Across-arc variation of the Famatinian magmatic arc (NW Argentina) exemplified by I-, S- and transitional I/S-type Early Ordovician granitoids of the Sierra de Velasco

GROSSE, P.; BELLOS, L.I.; DE LOS HOYOS, C.; LARROVERE, M.; ROSSI, J.N.; TOSELLI, A.J.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2011 vol. 32 p. 110 - 126

0895-9811

The Early Ordovician granitoids of the Sierra de Velasco reflect the across-arc compositional variation ofthe Famatinian magmatic arc (NW Argentina) developed along the proto-Andean margin of westernGondwana. This variation is characterized by means of field, petrographical, geochemical and isotopicstudies. The Sierra de Velasco contains three main types of Early Ordovician granitoids that are mineralogicallyand geochemically distinct, but generally conform a continuous high-K, magnesian and calcalkalicmagmatic series. I-type granitoids (IG) make up the southern part of the range. They are biotiteehornblendeetitanite metaluminous to weakly peraluminous granodiorites and tonalites typical ofa coastal I-type belt and were possibly formed by melting of mafic lower crust/lithospheric mantle, withminor assimilation of crustal metasediments. S-type granitoids (SG) crop out in the central and northernportions of the range. They are biotiteemuscovite and biotiteecordierite strongly peraluminous syenoandmonzogranites representative of an inland S-type belt and were possibly formed by large-scaleanatexis of metasedimentary crust and hybridization with more mafic lower crustal melts. Between theIG and SG, in the central parts of the range, transitional I/S-type granitoids (TG) are recognized thatconsist of biotite, biotiteemuscovite and subordinate biotiteetitaniteeallaniteeepidote moderatelyperaluminous monzogranites, granodiorites and tonalites. The TG show intermediate characteristics andwere possibly generated by less common mechanisms of mixing between I-type and S-type magmasand/or their parent mafic lower crustal and metasedimentary melts. The transition from IG to TG to SGtowards the continental interior reflects a compositional continuum related to a progressive variation inthe degrees of mixing between mafic and metasedimentary end-members.