Magmatic sources and tectonic setting of Gondwana margin Ordovician magmas, northern Puna of Argentina and Chile.

COIRA, B., KAY, S.M., PÉREZ, B., WOLL, B., HANNING, M. AND FLORES, P.

GEOLOGICAL SOCIETY OF AMERICA BULLETIN

GEOLOGICAL SOC AMER, INC

Lugar: Boulder; Año: 1999 vol. 336 p. 145 - 170

0016-7606

ABSTRACT  Understanding the sources of Ordovician magmatic rocks in the broad western Faja Eruptiva Occidental and eastern Faja Eruptiva Oriental magmatic belts in the northern Puna of Argentina and Chile is important to Ordovician geodynamic models for western Gondwana evolution and Gondwana-Laurentian terrane interactions. A critical evaluation of existing chemical and age data, along with new major trace element data, and field observations leads to a working model in which magmatism in the western belt progressively occurred in an active to waning are to collisional regime, whereas that in the eastern belt occurred in an oblique fault regime transitional to a subduction zone to the south. The model is hampered by data gaps in all areas, and particularly by lack of ages in western and southern regions. The best understood area is the northern Faja Eruptiva Oriental where dacitic and mafic units with published ages of 476-467 Ma occur in linear fault-controlled trends. Their chemistry is consistent with dacitic magma sources being dominated by sedimentary-type protoliths melted in association with emplacement of mantle-derived alkaline mafic magmas. These dacitic units represent lava-dome complexes emplaced contemporaneously with outer shelf and slope basin sedimentation. Farther south where deeper crustal levels are exposed, volcanic/subvolcanic units grade into poorly studied plutonic facies. Trondhjemitic bodies are reported to the east. Contemporaneous units in the Faja Eruptiva Occidental are plutons and mafic to rhyolitic lavas in sequences of volcaniclastic sediments. Chemical signatures in the poorly dated Cordón de Lila lavas and Choschas dioritic to leucogranodioritic pluton in Chile are consistent with emplacement in a magmatic arc on thinned continental or oceanic crust. Mafic volcanic units in volcanic-sedimentary sequences just to the east could represent arc volcanism behind the front. An Arenig-Llanvirn change to bimodal and predominantly silicic magmatism, the emplacement of shoshonitic and mafic andesitic flows with weak arc signatures followed by alkaline dikes in the eastem Faja Eruptiva Occidental, and the formation of the Faja Eruptiva Oriental dacitic-mafic sequences could signal a change to a very oblique subduction regime. This change would be roughly contemporaneous with the arrival of the Laurentia-derived Precordillera terrane to the south. A post-Arenig-Llanvirn compressional regime seems required to explain Ocloyic deformation in the east and late plutons to the west. The nature of the eastern boundary of a peri-Gondwana Arequipa terrane and the existence of a peri-Gondwana Famatina-Puna Oriental terrane remain unclear.