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

Small, tectonically isolated fragments of continental crust (microcontinents) are commonly accreted to orogenic belts. Possible mechanisms for the formation of microcontinents include strike-slip translation; for example, Baja California has been translated by hundreds of kilometres along the Gulf of California (Larson et al. 1968). Another possible mechanism for microcontinent formation is re-rifting of a continental margin, which has the effect of isolating a passive margin segment within a tract of oceanic crust (Vink et al. 1984; M�ller et al. 2001). In both of these scenarios, the resultant microcontinental blocks are likely to be preserved in any future accretion event because of their buoyancy. We favour the second scenario for the outboard location of Tyrone Central Inlier relative to the Laurentian margin. A similar tectonic model was envisaged for isolation of the Dashwoods block from the Humber zone during opening of Iapetus on the Newfoundland segment of the Laurentian margin (Cawood et al. 2001; Waldron & van Staal 2001).

The Tyrone Central Inlier is envisaged as a Laurentian microcontinental block produced during opening of the Iapetus Ocean at c. 570 Ma (Fig. 7a). The Iapetus Ocean started to close during the Late Cambrian-Early Ordovician and an intra-oceanic arc, such as the Lough Nafooey Arc in western Ireland, was initiated during the Early Tremadoc (Dewey & Mange 1999). This arc had encountered the Laurentian margin by c. 490 Ma (Chew et al. 2007). An along-strike equivalent to the Lough Nafooey Arc is inferred for the Tyrone segment of the Laurentian margin (Fig. 7b). Continued closure of the Iapetus Ocean caused obduction of suprasubduction-zone ophiolites such as the Highland Border and Shetland ophiolites and the Deerpark Complex in western Ireland (Dewey & Mange 1999), and the thrusting of the ophiolitic rocks of the Tyrone Igneous Complex over the Tyrone Central Inlier (Fig. 7c; Hutton et al. 1985). At this time, the Tyrone Central Inlier high-grade metamorphism is envisaged to have commenced. A reversal in subduction polarity is inferred at around 470 Ma in western Ireland (Dewey & Mange 1999); a similar situation is envisaged here to produce a continental arc in the Tyrone Central Inlier (Fig. 7d), with continued high-grade metamorphism and deformation (including leucosome and pegmatite development) in the roots of this deforming arc. The Tyrone Central Inlier-Tyrone Plutonic Group were then intruded by a series of stitching tonalitic-granodioritic plutons at 470-465 Ma (Cooper & Mitchell 2004) and accompanied by the extrusion of arc lavas (Fig. 7d). Final juxtaposition of the inlier with the Laurentian margin occurred during regional SE-directed D^sub 3^ thrusting of the Dalradian units over the Tyrone Central inlier along the Omagh Thrust (Fig. 7e) (Alsop & Hutton 1993).

T. Johnston of the Geological Survey of Northern Ireland is thanked for advice concerning the sampling localities. R. Spikings, C. Ginibre and M. Murphy are thanked for technical assistance with the ^sup 40^Ar-^sup 39^Ar, electron microprobe and isotope dilution thermal ionization mass spectrometry analyses, respectively. A. Wood and V Pashley at NIGL are thanked for technical assistance with mineral separation and LA-MC-ICP-MS work, respectively. S. Noble is thanked for providing access to sample JTP-210. The warm hospitality of 'Wee Davey' and the staff at the Royal hotel, Cookstown, were simply the best. The NordSIMS facility is operated under an agreement between the research councils of Denmark, Norway, and Sweden, the Geological Survey of Finland, and the Swedish Museum of Natural History. This is NordSIMS contribution 201. The careful and insightful reviews by Grahame Oliver, Craig Storey and Associate Editor Peter Cawood are most gratefully acknowledged.