Discovery of a Tertiary Granite Pluton, Northern Victoria Land

Home , Mount Melbourne

Discovery of a Tertiary granite pluton, percent orthoclase, 10 percent albite (0rq iAbi0), that the albite is Ab98 and that the chlorite is iron-rich. Rims of the magnetite are northern Victoria Land 95 percent megnetite, 5 percent ulvospinel (MtqUvsp), but their cores are Mt70Uvsp30, perhaps indicating primary and secondary phases of this mineral. EDMUND STUMP, The original assemblage appears to have contained two feld- JOHN R. HOLLOWAY, spars, plus quartz. The chlorite presumably was biotite. The and allanite apparently has been undisturbed by secondary effects. SCOTT C. BORG The granite has a minimum melt bulk composition with 75 weight percent silica (Si02) and very low magnesium oxide Department of Geology (MgO) and calcium oxide (CaO). It is slightly peraluminous Arizona State University with 0.9 weight percent normative corundum. Tempe, Arizona 85281 The data from the rubidium/strontium (Rb/Sr) analyses are summarized in the table (X = 1.42 x 10 11 for Rb). Although the RICHARD LEE ARMSTRONG

Department of Geological Sciences University of British Columbia Vancouver, British Columbia V6T 1W5 Canada Analytical data

Initial 87 Rb 87 Sr Sample Sra Rb Age 87S During the 1980-1981 field season our party undertook sys- 86S 86S 865r tematic collection of early-middle Paleozoic granitoids throughout northern Victoria Land (Stump et al. 1982). In the course of a subsequent strontium isotope study, we discovered ETTC 16.2 218 39.1 0.7195 that one intrusion is Tertiary. whole rock This unique pluton is exposed for about 5 kilometers along 9 ± 1 0.7148 the southwestern side of Mariner Glacier, immediately to the ETT 43.4 179 11.93 0.7162 east of the confluence of Meander Glacier (figure), where it chlorite intrudes Bowers Supergroup at a steep angle, showing very little contact effects. a In parts per million. The rock is a medium-grained granite with numerous In millions of years. miarolitic cavities, suggesting a shallow level of emplacement. "ETT" denotes sample number. It is composed of a secondary assemblage of quartz, K-feldspar, albite, chlorite, and accessory amounts of allanite and magnetite. Microprobe analysis indicates that the K-feldspar is 90

date of 9 ± I million years may not represent the primary 6/El 0 crystallization of the magma, there is no doubt that this rock is of Tertiary age and not a part of the Cambro-Ordovician Granite MALTA Harbor Intrusives or the Devonian Admirality Intrusives. Even \ ), PLATEAU if we assume a lowest possible initial ratio (0.704) the age of the whole rock cannot exceed 28 million years. The high initial Sr ratio (0.715) may indicate that the source of the magma was partially melted lower crust, with little or no contamination from mantle basalt. Alkaline basalts and trachytes of the McMurdo Volcanics, locally part of the Hallett Volcanic province, crop out at numerous localities from Cape Adare to Mount Melbourne (Hamilton C N C1 1972; Nathan and Schulte 1967, 1968; Riddols and Hancox 1968). Locally much of the Malta Plateau, across Mariner Glacier from Tertiary P/u ton the Tertiary pluton, is bordered by these volcanics. Dates on McMurdo Volcanics throughout northern Victoria Land range from less than 100,000 years to 8 million years, with one ques- tionable date of 15 million years in the Nathan Hills (Armstrong 1978; Kreuzer et al. 1981). -- C 7 It seems likely that the McMurdo Volcanics provided heat for generation of the granite melt, but the source regions for the mafic and silicic magmas may have been distinct (mantle vs. lower crust). Location map of Tertiary-aged granite, Mariner—Meander Glaciers This research was supported by National Science Foundation area. grant DPP 80-19991.

1983 REVIEW 17

References on Mt. Melbourne, northern Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 10, 422-430. Armstrong, R. L. 1978. K-Ar dating: Late Cenozoic McMurdo Volcanic Nathan, S., and F. J. Schulte. 1968. Geology and petrology of the Camp- Group and dry valley glacial history, Victoria Land, Antarctica. New bell-Aviator divide, northern Victoria Land, Antarctica: Part 1. Post- Zealand Journal of Geology and Geophysics, 21, 686-698. Paleozoic rocks. New Zealand Journal of Geology and Geophysics, 11, Hamilton, W. 1972. The Hallett Volcanic province, Antarctica (U.S. Geo- 940-975. logical Survey Professional Paper 456-C). Washington, D.C.: U.S. Riddols, B. W., and C. T. Hancox. 1968. Geology of the upper Mariner Government Printing Office. Glacier region, north Victoria Land, Antarctica. New Zealand Journal of Kreuzer, H., A. HOhndorf, H. Lenz, U. Vetter, F. Tessensohn, P. Muller, Geology and Geophysics, 11, 881-899. H. Jordon, W. Harre, and C. Besang. 1981. K/Ar and Rb/Sr dating of Stump, E., J. R. Holloway, S. G. Borg, and K. F. Lapham. 1982. Geo- igneous rocks from northern Victoria Land, Antarctica. Geologisches logical investigations on early to middle Paleozoic magmatic rocks, Jahrbuch, B41, 267-273. northern Victoria Land, Antarctica. Antarctic Journal of the U.S., 17(5), Nathan, S., and E J. Schulte. 1967. Recent thermal and volcanic activity 17-18.

Transportation was primarily by motor toboggan-drawn Paleontology of the Lower Paleozoic sledge and during the period 17 November 1981 through 3 of northern Victoria Land: January 1982 we covered some 1,200 kilometers. Our fieldwork was aided substantially by the availability of helicopter support Brachiopods with Australian and New for establishing food and fuel depots and for geological recon- Zealand affinities in the Spurs naissance of outlying areas. Formation Our principal objective was to make fossil collections that would better constrain the age of the Bowers Supergroup and its various formations. In this we were successful and short accounts are available (Cooper, Jago, and Rowell 1982b; in A. J. ROWELL press). In outline, it is now apparent that the oldest beds of the supergroup are much younger than had previously been Department of Geology thought. The base of the Sledgers Group (figure 1) is unlikely to and be older than Middle Cambrian. Earlier views regarded these Museum of Invertebrate Paleontology University of Kansas basal beds of the "Bowers Trough" as Vendian (Cooper et al. Lawrence, Kansas 66045 1982a; Laird 1981; Laird, Bradshaw, and Wodzicki 1982; Tessen- sohn et al. 1981), an age based upon acritarchs that are now R. A. COOPER known to range into post-Vendian rocks (Vidal in Cooper et al. in press). This revised date for the initiation of deposition in the New Zealand Geological Survey trough has considerable paleogeographic implication. Further- Lower Hutt, New Zealand more, it alters our perception of the magnitude of the strati-

J. B. JAGO

South Australia Institute of Technology MIDDLE CAMBRIAN I UPPER CAMBRIAN LORD. Ingle Farm, South Australia SLEDGERS MARINER LEAP YEAR P. BRADDOCK GROUP GROUP GROUP

Picton, New Zealand o 0 M 3o 3Ct) C CM C)

During the austral summer of 1981-1982, we investigated the o 0 I N

paleontology of the Bowers Supergroup of northern Victoria CD Land (figure 1) as event K20 of the New Zealand Antarctic tic)0 Research Program. Although it was under New Zealand spon- C) 3 sorship and had two New Zealand members, the team had an international composition because it also included an Aus- H 11 U 0 3 tralian and an American. Much of the season we worked in Cn close cooperation with members of event K19 (M.G. Laird, J . D. Bradshaw, C. J. D. Adams, and K. Sullivan) who were studying the sedimentology and physical stratigraphy of the same se- Figure 1. Stratigraphic subdivision and age of the Bowers Super- quence of Lower Paleozoic rocks. group (after Cooper et al. in press).

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