Geochemistry of sands along the San Nicolás and San Carlos beaches, Gulf of California, Mexico: implications for provenance and tectonic setting

Authors: JOHN SELVAMONY ARMSTRONG ALTRIN, RAMASAMY NAGARAJAN, YONG IL LEE, JUAN JOSÉ KASPER ZUBILLAGA, LESLIE PATRICIA CÓRDOBA SALDAÑA

Abstract: The weathering conditions, provenance, and tectonic setting of sands from the San Nicolás (SN) and San Carlos (SC) beaches along the Gulf of California, Mexico, have been studied using mineralogy, major element, and trace element data. The compositional similarity among 4 independent groups (each beach area consists of 2 grain-size groups, i.e. medium- and fine-grained sands) was tested statistically by the application of analysis of variance at the 99% confidence level to avoid misinterpretation. The X-ray diffraction and SEM-EDS data revealed that the fine-grained SN sands were abundant in rutile and zircon minerals. The higher SiO_2/Al_2O_3 ratio of the SN sands than the medium- and fine-grained SC sands indicated that the compositional maturity was greatest for the SN sands (F_{calc} = 366.756151 and (F_{crit})_{99%} = 5.065158, where F_{calc} > (F_{crit})_{99%} indicates that the data populations are significantly different at 99% confidence level). The chemical index of alteration values for the SN (ca. 41-45) and SC (ca. 48-51) sands indicated low to moderate weathering intensity in the source region. The significant enrichment of the low rare earth element and the flat heavy rare earth element patterns of the SN sands indicated that the sources were largely felsic rocks. The low positive Eu anomaly in the SC sands was probably due to the contribution of sediments from intermediate rocks between felsic and mafic compositions. The comparison of rare earth element data of the sands with rocks located relatively close to the study areas revealed that the SN sands received a major contribution from felsic rocks and SC sands from intermediate rocks. The compositional difference between the SN and SC beach areas indicated that longshore currents played a less significant role. Discriminant function-based major element diagrams for the tectonic discrimination of siliciclastic sediments revealed a rift setting for the Gulf of California, which is consistent with the general geology of Mexico.

Keywords: Beach sand, SEM-EDS, X-ray diffraction, weathering, rift setting, rare earth element

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