bluuebunny/crossref_metadata_2025 huggingface dataset:
Cryptalgalaminate dolomite of the Dunphy Formation, Labrador Trough: diagenetic and tectono-metamorphic evolution related to copper mineralization
Authors: Chevé S. R. , Schrijver K. , Tassé N.
Published: April 2011

The strata-bound Cu–Fe–S mineralization of the Lower Proterozoic Dunphy Formation forms the largest known sediment-hosted copper deposit of the Labrador Trough. Occurring in three thin stratiform dolomitic intervals, the sulfides are mainly (1) disseminated in a primary porosity (birdseyes) cemented by dolospar and (2) vein bound in early tectonic spar-filled tension gashes. The first of these formed during diagenesis, the second during synkinematic metamorphism. Their parageneses and chemical and sulfur-isotopic compositions are similar.The history of cementation of the host dolomite of one of the intervals has been traced by cathodoluminoscopy of dolospar(–sulfide)-filled birdseyes. Four concentric rim cements, locally disrupted, are followed by two to three central cements. Electron-probe microanalysis shows increasing Fe and Mn in solid solution in these cements from rim to center, pointing to a decrease in Eh toward the center during cementation, corroborated by the presence of semi-centrally located sulfides.The degradation of Recent cyanobacterial mats is used, with few modifications, as a model for the diagenetic evolution of the dolomite. A syndepositional stage of microbial calcification was penecontemporaneous with biochemical dolomitization and followed by microbial diagenetic sulfate reduction of pore waters. The fenestral fabrics of the mats were the major channels of transport of cement-forming substances and the only channels that governed the lateral component of movement of the major pulses of metalliferous fluids.Copper and iron, presently bound to sulfur, were possibly provided by leaching of underlying redbeds and channelled by synsedimentary faults to the porous, degrading cyanobacterial mats.
Metal fixation by bacterial cell walls
Authors: Beveridge T. J. , Fyfe W. S.
Published: April 2011

All biomass contains a significant quantity of metallic constituents, and mineralization in living and dead biodebris may contribute to element transport from the hydrosphere into sediments. The anionic cell walls of bacteria are remarkable in their ability to fix metals and provide sites for nucleation and growth of minerals. Results presented show the types of cell wall polymers that are responsible for metal binding in walls of Gram-positive and Gram-negative bacteria.
Low-temperature sulphate reduction: biological versus abiological
Authors: Trudinger P. A. , Chambers L. A. , Smith J. W.
Published: April 2011

Sulphate is considered to have been a major source of sulphide in strata-bound and stratiform base-metal sulphide deposits. Many of these deposits, however, appear to have been formed at moderate temperatures (<200 °C), which poses the question, By what mechanism(s) was sulphate reduced to sulphide? Two modes of reduction have been established experimentally: (1) catalysis by sulphate-reducing bacteria, which at present is only known to occur below ca. 100 °C; and (2) abiological reduction by ferrous iron or organic matter, which has only been clearly shown above ca. 250 °C.Several attempts have been made to demonstrate abiological reduction below 200 °C, and some new data are presented here. Although the results do not exclude the possibility that such a reaction may be geochemically significant, there has been no unequivocal demonstration of nett sulphide formation from sulphate at these temperatures.Recent studies of the microbiology of hydrothermal regions have opened up the prospect of bacterial sulphate reduction at much higher temperatures than had earlier been thought possible.
U–Pb zircon ages for magmatism in the Red Lake greenstone belt, northwestern Ontario
Authors: Corfu F. , Wallace H.
Published: April 2011

U–Pb dating was carried out on nine volcanic rocks and two felsic intrusions from the Red Lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization.The data indicate a protracted period of igneous activity spanning at least 270 Ma. Felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages of [Formula: see text] and [Formula: see text]. A third unit, dated at [Formula: see text], contains inherited zircons older than 2982 Ma, which casts some uncertainty on the validity of the inferred intercept age. Rocks in the western part of the belt, previously believed to form a relatively young calc-alkalic sequence but now known to be dominantly tholeiitic, are shown to be relatively old, with ages of [Formula: see text] and [Formula: see text]. These two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units.Another volcanic horizon in the centre of the belt is dated at 2830 ± 15 Ma, and calc-alkaline volcanic sequences on the southern and northern flanks of the belt yield ages of 2739.0 ± 3.0 and [Formula: see text], respectively. An age of [Formula: see text] was determined for tholeiitic pyroclastic rocks near the base of the predominantly calc-alkaline Heyson sequence.The major gold deposits of the Red Lake belt appear to be present dominantly within older supracrustal sequences. On the other hand, they are also associated with late deformation zones that postdate the intrusion of the Dome Stock dated at 2718.2 ± 1.1 Ma ago. The time of an earlier folding event is bracketed by this age and by the age of [Formula: see text] for an isoclinally folded felsic dike.
U–Pb zircon ages for rocks from the Island Lake greenstone belt, Manitoba
Authors: Turek A. , Carson T. M. , Smith Patrick E. , Schmus W. R. Van , Weber W.
Published: April 2011

The Archean Hayes River Group of the Island Lake greenstone belt (Superior Province, Sachigo Subprovince) comprises mafic to felsic metavolcanics, subvolcanics, and associated metasedimentary rocks. The Hayes River Group is intruded by granitoid rocks belonging to the early intrusive complex. One such pluton, the Bella Lake tonalite, is intrusive into the metabasalt of the Hayes River Group and has a U–Pb zircon age of 2886 ± 15 Ma. Similar intrusives of this complex, either internal or marginal to the greenstone belt, yield zircon ages of 2801 ± 8 Ma (Pipe Point tonalit) and 2768 ± 22 Ma (Linklater Island prophyry). This suggests that the early intrusive complex was emplaced over an ~ 120 Ma long interval by at least three separate intrusive episodes.Subsequent to the emplacement of the early intrusive complex, the isoclinally folded Hayes River Group and the early intrusive complex were uplifted, eroded, and followed by the unconformable deposition of the Island Lake Group, comprising fluvial to marine metasedimentary rocks. The stratigraphically lower part of the Island Lake Group is bracketed by the 2768 ± 22 Ma age of the Linklater Island porphyry and the 2729 ± 3 Ma age obtained for the late tectonic suite—the Pipe Point quartz diorite and feldspar porphyry. A feldspar quartz porphyry belonging to the post-tectonic intrusive rocks intrudes higher stratigraphic levels and has been dated at 2699 ± 4 Ma (Horseshoe Island quartz feldspar porphyry).
Windsor Group (Lower Carboniferous) conodont biostratigraphy and palaeoecology, Magdalen Islands, Quebec, Canada
Authors: Plint Hilary A. , Bitter Peter H. von
Published: April 2011

The Taphrognathus transatlanticus, Clydagnathus windsorensis, and Gnathodus zones, initially defined in the Codroy Group of Newfoundland, were identified in the Havre-aux-Maisons Formation of the Windsor Group of the Magdalen Islands, Quebec, Canada. Recognition of these zones permits correlation within the islands and with Newfoundland and Nova Scotia. The Taphrognathus transatlanticus and Clydagnathus windsorensis zones correlate with the lower and upper macrofaunal B Subzone, respectively, whereas the Gnathodus Zone corresponds to the C, D, and E macrofaunal subzones.Tectonism and plastic movement of sulphates and chlorides have resulted in fragmentation and transport of parts of the Havre-aux-Maisons Formation. The conodont zonation complements the macrofaunal zonation permitting correlation of fragmented blocks. When macrofaunas are scarce or absent the conodont zonation provides an alternative to one based on macrofauna only. It has been used effectively in assessing previous zonal determinations based on macrofauna.Conodonts of the Magdalen Islands are dominated by Clydagnathus, Cavusgnathus, Mestognathus, and Taphrognathus, all asymmetric cavusgnathiform genera. This dominance, the total lack of Gnathodus, abundant sulphates and chlorides, as well as foraminiferal and algal evidence, suggest that Lower Carboniferous conodonts of the Magdalen Islands lived in shallow-water shelf environments under conditions of fluctuating salinity.
Crustal section across the polar continent–ocean transition in Canada
Authors: Sobczak L. W. , Mayr U. , Sweeney J. F.
Published: April 2011

The Canadian Arctic Transect extends northwards from the Canadian Shield across a thick (about 18 km) and wide (over 800 km) sedimentary section consisting of four overlapping basins. These overlie a continental crystalline crust that thins from 48 to 8 km towards the Canada Basin. The latter overlies a relatively thin (5 – 10 km thick) oceanic crust below the Arctic Ocean. The calculated gravity effects of the upper sedimentary section were stripped away from the observed gravity anomaly, and the residual anomalies were used to determine the boundaries between sediment and crystalline crust and between crust and mantle. Residual anomalies with short wavelength and steep gradients were used to modify the initial near-surface structural model and to identify zones of evaporite and mafic rocks within the sedimentary rock column.Some interesting results emerge from this analysis: (1) analysis of the gradients of the shelf and slope suggests that shelf subsidence is hinged about a line near the central axis of the Sverdrup Basin; (2) continental crystalline crust thins oceanward from 48 km to 8 km at the transition zone over a distance of 825 km and appears to have stretched from an original width of 543 km, for an apparent stretch factor of about 1.5; (3) sediment thickness is usually inversely related to the crystalline crustal thickness; (4) the mantle below the ocean appears to be less dense than below the continental crust, with an assumed significant vertical density boundary between the two below the continental shelf (transition zone); (5) this analysis supports the concept that evaporites occur along the axis of the Sverdrup Basin, and mafic rocks appear to be concentrated along the flanks of the Sverdrup Basin; and (6) seismicity usually occurs over areas of relatively positive gravity anomalies that are considered to be the result of uncompensated sedimentary loads or mafic igneous intrusions or are areas of uplifted and folded rocks.
The relationship between sedimentary facies and faunal associations in the Llandovery siliciclastic Ross Brook Formation, Arisaig, Nova Scotia
Authors: Hurst J. M. , Pickerill R. K.
Published: April 2011

The Llandovery siliciclastic Ross Brook Formation of Arisaig, Nova Scotia, comprises three broadly defined sedimentary facies. These are a mottled silty mudstone facies (facies A), a laminated shale facies (facies B), and a laminated siltstone facies (facies C). Facies A consists of variably bioturbated silty mudstones, muddy siltstones, and fissile shales. It developed in relatively shallow water conditions; mudstones were presumably deposited from suspension, and siltstone laminae and thin layers were formed by increased wave and current activity. Mottling resulted from bioturbation by epifaunal and, particularly, infaunal benthic organisms. Within facies A, two subfacies are recognized: A(i) is mudstone dominated, and A(ii) is fine-grained siltstone dominated. Facies B consists of alternating laminae of undisturbed mudstone and fine-grained siltstone probably produced as a result of deposition from suspension during a temporary upward expansion of the oxygen-minimum layer. Facies C consists of 0.5–30 cm thick fine- to coarse-grained siltstones, which occur in lenses or layers of single, composite, or amalgamated units. Internally they are extremely variable, but all are interpreted as a result of deposition from storm-generated currents.The Ross Brook Formation formed on a shallow-marine, storm-influenced, subtidal inner–mid muddy shelf and is dominated by extensive but stratigraphically variable developments of facies A and facies C. Absolute water depth per se is difficult to assess, and although fluctuations occurred, much of the sequence is believed to have accumulated at or in the immediate vicinity of fair-weather wave base in water depths estimated to have been between 30 and 60 m. Silt supply was generally low, possibly reflecting great distance from source or the presence of a mud-dominated shoreline.Five brachiopod-dominated associations, which are stratigraphically the Eocoelia hemisphaerica, the Eocoelia intermedia – Eocoelia curtisi, the Visbyella nana, the "Camarotoechia" rossonia, and the Eocoelia sulcata associations, occur through the sequence. Associations change where die sum of the facies characteristics change, suggesting that the major physical controlling factor was substrate type and related environmental parameters. The development of discrete but intergrading associations is viewed as a consequence of the long-term persistence of a set of conservative animal–sediment relationships, superimposed on which is the evolutionary pattern of immigration and extinction of individual species.
Inverse-graded units within till in drumlins near Caledonia, southern Ontario
Authors: Menzies J.
Published: April 2011

Examination of several drumlins in the Caledonia drumlin field of southern Ontario reveals inverse-graded units occurring from the surface to near bedrock. These units were found within a two-till, conformable sequence. Evidence indicates that the two tills, on the basis of provenance and geochemical and geotechnical characteristics, are subfacies of a single depositional phase. Fine subunits within each inverse-graded unit have above average calcium and magnesium contents and exhibit pore-wall cutan formation. Below average calcium and magnesium contents occur within coarse subunits, and no cutans were observed. In explaining this differentiation and the cause of inverse grading, a hypothesis is suggested of pore-water migrating contemporaneously with localized and episodic till "lodging" or deposition from a mobile, basal tractive load. For the drumlins revealing these inverse-graded units, it is thought that an accretion–depositional model is most likely for formation from an initiating nucleal patch of immobilized tractive load. It is suggested that subglacial environments similar to that in the Caledonia area must have existed over widespread lowland areas beneath the Quaternary ice sheets.
Limestone diagenesis of Upper Devonian Nisku carbonates in the subsurface of central Alberta
Authors: Machel Hans-G.
Published: April 2011

Limestone diagenesis of the Upper Devonian Nisku Formation in the subsurface of central Alberta began syndepositionally. Most early calcite cements are fibrous, bladed, and columnar and form isopachous precipitates in primary voids. Partial syndepositional induration of the lime matrix (sand and mud) is indicated by angular limestone intraclasts in deposits flanking some buildups, hardgrounds, and neptunian dikes in the reefal facies. Some buildups contain intertidal fades, possible vadose cements, and some early dissolution molds and vugs in their upper parts, suggesting shallow influence of meteoric pore waters towards the end of Nisku time. Postdating these features, precipitation of fibrous and subequant calcite cements, neomorphism, mechanical compaction, and incipient stylolitization resulted in complete induration and lithification of the buildups and associated marly strata.Trace-element compositions of the various calcites are variable within individual petrographic groups and may have originated from meteoric and (or) marine-phreatic diagenesis. However, isotopic and petrographic data of most cements suggest marine-phreatic diagenesis. Therefore, most cements were probably formed and (or) recrystallized in pore waters of marine parentage. The oxygen-isotope data suggest that limestone diagenesis and lithification were essentially completed at a depth of about 300 m. This burial depth was achieved near the end of Famennian or in Early Mississippian time.
A geophysical interpretation of the onshore and offshore geology of the southern Avalon Terrane, Newfoundland
Authors: Miller H. G.
Published: April 2011

This paper presents new land and underwater gravity data for the southern portion of the Avalon Terrane of Newfoundland. These gravity data are combined with existing airborne and marine magnetic data to produce a detailed geophysical interpretation of the geology of the southern Avalon Peninsula and the adjacent portions of Placentia and St. Mary's bays.Qualitative interpretation of the geophysical data in conjunction with the known geology reveals that the major faults previously mapped only on land and inferred beneath St. Mary's Bay can be traced offshore geophysically. In addition, the previously unknown offshore limits of a small sedimentary basin in the southeast part of the area can be delineated.Quantitative modelling demonstrates that (1) Precambrian mafic to ultramafic rocks underlie the whole area, (2) thrust faulting in the St. Mary's Peninsula and elsewhere has led to the juxtaposition of Bull Arm volcanic rocks over Harbour Main volcanic rocks, and (3) the evolution of the area may be explained by a combination of earlier volcanic island and extensional models.The mapping has also revealed the presence of previously unknown northwest–southeast lineations in the geophysical patterns, which may be related to the late wrench faulting.
Primitive clay precursors formed on feldspar
Authors: Tazaki Kazue , Fyfe W. S.
Published: April 2011

High-resolution transmission electron microscopy of clay formation on K-feldspar has revealed the existence of intermediate states between feldspar and crystalline clay products. During the earliest weathering stages of K-feldspar, a primitive clay precursor forms on the feldspar surface that is spotted by ion oxides. This reactive iron is incorporated into the primitive clay precursors, which have an ultrathin 150–200 Å (1 Å = 0.1 nm) circular form and 14–20 Å lattice images or long, curled fiber forms with varied lattice image spacings. The electron diffraction patterns of primitive clay precursors show diffuse rings at 2.65, 2.04, and 1.51 Å, suggesting low crystallinity, random orientation, and partial inheritance of the original structure. EDX step scanning analysis showed that the major-element concentrations of Si, Al, and K tend to decrease from unaltered parts to altered parts of precursors with substantial increase in Fe. Auger depth profiling showed the thickness of the primitive clay layers is 150–300 Å. The primitive clay precursors may well precede formation of spheroidal particles of halloysite, squat cylinders of halloysite or hexagonal crystalline, and tabuler halloysite (7 Å). SEM, XRD, SIMS, and SAM data support the TEM results.
Analysis of orientation diagrams derived from a simulated quartz fabric
Authors: Starkey John
Published: April 2011

A simulated quartz fabric is generated in which the c axes tend to be parallel to a single direction. A set of pole figures is computed corresponding to the pole figures that would be measured by X-ray fabric analysis. Comparisons are made between orientation diagrams derived from the "measured" pole figures both directly and via the orientation function. The derived pole figures and inverse pole figures and the crystal orientation matrix obtained by the direct method correlate more closely with the original, simulated data. The inverse pole figures derived from the orientation distribution function display spurious symmetry indicating nonexistent differences in equivalent specimen directions and in the orientation patterns of positive and negative crystallographic forms. It is demonstrated that the analysis and comparison of pole figures require that the number of faces belonging to the crystallographic forms represented in the pole figure must be taken into consideration. Further, where more than one form is present and the data are obtained by X-ray diffraction, the relative diffracting intensities must also be considered. This leads to the formulation of an effective multiplicity factor.
Thorium and uranium contents of gneisses and trondhjemites in the Western Dharwar craton, India
Authors: Callahan Elaine J. , Rogers John J. W.
Published: April 2011

Widespread metasomatic addition of U has apparently occurred in Archean tonalitic–trondhjemitic (gray) gneisses in the Western Dharwar craton of southern India. Gneisses of 3200–3000 Ma age near the Holenarasipur schist belt contain average concentrations of about 10 ppm Th and 3.5 ppm U. The U is considerably more abundant than average U (1.5 ppm) in Archean gray gneisses of the Superior Province of the Canadian Shield, and consequently the Th/U ratios are low in the Indian gneisses. Comparatively massive trondhjemitic plutons associated with a 3000 Ma event in the Dharwar craton are very depleted in both Th and U, and we propose that the metasomatism of the gneisses occurred while the plutons were being emplaced and were expelling fluids from the magmas. There is no evidence for metasomatism 2500 Ma ago, the age of extensive granulite metamorphism in southern India.
Evolution of the triangle zone in the Rocky Mountain Foothills near Coalspur, central Alberta
Authors: Charlesworth H. A. K. , Johnston S. T. , Gagnon L. G.
Published: April 2011

A triangle zone, which commonly occurs along the external margin of a foreland thrust and fold belt with a buried thrust front, is underlain by a subhorizontal, blind, foreland-verging thrust that ends against a foreland-dipping, hinterland-verging thrust. These contemporaneous thrusts, active towards the end of orogenesis, enclose an intercutaneous wedge that moved towards the foreland. During orogenesis, a triangle zone evolves through periodic replacement of faults bounding the active wedge. Replacements occur in cycles during each of which a lower fault tends to be replaced by one in a lower stratigraphic horizon, an upper fault by one farther away from the foreland. Each cycle ends with the lower fault moving to a younger horizon where it joins a new, more external upper fault.Near Coalspur, the triangle zone exposes the remnants of several wedges involving Upper Cretaceous and Paleocene molasse. Most of these wedges developed during the last cycle but one and have a combined displacement of about 5 km. Within this cycle, the younger the wedge, the older the strata at its extremity. The upper fault of each wedge cuts the lower fault of the preceding wedge. The upper fault of one of the wedges has a lateral ramp.
Archaeomagnetic intensity determination: a nineteenth century pottery kiln near Jordan, Ontario
Authors: Aitken M. J. , Allsop A. L. , Bussell G. D. , Winter M.
Published: April 2011

Evaluation of the ancient geomagnetic field at the time that this kiln last cooled down, around A.D. 1840, has been made by means of the Thellier technique applied to bricks from the floor. The value obtained, 56 ± 1 μT, is significantly lower than the value expected from contemporary observatory measurements, 64 μT. The difference is consistent with the demagnetizing field expected from the rather strong magnetization of the baked clay.
Upper Silurian and Lower Devonian graptolites and graptolite biostratigraphy, northern Yukon, Canada
Authors: Lenz Alfred C.
Published: April 2011

Upper Silurian and Lower Devonian graptolite biostratigraphy of the northern Canadian Cordillera is revised in the light of new data. The zones, listed in order of oldest to youngest, are Lobograptus progenitor, Saetograptus fritschi linearis, and Bohemograptus bohemicus tenuis: Ludlow; Monograptus formosus, Pristiograptus ultimus, Pristiograptus chelmiensis, Monograptus bouceki, and Pristiograptus transgrediens praecipuus: Pridoli; Monograptus uniformis and M. hercynicus: Lochkov; and Monograptus falcarius?, M. fanicus, M. thomasi?, and M. yukonensis: Prag.The following taxa, inclusive of new species, are described: Pristiograptus separabilis Teller, Pseudomonoclimacis tetlitensis n. sp., P. richardsonensis n. sp., P.? bispinosus n. sp., Monograptus ceratus n. sp., M. aequabilis bardoensis Porebska, M. cf. anerosus Koren', M. fanicus Koren', M. hercynicus hercynicus Perner, M. uniformis parangustidens Jackson and Lenz, Saetograptus fritschi linearis (Bouček), and Abiesgraptus? sp.
The status of Salterella as a Lower Cambrian index fossil
Authors: Fritz W. H. , Yochelson Ellis L.
Published: April 2011

In the North American Cordillera there is strong evidence that Salterella is restricted to the medial part of the Bonnia–Olenellus Zone. Reported younger and older occurrences are attributed to either stratigraphic error or misidentification. An unnamed genus with a superficial resemblance immediately postdates Salterella, but it lacks an inner laminar cone. In the Cordillera Salterella occurs in inner detrital and middle carbonate belt strata that were deposited in the upper half of Grand Cycle B and the lower half of Grand Cycle C.In and near the Appalachian Mountains of North America and in Greenland, Spitsbergen, and Scotland, occurrences of Salterella are consistent with a medial Bonnia–Olenellus Zone age; however, this cannot be as rigorously tested as in the Cordillera, because of fewer taxa associated with the genus and in super- and subjacent strata. Outer detrital (basinal) strata of the Taconic Allochthon contain Salterella, but it is uncertain whether the fossils are of local or derived origin.Speculations that Campitius titanius Firby and Durham of California belongs in the genus Salterella are doubtful, and C. titanius is clearly older. The relationship between Salterella and Volborthella and their relative ages within the Early Cambrian are still unclear.
Late Precambrian to Late Devonian mafic magmatism in the Antigonish Highlands of Nova Scotia: multistage melting of a hydrated mantle
Authors: Murphy J. Brendan
Published: April 2011

Five suites of alkalic basalt ranging in age from Late Precambrian to Late Devonian are found in the Antigonish Highlands of Nova Scotia. In contrast, on neighbouring Cape Breton Island, alkalic basalts are rare even in suites that are contemporaneous with those in the Antigonish Highlands. Late Precambrian alkalic basalts in the Antigonish Highlands are genetically associated with calc-alkalic rocks and are probably subduction related, whereas the younger suites are continental, rift related, and within plate. Major and compatible trace-element abundances can be explained by crystal fractionation of olivine ± clinopyroxene ± orthopyroxene ± spinel ± garnet. However, incompatible trace-element concentrations are strongly influenced by mantle metasomatism that occurred prior to, or synchronously with, the oldest alkalic rocks. The metasomatic event enriched the mantle in Fe, Ti, P, Zr, and light rare-earth elements. The trace-element composition of the younger suites is similar to that of the oldest alkalic rocks and may have been strongly influenced by the Late Precambrian metasomatic event. The anomalously low Nb/Y ratio (generally less than 1 in all suites) and application of phase-equilibria studies indicate that the metasomatic fluid was probably rich in H2O. This fluid may have been derived from dehydration of the subducting slab in Late Precambrian time, resulting in metasomatism of the overlying mantle wedge in the Late Precambrian. It is proposed that the younger suites obtained their fluids by dehydration of the previously metasomatized mantle associated with the generation of local pull-apart basins. Thus, the metasomatic fluid was exotic with respect to the oldest basalts but indigenous with respect to the younger basalts. In the younger basalts, the indigenous fluid was probably focussed at the site of melting by structural events (i.e., rifting). In situations in which the chemistry of mafic magmas is predetermined by earlier metasomatic events, caution is advised in using trace-element criteria to evaluate the tectonic setting.
Rare-earth-element geochemistry of some Archean iron formations north of Lake Superior, Ontario
Authors: Barrett T. J. , Fralick P. W. , Jarvis I.
Published: April 2011

Rare-earth-element (REE) compositions of iron formation from two Archean terrains in western Ontario have been determined in order to assess the possible influence of hydrothermal activity on the REE patterns of chemical sediments. One terrain is characterized by sulfide-facies iron formation in association with volcanic flows and volcaniclastic sediments, whereas the other is dominated by oxide-facies iron formation intercalated within submarine-fan clastic sediments. Mineral separates of chert, magnetite, and pyrite from the iron formations have low ΣREE concentrations (< 20–30 ppm) and display moderate to strong positive Eu anomalies (relative to Archean shale). The positive anomalies (and lack of negative Ce anomalies) indicate that Archean sea water from which iron formation locally precipitated was reduced, although to varying degrees.The REE patterns of mineral separates from a given locality are almost identical, but the patterns for various localities differ in detail. A number of iron-formation samples interbedded within volcanics and at the volcanic–sediment interface show a distinct positive La anomaly and near-flat to slightly heavy-REE (HREE)-enriched patterns. The only modern environment where metalliferous sediments are accumulating with these combined characteristics is the Red Sea brine deeps. By contrast, limited data from iron formation interbedded within the clastic submarine fan suggest a fairly flat pattern with a moderate positive Eu anomaly and no La enrichment. We therefore suggest that the latter pattern typifies nonhydrothermal Archean seawater.Where seawater was influenced by a direct hydrothermal contribution, La enrichment and enhancement of the Eu anomaly could result. However, since periods of low-intensity discharge and (or) bottom-water mixing could eliminate the hydrothermal signal, not all samples from volcanic associations need show these features. By analogy with the Red Sea, preservation of a hydrothermal signal is most likely where circulation in the depositional basin is restricted and bottom waters are strongly reducing. Evidence for such conditions in the volcanic association is provided by the nature of the associated sediments (e.g., carbonaceous slates and unreworked distal turbidites).
Magmatic interaction between mantle and crust during the evolution of the Archean Lac Guyer greenstone belt, New Quebec
Authors: Seymour Karen St. , Francis Don
Published: April 2011

The Lac Guyer greenstone belt was one of a series of volcanic troughs active during the Archean in the James Bay territory of the Superior Province of Quebec. The belt consists of a succession of isoclinally folded volcanic rocks comprising a lower sequence of basalts overlain by felsic tuffs and rhyodacites that are in turn succeeded by an upper sequence of basalt and komatiite. Plutons of granodioritic composition syntectonically intrude the volcanic succession. The development of this volcanic succession can be interpreted in terms of a model involving an intimate interaction between a differentiated crust and Mg-rich magmas rising from the mantle. Although some of these magmas reached the surface to erupt as komatiites, the majority were trapped at the base of the crust and fractionated towards basaltic compositions. This process caused partial melting of the base of the crust, which was probably mafic in composition, and produced granodioritic magmas whose derivative liquids erupted as rhyodacites.
Paleomagnetism and orientation of Precambrian dykes, eastern Lake Superior region, and their use in estimates of crustal tilting
Authors: Halls H. C. , Shaw E. G.
Published: April 2011

Archean rocks form the eastern margin of the 1.1 Ga old Central North American rift along the eastern shore of Lake Superior and have been tilted westwards in response to rifting. Paleomagnetic and structural data from 2.6 Ga old Matachewan dykes suggest a westward crustal tilt of about 60°, which agrees well with dips recorded in nearby Keweenawan volcanics that rest directly on basement rocks. The Matachewan dyke swarm occurs throughout the east shore region of Lake Superior, whereas Keweenawan supracrustal sequences, which give a more precise estimate of tilt, are restricted to a few isolated shoreline patches. Estimates of crustal tilt can be obtained from the dykes on a regional basis, thus generating a more complete picture of basement deformation adjacent to a major intracratonic rift.
Crustal structure across the Southwest Newfoundland Transform Margin
Authors: Todd B. J. , Reid I. , Keen C. E.
Published: April 2011

A seismic-refraction survey providing deep crustal structure information of the continent–ocean boundary across the South-west Newfoundland Transform Margin was carried out using large air-gun sources and ocean-bottom seismometer receivers. Continental crust ~30 km thick beneath the southern Grand Banks (P-wave velocity = 6.2–6.5 km/s) thins oceanward to a 25 km wide transition zone. In the transition zone, Paleozoic basement of the Grand Banks (5.5–5.7 km/s) is replaced by a basement of oceanic volcanics and synrift sediments (4.5–5.5 km/s). Seaward of the transition zone the crust is oceanic in character, with a velocity gradient from 4.7 to 6.5 km/s and a thickness of 7–8 km. Oceanic layer 3 is absent. No significant thickness of intermediate-velocity (>7 km/s) material is present at the continent–ocean transition, indicating that no under-plating of continental crust has taken place. The continent–ocean transition across the transform margin is much narrower than across rifted margins, supporting the theory that formation of the transform margin is by shearing of continental plates.
Crustal structure beneath the southern Grand Banks: seismic-refraction results and their implications
Authors: Reid I.
Published: April 2011

A seismic-refraction profile was shot on the southern Grand Banks using large air-gun sources and an array of ocean-bottom seismograph receivers. A sediment column 1–2 km thick directly overlies Paleozoic basement with velocity structure similar to that of the Meguma Zone of Nova Scotia. The main crustal layer is 27 km thick, with seismic velocity of 6.3 km/s increasing to about 6.5 km/s in the lowest few kilometres. Complexity is apparent in the crust–mantle transition around 32 km depth. Comparison with deep multichannel reflection data suggests that the increased velocity in the lower part of the crust may be associated with a reflective zone and shows the Mohorovičić discontinuity to be delineated by a well-defined reflection. The absence of a major lower crustal layer of intermediate velocity (> 7 km/s) is consistent with observations elsewhere in the region.
Labrador shelf benthic Foraminifera and stable oxygen isotopes of Cibicides lobatulus related to the Labrador Current
Authors: Vilks G. , Deonarine B.
Published: April 2011

Foraminifera from 38 samples of surface sediments contained 22 calcareous and 17 agglutinated species. Eight stenotopic species relate to different waters of the Labrador Current: Cassidulina reniforme, Reophax arctica, and Cribrostomoides crassimargo are associated with the inner shelf–estuarine environment; Islandiella helenae and Haynessina orbiculare are associated with the cold and diluted inner shelf waters; Buccella frigida is related to the slope waters; and Melonis zaandamae is related to the deep waters of the basins and the outer Labrador Shelf. The distribution of these species down core in core 13, collected from one of the basins on the Labrador Shelf, indicates the presence of inshore waters in the bottom of the basin about 15 000 years BP.The ratios of the two stable oxygen isotopes 18O and 16O were determined in the tests of Cibicides lobatulus from surface sediment samples collected at 28 localities on the Labrador Shelf. The range of bottom salinities at station locations was 28.6 to 34.8‰, and the corresponding range in bottom temperatures was 4 to −1.5 °C. The range in δ18O measured in the tests of C. lobatulus was −0.5 to 2.4‰. The significance of the linear regression between salinity and δ18O is better than a 99.9% level of probability.The down-core δ18O profile and the proportion of estuarine species in core 13 show a trend that corresponds to lower paleosalinities on the Labrador Shelf. The δ18O values become more negative, approaching the mean value of the present-day inshore waters, and the total assemblage of Foraminifera also changes towards a greater proportion of estuarine species. The paleoceanographic boundary is estimated to be slightly younger than 15 000 years BP, according to extrapolation of dated lithologic and faunal boundaries from nearby cores.We demonstrate that the stable oxygen isotope ratios from tests of the epibenthic foraminifer Cibicides lobatulus are useful in establishing paleosalinities on a subarctic continental shelf, where waters have been diluted with fluvial or glacial runoff.
Open and filled karst features on the Cayman Islands: implications for the recognition of paleokarst
Authors: Jones Brian , Smith Duncan S.
Published: April 2011

The dolostones of the Oligocene–Miocene Bluff Formation on the Cayman Islands are characterized by well-developed surficial, interface, and subsurface karst features that are open or filled with speleothems and sediment.Some of the open caves show evidence of a minimum of two stages of development related to dissolution in the phreatic zone and the vadose zone. The development of speleothemic formations (e.g., stalactites, stalagmites, columns) is limited in some caves but extensive in others. Some of the caves have limited amounts of sediment on their floors. In these caves, the speleothems and sediments are still formed of CaCO3 and thus contrast sharply with the dolostone of the Bluff Formation in which they occur. Some of the joints, sinkholes, and caves are filled with breccia, caymanite, terra rossa, terra rossa breccia, pisolitic limestone, and speleothems. Although filled, such karst features are analogous to the open karst features that occur elsewhere on the islands. The filling of joints, sinkholes, or caves is not directly related to the age of the karst development, since caves of the same age may be open or filled. This suggests that local conditions, such as the availability of sediment or the nature of the waters flowing through the caves, played an important role in determining whether a karst feature is filled.Surficial karst features have a low preservation potential, whereas interface and subsurface karst features have a high preservation potential in some circumstances. In the case of sinkholes this assumes that later transgressions did not remove the upper part of the rock body that contained the filled sinkholes. For caves this assumes that they were filled with speleothems and sediment prior to their reaching such a size that collapse of the overlying strata occurred because of the lack of support. If collapse occurs, the presence of caves can only be inferred from the resultant collapse breccias. A potentially valuable criterion for the recognition of paleokarst may lie in the fact that the rocks filling open karst features (e.g., joints, sinkholes, caves) may contrast sharply with the host strata in terms of both lithology and age.
Basal-flow conditions at the northeastern margin of the Laurentide Ice Sheet, Lancaster Sound
Authors: Klassen R. A. , Fisher D. A.
Published: April 2011

A major moraine system developed by outlet glaciers of the Laurentide Ice Sheet on coastal mountains of Bylot Island provides a basis for reconstructing paleoglacier surfaces and estimating basal shear stresses within the ice. The system, named Eclipse moraine, lies between elevations of 100 and 500 m asl and can be traced almost continuously over distances of 50–70 km. Where the glaciers flowed inland within valleys, the outline of the moraine on valley walls is used to portray the profile of the glacier along the valley median. If ideal plastic flow is assumed, ice thicknesses and surface slopes derived from the reconstructed profiles indicate yield stresses of about 125 ± 46 kPa. Reconstructed profiles of the main outlet glaciers entering Baffin Bay from Lancaster Sound and Pond Inlet indicate lesser yield stresses of about 55–80 kPa, respectively, within the channels; estimates are 35–60 kPa if isostatic depression is assumed. Calculations indicate mat basal shear stresses in the channels were near the low end of the range characteristic of modern ice sheets and ice caps.
Massive ground ice and ice-cored terrain near Sabine Point, Yukon Coastal Plain
Authors: Harry D. G. , French H. M. , Pollard W. H.
Published: April 2011

Massive ground ice, 5–6 m in thickness, is exposed within retrogressive thaw flow slides near Sabine Point, Yukon Territory. The ice is present near the upper surface of Buckland Till and is overlain and thaw truncated by mudflow sediments and a thick unit of peat and organic silt. Cryotextural and petrographic analyses suggest that the ice formed primarily by segregation processes. The ice occurs within an area of rolling terrain, surrounded by lacustrine basins. This may form a remnant of an initial post-Buckland surface, degraded by multiple cycles of thermokarst during the period 14 000 to 8000 years BP.
Fluid-inclusion evidence of basinal brines in Archean basement, Thunder Bay Pb–Zn–Ba district, Ontario, Canada
Authors: Haynes Frederick M.
Published: April 2011

Fluid inclusions from three quartz–galena–sphalerite–barite–calcite veins in the Thunder Bay district of western Ontario contain liquid + vapor ± halite and homogenize by vapor disappearance or halite dissolution at temperatures of 90–200 °C. Cyclically frozen, liquid + vapor (type I) inclusions undergo four melting events upon gradual warming (initial melting at −55 to −46 °C; ice disappearance at −30.2 to −25.4 °C; inversion of hydrohalite to halite at −8.0 to 0.7 °C; and halite melting at 14.0 to 56.3 °C. Liquid + vapor + halite (type II) inclusions behave similarly but have higher Tm ice (−27.2 to −21.7 °C) and Tm halite (105–203 °C). Scanning electron microscopy and energy dispersive analysis of fluid-inclusion-derived decrepitates indicate that the solutes consist of NaCl > CaCl2 [Formula: see text] KCl and are consistent with the low-temperature phase observations in that they define two distinct populations based on CaCl2/(CaCl2 + NaCl) ratios.The temperatures and compositional trends defined by the inclusion results are similar to those documented for basinal brines and from fluid inclusions in Mississippi Valley type ore deposits. The Thunder Bay veins cross the basal unconformity of the Middle Proterozoic Sibley basin and extend into Archean basement granites, such that the fluid inclusions results provide direct evidence that basinal waters infiltrated basement rock in western Ontario. The inclusion fluids and associated mineralization are thought to result either from dewatering of the Sibley basin during Keweenaw age rifting or from the introduction of exotic Paleozoic basinal waters when the Michigan basin extended over the region.
Pleistocene epigenetic and syngenetic frost fissures, Belchatów, Poland
Authors: French H. M. , Gozdzik J. S.
Published: April 2011

Pseudomorphs of cryogenic fissures in stratified medium to fine silty sand and sand at Belchatów, Poland, indicate both epigenetic and syngenetic freezing of sediments at various times during the Pleistocene. Syngenetic fissures are more frequent. They are typically 0.5–2.0 m in vertical dimension, range in width from 5 to 20 cm, exhibit both upwarps and downwarps of enclosing sediments, and are offset from each other in the stratigraphic sequence. Epigenetic fissures are larger, generally increase in width upwards, and are related to an inferred ground surface. It is difficult to distinguish between syngenetic fissures that penetrate permafrost and soil (ground) veins or wedges that require only deep seasonal frost for their formation. Cold, continental environments characterized by rapid sedimentation and permafrost aggradation appear most conducive for formation of syngenetic fissures. Today, both large and small syngenetic fissures are known to exist in the permafrost regions of Siberia but are rarely reported from the western Arctic lowlands of North America.
U–Pb geochronology of reactivated Archean basement and of Hudsonian metamorphism in the northern Labrador Trough
Authors: Machado N. , Goulet N. , Gariépy C.
Published: April 2011

The Labrador Trough is a Proterozoic orogenic belt bordering the eastern margin of the Archean Superior Province. The volcano-sedimentary sequences of the trough were deformed and metamorphosed during the Hudsonian Orogeny (ca. 1800 Ma). The eastern basement, present as domal inliers, was remobilized during this orogeny, whereas the western basement remained unaffected.In this study we present U–Pb ages of the western and eastern basement of the Proterozoic Labrador Trough orogen between Leaf River and Koksoak River and of the northernmost extension of the DePas batholithic complex farther east. From the western basement, the Leaf Bay granodiorite has an age of 2721 ± 3 Ma (zircon). From the gneiss domes of the eastern basement the ages are as follows: Lac Moyer—2883 ± 6 Ma (zircon), 1793 ± 5 Ma (monazite), and 1746 Ma (minimum age, rutile); Lac Boulder—an upper-intercept age of 2868 ± 8 Ma and a lower-intercept age of 1783 ± 11 Ma (zircon–titanite regression) and an age of 1740 ± 5 Ma (rutile); Lac Olmstead Lake—2721 ± 4 Ma (zircon) and 1774 ± 5 Ma (titanite); Leaf Strait—2719 ± 7 Ma (zircon) and 1783 ± 2 Ma (monazite). The DePas Batholith contains four generations of zircon: the minimum ages of the three oldest are 2688, 2779, and 2922 Ma; we also obtained a monazite age of 1808 ± 2 Ma.These results lead us to the following conclusions: (i) The age of the Leaf Bay granodiorite, 2721 ± 3 Ma, is a minimum age of the gneisses of the Minto Subprovince, (ii) The westernmost gneiss domes of the eastern basement (Lac Moyer and Lac Boulder), bounded by two major faults, contain zircon formed during metamorphic events dated at 2868 ± 8 and 2883 ± 6 Ma and may be part of an allochthon. (iii) The Lac Olmstead and Leaf Strait domes contain metamorphic zircon dated at 2719–2721 Ma. (iv) The minimum age of migmatization of the DePas complex is 2688 Ma. (v) The monazite ages date the latest phase of metamorphism in the area, (vi) The titanite ages probably represent post-deformational cooling, whereas the rutile ages could represent late-metamorphic hydrothermal activity, (vi) The available data suggest the presence of continuous Archean basement—remobilized during the Hudsonian Orogeny—between the Labrador Trough and the DePas Batholith, which could represent a collision zone. The final stages of this collision could be slightly older (1808 Ma) than the equivalent activity in the Labrador Trough. These speculations raise the possibility that the evolution of the Labrador Trough is closely related in time and space to events occurring in its "hinterland".