The mesoproterozoic U-PB (LA-ICP-MS) age of the Loma Alta gneissic rocks: Basement remnant of the San Rafael Block, Cuyania Terrane, Argentina

Abstract

The main Mesoproterozoic basement outcrop within the San Rafael Block is a tectonic sliver of 10 km by 2 km, oriented NNW to SSE from Arroyo Ponón Trehué and Río Seco de los Leones know as the Cerro La Ventana Formation. This unit consists of amphibolites, quartz micaschists, gneissic rocks and pegmatite to aplitic dykes, affected by heterogeneous ductile shear zones and is covered by carbonate-siliciclastic rocks bearing Ordovician fossils. The basement assemblage suggests the metamorphosed volcano-plutonic complex showing tight to isoclinal folding. The similar microgranitoid enclave and chemical features suggest a juvenile character as TTG-series like Las Matras Pluton towards the south. Mesoproterozoic crystallization ages (c. 1.2 Ga) of the Cerro La Ventana basement are based in Rb-Sr, TIMS U-Pb and Sm-Nd, with eNd range of +2.8 to +4.7. At the region of Loma Alta (Lomas Orientales sector) about 50 km to the NW of Cerro Ponón Trehué, within the so-called ‘Nihuil basic unit’ extending for around 20 km to the NNE of the El Nihuil dam, outcropped a meter-scale relics of orthogneissic rocks of intermediate compositions bear a foliation with orientations similar to Cerro La Ventana type section. The in situ U-Pb (LA-ICP-MS) zircon data on these rocks show also a Mesoproterozoic crystallization age: mean 207Pb/208Pb is 1266±15 Ma (n=34) and plotted in a Concordia diagram record an upper intercept at 1278±21 Ma (MSWD: 13). With these isotopic data we record a new presence of Mesoproterozoic (Ectasian-age) rocks at the region of Loma Alta of the San Rafael Block as a part of the Laurentian affinity Cuyania terrane in SW Gondwana.