摘要:

The chrysotile asbestos deposits of southeastern Quebec occur in serpentinized peridotites of Ordovician ophiolites. The asbestos is localized in veins within blocks of partly serpentinized peridotite bounded by zones containing schistose serpentinite and granitic masses. It is thought that these zones localized shear and provided the channelway s by which water entered the peridotite. Different generations of chrysotile veins can be identified locally by cross-cutting relationships, but these observations cannot be generalized regionally. The asbestos was produced late during the deformation of the peridotite, filling extensional fractures in the peridotite induced by stresses resulting from motion along the shear zones.The factors that control the formation of the chrysotile veins are the degree of serpentinization of the peridotite and the size of the peridotite mass. Other factors being equal, the smaller the peridotite mass is, the shorter the time necessary to serpentinize it. Smaller masses of peridotite, once serpentinized, weaken, develop a foliation, and will not develop cross-fiber veins. Larger peridotite masses will be less serpentinized and stronger and will develop cross-fiber veins.

ISSN:0008-4077    

DOI:10.1139/e87-001

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The McClean group of uranium deposits consists of elongate pods of high-grade uranium mineralization (width = ~ 15–40 m) tightly confined to within ±40 m of the basal unconformity. Uraninite–coffinite–sulphide/arsenide–chlorite–siderite mineralization at McClean is surrounded by a muscovite/illite ± haematite hydrothermal alteration halo,which can contain coffinite–pararammelsbergite (NiAs2) – muscovite/illite nodules. Ten laser probe40Ar/39Ar dates, two of which are step-heat runs showing good plateaus, and 10 conventional K – Ar dates for this material show a distribution with asharp beginning at ~ 1320 Ma, a marked peak in the 1250–1200 Ma class interval, and a tail to dates as young as 1002 ± 33 (1σ) Ma. These determinations are in complete agreement with direct (U–Pb and Sm–Nd) dates on uraninite at the Midwest (e.g., 1328 ± 9 and 1110 ± 28 Ma), Key Lake (e.g., 1350 ± 4 and 1281 ± 6 Ma), and Collins Bay B deposits(e.g., 1281 ± 80 Ma). Since estimated depositional ages for the Athabasca sedimentary sequence are in the 1470 ± 15 to1428 ± 15 Ma range, uranium mineralization and associated hydrothermal alteration started ~ 100–150 Ma after Athabasca sedimentation, a result consistent with fluid-inclusion data, which indicate that mineralization took place at ~ 160–220 °C beneath ~ 3000 m of cover at a relatively advanced stage in the evolution of the basin. It is suggested that the similar initiation dates for uranium mineralization might reflect a widespread faulting event that affected the eastern part of the basin. A muscovite/illite closure temperature calculated from a measured argon diffusion activation energy of 36 ± 4 kcal/mol(1 kcal = 4.1868 kJ) indicates that the base of the Athabasca Basin in the McClean area has not been disturbed by temperatures greater than ~ 140 °C for 1.1–1.0 Ga. It is suggested that mineralization ceased when fracture permeability had been sealed by crystallization of secondary minerals. The duration of mineralization may have been ~ 150 Ma, a relatively long time interval not unreasonable for the base of a sedimentary basin.Secondaryillites interstitial to quartz grains from the HLM1 stratigraphic borehole give40Ar/39Ar ages of 1459 ± 4, 1341 ± 4, and 1113 ± 11 Ma, indicating that formation of diagenetic sheet silicatespredateduranium mineralization. Recrystallization or formation of sheet silicates in relict sedimentary layers and in subunconformity altered basement referred to as "regolith" started at approximately the same time, since dates of 1484 ± 55 Ma (sedimentary layer), 1482 ± 49 Ma (regolith), and 1453 ± 49 Ma (regolith) have been obtained. Resetting of interstitial, sedimentary layer, and regolith sheet-silicate dates continued to ages of, for example, 1113 ± 11 Ma (interstitial) and 1038 ± 55 Ma (sedimentary layer), which exactly coincide with the youngest ages obtained for the alteration halo associated with mineralization.The youngest date obtained is a40Ar/39Ar plateau age of 673 ± 3 Ma. The sample (2045-517) was obtained from within 2 mm of a concentrated pitchblende nodule and may have been disturbed in some way by its proximity to uranium.

ISSN:0008-4077    

DOI:10.1139/e87-002

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

Rb–Sr and U–Pb data from the Borden complex of northern Ontario, a carbonatite associated with the Kapuskasing Structural Zone, indicate a mid-Proterozoic age. A207Pb/206Pb age of 1872 ± 13 Ma is interpreted as the emplacement age of this body, grouping it with other ca. 1900 Ma complexes that are the oldest known carbonatites associated with the Kapuskasing structure. A206Pb–238U age of 1894 ± 29 Ma agrees with the Pb–Pb age but has a high mean square of weighted deviates (MSWD) of 42. A Rb–Sr apatite–carbonate–mica whole-rock isochron date of 1807 ± 13 Ma probably indicates later resetting of the Rb–Sr system.An ϵSr(T) value of −6.2 ± 0.5 (87Sr/86Sr = 0.70184 ± 0.00003) and an ϵNd(T) value of +2.8 ± 0.4 for Borden indicate derivation of the Sr and Nd from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. These closely resemble the ϵ values for Sr and Nd from the Cargill and Spanish River complexes, two other 1900 Ma plutons. The estimated initial207Pb/204Pb and206Pb/204Pb ratios from Borden calcites plot significantly below growth curves for average continental crust in isotope correlation diagrams, a pattern similar to those found in mid-ocean ridge basalts (MORB) and most ocean-island volcanic rocks, again suggesting a source depleted in LIL elements. The combined Nd and Sr, and probably Pb, data strongly favour a mantle origin for the Borden complex with little or no crustal contamination and support the model of Bellet al. that many carbonatites intruded into the Canadian Shield were derived from an ancient, LIL-depleted subcontinental upper mantle.

ISSN:0008-4077    

DOI:10.1139/e87-003

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate. In this paper, we interpret two LITHOPROBE profiles from southernmost Vancouver Island that were shot across three important terrane-bounding faults—Leech River, San Juan, and Survey Mountain—to determine their subsurface geometry and relationship to deeper structures associated with modem subduction.The structure beneath the island can be divided into an upper crustal region, consisting of several accreted terranes, and a deeper region that represents a landward extension of the modern offshore subduction complex. In the upper region, the Survey Mountain and Leech River faults are imaged as northeast-dipping thrusts that separate Wrangellia, a large Mesozoic–Paleozoic terrane, from two smaller accreted terranes: the Leech River schist, Mesozoic rocks that were metamorphosed in the Late Eocene; and the Metchosin Formation, a Lower Eocene basalt and gabbro unit. The Leech River fault, which was clearly imaged on both profiles, dips 35–45 °northeast and extends to about 10 km depth. The Survey Mountain fault lies parallel to and above the Leech River fault and extends to similar depths. The San Juan fault, the western continuation of the Survey Mountain fault, was not imaged, although indirect evidence suggests that it also is a thrust fault. These faults accommodated the Late Eocene amalgamation of the Leech River and Metchosin terranes along the southern perimeter of Wrangellia. Thereafter, these terranes acted as a relatively coherent lid for a younger subduction complex that has formed during the modem (40 Ma to present) convergent regime.Within this subduction complex, the LITHOPROBE profiles show three prominent bands of differing reflectivity that dip gently northeast. These bands represent regionally extensive layers lying beneath the lid of older accreted terranes. We interpret them as having formed by underplating of oceanic materials beneath the leading edge of an overriding continental place. The upper reflective layer can be projected updip to the south, where it is exposed in the Olympic Mountains as the Core rocks, an uplifted Cenozoic subduction complex composed dominantly of accreted marine sedimentary rocks. A middle zone of low reflectivity is not exposed at the surface, but results from an adjacent refraction survey indicate it is probably composed of relatively high velocity materials (~ 7.7 km/s). We consider two possibilities for the origin of this zone: (1) a detached slab of oceanic lithosphere accreted during an episodic tectonic event or (2) an imbricated package of mafic rocks derived by continuous accretion from the top of the subducting oceanic crust. The lower reflective layer is similar in reflection character to the upper layer and, therefore, is also interpreted as consisting dominantly of accreted marine sedimentary rocks. It represents the active zone of decoupling between the overriding and underthrusting plates and, thus, delimits present accretionary processes occurring directly above the descending Juan de Fuca plate. These results provide the first direct evidence for the process of subduction underplating or subcretion and illustrate a process that is probably important in the evolution and growth of continents.

ISSN:0008-4077    

DOI:10.1139/e87-004

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

An analysis of the subaqueous morphology of the Bella Coola delta, British Columbia, based on fathometer profiles, seismic and sidescan sonar surveys, gravity cores, and bottom grabs, defines a proximal, sandy delta front and a distal silt–clay prodelta. The former extends 3 km offshore to water depths of 275 m; the latter, 3 – 15 km down fjord to a depth of 550 m. Large chutes, succeeded downslope by depositional zones, characterize the delta front. Deep-seated rotational slides occur in the proximal part of the prodelta, indicating that the delta front and prodelta consist of separate mass-movement systems.

ISSN:0008-4077    

DOI:10.1139/e87-005

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

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

ISSN:0008-4077    

DOI:10.1139/e87-006

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

Brachiopods from shallow-water limestone boulders in beds 12, 13, and 14 of the allochthonous Cow Head Group belong in the earliest WhiterockianOrthidiellaZone. Brachiopods of the autochthonous intertidal to deep-water Table Head Group are equivalent to those of the younger WhiterockianAnomalorthisZone. The Cow Head is older than the Table Head.Integration of the brachiopod data from western Newfoundland with those from the type Whiterockian in Nevada, combined with the evidence of graptolites from both areas, indicates that theOrthidiellaZone spans the Australian Castlemainian, Yapeenian, and lower Darriwilian Dal zones.

ISSN:0008-4077    

DOI:10.1139/e87-007

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The main petrological, mineralogical, and geochemical features of the metavolcanites belonging to the Late Proterozoic tectono-sedimentary units of southern Mauritanides (Termessé and Niokolo Koba – Koulountou) and western Hoggar, Algeria (Timesselarsine), permit a reconstruction of the geodynamic environment that controlled the evolution of the Pan-African ranges of western Africa. Flows and pyroclastic beds metamorphosed under greenschist-facies conditions are interbedded within detritic and carbonate platform sediments, suggesting an unstable environment in the vicinity of a continental margin.The association of oceanic tholeiites slightly enriched in LREE and basalts more enriched in TiO2, Zr, Nb, and LREE displaying alkalic affinities suggests intracontinental rifting succeeded by expansion. The lack of ultrabasic – basic complexes (oceanic paleosutures) in southern Mauritanides and their occurrence in the Hoggar suggest that the opening was bigger in the north than in the south.The later calc-alkalic sequences of the Niokolo Koba – Koulountou units are tectonically associated with the Termessé unit. In the Timesselarsine unit, calc-alkalic acid rocks coexist with the basic lavas previously mentioned. Thus, the closure of the rift related to a continental collision appears to be later in southern Mauritanides than in the Hoggar.

ISSN:0008-4077    

DOI:10.1139/e87-008

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The Otish gabbro sills intrude Aphebian clastic rocks lying uncanformably on the Archean rocks of the Superior Province close to its juncture with the Grenville Province. The sills are undated but by inference may be ca. 1750 Ma. Two dyke swarms are known in the vicinity, the 1950 Ma, northwest-trending Mistassini dykes and a northeast-trending swarm of unknown age extending 600 km from Senneterre to the Otish Mountains and possibly another 300 km to the northeast. The trends of feeder dykes to the Otish sills are physically compatible with the dominant northeast dykes, which are therefore considered to be the feeders and should be called the Otish dykes.The Otish sills appear to be a unique occurrence along the 900 km dyke trend, possibly, but not entirely because of the chances of preservation. The general form of the Otish sill complex is a triangle bounded on the north by the east–west lip of the sedimentary basin, on the southwest by a northwest-trending Otish feeder dyke, and on the southeast by the underlying northeast feeder dykes. These dykes segment the sills into a series of four or five separate intrusive complexes, small in the northwest and becoming larger to the southeast. The regular inclination of tension fractures in the basal chilled margin of the sills suggests a crude pattern of flow from the feeder dykes inward to the centre of the sheets.Interpretation of the sedimentary sequence indicates that the Otish clastics were deposited higher on the paleoslope than the Mistassini carbonates. Although few dykes intrude the deeper basin, the magma rose and formed sills within the higher sequence. This variation may be explained by the different mechanical character of the two types of cover rock controlling the dyke behaviour. The relatively plasto-viscous Mistassini carbonate–shale sequence resisted the formation of tension fractures, whereas the brittle elastics opened easily, allowing the magma to rise into the stratified sequence, forming the sill complexes.

ISSN:0008-4077    

DOI:10.1139/e87-009

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The Ordovician–Silurian strata of the Percé area are assigned to the Matapédia Group. They occur in a southwestern monoclinal sequence, unconformably overlying Cambrian strata, and are assigned to the Pabos and White Head formations. The Pabos Formation is preponderantly a terrigenous sequence, whereas the White Head is preponderantly a carbonate sequence. The Pabos strata of this area are included in the new Rouge Member, in which four brachiopod-dominated communities are recognized: theDalmanella,Catazyga,Sowerbyella, andEpitomyoniacommunities, within which trilobites occur sporadically. The White Head Formation is divided into three new limestone members and a new mudstone member. The basal Burmingham Member has yielded aCatazygaCommunity and aSowerbyella-like Community. The Côte de la Surprise Member is composed of mudstones with the previously describedHirnantiaCommunity. The Rouge, Burmingham, and Côte de la Surprise members are Ashgillian (Upper Ordovician), but the uppermost two members of the White Head Formation, the L'Irlande and Des Jean members, are Llandoverian. They yield anAcernaspisCommunity, assigned a paleoecological position intermediate between those of theClorindaand graptolite communities. The Matapédia Group limestones and shales in the structurally complex northeastern sequence are informally termed the Grande Coupe beds. These beds are partly or wholly time-correlative with the Rouge and Burmingham members but were deposited in deeper water. AStenopareiaCommunity includes the highly fossiliferous Grande Coupe beds, with a local development of theFoliomenaCommunity. The Percé area is unique within the Quebec Appalachians because the strata of the Matapédia Group are highly fossiliferous, with distinct European affinities in the Ordovician, and because the monoclinal sequence is a deepening-upward sequence, probably to the north and west of deeper water clastics.

ISSN:0008-4077    

DOI:10.1139/e87-010

出版商:NRC Research Press    

年代:1987

数据来源: NRC