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

439

ISSN 0016-7029, Geochemistry International, 2006, Vol. 44, No. 5, pp. 439–455. © Pleiades Publishing, Inc., 2006.Original Russian Text © E.N. Terekhov, T.F. Shcherbakova, 2006, published in Geokhimiya, 2006, No. 5, pp. 483–500.

INTRODUCTION

Long-term studies of high-grade metamorphic rocksin the eastern part of the Baltic Shield resulted in thefinding of their abundant acid varieties with positive Euanomalies. Rocks exposed at the Earth’s 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 Earth’s crust. Most of our sam-ples were collected at the Lapland–Belomorian 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 structural–geochemicalboundary of the Earth’s crust. Similar to the apparenthorizon, some boundaries in the Earth’s crust seem toalways occur at a distance. For example, the boundarybetween ductile and brittle deformations alwaysoccurs at approximately the same depth (10–15 kmfrom the surface), regardless of the processes that cantake place.

ANALYTICAL TECHNIQUES

All 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: [email protected]

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

KrKrKr

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

Caledonides

1

2

3

4

5

6

7

8

9

10

‡ b

‡ ‡

‡

‡

‡ ‡

b c

b

b b

b

b

AR


Vol. 44

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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 GEOLOGICAL–PETROGRAPHIC 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 1–50 cm or, occasionally, up to1.5 m. The granites sometimes occur as orthotectitesthat compose irregularly shaped patches or lenses 20–30 cm across. More rarely, orthotectite lenses arearranged along two perpendicular directions. Both thegranites and the orthotectites are very leucocratic rocksand contain no more than 2–5% 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. 2a–2c, 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 Por’ya 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 muscovite–quartz 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 1–2 mm, which are most typ-ical of these rocks, as they also contain smaller (0.1–0.2 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, and occasionalmuscovite and secondary aggregates of epidote-zoisite.The proportions of leucocratic minerals vary, particu-larly the amounts of microcline, which can be con-tained in the rocks in the form of antiperthitic inclu-sions, anhedral interstitial grains, or large porphyro-blasts. Some samples (sample 24/1) contain singleanhedral large (12

×

7 mm) microcline prisms. In thisrock, a large microcline grain contains small anhedraldomains of brown older plagioclase and myrmekiteswith small vermicular quartz grains that display simul-taneous extinction. The groundmass of this rock con-tains no microcline. Quartz is also unevenly distributedin the rock: the mineral can be contained in the form ofsmall equant or randomly and irregularly shaped grainsin the interstitial space or lens-shaped aggregates up to3–7-mm long. The quartz shows no traces of deforma-tions and was likely recrystallized after the cataclasisand mylonitization of the rock. Its amount varies from30% (of the total rock volume) in sample 24/1 to 50%in the samples 94103/4 and 9405/2. The strongly silic-

Fig. 1.

Schematic geological map of the Lapland–Belomorian Belt and surrounding structures with our sampling sites of acid rockswith positive Eu anomalies. (

1

) Alkaline intrusions: (

a

) Paleozoic, (

b

) Late Paleoproterozoic; (

2

) Paleoproterozoic granitoids:(

a

) postkinematic (1.8–1.7 Ga), (

b

) synkinematic (1.85–1.8 Ga), (

c

) prekinematic (2.6–2.4 Ga); (

3

) Sumian intrusive magmaticrocks (2.5–2.45 Ga) layered intrusions, coronites (drusites), and gabbro-anorthosites of the Lapland–Belomorian Belt); (

4

) volcano-sedimentary rocks of the Karelian Formation (2.6–1.8 Ga): (

a

) Ludicovian, Jatulian, (

b

) Sumian, Sariolian; (

5

) Late Archean(

a

) greenstone belts and (

b

) kyanite-bearing rocks of the Belomorian Belt; (

6)

Archean granite-gneisses of tonalite–trondhjemite–granodiorite composition; (

7

) Late Archean–Paleoproterozoic (

a

) migmatites and (

b

) garnet amphibolites; (

8

) granulites: (

a

) LateArchean and (

b

) Paleoproterozoic; (

9

) major faults and the Lapland–Belomorian detachment (heavy line), a normal fault that con-trolled the exhumation of the lower and middle crustal rocks to the surface; (

10

) sampling sites of acid rocks with positive Eu anom-alies (sample numbers correspond to those in (

a

) Tables 1, 2 and (

b

) in [2]). KM—Karelian Massif, BB—Belomorian Belt, LB—Lapland Belt, CKB—Central Kola Block, MB—Murmansk Block, PS—Pechenga Structure, I-V—Imandra–Varzuga Structure.

442


Vol. 44

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2006


Subalkaline granite

Maficgranulites

Acid granulite

Enderbite plagioclase migmatite developingGarnet amphibolite and plagioclase migmatite in the bottom part of the Lapland Belt

Granite-gneiss

charnockite

Subalkaline

granite

Granite-gneiss of the Archean basement

Migmatite

Plagioclase migmatite

Fine-grained

Vein granite

Pink and gray

100 m500

2 m10(‡) (b)

(c) (d)

(e) (f)

Granite-gneiss

BlastomyloniteMafic granulite

kyanite-bearing rock

granite-gneiss

after mafic granulite

(g) (h)


Vol. 44

No. 5

2006


ified rocks contain minor amounts of muscovite in theform of thin elongated (2–3 mm) laths. These granitesseem to be related to acid leaching zones and composetheir axial parts. In some instances, the granitesaffected by acid leaching are completely devoid of theiroriginal features and transformed into muscovite–quartz rocks (sample 9435/1) or preserve their originalfeatures in relict grains, as in sample 767/21, which is amuscovite–quartz rock with patches of old plagioclaseand skeletal kyanite prisms. The latter indicate that thepristine rock was kyanite–garnet–biotite gneissaffected by leaching. The mafic mineral of the veinrocks is dark olive biotite, whose contents vary fromsingle platelets to 2–5%. In places, the biotite isreplaced by zoisite or earthy aggregates of epidote.

It should be emphasized that the vein rocks some-times bear relics of hornblende–biotite (sample 24/1) orbiotite (sample N033/30) plagioclase migmatites, con-taining 15–20% biotite. Relics of these pristine rocksare also distinguishable by the occurrence of older pla-gioclase, which is completely saussuritized. The rocksoften display traces of cataclasis, partial mylonitiza-tion, and blastesis, which suggest brittle deformationsin the vein rocks during their final evolutionary stages.The data presented above and the fairly uneven distri-

bution of microcline in the vein rocks indicate that theywere formed in situ.

The geological nature of the vein granites with pos-itive Eu anomalies is uncertain and was interpretedvariably. To some extent, this is explained by the factthat these rocks often occur among gray gneisses,whose genesis is also disputable. Most foreignresearchers believe that gray gneisses were producedvia magmatic crystallization, and veins of granites withpositive Eu anomalies are the final products of this pro-cess [8]. Most Soviet and Russian geologists considergray gneisses to be the products of metasomatic alter-ations in mafic rocks, and the granites with positive Euanomalies are thought to correspond to the final stage ofgranitization: the microcline stage of this process [9].The results of our research indicate that granites withpositive Eu anomalies are present not only among graygneisses, but also among granulites, anorthosites, mas-sive and gneissose charnockites and enderbites, subal-kaline granites, quartz monzonites, and kyanite-bearingrocks, and hence, studying these rocks may shed lightonto global events in the evolution of the Earth’s crust.

The

blastomylonites

compose tectonic zones rang-ing from a few to a few hundred meters in thickness.These are pale, small-grained finely scaled rocks, often

Fig. 3.

Al

2

O

3

/(Na

2

O + K

2

O) vs. Al

2

O

3

/(CaO + Na

2

O +K

2

O) diagram for (

1

) potassic granite, (

2

) sodic granite, and(

3

) blastomylonite. Compositional fields of granites fromvarious geodynamic environments (after [11]): CAG—con-tinental arc granites, CCG—continental collisional gran-ites, RRG—rift-related granites, POG—postorogenic gran-ites, CEUG—continental epeirogenic granites.

Fig. 4.

(Na

2

O + K

2

O) vs. SiO

2

diagram. (

1

–

4

) Acid rockswith positive Eu anomalies: (

1,

2

) vein granite: (

1

) potassic,(

2

) sodic; (

3

,

4

) blastomylonite: (

3

) potassic, (

4

) sodic;(

5

) host rocks. (I) Tonalite; (II) plagiogranite, (III) granite,(IV) leucogranite; (V) subalkaline granite, (VI) subalkalineleucogranite. Compositional fields of granitoid series: Al—alkaline, Ca—calc–alkaline, Th

—

tholeiitic (after [12]).

CAG

POG

CEUG

RRG

CCG

Highly aluminousModerately

Alkaline

1

23

0.7 0.9 1.1 1.3 1.5 1.7 1.9

0.6

1.4

1.8

2.2

1.0

2.6

3.0

0.5Al

2

O

3

/(CaO + Na

2

O + K

2

O) (molar)

Al

2

O

3

/ (Na

2

O + K

2

O) (molar)

aluminous

Fig. 2.

Schematic vertical sections showing the structural settings of acid rocks with positive Eu anomalies: (a)–(f) vein granites,(g, h) blastomylonite zones in various complexes in the eastern part of the Baltic Shield. Sketches correspond to (a) the central,(b) upper rear, and (c) lower rear parts of the Lapland Belt; Belomorian Belt (d) western part (area with overprinted granulite mineralassemblages) and (e) central (amphibolite–migmatite) parts and (f) the pre-Karelian basement. The structural situations (g) and(h) can occur in any of the compositions in the eastern Baltic Shield metamorphosed to the granulite and amphibolite metamorphicfacies.

60

2

4

6

8

10

065 70 75

SiO

2

, wt %

Na

2

O + K

2

O, wt %

12

45

3

Al

Ca

Th

I

IV

VIV

II

III

444


Vol. 44

No. 5

2006


Tab

le 1

.

Che

mic

al c

ompo

sitio

n of

vei

n gr

anite

s w

ith p

ositi

ve E

u an

omal

ies

and

of r

ocks

ass

ocia

ted

with

the

gran

ites

in th

e ea

ster

n pa

rt o

f th

e B

altic

Shi

eld

Com

pone

ntK

79/8

K79

/11

K84

/5

K84

/8K

88

K89

/4

K89

/7

K10

2/4

K10

2/6

K10

4/13

K10

4/9

1

2

3

45

6

7

8

9

10

11

SiO

2

75.5

1

73.7

4

63.5

9

73.7

673

.98

65.2

272

.96

73.0

574

.32

73.2

170

.19

TiO

20.

120.

040.

710.

10.

030.

610.

050.

070.

030.

010.

06

Al 2

O3

12.9

614

.88

17.4

615

.46

13.6

916

.19

15.5

14.6

214

.89

14.7

915

.27

Fe2O

32.

762.

157.

031.

890.

795.

572.

042.

041.

241.

551.

27

MnO

0.10

0.02

0.08

0.02

0.01

0.05

0.03

0.04

0.01

0.03

0.17

MgO

0.40

0.15

3.27

0.52

0.04

2.82

0.4

0.38

0.03

0.22

0.42

CaO

2.95

2.97

2.2

2.87

0.21

3.07

2.53

1.81

3.02

1.34

2.03

Na 2

O4.

145.

342.

594.

271.

913.

643.

693.

394.

383.

592.

03

K2O

0.55

0.35

2.1

0.55

8.95

2.02

2.43

4.39

0.41

4.98

10.0

6

P 2O

50.

030.

020.

090.

130.

010.

060.

080.

050.

020.

040.

02

LO

I0.

490.

320.

920.

420.

220.

780.

210.

060.

330.

060.

26

Cr

4920

449

228

2316

922

1634

176

10

Ni

3413

188

235

9119

815

169

Co

426

456

n.f.

3111

42

323

V17

837

676

211

313

4440

115

47

Rb

348

7811

416

8034

8920

116

240

Ba

270

120

370

200

1700

700

1200

1200

300

600

2400

Sr15

037

020

034

033

031

035

027

041

017

051

0

Zr

108

7423

924

910

882

––

––

Y–

n.f.

–8

110

5–

n.f.

––

La

14.0

3.1

25.0

12.0

4.6

28.0

13.0

37.0

<3.

023

.05.

1

Ce

38.0

5.2

52.6

23.4

8.0

61.0

28.3

77.0

<5.

047

.04.

4

Nd

17.7

3.7

25.1

9.3

4.5

20.0

8.4

22.0

<5.

015

.03.

0

Sm3.

52.

6 5.

02.

0n.

f.3.

43.

45.

4<

2.0

3.4

1.0

Eu

0.5

0.4

1.2

0.5

0.6

0.9

1.4

1.3

<0.

50.

50.

25

Gd

3.0

0.8

3.0

1.5

n.f.

2.0

1.0

1.4

<1.

02.

40.

80

Er

3.4

0.4

3.4

1.0

n.f.

1.4

0.8

1.2

<1.

51.

50.

4

Yb

3.7

0.25

1.7

0.4

0.2

0.8

0.44

0.94

<0.

21.

10.

24

(La/

Yb)

n2.

810

1018

1622

1928

1415

Eu/

Eu*

0.49

1.5

0.89

1.28

3.5

0.96

1.7

1.09

0.47

1.6

GEOCHEMISTRY INTERNATIONAL Vol. 44 No. 5 2006


Tab

le 1

. (C

ontd

.)

Com

pone

nt24

/224

/194

05/1

9405

/294

103/

194

103/

4K

17/1

K17

/3K

17/6

K24

/2K

24/5

1213

1415

1617

1819

2021

22

SiO

248

.11

70.6

765

.66

74.6

274

.81

76.3

047

.74

75.6

70.4

972

.39

74.3

4

TiO

21.

010.

20.

690.

020.

130.

130.

840.

070.

230.

570.

01

Al 2

O3

14.4

515

.43

15.5

614

.27

13.3

913

.55

20.8

11.6

214

.56

12.7

815

.02

Fe2O

312

.59

1.37

5.23

0.86

2.5

1.54

11.4

31.

922.

315.

921.

42

MnO

0.23

0.02

0.07

0.01

0.03

0.01

0.2

0.02

0.06

0.05

0.02

MgO

7.13

0.37

2.24

0.06

0.41

0.43

5.61

1.07

1.0

2.27

0.36

CaO

11.8

41.

023.

151.

532.

213.

0811

.86

0.54

1.88

1.2

0.61

Na 2

O3.

032.

914.

363.

423.

354.

141.

881.

972.

761.

711.

94

K2O

0.74

6.97

2.07

4.87

2.7

0.58

0.3

5.90

6.23

2.55

5.93

P 2O

50.

080.

050.

270.

020.

030.

050.

430.

020.

010.

070.

14

LO

I0.

810.

460.

630.

180.

330.

230.

021.

250.

550.

560.

23

Cr

196

––

––

308

1112

08

Ni

132

944

716

2232

77

6014

Co

50–

––

–35

55

186

V27

0–

––

–40

06

2812

012

Rb

2411

313

612

856

148

9010

010

514

5

Ba

219

6660

395

1611

1115

358

100

1800

5440

570

1230

Sr14

071

632

933

130

828

982

013

040

015

024

0

Zr

2053

132

3793

190

4311

029

200

13

Y35

1119

5n.

f.6

––

––

–

La

7.9

19.0

14.0

5.8

47.0

12.0

9.0

32.5

23.0

38.0

10.9

Ce

16.0

28.0

25.0

6.7

80.0

19.0

23.4

65.0

26.3

74.5

14.6

Nd

11.0

7.4

14.0

1.8

24.0

7.9

15.8

23.2

4.7

35.3

6.5

Sm3.

41.

43.

50.

63.

41.

45.

25.

21.

17.

43.

6

Eu

0.9

1.6

0.64

0.7

0.89

0.5

1.4

0.7

2.4

1.2

1.9

Gd

4.2

1.2

2.5

0.5

0.98

0.25

3.8

2.6

1.6

5.0

0.9

Er

2.0

0.9

0.47

0.24

0.2

0.2

2.2

1.6

0.6

2.5

0.7

Yb

1.1

0.1

0.31

0.03

0.11

0.14

1.5

0.7

0.3

2.0

0.6

(La/

Yb)

n4.

812

040

170

280

604.

838

4512

14

Eu/

Eu*

0.77

4.6

0.63

3.75

1.13

1.48

0.92

0.5

60.

564.

1

446



Tab

le 1

. (C

ontd

.)

Com

pone

ntN

41/1

N41

/2N

42/4

N42

/2N

44/1

N44

/220

02/1

2002

/2N

033/

2N

033/

3

2324

2526

2728

2930

3132

SiO

265

.97

73.1

664

.35

74.4

763

.22

67.8

771

.81

71.3

671

.473

.06

TiO

20.

420.

040.

190.

030.

780.

050.

30.

030.

260.

13A

l 2O

315

.47

15.5

815

.51

14.1

614

.96

15.2

114

.73

16.2

814

.75

15.5

4Fe

2O3

3.97

0.86

2.39

0.84

5.76

0.67

2.59

0.74

2.52

1.73

MnO

0.05

0.01

0.02

0.01

0.06

0.01

0.06

0.01

0.04

0.02

MgO

1.97

0.58

1.37

0.28

2.94

0.39

0.5

0.05

1.13

0.48

CaO

4.0

1.87

1.55

0.76

1.71

0.78

1.23

0.21

1.55

1.49

Na 2

O4.

754.

83.

44.

633.

053.

573.

692.

883.

935.

03K

2O1.

374.

46.

214.

663.

637.

014.

238.

164.

813.

49P 2

O5

0.13

0.01

0.06

0.88

0.04

0.08

0.09

0.04

0.06

0.06

LO

I1.

050.

180.

470.

630.

071.

01.

00.

450.

050.

03N

i31

931

2468

1313

1411

11R

b83

9013

863

117

326

132

184

111

85B

a47

974

416

3928

010

6325

8816

2413

5518

7011

01Sr

302

304

334

138

340

–24

832

829

732

2Z

r20

026

131

5013

745

285

2111

510

0Y

118

1413

1613

346

1613

La

12.0

2.4

27.0

2.1

27.8

8.7

79.0

2.2

25.0

5.7

Ce

23.2

5.2

51.0

2.4

60.9

16.1

119.

03.

840

.016

.0N

d8.

41.

623

.01.

925

.45.

645

.01.

919

.04.

8Sm

2.1

0.2

5.3

0.2

4.31

1.39

8.0

0.2

3.3

0.77

Eu

0.39

0.16

1.0

0.3

0.93

1.01

1.3

0.6

0.96

0.45

Gd

1.03

0.5

1.9

0.68

3.38

1.12

4.7

0.8

1.4

0.97

Er

0.31

0.24

0.4

0.66

1.66

0.30

2.4

0.2

0.3

0.59

Yb

0.2

0.08

0.05

0.13

1.55

0.19

2.2

0.04

0.26

0.14

(La/

Yb)

n36

830

013

1120

2450

7018

Eu/

Eu*

0.73

2.1

0.79

2.1

0.72

2.4

0.6

3.5

1.16

1.61

Not

e:(1

–11)

Sam

ples

fro

m t

he c

entr

al p

art

of t

he B

elom

oria

n B

elt

[(1–

7) r

oadw

ay c

uttin

g of

the

St.

Pete

rsbu

rg–M

urm

ansk

hig

hway

]: (

1, 2

) N

igro

zero

Lak

e ar

ea, A

rctic

Cir

cle

(1—

plag

iocl

ase

mig

mat

ite, 2

—gr

anite

); (

3, 4

) 40

km

sou

th o

f N

igro

zero

Lak

e (3

—pl

agio

clas

e m

igm

atite

, 4—

gran

ite);

(5)

gra

nite

, wes

tern

par

t of

Niz

hnee

Kot

ozer

o la

ke; (

6, 7

) 10

km

sout

h of

Nig

roze

ro la

ke (

6—pl

agio

clas

e m

igm

atite

, 7—

gran

ite);

(8–

11)

near

the

villa

ge o

f Po

n’go

ma

(8, 1

0—ho

st m

igm

atite

-gra

nite

, 9, 1

1—ve

in g

rani

te);

(12

–17)

wes

tern

par

tof

the

Bel

omor

ian

Bel

t (1

2, 1

3—N

otoz

ero

Lak

e ar

ea:

12—

mafi

c gr

anul

ite, 1

3—gr

anite

, 14,

15—

nort

hwes

tern

ban

k of

Gab

ozer

o L

ake:

14—

char

no-e

nder

bite

, 15—

gran

ite;

16,

17—

nort

hern

sho

re o

f Vaz

henk

a L

ake:

16—

nebu

lar

plag

iocl

ase

mig

mat

ite, 1

7—gr

anite

); (

18–2

2) L

apla

nd G

ranu

lite

Bel

t, U

mba

str

uctu

re (

18–2

0—Sh

omba

ch C

ape:

18—

pris

tine

mafi

c gr

anul

ite,

19—

mig

mat

ite-c

harn

ocki

te;

20—

gran

ite;

21,

22—

15 k

m n

orth

of

the

settl

emen

t of

Um

ba;

21—

mig

mat

ite,

22—

gran

ite);

(23

–32)

nor

ther

n N

orw

ay (

23–2

6—K

irke

nes

area

: 23

—pl

agio

clas

e m

igm

atite

, 24—

gran

ite, 2

5—m

igm

atite

, 26—

gran

ite;

(27,

28)

Jar

fiord

mou

th:

27—

plag

iocl

ase

mig

mat

ite w

ith c

ordi

erite

, 28—

gran

ite;

29–3

2—K

aras

jok

area

: 29,

30—

20 k

m s

outh

of

it: 2

9—su

balk

alin

e gr

anite

, 30—

vein

gra

nite

, 31,

32—

35 k

m w

est o

f th

e to

wn

of K

aras

jok,

Arc

hean

Jar

gul b

asem

ent c

ompl

ex: 3

1—gr

aygn

eiss

, 32—

vein

gra

nite

. Das

hes

mea

n no

t ana

lyze

d; n

.f. m

eans

not

fou

nd d

ue to

low

con

cent

ratio

ns. P

rint

ed in

ital

ics

are

vein

gra

nite

s, n

orm

al f

ace—

host

roc

ks.



Tab

le 2

. C

hem

ical

com

posi

tion

of b

last

omyl

onite

s w

ith p

ositi

ve E

u an

omal

ies

and

thei

r ho

st r

ocks

Com

po-

nent

767/

176

7/2

767/

1876

7/21

9412

5/1

778/

194

22/2

9431

/194

35/1

751/

275

1/3

350

F33

/6

12

34

56

78

910

1112

13

SiO

250

.49

62.0

543

.07

75.9

076

.44

75.5

670

.48

74.2

875

.44

52.3

974

.05

63.4

761

.31

TiO

21.

080.

021.

180.

020.

040.

010.

040.

170.

052.

630.

140.

630.

67A

l 2O

321

.78

22.8

427

.06

15.3

513

.93

12.5

115

.33

14.2

713

.814

.63

14.4

316

.26

20.9

1Fe

2O3

11.5

31.

2311

.97

0.92

0.93

1.31

2.79

1.64

1.02

14.9

81.

78.

956.

16M

nO0.

010.

020.

150.

010.

010.

010.

030.

010.

010.

170.

020.

090.

06M

gO4.

950.

286.

410.

210.

10.

260.

850.

340.

093.

020.

213.

852.

78C

aO3.

994.

137.

192.

111.

150.

352.

721.

581.

447.

411.

392.

720.

54N

a 2O

5.21

8.99

2.36

3.03

4.75

1.97

4.21

4.42

2.66

3.6

4.4

2.47

1.85

K2O

0.91

0.16

0.3

1.51

2.00

7.44

2.65

2.73

5.25

0.88

3.3

1.04

4.97

P 2O

50.

080.

150.

020.

10.

040.

020.

110.

040.

10.

50.

030.

10.

11L

OI

0.16

0.11

0.25

0.79

0.52

0.35

0.39

0.37

0.1

0.15

0.25

0.42

0.07

Ni

9417

223

13Ì/

Ó9

1112

1243

1277

47R

b34

142

2760

172

7253

101

Ì/Ó

110

1811

9B

a40

130

210

460

8916

0080

013

9011

0935

055

165

615

40Sr

402

606

357

238

9019

745

748

032

147

135

061

626

0Z

r14

954

148

9n.

f.56

107

9741

169

9312

616

5Y

2314

3310

514

149

731

14–

32L

a22

.07.

029

.06.

85.

813

.013

.016

.04.

126

.012

.029

.031

.0C

e63

.015

.065

.07.

99.

324

.025

.026

.05.

072

.022

.047

.056

.0N

d22

.06.

729

.03.

83.

59.

97.

88.

72.

642

.05.

318

.022

.0Sm

4.4

1.0

6.9

1.1

0.59

1.8

1.6

2.2

0.5

8.9

2.1

4.3

4.7

Eu

1.0

1.1

1.3

0.39

0.5

0.9

0.63

0.6

0.48

2.7

1.2

1.9

1.8

Gd

4.2

1.6

4.8

0.57

1.0

1.2

0.77

0.4

0.5

7.7

1.9

4.6

3.4

Er

1.7

1.2

3.2

0.46

0.2

0.43

0.2

0.8

0.2

3.6

1.0

3.0

2.5

Yb

2.6

0.4

3.7

0.1

0.1

0.31

0.11

0.05

0.03

2.2

0.4

2.8

2.2

(La/

Yb)

n5.

213

4.9

4036

2878

230

124

815

79

Eu/

Eu*

0.7

2.62

0.67

1.42

2.5

1.8

1.51

1.27

2.8

0.97

1.8

1.3

1.34

Not

e:(1

–6)

Cen

tral

par

t of

the

Bel

omor

ian

Bel

t: (1

–4) V

arat

skoe

cor

undu

m o

ccur

renc

e, (

1) a

mph

ibol

ite, (

2) o

ligoc

lasi

te, (

3) k

yani

te-b

eari

ng a

mph

ibol

ite, (

4) q

uart

z–ky

anite

–mus

covi

tem

etas

omat

ic r

ock,

(5)

wes

tern

par

t of

the

Kop

at-o

zero

Lak

e ar

ea, q

uart

z–m

usco

vite

roc

k; (

6) s

outh

ern

part

of

Pezh

ostr

ov I

slan

d, n

ebul

ar m

igm

atite

; (7

–11)

wes

tern

par

t of

the

Bel

omor

ian

Bel

t, N

otoz

ero

Lak

e ar

ea, (

7–9)

bla

stom

ylon

ites

with

lent

icul

ar q

uart

z de

velo

ping

aft

er g

ray

gnei

sses

; (10

) int

rusi

ve e

nder

bite

and

(11)

bla

stom

ylon

ite d

evel

opin

g af

ter

it; (

12)

acid

gne

isso

se g

ranu

lite,

Bel

ozer

skay

a B

ay, K

olvi

tsa

stru

ctur

e; (

13)

anat

ectic

aci

d gr

anul

ite, 1

0 km

sou

thea

st o

f th

e se

ttlem

ent o

f In

ari,

nort

hern

Fin

land

. Pri

nted

in it

alic

sar

e bl

asto

myl

onite

s an

d ot

her

rock

s in

flui

d-re

wor

king

zon

es, n

orm

al f

ace—

host

roc

ks.

448



with augen textures and variable amounts and sizes ofthe augen. The schistosity of these rocks is accentuatedby the orientation of biotite flakes, with quartz lenseselongated conformably with this orientation. The blas-tomylonites most commonly contain augen of feldspars(which sometimes account for 25% of the rock by vol-ume) and, more rarely, garnet. The schistosity planes ofthe blastomylonites often bear slickensides, which tes-tify to their development at brittle deformations duringthe final stages of the process.

The microtexture of the rocks is blastomylonitic andis accentuated by small (0.3–0.5 mm) biotite flakes,some of which are recrystallized and reach 1.5 mm inlength. The texture is characterized by a fine-grained(<1 mm) groundmass with larger plagioclase relics,which were preserved during shattering and myloniti-zation. These relics are usually rounded or ellipsoidaland have patchy extinction. These can be porphyro-clasts (“augen”) and porphyroblasts. The latter are usu-ally large (up to 3–5 mm) grains of quartz and feldspar.The blastomylonites sometimes contain chains of small(<0.1 mm) plagioclase grains, which develop as com-plete or incomplete rims around larger grains. Someporphyroblasts are surrounded by biotite flakes. Sam-ple 9431/1 is a typical blastomylonite with a character-istic schistose–banded structure. The schistosity isaccentuated by systematically oriented biotite laths andflakes, and the banding is defined by thin (0.2–0.4 mm)equally spaced laminae of recrystallized quartz elon-

gated conformably with the schistosity, which impart alook of thin lamination to the rocks.

The blastomylonites consist of plagioclase, quartz,microcline, and biotite. The secondary minerals are epi-dote, epidote–zoisite aggregates, and earthy epidote–saussurite masses. Accessory minerals are generallyatypical. The quantitative proportions of major miner-als vary (with greater or lesser amounts of plagioclaseand microcline). The contents of biotite range from 3–5to 15%. Some mylonite varieties are more recrystal-lized, so that the mineral grains in them reach 1–2 mm,and traces of mylonitization can be identified by thepresence of single flattened plagioclase crystals oraggregates of small (<0.1 mm) plagioclase grains withragged outlines between larger grains of other minerals(samples 9422/2 and 9435/1). The mineralogy of theblastomylonites corresponds to that of biotite plagiomigmatites, their nebular varieties, and granites. Some15–20 years ago, most mylonites were considered to becomponents of stratified Early Precambrian complexes.Now their affiliation with tectonic zones is beyonddoubt, although some researchers still continue to classthese rocks with banded metasediments [10].

PETRO- AND GEOCHEMISTRY OF ACID ROCKSWITH A POSITIVE Eu ANOMALY

Regardless of their affiliation with certain tectono–stratigraphic complexes, all of the rocks with positiveEu anomalies are rich in SiO2 (70–76%), and most ofthem are highly aluminous (Fig. 3). The highest silicacontents were detected in rocks affected by acid leach-ing. These varieties are strongly depleted in ä2é (0.35–0.58 wt %) and are relatively rich in CaO (3–4 wt %).Note that most of the rocks bear as little as 1–1.5 wt %CaO. Most of these rocks are rich in ä2é, whose con-tents occasionally reach anomalously high values (7–10 wt %), a fact definitely suggesting the metasomaticintroduction of this element (Tables 1, 2). Another nota-ble feature of these rocks is their depletion in such ele-ments as Ti, Fe, Mn, Mg, and P, which makes the rocksleucocratic. According to their proportions of the sumof alkalis to silica, most of them can be classed withsubalkaline granites, with a few samples correspondingto leucogranites and subalkaline leucogranites (Fig. 4).Figure 4 also shows the compositional fields of rockseries, and it can be seen that the data points of the veingranites plot within the fields of the tholeiitic and alka-line series. At the same time, the host rocks and blasto-mylonites (which usually inherit compositions from thehost rocks) affiliate with the calc–alkaline series.

A notable feature in the distribution of trace ele-ments is the depletion of the rocks in rubidium, whoseconcentrations are 1.5–3.5 (and occasionally even 7–14 times) times lower than the average rubidium con-centration in lithospheric granite [13]. It should bementioned that some of the rocks have Rb concentra-tions approaching the clarkes for granites (184 as com-

CaO, wt %

Sr, ppm0

1

100

(a)

K2O, wt %

Ç‡, ppm

200 300 400 500 600 700

2

3

4

(b)

0

2

4

6

8

10

2500200015001000500 5500 6000 6600

~ ~

Fig. 5. (a) CaO vs. Sr and (b) K2O vs. Ba diagrams for acidrocks with positive Eu anomalies from the eastern part ofthe Baltic Shield.



pared with 210 ppm) or even slightly exceeding them(237 ppm), but these petrographic varieties are rare.Although we did not identify any clearly pronouncedtendencies in the distribution of Rb and K in the rocks,the highest Rb concentrations were found in their vari-eties with anomalously high concentrations of ä2é (8–9 wt %). The general poorness of rocks in Rb isexplained by their insignificant contents of biotite,since Rb contained in acid rocks is known [14] to bepreferably concentrated in micas compared to otherK-bearing minerals. It was demonstrated previouslythat the Rb concentration in biotite from granites andplagioclase granites from the White Sea area (Belomo-rie) is 370–510 ppm [15].

Zirconium. Similar to Rb, these rocks are depletedin Zr, whose contents are two–five times lower thangranite clarke. When granitization of mafic rocks wasstudied, it was determined that Zr is correlated with Ti[15]. Conceivably, the low contents of TiO2 in the gran-ites (mostly 0.03–0.13 wt %) and the absence of acces-sory zircon cause low Zr concentrations.

Barium and strontium. The distribution of Ba isknown to be controlled by the ability of this element toenrich K-bearing minerals. All of our rocks (exceptonly their occasional varieties) contain very littlebiotite, and their potassic feldspar is the only mineralable to concentrate Ba. The potassic feldspar of theBelomorian biotite granites contains 744 ppm Ba [15],whereas the concentration of this element in our rocksis often much higher. The Ba concentrations in the veinrocks vary from 120 to 6600 ppm (Table 1), and thecontents of this element in the blastomylonites are nota-bly lower (89–1600 ppm (Table 2)). As can be seenfrom the tables, all samples, including those of rockswith low K2O concentrations, are much higher in Bathan the average crustal granite (830 ppm). Thereseems to be a direct correlation between the concentra-tions of Ba and K2O in our rocks, although this correla-tion is not very strong (Fig. 5). All rocks with no morethan 1 wt % K2O are the poorest in Ba, but the varietieswith approximately 7% K2O have Ba concentrations ashigh as 1600, 2600, and 6600 ppm.

Fig. 6. Chondrite-normalized REE patterns for vein granites and their host rocks from various areas in the eastern Baltic Shield.Sample numbers correspond to those in Tables 1 and 2.

(‡)

(d)

(g) (h) (i)

(f)

(c)(b)

(e)

100

0.5

Rock/chondrite

ä79Nigrozero Lake area

k79/8

1.0

510

50

100

0.51.0

510

50

100

0.51.0

510

50 Plagioclase

Plagioclase Plagioclase Plagioclase

k89/4

K84Tedino area

Granite

GraniteGranite

Granite

Granite Granite Granite

Granite Granite

Granite

k24/1k104/9

94103/4k9405/1

k17/6

k94103/1

k89/7

k84/8

k88k79/11

K89

K104 24

9405 94103

Í17

K102

Pon’goma area

Migmatite k102/4

k102/6

Nebular migmatitek104/13

Notozero Lake areaTwo-pyroxene

(24/2)

Notozero Lake area

Charnoenderbite(9405/1)

Vazhenka Lake

Umba Structure,

Mafic

k17/1

Migmatitek17/3

La Ce Nd Sm Eu Cd Er YbLa Ce Nd Sm Eu Cd Er YbLa Ce Nd Sm Eu Cd Er Yb

Pon’goma area

migmatite migmatitek84/5

migmatite

granulite

migmatite

Shombach

granulite

450



All of the granitoids contain up to 1.5–6 Sp clarkestypical of lithospheric granites (except only occasionalsamples, whose Sp contents are close to the clarkes:90–150 ppm and 110 ppm, respectively) (Table 1). Sr iscommonly isomorphous with Ca in various Ca-bearing

minerals of magmatic rocks, mostly in plagioclase [16].As can be seen from Fig. 5, the increase in the Sr con-centrations in our rocks is not correlated with theincrease in their Ca contents. Similar to Ba, the accu-mulation of Sr in the feldspars of magmatic rocks isdirectly correlated with their crystallization tempera-ture [17]. Hence, the Ba and, particularly, Sr concentra-tions of the rocks are not correlated with the concen-trations of major “host” elements (K and Ca), and theenrichment of the former was likely controlled bysome other phenomena. One of the reasons for thesignificant enrichment of these elements in the gran-ites and blastomylanites could be their anomalouslyhigh temperatures, which is also corroborated byexperimental data [16].

Rare earth elements. The rocks in question are char-acterized by very low concentrations of REE (Figs. 6,7). Practically all the samples (except only two) bear 2–30 times less La, Ce, Nd, and Sm than crustal granites.The granitoids are depleted even more in HREE, whoseconcentrations in some samples are close to chondritic.

Fig. 7. Chondrite-normalized REE patterns for vein granites and their host rocks from (a–e) various areas at the Baltic Shield and(f–i) blastomylonites of acid composition.

Eu/Eu*

(La/Yb)n

6.0

5.0

4.0

3.0

2.0

1.0

0.920 40 60 80 100 120

„‡ÌËÚGranite Blastomylonite

Fig. 8. Magnitude of Eu anomalies (Eu/Eu*) as a functionof REE fractionation (La/Yb)n.

10050

105

1.00.5

10050

105

1.00.5

10050

105

1.00.5

LaCe

NdSm

EuCd

Er Yb LaCe

NdSm

EuCd

Er Yb LaCe

NdSm

EuCd

Er Yb

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

ä24—15 km north of the

Migmatite k24/2

Granite k24/5

GraniteN41/2

Plagioclase migmatiteN41/1

N41, Kirkenes area N42 Kirkenes area

MigmatiteN42/4

GraniteN42/2

N44 Jarfjord

Gneisses with

GraniteN44/2

2002 and N2033,

Subalkaline

N033/2Gray gneiss

Granite2002/2 N033/3

767 Chupa area

Amphibolite with 767/18

Amphibolite

Quartz

767/21 Oligoclasite

767/2

Anatectic granuliteF33/6

Acid granulite350

Nebular

778/1

Quartz-muscovite

94125/1

Silicified gray gneiss9431/1

Mylonite after

9422/1

Silicified

9435/1

751 eastern side

Enderbite751/2

Acid blastomylonite751/3

Rock/chondrite

settlement of Umba

cordierite (N44/1)

Karasjok area

granite 2002/1kyanite

metasomatic rock

migmatite

metasomatic rock

gray gneiss

muscovite granite

of Notozero Lake



The depletion of the granites in REE is particularly con-spicuous in comparison with the host acid rocks (Figs.6, 7). At the same time, the granites often have veryhigh (La/Yb)n ratios (>130). There is no clear correla-tion between the magnitudes of the positive Eu anoma-lies and the (La/Yb)n ratios (Fig. 8). The low HREEconcentrations are related to the leucocratic characterof the rocks, which contain relatively few mafic miner-als (2–5%), mostly biotite. The rocks typically bear noaccessory minerals (some of which are known to beenriched in REE). The aforementioned configurationsof REE patterns and positive anomalies are characteris-tic of plagiogranite and granites of mostly metamorphicor anatectic genesis [18], although our rocks span abroader petrographic spectrum: they are not only pla-giogranites and granites but also plagiomigmatites andmigmatites, their nebular varieties, plagioclasites, blas-tomylonites of granitic composition, and muscovite–quartz rocks from acid leaching zones. All of them havepositive Eu anomalies. As can be seen from Tables 1and 2 and from Figs. 8–10, the height of the Eu anom-aly in the rocks is independent of their chemical com-position.

All REE are trivalent, but Eu3+ is transferred intoEu2+ in reducing environments. Because of this, melt-ing in the presence of reduced fluids (hydrogen orhydrocarbons) is favorable for the preferential genera-tions of Eu2+. Sites available for Eu2+ are unavailablefor Eu3+. For example, feldspars more easily accommo-date Eu2+, whose ionic radius is closer to that of Sr2+,and Eu2+ can substitute Ba2+ in Ba-bearing minerals[20]. The ionic radius of Eu2+ is larger than that of Eu3+

[19], and, thus, the reduction of Eu3+ to Eu2+ is facili-tated, in addition to the presence of a reduced fluid, alsoby extensional environments.

Several researchers (for example, A.A. Beus)addressed themselves to the experimental data [16] andpointed out the preferable accommodation of Eu2+

(compared to Eu3+) in the feldspar structure. Thisimplies that the origin of granites with positive Euanomalies is associated with a shift of the Eu2+ – Eu3+

equilibrium toward Eu2+. According to this scheme, themagnitude of the Eu anomaly should be directly pro-portional to the Ca and Sr concentrations, but this is notthe case with our rocks (Figs. 9, 10). The absence ofcorrelations between high positive Eu anomalies andthe Sr concentrations was also mentioned by otherresearchers [21]. The highest Eu anomalies are typicalof granites enriched in Ba, i.e., an element whoseenrichment in a rock is often thought to be caused bydeep-seated fluids [22]. The vein granites also showclear correlations between the height of the positive Euanomalies and the Ba concentrations, but no such cor-relations were detected in the blastomylonites (Fig. 11).The Eu concentrations in the vein granites decreasecompared to the concentrations of this element in theamphibolite-facies rocks. At the same time, the granites

cutting across the rocks with granulite assemblages arepractically always notably enriched in Eu (Figs. 6, 7).

DISCUSSION

Granites depleted in LREE and with well-pro-nounced negative Eu anomalies are spread quitewidely. Depletion in LREE in the course of crystalliza-

Eu/Eu*

6.0

5.0

4.0

3.0

2.0

1.0

0.9100 200 300 400 500 600 700 800 900 1000

Sr, ppm

„‡ÌËÚGranite Blastomylonite

Eu/Eu*6.0

5.0

4.0

3.0

2.0

1.0

0.90.25 0.75 1.25 1.75 2.25 3.252.75

CaO, %

Granite Blastomylonite

Eu/Eu*6.0

5.0

4.0

3.0

2.0

1.00.9

„‡ÌËÚ

200

400

600

800

1000

1200

1400

1600

1800

2000

3000

4000

5000

6000

Ba, ppm

Granite Blastomylonite

Fig. 9. Magnitude of Eu anomalies (Eu/Eu*) as a functionof Sr concentration in the rocks.

Fig. 10. Magnitude of Eu anomalies (Eu/Eu*) as a functionof CaO concentration in the rocks.

Fig. 11. Magnitude of Eu anomalies (Eu/Eu*) as a functionof Ba concentration in the rocks.

452



Fig. 12. Structural setting of acid rocks (granites and blastomylonites) with positive Eu anomalies in the vertical section of theEarth’s crust during the development of a normal fault and changes in the fluid regime.

Setting of fluid systems before the development of the normal fault

Setting of fluid systems during the evolution of the normal fault

Oxidized fluids

Boundary between brittle and ductile deformations

Boundary of fluid systems Would-be fault

Reduced fluids

Would-be degassingpathways

Would-be sites of shockdecompression

Upper crust Normal fault

Oxidized fluids

Intrusion in the lowercrust

Would-be boundary between fluid systems

Lower crust

Reduced fluids

Upper crust

Middle crust

Mantle and

Asthenosphere

Boundary between the brittle

Boundary between the fluid

Veins of granites

Zones of blastomylonites

(brittle deformationzone)

(plastic deformation zone)

lithosphere

and ductile deformation zones

systems during the evolution of the normal fault and the destruction of the transition zone

with positive Eu anomalies

with positive Eu anomalies (not to scale)



tion in the sequence from granodiorites to granites withthe development of negative Eu anomalies is usuallyexplained by the crystallization of minerals concentrat-ing REE (such as monazite and apatite) and theirremoval from the magma [23]. Granites with positiveEu anomalies are rarer and occur mostly in the Precam-brian TTG series or, occasionally, in modern geody-namic environments [12]. With rare exceptions [9, 18],their genesis is also thought to be related to magma dif-ferentiation [3, 8]. Considering REE distribution inrocks, most researchers either do not explain the gene-sis of the Eu anomalies at all or, if the anomalies arepositive, relate them to high plagioclase contents in therocks (i.e., high concentrations of Sr, an element iso-morphous with Eu [1]). When the orthogneisses oftonalite–granodiorite composition in the InnerHebrides were studied, it was determined that granu-lite-facies gneisses have low REE concentrations andpositive Eu anomalies, whereas the amphibolite-faciesgneisses commonly have higher REE concentrationsbut negative Eu anomalies. The Eu enrichment in theserocks was explained by the formation of plagioclasecumulates, which appear at depths corresponding to thegranulite facies [24]. The study of trondhjemites insouthwestern Finland revealed a gradual transitionfrom gabbro to trondhjemites, as follows from the REEpatterns of these rocks. This rock series is characterizedby a decrease in the contents of LREE and HREE andan increase in the positive Eu anomalies with the tran-sition from varieties of intermediate composition(SiO2 = 65.7%) to trondhjemites (SiO2 = 72.4%). Thisbehavior of REE in the rocks from southwestern Fin-land can be explained by fractional crystallization [3].It was demonstrated that the granitoids of the Belomo-rian Belt are high in Al2O3 (15–16 wt %) and SiO2 (upto 74 wt %), bear much plagioclase (61–74%), and aredepleted in REE, but enriched in Eu [15]. According to[18], positive Eu anomalies were detected in ender-bites, plagiogranites, anorthosites, syenites, andleucogranites (orthotectites) of the Ol’khon metamor-phic complex in the western Baikal area. The anoma-lous behavior of Eu was explained by the ability of Eu2+

to form stable aluminosilicate complexes in stronglypolymerized water-poor melts. This fact confirms thepreservation of Eu concentrations at broad variations inthe contents of other REE, particularly HREE.

It is sometimes hypothesized that plagiograniteswith positive Eu anomalies could be produced by par-tial melting. Rocks of this type were found in the Kar-moy ophiolite complex in western Norway, in whichthey are associated with dikes and occur as thin (a fewcentimeters thick) layers parallel to shear zones or asbranching veinlets in brecciated mafic magmatic rocks[25]. The simulations of REE behavior indicate that theKarmoy plagiogranites were produced by partial melt-ing of amphibolites (40% hornblende and 60% plagio-clase). The aforementioned examples of positive Euanomalies pertained to plagioclase granites. However,analogous anomalies quite often occur in potassic

rocks. For example, the general decrease in the concen-trations of REE in the TTG series with increasing silic-ity and the concept that the granites of these series cor-respond to the final stages of the metasomatic alter-ations of basic rocks led some geologists to regard thepositive Eu anomalies as residual (all REE during thefinal stage were removed, because of a decrease in theamount of mafic minerals, whereas the Eu concentra-tions did not change) [2, 9]. In other natural materials,positive Eu anomalies were detected in oil, coal [19],phosphorites [20], and Archean BIF [26]. An Eu excessin these rocks is considered to be an indication ofstrongly reduced conditions. Inasmuch as the Eu2+/Eu3+

ratio is a function of the redox potential of the mineral-izing environment, it is reasonable to think that grani-toids with positive Eu anomalies were produced underreduced conditions [13].

The origin of high-Al granites was related for a longtime to the melting of crustal rocks during continentalcollision [8, 12]. However, the past 10–15 years wit-nessed the development of models of the origin of suchgranites via partial melting triggered by the emplace-ment of hot mantle magmatic masses into lower crustallevels (underplating) [27]. The genesis of small graniteveins scattered over vast areas in the eastern BalticShield (Fig. 1) cannot be explained within the scope ofeither of these models. The REE patterns of these gran-ites and blastomylonites can be most probablyaccounted for by the reduced conditions in the environ-ments where these rocks were formed. We propose thefollowing model of the geodynamic environment,where these rocks could be formed. The boundarybetween brittle and ductile deformation zones to whichthe origin of these rocks is thought to be restrictedlately attracted much attention from geologists, geo-physicists, and geochemists. This boundary is nowoften considered to be a significant discontinuity in theEarth’s crust (its “barrier zone”) that can control theuplift of fluids and plastic rocks to the surface [28].Under extreme conditions, for example, at crustalextension, this barrier is destroyed and becomes perme-able to deep, mostly reduced fluids.

A very important tectonic event in the evolution ofthe Lapland–Belomorian Belt was the exhumation ofdeep rocks to the surface, i.e., their transfer from thezone of ductile deformation to that of brittle deforma-tions. It was thought until quite recently that most Pre-cambrian complexes were exposed at the surface byerosion. However, now numerous lines of evidencewere obtained that these rocks were brought to the sur-face mostly by tectonic processes. There is still no con-sensus concerning the geodynamic environment ofthese processes. Many geologists are prone to relatethem to collision and the “squeezing” of deep rocks.The LBB, the largest structural feature in the easternBaltic Shield is often believed to be a collision zone,whose development was controlled by the convergenceof the Karelian and Kola massifs, a process that forcedout lower crustal rocks [2, 10]. However, modern stud-

454



ies in several territories, including such a classic colli-sional area as the Himalayas, indicate that deep rocksare brought to the surface in extensional environments[29]. Although the driving forces of the extensionalprocesses can vary, the exhumation of deep rocks to thesurface is related exclusively to extensional environ-ments, a concept that provokes no doubt in light of theresults of modern research [30]. The structural mecha-nism facilitating the exhumation of deep rocks to thesurface is a normal fault at gentle angles. During thisprocess, the rocks of the lying limb are brought to thesurface and pass from the zone of ductile deformationsto that of brittle deformations [31]. A principallyimportant component of the evolution of low-anglefaults is the decompressional melting of their foot-wall rocks and the spontaneous release of deep fluids[32]. The structure of the Lapland–Belomorian Belt canalso be considered in the context of extension pro-cesses: the moving of the Kola and Karelian massifs inopposite directions away from the axis of the belt. Thetectonic denudation of the 15- to 25-km-thick rockssequence overlying the Lapland–Belomorian Belt cre-ated conditions favorable for shock decompression atcertain depths and the release of huge amounts of flu-ids, which accompanied the post-tectonic magmatismin the area. The 1.8- to 1.7-Ga rocks of this stageintruded the rocks of the Lapland–Belomorian Beltwhen the latter were close to the surface, whereas ourgranites were produced somewhat earlier, at the bound-ary between the brittle- and ductile-deformation zones,when deep-seated rocks were brought to the surface.The blastomylonites were produced at depths greaterthan those where the granite veins were emplaced, andall of these rocks marked the exhumation of deep rocksto the surface. Judging from the occurrence of positiveEu anomalies in the rocks, the latter process was asso-ciated with a flow of reduced fluids. Diverse models forfluid transfer within the crust can be grouped into twomajor types: fluid migration along systems with openpores and cracks, and fluid movement along “viscous”tectonic boundaries [33], which can be compared ingeneral terms with the granite veins and blastomyloniti-zation zones. The early evolutionary stages of mostLBB rocks were marked by their transformations underthe effect of reduced fluids, which later gave way tooxidized fluids, i.e., the boundary between the fluid sys-tems continuously shifted downward. However, theshock decompression that occurs during normal fault-ing can be associated with the release of reduced fluids,which start to ascend along blastomylonite zones oropen fractures (in the zone of brittle deformations). Thereduced fluids typically bear and release much moreheat than oxidized fluids can [33], and this facilitatespartial melting along open fractures and the generationof granites. The latter are depleted in mafic components(which are removed by flows of hydrogen fluids) andhave positive Eu anomalies (an indication of a reducedenvironment). The ascending fluids oxidize and pro-duce various metasomatic rocks with negative Eu

anomalies (Fig. 12). This process results in the comple-mentary differentiation of Eu, which is concentrated inthe transitional zone, where reduced fluids are periodi-cally injected. After this, the fluids (that are alreadydepleted in Eu and oxidized) migrate upward andlargely control the composition of the upper crust.

CONCLUSIONS

(1) The eastern part of the Baltic Shield was foundto contain abundant acid rocks with positive Eu anom-alies. These rocks are mostly vein granites and, to alesser degree, plagiogranites and blastomylonites. Thegranites and blastomylonites have similar chemicalcompositions, but varying K2O concentrations.

(2) The rocks described in this publication have lowconcentrations of Rb and Zr, are strongly depleted inREE (whose concentrations approach those of basalts),and are enriched in Sr and Ba, a feature suggesting thatthe fluids came from a significant depth.

(3) The transition zone from ductile to brittle defor-mations was favorable for the origin of these rocks.Horizontal extension periods were characterized by theinflow of reduced fluids into the zone and the partialmelting of the host rocks along open fractures. Thisprocess facilitated the melting of the host rocks alongopen fractures and controlled the REE distribution inthe rocks and the occurrence of positive Eu anomaliesin them.

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Documents

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