Sequence, age and geochemistry of volcanic rocks of the Early Cretaceous Oyotún Formation in northwestern Peru: Revelation for the details of paleo-Farallon subduction

The Oyotún Formation volcanic rocks are extremely widely distributed Mesozoic volcanic rocks in Northwestern Central Andes, and have important implications for understanding the Early Cretaceous crust-mantle interactions in Central Andes. The volcanic rocks are exposed in a small area with thin thickness and uniform lithology in the west, and are outcropped extensively with large thickness and more complex lithofacies in the west. LA-ICP-MS zircon U-Pb dating shows that the Oyotún volcanic rocks are concentrated in the range of 139.7~131.0Ma. Geochemically, the Oyotún Formation volcanic rocks show the SiO₂ range from 54.44% to 72.33%, medium enrichment of MgO (0.56%~1.55%), metaluminous-strong peraluminous nature (A/CNK=0.98~3.51), enrichment of Cs, Rb, Th, U, Pb and LREE with indistinct negative europium anomalies, and depletion of Nb, Ta, Ti, P, suggesting obvious characteristics of volcanic arc. The ε_Hf(t) of Oyotún Formation volcanic rocks are between 3.37 and 8.42, with the ε_Hf(t) values at the bottom significantly lower than the values at the top, indicating that the rocks possibly originated from the process of low degree partial melting of the juvenile basaltic crust through the action of plate fluid, with the contamination of Archean ancient continent materials. The degree of contamination decreased when the ages became younger. The juvenile basaltic crust was probably derived from depleted mantle in an extension setting on the active continental margin.