New age constraints for the Ordovician Tyrone Volcanic Group, Northern Ireland

Journal of the Geological Society, Jan 2008 by Cooper, M R, Crowley, Q G, Rushton, A W A

Abstract:

New biostratigraphical evidence and a high-precision isotope dilution thermal ionization mass spectrometry U-Pb zircon age provide refined age constraint for the Ordovician Tyrone Volcanic Group of the Tyrone Igneous Complex. In a graptolite fauna from Slieve Gallion, the presence of Isograptus victoriae lunatus, the index fossil of the victoriae lunatus graptolite zone, indicates a correlation with the Australasian Castlemainian (Ca1) Stage. The U-Pb zircon age of 473 ± 0.8 Ma dates a volcanic arc-related rhyolite body that is stratigraphically below graptolitic mudstones of Slieve Gallion. The U-Pb isotopic and biostratigraphical age constraints closely match an interpolated age for the base of the Middle Ordovician and indicate a Whitlandian age for the upper Tyrone Volcanic Group, which supports the regional correlation with the Ballantrae Complex, Midland Valley Terrane, Scotland.

The Tyrone Igneous Complex extends over an area of c. 350 km^sup 2^ of Counties Tyrone and Londonderry in Northern Ireland. It is recognized as part of the basement of the Midland Valley Terrane of Scotland, Northern Ireland and Ireland (Fig. 1a) (Bluck et al. 1992). A strong regional link has been made between the Tyrone Igneous Complex and the Ballantrae Complex of Girvan (Bluck 1985), which together compose the Tyrone-Girvan sub-terrane. In a broader context, the Tyrone Igneous Complex appears to fit most closely with the Notre Dame Subzone of the Dunnage Zone of Newfoundland, as described by Van Staal et al. (1998).

Although the distinctive array of igneous and sedimentary lithologies present in the Tyrone Igneous Complex was recognized by the Geological Survey of Ireland in the 19th century, it was Hartley (1933) who first appreciated the lithological and stratigraphical similarities between it and the Arenig Ballantrae Igneous Group in the Girvan area of SW Scotland. This correlation was apparently dispelled when Hartley (1936) reported very rare graptolites from 'black shales' in the upper part of the Tyrone Igneous Complex on Slieve Gallion, which appeared to indicate a Llandeilo-early Caradoc age. Remapping of the Pomeroy area by the Geological Survey of Northern Ireland (1979) led to the recognition of the two groups of rocks that now constitute the Tyrone Igneous Complex, namely, the structurally lower Tyrone Plutonic Group overlain by the Tyrone Volcanic Group (Fig. 1b). The resurvey also identified the varied lithologies and temporal array of the major intrusive bodies that are present. However, no new evidence for the age of the Tyrone Igneous Complex was forthcoming and indeed the possibility of a Dalradian age for the basal Plutonic Group was suggested (Geological Survey of Northern Ireland 1979).

The geological significance of the Tyrone Igneous Complex again changed with the seminal work of Hutton et al. (1985), who recognized the ophiolitic affinity of the Tyrone Plutonic Group, and demonstrated a basic igneous association of layered, isotropic and pegmatitic gabbros, doleritic sheeted dykes and rare basaltic pillow lavas. In the overlying Tyrone Volcanic Group they identified the components of a volcanic arc sequence, comprising pillow lavas, tuffs of basic to intermediate composition, rhyolites, chert, siltstone and dark grey mudstone representing up to three volcanic cycles. Further graptolite specimens discovered by Hutton & Holland (1992) at the locality on Slieve Gallion pointed to a much older age for the Tyrone Volcanic Group within the Ordovician than proposed by Hartley (1936), but were still somewhat indefinite, being long-ranging forms of Arenig-Llanvirn age.

Although individually fault-bound, the Tyrone Plutonic Group and Tyrone Volcanic Group are considered to be contemporaneous (Hutton & Holland 1992; Parnell et al. 2000), and are pinned together by a suite of volcanic arc-related intrusions of Arenig to Llanvim age (S. R Noble et al., pers. comm.). A tonalite intrusion belonging to this suite, located at Craigballyharky NE of Pomeroy (Fig. 1b), with a published U-Pb zircon age of 471 2/-4Ma (Hutton et al. 1985), intrudes and mixes with gabbro (Angus 1962, 1977) thought to belong to the Tyrone Plutonic Group ophiolite. Such magma mixing demonstrates that the gabbro was still molten at the time of tonalite intrusion, and hence an Arenig age for part of the Tyrone Plutonic Group is conceivable.

This paper provides a more refined biostratigraphical age for mudstones within the Tyrone Volcanic Group, and presents a high-precision U-Pb isotopic age for an underlying volcanic arcrelated rhyolite body.

Biostratigraphy

Previous biostratigraphy

Biostratigraphical evidence for the Ordovician age of the Tyrone Volcanic Group of the Tyrone Igneous Complex has been based solely on small collections of fragmentary graptolites from one locality in the Sruhanleanantawey stream [IGR 27905 38790] (Fig. 1b). The stream section, on the NW slopes of Slieve Gallion [from IGR 2788 3886 to IGR 27901 38785], exposes a variety of lithologies including greenish grey tuffs and lava with hornblende and feldspar phenocrysts, chert, light grey, thin-bedded tuffaceous siltstone and dark grey, thin- to medium-bedded mudstone with variable amounts of tuffaceous debris and pyrite.