Palaeogeography of Prague Synform in Silurian times (Wenlock Ludlow...
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Palaeogeography of Prague Synform in Silurian times (Wenlock–Ludlow): insights from palaeomagnetism, geochemistry and biostratigraphy Tasáryová 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. – 3 Institute of Geology, Academy of Sciences, Rozvojová 269, Praha 6, 165 00, Czech Republic 1 Faculty of Science, Charles University, Albertov 6, Praha 2, 128 43, Czech Republic 2 Czech 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 Stage 439 436 435 430 425 440 443,7 Rhuddanian Aeronian T elychian Llandovery Wenlock Ludlow Lud. Gorstian Homerian Sheinwood. S. linearis S. chimaera - L. scanicus C. ludensis C. praedeubeli - C. deubeli P. parvus T. testis C. radians C. lund- greni 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 Graptolite biostratigraphy Motol Fm. Lithos. Kopanina Fm. Litohlavy Fm. Želkovice Fm. Lithology shalow deep graptolite shales volcanites tuffs, tuffites tuffitic shales, limestones clayey & calcareous shales silicites calcareous shales biosparite limestones cephalopod limestones brachiopod & trilobite shales C. rigidus C. ellesae-ramosus M. dubius M. riccartonensis C. insectus Ma lavas L. progenitor N. nilssoni C. centrifugus P. leptotheca D. pectinatus P. acuminatus sampled intrusions & effusions & modified after Kříž (1998) Sv. Jan, Kosov, Řeporyje Sv. Jan Řeporyje Suchomasty [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 SEGMENT CENTRAL SEGMENT CENTRAL SEGMENT WESTERN SEGMENT WESTERN SEGMENT SOUTHERN SEGMENT Svatý Jan Volcanic Centre Kosov Volcanic Centre EASTERN SEGMENT EASTERN SEGMENT Řeporyje Volcanic Centre Silurian Cretaceous Faults Major volcanic accumulation N 5 km tectonic segments, volcanic centres Prague Basin: Suchomasty Volcanic 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 INTRUSIVES 1000 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 Sample/NMORB 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) Sample/REE chondrite 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 WENLOCK 1000 100 10 1 Ce Nd Sm Gd Dy Er Yb La Pr Pm Eu Tb Ho Tm Lu INTRUSIVES 1000 100 10 1 Ce Nd Sm Gd Dy Er Yb La Pr Pm Eu Tb Ho Tm Lu LUDLOW 1000 100 10 1 alkali basalt basalt Nb/Y 0.005 0.02 Zr/Ti Pearce (1996) 1 2 0.01 bas. andes. 0.5 0.2 mg# 40 50 60 70 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Nd ε i 8.0 80 30 FC A Nb/Yb OIB EMORB NMORB Volcanic arc array NMORB-OIB array Th/Yb 0.1 1 0.01 0.1 1 10 100 Pearce (2008) Th Alk TiO2/Yb OIB array (deep melting) MORB array (shallow melting) OIB NMORB EMORB NE 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 Down Horizontal Vertical Unit = 2.58e+00 A/m 0 10 20 30 40 50 60 70 80 90 mT M/Mmax Mmax= 19.7e+00 A/m 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1 W W E E N S 0.8 Primary magnetization component C (Silurian) Secondary magnetization component B (Variscan) Viscous component (present-day) AF demagnetization: "Na Černidlech" site, Svatý Jan Volcanic Centre, Wenlock Down Up Up Down Horizontal Vertical Unit = 295.e-06 A/m 0 100 200 300 400 500 600 °C M/Mmax Mmax= 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 W W W E E N S 0 Thermal demagnetization: "Na Černidlech" site, Svatý Jan Volcanic Centre, Wenlock [3] 50 200 100 [km] 300 km 0 -100 ST crust ST mantle lithosphere TB crust TB mantle lithosphere MD crust MD mantle lithosphere BV crust BV mantle lithosphere Astenosphere Lithosphere - 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 3 dating (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. Mobilization of alkalis is due to sea floor weathering. Positive Ba, Sr anomalies reflect an overprint by Variscan fluids. Near linear courses of REE patterns lack any Eu anomaly (Eu/Eu* = 0.9–1.1). Note two most primitive 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 attenuation of the Neoproterozoic continental crust and asthenosphere upwelling. sources. Also, moderately positive values of Nd ε are typical of pristine OIB and/or EMORB. i Indeed t he 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) MAVACS Thermal Demagnetizer 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 Carboniferous Devonian Silurian Ordovician Cambrian -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 9,11 8 49V 48V 47V 46V 43V 38V 37V Palaeolatitude [°] 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 o palaeomagnetic declination step of D = 20 . Green lines indicate distribution of pole positions due to palaeotectonic rotations for the same palaeomagnetic inclination. P1 0 I=-40 0 I=-30 0 I=-50 T1 T2 P1 P2 C3 C2 C1 D3 D2 D1 -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 0 10 -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 o φ = 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 (Havlí č ek, 1981), rather isolated from other Gondwana-derived terranes of the Variscan Europe. Silurian orientation 170° counter clockwise or 190°clockwise rotation Present-day orientation Prague Synform Teplá-Barrandian Unit ~ Perunica microplate 425 Ma 422 Ma Karlštejn Karlštejn Sv. Jan Sv. Jan Kosov Kosov Černidla Černidla Vinařice Kosov Vinařice Kosov V 36V 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
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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
