Mineralogy and Petrology

, Volume 99, Issue 3–4, pp 185–199 | Cite as

Geochemistry of the Drahotín and Mutěnín intrusions, West Bohemian shear zone, Bohemian massif: contrasting evolution of mantle-derived melts

  • Lukáš Ackerman
  • Martina Krňanská
  • Wolfgang Siebel
  • Ladislav Strnad
Original Paper

Abstract

In western Bohemia, the Drahotín (gabbro-diorite) and Mutěnín (gabbronorite-diorite-syenite) intrusions show different origins and patterns of geochemical evolution. Parental magmas of the Drahotín intrusion were derived predominantly from enriched mantle sources, and the melts have undergone a significant degree of assimilation-fractional crystallization (AFC) during their ascent and/or emplacement into the crust. In contrast, the compositional variation of the complex Mutěnín intrusion cannot be explained by simple AFC processes, but more likely reflects the involvement of several parental magmas. The gabbronorite was derived from a depleted mantle source, whereas the diorite/syenite stem from a mixed mantle-crust reservoir. The contrasting evolution of the Drahotín and Mutěnín intrusions may be due to their melt derivation and magma emplacement under different tectonothermal regimes at different times.

Introduction

The southwestern part of the Bohemian Massif hosts numerous plutonic bodies of Cadomian (e.g., the Kdyně-Neukirchen Massif, 524–504 Ma) and Variscan age (e.g., the Babylon granite, 352–336 Ma; the Klatovy pluton, 347 ± 4 Ma; the Mutěnín intrusion, 341 ± 1 Ma; and the Drahotín intrusion, 328 ± 1 Ma) (Miethig 1995; Dörr et al. 1996, 1997, 2002; Dörr and Zulauf 2009). They are predominantly concentrated along the tectonic boundary between the Teplá-Barrandian and Moldanubian Units, i.e. the Bohemian shear zone (BSZ), which is composed of the Hohen Bogen shear zone (HBSZ) and the West/Central Bohemian shear zones (WBSZ and CBSZ, respectively; Fig. 1). These intrusions range in composition from gabbro through gabbro-diorite to syenite-granite. The magmatic complexes of Kdyně-Neukirchen, Mutěnín and Drahotín are composed of gabbro-diorite, locally rich in Fe and Ti. The Cadomian Kdyně-Neukirchen massif was investigated in detail by Vejnar (1984), Miethig (1995), Propach and Pfeiffer (1998), Svobodová (1999) and Bues et al. (2002). These studies have revealed that the parental magma for this body was most probably derived from a mantle source with a Mid-Ocean Ridge Basalt (MORB)-like signature, and the magma was subsequently modified by assimilation-fractional crystallization (AFC) processes after emplacement into the lower crust (Svobodová 1999).
Fig. 1

Geological map of the Western Bohemia (adopted and modified after Cháb et al. 2007) with position of Drahotín (D) and Mutěnín (M) intrusions. WBSZ—Western Bohemian shear zone, HBSZ—Hoher Bogen shear zone, CBSZ—Central Bohemian shear zone. B1—Bor granite, B2—Babylon granite, K—Klatovy pluton, S—Stod pluton, KN—Kdyně-Neukirchen Massif

The Variscan Mutěnín and Drahotín intrusions are composed of gabbro-gabbronorite-diorite-syenite (Vejnar 1975; Tonika 1979; Vejnar 1980).

In this paper, we present petrographic, major and trace element, and Sr-Nd isotopic data for the Mutěnín and Drahotín intrusions in order to establish the source of their parental magmas and to constrain their geochemical evolution during emplacement.

Geological setting

The Drahotín and Mutěnín intrusions are located on the western side of the West Bohemian shear zone (WBSZ), which represents the boundary between the Teplá-Barrandian and Moldanubian Units (Fig. 1). These two units differ in their igneous constituents, tectonometamorphic evolution, and age (e.g., Matte et al. 1990; Franke 1989). The WBSZ represents part of the complex Bohemian shear zone (BSZ), along which a minimum 10 km of vertical throw of the Teplá-Barrandian Unit against the Moldanubian Unit was suggested (Zulauf et al. 2002). Such elevator-style movement led to the juxtaposition of the supracrustal Teplá-Barrandian Unit against the high-grade rocks of the extruded Moldanubian orogenic root (Dörr and Zulauf 2009). In contrast, on the basis of differences in structural and metamorphic evolution between the Moldanubian and Teplá-Barrandian Units, Pitra et al. (1999) attributed their juxtaposition to late-orogenic strike-slip tectonics.

The Teplá-Barrandian Unit (TBU) represents the largest exposed block of Cadomian crust in the Bohemian Massif. The basement of the TBU consists mainly of greywackes, siltstones and volcanic rocks, all metamorphosed to low grade. These rocks are associated with higher grade metamorphic rocks, such as migmatite, micaschist and paragneiss in the NW and W parts of the TBU (Zulauf et al. 1997; Zulauf et al. 2002). Cambrian to Mid-Devonian unmetamorphosed sediments of the Barrandian syncline were deposited on this basement in the area between Prague and Plzeň. The TBU is intruded by a large number of plutonic rocks (e.g., Mračnice trondhjemite—523 ± 5 Ma, Zulauf et al. 1997; Stod granite—518 ± 8 Ma, Kreuzer et al. 1990) and Variscan age (e.g., Bor granite—315–337 Ma, Babylon granite—352–320 Ma; Dörr et al. 1996; Dörr and Zulauf 2009). Barrandian Mid-Devonian sediments contain abundant detrital zircons which yield ages between 530 and 500 Ma (Strnad and Mihaljevič 2005), suggesting that the Cadomian granitoids of the TBU were exposed in Givetian times.

The Moldanubian Unit located to the W of the WBSZ consists of high-grade metamorphic rocks, mostly biotite-cordierite paragneiss of the Monotonous (Ostrong) group with foliations dipping steeply to the ENE, subparallel to the WBSZ (Vejnar 1984; Zulauf 1994; Vrána et al. 1995; Fiala et al. 1995). These rocks have undergone regional metamorphism, followed by high-temperature/low-pressure contact metamorphism at 336–316 Ma (e.g., Kalt et al. 1999, 2000; Propach et al. 2000; O’Brien 2000; Teipel et al. 2004; Gerdes et al. 2006). Post-tectonic granites and gabbro-diorite plutons of Variscan age intruded the Moldanubian terrane, including the two plutons, Drahotín and Mutěnín, which are the objects of this study.

Drahotín and Mutěnín intrusions

The Drahotín intrusion is located in the Moldanubian Unit, with its eastern boundary delineated by the WBSZ (Fig. 1). Despite its small outcrop area of about 5 km2, strong petrographical variations make this intrusion one of the most interesting basic intrusions in the Bohemian Massif. The intrusion is layered, and Vejnar (1980) distinguished three different lithological zones: (1) a basal zone with predominant phlogopite-olivine gabbronorite, (2) a main zone consisting of amphibole gabbronorite, and (3) an upper zone composed of quartz-bearing biotite-amphibole diorite. According to Vejnar (1980), the Drahotín intrusion was formed from a tholeiitic magma which was generated at depths of about 50 km, and concordant U-Pb zircon data yield a Variscan age of 328 ± 1 Ma (Dörr and Zulauf 2009).

The Mutěnín intrusion, which underlies an area of approximately 7 km2, is located ∼3 km NW of Drahotín and occurs in a similar tectonic position (Fig. 1). The intrusion has a strong concentric internal zonation of alkaline rocks, in which the core consists of several isolated bodies of Fe-diorite, followed by an intermediate zone of biotite-amphibole diorite, and quartz-biotite-amphibole diorite along the margin. The E and NE parts are made up of Fe-rich syenite (Vejnar 1984; Tonika 1979). The intrusion has a contact aureole, in which Moldanubian paragneiss and amphibolite wall-rocks were thermally recrystallized (Vejnar 1984). The intrusion depth was estimated at 6.0 to 7.1 kbar (∼23 ± 4 km) by Zulauf et al. (2002), using Al-in-hornblende barometry. U-Pb zircon geochronology yields an age of 341 ± 1 Ma (Dörr and Zulauf 2009) whereas Ar-Ar age on amphibole from diorite yields 327 ± 7 Ma (Kreuzer et al. 1992).

Sampling and analytical methods

Because the Drahotín and Mutěnín intrusions are very poorly exposed, most samples investigated in this study were collected from blocks, although a few Mutěnín samples were taken from small outcrops (GPS co-ordinates of all samples are listed in Table 1). All samples were crushed and powdered in an agate mortar, and thin sections were prepared for petrographic studies.
Table 1

Summary of petrography of the Drahotín and Mutenín rocks and their GPS coordinates

Sample

Rock composition

Location

GPS location

Longitude (E)

Latitude (N)

06DR1

amp-bt diorite

Drahotín

49°31′10″

12°44′90″

06DR3B

amp-bt diorite

Drahotín

49°31′11″

12°45′81″

06DR5B

phl gabbro

Drahotín

49°30′53″

12°44′92″

06DR6A

amp-bt diorite

Drahotín

49°30′50″

12°44′94″

06DR7B

amp-bt diorite

Drahotín

49°30′22″

12°44′78″

06DR8

phl gabbro

Drahotín

49°30′15″

12°44′88″

06DR9

phl gabbro

Drahotín

49°30′13″

12°45′67″

06MU1

amp-bt diorite

Mutěnín

49°32′32″

12°44′09″

06MU2B

amp-bt diorite

Mutěnín

49°32′31″

12°44′10″

06MU3B

Gabbronorite

Mutěnín

49°32′45″

12°44′05″

06MU4

Gabbronorite

Mutěnín

49°32′45″

12°44′05″

06MU5

amp-bt diorite

Mutěnín

49°33′09″

12°43′31″

06MU6

amp-bt diorite

Mutěnín

49°33′10″

12°43′30″

06MU7

Syenite

Mutěnín

49°33′05″

12°44′42″

06MU9

amp-bt diorite

Mutěnín

49°33′06″

12°44′47″

06MU10

amp-bt diorite

Mutěnín

49°32′03″

12°43′57″

06MU11

Syenite

Mutěnín

49°32′03″

12°43′58″

Amp amphibole, bt biotite, phl phlogopite

Whole-rock major element analyses (wet-technique) and trace-element analyses (ICP-MS) were performed at the Faculty of Science, Charles University, Prague. The major elements were determined using total digestion with mineral acids (HF-HClO4) and/or sintering of sample (both in Pt dish and crucibles) and subsequent chemical analysis: Al2O3, Fe2O3, FeO, MgO, CaO were determined by volumetric analyses, P2O5 and TiO2 spectrophotometrically or by optical emission spectroscopy (ICP OES), SiO2, H2O were determined gravimetrically and Na2O, K2O using flame atomic absorption spectrometry (FAAS). Limits of detections vary between 0.01 and 0.1 wt. % depending on element concentrations. Analyses of international reference materials (BCR-2; Table 2) yield a total error (1-sigma) for whole-rock analyses of ±3%. Trace element ICP-MS analyses followed the methods of Strnad et al. (2005). The external reproducibility was monitored using the BCR-2 reference material (USGS), and the relative differences yielded accuracies better than 10% for all corresponding trace elements with respect to the recommended values (Dulski 2001; Wilson 1997). The GCDkit geochemical software program (Janoušek et al. 2006) was used for data handling and whole-rock/trace element plotting.
Table 2

Major element (wet-technique) analyses of USGS reference material BCR-2. Reference values from Wilson (1997)

 

BCR-2 (this study)

SD (1σ)

Reference value

SiO2

53.96

1.40

54.1

TiO2

2.37

0.06

2.26

Al2O3

13.24

0.20

13.5

Fe2O3

2.22

0.08

 

FeO

10.18

0.50

 

∑Fe2O3

13.53

0.28

13.8

MnO

0.19

0.00

0.20

MgO

3.80

0.12

3.59

CaO

7.15

0.27

7.12

Na2O

3.18

0.09

3.16

K2O

1.70

0.03

1.79

P2O5

0.38

0.02

0.35

H2O+

1.01

0.05

 

H2O−

n.d.

  

CO2

0.20

0.00

 

Total

99.58

  

∑Fe2O3 is not included in Total calculation

n.d. not detected

Sr-Nd isotopic analyses were performed at the Institute of Geosciences (Eberhard-Karls-University Tübingen). Some whole-rock powders (labelled * in Table 5) were spiked with a mixed 84Sr-87Rb and 149Sm-150Nd tracer solutions. All samples were digested in 48% HF for seven days at 180°C in a Teflon bomb surrounded by a steel jacket. Digested samples were dried and re-dissolved in 6 N HCl, dried again, and re-dissolved in 2.5 N HCl. Sr and light rare-earth elements were separated on quartz columns using conventional ion exchange chromatography with a 5 ml Bio Rad AG 50 W-X12 (200–400 mesh) resin bed. Nd was separated from other rare-earth elements on quartz columns using 1.7 ml Teflon powder coated with HDEHP, di(2-ethylhexyl) orthophosphoric acid, as a cation exchange medium. All isotopic measurements were made on a Finnigan MAT 262 thermal ionization mass spectrometer. Sr was loaded with a Ta-HF activator on pre-conditioned W-filaments and measured in a single-filament mode. Nd was loaded as a phosphate and measured in a double Re-filament configuration mode. The 87Sr/86Sr and 143Nd/144Nd isotope ratios were normalised to 86Sr/88Sr = 0.1194 and to 146Nd/144Nd = 0.7219, respectively. The La Jolla Nd standard gave a 143Nd/144Nd ratio of 0.511867 ± 0.000018 (2σ SD, reference value 0.511850) and JNdi-1 standard yielded 0.512157 ± 0.000008 (2σ SD, reference value 0.512115). The NBS 987 Sr standard yielded an 87Sr/86Sr ratio of 0.710266 ± 0.000020 (2σ SD, reference value 0.710240). Total procedural blanks (chemistry and loading) were <200 pg for Sr and <40 pg for Nd. Initial Sr and Nd isotope ratios were calculated from Rb-Sr and Sm-Nd concentrations by isotopic dilution (labelled * in Table 5) or determined by ICP-MS. Single stage depleted mantle model-ages (TDM) were calculated with depleted present-day parameters, 143Nd/144Nd = 0.513151 and 147Sm/144Nd = 0.219 (Liew and Hofmann 1988).

Petrography

The studied rocks are generally seen to be fresh in hand specimen, except for local alteration along grain boundaries. In total, eight samples from the Drahotín intrusion, including one from the WBSZ in contact with the intrusion, and ten samples from the Mutěnín intrusion were examined.

Drahotín

The Drahotín intrusion consists of (monzo) diorite and gabbro. The (monzo) diorite is less altered compared to gabbro, and dominantly consist of plagioclase (40 to 60 vol. %) and high but variable (up to 40 vol. %) proportions of amphibole and biotite (Fig. 2a). Plagioclase forms small tabular grains up to 1 mm in length, which are associated with anhedral grains of quartz and K-feldspar. Large, aligned biotite flakes, up to several millimetres in diameter, are accompanied by two generations of amphibole (brown and green). Needle-like apatite grains form abundant, local accumulations.
Fig. 2

Microphotographs of the studied rocks. (a) Drahotín diorite; (b) Drahotín gabbro; (c) Mutěnín diorite; (d) Mutěnín gabbronorite. Abbreviations: cpx—clinopyroxene, opx—orthopyroxene, phl—phlogopite, amp—amphibole, plg—plagioclase, bt—biotite

Phlogopite-bearing gabbro is composed of millimetre-scale grains of plagioclase (10 to 50 vol. %), clinopyroxene (<10 vol. %), amphibole (30–90 vol. %) and olivine (<5 vol. %). The rock is partially to completely recrystallized (Fig. 2b). Orthopyroxene is rare and, if present, forms relicts in amphibole cores. Clinopyroxene is strongly replaced by amphibole and contains abundant inclusions of plagioclase, which are also present in amphibole. Undulose extinction is common, especially in larger plagioclase grains. Accessory minerals comprise small, disseminated ilmenite grains (<0.1 mm), commonly accompanied by olivine. Large grains of phlogopite accompanied by ilmenite are almost completely replaced by chlorite.

Mutěnín

The Mutěnín intrusion is petrographically more diverse than the Drahotín intrusion, consisting of gabbronorite, diorite, monzonite and syenite with veins of granitic pegmatite. Gabbronorite with fine-grained cumulate texture is composed of plagioclase (60–70 vol. %), amphibole (25 to 30 vol. %) and less then 5 vol. % of orthopyroxene (Fig. 2d). Plagioclase occurs as small grains (<0.5 mm) with irregular shapes commonly showing undulose extinction. Two types of amphibole are present: primary brown amphibole associated with orthopyroxene, and secondary green amphibole, which contains relicts of clinopyroxene and occurs in millimetre-scale layers, or replaces rare individual clinopyroxene grains. Orthopyroxene is abundant in the form of long tabular grains. The rock contains a high content of ilmenite, comprising either irregular larger grains associated with brown amphibole or small euhedral inclusions in plagioclase.

The diorite consists of plagioclase (50 to 60 vol. %), K-feldspar (<5 vol. %), amphibole (10–30 vol. %), biotite (20 to 35 vol. %), and some contain subordinate amounts of quartz and titanite. Millimetre-scale plagioclase and K-feldspar form irregular and tabular grains with undulose extinction and optical zonation. Deformed grains of biotite are associated with brown or green amphibole, and these minerals occur in variable proportions in this rock type (Fig. 2c). Titanite and apatite, if present, occur in association with biotite and amphibole, where apatite forms rounded grains enclosed in biotite.

The monzonite and syenite occur in two different textural types: (1) a coarse-grained, equigranular texture dominated by K-feldspar, in which biotite and quartz are located at triple junction boundaries, and (2) a porphyroclastic texture with K-feldspar porphyroclasts in a matrix of plagioclase, and aligned amphibole, which imparts a strong foliation to the rock. Both textural types contain high contents (up to 8%) of prismatic zircon and allanite.

Whole-rock chemistry

Major and trace element analyses of Drahotín and Mutěnín rock types are given in Tables 3 and 4, respectively.
Table 3

Whole-rock major and trace element composition of the Drahotín rocks

Sample

06DR1

06DR3B

06DR5B

06DR6A

06DR7B

06DR8

06DR9

Rock type

Diorite

Diorite

Diorite

Diorite

Diorite

Gabbro

Gabbro

SiO2 (wt. %)

55.48

54.06

52.68

53.09

56.78

50.69

50.82

TiO2

1.88

1.39

0.77

1.68

1.55

1.09

0.68

Al2O3

17.40

16.33

14.65

17.43

16.81

8.94

8.85

Fe2O3

2.93

2.65

1.08

2.35

1.01

2.04

1.79

FeO

6.20

4.94

4.38

6.45

5.10

6.46

6.62

MnO

0.12

0.11

0.11

0.13

0.10

0.20

0.15

MgO

3.13

4.50

7.59

4.52

4.61

13.02

15.17

CaO

4.65

5.73

14.68

6.69

5.79

13.75

11.39

Na2O

2.55

2.87

1.52

3.02

1.89

0.93

0.81

K2O

3.54

4.32

0.64

2.35

3.61

0.47

0.48

P2O5

0.20

0.24

0.02

0.04

0.12

0.05

0.04

H2O−

0.18

0.10

0.12

0.14

0.14

0.10

0.14

H2O+

1.50

1.73

1.29

1.47

1.82

1.68

2.36

CO2

0.08

0.11

0.10

0.11

0.06

0.23

0.37

Total

99.84

99.08

99.63

99.47

99.39

99.65

99.67

Sc (ppm)

24

19

40

33

22

73

53

V

104

128

204

140

134

535

332

Cr

38

120

292

59

94

1,595

1,061

Co

22

23

33

26

25

48

66

Ni

21

54

80

19

35

138

160

Cu

29

25

41

22

29

58

79

Zn

116

87

50

106

72

65

68

Rb

92

51

5.2

72

21

7.9

17.3

Sr

501

1,620

395

538

617

308

323

Y

23.4

37.4

17.7

29.9

18.9

22.5

15.4

Zr

448

500

60

117

143

113

84

Nb

22.8

20.2

2.43

15.1

10.1

2.52

2.28

Cs

2.45

1.23

0.14

2.30

0.31

0.26

0.54

Ba

1,506

6,400

326

1,712

3,265

374

280

La

17.5

356

11.3

45

30

16.5

13.6

Ce

71

502

27.6

152

72

42

33

Pr

6.06

65

3.92

14.7

8.91

6.30

4.81

Nd

27

242

19

63

39

31

23

Sm

6.77

32

4.89

11.6

7.62

7.11

5.46

Eu

2.65

6.74

1.38

3.15

2.67

1.80

1.41

Gd

6.44

24.5

4.72

10.2

6.40

6.62

4.59

Tb

0.93

2.22

0.70

1.34

0.85

0.92

0.65

Dy

5.01

8.56

4.02

6.77

4.23

5.07

3.59

Ho

0.98

1.39

0.75

1.24

0.82

0.91

0.65

Er

2.69

4.01

2.07

3.41

2.21

2.54

1.76

Tm

0.36

0.46

0.29

0.46

0.30

0.35

0.23

Yb

2.16

2.82

1.68

2.79

1.83

2.08

1.48

Lu

0.32

0.42

0.24

0.40

0.27

0.31

0.21

Hf

11.3

13.65

1.83

3.09

3.88

3.39

2.48

Pb

23

66

4.4

32

14.9

2.2

23

Th

14.7

57

3.55

13.4

10.1

3.54

3.17

U

2.56

7.8

0.94

2.51

1.93

0.85

0.70

Table 4

Whole-rock major and trace element composition of the Mutěnín rocks

Sample

06MU1

06MU2B

06MU3B

06MU4

06MU5

06MU6

06MU7

06MU9

06MU10

06MU11

Rock type

Diorite

Diorite

Gabbro-norite

Gabbro-norite

Diorite

Diorite

Syenite

Diorite

Diorite

Syenite

SiO2 (wt. %)

48.52

48.86

50.30

50.40

43.98

45.27

57.05

49.34

50.36

61.54

TiO2

2.66

1.39

0.60

0.90

2.30

1.96

0.55

2.25

1.59

0.29

Al2O3

16.95

19.51

15.47

15.85

18.05

18.12

17.39

17.27

21.42

18.82

Fe2O3

3.01

3.85

3.35

2.50

3.42

3.86

3.56

2.84

2.74

1.54

FeO

8.06

7.34

8.10

6.95

9.00

9.85

5.21

7.78

6.11

2.60

MnO

0.16

0.14

0.20

0.18

0.14

0.17

0.17

0.16

0.10

0.08

MgO

4.28

2.85

9.11

8.00

4.99

3.99

0.22

4.28

2.10

0.40

CaO

7.65

5.31

9.03

10.86

8.78

7.35

3.29

7.35

6.58

1.64

Na2O

3.67

4.08

2.15

2.49

2.86

3.31

3.75

3.74

4.61

4.06

K2O

2.54

3.60

0.30

0.38

2.80

3.13

7.09

2.75

2.29

7.83

P2O5

0.20

0.29

0.04

0.04

0.42

0.36

0.04

0.20

0.15

0.02

H2O−

0.04

0.20

0.08

0.14

0.10

0.14

0.12

0.10

0.12

0.08

H2O+

1.56

1.84

0.92

1.21

1.52

1.55

0.91

1.29

1.19

0.66

CO2

0.16

0.23

0.27

0.03

0.09

0.07

0.16

0.33

0.11

0.11

Total

99.46

99.49

99.92

99.93

98.45

99.13

99.51

99.68

99.47

99.67

Sc (ppm)

33.3

14.4

36.6

48.2

34.8

26.8

25.5

31.8

6.2

7.9

V

237

85

245

283

249

112

4

238

47

4

Cr

92

21

147

272

13

3

7

12

62

6

Co

31

18

46

37

33

29

1

30

14

2

Ni

34

4

53

47

<1

<1

<1

4

25

<1

Cu

34

24

42

20

32

25

11

32

21

7

Zn

114

132

112

101

120

134

132

113

92

68

Rb

64.5

49.8

2.0

1.8

32.3

25.7

91.5

15.3

40.6

127.1

Sr

880

775

228

229

1,124

1,129

202

802

1,229

290

Y

41.4

32.1

22.8

30.5

46.0

43.0

73.1

39.4

15.2

16.2

Zr

495

497

31

50

371

149

1,673

384

1,538

593

Nb

20.1

16.3

1.6

2.7

17.9

21.9

37.3

18.8

10.9

11.6

Cs

2.24

0.78

0.14

0.15

0.44

0.25

0.96

0.14

0.80

1.50

Ba

2,162

4,781

337

831

3,975

4,003

1,585

2,284

3,552

1,914

La

101

175

6.84

10.8

133

136

496

119

84

286

Ce

219

382

16.8

29.1

306

315

1,196

256

156

544

Pr

27.6

37.3

2.62

3.97

41.1

41.7

105

30

17.0

59

Nd

112

139

13

20

171

172

364

115

62

199

Sm

17.9

19.5

4.00

5.79

27.2

26.2

49.3

17.9

8.15

24.8

Eu

4.45

5.21

1.33

1.51

6.02

5.90

4.78

4.33

5.78

3.48

Gd

15.1

16.5

4.65

6.58

21.7

20.3

40.7

15.2

7.01

18.0

Tb

1.84

1.70

0.77

1.13

2.34

2.21

4.19

1.79

0.69

1.40

Dy

8.95

7.53

4.80

6.55

11.0

10.3

19.1

8.86

3.16

4.40

Ho

1.69

1.37

0.99

1.33

1.93

1.84

3.45

1.60

0.61

0.69

Er

4.59

3.71

2.95

3.77

5.29

4.99

10.0

4.69

1.85

2.16

Tm

0.62

0.45

0.41

0.50

0.66

0.61

1.29

0.62

0.27

0.24

Yb

3.95

2.75

2.71

3.17

3.95

3.67

8.56

3.85

1.93

1.64

Lu

0.58

0.41

0.42

0.48

0.58

0.52

1.32

0.57

0.36

0.27

Hf

11.5

9.9

0.84

1.22

8.8

4.39

35.2

9.8

31.2

13.3

Pb

32.0

34.1

1.1

2.3

20.5

17.2

56

21.0

20.3

65

Th

11.9

22.8

0.18

1.60

6.90

6.12

136

16.5

8.32

114

U

3.16

1.40

0.04

0.12

1.74

1.46

6.47

2.79

3.77

4.86

Major elements

The analyzed Drahotín rocks plot in the fields of gabbro, diorite and monzonite in the TAS classification diagram (Middlemost 1994). Two separate groups can be distinguished based on alkali contents: (1) subalkaline rocks (gabbro and gabbrodiorite) containing less than 3 wt. % alkalies and (2) alkaline rocks (diorite, monzodiorite and monzonite) with 5–8 wt. % Na2O + K2O (Fig. 3). In contrast, the Mutěnín rocks range in composition from subalkaline Fe-rich gabbro (SiO2 = 50 wt. %, Na2O + K2O = 2.5–2.9 wt. %) through monzogabbro/diorite to high-alkali monzonite and syenite (Fig. 3).
Fig. 3

Total Alkali vs Silica (TAS) diagram after Middlemost (1994)

Major elements plotted against SiO2 (Fig. 4) illustrate significant compositional differences between the Drahotín and Mutěnín intrusions. Drahotín gabbro and gabbrodiorite display similar SiO2 contents (51–57 wt. %), and variable MgO (3–15 wt. %) and CaO (5–15 wt. %) contents. No correlation is found between SiO2 and FeOT and P2O5, but pronounced positive correlations exist between SiO2 and TiO2 and K2O and negative correlations between SiO2 and MgO and CaO (Fig. 4). A well-developed positive correlation also exists between FeOT/MgO and TiO2 (Fig. 5). In contrast, the Mutěnín intrusion consists predominantly of K2O-rich alkaline rocks with basic to acidic character (SiO2 = 44–62 wt. %, Na2O/Na2O + K2O from 0.3 to 0.6) and subalkaline Fe-gabbronorite (SiO2 = 50 wt. %, Na2O/Na2O + K2O ∼ 0.9). The gabbronorite, diorite and syenite form three distinct groups in terms of their major element concentrations (Fig. 4) and gabbronorite/diorite have moderate to high FeOT (8.6–13.3 wt. %) and TiO2 (0.5–2.7 wt. %) contents.
Fig. 4

Variations of whole-rock major oxides with SiO2 contents for Drahotín and Mutěnín rocks. Mutěnín rocks are distinguished into three groups: (a) gabbronorite, (b) diorite and (c) syenite-pegmatite. Symbols as in Fig. 3

Fig. 5

FeOT/MgO vs TiO2 plot showing well-developed positive correlation for Drahotín rocks, but absence of any correlation for Mutěnín rocks. Symbols as in Fig. 3

Minor and trace elements

The transition metals (Sc, V, Cr, Co, Ni, Cu and Zn) decrease with increasing SiO2, in agreement with their compatible behaviour during the crystallization of mafic magmas, e.g. by olivine and clinopyroxene fractionation.

Trace elements, some minor elements (e.g., K, P, Ti; normalized to primitive mantle; McDonough and Sun 1995) and rare-earth elements (REE; normalized to chondrite; Boynton 1984) for the Drahotín and Mutěnín intrusions are plotted in Fig. 6. Both intrusions are significantly enriched in REE (10–1,000 times) compared to chondrite with LaN/YbN ratio ranging from 1.8 to 118. Drahotín rocks show similar REE patterns, except for one sample (06DR3), which is markedly enriched in LREE (LaN/YbN = 85) due to a high content of apatite. For Mutěnín samples, two types of REE patterns can be distinguished: (1) a primitive, relatively unfractionated REE distribution (LaN/YbN = 1.7–2.3) in the subalkaline gabbronorite (06MU3B, 06MU4), and (2) a strong LREE-enrichment (LaN/YbN = 17.2–117.6) in the alkaline diorite/syenite, which reflects the relatively high apatite and allanite contents in diorite and syenite, respectively. Most samples have negative Eu anomalies (Eu/Eu* = 0.3–0.9), but some diorites have positive Eu anomalies.
Fig. 6

Whole-rock REE and other minor/trace element distributions normalized to chondrite (Boynton 1984) and primitive upper mantle values (McDonough and Sun 1995), respectively

Compared to primitive mantle in a plot of extended trace elements (Fig. 6), all Drahotín and Mutěnín rocks show similar enrichment in large-ion lithophile elements (LILE), such as Ba and Rb, although U and Th are markedly depleted compared to the other LILE. In such a plot the high field strength elements (HFSE) show negative anomalies and have relatively high values for the ratios of ThN/NbN (up to 81), NdN/ZrN (up to 9.7) and EuN/TiN (up to 15.7).

Sr-Nd isotopic compositions

Sr and Nd isotopic data for whole-rocks from the Drahotín (8 samples) and Mutětín (10 samples) intrusions are given in Table 5. Initial Sr and Nd isotopic ratios calculated at 330 Ma are plotted in Fig. 7.
Table 5

Rb-Sr and Sm-Nd concentration and isotopic data for Drahotín and Mutěnín rocks

Sample

Locality

Rock

Rb (ppm)

Sr (ppm)

87Rb/86Sr

87Sr/86Sr

87Sr/86Sr (i)

Sm (ppm)

Nd (ppm)

147Sm/144Nd

143Nd/144Nd

143Nd/144Nd (i)

eNd (i)

TDM (Ga)

06DR1

Drahotín

Diorite

193*

413*

1.353

0.7144 ± 10

0.7080

4.1*

16.5*

0.1513

0.512381 ± 9

0.512054

−3.1

1.7

06DR2B

Drahotín

Migmatite

63

117

1.553

0.7310 ± 9

0.7237

8.0

40

0.1203

0.512121 ± 8

0.511861

−6.9

1.6

06DR3B

Drahotín

Diorite

51

1,620

0.092

0.7077 ± 10

0.7073

32.2

242

0.0809

0.512218 ± 9

0.512043

−3.3

1.0

06DR5B

Drahotín

Diorite

5.2

395

0.038

0.7084 ± 8

0.7082

4.9

19.3

0.1540

0.512271 ± 10

0.511938

−5.4

2.1

06DR6A

Drahotín

Diorite

114*

486*

0.678

0.7105 ± 10

0.7073

11.1*

57*

0.1170

0.512357 ± 7

0.512104

−2.1

1.2

06DR7B

Drahotín

Diorite

21

617

0.098

0.7097 ± 9

0.7092

7.6

39

0.1198

0.512170 ± 7

0.511911

−5.9

1.5

06DR8

Drahotín

Gabbro

19*

303*

0.182

0.7080 ± 9

0.7071

6.9*

29*

0.1432

0.512427 ± 5

0.512118

−1.9

1.5

06DR9

Drahotín

Gabbro

24*

302*

0.226

0.7082 ± 10

0.7071

4.8*

20.7*

0.1418

0.512420 ± 8

0.512114

−1.9

1.4

06MU1

Mutěnín

Diorite

85*

770*

0.321

0.7074 ± 10

0.7059

15.3*

94*

0.0989

0.512453 ± 10

0.512239

+0.5

0.9

06MU2B

Mutěnín

Diorite

50

775

0.186

0.7081 ± 7

0.7072

19.5

139

0.0855

0.512278 ± 8

0.512093

−2.3

1.0

06MU3B

Mutěnín

Gabbronorite

2.0

228

0.026

0.7041 ± 9

0.7040

4.0

13.4

0.1811

0.512665 ± 6

0.512274

+1.2

1.9

06MU4

Mutěnín

Gabbronorite

4.0*

212*

0.055

0.7041 ± 10

0.7038

5.3*

16.9*

0.1917

0.512808 ± 5

0.512394

+3.5

1.9

06MU5

Mutěnín

Diorite

84*

1,113*

0.218

0.7073 ± 9

0.7063

25.1*

160*

0.0950

0.512375 ± 8

0.512170

−0.8

1.1

06MU6

Mutěnín

Diorite

26

1,129

0.066

0.7075 ± 9

0.7072

26.2

172

0.0922

0.512264 ± 8

0.512065

−2.9

1.1

06MU7

Mutěnín

Syenite

91

202

1.308

0.7133 ± 7

0.7072

49.3

364

0.0823

0.512221 ± 8

0.512043

−3.3

1.0

06MU9

Mutěnín

Diorite

15

802

0.055

0.7072 ± 7

0.7069

17.9

115

0.0946

0.512285 ± 7

0.512081

−2.6

1.1

06MU10

Mutěnín

Diorite

41

1,229

0.096

0.7071 ± 7

0.7067

8.2

62

0.0796

0.512244 ± 10

0.512072

−2.8

1.0

06MU11

Mutěnín

Syenite

144*

290*

1.435

0.7130 ± 9

0.7063

19*

154*

0.0749

0.512321 ± 9

0.512159

−1.1

0.9

(i)—initial ratios calculated back to 330 Ma. *concentration determined by isotopic dilution. Estimated uncertainties of the 87Rb/86Sr and 147Sm/143Nd isotopic ratios were better than 1% and 0.2%, respectively. For details on calculations see “Sampling and analytical methods” section

Fig. 7

Variation in εNd and 87Sr/86Sr calculated back to 330 Ma. Note homogenous Sr-Nd composition of alkaline rocks from Mutěnín compared to the large variation in Sr-Nd isotopic composition of Drahotín rocks

Drahotín diorite and gabbro have variable, radiogenic Sr isotopic compositions (87Sr/86Sr(330 Ma) = 0.7073–0.7092) and unradiogenic Nd isotopic compositions (143Nd/144Nd(330 Ma) = 0.511911–0.512118), corresponding to εNd 330 Ma values between −5.4 to −1.9 for gabbro and −5.9 to −2.1 for diorite. The WBSZ migmatite has a very radiogenic 87Sr/86Sr(330 Ma) ratio (0.7237), while its Nd isotopic composition is similar to some of Drahotín rocks (εNd(330 Ma) = −6.9). Samples show significant negative correlation between 87Sr/86Sr and εNd (Fig. 7).

Mutěnín diorite and syenite have moderately radiogenic Sr, and unradiogenic to slightly radiogenic Nd isotopic compositions (87Sr/86Sr(330 Ma) = 0.7059–0.7072; 143Nd/144Nd(330 Ma) = 0.512043–0.512239; εNd(330 Ma) from −3.3 to +0.5). In contrast to all these rocks, the Mutěnín Fe-gabbro has a less radiogenic Sr-Nd isotopic composition (87Sr/86Sr(330 Ma) = 0.7038–0.7040) and more radiogenic Nd isotopic composition (143Nd/144Nd(330 Ma) = 0.512274–0.512394; εNd(330 Ma) = +1.2 to +3.5). There is a pronounced negative correlation trend between 87Sr/86 Sr and εNd (Fig. 7) for the suite of samples from the Mutenín and Drahotín intrusions

Discussion

The Drahotín and Mutěnín intrusions share similar characteristics in petrography, rock types (gabbro-diorite) and ages (Drahotín 328 ± 1 Ma, Mutěnín 341 ± 2 Ma; Dörr and Zulauf 2009). Additionally, a number of characteristics point to a similar evolution of these two intrusions: (1) similar Fe-Ti enrichment, (2) elevated (∼5%) quartz contents in diorites from both intrusions, (3) enrichment in large ion lithophile elements (LILE), coupled with depletion in Nb and Zr, and (4) radiogenic Sr and unradiogenic Nd isotopic compositions of most rocks, coupled with negative correlation trends between 87Sr/86Sr and εNd. On the other hand, major and trace element compositions reveal some important differences. For example, compared to Drahotín, the Mutěnín intrusion has a more alkaline character and, except for the Fe-gabbros, a more highly fractionated LREE enriched pattern (LaN/YbN = 17.3–118). Drahotín rocks show a negative correlation between SiO2 and MgO, CaO and a positive correlation between FeOT/MgO and TiO2 (Fig. 4 and 5). Such trends are consistent with a process of fractional crystallization (e.g., olivine, feldspar and apatite fractionation) and with petrographic observations. In contrast, the Mutěnín rocks do not show such elemental variations. In addition, the Mutěnín intrusion has a much more variable Sr-Nd isotopic composition compared to the Drahotín suite (Fig. 7).

Geochemical evolution of the Drahotín and Mutěnín intrusions

Compositional trends of the Drahotín and Mutěnín intrusions indicate the important but different roles of enriched mantle and/or continental crust in their evolution. In the following, the evolution of the Drahotín and Mutěnín intrusions will be discussed separately.

The Drahotín intrusion shows major element and petrographic evidence for fractional crystallization. For example, negative correlations between SiO2 vs MgO and positive correlations between SiO2 vs K2O are evident in Fig. 4. These correlations are coupled with an increase in biotite over amphibole (from 10/30 vol. % to 50/10 vol. %) in diorites. To provide more constraints on the role of continental crust, the importance of fractional crystallization (FC) and assimilation-fractional crystallization (AFC) processes were examined using the Rayleigh fractionation model for FC process (e.g., Allègre et al. 1977) and the AFC model for trace element and Sr-Nd isotopic composition of DePaolo (1981). Partitioning coefficients were taken from Aignertorres et al. (2007), Adam and Green (2006), Bindeman et al. (1998) and McKenzie and O’Nions (1991). The mass ratio of assimilant/mass crystallized for AFC model was estimated to be 0.5. Elements with different degrees of compatibility (Sc vs Th and La) during magma fractionation were selected for trace element AFC model. Taking into account the rock compositions, the most primitive rock from Drahotín (sample 06DR8; gabbro) containing 60% amphibole, 30% plagioclase and 10% phlogopite was chosen to represent the composition of the modelled parent magma. The composition of assimilant is difficult to estimate because the rocks surrounding the intrusion are highly variable in composition. Therefore, the composition of lower continental crust (Rudnick and Gao 2003) was chosen to represent the assimilant. The calculated FC and AFC paths are shown in Fig. 8. The Drahotín rocks do not show ideal fits with the calculated curves for either process (Fig. 8), most likely because of uncertainties in partitioning coefficients and/or parent magma and assimilant compositions. However, it is clear that the Drahotín compositional trends cannot be explained by simple FC processes; although sample 06DR6 fits the FC trend for La-Sc, albeit at an unrealistically high degree of melt fractionation (70–80% of parent melt crystallization). On the other hand, AFC processes could have played an important role in the evolution of the Drahotín rocks. The gabbros can be explained by ∼20–30% of parent magma crystallization (e.g., olivine crystallization during FC as can be suggested from decreasing olivine content in the gabbros along AFC trend), whereas most of the diorites point to ∼40–60% crystallization. One sample of Drahotín diorite (06DR3) lies off the AFC trend (Fig. 8), shows a high enrichment in Sr-Th-U-LREE, and contains ∼5 vol. % of apatite, suggesting that this sample may represent the highest evolved member of the magmatic suite. The Sr-Nd isotopic composition and modelling AFC processes involving Sr-Nd isotopic composition provides further constraints on the evolution of the Drahotín rocks (Fig. 9). The gabbros (06DR8, 06DR9) most depleted with respect to trace elements have also the most depleted Sr-Nd isotopic signature, but are still enriched compared to bulk silicate Earth (Fig. 7). The composition of Drahotín rocks could be produced by the effects of the AFC process on a primary gabbroic magma (samples 06DR8, 06DR9), with a crustal assimilant having a Sr-Nd composition similar to that of Moldanubian micaschist (Janoušek et al. 1995). However, with a different percentage of parent magma crystallization comparing to the trace element AFC model.
Fig. 8

Th and La vs. Sc variations diagrams. The fractional crystallization (FC; dashed line) and assimilation-fractional crystallization (AFC; solid line) trends are constructed for Drahotín intrusion using the most primitive rock (06DR8; black star) as a parent composition and lower continental crust (Rudnick and Gao 2003) as an assimilant. The numbers along the lines represent percentage of parent magma crystallization during FC and AFC processes. Absence of well-defined trends for Mutěnín suggests either limited role of simple FC and AFC processes during the evolution of the Mutěnín intrusion, or a more complex evolution. For more details see the text

Fig. 9

The Sr-Nd isotope assimilation-fractional crystallization (AFC) model for Drahotín intrusion using the most primitive rocks (06DR8-06DR9; black star) as a parent magma composition and Moldanubian micaschist (sample CR-5; Janoušek et al. 1995) as an assimilant. The numbers along the line represent percentage of parent magma crystallization during AFC process

The Mutěnín intrusion consists of two contrasting rock series—predominant alkaline diorite, syenite and rare subalkaline gabbronorite, which form small bodies within the diorites (Vejnar 1984). While it appears that there are some inverse correlations of MgO, CaO and P2O5 with increasing SiO2 for alkaline rocks (Fig. 4), these patterns are not consistent with fractional crystallization trends and sample petrography. However, some negative correlations within individual rock groups (e.g., negative correlations between P2O5 and SiO2 for diorites) could be attributed to fractional crystallization. Moreover, the absence of any fractionation trend for Sc and La-Th (Fig. 8) points either to the limited role of simple FC and AFC processes during the evolution of the Mutěnín intrusion, or to a more complex evolution. More importantly, the high concentrations of lithophile elements in most of the alkaline gabbro and syenite samples cannot be produced from the subalkaline gabbros by any realistic FC or AFC process. In turn, this implies that these two rock series represent or were derived from independent parent magmas. The primitive REE distribution of the subakaline gabbros (Fig. 6) is most likely attributed to their cumulate character (i.e., products of early fractionation from their parent magma; see cumulate texture in Fig. 2d). Moreover, compared to the Mutěnín alkaline rocks, the Sr-Nd isotopic compositions of gabbronorite samples 06MU3B and 06MU4 are distinctly different, having unradiogenic ratios for 87Sr/86Sr(330 Ma) (0.7038–0.704) and εNd(330 Ma) (+1.2 to +3.5) (Fig. 7). Such values suggest an origin for the Mutěnín gabbronorites from a mantle-derived basaltic magma with a depleted Sr-Nd isotopic signature. In contrast, the Mutěnín alkaline rocks have similar, but more pronounced, general characteristics as those of the Drahotín diorites: LREE and LILE enriched character, coupled with significant negative Nb-anomalies (ThN/NbN = 2.3 to 81) and variable Sr-Nd compositions. In the absence of evidence for a simple AFC process, such a combination of features may represent the contamination of a parental magma by compositionally diverse crustal rocks or derivation from a mixed mantle-crust reservoir (e.g., Schulmann et al. 2002; Holub et al. 1997).

Implications for evolution of the Bohemian shear zone (BSZ)

The ages of the Drahotín and Mutěnín intrusions (328 ± 1 Ma and 341 ± 1 Ma, respectively; Dörr and Zulauf 2009) are similar to those for other BSZ related magmatic rocks (e.g., the Bor, Nýrsko, Babylon, Klatovy, and Kozlovice granites; Dörr and Zulauf 2009 and references therein). However, except for the older Teufelsberg diorite (359±2 Ma; Bues et al. 2002) in the Kdyně intrusion, the Drahotín and Mutěnín gabbro to diorite intrusions differ in composition from the more prevalent granodiorite to granite plutons associated with the BSZ.

The two contrasting geochemical patterns of the Drahotín and Mutěnín intrusions may be related to the influence of different tectonothermal regimes during their times of emplacement. At the time of the Mutěnín intrusion (341 Ma), the possible existence of higher thermal gradients (e.g., Dörr and Zulauf 2009) may have promoted extensive mantle-crust interactions (i.e., mixing and/or mingling), which resulted in the wide compositional range of the intrusion. In contrast, the Drahotín intrusion was emplaced at 328 Ma, when thermal gradients may have been lower resulting in less extensive parent magma modification via AFC processes.

Conclusions

The Variscan Drahotín and Mutěnín plutons intruded gneisses of the Moldanubian Unit and are associated with the Western Bohemian shear zone (WBSZ), which is part of the complex tectonic boundary between the Teplá-Barrandian and Moldanubian Units. In spite of several similar characteristics, major element, trace element, and Sr-Nd isotopic compositions document different origins and evolutions for the two plutons. The parental magmas of the Drahotín intrusion (gabbro) can be explained by derivation from an enriched mantle source. The melts were modified by subsequent crustal assimilation and fractional crystallization (AFC) during magma transport and/or emplacement (diorite). The Mutěnín intrusion is a composite gabbronorite-diorite-syenite body. Trace element and Sr-Nd isotopic compositions demonstrate that this rock association cannot be explained by simple AFC processes from the most primitive member (gabbronorite). It is more likely that gabbronorite and the diorite to syenite suite represent two independent parent magmas. The primitive trace element and Sr-Nd isotopic composition of gabbronorite argues for derivation from depleted mantle, whereas the diorites to syenite suite was most likely derived from a mixed mantle-crust reservoir. Different evolutionary trends of the Drahotín and Mutěnín intrusions are likely related to the existence of different tectonothermal regimes during their emplacement.

Notes

Acknowledgements

The authors are greatly indebted to Zdeněk Vejnar for the help in the field, Jana Rajlichová for technical assistance, Gordon Medaris for grammatical corrections and Elmar Reitter for Rb-Sr and Sm-Nd isotope analyses. This research was supported by the Grant Agency of the Academy of Sciences, project No. KJB300130612, the Scientific Programme CEZ: AV0Z30130516 of the Institute of Geology, Acad. Sci. CR and the Scientific Programme of Ministry of Education: MSM0021620855.

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Lukáš Ackerman
    • 1
    • 2
  • Martina Krňanská
    • 3
  • Wolfgang Siebel
    • 4
  • Ladislav Strnad
    • 5
  1. 1.Institute of Geology v.v.i.Academy of Sciences of the Czech RepublicPraha 6Czech Republic
  2. 2.Czech Geological SurveyGeologická 6Praha 5Czech Republic
  3. 3.Faculty of Science, Institute of Geochemistry, Mineralogy and Mineral ResourcesCharles UniversityPraha 2Czech Republic
  4. 4.Institute of GeosciencesEberhard-Karls-University TübingenTübingenGermany
  5. 5.Faculty of Science, Laboratories of the Geological InstitutesCharles UniversityPraha 2Czech Republic

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