U-PB DETRITAL ZIRCON GEOCHRONOLOGY AND PROVENANCE STUDY OF THE SILICICLASTIC MOJINA FORMATION IN CHIHUAHUA, MEXICO: AN INDIRECT APPROACH TO DELIMITING PRECAMBRIAN BASEMENT PROVINCES IN SW LAURENTIA

Sierra El Cuervo and Cerro Carrizalillo are the only outcrops of Precambrian igneous and metamorphic rocks in Chihuahua. This complicates any attempt to delimiting Precambrian basement provinces along the SW margin of Laurentia. However, using U-Pb detrital zircon geochronology from younger siliciclastic units and dating clasts of basement rocks present in conglomeratic layers are indirect but useful approaches to reduce this rock scarcity problem.

In Sierra Mojina, located in north-central Chihuahua, there is a siliciclastic fluvial succession, the Mojina Fm, made up of ~200 m of shale, sandstone and polymictic conglomerate. Previous studies suggested that the Mojina Fm was Late Permian (SGM, 1998), but our detrital zircon (DZ) geochronology supports the Cretaceous age initially proposed by Haenggi (2001). Our clast imbrication measurements also suggest a south- to north-directed transport for the detrital components. Detrital zircon (DZ) grains (150 points) dated from sandstone of the Mojina Fm include concordant grains with Archean (~2.7–2.5 Ga), Paleoproterozoic (~1.85 Ga and ~1.75 Ga), Mesoproterozoic (~1.4 Ga and ~1.2–1.1 Ga), Neoproterozoic (~900–565 Ma), Cambrian to Devonian (~500–410 Ma), Permo-Triassic (~283–210 Ma), Late Jurassic (~162 Ma) ages and two younger grains at 148 and 144 Ma. These Berriasian zircon grains are interpreted as the maximum depositional age for the Mojina Fm, which is thus correlative with Lower Cretaceous Las Vigas Fm. Aptian marine limestones of the Cupido and La Peña Formations concordantly overlie the Mojina Fm rocks. We dated zircons from two conglomerate clasts, a metarhyolite with a crystallization age at 1071 ± 15 Ma and a subarkosic metasandstone with two DZ populations at ~1.12 Ga y ~1.27 Ga and with inherited zircon cores at ~1.4 y ~1.6 Ga. Based on this DZ signature and petrography, the metasandstone could be a metamorphosed equivalent to the Upper Cambrian-Lower Ordovician Falomir Sandstone present in Cerro Carrizalillo (Bliss sandstone equivalent). This study indicates that Precambrian basement and lower Paleozoic supracrustal rocks were exposed south of Sierra Mojina during the Early Cretaceous; however, the DZ signature from the Mojina sandstone requires provenance from both SW Laurentian and Perigondwanan basement sources.