U-Pb geochronology and coupled Hf-Nd-Sr isotopic-chemical constraints of the Cassiterita Orthogneiss (2.47–2.41-Ga) in the Mineiro belt, São Francisco craton: Geodynamic fingerprints beyond the Archean-Paleoproterozoic Transition
Abstract The Cassiterita Orthogneiss is the oldest pluton so far found within the Mineiro belt. In situ zircon U-Pb ages constrain the crystallization of the Cassiterita Orthogneiss to between 2472 ± 11 and 2414 ± 29 Ma. Zircon overgrowths are dated at 2024 ± 54 Ma, which matches the timing of the regional metamorphic overprint in the main belt. One metatonalite dike yields a zircon U-Pb age of 2156 ± 17 Ma, while a single inherited core is concordant at 2426 ± 17 Ma. The studied rocks exhibit low to medium grade metamorphism, and show tonalitic and granodioritic composition akin to TTG suites (tonalite-trondhjemite-granodiorite). The trace element patterns suggest that partial melting and fractional crystallization operated in the genesis. The La/Yb N , Yb N , Sr/Y, Nb/Ta and Zr/Sm values point to a garnet amphibolite residue. The positive e Nd(t) (+5.2 to +1.3) and low 87 Sr/ 86 Sr i (0.700–0.702) isotopic signatures suggest a LREE- and/or LILE LILE-enriched MORB mantle source. The zircon e Hf(t) parameters (-8.0 to +2.9) of the studied samples indicate some crustal assimilation during magma genesis. The chemical and isotopic signatures of the Cassiterita Orthogneiss are similar to nearby (ca. 2.35 Ga) TTG plutons, interpreted as cogenetic. This peculiar plutonism (from 2.47 to 2.35 Ga) marks the earliest stage of a long-lived accretionary regime (2.47–2.00 Ga) that built the Mineiro belt. This earliest orogenic event is roughly coeval with the (chemical) sedimentary infill of the passive basin (Minas Supergroup), previously dated at ca. 2.42 Ga. The age matches suggest a geodynamic link between the oceanic arc magmatism and penecontemporaneous basin infill in the continental margin. Early Paleoproterozoic TTG plutons such as the Cassiterita Orthogneiss have been related to the earliest accretionary processes that developed away from continental margins, where roughly coeval dike swarms and A-type plutons are emplaced along with the onset of passive basins. We explore the potential association and temporal interplay between convergent and intraplate settings and other major changes beyond the Archean-Paleoproterozoic Transition.