Lithogeochemical and isotope (Rb-Sr, Sm-Nd, Pb-Pb whole rock and Lu-Hf zircon) composition of samples from the Shackleton Range, East Antarctica

Three distinct, spatially separated crustal terranes have been recognised in the Shackleton Range, East Antarctica: the Southern, Eastern and Northern Terranes. Mafic gneisses from the Southern Terrane provide geochemical evidence for a within-plate, probably back-arc origin of their protoliths. A plume-distal ridge origin in an incipient ocean basin is the favoured interpretation for the emplacement site of these rocks at c. 1850 Ma, which, together with a few ocean island basalts, were subsequently incorporated into an accretionary continental arc/supra-subduction zone tectonic setting. Magmatic underplating resulted in partial melting of the lower crust, which caused high-temperature granulite-facies metamorphism in the Southern Terrane at c. 1710-1680 Ma. Mafic and felsic gneisses there are characterised by isotopically depleted, positive Nd and Hf initials and model ages between 2100 and 2000 Ma. They may be explained as juvenile additions to the crust towards the end of the Palaeoproterozoic. These juvenile rocks occur in a narrow, c. 150 km long E-W trending belt, inferred to trace a suture that is associated with a large Palaeoproterozoic accretionary orogenic system. The Southern Terrane contains many features that are similar to the Australo-Antarctic Mawson Continent and may be its furthermost extension into East Antarctica. The Eastern Terrane is characterised by metagranitoids that formed in a continental volcanic arc setting during a late Mesoproterozoic orogeny at c. 1060 Ma. Subsequently, the rocks experienced high-temperature metamorphism during Pan-African collisional tectonics at 600 Ma. Isotopically depleted zircon grains yielded Hf model ages of 1600-1400 Ma, which are identical to Nd model ages obtained from juvenile metagranitoids. Most likely, these rocks trace the suture related to the amalgamation of the Indo-Antarctic and West Gondwana continental blocks at ~600 Ma. The Eastern Terrane is interpreted as the southernmost extension of the Pan-African Mozambique/Maud Belt in East Antarctica and, based on Hf isotope data, may also represent a link to the Ellsworth-Whitmore Mountains block in West Antarctica and the Namaqua-Natal Province of southern Africa. Geochemical evidence indicates that the majority of the protoliths of the mafic gneisses in the Northern Terrane formed as oceanic island basalts in a within-plate setting. Subsequently the rocks were incorporated into a subduction zone environment and, finally, accreted to a continental margin during Pan-African collisional tectonics. Felsic gneisses there provide evidence for a within-plate and volcanic arc/collisional origin. Emplacement of granitoids occurred at c. 530 Ma and high-temperature, high-pressure metamorphism took place at 510-500 Ma. Enriched Hf and Nd initials and Palaeoproterozoic model ages for most samples indicate that no juvenile material was added to the crust of the Northern Terrane during the Pan-African Orogeny but recycling of older crust or mixing of crustal components of different age must have occurred. Isotopically depleted mafic gneisses, which are spatially associated with eclogite-facies pyroxenites, yielded late Mesoproterozoic Nd model ages. These rocks occur in a narrow, at least 100 km long, E-W trending belt that separates alkaline ocean island metabasalts and within-plate metagranitoids from volcanic arc metabasalts and volcanic arc/syn-collisional metagranitoids in the Northern Terrane. This belt is interpreted to trace the late Neoproterozoic/early Cambrian Pan-African collisional suture between the Australo-Antarctic and the combined Indo-Antarctic/West Gondwana continental blocks that formed during the final amalgamation of Gondwana

Palaeoproterozoic to Palaeozoic magmatic and metamorphic events in the Shackleton Range, East Antarctica: Constraints from zircon and monazite dating, and implications for the amalgamation of Gondwana

A comprehensive set of new geochronological data from different parts of the Shackleton Range in East Antarctica, comprising U–Pb single zircon and Th–U–Pb single and multi-grain monazite data, combined with published results, reveal a complex tectono-thermal history of the Shackleton Range. Three distinct, spatially separated terranes or units with different magmatic and metamorphic history are now recognised: (i) the Southern Terrane (Unit I) contains detrital components as old as 2850 Ma, experienced magmatism between 1850Ma and 1810Ma and underwent amedium- to high-grade metamorphic event at 1710–1680Ma and, locally, again at 510 Ma; (ii) the Eastern Terrane (Unit II) occurs in the easternmost part of the Shackleton Range and contains c. 1060Ma old Grenvillian granitoids, which experienced metamorphism at c. 600 Ma; and (iii) the Northern Terrane (Unit III) is characterised by 530Ma old granites and diorites, which are hosted within paragneisses as well as mafic and ultramafic rocks. All rocks of Unit III experienced upper amphibolite- to granulite-facies and, locally, eclogite-facies metamorphism at 510–500 Ma. The geologic features of Palaeoproterozoic tectonism in the Southern Terrane are very similar to those of the Australo-AntarcticMawson Continent. Thismay indicate that theMawson Continent extends across the East Antarctic Shield into the Shackleton Range. The 1060Ma and 600Ma events in the Eastern Terrane have not been documented for any part of the Shackleton Range before and are correlated with Grenvillian and Pan-African tectonism in Dronning Maud Land. By implication, this suggests that the Pan- African Mozambique/Maud Belt continues into the Shackleton Range. The associated suture is located in the easternmost Shackleton Range and is related to the amalgamation of the Indo-Antarctic plate with West Gondwana. This was followed by further collision of the combined Indo-Antarctic/West Gondwanan block with East Gondwana at approximately 510Ma in theNorthern Terrane. A suture related to this latter collision can be traced in the Northern Shackleton Range and may continue northwards to the Sør Rondane Mountains and the Lützow Holm Bay area. Our data support the model that East Antarctica finally assembled during the Pan-African orogeny, rather than during earlier Mesoproterozoic events