tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?, The

Journal of the Geological Society, May 2008 by Chew, D M, Flowerdew, M J, Page, L M, Crowley, Q G, Daly, J S, Cooper, M, Whitehouse, M J

The predominantly extrusive rocks of the Tyrone Volcanic Group consist of basaltic pillow lavas and andesitic to rhyolitic lavas of presumed arc affinity (Cooper & Mitchell 2004 and references therein). Associated with these extrusive rocks are volcaniclastic rocks with local chert and mudstone horizons. The mudstone units associated with the volcanic series have yielded graptolite fragments originally interpreted as Llandeilo-Caradoc in age (Hartley 1936). Subsequent reinvestigation of the same locality by Hutton & Holland (1992) yielded an Arenig-Llanvirn graptolite fauna. More recently, Cooper et al. (2008) document the presence of Isograptus victoriae lunatus in graptolitic mudstones from Slieve Gallion, along with a U-Pb zircon age of 473 �0.8 Ma for an extrusive rhyolite that sits stratigraphically below the graptolitic mudstones. Isograptus victoriae lunatus is the index fossil of the victoriae lunatus graptolite zone, and indicates a correlation with the Australasian Castlemainian (Cal) Stage. The U-Pb isotopic and biostratigraphical age constraints match closely with the interpolated age for the base of the Middle Ordovician (471.8 � 0.8 Ma, Cooper & Sadler 2004).

Field relationships within the Tyrone Central Inlier

Structure and metamorphism

The Tyrone Central Inlier is composed of psammitic and semipelitic paragneisses, which are cut by various acidic intrusive rocks. Further subdivision of these gneissic rocks is hampered by a combination of poor exposure and a lack of lithological diversity. The paragneisses have experienced polyphase deformation. An early, bedding-parallel S^sub 1^ foliation is folded by tight to isoclinal F^sub 2^ folds (Fig. 2a). An axial-planar S^sub 2^ foliation is well developed in the F^sub 2^ fold hinges, but is commonly co-planar to bedding and S^sub 1^ on the F^sub 2^ fold limbs. Hence at many localities and in thin section, only one foliation (regarded as an S^sub 1^-S^sub 2^ composite foliation) may be recognized. This main fabric is typically subhorizontal, and usually dips shallowly to the NW. Locally developed small-scale, upright F^sub 3^ folds affect the main fabric and are associated with the development of a S^sub 3^ crenulation cleavage.

In contrast to both the Dalradian Supergroup of NW Ireland and the Slishwood Division, no metamorphosed basic rocks have been recorded within the Tyrone Central Inlier. Therefore the metamorphic grade is based entirely on assemblages observed within metasedimentary lithologies. A prograde assemblage of biotite plagioclase sillimanite quartz � muscovite � garnet is typically observed in pelitic lithologies. Cordierite has also been observed locally (Hartley 1933). The absence of K-feldspar suggests that the second sillimanite isograd has not been reached, and hence the limited amount of prograde muscovite is probably due to it having been consumed at lower grades.

Field relationships of the minor igneous intrusive rocks

The main (composite S^sub 1^-S^sub 2^) upper amphibolite-facies fabric in the Tyrone Central Inlier is cut by a variety of minor acidic igneous intrusive rocks. These are best seen at Corvanaghan Quarry [H719813] where the exposure is relatively fresh. Here, quartz-plagioclase leucosomes up to 10 cm wide contain biotite-rich patches up to 5 cm across that are observed to cut the 82 fabric in dark psammitic gneisses (Fig. 2a). These leucosomes are in turn cut by muscovite-bearing pegmatites (Fig. 2b). At other localities in the Central Inlier, the main high-grade fabric is cut by both quartz-K-feldspar porphyritic dykes and coarse-grained quartzo-feldspathic sheets, up to 50 cm wide, which contain large (up to 10 cm long) biotite-rich patches. The quartzo-feldspathic sheets represent development of substantial leucosomes and are often cut by pegmatite veins. Their relationship with the quartz-K-feldspar porphyries is uncertain, but it is possible that these quartz-K-feldspar porphyries are coeval with the acidic hypabyssal igneous intrusions, which cut both parts of the Tyrone Plutonic Complex.