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Home , Black Knob, Castle Rock (Antarctica), Hut Point Peninsula, McMurdo Sound, Mount Erebus, Ross Island

significantly below their Curie temperatures (approxi- Wilson, R. L., and N. D. Watkins. 1967. Correlation of mately 550°C.). petrology and natural magnetic polarity in Columbia Plateau basalts. Geophysical Journal of the Royal Astro- Previous work (Pucher, 1969; Stacey and Banerjee, nomical Society, 12(4): 405-424. 1974) indicates that the CRM intensity acquired in a low field is significantly less than the TRM intensity. It thus would appear that if a CRM induced at temperatures considerably below the Curie tempera- Geology of , ture, contributes a significant proportion to the ob- served NRM intensity, too low an intensity value will be assigned to the ancient field. Although it is too early to report a firm value for PHILIP R. KYLE the intensity of the ancient field during the imprint- Department of Geology ing of unit 13 and related flows, we think that the Victoria University strength of the ambient field was more likely to Wellington, have been about 0.5 oe (based on samples at about 141 meters) than about 0.1 oe (based on samples SAMUEL B. TREVES 122.18 and 126.06 meters). The virtual dipole Department of Geology moment (Smith, 1967b) calculated for an estimated University of Nebraska field intensity of 0.5 oe at the site is 7 X 10 25 gauss Lincoln, Nebraska 68508 cubic centimeters. This is larger than the value of 5.5 X 1025 gauss cubic centimeters (Smith, 1967b) Hut Point Peninsula is about 20 kilometers long calculated on the basis of paleointensity experiments and 2 to 4 kilometers wide. It consists of a series of made on some Japanese andesites and basalts esti- en echelon lines of volcanic cones that extend in a mated to be 1 million years old. south-southwest direction from , Ross The authors thank the National Science Founda- Island, . The cones are composed of basan- tion and Dr. L. D. McGinnis, U.S. coordinator of jte and basanitoid with lesser amounts of the Dry Valley Drilling Project, Northern Illinois hawaiite and . Most of the volcanic hones University, for making the samples available for study. of Hut Point Peninsula are on the western side of the peninsula where they constitute a well defined lineament. A subparallel, older, and less well defined References lineament occurs to the east and is traceable frm a point just east of to Cape Armitage. Cox, Allen V. 1966. Paleomagnetic research on volcanic The youngest lineament, however, is transverse, rocks of McMurdo Sound. Journal of the U.S., 1(4): 136. almost at right angles, to the older trends and passes Forbes, R. B., D. L. Turner, and J . R. Carden. 1974. Age from through to Crater of from Ross Island, Antarctica. Geology, 2(6) Hill. Wellman (1964) describes it as a fault. 297-298. Cole et al. (1971) and Kyle and Treves (1973) Gromme, C. S., T. L. Wright, and D. L. Peck. 1969. Magnet- ic properties and oxidation of iron-titanium oxide minerals briefly describe the geology of Hut Point Penirsula. in Alae and Makaopuhi lakes, Hawaii. Journal of This report updates and expands those earlier r4orts Geophysical Research, 74(22) : 5277-5293. and incorporates recent findings (Forbes et al., 974; Jaeger, J. C. 1957. The temperature in the neighborhood of Kyle, 1974; Treves and Au, 1974) and the Msults a cooling intrusive sheet. American Journal of Science, of Dry Valley Drilling Project (DVDP) drilling ir this 255(4): 306-318. Pucher, Rudolf. 1969. Relative stability of chemical and area (Treves and Kyle, 1973; Kyle and Trevs, in thermal remanence in synthetic ferrites. Earth and Plane- press), which greatly enhanced our knowledge cf the tary Science Letters, 6(2): 107-111. subsurface geology of Hut Point Peninsula and our Smith, P. J . 1967a. On the suitability of igneous rocks for understanding of the surface geology re1ationsFips. ancient geomagnetic field intensity determinations. Earth and Planetary Science Letters, 2(1): 99-105. Paleomagnetic measurements (table 1) were rnade Smith, P. J . 1967b. The intensity of the ancient geomagnetic on 1-inch diameter core samples of surface expoures. field: a review and analysis. Geophysical Journal of the Remanent magnetism was measured with a flugate Royal Astronomical Society, 12(4) : 321-362. spinner magnetometer. The samples were not clened. Stacey, F. D. 1967. The Koenigsberger ratio and the nature Instrumental and field orientation errors may be of thermoremanence in igneous rocks. Earth and Planetary Science Letters, 2(1): 67-68. ±20° for declination (D), ±10° for inclination I (I), Stacey, F. D., and S. K. Banerjee. 1974. The Physical Prin- and ±20° for magnetic intensity (J). These weas- ciples of Rock Magnetism. New York, Elsevier. 195p. urements, however, are satisfactory for determining Treves, S. B., and P. R. Kyle. 1973. Geology of DVDP 1 normal and reversed polarity. and 2, Hut Point Peninsula, Ross Island, Antarctica. In: Dry Valley Drilling Project Bulletin 2. DeKalb, Northern The younger olivine-augite basanitoid lavas, the Illinois University. 11-82. hawaiite flows from Half Moon Crater, and the

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basanitoid flow of Crater Hill are normally polarized. Table 1. Paleomagnetic measurements of Hut Point The Observation Hill phonolite is reversely polarized Peninsula volcanic rocks. (Cox, 1966) and, hence, is older than 0.69 million years. (Cox, 1969). Location N J I D K R 95 Polar- ity The volcanic sequence at Hut Point Peninsula is inferred from geologic mapping, potassium-argon North dates, paleomagnetism, and geomorphic evidence. side, Twin The surface flows have been divided into five Crater 6 6.5 -23 322 870 5.994 2.3 N (?) informal sequences. The earlier, preliminary eruptive sequence (Kyle and Treves, 1973) still is the basis South end, of our stratigraphy, but it has been modified to reflect 8 4.0 -80 208 537 7.987 2.4 N new analytical data. Potassium-argon determinations (table 2) indicate that the volcanic activity that built South end, Hut Point Peninsula occurred over a period ranging Half Moon from 0.4 to more than 1.2 million years ago. Crater 1 10.2 -78 61 - - - N The Twin Crater sequence lavas consist of olivine- augite basalt. The flows show little erosional effects Observation Hill, near and are normally polarized. The Black Knob lavas nuclear (figure) of this sequence are 0.43 million years old power and probably are the youngest volcanic rocks of the plant 9 2.7 84 319 89 8.910 5.5 R peninsula (Wellman, 1964). The paleomagnetic data and the age determinations indicate that the rocks Flows, 250 of this sequence were erupted between 0.43 and 0.69 meters north of million years ago. 9 8.7 -88 196 1836 8.996 1.2 N The Half Moon Crater sequence consists of hawai- ite. A flow from Half Moon Crater gives a potassium- X 10 emu/cc argon date of 1.0 ± 0.2 million years (table 2) and N: number of samples; J: intensity of magnetization; D and I: shows normal magnetic polarity (table 1). It is mean declination and mean inclination of remanent magne- tism, respectively; K: precision constant; R: resultant vector; suggested here that Half Moon Crater volcanism 95: semi-vertical angle of 95 percent confidence cone; N: occurred during the Jararnillo Event (Cox, 1969), normal; R: reversed a period of normal polarity that occurred 0.90 to 0.95 million years ago. The Castle Rock sequence consists of olivine-augite, sequence, they may have been erupted during the basanitoid hyaloclastite. A potassium-argon date of Gilsa Event, a period of normal magnetic polarity 1.12 ± 0.4 million years was determined on an 1.61 to 1.79 million years ago (Cox, 1969). olivine-augite basanitoid dike that cuts the hyalo- The hyaloclastite of Castle Rock consists of olivine- clastite of Castle Rock. This age determination is augite basanitoid that, petrographically, resembles the not completely satisfactory (table 2). The hyalo- rocks of the Crater Hill sequence. If they reflect the cl2stite of this sequence is considered to be submarine same general period of igneous activity, the hyalo- or subglacial. clastite probably is an earlier phase of the Crater Hill sequence. The Observation Hill sequence consists of kaer- sutite phonolite and older, benmoreite-mugearite The hyaloclastite of Castle Rock is a sub- lavas. The latter have little surface expression but are marine or subglacial deposit. The turret shape of abundant in the subsurface, as the cores of DVDP holes 2 and 3 show (Kyle and Treves, in press). Table 2. Whole rock potassium-argon age determinations of Hut Point Peninsula volcanic rocks. Forles et al. (1974) indicate that the phonolite of Observation Hill is 1.18 ± 0.03 million years old Sample Location Age Reference (table 2), a date that is consistent with the reversed (million years) polarity (Cox, 1966). The Crater Hill sequence consists of olivine-augite 22892 Black Knob 0.43 ±0.1 R. L.Armstrong 22900 Southwest of Black basanitoid. These lavas show a moderate amount of Knob 0.58±0.06 R.L. Armstrong erosion and are overlain by phonolite lavas of the 22878 Half Moon Crater 1.0 +0.2 R. L. Armstrong Observation Hill sequence at The Gap and at Cape 22879 Dike, Castle Rock 1 . I ±0.4 This paper Armitage. Crater Hill lavas are normally polarized Observation Hill 1.18±0.03 Forbes et al., 1974 near Scott Base. Since they are older than the Victoria University number. reversely polarized lavas of the Observation Hill Written communication.

September-October 1974 233 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- 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 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. (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 . 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 , 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 . 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

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