A stable isotope study of retrograde alteration in SW Connemara, Ireland

Jenkin, Gawen R.T.; Fallick, Anthony E.; Leake, Bernard Elgey

doi:10.1007/bf00310743

Cited by 31 publications

(36 citation statements)

References 54 publications

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“…Most work on plagioclase alteration in granitic rocks has dealt with saussuritization, sericitization and albitization processes (Jenkin et al 1992;Garcia et al 1996;Fiebig and Hoefs 2002;Boulvais et al 2007), whereas less attention has been paid to the replacement of plagioclase by hydrothermal K-feldspar . The aim of this paper is to highlight and discuss the types, timing and conditions of hydrothermal alteration of magmatic plagioclase in Proterozoic granitic rocks from SE Sweden (Figure 1).…”

Section: Introductionmentioning

confidence: 99%

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

et al. 2009

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Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel the mechanisms and conditions of plagioclase alterations. These alterations include saussuritization, albitization and replacement of plagioclase by K-feldspar. The hydrothermal alterations, which are inferred to have occurred at ca. 250-4008C, resulted in concomitant formation of Alrich titanite, epidote, calcite, pumpellyite, prehnite and iron oxides. Replacement of plagioclase by K-feldspar occurs in red-stained zones, which have developed close to thin fractures owing to the precipitation of tiny Fe-oxide pigment particles within the altered plagioclase, whereas saussuritized plagioclase has less systematic spatial relationships to these fractures. Albitization of plagioclase occurred in rocks that are poor in biotite compared to rocks that suffered extensive saussuritization. The chemical and textural characterization of various types of plagioclase alterations allows elucidation of the granitic hydrothermal systems. Features of feldspar alteration in the granitic rocks are similar to those encountered in feldspathic sandstones and should hence be considered in studies on diagenetic changes of siliciclastic successions during basin evolution.

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Section: Introductionmentioning

confidence: 99%

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

et al. 2009

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“…462 Ma metasomatic deposits in Connemara (Yardley et al 1991;Cliff et al 1993). Such a large regional extent of hydrothermal activity related to emplacement of the Galway batholith has been suggested by Jenkin et al (1992) for retrograde alteration in southern Connemara.…”

Section: Category 3 Agesmentioning

confidence: 82%

“…At this time, a major post-Grampian granitic intrusion-the Galway batholith-intruded older plutons of the Connemara igneous complex (e.g., Burke 1957;Leake and Tanner 1994). The thermal pulse associated with batholith intrusion led to the development of a regionally extensive hydrothermal system (Jenkin et al 1992), which may be responsible for local retrogression of M2-M3 metamorphic assemblages throughout Connemara.…”

Section: Post-grampian Extensional Faulting Erosion Sedimentation mentioning

confidence: 99%

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Geological significance of ⁴⁰Ar/³⁹Ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions

Friedrich

Hodges

2016

Can. J. Earth Sci.

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Abstract:The Connemara region is a world-class example of a regional-scale, high-temperature metamorphic terrain. Its rock record documents formation of a bi-vergent orogenic wedge and associated calkalkaline magmatism in a an arc-continent collisional setting (Grampian orogeny), for which a protracted evolution was inferred based on a >75 Ma spread in U-Pb, Rb-Sr, and K-Ar mineral ages. In contrast, geological field observations imply a simple relationship between syntectonic magmatism, bi-vergent deformation, and Barrovian-type metamorphism. We explore the significance of the spread in apparent cooling ages using 40 Ar/ 39 Ar mica thermochronometers of varying grain sizes and composition, collected across metamorphic grades ranging from staurolite to upper sillimanite. We integrated geological and previously published geochronological evidence to identify a 32 Ma range (ca. 475-443 Ma) of permissible cooling ages and distinguished them from those dates not related to cooling after high-temperature metamorphism. Variations in 40 Ar/ 39 Ar dates at a single locality are ≤10 Ma, implying rapid cooling (≥6-26°C/Ma) following metamorphism and deformation. A distinct cooling age variation (≥15 Ma) occurs on the regional scale, consistent with spatial differences in the metamorphic, magmatic, and deformational evolution across Connemara. This cooling record relates to a lateral thermal gradient (30°C/km) in an evolving arc-continent collision, rather than to differential unroofing of the orogen. Our results imply that the large (≥50 Ma) spread in thermochronometers commonly observed in orogens does not automatically translate into a protracted cooling history, but that only a small number of thermochronometers supply permissible cooling ages.Résumé : La région de Connemara des Calédonides irlandaises est un exemple de classe mondiale de terrain métamorphique de haute température d'envergure régionale. Sa formation est reliée au magmatisme calcoalcalin dans le contexte d'une collision arc-continent, pour laquelle une évolution prolongée a été inférée à la lumière de la distribution sur plus de 75 Ma d'âges U-Pb, Rb-Sr, et K-Ar sur différents minéraux. Une telle évolution ne correspond pas aux observations géologiques sur le terrain, qui indiqueraient plutôt une relation simple entre un magmatisme, une déformation et un métamorphisme de type barrovien syntectoniques. Nous avons examiné la signification de la distribution des âges apparents de refroidissement à l'aide de thermochronomètres 40 Ar/ 39 Ar sur micas de granulométries et compositions variées, prélevés à différents degrés de métamorphisme allant de la zone à staurotide à la zone à sillimanite supérieure. Nous avons intégré les données géologiques et de l'information géochronologique déjà publiée pour délimiter une fenêtre de 32 Ma (de 475 à 443 Ma environ) pour les âges de refroidissement possibles et avons distingué ces derniers des âges non reliés au refroidissement ayant suivi le métamorphisme de haute température. La variation des âges 40 Ar/ 39 Ar en un ...

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“…1) and gold genesis is still controversial (Pattrick et al 1988;Craw 1990;Curtis et al 1993). Late state meteoric incursion into the Dalradian belt has been documented previously from studies of post-metamorphic mineralogy, stable isotopes, and fluid inclusions (Craw 1990;Jenkin et al 1992;Curtis et al 1993). The present paper describes the regional extent of meteoric involvement in late stage vein formation, and examines possible relevance of this meteoric incursion to gold mineralization.…”

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confidence: 99%

Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

Craw

Chamberlain

1996

Mineral. Deposita

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Post-metamorphic quartz veins which occur over hundreds of square kilometres in the biotite zone of the Dalradian metamorphic belt consist of three principal types: anhedral quartz with pyrite, anhedral quartz with hematite, and prismatic quartz with hematite or rutile. The oxide minerals in anhedral veins have formed by oxidation of pre-existing sulphides, and gold was mobilized during this oxidation. Anhedral quartz veins formed from an aqueous fluid with up to 5 wt% dissolved salts and 16wt% CO2 at about 300~ Texturally later prismatic quartz crystals formed from a compositionally similar fluid which was undergoing phase separation at the H20-CO2 solvus at 160-200 ~ and 500 to 1200 bars fluid pressure. Oxygen isotope ratios for quartz from the veins range from 12.0 to 15.3% o, with hematite-bearing veins generally isotopically heavier than pyrite-bearing veins. Calculated fluid oxygen isotope ratios range from + 8%0 for pyrite-bearing veins to -2%o for late prismatic crystals. The mineralizing fluid contained a substantial component of meteoric water whose isotopic and chemical composition evolved with progressive water-rock interaction. Evolution of meteoric fluid composition involved migration of oxidation and oxygen isotope fronts in the down-flow direction as head-driven water passed through structurally controlled fractures in the schist pile. A gold solubility trough occurs for the observed fluid in the oxidation frontal zone. Gold remobilization and reprecipitation occurred progressively as the oxidation front migrated through the schist pile.

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A stable isotope study of retrograde alteration in SW Connemara, Ireland

Cited by 31 publications

References 54 publications

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Geological significance of ⁴⁰Ar/³⁹Ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions

Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

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A stable isotope study of retrograde alteration in SW Connemara, Ireland

Cited by 31 publications

References 54 publications

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Geological significance of 40Ar/39Ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions

Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

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Geological significance of ⁴⁰Ar/³⁹Ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions