Pre-Pampean metasedimentary rocks from the Argentinian Puna: Evidence for the Ediacaran margin of Gondwana or the Arequipa–Antofalla–Western Pampeanas block

Abstract

Quartzites exposed in the southern Puna (northwest Argentina) form faulted contacts with Late Ordovician volcaniclastic rocks and are discordantly overlain by Permo-Carboniferous red-beds. These strongly recycled rocks lack sedimentary structures and fossils. Therefore, a geochemical approach was preferred for this study. U–Pb isotope analysis of detrital zircon yielded an unusual age population. Ten grains lie within error of 570 and 590Ma—with the youngest detrital zircon at 571±14Ma—and are therefore interpreted as defining the maximum sedimentation age range.There is no evidence of Pampean orogeny event detritus (c. 550–520Ma). Therefore, we argue that these rocks are unlikely to be syn-tectonic with the event, or even younger. Alternatively, it can be argued that this component is absent due to differences in paleocurrent direction, sorting during transportation, or a hypothetical structural high; all of which could have blocked the Pampean provenance signal from reaching the depositional basin. Sorting related to the depositional environment can be excluded as the sample is a combination of all occurring grain sizes. There is no preserved geological evidence of an obstruction, which allows us to envisage different scenarios for deposition of these rocks.The most likely interpretation is that these successions predate the Pampean orogeny and deposition of the Cambrian Puncoviscana Formation. Consequently, the rocks could either have been deposited within the (passive) margin of the Arequipa–Antofalla–Western Pampeanas terrane (western Gondwana) or MARA block; or are unusual, reworked sedimentary rocks related to the active continental margin of the Pampean orogen.