Generation of high-silica rhyolite: A Nd, Sr, and O isotopic study of Sierra La Primavera, Mexican Neovolcanic Belt

Abstract

High-silica rhyolites of the Sierra La Primavera, a late Pleistocene center near Guadalajara, are extremely Sr-poor (0.3–1.3 ppm), yet (with one exception) values of 87 Sr/ 86 Sr i are relatively low at 0.7041–0.7048. Values of 143 Nd/ 144 Nd for all the rhyolites are (within errors) identical to a basalt at 0.5129. These surprisingly primitive values, along with feldspar ∂ 18 O of +6.6‰, are consistent with an origin by fractional crystallization of mantle-derived basalt. However, absence of the large volume of associated intermediate rocks that would be expected if the 40 km 3 of erupted rhyolite were produced mainly by fractional crystallization suggests alternative processes involving partial melting of Mesozoic or Tertiary mafic intrusive rocks (or lower-crustal metamorphic equivalents). The latter interpretation is preferred, especially in light of comparative data for other North American, Cenozoic, high-silica rhyolites. Isotopic compositions correlate with basement age, but generally lie between values for associated basalts and the underlying crust. Nearly all can be interpreted as containing both a young mantle-derived component and a crustal component, probably derived by partial melting at intermediate to deep levels of the crust. No matter what the proportions of mantle- and crust-derived material in parental magmas, the extremely low concentrations of Sr and Ba in the high-silica rhyolites require extensive fractional crystallization of feldspar-rich assemblages after parental liquids attain rhyolitic compositions.