Fractionation Of Heavy Rare Earth Elements Research Articles

RARE-EARTH elements in the topsoils of a Russian industrial city: Sources and human health risk assessment

Research on rare-earth elements (REEs) in urban soils of Russian industrial cities is extremely limited. This study investigates the potential sources and human health risks of REEs contained in the topsoils of the industrial Russian city of Chelyabinsk. The study also takes into account natural sources of REE as the city is located on the border of granites (Urals) and sedimentary rocks (Western Siberia). We analyzed the concentration and distribution of REEs in the soils of four types of locations: residential courtyards, city parks, roadsides, and industrial locations. The total REE concentrations ranged from 44 to 255 mg/kg, with average concentrations of 140, 124, 113 and 89 mg/kg in the courtyards, roadsides, industrial location and city parks, respectively. The REE content in courtyard soils could be influenced by poor cleaning of fallen leaves. The ratios of light REE (LREE) to heavy REE (HREE) ranged from 9.5 to 10.1, revealing an obvious fractionation between them. The fractionation of LREE and HREE, and the REE/ES (European Shale) pattern showed that REE accumulation in Chelyabinsk soils has been disturbed by human activities. It was shown that the dust from industrial emissions was the main anthropogenic source of REE accumulation in urban soil. The largest amounts of REEs are emitted from an electrometallurgical plant and zinc production plant. Fortunately, the estimated daily intakes of REE from soils for children and adults were well below the safety thresholds. At the same time, in order to prevent social tension and reduce the anthropogenic load on the urban area, it is recommended to use phytoremediation technologies, smart landscaping of industrial and residential areas, more thorough cleaning of fallen leaves and road dust. It is also recommended to move the most dangerous production processes outside the urban area.

Ordovician−Early Devonian granitic magmatism as the consequence of intracontinental orogenic activity along the Qinhang belt in South China

The early Paleozoic tectono-magmatic activity within the South China block, which is well illustrated by Ordovician−Devonian granites in the western Qinhang belt, was the response to closure of the Proto-Tethys Ocean and convergence of continental blocks. The spatiotemporal distribution and source characteristics of the granites provide us the opportunity to understand the processes and driving mechanisms of intracontinental orogeny. As an example, the Miaoershan-Yuechengling granite batholith in northern Guangxi, located along the western margin of the Qinhang orogenic belt, is mainly composed of quartz monzonite and monzogranite. All the granitic rocks from Miaoershan-Yuechengling batholith are composed of K-feldspar, quartz, plagioclase, biotite, and hornblende. Geochronologic dating indicates that the Miaoershan-Yuechengling batholith was emplaced during the late Silurian and Early Devonian, respectively. The rocks have high SiO2, with an average value of 73.29 wt%, and total alkalis (Na2O + K2O = 7.21−10.03 wt%), but low Al2O3 (12.96−15.51 wt%), showing characteristics of the high-potassium calc-alkaline series of S-type peraluminous granites (Al2O3/[CaO + Na2O + K2O] = 1.03−1.22). Trace elements in the Miaoershan-Yuechengling granitic rocks are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements. Their rare earth element (REE) trends are characterized by relatively flat distribution patterns with weak light REE enrichment, weak heavy REE fractionation, and negative Eu anomalies. Zircons from the rocks have negative εHf(t) values ranging from −13.24 to −5.1, with crustal model ages (THf2) of 2.2−1.7 Ga. These features indicate that they are S-type granites with parental magmas originating from partial melting of sandy argillaceous sources of Paleoproterozoic lower continental crust. The thermal budget for Ordovician to Early Devonian magmatism is attributed either to crustal thickening in relation to intracontinental orogenic compression or to crustal thinning due to postorogenic tectonic extension during assembly and breakup of Greater Gondwana. This study reveals that the change in mantle convection systems during plate interactions acted as a major driving force for the early orogenic processes, late collapse of the orogenic belt, and massive syncollisional to postorogenic magmatism.

Recognising Mineral Deposits from Cover; A Case Study Using Zircon Chemistry in the Gawler Craton, South Australia

Detrital zircon grains preserved within clasts and the matrix of a basal diamictite sequence directly overlying the Carrapateena IOCG deposit in the Gawler Craton, South Australia are shown here to preserve U–Pb ages and geochemical signatures that can be related to underlying mineralisation. The zircon geochemical signature is characterised by elevated heavy rare-earth element fractionation values (GdN/YbN ≥ 0.15) and high Eu ratios (Eu/Eu* ≥ 0.6). This geochemical signature has previously been recognised within zircon derived from within the Carrapateena orebody and can be used to distinguish zircon associated with IOCG mineralisation from background zircon preserved within stratigraphically equivalent regionally unaltered and altered samples. The results demonstrate that zircon chemistry is preserved through processes of weathering, erosion, transport, and incorporation into cover sequence materials and, therefore, may be dispersed within the cover sequence, effectively increasing the geochemical footprint of the IOCG mineralisation. The zircon geochemical criteria have potential to be applied to whole-rock geochemical data for the cover sequence diamictite in the Carrapateena area; however, this requires understanding of the presence of minerals that may influence the HREE fractionation (GdN/YbN) and/or Eu/Eu* results (e.g., xenotime, feldspar).

Paleobasinal to band-scale REE + Y distribution in iron formations from Carajás, Amazon Craton, Brazil

Banded Iron Formations (BIFs) from the Carajás Mineral Province (CMP), southeastern Amazonian Craton, are the protolith of the world-class iron deposits of the CMP. Besides being one of the largest iron ore deposits in the world, they recorded important paleoenvironmental and regional geodynamic markers across the Neoarchean-Paleoproterozoic transition. We investigate the secular variation of geochemical signature, from several deposit of Carajás Province. The low contents of Al2O3 and of 19 other immobile trace elements (e.g., Zr, Th, Hf, Ti) in 26 high-resolution detritus-free BIF samples indicate their detritus- free nature. Their positive shale-normalized (subscript SN) La and Gd anomalies are similar to those of modern seawater; whereas their Y/Ho ratios are higher. These BIF samples also display large positive chondrite-normalized (subscript CN) Eu anomalies, reflecting the influence of high-temperature hydrothermal vents and bottom solutions. In contrast to modern seawater and to most other Neoarchean/Paleoproterozoic detritus-free BIFs, the Carajás BIFs do not show the typical heavy rare earth element (REE) enrichment relative to light REE; instead they do show variable light to heavy REE fractionation trends (YbSN/PrSN ratios: 0.05–4.91). The Th/U ratios of both bulk BIFs and mesobands are generally lower than the upper-crustal ratio. Although the bulk BIFs show mostly chondritic values, the individual Fe and Si bands exhibit distinctly higher ratios. The general absence of negative CeSN anomalies attests for rather reducing paleoenvironmental conditions. Unlike the great majority of the data which lack negative anomalies of Ce, a few samples do display such anomalies, which may be compatible to the oxidizing conditions shown by the low Th/U ratios of most the Carajás BIFs. Such low Th/U ratios may register terrestrial weathering conditions, which cause preferential U input into the oceans in response to the oxidation of immobile U4+ to more mobile U6+.

Geochemistry of NW-SE trending Palaeoproterozoic mafic dyke intrusions in the Bundelkhand Craton, India and subcontinental lithospheric mantle processes

NW-SE trending mafic dykes in the Bundelkhand Craton of the Indian shield manifest Palaeoproterozoic igneous activity. These dykes are Fe-rich tholeiitic basalts with compositions varying from near primary melts to more evolved magmas (Mg#: 0.64–0.35) and show enrichment of large-ion lithophile and light rare earth elements relative to primordial mantle values. Despite very subtle variations of elemental abundances and similar structural trend, at least two groups are identified, mainly based on incompatible element ratios and rare earth element patterns. Group 1 samples are characterized by relatively low Ti/Y (av. 222 ± 22) and Ti/Nb (av. 609 ± 108), high Zr/TiO2 (av. 117 ± 15), more light to heavy rare earth element fractionation (La/Yb)CN: 2.7–5.1) and mild negative Eu anomaly (Eu* = 0.77 ± 0.10). Group 2 samples possess high Ti/Y (av. 334 ± 46) and Ti/Nb (av. 1349 ± 198), low Zr/TiO2 (av. 70 ± 7) and minor light to heavy rare earth element fractionation ((La/Yb)CN: 1.5–2.7) without any Eu anomaly (Eu* = 0.97 ± 0.04). Both groups show relative depletions in Nb, Sr and P while the Group 1 also shows Ti depletion. A third group is apparent but is less certain. Group 2 dykes constitute the 1.98 Ga Jhansi swarm, whilst the Group 1 and unclassified samples are likely to be of older age (~2.18–2.20 Ga and ~2.37 Ga) based on available U-Pb ages and palaeomagnetic considerations. Despite the compositions indicate different batches of magma, the dykes have near similar petrogenetic evolutionary pattern as if the entire population constitutes a single clan. Samples of both groups (a) do not indicate significant crustal contamination (b) derived from two different batches of magmas formed by ~10–12% fractional melting of mantle near the spinel-garnet transition zone and (c) evolved through initial fractionation of olivine and thereafter clinopyroxene and plagioclase became important fractionation phases. The more primitive compositions of both groups indicate mantle potential temperatures of ~1550–1600 °C suggesting thermal anomaly. The dyke magmas inherited variably enriched compositions as a result of interaction of upwelling mantle melts with metasomatised subcontinental lithospheric mantle. Low H2O-CO2-rich silicate melts/fluids are likely the dominant metasomatic agents to develop the Indian subcontinental lithosphere in the Archaean (c. 3.0 Ga).

Early Cretaceous redbeds from the Minle Basin, Hexi Corridor, northwest China: Mineralogy and geochemistry implications for paleoweathering, provenance, and tectonic settings

Redbeds with a large thickness in the lower Cretaceous record abundant geologic information in the Minle Basin. We have conducted the paleoweathering conditions, provenance, and tectonic settings based on mineralogy and geochemistry. Our results indicate that mudstone samples are characterized by abundant illite with negligible amounts of K-feldspars and analcime. The lower part of the lower Cretaceous is rich in quartz, whereas the upper part is dominated by dolomite and analcime. We suggest that this is caused by the decreasing input of the clastic influx during the middle-late early Cretaceous. High index of compositional variation values (average 1.33) indicate first-cycle sediment supply, suggesting an overall compositional immaturity and short-distance transportation. These characteristics are consistent with an active regional extension tectonic setting. The [Formula: see text] system ([Formula: see text];[Formula: see text];[Formula: see text]) and Th/U versus Th consistently reveal that the lower Cretaceous experienced a positive gradient in chemical weathering from young to old formations. Although the patterns of trace elements in three formations of the lower Cretaceous are different, those of the rare earth elements (REEs) tend to be consistent. The significant enrichment of light REEs, heavy REEs fractionation, and distinctive negative Eu anomalies suggest derivation from an old, upper continental crust composed of predominantly felsic sediments. This interpretation is supported by several discrimination diagrams such as titanium dioxide-nickel ([Formula: see text]), which shows the characteristics of immature recycled sediments. A few sensitive elements, ratios, and normalized REE patterns indicate a provenance of an active continental margin and a continental island arc (CIA). The La-Th-Sc, Th-Co-Zr/10, and Th-Sc-Zr/10 discrimination plots further confirm the CIA signature. Thus, we conclude that the early Cretaceous redbeds in the Minle Basin, Hexi Corridor, were deposited in a dustpan-shaped half-graben basin in a CIA setting when northwest China was influenced by intense regional extension.

Petrogenesis of the Early Cretaceous Kada igneous rocks from Tethyan Himalaya: Implications for initial break‐up of eastern Gondwana

Early Cretaceous magmatic rocks associated with the Kerguelen mantle plume are widespread in the Comei area with a chronology concentrated from 134 to 130 Ma. However, less data for the earliest Early Cretaceous magmatic rocks cropping out in the Comei area have been reported. The earliest Early Cretaceous magmatic rocks from the Kada area comprise mafic rock (diabase) and felsic rocks (granite, dacite, and quartz monzonite). This paper presents a detailed LA‐ICP‐MS zircon U–Pb chronology with major and minor (trace) elements and Lu–Hf isotope data from Kada igneous rocks to identify the extent of the early activity of the Kerguelen mantle plume and the initial break‐up of eastern Gondwanaland. The LA‐ICP‐MS zircon U–Pb dates reveal that the age of Kada magmatic rocks range from 136.9 to 143.5 Ma. Kada mafic rocks exhibit both OIB (Samples KD3 and KD06) and N‐MORB (Sample KD7‐1) characteristics. The N‐MORB‐type mafic rocks were derived from partial melting of depleted spinel lherzolite. However, the OIB‐type mafic rocks were derived from partial melting (~60–80 km) of spinel + garnet lherzolite. Kada felsic rocks show obvious enrichment of light rare earth elements and negative Eu anomaly features. Among these, dacite and quartz monzonite rocks show flat heavy rare earth elements and clear Nb, Ta, P, and Ti negative anomalies with depleted εHf(t) characteristics (−11.0 to −21.0). This is indicative of partial melting of specific crustal rocks in an intraplate environment caused by underplating of coeval basaltic magmas. However, granite is characterized by high heavy rare earth element fractionation and high and dispersed εHf composition (−1.5 to +8.9), thus indicating that granite is derived from extensive fractional crystallization of mantle‐derived melts from an AFC process. The Kada felsic rocks show A‐type granitic characteristics in an extensional environment. Based on the evidence from palaeomagnetism reconstructions, Kada rocks and Bunbury basalts are both distributed in and influence the scope of the Kerguelen mantle plume. They are also coeval with the initial break‐up of eastern Gondwana. We suggest that Kada magmatic rocks in southern Tibet represent the earliest activity of the Kerguelen mantle plume and that they played a role in the initial break‐up of eastern Gondwana.

Petrogenesis of Mesozoic volcanic rocks in the Zha'ertai area, Inner Mongolia: Constraints from trace elements and Sr–Nd–Pb isotopes

The Mesozoic acid volcanic rocks of the Zha'ertai area, located in Inner Mongolia, were formed in the late Early Cretaceous and resulted in polymetallic deposits of gold, silver, and other precious metals with huge commercial value. The more developed acid volcanic rocks include subvolcanic and pyroclastic rocks and volcanic lava, with SiO2 of 73.18–83.82%, Na2O + K2O of 6.17–9.03%, and K2O/Na2O of 1.28–70.27%. These rocks are enriched in large‐ion lithophile elements and high‐field strength elements. They are also enriched in light rare earth elements (REEs), with light to heavy REE ratios of 2.91–14.30; light to heavy REE fractionation is significant, and there are obvious negative Eu anomalies (δEu = 0.02–0.25). The acid volcanic rocks have extremely low TiO2, MgO, P2O5, and TFe2O3 contents, exhibit significant enrichment in Th, U, and Pb, and are strongly depleted in Ba and Sr, showing the trace elements feature of A‐type rhyolite. The rocks have smaller (87Sr/86Sr)i, lower εNd (t), younger TDM2, lower (206Pb/204Pb)i,(207Pb/204Pb)i, and (208Pb/204Pb)i. The acid volcanic rocks differ considerably from the intermediate to basic volcanic rocks in geochemical composition, and there is a compositional hiatus present in the succession. This indicates that neither endmember of these Mesozoic bimodal volcanic rocks is cogenetic and that rhyolitic rocks might not be a product of basaltic magma crystallization differentiation. Instead, they may result from partial melting of crust‐derived rock, stemming from partial melting of rocks in the pre‐existing crust underplated by mantle‐derived magma. This suggests that the acid volcanic rocks were formed in a lithospheric extensional setting during the Early Cretaceous.

Geochronology and geochemistry of late Mesozoic volcanic rocks in the Wenkutu area, Great Xing'an Range, China

This paper reports on the petrography, geochemistry, and geochronology of late Mesozoic volcanic rocks in the Wenkutu area of the northern Great Xing'an Range, North‐east China, and discusses the formation age, petrogenesis, and tectonic environment of the Manketouebo, Manitu, Baiyingaolao, and Meiletu formations. Results of Zircon U–Pb dating show that the volcanic rocks in the Wenkutu area were formed during the Early Cretaceous (Manketouebo Formation at 145 ± 1 Ma, Manitu Formation at 141.3 ± 1.7 Ma, Baiyingaolao Formation at 128.7 ± 1.1 Ma, and Meiletu Formation at 129 ± 1 Ma). The Manketouebo, Manitu, and Baiyingaolao formations are dominated by intermediate to acid volcanic rocks that belong to the high‐K calc‐alkaline series, with Eu/Eu* values of 0.38–0.84.They are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE). The Meiletu Formation is dominated by intermediate to basic volcanic rocks, belongs to the high‐K calc‐alkaline series, has Eu/Eu* values of 0.89–0.97, and is enriched in Ba, K, Th, U, La, Ce, Sr, Nd, and Hf but relatively depleted in Nb, Ta, Zr, P, and Ti, with no obvious fractionation of LREE and HREE. Results indicate that the magma of the Manketouebo, Manitu, and Baiyingaolao formations had a crustal origin, whereas the Meiletu Formation was sourced from the mantle and subsequently contaminated by crustal materials. The late Mesozoic volcanic rocks in the study area were formed during an orogenic stage, and the compressional environment was caused by subduction of the paleo‐Pacific Plate below the Asian continental plate.

Determination of trace elements and lanthanide (REE) signatures in uranium mine products in South Africa by means of inductively coupled plasma mass spectrometry

The lanthanides are Rare Earth Elements (REE) that exhibit consistent patterns under varying geochemical conditions. Therefore they can be used as signatures to determine the chemical and physical change that has occurred on a uranium ore since it was mined and processed. The objective of this work was to determine the trace elements and REE (Lanthanides patterns) in uranium ore samples using a quadrupole inductively coupled plasma mass spectrometer. The REE fingerprint results presented here suggests that uranium is embedded in uraninite, phosphorite deposit in Apatite & fluorapatite minerals. The uranium is recovered from phosphoric acid, while lime used for precipitation of impurities and magnesia is used for precipitation of uranium. The REE patterns normalized to the mean of CI-Chondrites from this study, show LREE uniformity and HREE fractionation with strong positive Tb anomaly. The ΣLREE/ΣHREE, and La/Sc ratios are relatively constant in these samples. The average composition based on this study are(in μgg−1): Sc=0.000171±0.000029, Y=0.00256±0.000046, La=0.00736±0.00206, Ce=0.00648±0.00213, Pr=0.00714±0.0058, Nd=0.00462±0.00163, Sm=0.0493±0.00328, Eu=ND, Gd=0.0125±0.00643, Tb=30±13, Dy=0.0165±0.00747, Ho=ND, Er=0.00515±0.00265, Tm=ND, Yb=ND and Lu=ND. The REE pattern in the fissure water from underground typically shows enrichment in light REE (LREE), depletion in heavy REE (HREE). ND=not detected.Average values for the trace elements were: Al (567.11), Cr (4.42), Cu (0.47), Fe (1781.69) and Zn (0.46) mg/L. Based on this elemental concentration, Al, Cr, Cu and Zn are at trace level specifically for this site. This family of elements is indicative of a carry through from a precursor ore. V (0.25), Ti (1.13) and Fe represent a type of in-situ leach solution. The uranium concentrations in the Carletonville gold fields suggests strongly that gold and uranium, in detrital pyrites and uraninite, were hydrothermally recycled (uranium may have been recycled by hydrocarbon fluids) from the Witwatersrand Reefs. Also the U concentrations in the water samples is much higher than that in the tailing (except for tailing 4 which is 100years old), and is a signature for uranium processing. Also the results show a Tb-U correlation signature distinguishing between raw mineral ore and tailing (processed) uranium samples. The U concentrations are between 1.40 and 6.24ppm while that of Tb is between 0.84 and 1.58ppm in the fissure water.

Geochemical and Hf isotopic compositions of Late Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China: petrogenesis and tectonic implications

ABSTRACTLate Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China, are located along the southern margin of the Mongol–Okhotsk orogenic belt. They comprise granodiorite, monzogranite, syenogranite, and lesser gabbro–diorite, of adakitic and calc­alkaline affinity. The adakite-like and calc­alkaline granites share similar light rare earth elements (LREE) characteristics; however, their heavy rare earth elements (HREE) trends differ from one another. The relative abundances of HREE in the calc­alkaline granites are relatively consistent and are similar to those of intrusive rocks formed from dehydration melting of garnet-free amphibolitic source rocks at relatively low pressures. In contrast, the adakite-like granites show more prominent HREE fractionation trends, indicating that they crystallized at higher pressures, where garnet in the source rocks was stable. At least two isotopically distinct sources were involved in the petrogenesis of the granites, but the extent to which they contributed varies between plutons. Most intrusions have incorporated an isotopically primitive component, possibly juvenile mafic crust. The other sources include a small proportion of old continental crustal material and isotopically evolved wall rocks. The gabbro–diorites have high MgO contents (>7 wt.%), a high Mg# (>0.6), and show moderate LREE and HREE fractionation, indicating they formed from the melting of subducted metasomatized lithospheric mantle. All of the intrusions in the study area are characterized by a relative enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicating they were emplaced in an Andean-type active continental margin setting related to southward subduction of the Mongol–Okhotsk oceanic plate.

Ages and tectonic implications of Neoproterozoic ortho- and paragneisses in the Beishan Orogenic Belt, China

The Beishan Orogenic Belt (BSOB) in China plays a pivotal role in understanding the tectonic evolution of the southern Central Asian Orogenic Belt. Despite numerous studies primarily emphasizing on Phanerozoic events, the existence and significance of Precambrian rocks in the BSOB still remain controversial. Here we report new geochemical and zircon U–Pb–Hf isotopic data for the Gubaoquan ortho- and paragneisses in the BSOB and the comparable Xingxingxia orthogneisses in the Central Tianshan Orogenic Belt to constrain their age, petrogenesis and tectonic implications.New LA-ICPMS U–Pb zircon dating results of zircon reveal that the protoliths of the Gubaoquan orthogneisses crystallized at 905–871Ma, while the Gubaoquan paragneisses yielded an age peak at 1040–910Ma with a maximum depositional age of ∼871Ma. They are the oldest rocks discovered in the BSOB, mainly distributed in the southern units and interpreted as primary sources for the local Phanerozoic igneous and sedimentary rocks. The intrusion age of the Xingxingxia orthogneisses was ∼1014 Ma. All the Gubaoquan and Xingxingxia orthogneisses show low A/CNK, Rb/Sr and Rb/Ba ratios and similar depletion of Nb, Ta and Ti, akin to I-type granitoids related to subduction. However, the former have much lower Sr contents and Sr/Y ratios, lower light to heavy rare earth element fractionation (LREE/HREE=4.43–10.12), and weaker negative Eu anomalies, related to a back-arc setting at low pressures, whereas the latter formed at high pressures in a continental arc setting. Zircon Hf-isotope compositions in the Gubaoquan ortho- and paragneisses exhibit a large range of ɛHf(t) values from −3.77 to +5.29, and two-stage model ages (TCDM) of 1.98–1.44Ga, while those in the Xingxingxia orthogneisses are clustered with slightly higher ɛHf(t) values (+4.02 to +8.53) and younger TCDM ages (1.60–1. 34Ga). The protoliths of both the Gubaoquan and Xingxingxia orthogneisses were derived from the mixing of basaltic magmas and those derived from the partial melting of meta-greywackes. Based on a review of Neoproterozoic events, the BSOB tends to closely link with the Chinese Tianshan Orogenic Belt and Tarim Craton. They were most likely located in the periphery of the already assembled Rodinia supercontinent.

Zircon U-Pb ages, geochemistry, and Sr-Nd-Pb-Hf isotopic compositions of the Pinghe pluton, Southwest China: implications for the evolution of the early Palaeozoic Proto-Tethys in Southeast Asia

The geological record of the Neoproterozoic to early Palaeozoic Proto-Tethyan Ocean in Southeast Asia is not clear. To better constrain the evolution of the Proto-Tethys, we present new geochronology, geochemistry, and petrology of the late Cambrian to Ordovician Pinghe pluton monzogranite from the Baoshan block, western Yunnan, southwest China. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of four zircon samples yield ages of 482–494 and 439–445 Ma for the pluton, interpreted as two episodes within one magmatic event accompanying the whole process of subduction–collision–orogeny between buoyant blocks and oceanic crust of the Proto-Tethys. The monzogranite belongs to the strong peraluminous, high-K, calc-alkaline series and shows characteristics of both I-type and S-type granitic rocks. It is characterized by extremely high Rb/Sr and Rb/Ba but low TiO2, MgO, FeOt, and CaO/Na2O ratios. The monzogranite is also moderately enriched in light rare earth elements (LREEs), depleted in heavy rare earth elements (HREEs), lacks HREE fractionation, and has strongly negative Eu (Eu/Eu* = 0.06–0.49), Ba, Nb, Ta, Sr, and Ti anomalies. Whole-rock εNd(t) and εHf(t) values range from −8.7 to −11.6 and −5.55 to −9.58, respectively. Nd and Hf two-stage model ages range from 1.66 to 2.06 Ga and 2.14 to 3.00 Ga, respectively, with variable radiogenic 206Pb/204Pb(t) (16.547–18.705), 207Pb/204Pb(t) (15.645–15.765), and 208Pb/204Pb(t) (38.273–38.830). These signatures suggest that the monzogranite magma was derived from partial melting of heterogeneous metapelite, which was generated from Neoarchean to Palaeoproterozoic materials mixed with basaltic magma. The monzogranite magma underwent crystallization differentiation of plagioclase, K-feldspar, and ilmenite. Magmatism to form the Pinghe pluton occurred in a post-collisional setting. Based on the comparison of coeval granites throughout adjacent regions (e.g. Himalayan orogen, Lhasa Terrane, and parts of Gondwana supercontinent), we propose that the Baoshan block was derived from the northern Australian Proto-Tethyan Andean-type active continental margin of Gondwana and experienced subduction of the Proto-Tethyan oceanic crust and accretion of an outboard micro-continent. The Pinghe pluton could have formed when a subducting oceanic slab broke off during collision.

Geochemistry and mineralizations of the Wadi Ghadir younger granites and associated pegmatites, South Eastern Desert, Egypt

The study area is covered by quartz diorites, monzogranites, and pegmatites. Geochemically, the monzogranites and pegmatites have peraluminous to slightly metaluminous calc-alkaline to alkaline affinity, emplaced in volcanic arc setting and crystallized under moderate water-vapor pressure (3–5 kb) and temperature of 800–850 °C for monzogranites and 700–750 °C for pegmatites. The monzogranites reveal small to moderate negative Eu anomaly (Eu/Eu* = 0.62 to 1.0) and averages Eu/Sm = 0.168 and light rare earth elements (LREEs)/heavy rare earth elements (HREEs) = 48.78. The pegmatites reveal moderate to large negative Eu anomaly (Eu/Eu* = 2.39 to 2.96) patterns and have averages Eu/Sm = 0.075 and LREEs/HREEs = 32.22. The monzogranites are altered along two fracture zones exhibiting propylitic, silicification and record high radioactive measurements. These alterations cause enrichment in SiO2, Fe2O3 t, Ni, Y, V, Co, Th, and REEs except Er and depletion in Al2O3, TiO2, CaO, MgO, Na2O, K2O, P2O5, Cu, Zn, Cr, Zr, Rb, Ba, Pb, Sr, Nb, Ga, Hf, Cs, Li, Sn, Be, Sc, U, and Ta. The normalized chondrite pattern denotes moderate REEs fractionation and shows a narrow range fractionation of HREEs in both fresh, altered monzogranites and pegmatites. The studied fresh and altered monzogranites do not show tetrad effect, while the pegmatites show tetrad effect. The average eU content in fresh monzogranites is 5.55 parts per million (ppm), and the average U content is 5.03 ppm, whereas the average eTh content is 18.93 ppm, and the average Th content is 20.90 ppm. The average eU content in pegmatites is 14.76 ppm, and the average U content is 4.75 ppm, whereas the average eTh content is 57.02 ppm, and the average Th content is 15.30 ppm. The eU and eTh in the alteration zones reach up to 16.50 and 296.20 ppm, respectively. These high radioactivities are attributed to the presence of thorite, bastnasite, fluorite, and columbite.

Petrography, magnetic susceptibility and geochemistry of the Rio Branco Granite, Carajás Province, southeast of Pará, Brazil

The Paleoproterozoic Rio Branco stock intrudes the RESUMO: Petrografia, suscetibilidade magnetica e geoquimica do Archean Cruzadao biotite monzogranite. It occurs west of the Canaa dos Carajas city, close to the Sossego copper mine in the Carajas Prov- ince. It is constituted by undeformed and isotropic, hololeucocratic syenogranites, showing equigranular texture. The granite is formed essentially by perthitic alkali feldspar, quartz, and plagioclase, with additional variable amounts of chloritized biotite. Fluorite, allanite, and zircon are common accessory minerals, whereas pyrite and chal- copyrite are scarces. Albitization and subordinate greisenization are the main alteration processes that affected the granite. The secondary mineralogy is represented by albite, fluorite, topaz, chlorite, musco- vite, siderophyllite, and iron oxides. Magnetic susceptibility values are systematically low and vary between 1.3 x 10 -5 a 6.9 x 10 -4 (SI). The Rio Branco Granite is metaluminous to peraluminous and shows high FeOt/(FeOt + MgO). It has geochemical affinities with ferroan, reduced A2-subtype granites. The rare earth element (REE) patterns are flat with little heavy REE fractionation and show accentuated negative Eu anomalies (Eu/Eu* = 0.08 – 0.13). These characteristics are typical of evolved granites, derived from volatile-enriched liq- uids responsible by the intense subsolidus hydrothermal alteration. The dating of the Rio Branco granite did not give conclusive results, but the obtained data, associated with geological evidence, suggest that it is of Paleoproterozoic age. The comparison between the Rio Bran- co granite and Paleoproterozoic A-type suites of the Carajas Province suggests that the Rio Branco granite has more affinity with the Velho Guilherme suite and, in lesser degree, with the Serra dos Carajas suite. On the other hand, is clearly different from oxidized A-type granites of the Jamon suite. Despite its similarities with tin-specialized gran- ites, the Rio Branco Granite is not associated with tin mineralizations.

Partitioning of rare earth elements between an aqueous chloride fluid phase and melt during the decompression-driven degassing of granite magmas

This paper reports the results of numerical simulation for the behavior of rare earth elements (REE) during decompression degassing of H2O- and Cl-bearing granite melts at pressures decreasing from 3 to 0.5–0.3 kbar under near isothermal conditions (800 ± 25°C). Fluid phase in equilibrium with the melt contains mainly chloride REE complexes, and their behavior during magma degassing is, therefore, intimately related to the behavior of chlorine. It was shown that the contents and distribution patterns of REE in the aqueous chloride fluid phase formed during decompression vary considerably depending on (1) the contents of volatiles (Cl and H2O) in the initial melt, (2) the redox state of the magma, and (3) the dynamics of fluid phase separation from magmas during their ascent toward the Earth’s surface. During decompressiondriven degassing, the contents of both Cl and REE in the fluid decrease, especially dramatically under opensystem conditions. The REE patterns of the fluid phase compared with those of the melt are characterized by a higher degree of light to heavy REE fractionation. A weak negative Eu anomaly may be present in the REE patterns of Cl-rich fluids formed during the early stages of degassing at relatively high pressures. At a further decrease in pressure and Cl content in the fluid, it is transformed into a positive Eu anomaly increasing during decompression degassing. Such an anomalous behavior of Eu during degassing is related to its occurrence in magmatic melts in two valence states, Eu3+ and Eu2+, whereas the other REE occur in melts mainly as (REE)3+. The Eu3+/Eu2+ ratio of melt is controlled by the redox state of the magmatic system. The higher the degree of melt reduction, the more pronounced the anomalous behavior of Eu during decompression degassing. The amount of REE extracted by fluid from melt during various stages of degassing does not significantly influence the content and distribution patterns of REE in the melt.

Mobility and fractionation of rare earth elements during supergene weathering and gossan formation and chemical modification of massive sulfide gossan

Primary massive sulfide gossans (MSG) in the Bathurst Mining Camp (BMC), New Brunswick, Canada, are characterized by relative enrichment of Au, Sb, and As, formation of jarosite group minerals (jarosite, plumbojarosite, and argentojarosite) and little or no fractionation in the rare earth elements (REE), including preservation of large positive Eu anomalies (average [Eu/Eu*] NASC = 4.14 in MSG; 6.61 in massive sulfide mineralization; 0.60 in host rocks). The chemical and mineralogical characteristics of MSG (e.g., Halfmile Lake deposit) imply low pH (<3) and relatively oxidizing conditions during gossan formation; oxidation of a volcanogenic massive sulfide body (comprising pyrite, pyrrhotite, sphalerite, galena, and chalcopyrite) with a falling water table. The lack of light REE or heavy REE fractionation and preservation of positive Eu anomalies characteristic of the original (465 Ma) hydrothermal fluid is consistent with relatively large water–rock ratios during massive sulfide mineralization oxidation, and removal of the REE predominantly as sulfate complexes (LnSO 4 +, Ln(SO 4) 2 −). Low pH groundwaters recovered from past producing mines in the BMC display REE patterns reflecting those inferred to have occurred during gossan formation. Gossan at the Restigouche deposit, in contrast to the Halfmile Lake deposit, displays mineralogical and chemical evidence for having been chemically reworked since primary gossan formation. Evidence for chemical reworking includes loss of primary massive sulfide mineralization textures, replacement of plumbojarosite with anglesite, almost complete removal of jarosite minerals, loss of Au, Sb, and As and apparent preferential removal of Eu, resulting in loss of positive Eu anomalies for most samples (average [Eu/Eu*] NASC = 1.21 in the gossan, with many displaying strong negative anomalies; 3.65 in massive sulfide mineralization; 0.54 in host rocks). Based on geochemical modeling, conditions inferred for the chemical reworking of the Restigouche deposit include near neutral conditions and either relatively oxidizing conditions with Eu 2+ hosted in a preferentially weathered mineral host (possibly through substitution for Pb in plumbojarosite and beudantite) or cycling between reduced and oxidized conditions during gossan reworking.

Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Tertiary volcanic rocks from the Westerwald region range from basanites and alkali basalts to trachytes, whereas lavas from the margin of the Vogelsberg volcanic field consist of more alkaline basanites and alkali basalts. Heavy rare earth element fractionation indicates that the primitive Westerwald magmas probably represent melts of garnet peridotite. The Vogelsberg melts formed in the spinel---garnet peridotite transition region with residual amphibole for some magmas suggesting melting of relatively cold mantle. Assimilation of lower-crustal rocks and fractional crystallization altered the composition of lavas from the Westerwald and Vogelsberg region significantly. The contaminating lower crust beneath the Rhenish Massif has a different isotopic composition from the lower continental crust beneath the Hessian Depression and Vogelsberg, implying a compositional boundary between the two crustal domains. The mantle source of the lavas from the Rhenish Massif has higher 206 Pb/ 204 Pb and 87 Sr/ 86 Sr than the mantle source beneath the Vogelsberg and Hessian Depression. The 30---20Ma volcanism of the Westerwald apparently had the same mantle source as the Quaternary Eifel lavas, suggesting that the magmas probably formed in a pulsing mantle plume with a maximum excess temperature of 100 � C beneath the Rhenish Massif. The relatively shallow melting of amphibole-bearing peridotite beneath the Vogelsberg and Hessian Depression may indicate an origin from a metasomatized portion of the thermal boundary layer.

Paleoproterozoic arc magmatism imposed on an older backarc basin: Implications for the tectonic evolution of the Trans-Hudson orogen, Canada

Compositional data from 72 granitoids and associated mafic rocks in the Paleoproterozoic Kisseynew domain, Trans-Hudson orogen, Canada, were used to evaluate the tectonic setting of magma generation between 1840 and 1820 Ma. These data thus provide insights into the development of the Trans-Hudson orogen just prior to amphibolite-grade metamorphism and the final stages of continental collision. The granitoid rocks are hornblende- or orthopyroxene-bearing, medium- to high-K calc-alkalic, and weakly peraluminous. The trends on Harker variation diagrams are consistent with crystal-fractionation processes, although the range in values is related to the temporal and spatial distribution of samples. The extent of LREE (light rare earth element) enrichment increases with increasing SiO 2 , as does variability in HREE (heavy rare earth element) fractionation and magnitude of Eu anomalies. Most samples have (Gd/Yb) n < 10 (suite Ia), although five have (Gd/Yb) n = 17-24, suggesting the presence of garnet residua (suite Ib). The range in rock types, calc-alkalic character, and negative Nb and Ti anomalies are all characteristic of arc environments. Magma generation within the Trans-Hudson orogen, including the Kisseynew domain, between 1.84 and 1.82 Ga was related to interaction between the Sask craton and the Flin Flon-Glennie complex. A decrease in subduction angle likely resulted from northward-directed subduction of more buoyant crust, with associated changes in thermal conditions leading to melting over a wide area. Most magmas formed in a slab-dehydration-dominated regime, although localized slab melts may have been generated. Ultimately subduction ceased, and the slab foundered into the mantle, potentially resulting in the generation of alkalic and potassic magmas within the Rae and Hearne cratons.

Alkaline intrusions in a near-trench setting, Franciscan Complex, California: Constraints from geochemistry, petrology, and 40Ar/39Ar chronology

Small gabbroic bodies intruding metagraywackes and cherts of the Late Mesozoic Franciscan Complex represent small-scale magmatic activity in a trench sedimentary environment or in the accretionary prism itself. At Leech Lake Mountain, located in the eastern Franciscan belt of northern California, interbedded radiolarian cherts and graywackes are locally intruded by Ti-rich alkalic diabase sills. The thicker sills are internally differentiated and display cumulate textures of olivine and augite. Although the diabases and surrounding graywackes were overprinted by blueschist metamorphism as indicated by jadeite-rich clinopyroxene and glaucophane, they show little deformation and good preservation of the mafic igneous minerals. The sills comprize alkali basaltic to hawaiitic compositions, all related by fractionation of olivine (Fo₈₄), augite, and titanomagnetite. The immobile trace elements Th, U, Nb, and the LREE (light Rare Earth Elements) are strongly enriched compared to MORB (Mid-Ocean Ridge Basalt), whereas Y and the HREE (heavy Rare Earth Elements) are depleted. Trace element modeling is consistent with either small degrees of melting of primitive mantle or with 6 to 8 percent of partial melting of depleted MORB mantle, which is enriched by up to 8 percent of a metasomatic melt. About 6 percent of garnet in the mantle source of the diabases can account for the observed HREE fractionation and suggests a depth of melting below the garnet-spinel transition zone at 60 to 80 km. This is in agreement with the reconstructed age and thickness of the oceanic plate the diabases intruded in the Early Cretaceous. Incompatible trace element ratios of the diabases are consistent with a HIMU (high μ)-type mantle source. The Early Cretaceous tectonic setting of the Leech Lake Mountain intrusive rocks along the California continental margin is dominated by rapid, highly oblique subduction. If melt is present beneath the oceanic lithosphere without a thermal anomaly, it may readily percolate upward through conduits opened by transtensional tectonics. Asthenospheric up- welling beneath a fracture zone that is migrating along the continental margin may be a viable alternative. ⁴⁰Ar/³⁹Ar dating of amphibole from a diabase olivine cumulate yields a plateau age of 119.0 ± 0.4 Ma, which is the first reported igneous age for the northern California Franciscan Complex. This age implies rapid subduction and metamorphism a few million years after intrusion of the diabases similar to the southern California Ortigalita Peak intrusion. However, the Leech Lake Mountain intrusive rocks are 25 my older and demonstrate that correlations on the basis of similar lithologies and metamorphic grade are problematic.