Lithogeochemistry and zircon U-Pb geochronology of the Granjeiro Complex and associated units, Curral Novo do Piauí, NW-Borborema Province, Brazil: implications for Archean to Paleoproterozoic crustal evolution
José Alberto Rodrigues do ValeLena Virgínia Soares MonteiroCamila Franco BastoVladimir Cruz de MedeirosDouglas Almeida SilveiraEvilarde Uchôa FilhoNilo Costa PedrosaJoseneusa Brilhante RodriguesTiciano José Saraiva dos Santos
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Abstract:
The Curral Novo Iron District, located in the center-east of the state of Piauí, is hosted in the Granjeiro Complex in its southwestern section, located in the Rio Piranhas-Seridó Domain, northern subprovince of Borborema Province.In this district, the banded iron rocks have an average content of approximately 27% Fe, with an estimated potential of more than one million tons of ore.Through the integration of petrographic, isotopic, geochemical and geochronological field data, a comprehensive characterization of the lithotypes present in the complex (orthogneisses, migmatites and a metavolcano-sedimentary unit) and associated units (syenogranitic orthogneisses and pegmatite granites) was carried out, focusing on the evolutionary discussion of this Archean complex, as well as the origin of metasomatism present in the iron district.Tonalitic and granodioritic orthogneisses include calcic to alkali-calcic rocks with low mafic mineral content.These rocks have low concentrations of lithophile elements and high field potential elements, significant fractionation of light rare earth elements and slightly negative Eu anomalies.U-Pb geochronology in tonalitic orthogneiss zircon from the Granjeiro Complex indicates the age of the protolith in the Paleoarchean at 3349 Ma.The metavolcano-sedimentary unit is made up of amphibolites, magnesian schists, serpentinites and banded iron rocks.Such rocks derived from mafic and ultramafic protoliths have a komatiitic and basaltic composition and have a genesis associated with the subduction phase of oceanic plates, presenting a geochemical affinity similar to that of modern island arc rocks.The iron rocks present sedimentary facies of the oxide and silicate types, with strong impoverishment of high field potential elements and rare earth elements and negative Ce anomalies.These units are intruded by syenogranitic plutons, collisional powders, type A2, aged 2651 Ma.Chemically, these granites are metaluminous to peraluminous, alkali-calcic and ferrous types, having a slight enrichment of light rare earth elements in relation to heavy rare earth elements and high contents of lithophile elements and high field potential elements.The rocks of the Granjeiro Complex show records of strong crustal reworking in the Paleoproterozoic, starting from the Riacian, with a metamorphic peak in metavolcanosedimentary rocks indicated by U-Pb dating in amphibolite zircon at 2200 Ma.An important magmatic event related to the global process of extension of continental masses during the Statherian period, aged 1759 Ma, represented by several intrusive plutons in all units of the area, promoted hydrothermal alteration in the surrounding amphibolites and iron rocks, causing silicification, albitization, potassification, carbonatization, sulfidation and reconcentration of iron.δ 34 S values = -3.11 to +2.3 per mil in hydrothermal sulfides suggest magmatic sources for the sulfur associated with the hydrothermal system.The incorporation of the analysis of these diverse geological data, made it possible to obtain a detailed understanding of the geological evolution of the Granjeiro Complex in the study area, as well as the origin of metasomatism in the iron district.Lithogeochemistry and zircon U-Pb geochronology of the Granjeiro Complex and associated units, Curral Novo do Piauí, NW-Borborema Province, Brazil: implications for Archean to Paleoproterozoic crustal evolution FIGURE 1. Tectonic framework of the Borborema Province and its division into North, Central (or Transversal) and South subprovinces, in addition to their respective domains (Medeiros et al. 2021).Keywords:
Protolith
Geochronology
Banded iron formation
Banded iron formation
Greenschist
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Archean banded iron formations (BIF) represent a major contributor to better constraining and assessing the paleogeography and evolution of Archean cratons. In this context, we conducted an exhaustive sampling and analysis campaign of BIF units in the Congo Craton, covering several greenstone belts within the Ivindo, Kelle-Mbomo, and Chaillu blocks. The REE + Y patterns suggest: (1) Interaction of seawater with Fe-oxyhydroxides, as illustrated by strong REE enrichment coupled with La and Y enrichment; (2) contributions from high-temperature (>250 °C) hydrothermal fluids, illustrated by positive Eu anomalies; and (3) detrital input as suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio. These observations suggest a typical environment of Algoma-type BIF deposition. Moreover, assessment of the Ce anomalies in a combination of HREE enrichment indicates that some basins in the Chaillu and Ivindo blocks may have known potential oxygen-rich episodes in the early Archean during the deposition of these BIFs.
Banded iron formation
Greenstone belt
Deposition
Early Earth
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Banded iron formation
Greenstone belt
Dharwar Craton
Stromatolite
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Similarities between Proterozoic (~ 1.8-2.5 Gyr) and Archean (> 2.5 Gyr) banded iron-formations are probably more significant than their differences. The contrasts largely reflect differences in the tectonic settings of Proterozoic and Archean terrains. Archean banded iron-formations are not as thick nor laterally as extensive as the major Proterozoic iron-formations. Nevertheless, some Archean iron-formations have strike lengths of over 150-200 km and may have been quite extensive prior to the deformation that has affected most Archean terrains. Stratigraphic sequences in which iron-formations occur are highly variable and indicate that iron-formations formed in many depositional environments. Sedimentary textures in the iron-formations are dominated either by granules and oolites or laminations (including microbanding) reflecting differences in their physical conditions of deposition. Granular and oolitic textures are abundant in only three Proterozoic depositional basins and most Precambrian iron-formations are laminated. Despite differences in associated lithologies and sedimentary textures Precambrian iron-formations have similar bulk compositions and mineral assemblages, implying that the chemical conditions of iron-formation deposition were similar through much of the Precambrian. The formation of banded iron-formation appears not to have reached a maximum around 1.8-2.0 Gyr but to have been an important process over a long period in the Precambrian.
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Riphean
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Archean rocks may provide a record of early Earth environments. However, such rocks have often been metamorphosed by high pressure and temperature, which can overprint the signatures of their original formation. Here, we show that the early Archean banded rocks from Isua, Akilia, and Innersuartuut, Greenland, are enriched in heavy iron isotopes by 0.1 to 0.5 per mil per atomic mass unit relative to igneous rocks worldwide. The observed enrichments are compatible with the transport, oxidation, and subsequent precipitation of ferrous iron emanating from hydrothermal vents and thus suggest that the original rocks were banded iron formations (BIFs). These variations therefore support a sedimentary origin for the Akilia banded rocks, which represent one of the oldest known occurrences of water-laid deposits on Earth.
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Early Earth
Hadean
Greenstone belt
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This thesis summarises the results of four original papers concerning U-Pb geochronology and geochemical evolution of Archaean rocks from the Kuhmo terrain and the Nurmes belt, eastern Finland. The study area belongs to a typical Archaean granite-greenstone terrain, composed of metavolcanic and metasedimentary rocks in generally NS trending greenstone belts as well as a granitoid-gneiss complex with intervening gneissic and migmatised supracrustal and plutonic rocks. U-Pb data on migmatite mesosomes indicate that the crust surrounding the Tipasjarvi-Kuhmo-Suomussalmi greenstone belt is of varying age. The oldest protolith detected for a migmatite mesosome from the granitoid-gneiss complex is 2.94 Ga, whereas the other dated migmatite protoliths have ages of 2.84–2.79 Ga. The latter protoliths are syngenetic with the majority of volcanic rocks in the adjacent Tipasjarvi-KuhmoSuomussalmi greenstone belt. This suggests that the genesis of some of the volcanic rocks within the greenstone belt and surrounding migmatite protoliths could be linked. Metamorphic zircon overgrowths with ages of 2.84–2.81 Ga were also obtained. The non-migmatised plutonic rocks in the Kuhmo terrain and in the Nurmes belt record secular geochemical evolution, typical of Archaean cratons. The studied tonalitic rocks have ages of 2.83–2.75 Ga and they have geochemical characteristics similar to low-Al and high-Al TTD (tonalite-trondhjemite-dacite). The granodiorites, diorites, and gabbros with high Mg/Fe and LILE-enriched characteristics were mostly emplaced between 2.74–2.70 Ga and they exhibit geochemical characteristics typical of Archaean sanukitoid suites. The latest identified plutonic episode took place at 2.70–2.68 Ga, when compositionally heterogeneous leucocratic granitoid rocks, with a variable crustal component, were emplaced. U-Pb data on migmatite leucosomes suggest that leucosome generation may have been coeval with this latest plutonic event. On the basis of available U-Pb and Sm-Nd isotopic data it appears that the plutonic rocks of the Kuhmo terrain and the Nurmes belt do not contain any significant input from Palaeoarchaean sources. A characteristic feature of the Nurmes belt is the presence of migmatised paragneisses, locally preserving primary sedimentary structures, with sporadic amphibolite intercalations. U-Pb studies on zircons indicate that the precursors of the Nurmes paragneisses were graywackes that were deposited between 2.71 Ga and 2.69 Ga and that they had a prominent 2.75–2.70 Ga source. Nd isotopic and whole-rock geochemical data for the intercalated amphibolites imply MORB sources. U-Pb data on zircons from the plutonic rocks and paragneisses reveal that metamorphic zircon growth took place at 2.72–2.63 Ga. This was the last tectonothermal event related to cratonisation of the Archaean crust of eastern Finland.
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Precambrian iron-formation, metamorphosed Archean deposits, unmetamorphosed Proterozoic deposits, genesis, Western Australia, South Australia, Queensland
Banded iron formation
Iron ore
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Rare earth element (REE) analyses of Precambrian banded iron formations (BIFs) show that distinct negative Ce anomalies, although rather weak or moderate (Ce/Ce* = 0.5–0.9), are commonly present in Algoma-type BIFs of the Early and Middle Archean, and even in the 3.8–3.7 Ga Isua iron formation (IF). This indicates that the seawater columns from which the BIFs precipitated were not entirely anoxic and that Ce oxidation mechanisms already existed in the 3.8–3.7 Ga oceans. The presence of pronounced negative Ce anomalies (Ce/Ce* = 0.1–0.5) in Late Archean (2.9–2.7 Ga) Algoma-type BIFs suggests that strongly oxygenated oceanic conditions like today emerged by 2.9–2.7 Ga. This suggestion is consistent with geologic evidence that small but widespread Mn deposits formed during Late Archean time. The Hamersley and Transvaal IFs (2.7–2.4 Ga in age) have less noticeable Ce anomalies (Ce/Ce* = 0.7–1.0). These BIFs were deposited on an evolving rift in a land-locked ocean that became anoxic due to intense hydrothermal activity. The 2.2–2.1 Ga Superior-type IFs exhibit distinct negative Ce anomalies (Ce/Ce* = 0.2–0.7), but the ca. 1.9 Ga IFs and the ca. 0.7 Ga IFs have less distinct Ce anomalies. These variations in Ce/Ce* values of the post-2.7 Ga BIFs may reflect the episodicity in global mantle plume activity that created locally anoxic basins.
Banded iron formation
Rare-earth element
Early Earth
Mantle plume
Atmospheric oxygen
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Greenstone belt
Banded iron formation
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Syncline
Banded iron formation
Felsic
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