PETROLOGY OF THE ARDARA APPINITES, COUNTY DONEGAL, NW, IRELAND

In northwest Donegal, Ireland, a large number of appinitic (hornblende/ plagioclase-rich) and lamprophyric intrusions occur around the ca. 405 Ma Ardara granitic pluton. They form part of a suite of coeval, plutonic and hypabyssal rocks, ranging in composition from ultramafic to felsic, in which hornblende is the dominant mafic mineral and typically occurs as large prismatic phenocrysts within a finer grained matrix. Geochemical analyses show the mafic rocks to contain high abundances of major elements including MgO, FeO and the trace elements such as Ni, indicating a strong mantle influence in the origin of the appinitic and lamprophyric magma. 40Ar/39Ar thermochronology performed on hornblende separated from two samples of appinite yield mid-Silurian (432.0 1.2 Ma) and (434.7 1.4 Ma) cooling ages that are interpreted to closely date the time of their respective intrusion. Sm-Nd isotopic analyses, in concert with the age data, yield positive and negative (+3.1 to -4.8) initial Nd values (t = 435 Ma), indicating that they were derived from a LREE depleted mantle reservoir later modified by either mixing with less-depleted mantle or with recycled LREE enriched, negative Nd crust. This suggests an origin through melting of the sub-continental lithospheric mantle and/or metasomatized underplated mafic crust, with only limited crustal contamination during magma ascent. The compositional range of the appinitic intrusions is interpreted to reflect their emplacement along deep-seated crustal fractures that allowed for coeval mafic and Ardara granitic magma to mix and mingle.