POST-VARISCAN HOT DOLOMITES IN THE PALEOZOIC OF THE CANTABRIAN ZONE, NW SPAIN: INDICATORS OF A CRUSTAL-SCALE HYDROTHERMAL PALEOFIELD?

A large-scale dolomitisation process affected Late Carboniferous and to a minor degree Early Cambrian carbonates in the Cantabrian Zone, NW Spain, which represent the thin-skinned foreland of the Variscan Iberian Massif. The burial nature of the often massive dolomite is indicated by coarse crystallinity, only rare preservation of precursor fabrics, often non-planar textures and irregular morphology. A replacive (generation A, rarely saddle dolomite) and a void-filling (generation B, always saddle dolomite) dolomitising event can be distinguished. In Carboniferous rocks, both generations display very similar characteristics: dull red and unzoned CL, depletion in Sr and enrichment in Fe and Mn relative to the precursor carbonates, ample spread of d¹⁸O values (-3 to -12 ‰ VPDB -distinctly lower than the precursors-), d¹³C values (1,7-5,4 ‰ VPDB) strongly buffered by the precursor carbonates. These dolomites formed from hot (100-170 °C) and saline (around 31 equiv. wt % NaCl) fluids, slightly radiogenic and strongly enriched in ¹⁸O relative to Late Paleozoic seawater. The volumetrically minor dolomites in the Cambrian show very similar features, where the d¹³C values have been buffered by the Cambrian limestone and early diagenetic dolomite. A continuous dolomitisation process, which evolved from replacive to void-filling is indicated by the uniform CL petrography and missing reaction borders between the two main dolomite phases. The dolomitising fluids were hydrothermal and hypersaline, representing strongly evaporated seawater, possibly already residing in the surrounding rock column. Dolomitisation post-dates the main Variscan compressional events in the area, and pre-dates precipitation of ore minerals (~280-260 Ma). It is apparently synchronous to the formation of the Cantabrian Arc (latest Carboniferous/earliest Permian), when a delamination of the lower crust (Fernández-Suárez et al., Journal of the Geological Society, 157, 2000) was accompanied by increased heat flow. Main pathways for the dolomitising fluids were Variscan thrust and fault planes.