Electron Microprobe Dating Of Monazite Research Articles

Ordovician (Cenerian) metamorphism in the Western Carpathians: Evidence from EMP monazite dating of polymetamorphosed granitoids in the Veporic unit, Slovakia

Late Ediacaran to Cambrian metagranitoids from the northern Veporic unit of the Western Carpathians show imprints of three metamorphic events, which can be assigned to the Cenerian, Variscan, and Alpine orogenies based on electron microprobe dating of monazite. Metamorphic monazites in the metagranitoids are mostly of the Lower to Middle Ordovician age (480–460 Ma). The Ordovician monazite formed in equilibrium with the metamorphic assemblage garnet, biotite, kyanite, ilmenite and quartz at P-T conditions of 6–7 kbar and 550–570 °C, thus providing clear evidence for Cenerian metamorphism in the Western Carpathians. Variscan metamorphism caused minor monazite growth/recrystallization at 364 ± 13 Ma and produced a new mineral assemblage garnet, kyanite, rutile and phengite at P-T conditions of 20–22 kbar and 670–690 °C. The low-grade Alpine metamorphism is recorded by monazite of Cretaceous age (96 ± 23 Ma), but only in orthogneiss of extremely low-Ca composition.The Veporic metagranitoids show incomplete transformation from magmatic stage, still preserving remnants of plagioclase, K-feldspar and high-Ti biotite. This indicates that the metagranitoids remained relatively dry and failed to complete metamorphic reactions due to kinetic factors at fluid-deficient conditions. Nevertheless, the metagranitoids likely underwent significant geochemical changes during their metamorphic evolution including a severe loss of CaO and Na2O.

Electron microprobe dating of monazites from rhyolites of the Veľká Stožka Massif (Muráň nappe, Western Carpathians) – implications for the Permian volcanic evolution in Internal Western Carpathians

New results of field research, as well as petrographical and geochronological studies are presented in the form of a geological map, tectonostratigraphic scheme and new magmatic age data of monazite crystallization from the rhyolite body located on SW slopes of the Veľká Stožka Massif, Dudlavka Valley (Muráň Plateau). The rhyolite bodies in the Veľká Stožka area are tectonically incorporated into the Lower Triassic formations at the base of the Muráň nappe. The electron microprobe dating of monazites yields magmatic / volcanic age 281 ± 4.5 Ma (Artinskian – Kungurian), which complements geochronological data from the same rhyolite body below the Veľká Stožka Massif, as well as other rhyolite bodies from Telgárt, Poniky and Tisovec, which, originally erupted in a Permian volcanically active fault zone (VAFZ) geodynamical setting. The comparison of geochronological data 281 ± 4.5 Ma and 269.5 ± 1.8 Ma from the studied locality suggest a geodynamic evolution, where not only one, but several separate volcanic eruption phases acted in the original domain of rhyolitic volcanism. The geochronological correlation of the rhyolite bodies in the Upper Hron Valley, active in time span 281–263 Ma, allowed to identify a volcanic hiatus between 280–271 Ma. Such eruptional pause correlates very well with other volcanic provinces such as Harnobis volcanogenic horizon of the North Veporic realm, the Petrova hora and Novoveská Huta Fm. of the Northern Gemericum, which erupted during hiatus of VAFZ and their activities have ended in during recovering rhyolite eruptions in VAFZ. Reactivation of rhyolitic volcanic activity in VAFZ 271–263 Ma is a result of paleostress reorganisation in Pangea lithosphere. The revealed paleovolcanic relationships highlight interconnection of paleotectonic and paleovolcanic events in the known paleotectonic units of Internal Western Carpathians during Permian.

The Pocologan metamorphic suite of southern New Brunswick, Canada: New constraints on age and conditions of medium- to high-pressure metamorphism on the Ganderian margin of the Rheic Ocean

Geologically diverse fault-bounded crustal slices form the present-day southern margin of Ganderia in southern New Brunswick. One such slice, the Pocologan metamorphic suite (PMS), was previously shown to contain high-pressure metamorphic rocks and interpreted as part of the forearc of the adjacent Silurian Kingston arc terrane. In this study, the protolith age of metasedimentary rocks in the PMS has been constrained by U-Pb dating of detrital zircon to less than about 470 Ma. The age spectrum is dominated by 500 to 600 Ma ages and sparse older ages, typical of other Lower Paleozoic sedimentary components of Ganderia. An orthogneiss sample yielded a U-Pb concordia age of 433.5 ± 0.7 (2σ) Ma, interpreted as the igneous crystallization age and further supporting a link to the Kingston arc. Electron-microprobe dating of monazite from mica-schist samples yielded a mean age of 388.3 ± 2.8 (2σ) Ma, an older population at about 417 Ma, and older detrital grains at 450 Ma. Pseudosection modeling and quartz-in-garnet barometry showed that garnet cores in these mica-schist samples formed at 7 kbar and 550 °C; similar modeling combined with phengite geobarometry showed a second event at similar pressure but higher temperature. This event was followed by pressure decrease to 5 kbar and slight cooling, when biotite and staurolite porphyroblasts formed. Mylonitization started at the peak of the second metamorphic event and ended during early exhumation. The P-T and monazite age data suggest that the PMS was metamorphosed in the Early Devonian by burial to middle-lower crustal levels during the collision of Avalonia with Ganderia and metamorphosed again about 30 million years later, also at middle-lower crustal levels, during dominantly transcurrent tectonic activity. During this time different crustal slices were juxtaposed to form the PMS and probably the entire fault-bounded Ganderian and Avalonian terrane assembly in southern New Brunswick.

Electron-microprobe dating of monazite: The story

This paper is a testimony of how electron microprobe dating of monazite was developed in the early days of this technique, in the 90's. Five contributions have been collected from Nagoya, Clermont-Ferrand, Orléans, Salzburg, and Amherst. The technique was designed in Nagoya by Professor Suzuki, using an old JXA-5A JEOL electron microprobe. The first results were published in 1991, with great difficulty. Professor Suzuki also created the isochron-like Pb-Th* diagram to present the data. Progressive improvement of the method finally allowed the Nagoya team to date Cenozoic monazite. The technique was re-discovered in Clermont-Ferrand using a Cameca Camebax microprobe. This team demonstrated the accuracy of the method, and proposed statistical tools to improve the precision and to handle multimodal populations. The first results were published in 1993. In Orléans, the French Geological Survey (BRGM) developed the method with a Cameca SX50 to support mapping projects. They developed a new method for calculating the age, and showed that this technique was very efficient for dating monazite in migmatites. In Salzburg, the first results were obtained in 1996, using a Jeol JX 8600. It proved to be very efficient in the Alps to distinguish Cadomian, Ordovician, Variscan and Alpine events. It allowed this team to directly date a monazite-allanite reaction they described for the first time. Finally, the Amherst team investigated in detail the technical issues of electron microprobe monazite dating. They showed that the best analytical procedures must include at least: detailed chemical mapping, several interference corrections and special care for background measurements. With the help of Cameca they designed a specific microprobe for dating, named “Ultrachron”. With this machine, high precision ages can be obtained on well characterized individual grains in thin section, giving birth to a new era in geochronology of complex metamorphic rocks.

Thermal history of a Late Mesoproterozoic paired metamorphic belt (?) during Rodinia assembly: New insight from medium-pressure granulites from the Aravalli-Delhi Mobile Belt, Northwestern India

In this study, we investigate the possible record of a Late Mesoproterozoic paired metamorphic belt in the Aravalli-Delhi Mobile Belt (ADMB), NW India using a suite of supracrustal and metaigneous granulites from the Pilwa-Chinwali granulite terrain at the north-western margin of the ADMB. Using metamorphic reaction textures, mineral chemistry, metamorphic reaction history, geothermobarometric computations and electron microprobe dating of monazite in 5 samples of pelitic granulite, leptynite gneiss, enderbite and charnockite, we have deduced a medium-pressure granulite facies metamorphism (P between 4.9 and 6.8 kbar, T > 760–815 °C) along a heating-cooling, counterclockwise P-T path between 1.09 and 1.01 Ga. When collated with published metamorphic and chronological constraints and geological settings of the adjoining crustal domains of the ADMB, these findings provide new insights into the developments of two tectonic domains of contrasting thermal gradients at ca. 1.0 Ga, consistent with metamorphic transformations in tectonically thickened middle-lower crustal sections during continental collision to continental subduction and in the root zones of spatially adjacent island arc, as part of the Rodinia supercontinent assembly event.

Grenvillian age high-pressure upper amphibolite-granulite metamorphism in the Aravalli-Delhi Mobile Belt, Northwestern India: New evidence from monazite chemical age and its implication

In this study, we deduce the age of the collisional orogeny in the Aravalli-Delhi Mobile Belt, using electron microprobe (EMP) dating of monazites of two suites of metapelitic to metapsammitic rocks from the Banded Gneissic Complex of Central Rajasthan. These metasediments of the Sandmata and the Mangalwar Complexes record a high-pressure, upper amphibolite ( P ∼ 10.3 ± 0.8 kbar, T ∼ 675 ± 60 °C) to granulite facies ( P ∼ 12–14 kbar, T ∼ 815 °C) peak metamorphism (M 2P stage) and a post-peak decompression to ∼5 kbar, 575–660 °C (M 2R stage), following a clockwise P– T path (M 2 event). The metapelites of the Sandmata Complex, however, record an older, medium-pressure, sillimanite-facies granulite metamorphism (M 1 event). Electron microprobe dating of monazites, occurring in the different textural settings of three metapelite granulite samples of the Sandmata Complex yields three mean spot age populations at ∼1700 ± 3 to ∼1733 ± 15 Ma, ∼1597 ± 6 to ∼1629 ± 23 Ma and ∼882 ± 15 Ma. While Late Palaeoproterozoic age populations are correlated with the timing of the M 1 metamorphism, the Early Neoproterozoic ages from garnet-breakdown textural domain constrain the timing of the overprinting M 2R event. Interpretation of the Late Mesoproterozoic age component is problematic. It may reflect either a mixed age due to partial Pb loss of the Palaeoproterozoic monazites by a younger high-grade event or may define the timing of a high-grade tectonothermal event, the imprints of which have been registered in the Proterozoic mobile belts of central, eastern, north-eastern and south-eastern India. Monazites from the metapelites of the Mangalwar Complex yield characteristic Late Mesoproterozoic to Neoproterozoic ages with clusters at 946 ± 6, 909 ± 9 and 865 ± 10 Ma. The metagranites intrusive into the M 1 granulites additionally register an emplacement age of ∼978 ± 18 Ma. The age span of ∼0.98–0.95 to ∼0.88–087 Ga is taken as the age of the M 2 event. Based on these geochronologic and metamorphic findings, we assign a Grenvillian age for the collisional orogeny in the Aravalli-Delhi Mobile Belt. The extent of propagation of the Grenvillian orogenic front in the Aravalli-Delhi orogen and its implication in Rodinia reconstruction are also discussed.

Age constraints on Late Paleozoic evolution of continental crust from electron microprobe dating of monazite in the Peloritani Mountains (southern Italy): another example of resetting of monazite ages in high-grade rocks

In situ monazite microprobe dating has been performed, for the first time, on trondhjemite and amphibolite facies metasediments from the Peloritani Mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the Hercynian Belt. All samples show single-stage monazite growth of Hercynian age. One migmatite and one biotitic paragneiss yielded monazite ages of 311 ± 4 and 298 ± 6 Ma, respectively. These ages fit with previous age determinations in similar rocks from southern Calabria, indicating a thermal metamorphic peak at about 300 Ma, at the same time as widespread granitoid magmatism. The older of the two ages might represent a slightly earlier event, possibly associated with the emplacement of an adjacent trondhjemite pluton, previously dated by SHRIMP at 314 Ma. No evidence for pre-Hercynian events and only a little indication for some monazite crystallization starting from ca. 360 Ma were obtained from monazite dating of the metasediments, suggesting either a single-stage metamorphic evolution or a significant resetting of the monazite isotope system during the main Hercynian event (ca. 300 Ma). Rare monazite from a trondhjemite sample yields evidence for a late-Hercynian age of about 275 Ma. This age is interpreted as representing a post-magmatic stage of metasomatic monazite crystallization, which significantly postdates the emplacement of the original magmatic body.

The metamorphic evolution of migmatites from the Ötztal Complex (Tyrol, Austria) and constraints on the timing of the pre-Variscan high-T event in the Eastern Alps

Within the Otztal Complex (OC), migmatites are the only geological evidence of the pre-Variscan metamorphic evolution, which led to the occurrence of partial anatexis in different areas of the complex. We investigated migmatites from three localities in the OC, the Winnebach migmatite in the central part and the Verpeil- and Nauderer Gaisloch migmatite in the western part. We determined metamorphic stages using textural relations and electron microprobe analyses. Furthermore, chemical microprobe ages of monazites were obtained in order to associate the inferred stages of mineral growth to metamorphic events. All three migmatites show evidence for a polymetamorphic evolution (pre-Variscan, Variscan) and only the Winnebach migmatite shows evidence for a P-accentuated Eo-Alpine metamorphic overprint in the central OC. The P-T data range from 670–750 °C and < 2.8 kbar for the pre-Variscan event, 550–650 °C and 4–7 kbar for the Variscan event and 430–490 °C and ca. 8.5 kbar for the P-accentuated Eo-Alpine metamorphic overprint. U-Th-Pb electron microprobe dating of monazites from the leucosomes from all three migmatites provides an average age of 441 ± 18 Ma, thus indicating a pervasive Ordovician-Silurian metamorphic event in the OC.

Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt, Bahia, Brazil

The Salvador–Curaçá Belt, located in São Francisco Craton, Brazil, was subjected to granulite facies metamorphism during the Paleoproterozoic orogeny ( c. 2.0 Ga). Well preserved in enclaves of silica-undersaturated sapphirine-bearing granulite occur in a charnockite outcrop located along a kilometric-scale shear zone. The sapphirine-bearing granulite preserves domains with distinct mineral assemblages that record interactions between melt and peritectic phases (orthopyroxene 1 + spinel 1 + biotite 1). Sapphirine was crystallized in the Si-poor cores of the enclaves, sillimanite and spinel–cordierite symplectites in the intermediate Si-rich domains between cores and margins, and garnet and quartz-bearing cordierite/biotite symplectites in Si-rich margins of the enclaves. Melt-rock interactions and metamorphism occurred at ultrahigh temperatures of 900–950 °C at 7.0–8.0 kbar pressures. The mineralogical evolution of the domains reflects not only the influence of changes in bulk composition in the equilibrium volume of the reactions but also P– T changes during orogeny evolution. Electron microprobe dating of monazite both in the sapphirine-bearing granulite and charnockite indicates UHT metamorphism timing at c. 2.08–2.05 Ga that is related to global Paleoproterozoic UHT metamorphic events that occurred during the Columbia supercontinent assembly.

Electron microprobe monazite geochronological constraints on the Late Palaeozoic tectonothermal evolution in the Chinese Tianshan

Electron microprobe monazite data from the early Palaeozoic granitoid gneisses in the Central Tianshan Tectonic Zone provide two main ages: (1) an age of 355 ± 12 Ma obtained on the chemically complex grains, which is good agreement with the high-pressure metamorphic age obtained on eclogite and blueschist rocks from the south Tianshan Tectonic Zone; (2) an age of 323 ± 7 Ma obtained on the complex and the monogenetic grains, which is also closely consistent with the age for a major episode of cooling–recrystallization of these high-pressure rocks. Monazite from the schist of the Xingxingxia complex yields a younger Th/Pb–U/Pb centroid age of 282 ± 9 Ma. This younger age is interpreted as being linked to a magmatic episode, possibly related to underplating–extension, and/or shear activities. These data provide timing constraints on the episodes of metamorphism and high-pressure metamorphism and evidence for transition from compression to extension during the Carboniferous to early Permian, for the Chinese Tianshan region. Moreover, this study shows that electron microprobe dating of monazite, together with recent statistical treatment, can provide a reliable method to reveal a sequence of events within an orogenic cycle.

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.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Aureoles of Pb(II)-enriched feldspar around monazite in paragneiss and anatectic pods of the Napier Complex, Enderby Land, East Antarctica: the roles of dissolution-reprecipitation and diffusion

Extraordinarily high Pb content in K-feldspar and plagioclase has been found contiguous to monazite in two occurrences in the ultrahigh-temperature Napier Complex of Antarctica. Monazite shows a variety of textures and compositions. In a garnet-sillimanite-orthopyroxene paragneiss at Mount Pardoe (Amundsen Bay), grains range 80–150 μm across and are anhedral; two grains are Th- and Si-dominant. In pods that crystallized from anatectic melts at 2500 Ma at Zircon Point, Casey Bay, monazite grains range 0.05 mm–1 cm in length and are highly variable in texture. The coarsest grains (>0.7 cm) are skeletal and euhedral, whereas the smallest grains are anhedral and associated with fine- to medium-grained quartz, K-feldspar, plagioclase, garnet, sillimanite and rutile in aggregates that form interstitial veinlets interpreted to be a second generation of anatexis during an event at 1100 Ma. The huttonite component (ThSiO4) reaches 30 mole% in the cores of the coarsest skeletal grains, whereas other grains, particularly smaller ones, show complex and irregular zoning in Th and U. The latter zoning is attributed to dissolution-reprecipitation, which also resulted in complete Pb loss during the 1100 Ma event. In the paragneiss at Mount Pardoe, K-feldspar and myrmekitic plagioclase (An16) are found in a 70–80 μm band between monazite and orthopyroxene and contain up to 12.7 wt.% and 2.7 wt.% PbO, respectively, corresponding to 18.5% and 3.4% PbAl2Si2O8 component, respectively. Cathodoluminescence of both feldspars increases with distance from a nearby monazite grain and is not correlated with Pb content. Incorporation of Pb in K-feldspar and plagioclase could be a result of diffusion, even though the monazite adjacent to feldspar apparently lost little Pb, i.e., Pb could have been transported by fluid from the Th-rich grains, which did lose Pb. In contrast to the paragneiss, cathodoluminescence correlates with Pb content of K-feldspar in aureoles surrounding skeletal monazite grains 0.7–1 cm across in anatectic pods at Zircon Point. Pb content of K-feldspar decreases monotonically to near detection limits within several millimetres of monazite grains; the greatest PbO concentration is attained in K-feldspar inliers and embayments in monazite, 8.8 wt.%, corresponding to 11.7% PbAl2Si2O8 component. Fine-grained quartz in the K-feldspar suggests that the mechanism for Pb incorporation involved breakdown of feldspar: Pb2+ + 2(K,Na)AlSi3O8 → PbAl2Si2O8 + 4SiO2 + 2(K,Na)+ . The smooth decrease of Pb in the aureoles is not characteristic of dissolution-reprecipitation, which is characterized by abrupt changes of composition, and it seems more likely that Pb was incorporated in K-feldspar by diffusion at 1100 Ma. We suggest a model whereby fluid introduced during the 1100 Ma event flowed along grain boundaries and penetrated mineral grains. Temperatures were sufficiently high, i.e., 700°C, assuming burial in the mid-crust, for the fluid to induce localized melting of quartzofeldspathic matrix of the anatectic pods. Loss of radiogenic Pb was complete. Some penetration of K-feldspar by aqueous fluid is suggested by the presence of scattered galena specks and by rays of turbidity emanating from monazite. Aqueous fluid or water-rich granitic melt may have mediated the diffusion of Pb in feldspar, but it did not cause dissolution-reprecipitation. Although Pb was mobilized by aqueous fluid or water-rich granitic melt, it was not entirely flushed from the immediate vicinity of the monazite, but nearly half was incorporated in adjacent feldspar. Fluid activity that could cause Pb loss in monazite does not always leave an obvious trace, i.e., hydrous minerals, such as sericite, are very sparse, and biotite is absent in the anatectic pods at Zircon Point. Nonetheless, electron microprobe dating of monazite from the pods could not detect the 2500 Ma age of original crystallization determined by isotopic dating.

Discovery of sapphirine-bearing Mg–Al granulites in the North China Craton: Implications for Paleoproterozoic ultrahigh temperature metamorphism

We report here for the first time the occurrence of Mg–Al granulites within the khondalite belt of the North China Craton and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT indicator assemblages in these rocks include equilibrium sapphirine + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature perthites. The surrounding khondalites also carry spinel + quartz equilibrium assemblage. The sapphirines occurring in various microstructural associations are markedly magnesian ( X Mg = 0.69–0.77) with a composition varying from 7:9:3 to 2:2:1 end members. Spinel is a solid solution of Mg-spinel and hercynite ( X Mg = 0.41–0.54) with ZnO content varying from 0.48 to 1.79 wt.%. Garnet shows a compositional range between pyrope and almandine ( X Mg = 0.39–0.48) with minor grossular (2.6–3.8 mol%) and spessartine (0.5–1.8 mol%). The orthopyroxenes display marked compositional variations in terms of Al 2O 3 content, with the core of the medium-grained orthopyroxene adjacent to sillimanite in the matrix showing the highest Al 2O 3 content (8.4 wt.%). The history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine, spinel, quartz and sillimanite within garnet. Sapphirine formed during the peak stage through the reaction: Spl + Qtz + Sil → Spr. Occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction: Spr + Qtz + Grt → Opx + Sil. A series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse-grained minerals such as: Opx + Sil + Qtz → Crd, Grt + Sil + Qtz → Crd, Grt + Qtz → Crd + Opx, Grt + O 2 → Crd + Qtz + Fe 2O 3 in Mag or Ilm, and finally Opx + Kfs + H 2O → Bt + Qtz. Whereas the stability of sapphirine + quartz and orthopyroxene + sillimanite + quartz in the matrix assemblage suggests T > 1000 °C and P > 10 kbar peak metamorphism, conventional geothermobarometric estimates confirm the ultrahigh-temperature nature and also trace the retrograde conditions (930–970 °C at P > 10 kbar — Al in Opx; 900–1000 °C — perthite; 930–990 °C — Grt–Opx; and 910–940 °C — sapphirine–spinel). Electron microprobe dating of monazites in textural association with the UHT assemblages yield a sharply defined isochron age of 1917 ± 48 Ma, which correlates with the 1927 ± 11 Ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel + quartz assemblage. An evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and we interpret these trajectories to be part of an overall anti-clockwise P– T evolution. The study area, located within the northern margin of the North China Craton, defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen. Evidence for the extensional phase is provided by the several mafic magmatic intrusives of Paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle-derived magmas. The UHT metamorphism in North China Craton coincided with the assembly of the Paleoproterozoic supercontinent Columbia.

U-Th-total Pb dating of monazite from orthogneisses and their ultra-high temperature metapelitic enclaves: implications for the multistage tectonic evolution of the Madurai Block, southern India

Electron microprobe dating of monazite was performed on enclaves of sapphirine-bearing granulites and their host granites from the southeastern part of the Madurai Block in India. The granulites are strongly migmatitic and comprise quartzofeldspathic leucosome veins and restitic domains of sapphirine, garnet, orthopyroxene, sillimanite and cordierite. Inclusions of sapphirine-quartz intergrowths in garnet testify to an UHT stage of metamorphism which was followed by a stage of decompression during which garnet was replaced by coronitic opx-sill intergrowths. Monazite inclusions in garnet, orthopyroxene and quartz often yield a bimodal age distribution of Mid- to early Neoproterozoic (1500-850 Ma) ages in the cores which are rimmed by late Neoproterozoic age domains (630-580 Ma). Monazites in the granite and the charnockite exclusively yield Pan-African values. The occurrence of sapphirine-quartz intergrowths and the early Neoproterozoic age population in the same petrographical position infers that the crystalline basement of the Madurai Block already experienced a stage of UHT metamorphism prior to the Pan-African orogenic event.

Electron microprobe dating of monazite from high- T shear zones in the São José de Campestre Massif, NE Brazil

The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperature metamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provoked generalized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean to Paleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatite and leuconorite within the high- T shear zones that make up the eastern continuation of the huge E–W Patos shear belt. These data were also constrained by using the Sm–Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The results suggest an age of about 578 to 574 Ma for the peak of the widespread high- T metamorphism. This event is best recorded by Sm–Nd garnet-whole rock ages. The U–Th–Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553 ± 10 Ma. When compared to the Sm–Nd garnet-whole rock ages, the U–Th–Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peak of the high- T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, the U–Th–Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete re-opening and resetting of the U–Th–Pb isotopic system produced by the action of low- T Ca-rich fluid.

Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

The paragenesis of monazite in metapelitic rocks from the contact aureole of the Harney Peak Granite, Black Hills, South Dakota, was investigated using zoning patterns of monazite and garnet, electron microprobe dating of monazite, bulk-rock compositions, and major phase mineral equilibria. The area is characterized by low-pressure and high-temperature metamorphism with metamorphic zones ranging from garnet to sillimanite zones. Garnet porphyroblasts containing euhedral Y annuli are observed from the garnet to sillimanite zones. Although major phase mineral equilibria predict resorption of garnet at the staurolite isograd and regrowth at the andalusite isograd, textural and mass balance analyses suggest that the formation of the Y annuli is not related to the resorption-and-regrowth of garnet having formed instead during garnet growth in the garnet zone. Monazite grains in Black Hills pelites were divided into two generations on the basis of zoning patterns of Y and U: monazite 1 with low-Y and -U and monazite 2 with high-Y and -U. Monazite 1 occurs in the garnet zone and persists into the sillimanite zone as cores shielded by monazite 2 which starts to form in the andalusite zone. Pelites containing garnet porphyroblasts with Y annuli and monazite 1 with patchy Th zoning are more calcic than those with garnet with no Y annuli and monazite with concentric Th zoning. Monazite 1 is attributed to breakdown of allanite in the garnet zone, additionally giving rise to the Y annuli observed in garnet. Monazite 2 grows in the andalusite zone, probably at the expense of garnet and monazite 1 in the andalusite and sillimanite zones. The ages of the two different generations of monazite are within the precision of chemical dating of electron microprobe. The electron microprobe ages of all monazites from the Black Hills show a single ca. 1713 Ma population, close to the intrusion age of the Harney Peak Granite (1715 Ma). This study demonstrates that Y zoning in garnet and monazite are critical to the interpretation of monazite petrogenesis and therefore monazite ages.

Petrology and in situ U-Th-Pb Monazite Geochronology of Ultrahigh-Temperature Metamorphism from the Andriamena Mafic Unit, North-Central Madagascar. Significance of a Petrographical P-T Path in a Polymetamorphic Context

Petrological studies and electron microprobe dating of monazite from the mafic Andriamena unit, north–central Madagascar, indicate that an apparently continuous P–T path inferred for Mg-granulites is actually discontinuous, resulting from the superposition of two distinct metamorphic events at 2 5 Ga and 750Ma. The late Archaean event corresponds to an ultrahigh-temperature metamorphism (1000 C, 10 5 kbar) characterized by a sapphirine–garnet–orthopyroxene–quartz assemblage. Neoproterozoic ages are associated with the development of a sapphirine–cordierite-bearing assemblage, symplectites of orthopyroxene–sillimanite and partial melting at 850 C and 7 kbar. This sequence of reactions and mineral assemblages could be interpreted as the result of near-isothermal decompression to about 4 kbar followed by isobaric cooling to 650 C. However, geodynamic constraints suggest that the granulites underwent a phase of cooling to the stable geotherm following the ultrahightemperature metamorphism at 2 5 Ga. Consequently, we suggest that the ‘petrographical path’ inferred from the Mg-granulites is not representative of the actual P–T–t path. The decompression, in particular, is an artefact of the P–T path with no geological meaning; it results from the equilibration of the refractory late Archaean ultrahigh-temperature assemblages at a lower pressure during the middle Neoproterozoic event.

An Assessment of Precision and Accuracy of Electron Microprobe Dating of Monazite

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield

To obtain the chemical Th ∗–Pb isochron ages and surface maps of monazite crystals in igneous and metamorphic rocks from the southern Brazilian Shield, we employ Th–U-total Pb dating by an electron probe microanalyzer. The ages of two Trans-Amazonian metamorphic events are given by a felsic, garnet-bearing granulite from the Santa Maria Chico granulitic complex. The age of the first event, at approximately 2.35 Ga, was obtained by surface mapping in a grain included in garnet. The dating of the second event, 1899±43 Ma, is in agreement with previous data obtained in zircon crystals with sensitive high-resolution ion microprobe. Other determinations belong to the Brasiliano cycle. In the São Gabriel block, an age of 643±129 Ma was obtained on monazite from a staurolite-garnet schist of the Cambaizinho Formation, whereas a staurolite-bearing schist from the Passo Feio complex yielded a 510±68 Ma age. Several units in the Dom Feliciano belt were dated, including the biotite-sillimanite gneisses of the Várzea do Capivarita complex (552±90 Ma), the sillimanite-garnet gneisses of Camboriú complex (565±77 Ma), the Três Figueiras granite (558±57 Ma), and the Plaza Itapema granite (545±55 Ma). The ages presented in this study, obtained through monazite chemical dating, are confirmed through comparison with previous data regarding zircon crystals from the same geological units.