⁴⁰Ar/³⁹Ar geochronology and petrogenesis of the Sierra de San Miguelito Volcanic Complex, Mesa Central, Mexico

The southern part of the Mesa Central (MC) province, Mexico, is formed of several Cenozoic volcanic complexes. The Sierra de San Miguelito Complex (SSMC) is in the south-eastern part of the MC. The SSMC consists of: (1) mafic volcanic rocks of porphyritic texture and trachybasalt/basalt compositions; (2) intermediate volcanic rocks of porphyritic texture and basaltic-trachyandesite, basaltic andesite and andesite compositions; and (3) silicic volcanic rocks of porphyritic texture and rhyolite composition. New 40Ar/39Ar dating results, in combination with major- and trace-element data, and Sr-Nd-Pb isotope data, are used to investigate the petrogenesis and geodynamic evolution of SSMC. The 40Ar/39Ar radiometric age data constrains the magmatic events in the SSMC to between 34 and 21 Ma. Chondrite-normalized rare-earth element patterns are distinct for each volcanic succession; mafic and intermediate lavas have relatively flat light rare earth element (LREE) and large ion lithophile element (LILE) patterns, whereas the silicic volcanic rocks show enrichment in LREE and high field strength elements (HFSE). Within each volcanic phase, the total rare-earth element concentrations increase from mafic to silicic, and the size of the negative Eu anomalies progressively increase (Eu/Eu* from 0.02 to 1.04). The initial 87Sr/86Sr ratios are widely distributed (from 0.70344 to 0.71973) whereas the initial 143Nd/144Nd ratios are somewhat low and show a narrower range (0.51245 to 0.51287), indicating the mafic magmas derived from a slightly heterogeneous source. Geochemical modelling of the mafic volcanic rocks reveals two sources of magma: (1) a parental magmas generated from melting underlying lithospheric mantle; and (2) a second lithospheric melt contaminated by lower crust. Intermediate magmas evolved from assimilation and fractional crystallization (AFC) processes of both lithospheric melts, at shallower levels. The silicic volcanic rocks in the area, however, were probably derived from partial melting of sedimentary rocks within the upper–middle continental crust. New multidimensional tectonic discrimination diagrams, combined with the magmatic model, indicates that volcanic activities in the region were generated in an extensional environment.