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Laurence Page - One of the best experts on this subject based on the ideXlab platform.

  • Ar-40/Ar-39 Hornblende geochronology from the Forsmark area in central Sweden : constraints on late Svecofennian cooling, ductile deformation and exhumation
    Precambrian Research, 2008
    Co-Authors: Tobias Hermansson, Michael B. Stephens, Laurence Page
    Abstract:

    At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 Hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 Hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in Hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of Hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in Hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved. (Less)

  • ar 40 ar 39 Hornblende geochronology from the forsmark area in central sweden constraints on late svecofennian cooling ductile deformation and exhumation
    Precambrian Research, 2008
    Co-Authors: Tobias Hermansson, Michael B. Stephens, Laurence Page
    Abstract:

    At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 Hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 Hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in Hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of Hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in Hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved. (Less)

Tobias Hermansson - One of the best experts on this subject based on the ideXlab platform.

  • Ar-40/Ar-39 Hornblende geochronology from the Forsmark area in central Sweden : constraints on late Svecofennian cooling, ductile deformation and exhumation
    Precambrian Research, 2008
    Co-Authors: Tobias Hermansson, Michael B. Stephens, Laurence Page
    Abstract:

    At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 Hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 Hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in Hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of Hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in Hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved. (Less)

  • ar 40 ar 39 Hornblende geochronology from the forsmark area in central sweden constraints on late svecofennian cooling ductile deformation and exhumation
    Precambrian Research, 2008
    Co-Authors: Tobias Hermansson, Michael B. Stephens, Laurence Page
    Abstract:

    At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 Hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 Hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in Hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of Hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in Hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved. (Less)

Michael B. Stephens - One of the best experts on this subject based on the ideXlab platform.

  • Ar-40/Ar-39 Hornblende geochronology from the Forsmark area in central Sweden : constraints on late Svecofennian cooling, ductile deformation and exhumation
    Precambrian Research, 2008
    Co-Authors: Tobias Hermansson, Michael B. Stephens, Laurence Page
    Abstract:

    At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 Hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 Hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in Hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of Hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in Hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved. (Less)

  • ar 40 ar 39 Hornblende geochronology from the forsmark area in central sweden constraints on late svecofennian cooling ductile deformation and exhumation
    Precambrian Research, 2008
    Co-Authors: Tobias Hermansson, Michael B. Stephens, Laurence Page
    Abstract:

    At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 Hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 Hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in Hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of Hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in Hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved. (Less)

Mark T Brandon - One of the best experts on this subject based on the ideXlab platform.

  • regional tilt of the mount stuart batholith washington determined using aluminum in Hornblende barometry implications for northward translation of baja british columbia
    Geological Society of America Bulletin, 1996
    Co-Authors: Jay J Ague, Mark T Brandon
    Abstract:

    We have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum-in-Hornblende (AH) barometer. The method is used to correct previously published paleomagnetic data from the 93‐96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting. AH barometry was done on 46 samples from the batholith using the compositions of Hornblende rims coexisting with the full mineral assemblage required for pressureestimation.High-contrastback-scatteredelectronimagingwas usedtoensurethattheanalyzedHornblendeswerenotsignificantly affected by subsolidus alteration. The success of the AH barometry is indicated by two observations. First, increases in the Al content of the Hornblendes are governed almost entirely by a pressure-sensitive tschermak-type substitution. Second, amphibole-plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions (’650 &C) and that temperature has a negligible effect on our pressure estimates. AH barometry results indicate that the depth of crystallization across the batholith decreases systematically from ’0.3 GPa in the northwest to ’0.15 GPa in the southeast,consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations. Using a best-fit planar-tilt model and bootstrap analysis of uncertainties, we estimate that the paleohorizontal plane has a strike of 43& 630.4&and dip of 7& 62.0&southeast (695% confidence). Our estimated paleohorizontal allows us to restore the paleomagnetic data of Beck et al. (1981) and to estimate the original paleolatitude of the Mount Stuart batholith. Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanence. The carrier of this remanence was not resolved, but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith. (1) The ‘‘stable’’ sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our AH results and concordant K/Ar ages for Hornblende/biotite pairs. (2) The high coercivity component was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal. (3) Sites from the batholith and the contact aureole gave similar directions. (4) The directions show no indication of tilt-related smearing. After restoration, the paleomagnetic data indicate 42& 6 11& clockwise rotation and 3100 6 600 km of northward offset

John Fitz D Gerald - One of the best experts on this subject based on the ideXlab platform.

  • diffusion of 40ar in metamorphic Hornblende
    Contributions to Mineralogy and Petrology, 1990
    Co-Authors: Suzanne L Baldwin, Mark T Harrison, John Fitz D Gerald
    Abstract:

    Isothermal, hydrothermal experiments were performed on two compositionally contrasting Hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic Hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 μm. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 μm. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic 40Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 μm, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of ∼ 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on Hornblendes. A comparison of results for 34 μm splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic Hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 μm and 34 μm splits of 40Ar/39Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of 40Ar in Hornblende (∼60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic Hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for 40Ar in Hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for 40Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.