Geology

The Cree Lake Zone in the southwestern part of the Hearne craton, reworked during the Paleoproterozoic Trans-Hudson orogeny, includes the Wollaston Group, a sequence of Paleoproterozoic metasedimentary rocks. As these rocks have been multiply deformed and metamorphosed to high-grade, a geochemical and isotopic study was initiated to see through the overprinting events to better constrain the provenance and evolution of the Wollaston Group.

Polydeformed high-grade rocks of the Beaverlodge domain provide insights into the multi-stage tectonic history of southwestern Rae Province. Structural analysis indicates that the Murmac Bay Group (MBG) occupies the highest structural level in the domain and was translated over older, deeper level rocks during Taltson(-Thelon) orogeny, which peaked at ~1.93 Ga. Two subsequent episodes of deformation, at 1.90 Ga and 1.80 Ga, are respectively linked to tectonic activity along the Snowbird tectonic zone (STZ) and the terminal stages of Trans-Hudsonian orogeny. The results presented herein indicate that tectono-metamorphic effects of the Taltson orogeny extend eastward across the Rae Province from the Taltson domain to the STZ. They also highlight fundamental differences in crustal history of potentially equivalent rocks in the Beaverlodge and Tantato domains. Whereas Archean rocks of the Tantato domain underwent high-P metamorphism at 2.6 to 2.5 Ga and again at ca. 1.9 Ga but remained ...

Coronite derived from olivi-neand Fe-Ti-oxide-bearing diabase or gabbro is common in high-grade metamorphic terr-anes, butits progressive development from completely unmetamorphosed protolith has rarely been fully do"um"nted. Otvine diabase of the 1.24-Ga Sudbury continental dyke swarm, exposed across the Grenville Front iouthwest of Sudbury, Ontario, provides su,ch docnmentation. Progressive textural and chemical changes are recorded by primary igneous and secondary metamorphic mine14l5, in rocks whose bulk chemistry has not changed, from unmetamorphosed diabase to coronitic metadi;base in which metamorphic conditions approached the granulite facies (>700oC at >8.0 kbar) during Grenvillian orogeny at-1.0 Ga. Southeast-trending Sudbury dykes are undisturbed in the Southern and Superior province foreland of the GrenviJle orogen, but approaching the Grenville Front are offset by faults and locally contoin greenschist-grade metamorphic assemblages. mforneO metadiabase dykes in the uplifted margin of the Grenville orogen, known fromiock chemistry and U-Pb age of primary baddeleyite to be correlative with the Sudbury swarm, display reactions of both olivile and Fe-Ti oxide with plagioclase, leading to classic coronite development within a few kilometers of the front. Textural relationships indicate that peak metamorphism outlasted early deformation, compatible with a prograde P-T-t path. Of particular interest is comparison of the reactions between Fore olivine and An66 plagioclase (xenocrysts) and Fo' and Aq, (matrix) in the same rocks, and increase of Ti in biotite, Na and Al in clinopyroxene toward higher grade. Comparison of metamorphism in the dykes and their host rocks indicates telescoping of pre-Grenvillian isograds across the front.

The Red Lake mine trend, a deformation zone in the Archean Red Lake greenstone belt that hosts the world-class Campbell-Red Lake gold deposit, is characterized by abundant foliation-parallel iron-carbonate ± quartz veins with banded colloform-crustiform structures and cockade breccias overprinted by silicification and gold mineralization. There is an apparent incompatibility between the cavity-fill structures of the veins and breccias (typically developed at shallow crustal depths) and the upper greenschist to lower amphibole facies metamorphic conditions recorded in the host rocks (indicating relatively deep environments). This, together with the development of veins along the foliation plane, represents an enigmatic problem that may be related to the interplay between fluid dynamics and stress field. We approach this problem through systematic study of fluid inclusion planes (FIPs) in the vein minerals, including the orientations of the FIPs and the pressure-temperature conditions inferred from fluid inclusion microthermometry. We find that fluid inclusions in the main stage vein minerals (pregold mineralization ankerite and quartz and syn-ore quartz) are predominantly carbonic without a visible aqueous phase, whereas many inclusions in the postore stage contain an aqueous phase. Most FIPs are subvertical, and many are subparallel to the foliation. High fluid pressure coupled with the high wetting angles of the water-poor, carbonic fluids may have been responsible for the abundance of brittle deformation features. The development of subvertical FIPs is interpreted to indicate episodic switching of the maximum principal compressive stress (r 1) from subhorizontal (perpendicular to the foliation) to subvertical (parallel to the foliation) orientation. The subvertical r 1 is favorable for the formation of foliation-parallel veins, as fractures are preferentially opened along the foliation in such a stress regime, the origin of which may be linked to the fluid source.

The Proterozoic Athabasca Basin in Canada is known for its world-class, high-grade uranium deposits developed near the unconformity between the basin and the basement. The Douglas Formation in the upper part of the preserved strata in the basin contains total organic carbon (TOC) of up to 3.56 wt.%. Previous studies of organic matter maturation and hydrocarbon inclusions indicate that oil and gas have been generated from this formation, and that some hydrocarbons found in the unconformity-related uranium deposits were derived from the Douglas Formation. This study aims to evaluate how the oil and gas generation processes may have affected the fluid overpressure development in the basin, and whether or not the hydrocarbons generated in the Douglas Formation could migrate downward to stratigraphically lower intervals and eventually to the sites of mineralization near the unconformities. We carried out a series of numerical experiments to examine fluid overpressures, flow directions, temperatures, and oil and gas generation processes using a two-dimensional conceptual model derived from a geologic cross-section from the basin center to the eastern margin. An additional 5 km strata were added to current basin stratigraphy to account for observed paleogeothermal data. Variation studies were undertaken to account for uncertainties in the lithologies of the eroded strata, and the wide ranges of possible permeabilities of different lithologies and kinetic parameters of oil and gas generation. It is found that, if moderate permeabilities are used in the modeling for each lithology (known as the base model), oil and gas generation processes contribute little to the development of fluid overpressure, and fluid pressure in the basin is close to hydrostatic regardless of whether or not hydrocarbon generation in the Douglas Formation is included in the modeling. However, if permeabilities are assigned values one order of magnitude lower than in the base model, significant fluid overpressures are developed in the eroded strata in the upper part of the model. In the base model, oil generated in the Douglas Formation may migrate downward, driven by an overpressure zone situated above the Douglas Formation, but gas migrates upward. In the low-permeability model, however, the overpressures developed above the Douglas Formation are so high that both oil and gas generated in the Douglas Formation migrate downward. The numerical modeling results thus indicate that it is hydrodynamically possible for oil and gas generated in the Douglas Formation to migrate to the base of the basin and reach the sites of the unconformity-related uranium deposits.

In the Southern Province, Ontario, southeast-striking diabase dykes of the 1235 Ma Sudbury continental swarm intrude folded Huronian strata and Paleo-and Mesoproterozoic plutons. The dykes are truncated at the Grenville Front, southeast of which irregularly oriented metadiabase dykes, correlative in age and chemical composition with the Sudbury swarm, cut gneissic fabrics in the Grenville Province. Some irregularities in dyke trend, including marked deflections in strike at and southeast of the Grenville Front mylonite zone, are primary features, resulting from dyke propagation across pre-existing structure and an associated change in paleostress field. However, other irregularities, in particular the markedly sinuous traces of many dykes, analogous to classic buckle fold forms, are attributable to superimposed (Grenvillian) deformation. The deformation history of the dykes is constrained by a distinctive sequence of macroscopic and microscopic structures. Early deformation, related to northwest-southeast compression and accompanying overthrusting, resulted in layer-parallel shortening and buckling of the dykes in a regime with elements of both pure and simple shear, a process accommodated internally by alignment, bending and kinking of plagioclase and other primary minerals. Coeval metamorphism, manifested by reaction coronas around primary olivine and Fe-Ti oxide, outlasted early deformation. A later period of deformation resulted in tightening of the buckle folds, superseded by faulting, in response to further strain accumulation. The equivalent microscopic-scale structures are late fractures and brittle-ductile microfaults which postdate plastic strain. A cause-and-effect relationship, whereby pre-metamorphic microstructures strongly influenced the location and development of syn-to post-metamorphic microstructures, suggests that the two phases of dyke deformation represent more or less a continuum, separated only by a period of decreased strain rate coinciding with the peak of metamorphism. In relation to existing models for the tectonic development of the northwestern Grenville orogen, the progression of dyke structures and their relationship to metamorphism are best explained by three principal stages: (1) at > 1035 Ma, an early stage of penetrative shortening and overthrusting, correlated with buckling of the dykes and their depression to lower crustal levels; (2) an intermediate stage, commencing some time before 1020 Ma, when compressive stresses waned, enabling thermal relaxation, and ductile extension took place in the interior of the orogen, initiating passive uplift of the dykes; (3) at 1010-980 Ma, a stage of renewed thrusting during which the dykes were further deformed as they were uplifted across the ductilebrittle transition, documenting the final advance of the Grenville orogen toward its foreland.

The Cigar Lake uranium deposit in the Athabasca Basin is the world's second largest high-grade unconformity-related uranium deposit. Its distinct geological architecture includes heterogeneous basement lithologies, a local basement high and sub-vertical faults. The sub-vertical faults are classified further into two sub-types: faults that are restricted to the basement, termed 'basement faults', and faults that are distinctly related to post-Athabasca fault reactivation, in that they extend upward into the sandstone, termed 'extended basement faults'. This study aims to evaluate the effects of the aforementioned geological factors on the fluid flow patterns under two different driving forces, buoyancy due to variation of fluid density or 'thermal convection' and deformation or a combination of them, and to determine the most probable fluid flow scenarios for the formation of the deposit. The numerical results show that fluid flow is strongly affected by the two types of faults. While the basement faults represent fluid paths with complex fluid flow patterns, depending on the driving forces, providing favourable physical conditions for different chemical processes, such as fluid-fluid and fluid-rock interactions, the extended basement faults enhance the permeability of an E-W corridor within the sandstone and significantly strengthen the overall upwelling flow above the basement faults, promoting sandstonehosted mineralization. The numerical results also suggest that the main deposit likely formed via fluid convection during tectonically quiet periods, although faulting played a critical role in increasing permeability, in turn, enhancing thermal convection.

The Taltson magmatic zone is inferred to be the southern extension of the Thelon tectonic zone; together they are thought to have developed during a ca. 2.02-1.90 Ga orogeny that accreted the Rae and Slave cratons. The continuity between the Thelon tectonic and Taltson magmatic zones is predicated upon the similarity of the ages of igneous rocks and a linear aeromagnetic high that apparently links the two. The Taltson magmatic zone contains two suites of metaplutonic rocks: ca. 1.986-1.1959 Ga continental arc type plutons; and 1.955-1.910 Ga peraluminous plutons. They intruded a basement complex dominated by Mesoarchean to Paleoproterozoic orthogneisses and granitoid rocks. Peak metamorphic conditions were between 1.94 and 1.93 Ga. New mapping south of the western Athabasca Basin indicates that plutons of generally intermediate composition extend from the Virgin River shear zone beneath the western Athabasca Basin. The limited available geochronological data supports that interpretation as rocks with ages similar to the continental arc type plutons in the Taltson magmatic zone are found near the Virgin River shear zone, beneath the western Athabasca Basin and along basin"s western margin. Plutons of similar age and composition to the peraluminous plutons of the Taltson magmatic zone and similar metamorphic ages are found in the basement to the western Athabasca Basin and the exposed Shield to the south. These data indicate that the Taltson magmatic zone extends into Saskatchewan, and is not spatially related to the Taltson aeromagnetic high, which represents the signature of the Taltson basement complex and not the igneous rocks of the Taltson magmatic zone. The wide distribution of rocks similar in age to those of the Taltson magmatic zone suggest that the Thelon orogeny is more complex than implied in most published models and likely included multiple magmatic arcs and accretionary events. This inference is supported by differences structural characteristics and metamorphic ages along the proposed orogenic belt.

A paleolimnological and environmental reconstruction of the volcanic Lake Boquete, in the northern highlands of Panama, was produced based on diatom, sediment, and charcoal analyses. The main aim of this study is to contribute to the understanding of the long-term anthropogenic effect in Lake Boquete and its watershed, a unique space (Gran Chiriquí region) where human occupation and nature have interacted since ~7000 yr BP (years Before Present). Our results indicate two main periods of environmental change. In the first period, from ~2840 to 1740 cal yr BP (calibrated years Before Present), the lake was under the influence of high-energy flow regimes and high erosion rates. Water pH was gradually changing from neutral to alkaline. There’s evidence of fires with four main events, some appear to be natural as indicated by the proxies, while others seem to be associated with anthropic causes most likely related to intense periods of cultivation; a particularly intense fire event is identified at ~2607 cal yr BP. We infer that during this period the lake and its watershed were under intense human impact. In the second period, from ~1740 to 197 cal yr BP, the lake was under a steady low-energy flow regime, submerged vegetation was more abundant, and pH became more acidic, fire events were less frequent. We infer that these conditions were the result of land abandonment due to human migration to the lowlands. Notwithstanding intense and frequent dry climate spells during the second period, Lake Boquete water levels were not affected, and thus, this record suggest that the main cause of environmental and limnological change in Lake Boquete and its watershed has been anthropogenic activities.

Keywords: Paleolimnology, late Holocene, diatoms, charcoal, sediment, Panama.

Lake Victoria (LV), Africa’s largest lake is situated in the African Great Rift Valley. Due to its shallowness (max.68 m; mean 40 m) and limited river inflow, LV is very sensitive to variations in climate and lake level fluctuations. As a result, LV has undergone repeated low stand periods, or even complete desiccation during the Late Pleistocene with profound effects on the aquatic ecosystem. One example is the emergence of a unique biodiversity of endemic cichlid species following the lake’s last desiccation event during the last glacial and subsequent refilling commencing ~15,000 years ago.

Fire regimes differ across tropical and subtropical biomes depending on multiple parameters whose interactions and levels of importance are poorly understood, particularly at multidecadal and longer time scales. In the catchment of Lake Victoria, savanna, rainforest, and Afromontane vegetation have interspersed over the last 17,000 years, which may have influenced the fire regime and vice versa. However, climate and humans are most often the primary drivers of fire regime changes, and analysing their respective roles is critical for understanding current and future fire regimes. Besides a handful of radiocarbon dates on grassy charcoal, the timescales of published studies of Lake Victoria sediment chronologies rely mostly on dates of bulk sediment, and chronological disagreements persist, mainly due to variation between estimations of the 14 C reservoir effect. Here, we provide independent 14 C chronologies for three Late Glacial and Holocene lacustrine sediment cores from various water depths and compare them with the biostratigraphy to establish a new chronological framework. We present the first continuous sedimentary charcoal records from Lake Victoria; these suggest that fire activity varied substantially during the past 17,000 years. Our new pollen records reveal the long-term vegetation dynamics. The available evidence suggests that before human impact increased during the Iron Age (ca. 2400 yr BP), biomass burning was linked to climate and vegetation reorganizations, such as warming, drying, and the expansion of rainforests and savannas. Our results imply that climate can trigger substantial fire regime changes and that vegetation responses to climate change can co-determine the fire regime. For instance, biomass burning decreased significantly when the rainforest expanded in response to increasing temperatures and moisture availability. Such insights into the long-term linkages between climate, vegetation, and the fire regime may help to refine ecosystem management and conservation strategies in a changing global climate.

The emplacement age of the Muntele Mare Variscan granite (Apuseni Mountains, Romania)Like the Alps and Western Carpathians, the Apuseni Mountains represent a fragment of the Variscan orogen involved in the Alpine crustal shortenings. Thus the more extensive Alpine tectonic unit in the Apuseni Mountains, the Bihor Autochthonous Unit is overlain by several nappe systems. During the Variscan orogeny, the Bihor Unit was a part of the Someş terrane involved as the upper plate in subduction, continental collision and finally in the orogen collapse and exhumation. The Variscan thermotectonic events were marked in the future Bihor Unit by the large Muntele Mare granitoid intrusion, an S-type anatectic body. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating yielded a weighted mean age of 290.9 ± 3.0 Ma and a concordia age of 291.1 ± 1.1 Ma. U-Pb isotope dilution zircon analyses yielded a lower intercept crystallization age of 296.6 + 5.7/-6.2 Ma. Thes...

Despite the rise of marine diatoms in the world's oceans throughout the Cretaceous, only a handful of fossil localities worldwide detail invasion of freshwater habitats by diatoms commencing in the Late Cretaceous. We report on the occurrence of numerous freshwater diatom specimens and species from the Battle Formation, an extensive freshwater locality in western Canada that formed in the Late Cretaceous approximately 66.5 million years ago (Ma). The formation represents one of the oldest known localities worldwide harboring definitive remains of freshwater diatoms, contains the oldest known freshwater specimens of the centric diatom genus Aulacoseira, and confirms that these early Aulacoseira colonizers formed filaments linked together with interdigitating spines. We further document a high diversity of araphid pennate diatoms belonging to the order Fragilariales. Seven pennate morphotypes were uncovered, six of which definitively lacked a raphe and whose closest modern relatives are in the genera Fragilariforma, Fragilaria, and Stauroforma. Given the extensive coverage of the Battle Formation, it is possible that it represents a network of numerous smaller shallow waterbodies that collectively offered a diversity of environments for colonization, making it a unique deposit for examination of early freshwater diatoms.

The Biharia sequence from the Apuseni Mountains is a component of the Biharia Paleozoic terrane. The Biharia terrane probably evolved as an island arc between the Cadomian Someş and Baia de Arieş terranes. A gneissic metagranodiorite associated with metabasites from the Valea Ierţii creek, was sampled for U/Pb and Pb/Pb zircon age determination. The zircons extracted out of the sampled rock were subjected both to dilution and evaporation methods. Dilution method offered Concordia intercepts at 227±23 Ma, 312±13 Ma, 465.7+8.4/-8.0 Ma, 703±21 Ma and 1604±45 Ma. Evaporated zircon grains gave 450±20 Ma and 543±17 Ma. The 227±23 Ma age and 312±13 Ma age have been interpreted as Pb loss due to the final effect of the Permian widespread magmatism and late Variscan anatexis respectively. The 465.7+8.4/-8.0 Ma and 450±50 Ma ages probably represent the protolith generation time. The 543±17 Ma is viewed as an inherited Cadomian age and the 703±21 and 1604±45 Ma might represent Cadomian and Saharan detrital zircon ages.