The Tennant Creek porphyry revisited: A synsedimentary sill with peperite margins,Early Proterozoic,Northern Territory

Isolated outcrops of coarse‐ and medium‐grained, quartz‐feldspar porphyry occur for some 60 km from Nobles Nob to near Warrego in the Early Proterozoic Tennant Creek Block of the central Northern Territory. The outcrops appear to be part of an approximately conformable sheet of variable thickness (several tens to a few hundred metres), enclosed by sedimentary rocks of the Warramunga Group. The porphyry is characterized by euhedral, complete, evenly distributed crystals up to 2 cm across, in a microcrystalline groundmass. In some samples the groundmass shows relict perlitic cracks and was formerly coherent glass. In detail, upper and lower contacts of the porphyry sheet are highly irregular: porphyry contains wisps, blocks and large rafts of sandstone, and sandstone encloses single euhedral crystals and blebs, tongues and lobes of porphyry. Clasts of each rock type commonly have fluidal shapes. Sedimentary rocks at both the upper and the lower contacts are indurated and silicified. Close to the contacts, bedding is not easily identified, or else is disturbed. The quartz‐feldspar porphyry is interpreted to have cooled and solidified from phenocryst‐rich magma that intruded approximately parallel to bedding in the enclosing sedimentary sequence. The peperite margins of the sill suggest that the porphyritic magma invaded relatively weak, poorly consolidated, wet sediments rather than solid sedimentary rock. Rapid expansion and movement of heated pore fluid would have temporarily disrupted particle packing in the sediments at the contacts, and allowed intricate penetration of magma accompanied by quenching and fragmentation. In addition to relatively short‐term and local effects on the pore fluid chemical and physical properties, the intrusion was probably responsible for premature and permanent dewatering of the adjacent sediments. The sill together with its bordering zone of peperite and indurated sedimentary rocks may have constituted a significant physical barrier to any subsequent fluid transport in the enclosing sedimentary sequence.     

Proterozoic – Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: The Selwyn Block and its tectonic implications

Aeromagnetic and field data suggest that meta‐igneous rocks exposed on the south coast of central Victoria at Waratah Bay, Phillip Island, Barrabool Hills and inland near Licola, are continuous—beneath Bass Strait—with Proterozoic/Cambrian igneous rocks in King Island and Tasmania. This correlation is supported by a pre‐Early Ordovician unconformity above gabbro protomylonite at Waratah Bay, age equivalent to the Tasmanian Tyennan unconformity. Cambrian volcanics at Licola and unusual features of the Melbourne Zone sequence indicate that Tyennan continental crust extends north as basement to the central Victorian portion of the Lachlan Fold Belt. In contrast, adjacent parts of the Lachlan Fold Belt in Victoria contain conformable sea‐floor sequences that span the Early Cambrian to Late Ordovician, with no evidence of either Cambrian deformation or underlying continental basement. The block of Tyennan continental crust beneath central Victoria—the Selwyn Block—is fundamentally different, and has influenced temporal and spatial patterns of sedimentation, deformation, metamorphism and plutonism. Palaeogeographical reconstructions suggest that the block was a submarine plateau that lay outboard of the Australian craton, upon which a condensed Ordovician sequence was deposited. The sequence above the Selwyn Block unconformity at Waratah Bay is similar to widespread post‐Tyennan sediments in western Tasmania. During Late Ordovician and Early Silurian deformation, the Selwyn Block protected much of the overlying sedimentary sequence. Instead, shortening was focused into the Stawell and Bendigo Zones to the west. These zones were sandwiched between the Selwyn Block and the Australian craton in a ‘vice’ scenario reminiscent of some Appalachian orogenic events. The region above the Selwyn Block was downwarped adjacent to the overthrust Bendigo Zone as a foreland deep, into which a conformable clastic wedge of sediment was deposited in Late Ordovician to Devonian time, prior to final Middle Devonian deformation. The Selwyn Block includes the Cambrian calc‐alkaline Licola and Jamieson Volcanics that are correlated with the Tasmanian Mt Read Volcanics. In Victoria, these form a basement high controlling the unusual down‐cutting thrusts in the overlying Melbourne Zone and explaining the major structural vergence reversal between the Melbourne and Tabberabbera Zones. The Selwyn Block has exerted some control on the timing, chemistry and distribution of post‐orogenic granites, and on central Victorian gold mineralisation. Reactivated faults in the block influenced deposition, and continue to control the deformation of the portions of the Otway and Gippsland Basins that lie above it.     

Lamprophyric intrusions of probable carbonatitic affinity from South Australia

Primary carbonate‐bearing lamprophyric rocks have been recognised in the Walloway Diapir, South Australia. Their petrography and trace‐element geochemistry indicates that they have carbonatitic affinities. The rocks are magnetic and can be detected by ground magnetometer surveys.     

云南东川岔河铁矿成因探讨

岔河铁矿产于构造破碎带中,似层状产出,围岩和矿体中石英、方解石脉发育,矿床成因属浅海一滨海相沉积变质型褐铁矿床.     

No proof from carbon isotopes in the Francevillian (Gabon) and Onega (Fennoscandian shield) basins of a global oxidation event at 1980–2090 Ma following the Great Oxidation Event (GOE)

Highly depleted C isotope composition of organic matters from the Onega (Fennoscandian shield) and Francevillian (Gabon) basins are differently interpreted. Kump et al. (2011) suggested the occurrence of a massive and global oxidation event during the period of 1980–2090 Ma, which follows the Great Oxidation Event (2450–2320 Ma) (Bekker et al., 2004). Inversely, Gauthier-Lafaye and Weber (2003) invoke the possible action of methanotrophic microorganisms to explain the δ¹³C values as low as –46‰ measured in the Franceville basin. Here we present the isotope data available in the Franceville basin in order to discuss these two interpretations. The lack of any δ¹³C correlation between organic matter and carbonate in the Franceville basin does not allow the consideration of a massive and global oxidation event.     

http://www.sciencedirect.com/science/article/pii/S1674987112000898

In more than 4 Ga of geological evolution, the Earth has twice gone through extreme climatic perturbations, when extensive glaciations occurred, together with alternating warm periods which were accompanied by atmospheric oxygenation. The younger of these two episodes of climatic oscillation preceded the Cambrian “explosion” of metazoan life forms, but similar extreme climatic conditions existed between about 2.4 and 2.2 Ga. Over long time periods, changing solar luminosity and mantle temperatures have played important roles in regulating Earth's climate but both periods of climatic upheaval are associated with supercontinents. Enhanced weathering on the orogenically and thermally buoyed supercontinents would have stripped CO₂ from the atmosphere, initiating a cooling trend that resulted in continental glaciation. Ice cover prevented weathering so that CO₂ built up once more, causing collapse of the ice sheets and ushering in a warm climatic episode. This negative feedback loop provides a plausible explanation for multiple glaciations of the Early and Late Proterozoic, and their intimate association with sedimentary rocks formed in warm climates. Between each glacial cycle nutrients were flushed into world oceans, stimulating photosynthetic activity and causing oxygenation of the atmosphere. Accommodation for many ancient glacial deposits was provided by rifting but escape from the climatic cycle was predicated on break-up of the supercontinent, when flooded continental margins had a moderating influence on weathering. The geochemistry of Neoproterozoic cap carbonates carries a strong hydrothermal signal, suggesting that they precipitated from deep sea waters, overturned and spilled onto continental shelves at the termination of glaciations. Paleoproterozoic (Huronian) carbonates of the Espanola Formation were probably formed as a result of ponding and evaporation in a hydrothermally influenced, restricted rift setting. Why did metazoan evolution not take off after the Great Oxidation Event of the Paleoproterozoic? The answer may lie in the huge scar left by the ～2023 Ma Vredefort impact in South Africa, and in the worldwide organic carbon-rich deposits of the Shunga Event, attesting to the near-extirpation of life and possible radical alteration of the course of Earth history.     

Interpreting prograde-growth histories of Al2SiO5 triple-point rocks using oxygen-isotope thermometry: an example from the Truchas Mountains, USA

Oxygen‐isotope compositions of kyanite, andalusite, prismatic sillimanite and fibrolite from the Proterozoic terrane in the Truchas Mountains, New Mexico differ from one another, suggesting that these minerals did not grow in equilibrium at the Al₂SiO₅ (AS) polymorph‐invariant point as previously suggested. Instead, oxygen‐isotope temperature estimates indicate that growth of kyanite, andalusite and prismatic sillimanite occurred at c. 575, 615 and 640 °C respectively. Temperature estimates reported in this paper are interpreted as those of growth for the different AS polymorphs, which are not necessarily the same as peak metamorphic temperatures for this terrane. Two distinct temperature estimates of c. 580 °C and c. 700 °C are calculated for most fibrolite samples, with two samples yielding clear evidence of quartz‐fibrolite oxygen‐isotope disequilibrium. These data indicate that locally, and potentially regionally, oxygen‐isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation. Pressures of mineral growth that were extrapolated from oxygen‐isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during M1 at 5 kbar, and that andalusite, prismatic sillimanite and a second generation of fibrolite grew during M2 at 3.5 kbar. M1 and M2 therefore represent two distinct metamorphic events that occurred at different crustal levels. The ability of the AS polymorphs to retain δ¹⁸O values of crystallization make these minerals ideal to model prograde‐growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure–temperature histories of multiple metamorphic events.     

Genesis of intermediate igneous rocks at the end of the Sveconorwegian (Grenvillian) orogeny (S Norway) and their contribution to intracrustal differentiation

Abundant ferroan, metaluminous granitoids (970–950 Ma) emplaced at the end of the Sveconorwegian collisional orogeny (1130–900 Ma) are dominated by intermediate to silicic compositions with rare mafic facies. Both 73% fractional crystallization of an amphibole-bearing gabbroic cumulate substracted from the parent mafic composition and 30% non-modal batch melting of an amphibolitic source equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition. A better fit is obtained for the partial melting process. Both processes could have occurred simultaneously to produce mafic cumulates and restites. As there is no evidence for large volumes of dense mafic rocks in the Sveconorwegian upper crust, these dense mafic rocks were probably produced in the lower crust. Formation of these granitoids, thus, contributed to the vertical stratification of the Proterozoic continental crust and also to the transfer of water from the lower crust to the surface.     

Mixed messages in iron oxide–copper–gold systems of the Cloncurry district,Australia: insights from PIXE analysis of halogens and copper in fluid inclusions

Proterozoic rocks of the Cloncurry district in NW Queensland, Australia, are host to giant (tens to hundreds of square kilometers) hydrothermal systems that include (1) barren regional sodic–calcic alteration, (2) granite-hosted hydrothermal complexes with magmatic–hydrothermal transition features, and (3) iron oxide–copper–gold (IOCG) deposits. Fluid inclusion microthermometry and proton-induced X-ray emission (PIXE) show that IOCG deposits and the granite-hosted hydrothermal complexes contain abundant high temperature, ultrasaline, complex multisolid (type 1) inclusions that are less common in the regional sodic–calcic alteration. The latter is characterized by lower salinity three-phase halite-bearing (type 2) and two-phase (type 3) aqueous inclusions. Copper contents of the type 1 inclusions (>300 ppm) is higher than in type 2 and 3 inclusions (<300 ppm), and the highest copper concentrations (>1,000 ppm) are found both in the granite-hosted systems and in inclusions with Br/Cl ratios that are consistent with a magmatic source. The Br/Cl ratios of the inclusions with lower Cu contents are consistent with an evaporite-related origin. Wide ranges in salinity and homogenization temperatures for fluid inclusions in IOCG deposits and evidence for multiple fluid sources, as suggested by halogen ratios, indicate fluid mixing as an important process in IOCG genesis. The data support both leaching of Cu by voluminous nonmagmatic fluids from crustal rocks, as well as the direct exsolution of Cu-rich fluids from magmas. However, larger IOCG deposits may form from magmatic-derived fluids based on their higher Cu content.