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Abstract

A case of hydrothermal metamorphism is described from the Cameros Basin along the rio Cidacos transect. The highest grade is reached at the base of the Urbión Group, a fluvial sequence Upper Jurassic to Lower Cretaceous in age (Weald facies). Physical conditions at the thermal peak are estimated at abo 1 Kb (PL) and 326± 13°C (i.e. very low grade metamorphism). Fluids were largely water brines with minor amounts of gases, CO2 and CH4• f02 and f52 were close to the graphite and Po-Py buffers respectively. This metamorphism is spatially associated with a deformation band (5-6 km. ín wídth) wíth schístosíty ín the pelitíc beds and promínent tensíonal, quartz ínfílled veíns in the more samítíc ones. K-Ar dating on authígeníc illites has gíven ages from 108 to 86 M.a., i.e. Míddle Cretaceous, and thus younger than the basín fílling (Urbíón Group+Enciso F.+Olívan F.). Metamorphism migth be related wíth the circulatíon of hot mígratory fluíds (i.e. tectoníc brínes), along the deformation bando The tectonic sígníficance of this deformation band is slill uncertain.