40Ar/39Ar and K–Ar geochronology of magmatic and hydrothermal events in a classic low-suphidation epithermal bonanza deposit: El Peñon, northern Chile

The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and ⁴⁰Ar/³⁹Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (～59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (～55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (～48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. ⁴⁰Ar/³⁹Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (⁴⁰Ar/³⁹Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events.     

Spider impact structure,Kimberley Plateau,Western Australia: interpretations of formation mechanism and age based on integrated map-scale data

The centre of the 13?×?11 km Spider impact structure, Western Australia, displays an unusual system of eroded folds and imbricated thrusts surrounding a sandstone dome. As inferred from GIS-integrated remote sensing, geological and digital elevation data, the structural setting of the original crater was influenced by, and hence post-dates, the formation of the Mt Barnett Syncline, the east?–?west-oriented axis of which runs through the Spider structure. The syncline formed during the regional Yampi Orogeny (ca 900 Ma), thus constraining the maximum age of the impact event. The sandstone dome in the centre of Spider formed prior to the imbrication, as interpreted from the present setting that indicates a deflection of the southward moving material during the crater collapse. Two modes of formation are discussed in order to explain the south-directed shortening in the Spider impact structure: (i) impact into the bottom of a syncline-controlled palaeovalley leading to uplift of the central crater floor followed by gravity-driven asymmetric sliding preferentially from the northern crater wall and valley slope, respectively; and (ii) moderately oblique (～10?–?30°) impact from the north onto the axis of the syncline, producing a central uplift under the influence of downrange residual momentum and, thus, asymmetric deformation inside the uplift and farther downrange. Neither model alone explains all the observations, and only a combination of both may provide a satisfactory solution.     

Gnargoo: a possible 75 km-diameter post-Early Permian – pre-Cretaceous buried impact structure,Carnarvon Basin,Western Australia

The Gnargoo structure is located on the Gascoyne Platform, Southern Carnarvon Basin, Western Australia, and is buried beneath about 500 m of Cretaceous and younger strata. The structure is interpreted as being of possible impact origin from major geophysical and morphometric signatures, characteristic of impact deformation, and its remarkable similarities with the proven Woodleigh impact structure, about 275 km to the south on the Gascoyne Platform. These similarities include: a circular Bouguer anomaly (slightly less well-defined at Gnargoo than at Woodleigh); a central structurally uplifted area comprising a buried dome with a central uplifted plug; and the lack of a significant magnetic anomaly. Gnargoo shows a weakly defined inner 10 km-diameter circular Bouguer anomaly surrounded by a broadly circular zone, ～75 km in diameter. The north?–?south Bouguer anomaly lineament of the Giralia Range (a regional topographic and structural feature) terminates abruptly against the outer circular zone which is, in turn, intersected on the eastern flank by the Wandagee Fault. A <?28 km-diameter layered sedimentary dome of Ordovician to Lower Permian strata, surrounding a cone-shaped, central uplift plug of 7?–?10 km diameter, are inferred from the seismic data. Seismic-reflection data indicate a minimum central structural uplift of 1.5 km, as compared to a model uplift of 7.3 km calculated from the outer structural diameter. An interpretation of Gnargoo in terms of a plutonic or volcanic caldera/ring origin is unlikely as these features display less regular geometry, are typically smaller and no volcanic rocks are known in the onshore Gascoyne Platform. An interpretation of Gnargoo as a salt dome is likewise unlikely because salt structures tend to have irregular geometry, and no extensive evaporite units are known in the Southern Carnarvon Basin. Morphometric estimates of the rim-to-rim diameter based on seismic data for the central dome correspond to the observed diameter deduced from gravity data, and fall within the range of morphometric parameters of known impact structures. The age of Gnargoo is constrained between the deformed Lower Permian target rocks and unconformably overlying undeformed Lower Cretaceous strata. Because of its large dimensions, if Gnargoo is an impact structure, it may have influenced an environmental catastrophe during this period.     

Matt Wilson structure: record of an impact event of possible Early Mesoproterozoic age,Northern Territory

The Matt Wilson structure is a circular 5.5 km-diameter structure in Early Mesoproterozoic or Neoproterozoic rocks of the Victoria Basin, Northern Territory. It lies in regionally horizontal to gently dipping Wondoan Hill and Stubb Formations (Tijunna Group) and Jasper Gorge Sandstone (Auvergne Group). An outer circumferential syncline with dips of 5?–?40° in the limbs surrounds an intermediate zone with faulted sandstone displaying horizontal to low dips, and a central steeply dipping zone about 1.5 km across. Several thrust faults in the outer syncline appear to indicate outward-directed forces. The central zone, marked by steeply dipping to overturned Tijunna Group and possibly Bullita Group sandstone and mudstone, indicates uplift of at least 300 m. The rocks are intensely fractured with some brecciation, and contain numerous planar to subtly undulating surfaces displaying striae which resemble shatter cleavage. Thin-sections of sandstone from the central area show zones of intense microbrecciation and irregular and planar fractures in quartz, but no melt-rocks have been identified. The planar fractures occur in multiple intersecting parallel sets typical of relatively low-level (5?–?10 GPa) shock-pressure effects. Alternative mechanisms, i.e. igneous intrusion, carbonate collapse, diapirism and regional deformation processes, have been discounted. The circular nature, central uplift, faulting, shatter features and planar fractures are all consistent with an impact origin. The Matt Wilson structure is most likely a deeply eroded impact structure in which the more highly shocked rocks of the original crater floor have been removed by erosion. Estimates of the age of the Auvergne and Tijunna Groups range from Early Mesoproterozoic (which we favour) to Late Neoproterozoic. Early Cambrian Antrim Plateau Volcanics near the impact structure show no signs of impact effects, allowing the age of impact to be constrained between Early Mesoproterozoic and Early Cambrian. The presence of widespread soft-sediment deformation features, apparently confined to a single horizon in the Saddle Creek Formation some 700?–?1000 m stratigraphically higher in the Auvergne Group than the rocks at the impact site, and apparently increasing in thickness towards the Matt Wilson structure, lead us to speculate that this probable event horizon is related to the impact event: if correct the impact occurred during deposition of the Saddle Creek Formation.     

Microscopic morphology and elemental composition of size distributed atmospheric particulate matter in Urumqi, China

Microscopic morphology and elemental composition of atmospheric particulate matter (PM) in 13 different size fractions from 0.01 to 10 μm were studied using a Field Emission Scanning Electron Microscope with Energy-Dispersive Spectrometer (FESEM–EDX). The relative mass fractions exhibited a bimodal distribution with a major mode in the fine range (0.18–1 μm) and a minor mode in the coarse range (>1 μm), suggesting that the major pollution of PM is fine particles in this area of Urumqi atmosphere. The PM could be classified as follows: aluminosilicate/silica mineral, Si–Al rich fly ash, Fe oxide particle, Ti dominant particle, sulfate/carbonate crystal, carbonaceous aerosols (including soot, organic carbon, tar ball and irregularly shaped carbon). The soot and organic carbon with anthropogenic sources are dominant types in fine range samples (<1 μm). The natural source minerals and secondary synthesized sulfate/carbonate crystals were accumulated in the coarse range (>1 μm). Elemental composition of various types of particles (0.056–5.6 μm) was also analyzed by EDX. C, S, O, N, Si, Al, Fe, Ca, Na, K, Mg, Cl, F, Hg were detected in most samples. Si, Al and Ca accumulated in coarse fractions, while S and Hg mainly accumulated in fine fractions. Concentrations of 15 metallic elements in size range from 0.1 μm to 5.6 μm were divided into three groups based on their possible sources. (1) The crustal elements (Al, Mg, Fe, Mn and V), mainly present in coarse particles (>1 μm); and (2) the anthropogenic source elements (Ca, Ni, As, Cu, Pb, Cd and Hg). The concentrations of Ca and Ni increased with increasing particle size, while As, Cu, Pb, Cd and Hg showed opposite trends. As, Cu, Pb, Cd and Hg accumulated mainly in fine fraction (<1 μm). (3) The multi sources elements (Cr, Co and Se) possibly come from both natural and anthropogenic sources. High levels of heavy metals, especially Hg in nanosize particles, may pose great risk to human health.     

Traces from the past: the Cenozoic regolith and intraplate neotectonic history of the Gun Emplacement,a ferricreted bench on the western margin of the Mt Lofty Ranges,South Australia

The Gun Emplacement is a small but distinctive bench on the Eden–Burnside Fault Escarpment near Anstey Hill, in the northeastern suburbs of Adelaide, South Australia, occurring at an elevation of ～210–220 m asl. It is underlain by Middle Eocene North Maslin Sand and is capped by resistant, ferricreted colluvium. Paleomagnetic dating of hematitic mottles in the ferricreted colluvium, immediately underlying the emplacement, returned a Pliocene/Early Pleistocene age. This age is equivalent to that obtained for summit surface weathering. Fault scarps and exposures, including slickensides and fault gouge material, suggest that the Eden–Burnside Fault at this location has a strong en échelon pattern developed in response to reverse-sinistral oblique-slip faulting, reflecting continental stress fields. Remnants of ferricrete cappings forming stranded benches on the Eden–Burnside Fault Escarpment at elevations up to 25 m above the Gun Emplacement demonstrate recurrent tectonism of the South Mt Lofty Ranges related to intraplate deformation. There are at least four distinct ferricrete benches preserved on the eastern side of the active fault leading up from the Gun Emplacement surface. These benches demonstrate alternating periods of stability and tectonic activity disrupting and uplifting the ferricreted surfaces. A fresh surface rupture occurs and may be related to a recent seismic event.     

Acraman – Bunyeroo impact event (Ediacaran), South Australia,and environmental consequences: twenty-five years on

Multidisciplinary research during the past 25 years has established that the Acraman impact structure in the 1.59 Ga Gawler Range Volcanics on the Gawler Craton, and an ejecta horizon found 240?–?540 km from Acraman in the ??580 Ma Bunyeroo Formation in the Adelaide Fold Belt and Dey Dey Mudstone in the Officer Basin, record a Late Neoproterozoic (Ediacaran) event of major environmental importance. Research since 1995 has verified Acraman as a complex impact structure that has undergone as much as 3?–?5 km of denudation and which originally had a transient cavity up to 40 km in diameter and a final structural rim possibly 85?–?90 km in diameter. The estimated impact energy of 5.2?×?10⁶ Mt (TNT) for Acraman exceeds the threshold of 10⁶ Mt nominally set for global catastrophe, and the impact probably caused a severe perturbation of the Ediacaran environment. The occurrence of the impact at a low palaeolatitude (12.5 +?7.1/???6.1°) may have magnified the environmental effects by perturbing the atmosphere in both hemispheres. These findings are consistent with independent data from the Ediacaran palynology of Australia and from isotope and biomarker chemostratigraphy that the Acraman impact induced major biotic change. Future research should seek geological, isotopic and biological imprints of the Acraman?–?Bunyeroo impact event across Australia and on other continents.     

Uplift and denudation of Mt Sanqingshan Geopark,Jiangxi Province,China

Mt Sanqingshan, a global Geopark and world natural heritage site located in Jiangxi Province, China, is famous for its eroded granite peaks. The uplift and denudation history of the area has been reconstructed using fission track methods for the first time. Apatite fission track ages (AFTAs) cluster into three groups at ca. 25 Ma, 45–55 Ma, and 70 Ma. These ages can be related to ancient multilevel denudation planes at about 900, 1200, and 1500 m above sea level, respectively. The apatite data also reveal four cooling stages for the Mt Sanqingshan region, from ca. 90 to 65–60 Ma, 65–60 to 45 Ma, 45 to 20–15 Ma, and 20–15 Ma to the present, with cooling rates of 1.96°C, 1.18°C, 0.37°C, and 3.78°C per million years, respectively, and an average cooling rate of 1.80°C per million years. Calculated uplift rates are 0.055, 0.034, 0.011, and 0.11 mm year^?1 in the four stages, yielding uplifts of 4140, 570, 290, and 1940 m, respectively. The uplift rate of the last stage was significantly faster than that of the other three preceding stages, reflecting rejuvenation of Mt Sanqingshan, as a result of new tectonism. The average uplift rate at Mt Sanqingshan is 0.053 mm year^?1, and the average denudation rate is 0.048 mm year^?1, resulting in 3550 m of uplift and 2540 m of denudation relative to eustatic sea level. The 1010 m difference is very close to the average elevation of about 1000 m at present. A comparison of uplift–denudation histories for Mt Sanqingshan and Mt Huangshan shows that fission track results can be useful for defining geomorphological development stages.     

Metastable staurolite–cordierite assemblage of the Bossòst dome: Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossòst dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M₁, while coaxial conditions prevailed during later contact metamorphic M₂ growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–staurolite assemblage preserves conditions during M₁, estimated by AFM diagrams and P–T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that staurolite is not a stable phase in cordierite–andalusite assemblages of M₂, suggesting polyphase metamorphism and decompression along a clockwise P–T path for the staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).     

Late Cretaceous eclogitic high-pressure relics in the Bitlis Massif

A new occurrence of eclogites was found in the Kesandere valley in the eastern most part of the Bitlis complex, SE Anatolia. These high-pressure (HP) relics were preserved in calc-arenitic metasediments within the high-grade metamorphic basement of the Bitlis complex. The eclogitic parageneses were strongly overprinted during decompression and heating. These new eclogites locality complements the evidence of blueschist-facies metamorphism documented recently in the meta-sedimentary cover sequence of this part of the Bitlis complex. Thermodynamic calculations suggest peak conditions of ca. 480–540 °C/1.9–2.4 GPa. New U/Pb dates of 84.4 ± .9 and 82.4 ± .9 Ma were obtained on zircons from two Kesandere eclogite samples. On the basis of geochemical criteria, these dates are interpreted to represent zircon crystallization during the eclogitic peak stage. Kesandere eclogites differ from those previously described in the western Bitlis complex (Mt. Gablor locality) in terms of lithologic association, protolithic origin, and peak P–T conditions (600–650 °C/1.0–2.0 GPa, respectively). On the other hand, eclogitic metamorphism of Kesandere metasediments occurred shortly before blueschist-facies metamorphism of the sedimentary cover (79–74 Ma ⁴⁰Ar/³⁹Ar white mica). Therefore, the exhumation of Kesandere eclogites started between ca. 82 and 79 Ma, while the meta-sedimentary cover was being buried. During this short time span, Kesandere eclogite were likely uplifted from ~65 to 35 km depth, indicating a syn-subduction exhumation rate of ~4.3 mm/a. Subsequently, eclogite- and blueschist-facies rocks were likely retrogressed contemporarily during collision-type metamorphism (around 72–69 Ma). The Bitlis HP rocks thus sample a subduction zone that separated the Bitlis–Pütürge (Bistun?) block from the South-Armenian block, further north. To the south, Eocene metasediments of the Urse formation are imbricated below the Bitlis complex. They contain (post Eocene) blueschists, testifying separation from the Arabian plate and southward migration of the subduction zone. The HT overprint of Kesandere eclogites can be related to the asthenospheric flow provoked by subducting slab retreat or break off.     

Low‐temperature thermochronology of the Mt Painter Province,South Australia

Apatite fission track results are reported for 26 outcrop samples from the Mt Painter Inlier, Mt Babbage Inlier and adjacent Neoproterozoic rocks of the northwestern Curnamona Craton of South Australia. Forward modelling of the data indicates that the province experienced variable regional cooling from temperatures >110°C during the Late Palaeozoic (Late Carboniferous to Early Permian). The timing of this cooling is similar to that previously reported from elsewhere in the Adelaide Fold Belt and the Curnamona Craton, suggesting that the entire region underwent extensive Late Palaeozoic cooling most likely related to the waning stages of the Alice Springs or Kanimblan Orogenies. Results from the Paralana Fault Zone indicate that the eastern margin of the Mt Painter Inlier experienced a second episode of cooling (～40–60°C) during the Paleocene to Eocene. The entire region also experienced significant cooling (less than ～40°C) during the Late Cretaceous to Palaeogene in response to unroofing and/or a decrease in geothermal gradient. Regional cooling/erosion during this time is supported by: geomorphological and geophysical evidence indicating Tertiary exhumation of at least 1 km; Eocene sedimentation initiated in basins adjacent to the Flinders and Mt Lofty Ranges sections of the Adelaide Fold Belt; and Late Cretaceous ‐ Early Tertiary cooling previously reported from apatite fission track studies in the Willyama Inliers and the southern Adelaide Fold Belt. Late Cretaceous to Palaeogene cooling is probably related to a change in stress field propagated throughout the Australian Plate, and driven by the initiation of sea‐floor spreading in the Tasman Sea in the Late Cretaceous and the Eocene global plate reorganisation.     

Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps

The Lepontine dome represents a unique region in the arc of the Central and Western Alps, where complex fold structures of upper amphibolite facies grade of the deepest stage of the orogenic belt are exposed in a tectonic half-window. The NW-verging Mont Blanc, Aar und Gotthard basement folds and the Lower Penninic gneiss nappes of the Central Alps were formed by ductile detachment of the upper European crust during its Late Eocene–Early Oligocene SE-directed underthrust below the upper Penninic and Austroalpine thrusts and the Adriatic plate. Four underthrust zones are distinguished in the NW-verging stack of Alpine fold nappes and thrusts: the Canavese, Piemont, Valais and Adula zones. Up to three schistosities S1–S3, folds F1–F3 and a stretching lineation XI with top-to-NW shear indicators were developed in the F1–F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Alpe Bosa and Wandfluhhorn anticlines and synclines overprint the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the Adriatic indenter. The transverse folding F4 was followed since 30 Ma by the pull-apart exhumation and erosion of the Lepontine dome. This occurred coevally with the formation of the dextral ductile Simplon shear zone, the S-verging backfolding F5 and the formation of the southern steep belt. Exhumation continued after 18 Ma with movement on the brittle Rhone-Simplon detachment, accompanied by the N-, NW- and W-directed Helvetic and Dauphiné thrusts. The dextral shear is dated by the 29–25 Ma crustal-derived aplite and pegmatite intrusions in the southern steep belt. The cooling by uplift and erosion of the Tertiary migmatites of the Bellinzona region occurred between 22 and 18 Ma followed by the exhumation of the Toce dome on the brittle Rhone–Simplon fault since 18 Ma.     

Suckling Dome and the Australian–Woodlark plate boundary in eastern Papua: the geology of the Keveri and Ada'u Valleys

The Owen Stanley Fault Zone (OSFZ) is the low-angle thrust boundary between the Australian and Woodlark plates. The eastern extension of the OSFZ links with the Woodlark Basin spreading centre. Recent tectonic models of eastern Papua depict the OSFZ boundary passing through the Mt Suckling district, with the Keveri Fault a key component. Gravity data clearly show that the OSFZ and the Papuan Ultramafic Belt (PUB) pass north of Mt Suckling. Tectonised mafic and ultramafic rocks of the Mt Suckling district, previously referred to the PUB, are reassigned to the Awariobo Range Complex (new name). Extensive pillow basalts previously referred to the middle Eocene part of the Kutu Volcanics at the top of the PUB sequence are, in the map area, reassigned on lithological and biostratigraphic grounds to the late Oligocene–middle Miocene Wavera Volcanics. The detailed work reported here indicates that the Keveri Fault is unrelated to the OSFZ with no evidence for thrusting along the structure. The area's tectonic history has been dominated by large vertical displacements along the Keveri Fault. The commencement of late Miocene buoyant uplift of the Suckling Dome (new name), related to granite intrusion into thick crust of the eastern Papua region, marks the inception of the Keveri Fault and coincides with the initiation of Woodlark rifting. The fault facilitated much of the rapid vertical movement of the dome, with an estimated 8000 m of uplift (2.5 m/10³ a) since the late Miocene. Movement on the Keveri Fault is notably different from structures flanking other metamorphic core complexes in eastern Papua. There is no field evidence for the development of a low-angle, south-dipping detachment fault along the southern margin of the Suckling Dome. The Suckling Dome is the westernmost of the eastern Papua domes, localised within a broad extensional zone that continues to propagate westward along the OSFZ plate boundary.     

Eocene mafic volcanism in northern Anatolia: its causes and mantle sources in the absence of active subduction

Eocene mafic volcanic rocks occurring in an E–W-trending, curvilinear belt along and north of the Izmir–Ankara–Erzincan suture zone (IAESZ) in northern Anatolia, Turkey, represent a discrete episode of magmatism following a series of early Cenozoic collisions between Eurasia and the Gondwana-derived microcontinents. Based on our new geochronological, geochemical, and isotope data from the Kartepe volcanic units in northwest Anatolia and the extant data in the literature, we evaluate the petrogenetic evolution, mantle melt sources, and possible causes of this Eocene volcanism. The Kartepe volcanic rocks and spatially associated dikes range from basalt and basaltic andesite to trachybasalt and basaltic trachyandesite in composition, and display calc-alkaline and transitional calc-alkaline to tholeiitic geochemical affinities. They are slightly to moderately enriched in large ion lithophile (LILE) and light rare earth elements (LREE) with respect to high-field strength elements (HFSE) and show negative Nb, Ta, and Ti anomalies reminiscent of subduction-influenced magmatic rocks. The analysed rocks have ⁸⁷Sr/⁸⁶Sr_(i) values between 0.70570 and 0.70399, positive ?_Nd values between 2.7 and 6.6, and Pb isotope ratios of ²⁰⁶Pb/²⁰⁴Pb_(i) = 18.6–18.7, ²⁰⁷Pb/²⁰⁴Pb_(i) = 15.6–15.7, and ²⁰⁸Pb/²⁰⁴Pb_(i) = 38.7–39.1. The ⁴⁰Ar/³⁹Ar cooling ages of 52.7 ± 0.5 and 41.7 ± 0.3 Ma obtained from basaltic andesite and basalt samples indicate middle to late Eocene timing of this volcanic episode in northwest Anatolia. Calculated two-stage Nd depleted mantle model (T_DM) ages of the Eocene mafic lavas range from 0.6 to 0.3 Ga, falling between the T_DM ages of the K-enriched subcontinental lithospheric mantle of the Sakarya Continent (1.0–0.9 Ga) to the north, and the young depleted mantle beneath central Western Anatolia (0.4–0.25 Ga) to the south. These geochemical and isotopic features collectively point to the interaction of melts derived from a sublithospheric, MORB-like mantle and a subduction-metasomatized, subcontinental lithospheric mantle during the evolution of the Eocene mafic volcanism. We infer triggering of partial melting by asthenospheric upwelling beneath the suture zone in the absence of active subduction in the Northern Neotethys. The geochemical signature of the volcanic rocks changed from subduction- and collision-related to intra-plate affinities through time, indicating an increased asthenospheric melt input in the later stages of Eocene volcanism, accompanied by extensional deformation and rifting.     

Impact cratering and distal ejecta: the Australian record

The Australian continent has one of the best-preserved impact-cratering records on Earth, closely rivalling that of North America and parts of northern Europe, and the rate of new discoveries remains high. In this review 26 impact sites are described, including five small meteorite craters or crater fields associated with actual meteorite fragments (Boxhole, Dalgaranga, Henbury, Veevers, Wolfe Creek) and 21 variably eroded or buried impact structures (Acraman, Amelia Creek, Connolly Basin, Foelsche, Glikson, Goat Paddock, Gosses Bluff, Goyder, Kelly West, Lawn Hill, Liverpool, Matt Wilson, Mt Toondina, Piccaninny, Shoemaker, Spider, Strangways, Tookoonooka, Woodleigh, Yallalie, Yarrabubba). In addition a number of possible impact structures have been proposed and a short list of 22 is detailed herein. The Australian cratering record is anomalously biased towards old structures, and includes the Earth's best record of Proterozoic impact sites. This is likely to be a direct result of aspects of the continent's unique geological evolution. The Australian impact record also includes distal ejecta in the form of two tektite strewn fields (Australasian strewn field, ‘high-soda’ tektites), a single report of 12.1?–?4.6 Ma microtektites, ejecta from the ca 580 Ma Acraman impact structure, and a number of Archaean to Early Palaeoproterozoic impact spherule layers. Possible impact related layers near the Eocene?–?Oligocene and the Permian?–?Triassic boundaries have been described in the literature, but remain unconfirmed. The global K?–?T boundary impact horizon has not been recognised onshore in Australia but is present in nearby deep-sea cores.     

The Ligurian Helminthoid flysch units of the Emilian Apennines: stratigraphic and petrographic features,paleogeographic restoration and structural evolution

Abstract

This work deals with the Cretaceous-Tertiary Helminthoid flysch successions of the Emilian Apennines and related basal complexes (Mt. Caio, Val Baganza, Solignano, Mt. Venere-Monghidoro and Mt. Cassio Units): it is based on an integrated approach which included stratigraphic, petrographic and structural observations. Detailed stratigraphic sections measured in the various successions evidenced the specific features of the different flysch formations. The main framework composition analysis of the arenites pointed out a partly ‘oceanic’ alimentation for the Mt. Caio Flysch Fm; the Mt. Venere-Monghidoro, and Mt. Cassio Flysch Fms have been alimented exclusively by a terrigenous detritus mainly derived from continental basement source areas. The heavy mineral assemblage of the Mt. Caio Flysch Fm is characterized by picotite, that of the Mt. Venere-Monghidoro, Solignano and Mt. Cassio Flysch Fms commonly contains straurolite, garnet and chloritoid, generally considered to be typical products of the Adriatic continental margin. The calcareous nannofossils biostratigraphy indicated that the flysch sedimentation started during the Late Campanian and ended between the Paleocene (Mt. Cassio Flysch Fm and Mt. Venere-Monghidoro Fms) and the Middle Eocene (Mt. Caio Flysch Fm). We propose a schematic paleogeographic restoration for the External Ligurian Domain which implies a more internal position for the Mt. Caio succession and a more external one for the Mt. Venere-Monghidoro and Mt. Cassio successions. The Helminthoid flyschs sedimented after and during deformation and subduction phases in perched and fore-arc basins partly overlying the marginal part of the Adriatic plate. The External Ligurian nappes’ stacking consists, in the study area, from the bottom, of the following units: Caio Unit, Val Baganza Ophiolitic Unit, Monghidoro Unit, Cassio Unit. This pile of thrust-nappes, sealed by the Epiligurian succession, has been already realized before Late Eocene. In our opinion it was generated by a frontal west-verging frontal accretion process (offscraping), which let the flysch successions remain, in this phase, quite undeformed. This westverging thrusting phase, starting from the Middle-Late Eocene, has been followed by an important folding event which generated striking hectometric and kilometric ‘Apenninic’ reverse folds, sometimes associated with NE-verging thrust surfaces. The Oligocene and post-Oligocene evolution is characterized by a block-translation of the Ligurian staking over the Subligurian, Tuscan and Umbrian Domains, associated with a new generation of minor thrusts and thrust related Apenninic folds. © 2000 Éditions scientifiques et médicales Elsevier SAS     

New insights into the size and timing of the Lawn Hill impact structure: relationship to the Century Zn – Pb deposit

The Lawn Hill circular structure in northwest Queensland contains unambiguous evidence of an extraterrestrial impact, including planar deformation features in quartz, impact diamonds, widespread shatter cone formation and impact melt breccia in the Mesoproterozoic basement. The question of its relevance to ore genesis is investigated because the world-class Century Zn – Pb deposit is situated at the conjunction of the 100+ km Termite Range Fault and the previously defined margin of the impact structure. The impact structure is considered to be a 19.5 km wide feature, this constrained in part by the outer margin of an annulus of brecciated and highly contorted limestone. New evidence is presented indicating impact into this Cambrian limestone, including: (i) ‘dykes’ of brecciated Cambrian limestone extending hundreds of metres into the Mesoproterozoic basement; (ii) highly contorted bedding in the limestone annulus compared with essentially undeformed limestone away from the impact site; as well as (iii) a 1 Mt megaclast of Mesoproterozoic Century-like ore suspended in the limestone. Through aerial photograph analysis, large-scale convoluted flow structures within the limestone are identified, and these are interpreted to indicate that parts of the Cambrian sequence may have been soft or only semi-consolidated at the time of impact. This highly contorted limestone bedding is suggested to represent slump-filling of an annular trough in response to impact-induced partial liquefaction of a sediment veneer. The age of impact is therefore considered to be concurrent with limestone formation during the Ordian to early Templetonian, at 520 – 510 Ma. Formation of the Century deposit is found to be unrelated to impact-generated hydrothermal activity, although some minor hydrothermal remobilisation of metals occurred. However, there was macro-scale remobilisation of gigantic ore fragments driven by impact-induced lateral and vertical injection of limestone into the Proterozoic sediments. The limestone-filled annular trough surrounds a 7.8 km diameter central uplift, consistent with formation of a complex crater morphology.     

Assessment of chemical properties and hydro-geochemical coefficients at the Qareh Sou Basin,Golestan Province,Iran

This paper focuses on the Qareh Sou Basin in Golestan Province, Iran. Golestan Province is the third largest cereal producer in Iran and water scarcity and salinity are major problems in this area. This study attempts to facilitate the comprehension of system behavior with respect to water quality issues and hydro-geochemical coefficients within the Qareh Sou Basin. This study was carried out during the year 2010. Various parameters, such as pH, EC, chloride, sulfate, bicarbonate, sodium, potassium, calcium and magnesium have been determined for evaluation purposes. Then, Ca/Mg, Na/Cl, Mg/(Ca + Mg), Ca/HCO₃, (Ca + Mg)–(HCO₃ + SO₄), (Na + K)–Cl, (Ca + Mg + Na + K)–Cl, HCO₃ + SO₄, Ca + Mg and chloro-alkaline indices (CAI) were calculated. Results show that cation exchange probably is an important factor in the hydrochemistry and silicate mineral weathering. Also, CAI-1 plot against CAI-2 demonstrates that most of samples have positive values which suggest normal ion exchange in the system. The carbonic acid is the main agent of calcite, limestone and dolomite weathering which occurs in some stations. According to Chadha’s diagram, the type of water is determined as Ca–Mg–HCO₃.     

Phase relations in the carbon-saturated C–Mg–Fe–Si–O system and C and Si solubility in liquid Fe at high pressure and temperature: implications for planetary interiors

The phase and melting relations of the C-saturated C–Mg–Fe–Si–O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = ?1.7 to ?1.2, where ΔIW is the deviation of the oxygen fugacity (fO₂) from an iron-wüstite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < ?3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe–Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO₂, whereas the solubility of Si decreased with increasing fO₂. The carbon-bearing phases were graphite, Fe₃C, SiC, and Fe alloy melt (Fe–C or Fe–Si–C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < ?6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe–Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth’s core.     

Effects of mineralogical and textural characteristics of ilmenite concentrate on synthetic rutile production

The effect of mineralogy and texture of Qara-aghaj ilmenite concentrate on titanium dioxide prepared via reduction-slagging acid leaching process as a raw material in chloride route was investigated. The concentrate contains 44.5 % TiO₂ and its content in ilmenite lattice varies from 41.6–48 %. Hematite exsolved lamellae inside ilmenite which affect the reduction process positively are host of the most of the Cr and V as pigment colorizer metals. Apatite fine inclusions inside ilmenite as the source of Ca and P could have negative effects on synthetic rutile. Spinel ultrafine particles inside ilmenite containing Al and Si could also affect the synthetic rutile negatively. The other important elements which have been substituted in ilmenite lattice are Mg and Mn. The prepared titanium dioxide concentrate containing 91 % TiO₂ and 0.6 % Fe₂O₃ is mainly formed by rutile and small amount of anatase and Ti₂O₃ phases. The solid solution of rutile inside Ti₂O₃ was also observed. The content of Cr, V, Mn, and Al are decreased to permissible amount during slagging and leaching process while the quantity of other impurities such as Mg, Si, and Ca are relatively high in the product, and they cause some difficulties in pigment production via chloride route. The Mg and Ca sourced from ilmenite lattice and apatite inclusions, respectively, can affect the precipitation process. So, it is predicted that Qara-aghaj ilmenite concentrate will be suitable for sulfate route, but it is necessary to investigate comprehensively.