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Age of the Vanda porphyry dikes in Wright Valley, southern Victoria Land, Antarctica
EARTH AND PLANETARY SCIENCE LETTERS 3 (1967) 321-324. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM
AGE OF THE VANDA PORPHYRY DIKES IN WRIGHT VALLEY, SOUTHERN VICTORIA LAND, ANTARCTICA Lois M. JONES and G. FAURE
Department of Geology and Institute of Polar Studies, The Ohio State University, Columbus, Ohio 4.3210. USA Received 17 November 1967
Samples of the Vanda porphyry dikes from Wright Valley, Antarctica, have an age of 470 ± 7 m.y . by the Rb-Sr method. This age is consistent with field evidence . The age of 1000 m.y . obtained for these dikes by earlier investigators is interpreted to be a result of contamination with radiogenic strontium.
Wright Valley is one of several eastwest trending, ice-free valleys situated west of McMurdo Sound in southern Victoria Land . The geology of this region has been described by several investigators (1-51 . The following is a summary of the geology of Wright Valley, which is illustrated in table 1 . The basement complex consists of more than 5000 meters of tightly-folded metamorphosed sediments (Asgard Formation) and a variety of igneous rocks, including the Olympus granite-gneiss, the Dais granite, and the Vida granite. According to the work of McKelvey and Webb [2], the Asgard Formation is flanked on the east and west by the Olympus granite-gneiss. Although the contact is generally gradational, it is thought to be intrusive because inclusions of the Asgard schist occur within the Olympus gneiss in the contact zone. The foliation of the gneiss and the alignment of the inclusions are parallel to the schistosity of the Asgard Formation. The Olympus granite-gneiss is flanked in turn by the Dais granite whose relationship to the Olympus gneiss is not clear. The Vida granite intrudes the Dais and Olympus granites. These rocks are cut by swarms of the Vanda dikes consisting of lamprophyre and acid porphyry . Field relations in Wright Valley 121 indicate that the * Laboratory for Isotope Geology and Geochemistry Contribution No. 2.
porphyry dikes cut the lamprophyre dikes and therefore are presumed to be the youngest igneous rocks of the basement complex. The basement rocks were later uplifted and eroded to form the Kukri Peneplain. A thick sedimentary sequence (Beacon Group) was deposited unconformably during Devonian to mid-Mesozoic time [6] . The basement complex and the Beacon Group were intruded by the Ferrar Dolerite dikes and sills during the Jurassic period [7] . Finally, volcanic activity accompanied by extrusion of basalt occurred in late Cenozoic time [5] . Age determinations of rocks from the Basement complex of southern Victoria Land have been reported by several investigators [8-111 . In general, dates by the K-Ar and Rb-Sr methods on biotite range from 340 to 520 m.y. Recently Deutsch and Webb [ 12] published Rb-Sr age determinations of biotite and feldspar extracted from the Asgard Formation, the Olympus and Dais granites, and several other granitic intrusives of the region . The dates range from about 340 to 535 m.y., with a concentration of dates in the interval of 480-495 m.y. In addition, they analyzed in duplicate one total rock specimen as well as separated feldspar and biotite-hornblende fractions from one porphyry dike. Assuming an initial 875r/ 86Sr ratio of 0.709 ± 0.004, they calculated average ages of 942 ± 80 and 1000 ± 80 m.y . for the feldspar and total rock samples, respectively . The biotite-
L. ßy4. JONES and G. FAURE
322
Table 1 Table of formations of the Wright Valley area, Antarctica (after McKelvey and Webb Age
Group
121).
Formation
Quaternary Upper Triassic
McMurdo Volcanics Victoria Orogeny
Cretaceous Jurassic Mid-Mf.:sozoic (Jurassic) Mid-Paleozoic (Devonian)
Ferrar Dolerites
Beacon Sandstone
Kukri Peneplain Lower Paleozoic
Victoria lntrusives
Vanda Porphyry and Lamprophyre Vida Granite Theseus Granodiorite Loke Microdiorite Dais Granite Olympus Granite-gneiss
Wright Intrusives
Upper Cambrian
hornblende fraction indicated an age of 477 ± 15 m.y. 1This porphyry sample originated from a dike which cuts the Olympus granite-gneiss for which the same authors obtained biotite Rb-Sr ages of 481 ± 15 and 494 ± 15 m.y. The ages obtained for the porphyry dike and Olympus gneiss are clearly inconsistent with the field relationships. Deutsch and Grögler [111, using the U-Pb method, dated zircons from the Olympus granite-gneiss exposed in Victoria Valley, immediately to the north of :aright Vallley. These data, when plotted on a concordant diagram, indicate an age of 610 m.y. Although thin. age is significantly older than the Rb-Sr ages for the equivalient rock in Wright Valley, the discrepancy i)etween the apparent ages of the porphyry dike and the Olympus granite-gneiss still exists. We have dated two specimens of porphyry and ,heir potassium feldspar concentrates by the Rb-Sr method. These porphyry specimens are assumed to
ô
U E
Ross Orogeny Cambrian Precambrian
â
m Asgard Formation
be cogene-tic with the porphyry dated by Deutsch and Webb, as only one intrusive phase of these dikes has been identified [2] . The specimen dated by Deutsch and Webb originated from a dike approximately 4.8 km east of Bull Pass in Wright Valley . Our specimens were collected near the terminus of the Meserve Glacier approximately 6 km to the southeast from dikes which intrude the Asgard Formation. Here the porphyry dikes have an average width of 3 to 4 meters. The specimens analyzed by us were obtained as grab samples and could not be identified with any specific diike They are composed of phenocrysts of potassium feldspar (up to 1 cm) and minor amounts of quartz, hornblende, and biotite in a micro-crystalline matrix . The proportion of matrix to phenocryst is approximately 55-70% by volume . The analytical data are given in table 2 and are plotted in fig. 1 . On a strontium isochron plot, the total-rock and feldspar points establish an excellent isochron yielding
VANDA PORPHYRY DIKES IN WRIGHT VALLEY
323
Table 2
Rb/Sr analytical results for Vanda porphyry, Wright Valley .
TR = total rock, F = K-feldspar. Rb and Sr concentrations were measured by isotopic dilution using spikes enriched in 87 Rb and 86 Sr, respectively. Sample 238-TR 238-F 239-TR 239-F Eimer and Amend SrC03 isotope standard (14 measurements)
Rb (ppm)
Sr (ppm)
(87 Sr/86 Sr) *corr .
87Rb/86Sr
218.1 312.1 200.2 382.3
127.7 153.0 158.8 153.2
0.7443 0.7510 0.7358 0.7593
4 .96 5.92 3 .66 7 .26
0.7084 ± 0.0002
* Corrected for isotopic fractionation, assuming 87 Sr/86 Sr = 0.1194.
an age of 47017 m.y. (fig. 1). The initial 87 Sr/ 86 Sr ratio for this suite of rocks is 0.7119 ± 0.0006. Since the initial ratios of the two whole rocks and their ' feldspars are identical, we conclude that no redistribution of radiogenic strontium has occurred in these rocks since crystallization. Therefore, these results indicate that the porphyry dikes crystallized during the
Ordovician Period and have not been disturbed since. This age obtained for the Vanda porphyry dikes is significantly lower than the zircon age of the Olympus granite-gneiss [I 1 I and therefore is consistent with the field relationships in contrast to the 1000 m.y. date of Deutsch and Webb . Further evidence for an early Paleozoic age of the porphyry dikes can be de-
0.800
Sr% S
r
0 .750
0.700 Fig . 1 . Isochron plot of Rb-Sr data of Vanda porphyry dikes from Wright Valley, Antarctica . A87Rb - 1 .39 X 10 -11 yr - 1 .
324
L. M. JONES and G. FAURE
rived iva their relationship with the Vanda lamprophyre dikes which they cut [2] . Biotites from two samples oflamprophyre dikes from Taylor Valley, which lies immediately to the south of Wright Valley, give ages of 45$ m.y. [91 and 520 m.y. [101 by the K-Ar method. We have no explanation for the discrepancy of these two dates . The Precambrian date reported by Deutsch and Webb can be re-evaluated by plotting their data on an isochrcm diagram, also shown in fig. 1 . Their Precambrian elates, which ranged from 931 to 1030 m.y., are represKmted by a fan of isochrons, each established by the sample and their assumed initial g 7Sr/g 6 Sr ratio. However, when the biotite-hornblende concentrate from their porphyry is included in the array, a poorlydefined isochron may be drawn as shown by the upper solid line in fig. 1 . This isochron is compatible with an age of about 450 m.y. and suggests an initial g 7Sr/ 86 Sr ratio of about 0.764 for this rock . Deutsch and Webbb considered this possibility when they calculated that the initial g7 Sr/g6Sr ratio would have to be 0.76 in order to obtain an age of 500 m.y. for this rock. However, they regarded this value of the initial ratio as unusual and did not accept it. We suggest that the high initial g 7Sr/86 Sr ratio cf the porphyry of Deutsch and Wetb is due to contamination with radiogenic strontium. This contamination could have occurred (a) at the time of intrusion of the porphyry dike or (b) during the intrusion of the Ferrar Dolerites. The dolerite sill nearest to the porphyry sample lies about 100-130 m above the sample locality 1131 . The dolereites have beeh dated by the K-Ar method and yield ages in the range of 147-163 m .y . 171 . If the strontium of the porphyry dike had been homogenized isotopically by the intrusion of the dolerites, the biotite fraction should indicate an age of about 160 m.y. However, since the biotite-hornblc;nde concentrate gives an Ordovician date, the high initial 87 Sr/g6 Sr ratio was apparently incorporated into tlae rock at the time of emplacement and may be the reyult of contamination with radiogenic g7 Sr from the country rock . We conclude that C'ie Vanda }porphyry dikes in Wright Valley were intruded 47'.1 m.y . ago . This Ordovician date is consistent with the fi°:d relationships and previously determined ages of the basement complex.
The manuscript was reviewed by Drs. C. Bull and D. D. Elliott of the Institute of Polar Studies. This study was supported by NSF Grant No. GA 713. The porphyry samples dated in this study were generously provided by C. It. Carnein, who collected them during the austral summer of 1965-1966 .
References (11 P. N. Webb and B. C. McKelvey, Geological investigations in southern Victoria Land, Antarctica. Part 1 Geology of Victoria Dry Valley, N.Z . J . Geol . Geophys. 2 (1959) 120. (2) B. C. McKelvey and P. N. Webb, Geological investigations in southern Victoria Land, Antarctica . Part 3 Geology of Wright Valley, N.Z . J . Geol . Geophys. 5 (1962) 142. A. D. Allen and G. W. Gibson, Geological investigations in southern Victoria Land, Antarctica. Part 6 - Outline of the geology of the Victoria Valley region, N.Z . J. Geol . Geophys. 5 (1962) 234. [41 B. M. Gunn and G. Warren, Geology of Victoria Land between the Mawson and Mulock Glaciers, Antarctica, N.Z . Geol. Surv. Bull . No. 7 1 (1962) 157 p. T. R. Haskell, J . P. Kennett, W. M. Prebble, G. Smith and 1. A. G. Willis, The geology of the Middle and Lower Taylor Valley of South Victoria Land, Antarctica, Trans. Roy. Soc. N.Z ., Geology 2 (1965) 169. [61 P. N. Webb, Geological investigations in southern Victoria Land, Antarctica. Part 4 - Beacon Group of the Wright Valley and Taylor Glacier region, N.Z . J. Geol. Geophys. 6 (1963) 361. [71 1. McDougall, Potassium-argon age measurements on dolerites from Antarctica and South Africa, J . Geophys. Res. 68 (1963) 1535. [81 S. S. Goldich, A. O. Nier and A. L. Washburn, A40/K40 age of gneiss from McMurdo Sound, Antarctica, Trans. Am . Geophys. Union 39 (1958) 956. E. E. Angino, M. D. Turner and E. J. Zeller, Reconnaissance geology of lower Taylor Valley, Victoria Land, Antarctica, Bull. Geol . Soc. Am . 73 (1962) 155â . 1101 W. C. Pearn, E. E. Angino and D. Stewart, New isotopic age measurements from the McMurdo Sound area, Ant111 arctica, Nature 199 (1963) 685. 1 S. Deutsch and N. Gr6gler, Isotopic age of Olympus Granite-gneiss (Victoria Land - Antarctica), Earth Planet. Sci. Letters 1 (1966) 82. (121 S. Deutsch and P. N. Webb, Sr/Rb dating on basement rocks from Victoria Land ; evidence for a 1000 million year old event, Proceedings of the First International Symposium on Antarctic Geology, Cape Town 1963, ed . R. J. Adie (North-Holland, Amsterdam, 1964) p. 557. 1131 P, N. Webb, private communication (1967) .
AGE OF THE VANDA PORPHYRY DIKES IN WRIGHT VALLEY, SOUTHERN VICTORIA LAND, ANTARCTICA Lois M. JONES and G. FAURE
Department of Geology and Institute of Polar Studies, The Ohio State University, Columbus, Ohio 4.3210. USA Received 17 November 1967
Samples of the Vanda porphyry dikes from Wright Valley, Antarctica, have an age of 470 ± 7 m.y . by the Rb-Sr method. This age is consistent with field evidence . The age of 1000 m.y . obtained for these dikes by earlier investigators is interpreted to be a result of contamination with radiogenic strontium.
Wright Valley is one of several eastwest trending, ice-free valleys situated west of McMurdo Sound in southern Victoria Land . The geology of this region has been described by several investigators (1-51 . The following is a summary of the geology of Wright Valley, which is illustrated in table 1 . The basement complex consists of more than 5000 meters of tightly-folded metamorphosed sediments (Asgard Formation) and a variety of igneous rocks, including the Olympus granite-gneiss, the Dais granite, and the Vida granite. According to the work of McKelvey and Webb [2], the Asgard Formation is flanked on the east and west by the Olympus granite-gneiss. Although the contact is generally gradational, it is thought to be intrusive because inclusions of the Asgard schist occur within the Olympus gneiss in the contact zone. The foliation of the gneiss and the alignment of the inclusions are parallel to the schistosity of the Asgard Formation. The Olympus granite-gneiss is flanked in turn by the Dais granite whose relationship to the Olympus gneiss is not clear. The Vida granite intrudes the Dais and Olympus granites. These rocks are cut by swarms of the Vanda dikes consisting of lamprophyre and acid porphyry . Field relations in Wright Valley 121 indicate that the * Laboratory for Isotope Geology and Geochemistry Contribution No. 2.
porphyry dikes cut the lamprophyre dikes and therefore are presumed to be the youngest igneous rocks of the basement complex. The basement rocks were later uplifted and eroded to form the Kukri Peneplain. A thick sedimentary sequence (Beacon Group) was deposited unconformably during Devonian to mid-Mesozoic time [6] . The basement complex and the Beacon Group were intruded by the Ferrar Dolerite dikes and sills during the Jurassic period [7] . Finally, volcanic activity accompanied by extrusion of basalt occurred in late Cenozoic time [5] . Age determinations of rocks from the Basement complex of southern Victoria Land have been reported by several investigators [8-111 . In general, dates by the K-Ar and Rb-Sr methods on biotite range from 340 to 520 m.y. Recently Deutsch and Webb [ 12] published Rb-Sr age determinations of biotite and feldspar extracted from the Asgard Formation, the Olympus and Dais granites, and several other granitic intrusives of the region . The dates range from about 340 to 535 m.y., with a concentration of dates in the interval of 480-495 m.y. In addition, they analyzed in duplicate one total rock specimen as well as separated feldspar and biotite-hornblende fractions from one porphyry dike. Assuming an initial 875r/ 86Sr ratio of 0.709 ± 0.004, they calculated average ages of 942 ± 80 and 1000 ± 80 m.y . for the feldspar and total rock samples, respectively . The biotite-
L. ßy4. JONES and G. FAURE
322
Table 1 Table of formations of the Wright Valley area, Antarctica (after McKelvey and Webb Age
Group
121).
Formation
Quaternary Upper Triassic
McMurdo Volcanics Victoria Orogeny
Cretaceous Jurassic Mid-Mf.:sozoic (Jurassic) Mid-Paleozoic (Devonian)
Ferrar Dolerites
Beacon Sandstone
Kukri Peneplain Lower Paleozoic
Victoria lntrusives
Vanda Porphyry and Lamprophyre Vida Granite Theseus Granodiorite Loke Microdiorite Dais Granite Olympus Granite-gneiss
Wright Intrusives
Upper Cambrian
hornblende fraction indicated an age of 477 ± 15 m.y. 1This porphyry sample originated from a dike which cuts the Olympus granite-gneiss for which the same authors obtained biotite Rb-Sr ages of 481 ± 15 and 494 ± 15 m.y. The ages obtained for the porphyry dike and Olympus gneiss are clearly inconsistent with the field relationships. Deutsch and Grögler [111, using the U-Pb method, dated zircons from the Olympus granite-gneiss exposed in Victoria Valley, immediately to the north of :aright Vallley. These data, when plotted on a concordant diagram, indicate an age of 610 m.y. Although thin. age is significantly older than the Rb-Sr ages for the equivalient rock in Wright Valley, the discrepancy i)etween the apparent ages of the porphyry dike and the Olympus granite-gneiss still exists. We have dated two specimens of porphyry and ,heir potassium feldspar concentrates by the Rb-Sr method. These porphyry specimens are assumed to
ô
U E
Ross Orogeny Cambrian Precambrian
â
m Asgard Formation
be cogene-tic with the porphyry dated by Deutsch and Webb, as only one intrusive phase of these dikes has been identified [2] . The specimen dated by Deutsch and Webb originated from a dike approximately 4.8 km east of Bull Pass in Wright Valley . Our specimens were collected near the terminus of the Meserve Glacier approximately 6 km to the southeast from dikes which intrude the Asgard Formation. Here the porphyry dikes have an average width of 3 to 4 meters. The specimens analyzed by us were obtained as grab samples and could not be identified with any specific diike They are composed of phenocrysts of potassium feldspar (up to 1 cm) and minor amounts of quartz, hornblende, and biotite in a micro-crystalline matrix . The proportion of matrix to phenocryst is approximately 55-70% by volume . The analytical data are given in table 2 and are plotted in fig. 1 . On a strontium isochron plot, the total-rock and feldspar points establish an excellent isochron yielding
VANDA PORPHYRY DIKES IN WRIGHT VALLEY
323
Table 2
Rb/Sr analytical results for Vanda porphyry, Wright Valley .
TR = total rock, F = K-feldspar. Rb and Sr concentrations were measured by isotopic dilution using spikes enriched in 87 Rb and 86 Sr, respectively. Sample 238-TR 238-F 239-TR 239-F Eimer and Amend SrC03 isotope standard (14 measurements)
Rb (ppm)
Sr (ppm)
(87 Sr/86 Sr) *corr .
87Rb/86Sr
218.1 312.1 200.2 382.3
127.7 153.0 158.8 153.2
0.7443 0.7510 0.7358 0.7593
4 .96 5.92 3 .66 7 .26
0.7084 ± 0.0002
* Corrected for isotopic fractionation, assuming 87 Sr/86 Sr = 0.1194.
an age of 47017 m.y. (fig. 1). The initial 87 Sr/ 86 Sr ratio for this suite of rocks is 0.7119 ± 0.0006. Since the initial ratios of the two whole rocks and their ' feldspars are identical, we conclude that no redistribution of radiogenic strontium has occurred in these rocks since crystallization. Therefore, these results indicate that the porphyry dikes crystallized during the
Ordovician Period and have not been disturbed since. This age obtained for the Vanda porphyry dikes is significantly lower than the zircon age of the Olympus granite-gneiss [I 1 I and therefore is consistent with the field relationships in contrast to the 1000 m.y. date of Deutsch and Webb . Further evidence for an early Paleozoic age of the porphyry dikes can be de-
0.800
Sr% S
r
0 .750
0.700 Fig . 1 . Isochron plot of Rb-Sr data of Vanda porphyry dikes from Wright Valley, Antarctica . A87Rb - 1 .39 X 10 -11 yr - 1 .
324
L. M. JONES and G. FAURE
rived iva their relationship with the Vanda lamprophyre dikes which they cut [2] . Biotites from two samples oflamprophyre dikes from Taylor Valley, which lies immediately to the south of Wright Valley, give ages of 45$ m.y. [91 and 520 m.y. [101 by the K-Ar method. We have no explanation for the discrepancy of these two dates . The Precambrian date reported by Deutsch and Webb can be re-evaluated by plotting their data on an isochrcm diagram, also shown in fig. 1 . Their Precambrian elates, which ranged from 931 to 1030 m.y., are represKmted by a fan of isochrons, each established by the sample and their assumed initial g 7Sr/g 6 Sr ratio. However, when the biotite-hornblende concentrate from their porphyry is included in the array, a poorlydefined isochron may be drawn as shown by the upper solid line in fig. 1 . This isochron is compatible with an age of about 450 m.y. and suggests an initial g 7Sr/ 86 Sr ratio of about 0.764 for this rock . Deutsch and Webbb considered this possibility when they calculated that the initial g7 Sr/g6Sr ratio would have to be 0.76 in order to obtain an age of 500 m.y. for this rock. However, they regarded this value of the initial ratio as unusual and did not accept it. We suggest that the high initial g 7Sr/86 Sr ratio cf the porphyry of Deutsch and Wetb is due to contamination with radiogenic strontium. This contamination could have occurred (a) at the time of intrusion of the porphyry dike or (b) during the intrusion of the Ferrar Dolerites. The dolerite sill nearest to the porphyry sample lies about 100-130 m above the sample locality 1131 . The dolereites have beeh dated by the K-Ar method and yield ages in the range of 147-163 m .y . 171 . If the strontium of the porphyry dike had been homogenized isotopically by the intrusion of the dolerites, the biotite fraction should indicate an age of about 160 m.y. However, since the biotite-hornblc;nde concentrate gives an Ordovician date, the high initial 87 Sr/g6 Sr ratio was apparently incorporated into tlae rock at the time of emplacement and may be the reyult of contamination with radiogenic g7 Sr from the country rock . We conclude that C'ie Vanda }porphyry dikes in Wright Valley were intruded 47'.1 m.y . ago . This Ordovician date is consistent with the fi°:d relationships and previously determined ages of the basement complex.
The manuscript was reviewed by Drs. C. Bull and D. D. Elliott of the Institute of Polar Studies. This study was supported by NSF Grant No. GA 713. The porphyry samples dated in this study were generously provided by C. It. Carnein, who collected them during the austral summer of 1965-1966 .
References (11 P. N. Webb and B. C. McKelvey, Geological investigations in southern Victoria Land, Antarctica. Part 1 Geology of Victoria Dry Valley, N.Z . J . Geol . Geophys. 2 (1959) 120. (2) B. C. McKelvey and P. N. Webb, Geological investigations in southern Victoria Land, Antarctica . Part 3 Geology of Wright Valley, N.Z . J . Geol . Geophys. 5 (1962) 142. A. D. Allen and G. W. Gibson, Geological investigations in southern Victoria Land, Antarctica. Part 6 - Outline of the geology of the Victoria Valley region, N.Z . J. Geol . Geophys. 5 (1962) 234. [41 B. M. Gunn and G. Warren, Geology of Victoria Land between the Mawson and Mulock Glaciers, Antarctica, N.Z . Geol. Surv. Bull . No. 7 1 (1962) 157 p. T. R. Haskell, J . P. Kennett, W. M. Prebble, G. Smith and 1. A. G. Willis, The geology of the Middle and Lower Taylor Valley of South Victoria Land, Antarctica, Trans. Roy. Soc. N.Z ., Geology 2 (1965) 169. [61 P. N. Webb, Geological investigations in southern Victoria Land, Antarctica. Part 4 - Beacon Group of the Wright Valley and Taylor Glacier region, N.Z . J. Geol. Geophys. 6 (1963) 361. [71 1. McDougall, Potassium-argon age measurements on dolerites from Antarctica and South Africa, J . Geophys. Res. 68 (1963) 1535. [81 S. S. Goldich, A. O. Nier and A. L. Washburn, A40/K40 age of gneiss from McMurdo Sound, Antarctica, Trans. Am . Geophys. Union 39 (1958) 956. E. E. Angino, M. D. Turner and E. J. Zeller, Reconnaissance geology of lower Taylor Valley, Victoria Land, Antarctica, Bull. Geol . Soc. Am . 73 (1962) 155â . 1101 W. C. Pearn, E. E. Angino and D. Stewart, New isotopic age measurements from the McMurdo Sound area, Ant111 arctica, Nature 199 (1963) 685. 1 S. Deutsch and N. Gr6gler, Isotopic age of Olympus Granite-gneiss (Victoria Land - Antarctica), Earth Planet. Sci. Letters 1 (1966) 82. (121 S. Deutsch and P. N. Webb, Sr/Rb dating on basement rocks from Victoria Land ; evidence for a 1000 million year old event, Proceedings of the First International Symposium on Antarctic Geology, Cape Town 1963, ed . R. J. Adie (North-Holland, Amsterdam, 1964) p. 557. 1131 P, N. Webb, private communication (1967) .