Palaeozoic accretion of the microcontinent Chilenia, North Patagonian Andes: high-pressure

MARTÍNEZ JUAN CRUZ; DRISTAS A. JORGE; MASSONNE, H-J

INTERNATIONAL GEOLOGY REVIEW

TAYLOR & FRANCIS INC

Año: 2012 vol. 54 p. 472 - 490

0020-6814

Basement rocks of the Colohuincul Complex (CC) crop out in the eastern foothills of the North Patagonian Andes (latitude 41◦S). 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 (1.8 GPa and 440◦C) 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 580◦C (1.1 GPa), followed by cooling to 570◦C (0.9 GPa). In contrast, the migmatitic paragneiss underwent partial melting and subsequent P?T conditions of 610◦C and 0.5 GPa. 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 ages in migmatites and schists of the Colohuincul Complex with weighted average peaks at 391.7 ± 4.0 Ma (2ó) and 350.4 ± 5.8 Ma (2ó) are ascribed to the collisional and a later retrograde event.◦S). 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 (1.8 GPa and 440◦C) 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 580◦C (1.1 GPa), followed by cooling to 570◦C (0.9 GPa). In contrast, the migmatitic paragneiss underwent partial melting and subsequent P?T conditions of 610◦C and 0.5 GPa. 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 ages in migmatites and schists of the Colohuincul Complex with weighted average peaks at 391.7 ± 4.0 Ma (2ó) and 350.4 ± 5.8 Ma (2ó) are ascribed to the collisional and a later retrograde event.