The Neoproterozoic-Early Paleozoic metamorphic and magmatic evolution of the Eastern Sierras Pampeanas: an overview

STEENKEN, A.; LOPEZ DE LUCHI, M. G.; MARTINEZ DOPICO, C.; DROBE, M.; WEMMER, K.; SIEGESMUND, S.

INTERNATIONAL JOURNAL OF EARTH SCIENCES

SPRINGER

Lugar: Heidelberg; Año: 2011 vol. 100 p. 465 - 488

1437-3254

The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. Sm-Nd results for the metamorphic basement suggest that the TDM age interval of 1.8-1.7 Ga, which is associated with the less radiogenic values of epsilon Nd(540) (-6 to -8) can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger TDM age and a more positive epsilon Nd(540) value. In metaclastic rocks of different metamorphic age, predominance of Grenville detrital components is associated with a relatively more positive εNd(540) value. Granitoids emplaced after pre-peak metamorphism in the Pampean orogen form two clusters, one with TDM ages between 2.0 - 1.75 Ga and another between 1.6-1.5 Ga. Pampean post- 540 Ma granitoids exhibit more homogenous TDM ages ranging from 2.0 - 1.75 Ga. TDM values for the Ordovician Famatinian granitoids define a main interval of 1.8-1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger TDM ages ranging from 1.3-1.0 Ga. Achalian magmatism exhibits more radiogenic epsilon Nd(540) values that range between 0.5 to -4 and TDM ages younger than 1.3 Ga. Two types of Pampean related mafic rocks are recognized: one with a depleted mantle signature and another younger group with an enriched mantle signature, which is associated with the peak of metamorphism. Ordovician mafic-ultramafic rocks that were emplaced in an ensialic back-arc basin exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. The palaeotectonic scenario allows to speculate for a major rift event (Ediacaran) dividing in two a Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks) and producing juvenile crust. This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this margin lead to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block which reworks the same crust Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite facies metamorphismled to a final collision with the western block (Pampean orogeny). Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high grade metamorphism with mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. In Devonian times a new subduction regime was installed west of the Eastern Sierras Pampeanas and magmatism involves a transient heat anomaly associated with extensive melting of an enriched mantle source and probably of a crustal segment. In pre-Devonian times crustal reworking is dominant, whereas processes during the Achalian orogeny led to different geochemical and isotopic signatures that reflect a major input of juvenile magmatism.