Petrogenesis of protoliths and U–Th–Pb SHRIMP ages in zircons of the Cerro Negro Paleoproterozoic mylonitic igneous suites, Tandilia belt: Adakitics local fingerprints in the Rio de la Plata craton

Abstract

A dataset of 23 geochemical analyses of mylonitized igneous rocks from the Cerro Negro hill, at the northwestern-most tip of the Azul Megashear Zone of the Tandilia Belt (TB), was evaluated to characterize protoliths: mafic granulites, felsic granulites, garnet-bearing hornblende gneissic and garnet-bearing granite. The evaluation of major and trace elements points to rocks derived from an arc-related suite of metaluminous to peraluminous magmas. Felsic granulites and garnet-bearing granite displayed REE patterns with Eu/Eu* >1.5 and K/Rb ratios around 250, suggesting accumulation of plagioclase feldspars during differentiation of mafic underplated magmas. An exceptional adakite-like signature is reported based on high Sr/Y (20–100) for low Yb (<15 ppm) and high La/YbN (20–70) ratios for low YbN (<4) in a felsic granulite and a garnet-bearing granite. A garnet-bearing hornblende gneissic mylonite and a xenolith of garnet-bearing striped gneiss, were selected for U–Th–Pb SHRIMP in zircon. Crystallization ages of 2092 ± 2 Ma and 2163 ± 5 Ma were determined respectively. A lower intercept at 610 ± 40 Ma obtained in the discordia diagram of the garnet-bearing stripped gneiss unravels a Brasiliano overprint in TB. We propose a petrogenetic model of two stages of incremental melting of a subducting slab based on adakitic signatures seen in some of the studied rocks. LILE-depleted mafic granulites derived from an initial stage of dehydration of the slab with constant K/Rb ratio (217–346). Subsequent partial-melting of the remaining LILE-depleted eclogite led to adakitic magmas as source of the garnet-bearing granite (2163 ± 5 Ma) and felsic granulite protolith. Adakitic source shows increasing K/Rb (189–354) ratio with decreasing Rb. Finally, a calc-alkaline magmatism produced the younger granite (2092 ± 2 Ma) which intruded preexisting rocks prior to their high-grade mylonitization. This model is consistent with the exceptional conditions hypothesized for adakites petrogenesis in the context of the Archean-Paleoproterozoic transition.