%0 Journal Article
%A González Acebrón, Laura
%A Goldstein, Robert H.
%A Mas Mayoral, José Ramón
%A Arribas Mocoroa, José
%T Criteria for recognition of localization and timing of multipleevents of hydrothermal alteration in sandstones illustratedby petrographic, fluid inclusion, and isotopic analysisof the Tera Group, Northern Spain
%D 2011
%@ 1437-3254
%U https://hdl.handle.net/20.500.14352/42525
%X Stratigraphic relations, detailed petrography,microthermometry of fluid inclusions, and fine-scale isotopicanalysis of diagenetic phases indicate a complexthermal history in Tithonian fluvial sandstones and lacustrinelimestones of the Tera Group (North Spain). Twodifferent thermal events have been recognized and characterized,which are likely associated with hydrothermalevents that affected the Cameros Basin during the mid-Cretaceous and the Eocene. Multiple stages of quartzcementation were identified using scanning electronmicroscope cathodoluminescence on sandstones and fracturefills. Primary fluid inclusions reveal homogenizationtemperatures (Th) from 195 to 350 C in the quartz cementsof extensional fracture fillings. The high variability of Thdata in each particular fluid inclusion assemblage is relatedto natural reequilibration of the fluid inclusions, probablydue to Cretaceous hydrothermal metamorphism. Somesecondary fluid inclusion assemblages show very consistentdata (Th = 281–305 C) and are considered not to havereequilibrated. They are likely related to an Eocenehydrothermal event or to a retrograde stage of the Cretaceoushydrothermalism. This approach shows how multiplethermal events can be discriminated. A very steep thermalgradient of 97–214 C/km can be deduced from d18O valuesof ferroan calcites (d18O -14.2/-11.8% V-PDB) thatpostdate quartz cements in fracture fillings. Furthermore,illite crystallinity data (anchizone–epizone boundary) areout of equilibrium with high fluid inclusion Th. Theseobservations are consistent with heat-flux related to shortlivedevents of hydrothermal alteration focused bypermeability contrasts, rather than to regional heat-fluxassociated with dynamo-thermal metamorphism. Theseresults illustrate how thermal data from fracture systemscan yield thermal histories markedly different from hostrockvalues, a finding indicative of hydrothermal fluid flow.
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