DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE TOLIMAN SEQUENCES, CENTRAL MEXICO: POSSIBLE PROVENANCE SOURCES

The Toliman area is located in the central Queretaro state. Is a interesting geological zone because it is situated between two tectonics settings: the Sierra Madre Oriental and the Guerrero arc. The stratigraphy of Toliman area consists of Pre-Jurassic, foliated sandstone and shale (El Chilar Complex), overlain by a Early Cretaceous volcanosedimentary sequence (San Juan de la Rosa Fm.). These volcaniclastic rocks are overlain by early Late Cretaceous calcareous debris (Peña Azul Fm.). The whole sequence have experienced at least two phases of compressive deformation. Nine samples (four in sandstone, four in vulcanosedimentary rocks and one in calcareous debris) were analyzed by U-Pb geochronology on zircons, using LA-ICP-MS (Laser ablation–inductively coupled plasma–mass spectrometry), to constraints the depositional history and provenance of the sediments. The results obtained for sandstone samples shows three populations of concordant zircons at ~243-300 Ma, ~450-750 Ma and ~900-1250 Ma, with the youngest zircon grains at 243-250 ±3 Ma and oldest zircons at ~1600-2500 Ma. The youngest zircon population may have come from rocks derived from the Permo-Triassic continental arc of east and central Mexico. The second, less abundant zircon population, could be derived from Paleozoic-Panafrican units such those found elsewhere in the Acatlán Complex and/or Maya Block. The oldest and most abundant zircon population, may have been derived from the Oaxaquia Grenvillian basement. The Maximum depositional age for the volcanosedimentary rocks of San Juan de la Rosa Formation was ca. 130 Ma. Most of the zircons are Early Cretaceous to Late Jurassic (134-175 Ma) and igneous in origin. They are also devoid of inherited xenocrystic zircons, which possibly suggests that those zircons derived from a nearby source. The older zircon populations (280-500 Ma and 900-1100 Ma) are similar to those found in the underlying Chilar Complex. Finally, the maximum depositional age obtained for one sample of Peña Azul Formation is 126 ± 2 Ma, the most important zircon population averaging 136 Ma., which is also poor in older zircons as a possible consequence of sea transgression and minor erosion at that time.