Petrogenesis of the Limerick Igneous Suite: insights into the causes of post-eruptive alteration and the magmatic sources underlying the Iapetus Suture in SW Ireland

Abstract

The Limerick Igneous Suite (LIS) in County Limerick, SW Ireland, contains two distinct basaltic igneous units: the Knockroe and Knockseefin, which are expressed as hypabyssal intrusions, porphyritic dykes, diatremes, lava flows, agglomerates and tuffs. These units make up two distinct evolutionary trends: the Knockroe igneous units which range from alkaline basalts to trachyandesites, and the Knockseefin igneous units which range from alkaline basalts to basanites. U‒Pb dating of apatite establishes a primary crystallization age of c. 350 Ma for the Knockroe units. Strontium and Nd isotopes from the least-altered Knockroe bulk rock samples range over 0.70301–0.70454 and 0.512457–0.512493, respectively. The Sr isotopes for the least-altered Knockseefin samples are similar, ranging over 0.70325–0.70386, but the Nd values are slightly more radiogenic, spanning 0.512431–0.512437. Altered samples are buffered against changes in Nd, but some show excursions towards radiogenic Sr, suggesting contamination from Carboniferous seawater and/or introduction of Rb. Rb–Sr calculations conducted on altered Knockroe samples returned ages within uncertainty of the U‒Pb dates. The bulk rock isotope values, normalized trace element plots as well as Zr/Sm, Ce/Pb and Nb/U values demonstrate the LIS is comparable to ocean island basalts and Ce/Y and Zr/Nb values suggest the units likely originated from low degrees of partial melting likely caused by extension related to the amalgamation of Laurussia and Gondwana.