EFFECTS OF FLUID INFILTRATION ON WOLLASTONITE GENESIS AT THE MERIDA CONTACT-METAMORPHIC DEPOSITS, SW SPAIN

The Merida wollastonite deposits (SW Spain) occur in the inner part of a contact-metamorphic aureole developed on impure carbonate rocks of Lower Cambrian age, by the intrusion of a late Hercynian granite. Cont act metamorphism associated with the granite emplacement did not only cause an important thermic anomaly but also triggered a complex fluid- rock interaction driven by infiltration of externally-derived fluids. Petrographic and stable isotope data suggest that the aureole was infi ltrated by large amounts of aqueous fluids, emanating from the nearby granite, through lithological contacts. As a result of fluid infiltrat ion, an extensive dilution of the CO2-rich fluid phase evolved by prog rade devolatilization reactions took place, thus providing an environm ent suitable for wollastonite formation. Nearly pure wollastonite band s resulted from chert interlayered in Limestone by local exchange betw een CaO and SiO2 across bimetasomatic rims, in the presence of virtual ly pure water showing a magmatic-dominated isotope signature (delta(18 )O values as low as 11.8 parts per thousand are recorded for wollaston ites adjacent to the granite contact). Distribution and modal abundanc e of wollastonite provide insights for determining the fluid flow path s and the extent of infiltration. Fluid circulation was dominantly par allel to bedding and clearly focused along marble-metachert boundaries , which acted as channelways for migration of fluids in the direction of decreasing temperature. Conditions of wollastonite formation have b een estimated as follows: P=570-700 bars; T = 550-600 degrees C; and X -CO2 approximate to 0.05.