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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Authors: | Laura González-Acebrón R H Goldstein Ramón Mas José Arribas |
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Institution: | 1.Dpto. Estratigrafía, Facultad de Ciencias Geológicas (UCM), Instituto de Geología Económica (CSIC),Madrid,Spain;2.Department of Geology,University of Kansas,Lawrence,USA;3.Dpto. Petrología y Geoquímica, Facultad de Ciencias Geológicas (UCM),Instituto de Geología Económica (CSIC),Madrid,Spain |
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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 δ18O values of ferroan calcites (δ18O −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
short-lived 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 host-rock values, a finding indicative of hydrothermal fluid flow. |
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