Isotopic dating of overthrusting, collapse and related granitoid intrusion in the Grampian orogenic belt, northwestern Ireland

Abstract

In the Grampian (Ordovician) orogenic belt in northwestern Ireland the Dalradian Supergroup abuts and interleaves with an enigmatic psammite-dominant unit, with relict latest Precambrian granulite-facies assemblages, known as the Slishwood Division. These two units were most likely tectonically juxtaposed during the D 3 phase of Dalradian deformation. Using Ar-Ar, Rb-Sr and Sm-Nd methods, 53 new mineral ages have been obtained from both rock units, and from pegmatites intruding them, in an attempt to constrain the timing of their juxtaposition and the history they subsequently shared. Prior to D 3 (c. 480 Ma), retrograde hornblende poikiloblasts grew in granulite-facies metabasite pods within the Slishwood Division. Tectonic juxtaposition (D 3 ) with the Dalradian is loosely constrained between 479 Ma and 459 Ma but is likely to have occurred between 470 Ma and 459 Ma. Dalradian peak metamorphic conditions were attained shortly after D 3 , at around 460 Ma. Extensional collapse of the orogen, with rapid uplift and exhumation, is dated by an abundance of mineral cooling ages between 460 Ma and 450 Ma. Orogenic collapse is also dated, more precisely, by widespread pegmatite intrusion into both rock units at about 455 Ma. Initial 87 Sr/ 86 Sr ratios suggest that these pegmatites are partial melts from Dalradian metasediments. A second suite of pegmatites were intruded, along with the Ox Mountains Granodiorite, much later at or around 400 Ma during sinistral shearing. ‡Author for correspondence: michael.flowerdew@ucd.ie

Laser ablation 40 Ar/ 39 Ar dating of metamorphic fabrics in the Caledonides of north Ireland

“…Such a slow cooling rate contrasts with those determined by other workers for areas in Scotland and western Ireland (Dempster 1985;Friedrich et al 1999b;Flowerdew et al 2000) and stratigraphic evidence for rapid unroofing of the Grampian Terrane in adjacent Scotland during this period (Haughton et al 1990;Dewey & Mange 1999;Oliver 2001). Thus, either cooling-erosion rates were much less for this region or these data record a post Grampian Orogeny tectonometamorphic event related either to Avalonia-Laurentia interaction associated with the final closure of the Iapetus Ocean (the Acadian Orogeny) or to subduction of oceanic crust below the Laurentian margin preceding final closure.…”

Laurentian crustal recycling in the Ordovician Grampian Orogeny: Nd isotopic evidence from western Ireland

“…The North Mayo Dalradian sequence includes the late Proterozoic Grampian, Appin and Argyll groups, which consist of, respectively, schists and psammites, quartzites derived from mature sands overlain by interbedded shales and carbonates, and quartzites interspersed with metabasic volcanic rocks (Winchester & Max, 1996). SE-verging nappes within these Dalradian units in North Mayo, the Sperrin Mountains and the Ox Mountains inlier, have been shown to have originated during Grampian deformation, as the Dalradian sequence was overthrust over volcanic units and/or basement probably associated with the colliding Lough Nafooey Arc (Alsop & Hutton, 1993;Flowerdew et al 2000;Chew, 2003).…”

Preservation of terranes during the assembly of the Annieopsquotch Accretionary Tract: Inferences from the provenance of a Middle Ordovician ophiolite to arc transition, central Newfoundland Appalachians1Geological Survey of Canada Contribution 20100463.2This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

“…These cryptic unconformities, marked by Darriwilian conglomerates, are present throughout the AAT (e.g., Thurlow and Swanson 1981;Bostock 1988;Zagorevski et al 2006) but have rarely been investigated; thus, the geodynamic event that produced these unconformities is poorly understood. The unconformities may be related to continued TaconicGrampian convergence between the accreted terranes and the Laurentian margin which is marked by high-grade metamorphism (e.g., Friedrich et al 1999;Flowerdew et al 2000Flowerdew et al , 2005van Staal et al 2007;Chew et al 2008) and exhumation of deep metamorphic rocks in the inboard terranes (e.g., Dewey and Mange 1999). The identification of regional exhumation and erosion in Newfoundland, New England, and the Caledonides indicates that there was extensive and regional Darriwilian reorganization of the composite Laurentian margin.…”

“…The sediments record progressive unroofing of ophiolitic, plutonic and deep metamorphic Dalradian sources synchronous with arc magmatism in the Connemara arc (Dewey and Mange 1999; i.e., second stage of Notre Dame Arc: van Staal et al 2007;McConnell et al 2009). The position of the South Mayo inboard of the Notre Dame Arc and predominantly Dalradian metamorphic sources (e.g., Dewey and Mange 1999;Flowerdew et al 2000;Chew et al 2008;Clift et al 2009;McConnell et al 2009) distinguishes these from the outboard (i.e., part of the AAT) volcanic-derived conglomerate. The age of Connemara arc magmatism (e.g., Friedrich et al 1999) overlaps the provenance age of the conglomerate; however, the mafic and andesitic rocks characterized by high La/Sm ratio (i.e., type 1 and type 2, this study) are absent (Draut and Clift 2001), precluding Connemara Arc and correlatives as a direct source.…”