ISSN:0008-4077    

DOI:10.1139/e92-001

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

The Ag–Pb–Zn–Au vein and replacement deposits of the Kokanee Range, southeastern British Columbia, are hosted by the Middle Jurassic Nelson batholith and surrounding Cambrian to Triassic metasedimentary rocks in the hanging wall of the transcrustal Slocan Lake Fault, Field relations indicate that mineralization is younger than the Nelson batholith and a Middle Jurassic foliation in the Ainsworth area but coeval or older than Eocene unroofing of the Valhalla metamorphic core complex in the footwall of the Slocan Lake Fault. Lamprophyre and gabbro dykes are broadly coeval with mineralization and have biotite and hornblende K–Ar ages defining a short-lived Middle Eocene alkaline magmatic event between 52 and 40 Ma. An older, Early Cretaceous alkaline magmatic event (141 – 129 Ma) is possible but incompletely documented.K–Ar and step-heating40Ar/39Ar analyses on hydrothermal vein and alteration muscovite indicate that hydrothermal fluids were precipitating vein and replacement deposits 58–59 Ma ago. Crosscutting relationships with lamprophyre dykes indicate the Kokanee Range hydrothermal system lasted for more than 15 Ma. Eocene crustal extension resulted in a high heat flow and structures which were probably responsible for hydrothermal fluid movement and flow paths.A 100 Ma time interval is documented between batholith emplacement and spatially associated mineralization, ruling out any genetic link between the two. Similar large age differences between granite intrusion and peripheral mineralization have recently been documented for two world-sea le Ag–Pb–Zn vein districts, which suggest that spatial association between granite and Ag–Pb–Zn mineralization is not sufficient to infer a genetic link.

ISSN:0008-4077    

DOI:10.1139/e92-002

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

The Polaris Mississippi Valley-type (MVT) zinc–lead deposit is located in the Middle to Upper Ordovician Thumb Mountain Formation of the Cornwallis Fold Belt in the Canadian Arctic Archipelago. Paleomagnetic analysis was done on samples from 29 sites in the sphalerite–galena ore, the surrounding dolomitic envelope, and the adjacent host-rock limestones, using alternating-field and thermal step demagnetization, scanning electron microscopy with energy-dispersive analysis, and saturation isothermal remanente analyses. Excluding the modern viscous remanence component, the ore, dolomite alteration, and host limestones carry only a prefoldingAremanent magnetization component that resides in single- and pseudosingle-domain magnetite, both as discrete grains and as inclusions in the MVT ore minerals. All evidence indicates that the mineralization and magnetization events were coeval. The mean magnetization direction ofD = 145.8°,I = −31.1° (α95 = 5.9°,k = 26.2,N = 24) after tilt and a minor plunge correction gives a Late Devonian pole position of 28.6°N, 121.2°E (dp = 3.7°, dm = 6.6°,N = 24). The results indicate that the ore deposit was formed during the onset of the Ellesmerian Orogeny and tilted later in the orogeny.

ISSN:0008-4077    

DOI:10.1139/e92-003

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

Amphibolites (hornblende–plagioclase rocks withTpeak metamorphism = 650–700 °C) are abundant near the old Calumet Zn–Pb–Ag mine and Au prospect 90 km northwest of Ottawa. They lack primary structures and their original petrochemical character is obscured by metamorphism and profound alteration manifested by a variety of phlogopite–biotite-, garnet-, cummingtonite-, gahnite-, clinopyroxene-, carbonate scapolite-, and calcite-bearing assemblages. This alteration has modified the SiO2, alkali, and most alkaline earth distributions. Trace and minor element chemistry demonstrates that the amphibolites are metaigneous and bimodal, with two suites, one derived from differentiated arc tholeiites and the other from relatively undifferentiated rocks having transitional arc basalt – boninite chemistry. The complex rock package enclosing mineralization includes metatholeiites, whereas the boninitic metabasalts occur in a discrete interval in the structural hanging wall of the mineralization. Although these rocks represent a new geochemical assemblage in the Allochthonous Monocyclic Belt (Central Metasedimentary Belt), their implied geotectonic setting is comparable to those previously inferred for several other areas, including that near Montauban, Quebec, where there are very similar styles of mineralization.

ISSN:0008-4077    

DOI:10.1139/e92-004

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

Paleomagnetic results, and age estimates derived from them, arc presented for three diatremes, using as a basis of comparison the combined apparent polar wander (APW) path for North America and Europe of Van der Voo. The Cross diatreme of the Front Ranges of the Canadian Rocky Mountains has yielded a radiometric age of 241 Ma (earliest Triassic) and is hosted by the flat-lying Pennsylvanian Tunnel Mountain Formation. It has normal polarity magnetization and yields a paleopole correctly placed according to its radiometric age on the APW path. The Blackpool diatreme (for which no radiometric age is available), which is located in the Main Ranges of the Rocky Mountains, is known to be post-Late Ordovician because it is hosted by rocks of that age. It also has magnetization of normal polarity and yields a paleopole that, when calculated with respect to present horizontal, is coincident with the latest Cretaceous to Paleocene paleopole for North America. The paleopole, when calculated with respect to bedding, lies on the Middle Ordovician portion of the combined APW path. A clockwise rotation of 10° brings the paleopole into agreement with the latest Ordovician. Hence, from a paleomagnetic standpoint, a latest Cretaceous to Paleocene or latest Ordovician age is possible. The HP pipe (radiometric age 391 ± 5 Ma or Early Devonian), previously studied by D. T. A. Symons and M. T. Lewchuk, is hosted in limestones of Upper Cambrian to Middle Ordovician age. It has reversed polarity and yields a paleopole that, when compared with the combined APW path, suggests an age of mid-Permian, although errors are such that it could be somewhat younger, roughly coeval with the Cross diatreme. We conclude, therefore, that the radiometric age estimated for the HP pipe could be too old by about 130 million years.

ISSN:0008-4077    

DOI:10.1139/e92-005

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

Six rhyolitic tephra layers from ancient loess and related detritus in the Cypress Hills, Saskatchewan, represent separate volcanic eruptions from the Snake River Plain. Idaho, U.S.A. The weighted mean age and uncertainty of the youngest tephra bed is 8.3 ± 0.2 Ma, using the isothermal plateau fissiontrack technique on its hydrated glass shards. The loess that hosts five of these tephra beds extends across the Cypress Plain, which is the oldest (Middle Miocene) and highest depositional surface in the Interior Plains, and also occurs on four juxtaposed erosion surfaces. It appears that the first and maybe the second erosion surface began forming before 10 Ma, and that formation of the second, third, and fourth erosion surfaces was completed between 10 and 8.3 Ma.

ISSN:0008-4077    

DOI:10.1139/e92-006

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

Mazama tephra is a widespread mid-Holocene stratigraphic marker, dating to ca. 6845 BP, and is considered useful for the correlation of Holocene events in western North America. We present evidence from Copper Lake, Alberta, for a tephra layer that underlies Mazama tephra and appears to be a discrete tephra layer but that is indistinguishable from the conventional Mazama tephra by microprobe analysis of glass shards. Three alternative hypotheses are considered to explain this tephra underlying Mazama:in situposition of an earlier Mazama-like tephra, recycling of tephras, and settling of tephra slabs through the gyttja. The size distributions of birch pollen grains indicate that the Mazama-like tephra is in primary stratigraphic position in the early Holocene gyttja, and that sediment recycling has not destroyed the integrity of Copper Lake sediment stratigraphy. It is concluded that most probably a Mazama-like tephra was deposited between 9700 and 10 500 BP. If thisin situhypothesis is correct, the identification of a tephra as Mazama by major-element chemistry does not necessarily fix the age of the enclosing strata. Evidence to confirm the existence and plot the distribution of a Late Pleistocene – early Holocene, Mazama-like tephra is needed from other sites in northwestern America. A collateral result of this study is that ablation of valley-bottom ice in the vicinity of Castle Mountain took place prior to 10 500 BP.

ISSN:0008-4077    

DOI:10.1139/e92-007

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

A deep channel in bedrock extending to more than 25 m below sea level occurs along the north shore of the otherwise uniformly shallow Kingston basin of Lake Ontario. Bathymetric and subbottom acoustic data are used to map the topography of the channel and to reconstruct its late glacial and postglacial sedimentary history. The results are interpreted as showing that the large channel and smaller channels nearby were created by high-velocity subglacial meltwater flow. Acoustic facies assemblages of sediments deposited in the channels record patchy deposition, or deposition followed by partial erosion, of glacial sediments on the bedrock of the channel floor, followed by deposition and episodic erosion of glaciolacustrine sediment in a high-energy, ice-proximal lake. Palaeoslope analysis confirms that the early Holocene low-water phase of Lake Ontario resulted in the development of a fluvial system in part of the channel. Water level was controlled by a sill at Kingston. Kingston basin, the Bay of Quinte, and possibly, for a short time, a much larger area of the upper Great Lakes drained through the channel. However, for most of the period, until it was flooded by the rising waters of Lake Ontario, the channel was occupied by a small river on a wide floodplain or it was flanked by broad marshes.

ISSN:0008-4077    

DOI:10.1139/e92-008

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

A succession of Quaternary deposits in the Merritt basin of south-central British Columbia contains evidence for four glaciations and two interglaciations. Paleomagnetic signatures in these sediments are of three types: normal polarity, proposed to have been acquired during the Brunhes Normal Polarity Chron; reversed polarity, proposed to have been acquired during the Matuyama Reversed Polarity Chron; and reversed polarity (Matuyama age) all but obscured by a normally magnetized overprint (Brunhes age).Reversely magnetized deposits at the base of the succession include glacial lacustrine deposits, interpreted as representing two different glaciations, and a paleosol and a succession of nonglacial sediments, which are evidence of two interglaciations. As the reversed polarity of these deposits is proposed to have been acquired during the Matuyama Reversed Polarity Chron, they are older than 790 ka.Normally magnetized deposits, which make up the rest of the succession, contain evidence for only two glaciations, but traces of other glaciations may have been removed during the erosion interval encompassed by a major unconformity that underlies deposits of the last glaciation. All are referred to the Brunhes Normal Polarity Zone. In addition to these glacial and interglacial deposits, a series of normally magnetized Quaternary basalt flows forms a bench 90 m above the floor of the basin. These basalts were extruded after 790 ka but before the penultimate glaciation.

ISSN:0008-4077    

DOI:10.1139/e92-009

出版商:NRC Research Press    

年代:1992

数据来源: NRC

摘要:

Large submarine channels lead away from the coarse-grained fjord-head delta in Queen Inlet, Glacier Bay, Alaska. Coarse sediment loads from Carroll Glacier are transported down the delta foreslope and through the submarine channels, and finer material is dispersed throughout the fjord from overflow plumes. Slides and slumps occur intermittently on the delta foreslope. A recording current meter positioned in a submarine channel 1 m above the bottom and 2.7 km seaward from the delta brink indicated during July 1989 predominantly quasi-continuous downfjord currents, although several small surges were also observed. Flows occurred throughout the daily tidal cycle in both neap and spring periods for intervals of up to several hours, often beginning and ending rapidly. Current velocities ranged from 0 to 120 cm∙s−1, averaging 12 cm∙s−1over one 56 h period, and 16 cm∙s−1over a 13 h period. Suspended sediment concentrations 1 m above bottom commonly exceeded 2 g∙L−1and attained 8.2 g∙L−1. The density added to the bottom flow by high turbidity compensated for density lowered by a slight decrease of salinity attained through mixing with fresh meltwater.The contrast between sporadic surges on the delta foreslope and predominantly quasi-continuous turbidity currents downstream in the submarine channels is interpreted as due to attenuation and overlapping of sediment gravity flows of various sizes and velocities originating at the delta front. The relatively low-density, low-velocity, quasi-continuous currents transport large volumes of sediment through the channels, and probably play an important role in maintaining cha

ISSN:0008-4077    

DOI:10.1139/e92-010

出版商:NRC Research Press    

年代:1992

数据来源: NRC