The Famatinian mobile belt of NW Argentina: an Early Ordovician I- and S-type granite province on the proto-Andean margin of Gondwana and a counterpart of the Lachland Fold Belt

PANKHURST, R.J; RAPELA, C.W..; FANNING, C.M.

Clermont-Ferrand

Simposio; IV Hutton Symposium on the The Origin of Granites and Related Rocks; 1999

BRGM

New studies incorporating U-Pb SHRJMP zircon geochronology and geochemistry have been carried out on representative granitoids of the Famatinian mobile belt of NW Argentina. These are batholithic bodies dominated by granodiorite and granite, of which a high proportion (ca. 50% regionally) are moderately to highly peraluminous (terminology of Villaseca et al., 1998). A metaluminous 1-type suite consists of dominant hornblende-biotite granodiorite, associated with minor amounts of tonalite and gabbro and intruded by smaller bodies of monzogranite. This suite has a geochemically calcic trend. Minor amounts of S-type cordierite-granite occur within the metaluminous outcrops, where they are related to migmatization super-imposed on a low-P high-T regional metamorphism, and may be assigned to local melting of country rock pelites in the roof zone. However, S-type peraluminous granites occur on a much larger scale, especially in and to the north-east of the Sierra de Famatina, as batholithic megacrystic granites containing cordierite and, locally, andalusite. Contrary to previous data and interpretations, it is clear from the new results that emplacement of the I-type and S-type granitoids of this belt was essentially synchronous, a1nost entirely within the interval 495-470 Ma. Thus, Famatinian magmatism of both types was entirely Early Ordovician in age. This is as much as 50 Ma before the collision of the Laurentia-derived Precordillera terrane with Gondwana, the event to which the implied high degree of crustal melting was previously ascribed. The Famatinian granitoids, although somewhat older and more obviously formed during a single short-lived magmatic event, are a direct counterpart to the Lachian Fold Belt granitoids of SE Australia in compositional range and distribution. A Famatinian I-S boundary can be drawn which roughly parallels the earlier super-continental margin (with the S-type granites on the internal side). The regional tectonic environment of this magmatism is perhaps better defined than that of the Lachian Fold Belt. The Famatinian magmatic arc was associated with closure of an ensialic marginal basin developed on continental crust that had been accreted only 30 Ma earlier during the Pampean orogeny (Middle Cambrian).