Geochemistry of two Mexican tropical basins in an active margin and their influence on littoral sediments

Two exorreic basins from southern Mexico were studied in order to estimate sand composition and provenance in tropical latitudes based on sample suites collected under similar climate conditions and relief but with differences in parent rock type. The climate in these basins is warm to humid and the topography reflects young basins which belong to an active margin. The westward basin, La Tigra, has a greater outcrop area of plutonic rocks, whereas the Petatlan Basin is characterized by a greater abundance of metamorphic rocks. Chemical analysis of sediment samples show greater weathering in Petalan than in La Tigra Basin. Microprobe analysis of detrital magnetite grains from both basins yield evidence of a felsic plutonic/volcanic or metamorphosed mafic or ultramafic source. The average mineralogical maturity index (Mi = QF + RF) is very similar for both basins (0.30 in La Tigra and 0.35 in Petatlan), while the provenance index (Pi = FRF) shows average values of 0.46 for La Tigra Basin and 0.22 for Petatlan Basin. These differences seem to be mainly related to lithologic differences. Beach sand samples reflect a lithological admixture of sand derived from both basins, based on chemical analysis and on provenance index (0.34).