A dissertation submitted to the Faculty of Science, University of Witwatersrand in the fulfillment of the requirements for the degree of Master of Science (Geology) 2017 / The LG-6 chromitite layer is the thickest (0.90 to 1.20 m thick) chromitite layer in the Lower Group chromitites of the Bushveld Complex and is of economic significance owing to the relatively high Cr-content. It can be traced across the entirety of the western limb and is mined in both the western limb and the eastern limb. This study evaluates previously published models of chromitite formation using data from the LG-6 chromitite at Ruighoek Mine, western Bushveld Complex. Data includes petrographic studies of the reef and host rocks, whole rock analysis of the silicate host rocks and reef, and mineral chemistry for orthopyroxene, olivine and Cr-spinel using electron probe microanalysis (EPMA). In the Ruighoek region the LG-6 chromitite comprises up to 95 vol. % chromite and is typically hosted by orthopyroxenite. Borehole data indicated an area (about 250 m2 in size) where the LG-6 is entirely hosted by harzburgite (42% orthopyroxene, 11% Cr-spinel, 14% olivine, 32% serpentine and 1% other) rather than orthopyroxenite. The whole rock and mineral chemistry revealed that the LG-6 chromitite in this area has an exceptionally high Cr/Fe ratio, up to 2.1. The whole rock data also indicated several compositional reversals in terms of MgO, Al2O3, Cr2O3, FeO, and Mg# (Mg/ [Mg+Fe2+]) for the unenriched borehole 13R-3, and compositional reversals in Cr/Fe and Cr# (Cr/ [Cr+Al]) for the enriched borehole 13R-9 upwards through the chromitite layer. The hanging wall harzburgites are characterized by an increase in Mg# for the mineral chemistry of the Cr-spinel, orthopyroxene and olivine compared to those in the footwall harzburgite. Importantly, spatial 3D modelling of borehole data at Ruighoek mine (19 drill-cores) indicates that the elevated Cr/Fe ratio in LG-6 chromitite is coincident with a depression in the topography of the chamber floor at the time of formation of the LG-6 chromitite. These data are difficult to reconcile with existing models for chromitite formation in layered intrusions, such as the models for gravity settling, addition of a Cr-spinel crystal-laden magma, or a pressure increase. Thus, this work has developed a new model for formation of the LG-6 chromitite at Ruighoek Mine. The exceptionally high Cr/Fe ratio of LG-6 chromitite and its close association with harzburgite is attributed to multiple replenishments of the chamber by relatively primitive magmas. These are inferred to either be saturated in olivine and chromite, or chromite alone. The occurrence of relatively primitive rocks within the depression is suggested to be related to a local feeder situated within the depression. Injection of new, relatively dense magma pulses from the feeder are inferred to spread out across the chamber floor as basal flows owing to compositional stratification of the resident magma at the time of development of the LG-6 chromitite. The replenishing magmas contributed to the existing compositional stratification in the chamber, resulting in the most primitive composition within the depression of the chamber floor. Subsequent crystallisation of the most primitive magmas within the depression resulted in local development of LG-6 chromitite with exceptionally high Cr/Fe ratios together with the enclosing harzburgitic rocks. The thickness of the LG-6 chromitite is attributed to continuous replenishment by large volumes of new, chromite-saturated, magmas via the feeder channel located in the depression. This study suggests that magma stratification and the replenishment of the chamber by chromite-saturated magmas played an important role in the development of the chromitite layers of the Bushveld Complex. / MT 2018

Thesis advisor: J. Christopher Hepburn / The Nashoba terrane is a highly metamorphosed and sheared Paleozoic tectonic block in eastern Massachusetts. The metamorphic rocks that compose the terrane are intruded by a series of diorites, tonalites, and granites. The Andover Granite is a complex multiphase granitic suite found in the northern part of the Nashoba terrane and is composed of both foliated and unfoliated granites as well as a granodiorite phase. The Sgr Group of granites is a series of unfoliated granites exposed along the Nashoba-Avalon terrane boundary. New crystallization ages for the foliated Andover Granite and the Sudbury Granite, southernmost body of the Sgr Group of granites, are presented. CA-TIMS U-Pb geochronology on zircons collected from these granites yielded 419.43 ± 0.52 Ma and 419.65 ± 0.51 Ma crystallization ages for the foliated Andover Granite and a 420.49 ± 0.52 Ma crystallization age for the Sudbury Granite. Geochemical and petrographic analysis of these granites indicate that the foliated Andover Granite is a high-K calc-alkaline, peralmuminous, S-type, biotite + muscovite granite and the Sudbury granite is high-K calc-alkaline, metaluminous to slightly peraluminous, I-type, biotite granite. These two granites are interpreted to have formed from the anatexis of either Nashoba terrane metasedimentary rocks and/or its underlying basement just prior to the Acadian orogeny. It is proposed that when Silurian diorite/tonalite magmas intruded into the Nashoba terrane, the influx of magmatic heat was sufficient to trigger crustal melting and promote granite genesis. This petrogenetic scenario fits well with regional tectonic models showing the Silurio-Devonian convergence of Avalonia towards Ganderia (which formed the eastern side of composite Laurentia at the time) in the northern Appalachians. Prior to the collision of Avalonia to composite Laurentia, mafic and intermediate composition arc magmas intruded the eastern Ganderian margin. The large amount of heat that accompanied these intrusions is believed to have contributed to Acadian metamorphism and influenced the formation of granitic plutons along the margin. It is therefore proposed that the plutonic record of the Nashoba terrane shows that by the Late Silurian - Early Devonian, Avalonia was still outboard of Laurentia in the vicinity of southern New England. / Thesis (MS) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

>Magister Scientiae - MSc / The late- to post-collisional Cape Granite Suite (CGS) located in the southwest of South Africa is comprised of S-, I-, and A-type granites, mafic intrusives, and volcanic flows. The CGS is interpreted to have formed during the closing of the Adamastor Ocean during the Late-Proterozoic to Early-Cambrian. Recently, the S-type granites have received much attention concerning their petrogenesis and sources. However, the I- and A-type granites remain poorly understood and little studied. Therefore, with new geochemical and isotopic data the petrogenesis, sources, and tectonic settings of I- (Vredenburg Batholith) and A-type (Cape Columbine) granites of the CGS form the focus for this study. The major and trace element data presented in this thesis show that the granites from the Vredenburg Batholith are weakly peraluminous to metaluminous, ferroan, and alkali-calcic. Associated with the granites are metaluminous, magnesian, and calc-alkalic igneous enclaves. Formerly, the granites have been interpreted to have formed by fractionation. However, with new geochemical analyses and reassessment of such models, it can be shown that such processes are incompatible with accounting for the chemical variation displayed by the granites and their enclaves. Moreover, the I-type granites and enclaves exhibit positive linear trends between whole-rock major and trace elements vs. maficity (Fe + Mg), which can be explained by co-entrainment of peritectic and accessory phases. The lithogeochemical characteristics of the enclaves and host granite reflect melting of a heterogeneous source. Moreover, the granite and enclave"s ?Nd(t) values reflect melting of Paleoproterozoic-aged crustal sources. Finally, with tectonomagmatic discrimination diagrams, it can be shown that the tectonic setting of the granites indicates a transition from a collisional to extensional regime which corroborates the inferences of previous studies. The Cape Columbine Granites lithogeochemical characteristics are ferroan, calc-alkalic and weakly peraluminous. They show typical A-type granite characteristics in having high silica content, high Na + K values, REE enrichment as compared to S- and I-type granites and strong negative Eu anomalies. For this thesis, it can be shown that anatexis of quartzofeldspathic protolith in an extensional regime produced the chemical variation of the Cape Columbine Granite. Moreover, their isotope ratios are typically radiogenic, indicative of a crustal origin. With this new geochemical data evidence is provided against and in support of previous inferences made about the petrogenesis of the I- and A-type granites of the CGS. This also betters our understanding of the magmatic processes involved in the construction of the CGS over time.

Typescript. / Digitized by Kansas Correctional Industries

Thesis (Ph.D.), Memorial University of Newfoundland, 2000. / Bibliography: leaves R-1-R-17. Also available online.

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The Lac Guyer greenstone belt in the James Bay region of New Quebec was a zone of active volcanism in Archean times and constituted part of a system of "rifts" known as the La Grande superbelt. The rifting of continental crust initiated with the deposition of a sequence of volcanics dominated by pillowed basalts. An overlying volcanic sequence began with the deposition of rhyodacites and other felsic volcaniclastics, intercalated with mafic flows and tuffs. These, in turn, are overlain by peridotitic and pillowed pyroxenitic komatiite flows and then by pillowed basalts. The rocks in this Archean belt have experienced at least two periods of deformation and attained amphibolite facies. / The early voluminous basalts of the first sequence probably reflect the eruption of steady state liquids from periodically replenished, periodically tapped, magma chambers located near the interface of an early crust and Archean mantle. With time the mafic crust overlying these magma chambers, melted to produce tonalitic-granodioritic magmas. These magmas rose and experienced amphibole fractionation. Residual liquids produced by this fractionation erupted as rhyodacites contemporanously with later basalts. The eruption of komatiitic liquids indicates a major failure of the crustal screen enabling primitive magmas to reach the surface. The spectrum of primitive komatiitic compositions reflects variable degrees (15 to 40%) of partial melting of a garnet lherzolite in the Archean mantle. Pyroxenitic komatiite magmas represent the least degree of melting with garnet remaining in the mantle residue. Garnet was completely consumed, however, in the more extensive melting involved in the production of the peridotitic komatiite magmas. As the volcanic pile of the second volcanic sequence grew crustal magma reservoirs were re-established. Primitive komatiitic magmas held in these reservoirs underwent extensive crystal fractionation to yield residual steady state liquids which erupted as the upper basalts of the second volcanic sequence.