Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S₁). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north–south shortening (D₂), open to isoclinal south–southeast-oriented F₂ folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D₂ Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D₂ history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.     

1.8–1.5-Ga links between the North and South Australian Cratons and the Early–Middle Proterozoic configuration of Australia

The Archaean and Early–Middle Proterozoic (1.8–1.5 Ga) terranes of the North Australian Craton and the South Australian Craton are separated by 400 km of ca. 1.33–1.10-Ga orogenic belts and Phanerozoic sediments. However, there is a diverse range of geological phenomena that correlate between the component terranes of the two cratons and provide evidence for a shared tectonic evolution between approximately 1.8 and 1.5 Ga. In order to honour these correlations, we propose a reconstruction in which the South Australian Craton is rotated 52° counterclockwise about a pole located at 136°E and 25°S (present-day coordinates), relative to its current position. This reconstruction aligns the ca. 1.8–1.6-Ga orogenic belts preserved in the Arunta Inlier and the Gawler Craton and the ca. 1.6–1.5-Ga orogenic belts preserved in the Mount Isa Block and the Curnamona Province. Before 1.5 Ga, the South Australian Craton was not a separate entity but part of a greater proto-Australian continent which was characterised by accretion along a southward-migrating convergent margin (ca. 1.8–1.6 Ga) followed by convergence along the eastern margin (ca. 1.6–1.5 Ga). After 1.5 Ga, the South Australian Craton broke away from the North Australian Craton only to be reattached in its current position during the ca. 1.33–1.10 Ga-Albany–Fraser and Musgrave orogenies.     

Palynology of Archean microfossils (c. 3.0 Ga) from the Mount Grant area, Pilbara Craton, Western Australia: Further evidence of biogenicity

Palynological techniques have only rarely been applied to Archean rocks, mainly because of concerns about contamination. However, where microstructures are abundant and well documented, palynology can add considerably to interpretations based on thin-section examination. A modified palynological preparation method, which avoids vigorous physical or chemical methods that might cause the fragmentation of fragile specimens, was applied to black chert from the c. 3.0 Ga Farrel Quartzite of the Mount Grant area, Pilbara Craton, Western Australia, known to contain abundant microstructures of putative biogenic origin.Four main morphological types have been reported from chert thin sections: threads, films, hollow spheres and spindles. These microstructures exhibit morphological and chemical signatures consistent with a biological origin as well as showing features indicative of taphonomic degradation. Nevertheless, the possibility remains that they are artefacts. Similar structures have been attributed to physical processes, such as the accretion of fine particulate matter that has subsequently been redistributed as a result of crystal growth.By extracting structures palynologically, we have increased the probability of a biogenic origin. Firstly, examination of the morphology free of the encompassing matrix demonstrates that they are indeed three-dimensional structures with complex morphology; secondly, the extraction of entire specimens militates against them being particulate matter aggregations, which would simply disintegrate during preparation and thirdly, the chemicals used remove silicates and most other minerals, confirming the organic composition of the structures. To date, we have extracted films, hollow spheres and a few poorly preserved spindles, but preparation continues. The fact that the specimens retain their morphological integrity during palynological extraction argues strongly in favour of a biogenic origin for the microstructures.     

3.1 Ga tuff from the Sholl Belt in the West Pilbara: further evidence for d diachronous volcanism in the Pilbara Craton of Western Australia

An age of 3112 ± 6 (2σ) Ma, determined by conventional techniques on single zircons from a felsic volcanic rock from the Sholl Belt in the western part of the Archaean Pilbara Craton of Western Australia, is interpreted as the age of felsic volcanism. This is about 100 Ma older than felsic volcanic rocks in the nearby but unconnected Whim Creek Belt and is significantly younger than felsic volcanism in the East Pilbara, which took place during two distinct episodes at 3450 Ma and 3300 Ma. The present results rule out previous correlations between the felsic volcanic rocks of the Sholt Belt and the 3452 ± 16 Ma old Duffer Formation in the East Pilbara.     

Palaeomagnetic study of Archaean Banded Hematite Jasper Rocks from the Singhbhum-Orissa Craton, India

The Banded Hematite Jasper Formation within the Iron Ore Supergroup of the Singhbhum Craton in eastern India comprises fine alternating layers of jasper and specularite. It was deposited at 3000 Ma and deformed by a mobile episode at 2700 Ma. Hematite pigment (<1 μm) mixed with cryptocrystalline silica and specularite (> 10 μm) is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts (1–2%) of magnetite are present. Palaeomagnetic study identifies a dual polarity remanence resident in hematite (D/I = 283/60°, α₉₅ = 12°) which predates deformation. Studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite. The palaeopole (32°E, 24°N, dp/dm = 14/18°) records the earliest post-metamorphic magnetisation event in the Orissa Craton. A minimum apparent polar wander motion of the Orissa-Singhbhum craton of through 80° is identified during Late Archaean times (2900-2600 Ma).     

Controls on the emplacement and genesis of the MKD5 and Sarah’s Find Ni–Cu–PGE deposits,Mount Keith,Agnew–Wiluna Greenstone Belt,Western Australia

The Mount Keith (MKD5) nickel sulfide deposit is one of the largest komatiite-hosted nickel sulfide deposits in the world; it is hosted by a distinctive spinifex-free, cumulate-rich, ultramafic horizon/unit termed the Mount Keith Ultramafic (MKU). The Mount Keith Ultramafic shows significant variation along its lateral extent. The internal architecture is made up of adcumulate-textured pods and lenses, which are flanked by thinner meso- and orthocumulate-textured units, overlain by pyroxenitic and gabbroic horizons. The lateral and vertical changes in the geometry and internal architecture reflect variations in the lithological association and emplacement conditions along the strike extent of the belt. The chilled margins of the Mount Keith Ultramafic unit contain ∼1,200 ppm Ni. Olivine cumulates average ∼2,500–3,500 ppm Ni, with few exceptions (Ni > 4,500 ppm) reflecting occurrence of minor nickel sulfides, whereas pyroxenites and gabbros generally contain, respectively, ∼1,500–2,000 and ∼100–1,000 ppm Ni. Olivine cumulates generally contain low Cr concentrations (<2,500 ppm Cr), with the rare presence of chromite-rich intervals containing anomalously high values (>5,000 ppm Cr). The internal stratigraphy of the Mount Keith Ultramafic unit may be subdivided into two groups based on rare earth element distribution. The chilled margins and the internal units of the Main Adcumulate domain display LREE-enriched patterns [(La/Sm) _n  > 1–3] and negative Eu, Hf, Zr, Nb, and Ti anomalies. The internal units in the Western Mineralized Zone generally display flat chondrite-normalized REE patterns and only minor negative Nb anomalies. The pattern of platinum-group element (PGE) distribution varies greatly along the strike extent of the Mount Keith Ultramafic unit. The chilled margins display relatively low absolute concentrations [PGE (excl. Os) ∼16 ppb] and relatively fractionated patterns, with subchondritic Pt/Pd ratios (∼1.5), and superchondritic Pd/Ir ratios (∼3). The PGE trends in the thick adcumulate-textured pods containing widespread nickel sulfide mineralization display positive correlation with sulfide abundance, whereas fractionated pyroxenites and gabbros in the thinner domains display highly depleted PGE concentrations and generally show compatible PGE trends. The nickel sulfide ore typology and style vary greatly along the strike extension of the Mount Keith Ultramafic unit. Basal massive nickel sulfide mineralization (e.g., Sarah’s Find) occurs in the thinner meso- and orthocumulate-textured units, whereas stratabound disseminated nickel sulfide mineralization (e.g., MKD5 Ni Deposit) is hosted in the adcumulate-textured pods. We hypothesize that the very low PGE content of the initial liquid of the Mount Keith Ultramafic unit indicates that the initial magma pulse that penetrated through the dacite host-rock had already equilibrated with sulfides at depth and/or carried entrained immiscible sulfide blebs. We argue that upon emplacement, the intruding magma experienced a significant thermal shock at the contact with water-saturated volcaniclastic breccias. The sudden chilling would have increased the viscosity of the magma, possibly to the point where it was no longer able to sustain the suspension of the immiscible sulfide liquid. As a result, the sulfide blebs coalesced and formed the basal massive sulfide nickel sulfide mineralization at the base of the sill (i.e., Sarah’s Find). Prolonged focused high volume magma flow within the sill resulted in the emplacement of a thick, lens-shaped accumulation of olivine adcumulate. Local variations in intensive parameters other than crustal assimilation (e.g., T, fO₂, fS₂) may be principally responsible for sulfide supersaturation and controlled the local distribution of stratabound disseminated nickel sulfide mineralization (e.g., MKD5 Ni Deposit), generally localized within the core of the thicker dunite lenses.     

Microbial remains in some earliest Earth rocks: Comparison with a potential modern analogue

Carbonaceous matter (CM) from ca. 3.5 Ga hydrothermal black cherts of the Pilbara Craton of Western Australia and the Barberton Greenstone Belt of South Africa yielded transmission electron microscopy (TEM) images that are suggestive of microbial remains and possible remnants of microbial cell walls. These are compared to a potential modern analogue, the hyperthermophilic Methanocaldococcus jannaschii, derived from an active seafloor hydrothermal environment and cultured under similar conditions. A striking resemblance to the early Archaean forms was evident in wall structure and thermal degradation mode. Cell disintegration of the cultures occurred at 100 °C marking the limits of life. Complete disintegration, deformation and shrinkage occurred at 132 °C. A multidisciplinary approach to the characterisation of the CM was undertaken using organic petrology, TEM coupled with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and elemental and carbon isotope geochemistry. Reflectance measurements of the CM to determine thermal stress yielded a range of values corresponding to several populations, and pointing to different sources and processes. The δ¹³C values of Dresser Formation CM (−36.5 to −32.1‰) are negatively correlated with TOC (0.13–0.75%) and positively correlated with C/N ratio (134–569), which is interpreted to reflect the relative abundance of high R_o/oxidised/recycled CM and preferential loss of ¹²C and N during thermal maturation. TEM observations, inferred carbon isotopic heterogeneity and isotope fractionations of −27 to −32‰ are consistent with the activity of chemosynthetic microbes in a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the CM.     

The Mesoarchean emergence of modern-style subduction

Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated.In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process.     

A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India

The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of D_m = 148.1, I_m = + 48.6 (K = 22.2, α₉₅ = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.     

Linking uplift and mineralisation at the Mount Novit Zn-Pb-Ag Deposit,Northern Australia: Evidence from geology,U–Pb geochronology and sphalerite geochemistry

The subeconomic Mount Novit Zn-Pb-Ag deposit is located approximately 20 km south of Mount Isa, Queensland. In contrast to the nearby Mount Isa, Hilton and George Fisher Zn-Pb-Ag deposits, mineralisation at Mount Novit is situated to the west of the regional-scale Mount Isa Fault and is hosted in the Moondarra Siltstone as opposed to the Urquhart Shale. Lower-grade (<4 wt.% Zn + Pb) Zn-Pb-Ag mineralisation primarily replaces pre-existing carbonate alteration and veining and consists of pyrrhotite, pyrite and sphalerite with lesser galena. Higher-grade (>10 wt.% Zn + Pb) mineralisation occurs as a matrix supported breccia dominated by sphalerite and pyrrhotite with galena, pyrite, and magnetite. In-situ U–Pb geochronology was completed on apatite and two textural varieties of monazite. Fine-grained (<50 µm) subhedral to anhedral monazite is located within highly foliated biotite alteration directly adjacent Zn-Pb-Ag mineralisation and yields a mean weighted ²⁰⁷Pb/²⁰⁶Pb age of 1527 ± 18 Ma (MSWD = 1.06). This age is consistent with the formation of highly foliated biotite alteration during D₃ deformation of the Isan Orogeny. Apatite from the same fabric yields a lower intercept age of 1443 ± 29 Ma (MSWD = 1.30). Consistent with previous studies, this age is interpreted to represent the age of a major thrusting event along the Mount Isa Fault that resulted in the cooling of the Mount Novit area below ～375 °C. Coarse-grained monazite is coeval with Zn-Pb-Ag mineralisation and yields a mean weighted ²⁰⁷Pb/²⁰⁶Pb age of 1457 ± 11 Ma (MSWD = 0.28). Sphalerite from Mount Novit has low concentrations (<1 ppm) of Ge and Ga and a relatively high concentration of In (5 to >10 ppm), possibly reflecting the leaching of the metals from an underlying basement unit. The GGIMFis geothermometer (Frenzel et al., 2016) produced a mean formation temperature of 345 ± 52 °C. The timing and temperature of Zn-Pb-Ag mineralisation is consistent with the age and cooling temperature of apatite presented in this study. Based on these correlations, we suggest that Zn-Pb-Ag mineralisation at Mount Novit was emplaced during an episode of major thrusting along the Mount Isa Fault, with the precipitation of Zn-Pb-Ag mineralisation driven by the cooling of the Mount Novit area below ～375 °C. A key implication of this study is a new model for synorogenic Zn-Pb-Ag mineralisation to the south of Mount Isa, which contrasts with the widely accepted regional-scale syngenetic metallogenic model.     

The Archaean High-Mg Diorite Suite: Links to Tonalite-Trondhjemite-Granodiorite Magmatism and Implications for Early Archaean Crustal Growth

The 2·95 Ga Pilbara high-Mg diorite suite intrudes thecentral part of the Archaean granite–greenstone terrainof the Pilbara Craton, Western Australia, and shows many featurestypical of high-Mg diorite (sanukitoid) suites from other lateArchaean terrains. Such suites form a minor component of Archaeanfelsic crust. They are typically emplaced in late- to post-kinematicsettings, sometimes in association with felsic alkaline magmatism,and are either unaccompanied by, or post-date, tonalite–trondhjemite–granodiorite(TTG) magmatism, which comprises a much greater proportion ofArchaean felsic crust. The TTG series comprises sodic, Sr-richrocks with high La/Yb and Sr/Y ratios, thought to result frompartial melting of eclogite facies basaltic crust. High-Mg dioriteshares these characteristics but has significantly higher mg-number(     

Evidence for Early LREE-enriched Mantle Source Regions: Diverse Magmas from the c. 3{middle dot}0 Ga Mallina Basin, Pilbara Craton, NW Australia

The c. 2·97–2·95 Ga magmatic history ofthe Mallina Basin, in the Pilbara Craton of NW Australia, includeswhat is perhaps the most lithologically diverse magmatism ofany similar-sized Archaean terrain, and is unusual for similar-sizedterrains of any age. The magmatism includes light rare earthelement (LREE)-rich basaltic rocks, LREE-rich gabbros and rockswith boninite-like compositions (collectively the ‘Mallinamafic suite’), and high-Mg diorites (sanukitoids). TheMallina mafic suite is characterized by high primitive mantlenormalized (La/Nb)_PM (>3) and (La/Yb)_PM (>2), and non-radiogenicNd-isotopic compositions (     

Evidence for eukaryotic diversification in the 1800 million-year-old Changzhougou Formation, North China

Shales and silty shales of the 1800 million-year-old Changzhougou Formation (lowermost Changcheng Group) in the Pangjiapu Region of North China contain well-preserved, acritarchs. These microbial body fossils, although sphaeromorphs, have a size and fine-scale morphological complexity that would attribute them to the eukaryotic domain on the tree of life. Sphaeromorphs range in size from 62 μm to 216 μm. Shapes include spheroidal, ellipsoidal, and fusiform. Some have medial splits indicating excystment and others have a complex wall structure with three or more wall layers. Thirteen distinct morphological entities have been identified. These results indicate that eukaryotic microbial life was well established by close of the Paleoproterozoic and had already undergone a moderate amount of diversification.     

Age and autochthonous evolution of the Sunsás Orogen in West Amazon Craton based on mapping and U–Pb geochronology

The West Amazon Craton consists of rocks of the Sunsás Orogen and the Rondônia-Juruena Province. The Sunsás Orogen comprises the western part of the Amazon Craton in South America and is best exposed in eastern Bolivia and western Rondônia and Mato Grosso states of Brazil. The integration of available maps and isotopic data together with new U–Pb and Sm–Nd analyses from 20 samples (plus 55 earlier dates), establish the timing of geologic events in the West Amazon Craton from 1840 to 1110 Ma. To unravel the complex geologic history of the study area, we primarily sampled granitoids and gneisses to develop a better stratigraphy and secondarily to narrow the age gaps between known discordances. Four periods of orogenic activity are identified within the Sunsás Orogen: 1465–1427 Ma (Santa Helena orogeny), 1371–1319 Ma (Candeias orogeny), ca. 1275 Ma (San Andrés orogeny), and 1180–1110 Ma (Nova Brasilândia orogeny). Notable is the absence of an Ottawan orogeny (1080–1020 Ma) equivalent. In the Rondônia-Juruena Province three main orogenies are recognized: the Juruena (1840–1780 Ma), the Jamari (1760–1740 Ma) and the Quatro Cachoeiras (1670–1630 Ma). Post-Sunsás rocks include Rondônia tin granites, Palmeiral sandstones, Nova Floresta basalt, and alkalic pipes.All inherited U–Pb ages of zircon and all exposed pre-Sunsás rocks in Bolivia have ages that correlate well to the neighbouring Rondônia-Juruena Province. This fact, together with the absence of fragments of older, Archean and Trans-Amazonian crust, suggests that the Sunsás Orogen is autochthonous and evolved over a continental margin formed dominantly by rocks of the Jamari (1760–1740 Ma) and Quatro Cachoeiras (1670–1630 Ma) orogenies plus rocks of the post-tectonic Serra Providência Suite (1560–1540 Ma). Almost all granulites known in Eastern Bolivia and in neighbouring area in Brazil are not basement rocks, but were formed during the Mesoproterozoic and are mainly associated with the Candeias orogeny (1371–1319 Ma). Dated samples of the Chiquitania and Lomas Manechi Complexes in Bolivia revealed a variety of ages and types of ages (metamorphic, magmatic, and inherited) indicating that those two units require more study. There is no evidence for the existence of a Paraguá Craton or Paraguá Block, which is almost totally composed of arc-related granites also formed during the Candeias orogeny.The main difference between the Sunsás Orogen and the Grenville Orogen of Laurentia is the absence in Amazonia of an Ottawan-equivalent orogeny (1080–1020 Ma). The existence of age-equivalents of the Candeias and Santa Helena orogenies in Laurentia (Pinwarian orogeny and rocks of the Eastern Granite-Rhyolite Province and the Composite Arc Belt) indicates that the connection of the two continents may have started from about 1450 Ma. In addition, the two belts may not have been directly juxtaposed, but instead, that one may have been the extension of the other during the Mesoproterozoic. The possibility that Amazonia joined the southwestern part of Laurentia also provides a good fit for the Hudson-Tapajós and Mazatzal-Yapavai-Rondônia-Juruena Provinces. This possible link to Laurentia may have started during the formation of the Trans-Hudson Orogen and its correlative Rondônia-Juruena and Tapajós provinces from about 1900 Ma.     

Geochemistry of the Neoarchaean mafic volcanic rocks of the Geita area, NW Tanzania: Implications for stratigraphical relationships in the Sukumaland greenstone belt

Geochemical data are presented for a suite of mafic volcanic rocks from the Geita area in the Sukumaland greenstone belt (SGB) of northwestern Tanzania with the aim of constraining their petrogenesis, tectonic setting and to assess a possible genetic link with mafic volcanic rocks from the Rwamagaza area also from the SGB previously reported by [Manya, S., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchaean Sukumaland greenstone belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton. Precambrian Research 121, 35–45] and [Manya, S., 2004. Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland greenstone belt, northwestern Tanzania. Journal of African Earth Sciences 40, 269–279]. Mafic volcanic rocks from the two locations in the SGB show similar geochemical and Nd-isotopic compositions. Trace element and Nd-isotope compositions are consistent with their generation from a depleted MORB mantle (DMM) source which had been metasomatised by a subduction component in a late Archaean back arc setting at 2823 Ma.These findings are at variance with the previously proposed lithostratigraphical framework in the SGB which postulated an inner arcuate belt dominated by lower Nyanzian mafic volcanic rocks and an outer belt dominated by upper Nyanzian chemical sedimentary rocks, rare felsic flows and shales. The presence of mafic volcanic rocks flanking the outer belt which are of similar composition and age as those of the inner belt suggests that mafic volcanics in the SGB form discontinuous patches of rock distributed throughout the belt and separated by intervening granites. Furthermore, they corroborate previous evidence that both the rocks of the inner and outer belt formed more or less coevally and the subdivision of the volcano-sedimentary package of the SGB (and other greenstone belts of the Tanzania Craton) into a lower mafic volcanic dominated unit and an upper felsic volcanic and BIF dominated unit is not stratigraphically valid.     

Correlating multiple deformation events across the Mesoproterozoic NE Australia using foliation intersection axes (FIA) preserved within porphyroblasts

Foliation Intersection/Inflection Axes within porphyroblasts (FIAs) allow the chronological and kinematic linking of deformation episodes with associated metamorphism. Measurement of FIAs in the Mesoproterozoic Eastern Fold Belt (EFB) of the Mount Isa Inlier, NE Australia, has revealed phases of deformation and metamorphism that could not previously be distinguished from one another. Both the ‘asymmetry switch’ and ‘FitPitch’ FIA measurement techniques have been applied to key localities of polymetamorphosed and multiply deformed EFB, and they yielded the same result. These independent techniques have revealed (1) E–W trending structures that formed during N–S bulk shortening (D₁) and associated metamorphism (M₁) formed during a period of orogenesis (O₁) and N–S oriented structures that formed during E–W bulk shortening (D₂) and associated metamorphism (M₂) during a period of orogenesis (O₂), and (2) the crustal scale tectonic processes associated with polymetamorphism. Middle to upper amphibolite facies metamorphic conditions occurred during O₁ with crustal thickening followed by near-isothermal decompression leading to low-pressure/high-temperature (LP/HT) conditions with the emplacement of Williams and Naraku Batholiths around 1550 Ma. This was followed by a second period of middle-to upper-amphibolite facies metamorphism, O₂. This history not only correlates better across the EFB, but also with the tectono-metamorphic model recently proposed for the Mesoproterozoic Georgetown Inlier of the north Australian Craton.     

Transport of trace elements under different seasonal conditions: Effects on the quality of river water in a Mediterranean area

Concentrations of total and dissolved elements were determined in 35 water samples collected from rivers in Sardinia, a Mediterranean island in Italy. The overall composition did not change for waters sampled in both winter and summer (i.e., January at high-flow condition and June at low-flow condition), but the salinity and concentrations of the major ions increased in summer. Concentrations of elements such as Li, B, Mn, Rb, Sr, Mo, Ba and U were higher in summer with only small differences between total and dissolved (i.e., in the fraction <0.4 μm) concentrations. The fact that these elements are mostly dissolved during low flow periods appears to be related to the intensity of water–rock interaction processes that are enhanced when the contribution of rainwater to the rivers is low, that is during low-flow conditions. In contrast, the concentrations of Al and Fe were higher in winter during high flow with total concentrations significantly higher than dissolved concentrations, indicating that the total amount depends on the amount of suspended matter. In waters filtered through 0.015 μm pore-size filters, the concentrations of Al and Fe were much lower than in waters filtered through 0.4 μm pore-size filters, indicating that the dissolved fraction comprises very fine particles or colloids. Also, Co, Ni, Cu, Zn, Cd and Pb were generally higher in waters collected during the high-flow condition, with much lower concentrations in 0.015 μm pore-size filtered waters; this suggests aqueous transport via adsorption onto very fine particles. The rare earth elements (REE) and Th dissolved in the river waters display a wide range in concentrations (∑REE: 0.1–23 μg/L; Th: <0.005–0.58 μg/L). Higher REE and Th concentrations occurred at high flow. The positive correlation between ∑REE and Fe suggests that the REE are associated with very fine particles (>0.015 and <0.4 μm); the abundance of these particles in the river controls the partitioning of REE between solution and solid phases.Twenty percent of the water samples had dissolved Pb and total Hg concentrations that exceeded the Italian guidelines for drinking water (>10 μg/L Pb and >1 μg/L Hg). The highest concentrations of these heavy metals were observed at high-flow conditions and they were likely due to the weathering of mine wastes and to uncontrolled urban wastes discharged into the rivers.     

Contrasted tectonic styles for the Paleoproterozoic evolution of the North China Craton. Evidence for a 2.1 Ga thermal and tectonic event in the Fuping Massif

Structural analysis along with ⁴⁰Ar–³⁹Ar and U–Pb datings in the Fuping massif provide new insight into the evolution of the eastern part of the Trans-North China Belt (North China Craton), from 2.7 Ga to 1.8 Ga. D1 is responsible for the development of a dome-and-basin structure coeval with crustal melting giving rise to migmatite and Nanying gneissic granites at 2.1 Ga. This dome-and-basin architecture resulted from the interference between a N–S compression of a weak ductile crust and gravity-driven vertical flow, in a high thermal regime. The next events involved flat lying ductile thrusting (D2) and normal faulting (D3) dated at around 1880 Ma and 1830 Ma, respectively. The D2 and D3 events belong to the Trans-North China Orogeny that results in the final amalgamation of the North China Craton. The D1 deformation is considered as evidence for an earlier orogen developed around 2.1 Ga prior to the Trans-North China Orogeny. The change in the deformation style between the 2.1 Ga and 1.8 Ga could be viewed as a consequence of the cooling of the continental crust in the North China Craton.     

云南玉龙雪山第四纪冰期与冰川演化模式

云南玉龙雪山高峰区有现代冰川１９条,面积１１．６１Ｋｍ＾２,冰川融水汇入金沙江。在现代冰川外围有丰富的古冰川遗迹,可划分３次更新世冰期,即大理、丽江和云杉枰冰期。相当于欧洲阿尔卑斯的玉木里斯和民德冰期。其中以丽江冰期冰川规模最大,当时,河水冰期的白水河冰川长２．５Ｋｍ,冰川 演化模式与梅里雪山、贡嘎山东坡相同,均属藏东南型,而与青藏高原广大高山区以例数第三次冰不川最大规模有别。主要是它们位于青藏高     

Geochemistry of fine-grained clastic sedimentary rocks of the Neoproterozoic Ikorongo Group, NE Tanzania: Implications for provenance and source rock weathering

The Neoproterozoic Ikorongo Group, which lies unconformably on the late Archaean Nyanzian Supergroup of the Tanzania Craton, is comprised of conglomerates, quartzites, shales, siltstones, red sandstones with rare flagstones and gritstones and is regionally subdivided into four litho-stratigraphic units namely the Makobo, Kinenge, Sumuji and Masati Formations.We report geochemical data for the mudrocks (i.e., shales and siltstones) from the Ikorongo basin in an attempt to constrain their provenance and source rock weathering. These mudrocks are compositionally similar to PAAS and PS indicating derivation from mixed mafic–felsic sources. However, the siltstones show depletion in the transition elements (Cr, Ni, Cu, Sc and V) and attest to a more felsic protolith than those for PAAS and PS. The Chemical Index of Alteration (CIA: 52–82) reveal a moderately weathered protolith for the mudrocks. The consistent REE patterns with LREE-enriched and HREE-depleted patterns ((La/Yb)_CN = 7.3–38.3) coupled with negative Eu anomalies (Eu/Eu* = 0.71 on average), which characteristics are similar to the average PAAS and PS, illustrate cratonic sources that formed by intra-crustal differentiation.Geochemical considerations and palaeocurrent indications suggest that the provenance of the Ikorongo Group include high-Mg basaltic-andesites, dacites, rhyolites and granitoids from the Neoarchaean Musoma-Mara Greenstone Belt to the north of the Ikorongo basin. Mass balance calculations suggest relative contributions of 47%, 42% and 11% from granitoids, high-magnesium basaltic-andesites and dacites, respectively to the detritus that formed the shales. Corresponding contributions to the siltstones detritus are 53%, 43% and 4%.