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 Tertiary basalts. They reach a maximum thickness of 30 m and once provided a major source of iron and aluminium ore. An extensive database comprising major, minor, and trace elements has been compiled for 240 samples in order to study the effects of weathering in terms of the changes in whole-rock chemistry and mineralogy from basalt through to iron-rich crust. Percolating waters caused degradation of the parent basalt mineralogy and precipitation of neoformed phases, principally through incongruent dissolution processes. Primary olivine, plagioclase feldspar, and augite were successively broken down and replaced by a mineral assemblage consisting of hematite, gibbsite, goethite, anatase, meta-halloysite and kaolinite. Changes in mineralogy facilitated concomitant changes in element concentrations. Mass balance calculations indicate that all elements were depleted in the iron crust. Enrichment of Al, LOI, Cr, Cu, and V occurred in the laterite horizon, while enrichment of Al, LOI, Ba, Ce, Cr, Cu, Ni, and Rb occurred in the lithomarge. Notably, yttrium was found to be mobile indicating that weathered basalts should not be used in discrimination of original tectonic environments. The severe leaching conditions evidenced by yttrium depletion, local aluminium redistribution, and iron crust formation are indicative of weathering under a humid sub-tropical monsoon climate. (C) 2000 Elsevier Science B.V. All rights reserved.