bluuebunny/crossref_metadata_2025 huggingface dataset:
Processus de fractionnement dans les coulées komatiitiques archéennes: cas des laves à spinifex de la ceinture de roches vertes de Tipasjärvi (Finlande orientale)
Authors: Blais Sylvain , Auvray Bernard , Jahn Bor-ming , Taipale Kalle
Published: April 2011

Komatiitic flows (ultramafic lavas with MgO > 18%) from the small Tipasjärvi Archean greenstone belt (eastern Finland) are of two types: (1) thin lava flows with microspinifex textures showing no chemical variation along a vertical cross section; (2) a thicker flow with important chemical variation in which macroscopic spinifex textures are found within small veins in the upper part of this flow. Two different trends of fractional crystallization can be demonstrated by major- and some trace-element variations in the upper and the lower parts of the flow: olivine crystallizes and settles to the base of the flow; in the upper part, the major phase that crystallizes seems to be clinopyroxene.Such double-trend fractionation has also been shown in some other komatiitic flows of similar ages from Australia and Canada. However, some komatiitic flows indicate a simpler fractionation process involving olivine only.Rare-earth-element (REE) distribution patterns in the thick Finnish flow show two quite different patterns. The upper part of the flow has strongly LREE-depleted patterns, whilst those in the lower part of the flow are flat. It is suggested that the flow is composite and that two source regions are involved.
The geochemistry of komatiites and basalts from the Deadman Hill area, Munro Township, Ontario, Canada
Authors: Canil Dante
Published: April 2011

A sequence of Archean komatiites (> 18 wt.% MgO), komatiitic basalts (10–18 wt.% MgO), high-Mg tholeiites (6–10 wt.% MgO), and high-Fe tholeiites (< 8 wt.% MgO) is exposed in the Deadman Hill area of Munro Township, Ontario, Canada. Major- and trace-element analyses of 28 samples are used to assess their petrogenetic significance. The use of molecular proportion ratio plots shows the samples have maintained their primary SiO2, FeO*, MgO, TiO2, Al2O3, Ni, Cr, Zr, Y, and V contents. Secondary redistribution of Na2O, K2O, Rb, Sr, Ba, and, in some samples, CaO has occurred.Covariation in both major- and trace-element data suggests the komatiites are primary melts that equilibrated with a harzburgite residua at pressures of 3–6 GPa. Garnet did not have a major role in their petrogenesis or in the petrogenesis of spatially related komatiitic basalts and high-Mg tholeiites. Major- and trace-element variation in komatiitic basalts with 17–12 wt.% MgO requires that they be partial melts in equilibrium with clinopyroxene at pressures of < 3 GPa. They are unrelated to the komatiites. Both lower degree partial melting of the same source as lavas with 17–12 wt.% MgO and crystal fractionation of clinopyroxene from liquids with ~12 wt.% MgO can model the evolution of less magnesian komatiitic basalts and high-Mg tholeiites. The Zr/Y and Zr/Ti ratios of the high-Fe tholeiites indicate that they are unrelated to the komatiites, komatiitic basalts, and high-Mg tholeiites and were derived by partial melting of a garnet lherzolite source.
The use of automated image analysis in coal petrology
Authors: Goodarzi Fariborz
Published: April 2011

A comparison was made between the data obtained using the Zeiss image analyser (IBAS) and those obtained by conventional methods, using polished, particulate and blocks of coal. The results obtained by using the two methods are in good agreement.IBAS is an automatic system and reduces the data (reflectance measurement and point count analysis) collecting time substantially, compared with conventional methods. Seam profiles, based on polished blocks, can be constructed rapidly using IBAS to determine the paleoenvironment of coal deposition or the position of macerals resistant to grinding. Also, evaluation and presentation of large numbers of coal samples can be performed in a short time by using polished, particulate blocks.
Plant and insect fossils from Nipissing sediments along the Goulais River, southeastern Lake Superior
Authors: Warner Barry G. , Karrow Paul F. , Morgan Alan V. , Morgan Anne
Published: April 2011

Fossiliferous sediments have long been known to occur along the banks of the Goulais River. Radiocarbon dating has established their time of deposition as late in the Nipissing transgression. Sedimentary environments and fluvial conditions in the ancestral river channel shaped fossil assemblages. Pollen reflects regional upland forests of Pinus and Picea, with other trees and shrubs such as Quercus, Fraxinus, Acer, Ulmus, and Alnus. Forested peatlands, fens, and emergent and submerged aquatic communities were common in lowland valleys and along the floodplain of the river. These reconstructions are supported by aquatic insect fossils and hygrophilous taxa such as Micropeplus sculptus and Elaphrus olivaceus and also are confirmed by plant macrofossils such as Myrica gale, Triadenum virginicum, Dulichium arundinaceum, Typha, Brasenia schreberi, Potamogeton spirillus, and possibly Zizania palustris. There is no evidence for regional climatic change during this portion of Holocene time.
New Lower Jurassic ammonite faunas from the Fernie Formation, southern Canadian Rocky Mountains
Authors: Hall Russell L.
Published: April 2011

New ammonite faunas are described from sections along Bighorn and Scalp creeks in central-western Alberta where Lower Jurassic parts of the Fernie Formation are exposed. The first record of the upper Sinemurian Obtusum Zone from the Fernie is based on the occurrence of Asteroceras cf. stellare and Epophioceras cf. breoni in the basal pebbly coquina on Bighorn Creek. The overlying Red Deer Member has yielded Amaltheus cf. stokesi, representing the upper Pliensbachian Margaritatus Zone; in immediately superjacent strata the first North American examples of ?Amauroceras occur together with Protogrammoceras and ?Aveyroniceras. In the basal parts of the overlying Poker Chip Shale a fauna including Harpoceras cf. falciferum, Harpoceratoides, Polyplectus cf. subplanatus, Hildaites cf. serpentiniformis, and Dactylioceras cf. athleticum is correlated with the lower Toarcian Falciferum Zone.The upper parts of the Poker Chip Shale on Fording River in southeastern British Columbia contain a fauna representing some part of the upper Toarcian, but owing to poor preservation, generic identifications are only tentatively made.
Fossil evidence for fault-derived stratigraphic repetition in the northeastern Newfoundland Appalachians
Authors: Pluijm Ben A. van der , Karlstrom Karl E. , Williams Paul F.
Published: April 2011

Two types of bedding-parallel faults are common in the Dunnage Zone of eastern Notre Dame Bay. They are (i) early thrusts, which together with bedding were rotated by later folding into steep attitudes; and (ii) postfolding transcurrent faults. Both types of faults occur at all scales and give rise to repetitions in the stratigraphic sequence.A compilation of the ages of various rock types based on fossil evidence indicates a relatively simple stratigraphy for the Dunnage Zone. This interpretation appears to be at variance with many reported stratigraphic successions, until repetitions by bedding-parallel faulting are taken into account.Examples are given of sequences in which the repetition of various rock types, dated by means of fossils, is due to bedding-parallel faulting. We believe that the simple lithostratigraphy is a reliable aid for structural interpretations in areas where fossils are scarce or absent.
Gold–copper–bismuth mineralization in hedenbergitic skarn, Tombstone Mountains, Yukon
Authors: Brown Isobel J. , Nesbitt Bruce E.
Published: April 2011

Gold mineralization on the Marn property, Yukon, occurs in two pyroxene skarn bodies, which are adjacent to the Mount Brenner Stock in the Ogilvie Mountains. The skarns are separated by a 600 m wide monzonite intrusion and show contrasting mineralogical and geochemical characteristics in addition to quite different metal values. Significant but uneconomic Au, Ag, W, and Cu mineralization is found in skarn on the north side of the intrusion, while very low Au grades (0.052 g/t) occur at the southern contact. The mineral assemblages of both skarns are dominated by iron-rich pyroxenes. The iron content of the pyroxenes varies between Hd40 and Hd80 in the northern location and Hd80 and Hd100 in the southern skarn. A well-developed sequence of retrograde alteration affected only the northern skarn. This was probably the result of porosity and permeability differences in the early, high-temperature pyroxene skarn, which permitted greater fluid–rock interaction in the northern skarn during cooling. A small volume of diopsidic, aluminous, wollastonite-bearing skarn occurs in both the northern and southern localities. The relationship of this type of skarn to the hedenbergitic skarn is ambiguous, since there is no large-scale mineralogical zoning. The Marn is similar to hedenbergitic, auriferous skarns of Japan, where the oxidation state of the intrusive rocks is believed to be the controlling factor in the development of skarn mineralogy.
Pre-Silurian stratigraphy and tectonic significance of the St. Croix Belt, southeastern Maine
Authors: Ludman Allan
Published: April 2011

The St. Croix Belt of southeastern Maine and southwestern New Brunswick is part of a distinctive terrane situated between the Avalon platform and Miramichi arc—two important physiographic components of the Late Precambrian – early Paleozoic Iapetus Ocean. It bears lithologic and stratigraphic similarities to both of those tracts but is identical to neither. Formerly mapped entirely as the Cookson Formation, it is now divided into four formations of Cambrian through Early Orodovician age assigned to the Cookson Group. These rocks record periodic influxes of terrigenous debris into a deep anoxic basin and may have been part of the west-facing continental slope of the Avalonian continental block. The St. Croix Belt has been strongly affected by both pre-Silurian and Early Devonian (Acadian) folding. Facies changes rather than tectonic sutures are used to explain the relationships between the St. Croix Belt and its neighbouring pre-Silurian tracts.
On the age of the Queen Charlotte Group of British Columbia
Authors: Haggart James W.
Published: April 2011

The Cretaceous Queen Charlotte Group has traditionally been divided into three lithostratigraphic units in the type region: the Haida, Honna, and Skidegate formations. The sandstone member of the Haida Formation is commonly rich in fossil molluscs, and its Albian age is well documented. Megafossils are less abundant in the shale member of the Haida Formation and in the Honna and Skidegate formations; consequently, the ages of these units have not been well defined. In the last century, however, the early Turonian bivalve Mytiloides labiatus (s.l.) was identified from both the shale member of the Haida and the Skidegate Formation; unfortunately, the stratigraphic significance of these occurrences was not fully appreciated at that time. More recent collections of fossil molluscs from the Queen Charlotte Group have verified that both the Skidegate Formation and the upper part of the Haida shale member include strata of Turonian age, thus indicating the two units are, at least in part, lateral equivalents. Based on the presence of the megafossils Peroniceras (s.s.) sp. and Plesiotexanites sp., it is concluded that the greater part of the Honna Formation is of Coniacian age and caps the stratigraphic sequence.
Diagenesis and pyritization of crinoid ossicles
Authors: Brand Uwe , Morrison Joan O.
Published: April 2011

Diagenetic alteration of original high-Mg calcite crinoid skeletal material was achieved by reaction of the calcium carbonate with isotopically light meteoric water. The light δ18O values (−10.57 ± 0.74‰, PDB) are a reflection of the water source, whereas the δ13C values (−3.49 ± 2.63‰, PDB) are a consequence of the high reactive-organic-matter content in the sediments of the Brush Creek at Sewickley, Pennsylvania. High Fe (22 970 ± 30 440 ppm) and Mn (3760 ± 1450 ppm) contents suggest further that the diagenetic fluids were reducing waters that reacted with the crinoid ossicles in the shallow-burial marine and meteoric environments.Four different types of pyrite are found within crinoid ossicles. Framboidal pyrite, aggregate pyrite, and nanopyrite are generally found in the pores of the stereom. Micropyrite, which is present in the form of octahedral, dodecahedral, and pyritohedral crystals, and nanopyrite and aggregate pyrite replace the calcitic stereom. The degree of pyritization and the pore filling increase toward the outer periphery of the crinoid ossicles. Pyrite formation of the crinoid ossicles took place in two stages. In stage I, upon death, the crinoids disarticulated and were quickly buried. In this shallow sediment layer (1–10 cm) the pore water was both undersaturated with respect to calcite and oxidizing. This brought about oxidation of the organic tissue in the stroma of the crinoids and selective dissolution of the high-Mg calcite skeletons. With further burial in stage II, in the presence of reactive organic matter local iron was solubilized, marine sulphate was reduced to sulphide, and isotopically light carbon was produced by bacterial action. Reactive iron combined with sulphur to form framboidal and nanopyrite in the pores of the stereoms. With further burial the micropyrite formed in the crinoid ossicles. Termination of the pyritization process came about with depletion of the iron and (or) sulphur, and this process proceeded very rapidly under shallow-burial conditions while the crinoids resided in the microbial sulphate-reduction zone of the marine–phreatic environment.Fossils are preserved in different stages, with brachiopod valves preserved in their original low-Mg calcite mineralogy, whereas molluscs and crinoids show the complete trend from preserved aragonite and high-Mg calcite, respectively, to diagenetic low-Mg calcite. The preservation potential of fossils is closely linked to the thermodynamic stability of the skeletal carbonate in the presence of diagenetic fluids. Carbon/sulphur ratios support the assertion that Brush Creek sediments were deposited in normal marine waters and favour generally oxic redox conditions for Pennsylvanian seawater.
Heterochrony (progenesis) in the Silurian crinoid Homocrinus Hall
Authors: Eckert James D.
Published: April 2011

Homocrinus displays a number of characteristics inferred to be progenetic, including unusually small size, greatly elongated free brachials, enlarged proxistele, and absence of an anal tube. Accelerated ontogeny allowed Homocrinus to achieve early maturity and reproduction, ensuring survival in paleoenvironments characterized by frequent, episodic, high rates of sedimentation commonly unfavourable to other crinozoans.
Stable isotopes in sulphate minerals from the Salina formation in southwestern Ontario
Authors: Fritz P. , Lapcevic P. A. , Miles M. , Frape S. K. , Lawson D. E. , O'Shea K. J.
Published: April 2011

The oxygen and sulphur isotopic compositions of anhydrite and minor gypsum from the Salina formation in the Michigan Basin of southwestern Ontario were investigated to obtain information on depositional environments. The data base was also used to compare the isotopic compositions of these Late Silurian sulphates with global "age" curves for evaporites.The morphological and petrographical appearance of the sulphates suggests a shallow-water, possibly sabkha-type environment of deposition of the Salina anhydrites throughout most of the basin. δ18O values are rather consistent throughout the Salina but with short and substantial isotopic shifts in specific horizons. These rapid changes reflect the importance of "redox cycling" in shallow, restricted environments. No significant correlation between δ18O and δ34S is observed in massive and bedded anhydrites but is recognized in nodular and some chickenwire anhydrites. Biological reduction of sulphate in the environment of deposition must account for this.Comparison of 18O and 34S data with those from older and younger evaporites shows that the Salina sulphates are slightly depleted in 18O, if compared with the global, extrapolated curves for marine sulphates, but are enriched in 34S, possibly reflecting a major and rapid change in marine sulphate composition at the transition from the Silurian to Devonian times.
Rates of infiltration into frozen and unfrozen fine sand
Authors: Engelmark Helén
Published: April 2011

Laboratory experiments on infiltration into a fine sand under frozen and unfrozen conditions have been performed. The infiltration rate has been measured for different soil-temperature profiles. Different temperatures of the rain or meltwater have been simulated. Under frozen conditions, the infiltration rate was 1 – 2 mm/min, which is about one order of magnitude smaller than the infiltration rate under frozen conditions (10.6 mm/min). The results are discussed and compared with other data.
Petrology of a plagioclase-bearing olivine websterite from the Gorringe Bank (northeastern Atlantic Ocean)
Authors: Serri Giancarlo , Hébert Réjean , Hekinian Roger
Published: April 2011

Samples of plagioclase olivine websterite included in variously serpentinized harzburgite and lherzolite tectonites were collected by submersible on the northwest flank of the Gettysburg Seamount near 36°N in the northeastern Atlantic. The websterite exhibits a coarse porphyroclastic texture (70% porphyroclasts), and the composition of the pyroxene shows a progressive re-equilibration with decreasing temperature (and probably pressure) under anhydrous conditions from a near-solidus temperature of ~1250 °C (porphyroclasts) down to ~700 °C (granoblasts). In situ observations, as well as textural, mineralogical, and chemical evidence, suggest that the websterite can best be interpreted as a heteradcumulate rock that crystallized on the walls of narrow conduits from a MORB type magma flowing through the residual tectonites.The Mg #'s (= 100 × Mg/(Mg + Fe2+)) of coexisting clinopyroxene, orthopyroxene, and olivine (91.5–90.5, 90.0–89.7, and 89.5–88.5, respectively), the plagioclase composition (An88.7–87.9), and the mineral minor-element distribution are not compatible with the crystallization at low pressure of the websterite from N-MORB type parental magma. Instead, the calculated composition of the liquid in equilibrium with the websterite's most magnesian minerals and the bulk-rock composition are more akin to those of a poorly differentiated T-MORB than of N-MORB parental magma. Phase relations and mineral compositions produced by anhydrous melting experiments for synthetic and natural systems indicate that the pressure of crystallization of the plagioclase olivine websterite is constrained within the narrow range of 7–10 kbar. It is thought that fractionation of websterite is probably a minor but significant petrologic factor of MORB type magma differentiation. It could explain some of the compositional variations observed in magnesian MORB.
Lead-isotope compositions of galena in carbonate-hosted deposits of western Newfoundland: evidence for diverse lead sources
Authors: Swinden H. Scott , Lane T. E. , Thorpe R. I.
Published: April 2011

Carbonate-hosted zinc and lead deposits occur in two settings in the Humber Terrane of western Newfoundland: (i) associated with dolomitized units and breccias in Cambrian and Ordovician shelf-facies carbonate rocks; and (ii) as open-space, vein, and breccia fillings associated with calcite within or near the margins of Carboniferous basins. We report 11 new lead-isotope analyses from deposits of the former type on the Great Northern Peninsula (the GNP suite) and 8 new analyses from deposits of the latter type on the Port au Port Peninsula and the northern part of the Bay St. George subbasin (BSG suite).The isotopic composition of lead in galena from the GNP suite defines linear trends in which the lead-isotope ratios increase with stratigraphic height in the Cambro-Ordovician sequence. These trends are interpreted as mixing lines; samples at the non-radiogenic end have very low 207Pb/204Pb but high 208Pb/204Pb ratios relative to 206Pb/204Pb and are interpreted as representing substantial evolution in high-grade metamorphic rocks of the Grenville Structural Province that underlie the sedimentary succession. The more radiogenic source(s?) is probably located within the sedimentary pile and contributed metals to the fluids during their migration to the deposition sites.Lead isotopes in deposits in the Bay St. George area cluster near average crustal model growth curves. Metal sources for these deposits, considered by previous workers as diagenetic in origin, are interpreted as being clastic sediments within the sedimentary basin.
Pipe flow in James Bay coastal wetlands
Authors: Woo Ming-Ko , DiCenzo Peter
Published: April 2011

Pipes are common along the banks of streams that drain the wetlands of the James Bay coast. The primary source of water to the pipes is overland flow, which responds very quickly to rainfall events. Pipe flow, therefore, becomes insignificant shortly after the termination of surface runoff. An assessment of the relative contributions of pipe flow and Darcy's groundwater flow to a small coastal creek shows that whereas groundwater flow contributed less than 1% to summer stormflow, pipes provided about 10% of stream runoff.
Stable isotopes in planktonic and benthic foraminifera from Arctic Ocean surface sediments
Authors: Aksu A. E. , Vilks G.
Published: April 2011

Oxygen and carbon isotopic analyses have been performed on the tests of Planulina wuellerstorfi and three size fractions of sinistral Neogloboquadrina pachyderma recovered from 33 Arctic Ocean surface-sediment samples. Stable isotopic compositions of N. pachyderma are found to be dependent on the test size: larger specimens show considerable enrichment in both δ18O and δ18C. The difference between the isotopic compositions of the 63–125 and 125–250 μm size fractions in N. pachyderma can be explained by biogenic fractionation effects during foraminiferal test growth. Larger (250–500 μm) N. pachyderma displayed accretions of secondary calcite, i.e., the outermost shell contained significant amounts of inorganically precipitated magnesium calcite. Thus, larger foraminifera may not be suited for down-core stable isotopic studies. There is a difference of ~2‰ between δ18O values of surface samples from the eastern and western Arctic Ocean, reflecting large differences between surface-water salinity in these regions. Therefore, oxygen isotopic data may have limited use as a chronostratigraphic tool in down-core studies in the Arctic Ocean, but we can use them to infer past variations in surface-water salinities. Planulina wuellerstorfi also showed depletions of both δ18O and δ18C in its calcite tests relative to calcite precipitated in isotopic equilibrium with ambient sea water; these depletions ranged from −0.8 to −0.9‰ in δ18Oand −1.2 to −0.9‰ in δ18C. This taxon is found to deposit its shell very close to the δ18C of ΣCO2 of bottom waters.
Neogene geology of the Okanagan Highland, British Columbia
Authors: Mathews W. M.
Published: April 2011

Neogene basaltic successions, one of the Early(?) and Middle Miocene age, the other of latest Miocene and Pliocene age, rest on land surfaces characterized by broad, low-relief components, above which rose hills that may never have been totally covered by the basaltic rocks. Local stream deposits, in places auriferous or uraniferous, have accumulated in channels cut into or running on the subbasalt surface and have been covered by the basalts. A subsequent, Late Pliocene and post-Pliocene differential uplift has produced elongate, northerly trending topographic highs and has led to major incision by streams. A third basaltic succession, which locally extends into the incised valleys, is recognized but is not discussed in detail.
Convection tectonics: some possible effects upon the Earth's surface of flow from the deep mantle
Authors: Wilson J. Tuzo
Published: April 2011

Until a little more than a century ago the land surface not only was the only part of the Earth accessible to humans but also was the only part for which geophysical and geochemical methods could then provide any details. Since then scientists have developed ways to study the ocean floors and some details of the interior of the Earth to ever greater depths. These discoveries have followed one another more and more rapidly, and now results have been obtained from all depths of the Earth.New methods have not contradicted or greatly disturbed either old methods or old results. Hence, it has been easy to overlook the great importance of these recent findings.Within about the last 5 years the new techniques have mapped the pattern of convection currents in the mantle and shown that these rise from great depths to the surface. Even though the results are still incomplete and are the subject of debate, enough is known to show that the convection currents take two quite different modes. One of these breaks the strong lithosphere; the other moves surface fragments and plates about.It is pointed out that if expanding mid-ocean ridges move continents and plates, geometrical considerations demand that the expanding ridges must themselves migrate. Hence, collisions between ridges and plates are likely to have occurred often during geological time.Twenty years ago it was shown that the effect of a "mid-ocean ridge in the mouth of the Gulf of Aden" was to enter and rift the continent. This paper points out some of the conditions under which such collisions occur and in particular shows that the angle of incidence between a ridge and a coastline has important consequences upon the result. Several past and present cases are used to illustrate that collisions at right angles tend to produce rifting; collisions at oblique angles appear to terminate in the lithosphere in coastal shears, creating displaced terrane, but in the mantle the upward flow may continue to uplift the lithosphere far inland and produce important surface effects; collisions between coasts and mid-ocean ridges parallel to them produce hot uplifts moving inland. For a time these upwellings push thrusts and folds ahead of them, but they appear to die down before reaching cratons.
The Iceland Research Drilling Project crustal section: stable remagnetization below 3 km crustal depth
Authors: Hall James M. , Fisher Brian E.
Published: April 2011

A section at below 3.1 km depth in Icelandic crust, sampled in the 1978 Icelandic Research Drilling Project drill hole, contains a number of subaerially deposited lava flows showing both downwards and probably original upwards inclinations of cleaned, stable remanent magnetization. Such "mixed polarities" are inconsistent with an initial cooling thermoremanent origin for the magnetization. An attempt is made to identify the factors involved in producing these mixed polarities and to consider the possible wider importance of the results. The mixed-polarity flows have experienced intense hydrothermal alteration, followed by the widespread deposition of secondary magnetite. Secondary magnetite, which is formed in relatively anhydrous conditions associated with dike intrusion, dominates primary magnetite volumetrically where dike density locally exceeds about 30%.Where secondary magnetite is very dominant or is the only type of magnetite present, directional remagnetization appears to be uniform and complete. Where secondary and primary magnetite are both important, relatively high remanence and saturation magnetizations, total magnetite and primary magnetite grain size, and low deuteric oxidation state of primary magnetite are all associated with downwards directional remagnetization. It appears that a complex balance of the properties and history of primary and secondary magnetite, in addition to the relative abundances of these phases, controls the final stable polarity of samples.If the narrow transition zones between little-altered extrusives, greenschist-facies flows and dikes of the Troodos (Cyprus) ophiolite, and DSDP hole 504B are typical of oceanic crust, a narrow ~0.2 km interval of mixed polarities may be underlain in some locations by an intermediate crustal layer in polarity opposition with the uppermost, little-altered, extrusive layer.
Origin and U–Pb geochronology of amphibolite-facies metamorphic rocks, Miramichi Highlands, New Brunswick
Authors: Fyffe Les , Barr Sandra M. , Bevier Mary Lou
Published: April 2011

The Miramichi Highlands of New Brunswick are underlain by subgreenschist- to greenschist-facies sedimentary and volcanic rocks of the Cambro-Ordovician Tetagouche Group and by amphibolite-facies paragneisses, amphibolites, and felsic orthogneisses of the Trousers Lake and Sisson Brook suites. New field, geochemical, and geochronologic data for the amphibolites and felsic orthogneisses suggest that they are high-grade metamorphic equivalents of the Tetagouche volcanic rocks and their associated intrusions.Amphibolites in the Miramichi Highlands occur as striped and unstriped varieties that possess chemical characteristics indicative of an igneous origin. However, the two types are compositionally distinct: the striped amphibolites resemble volcanic-arc tholeiites, whereas the unstriped amphibolites are like within-plate tholeiites. The geochemically inferred tectonic origin of these amphibolites is compatible with a recently proposed intracontinental back-arc tectonic setting for the Tetagouche Group.Felsic orthogneisses (Fox Ridge augen granite and Trousers Lake felsic orthogneiss) exhibit concordant contacts with the unstriped amphibolites. U–Pb zircon ages for the Fox Ridge augen granite [Formula: see text] and Trousers Lake felsic orthogneisses [Formula: see text] indicate a Late Ordovician intrusive event. Thus, there is no evidence for Precambrian granite and orthogneiss in the Miramichi Highlands, as had been previously inferred from a correlation with purported Precambrian rocks in the Gander Zone of Newfoundland. The age of the unstriped amphibolites is interpreted as being the same as that of the felsic orthogneisses because these two rock types always exhibit close relationships in the field. The age of the striped amphibolites is less certain, although a correlation with Ordovician basalts of the Tetagouche Group is consistent with their field relationships and tectonic setting.
Geochemistry of the gneissic basement complex near Valemount, British Columbia: further evidence for a varied origin
Authors: Chamberlain V. E. , Lambert R. St J. , Duke M. J. M. , Holland J. G.
Published: April 2011

Rare-earth elements and other trace elements have been determined by activation analysis and X-ray fluorescence for representative samples from each of the four blocks of basement gneisses near Valemount, eastern British Columbia. Patterns in mafic and tonalitic gneisses are generally as expected, but the granite–gneisses have very large negative europium anomalies, up to Eu*/Eu = 18, indicating multistage histories involving plagioclase fractionation. Modelling shows that plagioclase fractionation alone is insufficient to account for these anomalies without intervention of other phases: apatite control is suggested, among other possibilities. The granite–gneisses also contain exceptionally low magnesium (0.1–0.2%), phosphorus (<300 ppm), scandium (<0.07 ppm), and cesium (<0.5 ppm). After partial melting is considered as a possible mode of origin, it is concluded that the granite–gneisses are final, small-scale fractionates from enriched tholeiite magmas. These might be associated with upper Proterozoic rifting processes. Previously published conclusions regarding the protolith of each subset of the gneisses are confirmed; likewise, the earlier conclusion that the gneisses cannot be correlated across the Southern Rocky Mountain Trench is substantiated in detail.
Thrust-related metamorphism beneath the Shetland Islands oceanic fragment, northeast Scotland
Authors: Spray John G.
Published: April 2011

A ≤400 m thick metamorphic sequence showing thermal inversion is present beneath a dismembered ultrabasic–basic complex in the Shetland Islands of northeast Scotland. The metamorphic grade changes from upper amphibolite facies in metabasites at the top of the sequence to low greenschist facies in metasediments at the base. Garnet–clinopyroxene thermometry yields temperatures of ~ 750 °C (at 300 MPa) for the highest grade assemblage. There is no evidence for high pressures of metamorphism, and maximum overburden may never have exceeded the original thickness of the overlying ultrabasic–basic complex, which is estimated to have been ~ 10 km.The internal structure and field relations of the ultrabasic–basic complex reveal that it is a displaced fragment of oceanic crust and upper mantle of Ordovician age. The chemistry of its basic lithologies suggests low-K tholeiite, suprasubduction zone, pre-arc affinities. In contrast, the underlying meteamorphic sequence possesses a mid-ocean ridge basalt (MORB) signature.Four K–Ar age determinations from amphibole mineral separates of the metamorphic sequence range from 479 ± 6 to 465 ± 6 Ma. The highest age is interpreted as the date of the onset of metamorphic sole formation and the initial tectonic displacement of the oceanic fragment.It is concluded that the metamorphic sequence was generated during intraoceanic thrusting during the destruction of a young, marginal oceanic basin located between a continental margin and the ocean lithosphère of Iapetus. Certain MORB lithologies were metamorphosed and transferred to the marginal basin hanging wall during the subduction of Iapetus. Apparent thermal inversion was caused during overthrusting by the gradual underplating of the hanging wall in close proximity to a suprasubduction zone spreading centre.
An 40Ar/39Ar geochronological study of the Elzevir batholith and its bearing on the tectonothermal history of the southwestern Grenville Province, Canada
Authors: Hanes J. A. , Clark S. J. , Archibald D. A.
Published: April 2011

Results of 40Ar/39Ar step heating for muscovite, potassium feldspar, and plagioclase from the ca. 1240 Ma Elzevir trondhjemite in the Central Metasedimentary Belt of the Canadian Grenville Province have been combined with earlier data and used to deduce a thermal history for the eastern Elzevir terrane following the Ottawan orogeny (1050–950 Ma). Muscovite yielded precise plateau dates of ca. 900 Ma, whereas the potassium feldspar displayed disturbed age spectra with ca. 730–760 Ma dates for broad "plateau" regions. This age difference could be explained by slow cooling or by thermal overprinting at 760–800 Ma that has updated the microcline. The plagioclase spectra provide evidence for a low-temperature hydrothermal event ca. 380 Ma ago that has generated sericite in saussurite and has caused partial argon loss from the microcline. This result agrees with the other plagioclase thermochronometry and paleomagnetism on the Cordova gabbro 25 km to the west but provides a lower estimate for the upper age bracket of this event. It is suggested that earlier models of protracted, post-700 Ma cooling of this part of the Grenville orogen may be a consequence of variable updating of plagioclase by this Devonian alteration event.Results of 40Ar/39Ar step heating for muscovite and microcline from the Bark Lake diorite in the Bancroft terrane are in agreement with earlier work in this area, and the cooling path of the Elzevir trondhjemite is seen to be indistinguishable, within the limitations of the method, from that of the Bark Lake diorite in the time period 1050–750 Ma.
Slave River delta: geomorphology, sedimentology, and Holocene reconstruction
Authors: Vanderburgh Sandy , Smith Derald G.
Published: April 2011

The Holocene Slave River delta (8300 km2) is a long (170 km), narrow (42 km average width) alluvial sand body, which extends north from the Slave River rapids at Fort Smith to Great Slave Lake, Northwest Territories. The delta is flanked by the Talston and Tethul rivers and Canadian Shield to the east and by the Little Buffalo River to the west. Wave-associated sedimentary structures in lithostratigraphic logs from river cutbanks indicate that the sandy delta was wave influenced. Most of the logs (34) consist of three facies: basal laminated mud (unknown thickness), interbedded mud and sand (2.5 m), and planar-tabular ripple sets interbedded with cross-laminated to flat-bedded sand (3.0–14.5 m).Eleven radiocarbon-dated wood samples from river cutbanks were used to reconstruct the delta paleoshoreface and to calculate the rate of progradation, which averaged 20.7 m/year from 8070 BP to the present. In the same period isostatic rebound of the delta region relative to the Liard River delta averaged 12 cm/km (a total rebound of 48 m). The data were calculated normal to the retreating Laurentide ice front.From the surface to depths of 59 m, the subaerial and subaqueous delta front exhibits barrier islands, lagoons, offshore bars or sand waves, tensional cracks, slumps and pressure ridges. The barriers and offshore bars consist of medium grain-sized sand, whereas the slumps and pressure ridges are interpreted as mud.
Northward motion of the Whitehorse Trough: paleomagnetic evidence from the Upper Cretaceous Carmacks Group
Authors: Marquis Guy , Globerman Brian R.
Published: April 2011

The Upper Cretaceous Carmacks Group (70.4 ± 2.4 Ma) comprises gently dipping basaltic and andesitic lava flows overlying volcaniclastic deposits of the Intermontane Belt in the Whitehorse Trough. The sampling area is in southern Yukon and northern British Columbia; it lies west of the Tintina – Northern Rocky Mountain Trench fault and Teslin Suture Zone and east of the Denali – Shakwak fault. Volcanic sections were sampled in three regions spread over 300 km, providing the first paleomagnetic data from pre-Tertiary volcanic rocks in the northern Canadian Cordillera. Alternating-field and thermal demagnetization revealed stable magnetization for 18 of the 27 sites collected. The overall mean direction (D = 166.7°, I = −71.4°, k = 53, α95 = 4.8°, N = 18 sites) is pre-folding and is most probably primary (latest Cretaceous). This gives a paleopole at 109.4°E, 82.1°N, K = 21, A95 = 7.8°. A critical evaluation of North American cratonic data yields a reference paleopole for the latest Cretaceous at 185.8°E, 77.7°N, A95 = 7.7°, implying 13.4 ± 8.5 °(1500 ± 950 km) northward displacement and 10.2 ± 20.7 °(not significant) clockwise rotation of the Whitehorse Trough. The displacement occurred between 70.4 ± 2.4 and 59 – 54 Ma, the "docking" time suggested by other paleomagnetic studies in Alaska. The amount and timing of this displacement are consistent with Gabrielse's estimate of cumulative dextral displacements along the Northern Rocky Mountain Trench fault and associated lineaments. As expected, it is intermediate between the low paleolatitudes observed from middle Cretaceous rocks and the concordant paleolatitudes observed in Middle Eocene rocks of the Western Cordillera farther south, in British Columbia and in northern Washington. The estimated displacement is consistent with those predicted by kinematic models of Engebretson and Umhoefer based on North Pacific Basin plate motions.
U–Pb zircon ages for the Rice Lake area, southeastern Manitoba
Authors: Turek A. , Keller R. , Schmus W. R. Van , Weber W.
Published: April 2011

The Archean Rice Lake greenstone belt in southeastern Manitoba is made up of mafic to felsic volcanic rocks and associated intrusive and metasedimentary rocks. The belt is flanked to the north by the Wanipigow River granitic complex and to the south by the Manigotagan gneissic belt. The Ross River quartz diorite pluton is intrusive into the centre of the greenstone belt. U–Pb zircon ages indicate a major volcanic and plutonic event in the area at 2730 Ma. Ages for two volcanic units of the Rice Lake Group are 2731 ± 3 and 2729 ± 3 Ma. The Ross River pluton yields an age of 2728 ± 8 Ma and the Gunnar porphyry gives an age of 2731 ± 13 Ma; both intrude rocks of the Rice Lake Group. Granitic rocks of the Wanipigow River granitic complex give ages of 2731 ± 10 and 2880 ± 9 Ma, while a post-tectonic granite in the Manigotagan gneissic belt has an age of 2663 ± 7 Ma.
Geology of unmineralized and gold-bearing iron formation, Contwoyto Lake – Point Lake region, Northwest Territories, Canada
Authors: Lhotka Paul G. , Nesbitt Bruce E.
Published: April 2011

Numerous gold occurrences, including the Lupin mine, exist in Archean iron formation within the Contwoyto Lake – Point Lake region of the Slave Province. Early studies suggested that gold was a syngenetic component of the iron formation; however, the present study suggests that the gold and sulphides are epigenetic.At both the Lupin mine and small-scale gold occurrences gold is associated with quartz veins and concentrations of pyrrhotite or pyrite and of arsenides (arsenopyrite ± loellingite) in iron formation. The quartz veins contain 0.03–1.00 ppm Au and comprise sulphide-poor quartz. A zoned sequence of hydrothermal alteration is present in iron formation adjacent to quartz veins. Immediately adjacent to the veins a calc-silicate lithology (0.03–1.00 ppm Au) is sometimes developed that comprises hedenbergite + quartz ± epidote ± scheelite ± grossular. Next is an arsenide-rich zone (5–30 ppm Au) comprising hornblende + quartz ± hedenbergite ± epidote ± actinolite. The next zone is an iron-sulphide zone (5–30 ppm Au), lacking abundant arsenides but containing pyrrhotite or pyrite and hornblende + quartz ± hedenbergite ± epidote ± actinolite. Farther from the veins, iron formation is unmineralized (≤0.03 ppm Au), lacks sulphides, and comprises grunerite + quartz ± magnetite. In the transition zone, hornblende replaces grunerite, and iron sulphides replace amphiboles and magnetite. The scale of the sequence of zones varies from millimetres to metres about individual veins. In well-mineralized portions of the Lupin mine, where quartz veins are closely spaced, unmineralized iron formation is absent between the veins.The symmetrical zonal pattern in the mineralogy and gold values about the veins at Lupin and at the small-scale occurrences indicates that mineralization at both scales of gold occurrence formed by an epigenetic process. Mineralization occurred by selective sulphidation of iron formation after most of the Archean deformation and was coincident, or nearly coincident, with peak thermal conditions. Gold was probably transported as an aqueous gold–sulphide complex and deposited as a result of sulphidation reactions.
Deformational history of an Archean fold belt, eastern Point Lake area, Slave Structural Province, N.W.T.
Authors: King J. E. , Helmstaedt H.
Published: April 2011

Archean metasedimentary rocks in the eastern Point Lake area of the Slave Structural Province preserve a sequence of Archean structures consisting of two generations of folds (F1 and F2) with little associated penetrative cleavage and two subsequent generations of cleavage (S3 and S4) with little associated folding. Gneissic layering in the high-grade margin of the belt is composed of transposed bedding and the S3 cleavage. Folding occurred prior to the thermal peak of metamorphism, whereas the develoment of subsequent cleavages spanned the thermal peak. The regional orientation of the folds and cleavages appears to be independent of the emplacement of granitoid intrusions, although their orientation is modified adjacent to syn- to late tectonic plutons. The supracrustal belt is interpreted as being part of a pre- to early metamorphic, west-verging fold (thrust?) belt whose strain pattern has been modified by post-folding, synmetamorphic shortening, and syn- to post-tectonic plutons. This deformation sequence is similar to those described in other supracrustal belts of the Slave Province and supports the concept that the Slave Province has undergone regional, horizontally directed compression before and during intrusion of large amounts of granitoids.
Multiple ages of Nipissing Diabase intrusion: paleomagnetic evidence from the Englehart area, Ontario
Authors: Buchan K. L. , Card K. D. , Chandler F. W.
Published: April 2011

Nipissing Diabase sills and baked host sediments of the Coleman Member of the Huronian Supergroup east of Englehart, Ontario, are shown to have a characteristic remanent magnetization direction (called N3) that is steeply up and to the west (D = 268.0°, I = −59.0°, k = 42, α95 = 6.0°). Petrographic study indicates that fresh pyroxene gabbro carries the N3 component at most sill sites. A baked contact test with the Coleman Member suggests that the magnetization is primary. The N3 magnetization direction is unlike either the N1 or N2 magnetization direction reported for Nipissing sills at other localities in the Southern Province. Three distinct ages of Nipissing sill emplacement are proposed. A single Nipissing sill site in the sampling area carries the N1 direction.A northeast-trending diabase dyke crosscuts both the Nipissing sills and Coleman sediments. It carries an N2 direction and has overprinted nearby intrusive and sedimentary rocks (D = 282.0°, I = 61.1°, k = 48, α95 = 5.8°). Several N3 sill sites far from the dyke may also carry a softer N2 overprint. However, the N3 and N2 directions and the direction of the present Earth's magnetic field fall near a great circle, making it difficult to separate the N2 and present-field components in such cases.
Sr and Nd isotope systematics of polymetamorphic Archean gneisses from southern West Greenland and northern Labrador
Authors: Collerson Kenneth D. , McCulloch Malcolm T. , Nutman Allan P.
Published: April 2011

Sr and Nd isotopic data for middle to late Archean polymetamorphic felsic gneisses from localities in the Nuuk area, West Greenland, are compared and contrasted with new isotopic results for early Archean Amîtsoq gneisses and with data for isotopically reworked Kiyuktok gneisses from the Saglek area, Labrador. Sr isotopic data for individual suites of felsic gneisses record the time-integrated effect of variable Rb–Sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity.Contrasting petrologic and Sr–Nd isotopic characteristics are the result of differences in level of exposure, caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post-shearing folding deformation. Sm–Nd systematics of felsic gneisses from Nordafar, Ikerasakitsup akornga, Tinissaq, and Kangimut sammisoq – Qasigianguit define a geologically meaningless ca. 3280 Ma Nd "isochron", which is the result of mixing of samples from unrelated suites and the effect of open-system behaviour. Gneisses lying on this pseudoisochron were variably affected by ca. 2800–2900 Ma prograde granulite-facies metamorphism and structurally controlled retrogression under amphibolite- to greenschist-facies conditions.The study shows that Sr–Nd isotope systematics of geologically identifiable units may be modified by open-system behaviour during prograde and retrograde metamorphism. Isotopic data from gneiss complexes metamorphosed under granulite-facies conditions may therefore yield equivocal information concerning isochron interpretation, significance of model ages, and estimates of crustal residence time.