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.

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.

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A thesis submitted in fulfilment of the requirements for the degree of PhD in Geology University of the Witwatersrand / Numerous small base-metal deposits occur in the acidic rocks of the Bushveld Complex, and modern exploration programs are currently re-examining this metallotect in an attempt to refine the current working hypothesis for mineralisation in these granites. The hypothesis proposed for the origin of mineralisation is multifaceted, encompassing both spatial and temporal relationships between at least three episodes of ore formation. The first episode of mineralisation (typified by the Zaaiplaats tin deposit) occurred at relatively high temperatures (>600'C to 4000' C), and resulted in the formation of orthomagmatic cassiterite, scheelite and an early generation of fluorite. At lower temperatures (200°C<T<400°C), where processes were essentially fluid dominated, a mesothermal Cu-Pb-Zn-As-Ag-Au assemblage was deposited (exemplified by the Spoedwel, Boschhoek and Albert copper and silver deposits). A third episode of mineralisation resulted in the formation of an Fe-U-F assemblage and is recognised at several, but not necessanly all, of the deposits examined (for example, the Albert silver deposit). The extended nature of this three-stage paragenetic sequence is considered to reflect widespread mixing between an early fluid derived by H20-saturation of the granitic magma and an external meteoric/connate fluid, circulation of which was stimulated by the long-lived high heat-productive capacity of the Bushveld granites, as well as exhumation of the metallotect; The early high-temperature Sn/W assemblage was precipitated while magmatic fluids dominated the system. With time, the pluton cooled and was subject to regional uplift. Fractures developed, acting as conduits for external fluids of meteoric and/or connate origin. The late magmatic fluids, enriched in incompatible metals (and volatiles), interacted with the latter fluid, resulting in the localised precipitation of a secondary, lower-temperature mineral assemblage (Cu-Pb-Zn) in the zone of fluid mixing. As the external fluid component became progressively more dominant, the paragenesis changed, forming the :final Fe-U-F assemblage. The formation of these three different, temporally separate assemblages is adequately explained in terms of a fluid mixing model, wherein the concentration ofmetaIs and localisation of ore deposits are controlled by lithology and structure. / Andrew Chakane 2018

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Reconnaissance work and high-density sampling of volcanic rocks at the Quaternary Tatara-San Pedro complex (TSPC) in the Southern Volcanic Zone (SVZ) of Chile has yielded one of the most complete eruptive chrono-stratigraphies and comprehensive geochemical datasets of any arc volcano on Earth. The TSPC is a large frontal arc stratovolcano within the SVZ that exhibits a wide compositional diversity of lavas from basalt to rhyolite, covering most of the ranges in major and trace element contents across the SVZ. The TSPC occupies a pivotal position within the SVZ, where it is "intermediate" in terms of geophysical and geochemical characteristics between northern and southern SVZ volcanoes. The large TSPC dataset and stratigraphic control provides a unique opportunity to elucidate magma source heterogeneity and distinguish between contributions from upper mantle, subducted slab, and crust in a volcanic complex overlying relatively thick continental crust. Furthermore, the results of this investigation provide important constraints about the role of various recycled materials in generating the SVZ mantle and implications for along-arc magmatism and geochemical variability. TSPC magmas least impacted by crustal contamination (evolved lavas are filtered out on the basis of > 56 wt.% SiO2 and Rb/Y >1.75) have compositions bounded by three chemically and isotopically distinct mantle-derived end-members. The `prevalent TSPC mantle' end-member, which includes the largest number of analyzed lavas, is interpreted to represent melts of the upper mantle below TSPC that has been modified by long-term subduction. A second end-member shows extreme depletions in incompatible high field strength elements (HFSE) and the lowest concentrations of fluid-immobile incompatible elements, but has the highest aqueous fluid-mobile/immobile element ratios at the volcano (e.g., Sr/Nd and Pb/Ce). The source of these `low HFSE' magmas is `prevalent TSPC mantle' that experienced previous melt extraction, followed by more recent melting due to infiltration of solute-rich fluid from the subducting basaltic Nazca oceanic crust. A third end-member is enriched in incompatible elements and has the lowest Nd-Hf and highest Sr isotope ratios. This `TE enriched' end-member has common chemical characteristics with behind-the-arc basalts, indicating derivation from trace element-enriched behind-the-arc South American mantle that has been advected trenchward into the convecting mantle wedge. Determining the composition and relative input of slab-derived components to the SVZ mantle wedge has remained elusive for the last 2+ decades because of inadequate datasets and the controversial role of crustal contamination within the thick Andean continental crust. The `prevalent TSPC mantle' magmas, which best represent melts of the subduction-modified mantle wedge composition beneath the TSPC, provide important constraints on both the composition of the "pre-subduction" mantle and geochemical modifications by way of subduction. Mass-balance modeling suggests that the source of `prevalent TSPC mantle' magmas has been generated by a two-stage, three component mixing process. Isotopic and trace element evidence indicate that ~7-11% bulk subducted Chilean trench sediment has been added to an `E-MORB-like' pre-subduction mantle composition. This mantle mixture is further infiltrated by ~4% solute-rich fluid derived from the subducted Nazca basaltic oceanic crust. Trace element patterns of end-member `prevalent TSPC mantle' magmas are best fit by a two-stage partial melting model whereby the residual mantle, after a small degree melt extraction (F = 0.1%), undergoes F = 22% partial melting. The high melt fraction appears to potentially correspond with large volumes of solute-rich fluid released from the subducted Mocha Fracture Zone (MFZ). Although geochemical attributes of mafic TSPC magmas suggest that they are all derived from the same general mantle framework operating below the complex, one lava sequence appears to deviate. The mantle origins of the Upper Placeta San Pedro Sequence (UPSPS) have remained elusive since first being studied. It is a well-characterized basaltic lava series that erupted over a short interval at ~235-240 ka with highly variable incompatible element abundances and a large xenocrystic cargo. The new comprehensive chemical and radiogenic isotope (Sr-Nd-Pb-Hf) dataset, along with stratigraphic control and understanding of the larger-scale geochemical variability at the TSPC, provides fresh perspectives about the mantle sources and evolution of UPSPS magmas. While the UPSPS magmas are derived from the same sources as other TSPC magmas, they have undergone a unique petrogenetic evolution. This is evident from decoupled trace element-isotopic trends that are difficult to reconcile with other mafic TSPC magmas. Based on the Nd-Hf isotope ratios and trace element ratios, the two UPSPS unit magmas are derived from depleted-`TE enriched' and `prevalent TSPC mantle' sources. High ratios of aqueous fluid-mobile/immobile elements, such as high Pb/Ce and Sr/Nd, as well distinctive Sr and Pb isotope ratios, indicate that the UPSPS magmas were generated through fluxing of the mantle wedge, already depleted by melt removal by an solute-rich fluid derived from the subducted Pacific oceanic crust as well as the overlying trench sediment, which caused it to melt.

Mantle xenoliths allow us to investigate the geochemical and dynamic evolution of the mantle beneath the western margin of Antarctica and reconstruct a timeline of geologic events that are obscured on the surface. For this study, mantle xenoliths, brought to the surface by recent volcanism, were collected along a transect from the rift shoulder and into the rift basin in the western margin of the West Antarctic Rift System (WARS), thus providing a recent snapshot of the lithospheric mantle after major episodes of rifting. The second chapter of my thesis focuses on determining the age and persistence of the mantle within the rift. The rhenium-osmium (Re-Os) isotope system has proven to be an invaluable tracer of the tectonic history of the lithospheric mantle and can constrain the age of melt extraction and subsequent stabilization of the lithospheric mantle. This allowed us to track the age of the lithospheric mantle across this rifted margin. Os isotopes, combined with major element compositions, reveal widespread Paleoproterozoic (1.7-2.4 Ga) stabilization of the lithosphere and subsequent preservation, suggesting the lithosphere has dynamically thinned in response to rifting. Major element data allowed us to place temperature (T) constraints on the mantle and characterize the thermal history in the WARS. This study also revealed the oldest lithosphere ages recorded in Antarctica (3.3 Ga) and is the first to report ages that coincide with adjacent crustal ages, thus confirming the coupled relationship between the lithospheric mantle and continental crust. An integral factor controlling the composition of magmas generated at Earth’s surface is the composition of the SCLM. Magmas generated at depth must pass through it, and subsequently may take on geochemical signatures of the lithosphere, or may leave behind geochemical imprints of the migrating magma in the SCLM. Trace elements provide a means to investigate both the depletion and re-enrichment history of the SCLM. The third chapter of my thesis investigates the metasomatic overprinting of the Paleoproterozoic SCLM. Metasomatism, which is the chemical alteration of a rock by a migrating melt and/or fluid, leaves behind diagnostic signatures of the metasomatizing agent (e.g. subduction related fluids or carbonated melts). This can occur cryptically, where a melt percolates through the rock, changing the composition of the rock, but not the lithology. Modal metasomatism produces new mineral phases that are not typically expected in the rock. In xenoliths, trace elements enable us to decode geochemical signatures, and determine the sources of metasomatism. The WARS lithosphere has experienced varying degrees of re-enrichment, broadly characterized by low high field strength element (HFSE) abundances and rare earth element (REE) enrichments that correspond with carbonatite metasomatism. In addition, the presence of secondary hydrous phases (e.g. amphibole and phlogopite) imparted distinct geochemical signatures, revealing that the SCLM beneath the WARS was modified by reactive porous flow with an evolving metasomatic fluid/melt. Widespread Cenozoic rift-related volcanism (<20 Ma) is observed throughout the western margin of the East Antarctic Craton. It has been proposed that the Cenozoic basaltic volcanism in the region of our study site originated from a SCLM source that had been metasomatized during subduction along the paleo-Pacific margin of Gondwana, and subsequent extension in the WARS during the Late Cretaceous (~90 Ma). The fourth chapter of my thesis utilizes strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopes to date depletion and refertilization events in the lithosphere, as well as understand the role of the SCLM in the formation of WARS volcanism. Together with lithologic features (e.g. presence of hydrous phase additions), Sr and Nd isotopic ratios in WARS xenoliths provide a geochemical link to the Cenozoic rift-related magmatism, and supports the SCLM’s role in the formation of diffuse alkaline magmatism throughout the region. Lu-Hf isotope model ages add a constraint on the timing of melt depletion, and establish a relationship between depleted and refertilized domains. Sr isotopes constrain a genetic link between the metasomatized Archean lithosphere sampled on the rift shoulder and the highly radiogenic character of the Ferrar flood basalts, and indicate long-term storage of subduction modified mantle domains in the SCLM. The Sm-Nd isotope system is variably overprinted by metasomatism throughout the WARS. The most highly metasomatized location produces a well-correlated isochron that indicates that the SCLM acquired its trace element metasomatic signature about 130 Ma ago, during the late stages of subduction along the paleo-Pacific margin of Gondwana.

Magma recharge events and subsequent mixing processes are understood to precede volcanic eruptions. Textural evidence of intrusion of hot, mafic magma into a cooler, rheologically locked silicic magma is commonplace. Solidified "blobs" of injected magma, called enclaves, are evidence of magma mixing, but the petrological and mechanical conditions during their formation are debated. Mount Hood, Oregon consistently erupts andesite bearing compositionally similar enclaves. These enclaves are evidence of mingling and mixing of two magmas. However, due to the compositional similarity between enclave and host lava (e.g. ~1-5 wt.% difference in SiO2), it is unclear whether the preserved enclaves represent; 1) partially hybridized mafic melt remaining after mixing with significant crystal exchange with the host magma or 2) the preserved remnants of the intruding magma during recharge, with no homogenization or crystal exchange with the host magma. The aim of this study is to understand how and why enclaves form in compositionally similar host magmas, such as those at Mount Hood. Building off previous research, we utilize a combination of field observations, chemical analyses, and numerical modeling to constrain the rheology of the magmas prior to and during mixing. The degree of magma mixing is dependent on the viscosity contrast between the host and intruding magmas. Since these magmas are similar compositionally, variations in other magmatic properties such as crystallinity, and therefore temperature, and density may drive the viscosity differences between the host and intruding magmas needed for enclave formation. The enclaves at Mount Hood are vesicular (13-28%), coarse-grained; made up of mainly groundmass crystals (200-450 µm) with sparse microlites (< 200 µm), glass (450 µm) proportions, and rarely contain quenched margins. Additionally, crystals within the host magma show preferential alignment along the margins between host and enclave, suggesting a fluid behavior of the host magma during mixing. Based on textural and compositional evidence, we hypothesize that the intruding magma was buoyant, viscous, and crystalline, due to decompression-induced crystallization and exsolution of volatiles, during recharge and ascent to the shallow magma reservoir. Injection and underplating of the viscous crystalline intruding magma into a hot convecting host magma induces enclave formation. Crystallization temperatures differ by only 6-15 °C between host and enclave lavas, derived by the two pyroxene geothermometry method by Putrika (2008). These crystallization temperatures are consistent with crystallization in compositionally similar magmas. However, with such similar crystallization and liquidus temperatures, maintaining a viscosity contrast between the mixing magmas for enclave survival after formation suggests other properties, apart from temperature, must explain the viscosity contrast needed for enclave survival after enclave dispersal and thermal equilibration occurs. The presence of bubbles, from exsolution during crystallization, within the enclave magma increases the viscosity while simultaneously decreasing the density. Therefore, the presence of bubbles increases the viscosity of the intruding magma and maintains the viscosity contrast during the mixing process after thermal equilibration occurs. Additionally, if degassing occurs, rapid crystallization maintains the high viscosity of the enclaves. The enclaves observed at Mount Hood represent the solidified remnants of the last recharge event prior to eruption. The presence of compositionally similar enclaves and host lavas suggest a transient precursor event just prior to eruption at Mount Hood and can be applied to other recharge-driven arc volcanic systems.

Thesis (M.S.)--University of Hawaii at Manoa, 2004. / Includes bibliographical references (leaves 159-185). Also available via World Wide Web.