geodynamic evolution of central europegeo.mff.cuni.cz/documents/2010-ml-dolejs.pdfmetamorphic rocks...

David DolejšInstitute of Petrology & Structural Geology

Charles University, Prague

[email protected]

Geodynamic evolutionof central Europe

Outline of the talk

• terrane assembly and basement units of central Europe

• paleogeography and plate dynamics in Proterozoic and PhanerozoicNeoproterozoic convergence: 700-550 Ma Cambrian-Ordovician extension and break-up: 530-480 MaLate Paleozoic convergence, collision and exhumation: 440-310 Ma

• crustal segments of central Europesedimentary, magmatic and metamorphic record of geodynamic processes

Bohemian (Teplá-Barrandian) zoneSaxothuringian zoneLugian (Sudetes) zoneMoldanubian zone

• open question in geodynamic scenariosburial mechanisms of diverse precursor lithologieslocation and spatial orientation of subduction zonesheat sources for collisional magmatic activity

Before we start …Observations and data sources for geodynamic interpretations

• sedimentary recorddepositional environments and basins

• estimates of pressure-temperature paths from metamorphic rocks reconstruction of burial and exhumation processes

• structural observations style, spatial orientation and polarity of plate motions

• geophysical data (seismology) current structure of the lithosphere

• geochemistry of magmatic rocks signature of rifting, subduction and collisional environments

• geochronological dating time frame for processes

Terrane assembly of the European continent

Linnemann and Romer (2010)

• Variscan orogen (late Paleozoic) formsthe backbone of western and central Europe

• closure of the Rheic ocean, when Laurussiaand Gondwana collided

• terrane fragments consist of Proterozoic crust with Cadomian volcano-plutonic arcs and voluminous volcanosedimentary deposits that formed in basins during extension and rifting of the Gondwana margin in early Paleozoic

McCann (2008)

Paleogeography: 750-550 Ma

• break up of the Rodinia supercontinent (~ 750 Ma) leads to separation of Baltica,Siberia, West and East Gondwana. Closure of these oceanic basins in Vendianleads to the formation of volcanic and plutonic arcs (“Cadomian” orogeny)

Neoproterozoic convergence and formation of arcs

Linnemann et al. (2007)

McCann (2008)


• gradual opening of the Iapetus ocean (starting at ~550 Ma) leads to diachronicextension and rifting events along the northern margin of Gondwana

Early Paleozoic rifting of the Gondwana margin


Cocks and Torsvik (2006)

• extension and heating (mantle upwelling?) during Cambrian and Ordovician


• progressive rifting of Baltica and Siberia vs. Gondwana leads to opening of theRheic ocean

opening of the Rheic ocean


Cocks and Torsvik (2005)

• formation of linear oceanic depressions and passive continental margins• carbonate and siliciclastic sedimentation, sea-floor mantle-derived igneous activity and ocean-floor exhalative mineralization


• in early Silurian Baltica and Laurentia begin to converge (→ Laurussia);Rheic ocean closes by northward subduction beneath the Avalonian margin ofLaurussia

closure of the Rheic ocean

Hofmann et al. (2009)

• gradual closure of oceanic basins• deep marine sedimentation along passive margins shows shallowing from siliciclastic

turbidites to shelf and reef carbonate environments

McCann (2008)


• from Silurian (~430 Ma) southward subduction zones initiate along the northern margin of Gondwana (Variscan orogeny). This orientation is implied by studiesof magmatic rocks and exhumation mechanisms of metamorphic rocks but notsupported by paleogeographic reconstructions

initiation of subduction along the northern Gondwana margin

• deltaic and terrestrial input (plant remnants) to shelf sediments from early Devonian (~402 Ma)

• closing of narrow oceans (e.g., “Saxothuringian” ocean) along the formerly rifted Gondwana margin

Hofmann et al. (2009)

Schulmann et al. (2009)


• kinematics of the Laurussia-Gondwana continental collision is very poorly understood Laurussia vs. Gondwana continental collision

Kroner et al. (2007)

• oblique convergence was accommodated by NW-SE-directed subduction as well asNE-SW and NNW-SSE-oriented strike-slip faults with vertical component

• marine sedimentation ceases at 390 Ma but deposition in orogenic foreland begins at 410 Ma

• intracontinental riverine sedimentation starts at 314 MaVariscan collisional orogeny

Kroner and Romer (2010)

A brief review …

750-540 Ma: convergence between Siberia, Baltica and Gondwanasouthward subduction of the Paleo-Pacific ocean leading to an Andean-type active continental margin (“Cadomian” orogeny)

530-480 Ma: rifting events related to Iapetus opening along the northern Gondwanamargin; bimodal igneous activity related to continental extension andheating (“late Cadomian” intrusives)

500-470 Ma: opening of the Rheic ocean, sea-floor magmatic activity, formation of passive continental margins, deep marine sedimentation

430-360 Ma: closure of the Rheic ocean, northward and southward subductions,collisions of crustal segments along the northern margins of Gondwana, slab break-offs, termination of marine deposition

360-315 Ma: oblique collision of continental margins of Laurussia and Gondwanaaccommodated by crustal-scale strike-slip movements with verticalcomponent, extensive mantle and crustal melting and emplacementof batholiths (continental arc, Variscan orogeny)

CRUSTAL SEGMENTS OF THE BOHEMIAN MASSIFAND PETROLOGICAL RECORD OF GEODYNAMICS

Geological units of the Bohemian Massif

Schulmann et al. (2009)


Kroner et al. (2007)

• Saxothuringian and Sudetes• Teplá-Barrandian (Bohemicum)• Moldanubian (including Bavaricum)• Brunovistulian with Moravosilesian

Teplá-Barrandian unit (Bohemicum)overview

• remnant of accreted Neoproterozoic basin on volcanic arc (Gondwana margin)• early Paleozoic extension (shelf sediments with volcanic activity)

• Neoproterozoic sediments (630-540 Ma): siliciclastic flysch (9 000 m thick) withsea-floor volcanics and chert and carbonate facies on volcanic elevations

Hajná et al. (2010)

Mašek (1982)

Teplá-Barrandian unit (Bohemicum)


• bedded shelf deposits (turbidites, graywackes, shales)• olistolites (cherts, carbonates)• submarine mafic volcanics (lava flows, pillow lavas, peperites)

lithology and structures of the Neoproterozoic basement

Teplá-Barrandian unit


Neoproterozoic volcanic arc (570-540 Ma)

• volcanic arc (andesites,dacites with subordinatebasalts and minor intrusions of tonalites and trondhjemites (570-560 Ma)

Teplá-Barrandian unit

coarse pebbly conglomerates (submarine fans)

Neoproterozoic volcanic arc (570-540 Ma)

bedded to laminated graywackes and siltstones (turbidites)

• intra-arc sedimentary deposits


Teplá-Barrandian unitPaleozoic sedimentary evolution

bedded sequence of cherts, sandstones and siltstones(Upper Ordovician) representing marine deposits when approaching maximum extension of the riftedmargin of Gondwana

shale-carbonate sediments (Silurian-Devonian boundary);transition into shallow-marine carbonate depositionduring Rheic ocean convergence

SaxothuringianKroner et al. (2010)

overview

• outboard autochtonous domaincomposed of unmetamorphosedor low-grade volcanosedimentary rocks from Neoproterozoic tomid-Devonian, deformed in awrench and thrust zone

• inboard allochtonous domain, a series of thrust sheets withhigh-grade metamorphic rocks(370-340 Ma) and voluminoussilicic batholiths (325-310 Ma)

Saxothuringian


autochtonous domainLinnemann et al. (2010)

bedded cherts of the Cadomian back-arc basin

conglomerates of the Cadomian retro-arc basin

Saxothuringian


autochtonous domain


• volcanosedimentary sequence of theoceanic rift to passive continentalmargin

• Cambro-Ordovician rift basin

• deep marine shelf sedimentation from Silurian to mid-Devonian

• allochtonous domain comprises the Saxonian Granulite Mts., high-pressuremetamorphic rocks of the Erzgebirge and several metamorphic nappes inBavaria, Saxony and Sudetes

• peak pressure of 2.2 GPa (~85 km) was reached at ~342 Ma followed byrapid exhumation (1 cm yr-1) and amalgamation with surrounding low-pressurevolcanosedimentary units at 335 Ma

Saxothuringianallochtonous domain

Rotzler and Romer (2010)

• high- to ultra-high pressure nappe stacks of the Erzgebirge Mts.

• Neoproterozoic and Paleozoic protoliths were buried deeper than 4 GPa (150 km)• narrow prograde and exhumation pressure-temperature paths are consistentwith burial in a subduction channel, detachment and exhumation along the baseof the wedge-arc lithosphere

Saxothuringianallochtonous domain

Rotzler and Romer (2010)

• Moldanubian zone is a complex tectonic assembly including medium- to high-gradevolcanosedimentary units, bodies and slices of ultrahigh-pressure mafic and silicicrocks, and remnants of magmatic arc as well as postcollisional mantle- and crust-derived batholiths

• it represents an original arc that underwent continental collision and has beenexhumed and eroded to mid-crustal level

Moldanubianoverview

Štípská et al. (2008)

Moldanubian

Štípská et al. (2008)

assembly of high- and mid-pressure metamorphic rocks

• older vertical fabric related to exhumation from thecontinental collisional root overprinted by youngersubhorizontal fabric recording the lateral flow in themiddle crust

Moldanubianassembly of high- and mid-pressure metamorphic rocks

• constraints on the depth (pressure) of the continental root andon tectonic amalgamation come from mineral equilibria

Štípská et al. (2008) Faryad (2009)

• mantle ultramafics and eclogitic oceanic crustwere probably assembled at 3.0-3.5 GPa

• felsic and intermediate granulites generallyrecord same exhumation paths from 2.0 GPa

• strong reworking and lateral flow at 0.7 GPa

Heading towards the open questions ...

Open questions about the Variscan evolution of central Europe

• how and how fast were various lithologies buried? variety of precursors of different ages - oceanic crust, ultramafics, intermediate arc intrusives, metamorphosed supracrustal rocks

• where are the subduction zones?when were they active and how many, role of slab break-off

• were the neighbors always neighbors?the role of strike-slip and elevator-style movements in assembling the crustalsegments

• is the collisional thermal regime consistent with subduction? heat sources for large-scale magma generation: radiogenic heat, mantle upwelling, heat advection during slab break-off

How were individual lithologies buried?evidence from mineral inclusions in granulites

• felsic and intermediate granulites are abundant in Moldanubian and Saxothuringian zones and Sudetes: they were interpreted as early Paleozoic intrusives (related to Iapetus extension) or as magmas crystallized near the base of the overthickened crust

• mineral inclusions (phengite, paragonite or sodic amphibole) preserved in prograde metamorphic garnet provide evidence for burial to low temperatures and high pressures, 620 oC and 2.2 GPa, consistent withsubduction of continental crust

Faryad et al. (2010)

How were individual lithologies buried?evidence from textures of ultramafic rocks

Exsolutions of garnet in pyroxene and vice versa that formed during cooling and decompression in garnet pyroxenites

Faryad et al. (2009)

• garnet peridotites and pyroxenites formed along oceanic geotherm (or a mantle adiabat) and were buried directly to 4.5-5.0 GPa (cratonic geotherm);this is consistent with of a back-arc or intra-arc basin floor burial beneath overthickened continental margin

Where were the subduction zones?Rhenohercynian

Zeh and Will (2010)

• the „Saxothuringian“ arc is themost outward arc (NW) of the Variscanorogeny

• reversal of subduction polarity of theRheic ocean at 390 Ma

• preserved as mid-German crystalline rise, its magmatic activity andsedimentary record is consistentwith southeast-directed subduction

Where were the subduction zones?Saxothuringian

• magmatic arc produced by closure of the Saxothuringian microocean beneath the Gondwana margin is often sought for in the Central Bohemian plutoniccomplex

• protracted subduction assumes that Teplá-Barrandian unit formed the arc/forearc basement asearly as 380 Ma (K. Schulmann); underplating-delamination model produces no magmatic arc but allows for igneous activity at 325-310 Ma near the Saxothuringian-Teplá-Barrandian boundary(H.-J. Massonne, G. Zulauf, F. Finger)

Schulmann et al. (2009) Massonne (2006)

Where were the subduction zones?Moldanubian

• the Central Bohemian plutonic complex (354-341 Ma) – a plutonic arc but whose?

Schulmann et al. (2009) Finger et al. (2007)

Dorr and Zulauf (2008)

There is little agreement on• derivation of the Central Bohemian arc• single- or double-sided subduction• relation of batholith emplacement to

strike-slip and elevator-style tectonicsat the Teplá-Barrandian margins

Where were the subduction zones?constraints from paleogeography

• paleogeographic reconstructions favor north- or northeastward subduction,a scenario that is opposite to geochemical or geophysical considerations

• Neoproterozoic terranes are sourced in the south,along the northern margin of Gondwana

• no development of active continental margin of Gondwana• oblique subduction leads to final assembly assisted by

crustal-scale strike slip faults (NNE-SSW at 370-340 Ma andNW-SE at 330-315 Ma)

Kroner and Romer (2010)

Thermal consequences of Variscan convergence

Finger et al. (2009)late orogenic magmatic activity: Saxo-Danubian belt

• five stages of magmatic activity, which predates to postdates Variscan continental convergence

Reconciling kinematics, magmatic pulses and agesHofmann et al. (2009)

Geochronology of Saxothuringian

• North Variscan complex (350-330 Ma): magmatic arc of southward-dipping Rhenohercyniansubduction

• Central Bohemian complex (360-335 Ma): magmatic arc of southeastward-dippingSaxothuringian subduction

• perpotassic plutons (durbachites, 340-335 Ma): mantle-derived magmas utilizing exhumationchannels with high-pressure rocks

• Saxodanubian complex (330-325 and 315-310 Ma): decompression melting related torapid crustal exhumation

• Sudetic complex (315-300 Ma)

NV

CB

durb

SD

Su

Heat sources

Lexa et al. (2011)

radiogenic heat production from felsic igneous crust

• underplated felsic igneous crust of Paleozoic age providing heat of 4 mW m-3 with thermal incubation of 10-15 Ma is sufficient to promote rheological thermal weakening and partialmelting

Summary

• Paleo-Pacific convergence between Siberia, Baltica and Gondwana (750-540 Ma)by southward subduction leading to an Andean-type active continental margin (“Cadomian” orogeny). Accreted ocean floor, arc and retro-arc basins preserved in Teplá-Barrandian and Saxothuringian zones

• diachronic incipient rifting events (540-520 Ma) related to Iapetus opening along the northern Gondwana produce bimodal igneous activity related to continental extension and heating (basalts, rhyolites and metagranites in Saxothuringian andSudetes)

• opening of the Rheic ocean (500-470 Ma): sea-floor volcanosedimentarysequences, formation of passive continental margins, marine and siliciclasticsedimentation (preserved in all crustal segments)

• closure of the Rheic ocean (430-360 Ma), collisions of microcontinents along Avalonia and northern margins of Gondwana, termination of marine deposition. Subduction organization is less clear: northward and possibly southward subductions, slab break-offs

• formation of magmatic arcs (mid-German Crystalline High, Central BohemianPlutonic Complex at 350-335 Ma) and exhumation of diverse high-pressure rocksfrom the continental root (Saxothuringian, Sudetes, Moldanubian)

Summary

• extensive and voluminous partial melting and emplacement of intrusive batholithsat 330-310 Ma (Saxodanubian complex in Saxothuringian and Moldanubian, Sudetes, Teplá-Barrandian). Heat sources are controversial but radiogenic heat from burriedearly Paleozoic silicic crust appears to be plausible

• spatial arrangement and life time of subduction zones after 380 Ma, assignmentof magmatic arcs and spatial relationships between crustal segments including therole of strike-slip and vertical movements during oblique collision are stillpoorly understood

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