RETROGRADE FORMATION OF NACL-SCAPOLITE IN HIGH-PRESSURE METAEVAPORITES FROM THE CORDILLERAS-BETICAS (SPAIN)

A Permo-Triassic pelite-carbonate rock series (with intercalated metab asitic rocks) in the Cordilleras Beticas, Spain, was metamorphosed dur ing the Alpine metamorphism at high pressures (P(min) near 18 kbar). T he rocks show well preserved sedimentary features of evaporites such a s pseudomorphs of talc, of kyanite-phengitetalc-biotite, and of quartz after sulfate minerals, and relicts of baryte, anhydrite, NaCl and KC l, indicating a salt-clay mixture of illite, chlorite, talc, and halit e as the original rock. The evaporitic metapelites have a whole rock c omposition characterized by high Mg/(Mg + Ca) ratios > 0.7, variable a lkaline and Sr, Ba, contents, but are mostly K2O rich (< 8.8 wt%). The F (< 2600 ppm), Cl (< 3600 ppm), and P2O5 (< 0.24 wt%) contents are a lso high. The pelitic member of this series is a fine grained biotite rock. Kyanite-phengite-talc-biotite aggregates in pseudomorphs develop ed in the high pressure stage. Albite-rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of t he uplift. Scapolite, rich in NaCl (Ca/(Ca + Na) mol% 24-40) and poor in SO4, with Cl/(Cl + CO3) ratios between 0.6 and 0.8, formed as porph yroblasts, sometimes replacing up to 60% of the rock in a late stage o f metamorphism (between 10 and 5 kbar, near 600-degrees-C). No reactio n with albite is observed, and the scapolite formed from biotite by: A l-biotite + CaCO3 + NaCl + SiO2 = Al-poor biotite + scapolite + MgCO3 + KCl + MgCl2 + H2O Calculated fluid composition in equilibrium with s capolite indicates varying salt concentrations in the fluid. Distribut ion of Cl and F in biotite and apatite also indicates varying fluid co mpositions.