study of the Pensacola Mountains, for much helpful discus- was established at the LC-130 ski-plane landing site near sion of the geologic relations here reported. Walker Peak, from which motor toboggan and ski traverses This research is supported by National Science Founda- were made to sites of investigations. The massif and nearby tion interagency agreement DDP 76-21663 with the nunataks are underlain by layered mafic igneous rocks of Geologic Division, U.S. Geological Survey. cumulus origin that make up the lower part of the Dufek intrusion, a Jurassic stratiform complex of immense size in the northern third of the Pensacola Mountains. References The U.S. Geological Survey (USGS) made reconnaissance studies using Army helicopters of all outcropping parts of the intrusion in the summer 1965-1966. This work showed Clarkson, P.D. 1972. Geology of the Shackleton Range: a that all of the Forrestal Range, located about 50 kilometers preliminary report. British Antarctic Survey Bulletin, 31: 1-15. southeast of Dufek Massif, is also underlain by part—the Craddock, Campbell. 1970. Tectonic map of Antarctica. Antarctic iron-enriched upper part - of the Dufek intrusion (Ford and Map Folio Series, 12: plate XXI. Boyd, 1968), and that the body probably has an area of at Ford, A.B. 1972. The Weddell orogeny—latest Permian to early least 24,000 square kilometers (Behrendt et al., 1974). As Mesozoic deformation at the Weddell Sea margin of the Trans. the thickness is estimated to be on the order of 8 to 9 antarctic Mountains. In: Antarctic Geology and Geophysics kilometers (Ford, 1976), the original magma volume may (Adie, R.J., editor). Oslo, Universitetsforlaget. 419-425. have been on the order of 300,000 cubic kilometers. Schmidt, D.L., J.H. Dover, A.B. Ford, and R.D. Brown. 1964. The 1976-1977 summer fieldwork was the beginning of a Geology of the Patuxent Mountains. In: Antarctic Geology (Adie, detailed geologic study to obtain data on parts of the intru- R.J., editor). New York, John Wiley. 276.283. sion not adequately covered in earlier field studies and to in- Schmidt, D.L., P.L. Williams, W.H. Nelson, and JR. Ege. 1965. vestigate questions raised by the reconnaissance and ensuing Upper Precambrian and Paleozoic stratigraphy and structure of laboratory studies. Principal results of that background the Neptune Range, Antarctica. U.S. Geological Survey profes- work are (1) a 1:1,000,000-scale geologic map showing the sional Paper, 525 D. D112.D119. . regional setting of the intrusion (Schmidt and Ford, 1969), Schmidt, D. L., and A.B. Ford. 1969. Geology of the Pensacola and (2) two 1:250,000-scale geologic maps showing major rock- Thiel Mountains. Antarctic Map Folio Series, 12: plate V. stratigraphic units within and near the intrusion (Ford et Schwab, F.L. 1975. Framework mineralogy and chemical composi- al., in press a, in press b), (3) the definition of units and tion of continental margin-type sandstone. Geology, 3: 487.490. measurement of general stratigraphy of the layered se- Stephenson, P.J. 1966. Geology. 1. Theron Mountains, Shackleton quence (Ford, 1976), (4) the recognition of the cumulus Range and Whichaway Nunataks. Trans-Antarctic Expedition origin of the layered rocks (Ford and Boyd, 1968), (5) the 1933-1938 Scientific Reports, 8: 1.79. analysis of rock magnetic properties and paleomagnetism of Thompson, M.R.A. 1972. Inarticulate brachiopoda from the the body (Beck and Griffin, 1971; Beck, 1972), (6) the deter- Shackleton Range and their stratigraphical significance. British mination of a strong iron-enrichment trend in rocks Antarctic Survey Bulletin, 31: 17-20. stratigraphically upward in the sequence (Ford, 1970), (7) Williams, P.L. 1969. Petrology of upper Precambrian and the determination of iron-enrichment fractionation trends Paleozoic sandstones in the Pensacola Mountains, Antarctica. Journal of Sedimentary Petrology, 39: 1455-1465. in orthopyroxene, inverted pigeonite, and augite- ferroaugite (Himmelberg and Ford, 1973, 1976), (8) using coexisting pyroxene geothermometry, the calculation that primary crystallization occurred in the temperature range 1180 0 to 1074°C (Himmelberg and Ford, 1976), (9) the Geological studies of the Dufek determination that compositions of the iron-titanium oxide minerals largely reflect subsolidus recrystallization in a intrusion, Pensacola Mountains, calculated temperature range of about 900° to 600°C, therefore showing little systematic change related to frac- 1976-1977 tionation except for apparent systematic variation in vanadium and aluminum concentrations in ilmeno- magnetites (Himmelberg and Ford, 1975, in press), (10) the ARTHUR B. FORD, CHRISTINE CARLSON2, determination of the crystallization sequence bytownite- or- GERALD K. CZAMANSKE, WILLIS H. NELSON, thopyroxene, labradorite-pigeonite- augite, labradorite- and CONSTANCEJ. NUTT2 pigeonite- augite-magnetiteilmenite, labradorite-ferro- augite- magnetite- fayalite-apatite (Himmelberg and Ford, U.S. Geological Survey 1976), and (11) the determination of a radiometric age of Menlo Park, California 94025 168 million years for the intrusion (R.W. Kistler, USGS, un- 2Department of Geology published data), showing it to be coeval with tholeiitic Stanford University Stanford, California 94305 Formed by gravity settling of minerals and their accumulation on the floor of the magma chamber. Such rocks are termed Geological studies by the authors and geophysical studies "cumulates" and named by prefixing names of settled minerals, in by A.W. England (USGS, Reston, Virginia) were focused in decreasing order of abundance, as in pyroxene-plagioclase the western part of the Dufek Massif (82 136S. 52130W.) cumulate" (Jackson, 1971). Common rock name may also be used, between 30 November and 30 December 1976. A base camp as in "gabbroic cumulate", if settled phases not specified.

90 ANTARCTIC JOURNAL Figure 1. Fifteen-meter-thick dark layer of pyroxene Figure 2. Repetitive thin dark layers of pyroxene cumulate on northern spur of Neuburg Peak. Shows sharp cumulates in gabbroic cumulates of Aughenbaugh Peak basal contact with light-colored plagioclase cumulate. near top of Dufek Massif. Gradational upper contact with gabbroic cumulate not ap- parent. igneous activity (Ferrar Dolerite) throughout the Trans- antarctic Mountains, and establishing an upper age limit of Middle Jurassic for the post-Permian Weddell orogeny in the Pensacola Mountains (Ford, 1972). The principal activities of the 1976-1977 geologic fieldwork were (1) the geologic mapping of the Dufek Massif area at 1:25,000 scale, for publication of 1:50,000-scale geologic maps showing distribution of the significant cumulate layers, (2) the measurement of additional stratigraphic sections in areas not reached in earlier fieldwork, and (3) the detailed sampling of selected stratigraphic intervals on Walker Peak, Neuburg Peak (figure 1), and Aughenbaugh Peak (figure 2) to investigate the possible presence of cyclic units suggested by the analytical work on pyroxenes (Himmelberg and Ford, 1976). During the geologic mapping, layered gabbroic rock of Brown Nunataks (located about 5 kilometers northwest of Dufek Massif) was found to be considerably different from gabbroic cumulates elsewhere. The rock is unusually heterogeneous and shows much more irregularity of layering than at Dufek Massif (compare figure 3 with figures 1 and 2). Large breccialike masses of noncumulus gabbro are Figure 3. Heterogeneous layering in gabbro of Brown locally abundant and are postulated to represent magmatic Nunataks. slump deposits from a nearby steep contact wall. The geological relations thus seem to support geophysical inter- significant amounts of copper and nickel, in order to begin a pretations of England and Nelson (1977) that the ice- study (by Czamanske) of these minerals and their variation covered contact of the intrusion lies much nearer Dufek with fractionation. Walker (1961, p. 208) reported that ac- Massif, probably not far north of Brown Nunataks, than in- cessory metallic minerals are common, locally forming con- ferred from the early reconnaissance surveys by Behrendt et centrations as much as 15 percent of the rock, and include at. (1973, 1974). bornite(?), chromite, ilmenite, magnetite, chalcopyrite, and Layered mafic intrusions often contain a variety of pyrite. We found that sulfide minerals are not common in metallic minerals (Wager and Brown, 1968), and therefore the Dufek Massif area, and we did not find concentrations of we searched carefully for any evidence of metal concentra- more than a few percent. We also were unable to confirm in tions, in particular, of sulfide minerals that might contain the field the reported occurrence of bornite and chromite.

October 1977 91 Minor amounts of sulfides are typically associated with Mountains, Antarctica. U.S. Geological Survey Professional country-rock xenoliths, probably as products of reaction Paper, 750-B, B117.B121. with the magma. Behrendt, j.C., W.L. Rambo, J.R. Henderson, R.E. Wanous, and Laurent Meister. 1973. Simple bouger gravity and aeromagnetic Preliminary results of chemical studies show that the maps of the Davis Valley quadrangle and part of the Cordiner stratigraphic variations in minor-metal concentrations are Peaks quadrangle and vicinity, Antarctica. U.S. Geological generally similar to those of other layered intrusions, at least Survey map GP-887. Scale 1:250,000. above their basal layers of ultramafic cumulates. (Basal Behrendt, J.C., J.R. Henderson, Laurent Meister, and W.L. ultramafic cumulates are probably present in the Dufek in- Rambo. 1974. Geophysical investigations of the Pensacola Moun- trusion, but they are not exposed.) These studies indicate tains and adjacent glacierized areas of Antarctica. U.S. that higher cumulates become enriched in sulfur and, sur- Geological Survey Professional Paper, 844: 28 p. prisingly, the platinum-group metals, abundances of which England, A.W., and Willis H. Nelson. 1977. Geophysical studies of are greater in the Forrestal Range than in the Dufek Massif. the Dufek intrusion, Pensacola Mountains, Antarctica. Antarctic Little published information is available for comparing Journal of the United States, XII(4): 93-94. fractionation trends of the platinum-group metals with Ford, A.B. 1970. Development of the layered series and capping other layered intrusions. Analyses completed are mostly of granophyre of the Dufek intrusion of Antarctica. In: Symposium on the Bushveld Igneous Complex and other layered intrusions typical cumulates, although some data are available on (Visser, D.J.L., and G. Von Gruenewaldt, editors). Geological minor metals in pyroxenes and iron-titanium oxides Society of South Africa Special Publication, 1: 492-510. (Himmelberg and Ford, 1976, in press). Ford, A.B. 1972. The Weddell orogeny—latest Permian to early Preliminary results include the following: (1) chromium Mesozoic deformation at the Weddell Sea margin of the Trans- abundance is 10 to 500 parts per million (ppm) in Dufek antarctic Mountains. In: Antarctic Geology and Geophysics Massif and decreases to below detectability limits (2 ppm) in (Adie, R.J., editor). Oslo, Universitetsforlaget. 419.425. the Forrestal Range, (2) nickel abundance is 10-150 ppm in Ford, A.B. 1976. Stratigraphy of the Dufek intrusion, Antarctica. Dufek Massif and decreases to below detectability limits (2 U.S. Geological Survey Bulletin 1405-D: 36 p. ppm) in the Forrestal Range, (3) platinum-group metal con- Ford, A.B., and W.W. Boyd, Jr. 1968. The Dufek intrusion, a ma- centrations are mostly below detectability limits (0.004 ppm jor stratiform gabbroic body in the Pensacola Mountains, Antarc- for palladium and 0.010 ppm for platinum) in Dufek Massif tica. 23rd International Geological Congress. Proceedings, 2: and increase somewhat in the Forrestal Range, where max- 213-228. imum known values reach only 0.044 ppm palladium and Ford, A.B., D.L. Schmidt, W.W. Boyd, Jr., and W.H. Nelson. In 0.010 ppm platinum, (4) copper concentration is 12 to 40 press a. Geologic map of the Saratoga Table quadrangle, Pen- ppm in Dufek Massif and increases to 90 to 2000 ppm in the sacola Mountains, Antarctica. U.S. Geological Survey map, A-9. Forrestal Range, (5) iron, titanium, and vanadium have Scale 1:250,000. moderate to strong positive correlation with stratigraphic Ford, A.B., D. L. Schmidt, and W.W. Boyd, Jr. In press b. Geologic height (correlation coefficient, r, of + 0.50 or greater), (6) map of the Davis Valley quadrangle and part of the Cordiner Peaks quadrangle, Pensacola Mountains, Antarctica. U.S. platinum, palladium, copper, cobalt, and sulfur have low to Geological Survey map A-b. Scale 1:250,000. moderate positive correlation (r = + 0.30-0.50) with Himmelberg, G.R., and A.B. Ford. 1973. Pyroxene compositional height, and (7) silica, magnesium, chronium, and the ratio trends in the Dufek intrusion, Pensacola Mountains, Antarctica. nickel/cobalt have moderate to strong negative correlations AntarcticJournal of the U.S., VIII(5): 260-263. with height. These studies will be expanded to include work Himmelberg, G.R., and A.B. Ford. 1975. Petrologic studies of the on samples collected in 1976-1977. Dufek intrusion: iron-titanium oxides. Antarctic Journal of the This project is supported by National Science Foundation U.S., X(5): 241-244. interagency agreements DPP 76-21663 and DPP 75-17682 Himmelberg, G.R., and A.B. Ford. 1976. Pyroxenes of the Dufek with the Geologic Division, U.S. Geological Survey. intrusion, Antarctica Journal of Petrology, 17: 219.243. Himmelberg, G.R., and A.B. Ford. In press. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist Jackson, E.D. 1971. The origin of ultramafic rocks by cumulus pro- References cesses. Fortschr[te Mineralogie, 48: 128-174. Schmidt, D. L., and A.B. Ford. 1969. Geology of the Pensacola and Thiel Mountains. Antarctic Map Folio Series, 12: plate V. Scale 1:1,000,000. Beck, M.E., Jr. 1972. Palaeomagnetism and magnetic polarity zones Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. in the Jurassic Dufek intrusion, Pensacola Mountains, Antarctica. Edinburgh, Oliver and Boyd. 588 p. Journal of the Royal Astronomical Society, 28: 49-63. Walker, P.T. 1961. Study of some rocks and minerals from the Beck, ME., Jr., and N.L. Griffin. 1971. Magnetic intensities in a Dufek Massif, Antarctica. IGY World Data Center A, Glaciology. differentiated gabbroic body, the Dufek intrusion, Pensacola Glaciology Report, 4: 195-213.

92 ANTARCTIC JOURNAL