Strontium Isotopic Signatures of Streams from Taylor Valley, Antarctica, Revisited: The Role of Carbonate Mineral Dissolution

We have collected and analyzed a larger set of stream waters for major ions and strontium isotopes in Taylor Valley, McMurdo Dry Valleys, Antarctica. These new data substantiate the concept that the dissolution of carbonate minerals is a significant source of strontium, even in polar desert environments where liquid water is primarily limited to stream channels. In Taylor Valley, most of the carbonate minerals present are the result of secondary processes, such as mineral precipitation and/or eolian deposition, and not through primary geologic sources, such as bedrock or till sources. In Von Guerard Stream (Lake Fryxell basin) and Andersen Creek (Lake Hoare basin), water samples were collected during the austral summer to determine short-term ⁸⁷Sr/⁸⁶Sr patterns. The observed variability in both time and space may be dependent on the relative amounts of primary and secondary carbonates present in the streambeds and hyporheic zone as well as contingent on the relative proportions of carbonate and aluminosilicate weathering.     

The Helium Isotopic Chemistry of Lake Bonney, Taylor Valley, Antarctica: Timing of Late Holocene Climate Change in Antarctica

To better understand the long-term climate history of Antarctica, we studied Lake Bonney in Taylor Valley, Southern Victoria Land (78°S). Helium isotope ratios and He, Ne, Ar and N₂ concentration data, obtained from hydrocasts in the East (ELB) and West (WLB) Lobes of Lake Bonney, provided important constraints on the lakes Holocene evolution. Based on very low concentrations of Ar and N₂ in the ELB bottom waters, ELB was free of ice until 200 ± 50 years ago. After which, low salinity water flowing over the sill from WLB to ELB, covered ELB and formed a perennial ice cover, inhibiting the exchange of gases with the atmosphere. In contrast to the ELB, the WLB retained an ice cover through the Holocene. The brine in the WLB bottom waters has meteoric N₂ and Ar gas concentrations indicating that it has not been significantly modified by atmospheric exchange or ice formation. The helium concentrations in the deep water of WLB are the highest measured in non-thermal surface water. By fitting a diffusional loss to the ³He/⁴He, helium, and Cl profiles, we calculate a time of 3000 years for the initiation of flow over the sill separating the East and West Lobes. To supply this flux of helium to the lake, a helium-rich sediment beneath the lake must be providing the helium by diffusion. If at any time during the last million years the ice cover left WLB, there would be insufficient helium available to provide the current flux to WLB. The variations in water levels in Lake Bonney can be related to climatic events that have been documented within the Southern Victoria Land region and indicate that the lakes respond significantly to regional and, perhaps, global climate forcing.     

Stable Strontium Isotopic Fractionation During Chemical Weathering in Drainage Basins: Mechanisms and Applications

The new non-traditional stable strontium (Sr) isotope has aroused great attention from academic scholars in terms of the continental weathering and marine Sr cycle. The analytical precision of stable Sr isotope using mass spectrometry is better than 0.03‰. The compiled δ⁸⁸/ ⁸⁶Sr values vary from -3.65‰ to 1.68‰ in natural reservoirs. Recent findings indicate that multiple processes can cause stable Sr isotope fractionation in Earth surface, including the incongruent dissolution of primary minerals, the formation and adsorption of secondary minerals, the precipitation of calcium carbonate, and the biological cycling. These processes lead to higher δ⁸⁸/ ⁸⁶Sr in the liquid phase and lower δ⁸⁸/ ⁸⁶Sr in the solid phase, and thus result in different geochemical behavior of stable Sr isotopes in water and sediment during the weathering processes. The δ⁸⁸/ ⁸⁶Sr values of river sediment decrease with the increase of weathering intensity, which has the potential to indicate chemical weathering intensity. Meanwhile, further study on the fractionation mechanisms and constraints of stable Sr isotopes in river water plays an important role in tracing chemical weathering processes within the watershed, which will lead to a better understanding of the global ocean Sr cycle.     

Chemical and Strontium Isotopic Compositions of the Hanjiang Basin Rivers in China: Anthropogenic Impacts and Chemical Weathering

The Hanjiang River, the largest tributaries of the Changjiang (Yangtze) River, is the water source area of the Middle Route of China’s South-to-North Water Transfer Project. The chemical and strontium isotopic compositions of the river waters are determined with the main purpose of understanding the contribution of chemical weathering processes and anthropogenic inputs on river solutes, as well as the associated CO₂ consumption in the carbonate-dominated basin. The major ion compositions of the Hanjiang River waters are characterized by the dominance of Ca²⁺ and HCO₃ ⁻, followed by Mg²⁺ and SO₄ ²⁻. The increase in TDS and major anions (Cl⁻, NO₃ ⁻, and SO₄ ²⁻) concentrations from upstream to downstream is ascribed to both extensive influences from agriculture and domestic activities over the Hanjiang basin. The chemical and Sr isotopic analyses indicate that three major weathering sources (dolomite, limestone, and silicates) contribute to the total dissolved loads. The contributions of the different end-members to the dissolved load are calculated with the mass balance approach. The calculated results show that the dissolved load is dominated by carbonates weathering, the contribution of which accounts for about 79.4% for the Hanjiang River. The silicate weathering and anthropogenic contributions are approximately 12.3 and 6.87%, respectively. The total TDS fluxes from chemical weathering calculated for the water source area (the upper Hanjiang basin) and the whole Hanjiang basin are approximately 3.8 × 10⁶ and 6.1 × 10⁶ ton/year, respectively. The total chemical weathering (carbonate and silicate) rate for the Hanjiang basin is approximately 38.5 ton/km²/year or 18.6 mm/k year, which is higher than global mean values. The fluxes of CO₂ consumption by carbonate and silicate weathering are estimated to be 56.4 × 10⁹ and 12.9 × 10⁹ mol/year, respectively.     

The Geochemistry of Supraglacial Streams of Canada Glacier,Taylor Valley (Antarctica), and their Evolution into Proglacial Waters

We have investigated the geochemistry of supraglacial streams on the Canada Glacier, Taylor Valley, Antarctica during the 2001–2002 austral summer. Canada Glacier supraglacial streams represent the link between primary precipitation (i.e. glacier snow) and proglacial Lake Hoare. Canada Glacier supraglacial stream geochemistry is intermediate between glacier snow and proglacial stream geochemistry with average concentrations of 49.1 μeq L⁻¹ Ca2+, 19.9 μeq L⁻¹ SO₄²⁻, and 34.3 μeq L⁻¹ HCO₃⁻. Predominant west to east winds lead to a redistribution of readily soluble salts onto the glacier surface, which is reflected in the geochemistry of the supraglacial streams. Western Canada Glacier supraglacial streams have average SO₄²⁻:HCO₃⁻ equivalent ratios of 1.0, while eastern supraglacial streams average 0.5, suggesting more sulfate salts reach and dissolve in the western supraglacial streams. A graph of HCO₃⁻ versus Ca2+ for western and eastern supraglacial streams had slopes of 0.87 and 0.72, respectively with R2 values of 0.84 and 0.83. Low concentrations of reactive silicate (> 10 μmol L⁻¹) in the supraglacial streams suggested that little to no silicate weathering occurred on the glacier surface with the exception of cryoconite holes (1000 μmol L⁻¹). Therefore, the major geochemical weathering process occurring in the supraglacial streams is believed to be calcite dissolution. Proglacial stream, Anderson Creek, contains higher concentrations of major ions than supraglacial streams containing 5 times the Ca²⁺ and 10 times the SO₄²⁻. Canada Glacier proglacial streams also contain higher concentrations (16.6–30.6 μeq L⁻¹) of reactive silicate than supraglacial streams. This suggests that the controls on glacier meltwater geochemistry switch from calcite and gypsum dissolution to both salt dissolution and silicate mineral weathering as the glacier meltwater evolves. Our chemical mass balance calculations indicate that of the total discharge into Lake Hoare, the final recipient of Canada Glacier meltwater, 81.9% is from direct glacier runoff and 19.1% is from proglacial Andersen Creek. Although during a typical, low melt ablation season Andersen Creek contributes over 40% of the water added to Lake Hoare, its overall chemical importance is diluted by the direct inputs from Canada Glacier during high flow years. Decadal warming events, such as the 2001–2002 austral summer produce supraglacial streams that are a major source of water to Lake Hoare.     

Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas,north Victoria Land,Antarctica

Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatrick Basalt and Ferrar Dolerite from north Victoria Land, Antarctica, are examined in order to address the role of crustal assimilation and the characteristics of their mantle source. Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava sequence [Mg-number, Mg/(Mg+Fe⁺²)=24, MgO=2.4%, SiO₂=57.1%, initial⁸⁷Sr/⁸⁶Sr=0.7087–0.7097, (ε_Nd=−4.3) conform previous reports that attribute variations in the concentrations of the more mobile elements and calculated initial⁸⁷Sr/⁸⁶Sr to mid-Cretaceous alteration and elevated δ¹⁸O to low-temperature interaction with meteoric water. The underlying lavas and the sills that are of the Mt. Fazio chemical type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number=40–65, MgO=3.7 7.5%; SiO₂=52.6–58.3%, initial⁸⁷Sr/⁸⁶Sr=0.7087–0.7117, ε_Nd=−5.6 to −4.8). The effects of rock alteration on apparent initial⁸⁷Sr/⁸⁶Sr are demonstrated by large differences between the initial ratio of mineral separates or leached fractions and whole rocks. Cretaceous alteration produced Rb and Sr redistribution within the lava sequence that results in erroneous calculated initial⁸⁷Sr/⁸⁶Sr ratios. These effects are responsible for the large initial⁸⁷Sr/⁸⁶Sr variations previousl7 proposed which, combined with the large range in whole-rock δ¹⁸O, were purported to show very large degrees of crustal assimilation. The variations in ε_Nd are restricted and indicate much smaller degrees of assimilation. The least altered of the MFCT rocks show good chemical and isotopic correlations that can be integrated into a model involving fractionation of pyroxene and plagioclase coupled with assimilation of material similar to early Paleozoic basement. The lower⁸⁷Sr/⁸⁶Sr and higher ε_Nd of the SPCT suggest that they were derived by extensive fractionation of a more primitive, less contaminated, precursor of the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial⁸⁷Sr/⁸⁶Sr and low initial¹⁴³Nd/¹⁴⁴Nd; this may reflect either earlier assimilation or an enriched source. The chemical and isotopic similarities, as well as the close geographic correspondence of the Ferrar Group to granitoids produced during the early Paleozoic Ross Orogeny suggest that in either case Ross-type material may have been involved in the development of the enriched isotopic signature. Editorial responsibility: I. Parsons     

Isotopic Evolution of Saline Lakes in the Low-Latitude and Polar Regions

Isotopic fractionations associated with two primary processes (evaporation and freezing of water) are discussed, which are responsible for the formation and evolution of saline lakes in deserts from both low-latitude and the Polar regions. In an evaporative system, atmospheric parameters (humidity and isotopic composition of water vapor) have strong influence on the isotopic behavior of saline lakes, and in a freezing system, salinity build-up largely controls the extent of freezing and associated isotope fractionation. In both systems, salinity has a direct impact on the isotopic evolution of saline lakes. It is proposed that a steady-state “terminal lake” model with short-term hydrologic and environmental perturbations can serve as a useful framework for investigating both evaporative and freezing processes of perennial saline lakes. Through re-assessment of own work and literature data for saline lakes, it was demonstrated that effective uses of the isotope activity compositions of brines and salinity-chemistry data could reveal dynamic changes and evolution in the isotopic compositions of saline lakes in response to hydrologic and environmental changes. The residence time of isotopic water molecules in lakes determines the nature of responses in the isotopic compositions following perturbations in the water and isotope balances (e.g., dilution by inflow, water deficit by increased evaporation, and/or reduction in inflow). The isotopic profiles of some saline lakes from the Polar regions show that they switched the two contrasting modes of operation between evaporative and freezing systems, in response to climate and hydrological changes in the past.     

Late Pleistocene glacial chronology of the Taylor Valley,Antarctica, and the global climate

Carbonate-rich lacustrine and deltaic deposits, containing thin beds of finely laminated carbonates and thick beds of silt, crop out at several sites in the Taylor Valley and have been encountered in cores obtained by the Dry Valley Drilling Project (DVDP). Fragments of the more indurated carbonate beds have widespread occurrence as part of the desert “lag gravel” which covers much of the valley floor. Analysis of the carbonates suggests that they were deposited as algal limestones from waters derived from the East Antarctic Ice Sheet via the Taylor Glacier at times which correspond to the previous three global interglacial periods, as evidenced by the ice volumes deduced from oxygen-isotopic analysis of oceanic cores. The lacustrine carbonates have been found up to 30 km beyond the present terminus of the Taylor Glacier, and up to 100 m above the level of Lake Bonney, into which the Taylor Glacier at present discharges. It is concluded that the Taylor Glacier has advanced during each of the previous three interglaciations, and it is suggested that this has been caused by a thickening of the East Antarctic Ice Sheet during the interglaciations.     

Petrogenesis of carbonate-bearing nepheline syenites and carbonatites from Southern Victoria Land, Antarctica: origin of carbon and the effects of calcite-graphite equilibrium

Chemically-evolved carbonate-bearing nepheline syenites are intruded into basement metasediments of the Koettlitz Group on Dismal and Radian Ridges in the Pipecleaner Glacier region of Southern Victoria Land, Antarctica. Whole rock XRF data from the Dismal Nepheline Syenite defines a broad trend which is consistent with the removal of a cumulate fraction of approximate composition 70% hedenbergite, 15% nepheline, 10% titanite and 5% apatite. Stable isotope, major and trace element and mineralogical characteristics of the syenites are very similar to those of cross-cutting calcite-rich dykes indicating derivation from closely-related source magmas. The general association of carbonatites and nepheline syenites with extensional environments, suggests that the Dismal and Radian Ridge nepheline syenites and carbonatite dykes indicate a period of early Paleozoic (531 Ma) rifting or intrusion into localised tensional structures in an overall compressional regime. Assimilation of marble by the syenite magmas is evidenced by abundant rafts and xenoliths within the Dismal Nepheline Syenite, however, carbon and oxygen isotopic ratios from syenite and carbonatite calcites are distinctly lighter than values from the marble country rock indicating a magmatic source. Graphite and calcite commonly occur as aggregates in the Dismal Nepheline Syenite suggesting equilibrium between these two carbon-bearing phases. Isotopic fractionation between calcite and graphite, via the equilibrium: C + O₂ = CO₂ has resulted in enrichment of the ¹³C isotope in calcites from the Dismal Nepheline Syenite.     

Segregation of leucogranite microplutons during syn-anatectic deformation: an example from the Taylor Valley, Antarctica

A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn-anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre-scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre-thick stromatic leucosomes. Vein- and dyke-like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome-to-rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite 'microplutons' up to 50 m across beneath refractory 'cap-rock' layers.
Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite-bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50-m-wide 'microplutons' of xenolith-rich leucogranite.
Lack of vapour-absent melting and consequent low melt-to-rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome 'microplutons' did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic-scale granitoids.     

Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt,north Victoria Land,Antarctica: implications for low-temperature alteration

Chemical and isotopic (Sr, O, H) variations have been examined in an iron-rich lava flow of the Kirkpatrick Basalt from the Mesa Range in north Victoria Land, Antarctica. The flow is homogeneous with respect to the less mobile elements, whereas variations observed in K, Na, Si, Fe, and Rb result largely from alteration of glassy matrix material. Whole-rock Rb–Sr isotope data fall along a poorly-defined 103 Ma array attributed to secondary mobilization of Rb during the mid-Cretaceous. Alteration at that time is suggested by paleomagnetic data and would also account for discordant K–Ar dates. Whole-rock ¹⁸O values vary from +5.8 to +8.2 and a plagioclase separate has a ¹⁸O value of +5.6, reflecting the original composition of the magma. The range of ¹⁸O values for the whole-rock samples results from low-temperature alteration occurring primarily in the Jurassic and/or mid-Cretaceous. Whole-rock D values (-201 to -243) are markedly depleted, approaching equilibrium with modern meteoric water. In light of these data, variable Sr and O isotopic ratios in the underlying sequence of flows, previously interpreted in terms of an assimilation-fractionation model, may largely reflect post-magmatic alteration.     

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.     

Chemical studies of H chondrites. II: Weathering effects in the Victoria Land,Antarctic population and comparison of two Antarctic populations with non-Antarctic falls

We report RNAA data for 14 siderophile, lithophile and chalcophile volatile/mobile trace elements (Ag, Au, Bi, Cd, Co, Cs, Ga, In, Sb, Se, Te, Tl, U, Zn) in interior portions of 45 different H4–6 chondrites (49 samples) from Victoria Land, Antarctica and 5 H5 chondrites from the Yamato Mts., Antarctica.Relative to H5 chondrites of weathering types A and B, all elements are depleted (10 at statistically significant levels) in extensively weathered (types B/C and C) samples, probably by leaching on the ice sheet surface. Contents of 8 elements in extensively weathered samples may provide a qualitative ranking for terrestrial surface residence. Chondrites of weathering types A and B seem compositionally uncompromised and as useful as contemporary falls for trace-element studies. When data distributions for these 14 trace elements in non-Antarctic H chondrite falls and unpaired samples from Victoria Land and from the Yamato Mts. (Queen Maud Land) are compared statistically, numerous significant differences are apparent. Concentrations of 8 elements differ significantly in the Victoria Land and non-Antarctic H5 chondrite populations. Essentially the same compositional differences and ⁵³Mn content and shock history differences appear when H4–6 populations are compared. Contents of 8 elements differ when Queen Maud Land and Victoria Land populations are compared and 5 for the Queen Maud Land/non-Antarctic comparison.These and other differences give ample cause to doubt that the various sample populations derive from the same parent population. The observed differences do not reflect weathering, chance or other trivial causes: a preterrestrial source must be responsible. The least unlikely of these involves a temporal variation in the source regions from which meteoroids derive.     

Chemical characterization of microbial-dominated soil organic matter in the Garwood Valley, Antarctica

Despite its harsh environmental conditions, terrestrial Antarctica contains a relatively large microbial biomass. Natural abundance carbon and nitrogen stable isotope signatures of organic materials in the dry valleys indicate mixed provenance of the soil organic matter (SOM) with varying proportions of contributions from lichens, mosses, lake-derived algae and cyanobacteria. Here we employed two complementary analytical techniques, biomarker measurements by gas chromatography/mass spectrometry and solution-state ¹H nuclear magnetic resonance spectroscopy, to provide further information at a molecular-level about the composition and possible source of SOM in the Garwood Valley, Antarctica. The predominance of branched alkanes and short-chain lipids in the solvent extracts indicates that the primary contribution to the SOM was microbial-derived. Chemical structures in the NaOH extracts from soils were also dominated by amide, peptides, and a CH₃-dominating aliphatic region that were characteristic of microbial signatures. Furthermore, the SOM in the Garwood Valley contained compounds that were different from those in the cyanobacteria-dominated mat from a nearby lake (including monoethyl alkanes and enriched side-chain protons). This observation suggests that easily degradable carbon sources from the nearby lake did not dominate the SOM, which is consistent with a fast turnover of the mat-derived organic matter found in the valley. This study highlights the important role of native soil microbes in the carbon transformation and biogeochemistry in terrestrial Antarctica.     

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.     

南极罗斯岛及泰勒谷地区火山岩的地球化学特征及成因讨论

利用微量元素和Ｓｒ,Ｎｄ,Ｐｂ同位素资料,得出了南极罗斯岛及泰勒谷地区火山岩来自于上地幔,五个地区火山岩具有相同的源区。岩石具有大洋岛屿玄武岩（ＯＩＢ）的特征。来自地幔的火山喷发的岩浆未遭受地壳物质的混染作用。地幔源区是具有高μ（＝２０６Ｐｂ／２０４Ｐｂ）比值的地幔组分ＨＩＭＵ与亏损地幔组分ＤＭＭ混合作用的结果,其中以ＨＩＭＵ组分为主,原生岩浆由具ＨＩＭＵ组分特征的深部地幔柱与浅部的ＤＭＭ混合而成。     

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.     

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 . , , , , : — - — , . , - , , , , .. ., , - , , , , .

     

Oxygen isotope geochemistry of the Granite Harbour Intrusives, Wilson Terrane, Northern Victoria Land, Antarctica

Summary The oxygen and strontium isotope compositions of the Cambro-Ordovician granitoids cropping out in the Wilson Terrane (Granite Harbour Intrusives–GHI) constrain the petrological evolution of the magmatism in Antarctica, related to the Ross Orogeny. The measured δ¹⁸O_WR values of these intrusives define three different compositional groups: the metaluminous rocks (MAG), with δ¹⁸O_WR ranging from 6.9 (olivine gabbro) to 11.4‰ (monzogranite); the unaltered peraluminous granites (PAG), having δ¹⁸O_WR values ranging from 10.6 to 13.2‰, and the foliated peraluminous leucogranites (SKG), characterised by δ¹⁸O_WR values above 14‰. The analysis of equilibrium mineral assemblages indicates that the high δ¹⁸O_WR values are magmatic and unaffected by low-temperature processes. A few peraluminous granites sampled in the vicinity of Cenozoic intrusions show anomalously low δ¹⁸O_WR, due to meteoric-hydrothermal alteration. The isotopic data indicate that the coeval and spatially related metaluminous mafic and felsic intrusives forming the GHI were not comagmatic: the mafic and intermediate rocks were likely derived from lower crustal contamination of a pristine basaltic magma; their δ¹⁸O_WR values were also increased during emplacement, due to the interaction with the adjacent ¹⁸O-rich hydrous felsic magmas (mixing). Oxygen isotope data indicate that the crustal sources producing the Granite Harbour intrusives were not homogeneous: the felsic metaluminous intrusives were produced by partial melting of fertile rock with possible igneous origin, whereas partial melting of a metapelitic source rock is claimed for the genesis of the peraluminous granites. Received February 9, 2001; revised version accepted August 10, 2001