Cannibalization of a late Cambrian backarc in southern Peru: New insights into the assembly of southwestern Gondwana

Abstract

In the Eastern Cordillera of Peru, observations related to the accretion of the Arequipa Terrane to the Amazon craton are scarce and reactivation of sutures in a backarc basin may make interpretation more difficult. Cambro-Ordovician backarc successions located in proximity to a proposed suture in the Eastern Cordillera of southern Peru were investigated at Umachiri and Ollantaytambo in order to disentangle the early Paleozoic tectonic history of the Arequipa Terrane. At Umachiri, newly identified fossil assemblages in the upper Cambrian Llallahue Formation (Fm) and in the Lower to Middle Ordovician Umachiri Fm constrain the age of these successions and allow correlation of these strata throughout the Central Andes. Using U–Pb geochronology on zircon, we establish that the Ollantaytambo Fm is an upper Carboniferous succession and that the Umachiri Fm instead correlates with the Lower to Middle Ordovician Verónica and San José formations, which form the base of a backarc succession in the Eastern Cordillera of southern Peru that extended into northwestern Argentina. Detrital zircons from the Umachiri Fm display age signatures characteristic of Arequipa basement and contain a prominent 540–510 Ma peak. Precise dating and trace element analysis yielded 539 Ma arc-derived and 522–510 Ma mantle-derived zircons. We infer that a Cambrian backarc system related to deposition of the lower Furongian (~494 Ma) Llallahue Fm developed following ~ 540 Ma Pampean arc magmatism. Closure of the backarc in late Cambrian to Early Ordovician time resulted in regional deformation, uplift, and erosion followed by renewed backarc subsidence in the Ordovician Period. In summary, our results support a new tectonic model of late Neoproterozoic to early Cambrian Pampean collision of an exotic Arequipa Terrane with Amazonia, opening and closing of a Cambrian backarc basin, the presence of parallel Ordovician continental arcs associated with Famatinian subduction, and the opening of an Ordovician–Devonian backarc basin.