The Cretaceous-Eocene Mexican Magmatic Arc: Conceptual framework from geochemical and geochronological data of plutonic rocks

Abstract

We present a study based on new and compiled geochronological and geochemical data to assess the tectonomagmatic evolution of Mexico from the Late Cretaceous and Eocene (~90–40 Ma). review of the literature highlights the striking geological differences observed in southwestern US and Mexico and particularly south of the classical Laramide province. Current tectonic models for western Mexico indicate that the continental margin of Mexico was fully consolidated by ~90 Ma, with the final accretion of the Alisitos and Guerrero arc terranes. Since then, east-faced subduction with a regular lowering of the slab dip persisted up to ca. 40 Ma, when compression and calc-alkaline magmatism ceased along the entire Cordillera. The sustained magmatic arc activity along the herein defined “Cretaceous-Eocene Mexican Magmatic Arc” (CEMMA), leads us to recommend abandoning the use of the term “Laramide magmatism” in Mexico. We define the CEMMA as an arc that trends NW-SE along most of western Mexico, but scattered outcrops occur also in central and eastern Mexico. The CEMMA composition ranges from gabbro to alkali granite, but hornblende ± biotite granodiorites are by far the most abundant type of rocks. Coeval volcanic rocks have been deeply eroded, but andesite-dominated sequences are partially preserved in the eastern part of the arc, particularly below the Cenozoic volcanic rocks of the Sierra Madre Occidental. The geochemical database that we present include 729 samples and shows that the CEMMA formed in a typical continental magmatic arc environment (calc-alkaline–metaluminous signatures and #Mg’s < 68). However, rocks from the eastern part of the arc reach more alkaline compositions, particulry due to larger K2O uptakes in the magma away from the trench. REE plots are characterized by negative slopes and negative to slightly positive Eu anomalies. Multielement spectra reveal negative anomalies of NbTa, Ti and P, coupled with positive anomalies of U, K and Pb. Initial values of 87Sr/86Sr and εNd range from 0.7031 to 0.7137 and from +6.2 to −8.3, respectively, suggesting mixing of variable proportions of crustal materials with the mantle-derived magmas. These elemental and isotopic features are typical of continental margin magmatic arcs. In this context, sustained generation of calc-alkaline magma under these conditions yielded outstanding conditions to form a diversity of metallic ores, particularly porphyry copper deposits, in the region of Arizona-New Mexico- Sonora, located just south of the classical Laramide province. Although the ages along the CEMMA can be roughly explained by partial melting of a metasomatized section of the mantle above the region of amphibole instability (~100 km), plutons with crystallization ages of ~90 Ma in eastern Sonora, and even more to the east in Durango and Zacatecas, are older than expected. We consider that these plutons were generated by more discrete magma pulses linked to the dehydration of the phlogopite deeper in the slab (~200 km).