Potassium-argon ages of Dufek intrusion and other ...
References Abel, K. D. 1978. Plagioclases of the Dufek intrusion, Antarctica. M.A thesis, University of Missouri. Albee, A. L., and L. Ray. 1970. Correction factors for electron probe microanalysis of silicates, oxides, carbonates, phosphates, and sulfates. Analytical Chemistry, 42: 1408-14. Bence, A. E., and A. L. Albee. 1968. Empirical correction factors for the electron microanalysis of silicates and oxides. Journal of Geology, 76: 382-403. Bottinga, Y., and D. F. Weill. 1970. Densities of liquid silicate systems calculated from partial molar volumes of oxide components. American Journal of Science, 269: 169-82. Campbell, I. H., P. L. Roeder, and J . M. Dixon. 1978. Plagioclase buoyancy in basaltic liquids as determined with a centrifuge furnace. Contributions to Mineralogy and Petrology, 67: 369-77.
Potassium-argon ages of Dufek intrusion and other Mesozoic mafic bodies in the Pensacola Mountains R. W. KISTLER
and A.
B. FORD
U.S. Geological Survey Menlo Park, Cal!fornia 94025
Conventional potassium-argon (K-Ar) and argon-40/ argon-39 (40Ar/39 Ar) age determinations show that the stratiform gabbroic Dufek intrusion (Ford, 1976) is about the same age as tholeiitic diabase sills of the Pecora Escarpment (85°37'S168 040'W) at the south end of the Pensacola Mountains. The igneous activity corresponds with the main Early Jurassic activity of the Ferrar Group of Grindley (1963) elsewhere in the Transantarctic Mountains. Samples from the Dufek intrusion dated in our study include plagioclase of three plagioclase cumulates (anorthosites) from widely spaced stratigraphic intervals in the Dufek Massif and Forrestal Range; pyroxene from a pyroxene-cumulate (pyroxenite) layer in the Dufek Massif; and fine-. grained noncumulus whole-rock gabbro from an inferred chilled contact zone near Mount Lechner in the Forrestal Range. In addition, we also determined ages on plagioclase, pyroxene, and whole-rock samples from diabase sills of the Pecora Escarpment and basalt and diabase dikes of the Cordiner Peaks. The sills intrude the Permian Pecora Formation, and the dikes cut the Devonian Dover sandstone. and the overlying Gale mudstone (Ford et al., 1978). Conventional K-Ar ages on samples from the Dufek intrustion range from 174.1 ± 4.4, 171.2 ± 4.3, and
Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. U.S. Geological Survey Bulletin, no. l405-D. Ford, A. B., R. L. Reynolds, Carl Huie, and S. J . Boyer. 1979. Geological investigation of the Dufek intrusion, Pensacola Mountains, 1978-79. Antarctic Journal of the United States (this issue). 1-Iimmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the Dufek intrustion, Antarctica. Journal of Petrology, 17: 21943. Himmelberg, G. R., and A. B. Ford. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62: 623-33. Irvine, T. N. 1978. Density current structure and magmatic sedimentation. In Carnegie Institution of Washington Yearbook, 77: 717-25. Wager, L. R., and G. M. Brown. 1968. Layered Igneous Rocks. Edinburgh, Scotland: Oliver & Boyd.
169.5 ± 4.2 million years for the plagioclase, through 111.9± 2.8 and 106.3 ± 2.7 million years for the wholerock gabbros, to 97.5 ± 2.4 million years for the pyroxene samples. In thin section, the plagioclase appears little altered, but the pyroxenes (augite and inverted pigeonite) show variable and commonly large effects of inversion and exsolution (Himmelberg and Ford, 1976). Kistler and Dodge (1966) suggested that the young K-Ar ages for pyroxene-.-compared to those for coexisting biotite, hornblende, plagioclase, and orthoclase from Sierra Nevada (California) quartz diorite plutons-may be related to ubiquitously observed exsolution lamellae, which decrease the effective diffusion dimensions and may permit low-temperature argon loss. Similarly, the considerably younger ages of Dufek intrusion samples containing pyroxene probably reflect low-temperature argon loss during or after subsolidus phase changes. To ascertain that these young pyroxene ages are not the result of inaccuracies in the measurement of lowK2 0 concentrations, pyroxene from two localities in the Dufek intrusion was also dated by the total-fusion 40Ar/ 39 Ar technique. Results showed that the young conventional K-Ar ages of samples containing pyroxene are probably not due to such analytical difficulties. The best present estimate for the age of the intrusion is therefore considered to be the average of the three plagioclase determinations: 171.6 ± 4.3 million years. This age is in close agreement with, and within the range of analytical uncertainties of, the value of 175 ± 5 million years reported by Elliot, Fleck, and Sutter (in press) for basalt flows of the Ferrar Group in the central Transantarctic Mountains. Samples of whole-rock holocrystalline chilled-contact basalt and of three plagioclase-pyroxene pairs from two diabase sills in the Pecora Escarpment yielded conventional K-Ar ages ranging from 223.1 ± 5.6 million years for the basalt to 177.7 ± 4.5 million years for a plagioclase. In contrast with the Dufek intrusion, the apparent ages of pyroxene in all three pairs are slightly greater than those of coexisting plagioclase and are probably affected by small but variable amounts of excess argon.
This interpretation is supported by an argon isochron plot of 40Ar/36Ar and 40K/36 Ar values (R. W. Kistler, unpublished data, 1978). The pyroxene shows much greater optical homogeneity than in the Dufek intrusion. The best estimate for the age of the sills is considered to be the average of the mineral pair showing the most concordant and, more importantly, the youngest ages of all analyzed pairs: 178.9 ± 4.5 million years (obtained from a plagioclase age of 177.7 ± 4.5 million years and a pyroxene age of 180.0 ± 4.5 million years). Analysis of a coexisting pyroxene-plagioclase pair from a dike in the Cordiner Peaks yielded discordant apparent ages of 307.9 ± 7.7 million years for pyroxene and 168.8 ± 4.2 million years for plagioclase. The plagioclase age is considered to provide the best estimate for the dike's age, in that the pyroxene, which is optically homogeneous, is inferred to contain excess argon. The above results indicate that the basaltic magmatism that produced the dikes and sills and the Dufek intrusion in the Pensacola Mountains occurred over a narrow time interval in the Early Jurassic. The bodies are correlated with the Ferrar Group in other parts of the Transantarctic Mountains (Elliot, Fleck, and Sutter, in press) on the basis of age equivalence and close chemical comparisons (A. B. Ford, unpublished data, 1978). The age of the Dufek intrusion reported here also provides an upper age limit of Early Jurassic for the latest major orogeny in the Pensacola Mountains, during which rocks as young as Permian were strongly folded (Ford, 1972). The fieldwork on which this study is based was sup-
ported by National Science Foundation grants G 2389 (in the Pecora Escarpment, 1962-63), GA 222 (on the Dufek intrusion, 1965-66), and AG 238 (in the Cordiner Peaks, 1973-74) to the U.S. Geological Survey. References
Elliot, D. H., R. j. Fleck, and J. F. Sutter. In press. K-Ar dating of the Ferrar Group, central Transantarctic Mountains. In Geology of the Central Transantarctic Mountains, eds. M. D. Turner and J . F. Splettstoesser. Antarctic Research Series, Memoir. Washington, D.C.: American Geophysical Union. Ford, A. B. 1972. The Weddell orogeny—latest Permian to early Mesozoic deformation at the Weddell Sea margin of the Transantarctic Mountains. In Antarctic Geology and Geophysics, ed. R. J . Adie, pp. 419-25. Olso, Norway: Universitetsforlaget. Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion. Antarctica. U.S. Geological Survey Bulletin, no. 1405—D. Ford, A. B., D. L. Schmidt, and W. W. Boyd, Jr. 1978. Geologic map of the Davis Valley Quadrangle and Part of the Cordiner Peaks Quadrangle, Pensacola Mountains, Antarctica (1:250,000 scale). U.S. Geological Survey Map A—b.
Grindley, G. W. 1963. The geology of the Queen Alexandra Range, Beardmore. Glacier, Ross Dependency, Antarctica: With notes on the correlation of Gondwana sequences. New Zealand Journal of Geology and Geophysics, 6: 307-47. Himmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the Dufek intrusion, Antarctica. journal of Petrology, 17: 219-43. Kistler, R. W., and F. C. W. Dodge. 1966. Potassium-argon ages of coexisting minerals from pyroxene-bearing granitic rocks in the Sierra Nevada, California. Journal of Geophysical Research, 71(8): 2157-61.
Geological field investigation of Dufek intrusion ARTHUR B. FORD U.S. Geological Survey Menlo Park, California 94025
RICHARD L. REYNOLDS U.S. Geological Survey Denver, Colorado 80225
CARL HUIE U.S. Geological Survey Menlo Park, California 94025
STEPHEN J. BOYER U.S. Geological Survey Denver, Colorado 80225
Between 4 November and 20 December 1978, the authors carried out geological field studies of the layered gabbroic Dufek intrusion (Ford, 1976). This work continued the detailed investigation of the unusually large igneous complex that was started in the western Dufek Massif, in the lower part of the body, in the summer of 1976-77 (Ford et al., 1977). In 1978, the authors focused on the iron-enriched upper part of the body, in the southern Forrestal Range. During November, fieldwork was done by snowmobile and ski traverses from a tent camp in May Valley. During December, work was also continued in the Dufek Massif from tent camps located on the Sallee Snowfield and near Aughenbaugh Peak. The purpose of this fieldwork was to investigate questions raised by laboratory and office studies following a 1965-66 reconnaissance of the complex (Ford and Boyd, 1968). Although topographic base maps had not been available at the time of that reconnaissance, the data obtained were adequate for compilation of two recently published 1:250,000-scale geologic maps of the body
Potassium-argon ages of Dufek intrusion and other Mesozoic mafic bodies in the Pensacola Mountains R. W. KISTLER
and A.
B. FORD
U.S. Geological Survey Menlo Park, Cal!fornia 94025
Conventional potassium-argon (K-Ar) and argon-40/ argon-39 (40Ar/39 Ar) age determinations show that the stratiform gabbroic Dufek intrusion (Ford, 1976) is about the same age as tholeiitic diabase sills of the Pecora Escarpment (85°37'S168 040'W) at the south end of the Pensacola Mountains. The igneous activity corresponds with the main Early Jurassic activity of the Ferrar Group of Grindley (1963) elsewhere in the Transantarctic Mountains. Samples from the Dufek intrusion dated in our study include plagioclase of three plagioclase cumulates (anorthosites) from widely spaced stratigraphic intervals in the Dufek Massif and Forrestal Range; pyroxene from a pyroxene-cumulate (pyroxenite) layer in the Dufek Massif; and fine-. grained noncumulus whole-rock gabbro from an inferred chilled contact zone near Mount Lechner in the Forrestal Range. In addition, we also determined ages on plagioclase, pyroxene, and whole-rock samples from diabase sills of the Pecora Escarpment and basalt and diabase dikes of the Cordiner Peaks. The sills intrude the Permian Pecora Formation, and the dikes cut the Devonian Dover sandstone. and the overlying Gale mudstone (Ford et al., 1978). Conventional K-Ar ages on samples from the Dufek intrustion range from 174.1 ± 4.4, 171.2 ± 4.3, and
Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. U.S. Geological Survey Bulletin, no. l405-D. Ford, A. B., R. L. Reynolds, Carl Huie, and S. J . Boyer. 1979. Geological investigation of the Dufek intrusion, Pensacola Mountains, 1978-79. Antarctic Journal of the United States (this issue). 1-Iimmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the Dufek intrustion, Antarctica. Journal of Petrology, 17: 21943. Himmelberg, G. R., and A. B. Ford. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62: 623-33. Irvine, T. N. 1978. Density current structure and magmatic sedimentation. In Carnegie Institution of Washington Yearbook, 77: 717-25. Wager, L. R., and G. M. Brown. 1968. Layered Igneous Rocks. Edinburgh, Scotland: Oliver & Boyd.
169.5 ± 4.2 million years for the plagioclase, through 111.9± 2.8 and 106.3 ± 2.7 million years for the wholerock gabbros, to 97.5 ± 2.4 million years for the pyroxene samples. In thin section, the plagioclase appears little altered, but the pyroxenes (augite and inverted pigeonite) show variable and commonly large effects of inversion and exsolution (Himmelberg and Ford, 1976). Kistler and Dodge (1966) suggested that the young K-Ar ages for pyroxene-.-compared to those for coexisting biotite, hornblende, plagioclase, and orthoclase from Sierra Nevada (California) quartz diorite plutons-may be related to ubiquitously observed exsolution lamellae, which decrease the effective diffusion dimensions and may permit low-temperature argon loss. Similarly, the considerably younger ages of Dufek intrusion samples containing pyroxene probably reflect low-temperature argon loss during or after subsolidus phase changes. To ascertain that these young pyroxene ages are not the result of inaccuracies in the measurement of lowK2 0 concentrations, pyroxene from two localities in the Dufek intrusion was also dated by the total-fusion 40Ar/ 39 Ar technique. Results showed that the young conventional K-Ar ages of samples containing pyroxene are probably not due to such analytical difficulties. The best present estimate for the age of the intrusion is therefore considered to be the average of the three plagioclase determinations: 171.6 ± 4.3 million years. This age is in close agreement with, and within the range of analytical uncertainties of, the value of 175 ± 5 million years reported by Elliot, Fleck, and Sutter (in press) for basalt flows of the Ferrar Group in the central Transantarctic Mountains. Samples of whole-rock holocrystalline chilled-contact basalt and of three plagioclase-pyroxene pairs from two diabase sills in the Pecora Escarpment yielded conventional K-Ar ages ranging from 223.1 ± 5.6 million years for the basalt to 177.7 ± 4.5 million years for a plagioclase. In contrast with the Dufek intrusion, the apparent ages of pyroxene in all three pairs are slightly greater than those of coexisting plagioclase and are probably affected by small but variable amounts of excess argon.
This interpretation is supported by an argon isochron plot of 40Ar/36Ar and 40K/36 Ar values (R. W. Kistler, unpublished data, 1978). The pyroxene shows much greater optical homogeneity than in the Dufek intrusion. The best estimate for the age of the sills is considered to be the average of the mineral pair showing the most concordant and, more importantly, the youngest ages of all analyzed pairs: 178.9 ± 4.5 million years (obtained from a plagioclase age of 177.7 ± 4.5 million years and a pyroxene age of 180.0 ± 4.5 million years). Analysis of a coexisting pyroxene-plagioclase pair from a dike in the Cordiner Peaks yielded discordant apparent ages of 307.9 ± 7.7 million years for pyroxene and 168.8 ± 4.2 million years for plagioclase. The plagioclase age is considered to provide the best estimate for the dike's age, in that the pyroxene, which is optically homogeneous, is inferred to contain excess argon. The above results indicate that the basaltic magmatism that produced the dikes and sills and the Dufek intrusion in the Pensacola Mountains occurred over a narrow time interval in the Early Jurassic. The bodies are correlated with the Ferrar Group in other parts of the Transantarctic Mountains (Elliot, Fleck, and Sutter, in press) on the basis of age equivalence and close chemical comparisons (A. B. Ford, unpublished data, 1978). The age of the Dufek intrusion reported here also provides an upper age limit of Early Jurassic for the latest major orogeny in the Pensacola Mountains, during which rocks as young as Permian were strongly folded (Ford, 1972). The fieldwork on which this study is based was sup-
ported by National Science Foundation grants G 2389 (in the Pecora Escarpment, 1962-63), GA 222 (on the Dufek intrusion, 1965-66), and AG 238 (in the Cordiner Peaks, 1973-74) to the U.S. Geological Survey. References
Elliot, D. H., R. j. Fleck, and J. F. Sutter. In press. K-Ar dating of the Ferrar Group, central Transantarctic Mountains. In Geology of the Central Transantarctic Mountains, eds. M. D. Turner and J . F. Splettstoesser. Antarctic Research Series, Memoir. Washington, D.C.: American Geophysical Union. Ford, A. B. 1972. The Weddell orogeny—latest Permian to early Mesozoic deformation at the Weddell Sea margin of the Transantarctic Mountains. In Antarctic Geology and Geophysics, ed. R. J . Adie, pp. 419-25. Olso, Norway: Universitetsforlaget. Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion. Antarctica. U.S. Geological Survey Bulletin, no. 1405—D. Ford, A. B., D. L. Schmidt, and W. W. Boyd, Jr. 1978. Geologic map of the Davis Valley Quadrangle and Part of the Cordiner Peaks Quadrangle, Pensacola Mountains, Antarctica (1:250,000 scale). U.S. Geological Survey Map A—b.
Grindley, G. W. 1963. The geology of the Queen Alexandra Range, Beardmore. Glacier, Ross Dependency, Antarctica: With notes on the correlation of Gondwana sequences. New Zealand Journal of Geology and Geophysics, 6: 307-47. Himmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the Dufek intrusion, Antarctica. journal of Petrology, 17: 219-43. Kistler, R. W., and F. C. W. Dodge. 1966. Potassium-argon ages of coexisting minerals from pyroxene-bearing granitic rocks in the Sierra Nevada, California. Journal of Geophysical Research, 71(8): 2157-61.
Geological field investigation of Dufek intrusion ARTHUR B. FORD U.S. Geological Survey Menlo Park, California 94025
RICHARD L. REYNOLDS U.S. Geological Survey Denver, Colorado 80225
CARL HUIE U.S. Geological Survey Menlo Park, California 94025
STEPHEN J. BOYER U.S. Geological Survey Denver, Colorado 80225
Between 4 November and 20 December 1978, the authors carried out geological field studies of the layered gabbroic Dufek intrusion (Ford, 1976). This work continued the detailed investigation of the unusually large igneous complex that was started in the western Dufek Massif, in the lower part of the body, in the summer of 1976-77 (Ford et al., 1977). In 1978, the authors focused on the iron-enriched upper part of the body, in the southern Forrestal Range. During November, fieldwork was done by snowmobile and ski traverses from a tent camp in May Valley. During December, work was also continued in the Dufek Massif from tent camps located on the Sallee Snowfield and near Aughenbaugh Peak. The purpose of this fieldwork was to investigate questions raised by laboratory and office studies following a 1965-66 reconnaissance of the complex (Ford and Boyd, 1968). Although topographic base maps had not been available at the time of that reconnaissance, the data obtained were adequate for compilation of two recently published 1:250,000-scale geologic maps of the body
