Chemical Metasedimentary Rocks Research Articles

Metallogenetic Mn-model of the Rhyacian-aged Buritirama Formation, Carajás domain (Amazon Craton)

The Mn-bearing Buritirama Formation is composed of clastic and chemical metasedimentary rocks representing a Rhyacian platformal basin (ca. 2.18–2.06 Ga) that limits the northern border of the Archean-Paleoproterozoic Carajás domain with the Paleoproterozoic Bacajá domain in the southern Amazon Craton, northern Brazil. The Buritirama Formation is divided into three main stratigraphic units. The Lower and Upper units consist of a basal quartzite followed by carbonate–silicate rocks (marls), dolomitic marbles (dolostone) and minor biotite schist while the Intermediate Unit consists of Mn-carbonate rocks topped by a supergene Mn-oxide deposit (Cenozoic age). These rocks were deformed and metamorphosed during the Transamazonian event (ca. 2.06 Ga).Geochemical data from dolomitic marbles (Lower Unit) exhibit negative Ce anomalies, high Y/Ho ratios (ca. 48) and seawater-like REE + Y patterns that reinforce an open platform setting for the Buritirama basin. Positive δ13C values (+3.2‰ to +5.0‰ V-PDB) for dolomitic marbles are considered to be an expression of the Lomagundi Carbon Isotopic Excursion. The metallogenesis of Mn-carbonate rocks is evaluated based on redox-sensitive trace elements (e.g. Zn and Ni), PAAS-normalized REE + Y patterns and δ13C values (−2.56‰ to +0.15‰ V-PDB) and point out to a classical multi-stage redox-controlled Mn-model involving: i) a primary hydrothermal fluid source related to ocean vents and/or normal faults; ii) microbially-mediated reduction of Mn-oxides particles at the ocean bottom; and iii) reactions between Mn2+ and CO32− (organic matter and seawater related), giving rise to diagenetic Mn-carbonates. The Rhyacian age, Mn deposits, and presence of Lomagundi-like δ13C allows to correlate the Buritirama Formation with the Águas Claras Formation within the Carajás domain further south, thus establishing an important metalliferous event that was up to now largely overlooked within the southern Amazon Craton.

Further evidence of Rhyacian arc magmatism in the basement of the Brasília Belt, western São Francisco pericraton

A recently mapped area in central Brazil revealed a slice of Paleoproterozoic basement within Neoproterozoic metasedimentary rocks of the Brasília Belt. The area, named here the Artulândia area for its proximity to the homonymous village, contains metatonalites and metagranodiorites in tectonic contact with a meta-volcanosedimentary sequence that encompasses felsic, intermediate and mafic metavolcanic rocks interlayered with clastic and chemical metasedimentary rocks. A sample of felsic orthogneiss from the metavolcano-sedimentary sequence, yielded a U–Pb zircon age of 2142 ± 4.4 Ma, which is interpreted as the crystallization age of the volcanic protolith. Samples from the metagranite unit returned U–Pb zircon ages of 2130 ± 7.4 Ma for a biotite metatonalite, 2157 ± 9 for a biotite metagranite, and 2147 ± 5 Ma for a sulfide-bearing metagranodiorite, indicating roughly contemporaneous plutonic activity. Whole-rock geochemistry shows calc-alkaline signatures for both units, and trace element results are compatible with an extensional setting, such as back-arc or intra-arc, for the metavolcano-sedimentary sequence, and a magmatic arc system for the metagranite unit. These rocks were generated in an episode of juvenile crust formation, as suggested by εNd(2.14Ga) values between 0 and + 3.18. Based on lithological association, age and chemical signature, we suggest that the studied rocks represent a lateral continuation of the Campinorte Arc beyond the Rio Maranhão Thrust. Thus, the occurrence of Campinorte Arc rocks on both sides of the thrust argues against the thrust being a Neoproterozoic collisional suture. The geology of the Artulândia area in the context of the Campinorte Arc attests to the complexity of the Rhyacian amalgamation that generated the São Francisco Paleocontinent during a Rhyacian Orogeny.

Geology, petrology and U-Pb geochronology of metavolcanic rocks in the Mundo Novo greenstone belt, eastern São Francisco Craton, NE Brazil: considerations about its tectonic setting

Abstract Field, petrological and LA-ICP-MS U-Pb geochronological data of metavolcanic rocks were used to interpret the petrological processes and to propose the tectonic setting for the Mundo Novo greenstone belt (MNGB) in the eastern Sao Francisco Craton. The metavolcanic rocks studied are metakomatiite, eastern and western metabasalts, and metadacite with subordinate metarhyolite, which host ocean floor hydrothermal alteration zones and are covered by ocean floor lithological associations composed of chemical metasedimentary rocks. Fractional crystallization and heterogeneous intraoceanic contaminations explain the mineralogical differences between the two metabasalts and the high (La/Yb)N ratio values of metakomatiite and metadacite. The metakomatiite and the eastern and western metabasalts feature a vector from the MORB-OIB array to the volcanic arc array in the Nb/Yb-Th/Yb diagram, similar to the Archean intraoceanic arc-basin systems. The geochemical pattern of the eastern and western metabasalts in the Zr-Zr/Y diagram suggests volcanism in nearby island arc and back-arc basin settings, respectively. The 2595 ± 21 Ma U-Pb zircon crystallization age of the metadacite allowed the determination of the timing of volcanism in the MNGB. Therefore, an intraoceanic provenance in an arc-basin system is proposed for the MNGB in the Neoarchean, which was later compressed between cratonic blocks during the Rhyacian-Orosirian event.

Neoarchaean banded iron formation within a ∼2620 Ma turbidite-dominated deep-water basin, Slave craton, NW Canada

Banded iron formations (BIFs) are iron- and silica-rich chemical metasedimentary rocks that span much of the Archaean and early Palaeoproterozoic rock record. Given that BIFs precipitated from ambient seawater, they provide an important archive of Earth’s ancient marine environments. Within the Slave craton occurs one of the largest and well-preserved Achaean turbidite basins in the world. Interstratified within the greywacke-mudstone turbidites is BIF that is commonly associated with ferruginous sediments and locally with a felsic tuff bed. Field investigations were conducted at four BIF locations from west to east across the Slave craton. These BIFs are composed of micro- and mesobands of quartz and magnetite, with various amounts of metamorphic grunerite, ferro-actinolite, ferro-hornblende as well as greenalite and stilpnomelane, the latter two are interpreted as retrograde metamorphic phases. As the BIF was deposited in a deep-water turbidite-dominated basin(s), the chemical component was occasionally diluted by clastic detritus, as illustrated by highly variable Al2O3 content, and Al2O3/TiO2 and Cr/Ni that are similar to the clastic sediments. On a layer-by-layer basis, the iron content in the BIF ranges from as low as 4wt.% and up to ∼75wt.% Fe2O3, whereas the SiO2 content ranges from 20wt.% to as high as ∼94wt.%. This, together with a hyperbolic germanium/silicon distribution, suggests that continentally-sourced silica comprised the background precipitation, which was periodically interrupted by pulses of hydrothermally-sourced iron. High-temperature hydrothermal fluids contributed, with an average shale-normalized positive Eu anomaly of 1.92, which indicates input of reduced Eu2+ to the ambient seawater. The BIF shows characteristic aqueous fluid-derived rare earths+yttrium fractionation (Pr/Yb<1) and positive La and Y anomalies. A negative Ce anomaly in parts of the BIF indicates the presence of an oxidizing agent able to fractionate Ce3+ to Ce4+. The age and trace element signature of the felsic tuff is similar to arc-type volcanic rocks, potentially linking the BIF and turbidite basin(s) to the Defeat Suite plutons (ca. 2630–2620Ma). The slow chemical rainout behind BIF deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump-generated debris flows, sourced and triggered by the tectonically active arc-terrain, episodically delivered clastic sediment.

Hidrogeoquímica das águas subterrâneas no município de Lençóis, Bahia, Brasil

This study aimed to point out the hydrogeochemistry groundwater in the municipality of Lençóis, State of Bahia, to classify them according to the content and types of dissolved salts. To this, twenty sampling points were listed in the groundwater in aquifers formed by metasediments, detritical covers and limestones, with the purpose of analyze the following parameters: electrical conductivity, temperature, pH, turbidity, total dissolved solids, HCO3, PO4, SO4, Cl-, NO3, Na+, K+, Ca2+, Mg2+, Fe3+, Mn4+, Sr2+ and Ba2+. The results showed that the groundwater in the municipality of Lençóis are distributed into two zones with different hydrogeochemistry, one of the largest controlled by geological clastic metasedimentary rocks typologies (sandstones, quartzites, conglomeratics, detritical covers) where the process of water-rock interaction allows occur little quantity salt water, slightly acidic, sweet, soft and predominantly sulfated sodic. The other zone is controlled by chemical metasedimentary rocks (limestones) as well as bicarbonates, where they occur hard water, slightly alkaline, sweets and sulfated calcic and mixed. The levels of metals and elements nitrate, phosphate and sulfate presented themselves generally at low concentrations, with some deviations detected in points of the urban area.

Detrital zircon sedimentary provenance ages for the Eoarchaean Isua supracrustal belt southern West Greenland: Juxtaposition of an imbricated ca. 3700 Ma juvenile arc against an older complex with 3920–3760 Ma components

The amphibolite facies Eoarchaean Isua supracrustal belt (northern part of the Nuuk region, southern West Greenland) is dominated by strongly deformed metabasalts, with chert, banded iron formation, felsic volcanic and volcano-sedimentary rocks and minor gabbro and sedimentary carbonates. It comprises a suture zone between a northern terrane formed at ca. 3700Ma and a southern one formed at ca. 3800Ma. At the junction between these two terranes is a strongly tectonised, thin unit of metachert, BIF and carbonate-bearing rocks with minor detrital components, named the dividing sedimentary unit. Away from the belt, the northern terrane is dominated by ca. 3700Ma tonalites and the southern one by ca. 3800Ma tonalites.The Isua supracrustal belt chemical metasedimentary rocks give low yields of small (commonly ≤50μm long), prismatic, oscillatory-zoned zircons with less common, somewhat rounded zircons of volcanic and/or detrital origin. The most siliceous of the felsic schists give a larger yield of volcanic or detrital zircons. The ages of these zircons are distinct in different parts of the belt. In the northern terrane, most have ages of 3710–3690Ma, with a few slightly older <3750Ma grains. In the southern 3800Ma terrane, a fuchsite-quartzite has 3900–3810Ma zircons. Three samples of the dividing sedimentary unit along the whole length of the belt yielded both 3920–3880Ma (in two samples) and ca. 3750Ma detrital zircons (in all three samples). The part of the ca. 3700Ma northern terrane in the Isua supracrustal belt is interpreted as lithologically diverse and imbricated assemblage of ca. 3700Ma arc-related volcanic rocks (boninitic, island arc tholeiite-picrite and andesite-dacite). In the associated metasedimentary rocks, the lack of >3750Ma detrital zircons suggests that these volcanic assemblages evolved remote from 3800Ma crust. In contrast, the up to 3940Ma detrital zircons from the dividing sedimentary unit and the southern fuchsite quartzite shows that the southern ca. 3800Ma terrane, although dominated by arc-related rocks, might contain older (pre-3800Ma) crust. We propose that the dividing sedimentary unit was deposited on top of already deformed southern terrane rocks, and then acted as a décollement when the northern terrane ca. 3700Ma arc complex collided with the southern terrane by ca. 3660Ma (from previous age constraints). Thus the detrital zircon age data is compatible with proposals from other evidence for juxtaposition of ca. 3700 and 3800Ma terranes in the Isua area.Some thin siliceous metasedimentary rocks along post-assembly mylonite zones in the Isua supracrustal belt yield some <3660Ma zircons. A tentative interpretation of these siliceous rocks (requiring confirmation) is that they could be younger, post-collisional sedimentary rocks, which were incorporated as tectonic lenses along some of the post-terrane-assembly <3660Ma mylonites which partition the collisional assemblage.

Geologic correlation between the Neoproterozoic Sergipano belt (NE Brazil) and the Yaoundé belt (Cameroon, Africa)

The Yaoundé belt (Cameroon) and the Sergipano belt (NE Brazil) belonged to a major and continuous Neoproterozoic orogen at the northern margin of the ancient Congo-São Francisco craton. The Yaoundé belt comprises schists, quartzites, gneisses and migmatitic gneisses grouped into three domains; the low-grade Mbalmayo Group in south and the medium- to high-grade Yaoundé and Bafia Group in north. The Sergipano belt is divided into six domains, the three southernmost of which are mostly made up of clastic and chemical metasedimentary rocks whereas the others are more diverse with a migmatite–gneiss complex, and two metavolcanicplutonic complexes. In general, the two belts show structural vergence and decrease of metamorphic grade towards the craton; three main deformation phases are recognized in the Sergipano belt in contrast with two described in the Yaoundé belt. The minimum age of Pan-African-Brasiliano collision in the Sergipano belt is constrained at 628 ± 12 Ma on syn-collision granites, whereas in the Yaoundé belt collision took place between 620 and 610 Ma, i.e. the age of granulite facies metamorphism. Sm–Nd isotope geochemistry and U–Pb age dating indicate that most clastic metasedimentary rocks in both belts were derived from sources to the north and, to a lesser degree, from the cratons to the south.

Sm–Nd studies at Mina III gold deposit, Crixás greenstone belt, central Brazil: implications for the depositional age of the upper metasedimentary rocks and associated Au mineralization

The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brası́lia belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe- and Al-rich metasedimentary rocks of the upper unit. Sm–Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00±0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show T DM model ages of mostly 2.33–2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite–garnet schist yields an age of 505±7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.

Geologia e geocronologia da suíte metamórfica colorado e suas encaixantes, SE de Rondônia: implicações para a evolução mesoproterozóica do SW do cráton Amazônico

Combined geochronological (U-Pb, 40Ar/39Ar, and Rb-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondônia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: 1) porphyritic monzogranite associated with amphibolite (bimodal magmatism), 2) interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation), 3) muscovite-garnet leucogranite, and 4) mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as undeformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0.18). The porphyritic ortogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 + 45 Ma, and 87Sr/86Sr i = 0.7040 + 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age of 1360 + 13 Ma. The 40Ar/39Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 + 3 Ma (grain 1), and 1303 + 2 Ma/1305 + 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 + 2 Ma. Hornblende from two amphibolite samples yielded 40Ar/39Ar plateau ages of 1313 + 4 Ma; 1313 + 6 Ma and 1312 + 3 Ma (RO-18), and 1325 + 3 Ma; 1326 + 2 Ma; 1330 + 3 Ma (RO-19). The weighted-mean age is 1319 + 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondônia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilândia Terrane (1215 - 1110 Ma). However, the unequivocal characterization of an orogeny refered to the time interval 1.36 - 1.32 Ga is still uncertain due to scarcity of geochronological data in the region. Similar ages are reported for correlatable intrusive rocks which cut the polydeformed basement rocks of central Rondônia. Thus, these ages are compatible with available Mesoproterozoic tectonic models for the SW Amazonian Craton.

Geologia de terrenos Arqueanos e Proterozóicos da região de Crixás-Cedrolina, Goiás

The paper describes theresult of a 1:25.000 scale geologic mapping of a 820 sq. km. area located at a portion of the contact between the Archean and Proterozic terrains between Crixas and Cedrolina, Goias, central Brazil. The Archean terrains comprise parts of the Crixas, Guarinos, and Pilar de Goias greenstone belts, the Caiamar Complex and the Moquem Block. The greenstone belts consist of different volumes of low-grade metakomatiites, metabasalts, siliciclastic and chemical metasedimentary rocks, which differ in structure and metamorphic style. The Caiamar complex consists of tonalite gneisses and a tonalite intrusion, subordinately mafic dikes metamorphosed under mid- to high-grade amphibolite facies in the former, and greenschist facies in the latter. The Moquem block consists of tonalite and granodiorite gneisses, as well as a mafic dike swarm and gabbro and pyroxenite intrusions metamorphosed under mid- to high-grade amphibolite facies. The Proterozoic rocks overthrust the Archean rocks and consist of metasedimentary and metavolcanosedimentary units. The latter may be subdivided into at least three thrust-slices that contrast in rock assemblage, structural pattern and metamorphic grade. The vergence of the Proterozoic associations is towards the south-southeast, in contrast with the general easterly vergence of the Brasilia Belt towards the Sao Francisco Craton.

Rare-earth element geochemistry of clastic and chemical metasedimentary rocks associated with hydrothermal sulphide mineralisation in the Olary Block, South Australia

Analysis of a variety of rock types from the South Australian portion of the Lower to Middle Proterozoic Willyama Inliers by ICP-MS demonstrate that primary rare-earth element (REE) distributions remained essentially unchanged, despite multiphase deformation and amphibolite grade metamorphism. Variable trends of preferential REE mobilisation are observed only where large amounts of fluid caused the rock to undergo strong retrogression and/or hydrothermal alteration. Except for these conditions, REE can be considered immobile and their distribution is generally independent of mineralogy, thus providing a powerful tool for the characterisation of provenance of protoliths in a high-grade metamorphic terrain. REE signatures and other geochemical constraints of some volcanogenic sediments, for example, point to an andesitic parentage, suggesting bimodal and gradually changing volcanism occurring in a developing intracontinental rift zone. REE distributions in metasediments associated with sulphide mineralisation reflect both physico-chemical characteristics of the ore-bearing fluids and rock compositions. Comparison of their REE profiles allows discrimination between: (a) strata-bound, redox-controlled deposits; (b) exhalative mineralisation; and (c) epigenetic vein-type mineralisation. REE distributions in the analysed rocks are in agreement with other lines of textural and geochemical evidence for that limited syn-tectonic remobilisation (low fluid/rock ratios, unchanged primary REE signatures) but widespread late-stage mobilisation of sulphides (high fluid/rock ratios, selective mobilisation of REE) occurred in the Olary Block. REE signatures of exhalative precipitates associated with sulphide mineralisation do not show a systematic stratigraphic variation but are indicative of fluctuating f O 2 conditions over short distances and display only moderately positive Eu anomalies. This contrasts with findings for the adjacent Broken Hill Block and highlights the dissimilarities of ore formation processes that were active in the Olary and the Broken Hill Block as well as the depositional differences between the two blocks.