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Geology of the Yécora area, northern Sierra Madre Occidental, Mexico

Jean-Jacques Cochemé
Jean-Jacques Cochemé
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Alain Demant
Alain Demant
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January 01, 1991

This study presents new field, structural, petrographic, and geochemical data on the Cenozoic volcanic rocks of the northwestern Sierra Madre Occidental (SMO). Since the Mesozoic, this region has been the site of three successive magmatic events: (1) A calcalkaline sequence (90 to 40 Ma), which in Sonora is characterized by the emplacement of the granitoids of the Cordilleran coastal batholiths coeval with the subduction of the East Pacific plate under the North American continent. (2) An Eocene-Oligocene (around 35 Ma) volcanic sequence represented by calcalkaline andesites and high-K ignimbrites. The plagioclase-biotite phenocrystic assemblage is typical of the ignimbrites of this sequence in the northern SMO. Some of these rocks are caldera-related but they are much thinner and restricted in exposure than those in the southernmost areas. This magmatic activity probably accounts for much of the mineralization in the region. (3) A bimodal Oligocene-Miocene sequence (30 to 17 Ma) consisting of successive eruptions of basalt and basaltic andesite associated with acidic calcalkaline tuffs and lavas. Basic rocks form the predominant outcrops in the northern area of the SMO. The trap-forming flows were extruded from fissures and small calderas. These mafic rocks are transitional between the calcalkaline and tholeiitic series and show a progressive change from orogenic to anorogenic affinities with time. This volcanic event is related to pre-Basin and Range normal-faulting tectonism. Extensional tectonism at 17 Ma accounts for the modern basin-range morphology, which is well developed in the northern SMO. The volcanism progressively disappeared along the central axis of the SMO during this time. Quaternary volcanism is rare in Sonora; there are scattered basaltic fields such as the Quaternary lavas of Moctezuma, which crop out in central Sonora. Basalts are typically anorogenic and coeval with strike-slip faults, which affected the whole western margin of the North American continent, and with the opening of the Gulf of California. Based on the age and geochemical characteristics of the studied rocks, it is possible to conclude that, since Mesozoic time, the magmatism in the northern SMO probably reflects successive changes in the geodynamic regime.

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