DANNEMORITE-BEARING COTICULES FROM THE OSSA-MORENA CENTRAL BELT (SOUTHERN IBERIAN MASSIF, SW SPAIN)

Two types of coticules are distinguished in an Upper Cambrian-Lower Or dovician, greenschist facies volcano-sedimentary sequence from the Sou th-Central belt of the Ossa-Morena Zone (Iberian Massif, SW Spain). Ty pe I coticules consist of very fine-grained spessartine with abundant inclusions of quartz and hematite. In the type 2 coticule garnetite ba nds alternate with bands consisting of spessartine, dannemorite, magne tite and quartz. Garnets from these coticules contain numerous inclusi ons of quartz, and even Mn-carbonate. Most of Mn and Fe appear in biva lent state in the whole rock composition and their high Al and Ti cont ents indicate an intermediate composition between original Mn-deposits , represented in the study area by braunite nodules of hydrogenous ori gin, and rocks of sedimentary and volcano-sedimentary origin. REE conc entration levels of the coticules are lower than those of the braunite nodules and do not have a positive Ce anomaly. The correlation of As, Fe and Mn suggests that part of the original hydrogenous Mn-deposits was leached by reduced As-rich fluids to produce Mn-carbonate precipit ation in carbonate-bearing protoliths. During the Hercynian metamorphi sm, important amounts of spessartine crystallized in protoliths charac terized by higher Al/Si ratios, resulting in the formation of coticule s. Whole-rock composition controls the compositional features of amphi boles and garnets of the studied coticules. Ca/(Ca+Mn) and Al/Fe ratio s of garnets correspond with the whole-rock ratios in comparison with other lithologies of the volcano-sedimentary sequence, whereas the Mn content in amphiboles seems to be controlled by the whole-rock Mn/Fe r atio. The presence of dispersed magnetite in the type 2 coticules indi cates fO(2) conditions below the hematite-magnetite buffer. Under such conditions tirodite-dannemorite series amphiboles and garnet with hig h almandine end-member content crystallized.