Juvenile basic magmas in the root of the Ordovician marginal arc of Gondwana

RAPELA, C.W.; CASQUET, C.; PANKHURST, R.J; BALDO, E.G.; GALINDO, C.; DAHLQUIST, J.A.

Medellín

Simposio; VIII South American Symposium on Isotope Geology; 2012

GEMMA

A remarkable difference between the typical Cretaceous to Recent Andean granites and those of the Proto-Pacific Early Paleozoic subduction-related magmatism is that the former are dominated by juvenile Sr and Nd isotopic signatures, while the latter plot outside the “mantle array” in the (87Sr/86Sr)0 – eNdt isotopic diagram, with eNdt < -2 (Rapela et al., 2008, 7th International Symposium on Andean Geodynamics, Nice, 4pp). The Cambrian (Pampean) and Early Ordovician (Famatinian) events included abundant amphibole-bearing and noritic gabbros with less than 50% SiO2 that share the same crustal signature as the intermediate rocks, both interpreted as reflecting the composition of a Proterozoic lithospheric source (Pankhurst et al. 2000, Trans. Roy. Soc, Edin., 91.1/2: 151-168). Most exposures of the extensive Famatinian magmatism are plutons emplaced at shallow and intermediate levels (2-5 kbar) and volcanic sequences. However, west of the Famatinian belt in Sierra de Valle Fértil (SVF), rocks that were accreted at the root of the arc are exposed (Las Chacras block). A large proportion of gabbro is characteristic of the SVF belt, and although typical asthenospheric Nd and Sr isotopic relations are absent, intermediate to acidic plutonic rocks are considered hybrid rocks formed by interaction between mantle-derived gabroic magmas and metasedimentary rocks (Otamendi et al., 2012, J. Petrol. doi: 10.1093/petrology/egr079). We report here chemically and isotopically juvenile garnetiferous amphibolites (SiO2 = 48-54%; εNdt= +4.7- to -0.1) from Las Chacras interlayered with metasedimentary gneisses metamorphosed at medium to high-P (12.1 ± 1.1 kbar and 780 ± 45 ºC), formed essentially coevally with arc-magmatism at 468 ± 4 Ma (Casquet et al., 2011, VII Hutton Symposium, Abstract p. 33). Mixing calculations based on a primitive arc component and the metasedimentary rocks suggest that a variable proportion of a primitive arc component is present in the Ordovician igneous rocks of the SVF belt.