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1.
40Ar/39Ar geochronology of muscovite and biotite grains genetically related to gold and Be–Ta–Li pegmatites from the Seridó Belt (Borborema province, NE Brazil) yield well-defined, reliable plateau ages. This information, combined with data about paragenetic and field relationships, reveals Cambro-Ordovician mineralization ages (520 and 500–506 Ma) for the orogenic gold deposits in the Seridó Belt. Biotite ages of 525±2 Ma, which represent the mean weighted results of the incremental heating analysis of six biotite single crystals, record the time of pegmatite emplacement and reactivation of Brasiliano/Pan-African strike-slip shear zones. These results, along with previous structural evolution studies, suggest that shear zones formed during the Brasiliano/Pan-African event were reactivated in the Upper Cambrian–Lower Ordovician. Mineralization occurs late in the history of the orogen.  相似文献   

2.
In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor Silurian to Lower Devonian zircons and very rare Archaean (2.5?Ga) and late Palaeoproterozoic (1.8-2.0?Ga) ones. No Siluro-Devonian source has yet been identified on the Kalahari Craton, but there are indications for such a source in southern Patagonia. Palaeozoic successions in eastern Argentina carry a similar detrital zircon population to that found here, including evidence of a Silurian to Lower Devonian magmatic event. The Kalahari and Río de la Plata Cratons were thus in all likelihood in close proximity until at least the Carboniferous.  相似文献   

3.
Summary  The Permo-Triassic Cape Fold Belt around the southern tip of Africa consists of a thick sequence of Palaezoic siliciclastic sedimentary and pre-Cape basement rocks believed to be of Pan-African age. Both the basement rocks and the supracrustal rocks of the Cape Supergroup display only low metamorphic grades. Application of chlorite, chlorite-chloritoid Fe-Mg exchange, and calcite-graphite carbon isotope geothermometry to rocks from the unconformable contact between pre-Cape basement and the Cape Supergroup made it possible to distinguish pre-Cape and syn-Cape metamorphic overprints. During Pan-African metamorphism temperatures of up to middle greenschist facies conditions (around 400 °C) were reached, whereas lowermost greenschist facies conditions (around 300 °C) were not exceeded during the 220–290 Ma Cape orogeny. In the past, most if not all of the pre-Cape basement rocks, which form the Pan-African Saldania Belt, were considered to be of Neoproterozoic age. A hiatus of about 100 °C observed between two adjacent limestone horizons that previously had been grouped together into a single formation at the bottom of the allegedly Neoproterozoic Kango Group indicates that almost all of this group is syn- to post-orogenic with respect to the Pan-African orogeny. A revision of the stratigraphy of the Kango Group is therefore suggested. Only its lowermost member is truly Pan-African and probably related to about 620–740 Ma post-Sturtian cap carbonates in other Pan-African belts of southern Africa. The remainder of the Kango Group reflects the successive development of two stages of orogen-related intra-continental basins: The older stage led to a typical syn-orogenic foreland basin related to tectonic loading in the Gariep and Damara orogenic belts further north(west) between 570 and 540 Ma; the younger is believed to have formed either a further foreland basin or an intra-orogen pull-apart basin caused by later tectonic loading in the Ross orogenic belt and its continuation into the southern Saldania Belt between 510 and 480 Ma. Received May 7, 2000;/revised version accepted January 15, 2001  相似文献   

4.
The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino?CSierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U?CPb ages of ca. 1,000?Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U?CPb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000?Ma inheritance formed at ca. 750?Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640?Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570?Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535?Ma of post-tectonic granitoids (Santa Teresa and José Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Río de La Plata Craton were in their present positions by ca. 535?Ma.  相似文献   

5.
Four slate samples from subduction complex rocks exposed on the south coast of New South Wales, south of Batemans Bay, were analysed by K–Ar and 40Ar/39Ar step‐heating methods. One sample contains relatively abundant detrital muscovite flakes that are locally oblique to the regional cleavage in the rock, whereas the remaining samples appear to contain sparse detrital muscovite. Separates of detrital muscovite yielded plateau ages of 505 ± 3 Ma and 513 ± 3 Ma indicating that inheritance has not been eliminated by metamorphism and recrystallisation. Step‐heating analyses of whole‐rock chips from all four slate samples produced discordant apparent age spectra with ‘saddle shapes’ following young apparent ages at the lowest temperature increments. Elevated apparent ages associated with the highest temperature steps are attributed to the presence of variable quantities of detrital muscovite (<1–5%). Two whole‐rock slate samples yielded similar 40Ar/39Ar integrated ages of ca 455 Ma, which are some 15–30 million years older than K–Ar ages for the same samples. These discrepancies suggest that the slates have also been affected by recoil loss/redistribution of 39Ar, leading to anomalously old 40Ar/39Ar ages. Two other samples, from slaty tectonic mélange and intensely cleaved slate, yielded average 40Ar/39Ar integrated ages of ca 424 Ma, which are closer to associated mean K–Ar ages of 423 ± 4 Ma and 409 ± 16 Ma, respectively. Taking into account the potential influences of recoil loss/redistribution of 39Ar and inheritance, the results from the latter samples suggest a maximum age of ca 440 Ma for deformation/metamorphism. The current results indicate that recoil and inheritance problems may also have affected whole‐rock 40Ar/39Ar data reported from other regions of the Lachlan Fold Belt. Therefore, until these effects are adequately quantified, models for the evolution of the Lachlan Fold Belt, that are based on such whole‐rock 40Ar/39Ar data, should be treated with caution.  相似文献   

6.
New age data on detrital zircons and micas are presented from key units within the Neoproterozoic Katanga Supergroup, which hosts the major stratiform Cu–Co deposits of the Central African Copperbelt. Detrital zircon ages indicate a mainly Palaeoproterozoic (between 2081 ± 28 and 1836 ± 26 Ma) provenance for the Katanga basin, derived from the Lufubu Metamorphic Complex of the Kafue Anticline and the Bangweulu Block to the north of the outcrop belt. Detrital zircons and clasts from the Grand Conglomerat glacial diamictite indicate a source from the Palaeoproterozoic metavolcanic porphyries and granitoids of Luina Dome region, which was a basement high during Nguba Group deposition. Minor zircons of Mesoproterozoic age may have been derived from the Kibaran belt. Finally, 40Ar/39Ar age data from detrital muscovites from Biano Group siltstones give a maximum age of sedimentation of 573 Ma, strongly supporting previous models that the Biano Group was deposited in a foreland basin of the Lufilian Orogen.  相似文献   

7.
The Anakie Metamorphic Group is a complexly deformed, dominantly metasedimentary succession in central Queensland. Metamorphic cooling is constrained to ca 500 Ma by previously published K–Ar ages. Detrital‐zircon SHRIMP U–Pb ages from three samples of greenschist facies quartz‐rich psammites (Bathampton Metamorphics), west of Clermont, are predominantly in the age range 1300–1000 Ma (65–75%). They show that a Grenville‐aged orogenic belt must have existed in northeastern Australia, which is consistent with the discovery of a potential Grenville source farther north. The youngest detrital zircons in these samples are ca 580 Ma, indicating that deposition may have been as old as latest Neoproterozoic. Two samples have been analysed from amphibolite facies pelitic schist from the western part of the inlier (Wynyard Metamorphics). One sample contains detrital monazite with two age components of ca 580–570 Ma and ca 540 Ma. The other sample only has detrital zircons with the youngest component between 510 Ma and 700 Ma (Pacific‐Gondwana component), which is consistent with a Middle Cambrian age for these rocks. These zircons were probably derived from igneous activity associated with rifting events along the Gondwanan passive margin. These constraints confirm correlation of the Anakie Metamorphic Group with latest Neoproterozoic ‐ Cambrian units in the Adelaide Fold Belt of South Australia and the Wonominta Block of western New South Wales.  相似文献   

8.
From the Archean to the end of the Neoproterozoic the Borborema Province, northeast Brazil went through a complex polycyclic geologic evolution, ending, between 660 and 570 Ma, with the Brasiliano/Pan-African orogeny that led to West Gondwana amalgamation. Evolution of the metasedimentary covers of the Province, from the beginning of their deposition up to their involvement in the Brasiliano/Pan-African collision, is a key element in understanding formation of Gondwana and in attempts in pre-drift correlation between South America and West Africa. One of these covers, the Ceará Group, is exposed in the Ceará Central domain. Aiming to unravel the history of the Ceará Group, we carried out a geochronologic study of representative samples, combining Sm–Nd isotopic data, conventional U–Pb TIMS dating of zircon and U–Pb SHRIMP age determination of detrital zircon grains. Our results show that sedimentation of the Ceará Group started around 750 Ma, following rifting of the Archean/Paleoproterozoic basement, associated with bimodal volcanism. The interlayered basic volcanic rocks, re-crystallized into garnet amphibolites, show a concordant age of 749 ± 5 Ma interpreted as the age of crystallization. About 90% of calculated Sm–Nd TDM model ages of metasedimentary rocks are Paleoproterozoic and more than 50% of the analyzed samples have TDM between 1.95 and 2.4 Ma, with strongly negative ɛNd, consistent with provenance mainly from the Paleoproterozoic basement. Strong contrast between Paleoproterozoic TDM with negative ɛNd and young TDM (Mesoproterozoic) with slightly positive ɛNd is interpreted as a consequence of changes in detritus provenance induced by geomorphologic alterations resulting from tectonic activity during rifting. Ages of detrital zircon grains obtained by SHRIMP U–Pb analyses show three main groups: about 1800 Ma, 1000–1100 Ma and ca. 800 Ma which corresponds to the bimodal magmatism associated, respectively to the Orós-Jaguaribe domain, Cariris Velhos event and Independência Group.  相似文献   

9.
Late Pennsylvanian sedimentary rocks in the Narragansett basin were metamorphosed (lower anchizone to sillimanite grade) during late Paleozoic regional metamorphism at ca. 275–280 Ma. Twenty-five variably sized concentrates of detrital muscovite were prepared from samples collected within contrasting low-grade areas (diagenesis — lower greenschist facies). Microprobe analyses suggest that the constituent detrital grains are not chemically internally zoned; however, some grains within several concentrates display very narrow (<25 m), compositionally distinct, low-grade, epitaxial peripheral overgrowths. Detrital muscovite concentrates from the lower anchizone are characterized by internally concordant 40Ar/39Ar age spectra which define plateau ages of ca. 350–360 Ma. These are interpreted to date post-Devonian (Acadian) cooling within proximal source areas. Concentrates from lower grade sectors of the middle anchizone display slightly discordant spectra in which apparent ages systematically increase from ca. 250–275 Ma to define intermediate- and high-temperature plateaus of ca. 360–400 Ma. Detrital muscovite within samples from higher grade sectors of the middle anchizone and the upper anchizone are characterized by systematic low age discordance throughout both low-and intermediate-temperature increments. High-temperature ages only range up to ca. 330 Ma. Six size fractions of detrital muscovite from a sample collected within the lower greenschist facies have similarly discordant spectra, in which, apparent ages increase slightly throughout the analyses from ca. 250 Ma to 275 Ma. The detrital muscovite results are interpreted to reflect variable affects of late Paleozoic regional metamorphism. However, it is uncertain to what extent the systematic low age spectra discordance reflects intracrystalline gradients in the concentration of 40Ar and/or experimental evolution of gas from relatively non-retentive epitaxial overgrowths. However, low age discordance occurs regardless of the extent of epitaxial overgrowth. Intermediate-temperature increments evolved during 40Ar/39Ar whole-rock analyses of five slate/phyllite samples are characterized by internally consistent apparent K/Ca ratios. These are attributed to gas evolved from constituent, very fine-grained white mica. Samples from lower grade portions of the middle anchizone are characterized by intermediate-temperature apparent ages which systematically increase from ca. 275–300 Ma to ca. 360–375 Ma before evolution of a high-temperature contribution from detrital plagioclase feldspar. This age variation may reflect partial late Paleozoic rejuvenation of very fine-grained detrital material with a source age similar to that for the detrital muscovites. Slate/phyllite samples from upper sectors of the middle anchizone and from the upper anchizone were completely rejuvenated during late Paleozoic metamorphism and record intermediate-and high-temperature plateau ages of ca. 270–290 Ma. These data document that metamorphic conditions of the lower to middle biotite zone (ca. 325–350 °C) are required to completely rejuvenate intracrystalline argon systems of detrital muscovite. Therefore, the 40Ar/39Ar dating method may be useful in determination of detrital muscovite provenance and in resolution of the metamorphic evolution of low-grade terranes.  相似文献   

10.
《Gondwana Research》2014,25(3-4):1051-1066
The Early Palaeozoic Ross–Delamerian orogenic belt is considered to have formed as an active margin facing the palaeo-Pacific Ocean with some island arc collisions, as in Tasmania (Australia) and Northern Victoria Land (Antarctica), followed by terminal deformation and cessation of active convergence. On the Cambrian eastern margin of Australia adjacent to the Delamerian Fold Belt, island arc and backarc basin crust was formed and is now preserved in the Lachlan Fold Belt and is consistent with a spatial link between the Delamerian and Lachlan orogens. The Delamerian–Lachlan connection is tested with new zircon data. Metamorphic zircons from a basic eclogite sample from the Franklin Metamorphic Complex in the Tyennan region of central Tasmania have rare earth element signatures showing that eclogite metamorphism occurred at ~ 510 Ma, consistent with island arc–passive margin collision during the Delamerian(− Tyennan) Orogeny. U–Pb ages of detrital zircons have been determined from two samples of Ordovician sandstones in the Lachlan Fold Belt at Melville Point (south coast of New South Wales) and the Howqua River (western Tabberabbera Zone of eastern Victoria). These rocks were chosen because they are the first major clastic influx at the base of the Ordovician ‘Bengal-fan’ scale turbidite pile. The samples show the same prominent peaks as previously found elsewhere (600–500 Ma Pacific-Gondwana and the 1300–1000 Ma Grenville–Gondwana signatures) reflecting supercontinent formation. We highlight the presence of ~ 500 Ma non-rounded, simple zircons indicating clastic input most likely from igneous rocks formed during the Delamerian and Ross Orogenies. We consider that the most probable source of the Ordovician turbidites was in East Antarctica adjacent to the Ross Orogen rather than reflecting long distance transport from the Transgondwanan Supermountain (i.e. East African Orogen). Together with other provenance indicators such as detrital mica ages, this is a confirmation of the Delamerian–Lachlan connection.  相似文献   

11.
40Ar/39Ar age data from the boundary between the Delamerian and Lachlan Fold Belts identify the Moornambool Metamorphic Complex as a Cambrian metamorphic belt in the western Stawell Zone of the Palaeozoic Tasmanide System of southeastern Australia. A reworked orogenic zone exists between the Lachlan and Delamerian Fold Belts that contains the eastern section of the Cambrian Delamerian Fold Belt and the western limit of orogenesis associated with the formation of an Ordovician to Silurian accretionary wedge (Lachlan Fold Belt). Delamerian thrusting is craton-verging and occurred at the same time as the final consolidation of Gondwana. 40Ar/39Ar age data indicate rapid cooling of the Moornambool Metamorphic Complex at about 500 Ma at a rate of 20 – 30°C per million years, temporally associated with calc-alkaline volcanism followed by clastic sedimentation. Extension in the overriding plate of a subduction zone is interpreted to have exhumed the metamorphic rocks within the Moornambool Metamorphic Complex. The Delamerian system varies from a high geothermal gradient with syntectonic plutonism in the west to lower geothermal gradients in the east (no syntectonic plutonism). This metamorphic zonation is consistent with a west-dipping subduction zone. Contrary to some previous models involving a reversal in subduction polarity, the Ross and Delamerian systems of Antarctica and Australia are inferred to reflect deformation processes associated with a Cambrian subduction zone that dipped towards the Gondwana supercontinent. Western Lachlan Fold Belt orogenesis occurred about 40 million years after the Delamerian Orogeny and deformed older, colder, and denser oceanic crust, with metamorphism indicative of a low geothermal gradient. This orogenesis closed a marginal ocean basin by west-directed underthrusting of oceanic crust that produced an accretionary wedge with west-dipping faults that verge away from the major craton. The western Lachlan Fold Belt was not associated with arc-related volcanism and plutonism occurred 40 – 60 million years after initial deformation. The revised orogenic boundaries have implications for the location of world-class 440 Ma orogenic gold deposits. The structural complexity of the 440 Ma Stawell gold deposit reflects its location in a reworked part of the Cambrian Delamerian Fold Belt, while the structurally simpler 440 Ma Bendigo deposit is hosted by younger Ordovician turbidites solely deformed by Lachlan orogenesis.  相似文献   

12.
K-Ar and 40Ar/39Ar geochronological studies were performed on selected minerals from the Mina III gold deposit, Crixás greenstone belt, state of Goiás, central Brazil. They include amphibole and biotite from amphibole schists; biotite from quartz-chlorite-carbonate-muscovite schists; chloritoid, muscovite, and paragonite from muscovite schists; and biotite from biotite-marbles. Analyses yielded ages between 750 and 500 Ma, indicating that the Brasiliano orogenic event affected rocks of the Archean greenstone belt. It is proposed that the gold mineralization, closely related to a post-metamorphic peak hydrothermal alteration and spatially associated with muscovite schists, is 505 ± 10 Ma, whereas older ages may indicate an excess of argon.  相似文献   

13.
<正>Thus far,our understanding of the emplacement of Xuebaoding granite and the occurrence and evolution of the Songpan-Garze Orogenic Belt has been complicated by differing age spectra results.Therefore,in this study,the ~(40)Ar/~(39)Ar and sensitive high resolution ion micro-probe(SHRIMP) U-Pb dating methods were both used and the results compared,particularly with respect to dating data for Pankou and Pukouling granites from Xuebaoding,to establish ages that are close to the real emplacements.The results of SHRIMP U-Pb dating for zircon showed a high amount of U,but a very low value for Th/U.The high U amount,coupled with characteristics of inclusions in zircons,indicates that Xuebaoding granites are not suitable for U-Pb dating.Therefore,muscovite in the same granite samples was selected for ~(40)Ar/~(39)Ar dating.The ~(40)Ar/~(39)Ar age spectrum obtained on bulk muscovite from Pukouling granite in the Xuebaoding,gave a plateau age of 200.1±1.2 Ma and an inverse isochron age of 200.6±1.2 Ma.The ~(40)Ar/~(39)Ar age spectrum obtained on bulk muscovite from Pankou granite in the Xuebaoding gave another plateau age of 193.4±1.1 Ma and an inverse isochron age of 193.7±1.1 Ma. The ~(40)Ar/~(36)Ar intercept of 277.0±23.4(2σ) was very close to the air ratio,indicating that no apparent excess argon contamination was present.These age dating spectra indicate that both granites were emplaced at 200.6±1.3 Ma and 193.7±1.1 Ma,respectively.Through comparison of both dating methods and their results,we can conclude that it is feasible that the muscovite in the granite bearing high U could be used for ~(40)Ar/~(39)Ar dating without extra Ar.Based on this evidence,as well as the geological characteristics of the Xuebaoding W-Sn-Be deposit and petrology of granites,it can be concluded that the material origin of the Xuebaoding W-Sn-Be deposit might partially originate from the Xuebaoding granite group emplacement at about 200 Ma.Moreover,compared with other granites and deposits distributed in various positions in the Songpan-Garze Orogenic Belt,the Xuebaoding emplacement ages further show that the main rare metal deposits and granites in peripheral regions occurred earlier than those in the inner Songpan-Garze.Therefore,~(40)Ar/~(39)Ar dating of Xuebaoding granite will lay a solid foundation for studying the occurrence and evolution of granite and rare earth element deposits in the Songpan-Garze Orogenic Belt.  相似文献   

14.
The Xinlu Sn‐polymetallic ore field is located in the western Nanling Polymetallic Belt in northeastern Guangxi, South China, where a number of typical skarn‐, hydrothermal vein‐type tin deposits have developed. There are two types of Sn deposits: skarn‐type and sulfide‐quartz vein‐type. The tin mineralizations mainly occur on the south side of the Guposhan granitic complex pluton and within its outer contact zone. To constrain the Sn mineralization age and further understand its genetic links to the Guposhan granitic complex, a series of geochronological works has been conducted at the Liuheao deposit of the ore field using high‐precision zircon SHRIMP U‐Pb, molybdenite Re‐Os, and muscovite Ar‐Ar dating methods. The results show that the biotite‐monzogranite, which is part of the Xinlu intrusive unit of the Guposhan complex pluton, has a SHRIMP U‐Pb zircon age of 161.0 ± 1.5 Ma. The skarn‐type ore has a 40Ar‐39Ar muscovite plateau age of 160 ± 2 Ma (same as its isochron age), and the sulfide‐quartz vein‐type ore yields an Re‐Os molybdenite isochron age of 154.4 ± 3.5 Ma. The magmatic‐hydrothermal geochronological sequence demonstrated that the hydrothermal mineralization took place immediately following the emplacement of the monzogranite, with the skarn metasomatic mineralization stage predating the sulfide mineralization stage. Geochronologically, we have compared this ore field with 26 typical Sn deposits distributed along the Nanling Polymetallic Belt, leading to the suggestion of the magmatic‐metallogenic processes in the Xinlu ore field (ca. 161–154 Ma) as a component of the Early Yanshanian large‐scale Sn‐polymetallic mineralization event (peaked at 160–150 Ma) in the Nanling Range of South China. Petrogenesis of Sn‐producing granite and Sn‐polymetallic mineralization were probably caused by crust–mantle interaction as a result of significant lithospheric extension and thinning in South China in the Late Jurassic.  相似文献   

15.
The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, São Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the São Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasília Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS U–Pb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasília Belt are discussed.The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the São Francisco Craton. On the other hand, samples from the Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed in the easternmost part of the belt and covering large areas of the São Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt.  相似文献   

16.
Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7?Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3?Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6?±?1.1?Ma, 310.5±1.2?Ma, and 303.3±0.2?Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300?Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360?Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences.  相似文献   

17.
LA-ICP-MS U-Pb analyses performed on zircon grains from the Lizio granite yielded an emplacement age of 316 ± 6 Ma. Typical S-C structures show that the Lizio granite was emplaced contemporaneously with dextral shearing along the northern branch of the South Armorican Shear Zone and that it was therefore active at that time. 40Ar/39Ar analyses performed on muscovite grains yielded plateau dates ranging between 311.5 and 308.2 Ma. Muscovite chemistry is typical of primary magmatic muscovite, which precludes a late fluids-induced resetting of the K-Ar isotopic system. 40Ar/39Ar dates thus likely correspond to the cooling ages below the argon closure temperature. Considering the uncertainties on the measured ages, we can propose that either the Lizio granite cooled down quickly in less than a million of years or that it remained in a hot environment for several millions of years after its emplacement. This latter scenario could have been sustained by shear heating during dextral shearing along the northern branch of the South Armorican Shear Zone.  相似文献   

18.
The Grove Mountains are the inland exposures of the Prydz Belt in East Antarctica. Although the 550–500 Ma orogenic event was recognized as the latest major magmatic–metamorphic activity in the Prydz Belt, its subduction–collision origin was not confirmed until the discovery of high-pressure (HP) mafic granulite erratic boulders in the glacial moraines from the Grove Mountains. Because no HP metamorphic bedrock is exposed in this area, an understanding the regional geology required a thorough study of the morainal debris mineralogy and detrital zircon U–Pb chronology. Detrital zircon U–Pb age histograms show 550–450 Ma, 900–800 Ma, and 1100–1000 Ma modes from three morainal deposits and one paleosol samples. The oldest ages were 2300 to 2420 Ma. Detailed electron probe microanalyses (EPMA) for the detrital mineral grains were compared with the minerals from the nearby exposed bedrock. The mineral chemistry indicates that the exposed bedrock in the Grove Mountains was not the sole source for morainal materials. This new U–Pb zircon geochronology and microprobe mineral data support the previous interpretation that the 550–500 Ma tectonic activity was the final collisional event that formed the Prydz Belt and amalgamated East Antarctica.  相似文献   

19.
Provenance studies have been performed utilising major and trace elements, Nd systematics, whole rock Pb–Pb isotopes and zircon U/Pb SHRIMP data on metasedimentary rocks of the Sierra de San Luis (Nogolí Metamorphic Complex, Pringles Metamorphic Complex, Conlara Metamorphic Complex and San Luis Formation) and the Puncoviscana Formation of the Cordillera Oriental. The goal was the characterisation of the different domains in the study area and to give insights to the location of the source rocks. An active continental margin setting with typical composition of the upper continental crust is depicted for all the complexes using major and trace elements. The Pringles Metamorphic Complex shows indications for crustal recycling, pointing to a bimodal provenance. Major volcanic input has to be rejected due to Th/Sc, Y/Ni and Cr/V ratios for all units. The εNd(540 Ma) data is lower for the San Luis Formation and higher for the Conlara Metamorphic Complex, as compared to the other units, in which a good consistency is given. This is similar to the TDM ages, where the metapsammitic samples of the San Luis Formation are slightly older. The spread of data is largest for the Pringles Metamorphic Complex, again implying two different sources. The whole rock 207Pb/206Pb isotopic data lies in between the South American and African sources, excluding Laurentian provenances. The whole rock Pb–Pb data is almost indistinguishable in the different investigated domains. Only the PMC shows slightly elevated 208Pb/204Pb values. Possible source rocks for the different domains could be the Quebrada Choja in the Central Arequipa–Antofalla domain, the Southern domain of the Arequipa–Antofalla basement, the Brazilian shield or southern Africa. Zircon SHRIMP data point to a connection between the Puncoviscana Formation and the Conlara Metamorphic Complex. Two maxima around 600 Ma and around 1000 Ma have been determined. The Nogolí Metamorphic Complex and the Pringles Metamorphic Complex show one peak of detrital zircons around 550 Ma, and only a few grains are older than 700 Ma. The detrital zircon ages for the San Luis Formation show age ranges between 590 and 550 Ma. A common basin can be assumed for the Conlara Metamorphic Complex and the Puncoviscana Formation, but the available data support different sources for the rest of the Complexes of the Sierra de San Luis. These share the diminished importance or the lack of the Grenvillian detrital peak, a common feature for the late Cambrian–early Ordovician basins of the Eastern Sierras Pampeanas, in contrast to the Sierras de Córdoba, the PVF and the Conlara Metamorphic Complex.  相似文献   

20.
Sung Won Kim   《Gondwana Research》2005,8(3):385-402
An understanding of the Okcheon Metamorphic Belt (OMB) in South Korea is central to unraveling the tectono-metamorphic evolution of East Asia. Amphibole-bearing rocks in the OMB occur as calcsilicate layers and lenses in psammitic rocks, in the psammitic rocks themselves, and in the mafic volcanic layers and intrusives. Most amphiboles fail to show 40Ar/39Ar plateau ages; those that do have ages ranging from 132 to 975 Ma. The disturbed age pattern and wide variation in 40Ar/39Ar ages can be related to metamorphic grade, retrograde chemical reactions, excess Ar and amphibole composition. The oldest age (975 Ma) can be interpreted either as an old igneous or metamorphic age predating sedimentation or a false age caused by excess Ar. The youngest age of 132 Ma and the disturbed age pattern found in amphiboles from rocks located close to Jurassic granitoids are the result of retrograde thermal metamorphic effects accompanying intrusion of the granitoids. Some medium- or coarse-grained amphiboles in the calcsilicates are aggregates of fine-grained crystals. As a result, they are heterogeneous and prove to be readily affected by excess Ar. A disturbed age pattern in amphiboles from the calcsilicates occurring in the high-grade metamorphic zone may also be the product of excess Ar. On the other hand, the disturbed pattern of amphiboles present in the calcsilicates from the low-grade metamorphic zone could arise from both excess Ar and mixed ages. However, amphiboles from psammitic rocks and some calcsilicates in the high-grade metamorphic zone and in intrusive metabasites display real plateau ages of 237 to 261 Ma. The temperature conditions in the high-grade metamorphic zone were higher than the argon closing temperature for amphibole, and the amphiboles in this zone give plateau ages only when they are homogeneous in composition, lack excess Ar, and have not been thermally affected by intrusion of the granitoids. The unmodified 40Ar/39Ar ages prove rather younger than the age of the Late Paleozoic metamorphic event of 280 to 300 Ma, but they are close to muscovite K-Ar ages of 263 to 277 Ma. These 40Ar/39Ar amphibole ages are interpreted as the time of cooling that followed the main regional, intermediate-P/T metamorphic climax. The results demonstrate that interpretation of 40Ar/39Ar amphibole ages in an area subjected to several metamorphic events can be accomplished only by undertaking a thorough tectono-metamorphic study, accompanied by detailed chemical analysis of the amphiboles.  相似文献   

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