Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

Hunt, Emma J.; O’Driscoll, Brian; Daly, J. Stephen

doi:10.1017/s0016756811000872

Cited by 7 publications

(4 citation statements)

References 62 publications

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“…This new mechanism necessitates profound reassessment of existing petrogenetic models for chromitite deposits. We also infer that this mechanism can be extrapolated to explain the podiform chromitites that are commonly observed in ophiolites as well as non-ophiolitic peridotite complexes 34 – 36 . In addition, we predict that the origin of other magmatic deposits can ultimately be resolved by examining the consequences of lithostatic pressure reduction on liquidus phase equilibria.…”

Section: Discussionmentioning

confidence: 69%

Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

et al. 2018

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Platinum-bearing chromitites in mafic-ultramafic intrusions such as the Bushveld Complex are key repositories of strategically important metals for human society. Basaltic melts saturated in chromite alone are crucial to their generation, but the origin of such melts is controversial. One concept holds that they are produced by processes operating within the magma chamber, whereas another argues that melts entering the chamber were already saturated in chromite. Here we address the problem by examining the pressure-related changes in the topology of a Mg2SiO4–CaAl2Si2O8–SiO2–MgCr2O4 quaternary system and by thermodynamic modelling of crystallisation sequences of basaltic melts at 1–10 kbar pressures. We show that basaltic melts located adjacent to a so-called chromite topological trough in deep-seated reservoirs become saturated in chromite alone upon their ascent towards the Earth’s surface and subsequent cooling in shallow-level chambers. Large volumes of these chromite-only-saturated melts replenishing these chambers are responsible for monomineralic layers of massive chromitites with associated platinum-group elements.

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Section: Discussionmentioning

confidence: 69%

Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

et al. 2018

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“…The Dawros site is situated near Letterfrack, Connemara, County Galway, western Ireland (53°34¢N; 9°58¢W). It is underlain by peridotite and the geology has been described by Rothstein (1957), Leake (1964) and Hunt et al (2012) among others. The vegetation is grazed grassland (Fig.…”

Section: Study Sitesmentioning

confidence: 99%

Geo‐ecological studies on two ultramafic sites in western Ireland

Brearley

2018

Ecological Research

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Ultramafic soils are found in many sites around the world where they can vary from exceptionally barren to reasonably fertile. Two ultramafic sites in western Ireland were studied: grassland at Dawros, County Galway and grassy heath near the base of Croagh Patrick, County Mayo. Rock and soil chemistry was examined along with foliar nutrients (at Dawros only). Ellenberg reaction values of all plant species recorded were determined. Two bioassays were conducted to determine relative differences in fertility between ultramafic and adjacent non‐ultramafic soils and to assess nutrient limitation in the Croagh Patrick soil. Both soils showed many of the chemical characteristics typical of other ultramafic sites including a moderately high nickel concentration; in general, soil metal concentrations were higher in Dawros soils. However, nitrogen, phosphorus and calcium (with a calcium:magnesium ratio c. 0.6) were all at high concentrations at Dawros leading to a fertile grassland with both calcicole and calcifuge species present spanning six Ellenberg reaction values. Foliar nutrient concentrations were not unusual although calcium:magnesium ratios were approximately double in non‐ultramafic soils compared to ultramafic soils. Croagh Patrick soil had lower concentrations of most nutrients and presented a grassy heath vegetation with more acidic reaction values. The bioassays showed plant growth to be reduced in this soil relative to that at Dawros and to be clearly limited by phosphorus availability. Whilst these two Irish ultramafic sites do not show the extreme features associated with other sites across the world they indicate the global diversity of serpentine ecologies.

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“…Bennett & Gibb (1983) alternatively suggested that it was possible the Dawros intrusion was emplaced as a separate body to the Currywongaun-Doughruagh gabbros. Hunt et al (2012) used mineral chemical and textural characteristics of Cr-spinel in chromitite layers from the Dawros intrusion to demonstrate that these rocks probably represent cumulates formed in an open-system, mid-to-lower crustal supra-subduction zone magma chamber.…”

Section: Geological Settingmentioning

confidence: 99%

Timescales of magmatism and metamorphism in the Connemara Caledonides: insights from the thermal aureole of the Dawros–Currywongaun–Doughruagh Complex, western Ireland

O’Driscoll

Chew

2021

Geol. Mag.

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Zircon separates from the contact aureole of the syn-tectonic Dawros–Currywongaun–Doughruagh Complex, western Ireland, are studied to constrain the nature and timing of magmatism associated with the early stages of the Grampian Orogeny. The samples analysed come from the uppermost part of the Dalradian Supergroup in northern Connemara (the Ben Levy Grit Formation), where a laterally extensive (>10 km) package of metamorphosed siliciclastic sedimentary rocks containing heavy mineral seams crops out. The seams mainly comprise magnetite, but zircon is also present in greater than accessory quantities. The seams have been locally reworked at granulite-facies metamorphic conditions during intrusion of the Dawros–Currywongaun–Doughruagh Complex magmas. Here we combine in situ mineral chemical and U–Pb geochronological analyses of zircons from samples of these heavy mineral seams collected at different locations in the Dawros–Currywongaun–Doughruagh Complex thermal aureole. An important finding is that the zircons studied have magmatic trace-element compositions, interpreted here as a function of their growth during contact metamorphic-induced partial melting. The zircons yield a range of U–Pb spot ages whose uncertainties suggest a maximum duration of zircon growth of ˜11 Ma, between 477.1 and 466.1 Ma, though it is likely that zircon growth occurred much more quickly than this. The age constraints revealed here match well with the range of 475 to 463 Ma previously proposed for the Grampian Orogeny overall in Connemara and lend useful support to models that argue for high-intensity, relatively short-lived Grampian orogenesis in the Connemara Caledonides.

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Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

Cited by 7 publications

References 62 publications

Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

Geo‐ecological studies on two ultramafic sites in western Ireland

Timescales of magmatism and metamorphism in the Connemara Caledonides: insights from the thermal aureole of the Dawros–Currywongaun–Doughruagh Complex, western Ireland

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