INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Middle Devonian magmatism of the Sierra de San Luis
LÓPEZ DE LUCHI, M. G.; IANNIZZOTTO, N.; GEUNA, S.; RAPALINI, A.E.; SIEGESMUND, S.; WEMMER, K.
Simposio; Seventh Hutton Symposium on Granites and related Rocks; 2011
Universidad de Granada, Politécnico de Avila, Universidad de Salamanca
Middle Devonian Achalian granitoids intruded the Eastern Sierras Pampeanas (ESP) basement ca. 600 km east of the inferred proto-Pacific margin of Gondwana margin in which a coeval crustal thickening event was recognized (López de Luchi et al, 2007 and references therein). In Sierra de San Luis voluminous Middle Devonian batholiths are made up by an I- to A-type hybrid monzonite?granite suite with metaluminous magnesian alkali-calcic (393-385 Ma) porphyritic to equigranular monzonite- quartz monzonite? granodiorite±calc-alkalic monzogranite and mildly peraluminous magnesian to ferroan alkali-calcic equigranular monzogranites. SiO2 (50?62%), high Mg# (50?60), Cr, Ni, V, LILE, LREE, Th, Zr and Ba, Sr, Sr/Y, (La/Yb)N (50-20), smooth Eu/Eu*, moderate Na2O (ca 3.5) and troughs at Nb and Ta for monzonitic rocks suggest that garnet was a major residual phase with a minor role of plagioclase and a relatively deep seated lithospheric mantle source which underwent subduction-related metasomatism. The arc-like signature reflected in the negative Nb-Ta anomalies could also indicate crustal contamination of the mantle-derived magmas ascending through the crust. The metaluminous biotite (±amphibole) porphyritic granodiorite-monzogranite and the mildly peraluminous biotite granites are characterized by relatively low CaO and 87Sr/86Sr, high LILE, Sr/Y, and (La/Yb)N, low Rb/Sr (1.5, ASI 1.05-1.18 and Ga/Al 2.6-3.9. Although most of the granites show Ga/Al higher than 2.5, contents of Nb, Zr, Ce, and Y are lower than for typical A-type granites. Emplacement is syn-kinematic with conjugate crustal scale shear zones that acted as conduits for enriched mantle derived mafic magmas and controlled the direction of flow (López de Luchi et al. 2007 and references therein). This syn-kinematic emplacement 600km inboard of the continental margin suggest that compression extended to the hinterland and may have favoured hybridization between coeval felsic and mafic magmas. Transition to A-type magmatism would record foundering of the slab or slab break-off. Middle Devonian magmatism would record an episode of crustal growth by mafic magma input during the Achalian orogeny.