Origin of andesite in the deep crust and eruption rates in the Tancitaro-Nueva Italia region of the central Mexican arc

Abstract

The Tancítaro-Nueva Italia region (~4400 km 2 ) is located within the west-central Mexican volcanic belt and spans ~80 km of arc length along the volcanic front. On the basis of 65 40 Ar/ 39 Ar ages, it is shown that a total volume of ~326 ± 57 km 3 of magma has erupted from >200 vents since 1.2 Ma. Nearly a third of this volume (103 ± 5 km 3 ) was erupted as crystal-rich andesite from a large stratovolcano, Volcán Tancítaro, whereas the remaining two-thirds (~223 ± 52 km 3 ) span the complete range from 51 to 63 wt% SiO 2 , are relatively crystal-poor, and were erupted from peripheral vents. The total proportions of erupted lava types are ~6%-7% basalt, ~18% basaltic andesite, and ~74%-76% andesite; there is no rhyolite and only a trace of dacite.There is no evidence that any signifi cant magma differentiation occurred in the upper crust along this arc segment. Based on phenocryst abundances and modes, majorand trace-element data, as well as phaseequilibrium experiments from the literature, the following mechanisms for formation of andesite in this region can be ruled out:(1) crystal fractionation of basalt/basaltic andesite in the upper crust, (2) magma mixing of basalt/basaltic andesite with dacite/ rhyolite, and (3) assimilation of granitoid by basalt/ basaltic andesite. Instead, the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting (>20%) of hornblende-rich (~40%) gabbronorite. The basaltic andesites are required, owing to their elevated Ni concentrations, to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two-pyroxene fractionation in the deep crust.

Reconstruction of the volcanic history of the Tacámbaro-Puruarán area (Michoacán, México) reveals high frequency of Holocene monogenetic eruptions

“…Eruption rate per area (or accumulation rate) over the last 1 Myr in the TAC is >2.5 km 3 /Myr/100 km 2 . This value is higher in the monogenetic area surrounding the Tancítaro stratovolcano to the NW (~4.2 km 3 /Myr/100 km 2 ; Ownby et al 2011), but this estimate includes the volume of buried flows (<70 % of the total volume estimate), which was not considered in the present study. Eruption rates in both regions may thus be similar.…”

“…By contrast, the process of generation of the crystal-poor andesites typically produced by monogenetic vents in the TMVB remains debated. Ownby et al (2008Ownby et al ( , 2011 propose an origin by partial melting of the deep crust, whereas other authors favor a process of polybaric fractionation±contam-ination of the more primitive, mantle-derived melts (Hasenaka and Carmichael 1987;Moore and Carmichael 1998;Siebe et al 2004b;Schaaf et al 2005). Clearly, additional geochemical data (especially radiometric dating) are needed to address the origin of temporal and spatial variations in the proportion of andesites and other rock types across the belt.…”

“…A general consensus exists that basalts and basaltic andesites in the TMVB are derived from hydrated mantle melts that underwent little modification by crystal fractionation and/or crustal contamination during transit to the surface (e.g., Moore and Carmichael 1998;Ownby et al 2008Ownby et al , 2011Carmichael 2002;Weber et al 2011). Their predominance in the Tacámbaro area is thus compatible with the observed high magma eruption rates, the elevated H 2 Ocontents of primitive melts generated in the southern part of the MGVF (Johnson et al 2009) and the existence of crustal structures that favor rapid magma ascent.…”

Extreme Volcanic Risks 1

“…The shield volcanoes of this region are very different from other parts of the world due to their chemical composition, size, and origin. Paracho volcano is an example of a composite volcano w60,000 years old, and Tancítaro volcano is an example of a stratovolcano, whose approximate age is 530,000 years BP but with Holocene eruptive activity (Ban et al, 1992;Hasenaka et al, 1994;Ownby et al, 2007;Ownby et al, 2011). The eruptive activity in this field is recurrent and further eruptions in the region are expected in the future, either by the emergence of new cinder cone, from a shield or composite volcanoes and stratovolcanoes.…”

40Ar/39Ar geochronology of Volcan Tepetiltic, western Mexico: Implications for the origin of zoned rhyodacite-rhyolite liquid erupted explosively from an andesite stratovolcano after a prolonged hiatus

“…This long-term rate, which may have included one or more episodes of rapid cone-building interspersed with lesseffusive time periods, falls within the broad range of those documented (0.1-0.7 km 3 /k.y.) at other arc central volcanoes (e.g., Hildreth and Lanphere, 1994;Gamble et al, 2003;Lewis-Kenedi et al, 2005;Bacon and Lanphere, 2006;Jicha and Singer, 2006;Hora et al, 2007;Singer et al, 2008;Schmidt and Grunder, 2009;Ownby et al, 2011).…”