Canadian Journal of Earth Sciences

The late Early to early Middle Eocene Okanagan Highlands fossil sites, spanning ~1000 km north–south (northeastern Washington State, southern British Columbia) provide an opportunity to reconstruct biotic communities across a broad upland landscape during the warmest part of the Cenozoic. Plant taxa from these fossil sites are characteristic of the modern eastern North American deciduous forest zone, principally the mixed mesophytic forest, but also include extinct taxa, taxa known only from eastern Asian mesothermal forests, and a small number of taxa restricted to the present-day North American west coast coniferous biome. In this preliminary report, paleoclimates and forest types are reconstructed using collections from Republic in Washington State, USA., and Princeton, Quilchena, Falkland, McAbee, Hat Creek, Horsefly, and Driftwood Canyon in British Columbia, Canada. Both leaf margin analysis (LMA) and quantitative bioclimatic analysis of identified nearest living relatives of megaflora indicated upper microthermal to lower mesothermal moist environments (MAT ~10–15 °C, CMMT > 0 °C, MAP > 100 cm/year). Some taxa common to most sites suggest cool conditions (e.g., Abies, other Pinaceae; Alnus, other Betulaceae). However, all floras contain a substantive broadleaf deciduous element (e.g., Fagaceae, Juglandaceae) and conifers (e.g., Metasequoia) with the bioclimatic analysis yielding slightly higher MAT than LMA. Thermophilic (principally mesothermal) taxa include various insects, the aquatic fern Azolla, palms, the banana relative Ensete, taxodiaceous conifers, Eucommia and Gordonia, taxa which may have occurred near their climatic limits. The mixture of thermophilic and temperate insect and plant taxa indicates low-temperature seasonality (i.e., highly equable climate).

The absolute ratio of ¹¹B/¹⁰B has been determined for boron from different terrestrial sources with a precision of 0.2% (two standard errors) and a reproducibility of 0.2% (half-range). Values fall in the range 4.108 to 3.987 (i.e. 3% variation) and give a corresponding range in the boron atomic weight of 10.814 to 10.810 (0.04% variation). The absolute ratios are 7% lower than those reported by early workers, but are in accord with the results of recent investigations. Igneous rocks and boric acid are found to have high isotopic ratios, whereas Tokyo Bay water possesses a value for ¹¹B/¹⁰B near the mean.

Monazite is an underutilized mineral in U–Pb geochronological studies of crustal rocks. It occurs as an accessory mineral in a wide variety of rocks, including granite, pegmatite, felsic volcanic ash, felsic gneiss, pelitic schist and gneiss of medium to high metamorphic grade, and low-grade metasedimentary rocks, and as a detrital mineral in clastic and metaclastic sediments.In geochronological applications, it can be used to date the crystallization of igneous rocks, determine the age of metamorphism in metamorphic rocks of variable metamorphic grade, and determine the age and neodymium isotopic characteristics of source materials of both igneous and sedimentary rocks. It is particularly useful in the dating of peraluminous granitic rocks where zircon inheritance often precludes a precise U–Pb age for magmatic zircon. The U–Pb systematics of the mineral are not without complexity, however. Being a mineral that favors incorporation of Th relative to U, it can contain considerable amounts of excess ²⁰⁶Pb derived from initially incorporated ²³⁰Th, an intermediate decay product of ²³⁸U. Corrections for this effect can be made using the Th/U ratio of the host rock, but these corrections may not always be valid. Monazite is known to be capable of preserving inheritance in a manner similar to that of zircon, and it can lose Pb during episodic or prolonged heating events of uppermost amphibolite and granulite facies metamorphic grades. Monazite is less retentive of Pb than zircon during high-temperature igneous and metamorphic processes, and a few studies of its behavior suggest that its closure temperature is approximately 725 ± 25 °C. Examples of U–Pb systematics from most of the above situations are presented in this paper to illustrate both the utility and complexity of monazite in geochronological studies in an attempt to encourage more widespread application of this dating method.

Five discrete accretionary events assembled fragments of continental and oceanic crust into a coherent Superior craton by 2.60 Ga. They exhibit similar sequences of events at ~10 million year intervals: cessation of arc magmatism, early deformation, synorogenic sedimentation, sanukitoid magmatism, bulk shortening, regional metamorphism, late transpression, orogenic gold localization, emplacement of crust-derived granites, and postorogenic cooling. The Northern Superior superterrane recorded 3.7–2.75 Ga events prior to 2.72 Ga collision with the 3.0 Ga North Caribou superterrane. Following 2.98 Ga rifting, the Uchi margin of the North Caribou superterrane evolved in an upper plate setting before 2.72–2.70 Ga collision of the <3.4 Ga Winnipeg River terrane, which trapped synorogenic English River turbidites in the collision zone. The Winnipeg River terrane was reworked in 2.75–2.68 Ga magmatic and tectonic events, including the central Superior orogeny (2.71–2.70 Ga) that marks accretion of the juvenile western Wabigoon terrane. In the south, the Wawa–Abitibi terrane evolved in a mainly oceanic setting until Shebandowanian collision with the composite Superior superterrane at 2.695 Ga. Synorogenic Quetico turbidites were trapped in the collision zone. The final accretionary event involved addition of the Minnesota River Valley terrane (MRVT) from the south, and deposition and metamorphism of synorogenic turbidites of the Pontiac terrane during the ~2.68 Ga Minnesotan orogeny. Seismic reflection and refraction images indicate north-dipping structures, interpreted as a stack of discrete 10–15 km thick terranes. A slab of high-velocity material, possibly representing subcreted oceanic lithosphere, as well as Moho offsets, support a model of progressive accretion through plate-tectonic-like processes.

The ¹⁸O/¹⁶O ratios of fresh unmetamorphosed basalts from the Reykjanes, Mid-Atlantic, and Gorda Ridges and the East Pacific Rise fall in a narrow range of 5.5–5.9‰ (SMOW), identical to those of basalts from oceanic islands. Large plagioclase xenocrysts in some of the basalts are not in isotopic equilibrium with coexisting olivine. The basalts react with sea water at the ambient sea-floor temperature to form clays, resulting in an increase in δ¹⁸O of the basalt of 0.25‰ per m.y. This provides a very sensitive method to detect early stages of weathering, which otherwise might go unnoticed. Advanced submarine weathering of basalt can remove nearly one half of its SiO₂, MgO, CaO, and Na₂O and lesser amounts of Al₂O_s and TiO₂. Weathering does not proceed deeply enough into the basaltic sea floor to affect the chemical composition of sea water. Since it is likely that large amounts of basaltic debris (less than a few cm in radius) are on the ocean floor, their weathering can affect the ocean-sediment budget of TiO₂, Al₂O₃, SiO₂, MgO, and CaO.

At ~2700 Ma volcanism spawned by northward-directed subduction under Wabigoon subprovince, Canadian Shield, fed sediment into an adjacent fore-arc basin – trench system. Volcanically active island arcs to the south (Wawa subprovince) were transported northward during this interval and at ~2690 Ma collided with the accretionary complex (Quetico subprovince). Metamorphosed sedimentary units intercalated with the ocean floor and island-arc volcanic rocks of northern Wawa subprovince provided the opportunity to amass data useful in extending the paleogeographic interpretation of the area. This study examines the McKellar Harbour Formation, a several kilometre thick assemblage of graded metasandstone beds outcropping to the north of Lake Superior. Its sedimentology, geochemistry, and detrital U–Pb zircon geochronology were investigated in conjunction with the same attributes of possible correlative units in the region. Similarities between the McKellar Harbour Formation and sedimentary rocks composing the fore-arc basin and Quetico trench to the north indicate that by 2696 Ma the trench had been overwhelmed by sediment, to the point where turbidity currents were overflowing onto the abyssal ocean plain, building a prograding submarine ramp–fan that was to become the McKellar Harbour Formation. The turbiditic ramp–fan assemblage comprising most of this extensive depositional system crosses much of southern Superior Province, highlighting the interrelated genesis of rocks formed during amalgamation of the individual subprovinces.

A complex history of volcano-sedimentary deposition, polyphase strain, multiple intrusive events, and various stages of porphyroblastesis is indicated for the Hemlo gold deposit area within the Hemlo greenstone belt. Structural elements can be assigned to at least six stages of development (D₁–D₆). D₁ generated small-scale folds and low-angle faults (thrusts?) with no planar fabric, except within strain aureoles around the earliest intrusions. D₂ was a progressive event resulting from northeast-directed compression, which generated regional, predominantly S-shaped folds (early D₂); penetrative planar and linear fabrics, overturned stratigraphy, and formation of an inflection in the strike of the greenstone belt (mid-D₂); and development of high-strain zones with dominant sinistral and local dextral shear sense (late D₂). D₃ was a distinctly separate progressive event resulting from northwest-directed transpression, which generated variably penetrative east- to northeast-striking foliation (S₃), ductile dextral shear fabrics, and small-scale Z-shaped folds (early D₃), followed by brittle–ductile to brittle development of cataclasite and pseudotachylite in layer-parallel zones (late D₃). D₄ resulted in contractional kinks and brittle fractures, locally in conjugate sets. D₅ and D₆ are represented by brittle to brittle–ductile faults, which overprint Paleoproterozoic and Mesoproterozoic dikes, respectively. Four granitoid magmatic events span the interval 2720–2677 Ma, with emplacement mainly during D₂, between ca. 2690 and ca. 2684 Ma. A protracted period of regional medium-grade metamorphism likely spanned the D₂–D₃ stages. The Hemlo gold deposit was emplaced during mid-D₂ and was largely controlled by D₂ structural elements and competency contrast between rock units.

The Gunflint Formation, a Paleoproterozoic chemical–clastic sedimentary assemblage outcropping to the immediate northwest of Lake Superior, became famous in 1954 as containing the oldest fossil assemblage known at that time. Older microfossils have since been discovered, but the Gunflint procaryotes remain one of the most diverse Precambrian fossil communities. The finding of possible multicellular organisms in correlative lithic units in Michigan has recently added to the need for an exact age of the Formation. Zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the Gunflint Formation. A euhedral zircon population has yielded a 1878.3 ± 1.3 million years BP U–Pb age, believed to be nearly synchronous with the depositional age. This not only provides a precise age for the community of organisms, but also strongly supports a back-arc extensional setting for the Animikie Basin, rather than a foreland trough.

Most submarine greenstones recovered from the North Mid-Atlantic Ridge range in δ¹⁸O from 2.8 to 6.8‰ (SMOW). An extremely light rock. (δ¹⁸O = −7.1‰), was also analyzed, but it might be an ice rafted erratic. The heavier ones, up to 8‰, are probably weathered. On the average, the δ¹⁸O of 14 greenstones is slightly lower than that of fresh unmetamorphosed submarine basalt. The ¹⁸O fractionations between coexisting metamorphic minerals of the greenstones are consistent with the independently inferred range of metamorphic temperatures of 200–300 °C. The δ¹⁸O of the possible metamorphic water can be estimated from this temperature range and the δ¹⁸O of the metamorphic minerals to be − 2 to + 2‰. This argues strongly that little or no 'juvenile' water (water outgassed from the mantle) participated directly in the metamorphism. The δ¹⁸O data are consistent with the participation of sea water in the metamorphism. Similar conclusions can be drawn from the ¹⁸O analyses of a submarine diabase and two gabbros. Disseminated carbonates in the greenstones have δ¹⁸C values typically found in carbonatites or partially decarbonated marine limestones.

Weathering profiles developed side-by-side on sandstone and a mafic dike provide an unusual opportunity to examine the role of parent rock bulk composition in the chemical evolution of middle Precambrian regoliths. Because the profiles are adjacent to one another, differences in topography can be eliminated in accounting for differences in the chemical evolution of the two profiles. Both profiles show upward increases in Al, Ti, K, and Rb and decreases in Mg, Ca, and Na. In addition, the mafic regolith increases upward in Zr and Nb and decreases in Zn and Ni. Total Fe decreases upward in both profiles, but the sandstone profile retains significantly more of its initial Fe than does the mafic dike. This difference in Fe loss is consistent with weathering in a low-oxygen atmosphere of rock types with very different initial Fe contents and therefore different atmospheric requirements for complete oxidation of the Fe present. The Fe in the sandstone was mostly oxidized and retained within the profile, whereas much of the Fe in the mafic dike was not oxidized and was removed from the profile in the more soluble ferrous state. Petrographic evidence indicates that both sandstone and mafic dike weathering profiles underwent preweathering diagenesis, postweathering K–Rb metasomatism, and very low-grade metamorphism. Mineral chemistry indicates that, in the absence of chlorite, white mica composition closely reflects variation in bulk composition. Where both white mica and chlorite are present, changes in bulk composition are accommodated by variations in the proportions of these two minerals rather than by variations in white mica composition.