Abstract

VALDEZ MORENO, Gabriel; ARANDA-GOMEZ, José Jorge  and  ORTEGA-RIVERA, Amabel. Geochemistry and petrology of the Ocampo volcanic field, Coahuila, Mexico. Bol. Soc. Geol. Mex [online]. 2011, vol.63, n.2, pp.235-252. ISSN 1405-3322.

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.

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