Origin of rhyolitic lavas in the Mesa Central, Mexico, by crustal melting related to extension

Abstract

The emplacement of a voluminous sequence of rhyolitic lava flows and domes characterizes Oligocene volcanism in the Mesa Central (MC) of Mexico. Its dominant effusive style of emplacement contrasts deeply with the predominantly explosive volcanism of the Sierra Madre Occidental Volcanic Province toward the west. Whole rock geochemical (major- and trace elements) and Sr–Nd isotopic data of the MC Oligocene rhyolitic lavas document a marked change in magma composition at around 30 Ma, allowing us to distinguish between a lower and an upper sequence. Lavas from the lower sequence are geochemically similar to the high-K rhyolitic rocks of the eastern Sierra Madre Occidental. Major- and trace-element variations are characteristic of mantle-derived magmas evolving through fractional crystallization. The initial 87Sr/ 86Sr and ϵ Nd values are nearly constant (0.70644–0.70770 and −1.2 to −2.1 respectively) and indicate some contribution from crustal material. Lavas from the upper sequence are high-silica, peraluminous rhyolites, with strong enrichment in fluorine and in some incompatible lithophile elements (Rb, La, Sm, Yb, Y, Th, U, Nb, Ta), and strong depletion in the feldspar-compatible elements Sr, Ba, Eu. Initial 87Sr/ 86Sr ratios of the upper sequence lavas are high and variable (0.70812–0.72190), and decrease as silica content increases, whereas the ϵ Nd values are relatively constant (−1.4 to −2.8). The trace element behavior indicates an origin by variable degrees of non-modal partial melting of granulitic low-crustal rocks and chemical disequilibrium during melting processes. The high and variable Sr isotopic ratios could also be related to isotope disequilibrium melting processes if the isotopic heterogeneities between individual mineral phases were preserved during heating of the source rocks. The changes in geochemical compositions are related to the onset of crustal extension at high strain rates documented for the MC. Crustal extension promoted crustal melting at high melting rates, high melt segregation rates, rapid ascent of low-viscosity fluorine-rich magmas, and inhibited melt stagnation in magma chambers. Such conditions favored the effusive volcanic style and support the possibility of melting under disequilibrium conditions.