THE BASEMENT GEOLOGY OF THE PORCUPINE HIGH – A KEY TRANSATLANTIC LINK BETWEEN THE CALEDONIDES AND APPALACHIANS (Invited Presentation)

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.