GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 291-1
Presentation Time: 1:30 PM


CHEW, David1, TYRRELL, Shane2, DALY, J. Stephen3, COGNÉ, Nathan4, SUN, Kerryming2 and BADENSZKI, Eszter5, (1)Department of Geology, Trinity College Dublin, Dublin, Dublin 2, Ireland, (2)Earth and Ocean Sciences (EOS), National University of Ireland Galway, Galway, H91 TK33, Ireland, (3)UCD School of Earth Sciences, University College Dublin, Dublin 4, Ireland, (4)Géosciences Rennes UMR 6118, Université Rennes 1, Rennes, 35042, France, (5)UCD School of Earth Sciences, University College Dublin, D04 N2E5, Ireland

The Porcupine High, 200 km W of Ireland, is a key underexplored link between the circum-Iapetus basement geology of NW Europe and Newfoundland. We present geochronological data from sampling campaigns (dredging, shallow drill cores) in the last decade. The dredge samples are likely near in-situ; outcrop ridges were targeted by side-scan sonar while the predominance of metamorphic clasts recovered is inconsistent with IRD derived from the Irish massif or nearby. All samples were collected N of the inferred trace of the Iapetus suture.

The southernmost sample is a grey-banded gneiss boulder which yields a 0.49 Ga Ma U-Pb zircon crystallization age with inherited zircons at 1.1-1.25 and 1.55 Ga. A 0.39 Ga apatite U-Pb age records post-Caledonian exhumation; K-feldspar Pb isotope data (Pb-Kfsp) imply a Caledonian affinity.

To the north dredging recovered >90% lower greenschist-facies psammite cobbles. U-Pb detrital zircon data imply a Moine or Grampian Group Dalradian affinity with the youngest population at 0.95 Ga. U-Pb detrital apatite ages (0.9-1.8 Ga) were unaffected by Caledonian orogenesis which is atypical for Tonian successions in the Caledonides.

Farther north, drilled granitic orthogneiss yields a 1.31 Ga U-Pb zircon crystallization age. Granulite-facies metamorphism (12 kbar, 740°C) is of likely Grenville age, supported by a 0.88 Ga apatite U-Pb cooling age. Zircon Hf isotopic and Pb-Kfsp data suggest an affinity with the Rockall Bank Palaeoproterozoic, delimiting the offshore extent of the Annagh Gneiss Complex (AGC) of NW Ireland.

Northernmost dredging recovered >98% metamorphic clasts. U-Pb detrital zircon ages from two low-grade metasediments imply a Moine/Grampian Group and an Upper Dalradian affinity respectively; Caledonian apatite U-Pb ages are prevalent. The high-grade rocks (orthogneisses and amphibolites) can be broken into three groups: clasts with 2.75 Ga U-Pb zircon crystallization ages, 1.75 Ga apatite U-Pb ages and a Lewisian Pb-Kfsp signature; clasts with 1.83-1.94 Ga U-Pb zircon ages, 1.75 Ga apatite U-Pb ages and an AGC Pb-Kfsp affinity, and AGC clasts with 1.55-2.02 Ga U-Pb zircon ages and U-Pb apatite ages recording post-Grenville cooling. All three groups were found at one dredge site. These new results will discussed in the context of paleogeographic models for the N Atlantic.