Characterisation of Irish volcanic ash layers using combined in-situ U-Pb and Lu-Hf analyses of zircon and/or apatite and its implication for the growth rate of the Waulsortian reef

The Waulsortian Limestone, a Carboniferous deep-water
mudbank bioherm-facies, is the main host rock for multiple
significant Zn-Pb sulfide deposits in Ireland. However, neither
the timing of mineralisation nor the age of host rock is well
constrained. This study contributes to our understanding of the
growth rate of the bioherm, by dating volcanic ash beds in
stratigraphic equivalent formations above and below the
Waulsortian Limestone. Samples are from regional drill cores
and are restricted to the Limerick Syncline (Stonepark) and
Lisheen/Rapla district. The altered volcanic ash beds are between
2 and 10 cm thick and are distinct from the surrounding
limestones due to their green-beige colour and high clay content.
Uranium-Pb isotopic and trace element analyses on zircon and
apatite grain mounts were conducted by LA-ICP-MS at Trinity
College Dublin (TCD). An aliquot of each sample was crushed
and analysed for 40 trace elements by solution-Q-ICP-MS.
Whole-rock trace-element characterisation of the ashes did not
allow for cross-correlation between Stonepark and
Lisheen/Rapla. However, apatite trace element analyses of the
tuffs underlying the Waulsortian indicate an evolved magmatic
source, with the closest known evolved source being the
Variscan Belt of Europe, >1000 km from Ireland. As such a
distal source seems unlikely, the volcanic source remains
uncertain. Further source characterisation using Hf isotopes on
zircon will clarify the ash layer sources and potentially contribute
to stratigraphic correlations in the Stonepark and the
Lisheen/Rapla district.
The mean age of the stratigraphically underlying ash beds is
350.5 ± 3.9 Ma, while the overlying volcanic ash is dated at
348.2 ± 2.4 Ma. The thickness of the Waulsortian Limestone at
Stonepark is up to 550 m [1]. Thus, the age of these ash layers
implies a growth rate of ~240m / Ma, albeit with a large
uncertainty. This agrees with the growth rate of a modern reef
equivalent, which is 1 Ma for 130 m [2]. CA-ID-TIMS dating of
these ash layers will yield significantly more precise age
constraints on the growth rates of the substantial Waulsortian
mud mounds.