Rauer Group Research Articles

(Th+U)-Pb monazite ages from Al-Mg-rich metapelites, Rauer Group, east Antarctica

The age of high-temperature granulite-facies metamorphism (>� 800-850 � C) in the Rauer Group, Prydz Bay, east Antarctica, is relevant for establishing the metamorphic and temporal architecture of the Prydz Bay mobile belt. Monazites within Al-Mg-rich granulite- facies metapelites give an overall tanh-estimated Pan- African age of � 511±4 Ma (2r) using in-situ electron microprobe-based (Th+U)-Pb chronology, consistent with existing U-Pb zircon geochronology from the Rauer Group and Prydz Bay. Monazite occurs primarily within cordierite-bearing coronae and symplectic min- eral reaction textures, and also within biotite. Pan- African granulite-facies metamorphism is preferred as responsible for the development of the cordierite-bearing microstructures, and probably (peak) coarse-grained assemblages, constrained using an integrated geologic, geochronologic and metamorphic framework. Thus, Pan-African granulite-facies metamorphism affected the Rauer Group, within the Prydz Bay mobile belt. Moreover, integrated monazite geochronology may be used to decipher the temporal metamorphic histories of potentially complex high-temperature terrains.

A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system

AbstractDue to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H2O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H2O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria.

Adélie penguin colonies in eastern Prydz Bay: ‘biological indicators’ of exploration history and political change

AbstractTemporally and spatially increasing information on the distribution of Adélie penguin (Pygoscelis adeliae) breeding sites is used as an index of various national activities in eastern Prydz Bay, East Antarctica. Recorded instances of such sites are used to indicate both exploration and enhanced local knowledge. While Norwegians discovered the area (1935), and revisited it (1937), reports of penguins were minimal. The 1938 Ellsworth expedition added few details and the potential of Operation Highjump (1947) photographs to delimit breeding sites was never realised. Observations by Australian expeditioners from the mid-1950s onwards, supplemented to some extent by those from the Soviet Union, increased information substantially. When Davis station was established (January 1957), at least five breeding sites were known around eastern Prydz Bay. By 1973 this had increased to 23 or 24 sites, mostly north of the Sørsdal Glacier, which had apparently acted as a barrier to land-based exploration. Data available to 1980 showed 20 sites in the Vestfold Hills, added two in the Rauer Group, and omitted some recorded earlier. Ground surveys of the Vestfold Hills (November 1973) increased known sites slightly, discounted erroneous records, and massively increased numbers of individual colonies. In 1981 an air survey recognised 24 sites in the Vestfold Hills and increased those known to 47. In approximately the same period, official Soviet records showed perhaps four sites in the Vestfold Hills and another in the Rauer Group. Early reports provided poor estimates of breeding population sizes — totals of some 130,000 (or 174,200) pairs in the Vestfold Hills in 1973 are compared with perhaps 196,600 in 1981, with another 129,000 pairs to the south. By 1983 locations of breeding sites in the Vestfold Hills were well established, and this was achieved in southern Prydz Bay following publication of 1981 survey results.Progression of information regarding breeding sites in eastern Prydz Bay was slow. Initial Australian activities were slight following acceptance of its Antarctic Territory (1933). However, a Soviet Antarctic whaling fleet, uncertainty regarding American and Soviet intentions, and the imminent International Geophysical Year increased Australian interest. A station was established, local search areas expanded, and enhanced details regarding penguin breeding sites and colonies followed. Data reviews and surveys followed increasing international interest in southern ecosystems. Improved knowledge regarding the species' local populations reflected changing political agendas. Indeed, ‘knowledge’ itself gave early support to territorial claims. Participation in international surveys became an acceptable scientific endeavour, anticipated under Treaty agreements and promoted by associated organisations. In such fora, surveys and monitoring are expected, although not necessarily furthering the strength of existing claims.

Aspects of the Ecology of South Polar Skuas Catharacta maccormicki at Hop Island, Rauer Group, East Antarctica

SummaryAspects of the ecology of South Polar Skuas Catharacta maccormicki were investigated at Hop Island, Rauer Group, East Antarctica between December 1990 and February 1991. Data were obtained from pairs across the island generally for comparison with skuas (fringe-dwellers) nesting away from Adélie Penguin Pygoscelis adeliae colonies. While most defended territories included colonies of Adélie Penguins, Antarctic Petrels Thalassoica antarctica or Southern Fulmars Fulmarus glacialoides, others held no surface-nesting seabirds; however, all skuas took penguin material. Activities of two fringe-dwelling pairs, whose territories included fulmars, and fulmars and petrels, were intensively monitored; behaviours were associated mostly with brooding, vigilance or foraging. Foraging trips were longer and less frequent where only fulmars bred, but hunting success was similar to that of the pair hunting in petrel and fulmar colonies. The small number of fringe-dwelling pairs produced proportionally fewer chicks than others; the need to forage away from nesting areas, and to obtain penguin material, even when feeding territories include Antarctic Petrels and or Southern Fulmars, indicates the lower quality of their territories (which may vary). Breeding and other activities are considered in relation to breeding chronology of other local seabirds: the breeding success of fringe-dwellers may be modified by that of neighbouring seabird colonies.

The evolution of a layered metaigneous complex in the Rauer Group, East Antarctica: evidence for a distinct Archaean terrane

The Rauer Group in Prydz Bay, East Antarctica, contains both Archaean and Proterozoic crustal components. Archaean geological relationships have been defined from the Scherbinina layered complex (SLC), a polydeformed and metamorphosed Fe-tholeiite intrusive cut by several generations of deformed metabasite dykes. The SLC and three of these dyke generations are intruded by tonalitic and trondhjemitic sheets representative of Archaean felsic orthogneisses present outside the complex. UPb shrimp dating of zircons precisely defines the magmatic age of the SLC as 2844 ± 6 Ma. A tonalite sheet that cuts the SLC preserves a complex zircon isotopic array dominated by inherited grains derived from source rocks up to 3470 ± 30 Ma old, but from field evidence was emplaced at ca 2800-2550 Ma. The presence of 3470 Ma source rocks is unique in the East Antarctic Shield, and this feature as well as the prominence of 2840-2800 Ma orthogneisses establishes the Rauer Group as a distinct Archaean terrane. The Archaean of the Rauer Group is not comparable with the nearby Vestfold Hills, where ca 2520-2480 Ma orthogneisses form the dominant rock units. It is likely that the Vestfold Hills and Rauer Group terranes were amalgamated at either 1000 or 500 Ma, rather than at the end of the Archaean. A mid-Proterozoic tectonothermal event documented in the Rauer Group from syn-metamorphic 1030-1000 Ma intrusives (Kinny, P.D., Black, L.P. and Sheraton, J.W., 1993. Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands. Antarctic Sci., 5, 193–206) has not been detected in the zircon UPb data obtained on the Archaean gneisses examined here. Instead, the SLC and intrusive tonalite show resetting of zircon UPb systematics at ca 520 Ma. This resetting reflects Pb-loss within irregular compositional zones and healed cracks in older zircon, and is attributed to the effects of fluid infiltration under greenschist to amphibolite facies conditions. These effects correlate in time with ca 530-510 Ma higher-grade deformation and metamorphism evident in Proterozoic paragneisses exposed in areas to the south-west of the Rauer Group, and may record amalgamation of the Rauer Group with these areas during the East Gondwanian Pan-African orogeny.

Ultrahigh temperature granulite metamorphism (1050 °C, 12 kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica

Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to XMg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al2O3 to rims with up to 10.6 wt% Al2O3 adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al2O3–SiO2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet XMg , with apparent higher‐pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near‐isothermal decompression (UHT‐ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back‐reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high‐grade events at 1000 and 530 Ma. Late‐stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT‐ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late‐Proterozoic or Pan‐African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.

Timing and mechanisms of carbon isotope exchange in granulite-facies calc-silicate boudins, Rauer Group, East Antarctica

Mineralogically zoned, decimeter-diameter calc-silicate boudins enclosed within paragneisses from Little Italy Island, Rauer Group, east Antarctica underwent granulite-facies metamorphism (P 5 700‐900 MPa, T 5 840 6 40 8C) followed by near-isothermal decompression to 200‐400 MPa. During decompression several mineral reactions occurred in the presence of a pore fluid ( 0.4). The calc-silicate boudins show a general XCO2 increase in calcite d 13 C values from core (as low as 217‰) to rim ( 2 9t o 2 10‰); by contrast, d 18 O values show little variation across the boudins. The d 13 C profiles are similar to those predicted to result from diffusion within a sphere that is surrounded by a homogeneous reservoir. Diffusion of carbon isotopes probably occurred synchronous with the post-peak metamorphic mineral reactions. At that time, centimeter-scale diffusion would have been facilitated by the presence of the fluid, while isotopic exchange between the minerals and the fluid would have been promoted by recrystallization. For metamorphic porosities of 10 25 2 10 23 , the d 13 C profiles could have formed in a few thousand to hundreds of thousands of years. Small (millimeter to centimeter) scale variations in d 13 C values that may have been initially present within the boudins would have been homogenized on much shorter timescales than those required to form the profiles. The calculated timescales may reflect the time over which metamorphic recrystallization occurred and a reactionenhanced porosity was present. Graphite formed locally in the paragneisses only at margins of the boudins probably reflects the local escape of CO 2-bearing fluids from the boudins into relatively low rocks. fO2

Crustal evolution of the East Antarctic mobile belt in Prydz Bay: continental collision at 500 Ma?

The East Antarctic mobile belt as exposed in Prydz Bay presents an excellent example of a poly-metamorphic terrain where complicated high-grade structures can be grouped on the basis of lineation direction, sense of shear and metamorphic grade. In this way local intricacies resulting from truncating and interfering foliations and folds can be simplified by ordering structures with respect to their kinematic connotation. The resulting deformation scheme is simple and facilitates regional correlations, and simultaneously places metamorphic textures in a kinematic—structural background. The mobile belt in Prydz Bay is exposed along 200 km of coastline, and consists of granulite-facies gneiss in which low- to mid-crustal, compressional D1–2 structures, and mid- to upper-crustal, extensional D3–6 structures and relateddecompression-cooling textures are best explained in an exhumation model involving extensional collapse of upper crust in an overall compressional tectonic setting. This implies that compression and extension structures are genetically related. However, D3–6 structures in the Larsemann Hills appear to have formed between 550 and 500 Ma, whilst D2 structures in the Rauer Group have been dated at 1100–1000 Ma. Reinterpretation of the latter dates is possible, implying that most high-grade deformation in Prydz Bay is early Palaeozoic.

Crustal evolution of the East Antarctic mobile belt in Prydz Bay: continental collision at 500 Ma?

The East Antarctic mobile belt as exposed in Prydz Bay presents an excellent example of a poly-metamorphic terrain where complicated high-grade structures can be grouped on the basis of lineation direction, sense of shear and metamorphic grade. In this way local intricacies resulting from truncating and interfering foliations and folds can be simplified by ordering structures with respect to their kinematic connotation. The resulting deformation scheme is simple and facilitates regional correlations, and simultaneously places metamorphic textures in a kinematic—structural background. The mobile belt in Prydz Bay is exposed along 200 km of coastline, and consists of granulite-facies gneiss in which low- to mid-crustal, compressional D 1–2 structures, and mid- to upper-crustal, extensional D 3–6 structures and relateddecompression-cooling textures are best explained in an exhumation model involving extensional collapse of upper crust in an overall compressional tectonic setting. This implies that compression and extension structures are genetically related. However, D 3–6 structures in the Larsemann Hills appear to have formed between 550 and 500 Ma, whilst D 2 structures in the Rauer Group have been dated at 1100–1000 Ma. Reinterpretation of the latter dates is possible, implying that most high-grade deformation in Prydz Bay is early Palaeozoic.

A Pan‐African granulite facies metamorphic episode in Prydz Bay, Antarctica: Evidence from Sm‐Nd garnet dating

The granulite facies gneisses along the Prydz Bay coastline, Antarctica, have long been regarded as part of an extensive Proterozoic (ca 1000 Ma) terrane, based on correlations with other parts of eastern Antarctica. New Sm‐Nd garnet‐whole rock dates for various rocks indicate that the main metamorphic episode that has affected the rocks in Prydz Bay is of Pan‐African age (515–490 Ma). Only in one locality, in the western part of the area, have older garnet ages been found. On Søstrene Island, where petrographic and analytical work suggests an earlier higher pressure metamorphic event, an age of 990 Ma correlates with some well‐established metamorphic ages from the Northern Prince Charles Mountains to the south, the Rayner Complex, Enderby Land to the southwest and part of the Rauer Group to the east. Evidence for reworking of the rocks on Søstrene is provided by the textures and by the 500 Ma ages which have been determined for interlayered pelitic and felsic gneisses. The notion of a continuous Late Proterozoic (ca 1000 Ma) terrane ranging from the Rayner Complex, Enderby Land, in the west to the Northern Prince Charles Mountains and the Rauer Group has to be abandoned. The predominantly pelitic gneisses of the Prydz Bay coast are post‐1000 Ma crust derived from Early Proterozoic sources (as indicated by Nd model age) and metamorphosed at 515–490 Ma. Portions of reworked Late Proterozoic (ca 1000 Ma) basement appear to be rare in this area and have so far only been positively identified on Søstrene Island and, by earlier workers, in the Rauer Group. The Pan‐African thermo‐tectonic event probably occurred as a result of the collision of an Archaean block, represented by part of the Rauer Group, with a composite terrane, consisting of basement, metamorphosed at 1000 Ma, and an overlying cover of predominantly pelitic rocks, represented by the Prydz Bay terrane.

Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, East Antarctica

AbstractMeta-sediments in the Larsemann Hills that preserve a coherent stratigraphy, form a cover sequence deposited upon basement of mafic–felsic granulite. Their outcrop pattern defines a 10 kilometre wide east–west trending synclinal trough structure in which basement–cover contacts differ in the north and the south, suggesting tectonic interleaving during a prograde, D1 thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D2 and D3, each defined by a unique lineation direction and shear sense. D2 structures which are associated with the dominant gneissic foliation in much of the Larsemann Hills, contain a moderately east-plunging lineation indicative of west-directed thrusting. D2 comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D3 structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S3 is the dominant gneissic layering and folds sequences resemble D2 folding. Outside the D3 high-strain zone occurs a low-strain D3 window, preserving low-strain D3 structures (minor shear bands and upright folds) that partly re-orient D2 structures. All structures are truncated by a series of planar pegmatites and parallel D4 mylonite zones, recording extensional dextral displacements.D2 assemblages include coexisting garnet–orthopyroxene pairs recording peak conditions of ∼ 7 kbar and ∼ 780°C. Subsequent retrograde decompression textures partly evolved during both D2 and D3 when conditions of ∼ 4–5 kbar and ∼ 750°C were attained. This is followed by D4 shear zones which formed around 3 kbar and ∼ 550°C.It is tempting to combine D2–4 structures in one tectonic cycle involving prograde thrusting and thickening followed by retrograde extension and uplift. The available geochronological data, however, present a number of interpretations. For example, D2 was possibly associated with a clockwise P–T path at medium pressures around ∼ 1000 Ma, by correlation with similar structures developed in the Rauer Group, whilst D3 and D4 events occurred in response to extension and heating at low pressures at ∼ 550 Ma, associated with the emplacement of numerous granitoid bodies. Thus, decompression textures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.

The structural evolution of the Rauer Group, East Antarctica: mafic dykes as passive markers in a composite Proterozoic terrain

Archaean gneisses in the Rauer Group of islands, East Antarctica, record a prolonged history of high-grade deformational episodes, many of which predate that identified in mid-Proterozoic gneisses. Eleven generations of mafic dykes, belonging to discrete chemical suites, have been used as relative time markers to constrain this deformational history. Based on the timing of intrusion with respect to structures, dykes in the Rauer Group have been correlated with largely undeformed and dated dyke suites in the adjacent Vestfold Hills. This has allowed absolute ages to be inferred for the early- to mid-Proterozoic mafic dyke suites in the Rauer Group, and a correlation of the interspersed structural events. Most structures in the Rauer Group, however, developed in response to high-grade progressive deformation at approximately 1000 Ma. During this deformational episode, strains were repeatedly partitioned into sub-vertical, noncoaxial, high-strain zones recording NW-directed sinistral transpression, that separated zones of lower strain dominated by coaxial folding with axes parallel to the shear direction. Three additional mafic dyke suites intruded during this deformation which was followed by three stages of brittle-ductile deformation and a final suite of lamprophyre dykes. Due to the numerous intrusive time markers, the Rauer Group serves as an excellent illustration of how complicated gneiss terrains may be.

Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica

ABSTRACTThe role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2‐rich fluids or generally vapour‐absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre‐scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.Forsteritic marble layers and associated calcsilicates preserve a range of 13C‐ and 18O‐depleted calcite isotope values (δ13C= ‐9.9 to ‐3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of 13C and 18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2 infiltration during granulite facies metamorphism, but does indicate localized fluid‐rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite‐like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid‐rock interaction occurred prior to high‐grade metamorphism; other fluid‐rock histories are not, however, ruled out by the available data. Coupled trends of 13C and 18O depletion are modified to even lower values by the superposed development of small‐scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high‐temperature (>1000d̀C) granulite facies metamorphism.

Reactions and textures in wollastonite-scapolite granulites and their significance for pressure-temperature-fluid histories of high-grade terranes

Reaction textures in wollastonite-scapolite calc-silicates provide important evidence for pressure-temperature-fluid histories in granulites. Pressure-temperature, T-aCO2 and P-aCO2 diagrams, calculated for appropriate mineral compositions using the internally consistent dataset of Holland and Powell (1990), are presented and used to provide a framework for interpretation of the reaction textures. Garnet rims replacing wollastonite and scapolite in calc-silicate gneisses is a common texture in many granulite terranes. This texture can plausibly be developed in post-peak P-T histories dominated by cooling (e.g., near-isobaric cooling) and does not generally imply the infiltration of hydrous fluids. Examples from the Arunta Complex, Australia, and the Northern Prince Charles Mountains, Antarctica, are consistent with cooling without fluid influx, at 7–8 kbar and aCO2 of 0.3–0.5. Contrasting textures involving the replacement of garnet by wollastonite+scapolite or wollastonite+plagioclase symplectites, and the growth of wollastonite+plagioclase rims or coronas on scapolite-quartz boundaries, can be interpreted as essentially closed-system features and modelled through modal analysis of reaction products. In the case of the Rauer Group, Antarctica, near-isothermal decompression from ca. 8 to 6 kbar at 850–800°C and aCO2 in the range 0.35–0.45 is indicated by these textures, based on calculated P-T-aCO2 grids in the CaOAl2O3SiO2CO2 (CASV) and more complex systems. Calculated reactions producing wollastonite+scapolite from garnet-bearing assemblages do not involve decarbonation, but may consume CO2 with increasing temperature in the absence of calcite. Hence, these calc-silicates may act as CO2 “sinks” on the prograde segments of clockwise P-T paths. Conversely, scapolite-wollastonite granulites will act as sources of post-peak CO2-rich fluids liberated upon cooling and potentially trapped elsewhere as post-peak carbonic fluid inclusions.

The Proterozoic deformation of the Vestfold Hills Block, East Antarctica: implications for the tectonic development of adjacent granulite belts

The Archaean Vestfold Block occurs at the margin of the 1000 Ma mobile belt of East Antarctica, and shows a prolonged and very complicated Proterozoic deformational history, in which at least seven extensional and compressional deformational episodes alternated with emplacement of at least fourteen different dyke swarms. These swarms can be grouped into five major suites, each bracketed by predominantly compressional deformational events. The most voluminous dyke suites 4 and 5 were emplaced around 1380 Ma and 1250 Ma, respectively. The former is associated with a progressive sequence of extensional structures that evolved from pseudotachylites to ductile normal shears as heat flow increased in response to extensional processes and dyke emplacement. Similar increased heat flows probably accompanied the emplacement of suite 5. The subsequent 1000 Ma compressional events and associated recrystallisation of earlier structures and dykes at amphibolite facies grade, show a marked intensity increase towards the SW Vestfold Hills, where a second pegmatite suite was associated with rehydration of the granulites. In the Vestfold Hills, for a long period during the Mesoproterozoic the compressional and extensional stages alternated along an E-W axis. In the Rauer Group this direction parallels the strain gradient associated with 1000 Ma events, and is approximately normal to the outcrop trace of the envelope to the complexly folded Archaean-Proterozoic boundary. It suggests that Mesoproterozoic extensional and compressional events, including the formation of the 1000 Ma mobile belt, occurred along similar structural trends. In the NE half of the Rauer Group a large number of mafic dykes truncating older gneiss foliations can be recognized. Going W across the Rauer Group, the number of dyke types decreases and the older dykes are increasingly reworked by the 1000 Ma events. Most of these dykes can be correlated with the multiple dyke suites in the Vestfold Hills, and therefore, some of the granulite events that have been described as part of the 1000 Ma events, but which pre-date the emplacement of most dykes, probably correlate with the extensional events that formed between 1250 and 1400 Ma in the Vestfold Hills. This has major implications for the interpretation of the described decompressional P-T path in the Rauer Group which connects an upper P-T section that corresponds to the pre- to syn-dyke granulite events with a lower P-T section representing post-dyke granulite events, dated at 1000 Ma. Comparison with the Vestfold Block suggests that the path may be the aggregate result of several unrelated events within an extensional-compressional tectonic cycle. The Vestfold Hills Block serves as an example that much information about high-grade mobile belts may be obtained from adjacent ‘stable’ terrains, and that extreme care is required in interpreting the ‘simple’ decompressional P-T-t -deformation paths which are generally described in the mobile belt of East Antarctica.

Granulite facies metasomatism: zoned calc-silicate boudins from the Rauer Group, East Antarctica

Calc-silicate boudins from the Rauer Group, East Antarctica, were metamorphosed under granulite facies conditions during late Proterozoic (ca. 1,000 Ma) M3 metamorphism. Boudin cores contain low to moderate aCO2 assemblages including wollastonite, grossularandradite (grandite) garnet, clinopyroxene, scapolite, plagioclase, quartz±calcite. Petrological and stable isotopic evidence suggests that these core assemblages resulted from pre-peak M3 infiltration of water-rich fluids; there is no evidence for a pervasive fluid phase under peak M3 conditions. The boudins are separated from the surrounding Fe-rich pelites and semi-pelites by a series of concentric, high-variance reaction zones developed under peak M3 conditions. Variations in mineral assemblage, mineral composition and whole rock composition across these zones suggest that they formed by diffusional masstransfer, controlled principally by a chemical potential gradient in Ca across the original calc-silicate-paragneiss lithological boundary. As a consequence of the nearcomplete decarbonation of the calc-silicatesbefore the M3 peak, development of the diffusion-controlled reaction zones did not liberate significant CO2 during granulite facies metamorphism. Similar calcite-poor, low aCO2 calc-silicate horizons in other granulite facies terrains are unlikely to have been important local fluid sources during deep crustal metamorphism.

Aspects of the Breeding Biology of Antarctic Petrels and Southern Fulmars in the Rauer Group, East Antarctica

Breeding phenology, success and nest attendance of Antarctic Petrels Thalassoica antarctica and Southem Fulmars Fulmarus glacialoides at the Rauer Group (68°51′S, 77°50′E), East Antarctica, are discussed. Most data were collected on Hop Island (68°50′S, 77°42′E) in January and February 1988, and from December 1988 to March 1989. Observations extended from the late stages of incubation to post-guard or fledging periods. Some annual breeding indices collected from 1983 onwards at census sites are compared with meteorological data and the extent of fast ice for the nearby Davis Station (68°35′E, 77°58′E). Both species had a restricted hatching period, reflecting a brief and synchronised egg-laying period, typical of other southem fulmarine petrels. Antarctic Petrel chicks hatched from 4 January (1989), and c. 90% appeared by 16 January (both years). Southern Fulmar hatching began on 21 January (1988) and almost all chicks appeared by 6 February (both years). Adult attendance at nests declined with increasing chick age. For Antarctic Petrels, this was most marked at about 11 days; no chicks had continuously attendant adults after 24 days, although adults returned to feed them. Incubation shifts in Southern Fulmars were longer than shifts following hatching and the post-guard period started, on average, 13 days after hatching. Egg and chick losses varied between years and sites. The South Polar Skua Catharacta maccormicki was apparently involved in the majority of losses. Nest sites of both species resemble those elsewhere: Southern Fulmars may require steeper sites, allowing a fall away from colonies. Antarctic Petrels are less affected by accumulation of snow or ice and shelter from katabatic winds may be important. Although weather may modify breeding success locally, annual success must depend on the ability of parents to produce eggs and feed chicks: this may be moderated by the extent and persistence of pack ice.

Foods and Aspects of Growth in the Antarctic Petrel and Southern Fulmar Breeding at Hop Island, Rauer Group, East Antarctica

Foods of adult Antarctic Petrels Thalassoica antarctica and Southern Fulmars Fulmarus glacialoides breeding on Hop Island, Rauer Group, East Antarctica, were obtained using water-offloading techniques, from late incubation to late post-guard periods. Both species took mainly krill, particularly Euphausia superba, and fish, almost entirely Pleuragramma antarcticum; other crustaceans and cephalopods were infrequent. The increase of E. crystallorophias, particularly in Southern Fulmars, was associated with increased P. antarcticum in the diet of both species. No significant differences in prey sizes were detected but the smaller Antarctic Petrel tended to contain more fish and less krill. Results are compared with previous studies; all show a restricted number of prey species, and a wide range of prey sizes. Variations in proportions of specific groups may reflec sampling methods, but more likely reflect prey abundance or availability. The importance of ice cover is discussed. Inshore feeding by Southern Fulmars, and differing feeding methods, may allow segregation of foraging: assuming similar rates of digestion, the more digested food in Antarctic Petrels suggests a greater foraging range. This is supported by differences in attendance shifts at nests. Growth rates of chicks deprived a meal were compared with those of chicks given supplementary meals, and controls. Deprivation apparently caused no immediate effects; additional meals tended to increase growth of Antarctic Petrel chicks. Small sample sizes confounded interpretation but suggested that Antarctic Petrels, which typically breed further south than Southern Fulmars, are better adapted to local conditions where sympatric with Southern Fulmars. The slightly larger volumes of oil retrieved with Antarctic Petrel food supports this since oil offers a more efficient method of transporting food to growing chicks. For these two fulmarine petrels, segregation of food requirements may be by foraging differences; where sympatric, segregation of peak demands may also result from separation of laying periods.

Wollastonite--Scapolite Assemblages as Indicators of Granulite Pressure-Temperature-Fluid Histories: The Rauer Group, East Antarctica

Wollastonite--Scapolite Assemblages as Indicators of Granulite Pressure-Temperature-Fluid Histories: The Rauer Group, East Antarctica

Pressure–temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica

Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica.Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre‐date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope‐rich) garnets (XMg= 0.71) yet found in granulite facies rocks. A high‐temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet XMg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower‐temperature (c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages.The major Late Proterozoic (c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe‐rich metapelites and associated paragneisses in which syn‐ to post‐deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.