Pumpellyite-actinolite facies quartzites from the ...

N. Jb. Miner. Abh.

178

2

153-172

Stuttgart, Marz 2003

Pumpellyite-actinolite facies quartzites from the eastern cover of the Żulova pluton (NE Bohemian Massif) Jacek Puziewicz, Wrocław, Kamilla Olejniczak, Wrocław, and Jiirgen Koepke, Hannover With 5 figures and 8 tables

Puzrnw1cz, J., OLEJNICZAK, K. & KoEPKE, J. (2003): Pumpellyite -actinolite facies quartzites from the eastern cover of the Żulova pluton (NE Bohemian Massif). - N. Jb. Miner. Abh. (178): 153 - 172; Stuttgart. Abstract: Detrital and metamorphic minerals occur in the feldspar-rich quartzites occurring in the eastern cover of the Żulova granitic pluton (northern part of the Moravo - Silesian Zone of the Bohemian Massif). The detrital minerals comprise quartz, K-feldspar, plagioclase (An 23 - 48) and scarce biotite. The metamorphic minerał assemblage consists of albitic plagioclase, pumpellyite, actinolite/ferroactinolite, clinozoisite, chlorite, muscovite, titanite, graphite and optically isotropic "allophane" representing chemically a mixture of silica, alumina and iron oxides/hydroxides. The crushing of detrital plagioclase grains triggered their decomposition into pumpellyite, clinozoisite, albitic plagioclase and, in places, muscovite. The plagioclase decomposition products are associated with actinolite/ferroactinolite and chlorite. The metamorphic minera! assemblage indicates sub-greenschist pumpellyite-actinolite facies; rocks beJonging to this facies were not described up to now from the Moravo- Silesian Zone. The pumpellyite - actinolite metamorphism was of prograde character with pressure probably exceeding 4kbar. Key words: Quartzite, pumpellyite-actinolite facies, East Sudetes, Moravo-Silesian Zone.

Introduction Pumpellyite is a common minera! in metamorphosed graywackes and volcanic rocks whose grade is !ower than that of the greenschist facies. It is present in the two facies series of TuRNER (1980): the low pressure one, in which the prograde facies sequence is zeolite ~ prehnite-pumpellyite ~ pumpellyiDOI: 10.11 27/ 0077-7757/ 2003/0178-0153

0077-7757/ 03/ 0178-0153

$ 5 .00

© 2003 E. Schweizerbart'sche Verlagsbuchhandlung, D-701 76 Stuttgart

154

J. Puziewicz, K. Olejniczak and J. Koepke

te-actinolite -7 greenschist and in the high-pressure one, characterised by the prograde facies sequence zeolite -7 pumpellyite-actinolite -7 blueschist. TuRNER's (1980) statement "Fine-grained pumpellyite is notoriously difficult to recognise by optical means" remains valid. This is because optical properties of the pumpellyite are variable and quite similar to those of other lowgrade metamorphic minerals (epidote and clinozoisite/zoisite, chlorite, decomposed biotite etc.). Consequently, pumpellyite may often be overlooked during microscopic studies of low-grade metamorphic rocks. Pumpellyite is typical of low-grade metamorphic assemblages involving basie to intermediate volcanic rocks and graywackes (TuRNER 1981, NYSTROM 1983, SCHMIDT et al. 1997). It occurs also in granites metamorphosed at a very low grade (ALDAHAN 1989) and was identified to be one of the products of post-magmatic alteration of intermediate to basie plutonie rocks (FREIBERGER et al. 2001). In this paper we present an example of pumpellyite-bearing feldspar quartzites. The quartzites occur in the northern part of the Moravo -Silesian Zone of the Bohemian massif. This is, to our knowledge, the first description of subgreenschist facies rocks in the Moravo - Silesian Zone and northeastern part of the Bohemian massif.

Geological background The quartzites described in this paper occur in the northern part of the Jesenfk Mountains. The Jesenfk Mountains form the northern part of the Moravo-Silesian Zone of the Bohemian Massif. The Moravo -Silesian Zone originated due to Variscan collision of the advancing from the west Moldanubian and Lugian units of the Bohemian Massif with the eastwards located Brunia microcontinent. Geodynamic reconstructions (for review see ScHULMANN & GAYER 2000) show the following sequence of events, which formed the northern (Jesenfk Mountains) part of the Moravo- Silesian Zone: in (early) Variscan time (Lower Devonian; CHLUPAĆ 1989) the basal conglomerates and sandstones were deposited on the Cadomian crystalline basement of the Brunia microcontinent. The initial transgressive sediments were followed by deep water pelites interbedded with volcanic rocks. The Carboniferous basin closure and collision was connected with deformation and metamorphism caused by relatively hot Lugian units which were thrusted from the west onto the sedimentary cover and underlying basement of the Brunia microcontinent. The resulting NE - SW trending orogenie zone consists of W-dipping intermingled tectonic slices of deformed and metamorphosed Cadomian basement and its Variscan sedimentary cover. The grade of the Variscan metamorphism is of staurolite zone close to the thrust region and decreases to that of chlorite zone in the east. The orogenie process was completed by the emplacement of the Żulova

155

Pumpellyite-actinolite facies quartzites

N::ejów~ ' •• Kamienna Góra

13 \)

•

Biskupów

\::)

~ •

Gierałcice

t·•••

•

Suplkovice '

•

Plsećnlt

-

granitcs of the Żulova massif

r""""l gneisscs (locally migroatitic) ~ wilh inłiers of rnica schists

o

quartzilcs

-

amphibolites

-maxbles

CJ

Cenozoic sedimenls

-

Sudetic Marginal Fault

- - • - - Polish - Czech state border

o

3 krn

Fig.1. Geological sketch map of the quartzite outcrops in the Fore-Sudetic part of the Cervenohorske sedlo bełt (after CHAB et al. 1994, DuMrcz 1961, BARANIECKI 1957, BARANIECKI et al. 1970 and SAWICKI 1970). Numbers indicate described exposures. The inlet shows outcrops of crystalline basement (lines) and intrusive rocks (crosses) in the Bohemian massif.

156


granitic pluton. The latter refoliated neighbouring rocks (the foliation planes are vertical or dip at high angles to the east) and caused the sillimanite zone metamorphism in the contact aureole (SouCEK 1978, CHA.B & ZA.CEK 1994). The quartzites described in this study are one of the slices of the metamorphosed transgressive Variscan sediments. They occur in the cover of the Zulova pluton together with medium-grade metamorphic rocks (Fig. 1), which are considered to be Cadomian (e.g. MALUSKI et. al. 1995) or Devonian (CHAB & ZA.CEK 1994). In the central and southern part of the outcrop the quartzites occur close (500-2000 m) to the granite (Fig. 1). OLEJNICZAK & PUZIEWICZ (2000 a, b) reported the occurrence of pumpellyite and actinolite in the quartzites and suggested that they belong to the pumpellyite-actinolite facies.

Analytical methods Chemical analyses of whole rocks were done at the laboratory of the Institute of Geological Sciences, University of Wrocław, by "wet" chemical methods. Chemical compositions of minerals and "allophane" were analysed by the CAMECA SX50 microprobe at the Institute of Mineralogy, University of Hannover, using standard WDS procedures and PAP correction program (PoucHou & PICHOIR 1984). Mineral compositions and calculated formulae are given in Tables 3-7. The analysis of "decomposed biotite" (Table 6) was not recalculated, since it probably represents a mixture of biotitic and other (chloritic?) structures. The amount of pistacite in epidote was calculated assuming it to be an end-member of the Ca2 Al 30(Si04 )(Si2 0 7)(0H) Ca2 Fe30(Si04 )(Si2 0 7)(0H) solid solution, similarly to e.g. SCHMIDT & THOMPSON (1996).

Petrography and minerał chemistry Rock composition and fabric The quartzites occurring in the eastern cover of the Zulova granitic pluton are rich in feldspars. The amount of potassium feldspar is usually larger than that of plagioclase, which is expressed by the ratio of normative K-feldspar to normative albite+anorthite (cf. samples M 1501, M 1502 and M 1509 in Table 1). In extreme cases potassium feldspar is the only feldspar in the rock (cf. sample M 1525 in Table 1). Varieties in which potassium feldspar and plagioclase occur in equal amounts or plagioclase is dominating occur as well (cf. samples M 1566 and RS 1 in Table 1). The primary fabric of the sedimentary protolith is usually poorly preserved or obliterated. However, on a thin section scale it is possible to distinguish the


157

Table 1. Chemical analyses (wt %) and CIPW norms of quartzites.

Sample no M1S2S Exposure no

MlSOl 2

MlS02 2

M1S13 2

M1S09 3

87.63 0.23 6.81 O.lS 0.10 O.Ol O.Ol 0.08 0.12 3.28 0.02 1.11 99.SS 7S.SO 19.73 1.03 0.27 3.01

87.00 0.24 6.3S 0.31 0.44 O.Ol 0.04 0.04 0.87 3.19 0.02 O.S7 98.88 70.S9 19.lS 7.46 0.12 1.44

88.21 0.24 S.9S 0.37 0.40 O.Ol 0.03 0.02 O.SS 2.84 0.02 1.01 99.63 7S.10 17.03 4.71

83.03 0.42 8.S2 1.07 0.17 0.02 0.06 2.61 2.16 0.3S 0.06 1.11 99.S8 64.78 2.11 18.S8 12.72

86.10 0.24 6.SS 0.64 O.lS 0.02 0.06 O.SS 0.67 4.22 0.02 OAS 99.67 6S.43 2S.17 S.71 2.39

o.os 0.03

0.28 0.46

0.13 O.S4

0.13

0.18 0.07

0.46 0.04

0.46 0.03

1.09 0.41 0.13

0.6S 0.36 0.04

Si02 Ti02 Al203 Fe203 FeO Mn O MgO CaO Na20 K20 P20s loi total

Q or ab an

c di hy mt he ilm ap

*

O.lS 0.24 0.04

Analyses of RozKOSNY & cation.

1.99

SouĆEK

MlS66 7

RSl*

RS2*

91.13 90.67 9S.82 0.10 0.12 0.11 4.31 3.12 l.S7 0.61 0.19 0.24 0.34 0.40 0.26 0.07 0.04 O.Ol 0.03 0.23 0.23 0.07 2.61 0.11 0.91 0.40 0.40 1.40 0.22 0.70 0.03 O.IS 0.03 0.43 1.44 O.SO 99.43 99.S9 99.98 81.lS 84.64 90.74 8.36 1.33 4.16 7.77 3.4S 3.40 O.IS 6.17 0.3S l.2S 0.02 2. 13 0.16 0.69 0.89 0.28 0.3S 0.19 0.07

0.23 0.33

0.21 0.07

(1989), Czech Republic, no details on sample Io-

detrital grains and the matrix and/or cement. Most of the quartzites are equigranular, suggesting that their protolith was an arkosic arenite. Pebbles ranging in size from a few millimetres to a few centimetres are recognisable in some places, indicating a conglomeratic protolith. The quartzites are in places Jayered due to an alteration of quartz and quartz-feldspar layers, the thickness of which ranges from a few millimetres to 1- 2 centimetres. Quartz, feldspars, and locally small amounts of partly decomposed biotite are the detrital minerals. Optically isotropic "allophane" is supposedly a diagenetic product. The metamorphic mineral assemblage consists of pumpellyite, actinolite/ferroactinolite, clinozoisite, chlorite, muscovite, titanite and graphite. Carbonate, ilmenite and prehnite were identified in one place only. The

158


Table 2. Metamorphic minera! assemblages and detrital plagioclase compositions. Locality (for numbers see Fig. I) (I)

Gierałcice

Sample number

(I*) "allophane", muscovite, graphite (2) amphibole, chlorite, titanite, "allophane'', graphite (2) Burgrabice - Apla MJ513 (I *) pumpellyite, titanite, graphite M 1501 , (2) pumpellyite, "allophane'', MJ502 clinozoisite, muscovite, graphite (3) Sławniowice M 1509 (1 *) amphibole, chlorite, titanite, graphite (4) Kamenny vrch Ml592 (I*) pumpellyite, amphibole, clinozoisite, titanite, graphite (5) Kfemenac M 19 13 amphibole, pumpellyite, clinozoisite, muscovite, titanite (6) Bfle kameny M 1858x ( I*) clinozoisite, chlorite, pumpellyite, titanite, ilmenite with Ti02 intergrowths 1805 (2) amphibole, pumpellyite, clinozoisite, chlorite, titanite secondary muscovite, goethite (7) Jehlan M 1566 ( I*) clinozoisite, chlorite, muscovite, titanite, ilmenite with Ti02 intergrowths secondary prehnite, calcite, goethite and muscovite

*

M 1525 M 1528

Metamorphic minera! assemblage ( + albitic plagioclase)

Detrital plagioclase composition (mole % of an) plagioclase absent nd 40-43, elear

43-48 34- 36 32- 33, elear nd 23 - 26, elear, unzoned

nd

35-39, elear unzoned

Minerał assemblages studied by electron microprobe; otherwise minera! determination is by optical microscope.

metamorphic minerals occur in the interstices of detrital grains, in fissures of crushed feldspars, or are decomposition products of feldspars. The metamorphic minerals occur in small amounts, their assemblage is variable on the scale of an individual exposure 1 as well as on the scale of the whole outcrop (Table 2). The proportions of minerals are variable within an single exposure on the scale of decimetres or metres. Locally (northern part of the outcrop), quartzites contain decimetre- to metre-sized Jenses of pegmatites consisting of quartz and feldspars and abundant millimetre-thick quartz veins. Fibrolite veins occur in the southern part of the outcrop (V. ŻACEK pers. inform.). 1

The terms exposure and outcrop are used in the strict sense, i.e:exposure- a place where the rocks can be seen in situ, outcrop - the total area over which a particular rock unit occurs at the surface (WHITTEN & BROOKS 1987).


159

Foliation in quartzites is parallel to layering in the northern (i.e. Polish) part of the outcrop; in places, a second foliation, oblique to the layering, is present (OLEJNICZAK & Puzrnw1cz 2000 b). The orientation of foliation is diversified. A lineation is not visible in the northern part of the outcrop (OLEJNICZAK & PuzrnwICZ 2000b). A quartz c-axes orientation reconnaissance study revealed no preferred orientation (M. KALIŃSKA unpubl. data). The orientations of magnetic foliation and lineation in the northern (Polish) quartzites are random (T. WERNER pers. comm.) SCHULMANN & GA YER (2000) showed that the Czech part of the eastern cover of the Żulova granite forms a zone of homogenous vertical eleavage (the Jesenik shear zone), but they presented no detailed data on the quartzite unit investigated by us. The detrital minerals and diagenetic "allophane"

The detrital feldspars are subhedral to rounded. Their habit varies from not fractured to heavily fractured. The unfractured or slightly fractured plagioelase is elear. Its anorthite content is constant within a hand-specimen, but varies from place to place (Table 2). The analysed grains are unzoned and one population of plagioelase is present in most of the studied samples (Table 2). However, two plagioelase populations (34-36 and 43-48 % anorthite) occur in the quartzite from Sławniowice. The heavily fractured plagioelase grains are dusty or sericitised under the microscope. Their anorthite content is highly variable, ranging from 6 % to the amount typical of elear grains. BSE (backscattered electron) images show them to consist of fragments of porous plagioelase of variable composition intergrown with elinozoisite, chlorite, pumpellyite and sparse muscovite. Fractures are filled with pumpellyite or Fe hydroxides. The study of feldspars from Sławniowice (OLEJNICZAK & Puzrnwicz 2000 b) revealed the presence of thin (:s;20 µm) rims of porous feldspar with different composition than the host grain. The rims on K-feldspar grains contain 3 mole % albite (9 % in the host), and those on plagioelase contain 2 - 8 mole % anorthite (34 - 36 or 43-48 in the host). The rims were interpreted by OLEJNICZAK & PUZIEWICZ (2000 b) as the products of a low-grade metamorphism. Biotite of detrital origin occurs in some of the quartzites. It is poor in potassium (analysis 1 in Table 6). Some of the brown plates of biotite appearance have the composition of chlorite (analyses 2, 3 in Table 6). Green chlorite plates with sparse biotite layers occur as well. The content of elinochlore may vary by 10 mole % on the scale of thin section (Table 6). The term "allophane" is used to describe brown or grey isotropic aggregates, occurring in most of the studied samples. Microprobe data show that they are dominated by silica and alumina, with various amounts of iron and water

160


Table 3. Representative chemical analyses and structural formulae of pumpellyite. Sample no

M 1Sl3

37.8S 0.04 27.91 3.79 0.29 1.46 21.76

o.os O.Ol 93.16

MlS92

2

3

4

s

6

7

8

38.27 0.02 28.8S 3.21 0.21

37.90 O.OO 27.16 2.48 0.21

37.6S 0.06 27.S4 3.37

36.Sl

2.6S

21.78 O.OO 0.02 92.61

2 1.69 O.OO O.Ol 92.10

1.90 2 1.43 O.OO 0.04 92.24

38.27 O.OO 26.36 2.42 0.4 1 3. 12 23. 19 O.OO O.OO 93.77

38.24 O.Ol 26.66

0.2S

37.26 O.Ol 26.39 2.10 0.48 3.30

3.0S

3.02 O.OO 2.48 0.23

Analysis no Si0 2 Ti02 Al2 0 3 FeO* Mn O MgO CaO Na20 K20 to tal

M 18S8x

0.2S

O.OO

26.2S 6.78 0.10 O.OO

22.2S

22.S2

O.OO O.Ol 91.80

O.OO O.Ol 92.17

3.S6 0.74

2.2S 23.02 0.03 0.04

94.SS

Structural formulae based on 8 cations Si Ti Al Fe2+* Mn Mg Ca Na K Fe/(Fe + Mg) an content of neighbouring plagioclase

3.04 O.OO 2.64

3.0S

0.S9

3. 10 O.OO 2.7S 0.22 O.O l 0.03 1.89 O.OO O.OO 0.88

O.OO 2.S8 0.17 O.O l 0.32 1.87 O.OO O.OO 0.34

17

38

4S

0.2S 0.02 0.18 l.97 O.Ol O.OO

3.04 O.OO 2.62 0.23 0.02 0.23 1.86 O.OO O.OO

3.00 O.OO

O.SO

0. 14 0.03 0.40 l.92 O.OO O.OO 0.26

3.00 O.OO 2.S4 0.47 O.O l O.OO l.98 O.OO O.OO 1.00

O.OO 2.44 0. 16 0.03 0.37 1.96 O.OO O.OO 0.30

O.OO O.OO 0.47

s

20

nd.

18

nd.

2.Sl

o.os 0.26

l.9S

* Total Fe as FeO; nd - no data.

which is indicated by low microprobe totals (Ta ble 7) . Hig h porosity m ay also be the reason for low totals, b ut we d id not observe it in micrometer-scale BSE images .

Metamorphic minerals Pumpe ll yite occurs near or in the pornus, decomposed plagioclase. Usually both minerals are intergrown (Fig . 2). U nde r the microscope p umpe llyite shows variable optical featu res. T he minerał is colourless or pale brown. Some colo urless p umpellyite forms acic ular crystals which look similar to fibrolite or tremolite. The brown pumpellyite looks similar to decomposed biotite. The composition of pumpellyite is variable even in adjacent crystals (Table 3), but the overall compositional variation of the minerał is small (Fig. 3). Optical


161

Fig. 2. BSE image of pumpellyite grains (pmp; bright-grey to white) in porous plagioclase (pl). The anorthite content of plagioclase varies from 6 % to 45 (parts highest in anorthite are brightest). Note almost euhedral form of the albitic (6 % an) grain indicated by an arrow (sample no Ml513).

Al M1513

90

M1592 M1858x

50

Fe 2 +

Mg

Fig. 3. Ternary Al - Mg - Fe2 diagram (atomie proportions) showing pumpellyite compositions.

162


Fig.4. (a) Optical image of amphibole aggregate (sample no Ml592); (b) BSE image of termination of amphibole aggregate; the darker parts of the crystal are enriched in alumina relative to the lighter ones, cf analyses 4 and 5 in Table 4 (sample no M 1509).

o oo o

actinolite o o o

0.60

o

c o

magnesiohornblende

:;::;

o

0.50

o o o oO oo o o

o

•

E

-+

•

o

0.40 ~ N

• ••

Q)

LL

+

•

••

•

•

-~

o

• ••

• ••• • •• •••

()

CO ,._

Ol

0.30

•

~ .._..

--

Ol

~

ferrohornblende

ferroactinolite

0.20

8.00

7 .50

7 .00

Si (atoms pfu) o M1513

+ M1592 Fig. 5. Compositions of amphiboles from the quartzites plotted m molar Mg/ (Mg+ Fe2 +) vs. Si classification diagram of LEAKE et al. (1997).

163

Pumpellyite-actinolite facies quartzites Table 4. Representative chemical analyses and structural formulae of amphibole.

Sample no

MlS09

Analysis no

S0.66 O.Ol 0.6S OAS 26.72 1.28 S.30 11.79 0.04 O.OS 1.91 98.86

Si02 Ti02 Al203 Fe203* FeO* Mn O MgO CaO Na20 K20 H20"l to tal

MlS92

2

3

4

s

6

7

8

47.93 0.16 2.99 4.87 21.76 1.10 6.02 11.17 0.37 0.21 1.92 98.SO

47.07 0.16 6.83 1.69 22.28 l.2S S.88 11.79 0.67 0.3S 1.96 99.93

Sl.23 O.OS 0.87 2.28 23.14 1.49 6.83 11.68 0.07 0.03 l.9S 99.62

48.21 0.21 S.09 2.12 19.48 0.72 8.14 11.88 0.37 0.21 1.96 98.39

Sl.68 0.07 3.4S 2.64 lS.22 1.38 l l.S9 12.43 0.29 0.12 2.0S 100.92

S3.14 0.02 0.63 2.39 16.6S 1.41 11.38 12.19 0.02 0.03 2.02 99.88

S2.74 O.IS 2.74 1.16 16.S8 1.39 11.19 12.14 0.23 0.12 2.04 100.48

Structural formulae based on 13 cations excluding Ca, Na, K Si Ti Al Fe3+* Fe2+* Mn Mg Ca Na K Mg/(Mg+ Fe2+)

7.93

o 0.12 O.OS 3.SO 0.17 1.23 1.98 O.Ol O.Ol 0.26

7.48 0.02 O.SS O.S7 2.84 O.lS 1.40 1.87 O.Il 0.04 0.33

7.20 0.02 1.23 0.19 2.8S 0.16 1.34 1.93 0.20 0.07 0.32

7.8S O.Ol 0.16 0.26 2.97 0.1 9 1.56 1.92 0.02 O.Ol 0.34

7.37 0.02 0.92 0.24 2.49 0.09 1.86 l.9S 0.11 0.04 0.43

7.SS O.Ol O.S9 0.29 1.86 0.17 2.S2 l.9S 0.08 0.02 0.42

7.87

o 0.11 0.27 2.06 0.18 2.Sl 1.93

o O.Ol OAS

7.73 0.02 0.47 0.13 2.03 0.17 2.4S 1.91 0.06 0.02 OAS

* Calculated by stoichiometry. 1, 2 - Small grains in aggregate; 3 - Small (10 µm) grain located in the margin of the aggregate; 4, S - Central part and margin of crystal located at the aggregate termination (cf Fig. 4 b); 6, 7 - Core and margin (respectively) of small grain; 8 - Small grain associated with pumpellyite.

properties are not correlated with the total iron content: the pale brown variety (analysis 5 in Table 3) is in places poorer in iron than the colourless one (analysis 6 in Table 3). The amphibole occurs in interstices among detrital minerals. lt forms short needles or prisms of pale-green to colourless pleochroic scheme. The amphibole crystals are often overgrown with decomposed biotite (see below) or iron hydroxides. BSE imaging at large magnifications shows many of the acicular aggregates to consist of anhedral to subhedral fine (10- 50 µm) grains or need-

164


les (Fig. 4). The amphibole occurring in the quartzites has the composition of actinolite/ferroactinolite, sometimes overlapping with silica-rich hornblende/ferrohornblende (Fig. 5). The Al content in amphibole amounts up to 6.8 wt% (corresponding to 1.25 atoms per formula unit). The fine grains and needles occurring in the aggregates contain usually 0.54 (SPRINGER et al. 1992). This is not the case in most of the quartzites described by us (cf. Table 6). The variation of pumpellyite composition (Fig. 3) is characteristic of the pumpellyite-actinolite facies (SCHMIDT et al. 1997). Therefore, we conclude that the quartzites belong to the pumpellyite - actinolite fa-


169

ci es. Prehnite was identified in one sample only (M 1566, cf. Table 2), and it post-dates the assemblage of clinozoisite, chlorite, muscovite and titanite, which suggests that the quartzites were metamorphosed at pressures where prehnite-bearing assemblages are not stable (~4kbar in the diagram of FREY et al. 1991).

Regional geological implications The quartzites occur among the gneisses and amphibolites forming the eastern cover of the Żulova granitic pluton (Fig. 1). According to SoucEK (1978) and CHAB & ŻA.cEK (1994) the gneisses were refoliated and metamorphosed in the stability field of sillimanite (± cordierite) and subjected to local partial melting during the granite intrusion, dated at 305 Ma (Nov AK et al. in press). This led CHAB & ZA.CEK (1994) to the conclusion that the quartzites share the same high metamorphic grade. The amphibolites are also considered to be of high metamorphic grade (KREJZLTKOV A. 2000). The data presented in this study, as well as the earlier data of OLEJNICZAK & Puzrnw1cz (2000 a, b) indicate that the quartzites are of much lower metamorphic grade than the neighbouring rocks and belong to the pumpellyite-actinolite facies. The only indication of higher-grade metamorphism in the quartzites are hornblende rims on ferroactinolite and small crystals of hornblende in the amphibole aggregates. Since hornblende occurs as thin rims and small grains only, and the ferroactinolite is the main amphibole in the rock, we suggest that the quartzites were subjected to a short thermal (contact metamorphic?) event which did not eliminate the sub-greenschist mineral assemblage. The occurrences of sillimanite in veins, as well as loca] pegmatite occurrences, are not related to a regional metamorphic event but to hydrothermal action of fluids released after the Żulova granite crystallization. Therefore, the observed low-grade metamorphic facies of the investigated quartzites indicates that the high-grade (sillimanite zone) metamorphism of the surrounding gneisses must have been occurred prior to the tectonic event which juxtaposed the gneisses and quartzites.

Conclusions The detailed study of the quartzites occurring in the eastern cover of the Żulova granitic pluton (northern part of the Moravo-Silesian zone of the Bohemian Massif) revealed them to belong to the pumpellyite- actinolite facies. The detrital minera! assemblage of the quartzites-quartz and smaller amounts of feldspars, with K-feldspar usually prevailing over plagioclase, indicates a granitic source of the detrital material. The low-grade, sub-greenschist metamorphic minera] assemblage originated mainly due to brittle deformation of

170


plagioclase, which crushing initiated its decomposition and enabled the crystallization of metamorphic minerals. Their chemical composition is variable on the thin section scale, indicating that individual reaction sites in the rock developed independently. The subgreenschist metamorphism of the quartzites took place at pressures exceeding the upper stability limit of prehnite in the P-T diagram, i.e. ca 4kbar. Acknowłedgements

JP and KO thank to Dr. VLADIMIR ZA.CEK of the Czech Geological Survey, who kindly introduced us to the Czech part of the eastern cover of the Zulova pluton. The review of Dr. ANNE FEENSTRA (GFZ Potsdam) significantly improved the fina] version of the text. The study is a part of the research project KBN 6 P04D 04912.

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Author's addresses: JACEK Puzrnwrcz, KAMILLA OLEJNICZAK, Institute of Geological Sciences, University of Wrocław, Cybulskiego 30, 50-205 Wrocław, Poland. E-mail: [email protected] JURGEN KOEPKE, Institut fiir Mineralogie, Universitat Hannover, Welfengarten 1, 30167 Hannover, Germany. E-mail: [email protected]