Constraints on the origin of alkaline and calc-alkaline magmas from the Tuxtla Volcanic Field,Veracruz, Mexico |
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Authors: | S A Nelson Erika Gonzalez-Caver T Kurtis Kyser |
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Institution: | (1) Department of Geology, Tulane University, New Orleans, LA 70118, USA, US;(2) Department of Geological Sciences, University of Saskatchewan, Saskatoon, S7N OWO SK, Canada, CA |
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Abstract: | Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma include primitive basanites and alkali basalts, mildly
alkaline Hy-normative mugearites and benmoreites, and calc-alkaline basalts and basaltic andesites. The primitive lavas are
silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, variable La/Yb with constant
Yb at 6 to 8 times chondritic, and low Sr and O and variable Pb and Nd isotopic ratios. The primitive magmas originated by
increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar, in the garnet stability field.
Another group of alkali basalts and hawaiites has lower Ni and Cr concentrations and higher Fe/Mg ratios, and was derived
from the primitive group by crystal fractionation at pressures of several kbar. Incompatible trace elements in these silica
undersaturated lavas show depletion in high field strength elements (HFSE) relative to large ion lithophile elements, similar
to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residual
HFSE-bearing phase in the source, but could be the result of generation from a source contaminated by fluids or melts from
the subducted lithosphere. The silica-saturated mugearites and benmoreites, and the calc-alkaline basalts and basaltic andesites,
were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations generally lower than in the silica-undersaturated
lavas, and thus could not have been derived by crystal fractionation from the silica-undersaturated alkaline magmas. Magmas
parental to the silica-saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas.
Melting may have been promoted by an influx of fluid from the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotopic
data suggest that three components are involved in the generation of TVF magmas: the mantle, a fluid from the subducted lithosphere,
and continental crust. TVF alkaline lavas are similar to those erupted in the back-arc region of the MVB and Japan, and show
characteristics similar to alkaline magmas erupted in the southern Andean volcanic arc. These low degree melts reach the surface
along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passage to the surface.
Received: 15 September 1994/Accepted: 14 February 1995 |
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