Leinster Granite Research Articles

A laboratory simulation of degradation of leinster granite by dry and wet deposition processes

The accelerating effect of pollutant gases and acids on the degradation of ‘fresh” Leinster granite has been simulated in laboratory-based dry deposition and wet deposition exposure chambers. Granite degradation has been followed by weight change, stone extract and sectional analyses, pH change and cation content of the run-off solution. General deterioration of the granite structure is revealed by petrographic analyses to occur by a series of mineralogical tranformations, of which kaolinization and seritization are the most clearly visible. Microcracking, predominantly in the alkali/microcline and plagioclase feldspar crystals, is evident on the exposed side, especially in simulated acid rain solutions but not in deionized water. Ion exchange of H + for Ca 2+, allied with mineralogical transformation and salt crystallization in pores, evidently promote the cracking. Calcite and gypsum deposition in intergranular regions and cracks are also observed and there is evidence of the removal and oxidation of iron species on the mineral surface.

Discussion on detrital magmatic muscovite from the Lower Carboniferous of southwest Ireland: buried granites uncovered?

G. D. Sevastopulo writes: Clipstone & Roycroft (1992) have argued that the occurrence of muscovite of magmatic origin in Lower Carboniferous (Courceyan; Dinantian) limestones near Kilmaclenine, north County Cork constitutes primary evidence of the presence of now buried granites in southwest Ireland. Ford et al. 1991 have interpreted the gravity field over the south of Ireland. Their low pass filtered gravity map (fig. 3) shows two large negative anomalies. The first extends southwestward to the neighbourhood of Fermoy, County Cork; Ford et al. interpreted it as the expression of a laccolith, probably the subsurface extension of the Leinster Granite, within the basement to the Upper Palaeozoic. Because of its location within the basement, this laccolith could not have been a source of detrital muscovite during the Dinantian. The second negative anomaly is centred southwest of Killarney. Ford et al. were unable to decide whether it is caused by a buried granite or by an exceptionally thick succession of Devonian sandstone. In their ‘granite model’ ( Ford et al. 1991 , fig. 6), the main mass of the granite was shown in the footwall of the Killarney–Mallow fault, overlain by Old Red Sandstone and Dinantian limestone. To the south of the fault the granite is modelled as being below the thick succession of Old Red Sandstone, which can be mapped on the Iveragh Peninsula. If a granite was exposed in this region during latest Devonian and Dinantian time, it must have been located in what is now the footwall of the Killarney–Mallow

Simulation of the degradation of coupled stones by artificial acid rain

The degradation of coupled building stones has been simulated in a laboratory chamber using an artificial acid rain solution (‘wet’ deposition) with wet/dry cycling. The reaction of individual and coupled stones with the artificial acid rain was assessed from the pH, and confirmed by measurement of calcium ion content in the run-off. For the coupled stones, it is evident that there is a build up of damaging reaction products in the region of the lower stone closely adjacent to the upper stone. This behaviour is related to the increased time of wetness due to retained solution, containing calcium ions from runoff and anion species from the artificial acid rain solution, which allows locally enhanced reaction with the lower stone and subsequent development of salts. Where Leinster granite is the lower stone of the couple, calcium ions in the run-off from the upper stone, Portland limestone, accumulate in these regions, leading to the eventual development of gypsum in the near-surface regions of the granite, with extensive resultant degradation. The laboratory chamber allows rapid ranking of various stone combinations in the couple, and early insight into the degradation process which may ultimately assist in the development of conservation procedures.

Genesis of lithium pegmatites from the leinster granite margin, southeast ireland: Geochemical constraints

AbstractExtensive and economically‐important lithium pegmatites have been described from the eastern flank of the large S‐type peraluminous Leinster Granite batholith in southeast Ireland. This flank area is also the locus of a major dip‐slip ductile shear zone which was active during emplacement of the Tullow Lowlands pluton of the batholith. All the known bedrock occurrences of lithium pegmatite are contained within this marginal deformation zone.Most previous studies have suggested derivation of the lithium pegmatites by fractionation from the Leinster Granite itself, though none are conclusive. More recently, an anatectic model has been proposed whereby the pegmatites were derived by melting of Li‐rich sediments adjacent to the batholithic contact. Our investigation has been concerned with a detailed study of the Rb‐Sr isotope systematics and trace element geochemistry of the pegmatites and their host granitoids in an attempt to resolve the problem of pegmatite genesis.The results suggest a strong geochemical and temporal coherence between the Leinster Granite and the lithium pegmatites. A model is proposed in which Li enrichment in residual magmas may be linked to the development of muscovite instead of biotite in the Leinster Granite itself. The Li excluded from entry to biotite then accumulates in highly mobile, low‐viscosity pegmatitic fluids which migrate into the marginal shear zone of the Tullow Lowlands pluton where they crystallize at considerably lower temperatures.

Cryptic Sr and Nd isotopic variation across the Leinster Granite, southeast Ireland

AbstractRb–Sr isotope whole-rock data from the end-Caledonian Leinster Batholith define an errorchron age of 464±26 Ma, appreciably older than the accepted emplacement age of c. 405 Ma. This anomalously old age is the consequence of a highly variable initial Sr isotopic composition. Initial Nd ratios and TDM model ages also show a wide range of values. However, these isotopic variations are neither randomly distributed nor related to petrographic changes but instead reflect, in a simple way, geographic position about the axis of the batholith. The isotopic pattern revealed in the Leinster Batholith means that isochron ages obtained from S-type granites may be largely dependent on the sampling scheme adopted. The variable isotopic composition of the granite most likely resulted from either a heterogeneous metasedimentary source or hybridization of coeval, dominantly crustal melts.

Zoned muscovite from the Leinster Granite, S.E. Ireland

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Isotopic U–Pb ages of zircon and monazite from the Leinster Granite, southeast Ireland

AbstractA more precise age for the synkinematic emplacement of the large S-type Leinster batholith (southeast Ireland) has been obtained by U–Pb dating of zircon and monazite separates from the two main granite varieties of the Northern Pluton. A concordant age of 405±2 Ma derived from monazite is considered to reflect both the age of intrusion and the main regional deformation event (D1) in southeast Ireland. The monazite age is in good agreement with an earlier, less precise whole-rock Rb–Sr isochron determination. The zircon data indicate recent Pb loss effects and the existence of a small component of inherited radiogenic Pb derived from a crustal precursor during the generation of the batholith.

Evolution of fluid phases associated with lithium pegmatites from SE Ireland

AbstractFluid inclusions in quartz from internally zoned barren and spodumene-bearing pegmatites associated with the Leinster granite of SE Ireland represent a variety of early and late hydrothermal fluids responsible for the development of pegmatites. Microthermometry and optical examination reveal two main populations of inclusions. The first (Type 1) comprises low-moderate salinity brines which homogenized at temperatures up to about 400 °C. The second (Type 2) appear to postdate the first population and are characteristically more saline and homogenized at temperatures mostly below 250 °C. Isochores for model type 1 inclusion fluids indicate that a late-magmatic/early-hydrothermal fluid developed from the Leinster granite at 675 °C. and 2.5 kbar and cooled isobarically into the spodumene stability field where complete crystallization of the pegmatites took place. Later, more saline, type 2 fluids of unknown origin may have contributed to the alteration of spodumene to muscovite and albite with the accompanying release of lithium from the lattice of spodumene.

Tourmalinite as a potential host rock for gold in the Caledonides of Southeast Ireland

Initial investigations designed to test the potential for gold in a series of lithologies associated with tourmalinites and coticules in the neighbourhood of the Caledonian Leinster Granite of southeast Ireland are reported. The results of geological mapping, geochemical analyses and panning of stream sediments indicate that significant levels of gold are associated with some tourmalinite zones. The possible exploration implication of this work is that tourmalinites have potential as target indicators, although secondary controls on mineralization are also important.

Genesis, mineralogy and geochemistry of uranium in the Gortdrum stratiform copper deposit, Ireland

The Gortdrum Cu-Ag-As-Sb-Hg-U orebody occurs in Lower Carboniferous strata on the downthrown side of an ENE transcurrent fault system in Southern County Tipperary, Republic of Ireland. The deposit comprises a linear discordant zone of brecciated carbonates and country rocks that are altered and mineralised. Research has indicated anomalous radioelement concentrations associated with the main copper orebody and in particular with the altered breccias within this zone. The metal association (Cu-Ag-As-Sb-Hg-U) is considered unique for Irish Carboniferous carbonate-hosted copper deposits. However, the low temperature mineral assemblage of early uranium-bearing minerals with later sulphide stages is reminiscent of many vein-type hydrothermal ore deposits. The geochemistry of uranium-enriched rocks indicates that the alteration of the basic dykes was a most significant event in the trapping of uranium within the limestones. Propylitization of the early basic dykes by CO2-bearing fluids produced the alteration which led to the development of hematite, leucoxene and clay minerals. The reducing environments attendant with such mineralogy created a favourable environment for uranium precipitation. The reduction of U+6 to U+4 by oxidation of reduced Fe in dolomitized brecciated limestone occurred prior to the main sulphide mineral sequences. The genesis of uranium in the deposit is linked to radioelement remobilization from uranium-bearing heavy minerals in sediments of upper Devonian-Lower Carboniferous age. The uranium became available to low temperature hydrothermal fluids which carried then deposited the uranium along a transcurrent fault system. The source of the heavy minerals is considered to be the Leinster Granite to the east, by which time in the Carboniferous, unroofing of the main pluton had commenced.

The geochemistry of the Blackstairs Unit of the Leinster Granite, Ireland

The Blackstairs Unit is the southernmost of the five units which comprise the Leinster Granite. Four granite types are recognized, which in order of decreasing age are: Graiguenamanagh Granite, Porphyritic Microcline Granite, Non-Porphyritic Granite and Type 1 Granite. Using whole-rock geochemical data, in conjunction with field and petrographic evidence, a model is proposed whereby partial melting of a Lower Palaeozoic immature sedimentary source produced a parental granitic melt which crystallized without further differentiation to form the Non-Porphyritic Granites of the northeastern end of the pluton. At the SW end, this parental melt fractionated to form the Porphyritic Microcline Granite leaving the melt depleted in K 2 O, which then crystallized to form a slightly more acidic Non-Porphyritic Granite than that at the NE end. A mixture of restite minerals plus melt from this partial melting process crystallized forming the Graiguenamanagh Granite. Fractionation of thorium into restite allanite in the Graiguenamanagh Granite accounts for the low Th and Th/U levels which characterize the Non-Porphyritic Granite and the Leinster Granite in general. A high degree of partial melting of similar source material produced the low volume Type 1 Granite magmas which were subsequently intruded along faults. A similar petrogenesis is envisaged for the whole of the Leinster Granite.

Tectonic relationships of the Leinster Granite, Ireland

AbstractThe Leinster Granite in southeast Ireland comprises five plutons, which were diapirically emplaced into the Lower Palaeozoic rocks of the Leinster Massif. The pre‐existing Caledonian cleavage was exploited during intrusion which has resulted in an en echelon development of plutons as the granite belt cuts obliquely across the regional cleavage trend. A foliation map of the Leinster Massif illustrates how the foliation trend is modified from the regional trend within a shear zone approximately coincident with the granite belt. The shear zone has a sinistral sense of shear and developed before the plutons were fully consolidated. The shear strain decreases from 5 at the northern end of the shear zone to 0·6 near its termination at the southern end of the granite.

Stable isotope studies of sulphide mineralisation on the Leinster Granite margin and some observations on its relationship to coticule and tourmalinite rocks in the aureole

Galena and sphalerite occur in brecciated veins along the eastern margin of the Caledonian Leinster Granite in south-east Ireland. This mineralisation is spatially related to the occurrance of spessartine quartzites (coticules) in the aureole rocks. Tourmalinites are closely associated with these coticules. A decrease in the temperature of mineralisation from about 500°C in the northern Unit I to 250°C in the central Unit III to possibly less than 200°C in the southern Unit V of the intrusion is suggested by decreasing sulphide δ34S values, increasing sulphate δ34S values and sulphide-sulphate fractionation values. Sulphur, carbon and oxygen isotope data suggest the involvement in the vein mineralisation of sulphur, carbon and mineralising fluids at least partially derived from the coticule and tourmaline bearing aureole schists.

Abundance and distribution of uranium and thorium in the northern units of the Leinster granite, Ireland

SummaryAbundance data, determined by epithermal neutron activation analysis, are reported for radioelements in the northern units of the Leinster granite. Compared to similar data for silicic igneous rocks and British Caledonian granites, the Leinster granite shows normal U and K contents but is low in Th. Inter-element correlations and fission-track results suggest that most Th and up to 40% of the U is bound in resistate accessory phases (zircon, sphene, apatite), but that the bulk of U is associated with late-stage alteration products and is located at interstitial labile sites within the host-rock matrix. Mobilization of U located at such sites in the Leinster granite by hydrothermal leaching could lead to redeposition and enrichment in marginal sediments of Old Red Sandstone or younger age.

Relationship of hydrothermal phenomena within the Leinster Granite to crustal fractures delineated from Landsat imagery

Since Landsat imagery has become available, it has been of exceptional value in displaying large-scale, structural elements of extensive regions. Such elements include regional joint and fracture systems. Frequently many of these fractures have been overlooked on the ground and even on aerial photographs. The purpose of this paper is to report the results of a preliminary investigation into the existence of hitherto unrecognised major, largely transverse linears, which are seen on recent Landsat-1 and -2 imagery of the Leinster Granite in southeast Ireland. Analysis suggests that a strong spatial correlation exists between many hydrothermal-related features within the granite and its surroundings and the surface traces of the transverse linears which are interpreted as fracture zones.

Lithium pegmatites in the Leinster Granite (southeast Ireland)

Vein-like pegmatites, aplites and granitic country rocks from the SE region of the Leinster Granite (SE Ireland) were studied petrographically and geochemically. The results are reported and, amongst other data, numerous mineral analyses of spodumene, muscovite, lithian muscovite and feldspar are given. The pegmatites consist of quartz, alkali feldspar and small amounts of muscovite; they contain trace quantities of oxide minerals of Nb and Ta. The pegmatites in the SE region are of special interest, because they include up to 60 wt.% of spodumene (Li 2O contents over 4 wt.%). Greisenized Li-pegmatites show partial or complete replacement of spodumene and microcline by new lithian muscovite (up to 60 wt.% in the rocks) with high concentrations of Li, Rb, Cs and F; albite in the original pegmatites was changed into cleavelandite. The Leinster Granite is particularly rich in alkalies and proves to be a granite—granodiorite, according to the calculated position in the Streckeisen diagram. Aplites which are frequently associated with pegmatites consist mainly of finegrained albite and subordinate quartz. The relatively rare occurrence of Li-pegmatites in the Leinster Granite is probably due to rather low concentrations of water in the magma, conditions which prevented the formation of larger quantities of a water-saturated rest melt, which is usually rich in Li, Rb, Cs, Be, etc.

Deformation studies in the Irish Caledonides

An interpretation of the major structure of parts of the Irish Caledonides is made, based on the results of mapping of the main early penetrative fabrics. Interpretation of major changes in fabric are attributed to strain variation, and models are developed to explain these changes. The regional nappe structures in the Dalradian rocks have been modified by (1) D 2 slide zones, and (2) a D 4 sinistral shear zone adjacent to the Main Donegal Granite in NW Donegal. In N Mayo the major structure is interpreted in terms of a steep dextral E-W shear zone modifying the early formed nappe pile. The strain history adjacent to the Iapetus suture in central Ireland is described and a wide zone of non-axial planar cleavage discussed. In SE Ireland the regional strain pattern is modified in the tectonic aureole of the Leinster Granite.

Silurian microfossils from west of the Leinster Granite

The upper part of the Lower Palaeozoic sequence west of the Leinster Granite in eastern Ireland is shown, by means of chitinozoa, to be Llandovery in age, with some of it perhaps younger. These Silurian rocks are part of a turbidite succession which extends down to the lower Ordovician west of the granite and down to the lower Cambrian on the east.

Some curious garnet clusters from the Garnetiferous Beds of the Leinster Granite Aureole

SummaryHollow, often ellipsoidal, clusters of small manganiferous garnets are described. They occur as discrete entities in the normal garnet–quartz coticule fabric of the Garnetiferous Beds, an important group of metamorphosed manganese-rich sediments found within the aureole of the Leinster Granite. The clusters are thought to represent small original sedimentary concretions, subsequently completely reconstituted.

Engineering geology of the Turlough Hill pumped storage scheme

The Turlough Hill Pumped Storage Scheme has been constructed in the Leinster Granite some 40 km south of Dublin. The upper reservoir has been partially excavated on the summit of Turlough Hill and water can be discharged through an inclined tunnel into the underground power station and to the lower reservoir of Lough Nahanagan. The geological conditions at the site are outlined with particular reference to the rock properties and the location of the underground works.