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1.
While recycling of subducted oceanic crust is widely proposed to be associated with oceanic island, island arc, and subduction-related adakite magmatism, it is less clear whether recycling of subducted continental crust takes place in continental collision belts. A combined study of zircon U–Pb dating, major and minor element geochemistry, and O isotopes in Early Cretaceous post-collisional granitoids from the Dabie orogen in China demonstrates that they may have been generated by partial melting of subducted continental crust. The post-collisional granitoids from the Dabie orogen comprise hornblende-bearing intermediate rocks and hornblende-free granitic rocks. These granitoids are characterized by fractionated REE patterns with low HREE contents and negative HFSE anomalies (Nb, Ta and Ti). Although zircon U–Pb dating gives consistent ages of 120 to 130 Ma for magma crystallization, occurrence of inherited cores is identified by CL imaging and SHRIMP U–Pb dating; some zircon grains yield ages of 739 to 749 Ma and 214 to 249 Ma, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and a Triassic tectono-metamorphic event in the Dabie–Sulu orogenic belt, respectively. The granitoids have relatively homogeneous zircon δ18O values from 4.14‰ to 6.11‰ with an average of 5.10‰ ± 0.42‰ (n = 28) similar to normal mantle zircon. Systematically low zircon δ18O values for most of the coeval mafic–ultramafic rocks and intruded country rocks preclude an AFC process of mafic magma or mixing between mafic and felsic magma as potential mechanisms for the petrogenesis of the granitoids. Along with zircon U–Pb ages and element results, it is inferred that the granitic rocks were probably derived from partial melting of intermediate lower crust and the intermediate rocks were generated by amphibole-dehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement. The post-collisional granitoids in the Dabie orogen are interpreted to originate from recycling of the subducted Yangtze continental crust that was thickened by the Triassic continent–continent collision. Partial melting of orogenic lithospheric keel is suggested to have generated the bimodal igneous rocks with the similar crustal heritage. Crustal thinning by post-collisional detachment postdated the onset of bimodal magmatism that was initiated by a thermal pulse related to mantle superwelling in Early Cretaceous. 相似文献
2.
Laser ablation ICPMS U–Pb and Lu–Hf isotope data on granitic-granodioritic gneisses of the Precambrian Vråvatn complex in central Telemark, southern Norway, indicate that the magmatic protoliths crystallized at 1201 ± 9 Ma to 1219 ± 8 Ma, from magmas with juvenile or near-juvenile Hf isotopic composition (176Hf/177Hf = 0.2823 ± 11, epsilon-Hf > + 6). These data provide supporting evidence for the depleted mantle Hf-isotope evolution curve in a time period where juvenile igneous rocks are scarce on a global scale. They also identify a hitherto unknown event of mafic underplating in the region, and provide new and important limits on the crustal evolution of the SW part of the Fennoscandian Shield. This juvenile geochemical component in the deep crust may have contributed to the 1.0–0.92 Ga anorogenic magmatism in the region, which includes both A-type granite and a large anorthosite–mangerite–charnockite–granite intrusive complex. The gneisses of the Vråvatn complex were intruded by a granitic pluton with mafic enclaves and hybrid facies (the Vrådal granite) in that period. LAM-ICPMS U–Pb data from zircons from granitic and hybrid facies of the pluton indicates an intrusive age of 966 ± 4 Ma, and give a hint of ca. 1.46 Ga inheritance. The initial Hf isotopic composition of this granite (176Hf/177Hf = 0.28219 ± 13, epsilon-Hf = − 5 to + 6) overlaps with mixtures of pre-1.7 Ga crustal rocks and juvenile Sveconorwegian crust, lithospheric mantle and/or global depleted mantle. Contributions from ca. 1.2 Ga crustal underplate must be considered when modelling the petrogenesis of late Sveconorwegian anorogenic magmatism in the region. 相似文献
3.
Striking characteristics of the western Neoproterozoic belt of Cameroon (NFBC) are the large volume of granitoids and crustal-scale shear zones. New structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the Atlantic Ocean.
Three different complexes have been identified in the study area: the migmatitic complex of Foumbot (MCF), the metagranitoid complex of Bangwa (BC), and the Batié pluton (BP). The MCF was intruded by the BC, while the BP cuts through the BC. U–Pb zircon dating of metaleucogranite and metagranodiorite of the BC yielded concordant to subconcordant ages of 638 ± 2 Ma and 637 ± 5 Ma, respectively. A concordant U–Pb zircon age of 602 ± 1.4 Ma has been obtained from porphyrogranite of the BP. These ages are interpreted as emplacement ages. Continuous deformation from magmatic to solid-state flow along the BP margins and the (sub)parallelism of the steep solid-state foliation in the BP margins with the foliation in the surrounding BC and MFC suggest synkinematic emplacement of the BP along crustal-scale NNE to ENE-trending strike–slip shear zones. Subhorizontal foliations in migmatitic-gneiss xenoliths found in the BC suggest that the major transcurrent motion was preceded by thrusting.
The new data confirm previous assumptions that the western NFBC is equivalent to parts of the Borborema province of Brazil. There are geochronological correlations between the studied (meta)granitoids and Brasiliano pre- to syn-transcurrent granitoids of the Borborema province. 相似文献
4.
Yuejun Wang Weiming Fan Guochun Zhao Shaocheng Ji Touping Peng 《Gondwana Research》2007,12(4):404-416
This paper presents new zircon U–Pb data and interpretations for the gneissic rocks in the Yunkai massif in order to constrain the timing and evolution of the Caledonian tectonothermal event in the South China Block (SCB). Magmatic and inherited zircons from the orthogneiss in the region, previously thought to be of Precambrian origin, yielded 206Pb/238U apparent ages of 421–441 Ma and 513–1343 Ma, respectively. Also a weighted mean 206Pb/238U age of 236.0 ± 3.1 Ma was obtained, interpreted as the metamorphic resetting age during the Indosinian tectonic event. Our analyses show that the paragneiss in the region, previously regarded as Proterozoic sedimentary rocks, contains detrital zircons of the Archaean to Paleozoic origin, of which the youngest zircons yielded the U–Pb age of 423 Ma. These data indicate that (1) the Proterozoic and Archaean components may exist beneath the Yunkai massif; (2) most of the metaigneous rocks are actually the Caledonian anatectic granites possibly overprinted by Indosinian ( 236 Ma) reactivation; (3) some paragneiss might have originally deposited during the Devonian time; and (4) a subsequently rapid uplifting took place after the emplacement of the Caledonian granites, revealed by the observation that the Devonian clastic strata uncomfortably overlie the Caledonian granites. In combination with other geochronological data and geological observations throughout the SCB, we propose that the Caledonian tectonothermal event around Silurian ( 450–400 Ma) might be a result from an intracontinental collision between the Yangtze and Cathaysian blocks in response to the subduction/collision between the North China block and SCB. 相似文献
5.
Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ18O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma. 相似文献
6.
Zircon U–Pb SHRIMP, petrographical and geochemical data lead to the first characterization of the Tonian plutonism (Salto da Divisa Granite Suite), ascribed to the continental rift stage of the precursor basin of the Araçuaí Orogen (Eastern Brazil). The suite includes batholitic plutons and comprises mainly fluorite-bearing, dominantly mesoperthitic hornblende–biotite leucogranites. The presence of mafic (tholeiitic) gabbroic enclaves and syn-plutonic dykes confers to the suite a bimodal character. The plutons were locally deformed and foliated under amphibolite facies conditions, in response to the Neoproterozoic collage of the Araçuaí Orogen against the São Francisco Cratonic margin. However, undeformed magmatic facies are well preserved at inner portions of the plutons. The granitoids are metaluminous, with high SiO2 and HFSE: Nb, Zr, Y, Ta and REE (except Eu); low CaO, Al2O3, Sc, Ba, Sr; high FeOt/MgO ratios, characterizing a chemical signature akin to the subalkaline, A-2 type granites. U–Pb SHRIMP data obtained on zircons from the main pluton yielded a magmatic crystallization age of 875 ± 9 Ma. Some inherited xenocrysts revealed ages of ca. 2080 Ma, corresponding to ages of the host rocks, a Paleoproterozoic basement. Nd isotopic evolution studies confirm the Paleoproterozoic influence on magma genesis with a TDM model age of ca. 1.6 Ga and εNd of − 5.58 at 880 Ma. The African counterpart, the West Congo Belt, encompasses thick rift-related alkaline volcanic-sedimentary basin (Zadinian and Mayumbian groups, and associated anorogenic granites), dated in the interval of ca. 1000–900 Ma. The age differences between the Salto da Divisa Suite intrusion and the anorogenic magmatic episode at the West Congo Belt suggests a westward migration (i.e. to the Brazilian side) of the thermal axis of the rift, ca. 30 Ma after the ending of the extensional process in Africa. 相似文献
7.
At Telões, a subaluminous medium- to coarse-grained porphyritic biotite granite, crops out along the Vila Real NNE–SSW fault. It is a post-tectonic granite of 299±3 Ma old given by U–Pb isotopic data on zircon. It contains metaluminous to subaluminous tonalitic, granodioritic and monzogranitic enclaves. All granitoids have Fe2+-biotite and some enclaves contain magnesiohornblende and subsolidus actinolite. Monzogranitic enclaves show obvious similarities to the host granite. Linear array between enclaves and host granite is observed in Rb–Sr typical isochron diagram and gives the age of 286±11 Ma and (87Sr/86Sr)0=0.7063±0.0011. Microgranular enclaves have δ18O values similar to those of the hosting granite. Microgranular enclaves are hybrid rocks probably formed by mixing between a tonalitic enclave magma and a host granite magma as supported by the modelling of major and trace elements. The similar isotopic signatures suggest a subsequent partial equilibration of the enclaves and granite magmas. 相似文献
8.
The major and trace elements and Sr–Nd–Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at 43 Ma at shallow depths ( 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO2 = 56.98–72.67 wt.%; Mg# = 36.8–50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO2 = 53.21–60.94 wt.%; Mg# = 44.4–53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb)N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd–Sr isotopic compositions for the host rocks are εNd(43 Ma) = − 0.6 to 0.8 and mostly ISr = 0.70482–0.70548. The Nd model ages (TDM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are (206Pb/204Pb) = 18.60–18.65, (207Pb/204Pb) = 15.61–15.66 and (208Pb/204Pb) = 38.69–38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with ISr ranging from 0.70485 to 0.70517, εNd(43 Ma) − 0.1 to 0.8 and with Pb isotopic ratios of (206Pb/204Pb) = 18.58–18.64, (207Pb/204Pb) = 15.60–15.66 and (208Pb/204Pb) = 38.64–38.77. The MMEs have TDM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr–Nd isotope data indicates that 77–83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime. 相似文献
9.
The boundary between the Archean cratons and the Eastern Ghats Belt in peninsular India represents a rifted Mesoproterozoic continental margin which was overprinted by a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats granulites over the cratonic foreland. The contact zone contains a number of deformed and metamorphosed nepheline syenite complexes of rift-related geochemical affinities. In addition to the nepheline-bearing rocks, metamorphosed quartz-bearing monzosyenitic bodies can also be identified along the suture in the region between the Godavari-Pranhita graben and the Prakasam Igneous Province. One such occurrence at Jojuru near Kondapalle is geochemically comparable to the nepheline syenites and furnishes a weighted mean concordant U–Th–Pb SHRIMP zircon age of 1263 ± 23 Ma (2σ), which provides a lower age bracket for the rift-related magmatic activity. The original igneous mineral assemblage in the monzosyenite was partially replaced by the formation of coronitic garnet during the Pan-African metamorphism of the rocks. P–T estimates of garnet corona formation at the interface between clinopyroxene–orthopyroxene–ilmenite clusters and plagioclase indicate mid to upper amphibolite facies condition (5.5–7.0 kbar and 600–700 °C) during the thrust induced deformation and metamorphism associated with the Pan-African collisional tectonics. 相似文献
10.
Robert Anczkiewicz Jacek Szczepa&#x;ski Stanis&#x;aw Mazur Craig Storey Quentin Crowley Igor M. Villa Matthew F. Thirlwall Teresa E. Jeffries 《Lithos》2007,95(3-4):363-380
Combining Lu–Hf garnet geochronology with in situ trace element analyses in garnet allowed us to gain new insight into the metamorphic evolution of UHP–UHT rocks in the Stary Gierałtów region, in the Polish Sudetes. Prograde garnet growth recorded by Rayleigh-type heavy REE (HREE) zoning in the felsic granulites indicates that the obtained 386.6 ± 4.9 Ma Lu–Hf age represents the time of garnet crystallization on a prograde UHP metamorphic path. The surrounding rocks were metamorphosed at the same time as indicated by 381.2 ± 6.7 Ma Sm–Nd garnet age obtained for the mid-crustal metapelites. The second metamorphic episode, which affected most of the lower crust in the Orlica–Śnieżnik Massif (OSM) occurred at ca. 340 Ma as determined by U–Pb zircon and Sm–Nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event, which took place on a retrograde path.
Trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and HREE in garnets from mafic and felsic protoliths over the course of the metamorphic evolution. This had strong impact on the isotopic dating results and led to “decoupling” of the Sm–Nd and Lu–Hf clocks, which recorded timing of the two different metamorphic episodes separated by as much as 40 Ma. Moreover, the preservation of the HREE growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 °C implies that the Lu–Hf system under relatively dry conditions does not undergo significant diffusional re-equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth. Under hydrous conditions, at least some resetting will take place, as documented by the partially relaxed HREE zonation profile in the amphibolitised mafic granulite, which yielded a 10 Ma younger age. The HREE distribution study appeared to be a particularly valuable and essential tool, which allowed us to distinguish garnet growth from post-growth complexities and hence, provide improved age interpretation. Medium REE, on the other hand, did not show any obvious correlation with the isotopic signature of garnet.
Two distinct metamorphic episodes recorded in the Stary Gierałtów region show that buoyancy-driven uplift of UHP rocks can be arrested at the base of a continental crust if not supported by any additional force. In our case study, the UHP rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime. The multistage, discontinuous uplift revealed by the UHP rocks of the OSM provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast-track exhumation histories recognized in UHP terranes elsewhere. 相似文献
11.
The Variscan Hauzenberg pluton consists of granite and granodiorite that intruded late- to postkinematically into HT-metamorphic rocks of the Moldanubian unit at the southwestern margin of the Bohemian Massif (Passauer Wald). U–Pb dating of zircon single-grains and monazite fractions, separated from medium- to coarse-grained biotite-muscovite granite (Hauzenberg granite II), yielded concordant ages of 320 ± 3 and 329 ± 7 Ma, interpreted as emplacement age. Zircons extracted from the younger Hauzenberg granodiorite yielded a 207Pb–206Pb mean age of 318.6 ± 4.1 Ma. The Hauzenberg granite I has not been dated. The pressure during solidification of the Hauzenberg granite II was estimated at 4.6 ± 0.6 kbar using phengite barometry on magmatic muscovite, corresponding to an emplacement depth of 16-18 km. The new data are compatible with pre-existing cooling ages of biotite and muscovite which indicate the Hauzenberg pluton to have cooled below T = 250–400 °C in Upper Carboniferous times. A compilation of age data from magmatic and metamorphic rocks of the western margin of the Bohemian Massif suggests a west- to northwestward shift of magmatism and HT/LP metamorphism with time. Both processes started at > 325 Ma within the South Bohemian Pluton and magmatism ceased at ca. 310 Ma in the Bavarian Oberpfalz. The slight different timing of HT metamorphism in northern Austria and the Bavarian Forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer. 相似文献
12.
Spinel granulite in Odesan area, South Korea: Tectonic implications for the collision between the North and South China blocks 总被引：4，自引：0，他引：4
Spinel granulite formed in the Fe–Al-rich layers in migmatitic gneiss adjacent to a late Paleozoic collision-related mangerite intrusion in the Odesan area, eastern Gyeonggi Massif, South Korea, contains the high-temperature (HT) assemblage Crd + Spl + Crn. Spinel and cordierite compositions indicate peak metamorphic conditions of 914–1157 °C. Retrograde metamorphism reached amphibolite facies where garnet and cordierite broke down to biotite, sillimanite and quartz. These conditions, and the reactions inferred from mineral textures, are consistent with a clockwise P–T path. Metamorphic zircon overgrowths in the spinel granulite and enclosing migmatitic gneiss, dated by SHRIMP U–Pb, yield Permo-Triassic ages of 245 ± 10 and 248 ± 18 Ma respectively, consistent with the metamorphism being a product of the late Paleozoic collision between the North and South China blocks within South Korea. The zircon core ages and textures suggest that the ultimate source of the spinel granulite was a Paleoproterozoic (1852 ± 14 Ma) igneous rock. The protolith of the host migmatitic gneiss was a sediment derived principally from 2.49, 2.16 and 1.86 Ga sources. The age and conditions of spinel granulite metamorphism are similar to those of spinel-bearing granulite in the Higo terrane in west-central Kyushu (250 Ma, > 950 °C at 8–9 kbar), consistent with a continuation of the Dabie-Sulu collision zone into Japan through the Odesan area. 相似文献
13.
A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution
Christoph Bauer Daniela Rubatto Kurt Krenn Alexander Proyer Georg Hoinkes 《Lithos》2007,99(3-4):207-228
Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains.
The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event.
The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization.
The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean. 相似文献
14.
Xenoliths collected from Prindle volcano, Alaska (Lat. 63.72°N; Long. 141.82°W) provide a unique opportunity to examine the lower crust of the northern Canadian Cordillera. The cone's pyroclastic deposits contain crustal and mantle-derived xenoliths. The crustal xenoliths include granulite facies metamorphic rocks and charnockites, comprising orthopyroxene (opx)–plagioclase (pl)–quartz (qtz) ± mesoperthite (msp) and clinopyroxene (cpx). Opx–cpx geothermometry yields equilibrium temperatures (T) from 770 to 1015 °C at 10 kbar. Pl–cpx–qtz geobarometry yields pressures (P) of 6.6–8.0 kbar. Integrated mesoperthite compositions suggest minimum temperatures of 1020–1140 °C at 10 kbar using solvus geothermometry. The absence of garnet in these rocks indicates a range of maximum pressure of 5–11.3 kbar, and calculated solidi constrain upper temperature limits. We conclude that the granulite facies assemblages represent relatively dry metamorphism at pressures indicative of crustal thicknesses similar to present day ( 36 km). Zircon separates from a single crustal xenolith yield mainly Early Tertiary (48–63 Ma) U–Pb ages which are considerably younger than the cooling ages of the high-pressure amphibolites exposed at the surface. The distribution of zircon ages is interpreted as indicating zircon growth coincident with at least two different thermal events as expressed at surface: (i) the eruption of the Late Cretaceous Carmacks Group volcanic rocks in western Yukon and adjacent parts of Alaska, and (ii) emplacement of strongly bimodal high level intrusions across much of western Yukon and eastern Alaska possibly in an extensional tectonic regime. The distributions of zircon growth ages and the preservation of higher-than-present-day (> 25 ± 3 °C km− 1) geothermal gradients in the granulite facies rocks demonstrate the use of crustal xenoliths for recovering records of past, lithospheric-scale thermal–tectonic events. 相似文献
15.
Hong-Fei Zhang Randall Parrish Li Zhang Wang-Chun Xu Hong-Lin Yuan Shan Gao Quentin G. Crowley 《Lithos》2007,97(3-4):323-335
The Songpan–Garze fold belt covers a huge triangular area (> 200,000 km2), confined by the South China (Yangtze), North China and Tibetan Plateau continental blocks. In the Songpan–Garze fold belt, Triassic adakitic granitoids have been identified. However, whether there are Triassic A-type granites is unclear. Here, we report our first finding of an A-type granite (Nianbaoyeche), which occurs in the central part of the Songpan–Garze fold belt. The Nianbaoyeche granite ( 820 km2) is characterized by arfvedsonite in its mineral assemblage. Using both LA-ICPMS and TIMS U–Pb zircon dating methods, we obtain a magma crystallization age of 211 ± 1 Ma, which is slightly younger than Triassic adakitic granitoids (216–221 Ma) in the Songpan–Garze fold belt. The Nianbaoyeche granite is enriched in Si, K, Na, Rb, REE, HFSE (Nb, Ta, Zr, Hf), with elevated FeOtot/(FeOtot + MgO) and Ga/Al ratios, but is depleted in Al, Mg, Ca, Ba and Sr. The REE compositions show moderately fractionated patterns with (La/Yb)N = 2.67–7.54 and Eu/Eu = 0.09–0.34. These geochemical characteristics indicate that the Nianbaoyeche granite has an A-type affinity. Geochemical data and U–Pb zircon age, combined with regional studies, show that the Nianbaoyeche granite formed in a post-collisional tectonic setting. Sr–Nd isotopic data for the granite exhibit ISr = 0.7090–0.7123 and εNd(t) = − 2.72 to − 4.26 with TDM = 1.15–1.51 Ga, suggesting that the magma has a dominantly crustal source, though a minor contribution from the mantle cannot be ruled out. Melting to produce an A-type granite may have resulted from Triassic lithospheric delamination after Triassic crustal thickening of the Songpan–Garze fold belt due to convergence between the Yangtze, North China and North Tibet continental blocks. The lithospheric delamination model also helps to explain the Triassic adakitic magma generation in the Songpan–Garze belt. We conclude that association of A-type granite and adakitic granitoids in post-collisional environment could be a useful indicator of lithospheric delamination. 相似文献
16.
Dicheng Zhu Xuanxue Mo Guitang Pan Zhidan Zhao Guochen Dong Yuruo Shi Zhongli Liao Liquan Wang Changyong Zhou 《Lithos》2008,100(1-4):147-173
The relationship between the breakup of eastern Gondwanaland and the Kerguelen plume activity is a subject of debate. The Cona mafic rocks are widely exposed in the Cona area of the eastern Himalaya of south Tibet, and are studied in order to evaluate this relationship. Cona mafic rocks consist predominantly of massive basaltic flows and diabase sills or dikes, and are divided into three groups. Group 1 is composed of basaltic flows and diabase sills or dikes and is characterized by higher TiO2 and P2O5 content and OIB-like trace element patterns with a relatively large range of Nd(T) values (+ 1.84 to + 4.67). A Group 1 diabase sill has been dated at 144.7 ± 2.4 Ma. Group 2 consists of gabbroic sills or crosscutting gabbroic intrusions characterized by lower TiO2 and P2O5 content and “depleted” N-MORB-like trace element patterns with relatively higher, homogeneous Nd(T) values (+ 5.68 to + 6.37). A Group 2 gabbroic diabase dike has been dated at 131.1 ± 6.1 Ma. Group 3 basaltic lavas are interbedded with the Late Jurassic–Early Cretaceous pelitic sediments; they have compositions transitional between Groups 1 and 2 and flat to slightly enriched trace element patterns. Sr–Nd isotopic data and REE modeling indicate that variable degrees of partial melting of distinct mantle source compositions (enriched garnet–clinopyroxene peridotite for Group 1 and spinel-lherzolite for Group 2, respectively) could account for the chemical diversity of the Cona mafic rocks. Geochemical similarities between the Cona mafic rocks and the basalts probably created by the Kerguelen plume based on spatial–temporal constraints seem to indicate that an incubating Kerguelen plume model is more plausible than a model of normal rifting (nonplume) for the generation of the Cona mafic rocks. Group 1 is interpreted as being related to the incubating Kerguelen plume–lithosphere interaction; Group 2 is likely related to an interaction between anhydrous lithosphere and rising depleted asthenosphere enriched by a “droplet” originating from the Kerguelen plume, while Group 3 may be attributed to thermal erosion resulting in the partial melting of lithosphere during the long-term incubation of a magma chamber/pond at a shallow crustal level. The Cona mafic rocks are probably related to a progressively lithospheric thinning beneath eastern Gondwanaland from 150–145 Ma to 130 Ma. Our new observations seem to indicate that the Kerguelen plume may have started its incubation as early as the latest Jurassic or earliest Cretaceous period and that the incubating Kerguelen plume may play an active role in the breakup of Greater India, eastern India, and northwestern Australia. 相似文献
17.
Michel Faure Alain Cocherie Eugne B Mzme Nicolas Charles Philippe Rossi 《Gondwana Research》2010,18(4):369
In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pbtot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale. 相似文献
18.
Simone Tumiati Gaston Godard Silvana Martin Urs Kltzli Damiano Monticelli 《Lithos》2007,94(1-4):148-167
The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones. 相似文献
19.
Y.J. Bhaskar Rao S.P. Kelley N.B.W. Harris B.L. Narayana C. Srikantappa 《Gondwana Research》2006,10(3-4):357-362
The age of pseudotachylite formation in the crustal-scale Cauvery Shear Zone system of the Precambrian Southern Granulite Terrain (South India) has been analyzed by laser-probe 40Ar–39Ar dating. Laser spot analyses from a pseudotachylite from the Salem–Attur shear zone have yielded ages ranging from 1214 to 904 Ma. Some evidence for the presence of excess Ar is indicated by the scatter of ages from this locality. The host gneiss preserves Palaeoproterozoic Rb–Sr whole rock–biotite ages (2350 ± 11 to 2241 ± 11 Ma). A mylonite in the Koorg shear, ca. 200 km to the north, yielded an age of 895 ± 17 Ma the consistency of the age distribution from spot analyses precludes the presence of significant excess Ar. Despite published evidence for the growth of high-grade minerals within some components of the Cauvery Shear Zone during the Pan-African event (700–550 Ma), the pseudotachylites in this study provide no evidence for Pan-African formation. Instead they document the first evidence for Mesoproterozoic tectonism in the Cauvery Shear Zone system, thus prompting a review of the correlation between the Cauvery Shear Zone system and the large-scale shear zones located elsewhere in eastern Gondwana. 相似文献
20.
Geologic and metamorphic evolution of the basement complexes in the Kontum Massif, central Vietnam 总被引：3，自引：0，他引：3
N. Nakano Y. Osanai M. Owada Tran Ngoc Nam T. Toyoshima P. Binh T. Tsunogae H. Kagami 《Gondwana Research》2007,12(4):438-453
This paper presents a regional scale observation of metamorphic geology and mineral assemblage variations of Kontum Massif, central Vietnam, supplemented by pressure–temperature estimates and reconnaissance geochronological results. The mineral assemblage variations and thermobarometric results classify the massif into a low- to medium-temperature and relatively high-pressure northern part characterised by kyanite-bearing rocks (570–700 °C at 0.79–0.86 GPa) and a more complex southern part. The southern part can be subdivided into western and eastern regions. The western region shows very high-temperature (> 900 °C) and -pressure conditions characterised by the presence of garnet and orthopyroxene in both mafic and pelitic granulites (900–980 °C at 1.0–1.5 GPa). The eastern region contains widespread medium- to high-temperature and low-pressure rocks, with metamorphic grade increasing from north to south; epidote- or muscovite-bearing gneisses in the north (< 700–740 °C at < 0.50 GPa) to garnet-free mafic and orthopyroxene-free pelitic granulites in the south (790–920 °C at 0.63–0.84 GPa). The Permo-Triassic Sm–Nd ages (247–240 Ma) from high-temperature and -pressure granulites and recent geochronological studies suggest that the south-eastern part of Kontum Massif is composed of a Siluro-Ordovician continental fragment probably showing a low-pressure/temperature continental geothermal gradient derived from the Gondwana era with subsequent Permo-Triassic collision-related high-pressure reactivation zones. 相似文献