ABSTRACT
In southern Mexico, the Tremadocian Tiñu Formation with a Gondwanan fauna rests unconformably upon the ~1 Ga Oaxacan Complex, and is unconformably overlain by Carboniferous sedimentary rocks containing a Laurentian fauna. The Tinu Formation is inferred to represent deposition along the southern margin of the Rheic Ocean in its early stages of development. The geochemistry of sandstones and shales of the formation show: (1) high iron, magnesium, and titanium, indicating chemical immaturity in some samples; (2) a positive correlation between Zr and SiO2; (3) derivation from crust with mafic and felsic components; (4) LREE enrichment with a moderate Eu anomaly; (5) low ɛNd(t) values (−7.0 to 7.8, calculated for t = 485 Ma); and (vi) Mesoproterozoic TDM ages (1.5–1.83 Ga). Taken together, these data indicate a proximal source with two main components, an ancient, felsic-intermediate, moderately differentiated continental component, and a minor mafic component. These data are consistent with derivation from the underlying Oaxacan Complex, with little or no input from distal or juvenile sources. This is in accord with: (1) published Nd data from the Oaxacan Complex that yielded TDM ages of 1.5–1.8 Ga; (2) detrital zircon data from the Tinu Formation, which gave 990–1200 Ma ages; and (3) the Gondwanan affinity of Silurian fauna from northeastern Mexico. The lack of a juvenile, Cambrian signature in these geochemical data is inconsistent with an origin in the iapetus Ocean, and is compatible with a rift-to-drift model involving opening of the Rheic Ocean.
