Geological Magazine

Original Article

Early Neoproterozoic limestones from the Gwna Group, Anglesey

JANA M. HORÁKa1 c1 and JANE A. EVANSa2

a1 Department of Geology, National Museum of Wales, Cardiff CF10 3NP, Wales, UK

a2 NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

Abstract

Limestone megaclasts up to hundreds of metres in size are present within the Gwna Group mélange, North Wales, UK. The mélange has been interpreted as part of a Peri-Gondwanan fore-arc accretionary complex although the age of deposition remains contentious, proposals ranging from Neoproterozoic to Early Ordovician. This paper uses strontium isotope chemostratigraphy to establish the age of the limestone blocks and thus provide a maximum age constraint on mélange formation. Results show that, although the carbonates are locally dolomitized, primary 87Sr/86Sr ratios can be identified and indicate deposition sometime between the late Tonian and earliest Cryogenian. This age is older than that suggested by stromatolites within the limestone and indicates that the limestone did not form as cap carbonate deposits.

(Received March 04 2009)

(Revised March 25 2010)

(Accepted March 26 2010)

(Online publication June 16 2010)

Keywords:

  • Neoproterozoic;
  • limestone;
  • Anglesey;
  • chemostratigraphy;
  • mélange

Correspondence:

c1 Author for correspondence: Jana.horak@museumwales.ac.uk

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