Mineralogy and geochemistry of sands along the Manzanillo and El Carrizal beach areas, southern Mexico: implications for palaeoweathering, provenance and tectonic setting

The provenance of sands from the Manzanillo (MN) and El Carrizal (CAR) beach areas along the Mexican Pacific coast (southern Mexico) was investigated based on their modal composition, mineralogy and geochemical data. The average quartz, feldspar and lithic fragment (QFL) ratios revealed that the proportion of quartz is higher in CAR sands than in MN sands. In comparison with MN, the CAR sands are enriched in SiO2 content with higher SiO2/Al2O3 ratio indicating that sediment recycling is greatest for the CAR sands. The variation in K2O/Al2O3 and Na2O/K2O ratios between the MN and CAR sands specified a difference in the relative proportion of plagioclase. The chemical index of alteration (CIA) and Plagioclase Index of Alteration (PIA) values revealed a moderate weathering intensity in the source areas. The MN sands are characterized by a low degree of rare earth element (REE) fractionation (La-CN/Yb-CN=similar to 6-7), whereas the CAR sands define fractionated REE patterns (La-CN/Yb-CN=similar to 8.8-16.8). The fractionated REE pattern with elevated total low REE (Sigma LREE) content of the CAR sands suggested that sediment sorting is higher for the CAR than MN sands. Small negative and positive Eu anomalies (Eu/Eu-*=similar to 0.91-1.23) of MN sands indicated the derivation of sediments from felsic-intermediate source rocks. The CAR sands showed higher concentration of Sigma LREE than total heavy REE (Sigma HREE) and characterized by a significant negative Eu anomaly (Eu/Eu-*=similar to 0.46-0.78), suggested that the source rock is felsic type. The result obtained by the comparison of REE patterns of the beach sands to the probable source rocks is also consistent with this observation. The compositional differences identified between the MN and CAR beach areas suggested that longshore current in the mixing and homogenization of sands is not significant. The new tectonic discriminant-function-based multidimensional diagrams showed an arc setting (active) for the MN sands and a rift setting (passive) for the CAR sands, which is consistent with the regional geology of southern Mexico. Copyright (c) 2016 John Wiley & Sons, Ltd.