CONSTRAINTS ON THE ORIGIN OF ALKALINE AND CALC-ALKALINE MAGMAS FROM THE TUXTLA VOLCANIC FIELD, VERACRUZ, MEXICO

Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma in clude primitive basanites and alkali basalts, mildly alkaline Hy-norma tive mugearites and benmoreites, and calc-alkaline basalts and basalti c andesites. The primitive lavas are silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, var iable La/Yb with constant Yb at 6 to 8 times chondritic, and low Sr an d O and variable Pb and Nd isotopic ratios. The primitive magmas origi nated by increasing degrees of melting with pressure decreasing from g reater than 30 kbar to 20 kbar, in the garnet stability field. Another group of alkali basalts and hawaiites has lower Ni and Cr concentrati ons and higher Fe/Mg ratios, and was derived from the primitive group by crystal fractionation at pressures of several kbar. Incompatible tr ace elements in these silica undersaturated lavas show depletion in hi gh field strength elements (HFSE) relative to large ion lithophile ele ments, similar to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residua l HFSE-bearing phase in the source, but could be the result of generat ion from a source contaminated by fluids or melts from the subducted l ithosphere. The silica-saturated mugearites and benmoreites, and the c alc-alkaline basalts and basaltic andesites, were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations genera lly lower than in the silica-undersaturated lavas, and thus could not have been derived by crystal fractionation from the silica-undersatura ted alkaline magmas. Magmas parental to the silica-saturated magmas or iginated by higher degrees of melting at lower pressures than the prim itive magmas. Melting may have been promoted by an influx of fluid fro m the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotop ic data suggest that three components are involved in the generation o f TVF magmas: the mantle, a fluid from the subducted lithosphere, and continental crust. TVF alkaline lavas are similar to those erupted in the back-are region of the MVB and Japan, and show characteristics sim ilar to alkaline magmas erupted in the southern Andean volcanic are. T hese low degree melts reach the surface along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passa ge to the surface.