East African Antarctic Orogen Research Articles

Multiple timings of garnet-forming high-grade metamorphism in the Neoproterozoic continental collision zone revealed by petrochronology in the Sør Rondane Mountains, East Antarctica

The Sør Rondane Mountains (SRM), East Antarctica is located in a key area of Gondwana formation where the East African-Antarctic Orogen and the Kuunga Orogen cross. Timing of peak metamorphism has been considered at ca. 650–600 Ma in the SRM, on which previous tectonic models were built. In this study, timing of garnet-forming metamorphisms is determined using in situ U-Pb dating of zircon and petrochronological approach utilizing distribution coefficient of rare earth elements (DREE) between zircon and garnet and REE patterns of zircon. As a result, we obtained several different periods of garnet-forming metamorphism at ≥600 Ma and ≤580 Ma both from the NE- and SW-terranes. This is the first report of multiple timings of garnet formation detected within single samples from the SRM. The latest garnet formation in each sample is probably caused by regional metamorphism, because the garnets accompany pressure shadows and are wrapped by the penetrative foliation defined mainly by the arrangement of biotite. Therefore, multiple timings of garnet formation during a single metamorphism or polymetamorphism are both likely. In the former case, the SRM may be affected by a single long-lived regional metamorphism. This implies that the regional metamorphism belongs only to the East African-Antarctic Orogeny or that the East African Orogeny continuously transformed into the Kuunga Orogeny in the deep crust. In the latter case, the former regional metamorphism corresponds to the East African(-Antarctic) Orogeny, whereas the latest metamorphism may correspond to the final stage of East African-Antarctic Orogeny or the Kuunga Orogeny. Any temperature drops during these two garnet-forming timings support the polymetamorphism, which needs to be cleared in the future works.

Protracted late Neoproterozic – early Palaeozoic deformation and cooling history of Sør Rondane, East Antarctica, from 40Ar/39Ar and U–Pb geochronology

Abstract40Ar/39Ar and U–Pb data from five structural domains constrain the late Neoproterozoic – early Palaeozoic tectonothermal history of the eastern part of the East African–Antarctic Orogen in Sør Rondane. A total of 27 new Ar/Ar ages span 570–474 Ma, roughly corresponding to the age range of three generations of syn- to post-tectonic granitoids. The ages are distinct for the five structural domains. The oldest cooling ages come from the weakly deformed southern part of the SW Terrane of Sør Rondane (SW Terrane S), a sliver of a Tonian island arc, which escaped much of the late Neoproterozoic accretionary deformation. This terrane was intruded by the oldest and largest granitoid complex at c. 640–620 Ma. The oldest Ar/Ar amphibole and biotite ages of 570–524 Ma are from the Main Shear Zone, along the northern margin of the SW Terrane S sliver. It hosts granites of age c. 584–570 Ma strung out along the shear zone. Two younger granitoid phases are recorded in the adjacent four terranes to the west, north and east of the SW Terrane S, and correlate with the younger group of Ar/Ar biotite ages spanning 513–474 Ma. We interpret the magmatic and cooling history of duration > 150 Ma to reflect repeated phases of accretion, magmatism and reactivation, that is, collage-style tectonism, partly pre-dating the incorporation of Sør Rondane into Gondwana. The study area first accreted to the cryptic Valkyrie Craton in Tonian times, was then ‘sandwiched’ between the Kalahari and Indo-Antarctica cratons, and experienced extensional tectonics and elevated heat flux due to lithospheric delamination, which resulted in slow cooling during the Pan-African Orogeny.

Neoproterozoic geodynamic evolution of easternmost Kalahari: Constraints from U-Pb-Hf-O zircon, Sm-Nd isotope and geochemical data from the Schirmacher Oasis, East Antarctica

Late Tonian (ca. 785–760 Ma) granodioritic to granitic orthogneisses of the Schirmacher Oasis region in Dronning Maud Land (DML), East Antarctica, are interpreted as recording an active continental margin setting at the periphery of Kalahari and Rodinia. The rocks probably represent exposures of a significant tectonic province hidden beneath the ice, the erosional remnants of which are recorded as detrital zircons in late Tonian-Cryogenian metasedimentary rocks throughout central and eastern DML, as well as in ice-rafted debris from recent sediments offshore Dronning Maud Land. The orthogneisses have single-stage Sm-Nd model ages of ca. 1.3–1.5 Ga and zircon Hf-signatures (εHft = +2 – +5), indistinguishable from the adjacent Grenville-age basement rocks of easternmost Kalahari. Their geochemistry suggests that they evolved in the late stages of a continental margin magmatic arc and possibly within a roll-back tectonic framework, suggestive of subduction of relatively old oceanic lithosphere. The eastern Kalahari continental arc is one of a number of continental arcs that characterize the western part of the fragmenting Rodinia and document the supercontinent “turning inside out” after its formation at ca. 1000 Ma and a period of relative tectonic quiescence between ca. 900 and 800 Ma.The rocks show an ultra-high temperature metamorphic overprint that was accompanied by syn-tectonic magmatism from ca. 650 to 600 Ma. The high temperature metamorphism is interpreted to relate to back-arc extension that also led to major anorthosite magmatism elsewhere, prior to continental collision in the region. The rocks lack the subsequent widespread high-grade metamorphic overprint at ca. 590–500 Ma which occurs in the adjacent regions due to Himalayan-style continental collision along the East African-Antarctic Orogen during Gondwana assembly.

The initial Gondwana break-up: A synthesis based on new potential field data of the Africa-Antarctica Corridor

The initial opening of the Africa-Antarctica Corridor, in the heart of Gondwana, is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the Jurassic. Therefore, in 2014, new ship-borne magnetic data were systematically acquired in the northern Mozambique Basin and across Beira High, which we merged with all accessible magnetic data in the Mozambique Basin. Herein, distinct magnetic lineations are observed, which allow a refined identification of a whole set of Jurassic magnetic spreading anomalies, constraining the timing of the onset of oceanization, beginning at M38n.2n (164.1 Ma). In combination with high-resolution potential field data from the conjugate Antarctic margin, well-expressed fracture zones can be traced throughout the Africa-Antarctica Corridor and allow the precise rotation of Antarctica back to Africa. The initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account. Within a tight Gondwana fit, the Beira High can be restored along the major sinistral Namama-Orvin Shear Zone of the East African-Antarctic Orogen. The Beira High represents a continental block, which was detached from Antarctica, by 157 Ma at the latest. Simultaneously, the Antarctic plate cleared the area of the MCP. However, the crustal nature of the southern MCP remains ambiguous. The Northern Natal Valley and the Mozambique Ridge consist of thick oceanic crust, being emplaced between M26r-M18n (157.1–144 Ma) and M18n-M6n (144–131.7 Ma), respectively. About the half of this crust was won from the Antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the Explora Wedge. A refined kinematic break-up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the Africa-Antarctica Corridor and the Somali Basin.

A low-angle brittle shear zone in the western Sør Rondane Mountains, Dronning Maud Land, East Antarctica — Implication for assembly of Gondwanaland

The Sør Rondane Mountains (SRM), East Antarctica, lie within the late Neoproterozoic–early Paleozoic collision zone related to the formation of the Gondwana supercontinent. Many studies have been carried out in the eastern SRM, whereas fundamental questions on the western SRM remain unanswered, e.g. detail metamorphic history, age, kinematics of sheared rocks and others. This paper describes lithology and structure of the western SRM, and the tectonic implications of a low-angle brittle shear zone within this area, the Kanino-tume Shear Zone (KSZ), is discussed.Rocks of the study area are divided into units 1–3 based on their lithology and structural position. Units 1 and 2 are composed mainly of Neoproterozoic gneiss, and unit 3 consists mainly of ca. 1000–800 Ma metatonalite. Units 2 and 3, both separated by a mylonite/ultramylonite zone formed by dextral shearing (Main Shear Zone: MSZ), tectonically overlie unit 1 with the KSZ. The KSZ, showing top-to-the south sense of shear, cuts MSZ (∼ca. 530 Ma) and is intruded by mafic dikes (ca. 560–440 Ma). Therefore, units 2 and 3, which had been juxtaposed by the dextral movement along the MSZ, rode together onto unit 1 along the KSZ by top-to-the southward movement at late Neoproterozoic–early Paleozoic time. The KSZ gives critical evidence for late Neoproterozoic–early Paleozoic movement in the SRM within the East African–Antarctic orogen.

Paleoproterozoic (ca. 1.8 Ga) arc magmatism in the Lützow-Holm Complex, East Antarctica: Implications for crustal growth and terrane assembly in erstwhile Gondwana fragments

The Lützow-Holm Complex (LHC) of East Antarctica forms a part of the latest Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb geochronological data on meta-igneous rocks from four localities (Austhovde, Telen, Skallevikshalsen, and Skallen) in the LHC, and evaluate the regional Paleoproterozoic (ca. 1.8Ga) arc magmatism in this terrane for the first time. The geochemical features reveal a volcanic-arc affinity for most of the meta-igneous rocks from Austhovde and Telen, suggesting that the protoliths of these rocks were derived from felsic to mafic arc magmatic rocks. The protoliths of two mafic granulites from Austhovde are inferred as non-volcanic-arc basalt such as E-MORB, suggesting the accretion of remnant oceanic lithosphere together with the volcanic-arc components during the subduction–collision events. The weighted mean 206Pb/238U ages of the dominant population of magmatic zircons in felsic orthogneisses from Austhovde and Telen show 1819±19Ma and 1830±10Ma, respectively, corresponding to Paleoproterozoic magmatic event. The magmatic zircons in orthogneisses from other two localities yield upper intercept ages of 1837±54Ma (Skallevikshalsen), and 1856±37Ma and 1854±45Ma (Skallen), which also support Paleoproterozoic magmatism. The earlier thermal events during Neoarchean to Early Paleoproterozoic are also traced by 206Pb/238U ages of xenocrystic zircons in the felsic orthogneisses from Austhovde (2517±17Ma and 2495±15Ma) and Telen (2126±16Ma), suggesting partial reworking of the basement of a 2.5Ga microcontinent during ca. 1.8Ga continental-arc magmatism. The timing of peak metamorphism is inferred to be in the range of 645.6±10.4 to 521.4±12.0Ma based on 206Pb/238U weighted mean ages of metamorphic zircon grains. The results of this study, together with the available magmatic ages as well as geophysical and lithological data from the region, suggest that the LHC can be divided into three units: Neoarchean (ca. 2.5Ga) unit in the southern LHC (Shirase Orthogneiss or “Shirase microcontinent”), Neoproterozoic (ca. 1.0Ga) unit in the northern LHC, and supracrustal unit in the central LHC with fragments of Paleoproterozoic (ca. 1.8Ga) and minor Neoarchean (ca. 2.5Ga) and Neoproterozoic (ca. 1.0Ga) magmatic arcs. The 1.8Ga arc magmatism inferred in this study has also been reported from adjacent Gondwana fragments such as the Highland Complex in Sri Lanka, and the Trivandrum and Nagercoil Blocks in southern India. Although the ca. 1.8Ga arc-magmatic event is coeval in these regions, the Paleoproterozoic supracrustal unit in the central LHC may not be contiguous with those in the Highland Complex of Sri Lanka because recent studies have shown that the Vijayan Complex in Sri Lanka and the ca. 1.0Ga northern LHC possibly were part of a single crustal unit (northern Lützow-Holm–Vijayan Complex) within the Kalahari Block. The supracrustal unit possibly marks part of a discrete suture formed by the collision of the ca. 2.5Ga southern LHC (Shirase microcontinent) and the ca. 1.0Ga northern Lützow-Holm–Vijayan Complex during the latest Neoproterozoic–Cambrian Gondwana amalgamation, which might be coeval with the collision of the Vijayan and Wanni Complexes and the formation of the Highland Complex in Sri Lanka. Our study provides new insights on crustal growth and terrane assembly in the ancient continental blocks of Gondwana.

LA-ICP-MS U–Pb dating of rutile inclusions within corundum (ruby and sapphire): new constraints on the formation of corundum deposits along the Mozambique belt

Direct dating of corundum and the establishment of age constraints for corundum crystal growth are restricted due to the absence of a geochronometer. Syngenetic mineral inclusions may help define minimum ages for corundum growth. Here, we present results of in situ LA-ICP-MS U–Pb dating applied to rutile inclusions in corundum from localities along the Mozambique belt. U–Pb dating of rutile inclusions gave ages of 533 ± 11 Ma (2 σ) for the Gitonga pit (John Saul ruby mine) and 526 ± 13 Ma for the Aqua Ruby mine, both in the Mangari area of Kenya, and an age of 499 ± 12 Ma for ruby from the Morogoro area, Tanzania. These ages are interpreted as cooling ages that set a minimum age for corundum growth following metamorphism along the East-African Antarctic Orogen, and might be used for petrogenetic tracing of corundum in placer deposits.

Cryptic sub-ice geology revealed by a U-Pb zircon study of glacial till in Dronning Maud Land, East Antarctica

We have targeted the southern side of the Dronning Maud Land (DML) mountains in search of moraine material that might reveal the geology in the ice-covered region of the East Antarctic polar plateau. Nine samples of unconsolidated moraine material, carried by the northward flowing East Antarctic ice sheet to the southern side of the DML escarpment, were collected and processed for U-Pb zircon analyses. The analyses resulted in ca. 1100 new U-Pb zircon spot ages between ca. 2000 and 500Ma. The oldest, Palaeoproterozoic rocks come from the easternmost localities with a probable source region between the Sør Rondane Mountains and Dome Fuji. The samples have major Stenian and early Tonian age peaks. Early Tonian rocks are well known from the SW Terrane in the Sør Rondane Mountains and characterise the major Tonian Oceanic Arc Super Terrane (TOAST). Stenian ages are far less common in the outcropping region. Although Stenian ages are common in both the Maud Province of western-central DML as well as in the Rayner Complex, our samples differ with respect to composition and/or isotope geochemistry; they are juvenile, subduction-related and resemble an early phase of oceanic arcs that was so far unknown in this region. In the westernmost sample, the oldest age peak is ca. 800–720Ma with possible counterparts in the Schirmacher Oasis. All samples show a protracted Late Neoproterozoic/Early Palaeozoic metamorphic overprint, accompanied by igneous addition, most likely related to the East African-Antarctic Orogen. This overprint appears most intense in clasts collected from the westernmost locality, lasted for ca. 150Ma and an eastward younging of metamorphic ages is observed. The new moraine samples together with previous outcrop studies reveal that this region has undergone two major phases of oceanic arc/terrane accretion; the first one from ca. 1100 to 900Ma is probably related to accretion tectonics outboard of Rodinia, the second one from ca. 850 to 580Ma occurred as a result of ocean closure and finally Gondwana amalgamation. No significant old components are found, but the high proportion of Late Neoproterozoic/Early Palaeozoic ages confirms that the East African-Antarctic Orogen transects DML and East Antarctica.

Neoarchean–Early Paleoproterozoic and Early Neoproterozoic arc magmatism in the Lützow–Holm Complex, East Antarctica: Insights from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

The Lützow–Holm Complex (LHC) of East Antarctica forms part of the Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data for meta-igneous rocks including charnockite, felsic gneiss, metagabbro, and mafic granulite from the LHC and evaluate the Neoarchean to Early Paleoproterozoic (ca. 2.5Ga) and Early Neoproterozoic (ca. 1.0Ga) arc magmatic events. The trace element geochemical signatures reveal a volcanic arc affinity for the charnockites from Sudare Rocks and Vesleknausen and felsic gneiss from Rundvågshetta, suggesting that the protoliths of these rocks were derived from felsic arc magmas. In contrast, metagabbros from Skallevikshalsen and Austhovde, occurring as boudins in metasediments, show non-arc signatures (within-plate basalt or mid-oceanic ridge basalt). The upper intercept ages of magmatic zircons in charnockite plotted on concordia diagrams yielded 2508±14Ma (Sudare Rocks) and 2490±18Ma (Vesleknausen), clearly suggesting a Neoarchean to Early Paleoproterozoic arc magmatic event. A subsequent thermal event during Early Neoproterozoic traced by 206Pb/238U age of oscillatory-zoned core of zircon in mafic granulite from Langhovde (973±10Ma) is consistent with a similar Early Neoproterozoic magmatic event reported from the LHC, suggesting a second stage of arc magmatism. The timing of peak metamorphism has been inferred from 206Pb/238U mean ages of structureless zircons in metagabbros from Skallevikshalsen and Austhovde, mafic granulite from Langhovde, and felsic gneiss from Rundvågshetta in the range of 551±5.4 to 584±5.0Ma. Zircon Lu–Hf data of Neoarchean charnockites from Sudare Rocks and Vesleknausen indicate that the protolith magma was sourced from Paleo- to Neoarchean juvenile components mixed with reworked ancient crustal materials. Protolith magmatic rock of the felsic gneiss from Rundvågshetta might have been sourced from reworked Paleoarchean crust. In contrast, metagabbros from Skallevikshalsen and Austhovde were derived from the Paleoproterozoic reworked crust. The results of this study suggest the occurrence of several discrete crustal fragments of ca. 2.5Ga and ca. 1.0Ga juvenile magmatic arcs in the southwestern part of the LHC, which amalgamated during the latest Neoproterozoic collisional event related to the final stage of assembly of the Gondwana supercontinent.

Zircons traced from the 700–500 Ma Transgondwanan Supermountains and the Gamburtsev Subglacial Mountains to the Ordovician Lachlan Orogen, Cretaceous Ceduna Delta, and modern Channel Country, central-southern Australia

We test the hypothesis that the Transgondwanan Supermountains at the collision of East and West Gondwanaland were the provenance of a vast turbiditic fan that stretched alongside the East Gondwanaland margin to eastern Australia which, in turn, became the provenance of sediment shed into interior Australia to the Cretaceous Ceduna Delta in central-southern Australia and the modern Channel Country of central Australia. We employ an integrated analysis (U–Pb, Lu–Hf isotopes and trace elements) of detrital zircons in the Ceduna Delta and Channel Country. The main properties of the detrital zircons are U–Pb ages of 700–500 Ma (model ages TDMC 2.5–1.0 Ga; εHf +10 to −20) and 1300–1000Ma ages (TDMC 2.7–1.3Ga; εHf +4 to −17), in hosts of mafic granitoids with alkaline affinity. Zircons with these properties can be traced back through the drainage/paleo-slope to the intermediate provenances of the Ordovician turbidites and S-type granitoids of the Lachlan Orogen, then up-paleoslope to the primary or secondary provenance of the ancestral Gamburtsev Subglacial Mountains, and finally to the primary provenance of the Transgondwanan Supermountains atop the 700–500Ma East African–Antarctic Orogen. Another primary provenance, the 140–95Ma Whitsunday Volcanic Province/New Caledonia arc in northeastern Australia, also shed sediment across Australia to the Ceduna Delta.We suggest that the primary sediment from the 700–500Ma East African–Antarctic Orogen and the ancestral Gamburtsev Subglacial Mountains was shed into a deep-sea super-fan to (1) Ordovician turbidites in southeast Australia, recycled by melting of the turbidites to (2) 450MaS-type granites in the Lachlan Orogen, and (3) finally deposited, together with volcanogenic sediment from northeast Australia, in the Ceduna Delta. Zircons in the Channel Country and the Ceduna Delta have essentially the same properties, and indicate that the northeastern Australian provenance was largely unchanged over the past 100Ma.

The Main Shear Zone in Sør Rondane, East Antarctica: Implications for the late-Pan-African tectonic evolution of Dronning Maud Land

Structural investigations in western Sor Rondane, eastern Dronning Maud Land (DML), provide new insights into the tectonic evolution of East Antarctica. One of the main structural features is the approximately 120 km long and several hundred meters wide WSW-ENE trending Main Shear Zone (MSZ). It is characterized by dextral high-strain ductile deformation under peak amphibolite-facies conditions. Crosscutting relationships with dated magmatic rocks bracket the activity of the MSZ between late Ediacaran to Cambrian times (circa 560 to 530 Ma). The MSZ separates Pan-African greenschist- to granulite-facies metamorphic rocks with “East African” affinities in the north from a Rayner-age early Neoproterozoic gabbro-tonalite-trondhjemite-granodiorite complex with “Indo-Antarctic” affinities in the south. It is interpreted to represent an important lithotectonic strike-slip boundary at a position close to the eastern margin of the East African-Antarctic Orogen (EAAO), which is assumed to be located farther south in the ice-covered region. Together with the possibly coeval left-lateral South Orvin Shear Zone in central DML, the MSZ may be related to NE directed lateral escape of the EAAO, whereas the Heimefront Shear Zone and South Kirwanveggen Shear Zone of western DML are part of the south directed branch of this bilateral system.

New detailed aeromagnetic and geological data of eastern Dronning Maud Land: Implications for refining the tectonic and structural framework of Sør Rondane, East Antarctica

The Sør Rondane Mountains (SRM) in eastern Dronning Maud Land (DML) are located in an area, where two apparent Pan-African (650–520Ma) orogenic mobile belts appear to intersect, the East African-Antarctic Orogen and the Kuunga Orogen. Hence, a better understanding of the tectonic structure of the Sør Rondane region is an important key for unravelling the complex geodynamic evolution of the eastern DML and adjacent regions of East Antarctica during the Late Neoproterozoic/Early Palaeozoic amalgamation of Gondwana. The SRM were recently (2011–2012) aerogeophysically investigated with a 5km flight line spacing, covering a total area of ∼140,000km2. The aeromagnetic data are correlated with ground-based magnetic susceptibility measurements and geological field data and allow to project tectonic terranes and individual structures into ice-covered areas. Magnetic anomalies and basement foliation trends are collinear in areas dominated by simple shear deformation, whereas an area of large-scale refolding correlates with a subdued small-scale broken magnetic anomaly pattern. The latter area can be regarded as a distinct tectonic domain, the central Sør Rondane corridor. It magnetically separates the SRM into an eastern, a central, and a western portion. This subdivision is presumably related to late Pan-African extensional tectonics and suggests that such a tectonic regime may play a larger role than previously assumed. Voluminous late Pan-African granitoids, which are mainly undeformed, correlate with positive magnetic anomalies between +30 and +80nT, while a strong magnetic high (+680nT) near the granitic intrusion at Dufekfjellet is caused by a highly magnetised enigmatic body. The recently discovered prominent magnetic anomaly province of southeastern DML continues into the southern part of the Sør Rondane region, where only a few outcrops are exposed. Findings at these westernmost nunataks of the SRM indicate that the subdued magnetic anomaly pattern of this southeastern DML province is most likely caused by the predominance of metasedimentary rocks of yet unknown age.

Plate tectonic gemstones

The gemstones jadeite and ruby generally form as a result of the plate tectonic processes subduction and collision. Jade made of jadeite (jadeitite) forms when supercritical fluids released from subducting oceanic crust condense in the overlying mantle wedge, 20–120 km deep in the Earth. Jadeitite deposits thus mark the location of exhumed fossil subduction zones. Ruby, the red gem variety of corundum, forms during amphibolite- and granulite-facies metamorphism or melting of mixed Al-rich and Si-poor protoliths, 10–40 km deep in the crust. Suitable conditions generally exist where passive-margin carbonates and shales are involved in continental collision. Most ruby deposits formed during Ediacaran-Cambrian (ca. 550 Ma) collisions that produced the East African–Antarctic orogen and the supercontinent Gondwana, or during Cenozoic collisions in south Asia. Ruby is thus a robust indicator of continental collision. As a result of these diagnostic properties, we propose the term “plate tectonic gemstones” (PTGs) for jadeitite and ruby. The PTGs are a new type of petrotectonic indicator that are mostly found in Neoproterozoic and younger rocks. The PTGs as petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion, although the possibility of destruction via retrogression needs to be further assessed. Recognition of the PTGs links modern concepts of plate tectonics to economic gemstone deposits and ancient concepts of beauty, and may aid in exploration for new deposits.

Late Neoproterozoic extensional detachment in eastern Sør Rondane Mountains, East Antarctica: Implications for the collapse of the East African Antarctic Orogen

A geological study of the Balchenfjella area of the eastern Sør Rondane Mountains forms the basis of a revised tectonic history for eastern Dronning Maud Land in Antarctica. The Balchenfjella area can be structurally divided into two metamorphic units that are separated by a SE-dipping ductile extensional shear zone (the Balchen detachment fault). The mafic- and ultramafic-dominant part of the footwall side is exposed as a tectonic window adjacent to the detachment. Metamorphic and structural data indicate that the eastern Sør Rondane Mountains were affected by four phases of deformation (D1–D4). Phases D1 and D2 are developed in the footwall side of the shear zone, whereas D3 and D4 are recorded in both the footwall and hangingwall sides. Mineral assemblages in mafic lithologies are characteristic of granulite facies (D1 and D2) to amphibolite facies (D2 and D3) metamorphism. D1 is characterized by recumbent folds and this phase is interpreted as a compressional tectonic setting coincident with granulite facies metamorphism. D2 represents a transpressional deformation phase characterized by upright folds and a large-scale dextral shear zone. The Balchen detachment fault was formed during the extensional tectonic setting of D3. The timing of D3 extensional deformation is bracketed by the age of D2-related metamorphism (at c. 600Ma) and the timing of post-kinematic intrusion of granites (at c. 550Ma). The present results demonstrate that the eastern Sør Rondane Mountains underwent Late Neoproterozoic extensional collapse similar to that observed in other parts of the East African–Antarctic Orogen including the Arabian–Nubian Shield and Madagascar.

Interpretation of new regional aeromagnetic data over Dronning Maud Land (East Antarctica)

The geology of Dronning Maud Land (East Antarctica) is so far deduced from isolated outcrops along the coast and from a major coast parallel escarpment surmounting the thick ice sheet. Large parts of Dronning Maud Land are, however, hidden underneath thick ice. In this study we attempt to connect geological information with aeromagnetic data in order to unveil the sub-glacial geology of this part of East Antarctica. During four austral summer campaigns (2001–2005) in Dronning Maud Land, new aeromagnetic data were gathered across an area of 1.2million square kilometers of which a portion of 65% was previously unexplored. In total 100,000 line kilometers were flown over Dronning Maud Land between 14°W/20° E and 70°S/78.5°S. A striking result was the discovery of a pronounced magnetic anomaly, named here Forster Magnetic Anomaly, east of the Jutulstraumen. It starts at approximately 72°S/007°E and strikes in southwesterly direction as far south as 75°S/1°W. The Forster Magnetic Anomaly likely forms a major tectonic block boundary and/or a suture zone within the East African–Antarctic Orogen (EAAO). The shape and distribution of other magnetic anomalies are discussed in the context of the Proterozoic to Mesozoic geological history of this part of Antarctica.

Postcollisional High-Grade Metamorphism, Orogenic Collapse, and Differential Cooling of the East African Orogen of Northeast Mozambique

Abstract The postcollisional tectonic development of northeast Mozambique and subsequent cooling from high-temperature metamorphism is delineated with an extensive new set of U-Pb titanite, 40Ar/39Ar hornblende, and 40Ar/39Ar mica analyses. The complex data suggest a polyphase metamorphic history from the late Neoproterozoic to the Ordovician within the East African–Antarctic Orogen (EAAO), with marked differences between the major constituent blocks. In all the data sets, samples from the basement south of the Lurio Belt show generally younger ages than those from the north, resulting from a late metamorphic event and slow cooling between ca. 520 and 440 Ma. The ages north and south of the Lurio Belt are consistently offset by ca. 30–70 Ma, a difference that is maintained and even appears to increase during cooling from very high temperatures to ca. 350°C. Based on the first-order assumption that all the ages are cooling ages, cooling rates in the south are estimated at ca. 7°–8°C/Ma, while those north o...

Age and composition of Antarctic sub-glacial bedrock reflected by detrital zircons, erratics, and recycled microfossils in the Ellsworth Land–Antarctic Peninsula–Weddell Sea–Dronning Maud Land sector (240°E–0°–015°E)

The age and composition of the 14×106km2 of Antarctica's surface obscured by ice is unknown except for some dated detrital minerals and erratics. In remedy, we present four new analyses (U–Pb age, TDMC, εHf, and rock type) of detrital zircons from Neogene turbidites as proxies of Antarctic bedrock, and review published proxies: detrital hornblendes analysed for Ar–Ar age and bulk Sm–Nd isotopes; Pb isotope compositions of detrital K-feldspars; erratics and dropstones that reflect age and composition; and recycled microfossils that reflect age and facies. This work deals with the 240°E–0°–015°E sector, and complements Veevers and Saeed's (2011) analysis of the 70°E–240°E sector. Each sample is located in its ice-drainage basin for backtracking to the potential provenance. Gaps in age between sample and upslope exposure are specifically attributable to the provenance. The major provenance of detritus west of the Antarctic Peninsula (AP) is West Antarctica, and of detritus east of the AP East Antarctica. We confirm that the Central Antarctic provenance about a core of the Gamburtsev Subglacial Mountains (GSM) and the Vostok Subglacial Highlands (VSH) contains a basement that includes igneous (mafic granitoids) and metamorphic rocks with peak U–Pb ages of 0.65–0.50, 1.20–0.9, 2.1–1.9, 2.8–2.6, and 3.35–3.30Ga, TDMC of 3.6–1.3Ga, and mainly negative εHf. The potential provenance of zircons of 650–500Ma age with TDMC ages of 1.55Ga, and of zircons of 1200–900Ma age with positive εHf lies beneath the ice in East Antarctica south and southeast of Dronning Maud Land within the Antarctic part of the East African–Antarctic Orogen. Zircons with the additional ages of 1.7–1.4Ga, 2.1–1.9Ga, and 3.35–3.00Ga have a potential provenance in the GSM.

Delamination-induced late-tectonic deformation and high-grade metamorphism of the Proterozoic Nampula Complex, northern Mozambique

Lithospheric thickening during protracted Ediacaran supercontinent assembly was dissipated in various ways along the “Pan-African” East African–Antarctic Orogen. In NE Mozambique, late-tectonic extension and plutonism south of the Lúrio Belt has been modelled in terms of lithosphere delamination, although rigorous testing of the hypothesis by structural analysis has not yet been undertaken.This study presents the first analyses of late-tectonic structures in both the Mesoproterozoic basement and the Cambrian cover sequences in the Nampula Complex, NE Mozambique, supported by, and integrated with, new geochronological data. Both late compressional and extensional fabrics overprint the main Pan-African collisional structures to a variable degree in identified structural domains. The long-lived northern boundary of the Nampula Complex, the Lúrio Belt high-strain zone, initiated in the Ediacaran, was reactivated and segmented during these later phases, with boundary-parallel shortening.U–Pb SIMS ages from selected latest-tectonic units in the Nampula Complex and the Lúrio Belt give consistent ages between 518±2 and 514±5Ma. They are coeval with migmatisation and granitoid plutonism in the Nampula Complex. U–Pb titanite (471±9Ma) and 40Ar/39Ar biotite (431±3Ma) data suggest subsequent slow cooling rates in the Nampula Complex, consistent with basal heating by asthenosphere uprise following lithosphere delamination. Consequently, we believe the new data suggest that the observed intense late polyphase deformation has preferentially occurred where the delaminated lithosphere was doubly weakened by loss of the mantle root and the resulting long-lived elevated temperature regime.

Early Palaeozoic orogenic collapse and voluminous late-tectonic magmatism in Dronning Maud Land and Mozambique: insights into the partially delaminated orogenic root of the East African–Antarctic Orogen?

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.

The Kalahari Craton during the assembly and dispersal of Rodinia

In this paper, we review the dimensions, geometry and architecture of the components of the Kalahari Craton and the various positions this important crustal block could have occupied within Rodinia. The Kalahari Craton was spawned from a small composite Archaean core which grew by prolonged crustal accretion in the Palaeoproterozoic along its NW side (Magondi–Okwa–Kheis Belt, Rehoboth Subprovince) to form the Proto-Kalahari Craton by 1750 Ma. From ca. 1400 to 1000 Ma, all margins of this crustal entity recorded intense tectonic activity: the NW margin was a major active continental margin between ca. 1400 and 1200 Ma and along the southern and eastern margins, the Namaqua–Natal–Maud–Mozambique Belt records a major arc–accretion and continent–collision event between ca. 1100 and 1050 Ma. By ca. 1050 Ma, the Proto-Kalahari nucleus was almost completely rimmed by voluminous Mesoproterozoic crust and became a larger entity, the Kalahari Craton. Apart from southern Africa, fragments of the Kalahari Craton are now exposed in East- and West-Antarctica, the Falkland Islands and possibly also in South America. Immediately prior to the onset of arc– and continent–continent collision along the Namaqua–Natal–Maud Belt (part of the widespread “Grenville-age” orogeny during which Rodinia was assembled), Kalahari was subjected to intraplate magmatism – the Umkondo–Borg Large Igneous Province – at ca. 1110 Ma. The post-Rodinia rift and drift history of the Kalahari Craton is best preserved along the western, south-western and north-western margin, where rift sediments and volcanics indicate rifting and break-up at ca. 800–750 Ma. The position of the Kalahari Craton in Rodinia is problematic, and there is no unique solution for its placement in the supercontinent. One set of models has the Kalahari Craton lying along the SW side of Laurentia with the Namaqua–Natal–Maud Belt facing either inboard (correlation with the Ottawan cycle of the Grenville orogen) or outboard (mainly for palaeomagnetic reasons). In this arrangement the relatively late rift history and the subsequent incorporation of Kalahari into Gondwana is problematic. Alternatively, Kalahari could have been attached to Western Australia. In this model the Namaqua–Natal–Maud Belt has no counterpart and, although the timing of rifting at ca. 750 Ma fits, the location of rifting is problematic—the Kalahari Craton would have had to undergo major rifting along its eastern, rather than its western side, which is not consistent with overservations. So the matter is as yet unresolved, and much of the evidence of rifting along the eastern side of the Kalahari Craton was obliterated due to high-grade overprint along the Late Neoproterozoic/Early Palaeozoic East African–Antarctic Orogen.