Petrography and geochemistry of the magnesites and dolostones of the EdiacaranudIbor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin

摘要

The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternatingudsiliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpretedudto have been formed by the replacement of primary peritidal limestones. Through petrographic andudgeochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2).udDespite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlapudwith δ18O values ranging from +15.45 to +17.51‰ (SMOW) and δ13C from −0.13 to 3.21‰ PDB. Sr isotopeudvalues for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher thanud+17.87.0‰, and δ13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope valuesudare between +18.91 and 19.61, and the carbon isotope values range are similar to the other diageneticudphases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, beingudhigher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacementudof fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemicaludfeatures as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1udand D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpretedudto have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite.udD3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesiteudformation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes ofudthe earlier dolomite.udThe study of these magnesites and related dolostones may offer new insights into the model of formation ofudsparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factorsudand mechanism that control the diagenetic evolution of these carbonates has a great importance in order toudunderstand and predicts porosity and permeability variations of rocks formed under similar geologicaludconditions.