Did the São Francisco Craton Split in the Rhyacian PaleoProterozoic?

Rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the mineiro belt, são francisco craton, brazil

Authors:

Ávila et al

Abstract:

The Tiradentes suite in conjunction with the coeval Estação Tiradentes metasedimentary sequence makes up a distinct tectonic element within the Paleoproterozoic framework of the Mineiro accretionary belt in the Southern São Francisco Craton. This suite is the second occurrence of volcanic and subvolcanic rocks in the Mineiro belt and shows petrographic and geochemical similarities with the nearby Serrinha suite (2.23-2.21 Ga) - the first reported Rhyacian volcanism in the Mineiro accretionary belt.

The Tiradentes suite is represented by mafic andesites, dacites, granophyres and tonalites and shows petrographic features of volcanic rocks, such as spherulites, amygdales, granophyric intergrowths and porphyritic textures with plagioclase phenocrysts. The rocks of this suite are metaluminous to peraluminous, enriched in alkaline elements, compatible with low-Al2O3 trondhjemites and show primary fractionation of plagioclase, apatite, hornblende, biotite and ilmenite. High HREE content coupled with incipient negative Eu anomaly and high Y values are consistent with a garnet-amphibolite protolith with plagioclase and hornblende as present in the residual phases. The samples of the Tiradentes suite show geochemical signatures of magmatic arcs, which is also observed in rocks from the Serrinha suite.

Rocks of the Tiradentes suite yield the following U-Pb zircon crystallization ages: i) mafic andesite: 2217 ± 23 Ma; ii) dacitic dike: 2204 ± 11 Ma; iii) subvolcanic tonalite: 2213 ± 9 Ma. The deposition of the Estação Tiradentes metasedimentary sequence probably took place after ca. 2088 ± 12 Ga (although it may be as young as 1990 ± 39 Ma), according to the U-Pb detrital zircon geochronology of selected samples (diamictite, ferruginous quartzite and carbonaceous phyllite). The Tiradentes rocks show three TDM model ages ranging between 2.4-2.3 Ga, indicating short crustal residence for the magmatic protoliths. This inference is also supported by the ɛNd(2.2Ga) values (+1.1 to +2.3), whereas the respective 87Sr/86Sr(t) ratios (0.7018 to 0.7032) are consistent with the involvement of low Rb/Sr material in the magma genesis, such as a Paleoproterozoic oceanic lithosphere.

From a tectonic perspective, the above data are consistent with the idea that a Paleoproterozoic seafloor developed outboard of the Archean - Siderian passive-margin setting (Minas Basin) of the São Francisco proto-Craton. The 2.23-2.21 Ga volcanic–subvolcanic rocks of the Tiradentes and Serrinha suites were associated with a Rhyacian oceanic arc. The Estação Tiradentes metasedimentary sequence (turbidites and distal phyllites) probably formed in back-arc setting concomitant with the Tiradentes suite, as indicated by detrital zircon geochronology