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1.
We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.  相似文献   

2.
Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/238U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D1), predominately top-to-N ductile shearing and crustal extension (D2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D3), and continued N–S contraction, E–W extension and doming (D4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.  相似文献   

3.
Precise U–Pb geochronology and Hf isotope tracing of zircon is combined with whole-rock geochemical and Sr and Nd isotope data in order to unravel processes affecting mafic to felsic calcalkaline magmas prior to and during their crystallization in crustal magma chambers along the southern border of Central Srednogorie tectonic zone in Bulgaria (SE Europe). ID-TIMS U–Pb dating of single zircons from felsic and mixed/mingled dioritic to gabbroic horizons of single plutons define crystallization ages of around 86.5–86.0, 85.0–84.5 and 82 Ma. Concordia age uncertainties are generally less than 0.3 Ma (0.35%–2σ), and as good as 0.08 Ma (0.1%), when the weighted mean 206Pb/238U value is used. Such precision allows the distinction of magma replenishment processes if separated by more than 0.6–1.0 Ma and when they are marked by newly saturated zircons. We interpret zircon dates from a single sample that do not overlap to reflect new zircon growth during magma recharge in a long-lived crustal chamber. Mingling/mixing of the basaltic magma with colder granitoid mush at mid- to upper-crustal levels is proposed to explain zircon saturation and fast crystallization of U- and REE-rich zircons in the hybrid gabbro.Major and trace-element distribution and Sr and Nd whole-rock isotope chemistry define island arc affinities for the studied plutons. Slab derived fluids and a sediment component are constrained as enrichment sources for the mantle wedge-derived magma, though Hf isotopes in zircon suggest crustal assimilation was also important. Inherited zircons, and their corresponding ε-Hf, from the hybrid gabbroic rocks trace the lower crust as possible source for enrichment of the mantle magma. These inherited zircons are about 440 Ma old with ε-Hf of − 7 at 82 Ma, whereas newly saturated concordant Upper Cretaceous zircons reveal mantle ε-Hf values of + 7.2 to + 10.1. The upper and middle crusts contribute in the generation of the granitoid rocks. Their zircon inheritance is Lower Palaeozoic or significantly older and crustal dominated with 82–85 Ma corrected ε-Hf values of − 28. The Cretaceous concordant zircons in the granitoids are mantle dominated with a ε-Hf values spreading from + 3.9 to + 7.  相似文献   

4.
Potential sources for alluvial gem corundum and zircon from the Rio Mayo area, near Mercaderes, Colombia are reviewed, based on U–Pb dating of syngenetic and protogenetic mineral inclusions in corundum samples and on a zircon megacryst. Corundum recovered from the region (approx. 99% sapphire, 1% ruby) commonly shows growth banding, includes colour change stones and exhibits overlaps in colour ranges and inclusion characteristics. This suggests a contiguous genetic suite. The U–Pb dating used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Because of the young ages and low-U contents of the dated zircons, the acquired data required a special statistical treatment. The results from zircon, fluorapatite and allanite-(Ce) inclusions provide a corundum crystallization age of 8 to 11 Ma, in relation to northern Andean Miocene uplift and magmatism. The zircon megacryst gave a younger crystallization age of c. 0.6 Ma, unrelated to the corundum genesis. Geochemical parameters (trace element and O isotope ranges) for corundum samples suggest a metamorphic/metasomatic origin. The age data rules out corundum genesis during the Late Cretaceous ophiolitic generation, but leave open possible later metasomatic interactions with this substrate. The Cr/Ga and Ga/Mg ratios and O isotope range for the corundum fall within the known limits for metasomatic, desilicated felsic/ultramafic ‘plumasitic’ associations, suggesting a possible parental source. Allanite, extremely rare as an inclusion in corundum elsewhere, may prove a characteristic inclusion for Rio Mayo corundum.  相似文献   

5.
Zircon SHRIMP U–Pb and in-situ Lu–Hf isotopic analyses via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of a tuff within the Upper Paleozoic from Western Beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the North China block (NCB). SHRIMP U–Pb zircon dating of the tuff yielded a 206Pb/238U weighted mean age of 296 ± 4 Ma (95% confidence, MSWD = 3.3), which is very similar to the emplacement age of the newly discovered Carboniferous calc-alkaline, I-type continental arc granitoid plutons in the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCB. In-situ Lu–Hf analysis results of most zircons from the tuff yielded initial 176Hf/177Hf ratios from 0.282142 to 0.282284 and εHf(t) values from − 15.9 to − 10.7. These Lu–Hf isotopic compositions are very similar to those of the Late Carboniferous granitoids in the IMPU, but are very different to those of the Central Asian Orogenic Belt (CAOB). Together with the sedimentary and tectonic analyses results, we inferred that the source area of the tuffs within the NCB is the IMPU instead of the CAOB. Therefore, some arc volcanoes once existed in the IMPU on northern margin of the NCB during the Late Carboniferous, but they were entirely eroded due to strong exhumation and erosion of the IMPU during the Late Carboniferous to Early Jurassic.  相似文献   

6.
L. Millonig  A. Zeh  A. Gerdes  R. Klemd 《Lithos》2008,103(3-4):333-351
The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P–T conditions of 830–860 °C/8–9 kbar followed by a pressure–temperature decrease to 750 °C/5–6 kbar. This P–T path is inferred from the application of P–T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet–biotite–sillimanite–cordierite–plagioclase–(K-feldspar)–quartz gneisses, two charnoenderbitic garnet–orthopyroxene–biotite–K-feldspar–plagioclase–quartz gneisses and an enderbitic orthopyroxene–biotite–plagioclase–quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U–Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structural-metamorphic–magmatic event at 2644 ± 8 Ma. This age is significantly older than an U–Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a ‘dynamic regional contact metamorphic milieu’, which perhaps took place in a magmatic arc setting.  相似文献   

7.
Zircon fission-track (FT) and U–Pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the Asuke Shear Zone (ASZ), Aichi Prefecture, Japan. The U–Pb ages of all four samples are  67–76 Ma, which is interpreted as the formation age of Ryoke granitic rocks along the ASZ. The mean zircon FT age of host rock is 73 ± 7 (2σ) Ma, suggesting a time of initial cooling through the zircon closure temperature. The pseudotachylyte zone however, yielded a zircon FT age of 53 ± 9 (2σ) Ma, statistically different from the age of the host rock. Zircon FTs showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone. Coupled with the new zircon U–Pb ages and previous heat conduction modeling, the present FT data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault. The age for the pseudotachylyte coincides with the change in direction of rotation of the Pacific plate from NW to N which can be considered to initialize the NNE–SSW trending sinistral–extensional ASZ before the Miocene clockwise rotation of SW Japan. The present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon FT thermochronology.  相似文献   

8.
In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma.The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities.The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.  相似文献   

9.
The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (206Pb/204Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (206Pb/204Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (206Pb/204Pb = 17.12 to 17.41, 207Pb/204Pb = 15.47 to 15.54, 208Pb/204Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (206Pb/204Pb = 18.20 to 24.28 and 18.32 to 20.06, 207Pb/204Pb = 15.69 to 16.44 and 15.66 to 15.98, 208Pb/204Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.  相似文献   

10.
The Permian–Jurassic Mahanadi and Pranhita–Godavari Rifts are part of a drainage system that radiated from the Gamburtsev Subglacial Mountains in central Antarctica. From 12 samples we analysed detrital zircons for U–Pb ages, Hf-isotopes, and trace elements to determine the age, rock type and source of the host magma, and TDM model age. Clusters, in decreasing order of abundance, are (1) 820–1000 Ma, host magmas felsic granitoids with alkaline rock, (2) 1500–1700 Ma felsic granitoids, (3) 500 to 700 Ma mafic granitoids with alkaline rock, (4) 2400–2550 Ma granitoids, and (5) 1000–1200 Ma felsic and mafic granitoids, mafic rock, and alkaline rock. TDM ranges from 1.5 to 3.5 Ga. Joint paleoslope measurements and zircon ages indicate that the Eastern Ghats Mobile Belt (EGMB) and lateral belts and conjugate Antarctica are potential provenances. Zircons from the Gondwana Rifts differ from those in other Gondwanaland sandstones in their predominant 820–1000 Ma and 1500–1700 Ma ages (from the EGMB and conjugate Rayner–MacRobertson Belt) that dilute the 500–700 Ma (Pan-Gondwanaland) ages. The 1000–1200 Ma zircons reflect the assembly of Rodinia, the 500–700 Ma ones that of Gondwanaland; the other ages reflect collisions in the region.  相似文献   

11.
Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent 207Pb/206Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.  相似文献   

12.
U–Pb (TIMS–ID and SIMS) and Sm–Nd analyses of zircons and garnet-whole rock pairs were applied on high-pressure granulite facies metapelites and metagranodiorite from Tcholliré and Banyo regions, respectively in the Adamawa–Yadé and Western Domains of the Central-African Fold Belt (CAFB) of Cameroon. Cathodoluminescence (CL) images of zircons reveal that they are made up of ubiquitous magmatitic xenocrystic cores, surrounded and/or overprinted by light unzoned recrystallized domains. U–Pb data on cores yield ages ranging from Paleoproterozoic to Neoproterozoic, which we consider as dating inheritances. Data on overgrowths and recrystallized domains give ages ranging between 594 and 604 Ma, interpreted as the time of HP granulite-facies metamorphism in the Tcholliré and Banyo regions. This is also supported by ages derived from Sm–Nd garnet-whole rock pairs. Sediments of the Tcholliré region were deposited after ca. 620 Ma from Paleoproterozoic, Mesoproteroszoic and Neoproterozoic protoliths, while those from the Banyo region were deposited after 617.6 ± 7.1 Ma essentially from Neoproterozoic protoliths.  相似文献   

13.
Laser ablation ICP-MS U–Pb zircon geochronology of detrital zircons from a contact metamorphic sample of the Brixen Quartzphyllite from the innermost part of the contact aureole adjacent to the Brixen granodiorite yielded three different Precambrian concordia ages: zircon cores and an older generation of zircons give a maximum age of 2,023 ± 31 Ma, zircon rims and a younger generation of single grains yield a concordia age of 882 ± 19 Ma. A third generation of single zircon grains yields an age of 638 ± 20 Ma. In contrast to Austroalpine quartzphyllite complexes from the Eastern Alps neither Cambrian/Ordovician (570–450 Ma) nor Carboniferous (360–340 Ma) ages on single zircons have been observed so far in these samples. These ages provide evidence of a complex pre-Variscan evolution of the Southalpine basement since these data suggest a possible affinity of the Southalpine basement to Gondwana-related tectonic elements as well as to a possible Cadomian hinterland. This study shows that dating detrital zircons of the Brixen Quartzphyllites has great potential for providing age constraints on the complex geological evolution of the Southalpine basement.  相似文献   

14.
The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751 ± 7 Ma for protolith crystallization, and two group ages of 213 ± 4 to 245 ± 17 Ma and 126 ± 4 to 131 ± 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative εHf(t) values of − 5.1 to − 2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive εHf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ18O values of − 3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ18O rocks. North Dabie shares the same age of Neoproterozoic low δ18O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.  相似文献   

15.
The eastern Amery Ice Shelf (EAIS) and southwestern Prydz Bay are situated near the junction between the Late Neoproterozoic/Cambrian high-grade complex of the Prydz Belt and the Early Neoproterozoic Rayner Complex. The area contains an important geological section for understanding the tectonic evolution of East Antarctica. SHRIMP U–Pb analyses on zircons of felsic orthogneisses and mafic granulites from the area indicate that their protoliths were emplaced during four episodes of ca. 1380 Ma, ca. 1210–1170 Ma, ca. 1130–1120 Ma and ca. 1060–1020 Ma. Subsequently, these rocks experienced two episodes of high-grade metamorphism at > 970 Ma and ca. 930–900 Ma, and furthermore, most of them (except for some from the Munro Kerr Mountains and Reinbolt Hills) were subjected to high-grade metamorphic recrystallization at ca. 535 Ma. Two suites of charnockite, i.e. the Reinbolt and Jennings charnockites, intrude the Late Mesoproterozoic/Early Neoproterozoic and Late Neoproterozoic/Cambrian high-grade complexes at > 955 Ma and 500 Ma, respectively. These, together with associated granites of similar ages, reflect late- to post-orogenic magmatism occurring during the two major orogenic events. The similarity in age patterns suggests that the EAIS–Prydz Bay region may have suffered from the same high-grade tectonothermal evolution with the Rayner Complex and the Eastern Ghats of India. Three segments might constitute a previously unified Late Mesoproterozoic/Early Neoproterozoic orogen that resulted from the long-term magmatic accretion from ca. 1380 to 1020 Ma and eventual collision before ca. 900 Ma between India and the western portion of East Antarctica. The Prydz Belt may have developed on the eastern margin of the Indo-Antarctica continental block, and the Late Neoproterozoic/Cambrian suture assembling Indo-Antarctica and Australo-Antarctica continental blocks should be located southeastwards of the EAIS–Prydz Bay region.  相似文献   

16.
The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.  相似文献   

17.
There is an increasing evidence for the involvement of pre-Neoproterozoic zircons in the Arabian–Nubian Shield, a Neoproterozoic crustal tract that is generally regarded to be juvenile. The source and significance of these xenocrystic zircons are not clear. In an effort to better understand this problem, older and younger granitoids from the Egyptian basement complex were analyzed for chemical composition, SHRIMP U–Pb zircon ages, and Sm–Nd isotopic compositions. Geochemically, the older granitoids are metaluminous and exhibit characteristics of I-type granites and most likely formed in a convergent margin (arc) tectonic environment. On the other hand, the younger granites are peraluminous and exhibit the characteristics of A-type granites; these are post-collisional granites. The U–Pb SHRIMP dating of zircons revealed the ages of magmatic crystallization as well as the presence of slightly older, presumably inherited zircon grains. The age determined for the older granodiorite is 652.5 ± 2.6 Ma, whereas the younger granitoids are 595–605 Ma. Xenocrystic zircons are found in most of the younger granitoid samples; the xenocrystic grains are all Neoproterozoic, but fall into three age ranges that correspond to the ages of other Eastern Desert igneous rocks, viz. 710–690, 675–650 and 635–610 Ma. The analyzed granitoids have (+3.8 to +6.5) and crystallization ages, which confirm previous indications that the Arabian–Nubian Shield is juvenile Neoproterozoic crust. These results nevertheless indicate that older Neoproterozoic crust contributed to the formation of especially the younger granite magmas.  相似文献   

18.
New U–Pb SHRIMP ages in zircon, Ar–Ar ages in micas and amphiboles, Nd–Sr isotopes, and major and REE geochemical analyses in granitic gneisses and granitic stocks of the Central Cordillera of Colombia indicate the presence of a collisional orogeny in Permo-Triassic times in the Northern Andes related to the construction of the Pangea supercontinent. The collision is recorded by metamorphic U–Pb SHRIMP ages in inherited zircons around 280 Ma and magmatic U–Pb SHRIMP ages in neoformed zircons around 250 Ma within syntectonic crustal granitic gneisses. Magmatic U–Pb SHRIMP and Ar–Ar Triassic ages around 228 Ma in granitic stocks indicate the presence of late tectonic magmatism related to orogenic collapse and the beginning of the breakup of the supercontinent. During this period, the Central Cordillera of Colombia would have been located between the southern United States and northern Venezuela, in the leading edge of the Gondwana supercontinent.  相似文献   

19.
B. Seth  S. Jung  B. Gruner   《Lithos》2008,104(1-4):131-146
Three dating techniques for metamorphic minerals using the Sm–Nd, Lu–Hf and Pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in NW Namibia. The combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating. Here, PbSL, Lu–Hf and Sm–Nd garnet ages obtained on low- to medium-grade metasedimentary rocks from the Central Kaoko Zone of the Neoproterozoic Kaoko belt (NW Namibia) indicate that these samples were metamorphosed at around 550–560 Ma. On the other hand, granulite facies metasedimentary rocks from the Western Kaoko Zone underwent two phases of high-grade metamorphism, one at ca. 660–625 Ma and another at ca. 550 Ma providing substantial evidence that the 660–625 Ma-event was indeed a major tectonothermal episode in the Kaoko belt. Our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems. However, a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation.  相似文献   

20.
Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.  相似文献   

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