Geochemistry of accreted metavolcanic rocks from the Neoproterozoic Gwna Group of Anglesey-Lleyn, NW Wales, U.K. MORB and OIB in the Iapetus Ocean

Takuya Saito, Masaoki Uno, Tomohiko Sato, Wataru Fujisaki, Satoru Haraguchi, Yi bing Li, Yusuke Sawaki, Shinji Yamamoto, Shigenori Maruyama

    Research output: Contribution to journalArticlepeer-review

    6 Citations (Scopus)

    Abstract

    The Gwna Group in Anglesey-Lleyn, NW Wales, UK, is a Neoproterozoic accretionary complex that consists of basalt, bedded chert, red claystone, and trench turbidite that have been intercalated in coherent and incoherent mélanges that are considered typical Ocean plate stratigraphy (OPS). The sediments in the OPS can be useful for constraining the geological environment in the Iapetus Ocean. Most basalts in this area have undergone hydrothermal alteration, greenschist facies regional metamorphism, and surface oxidation. This indicates that immobile elements such as Al2O3 and TiO2, Rare Earth Elements (REE) and High Field Strength Elements (HSFE) are appropriate for discriminating the origin of the basalts in the Gwna Group. Most basalts showing light REE-enriched pattern in CI chondrite-normalized spider diagrams in within-plate basalt (WPB) fields, and some have flat patterns in spider diagrams in mid-oceanic ridge basalt (MORB) fields. In view of these relations, we conclude that the former erupted in an oceanic island. Oceanic island basalts (OIB) are common in Phanerozoic accretionary complexes, and this study presents the first evidence of OIB in a Neoproterozoic accretionary complex of the Gwna Group in Anglesey-Llyen and Llyen area. The OIB-like basalts are locally capped by red hematite-rich claystones. This indicates that a fully oxic pelagic condition was present around the oceanic island in the Iapetus Ocean in the Neoproterozoic, which is consistent with the redox condition estimated from contemporaneous shallow marine sediments. On the other hand, the presence of black mudstones on top of MORB-like meta-basalts suggests that deep-sea anoxia conditions were prevalent during the end-Proterozoic.

    Original languageEnglish
    Pages (from-to)243-255
    Number of pages13
    JournalTectonophysics
    Volume662
    DOIs
    Publication statusPublished - Dec 25 2014

    Keywords

    • Anglesey-Lleyn region
    • Gwna Group
    • LA-ICP-MS
    • Meta-basalt
    • Neoproterozoic

    ASJC Scopus subject areas

    • Geophysics
    • Earth-Surface Processes

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