The Late Archean Magmatic Event (LAME), ca. 2.7 Ga, was the greatest crustal addition event in Earth history. My focus is the Superior Province of Canada, where LAME occurred ca. 2.75 – 2.65 Ga. Mantle plumes impinged on the Abitibi subprovince, where ~ 16 km regional thickness of tonalite-trondhjemite-granodiorite (TTG) melt was produced. Granites (sensu stricto) were the last magmatic phase of LAME, with a Superior-wide regional thickness of ~ 1 – 3 km. Assuming a crustal source for both TTG and granites, I use 1-D thermal models to investigate the origin of TTG in the Abitibi subprovince and that of late granites in the Superior Province. Melting curves appropriate to the source of TTG and granites are used to determine the thickness of melt produced in the models. I show that the incorporation of upward melt transfer into a standard model of lower crustal melting may increase the amount of predicted melt by ~ 1/(1-f), where f denotes the fraction of melt that is on average being extracted from the source rocks. Partitioning of heat producing elements between melt and restite reduces the amount of melt produced, but the effect is secondary compared to the increase in melt production through upward melt transfer. For the Abitibi subprovince, I show that the emplacement of a single plume coupled with the emplacement of a 12-km-thick greenstone cover can generate a maximum of ~ 9-km-thickness of TTG melt. However, the emplacement of a series of plumes, each coupled with the emplacement of a 3-km-thick greenstone cover and a 10-km-thick sill results in ~ 20-km-thickness of TTG melt. My model incorporates delamination of restitic eclogite. Finally, I show that late granites in the Superior Province may have resulted from thickening of a crust that had been “pre-heated” during earlier arc activity and that prolonged granitic magmatism observed in some areas of the Superior Province may be explained by late underthrusting of fertile source rocks into deeper and hotter regions of the crust.

What tectonic processes were operating in the Archean, and whether they were similar to the “modern-style” plate tectonics seen operating today, is a fundamental question about Archean geology. The Superior Province is the largest piece of preserved Archean crust on Earth. As such it provides an excellent opportunity to study Archean tectonic processes. Much work has been completed in the southern part of the Superior Province. A well-documented series of discrete, southward younging orogenies related to a series of northward dipping subduction zones, has been proposed for amalgamating this part of the Superior Province. The tectonic evolution in the northwestern Superior Province is much less constrained, and it is unclear if it is related to the series of subduction zones in the southern part of the Superior Province, or if it is related to an entirely different process. Such ideas need to be tested in order to develop a concise model for the Meso – and Neoarchean tectonic evolution of the northwestern Superior Province. To this end, a field mapping, U-Pb geochronology, Nd isotope, and lithogeochemistry study was undertaken in the Island Lake greenstone belt. This granite-greenstone belt is part of the northern margin of the North Caribou terrane, a larger reworked Mesoarchean crustal block located in the northwestern Superior Province. U-Pb TIMS zircon geochronology data shows that the Island Lake greenstone belt experienced a long and complex geological history that included the deposition of three distinct volcanic assemblages at ca. 2897 Ma, 2852 Ma, and 2744 Ma, as well as a younger clastic sedimentary group, the Island Lake group. All of these volcanic assemblages include felsic and mafic volcanic rocks, as well as a suite of contemporaneous plutonic rocks. The U-Pb data set shows that the Savage Island shear zone, a regional fault structure that transects the Island Lake greenstone belt, is not a terrane-bounding feature as correlative supracrustal assemblages are observed on both sides of it. The Nd isotope data shows that the volcanic assemblages and contemporaneous plutons have been variably contaminated by an older ca. 3.0 Ga crustal source. The mafic volcanic rocks in the assemblages have two distinct geochemical signatures, and show a pattern of decreasing crustal contamination with decreasing age. Together these data suggests that the Meso – and Neoarchean volcanic assemblages are part of an intact primary volcanic stratigraphy that were built on the same ca. 3.0 Ga basement and have autochthonous relationships with each other. This basement is the North Caribou terrane. The youngest sedimentary group in the belt, the Island Lake group, was deposited between 2712 Ma and 2699 Ma. It consists of “Timiskaming-type” sedimentary rocks, and is the youngest clastic sedimentary package in the belt. A detailed study of detrital zircons in units from the stratigraphic bottom to the top of the sedimentary group indicates an age pattern of detrital zircons that is most consistent with a scenario in which sediments were deposited in inter-diapiric basins created by diapirism and sagduction (i.e., vertical tectonic) processes. During the diapiric ascent of the felsic material, inter-diapiric basins were formed in the synclines between adjacent domes, into which sediments were deposited. U-Pb zircon TIMS geochronology identified two ages of deformation in the Island Lake greenstone belt. Two dykes that crosscut an older, D1 foliation place a minimum age of ca. 2723 Ma on the D1 deformation, and two syn-kinematic dykes date movement along two transpressional shear zones to 2700 Ma. Together all these data indicate that the tectonic evolution in the Island Lake greenstone belt and in the northwestern Superior Province took place in three main stages. The first two stages involved the generation of Meso – and Neoarchean volcanic assemblages and contemporaneous plutonic rocks due to southward dipping subduction under the North Caribou micro-continent. The third stage involved the deposition of late “Timiskaming-type” sediments during vertical tectonic processes in conjunction with horizontal tectonic movement along late transpressional shear zones at ca. 2.70 Ga. At the end of this process the North Superior superterrane was terminally docked to the North Caribou terrane along the North Kenyon fault. This study shows that while a version of horizontal or “modern” style plate tectonics were operating in the Archean, vertical tectonic processes were also occurring and that these processes operated synchronously in the Neoarchean.

The origin of tourmaline nodules, bizarre spherical to irregular textures documented worldwide, remains a geologic mystery. Although previously described by numerous researchers, the physical and chemical parameters that govern their formation have yet to be resolved. Commonly containing tourmaline, quartz, and occasionally feldspar, nodules are surrounded by a halo of leucocratic host rock, and are typically eight to ten centimeters in diameter. Tourmaline nodules of the present study are contained within the Paleoproterozoic Scrubber Granite of the southern Gascoyne Complex in Western Australia. </p> <p>This study integrated field observations, nodule petrography, tourmaline crystal chemistry, tourmaline fluid inclusion analyses, whole rock chemistry of nodule cores, leucocratic halo zones and host granite zones, stable and radiogenic isotope signatures of tourmaline separates as well as comparisons with other tourmaline nodule studies to propose the most scientifically sound theory for the formation of tourmaline nodules in the Scrubber Granite. </p> Numerous nodule morphologies, including spherical and C-shaped nodules, along with other features such as tube-like nodules and tourmaline veins occur in massive, porphyritic, foliated and sheared phases of the Scrubber Granite. Microscopically, tourmaline displays prismatic, sub-rounded and massive textures. Microthermometric studies completed on tourmaline fluid inclusions revealed that the nodule-forming fluid contained 14 to 15 weight percent NaCl + CaCl2. Based on stable isotope studies and homogenization temperatures, fluid temperatures were constrained between 450 and 700¢ªC. The ¥ä18O and ¥äD concentrations of the nodule-forming fluid at this temperature range plot above the typical magmatic water field. Epsilon Nd values indicate that the tourmaline nodules of the Scrubber Granite may have been disturbed by a later metamorphic event.</p>Tourmaline nodules of the Scrubber Granite are herein proposed to have formed from the exsolution and rise of buoyant pockets or bubbles of volatile fluid derived from the crystallizing Scrubber Granite magma.

The Barry gold deposit is an example of an Archean greenstone-hosted lode gold deposit located in the Urban-Barry greenstone belt in the Abitibi subprovince of Québec, Canada. Auriferous zones are spatially associated with NE-trending ductile shear zones with moderate south-easterly dip. Gold mineralization occurs within albite-carbonate-quartz veins that are straight N64ºE/64ºSE and folded N20ºE/60ºSE and within the surrounding carbonate-quartz-pyrite and locally biotite-carbonate alteration zones of the host mafic volcanic rocks. The deposit has gold resources indicated at 52,300 oz (385,000 mt at 4.23 g/t Au) and inferred at 126,600 oz (966,000 mt at 4.07 g/t Au). The host mafic volcanic rocks are part of the 2717 Ma Macho Formation that exhibit a geochemical signature transitional between mid-ocean and island arc. They are cut by pre-ore diorite, pre- and post-ore quartz-feldspar porphyry (QFP), and quartz monzonite dikes and plugs interpreted to have formed in a volcanic arc to syn-collisional setting. The auriferous veins comprise 5-15% volume of the mafic volcanic rocks, are 1-5cm wide, and locally pinch and swell or are boudinaged. Although the volcanic units strike N55-60ºE and dip 40ºSE, the ore envelope (>2 g/t Au) is constrained from surface to 30m depth in an antiformal shape. Free gold is found in albite-carbonate-quartz veins, syn-mineralization altered host rocks, and locally within quartz veins cutting early QFP dikes. The timing of gold mineralization at the Barry deposit is well constrained by U-Pb zircon dating of pre-mineralization diorite and post-mineralization QFP dikes. Analyses of single zircon grains by thermal ionization mass spectrometry (TIMS) give concordant and overlapping data with indistinguishable ages, yielding an average age of 2697 ± 0.6 Ma that is interpreted as the age of gold mineralization at the Barry deposit. This date indicates that gold mineralization was coeval with regional deformation and magmatism, and is, to our knowledge, the most precise age yet established for Archean lode gold mineralization. This confirms that the Barry lode gold deposit formed during an earlier, pre-2686, deformational period in the late Archean, similar to what was documented in the Kiena, Norlartic and Siscoe (Main Zone) mines in the Southern Abitibi greenstone belt. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2009-05-12 17:18:25.925

A suite of silica-undersaturated syenites outcrops along the margins of a monzogranite pluton emplaced in the Knee Lake greenstone belt at Cinder Lake, Manitoba. Alkali-feldspar syenitic pegmatite probably represents a cumulate unit derived from magma that subsequently evolved to fine-grained feldspathoid syenites. This evolution involved an increase in the degree of undersaturation from cancrinite-nepheline syenite to vishnevite syenite. Abundant calcite veinlets, showing a carbonatitic isotopic and trace-element signature, crosscut the pegmatite and are interpreted to have evolved from the syenites. The monzogranite and syenites gave similar radiometric ages (~2.72 Ga), but evidence of fenitization in the former suggests that the syenites are somewhat younger. The emplacement of these rocks was coeval with the collision of the North Caribou and North Superior superterranes during amalgamation of the Superior Province. The observed transition from granitic to alkaline magmatism is interpreted to mark the transition from a collisional to post-collisional tectonic regime.

A suite of silica-undersaturated syenites outcrops along the margins of a monzogranite pluton emplaced in the Knee Lake greenstone belt at Cinder Lake, Manitoba. Alkali-feldspar syenitic pegmatite probably represents a cumulate unit derived from magma that subsequently evolved to fine-grained feldspathoid syenites. This evolution involved an increase in the degree of undersaturation from cancrinite-nepheline syenite to vishnevite syenite. Abundant calcite veinlets, showing a carbonatitic isotopic and trace-element signature, crosscut the pegmatite and are interpreted to have evolved from the syenites. The monzogranite and syenites gave similar radiometric ages (~2.72 Ga), but evidence of fenitization in the former suggests that the syenites are somewhat younger. The emplacement of these rocks was coeval with the collision of the North Caribou and North Superior superterranes during amalgamation of the Superior Province. The observed transition from granitic to alkaline magmatism is interpreted to mark the transition from a collisional to post-collisional tectonic regime.

What tectonic processes were operating in the Archean, and whether they were similar to the “modern-style” plate tectonics seen operating today, is a fundamental question about Archean geology. The Superior Province is the largest piece of preserved Archean crust on Earth. As such it provides an excellent opportunity to study Archean tectonic processes. Much work has been completed in the southern part of the Superior Province. A well-documented series of discrete, southward younging orogenies related to a series of northward dipping subduction zones, has been proposed for amalgamating this part of the Superior Province. The tectonic evolution in the northwestern Superior Province is much less constrained, and it is unclear if it is related to the series of subduction zones in the southern part of the Superior Province, or if it is related to an entirely different process. Such ideas need to be tested in order to develop a concise model for the Meso – and Neoarchean tectonic evolution of the northwestern Superior Province. To this end, a field mapping, U-Pb geochronology, Nd isotope, and lithogeochemistry study was undertaken in the Island Lake greenstone belt. This granite-greenstone belt is part of the northern margin of the North Caribou terrane, a larger reworked Mesoarchean crustal block located in the northwestern Superior Province. U-Pb TIMS zircon geochronology data shows that the Island Lake greenstone belt experienced a long and complex geological history that included the deposition of three distinct volcanic assemblages at ca. 2897 Ma, 2852 Ma, and 2744 Ma, as well as a younger clastic sedimentary group, the Island Lake group. All of these volcanic assemblages include felsic and mafic volcanic rocks, as well as a suite of contemporaneous plutonic rocks. The U-Pb data set shows that the Savage Island shear zone, a regional fault structure that transects the Island Lake greenstone belt, is not a terrane-bounding feature as correlative supracrustal assemblages are observed on both sides of it. The Nd isotope data shows that the volcanic assemblages and contemporaneous plutons have been variably contaminated by an older ca. 3.0 Ga crustal source. The mafic volcanic rocks in the assemblages have two distinct geochemical signatures, and show a pattern of decreasing crustal contamination with decreasing age. Together these data suggests that the Meso – and Neoarchean volcanic assemblages are part of an intact primary volcanic stratigraphy that were built on the same ca. 3.0 Ga basement and have autochthonous relationships with each other. This basement is the North Caribou terrane. The youngest sedimentary group in the belt, the Island Lake group, was deposited between 2712 Ma and 2699 Ma. It consists of “Timiskaming-type” sedimentary rocks, and is the youngest clastic sedimentary package in the belt. A detailed study of detrital zircons in units from the stratigraphic bottom to the top of the sedimentary group indicates an age pattern of detrital zircons that is most consistent with a scenario in which sediments were deposited in inter-diapiric basins created by diapirism and sagduction (i.e., vertical tectonic) processes. During the diapiric ascent of the felsic material, inter-diapiric basins were formed in the synclines between adjacent domes, into which sediments were deposited. U-Pb zircon TIMS geochronology identified two ages of deformation in the Island Lake greenstone belt. Two dykes that crosscut an older, D1 foliation place a minimum age of ca. 2723 Ma on the D1 deformation, and two syn-kinematic dykes date movement along two transpressional shear zones to 2700 Ma. Together all these data indicate that the tectonic evolution in the Island Lake greenstone belt and in the northwestern Superior Province took place in three main stages. The first two stages involved the generation of Meso – and Neoarchean volcanic assemblages and contemporaneous plutonic rocks due to southward dipping subduction under the North Caribou micro-continent. The third stage involved the deposition of late “Timiskaming-type” sediments during vertical tectonic processes in conjunction with horizontal tectonic movement along late transpressional shear zones at ca. 2.70 Ga. At the end of this process the North Superior superterrane was terminally docked to the North Caribou terrane along the North Kenyon fault. This study shows that while a version of horizontal or “modern” style plate tectonics were operating in the Archean, vertical tectonic processes were also occurring and that these processes operated synchronously in the Neoarchean.

Archean greenstone belts typically form narrow sheared basins separating bulbous tonalo-trondjhemo-granodioritic (TTG) batholiths. The role played by gravity in the development of such dome-and-keel structures is a key question in Archean tectonics. The Pukaskwa batholith - Hemlo shear zone (HSZ) is a representative example of the dome-and-keel structures that are common in Archean terrains. This region has received considerable attention because the HSZ hosts several major gold deposits that are currently being mined. Late dextral strike-slip kinematics of the HSZ are well recorded by abundant strain markers in greenstone rocks, whereas the quartzofeldspathic coarse-grained rocks of the Pukaskwa batholith bear no macroscopically visible fabric. The goal of this study is to understand the structural history of this greenstone belt-batholith system. The Pukaskwa batholith is a heterogeneous assemblage of TTG gneisses bounded by the Hemlo greenstone belt to the north. The density of the Pukaskwa batholith rocks (density = 2700 kg/m3) is on average less than that of the Hemlo greenstone rocks (density = 3000 kg/m3). Since Archean geotherms were considered higher than modern equivalents, the effective viscosity of the TTG rocks might have been sufficiently low to allow their diapiric ascent through denser greenstone rocks. Alternatively, the emplacement of the TTG batholith might have been driven primarily by transpressive tectonics. The anisotropy of magnetic susceptibility (AMS) provides valuable information on the internal fabric of the Pukaskwa batholith. This study provides the kinematic information needed to support either the diapiric or the transpressive tectonic model. AMS recorded east-west trending prolate and plano-linear fabrics across the northern section along the contact, suggesting that transpressional forces from the Hemlo shear zone affected the emplacement of the Pukaskwa batholith. Away from the contact, fabrics are generally flattened, indicative of doming through diapiric processes. Also, in order to fully evaluate the diapiric hypothesis, it is necessary to obtain reliable data on rock densities across the Pukaskwa batholith. The density of about 360 representative specimens from the Pukaskwa batholith has been measured and will constitute a valuable database for future gravimetric investigations by mining companies. The significant degree of correlation between high-field magnetic susceptibility and density in the Pukaskwa batholith should be taken into account in geophysical exploration in Archean terrains, only as a proxy for iron content.

abstract: Banded iron formations (BIFs) are among the earliest possible indicators for oxidation of the Archean biosphere. However, the origin of BIFs remains debated. Proposed formation mechanisms include oxidation of Fe(II) by O2 (Cloud, 1973), photoferrotrophy (Konhauser et al., 2002), and abiotic UV photooxidation (Braterman et al., 1983; Konhauser et al., 2007). Resolving this debate could help determine whether BIFs are really indicators of O2, biological activity, or neither. To examine the viability of abiotic UV photooxidation of Fe, laboratory experiments were conducted in which Fe-bearing solutions were irradiated with different regions of the ultraviolet (UV) spectrum and Fe oxidation and precipitation were measured. The goal was to revisit previous experiments that obtained conflicting results, and extend these experiments by using a realistic bicarbonate buffered solution and a xenon (Xe) lamp to better match the solar spectrum and light intensity. In experiments reexamining previous work, Fe photooxidation and precipitation was observed. Using a series of wavelength cut-off filters, the reaction was determined not to be caused by light > 345 nm. Experiments using a bicarbonate buffered solution, simulating natural waters, and using unbuffered solutions, as in prior work showed the same wavelength sensitivity. In an experiment with a Xe lamp and realistic concentrations of Archean [Fe(II)], Fe precipitation was observed in hours, demonstrating the ability for photooxidation to occur significantly in a simulated natural setting. These results lead to modeled Fe photooxidation rates of 25 mg Fe cm-2 yr-1—near the low end of published BIF deposition rates, which range from 9 mg Fe cm-2 yr-1 to as high as 254 mg Fe cm-2 yr-1 (Konhauser et al., 2002; Trendall and Blockley, 1970). Because the rates are on the edge and the model has unquantified, favorable assumptions, these results suggest that photooxidation could contribute to, but might not be completely responsible for, large rapidly deposited BIFs such those in the Hamersley Basin. Further work is needed to improve the model and test photooxidation with other solution components. Though possibly unable to fully explain BIFs, UV light has significant oxidizing power, so the importance of photooxidation in the Archean as an environmental process and its impact on paleoredox proxies need to be determined. / Dissertation/Thesis / Masters Thesis Biochemistry 2017

abstract: Archean oxidative weathering reactions were likely important O2 sinks that delayed the oxygenation of Earth’s atmosphere, as well as sources of bio-essential trace metals such as Mo to the biosphere. However, the rates of these reactions are difficult to quantify experimentally at relevantly low concentrations of O2. With newly developed O2 sensors, weathering experiments were conducted to measure the rate of sulfide oxidation at Archean levels of O2, a level three orders of magnitude lower than previous experiments. The rate laws produced, combined with weathering models, indicate that crustal sulfide oxidation by O2 was possible even in a low O2 Archean atmosphere. Given the experimental results, it is expected that crustal delivery of bio-essential trace metals (such as Mo) from sulfide weathering was active even prior to the oxygenation of Earth’s atmosphere. Mo is a key metal for biological N2 fixation and its ancient use is evidenced by N isotopes in ancient sedimentary rocks. However, it is typically thought that Mo was too low to be effectively bioavailable early in Earth’s history, given the low abundances of Mo found in ancient sediments. To reconcile these observations, a computational model was built that leverages isotopic constraints to calculate the range of seawater concentrations possible in ancient oceans. Under several scenarios, bioavailable concentrations of seawater Mo were attainable and compatible with the geologic record. These results imply that Mo may not have been limiting for early metabolisms. Titanium (Ti) isotopes were recently proposed to trace the evolution of the ancient continental crust, and have the potential to trace the distribution of other trace metals during magmatic differentiation. However, significant work remains to understand fully Ti isotope fractionation during crust formation. To calibrate this proxy, I carried out the first direct measurement of mineral-melt fractionation factors for Ti isotopes in Kilauea Iki lava lake and built a multi-variate fractionation law for Ti isotopes during magmatic differentiation. This study allows more accurate forward-modeling of isotope fractionation during crust differentiation, which can now be paired with weathering models and ocean mass balance to further reconstruct the composition of Earth’s early continental crust, atmosphere, and oceans. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2020