Glauconitic laminated crusts from Jurassic pillow-lava deposits (Betic Cordillera, South Spain)

Resumen

An exceptional record of Jurassic glauconitic laminated crusts is reported from veins of spaces among pillow-lava bodies in the Middle Subbetic (Betic Cordillera). The veins are composed by green crusts with planar (<36 mm thick), columnar (<130 mm length) and oncoid-like morphologies growing from the pillow-lava walls. Mineralogical and geochemical analyses of the green crusts led us to identify lath-like crystals of glauconite and minority filmy veil-like smectites (saponite and beidellite), calcite and iron oxides. The composition of clay minerals and their association with carbonates reveal whether the alteration occurred under oxidizing or reducing conditions. The main phase of hydrothermal alteration occurred under low temperature oxidizing conditions with a high sea-water/rock ratio, in an open-circulation regime, with precipitation of glauconite. The late phase of alteration is typified by the formation of saponite and calcite, and took place under confined conditions due to the burial of pillow-lava by marine sediment.