Geodynamic Significance of the Mesoproterozoic Magmatism of the Udzha Paleo-Rift (Northern Siberian Craton) Based on U-Pb Geochronology and Paleomagnetic Data

The emplacement age of the Great Udzha Dyke (northern Siberian Craton) was determined by the U-Pb dating of apatite using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). This produced an age of 1386 +/- 30 Ma. This dyke along with two other adjacent intrusions, which cross-cut the sedimentary units of the Udzha paleo-rift, were subjected to paleomagnetic investigation. The paleomagnetic poles for the Udzha paleo-rift intrusions are consistent with previous results published for the Chieress dyke in the Anabar shield of the Siberian Craton (1384 +/- 2 Ma). Our results suggest that there was a period of intense volcanism in the northern Siberian Craton, as well as allow us to reconstruct the apparent migration of the Siberian Craton during the Mesoproterozoic.Peer reviewe

Pyrite Textures, Trace Elements and Sulfur Isotope Chemistry of Bijaigarh Shales, Vindhyan Basin, India and Their Implications

The Vindhyan Basin in central India preserves a thick (~5 km) sequence of sedimentary and lesser volcanic rocks that provide a valuable archive of a part of the Proterozoic (~1800-900 Ma) in India. Here, we present an analysis of key sedimentary pyrite textures and their trace element and sulfur isotope compositions in the Bijaigarh Shale (1210 ± 52 Ma) in the Vindhyan Supergroup, using reflected light microscopy, LA-ICP-MS and SHRIMP-SI, respectively. A variety of sedimentary pyrite textures (fine-grained disseminated to aggregates, framboids, lags, and possibly microbial pyrite textures) are observed reflecting quiet and strongly anoxic water column conditions punctuated by occasional high-energy events (storm incursions). Key redox sensitive or sensitive to oxidative weathering trace elements (Co, Ni, Zn, Mo, Se) and ratios of (Se/Co, Mo/Co, Zn/Co) measured in sedimentary pyrites from the Bijaigarh Shale are used to infer atmospheric redox conditions during its deposition. Most trace elements are depleted relative to Proterozoic mean values. Sulfur isotope compositions of pyrite, measured using SHRIMP-SI, show an increase in δ34S as we move up stratigraphy with positive δ34S values ranging from 5.99% (lower) to 26.08� (upper). We propose limited sulphate supply caused the pyrites to incorporate the heavier isotope. Overall, we interpret these low trace element signatures and heavy sulfur isotope compositions to indicate relatively suppressed oxidative weathering on land during the deposition of the Bijaigarh Shale

Generation of Silicic Melts in the Early Izu-Bonin Arc Recorded by Detrital Zircons in Proximal Arc Volcaniclastic Rocks From the Philippine Sea

A 1.2 km thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation, and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U, and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.Support for this research was provided by the IODP, the United States Implementing Organization, and the U.S. National Science Foundation through grant NSF OCE-1558830 to AP

Melanesian back-arc basin and arc development: Constraints from the eastern Coral Sea

The eastern Coral Sea is a poorly explored area at the north-eastern corner of the Australian Tectonic Plate, where interaction between the Pacific and Australian plate boundaries, and accretion of the world's largest submarine plateau – the Ontong Java Plateau – has resulted in a complex assemblage of back-arc basins, island arcs, continental plateaus and volcanic products. This study combines new and existing magnetic anomaly profiles, seafloor fabric from swath bathymetry data, Ar–Ar dating of E-MORB basalts, palaeontological dating of carbonate sediments, and plate modelling from the eastern Coral Sea. Our results constrain commencement of the opening of the Santa Cruz Basin and South Rennell Trough to c. 48 Ma and termination at 25–28 Ma. Simultaneous opening of the Melanesian Basin/Solomon Sea further north suggests that a single > 2000 km long back-arc basin, with at least one triple junction existed landward of the Melanesian subduction zone from Eocene–Oligocene times. The cessation of spreading corresponds with a reorganisation of the plate boundaries in the area and the proposed initial soft collision of the Ontong Java Plateau. The correlation between back-arc basin cessation and a widespread plate reorganisation event suggests that back-arc basins may be used as markers for both local and global plate boundary changes.We thank the Captain and crew of R/V Southern Surveyor and Australia's Marine National Facility (MNF) for the success of voyage SS2012_V06. Mineral separation was done by John Simes and Belinda Smith Lyttle. We thank Hugh Davies for his attempts to locate old Solomon Sea dredge samples and Geoscience Australia and New Caledonia agencies for voyage support. We acknowledge funding from the Australian Research Council through grants DP0987713 and FT130101564 (MS), FL0992245 (SEW), IH130200012 (SZ) and DP130101946 (KJM). Support from the New Zealand Government through core funding to GNS Science (NM) is also acknowledged. We thank Robert Holm and an anonymous reviewer for their comments, which improved the quality of the manuscript

Geology and geochronology of the Two-Thirty prospect, Northparkes district, NSW

The Northparkes district, central New South Wales, hosts several economic Cu–Au deposits associated with discrete, thin, porphyry intrusive complexes emplaced in the Late Ordovician during formation of the Macquarie Arc. The recently discovered Two-Thirty Cu–Au–(Mo) prospect is a mineralised magmatic–hydrothermal breccia complex that is hosted by the moderately east-dipping Goonumbla Volcanic Complex on the western limb of the Milpose Syncline ∼15 km south of the Northparkes porphyry district. Generation of the magmatic–hydrothermal breccia complex is interpreted to be related to the 448.0 ± 4.4 Ma emplacement of the Two-Thirty porphyry. However, Re–Os dating of molybdenite from the breccia complex indicates a potential for a ca 440 Ma mineralising event that has similar timing to economic porphyry mineralisation in the Northparkes district. The discovery of the Two-Thirty prospect has important implications for exploration in the Northparkes district and the broader Macquarie Arc. Two-Thirty is only the second known occurrence of magmatic-hydrothermal breccia-hosted mineralisation discovered within the Macquarie Arc, with the other being Cadia Quarry. Mineralisation at Two-Thirty is potentially older than the Northparkes and Cadia deposits, and younger than the epithermal and calc-alkaline deposits at Cowal, Marsden and Ridgeway.KEY POINTS: The Two-Thirty is a polyphase magmatic–hydrothermal breccia complex that hosts Cu–Au (Mo). The Two-Thirty is the first significant breccia-hosted mineralisation found in the Northparkes district. U–Pb zircon crystallisation ages of the causative intrusion at Two-Thirty pre-date mineralisation at Northparkes. Re–Os dates of molybdenite from the Two-Thirty breccia complex are coeval with syn-mineralisation at Northparkes, supporting the model of periodic release of melts and fluids from underlying magma chambers during the formation of porphyry mineralisation in the Northparkes district.This research is funded by Australian Research Council sponsors of the Lachlan ARC Linkage Project ‘LP160100483’ CMOC-Northparkes, Rio Tinto, Evolution Mining, IMEx Consulting, Heron Resources, Sandfire Resources NL, New South Resources, AngloGold Ashanti, Alkane Resources, Geoscience Australia, The University of Tasmania, Australian National University, University of Melbourne, CCFS, Curtin University, the New South Wales, Tasmanian and Victorian state governments

Styles of Cenozoic collisions in the western and southwestern Pacific and their applications to Palaeozoic collisions in the Tasmanides of eastern Australia

The western and southwestern Pacific preserve evidence of Cenozoic collisions that guide our understanding of processes and geometries involved in collisions in ancient orogens, in particular in this case, the Palaeozoic Tasmanides of southeastern Australia. Although several styles of collisions are present in the Pacific, ranging from arc-arc collision to arc-plateau collision, the dominant two are oblique and strike-slip collisions between island arcs and rifted continental fragments, and collisions between forearc lithosphere and continental fragments. The 58 Ma collision along the northern margin of the Australian plate in New Guinea, the 44-34 Ma collision preserved in New Caledonia and the 26-25 Ma collision in the North Island of New Zealand may be parts of a single plate boundary collision that migrated southwards along the plate boundary. They characterize the main style of deformation in which a collision between forearc crust and continental fragment produces subduction flip or rollback, thereby avoiding a classic arc-continent collision. Processes involved in, and geometries that have developed from, SW and W Pacific style collisions have been applied to the interpretation of the evolution of the Delamerian Orogen and Lachlan Orogen in the southern Tasmanides with varying degrees of success. The ophiolite obduction model has been successfully applied to the western Tasmania part of the Delamerian Orogen, although there is discussion about its applicability to the mainland. The best example of an arc accretion, that of the Ordovician Macquarie Arc in the eastern Lachlan Orogen, developed from rare geometry in the western Pacific wherein (with the constraint that no forearc or subduction complex has been identified) the arc lies on the continental plate, above a continental-dipping subduction zone. The multiple subduction zone model of Halmahera has been widely applied to the back arc of the Lachlan Orogen, but evidence for clear subduction zones or arcs is not present.20 page(s

Zircon megacryst ages and chemistry, from a placer, Dunedin volcanic area, eastern Otago, New Zealand

Zircon megacrysts from a sapphire-bearing placer near Glenore, South Otago, New Zealand, gave U-Pb ages linked to the Miocene Waipiata Volcanics. Yellow and brown (zoned) zircons gave ages almost within error of a mean age of 19.1 ± 0.2 Ma. Laser-ablation inductively coupled plasma mass spectrometry analysis suggests these two zircons crystallised from separate magmas. The Ti-in-zircon thermometer gave different formation temperatures, between 550-685°C for yellow zircon and 700-730°C for brown zircon. The zircons have typical felsic chemistry, with positive Ce anomalies, while negligible Eu depletion indicates little plagioclase fractionation in their source magmas. Incompatible element values are enriched in the brown zircon. Such zircon megacrysts are rare in New Zealand, but they resemble east Australian zircons from basalt fields and may come from metasomatised mantle melts or fractionated alkali basalt magmas

The key role of mica during igneous concentration of tantalum

Igneous rocks with high Ta concentrations share a number of similarities such as high Ta/Nb, low Ti, LREE and Zr concentrations and granitic compositions. These features can be traced through fractionated granitic series. Formation of Ta-rich melts begins with anatexis in the presence of residual biotite, followed by magmatic crystallization of biotite and muscovite. Crystallization of biotite and muscovite increases Ta/Nb and reduces the Ti content of the melt. Titanium-bearing oxides such as rutile and titanite are enriched in Ta and have the potential to deplete Ta at early stages of fractionation. However, mica crystallization suppresses their saturation and allows Ta to increase in the melt. Saturation with respect to Ta and Nb minerals occurs at the latest stages of magmatic crystallization, and columbite can originate from recrystallization of mica. We propose a model for prediction of intrusion fertility for Ta

Quantifying and Characterizing Metal Concentrations in Derwent Estuary Sediments using Portable X-Ray Fluorescence Spectrometry

The Derwent Estuary is highly enriched in potentially toxic elements such as Zn, Pb, Cu, As, Hg and Cd. This occurred due to inputs from historical industrial activity adjacent to the river, predominantly prior to strict environmental protection procedures introduced in the 1970s. Contaminants are now buried at shallow depths within the sediment profile, in one or two highly concentrated layers decreasing in concentration away from an electrolytic zinc refinery, regarded as the main source of the contaminants. Enriched metals (Zn, Pb, Cu, Cd and As) in the estuary were estimated from data collected from 37 sediment cores using a portable X-ray fluorescence spectrometer, validated against mass spectrometer analyses. The thickness of the metal and metalloid enriched layers ranges from 32.5 cm to 107.5 cm, with an average thickness of 63 cm. Sedimentation rates based on this layer and the time since the start of zinc processing are approximately 0.46 cm/year. Recent trends in sedimentation based on the thickness of sediments since maximum metal and metalloid concentrations produced rates between 0.17 – 1.64 cm/year. Based on these sedimentation rates, the average time it will take for surface sediments to return to background metal and metalloid concentrations is approximately 123 years