Late burial, hydrothermal dolomitization of the Cambrian Láncara Fm., Cantabrian Zone (NW Spain): origin of the dolomitizing fluid and relation to the geodynamic setting

Several petrographic and geochemical methods have been applied in order to distinguish and characterize the late epigenetic dolomitization event occurring in the Cambrian Carbonate succession of the Láncara Formation, Cantabrian Zone, NW Spain. The phenomenon is volumetrically not impressive but has a large spatial distribution, affecting different tectonic units. Furthermore, this dolomite type is petrographically similar to the volumetrically more extensive dolomite replacing the thick Namurian and Westfalian succession, which was the object of a previous Ph.D thesis (Gasparrini 2003). New data from dolomitized Carboniferous samples have also been integrated for comparison. The contact between dolomitized and undolomitized lithologies is very sharp in the outcrops, usually crosscutting the bedding surface. The dolomitization affects mostly the calcareous members of the Láncara Formation (glauconitic packstone, birdseye limestone). Rarely, the stromatolitic dolostone, which forms the upper part of the lower member of the Láncara Formation, is also replaced by the epigenetic event. The dolomite is fabric-destructive with only rare remnants of former sedimentary features left. At the hand specimen scale, two different generations of dolomite can be easily distinguished: a matrix-replacive dolomite (Dol A), which is usually grey to beige in colour, and a void-filling coarser, white saddle dolomite (Dol B). Usually, Dol B is volumetrically less important relative to matrix-replacive Dol A, but locally it may reach 30-35 % of the dolomitized bodies. The transition between the matrix-replacive Dol A and the void-filling Dol B is gradual, never corresponding to a reaction boundary. The polimodal grain size distribution allows subdivision of Dol B in different sub-generations, with the last one (Dol B II) consisting of a clear inclusion-free rim. In CL, Dol B is characterised by the same red to red-dull luminescence of Dol A, with the exception of the last Dol B II generation (the clear rim), which is non-luminescent. Millimetre-scale pores are abundant in the dolomitized bodies whereas they are practically absent in the undolomitized lithologies. This dolomite generation has a burial origin, as indicated by coarse cristallinity, abundant saddle dolomite cement, zebra structures, high Fe-Mn concentrations and low Sr concentrations, low δ18O, radiogenic 87Sr/86Sr and fluid inclusions TH between 80 and 120°C. Raman Spectroscopy combined with microthermometry and crush-leach analyses have been applied to identify the main ion species present in individual inclusions. Eutectic and other phase change temperatures are correctly established, in addition to the identification of metastable phase assemblages that can lead to erroneous interpretation of the salinity of the fluids. Crush-leach analyses reveal that the dolomitizing fluids are highly evaporated seawater which has been modified through water-rock interaction only at a minor degree. For comparison, crush-leach tests were performed on samples from the dolomitized Namurian and Westphalian limestones. The data reveal that a possible coeval fluid flow with similar composition affected both the Cambrian and the Carboniferous carbonate successions. It can be assumed that a local fluid source is mainly defined by the adjacent host rock; consequently, different dolomitized rock formations would reveal different fluid systems. The similarity between dolomitizing fluids affecting the Cambrian as well as the Carboniferous limestones favours an external fluid source, which flooded both rock types. This fluid flow event affected the Cantabrian Zone presumably during the late stages of the Variscan orogeny, as indicated by indirect geological evidences (i.e. temporal relationship with dated ore minerals and main Variscan structures). This period was characterized by an extensional tectonic regime and by a high geothermal gradient which might have favoured the onset of convective cells in the study area