ABSTRACT
Paleozoic rocks exposed in northeast Hidalgo State, Mexico, have traditionally been assigned to the Guacamaya Formation, a orogenic flysch assemblage associated with the collision of South and North America during the formation of Pangea. However, major differences exist in the stratigraphy of these rocks and those of the Guacamaya Formation at its type section. The rocks of Hidalgo are here redefined as the Tuzancoa Formation, and comprise interbedded submarine andesitic-basaltic lava flows, siliciclastic turbidites, volcaniclastic turbidites, calcareous debris-flows, and lenses of conglomerate. The Tuzancoa Formation contains abundant Permian (Wolfcampian–Leonardian) fossils.
The lavas and volcaniclastics are andesitic to basaltic in composition, and are related to subduction. Their REE patterns are similar to those of recent island arc magmas, with slight enrichment in LREE, flat HREE, and no Eu anomaly. The εNd(280) of +4.38 in a representative sample is also similar to island arcs and suggests little crustal contamination. However, field evidence indicates that the arc was built upon Precambrian continental crust. Thus, we infer that the “primitive” arc geochemical signatures reflect emplacement on thin crust or under extensional conditions. The Tuzancoa Formation is interpreted to have been deposited in an intra-arc basin, and is probably related to rocks of the Las Delicias Formation in Coahuila and to the Guacamaya Formation in Tamaulipas. All are thought to have formed within a continental arc that extended throughout east-central Mexico in the Early Permian.
