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The Tillotson Peak Complex (TPC) in northern Vermont records high-pressure (HP) subduction zone metamorphism that occurred during the Ordovician Taconic Orogeny, and subsequent retrograde metamorphism and deformation that occurred during the Silurian Salinic Orogeny. Previous studies have documented a polymetamorphic history, with peak metamorphic pressures possibly up to 2.5 GPa and temperatures of 550°C. Prior to this research, constraints on the timing of metamorphism in the TPC were limited to a single Middle Ordovician 40Ar/39Ar total fusion age for glaucophane. This study integrates 40Ar/39Ar step heating analyses of multiple mineral phases and U-Pb dating of titanite with field and microstructural observations to further constrain the subduction–exhumation history of the TPC. Microstructural and petrologic analyses in thin section on samples of felsic gneiss, pelitic schist, amphibolite, and blueschist suggest deformation during varied P-T conditions. The earliest and highest-pressure metamorphic event documented in the TPC samples is associated with inclusions in garnet and white mica in S1 quartz microlithons. Inclusions of paragonite, titanite, and omphacite in garnets, locally defining S1, suggest that some blueschist may have formed in the retrograde path in association with the S2 foliation. A greenschist-facies metamorphic overprint in most samples is also associated with S2, primarily defined by epidote, white mica, and chlorite. E-W trending F2 intrafolial folds are commonly rootless in outcrop, locally defined by blueschist–eclogite-facies fold noses. Kinematic indicators relative to S2 and L2 stretching lineations give a predominantly top-to-the-E shear sense. S3 crenulation cleavage development is related to folding about E-W trending F3 folds that define the map pattern of the Tillotson Peak Complex. Locally developed S4 crenulations are axial planar to the NW-trending Gilmore Antiform. Additionally, D4 deformation and metamorphism is recorded by microfractures in garnet and epidote, as well as chlorite pseudomorphs after garnet. 40Ar/39Ar step heating of multiple phases and U-Pb dating of titanite yielded ages corresponding with the Taconian to the Salinic orogenies. Ages of ~485–480 Ma are attributed to prograde–peak metamorphism (M1) and S1 development. Ages that span ~471–456 Ma are interpreted to document retrograde M2 metamorphism through greenschist to locally blueschist-facies metamorphic conditions during exhumation and S2 development. Correlation of D3 microstructures in these samples with map-scale folds suggest that E–W trending folds developed in the range of ~455–445 Ma, recorded by minimum apparent ages in the field area, and locally as plateau ages along the margin of the TPC. Younger ages ~435–405 Ma are observed locally in apparent age gradients, and are interpreted to reflect metamorphic overprinting that resulted in the chlorite pseudomorphs after garnet and the growth of actinolite, which may be related to the timing of folding about the Gilmore Antiform. Results presented here suggest the impact of Acadian retrograde metamorphism and deformation on rocks of the TPC may be less significant than previous work suggests.

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The behavior of U in near surface sedimentary environments has changed throughout geologic time. There is a marked shift in U (IV) mineral stability and deposit style at approximately 2200 Ma due to changes in the concentration of O in the atmosphere. For example, prior to 2200 Ma, fluvial U deposits could form and U(IV) were stable in surface and near-surface environments. In modern, anthropogenic sedimentary systems such as U tailings, U (IV) minerals are not stable and readily oxidize to U (VI) minerals. In addition, U is much more mobile in modern sediments relative sediments that were deposited prior to 2200 Ma.

The behavior of U in near surface sedimentary environments has changed throughout geologic time. There is a marked shift in U (IV) mineral stability and deposit style at approximately 2200 Ma due to changes in the concentration of O in the atmosphere. For example, prior to 2200 Ma, fluvial U deposits could form and U(IV) were stable in surface and near-surface environments. In modern, anthropogenic sedimentary systems such as U tailings, U (IV) minerals are not stable and readily oxidize to U (VI) minerals. In addition, U is much more mobile in modern sediments relative sediments that were deposited prior to 2200 Ma.

Recent regional bedrock mapping carried out on the Hall Peninsula, Baffin Island, Nunavut has revealed previously unknown rock units, structural relationships and metamorphic conditions achieved. Mafic and ultramafic rocks occur primarily intercalated with metasedimentary rocks in the east-central region of the Hall Peninsula. Focused mapping was carried out in the Quaqqanituaq Area (QA), now the type locality for metasedimentary-mafic/ultramafic occurrences on the Hall Peninsula. The QA records an east-west compression event, D1, which produced in the dominant regional S1 fabric and tight, nearly isoclinal F1 folds. The QA also records a north-south compression event, D3 (regional D2 event not observed in QA) resulting in an S3 crenulation cleavage and open F3 folds. U-Pb detrital zircon geochronology and U-Pb geochronology of a cross-cutting monzogranite dyke bracket sediment accumulation and mafic/ultramafic intrusion/extrusion between 2.13 and 1.87 Ga and place a maximum age on D1 at 1.87 Ga. Garnet – biotite – K-feldspar – ± sillimanite mineral assemblages observed in QA pelites indicate that upper amphibolite facies metamorphic conditions were reached. This observation is empirically confirmed by the implementation of winTWQ software which indicates that the peak pressure and temperature conditions reached in the QA were 4.8±2.1 kbars and 645±50°C. Whole-rock major and trace element geochemical data was obtained for 75 mafic and ultramafic samples from all regions of the Hall Peninsula including 20 from the QA. Mafic samples were classified on the basis of N-MORB normalized trace element geochemistry as being alkaline (La/Sm ~ 4.9; Gd/Yb ~ 2.7), calc-alkaline (La/Sm ~ 4.3; Gd/Yb ~ 2.4), tholeiitic (La/Sm ~ 2.1; Gd/Yb ~ 1.1), or transitional (La/Sm ~ 2.7; Gd/Yb ~ 1.6). A negative Nb anomaly, which is on average 5.4 times lower with respect to Th and Ce is observed in all mafic samples with the exception of alkaline mafic rocks. On the basis of their geochemical profiles and comparative study of adjacent terrains, the mafic rocks are concluded result from partial melting a subduction modified mantle source during plume-initiated rifting of the North Atlantic Craton.

187Re-187Os analysis of magnetite and other oxide minerals from various well dated geological settings (kimberlites, lamprophyres, Manicouagan impact melt rock, Great Bear magmatic zone iron ore deposits and Laramie Anorthosite complex) worldwide have been used to investigate the potential for precise geochronology. 187Re-187Os isotopes in groundmass oxide minerals from kimberlites and lamprophyres are not suitable for emplacement geochronology due to low Re contents and large quantities of un-radiogenic Os. TRD and TMA ages can be calculated however, and correlate well with previously published depletion ages and large magmatic events in the overlying crust, indicating the effectiveness of this technique. Geochronology using magnetite from the remaining settings is im-precise due to very low Re and Os contents but the ages produced are in the correct area. Young ages produced from the Laramie anorthosite complex indicate that magnetite may have a low closure temperature to Os diffusion (~200-300C).

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V-Th-Pb fractionation trends, and the associated variations in Pb-isotopes, provide important constraints on the nature, and age, of a wide range of geological processes. This thesis presents three detailed case studies from a carbonatite intrusive complex, lower crustal granulite xenoliths, and young volcanic rocks which represent a range of processes that fractionate V, Th and Pb. New major, trace element and Sr, Nd and Pb isotope results are presented for carbonatites and pyroxenites from the 130 Ma old Jacupiranga complex in southern Brazil. The data preclude simple models in which the Sr-Nd and Sr-Pb isotope arrays are the result of crustal contamination processes, or liquid immiscibility between the carbonatites and magmas similar to those from which the pyroxenites crystallised. The initial Sr, Nd and Pb isotope ratios in the Jacupiranga complex are similar to those in the Parana high-Ti basalts and to the oceanic basalts of the Walvis Ridge and Tristan da Cunha, and were apparently inherited from the mantle source regions associated with incipient magmatism of the Tristan da Cunha hotspot and the opening of the South Atlantic. Most carbonatites have high V/Pb and low Rb/Sr ratios and infiltration and/ or metasomatism by such melts is one process that may be responsible for the negative correlation of U/Pb and Rb/Sr inferred for the source of certain oceanic basalts such as Tristan da Cunha. Correlations between trace and major elements in mostly mafic granulite xenoliths from southern Africa probably result from magmatic processes such as fractional crystallisation and crystal accumulation. From Pb and Nd isotope systematics it can be inferred that the northern Lesotho xenoliths are Proterozoic in age, whereas Cape province xenoliths appear to have been fonned in the Archaean and to have had their Nd isotopes reset in the Proterozoic. Most Pb isotope ratios from the granulites xenoliths are unradiogenic relative to the Geochron, which indicates relatively low V /Pb ratios in the later parts of their histories. The correlation between Pb isotope ratios and K₂O/A1₂O₃ in the granulites from Markt is considered to reflect magmatic control of the V /Pb fractionation. The Archaean, and some of the younger mantle derived rocks on the Kaapvaal craton, plot on similar Pb isotope trends. It is argued that segments of the crust and uppennost mantle beneath southern Africa stabilised together in the Archaean, and that both were remobilised in subsequent magmatic and orogenic events. Volcanic rocks in the Northland-Auckland peninsula, New Zealand, range from silica-undersaturated basanites to tholeiites and andesites. Although all the volcanic fields are in intraplate settings, two types of mantle source enrichment can be recognised. One is subduction-related and characterised by high Sr and low Nd isotope ratios and negative Nb anomalies. The other type has geochemical features characteristic of intraplate enriched mantle sources with Sr and Nd isotope ratios and Nb/Ba, Ba/La, similar to HIMV OIB, which has been attributed to the deep recycling of oceanic crust. The differences in Pb isotopes between the two mantle sources, i.e. higher /lB/4 and 7/4, but lower ²⁰⁶Pb/²⁰⁴Pb in the subduction-related material than in the inferred HIMU source, indicate that V/Pb in shallow recycled crustal material tend to be lower than in the deeper recycled material. These case studies confirm that (i) metasomatism by small degree melts can have significant effects on mantle trace element characteristics, (ii) the lower crust is a relatively low 11 environment and crystal accumulation may be an important mechanism in the fractionation of Th and U from Pb, cUld (iii) incorporation of recycled crustal material at different levels can result in different V-Th-Pb fractionation and so, with time, different Pb isotope ratios.