Geochemical and Sr-Nd Isotopic Evidence for a Rift-related Origin of Magmas in Tizayuca Volcanic Field, Central Mexican Volcanic Belt

New geochemical and Sr and Nd isotope data are presented on six selected rock samples of hy-normative late Quaternary basalt, basaltic trachyandesite, basaltic andesite, andesite, and dacite from Tizayuca Volcanic Field (TVF), located in the State of Hidalgo, Mexico, at the central part of the Mexican Volcanic Belt (MVB). This province (MVB) is a modern volcanic zone (Miocene to Recent), which in spite of ongoing subduction, presents widespread magmas whose origin is shown to be unrelated to subduction. The initial isotopic ratios of the sampled rocks, practically identical to their measured ratios, range as follows ⁸⁷Sr/⁸⁶Sr 0 70390-0 70540 and ¹⁴³Nd/¹⁴⁴Nd 0 512598-0 512907. There is a genera] increase in ⁸⁷Sr/⁸⁶Sr and a decrease m ¹⁴³Nd/¹⁴⁴Nd with increasing Si0₂ and decreasing MgO contents. The combined geochemical and isotopic evidence supports the generation of these magmas from the mantle or even in the lower crust, and subsequent modification through crustal assimilation processes. In spite of ongoing subduction of the Cocos plate, the TVF seems to owe its origin to rift-related processes as is the case of many other areas of the central and eastern MVB. A broader implication of the isotopic data from the MVB is that the basic assumption of geochronology of the same initial isotopic ratios seems to fall apart because none of the so far studied areas of the MVB shows a uniform Sr or Nd isotopic ratio Indeed, the ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios vary according to the rock-type for rocks of practically zero age.