The geology of ordovician rocks at Balbriggan,County Dublin,Eire

A thick series of Caradocian volcanics is described. The volcanics are basic-intermediate and it is own that they have a number of characteristics which are peculiar to emplacement in a subaqueous environment. A new interpretation of the stratigraphy results from a consideration of the folding of the succession.     

The chemistry of appinitic rocks associated with the Ardara pluton,Donegal, Ireland

Seventeen chemical analyses are given of rocks from appinitic intrusions associated with the Ardara granitic pluton in Donegal, northwest Ireland. The analyses show a series of basic and ultrabasic compositions, the high Al₂O₃, low MgO members of which are closely comparable to basalts, apart from their high water contents. The Ardara appinites are compared with other Caledonian appinitic rocks, and with basic and ultrabasic igneous rocks of other associations, from which the appinitic rocks are shown to be chemically quite distinct. The formation of the appinites is ascribed to crystal accumulation in a basaltic magma enriched in water. The granites associated with appinitic rocks also have distinctive compositions, indicative of formation under a high water pressure.     

Petrology,structure and age of the Rough Point sill,Donegal

The Rough Point sill, which is one of the thickest of a large number of sills intruded into the Appin Group metasediments of the Dalradian sequence in north Donegal, contains a number of internal features including mafic:felsic banding, pegmatite bands and significant variations in grain size. There are also appreciable variations in mineralogy, particularly the proportions of plagioclase and amphibole, and in the major and trace element chemistry. Together these features indicate that the sill is, like the metasediments which it intrudes, inverted, and is therefore older than the Caledonian F₂ structures which caused the inversion. Disruption of the sediments at the sill contact is also earlier than the F₂ event. The timing of the basic magmatic episode together with the geochemistry of the sill invite comparison with the metadolerites of the southwest Highlands of Scotland.     

Chironomid‐inferred late‐glacial summer air temperatures from Lough Nadourcan,Co. Donegal,Ireland

Western Ireland, located adjacent to the North Atlantic, and with a strongly oceanic climate, is potentially sensitive to rapid and extreme climate change. We present the first high‐resolution chironomid‐inferred mean July temperature reconstruction for Ireland, spanning the late‐glacial and early Holocene (LGIT, 15–10 ka BP). The reconstruction suggests an initial rapid warming followed by a short cool phase early in the interstadial. During the interstadial there are oscillations in the inferred temperatures which may relate to Greenland Interstadial events GI‐1a–e. The temperature decrease into the stadial occurs in two stages. This two‐stage drop can also be seen in other late‐glacial chironomid‐inferred temperature records from the British Isles. A stepped rise in temperatures into the Holocene, consistent with present‐day temperatures in Donegal, is inferred. The results show strong similarities with previously published LGIT chironomid‐inferred temperature reconstructions, and with the NGRIP oxygen‐isotope curve, which indicates that the oscillations observed in the NGRIP record are of hemispherical significance. The results also highlight the influence of the North Atlantic on the Irish climate throughout the LGIT. Copyright © 2010 John Wiley & Sons, Ltd.     

Production of multi-décollement folding by pressure release in the formation of a microbreccia at Lough Doon,Donegal, eire

An unusual matrix-supported microbreccia of Caledonian age is described from the Dalradian rocks of Donegal. Field and textural relationships indicate that the breccia formed ‘in situ’ by a non-dilational mechanism during regional deformation. The presence of breccia veins of low competancy during this deformation caused multi-décollement folds to be formed adjacent to the breccias. A cyclical mechanism of formation of these breccias is proposed in which fluctuations in the overpressuring of pore fluids escaping from an adjacent crystallizing appinite mass caused periodic excursions from ductile to brittle deformation during the regional folding.     

Principal components analysis of the geochemistry and mineralogy of the Portaskaig Tillite and Kiltyfanned Schist (Dalradian) of Co. Donegal, Eire

Forty-eight samples of a slightly metamorphosed glaciomarine till (Portaskaig Tillite) and 14 samples of the overlying Kiltyfanned Schist in the Dalradian (? late Precambrian) of Co. Donegal have been subjected to Q-mode principal components analysis based on chemical and modal analyses of the metasediments. This proved useful in comparing the trends of major element variation with those of other analyzed sediments, and revealed features in the sedimentary variation of the mixtite units of the Portaskaig Tillite which were not obvious from the raw data. Good agreement was obtained between sample classifications derived from independent principal components analyses of the geochemical and mineralogical data for both formations. The technique has potential for the study of multivariate trends of variation in sedimentary rocks.     

Quartz-topaz-loellingite rocks near Eldorado,Victoria

Two small dykes consisting of a quartz-topaz-loellingite rock type have recently been discovered within the aplitic phase of the Pilot Range granite, near Eldorado, in NE Victoria. Minor biotite, muscovite, chlorite, kaolinite, anatase and pharmacosiderite are associated. Apart from the loellingite, the dykes are similar mineralogically to the ‘topazites’ from New England, NSW. These were considered to be magmatic in origin, based on field relationships and high homogenization temperatures for fluid inclusions in topaz (Eadington & Nashar 1978, Contrib. Mineral. Petrol. 67, 433–438). Although experimental evidence on F-enriched ‘granitic’ systems is inconclusive, the emplacement of the Eldorado topazite dykes most likely involved both magmatic and hydrothermal components operating essentially simultaneously. The topazite melt represented a F-rich residual granitic magma, from which aqueous alkali halide-rich solutions separated during high level intrusion. Separation of these aqueous solutions was responsible for miarolitic cavities into which topaz crystals grew. F-OH equilibration calculations for coexisting topaz-biotite pairs suggest the minerals equilibrated in the presence of hydrothermal solutions of variable composition (in terms of the HF/H₂O fugacity ratio), at temperatures around 550°C. Alteration of topaz to muscovite, the precipitation of loellingite and the formation of clay and fluorite in the cavities occurred at progressively lower temperatures. The widespread alluvial topaz in the Beechworth-Eldorado area may be derived from similar quartz-topaz dykes.     

A composite spessartite–appinite intrusion from Port-na-Blagh,County Donegal,Ireland

A sheet-like appinite from Breaghy Head, North Donegal, is closely associated with a spessartite intrusion. Field, mineralogical, and geochemical similarities indicate that the parent magmas were very similar, and the appinite magma may have been derived from the spessartite by small degrees of crystal fractionation. The appinite is divided into an amphibole-rich lower part and a felsic upper part; the mineralogical contrast between the two is best explained by two-pulse intrusion of a batch of spessartitic magma within which amphibole and pyroxene had already segregated due to crystal settling or flow sorting. Unlike other Caledonian appinites, the Breaghy Head intrusion is not one of a cluster, and appears to be an isolated development. Possible reasons for its location are discussed.     

Analysis of an Upper Palaeozoic transgressive sequence in southwest County Cork,Eire

The rocks described belong to the predominantly siliceous Cork Beds of Late Devonian and Early Carboniferous age. The basal unit of the Cork Beds shows a generally coarsening upwards sequence reflecting the gradual migration of barrier beach over tidal flat environments. After a minor non-depositional phase, this is succeeded by a sequence of offshore subtidal sediments which gradually fine upwards. In the higher parts of the succession subsidence exceeded sedimentation to give deeper water conditions in Visean times. In the area described the Cork Beds successions are much thinner than in areas to the east and northwest. No marked diachroneity is present and thus the existence of a relatively positive area is indicated.     

Palaeoecological investigations towards the reconstruction of the post-glacial environment at Lough Doo,County Mayo,Ireland

Results of palaeoecological studies involving pollen analytical, chemical and palaeomagnetic investigations on a 3 m core from Lough Doo, NE County Mayo, are presented. The record, which commences shortly after 7000 BP, shows a sharp Alnus expansion coinciding with a decline in Pius at 6400 BP, an elm decline taking place in the context of severe soil erosion and the final decline of Pinus occurring as the chemical record indicates onset of severe reducing conditions in the catchment. Pine becomes extinct at or shortly before 3200 BP in the context of increased farming activity and the expansion of blanket bog in the adjacent upland areas. A later feature (c. 1880 BP) is the expansion of Taxus to 7.1% of total pollen. On the basis of the palynological evidence, it is concluded that there were no major Neolithic Landnam phases, which contrasts sharply with the evidence for widespread woodland clearance and the laying out of extensive and regular field systems in the northern coastal part of County Mayo. In the upper part of the core a severe inversion of ¹⁴C dates is recorded. On the basis of the palaeomagnetic and pollen evidence this began at c. 1500 BP and is considered to have been initiated in the context of woodland clearance and a renewal in farming activity.     

新疆柯坪库木如吾祖克地区二叠纪火山岩

新疆柯坪库木如吾祖克地区二叠纪火山岩产于柯坪微地块中,为陆相火山岩,主要由基性熔岩 (辉石玄武岩 )和酸性凝灰岩组成,具双峰式火山岩系组合特征。火山岩为 3个火山喷发旋回的产物,从下至上基性熔岩主元素成分成规律性变化,表明基性熔岩为同源岩浆分异演化的产物。基性熔岩具有大陆火山岩性质,形成于大陆拉张环境。该二叠纪火山岩为天山造山带石炭—二叠纪大规模裂谷作用在柯坪古老微地块上的反映。     

Uppermost Triassic to Lower Jurassic stratigraphy in the Lough Foyle Basin of County Londonderry,Northern Ireland

The Lough Foyle Basin is a half-graben that straddles the border between Northern Ireland and the Republic of Ireland and contains sediments that range in age from Lower Carboniferous to Holocene. The basin’s post-rift succession is represented by sediments of the Penarth Group and Lias Group. The lithostratigraphy and biostratigraphy of that interval are revised using new borehole material and existing outcrop. Palaeontological data provide a chronostratigraphic framework and aid palaeoenvironmental interpretations. Foraminifera, ostracods, palynomorphs indicate a nearshore, marginal marine depositional setting throughout much of the Rhaetian with a more marine, shelf and nearshore depositional setting for the Lias Group. The Penarth Group succession is similar to that elsewhere in Northern Ireland but the Lias Group (Waterloo Mudstone Formation) differs in that five distinct members can be recognised (Clooney Mudstone, Drummans Siltstone, Gortmore Mudstone, Tircreven Sandstone and Ballyleighery Mudstone), four of which are newly described. The deltaic and shelf sandstones of the Tircreven Sandstone Member are the only such examples preserved in the Jurassic strata of Northern Ireland and are some of the oldest in the Jurassic of the UK and Ireland, providing evidence of the proximity of the nearby Irish Landmass and representing a useful comparison for Early Jurassic sandstone reservoirs in offshore basins.     

The Ballybofey Anticline: A solution of the general structure of parts of Donegal and Tyrone

A tight early fold, the Ballybofey anticline, trends south-eastwards across central Donegal. South-west of its axial trace the Dalradian strata are inverted so that the rocks high in the Series occur at the lowest structural level, next to, and facing downwards, the supposedly Moinian psammites in the core of the Lough Derg antiform. Rapid facies changes occur in the Dalradian rocks across the Caledonian strike.     

10Be chronology of the last deglaciation of County Donegal, northwestern Ireland

A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic ¹⁰Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 ¹⁰Be kyr to 20.9±1.3 ¹⁰Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 ¹⁰Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous ¹⁰Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 ¹⁰Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated ¹⁴C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago.     

The faunal-stratigraphy of the ludlovian rocks between Craven Arms and Bourton,near Much Wenlock,Shropshire

The Ludlovian Series has been mapped between the River Onny at Craven Arms and the village of Bourton which lies 2½ miles SW of Much Wenlock. Ten subdivisions are recognised on biostratigraphical and lithostratigraphical criteria. For these divisions the stratigraphical terminology established by Holland, Lawson and Walmsley (1959, 1963) in the Ludlow area has been utilised. Two breaks occur in the otherwise continuous succession of variable calcareous sediments: there is lithological and faunal discontinuity at the base of the Leintwardine Beds and within the Upper Leintwardine Beds. The latter can be recognised over a wide area of the Ludlovian shelf. Accordingly alternative correlations, to those presently accepted, are proposed for the Leintwardine Beds.     

Intrusive and polymeta morphic rocks of the darakht-bid area,near Mashhad,Iran

Mesozoic tonalites were emplaced in the axial zones of F₂ folds in low to medium grade metamorphic rocks. Tonalite has assimilated Fe-rich pelite and, at the contact, tonalite contains refractory garnet xenocrysts. Digestion of the xenocrysts to biotite-quartz-plagioclase assemblages has produced a decrease in amphibole, K-feldspar and garnet and an increase in biotite und quartz from the margins towards the centre of the tonalite mass. The magma probably intruded hot regionally metamorphosed rocks at high pressures. In the schistose thermal aureole, almandine porphyroblasts decrease in size and abundance away from the contact. Garnet grew from pre-existing nuclei in the regional metamorphic rocks and was armoured from reaction by quartz-sillimanite sheaths. In the aureole, chloritoid and staurolite are confined to rocks with a restricted bulk rock composition, sillimanite is present in rocks with a high iron content and andalusite is common in more aluminous pelites.

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Zusammenfassung Mesozoische Tonalite, die in den Achsenebenen von F₂-Falten hoch bis mäßig metamorphosierter Gesteine eingedrungen sind, assimilierten dabei die eisenreiche Pellite. Sie enthalten in der Kontaktzone Granat-Xenocrysten. Die Umwandlung dieser zu Biotit-Quarz-Plagioklas führte zu einer Abnahme der Amphibole, Kalifeldspäte und Granate. Gleichzeitig aber erfolgte eine Zunahme von Biotit und Quarz ausgehend von den Rändern nach dem Zentrum der Tonalitmasse hin. Es ist zu vermuten, daß das Magma in die einst heißen regionalmetamorphen Gesteine mit einem hohen Druck eingedrungen ist.In der thermischen Aureole nehmen Almandin-Porphyroklasten, von den Rändern nach außen hin in Häufigkeit und Größe ab. Die Granatbildung erfolgte aus präexistierenden Keimen in den regional metamorphen Gesteinen. Die gebildeten Kristalle wurden durch eine Hülle von Quarz-Sillimanit geschützt.In der Aureole treten Chloritoid und Staurolit lediglich in Gesteinen mit einer beschränkten Gesamtzusammensetzung auf. Dagegen befindet sich Sillimanit in Gesteinen mit hohem Eisengehalt, während Andalusit in aluminiumreichen Gesteinen vorkommt.

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Résumé Des tonalites mésozoïques, mises en place dans les zones axiales de plis F₂ affectant des roches faiblement à modérément métamorphisées, ont de ce fait assimilé des pélites riches en fer et renferment des xénocristaux de grenat. La digestion des xénocristaux en plagiochate — quartz — biotite a conduit à la dimination de la teneur en amphibole, feldspath K et grenat, et à l'augmentation en biotite et quartz à partir de la bordure jusqu'au centre de la masse tonalitique. Le magma a probablement été mis en place à des pressions élevées dans des roches chaudes issues du métamorphisme régional. Dans les schistes de l'auréole thermique, les porphyroblastes d'almandin de croissent en dimension et en abondance à partir du contact. Le grenat s'est développé à partir de moyaux préexistant dans les roches issues du métamorphisme régional et a été protégé de toute réaction par une couche de quartz — sillimanite. Dans l'auréole le chloritoïde et la staurotide se limitent à des roches à composition chimique restreinte; la sillimanite est présente dans des roches à teneur élevée en fer, et l'andalousite est commune dans des pélites plus alumineuses.

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A Petrogenetic Study of the Rosses Granite Complex, Donegal

The origin and evolution of the Rosses magma series are consideredin terms of the processes of partial fusion, assimilation, andcrystallization differentiation. Discussion of these processesis assisted by a consideration of the actual course of crystallizationat the level of emplacement The chemical variation in the Rossescomplex is shown to be inconsistent with crystallization differentiation,but can be explained by a combination of the other two processes.A scheme of magmatic evolution is proposed which explains boththe chemical variation in the complex and the relation betweencomposition and order of emplacement.