首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 78 毫秒
1.
Ediacaran and Early Cambrian sedimentary rocks from NW Iberia have been investigated for detrital zircon U–Pb ages. A total of 1,161 concordant U–Pb ages were obtained in zircons separated from four Ediacaran samples (3 from the Cantabrian Zone and one from the Central Iberian zone) and two Lower Cambrian samples (one from the Cantabrian Zone and one from the Central Iberian Zone). Major and trace elements including REE and Sm–Nd isotopes were also analyzed on the same set of samples. The stratigraphically older Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 600 Ma based on detrital zircon content and is intruded by ca. 590–580 Ma granitoids constraining the deposition of this part of the sequence between ca. 600 and 580 Ma. The stratigraphically younger Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 553 Ma. The Ediacaran sample from the Central Iberian Zone has an identical within error maximum sedimentation age of ca. 555 Ma. The detrital zircon U–Pb age patterns are very similar in all the Ediacaran samples from both zones including the main age groups ca. 0.55–0.75 Ga, ca. 0.85–1.15 Ga and minor Paleoproterozoic (ca. 1.9–2.1 Ga) and Archean (ca. 2.4–2.6 Ga) populations. Kolmogorov–Smirnov statistical tests performed on this set of samples indicate that they all were derived from the same parent population (i.e., same source area). The same can be said on the basis of Nd isotopes, REE patterns and trace element concentrations. The two Cambrian samples, however, show contrasting signatures: The sample from the Cantabrian Zone lacks the ca. 0.85–1.15 Ga population and has a high proportion of Paleoproterozoic and Archean zircons (>60 %) and a more negative ε Nd and higher T DM values than the Ediacaran samples. The Early Cambrian sample from the Central Iberian Zone has the same U–Pb detrital zircon age distribution (based on KS tests) as all the Ediacaran samples but has a significantly more negative ε Nd value. These data suggest apparently continuous sedimentation in the NW Iberian realm of northern Gondwana between ca. 600 and 550 Ma and changes in the detrital influx around the Ediacaran–Cambrian boundary. The nature and origin of these changes cannot be determined with available data, but they must involve tectonic activity on the margin as evidenced by the angular unconformity separating the Ediacaran and Lower Cambrian strata in the Cantabrian Zone. The absence of this unconformity and the apparent continuity of detrital zircon age distribution between Ediacaran and Cambrian rocks in the Central Iberian Zone suggest that the margin became segmented with significant transport and sedimentation flux changes in relatively short distances. As to the paleoposition of NW Iberia in Ediacaran–Early Cambrian times, comparison of the data presented herein with a wealth of relevant data from the literature both on the European peri-Gondwanan terranes and on the terranes of northern Africa suggests that NW Iberia may have lain closer to the present-day Egypt–Israel–Jordan area and that the potential source of the hitherto enigmatic Tonian–Stenian zircons could be traced to exposed segments of arc terranes such as that described in the Sinai Peninsula (Be’eri-Shlevin et al. in Geology 40:403–406, 2012).  相似文献   

2.
《Precambrian Research》2006,144(3-4):297-315
Geochemical data from clastic rocks of the Ossa-Morena Zone (Iberian Massif) show that the main source for the Ediacaran and the Early Cambrian sediments was a recycled Cadomian magmatic arc along the northern Gondwana margin. The geodynamic scenario for this segment of the Avalonian-Cadomian active margin is considered in terms of three main stages: (1) The 570–540 Ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust, a continental magmatic arc and the development of related marginal basins; (2) the Ediacaran–Early Cambrian transition (540–520 Ma) coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc; and (3) Gondwana fragmentation with the formation of Early Cambrian (520–510 Ma) shallow-water platforms in transtensional grabens accompanied by rift-related magmatism. These processes are comparable to similar Cadomian successions in other regions of Gondwanan Europe and Northwest Africa. Ediacaran and Early Cambrian basins preserved in the Ossa-Morena Zone (Portugal and Spain), the North Armorican Cadomian Belt (France), the Saxo-Thuringian Zone (Germany), the Western Meseta and the Western High-Atlas (Morocco) share a similar geotectonic evolution, probably situated in the same paleogeographic West African peri-Gondwanan region of the Avalonian-Cadomian active margin.  相似文献   

3.
《International Geology Review》2012,54(12):1492-1509
ABSTRACT

The Biarjmand granitoids and granitic gneisses in northeast Iran are part of the Torud–Biarjmand metamorphic complex, where previous zircon U–Pb geochronology show ages of ca. 554–530 Ma for orthogneissic rocks. Our new U–Pb zircon ages confirm a Cadomian age and show that the granitic gneiss is ~30 million years older (561.3 ± 4.7 Ma) than intruding granitoids (522.3 ± 4.2 Ma; 537.7 ± 4.7 Ma). Cadomian magmatism in Iran was part of an approximately 100-million-year-long episode of subduction-related arc and back-arc magmatism, which dominated the whole northern Gondwana margin, from Iberia to Turkey and Iran. Major REE and trace element data show that these granitoids have calc-alkaline signatures. Their zircon O (δ18O = 6.2–8.9‰) and Hf (–7.9 to +5.5; one point with εHf ~ –17.4) as well as bulk rock Nd isotopes (εNd(t) = –3 to –6.2) show that these magmas were generated via mixing of juvenile magmas with an older crust and/or melting of middle continental crust. Whole-rock Nd and zircon Hf model ages (1.3–1.6 Ga) suggest that this older continental crust was likely to have been Mesoproterozoic or even older. Our results, including variable zircon εHf(t) values, inheritance of old zircons and lack of evidence for juvenile Cadomian igneous rocks anywhere in Iran, suggest that the geotectonic setting during late Ediacaran and early Cambrian time was a continental magmatic arc rather than back-arc for the evolution of northeast Iran Cadomian igneous rocks.  相似文献   

4.
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   

5.
Lower Paleozoic volcanic members have been investigated by geological, petrographical and geochemical means in a traverse across the Ossa-Morena Zone (OMZ) in south-west Spain.The volcanism lasted from the Early Cambrian to the Early Ordovician, with a peak in the Middle Cambrian. The volcanism is bimodal, starting up with acidic and ending with basic compositions. From north to south, peralkaline rhyolites change to rhyolites, and strongly enriched alkali basalts change via transitional basalts to mid-ocean ridge basalt (MORB-type basalts). The geological and magmatic evolution suggests an extensive Early Paleozoic rifting with its center along the southern boundary of the OMZ. Temporal, spatial and crustal aspects of the rifting event are presented in a geodynamic model.  相似文献   

6.
The aim of this article is to examine the geochemistry and geochronology of the Cadomian Mishu granites from northwest Iran, in order to elucidate petrogenesis and their role in the evolution of the Cadomian crust of Iran. The Mishu granites mainly consist of two-mica granites associated with scarce outcrops of tonalite, amphibole granodiorite, and diorite. Leucogranitic dikes locally crosscut the Mishu granites. Two-mica granites show S-type characteristics whereas amphibole granodiorite, tonalities, and diorites have I-type signatures. The I-type granites show enrichment in large-ion lithophile elements (e.g. Rb, Ba and K) and depletion in high field strength elements (e.g. Nb, Ti and Ta). These characteristics show that these granites have been formed along an ancient, fossilized subduction zone. The S-type granites have high K, Rb, Cs (and other large ion lithophile elements) contents, resembling collision-related granites. U–Pb zircon dating of the Mishu rocks yielded 238U/206Pb crystallization ages of ca. 550 Ma. Moreover, Rb–Sr errorchron shows an early Ediacaran age (547 ± 84 Ma) for the Mishu igneous rocks. The two-mica granites (S-type granites) show high 87Sr/86Sr(i) ratios, ranging from 0.7068 to 0.7095. Their ?Nd values change between ?4.2 and ?4.6. Amphibole granitoids and diorites (I-type granites) are characterized by relatively low 87Sr/86Sr(i) ratios (0.7048–0.7079) and higher values of ?Nd (?0.8 to ?4.2). Leucogranitic dikes have quite juvenile signature, with ?Nd values ranging from +1.1 to +1.4 and Nd model ages (TDM) from 1.1 to 1.2 Ga. The isotopic data suggests interaction of juvenile, mantle-derived melts with old continental crust to be the main factor for the generation of the Mishu granites. Interaction with older continental crust is also confirmed by the presence of abundant inherited zircon cores. The liquid-line of descend in the Harker diagrams suggests fractional crystallization was also a predominant mechanism during evolution of the Mishu I-type granites. The zircon U–Pb ages, whole rock trace elements, and Sr–Nd isotope data strongly indicate the similarities between the Mishu Cadomian granites with other late Neoproterozoic–early Cambrian (600–520 Ma) granites across Iran and the surrounding areas such as Turkey and Iberia. The generation of the Mishu I-type granites could be related to the subduction of the Proto-Tethyan Ocean during Cadomian orogeny, through interaction between juvenile melts and old (Mesoproterozoic or Archaean) continental crust. The S-type granites are related to the pooling of the basaltic melts within the middle–upper parts of the thick continental crust and then partial melting of that crust.  相似文献   

7.
The Band-e-Hezarchah granitoids (BHG) is located in the northern margin of the central Iran, where the very old continental crust of Iran is found. The BHG mainly include granodiorite, granite and leucogranite. Small meta-gabbroic stocks and dykes are associated with BHG. U–Pb zircon dating of the BHG granites and metabasites yield 238U/206Pb crystallization ages of ca. 553.6 and 533.5 Ma respectively (Ediacaran–early Cambrian). The metabasites have calc-alkaline signature and their magmas seem to have originated from a mantle wedge above a subduction zone. These rocks are thought to be formed in a continental back-arc setting, related to the oblique subduction of Proto-Tethys oceanic lithosphere beneath the northern margin of Gondwanan supercontinent during Ediacaran–Cambrian time. The initial 87Sr/86Sr ratios and ɛNd (t) values for metabasites are change from 0.705 to 0.706 and −3.5 to −3.6 respectively. Sr–Nd isotope composition of metabasites indicates that these rocks were derived from a subcontinental lithospheric mantle source. The BHG and associated metabasites are coeval with other similar aged metagranites and gneisses from Iranian basements exposed in central Iran, Sanandaj-Sirjan and Alborz zones. These rocks were formed due to continental arc magmatism of Neoproterozoic–early Cambrian, bordering the northern active margin of Gondwana.  相似文献   

8.
The recently discovered Taolaituo porphyry Mo deposit and Aobaotu hydrothermal vein Pb–Zn deposit are both located in the Great Xing’an Range, Northeast China. Here we present new zircon U–Pb ages, whole-rock geochemical and Pb isotopic data, and molybdenite Re–Os ages for these two deposits. The Mo mineralization in the Taolaituo area occurred in quartz porphyry, which yields zircon U–Pb ages ranging from 138.5 ± 0.8 to 139.1 ± 0.5 Ma. Fine-grained granite representing pre-mineralization magmatic activity was formed at 145.2 ± 0.5 Ma. Molybdenite Re–Os dating indicates that Mo mineralization occurred at 133.8 ± 1.2 Ma. In the Aobaotu deposit, the ore-related granodioritic porphyry has a zircon U–Pb age of 140.0 ± 0.4 Ma. These geochronological data indicate that these magmatic and hydrothermal activities occurred during the Early Cretaceous. The mineralogical and geochemical features of the Taolaituo and Aobaotu granitoids suggest they can be classified as A1-type within-plate anorogenic granites and I-type granites, respectively. The Pb isotopic compositions suggest a mixed crust–mantle origin of the granitoids in these two deposits. The Taolaituo granitoids were formed by the partial melting of lower crust and crust–mantle interaction, with subsequent fractionation of apatite, feldspar, Ti-bearing phases and allanite or monazite. In contrast, the Aobaotu granites were derived primarily from lithospheric mantle that had been transformed or affected by the addition of subduction-related components. Combined with the regional geology, tectonic evolution and available age data from the literature, our results suggest that the Early Cretaceous (140–100 Ma) was likely to be the most important peak period for metallogenic mineralization in Northeast China. The Taolaituo and Aobaotu deposits formed under an extensional environment at an active continental margin in response to subduction of the Palaeo-Pacific oceanic plate.  相似文献   

9.
The medium- to high-grade polymetamorphic basement rocks of the Peloritani Mountains, northern Sicily, include large volumes of augen gneiss of controversial age and origin. By means of a geochemical and SHRIMP zircon study of representative samples, the emplacement age of the original granitoid protoliths of the augen gneisses and the most likely processes and sources involved in that granitoid magmatism have been determined. U–Pb dating of three samples from widely spaced localities in the Peloritani Mountains yielded igneous protolith ages of 565 ± 5, 545 ± 4 and 545 ± 4 Ma, respectively. These late Ediacaran/early Cambrian ages are much older than was previously assumed on geological grounds, and are typical of the peri-Gondwanan terranes involved in the geodynamic evolution of the northern Gondwana margin at the end of the Avalonian–Cadomian orogeny. Major and trace element compositions and Sr–Nd isotopic data, in combination with zircon inheritance age patterns, suggest that the granitoid protoliths of the Sicilian and coeval Calabrian augen gneisses were generated by different degrees of mixing between sediment- and mantle-derived magmas. The magmas forming the ca. 545 Ma inheritance-rich granitoids appear to have had a significant contribution from partial melting of paragneiss that is the dominant rock type in the medium- to high-grade Peloritanian basement. The closeness of the inferred deposition age of the greywacke protoliths of the paragneisses with the intrusion age of the granitoids indicates rapid latest Precambrian crustal recycling involving erosion, burial, metamorphism to partial melting conditions, and extensive granitoid magmatism in less than ca. 10 Ma.  相似文献   

10.
West of the southern, Archean, part of the Reguibat Rise of the West African Craton the Oulad Dlim Massif consists of metamorphic nappes stacked during the Mauritanides (Variscan) orogeny. In the Derraman region, about 12 km west of the nappes, we have found strongly deformed hypersolvus aegirine-riebeckite A1-type granites with SHRIMP zircon U–Pb ages of ca. 525 ± 3 Ma, ε(Nd)525Ma (− 5.2 to − 6.8.) and Nd model ages TCR ≈ 1.85 Ga. These granites define two km-sized bodies and a few smaller satellites. One body is emplaced within a 3.12 Ga leucocratic gneiss. The other body and its satellites are emplaced within an Archean low-grade metasedimentary sequence with detrital zircons that have ages that peak at 2.84 Ga, 2.91 Ga, and 3.15 Ga. These Archean gneisses and metapelite rocks define a tectonic unit, hereafter called the Derraman-Bulautad-Leglat (DBL) unit, which was formed from the Reguibat basement at the very margin of the WAC. The ~ 525 Ma Derraman granites are the oldest post-Archean rocks in this unit and were generated in an intraplate rifting environment from melting of crustal fenites during the ubiquitous Cambrian rifting event that affected this part of northern Gondwana. At the present level of knowledge, however, we cannot decide whether the “old” Nd isotope signature of Derraman granites resulted from melting of an old (Paleoproterozoic) fenite source or reflects the signature of the mantle-derived metasomatising fluids. The just-discovered Derraman granites are strikingly similar to other rift-related Cambrian–Ordovician hypersolvus aegirine–riebeckite granites widespread in North Gondwana. Understanding the potential connections between them would help to understand the Cambrian–Ordovician breakdown of northern Gondwana.  相似文献   

11.
The Pernambuco Alagoas (PEAL) domain shows the major occurrence of granitic batholiths of the Borborema Province, NE Brazil, with Archean to Neoproterozoic range of Nd TDM model ages, giving clues on the role of granites during the Brasiliano orogeny. SHRIMP U/Pb zircon geochronological data for seven granitic intrusions of the PEAL domain divide the studied granitoids into three groups: 1) early-to syn-collision granitoids with crystallization ages ca. 635 Ma (Serra do Catú pluton), 2) syn-collision granitoids with crystallization ages 610–618 Ma (Santana do Ipanema, Água Branca, Mata Grande and Correntes plutons) and 3) late-to post-collision granitoids with ages of ca. 590 Ma (Águas Belas, and Cachoeirinha plutons). The intrusions of group 1 and 2, except the Mata Grande and Correntes plutons, show Nd TDM model ages ranging from 1.2 to 1.5 Ga, while the granitoids from group 3, and Mata Grande Pluton and Correntes plutons have Nd TDM model ages ranging from 1.7 to 2.2 Ga. The studied granitoids with ages <600 Ma are high-K, calc-alkaline, shoshonitic and those with ages <600 Ma are transitional high-K calc-alkaline to alkaline. The volcanic arc signatures associated with the Paleoproterozoic Nd TDM model ages are interpreted as inherited from the source rocks. The oldest ages and lower Nd TDM model ages are recorded from granitoids intruded in the southwest part of the PEAL domain, suggesting that these intrusions are associated with slab-tearing during convergence between the PEAL and the Sergipano domains. Zircon oxygen isotopic data in some of the studied plutons, together with the available Nd isotopic data suggest that the Brasiliano orogeny strongly reworked older crust, of either Paleoproterozoic or Tonian ages. The studied granitoids are coeval with calc-alkaline granitoids of the Transversal Zone and Sergipano domains and rare high-K calc-alkaline granitoids from the Transversal Zone domain. Such large volumes of high-K granitoids with crystallization ages older than 600 Ma are not recorded in the Transversal Zone domains, suggesting that at least between 600 and 650 Ma, the granitic magmatism of these two areas were distinct. However, the studied granitoids (630–580 Ma) located in the north part of the PEAL domain, north of the Palmares shear zone are coeval with granitoids of similar geochemical compositions in the Transversal Zone domain. It suggests that the southeastern part of the Transversal Zone and the northern part of the PEAL domains belonged to the same crustal block during the Brasiliano/Pan-African orogeny.  相似文献   

12.
Detrital zircons are important proxies for crustal provenance and have been widely used in tracing source characteristics and continental reconstructions. Southern Peninsular India constituted the central segment of the late Neoproterozoic supercontinent Gondwana and is composed of crustal blocks ranging in age from Mesoarchean to late Neoproterozoic–Cambrian. Here we investigate detrital zircon grains from a suite of quartzites accreted along the southern part of the Madurai Block. Our LA-ICPMS U-Pb dating reveals multiple populations of magmatic zircons, among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic (ca. 2980–1670 Ma, with peaks at 2900–2800 Ma, 2700–2600 Ma, 2500–2300 Ma, 2100–2000 Ma). Zircons in two samples show magmatic zircons with dominantly Neoproterozoic (950–550 Ma) ages. The metamorphic zircons from the quartzites define ages in the range of 580–500 Ma, correlating with the timing of metamorphism reported from the adjacent Trivandrum Block as well as from other adjacent crustal fragments within the Gondwana assembly. The zircon trace element data are mostly characterized by LREE depletion and HREE enrichment, positive Ce, Sm anomalies and negative Eu, Pr, Nd anomalies. The Mesoarchean to Neoproterozoic age range and the contrasting petrogenetic features as indicated from zircon chemistry suggest that the detritus were sourced from multiple provenances involving a range of lithologies of varying ages. Since the exposed basement of the southern Madurai Block is largely composed of Neoproterozoic orthogneisses, the data presented in our study indicate derivation of the detritus from distal source regions implying an open ocean environment. Samples carrying exclusive Neoproterozoic detrital zircon population in the absence of older zircons suggest proximal sources in the southern Madurai Block. Our results suggest that a branch of the Mozambique ocean might have separated the southern Madurai Block to the north and the Nagercoil Block to the south, with the metasediments of the khondalite belt in Trivandrum Block marking the zone of ocean closure, part of which were accreted onto the southern Madurai Block during the collisional amalgamation of the Gondwana supercontinent in latest Neoproterozoic–Cambrian.  相似文献   

13.
New insights on the Paleozoic evolution of the continental crust in the North Patagonian Massif are presented based on the analysis of Sm–Nd systematics. New evidence is presented to constrain tectonic models for the origin of Patagonia and its relations with the South American crustal blocks. Geologic, isotopic and tectonic characterization of the North Patagonian Massif and comparison of the Nd parameters lead us to conclude that: (1) The North Patagonian Massif is a crustal block with bulk crustal average ages between 2.1 and 1.6 Ga TDM (Nd) and (2) At least three metamorphic episodes could be identified in the Paleozoic rocks of the North Patagonian Massif. In the northeastern corner, Famatinian metamorphism is widely identified. However field and petrographic evidence indicate a Middle to Late Cambrian metamorphism pre-dating the emplacement of the ca. 475 Ma granitoids. In the southwestern area, are apparent 425–420 Ma (?) and 380–360 Ma metamorphic peaks. The latter episode might have resulted from the collision of the Antonia terrane; and (3) Early Paleozoic magmatism in the northeastern area is coeval with the Famatinian arc. Nd isotopic compositions reveal that Ordovician magmatism was associated with attenuated crust. On the southwestern border, the first magmatic recycling record is Devonian. Nd data shows a step by step melting of different levels of the continental crust in the Late Palaeozoic. Between 330 and 295 Ma magmatism was likely the product of a crustal source with an average 1.5 Ga TDM (Nd). Widespread magmatism represented by the 295–260 Ma granitoids involved a lower crustal mafic source, and continued with massive shallower-acid plutono volcanic complexes which might have recycled an upper crustal segment of the Proterozoic continental basement, resulting in a more felsic crust until the Triassic. (4) Sm–Nd parameters and detrital zircon age patterns of Early Paleozoic (meta)-sedimentary rocks from the North Patagonian Massif and those from the neighboring blocks, suggest crustal continuity between Eastern Sierras Pampeanas, southern Arequipa-Antofalla and the northeastern sector of the North Patagonian Massif by the Early Paleozoic. This evidence suggests that, at least, this corner of the North Patagonian Massif is not allochthonous to Gondwana. A Late Paleozoic frontal collision with the southwestern margin of Gondwana can be reconcilied in a para-autochthonous model including a rifting event from a similar or neighbouring position to its post-collision location. Possible Proterozoic or Early Paleozoic connections of the NPM with the Kalahari craton or the western Antartic blocks should be investigated.  相似文献   

14.
Mafic microgranular enclaves (MMEs) in host granitoids can provide important constraints on the deep magmatic processes. The Oligocene-Miocene granitoid plutons of the NW Anatolia contain abundant MMEs. This paper presents new hornblende Ar-Ar ages and whole-rock chemical and Sr-Nd isotope data of the MMEs from these granitic rocks. Petrographically, the MMEs are finer-grained than their host granites and contain the same minerals as their host rocks (amphibole + plagioclase + biotite + quartz + K-feldspar), but in different proportions. The Ar-Ar ages of the MMEs range from 27.9 ± 0.09 Ma to 19.3 ± 0.01 Ma and are within error of their respective host granitoids. The MMEs are metaluminous and calc-alkaline, similar to I-type granites. The Sr-Nd isotopes of MMEs are 0.7057 to 0.7101 for 87Sr/86Sr and 0.5123 to 0.5125 for 143Nd/144Nd, and are similar to their respective host granitoids. These lithological, petrochemical and isotopic characteristics suggest that the MMEs in this present study represent chilled early formed cogenetic hydrous magmas produced during a period of post-collisional lithospheric extension in NW Anatolia. The parental magma for MMEs and host granitoids might be derived from partial melting of underplated mafic materials in a normally thickened lower crust in a post-collisional extensional environment beneath the NW Anatolia. Delamination or convective removal of lithospheric mantle generated asthenospheric upwelling, providing heat and magma to induce hydrous re-melting of underplated mafic materials in the lower crust.  相似文献   

15.
New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ～ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times.  相似文献   

16.
ABSTRACT

A systematic Sm-Nd isotopic study is reported for Paleoproterozoic to Late Paleozoic strata from the Dongchuan area in the southwestern Yangtze Block. The results, combined with the available detrital zircon U-Pb ages and Hf isotope data, constrain the provenances of these sedimentary rocks and further identify three important tectono-magmatic activities. Most of the Paleo-Mesoproterozoic samples (Dongchuan Group) display a wide TDM2 range of 1.92–2.52 Ga with corresponding εNd(t) values of +4.0 to ?3.5, suggesting Paleoproterozoic-dominated provenances mixed with mantle-derived materials. This corresponds to the ~1.7–1.5 Ga mafic magmatic activities commonly occurred in the southwestern Yangtze Block, which are related to the early breakup of the Columbia supercontinent. The obvious vale of TDM2 and apex of εNd(t) occurred in the Neoproterozoic strata (~0.8 Ga) of the southwestern margin over the whole Yangtze Block. This is consistent with the widely recognized mantle-derived magmatism around the Yangtze Block related to the breakup of Rodinia. However, the decreases in Nd model ages are different among various regions, indicating that the Neoproterozoic mantle inputs are more profound in the southwestern and central Yangtze Block than the southeastern Yangtze and the Jiangnan orogenic belt. The late Ediacaran to early Cambrian strata from the southwestern Yangtze exhibit a decrease in TDM2 (from 2.00 to 1.67 Ga) and increase in εNd(t) (from ?9.0 to ?5.2). This is in accordance with the coeval juvenile crustal materials discovered in the northwestern Yangtze, which were probably derived from the assembly of the Gondwana continent. Thus, a Gondwanan affinity is suggested for the southwestern Yangtze Block. Overall, the Nd isotopic studies of the Paleoproterozoic to Late Paleozoic sedimentary strata from the southwestern Yangtze Block identified three major episodes of magmatic activities, late Paleoproterozoic (~1.7 Ga), Neoproterozoic (~0.8 Ga) and late Neoproterozoic-early Cambrian (~0.55 Ga) in the context of Columbia, Rodinia and the subsequent Gondwana supercontinents.  相似文献   

17.
Mid-Devonian high-pressure (HP) and high-temperature (HT) metamorphism represents an enigmatic early phase in the evolution of the Variscan Orogeny. Within the Bohemian Massif this metamorphism is recorded mostly in allochthonous complexes with uncertain relationship to the major tectonic units. In this regard, the Mariánské Lázně Complex (MLC) is unique in its position at the base of its original upper plate (Teplá-Barrandian Zone). The MLC is composed of diverse, but predominantly mafic, magmatic-metamorphic rocks with late Ediacaran to mid-Devonian protolith ages. Mid-Devonian HP eclogite-facies metamorphism was swiftly followed by a HT granulite-facies overprint contemporaneous with the emplacement of magmatic rocks with apparent supra-subduction affinity. New Hf in zircon isotopic measurements combined with a review of whole-rock isotopic and geochemical data reveals that the magmatic protoliths of the MLC, as well as in the upper plate Teplá-Barrandian Zone, developed above a relatively unaltered Neoproterozoic lithospheric mantle. They remained coupled with this lithospheric mantle throughout a geological timeframe that encompasses separate Ediacaran and Cambrian age arc magmatism, protracted early Paleozoic rifting, and the earliest phases of the Variscan Orogeny. These results are presented in the context of reconstructing the original architecture of the Variscan terranes up to and including the mid-Devonian HP-HT event.  相似文献   

18.
The Khetri region forms a late Palaeoproterozoic igneous–metamorphic complex in NE Rajasthan, India. Seven granitoid plutons of the Khetri complex have been studied for zircon U–Pb and Pb–Pb dating along with whole-rock and Nd–Sr isotope geochemistry to provide new constraints on the Palaeoproterozoic magmatic activity in the Aravalli orogen of northwestern India. Most intrusives show evidence of moderate to extreme albitisation forming microcline–albite granite and albite granite, respectively. The rocks are metaluminous to weakly peraluminous, largely ferroan and intraplate A-type granites. The U–Pb zircon ages for four plutons cover a time span of 1732–1682 Ma, whereas Pb–Pb zircon evaporation data for three intrusives indicate minimum emplacement ages between 1671 and 1537 Ma. The Nd–Sr isotopic systematics suggest the involvement of Neoarchaean to Palaeoproterozoic crustal components in the petrogenesis of these granitoids. A regional survey of late Palaeoproterozoic ages in the Aravalli orogen provides evidence for a geographically widespread extension-related event in the northwestern Indian shield about 1720–1700 Ma ago. The record of comparable ages and the magmatic history reported in parts of North America and the North China Craton may indicate the significance of this event for the rift tectonics of the hypothetical supercontinent Columbia.  相似文献   

19.
U–Pb zircon dating of three metagranitoids, situated within a tilted crustal section at the northwestern border of the Teplá Barrandian unit (Teplá crystalline complex, TCC), yields similar Cambrian ages. The U–Pb data of zircons of the Teplá orthogneiss define an upper intercept age of 513 +7/–6?Ma. The 207Pb/206Pb ages of 516±10 and 511±10?Ma of nearly concordant zircons of the Hanov orthogneiss and the Lestkov granite are interpreted to be close to the formation age of the granitoid protolith. Similar to the Cambrian granitoids of the southwestern part of the Teplá Barrandian unit (Doma?lice crystalline complex, DCC) the Middle Cambrian emplacement of the TCC granitoids postdates Cadomian deformation and metamorphism of the Upper Proterozoic country rocks, but predates Variscan tectonometamorphic imprints. Structural data as well as sedimentological criteria suggest a dextral transtensional setting during the Cambrian plutonism, related to the Early Paleozoic break-up of northern Gondwana. Due to strong Variscan crustal tilting, the degree of Variscan tectonometamorphic overprint is strikingly different in the dated granitoids. It is lowest in the weakly or undeformed Lestkov granite, located in the greenschist-facies domain. The Teplá orthogneiss in the north underwent pervasive top-to-NW mylonitic shearing under amphibolite-facies conditions. There is no indication for a resetting of the U–Pb isotopic system of the Teplá orthogneiss zircons that could be attributed to this imprint. Radiation damages accumulated until recent have probably caused lead loss.  相似文献   

20.
The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号