The highly siderophile elements and Re–Os isotope geochemistry of Variscan lamproites from the Bohemian Massif: implications for regionally dependent metasomatism of orogenic mantle

Journal Pre-proof The highly siderophile elements and Re-Os isotope geochemistry of Variscan lamproites from the Bohemian Massif: Implications for regionally dependent metasomatism of orogenic mantle

Lukáš Krmíček, Lukáš Ackerman, Jakub Hrubý, Jindřich Kynický PII:

S0009-2541(19)30397-3

DOI:

https://doi.org/10.1016/j.chemgeo.2019.119290

Reference:

CHEMGE 119290

To appear in:

Chemical Geology

Received date:

5 May 2019

Revised date:

14 August 2019

Accepted date:

28 August 2019

Please cite this article as: L. Krmíček, L. Ackerman, J. Hrubý, et al., The highly siderophile elements and Re-Os isotope geochemistry of Variscan lamproites from the Bohemian Massif: Implications for regionally dependent metasomatism of orogenic mantle, Chemical Geology (2019), https://doi.org/10.1016/j.chemgeo.2019.119290

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Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

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The highly siderophile elements and Re–Os isotope geochemistry of Variscan lamproites from the Bohemian Massif: implications for regionally dependent metasomatism of orogenic mantle

Lukáš Krmíčeka,b,*, Lukáš Ackermana, Jakub Hrubýc, Jindřich Kynickýd

Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, CZ-165 02 Prague 6, Czech Republic

b

Brno University of Technology, Faculty of Civil Engineering, AdMaS Centre, Veveří 95, CZ-602 00 Brno,

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a

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Czech Republic

Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, CZ-611 37 Brno,

Central European Institute of Technology, Brno University of Technology, Technická 10, CZ-616 00, Czech

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d

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Czech Republic

Lukáš Krmíček

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*Corresponding author:

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Republic

Phone: +420 233 087 241 Fax: +420 220 922 670

e-mail: [email protected]

Keywords: Orogenic mantle; HSE; Re–Os; Lamproites; Saxo-Thuringian Zone; Moldanubian Zone

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

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Abstract Orogenic lamproites represent a group of peralkaline, ultrapotassic and perpotassic mantlederived igneous rocks that hold the potential to sample components with extreme compositions from highly heterogeneous orogenic mantle. In our pilot study, we present highly siderophile element (HSE) and Re–Os isotope systematics of Variscan orogenic lamproites sampled in the territories of the Czech Republic, Austria and Poland, i.e., from the

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termination of the Moldanubian and Saxo-Thuringian zones of the Bohemian Massif.

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Orogenic lamproites of the Bohemian Massif are distinguished by variably high contents of

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SiO2, high Mg# and predominant mineral associations of K-rich amphibole and Fe-rich

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microcline. The HSE show (i) consistently very low contents in all investigated orogenic lamproites compared to the estimated concentrations in majority of mid-ocean ridge basalts,

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hotspot-related volcanic rocks (e.g., ocean island basalts, continental flood basalts, komatiites,

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some intraplate alkaline volcanic rocks such as kimberlites and anorogenic lamproites) and arc lavas, and (ii) marked differences in relative and absolute HSE abundances between the

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samples from the Moldanubian and Saxo-Thuringian Zone. Such a regional dependence in HSE from mantle-derived melts is exceptional. Orogenic lamproites have highly variable and high initial suprachondritic

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Os/188Os values (up to 0.631) compared with rather chondritic

to subchondritic Os isotope values of the young lithospheric mantle below the Bohemian Massif. The highly radiogenic Os isotope component in orogenic lamproites may be derived from preferential melting of metasomatised vein assemblages sitting in depleted peridotite mantle. This process appears to be valid generally in the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area. As a specific feature of the orogenic lamproites from the Bohemian Massif, originally ultra-depleted mantle component correlative with remnants of the Rheic Ocean lithosphere in the Moldanubian Zone was metasomatised by a mixture of evolved and juvenile material, whereas the lithospheric mantle

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in the Saxo-Thuringian Zone was enriched through the subduction of evolved crustal material with highly radiogenic Sr isotope signature. As a result, this led to observed unique regionally dependent coupled HSE, Rb–Sr and Re–Os isotope systematics.

1. Introduction Lamproites are comparatively a rare group of peralkaline, ultrapotassic and perpotassic

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mantle-derived igneous rocks (Le Maitre, 2002). Although they are volumetrically

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insignificant, their widespread distribution on most continents can provide valuable insights

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into the nature of the subcontinental lithospheric mantle (e.g., Mitchell and Bergman, 1991 and references therein). Lamproites can occur in different terrains varying in age from the

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Archaean in Western Australia (Graham et al., 1999) to Quaternary in Gaussberg, Antarctica

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(Sushchevskaya et al., 2014; Tingey et al., 1983). Thus, these rocks can be considered as deep “probes” for tracing various processes related to the Wilson cycle throughout the Earth’s

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history. Reported occurrences of lamproites generally show large differences in mineralogy

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and geochemistry. However, they are characteristic to have low contents of Ca, Al and Na, but relatively high contents of Mg, high K/Al ratios and extremely high concentrations of many incompatible elements (namely Ba, Zr and Ti; Foley et al., 1987; Le Maitre, 2002). The platinum-group elements (PGE: Os, Ir, Ru, Rh, Pt and Pd), along with Re and Au, are grouped together as the highly siderophile elements (HSE), defined by their strong affinities for metals and sulphides relative to silicates (Brenan et al., 2016). These elements have variable partitioning behaviour between highly compatible Os, Ir, Ru and Rh relative to mildly compatible Pt and Pd and moderately incompatible Re and Au during melting and mafic melt crystallization (e.g., Day et al., 2013). This exceptional geochemical behaviour serves HSE as a powerful tool for tracing different mantle processes such as partial melting,

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melt percolation and mantle metasomatism/refertilization (e.g., Büchl et al., 2002; Lorand and Luguet, 2016; Lorand et al., 2008; Luguet et al., 2004, 2007). However, the behaviour of HSE in orogenic mantle is not well constrained because of a limited amount of available HSE data of compositionally highly variable mantle-derived orogenic melts and mantle xenoliths. This is intriguing as orogenic lamproites hold a potential to sample mantle domains with extreme geochemical (including isotope) composition from highly heterogeneous orogenic mantle. In

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spite of that, there are still only a few papers dealing with incomplete HSE systematics of

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lamproitic rocks till the present day (e.g., Conticelli et al., 2007; Prelević et al., 2015).

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Traditionally, lamproites can be found in both anorogenic and orogenic geodynamic

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settings, and it has been demonstrated that their mantle source can be enriched by interaction with different reservoirs within the lithospheric mantle, including subducted continental and

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oceanic material (Krmíček et al., 2016; Prelević et al., 2008, 2010a, 2010b, 2015; Santosh et

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al., 2018). This makes the distinction criteria between orogenic and anorogenic types difficult (e.g., Davies et al., 2006; Kullerud et al., 2011; Lustrino et al., 2016; Prelević et al., 2012;

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Talukdar et al., 2018; Tappe et al., 2008). Most of the previous studies focused on trace element concentrations and various mostly lithophile radiogenic isotope systems, such as Sr–Nd–Pb–Hf, argue for the heterogeneous source lithology of lamproitic melts. However, the platinum group elements and Re–Os isotope system can provide different, unique insights into their petrogenesis. Because of their behaviour, their concentrations are controlled mainly by PGE-bearing [e.g., laurite (RuS2)] and Fe–Ni–Cu sulphides, alloys and tellurides/arsenides (e.g., Lorand and Luguet, 2016) within the peridotitic mantle. This is contrary to traditional radiogenic isotope systems, which are mostly tied to silicate minerals. Furthermore, the compatible behaviour of Os (wherein it gets concentrated in the solid residue) and moderate incompatibility of Re

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(concentrated in the melt phase; Pearson et al., 2004; Shirey and Walker, 1998) during partial melting of the lithospheric mantle lead to significant variations in Os isotope composition as well as Re/Os ratios in crust and mantle, increasing their sensitivity as petrological mantleevolution tracers (e.g., Allègre and Luck, 1980; Carlson, 2005). In our study, we present the first complete dataset on HSE abundances in various orogenic (high-silica) lamproitic intrusions sampled in different zones of the Variscan

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Bohemian Massif on the territory of the Czech Republic, Austria and Poland, together with

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their Re–Os isotope compositions to provide insights into the nature of mantle source parental

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for lamproitic melts and characterise the metasomatic component preserved within the

2. Geological setting

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orogenic mantle and, in particular, possible regional dependence of these components.

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In the Bohemian Massif, the occurrences of peralkaline dykes of lamproitic

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composition are almost exclusively restricted to the eastern margin of the Moldanubian Zone and to the West Sudetes Domain of the Saxo-Thuringian Zone (e.g., Krmíček et al., 2011, 2016; Němec, 1970, 1975, 1987, 1988; Wierzchołowski, 2003; Fig. 1). These dykes are all orogenic in origin, being emplaced after their host geological units collided during the Variscan orogeny. Spatial distribution of these lamproites in a belt at the eastern termination of the Moldanubian and Saxo-Thuringian zones is genetically connected with the closure of the Rheic Ocean followed by the subduction of the Brunovistulian Terrane (Krmíček et al., 2016). Individual samples of orogenic lamproites contain or lack typical Ba–Ti–Zr-bearing accessory minerals (baotite ± benitoite, bazirite, henrymeyerite, priderite) and can be termed as lamproites sensu stricto or lamproites sensu lato, respectively (Table 1; Krmíček et al.,

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2016). Orogenic lamproites and their more evolved equivalents occur both in the Moldanubian and Saxo-Thuringian zones, however, leucocratic types (leucolamproites of Krmíček et al., 2016) predominate in the Saxo-Thuringian Zone (Table 1; Hrubý, 2017). Their compositions can be distinguished by predominant mineral associations of K-rich amphibole and Fe-rich microcline, which mineralogically correspond to a new variety of silica-rich lamproite (cf. Krmíček et al., 2011; Kullerud et al., 2011, 2013; Fig. 2).

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The dykes have not been directly dated (the samples were not suited for

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Ar/39Ar

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Ar/39Ar age determinations of other mafic intrusions with

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crystals), however, based on

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step-heating dating as dark mica is rare and commonly consumed by tiny alkaline amphibole

lamproitic affinity from the Bohemian Massif, their emplacement age is ~330 Ma

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(Awdankiewicz et al., 2009; Krmíček et al., 2011). This is attested by their cross-cutting

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relationship to the amphibole–biotite melagranite to melasyenite of the Třebíč Massif and the

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clinopyroxene–amphibole–biotite melagranite to melasyenite of the Kłodzko–Złoty Stok Massif, with presumed emplacement ages between ~340 and 331 Ma (cf. Holub et al., 1997;

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Kotková et al., 2010; Kusiak et al., 2010; Mikulski et al., 2013). Lamproitic dykes in the Moldanubian Zone strike predominantly in the NE–SW to E– W direction and are characterised by mineral association of Fe-rich microcline, K-rich equivalents of sodic–calcic to sodic amphiboles ± phlogopite together with Ba–Ti–Zr-bearing accessory minerals (Krmíček et al., 2011). They are particularly widespread near Raabs in Austria (Karlstein, Lexnitz and Thures dykes) and around Třebíč in the Czech Republic (Chlístov, Kožichovice, Střítež and Šebkovice dykes). In the Saxo-Thuringian Zone, lamproitic dykes are very rare and were documented only from the Orlice–Sněžník (Orlica– Śnieżnik) Unit in the territory of the Czech Republic (Krmíček et al., 2016 and references therein) and from the Kłodzko–Złoty Stok Massif in Poland (Wierzchołowski, 2003). They

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have the same mineralogy as the Moldanubian lamproites (Krmíček et al., 2016; Hrubý, 2017); the main difference is their lower Mg# and higher SiO2 contents. Whereas the Moldanubian lamproites can be predominantly found as thinner, rapidly chilled dykes, SaxoThuringian lamproites are usually connected with more evolved thicker dyke intrusions (Fig.

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2).

3. Analytical methods

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The concentrations of HSE and Re–Os isotope data were obtained at the Institute of

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Geology of the Czech Academy of Sciences in Prague following the methods described in

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detail elsewhere (e.g., Ackerman et al., 2019). Weighted aliquots of rock powders (~1–2 g) 185

Re-190Os and

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Ir-99Ru-105Pd-194Pt spikes

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were spiked with appropriate amounts of mixed

and then dissolved in 4 ml HCl and 5 ml HNO3 at 260 °C in Carius tubes for 3 days (Shirey

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and Walker, 1995). Afterwards, the osmium was extracted by the solvent extraction (Cohen and Waters, 1996) and purified by microdistillation using CrO3 and HBr as described in detail

exchange

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by Birck et al. (1997). Iridium, Ru, Pd, Pt and Re were separated from the solution by anion chromatography using

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AG

1×8

resin

(BioRad).

Osmium

isotope

measuremements were determined using the Thermo Triton Plus multicollector thermal ionization mass spectrometer (TIMS), while Ir, Ru, Pt, Pd and Re concentrations were analyzed by the sector-field ICP-MS (Element 2, Thermo), both housed at the Institute of Geology of the Czech Academy of Sciences in Prague. Full procedural blanks were measured as follows: Re = 1 pg, Os = 0.8 pg with 187Os/188Os of 0.275, Ir = 7 pg, Ru = 22 pg, Pt = 15 pg and Pd = 35 pg. Accuracy of the Re–Os method was checked by the analysis of BIR-1a basalt reference material (USGS) with the results given in Table 2 in excellent agreement with previous studies (e.g., Ishikawa et al., 2014). Duplicate analysis of the sample LK13 was

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performed for the purpose of testing the degree of variations in HSE and Re–Os isotope data (and the distribution of HSE-bearing phases) within an individual sample powder (Table 2). The highly siderophile elements and Re–Os isotope data were interpreted in the context of available (both published and unpublished) major, trace element and isotope data for Variscan

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lamproites from the Bohemian Massif. Published dataset is given in Supplementary material.

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4. Results

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4.1 Whole-rock HSE concentrations

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Contents of HSE in all samples show consistently very low values (ƩHSE mostly below 1 ppb; Table 2) with respect to the estimated concentrations both in the primitive upper

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mantle (PUM; Becker et al., 2006) and in the mantle-derived rocks such as mid-ocean ridge

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basalts, hotspot-related volcanic rocks (ocean island basalts, continental flood basalts, komatiites, intraplate alkaline volcanic rocks such as kimberlites and anorogenic lamproites)

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and arc lavas (e.g., Chalapathi Rao et. al. 2013; Day, 2013; Gannoun et al., 2016; Graham et al., 1999 and references therein), with the Moldanubian lamproites being recognised by higher values on average than the samples from the Saxo-Thuringian Zone (Table 2). Rhenium concentrations are the most different when comparing samples from both studied zones. In the Saxo-Thuringian lamproites, the measured values do not exceed 0.006 ppb, while in the samples from the Moldanubian zone, they reach up to 0.029 ppb. One sample (KRM-K) yield a very high Re content of 0.186 ppb suggesting likely Re–Os perturbation, perhaps due to late-stage Re addition. Palladium concentrations are below the detection limit (<0.1 ppb) in all samples from the Saxo-Thuringian Zone with the exception of the KRM-K sample. In comparison, four of seven samples (including one duplicate analyse) from the Moldanubian

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Zone have Pd abundances above the detection limit with widely variable values between 0.12–2.5 ppb. Ruthenium shows a broad range of abundances; two samples from the SaxoThuringian Zone fall below the detection limit (<0.022 ppb), but the remaining values range between 0.03 and 0.13 ppb and between 0.065 and 0.37 ppb for the samples from the SaxoThuringian and Moldanubian zones, respectively. Thus, the Moldanubian lamproites are characterised by relatively elevated (up to one order of magnitude higher) Ru abundances

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compared to the Saxo-Thuringian samples. Iridium demonstrates a similar behaviour as Ru

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with the most values above detection limit (<0.007 ppb) varying between 0.028 and 0.14 ppb and <0.011 and 0.080 ppb for the Moldanubian and Saxo-Thuringian Zone lamproites,

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respectively. Abundances of Os and Pt show the smallest variation between the samples from

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different zones (0.002–0.08 ppb Os and 0.04–0.28 ppb Pt in Saxo-Thuringian Zone samples;

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0.006–0.09 ppb Os and 0.090–0.64 ppb Pt in Moldanubian Zone samples), with only one sample (PK103) falling below the detection limit for Pt analysis (<0.020 ppb). When

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compared Re with SiO2 contents or Mg# [Mg # = 100 × Mg/(Mg + Fetotal)], two regionally

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contrasting trends (Moldanubian vs. Saxo-Thuringian) of lamproitic rocks can be recognised (Fig. 3). When Re shows positive correlation with whole-rock SiO2 and negative correlations with Mg# (Fig. 3) in Saxo-Thuringian samples, the samples from the Moldanubian Zone are characterised by large variation in Re at almost constant SiO2 and Mg# (Fig. 3). However, the other HSE yield rather scattered systematics (Fig. 3). Among iridium-group PGE (I-PGE; Os, Ir, Ru), iridium and Os are positively correlated in all samples, while Ru and Os show similar behaviour only for the samples from the Saxo-Thuringian Zone (Fig. 3). The primitive mantle-normalised HSE distributions exhibit fractionated patterns with progressive increase from I-PGE to platinum-PGE (P-PGE; Pt, Pd; Fig. 4) with PtN/OsN between 1.5 and 36, feature typical for mantle-derived melts (see Gannoun et al., 2016 and references therein), while Re is mostly similar or lower to Pt (Fig 2). The Moldanubian lamproites are

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characteristic by minor depletion of Os and Ir relative to Pt and Re (OsN/ReN = 0.03–1.3), and by a slight positive Pd anomaly in two samples (PK12 and PK99). On the other hand, the Saxo-Thuringian lamproite patterns are difficult to be described in detail due to many analyses falling below detection limits. 4.2 Re–Os isotope composition

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The present-day 187Os/188Os isotope ratios vary widely within the studied sample suite, ranging between 0.1571 and 0.6850, corresponding to variable radiogenic initial Os/188Os(330) values of 0.157–0.631 and consequently highly positive calculated γOs values

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Os/188Os and

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zones as they oscillate between the

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between +39 and +406. There is no significant difference between the samples from different 187

Re/188Os values close to the 187

Os/188Os both at relatively

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composition of PUM and a component with highly radiogenic

low and high 187Re/188Os (Fig. 5A). Such a trend is similar to the one indicated by the Re–Os

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isotope composition of the Mediterranean orogenic lamproites (Conticelli et al., 2007; Prelević et al., 2015), but is much more pronounced. Additionally to that, there is no 187

Os/188Os and Os abundances (expressed as 1/Os), even when

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relationship between

considering both the Moldanubian and Saxo-Thuringian zones separately (Fig. 5B).

5. Discussion 5.1 Orogenic lamproites with regionally contrasting HSE Variscan orogenic lamproites of the Bohemian Massif are strongly enriched in LILE relative to HFSE with several trace element and isotope characteristics (Sr–Nd–Pb–Li) that seem to be regionally distinct (Krmíček et al., 2016). Primary geochemical features of lamproites from the Moldanubian Zone are marked by negative Nb–Ta and Ti anomalies

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relative to chondritic abundances, whereas lamproites from the Saxo-Thuringian Zone are significantly less depleted in Ti, Nb and Ta. This could be attributed to the stabilisation of different Ti-phases (titanite/rutile) in the source (John et al., 2011 and references therein). Moreover, lamproites from the Saxo-Thuringian Zone are characterised by a negative Eu anomaly being considered to be a primary feature of its metasomatised source (Krmíček et al., 2016). Based on the new results of our study, HSE show consistently very low contents in all

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samples of orogenic lamproites. However, in more detail, there are distinct differences in

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relative and absolute HSE abundances between the samples from the Moldanubian and SaxoThuringian zones (Figs. 2, 3; Table 2). Such a regional dependence in HSE from mantle-

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derived melts is exceptional.

Due to contrasting behaviour of Os and Re during partial melting, mantle-derived

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melts should be preferably enriched in Re over Os leaving the mantle residues with

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subchondritic Re/Os ratios. Indeed, arc lavas and melts derived by partial melting of MORB or OIB sources exhibit Re-enriched compositions (usually >100 ppt of Re; Gannoun et al.,

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2016 and references therein). For the rest of HSE, during the mantle depletion caused by common partial melting degrees (<20 %), low-melting point (e.g., Cu-bearing), sulphides represent a predominant host for P-PGE while I-PGE are hosted by highly refractory phases such as alloys or high temperature sulphides (e.g., laurite, monosulfide solid solutions) leading to characteristic enrichment from Os through Pt to Re in mantle-derived melts (e.g., Alard et al., 2000; Day, 2013; Gannoun et al., 2016; Lorand and Luguet, 2016; Luguet et al., 2004). However, observed Re and other HSE abundances in orogenic lamproites from the Bohemian Massif require a mantle source overall largely replenished in all HSE, especially in Re (Fig. 6). The most possible explanation for this characteristics is a presence of large proportion of recycled, HSE-poor material (e.g., eclogite, pyroxenite; Ackerman et al., 2013; Aulbach et al., 2016; Wang and Becker, 2015) in the source parental for lamproitic magmas,

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which would be consistent with observed variable, but radiogenic

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Os/188Os signatures (see

below). Moreover, there is a unique regional dependence between relatively Re-enriched Moldanubian lamproites and those from the Saxo-Thuringian Zone (Fig. 6). This difference can be primarily caused by the presence of regionally contrasting depleted/enriched mantle source and/or by variable oxygen fugacity during partial melting or metasomatism of depleted mantle source by regionally contrasting material (cf., Andrews and Brenan, 2002; Krmíček et

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al., 2016). Additionally, Tertiary orogenic lamproites from the Mediterranean area display

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relative Re and Os enrichment compared to Variscan orogenic lamproites from the Bohemian Massif (Fig. 6). Such enrichment could be related to the interaction with a component derived

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from the convective mantle via volatile-rich carbonatite fluids (Alard et al., 2011). Presence

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of the component derived from the convective mantle was recognised in the source of

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Mediterranean orogenic lamproites (Prelević et al., 2010b), while it is not known in Variscan

2019).

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(Carboniferous) orogenic mantle-derived melts of the Bohemian Massif (cf. Dostal et al.,

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5.2 Ultra-depleted mantle component in the source of orogenic lamproites Orogenic lamproites of the Bohemian Massif show compositional features that are indicative for the presence of ultra-depleted component in their mantle source. Such component is depleted in CaO and Al2O3 and its highly refractory character is highlighted by the composition of lamproitic olivine, with extremely high forsterite contents (Fo up to 94%), enclosing Cr-rich spinel with high Cr/(Cr + Al) up to 0.88 (Krmíček et al., 2016). Olivine with similarly high forsterite contents, hosting Cr-rich spinel, has been found in orogenic lamproites from the Mediterranean area (Prelević and Foley, 2007; Prelević et al., 2010a, 2013). Based on comparison with the result of experimental melting studies of Jaques and Green (1980) and Matsukage and Kubo (2003), a depletion similar to the one inferred for a

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refractory mantle component in the source of orogenic lamproites requires ~40% extraction of basaltic melt from a peridotitic source. During extensive extraction of basaltic melt, clinopyroxene is preferentially consumed, which leads to the stabilisation of ultra-refractory mantle domains that are subsequently mixed with the relatively fertile mantle. Such a process would result in large compositional differences of mantle peridotites from lherzolite to harzburgite and dunite, similar to those observed for the mantle xenoliths sampled by

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Cenozoic volcanic rocks in the Bohemian Massif (e.g., Kochergina et al., 2016). In

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comparison, the resulting ultra-depleted mantle is sampled by xenoliths from the oceanic sub-

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arc mantle (e.g., Simon et al., 2008).

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The presence of ultra-depleted mantle component that is correlated with the remnants of the Rheic Ocean lithosphere in the source of orogenic lamproites from the Bohemian

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Massif was recently confirmed by study focused on 7Li compositions (Krmíček et al., 2016).

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In general, orogenic lamproites show an extremely broad range of δ7Li values and lack correlation between the δ7Li and Re (Fig. 7). Both, relatively Re-enriched Moldanubian

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samples and Re-depleted Saxo-Thuringian lamproites yield heavy δ7Li values that fall between the values typical for fertile mantle (~3.5‰) and altered oceanic crust (~10‰; Marschall et al., 2017) whereas light δ7Li values are known from the sedimentary rocks representing subducted North Gondwana upper crust (Fig. 7). 5.3 Origin of coupled, regionally contrasting Sr–Os isotope signature in orogenic mantle Studied orogenic lamproites have high and variable suprachondritic initial

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Os/188Os

values, much higher than predominantly only slightly superchondritic to subchondritic values found in the Bohemian upper mantle rocks (e.g., Ackerman et al., 2009; 2013; Kochergina et al., 2016) but similar to those found in pyroxenites (cf. Ackerman et al., 2016; Becker et al., 2004). The highly radiogenic 187Os/188Os component is sampled both by lamproites (with high

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Mg#) and leucolamproites, which, together with absence of xenoliths, excludes the possibility that radiogenic 187Os/188Os component is related to low-pressure contamination by continental crust with high 187Os/188Os (e.g., Rocha-Júnior et al., 2012). This is (i) in line with the results of geochemical modelling giving unreasonably high amount of crustal contamination (Prelević et al., 2015), and (ii) indicative for the presence of heterogeneous Variscan orogenic mantle below the Bohemian Massif.

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The process mentioned in 5.1 alone cannot explain the extreme variability in

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radiogenic 187Os/188Os values in orogenic lamproites. Prelević et al. (2015) and Krmíček et al.

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(2016) suggested that highly radiogenic Os-(Sr) isotope component in orogenic lamproites

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can be derived from preferential melting of metasomatised (phlogopite and/or K-amphibolebearing) mantle, rich in clinopyroxenite veins assemblages sitting in depleted peridotitic

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mantle as envisioned by a vein/wall-rock interaction melting model of Foley (1992).

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Pyroxenite veins should be characterised by high Re/Os ratios and time-integrated radiogenic Os/188Os (cf. Aulbach et al., 2016; Becker and Dale, 2016 and references therein). Presence

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of clinopyroxenite vein assemblages in the source of orogenic lamproites was attested by elevated Ni contents in olivine phenocrysts (Krmíček et al., 2016; Prelević et al. 2013). Veins are formed due to reaction between K-rich melts or fluids derived from subducted material and surrounding depleted peridotitic mantle. As a result of this local refertilisation, crystallisation of phlogopite/amphibole, orthopyroxene and clinopyroxene at the expense of the surrounding olivine took place (cf. Förster et al., 2017; Sekine and Wyllie, 1982). This is confirmed by the result of our geochemical modelling. The observed variability in 187Os/188Os can be explained by binary mixing between clinopyroxenite and ambient mantle. This process seems to be valid generally in the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area (Fig. 8A). However, as inferred from the position in

187

Os/188Os vs. Os diagram, the Mediterranean orogenic lamproites require higher

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

15

contribution from ambient peridotitic mantle, which can explain their generally less lamproitic character expressed as lower molar (K2O + Na2O)/Al2O3 (cf. Krmíček et al., 2011, 2016; Prelević et al., 2008, 2010a; Fig. 8A). On the other hand,

187

Os/188Os variations in

combination with Os abundances alone do not have sufficient sensitivity to distinguish possible variations in mantle sources (and thus the nature and extent of mantle metasomatism) of orogenic lamproites from the Moldanubian and Saxo-Thuringian zones. Therefore, we 187

Os/188Os and 87Sr/86Sr as the latter is very sensitive indicator

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introduce the combination of

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of subduction-related (fluid-enhanced) metasomatism. Using this approach, the composition Moldanubian and Saxo-Thuringian lamproites is best explained by above mentioned

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vein/wall-rock melting model requiring 60–70% contribution from the metasomatised mantle

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with different (regionally dependent) Sr-isotope composition (Fig. 8B). This is in agreement

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with assumption of Krmíček et al. (2016), that the originally ultra-depleted mantle component was metasomatised by a mixture of evolved and juvenile material (thinned continental crust?)

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in the Moldanubian Zone, whereas the lithospheric mantle in the Saxo-Thuringian Zone

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underwent subduction-related metasomatism by evolved (with pronounced negative Eu anomaly and highly radiogenic 87Sr/86Sr) crustal material. This resulted in the stabilisation of strongly metasomatised parts showing the presence of granitic melts within the mantle rocks in different zones of the Bohemian Massif (cf. Ferrero et al., 2018 and references therein). As a result, the above-mentioned processes have very probably led to the observed unique regionally dependent HSE and coupled Sr–Os isotope systematics of silica-rich orogenic lamproites.

6. Conclusions

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

16

Based on our pilot study of highly siderophile elements and Re–Os isotope geochemistry of orogenic lamproites from the Bohemian Massif, we present the following conclusions: 1) All HSE show consistently very low contents in all samples compared to the other mantle-derived volcanic rocks (e.g., mid-ocean ridge basalts, ocean island basalts, continental flood basalts, komatiites, kimberlites, anorogenic lamproites, arc lavas) as the result of high

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proportions of HSE-depleted material (e.g., pyroxenite, eclogite) in the source parental for

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orogenic lamproitic magmas.

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2) There are remarkable differences in relative and absolute HSE abundances between

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the samples from the Moldanubian and Saxo-Thuringian zones. Such a regional dependence

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of HSE systematics in mantle-derived melts is exceptional. 3) Tertiary orogenic lamproites from the Mediterranean area display relative Re and

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Os enrichment compared to Variscan orogenic lamproites from the Bohemian Massif. Such

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enrichment is likely related to the interaction with component derived from the convective mantle via volatile-rich carbonatitic melts/fluids. 4) Studied orogenic lamproites have highly variable and suprachondritic

187

Os/188Os

values overlapping the values found in mantle pyroxenites from the Bohemian Massif. Therefore, the highly radiogenic Os isotope component in orogenic lamproites may be derived from preferential melting of metasomatised mantle rich in clinopyroxenite vein assemblages sitting in depleted peridotite mantle. This process seems to be generally valid for the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area.

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

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5) Based on the combination of Os–Sr isotope signature, the composition of Moldanubian and Saxo-Thuringian lamproites can be best explained by vein/wall-rock melting model requiring 60–70% contribution from the metasomatised mantle with different (regionally dependent) 87Sr/86Sr composition.

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Acknowledgements

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We are grateful to Jana Ďurišová and Jan Rejšek (both Institute of Geology of the Czech

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Academy of Sciences) for ICP-MS analyses and maintenance of TIMS lab, respectively. This research was financially supported by the internal project of the Institute of Geology of the

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Czech Academy of Sciences (no. 9324) and the institutional project RVO 67985831 as well as

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by the BUT project LO1408 “AdMaS UP – Advanced Materials, Structures and Technologies”, supported by the Ministry of Education, Youth and Sports CR under the

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“National Sustainability Programme I” and by the EXPRO 2019 project of the The Czech

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Science Foundation (no. 19-29124X). We gratefully acknowledge N. V. Chalapathi Rao and an anonymous reviewer for their constructive and helpful comments and K. Johannesson for excellent editorial handling.

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Figure Captions

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179, 28–35. https://doi.org/10.1016/j.lithos.2013.07.020.

Fig. 1. Distribution of Cadomian and Variscan massifs in Western and Central Europe with supposed location of the Rheic Ocean suture (modified after Linnemann et al., 2007; Nance et al., 2010) together with the regional geological subdivision of the Bohemian Massif (modified after Krmíček et al., 2016). Major Cadomian and Variscan granitoid intrusions are shown for reference. Circles and squares with numbers show the distribution of the studied lamproites in a belt at the eastern termination of the Moldanubian and Saxo-Thuringian zones. Abbreviations: BT – Brunovistulian Terrane; MZ – Moldanubian Zone; STZ – SaxoThuringian Zone; TBU – Teplá–Barrandian Unit.

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

32

Fig. 2. Microscopic features of the Variscan lamproites from the Bohemian Massif. A – scanned thin section with visible very fine-grained contact of thinner lamproite dyke with flow-oriented acicular K-amphibole (sample PK101, Karlstein, Austria, plane-polarised light); B – Detail of elongated greenish-brown K-amphibole in microcline-rich matrix (sample KRM-B, Horní Rokytnice, Czech Republic, plane-polarised light). Fig. 3. Distributions of various HSE concentrations vs. whole-rock SiO2 contents, Mg# and

of

Os abundances. Data for SiO2 contents and Mg# taken from Krmíček et al. (2016) and Hrubý

ro

(2017).

-p

Fig. 4. HSE patterns normalised to the primitive upper mantle (Becker et al., 2006) for

re

Moldanubian (A) and Saxo-Thuringian (B) lamproites. Note, that Re content of sample KRM-

lP

K is clearly modified by late-stage processes and therefore, this sample is excluded from the subsequent Re–Os figures. Reference values for mid-ocean ridge basalts (MORB) and ocean

187

Os/188Os vs.

187

Re/188Os ratios for the studied lamproites and for the

Jo ur

Fig. 5. A – The

na

island basalts (OIB) taken from Day (2013) and references therein.

Mediterranean orogenic lamproites (MOL; Conticelli et al., 2007; Prelević et al., 2015). Composition for primitive mantle peridotites (PMP) from Carlson (2005) and the range of 187

Os/188Os for subcontinental lithospheric mantle of the Bohemian Massif from Ackerman et

al. (2009) and Kochergina et al. (2016). B –

187

Os/188Os(330) vs. 1/Os for the investigated

lamproites from the Moldanubian and Saxothuringian zones. Fig. 6. Re and Os abundances (ppt) of the studied lamproites plotted together with the Mediterranean orogenic lamproites (Conticelli et al., 2007; Prelević et al., 2015) and range of majority OIB (Day et al., 2009; Ireland et al., 2009, 2011), MORB (Gannoun et al., 2007; Yang et al., 2013) and arc lavas (Alves et al., 2002; Chesley et al., 2002).

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

33

Fig. 7. δ7Li (‰) vs Re (ppb) for the studied lamproites from the Bohemian Massif. The δ7Li values for orogenic lamproites from the Bohemian Massif are from Krmíček et al. (2016) and unpublished data and those for upper mantle and altered oceanic crust from Marschall et al. (2017). The range of δ7Li for sediments representing subductable North Gondwana upper crust from Romer et al. (2014). Fig. 8. A –

187

Os/188Os(330) vs. Os (ppb) for the studied lamproites and for Mediterranean

of

orogenic lamproites (Conticelli et al., 2007; Prelević et al., 2015) with suggested binary

Os/188Os(330) vs.

87

Sr/86Sr(330) isotope variations of the orogenic lamproites from the

-p

187

ro

mixing line between fertile clinopyroxenite (metasome) and ambient mantle. B –

re

Moldanubian and Saxo-Thuringian zone with calculated binary mixing lines between fertile clinopyroxenites (metasomes) with regionally dependent Sr-signature (metasome 1: Sr = 800 Sr/86Sr = 0.7085, Os = 0.035 ppb,

Sr/86Sr = 0.7105, Os = 0.035 ppb,

187

187

Os/188Os = 0.7; metasome 2: Sr = 1200 ppm,

Os/188Os = 0.7) and host peridotite rock (Sr = 1200

na

87

87

lP

ppm,

Jo ur

ppm, 87Sr/86Sr = 0.704, Os = 0.4 ppb, 187Os/188Os = 0.135). See text for discussion.

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

34

Table 1 Sample location, geological position and petrographic type of the studied Variscan dykes, Bohemian Massif, Austria, the Republic, and Poland. No. Sample Location Rock type Cou (N) (E) characteristic MOLDANUBIAN ZONE 1 PK101 Karlstein 48°53'17.617" 15°23'9.971" Large angular Lamproite s.s. Migm blocks, probably par E-W trending PK103

Thures (chilled margin)

48°54'47.012" 15°24'30.752"

Angular blocks

3

LK11

Lexnitz (dyke centre)

48°55'51.300" 15°19'34.860"

4

PK106

Šebkovice (dyke centre)

49°7'52.070"

5

LK13

Střítež

6

PK99

Kožichovice

7

PK12

Chlístov

8

WEST SUDETES DOMAIN OF THE SAXO-THURINGIAN ZONE PK17 Rogówek 50°22'25.800" 16°44'55.100" Dyke exposed in Leucolamproite s.l. (dyke centre) old quarry

of

2

Lamproite s.s.

Migm par

Leucolamproite s.s.

Biotitie

Dyke NE-SW trending, 1 m thick, excavation

Lamproite s.s.

Sillima par

Dyke NE-SW striking, 1 m thick, excavation

Lamproite s.s.

Mel (“dur Třeb

15°54'58.199"

Angular blocks

Lamproite s.s.

Mel (“dur Třeb

49°12'20.107" 15°43'55.229"

Angular blocks

Lamproite s.s.

Biotitie

ro

Angular blocks

re

-p

15°48'17.668"

lP

49°11'11.142" 15°53'39.900"

Jo ur

na

49°12'9.612"

Mela Kłodzko M

9-1

KRM-A

Horní Rokytnice (fine-grained marginal zone)

50°11'14.549"

16°28'8.490"

Thicker dyke, NW-SE trending, in old quarry

Leucolamproite s.l.

Sn orth

9-2

KRM-B

Horní Rokytnice (coarse-grained central part)

50°11'14.549"

16°28'8.490"

Thicker dyke, NW-SE trending, in old quarry

Lamproite s.s.

Sn orth

9-3

KRM-E

Horní Rokytnice (felsic "pegmatite" enclave)

50°11'14.549"

16°28'8.490"

Thicker dyke, NW-SE trending, in old quarry

Leucolamproite s.l.

Sn orth

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

9-4

KRM-F

Horní Rokytnice (mafic part with "baotite" veinlets)

50°11'14.549"

16°28'8.490"

Thicker dyke, NW-SE trending, in old quarry

35

Lamproite s.s.

Sn orth

Dyke, 10 m thick Lamproite s.s. Sn NW-SE trending orth Lamproite s.s. = relatively undifferentiated sample containing lamproitic Ba–Ti–Zr-bearing accessory minerals (baotite ± bazirite, henrymeyerite, priderite); leucolamproite s.s. = differentiated sample containing lamproitic Ba–Ti–Zr-bearing ac minerals similar to lamproite s.s.; lamproite s.l. = relatively undifferentiated sample without lamproitic accessory mineral leucolamproite s.l. = differentiated sample without lamproitic accessory minerals. Samples KRM-A to KRM-K represents findings. Sample numbers correspond to locations in Fig. 1. 50°11'39.055"

16°28'2.860"

lP

re

-p

ro

of

Kouty

na

KRM-K

Jo ur

10

Krmíček et al.: HSE and Re–Os isotope systematic of orogenic lamproites / Chemical Geology Journal Pre-proof

36

Table 2

HSE concentrations and Re-Os isotope data of the studied Variscan dykes of the Bohemian Massif (Austria, the Czech Rep

Sample

Location

Rock type

Re (ppb)

Os (ppb)

Ir (ppb)

Ru (ppb)

Pt (ppb)

Pd (ppb)

187

188

Re/

Os

187

Os

MOLDANUBIAN ZONE Karlstein

lamproite s.s.

0.019

0.006

<0.007

0.10

0.090

<0.10

16.37

0.685

PK103

Thures

lamproite s.s.

0.016

0.006

<0.007

0.12

<0.020 <0.10

13.29

0.390

LK11

Lexnitz

leucolamproite s.s.

0.004

0.045

0.036

0.065

0.15

<0.10

0.44

0.266

PK106

Šebkovice

lamproite s.s.

0.005

LK13

Střítež

lamproite s.s.

0.027

PK12

Chlístov

lamproite s.s.

ro

0.12

0.13

<0.10

0.97

0.211

0.010

0.033

0.11

0.17

0.27

13.61

0.480

0.022

0.041

0.10

0.17

0.12

1.63

0.613

0.009

0.085

0.14

0.37

0.64

1.22

0.52

0.333

0.029

0.070

0.10

0.12

0.53

2.46

2.02

0.184

lP

lamproite s.s.

0.028

-p

0.007 Kožichovice

0.025

re

duplicate PK99

of

PK101

WEST SUDETES DOMAIN OF THE SAXO-THURINGIAN ZONE Rogówek

leucolamproite s.l.

0.001

0.084

0.080

0.13

0.25

<0.10

0.06

0.163

KRM-A

Horní Rokytnice

leucolamproite s.l.

0.004

0.019

0.023

0.061

0.15

<0.10

1.15

0.382

KRM-B

Horní Rokytnice

lamproite s.s.

0.004

0.004

0.011

<0.022

0.28

<0.10

5.25

0.660

KRM-E

Horní Rokytnice

leucolamproite s.l.

0.006

0.002

<0.007

<0.022

0.039

<0.10

15.30

0.390

KRM-F

Horní Rokytnice

lamproite s.s.

0.002

0.006

0.017

0.032

0.12

<0.10

1.93

0.244

KRM-K

Kouty

lamproite s.s.

0.186

0.019

0.051

0.038

0.18

0.13

basalt

0.680

0.380

0.12

0.28

4.67

5.4

Jo ur

na

PK-17

0.157

Reference material BIR-1a

 Os (330 Ma) values were calculated using the chondritic composition of Shirey and Walker (1998).

8.63

0.134

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8