Publication: Criteria for recognition of localization and timing of multiple
events of hydrothermal alteration in sandstones illustrated
by petrographic, fluid inclusion, and isotopic analysis
of the Tera Group, Northern Spain
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2011
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Springer Science Business Media
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Abstract
Stratigraphic relations, detailed petrography,
microthermometry of fluid inclusions, and fine-scale isotopic
analysis of diagenetic phases indicate a complex
thermal history in Tithonian fluvial sandstones and lacustrine
limestones of the Tera Group (North Spain). Two
different thermal events have been recognized and characterized,
which are likely associated with hydrothermal
events that affected the Cameros Basin during the mid-
Cretaceous and the Eocene. Multiple stages of quartz
cementation were identified using scanning electron
microscope cathodoluminescence on sandstones and fracture
fills. Primary fluid inclusions reveal homogenization
temperatures (Th) from 195 to 350 C in the quartz cements
of extensional fracture fillings. The high variability of Th
data in each particular fluid inclusion assemblage is related
to natural reequilibration of the fluid inclusions, probably
due to Cretaceous hydrothermal metamorphism. Some
secondary fluid inclusion assemblages show very consistent
data (Th = 281–305 C) and are considered not to have
reequilibrated. They are likely related to an Eocene
hydrothermal event or to a retrograde stage of the Cretaceous
hydrothermalism. This approach shows how multiple
thermal events can be discriminated. A very steep thermal
gradient of 97–214 C/km can be deduced from d18O values
of ferroan calcites (d18O -14.2/-11.8% V-PDB) that
postdate quartz cements in fracture fillings. Furthermore,
illite crystallinity data (anchizone–epizone boundary) are
out of equilibrium with high fluid inclusion Th. These
observations are consistent with heat-flux related to shortlived
events of hydrothermal alteration focused by
permeability contrasts, rather than to regional heat-flux
associated with dynamo-thermal metamorphism. These
results illustrate how thermal data from fracture systems
can yield thermal histories markedly different from hostrock
values, a finding indicative of hydrothermal fluid flow.