Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (∼ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time.     

青海南部杂多地区超镁铁质-镁铁质岩石的特征及Ar—Ar定年

通过野外地质研究和室内岩相学、岩石地球化学研究,在位于青藏高原中部的青海杂多地区的双湖-澜沧江结合带中发现了一套超镁铁质岩、镁铁质岩。该超镁铁质岩、镁铁质岩沿北西—南东向区域构造线多呈岩脉状侵入在早石炭世杂多群中,局部呈构造透镜体分布在断裂带中或呈包体分布在白垩纪花岗岩中,出露规模不大,局部侵入体具有层状侵入杂岩体的特征。主要岩石类型为辉石橄榄岩、辉长岩、辉长辉绿岩,岩石地球化学具高Ti、Fe、Al、Ga和LREE中等富集的特征。橄榄岩中角闪石的~(40)Ar/~(39)Ar同位素测年得到275.3Ma±1.9Ma的坪年龄,表明这套超镁铁—镁铁质岩石可能为早、中二叠世弧后盆地拉张的产物。     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

Abstract

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS     

青藏高原北羌塘盆地杂多地区晚二叠世—早三叠世陆相火山岩的地球化学特征及其意义

在北羌塘盆地东部杂多县特龙赛等地首次厘定出晚二叠世—早三叠世火山岩。该火山岩形成于陆相环境,由4个爆发一溢流相火山韵律组成一个完整的火山旋回。这些火山岩主要由强蚀变杏仁状橄榄玄武岩、岩屑晶屑凝灰岩、杏仁状安山岩等组成,TiO_2含量(1.33％～1.76％)与峨眉山低Ti玄武岩相当,MgO含量较低(2.23％～7.52％,平均为5.02％),富集轻稀土元素(LREE／HREE=4.16～13.44,平均为6.5),无明显的Eu异常(δEu=0.85～1.02,平均0.96),Nb、Ta轻度负异常,地球化学特征与大陆拉斑玄武岩相似。各种微量元素判别图解显示这些火山岩形成于大陆拉张带或陆内裂谷区。结合区域研究成果,晚古生代时整个羌塘地区很可能处于一种伸展背景。     

Geochemical mechanisms of travertine formation from fresh waters in southern Siberia

The subject of study was the chemical composition of common fresh-water springs precipitating travertines in tectonically passive regions of the Kolyvan'-Tomsk folded area and northwestern Salair. Attention was paid to the specific character of manifestation, mineralogy, and petrography of the produced travertines. Results of the study of isotopic composition of carbon in hydrocarbonate ion of waters and carbonate travertines are reported. It is shown that the genetic type of CO₂ accompanying the formation of travertines is biogenic. Study of the equilibrium of the underground waters with aluminosilicate and carbonate minerals has shown that the travertines are the product of evolution of an equilibrium-nonequilibrium water-rock system. New mechanisms of travertine formation from cool fresh waters are proposed.     

Paleozoic evolution of continental crust in the Beaujolais-Lyonnais area,northeastern part of the Massif Central,France

In the Beaujolais-Lyonnais area of the northeastern Massif Central accretion of continental and possibly oceanic crustal fragments occurred between Cambrian (?) and early Carboniferous time. Three distinct lithotectonic units (terranes?) have been recognized. The southern (Lyonnais-) Unit consists of medium- to high-grade metamorphics and includes eclogites; it formed in the early Paleozoic. The Brévenne-Unit to the north contains low- to medium-grade metamorphic mafic and felsic volcanics and subordinate sedimentary rocks which possibly originated during the early Paleozoic until Devonian time, in a sialic back-arc environment or along an active continental margin. The Beaujolais-Unit is represented by volcanics on the south and predominantly shallow marine clastics and carbonates on the north. It developed in a late Devonian or early Carboniferous ensialic marginal basin. The peak of metamorphism in the Lyonnais-unit (HP/HT) was reached in Silurian time. Subsequent NW-SE to E-W oriented convergence produced mylonitic foliation, structural imbrication of the Lyonnais basement rocks with the Brévenne-Unit and SE-vergent folds accompanied by low- to medium-grade metamorphism. Late Visean to Namurian N-S to NW-SE directed N-vergent thrusting produced tectonic imbrication of the metamorphic northern Brévenne-Unit with the nonmetamorphic Beaujolais-Unit. In the southern Brévenne-Unit and in the Lyonnais-Unit updoming along right-lateral high-angle normal faults was followed by emplacement of voluminous granitic plutons of crustal origin. Late Carboniferous to early Permian crustal thinning in the Beaujolais-Lyonnais area was associated with N-S trending left-lateral strike-slip faults and E-W to NE-SW trending right-lateral strike-slip faults. Basins that developed along these faults contain continental red beds.     

Comendite-bearing subduction-related volcanic associations in the Khan-Bogd area,southern Mongolia: Geochemical data

The vertical section of volcanic rocks in the Khan-Bogd Late Paleozoic depression, southern Mongolia, in the belt of southern Mongolian Hercynides contains comendites. The basement of the depression is made up of Devonian ophiolites (older than 362 Ma) overlain by volcanic associations of an active continental margin (ACM) (dated at 330 Ma) and a bimodal association (dated at approximately 290 Ma), which is subdivided into a lower unit (BLU), dacites of the intermediate layer (IL), and a bimodal association of the upper unit (BUU). The volcanic associations of the Devonian and ACM are calc-alkaline and poor in TiO₂. The BLU rocks have higher alkalinity and TiO₂ concentrations and show a transition from the tholeiitic to calc-alkaline series in the course of differentiation with the origin of comendites and trachyrhyolites, including those with adakite characteristics. The IL dacites are analogues of calc-alkaline magmas of the ACM type. The BUU volcanic association is composed of tholeiite basalts with moderate Ti concentrations (of the MORB type), comendites, and trachyrhyolites, with a compositional gap at rocks of intermediate composition. The variations in the canonical ratios of incompatible trace elements and petrochemical parameters of the Khan-Bogd volcanic rocks show that their parental magmas were derived mostly from a source of basalts of the arc type (IAB) with the addition of variable proportions of a source of the MORB type. The greatest role of the latter is identified in the magmas of the bimodal association. BLU and BUU are separated by IL, a fact testifying that the bimodal volcanism occurred simultaneously with normal dacite continental-marginal volcanism. Although the geodynamic environments in which volcanic rocks were formed somewhat varied during the development of the Khan-Bogd depression, a subduction environment remained predominant, and the volcanic rocks were derived from an IAB-type source. The subduction volcanic associations produced thereby are differentiated and vary in composition from basites to dacite and rhyolite, which could be formed at the assimilation of continental crustal material (CC). Conceivably, the bimodal volcanic association was generated when the subduction zone was approached by a mid-oceanic ridge, whose material could be added in appreciable amounts to the subduction sources. The volcanic evolution of the Khan-Bogd depression shows an evolution of geodynamic environments and the composition of the volcanic rocks generally resembling those in the western margin of North America in the Cenozoic. The acid BLU and BUU rocks were most probably generated by different mechanisms. The BLU comendites and trachyrhyolites were likely formed by the crystallization differentiation of an arc basite magma of elevated alkalinity. The acid BUU rocks resulted from the anatexis of basites of this association, particularly spilitized ones (as well as any other basites) and the subsequent crystallization differentiation of the anatectic magmas.     

Hydrochemical evolution of Na-SO4-Cl groundwaters in a cold, semi-arid region of southern Siberia

The Shira region of Khakassia in southern Siberia exhibits many features governing the evolution of groundwater and surface-water chemistry that are common to other cold, semi-arid areas of the world: (1) a continental climate, (2) location in a rain shadow, (3) low density of surface-water drainage, (4) occurrence of saline lakes, and (5) occurrence of palaeo- and modern evaporite mineralisation. In lowland areas of Shira, the more saline groundwaters and lake waters have a sodium-sulphate (-chloride) composition. Results of thermodynamic modelling suggest that these evolve by a combination of silicate weathering and gypsum and halite dissolution, coupled with carbonate precipitation to remove calcium and bicarbonate ions. An approximately 1:1 sodium:sulphate ratio occurs even in groundwaters from non-evaporite-bearing aquifers. This may indicate the formation of secondary sodium sulphate evaporites (in or near saline lakes or in soil profiles where the water table is shallow), which are subsequently distributed throughout the study area by atmospheric transport. Several urban groundwaters are characterised by very high nitrate concentrations, conceivably derived from sewage/latrine leakage. Received, June 1998 /Revised, May 1999, August 1999 /Accepted, August 1999     

Geochemical specifics of ongonites in the Ary-Bulak Massif, eastern Transbaikalia

A genetic model for magmatic rocks of the Ary-Bulak Massif is discussed based on a detailed geological map of the massif (prepared by the authors) and on original data of the authors on the petrography of the massif, its compositional zoning, trace-element geochemistry, physicochemical parameters of its crystallization, and melt inclusions in its minerals. The Ary-Bulak Massif was determined to be zonal, with the predominance (approximately 70% by area) of porphyritic topaz ongonites (central facies), which grade toward contacts into weakly porphyritic ongonites bearing topaz and, occasionally, fluorite (margin facies). Aphyric rocks with fluorite (inner-contact facies) occur as a stripe 50–80 m wide at the southwestern inner contact of the massif. Analysis of petrographic and geochemical data indicates that subvolcanic rocks of the Ary-Bulak Massif differ from typical elvanes (as they occur in the Cornwall province) but are similar to classic ongonites in the central and marginal facies of the massif. Rocks in the southwestern inner-contact zone are unusual high-F and high-Ca varieties, whose analogues have never been found in any rare-metal provinces with ongonites and which provide evidence of a complicated evolutionary history of the Ary-Bulak Massif. The geochemical evolution of this massif was determined to be characterized by the enrichment of the older inner-contact facies rocks in CaO, K₂O, F, and Rb, Cs, B, Ba, Sr, Sn, and Ta, whose concentrations decrease in the ongonites of the central facies. The central-facies ongonites thereby have much higher Na₂O and Li concentrations than those in the inner-contact facies rocks. It is demonstrated that the intense heating and melting of crustal material in this region at the Jurassic-Cretaceous boundary could have been induced by subalkaline basaltic magma. The chemical composition of the rocks, which is unusual for typical ongonites in, for example, high Ca and Sr concentrations, could be caused by the possible assimilation by the magma of limestones, which occur in the territory at a certain depth in the Ust’-Borzya Formation that hosts the Ary-Bulak Massif. The genesis of most rocks in the massif was controlled by the magmatic differentiation of crustal granitic magma, with the residual melts forming Li-F granites enriched in several trace elements (Li, Rb, Cs, B, Ba, Sr, etc.) and ongonites as their subvolcanic analogues.     

Neoproterozoic tectonic evolution of the Hongseong area, southwestern Gyeonggi Massif, South Korea; implication for the tectonic evolution of Northeast Asia

Two types of Neoproterozoic metabasites occur together with regionally intruded arc-related Neoproterozoic granitoids (ca. 850–830 Ma) in the Hongseong area, southwestern Gyeonggi Massif, South Korea, which is the extension of the Dabie–Sulu collision belt in China. The first type of metabasite (the Bibong and Baekdong metabasites) is a MORB-like back-arc basin basalt or gabbro formed at ca. 890–860 Ma. The Bibong and Baekdong metabasites may have formed during back-arc opening by diapiric upwelling of deep asthenospheric mantle which was metasomatized by large ion lithophile element (LILE) enriched melt or fluid derived from the subducted slab and/or subducted sediment beneath the arc axis. The second type of metabasite (the Gwangcheon metabasite) formed in a plume-related intra-continental rift setting at 763.5 ± 18.3 Ma and is geochemically similar to oceanic island basalt (OIB). These data indicate a transition in tectonic setting in the Hongseong area from arc to intra-continental rift between ca. 830 and 760 Ma. This transition is well correlated to the Neoproterozoic transition from arc to intra-continental rift tectonic setting at the margin of the Yangtze Craton and corresponds to the amalgamation and breakup of Rodinia Supercontinent.     

藏南洛扎地区过铝质花岗岩的地球化学特征及构造背景

洛扎及其以南地区分布有规模不等、产状不同的过铝质二云母(白云母)二长花岗岩和电气石二长花岗岩,这些岩体侵位的时代为中新世,构成了高喜马拉雅花岗岩带的东延部分。岩体以高铝、低镁铁组分,高锶、氧同位素比值为特征,地球化学研究显示它们是泥质岩的深熔作用和岩浆的结晶分异作用所形成的产物。过铝质花岗岩是在后碰撞造山作用阶段的大规模伸展拆离作用背景下沿拆离构造带侵位的,形成于藏南拆离系韧性活动阶段的晚期,是一种典型的后碰撞过铝质花岗岩。     

Geochemistry of radioactive elements in the rocks of the Guli Massif,Polar Siberia

The study of radioactive element distribution in the rocks of the Guli Complex revealed an increase of uranium and thorium contents in the final products of magmatic differentiation. In the carbonatite complex, the radioactive elements are mainly accumulated in the early rocks—phoscorites, while their contents in the late phases, dolomitic carbonatites, decrease. The Th/U ratio increases from near-chondritic values in the weakly differentiated highly-magnesian primary magmas to the late rocks—phoscorites, calcitic carbonatites, and dolomitic carbonatites. The majority of radioactive elements are hosted in rare-metal accessory minerals: perovskite, pyrochlore, calzirtite, and apatite. Rock-forming minerals are characterized by extremely low contents of radioactive elements.     

Isotopic geochronology and biostratigraphy of Riphean deposits of the Anabar Massif,North Siberia

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic?geochronological study of globular phyllosilicates (GPS) of the glauconite?illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb?Sr and K?Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb?Sr, 1485 ± 13 Ma; K?Ar, 1459 ± 20 Ma) and Yusmastakh (Rb?Sr, 1401 ± 10 Ma; K?Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.     

Geochemical and Isotopic Geochemical Characteristics of PGE Mineralization in the Lukkulaisvaara Layered Massif,Karelia, Russia

The study of the Lukkulaisvaara layered massif from the Olang group of intrusions in northern Karelia corroborates the important role of supplementary intrusive phases of PGE mineralization. Injection and crystallization of new magma portions result in (1) the formation of potholelike depressions within intrusion and (2) a return to high-temperature olivine-bearing mineral assemblages in the mafic part of section. PGM formation is accompanied by crystallization of secondary minerals in a wide range of temperatures and pressures. To provide insight into these problems, a geochemical study of Nadezhda area has been performed and new data obtained for the distribution of isotopes in the Rb–Sr, Pb–Pb, and Sm–Nd systems.     

Rb-Sr geochronology of Vendian shales,the Staraya Rechka Formation,Anabar Massif,northern Siberia

Fine-grained clayey subfractions (SF) with particle sizes of 1–2, 0.6–1.0, 0.3–0.6, 0.2–0.3, 0.1–0.2, and <0.1 μm were extracted from shales of the Vendian Staraya Rechka Formation in the Anabar Massif and studied by XRD and Rb-Sr methods. All the clayey subfractions are represented by illite with high crystallinity indices, which are characteristic of the low-temperature diagenesis/catagenesis zone and grow with the decrease of the particle size. The Rb-Sr systematics in clayey subfractions combined with mineralogical data provide grounds for the conclusion that illite from clayey rocks of the Staraya Rechka Formation was forming during two periods: approximately 560 and 391–413 Ma ago. The first illite generation was likely formed in the course of lithostatic subsidence of the Staraya Rechka sediments and the second one, during the Devonian lithogenesis stage. It is assumed that age of the first generation (∼560 Ma) is close to that of the Staraya Rechka Formation. This inference is consistent with biostratigraphic, chemostratigraphic, and geochronological data obtained for both rocks of the Anabar Massif and Vendian sediments from other regions of Siberia.     

Garnet evolution in pre-Variscan pelitic rocks from the Lake Emosson area, Aiguilles Rouges Massif, Western Alps

Abstract The chemical evolution of garnets from pelitic rocks of probable Palaeozoic age corresponds to a complex metamorphic evolution of the host rocks.
Among the almandine-rich garnets (Alm_60–80), two main types of evolution can be distinguished. Early Mn-rich garnets coexisting with kyanite may be replaced by plagioclase and then, during a late stage, by biotite and/or sillimanite. The second type of evolution corresponds to an overgrowth of Mn-poor late-stage garnet on older Mn-rich garnets which corresponds to a thermal peak with sillimanite-type of metamorphism. This new garnet may appear either as an overgrowth with a strong discontinuity, or as small, new euhedral garnet or as skeletal garnet.
This chemical evolution of garnet corresponds to an early collisional stage of metamorphism (of high pressure type with high Mn values) of probable Ordovician age followed by uplift and a thermal peak (low Mn values) in Devonian times.     

Field and Geochemical Studies of the Melilite-Bearing Arydzhangsky Suite,and an Overall Perspective on the Siberian Alkaline-Ultramafic Flood-Volcanic Rocks

This paper presents the first comprehensive geologic, petrographic, geochemical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava suite, the last suite in the entire north Siberian, flood-volcanic sequence to require study by modern methods. Within this sequence, only the Arydzhangsky Suite includes melilite-bearing lavas; it is composed of melanephelinites to limburgites (both melilite-bearing and melilite-free), with rare melilitites and picrites. The lavas contain from 0 to 60 vol% melilite, with MgO contents ranging from 5.7 to 29.5 wt%. Nonetheless, these compositionally diverse lavas are all quite similar in incompatible-element geochemistry. They are distinct from all other Siberian alkaline-ultramafic lavas and, among these lavas, show the most resemblance to the Yakutian kimberlites.

With this contribution, all of the north Siberian, alkaline-ultramafic lavas will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on the basis of distinctive rare-earth-element (REE) patterns: melilitite-related, melanephelinite-related, meymechite-related, trachybasalt-related, and ankaramite-related. The REE ratios and patterns that distinguish the groups have not evolved by fractionation, because they display no relation to MgO content. Judging from the isotopic data, crustal contamination had little influence on magma evolution. All rock groups, despite their geochemical dissimilarities, show close geochemical linkages among themselves, and significant geochemical similarity to kimberlites of the Yakutian province and ocean-island basalts (OIB). Thus, all of these continental and oceanic magmas may have originated in the same part of the mantle. Geochemical distinctions among the five rock groups could have been caused by various degrees of partial melting and differing amounts of dissolved volatiles.     

Structure and tectonic evolution of the Pirin-Pangaion Structural Zone (Rhodope Massif,southern Bulgaria and northern Greece)

The Pirin-Pangaion Structural Zone occupies the south-western part of the Rhodope Massif. It consists of Proterozoic amphibolite facies metamorphic rocks of the Rhodopian Supergroup, and granitoids of Hercynian, Late Cretaceous and Palaeogene age. The pre-Hercynian structure of the zone is dominated by an interference pattern of three superimposed fold generations of NE-SW and NW-SE trends. These structures are cut by Hercynian granitoids, and the entire complex is affected by late Hercynian or early Alpine conical folds. The zone was overthrusted by the Ogražden and Kroussia Units (Serbo-Macedonian ‘Massif’) along the north-east vergent Mid-Cretaceous Strimon overthrust, and by the Central Rhodope Zone of the Rhodope Massif, along the south-west vergent Meso-Rhodopean Overthrust. With this thrusting event, the Pirin-Pangaion Structural Zone was brought together with the Serbo-Macedonian ‘Massif’ and the Central Rhodope Zone to form the Late Cretaceous Morava-Rhodope Zone, which acted as a ‘plateau’ along the southern edge of the Eurasian plate. Late Cretaceous granitoid magma of crustal origin intruded this zone, whereas north of it the Srednogorie volcanic island arc was the site of igneous activity with magmas originating in the upper mantle. The West Thrace Zone developed as a Palaeocene to Oligocene depression superimposed over the older basement obliquely to the southern periphery of the Rhodope Massif. In the Late Eocene and Early Oligocene, this depression represented a volcanic island arc with mantle-derived basic to intermediate magmas; contemporaneous granitoid magmas formed through crustal melting in the thickened crust of the Rhodope Massif (Pirin and Pangaion Units included). Early Miocene thrusting was most intense in the Pangaion Unit, and was followed by Late Miocene to Quaternary extension.     

Geochemical and hydrogeochemical evolution of groundwater in Ferdows area, northeast of Iran

The chemical analysis of 19 water wells in Ferdows area, Northeastern Iran, has been evaluated to determine the hydrogeochemical processes and ion concentration background in the region. In the study area, the order of cation and anion abundance is Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and Cl^? > SO ₄ ^?2  > HCO₃ ^? > NO₃ ^?, respectively, and the dominating hydrochemical types are Na–Cl. Most metal concentrations in water depend on the mineral solubility, and pH, Eh, and salinity of the solution. Their ΣREE concentrations showed excellent correlations with parameters such as TDS and pH. North American Shale Composite (NASC)-normalized REE patterns are enriched in the HREEs relative to the LREEs for all groundwaters. They all have positive Eu anomalies (Eu/Eu* = 0.752–3.934) and slightly negative Ce anomalies (Ce/Ce* = 0.019–1.057). Reduction–oxidation, complexation, desorption, and re-adsorption alter groundwater REE concentrations and fractionation patterns. The positive Eu anomalies in groundwaters are probably due to preferential mobilization of Eu²⁺ relative to the trivalent REEs in the reducing condition.