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• Thin beds of barite crystal fans occur in the basal Ediacaran, East Finnmark, Norway; • Barite formed in a shallow marine to subaerial, freshwater-dominated setting; • Barite's Δ17O and δ18O values are among the lowest in all palaeocontinents; • Barite in cracks and fissures is post-depositional. Basal Ediacaran barite, which has mass-anomalous depleted 17O, supporting Snowball Earth models, likely grew in shallow-marine settings, but sedimentological constraints lack details. Environmental conditions in the Varanger Palaeovalley during the basal Ediacaran are well preserved in the Nyborg Formation, where barite forms < 15 mm high syn syn-sedimentary crystal fans on Archean basement or thin intervening sediments. Sedimentary evidence suggests crystal fans formed in low-energy, very shallow-marine to subaerial facies. Rare earth element and yttrium signatures in associated carbonates suggest non-saline growth environments. Post-depositional fluid-flow replaced crystal fan barite with silica (now quartz) and redeposited barite as: <1 × 0.2 mm "dispersed" grains in sediments below fans; <5 mm grains replacing calcite filling desiccation cracks; <1 mm grains in basement fissures; <3 µm grains replacing matrix calcite; irregular grains in detrital sheet-silicate + calcite grains. Caledonian dissolution-reprecipitation reworked barite into brittle fractures and stylolites. Barite Δ17O values are amongst the most negative (-0.9 – -1.25 ‰) and δ18O values the lowest recorded (9.9 – 16.9 ‰) anywhere, consistent with a high latitude Baltica at ∼ 635 Ma. δ34S values (17.7 – 24.5 ‰) lie within the known range for basal Ediacaran barite. [ABSTRACT FROM AUTHOR]

The article discusses the age and correlation of Neoproterozoic glacial deposits in the Scandinavian Caledonides. Previously, only the glaciogenic Smalfjord Formation in East Finnmark was considered to have evidence for multiple late Precambrian glacial horizons. However, recent research using U-Pb data from the Ottfjället Dyke Swarm suggests that all glacial deposits in the Scandinavian Caledonides, including the Mortensnes Formation in East Finnmark, are of pre-596 Ma age. The author argues that this correlation is consistent with worldwide data from the Cryogenian and Ediacaran Periods and provides evidence to support this claim. The article concludes by stating that the concept of a Varangerian/Varangian Glaciation is no longer valid in the type area. [Extracted from the article]

Three models are evaluated for restoring basement rocks coring tectonic windows (Window-Basement) in the Scandinavian Caledonides; parautochthonous (Model I) and allochthonous (models II/III), with initial imbrication of the Window-Basement post-dating or pre-dating, respectively, that in the external imbricate zone (Lower Allochthon). In Model I, the Window-Basement comes from the eastern margin of the basin now imbricated into the Lower Allochthon, while in models II/III it comes from the western margin. In Model II, the Window-Basement formed a basement-high between Tonian and Cryogenian sediments imbricated into the Middle and Lower allochthons; in Model III deposition of the Lower Allochthon sediments commenced in Ediacaran times. Balanced cross-sections and branch-line restorations of four transects (Finnmark–Troms, Västerbotten–Nordland, Jämtland–Trøndelag, Telemark–Møre og Romsdal) show similar restored lengths for the models in two transects and longer restorations for models II/III in the other transects. Model I can result in c. 280 km wide gaps in the restored Lower Allochthon, evidence for which is not seen in the sedimentology. The presence of <3 km thick alluvial-fan deposits at the base of the Middle Allochthon indicates proximal, rapidly uplifting basement during Tonian–Cryogenian periods, taken as the origin of the Window-Basement during thrusting in models II/III. Model I requires multiple changes in thrusting-direction and predicts major thrusts or back-thrusts, currently unrecognized, separating parts of the Lower Allochthon; neither are required in models II/III. Metamorphic data are consistent with models II/III. Despite considerable along-strike structural variability in the external Scandinavian Caledonides, models II/III are preferred for the restoration of the Window-Basement. [ABSTRACT FROM PUBLISHER]

Three important lithologies occur in two drill wells from the Pan-African basement underlying the Mesozoic Sab’atayn Basin, in a previously undocumented area of the Pan-African, 83 and 90 km NE of known exposures in Yemen. Cores from well 1 include amphibolite, with basaltic to andesitic compositions, affected by crustal contamination during emplacement into a thickened crust. Deeper in the well, an unfoliated dark red monzogranite has a U–Pb zircon age of 628.8 ± 3.1 Ma and a Rb–Sr biotite cooling age of 591.6 ± 5.8 Ma (∼300 °C). Regional constraints suggest emplacement in a transitional tectonic setting with compressional terrane amalgamation followed by extensional collapse. Sm–Nd isotope analysis yields a T DM model age of 1.24 Ga with negative ε Nd values, suggesting the monzogranite is part of the Al Bayda island arc terrane. Cores from well 2 contains a weakly deformed, massive (unbedded) medium grey meta-arkose exhibiting essentially no geochemical signature of weathering and with an almost pure dacitic composition. This rock may have been directly derived from an (extrusive) granitoid that was emplaced prior to, or during terrane amalgamation. A (U–Th–Sm)/He zircon age of 156 ± 14 Ma constrains the time of basement cooling to ∼180 °C, synchronous with basin formation. These lithologies provide new insights in the development of the Pan-African basement of Yemen, extending our knowledge of the nearby surface geology to the subsurface. [ABSTRACT FROM AUTHOR]

This paper presents a new geological map of Kea, in the western Cyclades of Greece. The map shows the three-part tectonostratigraphy typical of many Cycladic islands: (i) Footwall of blue-grey calcitic marble mylonites and likely volcanoclastic-derived schists of the Cycladic Blueschist Unit, almost completely retrogressed to greenschist facies; (ii) Detachment zone of phyllonites, schistderived protocataclasites and foliated cataclasites, white calcitic marble ultramylonites and dolostone (ultra)-cataclasites; (iii) Hanging wall of dolostones, with a variable protocataclastic deformation, and occasional foliated calcitic marbles. In the footwall, relict blueschist facies assemblages are rare, being best preserved in gneisses and Fe-Mn-rich rocks (both uncommon lithologies). The bluegrey calcitic marble mylonites, which are generally thicker than elsewhere in the Western Cyclades are more common on the eastern side of the island. This may reflect a lateral ramp geometry between the footwall and overlying detachment-zone. Structural data indicate a consistent top-to-SW to S directed deformation, confirming that the detachment on Kea is part of the West Cycladic Detachment System. Few significant steep faults were observed on the island, although the linear coastlines and valleys suggest that many minor faults or fracture zones may be present. [ABSTRACT FROM AUTHOR]

Abstract: This paper documents a previously unrecorded type of leaf deposit, comprising essentially monospecific linear accumulations of Colophospermum mopane leaves on a point bar of the ephemeral Hoanib River, NW Namibia. In these ‘leaf-jams’, leaf laminae stand on edge, orientated more-or-less normal to bedding. Leaf-jams, which formed upstream of cobbles, clumps of grass and sticks wedged against the former two, were orientated subparallel to the adjacent meandering river-bed, such that over the 40m of their occurrence, their mean azimuth changed by 59° anticlockwise downstream. The longest leaf-jam was 50cm and contained approximately 500 leaves, as well as grass culms, twigs (C. mopane, Tamarix usneoides and unidentified) and medium- to fine-grained sand and silt. Individual leaf-jams were partially buried in the point bar sediments up to a depth of 3cm. Leaf-jam formation occurred in the austral summer of 2006, during the waning stage of a major flood caused by anomalous tropical to extra-tropical storms. Their monospecifity is due to the overwhelming preponderance of the zonal taxon C. mopane in the catchment area, although the Khowarib Gorge contains a quite diverse azonal plant association due to the presence of a permanent water-seep. During leaf-jam formation, the water depth was less than the height of the cobbles (0.1m), with stream flow-rates competent to transport medium-grained sand (velocity estimated at 0.5m s−1). Leaves must have been partially or fully waterlogged to inhibit buoyancy forces tending to lift them out of the developing leaf-jams, which propagated upstream in a manner comparable to longitudinal bars in a braided river. If fossilised, such deposits would probably lead to a very biased interpretation of the composition of the surrounding flora; the correct interpretation would be the one least favoured by palaeobotanists. [ABSTRACT FROM AUTHOR]

The 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi‐equilibration of their major components across the rock matrices during interface‐controlled, size‐independent garnet growth. There is microstructural evidence for foliation‐parallel, small‐scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian‐type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet‐forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes. [ABSTRACT FROM AUTHOR]

Manganese (Mn) dendrites are a common type of mineral dendrite that typically forms two-dimensional structures on rock surfaces. Three-dimensional (3-D) Mn dendrites in rocks have rarely been reported, and so their growth implications have largely escaped attention. Here, we combined high-resolution X-ray and electron-based data with numerical modeling to give the first detailed description of natural 3-D Mn dendrites (in clinoptilolite tuffs) and elucidate their growth dynamics. Our data show that 3-D dendrite growth occurred by accretion of Mn-oxide nanoparticles formed when Mn-bearing fluids mixed with oxygenated pore water. The geometry of the resulting structures is sensitive to ion concentrations, the volume of infiltrating fluid, and the number of fluid pulses; thus, 3-D dendrites record the hydrogeochemical rock history. [ABSTRACT FROM AUTHOR]

The equilibrium model has been tested using Barrovian garnet‐zone micaschists of the Kalak Nappe Complex. In our model, equilibrium in the MnNCKFMASHT system was established across the entire rock volume during prograde metamorphism except for garnet, which developed growth zoning preserved at levels controlled by the kinetics of intracrystalline diffusion. The preservation potential of the disequilibrium fluctuations required for nucleation of garnet has been considered in our simulations, using a moving boundary multicomponent diffusion and growth model. Results indicate that the core of garnet that crystallizes during regional metamorphism does not retain the major element composition of the nucleus but reflects the compositional signature of the immediate overgrowth. The differences between the metamorphic conditions of successive garnet growth steps of the samples indicate characteristic trends in their pressure‐temperature evolutions that can be predicted with the equilibrium model. There is some latitude with regard to the absolute metamorphic conditions due to the inherent uncertainty of the thermodynamic data and the approximation of the reactive volume composition. However, the slopes of the pressure‐temperature paths together with systematic trends in the lithological, geochemical, and Lu–Hf garnet whole‐rock isotopic properties of the rocks, as well as their garnet crystal size frequency distributions, enable the identification of the Veidnes, Bekkarfjord, and Kolvik Nappes involved in the Caledonian Orogeny and provide new insight into their metamorphic evolutions. According to our findings, the base of the Kalak Nappe Complex experienced wide‐spread Barrovian‐type metamorphism at c. 420 Ma with a gradient of ∼40 bar/°C and peak conditions of ∼560°C and 6.7 kbar in the Bekkarfjord and Veidnes Nappes, whereas the hinterland‐placed Kolvik Nappe was metamorphosed at peak conditions of ∼590°C and 7.5 kbar. This event was preceded by moderate‐pressure metamorphism at c. 423 Ma resulting in garnet crystallization exclusively in the Bekkarfjord Nappe, along a gradient of ∼20 bar/°C and peak conditions of ∼570°C and 6.0 kbar. We consider both of these metamorphic and deformational episodes to be different stages of the Scandian phase during the Caledonian Orogeny. An additional orogenic event at c. 464 Ma is preserved in the Veidnes Nappe, which was juxtaposed with the Scandian Nappes by repeated out‐of‐sequence thrusting associated with the final continent–continent collision. This nappe provides a unique insight into an earlier stage of the Caledonian Orogeny, indicating a low‐pressure Barrovian‐type metamorphic history with a gradient of ∼15 bar/°C and peak conditions of ∼560°C and 4.5 kbar. [ABSTRACT FROM AUTHOR]

Clinoptilolite is a natural micro-porous zeolite composed of silica and alumina tetrahedra that commonly forms as a devitrification product of glass in volcanic tuffs. Besides applications in the agriculture, building and manufacturing industries, clinoptilolite has been widely studied in veterinary and human medicine because of its positive effects on health. Based on field observations and microstructural investigations, we show, for the first time, strain localization features along faults in natural zeolite tuffs in a clinoptilolite-tuff open-pit mine at Nižný Hrabovec (Slovak Republic). The faults are localized along pre-existing joints with plumose structures that acted as a pathway for the infiltration of mainly Mn- and K-rich fluids. Displacement along the faults created structures that are indicative of both velocity hardening (accommodated by dissolution precipitation creep on SC/SCC′ foliation) and velocity weakening (accommodated by multi-generations of ultracataclasites along principal slip surfaces) behaviors. Rock-fluid interaction was characterized by a high-mobility of K-rich fluids, with K-feldspar decorating the SC/SCC′ foliation, infiltrating fractures in the damage zone of the faults and precipitating as idiomorphic crystals in open cavities and along the fault surface. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast-cortex grains in cataclasites and truncated grains along principal slip surfaces probably suggest that at least some of the faults formed during seismic slip. • We present for the first-time fault localization in clinoptilolite (zeolite).. • Field observations show that the faults in these porous rocks are extremely localized.. • Microstructures are indicative of velocity hardening (SC/SCC′ foliation) and velocity weakening (principal slip surfaces in ultra-cataclasites).. • Microstructures along principal slip surfaces might indicate seismic slip. [ABSTRACT FROM AUTHOR]

The island of Milos is part of the South Aegean volcanic arc. The volcanic processes on Milos, which started ca. 3.5 Ma, have been studied in detail, but the limited outcrops of basement rocks have not yet been adequately investigated. We present new structural, petrological, and geochronological data that suggest that the basement rocks on Milos (Kiriaki complex) belong to the Cycladic Blueschist Unit, which underwent blueschist metamorphism at ~8.5 kbar and 400 °C. The complex experienced pervasive ductile deformation in the Paleocene-Eocene, with top-to-the-S to -SW shearing. Recalculation of published data from eclogite pebbles that occur as detrital products in "Green Lahars" structurally above the Kiriaki complex yields contrasting metamorphic conditions of ~19.5 kbar at 550 °C, suggesting that two nappes, the exposed Lower Cycladic Nappe and eroded relicts of the Upper Cycladic Nappe, exist on Milos. These nappes, which also occur on other Cycladic islands, are separated by the newly defined synorogenic Trans Cycladic thrust. Since Milos is located above the West Cycladic detachment system, the rocks escaped the postorogenic middle-late Miocene low-angled extensional overprint. While rocks in the footwall of the West Cycladic detachment system were experiencing ductile shearing, sediments were being nonconformably deposited on metamorphic rocks of the Kiriaki complex on Milos in the late Miocene. Most importantly, the postoro-genic West Cycladic detachment system did not reactivate but instead crosscut synorogenic structures such as the Trans Cycladic thrust, which bends 90° at Milos, exhibiting a change from W-E- to N-S-trending stretching lineations. [ABSTRACT FROM AUTHOR]