are so similar in all other respects? The geographic feldspathic pyroxenite positions of these features reveal the answer. 90- (pyroxene cumulate) Minna Bluff (elev. 1,060 meters) juts into the Ross Ice Shelf like a giant snow fence and either traps or gabbroicpyroxenite (plagioclase—pyroxene diverts the snow that prevailing southerly winds 06. cumulate)..-. ----- sweep off the ice shelf. Black Island is directly north TO-- mafic gabbro of Minna Bluff and so does not get much blown •: Y. - (plogioclose—pyroxene t/i snow. White Island has nothing to the south of it CL cumulate)

but open ice shelf, so snow can be blown over and 50------...1%•1S7 ------deposited on this island. The difference in snow ac- gobbro cumulation on the two islands is perhaps accentuated (-pyroxene-plogioclos by the wedge shape of White Island with its wide • .- cumulate) 30------3•..... ------side to the south, which favors the collection of snow. onorthositic gabbro Black Island is shaped with a point to the south that - (pyroxene—plagioclase diverts the winds around it. cumulate) 10-VY•Z- ...... The difference in snow accumulation on Black Is- onorthosite land and White Island thus seems to be best explained (plogloclose cumulate) A: I I I I I I I by the geographic and meteorologic conditions de- 250 2.80 3.00 3.20 3A0 scribed, although additional factors may be involved. density

Figure 1. Variation of density, in grams per tubic centimeter, with pyroxene content, in volume percent, and rock type for the Density of the Dufek Massif section. stratiform Dufek intrusion, the basal zone, an intermediate zone estimated to be Pensacola Mountains, Antarctica on the order of 2 kilometers thick, and the roof are A. B. FORD not exposed, indirect evidence suggests that the total thickness is at least 7 kilometers (Ford, 1970) and U.S. Geological Survey that the layered mafic rocks extend for great distances Menlo Park, California beneath adjoining continental ice sheets, probably S. W. NELSON over an area of at least 34,000 square kilometers in all (Behrendt, 1971). These estimates indicate that Department of Geology the body is comparable in size to some of the largest University of Nevada layered mafic complexes in the world. Although such bodies typically occur in a Precambrian craton set- An immense layered gabhroic complex, the Dufek ting, the Dufek body lies in a recurrently active intrusion, makes up the entire northern one-third of orogenic belt marginal to the antarctic craton. The the Pensacola Mountains near the head of the Wed- latest major deformation in the belt took place in dell Sea. Discovered only in 1957 on an IGY traverse probable Triassic time (Ford, in press), as indicated from Ellsworth Station (Aughenbaugh, 1961; Walker, by the presence of Permian fossils in folded beds and 1961), the complex was mapped in entirety, geo- by Middle Jurassic potassium-argon dates (R. W. physically surveyed, and extensively sampled by a Kistler, written communication, 1969) for the post- team of U.S. Geological Survey geologists, geophysi- orogenic Dufek intrusion. Radiometric dating and cists, and topographic engineers in the austral sum- chemical characteristics suggest that the body is re- mer of 1965-1966 (Schmidt and Ford, 1966, 1969; lated to Ferrar diabase intrusive activity (Compston Ford and Boyd, 1968; Behrendt et al., 1966). Com- et al., 1968) elsewhere in the Transantarctic Moun- pilation and analysis of the field data and laboratory tains. studies of samples have continued; this report briefly The Dufek body, which is considerably more dif- summarizes some of this work. ferentiated than any known Ferrar diabase sheet, is Parts of the intrusive body are excellently exposed composed of a highly varied suite of layered rocks in enormous escarpments that provide two complete ranging from anorthosite and granophyre to pyroxe- sections for study: a lower one, 2 kilometers thick, in nite and magnetite. The great bulk, however, is the Dufek Massif, and an upper one, also about 2 gabbro with variable amounts of the principal min- kilometers thick, in the Forrestal Range. Although eral phases, plagioclase, pyroxene—both clinopyrox- ene and orthopyroxene—and magnetite or other iron- Publication authorized by the Director, U.S. Geological and titanium-rich oxides. Bulk-rock densities closely Survey. reflect the varying major mineral content (fig. 1), as

September-October 1972 147 stratigrophic height (kilometers)

UI a,)

DUFEK MASSIF SECTION FORRESTAL RANGE SECTION

N •N • N 0 ; . • a•I. • • N LA, N :IL N

J Figure 2. Variation of density with stratigraphic height in Dufek intrusion. x pyroxenite layers; y magnetitite layers; z anorthosite layers.

40— FORRESTAL variants in the complex (fig. 2). Physical processes

In RANGE that operated during consolidation of the immense C n: 223 magma reservoir, including lateral current activity 20- A B E and gravity settling, were analogous to processes that a, I- operate during accumulation of some types of water- lain clastic materials that form sedimentary rocks. U) F-I-fl-.;-.tI 0 F] a, 270 2.90 3.10 330 i504 The igneous Dufek "sediments" accumulated on the E 80 - chamber floor in a crystal rain, interrupted episodi- DUFEK cally by turbidity-current-like floodings across the 0 E 0 — MASSIF floor by crystal-rich magma currents presumably gen- C 60- erated by convection. Evidence of scour along the U — 364 floor, which rose as crystals accumulated, is clear at C B several levels. Early currents carried mainly pyrox- 40- enes, later ones mainly plagioclases. The many sharp 0 fluctuations in the density curve of fig. 2 correspond F to thin pyroxenitic and anorthositic layers formed 20- thereby, as well as to gravity-accumulated magnetite concentrations that occur mainly in the Forrestal Range section. 2IO The continual separation of crystals led to progres- density sive changes in melt composition, to accompanying chemical changes in later formed crystals, and to the Figure 3. Frequency distribution of rock densities in the Dufek appearance of new phases in successively higher cumu- intrusion. lates. Early separation of magnesium-rich pyroxene, and presumably olivine in the unexposed basal layers, well as chemistry, particularly total iron oxides in the resulted in increasing iron content of the originally rocks. Magnetic susceptibility (K) shows a general tholeiitic melt and eventually in the crystallization of positive correlation with density in each of the ex- magnetite, locally in great amounts. The melt be- posed sections (Griffin, 1969). Using the curve of came enriched in alkalies and silica by early, and fig. 1, density measurement can provide a rapid continual, fractionation of calcium-rich plagioclase means for preliminary classification of many Dufek and enriched in water owing to the anhydrous char- rocks. acter of all early phases. Such changes led, in the Many aspects of magmatic history are clearly re- final stage of consolidation, to the development of a flected in the vertical distribution of rock-density 300-meter-thick capping layer of alkaline granitic

148 ANTARCTIC JOURNAL residue of granophyre containing iron-rich clino- and Geophysics (R. J. Adie, ed.). Oslo, Universitets- pyroxene, hornblende, and biotite. forlaget. Ford, A. B., and W. W. Boyd, Jr. 1968. The Dufek intrusion, Densities measured on approximately 600 samples a major stratiform gabbroic body in the Pensacola Moun- range widely from as low as 2.65 grams per cubic tains, Antarctica. Proceedings of the 23rd International centimeter for granophyre and 2.70 g/cc for an- Geological Congress, vol. 2: 213-228. orthosite to as much as 3.30 g/cc for pyroxenite and Griffin, N. L. 1969. Paleomagnetic properties of the Dufek 3.50 g/cc or more for magnetitite. Most gabbros intrusion, Pensacola Mountains, Antarctica. MS Thesis. University of California, Riverside. 93 p. (pyroxene-plagioclase cumulates) lie in the range Jaeger, J. C. 1964. The value of measurements of density 2.80-3.20 g/cc (figs. 1 to 3). Weighted according to in the study of dolerites. Journal of the Geological Society layer thicknesses, the average density of the Dufek of Australia, 11. 133-140. Massif section is about 2.95 g/cc; of the Forrestal Schmidt, D. L., and A. B. Ford. 1966. Geology of the north- Range section, about 3.03 g/cc. The estimated aver- ern Pensacola Mountains and adjacent areas. Antarctic age for the entire body, taking into consideration the Journal of the U.S., 1(4): 125. probable densities of unexposed sections, approximates Schmidt, D. L., and A. B. Ford. 1969. Geologic Map of Antarctica (Pensacola and Thiel Mountains) (Sheet 5). that of R. A. Dalys average gahbro or norite (Daly Antarctic Map Folio Series, 12. et al., 1966), about 2.98 g/cc, and only slightly ex- Walker, P. T. 1961. Study of some rocks and minerals from ceeds that of about 2.95 g/cc measured on rocks from the Dufek Massif, Antarctica. International Geophysical little differentiated diabase sills in the southern Pensa- Year World Data Center A Glaciology. Glaciology Report, cola Mountains. The upward increase in average 4: 195-213. density, contrasting with general upward decrease common in thin diabase sills elsewhere (Jaeger, 1964), obviously reflects the strong trend of iron enrichment during fractionational crystallization of the Dufek magma. The Dufek body is a highly inhomogeneous mass, and such differences in density for different parts of the total stratigraphic section as indicated Rb-Sr and K-Ar dating of rocks here should be considered in future more detailed from southern Chile gravity studies when sub-ice terrain maps become and West Antarctica available. This work is supported by National Science Founda- MARTIN HALPERN tion grant AG-238. Geosciences Division University of Texas at Dallas References Geological and geophysical field programs in the Aughenbaugh, N. B. 1961. Preliminary report on the geology south of Chile (Halpern, 1970) and in West Antarc- of the Dufek Massif. International Geophysical Year World tica have provided the opportunity for collecting sam- Data Center A Glaciology. Glaciology Report, 4: 155-193. ples of igneous and metamorphic rocks for radiometric Behrendt, J . C. 1971. Interpretation of geophysical data in dating. The aim of this program was to establish the the Pensacola Mountains, Antarctica. Antarctic Journal of the U.S., VI(5): 196-197. chronology of principal rock units so that the geologic history of these remote regions of the earths crust Behrendt, J . C., L. Meister, and J . R. Henderson. 1966. Air- borne geophysical study in the Pensacola Mountains, could be understood. Rubidium-strontium isotopic Antarctica. Science, 153 (3742) : 1373-1376. age analyses were carried out at the University of Compston, W., I. McDougall, and K. S. Heier. 1968. Geo- Texas at Dallas and potassium-argon isotopic dating chemical comparison of the Mesozoic basaltic rocks of at the University of Leeds, England. Antarctica, South Africa, South America, and Tasmania. Geochemica et Cosmochimica Acta, 32(2): 129-149. In southern Chile, metamorphic rocks constitute Daly, R. A., G. E. Menger, and S. P. Clark, Jr. 1966. Den- the oldest known rocks. Gneiss from the basement sity of rocks. In: Handbook of Physical Constants (S. P. of the Magellan Basin at the Atlantic entrance to the Clark, Jr., ed.). Geological Society of America. Memoir, Strait of Magellan have been rubidium-strontium 97: 19-26. total rock dated at 306 ± 156 million years (Xf3 = Ford, A. B. 1970. Development of the layered series and 10 per year) with an initial strontium-87 capping granophyre of the Dufek intrusion of Antarctica. 1.47 x In: Symposium on the Bushveld Igneous Complex and to strontium-86 ratio of 0.7112 ± 0.0033. Biotite Other Layered Intrusions (D. J. L. Visser and G. von from a sample of the gneiss has been rubidium-stron- Gruenswaldt, eds.). Geological Society of South Africa, tium and potassium-argon dated as Permian, implying Special Publication, 1: 494-510. that the basement of the Magellan Basin has been Ford, A. B. In press. The Weddell orogeny-latest Permian involved in one or more Paleozoic geologic events. to early Mesozoic deformation at the Weddell Sea margin of the Transantarctic Mountains. In: Antarctic Geology Paraschists from the basement complex along Chiles

September-October 1972 149