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

STEENKEN, A., WEMMER, K., SIEGESMUND, S. AND LÓPEZ DE LUCHI, M.G., 2006.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Lugar: Amsterdam; Año: 2006

0895-9811

Abstract Geochronological data on the structural evolution of the Sierra de San Luis provide an important insight into the geodynamic history along the southwestern margin ofGondwana. 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 is accommodated by the reactivation of the Famatinian high-temperature shear zones at greenschist facies conditions. K/Ar Ms ages point to their activity up to the Early Carboniferous. A post-Pampean structural evolution of the Pringles Metamorphic Complex is supported by new Sm/Nd and 207Pb/206Pb data, which agrees with the already 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 Ma 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 pre-dates 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 cooling after the Famatinian metamorphic peak. Ms ages from large Ms-booklets suggest a common cooling of the basement of the sierra at ~445 Ma, whereas ‘normal’ sized muscovites from gneissic rocks (K/Ar ages at ~380 Ma) indicate slow cooling of the Pringles Metamorphic Complex with respect to the other basement domains. K/Ar Bt ages between 360 Ma and 340 Ma captured cooling below ~300°C of the basement. Differential cooling is interpreted to reflect the Achalian tectonic cycle as an event separated from the Famatinian process.