Brazilian Shield Research Articles

Paleoproterozoic (~2.0 Ga) volcano‐plutonism in the southeastern region of the Amazon Craton: Petrological aspects and geotectonic implications

Detailed studies on the volcano‐plutonic associations of the Amazon Craton (AC) have been conducted since the 1980s. Since then, several issues concerning the definition of the lithostratigraphic units and their role in the AC evolution have been improved. Recently, a volcano‐plutonic association of around 2.0 Ga was described in the south‐central part of the AC, named as Jarinã Formation (volcanic rocks) and Vila Rica Intrusive Suite (granitoids). The present work deals with the unexpected occurrence of this volcano‐plutonic association in the southeastern region of the Amazon Craton. In the study area (Santaninha region), the rocks of the Jarinã Formation comprise two volcanic successions with distinct petrographic and compositional features. The amount and shape of the phenocrysts and crystal fragments allow the individualization of effusive and pyroclastic rocks. The geochemical character indicates that the volcanic rocks belong to two distinct successions as follows: intermediate to acid magnesian I‐type and acid ferroan A‐type. The I‐type succession shows high MgO, TiO2, P2O5, and Sr content and low K2O, Zr, Nb, Ta, Y, and heavy rare earth element and consists of undeformed andesitic to dacitic ignimbrites and subordinate effusive rhyodacites. The A‐type succession present high K2O, FeOt, Zr, Nb, Ta, Y, and heavy rare earth element content and low TiO2, P2O5, and Sr and comprises undeformed rhyolitic ignimbrites and subordinate effusive rhyolites. The granitoids of the Vila Rica Intrusive Suite crosscut the volcanic rocks of the Jarinã Formation and include two phaneritic facies as follows: coarse‐ to medium‐grained granite and medium‐grained granite. The MgO, TiO2, P2O5, Sr, K2O, Zr, Nb, Ta, Y, and rare earth element content demonstrate that the medium‐grained granite facies present geochemical characteristic of I‐type granites. Laser‐ablation inductively coupled plasma mass spectrometry U–Pb zircon data indicate crystallization ages of 2,009 ± 9 Ma for the I‐type dacitic ignimbrite, 1,994 ± 16 Ma for the A‐type rhyolitic ignimbrite, and 1,995 ± 14 Ma for the I‐type medium‐grained granite. These ages agree with the previous data of the Jarinã Formation in the south‐central region of the Amazon Craton and of the Vilca Rica Intrusive Suite in the southeastern of the Amazon Craton. The understanding of the geological context of these units carry on meaningful implications to the contextualization of the ~2.0 Ga volcano‐plutonic event in the AC and, consequently, to the compartimentalization of its geotectonic provinces.

Andean Tectonics and Mantle Dynamics as a Pervasive Influence on Amazonian Ecosystem

The Amazonian landscape evolution is the result of the combined effect of Andean tectonism, climate and the Earth’s interior dynamics. To reconstruct the landscape evolution and its influence on paleoenvironmental variations within Amazonia since the Oligocene, we conducted numerical experiments that incorporate different surface and geodynamic processes, reproducing many paleogeographic features as inferred from the sedimentary record. We show that the evolution of the drainage pattern gradually reduced the area of sedimentation derived from the Guiana and Brazilian shields while expanded the Andean derived deposits during the Miocene, affecting the nutrient availability. First order biotic habitats were inferred from these paleogeographical reconstructions, showing an eastward expansion of várzea and terra firme forests and consequent retraction of igapó forests, with a millennial-scale reconfiguration of a mosaic of habitats in the lowlands. We conclude that this dynamism probably guided the observed patterns of speciation in the most biodiverse biome on Earth.

Earthquake relocation, focal mechanism and stress field determination in central Brazil

In this work, we perform a seismological investigation in the central Brazil region for focal mechanisms and stress. This large area (910.000 km2) is surrounded by several geological provinces: Eastern Amazon craton; southwestern Parnaíba basin; western São Francisco craton; northern Paraná basin and almost the whole Tocantins province. The area is crossed, in an SW-NE direction, by the continental-scale discontinuity - Transbrasiliano lineament (LTB). In the Brazilian seismic catalog (1720–2019), it is possible to distinguish a seismic belt following the LTB lineament and some dispersed events in the northern, western and eastern areas. The study area is characterized mainly by low magnitude events (M < 4); only 11 events reported in the catalog with M > 4 occurred, and among them just one with magnitude 5. Based on quality criteria a set of 118 events from 338 present in the catalog were selected for a thorough analysis. This work was divided in three parts: (i) Events relocation with the iLoc code and Regional Seismic Travel Time (RSTT) velocity model; (ii) Focal mechanisms determination of 10 events using waveform envelopes inversion and polarities, and; (iii) Inversion of focal mechanisms for stress field. The study shows that the seismicity of the area is mainly concentrated in two relatively narrow belts and that the major compressional stress axis of the whole zone is well resolved with azimuth about 133° which is compatible with previous studies of stress in central Brazil.

The Archean granulite-enderbite complex of the northern Carajás province, Amazonian craton (Brazil): Origin and implications for crustal growth and cratonization

The central portion of the Canaã dos Carajás domain (Ouro Verde area) exhibits a peculiar scenario involving two units of orthopyroxene-bearing sodic rocks: (i) the Mesoarchean Ouro Verde felsic granulite with protolith crystallization and metamorphism ages of 3.05–2.93 Ga and 2.89–2.84 Ga, respectively, and (ii) the Neoarchean Café enderbite, composed of tonalites, trondhjemites and scarce quartz diorites that crystallized at 2.75–2.73 Ga and show syntectonic character. While the orthopyroxene in the felsic granulites was formed by breakdown of primary biotite during metamorphism, in the enderbitic rocks, this mineral was formed by crystallization of a hot water-poor magma at deep crustal levels. This terrane evolved in three important geodynamic stages: (1) at 3.05–2.93 Ga, TTG crust was generated by partial melting of LILE-enriched basalts in a N–S subduction setting; (2) from 2.89 to 2.84 Ga, in a collisional setting, large volumes of anatectic granites formed and contributed to crustal thickening, which triggered granulite-facies ultrahigh-temperature metamorphism in the TTG crust, forming the Ouro Verde felsic granulite; and (3) in the Neoarchean, at 2.75–2.73 Ga, the lowermost mafic crust was delaminated during crustal thickening, causing crustal underplating followed by partial melting of Mesoarchean mafic granulites, which was responsible for the origin of the enderbitic magma. This area was subjected to a transpressional tectonic regime, leading to the development of plutons (E–W-trending) amalgamated linearly and emplaced along ancient suture zones (ductile-brittle transition). Lu-Hf isotope data of magmatic zircon cores from the Mesoarchean felsic granulite show Hf-TDM2 values of 3.44–3.15 Ga and εHf(t) values between −1.7 and 3.0, which suggest juvenile sources. The Neoarchean enderbites present Hf-TDM2 ages of 3.46–3.29 Ga and lower εHf(t) values (between −4.8 and −1.9), which indicate a longer crustal residence time for these rocks. All these findings lead us to consider the central portion of the Canaã dos Carajás domain as exhumed lower crust, and the tectonic juxtaposition of anatectic granites, enderbites and high-grade metamorphic rocks suggests that the Ouro Verde area represents middle and deep exposure levels of the same crust.

Seismic attenuation structure of southern Peruvian subduction system

Seismic attenuation provides key constraints on understanding the structure and dynamics of subduction-zone system. In this study, we provide the first three-dimensional P and S-wave attenuation models beneath the Nazca-South America subduction zone by inverting t* data from 397 local shallow and intermediate-depth earthquakes. The study area covers the southern part of the Peruvian flat-slab region (11°–15°S) (where the subducting Nazca Ridge is migrating), and the Central Andean Plateau (∼13°–18°S) (associated with the subducting Nazca Plate) and the Brazilian mantle lithosphere. We obtain five major features in our attenuation model with seismicity including (1) well-defined along-strike and along-dip Nazca slab structure in terms of low attenuation, (2) high attenuation associated with the passage of Nazca Ridge and present location of the oceanic ridges, (3) high attenuation in the backarc mantle wedge and continental crust, (4) clear image of Brazilian Shield in terms of low attenuation, underthrusting to the west, and (5) high attenuation related to the slab dehydration beneath the volcanic arc at 100 and 200km depths. In particular, prominent low-Q zones in the continental crust and mantle wedge beneath the active volcanic arc reflect source zones of arc magmatism caused by fluids from the slab dehydration. The observed along-arc differences in slab Q estimates can be attributed to different hydration states between the flat and normal-dip slabs due to the subduction of the Nazca Ridge.

Phylogeny and biogeography of the Brazilian–Guiana Shield endemic Corymbophanes clade of armoured catfishes (Loricariidae)

AbstractNumerous rivers, interrupted by large waterfalls and extensive rapids, drain the geologically ancient Guiana Shield Highlands. We describe a new armoured catfish genus and two new species endemic to the upper Ireng and Kuribrong rivers, respective tributaries of the Amazon and Essequibo basins in western Guiana. Corymbophanes ameliae sp. nov. is distinguished by having vermiculations on the abdomen, bands on the caudal fin, the anal fin i,5 and narrow caudal peduncle. Yaluwak primus gen. &amp; sp. nov. is distinguished by having evertible cheek odontodes, a plated snout, a tall caudal peduncle and absence of adipose fin and iris operculum. We present a new molecular phylogenetic analysis inclusive of these and several related genera that suggests that the Corymbophanes clade (Araichthys, Corymbophanes, Cryptancistrus, Guianancistrus, Hopliancistrus and Yaluwak) originated in the Guiana Shield with secondary dispersal to the Brazilian Shield. Within the Guiana Shield, relationships among Corymbophanes and Yaluwak are consistent with geodispersal between drainages via headwater capture, although an uplift-mediated relictual distribution cannot be ruled out. ND2 haplotype structure among C. ameliae populations suggests that ichthyofaunal diversity on the Guiana Shield escarpment is shaped not only by inter-, but also intrafluvial barriers to gene flow.

Serra Sul diamictite of the Carajás Basin (Brazil): A Paleoproterozoic glaciation on the Amazonian craton

Abstract This paper reports the discovery of glacial deposits of likely Siderian–Rhyacian age (2.58–2.06 Ga) in South America (Carajás Basin, Brazil), thereby expanding the potential reach of Paleoproterozoic glaciations to the Amazonian craton for the first time. Glacially derived diamictites are stacked within a hitherto unrecognized ∼600-m-thick siliciclastic succession, here named the Serra Sul Formation. Well-preserved textures, with evidence of glaciotectonism and ice rafting, indicate deposition in a coastal subglacial to glacial-fed submarine fan system, in which the immediately underlying units (banded iron formation and volcanic rock) were the main source and bedrock. The Serra Sul diamictite may be correlated with any of the known Paleoproterozoic glaciations, or with none of them.

Detrital zircon signatures in Precambrian and Paleozoic sedimentary units in southern New Brunswick – more pieces of the puzzle

Southern New Brunswick consists of a complex collage of fault-bounded belts of Late Neoproterozoic igneous and metamorphic rocks, Early Paleozoic sedimentary, metamorphic and igneous units, and overlying Carboniferous sedimentary rocks. The area also contains the boundary between the Avalonian and Ganderian terranes as interpreted in the northern Appalachian orogen. New detrital zircon ages reported here provide improved understanding of depositional ages and provenance of diverse Neoproterozoic to Carboniferous rocks in this complex area. Detrital zircon data from samples with Neoproterozoic maximum depositional ages indicate a dominantly Gondwanan provenance with a strong influence from the Amazonian craton. However, quartzite from The Thoroughfare Formation on Grand Manan Island contains dominanly 2 Ga zircon grains, consistent with derivation from the West African Craton. The age spectrum is similar to that from the Hutchins Island Quartzite in the Isleboro block in Penobscot Bay, Maine, strengthening the previously proposed correlation between the two areas. Cambrian samples also show prominent peri-Gondwanan provenance with strong influence from Ediacaran to Early Cambrian arc magmatism. The maximum depositional ages of these samples are consistent with previous interpretations of Cambrian ages based on fossil correlations and field data. A Carboniferous sample from Avalonia shows a significant contribution from Devonian magmatism as the youngest detrital component, although its depositional age based on field relationships is Carboniferous. The results exemplify the need to integrate multiple datasets in making interpretations from detrital zircon data.

Phanerozoic cooling history of Archean/Paleoproterozoic basement in the southern Espinhaço Range, southeastern Brazil, through apatite fission-track analysis

Apatite fission track thermochronology of basement rocks from the southern Espinhaço Range and Quadrilátero Ferrífero in southeastern Brazil unravels the tectonic history of this portion of the Brazilian Shield. The study area encompasses an Archean and Paleoproterozoic granitoid-gneiss basement, in the southern border of the São Francisco Craton, and an Archean to Paleo-Mesoproterozoic sedimentary cover. Apatite fission track ages (AFT) vary from 187 ± 18 to 91.8 ± 7.3 Ma and horizontal confined track lengths vary from 9.62 ± 1.81 μm to 12.85 ± 1.35 μm. Thermal history modeling shows an accelerated cooling episode starting in the Upper Devonian to Early Permian. After this event, quiescence lasted from 115 to 170 Ma. Apatite samples lack evidence of far-field effects from rifting and opening of the South Atlantic Ocean, and also evidence from alkaline and basaltic magmatism emplacement during Mesozoic-Cenozoic. The AFT thermochronometer remained undisturbed during Transminas mafic dyke swarm intrusion. Finally, the last cooling event in the Espinhaço Range-Quadrilátero Ferrífero basement records ascension to surface temperatures due to epeirogeny after new configurations of the Nazca, South American and African plates. A climatic origin for the ascension is supported by accentuated erosion rates, even though a weathering-prone phase is registered in weathering profiles.

Historical biogeography of a Brazilian lineage of Tillandsioideae (subtribe Vrieseinae, Bromeliaceae): the Paranaean Sea hypothesized as the main vicariant event

Abstract Subtribe Vrieseinae comprise four genera, Alcantarea, Stigmatodon, Vriesea s.s. and Waltillia, encompassing c. 20% of species in Tillandsioideae (Bromeliaceae), almost all of which are exclusive to Brazil. Here, we explore the biogeographic history of Vrieseinae, sampling 21 of the 22 genera of Tillandsioideae (130 terminals) and three DNA sequence markers (two plastid: rps16-trnK and matK; one nuclear: PHYC). We inferred a dated phylogeny and the ancestral areas of this lineage through RASP (reconstruct ancestral state in phylogeny) analyses. Vrieseinae were recovered as monophyletic, but tribe Vrieseeae (subtribe Vrieseinae + subtribe Cipuropsidinae) were not. A vicariant event between the Andes and Brazilian Shield probably occurred c. 10 Mya, when two clades, Cipuropsidineae + Tillandsieae and Vrieseineae, diverged. The Atlantic Forest plus the Chacoan region is recognized as the ancestral area for Vrieseinae. The results confirmed the recent origin of genera of Vrieseinae, with estimated crown ages in the Pliocene (5.3–2.6 Mya). We propose that the Paranaean Sea influenced the divergence of the main clades; Pleistocene events were probably responsible for the diversification of the most recent clades. This study sheds light on the biogeographic history of a key group of Neotropical plants, providing a new hypothesis for the evolution of bromeliads.

New U–Pb age constraints on the geological history of the Ganderian Bras d’Or terrane, Cape Breton Island, Nova Scotia

The northern Appalachian orogen preserves evidence of a complex history of amalgamation of terranes with both Laurentian and Gondwanan affinities. The Ganderian Bras d’Or terrane of central Cape Breton Island is not well represented elsewhere in the orogen and its relationship to other Ganderian terranes is enigmatic, particularly with respect to its pre-Neoproterozoic history. The Boisdale Hills and Kellys Mountain areas contain the oldest metamorphic rocks in the Bras d’Or terrane. Quartzite units in the Boisdale Hills have detrital zircon populations with ages ranging from 3.2 to ca. 1 Ga. Paragneiss units from the Kellys Mountain area contain Meso- to Neoproterozoic detrital zircons, in which the youngest grains indicate that the maximum depositional age is &lt;600 Ma. The detrital zircon populations of rocks from both areas are consistent with Gondwanan provenance for the protoliths, most likely the Amazonian craton. New U–Pb dates for subduction-related dioritic to granodioritic plutons in the Boisdale Hills yielded ages of 560 to ca. 540 Ma. Sedimentary, bimodal volcanic and plutonic rocks from the Bourinot belt in the Boisdale Hills and related plutons in the Kellys Mountain area have ages of ca. 510–490 Ma and are interpreted to have formed during extension related to separation of Ganderia from Gondwana. The southeastern Bras d’Or terrane preserves rocks formed in Pan-African subduction zones on a former passive margin of Gondwana as well as rocks formed during the initial stages of rifting of Ganderia from Gondwana and the subsequent opening of the Rheic Ocean.

Geochemical characterization of a reconstructed 1110 Ma Large Igneous Province

1110 Ma Large Igneous Province (LIP) fragments in the Kalahari craton, southern Africa (Umkondo LIP); Dronning Maud Land, Antarctica; Bundelkhand portion of Indian craton (Mahoba dolerite dykes); Congo craton (Huila-Epembe dolerite dykes); and Amazonia (Rincon del Tigre-Huanchaca LIP) have been reconstructed as a single LIP with plume centre beneath the NW part of the Kalahari Craton. This paper offers the best estimates for the paleoposition of the Indian and Amazonian cratons along with conjoined Kalahari-SF/Congo reconstruction. This 1110 Ma mafic magmatism is dominantly tholeiitic, ranging from basalt to andesitic basalt in composition, generated over a range of mantle melting depths [(Gd/Yb)N = 1.2–2.3], exhibit low to high contamination with crustal components (negative Nb anomalies, eNd (0 to −12), and elevated Th/Yb). The data fall into two Groups based on TiO2 content, with Group 1 (low Ti) of andesitic basalt composition, and Group 2 (high Ti) exhibiting a basaltic affinity. Group 1 magmas were generated in the spinel lherzolite field followed by significant contamination likely during passage through metasomatised lithospheric mantle in crustal magma chambers. A greater melting depth reaching into the garnet lherzolite field is proposed for the Group 2 magmas. The majority of Group 2 units are located in the Kalahari and Congo cratons, and this reflects onset of deeper melting closer to the interpreted plume axis in our reconstruction.

Fluid geochemistry of the Serra Pelada Au-Pd-Pt deposit, Carajás, Brazil: Exceptional metal enrichment caused by deep reaching hydrothermal oxidation

Serra Pelada is a high-grade hydrothermal Au-Pd-Pt deposit within the Carajás mineral province, eastern Amazon craton, which shows an exceptional metal enrichment at carbon-enriched hosted ores (>1000’s ppm Au + Pt + Pd). The main orebody is structurally hosted at the hinge of a recumbent syncline, consisting of dolomitic metasandstones at the outer part of the fold, a layer of carbonaceous metasiltstone in which most orebodies are located, and red siltstone in the core of the fold. Distal hydrothermal alteration comprises spatially separated chlorite – carbon (reduced) and hematite (oxidized) alteration zones. The mineralized zones include silicification and argillic (carbon- kaolinite-, and hematite-rich) alteration zones. Fluid inclusions hosted by quartz and dolomite crystals from distal alteration zones and silicified portions within the mineralized zone record a wide range of salinity (5–22 wt% NaCleqv.) but relatively constant homogenization temperatures (170 ± 25 °C). Pressure and temperature estimations based in the fluid inclusion microthermometry and selenide thermometry constrain the formation of the deposit at 250–270 °C and 1.2–1.7 kbar. Element concentrations of alkali metals, Pb, Cs and to a lesser extent Ba correlate with salinity, whereas redox-sensitive elements including U and Ce vary widely. Gold concentrations from 50 to 500 ppb detected in some inclusions are consistent with the highly oxidizing conditions defined by selenide mineralogy. The presence of uranium and gold within fluid inclusions of the distal hematite alteration zone confirms the oxidizing, metal carrier fluids. The occurrence of hydrothermally precipitated carbon implies that a reducing CH4-bearing fluid was also present, and the structural relationships suggest that mixing between this reducing fluid (and interaction with carbon-upgraded rocks) with an acidic, oxidizing and metal-carrying fluids was the main process driving ore metal precipitation. The genesis of the Serra Pelada Au–Pd-Pt deposit shows similarities with sediment-hosted copper deposits and unconformity-related U ± Au ± PGE deposits, having in common the downflow of highly oxidizing fluids into the hydrothermal domain of the Earth's upper crust. Major differences in metal proportions of these broad classes of ore deposits and other sediment-hosted systems in different ore provinces are primarily caused by differences in source rocks available for oxidative leaching and variations in the geometry of the hydrothermal system.

Paleoproterozoic volcanic caldera in the Amazonian craton, northern Brazil: Stratigraphy, lithofacies characterization, and lithogeochemical constraints

In the Vila Tancredo Neves locality, Amazonian craton, northern Brazil, occur Paleoproterozoic well-preserved volcano–plutonic centers. The lower 1.88 Ga Sobreiro Formation has overlapped lavas comprising andesite at the base; andesi-basalt and latite at middle; and quartz-latite and rhyolite at the top; besides associated pyroclastic and autoclastic lithofacies. Reveals high-K calc-alkaline and metaluminous signature. This unit shows K, Ba, Sr, and Rb enrichments; and Nb and Ta negative anomalies. Occurs an LREE-enriched pattern relative to HREE. Eu anomaly is absent in the most primitive rocks, whereas the evolved rocks reveal a minor negative Eu anomaly. These chemical characteristics are compatible with arc-related andesites. The upper, fissure-controlled, 1.87 Ga Santa Rosa Formation has peraluminous rhyolites and intrusions of rhyolitic porphyries and granites. These alkali rhyolites erupted in several vents within greater brittle faults, with outflows of ignimbrites and volcaniclastic deposits with variable textures. They reveal HFSE enrichment (Nb and Ta) and strong negative anomalies of LILE (Ba and Sr). Some evolved rhyolitic samples usually attributed to the Santa Rosa Formation are calc-alkaline and have geochemical constraints closer to those of the Sobreiro Formation andesitic rocks. Observations of the volcano–plutonic complex point to a Paleoproterozoic volcanic caldera related to several pulses of felsic and andesitic–rhyodacitic–rhyolitic lavas that comprise a stratovolcano evolution in the pre-caldera stage. Pyroclastic sin-caldera deposits with ignimbrite, ash tuff, crystal-rich tuff, and minor polymictic breccia are less preserved. Later evolved domes, dikes, and volcaniclastic rocks represent the recurrent magmatism in the post-caldera stage. This work supplies new support for more systematic stratigraphic and geochemical studies of these poorly studied, but promising, Proterozoic units.

Three additional new genera of acidocerine water scavenger beetles from the Guiana and Brazilian Shield regions of South America (Coleoptera, Hydrophilidae, Acidocerinae).

Recent study of the water scavenger beetle subfamily Acidocerinae in the Neotropical region has uncovered numerous undescribed species that are not able to be placed in existing genera. Here, we describe three new genera to accommodate 17 of these new species from South America: Aulonocharesgen. nov. for Aulonochareslingulatussp. nov. (French Guiana, Suriname), Aulonocharesnovoairensissp. nov. (Brazil), and Aulonocharestubulussp. nov. (Brazil, Guyana, Suriname, Venezuela); Ephydrolithusgen. nov. for Ephydrolithushamadaesp. nov. (Brazil), Ephydrolithusminorsp. nov. (Brazil), Ephydrolithusogmossp. nov. (Brazil), Ephydrolithusspiculatussp. nov. (Brazil), and Ephydrolithustelisp. nov. (Brazil); and Primocerusgen. nov. for Primoceruscuspidissp. nov. (Venezuela), Primocerusgigassp. nov. (Venezuela), Primocerusneutrumsp. nov. (Guyana, Suriname, Venezuela), Primocerusocellatussp. nov. (Venezuela), Primoceruspetilussp. nov. (Brazil), Primoceruspijiguaensesp. nov. (Venezuela), Primocerusmaipuresp. nov. (Venezuela), Primocerussemipubescenssp. nov. (Guyana), and Primocerusstriatolatussp. nov. (Suriname). The genus Ephydrolithusgen. nov. is currently known to be restricted to seepages in the mountainous regions of the Brazilian Shield. Aulonocharesgen. nov. and Primocerusgen. nov. are both currently only known from the Guiana Shield, though widespread in that region where they are associated with streams and seeps. We present differential diagnoses, maps, habitat details, and illustrations of all new genera and species here described.

The tectonic controls on the Paleoproterozoic volcanism and the associated metallogeny in the South Amazonian craton, Brazil: Sr–Nd–Pb isotope constraints

Whole-rock Sr–Nd–Pb isotope data for the Sobreiro and Santa Rosa volcanic sequences and Archean basement rocks near São Félix do Xingu city, northern Brazil, provide constraints for the colossal Proterozoic effusive and explosive well-preserved volcano–plutonic magmatism that covers about 1,500,000 km2 of the Amazonian craton. The basal, 1.88 Ga high-K calc-alkaline andesitic–dacitic Sobreiro Formation shows narrow variations in isotopic ratios [(87Sr/86Sr)i = 0.70352 to 0.70518; (143Nd/144Nd)i = 0.509692 to 0.509972; (206Pb/204Pb)i = 16.0492 to 16.3041; and (207Pb/204Pb)i = 15.6265 to 15.7505] that suggest mixing between the DMM-B and EM-1 and/or EM-2 domains in the melting source. The integration with published data reveals geochemical affinity of the Sobreiro Formation with Andean-type subduction; episodes of subduction angle flattening; and forearc subduction erosion. The latter caused the input of crustal-derived material and oceanic sediments in the trench and mixing with mantle-sourced magmas, a common mechanism active in modern orogenic environments. The upper, fissure-controlled, 1.87 Ga A-type rhyolitic Santa Rosa Formation reveals a high initial ratio [(87Sr/86Sr)i = 0.70949 and a large range of values for (143Nd/144Nd)i = 0.509624 to 0.509909; (206Pb/204Pb)i = 16.7271 to 21.7086; and (207Pb/204Pb)i = 15.8421 to 17.2780], compatible with partial melting of several Archean crustal sources and terrigenous sediments. We supply interpretations relative to tectonism, magma origin and evolution, and formation of arc-related world-class precious and base metals deposits, analogous to the events that occurred in the Andean belt and western North America. It represents new support for more systematic mineral exploration with modern approaches of these poorly studied, but nonetheless promising, Amazonian Proterozoic units.

Hydrothermal dolomitization and porosity development: An example from Precambrian dolomitic rocks of Água Clara Formation, Ribeira Belt, southern Brazil

Dolomitic rocks from the Rio Bonito Quarry, located in the Água Clara Formation, record multistage dolomitization processes linked with porosity development in the southern Brazilian Shield. Dolomitic marble and breccia samples from this unit contain three textural types of dolomite that either are fabric destructive or void-filling phases. To study the role of dissolution in the porosity development in these dolomite types - from sedimentary to metamorphic domains - we combine petrographic methods, cathodoluminescence, Raman spectroscopy, micro-computed tomography, stable isotopes of carbon and oxygen, and fluid inclusion analysis. The protoliths of the dolomitic rocks of Rio Bonito Quarry are assumed as Mesoproterozoic (ca. 1.5 Ga) marls presenting shallow shelf to deep water lithofacies. The first dolomitization stage, forming a nonmimic replacement of the primary carbonate (Dol1) is fabric destructive and related to a burial setting (δ18O: -12.42 to - 9.68‰; δ13C: -2.29–0.40‰). The development of bedding-parallel stylolites of low amplitude led to increasing of porosity (up to 0.2%) and promoted the fluid circulation. During the Neoproterozoic Brasiliano tectonic events (ca. 650-550 Ma), the metamorphism of marls into dolomitic marble is intimately connected to the development of second replacement dolomite (Dol2). Penecontemporaneous brecciation is assumed as triggering factor to the fluid change, where Dol2 is akin to dolomites from MVT-deposits (δ18O: -19.12 to −12.03‰; δ13C: -0.07 to −2.17‰), and the total porosity is enhanced to 2%. The ubiquitous tectonic stress boosted the hydraulic fracturing, providing conditions to saddle dolomite (Dol3) precipitation, occurring as a replacement and void-filling phase. Lower isotopic values of Dol3 (δ18O: -14.93 to −10.77‰; δ13C: -3.36 to −1.89‰) suggest disequilibrium between the host-rock and the dolomitization fluid, while the porosity is augmented to 9%. The subsequent dedolomitization, generating void-filling calcite, is associated to meteoric phreatic input (δ18O: -8.31 to −5.56‰; δ13C: -5.59 to −2.56‰), similarly to the development of calcretes; therefore, diminishing the porosity towards 4%. Microthermometric data shows different signatures in the dolomitization fluid, which started at 100–150 °C and moderate-salinity (3–4 wt% eq.NaCl) in a burial setting, reaching up to 270 °C and low-salinity (0-2 wt% eq.NaCl) as a response to a shifting tectonic environment or fluid mixing. Additionally, the temperature shift between the host rock (120–130 °C) and the different dolomitization stages (from 140-150° to 210–270 °C), above 10 °C offset, point to a hydrothermal setting.

Diversidad de escarabajos coprófagos (Coleoptera: Scarabaeidae) de la zona norte de la Serranía de la Lindosa, Guaviare-Colombia

Se estudió la composición faunística de los escarabajos coprófagos en cuatro sitios de muestreo en la zona norte de la Serranía de La Lindosa en el Departamento del Guaviare, evaluando la diversidad y la estructura de los ensamblajes que se compararon con trabajos realizados en otras zonas de la Guyana biogeográfica. Los resultados muestran valores bajos en la diversidad, propios de zonas con baja disponibilidad de recursos y gran exigencia climática y el predominio de especies de hábito de nidificación cavadora sobre especies residentes y rodadoras. La composición de la fauna presenta gran número de especies comunes con otras áreas de la región, similares a las afinidades encontradas para otros taxones que apoyan los nexos de estas serranías colombianas con los escudos guyanés y brasileño.

New loricariid fishes from headwaters on Serra da Mantiqueira and Complexo do Espinhaço, Minas Gerais State, Brazil (Teleostei: Siluriformes: Loricariidae).

Three new species, of three different genera of loricariids, are described from the headwaters of Serra da Mantiqueira and Cadeia do Espinhaço, in Southeastern Minas Gerais State, Brazil. In this region, the Serra da Mantiqueira and the Complexo do Espinhaço are the highest points of the Brazilian Shield and delimits the headwaters of four drainages: São Francisco, Paraná, Doce and Paraíba do Sul basins. Harttia intermontana, n. sp., is described from the headwaters of the Rio Doce basin and is the first record of the genus in this basin. The new species can be distinguished from its congeners by the pattern of abdominal covering, presence of preanal plates, presence and pattern of ornamentation of canal plate, and some characteristics related to sexual dimorphism. Pareiorhaphis togoroi, n. sp., is described from the headwaters of the Rio das Mortes basin, tributary to the Rio Grande, in the upper Rio Paraná drainage, and represents the first record of the genus to this drainage, thus expanding its geographic distribution. It can be distinguished from most congeners by the absence of preadipose azygous plates, and characteristics related to secondary sexual characters of mature males: presence of odontodes on the lateral margin of head plus the absence of long hypertrophied odontodes on pectoral-fin spine. Neoplecostomus pirangaensis, n. sp., is described from the headwaters of the Rio Piranga, Rio Doce basin. The new species differs from all congeners by the much-reduced dermal platelets on the abdomen, devoid of developed odontodes between the insertions of the pectoral and pelvic fins. Neoplecostomus pirangaensis can also be distinguished from all congeners, except N. botucatu and N. paranensis, due to the complete absence of vestiges of the adipose fin (vs. vestiges, or adipose fin moderate to well developed and always present). The new species differs from N. botucatu by the absence of conspicuous dark spots all over the body and the presence of common dorsal bands in juveniles, and almost totally black in adults. It differs from N. paranensis by the bigger and less numerous teeth. Additionally, the new species differs from Neoplecostomus doceensis by the absence of enlarged fleshy folds between dentaries, and absence of a lateronasal plate.

Sediment provenance of Triassic and Jurassic sandstones in central Mexico during activity of the Nazas volcanic arc

We have investigated the provenance of Upper Triassic and Jurassic siliciclastic sediments (Zacatecas, Nazas and La Joya formations) in the Mesa Central region (areas of La Ballena, Charcas and Real de Catorce) in central Mexico. The sediments were deposited during activity of the Nazas volcanic arc at the western margin of Pangaea. Provenance characteristics are essential for reconstructing the palaeotectonic setting of the region. Overall, 26 sediment samples and 3 volcanic rock samples from the Nazas volcanic arc were analyzed. The latter are potential source lithologies. Petrographic analysis reveals that the analyzed sediments are mainly composed of monocrystalline quartz, polycrystalline quartz (chert and composite quartz), feldspar, and rock fragments of mainly volcanic origin. Th/Sc vs. Zr/Sc values illustrate low sediment recycling in the source area(s). For the majority of the analyzed samples low Cr and Ni values exclude input from a mafic/ultramafic (ophiolitic) source. The majority of the translucent heavy minerals (Pxn, Amp, Ep, Spn, Grt, Chl, Tur, Ant, Rt) are scarce in almost all samples. The most abundant translucent heavy minerals occurring in the samples are apatite and zircon. Apatite grains decrease from the southeast (La Ballena) towards the northeast (Real de Catorce) either indicating stronger weathering conditions in the area of Real de Catorce or a source area that delivered detritus exclusively for the zone of Real de Catorce. Sediments of the Zacatecas Formation illustrate maximum depositional ages ranging from Norian (Late Triassic) to the Hettangian/Sinemurian boundary (Early Jurassic). Detrital zircon U–Pb ages suggest that they received detritus from the Amazonian craton (∼1780–1300 Ma), the Oaxaquia microcontinent (∼1290–900 Ma), the Maya (Yucatan–Chiapas), Oaxaquia, Coahuila and possible Chortis, and Florida blocks (∼720–450 Ma) and the Permian–Triassic magmatic arc (∼300–240 Ma). The Acatlán Complex and the Maya (Yucatan–Chiapas) block are likely source areas for ∼445–310 Ma-old zircon grains. The source of the ∼240–200 Ma-old detrital zircons are likely magmatic rocks related to the early disassambly of western Pangaea. Detrital zircons from sediments of La Joya Formation provide maximum depositional ages from the Pliensbachian (Early Jurassic) to Oxfordian (Late Jurassic). The most probably source areas are the Oaxaquia microcontinent (∼1290–900 Ma), and the Maya (Yucatan–Chiapas), and Coahuila blocks (∼720–450 Ma). Zircon ages ranging from ∼200 to 150 Ma are indicative of detritus from the Nazas volcanic arc, which explains the presence of the volcanic fragments in La Joya Formation deposits.