LAUSR

Abstract In this paper new geochemical, geochronological and isotopic Lu–Hf data from the Siempre Amigos Hills (SA), western Tandilia, Argentina, is presented. Outcrops in this small area include representative lithologies of the basement Rio de la Plata craton (RPC) known as Tandilia Terrane. The new geochemical data characterize a heterogeneous Paleoproterozoic crust with a similar U–Pb SIMS magmatic age (~2.1 Ga) represented by a suite of mafic (amphibolites, charnockitic granulites), intermediate and felsic compositions (diorites, granodiorites and granitic gneisses), as well as intrusive granitic rocks (with Neoproterozoic age of ~652 Ma in zircon). Geochemical features suggest processes of magmatic differentiation from a mafic igneous protolith associated with the ‘Camboriu orogeny’ of the Trans-Amazonian–Eburnean cycle. Some amphibolites may represent sanukitoid magmas similar to those formed at the Archean-Proterozoic transition. This paper examines the hypothesis of a sanukitoid precursor for RPC Rhyacian magmatism. The potassic and calc-alkaline character of magmatic series, together with negative Nb–Ta–Ti anomalies and REE patterns, are compatible with classical modern subduction-related magmas. Precise Lu–Hf analyses of zircons collected from a granodiorite and a gneiss indicate a predominantly juvenile source (+eHf), with minor crustal recycling (-eHf). An inheritance age is recorded in a single Neoarchaean zircon core from a granodiorite (2771 ± 15 Ma), similar to calculated maximum Hf TDM (2.8 Ga). Considering the new available data from this region of the RPC, a sustainable petrogenetic model is proposed to explain the presence of geochemical and isotopic evidence of sanukitoid magmatism which may have occurred after 2.7 Ga, which seems to constitute a magmatic series which is a precursor to the development of the Rhyacian magmatic arc in the Tandilia Terrane. Medium-to high-grade metamorphic conditions probably resulted from Paleoproterozoic subduction followed by the intrusion of huge volumes of Rhyacian calc-alkaline magmatism. This model is compatible with RPC configuration as a collage of terranes, and also consistent with Trans-Amazonian–Eburnean amalgamation of various microcontinents in the formation of Atlantica.