THE CHIVILLAS FORMATION IN SOUTHEAST OF MEXICO: MAGMATIC AND STRATIGRAPHIC EVIDENCE FOR BARREMIAN RIFTING OF THE GULF OF MEXICO

Tectonosedimentary information from geochemical and stratigraphic data in central southeast Mexico allowed us, propose a proto-oceanic rift that might have been the inland extension of the Gulf of Mexico Rift. The Chivillas Formation is a litostratigraphic key unit to decrypt the record of the Cuicateca Terrane; this formation accumulated in Tehuacán Basin. The succession of Chivillas formation consists of thick intervals of pillow lavas interbedded with siliciclastic turbidites and debrites containing clasts derived from metamorphic and sedimentary sources. Clast composition and detrital zircon geochronology indicate a continental origin, with sources located south of the studied area. Detrital zircon ages range from 1.573 ± 60 to 125 ± 1.6 Ma. We interpret the ~126 Ma (Barremian) age obtained for the youngest zircon population as the maximum depositional age, possibly associated with contemporary volcanism. Jurassic detrital zircons were probably derived from the Sierra de Juárez mylonitic complex. Other peaks suggest a source from the Permo-Triassic arc, represented by the granitic bodies of Altotonga, La Mixtequita and Chiapas that outcrop both north and south of the study area; and Grenvillian basement of Zapoteco Terrane.

Pillow lavas of the Chivillas formation are mostly alkaline basalts, with SiO2 from 46% to 53%, and alkalis (K2O+Na2O) from 5 to 8 wt. %; all samples have low-TiO2 (<1.6 wt.%) and low V (180–242 ppm), with Ti/V between 30 and 50. 206Pb/204Pb isotopic ratios vary between 18.6 and 20.5, and 208Pb/204Pb from 38.4 to 40.3, within OIB and MORB ranges. Initial εNd(126) values are 0.3 to 4.1, and TDM are 632–1520 Ma. Lava compositions are similar to alkaline basalts along the margins of the Atlantic Ocean, particularly to basalts from the Central Atlantic Magmatic Province (CAMP), and to the Cretaceous Peri-Atlantic Alkaline Pulse (PAAP).

We suggest that the Tehuacán rift basin might be the last jump of the Gulf of Mexico Ridge. Thus Sierra de Juárez Complex was a transform fault that accommodated the right-lateral displacement in relief of the Western Main Transform. In this model, the last stage of extension for the Gulf of Mexico might have been as young as Barremian in age.