Geochemistry of Mesozoic volcanic rocks from the Fresnillo area (Chilitos Formation), Zacatecas, Mexico: Implications for the magma source and tectonic setting

Abstract

Geochemical data from Early Cretaceous volcanic rocks (ultra-basic, basic, intermediate) from the Fresnillo area (Chilitos Formation), Zacatecas, are presented in this study. These rocks are characterized by variable SiO2 = 38.1–61.7 wt %, Al2O3 = 17.5–27.2 wt %, and MgO = 0.21–6.4 wt % with the Mg# = 20.6–78.9. They are influenced by variable degrees of hydrothermal alteration, being the propylitic more dominant than argillic alteration. The geochemistry displays strong enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). Moreover, a depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and nearly flat heavy rare earth element (HREE) patterns are observed. Negligible or absence negative Eu anomalies indicate that fractional crystallization of plagioclase played an insignificant role during the magma evolution. The geochemical features suggest that volcanic rocks are derived from partial melting of an enriched mantle source that has experienced assimilation (low degree) and fractional crystallization. The low La/Nb (0.68–2.23) and Nb/Y (0.19–0.69) ratios, relatively high Zr/Y (3.68–10.56) and Th/Ta (1.03–3.65) ratios, and progressively enriched normalized patterns clue an island arc and most likely a back-arc basin environment for the Chilitos Formation in the Fresnillo area. New multidimensional discrimination diagrams indicate a mid-ocean ridge to island arc setting. A new tectono-magmatic model combined with previous geological observations indicate that the Chilitos Formation was formed in a back-arc tectonic setting and was part of the Guerrero terrane during the Late Jurassic to Early Cretaceous time.