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1.
Ruairidh J. Mitchell Tim E. Johnson Chris Clark Saibal Gupta Michael Brown Simon L. Harley Richard Taylor 《Journal of Metamorphic Geology》2019,37(7):977-1006
The time‐scales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel, and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole‐rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1,000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with <1 kbar of decompression. Concordant U–Pb ages obtained by SHRIMP from zircon (spread 1,050–800 Ma) and monazite (spread 950–800 Ma) demonstrate that crystallization of these minerals occurred during an interval of c. 250 Ma. By combining LA‐ICP‐MS U–Pb zircon ages with Ti‐in‐zircon temperatures from the same analysis sites, we show that the crust may have remained above 900°C for a minimum of c. 120 Ma between c. 1,000 and c. 880 Ma. Overall, the results suggest that, in the interval 1,050 to 800 Ma, the evolution of the Vizianagaram granulites culminated with UHT conditions from c. 1,000 Ma to c. 880 Ma, associated with minor decompression, before further zircon crystallization at c. 880–800 Ma during cooling to the solidus. However, these rocks are adjacent to the Paderu–Anantagiri–Salur crustal block to the NW that experienced counterclockwise P–T–t paths, and records similar UHT peak metamorphic conditions (7–8 kbar, ~950°C) followed by near‐isobaric cooling, and has a similar chronology during the Neoproterozoic. The limited decompression inferred at Vizianagaram may be explained by partial exhumation due to thrusting of this crustal block over the adjacent Paderu–Anantagiri–Salur crustal block. The residual granulites in both blocks have high concentrations of heat‐producing elements and likely remained hot at mid‐crustal depths throughout a period of relative tectonic quiescence in the interval 800–550 Ma. During the Cambrian Period, the EGP was located in the hinterland of the Denman–Pinjarra–Prydz orogen. A later concordant population of zircon dated at 511 ± 6 Ma records crystallization at temperatures of ~810°C. This age may record a low‐degree of melting due to limited influx of fluid into hot, weak crust in response to convergence of the Crohn craton with a composite orogenic hinterland comprising the Rayner terrane, EGP, and cratonic India. 相似文献
2.
Ti-in-zircon thermometry: applications and limitations 总被引:16,自引:5,他引:11
Bin Fu F. Zeb Page Aaron J. Cavosie John Fournelle Noriko T. Kita Jade Star Lackey Simon A. Wilde John W. Valley 《Contributions to Mineralogy and Petrology》2008,156(2):197-215
The titanium concentrations of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga were measured by ion microprobe. Samples come
from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons
(32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most
of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon
thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon
saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (∼15% melt residue
for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous
rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually,
the effects of reduced or , variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low
temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous
melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While
some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included
within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in
Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once.
However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio,
or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and , are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide
overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock
SiO2 and HfO2 for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO2 and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks
average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures
of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons
have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons
from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent
Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration
in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
3.
Ti-in-zircon thermometry with SHRIMP II multi-collector has been applied to two well-documented Archean igneous and metamorphic samples from southern West Greenland. Zircons from 2.71 Ga partial melt segregation G03/38 formed in a small (< 1 m3), closed system within a mafic rock under high pressure granulite facies conditions. Results of 14 Ti analyses present a mean apparent zircon crystallization temperature of 679 ± 11 °C, underestimating independent garnet-clinopyroxene thermometry by 20–50 °C but consistent with reduced aTiO2 in this system. 36 spot analysis on 15 zircons from 3.81 Ga meta-tonalite G97/18, with an estimated magmatic temperature > 1000 °C, yield a low-temperature focused normal distribution with a mean of 683 ± 32 °C, further demonstrated by high resolution Ti mapping of two individual grains. This distribution is interpreted to represent the temperature of the residual magma at zircon saturation, late in the crystallization history of the tonalite. Hypothetically, Ti-in-zircon thermometry on Eoarchaean detrital zircons sourced from such a high temperature tonalite would present a low-temperature biased image of the host magma, which could be misconstrued as being a minimum melt granite. Multiple analyses from individual zircons can yield complex Ti distributions and associated apparent temperature patterns, reflecting cooling history and local chemical environments in large magma chambers. In addition to inclusions and crystal imperfections, which can yield apparent high temperature anomalies, zircon surfaces can also record extreme (> 1000 °C) apparent Ti temperatures. In our studies these were traced to 49Ti (or a molecular isobaric interference) contamination derived from the double sided adhesive tape used in sample preparation, and should not be assigned geological significance. 相似文献
4.
R. B. Ickert I. S. Williams D. Wyborn 《Contributions to Mineralogy and Petrology》2011,162(2):447-461
The understanding of zircon crystallization, and of the Ti-in-zircon thermometer, has been enhanced by Ti concentration measurements
of zircon from a small, concentrically zoned pluton in south-eastern Australia, the Boggy Plain zoned pluton (BPZP). Zircon
crystals from rocks ranging in composition from gabbro to aplite were analysed for U–Th–Pb dating and Ti concentrations by
an ion microprobe. Geochronological data yield a 206Pb/238U age of 417.2 ± 2.0 Ma (95% confidence) and demonstrate the presence of older inherited or xenocrystic zircon. Titanium measurements
(n = 158) yield a mean Ti concentration of 11.7 ± 6.1 ppm (2SD) which corresponds to a mean crystallization temperature of 790°C
for an α-TiO2 = 0.74 (estimated using mineral equilibria), or 760°C for an α-TiO2 = 1.0. Apparent zircon crystallization temperatures are similar in all intrusive phases, although the gabbro yields slightly
higher values, indicating that crystallization occurred at the same temperature in all rock types. This finding is consistent
with previous work on the BPZP, which indicates that liquid–crystal sorting (crystal fractionation) was the dominant control
on chemical differentiation, and that late, differentiated liquids were similar in composition for all rock types. A simple
forward model approximately predicts the range of crystallization temperatures, but not the shape of the distributions, due
to sampling biases and complexities in the cooling and crystallization history of the pluton. The distribution of Ti concentrations
has a mode at a higher Ti (higher temperature) than the sample set of Hadean detrital zircon. This is consistent with the
hypothesis that the skew to low-T in the Hadean dataset is due to the presence of zircon that crystallized from wet anatectic
melts. 相似文献
5.
High‐ to ultrahigh‐temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand 下载免费PDF全文
Jean‐Baptiste Jacob James M. Scott Rose E. Turnbull Matthew S. Tarling Matthew W. Sagar 《Journal of Metamorphic Geology》2017,35(8):831-853
Lower crustal xenoliths erupted from an intraplate diatreme reveal that a portion of the New Zealand Gondwana margin experienced high‐temperature (HT) to ultrahigh‐temperature (UHT) granulite facies metamorphism just after flat slab subduction ceased at c. 110–105 Ma. P–T calculations for garnet–orthopyroxene‐bearing felsic granulite xenoliths indicate equilibration at ~815 to 910°C and 0.7 to 0.8 GPa, with garnet‐bearing mafic granulite xenoliths yielding at least 900°C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re‐integration of exsolution in feldspar (~900–950°C at 0.8 GPa), Ti‐in‐zircon thermometry on Y‐depleted overgrowths on detrital zircon grains (932°C ± 24°C at aTiO2 = 0.8 ± 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (≥850°C at 0.7 to 0.8 GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U–Pb pooled age of 91.7 ± 2.0 Ma. The cause of Late Cretaceous HT‐UHT metamorphism on the Zealandia Gondwana margin is attributed to collision and partial subduction of the buoyant oceanic Hikurangi Plateau in the Early Cretaceous. The halt of subduction caused the fore‐running shallowly dipping slab to rollback towards the trench position and permitted the upper mantle to rapidly increase the geothermal gradient through the base of the extending (former) accretionary prism. This sequence of events provides a mechanism for achieving regional HT–UHT conditions in the lower crust with little or no sign of this event at the surface. 相似文献
6.
冀东双山子群变质火山岩的地球化学、锆石U-Pb年代学及其对岩石成因和构造背景的制约 总被引:6,自引:5,他引:1
冀东双山子群是一套出露于青龙县东部变质程度较低的火山沉积地层,其中火山岩地层主要由变质玄武岩、安山岩和英安岩-流纹岩组成。全岩地球化学分析表明变玄武岩呈现拉斑玄武岩的地球化学特征,起源于尖晶石二辉橄榄岩12%~25%的部分熔融,变质安山岩与变质英安岩-流纹岩形成于弧下地幔部分熔融,该熔体受到地壳物质混染。LA-ICPMS锆石U-Pb定年表明本群变质安山岩形成于2514±16Ma,而变质流纹岩形成于2522±8Ma,二者均受到~2450和~2300Ma的后期热事件扰动。结合本群变质火山岩的岩石组合、地球化学特征和岩石成因,该套岩石可能形成于活动大陆边缘弧相关构造背景。 相似文献
7.
Thomas Bader Lifei Zhang Xiaowei Li Bin Xia Leander Franz Christian de Capitani Qingyun Li 《Journal of Metamorphic Geology》2020,38(4):421-450
High-pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging-wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution. 相似文献
8.
Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon,monazite, rutile and titanite from the Kalak Nappe Complex,northern Norway 下载免费PDF全文
D. Gasser P. Jeřábek C. Faber H. Stünitz L. Menegon F. Corfu M. Erambert M. J. Whitehouse 《Journal of Metamorphic Geology》2015,33(5):513-534
This study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high‐U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high‐grade metamorphism, or represent partially reset ages due to high‐T diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1‐forming event lost its Zr‐in‐rutile and U–Pb signatures during the S2‐forming event. It records Zr‐in‐rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr‐in‐rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path. 相似文献
9.
Phase relations of phlogopite with magnesite from 4 to 8 GPa 总被引:2,自引:2,他引:0
To evaluate the stability of phlogopite in the presence of carbonate in the Earth’s mantle, we conducted a series of experiments
in the KMAS–H2O–CO2 system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl90-mag10; wt%) and run at pressures from 4 to 8 GPa at temperatures ranging from 1,150 to 1,550°C. We bracketed the solidus between
1,200 and 1,250°C at pressures of 4, 5 and 6 GPa and between 1,150 and 1,200°C at a pressure of 7 GPa. Below the solidus,
phlogopite coexists with magnesite, pyrope and a fluid. At the solidus, magnesite is the first phase to react out, and enstatite
and olivine appear. Phlogopite melts over a temperature range of ~150°C. The amount of garnet increases above solidus from
~10 to ~30 modal% to higher pressures and temperatures. A dramatic change in the composition of quench phlogopite is observed
with increasing pressure from similar to primary phlogopite at 4 GPa to hypersilicic at pressures ≥5 GPa. Relative to CO2-free systems, the solidus is lowered such, that, if carbonation reactions and phlogopite metasomatism take place above a
subducting slab in a very hot (Cascadia-type) subduction environment, phlogopite will melt at a pressure of ~7.5 GPa. In a
cold (40 mWm−2) subcontinental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate and can coexist
with a fluid that becomes Si-rich with increasing pressure. Ascending kimberlitic melts that are produced at greater depths
could react with peridotite at the base of the subcontinental lithospheric mantle, crystallizing phlogopite and carbonate
at a depth of 180–200 km. 相似文献
10.
Testing the fidelity of thermometers at ultrahigh temperatures 总被引:1,自引:0,他引:1
Chris Clark Richard J. M. Taylor Tim E. Johnson Simon L. Harley Ian C. W. Fitzsimons Liam Oliver 《Journal of Metamorphic Geology》2019,37(7):917-934
A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H2O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO2 that is lower than the assumed value of unity. 相似文献
11.
Niels Jöns Wolfgang Bach Timothy Schroeder 《Contributions to Mineralogy and Petrology》2009,157(5):625-639
We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones
are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite,
and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic
intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization
of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered
peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization,
thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major
pathway for fluids through the oceanic crust, as evidenced by extremely low δ18O of altered plagiogranite veins (+3.0–4.2‰) and adjacent serpentinites (+ 2.6–3.7‰). The uniform oxygen isotope data indicate
that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
12.
Autochthonous inheritance of zircon through Cretaceous partial melting of Carboniferous plutons: the Arthur River Complex,Fiordland, New Zealand 总被引:1,自引:0,他引:1
Andrew J. Tulloch Trevor R. Ireland David L. Kimbrough William L. Griffin Jahandar Ramezani 《Contributions to Mineralogy and Petrology》2011,161(3):401-421
TIMS and SHRIMP U–Pb analyses of zircons from Milford Orthogneiss metadiorite (P = 1–1.4 GPa; T ≥ 750°C) of the Arthur River Complex of northern Fiordland reveal a bimodal age pattern. Zircons are predominantly either
Paleozoic (357.0 ± 4.2 Ma) and prismatic with oscillatory zoning, or Cretaceous (133.9 ± 1.8 Ma) and ovoid with sector or
patchy zoning. The younger age component is not observed overgrowing older grains. Most grains of both ages are overgrown
by younger Cretaceous (~120 Ma) metamorphic zircon with very low U and Th/U (0.01). We interpret the bimodal ages as indicating
initial igneous emplacement and crystallisation of a dioritic protolith pluton at ~357 Ma, followed by Early Cretaceous granulite-facies
metamorphism at ~134 Ma, during which a significant fraction (~60%) of the zircon grains dissolved, and subsequently reprecipitated,
effectively in situ, in partial melt pockets. The remaining ~40% of original Paleozoic grains were apparently not in contact
with the partial melt, remained intact, and show only slight degrees of Pb loss. Sector zoning of the Cretaceous grains discounts
their origin by solid state recrystallisation of Paleozoic grains. The alternative explanation—that the Paleozoic component
represents a 40% inherited component in an Early Cretaceous transgressive dioritic magma—is considered less likely given the
relatively high solubility of zircon in magma of this composition, the absence of 134 Ma overgrowths, the single discrete
age of the older component, equivalent time-integrated 177Hf/176Hf compositions of both age groups, and the absence of the Cambrian-Proterozoic detrital zircon that dominates regional Cambro-Ordovician
metasedimentary populations. Similar bimodal Carboniferous-Early Cretaceous age distributions are characteristic of the wider
Arthur River Complex; 8 of 12 previously dated dioritic samples have a Paleozoic component averaging 51%. Furthermore, the
age and chemical suite affinity of these and several more felsic rocks can be matched with those of the relatively unmetamorphosed
Carboniferous plutonic terrane along the strike of the Mesozoic margin in southern Fiordland, also supporting the in situ
derivation of the Carboniferous “inherited” component. 相似文献
13.
Zircon U-Pb SIMS dating combined with in-context (in thin section) monazite and xenotime U + Th-total Pb dating was used to clarify the Palaeozoic evolution of the ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia). Four distinct zircon, monazite and xenotime age domains testify to a prolonged evolution from igneous formation to multi-stage metasomatism and hydrothermal overprinting. The geological interpretation of age patterns from ‘cold’ granites, expected to have low zircon saturation temperatures (<800 °C) and relatively high amounts of zircon inheritance, requires special care, especially for what concerns proper attribution of zircon inheritance and igneous growth ages. These issues can be resolved using zircon saturation temperatures (TZrn) as proxy for the amount of zircon inheritance in combination with the temperature differences between TZrn and the granite solidus. In this respect, the Chopok granite is an atypical ‘cold’ granite. Due to TZrn being substantially lower (ca. 80 °C ± 50 °C) than the granite solidus temperature, practically no zircon inheritance was found. The zircon age data indicates that the Chopok granite is a product of an Early Ordovician (475.8 ± 3.3 Ma) magmatic event, corresponding with the widespread Early Palaeozoic magmatism recorded throughout the European Variscan belt. This is further corroborated by phosphate mineral ages. The post-magmatic activity recorded in the U-Pb systematics of zircon and phosphates overgrowths can be related to the different phases of the evolution of the Variscan orogen: Early Carboniferous (ca. 352 Ma) metasomatism documents the main Variscan orogenic event, whereas the Permo-Triassic age (ca. 255 Ma) reflects thermo-tectonic activity associated with large-scale crustal extension, contemporaneous with the initial continental leading to the break-up of Pangea. 相似文献
14.
The significance of Cenozoic magmatism from the western margin of the eastern syntaxis, southeast Tibet 总被引:6,自引:0,他引:6
Hongfei Zhang Nigel Harris Liang Guo Wangchun Xu 《Contributions to Mineralogy and Petrology》2010,160(1):83-98
The major and trace-element geochemistry, Sr–Nd bulk-rock isotopes, U–Pb zircon chronology and Lu–Hf isotopic compositions
are described for three granitic bodies which intrude the Nyingchi gneisses (Lhasa terrane) along the western margin of the
eastern Himalayan syntaxis. The Bayi two-mica granite and Lunan granite–granodiorite were intruded at 22 ± 1 and 25.4 ± 0.3 Ma,
respectively, whereas the Confluence biotite granite was emplaced at 49.1 ± 0.4 Ma. All share strong depletions in Y and HREE
requiring a garnet-bearing source both during and following the Eocene collision of the Indian plate with the Lhasa terrane.
The isotope geochemistry of these intrusives (ε
Nd(t) = −3 to −5, 87Sr/86Sr(t) = 0.706–0.707) indicates a crustal source within the Lhasa terrane. Sr–Nd systematics of the garnet-bearing Nyingchi gneisses
together with the U–Pb and Lu–Hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential
melt source. The combined element and isotope geochemistry of the plutons indicate a mixed source; the gneisses provide the
older component whereas the Gangdese batholith provides a younger, siliceous component. The involvement of garnet-bearing
crustal material in melt sources from the Cretaceous (80 Ma) to the Miocene (20 Ma) is consistent with the presence of a thicker
continental crust in the eastern Lhasa terrane, as is the presence of magmatic epidote in several plutons which indicates
a regional deepening level of exposure eastwards. Post-collision crustal melting is synchronous with proposed slab break-off
during the early Miocene, suggesting advective heating by rising asthenospheric melts. 相似文献
15.
Zircon Inheritance Reveals Exceptionally Fast Crustal Magma Generation Processes in Central Iberia during the Cambro-Ordovician 总被引:3,自引:0,他引:3
The Variscan basement of the Central Iberian Zone contains abundantCambro-Ordovician calc-alkaline to peraluminous metagranitesand metavolcanic rocks with two notable features: first, theywere apparently produced with no connection to any major tectonicor metamorphic event; second, they have an unusually high zirconinheritance. U–Pb dating combined with cathodoluminescenceimaging reveals that about 70–80%, in some samples nearer100%, of the zircon grains contain inherited pre-magmatic cores,despite the temperature reached by the magmas (about 900°C,calculated using the Ti-in-zircon thermometer) being high enoughto dissolve all the available zircon (from the rock's zirconsaturation temperature, 770–860°C). The fact thatthe dissolution of zircon was so incomplete can only be attributedto the kinetics of heat transfer to and from the magmas. Three-dimensionalmodeling of zircon dissolution behavior in melts with a compositionsimilar to the Iberian Cambro-Ordovician magmas indicates thatthe survival of zircons from the suggested late Pan-Africanprotolith would be possible only if melt production was rapid,specifically less than 104 years, and probably about 2 x 103years, from the beginning of melting (700°C) to the thermalpeak (900°C). Melt production was followed by fast magmatransfer to upper crustal levels resulting either in surfaceeruption or in the emplacement of small (< 400 m thick) sillsor laccoliths. We suggest that these elevated rates of crustalmelting could only have been caused by intrusion of mantle-derivedmafic magmas, most probably at the base of the crust. This scenariois consistent with a rifting regime in which crust and mantlewere mechanically decoupled; this would explain the scarcityof contemporaneous crustal deformation. Furthermore, fast meltingrates in the lower crust followed by fast melt transportationto the upper crust could also explain the lack of contemporaneousmetamorphism. The speed of the partial melting process resultedin the production of felsic magmas that inherited the geochemicalcharacteristics of their granitoid crustal protolith. This explainsthe apparent contradiction between the calc-alkaline to peraluminousgeochemical characteristics of the magmas and the inferred extensional(i.e. rift-related) tectonic setting. Our model is compatiblewith the hypothesis of fragmentation and dispersal of terranesfrom the northern margin of Gondwana that led to the openingof the Rheic and Galicia–South Brittany oceans and, ultimately,caused the detachment of the Iberian microplate from Armoricaand Gondwana during the early Paleozoic. KEY WORDS: igneous petrology; migmatite; granite; geochemistry; crustal contamination; ICP-MS; laser ablation 相似文献
16.
《地学前缘(英文版)》2018,9(6):1921-1936
The origin of zircon grains, and other exotic minerals of typical crustal origin, in mantle-hosted ophiolitic chromitites are hotly debated. We report a population of zircon grains with ages ranging from Cretaceous (99 Ma) to Neoarchean (2750 Ma), separated from massive chromitite bodies hosted in the mantle section of the supra-subduction (SSZ)-type Mayarí-Baracoa Ophiolitic Belt in eastern Cuba. Most analyzed zircon grains (n = 20, 287 ± 3 Ma to 2750 ± 60 Ma) are older than the early Cretaceous age of the ophiolite body, show negative εHf(t) (−26 to −0.6) and occasional inclusions of quartz, K-feldspar, biotite, and apatite that indicate derivation from a granitic continental crust. In contrast, 5 mainly rounded zircon grains (297 ± 5 Ma to 2126 ± 27 Ma) show positive εHf(t) (+0.7 to +13.5) and occasional apatite inclusions, suggesting their possible crystallization from melts derived from juvenile (mantle) sources. Interestingly, younger zircon grains are mainly euhedral to subhedral crystals, whereas older zircon grains are predominantly rounded grains. A comparison of the ages and Hf isotopic compositions of the zircon grains with those of nearby exposed crustal terranes suggest that chromitite zircon grains are similar to those reported from terranes of Mexico and northern South America. Hence, chromitite zircon grains are interpreted as sedimentary-derived xenocrystic grains that were delivered into the mantle wedge beneath the Greater Antilles intra-oceanic volcanic arc by metasomatic fluids/melts during subduction processes. Thus, continental crust recycling by subduction could explain all populations of old xenocrystic zircon in Cretaceous mantle-hosted chromitites from eastern Cuba ophiolite. We integrate the results of this study with petrological-thermomechanical modeling and existing geodynamic models to propose that ancient zircon xenocrysts, with a wide spectrum of ages and Hf isotopic compositions, can be transferred to the mantle wedge above subducting slabs by cold plumes. 相似文献
17.
S. Jung K. Mezger O. Nebel E. Kooijman J. Berndt F. Hauff C. Münker 《Contributions to Mineralogy and Petrology》2012,163(1):1-17
Leucocratic granites of the Proterozoic Kaoko Belt, northern Namibia, now preserved as meta-granites, define a rock suite
that is distinct from the surrounding granitoids based on their chemical and isotopic characteristics. Least evolved members
of this ~1.5–1.6-Ga-old leucogranite suite can be distinguished from ordinary calc-alkaline granites that occur elsewhere
in the Kaoko Belt by higher abundances of Zr, Y, and REE, more radiogenic initial εNd values and unradiogenic initial 87Sr/86Sr. The leucogranites have high calculated zircon saturation temperatures (mostly > 920°C for least fractionated samples),
suggesting that they represent high-temperature melts originating from deep crustal levels. Isotope data (i.e., εNdi: +2.3 to –4.2) demonstrate that the granites formed from different sources and differentiated by a variety of processes including
partial melting of mantle-derived meta-igneous rocks followed by crystal fractionation and interaction with older crustal
material. Most fractionation-corrected Nd model ages (TDM) are between 1.7 and 1.8 Ga and only slightly older than the inferred intrusion age of ca. 1.6 Ga, indicating that the precursor
rocks must have been dominated by juvenile material. Epsilon Hf values of zircon separated from two granite samples are positive
(+11 and +13), and Hf model ages (1.5 and 1.6 Ga) are similar to the U–Pb zircon ages, again supporting the dominance of juvenile
material. In contrast, the Hf model ages of the respective whole rock samples are 2.3 and 2.4 Ga, demonstrating the involvement
of older material in the generation of the granites. The last major tectonothermal event in the Kaoko Belt in the Proterozoic
occurred at ca. 2.0 Ga and led to reworking of mostly 2.6-Ga-old rocks. However, the presence of 1.6 Ga “post-collisional”
granites reflects addition of some juvenile mantle-derived material after the last major tectonic event. The results suggest
that similar A-type leucogranites are potentially more abundant in crustal terranes but are masked by AFC processes. In the
case of the Kaoko Belt, it is suggested that this rock suite indicates a yet unidentified period of mantle-derived crustal
growth in the Proterozoic of South Western Africa. 相似文献
18.
The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO2 inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO2 densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO2 inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO2 prior to analysis. This indicates a close relationship between CO2 and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO2-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills. 相似文献
19.
大洋斜长花岗岩是指分布在蛇绿岩或现今洋壳中的少量长英质侵入体,可形成在洋中脊及俯冲带等多种构造环境,虽然其在洋壳和蛇绿岩中所在体积很小,但对探究大洋岩石圈演化、俯冲起始及洋-陆转化机制具有重要意义。本文对北秦岭造山带西段商丹缝合带内新识别出的斜长花岗岩开展了岩石学、地球化学和锆石U-Pb年代学的综合研究。全岩地球化学结果指示花岗闪长岩具有高的SiO2、Al2O3含量以及较低的K2O、TiO2含量,属于低钾亚碱性、准铝质到弱过铝质的岩石。微量元素相对富Sr、贫Nb、Ta和Ti,具有平坦的稀土元素配分模式,是弧前环境中由俯冲板片在浅部层次部分熔融成因的大洋斜长花岗岩。3个斜长花岗岩样品分别记录了526±4 Ma、515±4 Ma和517±6 Ma的锆石U-Pb年龄,结合前人同时代的玻安岩、高镁安山岩的报道,西秦岭商丹缝合带保留有与伊豆-小笠原-马里亚纳(IBM)俯冲带相似的弧前岩石组合,这些弧前的玻安岩、高镁安山岩和斜长花岗岩表明商丹洋在早寒武世发生了洋壳的初始俯冲。结合区域地质资... 相似文献
20.
Daniela Rubatto Jörg Hermann Alfons Berger Martin Engi 《Contributions to Mineralogy and Petrology》2009,158(6):703-722
The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated
using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb
ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital
cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization
temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early
garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites,
and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths
and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature;
that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated
in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local
rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available.
The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted
melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling
in the order of 100°C/Ma to upper crustal levels. 相似文献