Controls on kaolinite and dickite distribution, Highland Boundary Fault Zone, Scotland and Northern Ireland

Kaolinite and dickite occur widely in central Scotland and Northern Ireland . Stable hydrogen and oxygen isotope compositions of both minerals are simi lar, suggesting that the formation of kaolinite occurred first at temperatu res of <50 degrees C from meteoric water, probably as a result of alteratio n of Lower Carboniferous volcanic rocks, and that dickitization followed lo cally as a result of local heating that accompanied the intrusion of dykes during Late Carboniferous-Permian times. This mechanism of dickite formatio n explains why the higher-temperature polytype dickite occurs in a region f rom the Firth of Clyde to Perthshire, in association with dyke swarms, whil st kaolinite occurs elsewhere. The original kaolinite precipitation was foc used along the high permeability fault zone now marked by the Highland Boun dary Fault Zone and its presumed trace in Northern Ireland.