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New SHRIMP zircon data from Gjelsvikfjella and Mu¨hlig-Hofmann-Gebirge indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the northeastern continuation of the Namaqua-Natal-Maud Belt. High-grade overprinting occurred between c. 560 and 490 Ma. This period is considered to bracket the collision of E- and W-Gondwana along the East African/Antarctic Orogen (EAAO) and also the subsequent collapse of the orogen. The onset of collision probably predates our oldest dates. We can clearly differentiate between an early compressional stage (Pan-African I) and a later dilational episode (Pan-African II). The Pan-African I event is characterised by the intrusion of leucogranites, and the formation of medium-U metamorphic zircon overgrowth. A sample of a syn-tectonic leucogranite gave a well-constrained crystallisation age of 558.4±5.6 Ma. These rocks are intensely deformed and have formed a second gt-bearing leucosome after their emplacement. Identical ages come from low-U metamorphic zircons and low-U rims from another leucosome at Festninga (557±13 Ma). The Pan-African II event (c. 530 and 490 Ma) is characterised by widespread melting and only little deformation. It is associated with charnockite magmatism, charnockitisation and high-temperature metamorphism. Wide, very high-U zircon rims form during this period. The early Pan-African II overprint between c. 530 and 520 Ma is recorded in almost all samples. An undeformed mafic dyke with a zircon crystallisation age of 523.2±4.8 indicates that there is no or minor deformation after its intrusion. This strongly metasomatised dyke, however, also indicates that although deformation ceased, intense fluid circulation continued. The youngest crystallisation ages come from a post-tectonic granite sheet which yields a crystallisation age of 486.9±3.8 Ma. Final cooling to temperatures below c. 600 °C is recorded by a titanite age of c. 483±11 Ma from the Stabben gabbro. Late to post-tectonic granitoids are voluminous and intrude over a period of c. 30 Ma, between c. 520 and 490 Ma. They are alkaline in composition and can be classified as A2-type. This indicates that they are probably formed by melting of a tonalitic to granodioritic lower crust in the cause of crustal thickening, or from underplated lower crust. [Copyright &y& Elsevier]

U–Pb sensitive high resolution ion microprobe (SHRIMP) zircon analyses have been used to place time constraints on the Proterozoic reworking of Archaean rocks during the Rayner Structural Episode in the Oygarden Group of islands, Kemp Land, East Antarctica. Felsic orthogneiss that preserves evidence for foliations that pre-date the Rayner Structural Episode, was deformed by two events during the Rayner Structural Episode: the first characterised by east-directed sub-horizontal thrusting, and the second by a regional scale south-dipping extensional shear zone. Metamorphic overgrowths on zircon grains from felsic orthogneiss, which have 929±12 and 924±17 Ma ages, are interpreted to have formed during granulite facies metamorphism that accompanied the thrusting event. Crystallisation of zircon in migmatite that also formed during this event, occurred in two stages: 904±16 and 884±24 Ma. Overgrowths on zircon in a pegmatite that intruded during the second event formed at 931±14 Ma. Charnockite that was emplaced in the Stillwell Hills prior to a regional thrusting event, was metamorphosed at 913±8 Ma. However, the distribution of isotopic data suggest the charnockite may have intruded at ca. 960 Ma. The ages for regional thrusting in the Oygarden Group and the Stillwell Hills correlate, suggesting that the Rayner Structural Episode did not affect Kemp Land until ca. 930 Ma. This age for peak metamorphism in Kemp Land is therefore 50 Myr younger than the age inferred for the Rayner Structural Episode further to the east on the Mawson Coast and south in the northern Prince Charles Mountains. The ages from the Oygarden rocks correlate with the second half of the orogenic episode seen on the Mawson Coast and northern Prince Charles Mountains, consistent with the possibility of two phases of collision in a transition from an Andean-style margin to continent–continent collision between 1000 and 900 Ma. Previously unreported ca. 1600–1650 Ma ages were also discovered in orthogneiss and pegmatite samples, indicating a thermal event in Kemp Land at this time. The 1600–1650 and 930–900 Ma ages from the Oygarden Group correlate with metamorphic and magmatic ages from the Eastern Ghats, placing further constraints on the correlation between these metamorphic belts. [Copyright &y& Elsevier]

New structural and metamorphic data coupled with U–Pb SHRIMP zircon and Rb–Sr step-leach biotite ages help constrain a period of Early Palaeozoic (Pan-African) deformation recognised in the northern Prince Charles Mountains, east Antarctica. This period of deformation is accommodated along discrete northeast trending mylonites that preserve up-dip reverse kinematics with dominantly southeast over northwest vergence. Ambient P–T conditions of 524±20°C and 7.6±4 kbar accompanied deformation. This phase of deformation post-dated the intrusion of planar felsic dykes that yield ages of c. 550 Ma and pre-dated Rb–Sr biotite ages of c. 475 Ma that record cooling of the terrane below c. 300°C. These mylonites are identical in age to continental collisional events recognised in the southern Prince Charles Mountains and Prydz Bay, which lie to the south and east of the northern Prince Charles Mountains, and similar in age to orogenesis recognised to the west in Lu¨tzow-Holm Bay. These belts represent sutures between the component lithospheric blocks of east and west Gondwana. The northern Prince Charles Mountains lie between these sutures. Consequently, the mylonites we report here are interpreted to have formed in an intraplate setting and developed in response to stresses applied along the plated margins as a consequence of continental collision during the amalgamation of Gondwana. [Copyright &y& Elsevier]

Examines the Paleozoic tectonism within the East Antartic craton. Definition of lithospheric block margins; Suturation of the Indo-Antartic continent with west Gondwana; Existence of a laterally continuous orogenic belt.

In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665 ± 50 °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels. To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U–Pb zircon geochronology and 40 Ar/ 39 Ar hornblende and biotite thermochronology. U–Pb zircon crystallization ages for the youngest granites are 102 Ma. Three hornblende ages are 103 to 100 Ma and 12 biotite ages are 101 to 99 Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40 Ar/ 39 Ar cooling ages indicates cooling rates > 100 °C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration. The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings. [ABSTRACT FROM AUTHOR]