Castle Rock resembles table mountains of Iceland Kyle, P. R., and S. B. Treves. In press. Geology of DDP 3, that are subglacially formed. If it is assumed that no Hut Point Peninsula, Ross Island, Antarctica. In: Dry Valley Drilling Project Bulletin Number 3. Dekalb, North- large amount of isostatic uplift occurred, the 413- ern Illinois University. meter elevation (above sea level) of Castle Rock, the McSaveney, M. J . , and E. R. McSaveney. 1972. A reapprisal shape of Castle Rock, and the lithology all suggest a of the pecten glacial episode, Wright Valley, Antarctica. subglacial origin. Antarctic Journal of the U.S., VII(5): 235-240. Treves, S. B., and M. Z. Ali. 1971. Geology and petrography The dated olivine-augite basanitoid dike of Castle of DVDP 1, Hut Point Peninsula, Ross Island, Antarctica. Rock probably is a feeder dike that fed lava to the Dry Valley Drilling Project Seminar 1. Paper, 29 (ab- ice contact zone where the hyaloclastite was formed. stract). In fact, the upper part of the dike is brecciated. This Treves, S. B., and P. R. Kyle. 1973. Geology of DVDP 1 feature suggests that the age of the hyaloclastite is about and 2, Hut Point Peninsula, Ross Island, Antarctica. Dry Valley Drilling Project Bulletin Number 2. Dekalb, the same age as the dike (1.12 ± 0.4 million years Northern Illinois University. 11-82. old). These data suggest that the Ross Ice Shelf may Webb, P. N. 1972. Wright Fjord, Pliocene marine invasion have expanded considerably about 1.1 million years of an antarctic dry valley. Antarctic Journal of the U.S., ago and that it must have been about 400 or so VII(5): 227-234. Wellman, H. W. 1964. Later geological history of Hut Point meters thick. This period of glaciation could corre- Peninsula, Antarctica. Transactions of the Royal Society late with invasions of the dry valleys by ice of the of New Zealand, 2: 149-154. Ross Sea (Denton et at., 1971; Calkin and Bull, 1972). There is some indirect evidence, however, that the pecten glaciation may have occurred more than 3 million years ago (Webb, 1972; McSaveney Genesis of McMurdo volcanics on and McSaveney, 1972). Ross Island It is clear that the hyaloclastite of Castle Rock may be, at least in part, subglacially formed. The SHINE-SOON SUN and GILBERT N. HANSON other hyaloclastite deposits of Hut Point Peninsula, Department of Earth and Space Sciences such as those that occur at Boulder Cones and in State University of New York at Stony Brook the DVDP drill holes (Treves and Kyle, 1973), may be Stony Brook, New York 11794 of submarine or subglacial origin. An exact deter- mination of their nature may contribute to a more exact glacial chronology in the area. The McMurdo volcanics on Ross Island consist of The authors thank Dr. R. L. Armstrong for per- a basanitoid-trachybasalt-phonolite sequence that is mission to use unpublished potassium-argon ages, and part of the Cenozoic volcanic province extending from Dr. C. Adams for the potassium-argon determination Mount Weaver to the Balleny Islands, a distance of of rocks from Castle Rock. This research was done nearly 2,000 kilometers. A suite of 17 volcanic rocks under the partial support of National Science Founda- from Ross Island have been analyzed for rare earth tion grant GV-36950. elements (REE). The upper and lower limits are shown normalized to chondrites in fig. 1. In order to References appreciate the details and to take advantage of the Calkin, P. E., and C. Bull. 1972. Interaction of the East 1 percent analytical uncertainty, fig. 2 shows two Antarctic Ice Sheet, alpine glaciations and sea level in the basanitoids, a trachybasalt, an anorthoclase phonolite, Wright Valley area, southern Victoria Land. In: Antarctic and two kaersutite-pyroxene-anorthoclase phonlites (Adie, R. J , editor). Oslo, Geology and Geophysics . normalized to the basanitoid with the lowest; ME Universitetsforlaget. 435-440. concentration. If the basanitoids are simply related Cole, J . W., P. R. Kyle, and V. E. Neall. 1971. Contributions to Quaternary geology of Cape Crozier, White Island, and by magmatic differentiation of olivine, clinopyroicene, Hut Point Peninsula, McMurdo Sound region, Antarctica. and spinel from a common parent, there should be New Zealand Journal of Geology and Geophysics, 14: a direct correlation between ME and silica conttent. 528-546. Cox, A. V. 1966. Paleomagnetic research on volcanic rocks Since no such correlation exists among five basanitoids of McMurdo Sound. Antarctic Journal of the U.S., 1(4): we conclude that some of the variability in the ME 136. must be related to the extent of partial melting, to Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. The variation in the residual mineralogy of the mantle late Cenozoic glacial history of Antarctica. In: The Late during melting, or to inhomogeneities of the 1E Cenozoic Glacial Ages (Turekian, K. K., editor). New Haven, Yale University Press. 267-306. in the mantle. Forbes, R. B., D. L. Turner, and J . R. Carden. 1974. Age of Goldich et al. (1973) show that the variations of trachyte from Ross Island, Antarctica. Geology, 2: 297- the major and trace element concentration of the 298. Ross Island volcanics can be explained by magmatic Kyle, P. R. 1974. Petrology and mineralogy of DVDP 1 and 2 core samples. Dry Valley Drilling Project Seminar 1. differentiation of minerals that occur as phenocrysts. Paper, 15 (abstract). We thus assume that differentiation is the most 234 ANTARCTIC JOURNAL important process for genesis of the trachybasalts and 4 phonolites, and we explain the REE data for the silica- enriched samples in terms of a differentiation model. Trachybasalts relative to the reference basanitoid have more enrichment of light REE than heavy REE. Using distribution coefficients determined on megacrysts of kaersutite, clinopyroxene ) and anorthoclase, and dis- 0 tribution coefficients from the literature, an nr F- pattern similar to that of trachybasalt can be cal- z culated by separating olivine, clinopyroxene, kaersut- (I) ite (hornblende), apatite, and spinel from basanitoid (fig. 3). The pronounced enrichment of the middle UJ a- REE in kaersutite makes it easy to recognize its im- portance in the differentiation process by the result- (I) ing epletion of middle iiE in the differentiates. Differentiation of kaersutite, however, is not as easily recognized using major elements because it has a bulk chemistry similar to basanitoid. Compared to the trachybasalts, anorthoclase phono- litcs have less middle REE depletion and more heavy REE nrichment. It appears that these volcanics have accu ulated a small percentage of anorthoclase as LaCe Nd SmEuGd Dy Er YbLu sho n by the 10 percent positive europium anomaly (fig. 2). Two kaersutite-pyroxene-anorthoclase pho- Figure 2. REE plot of representative samples of Ross Island vol. nolites—both with about 58 percent silica—have very canics normalized to the basanitoid with the lowest REE concentra- tion. Numerical values are the silica contents in percent (from different REE patterns (fig. 2). The sample with the Goldich, personal communication, 1973). Sample description: 43.5 large negative europium anomaly can be explained percent silica, reference basanitoid; 41.7 percent silica, basanitoid; by separation of more feldspar and the sample with 47.2 percent silica, trachybasalt; 55.7 percent silica, anorthoclase the strongly depleted middle REE can be explained by phonolite; 57.9 and 58.1 percent silica, kaersutite.pyroxene.anor- separation of more kaersutite (hornblende) (fig. 3). orthoclase phonolites. 4 a 3 0 I- U) z Ui I-. Cl) 0 CD 0 w I -J C-) a- (I) LaCe Nd SmEuGd Dy Er YbLu LaCe Nd SmEuGd Dy Er YbLu Figure 3. Preliminary results of model calculations for crystal fractionation. The basanitoid with the lowest REE was used as Figure 1. Upper and lower limits of rare earth element (REE) eon- the parent. Solid lines are the analyzed samples, dashed lines are centrations for McMurdo (Ross Island) volcanics normalized to the calculated differentiates. Percentages of the various minerals chondrites. removed are given for each calculated curve. Ser tember-October 1974 235 Pb207 Pb204 sequence therefore is most likely derived from a part 5.7 of the mantle (plume?) similar to that from which REUNION ASCENSION oceanic island volcanics are derived. Apparently the TRISTAN / Ross Island volcanics only are slightly, if at all, GUADALUPE G. ANARYISAZZO affected by the underlying continental crust or con- MID-OCEAN r tinental mantle. 55 RIDGES ) i—i1----- ICELAND We are indebted to Drs. S. S. Goldich, J. S. (,_— WAIIAN IS. Stuckless, S. B. Treves, and M. G. Mudrey, Jr., for samples and invaluable discussions. This study was 15.3 supported by National Science Foundation rant Gv-040762. Pb20"Pb204 Figure 4. Lead isotope plot of Ross Island volcanic data. Black References dots are basanitoid, open circles are trachybasalt, and squares are phonolite. The fields for the lead data for volcanic rocks from Frey, F. A. 1970. Rare earth and potassium abundan es in oceanic islands and mid-oceanic ridges are from Sun (1973). Saint Pauls rocks. Earth and Planetary Science Litters, 7: 351-360. Goldich, S. S., J . S. Stuckless, N. H. Suhr, and S. B. reves. 1973. Geochemistry of volcanic rocks of the Ross "sland 4 area, Antractica. Geological Society of America, Abstracts, 5: 638-639. Hubbard, N. J . 1973. Some chemical features of lavas from the Manua Islands, Samoa. Pacific Science, 25: 1713-187. Kay, R., and P. W. Gast. 1973. The rare earth content and origins of alkali-rich basalts. Journal of Geology, 81: 653-682. (I) Sun, S. S. 1973. Lead isotope studies of young volcanic w I— rocks from ocean islands, mid-ocean ridges and sland arcs.