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1.
The Southern Gemericum basement in the Inner Western Carpathians experienced a polyphase regional deformation. Differences in the pre-Alpine and Alpine events have been constantly discussed. To address this, monazites from metapelites and acid metavolcanic rocks were dated using the Th–U–Pb electron microprobe method. Three monazite generations, such as Precambrian, Early Paleozoic, and Alpine, have been recognized in the greenschist facies pelites and acid metavolcanic rocks of the Southern Gemericum basement. Both inherited magmatic monazite grains in metavolcanites and rare relics of detrital monazites within the polyphase monazite grains in metapelites yielded the Precambrian age in the time span of 550–660 Ma. They prove the provenance and derivation from deeper crustal Cadomian fragments. High-Y magmatic monazites of Early Paleozoic age (444 ± 13 and 477 ± 7 Ma) have been recorded in the acid metavolcanites and their metavolcaniclastics. These ages roughly fit within the previously published magmatic zircon age determinations (at 494 ± 1.7 and 464 ± 1.7 Ma) that clearly indicate two-phase volcanic activity in the Early Paleozoic Southern Gemericum basin. The Early Paleozoic magmatic monazites were partly overprinted by the low-Y Alpine monazites (133 ± 5 and 184 ± 16 Ma) at their rims. In Al-rich metapelites, the newly formed low-Y monazites of Alpine age commonly occur, reflecting the polystage compression geodynamic evolution with three distinct peaks at 100 ± 8, 133 ± 5, and 190 ± 16 Ma, respectively. No data as the evidence of the pre-Alpine metamorphic events were observed in metapelites. Only some monazites yield the age indications for the Permian extensional thermal re-heating (260–290 Ma). The monazite age data from the Southern Gemericum basement indicate the strong overprinting due to the polyphase Alpine deformation at least in the greenschist facies conditions. 相似文献
2.
We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337). 相似文献
3.
An investigation of the chemical composition of monazite from a number of localities has been carried out. Samples used include
monazites from metamorphic rocks, granitic rocks and a hydrothermal ore deposit. The REE distribution pattern of monazite
varies greatly in accordance with its geological environment. A remarkable feature of the monazites studied is that their
chondrite-normalised REE distribution patterns are mostly uniform between grains within the same sample, but differ significantly
from sample to sample. This characteristic apparently indicates that there is an important effect on the REE distribution
of monazite exerted by the host rock or source material from which monazite crystallised. Another important feature shown
by the monazites studied is that monazites in rocks containing garnet as a major mineral show extreme depletion of HREE, whereas
monazites in rocks without garnet or monazite that formed after the garnet breakdown contain significantly higher amounts
of Y and HREE. This suggests that the phase assemblage, especially garnet, plays an important role in the REE distribution
of monazites in these rocks. The value of REE distribution in monazite is exemplified with regard to the origin of monazite
in the Lewisian metamorphic rocks, which is a fundamental issue in monazite geochronology.
Received: 17 March 1999 / Accepted: 16 July 1999 相似文献
4.
U-Pb isotope analyses, rare earth and trace element analyses, and petrographic observations are presented for monazites from
the Wepawaug Schist in southern Connecticut, USA. Two samples of kyanite zone schist were collected less than a meter apart.
Each sample contains a different variety of monazite with distinct morphology, chemistry, and Pb isotopic composition. One
sample preserves a largely amphibolite facies mineralogy, including kyanite, staurolite, garnet, biotite, and chlorite, with
little textural evidence of later shearing. Monazites from this sample are xenoblastic with about 1 wt% ThO 2, 0.3 wt% CaO, and a more LREE enriched pattern than monazites from the second sample. These xenoblastic monazites preserve
textural evidence of a retrograde reaction to apatite which involves chlorite, indicating that these monazites became unstable
during retrograde chloritization of biotite. These monazites give strongly discordant U-Pb ages which fit a chord with an
upper intercept age of 411±18 Ma, interpreted as the minimum growth age of these xenoblastic monazites, perhaps during amphibolite
facies metamorphism. The second sample contains S-C banding, evidence of dynamic recrystallization, and abundant retrograde
chlorite. This sample contains idioblastic monazites with about 3 wt% ThO 2, 0.8 wt% CaO, and with less fractionated REE patterns. These monazites give close to concordant U-Pb ages with a mean 207Pb*/ 206Pb* age of 388 ± 2 Ma. This age is interpreted as probably representing the time of monazite growth during retrogression of
the sample from an amphibolite to greenschist facies mineralogy.
Received: 26 June 1995/Accepted: 25 May 1996 相似文献
5.
Mt. Narryer and Jack Hills meta-sedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia are
of particular importance because they yield Hadean detrital zircons. To better understand the tectonothermal history and provenance
of these ancient sediments, we have integrated backscattered scanning electron images, in situ U–Pb isotopic and geochemical
data for monazites from the meta-sediments. The data indicate multiple periods of metamorphic monazite growth in the Mt. Narryer
meta-sediments during tectonothermal events, including metamorphism at ~3.3–3.2 and 2.7–2.6 Ga. These results set a new minimum
age of 3.2 Ga for deposition of the Mt. Narryer sediments, previously constrained between 3.28 and ~2.7 Ga. Despite the significant
metamorphic monazite growth, a relatively high proportion of detrital monazite survives in a Fe- and Mn-rich sample. This
is likely because the high Fe and Mn bulk composition resulted in the efficient shielding of early formed monazite by garnet.
In the Jack Hills meta-sediments, metamorphic monazite growth was minor, suggesting the absence of high-grade metamorphism
in the sequence. The detrital monazites provide evidence for the derivation of Mt. Narryer sediments from ca. 3.6 and 3.3 Ga
granites, likely corresponding to Meeberrie and Dugel granitic gneisses in the Narryer Gneiss Complex. No monazites older
than 3.65 Ga have been identified, implying either that the source rocks of >3.65 Ga detrital zircons in the sediments contained
little monazite, or that >3.65 Ga detrital minerals had experienced significant metamorphic events or prolonged sedimentary
recycling, resulting in the complete dissolution or recrystallization of monazite. 相似文献
6.
Detrital monazites can be used to reconstruct the tectonothermal history of their provenance. Dating of beach monazites from Alleppey by EPMA U-Th-Pb total technique using the centroid method demonstrates that the ages recorded by even a small number of 28 analyses from a single thin section grain mount can potentially reveal a considerable portion of the tectonothermal history of the Trivamdrum block. Three monazite populations were identified that yielded ages 605 ± 9, 575 ± 23 and 548 ± 11 Ma, which have their more or less exact counterparts reported from the Trivandrum Block. One monazite grain yielded random Paleoproterozoic ages ranging from 1756 Ma to 1345 Ma representing similar ages which earlier have been interpreted as due to differential Pb-loss from >2000 Ma monazites during the late Neoproterozoic-Cambrian metamorphism. 相似文献
7.
The pre-Mesozoic, mainly Variscan metamorphic basement of the Col de Bérard area (Aiguilles Rouges Massif, External domain)
consists of paragneisses and micaschists together with various orthogneisses and metabasites. Monazite in metapelites was
analysed by the electron microprobe (EMPA-CHIME) age dating method. The monazites in garnet micaschists are dominantly of
Variscan age (330–300 Ma). Garnet in these rocks displays well developed growth zonations in Fe–Mg–Ca–Mn and crystallized
at maximal temperatures of 670°C/7 kbar to the west and 600°C/7–8 kbar to the east. In consequence the monazite is interpreted
to date a slightly pressure-dominated Variscan amphibolite-facies evolution. In mylonitic garnet gneisses, large metamorphic
monazite grains of Ordovician–Silurian (~440 Ma) age but also small monazite grains of Variscan (~300 Ma) age were discovered.
Garnets in the mylonitic garnet gneisses display high-temperature homogenized Mg-rich profiles in their cores and crystallized
near to ~800°C/6 kbar. The Ordovician–Silurian-age monazites can be assigned to a pre-Variscan high-temperature event recorded
by the homogenised garnets. These monazite age data confirm Ordovician–Silurian and Devonian–Carboniferous metamorphic cycles
which were already reported from other Alpine domains and further regions in the internal Variscides. 相似文献
8.
The evolution of the basement of southern Madagascar north and south of the Ranotsara shear zone was investigated using (U + Th)/Pb electron probe monazite age dating in combination with petrographic constraints. Several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic, metamorphic and/or fluid events. The oldest ages, ranging from 630–2400 Ma, occur relatively rare in relic cores. A first, clear age-population is dated at 550–560 Ma. Most ages fall in two populations at 420–460 and 490–500 Ma, which in some samples overlap in error. We interprete these ages as dating low-pressure and high-temperature metamorphism. We have also clear evidence for Carboniferous (300–310 Ma) monazite overgrowth rims, which can not directly be related to macroscopic structures or metamorphic parageneses. In combination with literature data, we propose that the observed monazite age populations are related to Gondwana amalgamation and subsequent rifting events during the break up of Gondwana. Our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of Madagascar. 相似文献
9.
The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO 2, and ∼0.94 wt. % UO 2. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation. 相似文献
10.
This paper reports the results of CHIME (chemical Th–U–Pb isochron method) dating of detrital monazites from Carboniferous sandstones in the Upper Silesia Coal Basin (USCB). A total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence. Age distributions were identified in detrital monazites from the USCB sequence and correlated with specific dated domains in potential source areas. Most monazites in all samples yielded ca. 300–320 Ma (Variscan) ages; however, eo-Variscan, Caledonian and Cadomian ages were also obtained. The predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic Bohemian Massif (BM), which suggests that various crystalline lithologies in the BM were the dominant sources of USCB sediments. 相似文献
11.
以化学法独居石电子探针定年的原理以及前人改良过的年龄计算公式为基础,利用全微分和最大误差原理,确定了年龄误差计算新方法。并用Visual C语言在Windows平台下编制出了计算年龄和年龄误差的计算机程序。运用此程序对前人公开发表的分析资料和计算的年龄以及年龄误差数据进行了重新计算,验证了给出的误差公式的可靠性。同时,利用JXA—8100电子探针仪对新疆东天山变质沉积岩的两个样品(KM2127—5,DK2107—2)中的独居石进行了电子探针微区U—Th—Pb成分分析。结果表明,样品DK2107—2记录了两期变质作用,峰期年龄分别是341.0±3.9Ma和255.2±3.3Ma,其中最主要的变质峰期年龄为341.0±3.9Ma,该期的矿物组合为Ky+St+Bt+Pl+Q+/-Or;而255.2±3.3Ma和样品KM2127-5记录的变质峰期年龄262.3±4.4Ma为次要变质峰期年龄,该期矿物组合是Cord+Bt+Pl+Or+Q。分析结果与前人用^40Ar—^39Ar法得到的结果相吻合,表明独居石电子探针定年技术是一种可靠有效的测年方法。 相似文献
12.
本文报道了大兴安岭南段林西地区新发现的那斯嘎吐云英岩型铍矿点的独居石U-Pb年龄、主微量元素及Nd同位素组成,并与大兴安岭南段同期的经棚高分异碱长花岗岩中的独居石地球化学数据进行对比,以探讨云英岩型铍矿化的形成时代与成因。那斯嘎吐云英岩主要呈带状赋存于早白垩世(~140Ma)碱长花岗岩岩体顶部,矿物组成包括石英、白云母、萤石、独居石以及锡石-绿柱石等铍-锡稀有金属矿物,其全岩Be含量可达397×10-6~13908×10-6。那斯嘎吐富铍云英岩-云英岩化花岗岩与经棚贫铍碱长花岗岩中的独居石均具有较高的ThO2(平均值为3.01%~6.88%)和较低的UO2含量(< 0.55%),并呈现出富集轻稀土元素、亏损重稀土元素和强烈Eu负异常(δEu < 0.08)的地球化学特征。与经棚岩浆成因独居石(TE1,3=0.95~1.00)相比,那斯嘎吐云英岩化相关的独居石具有明显高的TE1,3值(0.98~1.26)。岩相学和矿物地球化学研究结果表明,那斯嘎吐独居石形成于岩浆-热液过渡阶段,起源于高演化花岗质岩浆的富钠钙氟流体在其形成过程起到了关键作用。独居石LA-ICP-MS U-Pb同位素定年结果显示,那斯嘎吐云英岩型铍矿化形成于早白垩世(~139Ma),与赋矿碱长花岗岩时代一致。那斯嘎吐独居石的εNd(t)值为-0.29~0.95,与经棚独居石的εNd(t)值(-2.38~-1.68)和大兴安岭南段早白垩世高分异花岗岩的全岩εNd(t)值(-2.51~2.11)基本相当。独居石年龄和Nd同位素结果指示,那斯嘎吐富铍云英岩的形成与早白垩世花岗质岩浆高度分异晚期熔-流体相互作用密切相关,源区主要由新生和古老地壳物质共同组成。综合区域地质资料,查明早白垩世是大兴安岭南段Sn-W-Li-Be等稀有金属成矿作用的一个重要时期,其形成于古太平洋板块回卷诱发的岩石圈伸展环境。那斯嘎吐云英岩型铍矿的发现表明大兴安岭南段具有铍成矿的良好前景,林西地区是未来铍矿产勘查的重点靶区,值得在今后研究过程中予以充分关注。 相似文献
13.
独居石U-Th-Pb定年在地质年代学研究中占有重要地位,但其成因类型的识别和年龄值的合理地质解释始终是研究的难点。论文通过对比分析岩浆、热液和沉积成因独居石的内部结构、矿物组合以及稀土和微量元素地球化学特征,厘定了不同成因类型独居石的鉴别标志;并以三江特提斯构造演化研究为例,探讨了独居石的U-Th-Pb定年及其合理地质应用。不同地质环境中形成的独居石具有不同的内部结构和矿物组合及元素组成:岩浆独居石可能存在较宽且平直的岩浆环带,常与磷钙钍石共生;相对富集重稀土,具有较高的Y、Sc、Th、U、Sm、Gd等含量和Th/U比值,较低的La、Ce含量和稀土总量,显示为Eu强烈亏损的右倾斜分布模式。而热液独居石可能存在与流体作用有关的扇形环带,常与斜钍石共生;沉积独居石形态多不规则,与自生矿物相伴生。它们具有典型的四分组稀土分布模式,稀土总量较高,相对富集轻稀土(如La、Ce),贫Y、Sc、Th、U、Sm、Gd等,Th/U比值较低。独居石U-Th-Pb定年限定了松潘-甘孜造山带204~190Ma的早期变质作用和168~158Ma的局部热干扰,约140~130Ma冈底斯浅色花岗岩的形成与地壳的缩短加厚及快速隆升作用,约33~22Ma 红河断裂带与伸展作用有关的岩体侵位和左行走滑运动年代(且北部的去顶剥蚀作用要比中部早9Ma),约8~11Myr的东喜马拉雅构造结"挤出"构造变形时限。 相似文献
14.
The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperature metamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provoked generalized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean to Paleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatite and leuconorite within the high- T shear zones that make up the eastern continuation of the huge E–W Patos shear belt. These data were also constrained by using the Sm–Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The results suggest an age of about 578 to 574 Ma for the peak of the widespread high- T metamorphism. This event is best recorded by Sm–Nd garnet-whole rock ages. The U–Th–Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553 ± 10 Ma. When compared to the Sm–Nd garnet-whole rock ages, the U–Th–Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peak of the high- T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, the U–Th–Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete re-opening and resetting of the U–Th–Pb isotopic system produced by the action of low- T Ca-rich fluid. 相似文献
15.
Microanalytical studies of basement rocks below the Athabasca sandstone basin indicate that monazite is the dominant uranium-bearing
mineral in the study area. Drill core samples of hydrothermally altered basement show that monazite is commonly altered to
a Th–silicate phase, and uranium has been significantly mobilized. On average, 75% of the uranium bound to monazite is leached
out during monazite alteration. In contrast, no substantial mobilization of uranium from detrital minerals (e.g. zircon) has
yet been observed in the Athabasca sandstones. It is suggested that hydrothermal alteration of granitic rocks (especially
potassic pegmatoids and potassic orthogneisses) of the sub-Athabasca basement, represents the most important uranium source
for the unconformity-type deposits.
Received: 3 December 1999 / Accepted: 24 May 2000 相似文献
16.
Zircons of 10–100μm size and monazites of up to 10μm size are present in rhyolite and trachyte dikes associated with Deccan
basalts around Rajula in the southern Saurashtra Peninsula of Gujarat. On the basis of structural conformity of the felsic
and basaltic dikes, K-Ar ages and trace element considerations, a previous study concluded that the felsic rocks are coeval
with the Deccan Volcanics and originated by crustal anatexis. The felsic rocks contain two populations of zircons and monazites,
one that crystallized from the felsic melt and the other that contains inherited crustal material. Trace element variations
in the rhyolites and trachytes indicate that zircons and monazites crystallized from the felsic melts, but compositional analysis
of a zircon indicates the presence of a small core possibly inherited from the crust. Hf compositional zoning profile of this
zircon indicates that it grew from the host rhyolitic melt while the melt differentiated, and Y and LREE contents suggest
that this zircon crystallized from the host melt. Pb contents of some monazites also suggest the presence of inherited crustal
cores. Hence, any age determination by the U-Th-Pb isotopic method should be interpreted with due consideration to crustal
inheritance. Temperatures estimated from zircon and monazite saturation thermometry indicate that the crust around Rajula
may have been heated to a maximum of approximately 900°C by the intruding Deccan magma. Crustal melting models of other workers
indicate that a 1–2 million year emplacement time for the Deccan Traps may be appropriate for crustal melting characteristics
observed in the Rajula area through the felsic dikes. 相似文献
17.
Based on particular examples, this paper considers the capabilities and constraints of the step-leaching Pb-Pb dating (PbSL)
of metamorphogenic minerals. It was shown that stepwise leaching allows the separation of fractions enriched in uranogenic
and thorogenic Pb isotopes, which can be used for the determination of mineral ages and the time of crystallization of coexisting
equilibrium and, occasionally, disequilibrium monazites. The presence of monazite in a mineral is indicated by a high Th/U
ratio similar to that of monazite. The main limitation of the method is related to the presence in minerals of both disequilibrium
domains of the mineral matrix and disequilibrium monazite microinclusions. By the example of minerals studied, we discussed
three scenarios for the development of the U-Th-Pb isotopic systems of metamorphogenic minerals. 相似文献
18.
A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists. 相似文献
19.
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
20.
The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and 208Pb/ 232Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th-Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th-Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3Dž.5 Ma in the Buchan terrain, 352.9NJ.9 Ma in the Barrovian terrain, and 403.4LJ.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1Nj.1 Ma. Interpretation of Th-Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism. 相似文献
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