Monazite Ages Research Articles

Chemical compositions and a CHIME age of monazite in a granitic pegmatite from the Ishikawa district, Fukushima Prefecture, Japan

Monazite in a granite pegmatite from the Ishikawa district, Fukushima Prefecture, Japan has been examined in terms of U, Th, Pb, and rare earth element (REE) chemistry by electron microprobe analysis. Oscillatory and sector zoning were distinguished on the basis of back-scattered electron images. An averaged empirical formula was (Ce0.383Nd0.204La0.136Y0.062Sm0.061Pr0.050Gd0.040Dy0.014Er0.002Th0.046U0.005Ca0.011)∑1.014(P0.952Si0.040)∑0.992O4. Compositional variations suggest that the (Th + U) contents are controlled mainly by the coupled substitution (Th, U)+ Si = REE + P. Analytical data for clear portions of the monazite crystal were arrayed linearly on the PbO-ThO2* (ThO2 plus the equivalent of UO2) diagram and formed a single isochron for a CHIME age of 103 ± 26 Ma (2σ) that is in accord with previously reported radiometric ages for the granitic rocks in the Ishikawa district.

Documenting Exhumation in the Central and Northern Menderes Massif (Western Turkey): New Insights from Garnet-Based P-T Estimates and K-Feldspar 40Ar/39Ar Geochronology

AbstractThe Menderes Massif (Turkey) is a metamorphic core complex that records Alpine crustal shortening and extension. Here, nine garnet-bearing schist samples in the Central Menderes Massif (CMM) from below the Alaşehir detachment (AD) were studied to reconstruct their growth history. P-T estimates made using a chemical zoning approach, and petrological observations, indicate garnet grew between ~6 kbar and 550°C and 7.5-9 kbar and 625-650°C. Two P-T path shapes from two samples emerged (isobaric and burial), suggesting that either separate garnet-growth events occurred, or different garnet generations from the same metamorphic event were sampled. Despite observable diffusional modification in most garnets, thermobarometric estimates for crystal-rim growth yield P-T estimates similar to those reported elsewhere in the region. Ion microprobe monazite ages, paired with textural observations, from three of the samples time early retrograde metamorphism (~36-28 Ma). To better understand Neogene extension/exhumation, K-feldspar 40Ar/39Ar ages were obtained from two synextensional granites (Salihli and Turgutlu) exposed along the AD and two from the northern Simav detachment (Koyunoba and Eğrigöz). This data suggests the Simav detachment footwall rapidly exhumed at ~20 Ma, whereas the AD experienced two periods of exhumation/cooling (~14 Ma and~5 Ma). AD ages support a pulsed exhumation model for the massif.

Chemical composition and age of monazite-(Ce) in granitoids of the crystalline basement from the South Yamal

The relevance of the work is due to the need to improve the method of chemical dating as applied to high-thorium accessory minerals, which are difficult to date by isotope research methods. Purpose of the work: study of the chemical composition of accessory monazite from granitoids of the crystalline basement of the South Yamal and determination of its age. Research methodology: quantitative analysis of the chemical composition of monazite was carried out using X-ray spectral electron probe microanalyzer CAMECA SX 100 (electron beam diameter from 1 μm, BSE, SE, Cat modes, determination of elements from beryllium to uranium). The spectra were obtained with the help of inclined wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. The age calculation was carried out according to the well-known methods of foreign authors in addition to some developments of the author. Results. The chemical composition of monazite makes it possible to classify it as a cerium variety, the content of radiogenic components varies greatly (in wt.%): ThO2 – 5.37–16.31, UO2 – 0.40–0.81, PbO – 0.08–0,19. There are significant concentrations of SiO2 (up to 3.5 wt.%), Y2 O3 (up to 1.8 wt.%) and CaO (up to 1.2 wt.%). It turns out that monazite implements hattonite (Th4+(U4+)+Si4+ → REE3++P5+) and cheralite (Th4+(U4+)+Ca2+(Sr2+,Ba2+,Pb2+) → 2REE3+) isomorphism types. The decent content of lead and high crystallinity of the substance makes it possible to use this mineral as a geochronometer mineral. Conclusions. New data on the chemical composition of monazite have been obtained, and the late Permian age of granitoids has been determined by microprobe dating. The values of the point U–Th–Pb ages of monazite together give a weighted average age of 256 ± 10 Ma (MSWD = 0.15) and an isochron of 254 ± 19 Ma (MSWD = 0.28), which almost ideally coincides with the results of isotopic U–Pb zircon dating from the same rock, 254 ± 3 Ma.

Seeking the Sources of Dust: Geochemical and Magnetic Studies on “Cryodust” in Glacial Cores from Southern Spitsbergen (Svalbard, Norway)

Natural mineral particulate matter deposited from aerosols and trapped in glaciers—herein defined as “cryodust”—may be an excellent indicator of atmospheric circulation, if terrestrial sources of dust can be identified. In this study, we analyzed the composition of cryodust in shallow ice cores taken from five glaciers in Southern Spitsbergen (Svalbard Archipelago, Northern Norway). The chemical composition, magnetic properties and radiogenic ages of individual grains were measured, where possible, to provide indicators of source areas. To identify mineral and rock fragments, solid particulates were examined by Scanning Electron Microscope fitted with a backscattered electron and Energy Dispersive Spectroscopic detectors. An Electron MicroProbe was employed for the U-Th-Pb chemical dating of monazite grains. Magnetic measurements comprised analyses of magnetic susceptibility (κ) vs. temperature (T) variations and determination of magnetic hysteresis parameters. Monazite ages span 445–423 Ma, consistent with mineral growth during the Caledonian orogeny. Caledonian rocks are exposed in the Nordaustlandet area of North-Eastern Svalbard, and this is the most probable source for monazite grains. Magnetic analyses show a predominance of ferrous (FeII) over ferric (FeIII) phases, consistent with a lack of input from subtropical sources. The results from both methods are consistent with local sources of dust from exposures in the Svalbard archipelago.

PRECISE AGES FOR LODE GOLD MINERALIZATION IN THE XIAOQINLING GOLD FIELD, SOUTHERN MARGIN OF THE NORTH CHINA CRATON: NEW CONSTRAINTS FROM IN SITU U-Pb DATING OF HYDROTHERMAL MONAZITE AND RUTILE

Abstract The Xiaoqinling gold field, located along the southern margin of the North China craton, is the second largest gold producer in China, which comprises more than 1,200 auriferous quartz veins with a proven gold reserve of at least 800 tons. Previously, the absolute age of the gold metallogenesis in this area has not been well defined due to the lack of suitable dating minerals. This study presents new in situ laser ablation-inductively coupled plasma-mass spectrometry U-Pb ages of coexisting hydrothermal monazite and rutile for the Fancha gold deposit in this area, which yielded 206Pb/238U ages of 127.5 ± 0.7 Ma (n = 65, mean square of weighted deviates [MSWD] = 1.8) and 129.7 ± 4.3 Ma (n = 37, MSWD = 1.4), respectively. Both ages overlap within analytical uncertainty at the 2σ level of significance, suggesting that both gold-bearing veins were emplaced at ca. 128 Ma. Mineralogical observations indicate that the monazite and rutile precipitated simultaneously with gold from the hydrothermal fluid. Our new data, combined with recently published monazite age, define a more precise gold episode, demonstrating that the gold endowment of the Xiaoqinling area was formed during a relatively brief period at ca. 130 to 127 Ma. We suggest that auriferous fluids were generated as a result of interactions between the enriched mantle and the lower crust, which was driven by westward flat slab subduction of the Paleo-Pacific plate during the late Mesozoic. The peak of lithospheric thinning during the postsubduction may have led to the rapid release of gold from the fertilized mantle. Consequently, the large number of gold-bearing veins in the Xiaoqinling area may ultimately be related to the tectonic evolution and mantle fluid processes that occurred during Early Cretaceous lithospheric extension.

The Age of Monazite from the Ichet’yu Occurrence, Middle Timan (CHIME and LA–ICP–MS Methods)

Abstract—A study of the morphology, composition, and age (by the CHIME and LA–ICP–MS methods) of monazite from the Ichet’yu occurrence, located in Middle Timan, revealed some principal differences in the typomorphic features and genesis of its two varieties. The common yellow monazite from this occurrence is monazite-(Ce), in which the content of La is higher than that of Nd. The time of its crystallization (recrystallization) is estimated by the CHIME method as 518 ± 40 Ma. Kularite (a grayish-brown globular variety of monazite, in which Nd is higher than La) was dated at 978 ± 31 Ma. The age of some kularite grains is 520 ± 27 Ma, which may be interpreted as the date of a hydrothermal event that resulted in the simultaneous recrystallization of both monazite and kularite. However, these two monazite varieties were formed in two completely different sources and were combined in the mineral parasteresis of the Ichet’yu occurrence. The CHIME age of monazite from the Ichet’yu occurrence that belongs to two intervals (500–600 and 960–1000 Ma) is consistent with the age determined in the same monazite grains using the LA–ICP–MS technique.

High‐Resolution P‐T‐Time Paths Across Himalayan Faults Exposed Along the Bhagirathi Transect NW India: Implications for the Construction of the Himalayan Orogen and Ongoing Deformation

AbstractPressure‐temperature (P‐T) conditions and high‐resolution paths from 11 garnet‐bearing rocks collected across Himalayan fault systems exposed along the Bhagirathi River (Uttarakhand, NW India) reveal the tectonic conditions responsible for their growth. A garnet from the Tethyan metasedimentary unit has a 50.3 ± 0.6 Ma (Th‐Pb, ±1σ) monazite inclusion, suggesting that ductile mid‐crustal metamorphism occurred synchronously or soon after (<10 Myr) India‐Asia collision, depending on timing. High‐resolution garnet P‐T paths from the same rock show ∼1 kbar fluctuations in P as T increases over a ∼20°C interval, consistent with a period of erosion. We report garnets from the Main Central Thrust (MCT) hanging wall that have Eocene to Miocene monazite ages, and one garnet yields paths consistent with motion along the Main Himalayan Thrust (MHT) décollement. Most high‐resolution MCT footwall P‐T paths fluctuate in P (±1 kbar) as T increases, consistent with imbrication and paths from equivalent structural assemblages in central Nepal. Like those rocks, MCT footwall (Lesser Himalayan Formation, LHF) monazite ages are Early Miocene (9.3 ± 0.6 Ma) to Pliocene (3.0 ± 0.2 Ma). The results demonstrate the consistency in timing and conditions across the MCT at locations ∼650 km apart. If the present‐day Himalayan tectonic framework has not significantly changed since the Pliocene, the LHF duplex can be considered when attributing seismic events to particular faults. The MHT is undisputedly the significant factor in accommodating Himalayan seismic activity, but MCT footwall faults may explain some shallower hypocenters, without the need for unusual MHT geometries.

Unraveling the origins and P-T-t evolution of the allochthonous Sobrado unit (Órdenes Complex, NW Spain) using combined U–Pb titanite, monazite and zircon geochronology and rare-earth element (REE) geochemistry

Abstract. The Sobrado unit, within the upper part of the Órdenes Complex (NW Spain) represents an allochthonous tectonic slice of exhumed high-grade metamorphic rocks formed during a complex sequence of orogenic processes in the middle to lower crust. In order to constrain those processes, U–Pb geochronology and rare-earth element (REE) analyses of accessory minerals in migmatitic paragneiss (monazite, zircon) and mylonitic amphibolites (titanite) were conducted using laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). The youngest metamorphic zircon age obtained coincides with a Middle Devonian concordia monazite age (∼380 Ma) and is interpreted to represent the minimum age of the Sobrado high-P granulite facies metamorphism that occurred during the early stages of the Variscan orogeny. Metamorphic titanite from the mylonitic amphibolites yield a Late Devonian age (∼365 Ma) and track the progressive exhumation of the Sobrado unit. In zircon, cathodoluminescence images and REE analyses allow two aliquots with different origins in the paragneiss to be distinguished. An Early Ordovician age (∼490 Ma) was obtained for metamorphic zircons, although with a large dispersion, related to the evolution of the rock. This age is considered to mark the onset of granulite facies metamorphism in the Sobrado unit under intermediate-P conditions, and related to intrusive magmatism and coeval burial in a magmatic arc setting. A maximum depositional age for the Sobrado unit is established in the late Cambrian (∼511 Ma). The zircon dataset also record several inherited populations. The youngest cogenetic set of zircons yields crystallization ages of 546 and 526 Ma which are thought to be related to the peri-Gondwanan magmatic arc. The additional presence of inherited zircons older than 1000 Ma is interpreted as suggesting a West African Craton provenance.

The Conlara Metamorphic Complex: Lithology, provenance, metamorphic constraints on the metabasic rocks, and chime monazite dating

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.

Igneous-metamorphic basement of Taquetrén Range, patagonia, Argentina: A key locality for the reconstruction of the paleozoic evolution of patagonia

In this contribution, we present the stratigraphy of the igneous and metamorphic rocks of the Taquetrén Range, a sector located in the southernmost margin of the North Patagonian Massif (42°42′00″ S - 69°30′00″ W). Its igneous and metamorphic basement is composed of the newly defined “Lagunita Salada Igneous-Metamorphic Complex” (LSIMC), “Paso del Sapo Plutonic Complex” (PSPC) and “Sierra de Taquetrén Plutonic Complex” (STPC). The LSIMC comprises gneisses, schists, amphibolites and migmatites, which share a S1–S2 penetrative foliation with a mean orientation of 300°–330°/40°–60° NE. Based on mineral paragenesis, metamorphic conditions of these rocks are the result of Barrovian-type metamorphism in the upper amphibolite to granulite facies. EPMA Th–U–Pb ages of monazites display two isochron main populations at 379 ± 5 Ma and 323 ± 5 Ma, which suggest long-term high-temperature conditions for the region between Late Devonian and Carboniferous times. The Complex is intruded by concordant tonalites, granodiorites, porphyric granites and minor pegmatites and felsic dykes, which are grouped in the PSPC. Both the LSIMC and PSPC are intruded by unfoliated peraluminous granitoids grouped in the STPC. Based on field and microstructural data, the pervasive foliation identified in the PSPC was caused by processes ranging from magmatic flow to solid-state deformation, indicating a syntectonic emplacement. Zircon U–Pb analysis by LA-ICP-MS in the PSPC shows two distinguishable groups with concordia ages of 314.1 ± 2.2 Ma and 302.8 ± 2.2 Ma, interpreted as the crystallization and subsequent deformation age respectively, related to protracted high-strain conditions. The outcrops in this area represent an almost full tectonic cycle encompassing from medium-high grade metamorphic rocks and syn-tectonic intrusions to post-tectonic intrusions, therefore configuring a key locality for the analysis of North Patagonian Paleozoic evolution. Moreover, based on the compilation of U–Pb zircon ages, a ~20 My magmatic gap period (360-340 Ma) is recognized in the southwestern margin of the North Patagonian Massif coeval with amphibolite-granulite facies metamorphism in different sectors of the Central Patagonian Igneous-Metamorphic Belt, presenting thus important implications for the tectonic evolution of the area.

Tectonothermal evolution of a collisional orogen in the Khammam region, southeastern India: insights from structures, phase equilibria modeling and U–Th–(total) Pb monazite geochronology

The Khammam Schist Belt (KSB), southeastern India, represents a part of the collision zone that is sandwiched between the Eastern Dharwar Craton to the west and the Eastern Ghats Mobile Belt (EGMB) to the east. Quartzofeldspathic gneisses of the EGMB and garnet–kyanite metapelites of the KSB have been investigated to characterize P–T–t evolution of the Khammam region. Pseudosection modeling reveals that the melt-bearing quartzofeldspathic gneiss experienced peak and post-peak metamorphism at P–T conditions of 7.9–8.1 kbar/790–810 °C and ~ 7.0 kbar/740–750 °C, respectively. In contrast, metapelite witnessed peak metamorphism at 7.4–7.8 kbar/600–640 °C, followed by post-peak retrogression at 6.1–6.6 kbar/590–625 °C. U–Th–(total) Pb monazite ages from the gneiss constrain the peak and retrograde metamorphic episodes at 1.63–1.53 Ga and 1.48–1.38 Ga, while those in the metapelites were determined at 1.25–1.20 Ga and 1.18–1.10 Ga, respectively. These P–T–t estimates indicate that the KSB, Vinjamuru, and Ongole domains evolved distinctly during Late Paleoproterozoic–Late Mesoproterozoic. The younger ages (0.90–0.81 Ga) were ascribed to the formation of the Eastern Indian Tectonic Zone, implying its extension beyond the western margin of the EGMB. Besides, the distinct Neoarchean ages (2.79–2.45 Ga) are related to the Archean protolith of the quartzofeldspathic gneisses, which were likely derived from the Archean crust of the Napier–Rayner Complex. This tectonothermal restoration is new and characterizes the Khammam region as the hot and composite collision zone with protracted geological history. (250 words)

Late Neoproterozoic P-T-t paths of syn- and post-collisional metamorphism in the Paranaguá Terrane, Ribeira Belt (Brazil): implications for West Gondwana assembly

ABSTRACT Phase diagram modelling and U-Pb monazite ages reveal a clockwise Pressure-Temperature-time metamorphic evolution in the Paranaguá Terrane (Ribeira Belt – SE Brazil) and characterized important tectonic events. Comparisons of our dataset with the Dom Feliciano and Kaoko belts indicate a contrasting metamorphic evolution of South American and African units during West Gondwana assembly. The Paranaguá Terrane is considered an arc-related unit formed during the western Gondwana accretion, comprising Paleoproterozoic basement inliers, metasedimentary rocks and late Neoproterozoic granites. Metapelites mineral assemblages and chemistry combined with isotopic data indicate three tectonometamorphic stages for the terrane. M1 (640–610 Ma) is associated with collisional processes that resulted in a greenschist to lower amphibolite facies regional metamorphism (540–580°C and ~6-8 kbar). The progression of the metamorphism followed a decompression path of c. 2–4 kbars and reached P-T conditions of ~725°C at ~5 kbars (M2 – 610-580 Ma). M2 is associated with post-collisional processes related to slab-break off and partial exhumation of deeply buried sequences. Metapelites low-temperature retrograde conditions define a late M3 (540–500 Ma) metamorphic event. Our results demonstrate the similarities and differences of the Paranaguá-Dom Feliciano-Kaoko orogenic system. The metamorphic evolution of the Paranaguá Terrane and Northern Dom Feliciano Belt exhibits a geothermal gradient controlled by broad magmatic activity and post-collisional processes. In contrast, their suggested African counterpart (Kaoko Belt) shows a Barrovian metamorphic trend. This dataset permitted a refined time-space evolution of these southwest Gondwana units.

Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Brian\xe7onnais Zones

Ion probe 208Pb/232Th fissure monazite ages from the Argentera External Massif and from the high-pressure units of the Western Alps provide new insights on its Cenozoic tectonic evolution. Hydrothermal monazite crystallizes during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Monazite growth domains visualized by BSE imaging all show a negative Eu anomaly, positive correlation of Sr and Ca and increasing cheralite component (Ca + Th replacing 2REE) with decreasing xenotime (Y) component. The huttonite component (Th + Si replacing REE and P) is very low. Growth domains record crystallization following chemical disequilibrium in a fissure environment, and growing evidence indicates that they register tectonic activity. Fissure monazite ages obtained in this study corroborate previous ages, recording crystallization at ~ 36 Ma, ~ 32–30 Ma, and ~ 25–23 Ma in the high-pressure regions of the Western Alps, interpreted to be respectively related to top-NNW, top-WNW and top-SW thrusting in association with strike-slip faulting. During this latter transpressive phase, younger fissure monazite crystallization is recorded between ~ 20.6 and 14 Ma in the Argentera Massif, interpreted to have occurred in association with dextral strike-slip faulting related to anticlockwise rotation of the Corsica-Sardinia Block. This strike-slip activity is predating orogen-parallel dextral strike-slip movements along and through the internal part of all other External Crystalline Massifs (ECM), starting only at ~ 12 Ma. Our combined compositional and age data for hydrothermal monazite track crystallization related to tectonic activity during unroofing of the Western Alps for over more than 20 million years, offering chronologic insights into how different tectonic blocks were exhumed. The data show that fissures in the high-pressure units formed during greenschist to amphibolite facies retrograde deformation, and later in association with strike-slip faulting.

Exploring laser ablation U–Pb dating of regional metamorphic garnet – The Straits Schist, Connecticut, USA

Garnet is a prominent recorder of metamorphic processes and plays a pivotal role in deciphering the pressure-temperature evolution of metamorphic rocks. However, the potential of regional metamorphic garnet for laser ablation U–Pb dating remains unexplored, as variable and low U (<0.1 μg/g) coupled with elevated initial Pb contents make analyses challenging, and the presence of U-rich inclusions can affect results.We applied LA-ICP-MS U–Pb dating to garnet and zircon, monazite, and xenotime from The Straits Schist, western Connecticut, USA, to constrain the timing of garnet growth within the overall Acadian-age metamorphic history recorded by this unit. We assessed the effects of U-rich inclusions on garnet U–Pb data using high-resolution microbeam sampling, grain size analysis, and calculations assessing how inclusion size and abundance affect U concentrations measured in garnet.When analysing garnet, our results demonstrate that by scrutinizing the time-resolved U-signal, U-rich inclusions can be identified and effectively removed during data evaluation. If U-rich inclusions are unaccounted for, ‘garnet’ analyses have elevated U contents compared to pure garnet and in a Tera-Wasserburg diagram, data points are shifted to the left, right, or along the garnet regression line, for inclusions older, younger or of the same age as the garnet, respectively.Garnet from two metapelitic samples yielded lower intercept ages of 409 ± 10 Ma and 406 ± 15 Ma, and a combined U–Pb age of 406.1 ± 8.3 Ma. Metamorphic zircon yielded age clusters at 446.2 ± 4.2 Ma, 413.3 ± 3.0 Ma, and 382.9 ± 2.8 Ma, monazite ages range between 390 and 355 Ma, and xenotime yielded 370.1 ± 5.6 Ma. The ∼446 Ma age peak is attributed to detrital zircon formed during the waning stages of the Taconian orogeny. Early Acadian metamorphism is recorded by garnet and ∼413 Ma zircon, whereas monazite and zircon ages of 390 to 383 Ma, coincide with peak metamorphic conditions. Early Carboniferous monazite and xenotime ages could reflect the incursion of fluids into The Straits Schist during Neo-Acadian metamorphism, coeval plutonism in New England, or breakdown of garnet during regional exhumation.

Late Cretaceous to Paleocene Tectonometamorphic Evolution of the Blanchard River Assemblage, Southwest Yukon: New Insight into the Terminal Accretion of Insular Terranes in the Northern Cordillera

Abstract The Intermontane-Insular terrane boundary stretches over 2000 kilometers from British Columbia to Alaska in the western Cordillera. Juxtaposed between these terranes is a series of Jura-Cretaceous basinal and arc assemblages that record a complicated and contested tectonic evolution related to the Mesozoic-Paleocene accretionary history of northwestern North America. In southwest Yukon, west-verging thrust faults facilitated structural stacking of the Yukon-Tanana terrane over these basinal assemblages, including the Early Cretaceous Blanchard River assemblage. These previously undated compressional structures are thought to be related to the final collapse of the Jura-Cretaceous basins and the tectonic burial of the Blanchard River assemblage resulting in amphibolite facies metamorphism. New in situ U-Th-Pb monazite ages record at least three tectonic events: (1) the tectonic burial of the Blanchard River assemblage to amphibolite facies conditions between 83 and 76 Ma; (2) peak burial was followed by regional exhumation at ca. 70-68 Ma; and (3) intense heating and ca. 63-61 Ma low-pressure contact metamorphism attributed to the intrusion of the voluminous Ruby Range suite, which is part of the northern Coast Mountains batholith. The tectonometamorphic evolution recorded in the Blanchard River assemblage can be correlated to tectonism within southwest Yukon and along the length of the Insular-Intermontane boundary from western British Columbia through southwestern Yukon and Alaska. In southwest Yukon, these results suggest an asymmetric final collapse of Jura-Cretaceous basins during the Late Cretaceous, which relates to the terminal accretion of the Insular terranes as they moved northward.

Non‐Matrix‐Matched Determination of Th‐Pb Ages in Zircon, Monazite and Xenotime by Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry

Similar to the U‐Pb system, the 232Th‐208Pb decay scheme is also widely utilised in geochronology of Th‐rich accessory minerals. The matrix‐dependent systemic deviations observed between different accessory minerals and the paucity of well‐characterised reference materials for some minerals of interest (e.g., xenotime and allanite) are the major obstacles to yield accurate and precise Th‐Pb ages with laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). In this study, we investigated the effects of the addition of nitrogen, oxygen and water vapour before the ablation cell on the accuracy of Th‐Pb dating of accessory minerals (i.e., zircon, monazite and xenotime) using NIST SRM 610 glass as the calibrating reference material. Our results demonstrate that the measured 208Pb/232Th ages in zircon, monazite and xenotime were approximately 9–24% lower than their isotope dilution thermal ionisation mass spectrometry (ID‐TIMS) ages in normal ablation (without addition of additional gases). The measured 208Pb/232Th ages of zircon (Plešovice, M257 and Qinghu), monazite (44069 and Trebilcock) and xenotime (BS‐1 and MG‐1) showed excellent agreement with their respective reference values with the addition of small amounts of water vapour before the ablation cell. This may be due to the dramatically reduced biases for the down‐hole fractionation of Pb/Th intensity ratios in NIST SRM 610 and accessory minerals in the presence of water vapour. The Th‐Pb ages of zircon, monazite and xenotime were successfully analysed using NIST SRM 610 glass as the calibrating reference material with both 193 nm ArF excimer laser ablation‐quadrupole‐inductively coupled plasma‐mass spectrometry (LA‐Q‐ICP‐MS) and 213 nm Nd:YAG laser ablation‐sector field‐inductively coupled plasma‐mass spectrometry (LA‐SF‐ICP‐MS) in combination with the developed water vapour‐assisted method. This method is an effective and notably simple approach for non‐matrix‐matched analysis of Th‐Pb ages in zircon, monazite and xenotime by LA‐ICP‐MS.

Genesis of monazite-rich, orthopyroxene-bearing veins in the Kliphoog area of the Concordia Granite, Springbok, Namaqualand, South Africa: Sources, fluids, and the mobility of actinides and REE

A network of monazite-rich, coarse-grained veins and fine-grained veins permeating the mid-Proterozoic Concordia Granite in the Kliphoog area near Springbok, Namaqualand, North Cape Province, South Africa, are described with respect to their whole-rock chemistry, petrography, mineralogy, and mineral chemistry along with zircon and monazite geochronology. Chondrite-normalized trace element data indicate that the coarse-grained veins are enriched in LREE relative to the Concordia Granite with HREE enrichment being somewhat variable. HREE in altered Concordia Granite, at the contact with the coarse-grained vein, are heavily depleted compared to unaltered Concordia Granite while the LREE show the same approximate abundances. Chloritization is common in the coarse-grained veins along mineral grain boundaries with chlorite replacing most of the orthopyroxene megacrysts, though occasional remnants of orthopyroxene remain in the core. A traverse across one of the coarse-grained veins showed no obvious mineral chemical trends between the vein and surrounding granite. Relic Fe-rich megacryst orthopyroxene has an Al2O3 content of around 2.0 wt% suggesting formation at around 700 °C. The Fe-rich biotite has mean Ti values ranging from TiO2 = 4.1 to 1.2 wt% indicating formation at 700–800 °C. The coarse-grained veins and fine-grained veins are characterized by abundant accessory Th-rich monazite, heterogeneously distributed in the veins. Lesser amounts of monazite are found in the granite. Other accessory minerals in the veins and granite include zircon and rare fluorapatite. Back-scattered electron imaging shows that the anhedral to euhedral monazite grains (up to more than 1500 μm in size) tend to be complexly zoned with light and dark areas. Lighter areas are more enriched in Th and/or Ce than darker areas. This complex zoning can occur as magmatic/sector zoning and more commonly as a complex series of metasomatic alteration events. Metasomatic textures, which are due to variable fluid-induced mobility of Th, U, and REE, are the result of a coupled dissolution-reprecipitation process and have been reproduced experimentally using high pH alkali-bearing fluids. The analysis of seven zircon cores from the Concordia Granite yields a concordia age of 1173 ± 15 Ma suggesting that an age of ca. 1170 Ma most likely represents the true emplacement age of the Concordia Granite. The mean 207Pb/206Pb age of 1042 ± 9 Ma obtained on zircon from the main coarse-grained vein is interpreted as the crystallization age of the vein. 207Pb/235U monazite ages are highly variable in the veins and Concordia Granite host, ranging from ca. 1050 to 1000 Ma. This supports the petrographic and mineral chemistry evidence that the monazite U--Pb system was variably affected by a continuous or multi-pulse metasomatic/metamorphic event(s) occurring between 1050 and 1000 Ma. U--Pb analyses of deformed monazite crystals from a radioactive sinistral shear zone of the Kliphoog area yield a mean 207Pb/235U age of ca. 1006 ± 9 Ma, indicating that the late circulation of Th-LREE-rich fluids synchronous with strike-slip deformation occurred at ca. 1000 Ma. On a regional scale, the intrusion of the Kliphoog coarse-grained vein is contemporaneous with granulite-facies metamorphism as well as the A-type granitoid magmatism of the Spektakel Suite, mafic magmatism of the Koperberg Suite, and emplacement of the monazite vein-type deposit at Steenkampskraal. Therefore, a genetic relationship between the formation of monazite-rich Kliphoog veins and these metamorphic, magmatic, and metallogenic events is proposed.

Geochronology, mineral chemistry and genesis of REE mineralization in alkaline rocks from the Kohistan Island Arc, Pakistan

Alkaline igneous-related rare earth element (REE) deposits constitute a major global economic resource of REE. However, our understanding of their geodynamic settings and magmatic-hydrothermal processes remains uncertain. Here, we present an investigation of petrography, geochronology and mineral chemistry of newly discovered REE mineralization in the alkaline syenite and pegmatite from the Kohistan Batholith in the Kohistan Island Arc, Pakistan. The pegmatite contains abundant chevkinite and monazite (up to 10–35% in volume), which formed from crystallization of a REE-oversaturated evolved melt and are the main REE-bearing phases. U-Th-Pb isotopic dating of zircon from the syenite yields the emplacement age of 31.2 ± 0.3 Ma, which closely matches the ages of zircon and monazite from the pegmatite (30.1 ± 0.3 Ma and 30.9 ± 0.3 Ma, respectively). Zircons from the syenite and the pegmatite have similar ɛHf(t) values, ranging from + 0.2 to + 2.5, indicating that the syenite and the pegmatite are co-magmatic in origin. The alkaline magma likely formed by partial melting of the enriched lithospheric mantle induced by the asthenospheric upwelling, which was related to slab break-off at the post-collisional stage following the collision of the Kohistan Island Arc with the Indian Plate. Zircons show the evolutional trend in trace element compositions from the syenite to the pegmatite, marked by increases in REE, Hf, Y, U, volatiles and fo2. These changes reflect prolonged fractional crystallization processes of the alkaline magma, which were responsible for the REE enrichment in the residual pegmatitic melt. Post-collisional alkaline magmatic rocks are widely distributed in many places along the Tethyan orogenic belt. These observations suggest that these post-collisional alkaline rocks may be good targets for REE exploration.

U–Pb geochronology reveals evidence of a Late Devonian hydrothermal event, and protracted hydrothermal–epithermal system, within the Mount Painter Inlier, northern Flinders Ranges, South Australia

The Mount Gee Sinter and the Radium Ridge Breccia within the Mount Painter Inlier, South Australia, preserve evidence of a hydrothermal event peaking during the Late Devonian (ca 365 Ma). Prior to this study, limited data relating to this event were available, but our results of 846 LA-ICPMS U–Pb monazite analyses indicate the timing of this hydrothermal event. The dominant monazite population age of ca 363 Ma for the Mount Gee Sinter represents earlier phases of a protracted hydrothermal–epithermal system, whereas a later epithermal phase cross-cuts rocks hosting ca 220 Ma age zircon. Parts of the Radium Ridge Breccia have been recently interpreted as a series of Early Cretaceous glacial events. Zircon ages (604 zircon analyses) from the Radium Ridge Breccia define a detrital population dominated by ca 1585 Ma ages, consistent with derivation of clasts within the breccia from local granitic and metasedimentary basement. The Radium Ridge Breccia is, however, dominated by a ca 367 Ma aged monazite population, probably reflecting overprinting of the local Mesoproterozoic granitic basement rock during the same hydrothermal event as formed the earlier phase of the Mount Gee Sinter. It is interpreted that the monazite ca 365 Ma age reflects a significant Late Devonian hydrothermal event, evidenced locally in the southern Mount Painter Inlier, resulting from localised heat flow but with thermal implications for the regional geology of the northwestern Curnamona Province.

Eocene migmatite formation and diachronous burial revealed by petrochronology in NW Himalaya, Zanskar

AbstractIn this contribution, we highlight the importance of in‐situ monazite geochronology linked to P−T modelling for identification of timescales of metamorphic processes. Barrovian‐type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis. P−T−t paths are constrained through combined pseudosection modelling and in‐situ and in‐mount monazite and xenotime laser ablation–split‐stream inductively coupled plasma‐mass spectrometry. Petrography and garnet zoning combined with pseudosection modelling show that garnet‐staurolite schists record burial from ~530 to 560°C and 5.5 kbar to ~630 to 660°C and 7 kbar; staurolite‐kyanite schists from ~530 to 560°C and 5 kbar to ~670 to 680°C and 7−9 kbar; and garnet‐kyanite migmatites from 540−570°C and 5 kbar to ~680 to 750°C and 7−10 kbar, probably also to &gt;750°C and &gt;9 kbar above the muscovite stability field. The decompression paths of garnet‐staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re‐equilibration at ~600 to 670°C and 4−6 kbar in some of the staurolite‐kyanite schists, and at ~670 to 700°C and 6 kbar in garnet‐kyanite migmatites. Some of the staurolite‐kyanite schists and garnet‐kyanite migmatites also contain andalusite or andalusite‐cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts. BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence of c. 44 − 37 Ma ages. The first group of samples—two garnet‐staurolite schists—recorded only c. 31 − 27 Ma ages in porphyroblasts and no c. 40 Ma ages. The second group (samples of staurolite‐kyanite schist, garnet‐kyanite migmatites, augengneiss) have both the older, c. 44 − 37 Ma monazite ages in porphyroblasts and younger ages down to c. 22 Ma. These significantly different ranges of ages from porphyroblasts of 44−37 Ma, and 31−27 Ma, are interpreted as the duration of prograde P−T paths in Eocene and Oligocene, and indicate diachronous two‐stage burial of rocks. Early migmatization occurred at 38 Ma. The c. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26−24 Ma. The youngest ages of 23−22 Ma are related to leucogranite emplacement.