bluuebunny/crossref_metadata_2025 huggingface dataset:
Geophysical Studies of North African Cenozoic Volcanic Areas: III. Garian, Libya
Authors: Ade-Hall J. M. , Gerstein Susann , Gerstein Robert E. , Reynolds Peter H. , Dagley P. , Mussett A. E. , Hubbard T. P.
Published: April 2011

Paleomagnetic and K/Ar whole rock absolute age data are described for material from the Garian area of Libya, centered at 13°E, 32°N. Within-unit cleaned paleomagnetic directions from the essentially unaltered lavas are very well defined and can almost certainly be taken as reliable measurements of the geomagnetic field direction during the initial cooling of each flow. However, the distributions of mean direction, from which the effect of repeated sampling of the field at one time has been removed, does not suggest that a reversing axial dipole field has been recorded in a representative manner. Both N and R groups of directions are azimuthally elongated, and the average poles for the N and R groups differ by 21°, or four times the 95% level uncertainty for each average pole. A number of possible physical explanations for the paleomagnetic results are discussed. The conventional overall average pole at 88°N, 123°E, δp: 3°, δm: 7 °does not differ significantly from the geographic pole, a result which agrees closely with that of Schult and Soffel (1973). However, the value of these overall average poles in estimating absolute plate motion must await an understanding of the sources of the asymmetries in the paleomagnetic data.
Early Precambrian Rocks of Granitic Composition
Authors: Krupicka J.
Published: April 2011

The granitoids and the corresponding gneisses of the old sialic crust are not exclusively represented by rocks of tonalitic and granodioritic composition. Potassium-rich rocks of true granitic composition occur even in the oldest known complexes of the Earth's crust.The petrography of six samples (39 individual specimens) from the southwestern part of the Early Precambrian Amitsoq gneiss complex in the Godthaab area in Greenland, collected for dating purposes, and of 12 Early Precambrian granitoid pebbles from the basal Moodies conglomerate in South Africa shows that (a) half of the investigated samples of the Amitsoq gneiss, and (b) ten of the Moodies granitoid pebbles are rocks of true granitic composition. The rocks show no evidence of sedimentogenic origin.
Dispersal and Geochemistry of Surface Sediments in Halls Bay, North-Central Newfoundland: Application to Mineral Exploration
Authors: Slatt Roger M.
Published: April 2011

Surficial palimpsest sediments in Halls Bay, north-central Newfoundland, are mixtures of gravel, sand, and mud deposited from a number of sources in varying quantities from late Wisconsinan to the present time. Shallow water gravel originated as till and glacio-fluvial outwash. Gravel in deep water probably is ice-rafted. Sand and mud, which occurs with shallow water gravel and in deeper water, is a combination of fluvial material and material winnowed out of till and outwash by shallow water waves and currents during early marine transgression. There also may be a contribution of fine-grained sediment from the adjacent shelf.Gravel (coarser than [Formula: see text]), very fine sand (3 to [Formula: see text]) and coarse silt (4 to [Formula: see text]) modal grain-size classes predominate in the sediments. The very fine sand mode occurs on the west side of the inlet and the coarse silt mode occurs on the east side regardless of water depth, indicating net or active easterly dispersal of fine-grained sediment. This dispersal path may result from the presence in Halls Bay of a counterclockwise gyre of the Labrador Current that has developed since early transgression, which suggests the sediment surface is adjusting to the Halls Bay modern hydraulic regime.Sandy and muddy sediments are composed of quartz, feldspar, amphibole, illite, chlorite, montmorillonite, organic matter, CaCO3, and FeS. Major, minor, and trace element concentrations vary with grain-size, owing to the different proportions of these components in different size fractions. Calculation of an average chemical composition of sediments is biased because of this grain-size effect. The grain-size effect on chemistry of a suite of sediments can be accounted for by ratioing element concentrations to clay content.Plots of the ratio trace metal concentration/clay content vs. clay content for six trace metals indicate anomalous Cu concentrations occur in surface sediments along the east side of Halls Bay in the direction of fine-grained sediment dispersal. The anomalous Cu is derived from onshore mineralization in Lushs Bight Group volcanic rocks, which Occur along the west side of the inlet.The results provide an example of the applicability of marine sedimentologic/sedimentary geochemical investigations to mineral exploration. Local geochemical anomalies in sediments can be detected by routine analysis of total metal content of bulk samples provided the grain-size effect on chemistry is accounted for. The anomalous metal can be traced to its onshore source by evaluating sediment dispersal paths from textural variations.
Imbricate Subduction Zones and their Relationship with Upper Cretaceous to Tertiary Porphyry Deposits in the Canadian Cordillera
Authors: Godwin C. I.
Published: April 2011

A plate-tectonic model relates the genesis of Upper Cretaceous-to-Tertiary porphyry-type deposits to the evolution of the western and central Canadian Cordillera. Existence of two Benioff zones is assumed from definition of two distinct younging trends of intrusive centers. The first Benioff zone, initiated west of the Queen Charlotte Islands near the Middle Triassic, continued activity until the early Tertiary, when 50 m.y.-old granitic rocks and associated porphyry deposits near the eastern boundary of the Coast Crystalline Belt were formed. The second Benioff zone, initiated near the earliest Cretaceous, extended under the western margin of the North America plate and produced intrusive stocks and associated porphyry deposits that become younger from west to east across the Intermontane Belt. Intrusive activity associated with both Benioff zones ceased at about the same time, 50 m.y. ago, implying that the Benioff zones became imbricated. As a result, the North America plate overrode the Insular plate. Doubling of these plates is reflected in the late Mesozoic and Tertiary uplift and erosion of the Coast Crystalline Belt.
Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec)
Authors: Seguin M. K. , Laurent R.
Published: April 2011

A detailed examination was made to evaluate the variations of the remanent magnetization and magnetic susceptibility through four ophiolitic pillow lavas of Cambrian age. These pillowed metabasalts, which are covered by a thin layer of cherty argillite, derive from an olivine tholeiite of probable oceanic origin. They have been metamorphosed in the greenschist facies under a regime of low pressure, moderate temperature, and in the presence of water but absence of significant stress. They still display their primary structural zoning characterized by a thin outer shell, a much wider globulitic intermediate zone, and a porphyritic core. Morphologies of quenched microphenocrysts of olivine and plagioclase are well preserved.The magnetization resides in very fine-grained magnetite and, in spite of a very low remanent magnetization, the primary magnetic imprint appears to be still discernible. Remanent magnetization and Koenigsberger ratio vary from the pillow margin to its center in a fashion similar to the magnetic signature of some recent and fresh oceanic basalts. The magnetic zoning matches the textural, mineralogical, and chemical zoning characteristic of these pillow lavas. We found also that the N.R.M. vector is consistent and relatively stable within the pillow inner part of the intermediate zone and the outer part of the core and therefore that samples from these zones can be used for a regional paleomagnetic study of the ophiolitic complex.
Rb–Sr Geochronology of the Republic Area, Marquette County, Michigan
Authors: Schmus W. R. Van , Woolsey L. L.
Published: April 2011

Rb–Sr geochronologic studies have been carried out on Lower and Middle Precambrian rocks exposed in Marquette County, Michigan. These rocks were subjected to varying degrees of metamorphism during formation of the Republic metamorphic node. Samples of the Lower Precambrian granitic gneiss generally yield whole-rock model ages of 2500 to 2800 Ma for metamorphic grades ranging from chlorite through sillimanite zones, indicating relatively minor net flux of Rb and Sr on a scale of several tenths of a metre during the metamorphism. Mineral ages show partial to complete internal re-equilibration of the Rb–Sr systems about 1665 ± 50 Ma ago. Muscovite Rb–Sr ages from cross-cutting pegmatites are also about the same age. Data on minerals from metamorphosed Middle Precambrian metasediments of the Michigamme Formation are consistent with this age but are not definitive.Published structural studies indicate that the main deformation in the Republic node occurred essentially at the same time as the peak of the major mineral-forming metamorphic event. The age of 1665 Ma is inconsistent with the regional tectonics and chronology, which indicate that the major deformation and metamorphism in Upper Michigan occurred about 1900 Ma ago. However, the age of 1665 Ma does correspond to a time of more recent metamorphism prevalent throughout the western Great Lakes area, and we interpret the results from the Republic area as a general overprint younger than the main period of deformation and metamorphism.Two of the Lower Precambrian samples yield model whole-rock ages in excess of 3000 Ma. Detailed study of the rocks involved show that the apparently old ages are due to Sr and Rb redistribution on a larger scale during another event which occurred about 2100 to 2200 Ma ago.
The Petrochemistry of Altered Volcanic Rocks Surrounding the Louvem Copper Deposit, Val d'Or, Quebec
Authors: Spitz Guy , Darling Richard
Published: April 2011

The Louvem copper deposit, a carrot-shaped body of mineralized silicic pyroclastic rock, appears generally conformable with surrounding, steeply dipping volcanic rocks, but otherwise closely resembles the cross-cutting feeder pipes that underlie many Archean stratiform volcanogenic massive sulfide deposits. It is, like many such deposits, associated with peraluminous and calc-alkaline rocks in the felsic upper portion of a volcanic sequence.Naming of the Louvem volcanic host rocks by means of their chemical composition is rendered difficult by intense local alteration which has changed their original compositions. Of the four classification schemes tried, that based on sample SiO2 content appears to provide results that are least affected by this alteration and which therefore reflect most clearly the original compositions of the rocks surrounding the ore deposit.The calc-alkaline nature of Louvem volcanic rocks is apparent even for very altered near-ore samples. This is revealed by Ol–Ne–Qz and AFM diagrams, which appear to be suitable for the genetic classification of such altered rocks.The chemical nature of the wallrock alteration in and around the deposit is revealed by certain petrologic diagrams. All rocks in the study area show magnesium enrichment, but no petrologic diagram illustrates this very clearly. Outside the orebody, the alteration consists mainly of Na and Ca depletion, and those diagrams which show such depletion are the most useful. Of these, the AKF, AFM, and ACF plots appear to be most practical.
Alternative Interpretations for the Casino Complex and Klotassin Batholith in the Yukon Crystalline Terrane
Authors: Godwin Colin I.
Published: April 2011

A small part of the Klotassin batholith, that intrudes the Yukon Metamorphic Complex, has been studied in detail near the Casino porphyry copper–molybdenum deposit, Yukon Territory. Eleven potassium–argon model ages, including concordant biotite and hornblende ages, indicate a mid-Cretaceous (100 Ma) age for this batholith. All ages are indistinguishable statistically. This age, therefore, is interpreted as the age of emplacement of the Klotassin batholith. Older dates can be expected from rocks of the Yukon Metamorphic Complex, the Carmacks batholith, and Aishihik batholith.The Casino complex, host to the Casino deposit, is dated at latest-Cretaceous (70 Ma), based on potassium–argon model ages from two samples of biotite. The complex is clearly intrusive into and younger than the Klotassin batholith. Previously, this unit was interpreted as Early Tertiary. Possibly the several so called Early Tertiary volcanic and hypabyssal rocks in Yukon are not strictly contemporaneous.
Preliminary report on the Borgia meta-anorthosite (La Tuque, Québec, Canada)
Authors: Baer A. J.
Published: April 2011

A 10 km wide, subcircular body of highly recrystallized anorthosite (meta-anorthosite) located north of La Tuque, Québec, is partially surrounded by an aureole of gneisses in the granulite facies. It is foliated near the edges, but lineated in the center. Pegmatites and mafic dikes were intruded into the anorthosite after the major phase of emplacement, but before a later, minor phase of deformation. Younger pegmatites post-date all deformation. The pluton appears to have been emplaced like a hot diapir, possibly during the Grenvillian thermostructural event.
A new Devonian heteractinid sponge from southwestern Ellesmere Island, Arctic Canada
Authors: Rigby J. Keith
Published: April 2011

The new heteractinid astraeosponge Stellarispongia aspera is described from the Devonian Emsian part of the upper Blue Fiord Formation. The sponges were collected from near Sör Fiord in southwestern Ellesmere Island, District of Franklin. The new sponge is included in the Astraeospongiidae and is a thick walled, saucer shaped form, which has moderately large radiating canals that are cross-connected by intermediate sized, small canals. Spicules are octactine based, of at least three ranks, and larger spicules have robust short rays. Spicules are irregularly oriented in a felted arrangement.
The formation and evolution of Point Pelee, western Lake Erie
Authors: Coakley J. P.
Published: April 2011

Point Pelee originated as a sandy foreland enclosing a marsh approximately 4000 years BP. A reconstruction of paleoenvironmental conditions prior to that time suggests strongly that it was formed initially by the progressive merging of beach ridges and dunes formed on opposite sides of the Pelee-Lorain moraine when rising lake levels in the western basin of Lake Erie were 3 to 4 m below present lake datum (173.4 m above sea level). Since that time, it has retreated to its present position and orientation under the influence of slowly rising lake levels and increasing wave energy from both sides.Such an evolution, though different from conventional mechanisms of spit formation, is supported by evidence derived from the local post glacial stratigraphy and the geomorphology of relict features preserved on the Point and on the shoal area to the south.
Un diagramme pollinique au Mont des Éboulements, région de Charlevoix, Québec
Authors: Richard Pierre , Poulin Philippe
Published: April 2011

The history of vegetation has been registered in the sediments of lake Mimi since about 11 000 BP, The initial vegetation traced is a tundra which, under severe climatic conditions, lasted for about 1000 years. The herb tundra was progressively replaced by shrub tundra: a willow phase (Salix). followed by a dwarf birch phase (Betula cf. glandulosa) have been traced. These were followed by an afforestation phase characterized by an aspen community (Populus tremuloides) al about 10 000 BP. Spruce succeeded the aspen community, probably as an open black spruce (Picea mariana) community with some dwarf birch and green alder (Alnus crispa). An outstanding Alnus cf. crispa pollen peak (48%), supported by the annual pollen influx values, at the end of the spruce phase, could be interpreted as a return of colder climate that favored the expansion of this shrub over forest. This event would date about 9750 BP. An open fir (Abies balsamea) forest followed, and changed to the balsam fir – white birch (Betula papyrifera) forest (climax domain), which prevailed until now. The richer sites supported sugar maple (Acer saccharum) – yellow birch (Betula alleghaniensis) community and fir – yellow birch stands since 6200 BP. Six radiocarbon dates and annual pollen influx values are offered, and some ecological problems related to the interpretation of the pollen diagram are discussed.
Pelecypods of the Hell Creek Formation (Uppermost Cretaceous) of Garfield County, Montana
Authors: Russell Loris S.
Published: April 2011

The Hell Creek Formation of eastern Montana, which forms the top of the Cretaceous section, contains in addition to a rich vertebrate fauna a distinctive assemblage of pelecypods, made up of three species of Plesielliptio, one of Rhabdotophorus, two of Plethobasus, one of Quadrula, nine of Proparreysia, one of Obovaria?, one of Corbicula, and one of Sphaerium. Many of the species show distinctive ornamentation, analogous to that of living Unionacea in the Mississippi drainage. One species of Plesielliptio, one of Proparreysia, and one of Sphaerium are new. There is some community of species with the faunas of the Lance and Black Buttes formations of Wyoming, less with the older Cretaceous assemblages, and none with the abundant mollusks of the Paleocene Fort Union formations.
Large values of the tidal potential and the occurrence of extreme tides and extreme water levels
Authors: Godin Gabriel
Published: April 2011

Large values of the tidal potential are not necessarily reflected by extreme tides or water levels at a given locality.
Analysis of heat flow data—correlation of thermal resistivity and shock metamorphic grade and its use as evidence for an impact origin of the Brent Crater
Authors: Beck A. E. , Hamza V. M. , Chang C. C.
Published: April 2011

Measurements of heat flow and radioactive heat generation values have been made at small intervals using a completely cored, 1 km deep borehole located close to the center of the Brent Crater. A remarkable correlation is observed between the thermal resistivity values and shock metamorphic grade. This is interpreted as due to residual shock effects on the lattice conductivity. These effects are preserved over long periods of time, probably because of rapid cooling that prevented thermal annealing and recrystallization. The highly shock metamorphosed samples also have characteristically low heat production values, resulting from a depletion of thorium and uranium; potassium distribution is fairly uniform except at the boundaries of metamorphic zones, where it is enriched to a considerable degree. These results support the meteorite impact hypothesis for the origin of the crater.Mean heat production and corrected heat flow values obtained for this borehole are 1.51 ± 0.57 μWm−3 (3.6 ± 1.4 HPU) and 41.9 ± 6.3 mWm−2 (1.00 ± 0.15 HFU), respectively. These values indicate that this region may belong or be similar to the Central Stable region heat flow province.
Radioelement content and variation in some granitoid units of southeastern Manitoba and adjacent Ontario
Authors: Farquharson R. B.
Published: April 2011

This describes a reconnaissance study of K, U, and Th distribution in six granitoid units of diverse lithology, within the southwestern section of the English River gneissic belt. The various units are distinguishable on the basis of mean abundances of radioelements and proportions of those means. The data reveal anomalous enrichment in Th in a large mass of pink granodiorite, and U enrichment in both pegmatitic differentiates and quartz monzonite of possible anatectic derivation.
Development of a gully-flow near Sept-Iles, Quebec
Authors: Dredge L. A. , Thom B. G.
Published: April 2011

A gully system [Formula: see text] long and 30 m deep has developed on terraces adjacent to the Moisie River, Quebec, in a catastrophic manner following two periods of heavy rainfall. Raised postglacial deltaic sands and silts in which the gully developed were initially deposited in a loose to medium-dense state, and failed by spontaneous liquefaction in 1959 and 1966. Although liquefaction of the sands explains the mechanism of failure, it does not account for the pattern that ensued. The reticulated gully pattern is attributed to piping, which was enhanced by a network of lumber tracks covering the surface of the old delta. This is the first recorded example of sub-aerial gullying induced by catastrophic mass movement in sandy sediments. The location and recurrent development of this gully phenomenon is especially significant since the only road linking the settlements of the Quebec North Shore bridges the Moisie River at this site.
Archean rocks from the eastern Lac Seul region of the English River Gneiss Belt, northwestern Ontario, part 1. Petrology, chemistry, and metamorphism
Authors: Harris N. B. W. , Goodwin A. M.
Published: April 2011

The eastern Lac Seul region of the English River Gneiss Belt is divided into two domains defined by contrasting petrology and structure. The northern domain is underlain by east-trending, steeply south-dipping, migmatized metasediments, intruded by occasional granite sills, and the southern domain by gneissic tonalite and trondhjemite, with abundant amphibolite inclusions, intruded by granite dykes and diapirs: this domain has a complex structure with gently east-plunging open folds of about 5 km wavelength. Field evidence suggests that metasediments of the northern domain have been deposited on the tonalite trondhjemite basement, which was subsequently mobilized, thereby producing the steeply dipping paragneiss belt of the northern domain.The grade of metamorphism throughout the region lies in the upper amphibolite facies, rising locally to the granulite facies. Within 15 km of the southern margin of the gneiss belt, the metamorphic grade decreases to the greenschist facies.U–Pb dating of zircons indicates that the tonalite gneiss was emplaced at least 3040 m.y. ago, and the granite plutons at 2660 m.y., coeval with migmatization and upper amphibolite facies metamorphism. Late pegmatites were emplaced at 2560 m.y.
Preferred crystallographic orientation of quartz from deformed quartzite breccia of the Onaping Formation, Sudbury Basin
Authors: Rousell D. H.
Published: April 2011

Deformed quartzite breccia of the Onaping Formation, from the northeastern end of the South Range, contains quartz that occurs as elongate ribbons displaying extreme undulatory extinction and small recrystallized grains. The quartz optic axes of the ribbon grains and the recrystallized grains lie on small circle girdles, with the girdle axes approximately normal to the foliation. The angle between the girdles and the girdle axes is approximately 35°. The rocks display a prominent tectonic foliation and this is due to the dimensional preferred orientation of fragments. The quartzite breccia appears to be a natural analog of quartzites experimentally deformed under conditions of intermediate temperature and strain rate.
The reduction of statistical noise in airborne radiometric data
Authors: Tammenmaa J. , Grasty R. L. , Peltoniemi M.
Published: April 2011

An analytical expression for the Fourier energy spectrum of a radioactive point source is found to satisfactorily define that part of the spectrum in which all radiometric features are located. It is shown that the decreased signal-to-noise ratio arising from a shorter measurement time can be regained by the application of a suitable filter, resulting in better ground resolution and less distortion of radiometric anomalies.
Pleistocene and Recent sediments of northern Placentia Bay, Newfoundland
Authors: Stehman Charles F.
Published: April 2011

Study of the sediments found in northern Placentia Bay, Newfoundland, using MS-26-B echosounder profiles and 87 grab samples, reveals that sediments in that area are derived principally from glacial sources. Sediment facies distribution patterns are contrastingly different in different regions of the bay. The data suggest that surficial sediments associated with glacial drift, transgressive marine deposits, and modern sedimentation occur in a complex mosaic of seven facies distributed throughout the region. There is some evidence of relationships between sedimentary facies and both bathymetry and geography.
Evidence in support of a meteorite impact crater at Poplar Bay, Lac du Bonnet, southeastern Manitoba, Canada
Authors: Trueman D. L.
Published: April 2011

Poplar Bay, in southeastern Manitoba, is a circular lake approximately 3 km in diameter, 21 m in depth, and is framed by radial and arcuate photolineaments. Geophysical study of Poplar Bay shows a unique magnetic signature, and results of a gravity survey indicate a profound deficiency of mass under the lake. The form of the gravity anomaly is circular, and it has a maximum value of −7 mGal.There is no evidence of an igneous origin for the feature and data indicate the plausibility of a meteorite impact origin for Poplar Bay.
Expansion of the Earth due to a secular decrease in G—evidence from Mercury
Authors: Crossley D. J. , Stevens R. K.
Published: April 2011

The known surface of Mercury is unique among the photographed portions of the terrestrial planets in that there are no obvious tensional features, but instead some compressional wrinkling. If, as has been variously proposed, the Earth has undergone expansion due to a secular decrease in the gravitational constant G, then presumably so will all astronomical bodies. Using two alternative geophysical models for the interior of Mercury, the estimated expansion due to a decrease in G is probably in conflict with present observation. This constraint on the size of the change in G may be significant for planetary evolution.
Gravity evidence against a high-angle fault crossing the Rocky Mountain Trench near Radium, British Columbia
Authors: Spence G. D. , Ellis R. M. , Clowes R. M.
Published: April 2011

One of three explanations of a prominent time delay in the 6.5 km/s branch of a recent seismic refraction survey in the Rocky Mountain Trench suggested a high-angle crustal fault crossing the trench near Radium, British Columbia. If the density contrast between basement and cover rocks is 0.1 g/cm3, a gravity anomaly of approximately 18 mGal should be observed. To test the fault hypothesis, a gravity survey has been carried out in and adjacent to the trench in the Radium area. The resultant data are not consistent with the proposed fault model. The principal feature of the data is a pronounced low, which coincides with the trench throughout the survey area. The low is due to Cenozoic fill and interpretation by two-dimensional modelling indicates the thickness of fill is about 550 m to the north and 420 m to the south of Radium. As a result of this survey, the two alternative hypotheses to explain the seismic data must be reconsidered. These are (1) the existence of a crustal low velocity zone, and (2) a major deformation of the basement and overlying rocks due to the trench being an ancient zone of weakness, which coincides with the western limit of the continental Precambrian craton. As reflections from the top of a low velocity zone are not observed, the second alternative is preferred.
A reconnaissance paleomagnetic survey of the central Labrador Trough, Quebec
Authors: Park J. K.
Published: April 2011

Paleomagnetic studies have been carried out on tower Proterozoic (Aphebian) rocks of the Labrador Trough in central Quebec. The magnetizations are complex. Most magnetizations are secondary and none are demonstrably original, but their relative order can be estimated from their associated blocking temperatures. The oldest magnetizations (A) are thought to have been acquired at about 1800 Ma following deposition but not necessarily long afterwards. These magnetizations have blocking temperatures of about 600 °C and occur in the Seward Subgroup (mainly arkoses), the Menihek Formation (argillites), and the Retty Formation (peridotites). The Castignon carbonatitic complex also has A type magnetizations which are referred, tentatively, to a K–Ar date of 1875 ± 50 Ma, The next youngest magnetizations (B) have blocking temperatures commonly distributed between 300 and 500 °C and are considered to have been acquired at about 1600 Ma during uplift following the Hudsonian orogeny. B magnetizations are the commonest magnetizations and are found in the Seward, the Retty, and the Menihek. The youngest magnetizations (C) have low blocking temperatures predominantly less than 300 °C and were probably acquired long after the rocks were formed. The C magnetizations are found in the Seward Subgroup, the Willbob Formation (basalts), and the Wakuach Formation (gabbros). The age of the C magnetization appears to be about 1100 Ma, but its origin is not known. This chronology of magnetizations is consistent with the previously constructed path of apparent polar wandering for Laurentia.
The contouring of orientation data represented in spherical projection
Authors: Starkey John
Published: April 2011

The inadequacy of the current method of contouring orientation data in terms of the concentration of points occurring in 1% of the projected area is demonstrated. In particular, the maximum concentration and the areas occupied by the different concentrations are shown to be dependent on sample size. Using a counting area of 100/n% of the projection, where n is the sample size, yields meaningful orientation patterns which are comparable for different sample sizes. This is illustrated with reference to computer generated random and non-random orientation patterns. The data obtained are compared with predicted values and form a basis for comparison with measured orientation patterns to test their significance.
Archean variolites: source of iron in the Precambrian environment?
Authors: Brooks C.
Published: April 2011

Archean variolitic lavas contain matrix fractions possessing spectacularly high iron contents (up to 24 wt.% total FeO). The amount of iron contained in a single variolite horizon is greater than 1 × 1012 kg, a source of magmatic iron large enough to account for the iron required to form banded iron formation (BIF) early in the earth's history. The time distribution of BIF and variolites may be related, via the liquidus-depressant role of water in immiscible processes, to the increased likelihood of encountering 'wet', immiscibility-prone, partial-melt mantle derivates. Alternatively BIF may simply be the product of chemical degradation of these variolites.
Volcanic rocks of the Blake River Group, Abitibi Greenstone Belt, Ontario, and their sulfur content
Authors: Naldrett A. J. , Goodwin A. M.
Published: April 2011

Six hundred and ninety samples of volcanic rocks from the Blake River Group of the Abitibi Greenstone Belt have analysed for sulfur on a Leco sulfur analyser. Basaltic rocks have been subdivided into komatiites, Fe-rich tholeiites, Al-rich basalts, and intermediate basalts with more than 1% TiO2 and with less than 1% TiO2. Andesites have been subdivided into Fe-rich types, Al-rich types, and others. All dacites are grouped together as are all rhyolites. Rocks of many of these subdivisions occur at more than one level within the Blake River stratigraphy. Within a given rock subdivision, the sulfur content is distributed log normally. When the geometric mean of the sulfur content of each of the subdivisions outlined above is plotted against the arithmetic mean of the FeO content, a smooth curve is obtained, with sulfur increasing markedly with increase in FeO. The data give no indication of any change in sulfur content of a given rock subdivision with stratigraphic height. The arithmetic mean of the sulfur content of each rock subdivision also increases with the mean FeO content, although less smoothly than the geometric mean. The arithmetic means of sulfur content fall within the scatter of points obtained experimentally for the sulfur content of sulfur saturated basalts, supporting the contention that the Blake River rocks may have been saturated with sulfur at the time of their extrusion.
Effect of vegetation on limestone solution in a small High Arctic basin
Authors: Woo Ming-Ko , Marsh Philip
Published: April 2011

To evaluate the effect of tundra vegetation on limestone solution processes, the present study was carried out in a small basin in southwestern Ellesmere Island, N.W.T. A test reach was selected along the stream, and water samples were collected at regular intervals from a seepage point entering the reach, a soil water pit at the bottom of a vegetated slope along the test reach, and from the stream at the outlet of the reach. Hydrochemical characteristics of the samples were described by several measured and calculated variables including water temperature, pH, calcium and total hardness, bicarbonate concentration, equilibrium partial pressure of carbon dioxide, and indices of saturation with respect to calcite and dolomite. Throughout the growing season of 1975, all samples indicated higher concentrations in water hardness and in bicarbonate than those reported in nonvegetated areas of the Arctic. A rising trend was apparent in these data, with the concentrations reaching a seasonal maximum in late summer. These phenomena are attributed to the production of biogenic carbon dioxide, which increased the aggressiveness of the water. The partial pressure of carbon dioxide in soil water was directly increased by this process, while the addition of soil water to the stream caused noticeable downstream increase in partial pressure of carbon dioxide and a corresponding reduction in saturation with respect to calcite and to dolomite. The influence of vegetation was therefore very marked in both surface and in subsurface flows.
The Triassic unconformity of south-central British Columbia
Authors: Read P. B. , Okulitch Andrew V.
Published: April 2011

At five localities investigated in south-central British Columbia, Upper Triassic rocks are observed or inferred to unconformably overlie upper Paleozoic and older rocks. Paleozoic rocks beneath the unconformity show polyphase deformation and low-grade regional metamorphism which are absent in overlying rocks. Data from these and other localities define a regional angular unconformity of Late Permian or Early Triassic age on the western and southern margins of the Shuswap Metamorphic Complex. Permian and Triassic rocks preserve evidence of structural, sedimentary, and metamorphic events which permits separation of Triassic rocks into three fault-bounded tectonostratigraphic belts. The Eastern Belt contains the transition from miogeoclinal sedimentation throughout Triassic time in the Canadian Rockies to island arc volcanism in the Late Triassic to the west. Basal beds of the Triassic sequence become younger southwest-ward from the axis of the Early to Middle Triassic depocentre lying west of the Rockies. Rocks preserving Early Triassic deformation and metamorphism are restricted to the southwest corner of the belt and are truncated by the Pasayten Fault. The Central Belt, dominated by the products of Late Triassic volcanism in northern and central British Columbia, consists mainly of Middle (?) and Upper Triassic sediments in the south. Meagre evidence indicates that widespread deformation and low-grade regional metamorphism occurred just prior to the Late Triassic. Evidence for these events is not found beyond the faulted margins of the Central Belt. In the Western Belt, an Upper Triassic sequence of tholeiitic basalt and overlying calcareous sediments disconformably overlies Permian rocks. In the western Cordillera, low-grade regional metamorphism and minor plutonism characterize Triassic orogenies. Early Triassic orogenesis in the southwestern corner of the Eastern Belt is coeval with the Sonoma Orogeny and the Middle–Late Triassic orogenesis of the Central Belt represents the Tahltanian Orogeny.
Mid-Atlantic Ridge at 37° N: Geophysical anomalies in the area of Leg 37 drilling
Authors: Ross D. I. , Falconer R. K. H.
Published: April 2011

Geophysical data collected as part of Leg 37 are compiled with more recent data collected for new IPOD drilling site surveys. Bathymetric and magnetic maps covering the area of sites 332–335 are presented. On the basis of magnetic anomaly correlations it is suggested that site 334 is on normal crust between fracture zones A and B and not closer than 15 km to either fracture zone. Magnetic anomaly inversion is applied to a composite profile, extending from the ridge crest out to beyond anomaly 5. It shows a definite change in spreading rate at 4.7 ± 0.5 Ma. Average rates for the periods 0–4.7 Ma and 4.7–10 Ma are 10.2 ± 0.9 mm/yr and 14.0 ± 1.9 mm/yr respectively. The inversion results are consistent with a simple magnetic source layer 2 km thick.
Sulfides in Leg 37 drill core from the Mid-Atlantic Ridge
Authors: MacLean Wallace H.
Published: April 2011

Sulfides in 27 samples of drill core from four Leg 37 drill holes have been studied. Pyrite is the most abundant sulfide, averaging 0.39 ± 0.51% of basalt samples. Also present in basalt are sparse (0.0001%) and small (< 8 μm) magmatic globules of finely intergrown pyrrhotite, pentlandite, and chalcopyrite-iss, as well as a few globules composed of only one of these minerals. Larger and more abundant magmatic globules are present in gabbro and peridotite; pyrite is a minor phase in these rocks.Fe, Ni. Cu. and S are the major elements, and Co a minor element in the globules. Averaged Ni/Cu ratios for globules are: 3.3 (peridotite), 1.0 (gabbro). and 0.4 (basalt), the decrease in ratio indicating less normative olivine in the respective parent magmas when the globules were formed.Sulfides in globules in peridotite, and to a lesser extent in gabbro, form distinct grains free of inclusions, and appear to have equilibrated to below 300 °C. In basalt the minerals are finely intergrown and indicate quenching above about 600 °C, with local equilibration at lower temperatures.Three analyses of sulfur in basalt glass ranged from 930 to 1160 ppm, in good agreement with other analyses of deep-ocean basalts.
Calcareous nannofossils from Mid-Atlantic basement rocks of DSDP Leg37
Authors: Howe Robert C. , Ellis C. Howard
Published: April 2011

During Leg 37, Deep Sea Drilling Project, several holes were drilled on the northwest flank of the Mid-Atlantic Ridge. Calcareous nannofossils can be used to define age limits for the upper portion of basement rocks at sites 332 and 334. Nannofossil oozes recovered from immediately above basement basalts in holes 332A and 332B are of early Late or late Early Pliocene age (Discoaster tamalis or D. asymmetricus Subzone). Nannofossil chalks recovered from as deep as 180 m below the top of basalt in these holes are of late Early Pliocene age (D. asymmetricus Subzone). Nannofossils recovered from sediments immediately overlying basement basalts in hole 334 are of early Late Miocene age (D. bellus Subzone). Nannofossil chalks recovered from 50 m below the top of basalt in this hole are of late Middle Miocene age (Catinaster calyculus Subzone). Nannofossils recovered from the calcareous matrix of a gabbro and periodotite breccia occurring 68 m below the top of the basalt are also of late Middle Miocene age (C. coalitus Zone).The results indicate that the upper 180 m of basalt at site 332 were formed during an interval of time not exceeding 1 million years. At site 334 the upper 50 m of basement basalts were formed during an interval of time not exceeding 4.5 million years.
Magnetization and suitability for paleomagnetic field intensity measurements of selected samples from sites 332, 334, and 335 of Leg 37 DSDP
Authors: Carmichael C. M.
Published: April 2011

The magnetic properties of 31 small samples from sites 332, 334, and 335 of Leg 37 DSDP were measured, and attempts were made to determine paleomagnetic field intensity. Values of paleointensity of 0.5 and 0.7 times the present field were obtained using one sample from site 332 and one from Site 335 respectively. All of the remaining samples proved unsuitable for paleointensity determinations owing to alteration on heating. This alteration is of two types: (1) destruction of maghemite, which reduces the magnetization of the samples, and (2) a process that markedly increases their thermal and saturation remanences and which may involve precipitation of new small magnetic grains out of glass. A 19 m thick layer at a depth of about 600 m in hole 332B, which appears to be a single unit on the basis of visual examination and chemical analysis of samples, contains a magnetic polarity transition. This may be due to a field reversal, or a partial reversal, while the unit was being implaced. Only samples from site 335 have a magnetization that can account for the surface anomaly by the remanence of the layers drilled.
Oxygen isotope geochemistry of rocks from DSDP Leg37
Authors: Muehlenbachs K.
Published: April 2011

The isotopic compositions of minerals separated from DSDP Leg 37 samples indicate that the primary, unaltered δ18O of both the intrusive and extrusive rocks are identical (~5.7 ‰, SMOW) to those of unaltered basalts dredged from mid-ocean ridges. All of the analyzed basalts (6 to 10 ‰) have been enriched in 18O due to weathering by cold seawater, whereas the intrusive rocks (2.4 and 5.0 ‰) are depleted of 18O probably as a result of exchange with hot seawater at the mid-ocean ridge. Both kinds of altered rock are also known from the study of dredged materials. 18O is preferentially removed from seawater by the first process, but is added to seawater by the second. Exchange of oxygen between oceanic crust and seawater must be considered in any discussion of the evolution of the isotopic composition of the oceans, because large volumes of rock are altered each year as the oceanic crust is formed.
Palladium, iridium, and gold contents of mafic and ultramafic rocks drilled from the Mid-Atlantic Ridge, Leg 37, Deep Sea Drilling Project
Authors: Crocket James H. , Teruta Yuko
Published: April 2011

Basalts and ultramafic rocks from holes 332B and 334 of Leg 37, DSDP, were analyzed for palladium, iridium, and gold by neutron activation. Averages obtained for the basalts are: Pd < 0.69 ppb, Ir < 0.025 ppb, and Au = 2.9 ppb. The samples include one representative of each of the main basaltic lithological units except for Unit III, hole 332B, which is represented by a suite of 14 samples. Averages of 43, 0.60, and 16 ppb were obtained for Pd, Ir, and Au respectively in two ultramafic rocks and a gabbro from hole 334.In comparison with basalts from mid-ocean islands such as Hawaii, Leg 37 sea-floor basalts are significantly depleted in Ir and Pd, but are similar in gold content. These differences may be related to differing degrees of alteration sustained by rocks from these differing environments.The depth profiles for Ir and Pd in hole 332B show no significant trends, but a weak trend of increasing gold content with depth is suggested. Variation of Ir and Pd in adjacent lithological units may be as great as a factor of 50. The Pd distribution in 14 samples from Unit III suggests that these differences probably reflect real differences in average metal content rather than sampling effects. Although alteration of Leg 37 basalts is weak, the low Pd and Ir contents and the variability from flow to flow suggest that reaction with sea water has leached Pd and Ir from these rocks. A possible complementary enhancement of some noble metals including Pd has been noted in the sediments immediately overlying mid-ocean rises. Although gold does not seem to be depleted in Leg 37 basalts there is evidence that the metal is easily mobilized by local heating accompanying igneous intrusive rocks.The average Au/Ir ratio of the Leg 37 basalts is four times higher than that of the ultramafic–mafic rocks of hole 334. Although part of this effect is probably due to secondary alteration, it is considered unlikely that the Leg 37 basalts and ultramafic rocks are petrogenetically related.
Low temperature fluid alteration of oceanic layer 2 basalts, DSDP Leg 37
Authors: Andrews A. J.
Published: April 2011

Penetration of cold seawater into layer 2 of the oceanic crust occurs to at least 600 m, the maximum depth drilled below the sediment-acoustic basement boundary during Leg 37 of the DSDP. The main alteration phases are Mg-saponite, Fe-K rich celadonite, phillipsite, calcite, and hematite. The only exception to this occurs in the form of intensely hydrothermally altered gabbro breccias from an intrusive complex at site 334. There is no mineralogical evidence to suggest an increase in metamorphic grade with depth; however, a variation in mineralogy and alteration intensity, occurs as a function of structure, permeability, and the chemical nature of rock-fluid interaction. On this basis three types of alteration are defined as follows: (A) fracture focussed, oxidative; (B) palagonitic; and (C) non-oxidative, pervasive.Electron microprobe analyses reveal that alteration of fresh glass to palagonite involves the addition of H2O, an increase in total Fe, K2O, TiO2, and possibly SiO2, and the loss of CaO, MgO, Na2O, and MnO.Detailed examination of the mineralogy and chemistry of oxidative alteration suggests that during low temperature sea water–basalt interaction, basalt experiences a net gain in CaO, total Fe and K2O, while SiO2 and MgO appear to have been locally remobilized. These trends are generally consistent with the distribution of secondary phases. The distributions of Mn, Cu, Ni, Zn, Co, and Sr do not appear to have been significantly affected during this process.It is apparent that seawater must have experienced significant Eh lowering during this interaction; however, the occurrence of hematite–magnetite and absence of sulphide in this particular environment requires that seawater was either highly depleted in total sulfur or experienced a significant rise in pH.
A possible Proterozoic plate boundary in North America
Authors: Camfield P. A. , Gough D. I.
Published: April 2011

A long, narrow belt of rocks of very high electrical conductivity has been discovered and mapped by means of large arrays of recording magnetometers, over a distance of 1400 km from southeastern Wyoming to the edge of the Canadian Shield in Saskatchewan. Evidence that the conductive belt might be associated with conductive minerals in metamorphosed and fractured rocks in the basement has been discussed in earlier papers. Recent results on the Precambrian geology at both ends of the conductor, in the Churchill Province of the Shield and in southeastern Wyoming, support the hypothesis that the anomaly in electrical conductivity traces a major fracture zone in the lithosphere of Precambrian North America. This paper presents and relates various lines of evidence which together tend to substantiate such a fracture zone or mobile belt. From the age and composition of rocks near the southern end of the structure, Hills and others suggest that a Proterozoic subduction zone is located there. It is possible that the entire fracture zone from the Southern Rockies to the vicinity of Hudson Bay is a Proterozoic continental collision zone or geosuture.