The petrochemistry of the basic volcanic rocks of the South Connemara Group (Ordovician), western Ireland

Abstract

Summary16 samples of Ordovician basic volcanic rocks of the South Connemara Group, which abut the southern side of the metamorphic rocks of the Connemara massif in western Ireland, have been analysed for both major and trace elements. Although subject to low grade regional metamorphism and subsequently hornfelsed by the Galway Granite (400 Ma), their immobile element contents do not appear to be significantly disturbed. These elements characterise the metabasites of the South Connemara Group as ocean floor basalts having their origins in a marginal basin. The Skird Rocks Fault, separating the South Connemara Group from high grade metamorphic rocks of the Connemara massif, is consequently regarded as the northern margin of the vestiges of the lapetus Ocean which can be traced into, and along, the Southern Uplands Fault.

Strike-slip terranes and a model for the evolution of the British and Irish Caledonides

“…In westernmost Ireland the South Connemara Group, which occurs as an isolated unit on the north side of Galway Bay (Fig. 2) has been compared with the Northern Belt (Ryan, Max & Kelly, 1983;Ffrench & Williams, 1984). It contains low-grade basic pillow lavas, volcaniclastics, shales and cherts, which have been attributed to a back-arc marginal basin setting (Ffrench & Williams, 1984).…”

“…It contains low-grade basic pillow lavas, volcaniclastics, shales and cherts, which have been attributed to a back-arc marginal basin setting (Ffrench & Williams, 1984). The age of these rocks is not known, but they are widely believed to be Ordovician (Max & Ryan, 1975;Ryan, Max & Kelly, 1983;Ffrench & Williams, 1984). Their contact with the Connemara Terrane is a fault (Skird Rocks Fault; Max & Ryan, 1975) but the contact is intruded to the east by the Galway Granite (Fig.…”

“…The back arc was deformed and weakly metamorphosed post-late Ordovician/pre-early Silurian. In Ireland the South Connemara Group, for which a back-arc origin on both lithological and geochemical grounds has been independently suggested (Ffrench & Williams, 1984;Ryan, Max & Kelly, 1983), is correlated with the Northern Belt to produced a long thin terrane. This seals the outboard contact of the Midland Valley Terrane with the amalgamated Connemara-South Mayo Terrane at or before 398 + 12 Ma (the age of the stitching Galway Granite).…”

Caledonian strike-slip terrane accretion in W. Ireland: insights from very low-grade metamorphism (illite–chlorite crystallinity and b0parameter)

“…This comprises about 3 km of ocean-floor basalts, frequently pillowed, interbedded with and overlain by Lower to Middle Ordovician cherts and continental turbidites (South Connemara Group; Ryan, Max & Kelly, 1983;Ffrench & Williams, 1984;Williams, Armstrong & Harper, 1988) representing a seamount collision with a N-directed subduction zone (Dewey & Ryan, 2004). The deformation age is constrained by the c. 400 Ma Galway Granite (Crowley & Feely, 1997).…”

The South Connemara Group reinterpreted: a subduction-accretion complex in the Caledonides of Galway Bay, western Ireland

“…However, in addition to the enigma of the age of this Group discussed above, the published stratigraphies place a sedimentary thickness of some 1400 m stratigraphically beneath a thick sequence (Gorumna Formation) exhibiting MORB geochemistry (Ryan et al, 1983). It is, of course, possible for basalts to erupt in a trench by ridge subduction and/or at a migrating triple junction involving a ridge and trench as two of the three components.…”