Geocronologia e considerações acerca da evolução metamórfica da Formação Ferrífera São João Marcos (Criogeniano) e rochas metamáficas e metaultramáficas associadas, zona de interferência entre os orógenos Brasília e Ribeira

The study area is inserted in the context of the interference zone between the Brasília and Ribeira orogens. Based in microscopic evidence, four metamorphic stages were identified in the São João Marcos Iron Formation (SJ MIF) and associated metamafic and metaultramafic rocks. The first (E1), characterized by pyroxene relicts, marks the metamorphid peak in granulite facies. The second (E2) registers transition from granulite to amphibolite facies conditions, with the development of amphiboles (mainly hornblende) and plagioclase. The final moments of this stage is marked by the development of garnet coronas between hornblende and plagioclase, in a IBC (isobaric cooling) path. The third (E3) stage is defined by symplectites developed in a ITD (isothermal decompression) path, possibly related to the orogenic collapse. Finally, cummingtonite-grunerite appears marking the fourth (E4) stage, suggesting higher temperatures in relation to the previous stage. Regarding the geochronology, the metamafic and metaultramafic rocks present mostly archean and paleoproterozoic provenance, with subordinate neoproterozoic grains. 641,5 ± 2,9 Ma was calculated as crystallization, as well as 3 groups of metamorphic ages: 622 – 613 Ma; 599 – 598 Ma and 587 – 586 Ma. The associated metassedimentary rocks present archean to neoproterozoic provenance ages, with maximun depositional age in 686 Ma and 3 groups of metamorfic ages: 647 – 640 Ma, 609 – 605 Ma and 599 Ma. The stage E1 is related to a metamorphism between 647 – 640 Ma, contemporary to the crystallization of the metamafic and metaultramafic rocks; E2 and E3 are contemporary to metamorfic events between 622 – 598 Ma, which are related to Southern Brasília belt evolution.