Timescales of magmatism and metamorphism in the Connemara Caledonides: insights from the thermal aureole of the Dawros–Currywongaun–Doughruagh Complex, western Ireland

Brian O’Driscoll,David M Chew

doi:10.1017/s0016756821000649

Abstract

AbstractZircon separates from the contact aureole of the syn-tectonic Dawros–Currywongaun–Doughruagh Complex, western Ireland, are studied to constrain the nature and timing of magmatism associated with the early stages of the Grampian Orogeny. The samples analysed come from the uppermost part of the Dalradian Supergroup in northern Connemara (the Ben Levy Grit Formation), where a laterally extensive (>10 km) package of metamorphosed siliciclastic sedimentary rocks containing heavy mineral seams crops out. The seams mainly comprise magnetite, but zircon is also present in greater than accessory quantities. The seams have been locally reworked at granulite-facies metamorphic conditions during intrusion of the Dawros–Currywongaun–Doughruagh Complex magmas. Here we combine in situ mineral chemical and U–Pb geochronological analyses of zircons from samples of these heavy mineral seams collected at different locations in the Dawros–Currywongaun–Doughruagh Complex thermal aureole. An important finding is that the zircons studied have magmatic trace-element compositions, interpreted here as a function of their growth during contact metamorphic-induced partial melting. The zircons yield a range of U–Pb spot ages whose uncertainties suggest a maximum duration of zircon growth of ˜11 Ma, between 477.1 and 466.1 Ma, though it is likely that zircon growth occurred much more quickly than this. The age constraints revealed here match well with the range of 475 to 463 Ma previously proposed for the Grampian Orogeny overall in Connemara and lend useful support to models that argue for high-intensity, relatively short-lived Grampian orogenesis in the Connemara Caledonides.

Highlights

It has been suggested that arc–continent collision leads to relatively short timescale orogenic events (Dewey, 2005) compared to other styles of mountain building events
The Grampian orogen of the British and Irish Caledonides formed during the early stages of Iapetus Ocean closure and extends from Connemara, through NW Ireland and mainland Scotland to Shetland (Lambert & McKerrow, 1976; Dewey & Shackleton, 1984; Chew & Strachan, 2014)
We report new U–Pb ages from zircons in the thermal aureole of the Dawros–Currywongaun– Doughruagh Complex (DCDC)

Summary

Introduction

It has been suggested that arc–continent collision leads to relatively short timescale orogenic events (Dewey, 2005) compared to other styles of mountain building events (e.g. continent– continent collision or Andean-style ocean–continent plate collision). The Grampian orogen of the British and Irish Caledonides formed during the early stages of Iapetus Ocean closure and extends from Connemara (western Ireland), through NW Ireland and mainland Scotland to Shetland (Lambert & McKerrow, 1976; Dewey & Shackleton, 1984; Chew & Strachan, 2014) It is one of the most intensively studied arc–continent collision systems in the world, and a growing body of evidence in recent decades has pointed to a relatively short duration for deformation (Wellings, 1998), Barrovian-style metamorphism (Yardley et al 1987; Friedrich et al 1999b), magmatism (Friedrich et al 1999a,b) and exhumation (Friedrich et al 1999b) associated with orogenesis.

B O’Driscoll and DM Chew

Geological setting

Sample descriptions

Analytical techniques

Results

Discussion and conclusions