GEOCHEMICAL AND GEOCHRONOLOGICAL EVIDENCES FOR VOLCANIC ROCKS OF THE SIERRA DE SAN MIGUELITO COMPLEX, SAN LUIS POTOSI, MEXICO

The Sierra de San Miguelito Complex (SSMC) is situated in the southeastern part of the Mesa Central province, Mexico. It consists dominantly of three main volcanic successions: (1) basic volcanism composed of trachybasalt and basaltic lavas; (2) intermediate volcanism composed of basaltic-trachyandesite, basaltic andesite and andesitic lavas and (3) acid volcanism composed mainly of rhyolitic domes. New ⁴⁰Ar/³⁹Ar ages data indicate that SSMC volcanism was active during Oligocene to Early Miocene (34-21 Ma) in three main episodes. Chondrite-normalized rare-earth element patterns are distinct for each volcanic succession where basic and intermediate volcanisms have relatively flat light rare earth elements and large ion lithophile elements patterns, whereas acid volcanism shows enrichment in LREE and high field strength elements. Within each volcanism, total rare-earth element concentrations increase from basic to acid, and Eu anomalies become progressively more negative from basic to acid (Eu/Eu*=1.03-0.03). The observed variations are consistent with the effects of crustal contamination where lower crust may have contributed to evolved magmas. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios suggest that volcanic rocks from the SSMC derived from mantle source and evolved through magma mixing associated with crustal assimilation. Geochemical modelling suggest that intermediate rocks experience 45% of mixing process between basic and acid end-members. In contrast, partial melting model reveals that acid rocks show a 35% of melting from the upper continental crust. New multi-dimensional discrimination and isotope-isotope diagrams indicate an extensional system prevalent in this area.