones of Elk Point Group, while feldspar overgrowths are formed as a result of retention of pore-fluids due to the formation of a closed system controlled by marine flooding surfaces.

This study focuses on the geology of the Nautanen area. It is part of the SGU-financed project, Metamorphic Map of Sweden, which aims to compile metamorphic data from Sweden and takes the form of a number of Bachelor and Master projects. The main metamorphic event in the Nautanen area is the Svecokarelian Orogeny (1.96 – 1.75 Ga). The samples are metamorphosed sedimentary and volcanic rocks, which were intruded by intermediate to mafic intrusions and a later granite intrusion. The supracrustal rocks are folded and the Nautanen Deformation Zone (NDZ) traverses the area in a NW to SE direction. Petrographic studies, XRF analysis and THERMOCALC were used to estimate pressure and temperature and to elucidate evidence of fluid mobility. The average pressure was assumed to be below 4 kbar because of the presence of andalusite. The lowest and highest temperatures for metamorphism were 474±43°C and 681±14°C, with highest temperatures recorded nearest to the granite intrusion. XRF analysis revealed high concentrations of Ba (up to 7000 ppm) in the NDZ. High concentrations of Ba, skarn formation and replacement of garnet by chlorite indicate fluid-controlled metamorphism in the NDZ. The increasing temperature towards the granite intrusion suggests regional or contact prograde metamorphism that need not be related to the NDZ. / Metamorphic Map of Sweden, SGU-project

<p>Samples from the Whitefish Falls Area were examined petrographically and chemically to determine the regional grade of metamorphism. Mineralogy from the Lorrain formation (quartzities) and Gowganda Formation (argillites) were apparently incongrous. The resolution of this problem formed the major part of the study.</p> <p>Mineralogically the quartzites contained a kyanitequartz paragenesis with a subordinate kyanite-chloritoidquartz paragenesis. The argillites showed a chlorite-biotite -plagioclase-calcite-quartz paragenesis with only minor epidote present. From textural evidence possible reactions were deduced which were used to set the upper and lower limits for pressure and temperature. These boundaries were 470°C and 3.9 kb., lower limit and 550°C with a variable pressure for the upper limit.</p> Chemically, whole rock analysis was used to determine the parameters for various chemographic diagrams. AFM, ACF and AKF plots for the argillites showed a very restrictive composition which excluded the almandine field and promoted the biotite field. The bulk composition of the argillites was cited as the main reason for the inhibition of high grade index minerals, thus appearing to be lower grade than their quartzite counterparts. AFM and AKF diagrams for the quartzites verified their mineralogy and proved their high grade nature. / Thesis / Bachelor of Science (BSc)

Granulites are the dense, strong metamorphic rocks that are produced during high- (HT) to ultrahigh-temperature metamorphism (UHT) and partial melting of Earth's crust. Granulites are ubiquitous in exhumed Archean cratons and are thought to comprise much of Earth's stable lower crust. Understanding the mechanisms responsible for crustal heating in Archean terranes is thus paramount to understanding the stabilisation of early continental crust, and whether such mechanisms resemble modern tectonic processes. It is therefore important to quantify the pressure–temperature–time (P–T–t) paths of Archean granulites, as such paths can be diagnostic of heating mechanism. This dissertation explores: 1) novel approaches to reconstructing the P–T–t paths of granulites, and 2) what the deciphered P–T–t paths of rocks from two Archean granulite terranes reveal about Archean crustal heating. The first chapter shows how petrologic modelling at multiple scales from a texturally heterogeneous granulite can provide "snapshots" of the P–T path, which would be difficult to reconstruct otherwise. The remaining chapters are focused on reconstructing the P–T–t paths of two Archean granulite terranes: the Beartooth Mountains, and the Pikwitonei granulite domain (PGD). The second and third chapters present evidence for cryptic HT metamorphism of the Beartooth granulites at ~2.7 Ga characterized by rapid (< 1 Ma) exhumation at HT and fast cooling (~10-100 C/Ma) in the middle crust. This suggests advective/conductive heating over short length-scales. In the fourth chapter, thermobarometric data suggest the western PGD experienced UHT decompression followed by cooling in the lower crust. High-precision zircon and monazite dates reveal apparently episodic crystallization over at least ~24 Ma. This episodicity could reflect multiple thermal cycles or the control of local reactions on zircon/monazite crystallization during cooling. High-spatial resolution petrochronology provides temporal constraints on prograde metamorphism. These data suggest metamorphism in the PGD was driven by a long-lived heat source over large length-scales near the base of the lithosphere. Disparities in the timescales, length-scales, and the depth and amount of heating between the terranes may suggest different crustal heating mechanisms in each, and that the late Archean Earth may have been tectonically diverse. / Ph. D. / Granulites are the dense and strong rocks that are produced during extreme heating, high- (HT) to ultrahigh-temperature (UHT) metamorphism, and partial melting of Earth’s continental crust. Granulites are common in exhumed fragments of Earth’s ancient, Archean (> 2.5 billion yrs old) crust and are thought to comprise much of Earth’s stable lower crust. Understanding the mechanisms responsible for extreme heating of the crust is thus crucial to understanding how Earth’s early continents were stabilised, and how plate tectonic processes have evolved through Earth history. Metamorphic rocks preserve a commonly cryptic record of the path(s) they took through the crust, which are controlled in part by the heating mechanism(s) and tectonic settings involved in their genesis. It is therefore important to quantify the pressure–temperature–time (<i>P– T–t</i>) paths of Archean granulites, as such paths may be diagnostic of particular crustal heating mechanisms. This dissertation explores: 1) novel approaches to reconstructing the pressure– temperature–time (<i>P–T–t</i>) paths of granulites, and 2) what the deciphered <i>P–T–t</i> paths of rocks from two Archean granulite terranes reveal about Archean crustal heating. The first chapter presents a novel way to reconstruct the <i>P–T</i> path of a texturally complex and heterogeneous rock, which reveals a more detailed history than conventional methods would. The second, third, and fourth chapters are focused on reconstructing the <i>P–T–t</i> paths of granulites from two Archean terranes that record HT/UHT metamorphism ~2.7 billion years ago: the Beartooth Mountains, and the Pikwitonei granulite domain (PGD). These studies show that crustal heating of the Beartooth granulites was brief (< 1 million years duration) and seems to have occurred over short length-scales in the middle crust, while crustal heating of the PGD was long-lived (> 24 million years duration) and occurred over large length-scales near the base of the crust. The disparities in the timescales, length-scales, and the depth and amount of heating between the two terranes may suggest different crustal heating mechanisms operated in each, and that the Earth may have been tectonically diverse ~2.7 billion years ago.

Thesis (Ph.D.) - Carleton University, 2002. / Includes bibliographical references (p. 209-215). Also available in electronic format on the Internet.

Thesis (M.S.)--Ohio University, June, 2004. / Includes bibliographical references (p. 66-77).

Thesis (M.S.)--Ohio University, June, 2004. / Title from PDF t.p. Includes bibliographical references (p. 66-77)

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

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