Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

Abstract

The causes of hydrothermal alteration in dolerite dykes intruding Caledonian rocks of W Connacht are investigated using stable isotope, water content and K–Ar data for whole rocks and mineral separates. Using an isochron approach the Logmór dyke in the north is re-dated to 308±4 Ma; previously determined older whole-rock ages reflect excess 40Ar. The ∼ 305 Ma age previously proposed for the Teach Dóite suite in the south is reinforced by a 305 Ma age on a pyroxene separate, although the severe resetting of most samples is emphasised by other pyroxene and plagioclase ages of ∼210 Ma. Pyroxene δ18O values for these Upper Carboniferous dykes are mostly 5·5 to 6·1%, indicating negligible crustal contamination. Logmór whole-rock samples have water contents of 1·7–2·1 wt.%, δ5D= 59 to –47‰ and δ18O = 9·4 to 9·6‰; plagioclase shows little mineralogical alteration but its δ18O is 9·7‰. Hydrothermal alteration involving a local formation or metamorphic water took place at high fluid/rock ratios and high temperature during cooling after intrusion, most probably in a thermally-driven convection system. Teach Dóite dykes have water contents of 2·0–4·2 wt.%. δD= –58 to –38‰ and δ18O = 3·6 to 9·2‰, and were mostly altered in two stages; hydration upon intrusion to ∼ 2 wt.% water by contemporaneous meteoric water at low fluid/rock ratios was followed by extensive chemical and isotopic alteration at ∼210 Ma (Upper Triassic) by surface waters. This latter event could also have caused the extensive alteration observed in the host rocks.

Kaolin polytype evidence for a hot-fluid pulse along Caledonian thrusts during rifting of the European Margin

“…This trend overlaps the meteoric water line, using either the fractionation equations of Savin and Lee (1988) or Sheppard and Gilg (1996). The heavier end-member in a mixture would have a d 18 O composition close to zero and could be a surface/meteoric water of late Palaeozoic age (regional data for this time reported in Hall et al, 1989;Jenkin et al, 1998). However, as in other studies a mechanism of generating low dD values in the other end-member is not apparent (Fallick et al, 1993).…”

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

“…These veins are also comparable to Bohaun in terms of containing brecciated wall rock fragments, comb quartz and vugs. In addition, Jenkin et al (1998) document a strong 210 Ma regional fluid event that overprints Carboniferous dikes in the area. One of these altered dikes is within 20 km of Bohaun.…”

“…A potential source of brines, containing sulfate consistent with the mineralogy observed at Bohaun (late carbonate and baryte), is dewatering of proximal Carboniferous basin sediments. Fluids of this type would have been available from the Carboniferous onwards and have been recognized elsewhere in western Ireland (O'Reilly et al, 1997;Jenkin et al, 1998). Evaporites are reported from Carboniferous basins both to the east and west of Connemara providing a possible source of sulphate (Anderton et al, 1979;Robeson et al, 1988).…”

Successive geothermal, volcanic-hydrothermal and contact-metasomatic events in Cenozoic volcanic-arc basalts, South Shetland Islands, Antarctica

“…Mendoza, 2000). More recently, propylitization has also been attributed to fluid flow during tectonic uplift (Rose & Bird, 1994), extensional tectonism (Mensing & Faure, 1996;Jenkin et al 1998) or interaction with marine (Levi et al 1989), meteoric (Sorensen et al 1998), salinelacustrine (Battles & Barton, 1995) or saline-formation waters (Dilles, Farmer & Field, 1995). These result from the major convective flow of meteoric groundwaters during pluton emplacement, and may lead to the formation of epithermal veins.…”