Nature of the crust beneath west-central Chihuahua, Mexico, based upon Sr,Nd, and Pb isotopic compositions at the Tomochic volcanic center

In regions where crystalline basement is not well exposed, radiogenic isoto pe data from high-level igneous rocks can provide important information abo ut the underlying crust, even if only modest amounts of crustal assimilatio n have occurred. One such region is northwestern Mexico, where col er seque nces and younger tectonic features obscure the southern margin of the North American craton. Within the heart of the Sierra Madre Occidental volcanic field in northwestern Mexico, ranges of isotope ratios from the 38-29 Ma To mochic volcanic center in west-central Chihuahua, are epsilon Nd, +2.3 to - 5.2 (with only one value < -3.0); Sr-87/Sr-86, 0.7043-0.7089 (with only one value > 0.7070); Pb-206/Pb-204, 18.49-18.80; Pb-207/Pb-204, 15.56-15.62; a nd Pb-208/Pb-204, 38.35-38.79. These values are unlike those of xenolith-be aring, primitive, mantle derived magmas from either the southwestern United States or northern Mexico, and are interpreted to reflect modest crustal i nteraction, even in the least evolved compositions (basaltic andesite). Vol canic rocks of the Tomochic volcanic center have less radiogenic Sr and mor e radiogenic Nd than rocks from calc-alkaline, mid-Tertiary, ash-flow tuff provinces in the southwestern United States that were erupted through the 1 .8-1.0 Ga North American Precambrian craton (Laurentia). The Ph isotopic co mpositions of the Tomochic volcanic center have a restricted range and clos ely resemble those for volcanic rocks in eastern Chihuahua and west Texas t hat were erupted through the basement of the Coahuila block, which is outbo ard of the Ouachita front in the region. These results eliminate the possib ility that Laurentia continues southward beneath the Tomochic area. We sugg est instead that the younger basement to the south of the Ouachita front co ntinues into western Chihuahua, although a regional isotopic study is neede d to demonstrate the continuity of this configuration.