BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA Vol 63, Núm. 2, 2011, P. 235-252. http://dx.doi.org/10.18268/BSGM2011v63n2a7

Geoquímica y petrología del campo volcánico de Ocampo, Coahuila, México

Geochemistry and petrology of the Ocampo volcanic field, Coahuila, Mexico

Gabriel Valdez Moreno^1,*,+, José Jorge Aranda–Gómez² y Amabel Ortega–Rivera³

¹ Posgrado en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. 04510. ⁺ Dirección Actual: Unidad Académica de Ciencias de la Tierra, Universidad Autónoma de Guerrero. Ex–hacienda de San Juan Bautista, s/n, Taxco El Viejo, Guerrero, 40200, México.
² Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Querétaro, 76230, México.
³ Instituto de Geología, Estación Regional del Noroeste, Apartado Postal 1039, Universidad Nacional Autónoma de México, Hermosillo, Sonora, 83000 México.

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 Abstract

A set of mafic scoria cones and associated lava flows occur in the valley located between the towns of Cuatro Ciénegas and Ocampo in Coahuila. These structures form the Plio–Quaternary Ocampo volcanic field. The isotopic age of these volcanoes ranges between 1.82 ± 0.20 Ma and 3.41± 0.55 Ma (⁴⁰Ar/³⁹Ar, matrix). The Ocampo volcanoes define a regional WNW–ESE lineament of intraplate type, mafic volcanic rocks, which is roughly parallel to a magnetic anomaly in the basement. At a local scale, five scoria cones located in the Cuatro Ciénegas – Ocampo valley define a NNW–SSE trending, 8 km long lineament. The lavas issued from two volcanoes located immediately west of the town of Ocampo form a 50 m thick sequence. The location of the vents is marked by the presence of near–vent pyroclastic deposits. The studied volcanic rocks are hypocristalline with a pilotaxitic or intergranular texture in the matrix. Phenocryst assemblage in these rocks is always olivine + clinopyroxene + plagioclase + titanomagnetite. Some samples may contain in addition partially resorbed plagioclase and/or potassium feldspar xenocrysts. Based on their chemical composition, the Ocampo rocks are classified as hawaiite, basanite or alkali basalt. Multielement diagrams have a concave form, characteristic of intraplate–type magmas. REE patterns have steep slopes (La/Yb = 13.5 to 32.8), which are interpreted as the result of garnet in the source area of the magmas. Isotopic data (⁸⁷Sr/⁸⁶Sr = 0.70336 to 0.70346; εNd = 6.01 to 6.14; ²⁰⁸Pb/²⁰⁴Pb = 38.07 to 38.18, ²⁰⁶Pb/²⁰⁴Pb = 18.45 to 18.48) are homogenous and consistent with an OIB–type source. Geochemical variations observed in the studied samples can be explained with an AFC process. Crystal fractionation of olivine + clinopyroxene + plagioclase and simultaneous assimilation or crustal material during a relatively slow ascent to the surface seems to have played an important role in the evolution of the magma.

Keywords: Coahuila, geochemistry, intraplate, Basin and Range Province.