REGIONAL RETROGRADE ALTERATION OF SUB-GREENSCHIST FACIES CHLORITE TO SMECTITE

Dioctahedral smectite is present as a retrograde alteration product of chlorite in Permian-Triassic red slates of the Malaguide Complex in S ierra de Espuna (Betic Cordillera). Mineral assemblages and textures, illite ''crystallinity'' indices, and fluid inclusion data indicate su b-greenschist facies conditions that reached at least 180-degrees-C in the higher-grade tectonic unit of the Malaguide Complex, preceding fo rmation of smectite. Smectite, having K as the dominant interlayer cat ion, occurs ubiquitously intercalated with trioctahedral chlorite as t hin packets of layers and as individual layers that commonly change to chlorite along layers. Although some chlorite is typically homogeneou s and trioctahedral, much chlorite shows signs of alteration and has c ompositions corresponding to different degrees of smectite contaminati on. The incompatibility of metamorphic grade with the occurrence of sm ectite, the general association of chlorite and smectite, and the text ural relations collectively show that dioctahedral smectite is derived through replacement of trioctahedral chlorite. Such replacement occur s on a regional basis and demonstrates that caution must be used in in terpreting the occurrence of smectite in pelites as being due to progr ade processes. Alteration of trioctahedral chlorite under oxidizing co nditions due to introduction of phreatic water after uplift of the Bet ic Cordillera is proposed as the cause of formation of smectite.