GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF VEINS ASSOCIATED WITH SHORTENING STRUCTURES IN THE MEXICAN FOLD-THRUST BELT (CENTRAL MEXICO)

Calcite veins in the central Mexican fold-thrust belt have been analyzed to assess the provenance of fluids and rock-fluid interaction during deformation. Samples of calcite veins and host rock were taken along a 150 km transect in an almost complete cross section of the belt, which includes carbonate rocks from four Cretaceous paleogeographic units, from west to east: El Doctor Platform, Zimapan Basin, Valles-San Luis Platform and Tampico-Misantla Basin.

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 scale–presumably along faults or other fractures–providing material for the open veins.