Abstract

The Mesozoic-Cenozoic tectonic evolution of the Peruvian continental margin of South America is dominated by periods of intracontinental rifting and basin formation. The Andean cycle in this region starts in the Cretaceous with intrusion of the tonalite-dominated Coastal Batholith (CB) into an Albian marine basin forming part of a major extensional system extending from Columbia to the Antarctic. Basaltic material at the bottom of the basin was remelted to produce mainly tonalitic magmas of the CB, which were intruded vertically up axial fractures to form thin, trench-parallel, horizontal intrusions near the surface. Gravity modelling of the CB and its envelope shows the intrusion is tabular, with an aspect ratio close to 20. Individual plutons have aspect ratios close to 5. A thick (10-km) root zone to the west is interpreted as a multiple dyke feeder system. It is likely that much of the CB was intruded under a shallow sea that covered the basinal rocks prior to inversion. From Eocene times onwards, volcanism and plutonism migrated inboard, culminating in the intrusion of the Cordillera Blanca Batholith (CBB) and Yungay ignimbrites at c. 6 Ma. Situated above the now thickened crustal root of the Andes (c. 60 km), the CBB was intruded into Jurassic basinal shales which form part of the much larger West Pervian Trough, and its emplacement overlaps with intense uplift and exhumation along the Andean margin at c. 15-10 Ma. The western margin of the basin and CBB terminate at the Cordillera Blanca fault complex, a deeply disecting, trench-parallel crustal lineament. In contrast to traditional models of cordilleran magmatism, which often predict 'S'-type magmas inboard of the main arc, the CBB rocks are high Na, high Sr/Y types similar to Archean trondhjemites that contrast markedly with the calc-alkaline magmas of the CB. Similar calc-alkaline to adakite-like compositional trends are seen in spatially related volcanic rocks in land from the trench. In the Peru arc, the generation of large volumes of granitic (s.l.) melts is related to periods of crustal extension, with the pre-existing (Gondwana) structural template of the continental margin controlling both location and style of intrusion. The source material for the majority of plutonic rocks was newly generated basaltic lower crust. Volume constraints imply that the crustal column beneath the western Peruvian arc evolved significantly from c. 20 to 5 Ma, with vertical thickening driven by an elevated flux of mantle-derived magma into the lower crust, followed by rapid intracrustal remelting and chemical differentiation. It is likely that flare-up of insertial magmatic activity combined with rifting has led to periodic thermal weakening of the entire arc, resulting in fluctuating bulk rheology of arc lithosphere over time.