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 46 毫秒
1.
U–Pb sensitive high resolution ion microprobe (SHRIMP) zircon geochronology, combined with REE geochemistry, has been applied in order to gain insight into the complex polymetamorphic history of the (ultra) high pressure [(U)HP] zone of Rhodope. Dating included a paragneiss of Central Rhodope, for which (U)HP conditions have been suggested, an amphibolitized eclogite, as well as a leucosome from a migmatized orthogneiss at the immediate contact to the amphibolitized eclogite, West Rhodope. The youngest detrital zircon cores of the paragneiss yielded ca. 560 Ma. This date indicates a maximum age for sedimentation in this part of Central Rhodope. The concentration of detrital core ages of the paragneiss between 670–560 Ma and around 2 Ga is consistent with a Gondwana provenance of the eroded rocks in this area of Central Rhodope. Metamorphic zircon rims of the same paragneiss yielded a lower intercept 206Pb/238U age of 148.8±2.2 Ma. Variable post-148.8 Ma Pb-loss in the outermost zircon rims of the paragneiss, in combination with previous K–Ar and SHRIMP-data, suggest that this rock of Central Rhodope underwent an additional Upper Eocene (ca. 40 Ma) metamorphic/fluid event. In West Rhodope, the co-magmatic zircon cores of the amphibolitized eclogite yielded a lower intercept 206Pb/238U age of 245.6±3.9 Ma, which is interpreted as the time of crystallization of the gabbroic protolith. The metamorphic zircon rims of the same rock gave a lower intercept 206Pb/238U age of 51.0±1.0 Ma. REE data on the metamorphic rims of the zircons from both the paragneiss of Central Rhodope and the amphibolitized eclogite of West Rhodope show no Eu anomaly in the chondrite-normalized patterns, indicating that they formed at least under HP conditions. Flat or nearly flat HREE profiles of the same zircons are consistent with the growth of garnet at the time of zircon formation. Low Nb and Ta contents of the zircon rims in the amphibolitized eclogite indicate concurrent growth of rutile. Based on the REE characteristics, the 148.8±2.2 Ma age of the garnet–kyanite paragneiss, Central Rhodope and the 51.0±1.0 Ma age of the amphibolitized eclogite, West Rhodope are interpreted to reflect the time close to the (U)HP and HP metamorphic peaks, respectively, with a good approximation. The magmatic zircon cores of the leucosome in the migmatized orthogneiss, West Rhodope, gave a lower intercept 206Pb/238U age of 294.3±2.4 Ma for the crystallization of the granitoid protolith of the orthogneiss. Two oscillatory zircon rims around the Hercynian cores, yielded ages of 39.7±1.2 and 38.1±0.8 Ma (2σ errors), which are interpreted as the time of leucosome formation during migmatization. The zircons in the leucosome do not show the 51 Ma old HP metamorphism identified in the neighboring amphibolitized eclogite, possibly because the two rock types were brought together tectonically after 51 Ma. If one takes into account the two previously determined ages of ca. 73 Ma for (U)HP metamorphism in East Rhodope, as well as the ca. 42 Ma for HP metamorphism in Thermes area, Central Rhodope, four distinct events of (U)HP metamorphism throughout Alpine times can be distinguished: 149, 73, 51 and 42 Ma. Thus, it is envisaged that the Rhodope consists of different terranes, which resulted from multiple Alpine subductions and collisions of micro-continents, rather similar to the presently accepted picture in the Central and Western Alps. It is likely that these microcontinents were rifted off from thinned continental margins of Gondwana, between the African and the European plates before the onset of Alpine convergence.  相似文献   

2.
阿尔金江尕勒萨依榴辉岩及其直接围岩——石榴子石黑云母片麻岩锆石的阴极发光图像、微区原位LA-ICP-MS微量元素分析研究表明,榴辉岩锆石内部结构比较均匀,少数颗粒保留斑杂状残核;位于锆石斑杂状残核测点的重稀土相对富集,Th/U比值多大于0.4,为岩浆锆石的特征;位于锆石边部与内部结构均匀颗粒上的测点显示HREE近平坦型或弱亏损型的稀土配分模式,显示了与石榴石平衡共生的变质锆石特征;而石榴子石黑云母片麻岩的锆石具有核-幔-边结构,核部为碎屑锆石,幔部则为与石榴石平衡共生的变质锆石。LA-ICP-MS微区定年获得榴辉岩的变质年龄为(493±4.3)Ma,其原岩形成年龄为(754±9)Ma;石榴子石黑云母片麻岩的变质年龄为(499±27)Ma。榴辉岩的变质年龄滞后于其原岩的形成年龄约250Ma,并且榴辉岩与其直接围岩副片麻岩的变质年龄几乎完全一致,充分表明该超高压榴辉岩的形成是陆壳深俯冲作用的产物。  相似文献   

3.
An eclogite–mafic granulite occurs as a rare boudin within a felsic kyanite–K‐feldspar granulite in a low‐strain zone. Its boundary is marked by significant metasomatism–diffusional gain of potassium at the centimetre‐scale, and probable infiltration of felsic melt on a larger scale. This converted the eclogite–mafic granulite into an intermediate‐composition, ternary‐feldspar‐bearing granulite. Based on inclusions in garnet, the peak P–T conditions of the original eclogite are 18 kbar at 850–950 °C, with later matrix re‐equilibration at 12 kbar and 950 °C. Four samples from the transition of the eclogite–mafic granulite through to the intermediate granulite were studied. In the eclogite, REE patterns in the garnet core show no Eu anomaly, compatible with crystallization in the absence of plagioclase and consistent with eclogite facies conditions. Towards the rim of garnet, LREE decrease, and a weak negative Eu anomaly appears, reflecting passage into HP granulite facies conditions with plagioclase present. The rims of garnet next to ternary feldspar in the intermediate granulite show the lowest LREE and deepest Eu anomalies. Zircon from the four samples was analysed by LASS (laser ablation–split‐stream inductively coupled plasma–mass spectrometry). It shows U–Pb ages from 404 ± 4.0 to 331 ± 3.3 Ma, with a peak at 340 ± 4.0 Ma corresponding to the likely exhumation of the rocks to 12 kbar. Older ages from zircon with steep HREE patterns indicate the minimum age of the protolith, and ages <360 ± 4.0 Ma are interpreted to correspond to the eclogite facies metamorphism. Only some zircon grains ≤350 ± 4.0 Ma have flat HREE patterns, suggesting that these are primarily modified protolith grains, rather than new zircon crystallized in the eclogite‐ or granulite facies. The metasomatic processes that converted the eclogite–mafic granulite to an intermediate granulite may have facilitated zircon modification as zircon in the intermediate granulite has flat HREE and ages of 340 ± 4.0 Ma. The difference between the oldest and youngest ages with flat REE patterns indicates a 16 ± 5.6 Ma period of zircon modification in the presence of garnet.  相似文献   

4.
《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   

5.
U–Pb and Rb–Sr dating was undertaken in combination with P–T estimates to (1) constrain the time of ultrahigh-pressure (UHP) eclogite formation in the Stadlandet UHP province of Norway, (2) date later crustal melting–migmatization of the eclogite country gneisses, and (3) temporally trace post-migmatite cooling and retrogression under amphibolite facies metamorphic conditions. In contrast to earlier U–Pb studies which used accessory minerals from the gneisses, we focused on the direct dating of minerals defining the HP assemblage. For the eclogite, rutile and omphacite fractions were analyzed for U–Pb, and from an adjacent migmatite leucosome titanites and K-feldspar. For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex. A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78). The 389-Ma age is interpreted to date equilibration and freezing of the eclogite paragenesis at maximum temperatures of 770 °C, reached during decompression to 1.8 GPa. Decompression from 2.8 to 1.8 GPa occurred in the partial melting domain of granitic crust, with the migmatites being dated at 375±6 Ma by titanite and K-feldspar from an eclogite-adjacent granitic leucosome. This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C. After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level. In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma. For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year. First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation. This period of extension is followed by a much younger, decoupled thermal phase at 327±5 Ma, occurring under static conditions within very restricted zones, most likely in association with the circulation of fluid phases along old discontinuities. Initial isotopic signatures of Sr and Pb substantiate Paleo- to Meso-Proterozoic crust formation times of the Stadlandet UHP province precursor lithologies.  相似文献   

6.
Coesite‐bearing eclogites from >100 km2 in the southern Dulan area, North Qaidam Mountains (NQM) of western China, contain zircon that records protolith crystallization and ultra high pressure (UHP) metamorphism. Sensitive High‐Resolution Ion Microprobe (Mass Spectrometer) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb analyses from cathodoluminescence (CL)‐dark zircon cores in a coesite‐bearing eclogite yield an upper intercept age of 838 ± 50 Ma, and oscillatory zoned cores in a kyanite‐bearing eclogite gave a weighted mean 206Pb/238U age of 832 ± 20 Ma. These zircon cores yield steep heavy rare earth element (HREE) slopes and negative Eu anomalies that suggest a magmatic origin. Thus, c. 835 Ma is interpreted as the eclogite protolith age. Unzoned CL‐grey or ‐bright zircon and zircon rims from four samples yield weighted mean ages of 430 ± 4, 438 ± 2, 446 ± 10 and 446 ± 3 Ma, flat HREE patterns without Eu anomalies, and contain inclusions of garnet, omphacite, rutile, phengite and rare coesite. These ages are interpreted to record 16 ± 5 Myr of UHP metamorphism. These new UHP ages overlap the age range of both eclogite and paragneiss from the northern Dulan area, suggesting that all UHP rock types in the Dulan area belong to the same tectonic unit. Our results are consistent with slow continental subduction, but do not match oceanic subduction and diapiric exhumation UHP model predictions. These new data suggest that, similar to eclogites in other HP/UHP units of the NQM and South Altyn Tagh, protoliths of the eclogites in the Dulan area formed in a continental setting during the Neoproterozoic, and then subducted to mantle depth together with continental materials during the Early Palaeozoic.  相似文献   

7.
SHRIMP U–Pb dating and laser ablation ICP‐MS trace element analyses of zircon from four eclogite samples from the north‐western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events. Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312 ± 5, 307 ± 4 and 311 ± 17 Ma, with a mean age of 309 ± 3 Ma. One sample from the Hong'an Group, 1 km south of the shear zone yields a late Triassic age of 232 ± 10 Ma, similar to the age of ultra‐high pressure (UHP) metamorphism in the east Qinling–Dabie orogenic belt. REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions. Zircon also grew during later retrogression. Zircon cores from the Huwan shear zone have Ordovician to Devonian (440–350 Ma) ages, flat to steep heavy‐REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks. Cores from Hong'an Group zircon are Neoproterozoic (780–610 Ma), consistent with derivation from the South China Block. In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone. The major Triassic continent–continent collision occurred along a suture at the southern boundary of the shear zone. The first collision produced local eclogite facies metamorphism in the Huwan shear zone. The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains–Sulu terrane and a lower grade overprint in the shear zone.  相似文献   

8.
Multiple repetitions of migmatization processes are an important indication of the polychronous evolution of Precambrian Mobile Belts: this is certainly true for the Belomorian Belt. In the Belomorian Province of the Fennoscandian Shield, newly obtained data demonstrate the effect of two stages of melting of the Earth’s crust under conditions of higher pressure up to 8–14 kbar. The early stage of the migmatization and genetically related leucogranite formation took place in the Neoarchaean (2710 ± 15 and 2706 ± 14 Ma, U–Pb zircon ages), while the younger one happened in the Palaeoproterozoic (1944 ± 12 and 1882 ± 9 Ma, U–Pb zircon ages of leucosomes). The early stage of crust melting is related to collision in the Belomorian Neoarchaean orogen, while the later stage occurred during formation of the Lapland–Kola orogen.  相似文献   

9.
The amalgamation of South (SCB) and North China Blocks (NCB) along the Qinling‐Dabie orogenic belt involved several stages of high pressure (HP)‐ultra high pressure (UHP) metamorphism. The new discovery of UHP metamorphic rocks in the North Qinling (NQ) terrane can provide valuable information on this process. However, no precise age for the UHP metamorphism in the NQ terrane has been documented yet, and thus hinders deciphering of the evolution of the whole Qinling‐Dabie‐Sulu orogenic belt. This article reports an integrated study of U–Pb age, trace element, mineral inclusion and Hf isotope composition of zircon from an eclogite, a quartz vein and a schist in the NQ terrane. The zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning, high Th/U and 176Lu/177Hf ratios, pronounced Eu anomalies and steep heavy rare earth element (HREE) patterns. The zircon cores yield an age of 796 ± 13 Ma, which is taken as the protolith formation age of the eclogite, and implies that the NQ terrane may belong to the SCB before it collided with the NCB. The ?Hf(t) values vary from ?11.3 to 3.2 and corresponding two‐stage Hf model ages are 2402 to 1495 Ma, suggesting the protolith was derived from an enriched mantle. In contrast, the metamorphic zircon rims show no zoning or weak zoning, very low Th/U and 176Lu/177Hf ratios, insignificant Eu anomalies and flat HREE patterns. They contain inclusions of garnet, omphacite and phengite, suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions, and their weighted mean 206Pb/238U age of 485.9 ± 3.8 Ma was interpreted to date the timing of the eclogite facies metamorphism. Zircon in the quartz vein is characterized by perfect euhedral habit, some oscillatory zoning, low Th/U ratios and variable HREE contents. It yields a weighted mean U–Pb age of 480.5 ± 2.5 Ma, which registers the age of fluid activity during exhumation. Zircon in the schist is mostly detrital and U–Pb age peaks at c. 1950 to 1850, 1800 to 1600, 1560 to 1460 and 1400 to 1260 Ma with an oldest grain of 2517 Ma, also suggesting that the NQ terrane may have an affinity to the SCB. Accordingly, the amalgamation between the SCB and the NCB is a multistage process that spans c. 300 Myr, which includes: the formation of the Erlangping intra‐oceanic arc zone onto the NCB before c. 490 Ma, the c. 485 Ma crustal subduction and UHP metamorphism of the NQ terrane, the c. 430 Ma arc‐continent collision and granulite facies metamorphism, the 420 to 400 Ma extension and rifting in relation to the opening of the Palaeo‐Tethyan ocean, the c. 310 Ma HP eclogite facies metamorphism of oceanic crust and associated continental basement, and the final 250 to 220 Ma continental subduction and HP–UHP metamorphism.  相似文献   

10.
A migmatitic orthogneiss in the Western Segment in the Sveconorwegian Province of the Baltic Shield was dated using the ion-probe U–Pb method on zircon grains, which were also analysed for rare earth elements. Mesosome zircons have 1.605±0.010 Ga magmatic cores, which places the gneiss protolith in the same 1.61–1.59 Ga time bracket as continental arc-related gneisses, abundant in this part of the Sveconorwegian Province. These cores show REE profiles with strong HREE enrichment, positive Ce- and negative Eu-anomalies, typical of magmatic zircon. Migmatite leucosomes are folded and parallel with or slightly discordant to the fabric. They contain a small population of zircon with cores and metamorphic rims, which are interpreted as xenocrysts incorporated in the leucosome during melting of the mesosome. CL-bright metamorphic embayments and rims on xenocrysts reflect 1.01±0.05 Ga Sveconorwegian metamorphic reworking. Ce-anomalies are nearly absent and Eu-anomalies are reduced relative to igneous spots. This is probably a feature of fluid controlled environments where Ce and Eu oxidation states are buffered by the metamorphic fluid. From this and discordant rims from the mesosome we also conclude that the rims formed by reworking of the older zircon where the Pb-loss was also fluid induced. In the leucosome veins, magmatic acicular zircon gives 0.92±0.01 Ga, ascribed to the crystallisation of the veins. They originated by local melting, probably augmented by magma that formed at a deeper level. Widespread granitic and noritic late-Sveconorwegian magmatism close to 0.92 Ga in other parts of the Western Segment has equivalents in the Norwegian sectors of the Sveconorwegian Province. Leucosome formation was therefore part of a regional event related to exhumation of the Sveconorwegian Eastern Segment. We also provide the first unequivocal evidence for ductile deformation related to late-Sveconorwegian magmatism.  相似文献   

11.
Progressive Early Silurian low‐pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U–Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U–Pb ages survived through mid‐amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 ± 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of {211} crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 ± 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 ± 3.1 Ma. Detrital zircon U–Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ～12°C/106y from ～730 to ～170°C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600–500 Ma and 1.2–0.9 Ga old) indicate its derivation from a source remote from the Australian craton.  相似文献   

12.
Both oceanic and continental HP rocks are juxtaposed in the Huwan shear zone in the western Dabie orogen, and thus provide a window for testing the buoyancy‐driven exhumation of dense oceanic HP rocks. The HP metamorphic age of the continental rocks in this zone has not been well constrained, and hence it is not known if they are of the same age as the exhumation of the HP oceanic rocks. In situ laser ablation (multiple collector) inductively coupled plasma mass spectrometry (LA‐(MC‐)ICP‐MS), U–Pb, trace element and Hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the Huwan shear zone. U–Pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 ± 4 Ma. The zircon in this sample displays εHf (t) values of +6.1 to +14.4. The positive εHf (t) values up to +14.4 suggest that the protolith was derived from a relatively depleted mantle source, most likely Palaeotethyan oceanic crust. A granitic gneiss and the other eclogite yield protolith U–Pb ages of 738 ± 6 and 700 ± 14 Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the granitic gneiss has low εHf (t) values of ?14.2 to ?10.5 and old TDM2 ages of 2528–2298 Ma, suggesting reworking of Palaeoproterozoic crust during the Neoproterozoic. The zircon in the eclogite has εHf (t) values of ?1.0 to +7.4 and TDM1 ages of 1294–966 Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat heavy rare earth element patterns, weak negative Eu anomalies and low 176Lu/177Hf ratios. All these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar, and thus under eclogite facies conditions. The metamorphic zircon yields U–Pb ages of 310 ± 3 and 306 ± 7 Ma. Therefore, both the oceanic‐ and continental‐type eclogites share the same episode of Carboniferous eclogite facies metamorphism. This suggests that high‐pressure continental‐type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic‐type eclogites through buoyancy‐driven uplift.  相似文献   

13.
This paper provides further evidence for the ongoing discussion as to whether the Dabie UHPM belt formed in Triassic or Palaeozoic time, and whether the Sulu UHPM belt formed in Triassic or Neoproterozoic time. Combined use of laser Raman spectrometer (LR), cathodoluminescence imaging (CL), and ion probe U–Pb in‐situ dating (SHRIMP) provided accurate ages of UHPM from rocks collected from Weihai, NE Sulu UHPM belt. LR was used to identify coesite and other UHP minerals as inclusions in zircon separates from an amphibolized peridotite and an eclogite. CL was used to examine the zoning structure of these zircon, and SHRIMP dating was performed on specific spots on zircon to obtain ages of different geological events. An age of 221 ± 12 Ma was obtained for coesite‐bearing zircon from the amphibolized peridotite; an age of 228 ± 29 Ma for eclogite was obtained from the lower intercept of a concordia plot. These ages are interpreted as the time of UHPM in the Weihai region. Ultramafic rocks to the east of Weihai yield a magmatic age at 581 ± 44 Ma. The zircon in the ultramafic rocks possibly also records a thermal event at c. 400 Ma, but no independent geological evidence for this event has been found. The eclogite protolith formed in the Middle Proterozoic (1821 ± 19 Ma), which is similar to the age of country rock gneisses of 1847–1744 Ma. The new geochronological data confirm that UHPM occurred in the Triassic in the Sulu area when subduction took the ultramafic body and the eclogite protolith, together with the adjacent supracrustal rocks, to mantle depths.  相似文献   

14.
In order to decipher the origin of eclogite in the high‐P/T Sanbagawa metamorphic belt, SHRIMP U–Pb ages of zircons from quartz‐bearing eclogite and associated quartz‐rich rock (metasandstone) were determined. One zircon core of the quartz‐rich rock yields an extremely old provenance age of 1899 ± 79 Ma, suggesting that the core is of detrital origin. Eight other core ages are in the 148–134 Ma range, and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139–135 Ma from the Sanbagawa schists. Ages of metamorphic zircon rims (132–112 Ma) from the quartz‐rich rock are consistent with metamorphic zircon ages from the quartz‐bearing eclogite, indicating that eclogite facies metamorphism peaked at 120–110 Ma. These new data are consistent with both the Iratsu eclogite body and surrounding highest‐grade Sanbagawa schists undergoing coeval subduction‐zone metamorphism, and subsequent re‐equilibration under epidote amphibolite facies conditions during exhumation.  相似文献   

15.
刘福来  刘平华 《岩石学报》2009,25(9):2113-2131
北苏鲁仰口地区出露超高压的变辉长岩.锆石阴极发光图像和其内部矿物包体激光拉曼测试的联合研究结果表明,变辉长岩锆石具有弱发光效应的岩浆韵律环带的核和被改造的强发光效应的边.岩浆韵律环带的核部保存大量而复杂的矿物包体,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(P1)+石英(Qtz)+黑云母(Bt)+钛铁矿(Ilm)+磷灰石(Ap);边部保存的矿物包体则相对较少,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(Pl)+磷灰石(Ap).尽管岩浆韵律环带核部的稀土元素总合量比被改造的锆石边部明显偏高,但二者稀土元素配分模式具有明显的相似性,主要表现为轻稀土相对亏损,而重稀土明显富集,相应的(La/Yb)N=0.00015～0.00039,并具有明显的负Eu异常(Eu/Eu*=0.20～0.26)、相对明显的正Ce异常(Ce/Ce*=71.5～147.4)和较高的Th/U比值(1.97～2.69).上述特征表明,仰口地区变辉长岩中的锆石均为继承性的岩浆锆石,而没有新生的变质锆石.LA-(MC)-ICP-MS锆石原位U-Pb定年和Lu-Hf同位素分析结果表明,两件锆石样品Y1和Y2的年龄数据所构成的不一致线显示了十分接近的上交点和下交点年龄.其上交点年龄分别为785±15Ma(2σ)和784±12Ma(2σ),应代表原岩的形成时代,表明变辉长岩的原岩与Rodinia超大陆裂解的岩浆事件存在密切的成因关系;而下交点年龄分别为226±24Ma(2σ)和228±26Ma(2σ),与苏鲁其它类型超高压岩石中含柯石英锆石微区记录的变质年龄十分吻合,应代表变辉长岩的超高压变质时代.岩浆结晶锆石的核部具有明显偏高的176Lu/177Hf(0.00044～0.00291)和176Yh/177Hf(0.0165～0.1168)比值,而176Hf/177Hf比值变化于0.281956～0.282048之间,相应的εHf(t)=-8.5～-14.0,tDM2=2.03～2.32Ga,表明仰口地区变辉长岩的原岩起源于古元古代时期的富集地幔或发生部分熔融的下地壳残留体.被改造的岩浆结晶锆石的边部则具有明显偏低的176Lu/177Hf(0.00029～0.00060)和176Yh/177Hf(0.0112～0.0200)比值,而176Hf/177Hf(t)比值变化于0.281953～0.282002之间,相应的εHf(t)=-10.2～-11.9,tDM2=2.12～2.21Ga.与岩浆结晶锆石核部相比,被改造的岩浆锆石边部的176Lu/177Hf、176Yb/177Hf、176Hf/177Hf(t)比值和εHf(t)和tDM2值的变化范围更小,表明中-新三叠纪的超高压变质作用使岩浆结晶锆石边部的Lu-Hf同位素体系发生调整,更趋向于均一化.  相似文献   

16.
The microgeochemistry of zircon was studied in three samples: charnockite gneiss (1594), charnockite (1594a), and migmatite leucosome Lc4 (1594c). Prismatic (Zrn I) and oval (Zrn II) zircon morphotypes are distinguished in the first two samples. Most zircon grains consist of two-phase cores and overgrowth rims variable in thickness. The average weighted concordant U–Pb age of Zrn II cores from charnockite gneiss is 2436 ± 10 Ma. The concordant ages of Zrn I and Zrn II cores from charnockite are 2402 ± 16 Ma and 2453 ± 14 Ma, respectively. Some overgrowth rims are 1.9–2.1 Ga in age. In leucosome Lc4, all measured prismatic zircon crystals yielded a discordant age of 1942 ± 11 Ma (the upper intersection of discordia with concordia). These zircons are strongly altered and anomalously enriched in U and Th. Zrn I grains are enriched relative to Zrn II in REE, Li, Ca, Sr, Ba, Hf, Th, and U. Zrn I is considered to be a product of melt crystallization or subsolidus recrystallization in the presence of melt. Zrn II is relict or crystallizing from melt and then partly fused again. Zrn I from charnockite gneiss and especially from charnockite are markedly altered and have a more discordant age than Zrn II. This is probably related to concentration of fluid in the residual melt left after zircon crystallization.  相似文献   

17.
Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra‐high‐P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.  相似文献   

18.
陈丹玲  刘良 《地学前缘》2011,18(2):158-169
在岩相学观察和锆石CL图像研究的基础上,利用LA ICP MS原位分析方法,对北秦岭官坡超高压榴辉岩和伴生的石榴石角闪岩(榴闪岩)进行了详细的锆石微区微量元素和U Th Pb同位素分析,在榴辉岩样品中得到变质年龄为(502±11)Ma,原岩结晶年龄>(657±18)Ma;在榴闪岩样品中得到原岩结晶年龄为(791±6)Ma,变质年龄为487~503 Ma,角闪岩相退变质年龄为(366±4)Ma。岩石地球化学研究显示,北秦岭官坡地区的榴闪岩具有低Si(SiO2质量分数为4916%~5078%),高Ti(TiO2质量分数为228%~283%)、富集LREE、LILE和大部分的HFSE元素,不显Nb、Ta负异常的板内玄武岩特征,与北秦岭超高压榴辉岩地球化学特征一致。结合两者的野外产状、岩相学特征、锆石形貌和年代学研究结果,表明本文研究的官坡地区的榴闪岩是超高压榴辉岩在抬升过程中在角闪岩相条件下退变质的产物。综合两者的年代学研究结果,得到北秦岭地区超高压榴辉岩的变质年龄为(502±11)Ma,原岩结晶年龄为(791±6)Ma,角闪岩相退变质年龄为(366±4)Ma。研究得到的(502±11)Ma的榴辉岩相变质年龄与前人得到的该榴辉岩围岩超高压泥质片麻岩的变质年龄(507±38)Ma以及北秦岭松树沟地区的超高压长英质片麻岩的变质年龄485~514 Ma一致,表明它们经历了同期超高压变质作用。而且,榴辉岩(502±11)Ma的变质年龄与其原岩的结晶年龄(791±6)Ma存在近300 Ma的时间间隔,表明原岩具有板内玄武岩性质的北秦岭官坡超高压榴辉岩不可能是秦岭古生代大洋板块深俯冲的产物,而可能是已构造就位的古洋壳或裂谷火山岩在古生代随陆壳一起发生大陆深俯冲作用的产物。  相似文献   

19.
柴北缘超高压地体折返过程中地壳深熔的岩石学研究   总被引：2，自引：2，他引：0  
宏观、微观岩石学、地球化学和年代学研究表明,柴北缘锡铁山和绿梁山单元富含斜长石的浅色体和富含钾长石的浅色体是超高压地体折返过程中榴辉岩和片麻岩部分熔融的产物。阴极发光图像显示富含斜长石的浅色体中锆石具有明显的核-边双层结构,锆石核部无明显分带特征,并呈现出重稀土平坦和无Eu异常的稀土配分模式,～450Ma的年龄结果与区域上榴辉岩峰期变质时代一致;发光较弱的锆石边部具不明显的环带结构和较低的Th/U比值,～426Ma年龄结果代表了熔体的结晶时代。富含钾长石的浅色体中的锆石U-Pb定年结果记录的～910Ma、～450Ma和～426Ma三组年龄分别代表了片麻岩原岩结晶时代、高压-超高压变质作用时代和熔体结晶时代。富含斜长石的浅色体具有高SiO_2、Al_2O_3、CaO、Na_2O、Sr和LREE,而低MgO、FeO~T、K_2O、Y、Yb和HREE的英云闪长岩-奥长花岗岩的地球化学特征;而富含钾长石的浅色体具有高的SiO_2、Al_2O_3和K_2O+Na_2O,而较低的CaO、MgO、REE的花岗岩地球化学特征。黝帘石和少量的多硅白云母的脱水分解是触发超高压榴辉岩发生部分熔融形成富含斜长石的浅色体的主要机制;而多硅白云母的脱水分解则是触发超高压片麻岩部分熔融形成富含钾长石浅色体的主要机制。这些浅色体显著的促进了柴北缘超高压地体的快速折返,并对大陆俯冲隧道中的元素迁移和壳-幔作用具有重要的影响。  相似文献   

20.
Metasediments of the Rantasalmi-Sulkava area (Finland) showprogressive regional metamorphism with migmatization. The metasedimentsare represented by various types of metapsammites (plagioclase-rich,quartz-rich, and layers of granitic compositions—somerich in microcline and others in plagioclase) and metapelites(dark and light layers). The migmatites of this area are of stromatic type. They consistof leucosomes, mesosomes, and light-coloured plagioclase-richlayers which do not fit the definition of leucosome. Melanosomes,which usually separate leucosomes and mesosomes in stromaticmigmatites, are almost absent. The leucosomes are of three types: (i) quartz-rich; (ii) cordierite-rich;and (iii) granitic. The quartz-rich leucosomes formed firstat subsolidus temperatures through recrystallization. The graniticleucosomes are considered to have developed via partial melting.The cordierite-rich leucosomes are formed—like the graniticones—at supersolidus conditions, but the role of partialmelting is not clear. The mesosomes are the metamorphic portions of the migmatiteswhich are not transformed into leucosomes. They include metapsammiticlayers and light-coloured metapelitic layers, both rich in plagioclase. Besides mineral reactions resulting in new assemblages duringregional metamorphism, the main process changing the protolithsinto migmatites is the conversion of some of the protolith layersinto leucosomes, through (as we believe) an almost isochemicalpartial melting. The migmatites of the Rantasalmi-Sulkava area differ from othermigmatites investigated by the authors in having two differentgenetic types of leucosomes: one formed via partial meltingand the other through subsolidus recrystallization as mentionedabove. The process of migmatization is described and modelledin three steps. Reprint requests to W. Johannes  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