First Precambrian palaeomagnetic data from the Mawson Craton (East Antarctica) and tectonic implications

A pilot palaeomagnetic study was conducted on the recently dated with in situ SHRIMP U-Pb method at 1134 ± 9 Ma (U-Pb, zircon and baddeleyite) Bunger Hills dykes of the Mawson Craton (East Antarctica). Of the six dykes sampled, three revealed meaningful results providing the first well-dated Mesoproterozoic palaeopole at 40.5°S, 150.1°E (A95 = 20°) for the Mawson Craton. Discordance between this new pole and two roughly coeval poles from Dronning Maud Land and Coats Land (East Antarctica) demonstrates that these two terranes were not rigidly connected to the Mawson Craton ca. 1134 Ma. Comparison between the new pole and that of the broadly coeval Lakeview dolerite from the North Australian Craton supports the putative ~40° late Neoproterozoic relative rotation between the North Australian Craton and the combined South and West Australian cratons. A mean ca. 1134 Ma pole for the Proto-Australia Craton is calculated by combining our new pole and that of the Lakeview dolerite after restoring the 40° intracontinental rotation. A comparison of this mean pole with the roughly coeval Abitibi dykes pole from Laurentia confirms that the SWEAT reconstruction of Australia and Laurentia was not viable for ca. 1134 Ma

Constraints on the timing and conditions of high-grade metamorphism, charnockite formation and fluid--rock interaction in the Trivandrum Block, southern India

Incipient charnockites have been widely used as evidence for the infiltration of CO2-rich fluids driving dehydration of the lower crust. Rocks exposed at Kakkod quarry in the Trivandrum Block of southern India allow for a thorough investigation of the metamorphic evolution by preserving not only orthopyroxene-bearing charnockite patches in a host garnet–biotite felsic gneiss, but also layers of garnet–sillimanite metapelite gneiss. Thermodynamic phase equilibria modelling of all three bulk compositions indicates consistent peak-metamorphic conditions of 830–925 °C and 6–9 kbar with retrograde evolution involving suprasolidus decompression at high temperature. These models suggest that orthopyroxene was most likely stabilized close to the metamorphic peak as a result of small compositional heterogeneities in the host garnet–biotite gneiss. There is insufficient evidence to determine whether the heterogeneities were inherited from the protolith or introduced during syn-metamorphic fluid flow. U–Pb geochronology of monazite and zircon from all three rock types constrains the peak of metamorphism and orthopyroxene growth to have occurred between the onset of high-grade metamorphism at c. 590 Ma and the onset of melt crystallization at c. 540 Ma. The majority of metamorphic zircon growth occurred during protracted melt crystallization between c. 540 and 510 Ma. Melt crystallization was followed by the influx of aqueous, alkali-rich fluids likely derived from melts crystallizing at depth. This late fluid flow led to retrogression of orthopyroxene, the observed outcrop pattern and to the textural and isotopic modification of monazite grains at c. 525–490 Ma

A cryptic Gondwana-forming orogen located in Antarctica

The most poorly exposed and least understood Gondwana-forming orogen lies largely hidden beneath ice in East Antarctica. Called the Kuunga orogen, its interpolation between scattered outcrops is speculative with differing and often contradictory trends proposed, and no consensus on the location of any sutures. While some discount a suture altogether, paleomagnetic data from Indo-Antarctica and Australo-Antarctica do require 3000-5000 km relative displacement during Ediacaran-Cambrian Gondwana amalgamation, suggesting that the Kuunga orogen sutured provinces of broadly Indian versus Australian affinity. Here we use compiled data from detrital zircons offshore of East Antarctica that fingerprint two coastal subglacial basement provinces between 60 and 130°E, one of Indian affinity with dominant ca. 980-900 Ma ages (Indo-Antarctica) and one of Australian affinity with dominant ca. 1190-1140 and ca. 1560 Ma ages (Australo-Antarctica). We combine this offshore compilation with existing and new onshore U-Pb geochronology and previous geophysical interpretations to delimit the Indo-Australo-Antarctic boundary at a prominent geophysical lineament which intersects the coast east of Mirny at ~94°E

East Antarctic rifting triggers uplift of the Gamburtsev Mountains

The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 19581. The preservation of Alpine topography in the Gamburtsevs2 may reflect extremely low long-term erosion rates beneath the ice sheet3, but the mountains’ origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root4 beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens5, 6 and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa7, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere

Retention of Sm-Nd isotopic ages in garnets subjected to high-grade thermal reworking: implications for diffusion rates of major and rare earth elements and the Sm-Nd closure temperature in garnet

Accepted: 21 June 2009. Published online: 10 July 2009Garnet is a vital mineral for determining constrained P–T–t paths as it can give both the P–T and t information directly. However, estimates of the closure temperature of the Sm–Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. In this study, five igneous garnets from an early Proterozoic 2414 ± 6 Ma garnet—cordierite bearing s-type granite—which was subjected to high-T reworking have been dated to examine their diffusional behaviour in the Sm–Nd system. Garnets 8, 7, 6 and 2.5 mm in diameter were compositionally profiled and then dated, producing two-point Sm–Nd isochron ages of 2412 ± 10, 2377 ± 5, 2370 ± 5 and 2365 ± 8 and 2313 ± 11 Ma, respectively. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element EMPA and LA-ICPMS REE traverses reveal homogenous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggest that diffusion rates of REEs are considerably slower than that of the major cations. The retention of REE zoning and the lack of resetting in the largest grains suggest that Sm–Nd closure temperature in garnet is a function of grain size, thermal history and REE zoning in garnetRian Dutch and Martin Han