共查询到20条相似文献，搜索用时 31 毫秒
1.
Thomas Pettke Andreas Audtat Urs Schaltegger Christoph A. Heinrich 《Chemical Geology》2005,220(3-4):191-213
The Sn–W mineralized Mole Granite in Eastern Australia hosts zircon populations that crystallized at several stages during a protracted magmatic to hydrothermal evolution. Thirty-four elements have been quantified by laser-ablation inductively-coupled-plasma mass-spectrometric microanalysis with the aim of relating the chemistry of zircon to its growth environment. Trace element contents are highly variable for all textural occurrences. Zircon inclusions in earliest quartz phenocryst suggest that zircon was a liquidus phase that crystallized probably deep in the crust. Trace element contents are conspicuously high, showing only a slight positive Ce anomaly but a pronounced negative Eu-anomaly. Successive crystallization stages of magmatic zircon are characterized by progressive depletion in trace element contents, notably the rare earth elements, with an increasingly important positive Ce-anomaly. This evolution reflects saturation of REE accepting minerals such as monazite, thorite, xenotime and possibly apatite and is affected little by the exsolution of a magmatic–hydrothermal fluid. Zircon that is interpreted to have precipitated from aqueous fluids in Sn–W-bearing quartz veins shows REE patterns indistinguishable from those of late magmatic zircon. When combined with experimental evidence on the fluid–melt partitioning of REE, it indicates that the REE distribution coefficients for zircon/melt and zircon/fluid are largely comparable.
The second example of hydrothermal zircon crystallized some 2 My after the host granite. These crystals reveal an intragranular zonation of increasing trace element concentrations from core to rim. Therefore, REE abundances and patterns alone are not conclusive indicators of the geological environment in which zircon crystallized. Nevertheless, variations in trace element contents of zircon that relate to the chemistry of the melt or fluid from which zircon crystallized, as measured in cogenetic melt and fluid inclusions, are promising for future petrogenetic modeling.
Lead and Cs are strongly incompatible in hydrothermal zircon, with estimated zircon–fluid distribution coefficients D ≤ 0.001, while Sn and Li are moderately incompatible, DSn 0.6 and DLi 0.1, and Ce is compatible, DCe 14. Moreover, hydrothermal zircon has a more pronounced negative Eu-anomaly and higher Ta/Nb and U/Th ratios than the magmatic zircons of the Mole Granite. 相似文献
2.
After a decade of studies and development, it is now accepted that reliable U–Th–total Pb isochron ages can be calculated for monazite using an electron microprobe at μm scale, either directly on thin sections or on separated grains mounted in polished section. The potential for determining U–Th–Pb chemical ages from other U- and Th-enriched phases has been investigated compared to chemical monazite-dating results for which individual spot-age precisions of 20 to 100 Ma can be achieved from individual spot analyses. Using isochron plots for monazite, the age homogeneity of a given population of data can be assessed and, depending upon the number of analyses (n 50), a precision of 5 to 10 Ma can be obtained. The U content in xenotime widely varies from less than 0.1 wt.% up to 3 wt.%, but Th rarely exceeds 1 wt.%. As a consequence, the amount of radiogenic Pb produced during a given period remains significantly lower for xenotime than for monazite, leading to a lower precision (± 20 Ma) on the mean ages. Xenotime, however, appears to remain as a closed system, but common Pb must be carefully checked. Furthermore, the electron-microprobe technique (EPMA) allows controlling any age discrepancy on xenotime grains as small as 10–20 μm that cannot be dated by other isotopic methods. Such xenotime ages can be useful when studying the monazite–xenotime equilibrium. The electron microprobe is not the most reliable method for dating zircon since U and Th concentrations are generally low and common Pb is not negligible. Nevertheless, the spatial resolution of EPMA coupled with isotope methods allows conclusive in situ studies about radiogenic Pb mobility and metamictization. Thorite does not seem suitable for dating with either isotope methods or EPMA because of continuous radiogenic Pb loss. Conversely, the oxide phases, thorianite and baddeleyite are robust minerals with closed systems. They are rather rare and seem to incorporate negligible common Pb, making EPMA a method of choice for dating them. For thorianite, the precision on the mean age can be similar as that obtained for monazite, or even better, while the precision for baddeleyite cannot be significantly better than 20 to 50 Ma due to the limited amount of U ( 0.1%) and the lack of Th. 相似文献
3.
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
4.
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
5.
Zircon U–Pb ages and trace elements were determined for granulites and gneiss at Huangtuling, which are hosted by ultrahigh-pressure metamorphic rocks in the Dabie Orogen, east-central China. CL images reveal core–rim structure for most zircons in the granulites. The cores show oscillatory zoning, relatively high Th/U ratios, and HREE enriched patterns, consistent with a magmatic origin. They gave a weighted mean 207Pb/206Pb age of 2766 ± 9 Ma, interpreted as dating magma emplacement of the protolith. The rims are characterized by sector or planar zoning, low Th/U ratios, negative Eu anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U–Pb dating yields a weighted mean 207Pb/206Pb age of 2029 ± 13 Ma, which is interpreted to record a metamorphic event, possibly during assembly of the supercontinent Columbia. The gneiss has a protolith age of 1982 ± 14 Ma, which is younger than the zircon age of the granulite-facies metamorphism, suggesting a generally delay between HT metamorphism and the intrusion of post-collisional granites. A few inherited cores with igneous characteristics have 207Pb/206Pb ages of 2.90, 3.28 and 3.53 Ga, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants in the Yangtze Craton. A few Cretaceous metamorphic ages were also obtained, suggesting the influence of post-collisional collapse in response to Cretaceous extension of the Dabie Orogen. It is inferred that the recently discovered Archean basement of the Yangtze Craton occurs as far north as the Dabie Orogen. 相似文献
6.
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
7.
Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A (δ18O = 7 ± 1‰) preserves the δ18O value of the magmatic core (6.2 ± 0.8‰), whereas rim B is characterised by higher δ18O values (7.8 ± 1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ18O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ18O values measured in zircon overgrowths (11.8 ± 1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4 ± 1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ18O values are similar in garnet cores (14.8 ± 0.2‰) and in zircon metamorphic overgrowths (14.2 ± 0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event.
As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis. 相似文献
8.
Joseph M. Pyle 《Lithos》2006,88(1-4):201-232
Analysis of monazite-bearing lithologies from the Precambrian Honey Brook Upland (HBU) and overlying metasedimentary Paleozoic Chester Valley Sequence (CVS) (SE PA, USA) reveals overprinting of primary major and accessory phase parageneses by texturally and compositionally disparate secondary accessory phase parageneses. Two-pyroxene temperatures of 915–945 °C for reconstituted pyroxene reflect emplacement temperatures of felsic plutonic rocks (opdalite, charnockite) prior to Mesoproterozoic metamorphism. Monazite in metavolcanic felsic gneiss yields three age domains at 1009 ± 4 Ma (2 s.e.), 965 ± 6, and 876 ± 10 Ma. The first two domains record metamorphism of the HBU after anorthosite intrusion; peak monazite–xenotime temperatures for the monazite core domain are 700 °C, and high Th/U values in the second (overgrowth) age domain likely reflect a second high-T monazite growth episode. Formation of cummingtonite coronas on orthopyroxene in opdalite constrains maximum 1010 Ma metamorphic temperatures in the “granulite-facies” terrane to 730–740 °C. Evidence of increased Cl fluid activity in the 965 Ma metamorphism includes higher Cl content of matrix apatite relative to garnet-included apatite (metavolcanics), and Cl-bearing K-hornblende succeeding cummingtonite in coronal overgrowths (opdalite). Extreme monazite Th/U values (75–250) in the rim domain suggest growth during low-T hydrothermal alteration. In the opdalite, secondary singe-grain monazite and monazite + xenotime metasomites in apatite yield ages of 714 ± 24 and 586 ± 88 Ma, temperatures of 325–425 °C, and are interpreted to reflect thermal disturbances associated with late Proterozoic plutonic and volcanic activity in the Upland. This thermal disturbance may be recorded by Rb–Sr age of 567 Ma for biotite from a HBU gneiss. Monazite age domains in metaquartzite (378 ± 28, 272 ± 44 Ma) suggest that low-grade metamorphism (260–320 °C, Mnz–Xno thermometry) of the CVS is not a result of Taconian orogenesis. 相似文献
9.
Ian S. Buick Jrg Hermann Ian S. Williams Roger L. Gibson Daniela Rubatto 《Lithos》2006,88(1-4):150-172
The Central Zone of the Limpopo Belt (South Africa) underwent high-grade metamorphism at 2.7–2.5 and 2.03 Ga. Quartz-rich, garnet-, cordierite-, biotite- and orthoamphibole-bearing, feldspar-free gneisses from the western Central Zone reached granulite-facies conditions (800 °C at 8–10 kbar) followed by decompression. Garnet from one such sample shows significant zonation in trace elements but little zonation in major elements. Zoning patterns suggest that the early prograde breakdown of REE-rich accessory phases contributed to the garnet trace element budget. Monazite from the sample yields a SHRIMP weighted mean 207Pb–206Pb age of 2028 ± 3 Ma, indistinguishable from a SHRIMP zircon age of 2022 ± 11 Ma previously measured on metamorphic overgrowths on 2.69 Ga igneous zircon cores. New zircon and monazite formed before, or at, the metamorphic peak, and occur as inclusions in garnet. Monazite appears to have formed through the breakdown of early allanite ± xenotime ± apatite. Trace element zoning patterns in garnet and the age of accessory phases are most consistent with a single tectonometamorphic event at 2.03 Ga.
The plagioclase and K-feldspar-free composition of the garnet–cordierite–orthoamphibole gneisses requires open system processes such as intense hydrothermal alteration of protoliths or advanced chemical weathering. In the studied sample, the 2.69 Ga igneous zircons show a prominent negative Eu anomaly, suggesting equilibrium with plagioclase, or plagioclase fractionation in the precursor magma. In contrast, the other minerals either show small negative (2.03 Ga monazite), no (2.02 Ga zircon and garnet) or positive Eu anomalies (orthoamphibole). This suggests that the unusual bulk compositions of these rocks were set in after 2.69 Ga but before the peak of the 2.03 Ga event, most probably while the protoliths resided at shallow or surficial crustal levels. 相似文献
10.
The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial 87Sr/86Sr ratios (0.707–0.708) and inhomogeneous negative Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ18O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial 87Sr/86Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.
Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level. 相似文献
11.
Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France 总被引：4，自引：0，他引：4
U–Th–Pb dating of monazite with the electron probe microanalyser (EPMA) is increasingly documented as a reliable geochronological method offering high spatial resolution. This method has been applied on monazite from the Cévennes migmatites and granitoids from the southeast of the French Massif Central. Measurements were performed on separated grains after systematic back-scattered electron (BSE) imaging. Monazites from migmatites record two main ages: (i) a protolith age of about 550–543 Ma obtained on inherited cores, and (ii) a migmatization event between 329 ± 5 and 323 ± 3 Ma recorded by monazite rims and all other monogenetic grains. Monazite from the peraluminous Rocles pluton yields a 318 ± 3 Ma age. Finally, three granite dykes are dated at 333 ± 6, 318 ± 5 and 311 ± 5 Ma; the older dyke is the most deformed of them and is interpreted as linked to the migmatization event; the two other dykes are geochronologically, petrologically and structurally coeval with the Rocles pluton. The data constrain the timing of crustal melting following Variscan thickening in the northern Cévennes area. Migmatization of Ordovician protoliths took place at 329–323 Ma and was shortly followed by intrusion of leucogranite at 318–311 Ma. The study shows that EPMA dating of monazite can be successfully used to resolve a close succession of regional melting events. 相似文献
12.
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
13.
Monazite [(LREE)PO4], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements.
The U–Th–Pb systematic was applied to monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of monazite from a variety of settings and ages. The three independent 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant monazite as external standard.
This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise 208Pb/232Th, 206Pb/238U and 207Pb/235U ratios for dating Palaeozoic to Precambrian monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for 208Pb/232Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context. 相似文献
14.
In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U–Pb zircon and monazite geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313±2 Ma, 319±4 Ma, 316±2 Ma and 314±2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309±3 Ma for the Braga granite, 309±1 Ma for the Gonça granite and 311±5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeirós plutons (307±3.5 Ma and 306±2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300±1 Ma), followed by the emplacement of the Peneda–Gerês massif (Gerês, Paufito, Illa and Carris granites). The Gerês granite, emplaced at 296±2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290±2.5 Ma. For the Carris granite a minimum emplacement age of 280±5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313–319 Ma; (2) late-D3 biotite-dominant granitoids, 306–311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290–296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300–320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D4) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290–296 Ma. 相似文献
15.
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
16.
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
17.
SHRIMP U–Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianópolis Batholith, southern Brazil. The results, supported by SEM backscattered and cathodoluminescence imagery, identify inherited zircon populations and confirm the long-lived, crustal recycling processes responsible for the accretion of the batholith. The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations, and enables discrimination to be made between inherited cores and melt-precipitated overgrowths. New dating of two post-tectonic plutons (samples 1 and 2) revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths. The U/Pb isotopic results from both samples spread along concordia by ca. 40 m.y. (sample 1) to 100 m.y. (sample 2), clustering in two closely spaced (bimodal), partially overlapping peaks. Melt-precipitated rims and homogeneous new grains, dated at ca. 600 Ma, furnish the crystallisation age of the plutons. The magmatic textured cores and xenocrysts dated at ca. 630–620 Ma are interpreted as inherited restitic material from supposedly short-lived (meta)granitic sources. The reassessment of previous SHRIMP data of two banded anatectic granitoids (samples 3 and 4) revealed more complex morphological patterns, in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims, dated at ca. 600 Ma and 592±2 Ma, respectively. Major populations of magmatic-textured inherited cores dated at 2006±3 Ma and 2175±13 Ma characterise samples 3 and 4, respectively. The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages (ca. 2900–620 Ma), suggesting partial melting of metasedimentary components. The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent (meta)tonalitic gneiss and amphibolitic country-rock (paleosome). The recognition of the (melt-precipitated) Neoproterozoic overgrowths and new crystals, and the restite provenance of the cores, supplants a previous interpretation of Paleoproterozoic magmatism (cores) and Neoproterozoic (solid-state) metamorphic overprint. As a major consequence of the former interpretation, the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianópolis Batholith/arc. 相似文献
18.
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
19.
Fernando Corfu 《Lithos》2000,53(3-4):279-291
Stepwise HF-dissolution experiments on five Archean zircon populations reveal very systematic patterns of Pb discrimination, releasing at one point Pb with artificially too-old 207Pb/206Pb ages. The experiments involved a first HF dissolution step for 1 h at room temperature and evaporation on a hot-plate that produces Pb with young 207Pb/206Pb ages, followed by a 4 1/2 h HF dissolution step in an oven at 190°C that liberates the excessively old Pb. The final residue yields in most cases U–Pb data that are consistent with the results obtained by the normal selection, abrasion, and total dissolution procedure. In these examples, the too-old ages cannot be easily explained by the presence of an inherited core component but are more likely to indicate segregation of Pb in zircon sub-domains during thermal annealing episodes early in its history, as has been proposed by other workers. Aside from shedding light on these particular aspects of zircon U–Pb systematics, the combined results also provide geologically relevant information concerning the regional evolution of the western Superior Province in Ontario. An age of 2718±3 Ma for a gabbroic unit from the Quetico Subprovince shows that this was coeval to 2722–2718 Ma ultramafic to felsic igneous rocks in the adjacent Shebandowan greenstone belt, including a gabbro body dated in this study at 2725+17/−11 Ma. These age relationships suggest that volcanic units of the Shebandowan greenstone belt were tectonically imbricated in younger sedimentary rocks of the Quetico basin during late Archean convergence. The other three samples represent felsic intrusive units from Geraldton in the Wabigoon subprovince. An age of 2699±1 Ma for an Au-mineralized feldspar porphyry dyke and identical ages of 2690±1 Ma for two phases of the syn-tectonic Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt. 相似文献
20.
Emilie Janots Franois Negro Fabrice Brunet Bruno Goff Martin Engi Mohamed L. Bouybaoune 《Lithos》2006,87(3-4):214-234
REE mineralogy was characterized at the micrometer scale (using scanning electron microscopy) in the four tectono-metamorphic units of the Beni Mzala window, Sebtide Complex, Internal Rif, Morocco, which sample a HP-LT metamorphic gradient from subgreenschist to blueschist/eclogite facies. These tectonic units are composed of garnet-free metapelites (and associated synmetamorphic veins) from the same homogeneous aluminium-rich and calcium-poor protolith. In that context, mineralogical differences between units are assumed to result from contrasted pressure and temperature histories. Light REE-bearing Al-phosphates of the crandallite group with florencite-rich composition and variable goyazite content are the dominant LREE minerals in the lowest-grade units, Tizgarine (300 °C, 3 kbar as peak conditions) and Boquete de Anjera (350 °C, 10 kbar). In the latter unit, metamorphic florencite is associated with MREE-rich monazite. In the schist of the highest-grade units, Beni Mzala 2 (420–450 °C, 12 kbar) and Beni Mzala 1 (550 °C, 16 kbar), associations of allanite-rich epidote and synchisite (a LREE-fluorocarbonate) are found whereas florencite and monazite are no longer observed. At the wall of the quartz–kyanite veins, retrograde monazite-(Ce) with grain size of 20 to 50 μm is the only LREE-bearing mineral. Th–Pb and U–Pb SIMS data on some of these grains yield ages of 21.3 ± 1.7 Ma and 20.9 ± 2.1 Ma, respectively, consistent with previous K–Ar ages obtained on retrograde clay–mica mixtures in rocks from the same locality. The identification of a stability field for monazite in high-pressure aluminous metapelites with an upper thermal-limit below 450 °C as well as the derivation of meaningful U–Pb and Th–Pb SIMS ages demonstrates the monazite potential for dating HP-LT metapelites. However, since, under these conditions, monazite growth occurs below its admitted closure-temperature, these ages must be interpreted on the basis of monazite textural relationships. 相似文献