A window to the operation of microplate tectonics in the Tethys Ocean: the geochemistry of the Samothrace granite,Aegean Sea

Summary The island of Samothrace, northeastern Aegean Sea, consists of five main geological units: (i) A basement unit consisting of low grade metamorphic rocks (metapelites, marbles, metavolcanic rocks, and a metaconglomerate); (ii) an ophiolitic complex with K-Ar hornblende date of 154 ± 7 and 155 ± 7 Ma; (iii) A granite intrusion with biotite K-Ar dates of 14.5 ± 0.3 and 14.5 ± 0.5 Ma, and a contact metamorphic event dated at 40.9 + 2.2 Ma; (iv) a unit of Cenozoic volcanic rocks: orogenic volcanism apparently occurred in two cycles with Upper Eocene tholeiitic to calc-alkaline volcanic rocks and post-Eocene high-K andesites to trachytes. (v) Quaternary clastic sedimentary rocks which occur around the peripheral parts of the island. The granitic intrusion is predominantly a hornblende-biotite granite, granodiorite or quartz monzonite, with porphyritic variants and mafic enclaves. The pluton is cut by granophyre, aplite and rare granodioritic veins. All lithological units of the Samothrace intrusion show smooth and continuous major element trends and similar chondrite- and Ocean Ridge Granite-normalized incompatible element profiles. ORG-normalized incompatible element contents of Hf, Zr, Sm are explained with fractionation close to the normalizing values Y and Yb contents combined with high K/Yb ratios; Rb and Th are significantly enriched relative to Nb and Ta. In Y-Nb and Rb-SiO₂ space most samples of the Samothrace granite, plot in the volcanic arc and the syn-collisional granite fields. In Y + Nb-Rb space they are equally distributed within and transgress these two domains. The geochemical and regional data suggest a subduction or collision environment but biotite mineral data do not support a collisional setting for magma genesis. The Samothrace granite was probaby associated with a post-collisional domain after the closure of the Axios section of the Tethys Ocean.

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Ein Einblick in das Wirken von Mikroplattentektonik in der Tethys—Die Geochemie des Samothrake Granites, Agäisches Meer

Zusammenfassung Die Insel Samothrake in der nordöstlichen Ägäis besteht aus fünf geologischen Haupteinheiten: (i) einem schwach metamorphen Basement (Metapelite, Marmore, Metavulkanite und Metakonglomerate); (ii) einem ophiolithischem Komplex, der mit K-Ar Datierungen an Hornblende ein Alter von 154 ± 7 und 155 ± 7 Ma ergab; (iii) ein granitischer Intrusionskörper mit K-Ar Altern an Biotit von 14.0 ± 0.3 und 14.5 ± 0.5 Ma und einem kontaktmetamorphen Ereignis, das mit 40.9 ± 2.2 Ma datiert ist; (iv) eine Abfolge känozoischer Vulkanite, wobei der orogene Vulkanismus offensichtlich in zwei Zyklen ablief mit tholeiitischen bis kalkalkalischen Vulkaniten im oberen Eozän und high-K Andesiten bis Trachyten im post-Eozän; (v) quartären klastischen Sedimentgesteinen, die im Randbereich der Insel auftreten. Die Granitintrusion setzt sich hauptsächlich aus Hornblende-Biotitgraniten, Granodioriten oder Quarzmonzoniten mit teilweise porphyrischen und mafischen Enklaven enthaltenden Varietäten zusammen. Der Pluton wird von Granophyren, Apliten und seltener von granodioritischen Gängen durchschlagen. Alle lithologischen Einheiten der Samothrake Intrusion zeigen kontinuierliche Hauptelementtrends und ähnliche Chondrit und ORG-normalisierte inkompatible Elementprofile. Die Gehalte an den inkompatiblen Elementen Hf, Zr, Sm sind sehr ähnlich denen von ozeanischen Graniten (ORG). Die niedrigen Y und Yb-Gehalte und die hohen K/Yb Verhältnisse werden durch Fraktionierung erklärt. Rb und Th sind signifikant angereichert im Vergleich zu Nb und Ta. In Y-Nb und Rb-SiO₂ Diagrammen plotten die meisten Proben des Samothrake Granites im Feld der vulkanischen Inselbogen- und Synkollisionsgranite. Im Y + Nb-Rb Diagramm zeigt sich eine gleichmäßige und überlappende Verteilung. Die geochemischen und regionalen Daten weisen auf einen Subduktions- oder Kollisionsbereich hin, obwohl die Biotitzusammensetzungen nicht für eine Bildung der Magmen in einem Kollisionsbereich sprechen. Die Bildung des Samothrake Granites steht möglicherweise mit post-Kollisionstektonik nach dem Schließen der Axioszone in der Tethys in Zusammenhang.

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With 7 Figures     

Landslide Research at the British Geological Survey:Capture,Storage and Interpretation on a National and Site-Specific Scale

Abstract: Landslide research at the British Geological Survey (BGS) is carried out through a number of activities, including surveying, database development and real-time monitoring of landslides. Landslide mapping across the UK has been carried out since BGS started geological mapping in 1835. Today, BGS geologists use a combination of remote sensing and ground-based investigations to survey landslides. The development of waterproof tablet computers (BGS·SIGMAmobile), with inbuilt GPS and GIS for field data capture provides an accurate and rapid mapping methodology for field surveys. Regional and national mapping of landslides is carried out in conjunction with site-specific monitoring, using terrestrial LiDAR and differential GPS technologies, which BGS has successfully developed for this application. In addition to surface monitoring, BGS is currently developing geophysical ground-imaging systems for landslide monitoring, which provide real-time information on subsurface changes prior to failure events. BGS’s mapping and monitoring activities directly feed into the BGS National Landslide Database, the most extensive source of information on landslides in Great Britain. It currently holds over 14?000 records of landslide events. By combining BGS’s corporate datasets with expert knowledge, BGS has developed a landslide hazard assessment tool, GeoSure, which provides information on the relative landslide hazard susceptibility at national scale.     

Controls on regional variability in marine pore-water diagenesis below the seafloor in Upper Jurassic–Lower Cretaceous prodeltaic sandstone and shales,Scotian Basin,Eastern Canada

Diagenesis in the uppermost Jurassic to Lower Cretaceous deltaic sandstones and shales of the Scotian Basin is an important control on reservoir quality. Ferruginous zone (sub-oxic) marine pore-water diagenesis controls the initial formation of Fe²⁺-silicates that are the precursors of grain-rimming chlorite that preserves porosity. This study assesses the regional controls on the type of marine pore-water diagenesis by studying the sedimentology, mineralogy, and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin. Coated grains preserve a record of whether marine pore-water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone. Four types of coated grain were distinguished, each with a different mineral paragenesis. Mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite, and the correlation of P with Fe or Ti. Pyrite and Fe-calcite are found where the sulphidic zone is more significant than the ferruginous zone. Ferruginous zone diagenesis was common in low-sedimentation rate retrogradational sediments with low organic carbon, and in delta-front turbidites and river-mouth sandstones. Estuarine, tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower Fe:Ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products. The relative contribution of colloidal iron (hydr)-oxides appears greater in distal low-sedimentation rate environments. Where large changes in sedimentation rate occurred at ravinement surfaces, the underlying progradational rocks have evidence of ferruginous zone diagenesis, whatever their facies. Rapid upward migration of the pore-water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore-water zones. This study has shown that the availability of Fe and organic carbon varying in a complex manner in marine deltaic sediments, but that the resulting diagenesis by marine pore-water can be predicted from facies and paleogeographic setting.     

Geochemistry, tectonic setting and metamorphism of mid-triassic volcanic rocks of Greece

Mid-Triassic volcanic rocks are preserved in about eight localities in the nappes of the external Hellenides, and are also widespread in the Pelagonian zone of central Greece. Most have suffered low-grade metamorphism. Immobile trace elements from 35 samples, 80 major element analyses, and pyroxene analyses show that these rocks are the product of an eastward dipping subduction zone. The subducted ocean was perhaps analogous to the Pennine zone of the Alps. The Othris and Pindos ophiolites are probably contemporaneous products of related back-arc extension.Metamorphic mineral assemblages in volcanic rocks of the Phyllite Series of the external Hellenides indicate high-pressure, low-temperature conditions, probably related to a second subduction that occurred prior to the main phase of Hellenide nappe emplacement in the Miocene.     

Multiply exsolved clinopyroxene megacrysts from the Frank Smith Mine,Cape Province,South Africa

Two clinopyroxene megacrysts from the Frank Smith kimberlite show multiple exsolution of garnet, orthopyroxene and oxide phases. In one of the megacrysts the lamellar texture has partly recrystallized to a granular one. The chemical compositions of the exsolved phases are similar to those of discrete nodules and of granular pyroxenites found in the same kimberlite. Conditions of formation of the original homogeneous clinopyroxene solid solution are estimated to be ～1500°C and 37 kb, whereas the re-equilibration conditions are closer to 900°C and 35 kb. The megacrysts may have belonged to the Karroo age magnetism which predated kimberlite activity and have been brought to the surface as accidental xenocrysts by the Frank Smith kimberlite.     

The relationship between length and width of plutons within the crustal-scale Cobequid Shear Zone, northern Appalachians, Canada

The lengths and widths have been measured for 69 component bodies of composite plutons along the Cobequid Shear Zone. Plutons on major fault strands, those with mylonite zones >0.1 km wide, exhibit evidence of multiple intrusion of magma batches. Small plutons along short faults in stepover zones appear related to rapid emplacement of magma in bodies 1.5–4 km long by 0.1–2 km wide. Such small plutons show low enrichment in incompatible elements in older component bodies, but increasing amounts in younger bodies as a result of progressive magma expulsion from crystal mush during crystallization and shear-enhanced compaction in fault zones. Wider plutons generally occur along longer fault strands accommodating more strain and penetrating deeper into the crust and show enrichment in incompatible elements. The width of the mylonitic fault zone is about 15% of the width of these plutons. The length-to-width ratio of component bodies and composite plutons varies between 2 and 11. The best-fit line describing these data has a slope of 1.056, which implies scaling behavior between plutonism and tectonic processes. Scalar properties of plutonic bodies are similar to those of faults, but scalar relationships observed in component bodies do not apply to composite plutons.     

Triassic rift-related komatiite, picrite and basalt, Pelagonian continental margin, Greece

A wide range of Triassic rift-related volcanic rocks is preserved on the extensional continental margin of the central part of the Pelagonian micro-continental block of Greece, in the region of northern Evia Island and Othris. This part of the continental margin is unusual for the abundance of lavas, the wide range of lava types, and the presence of komatiitic lavas. The predominant rock types are subalkaline basalt and basaltic andesite, mildly alkaline basalt and picrite, and minor komatiite. Four groups of mafic rocks are distinguished from the 32 samples analyzed on the basis of variation in incompatible trace elements, with all but one group including both picritic and basaltic rocks. The geochemical character of the volcanic rocks suggests derivation from inhomogenous, spinel-bearing, lithospheric mantle. This mantle source was variably depleted and moderately refertilized by subduction-related fluids during earlier Hercynian subduction. The picrites were generated by variable degrees of partial melting, which are inferred to inversely correlate with pressure, thus suggesting decompression melting of their source. Evidence is lacking for a major mantle plume, but the enhanced magmatism in this sector of the rifted margin suggests that melting was induced by a large amount of hydration, which is appropriate to generate melts at lower temperatures. Consequent fractionation coupled with crustal assimilation generated the wide range of subalkaline to mildly alkaline mafic rocks.     

Late Paleozoic peperites in West Junggar,China, and how they constrain regional tectonic and palaeoenvironmental setting

Late Paleozoic peperites have been identified for the first time at the bottom of Tailegula Formation in West Junggar, China. This finding is significant for the reconstruction of Late Paleozoic evolution in the Junggar region. The peperites form successions up to 500 m thick interbedded with basaltic lava and sedimentary rocks. Four types of peperites are described and interpreted as resulting from basaltic lava bulldozed into wet, unconsolidated sediments at their basal contacts. Zircon LA-ICP-MS U–Pb dating of a tuff lens enclosed by lava showed that the peperites formed in the Late Devonian (ca. 364 Ma). The peperite-bearing units probably formed at a water depth of less than 3 km and are generally undeformed, occurring in continuous stratigraphic sections distributed regionally over a distance of 100 km on either side of the Darbut and Baijiantan ophiolitic belts, in contrast to the highly deformed slices of ophiolite. They demonstrate that the Darbut and Baijiantan ophiolitic belts should not be interpreted as significant plate boundaries and represent the underlying ocean crust uplifted along tectonic lineaments within a continuous shallow remnant ocean basin. The peperites formed during the spreading phase of the remnant ocean basin and represent the final stages of creation of oceanic crust.