Clay minerals as alteration products in basaltic volcaniclastic deposits of La Palma (Canary Islands, Spain)

Clay minerals from several volcaniclastic environments including pyroclastic (tuffs), epiclastic (lacustrine, alluvial terraces, marine fan delta) and unconformity-related paleosols in La Palma (Canary Islands) were studied by XRD, SEM, TEM, HRTEM imaging and AEM. Clay minerals and their assemblages allowed us to distinguish between primary volcaniclastic basaltic material produced directly by pyroclastic eruptions and epiclastic volcaniclastic material derived from erosion of pre-existing volcanic rocks. The clay fractions consist mainly of smectite with minor chlorite, mica, chlorite-smectite mixed-layers and talc. Phyllosilicates of the epiclastic units display wider compositional variations owing to wide variations in the mineralogical and chemical composition of the parent material. Most of the phyllosilicates (mica, corrensite, talc and chlorite) are inherited minerals derived from the erosion of the Basement Complex Unit, which had undergone hydrothermal alteration. Smectites of the epiclastic units are saponite and beidellite-montmorillionite derived from the hydrothermal Basement Complex Unit and from volcanic materials altered in the sedimentary environment. Conversely, clay minerals of unconformity-related paleosols are dominated by smectite composed of variable mixtures of saponite and beidellite, which were formed by pedogenetic processes with later hydrothermal influence. The mineralogical association in the pyroclastic unit is dominated by hydrothermally formed smectite (beidellite-montmorillonite), zeolites and calcite. This paper contributes to the differentiation between pyroclastic and epiclastic volcaniclastic rocks of several depositional settings in a basaltic volcanic complex by their clay minerals characterization. (c) 2004 Elsevier B.V. All rights reserved.