MASSIVE DOLOMITIZATION OF A LATE MIOCENE CARBONATE PLATFORM - A CASE OF MIXED EVAPORATIVE BRINES WITH METEORIC WATER, NIJAR, SPAIN

Late Miocene platform carbonates from Nijar, Spain, have been extensiv ely dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically hig hest outcrops, suggesting that dolomitizing fluids were derived from t he adjacent Nijar Basin. The dolomite crystals range from <10 to appro ximate to 100 mu m existing as both replacements and cements. Na, Cl a nd SO4 concentrations in the dolomites range from 200 to 1700 p.p.m., 250-650 p.p.m., and 600-7000 p.p.m., respectively, comparable with oth er Tertiary and modern brine dolomite values, and also overlapping val ues from mixing-zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are est imated to range between 7x seawater brine to freshwater ratios. The de lta(18)O and delta(13)C of the dolomites range from -1.0 to +4.2 parts per thousand PDB, and -4.0 to +2.0 parts per thousand PDB, respective ly. Sr-87/Sr-86 values (0.70899-0.70928) of the dolomites range from l ate Miocene seawater to values greater than modern seawater. Mixtures of freshwater with seawater and evaporative brines probably precipitat ed the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. delta (18)O and Na/Ca vs. delta(18)O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, n onstoichiometry and quantitative water-rock interaction modelling of c ovariations of Na vs. Sr and delta(18)O vs. delta(13)C. The possibilit y of multiple dolomitization events induced by evaporative brines, sea water and freshwater, respectively, is consistent with mineral-mineral mixing modelling. The basin-derived dolomitizing brines probably mixe d with freshwater in the Nijar Basin or mixed with fresh groundwater i n the platform, and were genetically related either to deposition of t he Yesares gypsum or the Feos gypsum. Dolomitization occurred during e ither the middle Messinian or the early upper Messinian. Nijar dolomit ization models may be applicable to dolomitization of other late Mioce ne platform carbonates of the western Mediterranean. Moreover, the Nij ar models may offer an analogue for more ancient evaporite-absent plat form carbonates hinging evaporite basins.