(oral p.) D. Bolshiyanov
Arctic and Antarctic Research Institute, Bering 38, 199397 St. Petersburg, Russia, <bolshiyanov@aari.nw.ru>.
The Bunger Hills are surrounded from all sides by ice of different type, ice shelf from the north, outlet glacier from the south and west, ice sheet from the east and passive glaciers from the south. This is in fact an island contacting the sea basin in the west and north with glaciers overlying the seabed from the east and south. The modern glacial regime provides the ice discharge at which the entire mass of discharged ice passes round the oasis.
The most widespread type of glaciers and firn fields directly in the territory of the oasis is the snow-ice dam forming at snow redistribution as a result of action of prevailing easterly winds. Snow fields and glaciers form in the wind shade of hillocks and ridges in the form of dams elongated in the western direction with a length up to several hundreds of meters and a thickness of tens of meters.
Due to cyclic oscillations of climatic parameters in the oasis, dams appear and disappear with periodicities of 5, 11 and 23 years, which was detennined from the occurrence of sand and gravel interlayers in the sections of bottom deposits of near-glacial lakes. Disappearance of dams leads to catastrophic events in the form of drain of the water basins embanked by them. The best examples of the dead ice dams are the ice bodies developed along the southern boundary of the oasis. They dam there the perennially ice-covered Lakes of White Smoke and Polyanskogo.
An analysis of spreading and the age of the stomach fat accumulations of Snow petrels (the so-called Antarctic "mumiye") in the Bunger Oasis (VERLKULICH et al. 1999) indicates the thickest and most ancient deposits of this substance in the center of the oasis. This means that the retreat of glaciers in the Bunger Oasis in the Late Pleistocene was in the direction from the center to the margins. The largest size of crustaceous lichen Buellia frigida and hence its most ancient age is observed in the central part of the Oasis. It is obvious that the age of lichens cannot be the same as the age of mumiye (up to 10 ka). Their age comprises merely tens and hundreds of years. However, the spatial regular features of the location of lichens suggests that after the oasis became free from glaciers in the early Holocene, the next glaciation stages were of the same character - accumulation and melting of latitudinally oriented passive glaciers. The last glaciation stage of the oasis also ended in the oasis becoming ice-free from its center. In addition, the largest lichen individuals are confined to the tope of the hills. At the slopes, they regularly decrease with moving downward. Thus. the tops of the hills were the first to become free of the ice and the snow cover.
These facts along with the revealed typical features about the structure and spreading of modem glacial bodies suggest that the Bunger Oasis was not occupied by the ice sheet in the Late Pleistocene contrary to modern concepts (lNGOLFSSON et al. 1998). It had its own local glaciation in the form of dead glacial and firn fields. The last datings of the deposits in the Bunger Oasis by the OSL method (GORE et al. 200 l) also showed that during the Last Glacial Maximum, the Antarctic ice sheet did not occupy the territory of the Oasis. In the Late Pleistocene, ice of the Antarctic ice sheet similar to the present time moved around the Bunger hills in the form of outlet glaciers. The Antarctic ice sheet dimensions could not be greater than the current ones. The east-western orientation of periodically accumulating local passive glaciers of the oasis was preserved over the last 10 ka. Hence, the wind regime above this territory in the Holocene was more or less stable.
Gore. D.B. et al. (2001): Geology .29/12: 1103-1106.
Ingolfsson, 0. et al. (1998): Antarctic Science 10/3: 326-344.
Verkulich. S.R. et al. (1999): lnfonn. Bui. of Russian Ant. Exp. 119: 92-104.
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-Chemical response of zircon to fluid infiltration and high-T deformation: Howard Peaks Intrusive Complex (northern Victoria Land, Antarctica), a case study
(poster p.)
R.M. Bomparola1, C. Ghezzo1, E. Belousova2, L. Dallai3, W.L. Griffin2 & S.Y.O'Reilly2
1Dipartimento di Scienze della Terra, University of Siena, Italy; <bomparola@unisi.it>, <ghezzo@unisi.it>;
?GEMOC ARC Nat. Key Centre. Dept. Earth Planet. Sci., Macquarie University. Australia;
<ebeloous@laurel.ocs.mq.edu .au>, <wgri ffin@laurel.ocs.mq.edu.au>. <sue.oreil I y@rnq.edu .au>;
3CNR-Istituto Geologia Arnbientale e Geoingegneria. Roma. Italy: <l.dallai@igag.cnr.it>.
The use of zircon U-Pb geochronology has proved indispensable in constraining the age, origin and thermal history of crustal rocks. However the high complexity of zircon's internal structures, associated to a frequent disturbance of the U-Pb system in many zircons of metamorphic and igneous rocks, makes often the interpretation of the radiometric data ambiguous. The application of high-precision in-situ laser ablation tecniques to obtain trace-element, U-Pb and Lu-Hf compositions in the same zircon grains, combined with a detailed study of the zircon's internal structures, is indispensable to discriminate between different types of zircon growth and alteration mechanisms and to attribute them to different geological processes. This approach has been applied to zircons extracted from six samples collected from foliated metaluminous, high-K monzogranites, granodiorites and tonalites of the Howard Peaks Intrusive Complex (Deep Freeze Range, Antarctica) emplaced during the Late Cambrian-Early Ordovician Ross Orogenesis and deformed under high-temperature solid-state conditions (MUSUMECI & PERTUSATI 2000). The aim of the present study is to define the emplacement age of the Howard Peaks Intrusive Complex and to constrain the source and subsequent evolution of the parent magmas.
The analyzed zircons show a complex pattern of internal structures with inherited components, euhedral concentric zonations, convoluted zones and patches of unzoned zircons sometimes retaining ghost zones. Ignoring the inherited components, a wide scatter in concordant and discordant ages between 518 and 440 Ma is observed, and two or three main age populations are found in aJI the analyzed samples. Discordant ages are often exhibited by domains characterized by the occurrence of bright zones in BSE images, while grains with convoluted zones, weak zoning or no zoning at all may show a wide range of concordant ages. A wide spread of trace-element compositions in the analyzed zircons is also observed, with large variations in U, Th, Y and LREE contents, and limited variability in HREE. Light REE enrichment is often related to the occurrence of bright domains and/or relatively younger ages in zircon. The observed trace-element variations cannot be related to fractional crystallization processes as indicated by the lack of a positive correlation between Yb and Th/U.
Moreover, the observed variations in trivalent and tetravalent cations caanot be explained by a simple volume diffusion, as it should produce larger variations in HREE relative to LREE and, a higher mobility of Hf than of U and Th, which is not observed. Lu-Hf analyses define, on the contrary, a relatively narrow range of mean 176Hf/177Hf around 0.2823 in distinct age and structural grou~s within each sample, suggesting that the Lu-Hf system was left relatively undisturbed. Similar low 17 Hf/177Hf ratios in all the analyzed samples support a common parental derivation from old recycled crust for the different selected intrusives. The limited scatter in 176Hf/177Hf could be related to different crustal components involved in the melting events. Less radiogenic ratios in old inherited cores, not related to the young population by a simple crustal evolution, are a confirmation of the composile nature of the crust.
The wide spread in zircon age signatures, internal structures and trace-element compositions is here related to post-crystallization alteration mechanisms that produced a strong modification of the trace-element distribution and the U-Pb system. The mobility of elements during the secondary processes was variable and probably related to different degree of fluid access during the high-lemperature ductile deformation related to the NE-SW dextral strike-slip shear zone in this area.
35
-Musumeci G. & Pertusa1i P.C. (2000): Anlarclic Sci. 12: 89-104
U-Pb geochronology of the Granite Harbour Intrusives from the Wilson Terrane, northern Victoria Land, Antarctica
(oral p.) R.M. Bomparola
Dipartimento di Scienze della Terra, University of Siena, Italy; <bomparola@unisi.it>.
Zircon crystals from intrusive rocks from the Wilson Terrane in northern Victoria Land (Antarctica), along a transect nearly orthogonal to the orogenic belt over a lenght of about 200 km, have been analyzed. The selected plutons cover a wide compositional range, from metaluminous and peraluminous granitoids to minor mafic rocks of gabbro-dioritic composition. Emplaced during the Late Cambrian-Early Ordovician Ross Orogenesis (TONARINI & ROCCHI 1994), they belong to the Granite Harbour Intrusives. Zircons have been used to estimate the emplacement age of crustal and mantle melts and the nature of the deep continental crust involved in the melting events. High-precision in-situ laser ablation tecniques have been used to obtain trace-element, U-Pb and Lu-Hf compositions in the same zircon grains.
Preliminary results obtained on 23 selected samples of the largest plutons (with the exception of minor intrusions and of the late orogenic stocks and dikes) allow to recognize the following regional distribution of U-Pb isotopic data for the Granite Harbour Intrusives of northern Victoria Land:
• The oldest felsic magmatic pulses are represented by the metaluminous monzogranitic-granodioritic intrusions of the Mt. Baxter in the Eisenhower Range and Mt. Jiracek in the Southern Cross Mountains and by some quanz-monzonite intrusions of the shoshonitic suite from the Terra Nova Intrusive Complex (Teall Nunatak), with mean ages around 512 Ma;
• The metaluminous shoshonitic monzogranites and quartz-monzonites cropping out in the Deep Freeze Range and northern Foothills display emplacement ages in the interval between 503 and 492Ma;
• The foliated amph-bearing tonalitic intrusions occurring in the same area show slightly older emplacement ages in the range 506-502 Ma; the mafic intrusions of gabbroic and dioritic composition show relatively younger ages spanning from 495 to 489 Ma;
• The peraluminous intrusions of the Tinker Glacier area in the Southern Cross Mountains emplaced between 494 and 488 Ma;
• The metaluminous monzogranitic intrusions of the Mountaineer Range show an age spectrum similar to the peraluminous granites of the Tinker Glacier area, between 495 and 490 Ma;
• The Keinath pluton, in the Deep Freeze Range, with a mean age of 485 Ma, represents the youngest large granitic intrusion in the studied area.
The geochronological data presented above indicate that during the Ross Orogenesis, in the studied area, the emplacement of the main intrusive complexes covered a time span of at least 25 Ma. Results obtained on the peraluminous granites of the Tinker Glacier area indicate their coeval generation and emplacement with the main metaluminous intrusive sequences.
Moreover, a wide range of inherited cores is present in some samples of both peraluminous and metaluminous granitoids with the following main age clusters: 2.2 Ga; 0.9-1.1 Ga; 800 Ma; 700 Ma;
570-61 O Ma; a major peak in the interval between 0.9 and I. I Ga. Some of the observed ages match known events documented in the Proterozoic crust of the East Antarctic Craton. Few Lower Archean cores are also present showing usually discordant ages.
36
-Tonanni, S. & Rocchi. S (1994): Terra An1arc11ca I; 46-50