Age Determinations from Andean Peru: A Reconnaissance Survey

More than 60 new K-Ar age determinations for mica, hornblende, and potassium feldspar together with a few Rb-Sr determinations on mica and potassium feldspar are presented.

Micas from two gneiss samples within the Arequipa batholith intrusive belt have yielded ages of ∼660 m.y. Five samples of granitic rocks, widely scattered in the Coastal and Eastern Cordilleras, have given ages ranging from 447 to 251 m.y. In the Eastern Cordillera, stratigraphic control is sufficient in one case to confirm intrusion in Late Ordovician or Early Silurian times and, in another case, Late Permian time.

Triassic and Early Jurassic apparent ages are known from the Coastal, Western, and Eastern Cordilleras; the parent rocks are both igneous and metamorphic. The metamorphic rocks are probably Paleozoic or older, and the apparent ages must be the result of Mesozoic or later disturbances. It is tentatively suggested that intrusion of granitic rocks occurred in Late Triassic or Early Jurassic times.

The Coastal batholith yields a spectrum of apparent ages from ∼100 to 10 m.y. Fewer ages are available from the Arequipa batholith, and they show a more limited range – from 80 to 50 m.y. The Cordillera Blanca batholith gives very young apparent ages of from 12 to 3 m.y.

Mineral ages from across the Coastal batholith get progressively younger from southwest to northeast. However, because virtually all the Andean ages have been-determined by the K-Ar method, they do not necessarily indicate a northeastward migration of intrusive foci. Very limited Rb-Sr data indicate initial ⁸⁷Sr/⁸⁶Sr ratios in the range 0.704 to 0.706 for the granitic rocks of the Coastal batholith.