scholarly journals IDADE U-Th-Pb DA MONAZITA E CORONAS DE REAÇÃO FOSFATO-SILICATO DE METACHARNOENDERBITO NEOARQUEANO DO SUL DO CRÁTON SÃO FRANCISCO (MG)

2016 ◽  
Author(s):  
Daniel Andrade Miranda ◽  
Alexandre de Oliveira Chaves
Keyword(s):  
Electron Microprobe ◽  
Granulite Facies ◽  
Amphibolite Facies ◽  
Metamorphic Fluids ◽  
Phosphate Silicate ◽  
Chemical Dating

ResumoNa porção Sul do Cráton São Francisco (MG), a oeste da cidade de Bom Sucesso, ocorrem litotipos neoarqueanos metamorfizados que variam desde termos ultramáficos, gabro-noríticos, enderbíticos a charnockíticos, e graníticos. Proveniente desta associação, um metacharnoenderbito cristalizado e metamorfizado em fácies granulito por volta de 2700 Ma, guarda cristais de monazita com coronas de apatita, allanita e epidoto surgidas pela interação da monazita com fluidos metamórficos tardios contendo Ca, Fe, Si e Al. Através da microssonda eletrônica, as composições dos cristais de monazita foram determinadas e seus teores de U, Th e Pb permitiram a obtenção de uma idade química neoarqueana média de 2657 ± 36 Ma. Sugere-se que ela corresponda à idade do retrometamorfismo de fácies anfibolito do metacharnoenderbito desenvolvido durante o período tardi-colisional Neoarqueano do evento Rio das Velhas, momento durante o qual as coronas de reação fosfato-silicato teriam se desenvolvido contemporaneamente ao crescimento de anfibólio e biotita da referida rocha.Palavras Chave: monazita, datação química U-Th-Pb, coronas de reação fosfato/silicato, Neoarqueano, Cráton São Francisco.AbstractMONAZITE U-Th-Pb AGE AND PHOSPHATE-SILICATE REACTION CORONAS OF A NEOARCHEAN METACHARNOENDERBITE FROM SOUTHERN SAO FRANCISCO CRATON (MG). In the southern portion of the São Francisco Craton (MG), west of the Bom Sucesso city, neoarchean metamorphosed lithotypes outcrop, which include ultramafic bodies, gabbro-norites, enderbites to charnockites, and granites. From this association, a metacharnoenderbite crystallized and metamorphosed under granulite facies around 2700 Ma yielded monazite crystals with apatite, allanite and epidote coronas arising due to the interaction of monazite with metamorphic fluids containing Ca, Fe, Si and Al. By electron microprobe, an average chemical age of 2657 ± 36 Ma was obtained. It is suggested that it corresponds to the age of amphibolite facies retrometamorphism developed during the Neoarchean late-collisional period of the Rio das Velhas event, during which time the phosphate-silicate reaction coronas would have developed contemporarily to the growth of amphibole and biotite in such rock.Keywords: Monazite, U-Th-Pb chemical dating, phosphate/silicate reaction coronas, Neoarchean, São Francisco Craton.

scholarly journals DESENVOLVIMENTO DO MÉTODO DE DATAÇÃO QUÍMICA U-Th-Pb DE MONAZITA POR MICROSSONDA ELETRÔNICA NA UFMG

2013 ◽  
Author(s):  
Alexandre de Oliveira Chaves ◽  
Elizabeth Kerpe de Oliveira ◽  
Luiz Rodrigues Armoa Garcia
Keyword(s):  
Electron Microprobe ◽  
Chemical Data ◽  
Common Lead ◽  
Icp Ms ◽  
Dating Method ◽  
Physics Department ◽  
Chemical Dating

O método de datação química U-Th-Pb (não-isotópica) de monazita por microssonda eletrônica vem sendo desenvolvido há pelomenos 20 anos e já tem o reconhecimento da comunidade geológica por apresentar resultados que se equivalem à geocronologia isotópicaU-Pb. Este mineral contém quantidades negligenciáveis de chumbo comum, guardando apenas Pb radiogênico proveniente do Th e U destemineral. O desenvolvimento deste método no Laboratório de Microanálises do Departamento de Física da Universidade Federal de MinasGerais mostra que os dados químicos de U, Th e Pb de cristais de monazita fornecidos por sua microssonda eletrônica produzem idadesnão-isotópicas para eles que se equiparam às idades isotópicas U-Pb produzidas pela técnica LA-ICP-MS. Grãos de monazita de placersmarinhos de Buena (RJ) isotopicamente datados pelo método U-Pb com idades entre 530 e 580 Ma foram quimicamente datadas na UFMGentre 505 e 580 Ma. Estes resultados são consideravelmente compatíveis e colocam o referido laboratório a disposição da comunidadegeocientífica para obtenção de idades de cristais de monazita.Palavras-Chave: MONAZITA, DATAÇÃO QUÍMICA, MICROSSONDA ELETRÔNICA, UFMG ABSTRACTDEVELOPMENT OF THE MONAZITE U-Th-Pb CHEMICAL DATING METHOD BY USING ELECTRON MICROPROBE AT UFMG. Themonazite U-Th-Pb chemical dating method (non-isotopic) by electron microprobe has been developed for about 20 years and has theacceptance of the geological community by presenting results that are equivalent to the isotope U-Pb geochronology. This mineral containsnegligible amounts of common lead, keeping only radiogenic Pb from the Th and U of this mineral. The development of this method in themicroanalysis laboratory of the Physics Department- UFMG shows that the monazite U, Th and Pb chemical data provided by itsmicroprobe produce non-isotopic ages for it that are similar to the U-Pb isotopic ages produced by LA-ICP-MS technique. Monazite grainsfrom marine placers of Buena (RJ) isotopically dated by method U-Pb between 530 and 580 Ma were chemically dated at UFMG between505 and 580 Ma. These results are consistent each other and put the laboratory available to the geoscience community as a tool inobtaining monazite ages.Keywords: MONAZITE, CHEMICAL DATING, ELECTRON MICROPROBE, UFMG

scholarly journals Zircon U–Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

2021 ◽  
Vol 2021 ◽  
pp. 1-11
Author(s):  
Xu Kong ◽  
Xueyuan Qi ◽  
Wentian Mi ◽  
Xiaoxin Dong
Keyword(s):  
Regional Metamorphism ◽  
Granulite Facies ◽  
Amphibolite Facies ◽  
Metamorphic Event ◽  
Isotopic Data ◽  
Metamorphic Condition ◽  
Eu Anomaly ◽  
Facies Metamorphism ◽  
Eclogite Facies ◽  
Plagioclase Gneiss

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

scholarly journals Mapping in the Fiskefjord area, southern West Greenland

1982 ◽  
Vol 110 ◽  
pp. 55-57
Author(s):  
A.A Garde ◽  
V.R McGregor
Keyword(s):  
Granulite Facies ◽  
Amphibolite Facies ◽  
Isotopic Age ◽  
Supracrustal Rock ◽  
Granulite Facies Metamorphism ◽  
West Greenland ◽  
Facies Metamorphism ◽  
Geological Work ◽  
Age Determinations

Previous geological work on the 1:100000 map sheet 64 V.l N (fig. 15) includes published maps of smaller areas by Berthelsen (1960, 1962) and Lauerma (1964), mapping by Kryolitselskabet Øresund A/S (Bridgwater et al., 1976) and mapping by GGU geologists for the 1:500000 map sheet Frederikshåb Isblink - Søndre Strømfjord (Allaart et al., 1977, 1978). The Amltsoq and Niik gneisses and Malene supracrustal rock units south and east of Godthåbsfjord have not so far been correlated with rocks in the Fiskefjord area. Godthåbsfjord separates the granulite facies gneisses in Nordlandet from amphibolite facies Nûk gneisses on Sadelø and Bjørneøen; the granulite facies metamorphism occurred at about 2850 m.y. (Black et al., 1973), while no published isotopic age determinations from the Fiskefjord area itself are available.

Apatite composition and the fugacities of HF and HC1 in metamorphic fluids

1985 ◽  
Vol 49 (350) ◽  
pp. 77-79 ◽  
Author(s):  
Bruce W. D. Yardley
Keyword(s):  
Experimental Data ◽  
Amphibolite Facies ◽  
Lower Grade ◽  
Two Samples ◽  
Metamorphic Fluids ◽  
Independent Estimate ◽  
Grade Rock ◽  
Acceptable Agreement

AbstractMicroprobe analyses of the halogen contents of apatites from two samples of amphibolite-facies schist from Connemara, Ireland, have been used to calculate the fugacity ratios fHCl/fH2O and fHF/fH2O using the experimental data of Korzhinskiy. The results imply fugacities for both acids in the range 0.03 to 0.1 bars, but whereas for the lower grade rock fHF > fHCl, the migmatitic sample gives fHF⋍fHCl. An independent estimate of fHF/fH2O from the biotite composition in one sample is in acceptable agreement with the result obtained from apatite.

scholarly journals Depositional constraints and age of metamorphism in southern India: U–Pb chemical (EMPA) and isotopic (SIMS) ages from the Trivandrum Block

2005 ◽  
Vol 142 (3) ◽  
pp. 255-268 ◽  
Author(s):  
M. SANTOSH ◽  
A. S. COLLINS ◽  
T. MORIMOTO ◽  
K. YOKOYAMA
Keyword(s):  
Electron Microprobe ◽  
Secondary Ion Mass Spectrometry ◽  
Granulite Facies ◽  
Southern India ◽  
Biotite Gneiss ◽  
Metamorphic History ◽  
Gondwana Supercontinent ◽  
Late Neoproterozoic ◽  
Formed Part ◽  
Tectonothermal Event

We report U–Pb electron microprobe (zircon and monazite) and Secondary Ion Mass Spectrometry (SIMS) U–Pb (zircon) ages from a granulite-facies metapelite and a garnet–biotite gniess from Chittikara, a classic locality within the Trivandrum Block of southern India. The majority of the electron-microprobe data on zircons from the metapelite define apparent ages between 1500 and 2500 Ma with a prominent peak at 2109±22 Ma, although some of the cores are as old as 3070 Ma. Zircon grains with multiple age zoning are also detected with 2500–3700 Ma cores, 1380–1520 mantles and 530–600 Ma outer rims. Some homogeneous and rounded zircon cores yielded late Neoproterozoic ages that suggest that deposition within the Trivandrum Block belt was younger than 610 Ma. The outermost rims of these grains are characterized by early Cambrian ages suggesting metamorphic overgrowth at this time. The apparent ages of monazite grains from this locality reveal multiple provenance and polyphase metamorphic history, similar to those of the zircons. In a typical case, Palaeoproterozoic cores (1759–1967 Ma) are enveloped by late Neoproterozoic rims (562–563 Ma), which in turn are mantled by an outermost thin Cambrian rim (∼515 Ma). PbO v. ThO*2 plots for monazites define broad isochrons, with cores indicating a rather imprecise age of 1913±260 Ma (MSWD=0.80) and late Neoproterozoic/Cambrian cores as well as thin rims yielding a well-defined isochron with an age of 557±19 Ma (MSWD=0.82). SIMS U–Pb isotopic data on zircons from the garnet–biotite gneiss yield a combined core/rim imprecise discordia line between 2106±37 Ma and 524±150 Ma. The data indicate Palaeoproterozoic zircon formation with later partial or non-uniform Pb loss during the late Neoproterozoic/Cambrian tectonothermal event. The combined electron probe and SIMS data from the metapelite and garnet–biotite gneiss at Chittikara indicate that the older zircons preserved in the finer-grained metapelite protolith have heterogeneous detrital sources, whereas the more arenaceous protolith of the garnet–biotite gniess was sourced from a single-aged terrane. Our data suggest that the metasedimentary belts in southern India may have formed part of an extensive late Neoproterozoic sedimentary basin during the final amalgamation of the Gondwana supercontinent.

Proterozoic geological evolution of the northern Vestfold Hills, Antarctica

1991 ◽  
Vol 128 (4) ◽  
pp. 307-318 ◽  
Author(s):  
C. W. Passchier ◽  
R. F. Bekendam ◽  
J. D. Hoek ◽  
P. G. H. M. Dirks ◽  
H. de Boorder
Keyword(s):  
Large Scale ◽  
Structural Evolution ◽  
Shear Zones ◽  
Granulite Facies ◽  
Strike Slip ◽  
Amphibolite Facies ◽  
Tectonic Event ◽  
Vestfold Hills ◽  
Two Phases ◽  
Brittle Faulting

AbstractThe presence of polyphase shear zones transected by several suites of dolerite dykes in Archaean basement of the Vestfold Hills, East Antarctica, allows a detailed reconstruction of the local structural evolution. Archaean and early Proterozoic deformation at granulite facies conditions was followed by two phases of dolerite intrusion and mylonite generation in strike-slip zones at amphibolite facies conditions. A subsequent middle Proterozoic phase of brittle normal faulting led to the development of pseudotachylite, predating intrusion of the major swarm of dolerite dykes around 1250 Ma. During the later stages and following this event, pseudotachylite veins were reactivated as ductile, mylonitic thrusts under prograde conditions, culminating in amphibolite facies metamorphism around 1000–1100 Ma. This is possibly part of a large-scale tectonic event during which the Vestfold block was overthrust from the south. In a final phase of strike-slip deformation, several pulses of pseudotachylite-generating brittle faulting alternated with ductile reactivation of pseudotachylite.

Archean rocks from the eastern Lac Seul region of the English River Gneiss Belt, northwestern Ontario, part 1. Petrology, chemistry, and metamorphism

1976 ◽  
Vol 13 (9) ◽  
pp. 1201-1211 ◽  
Author(s):  
N. B. W. Harris ◽  
A. M. Goodwin
Keyword(s):  
Complex Structure ◽  
Granulite Facies ◽  
Metamorphic Grade ◽  
Amphibolite Facies ◽  
Northwestern Ontario ◽  
Southern Margin ◽  
Field Evidence ◽  
Facies Metamorphism

The eastern Lac Seul region of the English River Gneiss Belt is divided into two domains defined by contrasting petrology and structure. The northern domain is underlain by east-trending, steeply south-dipping, migmatized metasediments, intruded by occasional granite sills, and the southern domain by gneissic tonalite and trondhjemite, with abundant amphibolite inclusions, intruded by granite dykes and diapirs: this domain has a complex structure with gently east-plunging open folds of about 5 km wavelength. Field evidence suggests that metasediments of the northern domain have been deposited on the tonalite trondhjemite basement, which was subsequently mobilized, thereby producing the steeply dipping paragneiss belt of the northern domain.The grade of metamorphism throughout the region lies in the upper amphibolite facies, rising locally to the granulite facies. Within 15 km of the southern margin of the gneiss belt, the metamorphic grade decreases to the greenschist facies.U–Pb dating of zircons indicates that the tonalite gneiss was emplaced at least 3040 m.y. ago, and the granite plutons at 2660 m.y., coeval with migmatization and upper amphibolite facies metamorphism. Late pegmatites were emplaced at 2560 m.y.

An Evaluation of Dating of Diagenetic Xenotime by Electron Microprobe.

2000 ◽  
Vol 6 (S2) ◽  
pp. 408-409 ◽  
Author(s):  
Brendan J. Griffin ◽  
Duncan Forbes ◽  
Neal J. McNaughton
Keyword(s):  
Electron Microprobe ◽  
Detrital Zircon ◽  
Microprobe Analysis ◽  
Electron Microprobe Analysis ◽  
Decay Rates ◽  
Wide Range ◽  
Isotopic Geochronology ◽  
Backscattered Electron ◽  
Dating Method ◽  
Chemical Dating

Xenotime is an igneous mineral commonly present in pegmatite and fractionated granite. Recent studies reveal that it also forms as a diagenetic mineral. Minute (0.1-5 μm) xenotime overgrowths typically crystallise on the surfaces of detrital zircon shortly after sedimentation, in a wide range of siliciclastic sedimentary units. For example, in backscattered electron (BSE) imaging using a scanning electron microscope (SEM), two minute, euhedral, pyramidal, xenotime overgrowths on an oscillatory-zoned detrital Ambergate zircon are evident (figure 1).Electron microprobe analysis (EMPA) geochronology is a chemical dating method that uses precisely measured concentrations of U, Th, and Pb, and the decay rates of U238, U235, and Th232, to calculate an age for a mineral. The EMPA dating method used in this study to date igneous xenotime and igneous-metamorphic monazite is the chemical isochron method (CHIME). EMPA geochronology is not a widely used technique because of the higher precision of isotopic geochronology.

Sign in / Sign up

Export Citation Format

Share Document