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Petrographic and Field Characteristics of Marie Byrd Land Volcanic Rocks Volcanic Rocks of the Ross Island Area

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Petrographic and Field Characteristics of Marie Byrd Land Volcanic Rocks Volcanic Rocks of the Ross Island Area References flows in the basal sequences, but ultramafic nodules Cast, Paul W., G. R. Tilton, and Carl Hedge. 1964. Iso- are much more common in the parasitic cones. Most topic composition of lead and strontium from Ascension of the nodules examined contain about 50 percent and Cough Islands. Science, 145(3637): 1181-1185. olivine, plus variable proportions of orthopvroxene, Halpern, M. 1968. Ages of antarctic and Argentine rocks clinopyroxene, and a brown garnet tentativel y identi- bearing on continental drift. Earth and Planetary Science Letters, 5: 159-167. fied as melanite. Harrington, H. J . 1958. Nomenclature of rock units in the Stratovolcanoes, many exceeding 4,000 m (13,000 Ross Sea region, Antarctica. Nature, 182(4631): 290. feet) in height, make up most of the Flood, Ames, Jones, L. M. and G. Faure. 1968. Origin of the salts in and Executive Committee Ranges, Mount Takahe, Taylor Valley. Antarctic Journal of the U.S., 111(5): Toney Mountain, and the Crary Mountains. They are 177-178. Wade, F. Alton. 1967. Geology of the Marie Byrd Land composed of trachyandesite flows and tuff breccias, coastal sector of West Antarctica. Antarctic Journal of and apparently lesser amounts of trachyte and the U.S., 11(4): 93-94. rhyolite. Mounts Waesche and Hartigan are stratovol- canoes that are exceptional in that each is composed of a large proportion of basalt. The trachyandesites are rich in olivine and soda-iron pyroxene, which may be found in the groundmass or as phenocrysts. Some Petrographic and Field Characteristics of these rocks carry modal nepheline and socialite. of Marie Byrd Land Volcanic Rocks Feldspar phenocrysts in the trachyandesites are most commonly anorthoclase, similar to that described by \VESLEY E. LEMASURIER Boudette and Ford (1966). University of Colorado The more acid rocks are marked by the disappear- Denver Center ance of olivine, the appearance of alkali amphibole in some samples, and by sanidine as phenocrysts in place Preliminary petrographic studies of samples col- of anorthoclase. Aegerine-augite appears as pheno- lected during the Marie Byrd Land Survey of 1967- crysts in even the most acid rocks, and quartz pheno- 1968 indicate that the lavas in this entire region are crvsts are found in some of the rhyolites. alkaline. Alkalinity is indicated petrographically by the The trachytic and rhyolitic rocks all appear to be presence of alkali pyroxenes in virtually all rocks peralkaline, and the entire petrographic province is except basalts, and by the predominance of alkali unusual in that both silica-undersaturated and silica- feldspar as phenocrysts in the andesites and the more oversaturated varieties of peralkaline rock are found. acid rocks of the region. Similar observations were made by petrographers that accompanied the earlier References oversrinw traverses to various parts of this area (An- Anderson, V. 11. 1960. The Petrography of Some Rocks from Marie Byrd Land, Antarctica. Ohio State Univer- derson, 1960: Doumani and Ehlers, 1962). sity. USNC-IGY Antarctic Glaciological Data: Field Fhc basaltic rocks in central Marie Byrd Land are Work 1958 and 1959. Report 825-2, Part VIII. 27 p. Boudette, E. L. and A. B. Ford. 1966. Physical properties characterized inineralogicallv by olivine and titan- of anorthoclase from Antarctica. American Mineralogist. atigite in the groundmass, and by plagioclase pheno- 51(9-10): 1374-1387. crysts and groundmass grains with broad, soda-rich Doumani, C. A. and E. C. Ehlers. 1962. Petrography of rims. These rocks are found principally in two geo- rocks from mountains in Marie Byrd Land, West Ant- logic occurrences: (1 a basal sequence of horizontal arctica. Geological Society of America. Bulletin. 73(7) 877-882. flows or hyaloclastite beds, and (2) parasitic cones on the flanks of stratovolcanoes. The basal sequence rests on a nearly flat bedrock erosion surface which is exposed in various ni.inataks along the Hobbs Coast, at Mounts Aldaz and Murphy, and in the Kohler Volcanic Rocks of the Ross Island Area Range. In the Crary Mountains, and apparently also at Toney Mountain, a basalt sequence forms a foun- SAM cci. B. IREVES dation upon which the stratovolcanoes of these ranges Department of Geology were built. The basaltic character of hyaloclastites in Unizersitv of Nebraska the basal sequences is recognizable in clasts that are composed of sideromnelane or tachylite, with pheno- During parts of 1968 and 1969, the data and rocks crysts of olivine, titanaugite and labradorite. collected during the 1967-1968 field season in the The parasitic cones are composed of beds of cinders Ross Island area were studied. The field observations and bombs, interbedded with basalt flow rock. The and mapping and subsequent work on specimens and flows are petrographically indistinguishable from thin sections indicate the following at this time September—October 1969 207.
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