Hybridization of Basic Wall Rocks in Xenolithic Igneous Complexes

Abstract

The igneous complexes at Glen Dubh (Isle of Arran), Bolton Hill (Pembrokeshire) and Garabal Hill (Argyllshire) show a variety of basic rock types intruded by later granitic rocks. In each case the basic rocks show evidence of being assimilated by granite producing a range of dark, mottled, heterogenous hybrid rocks of intermediate composition. Basic rocks show complex veining by acidic material whilst granites contain xenoliths of the basic wallrocks to varying extents before the reaction with the granite took place. Xenoliths occur in all stages of breakdown, with sharp or diffuse outlines, to xenocrysts orxenocrystic clumps. In places complete equilibrium may produce apparently "normal" intermediate rocks with no evidence of hybridization. Metasomatism has resulted in the transfer of certain constituents to the wallrocks, changing their chemistry, with the loss or localized distribution of existing constituents and causing zoning and replacement of pre-existing minerals. Geochemical analysis of the rock series show that the intermediate rocks form along generally linear compositional trends on variation diagrams between basic and acidic endmembers for most elements. Hybridization is also demonstrated by the use of statistics and mixing calculations.