Gabriela Solís-Pichardo

Tertiary arc-magmatism of the Sierra Madre del Sur, Mexico, and its transition to the volcanic activity of the Trans-Mexican Volcanic Belt

The Tertiary magmatic rocks of the Sierra Madre del Sur (SMS) are broadly distributed south of the Trans-Mexican Volcanic Belt (TMVB) and extend to the southern continental margin of Mexico. They represent magmatic activity that originated at a time characterized by significant changes in the plate interactions in this region as a result of the formation of the Caribbean plate and the southeastward displacement of the Chortis block along the continental margin of southwestern Mexico. The change from SMS magmatism to an E‐W trending TMVB volcanism in Miocene time reflects the tectonic evolution of southwestern Mexico during these episodes of plate tectonic rearrangement. The distribution and petrographic characteristics of the magmatic rocks of the SMS define two belts of NW orientation. The first is represented by the nearly continuous coastal plutonic belt (CPB), which consists of batholiths and stocks of predominantly felsic composition. The second belt is inland of the first and consists of discontinuously distributed volcanic fields with piles of andesitic to rhyolitic flows, as well as epiclastic and pyroclastic materials. These two belts were emplaced along a continental crust segment constituted by a mosaic of basements with recognizable petrologic and isotopic diAerences. These basements originated during diAerent tectono-thermal events developed from the Proterozoic to the Mesozoic. Major and trace element data of the SMS magmatic rocks define a clear sub-alkaline tendency. Variations in the general geochemical behavior and in the Sr and Nd isotopic ratios indicate diAerent degrees of magmatic diAerentiation and/or crustal contamination. These variations, specially in the inland Oligocene volcanic regions of Guerrero and Oaxaca states, seem to have been controlled by the particular tectonic setting at the time of magmatism. In northwestern Oaxaca greater extension related to transtensional tectonics produced less diAerentiated volcanic rocks with an apparently lower degree of crustal contamination than those of northeastern Guerrero. The geochronologic data produced by us up to now, in addition to those previously reported, indicate that the Tertiary magmatic rocks of the SMS range in age from Paleocene to Miocene. The general geochronologic patterns indicate a southeastward decrease in the age of igneous activity, rather than a gradual northeastward migration of the locus of magmatism toward the present-day TMVB. SMS magmatic rocks exposed to the west of the 1008W meridian are dominantly Late Cretaceous to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasing trend. Paleocene and Eocene magmatic rocks of the western region of the SMS seem to keep a general NNW trend similar to that of the Tertiary magmatic rocks of the Sierra Madre Occidental (SMO). In the eastern region of the SMS the Oligocene magmatic rocks show a trend that roughly defines an ESE orientation. The change in the trend of arc magmatism may be the eAect of the landward migration of the trench, for a given longitude, as a result of the displacement of the Chortis block. The

Miocene andesitic lavas of Sierra de Angangueo: a petrological, geochemical, and geochronological approach to arc magmatism in Central Mexico

ABSTRACT The early and middle Miocene andesitic lavas of the Sierra de Angangueo (MALSA) represent one of the most prominent landforms of intermediate magmatism in Central Mexico. In this paper, we present new petrological, geochemical, thermobarometric, and geochronological data in order to discuss the conditions of MALSA’s emplacement in the lithosphere. MALSA comprises a voluminous eroded early Miocene andesitic structure, emplaced on a Mesozoic basement. MALSA shows a wide variety of textures, from glassy and aphanitic to porphyritic, mainly composed of plagioclase (An10-55), clinopyroxene (Wo60-65; En35-40), amphibole (Mg-hornblende/edenite), and rarely olivine (Fo75–86) or orthopyroxene (En72-80). Major and trace elements plots follow a typical calc-alkaline trend with relatively increasing fractionation. The 87Sr/86Sr isotopic signatures range between 0.703343 and 0.704459 and εNd values from +1.37 to +4.84; apparently without a significant contribution of an old, thick, or highly radiogenic continental crust. Lead isotope values 206Pb/204Pb vary between 18.68 and 18.83, 207Pb/204Pb from 15.57 to 15.65, and 208Pb/204Pb from 38.39 to 38.67, suggesting contamination of magmas by juvenile crust. Thermodynamic calculations indicate equilibrium conditions at ≤900°C and ~2 kbar and an average log ƒO2 ≈ −10. Ar–Ar and K–Ar dating carried out on whole rock, matrix, amphibole, and plagioclase phenocrysts yielded ages between 13.0 ± 0.5 and 23.9 ± 0.3 Ma. These dates indicate a series of pulses with maximum magmatic activity between 24 and 18 Ma. The geochemical and petrologic data suggest partial melting processes in the lower or middle crust associated with possible magma mixing during its ascent; such a mechanism could explain a hybrid mantle-lower crust origin of this large volume of andesites. The MALSA, as well as the early to middle Miocene magmatism from Western Mexico, could represent two coeval and independent magmatic arcs prior to the present Trans-Mexican Volcanic Belt (TMVB) in Central Mexico.

Strontium isotopes and mobility of a Columbian mammoth (Mammuthus columbi) population, Laguna de las Cruces, San Luis Potosí, México

By using strontium isotopic ratios of dental enamel from molars, we were able to reconstruct the migration context for three individuals of a Columbian mammoth population ( Mammuthus columbi ) around Laguna de las Cruces, San Luis Potosi, central Mexico. A three-step leaching procedure was applied to eliminate secondary Sr contributions in the molar enamel. One of the studied individuals showed 87Sr/86Sr ratios similar to those obtained from soils and plants from Laguna de las Cruces and was identified as local, whereas the other two mammoths had different molar 87Sr/86Sr values, indicative of migration and mobility contexts.