Juan C. Martínez

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

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 (2σ) and 350.4 ± 5.8 Ma (2σ) are ascribed to the collisional and a later retrograde event.

Geochronology of igneous rocks in the Sierra Norte de Córdoba (Argentina): Implications for the Pampean evolution at the western Gondwana margin

U-Pb zircon data (secondary ion mass spectrometry [SIMS] and thermal ionization mass spectrometry [TIMS] analyses) from igneous rocks with differing structural fabrics in the Sierra Norte de Cordoba, western Argentina, suggest that the sedimentary, tectonic, and magmatic history in this part of the Eastern Sierras Pampeanas spans the late Neoproterozoic–Early Cambrian. A deformed metarhyolite layer in metaclastic sedimentary rocks gives a crystallization age of 535 ± 5 Ma, providing a limit on the timing of the onset of D 1 deformation and metamorphism. The new data coupled with published Neoproterozoic zircon dates from a rhyolite beneath the metaclastic section and detrital zircon ages from the section indicate a late Neoproterozoic to Early Cambrian depositional age, making this section time equivalent with the Puncoviscana Formation (sensu lato) of northwest Argentina. A synkinematic granite porphyry gives a crystallization age of 534 ± 5 Ma, providing a limit on the age of dextral mylonitization in the Sierra Norte area (D 2 event). The new age is consistent with ages of 533 ± 4 Ma from a mylonitic granite with dextral sense-of-shear fabrics and 531 ± 4 Ma from a late-synkinematic dacitic porphyry, which broadly indicates the final age of dextral deformation. A crystallization age of zircons from the postkinematic, high-level El Tio granite (530 ± 4 Ma) suggests that both stages of Pampean deformation and regional metamorphism, accompanied by synkinematic intrusions, were followed by uplift and took place during a very short time span in the Early Cambrian. This is supported by zircon dates of 523 ± 5 Ma from a rhyolite to dacite in the western part of the Rodeito area and dates from the undeformed El Escondido rhyolite and granite of 519 ± 4 Ma and 521 ± 4 Ma, respectively. These three crystallization ages also indicate that ductile dextral shearing and mylonitization associated with the Pampean D 2 event terminated in the Early Cambrian. Both stages of Pampean deformation in this segment of the western pre-Andean Gondwana margin seem to represent a continuous event that can be related to oblique dextral convergence between the overriding plate in the east and the subducting and finally colliding plate in the west. The postkinematic intrusions and extrusions are related to the late stage of the Pampean magmatic history, which terminated before Early Ordovician (Famatinian) time.