bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Planes-San Antonio pyritic deposit of Rio Tinto, Spain: its nature, environment and genesis
Authors: Williams D. , Stanton R. L. , Rambaud F. 
Published: May 2008

Summary
The San Antonio stratiform cupriferous deposit of Rio Tinto, discovered in the early 1960s, is now being exploited as an eastward extension of the bedded pyritite sheet in the old Planes mine, which is there underlain by a stockwork feeder pipe.
The Planes pipe consists of sulphur-rich—copper-poor sulphide ore formed by the almost complete replacement of a stockwork conduit which traverses Lower Carboniferous felsic pyroclastic rocks. Overlying the pipe, and extending far beyond its margin, is the genetically related Planes-San Antonio cupriferous pyritite, a ‘sedimentary-volcanogenic’ deposit precipitated as a chemical sediment derived from sea-floor hot springs associated with the Lower Carboniferous volcanic island arc of southwest Iberia. Most of the pyritite bodies of Rio Tinto were deposited directly above or close to their underlying feeder stockworks. The Planes-San Antonio pyritite displays well-preserved stratification, slump structures and interdigitated tuff bands, and was mainly precipitated comparatively distant from the nourishing Planes stockwork.
Across-layer base-metal zoning in the Planes-San Antonio sheet is usually characterized by an increase in copper, and to a lesser extent in zinc and lead, towards the base of the deposit. Much of this enrichment may have been due to late hypogene processes involving the ascent of juvenile-meteoric solutions which leached base metals from the underlying volcanic pile and deposited them as chalcopyrite, sphalerite and galena by partial replacement of the lower part of the pre-existing pyrite-rich stratiform sulphide sheet.

A notable feature of the pyritite is the abundance of colloform and framboidal pyrite. The mean isotopic composition of the sulphide sulphur of the Planes stockwork (
δ
34
S ≈ + 10‰) is distinctly heavier than that of the stratiform Planes-San Antonio pyrite (
δ
34
S ≈ + 2.7‰).

The cupriferous pyritic mass of Planes-San Antonio exhibits some of the features characteristic of pyritic deposits formed during the initial stages of arc development (‘Cyprus’ or ‘ophiolite’ type ores). It was actually formed, however, at a late stage of arc evolution, during a period of relative quiescence within the waning episodes of submarine explosive felsic volcanism, and it displays environmental traits analogous to those of the Japanese ‘Kuroko’ ores.
Kuroko deposits: their geology, geochemistry and origin
Authors: Sato Takeo 
Published: May 2008

Synopsis
The Kuroko deposits of Japan are predominantly massive, stratiform sulphide ores with associated underlying stockwork ores of Miocene age in the Green Tuff volcanic sequences. They formed during a very limited period around 13 m.y. ago in association with the late-stage felsic rocks of the mafic to felsic volcanic cycle that began in the late Oligocene or early Miocene. They occur in narrow zones parallel to island arcs. The Benioff zone is at about 150-km depth below the Kuroko zones, which were areas of high heat flow during the Miocene.
The major constituents of the deposits are pyrite, chalcopyrite, sphalerite, galena, barite and quartz. They are zoned vertically. Gypsum and/or anhydrite commonly occur as separate bodies beside the sulphide-barite bodies. It is thought that the ores were precipitated on the Miocene sea-floor from saline solutions in a weakly acidic, reducing environment at about 200–250°C as a result of falling temperatures. The zoning of minerals may have resulted from changes in density of the ore solution-sea water mixtures as precipitation proceeded.
Studies of light stable isotopes suggest that the mineralizing solutions were predominantly coeval sea water. Lead isotope data, together with base-metal contents of the host rocks, however, indicate that leaching by circulating sea water cannot have been the dominant mechanism. So, magmatic water probably played an important role in the mineralization. The following model is therefore proposed. About 20–25 m.y. ago, as the Pacific plate was being subducted, a large amount of basic to intermediate magma was generated and differentiated in the lower crustal region. Magmatic fluids produced at the last stage of differentiation, about 13 m.y. ago, rose to the sea-floor and, on mixing with sea water, produced the Kuroko deposits. Subsidence of the sedimentary basins before the felsic volcanism could have been the result of volume decrease during solidification of underlying magma.
Volcanogenic mineralization at Avoca, Co. Wicklow, Ireland, and its regional implications
Authors: Platt J. W. 
Published: May 2008

Abstract
A description is given to the geological setting of the stratiform pyritic orebodies at Avoca in Ireland and a volcanogenic origin is proposed on the basis of comparison with stratiform orebodies located in similar volcanic host rocks in other parts of the Caledonian orogen and elsewhere. The Ordovician volcanic sequence in southeast Ireland is interpreted as a fossil island arc complex, which was formed above a contemporary margin of southeasterly subduction, which locally defined the boundary of the evolving Caledonian orogenic belt in southeast Ireland.
The Avoca ore deposits are located within a sequence of calc-alkaline lavas, pyroclastics and sediments at a level marked by the predominance of felsic eruptives. The stratigraphy of the host rocks is described in some detail and related to the general geology of the Avoca belt. Massive pyritic ores and disseminated sulphide ores of ‘stringer type’ can both be recognized at Avoca. The massive ore shows a compositional gradation suggestive of a primary stratigraphy; a Cu-rich pyritic zone is overlain by a zone of Pb-Zn-Cu pyritic ore.
The interpretation of the genesis of the orebody is complicated by its deformed state, which is typical of other volcanogenic orebodies within the various Caledonian provinces. A genetic model can be proposed, however, by using the descriptions of the undeformed ‘Kuroko’ deposits in Japan as a standard for comparison.
The individual orebodies of Avoca type contain 5 000 000–6 000 000 tons of ore, and these are associated with extensive zones of low-grade disseminated ore in acid pyroclastic host rocks. Sequences of manganiferous and dolomitized sediments are stratigraphically closely related to the ores.
The principal geological features of the Avoca orebodies having been defined, it is deduced that there is a strong possibility that similar orebodies lie hidden in the volcanic rocks of the southeast Irish Caledonides and that an aggressive exploration programme could lead to new discoveries.
Rifting in east Africa and large-scale tectonic processes
Authors: Oxburgh E. Ronald 
Published: May 2008

The East African Rift System has many of the geophysical characteristics of a mid-ocean ridge and is laterally continuous with the Red Sea and Gulf of Aden spreading centres. Although all three were initiated simultaneously on continental crust in pre-Miocene times, that in East Africa did not evolve oceanic crust.
Seismic, thermal and geochemical evidence suggest that the lithosphere under much of Africa is about 300 km thick, whereas under the rift it is today 40–50 km. Thin lithosphere can result either from mechanical necking or from the upward migration of the isotherm defining the lithosphere/asthenosphere boundary. Both processes are important in the evolution of a continental rift into a spreading ocean, but the limited extension in East Africa indicates that here the latter must dominate. It is, however, difficult to transfer sufficient heat into the lithosphere from below in the time available; conductive processes are too slow, and transfer by magma, although fast enough, requires more mass addition to the lithosphere than is compatible with the limited surface extension. Hot volatiles might transfer heat to the lithosphere from below and depress melting temperatures without adding significantly to its mass.
Rifts have been attributed to convective mantle plumes or to membrane stresses in the lithosphere. If the lithosphere is thick, mantle plumes are unlikely to generate rifts in the time available.
Structural and volcanic evolution of the Gregory Rift Valley
Authors: King Basil C. 
Published: May 2008

The East African Rift System extends southwards from the Red Sea for a distance of about 3000 km. Within East Africa it is represented by the Western Rift and the Eastern Rift, of which the Gregory Rift Valley is that part lying within Kenya and northern Tanzania.
The Gregory Rift Valley was initiated in early Miocene times as a downwarp along the continental watershed on a land surface having considerable relief (up to 2000 m). Subsequent faulting produced a graben about 80 km wide and some 450 km in length, to the north and south of which the faults splay outwards over much broader zones.
Volcanic activity was not confined to the Rift, but developed both to the west in the Miocene and to the east from Plio-Pleistocene to Recent times. Within the Rift itself volcanism commenced in the north and extended to the south in the Pliocene, while both tectonic and volcanic activity tended later to restriction within a central zone, which from Lake Hannington northwards formed a narrow axial trough.
Episodes of volcanism and sedimentation largely or wholly infilled the downwarping trough, but only in the Pleistocene, by uplift of the shoulders and downfaulting of the floor, was the morphology of the present Rift Valley achieved. The altitude of the sub-volcanic surface averages about 1500 m along the shoulders of the Rift; its greatest depth below the floor may be southwards from near Lake Baringo, corresponding to an absolute displacement of some 4 km. About 8 km of crustal extension over about 8o km of rift width may have occurred, but there is no evidence for crustal separation.
Geophysical investigations and the Rift Valley geology of Kenya
Authors: Khan M. Aftab , Swain Christopher J. 
Published: May 2008

During the last decade there has been considerable activity in geophysical investigations in the East African Rift under the auspices of the Upper Mantle Project. Studies in the fields of explosion and earthquake seismology, gravity, magnetism, heat flow and electrical conductivity have considerably increased our understanding of the deep structure and processes beneath the Rift. The gravity data also throw light on the near-surface geology. A gravity map of Kenya at 1:2000000 has just been compiled at the University of Leicester using data from over 6500 stations, most of which have been recently established in the Rift area. The regional field can be represented by a seventh order trend surface with a ‘saddle’ centred at Marsabit in the north. From the centre of this, the regional field rises north-westwards towards Lake Turkana and falls south-westwards towards a pronounced bowl-shaped minimum centred in the Narok-Naivasha region near the culmination of the Kenya dome. The trend surface correlates inversely with the topography and may be attributed to deep structure. The residual gravity field of the Rift Valley region contains a number of features which are clearly related to the visible geology.
The axial region is associated with an undulating high along which lie a number of prominent caldera volcanoes—Suswa, Menengai, Silali, Emuruangogolak and Lomi. Further north the most spectacular high occurs at the syenite plug, El Moiti, which is probably the remnant of a trachyte volcano. The flanking negative anomalies can usually be correlated with low density lavas and sediments—notably at Lake Turkana, near Lokichar, the Kerio Valley and further south at Londiani and Ol Kalou, but lows are notably absent over the Baringo and Suguta basins. Quantitative interpretation requires an estimate of the true regional, as opposed to the synthetic regional of trend surface analysis, and is often extremely difficult to make, due to lack of geologic data. Nevertheless, several east-west profiles have been modelled, yielding, amongst other parameters, estimates of the thickness of sediments and volcanics.
A statistical approach to temporal biostratigraphy
Authors: Shuey R. T. , Brown Frank H. , Eck G. G. , Howell F. Clark 
Published: May 2008

Various statistics (similarity coefficients) can be constructed for faunal assemblages which vary more or less regularly as these assemblages change. Methods are described for placing assemblages on an evolutionary scale, the units of which are equal amounts of evolutionary transformation in the species makeup of assemblages. The greatest single factor involved in the scaling appears to be time; the effects of zoogeography are also discussed. Characteristics of different similarity coefficients differ, and some of these characteristics are discussed.
Applying our method to faunal assemblages from East African hominid sites, reasonable correlations can be established between Olduvai Gorge, Tanzania, the Shungura Formation, Ethiopia, and the Koobi Fora Formation, Kenya.
Allometry and Hominid studies
Authors: Wood Bernard A. 
Published: May 2008

When the shape of a bone changes during growth the relationship between its shape and its size can be expressed mathematically and the phenomenon is known as allometric or relative growth. The same concept applies to samples of adult bones of different mean overall size. It is likely that some of the morphological distinctions between early hominids are due to size related allometric features. However, even if such differences can be credited with an allometric basis their significance for hominid taxonomy is uncertain.
This paper reviews the development of allometric theory and deals with technical advances which have improved our understanding of the theory and application of allometry.
Three relevant applications of allometry are considered. These are its use in attempts to compensate for allometrically based intra-group differences which distort attempts to differentiate between taxonomic groups; its role in taxonomy and its use as a tool to increase our understanding of the relationships between function and morphology in hominids.
Caledonian correlations: Britain and Scandinavia
Authors: Nicholson Robin 
Published: May 2008

Summary
While there is widespread evidence of climactic late-Silurian thrusting in the southern Scandinavian Caledonides the scale of comparable early Ordovician (Grampian) movements is not yet established. Only in the lower, eastern, marginal nappes is there evidence of continuous sedimentation from late Precambrian up into lower Palaeozoic. In the metamorphic allochthon no fossils older than Tremodocian have been discovered. Barrovian metamorphism which seems to vary from nappe to nappe is widely developed: no genuine LT/HP metamorphism is recorded. The North Sea basin, largely founded on Old Red Sandstone (ORS) deposits of the terminal Caledonian stage, covers the junction between the two distinct Caledonian provinces of Britain and Scandinavia.
East Greenland Caledonides—an extension of the British Caledonides
Authors: Higgins Anthony K. , Phillips W. E. Adrian 
Published: May 2008

SummaryThe major part of the Caledonian fold belt in East Greenland comprises gneiss complexes, which in the south have yielded Archaean and early Proterozoic isotopic ages; their features may be compared to those of the Scourian and Laxfordian divisions of the Lewisian. They are overlain by middle Proterozoic supracrustal sequences, which in some areas are involved in migmatitic activity and granite emplacement of Grenville age; parallels may be drawn with events recorded in the Morar Division of the Moine in Scotland, and in Ireland with those in the Annagh gneiss complex in N Mayo and perhaps in the Ox Mountains sequence. The late Precambrian lower Eleonore Bay Group is similar in development to the Grampian Group (Dalradian) of the British Isles, and the upper Eleonore Bay Group bears comparison with the Appin Group of the Dalradian. The Vendian glacial event is well represented in both regions. The early Palaeozoic succession in Greenland shows marked similarities with the Durness Limestone succession. The pattern of Caledonian deformation, metamorphism and plutonic activity show many similarities but also differences. The marked marginal thrust zone in the west of the E Greenland fold belt is the logical continuation of the Moine thrust belt. Late Palaeozoic continental deposits overlie Caledonian folded units in both regions.
The continuation of the Canadian Appalachians into the Caledonides of Britain and Ireland
Authors: Kennedy M. J. 
Published: May 2008

Summary
Three zones can be recognised in the Canadian Appalachian-British Caledonian belt. The NW Marginal Zone extends from NW Scotland and Ireland through N Newfoundland to Quebec. It consists of late Precambrian to early Palaeozoic clastic sequences and an extensive Cambro-Ordovician carbonate platform. Orthotectonic deformation and metamorphism occurred in the early Ordovician, and middle Ordovician allochthons and obducted ophiolite characterise this zone in N America. The SE Marginal Zone extends from Wales and the English Midlands through SE Ireland to E Newfoundland, Nova Scotia and S New Brunswick. It is characterised by extensive late Precambrian volcanicity, late Precambrian orthotectonic deformation and metamorphism and the development of lower Palaeozoic basins in Wales and Nova Scotia. Late Precambrian orogenic activity locally involved ophiolite. The Axial Zone can be subdivided into a number of subzones, some of which do not extend for great distances along strike. It contains local ophiolites, trench complexes and calc-alkaline volcanics ranging in age from Cambrian to Silurian. Volcanicity was most pronounced in the Ordovician on both sides of this zone and extended into the marginal zones. Middle Ordovician deformation and metamorphism is locally intense but elsewhere Ordovician and Silurian sequences are apparently conformable. The Axial Zone and adjacent parts of the marginal zones have all suffered Siluro-Devonian, generally paratectonic, deformation and metamorphism. The history of the whole belt indicates that subduction was active on the SE in the late Precambrian, on the NW and in the Axial Zone in the lower and middle Ordovician and extended into the Silurian. Final closure of the Iapetus ocean was delayed until the upper Silurian-Devonian. Plate-tectonic models must involve multiple subduction zones.
Observations on eclogite- and granulite-facies rocks in the basement of the Caledonides
Authors: Sanders Ian S. 
Published: May 2008

Summary
Eclogite-facies metamorphism in the Glenelg inlier, NW Scotland, occurred after a phase of basic dyke intrusion (?Scourie dykes), and before a period of ?Caledonian mylonitisation. It possibly occurred during the Precambrian Morarian event since banded eclogitic rocks share the same ESE-dipping foliation as the overlying Moines. P-T conditions of metamorphism estimated as 750–600°C and 10–15 kb, suggest that the inlier is an upthrust segment of stable continental lower crust. Granulite-facies rocks of the NE Ox Mountains inlier, W Ireland, show evidence of Caledonian retrogression. Inferred P-T conditions (900–750°C, >10 kb) are those of a heated, lower crustal environment. The rocks are very similar to xenolithic material from the Scottish Midland Valley.
Basement-cover relationships at Lewisian inliers in the Moine rocks
Authors: Rathbone Paul A. , Harris Anthony L. 
Published: May 2008

Summary
Lewisian basement of the N Highlands occurs (a) as an autochthonous massif, the Strathy Complex, (b) in parautochthonous fold cores, e.g. Morar, and (c) as slices along zones of high strain, e.g. the Sgurr Beag Slide. Structural features, within the Morar Division psammites adjacent to the Glenfinnan Division and to Lewisian inliers, have been used to define zones of increasing strain within some of which Lewisian strips now occur. The psammites show evidence of high strain where slides occur and massive, cross-bedded psammites give way to thinly banded platey psammites with a total absence of sedimentary structures. Folds progressively tighten. Quartz veins are re-orientated into parallelism with the foliation in the psammite and discordant fabrics are lost. In contrast to psammites at slides, those adjacent to Lewisian rocks in parautochthonous fold cores show only some of these features and those adjacent to the autochthonous massif show none of these features.
Dating of the tectono-metamorphic history of the southwestern Moine, Scotland
Authors: Brewer Martin , Brook Maureen , Powell Derek 
Published: May 2008

Summary

New Rb-Sr whole-rock data are presented which confirm a Grenvillian age for widespread, early, regional metamorphism of the Moine in the SW Highlands of Scotland. It further suggests that Caledonian metamorphic activity occurred between
c.
470 and 410 Ma ago. Previous isotopic analyses of mineral separates from schists, gneissose pelites and pegmatites are reviewed in the light of the new data, and the work of Purdy & Jaeger (1976). This suggests a cooling history, following the Caledonian metamorphism, which involved sequential uplift of crustal segments.
The ‘Central Highland Granulites’: cover-basement tectonics in the Moine
Authors: Piasecki M. A. J. , van Breemen O. 
Published: May 2008

Summary
The Central Highland Granulites have been split into a cover assemblage of medium grade, little disturbed metasediments named the Grampian group, separated by a zone of sliding referred to as the Grampian Slide, from an underlying basement assemblage of high grade gneissose and migmatitic metasediments called the Central Highland Division. The Central Highland Division rocks are correlated on lithological, tectonic, metamorphic and radiometric grounds with the high grade Grenvillian rocks of the western Highland Moine, and are believed to represent their extension southeast of the Great Glen fault.

The discovery of Precambrian (
c
.720 Ma) pegmatites sheared by Caledonian (<570 Ma) sliding in the Central Highlands, enables the separation of Caledonian from Precambrian orogenic events in the rocks of both assemblages; their Precambrian orogenic history is so different, that the Central Highland Division rocks are considered to form a Grenvillian basement to the younger (Morarian) cover succession of the Grampian group. The western Highland Moine region is examined in the light of the tectonic cover-basement relationship postulated for the Central Highlands. The Loch Eil Division contains rock assemblages indistinguishable from the Central Highland Division, and others similar to the Grampian group. Evidence is put forward for a post Grenvillian, Morarian age for the Morar Division.

The distribution of metamorphic zones in the Moine would seem to reflect the distribution of high grade basement and lower grade cover rocks, and may be in part determined by movements on a series of Caledonian slides, though this has not been demonstrated.
The status of the Banff nappe
Authors: Ramsay Donald M. , Sturt Brian A. 
Published: May 2008

Summary
The status of the major horizon of gneisses (Cowhythe, Donside, Ellon and Inzie Head gneisses) in NE Scotland has been reinterpreted, on the basis of geochronological and fabric evidence, as the basal element of a major nappe, the Banff nappe. It is suggested that these gneisses represent a pre-Caledonian basement gneiss-complex, thrust into its present structural position during the Grampian phase of the Caledonian orogeny, after the formation of the Tay nappe. The basal zone of this unit is characterised by the development of distinctive mylonitic assemblages. In this interpretation, the rocks of the overlying Banff division form part of the allochthonous assemblage, as a Caledonian cover sequence, uncoupled from the basement along the Boyne line during emplacement.