International Geology Review, Vol. 45, 2003, p. 724–752. Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved. 0020-6814/03/681/724-29 $10.00 724 Reappraisal of the Geology and Geochemistry of Volcán Zamorano, Central Mexico: Implications for Discriminating the Sierra Madre Occidental and Mexican Volcanic Belt Provinces SURENDRA P. V ERMA 1 Centro de Investigación en Energía, UNAM, Priv. Xochicalco s/no., Col. Centro, Apartado Postal 34, Temixco, Mor. 62580, Mexico AND GERARDO CARRASCO-NÚÑEZ Centro de Geociencias, UNAM, Campus Juriquilla, Apartado Postal 1-472, Querétaro, Qro., Mexico Abstract We report new geological, 40 Ar/ 39 Ar 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 sys- tems, such as the NNW-SSE system, has occurred in more recent times. The new geological data from the study area are consistent with the 40 Ar/ 39 Ar 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: ( 87 Sr/ 86 Sr) i 0.70362–0.70474; ( 143 Nd/ 144 Nd) i 0.51258–0.51286; 206 Pb/ 204 Pb 18.38–18.81; 207 Pb/ 204 Pb 15.57–15.63; and 208 Pb/ 204 Pb 38.27–38.70. For SMO ignim- brites (~30–28 Ma) cropping out in the Zamorano area, ( 87 Sr/ 86 Sr) i are slightly higher (0.70407– 0.70557), and ( 143 Nd/ 144 Nd) 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 isotopi- cally 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. Introduction THERE ARE TWO major Cenozoic volcanic provinces in Mexico (Fig. 1): the Sierra Madre Occidental (SMO) province of mostly rhyolites and rhyolitic ignimbrites erupted from Eocene (~51 Ma) to mid- dle Miocene (~20 Ma) time (McDowell and Keizer, 1977; Nieto-Samaniego et al., 1999), and the younger Mexican Volcanic Belt (MVB) province of lava flows of dominantly basic and intermediate compositions (Aguilar-Y-Vargas y Verma, 1987). The questions of when eruptions in the MVB started, what the physical and temporal boundaries between the SMO and the MVB provinces are, and how the structural styles of these provinces differ, remain as yet unsatisfactorily answered. Volcán Zamorano is located in the area where the mid-Tertiary SMO and Neogene MVB magmatic provinces overlap, and therefore is a key piece to understanding the spatial and temporal relation- ships between these two provinces (Fig. 1). This vol- cano, located in the north-central part of the MVB, lies at the intersection of major physiographic prov- inces (Mesa Central, SMOr—Sierra Madre Oriental, and MVB). It is one of the most distal volcanoes from the Middle America Trench, about 470 km, and one of the few MVB volcanoes that directly lie on SMO rocks. Volcán Zamorano belongs to a poorly 1 Corresponding author; email: spv@mazatl.cie.unam.mx