MINERALOGY AND GEOCHEMISTRY OF REDUCED MANGANESE CARBONATE SILICATE ROCKS FROM THE ARACENA AREA (IBERIAN MASSIF, SW SPAIN)

Reduced and oxidized metamorphosed, manganese-rich rocks occur within a volcano-sedimentary sequence in the Aracena area (Southern Iberian M assif, SW Spain). The reduced facies of carbonate lithologies are char acterized by the presence of spessartine, rhodonite, Ca-tiroctite, cal cite, hausmannite, tephroite, and Mn-phlogopite. The silicate-rock ass emblages are composed of spessartine, Mn-actinolite, and rhodonite. Th e crystallization of several of these phases is regulated by the bulk composition of the rocks. Carbonate crystallization is controlled by X (CO2), rhodonites and tephroites by X(Mn), and amphiboles by X(Fe). Th e presence of carbonates and amphiboles determines the cation distribu tion among the mineral phases. Ca fractionation by calcite causes a sc arcity of this element in the paragenetic phases (amphibole and garnet ) while amphibole has the same effect on the Fe. The bulk composition of the manganese-rich reduced rocks studied is characterized by high b ut variable amounts of Mn, Ba and Mn/Fe, a high Al, Ti, Zr, As, Sb, an d Zn content and a low Cu, Ni, Co, V, and REE content. The relationshi ps between these elements allow them to be classified into three group s with different origins. Mn, Zn, LREE, As, and Sb have a hydrothermal origin; Si, Zr, Al, Ti, and HREE have a detrital origin, and CO2. and Ca are biogenic. These associations suggest periods of carbonate-detr ital sedimentation accompanied by pulses of hydrothermal activity. The mineral assemblages indicate metamorphic conditions corresponding to temperatures of 450-degrees-C - 500-degrees-C, a pressure below 4 kb, and fO2 lower than 10(-11) bars.