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Electron-microprobe Th-U-Pb monazite dating in Early-Palaeozoic high-grade gneisses as a completion of U-Pb isotopic ages (Wilson Terrane, Antarctica)

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Abstract

The electron microprobe (EMP) Th-U-Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of this method to isotopic U-Pb-SHRIMP ages for the evaluation of metamorphic processes required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th (e.g. VK-1) is unsuitable for EMP data evaluation; b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone (e.g. MPN) is partly useful; and c) Monazite with constant Th/Pb at high Th (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680 Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineral-chemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be bracketed between 510 and 450 Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th-U-Pb and SHRIMP U-Pb monazite data uniformly signal a major metamorphic event with a medium-pressure granulite facies peak between 512 and 496 Ma. Subsequent isothermal uplift and then amphibolite-facies conditions between 488 and 466 Ma led to crystallisation of pristine monazite. The high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more than 600 km along strike in the Wilson Terrane.

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... Analyses with measurable contents of Al were omitted from the dataset as well as analyses with totals outside the range from 98.0-101.5 wt% (Table 2). A reference monazite labelled as Madmon, with validated special ThO 2 *-PbO characteristics (Schulz & Schüssler 2013) was used for offline recalibration of ThO 2 , as well as for the control of data by repeated measurements at the beginning, midterm and end of analytical sessions. Interference of YLγ on the PbMα line was corrected by linear extrapolation of correction factors gained from analysis of various Ybearing standards, as proposed by Montel et al. (1996). ...
... However, such low monazite XYPO 4 are often observed in higher grade metamorphic terrains (e.g. Schulz et al. 2007;Schulz & Schüssler 2013). For both XGdPO 4 and XYPO 4 parameters, the igneous monazites appear in segmented populations for the monzogranite and alkali feldspar granite (Fig. 4d). ...
... Such monazite neoblast structures were not found in the present study and were also not reported from the Nördlinger Ries crater by Erickson et al. (2019). Like the internal structures in BSE images, the monazite mineral chemistry follows compositions and trends which are well-known from igneous and metamorphic rocks (Pyle et al. 2001;Spear & Pyle 2002) and described in numerous single reports (Finger & Clemens 1995;Förster 1998;Schulz & Schüssler 2013). It has been reported that monazite may undergo partial replacement and recrystallisation via fluid-mediated coupled dissolution-precipitation processes during metasomatism, metamorphism and postmagmatic processes (Harlov et al. 2011;Budzyń et al. 2011Budzyń et al. , 2017. ...
Article
Monazite in lithoclasts of suevite impact breccia in the Nördlinger Ries (Bavaria, Germany) and its Th-U-Pb dating by electron probe microanalysis Bernhard Schulz, Jan-Michael Lange, Joachim Krause, Dana Czygan Abstract In the Lehenberg (Lehberg or Limberg North) quarry in the NW part of the Megablock Zone of the Nördlinger Ries impact crater, granite and micaschist lithoclasts occur in a polymict suevite impact breccia. The lithoclasts display the petrographic characteristics of the shock metamorphism scale, as cavities filled with diaplectic glass, decorated planar elements in quartz grains, and severly kinked mica. In petrographic thin sections, monazite grains were detected by scanning electron microscope based automated mineralogy methods of spectral mapping. In backscattered electron imaging (BSE), monazite revealed the typical crystal shapes, and internal Th zoning and distribution pattern as known from igneous and metamorphic crystallization. Intragrain signs of shock metamorphism in a minority of monazite grains are strictly straight and parallel crack pattern resembling lamellae structures. Monazite mineral chemistry and bulk chemical Th-U-Pb ages were investigated by electron probe microanalyser (EPMA). The igneous and metamorphic monazites display contrasting and typical mineral-chemical properties. Metamorphic monazites follow strictly the cheralite substitution trend in Th + U vs Ca coordinates. Igneous monazite in an alkalifeldspar granite has the highest Y2O3 contents (~2 wt%) among all studied samples. In ThO2* vs PbO coordinates the monazite data define isochrones. Micaschist lithoclasts yielded 328 ± 3 Ma, 326 ± 6 Ma and 324 ± 5 Ma, interpreted to represent the thermal peak and post-peak age of metamorphic monazite crystallization. The 328 ± 5 Ma age of igneous monazite in the alkalifeldspar granite in contact to micaschist is interpreted to date the crystallisation of a synmetamorphic anatectic melt. This contrasts the 313 ± 3 Ma monazite crystallization age in a post-tectonic monzogranite. No indications of bulk Pb loss in monazite by shock metamorphism have been observed. The EPMA Th-U-Pb monazite ages from the lithoclasts match data from granites and meta-psammopelites in the outcropping pre-Mesozoic basement in the Western Bohemian Massif and the Black Forest. They confirm that the bottom of the Nördlinger Ries impact crater is situated in crystalline basement rocks belonging to the Moldanubian Zone. Kurzfassung Im Steinbruch am Lehenberg (auch Lehberg oder Limberg-Nord) im NW-Teil der Megablock-Zone des Nördlinger Rieskraters kommen Granit- und Glimmerschiefer-Lithoklasten in einer polymikten Suevit-Impaktbrekzie vor. Die Lithoklasten zeigen die petrographischen Merkmale der Stoßwellenmetamorphose-Skala, wie Einschlüsse mit diaplektischem Glas, Quarzkörner mit dekorierten planaren Elementen und stark geknickte Glimmer. In petrographischen Dünnschliffen wurde Monazit durch Spektralkartierung mit einem automatisierten Rasterelektronenmikroskop detektiert. Im Rückstreuelektronenbild (BSE) zeigt Monazit die für magmatische und metamorphe Kristallisation typischen Kristallformen sowie die internen Zonierungen und Verteilungsmuster von Th. Nur wenige Monazitkörner zeigen die für Stoßwellenmetamorphose typischen Internstrukturen wie scharf parallel angeordnete Risse die Lamellen bilden. Mineralchemie und Th-U-Pb-Alter der Monazite wurden mit der Elektronenstrahlmikrosonde bestimmt. Magmatische und metamorphe Monazite zeigen unterschiedliche Zusammensetzungen. In Th + U vs Ca Koordinaten folgen die metamorphen Monazite dem Cheralith-Substitutions-Trend. Magmatischer Monazit im Alkalifeldspat-Granit hat die höchsten Y2O3-Gehalte (~2 wt%) der untersuchten Proben. In ThO2* vs PbO Koordinaten definieren die Monazitanalysen Isochronen. In den Glimmerschiefer-Lithoklasten liegen die Th-U-Pb-Monazitalter bei 328 ± 3 Ma, 326 ± 6 Ma und 324 ± 5 Ma und werden als Alter der Metamorphose-Maximaltemperatur und nachfolgender Abkühlung interpretiert. Die Monazit-Isochrone von 328 ± 5 Ma im Alkalifeldspat-Granit im Kontakt zum Glimmerschiefer wird als Kristallisationsalter einer synmetamorphen anatektischen Schmelze interpretiert. Sie unterscheidet sich deutlich vom jüngeren Kristallisationsalter des posttektonischen Monzogranits mit seinen 313 ± 3 Ma alten Monaziten. Es ergeben sich keine Anzeichen für Verlust von Gesamt-Pb in Monazit durch die Stosswellenmetamorphose. Die Elektronenstrahlmikrosonden-Th-U-Pb-Alter der Monazite in den Lithoklasten gleichen denen von Graniten und Metapsammopeliten im prä-mesozoischen Basement der westlichen Böhmischen Masse und des Schwarzwalds. Dies belegt dass im Untergrund des Rieskraters die kristallinen Basementgesteine der Moldanubischen Zone anzutreffen sind. Keywords: suevite impact breccia, micaschist, granite, monazite, microstructures, Th-U-Pb age dating, Moldanubian Zone Schlüsselwörter: Suevit-Impaktbrekzie, Glimmerschiefer, Granit, Monazit, Mikrostrukturen, Th-U-Pb-Altersbestimmung, Moldanubisches Basement
... Suzuki et al. 1994;Montel et al. 1996). As Th concentrations in magmatic and metamorphic monazite are generally high, a sufficient amount of radiogenic Pb that is measureable by the EMP analysis accumulates in monazite within >100 myr (Schulz and Schüssler 2013). Thus, EMP analyses of bulk Th, U and Pb concentrations in monazite can be used for the calculation of a chemical model age and the associated error (e.g. ...
... The U was calibrated on a glass standard with 5 wt% UO 2 . Following the procedure outlined by Schulz and Schüssler (2013), monazite from a pegmatite in Madagascar (Madmon) was used for the calibration of ThO 2 and as a reference for the EMP analyses. The analytical results of the standard measurements are given in Electronic Supplement Table S3. ...
... The analytical results of the standard measurements are given in Electronic Supplement Table S3. As summarised by Schulz and Schüssler (2013), the age of the Madmon reference standard is well known and was determined by different techniques providing ages of 496 ± 9 Ma (concordant SHRIMP U-Pb age), 497 ± 2 Ma (TIMS Pb-Pb evaporation age) and 502 ± 6 Ma and 503 ± 6 Ma, respectively (EMP chemical model ages). The minor Y interference on the Pb Mα1 line was corrected by linear extrapolation after measuring several Pb-free yttrium glass standards with 5 and 12 wt% Y 2 O 3 (Montel et al. 1996). ...
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New in situ electron microprobe monazite and white mica 40Ar/39Ar step heating ages support the proposition that the Odenwald–Spessart basement, Mid-German Crystalline Zone, consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition. The monazite ages constrain tectonothermal events at 430 ± 43 Ma, 349 ± 14 Ma, 331 ± 16 Ma and 317 ± 12 Ma/316 ± 4 Ma, and the 40Ar/39Ar analyses provide white mica ages of 322 ± 3 Ma and 324 ± 3 Ma. Granulite-facies metamorphism occurred in the western Odenwald at c. 430 and 349 Ma, and amphibolite-facies metamorphism affected the eastern Odenwald and the central Spessart basements between c. 324 and 316 Ma. We interpret these data to indicate that the Otzberg–Michelbach Fault Zone, which separates the eastern Odenwald–Spessart basement from the Western Odenwald basement, is part of the Rheic Suture, which marks the position of a major Variscan plate boundary separating Gondwana- and Avalonia-derived crustal terranes. The age of the Carboniferous granulite-facies event in the western Odenwald overlaps with the minimum age of eclogite-facies metamorphism in the adjacent eastern Odenwald. The granulite- and eclogite-facies rocks experienced contrasting pressure–temperature paths but occur in close spatial proximity, being separated by the Rheic Suture. As high-pressure and high-temperature metamorphisms are of similar age, we interpret the Odenwald–Spessart basement as a paired metamorphic belt and propose that the adjacent high-pressure and high-temperature rocks were metamorphosed in the same subduction zone system. Juxtaposition of these rocks occurred during the final stages of the Variscan orogeny along the Rheic Suture.
... Depending on the abundance and grain size, a further detailed documentation of ∼20 larger grains per sample and also by additional wavelength dispersive spectroscopy element mapping is recommended. One has to be aware that monazite grain sizes and internal microstructures as observed in situ in petrographic thin sections may considerably differ from monazite in related heavy mineral concentrates, especially when migmatites are studied (Oyhantçabal et al., 2012;Schulz and Schüssler, 2013). ...
... Parts of such large monazites can show variable Th contents and traces of alteration with tiny holes along cracks, so that domains for reference measurements should be carefully selected (Figures 2c,d). However, it is important to emphasize that natural reference monazites involved in the calibration and adjustment of the monazite Th-U-Pb SIMS and SHRIMP age dating protocols are not necessarily suitable as reference material for Th-U-Pb EPMA monazite dating (Schulz and Schüssler, 2013). Reference monazite with perfectly homogeneous BSE gray tone as VK-1 (Figure 2b) has a large variation in PbO at constant Th + U and is thus inadequate as reference for the Th-U-Pb EPMA dating method. ...
... The MPN monazite (Figure 2d) can be used under the restriction that only the unaltered domains are taken into account. Judging from its ThO 2 * -PbO characteristics, monazite like the Madmon (Figure 2c) appears as suitable for the correlation of U-Pb isotopic and Th-U-Pb bulk chemical age dating methods, when homogeneous unaltered domains and the comparably high Th contents are considered (Schulz and Schüssler, 2013). ...
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The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO 4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.
... Eclogite-facies metamorphism in the Lanterman Range took place at ca. 500 Ma, amphibolite-facies regression at 490-486 Ma, and shear deformation under amphibolite to greenschist conditions at ca. 480-460 Ma (Di Vincenzo et al., 1997, 2001Palmeri et al., 2012;Di Vincenzo et al., 2014). A comparable metamorphic history is recorded by migmatites in the northern Wilson Terrane: a major high-grade metamorphic event with a granulite-facies peak between 512 and 496 Ma was followed by subsequent isothermal uplift and amphibolite-facies conditions between 488 and 466 Ma Schüssler et al., 2004;Schulz and Schüssler, 2013). ...
... The Schulz and Schüssler, 2013). It should be considered that high-temperature granulite-facies conditions (N 800°C; Schüssler et al., 2004) could have affected the isotope system of zircons. ...
... Henjes-Kunst et al. (2004) report some SHRIMP U-Pb ages for inherited zircons in migmatite samples from the Wilson Hills in the range of 1100-500 Ma with a dominant age group of 680-545 Ma (n = 14) in one of these samples. Detrital monazites in Wilson Hills migmatites and orthogneisses dated by the Th-U-Pb electron microprobe method yielded isochrone ages between 582 ± 19 Ma and 658 ± 46 Ma(Schulz and Schüssler, 2013). Despite the large scatter, these ages correspond well with the youngest detrital zircon ages of the Priestley Formation samples. ...
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The Ross-Delamerian orogenic belt was formed along the eastern side of the Australian-East Antarctic continent during west-directed subduction of the Palaeo-Pacific Ocean in the early Palaeozoic. Northern Victoria Land (NVL) in Antarctica was located at a central position of the Ross-Delamerian system. Its metamorphic basement is formed by three lithotectonic units formerly interpreted as terranes: the Wilson, Bowers and Robertson Bay terranes (from west to east). Dating of detrital zircons from 14 meta¬sedimentary samples of these terranes combined with petrographical and whole-rock geochemical studies give new insights into the stratigraphic and tectonic evolution of NVL. All samples show very similar zircon age spectra with two main intervals, a Ross/Pan-African-age interval (470–700 Ma) and a Grenville-age interval (900–1300 Ma), as well as subordinate craton-related ages dispersed over the range of ca. 1600–3500 Ma. The Ross/Pan-African-age zircon population tends to get more dominant from the Priestley Formation of the Wilson Terrane to the Molar Formation of the Bowers Terrane, and finally to the Robertson Bay Group, whereas the number of craton-related ages diminishes in this direction. A common East Antarctic source area is indicated for all analyzed samples. The Priestley Formation was deposited on the Palaeo-Pacific passive continental margin of East Gondwana in the late Neoproterozoic after Rodinia breakup. The sequence was subsequently metamorphosed and intruded by the Granite Harbour Intrusives during the Ross Orogeny. The Molar Formation of the Bowers Terrane is interpreted as a turbiditic sequence deposited in an accretionary setting on the active continental margin in the Late Cambrian during and after accretion of the Glasgow island arc allochthon. The thick, homogeneous sequence of the Robert¬son Bay Group resulted from continuous turbiditic sedimentation in an accretionary wedge in front of the Ross Orogen after docking and imbrication of the Glasgow island arc in the Early Ordovician.
... Analytical errors of 2σ at 20 kV acceleration voltage, 100 nA beam current, 5 µm beam diameter and counting times of 320 s (Pb), 80 s (U) and 40 s (Th) on peak have been considered for the calculation of ages. For Pb, the error ranges typically from 0.016 to 0.024 wt% for the given dwell time, based on measurement on a reference monazite ("Madmon"; Schulz and Schüssler 2013). Detection limits (1σ) calculated from counting rates for elements with low concentrations in monazite are in element %: Si 0.004; Ca 0.005; Y 0.020; Pb 0.006; U 0.016; Sm 0.060. ...
... The U was calibrated on a metal standard. The reference monazite labelled "Madmon" with special ThO 2 *-PbO characteristics (Schulz and Schüssler 2013) was used for calibration and offline re-calibration of ThO 2 as well as for the control of data. Interference of YLγ on the PbMα line was corrected by linear extrapolation as proposed by Montel et al. (1996). ...
... A reference monazite labelled as Madmon, with validated special ThO 2 *-PbO characteristics (Schulz and Schüssler 2013) was used for offline re-calibration of ThO 2 , as well as for the control of data by repeated measurements at the beginning, midterm and end of analytical sessions. Interference of YLc on the PbMa line was corrected by linear extrapolation of correction factors gained from analysis of various Y-bearing standards, as proposed by Montel et al. (1996). ...
... Monazite ages from single analyses are given with 2r error, see text. Mnz monazite single grain; data from reference standard monazite Madmon(Schulz and Schüssler 2013) is weighted average of 20 single analyses performed during the sessions on the samples. The monazites were analysed with a JEOL JXA-8530F at Helmholtz Institute Freiberg of Resource Technology. ...
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Monazite dating in metapelites is an emerging method to investigate polymetamorphic areas. A protocol for Th–U–Pb dating of monazite by electron microprobe was adopted for a JEOL JXA-8530F. It was applied to the Variscan and Early-Alpine metamorphic Austroalpine Oetztal-Stubai Complex (OSC). In the Alpeiner Valley in the Stubai region, the Schrankogel complex is the eastern succession of the Central Metabasite Zone. In this part, metabasites are alternating with metapelites. In 4 samples from micaschist lenses, dominantly Carboniferous monazite isochrone ages at 335 ± 4 Ma, 320 ± 4 Ma; 319 ± 4 Ma and 319 ± 4 Ma were obtained. The micaschist samples with diverse modal compositions and variable bulk rock Ca contents of calculated assay, display distinct monazite microstructures, as quantified by automated SEM-MLA (mineral liberation analysis) routines. Clusters of small monazite could indicate new crystallization and yielded isochrones at 313 and 304 Ma. In contrast, corona structures of apatite and allanite around large monazites with isochrones between 350 and 315 Ma suggest a decomposition during decreasing temperature. Garnets in metapelitic assemblages display growth zonations with low pyrope contents in the cores and pyrope-rich rims. A prograde metamorphism with high pressure amphibolite-facies peak conditions at ~ 12 kbar and ~ 680 °C, and a post Pmax path with decompression to 4 kbar and 640–600 °C was estimated from the micaschists and from zoned Ca-amphiboles in retrogressed amphibolitized eclogites. The P–T path entered the monazite stability field during the decompression. This signals a Carboniferous age of the metamorphism. A minor population in one sample is composed of sporadic Permian single monazite ages. A Cretaceous monazite population is lacking. In the wide parts of the Austroalpine basement with Carboniferous-to-Cretaceous mica mixing ages, monazite age populations allow to discriminate a distinct Permian metamorphic event.
... A reference monazite labelled as Madmon, with validated special ThO 2 *-PbO characteristics (Schulz and Schüssler 2013) was used for offline re-calibration of ThO 2 , as well as for the control of data by repeated measurements at the beginning, midterm and end of analytical sessions. Interference of YLc on the PbMa line was corrected by linear extrapolation of correction factors gained from analysis of various Y-bearing standards, as proposed by Montel et al. (1996). ...
... Monazite ages from single analyses are given with 2r error, see text. Mnz monazite single grain; data from reference standard monazite Madmon(Schulz and Schüssler 2013) is weighted average of 20 single analyses performed during the sessions on the samples. The monazites were analysed with a JEOL JXA-8530F at Helmholtz Institute Freiberg of Resource Technology. ...
... The EMP analytical setup for monazite dating in both Salzburg and Erlangen follows the recommendations of Pyle et al. (2002) and Jercinovic and Williams (2005) with reference to the optimal choice of spectral lines, as well as background and line overlap corrections. More details, regarding calibration standards etc. can be found in Krenn et al. (2008) and Schulz and Schüssler (2013). The comparability of the dating results of both laboratories was controlled by measurements of the monazite standard Madmon (Schulz and Schüssler 2013). ...
... More details, regarding calibration standards etc. can be found in Krenn et al. (2008) and Schulz and Schüssler (2013). The comparability of the dating results of both laboratories was controlled by measurements of the monazite standard Madmon (Schulz and Schüssler 2013). ...
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Allanite-fluorapatite reaction coronas around monazite are abundant in metamorphic rocks. We report here special cases where a new generation of “satellite” monazite grains formed within these coronas. Using examples from different P-T regions in the eastern Alps, we examine the origin and the petrological significance of this complex mineralogical association by means of the electron microprobe utilizing Th-U-Pb monazite dating and high-resolution BSE imaging. Satellite monazite grains form when a monazite-bearing rock is metamorphosed in the allanite stability field (partial breakdown of first generation monazite to fluorapatite plus allanite), and is then heated to temperatures that permit a back reaction of fluorapatite plus allanite to secondary satellite monazite grains surrounding the remaining original first generation monazite. Depending on the whole-rock geochemistry satellite monazites can form under upper greenschist- as well as amphibolite-facies conditions. In each of the three examples focused on here, the inherited core monazite was resistant to recrystallization and isotopic resetting, even though in one of the samples the metamorphic temperatures reached 720 °C. This shows that in greenschist- and amphibolite-facies polymetamorphic rocks, individual grains of inherited and newly formed monazite can and often will occur side by side. The original, inherited monazite will preferentially be preserved in low-Ca, high-Al lithologies, where its breakdown to allanite plus fluorapatite is suppressed. Conversely, a medium- or high-Ca, monazite-bearing rock will become particularly fertile for secondary monazite regrowth after passing through a phase of strong retrogression in the allanite stability field. Based on this knowledge, specific sampling strategies for monazite dating campaigns in polymetamorphic basement can be developed.
... Th-U-Pb monazite dating method yielded an isochrone age of 335 ± 3 Ma (n = 164, Fig. 8, supplementary Table S5, using the protocol of Schulz and Schüssler, 2013). Two large monazite grains enclosed in the same garnet yielded a weighted average age of 332 ± 9 Ma (8 spot analyses) and 342 ± 13 Ma (6 spots; Fig. 7a). ...
... Weighted average ages (Ma) with MSWD and minimal 2σ error are calculated according to Ludwig (2001). Analyses from Madmon monazite (Schulz and Schüssler, 2013) are used as reference and for calibration. Table S5, Fig. 8) agrees with published ages of metamorphism (~340-330 Ma; compare Section 2) and supports rapid subduction and exhumation of UHP rocks in the Erzgebirge (e.g. ...
Article
In the central Erzgebirge within the Bohemian Massif, lenses of high pressure and ultrahigh pressure felsic granulites occur within meta-sedimentary and meta-igneous amphibolite-facies felsic rocks. In the felsic granulite, melt rich parts and restite form alternating layers, and were identified by petrology and bulk rock geochemistry. Mineral assemblages representing the peak P-T conditions were best preserved in melanocratic restite layers. In contrast, in the melt rich leucocratic layers, garnet and related HP minerals as kyanite are almost completely resorbed. Both layers display differences in accessory minerals: melanosomes have frequent and large monazite and Fe–Ti-minerals but lack xenotime and apatite; leucosomes have abundant apatite and xenotime while monazite is rare. Here we present a detailed petrographic study of zircon grains (abundance, size, morphology, inclusions) in granulite-facies and amphibolite-facies felsic gneisses, along with their oxygen and hafnium isotope compositions. Our data complement earlier UPb ages and trace element data (REE, Y, Hf, U) on zircons from the same rocks (Tichomirowa et al., 2005). Our results show that the degree of melting determines the behaviour of zircon in different layers of the granulites and associated amphibolite-facies rocks. In restite layers of the granulite lenses, small, inherited, and resorbed zircon grains are preserved and new zircon formation is very limited. In contrast, new zircons abundantly grew in the melt rich leucocratic layers. In these layers, the new zircons (UPb age, trace elements, Hf, O isotopes) best preserve the information on peak metamorphic conditions due to intense corrosion of other metamorphic minerals. The new zircons often contain inherited cores. Compared to cores, the new zircons and rims show similar or slightly lower Hf isotope values, slightly higher Hf model ages, and decreased oxygen isotope ratios. The isotope compositions (Hf, O) of new zircons indicate partial Hf isotope homogenization in the melt, and melt infiltration from an external source. New zircon was most likely formed by a peritectic reaction with melt above the wet solidus (peritectic zircon). Conversely, the amphibolite-facies host gneisses lack indications of significant melt production. Pre-metamorphic zircons experienced mainly solid-state recrystallization and variable Pb loss with only minor new zircon formation. However, subtle changes in cathodoluminescence pattern, in the Hf and O isotopes, and in the Lu/Hf, Yb/Hf ratios of zircons suggest that small volumes of melt were locally present. In difference to granulites, melt was internally produced. The detection of low degree melts (inferred from zircon geochemistry) is extremely important for the rheology because these amphibolite-facies rocks could act as large scale ductile shear zones. The new zircon data support a different P-T path for closely spaced amphibolite- and granulite-facies rocks.
... Analytical errors of 2σ at 20 kV acceleration voltage, 100 nA beam current, 5 µm beam diameter and counting times of 320 s (Pb), 80 s (U) and 40 s (Th) on peak have been considered for the calculation of ages. For Pb, the error ranges typically from 0.016 to 0.024 wt% for the given dwell time, based on measurement on a reference monazite ("Madmon"; Schulz and Schüssler 2013). Detection limits (1σ) calculated from counting rates for elements with low concentrations in monazite are in element %: Si 0.004; Ca 0.005; Y 0.020; Pb 0.006; U 0.016; Sm 0.060. ...
... The U was calibrated on a metal standard. The reference monazite labelled "Madmon" with special ThO 2 *-PbO characteristics (Schulz and Schüssler 2013) was used for calibration and offline re-calibration of ThO 2 as well as for the control of data. Interference of YLγ on the PbMα line was corrected by linear extrapolation as proposed by Montel et al. (1996). ...
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The Hamadan high-grade metapelites in the northwestern part of the Sanandaj–Sirjan zone, Iran, show a polymetamorphic evolution with relics of a garnet-bearing metamorphic mineral assemblage (M1), a contact metamorphic overprint (M2) related to the emplacement of the Middle to Late Jurassic Alvand composite pluton and a Buchan-type regional metamorphic event (M3) marked by 40Ar/39Ar ages in the 80–70 Ma range that is associated with penetrative ductile deformation producing a foliation and a thermal overprint onto the M2 assemblages. The M1 event is exclusively preserved as small garnet grains and mineral inclusions contained therein, incorporated into M2-stage cordierite porphyroblasts. Distinct metamorphic zones are developed over a region of ~ 600 km2, which are partly correlated with distance to the composite pluton: zones (1) cordierite + K-feldspar hornfels, and (2) andalusite ± cordierite hornfels that surround the Alvand composite pluton at a distance of up to 5 km. These two zones are clearly related to M2 metamorphism associated with pluton emplacement. Zones (3) staurolite schist, (4) andalusite schist, and (5) sillimanite schist are found outside of the contact aureole and are considered to be the result of regional M3 metamorphism in the eastern part distant to the Alvand composite pluton. Conventional thermobarometry shows that temperatures in the area vary between ~ 560 and 660 °C for zones 1 and 2 and ~ 490 and 690 °C for zones 3–5. Phase equilibria modelling in the MnNCKFMASHT system indicates two distinct isobaric prograde paths at low pressures, at ~ 2.7 kbar for zones 1 and 2 and slightly higher pressures of around 3.5–5.5 kbar for zones 3–5. U–Th–Pb monazite geochronology revealed overlapping ages of 168 ± 11 Ma and 149 ± 19 Ma in the hornfels (1 and 2) and schistose (3–5) zones, respectively. These ages are similar to the intrusion age of the Alvand composite pluton (153.3 ± 2.7 to 166.5 ± 1.8 Ma) and are interpreted to reflect heating due to the emplacement of the composite pluton (M2 contact metamorphic event). However, 40Ar/39Ar dating of white mica and amphibole yielded plateau ages ranging from 80 to 69 Ma over the entire transect. The formation of schistosity in zones 3–5 postdates the intrusion and is thus related to M3 metamorphism. The white mica fabric indicates formation of the foliation during M3 garnet growth, which is followed by local retrogression of garnet to chlorite during exhumation. Consequently, the 40Ar/39Ar white mica and amphibole ages likely indicate reheating during M3 to more than ca. 500 ± 25 °C (argon retention temperature in amphibole). These data establish the occurrence of a Cretaceous, Buchan-style regional metamorphic event that had not been firmly identified before. Subsequent Late Cretaceous exhumation of the Hamadan complex with its high-grade metapelites is due to extension along the Tafrijan–Mangavi–Kandelan fault, which represents a major ductile low-angle normal fault. Metamorphic temperatures coupled with mineral ages from this and published work suggest a fast stage of cooling with a rate of ~ 6 °C/Ma during exhumation after M3 metamorphism.
... The U was calibrated on a metal reference material. The Th was calibrated on a reference monazite labelled as Madmon, with validated special ThO 2 *-PbO characteristics (Schulz et al., 2007(Schulz et al., , 2019Schulz and Schüssler 2013). The Madmon reference monazite was also used for offline re-calibration of ThO 2 , as well as for the control of age data. ...
... Monazite ages from single analyses are given with 2σ error, see text. Mnz -monazite single grain; Data from reference monazite Madmon (Schulz and Schüssler 2013) is weighted average of 20 single analyses performed during the sessions on the samples. The monazites were analysed with a JEOL JXA-8230 at Institute of Material Science of the TU Bergakademie Freiberg/Saxony, Germany. ...
Article
The Conlara Metamorphic Complex, the easternmost complex of the Sierra de San Luis, is a key unit to understand the relationship between the late Proterozoic-Early Cambrian Pampean and the Upper Cambrian-Middle Ordovician Famatinian orogenies of the Eastern Sierras Pampeanas. The Conlara Metamorphic Complex extends to the east to the foothills of the Sierra de Comechingones and to the west up the Río Guzmán shear zone. The main rock types of the CMC are metaclastic and metaigneous rocks that are intruded by Ordovician and Devonian granitoids. The metaclastic units comprise fine to medium-grained metagreywackes and scarce metapelites with lesser amounts of tourmaline schists and tourmalinites whereas the metaigneous rocks encompass basic and granitoids rocks. The former occur as rare amphibolite interlayered within the metasedimentary rocks. The granitic component corresponds to a series of orthogneisses and migmatites (stromatite and diatexite). The CMC is divided in four groups based on the dominant lithological associations: San Martin and La Cocha correspond mainly to schists and some gneisses and Santa Rosa and San Felipe encompass mainly paragneisses, migmatites and orthogneisses. The Conlara Metamoprphic Complex underwent a polyphase metamorphic evolution. The penetrative D2-S2 foliation was affected by upright, generally isoclinal, N-NE trending D3 folds that control the NNE outcrop patterns of the different groups. An earlier, relic S1 is preserved in microlithons. Discontinuous high-T shear zones within the schists and migmatites are related with D4 whereas some fine-grained discontinuous shear bands attest for a D5 deformation phase. Geochemistry of both non-migmatitic metaclastic units and amphibolites suggest that the Conlara Metamorphic Complex represents an arc related basin. Maximun depositional ages indicate a pre- 570 Ma deposition of the sediments. An ample interval between sedimentation and granite emplacement in the already metamorphic complex is indicated by the 497 ± 8 Ma age of El Peñon granite. D1-D2 history took place at 564 ± 21 Ma as indicated by one PbSL age calculated for the M2 garnet of La Cocha Group. D3 is constrained by the pervasively solid-state deformed Early Ordovician granitoids which exhibits folded xenoliths of the D1-D2 deformed metaclastic rocks. Pressure-temperature pseudosections were calculated for one amphibolite using the geologically realistic system MnNCKFMASHTO (MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3). Peak metamophic conditions (M2) indicate 6 kbar and 620 °C. Late chlorite on the rims and in cracks of garnet, along with titanite rims on ilmenite and matrix plagioclase breaking down to albite suggests that the P-T path moved back down. Monazite analyses yield isochron Th–U–Pb ages ranging from 446 to 418 Ma. The oldest age of 446 ± 5 Ma correspond to a migmatite from the Santa Rosa Group. Monazites in samples from the La Cocha and the San Martin group crystallized at decreasing temperatures, followed by the 418 ± 10 Ma low-Y2O3 monazites in one sample of the la Cocha Group that was also obtained from a migmatite, and would likely mark a later stage of a retrograde metamorphism New CHIME monazite ages presented here likely represent post-peak fluid assisted recrystallization that are similar to amphibole and muscovite cooling ages. Therefore the monazite ages may represent a re-equilibration of the monazite on the cooling path of the basement complex.
... UO 2 , and for the ThO 2 concentration a reference monazite labelled Madmon was used. It has a SHRIMP age of 496 ± 9 Ma and a monazite PbePb TIMS evaporation age of 497 ± 2 Ma (Schulz and Schüssler, 2013). Data of Madmon was also used for offline recalibration of ThO 2 in each analytical session. ...
... The interference of the Y Lg-with the Pb Ma-line was adjusted after Montel et al. (1996) by linear extrapolation after the measurement of lead-free yttrium reference glasses with 5% and 12% Y 2 O 3 . Furthermore, an empirical correction of the Th Mg-on the U Mbline was applied (Schulz and Schüssler, 2013). These parameters matched the analytical problems discussed in Williams et al. (2006) in the best way. ...
... The U was calibrated on a glass standard with 5 wt% UO 2 . The reference monazite Madmon, with special ThO 2 *-PbO characteristics (Schulz & Schüssler 2013) was used for calibration of ThO 2 , for offline re-calibration of Th, and for control of data by repeated measurements at the beginning and end of analytical sessions. Synthetic REEPO 4 orthophosphates from the Smithsonian Institution (Jarosewich & Boatner 1991) were used as standards for the REE. ...
... M -monazite single grain; MCor -monazite in corona structure; MinGrt -monazite grains enclosed in garnet; MinIlm -monazite enclosed in ilmenite; Msm -small monazite grain. Data from reference monazite Madmon(Schulz & Schüssler 2013) is mean of 110 single analyses performed during the sessions on the samples. ...
Article
Garnet micaschists from the parautochthonous Kliening and the overlying Preims complexes in the lower part of the Austroalpine Saualpe basement in Carinthia (Austria, Eastern Alps) display a polymetamorphic microstructural and mineral-chemical evolution. SEM automated mineralogy methods and analysis by EPMA were applied for garnet mineral-chemical characterisation and monazite dating. Two garnet generations with monophase and polyphase porphyroblasts and two monazite age populations have been revealed in low-Ca and high-Al-metapelites. Garnet 1 in both Kliening and Preims complexes has decreasing Mn, low Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallisation along a M1 prograde metamorphic P-T path culminating at 600 - 650 °C/7 kbar, followed by decompression. In the Preims Complex a subsequent crystallisation of a ~265 Ma monazite 1 population with high-Y and high-Gd indicates a Permian minimum age for the M1 metamorphic event. Monazite 1 crystallisation coincides with the intrusion of Permian and Early Triassic pegmatites. In the Kliening Complex where pegmatites are lacking, no Permian monazite is observed. Resorption of garnet 1 rims and corona structures of apatite and allanite around Permian monazite in the Preims Complex signal a retrogressive stage. An M2 metamorphic event with garnet 2 postdates the monazite corona structures. Garnet 2 displays always low Mn at high Ca and Mg. The XCa is increasing and then decreasing, while XMg is decreasing and then increasing toward the rims. The garnet 2 assemblages record a prograde P-T path from 530 °C/8 kbar to 700 °C/10 - 13 kbar in the Kliening Complex, and from 620 °C/10 - 12 kbar to 700 °C/14 - 15 kbar in the Preims Complex. Monazite 2 populations with ages between 95 and 81 Ma, and with lower Y and Gd contents crystallised at decreasing pressure subsequent to garnet 2 and indicate a Cretaceous (Eo-Alpine) age for the metamorphic event M2. In the histogram view, the monazite age pattern in the Preims Complex and the overlying Saualpe Eclogite Complex are quite similar with Permian and Cretaceous age maxima. These patterns differ from other Austroalpine basement areas with unimodal Carboniferous or bimodal Carboniferous and Permian monazite age populations.
... Despite its low modal abundance and generally small size in most crustal rocks, monazite is nevertheless a powerful tracer of geological processes. In metamorphic rocks, it is widely used for constraining the age of deformation and pressure-temperature conditions of distinct metamorphic events (Suzuki and Adachi, 1991;Williams et al., 1999;Montel et al., 2000;Foster et al., 2004;Just et al., 2011;Schulz and Schüssler, 2013;Finger et al., 2016;Engi, 2017). For magmatic rocks, where zircon ages appear especially unreliable due to significant Pb loss, monazite geochronology may be used to date the pluton emplacement (e.g. ...
... For calibration of REE with the exception of La, which was only available from the Smithsonian Institution (Jarosewich and Boatner, 1991), an alternative set of Pb-free REE-ultraphosphates from ASTIMEX Ltd. A reference monazite labelled as Madmon, with validated special ThO 2 * -PbO characteristics (Schulz and Schüssler, 2013) was used for offline recalibration of ThO 2 , as well as for the control of data. Was used as reference materials. ...
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Electron microprobe (EPMA) monazite Th – U – Pb ages have been determined from a suite of granulite and granitoids rocks of the Sangmelima granite-greenstone Belt (SGB) of the Ntem Complex in Cameroon or NW Congo Craton. The monazite-bearing rocks include charnockitic gneiss, migmatitic tonalite-trondhjemite-granodiorite (TTG) gneisses, granodiorite, monzogranite and a late pegmatite intrusion. The monazites of Sangmelima can be classified as monazite-Ce. Based on published ages and EPMA Th – U – Pb monazite age data from the SGB, the geodynamic evolution of the NW Congo Craton was refined through its tectonometamorphic and magmatic events. The major charnockitic and TTG magmatism event in Ntem Complex occurred between ∼2950–2850 Ma. This Mesoarchean crystalline basement underwent two major ductile tectonothermal events M1 – D1 and M2 – D2. (1) The onset of M1 – D1 corresponds to the migmatization of TTG at 2843 Ma coeval with the emplacement of syn-kinematic granite dykes (monzogranites) at 2840–2825 Ma. The S1 regional penetrative foliation (D1) was progressively generated during the M1 metamorphic event, which led to the formation of associated hypersthene-garnet-bearing granulitic mineral assemblage that peaked at ∼2830 Ma. (2) The M2 – D2 event between 2760–2740 Ma corresponds to the folding of S1 regional foliation by F2 folds and development of C2 shear zones synchronous with the anatexis event that generated the high-K granitoids at 2750 Ma. Late Neoarchean granitic pegmatite/aplite dikes and/or veins emplaced during the brittle deformation D3 and protracted cooling of the Ntem Complex between ∼2700–2550 Ma. Our new EPMA monazite U – Th – Pb ages from the SGB highlight similar magmatic and tectonothermal events of the adjacent northern Gabonese domain and the Chaillu Massif in Gabon part of the NW Congo craton (ca. 3.0–2.5 Ga).
... Despite its low modal abundance and generally small size in most crustal rocks, monazite is nevertheless a powerful tracer of geological processes. In metamorphic rocks, it is widely used for constraining the age of deformation and pressure-temperature conditions of distinct metamorphic events (Suzuki and Adachi, 1991;Williams et al., 1999;Montel et al., 2000;Foster et al., 2004;Just et al., 2011;Schulz and Schüssler, 2013;Finger et al., 2016;Engi, 2017). For magmatic rocks, where zircon ages appear especially unreliable due to significant Pb loss, monazite geochronology may be used to date the pluton emplacement (e.g. ...
... For calibration of REE with the exception of La, which was only available from the Smithsonian Institution (Jarosewich and Boatner, 1991), an alternative set of Pb-free REE-ultraphosphates from ASTIMEX Ltd. A reference monazite labelled as Madmon, with validated special ThO 2 * -PbO characteristics (Schulz and Schüssler, 2013) was used for offline recalibration of ThO 2 , as well as for the control of data. Was used as reference materials. ...
Article
Electron microprobe (EPMA) monazite Th–U – Pb ages have been determined from a suite of granulite and granitoids rocks of the Sangmelima granite-greenstone Belt (SGB) of the Ntem Complex in Cameroon or NW Congo Craton. The monazite-bearing rocks include charnockitic gneiss, migmatitic tonalite-trondhjemitegranodiorite (TTG) gneisses, granodiorite, monzogranite and a late pegmatite intrusion. The monazites of Sangmelima can be classified as monazite-Ce. Based on published ages and EPMA Th–U – Pb monazite age data from the SGB, the geodynamic evolution of the NW Congo Craton was refined through its tectonometamorphic and magmatic events. The major charnockitic and TTG magmatism event in Ntem Complex occurred between ~2950 and 2850 Ma. This Mesoarchean crystalline basement underwent two major ductile tectonothermal events M1 – D1 and M2 – D2. (1) The onset of M1 – D1 corresponds to the migmatization of TTG at 2843 Ma coeval with the emplacement of syn-kinematic granite dykes (monzogranites) at 2840–2825 Ma. The S1 regional penetrative foliation (D1) was progressively generated during the M1 metamorphic event, which led to the formation of associated hypersthene-garnet-bearing granulitic mineral assemblage that peaked at ~2830 Ma. (2) The M2 – D2 event between 2760 and 2740 Ma corresponds to the folding of S1 regional foliation by F2 folds and development of C2 shear zones synchronous with the anatexis event that generated the high-K granitoids at 2750 Ma. Late Neoarchean granitic pegmatite/aplite dikes and/or veins emplaced during the brittle deformation D3 and protracted cooling of the Ntem Complex between ~2700 and 2550 Ma. Our new EPMA monazite U–Th – Pb ages from the SGB highlight similar magmatic and tectonothermal events of the adjacent northern Gabonese domain and the Chaillu Massif in Gabon part of the NW Congo craton (ca. 3.0–2.5 Ga).
... Th-U-Pb dating of monazite by electron probe microanalysis (EPMA) is based on the given fact that common Pb in monazite (LREE, Th)PO 4 is negligible compared to radiogenic Pb resulting from the decay of Th and U [9]. Analysis of the bulk Th, U and Pb concentrations in monazite by EPMA, at a constant 238 U/ 235 U, allows for the calculation of an isochron age and/or for single-domain ages with a considerable error [9][10][11][37][38][39]. Monazite analyses were performed with instruments JEOL JXA-8200, JXA-8530F and JXA-8230 (JEOL Ltd., Akishima, Japan) with individually adopted analytical protocols [40,41]. Potential problems of data comparability were avoided by using the same reference monazite, labelled as Madmon, with validated special ThO 2 *-PbO characteristics [40] for offline re-calibration of ThO 2 , and for the data control by repeated measurements during the analytical sessions with each instrument. ...
... Analysis of the bulk Th, U and Pb concentrations in monazite by EPMA, at a constant 238 U/ 235 U, allows for the calculation of an isochron age and/or for single-domain ages with a considerable error [9][10][11][37][38][39]. Monazite analyses were performed with instruments JEOL JXA-8200, JXA-8530F and JXA-8230 (JEOL Ltd., Akishima, Japan) with individually adopted analytical protocols [40,41]. Potential problems of data comparability were avoided by using the same reference monazite, labelled as Madmon, with validated special ThO 2 *-PbO characteristics [40] for offline re-calibration of ThO 2 , and for the data control by repeated measurements during the analytical sessions with each instrument. The electron beam was set at 20 kV acceleration voltage, 50 nA beam current for calibration, 100 nA beam current for measurements and 5 µm beam diameter. ...
Article
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Garnet-bearing metapelites in the Helvetic and Austroalpine pre-Mesozoic polymetamorphic basement are characterised by pressure-temperature path segments reconstructed by microstructurally controlled geothermobarometry, and the Th-U-Pb monazite age distribution pattern revealed by the electron probe microanalyser (EPMA). In the Helvetic Aiguilles Rouges Massif and the Austroalpine Oetztal-Stubai basement to the NW an Ordovician-to-Silurian high temperature event preceded a pressure-dominated Carboniferous metamorphism. In the Austroalpine basement units to the south of the TauernWindow, the maximal pressures of the Carboniferous amphibolite-facies metamorphism range from 12 to 6 kbar. The decompressional P-T path segments signal a transition to low pressure conditions. A subsequent high pressure overprint is restricted to the Prijakt Subgroup unit in the Schobergruppe and documented by Cretaceous monazite crystallisation at 88 +- 6 Ma. In the Austroalpine Saualpe basement to the SE, a distinct early Permian metamorphism which started at low pressures of ~4 kbar/500 °C and reached maximal 6 kbar/600–650 °C predated the intrusion of Permian pegmatites. Permian monazite crystallised in line with the intrusion of pegmatites. Corona microstructures around the Permian monazites indicate retrogression previous to a Cretaceous high pressure metamorphism. That way, pressure-temperature-time paths resolve the spatial and temporal evolution in the polymetamorphic Alpine basement prior to the Tertiary collision.
... The U and Th were calibrated on a metal standard. The reference monazite Madmon, with special ThO 2 *-PbO characteristics (Schulz and Schüssler, 2013) and comparable age was used for calibration of ThO 2 , and as a control reference for the EMP data. Interference of YLγ on the PbMα line was corrected by linear extrapolation as proposed by Montel et al. (1996). ...
... Weighted average ages (Ma) with MSWD and minimal error of 2σ are calculated from single analyses according to Ludwig (2001). Madmon marks data from Madagascar reference Monazite (Schulz and Schüssler, 2013). average upper continental crust. ...
Article
In the Wadi Um Had area, Central Eastern Desert, Egypt, NE-trending metapelitic and molasse-type successions are exposed. The metasediments bear the geochemical signature of a first depositional cycle in two distinct continental island arc settings that derived from incipiently-to moderately-weathered intermediate to felsic sources under generally warm and humid conditions. The metapelitic succession records three distinct episodes of metamorphism, M1–M3, whereas the molasse-type succession records only the last metamorphic episode, M3. M1/D1 records an amphibolite facies tectono-metamorphic event that has been dated at 625 ± 5 Ma, whereas M2/D2 records a greenschist facies subduction-related event. Collision of the two domains during a NE–SW shortening D3, culminated in formation of the macroscopic NW–SE-trending folds. D2 and D3 correlate with the gneiss-forming event, which is constrained at <609 Ma, and doming of the nearby Meatiq gneiss dome, respectively. M3 is a hornblende hornfels facies thermal metamorphism related to the intrusion of the post-orogenic, Neoproterozoic (596.3 Ma) Um Had granite. This study records, for the first time, a tectono-metamorphic phase predating the gneiss-forming event in the Meatiq gneiss dome, and pushes the boundary of the Late Ediacaran terminal collision between East and West Gondwana to ≤600 Ma.
... %). The amounts of U and Th in zircons were very low, i.e. below the threshold of 1 wt% in accord with literature data (Deer et al., 1994;Kusiaka et al., 2014;Schulz and Schüssler, 2013). ...
... The beam current was set 150 nA at a beam diameter of 5 μm. Madmon, a monazite from a pegmatite in Madagascar, acts as reference for monazite data and offline recalibration of ThO 2 (U-Pb-SHRIMP Madmon age of 496 ± 9 Ma, around 10 wt% ThO 2 ; Schulz et al., 2007;Schulz and Schüssler, 2013). The calibration of PbO was realized on a crocoite standard, while U was calibrated with a U-metal. ...
Article
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The Capelinha Formation (Macaúbas Group) consists of a lower quartzitic unit with metamafic intercalations and an upper metapelitic sequence. It occurs in a complex tectono-metamorphic sector of the Araçuaí orogen, where post-collisional collapse-related structures superimposed collisional structures. The garnet-bearing assemblages started crystallization in the collisional deformation stage that formed the main regional foliation around 570 Ma. Garnet porphyroblasts display a well-developed growth zonation of Fe-Mg-Ca-Mn and show, from core to rim, increasing almandine and pyrope contents in contrast with decreasing grossular and spessartine contents. Mineral relations and microstructures provide criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The P-T values calculated from cores to rims of garnets, aligned along clockwise trends, resulted in increasing temperatures (from 500 ºC up to 620 ºC) under decompression conditions (from 8.0 kbar to 4.5 kbar). The Th-U-Pb dating of homogeneous monazites by electron microprobe revealed a recrystallization period at around 490 - 480 Ma. These ages can be related to the tectono-thermal event associated with the gravitational collapse, constraining the youngest time limit for metamorphic processes in the Araçuaí orogen.
... Electron-microprobe monazite Th-U-Pb dating is based on the assumption that concentrations of common Pb in monazite (LREE, Th)PO 4 are negligible when compared to radiogenic Pb resulting from the Th and U decay (Montel et al. 1996;Cocherie et al. 1998;Cocherie and Albarede 2001). Electron-microprobe analysis of the Th, U and Pb concentrations in monazite, at a constant 238 U/ 235 U, allows for the calculation of a chemical model age (CHIME), albeit typically with a considerable error (Suzuki et al. 1994;Montel et al. 1996 ;Pyle et al. 2005;Jercinovic et al. 2008;Suzuki and Kato 2008;Spear et al. 2009 Schulz et al. 2007;Schulz and Schüssler 2013). The Madmon was used for calibration and offline re-calibration of ThO 2 as well as for checking the data. ...
Article
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In the Haut-Allier region (French Massif Central), a Variscan inverted metamorphic sequence is made up by crustal nappes. A high-grade Upper Gneiss Unit (UGU) was thrusted over a Lower Gneiss Unit (LGU) and an amphibolite-facies para-autochthonous Micaschist Unit (MU). Growth-zoned garnets with distinct Mn–Mg–Fe–Ca and trace-element zonation trends occur in kyanite garnet gneisses at Agnat (UGU). The porphyroblasts have been characterised by automated energy-dispersive X-ray spectral mapping with SEM, by electron microprobe and LA-ICPMS analyses. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct a syn-deformational and clockwise P–T path for garnet crystallization in the UGU. The P–T path passed maximal pressures at 700 °C/13 kbar and then maximal temperatures at ~800 °C/11 kbar in the stability field of kyanite + K-feldspar. The final stage of the P–T path is a marked decompression from 10 to 5 kbar at 700–750 °C. The timing of this P–T evolution in the UGU has been constrained by electron-microprobe Th–U–Pb monazite dating (CHIME). A detailed interpretation of ages and Y-contents of monazites enclosed in a syncrystalline-rotated Mg-rich garnet allowed to relate a marked pressure decrease along a late stage of the P–T path to ~330 Ma. Simple 1D forward numerical modeling with variations of vertical velocity and geothermal gradient confirmed that the onset of monazite crystallization at ~360 Ma should give a maximal age of the early P–T evolution. A distinct group of Y-rich monazites could be the relic of this Late Devonian event. Most monazite ages from the UGU, the LGU and the MU in the Haut-Allier region range between 360 and 320 Ma, with isochrons for single samples giving ages from 332 to 338 Ma. The prograde–retrograde P–T–t evolution in the UGU appears as an independent Early Carboniferous metamorphic cycle, which was related to a continental collision. It post-dated a Silurian HP–(UHP) event and a subsequent Early Devonian migmatization in the UGU.
... While the metamorphic mineral assemblage depends on metamorphic grade and whole-rock composition, garnet and monazite commonly occur in high-grade metamorphic rocks (e.g., Spear, 1993;Schulz and Schüssler, 2013) and also in medium-grade metamorphic rocks with peraluminous compositions (e.g., Finger et al., 2002;Wing et al., 2003;Fitzsimons et al., 2005;Krenn and Finger, 2007). It is therefore reasonable to expect that the detrital monazites highly depleted in H-REEs ([Gd/Lu] N of >500) are originated mainly from garnet-bearing metamorphic rocks with typically medium-to high-grades. ...
Article
The African continent consists of several cratons rimmed by orogenic belts, which were mainly formed by collisions of the cratons during the Gondwana supercontinent assembly. Understanding of the timing and nature of the Pan-African Orogeny is essential to decipher the development of the African continent and Gondwana supercontinent. We carried out U–Pb dating and geochemical analyses of approximately 500 detrital monazites from the Nile, Niger, Congo, Zambezi and Orange Rivers. The U–Pb dating defined age peaks at ∼600 Ma for the Nile, ∼580 Ma for the Niger, ∼630, ∼610 and ∼550 Ma for the Congo, ∼500 Ma for the Zambezi and 1200–1000 Ma for the Orange. All but the Orange age peaks correspond to the period of the Pan-African Orogeny. In the Niger, Congo and Zambezi, the Pan-African age peaks are 20–40 Myr younger than those defined by the detrital zircons from the same rivers. In the Nile, however, the detrital monazite and zircon age peaks are contemporaneous. The geochemical analyses revealed significant variations in Eu/Eu∗, Gd/Lu and Th/U ratios among the detrital monazites, which can be attributed to elemental partitioning with the co-existing feldspars, garnet, and zircon/rutile, respectively. Considering that typical abundances of these minerals vary among igneous and metamorphic rocks, these geochemical data can be potentially linked to the genesis of the detrital monazites. The geochemical systematics, together with the discrepancy between the detrital monazite and zircon ages, suggest that the Pan-African monazite age peaks essentially reflect the timing of syn- to post-collisional metamorphic events in the source terranes, whereas the detrital zircons provide a more representative record of magmatic events. The results imply that the Pan-African Orogeny was promoted by pre- to syn-collisional felsic magmatism during the early stages and, 20–40 Myr later, it was dominated by syn- and/or post-collisional metamorphism.
... The most commonly distributed U-Pb reference material for monazite is 44069, which has a ThO 2 content of approximate 3-4 wt% (Aleinikoff et al. 2006;Li et al. 2013). Schulz et al. (2007) and Schulz and Schüssler (2013) recommended Madmon monazite as the EMPA Th-U-Pb monazite bulk chemical dating reference material, which has an age of 500 Ma with ThO 2 of~11 wt.%. Recently, Gonçalves et al. (2016) introduced several monazite crystals with similar ThO 2 content of 7-8 % as U-Pb isotope reference material for microanalysis. ...
Article
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Well-characterized matrix-matched natural mineral references of known age are an important prerequisite for SIMS (secondary ion mass spectrometry) U–Th–Pb dating. We have characterized RW-1, a 44 g yellowish-brown single monazite specimen from a Norwegian pegmatite as an excellent hi-Th reference material for secondary ion mass spectrometric U–Th–Pb dating. A total of 206 secondary ion mass spectrometric analyses over six analytical sessions were performed on different monazite fragments of RW-1. The analyses resulted in 207Pb-based common lead corrected 206Pb/238U ages and Th–Pb ages with overall 2 % (2 SD = standard deviation) variations, indicating the good U–Th–Pb system homogeneity. The homogeneity of Th content of 11.8 ± 1.0 wt% (2 SD) and Th/U of 42 ± 3 (2 SD) make this crystal also a good compositional reference material. We used the combined ID–TIMS(Pb)/ID–MC–ICP–MS(U) technique (i.e. isotope dilution thermal ionization mass spectrometry for Pb, and isotope dilution multi-collector inductively-coupled plasma mass spectrometry for U) to determine U–Pb ages of the monazite samples studied. The mean 207Pb/235U age of 904.15 ± 0.26 Ma (95 % confidence level) is recommended as the best estimate crystallization age for RW-1 monazite. Considering that the most commonly distributed U–Pb monazite reference materials have rather low ThO2, we suggest that this RW-1 monazite with its ThO2 of 13.5 wt% is a suitable reference material providing investigators more confidence when dating high-Th monazite unknowns.
... The diffusional closure temperature of monazite is relatively high (800-900°C) and the U-Th-Pb system should be preserved unchanged in a wide spectrum of metamorphic conditions (Cherniak et al., 2004;Gardes et al., 2006). However, metasomatic interaction of monazite with fluid may result in age discordance (Didier et al., 2013;Kelly et al., 2012;Schulz and Schüssler, 2013). Several experimental studies have shown that such alterations via fluidmediated coupled dissolution-reprecipitation processes can significantly disturb the Th-U-Pb system in monazite (Harlov and Hetherington, 2010;Harlov et al., 2011;Seydoux-Guillaume et al., 2002;Teufel and Heinrich, 1997), even well below the diffusional closure temperatures Williams et al., 2011). ...
Article
New research indicates that there is stratigraphic and structural continuity between the Młynowiec and Stronie Formations of the Orlica-Śnieżnik Dome (OSD), Bohemian Massif, and reveals the primary importance of the collision of Armorican Terrane Assemblage and Brunovistulian terrane in Variscan development of the OSD. The Młynowiec Formation is a partially migmatized metasedimentary unit and understanding its tectonometamorphic history is crucial for making regional-scale Variscan reconstructions. Structural studies, pseudosection modeling in MnNCKFMASH system and microprobe monazite dating revealed that this Variscan collision led to crustal thickening of the OSD, tectonic burial of the Młynowiec Formation to depths corresponding to 10-11 kbar and progression of regional metamorphism at 361±5 Ma. Continuous underthrusting of the Brunovistulian terrane led to exhumation of the OSD metasediments and the formation of subhorizontal, N-S-trending tight folds and a peak-metamorphic axial planar foliation. This episode took place at 650°C and 6-7 kbar in the non-migmatized rocks of structurally upper parts of the Młynowiec Formation, and at 700°C and 6-7 kbar in its structurally lower parts, where metamorphism was accompanied by localized in-situ migmatisation. During final consolidation of a mosaic of terranes, top-to-the-NE tectonic movements reactivated former fabrics, produced further uplift and cooled the rocks to greenschist facies. This pervasive shearing could be also responsible for the metasomatic event at ~ 330 Ma. The continuation of the NE-transpression of the OSD involved its interactions with adjacent crustal units in the east and in the north, which led to large-scale folding represented now by synforms and associated antiforms. The metamorphic structure of the rocks subjected to the Barrovian-type metamorphism was rebuilt so that the migmatitic rocks outcrop in the cores of large-scale antiforms, and the lithological contacts, penetrative foliation and metamorphic isograds dip locally at different angles (shallow, moderate to high) and at different azimuths (E-, NE-, N, NW and W). http://authors.elsevier.com/a/1PWz~98wdVUYZ
... To increase confidence in monazite dating some laboratories conduct re-measurements of internal comparative monazite ARMs (e.g. Finger and Helmy, 1998;Martin et al., 2007;Vlach, 2010;Allaz et al., 2013;Schulz and Schüssler, 2013;Bowles, 2015). However, the use of a single age standard may not be sufficient, particularly when the composition of unknown monazite differs considerably from the standard. ...
Article
This article proposes an improved approach to monazite dating by electron microprobe that includes a “monazite age reference correction” (MARC). During analysis, a set of differing monazite standard reference materials with established isotopic ages are measured at the start of the session. These measurements are used to test the analytical set-up and, if necessary, to calculate MARC factors that can be applied to monazite samples. The MARC is not intended as a way to correct systematic errors due to problems in set-up, but rather as a fine-scale adjustment for factors that cannot be readily assessed during single sessions. Long-term, multi-session calculation of MARC factors allows for precise monitoring of anomalous behavior among monazite age reference materials during individual sessions. The method can also assist in the identification of chemical inhomogeneity in monazite, such as that commonly produced by interaction with metasomatic fluids. A representative set of electron microprobe monazite age reference materials are presented, including two ‘reference monazites’ that are good examples of monazite with age disturbance induced by metasomatism. Additional modifications to analytic protocols are proposed, including a) corrections for count rate increases during long beam dwell times, and b) improved estimation of background values at line positions by accounting for the effect of mean atomic number.
... Depending on bulk rock compositions, metamorphic monazite potentially crystallizes in Ca-poor and Al-rich metapelites at temperatures above 300°C (Spear and Pyle, 2002;Janots et al., 2007;Spear, 2010). Apart from rock bulk composition and metamorphic temperature at medium to low pressures, the (re)crystallization of monazite is apparently controlled by the presence of various fluids, as has been documented in field and experimental studies (McFarlane and Harrison, 2006;Just et al., 2010;Budzyń et al., 2011Budzyń et al., , 2015Budzyń et al., , 2017Harlov et al., 2011;Williams et al., 2011;Schulz and Schüssler, 2013;Harlov, 2015). As a consequence, the Th-U-Pb dating of monazite has a significant potential to constrain geological events in shear zones and the involved protoliths, even below T c . ...
Article
Shear zones play a major role in the deformation of the crust at a variety of scales, as expressions of strain localization during orogeny and rifting, and also as reactivated structures. They influence the geometry and evolution of orogenic belts and rifts, crustal rheology, magma ascent and emplacement, and fluid flow. Consequently, assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere. The interpretation of thermochronologic data from shear zones is, however, not straightforward. In the first place, closure temperatures depend on a number of factors (grain size, cooling rate, mineral composition and pressure, among others). On the other hand, deformation-related processes such as dynamic recrystallization, neocrystallization and fluid circulation seem to be crucial for isotopic systems and, thus, the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks. For this reason, geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo- or recrystallization, fluid-assisted deformation and inheritance of the protolith age(s). In order to robustly reconstruct P-T-ε-t paths of long-term crustal-scale shear zones, structural, microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems. In addition, geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications. One of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion, particularly the coupling with deformation processes, based on natural, theoretical and experimental data. Future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks.
... ThO 2 * is ThO 2 plus O 2 equivalents after . Madmon, a monazite from a pegmatite in Madagascar, acted as a reference for monazite data (U-Pb SHRIMP age at 496 ± 9 Ma, around 10 wt% ThO 2 ; Schulz et al. 2007, Schulz & Schüssler 2013. ...
Article
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In the Southeastern Quadrilátero Ferrífero, a package of metapelitic rocks previously attributed to the Archean Rio das Velhas Supergroup crops out in Piranga locality. This study presents the mineral chemistry and U-Pb-Hf zircon geochronology on foliated staurolite-garnet mica schists. Garnet and staurolite index minerals are syn-to post-kinematic towards the main schistosity. Garnet porphyroblasts display well-developed compositional zoning of Mg-Fe-Mn-Ca, with increase of almandine and pyrope and decrease of spessartine towards the rim, implying in prograde metamorphic pattern. Estimates of P-T values for the metamorphic peak resulted in temperatures between 630 to 650ºC and pressure around 7 kbar. Pseudosections show well-defined stability fields in amphibolite facies, with a metamorphic path displaying progressive increase in P-T conditions. Maximum depositional age of 1,875 ± 51 Ma is established for the Piranga mica schists pointing to a depositional history that is younger than those previously described. Metamorphic Cambrian ages characterize the strong influence of deformational processes related to the final stages of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero.
... ThO 2 * is ThO 2 plus O 2 equivalents after . Madmon, a monazite from a pegmatite in Madagascar, acted as a reference for monazite data (U-Pb SHRIMP age at 496 ± 9 Ma, around 10 wt% ThO 2 ; Schulz et al. 2007, Schulz & Schüssler 2013. ...
... Orthophosphates of the Smithsonian Institution were used as standards for REE analysis (Donovan et al., 2003;Jarosewich and Boatner, 1991). Madmon, a monazite from a pegmatite in Madagascar, acts as reference for monazite data and offline recalibration of ThO 2 (U-Pb-SHRIMP Madmon age of 496 ± 9 Ma, around 10 wt% ThO 2 ; (Schulz et al., 2007;Schulz and Schüssler, 2013). The monazite chemical model ages were determined using the ThO 2 *-PbO isochrone method (CHIME) of and Adachi (1994, 1991), where the age is calculated from the slope of the regression line in ThO 2 * vs PbO coordinates forced through zero, in a JEOL software (MonaziteAge, program for calculating monazite ages, version 2.03 -McSwiggen & Associates 2010). ...
Article
Abstract In this contribution we investigate the exhumation T-t path of a nappe system located in the southernmost edge of the Brasília Orogen, southeast Brazil, developed during the West Gondwana assembly. The allochthons represent an inverted metamorphic pile of nappes and were deformed during the collision between the Paranapanema (active margin) and São Francisco (passive margin) paleocontinents. The nappe system comprises UHT rocks of a magmatic arc root (upper Socorro-Guaxupé Nappe), the accretionary wedge – foreland units metamorphosed under high-pressure conditions (intermediate Andrelândia Nappe System) and the lower passive margin metasedimentary sequences (the high-pressure Carrancas Nappes System and the Lima Duarte Nappe). New U-Th-PbT in monazite and 40Ar/39Ar in hornblende, biotite and muscovite ages combined with previously published data indicate different patterns of cooling for each allochthon. The upper nappes register a collision to exhumation/cooling path, from 630–625 Ma to 590–580 Ma, which indicates that the geological process active during the West Gondwana amalgamation involved fast exhumation tectonics. Cross-sections along the main transport direction of the nappes indicate a progressive decrease of metamorphic age peaks (630–625 Ma to 590–570 Ma) and 40Ar/39Ar cooling ages (600 Ma to 540 Ma) from the internal regions (SW) to the front (NE) of the nappe system, which indicate that the propagation of the nappe pile advanced progressively from the upper to the lower nappes.
... The U was calibrated on a metal reference material. The Th was calibrated on a reference monazite labelled as Madmon, with validated special ThO 2 *-PbO characteristics (Schulz et al. 2007(Schulz et al. , 2019Schulz and Schüssler 2013). The Madmon reference monazite was also used for offline re-calibration of ThO 2 , as well as for the control of age data. ...
Article
Erinpura Granites form the basement for the Neoproterozoic Malani Igneous Suite in NW India. Based on this study, the Erinpura granites can be divided into Erinpura-East (gneissic fabric), a belt parallel to the southern sector of the South Delhi Fold Belt, and Erinpura-West (magmatic fabrics). EPMA dating on monazites gives a time frame of 890 to 860 Ma for crystallization of both types. The geochemically homogeneous peraluminous S-type granites with ε Nd values of −2.1 to −10.8 are interpreted as melting products of Archaean crust. REE pattern follows the pattern of the average continental crust, but Erinpura-East samples are more fractionated with steeper HREE depletion indicating melting in a deeper crustal level. A thermal pulse between 835 and 820 Ma constrains the timing of deformation in granite-gneisses during uplift along thrust planes, coeval with shear-bound exhumation of high-grade metamorphic rocks and initiated by delamination of the lower crust in this southern sector of the South Delhi Fold Belt. This is in contrast to the northern sector of the SDFB with arrested orogenic development and without considerable delamination or erosion of the lower crust. Latest movement related to the 200 Ma tectono-magmatic history overlaps with initiation of rifting during the Malani igneous event. A change from S-type to A-type granites and shift of isotopic signatures to ε Nd values of −2.8 to −1.7 indicate substantial contribution of asthenospheric material in the Malani melting process. ARTICLE HISTORY
Chapter
The alkali metal Cesium was first described by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff during the investigation of mineral waters from Dürkheim. It is a silvery-gold, soft extremely reactive and pyrophoric metals, has a large ionic radius and a rather low melting point. Cesium belongs to the large ion lithophile elements. The Cesium market is very small. As a result, data on Cs resources and production are not available or very limited. According to the USGS Cs is currently only mined from massive pollucite mineralisation bearing pegmatites in Canada (Tanco) and Zimbabwe (Bikita). Cesium is a typical trace element and normally occurs in the level of a few to some tens of ppm. As Cs is almost incompatible during magmatic crystallisation, it becomes enriched in residual melts. Therefor Cs occurs as trace element in feldspar and mica. However, certain geological processes are capable to enrich Cs to several thousand ppm so that specific conditions can lead to the formation of discrete Cs minerals. At present 31 Cs are known and approved by the International Mineralogical Association. Most of them crystallise in granitic pegmatites or in alkaline complexes at late stage magmatic to hydrothermal processes. However, only the zeolite Cs mineral pollucite as part of the analcime-pollucite series is known to occur in larger and thus economic quantities. Pollucite is classified as tectosilicate and belongs to the zeolite group. It has a general composition of (Cs, Na)2Al2Si4O12xH2O and is isostructural to analcime NaAlSi2O6xH2O. Although it can crystallise in gem stone quality cubic, dodecahedral to trapezohedral crystals with various colours pollucite is commonly developed as glassy, colourless to white polycrystalline masses. Pollucite occurrences are reported from approximately 140 locations worldwide. However, at present only two LCT pegmatite deposits are known to host economic quantities of massive pollucite mineralisations. These are the Bikita LCT pegmatite in Zimbabwe and the Tanco LCT pegmatite deposit in Canada. Both pegmatite deposits have a comparable regional background, as they are hosted within greenstone belts on Archean Cratons and yield Neo-Archean ages of ~2625 Ma. Although several studies focused on the mineralogical and geochemical characteristics of pollucite are done, no systematic investigation focusing on the genesis of these massive pollucite mineralisations was performed so far. Therefore, the massive pollucite mineralisation at Bikita were studied in detail to understand the genetic concepts on their formation. Major Portions of Western Australia consists of Meso- to Neoarchean crustal units (e.g., Yilgarn Craton, Pilbara Craton) that are known to host a large number of LCT pegmatite systems. Among them are the LCT pegmatite deposits Greenbushes (Li, ±Ta) and Wodgina (Ta, Sn ±Li). In addition, small amounts of pollucite were recovered from a single diamond drill core at the Londonderry pegmatite field and more recently from the Sinclair LCT pegmatite, both located within the Yilgarn Craton. Despite that, no systematic investigation and/or exploration were conducted for the mode of occurrence of Cs and especially that of pollucite in Western Australia. Thus, Western Australia represents prospective area that may host one more deposit of a yet merely unique mineralisation type on the planet Earth.
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Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Kärnten, Austria, Eastern Alps) display complex microstructural and mineral-chemical relationships. Automated scanning electron microscopy routines with energy dispersive X-ray (EDX) spectral mapping were applied for monazite detection and garnet mineral-chemical characterisation. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron microprobe (EMP) analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low-Ca and high-Al-metapelites. Garnet 1 has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism at ~ 650°C/6 - 8 kbar. Sporadic monazite 1 crystallization started at c. 320 Ma. Subsequent pervasive 300 - 250 Ma high-Y and high-Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320 - 250 Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous-to-Early-Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate P/T gradient has not yet been described from the Saualpe, and preceded a Permo-Triassic low pressure stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing XCa at decreasing XMg, then increasing XCa and XMg, and finally decreasing XCa with increasing XMg, always at high Ca and Mg, and low Mn. This records a P-T evolution which passed through eclogite-facies conditions and reached maximum temperatures at ~ 750 °C/14 kbar during decompression-heating. A monazite 2 population (94 - 86 Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo-Alpine) metamorphism M2 at a high P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P-T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas. This article is protected by copyright. All rights reserved.
Chapter
After a description of the main geological units and the present-day geotectonic setting before Gondwana amalgamation, this chapter summarises the tectonic evolution of the Antarctic continent from its inclusion as part of the Gondwana supercontinent to the breakup of this landmass and the repositioning of Antarctica at southern polar latitudes since the Early Cretaceous. The geological evolution of the Antarctic continent is then described considering two main time periods: (1) c. 600–450 Ma, covering the processes which were active immediately before and during the amalgamation of Gondwana; and (2) c. 450–180 Ma, including all the major events that occurred after the final stage of Gondwana amalgamation to the time immediately before the Gondwana breakup phase. A subsequent section is devoted to the 180 Ma to recent time window during which present-day Antarctica and the other southern continents and surrounding oceanic basins formed as consequence of the fragmentation of Gondwana, and when tectonic processes led to the drift and dispersion of the various continental fragments. After a general overview of the most significant plate tectonic stages, and coeval magmatic products, the chapter reviews the main geological findings from the Ross Embayement region – one of the most investigated regions in Antarctica – the Transantarctic Mountains and the Ross Sea sector of the Western Antarctic Rift System. Persistent open problems, and potential research themes, are discussed in the Conclusions.
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Table A1 Overview of Cesium minerals. Fig. A2 Two contrasting tectonic models for the Archean development of the Yilgarn Craton. Table A3 Zonation and mineral assemblages of the Londonderry Feldspar Quarry pegmatite. Table A4 Zonation and mineral assemblages of the Cattlin Creek pegmatite. Fig. A5 Tectono-magmatic model for the Archean evolution of the Pilbara Craton. Fig. A6 Petrographical and mineralogical characteristics of albite. Fig. A7 Petrographical and mineralogical characteristics of K-feldspar. Fig. A8 Petrographical and mineralogical characteristics of quartz. Fig. A9 Petrographical and mineralogical characteristics of muscovite. Fig. A10 Petrographical and mineralogical characteristics of lepidolite. Fig. A11 Petrographical and mineralogical characteristics of tourmaline. Fig. A12 Petrographical and mineralogical characteristics of apatite. Fig. A13 Petrographical and mineralogical characteristics of cassiterite. Fig. A14 Petrographical and mineralogical characteristics of tantalite and columbite. Fig. A15 Petrographical and mineralogical characteristics of secondary Ta-and Nb-oxides. Fig. A16 Comparison of Cs vs. selected major oxides. Fig. A17 Comparison of Cs vs. selected minor and trace elements. Fig. A18 Chondrite normalised REE distribution pattern of selected samples. Fig. A19 Results of LA-ICP-MS dating of Ta-, Nb-and Sn-oxide minerals from Wodgina.
Article
A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif. Based on the textural relations and compositions of minerals, especially of zoned garnet and potassic white mica, a P-T path was reconstructed using contoured P-T pseudosections. The U-Th-Pb dating of monazite in the micaschist was undertaken with the electron microprobe. The micaschist experienced P-T conditions along a clockwise path between 16 kbar at 510 °C and 5 kbar at 555 °C followed by isobaric heating to about 600 °C. Monazite ages range between 315 and 480 Ma with the most prominent maxima and side maxima at 346.0±1.1 (2σ), 357.3±2.3, and 368.3±1.7 Ma. Ages older than 380 Ma were related to detrital monazite pointing to a Devonian sedimentary protolith. Other ages around 325 Ma were assigned to the isobaric heating by nearby post-tectonic granites. The high-pressure event, being the result of the collision of Laurussia and Gondwana after closure of the Rheic Ocean, occurred in the Late Devonian. The exhumation to 15–20 km (5 kbar) ended probably in the Early Carboniferous. The high-pressure micaschists from the Fichtelgebirge to the Erzgebirge crystalline complexes are suggested to represent a single nappe within a metamorphic nappe pile. This nappe is composed of metasedimentary slices, which experienced different peak pressures rather than representing a coherent crustal section.
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Mafic and ultramafic rocks intercalated with metamorphosed deep-marine sediments in the Glenelg River Complex of SE Australia comprise variably tectonized fragments of an interpreted late Neoproterozoic–earliest Cambrian hyper-extended continental margin that was dismembered and thrust westwards over the adjacent continental margin during the Cambro-Ordovician Delamerian Orogeny. Ultramafic rocks include serpentinized harzburgite of inferred subcontinental lithospheric origin that had already been exhumed at the seafloor before sedimentation commenced, whereas mafic rocks exhibit mainly enriched- and normal-type mid-ocean ridge basalt (E- and N-MORB) compositions consistent with emplacement in an oceanic setting. These lithologies and their metasedimentary host rocks predate deposition of the Cambrian Kanmantoo Group and are more likely to represent temporal equivalents of the older Normanville Group or underlying Neoproterozoic Adelaide Supergroup. The Kanmantoo Group is host to basaltic rocks with higher degrees of crustal contamination and yields detrital zircon populations dominated by 600–500 Ma ages. Except for quartz greywacke confined to the uppermost part of the sequence, metasedimentary rocks in the Glenelg River Complex are devoid of detrital zircon, and are interstratified with subordinate amounts of metachert and carbonaceous dolomitic slate suggestive of deposition in a deep-marine environment far removed from any continental margin. Seismic reflection data support the idea that the Glenelg River Complex is underlain by mafic and ultramafic rocks, and preclude earlier interpretations based on aeromagnetic data that the continental margin incorporates a thick pile of seawards-dipping basaltic flows analogous to those of volcanic margins in the North Atlantic. Correlative hyper-extended continental rift margins to the Glenelg River Complex occur along strike in formerly contiguous parts of Antarctica. Supplementary material Geochemical data for mafic and ultramafic rocks in the Glenelg River Complex and correlative terranes, and U–Th–Pb data for western Victoria gabbros are available at http://www.geolsoc.org.uk/SUP18821
Conference Paper
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Classic tectonic models interpret the Ross-orogenic structural edifice of northern Victoria Land in terms of three terranes, which formed due to westward subduction of the Palaeo-Pacific Ocean underneath the East Gondwana active continental margin (Kleinschmidt & Tessensohn 1987). These comprise from west to east the Wilson Terrane (low- to high-grade metamorphic rocks and voluminous granitoids), the Bowers Terrane (very low- to low-grade metamorphic island arc-type volcanic and sedimentary rocks), and the Robertson Bay Terrane (very low- to low-grade turbidites). U-Pb ages of detrital zircons separated from meta-sedimentary rocks of these three terranes were dated by LA-ICP-MS in order to get information on provenance and age of the rocks. We analyzed 4 samples from the Priestley Formation (Wilson Terrane), 3 samples from the Molar Formation (Bowers Terrane), 2 samples from the Millen Schists along the Bowers-Robertson Bay terrane boundary, and finally 3 samples from the Robertson Bay turbidites. The youngest concordant zircon ages of the Priestley Formation were around ca. 592 Ma, which is interpreted as a maximum age of deposition of the sediments. Ross-orogenic maximum ages were derived from the Molar Formation (ca. 495 Ma), the Millen Schists (ca. 485 Ma), and the Robertson Bay turbidites (ca. 470 Ma). The late Cambrian maximum age of the Molar Formation does not conflict with the middle Cambrian age of fossils that occur in exotic limestone blocks (Wolfart 1994). The following zircon-age groups are present in the samples with different percentages: a) 3500–1600 Ma: Archean-Palaeoproterozoic cratonic areas; b) 1300–900 Ma: Grenville Orogen; c) 700–580 Ma: Pan-African collision belts or pre-Ross-orogenic rifted-margin of East Gond-wana; d) 530–470 Ma (except Priestley Formation): Ross Orogeny. These results imply: (1) the existence of Grenville-age and Pan-African mobile zones in the ice-covered hinterland of the modern Transantarctic Mountains, and (2) a common East Antarctic source area for all analyzed formations. The Priestley Formation was deposited on the passive continental margin of East Gondwana in late Neopro¬tero¬zoic times, but before the onset of Ross-orogenic subduction of the Palaeo-Pacific Ocean. It was metamorphosed in Ross-orogenic times and intruded by syn- to late-orogenic granitoids (Granite Harbour Intrusives). The Bowers Terrane formed in an intra-oceanic island arc setting in the Cambrian and was accreted to the Ross-age active continental margin not later than ca. 495 Ma. The Robertson Bay turbidites formed in an accretionary setting in front of the now combined Wilson-Bowers terrane active continental margin of East Gondwana due to on-going westward subduction of the Palaeo-Pacific Ocean (e.g., Roland et al. 2004). References: Kleinschmidt, G. & Tessensohn, F. (1987): Early Paleozoic westward directed subduction at the Pacific continental margin of Antarctica. – In: McKenzie, G. (ed.): Gondwana Six. AGU Geophys. Monogr., 40: 89-105. Wolfart, R. (1994): Middle Cambrian Faunas, North Victoria Land, Antarctica. – Geol. Jb., B 84: 1-164; Hannover. Roland, N.W., Läufer, A.L. & Rossetti, F. (2004): Revision of the Terrane Model of Northern Victoria Land (Antarctica). – Terra Antarctica, 11: 55-65; Siena.
Article
Because of their essential information for constraining the timescales of high-temperature processes, zircon and monazite are the two most important minerals for understanding crustal processes. Zircon and monazite are common accessory minerals in pelitic granulites (Sulu orogen, eastern China). The pelitic granulites have a peak mineral assemblage of garnet + sillimanite + antiperthite + plagioclase + quartz + rutile ± biotite. Zircons from the granulite have low Th/U ratios (<0.1 mostly), flat HREE patterns with negative Eu anomalies, suggesting a typical of granulite-facies metamorphic zircon. Ti-in-zircon thermometers give temperature estimates ranging from 911 to 954 °C with a weighted mean of 928 ± 10 °C, suggesting a ultrahigh temperature (UHT) metamorphism. These metamorphic zircons yield a concordant ²⁰⁶Pb/²³⁸U age of 1843 ± 17 Ma (n = 26) and an upper intercept age of 1842 ± 15 Ma (n = 28), suggesting a Paleoproterozoic UHT metamorphism. The monazites in the pelitic granulite record the same Paleoproterozoic age (ca. 1840 Ma) as the zircons. The monazites generally exhibit a zoned structure in BSE images: gray core and bright rim. Both the cores and rims display significant depletion in HREEs and Y, and negative Eu anomalies, indicating their formation under granulite-facies metamorphic conditions. Notably, the bright rims are more depleted in HREEs and Y, and more negative Eu anomalies than the gray cores. Compared with the gray cores (Th = 45,429–165,236 ppm; U = 2227–6044 ppm; and Th/U = 18–28), the bright rims have high Th (194,616–285,224 ppm) but low U (1145–3209 ppm) contents, resulting in very high Th/U ratios (69–217). These results establish zircon-monazite-garnet- feldspar REE equilibrium and their U–Th–Pb system under UHT metamorphic processes. Paleoproterozoic UHT granulite-facies metamorphism in the Sulu orogen provides a crutial clue to understand its early geodynamic setting and tectonic evolution.
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Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately mature shales. Highly mature lithotypes with Chemical Index of Weathering values of ≥ 95 are typically absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very homogeneous series lacking significant compositional variations. Major variations are only found in chemical parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical sorting of the detritus. Geochemical data, 87Sr/86Sr t=490Ma ratios of 0.7120 - 0.7174, εNd,t=490Ma values of -7.6 to -10.3 and single-stage Nd-model ages of 1.7 - 1.9 Ga are indicative of an origin from a chemically evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for significant sedimentary infill from primitive "ophiolitic" sources. Metasediments of the Middle Cambrian Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace element data and Sm-Nd isotope data (εNd,t=500Ma values of -14.3 to -1.2 and single-stage Nd-model ages of 1.7 - 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor rocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from more western inboard areas of the Wilson Terrane (εNd,t=510Ma -7.0 to -14.3) indicate a relatively high proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age. Metasedimentary series in an eastern, more outboard position (εNd,t=510Ma -5.4 to -10.0; single-stage Nd model ages 1.4 - 1.9) on the contrary document stronger influence of a more primitive source with younger formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope geochemistry for an island-arc component in these series.
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High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a signiÞ cant number of individual spot analyses is required to reach such precise information (i.e., more than 30–40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efÞ cient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting signiÞ cant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but signiÞ cant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the Þ rst case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.
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New field data from the Lanterman Range - Mountaineer Range region suggest that the easternmost portion of the Wilson Terrane is composite, including four major metamorphic complexes, each of them characterized by distinct lithological assemblages and metamorphic patterns. These data, in conjuction with available geochronological constraints, support the subdivision of the Ross Orogen into an inner, broad, low P - high T belt hosting the magmatic arc, and an outer narrow belt including three distinct tectonic units composed by medium- to high-pressure rocks, some of them of mafic composition and retaining a MORB-like geochemical affinity. An orogenic model involving subduction-accretion at a continent/ocean plate boundary consistently integrates the Late Precambrian-Ordovician petrological, geochronological and structural records of the northern Victoria Land segment of the Antarctic paleo-Pacific margin of eastern Gondwana.
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The Léon domain adjacent to the Cadomian realm in the North Armorican domain appears to be a displaced crustal block, as its metamorphism and rock types bear a resemblance to the South Armorican domain of the internal Variscan belt. The amphibolite-facies Conquet-Penze Micaschist unit overlies the high-grade Lesneven Gneiss unit in the central part of the Léon. Timing and conditions of the metamorphic evolution have been evaluated. At the base of the Lesneven Gneiss unit, a high-pressure eclogite-facies stage (700 °C at >13 kbar) was followed by a high-temperature event (800 °C at 8 kbar), which is characterized by the crystallization of garnet-cor-dierite assemblages in aluminous paragneisses. Maximal temperatures in the upper parts of the Lesneven Gneiss unit were 630 °C at 6 kbar. Zoned garnet in assemblages with staurolite recorded prograde P-T paths from 490-610 °C at 5-8 kbar in the upper and at 6-9 kbar in the lower parts of the Conquet-Penze Micaschist unit. Garnet Y, heavy rare earth elements, and Li are low in high-grade gneisses and display strong zonations in the micaschists. A younger population of monazite with a broad range of Y contents displays Th-U-Pb ages between 340 and 300 Ma. It crystallized subsequent to formation of foliations S1-S 2 and Variscan peak metamorphic assemblages. In contrast, an older population of Cadomian monazite at 552-517 Ma is uniformly rich in Y, suggesting an earlier crystallization than garnet, however, at elevated temperatures. The findings do not support a South Armorican provenance of the Léon domain. The Léon units appear as part of a Cadomian crust at the northern margin of the former Armorican microplate. During a Variscan collision, this crust was strongly overprinted by underthrusting toward the southeast or east beneath the Central Armorican domain and by later uplift accompanied by Late Carboniferous dextral shear tectonics. The features are typical of the Variscan Saxo-Thuringian zone, which faced the Rheic Ocean to the north.
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Prograde suites of pelitic rocks were examined with electron microprobe and laser ablation inductively coupled plasma mass spectrometry to determine the systematics of element partitioning between coexisting monazite, xenotime, and garnet. Monazite grains that grew in equilibrium with xenotime are enriched in γ and Dy compared with monazite that grew in xenotime-absent assemblages. γ and heavy rare earth element contents of monazite coexisting with xenotime increase with rising temperature. Monazite-xenotime γ-Gd and γ-Dy partitioning is systematic within a metamorphic grade, and increases slightly with increasing metamorphic grade, suggesting that monazite-xenotime pairs approached partitioning equilibrium. Garnet and monazite in both xenotime-bearing and xenotime-absent assemblages show a strong (R2 = 0.94) systematic relationship between inverse temperature and In (KFA) for the net-transfer equilibrium γAG + OH-Ap + (25/4) Qtz = (5/4)Grs + (5/4)An + 3γPO4-Mnz + 1/2H2O, suggesting that garnet and monazite crystallized in compositional equilibrium. The following temperature-KFA relationship for the equilibrium above has been derived: T(°C = [-1·45p(bars) + 447772 (± 32052) / 567 (± 40)- Rln (KEq)] - 273·15 with a precision of some ± 30°C for temperature estimates. Our observations suggest that major and accessory phases interact in a coupled fashion during metamorphism, and also approach a state of compositional equilibrium as reactions proceed.
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The in-situ “chemical” Th–U–Pb dating of monazite with the electron microprobe is used to unravel the Neoproterozoic tectono-thermal history of the “Erinpura Granite” terrane in the foreland of the Delhi Fold Belt (DFB) in the NW Indian craton. These granitoids are variably deformed and show effects of shearing activity. Monazites from the Erinpura granitoids recorded two main events; (1) protolith crystallization at 863±23Ma and (2) recrystallization and formation of new Th-poor monazite at 775±26Ma during shear overprint. Some components of the Erinpura granitoids, such as the Siyawa Granite and granites exposed near Sirohi town, show evidence of migmatization. This migmatization event is documented by anatexis and associated monazite crystallization at 779±16Ma. The age data indicate an overlap in timing between anatectic event and ductile shear deformation. The end of the tectono-thermal event in the Sirohi area is constrained by a 736±6Ma Ar–Ar muscovite age data from the ductile shear zone.
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Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished from high-grade zones to the east and west. In the central zone, P–T estimates of 8 kbar and 800°C derive from the relic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C, as derived from mineral reactions and thermodynamic calculations on the assemblages (2) Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P–T–t path are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd.
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Accessory monazite crystals in granites are commonly unstable during amphibolite fa- cies regional metamorphism and typically become mantled by newly formed apatite-allan- ite-epidote coronas. This distinct textural feature of altered monazite and its growth mech- anism were studied in detail using backscattered electron imaging in a sample of metagranite from the Tauern Window in the eastern Alps. It appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite inter- growths in connection with Ca supply through metamorphic fluid. Around the apatite zone, a proximal allanite ring and a distal epidote ring developed. This concentric corona struc- ture, with the monazite core regularly preserved in the center, shows that the reaction kinetics were diffusion controlled and relatively slow. Quantitative electron microprobe analyses suggest that the elements released from mon- azite breakdown (P, REE, Y, Th, U), were diluted and redistributed in the newly formed apatite, allanite, and epidote overgrowth rings and were unable to leave the corona. This supports the common hypothesis that these trace elements are highly immobile during metamorphism. Furthermore, microprobe data suggest that the preserved monazite cores lost little, possibly none of their radiogenic lead during metamorphism. Thus, metastable monazite grains from orthogneisses appear to be very useful for constraining U-Th-Pb protolith ages. On the basis of these findings and a review of literature data, it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic compo- sition. While breaking down in granitoids, monazite may grow during prograde metamor- phism in other rocks such as metapelites.
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Monazite has become an important tool for geochronology, but it commonly exhibits complex internal zoning of composition and age. Experiments were conducted to characterize the textural development and the rate and mechanism of growth of finely powdered (2 O at 1.0 GPa and 1000 degrees C. Coarsely crushed quartz crystals 500 mu m in diameter grew rapidly and progressively engulfed monazite crystals to form arrays of monazite inclusions. The mean diameter of all monazite crystals decreased in the first 24 h, then increased at a constant rate consistent with growth by grain boundary diffusion-controlled Ostwald ripening with a minimum rate constant K1/4 = 4.41X10 (super -2) mu m/s 4 . Using small quartz crystals of uniform diameter ( approximately 0.5 mu m) in the starting material reduced quartz grain boundary mobility and limited the development of inclusions. Monazite grew by matrix volume diffusion-controlled Ostwald ripening with K1/3 = 1.02X10 (super -2) mu m/s 3 . In all run products, matrix coarsening produced linear crystal-size distributions that reflect continuous recrystallization and nucleation. Textural evidence suggests that matrix coarsening-induced coalescence was also an important growth mechanism. During annealing of fluid-filled rock, growing host crystals may occlude small monazite crystals, preserving their isotopic composition. Large monazite crystals may pin grain boundaries, while smaller crystals may move with grain boundaries by recrystallizing, a process that resets isotopic systems. Monazite crystals on grain boundaries may grow by Ostwald ripening to form rims and by coalescence. Accurate interpretations of monazite ages therefore require knowledge of the texture/growth history of the rock and its dated grains.
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Hydrothermal monazite is one of the most reliable geochronometers for U-Pb dating of epigenetic or hydrothermal ore deposits because its blocking temperature, at ca. 700degreesC, is higher than that of most metamorphic conditions. Although igneous monazite is routinely used for age dating igneous rocks, only a few ore deposits have been dated by hydrothermal monazite. This is due to the similar morphology of hydrothermal and igneous monazite, which prevents the reliable distinction of hydrothermal monazite, in particular. We demonstrate that hydrothermal monazite can be distinguished from igneous monazite by its unique geochemical signature, its local abundance, and to common association with hydrothermal mineral assemblages. Our data and the data of others reported in the literature suggest that the low ThO2 content of hydrothermal monazite (0-1 wt %) is distinct from that of igneous monazite (3 to >5 wt %) and may be used to determine their genesis.
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The main focus of this paper is on the petrological evolution of medium- to high-grade metamorphic units in the Wilson Terrane, the westernmost lithotectonic unit of the Ross Orogen in northern Victoria Land. The petrological data set is reviewed for all areas where P-T-t paths have been reconstructed and geochronological data are sufficiently complete to provide an overview of the regional metamorphic evolution of a ca. 600 km long segment of the Ross Orogen, from its termination along the Pacific coast to the Eisenhower Range near the Ross Sea coast. Petrological evidence reveals that different lithological units of the Wilson Terrane equate with distinct lithotectonic metamorphic complexes with partly independent P-T-t histories. In spite of the wide range of estimated peak metamorphic conditions, and variability in both shape of the P-T path (clockwise or counter-clockwise) and type of retrograde evolution (isobaric cooling or cooling/unloading), the reviewed P-T-t trajectories consistently support a setting of evolving subduction and accretion in the context of a Palaeozoic cordilleran-type active margin.
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The high-grade basement of the northwestern Wilson Terrane at Oates Coast is subdivided into three roughly north-south trending zones on the basis of tectonic thrusting and differences in metamorphic petrology. New results of detailed petrological investigations show that metamorphic rocks of the central zone were formed in course of one single, clockwise directed P-T evolution including a medium-pressure and high-temperature granulite-facies stage at about 8 kbar and >800° C, a subsequent isothermal decompression and a final stage with retrograde formation of biotite + muscovite gneisses. In the eastern and western zones the majority of metamorphic rocks experienced clockwise oriented metamorphism at somewhat lower P-T conditions of about 4-5.5 kbar and 700-800°C. While some rocks in both zones did not reach the upper stability limit of muscovite + quartz, granulite-facies rocks detected in parts of the western zone were formed under P-T conditions similar to those of the central zone. New SHRIMP data support an age for the metamorphic peak of 496-500 Ma in the central zone (Henjes-Kunst et al., 2004). 40Ar-39Ar dating of amphiboles and micas indicate a general trend to younger ages from the west to the east of the basement complex, i.e. from 488-486 Ma to 472-469 Ma for amphiboles and from 484-482 Ma to 466 Ma for micas. This is explained by temporal differences in the retrograde metamorphic evolution of the three zones in the course of the late-Ross-orogenic thrust-related uplift of the basement complex, with the western zone being exhumed earlier than the eastern zone.
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High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 ± 10 Ma and 493 ± 9 Ma. Coexisting zircon gave ages of 500 ± 4 Ma and 484 ± 5 Ma for a metatexite (two age populations) and 475 ± 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 ± 6 Ma and 482 ± 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages. While there is a component similar in age and in low Th/U ratio to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the metatexite, a major detrital contribution from 545 - 680 Ma old source rocks can be identified. The new age data support the model that granulite- to high- amphibolite-facies metamorphism and related igneous processes in basement rocks of northwestern Oates Land were confined to a relatively short period of time of Late Cambrian to early Ordovican age. An age of approximately 500 Ma is estimated for the Ross-orogenic granulite-facies metamorphism from consistent ages of monazite from two migmatites and of the older zircon age population in one metatexite. The variably younger zircon ages are interpreted to reflect mineral formation in the course of the post-granulite-facies metamorphic evolution, which led to a widespread high-amphibolite-facies retrogression and in part late-stage formation of ms+bi assemblages in the basement rocks and which lasted until about 465 Ma. The presence of inherited zircon components of latest Neoproterozoic to Cambrian age indicates that the high- to very-grade migmatitic basement in northwestern Oates Land originated from clastic series of Cambrian age and, therefore, may well represent the deeper-crustal equivalent of lower-grade metasedimentary series of the Wilson Terrane.
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In the Robertson Bay group very low grade metamorphism is widespread (175km width). Only near the western border in the Bowers Mountains is low grade reached. In the southern USARP Mountains a high temperature type was the only metamorphic event, which could be proved by the occurrence of andalusite-fibrolite-sillimanite + anatexis. The increase of metamorphic grade runs from W to E here, contrary to the increase in the Robertson Bay Group. However the metamorphic event in both units may be the same because of known age determinations and of identical parent rocks. The possibility will have to be examined in the intermediate mountain chains, e.g. the polymetamorphic Lanterman Range.-from Current Antarctic Literature
Conference Paper
Kinematic data from the Rennick Glacier area indicate the presence of two intra-Wilson Terrane late-Ross opposite-directed high-strain reverse shear systems. High-grade rocks are W- and E-ward displaced over low-grade rocks and shallow-level intrusions. The shear zones are offset in a step-like pattern suggesting the presence of ENE trending right-lateral faults. The structural pattern accounts for a relationship between the Exiles and Wilson thrusts in Oates Land,which in our opinion can be traced from the Pacific coast to the Ross Sea. The western front of the Ross Orogen towards the East Antarctic Craton is best interpreted as a broad W-vergent fold-and-thrust belt, along which the intra-Wilson Terrane arc was detached and thrust onto the craton. The shear zones related to the Exiles Thrust system represent the internal, easternmost thrusts of this belt. Based on our data in combination with recent geophysical and geochronological results,the craton-orogen boundary must be located significantly further W than previously inferred. The boundary and hence the inferred termination of the proposed fold-and-trust belt may roughly lie in the area between Mertz and Ninnis Glaciers in George V Land, taking into account a considerable amount of likely post-Ross crustal extension possibly related to the Wilkes Subglacial Basin.
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A metapelite from the easternmost Wyoming craton (Black Hills, South Dakota) has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800–1700 Ma assembly of southern Laurentia. Three deformational fabrics are recognized in oriented thin sections: an ENE-trending S1 fabric, preserved as oblique inclusion trails in garnet porphyroblasts; a NNW-trending S2 fabric, preserved as microlithons in the rock matrix; and a flattening fabric, S3, which transposed S1-S2 and dominates the matrix. A complex monazite porphyroblast has been analyzed in situ with the electron microprobe (Ultrachron) to constrain the timing of S1-S3 fabric formation associated with monazite growth. The core of this grain uniquely preserves the S1-S2 fabrics as sigmoidal inclusion trails. The mean total-Pb age of this domain is 1750 ± 10 Ma (all dates reported at 95% confidence; n = 39 spots), which is equivalent to the published ²⁰⁷Pb/²⁰⁶Pb age for the same domain. These results validate the total-Pb dating method in general and the Ultrachron in particular, for reliable age determination in low-Th monazite, and are interpreted as 1750 Ma minimum ages for the S1-S2 fabrics and sequential, D1-D2 collisional events that imposed them (~N-directed arc accretion and ~E-W continental collision, respectively). A higher-Th, Y rim of this same “Rosetta” grain truncates the S1-S2 sigmoid, and is associated with resorption textures in garnet porphyroblasts, coupled release of Y, and an S3 fabric that pervasively overprinted S1-S2 in the rock matrix. The mean Ultrachron date of this domain is 1692 ± 5 Ma (n = 17 spots), which is slightly younger that the published isotopic age for all monazite rims combined. These results support a ~1715–1690 Ma timeframe for localized doming (D3) related to granite magmatism, the onset of which has been dated independently at 1715 ± 3 Ma. The timing of post-D3 cooling through 350 and 300 °C is constrained by ⁴⁰Ar/³⁹Ar dates of ~1610 and ~1480 Ma obtained for separates of D3 matrix muscovite and biotite, respectively, which are interpreted as closure ages. This study shows that fabrics in poly-deformed rocks can be dated by linking monazite spot ages to key microtextures. Further, the results of this micrometer-scale study enhance previous knowledge of local thermotectonism (Black Hills) and regional terrane assembly (Laurentia).
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Previous theoretical considerations on the chemical U-Th-total Pb dating method failed to distinguish between thorogenic and uranogenic lead. However, it can be shown that the data points are located on a plane in the three-dimensional ThO2, PbO, UO2 space. The calculation of the best-fit plane yields a slope in the ThO2-PbO and UO2-PbO coordinate projections, and an initial PbO value. From the two slopes, Th/Pb- and U/Pb-ages can be calculated independently.The method described in this paper is applied to monazites from the Hercynian G4-granite of the Fichtelgebirge (Germany). Th/Pb- and U/Pb-ages were calculated at 323 ± 20 Ma and 304 ± 15 Ma, respectively. The intercept value close to zero indicates that no significant amounts of common lead are present in the monazites studied.
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We report measurements of Pb diffusion in both synthetic (CePO4) and natural monazites run under dry, 1-atm conditions. Powdered mixtures of prereacted CePO4 and PbZrO3 were used as the source of Pb diffusant for “in-diffusion” experiments conducted in sealed Pt capsules for durations ranging from a few hours to several weeks. Following the diffusion anneals, Pb concentration profiles were measured with Rutherford Backscattering Spectroscopy (RBS) and supplemented by measurements with secondary ion mass spectrometry (SIMS). In order to evaluate potential compositional effects upon Pb diffusivity and simulate diffusional Pb loss that might occur in natural systems, we also conducted “out-diffusion” experiments on Pb-bearing natural monazites. In these experiments, monazite grains were surrounded by a synthetic zircon powder to act as a “sink.” Monazites from these experiments were analyzed with SIMS. Over the temperature range 1100 to 1350°C, the Arrhenius relation determined for in-diffusion experiments on synthetic monazite is given by: D=0.94 exp (−592±39kJ mol−1/RT)m2s−1 Diffusivities for synthetic and natural monazites are similar, as are results of measurements made on the same samples using both RBS and SIMS. The activation energy for Pb diffusion we determined is more than three times that for natural monazite reported previously, with Pb diffusivities at
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Calculations of the transition from allanite to monazite-bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses. Calculations in the CFASHPCe and MnNCKFMASHPCe systems place the allanite to monazite transition in the middle amphibolite facies (525–600°C) for a bulk composition similar to Shaw's average pelite. The temperature of the transition is pressure dependent, and strongly dependent on the bulk rock CaO content, consistent with inferences from natural parageneses. The transition is also a function of the bulk Al2O3 content, although the calculated result is opposite to that inferred from natural samples. Comparison with published results in the La–Mg system suggest that the nature of the REE phosphate does not greatly influence the conditions of the transition.
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High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30–40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5 − 10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.
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The Delamerian orogen (southeast Australia) and the Wilson terrane (northern Victoria Land, Antarctica) constitute a formerly continuous lower Paleozoic fold-and-thrust belt developed along the paleo-Pacific margin of eastern Gondwana. Major folds and thrust faults in these regions, rooted in mid-crustal detachment zones, transported Cambrian-Ordovician granites and high-temperature- low-pressure metamorphic rocks divergently toward their respective western craton margins and eastern orogen margins and associated cover sequences. The structural imprints are related to the accretion of lower Paleozoic terranes at the eastern margin of the Delamerian orogen and the Wilson terrane. The continuity of the contemporaneous structure patterns in Australia and Antarctica is evidence for continuous convergent tectonism along the lower Paleozoic-Pacific margin of Gondwana.
Article
Two major thrust systems with contrasting senses of displacement transect the Wilson terrane crust of northern Victoria Land, Antarctica. Along both mylonitic shear zones the central high-grade metamorphic basement is detached and thrust divergently toward the west and east over synorogenic, lower grade fore-arc and back-arc basin sedimentary rocks, respectively. Deformation was preceded by pervasive high-temperature-low-pressure metamorphism. Granites intruded the basement prekinematically and postkinematically. The structures are interpreted as results of early Paleozoic subduction of the paleo-Pacific oceanic crust under the Antarctic craton.
Article
This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca+Si)/(Th+U+Pb+S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO4. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.
Article
Based on microstructural and compositional criteria, three generations of monazite can be identified in a peraluminous, garnet-rich, high-pressure metamorphic rock from the Bohemian Massif. The first monazite generation (M1) formed probably on the prograde part of the clockwise PT loop, a second (M2) during the high-pressure stage (26±3 kbar and 830±30 °C) and a third generation (M3) during subsequent near-isothermal decompression of the rock to 8±2 kbar.The high-pressure, M2 monazites are volumetrically dominant and form 100–300 μm grains. They are characterised by an unusual Sr–brabantite component (SrO up to 2 wt.%) and grew together with accessory Sr-rich fluorapatites (SrO up to 33 wt.%). The Sr contents of both minerals are indicative of high-pressure conditions and result from the breakdown of plagioclase and the release of Sr that occurred during this process.M3 monazites occur exclusively as small grains (10–100 μm) within and at the rims of apatite. Microstructures show that they grew at the expense of the apatite extracting LREEs from the latter under retrograde conditions. These “parasite monazites” in apatite are characterised by very low Th contents, which is consistent with having been derived primarily from an apatite source.M1 monazites are represented by resorbed, irregular shaped cores, which are occasionally preserved within the Sr–brabantite rich, high-pressure monazites (M2). These M1 cores are generally Sr-poor, indicating that they should have formed prior to the high-pressure stage, in the presence of plagioclase. The dissolution of M1 monazite and the reprecipitation of a Th, U-enriched M2 monazite during the high-pressure stage implies that LREEs were taken up by the high-pressure Sr-rich fluorapatite to some extent, and not completely processed for the formation of the M2 monazite. This would also explain the widespread retrograde growth of M3 monazite within the apatite. The ability of apatite to act as an interim host for LREEs at peak-PT conditions could be one of the reasons why monazite growth during regional metamorphism is often periodic.
Article
Determination of U-Th-Pb ages using the electron probe microanalyser (EPMA) is an inexpensive alternative method for dating monazite. The method is rapid and reliable, both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization. The main limitation of the method is its rather poor precision, i.e., ± 45 to ± 120 Ma for ages ranging from 300 to 3000 Ma calculated on each spot. The precision is limited by Pb content and by counting statistics, which cannot give precision better than 2% for individual determinations on U, Th, and Pb even at high levels. A procedure that uses the new Th/Pb vs. U/Pb diagram to improve the calculated precision on U-Th-Pb ages gives results to within ± 5 to ± 15 Ma for ages ranging from 300 to 3000 Ma. With complex polygenetic monazite, in which either the points show large scattering indicated by a large MSWD or the regression line exhibits a slope very different from neighbour theoretical isochrons, the procedure must be applied separately on homogeneous domains only. This makes it possible to distinguish between events separated by a gap of ∼20 to 60 Ma, according to the range of ages concerned (i.e., 300 to 3000 Ma). Several examples are given to illustrate these systematics.
Article
In carbonaceous shales of the Palaeoproterozoic Maraloou Formation, Capricorn Orogen, Western Australia, the intrusion of subvolcanic dolerite sills related to interlayered basaltic lavas (Killara Formation) caused hydrothermal fluid circulation that resulted in the growth of sub-millimetre-sized monazite crystals. In situ U–Pb analysis of monazite from two shale samples about 50 m apart in a diamond drill-hole gives 207Pb/206Pb ages of 1843±14 Ma and 1841±22 Ma. Combined, the data yield an age of 1843±10 Ma (MSWD=1.11; n=19 of 22), which is interpreted as the age of intrusion by the sills. This date provides a minimum age for the Maraloou Formation, previously only constrained between 2200 Ma and 1800 Ma. The presence of peperites, along with the development of fine coke mosaics in intruded kerogenous shale indicates that the magma intruded wet, shallowly buried sediments, and thus, that the 1843 Ma date probably approximates the depositional age of the Maraloou Formation. Our results demonstrate the potential of dating mafic intrusive rocks by U–Pb analysis of low-temperature metamorphic monazite in contact aureoles.
Article
In spite of the major importance of monazite as a repository for the rare earths and Th in the continental crust, for U-Th-Pb geochronology, and as a possible form for high-level nuclear waste, very little work has been carried out so far on the behaviour of this mineral during fluid-rock events. This contribution describes two contrasting examples of the hydrothermal alteration of monazite. The first case comes from a sample of the Carnmenellis granite (Cornwall, Southwest England), chloritized at 284 ± 16°C, whereas the other occurs in the Skiddaw granite (Lake District, Northwest England), which underwent greisenization at 200 ± 30°C.
Article
Electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite-sillimanite transition (620 +/- 15 C) and sillimanite-orthoclase (680 +/- 15 C) zones of the Cretaceous Ryoke metamorphic belt, southwest Japan. Monazites from pelitic gneisses are of metamorphic origin, euhedral to subhedral and chronologically homogeneous, giving chemical Th-U-total Pb isochron (CHIME) ages of 98.8 +/- 3.3 - 98.0 +/- 3.2 Ma. Two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca. 1700 Ma. Most detrital monazite grains are heterogeneous in the ThO2 and UO2 concentrations and have multiple or single rims as young as ca. 100 Ma. Several detrital monazite grains are well rounded in form, exhibit homogeneous Th and U distributions and show a Pb diffusion profile in the margin. The width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss: 18-22 micrometers for 620 C and 48-58 micrometers for 680 C. Assuming the isothermal diffusion of Pb from homogeneous monazite spheres during a 5 Ma duration of peak metamorphism, we obtain diffusion coefficients of 1.9 (+/- 0.3) x 10-21 and 1.5 (+/- 0.3) x 10-20 sq cm/s at 620 C and 680 C, respectively. These data derive an activation energy of 2.44 (+2.85/-1.26) x 105 J/mol and a frequency factor of 3.4 x 10-7 (8.5 x 10-12 - 2.2 x 107 sq cm/s, taking account of uncertainties of +/- 15 C in the temperatures and of +/- 20% in the diffusion coefficients. The diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for Pb in monazite that has been poorly understood so far.
Article
The Eastern Ghats Belt of peninsular India exposes the deeply eroded high-grade parts of a composite orogenic belt that, including the Rayner Complex in Antarctica, once formed an important crustal component of Proterozoic East Gondwana. Recent work established four major crustal units separated by tectonic boundaries: the Late Archaean Jeypore and Rengali Provinces (northwest and north), the Late Palaeoproterozoic Ongole Domain (southwest) and the Meso-Neoproterozoic Eastern Ghats Province (east). To identify the timing and regional distribution of tectonothermal events within the composite belt, monazites in 66 pelitic and felsic granulite samples from 26 localities were studied with the electron microprobe chemical dating technique. In most cases the monazites are polygenetic with rock- and grain-scale chemical and chronological heterogeneities that document growth during episodes of partial melting and significant modifications through strain- and fluid-induced recrystallisation and replacement.
Article
Zircons from an S-type granite in Antarctica originally analysed by conventional U-Pb procedures have been reanalysed by ion-microprobe. The new data show that the Proterozoic age previously derived for granitoid emplacement is not correct but reflects a meaningless value between the magmatic age of the granite (544±4 Ma) and that of the provenance of its source rocks. Subsequent growth of metamorphic zircon has compounded the complexity. A total of six different episodes of zircon growth are documented within this rock. These occurred at 469 ± 4 Ma, 544 ± 4 Ma, 1130 ± 50 Ma, about 2000 Ma, about 2500 Ma and 2825 ± 100 Ma. The data illustrate the ever-present danger of extrapolating isotopic data alignments to concordia, especially when they form a ‘reverse’ discordia, in which the data points more closely approach the younger part of the concordia than the upper part. Although not itself of Precambrian age, the granite, which forms part of the Wilson Plutonic Complex of northern Victoria Land, Antarctica, formed by melting of rocks of currently unknown depositional age, but which were apparently derived from a dominantly ∼ 1100 Ma provenance.
Article
Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this.Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbMα peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3bar) detectors show larger differences in exponential vs. linear peak-minus-background (P-B) values than the low-pressure (1bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ±2–3% for monazites with around 1500–2000ppm total Pb, or an additional absolute uncertainty of 20–50ppm Pb.
Article
Monazite is an underutilized mineral in U–Pb geochronological studies of crustal rocks. It occurs as an accessory mineral in a wide variety of rocks, including