An Ordovician basalt to peralkaline rhyolite fractionation series from Avoca, Ireland

Abstract

The Upper Ordovician Avoca volcanic rocks host a Kuroko-type massive sulphide deposit in southeast Co. Wicklow, Ireland. Immobile element geochemistry of lavas shows a bimodal subdivision into acid and basic groups. Basalts have characteristics of magmas from both subduction zone and enriched mantle sources, and are interpreted as products of the rifting of a volcanic arc. Rhyolites are highly enriched in HFS-elements and were originally peralkaline. Constant incompatible element ratios throughout the compositional spectrum indicate a fractional crystallization relationship between the basic and acid end members. The basaltic magmas evolved by crystallization of an assemblage in which plagioclase feldspar was an important component, producing large negative Eu anomalies. The transition to peralkalinity is marked by LREE depletion and no further increase in the Eu anomaly, which may represent the interval of minor phase precipitation and fO 2 variation seen in other peralkaline series. A group of acid pyroclastic Avoca volcanic rocks are subalkaline and are probably unrelated to the main evolutionary series. The Avoca volcanic rocks were erupted in an extensional tectonic setting within a continental margin volcanic arc, similar to the contemporaneous Welsh marginal basin.

Distribution and tectonic setting of Ordovician K-bentonites in the United Kingdom

“…In southeast Ireland, Caradoc bimodal (basalt/ rhyolite) volcanism also occurred in an extensional setting (McConnell, Stillman & Hertogen, 1991). Although this transition has been widely interpreted as the closure of Iapetus, some recent interpretations suggest that the Welsh back-arc basin is related to the southwestward subduction of Tornquist Sea oceanic crust (McKerrow, Dewey & Scotese, 1991).…”

Actively forming Kuroko-type volcanic-hosted massive sulfide (VHMS) mineralization at Iheya North, Okinawa Trough, Japan

“…Leat et al, 1986;McConnell et al, 1991;Piercey, 2011). In some instances the period of VHMS mineralization may be associated with a shift in immobile element characteristics of the host lithologies (i.e.…”

Initiation of Iapetus subduction under Irish Avalonia

“…Similar basalts occur in rifted arcs and back-arc spreading centres, where plumes of fertile mantle are added to the metasomatized, depleted mantle wedge (Lin et al 1990;Saunders & Tarney, 1984). Arc rifting has been recognized in the Caradoc of southeastern Ireland (McConnell, Stillman & Hertogen, 1991) but the Dowery basalts are much older and there is no evidence of preceding arc magmatism to which a plume component could have been added. Instead, we envisage that the Dowery basalts were generated in the earliest stages of a volcanic arc, by partial melting of a mantle source that had not been depleted by previous partial melting, but that had been enriched in light rare earth elements (and presumably alkalies and alkali earths) by slab-derived fluid fluxing.…”

“…Phillips, Stillman & Murphy, 1976;Kokelaar et al 1984;Stillman, 1988). However, neither the Ordovician configuration of the three areas nor the relative subduction polarity is well constrained and other scenarios are possible (Kokelaar, 1988;McConnell, Stillman & Hertogen, 1991). Initiation of Iapetus subduction and arc magmatism in Wales is recorded by late Tremadoc tectonism and eruption of the Rhobell Volcanic Complex (Kokelaar, 1986(Kokelaar, , 1988.…”