Kinematics and U-Pb dating of detrital zircons from the Sierra de Zacatecas, Mexico

The Mesozoic succession of the Sierra de Zacatecas (SZ) was originally described in two sequences. The older is the La Pimienta Phyllite or sometimes termed Zacatecas Formation (ZF) of presumably Late Triassic age based on paleontological constraints obtained at the beginning of the twentieth Century. Overlying the ZF is Las Pilas Volcanosedimentary Complex (LPYC) of unknown age, whose contact has been interpreted to be tectonic. We have studied these sequences in detail in order to constrain their stratigraphic relationships, deformation style, composition, and maximum depositional age. The ZF has been metamorphosed under greenschist facies conditions and its protolith is composed of sandstone, mudstone, limestone and rare conglomerates interlayered with lava flows and tuffs. This unit is intruded by dikes, sills, laccolithic bodies and we interpret the presence of hydrothermal vents at the type locality in the Arroyo El Bote. The ZF is in our interpretation in gradual contact with the LPVC. The LPVC is an igneous assemblage mainly made up of mafic pillowed to massive flows, commonly foliated and/or deformed. Contemporaneous laccolithic-like bodies, dikes and sills are observed. Feldespatic greywackes and wackes, mudstone and minor tuffs and limestone are interbedded within the lava flows. U-Pb dating of detrital zircons from ZF and LPVC from the Arroyo El Bote and the Sauceda de la Borda areas, respectively, yielded similar to 132 Ma and similar to 160 Ma old ages. Approximately 30% of the zircons from the ZF represent Paleozoic and Proterozoic inheritance, which is lacking in the LPVC, and suggests a change in the sedimentary source. The maximum depositional age for both sequences in this area is therefore interpreted as Early Cretaceous (Hauterivian), in contrast to the previous proposed Late Triassic age. According to the distribution and types of rocks, the volcano-plutonic and sedimentary assemblage may have been emplaced in a volcanic field that developed in an intra-arc or back-arc basin with sedimentary supply derived from a nearby continental block and a magmatic arc. The geometric analysis of foliation shows that the average poles of the ZF and LPVC are 86 degrees/235 degrees and 86 degrees/244 degrees respectively. The paleostress analysis yield an average sigma 1 for the thrust faults at 01 degrees/219 degrees suggesting that foliation and reverse faulting should have been developed contemporaneously during the Laramide Orogeny. Normal faults formed during a later extensional regime are associated to an average sigma 3 vector oriented 00 degrees/029 degrees. Since normal faults always cut the thrust faults, we interpret that normal faults developed in a post-orogenic event.