PETROGRAPHY AND GEOCHEMISTRY OF CERRO SAN MARTÍN AND SIERRA DE LOS CABALLOS MIOCENE VOLCANISM, NORTH-CENTRAL TRANS-MEXICAN VOLCANIC BELT

We report the results of a field, petrographic and geochemical survey conducted in the Cerro San Martín (CSM) and Sierra de los Caballos (SLC), in the state of Querétaro, México. These areas are among the northernmost exposures of the central Trans-Mexican Volcanic Belt (TMVB), and are located at the greatest distance from the modern trench (ca. 450 km). A middle to late Miocene age for these rocks was suggested by some scarce ages reported for the SLC and by correlation with nearby exposures. We dated an andesite lava from CSM at 9.0 ± 0.1 Ma (Ar-Ar, matrix),

Outcrops mainly consist of highly eroded lava flows and domes characterized by medium- to fine-grained, and porphyric textures. Rocks from CSM have phenocrysts of plagioclase, amphibole and clinopyroxene, and scarce orthopyroxene. In comparison, most samples from SLC have more abundant plg and opx phenocrysts, whereas amphibole can be absent. Only one plg- and opx-free, amph- and cpx-rich sample was found in this area.

Samples are classified as subalkaline, medium-K andesite and dacite. Trace elements allow recognizing a group of samples with higher Ni, Sr, La, La/Yb, Gd/Yb and Sr/Y, which in normalized multielement diagrams display marked negative Nb-Ta-Ti anomalies enrichments in LREE and MREE with respect to HREE, and small positive Sr and Pb anomalies. These samples also have higher Epsilon Nd (5.7-6.54), and lower ⁸⁷Sr/⁸⁶Sr_i (0.703303) values. These features, which are similar to those reported for adakitic rocks, dominate in samples from Cerro San Martín but were found only in one plg-free sample from Sierra Los Caballos. The adakitic character of these rocks is interpreted as the result of slab melting promoted by flat subduction; progressive slab flattening between early and late Miocene time would also account for the migration of volcanism away from the trench.

The characteristics of the second group, with lower Ni, Sr, La, La/Yb, Gd/Yb, less LREE enrichment, and marked positive Pb and Sr, and negative Nb-Ta anomalies, as well as lower Epsilon Nd (0.51-2.83), and higher ⁸⁷Sr/⁸⁶Sr_i (0.703455, 0.703606) values, indicates a different origin with a more important role of assimilated crustal material and minor contributions of fluids derived from the largely dehydrated flat subducting slab.