Accretionary lapilli andlithophysal spherulites from theTaylor Formation, Queen Maud

Mountains

8430'S

EDMUND STUMP*Institute of Polar StudiesThe Ohio State UniversityColumbus, Ohio 43210

During the 1970-1971 field season, we found oc-currences of accretionary lapilli and lithophysalspherulites, both features of volcanic rocks, in theEarly Cambrian Taylor Formation adjacent toShackleton Glacier (Stump, 1976) (figure 1).

Moore and Peck (1962), in summarizing accre-tionary lapilli, conclude that they are formed byconcentric accretion of moist ash in an airbornecloud during explosive volcanic eruptions.

A specimen of the Taylor Formation containingaccretionary lapilli was found high on a ridge crestnorth of Mount Orndorf, and though it was not inplace it probably was locally derived. The accre-tionary lapilli occur as usually intact, aimed ellip-soids with long axes to 5-10 millimeters in length,and packed so that adjacent ellipsoids are in con-tact (figure 2). Compositionally the lapilli consist ofmicrocrystalline quartz and feldspar, calcite, andminor amounts of opaque minerals. In additionsome small (to 0.1 millimeter) crystal fragments ofquartz and plagioclase occur with long axes ar-ranged concentrically within the ellipsoids. A slightdecrease of grain size occurs outwardly in the ellip-soids, and most have an outer shell (5 millimeters)of slightly darker material. The surroundingmatrix is composed of similar material, along withbroken crystals of plagioclase and quartz (to 1 centi-meter) and patches of sparry calcity. Also presentare forms suggesting devitrified glass shards.

Although this is the first report of accretionarylapilli from the "basement" rocks of the Transant-arctic Mountains, they have been found in the lowerMesozoic Prebble Formation of the BeardmoreGlacier area (Barrett, 1969; Barrett and Elliot,1972), and in Cenozoic volcanic rocks of the Execu-

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*Now at: Department of Geology, Arizona State University,Tempe, Arizona 85281. Figure 1. Location map.

246 ANTARCTIC JOURNAL

Figure 2. Accretionary lapilli. North of Mount Orndorf, sampleES-02.

tive Committee range in Marie Byrd Land (Dou-mani, 1960).

Spherulites are spherical masses of elongate crys-tals radiating from a single point. They often growduring devitrification of glassy, volcanic rocks andare found in various units of the Taylor Formation(Stump, 1974). Under certain circumstances sphe-rulites are found with radial or concentric cavities,called lithophysae, which may or may not be filledin by later mineral growth. The "thunder eggs" ofOregon are a type of lithophysal spherulite fam-iliar to many rockhounds.

In discussing the origin of these features, Ross(1941) concluded that volatiles held in solution involcanic glass are released during spherulitic crys-tallization of anhydrous feldspar and cristobalite,and that the increasing pressure of these releasedvolatiles combined with shrinkage due to cooling ofthe enclosed material causes the opening of thecavity.

Spherulites with both radial and concentric litho-physae were found at Taylor Nunatak, the informaltype locality of the Taylor Formation (Wade et al.,1965). They occur in porphyritic felsites containingembayed quartz and euhedral plagioclase pheno-crysts.

The concentric spherulites are 1-2 centimeters indiameter and are bright pink, in contrast to thedark-gray country rock enclosing them (figure 3.Microscopically it can be seen that the spherulittare composed of very fine, radiating crystals olquartz and feldspar. The lithophysae, which ai unot developed in all spherulites of this type, are1-2 millimeter-wide crescentic openings, arrangedroughly parallel to the margins of the spherulitesand filled in by anhedral quartz and in some casesa little calcite.

The spherulites with radial lithophysae are 1-3centimeters in diameter and are dark-brown bodies

December 1976

Figure 3. Spherulites with concentric lithophysae. TaylorNunatak. Polished slab, sample ES-01.

set in a medium-brown groundmass (figure 4). Inthin section, the spherulites appear as recrystallizedmosaics of K-feldspar and perhaps quartz. Quartz,calcite, and minor muscovite have grown in thestar-shaped lithophysae.

The discovery of accretionary lapilli and litho-physal spherulites in rocks of the Shackleton Gla-cier area serves to further substantiate the volcanicnature of much of the Taylor Formation foundthere (Wade, 1974).

This research was supported by National ScienceFoundation grant GV-26652.

ReferencesBarrett, P. J . 1969. Stratigraphy and petrology of the mainly

fluviatile Permian and Triassic Beacon rocks from Beard-

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Figure 4. Spherulites with radial lithophysae. Taylor Nunatak.Polished slab, sample ES-b.

247

more Glacier area, Antarctica. Columbus, The Ohio StateUniversity, Institute of Polar Studies. Report, 34. 132p.

Barrett, P. J . , and D. H. Elliot. 1972. The early Mesozoic vol-caniclastic Prebble Formation, Beardmore Glacier, Antarctica.In: Antarctic Geology and Geophysics (R. J . Adie, editor). Olso,Universitetsforlaget. 403-409.

Doumani, G. A. 1960. Geological observations in West Antarc-tica during recent oversnow traverses. New York, AmericanGeophysical Union. Transactions, 41: 706-710.

Moore, J . G., and D. L. Peck. 1962. Accretionary lapilli in vol-canic rocks of the western continental United States. Journalof Geology, 70: 182-193.

Ross, C. S. 1941. Origin and geometric form of calcedony-filledspherulites from Oregon. American Mineralogist, 26: 727-732.

Stump, Edmund. 1974. Volcanic rocks of the Early CambrianTaylor Formation, central Transantarctic Mountains. Antarc-tic Journal of the U.S., IX(5): 228-229.

Stump, Edmund. 1976. On the late Precambrian-early Paleozoicmetavolcanic and metasedimentary rocks of the Queen MaudMountains, Antarctica, and a comparison with rocks of similarage from southern Africa. Columbus, The Ohio State Uni-versity, Institute of Polar Studies. Report, 62.

Wade, F. A. 1974. Geological surveys of Marie Byrd Land andthe central Queen Maud Range. Antarctic Journal of the U.S.,IX(5): 241-242.

Wade, F. A., V. L. Yeats, J . R. Everett, D. W. Greenlee, K. E.LaPrade, and J . C. Shenk. 1965. The geology of the centralQueen Maud Range, Transantarctic Mountains, Antarctica.Antarctic Research Report Series, 65-1. Lubbock, Texas Tech-nological College. 54p.

Weathering and mineral synthesisin antarctic soils

F. C. UG0LINICollege of Forest ResourcesUniversity of Washington

Seattle, Washington 98195

In the polar regions emphasis has been placed onthe prevalence of physical versus chemical weather-ing, and in Antarctica the results of a study of Kellyand Zumberge (1961) would seem to preclude anypossibility of chemical weathering of significancetoday. Other studies conducted in the Mirnyy oasisand in the ice-free areas of southern Victoria Landadvanced evidence that mineral alteration is pres-ently occurring (Glazovskaia, 1958; Ugolini, 1964;Claridge, 1965; Linkletter, 1971; Behling, 1971;Bardin and Konopleva, 1975). Direct evidence of(1) ionic migration and (2) a continuous unfrozenfilm of water at the surface of the soil particles inthe frozen antarctic soils support the possibility ofcontemporary chemical weathering (Ugolini and

Grier, 1969; Ugolini and Anderson, 1973). Pre-vious studies in southern Victoria Land (Ugoliniand Bull 1965; Linkletter, 1971; Behling, 1971;Ugolini et al., 1973) have shown that silt and claypercentages increase from younger to older soils.These results apparently indicate that weatheringis occurring and that there is a weathering-timerelationship. These findings suggest two possiblemechanisms for the formation of clay: (1) synthesisof new minerals and/or (2) comminution or fractur-ing of existing minerals. To establish whether theincrease of silt and clay is due to either synthesesof clay minerals or to comminution or both, threesoils from Wright Valley, southern Victoria Land,were selected: one relatively young, one of inter-mediate age, and one old. The particle-size analy-sis performed on these samples emphasizes theimportance of the parent material on the mechani-cal constituents of soils. The old soil derived fromgranitic bedrock has almost as much clay as theintermediate soil formed on the moraine where theglacier had provided the initial grinding of theminerals. The young soil, however, despite deriva-tion from glacial deposits, has the lowest clay con-tent. Free-iron oxides, extracted either with oxalate(Fe.) or with dithionate (Fed), are used as indices ofchemical weathering. The Fe O/Fed ratio is related tothe degree of crystallinity of the iron oxides lib-erated through weathering. The lowest ratio re-corded in the old soil indicates a high degree ofcrystallinity, long exposure, and high intensity ofweathering. Thin sections of selected rock frag-ments at different depths from the old soil showthat feldspar is altering to mica and that the de-gree of weathering increases toward the surface.The best documented case of in situ clay synthesiswas found in the old profile (Jackson et al., in press).Here the feldspar in the granitic rock was weath-ered, in the soil, into a clay mica that, in turn, wasweathered into montmorillonite that subsequentlywas interlayered with iron, forming chloritizedmontmorillonite. The mineralogy of the young pro-file shows that the feldspar amount decreases withparticle size but still persists in the fine clay. Themajor minerals in this fraction include montmoril-lonite and mica vermiculite intergrade. Bothminerals are considered to be authigenic. In theintermediate profile, montmorillonite and mica-vermiculite intergrade occur in the coarse clay frac-tion; both minerals are considered to be authigenic.In this profile the prevailing minerals in the fineclay fraction are mica- vermiculite intergrades;whereas a vermiculite- montmorillonite intergradeis the major constituent of the coarse clay fraction.Both mineral assemblages are considered to beauthigenic.

Although not fully documented, the weatheringsequence in the young and intermediate soils seems

248 ANTARCTIC JOURNAL