Anomalous reverse zoning of saponite and corrensite caused by contact metamorphism and hydrothermal alteration of marly rocks associated with subvolcanic bodies.

The effects of a subvolcanic intrusion on its host rocks in the Priego de Córdoba area (SE Spain) was studied by optical microscopy, X-ray diffraction and electron microscopy. The intrusion of a laccolith of stratiform dolerite in partially consolidated marly sediments at quite shallow depths below the ocean floor took place during the intracontinental rifting phase of the Subbetic zone of the Betic Cordilleras. In the first stage, contact metamorphism caused the crystallization of calc-silicate consisting of grossular andradite garnet, diopside, vesuvianite and titanite for which the upper temperature limit was 500°C. Phyllosilicates are found in a network of veins cross-cutting the carbonate and calc-silicate matrix, indicating their formation during a process of hydrothermal alteration superimposed on the contact aureole. In the area closest to the subvolcanic rocks, saponite is the main phyllosilicate although some dispersed chlorite is found. In the zone furthest from the contact, corrensite together with saponite and beidellitic smectites were observed. The presence of low-temperature phases (saponite) in the area closest to the laccolith reveals the evolution of the hydrothermal system toward cooler conditions. In the early stage, the circulation of a hot hydrothermal fluid caused the crystallization of chlorite in the area surrounding the subvolcanic body and corrensite in the more distal area, which might have begun even during the contact metamorphism stage. A cooling phase followed, resulting in the crystallization of saponite in the host rocks, and the crystallization of dioctahedral and trioctahedral smectites inside the subvolcanic body.