U–Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana.Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin.Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt.Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt.Whilst continent–continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595–560 Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age.The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50–100 million years.     

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.     

Sm–Nd isotopic compositions as a proxy for magmatic processes during the Neoproterozoic of the southern Brazilian shield

Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).     

U/Pb and Sm/Nd geochronologic studies of the eastern Borborema Province, Northeastern Brazil: initial conclusions

The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma.The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province.Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (T_DM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (T_DM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group.Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ˜ 1.0 ± 0.1 Ga, based on U/Pb ages of zircons. Nd model ages (T_DM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments.Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.     

Neoarchean and Paleoproterozoic granitoids marginal to the Jeceaba-Bom Sucesso lineament (SE border of the southern São Francisco craton): Genesis and tectonic evolution

The sialic crust of the southern São Francisco craton along the Jeceaba-Bom Sucesso lineament, central-southern part of Minas Gerais (Brazil), encompasses, among other rock types, Neoarchean and Paleoproterozoic granitoids. These granitoids, according to their petrographic, lithogeochemical and geochronologic characteristics, were grouped into two Neoarchean suites (Samambaia-Bom Sucesso and Salto Paraopeba-Babilônia) and three Paleoproterozoic suites (Cassiterita-Tabuões, Ritápolis and São Tiago). Varied processes and tectonic environments were involved in the genesis of these suites. In particular, the lithogeochemistry of the (Archean and Paleoproterozoic) TTG-type granitoids indicates an origin by partial melting of hydrated basaltic crust in a subduction environment. In the Neoarchean, between 2780 and 2703 Ma, a dominant TTG granitoid genesis related to an active continental margin was followed by another granite genesis related to crustal anatexis processes at 2612–2550 Ma. In the Paleoproterozoic, the generation of TTG and granites s.s. occurred at three distinct times: 2162, 2127 and 1887 Ma. This fact, plus the rock-type diversity produced by this granite genesis, indicates that the continental margin of the southern portion of the São Francisco craton was affected by more than one consumption episode of oceanic crust, involving different island arc segments, and the late Neoarchean consolidate continent. A Paleoproterozoic tectonic evolution in three stages is proposed in this work.     

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.     

Closure of the Clymene Ocean and formation of West Gondwana in the Cambrian: Evidence from the Sierras Australes of the southernmost Rio de la Plata craton,Argentina

The formation of Gondwana took place across a series of Brasiliano–Pan African suture zones that record late Neoproterozoic to earliest Paleozoic collisions between Precambrian cratons. In South America, an internal suture zone marks the disappearance of the Clymene Ocean that separated the Amazon craton from the São Francisco and Rio de la Plata cratons. New geochronological data from the southern end of this massive collision zone in the Sierras Australes of central-eastern Argentina document Paleoproterozoic crust and suggest an Ediacaran age for the oldest sedimentary rocks. These two observations extend the known limit of the Rio de la Plata craton at least 200 km SW of previous estimates. New data also confirm the occurrence of late Ediacaran to late Cambrian magmatism in the Sierras Australes. The age of these hypabyssal to volcanic rocks corresponds to igneous events in the Pampean belt along the western margin of the Rio de la Plata craton, although the shallow level magma emplacement in the Sierra da Ventana study area contrasts with the deeply exhumed rocks of the Pampean orogeny type locality. These new age data are compared with a broad compilation of geochronological age Clymene collision belts to the north, the Paraguai and Araguaia belts. The close overlap of the timing of orogenesis indicates the age of Clymene ocean closure in its northern reaches. In the south, the Pampean belt was unconfined, allowing continued tectonic activity and crustal accretion throughout the Paleozoic.     

African, southern Indian and South American cratons were not part of the Rodinia supercontinent: evidence from field relationships and geochronology

We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ˜1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ˜1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa.Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goíás Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil.Furthermore, there is evidence for extensive granitoid magmatism in the period ˜840 to <600 Ma whose predominant calc-alkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern hemisphere, such as the São Francisco/Congo, Kalahari and Rio de La Plata cratons, as well as the Borborema/Trans-Saharan, Central Goiás Massif and Paraná blocks. Moreover, many extensional tectonic events detected in the southern hemisphere continental masses, but also many radiometric ages of granitois that are already associated with the process of amalgamation of Gondwana, are comprised within the 800–1000 age interval. This seems incompatible with current views on the time of disintegration of Rodinia, assumed to have occurred at around 750 Ma.     

Evidence for Neoproterozoic Continental Arc Magmatism in the Santa Quitéria Batholith of Ceará State,NW Borborema Province,NE Brazil: Implications for the Assembly of West Gondwana

Recent field investigations and geochronological studies of Neoproterozoic rocks in the northwestern part of the Borborema Province, Ceará State, NE Brazil provide important clues pertaining to the nature of convergence between the Borborema Province and the West African-São Luis craton during the assembly of West Gondwana. U-Pb zircon data indicate that the earliest evidence of convergent magmatism along the northwest margin of the Borborema Province occurred around 777 Ma, and was followed by the development of a large continental arc batholith (Santa Quitéria batholith) between ca. 665 and 591 Ma within the central part of Ceará State. These findings, along with supporting geophysical data, suggest that convergence between the Borborema Province and the West African-São Luis craton involved closure of an oceanic realm with subduction polarity to the southeast beneath the northwestern part of the province. Consequently, it seems likely that the Pharusian Ocean was continuous from the Hoggar Province in West Africa into South America during the late Neoproterozoic and additional data suggests that it may have even been connected with the Goianides Ocean of the Brasília Belt farther to the southwest.     

AGES AND DYNAMICS OF DEVELOPMENT OF THE ACTIVE VOLCANOES OF THE KURILE-KAMCHATKA REGION

The deformations of the West Congolian belt (about 600 Ma) and its foreland are described in terms of geometry and dynamics. Their transcurrent regimes, characterized by dextral movements within the belt and transverse sinistral movements in its foreland, are emphasized. On the basis of a Lower Cretaceous fit of the South American and the African continents and of the structural framework of the Brasiliano (Pan-African) domains, links between the West Congo belt and eastern Brazil Brasiliano belts (Sergipano, Rio Pardo) are proposed. In late Proterozoic reconstruction of these domains a geodynamic model of indentation tectonics by the Sao Francisco craton is proposed.     

Plate tectonic settings for Precambrian basic rocks from Brazil by multidimensional tectonomagmatic discrimination diagrams and their limitations

Multidimensional discrimination diagrams (2006–2011) for basic and ultrabasic igneous rocks were applied to Precambrian rock suites from the Amazonian and São Francisco cratons, and the Tocantins Province, Brazil, to infer their possible tectonic settings. The chosen study cases in the Amazonian craton include the ca. 3.0 Ga metabasalts of the Identidade greenstone belt, 1.87–1.80 Ga Parauapebas anorogenic basalt-rhyolite dikes, 1.86–1.82 Ga Rio Branco anorogenic gabbro-basalt association, ca. 1.76–1.74 Ga Aripuanã and Teles Pires intracratonic basalt-felsic volcanic associations, and 1.76–1.74 Ga Jamari and 1.60–1.53 Ga Serra da Providência arc-related gabbroic rocks. In the São Francisco craton, we selected 1.48 Ga arc-related amphibolites of the Rio Capim greenstone belt, continental mafic dikes of Uauá (2.6 Ga), Curaçá and Chapada Diamantina (1.5 Ga), and Espinhaço (ca. 1.0 Ga). In the Tocantins Province, ca. 3.0 Ga komatiites associated with oceanic basalts of the Crixás and Guarinos greenstone belts were studied. The application of the diagrams generally provided consistent results with the authors’ proposed tectonic settings based on field relationships and geochemical data. The exceptions are some within-plate (continental) mafic dikes and basalts for which our diagrams do not work well. For comparison, we also used two ternary and two bivariate traditional discrimination diagrams for the data from the Amazonian craton, whose results were poorer than the newer multidimensional diagrams.     

U-Pb detrital zircon geochronology and provenance of Neoproterozoic sedimentary rocks in southern Siberia: New insights into breakup of Rodinia and opening of Paleo-Asian Ocean

We present the synthesis of new data on detrital zircon geochronology of the Neoproterozoic strata of the southern part of the Siberian craton as well as a comprehensive analysis of previously published stratigraphic, sedimentological and geochronological (LA-ICP-MS) data obtained for key sections in this area that allows us to trace the process of birth and early stages of development of the Paleo-Asian Ocean (PAO). Before the break-up of Rodinia and opening of PAO, Tonian – Cryogenian intracontinental sedimentary basin existed between southern Siberia and northern Laurentia. The detachment of the southern flank of the Siberian craton from northern Laurentia and opening of the PAO between these cratons took place in Cryogenian. The detrital zircon ages from lower parts of Neoproterozoic successions suggest the Siberian craton as the sole provenance area right after the opening of the PAO. The age constraints on the lower parts of the studied Neoproterozoic successions, which are based on correlation of their tillite horizons with the Marinoan glaciation, suggest the late Cryogenian age for these sedimentary rocks. A clear change in the age spectra of detrital zircons from “unimodal” (Early Precambrian only) in older sedimentary rocks to “bimodal” (Early Precambrian as well as Neoproterozoic) in younger sequences of the studied successions marks the next stage of the PAO evolution. The abundance of youngest (630–610 Ma) detrital zircons in the upper parts of the studied sequences reflects a shrinkage of the oceanic basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleo-Asian Ocean. We suggest that a passive oceanic margin along the southern margin of the Siberian craton has been transformed into a series of foreland basins at ~610 Ma.     

Deep seismic refraction and gravity crustal model and tectonic deformation in Tocantins Province, Central Brazil

Interpretation of seismic refraction data in the central sector of Tocantins Province, Central Brazil, has produced a seismic crustal model with well-defined upper, intermediate, and lower crust layers having smooth velocity gradient in each layer. The depths to Moho vary from 32 to 43 km, and mean crustal P velocity varies from 6.3 km/s, beneath Goiás magmatic arc on the western side, to 6.4 km/s, below Goiás massif in the central portion and the foreland fold-and-thrust belt on the eastern side. The behaviour of the lower crust layer allows an improved understanding of regional gravimetric features of the central and northern sectors of Tocantins Province and suggests subduction of the Amazon plate in Central Brazil. In the southeastern sector, the refraction experiment resulted in the detection of a thinner crust (38 km) below Brasília fold belt and a thicker crust (41 km) below Paraná basin and São Francisco craton (42 km). The upper crust beneath Paraná Basin is around 20 km thick, whereas it is less than 10 km thick below the craton. These results bring new insights into the geological history of the central and southeastern sectors of Tocantins Province.Gravimetric measurements in the central sector of Tocantins Province delineate a high and a low anomaly separated by a steep gradient with a NE direction. The axis of the gradient seems to bend still further to NE in the northern sector of that province, whereas the gravimetric high continues northwards, defining a separation between them. This suggests that those features belong to different tectonic processes that occurred during Tocantins Province orogenesis. The gravimetric model, which incorporates seismically resolved structure beneath Tocantins Province, better matches the observed gravimetric data.Although tectonic movements have only been monitored with high-precision GPS for short time interval (1999–2001), the results suggest observable deformations. The main seismicity of Central Brazil, the Goiás–Tocantins seismic belt, seems to be spatially associated with the large gravimetric high anomaly and with the observed tectonic deformation.     

Flexural modeling of the neoproterozoic Araguaia belt,central Brazil

Flexural modeling of bending of the southern and southeastern borders of the Amazon lithospheric plate under the western border of the Goiás Massif and western Parnaı́ba basin was constrained by 1070 gravity stations between 5°–14°S and 46°–52.5°W. Topography and aeromagnetic data were also used to estimate the loads of the Araguaia thrust belt. A sequence of Bouguer gravity anomaly lows (−80 to −40 mGal) is located over the Araguaia thrust belt and Cenozoic sediments of the Ilha do Bananal basin. Bouguer anomalies over the Amazon craton, to the west of the thrust belt, are higher than −20 mGal. Towards the east, over the Goiás Massif, the São Francisco craton and the Paleozoic to Mesozoic Parnaı́ba basin, anomalies range from −70 to −20 mGal. Comparison between topography and gravity along profiles perpendicular to the cratonic borders and across the Araguaia thrust belt shows that the long-wavelength gravity anomalies are best explained by bending of the Amazon plate caused by loads such as the observed topography, the thrust-sheets of the Araguaia belt and the remnants of ancient island-arc system in the Goiás massif. The thickness of the Araguaia thrust belt together with the Cenozoic sediments was estimated using aeromagnetic data and it ranges from 6 to 8 km. This load was used to calculate the minimum effective elastic thickness T_e for the Amazon plate. T_e=80 km was estimated by comparing the observed Bouguer anomalies with the gravity anomalies caused by bending of the crust-mantle interface of a broken elastic plate model. These results support the proposition that the Araguaia belt formed during the collision and suture of the Amazon and the São Francisco lithospheric plates, in late Proterozoic times.     

Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: Significance for the origin of crustal fragments in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt contains many Precambrian crustal fragments whose origin is unknown, and previous speculations suggested these to be derived from either Siberia, Tarim or northern Gondwana. We present an age pattern for detrital and xenocrystic zircons from Neoproterozoic to Palaeozoic arc and microcontinental terranes in Mongolia and compare this with patterns for Precambrian rocks in southern Siberia, the North China craton, the Tarim craton and northeastern Gondwana in order to define the most likely source region for the Mongolian zircons. Our data were obtained by SHRIMP II, LA-ICP-MS and single zircon evaporation and predominantly represent arc-related low-grade volcanic rocks and clastic sediments but also accretionary wedges and ophiolitic environments.The Mongolian pattern is dominated by zircons in the age range ca. 350–600 and 700–1020 Ma as well as minor peaks between ca. 1240 and 2570 Ma. The youngest group reflects cannibalistic reworking of the Palaeozoic arc terranes, whereas the Neoproterozoic to late Mesoproterozoic peak reflects both reworking of the arc terranes as well as Neoproterozoic rifting and a Grenville-age crust-formation event.The 700–1020 Ma peak does not exist in the age spectra of the Siberian and North China cratons and thus effectively rules out these basement blocks as potential source areas for the Mongolian zircons. The best agreement is with the Tarim craton where a major Grenville-age orogenic event and early Neoproterozoic rifting have been identified. The age spectra also do not entirely exclude northeastern Gondwana as a source for the Mongolian zircons, but here the Neoproterozoic age peak is related to the Pan-African orogeny, and a minor Grenville-age peak may reflect a controversial orogenic event in NW India.Our Mongolian detrital and xenocrystic age spectrum suggests that the Tarim craton was the main source, and we favour a tectonic scenario similar to the present southwestern Pacific where fragments of Australia are rifted off and become incorporated into the Indonesian arc and microcontinent amalgamation that will evolve into a future orogenic belt.     

Gravity anomalies and the Indian lithosphere: Review and analysis of existing gravity data

The upper part of the lithosphere has been actively involved in various exogenic and endogenic processes which have left their imprint on the gravity field on the Indian Peninsula and the Himalaya. Analysis of the gravity field over the Dharwar craton shows that the greenstone belts of this craton have been formed as a result of development of deep fractures in the earth's crust during Archaean times. Precambrian mountain ranges such as the Aravallies, Vindhyans, Satpura and Eastern Ghats are located peripheral to Archaean cratons. Most of these mountain belts are characterized by gravity highs suggesting that the underlying crust is of higher than normal density. These mountain ranges with the exception of the Eastern Ghats do not appear to be locally compensated. Regional compensation seems to prevail over all these areas. Eastern Ghats ranges are also underlain by a crust of higher than normal density relative to the Dharwar and Bastar cratons and exist with a sharp contact with the cratons in the West. Isostatic compensation in the Eastern Ghats appears to have been achieved by thickening of the underlying crust. The Himalaya has attained a fairly high degree of isostatic compensation.     

Reassembly of the Dharwar and Bastar cratons at ca. 1 Ga: Evidence from multiple tectonothermal events along the Karimnagar granulite belt and Khammam schist belt,southern India

The northern part of the Nellore–Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U–Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at \(\sim \)1900 and \(\sim \)1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita–Godavari (P–G) valley basin at \(\sim \)1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.     

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300?million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The S?o Gabriel block in the west is characterized by c.760?C690?Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880?Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ??Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the S?o Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D₃) yield an U?CPb zircon age (LA-ICP-MS) of 701?±?10?Ma giving the approximate minimum age of the S?o Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789?±?7?Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234?±?28?Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100?C2,000?Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800?C700?Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and S?o Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880?Ma (Passinho event) and (ii) accretion of the 760?C700?Ma Cambaí/Vila Nova magmatic arc (S?o Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW?CNE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590?Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530?Ma and can be assigned to final stages of the amalgamation of West Gondwana.     

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.