Goias Massif Research Articles

Granulite-facies metamorphism at ca. 570-580 Ma in the Porangatu Granulite Complex, central Brazil: implications for the evolution of the Transbrasiliano Lineament

The Porangatu Granulite Complex is exposed in the central part of the Neoproterozoic Tocantins province in central Brazil, along the boundary between the Brasilia Belt to the east and the Araguaia Belt to the west. This is part of the transcontinental Transbrasiliano-Kandi shear system. The complex includes garnet-rich enderbite and charnockite, high-grade gneisses as well as lenses of garnet-bearing mafic granulite or amphibolites, and in situ anatectic charnockite, elongated in the NNE-SSW direction along the Talisma Shear Zone (TSZ). These rocks represent suites of ortho-derived rocks of calc-alkaline affinity and small contributions of tholeiitic basalts and aluminous paragneisses. The structural framework records thrust components probably related to the early stages of an oblique collision during the evolution of Neoproterozoic Brasiliano orogens, and can be understood as involving a collisional system of two crustal blocks, initially with thrust components which in its final stage evolved to a transcurrent system with dextral movement. This led to intense imbrication, generation of mylonitic foliation, stretching lineation, tectonic banding and rotation of structures and minerals. The heterogeneous and progressive ductile deformation was accompanied by metamorphic re-equilibrium in late Neoproterozoic time. Granulite facies conditions reached a metamorphic maximum at temperature and pressure above 850°C and 10 kbar, in an almost anhydrous environment, with or without anatexis. Zircon U-Pb SHRIMP analyses for two selected rock samples indicated the combined age of 580 ± 7 Ma for a charnockite and 548 ± 48 Ma for a mafic granulite from which the charnockite is throught to have been derived. The mafic granulite contains zircon grains of ca. 2.1 Ga, indicating Paleoproterozoic igneous protoliths involved in Neoproterozoic high-grade metamorphism. In addition, older inherited zircon grains of ca. 3.1 and 2.0 Ga ( 207 Pb/ 206 Pb ages) in charnockite also confirm the existence of older Archaean and Paleoproterozoic material in this region, possibly derived from the Goias Massif. A 0.88 Ga inherited zircon grain is suggestive of derivation from the Goias Magmatic Arc. This Neoproterozoic age for the high-grade metamorphism is substantially younger than those reported for other granulites in the Brasilia Belt (ca. 0.65 Ga), suggesting that the Porangatu Granulite Complex is more probably associated with the evolution of the younger Araguaia Belt. The new field, structural, petrographic and geochronological data suggest that the Porangatu Granulite Complex was involved in a high-temperature ductile strike-slip shear zone juxtaposing terrains of different ages (Archaean, Paleoproterozoic, Neoproterozoic), crustal nature and level (lower and middle continental crust), strongly reworked during the final stages of the Brasiliano orogeny, and represents the exposed roots of the Tocantins orogen.

Proveniência das rochas metassedimentares do Cinturão Araguaia com base em datações Pb-Pb em zircão e idades-modelo Sm-Nd

Provenance of metassedimentary rocks of the Araguaia belt based on Pb-Pb zircon ages and Sm- Nd model ages. This paper presents single zircon Pb-evaporation ages, obtained in detrital zircon grains, and Sm-Nd (T DM ) for metassedimentary rocks of the Araguaia belt, to investigate the possible source areas of these rocks, and to contribute to understanding the geologic evolution of this belt in the context of the West Gondwana amalgamation. Quartzite of the Morro do Campo Formation, located in the north segment of the belt (Xambioa region) gave Archean 207 Pb/ 206 Pb zircon ages (2.65-3.0 Ga), while in the south segment (Paraiso do Tocantins region), the 207 Pb/ 206 Pb zircon ages were mainly Meso-Neoproterozoic (1.25-0.85 Ga), and, secondarily, Paleoproterozoic (1.85-1.70 Ga). These results suggest the existence of different sources areas for this quartzite. The Sm-Nd (T DM ) model ages of metapelites (slate, phyllite and micaschist) present bimodal distribution with the main population occurring between 2.1 and 1.4 Ga (mode in 1.6-1.7 Ga), and a less abundant population between 2.7 and 2.4 Ga. These data have been interpreted as mixing of Paleoproterozoic (or even Archean) sources with younger, probably Meso-Neoproterozoic, sources. The main candidates to be the sources of the metassedimentary rocks of the Araguaia belt would be those crustal segments located in the southeastern (Sao Francisco Craton, Goias Massif, Goias Magmatic Arc). The sedimentary succession deposited in the Araguaia oceanic basin and the associated magmatic rocks were transported, later, towards the east margin of the Amazonian Craton, during the main tectonic that led to the structural development of the Araguaia belt as a result of the West Gondwana amalgamation.

Geologia, geocronologia e litoquímica dos granitos paleoproterozóicos do tipo A no setor setentrional da Província Tocantins

This work presents the geologic characterization of the Ipueiras Intrusive Suite, which comprises four granitic plutons, named do Carmo, Areias, Ipueiras and Italia, located in Goias Massif, northern sector of the Tocantins Province. New petrographic, litogeochemical, geocronological and isotopic data had allowed to discriminate these plutons of other granites of the region. Lithochemical data showed that they are peraluminous subalkaline granites, with A-type affinity. Single zircon Pb-evaporation data indicate an age of 2,08 Ga for this magmatism. Sm-Nd (T DM) ages between 2,15 and 2,19 Ga, and positive eNd(2,08 Ga) values (+2,5 to +2,9) for the Areias and Ipueiras granites suggest an origin involving juvenile mantle components. This granitogenesis may be related to the last magmatic pulses of the Transamazonian thermotectonic event, well defined in the region.

Dados geocronológicos preliminares sobre o enxame de diques Morro Agudo de Goiás

The Morro Agudo Dyke Swarm consists of mafic and ultramafic dykes and stocks that intrude archean granite-gneiss terranes of the Goias Massif. The dyke swarm was subdivided into five rock groups: Group I-basaltic andesite and metandesite in dykes; Group II-metadiabase, diabase, metagabbro and amphibolite in dykes; Group III-metadiabase associated with-meta ultramafic rocks in the same dyke; Group IV-meta ultramafic rocks in dykes; Group V-meta ultramafic rocks in stocks. Four basaltic andesite dyke samples were dated using the K-Ar whole-rock method, yielding 2,412 ± 30 Ma, 2,403 ± 48 Ma, 2,254 ± 25 Ma and 2,006 ± 45 Ma ages. A Sm-Nd isochronic diagram for Group III mafic-ultramafic dyke yielded a 2,331 ±101 Ma age with 0.50976 initial ratio. It is believed that this age may be extensive to the other mafic-ultramafic dykes of the dyke swarm.

Pb-Pb zircon ages of the porto nacional high-grade metamorphic terrain, northern portion of the Goiás massif, central Brazil

Single zircon Pb-evaporation ages were determined for a mafic granulite, two enderbites and a kinzigite of the Porto National High-Grade Metamorphic Terrain (PNHGT) in the Goias Massif. Zircons from mafic granulites and one of the enderbites yielded average 207 Pb/ 206 Pb ages of 2125 ± 3 Ma and 2153 ± 1 Ma, respectively, being interpreted as minimum ages of the igneous prptoliths. The other enderbite, whose zircons present round terminations, yielded an average 207 Pb/ 206 Pb age of 2097 ± 2 Ma. Zircons from the kinzigite, displaying cores and rims overgrowth, presented 207 Pb/ 206 Pb ages ranging between 2072 and 2115 Ma. However, a very homogeneous zircon crystal without overgrowth yielded an age of 2100 ± 2 Ma, with a distinctive low Th/U value (0.02). This zircon is interpreted as a metamorphic crystal and its age probably sets the age of the granulite facies metamorphism, indicating that the PNHGT is a result of the Tranzamazonian thermotectonic event.

THE PIRINEUS SYNTAXIS: AN EXAMPLE OF THE INTERSECTION OF TWO BRASILIANO FOLD-THRUST BELTS IN CENTRAL BRAZIL AND ITS IMPLICATIONS FOR THE TECTONIC EVOLUTION OF WESTERN GONDWANA

The Pirineus Syntaxis (PSX) is a concave to the foreland (toward the craton) curve in the otherwise NS-trending structural grain of the erogenic province bordering the west side of the Sao Francisco craton. Its geometry reflects the intersection between two non-parallel foldthrust belts of different ages and style, formed during two separate non-coaxial deformation episodes during the Brasiliano Orogenic period of Neoproterozoic (probably around 900-600 Ma). These ages are not well constrained, since very little radiometric dating is available. The two limbs of the syntaxis are here interpreted as the younger and narrower Northern Brasilia belt (NBB), and the older and more complete Southern Brasilia belt (SBB). Rocks involved in the deformation that formed the PSX include the Paranoa, Canastra and Araxa Groups, generally accepted as passive margin environment and more restricted to the SBB. The Rio do Peixe Group is a volcanic-sedimentary sequence that prevalently occurs in the SBB, while the Intrusive Rock Suite and Related Metasedimentary Rocks predominantly occur in the NBB. The Granite-gneiss complex of the Goias Massif (GCM) to the west, comprises the metamorphic core of the syntaxis. The collision of the Rio de la Plata (Paranapanema?) craton with the Sao Francisco craton formed the SBB, that preceded the final collision and accretion of the GCM with the SFC that formed the NBB. A weak collision of GCM with the southern border of the Amazonian craton determined the last deformation phase in the syntaxis. This is the first attempt to subdivide the Brasiliano of central Brazil and thus contributed to a better understanding of the evolution of western Gondwana. The conclusions of this paper were primarily based on field structural analysis.

Dados Rb-Sr e Sm-Nd da região de Jussara-Goiás-Mossâmedes (GO), e o limite entre terrenos antigos do Maciço de Goiás e o arco magmático de Goiás

The nature of the limits between the Archean/Paleoproterozoic terrains of the Goias Massif and the Neoproterozoic (ca. 900 - 650 Ma) juvenile terrains of the Goias Magmatic Are is still poorly understood. In this study we have investigated southwestern and southern areas of the massif, in the region between the towns of Goias, Jussara and Mossâmedcs. Several granitic, gneissic and mylonitic rock were recognized and dated (Rb-Sr and Sm-Nd methods) units in the transect between Goias, situated in the Goias Massif and Jussara, in the magmatic are. From east to west they are: (i) Uva tonalitic gneiss (GT) (2564 ± 140 Ma; i.r. = 0.7017 ± 0.0003; T DM =3.0-3.5 Ga); (ii) Uva granite (GU) (2851 ± 180 Ma; e Nd (T) = +0.3; T DM = 3.0-3. l Ga); (iii) Itapirapua mylonitic gneiss; (GMI) (iv) syn-tectonic coarse-grained porphyritic granite (GPG) (759 ± 62 Ma; i.r. = 0.7053 ± 0.0002; T DM = 2.0-2.3 Ga); (v) Granitic gneiss (GG) (2601 ± 209 Ma; i.r. = 0.7016 ± 0.0007; T DM = 2.4 - 2.5 Ga), (vi) Mylonitic gneiss (GM). GT and GU display predominantly E-W tectonic foliation, with a local and weak superimposed N-S deformation. N-S structures become progressively more important towards the west, with the development of wide dextral shear zones, which are typical in the rocks of the magmatic are. The structural and geochronologic data indicate that the E-W deformational fabric is Archaean or Paleoproterozoic in age, and the N-S fabric is Neoproterozoic. Felsic metavolcanic rocks from the Mossâmedes area, soutlvof the Goias Archaean terrains, yielded Rb-Sr isochron ages of 1978 ± 55 Ma and 1582 ± 101 Ma, with initial 87 Sr/ 86 Sr ratios of ca. 0.7023 and 0.7053, respectively. Therefore, these rocks are not part of the Neoproterozoic magmatic are, and are better interpreted as part of the southern edge of the Goias Massif. The data presented suggest that the rocks investigated here are part of the southern and southwestern edges of the Sao Francisco-Congo continental plate. According to this reconstruction the Goias massif represented an indentation of that plate during the continental collision that originated the Brasilia Belt.

REGIONAL DISTRIBUTION AND ESTRUCTURAL CONTROL OF THE GOLD OCCURRENCES/DEPOSITS IN THE GOIÁS MASSIF AND BRASÍLIA BELT

The distribution of gold occurrences/deposits in the GoiAs Massif and Brasilia Belt reflects the tectonic controls that operated during their geologic evolution. Their distribuiton shows three main regional trends: northwest-southeast, northeast-southwest and north-south, which correspond to tectonic discontinuities generated during the Brasiliano event. Since the regional structural features are an important control for the location of the main gold occurrences/deposits in both massif and belt, further studies of the regional tectonic regime and the establishment of structural models for these geotectonic units are essential to mineral exploration programs for gold in central Brazil.