Plagioclase compositional variations in anorthosites of ...
quartz tholeiite composition requires pressures of at least 8 to 9.5 kilobars for these temperatures (Green and Ringwood 1967). Newton (1983) suggested that cordierite would be stable in pelitic rocks at pressures less than 6.5 kilobars to 7 kilobars at 700 to 800°C. In summary, a maximum pressure of 7 kilobars is indicated for metamorphism of the Jetty Peninsula rocks. The stabilization of the garnet-clinopyroxene and olivine-plagioclase assemblages at pressures outside the ranges implied by Green and Ringwood's (1967) experiments is probably due to differences in bulk rock compositions between the Jetty Peninsula rocks and the rocks used in Green and Ringwood's (1967) experiments. Mineralogical features, notably the occurrence of graphite, aluminous minerals, and calcium-rich minerals suggest that the quartzofeldspathic gneisses and calc-silicate rocks have sedimentary precursors. The pyroxene granulites may be metabasalts. The ultramafic rocks are most likely derived from localized intrusives. The dominance of sedimentary precursors is consistent with Sheraton and Black's (1983) conclusion that the late Proterozoic gneisses of Mac. Robertson Land are largely derived from sedimentary protoliths. Samples of gneiss have been sent to K.D. Collerson (University of Regina, Saskatchewan) for whole-rock, trace-element, and samariumneodymium analyses, which should clarify the nature and ages of the precursors to the gneisses.
This research was supported by National Science Foundation grant DPP 84-14014 to the University of Maine.
Plagioclase compositional variations in anorthosites of the lower part of the Dufek intrusion
Anorthosite (and leucogabbro) also occur in large, rounded inclusions in gabbro in the Forrestal Range, but in this mode it does not show cumulus textures. Anorthosites of these types are common in layered mafic intrusions (Wager and Brown 1968), and their occurrences pose petrologic problems that were pointed out by Hess (1960) and still remain to be resolved. Czamanske and Sheidle (1985) provide a recent summary of origins proposed for such rocks. Anorthositic layers that show cycliclike stratigraphic repetition are particularly difficult to explain (Irvine, Keith, and Todd 1983). Earlier studies of plagioclase (Abel, Himmelberg, and Ford 1979) and other cumulus minerals (Himmelberg and Ford 1976, 1977) documented that their overall chemical variation and stratigraphic range in the Dufek intrusion are like those in other layered intrusions (Ford and Himmelberg in press) that have been interpreted in terms of fractional crystallization of tholeiitic magma and accumulation primarily from the base upward (Wager and Brown 1968). The studies suggested that small-scale reversals occur in the stratigraphic variation of mineral compositions in the vicinity of some anorthosite layers. The earlier studies were reconnaissances using widely spaced samples to determine overall variations. Except for the Walker Anorthosite, anorthosites were not included in the suite studied and therefore the origin of anorthosite layers was not addressed. We have begun detailed study of suites of samples spaced closely across several anorthosite layers and extending into gabbro above and below to document the nature of the mineral composition variations. We thus far have obtained plagioclase compositional data for the Walker Anorthosite and the lower anorthosite member and Spear Anorthosite Member of the
J.M. HAENSEL, JR. and G.R. HIMMELBERG Department of Geology University of Missouri Columbia, Missouri 65211
A. B. FORD U.S. Geological Survey Menlo Park, California 94025
The unusually large, differentiated Dufek intrusion (82°30'S 50°W) of Jurassic age consists dominantly of layered gabbro (plagioclase-pyroxene cumulate and plagioclase-pyroxenemagnetite cumulate). The stratigraphy and rock types are described by Ford (1976). Anorthosites occur throughout most exposed stratigraphic parts of the intrusion and in a variety of modes. Figure 1 shows the principal units of the rock. Most anorthosites are plagioclase cumulates in layers a few meters to a few tens of meters thick in the gabbro. In two units (Spear and Stephens anorthosite members, figure 1), they form cycliclike repeated layers, each with a sharp basal contact and most with a gradational contact with overlying gabbro. The much thicker Walker Anorthosite has a sharp contact with overlying gabbro. 1986 REVIEW
References
Green, D. H., and A. E. Ringwood. 1967. An experimental investigation of the gabbro to eclogite transformation and its petrological applications. Geochimica and Cosmochiinica Acta, 31, 767-833. Grew, E.S. 1985. Field studies on the Jetty Peninsula (Amery Ice Shelf area) with the Soviet Antarctic Expedition. Antarctic Journal of the U.S., 20(5), 52-53. Grew, E.S. 1986. An austral summer field season with the 30th Soviet Antarctic Expedition, 1984-1985. Antarctic Journal of the U.S., 21(1), 17-19. Loomis, T.P. 1976. Irreversible reactions in high-grade metapelitic rocks. Journal of Petrology, 17, 559-588. Newton, R.C. 1983. Geobarometry of high-grade metamorphic rocks. American Journal of Science, 283—A, 1-28. Ravich, MG., D.S. Soloviev, and L. Fedorov. 1978. Geological Structure of Mac. Robertson Land (East Antarctica). Leningrad: Gidrometeoizdat. (In Russian) Sheraton, J.W., and L.P. Black. 1983. Geochemistry of Precambrian gneisses. Relevance for the evolution of the East Antarctic Shield. Lithos, 16, 273-296. Tingey, R.J. 1982. The geologic evolution of the Prince Charles Mountains—An Antarctic Archean cratonic block. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press.
61
r --
I Eroded section
1500m
0 6. .0 .0 Co
LexmonGranophy;e
CD
0. 0 6.
Upper inclusion member
C 0 0
CD
..150m .
a) .0 0)
a)
(I)
000 1
CD
a)
0 6. .0 .0
0) C Co
CD
Co Cl)
0)
500m LL
0 U-
a) I-
Co Cl) Stephens Anorthosite Member
50 60 70 An (mole %)
a) 6. 6.
.2 Lower inclusion member
0
0) Co
I
L_------------I.-.-- -'
Sallee Snowfield section (hidden)
El
Spear Anorthosite Member
1500m I
0
6. .0 .0
0. 10 CD
g
6.
000 1
CD e
0
CD -
10< - 0
500m
a) 6. 6.
0
a)
Frost Pyroxenite Member
U-
.
a)
lNeubura Pvroxenite Member
o I I Walker Anorthoelte H---------Basal section (hidden)
Figure 1. Stratigraphic units of the Dufek intrusion, from Ford (1976). ("m" denotes "meters:')
Aughenbaugh Gabbro (figure 2). Compositions were obtained with an electron microprobe by step traversing and analyzing 10-25 points in each of at least three grains per rock sample. 62
60 70 80 An (mole %) Figure 2. Plagioclase compositions (mole percent anorthite) in anorthosite and adjacent gabbro layers in the Dufek Massif section. For each sample, "x" shows average composition, and horizontal line indicates compositional range. "LAM" denotes "lower anorthosite member." "SpAM" denotes "Spear Anorthosite Member:' ANTARCTIC JOURNAL
We found all plagioclase grains to be compositionally zoned, whether in anorthosite or adjacent gabbro. A maximum compositional range of grains within a single sample was found to be 5 to 10 mole percent anorthite; lesser ranges may in some cases have resulted from missing innermost parts of grains in thin section cuts. Although the range of anorthite content of individual grains varies from grain to grain, the difference in average composition of all grains in a sample is always less than 2 mole percent anorthite. Compositional averages and ranges for each analyzed sample are shown in figure 2. Single grains commonly exhibit compositional ranges about the same as the total range in the rock. Our results confirm Abel's et al. (1979) finding that plagioclases of the Dufek Massif section show an overall upward decrease in anorthite content, but they are presently inadequate to define closely stratigraphic trends in compositions within the anorthositic layers being studied. Plagioclase compositions of anorthite-81 and anorthite-78 occur respectively in lowest and highest parts of the Walker Anorthosite; and compositions are in the ranges anorthite-70--67 and anorthite-63-60, respectively, in the lower anorthosite member and the Spear Anorthostie member (figure 2). The generally large compositional range within a sample is a chief difficulty in discernment of trends within a layer. Moreover, in the Spear member, plagioclase of two samples from the same height have compositions with significantly different averages and only slightly overlapping ranges (figure 2). The significance of the smallscale reversals apparent in trends of average plagioclase compositions from gabbro into overlying anorthosite is therefore presently uncertain. Plagioclase zoning patterns are complex. They are characterized by an oscillatory zonation that is commonly nonsymmetrical (zone width unequal around grain core). The zoning patterns of different grains within each of the two Walker Anorthosite samples are similar. In contrast, however, there are significant grain-to-grain zoning pattern differences within individual samples of other anorthosite and gabbro, which suggest that the different grains of a rock did not crystallize in equilibrium. Plagioclases of anorthosites and adjacent gabbros show equally complex crystallization histories. Although the thin, widespread layers of anorthositic cumulate and of gabbro-containing noncumulus anorthositic inclu sions are volumetrically minor, we believe they can provide
Strontium and oxygen-isotope study of the Dufek intrusion A.B. FORD, R.W. KISTLER, and L.D. WHITE U.S. Geological Survey Menlo Park, California 94025
The mostly gabbroic Dufek intrusion (82°30'S 50°W) is one of the world's largest layered igneous complexes. It is more than 1986 REVIEW
important insights into the solidification history of the intrusion. In our continuing study of them, we have begun to analyze plagioclases of much more closely spaced (1 meter or less) samples from across four cyclic units of anorthosite and gabbro of the Stephens Anorthosite member (figure 1). The study will include cumulus and post-cumulus pyroxenes and will incorporate isotopic data (Ford, Kistler, and White, Antarctic Journal, this issue) that can provide insight into some magmatic processes not possible from major- and minor-element compositions alone. Similar to the conclusions drawn from the study of isotopic compositions of rocks and minerals, our study of plagioclases indicates that the crystallization history of the intrusion was complex. This work was supported in part by National Science Foundation grant DPP 80-20753 to the U.S. Geological Survey. References Abel, K.D., G.R. Himmelberg, and A.B. Ford. 1979. Petrologic studies of the Dufek intrusion: Plagioclase variation. Antarctic Journal of the U.S., 14(5), 6-8. Czamanske, G.K., and D.L. Schiedle. 1985. Characteristics of the banded-series anorthosites. In G.K. Czamanske and M.L. Zientek (Eds.), The Stillwater Complex, Montana: Geology and guide. (Montana Bureau of Mines and Geology Special Publication, Vol. 92.) Ford, A.B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. U.S. Geological Survey Bulletin, 1405(D), D1—D36. Ford, A.B., and G.R. Himmelberg. In press. Geology and crystallization of the Dufek intrusion. In R.J. Tingey (Ed.), Geology of Antarctica. Oxford: Oxford University Press. Ford, A.B., R.W. Kistler, and L.D. White. 1986. Strontium and oxygenisotope study of the Dufek intrusion. Antarctic Journal of the U.S., 21(5). Hess, H. H. 1960. Stillwater igneous complex, Montana—A quantitative mineralogical study. Geological Society of America Memoir, 80, 230. Himmelberg, G. and A.B. Ford. 1976. Pyroxenes of the Dufek intrusion, Antarctica. Journal of Petrology, 17(2), 219-243. Himmelberg, G.R., and A.B. Ford. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62, 623-633. Irvine, TN., D.W. Keith, and S.C. Todd. 1983. The J-M platinumpalladium reef of the Stillwater Complex, Montana: II. Origin by double-diffusive convective magma mixing and implications for the Bushveld Complex. Economic Geology, 78(7), 1287-1334. Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. Edinburgh: Oliver and Boyd.
50,000 square kilometers in area (Behrendt et al. 1981) and possibly 8-9 kilometers thick at its southern end (Ford 1976). It is about the same age as the dominant phase of Jurassic Ferrar Group tholeitic igneous activity that occurred throughout the Transantarctic Mountains (Ford and Kistler 1980). The stratigraphy and rock types are described by Ford (1976). We have begun a systematic study of the isotopic compositions of oxygen, strontium, and neodymium in rocks and minerals of the intrusion to investigate many questions about its crystallization history raised by previous studies. Work on other layered intrusions (Pankhurst 1969; Taylor and Forester 1979; Gray and Goode 1981; Palacz and Tait 1985; DePaolo 1985) shows that isotopic compositions are more sensitive indicators of many magmatic processes than are major- and minor-element compositions of rocks and minerals. 63
This research was supported by National Science Foundation grant DPP 84-14014 to the University of Maine.
Plagioclase compositional variations in anorthosites of the lower part of the Dufek intrusion
Anorthosite (and leucogabbro) also occur in large, rounded inclusions in gabbro in the Forrestal Range, but in this mode it does not show cumulus textures. Anorthosites of these types are common in layered mafic intrusions (Wager and Brown 1968), and their occurrences pose petrologic problems that were pointed out by Hess (1960) and still remain to be resolved. Czamanske and Sheidle (1985) provide a recent summary of origins proposed for such rocks. Anorthositic layers that show cycliclike stratigraphic repetition are particularly difficult to explain (Irvine, Keith, and Todd 1983). Earlier studies of plagioclase (Abel, Himmelberg, and Ford 1979) and other cumulus minerals (Himmelberg and Ford 1976, 1977) documented that their overall chemical variation and stratigraphic range in the Dufek intrusion are like those in other layered intrusions (Ford and Himmelberg in press) that have been interpreted in terms of fractional crystallization of tholeiitic magma and accumulation primarily from the base upward (Wager and Brown 1968). The studies suggested that small-scale reversals occur in the stratigraphic variation of mineral compositions in the vicinity of some anorthosite layers. The earlier studies were reconnaissances using widely spaced samples to determine overall variations. Except for the Walker Anorthosite, anorthosites were not included in the suite studied and therefore the origin of anorthosite layers was not addressed. We have begun detailed study of suites of samples spaced closely across several anorthosite layers and extending into gabbro above and below to document the nature of the mineral composition variations. We thus far have obtained plagioclase compositional data for the Walker Anorthosite and the lower anorthosite member and Spear Anorthosite Member of the
J.M. HAENSEL, JR. and G.R. HIMMELBERG Department of Geology University of Missouri Columbia, Missouri 65211
A. B. FORD U.S. Geological Survey Menlo Park, California 94025
The unusually large, differentiated Dufek intrusion (82°30'S 50°W) of Jurassic age consists dominantly of layered gabbro (plagioclase-pyroxene cumulate and plagioclase-pyroxenemagnetite cumulate). The stratigraphy and rock types are described by Ford (1976). Anorthosites occur throughout most exposed stratigraphic parts of the intrusion and in a variety of modes. Figure 1 shows the principal units of the rock. Most anorthosites are plagioclase cumulates in layers a few meters to a few tens of meters thick in the gabbro. In two units (Spear and Stephens anorthosite members, figure 1), they form cycliclike repeated layers, each with a sharp basal contact and most with a gradational contact with overlying gabbro. The much thicker Walker Anorthosite has a sharp contact with overlying gabbro. 1986 REVIEW
References
Green, D. H., and A. E. Ringwood. 1967. An experimental investigation of the gabbro to eclogite transformation and its petrological applications. Geochimica and Cosmochiinica Acta, 31, 767-833. Grew, E.S. 1985. Field studies on the Jetty Peninsula (Amery Ice Shelf area) with the Soviet Antarctic Expedition. Antarctic Journal of the U.S., 20(5), 52-53. Grew, E.S. 1986. An austral summer field season with the 30th Soviet Antarctic Expedition, 1984-1985. Antarctic Journal of the U.S., 21(1), 17-19. Loomis, T.P. 1976. Irreversible reactions in high-grade metapelitic rocks. Journal of Petrology, 17, 559-588. Newton, R.C. 1983. Geobarometry of high-grade metamorphic rocks. American Journal of Science, 283—A, 1-28. Ravich, MG., D.S. Soloviev, and L. Fedorov. 1978. Geological Structure of Mac. Robertson Land (East Antarctica). Leningrad: Gidrometeoizdat. (In Russian) Sheraton, J.W., and L.P. Black. 1983. Geochemistry of Precambrian gneisses. Relevance for the evolution of the East Antarctic Shield. Lithos, 16, 273-296. Tingey, R.J. 1982. The geologic evolution of the Prince Charles Mountains—An Antarctic Archean cratonic block. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press.
61
r --
I Eroded section
1500m
0 6. .0 .0 Co
LexmonGranophy;e
CD
0. 0 6.
Upper inclusion member
C 0 0
CD
..150m .
a) .0 0)
a)
(I)
000 1
CD
a)
0 6. .0 .0
0) C Co
CD
Co Cl)
0)
500m LL
0 U-
a) I-
Co Cl) Stephens Anorthosite Member
50 60 70 An (mole %)
a) 6. 6.
.2 Lower inclusion member
0
0) Co
I
L_------------I.-.-- -'
Sallee Snowfield section (hidden)
El
Spear Anorthosite Member
1500m I
0
6. .0 .0
0. 10 CD
g
6.
000 1
CD e
0
CD -
10< - 0
500m
a) 6. 6.
0
a)
Frost Pyroxenite Member
U-
.
a)
lNeubura Pvroxenite Member
o I I Walker Anorthoelte H---------Basal section (hidden)
Figure 1. Stratigraphic units of the Dufek intrusion, from Ford (1976). ("m" denotes "meters:')
Aughenbaugh Gabbro (figure 2). Compositions were obtained with an electron microprobe by step traversing and analyzing 10-25 points in each of at least three grains per rock sample. 62
60 70 80 An (mole %) Figure 2. Plagioclase compositions (mole percent anorthite) in anorthosite and adjacent gabbro layers in the Dufek Massif section. For each sample, "x" shows average composition, and horizontal line indicates compositional range. "LAM" denotes "lower anorthosite member." "SpAM" denotes "Spear Anorthosite Member:' ANTARCTIC JOURNAL
We found all plagioclase grains to be compositionally zoned, whether in anorthosite or adjacent gabbro. A maximum compositional range of grains within a single sample was found to be 5 to 10 mole percent anorthite; lesser ranges may in some cases have resulted from missing innermost parts of grains in thin section cuts. Although the range of anorthite content of individual grains varies from grain to grain, the difference in average composition of all grains in a sample is always less than 2 mole percent anorthite. Compositional averages and ranges for each analyzed sample are shown in figure 2. Single grains commonly exhibit compositional ranges about the same as the total range in the rock. Our results confirm Abel's et al. (1979) finding that plagioclases of the Dufek Massif section show an overall upward decrease in anorthite content, but they are presently inadequate to define closely stratigraphic trends in compositions within the anorthositic layers being studied. Plagioclase compositions of anorthite-81 and anorthite-78 occur respectively in lowest and highest parts of the Walker Anorthosite; and compositions are in the ranges anorthite-70--67 and anorthite-63-60, respectively, in the lower anorthosite member and the Spear Anorthostie member (figure 2). The generally large compositional range within a sample is a chief difficulty in discernment of trends within a layer. Moreover, in the Spear member, plagioclase of two samples from the same height have compositions with significantly different averages and only slightly overlapping ranges (figure 2). The significance of the smallscale reversals apparent in trends of average plagioclase compositions from gabbro into overlying anorthosite is therefore presently uncertain. Plagioclase zoning patterns are complex. They are characterized by an oscillatory zonation that is commonly nonsymmetrical (zone width unequal around grain core). The zoning patterns of different grains within each of the two Walker Anorthosite samples are similar. In contrast, however, there are significant grain-to-grain zoning pattern differences within individual samples of other anorthosite and gabbro, which suggest that the different grains of a rock did not crystallize in equilibrium. Plagioclases of anorthosites and adjacent gabbros show equally complex crystallization histories. Although the thin, widespread layers of anorthositic cumulate and of gabbro-containing noncumulus anorthositic inclu sions are volumetrically minor, we believe they can provide
Strontium and oxygen-isotope study of the Dufek intrusion A.B. FORD, R.W. KISTLER, and L.D. WHITE U.S. Geological Survey Menlo Park, California 94025
The mostly gabbroic Dufek intrusion (82°30'S 50°W) is one of the world's largest layered igneous complexes. It is more than 1986 REVIEW
important insights into the solidification history of the intrusion. In our continuing study of them, we have begun to analyze plagioclases of much more closely spaced (1 meter or less) samples from across four cyclic units of anorthosite and gabbro of the Stephens Anorthosite member (figure 1). The study will include cumulus and post-cumulus pyroxenes and will incorporate isotopic data (Ford, Kistler, and White, Antarctic Journal, this issue) that can provide insight into some magmatic processes not possible from major- and minor-element compositions alone. Similar to the conclusions drawn from the study of isotopic compositions of rocks and minerals, our study of plagioclases indicates that the crystallization history of the intrusion was complex. This work was supported in part by National Science Foundation grant DPP 80-20753 to the U.S. Geological Survey. References Abel, K.D., G.R. Himmelberg, and A.B. Ford. 1979. Petrologic studies of the Dufek intrusion: Plagioclase variation. Antarctic Journal of the U.S., 14(5), 6-8. Czamanske, G.K., and D.L. Schiedle. 1985. Characteristics of the banded-series anorthosites. In G.K. Czamanske and M.L. Zientek (Eds.), The Stillwater Complex, Montana: Geology and guide. (Montana Bureau of Mines and Geology Special Publication, Vol. 92.) Ford, A.B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. U.S. Geological Survey Bulletin, 1405(D), D1—D36. Ford, A.B., and G.R. Himmelberg. In press. Geology and crystallization of the Dufek intrusion. In R.J. Tingey (Ed.), Geology of Antarctica. Oxford: Oxford University Press. Ford, A.B., R.W. Kistler, and L.D. White. 1986. Strontium and oxygenisotope study of the Dufek intrusion. Antarctic Journal of the U.S., 21(5). Hess, H. H. 1960. Stillwater igneous complex, Montana—A quantitative mineralogical study. Geological Society of America Memoir, 80, 230. Himmelberg, G. and A.B. Ford. 1976. Pyroxenes of the Dufek intrusion, Antarctica. Journal of Petrology, 17(2), 219-243. Himmelberg, G.R., and A.B. Ford. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62, 623-633. Irvine, TN., D.W. Keith, and S.C. Todd. 1983. The J-M platinumpalladium reef of the Stillwater Complex, Montana: II. Origin by double-diffusive convective magma mixing and implications for the Bushveld Complex. Economic Geology, 78(7), 1287-1334. Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. Edinburgh: Oliver and Boyd.
50,000 square kilometers in area (Behrendt et al. 1981) and possibly 8-9 kilometers thick at its southern end (Ford 1976). It is about the same age as the dominant phase of Jurassic Ferrar Group tholeitic igneous activity that occurred throughout the Transantarctic Mountains (Ford and Kistler 1980). The stratigraphy and rock types are described by Ford (1976). We have begun a systematic study of the isotopic compositions of oxygen, strontium, and neodymium in rocks and minerals of the intrusion to investigate many questions about its crystallization history raised by previous studies. Work on other layered intrusions (Pankhurst 1969; Taylor and Forester 1979; Gray and Goode 1981; Palacz and Tait 1985; DePaolo 1985) shows that isotopic compositions are more sensitive indicators of many magmatic processes than are major- and minor-element compositions of rocks and minerals. 63
