Carboniferous retro-arc, NW Argentina: asymmetrical hot lithosphere controlled by a crustal discontinuity?

P.H. ALASINO; C. CASQUET; R. PANKHURST; C.W. RAPELA; C. GALINDO; J.A. DAHLQUIST

Avila

Simposio; 7th Hutton Symposium on Granites and Related Rocks; 2011

Universidad de Granada

Carboniferous magmatism in the western Gondwana margin (28°?32° S.) exhibits a compositional progression in space and time that is interpreted as reflecting different juxtaposed basement blocks. (1) Over more than 1000 km in the Eastern Sierras Pampeanas (ESP) minor but widespread anorogenic A-type magmatism (355?335 Ma) produced mainly mid-alkaline K-feldspar porphyritic granite and coeval alkaline mafic dykes (the latter geochemically classified as back-arc basin basalt). These granites occur as scattered plutons (at least 15 have been identified), usually with sub-circular forms, intruding Early Palaeozoic metamorphic and plutonic belts. (2) In the Western Sierras Pampeanas (WSP), where the basement is of Grenville age, the Carboniferous magmatism is represented by a calc-alkaline magmatic arc (<330 Ma). Some calc-alkaline plutons located near the inferred boundary of the two blocks show transitional chemical features. Based on existing structural, geochemical and geochronological data we propose a push-pull tectonothermal scenario (e.g., Lister and Forster 2009) whereby horizontal lithospheric shortening in the Devonian (Achalian phase; Sims et al., 1998) gave way to extension in the Early Carboniferous, probably related to roll-back of the subduction hinge at the active Gondwana margin farther west. The extensional phase resulted in an ensialic back-arc rifting in the ESP with asthenospheric upwelling and melting of underplated basaltic material in the lower crust (absence of garnet in the source and/or residuum) to yield the A-type magmas. Based on new and published whole-rock geochemistry and Nd isotope analyses of six A type plutons (Dahlquist et al., 2010 and reference therein), we conclude that even within the ESP outcrops there is significant mineralogical and geochemical gradation. In the western ESP (e.g., Sierra de Famatina, 68°W - near the border with the WSP) a minimal contribution from evolved upper crustal material is inferred for amphibole-bearing granitoids with åNd values close to zero. In the eastern ESP (~ 67°W, e.g., Sierras de Velasco and Zapata) the crustal contribution was more important: åNd values range down to -2, biotite is the only mafic mineral and primary muscovite is common. Zircon thermometry for the studied A-type granitoids reveals high crystallization temperatures of up to ca. 815° C. The variations in the contrasting types of Carboniferous magmatism between the WSP and the ESP suggest that the igneous province boundary occurs along a major crustal discontinuity where widespread silicic magmatism with a juvenile input coincided with >2000 thick alluvial conglomerates and shallow-marine strata of Mississippian age (Martina et al., 2011 and reference therein). Carboniferous A-type magmatism in the ESP was probably controlled by an asymmetrical stretched lithosphere with continental crust thickening eastwards to the foreland.