(oral p.) D.X. Beltont, F. Lisker2, H.J. Gibson3 & C.J.L. Wilson4
'School of Earth Sciences, The University of Melbourne Vic 30 I 0 Australia;
<dxbelton@unimelb.edu.au>
2Fachbereich Geowissenschaften. Universitat Bremen, D-28334 Bremen, Germany, <flisker@uni-bremen.de>
3Geotrack International, 37 Melville Rd, Brunswick West Vic 3055, Australia, <daisy@on2k.com.au>
4School of Earth Sciences, The University of Melbourne Vic 30 I 0 Australia, <cj I w@unimelb.edu.au>
The Phanerozoic to recent crustal evolution of the southern Prince Charles Mountains has until now been shrouded in mystery. Unlike similar basement exposures in the nPCMs, the sPCMs lack any recognised sedimentary deposits young enough to have provided some insight into the regions recent geological development. In the absence of traditional lithological and (bio)stratigraphic markers, thermochronological techniques provide an unparalleled opportunity to extract information on the Phanerozoic burial and/or exhumation phases of often very old basement rocks. Because of their low temperature sensitivity, shallow crustal thicknesses of up to -4 km are best characterised by the complimentary techniques of apatite fission track analysis and apatite (Uffh)/He analysis.
23
-Here we report on a series of samples based on material collected during ASAC Project 2137, as a preliminary to the recent PCMEGA program. While initial sampling was restricted to three vertical profiles retrieved from the southern Mawson Escarpment, the work, however, provided a framework for a far more extensive sampling program during PCMEGA. This recent expedition focussed on key topographic and structural features and their likely relationship to currently exposed regional landforms as well as geophysical anomalies detected beneath the present ice cover.
The East Antarctic Shield has long been considered to be a key lithospheric block in the former Rodinia and Gondwana. It formed the nucleus for growth and subsequent dispersion of both supercontinents. In the southern PCM, the shield terrain is dominated by Archaean rocks consisting of a gneissic basement complex overlain by a number supercrustal sediments and volcanic units. The rocks are characterized by pervasive ductile deformation and metamorphic grades up to amphibolite facies. These overprints are attributed to episodes of orogenesis related to the suturing of the proto-Indian and proto-Antarctic continents occurred during the Mesoproterozoic. Deep-seated tectono-metamorphic activity appears to have ceased with the conclusion of the Pan-African event. In contrast, the Phanerozoic history of the sPCMs is characterised by a series of rifting/denudation phases, most of was associated with the development of the Lambert Graben and the break-up of Gondwana.
Fission track analysis of samples from the southern PCM suggests a Phanerozoic history dominated by a Late Paleozoic initial rifting of the Lambert, Triassic to Early Cretaceous subsidence, followed by a Late Cretaceous to recent(?) rifting. The period of Late Cretaceous exhumation is particularly constrained by the presence of a remarkably widespread (pre-glacial - early Tertiary?) erosion surface that extends virtually the entire length of the Mawson Escarpment. Many of the massifs and smaller nunataks bordering the Lambert Graben preserve a similar palaeosurface at their summits. Results suggest that during the Permo-Triassic cooling (early rifting phase), the southern PCM experienced denudation of -3-4.5 km, while Cretaceous cooling (reactivation of the Lambert Graben?) generated -1.7-2.5 km of exhumation. The timing of these events duplicates the style of cooling histories previously interpreted for the northern PCM, however the overall magnitudes of section removed appear more modest in the southern PCM.
Sm-Nd ages of metamorphosed volcanic and plutonic rocks from Mount Ruker, the southern Prince Charles Mountains, East Antarctica
(poster p.) B.V. Belyatsky1, E.N. Kamenev2, a.A. Laiba3 & E.V. Mikhalsky2
1 Institute of Geology and Geochronology of Precambrian, St. Petersburg, Russia;
2VNII0keangeologia, St. Petersburg, Russia;
3Polar Marine Geological Research Expedition, Lomonosov, Russia.
Low grade metamorphic volcanic-sedimentary sequence at Mount Ruker includes presumably meta-volcanic members and layers of green schist (fine-grained rocks composed of chlorite, actinolite, hornblende, mica, carbonate, quartz, plagioclase in varying proportions; the color index is mostly between 40 and 80) of basaltic to dacitic composition (bimodal distribution from available data) and voluminous metasedimentary slate, schist, and quartzite. The sequence was intruded by mafic sills composed of meta-gabbro-dolerite (compared with the metavolcanics, it is essentially coarser-grained metamorphic rock with commonly preserved magmatic textures). Metavolcanics predominate in the southern, poorly exposed slopes, while metasediments make up most of the northern, steeper and much better exposed slopes. Later metadolerite and dolerite dykes (partly altered due to autometa-morphic processes) of tholeiitic composition and larger gabbroic bodies cut across the sequence, and dykes were also found to cut easterly located granite pluton dated at ca 3000 Ma ..
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-Three rock groups were studied for Sm-Nd isotope compos1t1on: metavolcanics, metagabbro-dolerites, and unmetamorphosed gabbro and dolerite. Metavolcanic rocks (two metabasalts, metaan-desite, and metadacite) define an isochron with an age of 2917 ±82 Ma (MSWD = 0.33; Nd; = 0.508446 ±0.000080). Three metagabbro-dolerite samples define an isochron with similar age of 2832
±140 Ma (MSWD = 0.25) but with somewhat lower Nd; = 0.50801 ±0.00014. Mineral separates (plagioclase, biotite) from one metagabbro-dolerite plot exactly along this reference line, which changes the isochron age to 2878 ±65 Ma (MSWD
=
0.639; Nd;=
0.507985). Both plagioclase and biotite are thought to be metamorphic minerals, thus the constrained age of c 2900 Ma is most I ikely to reflect the age of metamorphism, though low Nd; value precludes long premetamorphic crustal residence time. The rnetavolcanics are likely to be roughly co-eval with the metagabbro-dolerites, but their slightly higher Nd; value suggests that they originated from somewhat different mantle source or experief)ced more pronounced crustal contamination .. Granophyric gabbro collected in the southern slopes of Mount Ruker is one of only a few unaltered rocks. Its whole rock and mineral separates compositions (plagioclase, clinopyroxene) define a three-point reference line corresponding to an age of 2365 ±65 Ma (MSWD = l.8; Nd;= 0.508951 ±0.000062). Two dyke dolerite samples plot roughly along this line, putting five-point isochron age to 2400 ±200 Ma (MSWD=
4.33; Nd;=
0.508934).The initial Nd ratio is somewhat higher than in other measured rock types, providing evidence that these plutonic rocks derived from different (less enriched) mantle source. The age of these mafic plutonic rocks correspond to the Rb-Sr ages of high-Mg dykes in the Vestfold Hills (c 2400 Ma, COLLERSON & SHERATON 1986) and Enderby Land (c 2350 Ma, SHERATON & BLACK 1981), though later U-Pb studies showed younger age of the Vestfold Hills dykes (2240 Ma, LANYON et al. 1993).
Individual rock samples have rather low £Nd(T = 2.9 Ga) values (-8-9 for metavolcanic and fresh plutonic rocks, -18 for matagabbro-dolerites), which is significantly lower than £Nd values for granites and granite/plagiogranite gneisses in the southern Mawson Escarpment and Mount Ruker (0- -5) (MIKHALSKY et al. 2001 ), which implies that these rocks are not likely to contribute to the studied rocks isotopic compositions. The studied rocks were probably derived from variously enriched mantle sources with no significant crustal contamination, unless essentially older and yet undiscovered crustal protolith is involved.
Collerson, K.D. & Sheraton, J.W. (1986): J. Petrol. 27: 853-886.
Lanyon. R., Black, L.P. & Seitz, H.M. (1993): Contr. Mineral. Petrol. 115:184-203.
Mikhalsky, E.V., Sheraton, J.W .. Laiba. A.A. et al. (2001): AGSO Bulletin 247.
Sheraton, J.W. & Black, L.P. (1981 ): Comr. Mineral. Petrol. 78: 305-317.
Holocene history of George VI ice shelf, Antarctic Peninsula:
inferences from lake sediments (oral p.)
M. Bentley,1 J.A. Smith1'2
, D. Hodgson2, S. Roberts3, M. Barrett2, C. Bryant4, M. Leng5, P. Noon2, D.
Sugden3 & E. Verleyen6
1Department of Geography, Univ of Durham, Science Laboratories, South Road, Durham, DHI 3LE, UK;
<m.j.bentley@durham.ac.uk>;
2British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, UK;
3School of Geosciences, Univ of Edinburgh, Drummond Street, Edinburgh, EH8 9XP, UK;
4NERC Radiocarbon Lab, Scottish Enterprise Technology Park, East Kilbride, G75 OQF, UK;
5NERC Isotope Geoscience Laborat., Brit. Geol. Surv., Keyworth, Nottingham, NG 12 5GG, UK;
6Department of Biology, Univ of Ghent, Krijgslaan 281 S8, B-9000 Gent, Ghent, Belgium.
The recent collapse of several Antarctic Peninsula ice shelves has highlighted the need for a long-term perspective on ice shelf stability. An opportunity to obtain a highly detailed late-Quaternary history of one ice shelf is provided by Moutonnee Lake (ML). ML is a large, 50 m deep epishelf lake
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-impounded on the eastern side of Alexander Isl. by George VI Ice Shelf, which flows across George VI Sound from the Antarctic Peninsula. Detailed water chemistry measurements have shown ML to be clearly stratified. A distinct halocline occurs at 30 m, where modem freshwater overlies marine water. Our hypothesis is that any changes in the stability of the ice shelf should leave distinct biological, chemical and lithological signatures in the lake sedimentary record. Biologically, ice shelf loss would see the present stratified water column replaced by a purely marine one. Significant changes in sedimentation are also likely to follow any change in the configuration of George VI Ice Shelf. Outcrops of igneous and metamorphic rocks with distinctive chemical and isotopic signatures are found on the western coast of the Antarctic Peninsula. These differ from the rocks of Alexander
Isl., which are predominantly sedimentary in origin. Restricted ranges of igneous and metamorphic
clasts, transported through George Vl Ice Shelf to Alexander Isl., and deposited with locally derived sedimentary clasts, would be replaced, during periods of ice shelf loss, by a wide lithological assemblage dominated by ice-rafted debris and locally derived sedimentary clasts.
To test this hypothesis, high-resolution studies have been performed on two sediment cores extracted from ML. Analyses have included physical (MS, LOI and CaC03), biological (diatom and foraminifera) and isotopic measurements performed on both bulk sediment samples and individual foraminifera. Results demonstrate the existence of two distinct zones, where marine organisms (foraminifera and marine diatoms) and local sedimentary clasts dominate. Both zones coincide with elevated
8
13C values, interpreted here as enhanced marine activity. Taken together these data imply that George VI Sound has been free of the ice shelf on at least one occasion during the Holocene.Chronological control will be provided by AMS 14C dates performed on individual foraminifera.