Abstract In both palaeoenvironmental and palaeogeographical studies, Antarctica plays a unique role in our understanding of the history of the Earth. It has maintained a unique geographical position at the South Pole for long periods. As the only unpopulated continent, the absence of political barriers or short-term economic interests has allowed international collaborative science to flourish. Although 98% of its area is covered by ice, the coastal Antarctic region is one of the well-studied regions in the world. The integrity and success of geological studies lies in the fact that exposed outcrops are well preserved in the low-latitude climate. The continuing programme of the Japanese Antarctic Research Expedition focuses on the geology of East Antarctica, especially in the Dronning Maud Land and Enderby Land regions. Enderby Land preserves some of the oldest Archaean rocks on Earth, and the Mesoproterozoic to Palaeozoic history of Dronning Maud Land is extremely important in understanding the formation and dispersion of Rodinia and subsequent assembly of Gondwana. The geological features in this region have great significance in defining the temporal and spatial extension of orogenic belts formed by the collision of proto-continents. Present understanding of the evolution of East Antarctica in terms of global tectonics allows us to visualize how continents have evolved through time and space, and how far back in time the present-day plate-tectonic regime may have operated. Although several fundamental research problems still need to be resolved, the future direction of geoscience research in Antarctica will focus on how the formation and evolution of continents and supercontinents have affected the Earth's environment, a question that has been addressed only in recent years.

Abstract In eastern Dronning Maud Land (DML), East Antarctica, there are several discrete, isolated magmatic and high-grade metamorphic regions. These are, from west ( c. 20°E) to east ( c. 50°E), the Sør Rondane Mountains (SRM), Yamato–Belgica Complex (YBC), Lützow-Holm Complex (LHC), Rayner Complex (RC) and Napier Complex (NC). To understand this region in a Gondwanan context, one must distinguish between Pan-African and Grenvillian aged magmatic and metamorphic events. Sensitive high-resolution ion microprobe U–Pb zircon ages and Nd model ages for metamorphic and plutonic rocks are examined in conjunction with published geological and petrological studies of the various terranes. In particular, the evolution of the SRM is examined in detail. Compilation of Nd model ages for new and published data suggests that the main part of eastern Dronning Maud Land, including the SRM, represents juvenile late Mesoproterozoic ( c. 1000–1200 Ma) crust associated with minor fragments of an older continental component. Evidence for an Archaean component in the basement of the SRM is lacking. As for central DML, 1100–1200 Ma extensive felsic magmatism is recognized in the SRM. Deposition of sediments during or after magmatism and possible metamorphism at 800–700 Ma is recognized from populations of detrital zircon in metasedimentary rocks. The NE Terrane of the SRM, along with the YBC, was metamorphosed under granulite-facies conditions at c. 600–650 Ma. The SW and NE Terranes of the SRM were brought together during amphibolite-facies metamorphism at c. 570 Ma, and share a common metamorphic and magmatic history from that time. High-grade metamorphism was followed by extensive A-type granitoid activity and contact metamorphism between 560 and 500 Ma. In contrast, T DM and inherited zircon core ages suggest that the LHC is a collage of protoliths with a variety of Proterozoic and Archaean sources. Later peak metamorphism of the LHC at 520–550 Ma thus represents the final stage of Gondwanan amalgamation in this section of East Antarctica.

Abstract The late tectonic history of the southern part of the Late Neoproterozoic–Early Palaeozoic East African–Antarctic Orogen (EAAO) is characterized by lateral extrusion, extensional collapse and large volumes of high-temperature A 2 -type granitoids. This late-tectonic igneous province covers an area more than 15 000 km 2 of the EAAO in Dronning Maud Land (East Antarctica) and its northerly continuation as the Nampula Complex of NE Mozambique. The magmatic province is bounded in the north by the Lurio Belt. New secondary ionization mass spectrometry (SIMS) U–Pb analyses of zircons from two major late-tectonic granitoid intrusions from Dronning Maud Land indicate crystallization ages of 501±7 and 499±4 Ma, whereas a major extensional shear zone was dated at 507±9 Ma. New SIMS zircon U–Pb analyses of late-tectonic granitoid sheets and plutons from the Nampula Province indicate ages of 512±4, 508±4, 508±2 and 507±3 Ma. Consequently, the late-tectonic magmatism can be bracketed between c . 530 and 485 Ma. It started with small gabbro bodies emplaced at c . 530–520 Ma, culminated with the intrusion of major granite–charnockite plutons at c . 510–500 Ma and terminated with the introduction of small volumes of sheet-like granite at c . 485 Ma. The new dates demonstrate that extensional shearing and granitoid intrusion are synchronous, and that orogenic collapse and the magmatism are related. We ascribe the distribution, structural style, geochemical composition and age of the late magmatic province to a process of partial delamination of the orogenic root in the southern third of the EAAO. It remains to be tested whether there is a relationship between orogenic collapse–granitoid magmatism and south-directed escape tectonics in the southernmost EAAO.

Abstract Analysis of new lithological, structural, metamorphic and geochronological data from extensive mapping in Mozambique permits recognition of two distinct crustal blocks separated by the Lurio Belt shear zone. Extrapolation of the Mozambique data to adjacent areas in Sri Lanka and Dronning Maud Land, Antarctica permits the recognition of similar crustal blocks and allows the interpretation that the various blocks in Mozambique, Sri Lanka and Antarctica were once part of a mega-nappe, forming part of northern Gondwana, which was thrust-faulted c. 600 km over southern Gondwana during amalgamation of Gondwana at c. 590–550 Ma. The data suggest a deeper level of erosion in southern Africa compared with Antarctica. It is possible that this thrust domain extends, through the Zambezi Belt or Valley, as far west as the Damara Orogen in Namibia with the Naukluft nappes in Namibia, the Makuti Group, the Masoso Suite in the Rushinga area and the Urungwe klippen in northern Zimbabwe, fitting the mega-nappe pattern. Erosional products of the mountain belt are now represented by 700–400 Ma age detrital zircons present in the various sandstone formations of the Transantarctic Mountains, their correlatives in Australia, as well as the Urfjell Group (western Dronning Maud Land) and probably the Natal Group in South Africa.

Abstract Subsequent to the reconnaissance fieldwork in 1982, the Japanese Antarctic Research Expedition (JARE) carried out extensive geological studies that focused on structural and tectonic aspects, petrology, geochemistry and geochronology of the Napier Complex in Enderby Land, East Antarctica. Detailed field investigations in several key areas, including geological mapping of the Mt. Riiser-Larsen area and Tonagh Island, revealed that the Napier Complex comprises layered and massive gneiss units, of which the layered unit is composed of garnet felsic gneiss, orthopyroxene felsic gneiss, pelitic and basic gneisses, impure quartzite, and minor metamorphosed banded iron formation, whereas the massive unit consists mainly of orthopyroxene felsic gneiss. The boundary between the units is transitional in the Mt. Riiser-Larsen area, in which metamorphosed anorthosite and ultramafic rocks occur as thin layers, or blocks or pods, but on Tonagh Island the boundary is closely associated with the shear zone. Nine deformation episodes (D 1 –D 9 ) were suggested for Tonagh Island. These results of fieldwork were presented in detail in two geological maps. Geochemical studies showed that (1) garnet–sillimanite gneisses and garnet-rich felsic gneisses were derived from mudstone and sandstone, respectively, both enriched in MgO, Cr and Ni; (2) orthopyroxene felsic gneisses have a close REE affinity with Archaean tonalite–trondhjemite–granodiorite (TTG); (3) basic gneisses were derived from light rare earth element (LREE)-enriched or -depleted basalts; (4) meta-ultramafic rocks are comparable with komatiite and related depleted mantle peridotite. This suite of protoliths is reminiscent of Archaean greenstone–granite belts. Precise analyses of physical conditions of metamorphism were carried out by using reliable approaches such as feldspar thermometry, alumina content of orthopyroxene, inverted pigeonite and bulk-rock compositions, and clino- and orthopyroxene compositions with different textures (porphyroblastic and neoblastic), and the results suggested that the maximum metamorphic temperature might have reached 1130 °C (i.e. ultrahigh-temperature (UHT) metamorphism). P – T evolution of the Napier UHT metamorphism was examined by analyses of reaction textures combined with fluid inclusion studies, suggesting both clockwise (Bunt Island) and counterclockwise (Mt. Riiser-Larsen and Tonagh Island) P – T – t paths. U–Pb sensitive high-resolution ion microprobe and secondary ionization mass spectrometry zircon ages from the Mt. Riiser-Larsen area and Tonagh Island indicate three stages of protolith formation at around 3.28–3.23, 3.07 and 2.68–2.63 Ga, and two contrasting ages for the timing of peak UHT metamorphism at either c. 2.55 or c. 2.51–2.45 Ga. On the basis of these results, more comprehensive studies on the Napier Complex are essential in the future for understanding (1) the role and age of TTG protolith and (2) the origin and timing of UHT metamorphism.

Abstract This paper presents a form-line map of the Napier and Rayner Complexes, East Antarctica, constructed from attitude data for foliations shown on published geological maps, and discusses the macroscopic geological structures. The form-line map shows that the two complexes consist of several, structurally distinct, units or blocks bounded by east–west-, NE–SW- and NW–SE-striking faults. The major boundary between the two complexes, as indicated on the published geological maps, is a structural discontinuity shown as a large fault on the form-line map. On the form-line map, east–west- and NE–SW-trending folds are abundant and NW–SE-trending ones occur locally in both complexes. North–south-trending folds are also abundant in the Napier Complex. Dome-and-basin fold patterns on a regional scale occur in some regions. The regional strikes, macroscopic structures, and the major boundary between the two complexes are considered to have resulted from the same later deformation episode. The form-line map and distribution map of key mineral assemblages show that the Napier Complex is not uniform and includes at least two types of metamorphic units or fragments of the Archaean crust that were formed through distinct P – T – t evolutionary processes and divided by several faults.

Abstract C, O and Sr isotope geochemistry of high-grade marbles from the Lützow-Holm Complex, East Antarctica, has given clues on the depositional ages and post-depositional alterations. Dolomitic and calcitic marbles occur as thin layers with varying thickness (up to 100 m) in several outcrops in eastern Dronning Maud Land, most of which underwent post-depositional geochemical alterations. In particular, the Sr and O isotope alterations are extensive, with 87 Sr/ 86 Sr (550 Ma) ratios as high as 0.758 and δ 18 O values as low as −5‰. These data suggest that multiple stages of fluid–rock interaction processes during diagenesis, prograde to peak and retrograde metamorphic events have altered the depositional isotopic signatures. However, some of the marble layers, exceptionally, preserve pre-metamorphic geochemical characteristics, such as low Sr isotope ratios, high δ 18 O and δ 13 C values, and well-equilibrated unaltered trace and rare earth element patterns. Lowest 87 Sr/ 86 Sr isotopic ratios of 0.7066 and 0.7053 with high δ 13 C and δ 18 O values suggest an apparent age of deposition around 730–830 Ma, although total geochemical resetting of carbonates by seawater of this age cannot be ruled out. The apparent depositional ages are consistent with carbonate deposition in the ‘Mozambique Ocean’ that separated East and West Gondwana.

Abstract Rb–Sr and Sm–Nd mineral dating of metamorphic rocks from Skallen, Skallevikshalsen and Rundvågshetta, in the southwestern part of the Lützow-Holm Complex, Dronning Maud Land, assists in constructing a thermal history after peak metamorphism. The results fall into two groups: (1) a record of regional cooling after peak metamorphism (524–488 Ma); (2) local resetting 50–80 Ma after peak metamorphism (474–446 Ma). This grouping is consistently observed in published ages from various localities in the Lützow-Holm Complex. A Sm–Nd age of 524 Ma is indistinguishable from published zircon and monazite ages. Ages of 511 and 488 Ma are related to cooling after peak metamorphism. The younger age group overlaps with ages of post-metamorphic magmatism and related hydrothermal activity reported from localities throughout East Antarctica. This intracontinental, post-orogenic igneous activity continued after the tectonic assembly of Gondwana.

Abstract Ultrasonic measurements of P-wave velocity ( V p ) and S-wave velocity ( V s ) were conducted at high pressures up to 1.0 GPa and high temperatures up to 400 °C for ultrahigh-temperature (UHT) metamorphosed rocks from Mount Riiser-Larsen, in the Archaean Napier Complex of Enderby Land. The results at 1.0 GPa and 400 °C are V p =7.17 km s −1 , V s =4.24 km s −1 , V p / V s =1.69, Poisson's ratio (σ)=0.23 for pyroxenite (SiO 2 =44.2 wt%, density (ρ)=3.41 g cm −3 ); V p =6.93 km s −1 , V s =3.81 km s −1 , V p / V s =1.82, σ=0.28 for mafic granulite (SiO 2 =52.2 wt%, ρ=3.02 g cm −3 ); V p =6.88 km s −1 , V s =3.72 km s −1 , V p / V s =1.85, σ=0.29 for mafic granulite (SiO 2 =49.5 wt%, ρ=2.88 g cm −3 ); and V p =6.17 km s −1 , V s =3.59 km s −1 , V p / V s =1.72, σ=0.24 for orthopyroxene-bearing felsic gneiss (SiO 2 =65.4 wt%, ρ=2.68 g cm −3 ). V p and V p / V s of these UHT rocks are not comparable with the previously proposed seismic velocity model ( V p =6.56 km s −1 , V p / V s =1.70) for the lower crust beneath the Mizuho Plateau of eastern Dronning Maud Land. Combining the available measured velocity and density data with the seismic velocity profile defined for the Mizuho Plateau, we suggest that relatively low V p and V s characteristics of the lower crust beneath the Mizuho Plateau may be attributed to higher abundance of biotite in the mafic lower crustal rocks. It is proposed that the biotite-bearing lower crustal rocks were formed by metasomatic processes associated with Pan-African orogeny.

Abstract NE–SW- and north–south-striking dykes were emplaced into ultrahigh-temperature (UHT) granulites apparently after UHT metamorphism in the Mt. Riiser-Larsen area of the Archaean Napier Complex, East Antarctica, of which the north–south-striking dykes interrupt the NE–SW-striking ones. The NE–SW-striking dykes are tholeiite basalt (THB) and high-magnesian andesite (HMA) in composition. The THB dykes display relict doleritic textures, whereas the HMA dykes shows blastoporphyritic textures characterized by phenocrysts of clinopyroxene and plagioclase. Both sets of dykes exhibit large ion lithophile element and light rare earth element enrichment and negative anomalies of Nb, Ti and/or P in a spider diagram normalized to primitive mantle, which is reminiscent of modern subduction-related arc volcanism or continental flood volcanism. The isotope ratios of the THB dykes define isochron ages of 2.0–1.9 Ga: 1979±80 Ma in the Rb–Sr system (initial ratio ( I 0 ): 0.70239±0.00035) and 2078±104 Ma in the Sm–Nd system ( I 0 : 0.50964±0.00012). Such moderate 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd initial ratios may represent source materials closely related to the mantle wedge of a subduction zone. The north–south-striking dykes are compositionally divided into two basalt types. One is an alkaline basalt (AL) showing intergranular texture and characterized by high concentrations of incompatible elements, similar to those of ocean island basalt. They yield an isochron age of c. 1.2 Ga: 1161±238 Ma in the Rb–Sr system ( I 0 : 0.7047±0.0012). The other type (THB-m) is doleritic (ophitic) in texture, and has a tholeiitic affinity with a flat chondrite-normalized REE pattern, which is comparable with that of enriched mid-ocean ridge basalt. A comparison with dykes reported from other areas of the Napier Complex suggests that the north–south-striking dykes occur in restricted areas, whereas the NE–SW-striking dykes are more regional in occurrence. The 2.0–1.9 Ga magmatism of the NE–SW-striking dykes may have been related to the formation of continental crust of the Rayner Complex.

Abstract Metabasites from the Lützow-Holm Complex, East Antarctica, are the equivalent of metamorphosed ultramafic and mafic rocks with ultrabasic to intermediate compositions, which occur as layers and blocks in the quartzo-feldspathic or metasedimentary gneisses. Field occurrences and whole-rock geochemistry suggest that the ultramafic rocks are all cumulitic protoliths, whereas the mafic rocks are mostly basaltic protoliths including some cumulates. Moreover, in a regional context, the geochemistry of metabasites shifts from island arc to ocean-floor affinities in a southwesterly direction from the Prince Olav Coast to the Lützow-Holm Bay area. Neodymium isotopic data suggest that the metamorphic rocks from the Prince Olav Coast and the northern Lützow-Holm Bay areas were derived from immature continental crust formed by active Mesoproterozoic crustal growth, whereas those from the southern Lützow-Holm Bay area were derived from mature continental crust and oceanic crust of older age. Thus, these results suggest that the Lützow-Holm Complex includes lithological units with various origins and ages that were amalgamated by multiple subduction, and underwent high-grade metamorphism as a result of the final collision of East and West Gondwana during the Pan-African orogeny.

Abstract The region comprising central to eastern Dronning Maud Land (2°W to 40°E), East Antarctica, is underlain by Mesoproterozoic to Cambrian metamorphic rocks and post-kinematic intrusive rocks with varied compositions. The post-kinematic mafic dykes linked to the Pan-African orogen include various types of lithologies: lamprophyre and lamproite in Mühlig-Hofmannfjella in central Dronning Maud Land and lamprophyre and high-K dolerite in the Sør Rondane Mountains in eastern Dronning Maud Land. Most of the mafic dykes have been weakly affected by low-grade metamorphism, but clearly preserve their igneous textures. The mafic dykes show a high abundance of Rb, Ba, Sr and light rare earth elements with negative anomalies of Nb, Ta and Ti in a multi-element primitive mantle-normalized diagram. The geochemical characteristics of the mafic dykes suggest that they were derived from a metasomatized mantle source leaving phlogopite, rutile and/or titanite as residual phases. Considering Sr and Nd isotopic systematics of the mafic dykes and the host metamorphic rocks and coeval felsic intrusive rocks, a large crustal boundary potentially related to a suture zone of West and East Gondwana should pass between Mühlig-Hofmannfjella and the Sør Rondane Mountains.

Abstract Mt. Riiser-Larsen is the largest outcrop in the Archaean–early Proterozoic Napier Complex, East Antarctica. The area is structurally divided into the Main and the Western Blocks by the subvertical Riiser-Larsen Main Shear Zone (RLMSZ) of about 200 m width composed of mylonite and pseudotachylite. Mineral parageneses including sapphirine+quartz and osumilite, diagnostic of ultrahigh-temperature (UHT) metamorphism, are found in Mg-rich aluminous, siliceous and quartzo-feldspathic gneiss layers in both the Main and the Western Blocks of the Mt. Riiser-Larsen area. Some of the sapphirine–quartz associations are accompanied by retrograde reaction textures, which include growth of cordierite and/or garnet between sapphirine and quartz in the Main Block, and of orthopyroxene+sillimanite in the Western Block. These textures indicate the reaction 1 and 2 in the Main Block and 3 in the Western Block. Phase equilibria and P – T pseudosections for sapphirine+quartz-bearing associations suggest that these three reactions took place during a temperature drop from 1100 °C to 1000 °C at pressures of 0.6–0.8 GPa in the Main Block and 0.8–0.9 GPa in the Western Block. The geological structure and distribution of the UHT rocks provide an insight into the vertical extent of the>1000 °C UHT metamorphic zone: a minimum thickness of 4–5 km of the UHT-metamorphosed layers, which become deeper towards the west in the Main Block. The Western Block represents a c. 0.1–0.3 GPa ( c. 3–10 km) deeper structural level than the Main Block. In addition to the extent of the horizontal distribution of UHT metamorphism in the Napier Complex, our results on the vertical component provide new constraints for modelling the heat source and tectonic process of the unusually high-temperature regional metamorphism in the late Archaean–early Proterozoic. Electron microprobe monazite U–Th–Pb dating for hydrated and mylonitized sapphirine–quartz gneiss gave a wide spectrum of monazite age distribution between 2300 and 800 Ma, suggesting the tectonic uplift and juxtaposition of the two blocks in the Mt. Riiser-Larsen area later than the mid–late Proterozoic.