A quaternary monogenetic volcanic field in the Xalapa region, eastern Trans-Mexican volcanic belt: Geology, distribution and morphology of the volcanic vents

The most abundant volcanic manifestations along the east-west trending Trans-Mexican Volcanic Belt (TMVB) are the scoria cones. These have been grouped by other authors in extended monogenetic volcanic fields such as Michoacan-Guanajuato, Chichinautzin. Apan and Los Tuxtlas. Here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the Cofre de Perote volcano (CP), around the city of Xalapa in the state of Veracruz, Mexico. Within an area of about 2400 km(2), the "Xalapa Monogenetic Volcanic Field" (XMVF) contains over 50 late Quaternary volcanoes. Most of them are scoria cones, but small shield volcanoes and tuff rings also occur. The lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of CP, between 3000 and 500 m a.s.l. The representative rocks of the different volcanic centers include olivine basalt, basaltic andesite with phenocrysts of plagioclase. pyroxene and minor olivine, and andesite with phenocrysts of plagioclase and pyroxene. SiO(2) and Al(2)O(3) contents of the rocks vary between 45 and 62 wt% and 15 to 18 wt%, respectively. Most of the basaltic rocks have MgO contents between 4.2 and 9 wt%, Ni and Cr concentrations between 23 and 180 and 10 to 380 ppm, respectively, with a typical calc-alkaline behavior. Trace elements suggest two types of magmas; the most abundant are characterized by an enrichment of LILE and LREE with negative anomalies of Nb and Ti, which denote a calc-alkaline affinity. Others are LILE depleted and show high concentrations of MgO, Cr, and Ni, which is typical of primary calc-alkaline magmas. The mean scoria cone morphological values are: cone height (Hco) = 90.8 m, cone diameter (Wco) = 686.38 m, crater diameter (Wcr) = 208.49 m and 0.12 km(3) for the cone volume. We dated twelve different scoria cones using the (40)Ar/(39)Ar method; for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter (Hco/Wco) and maximum slope angle. Additionally, vegetation type, and cone and lava flow morphologies were also considered in order to determine rough relative ages. In general, the ages fall into three groups. those older than 2.0 Ma, those between 0.25-0.40 Ma and those less than approximately 0.1 Ma. (C) 2009 Elsevier B.V. All rights reserved.