PROVENANCE OF TRIASSIC(?)–JURASSIC RIFT-BASIN SUCCESSIONS, HUIZACHAL–PEREGRINA AND HUAYACOCOTLA UPLIFTS OF EASTERN MEXICO

Detrital zircon U-Pb data, including depth profiling of individual zircon grains, of Triassic(?)–Middle Jurassic rift successions in northeastern and east-central Mexico indicate dominant sediment sources in local rift-flanking uplifts. The Upper Triassic(?)–Middle Jurassic El Alamar and La Boca formations in the Huizachal-Peregrina uplift of northeastern Mexico underlie Middle to Late Jurassic conglomerate and evaporite. In the Huayacocotla uplift to the south, correlative strata of the Huizachal, Huayacocotla, and Cahuasas formations underlie Oxfordian carbonate strata. Sandstone is consistently lithic-quartzose to quartzolithic with 10-50% metamorphic lithic grains, which range from weakly foliated pelites to quartz-white mica phyllite and schist fragments. Sedimentary lithic fragments, commonly siltstone with abundant hematite cement, dominate some samples of the upper Cahuasas Formation. Volcanic lithic fragments are abundant in the lower La Boca Formation, but elsewhere are uncommon. Sandstone composition thus demonstrates sediment sources in metamorphic basement rocks, sedimentary rocks, and local silicic to intermediate volcanic rocks, consistent with derivation from local rift blocks, some with incompletely eroded Paleozoic sedimentary and volcanic sections.

Detrital zircon U-Pb ages indicate Gondwanan and peri-Gondwanan sediment sources for the rift successions. Zircon ages fall into five age groups: (a) Grenville grains (~1200-950 Ma), dominant in most samples; (b) Neoproterozoic grains (~900-500 Ma), usually present, and common in some Huayacocotla and Cahuasas samples; (c) early Paleozoic grains (~490-440 Ma), present in all samples, and abundant in some; (d) Permian grains (~300-250 Ma), common in most samples; and (e) Early and Middle Jurassic grains (~200-160 Ma), common in Cahuasas and La Boca samples. Petrography and zircon U-Pb grain ages thus corroborate local sediment sources in Grenville basement of Oaxaquia (eastern Mexico), the northern extent of the early Paleozoic Famatinian arc, the Permian-Early Triassic East Mexican arc, and Jurassic backarc magmatic sources. Young grain ages are typically absent from the lower part of the rift succession; therefore, maximum depositional ages must be viewed with caution as indicators of true depositional ages.