MN-FE SPINELS AND SILICATES IN MANGANESE-RICH ROCKS FROM THE OSSA-MORENA ZONE, SOUTHERN IBERIAN MASSIF, SOUTHWESTERN SPAIN

Manganese-rich rocks occur in a greenschist-facies volcano-sedimentary complex of the Ossa-Morena Zone, in the Iberian Massif, southwestern Spain. Four unusually Fe-rich manganese associations containing Mn-Fe spinels and silicates have been identified: I) magnetite + pyroxmangit e + spessartine + quartz, II) manganian jacobsite + pyroxmangite + aeg irine + quartz, III) jacobsite + pyroxmangite + tephroite + spessartin e + rhodochrosite, and IV) manganoan magnetite + rhodonite + ferroan t ephroite + spessartine + rhodochrosite. Major compositional variations of these minerals are complex functions of several factors. Oxygen fu gacity determines the Fe content of pyroxenoids in such a way that it is very low in the pyroxmangite of associations bearing Mn3+ spinels ( <0.4% FeSiO3) and reaches 30% FeSiO3 in associations with Fe2+-dominan t spinels. The presence of tephroite indicates X(CO2) values lower tha n 0.2, a condition that evolved toward higher values, as indicated by poikiloblasts of rhodonite. The most important effect of the whole-roc k composition is the crystallization of tephroite in rocks with a Mn:S i ratio higher than 1.7. Local Ca availability determines the crystall ization of rhodonite or pyroxmangite. In rocks with a low Mn:Fe ratio, the formation of tephroite is favored only when the accompanying pyro xenoid is rhodonite. In addition, in the absence of tephroite, the pyr oxmangite is Fe-enriched (30% FeSiO3), but in the absence of pyroxmang ite, the tephroite may contain approximately 20% Fe2SiO4. The iron con tent of the garnet is controlled by coexisting minerals that preferent ially partition Fe.