COMPOSITIONAL VARIATION OF PIEMONTITES FROM DIFFERENT MN-RICH ROCK-TYPES OF THE IBERIAN MASSIF (SW SPAIN)

Four types of piemontite-bearing Mn-rich assemblage are studied from t he Ossa-Morena Zone (Iberian Massif, SW Spain): silicate rocks contain ing braunite (SRB), silicate rocks containing hematite (SRH), carbonat e rocks containing braunite nodules (CRB) and carbonate rocks containi ng thin hematite beds (CRH). These rock-types are found in greenschist facies volcano-sedimentary units. High oxygen fugacity is crucial in the stability of piemontite in the studied assemblages. Variable piemo ntite end-member contents in the CRB and CRH rocks result from differe nces in the fO2; the molar proportion of piemontite in the CRB piemont ites (ca. Pm17) indicates higher fO2 conditions than those of CRH mang anian epidotes (Pm6Ps24). The Fe/Ca ratio varies as a function of the fO2, ranging from values of about 0.2 in many CRB piemontites to value s higher than 0.4 in the CRH manganian epidotes. In the silicate rocks , variations in the Mn3+/Fe3+ ratio of piemontite are thought to be ma inly due to the presence of braunite and hematite which fractionate Mn 3+ and Fe3+, respectively. The absence of braunite in SRH results in M n3+/Fe3+ ratios even greater than 5 in these piemontites (Pm20-29Ps6-1 7), as against 1.5 in the SRB piemontites (Pm19Ps13), despite the fact that, the whole-rock Mn3+/Fe3+ ratio is higher in SRB than in SRH. Th e Sr content of the piemontites in the studied rocks directly related to the total Sr content of the whole-rock.