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1.
In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U–Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma (Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567±5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance of the Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gföhl gneiss was recognised as a 488±6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456±3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440–450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian meta-granitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65–1.8 Ga, indicating a derivation from the South American part of Gondwana.  相似文献   

2.
Northwestern Fujian Province is one of the most important Pre-Palaeozoic areas in the Cathaysia Block of South China. Metavolcano-sedimentary and metasedimentary rocks of different types, ages and metamorphic grades (granulite to upper greenschist facies) are present, and previously were divided into several Formations and Groups. Tectonic contacts occur between some units, whereas (deformed) unconformities have been reported between others. New SHRIMP U–Pb zircon ages presented here indicate that the original lithostratigraphy and the old “Group” and “Formation” terminology should be abandoned. Thus the “Tianjingping Formation” was not formed in the Archaean or Palaeoproterozoic, as previously considered, but must be younger than its youngest detrital zircons (1790 Ma) but older than regional metamorphism (460 Ma). Besides magmatic zircon ages of 807 Ma obtained from metavolcano-sedimentary rocks of the “Nanshan Formation” and 751–728 Ma for the “Mamianshan Group”, many inherited and detrital zircons with ages ranging from 1.0 to 0.8 Ga were also found in them. These ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break-up of the Rodinia supercontinent. The new zircon results poorly constrain the age of the “Mayuan Group” as Neoproterozoic to early Palaeozoic (728–458 Ma), and not Palaeoproterozoic as previously thought. Many older inherited and detrital zircons with ages of 3.6, 2.8, 2.7, 2.6–2.5, 2.0–1.8 and 1.6 Ga were found in this study. A 3.6 Ga detrital grain is the oldest one so far identified in northwestern Fujian Province as well as throughout the Cathaysia Block. Nd isotope tDM values of eight volcano-sedimentary and clastic sedimentary rock samples centre on 2.73–1.68 Ga, being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation. These rocks underwent high grade metamorphism in the early Palaeozoic (458–425 Ma) during an important tectono-thermal event in the Cathaysia Block.  相似文献   

3.
This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua ‘Group’ gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84 ± 0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua ‘Group’, gives a magmatic zircon age of 2.14 ± 0.02 Ga and a metamorphic zircon age of 1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua ‘Group’ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.  相似文献   

4.
The Aguablanca Cu–Ni orthomagmatic ore deposit is hosted by mafic and ultramafic rocks of the Aguablanca stock, which is part of the larger, high-K calc-alkaline Santa Olalla plutonic complex. This intrusive complex, ca. 338 Ma in age, is located in the Ossa-Morena Zone (OMZ) of the Iberian Variscan Belt. Mineralization consists mainly of pyrrhotite, pentlandite and chalcopyrite resulting from the crystallization of an immiscible sulphide-rich liquid. Isotope work on the host igneous rocks (Sr, Nd) and the ore (S) suggests that contamination with an upper-crustal component took place at some depth before final emplacement of the plutons (Nd338=−6 to −7.5; Sr(338)=0.7082 to 0.7100; δ34S(sulphides) near +7.4‰). Assimilation–fractional crystallization (AFC) processes are invoked to explain early cumulates and immiscible sulphide-magma formation. Intrusion took place at the beginning of the type-A oblique subduction of the South Portuguese Zone under the Ossa-Morena Zone and was probably driven by transpressive structures (strike-slip faults). The mineralization is thus synorogenic.Aguablanca is probably the first case referred to in the literature of a magmatic Cu–Ni ore deposit hosted by calc-alkaline igneous rocks.  相似文献   

5.
The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains 242 Ma metamorphic zircons, as well as 440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have tDM model ages of 2.17–1.91 Ga and εNd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record tDM model ages of 2.13–1.42 Ga and εNd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.  相似文献   

6.
A late-Variscan rhyodacite is exposed at the contact between the Ossa Morena Zone and the Central Iberian Zone of the Iberian Massif, Central Portugal. Dykes of rhyodacite intruded the Série Negra Unit and the Sardoal Complex that are part of the Cadomian basement. The igneous crystallization age of the rhyodacite (308 ± 1 Ma) was obtained on igneous monazite by the ID-TIMS U-Pb method. It is broadly coeval with the emplacement of late-Variscan granitoids during the last deformation phase of the Variscan Orogeny (ca. 304–314 Ma) and with the development of the large late-Variscan strike-slip shear zones (ca. 307 Ma). The rhyodacite samples are calc-alkaline, show identical composition and belong to the same magmatic sequence. The rhyodacite isotopic signatures (Sm-Nd and δ18O) are consistent with depleted-mantle juvenile sources and the contribution of the meta-igneous lower crust. The input of mantle juvenile sources is related to Variscan reactivation of lithospheric fractures. The inherited Neoproterozoic (ca. 619 Ma) and Mesoproterozoic (ca. 1054 Ma) zircon ages, are similar to those of the Central Iberian Zone. This suggests that lower crust of the Central Iberian Zone was involved in the magma generation of the rhyodacite. Coeval late-Variscan magmatic rocks display a larger contribution from ancient crustal components, which may be attributed to the smaller volume and faster cooling rate of the rhyodacite and consequent lower melting of the crust. Mixing of juvenile mantle-derived melts with melts from the lower continental crust was followed by fractional crystallization of garnet and amphibole that remained in the source. Fractional crystallization of plagioclase, biotite, quartz and zircon occurred in shallower magma chambers. Fractional crystallization of zircon was not significant.  相似文献   

7.
In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pbtot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.  相似文献   

8.
In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS 207Pb–206Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.  相似文献   

9.
The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two well-known orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3 ± 1.5 and 187.2 ± 1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic. Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages.The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergence developed in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks, but an “intracontinental” thrust that developed after their juxtaposition.  相似文献   

10.
The Jinshajiang Suture Zone is important for enhancing our understanding of the evolution of the Paleo-Tethys and its age, tectonic setting and relationship to the Ailaoshan Suture Zone have long been controversial. Based on integrated tectonic, biostratigraphic, chemostratigraphic and isotope geochronological studies, four tectono-stratigraphic units can be recognized in the Jinshajiang Suture Zone: the Eaqing Complex, the Jinshajiang Ophiolitic Melange, the Gajinxueshan “Group” and the Zhongxinrong “Group”. Isotope geochronology indicates that the redefined Eaqing Complex, composed of high-grade-metamorphic rocks, might represent the metamorphic basement of the Jinshajiang area or a remnant micro-continental fragment. Eaqing Complex protolith rocks are pre-Devonian and probably of Early–Middle Proterozoic age and are correlated with those of the Ailaoshan Complex. Two zircon U–Pb ages of 340±3 and 294±3 Ma, separately dated from the Shusong and Xuitui plagiogranites within the ophiolitic assemblage, indicate that the Jinshajiang oceanic lithosphere formed in latest Devonian to earliest Carboniferous times. The oceanic lithosphere was formed in association with the opening and spreading of the Jinshajiang oceanic basin, and was contiguous and equivalent to the Ailaoshan oceanic lithosphere preserved in the Shuanggou Ophiolitic Melange in the Ailaoshan Suture Zone; the latter yielded a U–Pb age of 362±41 Ma from plagiogranite. The re-defined Gajinxueshan and Zhongxinrong “groups” are dated as Carboniferous to Permian, and latest Permian to Middle Triassic respectively, on the basis of fossils and U–Pb dating of basic volcanic interbeds. The Gajinxueshan “Group” formed in bathyal slope to neritic shelf environments, and the Zhongxinrong “Group” as bathyal to abyssal turbidites in the Jinshajiang–Ailaoshan back-arc basin. Latest Permian–earliest Middle Triassic synorogenic granitoids, with ages of 238±18 and 227±5–255±8 Ma, respectively, and an Upper Triassic overlap molasse sequence, indicate a Middle Triassic age for the Jinshajiang–Ailaoshan Suture, formed by collision of the Changdu-Simao Block with South China.  相似文献   

11.
This paper aims to discuss the structural evolution of the Iberian Pyrite Belt during the Variscan Orogeny. It provides new structural data, maps and cross sections from the eastern part of the Iberian Pyrite Belt. Regional geology of the South Portuguese Zone and lithostratigraphy of the Iberian Pyrite Belt are first briefly summarised. Three roughly homoaxial deformation phases are distinguished, and are mainly characterised by south-verging multi-order folds, axial planar cleavages and thrusts. Three structural units are distinguished: the La Puebla de Guzmán and Valverde del Camino antiforms are rooted units related to the propagation of southward-directed thrust systems that may branch onto the lower décollement level of the South Portuguese Zone; El Cerro de Andévalo is a structurally higher unit, mainly composed of allochthonous D1 thrust nappes. No evidence of sinistral transpression has been found in the transected cleavage and the strike of S3 with respect to S2. Better evidence of transpression is the moderately to steeply westerly plunging folds that show S-type asymmetry in down-plunge view. Variscan deformation in the Iberian Pyrite Belt is defined as the combination of a dominant southwards shear and a sinistral E-shear caused by oblique continental collision between the South Portuguese plate and the Iberian Massif.  相似文献   

12.
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   

13.
Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogenesis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran-Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.  相似文献   

14.
Conventional multi-grain and ion-microprobe dating of detrital zircons from a very low grade metagraywacke of the Tentudía Group (upper part of the Serie Negra, Ossa-Morena Zone, SW Spain) reveals an uppermost Vendian age for the deposition of the metagraywacke. The youngest detrital zircon grain provides a maximum depositional age of about 565 Ma. Thus, these data contradict earlier Middle to Upper Riphean (ca. 1350-850 Ma) estimates on the age of the Tentudía Group and favour a Precambrian/Cambrian boundary falling into the range of 540 to 530 Ma. The presence of about 20% of Pan-African detrital zircons ranging from about 700 to 550 Ma indicates the derivation from Gondwana. From the upper intercept ages of the fan-shaped data field defined by conventionally determined zircon fractions, it can be deduced that 2.1 Ga old zircons as well as Archean zircons existed in the provenance(s) of the Serie Negra sediments. This mixing of crustal components of different ages is in line with the Nd crustal residence age of 1.9 Ga. The latter value, as well as other model ages of the Iberian Massif, indicates unusually high amounts of ancient crust to be present in the strata. This is different to other (meta)sediments of the European Hercynides and suggests that the Iberian strata of uppermost Precambrian age may contain the detritus of more internal, older parts of Gondwana than other European strata of comparable ages. Geochemical data on the analysed sample and further metagraywackes of the Tentudía Group argue for a deposition in an arc environment. Such a scenario would conform with the syn- to post-orogenic shallow marine deposition of the studied sediments. Furthermore, an upper time limit for the pre-Lower Cambrian deformational history, including two phases of regional deformation, is given by the maximum age of deposition, implying a very short time interval for deposition and deformation of the Tentudía Group. Concerning the U-Pb systematics of detrital zircon fractions, it is probable that numerous, previously published conventional multi-grain zircon data on (meta)sedimentary rocks of the European Hercynides readily can be explained by the presence of up to 20% of Pan-African detrital zircons and later Phanerozoic lead loss during metamorphic transformation of the sedimentary protoliths. Moreover, this implies that such metasediments originated from post-Pan-African sedimentary precursors.  相似文献   

15.
洪涛  游军  吴楚  徐兴旺 《岩石学报》2015,31(9):2583-2596
扬子板块西缘滇西地区是否存在古老基底一直存在争议。本文对滇西桃花地区花岗斑岩进行了岩石学、地球化学和锆石SHRIMP U-Pb年代学研究。形成于晚造山-后碰撞背景的桃花花岗斑岩具岛弧花岗岩地球化学特征,其成因可能与:1)俯冲拆离的洋壳俯冲拆离的洋壳或富集地幔重熔作用;2)加厚的地壳部分熔融。花岗斑岩中的继承锆石有两种类型:一类是发育具有密集振荡环带的岩浆锆石;另一类是次浑圆状锆石。测年结果显示,花岗斑岩的岩浆锆石年龄为36.35±0.35Ma,环带发育的继承锆石年龄介于167~891Ma之间;而次浑圆状继承锆石可以分为两组,其207Pb/206Pb加权平均年龄分别为1851±22Ma与2499±32Ma。新的锆石测年结果表明着滇西桃花地区不仅存在古金沙江洋东向俯冲形成的晚古生代弧岩浆记录,还发现新元古代岩浆活动信息,及早古元古代和新太古代的锆石记录。推测1.8Ga与2.5Ga锆石可能是捕获自地壳或围岩(石鼓片岩),表明滇西地区可能存在古老基底。  相似文献   

16.
An arcuate structure, comparable in size with the Ibero-Armorican arc, is delineated by Variscan folds and magnetic anomalies in the Central Iberian Zone of the Iberian Massif. Called the Central Iberian arc, its sense of curvature is opposite to that of the Ibero-Armorican arc, and its core is occupied by the Galicia-Trás-os-Montes Zone of NW Iberia, which includes the Rheic suture. Other zones of the Iberian Massif are bent by the arc, but the Ossa-Morena and South Portuguese zones are not involved. The arc formed during the Late Carboniferous, at final stages of thermal relaxation and collapse, and an origin related with right-lateral ductile transpression at the scale of the Variscan belt is proposed. The Central Iberian arc explains the width of the Central Iberian Zone, clarifies the position of the allochthonous terranes of NW Iberia, and opens new perspectives for correlations with the rest of the Variscan belt, in particular, with the Armorican Massif, whose central zone represents the continuation of the southwest branch of the arc detached by strike-slip tectonics.  相似文献   

17.
《Gondwana Research》2013,23(3-4):855-865
The ages of detrital zircon grains from one paragneiss and inherited zircon cores from two augen gneisses from the amphibolite facies basement of the Peloritani Mountains (southern Italy) measured by SHRIMP U–Pb constrain the previously unknown deposition age of the original sediments and help to elaborate a model for their provenance and subsequent evolution. The deposition age is latest Neoproterozoic to Cambrian (~ 545 Ma), bracketed by the combined ages of the youngest detrital/inherited zircon populations and of zircon from virtually coeval granitoids that intrude the metasediments. This is consistent with the subgreenschist facies Palaeozoic volcano–sedimentary sequences exposed in the southern Peloritani Mountains being the original cover rocks of the northern Peloritani late Neoproterozoic to early Cambrian basement. The age spectra of the detrital/inherited zircon grains show that the Neoproterozoic/Cambrian sediments were derived from the erosion of sources dominated by Neoproterozoic rocks with ages in the range of 0.85–0.54 Ga, with other main components aged 1.1–0.9 and ~ 2.7–2.4 Ga, and a minor one aged ~ 1.6 Ga, as typically found in peri-Gondwanan terranes. The presence of a large amount of Grenvillian-aged zircon contradicts previous models that propose a West African affinity for the Calabria–Peloritani Terrane, and the absence of 2.2–1.9 Ga Trans Amazonian/Tapajós–Parima/Eburnean zircon rules out an Amazonian provenance. The age spectra are more consistent with the basement sediments having an East African origin, similar to that of the early Palaeozoic sandstones in southern Israel and Jordan, part of a “provenance regionality” shared with other terranes currently located in the eastern Mediterranean area.  相似文献   

18.
《International Geology Review》2012,54(12):1446-1461
ABSTRACT

Meta-pelitic rocks with interlayers of meta-psammites within the inner thermal aureole of the Alvand plutonic complex (Sanandaj-Sirjan Zone (SaSZ), western Iran) underwent partial melting; generating various types of migmatites. The mesosome of the Hamedan migmatites is classified into two groups: (1) cordierite-rich and Al-silicate-poor mesosomes and (2) cordierite-poor, Al-silicate-rich groups. Leucosomes are also variable, ranging from plagioclase-rich to K-feldspar-rich leucosomes. Mineral-chemical studies and thermobarometric estimations indicate temperature and pressure of 640–700°C and 3–5 kbar, respectively, for the formation of mesosomes. U–Pb zircon geochronology on 214 grains from the mesosome of migmatites indicates ages of 160–180 Ma (ca ~170 Ma) for zircon metamorphic rims and variable ages of 190–2590 Ma for the inherited detrital zircon cores. Inherited core ages show various age populations, but age populations at 200–600 Ma are more frequent. The age populations of the detrital zircons clarify that the provenance of the younger zircon grains (200–500 Ma) was more likely the Iranian plate, whereas the older grains (600 Ma to >2.5 Ga) may be sourced from both northern Gondwana (such as Arabian-Nubian Shield) and the neighbouring, old cratons like as Africa. We suggest that magmatic activities, especially mafic plutonism at ~167 Ma, are the main triggers for the heat source of metamorphism, partial melting, and migmatization. In contrast to a presumed idea for a Cretaceous regional metamorphic event in the NW parts of the SaSZ, this study attests that the metamorphism should be older and can be associated with Jurassic magmatic pulses.  相似文献   

19.
The Late Precambrian–Early Paleozoic metamorphic basement forms a volumetrically important part of the Andean crust. We investigated its evolution in order to subdivide the area between 18 and 26°S into crustal domains by means of petrological and age data (Sm–Nd isochrons, K–Ar). The metamorphic crystallization ages and tDM ages are not consistent with growth of the Pacific margin north of the Argentine Precordillera by accretion of exotic terranes, but favor a model of a mobile belt of the Pampean Cycle. Peak metamorphic conditions in all scattered outcrop areas between 18 and 26°S are similar and reached the upper amphibolite facies conditions indicated by mineral paragensis and the occurrence of migmatite. Sm–Nd mineral isochrons yielded 525±10, 505±6 and 509±1 Ma for the Chilean Coast Range, the Chilean Precordillera and the Argentine Puna, and 442±9 and 412±18 Ma for the Sierras Pampeanas. Conventional K–Ar cooling age data of amphibole and mica cluster around 400 Ma, but are frequently reset by Late Paleozoic and Jurassic magmatism. Final exhumation of the Early Paleozoic orogen is confirmed by Devonian erosional unconformities. Sm–Nd depleted mantle model ages of felsic rocks from the metamorphic basement range from 1.4 to 2.2 Ga, in northern Chile the average is 1.65±0.16 Ga (1σ; n=12), average tDM of both gneiss and metabasite in NW Argentina is 1.76±0.4 Ga (1σ; n=22), and the isotopic composition excludes major addition of juvenile mantle derived material during the Early Paleozoic metamorphic and magmatic cycle. These new data indicate a largely similar development of the metamorphic basement south of the Arequipa Massif at 18°S and north of the Argentine Precordillera at 28°S. Variations of metamorphic grade and of ages of peak metamorphism are of local importance. The protolith was derived from Early to Middle Proterozoic cratonic areas, similar to the Proterozoic rocks from the Arequipa Massif, which had undergone Grenvillian metamorphism at ca. 1.0 Ga.  相似文献   

20.
The Precambrian basement of northern Wuyishan (southern Zhejiang Province, eastern Cathaysia Block, South China), consists mainly of Paleoproterozoic granites and metamorphic rocks of the Badu Complex, which are the oldest rocks found in the Cathaysia Block. LA-ICPMS zircon U–Pb ages for a gneiss and five gneissic granites from the Tianhou, Danzhu, Xiaji and Lizhuang plutons indicate that magmatism and metamorphism took place between 1888 and 1855 Ma. The Xiaji (1888 ± 7 Ma) and Lizhuang (1875 ± 9 Ma) granites have high SiO2, K2O and Rb contents, high A/CNK (1.09–1.40) and Rb/Sr, and low contents of Sr, REE and mafic components (Mg, Fe, Ti, Mn and other transition metals). They have the geochemical signature of S-type granites, and a sedimentary protolith is confirmed by the presence of abundant inherited zircons with a range of ages and Hf-isotope compositions. The Tianhou and Danzhu granites are metaluminous to weakly peraluminous (A/CNK = 0.80–1.07), and have low SiO2 contents, high Ga/Al and FeO/(FeO + MgO) ratios, and Zn and HFSE concentrations typical of A-type granites. They also record high crystallization temperatures (885–920 °C), consistent with A-type granites. High Y/Nb ratios (>1.4) indicate that they belong to the A2 subgroup, suggesting that they probably formed in a post-orogenic tectonic setting. Their ages range from 1867 to 1855 Ma, slightly later than the syn-collisional Lizhuang and Xiaji S-type granites. These granitic rocks and the metamorphic rocks of the Badu Complex define a late Paleoproterozoic orogenic cycle in the area. All the 1.86–1.90 Ga zircons, whether derived from S- or A-type granites, show similar Hf-isotopic compositions, with Hf model ages clustering at 2.8 Ga. These model ages, and inherited zircons (ca. 2.5–2.7 Ga) found in some rocks, indicate that the late Paleoproterozoic magmatism and tectonism of the eastern Cathaysia Block represent an overprint on an Archaean basement. This Paleoproterozoic orogeny in the Wuyishan terrane coincides with the assembly of the supercontinent Columbia, suggesting that the Wuyishan terrane was the part of this supercontinent.Zircon ages also record an early Mesozoic (Triassic) tectonothermal overprint that was very intensive in the northern Wuyishan area, leading to high-grade metamorphism of Paleoproterozoic basement, Pb loss from Paleoproterozoic zircons and overgrowth of new zircon. The central and southern parts of Wuyishan and the Chencai area (northern Zhejiang Province) also experienced strong reworking in Neoproterozoic and early Paleozoic times. The Wuyishan terrane (especially in the north) represents a long-lived remnant of the old craton, which has survived for at least one billion years. The compositions of the basement rocks, the Paleoproterozoic orogeny and the Triassic tectonothermal imprint in the Wuyishan terrane are similar to those recognized in the Yeongnam massif of South Korea, suggesting that the two terranes may have been connected from Paleoproterozoic to Triassic time.  相似文献   

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