Ordovician bimodal volcanism in SW Wales: geochemical evidence for petrogenesis of the silicic rocks

Abstract

Rhyolitic lavas, pyroclastic, and high-level intrusive rocks form an important component of a bimodal tholeiitic basalt-rhyolite association of early Ordovician (Llanvirn) age exposed in North Pembrokeshire, on Ramsey Island, in the Fishguard area, and on Mynydd Preseli. In the same region, a 400 m thick tholeiitic layered differentiated sill, the St David's Head intrusion, considered to be co-genetic with this association, comprises a series of units ranging from basic (olivine + orthopyroxene-bearing gabbros) through intermediate (granophyric gabbro) to acidic (aplite) compositions. Consideration of the major and trace element concentrations in these various intrusive units suggests that they are related largely through a crystal fractionation process. The principal fractionating phases were clinopyroxene and plagioclase feldspar, with olivine and orthopyroxene in the more mafic compositions, substantial Fe-Ti oxide and minor apatite in the intermediate compositions, and minor apatite and titanite in the most evolved compositions. A comparison between the composition of a typical aplite from the St David's Head intrusion and compositions of the various silicic rocks from the bimodal suites in North Pembrokeshire suggests these latter rocks have also been derived from a more basic tholeiitic magma via crystal fractionation. Potential parental magmas are the various basaltic lavas and related doleritic intrusions of the Fishguard Volcanic Complex. However, certain silicic lavas and intrusions from the Porth Maen Melyn area appear not to have been derived from a tholeiitic parental magma, but rather have a calc-alkaline type chemistry, similar to older lavas of the Trefgarn Volcanic Group exposed a short distance to the south of Fishguard. These findings support the contention that large volume silicic units in bimodal associations can be derived by crystal fractionation, despite the obvious geochemical discontinuity, and the apparently anomalous volume relationships commonly present.