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Manuscript ID: 20170106. Received in: 09/08/2017. Approved in: 12/10/2017. ABSTRACT: Sr-Nd-Pb isotopic signatures were determined for silica-undersaturated hypabyssal alkaline basic rocks of the Los Baldecitos Formation in the Cerro Morado area, situated in the Ischigualasto — Vale de la Luna Provincial Park (northernmost San Juan Province, Argentina). The basic rocks show slight variations of the Sr-Nd-Pb isotopes and similar behavior of whole-rock geochemistry, which suggest a single source for the Cerro Morado volcanic rocks. The present-day Pb isotopic data show moderately radiogenic Pb compositions (206Pb/204Pb = 18.31 – 18.35), in addition to low values of 87Sr/86Sr initial ratios of 0.70314 – 0.70386 and highly radiogenic Nd initial isotopic compositions (+7.1 < εNd (228 Ma) < +9.3), which point to a PREMA mantle reservoir signature for the alkaline basalts with little evidence of crustal contamination. The involvement of a HIMU component in the source, previously suggested for the Cerro Morado volcanics, should be disregarded in the light of the Pb isotopic signature. The Sr-Nd signature together with Nd-TDM model ages not older than 295 Ma does not support the involvement of an ancient crust.

brazilian journal of geology

Pb isotope geochemistry and reappraisal of Sr-Nd isotopes of the Cerro Morado basic magmatism (Ischigualasto-Villa Union Triassic basin, NW Argentina): Implications for the mantle sources