AGE AND PROVENANCE OF THE NORTHWEST IRELAND APPINITES

In northwest Donegal, Ireland, a large number of appinitic (hornblende/intermediate plagioclase-rich) intrusions occur around the ca. 405 Ma Ardara granitic pluton. They form a group of coeval, plutonic and hypabyssal rocks, ranging in composition from ultramafic to felsic, in which hornblende is the dominant mafic mineral and typically occurs both as large prismatic phenocrysts and in the finer grained matrix. Chemical analyses show the mafic rocks to contain high abundances of Mg, Ni, Si and Fe, indicating a strong mantle influence on the mafic components of the appinitic magma. ⁴⁰Ar/³⁹Ar thermochronology performed on hornblende separated from 4 samples yield early Devonian cooling ages that are interpreted to closely date igneous emplacement. Sm-Nd isotopic analysis, in concert with the age data, yield quite strongly positive initial eNd values, indicating an important juvenile component in the source and a likely origin through melting of the sub-continental lithospheric mantle and/or metasomatized underplated mafic crust, with only limited fractionation 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 felsic magma to mix and mingle.