Palaeozoic accretion of the microcontinent Chilenia, North Patagonian Andes: high-pressure metamorphism and subsequent thermal relaxation

Abstract

Basement rocks of the Colohuincul Complex (CC) cropout in the eastern foothills of the North Patagonian Andes(latitude 41ºS). We studied the chemical composition of mineral phases in a mica-schist and a migmatite of this complex and constructed P?T pseudosections contoured by various chemical parameters of minerals. The P?T metamorphic path of the mica-schist is characterized by a high-pressure,low-temperature event (1.8GPa and 440ºC) indicated by a spessartine-rich core in prograde-zoned garnet and phengite relicts with high Si contents (3.40 pfu).The increase of Xpyrope (from 0.02 to 0.08) towards the garnet rim and the decrease of Si(to 3.16) in phengite reflect decompression accompanied by heating to 580ºC (1.1GPa), followed by cooling to 570ºC (0.9GPa). Incontrast, the migmatitic paragneiss underwent partial melting and subsequent P?T conditions of 610ºC and 0.5GPa. Thermal relaxation after crustal thickening deduced from the mica-schist is interpreted to be the result of collision as the microcontinent Chilenia was thrust under the western South American part of Gondwana. Mid-upper crustal P?T conditions of the migmatite reflect its location within the Gondwanan crust. Two populations of monazite Th?U?Pb agesin migmatites and schists of the Colohuincul Complex with weighted average peaks at 391.7 ± 4.0 Ma (2σ) and 350.4 ± 5.8 Ma (2σ) are ascribed to the collisional and a later retrograde event.