Gray, G.M., and A.D.T. Goode. 1981. Strontium isotopic resolution ofmagma dynamics in layered intrusion. Nature, 294(5837), 155-158.

Haensel, J.M., Jr., G.R. Himmelberg, and A.B. Ford. 1986. Plagioclasecompositional variations in ariorthosites of the lower part of theDufek intrusion. Antarctic Journal of the U.S., 21(5).

Himmelberg, G.R., and A.B. Ford. 1976. Pyroxenes of the Dufek intru-sion, Antarctica. Journal of Petrology, 17(2), 219-243.

Himmelberg, G.R., and A.B. Ford. 1977. Iron-titanium oxides of theDufek intrusion Antarctica. American Mineralogist, 62, 623-633.

Hunter, D. 1978. The Bushveld Complex and its remarkable rocks.American Scientist, 66, 551-559.

Irvine, T.N. 1980. Magmatic density currents and cumulus processes.American Journal of Science, 280A, 1-58.

Irvine, T.N., D.W. Keith, and S.G. Todd. 1983. The J-M platinum-palladium reef of the Stillwater Complex, Montana: II. Origin bydouble-diffusive convective magma mixing and implications for the

Bushveld Complex. Economic Geology, 78(7), 1287-1334.McCarthy, T.S., and R.G. Cawthorn. 1980. Changes in initial "Sr/'Sr

ratio during protracted fractionation in igneous complexes. Journal ofPetrology, 21(2), 245-264.

Palacz, Z. A., and S. Tait. 1985. Isotopic and geochemical investigationof unit 10 from the Eastern Layered Series of the Rhum intrusion,northwest Scotland. Geological Magazine, 122(5), 485-490.

Pankhurst, R.J. 1969. Strontium isotope studies related to petrogenesisin the Caledonian basic igneous province of NE Scotland. Journal ofPetrology, 10(1), 115-143.

Taylor, H.P., Jr., and R.W. Forester. 1979. An oxygen and hydrogenisotope study of the Skaergaard intrusion and its country rocks: Adescription of a 55-my. old fossil hydrothermal system. Journal ofPetrology, 20(3), 355-419.

Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. Edinburgh:Oliver and Boyd.

Apatites of the Dufek intrusion, apreliminary study

J.L. DRINKWATER, A.B. FORD, and G.K. CZAMANSKE

U.S. Geological SurveyMenlo Park, California

As part of a continuing study of the mineralogy of the layeredgabbroic Dufek intrusion (82°30'S 50°W), we have begun aninvestigation of the occurrence, distribution, and chemistry ofapatite, the only previously unstudied cumulus mineral in theintrusion. Apatite is generally a minor mineral in layered maficintrusions such as the Dufek and mostly crystallized at a latestage of differentiation from magma enriched in phosphorus(Wager and Brown 1968). The stratigraphy and rock types of theDufek intrusion are described by Ford (1976) and the termi-nology we generally follow is discussed by Irvine (1982).

In the Dufek intrusion, apatite has three principal modes ofoccurrence: (1) cumulus crystals with euhedral, prismaticshape, lying more or less parallel to lamination and layering; (2)postcumulus crystals of subhedral to anhedral shape, occurringin interstices between cumulus silicate and oxide grains; and (3)noncumulus crystals of commonly acicular shape, occurring ingranophyre and other noncumulus rock. Postcumulus apatitevaries up to about 2 millimeters by 4 millimeters in size and iscommonly much larger than cumulus apatite, which generallyranges between 1-3 millimeters in length and 0.05-0.2 milli-meters in breadth. Apparent length/breadth ratios are generallyless than 10 for postcumulus grains but 12-34 for cumulusgrains.

Our search of more than 1,000 thin sections representing allexposed rock types and stratigraphic parts of the intrusionshows that apatite of any type is rare in the lower 1.8 kilometersof stratigraphic thickness exposed in the Dufek Massif, but iscommon in the upper 1.7 kilometers of thickness exposed in theForrestal Range (figure 1). Based on cursory notations duringprevious studies of other minerals (Himmelberg and Ford 1976,1977; Abel, Himmelberg, and Ford 1979), we concluded thatapatite first appeared as a cumulus phase in the stratigraphic

succession about 400-500 meters below the (erosional) top ofthe intrusion. Our more detailed study shows that majoramounts (more than 1.5 volume percent) of cumulus apatitefirst occur about 450 meters below the top (figure 1) but thatminor (0.1-1.5 volume percent) or trace (less than 0.1 volumepercent) amounts occur at lower stratigraphic levels. We havefound local minor amounts of euhedral, cumulus-appearingapatite in an anorthositic (plagioclase cumulate) layer of theSpear Anorthosite Member of the Aughenbaugh Gabbro nearthe top of the Dufek Massif section, which is the lowest occur-rence known. However, gabbro above the layer appears to befree of cumulus apatite. In the lower part of the Forrestal Rangesection, above the Dufek Massif section, trace or minor amountsof cumulus-appearing apatite also locally occur in some pla-gioclase cumulate layers of the Stephens Anorthosite Memberof the Saratoga Gabbro (figure 2) and in overlying gabbro, butstratigraphic occurrences are discontinuous.

The onset of crystallization of major amounts of cumulusapatite occurred ("Ap +" on figure 1B) just before depostion of athick layer of gabbro containing abundant, large inclusions ofnoncumulus anorthosite and leucogabbro. The layer has theappearance of a "megabreccia" and may have formed duringsome kind of disruption event in the magma chamber (Ford andHimmelberg in press). The greatest concentrations of cumulusapatite occur in a 65-meter-thick gabbroic cumulate above theinclusion-bearing layer (figure 1A). Overlying that unit, majoramounts of noncumulus apatite occur in a 25-meter-thick unitof noncumulus mafic and intermediate rock below the Lex-ington Granophyre. Concentrations of apatite (noncumulus)decrease systematically upward in the Lexington at four lo-calities where the granophyre was studied. The stratigraphicvariation in apatite abundances, where known, is closely re-lated to P205 content of the rocks (figure 1B). The relationsshown in figure 1 suggest a record of apatite crystallization inthe Dufek instrusion generally similar to that of the Skaergaardinstrusion of Greenland, in which cumulus apatite crystalliza-tion occurred after 90-98 percent of the magma had crystallized(Brown and Peckett 1977).

An electron microprobe analysis of cumulus apatite (table)from near the base of the gabbroic cumulate unit of figure 1Ashows it to be fluorapatite with a composition similar to those ofsome Skaergaard fluorapatites (Brown and Peckett 1977; Nash

66 ANTARCTIC JOURNAL

"—i" LG

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.9-

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100

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noncumulus

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Figure 1. A. Columnar section showing rock types and apatite abundance near the top of the Dutek intrusion at Sorna Bluff. Height is from top ofinclusion-bearing gabbro layer. B. General columnar section of the Forrestal Range showing location of section A and stratigraphic variation inP205 content of rocks. "LG" is the Lexington Granophyre, and "SAM" is the Stephens Anorthosite Member of the Saratoga Gabbro (Ford 1976)."Ap +" marks the lowest occurrence of abundant cumulus apatite and "Ap + +," the base of the gabbroic cumulate unit which contains anoverall abundance of cumulus apatite.

1976). Preliminary optical and X-ray diffraction studies suggestthat fluorapatite is typical of the Dufek intrusion. Variations inthe compositions of Skaergaard fluorapatites are believed toreflect magmatic processes such as volatile transfer of inter-cumulus liquids and liquid immiscibility (Brown and Peckett1977). To study such processes that may have operated in theDufek intrusion, we plan to analyze a suite of apatites thatrepresents the stratigraphic range of apatite occurrence and thevariety of cumulus, postcumulus, and noncumulus types.

Iron-titanium oxide minerals are locally abundant in the up-per part of the Dufek intrusion (Himmelberg and Ford 1977),but we have not found them to be in any general or particularassociation with apatite, such as found in parts of the Bushveld

Complex of South Africa which contain oxides intergrown with20-40 percent apatite (Reynolds 1985).

An unusual variety of cumulus-appearing apatite that con-tains slender central inclusions of pyroxene or other mineralsparalleling the c axis has been found at several localities of theStephens Anorthosite Member. They resemble the 'infilled hol-low" apatites of a Norwegian layered gabbro instrusion thatGardner (1972) suggests are skeletal crystals containing crys-tallized trapped liquid in interior tubes, formed by rapid growthin a supercooled roof zone, which were recirculated by con-vection until they reached a sufficient size for deposition. Insynthetic systems, apatite formed by quenching frequently hasa central cavity along the length of the crystal (Wyllie, Cox and

1986 REVIEW 67

S...-..-' —.05)—(t r)-5-- --

- (t r)--S.--hcontact — ,- —..----- — .,.- '.... 7-.— .301.60)

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STEPHENSANORTHOSITEMEMBER

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Figure 2. Geologic cross section of the lowest of four anorthositic (plagioclase cumulate) layers of the Stephens Anorthosite Member ("SAM,"figure 113) on west spur of Mount Stephens, showing variation in apatite content. This normally uniform layer consists here of a finer grainedupper sublayer with a channel structure that locally cuts a coarser grained lower sublayer. The unusually abundant apatite (0.35 percent) at onelocality of upper unit is in a plagioclase cumulate also containing unusually abundant postcumulus pyroxene. Apatite contents are dominantlycumulus in the lower sublayer.

Biggar 1962). The occurrence and significance of such apatite inthe Dufek intrusion is presently little known. Support for amagmatic convection-current origin of the plagioclase-cumu-late layers (Ford and Himmelberg in press), such as shown infigure 2, would be provided if this type of apatite is found in our

Electron microprobe analysis of a cumulus apatite from the Dufekintrusiona

Compound

Weight percent

Si02 0.15FeO .33CaO

54.7P205 41.5Ti02 .01La203 <.01Ce203 .14Sm203 <.01Dy203 <.01Cl .10F

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1.39

Total

98.84

a Analysis in weight percent. Sample from near base of gabbro cumulatelayer shown in figure 1A.

continuing study to be generally associated with the pla-gioclase-cumulate layers.

The principal crystallization of apatite in the Dufek intrusionoccurred in the latest stages of magma differentiation (figure 1),but earlier and discontinuous periods of crystallization are indi-cated by the sporadic occurrences of cumulus-appearing apatiteat lower stratigraphic levels, such as in the StephensAnorthosite Member (figure 2) and the Spear AnorthositeMember of the Dufek Massif section. The origin of anorthositic(plagioclase cumulate) layers in layered intrusions is controver-sial (Czamanske and Schiedle 1985), and our studies of apatiteoccurrences in the Dufek intrusion will contribute, in conjunc-tion with isotopic and other petrologic studies in progress(Ford, Kistler and White, Antarctic Journal, this issue; andHaensel, Himmelberg and Ford, Antarctic Journal, this issue), toan understanding of the origin of such anorthosite layers.

This work was supported in part by National Science Founda-tion grant DPP 80-20753 to the U.S. Geological Survey.

References

Abel, K.D., G.R. Himmelberg, and A.B. Ford. 1979. Petrologic studiesof the Dufek intrusion: Plagioclase variation. Antarctic Journal of theU.S., 14(5), 6-8.

Brown, G.M., and A. Peckett. 1977. Fluorapatites from the Skaergaardintrusion, East Greenland. Mineralogical Magazine, 41, 227-232.

Czamanske, G. K., and D. Schiedle. 1985. Characteristics of the band-ed-series anorthosites. In G.K. Czamanske and M.L. Zientek (Eds.),The Stillwater Complex, Montana: Geology and guide. (Montana Bureauof Mines and Geology Special Publication, Vol. 92.)

68 ANTARCTIC JOURNAL

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 crystalliza-tion 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).

Gardner, P.M. 1972. Hollow apatites in a layered basic intrusion, Nor-way. Geological Magazine, 109(5), 385-392.

Ilaensel, J.M., Jr., G.R. Himmelberg, and A.B. Ford. 1986. Plagioclasecompositional variations in anorthosites of the lower part of theDufek intrusion. Antarctic Journal of the U.S., 21(5).

Flimmelberg, G. R., and A. B. Ford. 1976. Pyroxenes of the Dufek intru-sion, Antarctica. Journal of Petrology, 17(2), 219-243.

Himmelberg, CR., and A.B. Ford. 1977. Iron-titanium oxides of theDufek intrusion. American Mineralogist, 623-633.

Irvine, T.N. 1982. Terminology for layered intrusions. Journal of Pe-trology, 23(2), 127-162.

Nash, W.P. 1976. Fluorine, chlorine, and OH-bearing minerals in theSkaergaard intrusion. American Journal of Science, 276, 546-557.

Reynolds, I.M. 1985. Contrasted mineralogy and textural relationshipsin the uppermost titaniferous magnetite layers of the Bushveld Com-plex in the Bierkraal area north of Rustenburg. Economic Geology, 80,1027-1048.

Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. Edinburgh:Oliver and Boyd.

Wyllie, P.J., K.G. Cox, and G.M. Biggar. 1962. The habit of apatite insynthetic systems and igneous rocks. Journal of Petrology, 3(2),238-243.

Aerogeophysical survey yields newdata in the Weddell Sea

J.LABRECQUE, S. CANOE, R. BELL, and C. RAYMOND*

Lamon t- Doherty Geological ObservatoryPalisades, New York 10964

J. BioziNA

Naval Research LaboratoryWashington, D.C. 20,375

M. KELLER*

Inst it uto Antartico ArgentinaBuenos Aires, Argentina

J.C. PARRA and C. YANEZ*

Servicio Nacional de Geologia y Miiu'riaSantiago, Chile

Plate tectonic models attempting to describe the tectonic evo-lution of west Antarctica have been handicapped by a lack ofdata in the ice-covered regions surrounding the Antarctic Pen-insula. While marine geophysical data have been collected andthe general pattern of seafloor spreading determined for mostof the basins surrounding the Antarctic Peninsula, details of thedevelopment of critical passageways and the timing of riftinghave remained obscure. This situation is now changing as aresult of a state-of-the-art aerogeophysical survey initiated dur-ing the 1985-1986 austral summer. The aerosurvey is designedto gather magnetic and gravity data required to address theseproblems. The 3-year program complements surveys con-ducted in previous years by the British Antarctic Survey (BAS)(Renner, Sturgen, and Garrett 1985) and workers in the SovietUnion (Masolov 1980), as well as geophysical data gathered byprevious marine expeditions of various countries. The first yearof the U.S-Argentine-Chilean (usAc) aerogeophysical survey

* Members of the U.S. -Argentine-Chilean (usAc) survey team

has gathered nearly 74,000 kilometers of magnetics data overthe Weddell Sea, Bransfield Straits, and Drake Passage, and20,000 kilometers of gravity data in the Weddell Sea. Flight linesare shown superimposed on a bathymetric map of the region infigure 1.

The aerosurvey covered much of the unsurveyed westernWeddell Basin. Preliminary inspection of the data reveals theextension of the Late Cretaceous to Paleogene magnetic anoma-lly sequence into the western Weddell Basin where it disap-pears into a fossil subduction zone which borders the PowellBasin, the southeast Orkney platform and the Scotia Ridge(figure 2). Dense magnetics and gravity coverage of the regionhas been obtained to determine the development of the subduc-tion zone and support the planned Ocean Drilling Program(oDP) drill sites in the region. An abrupt transition to a smoothmagnetic pattern is noted in the westernmost Weddell Basin.This may be caused by deep sedimentation, fracture-zone off-sets, or continental crust. The anomaly lineation direction isobserved to change from a predominantly east-west direction inthe eastern Weddell to a northwest-southeast direction in thenorthwestern Weddell. The magnetic anomaly lineation patternsuggests the Late Cretaceous to Oligocene opening of the Wed-deli Basin occurred about a pole of rotation located in the south-western part of the basin. The Jurassic and Early Cretaceousanomaly sequence is difficult to map in the southwestern Wed-deli and further analysis of the magnetic anomaly data is beingcarried out to determine the pre-Cretaceous opening history ofthis key area.

Another clue to the early rifting of the Antarctic Peninsulaaway from Gondwana was discovered during the aerosurvey. Ahigh-amplitude, lineated magnetic anomaly bounds the south-ern Weddell Basin. The "Orion anomaly," as we refer to it,(named for the survey craft, a U.S. Navy P-3 Orion) was mappedby the aerosurvey and likely represents the ocean-continenttransition anomaly. The east-west lineation direction of the Ori-on anomaly at the southern boundary of the basin differs fromthe northwestern strike of the anomalies mapped in the north-western Weddell Basin, implying that at least two poles ofopening are required to describe the development of the Wed-deli Basin. The shape of the Orion anomaly should yield infor-mation on the possible post-rifting tectonism of west Antarcticaas well as the relative motion between east and west Antarctica.

High-altitude data, in which a clear anomaly sequence with adistinct central anomaly have been identified, were collected

1986 REVIEW 69