A U–Pb age for the Late Caledonian Sperrin Mountains minor intrusions suite in the north of Ireland: timing of slab break-off in the Grampian terrane and the significance of deep-seated, crustal lineaments

Abstract

An intrusion of trachy-andesite, representative of a newly discovered suite of high-K-Ba-Sr, calcalkaline minor intrusions (termed herein the Sperrin Mountains suite), hosted within the Grampian terrane in the north of Ireland, has been dated by U-Pb zircon at 426.69 ± 0.85 Ma (mid-Silurian; wenlock-Ludlow boundary). Geochemistry reveals a close association with the Fanad, Ardara and Thorr plutons of the Donegal Batholith and the Argyll and Northern Highlands Suite of Scotland. The deep-seated Omagh Lineament appears to have limited eastward propagation of the Sperrin Mountains suite from beneath the main centre of granitic magmatism in Donegal. A Hf depleted mantle model age (T DM Hf) of c. 800 Ma for trachy-andesite zircons indicates partial melting from a source previously separated from the mantle. whole-rock geochemistry of the suite is consistent with a model of partial melting, triggered by slab break-off, following thrusting of Ganderia-Avalonia under the Southern Uplands-Down-Longford accretionary prism (i.e. Laurentian margin). The new age constrains the timing of this event in the north of Ireland and is consistent with the petrogenesis of Late Caledonian high-K granites, appinites and minor intrusions across the Caledonides of northern Britain and Ireland.

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…The intersection between the Newry lineament and the Gala 7 + 2/7 + 3 boundary is also located close to the center of the Paleogene Slieve Gullion complex ( Fig. 9; Cooper et al, 2013), suggesting that the magma supply for this intrusion may have been controlled by the same crustal structures as parts of the Newry ig neous complex.…”

“…The siting of the Newry igneous complex on two deepcrustal lineaments, termed the Argyll and Newry lineaments, has thus been consid ered as a potential influence on its emplacement (Cooper et al, 2013). In this study, we inves tigated this possibility further, developing a model whereby deepcrustal lineaments interact with known uppercrustal structures.…”

“…Several deepseated crustal lineaments are thought to have influenced ascent and emplace ment of magma in the UK (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Stevenson et al, 2006;Cooper et al, 2013). These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture.…”

“…These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture. The origin of the lin eaments is unclear, although they are proposed to represent preCaledonian faults (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Cooper et al, 2013).…”

“…Since the Argyll and Newry lineaments rep resent deepseated structures (Cooper et al, 2013), it is proposed that these faults facili tated the ascent of magma from depth (Fig. 10).…”