Southeastern extent of the North American craton in Texas and northern Chihuahua as revealed by Pb isotopes
The geographic patterns of Pb isotopic compositions of Eocene to Miocene igneous rocks appear to delineate the southeastern edge of the North American Precambrian craton in Trans-Pecos Texas and northern Chihuahua, Mexico. The boundary parallels and lies southeast of the buried fault contact between Ouachita facies thrust sheets and Paleozoic cratonic sedimentary rocks. Lead isotopic data for basalts and granulite xenoliths suggests that the basalts contain a mixture of Pb from mantle and lower-crustal sources. On the northwest side of the boundary, this mixing involves 1.35 to 1.1 Ga cratonic lithosphere and yields linear arrays of 206Pb/204Pb versus 207Pb/204Pb and 208Pb/204Pb. Basaltic magmas on the southeast side of the boundary apparently interacted with more-radiogenic lithosphere accreted during the Ouachita orogeny. Basalts in a 50-km-wide central zone between the northwestern and southeastern provinces have Pb isotopic compositions that are intermediate between those of the northwestern and southeastern sources. In the northwestern province, lead isotopic compositions of intermediate to felsic rocks are less radiogenic than those of associated basalts, whereas in the southeast province they have more radiogenic compositions than the basalts. This isotopic divergence reflects assimilation of contrasting crust of each province.

Most felsic Tertiary rocks of the northwestern province have lower 207Pb/204Pb versus 206Pb/204Pb and somewhat elevated 208Pb/204Pb versus 206Pb/204Pb, suggesting assimilation of relatively high Th/U Precambrian crust. The central province also contains low 206Pb/204Pb and 207Pb/204Pb Precambrian components, but few igneous rocks have elevated 208Pb/204Pb. Southeastern-province crust appears dominantly Phanerozoic, although Precambrian basement with Pb isotopic compositions near model average crust may be present. The Pb isotopic zonation observed in West Texas is similar to that found in the Appalachians of the northern United States and Canada and in the Caledonides of Europe: that is, a progression from more-radiogenic outboard terranes to less-radiogenic inboard terranes.

Lead isotopic ratios from Tertiary igneous rocks of the northwestern part of Trans-Pecos Texas are similar to those of adjacent New Mexico and southeastern Arizona, although regional basement ages and isotopic characteristics differ. Extending the West Texas isotopic terranes to the southwest holds promise for testing and improving the understanding of the Sonora-Mojave megashear; Precambrian assembly of Gondwana; and correlation of Grenville-Caledonian terranes between the Appalachians, Texas, and eastern to southern Mexico.