Eventos igneos e metamórficos neoproterozoicos/eo-paleozoicos no arco magmático de Arenópolis, Goiás

Abstract

A proposta da tese é investigar a importante granitogênese de alto-K que aflora na região de São Luis de Montes Belos-Moiporá-Fazenda Nova-Mossâmedes, sudoeste de Goiás,compreendendo uma área aproximada de 5 600 km2 entre as coordenadas 510 000 N/ 8235 000 E e590 000 N/ 8150 000 E. Também investigou-se o significado geotectônico de quatro lineamentos estruturais de primeira ordem que ocorrem associados. Esses lineamentos são observados emimagem de satélite do sensor LANDSAT-5 (bandas 5 - 4 - 3) da região com destaque para olineamento Messianópolis-Novo Brasil que possui aproximadamente 120 km de extensão e cinturão de granitóides associado. Não se sabe se este sistema de lineamentos estruturais pode representar limites de blocos crustais em contato tectônico, zona de sutura ou apenas limite entre terrenos diferentes.Estudos geocronológicos pelos métodos U-Pb e Sm-Nd foram realizados para determinação da idade dos eventos ígneos e metamórficos e, eventualmente, do ambiente geotectônico que originou os gnaisses e granitóides da região. Investigou-se também os limites do Arco Magmático de Arenópolis e o significado das rochas com idades modelo antigas em meio a mais jovens que ocorrem nesta região.Para atingir os objetivos propostos foram realizadas várias etapas de campo para reconhecimento desses lineamentos e coleta de amostras. Nos afloramentos foram coletadas amostras que representassem de forma mais satisfatória possível as principais unidades. Foram utilizados mapas geológicos do SIG-Goiás 2009, na escala de 1/250.000 da região estudada.As análises U-Pb em zircão foram realizadas pelo método de diluição isotópica ID-TIMS no laboratório de geocronologia do IG-UnB e uma por SHRIMP no laboratório da Universidade Nacional da Austrália (ANU), em Canberra. O método U-Pb em zircão forneceu idades de cristalização dos granitoides, protólitos dos gnaisses e migmatitos.A sistemática Sm-Nd é usada como traçador de processos petrogenéticos e pode ser ferramenta importante para identificação dos ambientes tectônicos de origem. Idades modelo TDM em amostras de rocha total das rochas ígneas e metamórficas foram determinadas, bem como a idade do metamorfismo do ortognaisse de Sanclerlândia, a partir de uma isócrona de granada-RT. Todas estas análises foram realizadas no laboratório de geocronologia do IG-UnB. A combinação das idades obtidas pelos dois métodos permitiu estabelecer a cronologia da evolução crustal da região com vistas a estabelecer um quadro compatível com modelos baseados na tectônica global. Essa cronologia foi correlacionada aos principais eventos tectônicos já vi conhecidos na Faixa Brasília e no Arco Magmático de Arenópolis. O grande número de dados isotópicos obtidos nesta investigação permitiu estabelecer uma cronologia com intervalos menores destes eventos tectônicos.A tese foi elaborada na forma de 2 artigos, a serem submetidos à periódicos reconhecidos na área de tectônica e de circulação internacional. O primeiro artigo trata dos limites geográficos do Arco Magmático de Arenópolis e o significado de intercalações de ortognaisses e granitos de formados com idades modelo mais antigas intercaladas a mais jovens. O segundo artigo é sobre a idade dos eventos ígneos e metamórficos ocorridos no Arco Magmático de Arenópolis como agranitogenêse de alto-K pós colisional, uma das mais jovens da Faixa Brasília.Com o estabelecimento de um quadro geológico evolutivo mais claro desta região por meio da geocronologia e da geoquímica isotópica espera-se oferecer contribuição relevante ao conhecimento da evolução da crosta continental em parte da Faixa Brasília. _______________________________________________________________________________________ CAPÍTULO 2 - ABSTRACT

Data obtained in this work, combined with those available in the literature, allowed a better understanding of the crustal evolution in the southern portion of Brasilia Belt, where several rockunits belonging to the Neoproterozoic Arenópolis Magmatic Arc, as well as Archean granitic greenstone terrains represented by the Uvá Complex are exposed. Despite the large number of geochronological data obtained in the last two decades in rocks from this region, some questions regarding the chronology of igneous and metamorphic events and boundaries between the Neoproterozoic Arc and older units are not well established. Another aspect previously recognizedin this region is the presence of gneisses with TDM model age values predominantly between ca. 0.9and 1.2 Ga, in the midst of gneisses with Archean and Paleoproterozoic model age values. UsingTDM Sm-Nd model ages, _Nd (T) values and U-Pb crystallization ages in zircon from gneisses andgranites, it was possible to identify blocks of rock with coinciding boundaries, extensive morphostructural lineaments that stand out in Landsat satellite images: (i) Uvá Complex block withArchean model ages of about 2.9 Ga and _Nd (T) values of + 3.4 and crystallization age of 2.70 Ga;(ii) Sanclerlândia orthogneiss block has zircon U-Pb crystallization ages 828 ± 4 Ma and 822 ± 7Ma and TDM Sm-Nd model ages of 0.98 Ga and 1.44 Ga with positive _Nd (T) values. These data indicate that the protolith of this orthogneiss was formed by Neoproterozoic juvenile material; JG-22 sample represents the gneiss country-rocks of the post-collisional Córrego do Ouro granite andthat pre-collisional Buriti de Goiás granite on the southern edge of Uvá Complex. U-Pb analysis inzircon of this rock revealed206Pb/238U concordant age of ca. 614 Ma, interpreted as its crystallization age. Thus, it can be concluded that the crystallization age of the Buriti de Goiásgranite cannot exceed 614 Ma crystallization age of its country rocks. Sm-Nd isotopic data of Buritide Goiás granite revealed positive _Nd (T) values indicating that original magmas probably representre-melts of juvenile rocks of the Arenópolis Magmatic Arc while São Matheus granite revealednegative _Nd (614Ma) values indicating that original magmas represent the participation of olderrecycled material; (iii) Novo Brasil wedge block has deformed gneisses and granites with zircon UPb crystallization ages between 532 and 570 Ma and Sm-Nd model ages between 1.28 and 1.14 Gawith _Nd (T) values between -2.2 and -1 suggesting the important presence of Neoproterozoic juvenile material in the original magma composition. Only one sample JG-31 showed a model ageof 2.26 Ga and _Nd (2.1) of + 0.5; (iv) Ivolândia-Moiporá block is represented by a deformed granite represented by sample JG-138 with crystallization U-Pb age of 556 ± 5 Ma, model ages of 1.1 Gaand _Nd (T) of - 0.8 suggesting that the original magma represents partial melting of rocks from the Arenópolis Magmatic Arc. The similarity between geochronological and isotope data for the 3 Ivolândia-Moiporá and Novo Brasil wedge blocks, which are separated by the Moiporá-FazendaNova lineaments, demonstrate that this lineament does not separate rocks of different ages as it waspreviously believed. This lineament is probably formed by the foliation concentration and bandingof deformed granites exposed in the region. The Novo Brasil wedge is limited by theMessianópolis-Novo Brasil and Moiporá-Fazenda Nova lineaments. The first represents the largediscontinuity between two geological units of different ages. In its southern portion it separates the Sanclerlândia block from the Moiporá-Ivolândia block and its northern portion separates the Novo Brasil wedge from the Uvá Complex. The São Luis de Montes Belos lineament separates the Sanclerlândia block with orthogneisses crystallization age of around 825 Ma from the Turvânia-Fiminópolis-Palminópolis orthogneisses with U-Pb SHRIMP crystallization age of ca. 630 Ma. The Mossâmedes lineament has its concavity towards the southeast and separates the Uvá Complexfrom the Anicuns-Itaberaí volcano-sedimentary sequence. The discovery of 825 Ma crystallization ages for the Sanclerlândia orthogneiss, and between 530 and 590 Ma for gneisses and my loniticgranites of the Novo Brasil wedge are of great importance in understanding the geological evolutionof the southern Brasília Belt. In special the younger, Neoproterozoic to Early Cambrian ages, which indicate, at least locally, a late event of igneous and metamorphic rocks generation, previouslyunknown in the region. _______________________________________________________________________________________ CAPÍTULO 3 - ABSTRACT

In the region between Moiporá, Messianópolis, Córrego de Ouro in southwestern Goiás,there are several Neoproterozoic high-K late to post-orogenic biotite granite bodies as well as raremigmatite exposures, all associated with important N-S structural lineaments. These are I-Typegranites and some bodies only present foliation along their margins.This study investigates the relationship between granitic intrusions and the lineaments inorder to understand the roles of post-collisional granitogenesis and the thermal peak of theBrasiliano Cycle in the evolution of the Brasília Belt.SHRIMP U-Pb analysis of the Messianópolis granite and conventional TIMS zircon U-Pbdata for the Ivolândia granite revealed crystallization ages of 542 ± 5 Ma and ca 550 Ma,respectively. Sm-Nd analyses revealed TDM model ages that can be divided into two groups: (i) theolder group, includingthe Novo Brasil, Córrego do Ouro and Messianópolis granites, have modelages between 1.49 and 2.61 Ga and Nd (T) values that range, from - 4.6 to - 21.3, respectively, and(ii) the younger group, comprising the Serra do Impertinente and Ivolândia granites, have modelages between 1.15 and 1.35 Ga with less negative Nd (T) values of - 1.6 and - 3.2, respectively.The first group of granites, with older Sm-Nd TDM model ages is interpreted as having beenformed by magma that was generated from the re-melting of juvenile material belonging to the Arenópolis Magmatic Arc with strong contamination with ancient sialic crust material. The Córregodo Ouro granite with old Sm-Nd TDM model age (ca. 2.6 Ga) and a very negative Nd (T) value, canbe interpreted as the product of anatexis of ancient sialic crust.The second group of granites, which presented younger Sm-Nd TDM model ages andnegative Nd (T) values in comparison to those of the first group, can also be interpreted as derivedfrom re-melting of Arenópolis Magmatic Arc juvenile material in combination with some contamination with from ancient sialic crust.U-Pb dating of the zircon from leucosome JG-36B revealed an age of 535 5 Ma, which issimilar to the crystallization age of the high-K granites, indicating that they are part of the same granitogenesis.The crystallization of these granitic bodies and migmatites clearly post-dates the main phaseof the regional metamorphism and deformation (ca. 630 Ma) of the Brasília Belt. Localized meltingof the continental crust and the associated granitic magmatism may have been triggered by the latest movements of the lithospheric plates of the western portion of the Gondwana supercontinent in thelate Neoproterozoic and early Cambrian periods at approximately 545 Ma ago.Keywords: biotite granites, U-Pb zircon TIMS and SHRIMP, Sm-Nd, Arenópolis Magmatic Arc,Brasília Belt.