Monazite to the rescue: U-Th-Pb dating of the intrusive history of the composite Karkonosze pluton, Bohemian Massif

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Monazite to the rescue: U–Th–Pb dating of the intrusive history of thecomposite Karkonosze pluton, Bohemian Massif

Monika A. Kusiak a,b,c,⁎, Ian S. Williams d, Daniel J. Dunkley b,e, Patrík Konečny f, Ewa Słaby a, Hervé Martin g

a Institute of Geological Sciences, Polish Academy of Science, ul. Twarda 51/55, 00-818 Warsaw, Polandb Department of Applied Geology, Curtin University, PO Box U1987, WA 6845 Perth, Australiac Swedish Museum of Natural History, Box 50007, SE104 05 Stockholm, Swedend Research School of Earth Sciences, Australian National University, Acton, ACT 0200 Canberra, Australiae National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, 190-8518 Tokyo, Japanf Štátny geologický ústav Dionýza Štúra, Bratislava, Slovakiag Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, France

a b s t r a c ta r t i c l e i n f o

Article history:Received 10 September 2013Received in revised form 19 November 2013Accepted 20 November 2013Available online 28 November 2013

Editor: K. Mezger

Keywords:Bohemian MassifKarkonosze (Krkonoše) graniteU–Th–Pb geochronologyZirconMonazite

The large (~700 km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border betweenPoland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but contains arange of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism haveproved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to290 Ma, with more recent results converging to ~320–300 Ma. Dating of zircon and monazite from samples ofa variety of major andminor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby andMar-tin (2008), has narrowed the possible age range further. U–Pb agesmeasured on eight of ten zircon andmonazitesamples are in the range ~314–311 Ma. Zircon agesmeasured on the twomajor types of porphyritic granitoid are313 ± 3 and 311 ± 4 Ma, andmonazite ages are 312 ± 2, 313 ± 3 and 311 ± 3 Ma.Monazite from one hybridgranitoid has an age of 314 ± 3 Ma, and zircon from another an age of 314 ± 4 Ma. Zircon from a compositedyke has an age of 311 ± 6 Ma. The monazite U–Pb age of an equigranular granite, at 318 ± 6 Ma, is consistentwith geological evidence that it is older than the porphyritic granitoids but, because of the relatively large uncer-tainties, is not conclusive. Zircon fromonemicrogranular enclave is anomalously young, 302 ± 4 Ma. Evidence ismounting that the main porphyritic granitoids, hybrid granitoids and composite dykes were emplaced within ashort time interval between 314 ± 4 and 311 ± 3 Ma. Given the uncertainties, emplacement of these unitscould have been effectively simultaneous. The larger difference between the ages from the equigranular graniteand microgranular enclave, however, indicates that the whole Karkonosze thermal episode possibly lasted aslong as 15 Ma.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Karkonosze–Izera (Krkonoše–Jizera)Massif, on the Polish–Czechborder, contains the biggest granitoid body in the SudetyMountains, theKarkonosze pluton (Fig. 1). The Sudetes are located at the northernperiphery of the Bohemian Massif, the remains of a microcontinentthat was accreted to Laurussia following the closure of the Rheic Oceanbetween Laurussia and Gondwana during the Carboniferous VariscanOrogeny.

The Karkonosze pluton is a composite body containing a variety of li-thologies ranging from minor lamprophyre to voluminous leucogranite

(Słaby and Martin, 2008). The pluton intrudes a series of schistsand orthogneisses considered to have Late Precambrian protoliths(Mazur et al., 2006). The metamorphic rocks are interpreted as beingpart of a Late Devonian to Early Carboniferous nappe pile (Mazur andAleksandrowski, 2001) that was subsequently intruded by late- topost-collisional granites.

Because of its size and lithological complexity, the Karkonosze plu-ton has been the subject of several geochemical and geophysical studies(e.g., Franke et al., 2000; Słaby andMartin, 2008)which have focused onits structure and compositional zonation. The pluton has also attractedinterest because it is associated with minor W–Sn–Mo–Bi and Th–U–REE mineralization (Mikulski, 2007). Early attempts to date the plutonisotopically (e.g. Pin et al., 1987 — Rb–Sr; Kröner et al., 1994 — Pb–Pb;Marheine et al., 2002 — Ar–Ar) have shown that it is of broadlyCarboniferous age, but different techniques yielded inconsistent re-sults within this period. More recent dating by in situ zircon U–Pb

Chemical Geology 364 (2014) 76–92

⁎ Corresponding author.E-mail addresses: [email protected] (M.A. Kusiak),

[email protected] (I.S. Williams), [email protected] (D.J. Dunkley),[email protected] (P. Konečny), [email protected] (E. Słaby),[email protected] (H. Martin).

0009-2541/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.chemgeo.2013.11.016

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(e.g. Awdankiewicz et al., 2010; Kryza et al., 2012) has also produceda range of ages, such that the full range of estimates for the time ofpluton emplacement is now 330–290 Ma.

This raises the question as to whether the several magma batchesthat formed the composite pluton were emplaced over an extendedperiod of time.

We have addressed that question by dating a series of samples,previously analysed geochemically by Słaby and Martin (2008),representing a range of rock types including the main bodies ofequigranular and porphyritic granite, minor intrusions of hybrid rocks,composite dykes and microgranular magmatic enclaves. The samples

have been dated by SIMS zircon andmonazite U–Pb. As found by previ-ousworkers, the zircon data are complex and the interpretations are po-tentially ambiguous. The monazite data are much more clear-cut, andprovide a basis for explaining the dispersion in the zircon isotopiccompositions.

2. The Karkonosze pluton and its geological setting

The Karkonosze pluton is one of a series of Carboniferous graniticbodies emplaced into theBohemianMassif during theVariscanorogeny.These granites are scattered throughout the Bohemian Massif in the

803HR

804P, 812P

807Mi 808SPH, 809SPH

806HCh

805KA

810F 802MME

Recent sedimentary cover

Molasse

Hercynian granite

Serpentiniteand gabbro

Mylonite

Amphibolite

Metavolcanicrocks

Metasediments

Fault

Thrust

Orthogneiss

Micaschist

Cadomiangranite

State boundary

20 km

N

POLAND

CZECH REPUBLIC

GERMANY

Leipzig

Prague

Wroc awA

B

4 km

Fig. 1. A. Sketch geology map of the Sudetes. B. Enlargement of the Karkonosze pluton with sample location. Pink circles — zircon sample sites, yellow circles — monazite sample sites.A, adapted from Słaby and Martin (2008) and Žák et al. (2013).

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50µm

3.1 - 312±3 Ma Th/U = 0.7150µm

4.1 - 296±4 Ma Th/U = 0.38

A 803HR: Porphyritic granite, Liberec type

100µm

10.1 - 295±4 Ma Th/U = 0.22

50µm

3.1 - 314±5 Ma Th/U = 0.25

50µm

9.1 - 314±7 Ma Th/U = 1.36

50µm

7.1 - 278±7 Ma Th/U = 0.63

D 805KA: Composite dyke

14.1 - 303±5 Ma Th/U = 0.46

100µm

B 807MI: Porphyritic granite, Jizera type

50µm 50um 50µm

10.1 - 301±6 Ma Th/U = 0.82

E 802ER: Microgranular magmatic enclave

2.1 - 308±6 Ma Th/U = 0.91

50µm

6.1 - 313±5 Ma Th/U = 0.26

C 810F: Hybrid granodiorite

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internal central-European Variscides in Poland, Czech Republic, Austriaand Germany. The magmatic activity peaked between about 330 and290 Ma (Kryza et al., 2004; Mazur et al., 2006). The Bohemian Massif,an aggregation of Gondwana-derived micro terranes that migratednorthwards and collided with northern continental blocks in the EarlyPaleozoic (Vaughan and Pankhurst, 2008), is one of the largest andmost complex tectonic units of the European Variscides. The Sudetesconsists of a collage of Neoproterozoic to Early Carboniferous terraneslocated along the NEmargin of themassif. The complex ofmetamorphicrocks, separated by tectonic sutures and major faults/shear zones, wasdeformed in the Devonian to Carboniferous, producing a wide varietyof structural trends.

The Karkonosze-Izera Massif, in the NE Sudetes, consists predomi-nantly of Devonian to Carboniferous metasediments, orthogneissesand bimodal metavolcanic rocks. The orthogneisses, with Cambrian toOrdovician protoliths, are part of an extensive magmatic episodetypical of the Variscan belt throughout Europe. The large, compositeKarkonosze pluton (60 × 20 km) forming the core of the massif hasbeen the subject of several geological (e.g., Cloos, 1925), petrographical(e.g., Borkowska, 1966) and geochemical (e.g., Słaby and Martin, 2008)studies. The dominant rock types are porphyritic and equigranular gran-ites (Borkowska, 1966; Mierzejewski and Oberc-Dziedzic, 1990) thatcomprise approximately 90% of the outcrop area (Fig. 1). These are in-truded by dykes of lamprophyre and aplite, particularly in the easternpart of the pluton, and byminor granodiorite emplaced near the bound-ary between the major granite phases in the west. Small amounts oftwo-mica granite are exposed along the SW margin. From the outcroppattern, the two-mica granite was intruded first (Klomínský, 1969).This was followed by either two bodies of equigranular granite(Borkowska, 1966), or two large magma batches that produced twotypes of porphyritic granite, Liberec and Izera, with different magneticproperties, petrographic features and chemical compositions (Žák andKlomínský, 2007). The relative ages of the porphyritic and equigranulargranites remain unresolved. The composite dykes and granodiorite areyounger again. Borkowska (1966) concluded on structural groundsthat themain granitemasses in thewestern part of the pluton predatedthose in the east.

The Karkonosze has been interpreted as a late- to post-orogenicpluton (Duthou et al., 1991; Diot et al., 1994; Wilamowski, 1998).Following their detailed geochemical study, Słaby and Martin (2008)concluded that the pluton, classified as high-K calc-alkaline granite inthe scheme of Barbarin (1999), was of mixed origin. On the basis ofpetrology, and major and trace element chemical compositions, theyinterpreted the various intrusive phases in terms of an evolving geo-chemical system involving different degrees of mixing between felsicand mafic magmas. The end members of the system were inferred tobe the magmas that formed the felsic granites and the maficlamprophyres respectively (Słaby andMartin, 2008). The differentiatedequigranular granite was considered to have evolved by fractional crys-tallizationwithout interactionwithmaficmagma, all other componentsbeing the products of various degrees of magma mixing/mingling. Thedegree of mixing differed widely in time and place, but it was inferredthat magmas that evolved independently from mantle and crustalsources co-existed.

Lamprophyric and granitic magmas were considered to haveinteracted at an early stage, before much of the crystallization tookplace. This influenced the compositional range of the predominantporphyritic granite, and produced a series of volumetrically minorhybrid quartz diorite–monzodiorite–granodiorite rocks that intrudeit. The granite is also intruded by a swarm of composite dykes ofmonzodiorite to granodiorite composition, and locally includes a

range of microgranular magmatic enclaves with mostly lobate margins,possibly the result of magma mingling.

From the chemical evidence, these interactions were placed in atime sequence (Słaby and Martin, 2008); mixing of early lamprophyricmagmas with early porphyritic granite magmas to produce the hybridmagmas, then injection of magmas to produce the enclaves, then theintrusion of the composite dykes, some of which chilled against almosttotally crystallized granite. Equigranular granite, which escaped themixing, appeared to have crystallized over an extended period. Noneof the previous geochronology, however, provides a sufficient level ofdetail to test this model. Here we have dated selected samples fromthe Słaby and Martin (2008) study, and some additional material, inorder to do so.

3. Previous geochronology

A variety of isotopic techniques has been applied over many years,mainly in an attempt to date the porphyritic and equigranular granitefacies of theKarkonosze pluton, butwithout the emergence of a consen-sus as to the age of the magmatism. Whole rock Rb–Sr analyses of the‘central’ porphyritic granite by Pin et al. (1987) yielded an age of328 ± 12 Ma, and the ‘ridge’ equigranular whole rocks dated byDuthou et al. (1991) yielded 309 ± 3 Ma, demonstrating directly thatthe pluton was indeed of Late Carboniferous age. 40Ar/39Ar datingby Marheine et al. (2002) of a single biotite crystal from a sample ofporphyritic granite gave an age of 320 ± 2 Ma with the last twohigh temperature steps giving 315 Ma and 314 Ma. In contrast, the40Ar/39Ar dating of a single muscovite crystal from the structurallyolder two mica granite at the pluton margin gave 312 ± 2 Ma.

U–Pb and Pb–Pb dating, generally considered to be more robustisotopic systems that are less susceptible to alteration and post-emplacement thermal events, have also yielded a wider range of ageestimates than might be expected. Kröner et al. (1994) measured a sin-gle zircon Pb–Pb evaporation age of 304 ± 14 Ma on a monzogranitefrom the main porphyritic phase in the western part of the pluton.Machowiak and Armstrong (2007) attempted U–Pb dating of zirconfrom five samples of equigranular and porphyritic granite using in situanalysis by sensitive high resolution ion microprobe (SHRIMP). Theyencountered numerous problems, with high concentrations of U, locallyhigh common Pb contents and the widespread loss of radiogenic Pb,plus the presence of inherited older zircon, leading to widely dispersedisotopic compositions and discordance. Only three of the samplesyielded results that were sufficiently internally consistent to provide anestimate of the granite emplacement ages. These were 314.1 ± 3.3 Mafor porphyritic granite from Fajka, 314.9 ± 4.5 Ma for equigranulargranite from Miedzianka, and 318.5 ± 3.7 Ma for porphyritic granitefrom Radomierz. Based on these results they concluded that the granitein the northern and central parts of the pluton might be slightly olderthan that in the south-east.

Kryza et al. (2012) attempted to overcome the problem of radiogen-ic Pb loss by annealing the zircon grains from a sample of porphyriticgranite in advance of SHRIMP analysis, followed by a process of ‘chem-ical abrasion’ commonly used in preparing zircon for thermal ionisation(TIMS) analysis (Mattinson, 2005). Untreated zircon yielded a mean206Pb/238U age of 306 ± 4 Ma, but treated zircon, despite there stillbeing some evidence of radiogenic Pb loss, yielded 322 ± 3 Ma,whichwas considered to be the better estimate of themagmatic crystal-lization age.

Other studies have focused on minor magmatic phases within thepluton. Kusiak et al. (2009) SHRIMP dated zircon from a microgranularmagmatic enclave in the small body of equigranular granite in the

Fig. 2. Zircon BSE images from different types of granitoids: A) porphyritic granite, Liberec type, no contact granite from Hraniczna; sample 803HR; B) coarse-grained porphyritic granite,Jizera type, contaminated granite from Michałowice; sample 807Mi; C) hybrid rock; earliest hybrid granodiorite from Fojtka; sample 810F; D) composite dyke from Karpniki; sample805KA; E) microgranular magmatic enclave from Mrowiec Hill; sample 802ER.

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Table 1Ion microprobe U–Pb zircon isotopic data.

Spot ppm206Pb*

ppmU

ppmTh

232Th/238U 206Pb/204Pb ± %206Pbc

238U/206Pb ± 207Pb/206Pb ± 238U/206Pb⁎ ± 207Pb⁎/206Pb⁎ ± Age206Pb/238U

± Age207Pb/206P

±

Major rock types803HR-1.1 11.5 274 138 0.52 – – 0.04 20.42 0.27 0.05288 0.00079 20.42 0.27 0.05289 0.00079 308.2 3.8 324 34803HR-1.2 12.2 352 159 0.47 – – 0.20 24.70 0.49 0.05293 0.00069 24.69 0.49 0.05320 0.00069 255.9 5.0 337 30803HR-2.1 15.3 377 230 0.63 7.0E+03 2.2E+03 0.09 21.17 0.23 0.05303 0.00069 21.23 0.23 0.05094 0.00092 296.8 3.2 238 43803HR-3.1 20.7 485 335 0.71 1.8E+04 1.0E+04 0.13 20.12 0.22 0.05372 0.00059 20.14 0.22 0.05293 0.00074 312.4 3.3 326 31803HR-4.1 12.9 320 119 0.38 5.2E+03 1.6E+03 0.06 21.26 0.26 0.05278 0.00074 21.33 0.26 0.0500 0.0011 295.4 3.6 194 52803HR-5.1 21.0 580 200 0.36 4.60E+02 3.0E+01 3.52 23.71 0.26 0.08182 0.00065 24.62 0.27 0.0500 0.0023 256.7 2.8 193 106803HR-6.1 21.9 522 202 0.40 1.1E+05 3.5E+05 0.01 20.42 0.22 0.05257 0.00058 20.42 0.22 0.05244 0.00073 308.2 3.2 304 32803HR-7.1 22.7 540 194 0.37 2.4E+04 1.8E+04 0.22 20.41 0.22 0.05442 0.00060 20.42 0.22 0.05381 0.00075 308.2 3.2 363 31803HR-8.1 21.1 495 188 0.39 4.0E+03 1.3E+03 – 20.12 0.38 0.05232 0.00058 20.14 0.38 0.05154 0.00067 312.4 5.8 265 31803HR-8.1 14.0 313 202 0.67 1.89E+04 8.8E+03 0.28 19.12 0.25 0.05539 0.00078 19.20 0.25 0.0518 0.0014 327.3 4.1 275 63803HR-9.1 11.3 470 177 0.39 1.6E+04 1.3E+04 0.46 35.68 0.68 0.05332 0.00069 35.72 0.68 0.0524 0.0010 178.0 3.3 303 45803HR-9.2 23.5 1016 334 0.34 3.13E+03 6.6E+02 0.84 37.18 0.74 0.05619 0.00054 37.40 0.75 0.0515 0.0011 170.1 3.3 264 50803HR-10.1 108 2425 532 0.23 5.10E+03 5.4E+02 0.31 19.31 0.37 0.05536 0.00025 19.38 0.37 0.05249 0.00039 324.4 5.9 307 17803HR-11.1 17.3 403 165 0.42 – – 0.09 20.05 0.38 0.05338 0.00075 20.04 0.38 0.05380 0.00081 313.9 5.8 363 33803HR-13.1 49.5 1080 617 0.59 2.87E+03 4.1E+02 0.65 18.73 0.36 0.05831 0.00052 18.85 0.36 0.05321 0.00090 333.2 6.1 338 38803HR-14.1 11.8 266 114 0.44 7.8E+02 1.1E+02 2.85 19.42 0.41 0.07559 0.00091 19.88 0.42 0.0569 0.0029 316.3 6.6 489 112803HR-15.1 27.2 685 272 0.41 2.03E+04 9.4E+03 0.13 21.61 0.41 0.05316 0.00048 21.63 0.41 0.05244 0.00058 291.3 5.3 305 26803HR-16.1 31.0 728 425 0.60 1.24E+04 4.7E+03 0.02 20.15 0.38 0.05279 0.00043 20.18 0.38 0.05161 0.00062 311.8 5.7 268 28803HR-17.1 18.2 410 142 0.36 9.5E+03 5.2E+03 0.09 19.37 0.37 0.05362 0.00059 19.41 0.37 0.0521 0.0010 323.8 6.0 289 46803HR-18.1 29.8 1089 181 0.17 5.6E+03 1.1E+03 0.43 31.38 0.63 0.05358 0.00045 31.48 0.63 0.05097 0.00066 201.6 3.9 239 31803HR-19.1 39.2 1175 665 0.59 1.87E+02 1.8E+01 10.1 25.72 0.49 0.1310 0.0048 28.52 0.63 0.053 0.010 222.2 4.9 317 425803HR-20.1 32.8 738 355 0.50 2.22E+03 3.5E+02 0.83 19.33 0.37 0.05952 0.00051 19.49 0.37 0.0529 0.0012 322.5 5.9 326 50807Mi-1.1 93.4 2086 481 0.24 5.8E+04 2.4E+04 – 19.18 0.46 0.05199 0.00028 19.18 0.46 0.05174 0.00029 327.6 7.6 274 13807Mi-1.2 92.4 2113 470 0.23 2.78E+04 9.3E+03 – 19.65 0.31 0.05231 0.00028 19.67 0.31 0.05179 0.00033 319.7 5.0 276 15807Mi-1.3 97.7 2216 488 0.23 5.7E+04 3.5E+04 0.03 19.49 0.60 0.05309 0.00028 19.49 0.60 0.05284 0.00032 322.5 9.7 322 14807Mi-2.1 202 4299 1278 0.31 1.55E+03 4.3E+02 1.25 18.30 0.27 0.0633 0.0029 18.51 0.28 0.0538 0.0039 339.2 5.1 365 164807Mi-3.1 52.2 1225 842 0.71 4.8E+04 2.2E+04 0.06 20.18 0.30 0.05305 0.00038 20.18 0.30 0.05275 0.00040 311.7 4.6 318 17807Mi-4.1 62.2 1433 280 0.20 1.66E+03 1.6E+02 1.02 19.80 0.30 0.06088 0.00037 20.01 0.30 0.05206 0.00094 314.3 4.6 288 41807Mi-5.1 71.7 1721 376 0.23 4.22E+03 5.7E+02 0.52 20.61 0.31 0.05662 0.00032 20.70 0.31 0.05315 0.00058 304.1 4.5 335 24807Mi-6.1 72.3 1571 377 0.25 6.8E+03 1.5E+03 0.11 18.67 0.47 0.05402 0.00033 18.72 0.47 0.05186 0.00057 335.5 8.3 279 26807Mi-7.1 66.6 1627 599 0.38 2.04E+03 2.0E+02 0.84 20.98 0.31 0.05904 0.00038 21.16 0.32 0.05185 0.00078 297.6 4.4 279 35807Mi-8.1 81.4 1913 439 0.24 1.05E+04 2.8E+03 0.17 20.19 0.34 0.05393 0.00052 20.22 0.34 0.05254 0.00063 311.1 5.2 309 28807Mi-9.1 72.9 1721 390 0.23 1.70E+04 5.3E+03 0.08 20.28 0.30 0.05317 0.00033 20.30 0.30 0.05231 0.00043 309.9 4.6 299 19807Mi-10.1 109 2691 563 0.22 2.21E+03 1.9E+02 0.79 21.18 0.32 0.05856 0.00031 21.36 0.32 0.05192 0.00062 295.0 4.3 282 28807Mi-11.1 36.1 899 267 0.31 1.08E+03 1.1E+02 1.50 21.38 0.32 0.0642 0.0015 21.75 0.33 0.0506 0.0021 289.8 4.4 224 95807Mi-13.1 69.6 1625 446 0.28 1.56E+03 1.3E+02 1.21 20.05 0.32 0.06230 0.00038 20.28 0.32 0.05294 0.00085 310.2 4.8 326 37807Mi-14.1 79.7 1896 838 0.46 1.215E+03 8.3E+01 1.45 20.44 0.33 0.06406 0.00037 20.75 0.33 0.05200 0.00094 303.4 4.6 285 40

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Spot

ppm206Pb*

ppmU

ppmTh

232Th/238U 206Pb/204Pb ± %206Pbc

238U/206Pb ± 207Pb/206Pb ± 238U/206Pb⁎ ± 207Pb⁎/206Pb⁎ ± Age206Pb/238U

± Age207Pb/206P

±

Minor rock types810F-1.1 124 2720 542 0.21 1.58E+04 2.9E+03 0.04 18.84 0.26 0.05344 0.00021 18.86 0.26 0.05251 0.00027 333.0 4.7 308 12810F-2.1 30.5 788 180 0.24 7.1E+03 2.1E+03 0.37 22.18 0.35 0.05494 0.00046 22.24 0.36 0.05287 0.00079 283.6 4.3 323 33810F-3.1 48.8 1137 275 0.25 2.4E+04 1.2E+04 0.03 20.02 0.30 0.05291 0.00037 20.04 0.30 0.05230 0.00049 313.9 4.5 298 21810F-4.1 74.5 1687 415 0.25 2.29E+04 8.5E+03 0.07 19.46 0.29 0.05341 0.00029 19.47 0.29 0.05277 0.00037 322.9 4.6 319 16810F-5.1 49.6 1209 229 0.20 1.2E+05 1.0E+05 0.15 20.96 0.31 0.05350 0.00037 20.97 0.31 0.05338 0.00038 300.4 4.3 345 16810F-6.1 160 3744 943 0.26 1.29E+05 9.2E+04 0.02 20.09 0.32 0.05279 0.00022 20.09 0.32 0.05268 0.00023 313.1 4.7 315 10810F-7.1 35.8 851 366 0.44 3.3E+04 3.2E+04 0.12 20.41 0.31 0.05346 0.00044 20.42 0.31 0.05301 0.00064 308.2 4.4 329 27810F-8.1 77.2 1804 653 0.37 – – – 20.07 0.30 0.05258 0.00029 20.07 0.30 0.05265 0.00029 313.5 4.4 314 13810F-9.1 114 2637 891 0.35 5.11E+03 5.4E+02 0.35 19.79 0.30 0.05552 0.00034 19.86 0.30 0.05265 0.00045 316.7 4.5 314 20810F-10.1 46.9 1145 638 0.58 8.0E+03 1.7E+03 0.27 20.99 0.31 0.05452 0.00038 21.04 0.32 0.05269 0.00053 299.3 4.3 316 24810F-11.1 98.7 2308 916 0.41 1.23E+04 2.4E+03 0.16 20.10 0.30 0.05387 0.00026 20.13 0.30 0.05268 0.00035 312.6 4.4 315 15805KA-1.1 16.2 369 147 0.41 6.8E+03 3.6E+03 0.39 19.56 0.47 0.05596 0.00084 19.62 0.47 0.0538 0.0014 320.5 7.5 363 59805KA-1.2 12.8 306 113 0.38 3.6E+03 2.9E+03 0.51 20.50 0.53 0.0566 0.0018 20.61 0.54 0.0525 0.0038 305.5 7.9 306 166805KA-1.3 7.07 156 44 0.29 2.9E+03 1.2E+03 0.56 18.94 0.55 0.0575 0.0023 19.06 0.55 0.0526 0.0031 329.6 9.2 310 134805KA-2.1 185 4283 6256 1.51 3.72E+03 3.5E+02 0.37 19.90 0.48 0.05564 0.00025 20.00 0.48 0.05170 0.00044 314.6 7.2 272 20805KA-3.1 139 3368 3359 1.03 1.389E+03 8.2E+01 1.32 20.78 0.50 0.06295 0.00056 21.06 0.51 0.05241 0.00084 299.1 6.9 303 37805KA-4.1 118 2819 2571 0.94 3.79E+03 4.3E+02 0.54 20.48 0.49 0.05680 0.00031 20.58 0.49 0.05293 0.00053 305.8 7.0 326 23805KA-5.1 135 3101 3671 1.22 3.13E+03 3.1E+02 0.42 19.79 0.47 0.05607 0.00029 19.90 0.48 0.05139 0.00057 316.1 7.3 258 24805KA-6.1 192 4345 6265 1.49 4.28E+03 3.9E+02 0.33 19.43 0.47 0.05554 0.00024 19.51 0.47 0.05212 0.00040 322.2 7.4 291 17805KA-7.1 54.7 1421 872 0.63 1.081E+03 9.8E+01 1.56 22.32 0.54 0.06439 0.00050 22.70 0.54 0.0508 0.0013 277.9 6.5 232 61805KA-8.1 226 5339 9324 1.80 3.13E+03 3.3E+02 0.56 20.26 0.49 0.05706 0.00027 20.38 0.49 0.05237 0.00058 308.8 7.1 302 25805KA-9.1 175 4007 5267 1.36 9.49E+02 4.3E+01 1.92 19.66 0.47 0.06805 0.00037 20.05 0.48 0.05263 0.00079 313.8 7.2 313 35805KA-10.1 142 3447 3344 1.00 2.47E+03 2.7E+02 0.64 20.90 0.50 0.05749 0.00046 21.06 0.51 0.05154 0.00082 299.0 6.9 265 36802ER-1.1 245 5700 8446 1.53 5.74E+03 6.2E+02 0.29 19.99 0.40 0.05498 0.00024 20.05 0.40 0.05243 0.00036 313.7 6.2 304 16802ER-2.1 116 2743 2427 0.91 7.0E+03 1.7E+03 0.28 20.37 0.41 0.05480 0.00088 20.42 0.41 0.0527 0.0010 308.2 6.1 316 44802ER-3.1 133 3164 3386 1.11 5.18E+02 2.5E+01 3.40 20.43 0.41 0.0796 0.0010 21.18 0.44 0.0512 0.0018 297.4 6.0 251 81802ER-4.1 92.6 2254 1362 0.62 1.58E+03 2.8E+02 1.21 20.90 0.42 0.0620 0.0021 21.15 0.42 0.0528 0.0027 297.9 6.0 319 117802ER-5.1 118 2825 2921 1.07 1.51E+03 1.1E+02 1.27 20.60 0.41 0.06260 0.00036 20.86 0.42 0.05292 0.00079 301.9 6.0 325 35802ER-6.1 117 2646 2172 0.85 3.16E+02 1.9E+01 5.64 19.43 0.39 0.0978 0.0015 20.62 0.43 0.0514 0.0033 305.2 6.2 260 150802ER-7.1 82.6 2036 1198 0.61 6.61E+02 3.9E+01 3.00 21.18 0.42 0.0762 0.0011 21.78 0.46 0.0540 0.0017 289.4 5.8 373 72802ER-8.1 119 2261 1246 0.57 8.95E+01 1.7E+00 19.6 16.29 0.33 0.2105 0.0014 20.46 0.51 0.0455 0.0042 307.7 7.6 −29 224802ER-9.1 93.3 2285 1452 0.66 6.2E+03 1.3E+03 0.29 21.03 0.42 0.05466 0.00039 21.09 0.42 0.05231 0.00063 298.6 5.9 299 27802ER-10.1 103 2494 1983 0.82 6.8E+03 1.4E+03 0.28 20.83 0.42 0.05464 0.00037 20.89 0.42 0.05247 0.00058 301.5 6.0 306 25

Errors are 1-sigma; Pbc and Pb⁎ indicate the common and radiogenic portions, respectively.Error in standard calibration was 0.86% for sample 803HR; 0.65% for 807Mi; 1.19% for 805KA; and 0.94% for 802ER (not included in above errors but required when comparing data from different mounts).Common Pb corrected using measured 204Pb.Disc.% = % discordance between Pb–Pb and U–Pb ages.

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central south of the pluton. The U-rich zircon grains were highly com-plex, commonly consisting of an inclusion-rich central region, variouslysurrounded and/or cross cut by unzoned zircon interpreted as the prod-uct of hydrothermal recrystallization. The measured Pb/U ratios werewidely dispersed, with high Pb/U in the inclusion-rich regions andlower ratios elsewhere. Two data clusters were recognized representingunaltered igneous zircon and re-equilibrated zircon, with ages of314.2 ± 8.3 and 303.8 ± 2.2 Ma, interpreted as the ages ofmagmatismand late fluid percolation respectively.

Awdankiewicz et al. (2010) SHRIMP dated zircon from one of themonzodiorite dykes in the dyke swarm in the eastern part of the pluton.Unlike the zircon from the porphyritic granite and enclave discussedabove (Kusiak et al., 2009; Kryza et al., 2012), the dyke zircon grainshad well-preserved oscillatory igneous zoning, with scant evidence ofrecrystallization. The concordia age calculated from seven analyses ofigneous zircon was 313 ± 3 Ma. Considering previous U–Pb and Pb–Pbgeochronology on the pluton as a whole, however, this was interpretedas an underestimate, with an average of 318 Ma, based on two of theanalyses, being more likely to approximate the magmatic age.

4. Sampling strategy

The sampling strategy for the present study was designed to includeeach of the main rock types within the Karkonosze pluton, plusexamples of the minor hybrid intrusions, a composite dyke and amicrogranular magmatic enclave. The aim was to date the main mag-matic activity, to determine the duration of the igneous event as awhole, and to test whether the geochemical sequence proposed bySłaby and Martin (2008) was also a sequence in time. Because of theambiguities in the results of previous zircon geochronology monazite,which is not prone to radiogenic Pb loss following metamictization,was dated in preference to zircon wherever possible.

4.1. Major rock types

Most of the granitoids in the Karkonosze plutonic complex areperaluminous, a few are metaluminous. The dominant porphyritic bio-tite granitoids, the ‘central’ granite of Borkowska (1966), have a rangeof field textures indicating crystallization under disequilibrium condi-tions (e.g. abundant rapakivi feldspar). These have been interpreted asconsistent with magma mixing (Žák and Klomínský, 2007). There aretwo distinct types of porphyritic granite that differ in modal and chem-ical composition. The Jizera granite has large (50–70 mm) K-feldsparphenocrysts, relatively low bulk magnetic susceptibility, relativelyhigh Ca and Mg, and low Na and K. The Liberec granite, in contrast,has smaller K-feldspar phenocrysts (20–30 mm), 20 times higher mag-netic susceptibility, relatively high Na and K, and low Ca and Mg (Žákand Klomínský, 2007). The less abundant equigranular biotite granite,the ‘ridge’ granite of Borkowska (1966), is fine to medium grained andvery homogeneous, lacking megacrysts, microgranular enclaves andmafic schlieren. It is also extremely felsic (SiO2 mostly N76%: Słabyand Martin, 2008).

4.1.1. Porphyritic granites

4.1.1.1. Liberec granite. Three samples of the Liberec granite were stud-ied, one from the Hraničná quarry, Czech Republic (803HR: N50′ 46″,E15′ 09″) and two from Piec crag in the Janówka valley south ofJanowice Wielkie village, Poland (804P, 812P: N50′ 51″, E15′ 55″).Sample 803HR was studied by Słaby and Martin (2008). The Piec cragsamples were newly collected. The upper part of the outcrop (sample804P) consists of coarse-grained granite. The lower part (sample812P) is medium-grained, with some hexagonal quartz, interpreted byMierzejewski (2007) to indicate a crystallization temperature above800 °C. The upper granite, in contrast, probably crystallized at about700 °C.

4.1.1.2. Jizera granite. Two samples of the Jizera granitewere studied, onefrom Michałowice quarry, Poland (807Mi: N50′50″, E15′35″) and theother from the Szklarska Poręba Huta quarry, Poland (808SPH: N50′50″, E15′30″). Sample 807Mi was studied by Słaby and Martin (2008).The Szklarska Poręba Huta sample was newly collected from the northface of the quarry. The coarse-grained porphyritic granite consists oflarge (up to several centimetres), pink, euhedral K-feldspar phenocrystsin a matrix of quartz (commonly aggregated), K-feldspar, white plagio-clase and partly chloritized biotite. Rare plagioclase phenocrysts arerimmed by one or two biotite-rich rings. Accessory minerals includezircon, monazite and apatite.

4.1.2. Equigranular graniteThe sample of equigranular granite analysed was the same as that

studied by Słaby and Martin (2008). The sample (806HCh), from theHarrachov granite (Žák and Klomínský, 2007), came from HašlerovaChata in the Czech Republic (N50′47″, E15′31″). The granite is fine- tomedium-grained, very homogeneous and without megacrysts. Majorminerals are K-feldspar, plagioclase, quartz and biotite, with minormuscovite and accessory zircon, ilmenite and monazite (Borkowska,1966).

4.2. Minor rock types

4.2.1. Hybrid granitoidsTwo samples of hybrid granitoid, the Fojtka granodiorite of

Klomínský (1969), were analysed, one from near Fojtka, CzechRepublic (810F: N50′50″, E15′04″) and the other from the SzklarskaPoręba Huta quarry, Poland (809SPH: N50′50″, E15′30″). The Fojtkasample was studied by Słaby and Martin (2008). The hybrid granitoidsrange in composition from quartz diorite to granodiorite and occur asa series of small bodies intruded into porphyritic granite, mostly at ornear the boundary between the Jizera and Liberec phases of theKarkonosze pluton. The hybrid rocks are porphyritic to equigranular,with oval to almost subhedral–euhedral major mineral habits thathave been interpreted as evidence for magma mixing (Słaby andMartin, 2008). The major minerals are plagioclase, quartz, K-feldspar,amphibole and biotite, with accessory apatite, zircon, titanite, magne-tite, ilmenite ± monazite. The large pink K-feldspar phenocrysts aremantled by white plagioclase (rapakivi texture) and commonly spanthe boundary between the hybrid granitoid and its porphyritic host, afeature that has been interpreted by Słaby andMartin (2008) to indicatemechanical introduction of granite megacrysts into a hybrid magma.Hornblende- and biotite-mantled quartz ocelli are abundant.

4.2.2. Composite dykeThe sample of composite dyke from Karpniki, Poland (805KA: N50′

52″, E15′52″) was the same as that analysed by Słaby and Martin(2008). The dykes consist of broken and deformed mafic bodies2–3 m thick that intrude both the porphyritic and equigranular gran-ites. They aremedium-grained (2–3 mm)with a bimodal grain size dis-tribution. Anhedral quartz and alkali feldspar occur in a finer matrix ofsubhedral hornblende, biotite and plagioclase. Mafic clots of biotiteandhornblende are surrounded byquartzo-feldspathic rims. Plagioclasecrystals mantled by alkali feldspar (anti-rapakivi) and alkali feldsparmantled by plagioclase (rapakivi) are common. Accessory apatite,zircon, allanite and ilmenite are abundant.

4.2.3. Microgranular magmatic enclaveThe microgranular magmatic enclave sample (802ER) from

Mrowiec Hill in the Rudawy Janowickie mountains, Poland (N50′50″,E15′48″) was one of those studied by Słaby andMartin (2008). Enclavesof centimetre to metre size are common in the region. They consistmainly of a fine- to medium-grained assemblage of subhedral plagio-clase and biotite, with varied amounts of quartz and alkali feldspar.The alkali feldspar megacrysts have a narrow rapakivi rim. Accessory

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50µm

50µm

100µm

100µm

100µm

100µm

50µm

50µm

50µm

50µm

1.1 - 309±3 Ma Th/U = 35.5

A 804P: Porphyritic granite, Liberec type

B 812P: Porphyritic granite, Liberec type

C 808SPH: Porphyritic granite, Jizera type

D 806HCH: Equigranular granite

E 809SPH: Late hybritisation dyke

20.1 - 307±4 MaTh/U = 20.1

1.1 - 315±5 Ma Th/U = 21.7

12.1 - 308±4 Ma Th/U = 21.5

4.1 - 312±3 Ma Th/U = 35.8

11.1 - 310±3 MaTh/U = 15.5

5.1 - 313±3 MaTh/U = 16.1

6.1 - 313±4 Ma Th/U = 13.6

1.2 - 327±6 Ma Th/U = 49.5

2.1 - 337±4 MaTh/U = 35.3

2.2 - 322±3 Ma Th/U = 39.5

1.1 - 319±6 Ma Th/U = 40.2

Fig. 3. Monazite BSE images from different types of granitoids: A) porphyritic granite, Liberec type from Piec; sample 804P; B) porphyritic granite, Liberec type from Piec; sample 812P;C) porphyritic granite, Jizera type, from Szklarska Poreba Huta; sample 808SPH; D) equigranular granite from Haslerova Chata; sample 806HCh; E) late hybridized dyke from SzklarskaPoreba Huta; sample 809SPH.

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Table 2Ion microprobe U–Pb monazite isotopic data.

Labels ppm206Pb⁎

ppmU

ppmTh

232Th/238U 206Pb/204Pb ± %206Pbc

208Pb/206Pb ± 208Pb/232Th ± 207Pb/206Pb ± 238U/206Pb ± Age206Pb/238

± Age208Pb/232T

±

Major rock types804P-1.1 1450 2850 101,000 35.5 – – 0.04 10.928 0.084 0.01513 0.00022 0.0541 0.0014 20.33 0.21 308.8 3.2 303.4 4.4804P-2.1 1440 3600 96,000 26.6 2.80E+03 7.1E+02 0.65 8.349 0.062 0.01538 0.00026 0.0537 0.0019 20.43 0.27 307.6 3.9 308.3 5.3804P-3.1 1340 5290 84,000 15.8 4.5E+03 1.5E+03 0.40 4.859 0.031 0.01517 0.00023 0.0512 0.0018 20.28 0.24 310.8 3.5 304.5 4.7804P-6.1 1180 4230 73,000 17.3 – – 0.04 5.379 0.053 0.01548 0.00028 0.0526 0.0014 20.03 0.26 313.9 4.0 310.4 5.6804P-6.2 1180 1870 83,000 44.6 1.50E+03 5.9E+02 1.22 13.60 0.16 0.01521 0.00034 0.0434 0.0046 20.06 0.33 317.1 4.9 305.6 6.6804P-6.3 1580 4740 102,000 21.5 1.9E+04 1.3E+04 0.10 6.698 0.043 0.01542 0.00017 0.0533 0.0019 20.19 0.16 311.3 2.6 309.3 3.6804P-7.1 1460 4110 97,000 23.6 1.21E+05 6.3E+04 0.02 7.293 0.073 0.01518 0.00023 0.0508 0.0020 20.36 0.21 309.7 3.2 304.7 4.8804P-8.1 1360 4160 90,000 21.6 1.64E+03 5.7E+02 1.12 6.557 0.060 0.01505 0.00025 0.0475 0.0036 20.13 0.25 314.3 3.6 302.5 4.8804P-9.1 1010 3890 62,000 15.9 6.7E+03 4.5E+03 0.27 4.924 0.033 0.01549 0.00018 0.0508 0.0019 19.93 0.16 316.1 2.5 310.9 3.5804P-10.1 1030 3640 64,000 17.6 4.7E+03 1.6E+03 0.39 5.456 0.048 0.01557 0.00026 0.0512 0.0016 19.90 0.24 316.5 3.8 312.4 5.2804P-11.1 700 880 49,000 55.9 – – 0.04 17.38 0.23 0.01557 0.00042 0.0503 0.0023 19.98 0.40 315.6 6.3 312.4 8.5804P-12.1 1200 2160 85,000 39.4 1.95E+03 8.6E+02 0.94 12.09 0.14 0.01492 0.00031 0.0457 0.0039 20.55 0.31 308.8 4.4 299.8 5.9804P-13.1 950 1610 66,000 41.1 8.6E+02 2.0E+02 2.12 12.86 0.13 0.01532 0.00039 0.0372 0.0047 20.41 0.40 314.0 5.8 308.2 7.6804P-15.1 1070 3280 69,000 20.9 6.2E+03 3.3E+03 0.30 6.488 0.090 0.01548 0.00031 0.0501 0.0021 20.01 0.26 315.2 4.1 310.8 6.3804P-15.3 980 3280 62,000 18.9 7.5E+04 4.1E+04 0.02 5.877 0.045 0.01547 0.00029 0.0526 0.0013 20.06 0.29 313.5 4.4 310.3 5.9804P-17.1 1350 2490 94,000 37.6 8.1E+03 5.5E+03 0.23 11.436 0.095 0.01520 0.00025 0.0530 0.0021 20.01 0.25 314.2 3.8 304.9 5.0804P-16.1 1180 2550 79,000 30.9 4.6E+03 2.8E+03 0.40 9.711 0.078 0.01557 0.00025 0.0517 0.0025 20.22 0.25 311.4 3.7 312.2 5.0804P-18.1 950 3670 57,000 15.5 2.8E+04 1.8E+04 0.07 4.878 0.042 0.01576 0.00025 0.0514 0.0014 19.92 0.23 316.1 3.7 316.1 5.1804P-15.2 1540 4550 103,000 22.7 4.7E+04 3.5E+04 0.04 6.957 0.046 0.01499 0.00017 0.0540 0.0011 20.49 0.16 306.6 2.4 300.6 3.4804P-19.1 1290 4720 81,000 17.1 – – 0.04 5.281 0.055 0.01537 0.00027 0.0498 0.0012 20.05 0.26 314.7 4.0 308.6 5.5804P-20.1 1450 4680 94,000 20.1 1.21E+04 7.0E+03 0.15 6.271 0.087 0.01520 0.00031 0.0517 0.0017 20.49 0.28 307.5 4.2 305.0 6.1804P-21.1 1120 2130 77,000 36.2 – – 0.04 11.14 0.14 0.01524 0.00031 0.0520 0.0024 20.17 0.28 312.1 4.4 305.8 6.2812P-1.1 940 2760 60,000 21.7 – – 0.04 6.801 0.093 0.01566 0.00035 0.0540 0.0017 19.96 0.32 314.6 4.9 314.0 7.0812P-2.1 920 2050 61,000 29.8 3.7E+03 1.7E+03 0.49 9.08 0.10 0.01555 0.00028 0.0477 0.0023 19.63 0.25 322.3 4.0 312.3 5.6812P-3.1 840 1950 55,000 28.0 2.45E+03 6.7E+02 0.75 8.702 0.087 0.01574 0.00024 0.0570 0.0023 19.77 0.19 316.4 3.0 315.2 4.7812P-4.1 1020 2240 69,000 30.8 3.2E+03 1.4E+03 0.57 9.448 0.069 0.01532 0.00024 0.0492 0.0024 20.03 0.23 315.4 3.5 307.5 4.8812P-5.1 1150 3210 77,000 23.9 – – 0.04 7.366 0.049 0.01519 0.00024 0.0520 0.0017 20.33 0.24 309.8 3.7 304.7 4.9812P-6.1 830 1860 56,000 30.2 1.4E+04 1.6E+04 0.13 9.302 0.075 0.01525 0.00022 0.0523 0.0019 20.17 0.20 312.0 3.0 306.0 4.5812P-7.1 850 2670 59,000 22.0 2.03E+03 6.3E+02 0.90 6.460 0.052 0.01439 0.00032 0.0504 0.0027 20.40 0.28 309.4 4.0 289.0 6.3812P-8.1 1170 3500 75,000 21.4 – – 0.04 6.627 0.079 0.01551 0.00028 0.0518 0.0015 19.94 0.26 315.7 4.0 311.2 5.7812P-10.1 800 2070 53,000 25.7 – – 0.04 8.06 0.12 0.01530 0.00034 0.0525 0.0016 20.53 0.32 306.6 4.7 307.0 6.8812P-11.1 890 2070 60,000 29.0 – – 0.04 8.91 0.11 0.01521 0.00040 0.0522 0.0015 20.16 0.37 312.2 5.7 305.2 7.9812P-12.1 1270 3860 83,000 21.5 – – 0.04 6.705 0.080 0.01525 0.00032 0.0516 0.0013 20.45 0.29 308.1 4.3 306.0 6.4812P-13.1 7940 1590 95,000 59.6 2.9E+04 1.5E+04 0.06 17.66 0.11 0.0908 0.0012 0.1090 0.0016 3.267 0.030 1615 14 1742 22

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Labels

ppm206Pb⁎

ppmU

ppmTh

232Th/238U

206Pb/204Pb

± %206Pbc

208Pb/206Pb ± 208Pb/232Th ± 207Pb/206Pb ± 238U/206Pb ± Age206Pb/238

± Age208Pb/232T

±

812P-14.1 630 1600 42,000 26.4 3.8E+03 1.4E+03 0.48 8.31 0.15 0.01540 0.00049 0.0524 0.0028 20.41 0.43 308.4 6.4 308.9 9.9812P-15.1 910 2590 59,000 22.9 4.8E+03 3.3E+03 0.38 7.218 0.083 0.01550 0.00034 0.0495 0.0038 20.35 0.28 310.5 4.3 311.3 6.8812P-16.1 1300 3400 88,000 25.8 1.5E+04 1.9E+04 0.12 7.615 0.062 0.01503 0.00032 0.0506 0.0019 19.64 0.32 321.0 5.1 301.7 6.3812P-17.1 1120 3050 71,000 23.4 1.6E+05 1.6E+05 0.01 7.29 0.10 0.01582 0.00034 0.0529 0.0033 19.69 0.31 319.2 5.0 317.1 7.1808SPH-1.1 620 3690 34,000 9.2 7.5E+03 3.4E+03 0.25 2.865 0.029 0.01541 0.00025 0.0511 0.0014 20.11 0.21 313.4 3.2 309.5 4.9808SPH-2.1 320 1990 16,000 8.1 3.2E+03 1.6E+03 0.57 2.622 0.033 0.01607 0.00037 0.0480 0.0031 20.04 0.34 315.6 5.1 323.7 7.4808SPH-3.1 540 1760 35,000 19.6 2.7E+03 1.6E+03 0.67 6.237 0.063 0.01553 0.00033 0.0499 0.0037 20.45 0.33 308.8 4.7 311.8 6.5808SPH-4.1 1310 2530 90,000 35.8 9.3E+03 3.0E+03 0.20 11.050 0.083 0.01527 0.00022 0.0510 0.0014 20.21 0.22 311.9 3.3 306.5 4.5808SPH-5.1 1740 4910 114,000 23.3 2.7E+04 1.3E+04 0.07 7.222 0.059 0.01531 0.00027 0.0518 0.0013 20.25 0.27 311.1 4.0 307.2 5.3808SPH-6.1 660 3520 37,000 10.6 1.03E+05 8.6E+04 0.02 3.338 0.035 0.01563 0.00025 0.0523 0.0011 20.16 0.23 312.2 3.5 313.6 5.1808SPH-7.1 790 4050 45,000 11.1 1.14E+04 7.3E+03 0.16 3.494 0.031 0.01562 0.00029 0.0518 0.0015 20.10 0.28 313.3 4.2 313.6 5.8808SPH-8.1 930 2480 61,000 24.8 5.6E+03 3.2E+03 0.33 7.753 0.097 0.01530 0.00032 0.0523 0.0030 20.43 0.32 308.2 4.7 306.9 6.5808SPH-9.1 1600 4440 105,000 23.8 1.28E+04 9.6E+03 0.14 7.348 0.072 0.01535 0.00027 0.0502 0.0018 20.15 0.25 313.2 3.9 308.1 5.4808SPH-10.1 1140 3670 74,000 20.2 – – 0.04 6.151 0.064 0.01507 0.00027 0.0533 0.0014 20.18 0.26 311.6 4.0 302.3 5.4808SPH-11.1 1290 5170 80,000 15.5 – – 0.04 4.790 0.046 0.01520 0.00023 0.0541 0.0013 20.29 0.21 309.6 3.1 304.6 4.6808SPH-11.2 770 3890 44,000 11.4 – – 0.04 3.563 0.035 0.01538 0.00028 0.0536 0.0032 20.34 0.27 309.0 4.2 308.4 5.9808SPH-12.1 510 1510 33,000 21.6 2.7E+03 2.0E+03 0.69 6.97 0.12 0.01557 0.00046 0.0525 0.0054 20.70 0.43 304.2 6.1 312.4 9.0806HCh-1.1 1300 2180 88,000 40.2 7.0E+04 1.7E+04 0.03 12.50 0.17 0.01573 0.00041 0.0521 0.0017 19.74 0.37 318.8 5.8 315.4 8.2806HCh-1.2 890 1230 61,000 49.5 – – 0.04 15.05 0.14 0.01583 0.00041 0.0525 0.0020 19.22 0.38 327.1 6.3 317.4 8.3806HCh-2.1 1040 2020 71,000 35.3 9.35E+01 5.1E+00 19.55 10.27 0.14 0.01529 0.00036 0.033 0.011 19.06 0.34 337.5 4.3 308.3 5.3806HCh-2.2 930 1580 63,000 39.5 4.6E+04 2.8E+04 0.04 12.08 0.10 0.01579 0.00036 0.0583 0.0016 19.37 0.34 322.3 5.6 316.3 7.3806HCh-3.1 1370 2690 94,000 34.7 – – 0.04 10.59 0.14 0.01539 0.00029 0.0541 0.0013 19.81 0.23 317.0 3.7 308.6 5.7806HCh-3.2 1700 4990 109,000 21.8 2.2E+03 1.1E+03 0.84 7.907 0.074 0.01592 0.00022 0.0475 0.0038 22.82 0.21 278.3 2.2 319.8 4.1806HCh-7.1 1110 1870 75,000 40.0 – – 0.04 12.27 0.23 0.01575 0.00054 0.0542 0.0042 19.50 0.49 321.9 8.1 316 11806HCh-7.2 1450 3570 94,000 26.5 1.2E+04 1.2E+04 0.15 8.301 0.074 0.01568 0.00025 0.0523 0.0019 20.01 0.23 314.6 3.6 314.6 5.0

Minor rock types809SPH-1.1 750 4540 41,000 8.9 1.00E+04 5.7E+03 0.18 2.805 0.023 0.01552 0.00019 0.0496 0.0013 20.20 0.16 312.6 2.4 312.2 3.7809SPH-2.1 270 2670 11,000 4.1 3.81E+02 3.9E+01 4.80 1.291 0.017 0.01594 0.00031 0.0443 0.0046 19.78 0.25 321.2 3.7 324.9 5.4809SPH-3.1 660 4000 35,000 8.8 3.2E+04 1.3E+04 0.06 2.767 0.021 0.01574 0.00020 0.0510 0.0011 20.00 0.18 315.2 2.8 316.1 4.1809SPH-4.1 370 3120 17,000 5.4 – – 0.04 1.714 0.013 0.01574 0.00019 0.0540 0.0018 20.25 0.16 310.2 2.5 315.1 4.0809SPH-5.1 490 1860 30,000 16.1 1.95E+03 8.8E+02 0.94 5.091 0.047 0.01558 0.00023 0.0443 0.0039 20.31 0.22 312.9 3.1 313.8 4.3809SPH-6.1 510 2220 30,000 13.6 5.0E+04 5.0E+04 0.04 4.249 0.045 0.01554 0.00026 0.0507 0.0023 20.17 0.23 312.7 3.6 312.1 5.4809SPH-7.1 650 4150 34,000 8.1 6.6E+03 3.4E+03 0.28 2.551 0.025 0.01567 0.00027 0.0518 0.0022 20.01 0.25 314.7 3.8 314.6 5.5809SPH-8.1 660 4520 34,000 7.5 – – 0.04 2.322 0.020 0.01560 0.00028 0.0518 0.0014 19.92 0.27 316.2 4.2 313.1 5.7809SPH-9.1 660 4520 34,000 7.6 7.2E+03 4.6E+03 0.25 2.360 0.020 0.01538 0.00021 0.0500 0.0017 20.25 0.19 311.8 2.8 309.4 4.1809SPH-10.1 400 2970 19,000 6.4 2.29E+03 7.9E+02 0.80 2.015 0.027 0.01598 0.00032 0.0474 0.0028 19.77 0.26 320.1 4.1 322.6 6.4809SPH-11.1 440 3360 21,000 6.3 2.8E+03 1.0E+03 0.66 1.974 0.032 0.01558 0.00032 0.0470 0.0025 20.12 0.22 314.8 3.3 314.8 6.3809SPH-12.1 360 3090 16,000 5.2 3.9E+04 2.8E+04 0.05 1.621 0.030 0.01560 0.00042 0.0536 0.0017 20.10 0.35 312.6 5.3 312.3 8.4809SPH-13.1 260 1890 13,000 7.0 – – 0.04 2.185 0.052 0.01555 0.00056 0.0549 0.0029 20.22 0.51 310.4 7.7 311 11

“–” 204Pb below detection limit; average detection limit for 204Pb is b2 ppb.

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minerals include titanite, apatite, ilmenite, magnetite and zircon. Themargins of the enclaves are commonly rounded or lobatewhich, togeth-erwith themineral growth textures, have been interpreted as indicativeof the coexistence andmixing of twomagmas (Słaby andMartin, 2008).

5. Analytical methods

Zircon andmonazite grains were extracted from heavymineral con-centrates andmounted in epoxywith referencematerials, then polishedto expose mid-grain sections. The mounts were cleaned and carboncoated for SEM cathodoluminescence (CL) and BSE imaging of thezoning textures in zircon and monazite respectively, then cleaned andrecoated with Au for the SHRIMP U–Pb isotopic age determinations.

5.1. Monazite

Monazite was analysed for U–Th–Pb isotopes on the SHRIMP II andSHRIMP RG ion microprobes at the Research School of Earth Sciences,

ANU, Canberra, using procedures based on those of Williams andClaesson (1987) and Williams et al. (1996). A 10 kV primary ion beamof ~3 nA negative O2 was focused to a probe ~25 μm diameter and thesputtered secondary ions extracted at 10 kV, mass analysed at~5000R, and the isotopic species of interest measured on a single ETPelectronmultiplier by cyclic peak stepping. A small isobaric interferenceat 204Pb was removed by using weak energy filtering. Monazite grainswere mounted together with primary monazite standard ThompsonMine (radiogenic 206Pb/238U = 0.3152, U = ~2100 ppm) and second-ary standard 44069 (424.9 Ma; Aleinikoff et al., 2006). Common Pb cor-rections were made using 204Pb. Data were reduced using in-housePRAWN and LEAD software (T.R. Ireland) and ages calculated usingthe constants recommended by IUGS Subcommission on Geochronolo-gy (Steiger and Jäger, 1977). The monazite analyses were correctedfor the presence of excess 206Pb from excess initial 230Th, using theprocedure of Schärer (1984). A Th/U ratio of 4 was assumed for thefluid from which the monazite crystallized. Corrections to radiogenic206Pb/238U for low Th/U monazite were all less than 0.3% and most

B 804P: Porphyritic granite

311.5±2.2 Ma

313.0±3.0 Ma

C 812P: Porphyritic granite

317.6±5.6 Ma

F 806HCh: Equigranular granite

310.9±3.0 Ma

E 808SPH: Porphyritic granite

311.4±3.6 Ma D 807MI: Porphyritic granite

0.048

0.052

0.056

0.060

0.064

0.045

0.055

0.065

0.075

0.048

0.052

0.056

0.060

0.064

16 18 20 22 24 16 18 20 22 24

16 24 28 32 3620

238U/206Pb 207 P

b/2

06P

b

207 P

b/2

06P

b

207 P

b/2

06P

b

207 P

b/2

06P

b

207 P

b/2

06P

b

207 P

b/2

06P

b

238U/206Pb

238U/206Pb 238U/206Pb

238U/206Pb

238U/206Pb

300 280 320 340 300 280 320 340

300 280 320 340 300 280 320 340

300 280 320 340 260 220

300 340

313.0±2.8 Ma A 803HR: Porphyritic granite

n=11; MSWD = 1.3

n=22; MSWD = 0.9

n=15; MSWD = 1.2

n=10; MSWD = 1.6

n=13; MSWD = 0.4

n=6; MSWD = 0.8

Fig. 4. Concordia diagrams showing U–Pb analyses from themajor rock types: A–E) porphyritic granite, F) equigranular granite. All uncertainties 1 sigma; filled ellipses showdata used forage calculations, pink— zircon, yellow — monazite.

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≤ 0.2%. Corrections for a few monazite grains with high Th/U (up to 55in sample 804P) were up to 0.46%.

5.2. Zircon

Zircon U–Th–Pb isotopic compositions were measured using theSHRIMP II at the National Institute of Polar Research, Tokyo, Japan andtechniques following those of Williams (1998). Analytical spots were~30 μm diameter. The analyses were calibrated using zircon standardsSL13 (U = 238 ppm) and FC1 (1099 Ma; Paces and Miller, 1993).Common Pb corrections were based onmeasured 204Pb. Data reductionand processing were done using SQUID 1 and ISOPLOT 3.5 softwareprovided by K.R. Ludwig from the Berkeley Geochronology Center,University of California, Berkeley (Ludwig, 2003).

Analytical uncertainties in the zircon and monazite data tables andplots are 1σ, and in the pooled Concordia Age and weighted meanages are 95% confidence limits. The latter include the uncertainties inthe SHRIMP analyses of the standards, and in the decay constants.

6. Results

6.1. Porphyritic granitoids

6.1.1. Liberec granite

6.1.1.1. 803HR—Hraničná quarry. The zircon from sample 803HR occursas medium sized (50–100 μm diameter) subhedral simple prismaticgrains with aspect ratios of 2–3 and relatively few inclusions (Fig. 2A).CL imaging shows most grains to have weak concentric or bandedzonation, with no obvious cores. There is a wide range of U content(266–2425 ppm, most 300–1100 ppm) and Th/U (0.17–0.71, most0.35–0.60), normal values for zircon from a relatively felsic granite(Table 1). With a few exceptions, common Pb contents are relativelylow. The zircon radiogenic U–Pb isotopic compositions are all concor-dant or nearly so within analytical uncertainty, but there is a verywide range in radiogenic 206Pb/238U, 9 of the 22 analyses having206Pb/238U significantly lower than the main group (Fig. 4A). There isno strong correlation between 206Pb/238U and U or common Pbcontents.

Omitting the 9 low analyses assuming radiogenic Pb loss leaves 13determinations for which the range in 206Pb/238U is still a little largerthan expected from the analytical uncertainties (MSWD = 2.3). The ex-cess scatter is due to two determinations with slightly but significantlyhigher 206Pb/238U than the rest (8.1, 13.1), omitting which leaves 11analyses equal within uncertainty (MSWD = 1.3), giving a weightedmean 206Pb/238U age of 313.0 ± 2.8 Ma.

6.1.1.2. 804P— Piec crag. Themonazite recovered from sample 804P con-sists of clear, pale yellow, anhedral crystal fragments up to 100 μm di-ameter that, from their morphology and zoning textures, are clearlybroken pieces of much large grains, many of which had well-preserved crystal faces. BSE imaging shows simple concentric zoningwith well-developed sector zoning in some grains (Fig. 3A), consistentwith igneous monazite growth, and no subsequent recrystallization.Some grains contain a scattering of fine mineral inclusions.

The analysed grains have awide range of U contents (880–5290 ppm)and Th/U (15.5–44.6). The U range is the largest found in monazitefrom any of the samples in this study (Table 2). With a single excep-tion, the U–Pb isotopic analyses are concordant within analyticaluncertainty (Fig. 4B). All 22 measurements of radiogenic 206Pb/238Uare the same within uncertainty (MSWD = 0.9), giving a weightedmean age of 312.1 ± 2.2 Ma. Corrected for initial 230Th, this age be-comes 311.5 ± 2.2 Ma. Similarly, all measurements of radiogenic208Pb/232Th are the same within uncertainty (MSWD = 0.8), givinga weighted mean age of 307.2 ± 2.7 Ma. Both age estimates are thesame within analytical uncertainty.

6.1.1.3. 812P— Piec crag. Themonazite recovered fromsample 812P con-sists of clear, pale yellow, anhedral grains up to 100 μm diameter thatare fragments of larger grains. Unlike themonazite from804P, however,there is little evidence for the grains being broken. Instead, the crystalsare irregular in shape with cuspate margins consistent with partial dis-solution.Nevertheless, banded, concentric and sector zoning iswell pre-served, with little evidence for recrystallization, even at the grainmargins (Fig. 3B). Mineral inclusions are rare.

The analysed grains have a smaller range in U content (1595–3860)and Th/U (21.4–30.8, with one spot at 59.6) than those from 804P(Table 2), and lower common Pb on average. With a single exception(13.1), the U–Pb isotopic analyses are concordant within analytical un-certainty and tightly clustered (Fig. 4C). Except for analysis 13.1, all 15analysed spots have the same radiogenic 206Pb/238Uwithin analytical un-certainty (MSWD = 1.2), giving aweighedmean age of 313.6 ±3.0 Ma.Corrected for initial 230Th the age falls to 313.0 ± 3.0 Ma. There is morescatter in 208Pb/232Th, with two additional analyses (7.1, 17.1) beinglower and higher, respectively, than the main population. Omittingthese analyses leaves 13 208Pb/232Th measurements equal withinanalytical uncertainty (MSWD = 1.8), giving a weighted mean ageof 307.8 ± 3.4 Ma. Grain 13, with abnormally high Th/U (59.6), a206Pb/238U age of 1615 ± 14 Ma (σ) and 208Pb/232Th age of1742 ± 22 Ma (σ), is inherited from an older source.

6.1.2. Jizera granite

6.1.2.1. 807Mi — Michałowice quarry. The zircon from sample 807Mioccurs as relatively large (up to 150 μm diameter), euhedral, prismaticcrystals with high aspect ratios (4–5) and simple pyramidal termina-tions. Fine mineral inclusions are common, and some grains containfractures along which new minerals have grown. The centres of mostgrains have weak or indistinct patchy zoning, but towards the marginssimple concentric zoning is the norm (Fig. 2B). The U contents arehigh to very high (most 1500–2000 ppm, but up to 4300 ppm), butTh/U, at 0.2–0.7, is in the normal igneous range. Common Pb contentsare moderate to low (Table 1).

It has been observed elsewhere that SIMS estimates of Pb/U tend tobe biased upwards at zircon U contents over about 2500 ppm(Williamsand Hergt, 2000), and spot 2.1 from 807Mi, with a U content of4300 ppm, does have the highest measured radiogenic 206Pb/238U.Without other high-U analyses, however, it is not possible to estimatethe magnitude of the matrix effect in the sample.

All the U–Pb isotopic analyses are concordant or nearly sowithin an-alytical uncertainty (Fig. 4D), but there is a significant range in radio-genic 206Pb/238U (MSWD = 7.3). The scatter is due to several analysesboth above and below the median value, not simply a series of lowvalues due to radiogenic Pb loss. Omitting analysis 2.1, with the highestU, reduces the scatter significantly, but only with the omission of oneother high analysis and three low ones can the scatter be reducedenough to produce a set of analyses equal within uncertainty(MSWD = 1.6). Theweightedmean 206Pb/238U age from those 10 anal-yses is 311.4 ± 3.6 Ma.

6.1.2.2. 808SPH— Szklarska Poręba Huta quarry. The monazite grains re-covered from sample 808SPH are large (100–200 μm diameter), clear,pale yellow fragments of even larger grains. Well preserved faces onmany fragments and the mostly simple concentric zoning visible inBSE images (Fig. 3C) suggest that the original grains were mostlyeuhedral, although some original surfaces are slightly embayed. Thereis little evidence of recrystallization and inclusions are rare.

The U contents, at 1515–5165 ppm, are similar to those in monazitefrom the other porphyritic granite samples above, but the Th/U (mostly10–25) is somewhat lower. The U–Pb isotopic analyses (Table 2)are all concordant within analytical uncertainty and form a tightcluster (Fig. 4E) with no significant scatter in radiogenic 206Pb/238U(MSWD = 0.4). Theweightedmean 206Pb/238U age fromall 13 analyses

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is 311.3 ± 3.0 Ma. Corrected for initial 230Th, the age is 310.9 ± 3.0 Ma.There is also no significant scatter in the 13 analyses of radiogenic208Pb/232Th, which give a weighted mean age of 309.0 ± 3.8 Ma. Bothage estimates are equal within uncertainty.

6.2. Equigranular granitoid

6.2.1. Harrachov granite

6.2.1.1. 806HCh — Hašlerova Chata. The monazite grains from sample806HCh aremoderate sized (50–150 μmdiameter), subhedral, corrodedremnants of slightly larger grains with few preserved faces and com-monly rounded or embayed margins (Fig. 3D). There are few inclusions,but some grains have fractures along which they appear to have beenpartly dissolved. The remnant zoning that is visible in BSE ismostly oscil-latory but lacks the simple concentric structure seen in the monazitefrom the porphyritic granites. There is little evidence of recrystallization.

The U contents, at 1230–4990 ppm, are within the range observedin the porphyritic granites (Table 2), but Th is higher on average, givinga range of relatively high Th/U (21.8–49.5). With a single extremeexception (analysis 2.1), common Pb contents are mostly low. Theanalyses are all concordant within analytical uncertainty (Fig. 4F), butthere is a significant range in radiogenic 206Pb/238U mainly due to oneanalysis (3.2) that is much lower than the rest. Omitting these twoanalyses leaves 6 equal within uncertainty that give a weighed mean206Pb/238U age of 318.4 ± 5.6 Ma (MSWD = 0.8). Corrected for the rel-atively high initial 230Th, this drops to 317.6 ± 5.6 Ma. The 208Pb/232Thmeasurements are also scattered, the outliers being the data from thesame two spots above. Omitting these leaves 6 analyses equalwithin uncertainty (MSWD = 1.4), giving a weighted mean age of

314.6 ± 4.6 Ma. Both age estimates are equal within the relativelylarge uncertainties.

6.3. Hybrid granitoids

6.3.1. 810F — FojtkaThe zircon from sample 810F is medium-grained (50–100 μmdiam-

eter), and formsmostly stubby subhedral to euhedral prismatic crystalswith aspect ratios b 2 and simple pyramidal terminations (Fig. 2C).Some grains contain large mineral inclusions and most are finelyfractured. BSE images show that the zoning in the centre of most grainsis indistinct or patchy, but in the outer parts of the grains concentriczoning is well developed. In some areas there are cavities along partic-ular zones where zircon might have been dissolved out.

The zircon U contents are moderate to high (790–3740 ppm) andTh/U, although somewhat lower than in the zircon from themajor gran-ite types (0.21–0.58), is still within the normal igneous range (Table 1).Common Pb contents are mostly low. All the analyses are concordant ornearly so within analytical uncertainty (Fig. 5A), but there is a signifi-cant range in radiogenic 206Pb/238U (MSWD = 8.4). Three analysesare much lower than the rest, presumably because of radiogenic Pbloss. Omitting those does not eliminate the scatter (MSWD = 2.9).This is only achieved by also omitting one high analysis (1.1), leaving7 measurements equal within analytical uncertainty (MSWD = 0.9),giving a weighted mean 206Pb/238U age of 314.3 ± 4.3 Ma.

6.3.2. 809SPH — Szklarska Poręba Huta quarryThemonazite extracted from sample 809SPH consists of 100–150 μm

diameter anhedral to subhedral fragments of much larger grains(Fig. 3E). Some grains have no distinct crystal faces, because either theyhave been partly dissolved or were originally interstitial. Most, however,

D 802ER: Microgranular magmatic enclave 301.8±4.4 Ma

309.6±6.1 Ma

C 805KA: Composite dyke

314.3±4.3 Ma

A 810F: Hybrid granodiorite

313.7±2.7 Ma

B 809SPH: Hybrid granodiorite

16 18 20 22 24

0.048

0.052

0.056

0.060

0.064

16 18 20 22 24

0.048

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207 P

b/2

06P

b

207 P

b/2

06P

b

207 P

b/2

06P

b

207 P

b/2

06P

b

238U/206Pb

238U/206Pb 238U/206Pb

238U/206Pb

300 280 320 340

300 280 320 340 300 280 320 340

300 280 320 340

n=7; MSWD = 0.9

n=13; MSWD = 0.9

n=11; MSWD = 1.3

n=10; MSWD = 1.3

Fig. 5.Concordia diagrams showingU–Pb analyses fromminor rock types: A–B) hybrid granodiorite, C) composite dyke andD)microgranularmagmatic enclave. All uncertainties 1 sigma;filled ellipses show data used for age calculation, pink— zircon, yellow — monazite.

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have well-preserved remnant crystal faces. All grains have numerousfine inclusions, some aligned in trains parallel to the zoning. That zoning,as seen in BSE images, is weak and very broad and commonly truncatedat the grain margins.

The U contents of the monazite are similar to those seen inother monazite samples (Table 2), but the range is smaller (1865–4540 ppm). Th/U, on the other hand, is much lower (4.1–16.1, mostlyb9) because of consistently lower Th contents. With a single exception(analysis 2.1), common Pb contents are moderate to low. All 13 U–Pbanalyses are concordant within analytical uncertainty (Fig. 5B), and allhave the same radiogenic 206Pb/238Uwithin error (MSWD = 0.9), givingaweightedmean age of 313.8 ± 2.7 Ma. Following a trivial correction forinitial 230Th, the age becomes 313.7 ± 2.7 Ma. Similarly, all 208Pb/232Thmeasurements are equal within uncertainty (MSWD = 0.7), givinga weighted mean age of 314.6 ± 3.5 Ma. The two age estimates aremutually indistinguishable.

6.4. Composite dyke

6.4.1. 805KA — KarpnikThe zircon extracted from sample 805KA forms subhedral, prismatic

grains 50–100 μm diameter with aspect ratios of 1–2 (Fig. 2D).Although some crystal faces are reasonably well preserved, all grainsare deeply embayed, fractured and peppered with internal cavities,probably by partial dissolution. BSE imaging shows little evidence ofconcentric or oscillatory igneous growth zoning, although some frac-tures and cavities are aligned along what must have been originalgrowth zones (Fig. 2D). The common zoning texture consists of irregu-lar patches with sinuous, convolute or rounded boundaries which,particularly in the centres of the grains, have little or no relationshipto the external crystal shape.

The analysed spots mostly have high U contents (3100–4345 ppm)but one grain (grain 1) has lower U (155–370 ppm). Th/U is consistent-ly high (0.95–1.80), again except for grain 1 (Table 1), which has lowerTh/U (0.3–0.4). Common Pb contents are relatively high, particularly inthe most U-rich grains. All the U–Pb isotopic analyses are concordantwithin analytical uncertainty (Fig. 5C), but there is a significant scatterin radiogenic 206Pb/238U (MSWD = 3.6). Some of the U contents arehigh enough to expect a U-related matrix effect in the measurementof Pb/U (Williams and Hergt, 2000), but the scatter in the data issuch that the size of this effect cannot be defined with sufficient pre-cision to justify making a correction. The excess scatter is due to thelowest measured 206Pb/238U. Omitting that point, the remaining 11206Pb/238U determinations are equal within analytical uncertainty(MSWD = 1.6), giving a weighted mean age of 311.4 ± 5.0 Ma,the relatively large uncertainty reflecting the larger than normal uncer-tainty in the Pb/U calibration for the analytical session. Omitting thethree analyses of grain 1 as potentially anomalous gives an age of309.6 ± 6.1 Ma (MSWD = 1.3), indistinguishable from the resultobtained when those analyses are included.

6.5. Microgranular magmatic enclave

6.5.1. 802ER — Mrowiec HillThe zircon grains from sample 802ER have a superficial resemblance

to those from 805KA. They are 50–100 μm diameter, anhedral tosubhedral prismatic crystals with aspect ratios of 1–2. Most grains pre-serve at least some remnant crystal faces, although on some grainsthose faces are almost entirely destroyed (Fig. 2E). BSE imaging showsthat most grains preserve remnants of concentric igneous zoning, al-though sometimes only in small areas. Elsewhere the zoning is patchyand irregular, consistentwith partial recrystallization. The recrystallizedareas are commonly very rich in inclusions and cavities, in placesaligned parallel to original zoning. These features, and the embayedsurfaces of the crystals are consistent with their having been extensiverecrystallization and corrosion.

The U contents are mostly high (2040–3165 ppm), with one at5700 ppm. Th/U is moderate to high (0.57–1.53), the highest valuebeing in the highest-U grain (Table 1). Common Pb contents are highto very high, the highest measured in this set of samples. All 10 U–Pbisotopic analyses are concordant (Fig. 5D) and equal in radiogenic206Pb/238U within analytical uncertainty (MSWD = 1.3), giving aweighted mean age of 301.8 ± 4.4 Ma. Without the highest-U analysisthe age is effectively the same 300.5 ± 4.7 Ma (MSWD = 0.9).

7. Emplacement history of the Karkonosze pluton

Since the early field mapping by Cloos (1925), it has been knownthat the large (~700 km2) Karkonosze body is a composite pluton,consisting of a series of granitic intrusions that probably represent dif-ferent batches of magma. Following a detailed field and petrographicstudy of the body, Borkowska (1966) recognized three main compo-nents—equigranular (or ridge) granite, porphyritic (or central) graniteand aplogranite (granophyre). On the basis of field relationships, sheconcluded that the intrusion of the pluton tookplace in twomain stages,first the equigranular granite, then themainmass of porphyritic granite.

Following the first attempts to date the pluton isotopically, doubtwas cast on this interpretation. Pin et al. (1987) and Duthou et al.(1991) analysed the Rb–Sr compositions of whole rocks from the twomain granite types, constructing whole-rock isochrons from samplesobtained from across the pluton, and alternatively from samples collect-ed over a metre scale. The age calculated from samples of ‘central’ por-phyritic granite, 328 ± 12 Ma, was nearly 20 Ma older than thatobtained from samples of ‘ridge’ equigranular granite, 309 ± 3 Ma(Fig. 6). These dates, and the inference that, contrary to Borkowska(1966), the porphyritic granite predates the equigranular granite, havehad a strong influence on the interpretation of isotopic ages measuredsubsequently.

Awdankiewicz et al. (2010), for example, measured the SHRIMP zir-con U–Pb age of a micromonzodiorite dyke in the Liberec porphyritic

330 300 310 320 290 340 Ma

Major rock types

Minor rock types

U-Pb monazite

Present study: U-Pb zircon

Porphyritic granite

Equigranular granite

Hybrid granitoids

Composite dyke

Microgranular magmatic enclave

Rb-Sr Ar-Ar Pb-Pb U-Pb zircon

Previous studies:

Fig. 6. Published geochronological data from the Karkonosze pluton.Green triangles—whole rock Rb–Sr (Pin et al., 1987; Duthou et al., 1991); blue hexagons—biotite Ar–Ar (Marheine et al., 2002); grey circles — zircon Pb–Pb evaporation (Kröneret al., 1994); coral circles— in-situ zircon U–Pb (Kusiak et al., 2009; Awdankiewicz et al.,2010; Kryza et al., 2012); pink circles — SHRIMP zircon U–Pb (present study); yellowrhombus — SHRIMP monazite U–Pb (present study).

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granite near Bukowiec, 12 km SE of Jelenia Góra. Although theConcordia age calculated from 7 of the 9 analyses was 313 ± 3 Ma, amean age of 318 ± 3 Ma based on just the two highest 206Pb/238Umea-surements was preferred. It was argued that this result was in accordwith the Rb–Sr age of 328 ± 12 Ma previously measured on theporphyritic granite, as well as a SHRIMP zircon age of 318.5 ± 3.7 Mameasured on porphyritic granite by Machowiak and Armstrong(2007), and a similarly ‘old’ unpublished SHRIMP zircon age measuredby Kryza et al. (2012). A younger result on porphyritic granite(314 ± 3 Ma) also obtained by Machowiak and Armstrong (2007)was not considered by Awdankiewicz et al. (2010), nor was an age of315 ± 4 Ma that the former measured on a sample of structurallyolder equigranular granite (Machowiak and Armstrong, 2007).

Kryza et al. (2012) reported the results of a ‘chemical abrasion’ studyon zircon from a sample of porphyritic granite of the Jizera type collect-ed from the Szklarska Poręba Huta quarry. Nine of 12 SHRIMP U–Pbanalyses of untreated zircon formed a tight cluster yielding a weightedmean 206Pb/238U age of 306 ± 4 Ma. A group of sixteen analyses of zir-con from the same sample that was annealed for 60 h at 850 °C, thenleached with concentrated HF-HNO3 for 12 h in a bomb at 180 °C,yielded more scattered 206Pb/238U values, but about a higher meanvalue. After considering the analytical bias that might possibly be intro-duced by the annealing and leaching process and omitting 5 of theanalyses, both above and below the median value, a weighted mean206Pb/238U age of 322 ± 3 Ma was calculated and interpreted, in lightof previous geochronology, as the most accurate estimate of the zirconage (Kryza et al., 2012).

The zircon andmonazite dating in the present study, combinedwithprevious isotope geochronology (Pin et al., 1987; Duthou et al., 1991;Kröner et al., 1994; Machowiak and Armstrong, 2007; Kusiak et al.,2009; Awdankiewicz et al., 2010; Kryza et al., 2012), provides a muchclearer time frame for these processes and the formation of theKarkonosze pluton as a whole. The oldest monazite age was obtainedfrom equigranular granite sample 806HCh (Fig. 6), at 318 ± 6 Ma(Pb–U) and 315 ± 5 Ma (Pb–Th). This is not only consistent with thefield interpretation of Borkowska (1966) and the evolution model ofSłaby and Martin (2008), but also with the zircon age measured onthe same lithology by Machowiak and Armstrong (2007), 315 ± 4 Ma.

The zircon andmonazite agesmeasured on the two types of porphy-ritic granite aremutually consistent, and any age difference between thetwo types is smaller than the analytical uncertainties. Samples fromLiberec-type porphyritic granite yielded ages of 313 ± 3 Ma for zircon(803HR), and for monazite, ages of 313 ± 3 (Pb–U) and 308 ± 3 (Pb–Th) for sample 812P, and 312 ± 2 (Pb–U) and 307 ± 3 (Pb–Th) forsample 804P. Samples from Jizera-type porphyritic granite yieldedages of 311 ± 4 Ma for zircon (807Mi) and 311 ± 3 (Pb–U) and309 ± 4 (Pb–Th) for monazite (808SPH). These age estimates are inclose agreement with those of Machowiak and Armstrong (2007), butare younger than the Rb–Sr whole rock ages measured by Pin et al.(1987) and Duthou et al. (1991).

The ages obtained from the minor rock types are very similar. Agesmeasured on samples of hybrid granitoids, 314 ± 4 Ma for zircon(810F), and 314 ± 3 (Pb–U) and 315 ± 4 (Pb–Th) for monazite(809SPH), are irresolvable from the ages measured on the major rocktypes. Sample 810F comes from Fojtka (“Fajka”), the same localityfrom which the zircon dated by Machowiak and Armstrong (2007)was collected. Although their zircon sample was from porphyritic gran-ite, the age theymeasured (314 ± 3 Ma) is indistinguishable from oursfrom hybrid granite. The zircon age we obtained from a structurallyyounger composite dyke (311 ± 5 Ma) is also indistinguishable fromthe ages of the granitoids. The only indication of a younger componentin the pluton is a zircon age of 302 ± 4 Ma measured on a micro-granular magmatic enclave (802ER). Alone this age would be anoma-lous, since it contradicts the textural relationship with the host granite(the equigranular granite was dated at 318 Ma, so the microgranularmagmatic enclave must be that age or older). However, this enclave

age is similar to the zircon age of another enclave (304 ± 2 Ma)dated by Kusiak et al. (2009), which they interpreted as representingreequilibration by late magmatic fluids rather than original age ofgrowth of the magmatic zircon. In that study, the magmatic zircongrowth was loosely constrained at 314 ± 2 Ma.

Of the ten samples dated in the present study, both monazite andzircon from eight samples give Pb–U ages within a very narrow range,~314–311 Ma. The Pb–Th ages measured on the monazite from thosesamples have a larger range of ~315–307 Ma. Given that zircon isprone to metamictization and radiogenic Pb loss whereas monazite isnot because it self-anneals, the close agreement between the zirconandmonazite Pb–Uages gives confidence that bothminerals are record-ing mineral growth during the magmatic crystallization. The possiblesmall difference between the Pb–U, and Pb–Th ages, in three of themonazite samples remains to be resolved. Itmight be due to a smallma-trix effect in themeasurement of Pb/U (c.f. Fletcher et al., 2010) or to anunderestimation of the correction for initial 230Th. Itmight be due to theloss of a small amount of radiogenic 208Pb.

It is the similarity between the age estimates from multiple rocktypes that is the principal finding of this study. That similarity showsthat the emplacement of themainmagmabatches that formed the com-posite Karkonosze pluton took place within a very short period of time,less than the ~5 Ma resolution of our various dating methods. The pos-sible exception is the equigranular granite, which might predate themain intrusive phase by 5 Ma or more. Our results are both internallyconsistent and overlap, within the analytical precision, ages that othershave determined by variousmineral datingmethods. Higher ages basedon Rb–Sr whole-rock isochrons appear to be overestimates. It is worthconsidering, however, that magmatism need not be wholly restrictedto the narrow range indicated by our results. Earlier phases ofmagmatism might have occurred that are obscured by the main intru-sive event. If this is the case, the evidence for earlier magmatismmight only be preserved in minor plutons, or in minerals that survivedincorporation into later magmas. Thus some older zircon, such as thatfrom the micromonzodiorite dyke dated by Awdankiewicz et al.(2010), can be interpreted as being xenocrystic.

The newlymeasured ages enable a deeper understanding of the pet-rogenesis of the Karkonosze pluton. Following their detailed geochemi-cal study, Słaby and Martin (2008) developed a model for thegeneration of the various lithologies based on the mixing of mantle-derived magma with an evolving felsic, crustally-derived magma. Onlythe equigranular granite was considered to be free of an externalmantle-derived input. This model inherently implied a time sequencein which older magmas that originally produced lamprophyric rocksand equigranular granite separately, were followed by mixtures of thetwomagma types, both of whichwere evolving by progressive fraction-al crystallization (Fig. 26 in Słaby and Martin, 2008). These mixturesproduced the porphyritic granitoids, hybrid granites, microgranular en-claves and finally the composite dykes. This model was consistent withthe sequence suggested by Borkowska (1966). It is not inherent in themodel, however, that the sequence of events spanned a period longerthan that resolvable by our geochronology (~5 Ma) or that the produc-tion of either of the crustally-derived ormantle-derivedmagmas shouldoccur at one time; both could be produced inmultiple batches over a pe-riod longer than that which produced themain pluton. Thus our resultsare consistent with the model of Słaby and Martin (2008), which alsoallows for the possibility of batch magmatism producing similar buttemporally distinguishable varieties of magmatic rocks before andafter the main event.

8. Regional and wider context and implications

The Variscan orogeny in Central Europe produced several differenttypes of granitoids, commonly treated as a single “Variscan granites”group (Franke et al., 2000). Based on the age distribution, chemical com-positions, mineralogy and structural relationships, Finger et al. (1997)

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classified these granitoids into five main groups: 1) pre- to early colli-sional (370–340 Ma) ‘Cordilleran’ I-type granitoids (e.g. Košler et al.,1993; Holub et al., 1997); these early-Variscan intrusions include theOdenwald granitoids, Stare Sedlo orthogneisses, Cetic granitoidsand the Sázava suite; 2) syn-collisional (~340 Ma), deformed S-typegranitoids (e.g. Liew and Hofmann, 1988), mostly in the southeast-ern Bohemian Massif (Gföhl and Wolfshof gneisses); 3) late- topost-orogenic (340–310 Ma) S-type and high-K I-type granitoids(e.g. Schalteger, 1995); 4) post-orogenic (310–290 Ma) high-levelI-type plutons (e.g. Kröner et al., 1994; Petrik et al., 1994); granitoidsin this group are most common in the Alpine–Carpathian chain and inthe Southern Bohemian Batholith; and 5) post-orogenic to anorogenic(300–250 Ma) leucocratic, mostly A-type plutons (e.g. Uher andPushkarev, 1994).

Although there are fewgeochronological data from the Sudetic Bath-olith, based on work of Kröner et al. (1994) the I-type granites in thewestern part of the Karkonosze pluton were tentatively included byFinger et al. (1997) in Group 4. In a more recent study, Finger et al.(2009) classified the Sudetic Batholith as an independent group.

Both S-type and I-type plutons are typical of the later stages of theVariscan orogeny. These late-stage granitoids represent the largest vol-ume of Variscan plutonism, and are themost diverse in terms of miner-alogy and geochemistry (Finger et al., 2009). All of these varieties can bedivided into two groups on a geochronological basis. The older group(before 325 Ma) include the 340–335 Ma ‘durbachite’plutons of centraland eastern Bohemia (Holub, 1977; Kotková et al., 2010; Kusiak et al.,2010a). Of similar age (350–330 Ma) are granitoids in the CentralSudetes (Mazur et al., 2007) and plutons on the northern margin ofthe Bohemian Massif, including the plutonic complex of the MeissenMassif, where monzonites from Leuben and Heidenschanze have beendated by U–Pb (zircon) at 326 ± 6 and 330 ± 5 Ma, respectively(Nasdala et al., 1999). The younger group include granitoids in theWest Sudetes, dated at 322–300 Ma (Turniak et al., 2005), granitoidsof the Strzelin Massif in the East Sudetes, with zircon ages of 323 ± 3,313 ± 3 and 313 ± 2 Ma (Oberc-Dziedzic et al., 2013), and rocksfrom the East Bohemian Pluton, where zircon from the Postejn (Pot-štejn) granite of the Litice Massif was dated at 317 ± 6 Ma (Kusiaket al., 2010b). As for the Karkonosze pluton, earlier Rb–Sr data fromthe Strzelin Massif granitoids gave older ages than those obtained byzircon dating (347–330 Ma, Oberc-Dziedzic et al., 1996).

Based on our new age measurements, the Karkonosze pluton wasemplaced at ~315 Ma during the same late stage of the Variscan oroge-ny, at the same time as the youngest S- and I-type granites in the Centraland Southern part of the Bohemian Massif. This episode includes thelate stages of S-type dominated magmatism along the far southernmargin ofwhat Finger et al. (2009) define as the Saxo-DanubianGraniteBelt (SDGB). It represents a relatively minor late stage of magmatism inthe SDGB, however, that occurred predominantly between 330 Ma and320 Ma, and is attributed by Finger et al. (2009) to an episode of delam-ination in the underlyingmantle lithosphere. The absence of this earlier,I-type dominated magmatic pulse in the Sudetic granitioids on thenorthwesternmargins of the BohemianMassif suggests that themantledelamination model probably does not apply.

9. Conclusion

The Karkonosze pluton is comprised of a variety of rock types rang-ing from lamprophyre to leucogranite. Previous ambiguities concerningthe absolute age and time span of the magmatism have been resolvedby the spot-by-spot U–Th–Pb dating of zircon and monazite, not onlyshowing that monazite is a reliable chronometer in these rocks, butalso validating the interpretation of dates measured on selected do-mainswithin zircon grains that commonly are both U-rich and partiallyaltered. The age measurements indicate that the porphyritic granitoids,the most voluminous rocks in the Karkonosze pluton, the hybrid rocksand composite dykes were probably all emplaced within a narrow

time interval at ~312 Ma. The equigranular granite is possibly 5 Maolder. The significantly younger age of ~302 Ma measured on amicrogranular enclave does not record a magmatism, but post-magmatic modification due to late stage fluids as observed in Kusiaket al. (2009).

Our new geochronological results are in accord with the geochemi-cal model of Słaby and Martin (2008), but because the uncertaintieson themeasured ages (2–6 Ma) are larger than the time span of the em-placement of the various magmas, it is not possible to use the geochro-nology to place the magmas in an intrusive sequence. It does appear,however, that the equigranular granite is slightly older than the otherrock types.

Acknowledgements

We gratefully acknowledge two anonymous reviewers for their con-structive comments and Klaus Mezger for careful editorial handling.SHRIMP time for the monazite analyses was kindly provided by RSES,ANU and for the zircon analyses by NIPR. The research was supportedfinancially by a grant to M.A. Kusiak from the Foundation for PolishSciences (HOMING Programme).

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