Abstract

The late Miocene (6.4-4  0.1 Ma) Yungay volcanic rocks of the Cordillera Blanca massif, Peru (9S), represent the final magmatic components in this sector of the Andes. The magmas were erupted during a period of significant tectonic activity along the western continental margin of Peru, characterised by abnormal crustal thickening (> 50 km), rapid exhumation and shallowing of the Nazca Plate. New geochemical and petrological data show that the Yungay rocks differ markedly in composition from older Cretaceous and Eocene lavas in western Peru, which are typically calc-alkaline. Instead, the Yungay volcanics appear closer in composition (although not identical) to ‘adakites’, showing strong depletions in Y and heavy rare earth elements (La/Yb c. 30, Sr/Y 80-100, Na2O > 5 wt.%). Mg numbers range from 10-40. Matching chemical compositions are also present in the neighbouring Cordillera Blanca batholith, which overlaps in age with the Yungay rocks. Significantly, the Yungay ignimbrites are located immediately to the west of the Cordillera Blanca fault, a deeply penetrating crustal lineament. Both the batholith and Yungay magmas appear to have exploited this fault, implying a common structural link with their source regions. Similar relationships between adakite-like magmas and deep fault systems are characteristic of other adakite-like provinces (e.g. Philippines) and may play an important role in magmagenesis. In common with the batholith magmas, and despite their eruption age coinciding with shallowing of the Nazca plate from 30 to c. 5, we contend that the source region of the Yungay rocks is newly accreted mafic underplate material. We conclude that the presence of adakite-like rocks are not a definitive indicator of tectonic setting, either now or in the past.