bluuebunny/crossref_metadata_2025_split huggingface dataset:
First report of Pan-African Sm—Nd and Rb—Sr mineral isochron ages from regional charnockites of southern India
Authors: Unnikrishnan-Warrier C. , Santosh M. , Yoshida M. 
Published: May 2009

AbstractMineral and whole-rock isotope data for a massive charnockite from Kottaram in the Nagercoil Block at the southern tip of Peninsular India yield Sm—Nd and Rb—Sr ages of 517 ± 26 Ma and 484 ± 15 Ma respectively. The Nd model age calculated for the charnockite isc. 2100 Ma. Our study reports the first Pan-African mineral isochron ages from regional charnockites of Peninsular India, which are in good agreement with the recently obtained ages of incipient charnockites in the adjacent blocks, as well as alkaline plutons within the same block. Our results indicate that the Pan-African tectonothermal event in the granulite blocks south of the Palghat—Cauvery shear zone was regional, with terrain-wide rejuvenation. These results correlate with similar Pan-African tectono-thermal events reported from Sri Lanka and East Antarctica, and have an important bearing on Gondwana reconstructions.
Isotope studies of granitoids from the Bangenhuk Formation, Ny Friesland Caledonides, Svalbard
Authors: Johansson Å. , Gee D. G. , Björklund L. , Witt-Nilsson P. 
Published: May 2009

AbstractThe Caledonian Hecla Hoek succession in Ny Friesland, eastern Svalbard has been interpreted, for many decades, to be a continuous stratigraphic sequence. Early Palaeozoic and Neoproterozoic strata in its upper parts pass more or less conformably down into amphibolite facies rocks (Stubendorffbreen Supergroup) at depth. Recent isotopic age-determination and structural studies have indicated that the Stubendorffbreen succession is tectonostratigraphic and made up of at least three major thrust sheets. This paper provides new data from two meta-igneous units within the succession, the Bangenhuk and Instrumentberget gneisses. Both are granitoid sheets, consisting mainly of red, strongly lineated gneisses of monzogranitic composition; the Bangenhuk unit also contains some lenses of little deformed granitoids, as well as cross-cutting aplite dykes, amphibolitized dolerites and subordinate metasedimentary rocks. The latter are locally cut by granitoids. U—Pb zircon dating of six samples of variably deformed Bangenhuk granitoids, including one cross-cutting aplitic dyke, has yielded ages between 1720 and 1770 Ma, the higher values generally from the less deformed samples. The Instrumentberget gneissic granite yielded an age of 1737+46−41 Ma. These ages are interpreted to date the time of intrusion of the granitoids at around 1750 Ma; the younger ages may have been slightly lowered by Caledonian deformation, particularly those from specimens located close to a major fracture (the Billefjorden Fault Zone) in Wijdefjorden—Austfjorden. U—Pb dating of titanite from the least deformed granitoid also yields comparable Palaeoproterozoic ages; in the more deformed rocks, however, titanites give evidence of Caledonian ductile deformation at c. 410 Ma. The Rb—Sr system of the corresponding whole rock samples has been disturbed and yields an errorchron age of about 1650 Ma and, for some samples, an impossibly low initial Sr ratio. The Sm—Nd system may be more intact and yields initial εNd values of −2 to −3, suggesting some contribution from older crustal material to the granitoid magmas. The results indicate the presence of extensive units of Palaeoproterozoic granitic basement within the Lower Hecla Hoek succession of Ny Friesland, supporting the hypothesis that the latter is composed of tectonically intercalated basement and cover units.
Geochemical character of Neoproterozoic volcanic rocks in southwest Wales
Authors: Bevins R. E. , Pharaoh T. C. , Cope J. C. W. , Brewer T. S. 
Published: May 2009

AbstractVolcanic rocks of Neoproterozoic age form several fault-related inliers in Wales and the Welsh Borderland. Those in North Wales and the Borderland have been investigated recently, while those in southwest Wales remain little studied. In southwest Wales, basic to intermediate lavas and related volcaniclastic rocks and silicic tuffs in the St David’s area comprise the Pebidian Supergroup, which is intruded by minor basic intrusions, while the Coomb Volcanic Formation, exposed farther east around Llangynog, is composed of basic and silicic lavas and silicic tuffs, intruded by a single dacite sill and numerous dolerite sills. New geochemical data show that both the Pebidian and Coomb basalts have subduction-related characteristics. The Coomb basalts are geochemically similar to those of the Uriconian Group of the Welsh Borderland, while the Pebidian basalts bear a similarity to volcanic rocks recovered from the Bryn-teg Borehole, in the Harlech Dome. Uriconian-type volcanic rocks may underlie much of the Midlands Microcraton, with the Welsh area underlain by volcanic rocks of Pebidian type.
The crystalline units of the High Himalayas in the Lahul–Zanskar region (northwest India): metamorphic–tectonic history and geochronology of the collided and imbricated Indian plate
Authors: Pognante U. , Castelli D. , Benna P. , Genovese G. , Oberli F. , Meier M. , Tonarini S. 
Published: May 2009

AbstractIn the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallinessensu stricto(HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb–Sr and U–Th–Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the ‘cooler’ Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Fault.
Spinel lherzolite and other xenoliths from a dolerite dyke in southwest Donegal
Authors: Gallagher S. , Elsdon R. 
Published: May 2009

AbstractA highly xenolithic dolerite dyke is described which contains a large number of spinel lherzolite xenoliths. The petrology of the dolerite and the xenolith suite is described and electron probe analyses presented for the mineral phases in one of the spinel lherzolite xenoliths. The dyke geochemistry is consistent with a Permo-Carboniferous age.
The Solund–Stavfjord Ophiolite Complex and associated rocks, west Norwegian Caledonides: geology, geochemistry and tectonic environment
Authors: Furnes H. , Skjerlie K. P. , Pedersen R. B. , Andersen T. B. , Stillman C J. , Suthren R. J. , Tysseland M. , Garmann L. B. 
Published: May 2009

AbstractMetabasalts of the Upper Ordovician Solund-Stavfjord Ophiolite Complex of the westernmost Norwegian Caledonides, show N-to E-MORB affinity, with high Th/Ta (or Nb) ratios giving evidence of subduction influence. The Solund–Stavfjord Ophiolite Complex is overlain by a heterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which the Heggøy Formation of metasandstones and phyllites conformably overlies the metabasalts of the Solund–Stavfjord Ophiolite Complex. The Heggøy Formation contains, in places, abundant metabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund–Stavfjord Ophiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. This indicates that the Solund–Stavfjord Ophiolite Complex and Heggøy Formation developed in a marginal basin between a continental margin and an active subduction system, for which the present-day Andaman Sea may provide a realistic model. The other magmatic rocks of the Stavenes Group, showing both calc-alkaline and alkaline affinities, are less well time-constrained, but they are thought to represent an advanced stage of the island arc development, and ocean island build-up, respectively.
Upper Ordovician – lower Silurian sequences of the Bohemian Massif, central Europe
Authors: Štorch P. 
Published: May 2009

AbstractUpper Ordovician – lower Silurian sequences of the Bohemian Massif (in the Moldanubian Zone and Barrandian preserved in the Prague Basin; Rožmitál area; metamorphic ‘islets’ in the mantle of Central Bohemian Pluton; and in the Železné hory area), as well as along the northern margin of the Bohemian Massif in the Saxothuringian and Lugian (= West Sudetian) zones, show evidence of regressive–transgressive facies changes which can be related to glacio-eustatic sea-level fluctuations recognized elsewhere. Glacio-marine diamictites of late Ordovician (Hirnantian) age occur in the Prague Basin and Rožmitál area, and in north Bavaria and Thuringia. They provide evidence of temporary cooling of the region. Facies changes caused by synchronous sea-level changes allow stratigraphic correlation of even the faulted, weakly metamorphosed and biostratigraphically poorly dated sections in the Bohemian Massif. Correlation is based on sections in the Prague Basin in which the changes are best recorded and dated.
Palaeomagnetic and geochemical evaluation of a terrane-linking ignimbrite: evidence for the relative position of the Grampian and Midland Valley terranes in late Silurian time
Authors: Trench A. , Haughton P. D. W. 
Published: May 2009

AbstractThe proposed correlation of Lower Old Red Sandstone dacitic ignimbrites between the Northern Midland Valley and Grampian terranes of central Scotland has been investigated in a combined palaeomagnetic and geochemical study. Statistically indistinguishable late Silurian palaeolatitudes (Grampian terrane 27.0° S±3.5°; Northern Midland Valley terrane 22.9° S± 1.8°), comparable remagnetization histories, a shared ‘normal’ polarity for the primary magnetization and equivalent gyroremanence acquisition, favour correlation across the Highland Boundary Fault. Similar chemistry for biotite phenocrysts further corroborates correlation which was initially made on the basis of a similar field appearance and petrography.This correlation, together with evidence from provenance studies, suggests that the separation of the Northern Midland Valley and Grampian terranes inferred for the Ordovician period was largely removed by late Silurian times, with scope for relative displacements of the order of only tens of kilometres after Lower Old Red Sandstone deposition.
Metabentonites in the Moffat Shale Group, Southern Uplands of Scotland: Geochemical evidence of ensialic marginal basin volcanism
Authors: Merriman R. J. , Roberts B. 
Published: May 2009

AbstractMetabentonites occur extensively in the Moffat Shale Group of the Southern Uplands of Scotland. At Dob's Linn 135 metabentonite beds, 1–50 cm thick, occur in Ashgill to Llandovery strata, representing an aggregate thickness of 6 m of compacted ash accumulated over approximately 25 Ma. The metabentonites are characterized by relatively high concentrations of trace elements, including Ba, Cs, Hf, Nb, Rb, Ta, Th, U, Y, Zr and REEs, which were inherited from evolved vitric ash. Immobile trace element data indicate that a spectrum of silicic ash compositions accumulated, ranging from subalkaline to mildly peralkaline. In the late Ordovician N. gracilis to G. persculptus biozones, subalkaline ash falls predominated, whereas peralkaline ash falls predominated in the Llandovery (Silurian) P. acuminatus to M. convolutes biozones, giving way to predominantly subalkaline ash falls during accumulation of the M. sedgwickii to R. maximus biozones. Changeovers in the dominant ash types are marked by increased proportions of ash. The magmas from which the ash types evolved were generated in an ensialic arc transitional to a back-arc setting, and involved attenuated sialic crust and mantle characterized by variable depletion in HFS elements. Lithological, petrological and REE characteristics suggest that the Moffat Shale Group is not exclusively pelagic in origin and probably accumulated in a back-arc basin bordering an ensialic arc terrane.
Estimating rates of calcrete formation and sediment accretion in ancient alluvial deposits
Authors: Wright V. P. 
Published: May 2009

AbstractThe use of calcrete horizons as a means of assessing rates of sediment accretion in ancient alluvial sequences is reviewed. Any attempt to use such horizons in quantifying ancient alluvial deposition must appreciate that accurate data on rates of formation of Quaternary calcretes are limited. Such data show considerable variations in the rates and no reliable data exist at all for some calcrete types. The abundance of calcretes in the geological record warrants a new initiative to improve our understanding oftheir origins and rates of formation.
A Cretaceous Neo-Tethyan carbonate margin in Argolis, southern Greece
Authors: Clift Peter D. , Robertson Alastair H. F. 
Published: May 2009

AbstractThe Argolis Peninsula, southern Greece, is believed to form part of a Pelagonian microcontinent located between two oceanic basins, the Pindos to the west and theVardar to the east, in Triassic to Tertiary time. In eastern Argolis, two important units are exposed: (i) the Ermioni Limestones cropping out in the southwest; (ii) the Poros Formation, observed on an offshore island in the northeast, and on the adjacent mainland. Both these units comprise late Cretaceous (Aptian-Maastrichtian) pelagic limestones, calciturbidites, lenticular matrix- and clast-supported limestone conglomerates and slump sheets. However, the Poros Formation is distinguished from the Ermioni Limestones by the presence of bituminous micritic limestones and an increasing proportion of shale up sequence. These successions are deep-water slope carbonates that once formed the southeast-facing passive margin of the Pelagonian platform (Akros Limestone). Beyond this lay a late Cretaceous ocean basin in the Vardar Zone. This ocean was consumed in an easterly-dipping subduction zone in latest Cretaceous (?) to early Tertiary time, giving rise to an accretionary complex (Ermioni Complex). During early Tertiary (Palaeocene-Eocene) time the passive continental margin (Pelagonian Zone) collided with the trench and accretionary complex to the east. As the suture tightened, former lower-slope carbonates (Ermioni Limestones) were accreted to the base of the over-riding thrust sheets and emplaced onto the platform. Farther west, bituminous upper slope carbonates (Poros Formation) flexurally subsided and passed transitionally upwards into calcareous flysch and olistostromes in a foreland basin. These sediments were then overridden by the emplacing thrust stack and themselves underplated. Late-stage high-angle faulting then disrupted the tectonostratigraphy, in places juxtaposing relatively high and low structural levels of the complex.
The Emei Taphrogenesis of the upper Yangtze Platform in south China
Authors: Zhili Luo , Yizhong Jin , Xikui Zhao 
Published: May 2009

AbstractThe Yangtze Platform (Yangtze Palaeoplate) drifted into the area of southern China following late Silurian tectonism. In late Palaeozoic to early Mesozoic time the Yangtze Platform was subjected to strong extensional movements in its southeastern region within Yunnan, Guizhou, Guangxi and Hunan provinces, and along its northwestern margin in the Songpan-Ganzi area. Taphrogenesis (intracontinental extension) began in Devonian times, climaxed with the late Permian eruption of the Emeishan basalts, and ended in mid Triassic times. Therefore, the senior author (LZL) has named this extension the ‘Emei Taphrogenesis’, a phenomenon that was constrained by the neighbouring tectonic units of the Yangtze Platform. The platform has been substantially affected by the early Palaeozoic south China fold zone along its eastern margin, and by the late Palaeozoic opening of the Tethys Ocean on the northwestern margin. This paper delineates the tectonic patterns associated with the Emei Taphrogenesis.
The geology of the western part of the Fintona Block, Northern Ireland: evolution of Carboniferous basins
Authors: Mitchell W. I. , Owens B. 
Published: May 2009

AbstractThe western part of the Fintona Block is divided into four fault-bounded segments that contain red-bed sediments formerly assigned to the Lower Old Red Sandstone.Dating by miospores indicates the presence of deposits of early Devonian age in the Irvinestown Segment, late Viséan–early Silesian age in the Tempo–Lisbellaw Segment, and late Viséan–early Silesian and late Silesian ages in the Milltown Segment. Northward migration of the early Carboniferous marine transgression in the northern part of Ireland coincided with the sequential propagation of back-stepping faults and resulted in the development of diachronous facies belts between late Courceyan and Arundian times. Tectonic uplift, of a possible southwesterly extension of the Tyrone Igneous Complex, gave rise to the deposition of Asbian to Pendleian red-beds to the south of a massif. An interface between these red-beds and contemporaneous marine sediments farther to the south is recognized and dated. A new non-marine basin, containing Brigantian and Pendleian red-beds, also developed to the north of the massif A waterlogged floodplain that developed during Westphalian A times may be coeval with more widespread coal-bearing sequences elsewhere in Ireland. Alluvial fans prograded southwards over this plain during Westphalian B times when faults bordering a northern landmass were reactivated.
Factors controlling the kinetics of crystallization: supersaturation evolution in a porous medium. Application to barite crystallization
Authors: Prieto Manuel , Putnis Andrew , Fernandez-Diaz Lurdes 
Published: May 2009

AbstractThe nucleation of barite has been studied in a system involving the doublediffusion of Ba2+and SO42-in an Na-rich aqueoussolution, through a porous medium. The evolution of the concentration profiles in the medium, which is a column of porous silica gel, has been determined as a function of time by direct chemical analysis of the diffusion-controlled mass transfer. By measuring the pH evolution, a Debye–Huckel treatment of ionic complexing has enabled the supersaturation evolution to be determined. The location of barite precipitation in the column is controlled both by the need to exceed a threshold supersaturation, as well as achieve an ‘equality range’ in which [Ba2+]/[SO42−] is close to unity. The value of the threshold supersaturation is a kinetic parameter and depends on the rate at which supersaturation increases. The experimental system described here has wide application to the study of crystallization phenomena in rocks. Experiments on the effect of additives designed to inhibit nucleation of barite in North Sea oil wells are used to quantify the resultant increase in supersaturation threshold.
Milankovitch cyclicity and the Boom Clay Formation: an Oligocene siliciclastic shelf sequence in Belgium
Authors: Van Echelpoel E. , Weedon G. P. 
Published: May 2009

AbstractThe Boom Clay Formation of northeast Belgium consists of a sequence of alternating, laterally persistent beds of silt and clay. Power-spectral analysis of grain-size variations indicates regular sedimentary cycles with wavelengths of about 100 and 46cm that had periods of 124 and 57 thousand years (ka) or less. The lateral continuity ofthe beds, combined with the regularity and estimated periods of the cycles, suggests an indirect link to the 100 and 41 ka orbital (Milankovitch) cycles. The variations in meangrain size were caused by changing bottom-water turbulence as controlled by storms and/or glacio-eustatic water depth fluctuations. This study confirms that certain marinesiliciclastic sequences can provide proxy-records of palaeoclimatic variance.
Petrogenetic implications of mineral crystallization trends of Troodos cumulates, Cyprus
Authors: Thy P. 
Published: May 2009

AbstractHole number CY-4 of the Cyprus Crustal Study Project penetrated the lower sheeted dyke complex, gabbros and ultramafic cumulates of the Troodos ophiolite. The lower part of the drill core sampled a coarse-grained plutonic complex revealing phase and cryptic layering and one major magma chamber replenishment. This magma chamber intruded medium-grained gabbroic rocks showing intricate chemical evolution trends reflecting several magma replenishments. In the upper part of the core, the gabbroic cumulates are intruded by fine-grained dykes, which grade into the sheeted dyke complex and chemically can be correlated with the lavas of the lower pillow sequence. The upper pillow lavas are best correlated with the ultramafic cumulates. A study of coexisting plagioclase (An) and mafic mineral (opx Mg #) compositions in the drill core revealed three main rock types: (1) a primitive group (An95–98, Mg #75–85) represented by the lower, coarse-grained gabbroic and ultramafic cumulates. (2) an intermediate group (An86–95 and Mg #70–78) represented by the upper level gabbroic cumulates, and (3) the lower part of the sheeted dyke complex (An60–80 and Mg #60–70). The plagioclase of the gabbros and ultramafic cumulates have an unusually high An content. Numerical simulation of the expected anhydrous, one atmosphere, crystallization trends show that the Troodos trends cannot be reproduced from known spreading or subduction related glasses. Mineralogical evidence indicates that the extrusives and the cumulate sequences of the Troodos ophiolite are genetically related. Glasses from the extrusives, nevertheless, also fail to reproduce the mineral crystallization trends observed in the plutonics. Attempts to model high PH2O crystallization produced trends more consistent with those observed in the cumulates. The very sodium-poor nature of the plagioclase may therefore mainly reflect high PH2O crystallization. High water content is consistent with the inferred subduction zone basin origin for the Troodos ophiolite.
The Cretaceous—Tertiary boundary in Mangyshlak, U.S.S.R.
Authors: Naidin D. P. 
Published: May 2009

AbstractUpper Cretaceous carbonate sequences contain omission surfaces, hardgrounds and intercalations of ‘clays’. These ‘clays’ are largely associated with submarine biogeochemical carbonate dissolution that was caused by high biological productivity in the pelagic zone. The Maastrichtian-Danian ‘boundary clays’ probably accumulated during a maximum productivity that led to the exhaustion of nutrients, development of phenomena comparable to present-day red tides, and mass mortality of marine biota. There was a type of ecological break in the seas and oceans at the time of the Maastrichtian-Danian boundary.