The country rocks of Devonian magmatism in the North Patagonian Massif and Chaitenia

FANNING, C.M.; QUEZADA, P.; HERVE, F.; PANKHURST, R.J; CALDERÓN, M.; RAPELA, C.W.

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Lugar: Santiago de Chile; Año: 2018 vol. 45 p. 301 - 317

0718-7092

ABSTRACT. Previous work has shown that Devonian magmatism in the southern Andes occurred in two contemporaneousbelts: one emplaced in the continental crust of the North Patagonian Massif and the other in an oceanic island arcterrane to the west, Chaitenia, which was later accreted to Patagonia. The country rocks of the plutonic rocks consistof metasedimentary complexes which crop out sporadically in the Andes on both sides of the Argentina-Chile border,and additionally of pillow metabasalts for Chaitenia. Detrital zircon SHRIMP U-Pb age determinations in 13 samplesof these rocks indicate maximum possible depositional ages from ca. 370 to 900 Ma, and the case is argued for mostlyDevonian sedimentation as for the fossiliferous Buill slates. Ordovician, Cambrian-late Neoproterozoic and ?Grenvilleage?provenance is seen throughout, except for the most westerly outcrops where Devonian detrital zircons predominate.Besides a difference in the Precambrian zircon grains, 76% versus 25% respectively, there is no systematic variationin provenance from the Patagonian foreland to Chaitenia, so that the island arc terrane must have been proximal to thecontinent: its deeper crust is not exposed but several outcrops of ultramafic rocks are known. Zircons with devonianmetamorphic rims in rocks from the North Patagonian Massif have no counterpart in the low metamorphic grade Chileanrocks. These Paleozoic metasedimentary rocks were also intruded by Pennsylvanian and Jurassic granitoids.