Rubidium-strontium geochronology of granitic rocks from ...
This study attempts to demonstrate two important points concerning the origin and evolution of the crystalline basement rocks in this area of Antarctica: 1. There is a component to the crust in this region that is older than 950 million years and may be Archean. If the analyzed zircons in the gneiss are detrital, this older component may be the provenance for these zircons. If the zircons formed in situ, the 2,555 million years age may date an event that affected the gneiss. 2. Partial melting of ancient metasedimentary rocks is a plausible source for the vida granite. This study was supported by Office of Polar Programs, National Science Foundation, grants GV 0400762 and GV 36951.
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Deutsch, S., and N. Grogler. 1966. Isotopic age of olympus granitegneiss (Victoria Land—Antarctic). Earth and Planetary Science Letters, 1: 82-84. Faure, G., and L. M. Jones. 1974. Isotopic composition of strontium and geologic history of the basement rocks of Wright Valley, southern Victoria Land, Antarctic. New Zealand Journal of Geology and Geophysics, 17: 611-627. McKelvey, B. C., and P. N. Webb. 1962. Geological investigation in southern Victoria Land, Antarctic. 3. Geology of Wright Valley. New Zealand Journal of Geology and Geophysics, 5: 143-162. Stuckless, J . S., and R. L. Ericksen. 1975. Rb-Sr ages of basement rocks recovered from borehole DVDP 6, southern Victoria Land, Antarctica. Antarctic Journal of the US., 10: 302-307. Webb, P. N., and B. C. McKelvey. 1959. Geological investigations in south Victoria Land, Antarctica. Part I—Geology of Victoria Dry Valley. New ZealandJournal of Geology and Geophysics, 2: 120-136.
Rubidium-strontium geochronology of granitic rocks from Mt. Chapman, Whitmore Mountains, West Antarctica JACK KOVACH Department of Geology Muskingum College New Concord, Ohio 43762 GUNTER FAURE
institute of Polar Studies The Ohio State University Columbus, Ohio 43210
The Whitmore Mountains (centered approximately at 820 30'S.1040 30'W.) are composed predominantly of early October 1978
87R /Sr
Preliminary Rb-Sr isochron plot of whole-rock (wn), feldspar (F), and biotite (B) concentrates of coarse and fine-grained igneous rocks from Mt. Chapman, Whitmore Mountains. Mesozoic plutons of felsic composition that are intrusive into deformed metasedimentary rocks (Webers et al., in press). The geology and geologic history of the Whitmore Mountains are of particular interest because these mountains comprise one of the few presently exposed geologic ties between East and West Antarctica (Craddock and Webers, 1977). We present here a preliminary report on rubidium-strontium age determinations on whole-rock samples and mineral separates from a fine-grained equigranular granite (sample 120A) and a coarsely crystalline, porphyritic granite (sample 120B) from Mt. Chapman (82°35'S. 105°55'W.) in the Whitmore Mountains. The specimen was collected by W. E. Long during the 1957-58 field season. It weighed 1.075 kilograms and consisted of a fine-grained igneous rock of granitic composition in contact with a coarse-grained quartz monzonite. The contact shows no chilling or obvious intrusive relationships between the two varieties of rock. The coarse-grained phase of the igneous rock at Mt. Chapman may be equivalent to the Mt. Seelig granite, whereas the fine-grained phase may correlate with the Linck Nunataks granite described by Webers and others (in press). These authors reported K-Ar (biotite) dates of 190 ± 8 million years and 176 ± 5 million years for the Mt. Seelig and Linck Nunataks granites, respectively. Our age determination is based on analyses of whole-rock samples of both phases of the granitic rocks together with concentrates of biotite, muscovite, and feldspar. The interpretation of these data is preliminary because the analyses of the muscovites are still in progress. The available data define an isochron shown in the figure whose slope and intercept were estimated by an unweighted least-squares regression method. The results indicate a date of 173 million years and an initial strontium-87/strontium-86 ratio of 0.7 148, assuming a value of 1.42 x 10.11 per year for the decay constant of rubidium-87. The date is identical to the age of the Linck Nunataks granite reported by Webers and others (in press) and thus 17
supports the correlation with the fine-grained phase on Mt. Chapman. The coarse-grained phase may be older than 173 million years: it may have been homogenized isotopically by the intrusion of the fine-grained phase, assuming a relation ship between these rocks similar to that between the Mt. Seelig and the Linck Nunataks granites (Webers et al., in press). This interpretation is plausible because both samples were taken from a single hand specimen and therefore experienced similar temperature histories. However, our present data do not exclude the possibility that both phases of the granitic rock at Mt. Chapman formed at about the same time during a single episode of magmatic activity. The initial 87Sr/86Sr ratio (0.7 148) is high compared with that of strontium derived from the upper mantle (Faure, 1977). The presence of excess radiogenic 87 Sr indicates either that magma formed by partial melting of old sialic rocks in the underlying continental crust or that a mantle-derived magma was extensively contaminated with crustal 87 Sr. In either case, the resulting rocks are strongly affected isotopically, and presumably chemically, by the sialic crust. Thejurassic date (173 million years) and elevated initial 87 Sr/86 Sr ratio of the granitic rocks in the Whitmore Mountains are similar to the age and initial 87 Sr/86Sr ratio of the Kirkpatrick basalt and the Ferrar dolerite (Faure et at., 1972, 1974, in press). This coincidence also was pointed out by Ravich and Grikurov (1976), who suggested that the granitic stocks of the Whitmore Mountains and those located between the Ellsworth Mountains and the Thiel Mountains may be felsic derivatives of the tholeiite basalt magma that formed the basalt flows and dolerite sills in the Transantarctic Mountains. This interesting suggestion deserves further study. This research was supported by the Division of Polar Programs of the National Science Foundation through grant DPP 76-11871.
References
Craddock, C., and G. F. Webers. 1977. Geology of the Ellsworth Mountains to Thiel Mountains ridge. Antarctic Journal of the US., 12(4): 85. Faure, G. 1977. Principles of Isotope Geology. Wiley and Sons, New York. Faure, G.,J. R Bowman, D. H. Elliot, and L. M.Jones. 1974. Strontium isotope composition and petrogenesis of the Kirkpatrick Basalt, Queen Alexandra Range, Antarctica. Contributions to Mineralogy and Petrology, vol. 48, 153-169. Faure, G., R. L. Hill, L. M. Jones, and D. H. Elliot. 1972. Isotope composition of strontium and silica content of Mesozoic basalt and dolerite from Antarctica. In: Antarctic Geology and Geophysics (R. J. Adie, ed.). Universitetsforlaget, Oslo. pp. 617-624. Faure, G., K. K. Pace, and D. H. Elliot. In press. Systematic variations of 87 Sr/86Sr ratios and major element concentrations in the Kirkpatrick Basalt of Mt. Falla, Queen Alexandra Range, Transantarctic Mountains. In: Proceedings of the Third Symposium on Antarctic Geology and Geophysics, 22-27 August 1977 (C. Craddock, ed.). University of Wisconsin Press, Madison, Wisconsin. Ravich, M. G., and G. E. Grikurov, eds. 1976. Explanatory Notes to the Geologic Map of Antarctica (Scale 1:5,000,000). Research Institute of the Geology of the Arctic, Ministry of the Geology of the USSR, Leningrad. 18
Webers, G. F., C. Craddock, M. A. Rogers, and J . J. Anderson. In press. Geology of the Whitmore Mountains. In: Proceedings of the Third Symposium on Antarctic Geology and Geophysics, 22-27 August 1977, (C. Craddock, ed.). University of Wisconsin Press, Madison, Wisconsin.
Palynologic studies in the Transantarctic Mountains ROSEMARY A. ASKIN
Institute of Polar Studies The Ohio State University Columbus, Ohio 43210 JAMES M. SCHOPF2
Institute of Polar Studies and Department of Geology The Ohio State University Columbus, Ohio 43210 Further palynologic assemblages have been recovered from the Victoria Group, upper Beacon Supergroup, in the Nilsen Plateau, and Beardmore and Shackleton Glacier areas of Antarctica. Samples processed include carbonaceous mudstones, siltstones, and fine sandstones collected by past expeditions to these areas by the Institute of Polar Studies, The Ohio State University. Sample preparation included hot hydrofluoric acid treatment and flotation in zinc bromide solution. Oxidation and bleaching, to varying degrees for each sample, was required to clear the microfossils, whose state of preservation was generally poor (Kyle and Schopf, 1977). An informal palynostratigraphic zonation was proposed by Kyle (in press) for the Permian and Triassic palynologic succession in south Victoria Land. Though further refinement of zonal boundaries is necessary, this zonal scheme seems applicable to Victoria Group strata as far south as the Ohio Range (Kyle and Schopf, in press). Recovery of additional palynomorph assemblages has led to the recognition of species that occur in other Gondwana basins but previously were not known in the Transantarctic Mountains, and to the extension of known ranges of some species in the Transantarctic Mountains. Additional samples not yet examined include a collection from the Cumulus Hills, Shackleton Glacier area, made by the Institute of Polar Studies expedition during the 1977-78 season. This palynological study has enabled relatively precise correlation with the closely similar palynologic succession of eastern Australia (Kyle, in press; Kyle and Schopf, in press) and therefore has allowed finer age control of Victoria Group strata than previously was possible. It has also resulted in 'Formerly Rosemary A. Kyle. 2Deceased.
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References 0 200 400 600 800 1000 1200
Deutsch, S., and N. Grogler. 1966. Isotopic age of olympus granitegneiss (Victoria Land—Antarctic). Earth and Planetary Science Letters, 1: 82-84. Faure, G., and L. M. Jones. 1974. Isotopic composition of strontium and geologic history of the basement rocks of Wright Valley, southern Victoria Land, Antarctic. New Zealand Journal of Geology and Geophysics, 17: 611-627. McKelvey, B. C., and P. N. Webb. 1962. Geological investigation in southern Victoria Land, Antarctic. 3. Geology of Wright Valley. New Zealand Journal of Geology and Geophysics, 5: 143-162. Stuckless, J . S., and R. L. Ericksen. 1975. Rb-Sr ages of basement rocks recovered from borehole DVDP 6, southern Victoria Land, Antarctica. Antarctic Journal of the US., 10: 302-307. Webb, P. N., and B. C. McKelvey. 1959. Geological investigations in south Victoria Land, Antarctica. Part I—Geology of Victoria Dry Valley. New ZealandJournal of Geology and Geophysics, 2: 120-136.
Rubidium-strontium geochronology of granitic rocks from Mt. Chapman, Whitmore Mountains, West Antarctica JACK KOVACH Department of Geology Muskingum College New Concord, Ohio 43762 GUNTER FAURE
institute of Polar Studies The Ohio State University Columbus, Ohio 43210
The Whitmore Mountains (centered approximately at 820 30'S.1040 30'W.) are composed predominantly of early October 1978
87R /Sr
Preliminary Rb-Sr isochron plot of whole-rock (wn), feldspar (F), and biotite (B) concentrates of coarse and fine-grained igneous rocks from Mt. Chapman, Whitmore Mountains. Mesozoic plutons of felsic composition that are intrusive into deformed metasedimentary rocks (Webers et al., in press). The geology and geologic history of the Whitmore Mountains are of particular interest because these mountains comprise one of the few presently exposed geologic ties between East and West Antarctica (Craddock and Webers, 1977). We present here a preliminary report on rubidium-strontium age determinations on whole-rock samples and mineral separates from a fine-grained equigranular granite (sample 120A) and a coarsely crystalline, porphyritic granite (sample 120B) from Mt. Chapman (82°35'S. 105°55'W.) in the Whitmore Mountains. The specimen was collected by W. E. Long during the 1957-58 field season. It weighed 1.075 kilograms and consisted of a fine-grained igneous rock of granitic composition in contact with a coarse-grained quartz monzonite. The contact shows no chilling or obvious intrusive relationships between the two varieties of rock. The coarse-grained phase of the igneous rock at Mt. Chapman may be equivalent to the Mt. Seelig granite, whereas the fine-grained phase may correlate with the Linck Nunataks granite described by Webers and others (in press). These authors reported K-Ar (biotite) dates of 190 ± 8 million years and 176 ± 5 million years for the Mt. Seelig and Linck Nunataks granites, respectively. Our age determination is based on analyses of whole-rock samples of both phases of the granitic rocks together with concentrates of biotite, muscovite, and feldspar. The interpretation of these data is preliminary because the analyses of the muscovites are still in progress. The available data define an isochron shown in the figure whose slope and intercept were estimated by an unweighted least-squares regression method. The results indicate a date of 173 million years and an initial strontium-87/strontium-86 ratio of 0.7 148, assuming a value of 1.42 x 10.11 per year for the decay constant of rubidium-87. The date is identical to the age of the Linck Nunataks granite reported by Webers and others (in press) and thus 17
supports the correlation with the fine-grained phase on Mt. Chapman. The coarse-grained phase may be older than 173 million years: it may have been homogenized isotopically by the intrusion of the fine-grained phase, assuming a relation ship between these rocks similar to that between the Mt. Seelig and the Linck Nunataks granites (Webers et al., in press). This interpretation is plausible because both samples were taken from a single hand specimen and therefore experienced similar temperature histories. However, our present data do not exclude the possibility that both phases of the granitic rock at Mt. Chapman formed at about the same time during a single episode of magmatic activity. The initial 87Sr/86Sr ratio (0.7 148) is high compared with that of strontium derived from the upper mantle (Faure, 1977). The presence of excess radiogenic 87 Sr indicates either that magma formed by partial melting of old sialic rocks in the underlying continental crust or that a mantle-derived magma was extensively contaminated with crustal 87 Sr. In either case, the resulting rocks are strongly affected isotopically, and presumably chemically, by the sialic crust. Thejurassic date (173 million years) and elevated initial 87 Sr/86 Sr ratio of the granitic rocks in the Whitmore Mountains are similar to the age and initial 87 Sr/86Sr ratio of the Kirkpatrick basalt and the Ferrar dolerite (Faure et at., 1972, 1974, in press). This coincidence also was pointed out by Ravich and Grikurov (1976), who suggested that the granitic stocks of the Whitmore Mountains and those located between the Ellsworth Mountains and the Thiel Mountains may be felsic derivatives of the tholeiite basalt magma that formed the basalt flows and dolerite sills in the Transantarctic Mountains. This interesting suggestion deserves further study. This research was supported by the Division of Polar Programs of the National Science Foundation through grant DPP 76-11871.
References
Craddock, C., and G. F. Webers. 1977. Geology of the Ellsworth Mountains to Thiel Mountains ridge. Antarctic Journal of the US., 12(4): 85. Faure, G. 1977. Principles of Isotope Geology. Wiley and Sons, New York. Faure, G.,J. R Bowman, D. H. Elliot, and L. M.Jones. 1974. Strontium isotope composition and petrogenesis of the Kirkpatrick Basalt, Queen Alexandra Range, Antarctica. Contributions to Mineralogy and Petrology, vol. 48, 153-169. Faure, G., R. L. Hill, L. M. Jones, and D. H. Elliot. 1972. Isotope composition of strontium and silica content of Mesozoic basalt and dolerite from Antarctica. In: Antarctic Geology and Geophysics (R. J. Adie, ed.). Universitetsforlaget, Oslo. pp. 617-624. Faure, G., K. K. Pace, and D. H. Elliot. In press. Systematic variations of 87 Sr/86Sr ratios and major element concentrations in the Kirkpatrick Basalt of Mt. Falla, Queen Alexandra Range, Transantarctic Mountains. In: Proceedings of the Third Symposium on Antarctic Geology and Geophysics, 22-27 August 1977 (C. Craddock, ed.). University of Wisconsin Press, Madison, Wisconsin. Ravich, M. G., and G. E. Grikurov, eds. 1976. Explanatory Notes to the Geologic Map of Antarctica (Scale 1:5,000,000). Research Institute of the Geology of the Arctic, Ministry of the Geology of the USSR, Leningrad. 18
Webers, G. F., C. Craddock, M. A. Rogers, and J . J. Anderson. In press. Geology of the Whitmore Mountains. In: Proceedings of the Third Symposium on Antarctic Geology and Geophysics, 22-27 August 1977, (C. Craddock, ed.). University of Wisconsin Press, Madison, Wisconsin.
Palynologic studies in the Transantarctic Mountains ROSEMARY A. ASKIN
Institute of Polar Studies The Ohio State University Columbus, Ohio 43210 JAMES M. SCHOPF2
Institute of Polar Studies and Department of Geology The Ohio State University Columbus, Ohio 43210 Further palynologic assemblages have been recovered from the Victoria Group, upper Beacon Supergroup, in the Nilsen Plateau, and Beardmore and Shackleton Glacier areas of Antarctica. Samples processed include carbonaceous mudstones, siltstones, and fine sandstones collected by past expeditions to these areas by the Institute of Polar Studies, The Ohio State University. Sample preparation included hot hydrofluoric acid treatment and flotation in zinc bromide solution. Oxidation and bleaching, to varying degrees for each sample, was required to clear the microfossils, whose state of preservation was generally poor (Kyle and Schopf, 1977). An informal palynostratigraphic zonation was proposed by Kyle (in press) for the Permian and Triassic palynologic succession in south Victoria Land. Though further refinement of zonal boundaries is necessary, this zonal scheme seems applicable to Victoria Group strata as far south as the Ohio Range (Kyle and Schopf, in press). Recovery of additional palynomorph assemblages has led to the recognition of species that occur in other Gondwana basins but previously were not known in the Transantarctic Mountains, and to the extension of known ranges of some species in the Transantarctic Mountains. Additional samples not yet examined include a collection from the Cumulus Hills, Shackleton Glacier area, made by the Institute of Polar Studies expedition during the 1977-78 season. This palynological study has enabled relatively precise correlation with the closely similar palynologic succession of eastern Australia (Kyle, in press; Kyle and Schopf, in press) and therefore has allowed finer age control of Victoria Group strata than previously was possible. It has also resulted in 'Formerly Rosemary A. Kyle. 2Deceased.
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