Geoquímica isotópica (Sr y Nd) de las anortositas (“massif type anorthosites”) grenvillianas de las Sierras Pampeanas Occidentales (Argentina). Fuentes magmáticas e implicaciones geotectónicas

Abstract

Massif-type anorthosites are igneous complexes that consist for the most part of anorthosite, a plagioclase- rich plutonic rock. They are particularly abundant within or near the worldwide Grenvillian mobile belt (1.0-1.2Ga), which resulted from the assembly of the Mesoproterozoic supercontinent of Rodinia. In two of the most western Sierras Pampeanas (Sierras of Maz and Espinal, NW of Argentina), two anorthositic igneous complexes have been discovered recently (Casquet et al. 2005a). Coarse-grained anorthosites is the predominant rock type in these complexes. The age of crystallization is of 1070 ± 41 Ma (U-Pb SHRIMP, in zircons). These rocks have a superposed regional metamorphism of Ordovician age (431±30 Ma; U-Pb SHRIMP, in zircons). These complexes are the only ones mentioned in Argentina and are the first of recognized Grenvillian age in Sudamerica. Nd and Sr isotope composition suggest that primary magmas derived from a depleted source. Moreover moderate contamination with an enriched continental component is also inferred. Nd model ages suggest that the depleted source formed coeval with the anorthosite magmatism, which is compatible with a mantle plume probably related to the early break up of Rodinia.