Monazite Cores Research Articles

Zoned Monazite and Zircon as Monitors for the Thermal History of Granulite Terranes: an Example from the Central Indian Tectonic Zone

The growth and dissolution behaviour of detrital, metamorphic and magmatic monazite and zircon during granulite-facies anatexis in pelitic and psammo-pelitic granulites and in garnetiferous granite from the southern margin of the Central Indian Tectonic Zone (CITZ) have been investigated using reconstructed metamorphic reaction history, monazite electron microprobe dating and sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon geochronology. Whereas the pelitic granulites record medium-pressure granulite-facies metamorphism (BM1 stage), the psammo-pelitic granulite reached ultrahigh temperatures (TMax > 880°C at 8·7 kbar). The meta-psammite additionally records two stages of granulite-facies recrystallization (BM2 and BM3). Irrespective of variations in the bulk-rock compositions and peak metamorphic conditions, monazite is highly reactive during the BM1 event, producing complex, chemically zoned crystals. Textural, compositional and chemical ages of these grains indicate the stability of six compositional domains (CD1 to CD6 in the paragenetic sequence), of which CD1 represents pre-metamorphic detrital cores of Paleoproterozoic age. CD2 and CD3 (combined mean age of 1612 ± 14 Ma) mark two stages of recrystallization of detrital monazite cores during prograde events. Rims of CD4 monazite (ages between 1615 ± 14 and 1586 ± 14 Ma) on partially to completely equilibrated cores indicate melt crystallization at, or immediately following, peak BM1P metamorphism. CD5 monazite (age of 1574 ± 7 Ma) is restricted to the psammo-pelitic granulites, and marks final melt crystallization at the solidus during post-peak cooling (BM1R stage, where R represents retrograde metamorphism). The metamorphic rim of CD6 monazite (age of 1539 ± 24 Ma) around partially resorbed CD5 domains is linked to the decomposition of BM1 garnet during the terminal hydration event as part of a granulite-facies recrystallization event. Compositionally homogeneous monazite and rims of chemically zoned monazite grains in granite together record a magmatic crystallization age of 1604 ± 9 Ma. SHRIMP U–Pb zircon dating of the psammo-pelitic granulite and garnetiferous granite indicates detrital or inherited cores of Paleo- to Neoarchean age (3584 ± 3 to 2530 ± 3 Ma), which have been variously recrystallized and overgrown by new zircon: (1) at 1658 ± 12 Ma; (2) between 1595 ± 5 and 1590 ± 6 Ma; (3) at 1574 ± 9 Ma. These zircon dates are correlated with the timing of the following: (1) the protoliths of precursor sediments of the metasedimentary granulites, deposited between 2530 and 1658 Ma; (2) a short-lived high-grade event ∼65–70 Myr before the culmination of the BM1 granulite-facies event; (3) a high-T anatectic event, corresponding to the peak BM1P metamorphism at TMax > 900°C; (4) final crystallization of anatectic melt at the solidus (cf. BM1R metamorphic stage). These chronological constraints from monazite and zircon, when integrated with the metamorphic reaction history and published geochronological data, allow recognition of three episodes of granulite-facies metamorphism in the CITZ at 1658 Ma (pre-BM1 event), between 1612 and 1574 Ma (BM1 event), and between 1572 and 1539 Ma (combined BM2 and BM3 events), as part of a latest Paleoproterozoic to Early Mesoproterozoic orogenic event. This orogeny is linked to the growth of the Proto-Greater Indian Landmass.

Carboniferous inherited grain and age zoning of monazite and xenotime from leucogranites in far-eastern Nepal: Constraints from electron probe microanalysis

Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18-16 Ma in the High Himalaya. In garnet-bearing sillimanitemuscovite- biotite leucogranite, most monazite grains have inherited cores with the chemical dates of ~504-418 and ~342-272 Ma, which were overgrown by Early Miocene mantles and rims of 17.9 ± 1.5 Ma. Early Ordoviciain and Carboniferous ages are rarely found in the same euhedral-subhedral cores. In addition to the previously recognized Early Paleozoic magmatism, monazite cores with Carboniferous ages in Early Miocene leucogranites provide evidence for two periods of magmatism at the base of the High Himalaya prior to the Cenozoic Himalayan orogeny. In muscovite-biotite leucogranite, no inherited domains were observed in monazite and xenotime grains. They yielded the CHIME monazite and xenotime dates of 16.1 ± 2.0 and 19.8 ± 6.5 Ma, respectively. Monazite grains adjacent to xenotime have significantly lower concentrations of UO2 and Y2O3 compared to those isolated from xenotime. These results imply that xenotime influences Y and U contents in monazite, reflecting local equilibrium system. In aplitic leucogranite, monazite grains yielded the mean of apparent chemical date of 18.0 ± 2.2 Ma. The CHIME monazite ages of ~18-16 Ma in three leucogranites reflect the timing of melt crystallization.

Microprobe analysis and dating of monazite from the Potsdam Formation, New York: A progressive record of chemical reaction and fluid interaction

It has been recognized for several decades that REE-phosphates (monazite and xenotime) can grow during diagenesis and low-grade metamorphism. Growth of REE-bearing accessory phases at low-grade conditions commonly involves pervasive fluid-rock interaction, dissolution of detrital grains, transportation, and precipitation of REEs, typically facilitated by an increase in temperature. The occurrence of low-grade REE-phosphate offers a rare opportunity to date crystallization/mineralization and possibly fluid percolation. We report here the results of in situ dating by electron microprobe of Paleozoic authigenic and low-grade monazite and xenotime overgrowths on detrital monazite and zircon, respectively. Samples are from the Potsdam Formation, a basal sandstone deposited uncomformably on Proterozoic basement of the Adirondack Mountains of New York State. This study also focuses on the textural and chemical relationships of these REE-bearing accessory phases. Textures include rounded and fractured detrital monazite and zircon, which contrast with new sub-euhedral REE-phosphate overgrowths. Monazite overgrowths are enriched in LREE and depleted in HREE compared to detrital cores. The U and Th concentrations are low, typical of low-grade metamorphic conditions. Monazite core ages yield Proterozoic ages between 1.17 and 0.90 Ga (Shawinigan and Ottawan orogeny). Monazite overgrowth and xenotime ages indicate four to five major overgrowth events between ca. 500 Ma (shortly after the time of deposition) and ca. 200 Ma. As these ages are relatively young and the actinide content is low (∑ < 2 wt%), the radiogenic Pb content of monazite overgrowths and xenotime is low (<400 ppm). Therefore, EPMA dates have relatively large uncertainties. Nevertheless, the ages determined broadly correlate with major Paleozoic orogenic events recorded in the Appalachian Orogen to the East (Taconic, Salinic, Acadian, Neo-Acadian, and Alleghanian). Fluid percolation, driven by orogenic loading, may induce dissolution of detrital monazite and zircon. Subsequent precipitation of new monazite and xenotime probably results from changes in fluids or metamorphic conditions. This study demonstrates the power of the EMPA technique to resolve the fluid-related growth history of REE-phosphates in low-grade metasediments.

Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

Several metamorphic complexes in Southeast Asia have been interpreted as Precambrian basement, characterized by amphibolite to granulite facies metamorphism. In this paper, we re-evaluate the timing of this thermal event based on the large-scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central Vietnam. Most of the samples in this study are from metamorphic rocks (n=38) and granitoids (n=11) in the Kontum Massif. Gneisses (n=6) and granitoids (n=5) from the Hai Van Migmatite Complex and the Truong Son Belt, located to the north of the massif, were also studied. Two distinct thermal episodes (245–230Ma and 460–430Ma) affected Kontum Massif gneisses, while a single dominant event at 240–220Ma is recorded in the gneisses from the Hai Van Complex and the Truong Son Belt. Monazites from granitoids commonly yield an age of 240–220Ma. Mesoproterozoic ages (1530–1340Ma) were obtained only from monazite cores that are surrounded by c. 440Ma overgrowths. Thermobarometric results, combined with concentrations of Y2O3, Ce2O3, and heavy rare earth elements in monazite, and recently reported pressure–temperature paths suggest that Triassic ages correspond to retrograde metamorphism following decompression from high- to medium-pressure/temperature conditions. Ordovician–Silurian ages reflect low-pressure/temperature metamorphism accompanied by isobaric heating during prograde metamorphism. Some samples were affected by both metamorphic events. We conclude that high-grade metamorphism observed in so-called Precambrian basement terranes in central Vietnam occurred during both the Permian–Triassic and the Ordovician–Silurian, while peraluminous granitoid magmatism is Triassic. Additionally, our preliminary analyses for U–Pb zircon age and whole-rock chemistry of granitic gneisses from the Truong Song Belt suggests the presence of the Ordovician–Silurian volcanic arc magmatism in the region. Based on the pressure–temperature–time–protolith evolutions, metamorphic rocks from central Vietnam provide a continuous record of subduction–accretion–collision tectonics between the South China and Indochina blocks: in the Ordovician–Silurian, the region was characterized by active continental margin tectonics, followed by continental collision during the Late Permian to Early Triassic and subsequent exhumation during the Late Triassic. The results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other Southeast Asian terranes should be verified to achieve a better understanding of the Precambrian to Early Mesozoic tectonic history in Asia.

Integrated pressure–temperature–time constraints for the Tso Morari dome (Northwest India): implications for the burial and exhumation path of UHP units in the western Himalaya

AbstractNorthward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high‐pressure (HP) quartz‐eclogite to ultrahigh‐pressure (UHP) coesite–eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressure–temperature–time determinations within petrographically well‐constrained settings for zircon‐ and/or monazite‐bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion‐poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 ± 1.5 kbar and 535 ± 15 °C during early garnet growth and re‐equilibration. Sensitive high resolution ion microprobe (SHRIMP) U–Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 ± 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5–27.5 kbar and 630–645 °C using the assemblage garnet rim–omphacite–rutile–phengite–lawsonite–talc–quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P–T bracket. A SHRIMP U–Pb age determination of 50.8 ± 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet‐bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non‐fractionated isochemical phase diagram for the assemblage phengite–garnet–biotite–plagioclase–quartz–melt documents a restricted (M3) P–T stability field centred on 12.5 ± 0.5 kbar and 690 ± 25 °C. A second non‐fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet–cordierite–sillimanite–biotite–plagioclase–quartz–melt (M4) documents a narrow P–T stability field ranging between 7–8.4 kbar and 705–755 °C, which is consistent with independent multiequilibria P–T determinations. Th–Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 ± 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12‐mm a−1 rate of post‐pressure peak decompression at Tso Morari would appear less extreme.

Origin of an unusual monazite-xenotime gneiss, Hudson Highlands, New York: SHRIMP U-Pb geochronology and trace element geochemistry

A pod of monazite-xenotime gneiss (MXG) occurs within Mesoproterozoic paragneiss, Hudson Highlands, New York. This outcrop also contains granite of the Crystal Lake pluton, which migmatized the paragneiss. Previously, monazite, xenotime, and zircon from MXG, plus detrital zircon from the paragneiss, and igneous zircon from the granite, were dated using multi-grain thermal ionization mass spectrometry (TIMS). New SEM imagery of dated samples reveals that all minerals contain cores and rims. Thus TIMS analyses comprise mixtures of age components and are geologically meaningless. New spot analyses by sensitive high resolution ion microprobe (SHRIMP) of small homogeneous areas on individual grains allows deconvolution of ages within complexly zoned grains. Xenotime cores from MXG formed during two episodes (1034 ± 10 and 1014 ± 3 Ma), whereas three episodes of rim formation are recorded (999 ± 7, 961 ± 11, and 874 ± 11 Ma). Monazite cores from MXG mostly formed at 1004 ± 4 Ma; rims formed at 994 ± 4, 913 ± 7, and 890 ± 7 Ma. Zircon from MXG is composed of oscillatory-zoned detrital cores (2000-1170 Ma), plus metamorphic rims (1008 ± 7, 985 ± 5, and ∼950 Ma). In addition, MXG contains an unusual zircon population composed of irregularly-zoned elongate cores dated at 1036 ± 5 Ma, considered to be the time of formation of MXG. The time of granite emplacement is dated by oscillatory-zoned igneous cores at 1058 ± 4 Ma, which provides a minimum age constraint for the time of deposition of the paragneiss. Selected trace elements, including all REE plus U and Th, provide geochemical evidence for the origin of MXG. MREE-enriched xenotime from MXG are dissimilar from typical HREE-enriched patterns of igneous xenotime. The presence of large negative Eu anomalies and high U and Th in monazite and xenotime are uncharacteristic of typical ore-forming hydrothermal processes. We conclude that MXG is the result of unusual metasomatic processes during high grade metamorphism that was initiated at about 1035 Ma. This rock was then subjected to repeated episodes of dissolution/reprecipitation for about 150 m.y. during regional cooling of the Hudson Highlands.

Petrology and monazite dating of the Fe-rich gneisses from Kokava (Veporic Unit, Western Carpathians, Slovakia): Devonian sediments supplied from Gondwanan-sources metamorphosed in the Variscan times

*Corresponding author The Fe-rich gneisses from vicinity of Kokava nad Rimavicou (Central Slovakia) were studied by electron microprobe to resolve the origin of the iron ore bodies. Whole-rock chemistry of the ironstones shows the lack of CaO, MgO and partly elevated P 2 O 5 contents. This, together with the mineral assemblage of almandine + magnetite + grunerite + ilmenite + quartz + apatite + allanite + zircon + monazite suggests an origin by metamorphism of sedimentary protolith and disproves the older idea about the formation as a skarn. The Fe–Ti oxides thermometry suggests temperatures of 360–420 °C, and the average oxygen fugacity of ∆log fO 2 normalized to the FMαQ of +0.13. The reconstructed oxygen fugacity for temperatures 500–600 °C (amphibolite facies) yields an interval of c. +1 to +3 ∆log fO 2 (FMαQ). The presence of detrital zircons, and monazites dated in this study, reveals a participation of lithologies of the pan-African orogen. The source rocks of the Devonian ironstones sedimented probably as oolitic chamosite in lagoons. The dated detrital monazite cores (612–400 Ma) show an affinity to source rocks formed in northern peri-Gondwana due to rifting and opening of the Rheic Ocean and separation of Avalonia and Armorica microcontinents. The majority of monazite data from the rims of detrital grains and from unzoned metamorphic grains bear an evidence of Meso-Variscan metamorphism with ages clustering between 360 and 320 Ma (342 ± 4Ma – weighted mean ± 2σ). The younger population with elevated Th contents (18.0–20.5 wt. %) giving the ages of 310–240 Ma (279 ± 2 Ma – weighted mean) is connected with the collapse of Variscan orogen and/or the onset of the early Alpine continental rifting. The Alpine ages are very rare, but a few monazites and one Th–U–Si inclusion in quartz yielded Cretaceous ages (115–85 Ma).

Monazite stability, composition and geochronology as tracers of Paleoproterozoic events at the eastern margin of the East European Craton (Taratash complex, Middle Urals)

The Precambrian Taratash complex (Middle Urals) is one of the rare windows into the Palaeoproterozoic and earlier history of the eastern margin of the East European Craton. Monazite from intensively deformed rocks within a major amphibolite-facies shear zone in the Taratash complex has been investigated by means of electron-probe microanalysis and laser-ablation SF-ICP-MS. Metamorphic and magmatic cores of monazite from metasedimentary and metagranitoid rocks yield U–Pb ages of 2244 ± 19 and 2230 ± 22 Ma (± 2 σ) and record a previously unknown pre-deformational HT-metamorphic event in the Taratash complex. Subsequent dissolution–reprecipitation of monazite, during shear zone formation under amphibolite-facies conditions, caused patchy zonation and chemical alteration of the recrystallised monazite domains, leading to higher cheralite and huttonite components. This process, which was mediated by a probable (alkali + OH)-bearing metamorphic fluid also caused a total resetting of the U–Pb-system. The patchy domains yield concordant U–Pb-ages between 2052 ± 16 and 2066 ± 22 Ma, interpreted as the age of the shear zone. In line with previously published ages of high grade metamorphism and migmatisation, the data may point to a Palaeoproterozoic orogenic event at the eastern margin of the East European Craton. Post-deformational fluid-induced greenschist-facies retrogression caused partial to complete breakdown of monazite to fluorapatite, REE + Y-rich epidote, allanite and Th-orthosilicate.The retrograde assemblages either form coronas around monazite, or occur as dispersed reaction zones, indicating that the REE, Y, and Th were mobile at least on the thin section scale. The greenschist-facies metamorphic fluid was aqueous and rich in Ca. Monazite affected by advanced breakdown responded to the retrogression by incorporating the cheralite or huttonite components during a fluid-induced dissolution–reprecipitation process. This event did not reset the U–Pb-system but caused partial Pb loss reflected by discordant U–Pb-dates.

Growth of the Greater Indian Landmass and its assembly in Rodinia: Geochronological evidence from the Central Indian Tectonic Zone

The Sausar Mobile Belt at the southern margin of the Central Indian Tectonic Zone (CITZ) is a key unit in constraining the timing of continent–continent collision between the North and South Indian cratonic blocks. It consists of three domains – northern, central and southern – that record the history of this collision. Rocks from the northern and central domains were metamorphosed under medium pressure–medium temperature conditions and define a clockwise P–T trajectory. Monazite spot ages in garnet–cordierite migmatite from the western extremity of the northern domain yield a weighted mean age of 1043±18Ma (MSWD=0.26) for monazite cores and 955±11Ma (MSWD=0.43) for monazite rims. These ages are correlated with peak granulite facies metamorphism and post-peak decompression, respectively. Garnet–orthoamphibole gneiss and garnet–cordierite migmatite from the central and eastern sectors of the northern domain, respectively, yield weighted mean ages (for monazite) of 974±9Ma (MSWD=0.53) and 989±9Ma (MSWD=1.3). These ages mark the timing of retrograde metamorphism in the northern domain. Charnockite in the northern domain, which was emplaced during post-peak decompression, records a SHRIMP U–Pb zircon concordia age of 938±3Ma (MSWD=1.9). Lu–Hf isotopic analyses of these zircons show a restricted range of εHf(T) values (from −2.9 to −5.8 with a mean of −4.3) and have TDMc model ages from 1995 to 2163Ma (with a mean of 2064Ma), implying derivation of charnockitic magma from a Paleoproterozoic crustal source. In contrast, garnet–staurolite–kyanite schist and garnet–biotite–muscovite–quartz schist from the central domain of the Sausar Mobile Belt yield weighted mean monazite ages of 1062±13Ma (MSWD=1.2) and 993±19Ma (MSWD=0.16), respectively, which are broadly correlated with the peak and retrograde stages of metamorphism from this domain. Geochronological data thus tightly bracket the collisional orogeny in the CITZ at between 1.06Ga and 0.94Ga. When combined with recent data from collisional belts further east of the CITZ, namely the Chhotanagpur Gneissic Complex and Shillong Plateau Gneissic Complex, a uniformity of Mesoproterozoic to Early Neoproterozoic events is evident, implying final amalgamation of the North and South Indian blocks at this time along a ~1500km long orogenic belt. This means that two or more separate blocks were in existence at the time of incorporation within Rodinia, not a single block as suggested in most current models.

The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context: implications for the amalgamation of the Gondwana supercontinent

We examined the petrological characteristics of the Graphite group and Androyan group in Southern Madagascar, south of the prominent Ranotsara shear zone, and we performed U-Pb SHRIMP dating on zircon and U-Th-total Pb dating on monazite. Widespread high-temperature metamorphism is evidenced by Spl–Qtz assemblages occurring over ca . 75,000 km 2 in the whole Androyan group. The occurrence of symplectites consisting of Crd + Kfs + Qtz + Opx or Crd + Kfs + Q333tz + Bt, which are interpreted as pseudomorphs after osumilite, is restricted to a smaller area of about 250 km 2 . Furthermore, in some pelites Spr + Qtz + Sil or Opx + Sil + Qtz formed the peak-metamorphic assemblage, which broke down to Crd ± Spl. Orthopyroxene in metapelites is aluminous with Al 2 O 3 = 9–10 wt%. Peak-metamorphic conditions of T = 950–1000 °C and P = 8–11 kbar are followed by decompression at high temperatures, as shown by the formation of Crd + Opx 2 (Opx with 6–8 wt% Al 2 O 3 ) symplectites from Grt–Qtz–Opx 1 (8–9 wt% Al 2 O 3 ). The pressure decrease is furthermore constrained by Spr–Crd symplectites in SiO 2 -undersaturated metapelites, and extensive formation of late-stage cordierite in the whole Androyan group. During subsequent cooling, cordierite broke down to form And + Qtz + Carbonate/Chl. Throughout the Androyan group, ages of 560–530 Ma have been obtained from monazite (M 2 metamorphism). Samples which do not contain ultrahigh-temperature assemblages provide evidence for an earlier metamorphic event at 650–600 Ma (M 1 ) in monazite cores. Zircon generally shows both metamorphic ages. Therefore, the deduced clockwise P-T evolution of the UH T metamorphism is interpreted to correspond to the M 2 stage, which affected the whole Androyan group. P-T conditions of the older M 1 metamorphism are generally unrecognisable. High temperature metamorphic conditions during M 2 are likely caused by intense charnockite emplacement. The near-isothermal decompression points to subsequent rapid exhumation of the formerly overthickened crust during the M 2 metamorphism at 560–530 Ma. We interpret this metamorphic stage to reflect the assembly of the Gondwana supercontinent, most likely related to the collision of the Tanzania Craton with the Azania microcontinent subsequent to closure of the Mozambique Ocean.

Late Cambrian Reworking of Paleo-Mesoproterozoic Granulites in Shillong-Meghalaya Gneissic Complex (Northeast India): Evidence from PT Pseudosection Analysis and Monazite Chronology and Implications for East Gondwana Assembly

Pressure-temperature (PT) pseudosection analysis of garnet-cordierite-sillimanite metapelites indicates that the granulites in the central part of the Shillong-Meghalaya Gneissic Complex (SMGC) in Northeast India evolved along a clockwise PT path with near isobaric heating (prograde) between 635° and 730°C at ∼5.7–5.5 kb, peak PT conditions of approximately 730°C/5.5 kb, and cooling between 650° and 595°C at 3.4–3.2 kb. Chemical dating indicates that an overwhelming majority of monazite in the metapelites and granites intrusive into the metapelites formed at Ma . The retrograde PT segment of the Late Cambrian metamorphic PT path in the SMGC is similar to those estimated in the Prydz Bay area of East Antarctica. It is suggested that the Late Cambrian metamorphism, felsic magmatism, and deformation in central SMGC may be correlated with the Pan-African collision between India and Australia-Antarctica during the assembly of East Gondwana, consistent with paleogeographic reconstructions based on paleomagnetic data. By contrast, an overwhelming majority of chemical dates from monazite cores in western SMGC (Garo-Goalpara) metapelites are Paleo-Mesoproterozoic ( Ma, ), with younger rims of some matrix monazite grains dated at 1141–946, , and 649–524 Ma. PT pseudosection analysis of the Garo-Goalpara metapelites indicates a counterclockwise path of metamorphism that probably occurred at ∼1.6 Ga, with the end phase of retrogression possibly in the Neoproterozoic/Early Paleozoic. The final PT conditions at Garo-Goalpara (∼5.8 kb/630°C) are similar to the prograde conditions of Late Cambrian metamorphism in central SMGC. The 820-Ma dates from western SMGC correspond to high-grade metamorphism anatexis in the 876–784-Ma N-trending Eastern Indian Tectonic Zone at the eastern margins of Eastern Indian Precambrian gneissic complexes located further south in a reconstructed pre-Cretaceous configuration of the Indian shield.

Two episodes of the Indosinian thermal event on the South China Block: Constraints from LA-ICPMS U–Pb zircon and electron microprobe monazite ages of the Darongshan S-type granitic suite

The Darongshan granitic suite (~ 10,000 km 2) consists of five major units (Taima, Nadong and Jiuzhou plutons, and Pubei and Darongshan batholiths) typical of peraluminous S-type granitoids containing abundant granulite inclusions in the Cathaysia block, South China. Six samples from these plutons and batholiths have been investigated using both LA-ICPMS U–Pb age dating on zircon cores and EMP U–Th–Pb chemical age dating on monazite cores and rims. LA-ICPMS zircon results give similar major age populations ranging between 260 ± 3 and 250 ± 3 Ma for all units, with apparent older age peaks concentrated at 1020, 800, 430 and 330 Ma. On the other hand, EMP monazite results yield younger ages of 231–229 Ma for Nadong, Taima, Pubei and Darongshan and 224 Ma for Jiuzhou samples, with older age groups of 264 Ma for Taima and 256–250 Ma for Pubei units. Since the older monazite ages are similar to the majority of zircon ages, the latter are considered as inherited ages. Further because such zircon ages are similar with the emplacement time of the Emeishan large igneous province in western South China, they likely reflect the timing of metamorphism for the included fragments of granulitic crusts that had been formed by invasion of the Emeishan plume. The younger monazite ages, as present for all plutons and batholiths in the entire Darongshan area, are taken as the formation age of the host granites. Combining U–Pb zircon and EMP monazite ages known for Permo-Triassic high temperature and high pressure metamorphic rocks and granites in the Indochina block (e.g., the Kannack Complex of the Kontum massif), it is suggested that the Indosinian thermal activity had set records over both the Indochina (plus Simao) and South China blocks in two main episodes, one is 260–250 Ma and the other is 231–229 Ma. One plausible explanation is that these two blocks were one united continent before the Emeishan plume activity and an opening was triggered by this plume at ~ 260 Ma. Due to forces of the approaching Sibumasu block, both the South China and Indochina blocks were amalgamated again at ~ 230 Ma. We, therefore, advocate that double subduction of the plume-triggered oceanic crusts in opposite directions is responsible for the generation of the Darongshan granitic suite in the South China block and its counterpart in the Indochina block.

Fluid-mineral interactions and constraints on monazite alteration during metamorphism

AbstractClasts of metamorphosed Cadomian granites from the ∼50—60 Ma Carpathian flysch in Gródek near the Rożnowskie Lake (Silesian Unit, SE Poland) are studied. They are considered to represent the Silesian Ridge, one of the hypothetical, currently unexposed source areas that supplied Carpathian sedimentary basins with clastic material. The gneisses preserve several examples of corona textures that include cores of primary monazite surrounded by polygonal grains of secondary apatite with thorianite inclusions, with intermediate zones of lamellar grains of secondary monazite and outermost rims of clay minerals, or various combinations thereof. Preservation of the complete textures is rare with polygonal apatite with thorianite inclusions, lamellar grains of monazite and clay minerals being particularly prevalent. Locally, polygonal apatite with thorianite inclusions surrounded by allanite and REE-epidote corona with a bastnasite-synchysite phase occurs also. The textures observed developed during primary monazite breakdown and replacement by secondary minerals. The variation in reaction products indicates that alteration was strictly dependent on the local chemical system.

Reactions destroying detrital monazite in greenschist-facies sandstones from the Witwatersrand basin, South Africa

Detrital monazite in sandstone and conglomerate from the Witwatersrand Supergroup, South Africa, displays reaction textures indicating that monazite has undergone replacement after sediment deposition, and most probably during regional greenschist-facies metamorphism. In sandstones with moderate- to high-Ca bulk compositions, detrital allanite is preserved in heavy mineral bands, whereas co-existing detrital monazite is replaced by secondary allanite, apatite and ThSiO 4. In sandstones with low-Ca bulk compositions, detrital monazite is replaced by florencite, apatite and ThSiO 4 or pseudomorphed by metamorphic monazite, apatite and Th-silicate. The monazite alteration phases are commonly intergrown with metamorphic quartz, chlorite and sericite. Large (~ 0.5 mm) grains of metamorphic monazite are present in the same samples as detrital monazite pseudomorphs. This suggests that REE are mobile on a scale of millimetres to centimetres in these rocks, and that detrital grains are a likely source of components for metamorphic monazite. In other low-Ca sandstones, monazite forms composite grains that comprise a core surrounded by a pore-filling, inclusion-rich rim. The monazite cores have pronounced negative Eu anomalies and higher concentrations of U, Th, Ca and Y than the rims. Previous SHRIMP U–Pb dating of the cores gives dates of ~ 3.0 Ga and ~ 2.85 Ga, indicating a detrital origin. The monazite rims lack Eu anomalies and generally have lower Th contents than the cores. They result from dissolution and reprecipitation of monazite, and the ages of the rims suggest that replacement occurred at 2.045 Ga in the West Rand goldfield and at 2.12 Ga in the Welkom goldfield. Our results indicate that detrital igneous monazite is unstable under mid-greenschist facies conditions (~ 350 °C) and may be replaced by low-Th metamorphic monazite, allanite, apatite, florencite and ThSiO 4, as well as matrix minerals such as quartz, chlorite and sericite.

Application of electron probe microanalysis Th–U–total Pb geochronology to provenance studies of sedimentary rocks: An example from the Carpathian flysch

Two gneiss and one granulite cobble from the ca. 60 Ma Silesian Unit flysch rocks (Western Outer Carpathians, southern Poland) have been studied. These cobbles are considered to represent rocks derived from the Silesian Ridge, one of the internal source areas that supplied the Carpathian sedimentary basins with clastic material. This ridge developed at the boundary between Variscan and Cadomian crustal fragments of the European platform, and formed an elongated thrust belt rapidly uplifted and eroded during Late Cretaceous to Paleocene times. The pebble to cobble size clastic material present in the Carpathian flysch rocks provides the only opportunity to improve knowledge concerning the evolution of the Silesian Ridge. Electron probe microanalysis Th–U–total Pb monazite chronology has been used to constrain the timing of metamorphism and magmatic protolith of gneiss cobbles derived from the Silesian Ridge. Based on microtextural observations and chronological results four monazite populations were identified. The oldest age of ca. 592 Ma was obtained from the core and rim of grains with igneous concentric growth zonation in augen gneiss from Gródek. Monazite dates from gneiss from Izdebnik define two age populations of ca. 368 Ma and ca. 333 Ma. Similar results were obtained from a zoned monazite inclusion in garnet from granulite. Monazite cores, which pre-date garnet growth give an age of ca. 372 Ma, whereas the rims are ca. 336 Ma. The rims have lower Y content and reflect monazite growth after the onset of garnet growth. Results obtained in this study using improved electron microprobe analyses techniques are consistent with existing data, but much more tightly constrained. The increased precision is related to consideration of the complex compositional zonation and textural setting of monazite, and also to treating each compositional domain as a unique chrono-chemical system investigated in isolation by multi-point analyses. This study provides new geochronologic data for the Carpathian source terrain, and highlights the potential role for electron probe microanalysis of monazite to provenance studies.

Electron microprobe dating of monazites from an ultrahigh-temperature granulite in Southern India: Implications for the timing of Gondwana assembly

We report here monazite geochronology on a newly discovered high pressure (HP) and ultrahigh-temperature (UHT) granulite locality at Thoppur within the Palghat-Cauvery Shear Zone system in southern India. In situ dating of monazites associated with the UHT minerals using electron probe technique yielded PbO vs. ThO2* apparent isochron age of 544 ± 5 Ma. This is the first study from southern India where older monazite cores or complex age patterns are virtually absent, and all the monazite population in the studied samples crystallized/recrystallized during a single thermal maximum which we correlate with the timing of extreme crustal metamorphism during the final assembly of the Gondwana supercontinent.

Monazite begets monazite: evidence for dissolution of detrital monazite and reprecipitation of syntectonic monazite during low-grade regional metamorphism

Back-scattered electron (BSE) imaging and X-ray element mapping of monazite in low-grade metasedimentary rocks from the Paleoproterozoic Stirling Range Formation, southwestern Australia, reveal the presence of distinct, high-Th cores surrounded by low-Th, inclusion-rich rims. Previous geochronology has shown that the monazite cores are older than 1.9 Ga and overlap with the ages of detrital zircon grains (∼3.5–2.0 Ga), consistent with a detrital origin. Many cores have scalloped and embayed surfaces indicating partial dissolution of former detrital grains. Textural evidence links the growth of the monazite rims (∼1.2 Ga) to deformation and regional metamorphism during the Mesoproterozoic Albany-Fraser orogeny. These results indicate that high-Th detrital monazite is unstable under low-grade metamorphic conditions (<400°C) and was partially or completely dissolved. Dissolution was followed by near-instantaneous reprecipitation and the formation of low-Th monazite and ThSiO4. This reaction is likely to operate in other low-grade metasedimentary rocks, resulting in the progressive replacement of detrital monazite by metamorphic monazite during regional prograde metamorphism.

EMP chemical age dating of monazites from a complex terrain: The Paleo-Proterozoic of broken hill, Australia

A regional program of EMP age-dating has been undertaken on samples, chiefly from meta-sediments, covering all groups in the stratigraphy [1], main metamorphic assemblages and HT and LT shears (ca. 80 specimens, ca. 3000 spot ages). A cumulative plot of the spot (and isochron) ages reveals two concentrations: 1) a broad peak ca. 1720 to 1600 Ma and 2) twin peaks at ca. 520 & 480 Ma. All are independent of the position of the specimen in the stratigraphic sequence. The high grade metamorphic rocks and mylonites in 1) are characterized by any or all of the And, fibrolite (Fib) and Sil minerals. Texturally, on a regional basis, And is overprinted by Fib which is overprinted by at least two generations of prismatic Sil. The age spread 1) is thought to reflect the above textural complexity. Fib is often included in monazites within this age-peak, including those with ages of circa 1720 Ma. Monazite cores with ages in the older part of 1) are often rimmed by overgrowths in the younger part which concentrate in mylonites. A protracted thermo-tectonic event is indicated. Samples with monazites from the 520 & 480 Ma peaks have St + Ky ± Cld assemblages and are found mainly, but not exclusively, in retrograde shear zones. These monazites may be new grains or rim the older ones. All monazites corrode/alter when positioned in Bt-Chl-Ms-Ab assemblages. Monazites included in Qtz, Fsp or Grt may retain older ages, some older than 1720 Ma and which may be due to older metamorphic rocks. The spread of ages from 1720-1600 Ma is basically similar to that seen in the zircon-geochronology from the area [2]. This spread supports a thermo-tectonic origin [3] rather than one also involving deposition [4].

Two‐stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating

AbstractNew results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ∼900–950 °C and pressures of ∼9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ∼970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of &gt;25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.

Geochronology of the khondalite belt of Trivandrum Block, Southern India: Electron probe ages and implications for Gondwana tectonics

In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma. The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities. The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.