MINERALOGY AND GEOCHEMISTRY OF THE CARBONATES IN THE CALATAYUD BASIN (ZARAGOZA, SPAIN)

The Calatayud Basin is an elongated NW-SE depression in northeastern S pain. It is filled with continental Miocene sediments resulting from e rosion of Palaeozoic and Mesozoic rocks of the Castilian and Aragonese branches of the Iberian Range. Miocene series consist of lutitic, mar ry, carbonate and evaporitic levels characteristic of a short-lived sa lt lake or a low-salinity playa-lake depositional environment. The car bonates detected in the series were calcite, dolomite and magnesite. T he calcites generally show micritic textures, anhedral to subhedral mo rphologies and contain from 0 to 1 mole% MgCO3, and so can be consider ed ''low-Mg calcites''. Their mean isotopic values are delta(18)O = -7 .8 parts per thousand (PDB) and delta(13)C = -7.7 parts per thousand i ndicating precipitation in isotopic equilibrium with meteoric water an d an extensive contribution of CO, of organic origin to the total diss olved C, respectively. The calcites present Mg/Ca, Sr/Ca and Na/Ca mol ar ratios that suggest precipitation from Na-enriched meteoric water. Dolomites are non-stoichiometric (44.5-47.7 mole% MgCO3), disordered, micritic and anhedral to subhedral; their mean isotopic values [delta( 18)O = + 0.4 parts per thousand (PDB) and delta(13)C = -3.8 parts per thousand] indicate that they precipitated from more evolved water and with a higher contribution of atmospheric CO2 to the total dissolved C , respectively, than those of the calcites. Moreover, they present Mg/ Ca, Sr/Ca and Na/Ca molar ratios suggesting precipitation from water w ith similar composition to seawater. The isotopic composition of magne site [delta(18)O = + 4.6 parts per thousand (PDB) and delta(13)C = - 4 .0 parts per thousand], together with the higher concentration in Sr a nd Na, indicates that this phase probably precipitated from more evolv ed water. The isotopic values and trace-element contents of carbonates appear to confirm the mineralogical trend from calcite to dolomite an d to magnesite corresponding to the progressive increase of evaporitic concentration as the water, in a hydrologically closed system, was su bjected to more acutely arid conditions.