Geodynamic controls in the southernmost Northern Andes magmatic arc: Trace elements and Hf-O isotopic systematics in forearc detrital zircon

Abstract

U-Pb dating of single detrital zircon grains by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) paired with Hf and O isotopic and trace-element analyses provide first-order indicators of the Late Cretaceous−Cenozoic evolution of the southern Ecuadorian magmatic arc. Detrital zircon U-Pb ages define significant clusters that are tentatively interpreted as intense arc magmatism at ca. 72 Ma, ca. 60 Ma, and ca. 43 Ma. A major accretionary event in the Late Cretaceous (75−65 Ma) is marked by a broad range of zircon isotopic values (εHf[t] > 20 and δ18O > 8‰) that suggest melting of both the lower and upper crust (most likely of continental affinity) as well as enriched mantle components. Highly fractionated signatures in trace-element patterns and Eu/Eu* combined with mantle-like δ18O and juvenile εHf values characterize zircons from 60 to 45 Ma, suggesting that the Late Cretaceous−middle Eocene arc originated from an enriched mantle and likely reflects the persistence of overthickened crust previously attributed to the main Late Cretaceous accretionary period. Subsequently, negative shifts in εHf(t) isotopic composition from 45 to 30 Ma are paired with mantle-like δ18O values as well as decreases in U/Yb and Eu/Eu*. These signatures could be attributed to magma emplacement in a thinner crust and the existence of a broad extensional magmatic arc extending from the current forearc toward areas near the craton; however, other scenarios cannot be excluded. This event was characterized by enriched mantle melt sources with residence times pointing to known crustal events (Sunsás) in the Amazonian craton. From 30 to 10 Ma, the isotopic record slightly evolved toward a depleted mantle signature with a substantial increase in fractionation. Our results combined with previously published isotopic records from detrital zircon grains found in modern rivers suggest that, for at least the last 30 m.y., the southernmost Northern Andes magmatic arc has been segmented, with the emplacement of juvenile magmas to the north and more enriched magmas related to the recycling of ancient continental crust and/or subducted sediments to the south—aspects found in other Northern Andes settings in which the continental arc was constructed in both oceanic and continental crust.