Geochemistry of granites and metamorphic rocks
December 1985. The research was supported by National Science Foundation grant DPP 84-18445.
Qm
References Askin, R.A. 1987. Personal communication. Barrett, P.J., G.W. Grindley, and P.N. Webb. 1972. The Beacon Supergroup of East Antarctica. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. Berner, R.A. 1986. Personal communication. Coates, D.A. 1985. Late Paleozoic glacial patterns in the Central Transantarctic Mountains, Antarctica. Antarctic Research Series, 36(13), 325-338.
Collinson, J.C. In Preparation. The paleo-Pacific margin as seen from East Antarctica. Fifth International Symposium on Antarctic Earth Sciences.
Lt Figure 3. Triangular diagram showing petrographic composition of Pagoda Formation sandstones ("ss," N = 54) and diamictites ("dmct," N 21). ("Om" denotes monocrystalline quartz?' "F" denotes "feldspar?' "Lt" denotes "lithic fragments," including polycrystalline quartz.)
In summary, the terrestrial glacial facies assemblage, paleoice flow directions, and provenance data for the Pagoda Formation indicate deposition in a stable cratonic region distant from the paleo-Pacific margin of East Antarctica. There is no evidence for an active continental margin in the vicinity of the present central Transantarctic Mountains during Permo-Carboniferous time. These data, therefore, lend support to reconstructions of the Pacific margin of Gondwana which place microcontinental blocks, now in West Antarctica and New Zealand, adjacent to the present continental outline of East Antarctica (Daiziel and Elliot 1982). This study is based on data collected from 17 Pagoda sections in the Beardmore Glacier area visited during November and
Geochemistry of granites and metamorphic rocks: Central Transantarctic Mountains S.G. BORG, J.W. GOODGE, V.C. BENNETT, and D.J. DEPAOLO Department of Earth and Space Sciences University of California Los Angeles, California 90024
B.K. SMITH Department of Geology Arizona State University Tempe, Arizona 85287
1987 REVIEW
Dalziel,I.W.D., and D.H. Elliot. 1982. West Antarctica: Problem child of Gondwanaland. Tectonics, 1, 3-19. Dickinson, W.R., L.S. Beard, G.R. Brakenridge, J.L. Erjavec, R.C. Ferguson, K.F. Inman, R.A. Krepp, EA. Lindberg, and P.T. Ryberg. 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Geological Society of America Bulletin, 94,
222-235.
Elliot, D.H. 1975. Gondwana basins in Antarctica. In K.S.W. Campbell (Ed.), Gondwana geology, Canberra, Australia: National University Press. Frakes, L.A., J.L. Matthews, and J.C. Crowell. 1971. Late Paleozoic glaciation: Part Ill, Antarctica. Geological Society of America Bulletin, 82, 1581-1584. Lindsay, J.F. 1968. Stratgraphy and sedimentation of the lower Beacon rocks of
the Queen Alexandra, Queen Elizabeth, and Holland Ranges, Antarctica, with emphasis on Paleozoic glaciation. (Ph.D. Dissertation, Ohio State
University.) Lindsay, J.F. 1970. Depositional environment of Paleozoic glacial rocks in the Central Transantarctic Mountains. Geological Society of America Bulletin, 83, 1149-1172.
Miller, J.M.G. In preparation. Glacial advance and retreat sequences in a Permo-Carboniferous section, central Transantarctic Mountains. Miller, 1MG., and B.J. Waugh. 1986. Sedimentology of the Pagoda Formation (Permian), Beardmore Glacier area. Antarctic Journal of the U.S., 21(5), 45-46.
This note summarizes our work on the late Precambrian to early Paleozoic basement of the central Transantarctic Mountains from 1 June 1986 through 1 June 1987 and is intended to supplement two reports published in the 1986 review issue of the Antarctic Journal (Borg et al. 1986; Borg and DePaolo 1986). Current results of specific aspects of this work are available in three abstracts (Borg and DePaolo 1987a, 1987b; and Goodge and Borg 1987). Field program. Geologic mapping and sampling of granitic and metamorphic rocks was done in the Gabbro Hills in November 1986. Early Paleozoic plutonic rocks occupy a large proportion of the region (approximately 80-90 percent) and are mainly diorite, tonalite, and granodiorite with only one gabbroic pluton (compare with McGregor 1965). December was spent completing our mapping and sampling of the Miller Range which was begun in 1985-1986. Work on the structural geology of the Miller Formation and the Nimrod Group has confirmed and elaborated on our findings described last year (Borg et al. 1986). 21
Laboratory program. The laboratory program is directed toward using the isotopic and chemical compositions of granitic and metamorphic rocks to elucidate the tectonomagmatic evolution of the east antarctic craton (EAc) margin in Early Paleozoic time. This work has already yielded results that substantially modify previous models. Figure 1 is a neodymium evolution diagram showing 143neodymium/'neodumium in epsilon notation (€Nd) for granitic and metamorphic rocks calculated at the time of granite emplacement, approximately 500 million years ago. These data demonstrate that the Miller Formation is composed of Archean material. This was not clear from previous geochronological studies. Our data, when combined with structural relations, suggest that an Archean portion of the EAC is nearby under the polar ice sheet and strongly suggest that the bulk of the EAC may be of Archean age. The late Precambrian geoclinal turbidites (Goldie Formation) have isotopic compositions that require a sedimentary source other than the exposed preexisting basement rocks. This observation is important for reconstructions of the Paleozoic tectonics. Proterozoic basement (approximately 1.8-2.0 billion years old) is inferred to be present in the Miller Range on the basis of the isotopic compositions of the peraluminous granites. Metaluminous granites east
10
ENd()
0
0
Archean crust
Met-Al GHI o Per-AlGHI • Goldie Fm • Miller Fm D
-30 .. 0.0 1.0 2.0 3.0 Age (G a) Figure 1. Neodymium evolution diagram for granites of the central Transantarctic Mountains and related metamorphic rocks. ("Met-Al GHI" denotes "metaluminous Granite Harbor Intrusives." "Per-Al GHI" denotes "peraluminous Granite Harbor Intrusives." "Fm" denotes "formation:' END(i)denotes 143neodymium/1 neodymium composition in epsilon notation. "Ga" denotes "giga annum" or billion years.)
, Ross Ice Shelf
Mesozoic Ferrar Group and MesozoicPermian Beacon Super Group
/
7O°E
,
Cambrian Granite Harbour Intrusives Cambrian Byrd Group
ennox-King Glacier
Precambrian Beardmore Group Precambrian Nimrod Group Argosy Formation Miller Formation
East Antarctic Ice Sheet
( 5
f__I \;s 1,.
(y
\
o 25 50 75 100 I. I
kilometers
+
Axel !iIHeiberg Glacier
Figure 2. Geologic sketch map of the central Transantarctic Mountains. Sample locations are shown by circles. Section line A-B is perpendicular to structural trends of the Ross Orogeny and relates to figure 3.
22
ANTARCTIC JOURNAL
of the Marsh Glacier are inferred to be products of mixing between a depleted-mantle component and a crustal component. An important feature of the data is the pattern of variation over the region. Figure 2 shows the study area, locations of analyzed samples, and a cross-section line perpendicular to structural trends of the Ross Orogeny. Figure 3 shows ENd of several rock types (calculated for crystallization ages of 500 million years) projected onto the section line. Three crustal elements or "terranes" are indicated. The oldest is the Miller Formation, which is composed of Archean material but is confined to the upper plate of a major east-directed thrust zone in the Miller Range. This material was not involved in formation of the granites. A second terrane is crystalline crust of Proterozoic age located between the thrust zone and the Marsh Glacier. It is identified by the presence of peraluminous granites (wholly crustal melts) in the Miller Range, and its neodymium model age is fixed by the isotopic compositions of these granites (figure 1). The third crustal element, defined by the metaluminous granites in the axis and the eastern side of the Transantarctic Mountains, is characterized by continuous variation of initial ENd of the granites from ENd equals approximately -8 to 2 across the range. These granites are mixtures of mantle-derived magma and a Precambrian component similar to that which produced the peraluminous granites of the Miller Range. It is inferred that this region is underlain by thinned Proterozoic crust. The pattern of isotopic variation in the metaluminous granites is compatible with westward subduction of oceanic crust beneath the EAC at the time of granite genesis. The discon-
EP
El
El
t ---
ENdW -20
-30
Marsh Glacier
0
A
o Met-Al GHI o Per-Al GHI • Goldie Fm • Miller Fm
100 200 300 kilometers
B
Figure 3. Neodymium composition ( 143neodymium/1 neodymium in epsilon notation, N) at 500 million years ago plotted against distance across the Transantarctic Mountains. ("Met-Al GHI" denotes "metaluminous Granite Harbor Intrusives." "Per-Al GHI" denotes "peraluminous Granite Harbor Intrusives." "Fm" denotes "formation.")
1987 REVIEW
tinuity in the isotopic and chemical compositions of the granites at the Marsh Glacier is interpreted as a transition between thinned and normal-thickness crust. These data have important implications for the early Paleozoic tectonic evolution of this segment of Antarctica and are also important in defining the age, nature, and extent of the Precambrian basement of Antarctica. Our continuing analytical work is aimed at broadening the geographic coverage of the isotopic variations in the granitic rocks and better defining the isotopic signature of other metamorphic rocks of the region. The pattern of isotopic variation we are finding in the central Transantarctic Mountains is an important feature of the basement which we believe may be traceable northward. These isotopic "markers" will be valuable in working out the structural development of the Transantarctic Mountains. We would like to thank VXE-6, National Science Foundation, Polar Operations and Antarctic Services, Inc., for their efforts in support of our field work. This research was supported by National Science Foundation grant DPP 83-16807.
References Borg, S.C., and D.J. DePaolo. 1986. Geochemical investigations of lower Paleozoic granites of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 41-43. Borg, S.C., and D.J. DePaolo. 1987a. Isotopic studies of granites in the central Transantarctic Mountains (Abstract). LOS, 68,442. Borg, S.C., and D.J. DePaolo. 1987b. Isotopic studies of lower Paleozoic granites of the central Transantarctic Mountains (Abstract). Fifth Antarctic Earth Sciences Symposium, Cambridge, U.K., August, 1987. Borg, S.C., J.W.Goodge, D.J. DePaolo, and J.M. Mattinson. 1986. Field studies of granites and metamorphic rocks: Central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 43-45. Coodge, J.W,, and S.C. Borg. 1987. Metamorphism and crustal structure in the Miller Range, central Transantarctic Mountains (Abstract). Fifth Antarctic Earth Sciences Symposium, Cambridge, U.K., August, 1987. McGregor, V. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Range, Antarctica: Part 1—Basement complex, structure, and glacial geology. New Zealand Journal of Geology and Geophysics, 8,314-343. Borg, S.C., J.W.Goodge, D.J. DePaolo, and J.M. Mattinson. 1986. Field studies of granites and metamorphic rocks: Central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 43-45. Goodge, J.W., and S.C. Borg. 1987. Metamorphism and crustal structure in the Miller Range, central Transantarctic Mountains (Abstract). Fifth Antarctic Earth Sciences Symposium, Cambridge, U.K., August, 1987. McGregor, V.R. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Range, Antarctica: Part 1—Basement complex, structure, and glacial geology. New Zealand Journal of Geology and Geophysics, 8,314-343.
23
Qm
References Askin, R.A. 1987. Personal communication. Barrett, P.J., G.W. Grindley, and P.N. Webb. 1972. The Beacon Supergroup of East Antarctica. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. Berner, R.A. 1986. Personal communication. Coates, D.A. 1985. Late Paleozoic glacial patterns in the Central Transantarctic Mountains, Antarctica. Antarctic Research Series, 36(13), 325-338.
Collinson, J.C. In Preparation. The paleo-Pacific margin as seen from East Antarctica. Fifth International Symposium on Antarctic Earth Sciences.
Lt Figure 3. Triangular diagram showing petrographic composition of Pagoda Formation sandstones ("ss," N = 54) and diamictites ("dmct," N 21). ("Om" denotes monocrystalline quartz?' "F" denotes "feldspar?' "Lt" denotes "lithic fragments," including polycrystalline quartz.)
In summary, the terrestrial glacial facies assemblage, paleoice flow directions, and provenance data for the Pagoda Formation indicate deposition in a stable cratonic region distant from the paleo-Pacific margin of East Antarctica. There is no evidence for an active continental margin in the vicinity of the present central Transantarctic Mountains during Permo-Carboniferous time. These data, therefore, lend support to reconstructions of the Pacific margin of Gondwana which place microcontinental blocks, now in West Antarctica and New Zealand, adjacent to the present continental outline of East Antarctica (Daiziel and Elliot 1982). This study is based on data collected from 17 Pagoda sections in the Beardmore Glacier area visited during November and
Geochemistry of granites and metamorphic rocks: Central Transantarctic Mountains S.G. BORG, J.W. GOODGE, V.C. BENNETT, and D.J. DEPAOLO Department of Earth and Space Sciences University of California Los Angeles, California 90024
B.K. SMITH Department of Geology Arizona State University Tempe, Arizona 85287
1987 REVIEW
Dalziel,I.W.D., and D.H. Elliot. 1982. West Antarctica: Problem child of Gondwanaland. Tectonics, 1, 3-19. Dickinson, W.R., L.S. Beard, G.R. Brakenridge, J.L. Erjavec, R.C. Ferguson, K.F. Inman, R.A. Krepp, EA. Lindberg, and P.T. Ryberg. 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Geological Society of America Bulletin, 94,
222-235.
Elliot, D.H. 1975. Gondwana basins in Antarctica. In K.S.W. Campbell (Ed.), Gondwana geology, Canberra, Australia: National University Press. Frakes, L.A., J.L. Matthews, and J.C. Crowell. 1971. Late Paleozoic glaciation: Part Ill, Antarctica. Geological Society of America Bulletin, 82, 1581-1584. Lindsay, J.F. 1968. Stratgraphy and sedimentation of the lower Beacon rocks of
the Queen Alexandra, Queen Elizabeth, and Holland Ranges, Antarctica, with emphasis on Paleozoic glaciation. (Ph.D. Dissertation, Ohio State
University.) Lindsay, J.F. 1970. Depositional environment of Paleozoic glacial rocks in the Central Transantarctic Mountains. Geological Society of America Bulletin, 83, 1149-1172.
Miller, J.M.G. In preparation. Glacial advance and retreat sequences in a Permo-Carboniferous section, central Transantarctic Mountains. Miller, 1MG., and B.J. Waugh. 1986. Sedimentology of the Pagoda Formation (Permian), Beardmore Glacier area. Antarctic Journal of the U.S., 21(5), 45-46.
This note summarizes our work on the late Precambrian to early Paleozoic basement of the central Transantarctic Mountains from 1 June 1986 through 1 June 1987 and is intended to supplement two reports published in the 1986 review issue of the Antarctic Journal (Borg et al. 1986; Borg and DePaolo 1986). Current results of specific aspects of this work are available in three abstracts (Borg and DePaolo 1987a, 1987b; and Goodge and Borg 1987). Field program. Geologic mapping and sampling of granitic and metamorphic rocks was done in the Gabbro Hills in November 1986. Early Paleozoic plutonic rocks occupy a large proportion of the region (approximately 80-90 percent) and are mainly diorite, tonalite, and granodiorite with only one gabbroic pluton (compare with McGregor 1965). December was spent completing our mapping and sampling of the Miller Range which was begun in 1985-1986. Work on the structural geology of the Miller Formation and the Nimrod Group has confirmed and elaborated on our findings described last year (Borg et al. 1986). 21
Laboratory program. The laboratory program is directed toward using the isotopic and chemical compositions of granitic and metamorphic rocks to elucidate the tectonomagmatic evolution of the east antarctic craton (EAc) margin in Early Paleozoic time. This work has already yielded results that substantially modify previous models. Figure 1 is a neodymium evolution diagram showing 143neodymium/'neodumium in epsilon notation (€Nd) for granitic and metamorphic rocks calculated at the time of granite emplacement, approximately 500 million years ago. These data demonstrate that the Miller Formation is composed of Archean material. This was not clear from previous geochronological studies. Our data, when combined with structural relations, suggest that an Archean portion of the EAC is nearby under the polar ice sheet and strongly suggest that the bulk of the EAC may be of Archean age. The late Precambrian geoclinal turbidites (Goldie Formation) have isotopic compositions that require a sedimentary source other than the exposed preexisting basement rocks. This observation is important for reconstructions of the Paleozoic tectonics. Proterozoic basement (approximately 1.8-2.0 billion years old) is inferred to be present in the Miller Range on the basis of the isotopic compositions of the peraluminous granites. Metaluminous granites east
10
ENd()
0
0
Archean crust
Met-Al GHI o Per-AlGHI • Goldie Fm • Miller Fm D
-30 .. 0.0 1.0 2.0 3.0 Age (G a) Figure 1. Neodymium evolution diagram for granites of the central Transantarctic Mountains and related metamorphic rocks. ("Met-Al GHI" denotes "metaluminous Granite Harbor Intrusives." "Per-Al GHI" denotes "peraluminous Granite Harbor Intrusives." "Fm" denotes "formation:' END(i)denotes 143neodymium/1 neodymium composition in epsilon notation. "Ga" denotes "giga annum" or billion years.)
, Ross Ice Shelf
Mesozoic Ferrar Group and MesozoicPermian Beacon Super Group
/
7O°E
,
Cambrian Granite Harbour Intrusives Cambrian Byrd Group
ennox-King Glacier
Precambrian Beardmore Group Precambrian Nimrod Group Argosy Formation Miller Formation
East Antarctic Ice Sheet
( 5
f__I \;s 1,.
(y
\
o 25 50 75 100 I. I
kilometers
+
Axel !iIHeiberg Glacier
Figure 2. Geologic sketch map of the central Transantarctic Mountains. Sample locations are shown by circles. Section line A-B is perpendicular to structural trends of the Ross Orogeny and relates to figure 3.
22
ANTARCTIC JOURNAL
of the Marsh Glacier are inferred to be products of mixing between a depleted-mantle component and a crustal component. An important feature of the data is the pattern of variation over the region. Figure 2 shows the study area, locations of analyzed samples, and a cross-section line perpendicular to structural trends of the Ross Orogeny. Figure 3 shows ENd of several rock types (calculated for crystallization ages of 500 million years) projected onto the section line. Three crustal elements or "terranes" are indicated. The oldest is the Miller Formation, which is composed of Archean material but is confined to the upper plate of a major east-directed thrust zone in the Miller Range. This material was not involved in formation of the granites. A second terrane is crystalline crust of Proterozoic age located between the thrust zone and the Marsh Glacier. It is identified by the presence of peraluminous granites (wholly crustal melts) in the Miller Range, and its neodymium model age is fixed by the isotopic compositions of these granites (figure 1). The third crustal element, defined by the metaluminous granites in the axis and the eastern side of the Transantarctic Mountains, is characterized by continuous variation of initial ENd of the granites from ENd equals approximately -8 to 2 across the range. These granites are mixtures of mantle-derived magma and a Precambrian component similar to that which produced the peraluminous granites of the Miller Range. It is inferred that this region is underlain by thinned Proterozoic crust. The pattern of isotopic variation in the metaluminous granites is compatible with westward subduction of oceanic crust beneath the EAC at the time of granite genesis. The discon-
EP
El
El
t ---
ENdW -20
-30
Marsh Glacier
0
A
o Met-Al GHI o Per-Al GHI • Goldie Fm • Miller Fm
100 200 300 kilometers
B
Figure 3. Neodymium composition ( 143neodymium/1 neodymium in epsilon notation, N) at 500 million years ago plotted against distance across the Transantarctic Mountains. ("Met-Al GHI" denotes "metaluminous Granite Harbor Intrusives." "Per-Al GHI" denotes "peraluminous Granite Harbor Intrusives." "Fm" denotes "formation.")
1987 REVIEW
tinuity in the isotopic and chemical compositions of the granites at the Marsh Glacier is interpreted as a transition between thinned and normal-thickness crust. These data have important implications for the early Paleozoic tectonic evolution of this segment of Antarctica and are also important in defining the age, nature, and extent of the Precambrian basement of Antarctica. Our continuing analytical work is aimed at broadening the geographic coverage of the isotopic variations in the granitic rocks and better defining the isotopic signature of other metamorphic rocks of the region. The pattern of isotopic variation we are finding in the central Transantarctic Mountains is an important feature of the basement which we believe may be traceable northward. These isotopic "markers" will be valuable in working out the structural development of the Transantarctic Mountains. We would like to thank VXE-6, National Science Foundation, Polar Operations and Antarctic Services, Inc., for their efforts in support of our field work. This research was supported by National Science Foundation grant DPP 83-16807.
References Borg, S.C., and D.J. DePaolo. 1986. Geochemical investigations of lower Paleozoic granites of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 41-43. Borg, S.C., and D.J. DePaolo. 1987a. Isotopic studies of granites in the central Transantarctic Mountains (Abstract). LOS, 68,442. Borg, S.C., and D.J. DePaolo. 1987b. Isotopic studies of lower Paleozoic granites of the central Transantarctic Mountains (Abstract). Fifth Antarctic Earth Sciences Symposium, Cambridge, U.K., August, 1987. Borg, S.C., J.W.Goodge, D.J. DePaolo, and J.M. Mattinson. 1986. Field studies of granites and metamorphic rocks: Central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 43-45. Coodge, J.W,, and S.C. Borg. 1987. Metamorphism and crustal structure in the Miller Range, central Transantarctic Mountains (Abstract). Fifth Antarctic Earth Sciences Symposium, Cambridge, U.K., August, 1987. McGregor, V. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Range, Antarctica: Part 1—Basement complex, structure, and glacial geology. New Zealand Journal of Geology and Geophysics, 8,314-343. Borg, S.C., J.W.Goodge, D.J. DePaolo, and J.M. Mattinson. 1986. Field studies of granites and metamorphic rocks: Central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 43-45. Goodge, J.W., and S.C. Borg. 1987. Metamorphism and crustal structure in the Miller Range, central Transantarctic Mountains (Abstract). Fifth Antarctic Earth Sciences Symposium, Cambridge, U.K., August, 1987. McGregor, V.R. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Range, Antarctica: Part 1—Basement complex, structure, and glacial geology. New Zealand Journal of Geology and Geophysics, 8,314-343.
23
