Sources for the Ordovician Magmatism in northeastern Patagonia

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

Foz de Iguacu

Congreso; Meeting of the Americas; 2010

American Geophysical Union

The relation between North Patagonian Massif (NPM) and the rest of the terranes that formed West Gondwana at the Early Paleozoic is still controversial. Hypothesis of a common pre-Silurian evolution are in part based on the continuity of the Ordovician Famatinian arc magmatism. In the NE sector of the NPM three groups of Ordovician granitoids are represented. Group 1 (G1) comprises Amph-Bt granodiorite-tonalite with mafic enclaves emplaced in very low grade metaclastic rocks. Group 2 (G2) is bt-Ms pink monzogranites that appear as isolated plutons and Group 3 (G3) is a Ms leucogranite that appears either as isolated plutons intruding low to medium grade metaclastic rocks or as dykes intruding the other groups. G1 and G2 are located to the east whereas G3 is located to the west, inland in present coordinates. Major-element analysis of the sources for G1 indicates a mafic component which agrees with Sm-Nd data (Pankhurst et al. 2006) 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 would derive from a greywacke or igneous source that could represent the crustal/felsic component of G1. In any case G1 and G2 would involve sources scarcely affected by recycling processes. G3 indicates a pelite source specifically a Ms-rich pelite as indicated by the high Rb/Sr and low Al2O3/TiO2 especially for the western outcrops. Depletion in Sr and CaO and the slightly negative europium anomaly suggest that a low amount of plagioclase entered into the melting reaction. On the other hand dykes of G3 could correspond to highly evolved G2. Late stage magmatic fluids could have interacted with G2 in order to produce these dykes. Available ages of these granitoid groups indicate that they are mostly coeval. Source for G1 would represent an episode of crustal growth. The predominance of greywacke source rocks for G2 may suggest anatexis of an immature continental platform material. Towards the west dominating process for G3 generation probably indicate reworking of an older crustal segment. All along the Famatinian arc mafic rocks appear either as discrete gabbroic stocks or as enclaves chemical and isotopic signatures indicate a higher degree of interaction with the continental crust whereas sources for the more evolved granites are mostly feldspar-rich sources. Although the outcrops are relatively scarce in the eastern sector the exposed area for G2 is smaller relative to the metaluminous G1 granitoids which could indicate that significant heat input from a major batholith is required to generate a small amount of peraluminous magma at high levels of the crust.