摘要:

The New Russia gneiss complex in the northeastern Adirondack Highlands of New York includes meta-anorthosite gneiss and anatectic gneiss derived from metagabbro, mangerite, and charnockite. Metamorphic conditions during anatexis (850-950°C, highest near the anorthosite, with pressure ~750 MPa) are inferred from minerals and textures produced by dehydration melting of pargasitic hornblende and from ternary feldspar in anatectic segregations. The complex abuts and is crosscut by the eastern margin of the ~1130 Ma Marcy anorthosite massif. The crosscutting contact, the presence of meta-anorthosite gneiss within the complex and undeformed meta-anorthosite in the massif, and the occurrence of deformed and undeformed anatectic segregations within the New Russia gneisses indicate an approximate synchroneity of penetrative deformation, very high temperature metamorphism, and emplacement of anorthosite with both intrusion and anatexis outlasting deformation. The position of the New Russia gneisses, the metamorphic gradient within them, and the contemporaneity of anatexis with intrusion of anorthosite imply that the complex is the metamorphic aureole of the Marcy anorthosite

ISSN:0008-4077    

DOI:10.1139/e98-072

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

U-Pb zircon and monazite dates for granulite-facies basement xenoliths from the Popes Harbour dyke on the eastern shore of Nova Scotia provide strong evidence that the Meguma terrane overlies Avalonian basement. Slightly discordant (1.6%), "facetted" zircons from a mafic granulite indicate a minimum crystallization age of ~629 Ma, with near-concordant (0.7%) rounded zircons suggesting a maximum age for the last metamorphic event affecting the zircons at ~575 Ma. Two near-concordant (-0.9 to 0.4%) monazite fractions from a pelitic granulite indicate a major metamorphic disturbance at 378 ± 1 Ma, ~10 Ma prior to dyke entrainment and coincident with Meguma regional metamorphism. Projections from 378 Ma through four highly discordant (15-42%) metapelite zircon fractions give provenance ages between 880 and 1050 Ma and two others project to maximum ages of ~1530 Ma. Unlike Meguma sediments which lack Grenvillian-age (~1 Ga) detrital zircons and are dominated by ~2000 Ma detrital zircons, these dates indicate a dominantly Grenvillian-age provenance for the pelitic xenoliths. The "Avalonian" igneous, metamorphic, and provenance ages from the xenoliths suggest the Meguma rests on Avalonian basement. Because Avalonian sediments need a Grenvillian provenance and Meguma sediments lack such a source but require a2.0 Ga component missing in the xenoliths, it seems unlikely the Meguma was deposited on Avalonian crust. Thus the dating places on firmer footing the suggestion from earlier structural, seismic, and geochemical work that the Meguma structurally overlies Avalonian terrane. Thrusting occurred between the time of earliest Meguma deformation (~400 Ma) and intrusion of the xenolith-bearing dyke (~370 Ma).

ISSN:0008-4077    

DOI:10.1139/e98-079

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

40Ar/39Ar dates from eight muscovite separates and two whole-rock samples of slate, siltstone, and sandstone provide detrital, metamorphic, and post-metamorphic overprinting ages for the low-grade metamorphic rocks of the Meguma Supergroup in the Mahone Bay region of southern Nova Scotia. Variably reset detrital muscovite ages of 600-550 Ma agree with previously obtained U-Pb dates from detrital zircon. Separates of foliation-parallel muscovite yield precise40Ar/39Ar ages of 395-388 Ma. Petrographic and metamorphic data indicate that these are metamorphic mica growth ages. The range overlaps with the younger limit of previously reported whole-rock ages (410-380 Ma), but is more tightly constrained. An apparent inconsistency between previous40Ar/39Ar whole-rock ages as old as 410 Ma and regional deformation that affected sedimentary rocks as young as 385 Ma has been resolved by the younger ages reported here and by recent adjustments to the early Paleozoic time scale which suggest that the Lochkovian to Emsian Torbrook Group was deposited between 419 and 394 Ma. A muscovite plateau age of 376 Ma from one sample may be related to late-stage flexural slip deformation in the Ovens gold district.

ISSN:0008-4077    

DOI:10.1139/e98-081

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Textural relations between Al2SiO5phases, and deformation fabrics, provide constraints on the metamorphic history of the Hemlo gold deposit. Kyanite in the deposit is most common within and on the margins of boudinaged quartz ± realgar veins, and less commonly as rotated porphyroblasts within the matrix of schistose rocks. Kyanite predates the main (D2) schistosity. Sillimanite postdates kyanite, occurring irregularly as discrete knots and foliae that run parallel to, but sometimes cut across, the principal (D2) foliation, indicating that sillimanite postdates the D2 foliation. We regard kyanite to be part of the peak metamorphic assemblage, with sillimanite representing a partial later overprint most likely related to fluid infiltration. Rare andalusite occurs in two associations: as late-stage, clean, idioblastic crystals; and as large, fractured grains, locally overprinted by sillimanite, in boudinaged quartz-realgar veins. We suggest two possible origins for this second form of andalusite, one involving generally late growth, the second involving early growth prior to the development of peak metamorphic kyanite. Although not unambiguous, we prefer the second scenario.P-Tconditions from petrogenetic grid constraints, and new geothermobarometric estimates, indicate 6-7 GPa, 600-650°C for the peak kyanite grade metamorphism (ca. 2677 Ma?), and 4-5 GPa, 600°C for the later sillimanite overprint (ca. 2672 Ma?). We see these two events as part of an evolvingP-Tpath in a single metamorphic event. In our early andalusite scenario, the andalusite may have formed from pyrophyllite breakdown at 2-4 GPa, 450°C, possibly associated with emplacement of the regional suite of granodiorite plutons (ca. 2686 Ma). Late andalusite formed sporadically on the retrograde path. The occurrence of deformed andalusite and aligned kyanite in and on the margins of boudinaged auriferous realgar-stibnite-quartz veins provides evidence in support of a premetamorphic mineralization eve

ISSN:0008-4077    

DOI:10.1139/e98-078

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Over the past three decades, a significant number of small-magnitude and shallow earthquakes have occurred in the Western Canada Sedimentary Basin and are located along its western flank near areas of oil and gas production. One of the better documented examples is the swarm of earthquakes associated with the Strachan field, in the Alberta foothills. A model based on Segall's poroelastic theory is developed to account for the occurrence of earthquakes below the Strachan reservoir. Using this methodology, we show that the earthquake of 19 October 1996, underneath the Strachan field, was most probably triggered by gas extraction. The numerical model also implies that gas extraction would cause subsidence and localized changes in in situ stress magnitudes. There is a strong correlation between rates of production and the number of seismic events, but the onset of major seismic activity postdates the commencement of production by approximately 5 years. Poroelastic modelling can account neatly for this observed delay. The modelled stress changes due to gas extraction point to a regime which favours reverse or thrust faulting that is compatible with stress magnitude measurements in the area. The proposed mechanism involves volume changes which decrease the vertical stressSvand increase the larger horizontal stressSHmax. The mean stress increase beneath the reservoir appears to be small, but increasing the deviatoric stress permits Mohr-Coulomb failure. As a result, the initially high rate and long history of gas extraction appear likely to be the main trigger for the seismicity beneath the Strachan field.

ISSN:0008-4077    

DOI:10.1139/e98-080

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

U-Pb baddeleyite geochronology for two Borden diabase dykes of northern Baffin Island gives an intrusion age of ca. 720 Ma, coeval with the age established elsewhere for the Franklin igneous event. Thus, the Borden dykes belong to the Franklin dyke swarm, rather than forming a separate swarm that intruded at ca. 950-900 Ma, as has been suggested previously on the basis of paleomagnetism and K-Ar ages. As a result, the paleopole from the Borden dykes can no longer be utilized to help constrain the ca. 1050-850 Ma Grenville Loop of the North American polar wander path. Reevaluation of paleomagnetic data for the dykes of northern Baffin Island suggests that Borden dyke magnetizations resulted from superposition of a steeply directed component of chemical remanent magnetization on normal and reversed primary Franklin components. The overprint direction is consistent with a Cretaceous-Tertiary age and is likely related to normal faulting and graben development during the opening of Baffin Bay.

ISSN:0008-4077    

DOI:10.1139/e98-091

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Gravity data were obtained along two transects on the southern coast of Ungava Bay, which provide continuous gravity coverage between Leaf Bay and George River. The transects and the derived gravity profiles extend from the Superior craton to the Rae Province across the New Quebec Orogen (NQO). Interpretation of the transect along the southwestern coast of Ungava Bay suggests crustal thickening beneath the NQO and crustal thinning beneath the Kuujjuaq Terrane, east of the NQO. Two alternative interpretations are proposed for the transect along the southeastern coast of the bay. The first model shows crustal thickening beneath the George River Shear Zone (GRSZ) and two shallow bodies correlated with the northern extensions of the GRSZ and the De Pas batholith. The second model shows constant crustal thickness and bodies more deeply rooted than in the first model. The gravity models are consistent with the easterly dipping reflections imaged along a Lithoprobe seismic line crossing Ungava Bay and suggest westward thrusting of the Rae Province over the NQO. Because no gravity data have been collected in Ungava Bay, satellite altimetry data have been used as a means to fill the gap in data collected at sea. The satellite-derived gravity data and standard Bouguer gravity data were combined in a composite map for the Ungava Bay region. The new land-based gravity measurements were used to verify and calibrate the satellite data and to ensure that offshore gravity anomalies merge with those determined by the land surveys in a reasonable fashion. Three parallel east-west gravity profiles were extracted: across Ungava Bay (59.9°N), on the southern shore of the bay (58.5°N), and onshore ~200 km south of Ungava Bay (57.1°N). The gravity signature of some major structures, such as the GRSZ, can be identified on each profi

ISSN:0008-4077    

DOI:10.1139/e98-085

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The Eden Lake Complex intrudes granitic rocks located between the Lynn Lake and Leaf Rapids tectonic domains in the Reindeer Zone of the Trans-Hudson orogenic terrane in northern Manitoba. It consists of a number of magmatic phases. Of particular interest are aegirine-bearing monzonites and late rare earth element (REE)- and fluorine-enriched pegmatites. The aegirine monzonite ranges in SiO2content from 51 to 76 wt.%, showing a continuous fractionation trend from low to high silica. The CaO, Fe2O3T, and MgO contents are low (about 1-6 wt.%), TiO2contents range from 0.8 to about 0.1 wt.%, and total alkalis range from about 11 to 13 wt.%. The early evolution of the complex was influenced by pyroxene fractionation; during the later stages, K-feldspar fractionation had an increasing effect on the bulk composition of the magma. Accumulation of K-feldspar has obscured the tectonic signature of the complex, but elevated REE, high field strength element, and fluorine contents are consistent with an A-type granitoid association. The complex is associated with REE-bearing pegmatites. Total REE contents in the monzonites range from 1000 ppm in low-silica phases to 20 ppm in high-silica phases, with La/LuCNvalues ranging from 80 to 25. REE patterns reflect the fractionation of K-feldspar, pyroxene, and minor amounts of titanite, apatite, and zircon. Y/Nb and Yb/Ta values (relatively constant at ~1 and 1.5, respectively) are consistent with derivation from a depleted- mantle source enriched in REE; moreover, positive &egr;Ndvalues of 0.3-1.7 (with an assumed age of 1750 Ma) are consistent with derivation from an initially light rare earth element (LREE)-depleted source. Elevated &Sgr; REE contents and LREE-enriched characteristics of the complex suggest the source region was altered prior to generation of the magma(s).

ISSN:0008-4077    

DOI:10.1139/e98-089

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

This paper documents perennial spring activity at Expedition Fiord on western Axel Heiberg Island in the Canadian High Arctic. Two groups of mineralized springs occur near the McGill University Axel Heiberg Research Station located at 79°26'N, 90°46'W. The first is at Gypsum Hill, 3 km from the terminus of the White and Thompson glaciers, and the second site is at Colour Peak, approximately 10 km downvalley near the head of Expedition Fiord. Each spring group consists of 20-40 vents spread over several hundred square metres. The highly mineralized nature of the discharge causes a freezing-point depression of 7-10°C and produces a range of precipitates and travertine deposits. Year-round water temperature and discharge rate measurements have been obtained, demonstrating perennial activity at these sites. Results indicate that temperatures range from -4.0 to 6.6°C among the individual sources; however, water temperatures at the various outlets remain constant throughout the year despite a mean annual air temperature of -15°C. Although discharge from any one outlet is low (<0.5 to 2.0 L/s), the total discharge is substantial, each year producing several seasonal frost mounds and an icing 180 000 - 300 000 m2at the Gypsum Hill

ISSN:0008-4077    

DOI:10.1139/e98-097

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-4077    

DOI:10.1139/e99-032

出版商:NRC Research Press    

年代:1999

数据来源: NRC