bluuebunny/crossref_metadata_2025 huggingface dataset:
The origin and significance of crystal rich inclusions in pumices from two Chilean ignimbrites
Authors: de Silva S. L.
Published: May 2009

Crystal rich (∼ 70–98% phenocrysts) magmatic inclusions in pumices for compositionally heterogeneous ignimbrites from the Central Andes of northern Chile are interpreted as the products of crystal accretion at the sidewalls of the magma chambers. The inclusions are typically andesitic in composition and are found as ‘peppery textured’ lenses and bands, or as discrete ovoid ‘blobs’ within dacitic pumices from the early erupted portions of the ignimbrites. The inclusions have a bimodal gain size with large phenocrysts (> 1 mm), typical of those of the host pumice, set in a dominant finegrained framework (< 0.5 mm) of plagioclase, with lesser amounts of hornblende and biotite in equal proportions, and ubiquitous titanomagnetite in a matrix of vesiculated high-Si rhyolite glass. An igneous microgranular texture is defined by this framework. The mineralogy of the inclusions, as well as the compositions of the phenocrysts and glass, are very similar to those of the host pumices. These characteristics, in addition to available major, trace and REE data, are best reconciled if the inclusions represented samples of fractionated crystals and glass from the same magma as the host pumice. The restricted occurrence of these inclusions in the early erupted portions of the ignimbrites suggests that these crystal accumulations occurred in the upper portions of the magma chambers, at the sidewall; the dominantly fine grain size and crystal rich nature of the inclusions are considered to be the result of the higher thermal gradient in the boundary layer.

These inclusions may be an important link between the experimental and geochemical models for the origin of compositional layering in magma chambers by sidewall crystallization. The presence of similar inclusions in other ignimbrites and volcanics, as well as plutonics, suggest that they may be a common feature of silicic magmas.
A new genus of brissid echinoid from the Miocene of Australia
Authors: McNamara K. J. , Ah Yee C.
Published: May 2009

The brissid echinoid Amoraster (gen. nov.) is described from Miocene strata in southern Australia on the basis of two species: A. paucituberculata sp. nov., which ranges from the Batesfordian (latest Early Miocene) Bochara Limestone to the Bairnsdalian–Mitchellian (Middle–Late Miocene) Port Campbell Limestone in Victoria; and A. tuberculata sp. nov. from the Longfordian (Early Miocene) Mannum Formation in South Australia. Morphological changes which occurred with the evolution of A. paucituberculata from A. tuberculata are interpreted as being adaptations to the occupation of a finer grained sediment by the descendant species.
An application of cathodoluminescence microscopy to the study of textures and reactions in high-grade marbles from Connemara, Ireland
Authors: Yardley B. W. D. , Lloyd G. E.
Published: May 2009

Upper amphibolite facies dolomite and calcite marbles from the Dalradian Connemara Marble Formation of western Ireland display a wide range of textures when viewed in cathodoluminescence that are invisible in transmitted light. Examples observed include growth zones in calcite that appear to represent the infilling of a metamorphic secondary porosity, produced by the large reduction in solid volume accompanying growth of tremolite from dolomite and quartz. Retrograde phenomena are widespread, and include both carbonate pseudomorphs after tremolite and coarse veining, which is in optical continuity with peak metamorphic calcite but otherwise indistinguishable from it. Admixture of such widely differing calcite types may account for scatter in some stable isotope studies of marbles.
Structure, petrochemistry and evolution of a sheet swarm in an Icelandic central volcano
Authors: Gautneb Håvard , Gudmundsson Agust , Oskarsson Niels
Published: May 2009

Strike, dip, and thickness were measured for 504 sheets (inclined sheets and dykes) in the 4–6 Ma old Hafnarfjall central volcano in southwest Iceland. The average dip of sheets is 65°, 80% are less than 1.2 m thick, and the thickness tends to decrease with decreasing dip. In 0.5 km long traverses perpendicular to the average strike of sheets, the percentage of sheets ranges from about 6 to 11.

Of 140 chemically analysed sheets most are quartz-tholeiites; a few are intermediate or acid. The sheets are chemically more evolved than the host rock and were generated by a shallow crustal magma chamber at a mature stage of the central volcano, whereas the host rock was generated earlier before the chamber was established. Trace element results suggest that the sheet magmas evolved by low-pressure fractional crystallization as well as by mixing of primitive magmas and crustal melts.

A model is proposed where most of the sheets are generated by a growing shallow magma chamber. As the chamber grows its shape changes, and so does the local stress field associated with it. Because the sheets follow the stress trajectories of the local stress field, the potential pathways of the sheets change with the growth of the chamber, which may explain the common occurrence of cross-cutting sheets. From the evolved chemistry of the sheets, as well as from the pattern of the stress trajectories, it is concluded that the bulk of the sheets were injected from the upper part of the shallow magma chamber.
Resetting of the Rb–Sr whole-rock isotope system of an Ordovician microgranite during Devonian low-grade metamorphism
Authors: Evans J. A.
Published: May 2009

The Bwlch y Cywion microgranite intrudes an Ordovician sedimentary sequence in northeast Snowdonia, North Wales. Its age is recorded by hornfelses within its metamorphic aureole which give an Ordovician age of 454 ± 20 Ma. The whole-rock Rb–Sr isotope systems of the intrusion, however, and of an associated ash-flow tuff and a rhyolite of the Llewelyn Volcanic Group (Caradoc), were reset during low-grade metamorphism and give Devonian ages of 392±11 Ma, 392±5 Ma, and 405 ± 6 Ma, respectively.
The Skye Main Lava Series: liquid density and the absence of basaltic hawaiites
Authors: Cattell A. C.
Published: May 2009

Basaltic hawaiite lavas are virtually absent in the Eocene Skye Main Lava Series, in contrast to relatively abundant basalts and hawaiites. Fractional crystallization from basalt to basaltic hawaiite involves extraction of a large proportion of plagioclase, and liquid densities thereby increase. From basaltic hawaiite to hawaiite titanomagnetite is a significant fractionating phase, and liquid densities decline. The coincidence between a gap in erupted compositions and a density maximum implies that liquid density exerted a strong control on ‘eruptibility’ of magmas; basaltic hawaiites were too dense to be erupted. Density maxima occur in basalt suites if plagioclase fractionates before Fe–Ti oxides, and may explain compositional gaps in erupted magmas. Compositional gaps are not the inevitable result of density maxima; the density of the rock column above, and the fluid dynamics within, the magma chamber where differentiation occurs are also critical factors.
The Peak Trough – a major control on the geology of the North Yorkshire coast
Authors: Milsom J. , Rawson P. F.
Published: May 2009

Although the Mesozoic sediments of the Cleveland Basin (North Yorkshire) have generally not been strongly faulted, several approximately N–S trending faults have been identified along the coast. New seismic data from adjacent coastal waters has allowed the offshore extension to the fault system to be examined for the first time. The coastal faults from Peak (Ravenscar) to Red Cliff (Cayton Bay) are shown to form part of a linked system defining a narrow graben only some 5 km wide, the Peak Trough. Faulting has been complex, with decollement levels apparently developed in weak layers at various horizons in the Triassic and Permian strata: fault geometries and regional considerations suggest that extension has been dominant. Movement occurred intermittently from Triassic to latest Cretaceous or early Tertiary times.
The structure and evolution of the Northern Tyrrhenian Sea
Authors: Keller J. V. A. , Coward M. P.
Published: May 2009

Field studies on the island of Elba and seismic lines from the Northern Tyrrhenian Sea, Italy, indicate that major extensional displacements were accommodated along east-dipping low-angle detachment faults. The rifting and subsidence in the Northern Tyrrhenian Sea basin have followed convergence and collision of the Corso-Sardinian block and the Apulian microplate. This collisional episode produced the Northern Apennines fold-and-thrust belt. Major extensional faults cut down-section through the stratigraphy and pre-existing west-dipping thrust faults. West-dipping thrusts can also be reactivated and form antithetic faults to the east-dipping detachments. Brittle deformation conditions predominated during the extensional phase. The geometry, internal structure and the fabrics (brittle and penetrative) associated with a well-exposed low-angle extensional detachment in Elba are presented in this paper. A geometrical model for the brittle extensional faulting is presented in which regional extension was accommodated on a system consisting of two sets of simultaneously active antithetic faults. The east-dipping detachment faults appear to have started at steeper angles, based on field and seismic observations, and rotated counter-clockwise to lower dips. Due to this rotation, and for space accommodation, antithetic west-dipping faults formed and rotated clockwise. A tectonic model is proposed whereby slowing of the convergence between Apulia and Corsica, as well as Tethys oceanic crust and Apulian crust subduction, led to the delamination of the Apulian litho-spheric mantle away from the crust. Accompanying asthenospheric upwelling and intrusion at the crust—mantle interface beneath the Tyrrhenian Sea caused late orogenic crustal stretching in the Northern Apennines internal zone.
Pan-African volcanism: petrology and geochemistry of the Dokhan Volcanic Suite in the northern Nubian shield
Authors: Abdel-Rahman Abdel-Fattah M.
Published: May 2009

The Late Proterozoic Dokhan volcanic suite (620 Ma) of the northern Nubian shield is the product of Late Pan-African volcanism. The suite covers the entire spectrum from basalt to high-silica rhyolite and occurs as two units: a dark-coloured unit containing basalt-andesite-dacite, and a light-coloured unit encompassing dacite-rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ign-imbrites that are locally interstratified with basalt and andesite lava flows. The suite forms a continuum in composition with a wide range of Si02 (48–77 wt%), CaO (0.1–8.9 wt%), Sr (81–906 ppm), Zr (85–340 ppm) as well as most other elements, and is moderately enriched in incompatible elements, including rare earth elements (REE). The suite exhibits fractionated, subparallel REE patterns that are similar overall to Andean andesites and ignimbrites. Well-defined major and trace element trends and fractionated REE profiles are consistent with a fractionated basalt to rhyolite calc-alkaline magma series. It is a typical calc-alka-line orogenic complex and exhibits mineralogical-geochemical traits of arc-related volcanism. The suite neither resembles products of extensional nor transitional tectonic regimes as previously thought, but was produced in a subduction-related tectonic environment. The mafic nature of the least-evolved rocks of the suite, along with its relatively low initial 87Sr/86Sr ratio (0.7039) are considered to indicate a mantle source. A mantle-derived basaltic magma fractionated, with amphibole and plagioclase dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major and trace element fractionation modelling.
Reactivation history of the long-lived Billefjorden Fault Zone in north central Spitsbergen, Svalbard
Authors: McCann Andrew J. , Dallmann Winfried K.
Published: May 2009

New geological mapping has revealed further details of the tectonic and stratigraphic effects of Devonian and later reactivations of the Billefjorden Fault Zone, one of a number of important north—south trending lineaments in Svalbard. Analysis of offsets along the many steeply-dipping faults within the zone, and effects on the subsidence and deformation of the adjacent crustal blocks, is presented as a series of tectonic maps from the Late Devonian through to the Tertiary. Late Devonian sinistral transpression, suggested previously, cannot be ruled out, and Carboniferous reactivation was dominated by extension, with possibly a slight dextral strike-slip component. After Late Carboniferous to Early Cretaceous platform subsidence, during which the fault zone had little effect on sedimentation, development of the Tertiary West Spitsbergen Fold Belt (related to the opening of the Arctic Ocean) involved compressive (and transpressive?) reactivation of basement-seated structures further east, including the Billefjorden Fault Zone. In the Billefjorden—Austfjorden area this produced a large monoclinal fold across the fault zone, which was later cross-cut by extensional structures to produce the present day Billefjorden syncline. This localized late extension is related to a slight variation in the trend of the Billefjorden Fault Zone through this area.
The Neoproterozoic–lower Cambrian in northern Govi-Altay, western Mongolia: regional setting, lithostratigraphy and biostratigraphy
Authors: Khomentovsky V. V. , Gibsher A. S.
Published: May 2009

This paper provides a framework for the stratigraphy and correlation of key sections through the Neoproterozoic–lower Cambrian of the Govi-Altay area, western Mongolia, studied by members of IGCP Project 303. Dzabkhan Formation volcanic rocks at the base of the succession are suggested to have formed during rifting of the Baydrik–Tarbagatay microcontinent in the Baykalian interval (c. 850–650 Ma). Pre-Vendian accretion transformed this rift and adjacent microcontinental fragments into a marginal marine back-arc basin, in which accumulated relatively uninterrupted Neoproterozoic and earliest Palaeozoic sediments. Lithostratigraphy for the latter is defined, with detailed maps, at five sections (Bayan Gol northern and southern blocks, Tsagaan Gol, Salaany Gol and Tayshir). The Tsagaan Oloom Formation lower member contains two diamictites of possible Sturtian age. Middle and upper carbonates are compared with the Yudomian (Vendian) of Siberia, the former ending with a unit ofBoxonia grumulosastromatolites. In the upper member, phosphorite-shale is followed mainly by limestones, with the earliest skeletal fauna. The Bayan Gol Formation comprises siliciclastic–limestone intercalations with increasingly diverse invertebrate biota. Correlation of lithological marker beds across the study area produces a composite stratigraphic section to which the earliest skeletal fossil occurrences are tied. TheAnabarites trisulcatusZone of the Nemakit-Daldynian Stage begins near the base of the Tsagaan Oloom Formation upper member (unit 11). ThePurella antiquaZone begins near the base of the Bayan Gol Formation (unit 18). Assemblages typical of theNochoroicyathus sunnaginicusZone (base of the Tommotian Stage) range through units 19 to 23. Elements typical of theDokidocyathus regularisZone (middle Tommotian) appear in unit 24 near the top of the Bayan Gol Formation.
Glacial facies associations in a Neoproterozoic back-arc setting, Zavkhan Basin, western Mongolia
Authors: Lindsay J. F. , Brasier M. D. , Shields G. , Khomentovsky V. V. , Bat-Ireedui Y. A.
Published: May 2009

Diamictites, many of glacial origin, are globally distributed in the Neoproterozoic. Recently, two relatively thin diamictites in the Maikhan Uul Member at the base of the Neoproterozoic Tsagaan Oloom Formation from the Zavkhan Basin of western Mongolia have been identified as being of glacial origin. The Mongolian diamictites form a series of backstepping units within the transgressive systems tract of two major depositional sequences associated with sea-level changes. In each case the diamictites of the transgressive systems tract are abruptly overlain by deeper water, upward shoaling highstand systems tracts consisting of thinly bedded sandstones and shales in sequence 1 and thinly bedded, dark carbonates in sequence 3. The fact that the sequences conform closely to depositional models established at other localities suggests that all are related to major ice ages and that the depositional sequences they have generated provide a valuable tool for global correlation in this part of the stratigraphic column. Available stratigraphic and isotope geochemical information presented by Brasieret al.(1996, this issue) suggests that both diamictites are likely to be of Sturtian age. A riftogenic setting and Sturtian age for the diamictites provide a link with eastern Australia and western America. It is possible, therefore, that these diamictites formed during the breakup of a supercontinental assembly including Siberia, Australia and Laurentiac. 750–725 Ma BP.
Facies and sequence controls on the appearance of the Cambrian biota in southwestern Mongolia: implications for the Precambrian–Cambrian boundary
Authors: Lindsay J. F. , Brasier M. D. , Dorjnamjaa D. , Goldring R. , Kruse P. D. , Wood R. A.
Published: May 2009

Neoproterozoic–Cambrian rocks of the Zavkhan Basin (Govi-Altay, western Mongolia) comprise large-scale alternations of siliciclastic- and carbonate-dominated units (cf. ‘Grand Cycles’). Analysis of such depositional sequences near the base of the Cambrian confirms that the distribution of trace fossils, small shelly fossils and calcimicrobial structures was strongly controlled by ecology and taphonomy, corresponding to specific points in a sea-level cycle. Evolution of the Cambrian biota is thus viewed through aseries of narrow time windows, once only for each depositional cycle. Correlation of the Precambrian–Cambrian boundary level on the basis of the first appearance of thePhycodes pedumassemblage is also fraught with difficulty, since stratigraphic resolution may be limited to a single sea-level cycle(c. 1–5 Ma). It is suggested that, in many cases, basin analysis will need to be undertaken before this boundary can be drawn.
Early Cambrian bioconstructions in the Zavkhan Basin of western Mongolia
Authors: Kruse Peter D. , Gandin Anna , Debrenne Françoise , Wood Rachel
Published: May 2009

The Neoproterozoic–Cambrian succession in the Zavkhan Basin of western Mongolia preserves early Cambrian bioconstructions of Nemakit-Daldynian to Botomian age. As elsewhere (Siberia, Morocco), the Nemakit-Daldynian bioconstructions in the upper Tsagaan Oloom Formation to lower Bayan Gol Formation interval were purely calcimicrobial. Spectacular calcimicrobial ‘patch reefs’ of presumed Tommotian age are present in the lower Bayan Gol Formation. In contrast, late Atdabanian–early Botomian bioconstructions are varied and well developed in the upper Salaany Gol Formation including, in addition to calcimicrobial bioherms, Gordonophyton–Razumovskia crusts, radiocyathan—archaeocyathan bioherms and thickets of the ramose archaeocyath Cambrocyathellus.

These latter Zavkhan Basin buildups show some ecological and sedimentological features in common with coeval bioconstructions in South Australia, which also developed under a tectonically active regime.
Integrated chronostratigraphy of Proterozoic–Cambrian boundary beds in the western Anabar region, northern Siberia
Authors: Kaufman Alan J. , Knoll Andrew H. , Semikhatov Mikhail A. , Grotzinger John P. , Jacobsen Stein B. , Adams William
Published: May 2009

Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykai Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.
The gabbro–dyke transition zone demonstrated on Tviberg, Solund–Stavfjord Ophiolite Complex
Authors: Skjerlie K. P. , Furnes H.
Published: May 2009

The transition zone between 100 % dykes and high-level plutonic rocks of the Solund-Stavfjord Ophiolite Complex is complex due to the existence of many lithologies with different and variable contact relationships. The rocks of the plutonic complex vary in composition from FeTi basaltic to quartz dioritic, and the grain sizes vary from fine to pegmatitic. Felsic varieties are produced by fractional crystallization of basaltic magma as demonstrated by geochemical evolution and by gradual transition from gabbro to quartz diorite. Patches of fractionated dioritic rocks may show both gradual and intrusive relationships with the surrounding host gabbro. This demonstrates that late-stage liquids commonly left the source region and locally intruded the surrounding parent rocks. The high-level plutonic rocks are thoroughly epidotized and are cut by dykes consisting of granoblastic epidote and quartz. The high-level plutonic complex is associated with irregular bodies of fine- to medium-grained plagioclase-porphyritic diabase of high MgO content. These diabase bodies are intruded by dykes that become progressively more regular in shape. The plutonic complex locally shows intrusive relationships with the overlying 100% dyke complex, but is itself cut by two dyke swarms. The dykes of the first swarm formed while the plutonic complex experienced sinistral shear strain, and the dykes are generally less regular and thinner than the dykes of the second swarm. This indicates that the dykes of the first swarm intruded while the rocks of the plutonic complex were still hot, while the next dyke swarm intruded later when the rock complex was colder. Dykes of both swarms range in composition from slightly to strongly fractionated, suggesting that the magma chambers they were expelled from underwent significant fractionation in between magma replenishment. Numerous dykes of both swarms carry large quantities of glomeroporphyritic aggregates of plagioclase and altered clinopyroxene, indicating that the source area to the dykes very often was a crystal mush.
Very low- and low-grade metamorphism in the Trinity Peninsula Group (Permo-Triassic) of northern Graham Land, Antarctic Peninsula
Authors: Smellie J. L. , Roberts B. , Hirons S. R.
Published: May 2009

The Permo-Triassic Trinity Peninsula Group is a widespread, regionally metamorphosed metasedimentary sequence in northern Graham Land, Antarctica, which forms the local ‘basement’ to the mainly Jurassic–Cretaceous Antarctic Peninsula magmatic arc. The metamorphic grade, thermal evolution and pressure series of this major tectono-stratigraphical unit are largely unknown. Determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major phases, mainly in rare volcanic beds. However, diagnostic mineral parageneses are generally absent and the precise metamorphic grade is unknown or has to be inferred over large areas. Using white mica (illite) crystallinity of interbedded mudrocks, the Trinity Peninsula Group is now shown to have been pervasively altered mainly at anchizonal and epizonal grades. Conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest. Outwith thermal aureoles near plutonic intrusions, the alteration temperatures ranged mainly from 250 to 325 °C, exceeding 300 °C in the highest-grade (epizone/greenschist facies) parts of the sequence. The facies series, K-white mica b cell dimension measurements and mineral phases present are characteristic of an intermediate pressure series altered under moderate geothermal gradients (<35 °C/km), corresponding to burial depths of c. 7–10 km. Unroofing and substantial erosion of the Trinity Peninsula Group took place during polyphasal vertical tectonic movements linked to the development of the magmatic arc in northern Graham Land. The geological setting of the Trinity Peninsula Group is ambiguous and could have been a foreland (or back-arc) basin or the mid- to upper levels of an accretionary prism.
Orthoconic nautiloid cephalopod genera
Authors: Holland C. H.
Published: May 2009

Use of the nautiloid cephalopod generic namesOrthocerasandMichelinocerasis briefly reviewed. Two new genera,Imbricatoceras(upper Ordovician-Silurian) andRadnoroceras(Wenlock-Ludlow), are proposed for common taxa with distinctive ornamentation.
A new record of early Silurian land plant spores from the Paraná Basin, Paraguay (Malvinokaffric Realm)
Authors: Gray Jane , Boucot A. J. , Grahn Yngve , Himes Gregory
Published: May 2009

The first conclusive evidence for Silurian rocks in the Paraná Basin, Brazil, was reported in 1985. This evidence was based on organic-walled microfossils, principally spore tetrads, and associated phytoplankton (‘acritarchs’ and prasinophytes), the first recovered from the Vila Maria Formation on the northeastern rim of the Basin. The spore assemblage was typical of Gray's Microfossil Assemblage Zone I; size-frequency data for the tetrads suggested an early Silurian (early Llandovery; Rhuddanian) age. We now document a new MA Zone I occurrence of spore tetrads together with a few single trilete spores, from the southwestern rim of the Paraná Basin, Paraguay. The single spores are among the earliest known to have normally dissociated from a tetrahedral tetrad. Size-frequency data for the tetrads suggest a late Llandovery, pre-C5(mid-Telychian) age. These spores, together with phytoplankton and chitinozoans, were recovered from beds laterally equivalent to the Vargas Peña Shale at the type locality from which Llandovery diplograptids and monograptids are known. Llandovery age rocks on the southwesternandnortheastern borders of the Paraná Basin,c. 1400 km apart, suggest that early Silurian rocks were extensively distributed throughout southern Brazil and adjacent Paraguay. Close conformity between the age reference obtained with graptolites and spore tetrads again demonstrates the stratigraphic utility of early Silurian spores in providing reliable age determinations for otherwise unfossiliferous rocks. Spore tetrad assemblages available from the Malvinokaifric Realm (South America, Africa, Arabia) represent a distinct phytogeographic unit contrasted with spore tetrad assemblages from the North Silurian Realm of North America and parts of Europe.
Coal—magma interaction: an integrated model for the emplacement of cylindrical intrusions
Authors: Kent R. W. , Ghose N. C. , Paul P. R. , Hassan M. J. , Saunders A. D.
Published: May 2009

Olivine-bearing lamproite magmas intruded into Permian coal seams in northeast India occur as root-like cylinder stockworks, extending for up to several kilometres up-dip along the bedding planes of their sedimentary host. Clusters of eight or more conduits are typical, linked by thin tubular cross-branches. Cylindrical geometry may arise by injection of hot, low-viscosity fluid through a slot, with the development of multiple tube-like instabilities at the interface between the moving fluid and a higher-viscosity host. This behaviour appears more complex than the models of Chouke, van Meurs & van der Pod, and Saffman & Taylor, which predict the development of a single dominant tube in porous or layered flow. Cylinder emplacement may be an essentially passive process, in which the sediment column is reduced by expulsion of heated pore fluids at the head of the moving intrusion, creating a space into which the melt can propagate. Generation of a superheated vapour envelope by non-nucleated film boiling of these fluids around the hot lamproite magma (the Leidenfrost effect) allows melt flow to be maintained in a lengthening tube thermally insulated from the surrounding coal, in a manner analogous to submarine lava tubes. Cooling of the magma through the Nukiyama temperature (the temperature at which maximum evaporation of the heated fluid occurs) may give rise to violent surface boiling and the formation of large vapour bubbles at the magma–coal interface. Implosion of these bubbles could then result in the formation of shock breccias, comparable to hyaloclastites produced by bubble or surface film collapse in the vicinity of pillow lava tubes. The operation of such a process around lamproite magma is suggested by the presence of complex breccias composed of highly fragmented coal, sandstone, and lamproite, at the termini of certain cylinders.

Surface and subsurface exposures of the cylinders reveal the presence of a carbonate–chlorite–clay halo surrounding each intrusion, resulting from the alteration of mafic mineral phases by fugitive volatiles released from the protective vapour jacket. The coal seams proximal to intrusion clusters are relatively undeformed, with no evidence of assimilation by the invading melts. The coals have experienced extensive carbonization, probably as a result of slow conductive heating from the cooling lamproite bodies, or fluids derived therefrom. Field observations indicate that these thermal effects are not merely confined to the coal–melt interface, but occur for some considerable distance away from the intrusions, producing large areas of naturally coked coal.
Marine molluscan constraints on the age of Cretaceous fossil forests of Alexander Island, Antarctica
Authors: Kelly S. R. A. , Moncrieff A. C. M.
Published: May 2009

New evidence, provided by bracketing marine molluscan faunas, suggests a Late Albian age for fossil forests in the upper part of the Fossil Bluff Group of Alexander Island. These in situ forests are underlain by Late Aptian and Late Albian strata containing faunas including ammonites (the last of which is Lechites), inoceramid bivalves, dimitobelid belemnites and trace fossils. The tree-bearing levels occupied a fluvial dominated environment, subject to periodic major floods. They were transgressed during late Albian time by returning marine conditions with diverse faunas, including hamitid and puzosiid ammonites. The increased precision in dating of the fossil forests will improve palaeobotanical contributions to climate and environmental studies.
Glacial or non-glacial origin for the Bigganjargga tillite, Finnmark, northern Norway
Authors: Jensen P. A. , Wulff-Pedersen E.
Published: May 2009

Bigganjargga is the classic locality for the Varangian ice age in northern Scandinavia. The presence of two sets of striations on the underlying quartzite basement has been taken as evidence for a glacial origin of the Bigganjargga diamictite. However, soft-sediment surface structures on the underlying sandstone indicate that it was not consolidated when the diamictite was emplaced. Together with structures in the diamictite this shows that the deposit formed as a debris flow moving northwestwards over an unconsolidated substrate. There is no evidence for glacial origin or contribution for the diamictite material.
Neoproterozoic shear zone tectonics within the Icartian basement of Guernsey and Sark, Channel Islands
Authors: Tribe I. R. , Strachan R. A. , D’Lemos R. S.
Published: May 2009

The Channel Islands of Guernsey and Sark are amongst the few localities within the Neoproterozoic, Cadomian orogenic belt where Palaeoproterozoic basement is exposed. Basement units collectively referred to as ‘Icartian’ comprise orthogneisses (e.g. the c. 2000 Ma Icart granite gneiss), metasediments and amphibolites. On Guernsey, the protolith to the Icart granite gneiss was intruded into metasediments already deformed during a D1 deformation event. Both were variably deformed during a D2 event within an approximately north—south trending, steeply dipping, oblique dextral shear zone. On Sark, metasediments and amphibolites carry a D1 fabric (not necessarily correlatable with that on Guernsey) which was deformed during D2 to form recumbent—gently inclined folds and tectonic fabrics which locally carry top-to-the-south kinematic indicators. Early Cadomian quartz diorites which intruded the Icartian basement carry variably developed magmatic and moderate temperature (400–550 °C) LS solid-state fabrics which are similarly oriented to D2 basement structures. This indicates that the quartz diorites were emplaced and deformed during a protracted D2 event. Published 40Ar/39Ar amphibole cooling ages of c. 600 Ma from Guernsey and Sark quartz diorites and a host Icartian amphibolite are consistent with reworking of basement units during the Cadomian Orogeny. The contrasting kinematic pattern during Cadomian deformation implies that a flat-lying shear zone on Sark may have accommodated oblique dextral strike-slip movement on Guernsey with an overall southerly direction of tectonic transport. Deformation most likely occurred in the hanging-wall of a major south-dipping Neoproterozoic subduction zone.
The tectono-metamorphic evolution of a dismembered ophiolite (Tinos, Cyclades, Greece)
Authors: Katzir Yaron , Matthews Alan , Garfunkel Zvi , Schliestedt Manfred , Avigad Dov
Published: May 2009

The six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subduction zone (SSZ) affinity for the ophiolitic suite. ρ18O values of phyllites, gabbros and serpentinites range from 6 to 15%o. Model calculations indicate that such values are consistent with low temperature (50–200°C) alteration of parent rocks by sea-water at varying water/rock ratios. This would agree with the early low temperature mineralogy of the serpentinites, but the early high temperature alteration of the gabbros would require the presence of18O-enriched sea-water.The following overall history is suggested for Tinos ophiolitic slices. (1) Oceanic crust was generated at a supra-subduction zone spreading centre with high temperature alteration of gabbros. (2) Tectonic disturbance (its early hot stages recorded in an amphibolitic shear zone at the base of serpentinites) brought the already cooled ultramafics into direct contact with sea-water and caused low-Tserpentinization. (3) Tectonism after cooling involved thrusting which caused repetition and inversion of the original order of the oceanic suite. (4) Regional metamorphism of all the ophiolite components at greenschist-facies conditions (−450°C) overprinted the early alteration mineralogy. It was probably induced by continued thrusting and piling up of nappes. The Tinos ophiolite, dated as late Cretaceous and genetically related to other low pressure rock-units of the same age in the Aegean, differs in age and degree of dismemberment and metamorphism from ophiolites in mainland Greece.
The Kula Volcanic Field, western Turkey: the development of a Holocene alkali basalt province and the adjacent normal-faulting graben
Authors: Richardson-Bunbury J. M.
Published: May 2009

The Kula province of Western Turkey provides an excellent example of an alkali basalt province in an area of active rifting. This paper establishes the relationship between the generation of the basalts and the extension of the region. The wide, shallow, terrestrial basin, formed by early extension, was subsequently cut through by narrow, fault-bounded grabens. The genesis of alkali basalt magmas began soon after the concentration of the extension into two grabens ˜2 Ma. The basalts, in the form of some 80 small cinder cones and associated lava flows and fields, have a total volume of 2.3 km3. This small volume reflects the small amount of extension in the region (β < 1.2 where β = final length of crust: initial length of crust).The intercalation of sediments and basalts in the Kula area enables the establishment of a relationship between the extensional activity and the generation of the basalts. This is demonstrable using radiometric and stratigraphic techniques, notably Ar-Ar dating of amphibole phenocrysts.
Zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism
Authors: Kröner A. , Braun I. , Jaeckel P.
Published: May 2009

We report U—Pb and207Pb/206Pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at Ihosy, southern Madagascar. The granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650–700°C and 4–5 kbar. HigherP—Tconditions of 750–800°C and 6 kbar in the presence of low water activities have led to dehydration melting processes. The formation of granitic melts, which (partly) moved away from their source region, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bearing rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between ˜720 and ˜1855 Ma, suggesting a chronologically heterogeneous source region and a depositional age of less than ˜720 Ma for these rocks. High-grade metamorphism and anatexis are documented by zircon ages between 526 ±34 and 557 ±2 Ma with a mean age of about 550 Ma. The broad lithologies, metamorphic grades and ages recorded in the Ihosy rocks are similar to those in the Wanni Complex of northwestern Sri Lanka and in high-grade assemblages of southernmost India and support the contention that all these terrains were part of the Mozambique belt which formed as a result of collision of East and West Gondwana in latest Precambrian time.
Potassium–argon ages of volcanic rocks from northeast of Lake Turkana, northern Kenya
Authors: McDougall I. , Watkins R. T.
Published: May 2009

The Asille Group, a 1500 m succession comprising dominantly basaltic lavas but with interbedded sediments and silicic volcanics, lies unconformably upon metamorphosed basement northeast of Lake Turkana. Concordant K–Ar ages on alkali feldspars from pantelleritic units in the succession show that the Langaria Formation erupted 26.9±0.3 Ma ago in late Oligocene time, that Gum Dura Formation volcanism occurred at 15.8±0.2 Ma (early Miocene), and that there was widespread middle Miocene silicic volcanism at 13.0±0.3 Ma. Younger basaltic volcanism producing the flood lavas of the Gombe Group commenced at least 6.16±0.09 Ma ago in late Miocene time. These results, based upon 20 K–Ar age determinations, provide good numerical age control for much of the sequence developed within a little known sector of the Eastern Rift of Africa between the Ethiopian and Kenyan domes.
Multiphase magnetic overprints in the Moine Thrust Zone
Authors: Torsvik T. H. , Sturt B. A.
Published: May 2009

Palaeomagnetic studies from the southern part of the Moine Thrust Zone (MTZ) indicate a complex pattern of four major secondary remanence components (A–D). Their relative ages appear to vary from mid-Ordovician to Recent/Tertiary. The oldest remanence (A) is tentatively interpreted as associated with the formation of the Lochalsh Syncline (D1) and/or D2. However, the consistency between component A and the regional southeast-plunging lineation, makes it uncertain to what extent component A reflects the structural grain rather than a true palaeofield record. Component B appears to be associated with early Devonian uplift along the MTZ relating to localized shearing/high level brittle thrusting, and is almost synchronous with the emplacement of lamprophyre and felsic dykes.

In the inverted limb of the Lochalsh Syncline, and beneath the Balmacara Thrust, possible Permian (component C) and Recent/Tertiary magnetic overprints are recognized. Component C is partly of high magnetic stability and may indicate localized shears in the inverted limb, usually considered as Caledonian shearing. It is uncertain to what extent the youngest component (D), essentially recorded in rocks close to the Balmacara Thrust in mylonites and fault breccias, reflects important tectonic reactivation and magnetic resetting. This component is governed by low stability magnetic phases with blocking temperatures below 200 °C.
Mineralogy and geochemistry of Mesozoic black shales and interbedded carbonates, southeastern Sicily: evaluation of diagenetic processes
Authors: Azzaro E. , Bellanca A. , Neri R.
Published: May 2009

Upper Triassic/Lower Jurassic organic-rich shales and interbedded carbonates (Rhaetian → Sinemurian) are widespread in the subsurface of southeastern Sicily where important oil fields have been found hosted in Triassic reservoirs. Core samples from wells drilled offshore and onshore were studied from petrographie and geochemical viewpoints.

In the Hettangian/Sinemurian shale-carbonate sequences, which accumulated in a rapidly subsiding basin, the micritic aragonitic mud is still largely preserved. Mixed-layer I/S has remained randomly interstratified to a depth > 4000 m. Diagenetic carbonates are non-stoichiometric finely crystalline, pore-filling dolomite and/or calcite. The carbonate component exhibits a high Sr content and fair amounts of Fe and Mn. Carbon and oxygen isotopic values suggest a subsurface interstitial formation for the digenetic carbonates in an essentially closed system. Based on all accumulated data it is suggested that anoxic marine waters were retained in the sediment pores for a long time after deposition, thus enhancing the preservation of significant amounts of the original organic matter.

In contrast, Rhaetian tidal-flat deposits hosting black shales display a clay component characterized by ordered illite-rich I/S and a carbonate mineralogy dominated by low-Mg calcite in the uppermost beds and by near-stoichiometric dolomite in the lowermost ones. Petrographie, chemical and isotopic data indicate early cementation in an oxidizing phreatic environment and lower down in the sequence pervasive dolomitization in a sabkha-type environment.
Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones
Authors: Alsop G. I. , Hutton D. H. W.
Published: May 2009

The dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hitherto been regarded as a southeast-facing gently inclined F1 anticline, a gross geometry modelled on, and thought to be a possible correlative of, the Tay Nappe in Scotland. Remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up. However, a stratigraphie repetition coupled with a reversal in younging does occur in the Sperrin Mountains, much further south than previously realized. This hitherto unrecognized upward southeast-facing isoclinal Sperrin Nappe is, however, a D2 structure, traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km2. Following D2, a major 10 km thick D3 ductile shear zone resulted in translation towards the east-southeast. In the south, this deformation carried the Dalradian over Ordovician volcanics of the Tyrone Igneous Complex along the Omagh Thrust. Penecontemporaneity of magmatism with deformation clearly demonstrates that D3 is Caledonian (Arenig-Llanvirn). This deformation correlates with similar southeast-directed Caledonian thrusting in southern Donegal and Connemara. The apparent absence of Dalradian deformation of this age in southwestern Scotland may imply that Caledonian collision of outboard terranes with the miogeoclinal margin was initiated in Ireland and/or subsequent strike-slip has removed the evidence for deformation of this age from southwestern Scotland. The D3 shear zone in the Sperrin Mountains affects a very large volume of psammitic rocks. Within this shear zone the strain is not markedly higher than surrounding areas; however, its existence is demonstrated by the reorientation of mineral lineations over a large area. Such broad zones of only moderate strain may, we believe, be typical of translatory tectonics in areas of the mid-crust where there is little lithological diversity.
Deltaic sedimentation in an unstable tectonic environment – the Lower Limestone Group (Lower Carboniferous) of East Fife, Scotland
Authors: Fielding C. R. , Al-Rubaii M. , Walton E. K.
Published: May 2009

The Lower Limestone Group, which outcrops extensively on the east coast of Fife, southeast Scotland, consists of interbedded sandstones, siltstones and claystones with occasional coal seams, and limestone beds. Several of the coals have been mined economically in the past.

The sequence was deposited in a shallow-water, deltaic setting. Three facies associations have been distinguished; (1) laminated claystones, and marine bioclastic limestones, which were deposited in prodelta and marine shelf environments, (2) crudely coarsening-upward sandstone units, interpreted as delta front deposits of fluvially-dominated though wave-influenced, shallow-water deltas, and (3) interbedded clastic sedimentary rocks and coals, deposited in delta plain environments. The sequence as a whole accumulated by repeated progradation and abandonment of deltaic complexes.

Palaeosol profiles of four types are superbly preserved within the delta plain facies association. These are (1) organic soils (peats), now coal seams, (2) gley palaeosols, which formed in persistently waterlogged conditions, (3) freely drained soils which formed on a quartz sand-rich substrate, and (4) similar profiles formed by free drainage through variable substrates and involving B horizon accumulation of concretionary iron oxides. Compound palaeosols are common.

Deposition of the Lower Limestone Group was influenced by the tectonic and volcanic instability of the Fife area during Visean times. Frequent seismic events centred on active fault lines caused local uplift of parts of the normally waterlogged delta plain environment, allowing the formation of well-drained soils. Peats (coals) were best developed and least affected by oxidation in an elongate, fault bounded zone of enhanced subsidence.