The Nordpatagonian Massif evolution as compared to the adjacent Andean segment

CINGOLANI, C.; DALLA SALDA, L.H.; RAPELA, C.W.; HERVE, F; MUNIZAGA, F.; PARADA, M.A.

Belem

Congreso; VII Congreso Latinoamericano de Geología; 1988

Sociedade Brasileira de Geologia

The correlation of geologic units between the Nord Patagonian Massiff (NPM) and the adjacent Andes, including the Coast Ranges of Chile is investigated. The NPM is an extense region of the andean back arc between 38°30’ and 43°OO’South latitude which differs from the contiguous low lying areas of Patagonia because Precambrian and Paleozoic metamorphic and plutonic complexes are exposed in addition to the ubiquitous meso-cenozoic sedimentary and volcanic sequences. To the north, the NPM is covered by the Mesozoic sedimentary deposits of the Neuquen basin. To the west, it grades transitionally to the main range of the Andes. The Precambrian metamorphics the NPM have no equivalent in the Andes, where, however, they may be concealed beneath younger volcanic products. Cratonic South America possibly extends to the Liquiñe Ofqui fault in the chilean side of the Andes, west of which only the Late Paleozoic accretionary complexes are recognized. The greenstones and serpentinites in the latter have no counterpart in the more pelitic schists and gneisses of the Paleozoic metamorphic complexes of the Argentine Andes and NPM. The Late Paleozoic Southern Coastal Batholith (SCB) of the Coast Ranges of Chile differs greatly from the contemporaneous granitoids in the NPM. The former have a wider lithological spectrum with abundant more basic hornblende—rich varieties, while the latter are mainly restricted to biotite and two-mica leucogranitic types. Upheaval and erosion of these metamorphic and granitic complexes occurred before the deposition of the Permo—Triassic Choiyoi formation in the western border of the NPM, but only in the Late Triassic in the SCB. Scarce deformation of the Choiyoi and Late Triassic sedimentites of the NPM contrasts with the moderate to intense folding of the Late Triassic deposits in the SCB (Santa Juana\formation) and in the chilean slope of the Andes (Panguipulli formation). The extensive belts of Late Mesozoic to Cenozoic volcanics located over the western border of the MNP, have only locally developed counterparts in the Andes, which is mainly composed of inmature plutonic rocks of the Meso-Cenozoic north patagonian batholith. The young strato volcanoes which overlie the eroded plutonics, are similar chemically to them and suggest a stable subduction zone magma generating sys- tem since the Early Cenozoic with an apparent peak of magmatic and tectonic activity during the Miocene. Comparison between the geological constitution of the Andes and the NPM point to differences in the Paleozoic to Recent tectonic history of the NPM respect to the typical continental margin evolution of the Andes.