摘要:

Gamma-ray spectrometer measurements were obtained at and in the vicinity of 104 of the 124 U, Th, Mo, and rare-earth-element (REE) occurrences examined in the Central Metasedimentary Belt of the Grenville Province. Spatial, temporal, mineralogical, and geochemical relationships among granitic pegmatites, phlogopite – scapolite – Ca pyroxene skarns, and fluorite – apatite – calcite veins hosting U, Th, Mo, and REE minerals indicate a common magmatic–hydrothermal origin. Quartz–feldspar gneisses in the Central Metasedimentary Belt (n = 54) have low abundances of uranium (1–7 ppm) and thorium (4–27 ppm) suggesting that partial melting, fractional crystallization, and volatile phase separation were responsible for the enrichment of uranium (2–37 ppm) and thorium (5–102 ppm) in uncontaminated granitic pegmatites (n = 163) derived during ultrametamorphism. The U/Th ratio is probably inherited from the source quartz–feldspar gneiss protolith and enhanced during fractionation.Average U and Th concentrations and U/Th ratios at numerous localities show significant positive correlations among pegmatites, skarns, and veins, providing further evidence for a related origin. The interaction of the pegmatite-derived hydrothermal fluids with host rocks produced a spectrum of types and styles of alteration, which include (i) hybridization and (or) endoskarnification along pegmatite margins; (ii) marble- and clinopyroxenite-hosted exoskarn; and (iii) fluorite–apatite–calcite veins. The deposition of U, Th, Mo, and REE from the evolving hydrothermal fluid is responsible for the heterogeneous distribution of U, Th, and REE minerals and molybdenite within pegmatites, skarns, and veins at each locality. Secondary enrichment of uranium in association with hematitized sheared pegmatites and veins may be responsible for the observed large variation in U/Th ratios at some sites.

ISSN:0008-4077    

DOI:10.1139/e91-001

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A study of the sulphur-isotope and minor-element composition of gold ores and country rock from the Hemlo deposit, Ontario, has been carried out on 10 drill cores from the mineralization zone and two occurrences of stratiform barite west of Hemlo. The work confirms that Mo, As, and Sb correlate strongly with Au, whereas Ba, and by inference barite, does not. A preferential association of heavy rare-earth elements with higher grades of Au was also established. The isotopic compositions of barite and pyrite reflect a deposit-wide thermal equilibration at 504 °C. Whole-rock sulphur isotopic compositions can be modelled as a mix of sulphate and sulphide end members, the former marked by a relatively constant isotopic composition of roughly 10‰ and the latter by variable and negative isotopic compositions in mineralized zones.These isotopic characteristics are interpreted as the result of (i) preliminary fluid–rock reactions in which some isotope exchange between sedimentary barite and hydrothermal sulphide has occurred and (ii) a later stage metamorphic event in which sulphur-isotope equilibrium is established between the precipitated, distinctly isotopically light pyrite and bedded barite. These two fractionation events, separated in space and time, are required to explain both the variable and distinctly negative δ34S values observed for the pyrite in the Hemlo ores (−15.9 to +1.0%o) and the uniform Δδ34S value of 12.7 ± 1‰ obtained for the coexisting barite and pyrite pairs, in widely different proportions on a deposit-wide scale. Both the production of light isotopic sulphide and the later precipitation of Au are thought to have been influenced by an increase in fluid.

ISSN:0008-4077    

DOI:10.1139/e91-002

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Several Early Proterozoic Hearst–Matachewan (2.454 Ga), Kapuskasing (2.14 Ga), and Preissac (2.04 Ga) dykes were emplaced within the Archean crust surrounding the Kapuskasing structural zone (KSZ). The dykes are composed of moderately to highly fractionated tholeiitic basalts (Mg number = 24–55) that exhibit trace-element characteristics similar to those of intraplate basaltic magmas or ocean–island basalts (e.g., Zr/Nb = 6–21, Zr/Y = 2–5, high TiO2 = 0.9–3.2 wt.%, and (Fe2O3)t = 12.4–18.7 wt.%). Their initial Nd isotopic compositions display a range of depletedto enrichedvalues that are negatively correlated with the degree of light rare-earth element enrichment. We evaluate two models for the origin of these dykes: (i) The basaltic parental magmas were derived from two distinct sources, an isotopically depleted asthenospheric mantle (ϵNd = +4 and La/Sm = 2.7) and an isotopically enriched lithospheric(?) mantle (ϵNd = −4 to−8 and La/Sm = 5.1). The magmas subsequently underwent mixing and fractionation during ascent in the mantle or the lower crust. (ii) The parental magmas originated from a homogeneous Nd isotopically depleted asthenospheric mantle but later assimilated a substantial amount of Archean crustal material upon fractionation and ascent in the lower crust. Results derived for the latter model preclude any participation of the exposed crustal rocks in the KSZ, and the assimilation and fractional crystallization (AFC) model remains a viable hypothesis only if the parental magmas assimilated an older and perhaps more isotopically enriched crust than that represented in the KSZ.

ISSN:0008-4077    

DOI:10.1139/e91-003

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The La Galissonnière Pluton is a late-orogenic pluton located in the eastern Grenville Province of Quebec. The pluton contains biotite > hornblende, zircon, titanite, allanite, and apatite, along with secondary sericite and epidote. The pluton features elevated values of Nb, Y, and F. The normalized rare-earth element patterns have light rare-earth element concentrations of 400–500× chondrite, moderate slopes (10–20), and noticeable Eu anomalies. Trace-element discriminant diagrams suggest that the pluton formed in a late-orogenic environment and indicate a possible period of late Precambrian crustal attenuation in the eastern Grenville Province.

ISSN:0008-4077    

DOI:10.1139/e91-004

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The early middle Archaean Maggo gneisses of the southern Nain Province, Labrador, form the southwest portion of the once contiguous North Atlantic (Nutak) Craton (NAC). The gneisses and their late middle Archaean metamorphosed and migmatized equivalents are typical of grey gneiss terranes exposed worldwide. Geochemically the gneisses exhibit a continuous range of composition from 53.7 to 78.4 wt.% SiO2and straddle the boundary between low- and high-Al trondhjemites. Major-element distributions are comparable to those of other Archaean-aged NAC gneisses (Amîtsoq, Uivak, and Nûk gneisses), however, the Na2O and K2O contents are scattered. The gneisses are depleted in K, Rb, and Ba, are enriched in Sr, and have high-field-strength-element distributions similar to those for NAC gneisses.Rare-earth-element (REE) patterns of Maggo gneisses can be subdivided, on the basis of the nature of the Eu anomaly, into two groups of samples: (i) with negative Eu anomalies and higher Σ REE contents and (ii) with positive to normal Eu anomalies and lower Σ REE contents. The subdivision reflects differentiation processes in the parent magma of the Maggo gneisses. REE patterns are similar to those reported for NAC grey gneiss complexes. On the basis of the (La/Yb)Nand (Yb)Nvalues, the Maggo gneisses parent magma is interpreted as being derived by partial-melting processes from preexisting, high-grade (granulite to amphibolite facies) sialic continental crust equivalent to the early Archaean lithologies preserved elsewhere in the NAC.

ISSN:0008-4077    

DOI:10.1139/e91-005

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Geochemistry and Nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic, calc-alkalic, and lamprophyric suites in the Kamiskotia–Montcalm area. The Kamiskotia gabbroic complex (KGC) and cogenetic, bimodal volcanic rocks have ϵNd(t) = +2.2 to +2.6, consistent with a direct derivation from a long-term, light rare-earth element (LREE)-depleted mantle. The Montcalm gabbroic complex has decreasing ϵNd(t) upsection from +2.8 to +1.0, consistent with contamination by long-term, LREE-enriched (with respect to the long-term, LREE-depleted Abitibi mantle) crust during fractionation. Two calc-alkalic lamprophyre samples, characterized by large-ion lithophile element (LILE) and LREE enrichment and high MgO, Ni, and Cr contents, have ϵNd(t) of +2.5 and +2.8, indicating a derivation from a depleted mantle source that had undergone recent trace-element enrichment. A different lamprophyre suite is extremely LILE and LREE enriched and has an ϵNd(t) of +1.0, indicating a derivation from a slightly different source that had earlier LREE enrichment. Granitoid rocks internal and external to greenstone belt rocks have ϵNd(t) = +2.5 to +3.8 and +0.6 to −0.4, respectively. The lower values provide additional evidence for the existence of LREE-enriched crust in this area.Considering these data along with other radiogenic isotope studies, a petrogenetic and tectonic model is suggested for the crustal development of the southern Abitibi Subprovince. From >2740 to 2698 Ma—the major period of volcanic activity—komatiitic and tholeiitic suites and one lamprophyre suite were derived from a uniformly LREE-depleted mantle reservoir with ϵNd(t) = +2 to +3. Calc-alkalic granitoids were emplaced generally after 2700 Ma. Their long-term, LREE-depleted and LREE-enriched Nd isotopic signatures are similar to signatures in continental-arc settings (e.g., the Coastal Batholith of Peru). Form 2690 to 2670 Ma, when transpressional tectonism prevailed, mantle-derived magmatism was represented by long-term, LREE-enriched (ϵNd(t) = +1 to +2) lamprophyric and alkalic volcanic suites.The Kamiskotia suite has a seven-point, whole-rock – mineral isochron Sm–Nd age of 2710 ± 30 Ma, identical to U–Pb zircon ages for the suite, indicating closed-system behavior. An Rb–Sr mineral – whole-rock isochron age from one KGC sample is 2450 ± 30 Ma, identical to U–Pb ages for the Hearst–Matachewan dike swarm, a prominent feature in the KGC area. Regression of whole-rock and mineral-isotope data for one granitoid sample with a U–Pb zircon age of 2696 ± 1.5 Ma gives identical ages of 2530 ± 30 Ma in the Sm–Nd and Rb–Sr systems. The latter data add to an increasing body of evidence for cryptic, late thermal events after granitoid–greenstone belt development in the southern Abitibi Subprovince.

ISSN:0008-4077    

DOI:10.1139/e91-006

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A newly recognized suite of trondhjemite–tonalite and dacitic gneiss forms a 10 km wide belt of rocks within the Mount Holly Complex in the central part of the Green Mountain massif of Vermont. Field relationships and chemistry indicate that these gneisses are calc-alkaline, volcanic, and hypabyssal plutonic rocks older than the Middle Proterozoic regional deformation that affected the Mount Holly Complex. U–Pb zircon dates indicate ages as great as 1.35 Ga for crystallization of the volcanic protoliths and for intrusion of crosscutting trondhjemite. Tonalitic plutonism continued until 1.31 Ga.Map-scale contacts between the trondhjemitic–tonalitic–dacitic gneisses and the paragneiss sequence of the Mount Holly Complex are sharp, suggesting that the volcanic rocks of the trondhjemite–tonalite suite underlie the paragneiss units and do not intrude them. These relationships suggest that the trondhjemite–tonalite suite is either considerably older than, and unconformable beneath, the paragneiss cover rocks or represents a volcanic edifice slightly older than the deposition of the sedimentary precursor to the paragneiss units. The paragneiss and tonalite–trondhjemite gneisses are both intruded by younger granitoids that were intruded at about 1.25 Ga during strong dynamothermal metamorphism.The trondhjemitic gneisses of the Mount Holly Complex of Vermont have high Al2O3and low Yb contents and light rare-earth element enrichment patterns that are more characteristic of continental than oceanic volcanic arcs. The Mount Holly intrusives and volcanics may have formed during 1.35–1.31 Ga ensialic volcanic-arc activity, contemporaneous with ensimatic arc activity during the early part of the Elzevirian phase of the Grenville orogeny. In Vermont, later deformation and granite intrusion at about 1.25 Ga coincide with the major pulse of the Elzevirian orogeny and associated trondhjemitic plutonism in the Central Metasedimentary Belt of eastern Canada.

ISSN:0008-4077    

DOI:10.1139/e91-007

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

We have determined the seismic velocity structure of the crust in the vicinity of the Appalachian deformation front off western Newfoundland and the adjacent Gulf of St Lawrence. These measurements were made from two perpendicular wide-angle seismic refraction profiles, one of which is collinear with a previously recorded deep seismic reflection profile. The Grenville foreland crust, about 45 km thick, is characterized by velocities of 6.35 km/s in its upper part and 6.7 km/s in its lower part. Close to the coast of Newfoundland, a deep crustal reflective wedge is bounded by a northwest-dipping reflector and by the crust–mantle boundary, which is at only 39 km depth beneath the wedge. In the wedge, velocities of 7.2–7.3 km/s may indicate the presence of mafic and ultramafic rocks. We speculate that several processes could have caused the high velocities and the high reflectivity. The most attractive is perhaps crustal extension with consequent underplating during the formation of the Iapetus Ocean or during later reactivation by Carboniferous strike-slip movements.

ISSN:0008-4077    

DOI:10.1139/e91-008

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Cretaceous dinosaur footprints discovered in the J Sandstone of the South Platte Formation (Dakota Group) in Colorado and the St. Mary River Formation of southwestern Alberta exhibit the first reported foot pad skin impressions of large ornithopods. The Canadian tracks occur as sandstone casts preserved in mudstones, whereas the Colorado tracks are natural impressions in a sandstone bed overlain by shale. The South Platte Formation tracks occur as impressions in a widespread "dinoturbated" sandstone bed representing low-gradient, delta plain – coastal plain facies assemblages associated with the upper member of the group, the J Sandstone. Only one of the many iguanodontid trackways exhibits good skin impressions.The St. Mary River Formation palaeoenvironment is interpreted as an anastomosed fluvial system that flowed northeast over a low-gradient floodplain from Montana. Footprints, often preserved in trackways, were left as dinosaurs walked across lake and marsh sediments that were relatively well drained or in various stages of dewatering. The quality of preservation is variable, depending on the properties of the substrate, and only one hadrosaur footprint includes clear casts of skin patches on the bottom of the footprint. Similar track-rich facies assemblages, representing lowland coastal plain and deltaic environments, are found in both the Lower (Gething Formation, British Columbia) and Upper Cretaceous (Mesa Verde, Colorado) successions of western North America. Few substrates of these depositional environments were suitable for the preservation of skeletal remains, so the information derived from tracks is palaeontologically significant.

ISSN:0008-4077    

DOI:10.1139/e91-009

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The southern boundary of the Quetico metasedimentary subprovince of the Superior Province of the Canadian Shield near Kashabowie, Ontario, is a vertical, east–west feature affected by dextral transpression that had a north-northwest – south-southeast compressional component. A synmetamorphic,locallyD1microfabric and magnetic-susceptibility fabric with an east-directed extension lineation is kinematically compatible with this pattern. It shows the same bedding–cleavage relationship and the same direction ofstructural facingon D1in both the Quetico metasediments and the Shebandowan greenstone subprovince on the south side of the Quetico subprovince. In the low-grade rocks of the study area, there is a single phase of penetrative deformation, giving a nearly vertical schistosity at a consistent angle, anticlockwise with respect to the now nearly vertical east–west-striking strata. The absence of penetrative polyphase deformations may be due to the near parallelism of the subprovince boundary with the shear component of dextral transpression. Strain analysis indicates that the minimum shortening of the greywackes is 40% in a north–south direction. It is tentatively suggested that the shortening, the steepening of strata into a vertical position, and some of the S1fabric development may have occurred prior to the climax of metamorphism and transpression. If this sequence is correct, the strata would have dipped gently to the north prior to the steepening event, with the embryonic schistosity dipping to the west.

ISSN:0008-4077    

DOI:10.1139/e91-010

出版商:NRC Research Press    

年代:1991

数据来源: NRC