Geochronological evolution of the Pitangui greenstone belt, southern São Francisco Craton, Brazil: Constraints from U-Pb zircon age, geochemistry and field relationships

Abstract

The Pitangui greenstone belt (PGB) – located in the southern São Francisco Craton, (Brazil) – is a volcano-sedimentary assemblage that is tectonically imbricated over the orthogneiss and granitoid rocks collectively termed the Divinópolis Complex. One diorite gneiss sample from the northern Divinópolis Complex yields an igneous zircon age of 2878 ± 5.7 Ma, constraining the time of crystallization. The basal sequence of PGB predominantly comprise metabasalt with restricted bodies of metagabbro/metadiorite, metaultramafic volcanic rocks and BIF. The upper sequences of the PGB exhibit a maturation of the basin, grading to more felsic and sedimentary lithologies, such as clastic metasedimentary rocks, intermediate metavolcanic rocks, and metapyroclastic rocks. The first recognized deformation phase in the PGB (Dn) is associated with greenschist facies conditions and the development of the main tectonic foliation. The second deformation phase (Dn+1) is associated with amphibolite facies metamorphism, while a later (Dn+2) phase may be related to younger Proterozoic deformation (ca. 2.0 Ga). PGB metabasalts yield a crystallization age of 2740.2 ± 12 Ma and are characterized by a tholeiitic affinity and flat LREE patterns. Based on mixed MORB- and arc-affinities, a back-arc basin setting is suggested for the formation of PGB basalt. This mafic magmatism (2740 Ma) is slightly younger than the ages reported for the Rio das Velhas greenstone belt, with which the PGB is generally correlated. The metasedimentary rocks of the PGB have age intervals spanning from 2690 to 3250 Ma. One mica-quartz phyllite has a maximum depositional age of 2769.8 ± 8.9 Ma while a chlorite phyllite records and age of 2693.2 ± 7.9 Ma. The metasedimentary rocks are derived from a source area of mafic to felsic composition that is similar to TTGs, cratonic Archean shales and E-MORB. The predominant sources were likely the sialic Archean crust of the granite-gneiss complexes (e.g. the Divinópolis Complex and potassic granitoids of the Mamona Event) and the mafic sequences of the arc (e.g. PGB back-arc basalt). One late potassic pluton (Casquilho monzogranite) crosscutting the fabrics of the volcano-sedimentary assemblage and yields an intrusion age of 2694.7 ± 9 Ma, constraining the minimum sedimentation age assembly of the package. Collectively, a Neoarchean primitive island arc or back-arc basin is proposed to have generated the PGB, which is here interpreted as distinct from the Rio das Velhas greenstone belt. We suggest that PGB developed during an accretion-collision event in the southern São Francisco Craton before cratonic stabilization of the crust by the end of the potassic 2760-2680 Ma Mamona Event.