Rb 87/S r 86

Sr 87

Sr 8E

0.

0.

0.

0

0.

The Age of the Trollkjellrygg Volcanicsof Western Queen Maud Land

R. EASTIN and G. FAUREInstitute of Polar Studies

and Department of GeologyThe Ohio State University

and

D. C. NEETHLING

Geological SurveyPretoria, South Africa

The Trollkjellrygg Group consists of mafic to in-termediate lava flows with minor agglomerate andtuff. These rocks are exposed on Trollkjell Ridge,located west of Jutuistraumen Glacier in westernQueen Maud Land. The geology of this area hasbeen described by Neethling (1970) and Roots(1970), both of whom make reference to the earlierliterature.

We are reporting an age determination by thewhole-rock Rb-Sr method of volcanic rocks belong-ing to the Trollkjellrygg Group. Rb/Sr ratios weredetermined by X-ray fluorescence (Eastin, 1970),while the isotope composition of strontium was meas-ured by standard methods (Chaudhuri and Faure,1967) using a solid-source mass spectrometer. Thereproducibility of the Rb7/Srse ratio is ± 1.0% orbetter, and that of the Sr 87 /Sr86 ratio is ±0.001 orbetter. The data are presented in Table 1 and Figure1.

Eight of the nine rock specimens fit a straight linewithin experimental errors. The slope and intercept

Figure 1. Whole-rock Rb-Sr isochron for mafic to intermediate lavaflows of the Trollkiellrygg Group.

Table 1. Analytical data for the Trollkjellrygg VolcanicFormation of western Queen Maud Land.

OSUField Rb87Sr87No.No.LocalitySr86Sr86 N

atomic

Upper Member (860 m)489B8Bolten2.4960.7374491SN39Sn$kallen10.7560.7210486SN20Snttkallen2.7560.7446

(main nunatak)492SN6"0.8520.7205490SN5"1.6230.7285488SJ22Snp'kjerringa1.5020.7281487SJ11 5.2410.7725

Transitional or Middle Member (460 m)485SL1Nunatak 82021.5600.7284

Lower Member (260m)484U3Utkikken0.7610.7282

I Small satellite nunatak farthest to SE.'Exact location of this outcrop in the sequence is uncertain: it is

either transitional or middle member.All measured Sr87/Sr8" ratios have been normalized to Sr81/Sr88

=0.1194. Average Sr 7/Sr85 ratio of Eimer and Amend IsotopeStandard: 0.7082+0.0005 ((,). Rb/Sr ratios were determined b y XRFanalysis.

of this line were calculated by the least-squares cubicregression of York (1966). The best estimate of theage of these rocks is 856±30 million years and theirinitial Sr87/Sr86 ratio is 0.7097±0.0009 (ARb87 = 1.39 x 10"yr1).

Specimen #484 (from the lower member of theTrollkjellrygg Volcanics) does not fit the isochronand was therefore excluded from the statistical treat-ment of the data. Assuming an initial ratio of 0.710,a model age of 1.72 billion years can be calculatedfor this rock. This date is similar to that of the BorgMetamafics which intrude the Ahlmannrygg Groupand for which Allsopp and Neethling (1970) havereported an age of 1,700±130 million years.

We conclude that the Trollkjellrygg Volcanics arelate Precambrian in age and appear to be signifi-cantly younger than the Ahlmannrygg Group, whichis intruded by the Borg Metamafics.

Acknowledgements. This work was supported bythe National Science Foundation through Grant898—X. The rock samples were collected by B. R.Watters of the Antarctic Division, Geological Surveyof South Africa.

References

Allsopp, H. L. and D. C. Neethling. 1970. Rb-Sr isotopicages of Precambrian intrusives from Queen Maud Land,Antarctica. Earth and Planetary Science Letters, 8: 66-70.

Chaudhuri, S. and G. Faure. 1967. Geochronology of theKeweenawan rocks, White Pine, Michigan. EconomicGeology, 62: 1011-1033.

September—October 1970 157

Eastin, R. 1970. Geochronology of the Basement Rocks ofthe Central Transantarctic Mountains. UnpublishedPh.D. dissertation, Department of Geology, The OhioState University.

Neethling, D. C. 1970. Geology of the Ahlmann Ridge,western Queen Maud Land. Antarctic Map Folio Series,12, sheet 7.

Roots, E. F. 1970. Geology of western Queen Maud Land.Antarctic Map Folio Series, 12, sheet 6.

York, D. 1966. Least-squares fitting of a straight line.Canadian Journal of Physics, 44: 1079-1086.

of Gondwanaland are in preparation. A paper sum-marizing the significance of antarctic geology to theGondwanaland hypothesis appeared recently in thisjournal.

Reference

Craddock, C. 1970. Antarctic geology and Gondwanaland.Antarctic Journal of the U.S., V(3): 53-57.

Analysis of Geologic CollectionsCAMPBELL CRADDOCK

Department of Geology and GeophysicsUniversity of Wisconsin, Madison

Since 1959, the writer and his associates havecarried out eight antarctic geologic field programs,mainly in Ellsworth Land and Marie Byrd Land.This report summarizes progress during the past yeartoward completion and publication of the results ofthis work.

All fossil collections are now under study by spe-cialists. Professor Gerald Webers of Macalester Col-lege is working on the Paleozoic faunas from theEllsworth Mountains, and Dr. James Schopf of theU.S. Geological Survey is studying the Permian florasfrom these mountains. Precambrian stromatolites,collected in the Thiel Mountains in 1959, are in thehands of Mr. William Breed of the Museum ofNorthern Arizona. Fossils collected last year in theJones Mountains—probably plants of Mesozoic age—are under study at the University of Michigan.

Several manuscripts are presently in first draft.Dr. Bernhard Sporli of the University of Aucklandhas completed a preliminary report on the geology ofthe Ruppert Coast. Professor Robert Rutford of theUniversity of South Dakota and the writer havenearly finished a report on Tertiary glaciation in theJones Mountains. Mr. Craig White has completedthe petrographic study of rocks from the EightsCoast and has prepared a preliminary report on thegeology of that area.

The writer has invested most of his time in prepa-rations for the 1970 antarctic geology symposium inOslo and in the geology folio of the American Geo-graphical Society's Antarctic Map Folio Series. Areview paper on antarctic tectonics is in preparationfor the Oslo symposium. Compilation maps of theentire continent showing 1) fossil localities, 2) radio-metric age determination localities, and 3) bedrockgeology have been completed for the AGS folio, anda tectonic map of Antarctica and a reconstruction

Rocks of Coastal Enderby LandNear Molodezhnaya Station, Antarctica

P. B. MYERS, JR. and E. E. MACNAMARA

Department of Geological SciencesLehigh University

Under the auspices of the USARP exchange-scientist program, the junior author conducted pedo-logical, ecological, and geological surveys in coastalEnderby Land from March 1967 to March 1968 asa member of the XII Soviet Antarctic Expedition.Major studies and collections were made in thevicinity of the U.S.S.R. research station Molodezh -naya (67°40'S. 45°51'E.). This note describes rocktypes of the station environs.

All the rock exposures of the area are part of thePrecambrian crystalline basement of the antarcticplatform. The rocks are exposed in a series of ridgeswhich strike in a WNW direction. The principalrock types are finely to coarsely banded amphibolizedpyroxene-plagioclase gneisses and biotite leucogranitegneisses. Foliation in the gneisses strikes roughlyparallel to the exposed ridges and dips predominantlysouthward at steep angles to 700. Mineral assemblagessuggest that the basement gneisses were originallyelevated to granulite facies assemblages, but subsequent retrogressive metamorphism has producedassemblages characteristic of almandine-amphibolitefacies metamorphism throughout much of the area.Along the coast, in the vicinity of "Granat Point,"for example, there is little evidence remaining oforiginal granulite facies metamorphism.

In the granitized and amphibolized pyroxene-plagioclase gneisses of the basement rocks, the lighter-colored bands are composed of plagioclase, commonlyandesine averaging approximately An 3 , quartz, andone or all of the following colored minerals: biotite,hornblende, and pyroxene. The most common py-roxene exhibits a light pink to light green pleochroismand has all of the properties of ferrohypersthenewith the exception of an inclined extinction. Small,

158 ANTARCTIC JOURNAL