Palaeogeography of Prague Synform in Silurian times (Wenlock–Ludlow): insights from palaeomagnetism, geochemistry and biostratigraphyTasáryová Z. , Schnabl P. , Janoušek V. , Pruner P. , Štorch P. , Kletetschka G. , Čížková K. , Šlechta S. , Manda Š. , Erban V. & Frýda J. 1,2 3 1,2 3 3 1,3 3 3 1,2 1,2 1,2

BASALT GEOCHEMISTRY

INTRODUCTION GEOLOGICAL SETTING

Acknowledgements: This study has been funded by Grant Agency of the Czech Republic (P210-10-2351) and the research is part of the IGCP projects 580 and 591.

PALAEOMAGNETISM METHODS–

References:Babuška, V., Plomerová, J. (2012): Boundaries of mantle-lithosphere domains in the Bohemian Massif as extinct exhumation channels for high pressure rocks. Gondwana Research, 23, 973 987 .Boynton, W.V. (1984): Geochemistry of the rare elements: meteorite –studies. In: Henderson, P., (ed). Rare Earth Element Geochemistry. Elsevier, Amsterdam, 63 114. Cocks, L.R.M., Torsvik, T.H. (2006): European geography in a global context from the –Vendian to the end of the Palaeozoic. In: Gee, D.G., Stephenson, R.A., (eds). European Lithosphere Dynamics. Geol. Soc. London Memoirs 32, 83–95. Havlíček, V. (1981):Development of a linear sedimentary depression exemplified by the Prague Basin (Ordovician–Middle Devonian, Barrandian area–central Bohemia). Sbor. Geol. Věd, G 35, 7–48. Kirschvink, J.L. (1980): The least squares line and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society, 62, 699 718. –Krs, M., Pruner, P. (1995): Palaeomagnetism and palaeogeography of the Variscan formations of the Bohemian Massif, comparison with other European regions. J. Czech Geol. Soc. 40, 1-2, 3 46. Krs, M., Pruner, P., Man, O. (2001): Tectonic and paleogeographic –interpretation of the paleomagnetism of Variscan and pre-Variscan formations of the Bohemian Massif, with special reference to the Barrandian Terrane. Tectonophysics, 332, 93 114. Kříž J. –(1998): Silurian. In: Chlupáč I., Havlíček V., Kříž J., Kukal Z., Štorch P., (eds). Palaeozoic of the Barrandian (Cambrian to Devonian). Czech Geol. Survey, Prague, 149 164. Man, O. (2003): –Analysis of multi-component natural remanent magnetization based on the thermal demagnetisation spectrum. Studia Geophysica et Geodaetica, 47, 359 370. Patočka, F., –Pruner, P., Štorch, P. (2003): Palaeomagnetism and geochemistry of Early Palaeozoic rocks of the Barrandian (Teplá–Barrandian Unit, Bohemian Massif): palaeotectonic implications. Phys. Chem. Earth A 28, 735 749. Pearce, J.A. (1996): A user's guide to basalt discrimination –diagrams. In: Wyman, D.A. (ed.). Trace element geochemistry of volcanic rocks: application for massive sulphide exploration. Geol. Assoc. Canada, short course notes 12, 79 113. Pearce, –J.A. (2008): Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archaean oceanic crust. Lithos, 100, 14 48. Příhoda, K., Krs, –M., Pěšina, B., Bláha, J. (1989): MAVACS - a new system creating a nonmagnetic environment for palaeomagnetic studies. Cuadernos de Geologica Ibérica, 12, 223 250. –Stampfli, G., von Raumer, J., Borel, G.D. (2002): Palaeozoic evolution of pre-Variscan terranes: From Gondwana to the Variscan collision. In: Martínez Catalán, J.R., Hatcher, R.D., Jr., Arenas, R., Díaz García, F., (eds.). Variscan-Appalachian dynamics: The building of the late Palaeozoic basement. Geol. Soc. Am., Spec. Pap. 364, 263 280. Sun, S., McDonough, W.F. –(1989): Chemical and isotope systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (eds.). Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ. 42, 313 345. Tait, J., Bachtadse, V., Soffel, H. –(1994a): Silurian paleogeography of Armorica: New palaeomagnetic data from central Bohemia. J. Geophys. Res., 99, B2, 2897 2907. Tait, J., Bachtadse, V., Soffel, H. (1994b): –New palaeomagnetic constraints on the position of central Bohemia during Early Ordovician times. Geophys. J. Int., 116, 131 140. Tait, J., Bachtadse, V., Soffel, H. (1995): Upper –Ordovician palaeogeography for the Bohemian Massif: Implications for Armorica. Geophys. J. Int., 122, 211 218.–

3Institute of Geology, Academy of Sciences, Rozvojová 269, Praha 6, 165 00, Czech Republic

1Faculty of Science, Charles University, Albertov 6, Praha 2, 128 43, Czech Republic

2Czech Geological Survey, Klárov 3, Praha 1, 118 21, Czech Republic

Equator

30°

30°

60°

Perunica

Moldanubian

Saxothuringian

420 Ma [1] 420 Ma [2]

Cocks & Torsvik (2006) Stampfli et al. (2002)

A complex study of selected Silurian volcanic centres in the Prague Synform involved palaeomagnetic analyses of volcanites, their contact aureoles and surrounding sediments. It was combined with whole-rock and isotope geochemistry of volcanics and biostratigraphic dating. The obtained constraints on palaeoposition and geotectonic setting of the Prague Synform contribute to the refinement of Wenlock Ludlow palaeogeography of –peri-Gondwana terranes (see 1 Cocks & Torsvik 2006, and [ ][ ]2 Stampfli et al. 2002).

Palaeozoic volcanism of the TBU consits of two major volcanic episodes: After the first Cambro–Ordovician volcanic episode, volcanic centres shifted NE and basalts became dominant volcanic rock type in the Prague Basin during the Silurian–Devonian volcanic phase. Volcanic centres were controlled by ENE–WSW and NNW–SSE trending syn-volcanic deep-seated structures, which were reactivated during Variscan orogeny forming present-day structure of the basin – the Prague Synform [4]. Effusive products are restricted to Llandovery–Ludlow series [5], with maximum activity in Wenlock (Homerian, T. testis Biozone, c. 425 Ma) and Ludlow (Gorstian, S. chimaera Biozone, c. 422 Ma). Cessation of volcanism was marked by a short Devonian (Emsian) eruptive episode. Major volcanic accumulations – Svatý Jan, Kosov, Suchomasty, Řeporyje and basalts in Southern, Northern and Eastern tectonic segments [4, 5], were sampled for geochemical and palaeomagnetic analyses to contribute to Silurian palaeogeographic concepts.

Series

Sta

ge

439

436

435

430

425

440

443,7

Rhuddania

nA

ero

nia

nTely

chia

n

Lla

nd

overy

Wen

lock

Lu

dlo

w

Lud.

Go

rstia

nH

om

eria

nS

hein

woo

d.

S. linearis

S. chimaera -L. scanicus

C. ludensis

C. praedeubeli -C. deubeli

P. parvus

T. testis

C. radiansC.

lun

d-

gre

ni

C. murchisoni

M. belophorus

A. ascensus

C. vesiculosus

C. cyphus

D. triangulatus

D. simulans

L. convolutus

S. sedgwickii

R. linnaei

S. turriculatus

M. crispus

M. griestoniensis

T. tullbergi

O. spiralis

C. lapworthi

Graptolitebiostratigraphy

Mo

tol F

m.

Lith

os.

Ko

pa

nin

a

Fm

.L

itoh

lavy

Fm

.Ž

elk

ovi

ce F

m.

Lithologyshalow deep

graptolite shales

volcanites

tuffs, tuffites

tuffitic shales,limestonesclayey & calcareousshales

silicites

calcareousshalesbiosparitelimestones

cephalopodlimestones

brachiopod & trilobite shales

C. rigidusC. ellesae-ramosus

M. dubiusM. riccartonensis

C. insectus

Ma

lavas

L. progenitorN. nilssoni

C. centrifugus

P. leptotheca

D. pectinatus

P. acuminatus

sampled intrusions &effusions&

modified after Kříž (1998)

Sv.

Jan

, K

oso

v,

Řep

ory

je

Sv.

Jan

Řep

ory

je

Su

ch

om

asty

[5]

However, present-day geometry of mantle domains (microplates) in NW–SE cross-section through the Bohemian Massif [3], which is based on seismic aniso-tropy, suggests a distinct structure and deformation history of the TBU.

Prague Synform is a tectonically predisposed WSW–ENE trending trough, filled by Ordovician–Middle Devonian sequence and represents a part of Teplá-Barrandian Unit (TBU; Bohemian Massif). Silurian palaeogeographic con-cepts for the TBU involve either 1) existence of isolated microplate called Perunica (Havlíček, 1981; Cocks & Torsvik, 2006), or 2) no wide separation of the TBU from the adjacent Saxothuringian and Moldanubian domains (Stampfli et al., 2002).

PRAGUE

BEROUN

Czech Republic

Poland

Slovakia

Germany

Austria

Prague Synform:

N

5 km

Ordovician

Silurian

Devonian

Cretaceous

Faults

Effusions

Intrusions

Sampling sites:

NORTHERN

SEGMENT

SOUTHERN SEGMENTCENTRAL SEGMENT

CENTRAL SEGMENT

WESTERN

SEGMENTWESTERN

SEGMENT

SOUTHERN SEGMENT

Svatý Jan

Volcanic Centre

Kosov

Volcanic Centre

EASTERN

SEGMENTEASTERN

SEGMENTŘeporyje

Volcanic Centre

Silurian

Cretaceous

Faults

Major volcanic accumulation

N

5 km

tectonic segments, volcanic centres

PragueBasin:

SuchomastyVolcanic Centre

[4]

modified after Kříž (1998)

Cs Ba U K Ce Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Sr Nd Sm Ti Y Lu

1000

100

10

1

0.1

INTRUSIVES1000

Cs Ba U K Ce Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Sr Nd Sm Ti Y Lu

100

10

1

0.1

WENLOCK

Cs Ba U K Ce Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Sr Nd Sm Ti Y Lu

1000

100

10

1

0.1

LUDLOW

100

10

1

0.1

Sam

ple

/NM

OR

B

1000

Cs Ba U K Ce Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Sr Nd Sm Ti Y Lu

Sun & McDonough (1989)

OIB

EMORB

Cs Ba U K Ce Pr P Zr Eu Dy Yb

Rb Th Nb La Pb Sr Nd Sm Ti Y Lu

1000

100

10

1

0.1

Boynton (1984)

Sa

mp

le/R

EE

ch

on

dri

te

10

100

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu 1

Ce Nd Sm Gd Dy Er Yb

La Pr Pm Eu Tb Ho Tm Lu

WENLOCK1000

100

10

1

Ce Nd Sm Gd Dy Er Yb

La Pr Pm Eu Tb Ho Tm Lu

INTRUSIVES1000

100

10

1

Ce Nd Sm Gd Dy Er Yb

La Pr Pm Eu Tb Ho Tm Lu

LUDLOW1000

100

10

1

alkalibasalt

basalt

Nb/Y

0.005

0.02

Zr/

Ti

Pearce (1996)

1 2

0.01

bas.

andes.

0.50.2

mg#40 50 60 70

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Nd

εi

8.0

8030

FC

A

Nb/Yb

OIB

EMORB

NMORB

Volcan

ic a

rc

array

NMO

RB-OIB

arr

ay

Th

/Yb

0.1

1

0.01

0.1 1 10 100

Pearce (2008)

ThAlk

TiO

2/Y

b

OIB array

(deep melting)

MORB array

(shallow melting)

OIB

NMORB EMORB

N E

0.1 0.5 1 5 10 50

0.1

0.2

0.5

1

2

Nb/Yb Pearce (2008)

N

E

S

W

Down

Up

TILT CORR(Wulf)

Up

DownHorizontal Vertical Unit = 2.58e+00 A/m

0 10 20 30 40 50 60 70 80 90 mT

M/MmaxMmax= 19.7e+00 A/m

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.9

1

WW

EE

N

S

0.8

Primary magnetizationcomponent C (Silurian)

Secondary magnetizationcomponent B (Variscan)

Viscous component (present-day)

AF demagnetization: "Na Černidlech" site, Svatý Jan Volcanic Centre, Wenlock

Down

Up

Up

DownHorizontal Vertical Unit = 295.e-06 A/m

0 100 200 300 400 500 600 °C

M/MmaxMmax= 1.70e-03 A/m

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 °C

k [e-6 SI]2.5

TILT CORR(Wulf)

N

E

S

WWW

EE

N

S

0

Thermal demagnetization: "Na Černidlech" site, Svatý Jan Volcanic Centre, Wenlock

[3]

50

200

100

[km]

300 km0-100

ST crust

ST mantle lithosphere

TB crust

TB mantle lithosphere

MD crust

MD mantle lithosphere

BV crust

BV mantle lithosphere

AstenosphereLithosphere - Astenosphere Boundary

Moho

100

150

Saxothuringian (ST) Teplá-Barrandian (TB)Moldanubian

(MD)Brunovistulian (BV)

NW SE

exhumed rocks plutons After Babuška & Plomerová (2012)

x

x

Wenlock lava

Ludlow lava

Intrusive

LEGEND

The age resolution of the graptolite 3dating (10 years) allows tracing

volcanic events and correlation of individual volcanic centres through the entire Prague Synform. Silurian alkali basalts are characterized by high LILE abundances, high LREE/HREE ratios and lack HFSE depletion relative to the NMORB, suggesting an origin by a low-degree partial melting of a garnet peridotite.

Mobi l izat ion of alkalis is due to sea floor weathering. Positive Ba, Sr anomalies reflect an overprint by Variscan fluids.

Near linear courses of REE patterns l a c k a n y E u anomaly (Eu/Eu* = 0.9–1.1). Note two m o s t p r i m i t i v e picrite sills among intrusives.

Correlations of Nd isotopic data with independent fractionation parameter mg# – document a –general evolution towards less depleted mantle

Geotectonic setting of Silurian volcanism was most likely within-plate (continental rift with opening of oceanic basin), illustrating a progressive at tenuat ion of the Neoproterozoic continental crust and asthenosphere upwelling.

sources. Also, moderately positive values of Ndε are typical of pristine OIB and/or EMORB. i

Indeed the incompatible element ratios demonstrate no magma–crust interactions and document deep melting with garnet in residue. Transition from alkali to tholeiitic basalts correlates with time.

Pulse Magnetizer MMP10 JR 6A Spinner Magnetometer 2) LDA - 3 AF Demagnetizer

Superconducting Rock Magnetometer

1) MAVACSThermal Demagnetizer

300320340360380400420440460480500520540560580

CarboniferousDevonianSilurianOrdovicianCambrian

-60

-50

-40

-30

-20

-10

0

10

31 29

27

23

21

M2

M1

(D4)

(D3)

(D2)

(D1)19 18

17

16

13

12

10

9,118

49V48V

47V

46V43V

38V

37V

Pala

eo

lati

tud

e [

°]

Age [Ma]

-80

-70

M3

DATA PROJECTIONS

Primary remanent magnetization component C reflecting titanomagnetite or magnetite presence is in agreement with definition of Krs et al. (2001). This component (D = 189 196°, –I = -34 to -43°) was determined by temperature range of 280 480 °C (540 °C) and by alternating field AF range of –40 80(100) mT. Consequently, counter clockwise –palaeorotation of about 170° and/or clockwise palaeorotation of 190° is inferred for the Prague Synform. Secondary remanent magnetization component B, reflecting altered titanomagnetite or magnetite presence, was determined by temperature range of 80 280 °C (320 °C) and by alternating –field AF range of 5 25 (30) mT. Tilt-corrected mean of B is likely –to represent a Variscan overprint.

PALAEOMAGNETISM RESULTS–

(A): Virtual pole positions for Wenlock (pink) and Ludlow (blue) sampling sites.

(B): Apparent Polar Wandering Path inferred from East European Craton for Early Devonian to Middle Triassic time span is presented by a thick pink line (Patočka et al., 2003).

Empty green circles denote pole positions for the opalaeomagnetic declination step of D = 20 . Green lines

indicate distribution of pole positions due to palaeotectonic rotations for the same palaeomagnetic inclination.

P1

0

I=-4

0

0

I=-3

0

0

I=-5

0

T1T2

P1

P2C3

C2

C1

D3

D2D1

-40

-30

-10

0

10

20

30

40

50

60

70

100

110

120

130

140

150

160

170

-20

-70

-60

-50

-40

-30

-20

-10

010

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20 30

40

50

60

70

80

0

I=-40

0

I=-30

0

I=-50

Vinařice

Karlštejn

Černidla

Sv. Jan Kosov (Ludlow)

Kosov (Wenlock)

(A) (B)

(C): Prague Synform, Teplá-Barrandian Unit, Bohemian Massif: palaeolatitudes extrapolated from various sources, recalculated to a single reference point at

o oφ = 50 N, λ = 14 E. Black full circles belong to values derived from databases (Krs & Pruner, 1995; Patočka et al., 2003; Krs et al., 2001). Values M1, M2 and M3 follow Tait et al. (1994 a, b; 1995).

LEGEND

Wenlock (Svatý Jan & Kosov Volcanic Centres, Southern segment)

Ludlow (Suchomasty & Kosov Volcanic Centres)

(C)

CONCLUSIONS

Based on geochemical data along with biostratigraphical control on the timing of volcanic activity we infer that crustal attenuation associated with rifting of continental lithosphere resulted in the alkali to tholeiitic basaltic volcanism. The magma originated by a low degree of anatectic melting of a garnet peridotite source.Remanent magnetization component C (determined by a higher temperature than RM component B) allowed computation of mean tilt-corrected palaeolatitudes of 22.0° in southern hemisphere for Wenlock (Svatý Jan Volcanic Centre, Černidla); and of 24.4° in southern hemisphere for Ludlow (Suchomasty Volcanic Centre, Vinařice).

Berlin

Prague

Wien

Budapest

50°

20°15°10°

45°

55°

-24°

-25°

Berlin

Prague

Wien

Budapest

50°

20°15°10°

45°

55°

-21°

-22°

Wenlock - Svatý Jan Volcanic Centre, Černidla sampling site

Ludlow - Suchomasty Volcanic Centre, Vinařice sampling site The interpretation of secondary remanent

magnetization component B involves either 190° counter clockwise or 170° clockwise rotation of the Prague Synform during Variscan orogeny, which confirms results by Tait et al. (1994) and by Krs & Pruner (1995). Palaeolatitudes for Svatý Jan & Suchomasty Volcanic Centres in Wenlock and Ludlow concur with the palaeogeographic model of Cocks & Torsvik (2002). Moreover, the Silurian faunas of the Prague Synform correspond well with the inferred position near 25° S, SW of Baltica, i.e. far from peri-Gondwanan basins. Thus, our results are consistent with the original concept of independent Perunica microplate (Havlíček, 1981), rather isolated from other Gondwana-derived terranes of the Variscan Europe.

Silurianorientation

170° counter clockwise

or 190°clockwise rotation

Present-day orientation Prague Synform

Teplá-Barrandian Unit ~ Perunica microplate

425 Ma

422 Ma

KarlštejnKarlštejn

Sv. JanSv. Jan

Kosov Kosov ČernidlaČernidla

Vinařice

Kosov

Vinařice

Kosov

V36V

Laboratory experiments included: (1) progressive thermal demagnetization using the MAVACS (Magnetic Vacuum Control System, Příhoda et al. 1989) equipment with step interval of 40°C, (2) alternating field (AF) demagnetization using a Superconductiing Cryogenic Magnetometer (type 755 4 K) with steps at 5 mT to 20 mT, and (3) separation of remanent magnetization (RM) directions using principle component analysis as outlined in Kirschvink (1980) and Man (2003).

KLF 4A Kappabridge