a b s t r a c t a r t i c l e i n f oIn the Campo de Calatrava Volcanic Field (CCVF, Central Spain), the eruption of Pliocene-Pleistocene maar craters into two clearly distinct types of pre-volcanic rocks allows the observation and comparison of hardsubstrate and soft-substrate maar lakes. Hard-substrate maars formed when phreatomagmatic processes affected the jointed, Paleozoic igneous and metamorphic rocks (hard substrate), giving rise to funnel-like maar lake basins. Soft-substrate maars resulted from phreatomagmatic volcanic processes affecting poorlyconsolidated Pliocene sediments, forming bowl-like maar lake basins. Pre-volcanic bedrock determined the post-eruptive lacustrine architecture in the craters and favored a higher preservation of hard-substrate maars in comparison to soft-substrate maars. This is because the hard-substrate maars, surrounded by a deep stable crater wall, are more capable of collecting sediments in their basins. These sediments could be preserved for longer than similar deposits in broad, shallow maars with a soft substrate. Ancient soft-substrate maars do not usually preserve their original morphology well and can be identified only by their lacustrine deposits. Carbonate lacustrine/palustrine deposits surrounding a bowl-like depression are the remnants of this second type of maar lake, and allow reconstruction of the original morphology of ancient soft-substrate maar craters. Geophysical (electrical tomography ground surveys) and geomorphologic-geologic mapping techniques were combined with fieldwork and facies analysis in order to locate and accurately characterize the PliocenePleistocene soft-substrate maar volcanic structures of the CCVF.
Clay minerals from several volcaniclastic environments including pyroclastic (tuffs), epiclastic (Iacustrine, alluvial terraces, marine fan delta) and unconfonnity-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--montrnorillonite 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.
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