Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen,Antarctica

The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.     

Preliminary diagnoses of three new amphipod species from wellington harbour (note)

Preliminary diagnoses are given of Urothoe wellingtonensis n.sp. (Family Haustoriidae), Paraphoxus spinibasus n.sp., and P. chelatus n.sp. (Family Phoxocephalidae).     

Nitrate loss and transformation in 2 vegetated headwater streams

The processes of nitrate removal from the stream waters of 2 headwater catchments were studied. In study stretches where the stream channels were vegetated with thick mats of grass (Gly‐ceria fluitans), nitrate removal processes were particularly active. In such grassed stream channels, c. 75% of the nitrate removal was attributable to plant uptake and the remainder to denitrifica‐tion. Both of these nitrate removal processes were linearly dependent on nitrate concentration, resulting in an exponential decline of nitrate level from the springs along the stream's length. Regeneration of nitrogen, by plant decay within the stream channel, results in export of dissolved organic nitrogen and paniculate nitrogen from the catchments. The overall impact of vegetated stream channels in modifying nitrogen exports from catchments is discussed.     

Potassium‐argon ages on micas from the Precambrian region of North‐Western Queensland

Potassium‐argon measurements have been carried out on the separated micas of 27 samples, principally granites, from the Mount Isa‐Cloncurry region of north‐western Queensland. There is evidence for at least two tectonic periods within the “Lower Proterozoic” of the area. The first is represented only in the north‐western portion, with ages greater than 1,770 m.y. on the Ewen Granite, and on the granites of the Nicholson River area to the far north‐west. The second at 1,400–1,450 m.y. is manifested only to the south and east of the Kalkadoon‐Leichhardt complex, and including the Sybella Granite. The results may be further interpreted as lending support to the concept of a possible “metamorphic discontinuity” along the western flank of the Kalkadoon‐Leichhardt complex, postulated by Carter, Brooks and Walker; as suggesting possible contemporaneity of the Cliffdale Volcanics and the Argylla Formation; and as giving further evidence for the antiquity of stromatolites. Comparison with earlier work suggests that some deposits in this region may be contemporaneous with some of the Agicondian sediments of the Katherine‐Darwin area, and that the second tectonic period corresponds with the K‐Ar ages obtained on the Davenportian granites of Central Australia.     

On the age of uranium mineralisation at Nabarlek,northern territory,Australia

Discordant uranium‐lead isotope measurements are reported for a sample of Nabarlek pitchblende. These are compared with earlier measurements on uranium minerals from the South Alligator River area. New interpretations suggest that regional uranium mineralisation took place either 710 or 815 m.y. ago; the lower of these calculations is based on the assumption of modern episodic lead loss, and the higher assumes lead loss by continuous diffusion.     

Dating of glauconite from the Ngalia Basin,Northern Territory,Australia∗

Rb‐Sr and K‐Ar measurements have been made on five glauconite samples from the near basal Treuer Member of the Vaughan Springs Quartzite of the Ngalia Basin, Northern Territory, Australia. Comparison of results between and within the two groups of data demonstrates that variable losses of radiogenic strontium and argon have occurred, but allows a minimum age of 1280 m.y. to be calculated for the member. Sedimentation began in the Ngalia Basin shortly before the time of deposition of this member.

Regional correlations suggest that this minimum age applies to the adjacent Amadeus Basin as well.

Measurements were also made on glauconite from a single sample of the Lower Palaeozoic Djagamara Formation which is in the same sequence. It yields a mid‐Ordovician K‐Ar age which generally agrees with the broad range of post‐Lower Cambrian to pre‐Carboniferous age determined from fossil evidence in bounding formations. A low Rb/Sr ratio prevented calculation of a Rb‐Sr age, but the combination of K‐Ar age and Rb‐Sr measurements allowed an accurate initial ⁸⁷Sr/⁸⁶Sr ratio of .739 to be determined. This is much greater than ocean water values, and it appears that such information on young samples and/or those of low Rb/Sr ratio could help define the source material for glauconite formation.     

The Bajool earthquake sequence of 1991, and implications for the seismicity of central Queensland

The township of Bajool, situated 30 km south of Rockhampton, Queensland, experienced two minor earthquakes, each of Richter magnitude 2.9 on 10 June 1991. The foci were located 7 km north of Bajool, at a depth of less than 10 km. The felt intensity in Bajool was generally IV‐V, with minor damage occurring to several houses. There were several foreshocks and four aftershocks. The events may represent reactivation of a thrust fault, close to its junction with an inferred transform fault, the Fitzroy River Fault. These events demonstrate that the quiescence of central Queensland is apparent only, and that the recently installed University of Central Queensland Regional Network will provide a useful contribution to the understanding of earthquakes occurring along a passive continental margin.     

An initial formal model for spatial data infrastructures

The Commission on Spatial Data Standards of the International Cartographic Association is working to define formal models and technical characteristics of Spatial Data Infrastructures (SDI). To date, this work has been restricted to the Enterprise and Information Viewpoints from the ISO Reference Model for Open Distributed Processing standard. The Commission has developed models for these two viewpoints. These models describe how the different parts of an SDI fit together in the viewpoints in question. These models should be seen as a contribution towards the overall model of the SDI and its technical characteristics. During the model development process, the roles of the different Actors in an SDI in the Enterprise and Information Viewpoints have also been identified in Use Case diagrams of an SDI. All the models have been developed using the Unified Modeling Language.     

Jovian plasma torus interaction with Europa: 3D hybrid kinetic simulation. First results

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon-magnetosphere system with respect to variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa ( [Cassidy et al., 2007] and [Shematovich et al., 2005]). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream background ions). Non-thermal distributions of upstream plasma will be addressed in future work. Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider two models for background plasma: (a) with O⁺⁺ ions; (b) with O⁺⁺ and S⁺⁺ ions. The majority of O₂ atmosphere is thermal with an extended cold population (Cassidy et al., 2007). A few first simulations already include an induced magnetic dipole; however, several important effects of induced magnetic fields arising from oceanic shell conductivity will be addressed in later work.