EARLY AND MID-PROTEROZOIC CRUST OF THE COAHUILA TERRANE PRESERVED IN XENOLITHS AT CASCADA DE BASASEACHIC, SOUTHWESTERN CHIHUAHUA

We recognize two types of xenoliths in the Cascada tuff at Cascada de Basaseachic in southwestern Chihuahua. Type 1 xenoliths comprise the bulk of the xenoliths found. They are accidental igneous inclusions that preserve both plutonic and volcanic textures; the most common, however, are coarse-grained granitic xenoliths. These inclusions have ages coeval with that of the Cascada tuff (30 Ma). Type 2 xenoliths comprise a variety of metamorphic inclusions with variable degrees of gneissic layering and are presumed to represent samples of the basement. Analyzed Type 2 xenoliths have SiO2 between 66.9 and 73.7 %, mg# from 0.395 to 0.226, and are q, or, pl, hy, mt, il, ap and c normative. They are LREE enriched, with (La/Yb)N from 7.95 to 20.06 and moderate Eu anomalies. Spidergrams for Type 2 xenoliths are LIL enriched, exhibit positive Pb anomalies, and Nb, Ta, P and Ti depletions. Type 2 xenoliths have 87Sr/86Sr from 0.74018 to 0.74265, eNd from -14.61 to -15.70, 206Pb/204Pb from 17.851 to 18.759, 207Pb/204Pb from 15.572 to 15.681, and 208Pb/204Pb from 39.055 to 39.977. The Sr and Nd isotopic compositions of Type 2 xenoliths are considerably more evolved than those of either Type 1 xenoliths or of mid-Tertiary volcanic rocks of the region. The Type 2 xenoliths have Pb compositions that partially overlap that of Type 1 xenoliths and of the mid-Tertiary volcanic rocks in the region, although there is a strong tendency toward higher 207Pb/204Pb and 208Pb/204Pb relative to these other units. U-Pb geochronology of one Type 2 xenolith yielded a chord with intercepts at 43 Ma and 1.5 Ga, whereas another Type 2 xenolith yielded a chord with intercepts at 1.1 and 1.8 Ga. These data indicate that early and mid-Proterozoic crust is present at depth in southwestern Chihuahua and that portions of this crust have undergone strong metamorphism around 1.1 Ga. Although Grenvillian reworking of ca. 1.8 Ga crust is unusual in the SW United States and Sonora, it has been observed to the east in basement from the Coahuila terrane. The interpretation that Type 2 xenoliths are fragments of the Coahuila terrane is consistent with isotopic studies of Late Cretaceous and mid-Tertiary igneous rocks that also suggest that the Coahuila terrane extends in a swath across central Chihuahua and into eastern Sonora.