Ordovician Magmatism in the Northeastern North Patagonian Massif: Preliminary geochemical analysis in the context of the construction of the Gondwana margin during Early Paleozoic times

MARTÍNEZ DOPICO, C.I.; LÓPEZ DE LUCHI, M.G.; RAPALINI, A.E.

Buzios

Congreso; Gondwana 14. Reuniting Gondwana: East meets West; 2011

Universidade Federal do Rio de Janeiro,

The relation between the North Patagonian Massif (NPM) and other crustal blocks that formed West Gondwana in the Early Paleozoic is still controversial. Hypothesis of a common pre-Silurian evolution are partially based on the continuity of the Ordovician Famatinian arc and similar detrital zircon inheritance pattern in Early Cambrian metaclastic sequences with those from coeval units of the Sierras Pampeanas (Pampia terrane). Three groups of Ordovician granitoids are exposed in the northeastern corner of the NPM. Group 1 (G1) comprises Amph-Bt granodiorite-tonalites with mafic enclaves emplaced in very low grade metaclastic rocks (Arroyo Salado and Playas Doradas plutons, 41º32`S 65º10`W); Group 2 (G2) is a peraluminous Bt-Ms pink monzogranite that appears as isolated plutons (Punta Sierra, 40º31`S 64º59`W) and Group 3 (G3), composed of highly peraluminous Ms leucogranites that appear either as isolated plutons intruding medium grade metaclastic rocks (Valcheta pluton, 40º57`S 65º49`W) or as dykes intruding the other groups. Major-element analysis of the sources for G1 indicates a mafic component which agrees with relatively more radiogenic epsilon Nd and the microgranular mafic enclaves. Hybridization during a low pressure melting process involving the mixing of mafic and crustal /felsic precursors is indicated by different evolutionary paths. G2 could either derive from a greywacke (crustal melting) or fractional crystallization. In any case G1 and G2 would involve sources scarcely affected by recycling processes, if any. G3 sources might result from partial melting of Ms-rich pelite as indicated by the high Rb/Sr and high Al2O3/TiO2 especially (Valcheta locality). Moreover, G3 reveals a strong correspondence in its REE pattern with those of the encassing low grade metasedimenatary rocks . On the other hand, G3 dykes could also correspond to highly evolved G2. In that case, late stage magmatic fluids might have interacted with G2 in order to produce these dykes. Available ages of these granitoids indicate that they are mostly coeval, excepting some granitic dykes of G3 whose ages are uncertain. Source for G1would represent an episode of crustal growth compatible with a continental magmatic arc setting. The predominance of greywacke source rocks for G2 may suggest anatexis of an immature continental platform material or fractional crystallization from G1. Dominating process for G3 generation probably involves reworking and partial melting of an older crustal segment, which might be compatible with the low grade metasedimentary country rock. Although the Ordovician magmatic arc signatures and the chronological evidence support the continuity of the eastward subduction related Famatinian magmatic arc from the Sierras Pampeanas into the NPM, the location of the G3 magmatic bodies tens to one hundred kilometers west of the inferred magmatic arc (G1-G2) can be more easily explained as related to an hypothetical west dipping subduction zone.