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1.
The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones.  相似文献   

2.
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.  相似文献   

3.
The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goiás, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism, spanning ca. 140 m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785 Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3 Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707 Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3 Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0 Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550 Ma. One of the banded gneisses with a crystallization age of ca. 2700 Ma (2σ) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590 ± 10 Ma (2σ). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses.  相似文献   

4.
U–Pb SHRIMP results of 2672 ± 14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792 ± 11 and 2751 ± 9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612 + 3/− 2 Ma, it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution.  相似文献   

5.
The Pine Creek Orogen, located on the exposed northern periphery of the North Australian Craton, comprises a thick succession of variably metamorphosed Palaeoproterozoic siliciclastic and carbonate sedimentary and volcanic rocks, which were extensively intruded by mafic and granitic rocks. Exposed Neoarchean basement is rare in the Pine Creek Orogen and the North Australian Craton in general. However, recent field mapping, in conjunction with new SHRIMP U–Pb zircon data for six granitic gneiss samples, have identified previously unrecognised Neoarchean crystalline crust in the Nimbuwah Domain, the eastern-most region of the Pine Creek Orogen. Four samples from the Myra Falls and Caramal Inliers, the Cobourg Peninsula, and the Kakadu region have magmatic crystallisation ages in the range 2527–2510 Ma. An additional sample, from northeast Myra Falls Inlier, yielded a magmatic crystallisation age of 2671 ± 3 Ma, the oldest exposed Archean basement yet recognised in the North Australian Craton. These results are consistent with previously determined magmatic ages for known outcropping and subcropping crystalline basement some 200 km to the west. A sixth sample yielded a magmatic crystallisation age of 2640 ± 4 Ma. The ca. 2670 Ma and ca. 2640 Ma samples have ca. 2500 Ma metamorphic zircon rims, consistent with metamorphism broadly coeval with emplacement of the volumetrically dominant ca. 2530–2510 Ma granites and granitic gneisses. Neoarchean zircon detritus, particularly in the ca. 2530–2510 Ma and ca. 2670–2640 Ma age span, are an almost ubiquitous feature of detrital zircon spectra of unconformably overlying metamorphosed Palaeoproterozoic strata of the Pine Creek Orogen, and of local post-tectonic Proterozoic sequences, consistent with this local provenance. Neoarchean zircon is also a common detrital component in Palaeoproterozoic sedimentary units across much of the North Australian Craton suggesting the existence of an extensive, if not contiguous, Neoarchean crystalline basement underlying not only a large part of the Pine Creek Orogen, but also much of the North Australian Craton.  相似文献   

6.
The igneous complex of Neukirchen–Kdyn is located in the southwestern part of the Teplá–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the V epadly granodiorite and the Smr ovice diorite intruded at shallow crustal levels (<ca. 7 km depth) as was indicated by geobarometric data. K–Ar age data yield 547±7 and 549±7 for hornblende and 495±6 Ma for biotite of the Smr ovice diorite, suggesting that this pluton has remained at shallow crustal levels (T<ca. 350 °C) since its Cambrian emplacement. A similar history is indicated for the V epadly granodiorite and the Stod granite. In the SW, intermediate to mafic plutons of the Neukirchen–Kdyn massif (V eruby and Neukirchen gabbro, Hoher–Bogen metagabbro), which yield Cambrian ages, either intruded or were metamorphosed at considerably deeper structural levels (>20 km). The Teufelsberg ( ert v kámen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.  相似文献   

7.
The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along more than 300 km in the external zone of the Brasilia Fold Belt of the São Francisco Basin in east central Brazil. The sequence is about 4.8 km thick and contains a preserved glaciomarine diamictite unit (containing dropstone) at the top and a lower diamictite unit at the bottom. Previous C- and Sr-isotope profiles suggested the correlation of the upper diamictite unit with the “Sturtian” glacial event (ca. 750–643 Ma). However, new Re–Os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 ± 46 and 1100 ± 77 Ma. U–Pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 ± 15 and 1000 ± 25 Ma, respectively. Based on the Re–Os and U–Pb ages, the best age estimate of the Vazante Group is constrained to be 1000–1100 Ma and thus the two diamictite units are not correlative with the Sturtian glaciation(s) but most likely are records of glacial events that occurred during the late Mesoproterozoic.  相似文献   

8.
Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.  相似文献   

9.
The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.  相似文献   

10.
Nd and Sr isotope data were obtained for three plutonic suites (595–505 Ma) and distinct young granitoid intrusions (503 Ma), from the southern part of the Neoproterozoic Araçuaí Orogen. The Sr and Nd isotopes (87Sr/86Sr, eNd) and TDM values from the plutons and distinct basement rocks are used to constrain the magma genesis of the granitoid plutons. These isotopic parameters, with eNd values ranging from −4 to −24 and TDM ages from 1.3 to 2.8 Ga, for the granitoid suites, and −5 to −40 and 3.5 to 1.5 Ga, for the distinct Archean and Proterozoic basement complexes, suggest that the Jequitinhonha Complex metasediments are the main crustal source for most of these plutons, except for the youngest granitoid intrusions, which may have a protolith similar to the Mantiqueira and Guanhães complexes. Furthermore, the isotope data indicate a minor, but important, participation of Neoproterozoic oceanic lithosphere in the granite genesis, which corroborates with a confined orogenic model and a narrow oceanic consumption (B-subduction) for the Araçuaí Orogen.  相似文献   

11.
The Itacaiúnas Belt of the highly mineralised Carajás Mineral Province comprises ca. 2.75 Ga volcanic rocks overlain by sedimentary sequences of ca. 2.68 Ga age, that represent an intracratonic basin rather than a greenstone belt. Rocks are generally at low strain and low metamorphic grade, but are often highly deformed and at amphibolite facies grade adjacent to the Cinzento Strike Slip System. The Province has been long recognised for its giant enriched iron and manganese deposits, but over the past 20 years has been increasingly acknowledged as one of the most important Cu–Au and Au–PGE provinces globally, with deposits extending along an approximately 150 km long WNW-trending zone about 60 km wide centred on the Carajás Fault. The larger deposits (approx. 200–1000 Mt @ 0.95–1.4% Cu and 0.3–0.85 g/t Au) are classic Fe-oxide Cu–Au deposits that include Salobo, Igarapé Bahia–Alemão, Cristalino and Sossego. They are largely hosted in the lower volcanic sequences and basement gneisses as pipe- or ring-like mineralised, generally breccia bodies that are strongly Fe- and LREE-enriched, commonly with anomalous Co and U, and quartz- and sulfur-deficient. Iron oxides and Fe-rich carbonates and/or silicates are invariably present. Rhenium–Os dating of molybdenite at Salobo and SHRIMP Pb–Pb dating of hydrothermal monazite at Igarapé-Bahia indicate ages of ca. 2.57 Ga for mineralisation, indistinguishable from ages of poorly-exposed Archean alkalic and A-type intrusions in the Itacaiúnas Belt, strongly implicating a deep magmatic connection.A group of smaller, commonly supergene-enriched Cu–Au deposits (generally < 50 Mt @ < 2% Cu and < 1 g/t Au in hypogene ore), with enrichment in granitophile elements such as W, Sn and Bi, spatially overlap the Archean Fe-oxide Cu–Au deposits. These include the Breves, Águas Claras, Gameleira and Estrela deposits which are largely hosted by the upper sedimentary sequence as greisen-to ring-like or stockwork bodies. They generally lack abundant Fe-oxides, are quartz-bearing and contain more S-rich Cu–Fe sulfides than the Fe-oxide Cu–Au deposits, although Cento e Dezoito (118) appears to be a transitional type of deposit. Precise Pb–Pb in hydrothermal phosphate dating of the Breves and Cento e Dezoito deposits indicate ages of 1872 ± 7 Ma and 1868 ± 7 Ma, respectively, indistinguishable from Pb–Pb ages of zircons from adjacent A-type granites and associated dykes which range from 1874 ± 2 Ma to 1883 ± 2 Ma, with 1878 ± 8 Ma the age of intrusions at Breves. An unpublished Ar/Ar age for hydrothermal biotite at Estrela is indistinguishable, and a Sm–Nd isochron age for Gameleira is also similar, although somewhat younger. The geochronological data, combined with geological constraints and ore-element associations, strongly implicate a magmatic connection for these deposits.The highly anomalous, hydrothermal Serra Pelada Au–PGE deposit lies at the north-eastern edge of the Province within the same fault corridor as the Archean and Paleoproterozoic Cu–Au deposits, and like the Cu–Au deposits is LREE enriched. It appears to have formed from highly oxidising ore fluids that were neutralised by dolomites and reduced by carbonaceous shales in the upper sedimentary succession within the hinge of a reclined synform. The imprecise Pb–Pb in hydrothermal phosphate age of 1861 ± 45 Ma, combined with an Ar/Ar age of hydrothermal biotite of 1882 ± 3 Ma, are indistinguishable from a Pb–Pb in zircon age of 1883 ± 2 Ma for the adjacent Cigano A-type granite and indistinguishable from the age of the Paleoproterozoic Cu–Au deposits. Again a magmatic connection is indicated, particularly as there is no other credible heat or fluid source at that time.Finally, there is minor Au–(Cu) mineralisation associated with the Formiga Granite whose age is probably ca. 600 Ma, although there is little new zircon growth during crystallisation of the granite. This granite is probably related to the adjacent Neoproterozoic (900–600 Ma) Araguaia Fold Belt, formed as part of the Brasiliano Orogeny.Thus, there are two major and one minor period of Cu–Au mineralisation in the Carajás Mineral Province. The two major events display strong REE enrichment and strongly enhanced LREE. There is a trend from strongly Fe-rich, low-SiO2 and low-S deposits to quartz-bearing and more S-rich systems with time. There cannot be significant connate or basinal fluid (commonly invoked in the genesis of Fe-oxide Cu–Au deposits) involved as all host rocks were metamorphosed well before mineralisation: some host rocks are at mid- to high-amphibolite facies. The two major periods of mineralisation correspond to two periods of alkalic to A-type magmatism at ca. 2.57 Ga and ca. 1.88 Ga, and a magmatic association is compelling.The giant to world-class late Archean Fe-oxide Cu–Au deposits show the least obvious association with deep-seated alkaline bodies as shown at Palabora, South Africa, and implied at Olympic Dam, South Australia. The smaller Paleoproterozoic Cu–Au–W–Sn–Bi deposits and Au–PGE deposit show a more obvious relationship to more fractionated A-type granites, and the Neoproterozoic Au–(Cu) deposit to crustally-derived magmas. The available data suggest that magmas and ore fluids were derived from long-lived metasomatised lithosphere and lower crust beneath the eastern margin of the Amazon Craton in a tectonic setting similar to that of other large Precambrian Fe-oxide Cu–Au deposits.  相似文献   

12.
Detrital zircon provides a powerful archive of continental growth and recycling processes. We have tested this by a combined laser ablation ICP-MS U–Pb and Lu–Hf analysis of homogeneous growth domains in detrital zircon from late Paleozoic coastal accretionary systems in central Chile and the collisional Guarguaráz Complex in W Argentina. Because detritus from a large part of W Gondwana is present here, the data delineate the crustal evolution of southern South America at its Paleopacific margin, consistent with known data in the source regions.Zircon in the Guarguaráz Complex mainly displays an U–Pb age cluster at 0.93–1.46 Ga, similar to zircon in sediments of the adjacent allochthonous Cuyania Terrane. By contrast, zircon from the coastal accretionary systems shows a mixed provenance: Age clusters at 363–722 Ma are typical for zircon grown during the Braziliano, Pampean, Famatinian and post-Famatinian orogenic episodes east of Cuyania. An age spectrum at 1.00–1.39 Ga is interpreted as a mixture of zircon from Cuyania and several sources further east. Minor age clusters between 1.46 and 3.20 Ga suggest recycling of material from cratons within W Gondwana.The youngest age cluster (294–346 Ma) in the coastal accretionary prisms reflects a so far unknown local magmatic event, also represented by rhyolite and leucogranite pebbles. It sets time marks for the accretion history: Maximum depositional ages of most accreted metasediments are Middle to Upper Carboniferous. A change of the accretion mode occurred before 308 Ma, when also a concomitant retrowedge basin formed.Initial Hf-isotope compositions reveal at least three juvenile crust-forming periods in southern South America characterised by three major periods of juvenile magma production at 2.7–3.4 Ga, 1.9–2.3 Ga and 0.8–1.5 Ga. The 176Hf/177Hf of Mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile, mantle-derived magma, while that of zircon from the Guarguaráz Complex has a largely juvenile crustal signature. Zircon with Pampean, Famatinian and Braziliano ages (< 660 Ma) originated from recycled crust of variable age, which is, however, mainly Mesoproterozoic. By contrast, the Carboniferous magmatic event shows less variable and more radiogenic 176Hf/177Hf, pointing to a mean early Neoproterozoic crustal residence. This zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems, but probably derived from a more juvenile crust in their backstop system.  相似文献   

13.
New K–Ar and 40Ar/39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb)N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb)N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate–K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64–59 Ma for felsic dikes. Because Jurassic–Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

Resumo

Novos dados K–Ar e 40Ar/39Ar de enxames de diques toleíticos e alcalinos localizados no embasamento onshore da Bacia de Santos (SE Brasil) apontam para diferentes pulsos magmáticos ocorridos entre o Jurássico e o Terciário. Os diques de rochas toleíticas (basalto, diabásio e microgabro), são mais velhos, exibem altos teores de TiO2 (3,65% peso na média) e podem ser subdivididos em dois grupos magmáticos. O Grupo A aflora a norte da Bacia de Resende, compreende diques orientados na direção NW, com razões (La/Yb)N entre 15 e 32,3, e idades entre 192.9±2.2 e 160.9±1.9 Ma. O Grupo B aflora a sul e a leste da Bacia de Resende, engloba diques orientados na direção NNW e NNE, com razões (La/Yb)N entre 7 e 16, e idades obtidas entre 148.3±3 e 133.9±0.5 Ma. As rochas alcalinas (sienito, traquito, fonolito, basalto alcalino e lamprófiro) possuem teores médios de K, e afloram como diques, plugs e stocks.. As idades obtidas são de ca. 82 Ma para os diques lamprofíricos, de ca. 70 Ma para os plugs sieníticos, e entre 64 e 59 Ma para os diques félsicos. Como estas idades Jurássicas para diques toleíticos ainda não foram descritas para a região sudeste do Brasil, pode-se especular que durante o emplacement dos diques do GrupoA o cenário tectônico indicaria esforços extensionais anteriores à abertura do Oceano atlântico Sul, e contemporânea ao derrame basáltico do Karoo na África do Sul. Já os diques do Grupo B são contemporâneos ao magmatismo Serra Geral e ao enxame de diques de Ponta Grossa, e portanto este episódio está diretamente relacionado à separação entre o Brasil e África no Cretáceo. O magmatismo alcalino está associado a diversos episódios tectônicos que sucedem à abertura do Oceano Atlântico Sul e que resultaram no desenvolvimento das Bacias terciárias do sudeste brasileiro. Está provavelmente relacionado à chegada da Pluma de Trindade e as idades obtidas para a região em estudo indicam que o magmatismo lamprofírico é mais antigo (ca. 82 Ma), seguido pelos plútons sieníticos (ca. 70 Ma) e diques associados (64 a 59 Ma.)  相似文献   

14.
Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced  600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean 206Pb/238U age of 733 ± 7 Ma, indicating ophiolite formation at  730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt ( 810–780 Ma and  730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.  相似文献   

15.
SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic mélange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic mélange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of the Jinshajiang belt, a granitoid batholith (268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic mélange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the mélange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.  相似文献   

16.
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 (TDM) 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 (TDM) 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 (TDM) 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.  相似文献   

17.
Archean terrains of the Quadrilátero Ferrífero comprise a greenstone belt association surrounded by granitoid–gneiss complexes, mainly composed of banded TTG gneisses whose igneous protoliths are older than 2900 Ma. This early continental crust was affected by three granitic magmatic episodes during the Neoarchean: ca. 2780 to 2760 Ma; 2720 to 2700 Ma; and 2600 Ma. Dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 Ma, contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the Quadrilátero Ferrífero. Lead isotopic studies of lode–gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 Ma.Proterozoic evolution of the Quadrilátero Ferrífero comprises deposition of a continental-margin succession hosting thick, Lake Superior-type banded iron formations, at ca. 2500 to 2400 Ma, followed by deposition of syn-orogenic successions after 2120 Ma. The latter is related to the Transamazonian Orogeny. The western part of the Quadrilátero Ferrífero was also affected by the Brasiliano Orogeny (600 to 560 Ma).  相似文献   

18.
Dating and forward modelling of the fission-track data of apatite samples from the Dereli– ebinkarahisar region, south of Giresun in the Eastern Turkish Pontides, provides quantitative data on the regional tectonics resulting from the closure of neo-Thetys and the collision of Eurasia and Gondwana. The age vs. elevation profiles identified Senonian (80.7±3.2 to 62.4±2.5 Ma) slow uplift and denudation, interpreted as the result of the diapiric ascent of subduction-related plutons above the neo-Tethyan subduction zone beneath the Eurasian continent. This was followed by rapid differential uplift during the Palaeocene–Early Eocene (57.4±2.4 to 47.8±2.4 Ma), which juxtaposed granitoid units of different ages, compositions, and emplacement levels in the crust, and is thought to be related to the collision between the Pontide (Eurasian) and Anatolide (Gondwana) basements. The modelling results must be interpreted with caution, but appear to indicate a period of Mio-Pliocene (ca. 5 Ma) reheating related to volcanism associated with the westward escape of the Anatolian plate and uplift from the Pliocene (ca. 3.5 Ma) up to the present.  相似文献   

19.
Precise U–Pb geochronology and Hf isotope tracing of zircon is combined with whole-rock geochemical and Sr and Nd isotope data in order to unravel processes affecting mafic to felsic calcalkaline magmas prior to and during their crystallization in crustal magma chambers along the southern border of Central Srednogorie tectonic zone in Bulgaria (SE Europe). ID-TIMS U–Pb dating of single zircons from felsic and mixed/mingled dioritic to gabbroic horizons of single plutons define crystallization ages of around 86.5–86.0, 85.0–84.5 and 82 Ma. Concordia age uncertainties are generally less than 0.3 Ma (0.35%–2σ), and as good as 0.08 Ma (0.1%), when the weighted mean 206Pb/238U value is used. Such precision allows the distinction of magma replenishment processes if separated by more than 0.6–1.0 Ma and when they are marked by newly saturated zircons. We interpret zircon dates from a single sample that do not overlap to reflect new zircon growth during magma recharge in a long-lived crustal chamber. Mingling/mixing of the basaltic magma with colder granitoid mush at mid- to upper-crustal levels is proposed to explain zircon saturation and fast crystallization of U- and REE-rich zircons in the hybrid gabbro.Major and trace-element distribution and Sr and Nd whole-rock isotope chemistry define island arc affinities for the studied plutons. Slab derived fluids and a sediment component are constrained as enrichment sources for the mantle wedge-derived magma, though Hf isotopes in zircon suggest crustal assimilation was also important. Inherited zircons, and their corresponding ε-Hf, from the hybrid gabbroic rocks trace the lower crust as possible source for enrichment of the mantle magma. These inherited zircons are about 440 Ma old with ε-Hf of − 7 at 82 Ma, whereas newly saturated concordant Upper Cretaceous zircons reveal mantle ε-Hf values of + 7.2 to + 10.1. The upper and middle crusts contribute in the generation of the granitoid rocks. Their zircon inheritance is Lower Palaeozoic or significantly older and crustal dominated with 82–85 Ma corrected ε-Hf values of − 28. The Cretaceous concordant zircons in the granitoids are mantle dominated with a ε-Hf values spreading from + 3.9 to + 7.  相似文献   

20.
The West Amazon Craton consists of rocks of the Sunsás Orogen and the Rondônia-Juruena Province. The Sunsás Orogen comprises the western part of the Amazon Craton in South America and is best exposed in eastern Bolivia and western Rondônia and Mato Grosso states of Brazil. The integration of available maps and isotopic data together with new U–Pb and Sm–Nd analyses from 20 samples (plus 55 earlier dates), establish the timing of geologic events in the West Amazon Craton from 1840 to 1110 Ma. To unravel the complex geologic history of the study area, we primarily sampled granitoids and gneisses to develop a better stratigraphy and secondarily to narrow the age gaps between known discordances. Four periods of orogenic activity are identified within the Sunsás Orogen: 1465–1427 Ma (Santa Helena orogeny), 1371–1319 Ma (Candeias orogeny), ca. 1275 Ma (San Andrés orogeny), and 1180–1110 Ma (Nova Brasilândia orogeny). Notable is the absence of an Ottawan orogeny (1080–1020 Ma) equivalent. In the Rondônia-Juruena Province three main orogenies are recognized: the Juruena (1840–1780 Ma), the Jamari (1760–1740 Ma) and the Quatro Cachoeiras (1670–1630 Ma). Post-Sunsás rocks include Rondônia tin granites, Palmeiral sandstones, Nova Floresta basalt, and alkalic pipes.All inherited U–Pb ages of zircon and all exposed pre-Sunsás rocks in Bolivia have ages that correlate well to the neighbouring Rondônia-Juruena Province. This fact, together with the absence of fragments of older, Archean and Trans-Amazonian crust, suggests that the Sunsás Orogen is autochthonous and evolved over a continental margin formed dominantly by rocks of the Jamari (1760–1740 Ma) and Quatro Cachoeiras (1670–1630 Ma) orogenies plus rocks of the post-tectonic Serra Providência Suite (1560–1540 Ma). Almost all granulites known in Eastern Bolivia and in neighbouring area in Brazil are not basement rocks, but were formed during the Mesoproterozoic and are mainly associated with the Candeias orogeny (1371–1319 Ma). Dated samples of the Chiquitania and Lomas Manechi Complexes in Bolivia revealed a variety of ages and types of ages (metamorphic, magmatic, and inherited) indicating that those two units require more study. There is no evidence for the existence of a Paraguá Craton or Paraguá Block, which is almost totally composed of arc-related granites also formed during the Candeias orogeny.The main difference between the Sunsás Orogen and the Grenville Orogen of Laurentia is the absence in Amazonia of an Ottawan-equivalent orogeny (1080–1020 Ma). The existence of age-equivalents of the Candeias and Santa Helena orogenies in Laurentia (Pinwarian orogeny and rocks of the Eastern Granite-Rhyolite Province and the Composite Arc Belt) indicates that the connection of the two continents may have started from about 1450 Ma. In addition, the two belts may not have been directly juxtaposed, but instead, that one may have been the extension of the other during the Mesoproterozoic. The possibility that Amazonia joined the southwestern part of Laurentia also provides a good fit for the Hudson-Tapajós and Mazatzal-Yapavai-Rondônia-Juruena Provinces. This possible link to Laurentia may have started during the formation of the Trans-Hudson Orogen and its correlative Rondônia-Juruena and Tapajós provinces from about 1900 Ma.  相似文献   

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