Beacon Supergroup rocks of probable Devonian age, containing conglomerate clasts of lithologies unknown in outcrop in southern Victoria Land (SVL) occur in the St Johns Range to Bull Pass Region, SVL, Antarctica. The Lower Taylor Group sedimentary rocks, herein called the Sperm Bluff Formation, unconformably rest on the regionally extensive Kukri Erosion Surface that truncates local basement. The basement complex includes three Plutonic Suites, Dry Valley (DV) 1a, DVIb and DV2 of the Granite Harbour Intrusives that intrude metasedimentary rocks of the Koettlitz Group. Allibone et al. (1993b) suggested a SVL terrane accretion event may have occurred about the same time as accretion of a terrane known as the Bowers terrane in northern Victoria Land (NVL) based on changing chemistry of the CambroOrdovician granitoids. Further, it is suggested that conglomerate clasts of the Sperm Bluff Formation may have been derived from this postulated terrane (Allibone et al., 1993b; and Turnbull et al., 1994). Following extensive fieldwork provenance studies and basin analysis of the sedimentary Sperm Bluff Formation are used here to test these ideas. The Sperm Bluff Conglomerate of Turnbull et al. (1994) is re-interpreted as the Sperm Bluff Formation and described using a lithofacies-based approach. The Sperm Bluff Formation is divided into six lithofacies including 1) Conglomerate Lithofacies; 2) Pebbly Sandstone Lithofacies; 3) Crossbedded Sandstone Lithofacies; 4) Parallelbedded Lithofacies; 5) Low-angle Crossbedded Lithofacies; and 6) Interbedded Siltstone/Sandstone Lithofacies. The intimate field association of the Conglomerate, Pebbly Sandstone and Crossbedded Sandstone Lithofacies ties them to the Conglomerate Lithofacies Association whereas the other three units are independent. The Conglomerate Lithofacies Association is interpreted to represent a wavedominated deltaic environment, based on the presence of broad channels, pervasive crossbedding, paleocurrent and trace fossil data. Both Parallel-bedded and Low-angle Crossbedded Sandstone Lithofacies are interpreted as a foreshore-shore face shallow marine setting on the basis of low-angle crossbeds and trace fossil assemblages. The Interbedded Siltstone and Sandstone Lithofacies is interpreted as an estuarine environment based on alternating siltstone/sandstone beds and the presence of flaser and lenticular bedding, small crossbedded dune sets, mud drapes, syneresis cracks and diverse paleocurrent directions. An estuarine setting is tentatively favoured over a lagoonal setting due to the presence of syneresis cracks small channels and the proximity to a river delta. I suggest that the Sperm Bluff Formation is likely a lateral correlative of the Altar Mt Formation of the Middle Taylor Group, in particular the Odin Arkose Member. This interpretation is based on arkosic nature of the sedimentary rocks, regional paleocurrent patterns, the presence of salmon pink grits at Gargoyle Turrets and trace fossil assemblages. The upper most lithofacies at Mt Suess, the Low-angle Crossbedded Sandstone Lithofacies that only occurs at this site is- suggested as a lateral correlative to the Arena Sandstone, which stratigraphically overlies the Altar Mt Formation, based on quartzose composition, clay matrix, stratigraphic position and trace fossils present. Provenance analysis was carried out on sedimentary rocks and conglomerate clasts using clasts counts of conglomerates, petrographic analysis of clasts, point counts of sandstones and clasts, geochemistry and V-Pb detrital zircon analysis. Conglomerate clasts lithologies include dominantly silicic igneous clasts and finely crystalline quartzite clast amongst other subordinate lithologies such as vein quartz, schist, schorl rock, gneiss and sandstone. Despite past identification of granitoid clasts in the Sperm Bluff Formation (Turnbull et al., 1994), none were found. Rhyolitic clasts of the Sperm Bluff Formation have compositions typical of highly evolved subduction related rocks, although they have undergone post-emplacement silicification. Wysoczanski et al. (2003) date rhyolite and tuff clasts between 497±17 Ma and 492±8 Ma, placing them within error of all three Dry Valley Magmatic Suites and removing the likelihood of correlation to NVL volcanic rocks. Petrographic analysis suggests they are components of a silicic magmatic complex. Chemically the volcanic clasts appear to represent a single magmatic suite (Sperm Bluff Clast Suite), and are clearly related to the Dry Valley Plutonic Suites. Although clasts are not constrained beyond doubt to one Suite, DV2 is the best match. Quartzite clasts of the Sperm Bluff Formation are too pure and old to be derived from a local source. Detrital zircon V-Pb ages for the quartzite suggest zircons were derived from the East Antarctic Craton, and that the quartzite source rocks were deposited prior to the Ross-Delamarian Orogeny. Quartzite with a similar age signature has not been identified; however, the Junction Formation sandstone of northwest Nelson has a similar age spectrum. Sandstones from the Sperm Bluff Formation indicate derivation from a felsic continental block provenance, which contain elements of volcanic, hyperbyssal and plutonic rocks. They are arkosic to quartzose in composition and conspicuously lack plagioclase. Detrital zircon analyses give a strong 500 Ma peak in all 3 samples, characteristic of a Ross-Delamarian Orogen source, with few other peaks. The dominance of a single peak is highly suggestive oflocal derivation. The sedimentary rocks of the Sperm Bluff Formation are interpreted to be derived predominantly from the basement rocks they now overlie. The presence of the regionally extensive Kukri Erosion Surface at the lower contact of the Beacon Supergroup rocks implies an intermediate source must have existed. This most likely contained all components of the formation. I suggest that the DV2 Suite was emplaced in a subsiding, extensional intra-arc setting. Erosion of the uplifted arc region probably occurred from Late Ordovician to Silurian times with deposition of sediments in a subsiding intra-arc basin. Erosion of the rhyolitic complex in this region probably occurred, however, it is likely that some was preserved. Inversion of this basin prior to the Devonian probably provided the means for these sediments to be deposited as the Sperm Bluff Fonnation.

The first part of this dissertation deals with the timing of Mesozoic igneous activity in southern Cordillera of North America and its tectonic implications. A representative section in Santa Rita Mountains is dated using the zircon U-Th-Pb isotopic method. The oldest unit, the lower member of the Mt. Wrightson Formation, is concordantly dated at 210 ± 3 Ma. Initial basaltic andesite to andesite volcanism was followed by deposition of redbeds and associated volcanic rocks that are dated at 200 Ma. Felsic volcanism and eolian sand deposition may have spanned from 190 to 170 Ma. The Piper Gulch Granodiorite, representing the earliest Mesozoic intrusive equivalent, gives concordant dates of 188 ± 2 Ma. A second cycle of andesite and rhyolitic volcanism and sedimentation is dated at 151 ± 5 Ma using the whole-rock Rb-Sr isotopic method. The Hovatter Volcanics in the Little Harquahala Mountains, southwestern Arizona is dated at 165 Ma. Whole-rock Rb-Sr isotopic method on the same rocks gives a coherent reset isochron of 70 ± 3 Ma (Appendix III). A new stratigraphic correlation is proposed based on the dating data. Tectonic models proposed by previous workers to account for what seemed to be the lack of Triassic volcanic rocks are not necessary. This part of the Cordillera was an uplifted arc terrane during the Early Mesozoic (Appendix II) and may have provided volcanic detritus to the Late Triassic Chinle Formation in the Colorado Plateau. The second part of the dissertation deals with magma evolution and crust modification during arc magmatism. Rocks in southeastern Arizona have ƭ(N)(d) values of -3.4 to -6.4, while rocks to the west have ƭ(N)(d) values ranging from -8.5 to -9.2. An ƭ(N)(d) value of +2 for a Jurassic basalt indicates the presence of depleted mantle under the arc. Using lower crust and mantle end-members, 20 to 40% mantle input is estimated. This seems to argue for continuous growth model of the continental crust. Combined REE and isotopic data indicate that assimilation of lower crust by mantle melts followed by fractional crystallization took place. Detailed study indicates that the lower crust along sites of arc magmatism gets progressively hybridized by the mantle, becoming more mantle-like with time.

Equilibrium properties of the MgO-Al₂O₃-SiO₂ (MAS) system are modeled using techniques from statistical and quantum mechanics. The fundamental structural units in this model are the closed shell ions: Mg²⁺, Al³⁺, Si⁴⁺, and O²⁻. The equilibrium properties of the MAS system are determined by the interactions among these ions and by the environment (i.e. temperature and pressure). The interactions are modeled using coulombic, dispersion, and repulsive forces. Two parameters appearing in the repulsive terms for each cation-oxygen interaction are fitted from properties of quartz, corundum, and periclase crystals. The effects of the environment on the liquid and solid compositions found in this system are calculated using a Monte Carlo technique involving the generation of a Markov chain of configurations; each configuration being a "snapshot" of the particles in the liquid or solid material being studied. The properties of the material are derived from averaging appropriate quantities over all the configurations. Enthalpies of formation, heat capacities, and volumes of seven compositions in the MAS system have been calculated using this method. All are within three percent of the corresponding experimental values. Radial distribution functions for these runs show the competition among the cations for the common anion, oxygen, under charge and mass balance constraints. The electronic structure of several molecular clusters in the MAS system are examined using ab initio linear combinations of atomic orbitals (LCAO) techniques. The assumptions used in LCAO calculations are examined and a small, balanced basis set for the MAS system is presented. The Mg-, Al-, and Si-O interactions are all found to be highly ionic using this basic set. Using a first principles technique, the two body effective pair potentials assumed for the Monte Carlo calculations were shown to be physically reasonable.

Six alkalinity and oxidation classes of fresh igneous rocks were correlated with trace elements in rock chip samples from temporally and spatially associated ore deposits. Learning vector quantization and back-propagation artificial neural networks correctly classified 100 percent of whole rock oxides and 99 percent of mineralized samples; discriminant analysis correctly classified 96 and 83 percent, respectively. The high degree of correlation between chemistries of igneous rocks and related mineralization implies genetic links between magmatic processes or sources and the ore deposits studied. The petrochemical classification was evaluated by assigning 43 deposits to classes defined on eight variation diagrams, training neural networks to classify analyses of 569 igneous and 887 mineralized samples, and testing the networks on their ability to classify new data. Whole rock analyses were obtained from mining districts in which trace element geochemistry was also available. Half the data was eliminated using five alteration filter graphs. The K₂O and Fe₂O₃/FeO versus SiO₂ diagrams and iron mineralogy best defined alkalinity and oxidation classes. Neural networks trained with 90, 80, 70, or 50 percent of the samples correctly classified 81 to 100 percent of randomly withheld data. SiO₂/K₂O ratios of alkali-calcic igneous rocks are 14-20 and of calc-alkalic 20-30. Fe₂O₃/FeO ratios are >0.8 with abundant magnetite and sphene for oxidized, 0.5-1.2 with magnetite, sphene, and rare ilmenite for weakly oxidized, and <0.6 with ilmenite only in reduced subclasses. Lead-zinc-silver deposits as at Tombstone and Tintic are related to oxidized alkali-calcic igneous rocks. Polymetallic lead-zinc-copper-tin-silver deposits, such as Santa Eulalia and Tempiute, Nevada, are associated with weakly oxidized alkali-calcic rocks. Tin-silver deposits of Llallagua and Potosi are correlated with reduced alkali-calcic intrusives. Porphyry copper deposits as at Ray and Sierrita are connected with oxidized calc-alkalic plutons. Gold-rich porphyry copper deposits, such as Copper Canyon and Morenci are linked to weakly oxidized calc-alkalic plutons. Disseminated gold deposits, such as Chimney Creek, Nevada, are temporally and chemically correlated with reduced calc-alkalic igneous rocks, although physical connections between plutons and Carlin-type deposits remain unconfirmed. Magma series classification and neural networks have profound applications and implications to exploration, alteration and zoning studies, and metallogenesis.

This study investigates the relationship between the evolution of the igneous and hydrothermal systems in the Cananea mining district located in northern Sonora, Mexico. The Cananea district was chosen for this study because post-mineral uplift and erosion has tilted the Cananea Range ∼15° to the east and exposed an oblique section through approximately 6 vertical kilometers of a mineralized volcano-plutonic system and because porphyry-related stockwork, breccia, and pegmatitic silicate-sulfide mineralization are all well developed and well exposed. Major, trace element, and isotopic data suggest that the Laramide (∼64-56 Ma) igneous rocks represent a cogenetic calc-alkaline magmatic series ranging in composition from gabbro to granite. Neodymium isotope (εNd = +0.7 to -5.7) and strontium isotope (⁸⁷Sr/⁸⁶Sr(initial) = 0.70570 to 0.71037) values show a smooth inverse correlation and combined with the major and trace element data suggest that the Laramide rocks evolved from a mantle-derived parent melt by coupled assimilation and fractional crystallization. Phase petrology, mineral compositions, whole rock geochemistry, and alteration mineralogy indicate water, metal, chlorine, and sulfur content of the magmas increased with increasing differentiation and reached a maximum in late stage differentiates. The mineralized quartz-feldspar porphyries represent the rapid upward emplacement of this enriched differentiate. Porphyry-related mineralization in the Cananea district includes pegmatitic silicate-sulfide mineralization represented by the La Colorada and Maria deposits. Detailed field and geochemical work on the Maria deposit suggests that the fluids, alteration assemblages and paragenetic relations in silicate-sulfide pegmatites are similar to those seen in stockwork- and breccia pipe-hosted porphyry Cu systems, but that silicate-sulfide mineralization represents transitional conditions in which magmatic fluids are concentrated in a small volume and undergo efficient cooling and decompression leading to formation of high-grade telescoped mineralization.

Chemical sediments in a variety of geomorphic environments (pedogenic, fluvial, palustrine, lacustrine and pan) were investigated from seven regions (SW Kalahari, Kgalagadi, Kalkweissrand, Etosha, Linyanti, Okavango and Ngami) in the Kalahari of southern Africa. These primary and multi-phase sediments were characterised by a range of pure and intergrade silcrete, calcrete and dolocrete fabrics which contained an array of structures indicative of the crystalline and biogenic processes responsible for their precipitation, epigenesis and paragenesis. Petrography, mineralogy and isotope geochemistry provided significant insight regarding the environmental and geochemical conditions at the time of precipitation. Petrography indicated that the majority of chemical sediments were undergoing epigenetic modification as a consequence of the desiccation induced transition from phreatic to vadose diagenetic and geochemical conditions. The role of rapidly infiltrating meteoric water, associated with unstable wetting fronts, is believed to be instrumental in vadose diagenesis and the precipitation of crystalline/alpha fabric carbonate. Salinisation within the capillary fringe and deeper vadose zone is believed to be responsible for the sequence dolomitisation of crystalline calcite within mature sequences. Highly saline pan conditions instigate the precipitation of authigenic dolomite, calcite and K-feldspar within the surface sediments and authigenic silica at depth. Phreatic water beneath the islands, floodplains and fluvial systems of the Okavango, which undergoes evapotranspirational and transmission salinisation and ultimately terminal desiccation, are similarly precipitating silcrete. Pedogenic processes principally associated with C₄ vegetation are responsible for the gratification of carbonate mud within desiccating lacustrine, palustrine and pan sediments. Within the thin sandy Kalahari soils, pedogenesis is limited, but biogenic/beta fabric precipitation linked to mycorrihizae and tree/shrub savanna vegetation is instrumental in the formation of hardpans and the modification of upper calcrete horizons. The dominance of a distinct assemblage (smectite/kaolinite or sepiolite/palygorskite) of authigenic clay minerals present within all the environments provides evidence of semi-arid precipitation within Mg and Ca enriched saline/rapidly evaporating water or brackish/reduced permeability environments.

The lavas and pyroclastics of Ascension Island contain a suite of coarse grained igneous blocks which range in composition from olivine-gabbro to peralkaline-granite paralleling, but extending beyond the compositional range of the volcanics. The lavas range from alkali-basalt through hawaiite, trachybasalt, trachyandesite and trachyte to comendite. True basalt is relatively rare and there is a scarcity of analyses with 57 < Si02 < 63 wt %. No high pressure mineral assemblages and hence no possible mantle fragments have been found. Petrographic and isotopic data suggest that a suite of gabbros from Dark Slope Crater crystallised from a magma derived from a MORB-like source. The remaining blocks and all the lavas evolved from magmas derived from a less depleted source. The chemical variation seen in the lavas and blocks is best explained by crystal fractionation mechanisms in a relatively shallow magma chamber. The gabbroic blocks exhibit cumulus textures suggesting that they formed by accumulation of settling crystals. The intermediate to acid blocks compare much more closely in composition with the evolved lavas and are probably their slowly cooled equivalents. There is petrographic evidence that partial melting of intermediate coarse grained material gave rise to melts of granitic composition but these are not chemically equivalent to the acid lavas and blocks. A pegmatoid body crystallised in situ and closed system crystal fractionation alone resulted in a very similar sequence of mineral assemblages to the blocks and lavas and a peralkaline final liquid. High 87 S4/ 86 Sr ratios in the evolved lavas and blocks are attributable to contamination by a small quantity of highly radiogenic oceanic sediment. Comparison with other oceanic volcanoes suggests that these differentiation processes are much less important in determining the evolutionary path of the magma than its apparent starting composition.

No description available.

Deserted and disintegrating barns, houses, and silos have always perplexed me when driving through the country. I am fascinated by how this leisurely decay reveals their structural integrity in a slow, reverse process of construction. It is as if humanity and nature consciously collaborated to create these gigantic memento mori for a steady stream of highway viewers. These monumental tributes to inevitable decline, along with my own adventures in gardening, childrearing, eldercare, and travels, have led me to explore the universal cycles of life. The dilapidated buildings in my work are rendered in a tight, sharp, close-up viewpoint so that the viewer is forced to engage them. I will often layer images of seeds, leaves, and rocks on top of images of houses to symbolize the different stages of the life cycle. I see seeds and buildings as containers and incubators of potential. Any foliage represents a fulfillment of that potential, while rocks stand for the fossilized remains, or the achievements of one’s life accomplishments.

Ph.D. / Preserved Archean crust dominantly recording lower temperature conditions (greenschist to amphibolites facies), the earliest widespread record of ultrahigh- temperature metamorphism occur in the Neoarchean. Considering that, collisional tectonic setting has been postulated as a possible tectonic scenario for the generation of ultrahigh-temperature metamorphism, sites where Archean cratons underwent collision can be potential sites for preservation of ultrahigh-temperature metamorphic granulites. The Limpopo Complex is a high-grade metamorphic terrain considered to have formed by collision in Neoarchean time between the Archean Kaapvaal and Zimbabwe cratons.Detailed petrographic and mineral chemical characterization of representative high Mg-Al granulites from the Southern Marginal Zone, Central Zone and the Northern Marginal Zone – forming the three subzones of the Limpopo Complex – was carried out. Evidence for the preservation of mineral assemblages considered diagnostic of ultrahigh- temperature metamorphic conditions, such as orthopyroxene+sillimanite±quartz, high-Al/(MgTs) orthopyroxene, sapphirine+quartz, spinel+quartz, corundum+quartz and antiperthite, are shown from these high Mg-Al granulites. Most of these mineral assemblages are reported for the first time from the Limpopo Complex. In addition, two unique textures are also reported – one, the discovery of corundum lamellar intergrowth with orthopyroxene from a high Mg-Al granulite from the Southern Marginal Zone, and second, the rare occurrence of sapphirine+quartz post dating orthopyroxene+sillimanite±quartz from two Mg-Al granulites from the Central Zone. Pressure-temperature calculations including representative P-T phase diagrams computed for the bulk compositions of the granulites studied clearly indicate ultrahigh- temperature conditions for all the three subzones. In contrast to two previous studies, one each for the Southern Marginal Zone (~950°C) and the Central Zone (~930°C), this study presents higher temperature estimates of ~1050 to ~1100°C for the three subzones. Together with examples of ultrahigh-temperature metamorphic conditions reported by the two previous studies, this study shows that the ultrahigh-temperature event reported here has affected the length and breadth of the three subzones of the Limpopo Complex. Further, the high-pressure conditions inferred from the early composition of orthopyroxene from the unique orthopyroxene-corundum intergrowth and the P-T phase diagrams computed for representative granulites from the three zones suggest a common high pressure event in all the three sub zones of the Limpopo Complex.