Stratigraphic, geochemical and U–Pb zircon constraints from Slieve Gallion, Northern Ireland: a correlation of the Irish Caledonian arcs

Steven P Hollis,Mark R Cooper,Daniel J Condon,Garth Earls,Richard Herrington,Stephen Roberts

doi:10.1144/jgs2012-104

Abstract

Recent Ar–Ar and U–Pb zircon geochronology from across the British and Irish Caledonides has revealed a prolonged period of arc-ophiolite formation ( c . 514–464 Ma) and accretion ( c . 490–470 Ma) to the Laurentian margin during the Grampian orogeny. The Slieve Gallion Inlier of Northern Ireland, an isolated occurrence of the Tyrone Volcanic Group, records the development of a peri-Laurentian island arc–backarc and its obduction to an outboard microcontinental block. Although a previous biostratigraphic age constraint provides a firm correlation of at least part of the volcanic succession to the Ca1 Stage of the Arenig ( c . 475–474 Ma), there is uncertainty on its exact statigraphic position in the Tyrone Volcanic Group. Earliest magmatism is characterized by light rare earth element (LREE) depleted island-arc tholeiite. Overlying deposits are dominated by large ion lithophile and LREE-enriched, hornblende-phyric and feldspathic calc-alkaline basaltic andesites and andesitic tuffs with strongly negative ϵNd t values. Previously published biostratigraphic age constraints, combined with recent U–Pb zircon geochronology and new petrochemical correlations, suggest that the Slieve Gallion Inlier is equivalent to the lower Tyrone Volcanic Group. Temporal and geochemical correlations between the Slieve Gallion Inlier and Charlestown Group of Ireland suggest that they may be part of the same arc system, which was accreted at a late stage ( c . 470 Ma) in the Grampian orogeny. A switch from tholeiitic volcanism to calc-alkaline dominated activity within the Lough Nafooey Group of western Ireland occurred prior to c . 490 Ma, some 15–20 Myr earlier than at Tyrone and Charlestown. Supplementary materials: Sampling and geochemical results (major elements, loss on ignition, trace elements, REE and Nd isotopes) are available at www.geolsoc.org.uk/SUP18640 .

Highlights

Group of Ireland suggest they may be part of the same arc-system which was accreted at a late stage (c. 470 Ma) in the Grampian orogeny
The Caledonian-Appalachian orogen provides a rare window through the mid to lower crustal levels of an evolving orogenic belt
La/YbCN suggest a sudden and significant input of continental crust and/or detritus occurred at this time into the arc system; or the Derryganard Basalts represent an episode of volcanism associated with extensive back-arc rifting, such as the Beaghmore Formation of the lower Tyrone Volcanic Group

Summary

INTRODUCTION

The Caledonian-Appalachian orogen provides a rare window through the mid to lower crustal levels of an evolving orogenic belt. Non-arc type basalts of eMORB affinity are restricted to the SW of the Slieve Gallion Inlier, exposed between Sruhanleanantawey Burn and Letteran (Fig. 3), and around Mobuy Wood. Castle Lavas and Letteran Volcanics by a lack of hornblende phenocrysts It is not known if the Mobuy Wood Basalts are present on the E side of Slieve Gallion as these lavas were not sampled for geochemistry, augite bearing andesites have been reported on GSNI fieldsheets north of Tirgan. The Tawey Formation is restricted to the western side of Slieve Gallion It has been broadly divided into sequences dominated by lava (associated with high total magnetic intensity due to the presence of Fe-oxides; Fig. 3) and those dominated by tuff and sedimentary rocks. Limited CL imaging, of the zircons (including ones dated) indicates they are typical of magmatic zircons (including other samples dated in this study) based upon their external morphology and internal concentric zonation (Fig. 8b)

DISCUSSION

CONCLUSIONS