Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions |
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| Authors: | Paul J Wallace Ian S E Carmichael |
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| Institution: | (1) Ocean Drilling Program and Department of Geology and Geophysics, Texas A&M University, College Station, TX 77845, U.S.A. Tel.: (409) 845-0879; Fax: (409) 845-0876; e-mail: paul_wallace@odp.tamu.edu, US;(2) Department of Geology and Geophysics, University of California at Berkeley, Berkeley, CA 94720, U.S.A., US |
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| Abstract: | The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary
eruptive vents in addition to the large, composite volcanoes of Popocatépetl, Iztaccíhuatl, and Nevado de Toluca. The eruptive
vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous
in the Sierra Chichináutzin that forms the southern terminus of the Valley of Mexico. The Chichináutzin volcanic field (CVF)
is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The
crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc
worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites,
andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO,
and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of
incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline
rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle
wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements.
Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for
those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low
CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust.
In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type
alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the
hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from
behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial
variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities
that are related to previous extraction of alkaline to sub-alkaline basaltic melts.
Received: 23 June 1998 / Accepted: 23 December 1998 |
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