Quaternary iron-manganese deposits and associated pelagic sediments (radiolarian clay and chert, gypsiferous mud) from the Tyrrhenian Sea

Various types of pelagic sediments occur on a dolomitic basement located at some 1000 m depth on the eastern slope of the Tyrrhenian Sea, i.e. in the most internal, collapsed and presently submerged portion of southern Apennines. The deposits include laminated limonites, muds with manganosiderite nodules, radiolarian clays, opal chert, gypsiferous muds, lutites with calcareous plankton, and all are of Quaternary age. They are interpreted as products of the interactions between submarine hydrothermal activity and deep-sea sediments, and represent the first documented case of submarine hydrothermal sediments not directly connected to active ridges or volcanic buildings and deposits. The iron-rich sediments are very similar to many other reported examples of submarine thermal activity. The radiolarian clay displays very abundant and almost exclusively radiolarian tests, and no calcareous fossils, but shows numerous dissolution traces of carbonate skeletons. It has been interpreted as deriving from a primary bloom of siliceous plankton followed by an extensive leaching of the calcareous tests on the seafloor. The radiolarian chert shows a very early and yet unknown Opal-A cement. The immediate source of the cement is biogenous silica, whereas the ultimate source is the bloom of siliceous plankton triggered by the hydrothermal and volcanic activity. The gypsum muds are the result of a hydrothermal chemical remobilization of the Messinian sulphate beds which overlie the dolomitic basement.     

Palynological data on major Holocene climatic events in NW Iberia

Three NW Iberia Cantabrian Mountain pollen records are presented. They reflect the main Holocene climatic shifts in the North Atlantic region as recorded in the isotopic data from Greenland ice, Irish speleothems and reconstructed sea surface temperatures. Two brief forest regression episodes reconstructed from pollen may be synchronous with GH-11.2 and GH-8.2 events. At mid-altitude, two woodland expansion phases (7000-6000 ¹⁴C yr BP and 4000-2500 ¹⁴C yr BP) are separated by a phase of heaths and peat deposits. Major woodland declines occurred during the Galician-Roman Period (which includes the Bronze Age, the Iron Age and the Roman occupation) and from the end of the Medieval Period. The pollen data, backed up by archaeological and historical sources, suggest climatic impact of the Iron Age Cold Period, but are indecisive concerning the Little Ice Age. However, the pollen records do not support any significant 'Neoglacial' period (4000-3000 ¹⁴C yr BP) influence on NW Iberia.     

Mechanisms and Sources of Mantle Metasomatism: Major and Trace Element Compositions of Peridotite Xenoliths from Spitsbergen in the Context of Numerical Modelling

Mineral and whole-rock chemical data for peridotite xenolithsin basaltic lavas on Spitsbergen are examined to reassess mechanismsof melt–fluid interaction with peridotites and their relativerole versus melt composition in mantle metasomatism. The enrichmentpatterns in the xenoliths on primitive mantle-normalized diagramsrange from Th–La–Ce ‘inflections’ inweakly metasomatized samples (normally without amphibole) toa continuous increase in abundances from Ho to Ce typical foramphibole-bearing xenoliths. Numerical modelling of interactionbetween depleted peridotites and enriched melts indicates thatthese patterns do not result from simple mixing of the two end-membersbut can be explained by chromatographic fractionation duringreactive porous melt flow, which produces a variety of enrichmentpatterns in a single event. Many metasomatized xenoliths havenegative high field strength element and Pb anomalies and Srspikes relative to rare earth elements of similar compatibility,and highly fractionated Nb/Ta and Zr/Hf. Although amphiboleprecipitation can produce Nb–Ta anomalies, some of thesefeatures cannot be attributed to percolation-related fractionationalone and have to be a signature of the initial melt (possiblycarbonate rich). In general, chemical and mineralogical fingerprintsof a metasomatic medium are strongest near its source (e.g.a vein) whereas element patterns farther in the metasomatic‘column’ are increasingly controlled by fractionationmechanisms. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; trace elements; theoretical modelling     

The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1{middle dot}0 to 3{middle dot}2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Fluid-absent melting experiments on a zoisite- and phengite-bearingeclogite (omphacite, garnet, quartz, kyanite, zoisite, phengiteand rutile) were performed to constrain the melting relationsof these hydrous phases in natural assemblages, as well as themelt and mineral compositions produced by their breakdown. From1·0 to 3·2 GPa the solidus slopes positively from1·5 GPa at 850°C to 2·7 GPa at 1025°C,but bends back at higher pressures to 975°C at 3·2GPa. The melt fraction is always low and the melt compositionsalways felsic and become increasingly so with increasing pressure.The normative Ab–An–Or compositions of the initialmelts vary from tonalites at 1·0 GPa to tonalite–trondhjemitesat 1·5 GPa, adamellites at 2·1 and 2·7GPa, and to true granites at 3·2 GPa. At pressures <     

Effects of P, f(O2) and Mg/Fe Ratio on Dehydration Melting of Model Metagreywackes

We present results of dehydration melting experiments [3–15kbar, 810–950C f(O₂) QFM (quartz-fayalite-magetite)and Ni-NiO] on two Fe-rich mixtures of biotite (37%), plagioclaseAn₃₈ (27%), quartz (34%) and ilmenite (2%), which differ onlyin their biotite compositions (mg-number 23 and 0.4). Dehydrationmelting of metagreywackes of constant modal composition generatesa wide range of melt fractions, melt compositions and residualassemblages, through the combined effects of pressure, Fe/Mgratio and f(O₂). Crystallization of garnet is the chief controlon melting behavior, and is limited by two reactions: (1) thebreakdown of garnet + quartz to orthopyroxene + plagioclaseat low P, and (2) the oxidation of garnet to magnetite + anorthite+ quartz (enstatite), which is sensitive to both f(O₂) andP. Because of these reactions, melting of Mg-rich metagreywackesis rather insensitive to f(O₂) but strongly sensitive to P;the converse is true for Fe-rich metagreywackes. Garnet crystallizationrequires that plagioclase break down incongruently, liberatingalbite. This increases the Na₂O content of the melts and enhancesmelt production. Thus, melting of metagreywacke in a reducingdeep-crustal environment (with garnet stable) would producemore, and more sodic, melt than would garnet-absent meltingof the same source material in a relatively oxidizing, shallow-crustalenvironment. KEY WORDS: anatexis; metasediments; gneisses; granites; garnet ^*Corresponding author. Telephone: 706-542-2394; fax: 706-542-2425; e-mail: alpatino{at}uga.cc.uga.edu     

Tempestite facies models for the epicontinental Triassic carbonates of the Betic Cordillera (southern Spain)

This study focuses on storm deposits in the Muschelkalk facies of the Betic Cordillera (southern Spain) and interprets their deposition mechanisms. Three types of storm deposit are distinguished: (i) pot/gutter casts; (ii) tempestite beds; and (iii) storm‐winnowed deposits. Each deposit provides information about the carbonate platform environment in which it was deposited. The tempestite models proposed are: (i) the bypass‐zone tempestite model, occurring in a muddy ramp at the epicontinental basin margin. This model is characterized by potholes and gutters that form in a shoreline bypass‐zone during storms; (ii) the gradient‐current tempestite model in which frequent tempestite beds are related to storm gradient currents. Thickness and grain size decrease towards the deep distal ramp; and (iii) the winnowed deposit tempestite model whereby storm deposits are winnowed and deposited in the same environment with only short lateral transport having occurred. This model evokes more restricted and shallower conditions, lagoons or inland seas. The distribution of all these deposits in the stratigraphic sections studied corroborate the eustatic third‐order cycle identified, although the different features of the storm deposits and their positions in each section indicate a subsidence varying in time and space. In the transgressive stage, the margins of the epicontinental basin were a well‐developed ramp with potholes and gutters. In contrast, during the high sea‐level stage, storm deposits generated tempestite beds or storm‐winnowed deposits in the different areas. The epicontinental carbonate platform with ramp edges evolved into a complex depositional system of coastal and shallow‐marine environments with lagoons and restricted inland seas. Thus, the epicontinental platform underwent substantial change from the Late Anisian to the Late Ladinian and this is reflected in its storm deposits.     

Dolomite characteristics and diagenetic model of the Calcari Grigi Group (Asiago Plateau,Southern Alps – Italy): an example of multiphase dolomitization

A multidisciplinary study, conducted over the carbonate platform deposits of the Liassic Calcari Grigi Group (Southern Alps), highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks, to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history. This study was carried out in the Asiago Plateau (southernmost part of the eastern Southern Alps, northern Italy) which provides excellent outcrops of the Jurassic Calcari Grigi Group. The dolomitization of the Jurassic sequence is variable in terms of stratigraphic extension and geographic distribution. In the studied localities the dolomitization is generally limited to the Mount Zugna Formation and is characterized by an undulatory front, with ‘sub‐vertical dolomitic chimneys’ along the major faults. Within this unit, and often associated with faults, stacked high‐porosity and permeability bed‐parallel dolomitic bodies are developed that show excellent petrophysical properties. The dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals. Thin section, cathodoluminescence, isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities. The first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core. The porosity is thought to have increased after this stage, with dark blue luminescent dolomite accompanied by the corrosion of older crystals. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infilling of vugs and the remaining open pores with calcite cement. The diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores. Paragenetic and fluid‐inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation. The integration of the available data seem to indicate that the diagenetic evolution of the study area is related to: (i) the interplay between evolving fluids (from marine to brackish); (ii) the burial of the sequence (increasing temperature); and (iii) the evolution of the hydrogeological system (fault and fracture network, fluid mixing). This complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good, widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements.     

Partial Melting of Aluminous Metagreywackes in the Northern Sierra de Comechingones, Central Argentina

We describe a suite of metamorphic and migmatitic rocks fromthe Sierra de Comechingones (Sierras Pampeanas of Central Argentina)that include unmelted gneisses, migmatites and refractory granulites.The gneisses are aluminous greywackes metamorphosed in the amphibolitegrade and are likely to have been the protoliths for the higher-grademigmatites and granulites. Mineralogical characteristics andmajor and trace element compositions show that metatexite migmatites,diatexite migmatites and granulites are all melt-depleted rocks.The migmatites (both metatexites and diatexites) have undergoneH₂O-fluxed melting and lost