Altered volcanic ash layers of the Late Cretaceous San Felipe Formation, Sierra Madre Oriental (Northeastern Mexico): U[sbnd]Pb geochronology, provenance and tectonic setting

Abstract

A detailed petrographic, geochemical, and UPb geochronological study of altered volcanic ash layers, collected in eight outcrops of the Late Cretaceous San Felipe Formation (Sierra Madre Oriental, Northeastern Mexico), has been carried out. The main objectives have been: (1) to establish a deposit period, and (2) to propose a reliable provenance-transport-deposit-diagenetic model. These volcano-sedimentary strata represent the altered remains of vitreous-crystalline ash (main grains: quartz + K-feldspar (sanidine) + Na-plagioclase + zircon + biotite; groundmass: glass + calcite + clinochlore + illite) deposited and preserved in a shallow, relatively large in area, open platform environment. Major and trace element geochemistry indicate that parent volcanism was mainly rhyodacitic to rhyolitic in composition. Discrimination diagrams suggest a link to continental arc transitional to extension tectonic setting. UPb geochronology in zircon has revealed that the volcanic ash was released from their sources approximately during the range 84.6 ± 0.8 to 73.7 ± 0.3 Ma, being transported to the depocenters. Burial diagenesis process was marked by: (a) a limited recycling, (b) the partial loss of original components (mainly K-feldspar, plagioclase, biotite and glass), and (c) the addition of quartz, calcite, illite and clinochlore. The location of the source area remains uncertain, although the lack of enrichment in Zr/Sc ratio suggests that ashes were subjected to relatively fast and short-distance transport process. El Peñuelo intrusive complex, at 130–170 km west of the depocenters, is the nearest known zone of active magmatism during the Upper Cretaceous. This intermediate to felsic pluton, characterized by a geochemical affinity to post-orogenic tectonic setting, could be linked to the volcanic sources.