bluuebunny/crossref_metadata_2025 huggingface dataset:
Submarine ‘crystal tuffs’: their origin using a Lower Devonian example from southeastern Australia
Authors: Cas R. A. F.
Published: May 2009

Summary. The submarine crystal-rich volcaniclastics of the Lower Devonian Merrions Tuff contain concentrations of angular to euhedral volcanic quartz, plagioclase and orthoclase. Lithic fragments, largely altered vitriclasts, are minor components and the average matrix content is 37.6%. Associated dacite/andesite and rhyodacitic lavas have average groundmass contents of 64.2%. Rare shards in the graded, pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions. The sedimentology of the volcaniclastics suggests subsequent redeposition by cold-state mass-flow processes. The volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption.

The high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed, so concentrating the crystal fraction. The crystal-rich character of the crystal-tuffs is not simply due to explosive eruption. Several primary and secondary factors/processes could have interacted to ultimately produce the crystal-rich character, these being: (i) eruption of highly crystallized magmas (≤ 65% phenocrysts), (ii) concentration of crystals in primary eruption columns, (iii) concentration of crystals in any resulting pyroclastic flows, fine vitric ashes being elutriated out and being carried away in accompanying, trailing ash clouds, (iv) concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean, and (v) concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine/subaerial settings.
Rb-Sr isochron from Gboko and Ikyuen rhyolites and its implications for the age and evolution of the Benue Trough, Nigeria
Authors: Umeji A. C. , Caen-Vachette M.
Published: May 2009

Summary. Rhyolites overlying the basement granitic gneisses and granites adjacent to the selimentfilled Benue Trough were studied. Rb-Sr isochron age of 113 ± 3 Ma and initial 87Sr/86Sr rato of 0.7067 were obtained from two extrusive bodies which form Gboko and Ikyuen hills. It is shown that their eruption has some relation to crustal doming at the onset of the Benue rifting. The published age of the oldest exposed sedimentary infill of the trough is Albian. As a result of the investigation of these volcanic rocks from the basement margin, however, an earlier and absolute age for the initiation of the rift is implied and a late Barremian to early Aptian age is proposed for the formation of this continental rift.
Stratigraphy of the Upper Devonian argillite successions in north Cornwall
Authors: Beese A. P.
Published: May 2009

Some 880 m of grey, green and purple argillites are estimated to be present in complexly deformed Upper Devonian successions which occur in several overlapping coastal sections. Four lithostratigraphical formations are formally defined, in order of decreasing age: the Marble Cliff Limestone; Longcarrow Cove Slate; Harbour Cove Slate; and Polzeath Slate Formations. Improved correlation and biostratigraphical control is available for the two younger formations which are studied in detail. The formations range in age from lowest Frasnian to mid Famennian. Despite the intense deformation there are no major structural breaks in the successions.
Ordovician nautiloid faunas of Central and Southern Thailand
Authors: Stait B. A. , Burrett C. F.
Published: May 2009

Nautiloids from the shallow water Ordovician carbonates of Central and Southern Thailand can be grouped into five broad assemblages. 1. The Middle Ibexian fauna of indeterminate endocerids from Tarutao Island. 2. Hardmanoceras chrysanthimum (Kobayashi) of the Upper Ibexian age strata on Tarutao Island. 3. The Upper Ibexian Manchuroceras nakamense sp.nov. from Ron Phibum, Southern Thailand. 4. Wutinoceras sp., Chaohuceras? sp. of Lower to Middle Whiterockian age strata from Satun Province. 5. Wutinoceras sp., Armenoceras chediforme Kobayashi and Georgina sp. of Lower-Middle Whiterockian age strata from Kanchanaburi, Central Thailand. All of these genera occur in Australia and China.
Introduction to papers on the Precambrian–Cambrian boundary
Authors: Cowie J. W.
Published: May 2009

These papers arise from the conference in Bristol, England, of the above international Working Group, which was held in the University of Bristol conference centre from 9–13 May, 1983. Forty-five persons attended; thirty-two were members of the Working Group (about a third of the total membership which is drawn from about twenty countries in many parts of the world from Australia to Argentina and from Korea to western Canada.)
Assemblages of calcareous algae near the Precambrian/Cambrian boundary in Siberia and Mongolia
Authors: Riding Robert , Voronova Larisa
Published: May 2009

Calcareous algae make their first appearance near the base of the Tommotian. The oldest confirmed occurrences are of ?Gemma, Obruchevella and Renalcis in the Nemakit-Daldyn Horizon and late Yudomian of Siberia. A little higher in the sequence, in the Aldanocyathus sunnaginicus Zone of the earliest Tommotian, Angulocellularia, Bajanophyton, Batenevia, Botomaella, Girvanella, Hedstroemia, Korilophyton, Proaulopora, Subtifloria, and Tarthinia appear. Immediately above this, in the Dokidocyathus regularis Zone of the middle Tommotian, Chabakovia appears with the epiphytacean genera Epiphyton, Gordonophyton and Tubomorphophyton. There are only minor subsequent additions to this flora before the end of the Atdabanian. Kordephyton appears in the Dokidocyathus lenaicus Zone of the late Tommotian and Razumovskia in the early Atdabanian.

These fossils constitute three calcareous algae assemblages (CAA 1, 2, 3) characteristic of the Nemakit-Daldyn Horizon/late Yudomian, early Tommotian, and middle Tommotian/Atdabanian respectively. Most of the genera are long-ranging, but their rapid appearance and diversification near the base of the Tommotian repesents an important event in the history of calcareous algae and augments faunal data concerning evolution and biostratigraphy near the Precambrian/Cambrian boundary.
The Precambrian–Cambrian boundary problem and its prospects, Northwest Himalaya, India
Authors: Kumar G. , Raina B. K. , Bhargava O. N. , Maithy P. K. , Babu R.
Published: May 2009

The Precambrian–Cambrian boundary problem is being studied in an areno-argillaceous sequence in the Kashmir and Spiti Valley, Northwest Himalaya, India. In Kashmir, a rich and diversified microbiota – cryptarchs and algae of the Late Precambrian, and low Lower Cambrian trace fossils are recorded. In the Spiti Valley, the yield of microbiota is poor and the trace fossils are late Lower Cambrian.
Sand-wave immobility and the internal master bedding of sand-wave deposits
Authors: Allen J. R. L.
Published: May 2009

Sand waves are such comparatively immobile transverse bedforms because they occur in tide-induced oscillatory bottom boundary layers typified by a steady velocity-component that generally is small compared to the amplitude of the periodic part. Consequently, the net bed-material transport rates, responsible for the long-term translation of the sand waves, typically are very small compared with the larger of the instantaneous rates. Sand waves should, therefore, be marked internally by series of erosional or, under restricted circumstances, non-depositional master bedding surfaces, each such surface, together with an associated comparatively thin sediment increment, being attributable to one sand-driving tide. Studies of modern sand waves, and investigations in the stratigraphic record, lend support to this conclusion. A further consequence of the regime of intense reworking under which sand waves exist is that their component grains should be in all ways more mature, other things being equal, than particles transported the same net distance by rivers.
Hertzian fracture, a principal mechanism in the emplacement of the British Tertiary intrusive centres
Authors: Bahat Dov
Published: May 2009

In the British Tertiary intrusive centres emplacement of cone-sheets, ring-dykes and related magmatic bodies followed in shape and sequence the pattern of Hertzian fracture produced in the country rock by the magmatic diapir. In Ardnamurchan doming of the country rock, conic fracture and intrusion of cone-sheets started the development of the intrusive centre. Following that, pressure was reduced and strain relaxation occurred which resulted in arcuate and median fractures. These fractures were later on intruded by magma, hence ring-dykes (curved flanges) were formed. Resumption of pressure initiated the second cycle of cone fractures (and inner cone-sheets of centre 2).
The ontogeny of Failleana and the origin of the Bumastinae (Trilobita)
Authors: Ludvigsen R. , Chatterton B. D. E.
Published: May 2009

The metaprotaspid of the bumastine Failleana calva is similar to protaspids of scutelluids, whilst late meraspids and early holaspids of F. calva are similar to fully mature styginids. This suggests that the bumastines were derived from the styginids in the Early Ordovician and that the bumastines later gave rise to the scutelluids. Bumastus and similar Silurian trilobites are phyletic derivatives of Ordovician bumastines and not effaced scutelluids as claimed recently by Lane & Thomas. Revised diagnoses of Illaeninae and Bumastinae are presented.
Revised stratigraphy of the Mesozoic rocks of southern Cyprus
Authors: Swarbrick R. E. , Robertson A. H. F.
Published: May 2009

Recent resurgence of interest in the Mesozoic rocks of SW and southern Cyprus necessitates redefinition of the Mesozoic sedimentary and igneous rocks in line with modern stratigraphical convention. Two fundamentally different rocks associations are present, the Troodos Complex, not redefined, a portion of late Cretaceous oceanic crust, and the Mamonia Complex, the tectonically dismembered remnants of a Mesozoic continental margin. Based on earlier work, the Mamonia Complex is divided into two groups, each subdivided into a number of subsidiary formations and members. The Ayios Photios Group is wholly sedimentary, and records the evolution of a late Triassic to Cretaceous inactive continental margin. The Dhiarizos Group represents Triassic alkalic volcanism and sedimentation adjacent to a continental margin. Several other formations not included in the two groups comprise sedimentary mélange and metamorphic rocks. The Troodos Complex possesses an in situ late Cretaceous sedimentary cover which includes two formations of ferromanganiferous pelagic sediments, radiolarites and volcaniclastic sandstones. The overlying Cainozoic calcareous units are not redefined here.
A geomagnetic investigation of Carboniferous igneous rocks at Tickenham, County of Avon
Authors: Kearey P. , Allison J. R.
Published: May 2009

A geomagnetic survey indicates that the Carboniferous igneous rocks in the Tickenham area are more extensive than previously recognized. Inversions of the magnetic anomalies suggest the causative basaltic body is tabular in form, conforms to the regional dip and has a coplanar magnetization. The magnetization vector orientation indicates emplacement during a period of magnetic field reversal and thus at a time different from other Carboniferous igneous rocks in the region.
S-type characteristics of the Hercynian granitoids of the Central Palaeozoic Massif, Morocco
Authors: Mahmood A. , Bennani A.
Published: May 2009

This petrographic and chemical study of the Hercynian granitoids of the Central Palaeozoic Massif of Morocco covers the Zaër, Ment, Oulmès, and Moulay Bouazza plutons. These plutons are formed of biotite or two-mica, quartz-rich granodiorites and monzogranites containing reddish-brown, inclusion-rich biotites as the only essential mafic mineral; zoned plagioclase; and perthitic K-feldspar. The modal contents of micas and K-feldspar are highly variable. The granitoids are generally medium to coarse grained, massive; however foliated varieties are also commonly found, particularly at the contact of the' plutons with sedimentary rocks.

The strongly peraluminous character of the granitoids is shown by high normative corundum and high molar Al2O3/(CaO + Na2O + K2O) ratio, and is reflected by the occasional presence of aluminosilicates.

The mineralogical and chemical characteristics of the granitoids are similar to those of the S-type granites, and were inherited from the crustal source of the granitic magmas which are thought to have been generated by partial equilibrium fusion of the sandy argillaceous sediments of the Massif. The heat source for the fusion might have been provided by intrusion of mantle-derived material.
On the Bathonian (Middle Jurassic) ammonites Micromphalites Buckman and Gracilisphinctes Buckman from Kachchh, Western India
Authors: Jaitly A. K. , Singh C. S. P.
Published: May 2009

Systematic descriptions of specimens collected from Pachchham ‘Island’, Kachchh, Western India, belonging to the genera Micromphalites Buckman and Gracilisphinctes Buckman are given. Their affinities with the different forms belonging to the above genera are discussed and described as Micromphalites aff. hourcqui Collignon and Gracilisphinctes cf. arkelli Collignon respectively. The distribution of these two genera in other regions is also briefly discussed.
Garnet and associated minerals in the southern margin of the Menderes Massif, southwest Turkey
Authors: Ashworth J. R. , Evirgen M. M.
Published: May 2009

Assemblages with muscovite + quartz show a regular increase in grade from the Chlorite Zone at the base of the Lycian Nappe Complex to the Garnet Zone within the structurally underlying Menderes Massif. Biotite enters before garnet, which precedes oligoclase. Garnet-bearing assemblages in pelites are compared with those in re-equilibrated quartzofeldspathic gneisses, where garnet is unusually calcic (in one case approaching Gross50Alm50). Local retrograde effects are noted but no evidence is found for a polymetamorphic record in the mineral compositions. Garnet zoning, with Mn decreasing outwards, is interpreted as growth zoning; Ca decreases outwards in pelite garnets but shows the reverse effect in the gneisses. Chloritoid is common but rarely coexists with biotite, and garnet + chlorite + paragonite is found rather than chloritoid + albite. Garnet-biotite geothermometry, corrected for the effect of Ca in garnets with up to 29 mole % grossular, indicates temperatures of 530±5O°C near the garnet isograd. As in other areas, an attempt at muscovite-paragonite geothermometry gives an anomalous result. Metamorphic pressure isconsidered in the light of (i) Mn/Fe partition between garnet and biotite, (ii) Ca content of garnet coexisting with plagioclase + muscovite + biotite, (iii) Na in actinolite coexisting with albite + chlorite + magnetite, and (iv) celadonite content of muscovite which, however, shows variation due to disequilibrium within a specimen and does not provide an accurate geobarometer. Comparisons with published studies indicate a strong similarity to the Barrovian Dalradian of Scotland and lead to a tentative pressure estimate of approximately 5 kbar.
Tufted microbial (cyanobacterial) mats from the Proterozoic Stoer Group, Scotland
Authors: Upfold R. L.
Published: May 2009

Upward-thinning stellate and polygonal carbonate structures are described and compared to modern and ancient stromatolites. Although tufted microbial mats are common in modern settings they have rarely been described from ancient rocks owing to their poor preservation potential. The preservation of these from the Stoer Group is due to early replacement by calcite before their original tufted relief could be obliterated by compaction. Associated limestones with some similar features are interpreted as flat to mamillated microbial mats.
Polymetamorphism in the Dalradian of the central Scottish Highlands
Authors: Dempster T. J. , Harte B.
Published: May 2009

Systematic variations in both timing and conditions of metamorphism are observed in the Dalradian rocks of Central Perthshire, Scotland. Early (D2–D3 and syn-D3) relatively low-pressure metamorphism (550 °C, 7 kb) occurs in the Flat belt rocks of the Tay Nappe to the south, and later (post-D3) higher-pressure conditions (550 °C, 9 kb) dominate the Steep belt to the north. The latter appear to be superimposed on earlier low pressure assemblages (chloritoid–biotite) with the pressure increase probably relating to formation of the Tummel Steep Belt, possibly near 460 Ma. This polymetamorphism, in addition to the generally high pressures of Barrovian metamorphism throughout Perthshire, may account for the poor development of the metamorphic zonal pattern in this region compared to the eastern Dalradian.
The tectonic evolution of the Abitibi greenstone belt of Canada
Authors: Ludden John , Hubert Claude , Gariépy Clement
Published: May 2009

Based on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics.

The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma.

The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.
Some remarks on the continental margins in the Aquitaine and French Pyrenees
Authors: Boillot G.
Published: May 2009

From the Triassic to the Late Eocene, the Iberian Peninsula underwent three successive rotations with respect to the stable European plate, (a) Prior to the Late Aptian, a nearly 150 km southwestward motion resulted in stretching and thinning of the continental crust beneath the North Pyrenean zone, the Aquitaine Basin and the Bay of Biscay continental margins (rifting). Distensive structures trended 90° N to 130° N, and were shifted by 30° N to 50° N transform faults. (b) During the Late Aptian to Santonian interval, an approximately 400 km southeastward motion resulted in the opening of the Bay of Biscay and sinistral slipping of Iberia along the North Pyrenean transform zone (drifting), (c) During palaeocene–Eocene time, a 150 km northwestward convergent motion resulted in limited subduction of the oceanic crust of the Bay of Biscay beneath the Iberian plate, and folding of the Pyrenean chain. The folded belt resulted from squeezing of the former European and Iberian margins (rifted or transform margins, depending on the segment considered).
Palaeogene evolution of deformation in the northeastern Iberian Peninsula
Authors: Guimerá Joan
Published: May 2009

During the Palaeogene Alpine compression three units were differentiated in the studied area (south of the Ebro Basin): the Catalan Coastal Range, dominated by NE–SW major basement faults with a sinistral movement; The Iberian Range, where the important basement faults have a NW–SE direction and a reverse movement (often with a dextral component); the Linking Zone, between these two ranges, where an E–W dominant structural direction is marked by an array of folds and thrusts (with a northward vergence) in the Mesozoic cover.

From the analysis of both these major structures and the small-scale structures, it can be deduced that the compression in the studied area has evolved from a NW–SE direction (lower-middle Eocene) to a N–S direction and to a NE–SW one (uppermost Oligocene). The major structures are due to the N–S compression. Later, the stress-field progressively changed to a distensive regime.

We suggest a relation between the compressive phases and the displacement direction of the Iberian Plate with reference to the European Plate.
Subsidence history of the Aquitaine basin determined from subsidence curves
Authors: Brunet Marie-Françoise
Published: May 2009

The subsidence history of the Aquitaine basin has been determined from deep exploratory well data. The contribution of sediment loading to the subsidence has been evaluated and removed, including corrections for effects of compaction, water depth and changes of sea-level but not of flexural rigidity of the lithosphere. The tectonic subsidence curves obtained indicate that the basin underwent two periods of subsidence acceleration in the Trias–Early Lias and Late Jurassic–Early Cretaceous. They are interpreted in relation to the displacements of Iberia with respect to Europe as two periods of extension followed by periods of passive subsidence due to conductive cooling of the lithosphere; the formation of the ‘flysch trough’ in the southern part of the basin is then discussed.
Carboniferous North Atlantic palaeogeography: stratigraphic evidence for rifting, not megashear or subduction
Authors: Haszeldine R. S.
Published: May 2009

Three theories have been proposed for the origin of Carboniferous basins in Britain: megashear; tension from Rheic Ocean subduction; tension from rifting of the North Atlantic. The first two hypotheses are rejected because they do not explain the Carboniferous volcanism, Stephanian dyke swarm, Boreal marine transgressions during the late Carboniferous and Permian, Carboniferous sedimentary basin histories, basin types or basin orientations.

Carboniferous volcanics were rift-related due to crustal thinning, which also resulted in the formation of sedimentary basins in the British Isles and a marine transgression of Tethyan faunas. Newly formed Carboniferous fracture lines and basin orientations showed that tension varied between east–west and northwest–southeast. Crustal fracturing in the latest Dinantian, possibly due to collision of microcontinents in the closing Rheic and Phoibic oceans with North America, led to the synchronous initiation of rift basins in East Greenland, elevation of source areas for the Millstone Grit of the British Isles, and formation of transform fault zones near Svalbard and North Spain. The narrow, rapidly subsiding, quickly changing ‘fosse’ basins which formed in these transcurrent fault zones contrast with the coeval stable, slower subsiding rift-parallel ‘saucer’ basins of the British Isles. Variations of subsidence rates in all these basins allow interpretations of crustal stress history. Brittle fracturing in the Westphalian C formed the first oceanic crust, but free oceanic spreading from Spain to southwest of the Faeroes only occurred after Stephanian dyke intrusion and crustal thinning at the Faeroes. The Norwegian Sea underwent continental crustal thinning. Ocean spreading and crustal thinning ceased in latest Stephanian times.

Boreal marine transgressions advanced down the newly thinned Norwegian Sea, firstly along its northern part and then reaching the North Sea basins and Germany in the Rotliegendes and Zechstein. Synsedimetary ore deposits formed during early tensional fracturing of the crust; thick coal sequences formed in rift-parallel basins during clastic source decay of elevated rift margins; oil source rocks formed along narrow oceanic or continental rifts from the Westphalian C onwards.
Geochemical evidence for loss of Na and K from Moinian calc-silicate pods during prograde metamorphism
Authors: Tanner P. W. G. , Miller R. G.
Published: May 2009

A total of 111 major element analyses have been made of impure calcareous rocks from the Late Precambrian Moinian sequence in NW Scotland. Samples were collected along a 36 km traverse from three stratigraphic horizons which are repeated by folding and vary in metamorphic grade from lower to upper amphibolite facies. Anorthite content of the plagioclase, which increases progressively from albite to anorthite, was assessed by X-ray diffraction measurement of 2θ (131–11); this value is used as a ‘metamorphic index’ for rocks with a CaO/Al2O3 (wt. %) ration of 0.3–0.7 and Na2O/Al2O3, < 0.35. Na2O/Al2O3 and K2O/Al2O3 ratios plotted against this index, together with the spatial relationship between metamorphic/chemical isograds and stratigraphic boundaries show that both Na (0.24–4.82% Na2O) and K (0.01–3.28% K2O) are lost from the calc-silicate rocks during prograde metamorphism. These chemical changes result in a variation of the stable mineralogy with increasing metamorphic grade and take place through the medium of the intergranular fluid.
The fenitized aureole of the Newania carbonatite, Rajasthan
Authors: Viladkar S. G.
Published: May 2009

The Newania carbonatite is mainly ferruginous rauhaugite with subordinate ankeritic carbonatite and dykelets of sovite. In a zone of fenitization the surrounding granite-gneiss country rock has been converted into syenitic fenite, composed mainly of feldspar and ferri-eckermannite with small amounts of albite, anorthoclase and accessory apatite.
A note on the formation of fulgurites
Authors: Williams D. J. , Johnson W.
Published: May 2009

A phenomenon similar to the sand tube fulgurite has been observed in experimental work carried out to compact and sinter metal powders using high voltage discharges. The sand tube fulgurite is a fused quartz tube formed by the passage of a lightning discharge. Further experiments, in which a high voltage discharge was passed from a pointed electrode held above a large volume of copper powder, through to an earthed plate, give a copper product with the characteristic fulgurite morphology. This result suggests that the quartz sand can behave as a good conductor in the lightning discharge, perhaps due to conducting coatings on the surface of the sand particles.
Geology, geochronology and chemical evolution of the island of Pantelleria
Authors: Civetta L. , Cornette Y. , Crisci G. , Gillot P. Y. , Orsi G. , Requejo C. S.
Published: May 2009

Potassium–argon dating, field relations, geochemical and strontium-isotope compositions are reported for the island of Pantelleria (Strait of Sicily, Italy). These data support the following model for the genesis and evolution through time of the volcanic system: the peralkaline rocks originated from mantle-derived parental magmas; the trachytic magma differentiated in a low pressure magma chamber by crystal–liquid fractionation. This process led to a chemically zoned magma chamber tapped at different levels by successive eruptions. During low-pressure differentiation the 87Sr/86Sr ratios of some of the most evolved Sr-poor rhyolitic magmas increased from 0.703 up to 0.708 by contamination with crustal material.

The chemical variation displayed by the products of each of the defined eruptive cycles in the last 50000 years suggests an open system behaviour of the magma chamber which is episodically refilled by more mafic parent magma, differentiated at high rate and episodically erupted.
Zoning in highly alkaline magma bodies
Authors: Wolff John A. , Storey Michael
Published: May 2009

We present chemical data on magmatically heterogeneous pyroclastic deposits of late Quaternary age erupted from zoned magma systems underlying Tenerife (Canary Islands), Sao Miguel and Faial (Azores), and Vesuvius. The most fractionated magmas present at each centre are respectively Na-rich phonolite, trachyte, and K-rich phonolite. Within any one deposit, chemical variation is either accompanied by changes in the phenocryst assemblage (petrographic zonation) or is largely manifested in trace element abundances, unaccompanied by any petrographic change (occult zonation). Zoning is analogous to that in calc-alkaline systems where the most fractionated products are high-silica rhyolites. When a range of magma types are considered, a correlation emerges between roofward depletion of trace elements (especially REE) in the zoned system and compatability of those same trace elements in the accessory phenocryst phases present. Thus, allanite- or chevkinite-bearing rhyolitic systems are light-REE depleted roofwards, the sphene-bearing Tenerife system is middle-REE depleted roofwards, the melanite-bearing Vesuvius system is heavy-REE depleted roofwards, while the Azores systems, which lack these phases, display roofward REE enrichment. Therefore, the behaviour of trace elements may in each case be explained by fractionation of observed phenocryst assemblages. The resemblance between features of zoned magma systems and published work on the dynamic consequences of cooling saturated aqueous solutions prompts us to suggest that sidewall crystallization and consequent boundary-layer uprise to form a capping layer at top of the system may be a plausible mechanism for the generation of both petrographic and occult zonation. Reverse zoning occurs among the first-erupted tephra of some deposits, demonstrating that the most highly differentiated magma available is not always the first to be tapped during an eruption from a zoned system.
Ediacaran events: boundary relationships and correlation of key sections, especially in ‘Armorica’
Authors: Jenkins R. J. F.
Published: May 2009

The transitional interval between the Adelaidean and Ediacaran systems in the central Flinders Ranges, South Australia, comprises mainly basinal sediments, with some tuff beds (Bunyeroo Formation) and a widespread thin dolomite bed which apparently evidences a brief regressive episode (base of Wonoka Formation). Body imprints of metazoans, trace fossils and probable faecal pellets are present at various levels in the Ediacaran succession. Comparable assemblages occurring in key successions in southern Africa, northern Russia, Siberia, Newfoundland and England promise a global biostratigraphy. One Ediacaran body fossil occurs in the Yangtze Gorges section, China. Analysis of abundant late Precambrian radiometric data for the present North Atlantic margins suggests that the Ediacaran may be dated between about 590 Ma and ~ 545–540 Ma.
A note on the Birchi Beds, Lower Jurassic, at Lyme Regis, Dorset
Authors: Palmer C. P.
Published: May 2009

The chronic repetition of a small graphical error, initiated by H. B. Woodward and depicting the Birchi Beds (Lower Jurassic) dipping beneath Lyme Regis, needs rectifying since, by implication, it leads to a gross stratigraphical and structural error amounting to about 21 m (70 ft) at Lyme Regis, Dorset.
Graptolite ultrastructure: evolution of descriptive and conceptual terminology
Authors: Rickards R. B. , Dumican L. W.
Published: May 2009

Graptolite ultrastructural studies using SEM and TEM are now more than a quarter of a century old and have reached the stage of routine procedure where material of suitable preservation is to hand. (Specimens may be chemically isolated from the rock or, in SEM and STEM studies, may be retained in the matrix.) Description of ultrastructural elements has evolved from the purely empirical to a conceptual/biological classification following (a) the recognition of the role of collagen fibrils in skeletal construction, and (b) the realization that at least two models of skeletal secretion are possible, each essentially opposed to the other. In this paper old terminology is explained with reference to current practice and the latter is tabulated and referred to the increasing number of described ultrastructural elements. Some of the outstanding problems of interpretation are outlined; and a set of concise, in some cases revised, ultrastructural definitions is given together with illustration, where appropriate, of the ultrastructural element defined.
Geochemical evolution of the Jarawa Younger Granite complex and its related mineralization, northern Nigeria
Authors: Imeokparia E. G.
Published: May 2009

The Jarawa Younger Granite complex is composed of high silica alkali granites that were emplaced 161 Ma ago. The granites are characterized by high contents of Rb, Li, F, Sn, Nb, W above normal low-Ca granitic rocks and have typical S-type characteristics that are indicative of a substantial component of crustal melt.

Mineralization in the complex is associated with the biotite granite which was emplaced as a sheet-like body at relatively shallow depth and occurs as disseminations and as greisen lodes and veins.

Chemical studies of the granites have shown that the biotite granite represents a highly fractionated rock that crystallized from a residual magma from which the hornblende-biotite granite had previously crystallized. However the biotite granite is characterized by steep gradients in some minor and trace elements that apparently indicate that liquid-state differentiation and/or volatile complexing processes made significant contributions to their differentiation. Enrichment of Th, Li, Rb, Sn, W and Nb may be more closely linked to roofward migration of F.
Fossil Mediterranean molluscs as sea-level indicators
Authors: Richards G. W.
Published: May 2009

Intertidal molluscs are known to occur in similar positions or zones on shores throughout the world. However, little is known about the extent to which these distributions are preserved in the fossil record. This study attempts to define the relationship to present sea level of live and dead intertidal molluscs in the Mediterranean, and utilizes this information to interpret the position at which Quaternary shorelines were formed.

The preservation of zonation patterns in the fossil record is affected by post-mortem transport, breakage and burial of the shell material. To take account of such taphonomic changes, experimental studies of shell transport and burial under waves and unidirectional currents were made, and the distribution of dead molluscs on modern shores were compared with live distributions. It proved possible to predict the frequency with which certain species were deposited on the modern shore and, by analogy, the likelihood of their preservation in the fossil record. Thus death assemblages in fossil shorelines can be accurately related to past sea levels.
Reconnaissance potassium–argon geochronology of the Suregei–Asille district, northern Kenya
Authors: Fitch F. J. , Hooker P. J. , Miller J. A. , Mitchell J. G. , Watkins R. T.
Published: May 2009

Potassium–argon dating of 44 samples of basalt lavas and pantelleritic ignimbrites provides a provisional chronology for the 1900-metre thick Cenozoic succession of the Suregei–Asille district, northeast of Lake Turkana. Volcanic rocks of the Asille Group range in age from late Oligocene (Chattian) to late Miocene (Tortonian). Data obtained from conventional K–Ar total-rock analysis of basalt samples are appraised statistically to indicate the presence of three periods of enhanced volcanism centred around 27, 17 and 11.5 Ma. Equivalent ages obtained from the pyroclastic pantellerites by conventional K–Ar total-rock analysis and 40Ar/39Ar analysis of sanidine concentrates indicate that basic and acid eruptions were closely spaced in time. Continental tholeiite flood lavas belonging to the Gombe Group are of Pliocene age, but are difficult to date precisely because of their young ages and relatively high atmospheric contamination occurring as a result of the secondary alteration in the ubiquitous glass mesostasis. Those ages involving least atmospheric correction are considered most reliable, and are closely similar for the Chen Alia and Harr formations at around 4.85 Ma, although later flood eruptions may have occurred to the south and west of the Suregei–Asille district at about 3.85 Ma.
Panjal Trap chemistry and the birth of Tethys
Authors: Bhat M. I. , Zainuddin Syed M. , Rais A.
Published: May 2009

K2O, TiO2, P205, Zr, Y, Sr and Nb data from the Panjal Traps of Kashmir indicate that the lava erupted in a rift tectonic setting. This is also suggested by the other features of these rocks and volcanic history of the area. The time of eruption represents the initial phase of break-up of the Indian plate from the Siberian plate. This conclusion is in good agreement with the current views on the tectonic evolution of the area.
Rare earth abundances and origin of some Indian lamprophyres
Authors: Paul D. K. , Potts P. J.
Published: May 2009

Abundances of rare-earth elements (REE) and Th, Ta, Hf, Sc, Co, Ba determined by instrumental neutron activation analysis show similarity between lamprophyres and other potassic rocks including kimberlite. The Indian lamprophyres have fractionated REE patterns with enrichment of light REE. The light REEs show more variation than the heavy REEs within the group.

Lamprophyres occurring in intimate association with the alkaline basalt may be genetically related. The trace element characteristics of the abundant lamprophyre dykes in granitic terrain are unlikely to be generated by assimilation of alkaline basalt by granitic crust. Small amounts of partial melting in mantle pockets enriched in incompatible elements as a result of metasomatic processes would explain the elemental concentration observed in the lamprophyres.
The geology of central Isla Hoste, southern Chile: sedimentation, magmatism and tectonics in part of a Mesozoic back-arc basin
Authors: Andrews-Speed C. P.
Published: May 2009

Central Isla Hoste lies towards the Pacific side of a Mesozoic back-arc basin in southern Chile, behind a magmatic arc represented by the Patagonian batholith. The volcaniclastic sediments on Isla Hoste were derived from the magmatic arc and deposited in the back-arc basin by turbidity currents. These sediments may, in part, overlie mafic volcanic and intrusive rocks. Geochemical and lithologic data are used to suggest an ocean-floor origin for these mafic igneous rocks on central Isla Hoste. The sedimentary and mafic igneous rocks were deformed and uplifted during the middle Cretaceous Andean orogeny. However, the nature of the crust underlying most of the sedimentary rocks in the back-arc basin and the style of deformation in most of this basement are unknown. Therefore the role, if any, of subduction within the back-arc basin during the middle Cretaceous ‘closure’ of this basin is not certain. Post-kinematic intrusions within the back-arc basin are common. These intrusions will confuse attempts to determine by geophysical means the nature of the pre-kinematic basement of the back-arc basin and attempts to outline the present extent of the basin in offshore regions.
The primary crescumulates of the Lizard peridotite, Cornwall
Authors: Rothstein A. T. V.
Published: May 2009

Preserved in the deformed peridotites of the Lizard, Cornwall, are relics of intricate primary precipitates of olivine. The mineral compositions of those rocks with an abundance of primary relics are typical of the primary assemblage Lizard peridotite. In detail the olivine primary structures resemble, but are not identical to, the crescumulates described from various layered ultrabasic intrusions. The available evidence shows distinctive specific features in the Lizard crescumulates in the scale and morphology of the structures and in the conditions of growth.
Evidence for glaciation in the Ordovician rocks of western Ireland
Authors: Williams D. M.
Published: May 2009

The Maumtrasna Formation is a thick succession of coarse clastic rocks situated in the South Mayo Trough of western Ireland. Its upperage limit is not defined but it is post-Llanvirn in age and probably Ordovician. Several aspects of the conglomerates within the succession are compatible with the action of a glacial influence during its deposition. New evidence is presented which tends to confirm the glacial hypothesis. 10.7% of garnets separated from the matrix of the conglomerates exhibit chattermark trails. Published accounts of such trails indicate that they are only found in sediments which are glaciogenic or were derived from pre-existing glaciogenic sediments. It is suggested that the Ordovician glacial activity in the South Mayo Trough took the form of a highland glaciation which was maintained, possibly at sub-equatorial latitudes, by a continuous uplift of the source area.
Vendian rocks of the western part of the East European Platform: stratotypic range, boundaries and principles of their establishment
Authors: Makhnach A. S. , Veretennikov N. V. , Shkuratov V. I.
Published: May 2009

The Vendian in a stratotype area in the western part of the East European Platform includes three regional stratigraphic subdivisions: the Vilchanian, Volynian and Valdaian series, which clearly differ in structure and lithology. They are closely interconnected stratigraphically and geochronologically, and form an indivisible Late Baikalian structural stage of the platform sedimentary cover. The upper subdivision, the Valdaian Series, is palaeontologically substantiated and divided into horizons. The Vendian in such a composition has definite boundaries: the upper biostratigraphic, and lower lithostratigraphic ones, and is well correlated with the formations of the same age from other regions.
Sedimentation of the Vendian–Cambrian marine sequence, Poland
Authors: Lendzion K.
Published: May 2009

Anbstract

In the Lublin area of the East European Platform (southeast Poland) sedimentation of the Vendian–Cambrian marine sequence began in Late Vendian times. Vendian and Cambrian strata occur in sedimentary continuity and their features indicate sedimentation in the tidal zone of a shallow epicontinental sea. The Vendian–Cambrian boundary, drawn on the basis of mainly palaeontological data, passes between theVendotaeniaandSabellidites zones.
Upper Precambrian–Lower Cambrian biota from the northwest of Argentina
Authors: Aceñolaza Florencio Gilberto , Durand Felipe Ramón
Published: May 2009

The principal characters of the biota from Upper Precambrian–Lower Cambrian rocks in the northwest of Argentina are described. It is possible to observe two groups: the trace fossils and the medusoid impressions. At the same time, a regional palaeogeographical analysis is attempted as well as a comparison with outcrops of similar age from the Gondwanaland area and from North America.
Evolution of archaeocyaths and palaeobiogeography of the Early Cambrian
Authors: Zhuravlev Andrey Yu.
Published: May 2009

In the Early Cambrian, there were two peaks of the increase in number of new archaeocyathan genera. These diversification bursts are, perhaps, related to significant changes in Early Cambrian palaeogeography. The first burst at the beginning of the Atdabanian was, perhaps, connected with the initiation of archaeocyathan expansion beyond the Siberian Platform. During the second half of the Atdabanian, closely related archaeocyathan assemblages were established from North Africa to Australia because neither climate, nor geographic isolation could have affected their differentiation in that time. At the end of the Atdabanian and the beginning of the Botomian the number of archaeocyathan genera again increased sharply, and isolated provinces were established. The American–Koryakiyan province was formed in western North America (from Alaska to Sonora) and included the Koryakiya, while the Afro–Siberian–Antarctic province probably extended from North Africa and Western Europe to Siberia, Australia, and Antarctica. The main difference between the regular archaeocyathan assemblages lies in what skeletal elements filled the intervallum. There was a certain stability in the distribution of skeletal elements of high taxonomic rank and an interchangeability of elements of low taxonomic rank.
The stage scale of the Cambrian System
Authors: Spizharski T. N. , Zhuravleva I. T. , Repina L. N. , Rozanov A. Yu. , Tchernysheva N. Ye. , Ergaliev G. H.
Published: May 2009

A stage subdivision of the Cambrian System is proposed. In the lower series four stages are distinguished (the Tommotian, Atdabanian, Botomian, Tojohnian); in the middle series, two (the Amga and Maya stages); in the upper series, three (the Ayusakkanian, Sakian, and Aksayan stages). The stratotypes of the stages of the lower and middle series are located on the Siberian Platform, while the stratotypes of the stages of the upper series are found in Kazakhstan (the Maly Karatau). The lower boundary of the Cambrian is drawn along the bottom of the Al. sunnaginicus Zone, the lower boundary of the middle series along the Schistocephalus Zone, and that of the upper series along the Cormagnostus simplex Zone. The Cambrian–Ordovician boundary is not quite clear yet, but it is most likely to be drawn along the bottom of the C. proavus Zone.
Prospectatrix genatenta(Stubblefield) and the trilobite superfamily Cyclopygacea
Authors: Fortey R. A.
Published: May 2009

Cyclopyge genatentaStubblefield, 1927, is interpreted from new material, and established as the type species of a new cyclopygid genus,Prospectatrix. The morphology ofProspectatrixshows that the cyclopygids and nileids are closely related, and together with the Taihungshaniidae, these families are included in an expanded concept of the Cyclopygacea. The subfamily Ellipsotaphrinae Kobayashi and Hamada, 1971 is excluded from the Cyclopygidae. Pelagic habits arose on at least five separate evolutionary lines among Ordovician trilobites:Prospectatrixlies near the origin of one of these.CremastoglottosWhittard was derived from a robergioid ancestor by forward migration of the eyes.
The Ordovician trilobite fauna of the Great Paxton Borehole, Cambridgeshire
Authors: Rushton A. W. A. , Hughes C. P.
Published: May 2009

The Ordovician trilobite fauna of the Great Paxton Borehole, Cambridgeshire, consists of twelve species including the seven new species;Segmentagnostus stubblefieldi, Trinucleus bicallis, Bergamia artemis, Protolloydolithus salax, Ogygiocaris macrops, Merlinia anxiaandBohemilla tridens. The fauna recalls that from the lower Llanvirn of the Shelve area, Shropshire, but the possibility of an upper Llanvirn age remains.
The Ordovician brachiopods of Cornwall
Authors: Bassett Michael G.
Published: May 2009

The brachiopod fauna of the Gorran Quartzites of south Cornwall is described; it comprises five orthid genera. Three taxa are described under open nomenclature, and one new species is assigned to a new genus (Schalidomorthis stubblefieldi). The fauna supports the Llandeilo age suggested by accompanying trilobites. There is no brachiopod evidence to indicate that the quartzite pebbles of the Triassic Budleigh Salterton Pebble Bed are derived directly from the Ordovician of Cornwall.
Crustal growth of the Antarctic Peninsula by accretion, magmatism and extension
Authors: Storey B. C. , Garrett S. W.
Published: May 2009

A subduction-accretion model incorporating new geophysical data is presented to explain the geology of the Antarctic Peninsula from late Palaeozoic to Cenozoic time. According to the model, the peninsula consists of overlapping accretionary, magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form. The crust, which contains a small proportion of sialic basement, was mainly formed by accretionary and magmatic processes and modified to its present shape by extension. The Gondwanide Orogeny for the Antarctic Peninsula is interpreted in terms of the accretionary processes.