G 2389 This interpretation is supported by an argon isochron ported by National Science Foundation grants (in the Pecora Escarpment, 1962-63), GA 222 (on the plot of 40Ar/36Ar and 40K/36Ar values (R. W. Kistler, un- AG 238 (in the Cordiner published data, 1978). The pyroxene shows much Dufek intrusion, 1965-66), and greater optical homogeneity than in the Dufek intrusion. Peaks, 1973-74) to the U.S. Geological Survey. The best estimate for the age of the sills is considered References to be the average of the mineral pair showing the most Elliot, D. H., R. j. Fleck, and J. F. Sutter. In press. K-Ar dating concordant and, more importantly, the youngest ages of of the Ferrar Group, central Transantarctic Mountains. In all analyzed pairs: 178.9 ± 4.5 million years (obtained Geology of the Central Transantarctic Mountains, eds. M. D. from a plagioclase age of 177.7 ± 4.5 million years and Turner and J . F. Splettstoesser. Antarctic Research Series, a pyroxene age of 180.0 ± 4.5 million years). Memoir. Washington, D.C.: American Geophysical Union. Analysis of a coexisting pyroxene-plagioclase pair Ford, A. B. 1972. The Weddell orogeny—latest Permian to from a dike in the Cordiner Peaks yielded discordant early Mesozoic deformation at the Weddell Sea margin of apparent ages of 307.9 ± 7.7 million years for pyroxene the Transantarctic Mountains. In Antarctic Geology and Geo- and 168.8 ± 4.2 million years for plagioclase. The pla- physics, ed. R. J . Adie, pp. 419-25. Olso, Norway: Univer- sitetsforlaget. gioclase age is considered to provide the best estimate Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek for the dikes age, in that the pyroxene, which is opti- intrusion. Antarctica. U.S. Geological Survey Bulletin, no. cally homogeneous, is inferred to contain excess argon. 1405—D. The above results indicate that the basaltic magmatism Ford, A. B., D. L. Schmidt, and W. W. Boyd, Jr. 1978. Geologic that produced the dikes and sills and the Dufek intru- map of the Davis Valley Quadrangle and Part of the Cordiner sion in the Pensacola Mountains occurred over a narrow Peaks Quadrangle, Pensacola Mountains, Antarctica (1:250,000 time interval in the Early Jurassic. The bodies are cor- scale). U.S. Geological Survey Map A—b. Grindley, G. W. 1963. The geology of the Queen Alexandra related with the Ferrar Group in other parts of the Range, Beardmore. Glacier, Ross Dependency, Antarctica: Transantarctic Mountains (Elliot, Fleck, and Sutter, in With notes on the correlation of Gondwana sequences. New press) on the basis of age equivalence and close chemical Zealand Journal of Geology and Geophysics, 6: 307-47. comparisons (A. B. Ford, unpublished data, 1978). The Himmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the age of the Dufek intrusion reported here also provides Dufek intrusion, Antarctica. journal of Petrology, 17: 219-43. an upper age limit of Early Jurassic for the latest major Kistler, R. W., and F. C. W. Dodge. 1966. Potassium-argon ages orogeny in the Pensacola Mountains, during which rocks of coexisting minerals from pyroxene-bearing granitic rocks as young as Permian were strongly folded (Ford, 1972). in the Sierra Nevada, California. Journal of Geophysical Re- The fieldwork on which this study is based was sup- search, 71(8): 2157-61. Geological field investigation of STEPHEN J. BOYER Dufek intrusion U.S. Geological Survey Denver, Colorado 80225 ARTHUR B. FORD Between 4 November and 20 December 1978, the au- thors carried out geological field studies of the layered U.S. Geological Survey gabbroic Dufek intrusion (Ford, 1976). This work con- Menlo Park, California 94025 tinued the detailed investigation of the unusually large igneous complex that was started in the western Dufek Massif, in the lower part of the body, in the summer of RICHARD L. REYNOLDS 1976-77 (Ford et al., 1977). In 1978, the authors focused on the iron-enriched U.S. Geological Survey upper part of the body, in the southern Forrestal Range. Denver, Colorado 80225 During November, fieldwork was done by snowmobile and ski traverses from a tent camp in May Valley. Dur- ing December, work was also continued in the Dufek Massif from tent camps located on the Sallee Snowfield CARL HUIE and near Aughenbaugh Peak. The purpose of this fieldwork was to investigate ques- U.S. Geological Survey tions raised by laboratory and office studies following a Menlo Park, California 94025 1965-66 reconnaissance of the complex (Ford and Boyd, 1968). Although topographic base maps had not been available at the time of that reconnaissance, the data obtained were adequate for compilation of two recently published 1:250,000-scale geologic maps of the body (Ford, Schmidt, and Boyd, 1978; Ford et al., 1978). ing abundant included blocks of noncumulus anorthos- In the 1978-79 study, most of the larger outcrop ite and leucogabbro was found to occur at about the areas in the Forrestal Range were mapped at a scale of level of the chemical reversal. The apparent chaotic na- 1:25,000. This larger-scale mapping was needed to de- ture of the blocks (figure 3) gives the layer the appear- termine the stratigraphy and distribution of the many ance of a megabreccia in sedimentary rocks. A distinct stratiform units of anorthositic, leucogabbroic, and py- angular discordance of up to about 10 degrees was roxenitic cumulates interlayered within the mostly gab- found to exist between this horizon and the more steeply broic cumulates (figure 1). The term "cumulate" is dipping layered cumulates below. This angular relation used for igneous rocks formed by accumulative pro- occurs regionally across the body and is most pro- cesses involving currents or settling under gravity. Evi- dence for current erosion and deposition is strikingly displayed at a number of places by channellike struc- tures (figure 2) analogous to cut-and-fill channels in sed- imentary rock sequences. Laboratory studies (Himmelberg and Ford, 1976, 1977) show that mineral compositions vary systemati- cally and gradually upward through the complex to a level about one kilometer below the top, at which point the chemical trends show a sharp reversal before again resuming a differentiation trend similar to that below. Possible origins of this reversal include contamination; a major convective disturbance bringing in magma from another part of the chamber; and the injection of a large volume of new magma at a late stage of the bodys con- solidation. As the origin of the reversal is of great im- portance in interpreting the differentiation record, this was one of the principal issues studied during the 1978- 79 fieldwork. Figure 2. Anorthosite-filled channel In gabbrolc cumulates on Mount Stephens, Forrestal Range. The central part of the During the mapping, a major unit of gabbro contain- channel Is more than 15 meters thick. v ww EL wpq A Figure 1. Layer of leucogabbrolc cumulate, about 7 meters thick, in dark Figure 3. Anorthosite "megabreccia" layer, north rim of Sar- gabbrolc cumulate near May Valley, Forrestal atoga Table, Forrestal Range. (Note figure In lower center Range. Magnetite is abundant near the base of the layer. for scale.) FEll nounced in the axial part of the synform of the intru- References sion. It appears to be somewhat analogous to a regional Boyer, S. J . 1979. Glacial geologic observations in Dufek Massif angular unconformity in sedimentary rock sequences. and Forrestal Range. Antarctic Journal of the United States (this The discordance is tentatively interpreted as being the issue). result of strong axial subsidence or of lopolithic sagging, Ford, A. B. 1976. Stratigraphy of the layered gabbroic Dufek perhaps related to magma withdrawal from depth. If so, intrusion, Antarctica. U.S. Geological Survey Bulletin, no. the chemical reversal described above may be related to 1405-D. high-level emplacement of this "new" magma and mix- Ford, A. B. and W. W. Boyd, Jr. 1968. The Dufek intrusion, ing with residual magma of the chamber. The subsi- a major stratiform gabbroic body in the Pensacola Moun- dence was a comparatively sudden, perhaps cata- tains, Antarctica. 23rd International Geological Congress, Pra- gue. Proceedings, 2: 213-28. strophic, event resulting in the tearing loose from either Ford, A. B., Christine Carlson, G. K. Czamanske, W. H. Nelson, the chamber walls or roof of large blocks of anorthosite and C. J . Nutt. 1977. Geological studies of the Dufèk intru- and leucogabbro that were then spread as a basal me- sion, Pensacola Mountains, 1976-77. Antarctic Journal of the gabreccia deposit many kilometers across the chamber United States, 12(4): 90-92. floor. Several stratigraphic sections of this part of the Ford, A. B., D. L. Schmidt, and W. W. Boyd, Jr. 1978. Geologic body were measured and sampled in detail for addi- Map of the Davis Valley Quadrangle and Part of the Cordiner tional laboratory studies on the nature of the chemically Peaks Quadrangle, Pensacola Mountains, Antarctica (1:250,000 reversed zone and of mineralogic changes during the scale). U.S. Geological Survey Map A-b. Ford, A. B., D. L. Schmidt, W. W. Boyd, Jr., and W. H. Nelson. final crystallization stages of the body. 1978. Geologic Map of the Saratoga Table Quadrangle, Pensacola Studies of soils developed on a variety of bedrock lith- Mountains, Antarctica (1:250,000 scale). U.S. Geological Sur- ologies and of the glacial geologic history of the region vey Map A-9. also were carried out during the summer of 1978-79 Himmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the (Boyer, 1979). Dufék intrusion, Antarctica. journal o/Petrology, 17: 219-43. The authors work on this project has been supported Himmelberg, G.