ORIGIN OF LOWER PALEOZOIC E-LIKE MAGMATIC ROCKS IN THE LAURENTIAN-DERIVED CUYANIA TERRANE IN ARGENTINA

Mafic and ultramafic magmatic rocks with E-MORB-like major and trace element affinities and ages from about 576 to 418 Ma are well known between 28°S and 35°S latitude along the western margin of the Precordillera Range in the central Andes of western Argentina (e.g., Kay et al., 1984; Davis et al., 2000; Boedo et al., 2013). They consist of ophiolite complex lithologies dominated by variable age gabbro and diabase units in the Cortaderas region, Ordovician pillow lavas and sills intercalated with graywackes to the north, and late Silurian basalts intruded into sediments at 32°-32.5°S. All are related to a marine setting, have been subjected to low grade metamorphism and were emplaced along the Gondwana margin after collision of the Laurentian-derived Cuyania terrane at ~ 465 Ma. All have similar E-MORB-like major and trace element chemical characteristics and most have initial eNd ratios ranging from +6 to +7.5. Exceptions are Silurian basalts with eNd of +7.6 to +9.3 to the west. Indicators of their EMORB signatures are La/Ta ratios ~ 12 to 16; E-MORB (within plate) like ratios among Hf, Th, and Ta; and flat overall and REE patterns with La/Yb = 2-4 and La/Sm = 1.5-2. Ubiquitous augite grains have oceanic floor basalt-like Ti, Mn and Na compositions. Pyroxenite, ophitic diabase, and microgabbros near 31°20’S, 69°17’W also show an E-MORB-like chemistry. Graywackes in fault contact with the mafic rocks have arc-like La/Ta ratios and La, Hf, Sc and Th contents similar to turbidites derived from and deposited along a continental margin arc. Like Ordovician graywackes near 35°S with Nd model ages of 1.4–1.5 Ga, the graywackes were likely derived from the Cuyania terrane Grenville-age basement and deposited near an extending continental margin. The Precordillera E-MORB like basaltic rocks are largely interpreted to have been formed in the early stages of oceanic rifting along the Gondwana (Precordillera) continental margin with their E-MORB-like character reflecting an enriched mantle source near a continental margin. Overall, these E-MORB-like Precordillera mafic rocks have chemical features comparable to similar age mafic rocks along the Appalachian margin showing the similarities of processes along a long persisting relatively narrow Iapetus ocean.