摘要:

A zircon age ofhas been determined for the silica-saturated syenites of the Big Spruce Lake alkaline complex. Pb–Pb whole-rock ages ofand 2155 ± 16 Ma were obtained for the silica-undersaturated rocks of the complex and for the complex as a whole, respectively. The Sm–Nd whole-rock data yield an age of 2183 ± 75 Ma for the complex as a whole. By weighting the ages by the inverse square of the standard deviations, an average age for the complex of 2174 ± 20 Ma is determined. However, it is felt the zircon age is the most reliable for the time of crystallization of the complex. Minor contamination by country rock (Sm–Nd model ages of 2467–2595 Ma) and metasomatism contribute to scattered Sm–Nd data points and large errors in the ages obtained for the five individual units. The Pb–Pb data enable a limiting estimate of about 50 Ma for the maximum time span for the intrusion of centres 1–4.

ISSN:0008-4077    

DOI:10.1139/e86-001

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Pollen analysis of a 7.65 m sediment core from Eaglenest Lake (57°46′N, 112°06′W) provides evidence of the postglacial development of vegetation in the mixedwood section of the Boreal Forest. From the time of deglaciation (approximately 12 000 – 11 500 years BP) to 11 000 BP the area was colonized by mainly nonarboreal flora dominated byArtemisiaand Gramineae.Populus, the only tree taxon present, grew in sheltered areas, whileSalixand Cyperaceae were common on poorly drained sites. Open parkland prevailed under a dry, windy climate. Summer temperatures were rising rapidly from glacial minima in response to increased solar insolation. By 11 000 BPPicea(initiallyP.glauca) andBetula(mainly treeBetula) migrated to the area. Forest prevailed, but the canopy was likely more open than at present. By 9000 BP peak warm and dry conditions had passed, although conditions warmer and drier than at present persisted.Alnusmigrated to the area 8450 BP.Pinusreached the Birch Mountains about 7500 BP. No major changes in vegetation have occurred since that time, suggesting that modern climatic conditions were established by 7500 BP.

ISSN:0008-4077    

DOI:10.1139/e86-002

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

New40Ar/39Ar ages are presented for two hornblendes and one biotite from the dynamothermal aureole that is situated at the base of the Mont Albeit ophiolite. Analysis by the incremental release technique reveals a small component of excess40Ar in one hornblende, though both exhibit plateau-type spectra. The biotite was analyzed by the total fusion technique. All apparent ages are concordant and average 456 ± 3 Ma.Dynamothermal aureoles form during the obduction process, which juxtaposes ophiolites and amphibolite protoliths during a compressional tectonic event at a plate margin: in this instance, thrusting of the Mont Albert ophiolite over rocks of the Shickshock Group. The concordance of biotite and hornblende ages indicates rapid postmetamorphic cooling and firmly dates obduction at 456 ± 3 Ma ago, an age younger than any of the other northern Appalachian ophiolites. The high-pressure metamorphism of the amphibolite is inconsistent with the present high structural level of the composite Mont Albert – Shickshock allochthon. This implies that obduction and assembly of the allochthon took place in a different environment prior to emplacement in its present high-level position. The age presented here is therefore a maximum limit for the late thrusting event. A north–south diachronism of Humber Zone ophiolite obduction is supported by the new data.

ISSN:0008-4077    

DOI:10.1139/e86-003

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

U–Pb dating was carried out on nine volcanic rocks and two felsic intrusions from the Red Lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization.The data indicate a protracted period of igneous activity spanning at least 270 Ma. Felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages ofand. A third unit, dated at, contains inherited zircons older than 2982 Ma, which casts some uncertainty on the validity of the inferred intercept age. Rocks in the western part of the belt, previously believed to form a relatively young calc-alkalic sequence but now known to be dominantly tholeiitic, are shown to be relatively old, with ages ofand. These two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units.Another volcanic horizon in the centre of the belt is dated at 2830 ± 15 Ma, and calc-alkaline volcanic sequences on the southern and northern flanks of the belt yield ages of 2739.0 ± 3.0 and, respectively. An age ofwas determined for tholeiitic pyroclastic rocks near the base of the predominantly calc-alkaline Heyson sequence.The major gold deposits of the Red Lake belt appear to be present dominantly within older supracrustal sequences. On the other hand, they are also associated with late deformation zones that postdate the intrusion of the Dome Stock dated at 2718.2 ± 1.1 Ma ago. The time of an earlier folding event is bracketed by this age and by the age offor an isoclinally folded felsic dike.

ISSN:0008-4077    

DOI:10.1139/e86-004

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Palynologic analyses of four short cores collected along shallow- to deep-water transects in Second and Gignac lakes indicate two periods of forest clearance for farming. The first deforestation was by Huron Indians betweenA.D. 1450 and 1650, when a maple (Acer), beech (Fagus), and oak (Quercus) forest was cleared and corn (Zea) planted. This disturbance is identified by decreased tree pollen and increasedPteridium,Artemisia, and other herbs and is confirmed byZeapollen in Gignac Lake. From 1650 to 1875 there was a forest succession to oak, birch (Betula), and pine (Pinus). Following this recovery European loggers and farmers cleared this forest and attempted farming. Besides a reduction in tree pollen, a product of this deforestation includes the pollen of weedyAmbrosia,Gramineae, and introduced EuropeanRumexandPlantago. During the last 25 years an increase in tree pollen indicates local forest recovery consistent with present land use.By relating upland vegetation successions to lacustrine algal assemblage changes, probable ecological controls on algae during the last 600 years are identified.Peridinium willeiHuitfeldt-Kaas andPediastrumrespond to changes induced by forest clearance and agriculture.Aquadulcum awendaen. sp. andPeridinium wisconsinenseEddy prefer less alkaline water.Variations in palynomorph influx are related to basin morphology and water circulation. Gignac Lake, a steep sided and relatively deep lake, directs palynomorphs from shallow marginal banks to the deeper basin, whereas Second Lake, with a gently sloping shallow lake bottom, preferentially accumulates palynomorphs close to shore.

ISSN:0008-4077    

DOI:10.1139/e86-005

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Neutron-activation analysis, loss on ignition (LOI), and X-ray diffraction of eight cores collected in Second and Gignac lakes are correlated with historic and palynologic records to identify elements linked to erosion from deforestation and farming.Forest disturbance and farming are identified in cores of organic detritus sediment (gyttja) by decreased LOI and increased Na, Mg, Ba, Al, Ti, and Dy. LOI is not suitable for identifying forest disturbances in carbonate sediments. From neutron-activation analysis of carbonate mud only Na, Al, and Dy indicate erosion.Elements linked to the quantity of organic matter in sediments include U, V, and Cl, whereas Mn and I in surface sediments presumably correspond with variations in the oxidation potential and plant productivity, respectively.Sedimentation patterns relate to basin morphology. In Gignac Lake the basin is steep sided and relatively deep. Clastic detritus entering the lake is carried over the shallow, nearshore carbonate bank into deeper water. In Second Lake the basin is shallow and gently sloping. Minerals eroded from onshore are more equally distributed in this basin. In Second Lake the most rapid sedimentation is nearshore where submerged aquatic macrophytes produce and trap carbonate mud. In Gignac Lake the top of the shallow carbonate bank has few aquatic macrophytes; lime mud formed on the bank is washed into deep water offshore, where it dissolves.

ISSN:0008-4077    

DOI:10.1139/e86-006

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The Selamiut Range of the Torngat Mountains, northern Labrador, rises to over 1700 m above sea level and constituted a barrier to eastward-flowing Laurentide ice during former glaciations. Local and regional ice has left an assemblage of landforms exhibiting cross-cutting relationships. Three glacial phases are recognized in the area. The oldest and most extensive (the Ivitak phase) is marked by high-elevation till deposited when regional and local ice coalesced. At this time regional ice flowed eastward down Nachvak Fiord while interfiord highlands extended above the ice as nunataks. During a late Ivitak phase a large proglacial lake was dammed in the lower McComick Valley at 80 m above sea level when regional and local ice in Nachvak Fiord blocked the local drainage. During the subsequent Nachvak phase, lakes were dammed in the lower McCornick Valley at 67 and 53 m above sea level for ca. 3000 years by Laurentide ice alone. The Superguksoak had three subphases when local cirque glaciers expanded up to 5 km beyond their present limits. The Holocene marine limit around 33 m reflects the small ice load influencing Nachvak Fiord, or very late deglaciation of the Ivitak Cove area.The chronology of these glacial and proglacial deposits is based on their spatial relationships as well as pedology and lichenometry. Seventy-nine soil pits excavated to the base of the Cox horizon indicate the relative duration of weathering since deglaciation. Absolute ages for the soils are estimated to be > 40–40 ka BP for the Ivitak phase, 23–17 ka BP for the Nachvak phase, and 12–5 ka BP for Superguksoak I. Measurements ofRhizocarpon geographicumindicate that prominent Superguksoak-phase moraines bordering the present-day cirques were abandoned approximately > 4000, 3000, and 1500 years BP. Collectively these data are compared with earlier fieldwork in northern Labrador, and some discrepancies are discussed.

ISSN:0008-4077    

DOI:10.1139/e86-007

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

A new, small pelycosaur,Ianthasaurus hardestii, has been discovered in the well-known, richly fossiliferous locality in the Upper Pennsylvanian (Missourian) Stanton Formation, near Garnett, Kansas. It is represented by a partial, semi-articulated skeleton and scattered elements of at least three other individuals. The small, insectivorousIanthasauruspossesses a number of primitive pelycosaurian features but also shares several derived characters with the large Early Permian herbivoreEdaphosaurus, justifying its inclusion in the family Edaphosauridae and its recognition as the oldest and most primitive member of that group.

ISSN:0008-4077    

DOI:10.1139/e86-008

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The Archean Hayes River Group of the Island Lake greenstone belt (Superior Province, Sachigo Subprovince) comprises mafic to felsic metavolcanics, subvolcanics, and associated metasedimentary rocks. The Hayes River Group is intruded by granitoid rocks belonging to the early intrusive complex. One such pluton, the Bella Lake tonalite, is intrusive into the metabasalt of the Hayes River Group and has a U–Pb zircon age of 2886 ± 15 Ma. Similar intrusives of this complex, either internal or marginal to the greenstone belt, yield zircon ages of 2801 ± 8 Ma (Pipe Point tonalit) and 2768 ± 22 Ma (Linklater Island prophyry). This suggests that the early intrusive complex was emplaced over an ~ 120 Ma long interval by at least three separate intrusive episodes.Subsequent to the emplacement of the early intrusive complex, the isoclinally folded Hayes River Group and the early intrusive complex were uplifted, eroded, and followed by the unconformable deposition of the Island Lake Group, comprising fluvial to marine metasedimentary rocks. The stratigraphically lower part of the Island Lake Group is bracketed by the 2768 ± 22 Ma age of the Linklater Island porphyry and the 2729 ± 3 Ma age obtained for the late tectonic suite—the Pipe Point quartz diorite and feldspar porphyry. A feldspar quartz porphyry belonging to the post-tectonic intrusive rocks intrudes higher stratigraphic levels and has been dated at 2699 ± 4 Ma (Horseshoe Island quartz feldspar porphyry).

ISSN:0008-4077    

DOI:10.1139/e86-009

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Uncertainties in availability and formulation of exotic microfossils (e.g.,Lycopodiumspp.,Eucalyptusspp.) to be added to microfossil preparations may be resolved by the use of plastic microspheres (25.0 ± 1.0 μm), which are unaffected by standard pollen preparative treatments (KOH, HCl, HF, acetolysis, and organic solvents). The microspheres are sensitive to strongly oxidizing treatments (e.g., Schulze's reagent, H2O2: H2SO4reflux for diatom preparation) but can be added following digestion of organic material.Preparation of microsphere suspensions in a suitable electrolyte (e.g., NaCl – formic acid – water) permits calibration by Coulter counter of suitable dilutions (e.g., 5000 ± 100 mL−1) for delivery by "Tipet" volumetric flasks.The uniformity and distinctive appearance of the microspheres not only facilitate recognition but also provide a constant reference for both size (25 μm) and volume 8180 μm3), as well as a reference standard for computerized microfossil scanning.The microspheres are inert (vinyl-coated styrene polymer), featureless, opaque, available in a variety of sizes, and of a density (SG = 1.3) similar to those of pollen grains.A stainless steel volumetric sediment sampler (0.5–5.0 cm3) that facilitates subsampling in core or bulk samples is described.

ISSN:0008-4077    

DOI:10.1139/e86-010

出版商:NRC Research Press    

年代:1986

数据来源: NRC