Acid Volcanics Research Articles

The mineralogical and petrological factors in Alkali Silica Reactions in concrete

Reactive silicas which can cause Alkali Silica Reactions (ASR) are opal, Si02 rich volcanic glass, chalcedony (or fibrous quartz) and tridymite. Opal is a hydrous cryptocrystalline or colloidal form of silica (Si02 nHP) with a water content of around 6% to 10%. Silica-rich volcanic glass is a component of many young volcanic rocks. Chalcedony is a compact variety of silica composed of minute (crypto- or micro-crystalline) crystals of quartz (Si02 ) enclosing submicroscopic pores. Tridymite is a rare hexagonal high temperature polymorph of quartz with a S.G. of 2.28. Opaline silica and silica-rich volcanic glass are regarded as the most reactive. Chalcedony, tridymite and some forms of crypto- and microcrystalline quartz occurring in acidic volcanics, chert and flint are regarded as of intermediate reactivity. Strained macrocrystalline quartz can cause ASR but only under certain conditions. A factor which may influence the reactivity of particular aggregate is the proportion of reaction materials that is present in the aggregates. For a very reactive opaline silica the worst expansion may occur when it is found in the order of 2% to 5% (in the coarse aggregate), that is the pessimum. The pessimum for silica-rich volcanic glass is unknown. In the case of the strained macro-crystalline quartz, the pessimum being 100% that is quartzite containing wholly of strained quartz. The pessimum for the intermediate reactive silica is not known, but could well be higher than that of the opaline silica and definitely lower than that of strained quartz.

Review of Neogene and Quaternary volcanism of the Carpathian-Pannonian region

Development of the Pannonian Basins, as well as thrusting and folding of the Carpathians was accompanied by widespread volcanic activity. This volcanism can be divided into three main genetic types: 1. (1) Early Miocene mainly acidic volcanism (tuffs and ignimbrites). The recent spatial separation of these volcanics may be due to large right lateral displacement along the Mid-Hungarian shear zone during the Early Miocene. 2. (2) Middle Miocene-Pliocene calc-alkaline, mainly intermediate stratovolcanic complexes in the Inner Western Carpathians and in the East Carpathians, related to a subducted oceanic slab. Their geochemical compositions show a transitional character between active continental margin and island arc type magmatic rocks. The thickness of the crust increased with time and from west to east beneath this volcanic arc. 3. (3) Pliocene-Pleistocene alkali basaltic volcanism was related to an upwelling, then cooling asthenospheric dome. This is thought to have initiated the thermal regime from which these Melts ascended. The compositional difference in space and in time might reflect the existence of local individual small-sized diapiric bodies as well as several processes (e.g. fractional crystallization, mixing), which modified the original magma. The eruption of these basaltic melts was temporally associated with the final phase of development of the Pannonian Basin. This volcanism was associated with recent uplifts and tectonic rejuvenation of the area.

Mesozoic acid volcanics from southern Angola: petrology, Sr-Nd isotope characteristics and correlation with the acid stratoid volcanic suites of the Paranà basin (south-eastern Brazil)

The Angolan Mesozoic volcanic suites are concentrated along the continental margin and are characterized by voluminous sheet-like acid rock-types overlying stratoid tholeiitic basalts. The acid volcanics closely correspond in terms of stratigraphy, petrology and Sr-Nd isotope compositions to those of Lower Cretaceous age (135±3 Ma) in the northern Parana basin (SE-Brazil). The Mesozoic tholeiitic volcanic suites of Angola (and Namibia), which crop out close to the continental margin represent the easterly extension of the vast volcanic activity of the Parana basin which occurred before the opening of the S. Atlantic ocean

Apparent Magnetisation from Aeromagnetic Data

Magnetic effects associated with the terrain may cause great difficulty in the interpretation of aeromagnetic surveys, especially for draped surveys above a rugged magnetised terrain. A method has been developed that allows (gridded) magnetic field observations and topography (described by a digital terrain model) to be inverted to obtain a map showing the apparent magnetisation of the surficial rocks. The surficial rocks are modelled as either a surface shell of given thickness, or a layer with its upper surface defined by the topography and a flat lower surface of given elevation. Maps of apparent magnetisation, free from topographic effects, may then be used for interpretation, in particular to identify areas of abnormal near surface magnetisation. This method was used to analyse draped data from White Island, an active andesitic volcano in the Bay of Plenty, New Zealand. The analysis showed that although the magnetic data are dominated by topographic effects, several areas of abnormally high near-surface magnetisation are present, including two areas that appear to be associated with the active vent. Subsequent surface sampling has confirmed the presence of highly magnetised rocks in some of these areas. High magnetisation areas in the vicinity of the active vent are interpreted to be zones of magnetite that stabilized below the Curie temperature. The discovery of ejecta containing secondary magnetite supports this interpretation. Model calculations, using a topographic model with magnetisations derived from the inversion, demonstrate that the areas of anomalous magnetisation would not have been resolved in a constant-elevation survey. Magnetic data from an area in the Taupo Volcanic Zone, New Zealand, which is geologically covered by Recent acid volcanics (mainly rhyolites), were also analysed. For this case a digital terrain model was derived directly from the barometric and radar altimeter data recorded during the survey. Apparent magnetisations calculated using a shell model proved unrealistically high. However, a flat-based model of the topography gave apparent magnetisations that were within the range measured for other Taupo Volcanic Zone rhyolites.

Geochemistry, provenance and palaeoclimate of the Sleat and Torridon groups in Skye

Synopsis Arkoses of the Sleat and Torridon groups in south-east Skye form a continuous succession 4500 m thick. Palaeocurrents came from the west. The source rocks were hornblende-biotite-gneisses, granite or granite-gneisses, quartzose sediments and acid volcanics, all with upper crustal geochemical signatures. Detritus from the basic gneisses predominates in the lowest sediments, but is gradually replaced by granitic material stratigraphically up the succession. The lowest sediments accumulated in palaeovalleys with a relief of <200 m. They retain the geochemistry of the parental basic gneisses which formed the local basement. Subsequent rifting brought sediments tens of kilometres from the west, some from as far as the flanking highlands, where acid rocks were common. The climate was initially arid. But the development of rift highlands attracted rainstorms, and the higher runoff leached Ca from hornblende as it weathered to chlorite. Small amounts of Na may also have been lost from plagioclase during weathering. The ratio of material transported in solid form to that in solution is estimated to lie in the range 15–20 for the lowest formation, but to have been <10 higher in the succession. By analogy with very large modern catchments, this suggests latitudes of 22–40 deg. Palaeolatitudes of 30–45 deg. have been deduced for the Torridon Group on the mainland of Scotland, using palaeomagnetism.

Trace and rare earth elements in cassiterite ? sources of components for the tin deposits of the Mole Granite, Australia

Trace and rare earth elements have been determined for cassiterite from deposits associated with the Mole Granite and hosted by granite, metasediments and metavolcanics. The ΣREE of cassiterite is controlled by the ΣREE of the the ore fluid and the rocks through which this fluid circulated. The REE distribution factor and ΣLREE/HREE value of cassiterite is strongly influenced by the associated mineral assemblage, the fluid chemistry and the crystal chemical characteristics of the host mineral. Cassiterite from deposits hosted by granite have trace and rare earth element characteristics similar to those determined for the Mole Granite. Cassiterite from deposits hosted by metasediments or acid volcanics have most trace and rare earth element characteristics similar to those of the enclosing rocks and some characteristics similar to the Mole Granite. The ore fluid had chemical components derived from the parental granite and components acquired by passage through the metamorphosed aureole.

Geology of Los Azufres Caldera, Mexico, and its relationships with regional tectonics

Ferrari, L., Gardufio, V.H., Pasquar6, G. and Tibaldi, A., 1991. Geology of Los Azufres caldera, Mexico, and its relationships with regional tectonics. In: S.P Verma (Editor), Calderas: Genesis, Structure and Unrest. J. Volcanol. Geotherm. Res., 47: 129-148. The Los Azufres geothermal field is one of several silicic centres of the Mexican Volcanic Belt (MVB) for which a caldera structure was suggested. Geological and structural surveys in a wide area surrounding Los Azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism, consisting of four pyroclastic units and several dome complexes. Although a complete caldera structure is not morphologically evident, several facts suggest a long collapse history at Los A zufres: - the geothermal field lies at the centre of a subcircular depressed area (27 × 26 km in size) filled by a distinctive fluviolaeustrine sequence: - Middle to Late Miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it; - four large ignimbritic suites of latest Miocene and Pliocenc age outcrop outside the depression; - volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area: - no alternative caldera structures exist in a radius of 200 km from Los Azufres; - dacitic to rhyolitic lavas, principally extruded as dome complexes, were emplaced inside this depression during the Pleistocene. Los Azufres is therefore interpreted as a nested caldera of latest Miocene and Pliocene age, The recent regional tectonic evolution of the central sector of MVB comprises a Late Miocene-Early Pliocene left-lateral transcurrent phase, followed by a Late Pliocene-Quatern ary transtensional one. The silicic volcanism occurring between 6.1 and 2.8 Ma can be linked to the first phase, while in the adjacent areas of MVB a volcanic hiatus can be recognized. The following transtensional phase reached the Los Azufres area only during the Pleistocene and disrupted the caldera structure. Normal faults, developed during this period, controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle.

Ordovician alluvial fan to marine shelf transition in SW Sardinia, Western Mediterranean Sea: tectonically (“Sardic phase”) influenced clastic sedimentation

A localized Lower Ordovician, early Caledonian structural event (“Sardic phase”) is recorded in SW Sardinia by an angular and erosional unconformity, faulting, and mild folding. This event was accompanied by sedimentation of a transgressive, clastic unit (“Puddinga Beds”) which is particularly well developed along a major fault system (Gonnesa fault zone). There, the “Puddinga Beds” range from basal interfingering fans (some formed by coarse carbonate and schist-clasts others by schist-clasts only), to fluvio-lacustrine mud and gravelly sandstone, to coastal muddy and sandy deposits, and to fossiliferous restricted marine shelf sediments. The existence of the slightly compressive “Sardic phase” in SW Sardinia and quasi-coeval calc-alkaline volcanism and an extensional structural phase (“Sarrabese phase”) in central-east Sardinia, suggest transcurrent faulting. The calc-alkaline acid volcanism is also interpreted by some authors to indicate that the rifted basins developed in backarc position. The non-volcanic sequence of SW Sardinia analyzed in this paper was probably formed in basins developed on the northern margin of the Gondwanaland craton. This Sardinian system may be the eastward extension of transcurrent rifts which have also been recently reported from NW Spain. Structural activity has been the predominant control on sedimentation; however, worldwide, recurring, low sea-level stands may have influenced the sedimentation of the lower part of the “Puddinga Beds”, and the deglaciation of the Saharan Ice Sheet has contributed to the uppermost Ordovician-early Silurian deepening of the euxinic shelf.

The ophiolites of northeast India — a subduction zone ophiolite complex of the Indo-Burman orogenic belt

The ophiolites of northeast India are rootless blocks of various dimensions, floating in a matrix which belongs to the upper Cretaceous-Lower Tertiary Disang Group. They consist of diverse igneous, sedimentary and metamorphic rocks, of which ultramafics are the main component. They do not constitute a continuous sheet but are made up of units haphazardly juxtaposed along faults or they consist of lensoid slices interbedded with Disang Group rocks. The ultramafics are interpreted as slices of oceanic crust and upper mantle obducted onto the Indian continental margin. Associated blue schist is indicative of subduction zone tectonics. The occurrences of ultramafics showing intrusive contacts and the presence of intermediate — acid volcanics suggest an island arc — continent type of collision along the Benioff Zone coincident with the ophiolite belt. The tectonic history of the Indo-Burman orogenic belt can be tentatively summarised as follows: (1) Introduction of the subduction zone during Cretaceous times; obduction of oceanic crust and upper mantle and intermixing with deep oceanic sediments. (2) Deposition of the Disang sediments. Formation of an island arc separating the sedimentary basin into eastern and western sub-basins. The Barail sediments were deposited in both basins. The formation of the island arc was accompanied by plutonism and volcanism and by deformation and low grade metamorphism of the lower Disang sediments. (3) Gradual shallowing of the basin consequent upon the collision of the island arc with the easterly subducted India plate. (4) Continent-continent collision and rotation of the down-basin normal faults into low angle reverse faults (thrusts) along which the thrust sheets were piled up. The frontal areas of the thrust sheets were asymmetrically folded. This was followed by asymmetrical folding of the rocks of the foreland areas. (5) Formation of gravity faults and conjugate sets of strike-slip faults in the orogen and the foreland areas. The structures associated with the ophiolite belt are attributed first to the collision of the easterly subducting India plate with an island arc and subsequently with the Burma plate.

Phanerozoic Tectonic and Paleogeologic Evolution of Southern South America

Before pre-Variscan time, southern South America had a quasicratonic nature. New structural alignments were overimposed due to Gondwana breakup and South Atlantic opening. Phanerozoic tectonic history is divided as follows: (1) 'Caledonian Geosyncline.' Situated west of Brazilian shield and massives, consists of Cambrian to mid-Devonian clastics, carbonates, and intrusives. (2) Variscan Intracratonic Rifting and Foreland Basins. Alluvial and glaciolacustrine sediments accumulated in grabens and marine clastics accumulated in marginal basins along the western continental edge. This period ends in Late Jurassic time (Gondwana breakup) with basaltic extrusions along rifting zones. (3) Inter Malm Taphogenesis. Basement fracturing and Proto-Atlantic opening took place and marine flooding advanced from the south. Along the western margin, a magmatic arc existed since Early Jurassic. The back-arc basins were filled by marine clastics and evaporites, alternating with acidic volcanics and making up the 'Andine Geosyncline.' (4) Subhercynian Drifting. Sedimentary prisms were formed on the Atlantic margin. On the western margin, continental clastics were laid during the Middle and Late Cretaceous. Acidic extrusions and Andean batholith intrusion began during this time. (5) Laramic Flooding. An Atlantic transgression covered a great part of the extra-Andean region and Andean chains were formed. (6) Cenozoic Mountain Building. A new transgressionmore » expanded again over Argentine plains and Patagonia. New Andean uplifting phases, sub-Andean belt folding, and basement fracturing originated intermontane basins. Thick, alluvial, and piedmont sediments accumulated here.« less

Sedimentology of trench-arc sediments and its relation to ophiolite obduction

The eastern Himalaya in Xigaze District of southern Tibet has well developed Cretaceous-early Paleogene trench-arc sediments deposited coevally with Gandise magmatic/volcanic arc phase, generated by Tethyan subduction. The Congdu Formation, composed of ophiolitic greywacke turbidite interbeded with volcanic tuff and pelagic radiolarian chert, represents typical trench-floor sediments deposited on the top of a younger oceanic crust composed of pillow lava with thin-bedded chert cover of late Albian to early Cenomanian age. The Cretaceous olistostromes (Zongzhuo Formation) probably of Albian age might be earlier trench-slope deposits. The trench turbidite together with slope olistostromes may constitute the residual accretionary prism of a trench-subduction complex formed before collision. The well known Xigaze Flysch, partly submarine and partly subaerial, rests on the continental crust and was accumulated in the forearc basin as Arc-trench gap sediments during late Aptian to Maestrichtian. It is unlikely that a presumed oceanic crust would be the base of Xigaze Basin. The Yarlung Zangbo ophiolite is suggested to be an older oceanic crust and mantle of upper Jurassic to pre-Albian age because of the radiolarian microfossils associated with the thick-bedded red chert (Lhaze Group). Ophiolites occurring as large thrusted ultramafics, dismembered altered basalts and radiolarite slices, obducted along the suture and formed the tectonic ophiolite melange. The Zangbo Suture Zone involves mainly the Congdu trench turbidite and Zongzhuo olistostromes in addition to obducted ophiolite melange. In other words, the Zongbo Suture is mainly a trench-subduction complex with obducted ophiolite melange system. No evidence exist to suggest that the Xigaze forearc basin sediments were involved either in the ophiolite belt or in the suture zone. Qiuwu Molasse of Eocene age, which unconformably overlies the Gandise granitoids, indicates that the collision occurred during the end of the Cretaceous or at the beginning of the Paleocene. The red subaerial Liuqu Conglomerate of Oligocene to Miocene age that uncomformably overlying the ophiolite, further suggests the ophiolite obduction to be pre-Oligocene. It is nearly contemporary with the continental collision of the early stage. Gandise granitoids in the Trans- Himalaya might be the root of the magmatic arc that followed the present Suture Zone to the north. These granitoids range in age from 120 to 40 Ma and are coeval with intermediate to acidic volcanics of Lingzizong Formation occurring to the north of the Gandise plutons and may correspond to Dras island arc volcanics in Ladakh.

Cogeneticity of Tavidar volcanics: Evidence by mixing line calculation

The Tavidar volcanics belong to a trachybasalt-trachyandesite-rhyolite association. Distribution and behavior of major and trace elements and field relationships indicate that intermediate and acidic volcanics of the area are cogenetic, whereas the basaltic phase is younger. Cogeneticity also is confirmed by the mixing line calculation. In the present case, instead of two pole mixing, three pole mixing between end members andesite, quartztrachyte, and rhyolite has been considered. Results show that a perfect linearity in data exists between andesites and quartztrachyte and between quartztrachyte and rhyolites.

Origin of the acidic volcanics of the Tolfa district, Tuscan Province, central Italy: an elemental and Sr-isotopic study

Volcanic rocks from the southern part of the Tuscan Province, the Tolfa district, range in composition from dacites to rhyolites. They have relatively high contents of many incompatible elements and their 87Sr/86Sr ratios vary between 0.7129 and 0.7148 (28 analyses) with one analysis of 0.7112. The four major eruption centres of the district can be distinguished geochemically. It is shown that the variation of major and trace elements within each area can be explained by fractional crystallization without significant crustal assimilation. The geochemical patterns of the incompatible elements of the investigated volcanics have a close similarity to the alkaline undersaturated volcanics from the nearby Roman Province. The geochemical and isotopic similarity between the Tolfa volcanics and those of the Roman Province suggests a common origin for these components of the magmas. It is suggested that, while the origin of the LIL-element enrichment of the Roman Province magmas is a mantle which was metasomatised by fluids or melts derived from upper crustal material of sedimentary origin recycled into the mantle by subduction, the Tolfa volcanics were derived directly from the subducted material by melting.

七谷期以降の火山活動と石油・天然ガス鉱床

Deep-seated volcanic reservoirs of oil and gas in the Niigata basin consist of andesites of the Shiiya and Teradomari formations and rhyolite lavas of the Nanatani formation. Among these volcanic rocks the last one is most suitable as potential hydrocarbon reservoir.Volcanism of the Nanatani stage is bimodal and occured in trough of back-arc side after the opening the Japan Sea and is dominated by acidic volcanism in the Niigata basin. Acidic rocks of the latter half of the stage are widely distributed beneath the Niigata and Nagaoka Plains. Although acidic rocks of the Nanatani stage were recently ascertained by drilling near Itoigawa, these rocks may be not found in the Kubiki area where is occupied by so-called Nanbayama facies of submarine fan nature according to tectonic development of hinterland. Volcanic rocks erupted after the Teradomari stage are mostly andesite and dacite and are maldistributed.Recently from geochemical view point, it is stated by many petroleum geologists that maturation of source rocks started after the Teradomari stage and formation of reservoir and migration of oil and/or gas occured during and after the Nishiyama stage in the oil and gas fields having volcanic reservoir. Such views are due to the assumption that temperature capable of maturation and formation of secondary vug was attained by deep burial of thick sediments. However, it will be infered that heating of mudstone around rhyolite body and formation of secondarty vug by post-action alteration of rhyolite took place during and soon after the Nanatani stage.

VOLCANIC CONTRIBUTIONS TO SEDIMENTATION IN UPPER BURDIGALIAN-LOWER LANGHIAN SEDIMENTS OF THE VENETIAN MOLASSIC BASIN

In the Upper Burdigalian-Lower Langhian sediments of the Venetian molassic basin two vol­caniclastic layers are clearly recognizable, respectively within the Globigerinoides trilobus and Praeorbulina glomerosa zones of Bizon & Bizon (1972). The chemical composition of well-preserved glass shards, tested by electron microprobe, reveals rhy­olitic composition and provides information about contemporaneous acidic explosive volcanism. Due to the probably depositional mechanism (fallout), the location of the effusive centres remains unknown. Although many ash layers are quite well-known in the same stratigraphic interval throughout the Apennine chain, a ten­tative correlation still seems very difficult, due to different methods of analysis and dating criteria.

Intense acidic volcanism at the Cretaceous-Tertiary boundary

A 87Sr/ 86Sr spike in seawater strontium with amplitude 2 × 10 −4 and duration of order 2 Ma is superimposed on longer-term variations at the Cretaceous-Tertiary boundary. The anomaly has been attributed to increased continental runoff due either to meteorite impact-related acid rain or sea-level regression. We speculate here that the spike could have resulted from intense, explosive acid volcanism preceding the development of the Deccan traps. A good model, both in tectonic position and geochemical characteristics, for these as yet elusive acidic products is provided by granites in the Seychelles Islands and particularly Mahé.

Conditions of gold-bearing arsenopyrite crystallization in the Villeranges Basin, Marche-Combrailles shear zone, France; a mineralogical and fluid inclusion study

The Au-As-Sb deposits from the Marche-Combrailles (northwestern part of the French Massif Central district) are spatially associated to the Marche-Combrailles shear zone. Host rocks are either Visean volcano-sedimentary tuffs or formations of the Paleozoic basement presenting a greenschist facies assemblage which resulted from extended hydrothermal metamorphism. The strong macro- and microfracturing that affected the Villeranges basin as well as the surrounding formations is related to late tectonic reactivation of the Marche-Combrailles shear zone around 300-315 Ma which favored intense circulation of hydrothermal fluids. Solubilization, transport, and deposition of Au, As, and Sb occurred together with specific rock alteration along faulted zones on distances ranging from several centimeters to a few decimeters. Wall rocks are affected by complete bleaching resulting from chlorite replacement by phengite (T = 240 degrees + or - 30 degrees C) followed by complete replacement of the previous minerals by an illite-quartz-ankerite-pyrite assemblage. Gold is carried in a combined state with arsenopyrite, occurring either within a diffuse network of quartz + ankerite veinlets or as isolated crystals in the altered tuff. Arsenopyrite crystals are characterized by important enrichments either at their periphery or within microcrack networks affecting their whole mass. Mass balance calculations on fresh and altered rocks show that during the ore stage, hydrothermal fluids contributed H 2 O, CO 2 , and S (Au, Sb) to authigenic minerals and leached almost completely Na, Ba, Sr, Si, Fe, Ca, Mg, Al, Ti, and K exhibit similar contents in fresh and altered rocks and are redistributed among the authigenic minerals.Ore fluids are characterized by an aqueous composition, a rather low salinity, and temperatures ranging from 160 degrees to 200 degrees C. Ore deposition occurred at a low pH, indicated by the complete alteration of the host rocks into a quartz-illite association, and at a low f (sub O 2 ) intermediate between the f (sub O 2 ) fixed by the hematite-magnetite and Ni-NiO oxygen buffers. Gold deposition probably resulted from a destabilization of Au bisulfide or thioarsenide complexes during the sulfide crystallization.Thus, the Au-As-Sb ores in the Villeranges area result from a single hydrothermal stage and occur during a late stage of brittle deformation, significantly posterior to the continental basin formation and the acid volcanism. Ore fluid circulation is linked mostly to the fluid convection produced by the abnormal heat flow characterizing the Marche-Combrailles shear zone.

Iridium anomaly from the Acraman impact ejecta horizon: impacts can produce sedimentary iridium peaks

THE observation of anomalously high iridium concentrations at the Cretaceous/Tertiary (K/T) boundary1 triggered the still unresolved debate concerning the origin of sedimentary Ir anomalies. Several hypotheses have been presented, including meteorite impacts1,2 and intense volcanism3,4. Here we present data from the Acraman impact ejecta horizon, South Australia5, which conclusively links high sedimentary iridium concentrations to a major meteoroid-impact structure and its widely dispersed ejecta. An Ir anomaly (up to 2 parts per 109 (p.p.b.) Ir as well as Au, Pt, Pd, Ru and Cr anomalies) is present within the ejecta from the Acraman impact event preserved in late Precambrian (∼600 Myr BP) shales within the Adelaide Geosyncline. The highest Ir (and Cr) concentrations within the ejecta are associated with the coarsergrained clasts of acid volcanics (boulder to medium-sand size) suggesting that most of the Ir is carried by these fragments. The target rocks at the impact site are comparable acid volcanics that have very low Ir, Au, Pt, Pd, Ru and Cr concentrations, indicating a meteoritic origin for the anomalous Ir levels within the ejecta. To our knowledge, this is the first example in which an accepted, widely dispersed impact-ejecta blanket, itself uniquely linked to a major terrestrial impact structure5,6 has been shown to be anomalously high in cosmogenic siderophile elements. This finding strongly supports the hypothesis that Ir anomalies in sediments can be produced by terrestrial meteorite impact events.

P-T -fluid regimes of metamorphism and related magmatism with specific reference to the granulite-facies Sharyzhalgay complex of Lake Baikal

Summary Petrological studies, including geothermobarometry, of metamorphic mineral parageneses together with isotopic data, reveal differences in the P-T -fluid regimes of Precambrian shields and surrounding fold belts. Typical features of shields are CO 2 -rich fluids and preservation of retrograde portions of P-T trajectories, while in fold belts, prograde as well as retrograde stages of metamorphism are recorded in the minerals and the CO 2 contents of the fluids vary widely. Geothermobarometric estimates of prograde and retrograde stages in several metamorphosed fold-belt complexes are in accord with the distribution of Al 2 SiO 5 polymorphs. A model involving vertical redistribution is proposed for the granulite-facies Sharyzhalgay complex of Lake Baikal, which is interpreted to be the metamorphosed equivalent of the Karelian greenstone belt. Increased heat flow and burial leads to gravitational redistribution due to density differences, so that dome structures cored by granulite-facies gneiss occur within envelopes of down-sinking metabasites and meta-ultrabasites. The model is supported by the geochronology and by the P-T -fluid regimes deduced from geothermobarometry and fluid-inclusion studies of the polymetamorphic Sharyzhalgay complex. This model may apply to other granulite-facies complexes which appear to be the result of high-grade metamorphism of Archaean greenstone belts made up of tholeiitic and komatiitic volcanics. These rocks were originally underlain by a complex of acid volcanics (dacites and rhyolites).

Geochemistry of the collision related acid volcanism in eastern rhodopes, Bulgaria

Geochemistry of the collision related acid volcanism in eastern rhodopes, Bulgaria