Abstract

VELASCO-TAPIA, Fernando et al. Petrographic and geochemical study of the El Peñuelo intrusive (Concepción del Oro Intrusive Belt), Northeastern Mexico. Bol. Soc. Geol. Mex [online]. 2011, vol.63, n.2, pp.183-199. ISSN 1405-3322.

This work presents the petrographic and geochemical data for the El Peñuelo Plutonic Complex (EPPC), which is an intrusive that belongs to Concepción del Oro Intrusive Belt (COIB; Northeastern Mexico). The EPPC is a semi-circular structure, constituted by three emplacement centers, situated in the eastern part of the COIB and the northern extension of the Taxco-San Miguel de Allende fault system. However, the complex was not affected by the activity of this fault system. The EPPC is constituted by intrusive rocks varying from quartz monzodiorite to quartz syenite, the latter covering most of the complex surface. The EPPC was emplaced in Upper Cretaceous marine sedimentary rocks. The quartz syenite is cut by quartz monzodioritic and porphyritic meso-syenitic dykes. Also, this unit contains irregularly distributed monzodioritic microgranular enclaves. Additionally, pegmatitic dykes cut the rest of the lithologic units. The EPPC mineralogical assemblage shows variable quantities of plagioclase + alkaline feldspar + quartz ± amphibole + biotite ± orthopyroxene + clinopyroxene + Fe-Ti oxides. Intrusive rocks have a chemical composition of SiO₂ = 45.7-72.0 %, Mg# = 39.2-60.2, and n-Fe = 0.54-0.73. They show chondrite-normalized REE patterns enriched in light elements [(La/Yb)N = 6-11] with no Eu anomalies. Primitive Mantle-normalized multi-element diagrams show LILE enriched patterns relative to the HFSE. Their trace-element geochemistry is comparable to high-Ba-Sr granitoids: (a) high Ba concentration (= 594-2302 ppm) and Sr (= 444-2192 ppm); (b) low concentrations of Y (= 10-46 ppm) and Nb (= 6-17 ppm); and (c) high values for Sr/Y (= 25-85) and (La/Yb = 8.9-16.5) ratios. The origin of EPPC has been related to partial melting of an enriched lithospheric mantle, in a post-orogenic setting, followed by fractional crystallization coupled to crustal assimilation.

Keywords : Northeastern Mexico; high-Ba-Sr granitoids; post-orogenic magmatism.

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