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Published 1981 | Published
Journal Article Open

An overview of Precambrian rocks in Sonora

Abstract

The oldest stratified rocks recognized in NW Sonora (and in Mexico) are deformed muscovite-quartz schists, quartzites, and biotite-quartzofeldspathic gneisses near Caborca, which are cut by calcalkaline intrusives ranging from 1,710 to 1,750 m.y. in age. Southwest of Caborca, upper amphibolite facies layered quattzofeldspathic and amphibolitic gneisses were apparently deformed and metamorphosed at about 1,660 ± 15 m.y. ago, concealing original lithologies and ages. In northeastern Sonora, a younger belt of eugeosynolinal strata, about 1,680 ± 20 m. y. old was tightly folded and metamorphosed to greenschist facies about 1,650 m.y. ago. Numerous granitic plutons intruded into the older Precambrian crust about 1,410 to 1,440 m.y. ago. These major intrusive masses are not known to have been accompanied by regional sedimentation or deformation. Rare, small plutons of micrographic granite added to the Precambrian crystalline complexes about 1,100 m.y. ago, are the youngest Precambrian igneous rocks recognized. They limit the age of a thick miogeoclinal sequence of unmetamorphosed quartzose sandstones, carbonates with numerous stromatolite horizons, and shales which rest nonconformably on them. The sequence is overlain without unconformity by a fossiliferous Lower Cambrian section. The northwestern and northeastern Precambrian suits appear to be separated by a Jurassic magmatic arc and a postulated shear structure of large lateral displacement. Both suites correlate northward into related belts in the SW United States. To the east they are concealed by Phanerozoic cover. Abrupt termination of Precambrian exposures south and west suggests major younger tectonic features which we suspect played important but undefined roles in the apparent absence of Precambrian basement under much of northern and west-central Mexico.

Additional Information

© 1981 Universidad Nacional Autónoma de Mexico. Our investigations have benefited from logistical support at times from the Instituto de Geologia, Universidad Nacional Autónoma de Mexico, Universidad de Sonora, Consejo de Recursos Naturales No Renovables, and Instituto Nacional de Energia Nuclear. Conversations with Jaime Roldán-Quintana, Guillermo A. Salas, Claude Rangin, Richard Merriam, and Jack Eells have contributed to our efforts to decipher the history of Precambrian rocks. Mineral separations by Jaime Alvarez, Rico Dagonel and O. Shields and invaluable efforts by Gerri Silver and Maria Pearson toward polishing off great batches of chemistry and mass spectrometry are most kindly acknowledged. This work was supported by NSF grants: GA-15989 and EAR 74-00155 A01 (formerly GA-40858) awarded to Caltech and EAR 76-84138 awarded to the University of Pittsburgh.

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Created:
August 19, 2023
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October 25, 2023