Compositional variability of regoliths on equatorial highlands (East Timor). Source-rock control and competing effects of weathering and denudation

Abstract

• Non-mobile elements discriminate Si-rich and Si-poor regoliths formed on distinct rock units. • Si-poor suites tend to be more affected by weathering than Si-rich suites. • Weathering extent better assessed by MIA and α Al Mg in Si-poor regoliths. • Minor alteration and source rock variability compromise weathering assessments in Si-rich suites. • Chemical weathering is strongly limited by high denudation rates. The intensity of chemical decomposition increases with rainfall and temperature, being extreme under equatorial climate, but in tectonically active regions it can be limited by the rapid erosion of surface material when weathering reactions are still incomplete This paper presents mineralogical and geochemical data from weathering mantles exposed on the equatorial mountainous region of Maubisse (East Timor). Two major groups of regoliths, with contrasting Si-content and associated with different parent rocks, are discriminated by non-mobile elements contents. Although partially inherited from parent rock, the geochemistry (e.g., the values obtained for multi-element weathering parameters, such as CIA, CIX, MIAo, WIP, and α-indices of element mobility) and diverse clay assemblages with low kaolinite contents are in general indicative of minor weathering intensities despite wet (>2000 mm of annual rainfall) and warm (annual average temperature ~20 °C) conditions. These compositional data show that weathering reactions are not keeping up with physical denudation in this tectonically active region. Weathering intensity depends on local orographic and lithologic features, being highest on basaltic terranes due to the enrichment in weatherable silicates relative to sedimentary successions. The present research demonstrates the extent to which different factors can control regolith composition in mountainous equatorial regions and how the application of multiple compositional parameters can help to untangle the roles played by those factors.