Detrital Monazite Research Articles

Paleogeography of South China in the Early Paleozoic: evidence from U–Pb geochronology of detrital monazite

The tectonic affinity of South China in paleogeographic reconstructions of the Gondwana supercontinent during the Early Paleozoic remains controversial. Here, we present new detrital monazite U–Pb age data from Lower Devonian Posongchong Formation clastic sedimentary rocks in the Youjiang Basin in southwestern South China to explore the link between South China and Gondwana. U–Pb detrital monazite geochronology yields dominant Pan-African (657–460 Ma) and minor early Neoproterozoic (998–798 Ma) contents. Taking into account the direction of marine transgression, Northern India, Himalaya, Lhasa, Qiangtang terrane, Hainan Island, and Western Australia, located on the northern edge of East Gondwana, are the main provenances of the Youjiang Basin, with the Jiangnan orogen also providing some clastic materials. The source of the clastic material is also closely related to the Pan-African movement and the Neoproterozoic collision between the Yangtze Block and the Cathaysia Block. It is thus suggested that South China constituted part of East Gondwana in the Early Paleozoic. The unconformity between the Late Cambrian and Early Ordovician in South China, named the Yunan event, was a product of the final assembly of East Gondwana. Detrital monazite is an effective near-source indicator in tracing the provenance of the basin.

Metamorphic Evolution of Chloritoid-Bearing Micaschist from the Variscan Elstergebirge: Evidences for Stacking of High-Pressure Rocks in the Saxothuringian Zone of Central Europe

A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif. Based on the textural relations and compositions of minerals, especially of zoned garnet and potassic white mica, a P-T path was reconstructed using contoured P-T pseudosections. The U-Th-Pb dating of monazite in the micaschist was undertaken with the electron microprobe. The micaschist experienced P-T conditions along a clockwise path between 16 kbar at 510℃ and 5 kbar at 555℃ followed by isobaric heating to about 600℃. Monazite ages range between 315 and 480 Ma with the most prominent maxima and side maxima at 346.0±1.1 (2σ), 357.3±2.3, and 368.3±1.7 Ma. Ages older than 380 Ma were related to detrital monazite pointing to a Devonian sedimentary protolith. Other ages around 325 Ma were assigned to the isobaric heating by nearby post-tectonic granites. The high-pressure event, being the result of the collision of Laurussia and Gondwana after closure of the Rheic Ocean, occurred in the Late Devonian. The exhumation to 15-20 km (5 kbar) ended probably in the Early Carboniferous. The high-pressure micaschists from the Fichtelgebirge to the Erzgebirge crystalline complexes are suggested to represent a single nappe within a metamorphic nappe pile. This nappe is composed of metasedimentary slices, which experienced different peak pressures rather than representing a coherent crustal section.

Persistence of Grenvillian dominance in Laurentian detrital zircon age systematics explained by sedimentary recycling: Evidence from detrital zircon double dating and detrital monazite textures and geochronology

Abstract Grenvillian ages dominate Neoproterozoic to Paleozoic detrital zircon (DZ) populations across eastern Laurentia and persist through the present. The persistence of this dominance is inferred to result from recycling of DZ grains ultimately sourced from exceptionally Zr-rich and zircon-fertile Grenvillian granitoids. Pennsylvanian arenites of the Appalachian Basin (eastern United States) exhibit DZ U-Pb age distributions that are nearly identical to those of Neoproterozoic to Cambrian strata, and contain detrital diagenetic monazite grains formed via metamorphism or diagenesis of sedimentary rocks in the source region. Detrital zircon (U-Th)/He ages are mostly 475–300 Ma, yielding lag times [Δt = U-Pb age − (U-Th)/He age] of 500–1000 m.y. and 1200–2400 m.y. for Grenvillian and Paleoproterozoic to Archean DZ grains, respectively. Detrital monazite Th-Pb ages are comparable to (U-Th)/He cooling ages, reflecting formation of monazite during Paleozoic regional metamorphism of Neoproterozoic to Cambrian strata that reset the (U-Th)/He systematics of Grenvillian DZ grains within those metasediments. These results are either consistent with or prove recycling. Incorporation of other geological constraints permits definition of at least three (and potentially five) recycling events and their timing following initial post-Grenvillian exhumation and erosion (the “great Grenvillian sedimentation episode”). Recycling events include dispersal of post-Grenvillian sediment during deposition of Neoproterozoic to Cambrian strata (formation of the “Great Unconformity”: cycle 1), subsequent erosion of metamorphosed Neoproterozoic to Cambrian strata generating detritus for the Pennsylvanian arenites sampled here (cycle 2), and modern erosion of those arenites (cycle 3). Pancontinental river systems facilitated dispersal of sediment of ultimate Grenvillian age during or after each cycle.

A clearer view of crustal evolution: U-Pb, Sm-Nd, and Lu-Hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern China

AbstractMuch of the global picture of crustal evolution has been constructed using zircon. While this has revealed a rich and complex history, this view is necessarily incomplete because of the lithology-specific affinity of zircon and the high temperatures needed to reset the U-Pb and Lu-Hf systems inherent within it. Here we use a five mineral, multi-isotope system approach to compare the record of crustal evolution recorded by zircon versus the picture provided by monazite, titanite, apatite, and rutile from the Yong-Ding and Luan rivers, northern China. These other minerals sample more diverse lithologies and temperature-pressure conditions that reflect additional tectonothermal events to those recorded solely by zircon. Zircon from both studied rivers predominantly reflects magmatic features, yielding age peaks at 2.6–2.3, 2.0–1.8, and 0.38–0.13 Ga, corresponding to the major magmatic events in their catchments. However, the detrital zircon record from both catchments fails to record and detail several important tectonothermal events. Specifically, the detrital monazite U-Pb ages cluster into two Paleoproterozoic peaks of ca. 1.95 and 1.85 Ga, while detrital apatite and rutile ages document unimodal and protracted U-Pb age peaks at 1.9–1.6 Ga. The different U-Pb closure temperatures of monazite, apatite, and rutile likely record two metamorphic events and the subsequent cooling history—key details that are absent from or obscured in the zircon record. The Phanerozoic mineral U-Th-Pb ages correspond to multiple magmatic events between 0.40 and 0.24 Ga and subsequent 0.24–0.20 Ga metamorphism. The 0.60–0.25 Ga rutile U-Pb ages along with 0.33–0.24 Ga U-Pb ages in some zircon grains with radiogenic Hf isotope compositions from the Luan River do not match the geological records in the North China Craton, but instead reflect the protracted subduction-accretionary history of the Central Asian Orogenic Belt. In addition to their U-Th-Pb ages, Nd model ages of monazite, titanite, and apatite, plus zircon Hf model ages provide additional constraints on regional crustal evolution. The Nd model age information is blurred by the fact that the relationship between the Sm/Nd of these minerals and their former host rocks is not precisely known. Taken at face value, the monazite Nd model ages have two Neoarchean peaks at 2.9–2.7 and ca. 2.5 Ga, that may correspond to two crustal growth episodes, while the titanite Nd model ages with predominant peaks at 2.2–1.8 and 1.5–1.3 Ga broadly correspond with those derived from the whole-rock analyses of the wide spread Phanerozoic granitoids, and hence record extensive crustal reworking. In contrast, the zircon Hf model ages are strongly skewed to a 2.9–2.7 Ga period and fail to record the post-Archean evolution of this region. These data highlight the power of integrating the U-Th-Pb age and Lu-Hf/Sm-Nd isotope compositions of multiple detrital minerals, with a broad range in geochemical behavior and closure temperatures, to gain a more complete understanding of tectonothermal history and crustal evolution than zircon alone.

Geochemical Discrimination of Monazite Source Rock Based on Machine Learning Techniques and Multinomial Logistic Regression Analysis

Detrital monazite geochronology has been used in provenance studies. However, there are complexities in the interpretation of age spectra due to their wide occurrence in both igneous and metamorphic rocks. We use the multinomial logistic regression (MLR) and cross-validation (CV) techniques to establish a geochemical discrimination of monazite source rocks. The elemental abundance-based geochemical discrimination was tested by selecting 16 elements from granitic and metamorphic rocks. The MLR technique revealed that light rare earth elements (REEs), Eu, and some heavy REEs are important discriminators that reflect elemental fractionation during magmatism and/or metamorphism. The best model yielded a discrimination rate of ~97%, and the CV method validated this approach. We applied the discrimination model to detrital monazites from African rivers. The detrital monazites were mostly classified as granitic and of garnet-bearing metamorphic origins; however, their proportion of metamorphic origin was smaller than the proportion that was obtained by using the elemental-ratio-based discrimination proposed by Itano et al. in Chemical Geology (2018). Considering the occurrence of metamorphic rocks in the hinterlands and the different age spectra between monazite and zircon in the same rivers, a ratio-based discrimination would be more reliable. Nevertheless, our study demonstrates the advantages of machine-learning-based approaches for the quantitative discrimination of monazite.

Sources of U and Th in groundwater of the paleobeach aquifer at Cox's Bazar, Southeast Bangladesh

To explore the influence of radioactive heavy minerals on thorium (Th) and uranium (U) contamination in groundwater, we performed geochemical and mineralogical investigations of core samples (from the ground surface to 18.9 m depth) from a paleobeach area in Cox's Bazar, southeastern Bangladesh. Geochemical data document elevated levels of total Th (6.3–202.3 mg/kg, average 32.9 mg/kg) and U (1.1–33.4 mg/kg, average 5.2 mg/kg), and lithological observations document monazite concentrations as great as 2.36 wt% and zircon concentrations as great as 3.82 wt%. These minerals were found in organic-rich fine to very fine sand in the uppermost meter of the sediment column (down to 1.2 m depth) rather than in coarser sandy sediment. Energy-dispersive spectroscopy of several monazite and zircon grains indicated that alteration was greater in oxic sediment (down to 3 m depth) than reduced sediment (deeper than 3 m). Monazite and zircon had relatively high concentrations of Th (3395.9–3937.5 mg/kg and 275.5–318.4 mg/kg, respectively) and U (850.7–990.6 mg/kg and 256.3–290.5 mg/kg, respectively). Chemical weathering (dissolution and reprecipitation) of detrital monazite and zircon grains, which releases U and Th into the groundwater, may be a major and growing concern for drinking water in the study area. Shallow tubewells (<20 m depth) waters were enriched in U (0.09–9.57 μg/L) and Th (0.02–127.09 µg/L) and that the enrichment mostly occurred in highly oxic environments (Eh = 0.35–0.5 V). We suggest that hydrological and biogeochemical conditions in the aquifer that follow land-use changes influence the release of Th and U into the groundwater of the study area.

Detrital monazite from Upper Jurassic sediments in the central part of the Frolov megadepression, Western Siberia: chemical dating and provenances

Detrital monazite from Upper Jurassic sediments in the central part of the Frolov megadepression, West Siberian megabasin, is studied. Chemical composition of the mineral is studied and data on its age (chemical dating) are presented. Most monazite clasts are characterized by low roundness and their age corresponds to the Lower Paleozoic. Upper Jurassic sediments were likely derived not only from the local Early Paleozoic rock complexes (altaides), which make up the pre-Jurassic basement of the Frolov megadepression, but also from rocks located east and south of this megadepression.

A Multiproxy provenance approach to uncovering the assembly of East Gondwana in Antarctica

East Gondwana is generally thought to have assembled through the amalgamation of Indo-Antarctica and Australo-Antarctica along the Ediacaran–Cambrian Kuunga orogen. The location of a boundary between Indo-Antarctica and Australo-Antarctica within this key Gondwana-forming orogen remains controversial because extensive ice cover in East Antarctica precludes traditional characterization of terranes. Here, we integrated Pb-isotope analysis of detrital feldspar grains with U-Pb dating of detrital monazite and zircon grains from offshore sediments to infer the location of the onshore boundary between Indo-Antarctica and Australo-Antarctica. New and compiled data from onshore basement exposures highlight the different age and Pb-isotope signatures of Indo-Antarctica and Australo-Antarctica. Holocene sediments offshore from Mirny Station (Queen Mary Land, East Antarctica) have detrital feldspar Pb-isotope signatures and detrital monazite and zircon U-Pb ages that reflect contributions from both Indo-Antarctica and Australo-Antarctica. The presence of both Indo-Antarctic and Australo-Antarctic crust beneath ice cover near Mirny Station implies proximity to a fundamental terrane boundary within the Kuunga orogen, which could coincide with a geophysical lineament at ∼94°E (Mirny fault). The geophysical expression of this boundary extends into the subglacial interior of East Antarctica, where, prior to more recent rifting, it may have connected with one or more previously inferred Gondwana-forming sutures. The revised geometry of the Kuunga orogen suggests that the assembly of East Gondwana involved dominantly strike-slip motion in the Mirny region coupled with high-angle convergence between Indo-Antarctica and Australo-Antarctica to the west.

Detrital Monazite from Upper Jurassic Sediments in the Central Part of the Frolov Megadepression, Western Siberia: Chemical Dating and Provenances

Detrital monazite from Upper Jurassic sediments in the central part of the Frolov megadepression, West Siberian megabasin, is studied. Chemical composition of the mineral is studied and data on its age (chemical dating) are presented. Most monazite clasts are characterized by low roundness and their age corresponds to the Lower Paleozoic. Upper Jurassic sediments were likely derived not only from the local Early Paleozoic rock complexes (altaides), which make up the pre-Jurassic basement of the Frolov megadepression, but also from rocks located east and south of this megadepression.

An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

AbstractThe Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

Benefits of a Multiproxy Approach to Detrital Mineral Provenance Analysis: An Example from the Merrimack River, New England, USA

AbstractThe advantages in provenance research of U‐Pb dating different detrital minerals along with simultaneously analyzing trace elements is demonstrated in a study of sand from the mouth of the Merrimack River in New England, USA. Zircon ages record episodes of magmatism in the Early Paleozoic, peaking in the Early Devonian, followed by quiescence through the remainder of the Paleozoic and additional magmatic episodes in the Jurassic and Cretaceous. Simultaneous measurement of trace elements in zircons reveals a shift from arc magmatism to crustal melting associated with terrane collision in the Early Devonian, while many Jurassic grains are clearly derived from A‐type granites. Detrital monazites and rutiles have Devonian and Permian ages. Many of the older monazites have trace element characteristics suggestive of igneous origin, while Permian monazites are clearly metamorphic and record orogenesis that is absent from the detrital zircon record. Rutile grains have trace element chemistry indicative of mostly metasedimentary source rocks, and Zr thermometry indicates growth under amphibolite facies conditions. Age offsets between monazite and rutile populations provide information about the region's cooling history. Titanite grains have trace element chemistry mostly consistent with igneous origin and U‐Pb ages lining up with minor zircon age populations in the Ordovician‐Silurian and the Middle Devonian, suggesting that these magmatic episodes produced metaluminous compositions. These results show that combining trace element fingerprinting with dating and analyzing multiple detrital mineral species provide a more complete portrait of the geologic history of the sediment source region than U‐Pb dating of zircon alone.

Proof of recycling in clastic sedimentary systems from textural analysis and geochronology of detrital monazite: Implications for detrital mineral provenance analysis

Proof of recycling in clastic sedimentary systems from textural analysis and geochronology of detrital monazite: Implications for detrital mineral provenance analysis

Provenance analysis of detrital monazite, zircon and Cr‐spinel in the northern Tibetan Plateau: Implications for the Paleozoic tectonothermal history of the Altyn Tagh and Qimen Tagh Ranges

AbstractSediment provenance studies have proven to be an effective method to extract the sediment provenance and tectonic process information recorded by detrital minerals. In this contribution, we conducted detrital monazite and zircon U‐Pb geochronology and detrital Cr‐spinel major element chemistry analyses on samples from the Qaidam Basin to reconstruct the spatial and temporal evolution of the Altyn Tagh Range and the Qimen Tagh Range in the northern Tibetan Plateau. Based on the significant variation in [Th/U]N, [Gd/Lu]N and [Eu/Eu*]N and the U‐Pb ages of the monazite and zircon, the South Altyn Tagh subduction‐collision belt and the North Qimen Tagh Range were, respectively, the main provenances of the Ganchaigou section and the Dongchaishan‐Weitai section in the Qaidam Basin in the Cenozoic. Paleozoic peak metamorphism, retrograde granulite‐facies metamorphism and amphibolite‐facies metamorphism in the South Altyn Tagh subduction‐collision belt were well recorded by the detrital monazite. In comparison, the detrital zircon is a better indicator of igneous events than detrital monazite. Synthesizing the detrital monazite, zircon and Cr‐spinel data, we concluded that the South Altyn Tagh Ocean and Qimen Tagh Ocean existed in the early Paleozoic and that the Altyn Tagh terrane and Qimen Tagh terrane experienced different Paleozoic tectonothermal histories. The collision between the Qaidam terrane and the Azhong terrane occurred at ca. 500 Ma. The Middle Ordovician was the key period of transformation from the collision‐induced compressional environment to an extensional environment in the area of the South Altyn Tagh Range. In the early Paleozoic, the Qimen Tagh area was characterized by the subduction of oceanic crust.

Geochemistry and provenance of the Turquoise Bluff Slate, northeastern Tasmania: tectonic significance

The Ordovician Turquoise Bluff Slate in northeastern Tasmania is a 2 km-thick sequence of deep-marine siliceous black slates. It is dominated by meta-siltstones with bimodal grainsize distributions typical of turbidite TE-1 and TE-2 facies. The slates have high SiO2 indicating they are hemipelagites. The high Ba and V indicate they were deposited in an anoxic environment associated with high oceanic productivity. All these features are common in muddy turbidites. U–Th–Pb dating of detrital monazite and authigenic xenotime in the slates supports previous evidence that the dominant cleavage, in this unit, formed during the Benambran Orogeny. The whole-rock composition of the slates is similar to black slates in the Adaminaby Group, NSW. A review of Paleozoic whole-rock compositions from the Lachlan Orogen confirms they all have trace element contents similar to average Australian shale. However, there are subtle differences in composition. The Turquoise Bluff Slate and other Mathinna Supergroup rocks from the Eastern Tasmania Terrane have higher average Cr content than similar age turbidites from Victoria and NSW. This probably reflects a small contribution from Tasmania Cambrian ultramafic rocks in the provenance. If this were correct, northeastern Tasmania was closer to western Tasmania in the Paleozoic than other provinces of the Lachlan Orogen, southeastern Australia. Other subtle features of the whole-rock composition of Paleozoic sedimentary rocks from the Lachlan Orogen indicate it may be possible to recognise provincial variations in composition that will provide new constraints on tectonic models of southeastern Australia.

Pressure-temperature-time evolution of a Variscan garnet-bearing micaschist from the northeastern Fichtelgebirge, NW Bohemian Massif in central Europe

The studied micaschist from the northeastern Fichtelgebirge crystalline complex (FC) contains mm-sized garnet with a foam structure. Staurolite formed at the margin of garnet. Relics of staurolite are enclosed in mm-sized andalusite porphyroblasts. Garnet exhibits prograde concentric zonation with the pyrope component increasing from 1 to 6 mol%. Cores of potassic white-mica grains locally contain Si contents up to 3.15 per formula unit (pfu). The majority of this mica is characterized by Si contents close to 3.00 pfu. Pressure-temperature (P-T) pseudosections, calculated for the micaschist, indicate conditions of 10 kbar and 505 °C for an early metamorphic stage which was followed by a pressure decrease to 5 kbar and 535 °C. Late metamorphic P-T conditions recorded by garnet are around 4.5 kbar and 565 °C, compatible with the presence of staurolite. Andalusite might have metastably formed at this stage or somewhat lower pressures. The U-Th-Pb dating of monazite with the electron microprobe yielded subordinate ages younger than 315 Ma which were assigned to the nearby emplacement of granites in the FC. Some ages older than 395 Ma were related to detrital monazite in the sedimentary protolith of the micaschist. An age cluster at 384.5 ± 1.8 (2σ) Ma is preferentially assigned to the early high-pressure (HP) metamorphism resulting from the collision of Laurussia with a Peri-Gondwanan terrane. Exhumation to 15–20 km depths might have ended at 362.3 ± 1.0 Ma, but possibly the HP metamorphism occurred at this age and the 384.5 Ma age must be then referred to the provenance area of detrital monazite. The age cluster at 325.0 ± 0.7 Ma is related to a second collisional event that caused the overriding of the crystalline nappes of the FC onto the Laurussian crust.

Depositional age and correlation of the Oonah Formation: refining the timing of Neoproterozoic basin formation in Tasmania

ABSTRACTThe Proterozoic Oonah Formation comprises a thick sequence of turbidites and mafic rocks that is widely exposed in western and northern Tasmania. The regional significance of the Oonah Formation is poorly understood as its contacts are faulted and estimates of its depositional age range from 1450 Ma to 700 Ma. Here we refine the depositional age of the Oonah Formation and clarify its relationship to other Proterozoic sequences in Tasmania. Magmatic apatite from the syn-sedimentary Cooee Dolerite has a 207Pb-corrected 238U/206Pb age of 733 ± 9 Ma that provides a robust estimate of the depositional age of the Oonah Formation. The Oonah Formation contains abundant 750 Ma detrital monazite and has detrital zircon age populations at 1800–1700 Ma, 1590 Ma, and 1450 Ma. The new age and provenance data support the interpretation that the Oonah Formation is a lateral equivalent of the base of the Togari Group and correlatives in Tasmania. We demonstrate that the stratigraphy and provenance of Neoproterozoic strata in Tasmania differ from time-equivalent sequences in Adelaide Rift Complex but are similar to the Pahrump Group (Death Valley, California) and the Cobham Formation (Transantarctic Mountains), which supports a southwest Laurentia—Tasmania—East Antarctica connection within supercontinent Rodinia.

REE-Th-U and Nd isotope systematics of monazites in magnetite- and ilmenite-series granitic rocks of the Japan arc: Implications for its use as a tracer of magma evolution and detrital provenance

Monazite [(Ce, La, Th) PO4], a common accessory mineral in low-Ca felsic rocks and a wide range of metamorphic rocks, is suitable for in situ U–Pb dating, rare earth element (REE) geochemistry, and Nd isotope tracer studies. To link monazite composition with igneous petrogenesis and differentiation, we carried out comprehensive LA-ICP-MS measurements of REE-Th-U abundances and Sm-Nd isotope ratios in monazites from magnetite-series (oxidized type) and ilmenite-series (reduced type) granitic rocks across the Japan arc. The data revealed systematic differences in monazite composition between magnetite-series and ilmenite-series samples, and between pegmatites and granites. In ilmenite-series granitic rocks, monazites from pegmatites showed larger negative Eu anomalies, lower light-REE/middle-REE ratios, and higher middle-REE/heavy-REE ratios than those from granites. These geochemical variations were attributed to significant fractional crystallization of feldspars, monazite, xenotime, and garnet during differentiation in relatively reduced peraluminous granitic magmas. In contrast, there was no remarkable difference in the REE fractionation pattern between magnetite-series pegmatites and granites. Furthermore, the magnitudes of the negative Eu anomalies in the magnetite-series monazites were smaller than those observed in the ilmenite-series samples. These features were interpreted to reflect the suppression of monazite, xenotime, and garnet fractionation and limited Eu incorporation into fractionating feldspars in relatively oxidized and non-peraluminous magmas. In addition, our data demonstrated intra-grain variations in REE-Th-U compositions in a monazite from the Masaki ilmenite-series pegmatite, despite its homogenous Nd isotope composition. These variations suggest that the core and rim of the grain crystallized from distinct magmas that were generated from similar crustal rocks, but were differentiated under different conditions. A comparison of the present data with previously reported data indicates that igneous monazites from granitic rocks are distinct from a wide range of metamorphic monazites in REE-Th-U composition, especially due to their larger negative Eu anomalies. This finding highlights the potential utility of monazite REE-Th-U systematics as a provenance indicator for detrital monazites.

Beach Monazites from Alleppey: A Window to the Trivandrum Block, Southern India

ABSTRACT Detrital monazites can be used to reconstruct the tectonothermal history of their provenance. Dating of beach monazites from Alleppey by EPMA U-Th-Pbtotal technique using the centroid method demonstrates that the ages recorded by even a small number of 28 analyses from a single thin section grain mount can potentially reveal a considerable portion of the tectonothermal history of the Trivamdrum block. Three monazite populations were identified that yielded ages 605 ± 9, 575 ± 23 and 548 ± 11 Ma, which have their more or less exact counterparts reported from the Trivandrum Block. One monazite grain yielded random Paleoproterozoic ages ranging from 1756 Ma to 1345 Ma representing similar ages which earlier have been interpreted as due to differential Pb-loss from &amp;gt;2000 Ma monazites during the late Neoproterozoic-Cambrian metamorphism.

Tracing crustal evolution by U-Th-Pb, Sm-Nd, and Lu-Hf isotopes in detrital monazite and zircon from modern rivers

Abstract Detrital zircon U-Pb age and Hf isotope studies are useful for identifying the chemical evolution of the continental crust. Zircon, however, is typically a magmatic mineral and thus often fails to document the timing of low-grade metamorphism, and its survival through multiple sedimentary cycles potentially biases the crustal evolution record toward older events. In contrast, monazite typically records metamorphic events and is less likely to survive sedimentary recycling processes, thus providing information not available by zircon. Here, we demonstrate that monazite apparently faithfully records the Sm-Nd isotope composition of the bulk rock and can therefore track the record of crustal evolution and growth, similar to Hf isotopes in zircon. We examine the utility of detrital zircon and monazite for studies of crustal evolution through a comparison of age and tracer isotope information using sediments from two large rivers draining the South China block (SCB). Monazite and zircon grains yield mostly Mesozoic and Paleozoic U-Pb ages and depleted mantle model age peaks at ca. 1900–1300 Ma, indicating that both minerals preserve similar, yet critical, information on the crustal evolution of the catchment area. In contrast, zircon yields abundant Neoproterozoic and older U-Pb ages with a very large spread of model ages, preserving a history strongly skewed to older ages. Based on the lack of known rocks of this age in the catchments, ancient zircon was likely sourced from sedimentary rocks within the catchment area. This combined data set presents a more complete history of crustal evolution and growth in the SCB and demonstrates the advantages of an integrated approach that includes both detrital monazite and zircon.

The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence

Geohistory of the Yunkai massif in South China Block is important in understanding the geodynamics for the build-up of this block during the Phanerozoic orogenies. To investigate this massif, we conduct EMP monazite and U-Pb zircon geochronological determinations on mineral inclusions and separate for seventeen samples in four groups, representing metamorphic rocks from core domain, the Gaozhou Complex (amphibolite facies, NE-striking) and the Yunkai Group (greenschist facies, NW-striking) of this massif and adjacent undeformed granites. Some EMP monazite ages are consistent with the NanoSIMS results. Monazite inclusions, mostly with long axis parallel to the cleavage of platy and elongated hosts, give distinguishable age results for NW- and NE-trending deformations at 244–236Ma and 236–233Ma, respectively. They also yield ages of 233–230Ma for core domain gneissic granites and 232–229Ma for undefomed granites. Combining U-Pb zircon ages of the same group, ~245Ma and ~230Ma are suggested to constrain the time of two phases of deformation. Aside from ubiquity of Triassic ages in studied rocks, ages of detrital monazite in the meta-sandstone match the major U-Pb zircon age clusters of the metamorphic rock that are largely concentrated at Neoproterozoic (1.0–0.9Ga) and Early Paleozoic (444–431Ma). Based on these geochronological data, Triassic is interpreted as representing the time for recrystallization of these host minerals on the Early Paleozoic protolith, and the also popular Neoproterozoic age is probably inherited. With this context, Yunkai massif is regarded as a strongly reactivated Triassic metamorphic terrain on an Early Paleozoic basement which had incorporated sediments with Neoproterozoic provenances. Triassic tectonic evolution of the Yunkai massif is suggested to have been controlled by converging geodynamics of the South China and Indochina Blocks as well as mafic magma emplacement related to the Emeishan large igneous province (E-LIP).