FACTORS CONTROLLING THE GENESIS OF FE-MN CRUSTS IN STRATIGRAPHIC BREAKS OF THE EASTERN BETIC CORDILLERA (SE SPAIN) DEDUCED FROM NUMERICAL-ANALYSIS OF GEOLOGICAL DATA

Fe-Mn crusts occur in four stratigraphic breaks of Jurassic pelagic sw ell sequences in the eastern Betic Cordillera. Crusts were characteriz ed by quantitative and qualitative variables deduced from their strati graphic position (under-and overlying lithologies), texture (thin or b anded), mineralogical and chemical composition. Quantitative chemical variables were summarized in two new variables using principal compone nt analysis. Component A groups detrital elements, and Fe with chalcop hile elements, which suggests a seawater enrichment by detrital and hy drothermal contributions. Component B associates elements of hydrogeno us precipitation and adsorption during exposure of Fe-oxide to the sea water. Relations between quantitative and qualitative variables were e stablished by the use of correspondence analysis. Variables form two g roups: TX1 (variables representing thin crusts, type I) and TX2 (bande d crusts, type II). Thin crusts are related with chemical variables de noting high detrital element contents (Al), low Mn, REE, and transitio n metal element contents (B3), presence of detrital minerals (DET) and Mn-Co-poor goethite (GT). Banded crusts are associated with variables representing low detrital element contents (A2, A3), high Mn, REE, an d transition metal element contents (B1, B2), presence of Mn-Co-rich g oethite (MNG), Mn-oxyhydroxides (MNO), and REE-phosphates (PH). These associations indicate that, in the type II crusts, adsorption processe s have a greater importance than in the type I crusts. Correspondence analysis helps to clarify the geological variables determining the ext ent of the adsorption processes, since variables related with textural type I are associated to lithological variables (OCL: overlying chert y limestones; UML3: underlying thick rhythmite of marls and limestones ) indicating a geological context of pelagic swell slightly deeper tha n those associated with the textural type II (UCC: underlying crinoida l calcarenite; UML1: thin underlying marl-limestone rhythmite).