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U-total Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany

U-total Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany Lisowiec, K., Budzy, B., Slaby, E., Schulz, B., and Renno, A.D. 2014. Th-U- Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany. Acta Geologica Polonica, 64 (4), 457­472. Warszawa. Monazite from the Stolpen monzogranite (SE Germany) was studied to constrain the Th-U- Pb of pluton formation. Monazite grains demonstrate subtle to distinct patchy zoning related to slight compositional variations. Textural and compositional characteristics indicate that the monazite formed in a single magmatic event in a slightly heterogeneous system, and was only weakly affected by secondary alteration, which did not disturb the Th-U-Pb system. Chemical dating of the monazite gave a consistent of 299 ± 1.7 Ma. The current study presents the first geochronological data for the Stolpen granite. It provides evidence that Stolpen is the youngest Variscan granitic intrusion in the Lusatian Granodiorite Complex and indicates that magmatic activity related to post-collisional extension in this region lasted at least 5my longer than previously assumed. Key words: Monazite, Th-U-Pb chemical dating; Lusatian Granodiorite Complex; Stolpen Granite; Variscan granitoids. INTRODUCTION The convergence of Gondwana and Laurassia during the Paleozoic, including suuction and continental collision, produced a wide variety of magmatic and metamorphic rocks from the Bohemian Massif in the east to the Massif Central in the west. Due to intensive heating and melting of the crust and/or the mantle, caused by burial or decompression during late-orogenic extension, many granitic intrusions formed, often deriving their melts from heterogeneous sources (e.g. Finger et al. 1997; Finger et al. 2009; Siebel et al. 2003; Förster and Romer 2010). The granitoid bodies are most abundant in the Moldanubian Zone of the orogenic belt 458 KATARZYNA LISOWIEC ET AL. (the main part of the Bohemian Massif) and less abundant in the Saxo-Thuringian and Teplá-Barrandian zones (e.g. Finger et al. 1997; Oberc-Dziedzic et al. 2013). They differ in petrography, geochemistry and geochronology (Finger et al. 1997); therefore a careful study of all the types is crucial to understanding their evolution and global mantle-crust interactions related to orogenic and post-orogenic movements. The s of the Variscan granitoids have been constrained in numerous papers using various methods including the Single Zircon Evaporation method (e.g. Kröner et al. 1994; Siebel et al. 2003), whole rock RbSr (e.g. Kröner et al. 1994; Finger et al. 1997), the UPb method in zircon and monazite (e.g. Gerdes et al. 2003; Klein et al. 2008; Finger et al. 1997; ObercDziedzic et al. 2013; Kryza et al. 2012) and Th-U- Pb of uraninite and Re-Os of molyenite (Förster et al. 2012). All these methods are not interchangeable with each other and may document slightly different sts of the magmatic/metamorphic events. Fluid overprint further complicates the use of some of them, e.g. U-Pb s of zircon or monazite due to remobilization of Pb. One of the most rapid and widely used methods of determination is Th-U- Pb dating of monazite. Monazite is a LREE-rich phosphate [(REE,Th,U)PO4] which incorporates significant amounts of Ce, La, Sm and Nd, as well as other elements such as Y, Th and U. Thelast two are particularly important in terms of using monazite for Th-U-Pb dating. Because monazite occurs in various types of magmatic, metamorphic and sedimentary rocks, it can be used to constrain the timing of geological processes such as magma crystallization and metamorphism or to define the of protholith(s) (Williams et al. 2007). Diffusion of major and trace elements in monazite is very slow (Cherniak and Pyle 2008; Cherniak et al. 2004a; Cherniak et al. 2004b; Parrish 1990), therefore it can preserve compositional zoning which records different sts of crystallization or metamorphic deformation. Due to the fact that monazite contains negligible amounts of common Pb relative to radiogenic Pb (Parrish 1990), it is possible to use the chemical Th-U- Pb method employing an electron microprobe to constrain its (Jercinovic and Williams 2005; Jercinovic et al. 2008; Konecný 2004; Montel et al. 1996; Pyle et al. 2005; Spear et al. 2009; Suzuki and Adachi 1991, 1994; Suzuki and Kato 2008). Although chemical dating of monazite is mostly used in metamorphic petrology (Finger and Krenn 2007; Kohn et al. 2005; Liu et al. 2007; Rosa-Costa et al. 2008; Tickyj et al. 2004; Williams et al. 2007), it has also found applications in constraining the s of magmatic events with high precision (Just et al. 2011). The resistance of monazite to complete alteration and its ability to preserve its growth textures provide an opportunity to reach deep into the magmatic history. One region of the Variscan Orogenic Belt where granitic intrusions are rather scarce is the Lusatian Granodiorite Complex (LGC), located in the eastern part of the Saxo-Thuringian Zone. It experienced only minor metamorphism and deformation during the Variscan orogeny (Kröner et al. 1994). It contains only several late-Variscan granitoid bodies, most of which have been studied in terms of geochemistry and geochronology (Kröner et al. 1994; Hammer et al. 1999; Förster et al. 2012). However there is one pluton,­ the Stolpen granitoid,, which cannot be precisely situated within the intrusion sequence of the Lusatian Block due to a lack of geochronological data. This study reports monazite UTh-Pb timing constraints on the formation of the Stolpen pluton.. The analyzed monazite formed mostly at the magmatic sts of pluton formation; however, as infiltration by post-magmatic fluids has been already documented (Lisowiec et al. 2013), the samples were carefully studied to minimize the influence of fluid-alteration on the calculated s. GEOLOGICAL SETTING The granitoid pluton of Stolpen is located in the southern part of the Lusatian Granodiorite Complex which comprises the central part of the Lusatian Anticlinal Zone at the NE margin of the Bohemian Massif (Text-fig. 1). The pluton is slightly SE-NW elongated, which is the main direction of shearing during the Variscan orogenesis (Krentz in Kozdrój et al. 2001). Magma emplacement used tectonic faults that were formed during and after orogenic movements. The pluton intruded Cadomian to Early-Palaeozoic (600­490 Ma) magmatic ­ tonalitic to syenogranitic in composition, locally metamorphosed rocks. The envelope of the complex consists of upper-Proterozoic sedimentary rocks, mostly greywackes and pelites. The Stolpen granite belongs to the group of late- to post-Variscan intrusions in the Lusatian Granodiorite Complex which contains also the amphibole granites of Wiesa and Grossschweidnitz and the biotite granite of Königshain-Arnsdorf, with s constrained by zircon-evaporation method at 304 ± 10 Ma, 312 ± 10 Ma and Th-U- Pb dating of uraninite and molyenite at 327­328 Ma, respectively (Kröner et al. 1994; Förster et al. 2012). Knowledge of the petrogenesis of the Stolpen granite is very limited; however Hammer et al. (1999) sug- 459 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY Text-fig. 1. Sketch of the study area (after Kozdrój et al. 2001) with sampling locations. AR1, AR3 ­ 51°0'58.77" N, 14°7'27.71" E gest that it originated from a crustal magma. The melting process was induced by an upwelling mantle diapir preceded by a fluid front. The fluids were responsible for crustal magma enrichment in LILE and HFSE. An accessory mineral study was consistent with such an hypothesis but did not exclude other possibilities (Lisowiec et al. 2013). Hammer et al. (1999) place the pluton among other Variscan granitic intrusions but do not give an exact . It is therefore unknown where exactly it is positioned among other Variscan intrusions in the region. The pluton consists mostly of medium- to coarsegrained monzogranite of peraluminous character (Hammer et al. 1999). Whole-rock geochemistry was documented by Hammer et al. (1999) and is presented in Table 1. The authors also report an aver Th/U ratio of 4.4. Granite samples used in this study were taken from the SW part of the magmatic body, which consists of a quite homogenous, medium-grained facies. The mineralogy of the granite is quite typical, the main assembl containing quartz, alkali-feldspar, plagioclase, biotite and small amounts of muscovite. Quartz forms two populations: older large crystals and younger small crystals occurring as inclusions in other minerals or located interstitially. Alkali-feldspar is Krich with a subordinate Na-rich (anorthoclase) component, whilst the plagioclase composition is almost pure albite, rarely oligoclase (Ab<20%). The pure albitic composition may suggest secondary post-magmatic crystal-fluid interaction. Plagioclase often exhibits weak zonation. Alkali-feldspar shows a strong perthitization and is often replaced by plagioclase (albite) on the margins, which again may be related to reaction with fluids. Similarly the other phases show pristine magmatic compositions affected by interaction with fluids. Biotite underwent almost chloritization and its content reaches ~43 wt%. Rarely its margins are replaced by muscovite. Accessory minerals present in the granite are fluorite, zircon, monazite (with a high contribution from a cheralite component), titanite, allanite, apatite, xenotime, Y-rich silicates and Y-Ti-phases, Th-rich minerals (oxides and silicates), Nb-Ta minerals (mostly columbite), xides and secondary REE-carbonates. Fluorite is the most abundant accessory mineral and forms three populations: homogenous, more or less regularly zoned and patchy. Individual populations show no specific textural positions. Y-rich minerals are represented Element/ Content Element Content Element Content oxide TiO2 Fe2O3 MnO MgO Na2O K2 O P2 O5 Ba Co Cr Cs Cu Ga Hf Li Nb Ni Pb Rb Sc Sr Ta Th U V Y Zn Zr La Ce Nd Sm Eu Tb Yb Lu Table 1. Whole-rock chemical composition of the Stolpen monzogranite (from Hammer et al. 1999). Element oxides are given in wt [%], trace elements in [ppm]. 460 KATARZYNA LISOWIEC ET AL. mostly by strongly zoned hingganite­(Y) and aeschynite­(Y) (Lisowiec et al. 2013). Zircon, monazite and xenotime sometimes form intergrowths. The accessory mineral assembl (mostly zircon and monazite) and the evidence of magmatic and post-magmatic processes that it carries has been studied by Lisowiec et al. (2013). Some parts throughout the pluton have more aplitic or pegmatitic character with nearly the same mineral composition as the granite. In the area we can find also numerous andesitic (and one rhyolitic) dykes which are situated in the vicinity of the pluton or intruded within the granite. RESULTS AND DISCUSSION Textures and chemical composition of monazite Monazite is quite abundant in the accessory mineral assembl of the Stolpen granite (Lisowiec et al. 2013). Generally it forms sub- to anhedral 10­20 m inclusions in fluorite and K-feldspar. It often occurs also in the rock matrix as subhedral crystals up to 300 m in size. The whole population of monazite grains represents a wide spectrum of growth textures, from nearly homogenous to irregularly zoned, spongy and strongly dissolved (Text-fig. 2), evidence of fluid overprint (Lisowiec et al. 2013). The penetrating fluids were enriched in fluorine, Ca, Y and CO2, based on the high abundance of secondary fluorite and Y-rich silicates. Such a fluid composition enabled the remobilization of trace elements from the monazite grains which were later incorporated into secondary accessory phases. Alterations in monazite include mostly enhanced huttonite and cheralite substitutions. The monazite crystals forming inclusions in fluorite are partly corroded at the contact with the host mineral. Small monazite grains occasionally overgrow zircon margins. Because of the alteration, careful selection of the grains and evaluation of their chemistry had to be undertaken prior to any chemical dating analysis. Twelve monazite grains, which represent the most `pure' monazite end-member, were selected for determination (Text-fig. 3). The monazite grains show very subtle (Text-fig. 3abe) to distinctly patchy zoning (Text-fig. 3dgh). Dark patches in BSE imaging are often located along rims (Text-fig. 3cdghjl). Locations of the spot measurements were chosen carefully to avoid any contribution of potentially fluid-altered domains; therefore, ANALYTICAL METHODS Granite samples were initially crushed in a jaw crusher, than fragmented using a Selfrag high volt pulse power fragmentation. Afterwards, the two smallest fractions of 500­250 m and 80­250 m were used for separation in heavy liquids. The mineral separates were mounted in epoxy and polished. Backscattered electron (BSE) ims were made using a Quanta 600 FEG-MLA600F field emission scanning electron microscope (SEM) equipped with two energy dispersive spectrometers (EDS) at the Institute of Mineralogy, TU Bergakademie Freiberg, Germany. The analytical conditions were as follows: accelerating volt 20 kV, with some exceptions when 15, 25 or 30 kV were used, and a 200 A beam current with the beam focused on the sample coated with carbon. Analyses of Th, U, Pb for the calculation of monazite s, as well as Y, REE, Ca, Si, P, Sr, Al and As for corrections and evaluation of the mineral chemistry, were carried out using a Cameca SX-100 electron microprobe at the Department of Electron Microanalysis in the State Geological Institute of Dionýz Stúr in Bratislava. The analytical methods for determination followed procedures presented in Petrik and Konecný (2009). To obtain the optimum c/s/nA (counts per seconds divided by sample current) and to minimize surface dam the following analytical conditions were used: accelerating volt 15 KV, sample current 180 nA, counting times: Pb of 300 s, Th 35 s, U 80 s, Y 40 s, REE 10­50 s, except Lu 100 s, P, S, Al, Si and Ca 10 s, Sr 20 s, As 120 s. Calibrations were performed using synthetic and natural standards: REE and Y were taken from phosphates XPO4, Th from ThO2, Pb from PbCO3, U from , Ca and Si from wollastonite, As from GaAs, S from barite and Al from .The resulting s were calculated using the statistical approach of Montel et al. (1996). Text-fig. 2. T Representative BSE ims of monazite grains and their textures; a ­ monazite grain with the most homogenous texture showing only very subtle patchy zoning; b, c ­ monazite grains with more distinct patchy zoning; d ­ monazite grain with a very strongly resorbed texture 461 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY Text-fig. 3. BSE ims of twelve analyzed grains with measured points and calculated s; a, b, e, f and i ­ grains with subtle patchy zoning and slight variations in chemical composition; c, d, g, h, j, k and l ­ grains with more distinct patchy zoning and more significant variations in chemical composition; in grains c, d, g, h, j, k, l BSE-dark zones are located on the margins or along cracks where it was possible, at a safe distance from margins and cracks. However, both types of zones, BSE-bright and BSE-dark, were investigated in order to assess the potential difference in chemical composition and characteristics, which in some cases meant analyzing small patches close to margins or crevices. The chemical composition of the entire monazite population shows their affinity to the monazite-huttonite series, with the main substitution mechanism: Si4+ + Th4+ (U4+)= REE3+ + P5+ (Text-fig. 4ab). Grains with subtle patchy zoning show only slight differences in element concentrations (-mz1x, mz1 and mz3 in Table 2). Th, U and Pb contents in a single grain vary in the ranges 3.00 wt.%, 0.30 wt.% and 0.05 wt.%, respectively. Monazite grains showing more distinct zoning demonstrate stronger variations in composition, mostly in Th, U,Pb, Y and La. The highest chemical gradient can be observed in grain -mz9 where the BSE-dark rim is strongly depleted in Th and Pb (spot 4 and 5); and AAR3-mz2, where the BSE-bright patch 462 KATARZYNA LISOWIEC ET AL. Text-fig. 4. Chemical composition plots of monazite grains; a, b ­ plots showing the main substitution mechanism in the structure of monazite : Si4+ + Th4+ (U4+)= REE3+ + P5+; c ­ LREE vs. Th plot showing decreasing LREE content with increasing Th abundance; d ­ HREE vs. Th plot showing no correlation between these elements (spot 3) is highly enriched in Y, U and HREE, and depleted in LREE. BSE-dark parts of the grains are usually depleted in Th, U and Pb, interpreted as related to decreasing availability of Th and U during monazite growth. Depletion in these elements is coupled with enrichment in LREE. There is no correlation between Th and heavier lanthanides or yttrium (Text-fig. 4cd). The growth textures along with the chemical composition of monazite grains were studied carefully in terms of primary vs. secondary origin to ensure the quality of the data. Grains with very subtle patchy zoning and a low chemical gradient are undoubtedly of primary magmatic origin. In cases where the BSE intensity shows more distinct differences between the zones, the possibility of BSE-dark patches (depleted in Th, U and Pb) being altered by secondary hydrothermal processes has to be taken into account. These examples include mainly grains mz2, mz5, mz7 and mz10,where the patchy character is slightly more pronounced. Dark patches are usually associated with crystal margins or cracks. Several measurement points are located on the BSE-dark patches and margins; however their composition does not reveal any significant post-magmatic fluid overprint. These domains are indeed depleted in Th, U and Pb, but the degree of depletion is comparable for all three elements. Such a feature is not likely to take place during fluid alteration which usually results in preferential depletion (or enrichment) in one or two of these components, most usually only Pb (e.g. Williams et al. 2011; Harlov et al. 2011). The compositional variations, especially the Th-, Pb- and U-contents, can be therefore attributed most probably to fluctuations in melt composition during crystal growth. Binary plots carry further evidence of the negligible contribution of fluid overprint. Th vs. Si diagrams (Text-fig. 4a) are well correlated and almost all points lay within the thin correlation line. Furthermore, points representing both BSE-bright and BSE-dark domains form the same trend on the plots (Text-fig. 4abc). The only distinction of the BSE-bright zones relative to BSE-dark ones is the enrichment in light- and especially heavyrare earth elements, and Th, U and Pb. Numerous studies have shown that zones affected by post-magmatic fluid alteration display a distinct chemical pattern, distinguishable from the domains formed at the magmatic st and therefore allowing a straightforward location Table 2. Electron microprobe analyses of twelve analyzed grains (wt%). Sample mz1a 6 0.030 0.291 3.272 8.902 0.252 0.224 0.318 0.211 0.427 0.257 0.239 0.507 0.878 9.803 3.327 9.603 1.100 0.095 0.326 0.173 0.384 0.172 0.050 0.148 0.160 99.09 289 336 0.186 99.25 317 0.080 0.196 98.99 296 0.082 0.132 0.111 0.128 0.136 98.95 303 0.335 0.085 0.154 0.116 0.042 0.160 99.35 308 0.107 0.335 0.149 0.093 0.039 0.119 99.42 298 2.773 1.457 0.111 0.610 0.119 0.420 0.168 0.073 0.169 98.28 285 294 299 298 98.96 97.89 98.44 0.201 0.197 0.178 0.175 97.92 0.089 1.561 0.130 0.053 0.245 0.084 0.213 0.132 0.131 0.141 0.133 0.108 0.203 0.138 0.144 0.129 0.109 0.091 0.099 0.394 0.362 0.371 0.331 0.451 0.332 0.406 0.364 0.233 0.762 0.202 0.517 0.209 0.517 0.521 0.198 0.177 0.097 0.113 1.142 0.817 1.831 0.710 1.152 0.503 1.231 1.342 0.500 2.432 2.031 3.405 1.761 2.361 1.361 2.495 2.676 1.184 3.354 3.371 3.390 3.445 3.436 3.205 3.445 3.507 3.314 3.281 3.212 1.183 0.501 0.118 0.351 0.075 0.116 0.103 0.036 0.183 99.69 318 9.270 10.424 8.405 10.760 10.492 12.436 9.599 0.703 0.281 1.384 0.514 0.807 0.287 0.524 0.787 0.341 0.277 0.298 0.333 0.297 0.249 0.338 0.635 0.245 0.411 0.213 3.501 3.632 3.291 3.350 2.808 3.435 3.670 2.348 2.896 2.276 3.088 3.472 3.222 0.274 0.307 0.281 0.284 0.306 0.193 0.285 0.239 0.261 0.260 0.290 0.266 0.286 0.294 3.287 0.234 0.220 0.030 0.026 0.031 0.033 0.035 0.235 3.137 0.333 0.440 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 0.310 3.377 0.266 0.238 mz2 mz3 mz1 6 0.317 3.162 0.247 0.729 9.623 Grain Point no. SO3 ThO2 10.435 13.460 15.293 13.284 11.965 12.281 12.438 12.381 12.112 3.339 1.392 0.489 0.136 0.377 0.133 0.041 0.183 99.76 293 3.232 1.465 0.075 0.541 0.185 0.297 0.074 0.133 0.061 0.176 99.20 304 3.298 1.426 0.453 0.162 0.342 0.074 0.144 0.041 0.180 99.20 303 3.344 1.556 0.653 0.213 0.362 0.093 0.125 0.112 0.173 99.54 303 3.266 1.429 0.478 0.120 0.386 0.138 0.099 0.041 0.179 98.98 292 27.775 27.740 28.533 28.705 28.409 25.462 25.459 26.253 24.173 27.653 27.096 28.729 26.041 27.323 30.025 31.070 29.433 28.617 28.594 28.624 28.173 28.562 26.291 3.429 10.032 10.654 10.695 10.555 11.086 10.488 12.453 2.517 1.157 0.132 0.452 0.361 0.155 0.096 0.190 0.177 99.16 301 Nd2O3 Tm2O3 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY PbO ­ below detection limit 336 - s not included in the calculation Table 2. Electron microprobe analyses of twelve analyzed grains (wt%), continued. Sample mz5 5 24.362 0.231 2.945 11.232 0.457 0.262 14.370 29.939 3.219 9.487 0.962 0.321 0.123 0.290 0.096 0.116 0.120 0.047 0.154 98.92 293 290 297 295 98.59 98.72 98.74 0.191 0.192 0.099 0.144 98.39 297 0.156 99.28 297 0.095 0.105 0.113 0.194 0.204 0.105 0.089 0.121 0.130 0.165 0.167 0.137 0.154 0.159 0.136 0.220 0.142 99.17 309 0.104 0.085 0.087 0.376 0.442 0.374 0.373 0.368 0.379 0.452 0.084 0.109 0.085 0.345 0.199 97.21 290 0.250 0.584 0.609 0.625 0.609 0.585 0.439 0.101 0.139 0.173 0.128 0.191 0.171 0.312 0.089 0.072 0.149 97.63 296 0.710 1.103 1.440 1.438 1.528 1.500 1.151 0.538 0.075 0.355 0.337 0.145 0.094 1.861 0.186 96.98 353 1.556 2.428 3.013 2.865 2.854 2.867 2.518 1.454 1.232 10.066 11.718 13.647 13.135 12.917 13.265 12.110 10.312 9.799 3.082 3.455 3.637 3.610 3.510 3.485 3.266 3.257 3.154 3.387 11.499 2.043 0.852 0.189 0.389 0.114 0.141 0.140 0.151 98.22 305 27.509 26.595 28.144 26.391 26.079 26.845 24.918 29.158 28.022 28.057 11.751 9.704 10.572 9.605 9.611 9.750 9.504 13.255 12.630 11.765 13.841 30.439 3.240 9.582 1.338 0.689 0.173 0.372 0.070 0.465 0.103 0.089 0.144 100.56 313 0.538 0.544 0.764 0.852 0.816 0.844 0.774 0.256 0.394 0.528 0.534 0.049 0.861 10.071 27.409 3.691 13.504 2.966 1.541 0.148 0.631 0.402 0.078 0.474 0.107 0.230 0.115 99.85 293 0.294 0.732 0.257 0.196 0.200 0.203 0.802 0.196 0.272 0.265 0.286 0.173 14.843 13.140 7.343 11.019 11.999 10.404 13.885 11.548 11.792 11.034 10.150 8.951 3.619 3.547 1.883 2.654 2.815 2.431 4.039 2.988 3.507 2.767 2.572 2.111 2.332 9.652 0.177 0.827 10.039 27.074 3.510 13.053 2.828 1.464 0.097 0.542 0.392 0.435 0.258 0.126 98.22 296 0.281 0.259 0.296 0.264 0.291 0.287 0.260 0.257 0.284 0.273 0.266 0.277 0.275 22.903 23.624 26.052 24.435 24.742 25.384 21.993 23.576 22.574 24.423 26.020 25.975 24.820 0.032 0.027 0.029 22.659 0.243 3.774 15.642 0.184 0.496 11.451 26.259 3.172 11.055 1.771 0.881 0.269 0.333 0.080 0.498 0.156 0.202 99.41 298 6 1 2 3 4 6 1 2 3 4 1 2 3 4 5 0.028 23.823 0.276 2.957 12.677 0.163 0.721 10.923 26.439 3.266 11.785 2.168 1.053 0.401 0.382 0.480 0.234 0.160 98.27 291 mz7 mz8 Grain mz4 Point no. 6 23.658 0.294 3.169 11.588 0.207 0.002 0.546 11.490 28.167 3.387 11.225 1.818 0.787 0.249 0.379 0.072 0.463 0.173 0.362 0.036 0.154 98.51 302 SO3 ThO2 Nd2O3 KATARZYNA LISOWIEC ET AL. Tm2O3 PbO ­ below detection limit 336 - s not included in the calculation Table 2. Electron microprobe analyses of twelve analyzed grains (wt%), continued. Sample mz10 5 24.432 0.274 2.688 10.255 0.374 0.300 13.889 30.336 3.326 9.940 1.062 0.481 0.186 0.351 0.108 0.106 0.043 0.147 98.46 309 297 316 98.55 97.37 97.31 284 0.132 0.147 0.127 0.045 0.011 0.090 96.89 300 0.086 0.107 0.101 0.307 0.070 0.143 97.77 304 0.120 0.096 0.147 0.182 0.128 0.055 0.092 0.076 0.323 0.298 0.311 0.402 0.342 0.347 0.085 0.132 0.093 0.075 0.129 98.32 300 0.109 0.100 0.110 0.177 0.195 0.572 0.232 0.234 0.352 0.165 0.091 0.133 96.58 324 0.128 0.201 0.286 0.474 0.570 1.441 0.681 0.788 0.573 0.727 0.282 0.342 0.129 0.088 0.132 97.51 312 1.064 1.252 1.401 2.857 2.087 2.141 1.969 2.216 9.608 9.417 10.275 12.698 14.031 14.742 13.729 14.610 3.283 3.202 3.357 3.556 3.660 3.750 3.609 3.771 3.682 14.031 2.131 0.665 0.240 0.349 0.117 0.066 0.125 97.32 306 31.053 30.225 30.071 28.668 27.029 27.752 27.794 26.907 27.641 14.234 13.377 13.019 10.715 10.674 10.908 11.090 10.522 11.148 11.399 27.390 3.237 11.471 1.947 0.720 0.351 0.379 0.135 0.162 0.022 0.158 96.45 318 0.338 0.427 0.449 0.703 0.438 0.454 0.405 0.499 0.471 0.503 0.019 0.031 0.235 11.980 28.162 3.224 10.419 1.522 0.422 0.226 0.353 0.139 0.046 0.172 97.67 294 0.391 0.389 0.300 0.209 0.322 0.302 0.252 0.309 0.258 0.148 0.215 9.514 9.990 9.909 6.680 10.271 9.463 9.127 9.197 9.030 11.484 13.364 2.466 2.785 2.624 1.915 2.821 2.674 2.556 2.521 2.319 2.872 3.246 3.488 12.942 0.535 2.088 8.111 22.859 3.344 12.885 3.452 1.975 0.212 0.944 0.493 0.196 0.110 0.121 0.051 0.182 98.15 295 0.274 0.235 0.259 0.297 0.282 0.318 0.279 0.313 0.304 0.279 0.257 0.264 25.038 24.425 23.779 25.152 24.359 23.850 24.032 24.694 24.431 23.453 23.455 23.731 0.028 0.026 0.027 0.026 24.437 0.281 2.704 11.982 0.196 0.517 10.192 27.734 3.508 12.376 2.222 0.902 0.349 0.380 0.116 0.125 0.026 0.160 98.47 305 6 1 2 3 4 1 2 3 5 6 1 2 3 4 mz1x mz2 5 22.190 0.318 3.672 14.976 0.241 0.311 11.156 26.944 3.307 10.547 1.590 0.567 0.980 0.198 0.345 0.078 0.131 0.093 0.062 0.195 97.12 298 AAR3 Grain mz9 Point no. 6 22.393 0.287 3.700 14.473 0.246 0.374 11.021 26.934 3.218 10.824 1.676 0.571 0.195 0.322 0.087 0.171 0.083 0.193 96.98 304 SO3 ThO2 Nd2O3 Tm2O3 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY PbO 317 ­ below detection limit 336 - s not included in the calculation 466 KATARZYNA LISOWIEC ET AL. on binary diagrams (Poitrasson et al. 2000; Harlov et al. 2002; Williams et al. 2011; Seydoux-Guillaume et al. 2012). Unfortunately no measurement spots were located in thin BSE-dark domains near the cracks, so the nature of element depletion is unknown. However, as they were not included in the dating, their potential hydrothermal origin did not affect the calculations. Taking into account both the compositional and textural characteristics of the grains, a magmatic origin is suggested for the entire population of monazites used for chemical dating. Places where fluid overprint is a possibility (cracks or lobate margins) either show a composition which is not significantly altered or were carefully avoided in the selection of the measurement spots. Monazite s The monazite chemical dating yielded an uniform of 299 ± 1.7 Ma (2) (Text-fig. 5ab) for the whole population. Single spot s range from 281 Ma to 318 Ma. No systematic difference between the s calculated for BSE-dark and BSE-bright zones of the entire monazite population can be observed. Both compositional domains demonstrate similar distribution (289­318 Ma for BSE-ark zones and 291­318 Ma for BSE-bright zones). In single grains the s calculated for BSE-dark zones are either younger or older from those calculated for BSE-bright, depending on the grain. Text-fig. 5. Results of calculation (with two abnormal s substracted); a ­ histogram of monazite Th-U-Pb s; b ­ Pb vs. Th* (wt%) isochron diagram, where Th* is Th + U equivalents expressed as Th. Isochrons are calculated from regression forced through zero as proposed by Montel et al. (1996) A systematic difference between BSE-dark and BSE-bright zones might suggest an involvement of a secondary process affecting the monazite chemistry and, consequently, the s obtained; however, no such feature is observed. In cases where BSE-dark zones might have originated from fluid infiltration (indicated by an irregular, lobate boundary of the margin), e.g. in grain -mz5, point 2 (Text-fig. 3g), the younger of 295 Ma may be considered to be a result of selective leaching of Pb by F-bearing fluids (Williams et al. 2011), but no definite interpretation can be proposed based on only one analysis. In the case of grain AR-mz9, where the upper margin is depleted in Th, U and Pb, the calculated for spots 4 and 5 is older than for the rest of the grain as well as the majority of measured points. Such case could be explained by fluid-aided removal of Th from the grain margin, but, as in the previous example, there is not sufficient evidence to confirm or exclude this suggestion. Selective leaching of Th, U or Pb by hydrothermal fluids may disturb the Th-U-Pb system and, therefore, yield an unrealistic or even ly reset the Th-U-Pb clock (Bosse et al. 2009; Williams et al. 2011; Seydoux-Guillaume et al. 2012). The domains which are texturally suspected of being altered by postmagmatic fluids include BSE-dark cracks and some lobate grain margins. However, as mentioned above, compositional evidence of fluid-mediated alteration is scarce. As no measurement spots were located in the BSE-dark zones along the cracks, the potential disruption of the Th-U-Pb system by these domains was avoided. In the case of the margins, single examples show younger s (e.g. -mz5, point 2). However, their number is insufficient to visibly disturb the calculated for the whole population. Looking at textural, compositional and geochronological data, it can be stated that the studied monazite formed during one magmatic episode in a slightly heterogeneous magma and was moderately affected by post-magmatic fluids. Such an overlap of processes was already documented by the accessory mineral study of the Stolpen granite (Lisowiec et al. 2013). The selection of spots eliminated the effect of fluid alteration and thecalculated can be treated as the magmatic of monazite crystallization. However meaningful the obtained is (textural evidence, high precision and geotectonic context point to its high reliability), it must be stressed that electron microprobe dating of monazite is not the most precise dating method (compared to SHRIMP or TIMS). The precision depends on the precision of the microprobe measurement itself and there is a number of analytical factors influencing the measurement error, such as counting statistics, background measurements, peak overlap corrections etc. (Pyle et al. 2005; Williams et al. 467 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY 2006). Therefore the calculated and its precision must be treated with caution. Another important issue that must be taken into account when using minerals for dating is the st at what the mineral appears during magmatic differentiation. Monazite usually starts to crystallize in the middle to late sts so that it records only exactly this time. As mentioned earlier, monazite occurs both as inclusions in either feldspar of fluorite and as large crystals in the rock matrix, which represent subsequent monazite generations. However, chemical dating was performed on heavy mineral separates, so the textural context of the studied monazite grains is lost. Nevertheless, it can be assumed that the separated monazite crystals (which are not intergrown with any other minerals) represent most probably the `matrix' population, which is more prone to be released during crushing and heavy liquid separation. Consequently, this population reflects most likely the beginning of monazite crystallization. Therefore the of 299 ± 1.7 Ma records the early sts of monazite formation. However, as monazite usually starts to crystallize in the middle to late sts of magma differentiation, the calculated must be considered as a minimum of the intrusion as granitoid plutons may form over wide time spans. Variscan magmatism of Saxo-Thuringian zone of the Bohemian Massif The monazite is the first obtained for the Stolpen magmatic body. It confirms the previous suggestion that the Stolpen granite is one of the late-Variscan intrusions in the Lusatian Granodiorite Complex (Hammer et al. 1999). The magmatic activity started most probably earlier than the obtained ; as it lasted until at least 299 ± 1.7 Ma, the granite may be regarded as one of the youngest plutons in the whole intrusive sequence within the Saxo-Thuringian and Moldanubian zones. This information is particularly important for the determination of the whole path of evolution of the magmatism during the convergence of Gondwana and Laurussia (Matte 1986; Ziegler 1986; Finger and Steyrer 1990; Matte et al. 1990; Dallmeyer et al. 1995; Franke 2000; Franke et al. 2005). The Lusatian Complex belongs to the mid-European segment of the Variscan orogenic belt. The belt,which resulted from continent-continent collision, shows the emplacement of many granitic bodies (Finger et al. 1997). The greatest magmatic activity took place during the Late Carboniferous and was related to transpressional-transtensional tectonics (Finger and Steyrer 1990; Diot et al. 1995; Mazur and Aleksandrowski 2001). The plutons located at the northern extreme of the Bohemian Massif were emplaced dur- ing this period. They are all composite bodies of mixed mantle-crust origin (Gerdes et al. 2000; Janousek et al. 2004, Finger et al. 1997; Slaby and Martin 2008). Within these plutons, the Stolpen granite seems to present the final st of a long lasting magmatism. In general, two sts of granite emplacement within the Saxo-Thuringian and Moldanubian zones can be distinguished. Förster and Romer (2010) concluded that igneous activity in the Saxo-Thuringian Zone, including the northern and northwestern part of the Bohemian Massif, occurred at 335­320 Ma and 305­280 Ma. Some of the plutons, e.g. the granitoid pluton of Karkonosze, formed over several My, with the oldest rocks from this intrusion dated at 319­320 Ma (U-Pb in zircon, Zák et al. 2013), and the youngest at 302 ± 4 Ma (U-Pb in zircon, Kusiak et al. 2014). Finger et al. (2009) and Siebel et al. (2003) studied the Moldanubian part of the Bohemian Massif and also distinguished two major intrusive events; one more voluminous between 328­320 Ma, and the second one, less voluminous, between 317­310 Ma. Moreover, Finger et al. (2009) suivided Variscan granitoid intrusions into five groups of granite belts characterized by slightly different s, geotectonic settings and magma generation mechanisms. The oldest are: "North Variscan Granite Belt", "Central Bohemian Granite Belt" and "Durbachitic Granites", with s of ca. 330 to 350 Ma, 360 to 335 Ma and 335 to 340 Ma, respectively. Intrusions with a younger (330 to 310 Ma) include the south-western sector of the Bohemian Massif, and the granites from the western Erzgebirge and Fichtelgebirge. According to Finger et al. (2009) they form a coherent plutonic belt ("Saxo-Danubian Granitic Belt"), formed most probably due to the delamination of lithospheric mantle (Bird 1979). The fifth group, involving the youngest granites located in the Sudetes, is called the "Sudetic Granite Belt" (including e.g., Karkonosze Massif, Strzegom-Sobotka Massif, Strzelin Massif and Klodzko-Zloty Stok Massif; Mazur et al. 2007) and is dated at ca. 315 to 300 Ma. Gerdes et al. (2003) reported a bimodal timing of magmatism in the South Bohemian Massif, with the first pulse at 331­323 Ma (with a higher mantle input) and the second, less significant, at 319­315 Ma. According to Siebel et al. (2010), one of the youngest magmatic impulses in the Bohemian Massif was the Fichtelgebirge intrusive complex, with U-Pb zircon s ranging from 291.2 ± 6.4 Ma to 298.5 ± 3.9 Ma for different types of granites comprising the intrusion. Late-Variscan granitoids from the Erzgebirge fall within the older group of intrusions (Romer et al. 2010), whereas the younger magmatic event is absent. The s of the amphibole-bearing granitoids 468 KATARZYNA LISOWIEC ET AL. from the Lusatian Granodiorite Complex (granitoids from Wiesa ­ 304 ± 10 Ma and Klienschweidnitz ­ 312 ± 10 Ma) place these intrusions within the youngest st of magmatic activity. The granite of Königshain was dated first by Hammer et al. (1999) at 315 ± 6 Ma using zircon-evaporation method and would also belong to the younger set of intrusions. However Th-U Pb dating of uraninite and molyenite by Förster et al. (2012) yielded older s of 328.6 ± 1.9 Ma for uraninite and 327 ± 1.3 Ma, 327.6 ± 1.3 Ma for molyenite, indicating that the magmatic processes in the LGC started approximately at the same time as the older igneous events in other parts of the SaxoThuringian Zone and the Bohemian Massif. The granitoid pluton of Stolpen, with monazite of 299 ± 1.7 Ma seems to be younger than its neighbour and belongs to the second impulse of magmatic activity in the Saxo-Thuringian Zone (Förster and Römer 2010). The difference between the Stolpen and Königshain intrusions, which cannot be fully estimated based on present data, is difficult to explain, especially as these two plutons are located in one geotectonic unit. It is possible that the studied samples were taken from the youngest part of the pluton, whereas the main body formed some million years before. Nevertheless the Stolpen granite, or at least part of it, is the youngest intrusion in the Lusatian Granodiorite Complex, indicating that the magmatic activity in this region lasted at least 5 my longer than previously estimated. According to previous studies of the biotite-bearing granitoid intrusions from the Lusatian Granodiorite Complex, magma generation mechanisms involved melting of the lower crust triggered by a mantle diapir (enriching the granitoid rocks in LILE and HFSE), accompanied by post-collisional extension (Hammer et al. 1999) in the case of the Stolpen granite, and crust melting in a compressional regime in case of the Königshain granite (Eidam et al. 1991). Amphibole-bearing granitoids (from Wiesa and Kleinschweidnitz) formed due to melting of metasomatized mafic lower crust (probably tholeiitic, Hammer et al. 1999). The melting of the lower crust, in the case of both biotite- and amphibolebearing granites, was induced probably by delamination processes, as proposed by Hammer et al. (1999). A similar scenario for the Late Carboniferous ­ Early Permian magmatism in Central Europe is also suggested by more recent studies of Finger et al. (2009), Slaby et al. (2010) and Turniak et al. (2014). Finger et al. (2009) proposed a delamination model for the formation of the Saxo-Danubian granitoids, which extend along the NE and SW margins of the Bohemian Massif. Late Variscan Lusatian granitoids (including Stolpen) may be considered as the most northerly part of this belt, but the younger of the Stolpen granite is not in accord with the older rocks formed south-west of the pluton (e.g. in the Erzgebirge). Studies of the Strzegom­Sobotka Massif (Turniak et al. 2014) belonging to the Sudetic Granitic Belt, have suggested a close relationship to post-Variscan bimodal volcanism. The heat required for melting of the lower crust was supplied by the ascent of mantle-derived basaltic magmas. The mechanisms possibly responsible for melting of the lithospheric mantle include decompression related to lithospheric extension/rifting and delamination and the convective removal of the thickened mantle. Perhaps similar mechanisms operated in the LGC, which is a western prolongation of the Sudetic Granitic Belt. An interesting comparison can be also made with the granitic rocks (dated at ~300 Ma) associated with the Kraków-Lubliniec Fault Zone (located to the East of the Variscides) which is a prolongation of the Elbe Line (near which the Stolpen granite is located). Slaby et al. (2010) proposed a two-st origin, involving: (1) transpressional regime accompanied by crustal thickening, delamination of the lithospheric mantle and mantle metasomatism , and (2) transtensional regime causing partial melting of upper metasomatized mantle and lower mafic crust. Therefore, it seems that similar processes may have caused granitoid formation along the Elbe Zone and its extension to the Kraków-Lubliniec Fault Zone. The of the Stolpen granite agrees with such an assumption. Magmatism in the Bohemian Massif is characterized by magmas derived from at least two sources: mantle and crust (Finger et al. 1997; Janousek et al. 2004; Gerdes et al. 2000; Siebel et al. 2003; Slaby and Martin 2008). It is noticeable that with progressive evolution of the magmatism, the contribution of the mantle source diminished and the peraluminosity of magmas increased. However mantle activity did not disappear entirely; it is present in a form of late mafic dykes. The Mantle source also contributed continuously with fluids, whose signature is discernible in the granite alterations products and granite pegmatites (e.g. Martin 2006), as is also seen in the case of the Stolpen granite (Lisowiec et al. 2013). The delamination scenario supports mantle-crust interactions, which may involve mixing between crust- and mantle-derived melts (as suggested for some granites from the Saxo-Danubian Granitic Belt, Finger et al. 2009) or can be limited to heat transfer and influx of mantle-derived fluids. The Stolpen granite fits the general features of magma evolution in the Bohemian Massif. Both the obtained monazite and magma affinity fit to the late st of Variscan magmatism outline. The peraluminous character of the Stolpen granite and the only slight contribution of mantle fluids (Hammer et al. 1999; Lisowiec 469 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY et al. 2013) suggest that at the end of the emplacement of Variscan granitoids the interaction between the mantle and the crust was limited, but noticeable. The tectonic setting of the Stolpen pluton near the Stolpen-Klotzsche Fault indicates that the mechanism of emplacement along older shear zones was similar to those of other granites from the LGC, as e.g., the Königshain granite (Förster et al. 2012). acknowledge Daniel Harlov for his helpful discussion of the results. Igor Broska and Ray MacDonald, are thanked for insightful reviews, and Ray MacDonald also for linguistic corrections of the final version. This study was funded by a NCN grant 2011/01/N/ST10/04756. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Acta Geologica Polonica de Gruyter

U-total Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany

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de Gruyter
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ISSN
2300-1887
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2300-1887
DOI
10.2478/agp-2014-0024
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Abstract

Lisowiec, K., Budzy, B., Slaby, E., Schulz, B., and Renno, A.D. 2014. Th-U- Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany. Acta Geologica Polonica, 64 (4), 457­472. Warszawa. Monazite from the Stolpen monzogranite (SE Germany) was studied to constrain the Th-U- Pb of pluton formation. Monazite grains demonstrate subtle to distinct patchy zoning related to slight compositional variations. Textural and compositional characteristics indicate that the monazite formed in a single magmatic event in a slightly heterogeneous system, and was only weakly affected by secondary alteration, which did not disturb the Th-U-Pb system. Chemical dating of the monazite gave a consistent of 299 ± 1.7 Ma. The current study presents the first geochronological data for the Stolpen granite. It provides evidence that Stolpen is the youngest Variscan granitic intrusion in the Lusatian Granodiorite Complex and indicates that magmatic activity related to post-collisional extension in this region lasted at least 5my longer than previously assumed. Key words: Monazite, Th-U-Pb chemical dating; Lusatian Granodiorite Complex; Stolpen Granite; Variscan granitoids. INTRODUCTION The convergence of Gondwana and Laurassia during the Paleozoic, including suuction and continental collision, produced a wide variety of magmatic and metamorphic rocks from the Bohemian Massif in the east to the Massif Central in the west. Due to intensive heating and melting of the crust and/or the mantle, caused by burial or decompression during late-orogenic extension, many granitic intrusions formed, often deriving their melts from heterogeneous sources (e.g. Finger et al. 1997; Finger et al. 2009; Siebel et al. 2003; Förster and Romer 2010). The granitoid bodies are most abundant in the Moldanubian Zone of the orogenic belt 458 KATARZYNA LISOWIEC ET AL. (the main part of the Bohemian Massif) and less abundant in the Saxo-Thuringian and Teplá-Barrandian zones (e.g. Finger et al. 1997; Oberc-Dziedzic et al. 2013). They differ in petrography, geochemistry and geochronology (Finger et al. 1997); therefore a careful study of all the types is crucial to understanding their evolution and global mantle-crust interactions related to orogenic and post-orogenic movements. The s of the Variscan granitoids have been constrained in numerous papers using various methods including the Single Zircon Evaporation method (e.g. Kröner et al. 1994; Siebel et al. 2003), whole rock RbSr (e.g. Kröner et al. 1994; Finger et al. 1997), the UPb method in zircon and monazite (e.g. Gerdes et al. 2003; Klein et al. 2008; Finger et al. 1997; ObercDziedzic et al. 2013; Kryza et al. 2012) and Th-U- Pb of uraninite and Re-Os of molyenite (Förster et al. 2012). All these methods are not interchangeable with each other and may document slightly different sts of the magmatic/metamorphic events. Fluid overprint further complicates the use of some of them, e.g. U-Pb s of zircon or monazite due to remobilization of Pb. One of the most rapid and widely used methods of determination is Th-U- Pb dating of monazite. Monazite is a LREE-rich phosphate [(REE,Th,U)PO4] which incorporates significant amounts of Ce, La, Sm and Nd, as well as other elements such as Y, Th and U. Thelast two are particularly important in terms of using monazite for Th-U-Pb dating. Because monazite occurs in various types of magmatic, metamorphic and sedimentary rocks, it can be used to constrain the timing of geological processes such as magma crystallization and metamorphism or to define the of protholith(s) (Williams et al. 2007). Diffusion of major and trace elements in monazite is very slow (Cherniak and Pyle 2008; Cherniak et al. 2004a; Cherniak et al. 2004b; Parrish 1990), therefore it can preserve compositional zoning which records different sts of crystallization or metamorphic deformation. Due to the fact that monazite contains negligible amounts of common Pb relative to radiogenic Pb (Parrish 1990), it is possible to use the chemical Th-U- Pb method employing an electron microprobe to constrain its (Jercinovic and Williams 2005; Jercinovic et al. 2008; Konecný 2004; Montel et al. 1996; Pyle et al. 2005; Spear et al. 2009; Suzuki and Adachi 1991, 1994; Suzuki and Kato 2008). Although chemical dating of monazite is mostly used in metamorphic petrology (Finger and Krenn 2007; Kohn et al. 2005; Liu et al. 2007; Rosa-Costa et al. 2008; Tickyj et al. 2004; Williams et al. 2007), it has also found applications in constraining the s of magmatic events with high precision (Just et al. 2011). The resistance of monazite to complete alteration and its ability to preserve its growth textures provide an opportunity to reach deep into the magmatic history. One region of the Variscan Orogenic Belt where granitic intrusions are rather scarce is the Lusatian Granodiorite Complex (LGC), located in the eastern part of the Saxo-Thuringian Zone. It experienced only minor metamorphism and deformation during the Variscan orogeny (Kröner et al. 1994). It contains only several late-Variscan granitoid bodies, most of which have been studied in terms of geochemistry and geochronology (Kröner et al. 1994; Hammer et al. 1999; Förster et al. 2012). However there is one pluton,­ the Stolpen granitoid,, which cannot be precisely situated within the intrusion sequence of the Lusatian Block due to a lack of geochronological data. This study reports monazite UTh-Pb timing constraints on the formation of the Stolpen pluton.. The analyzed monazite formed mostly at the magmatic sts of pluton formation; however, as infiltration by post-magmatic fluids has been already documented (Lisowiec et al. 2013), the samples were carefully studied to minimize the influence of fluid-alteration on the calculated s. GEOLOGICAL SETTING The granitoid pluton of Stolpen is located in the southern part of the Lusatian Granodiorite Complex which comprises the central part of the Lusatian Anticlinal Zone at the NE margin of the Bohemian Massif (Text-fig. 1). The pluton is slightly SE-NW elongated, which is the main direction of shearing during the Variscan orogenesis (Krentz in Kozdrój et al. 2001). Magma emplacement used tectonic faults that were formed during and after orogenic movements. The pluton intruded Cadomian to Early-Palaeozoic (600­490 Ma) magmatic ­ tonalitic to syenogranitic in composition, locally metamorphosed rocks. The envelope of the complex consists of upper-Proterozoic sedimentary rocks, mostly greywackes and pelites. The Stolpen granite belongs to the group of late- to post-Variscan intrusions in the Lusatian Granodiorite Complex which contains also the amphibole granites of Wiesa and Grossschweidnitz and the biotite granite of Königshain-Arnsdorf, with s constrained by zircon-evaporation method at 304 ± 10 Ma, 312 ± 10 Ma and Th-U- Pb dating of uraninite and molyenite at 327­328 Ma, respectively (Kröner et al. 1994; Förster et al. 2012). Knowledge of the petrogenesis of the Stolpen granite is very limited; however Hammer et al. (1999) sug- 459 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY Text-fig. 1. Sketch of the study area (after Kozdrój et al. 2001) with sampling locations. AR1, AR3 ­ 51°0'58.77" N, 14°7'27.71" E gest that it originated from a crustal magma. The melting process was induced by an upwelling mantle diapir preceded by a fluid front. The fluids were responsible for crustal magma enrichment in LILE and HFSE. An accessory mineral study was consistent with such an hypothesis but did not exclude other possibilities (Lisowiec et al. 2013). Hammer et al. (1999) place the pluton among other Variscan granitic intrusions but do not give an exact . It is therefore unknown where exactly it is positioned among other Variscan intrusions in the region. The pluton consists mostly of medium- to coarsegrained monzogranite of peraluminous character (Hammer et al. 1999). Whole-rock geochemistry was documented by Hammer et al. (1999) and is presented in Table 1. The authors also report an aver Th/U ratio of 4.4. Granite samples used in this study were taken from the SW part of the magmatic body, which consists of a quite homogenous, medium-grained facies. The mineralogy of the granite is quite typical, the main assembl containing quartz, alkali-feldspar, plagioclase, biotite and small amounts of muscovite. Quartz forms two populations: older large crystals and younger small crystals occurring as inclusions in other minerals or located interstitially. Alkali-feldspar is Krich with a subordinate Na-rich (anorthoclase) component, whilst the plagioclase composition is almost pure albite, rarely oligoclase (Ab<20%). The pure albitic composition may suggest secondary post-magmatic crystal-fluid interaction. Plagioclase often exhibits weak zonation. Alkali-feldspar shows a strong perthitization and is often replaced by plagioclase (albite) on the margins, which again may be related to reaction with fluids. Similarly the other phases show pristine magmatic compositions affected by interaction with fluids. Biotite underwent almost chloritization and its content reaches ~43 wt%. Rarely its margins are replaced by muscovite. Accessory minerals present in the granite are fluorite, zircon, monazite (with a high contribution from a cheralite component), titanite, allanite, apatite, xenotime, Y-rich silicates and Y-Ti-phases, Th-rich minerals (oxides and silicates), Nb-Ta minerals (mostly columbite), xides and secondary REE-carbonates. Fluorite is the most abundant accessory mineral and forms three populations: homogenous, more or less regularly zoned and patchy. Individual populations show no specific textural positions. Y-rich minerals are represented Element/ Content Element Content Element Content oxide TiO2 Fe2O3 MnO MgO Na2O K2 O P2 O5 Ba Co Cr Cs Cu Ga Hf Li Nb Ni Pb Rb Sc Sr Ta Th U V Y Zn Zr La Ce Nd Sm Eu Tb Yb Lu Table 1. Whole-rock chemical composition of the Stolpen monzogranite (from Hammer et al. 1999). Element oxides are given in wt [%], trace elements in [ppm]. 460 KATARZYNA LISOWIEC ET AL. mostly by strongly zoned hingganite­(Y) and aeschynite­(Y) (Lisowiec et al. 2013). Zircon, monazite and xenotime sometimes form intergrowths. The accessory mineral assembl (mostly zircon and monazite) and the evidence of magmatic and post-magmatic processes that it carries has been studied by Lisowiec et al. (2013). Some parts throughout the pluton have more aplitic or pegmatitic character with nearly the same mineral composition as the granite. In the area we can find also numerous andesitic (and one rhyolitic) dykes which are situated in the vicinity of the pluton or intruded within the granite. RESULTS AND DISCUSSION Textures and chemical composition of monazite Monazite is quite abundant in the accessory mineral assembl of the Stolpen granite (Lisowiec et al. 2013). Generally it forms sub- to anhedral 10­20 m inclusions in fluorite and K-feldspar. It often occurs also in the rock matrix as subhedral crystals up to 300 m in size. The whole population of monazite grains represents a wide spectrum of growth textures, from nearly homogenous to irregularly zoned, spongy and strongly dissolved (Text-fig. 2), evidence of fluid overprint (Lisowiec et al. 2013). The penetrating fluids were enriched in fluorine, Ca, Y and CO2, based on the high abundance of secondary fluorite and Y-rich silicates. Such a fluid composition enabled the remobilization of trace elements from the monazite grains which were later incorporated into secondary accessory phases. Alterations in monazite include mostly enhanced huttonite and cheralite substitutions. The monazite crystals forming inclusions in fluorite are partly corroded at the contact with the host mineral. Small monazite grains occasionally overgrow zircon margins. Because of the alteration, careful selection of the grains and evaluation of their chemistry had to be undertaken prior to any chemical dating analysis. Twelve monazite grains, which represent the most `pure' monazite end-member, were selected for determination (Text-fig. 3). The monazite grains show very subtle (Text-fig. 3abe) to distinctly patchy zoning (Text-fig. 3dgh). Dark patches in BSE imaging are often located along rims (Text-fig. 3cdghjl). Locations of the spot measurements were chosen carefully to avoid any contribution of potentially fluid-altered domains; therefore, ANALYTICAL METHODS Granite samples were initially crushed in a jaw crusher, than fragmented using a Selfrag high volt pulse power fragmentation. Afterwards, the two smallest fractions of 500­250 m and 80­250 m were used for separation in heavy liquids. The mineral separates were mounted in epoxy and polished. Backscattered electron (BSE) ims were made using a Quanta 600 FEG-MLA600F field emission scanning electron microscope (SEM) equipped with two energy dispersive spectrometers (EDS) at the Institute of Mineralogy, TU Bergakademie Freiberg, Germany. The analytical conditions were as follows: accelerating volt 20 kV, with some exceptions when 15, 25 or 30 kV were used, and a 200 A beam current with the beam focused on the sample coated with carbon. Analyses of Th, U, Pb for the calculation of monazite s, as well as Y, REE, Ca, Si, P, Sr, Al and As for corrections and evaluation of the mineral chemistry, were carried out using a Cameca SX-100 electron microprobe at the Department of Electron Microanalysis in the State Geological Institute of Dionýz Stúr in Bratislava. The analytical methods for determination followed procedures presented in Petrik and Konecný (2009). To obtain the optimum c/s/nA (counts per seconds divided by sample current) and to minimize surface dam the following analytical conditions were used: accelerating volt 15 KV, sample current 180 nA, counting times: Pb of 300 s, Th 35 s, U 80 s, Y 40 s, REE 10­50 s, except Lu 100 s, P, S, Al, Si and Ca 10 s, Sr 20 s, As 120 s. Calibrations were performed using synthetic and natural standards: REE and Y were taken from phosphates XPO4, Th from ThO2, Pb from PbCO3, U from , Ca and Si from wollastonite, As from GaAs, S from barite and Al from .The resulting s were calculated using the statistical approach of Montel et al. (1996). Text-fig. 2. T Representative BSE ims of monazite grains and their textures; a ­ monazite grain with the most homogenous texture showing only very subtle patchy zoning; b, c ­ monazite grains with more distinct patchy zoning; d ­ monazite grain with a very strongly resorbed texture 461 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY Text-fig. 3. BSE ims of twelve analyzed grains with measured points and calculated s; a, b, e, f and i ­ grains with subtle patchy zoning and slight variations in chemical composition; c, d, g, h, j, k and l ­ grains with more distinct patchy zoning and more significant variations in chemical composition; in grains c, d, g, h, j, k, l BSE-dark zones are located on the margins or along cracks where it was possible, at a safe distance from margins and cracks. However, both types of zones, BSE-bright and BSE-dark, were investigated in order to assess the potential difference in chemical composition and characteristics, which in some cases meant analyzing small patches close to margins or crevices. The chemical composition of the entire monazite population shows their affinity to the monazite-huttonite series, with the main substitution mechanism: Si4+ + Th4+ (U4+)= REE3+ + P5+ (Text-fig. 4ab). Grains with subtle patchy zoning show only slight differences in element concentrations (-mz1x, mz1 and mz3 in Table 2). Th, U and Pb contents in a single grain vary in the ranges 3.00 wt.%, 0.30 wt.% and 0.05 wt.%, respectively. Monazite grains showing more distinct zoning demonstrate stronger variations in composition, mostly in Th, U,Pb, Y and La. The highest chemical gradient can be observed in grain -mz9 where the BSE-dark rim is strongly depleted in Th and Pb (spot 4 and 5); and AAR3-mz2, where the BSE-bright patch 462 KATARZYNA LISOWIEC ET AL. Text-fig. 4. Chemical composition plots of monazite grains; a, b ­ plots showing the main substitution mechanism in the structure of monazite : Si4+ + Th4+ (U4+)= REE3+ + P5+; c ­ LREE vs. Th plot showing decreasing LREE content with increasing Th abundance; d ­ HREE vs. Th plot showing no correlation between these elements (spot 3) is highly enriched in Y, U and HREE, and depleted in LREE. BSE-dark parts of the grains are usually depleted in Th, U and Pb, interpreted as related to decreasing availability of Th and U during monazite growth. Depletion in these elements is coupled with enrichment in LREE. There is no correlation between Th and heavier lanthanides or yttrium (Text-fig. 4cd). The growth textures along with the chemical composition of monazite grains were studied carefully in terms of primary vs. secondary origin to ensure the quality of the data. Grains with very subtle patchy zoning and a low chemical gradient are undoubtedly of primary magmatic origin. In cases where the BSE intensity shows more distinct differences between the zones, the possibility of BSE-dark patches (depleted in Th, U and Pb) being altered by secondary hydrothermal processes has to be taken into account. These examples include mainly grains mz2, mz5, mz7 and mz10,where the patchy character is slightly more pronounced. Dark patches are usually associated with crystal margins or cracks. Several measurement points are located on the BSE-dark patches and margins; however their composition does not reveal any significant post-magmatic fluid overprint. These domains are indeed depleted in Th, U and Pb, but the degree of depletion is comparable for all three elements. Such a feature is not likely to take place during fluid alteration which usually results in preferential depletion (or enrichment) in one or two of these components, most usually only Pb (e.g. Williams et al. 2011; Harlov et al. 2011). The compositional variations, especially the Th-, Pb- and U-contents, can be therefore attributed most probably to fluctuations in melt composition during crystal growth. Binary plots carry further evidence of the negligible contribution of fluid overprint. Th vs. Si diagrams (Text-fig. 4a) are well correlated and almost all points lay within the thin correlation line. Furthermore, points representing both BSE-bright and BSE-dark domains form the same trend on the plots (Text-fig. 4abc). The only distinction of the BSE-bright zones relative to BSE-dark ones is the enrichment in light- and especially heavyrare earth elements, and Th, U and Pb. Numerous studies have shown that zones affected by post-magmatic fluid alteration display a distinct chemical pattern, distinguishable from the domains formed at the magmatic st and therefore allowing a straightforward location Table 2. Electron microprobe analyses of twelve analyzed grains (wt%). Sample mz1a 6 0.030 0.291 3.272 8.902 0.252 0.224 0.318 0.211 0.427 0.257 0.239 0.507 0.878 9.803 3.327 9.603 1.100 0.095 0.326 0.173 0.384 0.172 0.050 0.148 0.160 99.09 289 336 0.186 99.25 317 0.080 0.196 98.99 296 0.082 0.132 0.111 0.128 0.136 98.95 303 0.335 0.085 0.154 0.116 0.042 0.160 99.35 308 0.107 0.335 0.149 0.093 0.039 0.119 99.42 298 2.773 1.457 0.111 0.610 0.119 0.420 0.168 0.073 0.169 98.28 285 294 299 298 98.96 97.89 98.44 0.201 0.197 0.178 0.175 97.92 0.089 1.561 0.130 0.053 0.245 0.084 0.213 0.132 0.131 0.141 0.133 0.108 0.203 0.138 0.144 0.129 0.109 0.091 0.099 0.394 0.362 0.371 0.331 0.451 0.332 0.406 0.364 0.233 0.762 0.202 0.517 0.209 0.517 0.521 0.198 0.177 0.097 0.113 1.142 0.817 1.831 0.710 1.152 0.503 1.231 1.342 0.500 2.432 2.031 3.405 1.761 2.361 1.361 2.495 2.676 1.184 3.354 3.371 3.390 3.445 3.436 3.205 3.445 3.507 3.314 3.281 3.212 1.183 0.501 0.118 0.351 0.075 0.116 0.103 0.036 0.183 99.69 318 9.270 10.424 8.405 10.760 10.492 12.436 9.599 0.703 0.281 1.384 0.514 0.807 0.287 0.524 0.787 0.341 0.277 0.298 0.333 0.297 0.249 0.338 0.635 0.245 0.411 0.213 3.501 3.632 3.291 3.350 2.808 3.435 3.670 2.348 2.896 2.276 3.088 3.472 3.222 0.274 0.307 0.281 0.284 0.306 0.193 0.285 0.239 0.261 0.260 0.290 0.266 0.286 0.294 3.287 0.234 0.220 0.030 0.026 0.031 0.033 0.035 0.235 3.137 0.333 0.440 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 0.310 3.377 0.266 0.238 mz2 mz3 mz1 6 0.317 3.162 0.247 0.729 9.623 Grain Point no. SO3 ThO2 10.435 13.460 15.293 13.284 11.965 12.281 12.438 12.381 12.112 3.339 1.392 0.489 0.136 0.377 0.133 0.041 0.183 99.76 293 3.232 1.465 0.075 0.541 0.185 0.297 0.074 0.133 0.061 0.176 99.20 304 3.298 1.426 0.453 0.162 0.342 0.074 0.144 0.041 0.180 99.20 303 3.344 1.556 0.653 0.213 0.362 0.093 0.125 0.112 0.173 99.54 303 3.266 1.429 0.478 0.120 0.386 0.138 0.099 0.041 0.179 98.98 292 27.775 27.740 28.533 28.705 28.409 25.462 25.459 26.253 24.173 27.653 27.096 28.729 26.041 27.323 30.025 31.070 29.433 28.617 28.594 28.624 28.173 28.562 26.291 3.429 10.032 10.654 10.695 10.555 11.086 10.488 12.453 2.517 1.157 0.132 0.452 0.361 0.155 0.096 0.190 0.177 99.16 301 Nd2O3 Tm2O3 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY PbO ­ below detection limit 336 - s not included in the calculation Table 2. Electron microprobe analyses of twelve analyzed grains (wt%), continued. Sample mz5 5 24.362 0.231 2.945 11.232 0.457 0.262 14.370 29.939 3.219 9.487 0.962 0.321 0.123 0.290 0.096 0.116 0.120 0.047 0.154 98.92 293 290 297 295 98.59 98.72 98.74 0.191 0.192 0.099 0.144 98.39 297 0.156 99.28 297 0.095 0.105 0.113 0.194 0.204 0.105 0.089 0.121 0.130 0.165 0.167 0.137 0.154 0.159 0.136 0.220 0.142 99.17 309 0.104 0.085 0.087 0.376 0.442 0.374 0.373 0.368 0.379 0.452 0.084 0.109 0.085 0.345 0.199 97.21 290 0.250 0.584 0.609 0.625 0.609 0.585 0.439 0.101 0.139 0.173 0.128 0.191 0.171 0.312 0.089 0.072 0.149 97.63 296 0.710 1.103 1.440 1.438 1.528 1.500 1.151 0.538 0.075 0.355 0.337 0.145 0.094 1.861 0.186 96.98 353 1.556 2.428 3.013 2.865 2.854 2.867 2.518 1.454 1.232 10.066 11.718 13.647 13.135 12.917 13.265 12.110 10.312 9.799 3.082 3.455 3.637 3.610 3.510 3.485 3.266 3.257 3.154 3.387 11.499 2.043 0.852 0.189 0.389 0.114 0.141 0.140 0.151 98.22 305 27.509 26.595 28.144 26.391 26.079 26.845 24.918 29.158 28.022 28.057 11.751 9.704 10.572 9.605 9.611 9.750 9.504 13.255 12.630 11.765 13.841 30.439 3.240 9.582 1.338 0.689 0.173 0.372 0.070 0.465 0.103 0.089 0.144 100.56 313 0.538 0.544 0.764 0.852 0.816 0.844 0.774 0.256 0.394 0.528 0.534 0.049 0.861 10.071 27.409 3.691 13.504 2.966 1.541 0.148 0.631 0.402 0.078 0.474 0.107 0.230 0.115 99.85 293 0.294 0.732 0.257 0.196 0.200 0.203 0.802 0.196 0.272 0.265 0.286 0.173 14.843 13.140 7.343 11.019 11.999 10.404 13.885 11.548 11.792 11.034 10.150 8.951 3.619 3.547 1.883 2.654 2.815 2.431 4.039 2.988 3.507 2.767 2.572 2.111 2.332 9.652 0.177 0.827 10.039 27.074 3.510 13.053 2.828 1.464 0.097 0.542 0.392 0.435 0.258 0.126 98.22 296 0.281 0.259 0.296 0.264 0.291 0.287 0.260 0.257 0.284 0.273 0.266 0.277 0.275 22.903 23.624 26.052 24.435 24.742 25.384 21.993 23.576 22.574 24.423 26.020 25.975 24.820 0.032 0.027 0.029 22.659 0.243 3.774 15.642 0.184 0.496 11.451 26.259 3.172 11.055 1.771 0.881 0.269 0.333 0.080 0.498 0.156 0.202 99.41 298 6 1 2 3 4 6 1 2 3 4 1 2 3 4 5 0.028 23.823 0.276 2.957 12.677 0.163 0.721 10.923 26.439 3.266 11.785 2.168 1.053 0.401 0.382 0.480 0.234 0.160 98.27 291 mz7 mz8 Grain mz4 Point no. 6 23.658 0.294 3.169 11.588 0.207 0.002 0.546 11.490 28.167 3.387 11.225 1.818 0.787 0.249 0.379 0.072 0.463 0.173 0.362 0.036 0.154 98.51 302 SO3 ThO2 Nd2O3 KATARZYNA LISOWIEC ET AL. Tm2O3 PbO ­ below detection limit 336 - s not included in the calculation Table 2. Electron microprobe analyses of twelve analyzed grains (wt%), continued. Sample mz10 5 24.432 0.274 2.688 10.255 0.374 0.300 13.889 30.336 3.326 9.940 1.062 0.481 0.186 0.351 0.108 0.106 0.043 0.147 98.46 309 297 316 98.55 97.37 97.31 284 0.132 0.147 0.127 0.045 0.011 0.090 96.89 300 0.086 0.107 0.101 0.307 0.070 0.143 97.77 304 0.120 0.096 0.147 0.182 0.128 0.055 0.092 0.076 0.323 0.298 0.311 0.402 0.342 0.347 0.085 0.132 0.093 0.075 0.129 98.32 300 0.109 0.100 0.110 0.177 0.195 0.572 0.232 0.234 0.352 0.165 0.091 0.133 96.58 324 0.128 0.201 0.286 0.474 0.570 1.441 0.681 0.788 0.573 0.727 0.282 0.342 0.129 0.088 0.132 97.51 312 1.064 1.252 1.401 2.857 2.087 2.141 1.969 2.216 9.608 9.417 10.275 12.698 14.031 14.742 13.729 14.610 3.283 3.202 3.357 3.556 3.660 3.750 3.609 3.771 3.682 14.031 2.131 0.665 0.240 0.349 0.117 0.066 0.125 97.32 306 31.053 30.225 30.071 28.668 27.029 27.752 27.794 26.907 27.641 14.234 13.377 13.019 10.715 10.674 10.908 11.090 10.522 11.148 11.399 27.390 3.237 11.471 1.947 0.720 0.351 0.379 0.135 0.162 0.022 0.158 96.45 318 0.338 0.427 0.449 0.703 0.438 0.454 0.405 0.499 0.471 0.503 0.019 0.031 0.235 11.980 28.162 3.224 10.419 1.522 0.422 0.226 0.353 0.139 0.046 0.172 97.67 294 0.391 0.389 0.300 0.209 0.322 0.302 0.252 0.309 0.258 0.148 0.215 9.514 9.990 9.909 6.680 10.271 9.463 9.127 9.197 9.030 11.484 13.364 2.466 2.785 2.624 1.915 2.821 2.674 2.556 2.521 2.319 2.872 3.246 3.488 12.942 0.535 2.088 8.111 22.859 3.344 12.885 3.452 1.975 0.212 0.944 0.493 0.196 0.110 0.121 0.051 0.182 98.15 295 0.274 0.235 0.259 0.297 0.282 0.318 0.279 0.313 0.304 0.279 0.257 0.264 25.038 24.425 23.779 25.152 24.359 23.850 24.032 24.694 24.431 23.453 23.455 23.731 0.028 0.026 0.027 0.026 24.437 0.281 2.704 11.982 0.196 0.517 10.192 27.734 3.508 12.376 2.222 0.902 0.349 0.380 0.116 0.125 0.026 0.160 98.47 305 6 1 2 3 4 1 2 3 5 6 1 2 3 4 mz1x mz2 5 22.190 0.318 3.672 14.976 0.241 0.311 11.156 26.944 3.307 10.547 1.590 0.567 0.980 0.198 0.345 0.078 0.131 0.093 0.062 0.195 97.12 298 AAR3 Grain mz9 Point no. 6 22.393 0.287 3.700 14.473 0.246 0.374 11.021 26.934 3.218 10.824 1.676 0.571 0.195 0.322 0.087 0.171 0.083 0.193 96.98 304 SO3 ThO2 Nd2O3 Tm2O3 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY PbO 317 ­ below detection limit 336 - s not included in the calculation 466 KATARZYNA LISOWIEC ET AL. on binary diagrams (Poitrasson et al. 2000; Harlov et al. 2002; Williams et al. 2011; Seydoux-Guillaume et al. 2012). Unfortunately no measurement spots were located in thin BSE-dark domains near the cracks, so the nature of element depletion is unknown. However, as they were not included in the dating, their potential hydrothermal origin did not affect the calculations. Taking into account both the compositional and textural characteristics of the grains, a magmatic origin is suggested for the entire population of monazites used for chemical dating. Places where fluid overprint is a possibility (cracks or lobate margins) either show a composition which is not significantly altered or were carefully avoided in the selection of the measurement spots. Monazite s The monazite chemical dating yielded an uniform of 299 ± 1.7 Ma (2) (Text-fig. 5ab) for the whole population. Single spot s range from 281 Ma to 318 Ma. No systematic difference between the s calculated for BSE-dark and BSE-bright zones of the entire monazite population can be observed. Both compositional domains demonstrate similar distribution (289­318 Ma for BSE-ark zones and 291­318 Ma for BSE-bright zones). In single grains the s calculated for BSE-dark zones are either younger or older from those calculated for BSE-bright, depending on the grain. Text-fig. 5. Results of calculation (with two abnormal s substracted); a ­ histogram of monazite Th-U-Pb s; b ­ Pb vs. Th* (wt%) isochron diagram, where Th* is Th + U equivalents expressed as Th. Isochrons are calculated from regression forced through zero as proposed by Montel et al. (1996) A systematic difference between BSE-dark and BSE-bright zones might suggest an involvement of a secondary process affecting the monazite chemistry and, consequently, the s obtained; however, no such feature is observed. In cases where BSE-dark zones might have originated from fluid infiltration (indicated by an irregular, lobate boundary of the margin), e.g. in grain -mz5, point 2 (Text-fig. 3g), the younger of 295 Ma may be considered to be a result of selective leaching of Pb by F-bearing fluids (Williams et al. 2011), but no definite interpretation can be proposed based on only one analysis. In the case of grain AR-mz9, where the upper margin is depleted in Th, U and Pb, the calculated for spots 4 and 5 is older than for the rest of the grain as well as the majority of measured points. Such case could be explained by fluid-aided removal of Th from the grain margin, but, as in the previous example, there is not sufficient evidence to confirm or exclude this suggestion. Selective leaching of Th, U or Pb by hydrothermal fluids may disturb the Th-U-Pb system and, therefore, yield an unrealistic or even ly reset the Th-U-Pb clock (Bosse et al. 2009; Williams et al. 2011; Seydoux-Guillaume et al. 2012). The domains which are texturally suspected of being altered by postmagmatic fluids include BSE-dark cracks and some lobate grain margins. However, as mentioned above, compositional evidence of fluid-mediated alteration is scarce. As no measurement spots were located in the BSE-dark zones along the cracks, the potential disruption of the Th-U-Pb system by these domains was avoided. In the case of the margins, single examples show younger s (e.g. -mz5, point 2). However, their number is insufficient to visibly disturb the calculated for the whole population. Looking at textural, compositional and geochronological data, it can be stated that the studied monazite formed during one magmatic episode in a slightly heterogeneous magma and was moderately affected by post-magmatic fluids. Such an overlap of processes was already documented by the accessory mineral study of the Stolpen granite (Lisowiec et al. 2013). The selection of spots eliminated the effect of fluid alteration and thecalculated can be treated as the magmatic of monazite crystallization. However meaningful the obtained is (textural evidence, high precision and geotectonic context point to its high reliability), it must be stressed that electron microprobe dating of monazite is not the most precise dating method (compared to SHRIMP or TIMS). The precision depends on the precision of the microprobe measurement itself and there is a number of analytical factors influencing the measurement error, such as counting statistics, background measurements, peak overlap corrections etc. (Pyle et al. 2005; Williams et al. 467 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY 2006). Therefore the calculated and its precision must be treated with caution. Another important issue that must be taken into account when using minerals for dating is the st at what the mineral appears during magmatic differentiation. Monazite usually starts to crystallize in the middle to late sts so that it records only exactly this time. As mentioned earlier, monazite occurs both as inclusions in either feldspar of fluorite and as large crystals in the rock matrix, which represent subsequent monazite generations. However, chemical dating was performed on heavy mineral separates, so the textural context of the studied monazite grains is lost. Nevertheless, it can be assumed that the separated monazite crystals (which are not intergrown with any other minerals) represent most probably the `matrix' population, which is more prone to be released during crushing and heavy liquid separation. Consequently, this population reflects most likely the beginning of monazite crystallization. Therefore the of 299 ± 1.7 Ma records the early sts of monazite formation. However, as monazite usually starts to crystallize in the middle to late sts of magma differentiation, the calculated must be considered as a minimum of the intrusion as granitoid plutons may form over wide time spans. Variscan magmatism of Saxo-Thuringian zone of the Bohemian Massif The monazite is the first obtained for the Stolpen magmatic body. It confirms the previous suggestion that the Stolpen granite is one of the late-Variscan intrusions in the Lusatian Granodiorite Complex (Hammer et al. 1999). The magmatic activity started most probably earlier than the obtained ; as it lasted until at least 299 ± 1.7 Ma, the granite may be regarded as one of the youngest plutons in the whole intrusive sequence within the Saxo-Thuringian and Moldanubian zones. This information is particularly important for the determination of the whole path of evolution of the magmatism during the convergence of Gondwana and Laurussia (Matte 1986; Ziegler 1986; Finger and Steyrer 1990; Matte et al. 1990; Dallmeyer et al. 1995; Franke 2000; Franke et al. 2005). The Lusatian Complex belongs to the mid-European segment of the Variscan orogenic belt. The belt,which resulted from continent-continent collision, shows the emplacement of many granitic bodies (Finger et al. 1997). The greatest magmatic activity took place during the Late Carboniferous and was related to transpressional-transtensional tectonics (Finger and Steyrer 1990; Diot et al. 1995; Mazur and Aleksandrowski 2001). The plutons located at the northern extreme of the Bohemian Massif were emplaced dur- ing this period. They are all composite bodies of mixed mantle-crust origin (Gerdes et al. 2000; Janousek et al. 2004, Finger et al. 1997; Slaby and Martin 2008). Within these plutons, the Stolpen granite seems to present the final st of a long lasting magmatism. In general, two sts of granite emplacement within the Saxo-Thuringian and Moldanubian zones can be distinguished. Förster and Romer (2010) concluded that igneous activity in the Saxo-Thuringian Zone, including the northern and northwestern part of the Bohemian Massif, occurred at 335­320 Ma and 305­280 Ma. Some of the plutons, e.g. the granitoid pluton of Karkonosze, formed over several My, with the oldest rocks from this intrusion dated at 319­320 Ma (U-Pb in zircon, Zák et al. 2013), and the youngest at 302 ± 4 Ma (U-Pb in zircon, Kusiak et al. 2014). Finger et al. (2009) and Siebel et al. (2003) studied the Moldanubian part of the Bohemian Massif and also distinguished two major intrusive events; one more voluminous between 328­320 Ma, and the second one, less voluminous, between 317­310 Ma. Moreover, Finger et al. (2009) suivided Variscan granitoid intrusions into five groups of granite belts characterized by slightly different s, geotectonic settings and magma generation mechanisms. The oldest are: "North Variscan Granite Belt", "Central Bohemian Granite Belt" and "Durbachitic Granites", with s of ca. 330 to 350 Ma, 360 to 335 Ma and 335 to 340 Ma, respectively. Intrusions with a younger (330 to 310 Ma) include the south-western sector of the Bohemian Massif, and the granites from the western Erzgebirge and Fichtelgebirge. According to Finger et al. (2009) they form a coherent plutonic belt ("Saxo-Danubian Granitic Belt"), formed most probably due to the delamination of lithospheric mantle (Bird 1979). The fifth group, involving the youngest granites located in the Sudetes, is called the "Sudetic Granite Belt" (including e.g., Karkonosze Massif, Strzegom-Sobotka Massif, Strzelin Massif and Klodzko-Zloty Stok Massif; Mazur et al. 2007) and is dated at ca. 315 to 300 Ma. Gerdes et al. (2003) reported a bimodal timing of magmatism in the South Bohemian Massif, with the first pulse at 331­323 Ma (with a higher mantle input) and the second, less significant, at 319­315 Ma. According to Siebel et al. (2010), one of the youngest magmatic impulses in the Bohemian Massif was the Fichtelgebirge intrusive complex, with U-Pb zircon s ranging from 291.2 ± 6.4 Ma to 298.5 ± 3.9 Ma for different types of granites comprising the intrusion. Late-Variscan granitoids from the Erzgebirge fall within the older group of intrusions (Romer et al. 2010), whereas the younger magmatic event is absent. The s of the amphibole-bearing granitoids 468 KATARZYNA LISOWIEC ET AL. from the Lusatian Granodiorite Complex (granitoids from Wiesa ­ 304 ± 10 Ma and Klienschweidnitz ­ 312 ± 10 Ma) place these intrusions within the youngest st of magmatic activity. The granite of Königshain was dated first by Hammer et al. (1999) at 315 ± 6 Ma using zircon-evaporation method and would also belong to the younger set of intrusions. However Th-U Pb dating of uraninite and molyenite by Förster et al. (2012) yielded older s of 328.6 ± 1.9 Ma for uraninite and 327 ± 1.3 Ma, 327.6 ± 1.3 Ma for molyenite, indicating that the magmatic processes in the LGC started approximately at the same time as the older igneous events in other parts of the SaxoThuringian Zone and the Bohemian Massif. The granitoid pluton of Stolpen, with monazite of 299 ± 1.7 Ma seems to be younger than its neighbour and belongs to the second impulse of magmatic activity in the Saxo-Thuringian Zone (Förster and Römer 2010). The difference between the Stolpen and Königshain intrusions, which cannot be fully estimated based on present data, is difficult to explain, especially as these two plutons are located in one geotectonic unit. It is possible that the studied samples were taken from the youngest part of the pluton, whereas the main body formed some million years before. Nevertheless the Stolpen granite, or at least part of it, is the youngest intrusion in the Lusatian Granodiorite Complex, indicating that the magmatic activity in this region lasted at least 5 my longer than previously estimated. According to previous studies of the biotite-bearing granitoid intrusions from the Lusatian Granodiorite Complex, magma generation mechanisms involved melting of the lower crust triggered by a mantle diapir (enriching the granitoid rocks in LILE and HFSE), accompanied by post-collisional extension (Hammer et al. 1999) in the case of the Stolpen granite, and crust melting in a compressional regime in case of the Königshain granite (Eidam et al. 1991). Amphibole-bearing granitoids (from Wiesa and Kleinschweidnitz) formed due to melting of metasomatized mafic lower crust (probably tholeiitic, Hammer et al. 1999). The melting of the lower crust, in the case of both biotite- and amphibolebearing granites, was induced probably by delamination processes, as proposed by Hammer et al. (1999). A similar scenario for the Late Carboniferous ­ Early Permian magmatism in Central Europe is also suggested by more recent studies of Finger et al. (2009), Slaby et al. (2010) and Turniak et al. (2014). Finger et al. (2009) proposed a delamination model for the formation of the Saxo-Danubian granitoids, which extend along the NE and SW margins of the Bohemian Massif. Late Variscan Lusatian granitoids (including Stolpen) may be considered as the most northerly part of this belt, but the younger of the Stolpen granite is not in accord with the older rocks formed south-west of the pluton (e.g. in the Erzgebirge). Studies of the Strzegom­Sobotka Massif (Turniak et al. 2014) belonging to the Sudetic Granitic Belt, have suggested a close relationship to post-Variscan bimodal volcanism. The heat required for melting of the lower crust was supplied by the ascent of mantle-derived basaltic magmas. The mechanisms possibly responsible for melting of the lithospheric mantle include decompression related to lithospheric extension/rifting and delamination and the convective removal of the thickened mantle. Perhaps similar mechanisms operated in the LGC, which is a western prolongation of the Sudetic Granitic Belt. An interesting comparison can be also made with the granitic rocks (dated at ~300 Ma) associated with the Kraków-Lubliniec Fault Zone (located to the East of the Variscides) which is a prolongation of the Elbe Line (near which the Stolpen granite is located). Slaby et al. (2010) proposed a two-st origin, involving: (1) transpressional regime accompanied by crustal thickening, delamination of the lithospheric mantle and mantle metasomatism , and (2) transtensional regime causing partial melting of upper metasomatized mantle and lower mafic crust. Therefore, it seems that similar processes may have caused granitoid formation along the Elbe Zone and its extension to the Kraków-Lubliniec Fault Zone. The of the Stolpen granite agrees with such an assumption. Magmatism in the Bohemian Massif is characterized by magmas derived from at least two sources: mantle and crust (Finger et al. 1997; Janousek et al. 2004; Gerdes et al. 2000; Siebel et al. 2003; Slaby and Martin 2008). It is noticeable that with progressive evolution of the magmatism, the contribution of the mantle source diminished and the peraluminosity of magmas increased. However mantle activity did not disappear entirely; it is present in a form of late mafic dykes. The Mantle source also contributed continuously with fluids, whose signature is discernible in the granite alterations products and granite pegmatites (e.g. Martin 2006), as is also seen in the case of the Stolpen granite (Lisowiec et al. 2013). The delamination scenario supports mantle-crust interactions, which may involve mixing between crust- and mantle-derived melts (as suggested for some granites from the Saxo-Danubian Granitic Belt, Finger et al. 2009) or can be limited to heat transfer and influx of mantle-derived fluids. The Stolpen granite fits the general features of magma evolution in the Bohemian Massif. Both the obtained monazite and magma affinity fit to the late st of Variscan magmatism outline. The peraluminous character of the Stolpen granite and the only slight contribution of mantle fluids (Hammer et al. 1999; Lisowiec 469 TH-U- PB TIMING OF THE GRANITOID PLUTON OF STOLPEN, GERMANY et al. 2013) suggest that at the end of the emplacement of Variscan granitoids the interaction between the mantle and the crust was limited, but noticeable. The tectonic setting of the Stolpen pluton near the Stolpen-Klotzsche Fault indicates that the mechanism of emplacement along older shear zones was similar to those of other granites from the LGC, as e.g., the Königshain granite (Förster et al. 2012). acknowledge Daniel Harlov for his helpful discussion of the results. Igor Broska and Ray MacDonald, are thanked for insightful reviews, and Ray MacDonald also for linguistic corrections of the final version. This study was funded by a NCN grant 2011/01/N/ST10/04756.

Journal

Acta Geologica Polonicade Gruyter

Published: Dec 1, 2014

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