A mafic-ultramafic cumulate sequence derived from boninite-type melts (Niagara Icefalls,northern Victoria Land,Antarctica)

The layered sequence from Niagara Icefalls (northern Victoria Land, Antarctica) is related to the Cambrian-Early Ordovician Ross Orogeny. The sequence consists of dunites, harzburgites, orthopyroxenites, melagabbronorites and gabbronorites of cumulus origin. The Mg# of olivine, spinel, orthopyroxene and clinopyroxene from these rocks yields positive correlations, thus indicating formation from melts that mainly evolved through fractional crystallisation. The following fractionation sequence was identified: olivine (up to 94 mol% forsterite) + Cr-rich spinel → olivine + orthopyroxene ± spinel → orthopyroxene → orthopyroxene + anorthite-rich plagioclase ± clinopyroxene. Clinopyroxenes retain the peculiar trace element signature of boninite melts, such as extremely low concentrations of HREE and HFSE, and LILE enrichment over REE and HFSE. U–Pb isotope data on zircons separated from a gabbronorite have allowed us to constrain the age of emplacement of the Niagara Icefalls sequence at ∼514 Ma. The occurrence of inherited zircons dated at ∼538 Ma indicates that the boninitic melts experienced, at least locally, crustal contamination. The Niagara Icefalls sequence can be related to a regional scale magmatic event that affected the eastern margin of the Gondwana supercontinent in the Middle Cambrian. We propose that the formation of the sequence was associated with the development of an embryonic back-arc basin in an active continental margin. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Geochemistry and Sr-Nd isotopes of amphibolite dykes of northern Victoria Land, Antarctica

Mafic dykes cutting the gneisses and migmatites in the Deep Freeze Range high-grade metamorphic complex of northern Victoria Land, Antarctica, have undergone strong recrystallization and deformation during amphibolite-facies metamorphism. Metamorphic mobility mostly affected the large-ion lithophile elements (LILE). Rare Earth (RE) and the high field-strength elements (HFSE) were essentially immobile during metamorphism. Together with the major-element geochemistry, this suggests primary characteristics of evolved tholeiitic magmas and mafic cumulates. No precise ages of intrusion are available for the dykes, but geological evidence suggest emplacement during the time interval 800 to 900 Ma. The Rb-Sr isotopic system in some of the dykes were also variably affected by a later thermal event, probably coincident with the time of amphibolite metamorphism, ca. 500–550 Ma ago. This event can be correlated with the Ross Orogeny in the Transantarctic Mountains. Nd isotopes and trace element abundances indicate that the dykes were derived by different degrees of partial melting and fractionation of heterogeneous sub-continental lithospheric mantle. The Nd isotopic compositions range from depleted to enriched signatures (ε^Nd computed back to 850 Ma = +4.5 to −11.61), and are coupled to different trace element normalized patterns characterized by a slight positive Nb anomaly in the former case to a strong negative Mb anomaly for the latter samples. On isotopic and chemical ground the depleted signature of the mantle source resembles that reported for E-type MORB. The nature of the enriched components cannot be uniquely stated; nevertheless, on the basis of isotopic and geochemical data, it could be represented by sediments recycled into the sub-continental mantle or by crustal contamination during underplating of mafic magmas, or a combination of the two processes.     

Topography,structural and exhumation history of the Admiralty Mountains region,northern Victoria Land,Antarctica

The Admiralty Mountains region forms the northern termination of the northern Victoria Land, Antarctica. Few quantitative data are available to reconstruct the Cenozoic morpho-tectonic evolution of this sector of the Antarctic plate, where the Admiralty Mountains region forms the northern termination of the western shoulder of the Mesozoic–Cenozoic West Antarctica Rift System. In this study we combine new low-temperature thermochronological data (apatite fission-track and (U-Th-Sm)/He analyses) with structural and topography analysis. The regional pattern of the fission-track ages shows a general tendency to older ages (80–60 ​Ma) associated with shortened mean track-lengths in the interior, and younger fission-track ages clustering at 38–26 ​Ma with long mean track-lengths in the coastal region. Differently from other regions of Victoria Land, the younger ages are found as far as 50–70 ​km inland. Single grain apatite (U-Th-Sm)/He ages cluster at 50–30 ​Ma with younger ages in the coastal domain. Topography analysis reveals that the Admiralty Mountains has high local relief, with an area close to the coast, 180 ​km long and 70 ​km large, having the highest local relief of >2500 ​m. This coincides with the location of the youngest fission-track ages. The shape of the area with highest local relief matches the shape of a recently detected low velocity zone beneath the northern TAM, indicating that high topography of the Admiralty Mountains region is likely sustained by a mantle thermal anomaly. We used the obtained constraints on the amount of removed crustal section to reconstruct back-eroded profiles and calculate the erosional load in order to test flexural uplift models. We found that our back-eroded profiles are better reproduced by a constant elastic thickness of intermediate values (Te ​= ​20–30 ​km). This suggests that, beneath the Admiralty Mountains, the elastic properties of the lithosphere are different with respect to other TAM sectors, likely due to a stationary Cenozoic upper mantle thermal anomaly in the region.     

The Cambrian Ross Orogeny in northern Victoria Land (Antarctica) and New Zealand: A synthesis

In the Cambrian, the paleo-Pacific margin of the East Gondwana continent, including East Antarctica, Australia, Tasmania and New Zealand, was affected by the Ross–Delamerian Orogeny. The evidence from geochemistry of volcanic rocks and petrography of clastic sediments in northern Victoria Land (Antarctica) reveals that orogenesis occurred during a phase of oblique subduction accompanied by the opening and subsequent closure of a back-arc basin. A similar sequence of events is recognized in New Zealand. In both regions Middle Cambrian volcanic rocks are interpreted as arc/back-arc assemblages produced by west-directed subduction; sediments interbedded with the volcanic rocks show provenance both from the arc and from the Gondwana margin and therefore place the basin close to the continent. Rapid back-arc closure in the Late Cambrian was likely accomplished through changes to the subduction system.     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.     

A sapphirine-cordierite-garnet-sillimanite granulite from Enderby Land,Antarctica: implications for FMAS petrogenetic grids in the granulite facies

A quartz-absent magnesian paragneiss layer from Mount Sones, in the Archaean Napier complex of Enderby Land, Antarctica, contains the stable divariant FMAS assemblage sapphirine (X _Mg=78) — cordierite (X _Mg=87) — garnet (X _Mg=51) — sillimanite. Rare green spinel (X _Mg=53.5, ZnO=2.65wt%) occurs as inclusions mainly within sapphirine, but also within sillimanite and garnet. Late thin coronas of cordierite (X _Mg=90.5) mantle sapphirine in contact with extensively exsolved anorthoclase. The mineral textures are interpreted to indicate the former stability of a hypersthene-quartz absent assemblage followed by the development of the FMAS equilibrium assemblage sapphirine-cordierite-garnet-sillimanite (sp, hy, qz) and further divariant reaction involving the consumption of sapphirine. The (sp, hy, qz) assemblage uniquely defines the stable P-T reaction topology appropriate to granulites from the Napier Complex, as this paragenesis is allowed in the grids of Hensen (1971, 1986) but is not possible in other grids which assume the stability of a sapphirine-absent ([sa]) FMAS invariant point involving the phases spinel, garnet, hypersthene, cordierite, sillimanite and quartz. The observed mineral assemblages and textures are consistent with peak metamorphism between the [sp] and [hy] invariant points of Hensen (1971), at temperatures of 930–990° C, followed by cooling on a lower dP/dT trajectory towards the (sp, qz) univariant line. The initial spinel-bearing assemblage was stabilized by Zn and to a lesser extent by Ni and Cr, and hence does not require a marked decrease in temperature and increase in pressure to produce the (sp, hy, qz) assemblage. It is inferred that fO ₂ conditions substantially lower than those used in the experiments of Annersten and Seifert (1981) prevailed in the high-grade metamorphism in the Napier Complex.     

Ammonium in granites and its petrogenetic significance

Ammonium is present as a trace constituent in all granites, with an average concentration of 45 ppm (NH₄⁺), equivalent to 35 ppm of elemental N. It shows wide variations related to petrography and region, but the only significant correlation between ammonium and other geochemical parameters is that it is most abundant in peraluminous granites and least abundant in peralkaline granites. These variations can be related to (a) the amount of sedimentary material in the magmatic source region, and (b) redox conditions in the source region. The ammonium content of granitic magmas can also be modified by fractionation or contamination. Hydrothermal alteration has a major effect on the ammonium content of granitic rocks, and variation due to this cause may exceed the magmatic variation. Most hydrothermally altered granites are enriched in ammonium as a result of the transfer of ammonium from sedimentary country rocks by the hydrothermal fluids.     

Evidence of magmatic CO2-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land,Antarctica)

Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO₂+H₂O inclusions + Al₂SiO₅ ± brines in garnet (type 1); early CO₂-rich inclusions (± brines) in quartz (type 2); early CO₂+CH₄ (up to 4 mol%)±H₂O inclusions + graphite + fibrolite in quartz (type 3); late CH₄+CO₂+N₂ inclusions and H₂O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO₂-rich, possibly with immiscible brines. CO₂-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease off _{O 2} can entering in the graphite stability field; at the same time, the feldspars reacted with CO₂-rich fluids to give secondary fibrolite + graphite. Decrease ofT andf _{O 2} can explain the progressive variation in the fluid composition from CO₂-rich to CH₄ and water dominated in a closed system (in situ evolution). The presence of N₂ the late stages indicates interaction with external metamorphic fluids.Contribution within the network Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis. CEC program: Human Capital and Mobility     

P–T evolution and episodic zircon growth in barroisite eclogites of the Lanterman Range,northern Victoria Land,Antarctica

Prograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M_1–2 and M₃. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M₁) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M₂ assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M₁, whereas the inclusion‐poor outer part (M₂) is typified by the outward decrease in Ca/Mg and X_Fe (=Fe²⁺/(Fe²⁺+Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M₁) to ~22–25 kbar and 630–650°C (M₂). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M_3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M_3A garnet, ubiquitous overgrowth of the M_3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 515 ± 4 Ma (tσ), representing the time of the M₂ stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M₃. Average burial rates estimated from the M₂ and M₃ ages are too low (<2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.     

The Dorn gold deposit in northern Victoria Land,Antarctica: Structure,hydrothermal alteration,and implications for the Gondwana Pacific margin

The Dorn gold deposit in northern Victoria Land, Antarctica is a fault related gold-only deposit and it represents the first described occurrence of gold mineralization in Paleozoic terranes of the Antarctic continent. The deposit is hosted in lower greenschist facies Middle Cambrian metavolcanic and metasedimentary rocks of the Bowers terrane. Gold-bearing veins are located in a brittle–ductile reverse high-strain zone, which was produced by transpressional deformation that overprints the structures of the Cambrian–Ordovician Ross Orogeny. The vein system is surrounded by a hydrothermal alteration zone that is as much as 300-m-wide, where the host rocks are partially to completely transformed into Fe–Mg carbonate-rich rocks with different degrees of replacement of the original mineralogy and texture.The type of host rock, the temperature estimates for mineralization from 290–320 °C, the quartz dominant vein system with sulfides and Fe-rich carbonates, and the controlling structures linked to a convergent margin tectonic setting together suggest that this mineralized vein system can be classified as an orogenic gold deposit. Close analogies are found with deposits of the Stawell zone in western Victoria, which is consistent with the correlation between units and hydrothermal events in northern Victoria Land and southeastern Australia.     

Zirconolite-bearing ultra-potassic veins in a mantle-xenolith from Mt. Melbourne Volcanic Field,Victoria Land,Antarctica

One mantle xenolith from a basanite host of the Mt. Melbourne Volcanic Field (Ross Sea Rift) is extraordinary in containing veins filled with leucite, plagioclase, clinopyroxene, nepheline, Mg-ilmenite, apatite, titaniferous mica, and the rare mineral zirconolite. These veins show extensive reaction with the dunitic or lherzolitic host (olivine+spinel+orthopyroxene+clinopyroxene). The reaction areas contain skeletal olivine and diopside crystals, plagioclase, phlogopite, aluminous spinel and ilmenite in a fine grained groundmass of aluminous spinel, clinopyroxene, olivine, plagioclase and interstitial leucite. The vein composition estimated from modal abundances and microprobe analyses is a mafic leucite-phonolite with high amounts of K, Al, Ti, Zr and Nb but low volatile contents. The melt is unrelated to the host basanite and was probably derived by smallscale melting of incompatible element-enriched phlogopite-bearing mantle material and must have lost most of its volatile content during migration, crystallization and reaction with the host dunite. While the veins are completely undeformed the dunitic host shows slight deformation. Vein minerals crystallized at high temperatures above 1000°C and pressures below 5 kbar according to the phase assemblage including leucite, nepheline and K-feldspar. Spinel/olivine geothermometry yielded 800–920°C for the re-equilibration of the host peridotite. Thus the xenolith must have been at shallow depth prior to and during the late veining event. Mantle material at shallow depths is consistent with rifting and the regional extreme displacement at the transition from the rifted Victoria Land Basin in the Ross Sea to the uplifted Trans-Antarctic Mountains.     

新疆百口泉闪长岩中高An值斜长石的成因及岩石学意义

岩浆成因的高An值斜长石(An80~100)是玄武质岩浆早期结晶的产物,由于其形成条件较为苛刻,它对于限定寄主岩浆条件和探讨岩石成因有重要的指示作用。在新疆西准噶尔百口泉地区发育含高An值斜长石(An80~90)的闪长岩,高An值斜长石呈不规则包裹体形式存在于闪长岩主体斜长石中(An40~60)。锆石SHRIMP定年显示百口泉闪长岩形成于316.9±2.9Ma(MSWD=1.5),闪长岩主、微量元素和Sr-Nd同位素与区域内同时期岩浆作用产物具有相似的演化趋势。根据闪长岩岩石学特征,结合高An值斜长石的实验研究成果,本文认为百口泉闪长岩中的高An值斜长石不是捕虏晶,也不是闪长岩的结晶产物,而是与闪长岩成岩过程相关的循环晶。依据角闪石铝压力计的估算,闪长岩结晶于1.2~2.9kbar,与实验条件下获得的高An值斜长石产出的压力范围相符。高An值斜长石的产出表明原始岩浆具有富水、亏损的特征,对于探讨整个岩浆系统的演化过程有重要指示意义。     

Petrogenesis of the Kirkpatrick Basalt,Solo Nunatak,northern Victoria Land,Antarctica, based on isotopic compositions of strontium,oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial ⁸⁷Sr/⁸⁶Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ¹⁸O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial ⁸⁷Sr/⁸⁶Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ³⁴S= -4.01 to +3.41‰ Variations in (δ³⁴S probably resulted from outgassing of SO₂ under varying oxygen fugacities. Laboratory for Isotope Geology and Geochemistry (Isotopia), Contribution No. 71     

粤北棉花坑铀矿床矿物共生组合特征及其意义

基于棉花坑矿床不同类型铀矿石中矿物共生组合关系研究,讨论了棉花坑矿床成矿作用过程中铀的沉淀富集机制.研究结果显示,棉花坑矿床铀矿体主要呈脉状充填或细脉浸染状产出;铀矿石类型包括萤石型、碳酸盐型、硅质脉型和红化型.虽然不同矿石类型中成矿期形成的脉石矿物种类存在差异,但均表现出以下共性特点:铀矿物赋存于脉状充填矿石的中心部...     

Multiple cosmogenic nuclides document complex Pleistocene exposure history of glacial drifts in Terra Nova Bay (northern Victoria Land, Antarctica)

Geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses (¹⁰Be, ²⁶Al, and ²¹Ne) allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the East Antarctic Ice Sheet (EAIS) and valley glaciers in the Terra Nova Bay region. Glacially scoured coastal piedmonts with round-topped mountains occur below the highest local erosional trimline. They represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 Ma (pre-dating the build-up of the Mt. Melbourne volcanic field). Since then, summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm/Ma. Complex older drifts rest on deglaciated areas above the younger late-Pleistocene glacial drift and below the previously overridden summits. The combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion. The areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers. The exposure ages of the older drift(s) indicate multiple Pleistocene glacial cycles, which did not significantly modify the pre-existing landscape.     

S- and I-type granitoids of North Victoria Land,Antarctica, and their inferred geotectonic setting

In the context of Gondwana North Victoria Land forms the Antarctic conjugate terrain to the East Australian Lachlan Fold Belt, where the distinction between S- and I-type granitoids was first worked out (Chappell &White 1974). Thus the area was considered a good testing ground for the hypothesis when the German Antarctic North Victoria Land Expedition (Ganovex) resumed fieldwork there in 1979/80.It was known at that time that there existed a Cambro/Ordovician and a Devonian/Carboniferous granitoid generation, which, in analogy to Australia, were taken to be related to two different orogenetic events. Our geological, petrographical and geochemical investigations, together with radiometric age dating, revealed that this is true only for the older granite generation. The younger generation is in fact anorogenic. At the same time, it became obvious that the S- and I-type classification did not completely fit the observed field data. At this pointPitcher's (1982) subdivision of the I-types into a »Cordilleran« and a »Caledonian« suite offered a solution to account for the observed irregularities.At about the same time, plate tectonic models based on evidence independent from the granite classification were developed for this part of the active Gondwana margin. This offers the opportunity to crosscheck the tectonic environment derived from the granite classification:The characteristics of the older, Cambro/Ordovician granitoids allow a perfect accommodation in the model of an active margin above a subduction zone as derived from other evidence.For the younger (Devonian/Carboniferous) granitoids, however, the postulated tectonic setting (post-collisional uplift and faulting) could not be verified in North Victoria Land.It is concluded thatPitchers classification is applicable in its petrographic and geochemical aspects but that the tectonic environment postulated for the production of »Caledonian« I-type granitoids may not be the same in all investigated areas.

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Zusammenfassung Im Gondwana Rahmen bildet Nord Victoria Land das antarktische Pendant zur ostaustralischen Lachlan Mobilzone, wo die Einteilung von Graniten in S- und I-Typen entwickelt wurde (Chappell &White 1974). Deshalb war eine überprüfung dieser neu entwickelten Hypothese eines der Arbeitsziele, als die deutschen Nord Victoria Land Expeditionen (Ganovex) dort 1979 ihre Arbeiten aufnahmen. An den bekannten zwei Granitgenerationen der Region wurden geologische, petrographische und geochemische Untersuchungen sowie Altersbestimmungen durchgeführt, deren Ergebnisse sich folgenderma\en zusammenfassen lassen.Die ältere, kambro-ordovizische Granitfolge steht, analog zur Situation in Australien, in enger Beziehung zu einer bedeutenden Orogenese, während sich zur jüngeren, devonisch-karbonischen Suite kein zugehöriges tektonisches Ereignis finden lie\. Dies steht im Gegensatz zur Situation in Australien/Tasmanien.Die S- und I-Typ Klassifikation, auf Nord Victoria Land angewendet, weist ähnliche Widersprüche auf, wie siePitcher (1982) zur Aufteilung der I-Typen in »kordillere« und »kaledonische« Sippen führten.Plattentektonische Modelle, die in den letzten Jahren für diese Region am mobilen Rand Gondwanas unabhängig von der Granitklassifikation nach geologischen und tektonischen Kriterien entwickelt wurden, erlauben es, die vonPitcher entwickelten tektonischen Bildungsmilieus für verschiedene Granit-Typen zu überprüfen:Die Charakteristiken der älteren (kambro-ordovizischen) Granite erlauben ein widerspruchsloses Einfügen in das aus anderen Kriterien abgeleitete Bild eines aktiven Kontinentalrandes über einer Subduktionszone.Für die jüngeren (devonisch-karbonischen) Granite dagegen, die im Charakter den »kaledonischen« I-TypenPitchers entsprechen, sind die für deren Bildung postulierten Vorgänge, nämlich Hebung und Dehnung nach einer Kollision, in Nord Victoria Land nicht erkennbar.Demnach erscheint zwar die KlassifikationPitchers sinnvoll, da sie in ihrer petrographisch-geochemischen Charakterisierung nachvollziehbar ist, aber das abgeleitete tektonische Milieu für die Bildung der »kaledonischen« Typen bedarf einer weiteren überprüfung.

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Résumé Dans le contexte du Gondwana, la Terre Victoria est le pendant antarctique de la zone plissée de Lachlan, en Australie Orientale, où la distinction entre granites S et I a été établie pour la première fois (Chappel &White 1974). C'est pourquoi la vérification de cette hypothèse a été portée au programme de l'expédition allemande à la Terre Victoria en 1979–80 (Ganovex).On connaissait à cette époque l'existence d'une génération cambro-ordovicienne et d'une génération dévono-carbonifère de granitoÏdes qui, par analogie avec l'Australie, avaient été rapportées à deux événements orogéniques différents. Nos investigations géologiques, pétrologiques, géochimiques et géochronologiques montrent que cette conclusion n'est vraie que pour les granites de la première génération. La seconde génération est, en fait, anorogénique. Il s'est avéré en mÊme temps que la répartition en types S et I ne correspond pas exactement aux observations de terrain. Par contre, la distinction introduite parPitcher (1982) parmi les granites I entre une série »de Cordillère« et une série »Calédonienne« peut expliquer les irrégularités que nous observons.Environ à cette époque, des modèles géodynamiques basés sur des considérations indépendantes de la classification des granites ont été proposés pour cette partie de la marge active du Gondwana. Ces modèles permettent de recouper les considérations sur l'environnement tectonique des granites, déduites de leur classification:Les caractères des granitoÏdes anciens, cambro-ordoviciens, s'accordent parfaitement au modèle d'une marge active surmontant une subduction.Par contre, pour les granitoÏdes jeunes, dévono-carbonifères, la situation tectonique postulée (soulèvement et fracturation post-collision) n'a pas pu Être confirmée sur la Terre Victoria.En conclusion, la classification dePitcher est applicable dans ses aspects pétrographiques et géochimiques, mais l'environnement tectonique qui préside à la formation de granitoÏdes de type »Calédonien I« n'est pas nécessairement le mÊme dans toutes les régions étudiées.

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- Lachlan, S I (Chappell & White, 1974). (GANOVEX) 1979 . , , . : - , , , , - , , - . S I , , (Pitcher, 1982) I . , , , , : — - — , . , - , , , , .. ., , - , , , , .

     

A new Holocene relative sea‐level curve for Terra Nova Bay,Victoria Land,Antarctica

More than 100 radiocarbon dates of penguin guano and remains, shells and seal skin afford ages for raised beaches adjacent to Terra Nova Bay, Antarctica. These dates permit construction of a new relative sea‐level curve that bears on the timing of deglaciation. Recession of the Ross Sea ice‐sheet grounding line from Terra Nova Bay occurred no earlier than 7200 ¹⁴C yr (8000 cal. yr) BP. Retreat along the Victoria Land coast may have been rapid, possibly contributing to eustatic sea‐level rise centred at ca. 7600 cal. yr BP. The presence of a significant amount of ice remaining in the Ross Sea Embayment in Holocene time lessens the chance that Antarctica contributed significantly to meltwater pulse 1A several thousand years earlier. Copyright © 2004 John Wiley & Sons, Ltd.