%0 Journal Article
%A Herrero Fernández, María Josefa
%A Martín Pérez, Andrea
%A Alonso-Zarza, Ana María
%A Gil Peña, Inmaculada
%A Meléndez Hevia, Alfonso
%A Martín García, Rebeca
%T Petrography and geochemistry of the magnesites and dolostones of the EdiacaranIbor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin
%D 2011
%@ 0037-0738
%U https://hdl.handle.net/20.500.14352/42556
%X The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternatingsiliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpretedto have been formed by the replacement of primary peritidal limestones. Through petrographic andgeochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2).Despite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlapwith δ18O values ranging from +15.45 to +17.51‰ (SMOW) and δ13C from −0.13 to 3.21‰ PDB. Sr isotopevalues for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher than+17.87.0‰, and δ13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope valuesare between +18.91 and 19.61, and the carbon isotope values range are similar to the other diageneticphases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, beinghigher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacementof fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemicalfeatures as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1and D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpretedto have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite.D3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesiteformation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes ofthe earlier dolomite.The study of these magnesites and related dolostones may offer new insights into the model of formation ofsparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factorsand mechanism that control the diagenetic evolution of these carbonates has a great importance in order tounderstand and predicts porosity and permeability variations of rocks formed under similar geologicalconditions.
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