Abstract

n our investigation, we reveal the existence of metaplagiogranites, genetically linked to Calymmian metabasic rocks, emplaced within siliciclastic sequences of the Perau Formation, Votuverava Group, Apiaí Terrain, Southern Ribeira Belt. The Perau Formation is primarily characterized as a metavolcano-sedimentary sequence, distinguished by three main units arranged from bottom to top: quartzites, calc-silicate rocks hosting Zn-Pb sulfide mineralization, and schists and phyllites interlayered with metabasic rocks. The basal unit could be related to the proximal area, considering the proximity to the continental basement blocks. Maximum sedimentation ages for the Perau Formation's siliciclastic sequences range from the late Statherian to early Calymmian. The metabasic rocks placed in the upper unit present Calymmian ages (U-Pb zircon: 1.45-1.5 Ga), but the tectonic setting of this magmatism event remains a topic of ongoing debate in the literature. Sodic granitoids, identified in two distinct outcrops within the Perau Formation, are characterized here as plagiogranites through petrography and geochemistry. These rocks are predominantly composed of plagioclase and quartz, with occurrences of hornblende, clinopyroxene, garnet, and opaque minerals. Their subalkaline acidic composition features SiO2 values ranging between 58.0 and 71.5 wt.%; Na2O, CaO, and K2O vary between 4.6 and 6.5 wt.%, 0.9 and 3.8 wt.%, and 0.04 to 0.17 wt.%, respectively. They exhibit trondhjemitic geochemical composition with tholeiitic, ferrous, calc-alkaline to alkali-calcic, and meta-aluminous signatures. Using the U-Pb zircon in-situ SIMS method, we obtained crystallization ages of 1444±10 Ma, confirming their coeval relationship with Calymmian basic magmatism. Based on the analysis in conjunction with its coeval basic magmatism, we infer that the metaplagiogranites have been formed through extensive fractional crystallization from a mantle source, supported by the REE-SiO2 and TiO2-SiO2-K2O systematic, REE and trace elements patterns, and the presence of a negative Eu anomaly. The reducing characteristics (tholeiitic and ferrous signatures) suggest an association with extensional MORB-type environments. While plagiogranites play a crucial role in ophiolitic sequences, their volumetric significance is comparatively limited. The emplacement of the Calymmian basic magmatism in the Perau Formation has been interpreted in the literature as sills interbedded with siliciclastic sequences, however the presence of cogenetic plagiogranites could link the basic magmatism to the development of an oceanic crust, in which deformation and metamorphism during the Neoproterozoic may have resulted in a dismembered ophiolitic sequence. The development of an ocean during the Calymmian period, associated with the Mesoproterozoic sequences of the Southern Ribeira Belt, provides further important evidence of coeval basic magmatism within various terranes and different orogenic belts, also identified in the Rio de la Plata, São Francisco, China, Congo, and Siberia cratons, which are now scattered remnants of the breakup of the Columbia Supercontinent.