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1.
We present a numerical study of point defects in crystalline zircon (ZrSiO4). Vacancies and interstitials of all the constituents of zircon have been considered. For each defect, the structure and the formation energies have been calculated. Calculations, using the supercell method, are based on the Density Functional Theory in the Local Density Approximation. Empirical potentials have also been considered for comparison with electronic structure results. We find a formation energy for the oxygen interstitial of 1.7 eV. This value is compatible with the experimental activation energy for oxygen diffusion in zircon, which proves an interstitial mechanism for the diffusion of oxygen in zircon. For all other defects the calculated formation energies lead to negligible thermal concentration at equilibrium. Received: 8 January 1999 / Revised, accepted: 14 May 1999  相似文献   

2.
《International Geology Review》2012,54(15):1835-1864
The Yinshan deposit is a large epithermal-porphyry polymetallic deposit, and the timing and petrogenesis of ore-hosting porphyries have been hotly debated. We present new results from geochemical, whole-rock Sr–Nd and zircon U–Pb–Hf–O isotopic investigations. Zircon U–Pb data demonstrate that the quartz porphyry, dacitic porphyry, and quartz dioritic porphyry formed at ?172.2 ± 0.4 Ma, ?171.7 ± 0.5 Ma, and ?170.9 ± 0.3 Ma, respectively. Inherited zircon cores show significant age spreads from ?730 to ?1390 Ma. Geochemically, they are high-K calc-alkaline or shoshonitic rocks with arc-like trace element patterns. They have similar whole-rock Nd and zircon Hf isotopic compositions, yet an increasing trend in ?Nd(t) and ?Hf(t) values typifies the suite. Older (inherited) zircons of the three porphyries display Hf compositions comparable to those of the Jiangnan Orogen basement rocks. In situ zircon oxygen isotopic analyses reveal that they have similar oxygen isotopic compositions, which are close to those of mantle zircons. Moreover, a decreasing trend of δ18O values is present. We propose that the ore-related porphyries of the Yinshan deposit were emplaced contemporaneously and derived from partial melting of Neoproterozoic arc-derived mafic (or ultra-mafic) rocks. Modelling suggests that the quartz porphyries, dacitic porphyries, and quartz dioritic porphyries experienced ?25%, ?10%, and ?10% crustal contaminations by Shuangqiaoshan rocks. Our study provides important constraints on mantle–crust interaction in the genesis of polymetallic mineralization associated with Mesozoic magmatism in southeastern China.  相似文献   

3.
Using empirical potentials derived from fitting the vibrational frequencies of a group of silicates and oxides (not including the mineral under study), a Born-von Karman rigid ion lattice-dynamical model has been applied to the whole Brillouin zone in phenakite (Be2SiO4), a silicate which can be described as a three-dimensional network of tetrahedral SiO4 and BeO4 groups strictly linked together by sharing the oxygen atoms at their corners. The atomic anisotropic displacement parameters (ADPs) derived from accurate crystal-structure refinement at room temperature agree very well with the corresponding theoretical estimates, thereby confirming the validity of the model and the procedure followed. The experimental values of the bond distances corrected for thermal motion show that the observed invariance of the Si-O bond length with respect to temperature is only apparent. Furthermore, the disagreement with the experimental values of thermodynamic functions such as the specific heat C p and entropy at room temperature is 1.3% and 0.9%, respectively, and such a disagreement remains below 3% for a wide range of temperature (200 to 1000 K). As for many oxides and silicates so far examined, the zero-point contribution to the ADPs of the Si and O atoms amounts to about one half of the corresponding value at room temperature, and that of the Be atoms is even higher (60%).  相似文献   

4.
Using empirical potentials derived from fitting the vibrational frequencies of a group of silicates and oxides (not including the mineral under study), a Born-von Karman rigid ion lattice-dynamical model has been applied to the whole Brillouin zone in phenakite (Be2SiO4), a silicate which can be described as a three-dimensional network of tetrahedral SiO4 and BeO4 groups strictly linked together by sharing the oxygen atoms at their corners. The atomic anisotropic displacement parameters (ADPs) derived from accurate crystal-structure refinement at room temperature agree very well with the corresponding theoretical estimates, thereby confirming the validity of the model and the procedure followed. The experimental values of the bond distances corrected for thermal motion show that the observed invariance of the Si-O bond length with respect to temperature is only apparent. Furthermore, the disagreement with the experimental values of thermodynamic functions such as the specific heat C p and entropy at room temperature is 1.3% and 0.9%, respectively, and such a disagreement remains below 3% for a wide range of temperature (200 to 1000 K). As for many oxides and silicates so far examined, the zero-point contribution to the ADPs of the Si and O atoms amounts to about one half of the corresponding value at room temperature, and that of the Be atoms is even higher (60%). Received: 8 December 1997 / Revised, accepted: 8 May 1998  相似文献   

5.
Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form?>?10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.  相似文献   

6.
This contribution evaluates the relation between protracted zircon geochronological signal and protracted crustal melting in the course of polyphase high to ultrahigh temperature (UHT; T?>?900 °C) granulite facies metamorphism. New U–Pb, oxygen isotope, trace element, ion imaging and cathodoluminescence (CL) imaging data in zircon are reported from five samples from Rogaland, South Norway. The data reveal that the spread of apparent age captured by zircon, between 1040 and 930 Ma, results both from open-system growth and closed-system post-crystallization disturbance. Post-crystallization disturbance is evidenced by inverse age zoning induced by solid-state recrystallization of metamict cores that received an alpha dose above 35 × 1017 α  g?1. Zircon neocrystallization is documented by CL-dark domains displaying O isotope open-system behaviour. In UHT samples, O isotopic ratios are homogenous (δ18O = 8.91?±?0.08‰), pointing to high-temperature diffusion. Scanning ion imaging of these CL-dark domains did not reveal unsupported radiogenic Pb. The continuous geochronological signal retrieved from the CL-dark zircon in UHT samples is similar to that of monazite for the two recognized metamorphic phases (M1: 1040–990 Ma; M2: 940–930 Ma). A specific zircon-forming event is identified in the orthopyroxene and UHT zone with a probability peak at ca. 975 Ma, lasting until ca. 955 Ma. Coupling U–Pb geochronology and Ti-in-zircon thermometry provides firm evidence of protracted melting lasting up to 110 My (1040–930 Ma) in the UHT zone, 85 My (ca. 1040–955 Ma) in the orthopyroxene zone and some 40 My (ca. 1040–1000 Ma) in the regional basement. These results demonstrate the persistence of melt over long timescales in the crust, punctuated by two UHT incursions.  相似文献   

7.
Petrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ18O values of ?0.6 to 0.1‰, but a few fall in the range ?5.2 to ?4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.  相似文献   

8.
The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5–4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760–930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051?±?9 Ma (2σ, MSWD?=?0.1) and 2,056?±?5 Ma (2σ, MSWD?=?0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity.  相似文献   

9.
U–Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700–500?Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460?Ma and a Variscan group at 340–335?Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340?Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of?~16 kbar/850?°C followed by isothermal decompression to 8–10 kbar/820–860?°C. In felsic granulite samples, the assemblage K-feldspar–garnet–kyanite–Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of?~13 kbar/925?°C, and a subsequent P–T decrease to 6.5–8.5 kbar/800–820?°C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P–T paths are used to discuss the significance of the U–Pb ages. In the monotonous unit, inherited zircon ages of 700–500?Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U–Pb zircon ages of ca. 340?Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.  相似文献   

10.
We report the results of an experimental calibration of oxygen isotope fractionation between quartz and zircon. Data were collected from 700 to 1000 °C, 10–20 kbar, and in some experiments the oxygen fugacity was buffered at the fayalite–magnetite–quartz equilibrium. Oxygen isotope fractionation shows no clear dependence on oxygen fugacity or pressure. Unexpectedly, some high-temperature data (900–1000 °C) show evidence for disequilibrium oxygen isotope partitioning. This is based in part on ion microprobe data from these samples that indicate some high-temperature quartz grains may be isotopically zoned. Excluding data that probably represent non-equilibrium conditions, our preferred calibration for oxygen isotope fractionation between quartz and zircon can be described by:
This relationship can be used to calculate fractionation factors between zircon and other minerals. In addition, results have been used to calculate WR/melt–zircon fractionations during magma differentiation. Modeling demonstrates that silicic magmas show relatively small changes in δ18O values during differentiation, though late-stage mafic residuals capable of zircon saturation contain elevated δ18O values. However, residuals also have larger predicted melt–zircon fractionations meaning zircons will not record enriched δ18O values generally attributed to a granitic protolith. These results agree with data from natural samples if the zircon fractionation factor presented here or from natural studies is applied.  相似文献   

11.
INTRODUCTIoNlmmobilizationofhigh-levelnuclearwastehaslongbeenaseriousproblemthatthecountries,especiallytheUSAandRussia(Weberetal.,1997,1996),whichhaveanuclearin-dustrytrytosolve.Glasswasteformasthefirstgenerationmaterialhasshownmanyshortcomings(Webertetal-,l997),andcrystallineceramicformshaveappearedorwillappearasthesecondgenerationwastehostform(Hayward,1989).Af-terbeingextensivelystudiedforthepurposeoftheimmobi-liZationinrecentyears(Weberetal.,l997lEwingetal.,l995;Murakamietal.,l986),…  相似文献   

12.
Zircon grains were separated from lunar regolith and rocks returned from four Apollo landing sites, and analyzed in situ by secondary ion mass spectrometry. Many regolith zircons preserve magmatic δ18O and trace element compositions and, although out of petrologic context, represent a relatively unexplored resource for study of the Moon and possibly other bodies in the solar system. The combination of oxygen isotope ratios and [Ti] provides a unique geochemical signature that identifies zircons from the Moon. The oxygen isotope ratios of lunar zircons are remarkably constant and unexpectedly higher in δ18O (5.61 ± 0.07 ‰ VSMOW) than zircons from Earth’s oceanic crust (5.20 ± 0.03 ‰) even though mare basalt whole-rock samples are nearly the same in δ18O as oceanic basalts on Earth (~5.6 ‰). Thus, the average fractionation of oxygen isotopes between primitive basalt and zircon is smaller on the Moon [Δ18O(WR-Zrc) = 0.08 ± 0.09 ‰] than on Earth (0.37 ± 0.04 ‰). The smaller fractionations on the Moon suggest higher temperatures of zircon crystallization in lunar magmas and are consistent with higher [Ti] in lunar zircons. Phase equilibria estimates also indicate high temperatures for lunar magmas, but not specifically for evolved zircon-forming melts. If the solidus temperature of a given magma is a function of its water content, then so is the crystallization temperature of any zircon forming in that melt. The systematic nature of O and Ti data for lunar zircons suggests a model based on the following observations. Many of the analyzed lunar zircons are likely from K, rare earth elements, P (KREEP)-Zr-rich magmas. Zircon does not saturate in normal mafic magmas; igneous zircons in mafic rocks are typically late and formed in the last most evolved portion of melts. Even if initial bulk water content is moderately low, the late zircon-forming melt can concentrate water locally. In general, water lowers crystallization temperatures, in which case late igneous zircon can form at significantly lower temperatures than the solidus inferred for a bulk-rock composition. Although lunar basalts could readily lose H2 to space during eruption, lowering water fugacity; the morphology, large size, and presence in plutonic rocks suggest that many zircons crystallized at depths that retarded degassing. In this case, the crystallization temperatures of zircons are a sensitive monitor of the water content of the parental magma as well as the evolved zircon-forming melt. If the smaller Δ18O(zircon–mare basalt) values reported here are characteristic of the Moon, then that would suggest that even highly evolved zircon-forming magmas on the Moon crystallized at higher temperature than similar magmas on Earth and that magmas, though not necessarily water-free, were generally drier on the Moon.  相似文献   

13.
北淮阳新开岭地区花岗岩锆石U-Pb年龄和氧同位素组成   总被引：9，自引：2，他引：7  
对大别造山带北麓的北淮阳新开岭地区岩浆岩进行了锆石阴极发光显微结构观察和SHRIMP法锆石微区UPb定年.在锆石阴极发光图像中, 一个花岗岩样品中的大部分锆石颗粒具有明显的初始岩浆振荡环带, 为典型的岩浆锆石, 少有蚀变的颗粒和/或区域; 而另一个花岗岩样品中的锆石虽然同样具有振荡环带, 但是大部分颗粒中心的初始岩浆环带被扰动, 指示这些锆石为岩浆锆石, 受到了较强的后期热液蚀变的改造.对锆石具有初始岩浆环带和溶蚀结构的区域分别进行SHRIMP法UPb微区定年结果表明, 这些岩浆岩的形成年龄为(820±4) Ma, 热液蚀变作用发生的时间为(780±4) Ma.新开岭地区新元古代花岗质岩石的形成和后期超固相热液蚀变作用分别对应于超大陆裂解之前的约830~795Ma岩浆活动和裂解过程中约780~745Ma的岩浆作用.单矿物激光氟化氧同位素分析结果表明, 这些岩浆岩具有非常低的δ18O值, 其中锆石为1.90‰~5.78‰, 石英为-2.88‰~-7.67‰, 斜长石为-4.01‰~-11.40‰.锆石和其他矿物之间表现出强烈的氧同位素不平衡, 而其他矿物之间则达到了氧同位素的再平衡.结合不同δ18O值锆石的内部结构特征, 认为该地区的热液蚀变作用为超固相条件下的高温热液蚀变.这一过程不但改变了石英等矿物的氧同位素组成, 同时也不同程度地改变了锆石的氧同位素组成, 所以这些样品中低δ18O值锆石可能是超固相条件下热液蚀变的结果.石英中具有异常低的δ18O值表明蚀变流体来源应为寒冷气候大气降水.所以, 新开岭地区亏损18O蚀变岩石的形成与裂谷岩浆作用和雪球地球事件相耦合的高温大气降水热液蚀变有关.   相似文献   

14.
Ab initio STO-3G molecular orbital theory has been used to calculate energy-optimized Si-O bond lengths and angles for molecular orthosilicic and pyrosilicic acids. The resulting bond length for orthosilicic acid and the nonbridging bonds for pyrosilicic acid compare well with Si-OH bonds observed for a number of hydrated silicate minerals. Minimum energy Si-O bond lengths to the bridging oxygen of the pyrosilicic molecule show a close correspondence with bridging bond length data observed for the silica polymorphs and for gas phase and molecular crystal siloxanes when plotted against the SiOSi angle. In addition, the calculations show that the mean Si-O bond length of a silicate tetrahedron increases slightly as the SiOSi angle narrows. The close correspondence between the Si-O bond length and angle variations calculated for pyrosilicic acid and those observed for the silica polymorphs and siloxanes substantiates the suggestion that local bonding forces in solids are not very different from those in molecules and clusters consisting of the same atoms with the same coordination numbers. An extended basis calculation for H4SiO4 implies that there are about 0.6 electrons in the 3d-orbitals on Si. An analysis of bond overlap populations obtained from STO-3G* calculations for H6Si2O7 indicates that Si-O bond length and SiOSi angle correlations may be ascribed to changes in the hybridization state of the bridging oxygen and (d – p) π-bonding involving all five of the 3d AO's of Si and the lone-pair AO's of the oxygen. Theoretical density difference maps calculated for H6Si2O7 show a build-up of charge density between Si and O, with the peak-height charge densities of the nonbridging bonds exceeding those of the bridging bonds by about 0.05 e Å?3. In addition, atomic charges (+1.3 and ?0.65) calculated for Si and O in a SiO2 moiety of the low quartz structure conform reasonably well with the electroneutrality postulate and with experimental charges obtained from monopole and radial refinements of diffraction data recorded for low quartz and coesite.  相似文献   

15.
Ion-microprobe analysis of zircons from an andalusite-bearing orthogneiss within the major Alpujárride nappe complex in the central part of the Betic Cordilleras has yielded a Hercynian age of 285?±?5 (2σ) Ma for euhedral rims, interpreted as the magmatic age of the andalusite-bearing biotite granite parent rock for the gneisses. Zircon age zoning systematics suggest a Paleozoic sedimentation age for the parent material for the anatectic source rock. Zircon cores represent several groups of ages: (1) Archean, c. 2.7?Ga; (2) Early Proterozoic, 2.2–2.0?Ga; (3) Middle Proterozoic, 1.1–0.9?Ga; (4) Pan-African, 0.8–0.5?Ga; including a well-defined event at 612?±?13 (2σ) Ma. Paragenetic and textural relations indicate that gneissification took place during a high-P (12–13?kbar) low-T (450–500?°C) collisional event during which the primary Alpine nappe pile was produced. The second and final Alpine tectono-metamorphic event led to reorganization of the primary nappe pile by extensional tetonics with coeval very fast rock uplift and cooling (from c. 8? to c. 1?kbar and c. 600 to 100?°C within the period 19.5–18.5?Ma). The fast uplift/cooling stage was triggered by slab break-off which is thought to have induced diapiric underplating by high-T asthenospheric material. This may have heated the collisional wedge, causing thermal weakening which might have advanced the late stage fast uplift/cooling. The Alpine events did not leave a zircon crystallization record. The inherited, Archean–Pan-African zircon age pattern corresponds to that established for the Hercynian basement in central and southern Europe which is considered as reworked Gondwana crust. Deeper levels of core complexes within the Betic-Rif belt thus belong to the pre-Triassic basement of the Tethyan realm (Betic-Ligurian lithosphere) and represent reworked material ultimately derived from Archean and Proterozoic rock complexes from the Gondwana crustal domain. This study implies that thermobarometry of rock complexes which went through several phases of tectono-metamorphic reworking may render ambiguous results if based upon field observations and petrography alone. Zircon ion-microprobe dating may provide additional constraints required to arrive at a feasible tectono-metamorphic history, that is P-T-t trajectory, for such rock complexes.  相似文献   

16.
Hydrothermal experiments were carried out at 2 kbar water pressure, 700 °–800 ° C, with the objective of determining the level of dissolved Zr required for precipitation of zircon from melts in the system SiO2-Al2O3-Na2O-K2O. The saturation level depends strongly upon molar (Na2O + K2O)/Al2O3 of the melts, with remarkably little sensitivity to temperature, SiO2 concentration, or melt Na2O/ K2O. For peraluminous melts and melts lying in the quartz-orthoclase-albite composition plane, less than 100 ppm Zr is required for zircon saturation. In peralkaline melts, however, zircon solubility shows pronounced, apparently linear, dependence upon (Na2O + K2O)/Al2O3, with the amount of dissolvable Zr ranging up to 3.9 wt.% at (Na2O + K2O)/Al2O3 = 2.0. Small amounts (1 wt.% each) of dissolved CaO and Fe2O3 cause a 25% relative reduction of zircon solubility in peralkaline melts.The main conclusion regarding zirconium/zircon behavior in nature is that any felsic, non-peralkaline magma is likely to contain zircon crystals, because the saturation level is so low for these compositions. Zircon fractionation, and its consequences to REE, Th, and Ta abundances must, therefore, be considered in modelling the evolution of these magmas. Partial melting in any region of the Earth's crust that contains more than 100 ppm Zr will produce granitic magmas whose Zr contents are buffered at constant low (< 100 ppm) values; unmelted zircon in the residual rock of such a melting event will impart to the residue a characteristic U- or V-shaped REE abundance pattern. In peralkaline, felsic magmas such as those that form pantellerites and comendites, extreme Zr (and REE, Ta) enrichment is possible because the feldspar fractionation that produces these magmas from non-peralkaline predecessors does not drive the melt toward saturation in zircon.Zircon solubility in felsic melts appears to be controlled by the formation of alkali-zirconosilicate complexes of simple (2:1) alkali oxide: ZrO2 stoichiometry.  相似文献   

17.
The lower crust of the Serre massif (Calabria, southern Italy) provides a window into the mid- to lower crust of the south European Variscan orogenic belt. Previously, zircon U-Pb ages were employed to date high-temperature processes affecting this portion of the Variscan crust. The present paper reports new LA-ICP-MS U-Pb data on the zircon of a deformed quartz-monzodiorite dike and of three mafic granulites sampled at the base of the lower crust section. Determination of trace elements on zircon, including rare earth elements (REE), has been also performed. The end of the Variscan exhumation, dated by anatectic zircon from migmatitic metapelites, and the growth-modification of zircon with respect to the growth of Variscan metamorphic garnet have been assumed as ??time markers??. The concordant zircon ages of the metamorphic basic rocks cover a range from 744?±?20 Ma to 231?±?10 Ma with high age density from 357?±?11?Ma to 279?±?10 Ma, a few ages comprised between 418?±?14 Ma and 483?±?12 Ma and between 505?±?11 Ma and 593?±?14 Ma. Zircon from the deformed quartz-monzodiorite dike evidences a minimum age of emplacement of 323?±?5 Ma. Most of the analysed zircon domains dated between 357?±?11 Ma to 279?±?10 Ma from garnet-bearing metabasic rocks show flat patterns of heavy rare earth elements (HREE), as expected in the case of their simultaneous growth with garnet. This allows to consider (1) zircon domains giving Variscan ages as ??metamorphic?? with specific geological significance, and (2) zircon domains with ages ranging from 564?±?17 Ma to 593?±?14 Ma as dating the emplacement of the magmatic protoliths as shown by internal microtextures, fractionated patterns of HREE and Th/U ratios (0.16?C0.19). The Variscan zircon ages (357?C279?Ma) reflect effects of crustal thickening, peak metamorphism and subsequent multistage Variscan decompression documented by the statistically significant clusters of ages around 347?C340?Ma, 323?C318?Ma, 300?C294?Ma and 279?Ma. The U-Pb zircon ages of the metabasic rocks suggest a period of about 60?C70?Ma for granulite facies metamorphism and anatectic conditions. Literature data indicate that the migmatitic metapelites of the upper part of the Serre crust section also underwent a long period, about 40?Ma, of granulite facies metamorphism and anatectic conditions. A diachronism emerges through the time comparison of the Variscan evolution between the upper and the lower portions of the Serre deep crust. The duration of the Variscan processes defined in Calabria is comparable to that of other south European Variscan blocks.  相似文献   

18.
Long-lived intra-oceanic arcs of Izu-Bonin-Marianas (IBM)-type are built on thick, granodioritic crust formed in the absence of pre-existing continental crust. International Ocean Discovery Program Expedition 350, Site U1437, explored the IBM rear arc to better understand continental crust formation in arcs. Detailed petrochronological (U–Pb geochronology combined with trace elements, oxygen and hafnium isotopes) characterizations of zircon from Site U1437 were carried out, taking care to exclude potential contaminants by (1) comparison of zircon ages with ship-board palaeomagnetic and biostratigraphic ages and 40Ar/39Ar geochronology, (2) analysing zircon from drill muds for comparison, (3) selectively carrying out in situ analysis in petrographic thin sections, and (4) minimizing potential laboratory contamination through using pristine equipment during mineral separation. The youngest zircon ages in Site U1437 are consistent with 40Ar/39Ar and shipboard ages to a depth of ~1390 m below sea floor (mbsf) where Igneous Unit Ig 1 yielded an 40Ar/39Ar age of 12.9 ± 0.3 Ma (all errors 2σ). One single zircon (age 15.4 ± 1.0 Ma) was recovered from the deepest lithostratigraphic unit drilled, Unit VII (1459.80–1806.5 mbsf). Site U1437 zircon trace element compositions are distinct from those of oceanic and continental arc environments and differ from those generated in thick oceanic crust (Iceland-type) where low-δ18O evolved melts are produced via re-melting of hydrothermally altered mafic rocks. Ti-in-zircon model temperatures are lower than for mid-ocean ridge rocks, in agreement with low zircon saturation temperatures, suggestive of low-temperature, hydrous melt sources. Zircon oxygen (δ18O = 3.3–6.0‰) and hafnium (εHf = + 10–+16) isotopic compositions indicate asthenospheric mantle sources. Trace element and isotopic differences between zircon from Site U1437 rear-arc rocks and the Hadean detrital zircon population suggest that preserved Hadean zircon crystals were probably generated in an environment different from modern oceanic convergent margins underlain by depleted mantle.  相似文献   

19.
本文报道了吉林南部金厂沟矿区黑云母闪长岩和正长花岗岩的锆石LA-ICP-MS U-Pb年龄、锆石Hf同位素和岩石地球化学资料,以确定该区岩体的形成时代、源区性质和构造背景。黑云母闪长岩和正长花岗岩的锆石U-Pb年龄分别为221.5±1.1 Ma和227.4±1.9 Ma,表明岩体形成于晚三叠世。正长花岗岩富硅、铝、碱和贫钙、钠、镁及铁,稀土元素配分曲线为右倾型,微量元素蛛网图上表现为大离子亲石元素(Rb、Ba、K)富集和高场强元素(Nb、Ta、Ti)及P亏损,锆石ε_(Hf)(t)介于–15.3~–9.0之间,Hf二阶段模式年龄(t_(DM2))介于1.83~2.22 Ga之间。以上特征表明,该期正长花岗岩为准铝质-过铝质钾玄岩系列,与S型花岗岩特征相似,岩浆起源于古元古代长英质下地壳在低压环境下的部分熔融。黑云母闪长岩具有富硅、铝、钾、钠和贫镁的特点,稀土元素配分曲线为右倾型,微量元素蛛网图上表现为富集大离子亲石元素Rb、Ba、K及活泼的不相容元素Th和U,相对亏损高场强元素Nb、Ta、P和Ti,具有高Sr(735×10~(–6)~1560×10~(–6)),低Yb(0.92×10~(–6)~1.23×10~(–6))的特征。锆石ε_(Hf)(t)为–12.9~–8.5,二阶段Hf模式年龄(t_(DM2))为1.82~2.07 Ga。结合前人研究成果,认为黑云母闪长岩起源于深部的古元古代镁铁质下地壳的部分熔融。综合分析吉南地区已有的年代学资料和区域构造研究成果,认为吉南中生代岩浆作用主要发生在晚三叠世、早中侏罗世和早白垩世,与辽东和胶东地区具有相同的年代学格架并构成一条北东向岩浆岩带。吉南地区晚三叠世黑云母闪长岩和正长花岗岩是扬子板块与华北板块碰撞拼合的产物。  相似文献   

20.
The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U–Pb, δ18O and LA-MC-ICPMS 176Hf isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two ~3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qôrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial εHf zircon values (n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one 207Pb/206Pb age, and correlated with 176Hf/177Hf compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite–trondhjemite–granodiorite (TTG) gneisses. Zircon from the ~3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic 176Hf/177Hf and mantle-like δ18O compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions (n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean δ18O values ranging from 5.8 ± 0.6 to 3.7 ± 0.5‰. The 3.64 Ga granitic augen gneiss sample displays the highest δ18O with a mildly supra-mantle composition of 5.8 ± 0.6‰. Inherited Eoarchaean TTG-derived zircon shows mantle-like values. Igneous zircon from all other samples, spanning more than a billion years of Archaean time, record low δ18O sub-mantle compositions. These are the first low δ18O signatures reported from Archaean zircon and represent low δ18O magmas formed by the remelting and metamorphism of older crustal rocks following high-temperature hydrothermal alteration by meteoric water. Meteoric fluid ingress coupled with crustal extension, associated high heat flow and intra-crustal melting are a viable mechanism for the production of the low δ18O granites, granodiorites and trondhjemites reported here. Both high and low δ18O magmas may have been generated in extensional environments and are distinct in composition from Phanerozoic I-type granitic plutonic systems, which are typified by increasing δ18O during intra-crustal reworking. This suggests that Archaean magmatic processes studied here were subtly different from those operating on the modern Earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement.  相似文献   

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