Whole-rock Rare Earth Element Research Articles

Contrasting U-Pb geochronology and geochemistry of uraninite from the Xianshi and Xiwang uranium deposits, South China: Implications for ore genesis

The Xiazhuang uranium ore field contains eighteen uranium deposits and is one of the most important granite-related uranium ore producers in South China. Silicified vein-type and intersection-type uranium deposits are the two main types of uranium deposits in this ore field. To constrain the mineralization ages and sources of uranium of these two types of uranium deposits, we present geochemistry and U-Pb isotopic data of uraninites from the representative Xianshi intersection-type and Xiwang silicified vein-type uranium deposits in the Xiazhuang ore field. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating on uraninite from the Xianshi deposit yielded the crystallization age of 89.9 ± 2.1 Ma, which is consistent with the emplacement age (∼90 Ma) of NNE-trending diabase porphyrite dykes in this area. Chondrite-normalized rare earth element (REE) patterns of uraninite from the Xianshi deposit are characterized by slight enrichment of light REE relative to heavy REE and weakly positive Eu anomalies. Our study suggests the involvement of mantle-derived metals in the formation of the Xianshi deposit. In situ LA-ICP-MS U-Pb dating on uraninite from the Xiwang deposit yielded a weighted mean age of 51.3 ± 0.8 Ma, which overlaps the timing of the 55–45 Ma crustal extension event in South China. Uraninite from the Xiwang deposit has much higher contents of W (5768–11305 ppm), Nb (294–1301 ppm), and Y (3013–6263 ppm) than those of uraninite from the Xianshi deposit (W = 291–371 ppm, Nb = 3–14 ppm, and Y = 1658–3149 ppm). Uraninite from the Xiwang deposit has “gull-winged” chondrite-normalized REE patterns with pronouncedly negative Eu anomalies, which resemble whole-rock REE patterns of the Maofeng and Xiazhuang granites. This study suggests that the Maofeng and Xiazhuang granites are probably the primary source of uranium for the Xiwang deposit.

Nature and origin of anorthosite enclaves within Proterozoic granite of Chotanagpur Granite Gneiss Complex of Eastern India

The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated, forming the Greater Indian Landmass. The Chotanagpur Granite Gneiss Complex (CGGC) represents the eastern extension of the CITZ and exposes several granite plutons. Enclaves of diverse origin are present as minor constituents within these granitic bodies. This study reports new major and trace element data for anorthosite enclaves hosted within granites in the Pattharkatti and Rajgir area from the northern margin of CGGC to get modern insights into the petrogenesis of anorthosites. Anorthosite enclaves show sharp contact with the host granite. They contain cumulus plagioclase (An87–94), intercumulus amphibole (magnesiohornblende and ferrotschermakite), and biotite (Mg-biotite and phlogopite) along with minor iron oxides. Amphibole crystallization pressure and temperature are constrained between 0.5 and 6.4 kbar and 653–780°C for the anorthosites. The studied anorthosites display a very gradual and steady increase in whole-rock rare earth element (REE) contents from Lu to La [(La/Yb)N = 1.22–13.08]. They also show a sharp decline in Fe2O3(t) and MgO, whereas Al2O3 increases with increasing silica contents from 45.69 to 51.16 wt%. In the chondrite normalized REE diagram, plagioclase exhibits LREE enriched patterns with strong positive Eu anomaly. The composition of parental liquid for anorthosite from the study area was estimated by adopting the equilibrium distribution method. Parental melt curves from Sm to La are near parallel and constrained broadly between trapped melt fractions (TMF) = ∼5%–15%. Anorthosites of the study area may have formed from the plagioclase-saturated basaltic melt.

Three Types of Mantle Eclogite from Two Layers of Oceanic Crust: A Key Case of Metasomatically-Aided Transformation of Low-to-High-Magnesian Eclogite

AbstractReconstructed whole-rock (RWR) and mineral major- and trace-element compositions, as well as new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis and evolution. On the basis of mineralogical and major-element compositions, the Catoca eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), and high-magnesian (high-Mg#) eclogites. The high-Al Catoca eclogites contain kyanite and corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns in garnets showing depleted LREEs, positive Eu anomalies (1.03–1.66), and near-flat HREEs; and high Sr contents in garnets and whole-rock REE compositions. All of these features point to a plagioclase-rich protolith (probably gabbro). RWR compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, and Pb) and δ18O of 5.5–7.4‰ of the low-Mg# Catoca eclogites are in good agreement with the compositions of picrite basalts and average mid-ocean ridge basalt (MORB). The depleted LREEs and NMORB-normalised Nd/Yb values of 0.07–0.41 indicate that the degree of partial melting for the majority of the low-Mg# eclogites protolith was ≥30%. The narrow δ18O range of 5.5–7.4‰ near the ‘gabbro–basalt’ boundary (6‰) obtained for the high-Al and low-Mg# Catoca eclogites reflects the influence of subduction-related processes. This case shows that mantle eclogites represented by two different lithologies and originating from different protoliths—plagioclase-rich precursor, presumably gabbro (for high-Al eclogites), and basalt (low-Mg# eclogites)—can provide similar and overlapping δ18O signatures on account of the influence of subduction-related processes. Chemical compositions of the high-Mg# eclogites indicate a complicated petrogenesis, and textural signatures reveal recrystallisation. The presence of Nb-rich rutile (8–12 wt% of Nb2O5) enriched with high field strength elements (HFSE) (Zr/Hf of 72.6–75.6) and multiple trace-element signatures (including RWR, NMORB-normalised Ce/Yb of 3.9–10.6 and Sr/Y of 5.8–9.6, MgO contents of 15.7–17.9 wt%, and high Ba and Sr) provide strong evidence for deep metasomatic alteration. High Cr contents in clinopyroxene (800–3740 ppm), garnet (430–1400 ppm), and accessory rutile (700–2530 ppm), together with extremely low Li contents of 1.0–2.4 ppm in clinopyroxene, may indicate hybridisation of the eclogites with peridotite. Comparison of the chemical compositions (major and trace elements) of (1) unaltered fresh cores of coarse-grained garnets from the low-Mg# eclogites, (2) secondary garnet rims (ubiquitous in the low-Mg# eclogites), (3) proto-cores in the coarse-grained garnet (high-Mg# eclogites), and (4) homogeneous recrystallised fine-grained garnets (high-Mg# eclogites) suggests that the high-Mg# eclogites formed through recrystallisation of low-Mg# eclogite in the presence of an external fluid in the mantle. Four of the five high-Mg# samples show that mantle metasomatism inside the Kasai craton mantle beneath the Catoca pipe occurred at a depth range of 145–160 km (4.5–4.8 GPa).

Enrichment of ion-exchangeable rare earth elements by felsic volcanic rock weathering in South China: Genetic mechanism and formation preference

Felsic volcanic rocks are an important source for generating ion adsorption-type rare earth element (REE) resources. To better understand the supergene enrichment of REEs related to felsic volcanic rock weathering, this study conducted an investigation of Indosinian felsic volcanic rocks from Guangxi, southwest China, which have been subjected to deep weathering. The weathering has formed large areas of thick regolith with the potential to host ion adsorption-type REE mineralization. The felsic volcanic rocks belong to the dacite–rhyolite series and are peraluminous, high-K, calc-alkaline, and REE-rich (325–376 ppm). Abundant REE-bearing accessory minerals are present in the felsic volcanic rocks, including titanite (average ΣREE = 15.1 wt%), allanite (average ΣREE = 14.9 wt%), and apatite (average ΣREE = 1510 ppm), and these three minerals contain an estimated 88.7% of the whole-rock REEs content. Significant REE enrichment is present in the felsic volcanic rock-derived regolith. A typical rhyolite-derived regolith profile has REE contents that increase from 376 ppm in the rhyolite to 1737 ppm in the regolith, representing an almost five-fold enrichment due to weathering. The REEs in the regolith are present in an ion-exchangeable form (iREEs), which accounts for 52%–87% of the total REEs present (TREEs). The occurrence of iREEs is closely linked to clay minerals, showing an affinity in the order halloysite > kaolinite > illite. Continuous operations of REEs by an eluviation–illuviation process from the source minerals (titanite + allanite + apatite) to sink minerals (kaolinite + halloysite + illite) results in an iREE-enriched zone in the middle and lower parts of the rhyolitic regolith. Notably, besides the studied Indosinian felsic volcanic rocks, there are multi-epochs of such lithology outcropped in South China. Their related iREEs mineralizations, however, are preferentially developed in the weathered terrains of early Yanshanian and Indosinian felsic volcanic rocks, and not in those of the Yanshanian and late Yanshanian felsic volcanic rocks. A comparison of the mineralized and barren units indicates that the iREEs mineralization hosted in felsic volcanic rocks regolith is controlled by some key endogenic and exogenic ore-forming factors. High initial REE concentrations in the unaltered felsic volcanic rocks, a suitable climate, and a relatively quiescent tectonic setting are favorable for the formation and preservation of iREEs mineralization.

Glauconite-bearing sedimentary phosphorites from the Tébessa region (eastern Algeria): Evidence of REE enrichment and geochemical constraints on their origin

Rare earth element (REE) analyses are reported on glauconite-bearing phosphorites from northeastern Algeria. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Petrographic examination indicates that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. In both deposits, glauconite grains are richer in REEs than other co-existing particles (pellets, coprolites, enameloid and dentine of marine fish teeth), but the glauconites of the Kef Essenoun deposit exhibit significantly higher REE concentration (min = 654 ppm, max = 1760 ppm, average = 1146 ppm) than those of Djebel El Kouif deposit (min = 543 ppm, max = 623 ppm, average = 584 ppm). The whole-rock REE concentrations also show substantial differences between the two deposits and the REE enrichment in the Kef Essenoun main sub-layer is more likely the result of the high glauconite content. PAAS normalized-REE patterns of the glauconite grains display similar patterns between the northern and southern localities with weak negative Ce anomalies and slight middle REE enrichments. On the other hand, the pellets and whole phosphorite grains from the northern deposit indicate a REE source from oxic-suboxic seawater, whereas those from the Kef Essenoun deposit exhibit slight middle REE enriched patterns with weak negative Ce anomaly suggesting a different environment of deposition (i.e. a tendency to reduced conditions). These geochemical results along with previous petrographic studies confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, where winnowing, transport and re-deposition of previously deposited phosphorites resulted in high accumulation and formation of glauconite grains. The glauconitization process of fecal pellets happened in two stages and was controlled by semi-confined micro-environments. This can explain the abundance of glauconite grains in the phosphorites from Kef Essenoun deposit and their restricted occurrence in the basal levels of the Djebel El Kouif outcrop.

Immature intra-oceanic arc-type volcanism on the Izanagi Plate revealed by the geochemistry of the Daimaruyama greenstones in the Hiroo Complex, southern Hidaka Belt, central Hokkaido, Japan

The Izanagi Plate is assumed to have underlain the western Panthalassa Ocean to the east of Eurasia, and to have been subducting under the Eurasian continent. Although the Izanagi Plate has been lost to subduction, the subduction complexes of the circum-Panthalassa continental margins provide evidence that subduction-related volcanism occurred within the Panthalassa Ocean, and not just along its margins. The Daimaruyama mass is a kilometer-sized allochthonous greenstone body in the Hiroo Complex in the southeastern part of the Nakanogawa Group in the southern Hidaka Belt, northern Japan. The Hiroo Complex is a subduction complex that formed within the Paleo-Kuril arc–trench system at 57–48 Ma. The Daimaruyama greenstones consist mainly of coarse volcaniclastic rocks with lesser amount of lava. Red bedded chert, red shale, and micritic limestone are also observed as blocks associated with the greenstones. The presence of Early Cretaceous (Aptian–Albian) radiolaria in red bedded cherts within the greenstones indicates that the Daimaruyama greenstones formed after this time. An integrated major and trace element geochemical dataset for whole-rocks and clinopyroxenes of the greenstones indicates a calc-alkaline magmatic trend with low TiO2 contents and increases in SiO2 and decreases in FeO* with increasing differentiation. Negative anomalies of Nb, Ta, and Ti in normal mid-ocean-ridge basalt type normalized patterns are interpreted as “arc-signatures”. Using “rhyolite-MELTS”, we conducted a numerical simulation of magmatic differentiation under conditions of 1.5 kbar and H2O = 3 wt% to reproduce the liquid line of descent of the Daimaruyama greenstones. Back-calculations of the equilibrium melt compositions from the trace element chemistry of the clinopyroxenes generally agree with the whole-rock rare earth element compositions of the Daimaruyama greenstones, therefore providing support for the conditions used for the rhyolite-MELTS calculations as well as the actual results. Clinopyroxene trace element compositions indicate substantial enrichment of Ba in the magma, reflecting the participation of shallow subduction components such as aqueous fluids. Geochemical investigations reveal that the Daimaruyama greenstones were probably submarine volcanic rocks that formed as a result of the subduction of the Izanagi Plate within the Thalassa Ocean (the Thalassa Ocean was the eastern realm of Panthalassa, and represents the proto-Pacific Ocean) after the Early Cretaceous (Aptian–Albian; 125–101 Ma), and they were eventually accreted onto the Paleo-Kuril arc–trench system at 57–48 Ma to form an allochthonous block as part of the mélange facies of the Hiroo Complex on the landward slope of the trench.

Controls on rare earth element distributions in ancient organic-rich sedimentary sequences: Role of post-depositional diagenesis of phosphorus phases

The processes that dictate the formation and preservation of rare earth element (REE) signatures in ancient marine black shales are incompletely understood, thus potentially limiting the utility of the REEs as geochemical tracers. Here, we report on whole-rock REE analyses of six black shale formations from different ages and depositional settings, which fall within three general groupings: Group 1 is characterized by low whole-rock REE content (ΣREE) (<250μg/g) and middle- and heavy-REE-enriched patterns (GdN/LaN<2, YbN/LaN<2.5); Group 2 is characterized by low ΣREE (<250μg/g) but greater middle-REE-enriched patterns (GdN/LaN>2, YbN/LaN<3); and Group 3 is characterized by high ΣREE (>250μg/g) and moderately middle-REE-enriched patterns (GdN/LaN~1.5–2.5, YbN/LaN~1). Representative samples from these groups were selected from the Marcellus and Hushpuckney shales and subjected to a multi-step leaching procedure, to isolate four extractable P-phases (reducible, authigenic carbonate-associated, detrital apatite, organic). In Group 1 samples with low carbonate content (<4% CaCO3), REEs were hosted mainly in the organic (50% of ΣREE) and siliciclastic fractions (37%); whereas in samples with greater carbonate content (>25% CaCO3), REEs were hosted mainly in carbonate-associated phases (53%). In Group 2, REEs were hosted mainly in the carbonate-associated (64%) and organic fractions (30%), and in Group 3, REEs were hosted mainly in the carbonate-associated (60%), detrital apatite (23%), and siliciclastic fractions (16%). Authigenic phases include minerals such as carbonate fluorapatite and authigenic calcite that, if present, tend to dominate the whole-rock REE inventory. In the absence of carbonate, whole-rock REE content depends on the relative REE contributions of the organic and siliciclastic fractions.

Provenance records of the North Jiangsu Basin, East China: Zircon U–Pb geochronology and geochemistry from the Paleogene Dainan Formation in the Gaoyou Sag

Detailed zircon U–Pb dating and whole-rock geochemical analyses were carried out on the sedimentary rocks of the Paleogene Dainan Formation from Gaoyou Sag in the North Jiangsu Basin, East China. Whole-rock rare earth element characteristics suggest that the provenance was mainly from the Late Proterozoic low-grade metamorphic felsic rocks in the Dabie–Sulu orogenic belt, with the parent rocks probably being the I-type high-potassium granite gneiss. Cathodoluminescence images indicate that most of the detrital zircons are originally magmatic. A few zircons show overgrowths, indicating multiple-episode tectonic events. The U–Pb age distribution patterns of the detrital zircons suggest four main magmatic episodes in the provenance: Late Archean–Early Proterozoic (2450–2600Ma), Early Proterozoic (1700–1900Ma), Late Proterozoic (700–850Ma), and Late Paleozoic–Mesozoic (100–300Ma). These zircon U–Pb age and whole-rock geochemical results suggest that the sediments of the Dainan Formation were mainly sourced from the recycled orogenic belts within and/or around the North Jiangsu Basin, including the basement of the Yangtze Block, the Neoproterozoic rocks in the Dabie–Sulu orogenic belt, and the Mesozoic igneous rocks in the south part of Zhangbaling Uplift.

A protracted timeline for lunar bombardment from mineral chemistry, Ti thermometry and U–Pb geochronology of Apollo 14 melt breccia zircons

New zircon U–Pb and trace element investigations from Apollo 14 lunar impact breccia sample #14311 reveal at least three distinct (Concordia, 2σ) age populations at 4334 ± 10, 4245 ± 10 and 3953 ± 10 Ma. Titanium-in-zircon thermometry (Tixln) results correlated with U–Pb ages range from ~800–1200 oC. Lattice strain models used to infer zircon versus whole-rock rare earth element contents, and partitioning calculations against lunar impact breccia component compositions, quantitatively constrain formation conditions for the different age populations. A compilation of new data with published work shows that Apollo 14 zircons older than ca. 4300 Ma formed by igneous processes associated with lunar crust formation. Compositional variability in the ca. 4240 Ma zircon age population is interpretable, however, via a mixture of inherited and melt-generated components from one or more large impacts perhaps related to a marked increase in bombardment flux. Ages from the youngest zircon group at ca. 3950 Ma coincide with the classical “late heavy bombardment” (LHB) as documented from previous lunar geochronologies. These results lend support to the idea that instead of a simple unimodal LHB scenario, or a monotonic decline in impacts, the Moon was battered by multiple cataclysms since ca. 4240 Ma. Such a “Picket fence”-like bombardment to the Moon best describes the mode and tempo of impacts that accompanied the late stages of solar system formation and giant planet migration.

Rare earth element signatures of economic and sub-economic porphyry copper systems in Urumieh–Dokhtar Magmatic Arc (UDMA), Iran

A wide variety of porphyry copper deposits have been reported along the Cenozoic Urumieh–Dokhtar magmatic arc in Iran. The formation of these deposits is attributed to closure of Neo-Tethys and the subsequent collisional tectonic regime during the Tertiary. This study presents whole-rock rare earth element (REE) data for the giant to small (Sarcheshmeh, Meiduk, Sungun, Darreh-Zerreshk, Dalli, Iju, Parkam and Ali-Abad) with sub-economic (Daraloo and Reagan) porphyry copper systems. The data indicate a temporal and geochemical evolution from sub-economic porphyry systems to economic deposits. All the economic and sub-economic porphyry copper systems exhibit LREE enrichment relative to HREE, whereas the economic deposits exhibit a relatively steep downward LREE to HREE profile, reflecting higher LREE abundance. The Eu anomalies vary from distinct negative in sub-economic deposits (Eu/Eu*=0.28–0.70) to either markedly less negative or positive anomalies (Eu/Eu*=0.45–1.67) in economic deposit. The economic porphyry deposits are characterized by relatively high La/Sm and Sm/Yb values, representing high crustal assimilation in a relatively thickened crust and provide insight into fractionation of hornblende with minor garnet in deep crustal parts (MASH zone). Compared with sub-economic deposits, the steep downward LREE to MREE and flatter to slightly upward MREE to HREE in economic deposits indicate hornblende involvement (magma evolution toward more volatile content). It seems that in an ongoing process of closure of Neo-Tethys, during compression and crustal shortening, the early Eocene–Oligocene sub-economic intrusions are followed by adakite-like hydrous Miocene (and younger) economic Cu-bearing intrusions.

Tectonic linkage between the Korean Peninsula and mainland Asia in the Cambrian: Insights from U–Pb dating of detrital zircon

The establishment of robust Paleozoic tectonic links between parts of the Korean Peninsula and cratonic blocks within China remains uncertain, despite their important implications for the overall tectonic evolution of East Asia. Here we provide new SHRIMP U–Pb detrital zircon geochronological constraints on correlations between sedimentary rocks of the Taebaeksan Basin (Cambrian–Ordovician) of South Korea and possible source rocks in the North and South China cratons. The detrital grain compositions and whole-rock rare earth element (REE) compositions of sandstones from the Cambrian formations in western (Sambangsan) and eastern (Myeonsan) parts of the Taebaeksan Basin suggest that the sedimentary provenance of these two formations is either a recycled orogen or a magmatic arc. The magmatic (eastern part) versus metamorphic (western part) origins inferred for the youngest populations (ca. 0.5–0.6Ga) of detrital zircon grains from the Cambrian formations collectively indicate that rapid uplift and erosion of the source rocks for the formations took place soon after the associated magmatic and metamorphic events. Thus, we propose that the paleo-sedimentary environments in which these two formations were deposited may have been retroarc basins with the detrital zircons originating from magmatic arcs. The Early Cambrian formation in the eastern Taebaeksan Basin contains populations of detrital zircon grains that show prominent age peaks at ca. 2.0Ga and ca. 1.8Ga, and a distinct lack of Meso- to Neoproterozoic ages, whereas the Middle Cambrian formation in the western Taebaeksan Basin exhibits age peaks at ca. 1.0Ga and ca. 0.7Ga. These age spectra imply that the western Taebaeksan area at the margin of the North China Block may once have been near Gondwana, whereas the eastern Taebaeksan area may have been situated at the margin of the North China Block more distant from Gondwana.

Mineral Chemistry and In Situ Dating of Allanite, and Geochemistry of Its Host Rocks in the Bagjata Uranium Mine, Singhbhum Shear Zone, India--Implications for the Chemical Evolution of REE Mineralization and Mobilization

The present study combines textures, compositions, and in situ dating of allanite and whole-rock geochemistry from the Bagjata uranium deposit in the Singhbhum shear zone, the most important uranium-producing belt in India, to investigate the chemical evolution of rare earth element (REE) mineralization. Allanite, volumetrically the most important sink of light rare earth elements (LREEs) in this deposit, occurs as disseminated grains, pockets, veins, and stringers in biotite schist and chlorite schist. Metasomatic addition of LREEs in the rock was associated with Ca-K-Fe ± B metasomatism, as evident from the vein mineralogy. The allanite-bearing metasomatized rocks are locally extremely enriched in REEs, reaching up to ~4.8 wt percent ∑REEs. The textures and compositions of allanite demonstrate that early REE-rich allanite was replaced by later, relatively less REE bearing allanite-epidote, and this alteration followed a substitution scheme of REE +3 + (Fe +2 , Mg +2 ) → Ca +2 + (Al, Fe +3 ). The presence of halite-bearing fluid inclusions in associated tourmaline, high Cl content of biotite, and other circumstantial evidence indicate that a high-salinity, Cl-rich fluid was likely responsible for initial LREE metasomatism. A later hydrothermal event altered preexisting allanite and removed REEs. The more oxidized nature of altered allanite and the presence of unidentified secondary REE minerals in association with sulfide might indicate the involvement of oxidized, sulfate-rich fluid in the alteration of allanite and the removal of REEs. Alternatively, fluoride complexing of REEs and selective removal of HREEs during this alteration are possible. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of allanite and monazite indicates that LREE metasomatism took place at ~1.88 Ga. The next hydrothermal event, which altered existing allanite, removed some REEs, and perhaps was associated with U (+HREE) addition, is dated at ~1.66 Ga. The youngest age determined from the thin rim of allanite is ~1.02 Ga. These three different ages, determined from allanite, overlap with the known thermotectonic events in and around the Singhbhum shear zone. Comparison between the calculated ∑REE content of allanite-rich rock prior to allanite alteration and whole-rock ∑REE composition of the same rock suggests that the addition of a large quantity of REEs was followed by significant REE removal. Strong variations in the modal abundances of the constituent minerals and in whole-rock compositions, including REEs, indicate open-system behavior. The whole-rock REE pattern suggests that decoupling between LREEs and HREEs might be possible during superimposed alteration. Therefore, petrogenetic modeling using an REE-based discrimination diagram and REE patterns should be done with caution for rocks that underwent multiple events of hydrothermal fluid flux.

Lithospheric Mantle Modification during Cenozoic Rifting in Central Europe: Evidence from the Księginki Nephelinite (SW Poland) Xenolith Suite

The Miocene Księginki nephelinite in SW Poland is part of the Central European Volcanic Province and occurs at the NE termination of the Cenozoic Ohře (Eger) Rift in the NW part of the Bohemian Massif. It contains numerous pyroxenite and peridotite xenoliths and clinopyroxene megacrysts. Pyroxenites (mostly olivine clinopyroxenites with no primary orthopyroxene) commonly have cumulate textures. Wehrlite and websterite xenoliths, texturally and chemically related to the pyroxenite suite, contain volcanic-textured intercumulus material similar to that of the host nephelinite. The Mg/(Mg + Fe) ratios of clinopyroxenes from the pyroxenite suite are 0·81–0·93; these are low-calcium]Ca 0·72–0·82 atoms per formula unit (a.p.f.u.)] and contain 0·22–0·37 a.p.f.u. Al. Clinopyroxene rare earth element (REE) patterns are slightly enriched in middle REE (MREE) and light REE (LREE) relative to heavy REE (HREE) (SmN/YbN = 2·4–4·3, LaN/YbN = 1·7–3·0) and are deflected downwards from Pr/Nd to La. The whole-rock REE patterns of the pyroxenite xenoliths are identical and show slight enrichment in LREE relative to HREE (LaN/Ybn = 3–5). The wehrlite cumulates are more LREE-enriched (LaN/Ybn = 13) and their REE patterns are identical to those of the host nephelinite. The peridotite xenoliths exhibit a range of textures from protogranular to mylonitic. Typically the peridotites (mostly harzburgites) consist of domains of olivine I (Fo88·9–91·9) and orthopyroxene I plus interstitial, texturally later clinopyroxene I [both with Mg/(Mg + Fe) = 0·89–0·93] and rare spinel [Cr/(Cr + Al) = 0·35–0·62]. The REE patterns of clinopyroxene I are similar to those of the pyroxenite suite. Fine grains of olivine II, commonly with forsterite contents identical to those of olivine I, but enriched in Ca, occur at the contacts between olivine I grains and in interstices or pools with glass. Clinopyroxene I commonly has spongy rims of clinopyroxene II, which are richer in Mg and poorer in Na. The peridotite xenoliths contain sparse fine-grained micrometre- to millimetre-scale aggregates consisting of (1) spinel, olivine II, clinopyroxene III and altered glass ± titanian biotite or (2) plagioclase, olivine II and clinopyroxene III ± glass. Olivine I in xenoliths containing these aggregates is typically Fo85·6–89·4, and the Mg/(Mg + Fe) ratio of pyroxenes is similar to that of olivine. The lithospheric peridotitic mantle beneath Księginki was infiltrated by alkaline silicate melts similar to the nephelinite lava prior to and during the volcanism. Pyroxenite cumulates crystallized from the melt in places where the flow was channelized. Locally melt infiltration was pervasive, leading to ‘Fe-metasomatism’ of the peridotites, which lowered the Fo content of the olivine to c. 86 %. The metasomatism was cryptic, and only the clinopyroxene trace element content was modified in peridotites not affected by pervasive melt infiltration or not in contact with pyroxenites. The mantle section affected by the metasomatism was located at a depth of ~35–50 km and was thermally equilibrated (temperatures of 1060–1120°C are recorded in both the pyroxenite and peridotite suites). The fine-grained aggregates are interpreted as remnants of the interstitial melt frozen after entrainment of the xenoliths during the eruption of the host nephelenite; they show that some of the peridotites resided at pressures allowing plagioclase crystallization, whereas the others were brought to the surface directly from spinel peridotite facies depths. The Księginki xenoliths provide a complete snapshot of the various effects of alkaline melt migrating through the lithospheric mantle during the initial stages of continental rifting.

MONAZITE, XENOTIME AND REE MINERALS IN ARCHEAN BANDED IRON-FORMATION FROM THE SHERMAN AND ADAMS MINES, ONTARIO, CANADA

We have used electron-microprobe analyses to establish the presence of a suite of accessory rare-earth-element (REE) minerals in Archean banded iron-formation (BIF, oxide facies) from the former Sherman mine, at Temagami, and the former Adams mine, south of Kirkland Lake, Ontario. The REE minerals identified are monazite and xenotime. Other REE-rich minerals were found, but their REE patterns and major-element compositions were not sufficiently distinctive to afford unambiguous identification. The grains of phosphate are small, 5 to 10 μm, and characterized by an anhedral morphology considered to reflect resorption. The monazite REE patterns are characterized by LREE enrichment similar to those seen in monazite from igneous and metamorphic rocks. However, all the monazite from BIF samples has a significant positive Eu anomaly, comparable to the Eu anomaly characteristic of the BIF host-rocks. These REE minerals contain no U or Th detectable by the electron microprobe. The positive Eu anomaly and the low U and Th contents argue against a volcaniclastic precursor. The accessory REE phosphate minerals probably arise from REE and phosphate that accompanied the deposition of the precursor oxide minerals that crystallized during the early diagenetic history of the iron formation, during the elimination and dehydration of pore fluids. A comparison of whole-rock REE content with that attributable to accessory minerals tentatively suggests that REE phosphates may significantly influence the bulk REE pattern of oxide facies iron-formation. Pore-water reflux from BIF sediment to the water column may represent a source of fluids with higher HREE proportions than original seawater, as possibly seen in REE profiles observed in xenotime.

REE-Y, Ti, and P Remobilization in Magmatic Rocks by Hydrothermal Alteration during Cu-Au Deposit Formation

Chondrite-normalized rare earth element (REE) plots of whole-rock geochemical analyses are commonly used to describe the history of magmatic systems. In the particular case of porphyry copper deposits, which are produced by ore-forming magmatic-hydrothermal systems, whole-rock REE concentrations commonly show an unusual depletion in the final phase of magmatism associated with the main ore-forming stage. Interpretation of REE signatures requires establishing whether they result from magmatic or hydrothermal fractionation processes, or a combination of both. We investigated whole-rock trace-element and REE patterns of samples from the giant Ok Tedi porphyry copper-gold deposit, Papua New Guinea, and found that REEs were most depleted in zones of greatest hydrothermal alteration. REE-rich accessory phases zircon, apatite, and titanite have lower modal abundances and become smaller and more anhedral in these intensely altered zones as compared to less altered rocks. This mineralogical change is accompanied by depletions in P, Ti, Y, and Zr, and deviations of Zr/Hf and Y/Ho from chondritic trace-element ratios—chemical signatures indicative of hydrothermal fractionation. We conclude that destruction of REE-rich accessory minerals, particularly apatite, has led to hydrothermal remobilization and depletion of REEs. This implies that intrusive rocks that experienced high-salinity, oxidized fluid-rock interaction at temperatures >400°C may have had their original magmatic REE signature modified.

Mantle metasomatism by melts of HIMU piclogite components: new insights from Fe-lherzolite xenoliths (Calatrava Volcanic District, central Spain)

Abstract Mantle xenoliths from the Calatrava Volcanic District (CLV), central Spain, are characterized by a wide compositional range that includes lherzolites (prevalent), as well as minor amounts of wehrlite, olivine (ol)-websterite and rare dunites. They generally have a bulk-rock Mg# of less than 89, lower than any primordial mantle estimates. Intra-suite variations in modal proportions are inconsistent with those predicted by melting models irrespective of the starting composition; mineral and bulk-rock variation diagrams show inconsistencies between the CLV compositions (anomalously enriched in Fe–Ti) and those predicted from the partial melting of primordial mantle material. Processes other than pure melt extraction are confirmed by the whole-rock REE (rare earth element) budget, typically characterized by LREE enrichments, with La N /Yb N (up to 6.7), probably related to pervasive metasomatism. CLV mantle clinopyroxenes (cpx) generally display fractionated REE patterns with upwards-convex shapes, characterized by low HREE (Tm–Lu) concentrations (typically &lt;6× chondrite) and enrichments in middle–light REE (MREE–LREE) (Nd N /Yb N up to 7, La N /Yb N up to 5). These ‘enriched’ cpx compositions either result from re-equilibration of primary mantle cpx with an incoming melt, or represent cpx crystallization directly from the metasomatic agent. The latter was plausibly generated at greater depths in the presence of residual garnet (from peridotite or eclogite starting materials). Separated cpx have homogeneous 87 Sr/ 86 Sr compositions between 0.7031 and 0.7032; 143 Nd/ 144 Nd ranges from 0.51288 to 0.51295 (εNd 4.74–6.07) and 176 Hf/ 177 Hf is in the range 0.28302–0.28265 (εHf −3.6 to 9.0). Unlike mantle xenoliths and alpine-type peridotites from other Iberian occurrences, which range in composition from the depleted mantle (DM) to the enriched mantle (EM), the CLV mantle cpx approach the composition of the HIMU mantle end member, the genesis of which is generally interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalent) via ancient subduction. A model is proposed for the mantle evolution under central Iberia, where sublithospheric convective instabilities – possibly triggered by the neighbouring subduction along the Betic collisional belt – could have remobilized deep domains from the mantle ‘transition zone’ (410–660 km), which may include relicts of older subducted slabs. Within these remobilized domains, characterized by the coexistence of peridotite and eclogite and referred to as a ‘piclogite’ association, the eclogites melt preferentially generating Fe–Ti rich melts characterized by a HIMU isotopic signature that infiltrates and metasomatizes the shallower lithospheric mantle.

The nature and timing of crustal thickening in Southern Tibet: Geochemical and zircon Hf isotopic constraints from postcollisional adakites

Rising as “the roof of the world” the Tibetan plateau is now underlain with the thickest continental crust on Earth. How and when was this crust formed, which would have exerted pivotal controls to the formation of the plateau, has long been an issue of hot debates. This paper reports zircon U–Pb ages and Hf isotope data for postcollisional (~ 30–9 Ma) adakites in the southern Lhasa terrane, southern Tibet. A comparative analysis of whole-rock rare earth element geochemistry and zircon Hf isotopes between the adakites and associated Gangdese igneous rocks suggests that the Tibetan crust underwent a major phase of tectonic thickening between ca. 45 and 30 Ma in the region. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the Late Cretaceous and Eocene time related to the Neotethyan subduction processes including breakoff of the subducted slab at ca. 50 Ma in the early stage of the India–Asia collision. These processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern Lhasa terrane. The indentation of India, consequently, caused distributed lithospheric thickening with formation of an orogenic root beneath southern Tibet. Root foundering during the Oligocene gave rise to the adakitic magmatism, regional topographic uplift, and onset of northward underthrusting of the Indian plate that has since played a key role in forming the entire Tibetan plateau.

Geochemical characteristics of rare earth element movement during incipient alteration of pyroxenes in Guatemalan corestones

AbstractEarth scientists currently use changes in major and minor elements as an indication of rock alteration. Work on spheroidal weathering has shown an increase in the rare earth element (REE) whole-rock abundancescan accompany minor changes in major elementsand highlights the need for new, more sensitive, geochemical techniques of detecting incipient alteration. Augites from the core and rind of a corestone system from the Tecuamburro complex in SE Guatemala are examined to determine their role in the whole-rock REE increases.

Geology and geochemistry of the Shuanggou ophiolite (Ailao Shan ophiolitic belt), Yunnan Province, SW China: Evidence for a slow-spreading oceanic basin origin

The early Carboniferous Shuanggou ophiolite lies in the middle segment of the Ailao Shan orogenic belt between the South China Block to the north and the Indochina Block to the south. The ophiolite consists of meta-peridotite, gabbro, diabase and basalt, capped by radiolarian-bearing siliceous rocks. No layered gabbros or sheeted dikes have been observed. The meta-peridotite underwent low degrees of partial melting, consistent with the low magma budget of this oceanic lithosphere. Whole-rock rare earth element analyses of gabbro indicate a geochemical affinity with normal mid-ocean ridge basalts, consistent with the crystallization order of plagioclase followed by clinopyroxene recognized in the gabbros. The ophiolite is believed to have formed in a small, slow-spreading oceanic basin. Collision of the Indochina Block with the South China Block in the late Paleozoic was responsible for the closure of the oceanic basin and emplacement of the ophiolite in the Ailao Shan orogenic belt.

Poster 37: A Case of Coexisting Neuropathologies

The Izanagi Plate is assumed to have underlain the western Panthalassa Ocean to the east of Eurasia, and to have been subducting under the Eurasian continent. Although the Izanagi Plate has been lost to subduction, the subduction complexes of the circum-Panthalassa continental margins provide evidence that subduction-related volcanism occurred within the Panthalassa Ocean, and not just along its margins. The Daimaruyama mass is a kilometer-sized allochthonous greenstone body in the Hiroo Complex in the southeastern part of the Nakanogawa Group in the southern Hidaka Belt, northern Japan. The Hiroo Complex is a subduction complex that formed within the Paleo-Kuril arc–trench system at 57–48 Ma. The Daimaruyama greenstones consist mainly of coarse volcaniclastic rocks with lesser amount of lava. Red bedded chert, red shale, and micritic limestone are also observed as blocks associated with the greenstones. The presence of Early Cretaceous (Aptian–Albian) radiolaria in red bedded cherts within the greenstones indicates that the Daimaruyama greenstones formed after this time. An integrated major and trace element geochemical dataset for whole-rocks and clinopyroxenes of the greenstones indicates a calc-alkaline magmatic trend with low TiO2 contents and increases in SiO2 and decreases in FeO* with increasing differentiation. Negative anomalies of Nb, Ta, and Ti in normal mid-ocean-ridge basalt type normalized patterns are interpreted as “arc-signatures”. Using “rhyolite-MELTS”, we conducted a numerical simulation of magmatic differentiation under conditions of 1.5 kbar and H2O = 3 wt% to reproduce the liquid line of descent of the Daimaruyama greenstones. Back-calculations of the equilibrium melt compositions from the trace element chemistry of the clinopyroxenes generally agree with the whole-rock rare earth element compositions of the Daimaruyama greenstones, therefore providing support for the conditions used for the rhyolite-MELTS calculations as well as the actual results. Clinopyroxene trace element compositions indicate substantial enrichment of Ba in the magma, reflecting the participation of shallow subduction components such as aqueous fluids. Geochemical investigations reveal that the Daimaruyama greenstones were probably submarine volcanic rocks that formed as a result of the subduction of the Izanagi Plate within the Thalassa Ocean (the Thalassa Ocean was the eastern realm of Panthalassa, and represents the proto-Pacific Ocean) after the Early Cretaceous (Aptian–Albian; 125–101 Ma), and they were eventually accreted onto the Paleo-Kuril arc–trench system at 57–48 Ma to form an allochthonous block as part of the mélange facies of the Hiroo Complex on the landward slope of the trench.