Time constraints on the Famatinian and Achalian structural evolution of the basement of the Sierra de San Luis (Eastern Sierras Pampeanas, Argentina)

STEENKEN, A.; SIEGESMUND, S.; WEMMER,K.; LÓPEZ DE LUCHI, M. G.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Lugar: Amsterdam; Año: 2008 vol. 25 p. 336 - 358

0895-9811

Geochronological data pertaining to the structural evolution of the Sierra de San Luis provide important insights into the geodynamic history along the southwestern margin of Gondwana. In the Pringles metamorphic complex, metamorphic fabrics (S1) were affected by two folding events (D2 and D3) related to the Ordovician approach of the Cuyania terrane. Contemporary formations of high-temperature shear zones record a prominent east-side-up displacement. Resumption of compression due to the Achalian collision with the Chilenia terrane was accommodated by the reactivation of the Famatinian high-temperature shear zones in greenschist facies conditions. K–Ar Ms ages point to their activity up to the Early Carboniferous. The post-Pampean structural evolution of the Pringles metamorphic complex is supported by new Sm–Nd and 207Pb/206Pb data, which agree with previously proposed early Cambrian sedimentation of the psammopelitic precursors. A Sm–Nd mineral isochron, together with published SHRIMP U–Pb zircon ages, bracket the emplacement of mafic and ultramafic intrusions that control the granulite facies metamorphism between 506 and 478 Ma. A late Cambrian emplacement of the crustal-derived Paso del Rey pluton is suggested by a 207Pb/206Pb zircon age at 491 ± 19 Ma. The syn-D2 emplacement of these granitoids indicates that the D1 to D2 structural evolution predates the generally assumed Middle Ordovician accretion of the Precordillera/Cuyania terrane. K–Ar Hbl, Ms, and Bt ages and some Rb–Sr mica data record the cooling after the Famatinian metamorphic peak. Ms ages from large Ms booklets suggest a common cooling of the basement of the sierra at approximately 445 Ma, whereas normal-sized muscovites from gneissic rocks (K–Ar ages of 380 Ma) indicate slow cooling of the Pringles metamorphic complex with respect to the other basement domains. K–Ar Bt ages between 360 and 340 Ma capture cooling below approximately 300 °C of the basement. Differential cooling is interpreted to reflect the Achalian tectonic cycle as an event separate from the Famatinian process.