Pervasive crustal reworking along the proto-Andean margin of northern Peru and southern Ecuador: Insights from U-Pb, geochemical, and isotopic analyses of zircon

Abstract

Continental growth at convergent margins is the result of complex interactions between mantle additions and crustal reworking. This study examines the pre-Andean tectono-magmatic evolution of the central-northern Andean margin (∼1°S to ∼6°S) using U-Pb geochronology, trace element analysis, and εHf_(t) and δ¹⁸O isotopic data from zircons embedded within detrital and bedrock samples. U-Pb dating revealed two primary clusters: a Late Neoproterozoic population (640–550 Ma) associated with the Brasiliano/Pan-African orogeny and a Permian–Triassic population (280–200 Ma) associated with the Gondwanides orogeny. There were also two minor clusters at ∼960 Ma and ∼345 Ma. Zircon grains dating between 1000 and 200 Ma crystallized during repeated episodes of crustal reworking, characterized by highly evolved εHf_(t) signatures (−10 to −30) at 640–510 Ma that are indicative of ancient Amazonian cratonic sources, with no evidence of Pampean or Famatinian contributions. The results also show a marked increase in juvenile input during the Carboniferous (380–310 Ma), followed by predominantly continental input as indicated by persistently high δ¹⁸O values up to 220 Ma. Although the U-Pb age spectra of these samples are similar to that of the central Andes, isotopic and geochemical data reveal that the central-northern Andean margin evolved within a tectonic context that is more closely aligned with central-eastern Gondwana (i.e., central Africa and eastern South America) rather than typical Andean settings.