Evolution of the Tyrone arc system during the Grampian (=Taconic) orogeny: field, geochemical, geochronological and isotope (Nd, Sr) constraints
Steven Philip Hollis1,2, Mark Cooper
3, Stephen J Roberts
2, Richard Herrington
4, Garth Earls
5, Daniel James Condon
6 and J Stephen Daly
7, (1)CSIRO, Perth, Australia, (2)University of Southampton, Southampton, United Kingdom, (3)Geological Survey of Northern Ireland, Belfast, United Kingdom, (4)Natural History Museum, London, London, United Kingdom, (5)University College Cork, Cork, Ireland, (6)NERC Isotope Geosciences Laboratory, Keyworth, United Kingdom, (7)University College Dublin, School of Earth Sciences, Dublin, Ireland
Contact First Author: Steven Philip Hollis; steven.hollis@csiro.au
Previously Published Material: This presentation details our current understanding of the Tyrone Igneous Complex and its role within the Grampian orogen based on recent PhD work by the lead author. The U-Pb zircon age constraints and geochemical data have been published in (Cooper et al. 2008, 2011 - J. Geol Soc, London; Hollis et al. 2012 - GSA Bulletin; Hollis et al. 2013ab - J. Geol Soc, London; Hollis et al. 2014 - Mineralium Deposita). Unpublished data includes new field constraints, whole rock geochemistry, and isotope (Nd, Sr) analyses. Several U-Pb zircon ages are in preparation from key localities across the complex. This work integrates ~350 whole rock geochemical analyses, 19 U-Pb zircon ages (+preliminary, if available), 26 Sr isotope and 55 Nd isotope constraints.
English Abstract:
The Tyrone Igneous Complex of Northern Ireland records the evolution of a relatively short-lived arc system and its accretion to the Laurentian margin during the Grampian orogeny. The ca. 484-479 Ma Tyrone Plutonic Group forms the structurally lowest levels of the complex and preserves a tectonically dissected suprasubduction ophiolite (ɛNdt +4.4 to +7.7; 87Sr/86Sri 0.7039-0.7106), obducted onto an outboard fragment of micro-continental crust (the Tyrone Central Inlier) prior to ca. 470 Ma. The structurally overlying ca. 475-469 Ma arc-related Tyrone Volcanic Group includes mafic to intermediate lavas, tuffs, rhyolite, banded chert, ironstone and argillaceous sedimentary rocks. Geochemical and isotopic signatures are consistent with its formation in an evolving peri-Laurentian island arc/backarc (ɛNdt -4.1 to -13.8) that was contaminated by continental crust and underwent several episodes of extension. Rift-related magmatism is characterized by the presence of (i) Fe-Ti rich eMORB (ɛNdt +0.6 to +5.9), island-arc tholeiitic basalt (ɛNdt +4.8) and FIV-affinity rhyolite breccias in the ca. 474-475 Ma lower Tyrone Volcanic Group; and (ii) OIB (ɛNdt +1.3), alkali basalt (ɛNdt +2.4) and FIII-affinity rhyolite in the ca. 473-469 Ma upper Tyrone Volcanic Group. Whereas calc-alkaline lavas and tuffs of the Tyrone Volcanic Group have strongly radiogenic 87Sr/86Sri values (0.7086-0.7212) between Middle Ordovician seawater and Laurentian continental crust, rift-related basalts display lower ratios (0.7055-0.7110) back towards primitive mantle. All units of the Tyrone Igneous Complex are stitched above the Tyrone Central Inlier by a suite of syn- to post-collisional I-type and arc-related intrusive rocks (ca. 470-464 Ma) which contain inherited Proterozoic zircons and have strongly contaminated isotopic signatures (ɛNdt -7.1 to -12.2; 87Sr/86Sri 0.7120-0.7196). Potential correlations across the British and Irish Caledonides will be discussed.
