Paleomagnetism of the Dufek Intrusion, Pensacola Mountains ...
Paleomagnetism of the Dufek Intrusion, Pensacola Mountains, Antarctica
I
UP EAST I
z
+
RUSSELL F. BURMESTER
Department of Geology Western Washington University Bellingham, Washington 98225 Sprsj
D. SHERIFF
Department of Geology University of Wyoming Laramie, Wyoming 82071
The paleomagnetism of the Dufek layered intrusion (83°S 50°W) was first investigated by Beck, Ford, and Boyd (1968) using oriented hand samples collected during the 1965-66 field season. About equal numbers of these samples had magnetizations that were directed upward (normal) and downward (reverse). The simplest explanation was that the polarity of the Earth's magnetic field had changed during the cooling of the intrusion, causing the intrusion to be divided into normal and reverse magnetozones (Beck 1972). We visited the intrusions's outcrops in the Dufek Massif and Forrestal Range during the 1978-79 field season primarily to sample the transition between magnetozones in order to document the behavior of the Earth's magnetic field during a field reversal. The search for transition zones was aided by a portable magnetometer capable of measuring the natural remanent magnetization (NRM) of small, oriented samples. Where we thought we found a transition zone, we cored the rock with a gasoline-powered diamond drill at 1-meter stratigraphic intervals over the available outcrop on ridges F-E and F-F near Soma Bluff in the Forrestal Range (Ford, 1976). Core orientation data obtained with a sun compass were reduced in the field with a programed pocket calculator. All equipment worked flawlessly despite subnormal temperatures. One shortcoming was that the ethylene glycol used as coolant for the drill bit left the cores, equipment, and operator slimy wet. We recommend trying an alcoholwater or another more volatile solution instead. Standard paleomagnetic cleaning, using alternating field (a.f.) demagnetization, showed that most samples from the transition zone have at least three magnetizations (figure 1). The component most easily demagnetized, or alternatively removed by grinding the cores' exteriors, is antiparallel to the core axis and was acquired during drilling (Burmester 1976). A second magnetization, demagnetized by a 40millitesla alternating field, is directed upward with a mean direction consistent with magnetization during a Jurassic normal interval. The characteristic magnetization that is isolated by a.f. demagnetization yields virtual geomagnetic 1980 REVIEW
MOMENT +5 Am'
I NORTH
Figure 1. Projection of magnetization vector onto horizontal plane (0) and north-south vertical plain (L). Solid lines are projections of core axis; dashed lines estimate the characteristic magnetization isolated by alternating field demagnetization. Numbers indicate peak field in millitesla (ml).
poles (vcr's) in the southern paleohemisphere which appear to define part of the VGP path during the reversal (figure 2). Preliminary work suggests that this charateristic magnetization resides in submicroscopic magnetite distributed in the plagioclase. There is a problem in determining whether the field was changing from reverse to normal or vice versa. Thermal demagnetization of representative samples shows a tendency for a component of normal magnetization to have a higher unblocking temperature than the reverse-transitional component (figure 3). Coarse-grained cumulate magnetite last equilibrated with exsolved illmenite lamellae above its Curie temperature (Himmelberg and Ford 1977). If the highest temperature magnetization resides in the cumulate magnetite and was acquired during cooling, then the transition must have proceeded from normal to reverse. This is opposite of what is indicated by inversion of aeromagnetic data (Behrendt, Drewry, Kowski, and Grim, in press). This discrepancy may be attributable to different responses of the coarse and fine magnetite to slow cooling. Such responses possibly allowed the coarse magnetite to change its magnetization after a reverse-to-normal transition was completed. To determine the "sense" of the transition we need to determine better where the various magnetic components reside and at what temperatures they were acquired. Beck (1972) corrected magnetic directions to what they would have been if the igneous layering had been 43
Behrendt, J. C., Drewry, D. J., Jankowski, E., and Grim, M. S. In press. Areomagnetic and radio echo ice-sounding measurements indicate a substantially greater area of the Dufek intrusion, Antarctica. Science. Burmester, R. F. 1976. Origin and stability of drilling induced magnetism. Geophysical Journal of the Royal Astronomical Society, 48, 1-14. Creer, K. M. 1970. A review of paleomagnetism. Earth Science Review, 6, 369-466. Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion of Antarctica. U.S. Geological Survey Bulletin, 1405-D, D1-D36. Ford, A. B., and Kistler, R. W. In press. K-Ar age, composition and origin of Mesozoic mafic rocks related to Ferrar Group, Pensacola Mountains, Antarctica. New Zealand Journal of Geology and Geophysics.
Figure 2. Portion of equal area plot of virtual geomagnetic poles (.) for ridge F-F magnetizations stable to alternating field demagnetization above 50 millitesla. Squares plotted in the northern hemisphere are Jurassic poles for Antarctica taken from compilations of McElhinny and Cowley (1978), Irving, Tanczyk, and Hastie (1976), and Creer (1970). Their mean (solid circle) is plotted in the southern hemisphere along with Beck's (1972) mean for the Dufek as published (X) and with tilt correction removed (+) and revised pole (Beck at al. 1979, A). The long dashed line is a paleo-meridian through VGPS, and short dashed lines are south paleolatitudes using 0 as pole. Dash-dot line is paleomeridian through the Dufek Intrusion (D.l.).
horizontal at the time of magnetization. Ford, Reynolds, Huie, and Boyer (in press) have since determined that the intrusion became synformal during the late stages of crystallization, so it now seems likely that no tectonic correction is required. A new paleomagnetic pole for the Dufek, recalculated from the previous data (Griffin 1969), with insight gained from our recent work and without a tilt correction, is 60°S 223°E, Ap = 10.5°, Am = 11.3° (figure 2) (Beck, Burmester, and Sheriff 1979). The age for this pole should also be changed to 172 ± 4 million years, in response to the change in the potassium-40 decay constant (Ford and Kistler in press). This work was supported by National Science Foundation grant DPI' 77-21904. We are indebted to A. B. Ford for his help and encouragement on this project from its inception.
Ford, A. B., Reynolds, R. L., Huie, C., and Boyer, S. J. 1979. Geologic field investigation of the Dufek intrusion. Antarctic Journal of the U.S., 14(5), 9-11.
I
NORTH UP
MOMENT15AnrI
cr
I
Z
Io
IL)
WEST toEAST tV c-' 0 \. 0o Al 0
References Beck, M. E., Jr. 1972. Paleomagnetism and magnetic polarity zones in the Jurassic Dufek intrusion, Pensacola Mountains, Antarctica. Geophysical Journal of the Royal Astronomical Society, 28, 49-63. Beck, M. E., Jr., Burmester, R. F., and Sheriff, S. D. 1979. Field reversal and paleomagnetic pole for Jurassic Antarctica, EOS, Transactions Of the American Geophysical Union, 60, 815.
Beck, M. E., Jr., Ford, A. B., and Boyd, W. A. 1968. Paleomagnetism of a stratiform intrusion in the Pensacola Mountains, Antarctica. Nature, 217, 534-535. 44
SOUTH DOWN Figure 3. Orthogonal projection of magnetization showing effect of cleaning in alternating fields to 25 millitesla followed by thermal demagnetization. Triangles are points on the section; dots are points on the map view. Thermal demagnetization removes a reverse component like one isolated In figure 1, allowing the residual magnetization to swing toward the normal Jurassic direction.
ANTARCTIC JOURNAL
Nowww
Griffin, N. L. 1%9. Paleomagnetic properties of the Dufek intrusion, Pensacola Mountains, Antarctica. Unpublished master's thesis, University of California-Riverside. Himmelberg, G. R., and Ford, A. B. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62, 623-633.
Irving, E., Tanczyk, E., and Hastie, J. 1976. Catalogue of paleomagnetic directions and poles-3rd issue. Paleozoic results 1949-1975. (Geomagnetic Series 5). Ottawa: Geomagnetic Service of Canada. McElhinny, M. W., and Cowley, J. A. 1978. Paleomagnetic directions and pole positions-XV. Pole numbers 15/1 to 15/232. Geophysical Journal of the Royal Astronomical Society, 52, 259-276.
Uranium-lead ages of zircons from Mount Provender, Shackleton Range, Transantarctic Mountains
sedimentary rocks of the Blaikiock Glacier Group (Clarkson 1972; Stephenson 1966). Solovyev and Grikurov (1978) and Clarkson, Hughes, and Thomson (1979) described a Middle Cambrian fauna from erratics of nearly unmetamorphosed rocks in moraine near Mount Provender. Grew and Halpern (1979) obtained rubidium-strontium (Rb-Sr) wholerock isochron ages of 583 ± 48 million years and 656 ± 66 million years on metamorphic rocks from this area. We present uranium-lead (U-Pb) isotopic data and ages on zircons (tables 1 and 2) separated from the samples analyzed by Grew and Halpern (1979); descriptions and precise locations of the samples are given by these authors. The zircons are clear, euhedral, elongated crystals. Most passed through 100-mesh sieve cloth but were retained by the 200-mesh cloth. No detrital cores were observed. Data on zircon from sample 1006-2 are concordant at 500 million years, while data on samples 10454 and 1029-7 lie close to a chord 0-500 million years and data on 10174 lie close to a chord 0-550 million years (see figure). The zircon U-Pb ages and total rock Rb-Sr isochron ages on samples 1006 and 1017 (no. 1-5) are consistent within the analytical
E. S. GREW
$Department of Earth and Space Sciences University of California Los Angeles, California 90024 W. I. MANTON
Department of Geosciences University of Texas-Dallas Richardson, Texas 75080
The bedrock exposures around Mount Provender (80023'S 30°W) consist of amphibolite-facies metamorphic rocks of the Shackleton Range Metamorphic Complex and of clastic
Table 1. Uranium and lead concentrations and isotopic ratios on rocks from the Mount Provender area, Shackleton Range, Antarctica Sample suite U Pb and no. (ppm) (ppm) 20613b/20412b 206Pb/207Pb 206Pb/208Pb 207PbI235U 206Pb/238U 1006-2 244 19.7 1.83 X 103 15.36 1017-4 136 33.7 6.87 X 103 16.45 1029-7 575 79.1 1.59 X 103 15.13 1045-4 743 71.3 4.19 X 10 16.35
0.8706 0.6327 0.0803 10.07 0.5403 0.0667 11.36 0.5679 0.0722 8.212 0.5899 0.0742
Common lead correction assuming a 500-million-year lead. Table 2. U-Pb ages of zircon., Rb-Sr isochron ages, and a biotite Rb-Sr age of rocks from the Mount Provender area, Sheckleton Range, Antarctica Age (million years) Rb-Sr age Rb-Sr age Sample isochron biotite suite Number 238U-206Pb 235U-207Pb 206Pb-20712b Number (million years) Number (million years)a 1006 2 515 512 497 1-5 '600 - 1017 4 431 451 556 1-5 583 ± 48 3 519 ± 15 1017 8b - 6-8 656±66 - 1029' 7 466 470 490 - - - 1045C 4 477 484 518 - - - • Grew and Halpern (1979). b Too few zircons for analysis. C Rb/Sr ratios are 0.06-0.19 and are too low for Rb-Sr whole-rock age determination (Grew and Halpern 1979).
1980 REVIEW
45
I
UP EAST I
z
+
RUSSELL F. BURMESTER
Department of Geology Western Washington University Bellingham, Washington 98225 Sprsj
D. SHERIFF
Department of Geology University of Wyoming Laramie, Wyoming 82071
The paleomagnetism of the Dufek layered intrusion (83°S 50°W) was first investigated by Beck, Ford, and Boyd (1968) using oriented hand samples collected during the 1965-66 field season. About equal numbers of these samples had magnetizations that were directed upward (normal) and downward (reverse). The simplest explanation was that the polarity of the Earth's magnetic field had changed during the cooling of the intrusion, causing the intrusion to be divided into normal and reverse magnetozones (Beck 1972). We visited the intrusions's outcrops in the Dufek Massif and Forrestal Range during the 1978-79 field season primarily to sample the transition between magnetozones in order to document the behavior of the Earth's magnetic field during a field reversal. The search for transition zones was aided by a portable magnetometer capable of measuring the natural remanent magnetization (NRM) of small, oriented samples. Where we thought we found a transition zone, we cored the rock with a gasoline-powered diamond drill at 1-meter stratigraphic intervals over the available outcrop on ridges F-E and F-F near Soma Bluff in the Forrestal Range (Ford, 1976). Core orientation data obtained with a sun compass were reduced in the field with a programed pocket calculator. All equipment worked flawlessly despite subnormal temperatures. One shortcoming was that the ethylene glycol used as coolant for the drill bit left the cores, equipment, and operator slimy wet. We recommend trying an alcoholwater or another more volatile solution instead. Standard paleomagnetic cleaning, using alternating field (a.f.) demagnetization, showed that most samples from the transition zone have at least three magnetizations (figure 1). The component most easily demagnetized, or alternatively removed by grinding the cores' exteriors, is antiparallel to the core axis and was acquired during drilling (Burmester 1976). A second magnetization, demagnetized by a 40millitesla alternating field, is directed upward with a mean direction consistent with magnetization during a Jurassic normal interval. The characteristic magnetization that is isolated by a.f. demagnetization yields virtual geomagnetic 1980 REVIEW
MOMENT +5 Am'
I NORTH
Figure 1. Projection of magnetization vector onto horizontal plane (0) and north-south vertical plain (L). Solid lines are projections of core axis; dashed lines estimate the characteristic magnetization isolated by alternating field demagnetization. Numbers indicate peak field in millitesla (ml).
poles (vcr's) in the southern paleohemisphere which appear to define part of the VGP path during the reversal (figure 2). Preliminary work suggests that this charateristic magnetization resides in submicroscopic magnetite distributed in the plagioclase. There is a problem in determining whether the field was changing from reverse to normal or vice versa. Thermal demagnetization of representative samples shows a tendency for a component of normal magnetization to have a higher unblocking temperature than the reverse-transitional component (figure 3). Coarse-grained cumulate magnetite last equilibrated with exsolved illmenite lamellae above its Curie temperature (Himmelberg and Ford 1977). If the highest temperature magnetization resides in the cumulate magnetite and was acquired during cooling, then the transition must have proceeded from normal to reverse. This is opposite of what is indicated by inversion of aeromagnetic data (Behrendt, Drewry, Kowski, and Grim, in press). This discrepancy may be attributable to different responses of the coarse and fine magnetite to slow cooling. Such responses possibly allowed the coarse magnetite to change its magnetization after a reverse-to-normal transition was completed. To determine the "sense" of the transition we need to determine better where the various magnetic components reside and at what temperatures they were acquired. Beck (1972) corrected magnetic directions to what they would have been if the igneous layering had been 43
Behrendt, J. C., Drewry, D. J., Jankowski, E., and Grim, M. S. In press. Areomagnetic and radio echo ice-sounding measurements indicate a substantially greater area of the Dufek intrusion, Antarctica. Science. Burmester, R. F. 1976. Origin and stability of drilling induced magnetism. Geophysical Journal of the Royal Astronomical Society, 48, 1-14. Creer, K. M. 1970. A review of paleomagnetism. Earth Science Review, 6, 369-466. Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion of Antarctica. U.S. Geological Survey Bulletin, 1405-D, D1-D36. Ford, A. B., and Kistler, R. W. In press. K-Ar age, composition and origin of Mesozoic mafic rocks related to Ferrar Group, Pensacola Mountains, Antarctica. New Zealand Journal of Geology and Geophysics.
Figure 2. Portion of equal area plot of virtual geomagnetic poles (.) for ridge F-F magnetizations stable to alternating field demagnetization above 50 millitesla. Squares plotted in the northern hemisphere are Jurassic poles for Antarctica taken from compilations of McElhinny and Cowley (1978), Irving, Tanczyk, and Hastie (1976), and Creer (1970). Their mean (solid circle) is plotted in the southern hemisphere along with Beck's (1972) mean for the Dufek as published (X) and with tilt correction removed (+) and revised pole (Beck at al. 1979, A). The long dashed line is a paleo-meridian through VGPS, and short dashed lines are south paleolatitudes using 0 as pole. Dash-dot line is paleomeridian through the Dufek Intrusion (D.l.).
horizontal at the time of magnetization. Ford, Reynolds, Huie, and Boyer (in press) have since determined that the intrusion became synformal during the late stages of crystallization, so it now seems likely that no tectonic correction is required. A new paleomagnetic pole for the Dufek, recalculated from the previous data (Griffin 1969), with insight gained from our recent work and without a tilt correction, is 60°S 223°E, Ap = 10.5°, Am = 11.3° (figure 2) (Beck, Burmester, and Sheriff 1979). The age for this pole should also be changed to 172 ± 4 million years, in response to the change in the potassium-40 decay constant (Ford and Kistler in press). This work was supported by National Science Foundation grant DPI' 77-21904. We are indebted to A. B. Ford for his help and encouragement on this project from its inception.
Ford, A. B., Reynolds, R. L., Huie, C., and Boyer, S. J. 1979. Geologic field investigation of the Dufek intrusion. Antarctic Journal of the U.S., 14(5), 9-11.
I
NORTH UP
MOMENT15AnrI
cr
I
Z
Io
IL)
WEST toEAST tV c-' 0 \. 0o Al 0
References Beck, M. E., Jr. 1972. Paleomagnetism and magnetic polarity zones in the Jurassic Dufek intrusion, Pensacola Mountains, Antarctica. Geophysical Journal of the Royal Astronomical Society, 28, 49-63. Beck, M. E., Jr., Burmester, R. F., and Sheriff, S. D. 1979. Field reversal and paleomagnetic pole for Jurassic Antarctica, EOS, Transactions Of the American Geophysical Union, 60, 815.
Beck, M. E., Jr., Ford, A. B., and Boyd, W. A. 1968. Paleomagnetism of a stratiform intrusion in the Pensacola Mountains, Antarctica. Nature, 217, 534-535. 44
SOUTH DOWN Figure 3. Orthogonal projection of magnetization showing effect of cleaning in alternating fields to 25 millitesla followed by thermal demagnetization. Triangles are points on the section; dots are points on the map view. Thermal demagnetization removes a reverse component like one isolated In figure 1, allowing the residual magnetization to swing toward the normal Jurassic direction.
ANTARCTIC JOURNAL
Nowww
Griffin, N. L. 1%9. Paleomagnetic properties of the Dufek intrusion, Pensacola Mountains, Antarctica. Unpublished master's thesis, University of California-Riverside. Himmelberg, G. R., and Ford, A. B. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62, 623-633.
Irving, E., Tanczyk, E., and Hastie, J. 1976. Catalogue of paleomagnetic directions and poles-3rd issue. Paleozoic results 1949-1975. (Geomagnetic Series 5). Ottawa: Geomagnetic Service of Canada. McElhinny, M. W., and Cowley, J. A. 1978. Paleomagnetic directions and pole positions-XV. Pole numbers 15/1 to 15/232. Geophysical Journal of the Royal Astronomical Society, 52, 259-276.
Uranium-lead ages of zircons from Mount Provender, Shackleton Range, Transantarctic Mountains
sedimentary rocks of the Blaikiock Glacier Group (Clarkson 1972; Stephenson 1966). Solovyev and Grikurov (1978) and Clarkson, Hughes, and Thomson (1979) described a Middle Cambrian fauna from erratics of nearly unmetamorphosed rocks in moraine near Mount Provender. Grew and Halpern (1979) obtained rubidium-strontium (Rb-Sr) wholerock isochron ages of 583 ± 48 million years and 656 ± 66 million years on metamorphic rocks from this area. We present uranium-lead (U-Pb) isotopic data and ages on zircons (tables 1 and 2) separated from the samples analyzed by Grew and Halpern (1979); descriptions and precise locations of the samples are given by these authors. The zircons are clear, euhedral, elongated crystals. Most passed through 100-mesh sieve cloth but were retained by the 200-mesh cloth. No detrital cores were observed. Data on zircon from sample 1006-2 are concordant at 500 million years, while data on samples 10454 and 1029-7 lie close to a chord 0-500 million years and data on 10174 lie close to a chord 0-550 million years (see figure). The zircon U-Pb ages and total rock Rb-Sr isochron ages on samples 1006 and 1017 (no. 1-5) are consistent within the analytical
E. S. GREW
$Department of Earth and Space Sciences University of California Los Angeles, California 90024 W. I. MANTON
Department of Geosciences University of Texas-Dallas Richardson, Texas 75080
The bedrock exposures around Mount Provender (80023'S 30°W) consist of amphibolite-facies metamorphic rocks of the Shackleton Range Metamorphic Complex and of clastic
Table 1. Uranium and lead concentrations and isotopic ratios on rocks from the Mount Provender area, Shackleton Range, Antarctica Sample suite U Pb and no. (ppm) (ppm) 20613b/20412b 206Pb/207Pb 206Pb/208Pb 207PbI235U 206Pb/238U 1006-2 244 19.7 1.83 X 103 15.36 1017-4 136 33.7 6.87 X 103 16.45 1029-7 575 79.1 1.59 X 103 15.13 1045-4 743 71.3 4.19 X 10 16.35
0.8706 0.6327 0.0803 10.07 0.5403 0.0667 11.36 0.5679 0.0722 8.212 0.5899 0.0742
Common lead correction assuming a 500-million-year lead. Table 2. U-Pb ages of zircon., Rb-Sr isochron ages, and a biotite Rb-Sr age of rocks from the Mount Provender area, Sheckleton Range, Antarctica Age (million years) Rb-Sr age Rb-Sr age Sample isochron biotite suite Number 238U-206Pb 235U-207Pb 206Pb-20712b Number (million years) Number (million years)a 1006 2 515 512 497 1-5 '600 - 1017 4 431 451 556 1-5 583 ± 48 3 519 ± 15 1017 8b - 6-8 656±66 - 1029' 7 466 470 490 - - - 1045C 4 477 484 518 - - - • Grew and Halpern (1979). b Too few zircons for analysis. C Rb/Sr ratios are 0.06-0.19 and are too low for Rb-Sr whole-rock age determination (Grew and Halpern 1979).
1980 REVIEW
45
