Abstract

The Silurian–early Devonian Donegal composite batholith of Ireland is a classic composite batholith that intruded Neoproterozoic Dalradian metasedimentary and metavolcanic rocks during and after the Scandian phase (c. 435–425 Ma) of the Caledonian Orogeny. The Scandian phase is widely attributed to the collision of Baltica, Ganderia and Avalonia with Laurentia during closure of the Iapetus Ocean. Isotopic and lithogeochemical data indicate episodic construction of the composite batholith from c. 430–400 Ma. The granitoid rocks are enriched in large-ion lithophile elements, such as Cs, Rb and Ba, and are depleted in high field strength (Nb, Ta, P and Ti) and heavy rare-earth elements. In particular, the chemistry of granitoid rocks from the Donegal composite batholith has Nb/Y >0.4, La/Yb >10, Sm/Yb >2.5 and Gd/Yb >2.0. These characteristics are compatible with a slab-failure origin for the batholith. Following collision, the Iapetus Ocean slab severed by detaching from the trailing continental crust. Upwelling asthenosphere filled the void and the advected heat melted the subcontinental lithospheric mantle and/or the base of the crust to generate the magmas that formed the Donegal composite batholith. The trace-element data indicate melt derivation from processes that left behind a garnet-bearing, plagioclase-absent residue.