TDM Model Ages Research Articles

Nd-isotope evidence for the distal provenance of the historical (c. <3000BP) lateritic surface cover underlying the Equatorial forest in Gabon (Western Africa)

Highlights: • The first Nd isotopic data on the lateritic surface cover (Cover Horizon) of western Equatorial Africa. • Clearly different Nd isotopic signatures between the Cover Horizon and underlying basement. • A consistent model attributing the Cover Horizon to the settling of aeolian particles derived from the Namib desert. Abstract: Surficial formations in Gabon, as well as in other places of western Central Africa include a ubiquitous, homogeneous and 1–3 m-thick clayey to sandy lateritic surface cover known as the ‘Cover Horizon’. From 14C radiometric dating it has been concluded that the emplacement of this unit was correlative with a major environmental crisis which affected Central Africa c. 3000–2000 years ago. 10Be and Nd-isotopic analyses have been performed to provide new constraints on the age and origin of this layer. Six samples from two depth profiles investigated for 10Be exhibit an almost constant concentration consistent with a very recent deposition age. Nd-isotopic analyses performed on the silt to clay fraction of eleven samples from widely spaced locations over Gabon attest for mildly radiogenic signatures (eNd = −23 to −17) in ten of them, and a slightly radiogenic signature (eNd = −9) in one sample. TDM model ages range from 1.6 to 2.6 Ga, and a perfect discrimination is observed between the Nd-isotopic signature of the Cover Horizon and that of the underlying Congo Craton. This makes an aeolian origin as the most probable for the Cover Horizon. The average eNd (c. −20) is however rather unusual for aeolian sediments or aerosols. A possible source of particles is therefore tested by considering the present-day atmospheric flux over Gabon and adjacent regions. Combined atmospheric modeling and Nd-isotopes leads to the conclusion that the fine fraction of the Cover Horizon could have originated from the northern part of the Namib desert.

Palaeoproterozoic (~1.89 Ga) felsic volcanism of the Iricoumé Group, Guyana Shield, South America: geochemical and Sm-Nd isotopic constraints on sources and tectonic environment

Hypabyssal and explosive volcanic rocks of the Iricoumé Group crop out in the Western Erepecuru–Trombetas Domain, in the Central Amazonian Province, south of the Guyana Shield. They are part of an extensive volcano–plutonic event, which marked the central part of the Amazonian Craton during the Orosirian. These volcanic rocks, formed at approximately 1.89 Ga, consist of pyroclastic rocks (ignimbrites, rheoignimbrites, surge-related lapilli-tuff, co-ignimbrite fall tuff) and subordinate felsic hypabyssal rocks. An older episode, with an age of approximately 1.99 Ga, occurs locally and is represented by latitic lava flows and spessartitic lamprophyres. The geochemical characteristics of the Iricoumé volcanic rocks suggest that they formed in a post-orogenic tectonic setting, while those of the older Orosirian magmatism suggest a subduction-related geodynamic setting. TDM model ages ranging from 2.18 to 2.39 Ga and slightly negative εNd(T) values for most of the Iricoumé volcanic rocks indicate dominant Rhyacian crustal sources for the parental magmas. The positive εNd(T) values exhibited by some Iricoumé rhyolites (TDM: 1.98 and 2.06 Ga) and by the 1.99 Ga lava flows and lamprophyre (TDM: 2.13 and 2.21 Ga) suggest a juvenile mantle source, although a minor contribution of Palaeoproterozoic crust cannot be ruled out. These new results furnish further evidence of the wide extent of the Iricoumé Group and coeval volcano–plutonic associations in the central part of the Amazonian Craton.

SHRIMP U–Pb zircon geochronology and Nd signatures of supracrustal sequences and orthogneisses constrain the Neoproterozoic evolution of the Pernambuco–Alagoas domain, southern part of Borborema Province, NE Brazil

The Borborema Province is the western part of a major Brasiliano belt that extends from Brazil through NW Africa in pre-drift reconstructions. This province resulted from convergence and collision among the West African, Congo-Sao Francisco, and Amazonian cratons about 600 Ma. This study focuses on the Pernambuco–Alagoas (PEAL) domain, which is a complex of magmatic, migmatitic, and metamorphic rocks, located in the southern part of the Borborema Province. U–Pb geochronology and Sm–Nd data for metasedimentary sequences (Rio Una, Inhapi) of the PEAL domain and a sample from a sequence of the Transversal Zone domain suggest that their deposition occurred during a Cryogenian extensional event, within the interval 850–631 Ma (or slightly younger). This extensional event occurred in the PEAL, Transversal Zone, and Sergipano domains before the onset of the Brasiliano collision and was followed by syn- and post-collisional magmatism. The Rio Una sequence and the sequence from Transversal Zone domain were deposited over a Rhyacian (ca. 2.0–2.2) basement having a juvenile Palaeoproterozoic Nd signature, whereas the Inhapi sequence was deposited over an Early Neoproterozoic (Tonian) basement. The deposition of the studied sequences is coeval with metasedimentary sequences to the north and south in other domains of the Borborema Province. However, differences in Nd isotopic signatures between the sequences from PEAL, Transversal Zone, and Sergipano domains suggest that they were formed in distinct basins. Metasedimentary sequences from the PEAL domain have Meso- and Palaeoproterozoic T DM model ages. These data suggest that the orogens where the metasedimentary sequences are located have a strong ensialic component. T DM model ages of ca. 1.0 Ga and eNd (0.6 Ga) values around zero recorded in granites from the southern part of the PEAL, suggesting that juvenile material was accreted to the southern part of the PEAL domain crust during the Tonian. The migmatites from the PEAL domain have a wide range of T DM ages along with a wide range of eNd (0.6 Ga) values. The transition between the northern and southern parts of the PEAL domain denotes the approximate northern margin of a Late Mesoproterozoic to Early Neoproterozoic rift.

From extension to shortening: Dating the onset of the Brasiliano Orogeny in eastern Borborema Province (NE Brazil)

In pre-drift reconstructions, the central and southern parts of the Borborema Province, northeastern Brazil, belong to a large Brasiliano-Pan-African orogenic realm situated to the north of the São Francisco-Congo Craton. In order to better understand the timing and geodynamic setting under which this orogenic system developed, a structural, geochemical and geochronological study was conducted across the east Pernambuco shear zone (EPSZ) system, which separates the Pernambuco-Alagoas Domain (PEAL) from the Central Domain. A sample of the Pinhões orthogneiss (GE-1), in the Central Domain, one sample of a syenitic orthogneiss (CA-34) wrapped by the EPSZ, and one sample of orthogneiss named Altinho (CA-40), in the northern portion of the PEAL, were dated by LA-ICP-MS. The Pinhões orthogneiss yielded an age of 869 ± 9 Ma, interpreted as the emplacement age of the protolith during a late Tonian magmatic episode. Samples CA-40 and CA-34 yielded 206Pb/238U weighted mean ages of 652 ± 6 Ma and 636 ± 3 Ma, respectively, which are interpreted as dating emplacement and crystallization of the magmatic protoliths. However, it is also possible that these rocks were formed during the same magmatic event in view of the identical ages of 646 ± 13 Ma and 646 ± 11 Ma, respectively, given by the less precise upper intercept of the discordia lines. The metaluminous and magnesian nature of the Altinho orthogneiss is akin to the calc-alkalic suite. However, some samples plot in the intraplate field in tectonic discrimination diagrams and the Nd TDM model age of 1.36 Ga is unlike that of juvenile magmas in convergent settings. The Altinho orthogneiss is quite similar in terms of trace elements geochemistry to the syenitic orthogneiss, which has a clearer intraplate affinity, and the dated samples have identical initial Sr isotope ratios (0.7047). Therefore, emplacement in an extensional setting is preferred over a convergent one. Two samples of paragneisses (SB-1 and BB-9) from the PEAL were also dated. The ages of the youngest zircon grains in sample BB-9 (655–642 Ma) overlap the crystallization age of the Altinho orthogneiss, implying that sedimentation is younger than or, at best, synchronous with magmatism. The age of low Th/U grains in samples CA-34 (615 ± 8 Ma) and SB-1 (587 ± 12 Ma) are related to a subsequent metamorphic overprint, which is loosely constrained between 580 and 620 Ma. These observations, combined with evidence provided by previous studies, suggest that the change from an extensional to a contracional setting occurred at ca. 640–630 Ma. In contrast with most collisional orogens, where a long period of oceanic subduction precedes collision, the inferred tectonic evolution suggests that the Brasiliano Orogeny resulted from inversion of continental and/or proto oceanic rifts.

Contributions to the petrography, geochemistry and geochronology (U-Pb and Sm-Nd) of the Paleoproterozoic effusive rocks from Iricoumé Group, Amazonian Craton, Brazi

The southernmost region of the Guyana shield, Amazonian craton, hosts large record of Paleoproterozoic effusive rocks of the Iricoume Group. They present remarkably well-preserved igneous textures and structures. The SiO2 contents reveal a bimodal association marked by a compositional gap between acid (SiO2 >; 67 wt%) and intermediate (SiO2 < 57.7 wt%) rocks. The acid effusive rocks are rhyolites to rhyodacites with high SiO2, alkali, Rb, Zr, Nb + Ta, La + Ce and 104 Ga/Al content and low Fe2O3tot, TiO2, CaO, Sr and Co content. They exhibit subalkaline, metaluminous-to-peraluminous compositions, and geochemically compatible to A-type magmatism emplaced in post-collisional to within-plate tectonic settings. The intermediate rocks are andesitic/basalt to andesite relatively high contents of TiO2, Al2O3, Fe2O3total, MgO, CaO, Sr and Co; low SiO2, K2O, Rb, Zr, Nb + Ta, La + Ce. They have subalkaline and metaluminous geochemical composition and plot on within-plate basalt field. The acid rocks crystallized at 1882 ± 11 Ma in U-Pb analyses for LA-MC-ICPMS zircon data. The Sm-Nd isotopic data on all rocks reveal a Nd TDM model ages between 2.59 and 2.16 Ga and eNd(t) values between -5.78 and 0.03, indicate that the magmatic evolution was related to the reworking of older Paleoproterozoic at the Rhyacian-Siderian period, continental crust (Transamazonian crust-forming event) with some mixing with a limited amount mantle-derived magmas or with contamination by Archean crust. The petrographic, geochemical and geochronological data presented in this paper suggest a within-plate to post-collisional tectonic setting for the Iricoume volcanism, involving lower crust uplift and generation of basalt magma in an extensional regime.

The 186 Ma Dashibalbar alkaline granitoid pluton in the north-Gobi Rift of central Mongolia: Evidence for melting of Neoproterozoic basement above a plume

The Jurassic Dashibalbar granitoid pluton (300 km 2 ) crops out in the Triassic North-Gobi rift of central Mongolia, just south of the 230 to 195 Ma Khentei batholith. The granitoids are shallow-seated dominantly, amphibole-bearing alkali feldspar granite that contain quartz-syenite/syenite enclaves. They are all composed of megacrystic mesoperthite, quartz, Ca-Na amphibole altered to biotite and rarely with pyroxene cores, magnetite and ilmenite. The pluton yielded a concordant U-Pb zircon age of 186 1 Ma, which is similar to a published 189 3M a 40 Ar/ 39 Ar amphibole age, and indicates rapid cooling through ca. 550 °C. This age is ca. 10 my younger than the 196 4 Ma age of the bimodal volcanic complex intruded by the pluton. The volcanic complex is composed of augite-phyric transitional basalt and rhyolite/ comendite. Both basalts and rhyolites/comendites are evolved within-plate varieties with positive Nd(t) ( 2.5) values. The granitoids are evolved alkaline, A-type granites and quartz-syenites/syenites that are enriched in light REE's, but show a distinct depletion in Eu, Sr and Ba, indicative of feldspar fractionation. The data are consistent with derivation of the granites from the syenites by an assimilation- fractional crystallization process involving a silicic crustal contaminant. The granitic rocks have Nd(t) values of 0.8 to 1.2, which are slightly lower than Nd(t) values 1.3 to 1.6 in the syenites, although both have similar TDM model ages (800-970 Ma). The 800 Ma model ages of the basalt and rhyolite/comendite are comparable to those of the intrusion and enclaves. The compositions of all these rocks, including Nd(t) and T DM , are within the range of A-type granites and volcanic complexes of the Early Mesozoic Mongolian-Transbaikalian igneous province. The results suggest deriva- tion of a parent magma of the granitoids and felsic volcanic rocks from underplated, enriched, Neoproterozoic mantle-derived basaltic rocks in the lower crust, whereas the Dashibalbar basalts were derived from Neoproterozoic subcontinental lithospheric mantle; Neoproterozoic megablocks crop out in adjacent parts of the Central Asian Orogenic Belt. Melting of lower crust and subcontinental lithospheric mantle implies a rising heat source. Although such a heat source is consistent with both rifting and passage over the Mongolian mantle plume, only the latter explains the west-to-east migration of the magmatism and rifting.

Rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the Mineiro belt, São Francisco Craton, Brazil

The Tiradentes suite in conjunction with the coeval Estação Tiradentes metasedimentary sequence makes up a distinct tectonic element within the Paleoproterozoic framework of the Mineiro accretionary belt in the Southern São Francisco Craton. This suite is the second occurrence of volcanic and subvolcanic rocks in the Mineiro belt and shows petrographic and geochemical similarities with the nearby Serrinha suite (2.23–2.21Ga) - the first reported Rhyacian volcanism in the Mineiro accretionary belt.The Tiradentes suite is represented by mafic andesites, dacites, granophyres and tonalites and shows petrographic features of volcanic rocks, such as spherulites, amygdales, granophyric intergrowths and porphyritic textures with plagioclase phenocrysts. The rocks of this suite are metaluminous to peraluminous, enriched in alkaline elements, compatible with low-Al2O3 trondhjemites and show primary fractionation of plagioclase, apatite, hornblende, biotite and ilmenite. High HREE content coupled with incipient negative Eu anomaly and high Y values are consistent with a garnet–amphibolite protolith with plagioclase and hornblende as present in the residual phases. The samples of the Tiradentes suite show geochemical signatures of magmatic arcs, which are also observed in rocks from the Serrinha suite.Rocks of the Tiradentes suite yield the following U–Pb zircon crystallization ages: (i) mafic andesite: 2217±23Ma; (ii) dacitic dike: 2204±11Ma; (iii) subvolcanic tonalite: 2213±9Ma. The deposition of the Estação Tiradentes metasedimentary sequence probably took place after ca. 2088±12Ga (although it may be as young as 1990±39Ma), according to the U–Pb detrital zircon geochronology of selected samples (diamictite, ferruginous quartzite and carbonaceous phyllite). The Tiradentes rocks show three TDM model ages ranging between 2.4 and 2.3Ga, indicating short crustal residence for the magmatic protoliths. This inference is also supported by the ɛNd(2.2Ga) values (+1.1 to +2.3), whereas the respective 87Sr/86Sr(t) ratios (0.7018–0.7032) are consistent with the involvement of low Rb/Sr material in the magma genesis, such as a Paleoproterozoic oceanic lithosphere.From a tectonic perspective, the above data are consistent with the idea that a Paleoproterozoic seafloor developed outboard of the Archean–Siderian passive-margin setting (Minas Basin) of the São Francisco proto-Craton. The 2.23–2.21Ga volcanic–subvolcanic rocks of the Tiradentes and Serrinha suites were associated with a Rhyacian oceanic arc. The Estação Tiradentes metasedimentary sequence (turbidites and distal phyllites) probably formed in back-arc setting concomitant with the Tiradentes suite, as indicated by detrital zircon geochronology.

Paleoproterozoic crust-formation and reworking events in the Tocantins Province, central Brazil: A contribution for Atlantica supercontinent reconstruction

The Natividade-Cavalcanti crustal block is a Paleoproterozoic sialic terrain which forms the basement of sedimentary sequences belonging to the Neoproterozoic Brasília Belt in central Brazil. Two main domains were recognized in this crustal block, the limits of which are not well constrained due to the lack of geological maps at an appropriate scale. The northeastern domain is characterized by narrow supracrustal belts which wrap around gneiss domes. At least three rock-forming events are recorded in the calc-alkaline, metaluminous basement gneisses of this domain: (i) in the Almas-Dianópolis area, granitoids were dated at c. 2.2Ga, (ii) early Paleoproterozoic magmatism was dated at c. 2.3–2.4Ga in the Natividade-Conceição do Tocantins area, and (iii) younger additions were dated at 2144±21Ma. TDM model ages range between 2.24 and 3.11Ga, and ɛNd(t) values range from +2 to close to zero, suggesting juvenile sources. Bordering this domain west- and southwards, extending from Cavalcante to Arraias, Paranã and Pindorama do Tocantins, mostly peraluminous granitoids of the Aurumina Suite are exposed. Geochronological data for Aurumina granitoid rocks of this sector point to magmatic events between 2.13 and 2.18Ga, whereas TDM model ages range between 2.21 and 2.92Ga with generally negative ɛNd(t) values. An exception is a largely undeformed tonalite sample from close to Arraias, dated at 2042±12Ma.

Changing provenance of Late Jurassic to Early Cretaceous rift-related sedimentary rocks of the South Atlantic Margin: LA-MC-ICPMS U–Pb and Lu–Hf isotopic study of detrital zircons from the Camamu Basin, Eastern Brazil

The Camamu basin is related to the breakup of Gondwana and belongs to the group of sedimentary basins that are part of the eastern Brazilian passive margin. We have determined the provenance of the precursor sediments by investigating the petrography of fluvial and eolian units from the early-rift Brotas Group and syn-rift Almada Group, coupled to in situ laser ablation MC-ICPMS U–Pb and Lu–Hf isotopic analyses of the detrital zircon grains. Detrital zircon grains from the Sergi Formation, Brotas Group, have a main population of U–Pb ages of 480–600Ma with smaller populations of ages of 900–1000Ma, 1900–2100Ma, 2500–2800Ma and 3000–3200Ma. The Almada Group zircon grains are characterized by a distribution of ages predominantly in the Paleoproterozoic and Archean. The Hf TDM model ages of the Brotas and Almada Group zircon grains range from ca. 1.1 to 4.0Ga, and the εHf(t) values vary from −31.4 to +9.6, providing evidence that some detrital zircon grains in the Camamu Basin were derived from juvenile sources but that the majority have reworked crustal sources or formed from juvenile magmatic rocks that were contaminated by older crustal material. Whole rock Sm–Nd isotope data collected previously are largely consistent with the new U–Pb, Lu–Hf zircon data but do suggest that zircon-poor detritus from the Neoproterozoic to Eopaleozoic Brasiliano Araçuaí-West Congo Belt contributed to particularly the Brotas Group sediments more than would be indicated by the zircon data alone. Thus, integrated petrographic, U–Pb, Lu–Hf zircon and Sm–Nd whole rock studies provide very important clues to the provenance history of the initial sedimentation on Camamu basin passive margin, where the early-rift phase is marked by the input of detritus from the Araçuaí-West Congo belt situated south of the basin, whereas syn-rift sedimentation received major contributions from Paleoproterozoic (Transmazonian cycle) to Archean (Jequié cycle) São Francisco craton units, located to the west.

Zircon U–Pb geochronology and Hf isotope data from the Yangtze River sands: Implications for major magmatic events and crustal evolution in Central China

In situ U–Pb and Hf isotope analyses were made on detrital zircons isolated from sands of the Yangtze River in order to understand the evolution and history of crust in that river basin. A total of 2277 detrital zircon grains were dated by the LA-ICP-MS U–Pb technique and 674 grains were also analyzed for Hf isotopes on the same spots of concordant grains. Our results show five major groups of U–Pb ages: at 2.4–2.6Ga, 1.8–2.0Ga, 700–850Ma, 400–550Ma and 200–300Ma, which correspond to known granitoid magmatic events within the drainage (Lüliangian, Jinningian, Caledonian, Hercynian and Indo-Sinian/Yanshanian). The initial Hf isotope ratios [εHf(t)] of the zircons exhibit a wide range from negative to positive for each of the five major age groups, revealing that existing crustal materials were mixing with variable volumes of new mantle melt at these times. Zircon Hf model ages (TDM2) display two major ranges of crustal growth, one being Mesoproterozoic (1.0–1.5Ga) and the other Archean–Paleoproterozoic (2.5–2.8Ga). These zircons were mainly sourced from the Yangtze Craton and the Cathaysia Block, respectively. Crustal growth rates based on TDM2 model ages suggest that 30% of the present crustal volume was formed by 2.5Ga, 48% by 1.8Ga, 70% by 1.4Ga and 96% was formed by 1.0Ga.

Mid-Cretaceous lamproite from the Kutch region, Gujarat, India: Genesis and tectonic implications

The Phanerozoic tectonic evolution of the Kutch region (NW India) has been attributed to rifting processes. In situ U-Pb perovskite dates (all data=124±4Ma, MSWD=42) obtained on three samples of a newly discovered lamproite dyke emplaced in the Kutch rift setting identify a previously unknown and relatively young (Mid-Cretaceous) magmatic event in this part of India. The dyke was emplaced into the Katrol Formation of Upper Jurassic to very early Cretaceous age. The presence of xenocrystic garnet and fractionated REE distribution patterns (low HREE/LREE) in the lamproite, along with high contents of transition metals and low Al2O3, suggest that it was derived from the garnet stability field in the mantle. Extreme impoverishment in HREE and HFSE suggest a mantle source region that has experienced both depletion and later enrichment. In situ analysis of Nd isotopes in perovskite of three samples gives a mean εNd=0.4±1 and a TDM model age of 598±64Ma, while in situ Sr-isotope analysis gives a mean 87Sr/86Sr=0.70388±2 (2SE), corresponding to εSr=8.6±0.3; both datasets suggest mixing between lithospheric-mantle and depleted-mantle components. The fault-controlled emplacement of this body highlights the role of deep-penetrating fault systems in providing a pathway and initiating decompressional melting of the mantle source rocks. The lamproite represents an earlier phase of rift related magmatism, different from the emplacement of the melanephelinite–basanites and basalts that dominate the igneous activity related to the Deccan volcanism in the Kutch mainland.

The Serra da Bolívia complex: The record of a new Neoproterozoic arc-related unit at Ribeira belt

The Neoproterozoic arc related rocks of the central segment of the Ribeira belt crop out at the Oriental Terrane that is subdivided into three thrust sheets verging northwestern ward, each of them with subduction related rocks. The recently discovered Serra da Bolívia complex crops out at the Cambuci domain that represents the lowermost thrust sheet of the Oriental terrane. The unit comprises orthogneisses of varied compositions, ranging from basic to acid rocks. Hornblende bearing coarse grained orthogneisses, with granodiorite to quartz-diorite compositions, is the most common association in the complex; followed by quartz poor rocks such as monzo-gabbros, monzodiorites, monzonites and syenites. Coarse grained gabbros (norites) and leucogabbros occur as lenses inside the orthogneisses.New geological, geochemical and geochronological data for recently described arc-related orthogneisses of the Serra da Bolívia complex is presented. The connection with other arc-related rocks of the Araçuaí and southern Ribeira belt in the context of West Gondwana reconstructions is also explored.Chemical data indicates at least three groups of sub-alkaline rocks and one group of alkaline to transitional character (SH). Among the sub-alkaline groups three calc-alkaline groups (CA1, CA2 and CA3) with progressive enrichment in Alkalis are discriminated. Tectonic discriminate diagrams suggest that the calc-alkaline groups display compositions compatible with arc settings, while the shoshonite group, with more evolved compositions indicates progressive maturity during arc development. Few basic rocks indicate CAB or IAT filiations.Obtained U–Pb ages for tonalites and gabbros yielded values between ca. 596 and 591Ma, but an older TIMS age pointed to ca. 620Ma. The youngest monzonite orthogneiss yielded ca. 574Ma, similar to the metamorphic ages previously described for the Ribeira belt. Sm-Nd data resulted on Tdm model ages of ca. 1.72Ga and 2.04Ga, except for three samples, which display Mesoproterozoic model ages. The initial ɛNd, calculated for 605Ma, which is the average age for the complex are between −8 and −12.Regional correlations suggest that the Serra da Bolivia Complex represents the inner cordilleran magmatic arc of the Ribeira belt that continues northward to the Galiléia arc (G1 association of the Araçuaí belt), and southwards to the Socorro arc at the interference zone of the Brasília belt. Combined U–Pb data indicate the development of a cordilleran arc between ca. 650 and 590Ma. Youngest syenitic to monzonitic bodies are coeval with the metamorphic episode and intrude the older rocks of the arc. The other two previously described arc related complexes, Rio Negro and Serra da Prata arcs are located easternmost at the belt and display contrasting more juvenile signatures and older ages reaching ca. 840M.Looking to the West Gondwana scenario, during Tonian to Cryogenian time, subduction all around older and stables plates resulted on the generation of large volumes of arc-related rocks. In most of the belts, only the plutonic zones of the magmatic arcs are exposed. Based on time span of rock crystallization of all the arc related rocks of the Ribeira belt, the subduction period lasted at least 240 million years. Even though subduction was breaking down into three individual zones, the tectonic scenario suggests oceanic plate consumption in a protracted evolution. The initial phases of most magmatic arcs are represented by more primitive and juvenile rocks, but at the end of the subduction period, at the Ediacaran, a more cordilleran setting was achieved. By the end of the Ediacaran period and the beginning of Cambrian, all the pieces were attached together in several diachronic collisions leading to the final amalgamation of West Gondwana.

Petrology of the high-Mg tonalites and dioritic enclaves of the ca. 2130 Ma Alto Maranhão suite: Evidence for a major juvenile crustal addition event during the Rhyacian orogenesis, Mineiro Belt, southeast Brazil

Combined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U–Pb zircon data are presented for the ca. 2130Ma Alto Maranhão suite. The suite, composed by allanite-bearing biotite hornblende tonalites and commingled dioritic mafic magmatic enclaves (MME), occupies an estimated area of >300km2 in the southern Quadrilátero Ferrífero region, Minas Gerais, Brazil. Tonalites and commingled MME are medium-K rocks, with high-Mg (Mg-number≥0.46) and high-Cr (≥55ppm) contents. They are also enriched in large-ion lithophile elements (LILE, Sr, Ba), some high-field-strength elements (HFSE, Th, U), and light rare earth elements (REE), but depleted in Nb, Ta, and heavy REE. TDM model ages for tonalites and MME are similar and range from 2.3 to 2.4Ga. The ɛNd(t) values are grouped around chondritic values (=0±1.0). Field relations and geochemical data indicate that tonalites and commingled dioritic MME crystallized as synchronous independent magma pulses, with limited mixing. Both magmas are compositionally equivalent to high-Mg andesites in modern subduction zones, implying that the suite was issued from the partial melting of a mantle wedge metasomatised to different degrees by slab-derived melts. Geobarometry (aluminium-in-hornblende) indicates crystallization at mid-crustal depths (i.e. 0.59–0.42GPa). The igneous suite evolved by fractional crystallization of hornblende, biotite and accessory minerals (zircon, apatite, allanite and Fe–Ti oxide). The ubiquitous occurrence of hornblende and allanite in tonalites and dioritic MME suggests that this mineral pair was largely responsible for the observed changes of light REE/heavy REE and Eu/Eu* ratios within the members of the suite. The geographical extent (>1000km2) and Paleoproterozoic ages of the Alto Maranhão suite (ca. 2130Ma) and associated granitoids of the Mineiro Belt (2.1–2.2Ga) constitute a major event of juvenile addition to the continental crust in the context of the southern São Francisco craton, and the ca. 2.2–2.1Ga Rhyacian orogenesis of the South American platform. The age and general geochemical characteristics of the Alto Maranhão suite and its emplacement after a ca. 2.35Ga juvenile, tholeiitic-source derived high-Al TTG suite, indicate a tectonic setting akin to that of Late-Archaean high-Mg granitoids.

The Cadomian Orogen: Neoproterozoic to Early Cambrian crustal growth and orogenic zoning along the periphery of the West African Craton—Constraints from U–Pb zircon ages and Hf isotopes (Schwarzburg Antiform, Germany)

The Cadomian Orogen in the NE Bohemian and the northern Armorican Massifs shows a distinct orogenic zoning from recent NW to SE consisting of (i) an outboard sitting continental crustal unit comprising Neoproterozoic rocks associated with c. 2.0Ga old Icartian Basement, (ii) a magmatic arc and a back-arc basin, (iii) a foreland or retro-arc basin, and (iv) the passive margin of the back-arc basin. New U–Pb zircon ages of detrital zircon of Neoproterozoic to Fortunian siliciclastics from the Schwarzburg Antiform in the Saxo-Thuringian Zone (NE Bohemian Massif) identify the West African Craton as the hinterland for the Cadomian Orogen as demonstrated by zircon populations dated at 1.8–2.2, 2.5–2.7, 3.0–3.1, and 3.4–3.5Ga. The dominant zircon population (c. 50–70% in each sample) is derived from a Cadomian magmatic arc in a time slice of c. 570–750Ma. The magmatic activity of the Cadomian arc stopped at c. 570Ma. Closure of the back-arc basin by arc-continent collision occurred between c. 570 and 542Ma under the formation of a foreland (retro-arc) basin. A short-living remnant basin existed between c.542 and 540Ma. Granitoid plutonism at 539–540Ma documents the final pulse of the Cadomian Orogeny. Hf isotope compositions, calculated ɛHfi values and TDM model ages for detrital and magmatic zircon show that during the c. 180Ma long Cadomian magmatic arc activity juvenile arc magmas were contaminated by recycling of Eburnian and Archaean crust. Mixing with an evolved continental crust is always present. The inferred geotectonic setting is a continental magmatic arc during the Neoproterozoic developed on a stretched Archaean and Palaeoproterozoic (Eburnian) crust. In the West African crustal evolution it can be demonstrated that during Eburnian orogenic processes (c. 1.8–2.2Ga) in most cases a 2.5–3.4Ga old basement became reworked. Archaean 2.5–2.9Ga magmas remelted a 3.0–3.4.Ga crust. Zircon grains with an age of 3.0–3.1 and 3.4Ga are derived from juvenile magmas. Two zircon grains dated at 2779±22 and 3542±28Ma imply reworking of pre-existing Eoarchean to Hadean crust and show TDM model ages of 3.98 and 4.29Ga, respectively.

The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana

This paper presents combined U/Pb, Th/U and Hf isotope analyses on detrital and magmatic zircon grains together with whole-rock geochemical analyses of two basement and eight sedimentary rock samples from the Namuskluft and the Dreigratberg in southern Namibia (Gariep Belt). The sedimentary sections evolved during the Cryogenian on the SW part of the Kalahari Craton and where therefore deposited in an active rift setting during the break-up of Rodinia. Due to insufficient palaeomagnetic data, the position of the Kalahari Craton within Rodinia is still under discussion. There are possibilities to locate Kalahari along the western side of Australia/Mawsonland (Pisarevski et al. in Proterozoic East Gondwana: supercontinent assembly and break-up, Geological Society, London, 2003; Evans in Ancient Orogens and modern analogues. Geological Society, London, 2009; and others) or together with the Congo-Sao Francisco and Rio de la Plata Cratons (Li et al. in Prec Res 45: 203–2014, 2008; Frimmel et al. in Int J Earth Sci (Geol Rundsch) 100: 323–354, 2011; and others). It is sill unclear which craton rifted away from the Kalahari Craton during the Cryogenian. Although Middle to Upper Cryogenian magmatic activity is known for the SE Kalahari Craton (our working area) (Richtersveld Suite, Rosh Pinah Fm), all the presented samples show no U/Pb zircon ages younger than ca. 1.0 Ga and non-older than 2.06 Ga. The obtained U/Pb ages fit very well to the exposed basement of the Kalahari Craton (1.0–1.4 Ga Namaqua Province, 1.7–2.0 Ga Vioolsdrif Granite Suite and Orange River Group) and allow no correlation with a foreign craton such as the Rio de la Plata or Australia/Mawsonland. Lu–Hf isotopic signatures of detrital zircon point to the recycling of mainly Palaeoproterozoic and to a smaller amount of Archean crust in the source areas. eHf(t) signatures range between −24 and +14.8, which relate to TDM model ages between 1.05 and 3.1 Ga. Only few detrital zircon grains derived from magmas generated from Mesoproterozoic crustal material show more juvenile eHf(t) signatures of +14, +8 to +4 with TDM model ages of 1.05–1.6 Ga. During Neoproterozoic deposition, only old cratonic crust with an inherited continental arc signature was available in the source area clearly demonstrated by Hf isotope composition of detrital zircon and geochemical bulk analysis of sedimentary rocks. The granodiorites of the Palaeoproterozoic basement underlying Namuskluft section are ca. 1.9 Ga old and show eHf(t) signatures of −3 to −5.5 with TDM model ages of 2.4–2.7 Ga. These basement rocks demonstrate the extreme uplift and deep erosion of the underlying Kalahari Craton at its western margin before general subsidence during Cryogenian and Ediacaran time. The sedimentary sequence of the two examined sections (Namuskluft and Dreigratberg) proposes the presence of a basin and an increasing subsidence at the SW part of the Kalahari Craton during the Cryogenian. Therefore, we propose the initial formation of an intra-cratonic sag basin during the Lower Cryogenian that evolved later to a rift basin at the cratonic margin due to increasing crustal tension and rifting together with the opening of the Adamastor Ocean. As the zircons of the sedimentary rocks filling this basin show neither rift-related U/Pb ages nor an exotic craton as a possible source area, the only plausible sedimentary transport direction providing the found U/Pb ages would be from the E or the SE, directly from the heart of the Kalahari Craton. Due to subsidence and ongoing sedimentation from E/SE directions, the rift-related magmatic rocks were simply covered by the input of old intra-cratonic material that explains the absence of Neoproterozoic zircon grains in our samples. The geochemical analyses show the erosion of a continental arc and related sedimentary rocks with an overall felsic provenance. The source area was a deeply eroded and incised magmatic arc that evolved on continental crust, without any evidence for a passive margin. All of this can be explained by the erosion of rocks related to the Namaqua Belt, which represents one of the two major peaks of zircon U–Pb ages in all analysed samples. Therefore, the Namaqua Belt was well exposed during the Cryogenian, available to erosion and apart from the also well-exposed Palaeoproterozoic basement of the Kalahari Craton one potential source area for the sedimentary rocks in the investigated areas.

Detrital zircon (U–Pb) and Sm–Nd isotope studies of the provenance and tectonic setting of basins related to collisional orogens: The case of the Rio Preto fold belt on the northwest São Francisco Craton margin, NE Brazil

The Rio Preto fold belt (RPFB), in northeastern Brazil, is represented by Precambrian metasedimentary sequences (Canabravinha and Formosa formations) that were accreted to the northwestern São Francisco Craton margin, during the collisional Brasiliano Orogeny (~630–500Ma). The analysis of the detrital zircon age spectra of samples from both formations revealed a broadly distinct sedimentary provenance for each one of them. While the Canabravinha Formation yielded late-Archean to Tonian zircons, the Formosa Formation yielded a relatively simple, bimodal detrital zircon age pattern at 1900–2200Ma and 2500–2650Ma. The Nd isotope compositions of each unit are also broadly distinct. The Canabravinha Formation shows variable TDM model ages ranging from 1.6 to 2.7Ga, with associated εNd(600) values of −8.7 to −19.5, thus reflecting a wide age range in the source areas. The Formosa Formation TDM distribution reflects the bimodal detrital zircon pattern, with two broad TDM peaks of 1.9–2.0 and 2.4–2.6Ga, and εNd(600) values from −12.9 to −26.5. Based on the detrital zircon and Sm–Nd isotope data, we suggest that the Canabravinha and Formosa formations represent two distinct sedimentary sequences in time and space. A sill-like amphibolite body interleaved within the Formosa Formation mica-schists yielded a U–Pb zircon age of 1962.6±9.9Ma that supports this interpretation. Thus, we suggest that the Formosa Formation represents part of the basement of the Neoproterozoic fold belt, presenting a very simple sedimentary provenance derived uniquely from the Cristalândia do Piauí Complex (Archean/Paleoproterozoic) to the north. The Canabravinha Formation, on the other hand, represents the infilling of a Neoproterozoic extensional (rift) basin (~900–600Ma), whose debris came mainly from the São Francisco Craton to the south and from secondary source areas further northeast in the Borborema Province, such as the Cariris Velhos belt (920–1000Ma). During the Brasiliano Orogeny, both the Proterozoic supracrustal sequences and the Archean/Paleoproterozoic basement were tectonically juxtaposed, deformed and metamorphosed under greenschist facies, thus complicating the original stratigraphic relationships between them.

Zircon U–Pb geochronology and Sr–Nd–Hf isotopic compositions of the Yuanzhuding granitoid porphyry within the Shi-Hang Zone, South China: Petrogenesis and implications for Cu–Mo mineralization

The Shi-Hang Zone is an important NE–SW-trending Mesozoic magmatic belt in South China, which is dominated by granites with relatively high εNd(t) values and young TDM model ages. Here, we present laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) zircon U–Pb ages, major and trace element compositions, and Sr–Nd–Hf isotope data for the Yuanzhuding porphyritic granitoids within the southwestern Shi-Hang Zone, and use these data to determine the origin of this granitoid and its relationship with Cu–Mo mineralization. Zircon U–Pb dating indicates that these granitoids were emplaced at 157.8±1.1Ma. They have initial 87Sr/86Sr ratios of 0.70941–0.71398, εNd(t) values of −3.15 to −2.02, and in situ zircon εHf(t) values of +1.71 to +6.17. Geochemically, most of them are high-K calc-alkaline and show an adakitic affinity. They are more likely emplaced in a continental arc setting related to westward subduction of the paleo-Pacific plate. Their parental magma originated chiefly from the overlying sediments of the downgoing slab, and subsequently interacted with the lithospheric mantle wedge at the temperatures of ~790°C. High oxygen fugacity during magmatic evolution played a crucial role in the development of Cu–Mo mineralization within the Yuanzhuding. The discovery of the Yuanzhuding deposit implies that the Chenzhou–Huaiji fault belt is most likely prospective for mineral exploration for porphyry Cu–Mo deposits.

Neoarchean–Paleoproterozoic multiple tectonothermal events in the western Alxa block, North China Craton and their geological implication: Evidence from zircon U–Pb ages and Hf isotopic composition

The Alxa block was traditionally considered to be part of the North China Craton, but its metamorphic basement has been poorly studied. Here we present a systematic zircon U–Pb and Hf isotopic investigation on four orthogneiss samples in the Beidashan area of the western Alxa block. The petrographic and geochemical data show that these rocks are granodioritic and trondhjemitic gneisses with TTG (tonalite–trondhjemite–granodiorite) characteristics. Zircons from the TTG gneisses display typical core–rim or core–mantle–rim structures. U–Pb datings and Hf isotopic analyses reveal two distinct age populations: the Latest Neoarchean (∼2.5Ga) and the Late Palaeoproterozoic (∼1.85Ga). The magmatic zircon cores and metamorphic mantles (rims) of the TTG gneisses were dated at similar ages around 2.5Ga, supporting the existence of Archean rocks in the western Alxa block. The short time interval between the Latest Neoarchean magmatism and the subsequent metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons have ɛHf(t) mainly between 0.8 and 5.0, TDM (Hf) model ages mainly between 2.6 and 2.8Ga (with a peak at ∼2.7Ga) and TDMC (Hf) model ages mainly between 2.7 and 3.0Ga (with a peak at ∼2.8Ga). The age of ∼1.85Ga obtained from two trondhjemitic gneisses is interpreted as the age of the Late Paleoproterzoic high-grade metamorphism. Our combined datasets show that the TTG gneisses in the Baidashan area of the western Alxa block experienced a main 2.7–2.8Ga crust growth, a ∼2.5Ga magmatic–metamorphic event and a ∼1.85Ga high-grade metamorphic event. The sequence of events is very similar to that of the other North China Craton. A Combination of the data of Paleoproterozoic metamorphic rocks in the Alxa block suggests that the Alxa block is the western extension of the Khondalite Belt rather than the Yinshan block.

The Grenvillian orogeny in the Altun–Qilian–North Qaidam mountain belts of northern Tibet Plateau: Constraints from geochemical and zircon U–Pb age and Hf isotopic study of magmatic rocks

Numerous Neoproterozoic magmatic and metamorphic events in the Altun–Qilian–North Qaidam (AQNQ) region record Grenvillian orogenesis and amalgamation of the supercontinent Rodinia. However, the tectonothermal regimes responsible for these Neoproterozoic events and the assumed position of the AQNQ in Rodinia remain controversial. Zircon U–Pb age data show that the orthogneiss and paragneiss/schist of the AQNQ experienced concurrent magmatism and metamorphism at 895–925Ma. Zircon Lu–Hf isotopic data indicate that the gneisses in the AQNQ have εHf (0.9Ga) values and tDM2 (Hf) model ages ranging from −5.6 to +3.9 and 1.4 to 1.9Ga. These data suggest that the early Neoproterozoic magma in the AQNQ was predominately derived from a late Paleoproterozoic–early Mesoproterozoic crustal source between 1.4 and 1.9Ga, marking an important episode of crustal growth in the AQNQ. The Neoproterozoic magmatism is geochemically characterized by (1) high SiO2, K2O, and low P2O5; (2) A/CNK ratios >1.0, ranging from 1.03 to 1.09; (3) enrichment in Rb, Th and U, and depletion in Ba, Nb, Ta, Sr, Ti, and Eu. Based on the geochemical resemblance to high-K calc-alkaline I-type granite and zircon Lu–Hf isotope signatures, the Neoproterozoic magmatism in the AQNQ was probably derived from ancient mafic-intermediate igneous rocks in an active continental margin. The Neoproterozoic tectono-magmatic–metamorphic history of the AQNQ, directly associated with the South China block (SCB) and the Tarim block (TB), indicates that the AQNQ and the TB coexisted as a single block in the early Neoproterozoic, which was temporarily connected to the SCB to the north or west in Rodinia during the late stages of the Grenvillian orogeny (950–900Ma).

The Pan-African Kekem gabbro-norite (West-Cameroon), U–Pb zircon age, geochemistry and Sr–Nd isotopes: Geodynamical implication for the evolution of the Central African fold belt

The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U–Pb zircon analyses provide an age of 576±4Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ).The REE patterns display enriched LREE (LaN/YbN=14.2–23.5) while HREE present a nearly flat profile (DyN/YbN=1.3–1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb–Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6–10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068–0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6–1.8Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust.The ca. 576Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.