2013_Kądziałko-Hofmokl et al._Paleomagnetism and magnetic mineralogy of metabasites and granulites...

7/29/2019 2013_Kdziako-Hofmokl et al._Paleomagnetism and magnetic mineralogy of metabasites and granulites from Orli

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Acta GeophysicaDOI: 10.2478/s11600-012-0092-y

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2012 Institute of Geophysics, Polish Academy of Sciences

Paleomagnetism and Magnetic Mineralogyof Metabasites and Granulites

from Orlica-nienik Dome (Central Sudetes)

Magdalena KDZIAKO-HOFMOKL1, Jacek SZCZEPASKI2,Tomasz WERNER1, Maria JELESKA1, and Krzysztof NEJBERT3

1Institute of Geophysics, Polish Academy of Sciences, Warszawa, Polande-mails: [email protected] (corresponding author),

[email protected], [email protected] of Geological Sciences, University of Wrocaw, Wrocaw, Poland

e-mail: [email protected] of Geochemistry, Mineralogy and Petrology, Warsaw University,

Warszawa, Poland; e-mail: [email protected]

A b s t r a c t

The results of palaeomagnetic, rock magnetic, and microscopicstudy of Early Paleozoic metabasites and granulites from the Orlica-nienik Dome (OSD, Sudetes) have been combined with geochro-nological data. In the eastern part of the OSD (nienik Massif, SM)ferrimagnetic pyrrhotite is prevalent, accompanied by various amounts ofFe-oxides. In the western part of the OSD (Orlica-Bystrzyca Massif,OBM) Fe-oxides dominate. All magnetic minerals originated duringhydrothermal and weathering processes. The palaeomagnetic studyrevealed the presence of three secondary components of natural rema-nence: Late Carboniferous, Late Permian, and Mesozoic. Two Paleozoiccomponents are related to volcanic activity in the Sudetes. They are car-ried by pyrrhotite and Fe-oxides and were isolated only in SM rocks. TheMesozoic component was determined in both parts of the OSD and is

carried by Fe-oxides. It covers a time span, from ~160 to ~40 Ma, corre-sponding to a long period of alteration.

Key words: Sudetes, Orlica-nienik Dome, Paleozoic, metabasites,paleomagnetism, rock-magnetism.


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M. KDZIAKO-HOFMOKL et al.

1. INTRODUCTIONThis paper is a continuation of a series of papers concerning paleomagnetismand rock-magnetism of rocks exposed in the Sudetes Mountains, SW Poland(easternmost part of the Bohemian Massif). The Sudetes are interpreted asa complex mosaic of terranes (Franke and elaniewicz 2000, Aleksandrowskiand Mazur 2002) composed of a deformed and metamorphosed, latest Pro-terozoic to middle Paleozoic, volcano-sedimentary succession intruded bypre-Ordovician granites. These complexes are covered by Late Devonian toCarboniferous sediments and intruded by voluminous Carboniferous gran-ites. The paleomagnetic study of the Sudetic Paleozoic rocks was undertaken

some years ago to help elucidate the tectonic history of the Sudetes and, asa consequence, the eastern part of the Variscan Belt. Results obtained till2002 served as the basis for constructing the Sudetic Apparent Polar WanderPath from Devonian to Permian (Jeleska et al. 2003 and references within).The oldest calculated point obtained by the spline method reaches 394 Ma,the youngest about 250 Ma. The results obtained later for rocks from theKodzko Metamorphic Complex (Kdziako-Hofmokl et al. 2003) are com-patible with the reference Sudetic Path for the Carboniferous-Permianboundary. On the whole, the Sudetic Path runs very close to the Apparent

Polar Wander Path for Baltica constructed by Torsvik and Smethurst (1994).Palaeomagnetic poles most often encountered in the studied Palaeozoic

Sudetic rocks are of Carboniferous and Permian and Mesozoic age; Devoni-an poles are less numerous. In many cases the Tertiary-Recent secondarycomponents of natural remanence, compatible with the reference data forStable Europe (Besse and Courtillot 2002), were also encountered. In orderto better define the oldest part of the Sudetic Path, isotopically dated ultraba-sic rocks from the Jordanw-Gogow Serpentinite Massif (JGSM) and theSowie Gry Block (GSB), belonging to the dismembered Sudetic ophiolite,were studied by Kdziako-Hofmokl et al. (2006, 2008). A geochronologicalstudy performed on zircons gave the minimum age of serpentinization of theJGSM peridotite at 400+4/-3 Ma (Dubiska et al. 2004) and the Sm-Ndisochrone revealed the cooling age of peridotite from the GSB at 4023 Ma(Brueckneret al. 1996). The palaeomagnetic results (Kdziako-Hofmokl etal. 2006, 2008) obtained for these ultrabasics revealed the presence of fourcomponents of natural remanence. Two of them were compatible with theCarboniferous and Permian segments of the Sudetic APWP, the third com-

ponent lay along the Tertiary-Recent part of the Master European APWPath. The fourth component, which was ascribed to the Early Devonian iso-topic ages quoted above, revealed a pole position that did not lie on the finalsegment of the APWP, but was displaced from it. This result, discussedthoroughly in the forementioned papers, implies that during the Early Devo-


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PALEOMAGNETISM OF ORLICA-NIENIK DOME

nian the JGSM + GSB block formed a common unit that accreted to theremaining Sudetic units not earlier than about 390 Ma.

The present study was undertaken in an attempt to find corroboration ofthese results in isotopically dated Palaeozoic metamorphic rocks from theOrlica-nienik Dome (OSD) the Polish part of the important Sudeticregion not yet studied palaeomagnetically.

The rocks in this structure experienced several tectonometamorphic epi-sodes from (U)HP/(U)HT through retrogression and hydrothermal altera-tions. For our study we have chosen metabasites (amphibolites and eclogites)and granulites to examine whether they had retained component/s of rema-

nent magnetization/s acquired during the times preceding the zircon andAr39/Ar40 metamorphic ages (370-330 Ma) recorded by rocks of the OSD.Bearing in mind that remagnetizations are very common in the Sudetes andthe primary components of age older than Late Carboniferous Early Devo-nian are difficult to find, we looked also for remagnetized components ofremanence to relate them to the post-Variscan history of Palaeozoic com-plexes in Sudetes. The Orlica-nienik Dome with its complex history seemsto be a good candidate for such trial.

We have attempted to combine our palaeomagnetic results with the

results of investigations of magnetic mineralogy and geochronological data.

2. GEOLOGICAL SETTING

The OSD, Central Sudetes, SW Poland (Fig. 1), comprises amphibolite-facies orthogneisses intercalated with a volcano-sedimentary succession. Themain textural varieties of the former rocks, the nienik and Gieratwgneisses, are derived from a similar granitic protolith dated at about 500 Ma(e.g., Turniak et al. 2000). The orthogneisses host inclusions of (U)HP

granulites and eclogites (Fig. 1b), whereas the surrounding volcano-sedimentary complex is composed of biotite to staurolite-grade micaschists and paragneisses with intercalations of marbles, acidic metavolcanicrocks and amphibolites. The supracrustal series represents a Neoproterozoicto Cambrian succession, based on micropaleontological evidence (e.g.,Gunia 1990) and detrital zircon ages (Mazuret al. 2012). In contrast, also onthe basis of detrital zircon ages, Jastrzbski et al. (2010) suggested that thewhole formation was laid down during Middle Cambrian Early Ordoviciantimes. The volcano-sedimentary complex contains relatively small bodies of

metabasic rocks of MOR affinity and shows a geochemical signature pro-duced by a subduction-related component (Ilnicki et al. 2012) or due tocrustal contamination (Floyd et al. 1996, Nowak and elaniewicz 2006).

The structure of the OSD is described as resembling that of gneiss domes(e.g., Chopin et al. 2012), with core part formed by SM and OBM mantled


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Fig. 1: (a) Geological sketch map of the eastern part of the Bohemian Massif,(b) geological sketch map of the Orlica-nienik Dome, with sampling localities andsample numbers in the Orlica-Bystrzyca Massif (OBM), and nienik Massif (SM).


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by the Nov Msto unit to the west, the Zbeh unit to the south and theStar Msto unit to the east. The NNE-SSW trending Star Msto Belt is

composed of Lower Ordovician amphibolites, tonalitic gneiss, paragneiss,and migmatites intruded by a Carboniferous granodioritic sill (Parry et al.1997, tpsk et al. 2001). The protholith of these rocks has been interpretedas a Lower Palaeozoic intracontinental rift succession, subsequently accretedonto the Variscan orogen (tpsk et al. 2001). Still farther east of the StarMsto Belt, the East Sudetic nappe pile comprises thrust units derived fromthe western margin of the Brunovistulian microplate (Schulmann and Gayer2000). To the south, the volcano-sedimentary complex of the Zbeh Unit is

composed of mica schists and paragneisses intercalated with acid and basicmetavolcanics. In the Nov Msto Unit (NMU) low grade metapelites andbasic and acidic metavolcanics of assumed Neoproterozoic age (e.g., Cha-loupsk et al. 1995) are intruded by Carboniferous granodiorites andtonalites (Bachliski and Haas 2002).

Numerous isotopic ages of metamorphism and subsequent cooling, pub-lished during the last two decades from the OSD and its neighbourhood,could be roughly grouped into four age intervals which may correspond toseparate thermal events: (i) 390-370 Ma, related most probably to HP/UHP

metamorphism (e.g., Anczkiewicz et al. 2007, Gordon et al. 2005), (ii) 350-320 Ma, being a record of the main phase of Variscan metamorphism (e.g.,Turniaket al. 2000, tpsk et al. 2004, Lange et al. 2005a,b, Schneideretal. 2006, Brcker et al. 2009, Chopin et al. 2012), (iii) 300-270 Ma, mostprobably a record of post-Variscan cooling (Maluski et al. 1995, Szczepa-ski 2002), and (iv) 120-80 Ma (Maluski et al. 1995, Marheine et al. 2002). Itis noteworthy that Permo-Carboniferous thermal events described from theOSD correlate well in time with the intense volcanism and sedimentation inthe adjacent Intra-Sudetic Basin (Awdankiewicz 1999, 2004). The most

recent isotopic ages obtained by means of U-Pb SHRIMP dating on zirconsdocument Late Carboniferous (about 310-300 Ma) and early Permian (294-283 Ma) ages of volcanism in this part of the Sudetes (Awdankiewicz andKryza 2010, Awdankiewicz et al. 2011, Kryza and Awdankiewicz 2012).Late Permian volcanic activity dated at about 294 Ma has also been docu-mented from the North-Sudetic Basin (Szczepara et al. 2011). Additionally,U-Pb zircon ages from the elaniak intrusion in the neighbouring KaczawaMts. were interpreted as the emplacement age (316-315 Ma) and a younger

event or episode of hydrothermal activity (about 269 Ma; Machowiaket al.2008). The above mentioned data indicate relatively strong Late Carbonifer-ous Early Permian magmatic activity preserved within the sedimentarybasins surrounding the OSD.

The OSD is cut by the Late Cretaceous Nysa Kodzka Graben (NKG)and divided into the eastern nienik Massif (SM) and the western Orlica-


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Bystrzyca Massif (OBM). Sedimentation started in the Late Cenomanian(e.g., Wojewoda 1997); however, as a fault-bounded basin the NGK was ini-

tiated in the Late Turonian / Early Coniacian (e.g., Don 1996). The devel-opment of the NKG is correlated well in time with the Late Cretaceousinversion of the Polish Basin, and the entire Central European Basin System(CEBS; Schecket al. 2002). The inversion in question also correlates withactivity of the Elbe Fault Zone (EFZ) constituting the SW boundary of theCEBS (Schecket al. 2002). The EFZ is about 800 km long and WNW-ESEtrending zone extending from the North Sea to SW Poland (Fig. 1a). Itsactivity involved three separate periods (Scheck et al. 2002 and references

therein): (i) Late Carboniferous, (ii) Late Jurassic / Early Cretaceous, and themost intense during (iii) Late Cretaceous / Palaeogene. The Late Cretaceousto Palaeogene uplift of Variscan massifs adjoining the EFZ from the SW wasrecorded by fission-track data from the Lusatian Thrust (Ventura et al.2009), the western Erzgebirge (Ventura and Lisker 2003) and the Sudetes(Aramowicz et al. 2006, Daniiket al. 2012).

3. SAMPLING AND EXPERIMENTAL METHODS

3.1 Sampling

Rocks chosen for our study are mainly amphibolites (metabasites) croppingout as small inclusions within the volcano-sedimentary complex. Moreover,some of the samples collected represent granulites hosted by the migmatizedGieratw orthogneisses, eclogites forming inclusions within the Gieratwtype orthogneisses, and metagabbro. The sampling localities were rathersmall and our collection comprises only 84 oriented hand samples from 12exposures: 6 lying in the eastern (SM) and 6 in the western (OBM) part ofthe OSD (see Fig. 1b). Nine localities contain amphibolites, one eclogites

(SM Midzygrze, MI), one granulites (SM Stary Gieratw, SG), andone metagabbros (OBM Lewin Kodzki 2, LK2). The LK2 andneighbouring amphibolite exposure, Lewin Kodzki 1 (LK1), are located inveins belonging to the NMU. The names of sampling sites with appropriateabbreviations used later in the text, numbers of samples and lithology andvalues of selected magnetic properties are summarized in Table 1. Handsamples were cut in the laboratory into standard cylinders for palaeomag-netic and rock magnetic investigations.

3.2 Methods of rock-magnetic investigations

The identification and characteristics of the mineral carriers of the magneticproperties of the study rocks were investigated using magnetic methods andreflected-light microscopy. The results obtained by the magnetic methodsled to the identification of magnetic minerals irrespective of their origin, and


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to an estimation of their domain state. The microscopic study helped to dis-tinguish magnetic and non-magnetic ore minerals of various origins and ap-

pearance, as well as their alteration. The combination of magnetic andmicroscopic results permitted the determination of the conditions andsequence of appearance of magnetic minerals of the same kind but differentorigin and, consequently, the probable origin and age of their magnetization.

3.2.1 magnetic methods

Magnetic methods comprised thermomagnetic analyses and investigation ofmagnetic hysteresis. Thermomagnetic analyses consisted of: (i) the thermaldecay of saturation remanence SIRM acquired in the field of 9 T, duringheating to 700C in non-magnetic space (TUS-Poland); (ii) continuousmeasurements of magnetic bulk susceptibility Kb, during heating in the low(196C room temperature, r.t.), and during heating and cooling in thehigh (r.t. 700C) temperature ranges (KLY-3 with low-temperature andhigh-temperature extensions CS3-L and CS3, accordingly, AGICO, CzechRepublic). Thermomagnetic experiments performed in the range of r.t. 700C reveal values of unblocking (Tub) and Curie (TC) temperatures charac-teristic of magnetic minerals, experiments performed in the 196C r.t.

range show temperatures of low-temperature transitions if present. Kb-Tplots may also reveal prevailing paramagnetic minerals; (iii) for a fewspecimens, measurements of magnetization induced in 450 mT, during heat-ing to 700C performed with a thermobalance in the Borok Laboratory of theInstitute of the Physics of the Earth of Russian Academy of Sciences; (iv)application of the Lowrie method (Lowrie 1990) consisting of step-likethermal demagnetization of three components of IRM acquired in the field of3 T (Z component), 0.4 T (Y component), and 0.15 T (X component). Thismethod reveals blocking temperatures of components of magnetic rema-

nences of various coercivities.Measurements of magnetic hysteresis were performed with a vibrating

magnetometer VSM (Molspin Ltd, UK) or, in the case of very weakly mag-netic specimens, with Micromag 2900 AGM (PMC, Princeton, USA) withthe highest available magnetic field of 1 T. For a few specimens the appa-ratus working in the Borok Laboratory (see above) was also used. Theobtained parameters: Mrs saturation remanence, Ms saturation magnetiza-tion, Hc coercivity, Hcr coercivity of remanence served for constructionof the Day Dunlop plot (Dunlop 2002), that helps to estimate the domainstate of magnetic minerals.

3.2.2 reflected-light microscopy

Identification of minerals in the examined metamorphic rocks and observa-tion of their textural intergrowths, permitting us to reconstruct the mineral


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succession scheme, were performed on a standard polarising microscopeECLIPSE E600W POL (NIKKON, Japan).

3.3 Methods of palaeomagnetic investigations

Palaeomagnetic investigations consisted of measurements of intensity anddirections of natural remanence NRM of specimens in their natural state andafter consecutive steps of demagnetization during thermal and alternatingfield demagnetization procedures. Measurements were performed with themagnetometer SQUID of DC type (2G Enterprises, USA) with an attachedalternating field demagnetizer with peak alternating field (AF) of 140 mT.

Thermal cleaning was performed with a nonmagnetic furnace of MagneticMeasurements (Great Britain) by heating specimens to 700C with demag-netization steps of 25, 50, or 100C (according to mineralogical compositionand temperature range). All the above equipment is located within a low-field cage of Magnetic Measurements compensating the external magneticfield. Magnetic susceptibilityKm of thermally demagnetized specimens wasmeasured in their natural state and after each heating step, with a KLY-2susceptibility bridge (Geophysica Brno, former Czechoslovakia) for moni-toring mineral changes implied by heating. All demagnetizing results were

analyzed with the PDA program package based on principal componentanalysis developed by Kirschvink (Kirschvink 1980). In some cases theRemasoft program of Chadima and Hrouda (2006) was used.

Bulk magnetic susceptibilityKm and its anisotropy AMS were measuredfor all untreated specimens cut for palaeomagnetic purposes. The results willbe the subject of another paper, aimed at an AMS study of the OSD metaba-sites and HP/UHP rocks. This will provide an opportunity for a detailed dis-cussion of the AMS data and tectonic models proposed by Pressler et al.(2007).

4. EXPERIMENTAL RESULTS AND INTERPRETATION

4.1 Results of rock-magnetic study of identification of magnetic

minerals

The plots of SIRM-T and plots of Lowrie reveal the dominance of ferrimag-netic (monoclinic) pyrrhotite with an unblocking temperature of about 330-350C in amphibolites from localities NG, LU, GO and in eclogites from MI(Fig. 2a,c,g,i) from SM. The presence of pyrrhotite is sometimes (Fig. 2b),but not always (Fig. 2h), observed also on Kb-T plots. The pyrrhotite phaseis often accompanied by various, but rather small, amounts of Fe-oxides(magnetite/maghemite, hematite with unblocking temperatures between 450and 670C and a phase with an unblocking temperature of 170-200C thatmay be connected with a goethite or maghemite coating formed on small


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Fig. 2. Thermomagnetic results for SM amphibolites and eclogites (SIRM-T plots left column, K-T plots central column, Lowrie experiments right column):(a) SIRM-T plot for NG amphibolite, (b) high-temperature Kb-T plot for NG am-

phibolite, (c) Lowrie plot for NG amphibolite, (d) SIRM-T plot for SN amphibolite,(e) high temperatures Kb-T plot for SN amphibolite, (f) Lowrie plot for NG am-

phibolite, (g) SIRM-T plot for MI eclogite, (h) high temperature Kb-T plot for MIeclogite, and (i) Lowrie plot for MI eclogite. SIRM means saturation remanenceacquired in the field of 9 T,K bulk susceptibility, T temperature, and mag/magh magnetite/maghemite.

magnetite grains during surface-oxidation at low temperatures (see, e.g.,Kdziako-Hofmokl 2001, and references within, zdemir and Dunlop2010). The same procedures performed on amphibolites from the remaininglocalities (in the SM and OBM parts) show a dominance of Fe-oxides: mag-netite/maghemite and hematite accompanied with small amounts of pyr-rhotite, in some cases visible on a Lowrie plots, and with goethite and/orsurface-oxidized magnetite (Fig. 2d-f for SM and Fig. 3a-h for OBM). Theunusual stability of isothermal remanences revealed by the Lowrie plot for

the specimen from LK1 (Fig. 3h) and metagabbro from LK2 suggests thepresence of hematite-ilmenite exsolutions in these rocks (McEnroe et al.2005). Granulites from SG contain hematite as a main magnetic phase;a 170-200C kink appears on SIRM-T and I(450 mT)-T plots in these rocksas well.


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Fig. 3. Thermomagnetic results obtained for OBM amphibolites and gabbro (SIRM-Tplots left column, K-T plots central column, Lowrie experiments right col-umn): (a) SIRM-T plots for GN amphibolite, (b) high temperature Kb-T plot for GNamphibolite, (c) SIRM-T plot for SZ amphibolite, (d) high temperature Kb-T plotfor SZ amphibolite, (e) Lowrie plot for SZ amphibolite, (f) SIRM-T plot for LK1amphibolite, (g) high temperature Kb-T plot for LK1 amphibolite, and (h) Lowrie

plot for LK1 amphibolite. Remaining notations as in Fig. 2.

Heating in air results in an increase of SIRM and magnetic susceptibilityin all studied rocks due to the formation of new magnetic minerals (magnet-ite).

The results clearly show differences in magnetic mineralogy between theeastern (SM) and western (OBM) rocks. In the SM ferrimagnetic pyrrhotiteis the main carrier of magnetic properties, whereas in the OBM (and inamphibolites from locality SN), the dominant magnetic minerals areFe-oxides. From this observation we decided to treat the results obtainedduring further study of rocks from the SM and OBM separately.

4.2 Hysteresis study

Measurements of hysteresis parameters were performed for 39 specimens(12 with Micromag, 2 in Borok, and 25 with VSM Nuvo). The dominantcontribution of paramagnetic phases is observed as a linear relationship of M


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Fig. 4. Results for magnetic hysteresis measurements. An example of hysteresis loopfor NG amphibolites: (a) measured with Micromag AGM 2900; large plots hyste-resis loops for whole signal (thin line) and ferromagnetic phase (bold line) in fieldup to 0.5 T, signal normalized; small inset plots IRM acquisition curves (right

part), and backfield demagnetization of IRM in fields up to 200 mT (left part);Hcr

means coercivity of remanence (in mT), Hc coercive force (in mT), Hcr/Hc andMrs/Ms ratios are included; (b) DunlopDay plot according to Dunlop (2002) plotsfor 38 samples; Mrs/Ms and Hcr/Hc hysteresis parameters for samples determinedwith VSM Nuvo or Micromag 2900 are plotted for SM metabasites and granulites aswell as for OBM metabasites (see attached legend). Results are compared to theoret-ical SD-MD, SD-SP, MD curves calculated by Dunlop (2002).


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versus H on the H-M plots; Fig. 4a shows the characteristic hysteresis plotsfor specimens from the SM and OBM metabasites.

The Day-Dunlop plot ofMrs/MsversusHcr/Hc (Dunlop 2002), (Fig. 4b),reveals the domain state of the magnetic minerals. Some scatter in the Mrs/Msand Hcr/Hc parameters is due to the low content of the ferrimagnetic phasewhich affects the accuracy, especially for data from the VSM magnetometer.Samples from the SM metabasites with prevalent pyrrhotite fall into tworegions most samples fall in the reference region for magnetite of SD grainsize (see especially NG) and some around the SD + MD mixing curve withvariable MD content (Fig. 4b). In two samples (LU, MI) a significant contri-

bution of SP grains is observed. Samples from the SM metabasites withFe-oxides are situated between the reference SD + MD and SD + SP mixingcurves of Dunlop (2002). This pattern may indicate a PSD state or, moreprobably, SD/MD with an SP contribution. The hysteresis parameters for theOBM metabasites are more scattered than that for the SM, caused by thescatter ofHc andHcr due to the varying contribution of hematite and the lowcontribution by a ferromagnetic phase. In general, they occupy two areaswhich differ in the SD, MD, and SP contributions in the Day plot.

4.3 Reflected-light microscopyA microscopic study was made of 15 samples selected from all sites exceptSG. The observed ferrimagnetic phases are present as accessory componentsand usually occur in association with non-magnetic opaque minerals or astiny inclusions within major rock-forming amphibole, plagioclase, garnet,and biotite. Microscopically visible opaque aggregates do not exceed 3 mmin diameter. They can be subdivided into two major mineral associations: anoxide association comprising rutile, ilmenite, low-Ti magnetite, hematite,goethite, two generations of leucoxene LEU I and LEU II (Fig. 5), and a sul-phide association that consists of pyrite, pyrrhotite, marcasite, post-pyrr-hotite magnetite, chalcopyrite, chalcocite, and graphite (Fig. 6). All texturalvarieties of Fe-hydroxide (goethite) and hematite grown during sulphideoxidation, often observed in all investigated samples, were included with theoxide association.

Evolution of oxide association: Rutile is the oldest opaque mineral pre-sent in this association (Fig. 5a). It broke down to ilmenite through retro-gressive metamorphic reactions with rock-forming minerals containing Fe

(Fig. 5c). Subsequently, during advancing retrogression, ilmenite was over-grown by titanite (Fig. 5b,c) with tiny lens-like inclusions of hematite withinilmenite grains (Fig. 5d). Such processes take place under conditions of highoxygen and/or H2O fugacities (Buddington and Lindsley 1964, Haggerty1991, Xirouchakis and Lindsley 1998, Frost et al. 2001, Harlov and Hansen


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Fig. 5. Fe-Ti oxides in metamorphic rocks of the OSD: (a) relic of rutile overgrownby coarse-grained leucoxene that originated due to retrogressive breakdown ofilmenite (LK1 amphibolite); (b) ilmenite aggregate overgrown by retrogresivetitanite (NG amphibolite); (c) ilmenite, partly replaced by coarse grained leucoxeneand rimmed by late titanite (LK2 metagabbro); (d) fine exsolutions of titanian hema-tite within host ilmenite (hemo-ilmenite); hemo-ilmenite is replaced by coarse-grained leucoxene (GN amphibolite); (e) magnetite grains within thin biotite-richzones, late hematite replace host magnetite along {111}; inset in upper-left corner of

the photograph shows another example of the euhedral magnetite from this somezone (SN amphibolite); and (f) hydrothermal hematite growing concurrently withthe thin quartz vein (LK1 amphibolite). Abbreviations: Hem hematite, Ilm ilmenite, Mag magnetite, Qz quartz, Rt rutile, Ttn titanite, HEM-ILM hemo-ilmenite, LEU (I) and (II) leucoxene, coarse-grained and fine-grained, re-spectively.


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Fig. 6. Mineralogy of sulphide association from metamorphic rocks of the OSD:(a) pyrrhotite crystallized and gradually transformed (MI eclogite); (b) pyrrhotite-chalcopyrite intergrowths developed during high-temperature external exsolution(NG amphibolite); (c) ilmenite, showing initial stage of leucoxenization (LEU II),associated with pyrite which containing small inclusions of pyrrhotite and magnetite(GN amphibolite); (d) secondary pyrite intergrown by large grains of magnetite andnumerous small pyrrhotite intergrowths that are relics of the large pyrrhotite grains

existed before pyritization of pyrrhotite (GN amphibolite); (e) pyrrhotite replaced bylate marcasite-goethite (NG amphibolite); and (f) goethite-hematite pseudomorph af-ter sulphide aggregate (SZ amphibolite). Abbreviations: Ccp chalcopyrite, Gth goethite, Hem hematite, Ilm ilmenite, Mag magnetite, Mrc marcasite, Po

pyrrhotite, Py pyrite, LEU II leucoxene II.


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2005). A continuation of the retrogressive processes connected with increas-ing oxygen fugacity led to a breakdown of ilmenite into aggregates of leuco-

xene (Ramdohr 1980, Mcke and Bhadra Chaudhuri 1991). Leucoxene,forming fine-grained, sometimes submicroscopic intergrowths, consists ofa mixture of Fe-Ti oxides, viz anatase and/or brookite, titanite, hematite,pseudorutile (Temple 1966, Mcke and Bhadra Chaudhuri 1991). Observedleucoxenes can be subdivided into two generations: coarser-grained LEU (I)formed earlier (Figs. 5a and 4c,d) and fine-grained/submicroscopic LEU (II)crystallized later (Fig. 6c).

LEU I was observed in the majority of analysed samples (Fig. 5a,c,d)

where its appearance and position suggest growth at the end of the transfor-mation path of the Fe-Ti oxides: rutile ilmenite titanite leucoxeneunder greenschist facies conditions within the temperature range 500-300C.LEU II developed due to Fe-leaching from primary ilmenite as very fine-grained to submicroscopic aggregates along ilmenite grain boundaries andfine fractures within ilmenite (Fig. 6c). This is typical of very low tempera-tures, not exceeding 50C (Temple 1966).

Evolution of the sulphide association: The sulphide association is dom-inated by pyrrhotite and pyrite. Pyrrhotite usually forms anhedral grains up

to 300 m in diameter (Fig. 6a,b,d,e). It appears in much greater amounts inthe SM part than in the OBM part of the OSD. At high temperatures (to 800-700C) it exists as a non-magnetic hexagonal form which alters due to pol-ymorphic transformation at temperatures of about 300C into ferrimagneticmetal-poor pyrrhotite (Fe7S8) with a monoclinic structure (4C polytype)(e.g., Tokonami et al. 1972, Morimoto et al. 1975, Schwarz and Vaughan1972, Kontny et al. 2000), which is unstable at high temperatures (Kissinand Scott 1982, Kullerud 1986). Similarly, magnetic investigations (seeabove) indicate an abundance of ferrimagnetic pyrrhotite in the SM part of

OSD, whereas in the OBM part it occurs only in minor amounts. During fur-ther cooling linked with an enhancement of sulphur activity, non-magnetichexagonal and ferrimagnetic monoclinic pyrrhotite were replaced by latepyrite and/or marcasite (Fig. 6c-e), processes widespread in the OBM part ofOSD. The breakdown of pyrrhotite to pyrite and/or marcasite is also respon-sible for the growth of two generations of late Ti-free magnetite (Ramdohr1980). This magnetite occurs as tiny inclusions in pyrite grains together withrelics of primary pyrrhotite (for details see Figs. 5e and 6c). The latest stage

of sulphide evolution reflects low-temperature hydrothermal alteration andweathering (Fig. 6e-f). These processes are documented in common goethitepseudomorphs after pyrrhotite and pyrite (Fig. 6e), which were commonlyobserved in a majority of samples. The oxidation of the sulphides to goethitetogether with the pyrrhotite breakdown to pyrite/marcasite (pyritization)


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effectively diminished the amount of ferrimagnetic pyrrhotite in the OSDmetabasites.

Summarizing, the results of reflected-light microscopy indicate thatmagnetic minerals present in the rocks were secondary, occurring in severalgenerations. They began to originate at temperatures slightly above 300C,during the final phases of retrogressive metamorphism (monoclinicpyrrhotite, magnetite) and formed during further cooling due to hydrother-mal processes and even weathering (monoclinic pyrrhotite, magnetite, hema-tite, goethite). These results support the findings from the rockmagneticstudies that ferromagnetic pyrrhotite is decidedly dominant in the SM part of

the OSD, whereas Fe-oxides dominate in the OBM part.4.4 Paleomagnetic experiments

The values of selected characteristic magnetic properties obtained for rocksfrom each locality are summarized in Table 1. The appropriate columns con-tain ranges of intensity of NRM, mean susceptibilityKm, median destructivefield (MDF), and unblocking temperatures Tub.

The values of NRM intensities in the majority of amphibolites from theSM and OBM parts of the OSD are lower than 100 mA/m. In only a few

cases are their values much higher (up to 2020 mA/m). Values ofKm rangebetween 90-260010

6 SI in the SM and 310-2090106 SI in the OBM,

with the exception of a few GN and SZ samples where Km reached valueshigher than 2500010

6 SI. Among the MI eclogites there are several sam-ples with intermediate Km (400-60010

6 SI) and low NRM (lower than10 mA/m), but there are also several with higher NRM, showing the hetero-geneous distribution of magnetic minerals in these rocks.

The results of the monitoring ofKm during heating experiments (meas-urements ofKm after each heating step) show that Km values do not changeup to 450-550C, and begin to increase, sometimes very quickly, when heat-ed to higher temperatures: between 450 to 600C in rocks from the SM, andbetween 550 and 600C in rocks from the OBM. This difference suggeststhat in rocks from the western part alteration proceeded further than in therocks from the eastern part.

Demagnetization experiments were performed for 84 specimens from theSM part of the OSD (51 with alternating field (AF) demagnetization AF,33 with thermal demagnetization TH), and 72 from the OBM part (44 with

AF and 28 with TH). Characteristic demagnetization plots are shown inFig. 7a-d for the SM and in Fig. 7e-h for the OBM part.Some samples responded better to the alternating field, others to the

thermal cleaning. The great differences in efficiency of the AF cleaning ofvarious specimens are reflected in the large range of values of MDF (see


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Fig. 7. Demagnetization plots for SM (a)-(d) and OBM (e)-(h) metabasites:(a) thermal demagnetization of NG amphibolite, (b) thermal demagnetization of SN

amphibolite, (c) thermal demagnetization of MI eclogite, (d) AF demagnetization ofMI eclogite, (e) thermal demagnetization of PO, (f) thermal demagnetization of GN,(g) AF demagnetization of LK1, and (h) thermal demagnetization of LK1. Irm/Inrmmeans intensity of NRM after each cleaning step/intensity of NRM before cleaning

procedure. Ovals denote direction of isolated component. Tick labels forX, Y, andZcomponents at Zijdervelds plots in mA/m.


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Table 1). In numerous specimens containing pyrrhotite and magnetite theyare low (less than 10 mT), in specimens containing hematite values are about

50-60 mT, whilst in some cases an AF of 140 mT does not influence NRMat all; such AF plots are characteristic of the presence of fine exsolutions ofhematite in an ilmenite host orvice versa (McEnroe et al. 2005). Accordingto these observations some specimens are demagnetized by alternating fieldof low intensity (Fig. 7d) and some not at all (Fig. 7g). Summarizing, the AFdemagnetization results revealed components of low (LF, 0 mT 10 mT), intermediate (IF, 10 mT


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Fig. 8. Stereographic projection of all characteristic directions isolated from OSMrocks: (a) for SM part, and (b) for OBM part. Density distribution counting contour

plots for these directions for SM part (c) and OBM parts (d). Density distributioncalculated by Spheristat (Pangaea Scientific, Canada). Clusters of directions aremarked (clusters 1-3 for SM part, cluster 3 of OBM part). Directions with negativeinclinations were inverted for density contours calculations.

low-intermediate inclinations), 2 (N-NE declinations, low-intermediate in-clinations), and 3 (N-NE declinations, intermediate-high inclinations),whereas in the OBM part (Fig. 8d) there is only cluster No. 3 (there was toofew directions forming a cluster of SW declinations for further investiga-tion). The remaining, randomly distributed directions were not taken intoaccount in further discussion. We suppose that they are a resultant of severalcomponents that we were not able to demagnetize.

Cluster 1 contains mainly directions of components isolated duringthermal cleaning of IT, HT, and VHT stability and only two results of the


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field cleaning of IF and HF stabilities. Half of the results forming cluster 2come from the thermal cleaning of IT and HT stability, and half from the AF

cleaning of IF and HF stability. The directions forming cluster 3 are resultsof the AF cleaning of LF, IF, and HF stability and the HT and VHT results.We imply that cluster 1 and perhaps 2 comprise components carried bypyrrhotite, maghemite/magnetite, and hematite, in various proportions,whereas cluster 3 is composed only of components carried by hematite andmagnetite/maghemite.

5. DISCUSSION

The results of the rock-magnetic experiments performed for rocks collectedin the OSD show distinct differences in magnetic mineralogy between theSM and OBM. In the majority of amphibolites and in all studied eclogitesfrom the SM ferrimagnetic pyrrhotite is dominant. Fe-oxides (hematite) areprevalent in the granulites and in one of the amphibolite localities (magnetitein SN). In the amphibolites and gabbros from the OBM, thermomagneticmethods reveal only traces of ferrimagnetic pyrrhotite, the dominating min-erals are magnetite/maghemite and hematite, often accompanied by goethite.The results of hysteresis the measurements led us to estimate the domain

state varying from SP through SD to MD (see DayDunlop plot, Fig. 4b).The domain state, connected with the size of grains, suggests that magneticminerals are present in a variety of sizes.

The microscopic results correspond to the rock-magnetic results, show-ing the presence of several generations of ferromagnetic pyrrhotite, magnet-ite, hematite, and goethite formed during retrogression, hydrothermalprocesses and finally weathering. Ferrimagnetic pyrrhotite is decidedly dom-inant in the SM part of the OSD, whereas Fe-oxides dominate in the OBMpart. The natural magnetic remanence of the identified mineral may be ofa chemical or thermochemical, perhaps sometimes thermal, character, beingacquired during growth of consecutive phases through their respective block-ing volume at enhanced temperature or when the temperatures decreasedbelow their blocking temperatures, or both.

The oldest magnetic mineral is ferromagnetic pyrrhotite, the temperatureof its formation from the previous hexagonal form, about 300C, corre-sponding to the final period of retrogression metamorphism. So, unfortunate-ly, the presence of components of remanence preceding or even equal to the

age of the metamorphic phase (330-350 Ma) is hardly possible.The palaeomagnetic results summarized in Table 2 comprise the meandirections of the components of remanence isolated at the respective locali-ties and included in each of the three clusters, together with appropriate polepositions, final mean directions calculated from locality means (where pos-


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sible), and mean directions and pole positions calculated from directionsobtained for all specimens used. As can be seen, the mean values calculated

in both ways are identical. Mean directions obtained for particular localitieswith their circles of confidence are presented in Fig. 9. In palaeomagneticpractice, the final results are usually calculated from locality means. But,taking into account the small number of directions of components of rema-nence isolated at respective localities, we think it justifiable to use, for fur-ther analysis, mean directions and appropriate pole positions calculated onthe basis of results obtained from specimens.

In order to assign proper ages of acquisition to each of the acquired

palaeomagnetic components, we compared the obtained pole positions 1, 2,and 3 (Fig. 10) with the reference Apparent Polar Wander Path (APWP) forthe Sudetes (Jeleska et al. 2003) and the Master Apparent Polar WanderPath for Europe for Triassic 20 Ma (Besse and Courtillot 2002). As waspredicted from the analysis of magnetic minerals, there is no component pre-ceding the metamorphic event.

The age of the oldest component 1 (recorded only at three localities inthe eastern part of the OSD, SM) corresponds to the Carboniferous segmentof the APWP lying between 320 and 307 Ma (Fig. 10) and will later be

Fig. 9. Site means for characteristic directions (see Table 2 for details). Three groupsof site means are observed. Directions with negative inclinations transposed with

positive inclinations to lower hemisphere.


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Fig. 10. Pole positions together with confidence ovals calculated from directions iso-lated for specimens against the reference APW paths for the Sudetes (394-218 Ma,Jeleska et al. 2003) and for Stable Europe (162 present, Besse and Courtillot2002): 1 pole C (Carboniferous) about 315 Ma, 2 pole P (Permian) about270 Ma, 3 pole M-C (Mesozoic-Cenozoic) about 140 Ma. Numbers along refer-ence APWP denote ages of respective poles in Ma.

denoted C. Its age is in between the main phase of Variscan metamorphismand times of post-tectonic cooling recorded in the OSD (see Geological set-

ting). On the other hand, this time span fits well with the age of volcanicactivity in the neighbouring Intra-Sudetic Basin and Kaczawa Mts., dated at316-300 Ma (see Geological setting), which must have resulted in extensivehydrothermal activity, and consequently, might have been the main source ofhydrothermal fluids that led to the formation of magnetic phases responsiblefor palaeomagnetic component C. Importantly, apatite fission-track data andzircon U-Th/He data collected from the adjacent Gry Sowie Massif(Aramowicz et al. 2006) and Rychlebsk hory Mountains region in the

Sudetes (Daniket al. 2012) indicate that the Variscan basement was con-cealed under a thick cover of Upper Carboniferous-Lower Permian molassesediments reaching 4-8 km and affected by temperatures higher than 200C.This is in good agreement with our observations suggesting that magneticminerals present in investigated samples started to form at temperaturesaround 300C.


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Our palaeomagnetic estimates of the time interval equivalent for compo-nent C (320-307 Ma) rather precludes its origin during the main Variscan

tectonometamorphic event (dated by various geochronological methods at350-320 Ma), as the time intervals do not overlap. Because this componentappeared only at the three localities in the eastern part of the OSD (SM), wepostulate that at the remaining localities this episode were reset by youngerthermal events.

Component 2 corresponds to the Permian segment of the APWP(between 290 and 260 Ma) and will be denotedP. Similarly to component C,it was found only in the eastern part of the OSD (SM), in eclogites (site MI),

and amphibolites (site SN). This time interval fits fairly well to the peak ofpost-orogenic volcanism in the Sudetes. A record of this volcanic activity ispreserved in the Intra-Sudetic Basin and extended at least over the time spanfrom 294 to 283 Ma (see Geological setting). It also coincides in time withthe palaeomagnetic pole position obtained for Permian volcanics in the Intra-Sudetic Basin (Westphal et al. 1987). This time interval was characterizedby a rapid unroofing related to uplift that resulted in cooling, which is docu-mented by Ar/Ar cooling ages obtained on amphiboles and micas from OSD(Schneider et al. 2006) and the neighbouring East Sudetes (Maluski et al.

1995, Szczepaski 2002). In contrast, relatively thick Lower Permian sedi-ments deposited, e.g., in the Intra-Sudetic Basin and North-Sudetic Basinindicate that the uplift was only local in character. This documents the activi-ty of faults which might have acted as channels for magmatic fluids.

Consequently, fault zones reactivated during the Permian postulated byMaluski et al. (1995) might serve as channels enabling penetration of rocksby magmatic fluids that could be responsible for growth of opaque mineralsobserved in the rocks of the OSD. We suppose that component P is a chemi-cal remanence acquired due to hydrothermal alterations related to activity of

magmatic fluids.In both parts of the OSD there are clusters of palaeomagnetic directions

labelled 3 corresponding to the Mesosoic segment of the Master EuropeanAPWP that will be denoted M. The pole position Mcombined for the SMand OBM parts encompasses times from 160 to about 40 Ma on APWP. Thistime span corresponds to periods of reactivation of the EFZ (Late Jurassic /Early and Late Cretaceous/Palaeogene; Scheck et al. 2002 and referenceswithin), activity of the Lusatian Thrust (Ventura et al. 2009), emergence of

NKG during Late Turonian/Early Coniacian (e.g., Don 1996), and theappearance of volcanics cropping out in the EFZ and the Sudetes. The mainpulse of Tertiary volcanism in the Sudetes started about 34 Ma in the EarlyOligocene (e.g., Birkenmajeret al. 2011) which is considerably later than thepostulated age of component M. However, the volcanism is considered asbelonging to the Central European Volcanic Province (CEVP) which extends


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over 700 km from the Rhenish Massif in the west through the Eger Grabento the East Sudetes in the east. Importantly, volcanism in this province start-

ed about 79 Ma in Late Cretaceous times and finished at 0.2 Ma (e.g.,Ulrych et al. 2011). Ulrych et al. (1996) reported a sample of pyroxenite col-lected near Dvr Krlov nad Labem (about 40 km west of the study area)dated using the K-Ar method at 69 4 Ma. Furthermore, Badura et al. (2006)dated using the K-Ar method basaltoids near Mokrzeszw and JewSudecki at 44.17.7 Ma and 58.75.9 Ma, respectively. Thus, it is possiblethat the volcanics forming the CEVP might serve as a source of fluids re-sponsible for the growth of opaque minerals documented in the study rocks.

According to Ventura and Lisker (2003), the Late Jurassic Late Creta-ceous thermal episode recorded in fission tracks from the Erzgebirge wasrelated to denudation due to wrench tectonics along the EFZ. This docu-ments the activity of the EFZ at that time, which was also postulated byScheck et al. (2002). Consequently, faults forming the EFZ might, in thetime span from 160 to 40 Ma, serve as channels for volcanic fluids responsi-ble for the palaeomagnetic component M reported in this study.

6. CONCLUSIONS

The results of the study show that the investigated rocks did not retaina component of remanence preceding the Variscan tectonometamorphic epi-sode but they retained three components of magnetic remanence acquiredduring the Late Carboniferous (C), Late Permian (P), and Mesozoic (M).

Component C, present only in the eastern part of OSD (SM), corre-sponds to the Late Carboniferous segment of the APWP located between 320and 307 Ma. The estimated time interval for component C rather precludesany relationship with the Variscan tectonothermal history of the OSD. Weinterpret it as linked to volcanic activity dated at 316-300 Ma in the Intra-Sudetic Basin and Kaczawa Mts.

The Late Permian component P (present only in the eastern part of OSD,SM) is a record of Late-Variscan volcanic activity in the neighbouring Intra-Sudetic Basin, dated at 283-294 Ma.

The youngest Mesozoic component M (present in both parts, SM andOBM) was associated with volcanism related to the formation of the CentralEuropean Volcanic Province, which was active from about 80 to 0.2 Ma.

The combined rock-magnetic and light reflected study indicated that the

main carrier of components of magnetic remanence of rocks from the easternpart of OSD (SM) is ferrimagnetic pyrrhotite that began to form from thenon-magnetic type at about 300C, during the final times of retrogression. Inthe western part (OBM) the components of the magnetic remanence are car-


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ried mainly by post-pyrrhotite Fe-oxides (magnetite, hematite) formed main-ly by hydrothermal processes.

This last conclusion implies that the rocks from the western part (OBM)were more affected by low temperature alteration than the rocks from theeastern part (SM).

Acknowledgments. This research was performed in the frameworkof project No. 5 (2008-2011) of the Institute of Geophysics, Polish Academyof Sciences. We acknowledge also support from the University of Wrocaw,grant No. 1017/S/ING for J. Szczepaski and from the Institute of Mineralo-gy, Petrology, and Geochemistry, University of Warsaw, project BStNo. 1536/4 for K. Nejbert for mineralogical investigations. The authors areindebted to Prof. Raymond Macdonald for corrections of English languagein the manuscript.

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Received 28 November 2011Received in revised form 31 August 2012

Accepted 19 September 2012

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