Sedimentary record of a Late Cretaceous volcanic arc in central Patagonia: petrography, geochemistry and provenance of fluvial volcaniclastic deposits of the Bajo Barreal Formation, San Jorge Basin, Argentina

Abstract

The Upper Cretaceous Bajo Barreal Formation (San Jorge Basin, Argentina) is a fluvial succession mostly composed of channel sandstones interbedded with thicker floodplain deposits dominated by tuffaceous strata. The goal of this contribution is to infer the provenance of the unit through combined petrographical and geochemical data of channel sandstones, primary tuffs and tuffo-psammites (reworked, no-mixed tuffs). Channel sandstones are dominantly litharenites and feldspathic litharenites with abundant participation of volcanic lithic fragments with different textures (porphyritic, eutaxitic, felsitic, pilotaxitic, trachytic and vitric-vitrophyric), pumice and plagioclase. K-feldspar, quartz and sedimentary rock fragments are scarce. QFLu diagram and several provenance indicators including dominance and textural types of rock fragments, K-feldspar/plagioclase ratio and inclusion-free quartz grains indicate a provenance from felsic to intermediate, pyroclastic-rich, arc-related volcanic rocks. This interpretation agrees with the values of various elemental ratios of trace elements such as La/Sc, Th/Sc, Cr/Th, Eu/Eu* and (La/Lu)N and discriminant functions based on major elements. Pyroclastic floodplain strata are commonly pumice-rich, occasionally glass shard-rich, vitric tuffs suggesting an origin from plinian-like eruptions. Subordinate components include volcanic lithic fragments, quartz, orthoclase and plagioclase. Zr/Ti versus Nb/Y and SiO2 versus K2O diagrams, as well as a multi-element diagram of selected trace elements shows that the pyroclastic rocks were originated from medium-K content, intermediate, arc related magmatic source. Considering the palaeoflow direction towards the east and south-east and presence of subduction processes along the western margin of Patagonia during the Late Cretaceous, it is interpreted that Bajo Barreal Formation constitutes the sedimentary record (distal facies of volcano-flanking fan or apron) of this coeval volcanic arc. Channel sandstones probably were mostly derived from the Divisadero Formation volcanic rocks because of similar petrographical features and REE pattern. In contrast, pyroclastic tuffaceous floodplain deposits would be derived from vents located over the actual outcrops of the Patagonian Batholith.