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GeochemistryofearlyPalaeozoicamphibolitesfromtheOrlica-Snieznikdome,BohemiaMassif:petrogenesisand...

ArticleinInternationalJournalofEarthSciences·April1996

DOI:10.1007/BF02422230

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P. A. Floyd (Y)7 J. A. WinchesterDepartment of Earth Sciences, University of Keele,Staffordshire, ST5 5BG, UKFax: c44-1782–715261E-mail: gga186keele.ac.uk

J. CiesielczukDepartment of Earth Sciences, Sosnowiec University, PL-41–200Sosnowiec, Poland

A. LewandowskaInstitute of Geological Sciences, Jagiellonian University,PL-30–063 Kraków, Poland

J. Szczepanski 7 K. TurniakInstitute of Geological Sciences, Warsaw University, Warsaw,Poland

Geol Rundsch (1996) 85 :225–238 Q Springer-Verlag 1996

ORIGINAL PAPER

P. A. Floyd 7 J. A. Winchester 7 J. CiesielczukA. Lewandowska 7 J. Szczepanski 7 K. Turniak

Geochemistry of early Palaeozoic amphibolites

from the Orlica-Snieznik dome, Bohemian massif:

petrogenesis and palaeotectonic aspects

Received: 21 July 1995 / Accepted: 29 January 1996

Abstract Orthoamphibolites within and marginal tothe Orlica-Snieznik dome in the eastern Bohemianmassif are associated with a series of Proterozoic-Low-er Palaeozoic supracrustal meta-sediments of the Stron-ie, Nové Mesto and Staré Mesto formations. Massiveand variably foliated amphibolites range from commonepidote–plagioclase–blue-green/green hornblende vari-eties to assemblages with diopside, garnet and brownamphibole. All the amphibolites have a basaltic compo-sition and are divided into three main chemical groupswhich are intimately associated in the field: main seriestholeiites, low-Ti tholeiites and alkali basalts. Allgroups exhibit a common enrichment in LIL ele-mentscNbcTa (relative to high-field-strength ele-ments) which reflects contamination by upper conti-nental crust and/or pelagic sediment compositions. Thegross chemistry of the tholeiites approaches that forMORB, although the geological environment and na-ture of crustal contamination suggests that the basalticprecursors were probably emplaced in a rifted ensialicbasinal environment. If the amphibolites are represent-ative of an early phase of the Lower Palaeozoic frag-mentation of Gondwanaland, then rifting did not fullydevelop ocean crust. The wider significance of thechemical interpretation implies that some of the varia-tion from Variscan Proterozoic-Palaeozoic metabasite

suites that is ascribed to a complex of different eruptivesettings could be accounted for by variable contamina-tion and/or source composition.

Key words L. Palaeozoic volcanics 7 Amphibolites 7Geochemistry 7 Petrogenesis 7 Crustal contamination7 Palaeotectonic environment 7 Orlica-Snieznik dome,Bohemian massif

Introduction

The Bohemian massif (Fig. 1) represents one of thelargest and most complex outcrops of the central Euro-pean Variscides and encompasses the classic Saxothu-ringian and Moldanubian tectonic zones (Kossmat1927). Recent reviews of this region (e.g. Chaloupský1989; Franke 1989a; Matte et al. 1990; Narebski 1993;Oliver et al. 1993a; Cymerman and Piasecki 1994) haveidentified various exotic terranes, separated by majorthrusts or ductile shear zones, that record the Palaeozo-ic collision between the continents of Gondwana andBaltica.

One of the general characteristic features of the Var-iscides has been the recognition of the progressive de-velopment and closure of sedimentary basins through-out the Palaeozoic (e.g. Franke 1989b). Major rifting ofcontinental crust has been recorded in both the Ordovi-cian and the Devonian, with both events accompaniedby massive bimodal volcanism which led locally to thedevelopment of basaltic ocean crust (Pin et al. 1988; Pin1990). For example, the Moldanubian zone of the east-ern Bohemian massif is often considered to represent alargely Lower Palaeozoic ocean basin (together withremnants of continental basement) overprinted by aVariscan tectonothermal event (Matte et al. 1990). Be-cause of the often high metamorphic grade of these re-gions, the previous existence of oceanic crust often re-lies on the geochemical characterization of numerousorthoamphibolite bodies which are considered repre-sentative of the original basaltic magmatism.

226

Fig. 1 Simplified map showing the location of the Orlica-Sniez-nik dome in the Bohemian massif relative to the classic tectoniczones of the central European Variscides. EFZ Elbe fracturezone; OFZ Odra fracture zone; SMF Sudetes marginal fault; TTLTornquist-Teisseyre line

Fig. 2 Simplified geologicalmap of the Orlica-Snieznikdome (modified after Don etal. 1990) showing the distribu-tion of the main amphibolitebodies within the Stronie,Staré Mesto and Nové Mestoformations. Generalized sam-pling locations (representing3–8 actual samples collected)are shown with symbols indi-cative of their respectivechemical group: low-Ti tho-leiites (squares), main seriestholeiites (circles) and alkalibasalts (triangles)

The objective of this work was to interpret thechemical variation within the amphibolites of the Orli-ca-Snieznik dome of the West Sudetes (Lugicum) in theBohemian massif (Fig. 2) in terms of their petrogenesis

and eruptive setting. Determination of the initial tec-tonic environment of the amphibolites is consideredparticularly important in the light of widespread LowerPalaeozoic continental rifting and subsequent subduc-tion-induced closure of the early sedimentary basins.However, current environmental interpretations sug-gested by central European Palaeozoic metabasite se-quences apparently include a complex variety of relictoceanic crust, volcanic arc basement, back-arc crust orbroadly “ophiolitic remnants” (reviewed by Perekalina1981; Pin 1990). More recent geochemical discrimina-tion of metabasite units within sectors of the Bohemianmassif has indicated that the original basalts were morelikely characteristic of an ensialic rifting regime withthe development of both within-plate basalts and de-pleted to enriched tholeiites, some with MORB-typefeatures (e.g. Narebski et al. 1986, 1988; Pin et al. 1988;Furnes et al. 1989, 1994; Opletal et al. 1990; Wojcie-chowska and Gunia 1993; Winchester et al. 1995). Also,on the basis of recent sedimentological and geochemi-cal data for Lower Palaeozoic metasediments (e.g.Gackowa formation) and associated acidic volcanics, asimilar continental rift setting is indicated (Kryza et al.1994).

We consider that our geochemical approach has rel-evance to understanding the initial fragmentation ofthe leading edge of Gondwanaland during the early

227

Palaeozoic, although its tectonic relationship to otherrifted continental segments (terranes) is by no meansclear. On the basis of the little age data available, it ispossible that the amphibolites of the Orlica-Snieznikdome may record an early stage in the main magmaticevent which culminated during the Ordovician riftingepisode.

Geological outline of the Orlica-Snieznik dome

The V-shaped outcrop of the Orlica-Snieznik domemetamorphic complex (Don et al. 1990) is situated inthe western Sudetes (Lugicum zone) straddling the Pol-ish–Czech Republic border (Fig. 2). Opinion is dividedas to the age of individual units and its deformation his-tory (e.g. see Zelazniewicz et al. 1995), and being cutoff from the Saxothuringian and Moldanubian by theElba fault zone (Fig. 1), its relationship to these classictectonic zones is debatable. For example, it has beencorrelated with the deeply eroded, high-grade Gföhlunit of the Moldanubian zone (Matte et al. 1990), adeeper structural level of the Tepla-Barrandian(Franke 1989), or a separate West Sudetes “unit” (Lu-gicum) of Proterozoic to Silurian metamorphics (fron-tispiece map in Dallmeyer et al. 1995). In terms of Bo-hemian massif terrane analysis, the Orlica-Snieznikdome is considered to represent the Gföhl terrane(Matte et al. 1990; Cymerman and Piasecki 1994), or asegment of the Sudeten Batholith terrane (Oliver et al.1993a), due to the occurrence of abundant graniticgneisses.

To the east the Orlica-Snieznik dome is separatedfrom the Jesenik metamorphic complexes (Czech Re-public) by the Ramzova line (Cymerman 1993) and/orthe Moravo-Silesian thrust zone (Zelazniewicz et al.1995), whereas to the west it is overlapped by the Up-per Carboniferous-Cretaceous sediments of the Intra-Sudetic depression. The northern margin has been af-fected by the ductile Central Sudetic fault (Cymermanand Piasecki 1994), which separates it geographicallyfrom late Variscan bodies, such as the Klodzko-ZlotyStok granodiorite pluton (289–262 Ma, K–Ar; Dep-ciuch 1972), which intrude the metamorphic basementto the northeast.

Various interpretations of the lithostratigraphy andstructural complexity of the Orlica-Snieznik dome haverecently been reviewed by Don et al. (1990), Urbaneket al. (1995) and Zelazniewicz et al. (1995). Briefly, theterrane is a late Precambrian-Lower Palaeozoic base-ment-cover sequence composed of granitic orthog-neisses, various paragneisses, with minor marbles andamphibolites, predominantly metamorphosed in theamphibolite facies, with rarer granulite and eclogite fa-cies equivalents. The high-grade regional metamor-phism represents a widespread Variscan tectonother-mal event younger than approximately 380–370 Ma(e.g. Rb–Sr, Borkowska et al. 1990; Ar–Ar, Oliver et al.1993b; Sm–Nd, Brueckner et al. 1991; Ar–Ar, Stelten-

pohl et al. 1993) superimposed on a varied late Protero-zoic to Ordovician sequence. Van Breemen et al.(1982) suggested 350 Ma as the age of the granulite fa-cies metamorphism, whereas Oliver et al. (1993a) con-sidered the metamorphism to be “pre-Upper Devon-ian” . However, the metamorphic age of the Orlica-Snieznik amphibolites and eclogites is more con-strained and shows a range from 341 Ma to 328(Brueckner et al. 1991; Steltenpohl et al. 1993) reflect-ing a Variscan cooling/exhumation pathway from eclog-ite to greenschist facies conditions.

The Orlica-Snieznik dome consists of four main“stratigraphic” units or groups (Don et al. 1990):1. Mlynowiec formation – homogeneous plagioclase

paragneisses (unknown age, but assumed to be Pro-terozoic)

2. Stronie formation – predominantly orthoamphibol-ites, with marbles and metasedimentary schists andgneisses (late Proterozoic-Cambrian, on the basis ofrecovered microfossils; e.g. Gunia 1984)

3. Snieznik augen gneisses – derived from granites in-truded around 487–514 Ma (Rb–Sr, Van Breemen etal. 1982; Sm–Nd, Liew and Hofmann 1988; U–Pb,Oliver et al. 1993a)

4. Gierałtów orthogneisses (partly migmatitic) withgranulites and eclogites; a precursor emplacementage of 464B18 Ma (Rb–Sr whole rock) has been re-corded by Borkowska et al. 1990. In the Czech Re-public, on the margins of the Orlica-Snieznik dome,are the Nové Mesto (to the west) and Staré Mesto(to the east) formations (Fig. 2), both of which con-tain discontinuous belts of amphibolitic rocks. Forthe purposes of this study they have also been in-cluded within the Orlica-Snieznik dome proper andcompared with the broadly lithologically similarStronie formation amphibolites.In summary, and within the constraints of the above

limited age data, the orthoamphibolites are associatedwith a series of Vendian-Ordovician supracrustal sedi-ments (Stronie formation, and also possibly the NovéMesto and Staré Mesto formations) that were invadedby granites (Snieznik augen gneisses, Gierałtów orthog-neisses) during the early Ordovician. Initial Sr isotoperatios for the granites vary between 0.707 and 0.715(Borkowska et al. 1990), and together with abundantinherited Precambrian zircons (Oliver et al. 1993a) in-dicate the presence of a strong crustal component.

Previous petrological and geochemical work on theamphibolites in the Orlica-Snieznik dome was based onlimited numbers of samples from selected areas. Theseearly data indicated that some of the orthoamphibolitesare subalkaline in character and display compositionssuggestive of a range of tectonic environments includ-ing island arcs and spreading centres with enrichedMORB characteristics (e.g. Wojciechowska et al. 1989;Opletal et al. 1990; Poubova and Sokol 1993; Wojcie-chowska and Gunia 1993). To the north of the Orlica-Snieznik dome is the Klodzko metamorphic unit, whichcontains greenschist facies metabasites that may be

228

marginally younger (late Silurian-early Devonian; Woj-ciechowska 1990) than the Orlica-Snieznik dome am-phibolites. According to Wojciechowska (1986, 1990)and Narebski et al. (1988) the eruptive setting inter-preted for these lower-grade metabasites is that of acontinental rifting environment, or a tectonically morecomplex situation involving some Red Sea-type oceancrust formation (Narebski et al. 1989), and clearly con-trasts with that previously envisaged for the tectonicallyadjacent amphibolites.

Sample selection and chemical analysis

A total of 169 orthoamphibolite samples were collectedfrom the Orlica-Snieznik dome and analysed for majoroxides and trace elements. Samples were selected fromsmall, scattered outcrops within the Stronie formation,as well as basic granulites and amphibolitized eclogitesfrom tectonically isolated rafts within the high-gradeGierałtów orthogneisses. Samples were also takenalong the almost continuous amphibolite belts withinthe Nové Mesto and Staré Mesto formations (Fig. 2) inthe Czech Republic. Due to strong ductile shearing andmetamorphism, and the lack of adequate field outcrop,it was not possible to demonstrate whether the or-thoamphibolites originally represented a series of lavasor sheet-like intrusions. Very rarely, in weakly foliatedcoarse-grained samples, relicts of subophitic and cumu-late textures were observed with large amphibole pseu-domorphs after original pyroxene. These amphiboliteswere probably derived from small intrusive units suchas dolerite sheets or gabbros (cf. Smulikowski 1979, forthe “gabbro-amphibolites” of the Trzebieszowicearea).

All the samples were analysed at the University ofKeele (UK) using an ARL 8420 X-ray fluorescencespectrometer, calibrated against both international andwithin-house geostandards (analytical method and pre-cision in Floyd and Castillo 1992). A subset of ten sam-ples were also analysed for Hf, Sc, Ta, Th, U and rareearth elements (REE) at Activation Laboratories Ltd.,Canada.

A representative selection of amphibolite analysesfrom the Orlica-Snieznik dome are presented in Ta-ble 1.

Petrographic outline

The amphibolites of the Orlica-Snieznik dome werelargely metamorphosed within the medium-pressureamphibolite facies of regional metamorphism, althougha localized relict of granulite and eclogite facies condi-tions is preserved in the Gierałtów orthogneisses atMatczyna Gora. Many of these higher-grade rocks haveundergone subsequent amphibolitization, and the com-plex as a whole displays variable retrogression with thedevelopment of secondary chlorite after mafics, saussu-

ritization of plagioclase and also late prehnite-quartzBcarbonate veins. Petrographic descriptions ofmany of the amphibolites, especially those aroundTrzebieszowice, have been documented by variousworkers (e.g. Don 1964; Smulikowski 1973; Wojcie-chowska and Gunia 1993). The nature of fluids in-volved during the amphibolite facies metamorphismhas been discussed by Klemd et al. (1995).

Relative to the dominant amphibolite facies dis-played by the majority of the Stronie and Staré Mestoformation amphibolites, a lower grade (greenschist fa-cies) is generally typical of the Nové Mesto formationmetabasites with the development of actinolite in thesouthwestern part of the outcrop (Opletal et al. 1980).This feature implies that metamorphic grade broadlyincreased across the area from the SW (greenschist fa-cies) to a maximum in the NE (eclogite/granulite fa-cies).

The gross fabric displayed by the amphibolites isgoverned mainly by the degree of deformation super-imposed on bodies of variable size and original texture,and shows gradation from highly foliated and bandedto massive, together with a corresponding variation ingrain size. The characteristic assemblage is plagio-clasechornblende, although colour variation in theamphibole from blue-green to green to pale brown im-plies that the full range of the amphibolite facies is rep-resented. This is also supported by a corresponding var-iation in amphibolite assemblages: (at relatively lowgrade) epidote or zoisitecplagioclase (An32–40)cblue-green hornblendeBbiotite; plagioclasecgreen horn-blendecdiopsideBgarnet; (at highest grades) plagio-clasecpale brown hornblendecgarnet. Granulitefacies metabasites typically contain little or noamphibole and are essentially garnet granulites with agarnetcgreenish clinopyroxenecquartzcplagiocla-se(An52–55) assemblage; biotite is rare and orthopyrox-ene subordinate.

Texturally, some amphibolites are characterized bythe development of large porphyroblastic or poikilo-blastic amphiboles surrounded by small equant grainsof similar aspect, together with a granular mosaic ofplagioclase. Rarely, larger laths of twinned plagioclasemay be present and the whole texture suggests the me-tamorphism of a relatively coarse-grained protolithsuch as a dolerite or gabbro. Original magmatic pyrox-ene is very rarely observed. In garnet-bearing amphi-bolites, garnet forms irregular porphyroblasts that maybe enclosed in large prismatic plates of brownish am-phibole, which are set in a granular blue-green horn-blende-plagioclase “matrix” . Common minor consti-tuents include biotite, apatite, sphene (mantling opa-ques) and quartz.

Chemical alteration

Under medium grades of metamorphism involving hy-drous fluids some degree of selective element mobility

229

Tab

le1

Rep

rese

ntat

ive

chem

ical

dat

a fo

r O

rlic

a-Sn

iezn

ik D

ome

orth

oam

phib

olit

es. C

hem

ical

gro

ups:

mai

n se

ries

tho

leiit

es (

sam

ples

1–1

0), l

ow-T

i tho

leiit

es (

sam

ples

11–

15)

and

alka

li ba

salt

s (s

ampl

es 1

6–20

). L

ocat

ion:

ref

ers

to a

ssoc

iate

d fo

rmat

ion

– St

rpSt

roni

e, S

Mp

Star

e M

esto

, NM

pN

ove

Mes

to

Sam

ple

no.

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ld n

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ion

1 S-20

Str

2 S-67

Str

3 Z-6

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r

4 Z-2

1PSt

r

5 S-24

SM

6 S-43

SM

7 S-55

SM

8 Z-4

3N

M

9 Z-2

2N

M

10 Z-6

4N

M

11 S-3

Str

12 Z-2

5KSt

r

13 S-32

SM

14 S-44

SM

15 S-54

SM

16 S-63

Str

17 S-75

Str

18 S-82

Str

19 S-99

Str

20 S-10

9St

r

Maj

or o

xide

s (w

t%)

SiO

2

TiO

2

Al 2

O3

Fe 2

O3t

MnO

MgO

CaO

Na 2

OK

2O

P2O

5

LO

I

48.4

71.

5715

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11.9

60.

177.

4610

.65

3.27

0.25

0.14

0.61

47.8

51.

1515

.98

10.4

80.

187.

5412

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2.81

0.67

0.09

0.97

44.9

52.

8816

.62

13.7

60.

175.

9710

.02

3.45

0.54

0.18

0.69

46.9

52.

7815

.73

13.3

80.

256.

839.

463.

140.

520.

350.

87

47.2

91.

7915

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12.0

20.

196.

4811

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3.31

0.66

0.17

1.04

47.6

91.

8415

.81

10.5

60.

167.

3911

.89

2.86

0.48

0.18

1.49

48.3

21.

9315

.80

12.2

70.

156.

999.

193.

400.

900.

210.

79

54.2

01.

5613

.35

10.1

80.

116.

976.

804.

100.

450.

181.

86

47.5

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7718

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11.0

90.

113.

274.

703.

750.

960.

165.

42

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1315

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12.8

20.

176.

5011

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2.53

0.68

0.28

1.61

50.0

80.

7415

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9.17

0.17

8.89

11.8

52.

740.

340.

031.

12

45.2

70.

9018

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14.6

60.

225.

4712

.30

0.94

0.41

0.00

10.

87

54.2

90.

9515

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10.4

60.

195.

947.

304.

330.

650.

060.

69

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11.8

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187.

6912

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1.85

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184.

6510

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2.77

0.69

0.62

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10.5

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174.

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900.

372.

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1.75

0.68

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12.7

80.

324.

4510

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1.27

2.03

0.75

3.31

Tot

al99

.64

99.9

799

.23

100.

2610

0.28

100.

3599

.95

99.7

610

0.63

99.1

810

0.24

99.5

899

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100.

3599

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100.

2910

0.24

99.8

899

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99.6

9

Tra

ce e

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ents

(pp

m)

Ba

Ce[

XR

F]

Cl

Cr

Cu

Ga

Hf

La[

XR

F]

Nb

Nd[

XR

F]

Ni

Pb

Rb

S Sc Sr Ta

Th

U V Y Zn

Zr

51 15 79 157 40 20 2.

23 5 17 74 12 7 54 34.0

332 0.

300.

400.

1028

3 29 84 109

49 9 84 358 56 14 1.

42 5 7

139 12 27 75 34.0

199 0.

300.

200.

5028

4 25 82 72

109 16 73 194 38 23 2.

22 7 11 66 12 19 93 27.9

302 0.

400.

500.

1046

3 27 84 90

124 44 323

216 17 21 3.

68 13 22 58 11 18 54 35.5

313 0.

601.

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3034

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186

82 25 112

245 31 19 2.

51 8 13 71 14 20 54 33.8

246 0.

300.

500.

1028

1 34 94 133

73 15 39 309 62 21 P4 9 19 109 13 17 53 P 287

P P P 288 26 76 130

143 24 67 382 34 18 P9 11 30 249 12 27 59 P 425

P P P 262 22 97 127

149 17 21 298 94 16 1.

92 5 15 198 10 11 92 18.1

332 0.30

0.60

0.10

210 23 84 101

218 16 58 60 33 22 P6 7 17 34 11 32 116

P 271

P P P 472 14 79 128

194 36 53 285 88 21 P7 14 7

207 11 20 76 P 406

P P P 279 31 111

165

27 5 81 986 42 14 1.

01 3 6 38 12 16 59 49.5

158 0.

300.

800.

4031

7 21 68 57

138 1 77 28 118 24 0.

42 3 17 18 11 28 180 43

.217

3 0.10

0.15

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613 9 55 31

203 10 36 124 38 17 1.

72 3 2 40 12 23 59 32

.821

7 0.30

0.30

0.20

305 27 88 75

53 12 40 336

125 14 P1 6 19 106 10 8 57 P 219

P P P 342 24 83 68

165 8 66 139 40 17 P6 3 4 52 10 114 55 P 385

P P P 236 26 75 59

272 76 177 7 27 23 5.

645 59 29 16 15 14 15

4 18.2

339 3.

004.

801.

5029

1 37 171

275

291 52 173 79 44 16 P 19 28 35 52 19 41 59 P 413

P P P 276 31 88 234

164 36 115

147 38 17 P9 24 37 88 10 30 55 P 326

P P P 419 33 100

200

34 39 45 467 59 21 P 19 27 27 281 12 16 73 P 287

P P P 245 24 118

154

372

113 73 160 43 25 P 55 78 55 95 15 58 57 P 486

P P P 252 34 101

366

Rar

e ea

rth

elem

ents

(pp

m)

La

Ce

Nd

Sm Eu

Gd

Tb

Yb

Lu

5.50

16.0

011

.00

3.07

1.13

3.50

0.70

2.31

0.31

2.60

8.00

6.00

2.05

0.79

2.70

0.50

2.15

0.30

6.20

18.0

012

.00

3.17

1.21

3.90

0.70

2.29

0.29

11.7

032

.00

20.0

05.

281.

756.

001.

004.

210.

59

6.50

18.0

013

.00

3.42

1.18

3.90

0.80

2.80

0.38

P P P P P P P P P

P P P P P P P P P

6.60

17.0

011

.00

3.37

1.22

4.20

0.70

1.91

0.26

P P P P P P P P P

P P P P P P P P P

3.30

9.00

5.00

1.77

0.71

2.60

0.50

2.11

0.29

1.10

3.00

2.00

0.60

0.33

0.80

0.20

0.82

0.12

2.80

9.00

6.00

2.15

0.77

3.00

0.60

2.61

0.35

P P P P P P P P P

P P P P P P P P P

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097

.00

44.0

07.

772.

645.

901.

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370.

31

P P P P P P P P P

P P P P P P P P P

P P P P P P P P P

P P P P P P P P P

230

Fig. 3a, b Distribution of mobile (Rb) and stable (Y) elementsrelative to Zr in the amphibolites of the Stronie formationgrouped according to hornblende colour. Garnet granulites fromthe higher grade Gierałtów formation plot on the same stable Zr–Y trend as the amphibolites

is to be expected especially for the large-ion-lithophile(LIL) elements (e.g. Thompson 1991). Characterizationand discrimination of the amphibolites into differentsuites and assignment to particular environmental set-tings has been on the basis of trace elements generallyconsidered relatively stable during alteration such asthe high-field-strength (HFS) elements and rare earthelements (REE; e.g. Floyd and Winchester 1978, 1983).Good linear coherence between pairs of stable incom-patible elements, and smooth normalized patterns forthe REE, or a sequence of incompatible elements, hassuggested that these elements mirror pre-metamorphicmagmatic compositional variations.

The Stronie formation amphibolites are representa-tive of the full range of the amphibolite facies and canbe used to determine if metamorphic grade has in-

Fig. 4 Stable element magma-type designation (after Winchesterand Floyd 1977) of amphibolites from the Stronie, Staré Mestoand Nové Mesto formations. Apart from some alkali basalts inthe Stronie formation, all amphibolites are metamorphosed sub-alkaline (tholeiitic) basalts

fluenced the chemistry of this single unit. Relative to Zrthe data scatter produced by Rb (Fig. 3a) reflects thegeneral mobility of the latter during metamorphism,whereas Y (Fig. 3b) is stable and produces a reasonablelinear relationship expected for a magmatic sequence.Moreover, the grade that the amphibolites reached (asindicated by the colour of the amphibole) does not ap-pear to have influenced the relative distribution of Zrand Y, because there is complete overlap between allcolour groups.

Chemical characterization of magmatic suites

On the basis of stable incompatible element ratios(Winchester and Floyd 1977) most amphibolites of theStronie, Staré Mesto and Nové Mesto formations rep-resent metamorphosed tholeiitic basalts; only theStronie formation also contains some meta-alkalic bas-alts (Fig. 4). As well as the magma type differencesnoted for the Stronie formation, the amphibolites herewere initially divided on a geographical basis into aneast group (directly to the northeast and east of Bystr-zyca Klodzka; partly described by Wojciechowska andGunia 1993) and a west group (the Polish continuationof the Nové Mesto formation amphibolite belt; Fig. 2).Within all three formations the wide spread of Nb/Yratios (Fig. 4) coupled with the range of TiO2 contents(Table 1) indicate that a number of chemical groups are

Fig. 5 Characterization of the Orlica-Snieznik dome amphibol-ites into low-Ti and main series tholeiites, and alkali basalts, onthe basis of V–Ti and Y–Zr distributions

P

231

232

present. Figure 5 illustrates the wide diversity in theamphibolites in terms of variable Ti/V and Zr/Y ratios.For example, the Stronie formation tholeiitic amphibol-ites can be divided into a low-Ti group (with Ti/V~20and Zr/Y ca. 3) and the bulk of the metatholeiites withhigher Ti/V (20–50) and Zr/Y (3–4) ratios. The Ti/V ra-tio allows some discrimination between the east andwest Stronie groups, although this is not so clear interms of Zr/Y. Both ratios clearly distinguish the Stron-ie meta-alkali basalts. Low-Ti tholeiites are also charac-teristic of the Staré Mesto formation amphibolites, al-though these were not found in the Nové Mesto forma-tion.

Due to the broad overlap between the amphiboliteswithin each formation, three chemical groups are re-cognized in terms of primary magmatic compositions:low-Ti tholeiites, main series tholeiites and alkali bas-alts. Apart from the distinguishing features mentionedabove, the low-Ti tholeiites generally display lowerHFS element contents and different normalized REEpatterns to the main series tholeiites. As seen in Fig. 6,the low-Ti tholeiites have essentially flat, undifferen-tiated patterns (LaN/YbN ca. 0.8–1.0), whereas the mainseries tholeiites exhibit light REE enriched patterns(LaN/YbN ca. 2–3). The Stronie alkali basalts are frac-tionated strongly with LaN/YbN ca. 20. Transition traceelement (Cr, Ni) abundances are broadly similar in allthree groups.

All chemical groups occur in the east Stronie forma-tion area, whereas both tholeiite groups are intimatelyadmixed in the Staré Mesto formation (Fig. 2). Theclose field association of different chemical groups,without obvious major tectonic boundaries dividing theamphibolites, is considered an important factor whentrying to discriminate chemically their relative eruptiveenvironments (see below).

Finally, for the Orlica-Snieznik dome as a whole, thebroad similarity of chemical characteristics suggeststhat the amphibolites within the three formations areprobably representative of the same magmatic event.

Petrogenetic features

Although the Orlica-Snieznik dome metabasalt groupshave been strongly metamorphosed, their chemical fea-tures allow a degree of petrogenetic interpretation. Theinternal chemical variation is governed largely by maficfractionation that produced a typical Fe-enrichmenttrend for the metatholeiite and alkalic basalt groups(Fig. 7), although the scatter in the FeO*/MgO ratio(especially for the low-Ti tholeiites) is partly a conse-quence of element mobility during metamorphism. De-creasing Cr contents relative to Zr (used as a fractiona-tion index) in all three chemical groups suggests thatpyroxeneBspinel were important phases during prima-ry fractionation. However, the wide spread of data forCr–Zr distributions (Fig. 7), and the variations alreadynoted for Ti/V and Zr/Y ratios (Fig. 5), suggests that anumber of partial melt batches may have been present

Fig. 6 Chondrite-normalized (values from Sun and McDonough1989) rare earth element patterns for the three chemical groupswithin the Orlica-Snieznik dome amphibolites. Top: main seriestholeiites; bottom: low-Ti tholeiites and alkali basalts

within each of the main chemical groups. Differentialpartial melting of a garnet-bearing mantle source wouldinfluence the proportion of Cr, V and Y released to themelt and thus cause variability in any ratios which in-volved these elements.

Although much of the group internal variation isgoverned by fractionation and (probably) variable par-tial melting, incompatible element patterns suggest thatthe three chemical groups were derived from separatesources with different compositions. The different nor-malized REE patterns (Fig. 6), ranging from unfrac-tionated chondritic to variably light REE enriched,imply that they are not related by simple magmaticprocesses. A direct comparison with a depleted mantlesource can be made via a MORB-normalized multi-ele-ment diagram (Fig. 8). This diagram also has the advan-tage that in comparing the chemical variation withMORB, the pattern produced can also be used to inter-pret the tectonic environment. The main series tho-leiites from each of the three formations exhibit similarnormalized patterns of progressive incompatible en-richment (Fig. 8) and in this sense are dissimilar to typ-ical depleted MORB compositions. In addition, twofeatures stand out: (a) a positive Sr anomaly, whichcould reflect some degree of primary plagioclase accu-

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Fig. 7 Influence of primary mafic fractionation in the Orlica-Snieznik dome amphibolite groups relative to Zr as a stable frac-tionation index. The gross increase in FeO*/MgO ratio suggeststhat all groups show Fe-enrichment trends with progressive frac-tionation (top), whereas the decrease in Cr contents indicates theinfluence of pyroxene and/or spinel fractionation (bottom)

Fig. 8 N-MORB normalized (values from Sun and McDonough1989) multi-element patterns for the three chemical groups withinthe Orlica-Snieznik dome amphibolites. Top: main series tho-leiites (mildly enriched pattern); bottom: low-Ti tholeiites (de-pleted “uniform” pattern) and alkali basalts (strongly enrichedpattern)

mulation, or possibly secondary mobility (although itappears to be a characteristic of most samples); and (b)there is a slight “step” in the profile between the HFSelements (relatively depleted and more MORB-like)and the stable LIL elements with NbcTa (relativelyenriched). This last feature appears to be a characteris-tic of all tholeiites from the Orlica-Snieznik dome. Forexample, although the low-Ti tholeiites exhibit a com-pletely different normalized pattern (Fig. 8), the en-riched LIL with NbcTa/depleted HFS feature is still

present. The differences between the patterns for themain series tholeiites (progressive enrichment) andlow-Ti tholeiites (flatter, undifferentiated) suggest thatdifferent sources were probably involved. The strongprogressive enrichment pattern of the alkali basalts(Fig. 8) is different again and has more in common withtypical enriched OIB compositions (e.g. Sun andMcDonough 1989).

Contamination effects

Although the two tholeiite chemical groups were de-rived from different sources, both exhibit a commonchemical feature of variable LIL with NbcTa, relativeto HFS elements. This relationship can be expressed interms of stable incompatible element ratios Nb/Y, Ce/Yand Ce/Zr, all of which are largely unaffected by closedsystem fractional crystallization processes. The widerange in values for all ratios throughout the data set(Fig. 9) suggests that a magmatic process may be caus-ing the variability, rather than highly heterogeneous

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Fig. 9a, b Modelled partial melting (PM) and assimilation withfractional crystallization (AFC) trends that could account for thelarge ratio variation between highly incompatible elements (Th,light REE, NbcTa) and HFS elements (Y, Zr) within the Orlica-Sniezik dome amphibolites. Partial melting trends represent thenon-modal melting of a spinel lherzolite composition in 5% inter-vals from 25–5%. The parental basalt composition is arbitary andtrends demonstrate high levels of contamination (assimilation/crystallization ratio rp0.8) of pelagic and upper continental crust(Up.CC) materials over a 60% crystallization range (Fp1.0–0.4)

source compositions. As seen in Fig. 9, all ratios aresusceptible to change caused by variable partial meltingand differential contamination during fractional crystal-lization (AFC process). In terms of modelling, the sec-ond of these processes is considered more importantbecause it can produce a far greater range of variabilitythat grossly matches that shown by the metatholeiites(Fig. 9). Parameters used in the modelling include thefollowing: (a) 5–25% range non-modal melting of a spi-nel lherzolite (ol50opx25cpx20sp5) with a primitive man-

tle composition (with 1.8 ppm Ce, 4.6 ppm Y, 11.2 ppmZr, 0.71 ppm Nb; after Sun and McDonough 1989) inproportions ol15opx30cpx50sp5. This melting rangechanges the ratios and covers some of the variation dis-played within both tholeiite groups. Similar non-modalmelting of a garnet lherzolite (ol55opx25cpx15gt5) makesvery little overall difference. (b) An arbitrary “paren-tal” basalt composition (with all ratios set at 0.1 and20 ppm Zr) was modelled under AFC conditions withaverage pelagic sediment (after Li 1991) and uppercontinental crust (after Taylor and McLennan 1985) ascontaminants. The rate of assimilation to fractionalcrystallization was set deliberately high (rp0.8) to rep-resent an extreme situation. Both contaminants in-crease the ratios and Zr contents considerably andbroadly encompass the ranges shown by the tholeiitegroups. It must be emphasized that the modelling illus-trated is only used to demonstrate and constrain thepossible effects of the AFC process in this situation,and not to provide a quantitative analysis of the actualprocess. However, it suggests that one of the character-istic chemical features of the tholeiite groups could beexplained by the contamination of the original basalticmelts by specific crustal materials. The lack of strongNb–Ta and Th negative anomalies in the normalizedmulti-element patterns (Fig. 7), which are generally in-dicative of the contamination of mantle melts by lowercontinental crust (e.g. Thompson et al. 1982), suggeststhat assimilation from this source is probably unlikely.On the other hand, upper continental crust and pelagicsediments have sufficiently high Nb and Ce contents toraise the ratios to the significant levels observed.

Tectonic environment discrimination

Major problems are involved in assessing the tectonicenvironment from amphibolitic rocks in the Bohemianmassif, or indeed, any structurally complex area. This islargely because many studies rely on limited samplescollected from selected areas for discrimination, suchthat a complex picture of multiple eruptive settings canemerge which is unsupported by the field geology. Inthis study we have attempted to sample most of the ma-jor amphibolite outcrops to provide a full cover of thechemical variation present. One of the most significantfeatures to emerge is the intimate field association ofthe three chemical groups identified and the correlationof the amphibolites in the mapped geological forma-tions. Also, the recognition of a common chemical fac-tor (relatively enriched LILcNbcTa with depletedHFS) which apparently characterizes this area is an ad-ditional feature. Both have implications for tectonicdiscrimination because the chemical groups are morelikely representative of processes superimposed onvariable source composition than different eruptive en-vironments.

Taken at face value the incompatible element nor-malized patterns (Figs. 6 and 8) could be interpreted as

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Fig. 10 Tectonic environment discrimination diagrams (based onHFS elements and Th) for the three chemical groups of the Orli-ca-Snieznik dome amphibolites. Hf–Ta–Th diagram after Wood(1980) with added back-arc basin basalt envelope from Floyd etal. (1991); Nb–Y–Zr diagram after Meschede (1986)

displaying features similar to MORB (for the main se-ries tholeiites) and island arc basalts (for the low-Titholeiites) especially when the selective enrichment dueto crustal contamination is removed. Most discrimina-tion diagrams involving HFS elements, such as Ti–V(Shervais 1982), Zr/Y–Zr (Pearce and Norry 1979),TiO2–Zr (Pearce 1982), and Nb–Y–Zr (Meschede1986) imply the co-existence of MOR and arc basalts.However, as seen in the illustrated discrimination dia-grams (Fig. 10), data plots in a number of fields includ-ing the MOR and the arc environments, and even over-laps with continental tholeiites from a within-plate set-ting. The crude trend towards the Th apex in the Hf–Ta–Th diagram (Fig. 10) is interpreted as due to thecontamination process (involving stable LIL elements)recorded above. Tectonic environment discriminationis not clearcut because none of the tholeiites have typ-ical MOR or arc basalt incompatible element patterns.In broad terms, however, both tholeiite groups could

be interpreted as displaying generally low incompatibleelement, MORB-like features with superimposed selec-tive crustal contamination. Differences between thetholeiite groups are probably the effect of different de-pleted mantle source compositions.

The difficulty in assigning a clear tectonic setting tothe tholeiitic groups suggests that the amphibolites arenot representative of the development of either trueoceanic crust or an arc which have apparently develop-ed within a supercrustal sequence (e.g. Stronie forma-tion). Instead, we consider that the metabasalt groupsrepresent the magmatic event which accompanied theinitial rifting of continental crust. The earlier(?) accu-mulation of shallow-water supercrustal sediments in amarine basin, the crustal contamination of the basalts,together with the continental derivation of the latergranites (Snieznik augen gneisses), all indicate that ear-ly Palaeozoic basin development was ensialic in nature,but (at least in this area) did not at that time developany true oceanic segment. The intimate association andvariable composition of the chemical groups is a reflec-tion of tapping sources of different composition anddepth in a subcontinental mantle.

Discussion and conclusions

The age of the Orlica-Snieznik amphibolites is not wellconstrained, especially if many of these bodies wereoriginally intrusive, and makes their correlation withsimilar bodies elsewhere in the Bohemian massif diffi-cult. However, their close stratigraphic association witha Vendian-Cambrian metasedimentary supracrustal se-quence suggests that they could be Lower Palaeozoicand possibly representative of the Ordovician magma-tism common to the Sudetes and elsewhere in Europe(Pin 1990; Oliver et al. 1993a). Previous geochemicalwork in the Orlica-Snieznik area is limited, but sug-gested that metavolcanics with MORB affinities (Ople-tal et al. 1990) occur in the southwest (largely NovéMesto formation), whereas in the north (Stronie forma-tion) a complex admixture of enriched MORB, islandarc and plume-related basalts are thought to be present(Wojciechowska 1986; Wojciechowska et al. 1989; Woj-ciechowska and Gunia 1993). On the other hand, am-phibolites from the Staré Mesto formation in the eastwere described as being “ophiolitic” (Mízar 1984) andcategorized as having an arc-type affinity (Poubova andSokol 1993). Many of the amphibolites studied by theabove workers show normalized incompatible elementpatterns and a range of TiO2 contents similar to the twotholeiite groups recognized in our work. As indicatedabove we believe that a variety of distinct tectonic set-tings is not present in this area, and that source compo-sition and selective contamination has been primarilyresponsible for the variation observed. Support for thisview comes from Sm/Nd isotope data on three amphi-bolites from the northwest (Wojciechowska et al. 1989)with low Nd values indicative of variably contaminated

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MORB. Also, because the tholeiitic basalt types arecommon to the Stronie, Staré Mesto and Nové Mestoformations, shear zones separating these formations arelikely to be of local, rather than regional, importance.

In geotectonic terms, and prior to Variscan meta-morphism and deformation, we envisage this segmentof the Bohemian massif as representative of the rela-tively early stages of continental rifting with the devel-opment of a shallow marine basin and associated vol-canism. The basalts (amphibolites) exhibit three chemi-cal groups, low-Ti tholeiites, main series tholeiites andalkali basalts, which are intimately associated in thefield, with only the relatively minor alkali basalts segre-gated within one area and formation. The different nor-malized incompatible element patterns for the tholeiitegroups are interpreted as due to the melts being de-rived from slightly different depleted mantle sources,whereas the alkali basalts suggest the tapping of an en-riched OIB-type source. The generally low incompati-ble element contents of the tholeiites are akin toMORB, but are not typical of depleted or N-typeMORB. Although the tholeiites have an underlyingprimitive MORB-like composition, their special charac-teristics (LILcNbcTa enrichment and HFSE deple-tion) have been influenced by high-level crustal con-tamination. Assimilation of upper continental crustand/or pelagic sediments (now possibly represented bythe basal Mlynowiec formation plagioclase parag-neisses) are thus considered to have produced some ofthe chemical variation which has been previously inter-preted as representative of different tectonic settings.

In summary, we suggest that magmatism took placein a strictly ensialic setting and that true ocean crust didnot develop (or has not been recorded) during the lateProterozoic-Lower Palaeozoic in the Orlica-Snieznikarea. This situation is similar to the early stage of theCambrian-Ordovician volcanism in the Kaczawa moun-tains (Furnes et al. 1994) where transitional tholeiitic-alkaline basalts were developed initially relative to late-stage depleted MOR-type basalts. The Snieznik tho-leiites are, however, more primitive than either thetransitional tholeiites or E-type MORB in the Kaczawamountains. A recent interpretation of Ordovician vol-canism in the adjacent Rudawy Janowickie complex(Winchester et al. 1995) also invokes an intracratonicrifting environment, but crustal attenuation was suffi-cient to allow the development of typical N-typeMORB (Leszczyniec formation tholeiites). However, inthe Orlica-Snieznik area, crustal attenuation had not(yet) reached this more advanced stage of developmentas exhibited by these other regions. The existence ofabundant underlying continental crust in the Orlica-Snieznik area is also suggested by the association of theamphibolites with a relatively shallow water sedimenta-tion regime and the large volumes of late intrusivegranitic material.

One further chemical feature to emerge is that manyof the Lower Palaeozoic volcanic sequences exhibit theassociation of both low- and high-TiO2 tholeiitic

groups, a feature which also appears to be characteris-tic of metavolcanics in the younger Klodzko metamorp-hic unit (Narebski et al. 1989) to the north of the Orli-ca-Snieznik dome. One common eruptive environmentwhich shows the close field association of low- andhigh-TiO2 groups is that of continental flood basaltprovinces (e.g. Gondwana CFB; Weigand and Ragland1970; Bellieni et al. 1984; Petrini et al. 1987; Cox 1988;Hawkesworth et al. 1992). Most models indicate thatthe variable Ti melts have been derived from differentmantle sources, e.g. Gondawa CFB low-Ti melts beingderived from the continental mantle lithosphere,whereas the higher Ti melts are sourced from the un-derlying asthenosphere (e.g. Hergt et al. 1991). Al-though the tectonic environments are not strictly analo-gous between Mesozoic CFB provinces and Lower Pal-aeozoic Bohemian massif rifts, the intimate field asso-ciation of the different chemical types in the latter situ-ation suggests a similar setting.

In conclusion, we consider that much of the LowerPalaeozoic magmatism in the Bohemian massif wasgenerated in an ensialic setting, and was dominated bythe production of basalts with broadly primitive chemi-cal features. However, variable rifting of continentalcrust only allowed the localized and limited productionof true ocean crust (dominated by depleted or N-typeMORB). Supposed arc tholeiites, recorded in previouswork, may instead be a consequence of selective crustalcontamination of the more primitive basaltic composi-tions.

Acknowledgements The authors thank Prof. W. Narebski (Mu-seum of the Earth, Kraków), and Dr. R. Kryza and colleagues(University of Wrocław) for enlightened discussions on Bohemianmassif geology. Special thanks are also due to Dr. Z. Cymermanand the officers of the PGI (Wrocław branch) for valuable fieldassistance and general guidence on Bohemian tectonics. Analyti-cal data at Keele University was provided by D. W. Emley and M.Aikin. Funding for the project by the EU (as part of the TEM-PUS educational scheme JEP no. 3656) is gratefully acknowl-edged. Knowledgeable comments by the referees (Profs. W.Franke and G. Worner) improved the text greatly.

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