Reappraisal of the Geology and Geochemistry of Volcán Zamorano, Central Mexico: Implications for Discriminating the Sierra Madre Occidental and Mexican Volcanic Belt Provinces

Abstract

We report new geological, 40Ar/39Ar geochronology, and geochemical data (major and trace elements, and Sr, Nd, and Pb isotopic ratios) on andesitic to dacitic rocks collected from Volcán Zamorano and on rhyolitic ignimbrites from the surrounding area. These data better constrain the relationship of two major volcanic provinces of Mexico: the mid-Tertiary Sierra Madre Occidental (SMO) and the Miocene to Recent Mexican Volcanic Belt (MVB). Structurally, the SMO and MVB show different extensional styles. The SMO was affected by east-west extension mainly during the Oligocene and middle Miocene, whereas the MVB was affected by extensional deformation during middle-to-late Miocene and Plio-Quaternary times, although reactivation of the older structural systems, such as the NNW-SSE system, has occurred in more recent times. The new geological data from the study area are consistent with the 40Ar/39Ar dates. The andesitic and dacitic rocks from Volcán Zamorano are about 10 Ma. These rocks, along with nearby andesitic and basaltic volcanoes, mark a relatively early phase of the MVB. The basaltic to dacitic rocks show the following ranges for Sr, Nd, and Pb isotopic ratios: (87Sr/86Sr)i 0.70362-0.70474; (143Nd/144Nd)i 0.51258-0.51286; 206Pb/204Pb 18.38-18.81; 207Pb/204Pb 15.57-15.63; and 208Pb/204Pb 38.27-38.70. For SMO ignimbrites (∼30-28 Ma) cropping out in the Zamorano area, (87Sr/86Sr)i are slightly higher (0.70407-0.70557), and (143Nd/144Nd)i are somewhat lower (0.51250-0.51253). Pb-isotope ratios for these ignimbrites are also higher (about 18.84-18.87, 15.65, and 38.78-38.82, respectively) than for basaltic to dacitic rocks from Volcán Zamorano and the surrounding area. Most studied rocks from both provinces seem to contain a crustal component. The ignimbrites are chemically and isotopically similar to rhyolitic and ignimbritic rocks sampled from other areas of the SMO. Our study of Volcán Zamorano shows that the transition from SMO to MVB was rather sudden and not a gradual process.