Archean evolution of the southern S?o Francisco craton (SE Brazil).

Programa de P?s-Gradua??o em Evolu??o Crustal e Recursos Naturais. Departamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto.Os continentes modernos s?o fundamentalmente diferentes de suas contrapartes Arqueanas, devido ? influ?ncia de um manto substancialmente mais quente sobre a produ??o e as propriedades reol?gicas da crosta no in?cio da Terra. Particularmente, a natureza e a composi??o do registro de rochas ?gneas sofreram modifica??es significativas no final do eon Arqueano, atestando mudan?as importantes nos processos geodin?micos. Este trabalho concentra-se nas rochas granit?ides posicionadas na por??o sul do craton S?o Francisco no sudeste do Brasil. Uma combina??o de observa??es de campo e petrogr?ficas, geoqu?mica de rocha total e an?lises de isot?picas (U-Pb, Lu- Hf, O e B) foram realizadas para um melhor entendimento nos mecanismos que levaram ? forma??o e estabiliza??o deste bloco crat?nico, al?m de compreender as modifica??es geodin?micas globais ocorridas durante esse per?odo. O embasamento do Quadril?tero Ferr?fero, no sul do cr?ton S?o Francisco, consiste em ortognaisses, intrudidos por abundantes pl?tons granit?ides e associados produtos magm?ticos (veios pegmat?ticos/apl?ticos, por exemplo). O embasamento do Quadril?tero Ferr?fero registra tr?s per?odos principais de magmatismo, denominados de Rio das Velhas I (RVI) e II (RVII) de 2.92-2.85 e 2.80-2.76 Ga, respectivamente, e o ?ltimo evento Mamona (2.75-2.68 Ga). Granit?ides e gnaissses dos tr?s complexos do embasamento estudados (Ba??o, Belo Horizonte e Bonfim) foram subdivididos em dois grupos, refletindo diferentes processos petrogen?ticos e/ou fontes: rochas de m?dio- e alto-K. Os gnaisses e granit?ides de m?dio-K foram formados durante os eventos Rio das Velhas I e II. Essas rochas apresentam algumas semelhan?as com os TTG Arqueanos, apesar de possu?rem um pequeno enriquecimento em SiO2 e K2O e deple??o em Al2O3, e foram interpretadas como o resultado da mistura entre um membro final derivado da fus?o parcial de uma rocha metabas?ltica e um fundido resultante da reciclagem de uma crosta mais antiga de composi??o TTG. Por outro lado, os granitos de alto-K colocados na crosta durante subsequente evento Mamona, assemelham-se aos t?picos granit?ides ? biotita do Arqueano tardio, com origem interpretada da fus?o parcial de metassedimentos imaturos. O envolvimento de metassedimentos na petrog?nese destes magmas de alto-K Neoarqueanos ? ainda suportado por: (i) uma tend?ncia para valores pesados de ?18O(Zrc), indicando equil?brio is?topico de O em condi??es de subsuperf?cie, e (ii) a presen?a fundidos magm?ticos ricos em boro e flu?dos intrudidos em 2.70-2.60 Ga, provavelmente derivados de um prot?lito metassedimentar rico em boro. Em geral, propomos que a evolu??o magm?tica e geodin?mica da por??o sul do craton S?o Francisco ocorreu da seguinte maneira. De ~ 3.50 a 2.90 Ga, fragmentos de crosta juvenil (possivelmente TTGs) foram produzidos, formando o prot?lito do atual sul do cr?ton S?o Francisco. Este ? um per?odo de crescimento crustal, como evidenciado pela modelagem isot?pica de hafnium ? partir de zirc?es detr?ticos. Em ~ 2.90 Ga, a crosta sofreu significativas modifica??es, devido ao in?cio de uma tect?nica colisional. Diversos fragmentos crustais com hist?rias distintas foram acrescidos progressivamente, construindo, por fim, um n?cleo continental r?gido e soerguido. Isto ? suportado por: (i) a transi??o para um regime dominado por reciclagem crustal, com diminui??o de contribui??o mant?lica em magmas rec?m-gerados, (ii) o envolvimento de componentes TTG mais antigos na gera??o de magmas de m?dio-K durante os eventos RVI e RVII e a simult?nea aus?ncia de materiais crustais preservados mais velhos que ~ 2.90 Ga na por??o sul do craton S?o Francisco, (iii) a abund?ncia de sistemas hidrotermais na por??o superior da crosta evidenciados por valores leves de ?18O(Zrc), indicando o soerguimento de grandes por??es da crosta continental durante o Neoarqueano, (iv) a progressiva matura??o crustal, registrada por uma tend?ncia para o magmatismo mais pot?ssico e rico em HFSE (High Field Strength Element). O soerguimento e r?pido soterramento de grandes bacias sedimentares dominadas por componentes cl?sticos no Neoarqueano (~ 2.75 Ga) levaram ? produ??o de granit?ides de alto-K durante o evento Mamona. Fluidos magm?ticos diferenciados portadores de turmalina percolaram a crosta e interagiram de forma generalizada com o greenstone belt adjacente. A presen?a de turmalina hidrotermal de origem magm?tica (e Neoarqueana) em associa??o espacial pr?xima aos complexos do embasamento sugere que a forma??o das estruturas domo e quilhas ocorreu antes do ?ltimo evento magm?tico (ou seja, antes de 2.70 Ga), o que est? em desacordo com a id?ia de que a regi?o adquiriu sua arquitetura atual durante o Paleoproteroz?ico.Modern continents are fundamentally different from their Archean counterparts, owing to the influence of a substantially hotter mantle on the production and rheological properties of the crust in the early Earth. Particularly, the nature and composition of the igneous rock record underwent significant modifications at the end of the Archean eon, attesting to important changes in geodynamic processes. This work focuses on the granitoid rocks that were emplaced in the southern tip of the S?o Francisco craton in southeast Brazil. Combined field and petrographic observations, whole rock geochemistry and isotope (U-Pb, Lu-Hf, O and B) analyses were performed in order to place better constraints on the mechanisms that led to the formation and stabilization of this cratonic block, and understand the global geodynamic modifications that occurred during this period. The basement of the Quadril?tero Ferr?fero district, in the southern S?o Francisco craton, consists of orthogneisses, intruded by abundant granitoid plutons and associated products of granitic magmatism (i.e. pegmatitic/aplitic veins). The basement in the Quadril?tero Ferr?fero records three main periods of magmatism, namely the Rio das Velhas I (RVI) and II (RVII) events at 2.92-2.85 and 2.80-2.76 Ga respectively, and the latest Mamona event (2.75-2.68 Ga). Granitoids and gneisses of the three studied basement complexes (Ba??o, Bonfim and Belo Horizonte) have been subdivided into two groups, reflecting different petrogenetic processes and/or sources: medium- and high-K rocks. Medium-K gneisses and granitoids were formed during the Rio das Velhas I and II events. These rocks have some similarities with Archean TTGs, despite a small enrichment in SiO2, K2O and depletion in Al2O3, and are thought to result from mixing between an end-member derived by partial melting of a metabasaltic rock and a melt resulting from recycling of older TTG crust. On the other hand, high-K granites emplaced during the later Mamona event resemble typical late Archean biotite- granites, and are inferred to result from the partial melting of immature metasediments. The involvement of metasediments in the petrogenesis of the Neoarchean high-K magmas is further supported by: (i) a trend towards heavy ?18O(Zrc) values, indicating O isotope equilibration at near-surface conditions, and (ii) the presence of boron-rich magmatic melts and fluids emplaced at 2.70-2.60 Ga, likely derived from a boron-rich metasedimentary protolith. Overall, we propose that the magmatic and geodynamic evolution of the southern S?o Francisco craton unfolded in the following way. From ~ 3.50 to 2.90 Ga, fragments of juvenile crust (possibly TTGs) were produced, forming the protolith of the present-day southern S?o Francisco craton. This period is one of net crustal growth, as evidenced by hafnium isotope modelling from detrital zircons. At ~ 2.90 Ga, the crust witnessed significant modifications, owing to the onset of collisional tectonics. Several crustal fragments with distinct histories were progressively assembled, ultimately building a strong and emerged continental nucleus. This is supported by: (i) the transition into a regime dominated by crustal reworking, with decreasing mantle input into newly generated magmas, (ii) the involvement of older TTG components in the generation of medium-K magmas during the RVI and RVII events, and the paralleled absence of preserved crustal materials older than ~ 2.90 Ga in the southern S?o Francisco craton, (iii) the abundance of upper-crustal hydrothermal systems evidenced by light ?18O(Zrc) values, indicating the emergence of large portions of the continental crust in the Neoarchean, (iv) the progressive crustal maturation, recorded by a trend towards more potassic and HFSE (High Field Strength Element)-rich magmatism. The emergence and rapid burial of large clastic sedimentary basins in the Neoarchean (~ 2.75 Ga) led to the production of high-K granitoids during the Mamona event. Contemporaneous widespread tourmaline-bearing differentiated magmatic fluids percolated the crust and interacted with the nearby greenstone belt. The presence of hydrothermal tourmaline of magmatic (and Neoarchean) origin in close vicinity of the basement complexes suggests that the formation of the dome-and-basin structures occurred prior to the last magmatic event (i.e. before 2.70 Ga), which is at odds with the current idea that the region acquired its present-day architecture in the Paleoproterozoic