GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF VEINS ASSOCIATED WITH SHORTENING STRUCTURES IN THE MEXICAN FOLD-THRUST BELT (CENTRAL MEXICO)
The sampled veins were associated with mesoscopic folds and formed at various stages during shortening. They include both closed (confined within layers) and open (crossing several layers) types. Closed veins were taken mostly from en echelon arrays with in single layers. They are assumed to have formed in a closed system, in other words, the material of the vein is provided by the surrounding rock. In contrast, the material in open veins is presumed to have come from a relatively foreign source. The selection of samples for stable isotopes (C-O) and REE analyses was based on detailed observations of mesoscopic structures and thin sections. The δ18O values vary between 15 and 27, and are slightly higher in both confined and open veins than in the host rock. The δ13C values vary between -3 and 3, and are higher in the host rock and confined veins than in the open veins, suggesting that the latter come from a different source. In addition, comparative analyses of REE of host rock, confined and open veins were carried out. The REE diagrams show a relative enrichment in LREE and a positive anomaly of Eu in the confined veins compared with the host rock, whereas the REE pattern in the open veins is very irregular, again suggesting a different provenance for these veins.
These results are consistent with solution transfer on a local scale being responsible for the confined veins, and transfer of fluids on a much broader scalepresumably along faults or other fracturesproviding material for the open veins.