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The Midcontinent Rift in the Lake Superior region of North America is one of the best preserved examples of an aborted Precambrian intercontinental rift, one that hosts a diverse suite of rock types in addition to the well-studied and voluminous rift-fill flood basalts. Although there is a growing database of high-precision age information for the main volcanic packages and the largest mafic intrusions, there is relatively little information available on the absolute timing of mafic-ultramafic intrusions, dyke swarms, and alkaline complexes, especially in the Ontario portion of the rift. We report new high-precision U–Pb ages for 29 samples, primarily collected in the Lake Nipigon area, Ontario. From these new age results, it is now possible to expand the known distribution of Geon 15 magmatism in the region, confirm an early stage of Midcontinent Rift mafic magmatism between 1150 and 1130 Ma, provide evidence that significant mafic–ultramafic magmatism occurred in the Lake Nipigon region slightly earlier (~1115–1110 Ma) than the main stage of rift magmatism (1108–1094 Ma), and further document synchronous ~1110–1100 Ma tholeiitic and alkaline magmatism.

Shear-wave seismic data recorded along four profiles during the SNoRE 97 (1997 Slave – Northern Cordillera Refraction Experiment) refraction – wide-angle reflection experiment in northwestern Canada are analyzed to provide S-wave velocity (Vs) models. These are combined with previous P-wave velocity (Vp) models to produce cross sections of the ratio Vp/Vs for the crust and upper mantle. The Vp/Vs values are related to rock types through comparisons with published laboratory data. The Slave craton has low Vp/Vs values of 1.68–1.72, indicating a predominantly silicic crustal composition. Higher values (1.78) for the Great Bear and eastern Hottah domains of the Wopmay orogen imply a more mafic than average crustal composition. In the western Hottah and Fort Simpson arc, values of Vp/Vs drop to ∼1.69. These low values continue westward for 700 km into the Foreland and Omineca belts of the Cordillera, providing support for the interpretation from coincident seismic reflection studies that much of the crust from east of the Cordilleran deformation front to the Stikinia terrane of the Intermontane Belt consists of quartzose metasedimentary rocks. Stikinia shows values of 1.78–1.73, consistent with its derivation as a volcanic arc terrane. Upper mantle velocity and ratio values beneath the Slave craton indicate an ultramafic peridotitic composition. In the Wopmay orogen, the presence of low Vp/Vs ratios beneath the Hottah – Fort Simpson transition indicates the presence of pyroxenite in the upper mantle. Across the northern Cordillera, low Vp values and a moderate-to-high ratio in the uppermost mantle are consistent with the region's high heat flow and the possible presence of partial melt.

The Walmsley Lake area of the southeastern Slave Province, Northwest Territories, exposes a crustal transect from greenschist through to upper amphibolite facies. Two tectonothermal crustal domains are proposed based on disparate tectonic histories and rock types. Within the upper tectonothermal domain, D1 deformation reached lower amphibolite facies prior to 2614 Ma and associated M1 metamorphic conditions outlasted D1. Peak M2 conditions reached middle amphibolite facies prior to 2603 Ma and outlasted D2 deformation. Deformation style and metamorphic sequencing during these two pre-2600 Ma events are consistent with regional crustal shortening and thickening in the upper tectonothermal domain. At mid-crustal levels in the lower tectonothermal domain, a third tectonothermal event (D3–M3) produced uppermost amphibolite-facies peak metamorphic conditions, transposed preexisting fabrics to shallow dips, and produced a subhorizontal foliation and recumbent folds at ca. 2583 Ma. D3 structures are not present in the upper tectono thermal domain, however, evidence of M3 heating is locally displayed in rocks at the base of the upper tectonothermal domain. Upper and lower domains were structurally decoupled during the D3–M3 event. In rare places where the zone between the two domains outcrops, it comprises a narrow high-strain zone. The metamorphic and structural topology of the Walmsley Lake area is consistent with the topology predicted from thermomechanical modeling and modern observations of gravitational accommodation in structurally thickened crust.

Eight hydrocarbon exploration wells in the southern Labrador Sea penetrated Precambrian basement rocks, adjacent to the course of a Lithoprobe marine seismic reflection survey. The rock types are mostly Early Proterozoic (1870–1800 Ma) calc-alkaline plutonic rocks or their metamorphic derivatives, but one example is a Middle Proterozoic (1270 Ma) anorogenic intrusion. These samples of buried basement document a transition from juvenile Proterozoic crust in the southeast (shown by εNd from +0.8 to +2.2) to ancient Archean crust in the northwest (shown by εNd from −2.7 to −7.0). However, initial Sr isotope ratios do not show such a clear pattern. The transition in εNd is similar to results of onland studies in Labrador and formerly adjacent south Greenland, and delimits the hidden edge of the North Atlantic Archean craton within the Early Proterozoic Makkovikian–Ketilidian Orogen. These results from close to the seismic profile confirm that regional reflectivity contrasts on either side of the isotopic boundary record fundamental differences between local Archean and Proterozoic crust. Southeast-dipping reflectors that broadly correspond with the boundary zone probably represent a collisional suture zone, along which juvenile terranes were probably placed structurally above the Archean craton.

In conjunction with the Lithoprobe southern Canadian Cordillera program, an extensive examination of geochemical indicators of origins, movement, chemical evolution, and economic significance of paleocrustal fluids was conducted. The study area covers approximately 360 000 km2from the Canadian Rockies to Vancouver Island. Research incorporated petrological, mineralogical, fluid-inclusion, δ18O, δD, δ13C, and Rb/Sr studies of samples of quartz ± carbonate veins and other rock types. The results of the study document a variety of pre-, syn-, and postorogenic, crustal fluid events. In the Rockies, a major pre-Laramide hydrothermal event was identified, which was comprised of a west to east migration of warm, saline brines. This was followed by a major circulation of meteoric water in the Rockies during Laramide uplift. In the southern Omineca extensional zone, convecting surface fluids penetrated to the brittle–ductile transition at 350–450 °C and locally into the underlying more ductile rocks. A principal conclusion of the study is that most quartz ± carbonate veins in metamorphic rocks in the southern Canadian Cordillera precipitated from deeply converted surface fluids. This conclusion supports a surface fluid convection model for the genesis of mesothermal Au–quartz veins, common in greenschist-facies rocks worldwide. The combination of our geochemical results with the results of other Lithoprobe studies indicates that widespread and deep convection of surface fluids in rocks undergoing active metamorphism is a commonplace phenomena in extensional settings, while in compressional-thrust settings the depth of penetration of surface fluids is more limited.

Episodic granitoid plutonism in the Early Proterozoic Makkovik Province of the easternmost Canadian Shield is revealed by high-precision U–Pb zircon studies of a range of intrusive rock types. The oldest granites yield ages of 1893 ± 2 and 1891 ± 5 Ma and document a previously unrecognized event that may correlate with an early migmatization of reworked Archean basement. These dates also constrain early structural reworking of the basement, the earliest deformational event grouped as part of the Makkovikian orogeny. Four samples have essentially identical zircon ages of 1801 ± 2, 1802 ± 2, [Formula: see text], and 1825–1799 Ma, and a fifth is slightly older, at [Formula: see text]. These ages suggest correlation with local volcanic sequences, dated in part at 1807 ± 4 Ma. The plutonic suites include both syn- and posttectonic granitoid assemblages and define the main magmatic pulse associated with the Makkovikian orogeny and constrain its final deformational episode. Distinctive, fluorine-enriched "A-type" granites yield an age of 1719 ± 3 Ma and represent a previously unrecognized late postorogenic to anorogenic magmatism of regional significance. Two layered, gabbro–diorite–monzonite–syenite suites yield identical zircon ages of 1649 ± 1 and 1649 ± 3 Ma. A regionally extensive granodioritic unit gives an age of 1647 ± 2 Ma, and a high-level alaskitic granite is dated imprecisely at 1640–1650 Ma. These plutonic suites correlate with volcanic rocks previously dated at 1649 ± 1 Ma.These data show that the plutonic evolution of the Makkovik Province is significantly more complex than previously supposed. The clustering of ages suggests episodic, rather than continuous, magmatism. The different age groupings can, to some extent, be correlated with compositional associations defined by major- and trace-element geochemistry. The new data also raise questions about the regional configuration of Early and Middle Proterozoic orogenic belts in Labrador. Previous correlations between the Makkovik Province, the Ketilidian Mobile Belt of Greenland, and the Svecofennian and Trans-Scandinavian belts of Sweden are supported and expanded by these new results.

Early and Middle Devonian fossils from the Sainte-Hélène Island diatreme breccias are criticially reviewed and analyzed stratigraphically. They include marine benthic faunas of Helderberg, Oriskany, Schoharie, Onondaga, and Hamilton ages. Brachiopods are the most abundant fossils; significant corals are also present and are discussed for the first time. These diatreme faunas from the Montréal region significantly augment the lithofacies–paleogeographic data known for this part of North America. For the first time, attention is paid to the lithologies in which the Sainte-Hélène Island faunas are embedded, as clues to the regional relations of these rock types. The absence of Silurian specimens is ascribed to nondeposition similar to that known in the Albany, New York, region to the south. The presence of marine, Hamilton age rocks and fossils at Montréal shows that the Acadian Orogeny and uplift into the nonmarine environment did not affect the area until at least some time in the Hamilton.

The Mount Peyton batholith of central Newfoundland is one of many Siluro-Devonian ("Acadian") plutons of the Appalachians with contrasting mafic and silicic compositions. The distributions of rare earth and other trace elements in 18 representative samples indicate that the contrasting rock types are not genetically related by any crystal–liquid fractionation process. Rather they support earlier suggestions that the granitic melt was generated by crustal melting due to intrusion of the mantle-derived mafic magma, and the two evolved independently through processes involving some crystal fractionation and contamination with country rocks during ascent.

Rare earth and other trace element data are presented for samples of the Sokoman Iron Formation, Labrador, and its associated sediments. The results show that the slates associated with the iron formation are typical in trace element contents compared to other argillaceous sediments except for the large Eu depletion characteristic of slates of their age. The iron formation, however, is fundamentally different in its trace element concentrations and patterns from those of the associated rocks. It is relatively enriched in the heavy REE and Eu and both the REE and Co, Cr, Sc, and Th concentrations bear no relationship to those of the slates and the dolomite.Trace element analyses of the various textural and mineralogic rock types in all cases substantiate the genetic conclusions of earlier workers based on field and petrographic observations. Silicate–carbonate facies samples show constant REE, Co, Sc, and Th distributions which are compatible with an origin as crystalline precipitates in equilibrium with sea water. Riebeckite-bearing iron formation is distinctive in that it reflects contamination by ordinary clastic material and (or) metamorphic solutions. Oxide facies rocks exhibit widely variable trace element abundances as is to be expected for rocks whose original trace element contents were controlled by adsorption processes. A group of iron-enriched oxide facies rocks show evidence of important heavy REE complexing associated with the migration of iron during diagenesis. Minor Ce anomalies in all facies of Sokoman Iron Formation indicate that oxidation of Ce to the +4 state was taking place at the time of iron deposition but probably not in close proximity to it.

In the northwestern part of the Canadian Shield upper Proterozoic rocks of Banks Island, Brock Inlier, Coppermine area (Rae Group), Minto Arch of Victoria Island, and some small inliers have all been considered as parts of the Amundsen Basin. Formational correlation among these areas has been hindered because different stratigraphic subdivisions have been used by geologists working in different regions and because no individual has previously studied the rocks of the basin as a whole. In general, the Proterozoic rocks of the mainland coastal region have been more finely subdivided than those of Victoria Island. Most of the upper Proterozoic rocks of the mainland regions and all of those at the southern tip of Banks Island are considered to be correlative with the Glenelg Formation of Victoria Island. The Glenelg Formation is divisible into at least four widespread mappable units, each of which has diagnostic rock types, sedimentary structures, and in some cases, stromatolites corresponding closely to those of the mainland areas.The stratigraphic succession in the Shaler Group of the Minto Arch area is remarkably similar to that of the sub-Rapitan Proterozoic sequence of the Mackenzie Mountain region about 500 km to the southwest. Approximate time equivalence is proposed. If this correlation is correct then the 'older' Proterozoic sequence in the northern Cordillera is between 1200 Ma and 700 Ma in age and is therefore Neohelikian or Hadrynian in age (or both). Other corollaries of the proposed correlation are that the Rapitan and younger Proterozoic rocks of the Cordilleran region are not represented in the northwestern part of the Canadian Shield and that the older Proterozoic rocks of the Mackenzie Mountains region are probably younger than at least part of the Belt–Purcell sequence of the Canada–U.S.A. border region.New paleocurrent measurements from sandstones in the western part of the Brock Inlier indicate that dominant sediment transport was to the north-northwest. Limited data from the older Proterozoic sequence in the northern part of the Canadian Cordillera suggest transport to the west. These data are consistent with the existence of a weak positive area to the south and east of the two depocentres with possible continuity of a shallow marine shelf sea around its northern margin.

The classical macrotextural subdivision of quartzo-feldspathic mylonitic rocks yields only three rock types: protomylonite, mylonite, and ultramylonite. This restriction impedes detailed mapping of the internal textural transitions common in wide, deep-seated, crustal-scale shear zones, where such transitions may occur over kilometres and involve several clearly mappable textural types. The introduction of two objectively defined field mapping terms, "homoclastic" and "heteroclastic," describing the macroscopic grain-size distribution within the porphyroclast population provides descriptive flexibility without changing the matrix–porphyroclast basis of the established classification. This allows the description of textural paths other than protomylonite → mylonite → ultramylonite and facilitates the consideration of textural paths in terms of strain partitioning between the constituent grains of the deforming aggregate, rather than as a simple function of finite strain.

A diverse suite of rocks was collected for paleomagnetic study from two formations in the Eastport area of northeastern Maine: 18 sites (103 samples) from redbeds and diabase sills of the Hersey Formation (uppermost Silurian to Lower Devonian) and 14 sites (74 samples) from lavas, redbeds, and calcareous nodules of the Eastport Formation (Lower Devonian; Rb/Sr age 412 Ma). Characteristic magnetizations based on AF and thermal demagnetization analyses give mean directions, after simple correction for bedding tilt, of D = 163.6°, I = 41.3°, α95 = 6.3°, for N = 16 sites in the Hersey, and D = 179.3°, I = 38.0°, α95 = 9.4°, for N = 14 sites in the Eastport Formation. Baked contact relations, the presence of opposite polarities, and the similarity in directions over different rock types and magnetic carriers point to an acquisition of magnetization near to the time of rock formation. The mean directions correspond to (south) paleopole positions at 19.8°S lat., 308.8°E long. (δp, δm = 4.7°, 7.7°) for the Hersey and 23.7°S lat., 293.7°E long. (δp, δm = 6.6°, 11.1°) for the Eastport Formation.These paleopoles are removed from the Upper Silurian Bloomsburg Formation pole, but are in reasonable agreement with similar age results from central Newfoundland, suggesting that the Acadia displaced terrain encompassed these areas. Siluro-Devonian paleopoles from intrusive rocks in northeastern North America are generally widely divergent and these may not be providing an accurate record of paleomagnetic field in all cases.

The average 18O/16O ratios of the major rock types of the surface crystalline rocks in different parts of the Canadian Precambrian Shield have been determined, using 47 composite samples prepared from 2221 individual rock specimens. The sampling areas include Baffin Island, northern and southwestern Quebec, Battle Harbour – Cartwright, northern District of Keewatin, Fort Enterprise, Snowbird Lake, Kasmere Lake, and Saskatchewan, covering approximately 1 400 000 km2. The granitic rocks from the Superior, Slave, and Churchill Provinces vary only slightly from region to region (δ18O = 6.9–8.4‰) and are significantly lower in 18O than similar rock types from the younger Grenville Province (δ = 9.2–10.0‰). The sedimentary and metasedimentary rocks have δ18O = 9.0–11.7‰ and hence are considerably lower than their Phanerozoic equivalents, possibly reflecting the presence of a high percentage of little-altered igneous rock detritus in the original sediments. The basic rocks in most regions fall within a δ18O range of 6.8–7.6‰, except in northern and southwestern Quebec where the δ-values are abnormally high (8.5–8.9‰). The overall average 18O/16O ratio of the surface crystalline rocks of the Canadian Shield is estimated to be 8.0‰, which represents an enrichment with respect to probable mantle derived starting materials by about 2‰.

This vacuum-assisted technique for colored impregnation of all rock types clearly defines porosity, even in low-permeability, very fine grained, or semifriable material. The technique is also suitable for use with water-soluble and heat-sensitive material. The technique is trouble-free and provides a practical way of visually reproducing rock pore space less than 1 μm in size.

There is no preferential partitioning of boron among the principal rock-forming minerals in high-grade rocks of the Kapuskasing Structural Zone (KSZ) and the Wawa Domal Gneiss region (WDG). Lithium is strongly concentrated in biotite and other ferromagnesian minerals but does not show consistent partitioning between these and the sialic minerals.The distribution of B and Li within a rock may be studied using an alpha-track image, which shows that the inconsistencies in partitioning may be largely attributed to disturbance of mineral equilibria by postmetamorphic low-grade alteration that deposited B and Li.Boron has similar concentrations in all the rock types studied, although it is an incompatible element that elsewhere accumulates in pegmatites. Lithium concentrations are low in the anorthositic rocks but are otherwise very variable. In some but not all rocks higher than usual B and Li can be attributed to introduction during alteration.Boron occurs at low concentrations (2–3 ppm) throughout both the KSZ and the WDG areas and has an abundance similar to that in other granulite terranes. It is significantly lower than in average upper crustal rocks (9–15 ppm), and this is attributed to loss by fluid transport during formation of lower crustal material. Lithium occurs at similar concentrations in upper crustal rocks (20–22 ppm) as in the WDG area (27 ppm) but is lower in the KSZ (13 ppm), suggesting again a loss by fluid transport in the deep crust. Both estimates of loss are minima because of the evidence of reintroduction of the elements during later alteration.Although there is field and petrological evidence of anatectic melting in the KSZ–WDG region the distribution patterns of B and Li show no evidence of this: this is not unexpected for elements that readily partition into a fluid phase.

Structural, mineralogical and textural characteristics of some layered granitic rocks are described. The layers result from the segregation of minerals into bands and lenses of contrasting color and grain size. They were formed near the roof of the pluton prior to final crystallization by flow sorting during episodic shearing generated during intrusion. Comparison of the mineralogy and textures of the layered rocks with those of the main rock types of the pluton shows that the differentiation of the pluton as a whole took place after the formation of the layers by interaction of felsic components concentrated in residual liquids with earlier formed crystals.

The relationship between mineralogy and bulk composition was examined in mylonites developed in a variety of rock types from the Grenville Front zone of eastern Labrador. Mylonites developed from olivine-, pyroxene-, and (or) amphibole-bearing protoliths are the most altered, typically being enriched in K2O and volatiles and depleted in CaO compared with the protolith. Bulk-compositional modification was accompanied by the formation of almandine-rich garnet and (or) relatively ferruginous hornblende in medium-grade (epidote–amphibolite to lower amphibolite facies) mylonites derived from both mafic and quartzo-feldspathic rocks. Protoliths containing biotite as the principal or sole Fe–Mg phase were the least altered during deformation. Apart from variations in Ti content, biotite in mylonitic assemblages derived from these rocks is compositionally similar to that in the low-strain precursor.Bulk-compositional changes recognized in narrow mylonite zones appear to have occurred in Grenvillian tectonites developed on a regional scale.

Since 1965, the Quebec Department of Natural Resources has conducted reconnaissance mapping in the Grenville Province, completing to date over 70 000 square miles (181 000 km2). In 1968 a computer-based data processing system was designed at Queen's University, and applied to this Grenville Project. Data are recorded in the field on input documents designed to recover a complete description of the structure and lithology of an outcrop in a standard and reproducible form. From these records a data bank, now with descriptions of over 5000 outcrops, has been constructed for the field seasons of 1968 and 1969. Lthological sorting programs based on the textural and mineralogical qualities used to define rock types have been written to provide listings of some 40 potential map-units. Outcrop maps identifying lithologies have been drawn on a computer-driven drum plotter to provide the geologist with a basic document from which to make geological interpretation. Structural data have been extracted and plotted in equal-area projections, and also plotted on maps directly by machine, using standard structural symbols for layering, foliation, and lineation. The system has upgraded the standard of data collection in the field, and provides a rapid and versatile means of handling data and of interpreting the geology. The data bank, when made publicly available, will enable users of government geological maps and reports to reinterpret the area in their own manner, or freely to add any proprietary data in the preparation of revisions.

Dr. Gadd's percipient comments on the Grand Manan paper are greatly appreciated and will certainly encourage the writer to continue his study of the glacial geology of the island as occasion permits. The questions raised by Dr.Gadd make the writer wish that he could have the benefit of his guidance in the field since the limitations of the written word in dealing with the features described are all too obvious. The possible relation of the exposures at the Lower Flock of Sheep and at Grand Harbour Brook (Figs. 9a and b , 5) had not occurred to the writer. He has no recollection of seeing at any time any trace of till in the immediate vicinity of the Flocks of Sheep but he will search again on his next visit. The scales, however, are different. From memory, aided by a close study of photographs, the level (wave-washed?) surfaces at the Flock of Sheep, Red Point Beach, and Grand Harbour Brook are, respectively, 8-9 m, 4.5 m, and about 2 m above HWOST.2 Careful search has been made for any clear evidence of the limit of marine submergence. (Dr. D. R. Grant had also raised this point with the writer.) No such evidence has yet been found but search for this will continue, as also for any indication of the sense of direction of ice movement in any new striae that may be found. It should have been made clear in the paper that the rock types found in the erratics that constitute the Flocks of Sheep are found also in the scattered individual erratics found elsewhere on the island. In the original (una