Geochemical investigations of lower Paleozoic granites of the ...
Green, P.F. 1981. A new look at statistics in fission track dating. Nuclear Tracks, 5, 77-86. Grindley, G. W. 1967. The geomorphology of the Miller Range, Transan-
tarctic Mountains with notes on the glacial history of neotectonics of
East Antarctica. New Zealand Journal of Geology and Geophysics, 10(2), 557-598. Grindley, G.W., and M.G. Laird. 1969. Geology of the Shackleton Coast.
Geochemical investigations of lower Paleozoic granites of the Transantarctic Mountains S.C. BORG and
D.J. DEPAOLO
Department of Earth and Space Sciences University of California Los Angeles, California 90024
This project is part of a long-term effort to investigate the granites in the Transantarctic Mountains with modern geochemical techniques. Work during the 1985-1986 season resulted in a comprehensive collection of large samples of granite and associated country rock from the region between the Nimrod and Good glaciers (see Borg et al., Antarctic Journal, this issue). Our sampling program will be extended to the Gabbro Hills in late 1986. The primary objective of this project is to develop a modern tectonic-petrogenetic model of the Paleozoic batholith and related rocks of the Transantarctic Mountains. The approach we are using is to characterize the granitic rocks with respect to their petrology, field relations and geochemistry, and in particular, with respect to their neodymium, strontium, and oxygen isotopic compositions in as many key areas of the range as possible. A pilot study of uranium-lead systematics in accessory phases is also underway. The chemical and isotopic patterns will yield information about Precambrian crustal structure, place constraints on Precambrian continental drift, and elucidate the nature of Paleozoic continental margin tectonics and magmatism. The patterns will also be directly applicable to unraveling post-plutonism tectonics. This type of study, which we refer to as "chemical tectonics" or "isotope tectonics," is a powerful means of establishing the large-scale structural, tectonic and petrogenetic characteristics of subcontinental-scale areas even where detailed geologic mapping is unavailable. In this light it is particularly well suited to studies of the antarctic basement because of limited exposures and obvious difficulties with access. The utility of this type of study to regional geologic problems in Antarctica has recently been demonstrated in northern Victoria Land (Borg 1984; Borg et al. in press). Considering the success of this granite study and the extensive areas of unstudied granitic basement in the range (see figure 1 of Borg et al., Antarctic Journal, this issue), it is clear that a systematic investigation of granitic rocks throughout the range is likely to improve our understanding of the evolution of the antarctic basement rocks by a very large factor. This work will dovetail 1986 REVIEW
(Antarctic Map Folio Series, Sheet 15, Folio 12.) New York: American Geophysical Society. Hayes, D.E., L.A. Frakes et al. 1975. Initial reports of the Deep Sea Drilling Project. Washington, D.C.: U.S. Government Printing Office.
Hurford, A.J., P.F. Green.
1982. A users' guide to fission track dating calibration. Earth and Planetary Science Letters, 59, 343-354.
with similar types of studies being carried out by workers in several countries on rocks from other continents, and will eventually allow us to understand the relationship of the antarctic basement rocks to those of other continents back into Archean time, and further our understanding of the evolution of continental crust on a global scale through Earth history. Exposed within the pre-Devonian basement of the Transantarctic Mountains is an extensive complex of granitic batholiths which was emplaced into Precambrian and early Paleozoic metasedimentary and metavolcanic rocks of the Ross Orogen. Sufficient age determinations have been done to establish a general emplacement history, but there presently exist little data bearing on the genesis of the granitoids. It has been suggested that the batholiths were emplaced along a margin of the east antarctic craton primarily during the Cambro-Ordovician Ross Orogeny and that this magmatism may have been related to a subduction zone dipping under East Antarctica (Elliot 1975; Gunner 1976; Stump 1976). The segment of the range between the Nimrod and the Beardmore glaciers (figure 1) is fundamental to regional petrogenetic studies of granites because this is the only area in the range which, according to existing maps, appears to straddle the boundary between old Precambrian craton and late Precambrian geosynclinal sediments (Grindley et al., 1964; Grindley and McDougall, 1969; Gunner, 1976). Studies of granites in this region will form a substantial foundation but data from granites in other parts of the range are necessary to a comprehensive understanding of the origin of this plutonic province. Initial analytical work was begun on powdered samples provided by C. Faure from rocks studied by Gunnerand Faure (1972) and Gunner (1976). Samarium-neodymium (Sm-Nd) and rubidium-strontium (Rb-Sr) isotopic measurements made on these sample confirm that there are large differences in isotopic composition between the granites on the Shackleton Coast and those in the Miller Range (figures 2 and 3). From these data we know there are substantial differences in crustal structure or age. However, the isotopic variations cannot be adequately interpreted until we understand the nature of the variations on a broader geographic scale. The extensive regional coverage available in our collection is an important feature as we begin to focus our lab work under this proposal toward establishing patterns of chemical and isotopic variation of the granites over the whole region. Within the reconnaissance analytical program underway we hope to establish a pattern of regional variation in the batholithic rocks across the range, and we will be able to see if the difference between the two clusters of points on figures 2 and 3 represents an abrupt discontinuity or if there is continuous variation between them. It is also possible that there is some combination of variation and discontinuity present. Whatever 41
I
Ross Ice Shelf
Mesozoic Ferrar Group and MesozoicPermian Beacon Super Group Cambrian Granite Harbour Intrusives •::i Cambrian Byrd Group
Lennox-King Glacier
Precambrian(?) Beardmore Group
5lI
rj
Precambrian Nimrod Group 0 25 50 75 100 I I I kilometers
East Antarctic Ice Sheet Isotopic compositions at 500 (sample : [€Nd : 87Sr/86Sr])
r1\6.s \
Miller Range A 72 MR: [-11.4: 0.73481 B 297 MR [-10.5:0.73321 C 315MR:[-11.8:0.7327] Shackelton Coast D 593 SC: [-2.80 7086) E 647SC:[_2.9:0.7112] F 672SC:[-4.8:0.71091
+ 86
6 5
Axel !I3Heiberg Glacier
+
180•
')'66ss
Figure 1. Geologic sketch map of the study area. Samarium and neodymium isotopic compositions of granites in the Miller Range in the lower Beardmore Glacier area are shown. Sigma-notation given in DePaolo (1980). The location of figure 2 of Borg et al. (Antarctic Journal, this issue) is indicated with a finger.
the case, this information will allow us to develop and modify models for the tectonic evolution of this part of Antarctica, provide constraints on the origin of the granites themselves (cf. DePaolo 1980, 1981), and allow us to constrain the place of the Precambrian basement of the Transantarctic Mountains vis a vis the other cratons extant in Proterozoic time. This research was supported by National Science Foundation grant DPP 83-16807.
References Borg, S.C. 1984. Granitoids of northern Victoria Land, Antarctica. Ph.D. thesis, Tempe: Arizona State University.)
Borg, S.C., E. Stump, B.W. Chappell, M.T. McCulloch, D. Wyborn, J.R. Holloway, and R.L. Armstrong. In press. Granitoids of northern Victoria Land, Antarctica: Implications of chemical and isotopic varia42
tions to regional crustal structure and tectonics. American Journal of Science.
DePaolo, D.J. 1980. Sources of continental crust: Neodymium isotope evidence from the Sierra Nevada and Peninsular Ranges. Science, 209, 684-687. DePaolo, D. J . 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters, 53, 189-202. Elliot, D.H. 1975. Tectonics of Antarctica: A review. American Journal of Science, 275—A, 45-106. Grindley, G.W., and I. McDougall. 1969. Age and correlation of the Nimrod Group and other Precambrian rock units in the central Transantarctic Mountains, Antarctica. New Zealand Journal of Geology and Geophysics, 12, 391-411. Gunner, J. 1976. Isotopic and geochemical studies of the pre-Devonian basement complex, Beardmore Glacier region, Antarctica. (Institute of Polar Studies, Report 41, Columbus: Ohio State University.) Gunner, J . , and C. Faure. 1972. Rb-Sr geochronology of the Nimrod Group, central Transantarctic Mountains. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. ANTARCTIC JOURNAL
Stump, E. 1976. On the Late Precambrian-Early Paleozoic metavolcanic and metasedimentary rocks of the Queen Maud Mountains, Antarctica, and a comparison with rocks of similar age from southern Africa. (Ph.D. thesis, Columbus: Ohio State University.)
10 8
6 2
10
•
CTM Shockleton Coast
£ (TM Miller Range
NVL fidmiralty Intrusives J NVL Granite harbour Intrusives
0
Depleted mantle evolution band
8
O)
6
EN dit) 0
l+cmscoaiil
-2
.lflldMRGFllI
+
Ev olution trend Model age for continental 'for average crust in the Miller Range continental crustal Nd
-8 10 -12
: 0 100 200 300 400 500
Age (Ga)
Figure 2. This figure shows clearly the different isotopic signature of the granites on either side of the central Transantarctic Mountains (Miller Range [CTM MR] to the west and Shackleton Coast [CTM SC] to the east). The peraluminous chemistry of the granites in the Miller Range indicates that they are pure crustal melts and so their isotopic composition reflects the isotopic composition of the Precambrian crust in this area. The neodymium model ages (based on a depleted mantle) for these samples give an estimate of about 2.0 billion years for the average age of this Precambrian crust. The Shackleton Coast samples have considerably higher initial isotopic signature as well as model ages of about 1.5 billion years. These characteristics may be explained by mixing between mantle magmas and Precambrian crust similar to that which produced the Miller Range granites. However, we cannot rule out the possibility that the origin of the Shackleton Coast granites may involve a Precambrian component which is completely different from that represented by the granites in the Miller Range.
Field studies of granites and metamorphic rocks: Central Transantarctic Mountains S.C. BORG, J.W. GOODGE, and D.J. DEPAOLO Department of Earth and Space Sciences University of California Los Angeles, California 90024
J.M. MATTINSON Department of Geological Sciences University of California Santa Barbara, California 93106
This article summarizes field studies of the late Precambrian to early Paleozoic basement of the central Transantarctic Mountains (see Borg and DePaolo, Antarctic Journal, this issue). During the 1985-1986 season, we used helicopter support from the Beardmore Camp to conduct reconnaissance mapping and 1986 REVIEW
ESr(t) Figure 3. This figure allows a comparison of both samarium and neodymium compositions of the granites of the central Transantarctic Mountains with granites from northern Victoria Land (NvL). Most of the samples of Granite Harbor Intrusives from NVL. are peraluminous rocks, like those from the Miller Range, and probably represent pure crustal melts. As such, their isotopic composition reflects the isotopic composition of the lower crust in that region. From this data it appears that the Precambrian crust in the Miller Range is older than the Precambrian crust which melted to produce the Granite Harbor Intrusives in NVL. Also, the Miller Range granites involve crustal rocks which are much more rubidium-rich and have evolved considerably more radiogenic strontium. The Shackleton Coast granites are isotopically similar to the Admiralty Intrusives in NVL but, until we have filled out our coverage of isotopic ratios in the Transantarctic Mountains we cannot specify the precise nature of the Precambrian crust involved in producing them.
sampling in the segment of the range between the Nimrod and Good glaciers (figure 1). In addition, several areas were mapped in detail, including the Campbell Hills and Cape Lyttelton area, the Mount Hope and Cape Allen area, and a portion of the western side of the Miller Range. Our detailed geologic mapping represents a substantial improvement over published maps. A comparison of our maps with published geologic maps indicates clearly that much work is necessary to produce an accurate map of the basement rocks of the region. In the Campbell Hills and Cape Lyttleton area, over 50 percent of the area was incorrectly depicted in the American Geographical Society Map Folio series (Grindley and Laird 1969). Similarly, in the Mount Hope and Cape Allen area approximately 20 percent is incorrectly represented on the Mount Elizabeth and Mount Kathleen Geologic Quadrangle Map (Lindsay, Gunner, and Barrett 1973). We recognize that earlier work relied heavily on interpretation of aerial photographs and that ground information was often not available. However, because these maps are the basis for the geologic framework in which current work is founded, it is important to remind ourselves the extent to which this work represents inferences or extrapolated information. Granites. Batholithic rocks assigned to the lower Paleozoic Granite Harbor Intrusives in this region include a variety of lithologic types ranging from diorite to tourmaline-bearing, 243
tarctic Mountains with notes on the glacial history of neotectonics of
East Antarctica. New Zealand Journal of Geology and Geophysics, 10(2), 557-598. Grindley, G.W., and M.G. Laird. 1969. Geology of the Shackleton Coast.
Geochemical investigations of lower Paleozoic granites of the Transantarctic Mountains S.C. BORG and
D.J. DEPAOLO
Department of Earth and Space Sciences University of California Los Angeles, California 90024
This project is part of a long-term effort to investigate the granites in the Transantarctic Mountains with modern geochemical techniques. Work during the 1985-1986 season resulted in a comprehensive collection of large samples of granite and associated country rock from the region between the Nimrod and Good glaciers (see Borg et al., Antarctic Journal, this issue). Our sampling program will be extended to the Gabbro Hills in late 1986. The primary objective of this project is to develop a modern tectonic-petrogenetic model of the Paleozoic batholith and related rocks of the Transantarctic Mountains. The approach we are using is to characterize the granitic rocks with respect to their petrology, field relations and geochemistry, and in particular, with respect to their neodymium, strontium, and oxygen isotopic compositions in as many key areas of the range as possible. A pilot study of uranium-lead systematics in accessory phases is also underway. The chemical and isotopic patterns will yield information about Precambrian crustal structure, place constraints on Precambrian continental drift, and elucidate the nature of Paleozoic continental margin tectonics and magmatism. The patterns will also be directly applicable to unraveling post-plutonism tectonics. This type of study, which we refer to as "chemical tectonics" or "isotope tectonics," is a powerful means of establishing the large-scale structural, tectonic and petrogenetic characteristics of subcontinental-scale areas even where detailed geologic mapping is unavailable. In this light it is particularly well suited to studies of the antarctic basement because of limited exposures and obvious difficulties with access. The utility of this type of study to regional geologic problems in Antarctica has recently been demonstrated in northern Victoria Land (Borg 1984; Borg et al. in press). Considering the success of this granite study and the extensive areas of unstudied granitic basement in the range (see figure 1 of Borg et al., Antarctic Journal, this issue), it is clear that a systematic investigation of granitic rocks throughout the range is likely to improve our understanding of the evolution of the antarctic basement rocks by a very large factor. This work will dovetail 1986 REVIEW
(Antarctic Map Folio Series, Sheet 15, Folio 12.) New York: American Geophysical Society. Hayes, D.E., L.A. Frakes et al. 1975. Initial reports of the Deep Sea Drilling Project. Washington, D.C.: U.S. Government Printing Office.
Hurford, A.J., P.F. Green.
1982. A users' guide to fission track dating calibration. Earth and Planetary Science Letters, 59, 343-354.
with similar types of studies being carried out by workers in several countries on rocks from other continents, and will eventually allow us to understand the relationship of the antarctic basement rocks to those of other continents back into Archean time, and further our understanding of the evolution of continental crust on a global scale through Earth history. Exposed within the pre-Devonian basement of the Transantarctic Mountains is an extensive complex of granitic batholiths which was emplaced into Precambrian and early Paleozoic metasedimentary and metavolcanic rocks of the Ross Orogen. Sufficient age determinations have been done to establish a general emplacement history, but there presently exist little data bearing on the genesis of the granitoids. It has been suggested that the batholiths were emplaced along a margin of the east antarctic craton primarily during the Cambro-Ordovician Ross Orogeny and that this magmatism may have been related to a subduction zone dipping under East Antarctica (Elliot 1975; Gunner 1976; Stump 1976). The segment of the range between the Nimrod and the Beardmore glaciers (figure 1) is fundamental to regional petrogenetic studies of granites because this is the only area in the range which, according to existing maps, appears to straddle the boundary between old Precambrian craton and late Precambrian geosynclinal sediments (Grindley et al., 1964; Grindley and McDougall, 1969; Gunner, 1976). Studies of granites in this region will form a substantial foundation but data from granites in other parts of the range are necessary to a comprehensive understanding of the origin of this plutonic province. Initial analytical work was begun on powdered samples provided by C. Faure from rocks studied by Gunnerand Faure (1972) and Gunner (1976). Samarium-neodymium (Sm-Nd) and rubidium-strontium (Rb-Sr) isotopic measurements made on these sample confirm that there are large differences in isotopic composition between the granites on the Shackleton Coast and those in the Miller Range (figures 2 and 3). From these data we know there are substantial differences in crustal structure or age. However, the isotopic variations cannot be adequately interpreted until we understand the nature of the variations on a broader geographic scale. The extensive regional coverage available in our collection is an important feature as we begin to focus our lab work under this proposal toward establishing patterns of chemical and isotopic variation of the granites over the whole region. Within the reconnaissance analytical program underway we hope to establish a pattern of regional variation in the batholithic rocks across the range, and we will be able to see if the difference between the two clusters of points on figures 2 and 3 represents an abrupt discontinuity or if there is continuous variation between them. It is also possible that there is some combination of variation and discontinuity present. Whatever 41
I
Ross Ice Shelf
Mesozoic Ferrar Group and MesozoicPermian Beacon Super Group Cambrian Granite Harbour Intrusives •::i Cambrian Byrd Group
Lennox-King Glacier
Precambrian(?) Beardmore Group
5lI
rj
Precambrian Nimrod Group 0 25 50 75 100 I I I kilometers
East Antarctic Ice Sheet Isotopic compositions at 500 (sample : [€Nd : 87Sr/86Sr])
r1\6.s \
Miller Range A 72 MR: [-11.4: 0.73481 B 297 MR [-10.5:0.73321 C 315MR:[-11.8:0.7327] Shackelton Coast D 593 SC: [-2.80 7086) E 647SC:[_2.9:0.7112] F 672SC:[-4.8:0.71091
+ 86
6 5
Axel !I3Heiberg Glacier
+
180•
')'66ss
Figure 1. Geologic sketch map of the study area. Samarium and neodymium isotopic compositions of granites in the Miller Range in the lower Beardmore Glacier area are shown. Sigma-notation given in DePaolo (1980). The location of figure 2 of Borg et al. (Antarctic Journal, this issue) is indicated with a finger.
the case, this information will allow us to develop and modify models for the tectonic evolution of this part of Antarctica, provide constraints on the origin of the granites themselves (cf. DePaolo 1980, 1981), and allow us to constrain the place of the Precambrian basement of the Transantarctic Mountains vis a vis the other cratons extant in Proterozoic time. This research was supported by National Science Foundation grant DPP 83-16807.
References Borg, S.C. 1984. Granitoids of northern Victoria Land, Antarctica. Ph.D. thesis, Tempe: Arizona State University.)
Borg, S.C., E. Stump, B.W. Chappell, M.T. McCulloch, D. Wyborn, J.R. Holloway, and R.L. Armstrong. In press. Granitoids of northern Victoria Land, Antarctica: Implications of chemical and isotopic varia42
tions to regional crustal structure and tectonics. American Journal of Science.
DePaolo, D.J. 1980. Sources of continental crust: Neodymium isotope evidence from the Sierra Nevada and Peninsular Ranges. Science, 209, 684-687. DePaolo, D. J . 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters, 53, 189-202. Elliot, D.H. 1975. Tectonics of Antarctica: A review. American Journal of Science, 275—A, 45-106. Grindley, G.W., and I. McDougall. 1969. Age and correlation of the Nimrod Group and other Precambrian rock units in the central Transantarctic Mountains, Antarctica. New Zealand Journal of Geology and Geophysics, 12, 391-411. Gunner, J. 1976. Isotopic and geochemical studies of the pre-Devonian basement complex, Beardmore Glacier region, Antarctica. (Institute of Polar Studies, Report 41, Columbus: Ohio State University.) Gunner, J . , and C. Faure. 1972. Rb-Sr geochronology of the Nimrod Group, central Transantarctic Mountains. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. ANTARCTIC JOURNAL
Stump, E. 1976. On the Late Precambrian-Early Paleozoic metavolcanic and metasedimentary rocks of the Queen Maud Mountains, Antarctica, and a comparison with rocks of similar age from southern Africa. (Ph.D. thesis, Columbus: Ohio State University.)
10 8
6 2
10
•
CTM Shockleton Coast
£ (TM Miller Range
NVL fidmiralty Intrusives J NVL Granite harbour Intrusives
0
Depleted mantle evolution band
8
O)
6
EN dit) 0
l+cmscoaiil
-2
.lflldMRGFllI
+
Ev olution trend Model age for continental 'for average crust in the Miller Range continental crustal Nd
-8 10 -12
: 0 100 200 300 400 500
Age (Ga)
Figure 2. This figure shows clearly the different isotopic signature of the granites on either side of the central Transantarctic Mountains (Miller Range [CTM MR] to the west and Shackleton Coast [CTM SC] to the east). The peraluminous chemistry of the granites in the Miller Range indicates that they are pure crustal melts and so their isotopic composition reflects the isotopic composition of the Precambrian crust in this area. The neodymium model ages (based on a depleted mantle) for these samples give an estimate of about 2.0 billion years for the average age of this Precambrian crust. The Shackleton Coast samples have considerably higher initial isotopic signature as well as model ages of about 1.5 billion years. These characteristics may be explained by mixing between mantle magmas and Precambrian crust similar to that which produced the Miller Range granites. However, we cannot rule out the possibility that the origin of the Shackleton Coast granites may involve a Precambrian component which is completely different from that represented by the granites in the Miller Range.
Field studies of granites and metamorphic rocks: Central Transantarctic Mountains S.C. BORG, J.W. GOODGE, and D.J. DEPAOLO Department of Earth and Space Sciences University of California Los Angeles, California 90024
J.M. MATTINSON Department of Geological Sciences University of California Santa Barbara, California 93106
This article summarizes field studies of the late Precambrian to early Paleozoic basement of the central Transantarctic Mountains (see Borg and DePaolo, Antarctic Journal, this issue). During the 1985-1986 season, we used helicopter support from the Beardmore Camp to conduct reconnaissance mapping and 1986 REVIEW
ESr(t) Figure 3. This figure allows a comparison of both samarium and neodymium compositions of the granites of the central Transantarctic Mountains with granites from northern Victoria Land (NvL). Most of the samples of Granite Harbor Intrusives from NVL. are peraluminous rocks, like those from the Miller Range, and probably represent pure crustal melts. As such, their isotopic composition reflects the isotopic composition of the lower crust in that region. From this data it appears that the Precambrian crust in the Miller Range is older than the Precambrian crust which melted to produce the Granite Harbor Intrusives in NVL. Also, the Miller Range granites involve crustal rocks which are much more rubidium-rich and have evolved considerably more radiogenic strontium. The Shackleton Coast granites are isotopically similar to the Admiralty Intrusives in NVL but, until we have filled out our coverage of isotopic ratios in the Transantarctic Mountains we cannot specify the precise nature of the Precambrian crust involved in producing them.
sampling in the segment of the range between the Nimrod and Good glaciers (figure 1). In addition, several areas were mapped in detail, including the Campbell Hills and Cape Lyttelton area, the Mount Hope and Cape Allen area, and a portion of the western side of the Miller Range. Our detailed geologic mapping represents a substantial improvement over published maps. A comparison of our maps with published geologic maps indicates clearly that much work is necessary to produce an accurate map of the basement rocks of the region. In the Campbell Hills and Cape Lyttleton area, over 50 percent of the area was incorrectly depicted in the American Geographical Society Map Folio series (Grindley and Laird 1969). Similarly, in the Mount Hope and Cape Allen area approximately 20 percent is incorrectly represented on the Mount Elizabeth and Mount Kathleen Geologic Quadrangle Map (Lindsay, Gunner, and Barrett 1973). We recognize that earlier work relied heavily on interpretation of aerial photographs and that ground information was often not available. However, because these maps are the basis for the geologic framework in which current work is founded, it is important to remind ourselves the extent to which this work represents inferences or extrapolated information. Granites. Batholithic rocks assigned to the lower Paleozoic Granite Harbor Intrusives in this region include a variety of lithologic types ranging from diorite to tourmaline-bearing, 243
