Genesis of positive Eu anomalies in acid rocks from the Eastern Baltic Shield

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  • 439

    ISSN 0016-7029, Geochemistry International, 2006, Vol. 44, No. 5, pp. 439455. Pleiades Publishing, Inc., 2006.Original Russian Text E.N. Terekhov, T.F. Shcherbakova, 2006, published in Geokhimiya, 2006, No. 5, pp. 483500.

    INTRODUCTIONLong-term studies of high-grade metamorphic rocks

    in the eastern part of the Baltic Shield resulted in thefinding of their abundant acid varieties with positive Euanomalies. Rocks exposed at the Earths surface mostcommonly have negative Eu anomalies (if any) [1], andthus, studying rocks with positive anomalies is of sig-nificant interest and may hopefully provide insight intothe behavior of Eu in the Earths crust. Most of our sam-ples were collected at the LaplandBelomorian Belt(LBB) and the closest surroundings (Fig. 1), althoughanalogous or fairly similar rocks with positive Euanomalies were also found elsewhere at the BalticShield [2, 3].

    The Eu anomaly is calculated by dividing the chon-drite-normalized Eu concentration of a rock by the half-sum of the normalized concentrations of Sm and Gd(the two neighboring elements) and is denoted Eu/Eu*.The deepest negative Eu anomalies, which likely deter-mine the composition of the upper crust, occur in gran-ites and metasomatic rocks, whose Eu/Eu* are some-times less than 0.01 [4], i.e., their Eu concentrations are100 or more times lower than they can be expectedjudging from the concentrations of other REE. No vol-canic rocks of mantle provenance show Eu depletion(i.e., Eu/Eu* = 0.65) typical of the average compositionof the upper crust [1], and MORB, within-plate volca-nics, and volcanic rocks of island arcs equally rarelydisplay positive and negative Eu anomalies. This putsforth the question as to where is Eu concentrated. Thewide occurrence of post-Archean rocks with negativeEu anomalies and, as a consequence, the typical REEpatterns of post-Archean sediments (the PAAS line

    reflects the average composition of the eroded rocks)provides indirect evidence of the occurrence of deep-seated (lower and middle crustal) rocks with comple-mentary REE patters, i.e., with positive Eu anomalies.However, most rocks of granulite complexes that arenow exposed at the surface (and could previously belocated in the lower crust) only occasionally have posi-tive Eu anomalies. Persistently present positive Euanomalies are typical only of anorthosites, whichaccount only for insignificant volumes in granulitecomplexes. Anorthosites commonly consist of cumulusplagioclase. Plagioclase, a mineral thought to beresponsible for the development of Eu anomalies [1], isknown to be unstable at depths of >40 km, and, hence,Eu anomalies should be related to some crustal pro-cesses. It is worth noting that rocks with positive Euanomalies are quite often found in Precambrian com-plexes and become progressively rarer in younger com-plexes, whereas the amount of rocks with negative Euanomalies increases. Hence, one of the mechanismsresponsible for Eu enrichment can be its temporaryaccumulation at a certain structuralgeochemicalboundary of the Earths crust. Similar to the apparenthorizon, some boundaries in the Earths crust seem toalways occur at a distance. For example, the boundarybetween ductile and brittle deformations alwaysoccurs at approximately the same depth (1015 kmfrom the surface), regardless of the processes that cantake place.

    ANALYTICAL TECHNIQUESAll of our rock samples were analyzed for La, Ce,

    Nd, Sm, Eu, Gd, Er, and Yb by plasma spectroscopy on

    Genesis of Positive Eu Anomalies in Acid Rocks from the Eastern Baltic Shield

    E. N. Terekhov and T. F. Shcherbakova

    Institute of the Lithosphere of Marginal and Epicontinental Seas, Russian Academy of Sciences, Staromonetnyi per. 22, Moscow, 109180 Russia

    e-mail: tereh@ilran.ru

    Received May 17, 2005

    Abstract

    The eastern part of the Baltic Shield contains an abundance of acid rocks with positive Eu anoma-lies. These rocks are vein granites and blastomylonites of similar chemical composition but with variable K

    2

    Oconcentrations. The rocks are depleted in Ti, Fe, Mg, Ca, Rb, Zr, and REE, but are enriched in Ba and Sr, a factsuggesting a deep-seated nature of the fluids that participated in the genesis of these rocks. A zone favorable forthe derivation of these rocks was transitional from brittle to ductile deformations. The rocks were produced dur-ing the tectonic exhumation of lower and middle crustal material a horizontal extension. Shock decompressionfacilitated the inflow of reduced fluids, which, in turn, ensured the partial melting of the host rocks along openfractures and controlled REE fractionation with the development of Eu maxima.

    DOI:

    10.1134/S0016702906050028

  • 440

    GEOCHEMISTRY INTERNATIONAL

    Vol. 44

    No. 5

    2006

    TEREKHOV, SHCHERBAKOVA

    N41N41N41N42N42N42

    N44N44N44

    PRPRPR222

    PRPRPR111

    PRPRPR111N033N033N033

    200220022002

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    LBLBLB

    ARARAR

    PSPSPS

    F33F33F33

    LBLBLB

    KpKpKp

    PRPRPR111

    BBBBBB

    BBBBBBARARAR222

    ARARAR222

    ARARAR

    ARARAR

    MBMBMB

    CKBCKBCKB

    PZPZPZ

    ARARAR222

    PRPRPR111I-V 0I-V 0I-V 0

    PRPRPR222

    k24k24k24KKMKKMKKM

    k17k17k17k84, 88, 89k84, 88, 89k84, 88, 89

    941259412594125

    778778778767767767

    941039410394103

    242424

    k79k79k79940594059405

    PRPRPR111

    Topozero

    Topozero

    Topozero

    PRPRPR111

    PRPRPR111

    ARARAR

    ARARAR222

    PRPRPR111

    ARARAR

    ARARAR222

    KMKMKM

    ARARAR222

    k102k102k102k104k104k104

    BBBBBB

    PRPRPR111

    ARARAR222

    ARARAR

    0 30 60 90 km

    Barents Sea

    WHITESEA

    Kem

    Belomorsk

    Caledo

    nides

    1

    2

    3

    4

    5

    6

    7

    8

    9

    10

    b

    b c

    b

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    b

    b

    AR

  • GEOCHEMISTRY INTERNATIONAL

    Vol. 44

    No. 5

    2006

    GENESIS OF POSITIVE Eu ANOMALIES IN ACID ROCKS 441

    a Monospec 1000 analyzer [5] at the Institute of theLithosphere of Marginal and Epicontinental Seas, Rus-sian Academy of Sciences. Ni, Rb, Ba, Sr, Zr, and Ywere determined on a TEFa device at the same institute.Conventional silicate analyses were conducted at theVinogradov Institute of Geochemistry, Siberian Divi-sion, Russian Academy of Sciences. The quality of theanalyses was checked by using internationally certifiedstandards and by comparing the results obtained by dif-ferent techniques for the same samples.

    BRIEF GEOLOGICALPETROGRAPHIC OVERVIEW

    According to their morphological characteristics,acid rocks with positive Eu anomalies from the easternBaltic Shield (Fig. 1) are classed into two groups(Fig. 2). One of them occurs as relatively thin veins ofgranites and orthotectites, which usually cut across thehost rocks or, more rarely, are conformable with them.The other group comprises linearized rocks (blastomy-lonites).

    The

    granites

    occur as veinlets a few centimetersthick and as veins up to 150 cm or, occasionally, up to1.5 m. The granites sometimes occur as orthotectitesthat compose irregularly shaped patches or lenses 2030 cm across. More rarely, orthotectite lenses arearranged along two perpendicular directions. Both thegranites and the orthotectites are very leucocratic rocksand contain no more than 25% biotite. A distinctivefeature of the orthotectites is their texture, which variesfrom coarse-grained to pegmatoid. Another of theirnoteworthy features is biotite rims along boundariesbetween the orthotectites and the host rocks, a featurethat reportedly [6] testifies that the rocks were formedby melting in situ. When the granites with positive Euanomalies occur among complicatedly folded graygneisses or dome structures of charnockites and ender-bites, they occur as randomly oriented veins (Figs. 2d, 2f).In the monoclinal sequences of the Lapland and Belo-morian complexes, the granites most commonlydevelop as veins with strikes parallel to those of thehost structures but with opposite dips (Figs. 2a2c, 2e).The granites usually compose of cutting veins, some ofwhich filled the fractures of brittle deformations. Someminerals of the host rocks, for example, hypersthene,bluish quartz, kyanite, and hornblende can be containedin the granite veins (although in smaller amounts and

    recrystallized). We are not aware of any data on theradiological ages of these granites, and their host rockswere dated at 3.1 (gray gneisses and amphibolites) to1.75 Ga (subalkaline granites) [2]. In the southeasternpart of the Lapland granulite belt (in the Porya Bayarea), the granites are intruded by 1.72-Ga lamproites[7]. Geological evidence indicates that the latter rockswere emplaced into material within a zone of brittledeformations and inherit the structural style from thegranite veins.

    The

    vein rocks

    that looked like granites in the expo-sures turned out to comprise of more than one petro-graphic variety (as can be seen in thin sections). Theseare biotite nebular plagioclase migmatites, leucocraticgranites, orthotectites, and muscovitequartz rocks.The rocks have granoblastic and, in places, hypidio-morphic-granular textures, and many of them exhibittraces of deformations, which are variably pronouncedin individual veins (samples 24/1, 9405/2, N41/2, andN033/3). Due to this, the textures of the rocks are cata-clastic: along with grains 12 mm, which are most typ-ical of these rocks, as they also contain smaller (0.10.2 mm) grains, which have uneven, ragged outlinesand occur between larger grains. Large plagioclasegrains are flattened and have a patchy extinction, andbiotite plates have stringy habits. All of the rock variet-ies have analogous compositions: they consists of pla-gioclase, quartz, microcline, biotite, an