Role of tectonism and fractional crystallization in the origin of lower Paleozoic epidote-bearing granitoids, northwestern Argentina

• Published in: Geology (Boulder) 1987-08, Vol.15 (8), p.709-713
• Main Authors: Saavedra, Julio, Toselli, Alejandro J, De Toselli, Juana N Rossi, Rapela, Carlos W
• Format: Article
• Language: English
• ISSN: 0091-7613
• DOI: 10.1130/0091-7613(1987)152.0.CO;2

In the northern Pampean Ranges a large group of peraluminous, two-mica Ordovician granitoids commonly contains epidote-bearing facies. This granitoid suite exhibits a wide compositional range within the calc-alkaline series, although the mineralogical and chemical characteristics do not fit the typical features of either I- or S-type granites. Many of these plutons show asymmetric compositional zoning, which is interpreted to be the product of fractional crystallization under tectonic stress. Tonalites occur only in the more fractionated plutons and are interpreted as cumulates. Magmatic epidote in the granitoids suggests a protracted crystallization period. The pressure and temperature conditions at the emplacement level are inferred to be moderate. Widespread tourmaline-bearing pegmatites indicate that the later facies crystallized from a water- and volatile-rich liquid. However, relict anhydrous minerals and a relatively low initial 87Sr/86Sr ratio (~0.705) suggest that the granitoids were derived from a water-undersaturated parental magma of deep origin.