EPISODIC CONTINENTAL ARC MAGMATISM IN THE PERUVIAN ANDES FROM U-PB ZIRCON GEOCHRONOLOGY

The Peruvian segment of the Andean continental arc (~5°–17°S) records over 1000 m.y. of intermittent magmatism along the western margin of Amazonia, part of the Earth’s oldest continuously active continental subduction margin. Existing geochronologic data include more than 300 bedrock and over 2000 detrital zircon U–Pb ages. We add about 560 new LA-ICP-MS detrital zircon U-Pb ages from three sandstone samples taken from an across-arc transect extending from the Pisco/Ica region in the west through Cusco and to the western edge of the Amazon. We also add 9 new bedrock ages from the Pisco/Ica region. When our data are combined with existing U-Pb zircon data for the entire country, five Phanerozoic periods of high magmatic additions are seen. Flare-ups occur in the Cenozoic and Cretaceous (21±8 and 65±15 Ma) related to the Andean cycle, Permian-Triassic (237±12 Ma) related to early Andean phase arc formation and the breakup of Pangea, Carboniferous-Permian (308±22 Ma) related to an arc formed during the assembly of Pangea, and Ordovician-Silurian (468±20) related to the Famatinian orogeny. Older small Proterozoic peaks of South American craton recycled zircons are at 1015±41, 1182±19, and 1816±30 Ma. Age correspondence between detrital zircon and bedrock age spectra is strong in Peru and yield a good data set for studying continental arc evolution. Average flare-up durations are ~50 m.y. Flare-up peaks are non-cyclic, with variable durations between them.

In the Pisco/Ica sample, the main peaks in the distribution of zircon ages are 16±8 and 88±8 Ma indicating input from local Cenozoic volcanism and Peruvian Coastal batholith magmatism. In the Cusco sample, peaks are at 73±11, 132±13, 575±60, 1030±49 and 1175±31 Ma reflecting local sourcing from the Cretaceous and Permo-Triassic flare-ups with some recycled Precambrian grains. The Amazon sample shows peaks at 260±15, 590±30, 1060±38, 1210±27 and 1370±40 Ma showing an increase in proto-Andean flare-ups and input from recycled Amazonian craton. This confirms that the Andes became a detrital zircon barrier no later than the Cretaceous.