Geochemical evolution of a palaeolaterite: the Interbasaltic Formation, Northern Ireland

The Interbasaltic laterites of Northern Ireland were formed during a period of relative volcanic inactivity by extensive chemical weathering of Tertia ry basalts. They reach a maximum thickness of 30 m and once provided a majo r source of iron and aluminium ore. An extensive database comprising major, minor, and trace elements has been compiled for 240 samples in order to st udy the effects of weathering in terms of the changes in whole-rock chemist ry and mineralogy from basalt through to iron-rich crust. Percolating water s caused degradation of the parent basalt mineralogy and precipitation of n eoformed phases, principally through incongruent dissolution processes. Pri mary olivine, plagioclase feldspar, and augite were successively broken dow n and replaced by a mineral assemblage consisting of hematite, gibbsite, go ethite, anatase, meta-halloysite and kaolinite. Changes in mineralogy facil itated concomitant changes in element concentrations. Mass balance calculat ions indicate that all elements were depleted in the iron crust. Enrichment of Al, LOI, Cr, Cu, and V occurred in the laterite horizon, while enrichme nt of Al, LOI, Ba, Ce, Cr, Cu, Ni, and Rb occurred in the lithomarge. Notab ly, yttrium was found to be mobile indicating that weathered basalts should not be used in discrimination of original tectonic environments. The sever e leaching conditions evidenced by yttrium depletion, local aluminium redis tribution, and iron crust formation are indicative of weathering under a hu mid sub-tropical monsoon climate. (C) 2000 Elsevier Science B.V. All rights reserved.