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1.
Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia 总被引:30,自引:1,他引:30
Daniela Rubatto Ian S. Williams Ian S. Buick 《Contributions to Mineralogy and Petrology》2001,140(4):458-468
We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central
Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence
and back-scattered electron imaging, supported by SHRIMP U–Pb dating, has revealed the different behaviours of zircon and
monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade,
at ∼600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both
partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite,
is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of
pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals
increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven
by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these
accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557–1587 Ma) in a stable
high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the
protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature
(750–800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found
in monazite, indicating that the closure temperature of the U–Pb system in relatively large monazite crystals can exceed 750–800 °C.
Received: 7 April 2000 / Accepted: 12 August 2000 相似文献
2.
虽然前人对澜沧岩群做了大量的研究,但缺乏同位素年代学方面的研究。此次研究针对澜沧江南段菖蒲塘-大田山地区澜沧岩群绢白云母石英片岩进行碎屑锆石U-Pb定年,为澜沧岩群的形成时代增添年代学证据。极大多数(129粒)锆石阴极发光(CL)图像显示其有明显的结晶振荡环带,指示其为岩浆成因的碎屑锆石,129粒碎屑锆石具有多组峰值年龄,最年轻一组年龄加权平均值为452±26Ma,表明澜沧岩群最早沉积时限不早于452±26Ma;极少数(1粒)锆石具弱阴极发光,缺乏内部结构特点,认为其为变质重结晶锆石,变质结晶锆石U-Pb年龄为255±3Ma,与前人研究所得澜沧岩群变质时代基本吻合。综合分析,澜沧岩群的沉积时限不早于452±26Ma,在二叠纪末期可能发生变质作用。 相似文献
3.
Geochronology and tectonic significance of Middle Proterozoic granitic orthogneiss, North Qaidam HP/UHP terrane, Western China 总被引:4,自引:0,他引:4
C. G. Mattinson J. L. Wooden J. G. Liou D. K. Bird C. L. Wu 《Mineralogy and Petrology》2006,88(1-2):227-241
Summary Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high
pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic,
mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe
U–Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ± Ma and 921 ± 7 Ma.
Zircon rims in one of these samples yield younger ages (397–618 Ma) compatible with partial zircon recrystallization during
in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area.
The similarity between a 2496 ± 18 Ma xenocrystic core and 2.4–2.5 Ga zircon cores in the surrounding paragneiss suggests
that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the
sediments. The magmatic ages of the granitic orthogneisses are similar to 920–930 Ma ages of (meta)granitoids described further
northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed
a coherent block prior to widespread Mid Proterozoic granitic magmatism. 相似文献
4.
Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps) 总被引:52,自引:2,他引:50
Gerhard Vavra Rolf Schmid Dieter Gebauer 《Contributions to Mineralogy and Petrology》1999,134(4):380-404
Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone
(IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments
in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons
in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic
variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite
facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types
in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the
leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived
event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite
(300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in
the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic
(anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the
adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation
of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration
and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled
alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during
extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at
210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower
Jurassic.
Received: 7 July 1998 / Accepted: 4 November 1998 相似文献
5.
Fernando Corfu Alan Crane Desmond Moser Graeme Rogers 《Contributions to Mineralogy and Petrology》1998,133(4):329-345
The Gruinard Bay area of the mainland Lewisian complex comprises a metamorphosed suite of Archaean trondhjemites and minor
granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves. The U-Pb zircon dating yields ages
of 2731 ±14 Ma and 2728 ±2 Ma for two trondhjemite and 2732 ±4 Ma for one granite sample, also revealing the presence of large
amounts of inherited xenocrystic zircons. Although the region has been pervasively overprinted by retrogressive events in
amphibolite to greenschist facies, the textural relations between biotite, hornblende, quartz and titaniferous minerals indicate
that these minerals are pseudomorphs of pyroxene and high-Ti amphibole formed in hornblende-granulite facies. Structural relations
link this metamorphism to a steep northeast-trending fabric coeval with the intrusion of the trondhjemites, dated at 2730 Ma.
Dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging
from 2825 to 2740 Ma. This age range reflects new growth during the 2730 Ma metamorphic/metasomatic events, superimposed on
older zircon phases which include combinations of xenocrystic cores, and magmatic and/or metamorphic growth phases whose mode
of formation cannot clearly be resolved by imaging techniques (e.g. cathodoluminescence) alone. A pegmatitic vein that escaped
the D3 strain and related isotopic disturbances yields a precise age of 2792 ±2 Ma, which constrains to some degree the earliest
orogenic events in the area. Age relationships displayed in the central block at Scourie–Badcall, and in the Gruinard Bay
area indicate that petrogenetic events in both areas were comparable about 2800 Ma and that both areas underwent trondhjemitic
magmatism about 2730 Ma. In contrast, at Gruinard Bay there is no isotopic evidence for a period of high-grade metamorphism
and magmatism at 2490–2480 Ma that drastically affected the Scourie block indicating that at this stage the two regions occupied
different levels of the crust.
Received: 23 October 1997 / Accepted: 20 July 1998 相似文献
6.
M. A. Yudovskaya V. V. Distler N. V. Rodionov A. V. Mokhov A. V. Antonov S. A. Sergeev 《Geology of Ore Deposits》2011,53(1):27-57
The formation conditions and age of the Sukhoi Log gold deposit are considered on the basis of new isotopic-geochemical data.
The U-Pb isotopic study of zircon and monazite from high-grade ore and host black slates at the Sukhoi Log deposit was carried
out with SIMS technique using a SHRIMP II instrument. Two generations of monazite are distinguished on the basis of optical
and scanning electron microscopy, cathodoluminescence, and micro X-ray spectroscopy. Monazite I is characterized by black
opaque porphyroblasts with microinclusions of minerals pertaining to metamorphic slates and structural attributes of pre-
and synkinematic formation. Monazite II occurs only within the ore zone as transparent crystals practically free of inclusions
and as rims around monazite I. The REE contents are widely variable in both generations. Porphyroblastic monazite I differs
in low U and Th (0.01–0.7 wt % ThO2) contents, whereas transparent monazite II contains up to 4 wt % ThO2. The average weighted U-Pb isotopic age of monazite
I is 650 ± 8.1 Ma (MSWD = 1.6; n = 9) and marks the time of metamorphism or catagenesis. The U-Pb age estimates of synore monazite II cover the interval of
486 ± 18 to 439 ± 17 Ma. Zircons of several populations from 0.5 to 2.6 Ga in age are contained in the ore. Most detrital
zircon grains have porous outer rims composed of zircon and less frequent xenotime with numerous inclusions of minerals derived
from slates. The peaks of 206Pb/238U ages in the most abundant zircon populations fall on 570 and 630 Ma and correspond to the age of newly formed metamorphic
mineral phases. The discordant isotopic ages indicate that the U-ThPb isotopic system of ancient detrital zircons was disturbed
470–440 Ma ago in agreement with isotopic age of monazite II and the Rb-Sr whole -rock isochron age of black slates (447 ±
6 Ma). The new data confirm the superimposed character of the gold-quartz-sulfide mineralization at the deposit. Black shales
of the Khomolkho Formation of the Bodaibo Synclinorium were affected by metamorphism over a long period; the peaks of metamorphism
and catagenesis are dated at 570 and 650–630 Ma. The high-temperature ore formation was probably related to a hidden granitic
pluton emplaced 450–440 Ma ago, that is, 200 Ma later than the events of greenschist metamorphism. Hercynian granitoid magmatism
(320–270 Ma) did not exert a substantial effect on the U-Th-Pb isotopic system in accessory minerals from the ore and could
not have been a major source of ore-forming fluids. 相似文献
7.
Granulite-grade, anorthositic and mafic xenoliths recovered from a Jurassic kimberlite pipe near Kirkland Lake, Ontario are
fragments of the lower crust that underlies the ca. 2.7 Ga Abitibi greenstone belt of the Superior craton. Cathodoluminescence
imaging and/or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization
history, characterized by two main stages of zircon growth. The age of the two stages has been constrained by combining imaging
results with isotope dilution U-Pb dating of grain fragments and single grains. Minimum ages for the first crystallization
stage in individual xeno liths are 2584 ± 7 Ma, 2629 ± 8 Ma, 2633 ± 3 Ma, whereas an approximate crystallization age for a
fourth sample is 2788 ± 57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic
zircon overgrowths. Times of solid-state zircon growth are most broadly constrained in three samples to the interval between
2.52 Ga to 2.40 Ga, and most precisely dated in a meta-anorthosite at 2416 ± 30 Ma. These complex zircons are intergrown with
garnet and clinopyroxene of the host granulite-facies assemblage, and thus the Paleoproterozoic ages of the metamorphic overgrowths
are interpreted to reflect an interval of isobaric, granulite-grade metamorphism of the lower crust beneath the greenstone
belt approximately 150 million years after craton formation. This interval of metamorphism is broadly coeval with the intrusion
of the Matachewan dyke swarm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin
sediments 200 km to the south during the opening of the Matachewan ocean. It is proposed that a significant volume of magma
intruded the crust-mantle interface during rifting, promoting isobaric metamorphism and zircon growth in the deep levels of
the Superior craton. Subsequent major rifting events along this margin apparently failed to produce a similar lower crustal
response. The results have important implications for the structure of lithosphere beneath Archean continental crust.
Received: 3 October 1995 / Accepted: 11 February 1997 相似文献
8.
Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps) 总被引:11,自引:2,他引:9
U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the
presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant
monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper
intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling
age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence
of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are
also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess 206Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism.
The age of growth of the new monazite crystals is approached by their 207Pb/235U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond
to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with
decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal
activity and alkaline magmatism.
Received: 7 July 1998 / Accepted: 4 November 1998 相似文献
9.
U-Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona 总被引:1,自引:0,他引:1
Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves
prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced
by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex
monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite
analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb
zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals
that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments
at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips
of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26
million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts
the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging
reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains
of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain.
Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance
appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite
data from the third sample exhibit dispersion in both the 207Pb/206Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by
incomplete dissolution or excess (thorogenic) 206Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either
Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve
exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the 207Pb/206Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent
literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note
for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve
a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise
absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes.
Received: 9 June 1996 / Accepted: 18 October 1996 相似文献
10.
Mazeno Pass Pluton and Jutial Pluton, Pakistan Himalaya: age and implications for entrapment mechanisms of two granites in the Himalaya 总被引:1,自引:0,他引:1
D. A. Schneider M. A. Edwards P. K. Zeitler C. D. Coath 《Contributions to Mineralogy and Petrology》1999,136(3):273-284
Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two
leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate
the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ± 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant
zircon and monazite ages suggesting crystallization at 9.45 ± 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite
dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness
of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby
shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (`deformation enhanced
ascent'). Together with new field observations, these results provide an insight on the spatial and temporal relationship
between plutonism and deformation relating to the development of the massif.
Received: 31 August 1998 / Accepted: 10 March 1999 相似文献
11.
Ion microprobe (SHRIMP) dating was carried out on different zircon domains from metamorphic rocks of the HP-HT terrane of central Rhodope, northern Greece, to constrain the timing of prograde and retrograde stages within a single tectono-metamorphic
cycle. A well determined P-T-t(relative) path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations.
Ion microprobe work was assisted by cathodoluminescence (CL) images of the zircon crystals. The geochronological results revealed
that Hercynian continental crust was subducted during the Eocene. Several stages of the Eocene tectono-metamorphic cycle –
including both the prograde and retrograde parts of the P-T path above ca 300 °C, 0.3 GPa – were dated using zircons from the following rock types: (1) A deformed quartz vein probably
formed at ca 300 °C, 0.3 GPa. Zircons in this vein precipitated from a hydrothermal fluid; they yielded an age of 45.3 ± 0.9 Ma
which corresponds to the time of a low-T prograde stage of metamorphism. (2) In kyanite eclogites, zircons were entirely reset during eclogite-facies metamorphism.
Resetting was very probably enhanced by the presence of fluids derived by H2O liberating reactions close to the P-peak. They yielded an age of 42.2 ± 0.9 Ma. (3) Orthogneisses surrounding the kyanite eclogites contained zircons with magmatic
oscillatory zoned cores, which yielded Hercynian ages of 294 ± 8 Ma (age of granitic protolith formation), whereas CL-bright,
metamorphic rims yielded, like the eclogite zircons, ages of 42.0 ± 1.1 Ma. Therefore, both the eclogites and orthogneisses
are interpreted to have approached maximum depth at around 42 Ma. (4) In a leucosome of a migmatized orthogneiss, oscillatory
zoned zircons yielded an age of 40.0 ± 1.0 Ma. At this time the rocks reached maximum temperatures during early decompression.
(5) A late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization
age of 36.1 ± 1.2 Ma. Thus, the whole tectono-metamorphic cycle above ca 300 °C, 0.3 GPa lasted from 45.3 ± 0.9 Ma to 36.1 ± 1.2 Ma,
that is 9.2 Ma with an extreme error value of 2.1 Ma. Based on combined SHRIMP and petrological data, the average rates of
heating and burial during subduction (above ca 300 °C, 0.3 GPa) are >94 °C/Ma and >15 mm/a, respectively. Rates of cooling
and exhumation (also above 300 °C, 0.3 GPa) are calculated to be >128° C/Ma and >7.7 mm/a. The Eocene age of metamorphism
in central Rhodope implies that the terrane of, at least, central Rhodope and the Cyclades very probably was part of the same
continental crust.
Received: 5 October 1998 / Accepted: 18 January 1999 相似文献
12.
点苍山-哀牢山杂岩带多期变质作用:嘎洒地区变沉积岩锆石微量元素与U-Pb年代学制约 总被引:2,自引:2,他引:0
点苍山-哀牢山杂岩带位于青藏高原东南缘,为云南三江地区一条重要的造山带,由扬子板块和印支板块于晚二叠世-中晚三叠世碰撞拼合而成。杂岩带主要由各类副片麻岩、片岩、石英岩、大理岩和斜长角闪岩构成,岩石发育强烈糜棱岩化和深熔作用。本文选取哀牢山北段新平嘎洒地区变沉积岩为研究对象,通过对变沉积岩锆石的阴极发光图像、微量元素、矿物包裹体组合、表面形态和U-Pb年龄的综合研究,揭示出嘎洒地区哀牢山杂岩经历了两期变质事件:其中,含石榴子石斜长二云母片岩中30颗变质锆石获得了较为一致的206Pb/238U年龄215±6Ma~227±5Ma,加权平均年龄为222.3±1.2Ma(n=30,MSWD=0.27),这些锆石具有浑圆状或椭圆状形态、较为均匀的阴极发光图像、平坦的HREE配分模式((Lu/Gd)N=0.73~4.08)和弱的负Eu异常,这些特征与典型的高级变质岩中变质锆石相似,而锆石的Th/U比值较为分散为0.06~0.84,平均值为0.45,可能与变质过程中富Th矿物独居石分解有关。变质年龄与杂岩带中点苍山和元阳地区变质岩中、晚三叠世变质年龄极为吻合,指示这期变质事件与中-晚三叠世古特提斯洋闭合-造山有关,标志着点苍山-哀牢山杂岩带为三江地区一条重要的古缝合线。此外,嘎洒地区夕线石榴黑云二长片麻岩的岩相学特征显示,岩石经历了石榴子石的转熔作用,除两颗锆石年龄为35.4Ma外,28颗锆石(增生边)给出了误差范围内较为一致的206Pb/238U年龄(27.3±0.5Ma~31.9±0.5Ma),加权平均年龄为29.4±0.53Ma(n=28,MSWD=2.0)。这些锆石的增生边中的矿物包裹体组合为夕线石+钾长石+黑云母+石英+独居石,且具有较低的Th/U比值(0.01~0.1),平坦的重稀土(HREE)配分模式((Lu/Gd)N=0.45~7.59)和中等程度的负Eu异常,这些特征指示该类锆石为典型的变质锆石。变质年龄与新生代红河-哀牢山剪切带内大量发现的同剪切岩浆岩、变质岩的年龄较为一致,指示这期年轻的变质事件与岩石圈尺度大规模剪切运动有关。此外,两类变沉积岩中6颗继承性碎屑锆石的年龄分布范围为528~783Ma,这些锆石具有锥形的锆石形态,清晰的振荡环带,表面发育蚀痕和凹坑,较高的Th/U比(0.1),陡倾的HREE配分曲线,表明这些锆石为经过剥蚀-搬运-沉积的岩浆锆石,具有继承性碎屑锆石的特征,说明哀牢山杂岩变沉积岩中至少应包含新元古代和早古生代的沉积物源,指示研究区哀牢山杂岩带部分岩石并不属于真正意义上的扬子结晶基底。 相似文献
13.
L. D. Ashwal R. A. Armstrong R. J. Roberts M. D. Schmitz F. Corfu C. J. Hetherington K. Burke M. Gerber 《Contributions to Mineralogy and Petrology》2007,153(4):389-403
Nepheline-bearing gneisses from the 75 km2 Tambani body in the Mozambique Belt of southern Malawi, are miaskitic biotite-nepheline monzodiorites, reflecting an absence
of K-feldspar, alkali amphiboles or pyroxenes, and contain euhedral zircon megacrysts up to 5 cm across. The zircons contain
U = 1–1,860 ppm, Th = 0–2,170 ppm and Y = 400–1,100 ppm, and very low concentrations of all other measured trace elements
except Hf (HfO2 = 0.53–0.92 wt. %). Cathodoluminescence images reveal oscillatory sector growth zoning and no evidence for xenocrystic cores,
indicating that the zircons represent primary magmatic crystallization products that have survived amphibolite grade metamorphism.
U-Pb isotopic analyses (by TIMS) yield an upper intercept age of 730 ± 4 Ma (MSWD = 1.7), which we interpret as the time of
magmatic crystallization of the zircons. This is coincident with 11 SHRIMP spot analyses, which yield a mean age of 729 ±
7 Ma (MSWD = 0.37). Metamorphism, at 522 ± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization
of zircon. Lu-Hf isotopic data for three whole-rock samples of nepheline-bearing gneiss are collinear with those for zircon
megacrysts, and correspond to an age of 584 ± 17 Ma (MSWD = 0.37. We interpret the Lu-Hf array to represent a mixing line
defined by the Hf isotopic signature of primary zircon and that of the rock-forming minerals reset during metamorphic (re-)crystallization;
hence the 584 Ma age is likely geologically meaningless. Given the well-defined association of nepheline syenites (and phonolitic
volcanic equivalents) with continental rifting, we suggest that the Tambani body represents a magmatic product formed at 730 Ma
during the break-up of the Rodinia supercontinent. The 522 Ma age is akin to other Pan-African metamorphic ages that record
collisional suturing events during the final assembly of Gondwana. Zircon-bearing nepheline gneisses thus preserve a record
of intra-continental rifting and of continental collision in southern Malawi. 相似文献
14.
O. Bruguier D. Bosch R. T. Pidgeon D. I. Byrne L. B. Harris 《Contributions to Mineralogy and Petrology》1999,136(3):258-272
Conventional and SHRIMP U-Pb analyses of zircon, monazite, titanite and apatite from the high grade rocks of the Northampton
Complex in Western Australia provide constraints on the timing of metamorphic processes and deformation events in the northern
Darling Mobile Belt (western margin of the Archean Yilgarn Craton). Paragneisses and mafic volcanics and/or intrusions have
undergone granulite facies metamorphism in a probable extensional tectonic setting prior to formation of W- to NW-verging
folds and thrusts cut by normal shears (interpreted as late collapse structures) during the main deformation event (D1). These structures are folded by open to tight folds with NW-striking axial surfaces developed in a second, NE-SW contractional
event (D2). Zircons from a mafic granulite provide an age of 1079 ± 3 Ma attributed to new zircon growth prior to, or at the peak of
regional granulite facies metamorphism. Metamorphic monazites extracted from a paragneiss yield an identical age of 1083 ± 3 Ma.
The similarity of ages between zircons from the mafic granulite (1079 ± 3 Ma) and monazites from the paragneiss (1083 ± 3 Ma)
is interpreted to reflect fast cooling and/or rapid uplift, which is consistent with thrusting of the gneissic units during
the first deformation event (D1) associated with the onset of retrograde metamorphism. Granitic activity at 1068 ± 13 Ma was followed by intrusion of post-D2 pegmatite (989 ± 2 Ma), which constrains the end of metamorphism and associated deformation. Cooling of the complex to about
500 °C is timed by the apatite age of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons from a paragneiss sample yield a maximum
age of 2043 Ma, with no evidence of an Archean Yilgarn signature. A majority of ages between 1.6 and 1.9 Ga are consistent
with derivation from the Capricorn Orogen on the northern margin of the Yilgarn Craton. Younger detrital zircons with 1150–1450 Ma
ages, however, indicate an additional source that had undergone early Grenvillian igneous or metamorphic event(s) and also
places a maximum age constraint upon deposition. The source of this clastic material may have been from within the southern
Darling Mobile Belt or from Greater India (adjacent to the Northampton Complex in Rodinia reconstructions). This study documents
an extended Grenvillian history, with basin formation, sedimentation, granulite facies metamorphism, contractional tectonics
(two periods with orthogonal directions of shortening) and late pegmatite emplacement taking place between 1150–989 Ma on
the western margin of the Yilgarn Craton. Ages recorded in this study indicate that the proposed global distribution of Grenvillian
belts during assembly of the Rodinia supercontinent should be reassessed to include the Darling Mobile Belt.
Received: 7 January 1998 / Accepted: 10 March 1999 相似文献
15.
乌拉山岩群是狼山地区最重要的前寒武纪变质基底之一,准确测定其原岩成岩与变质时代,对于进一步探讨狼山地区前寒武纪地质演化具有重要的意义。对狼山地区乌拉山岩群角闪黑云斜长片麻岩及其伴生的花岗质浅色脉体进行了岩石学和锆石U-Pb年代学研究。碎屑锆石U-Pb定年和野外地质调查表明,狼山地区乌拉山岩群角闪黑云斜长片麻岩碎屑锆石年龄介于2591~1800Ma之间,其中最小一组碎屑锆石年龄为1873Ma,结合其约270Ma的变质年龄,初步限定乌拉山岩群角闪黑云斜长片麻岩的原岩沉积年龄为1873~270Ma。综合新的研究资料,认为狼山地区乌拉山岩群除存在新太古代—古元古代变质岩外,可能还存在中元古代—晚古生代变沉积岩。锆石阴极发光图像与U-Pb定年结果综合表明,角闪黑云斜长片麻岩中发育大量变质锆石,获得的206Pb/238U年龄加权平均值为269±4Ma,代表狼山地区乌拉山岩群遭受晚古生代末期角闪岩相变质作用的时代,可能与华北板块与西伯利亚板块晚古生代末期碰撞造山作用有关。此外,采用预剥蚀方法,在乌拉山岩群高硅花岗质浅色脉体高U锆石中,获得的~(206)Pb/~(238)U年龄加权平均值为264±3Ma,被解释为乌拉山岩群花岗质浅色脉体的形成时代,代表本区晚古生代造山作用由同碰撞挤压向碰撞后伸展转换的时限。 相似文献
16.
蒙古戈壁阿尔泰巴音陶勒盖地区二长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义 总被引:1,自引:1,他引:0
蒙古戈壁阿尔泰省巴音陶勒盖地区地处南戈壁-阿尔泰构造带南缘,区内构造活动活跃,中酸性侵入岩比较发育。采用LA-ICP-MS锆石U-Pb测年技术,对区内二长花岗岩中的锆石进行同位素年龄研究。阴极发光图像显示,锆石颗粒多为自形-半自形,且有明显的初始岩浆振荡环带,少见蚀变微区,结合较高的Th/U值(0.65~1.79),断定其为典型的岩浆成因锆石;30颗锆石的~(206)Pb/~(238)U年龄加权平均值为300.2±0.9Ma,显示二长花岗岩成岩时代为晚石炭世,代表了华力西中期的一次构造-岩浆事件,为约束南戈壁-阿尔泰构造带晚石炭世深成岩浆活动时限提供了新证据。 相似文献
17.
秦岭富水杂岩的变辉长岩中斜锆石与锆石U-Pb同位素年龄的差异及其地质意义 总被引:4,自引:0,他引:4
秦岭富水杂岩体的一些变辉长岩含有粒度较大,U、Pb含量较高的斜锆石和锆石,是U-Pb同位素测年的极好矿物。本文对秦岭富水杂岩的中粗粒角闪黑云辉长岩中的斜错石和错石分别进行了SHRIMP法和TIMS法U-Pb同位素年龄测定,获得斜锆石和锆石的U-Pb同位素年龄分别为501.4±1.2 Ma和480.0±3.4 Ma,二者相差约20 Ma;对该岩石中的斜锆石和锆石的关系及锆石的成因进行了初步研究,认为斜锆石的U-Pb同位素年龄应可解释为秦岭富水杂岩中基性岩石的形成时代,而锆石的成因比较复杂,对其U-Pb同位素年龄地质意义的合理解释需作进一步的研究。 相似文献
18.
In the Rogaland–Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases
are reported: a phase of regional metamorphism linked to orogenic thickening (M1) and a phase of low-pressure thermal metamorphism
associated with the intrusion of the 931 ± 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite
facies to the west of the terrane and M2 culminated locally at 800–850 °C with the formation of dry osumilite-bearing mineral
associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples,
mainly from a suite of 1050 +2/−8 Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during
breakdown of allanite and titanite in upper amphibolite facies at 1012–1006 Ma. In the Feda suite and other charnockitic gneisses,
concordant to slightly discordant monazite at 1024–997 Ma probably reflects breakdown of biotite during granulite-facies M1
metamorphism. A spread of monazite ages down to 970 Ma in biotite ± hornblende samples possibly corresponds to the waning
stage of this first event. In the Feda suite, a well defined monazite growth episode at 930–925 Ma in the amphibolite-facies
domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite
was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex.
The M2 event was shortly followed by cooling through ca. 610 °C as indicated by tightly grouped U-Pb ages of accessory titanite
and titanite relict inclusions at 918 ± 2 Ma over the entire region. A last generation of U-poor monazite formed during regional
cooling below 610 °C, in hornblende-rich samples at 912–904 Ma. This study suggests: (1) that monazite formed during the prograde
path of high-grade metamorphism may be preserved; (2) that monazite ages reflect primary or secondary growth of monazite linked
to metamorphic reactions involving redistribution of REEs and Th, and/or fluid mobilisation; (3) that the U-Pb system in monazite
is not affected by thermal events up to 800–850 °C, provided that conditions were dry during metamorphism.
Received: 9 January 1997 / Accepted: 15 April 1998 相似文献
19.
20.
F. P. Florence R. S. Darling S. E. Orrell 《Contributions to Mineralogy and Petrology》1995,121(4):424-436
Garnet-sillimanite-biotite gneiss near Port Leyden, in the western Adirondack Highlands, New York, contains mineral assemblages
and textures that formed during high temperature metamorphism and anatexis at mid-crustal pressures. Evidence for melting
includes thin, plagioclase-rich veins, sieve textures in biotite, and the presence of small, euhedral garnet neoblasts. Hercynite-silicate
equilibria in combination with the solidus for biotite dehydration melting indicate metamorphic pressure was between 4 and
6.4 kbar at the temperature of melting (ca. 735° C). The gneiss is intruded by a small, discordant Fe-Ti oxide-apatite (nelsonite)
dike. Reported field occurrences of nelsonite demonstrate its common association with anorthosite plutons. Although no anorthosite
bodies are exposed in the Port Leyden region, the presence of nelsonite is evidence of anorthositic magmatism in the western
Adirondacks. Post-intrusion metamorphism has caused partial apatite recrystallization and produced a weak foliated texture
in the dike. U-Pb ages from zircon and monazite from both the gneiss and the nelsonite dike indicate that these rocks experienced
a complex, polymetamorphic history that we interpret to reflect two thermal episodes. An older event is recorded by discordant
zircons in the gneiss, which indicate a minimum age of 1129±6 Ma. A linear best fit to the data yields an upper intercept
at 1166±53 Ma. This range of ages coincides with anorthosite-suite magmatism in the Adirondacks. A minimum zircon age of 1104±3 Ma
was obtained from the nelsonite dike. Lead-loss or late zircon crystallization at about 1020 Ma affected the U-Pb systematics
of zircon in the dike. Monazite ages from both rocks also indicate high temperature metamorphism (>700° C) between 1040 and
960 Ma. The older zircon ages and textural relations in the metapelite are viewed as evidence for anatexis at ca. 1150 Ma,
and the presence of nelsonite suggests that the intrusion of anorthosite was coincident with partial melting in the gneiss.
P-T estimates of metamorphism, therefore, imply that anorthosite was emplaced to about 15 km depth in the western Adirondack
Highlands.
Received: 13 September 1994 / Accepted: 10 May 1995 相似文献