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U/Pb-Dating and Geochemical Characterization of the Brocken and the Ramberg Pluton, Harz Mountains, Germany

U/Pb-Dating and Geochemical Characterization of the Brocken and the Ramberg Pluton, Harz Mountains, Germany

GEOLOGICA SAXONICA Journal of Central European 56/1 (2010) 9 – 24

U/Pb-dating and geochemical characterization of the and the Pluton, Mountains,

Jana Zech 1 *, Teresa Jeff ries 2, Dominik Faust 3, Bernd Ullrich 4, Ulf Linnemann 5

1 University of Bern, Institute of Geography, Bern, Switzerland [email protected] 2 Natural Historic Science Museum London, London, UK 3 Technische Universität Dresden, Geographisches Institut, Dresden, Germany 4 Technische Universität Dresden, Institut für Geotechnik, Dresden, Germany 5 Senckenber Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie Dresden Königsbrücker Landstr. 159, 01109 Dresden, Germany

* Corresponding author

Abstract Th e Harz Mountains in Germany form a part of the Variscan basement in the southern part of the Rheno-Hercynian Zone. Th e area is situated close to the suture between Laurussia and Gondwana represented by the Mid-German Crystalline Zone. Th e Harz Mountains became intruded by a number of that are believed to be related to the Variscan orogeny. We dated zircons of two samples from the marginal facies of the two major granites Brocken and Ramberg using U/Pb single zircon dating by a Laser-ICP-MS. Th e measurements of the needle-shaped magmatic zircons show a concordia age of 283 ± 2.1 Ma for the Brocken and a concordia age of 283 ± 2.8 Ma for the Ramberg granite. Th ese ages are interpreted as the timing of intrusions, which seem to be related to the opening of the large molasse basins during the Lower and not, as assumed so far, to the Variscan orogeny. Th e event is characterized by extension and thinning of the crust during the formation of the molasse basins. Geochemical signatures and thin section microscopy support the assumption of a doming in the astenosphere and a resulting heat fl ow responsible to the formation of during the Lower Permian.

1. Introduction

The Harz Mountains are situated in the southern part 295 Ma and who assumed the intrusion of the melt in of the Rheno-Hercynian Zone, which are a part of the ~ 8 km depth in the lithosphere with temperatures around Central European Variscides. Th e two major Plutons are 300 °C. During the same time period like the Brocken the Brocken granite and the Ramberg granite. Th e orog- granite the intrusion of the Ramberg granite occurred. eny of this massif has been subject of scientifi c investiga- Petrographical investigations of Eidam and Seim (1963, tions for more than a century, particularly the timing of 1971, 1974) showed that the core facies of the granite the intrusion of both granites and the implications for the is enriched in plagioclas and depleted in kalifeldspar. geotectonic setting and the processes involved. Previous Furthermore Eidam and Seim assumed that the Ramberg results (Baumann et al. 1991 and references therein) sug- consists of two diff erent intrusions, which occurred at gest that the intrusion took place postcinematic after the diff erent times. Tietz (1996) investigated zircons of the variscian orogeny in the upper Rotliegend. It remains granitoid-xenolith of the Bodegang, one of the volcanic unclear whether the two plutons Brocken and Ramberg lodes in the Harz Mountains. His results show a petro- intruded cogenetic caused by similar geological origin. genetic relation between the Bodegang and the Ramberg According to Feldmann et al. (2002) both granites in- concerning the zircon types of the rocks. truded in the upper lithosphere. Th is is in agreement with In order to contribute to a better understanding of results of Baumann et al. (1991), who dated the intrusion the geological and geotectonic setting of the granite in- of the Brocken using titanite-isotope analyses to 293 – trusions in the Harz Mountains we (i) applied single zir-

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany

Fig. 1: Geographical setting of the Harz Mountains, Germany. con U/Pb-istope-dating to the marginal facies of both the Zone, Mid-German Crystalline Zone, the Saxothuringian Brocken and the Ramberg plutons, which should yield Zone and the (Walter 1995). Con- the crystallisation and the intrusion ages, respectively, and glomerations of small crustal blocks characterize the (ii) we analysed the geochemical composition and thin complex of the Variscian belt with diff erent origins along sections of samples from core and marginal facies to infer strike-slip and detachment systems. Th e Harz varies in its further information about the origin and crystallization of geological composition and the magmatic, tectonic, sedi- the magmas. mentary and erosional processes that were involved in its genesis. Hence the Harz can be divided from NW to SE in the , the middle Harz and the lower Harz (fi g. 1). Th e upper Harz comprises the granite, the 2. Geological setting Eckergneisscholle, the Harzburger Gabbronorit massive, Diabas and the western part of the Brocken. Th e middle Harz consists of carboniferic intrusions, Devonian schists, Th e Harz, representing a tectonically uplifted segment, is Diabas and as well as the eastern part of the located in the Rhenohercynian Zone, as one part of the Brocken granite and the Ramberg granite. Ordovizian Variscian belt in Central Germany. Th e Variscian belt and Devonian formations and molasse basins can be subdivided into four tectono-stratigraphic seg- of the lower Permian form the lower Harz (Mohr 1993). ments, which are from NW to SE: the Rhenohercynian Th e orogen of the Harz Mountains was caused by the col-

10 | GEOLOGICA SAXONICA Journal of Central European Geology 56/1 (2010) 9 – 24

Tab. 1: Geographical coordinates of the sample. ple RAM 1) was processed for single zircon U/Pb dating. After crushing and sieving to 0.063 – 2.0 mm, magnetic sample latitude longitude minerals were removed. Th e zircons were further puri- Brocken bg1 dat 51°47′40.67′′ 10°32′26.21′′ fi ed using a lithium tetraporat density solution. 100 – 150 ′ ′′ ′ ′′ bg 2 51°48 8.23 10°32 19.78 undisrupted zircons (30 – 230 μm) were picked under the bg 3 51°51′18.37′′ 10°32′49.32′′ microscope and placed into an epoxit resin mount. Th e U/ bg 5 51°46′26.95′′ 10°40′28.35′′ Pb isotope measurements were conducted at the Natural Ramberg ram 1 51°43′48.20′′ 11°1’43.66′′ History Museum London, UK, using a frequency quintu- rg 1 51°43′48.56′′ 11°1’43.44′′ pled Nd : YAG based laser ablation system (λ ~ 213 nm) rg 2 51°43′33.29′′ 11°2’0.39′′ and quadrupole based ICP-MS. Further details concerning the Laser-ICP-MS measurement technique are described in Jeff ries et al. (2003). 24 undisrupted zircons from the lision between Laurussia and Amorika during the Upper Brocken Pluton and 46 zircons from the Ram berg Pluton , which could be dated based on fl ysch were analysed. Each run of 12 unknowns bracketed by 8 formations (Wachendorf 2002, and references therein). determinations of the standard zircon 91500 (age 1062.5 Due to folding and exhumation and resultant volcan- ± 1.7Ma) for calibration and mass bias correction.Th e ism during the lower Permian (upper Rotliegend) a large 206Pb/238U, 207Pb/235U and 207Pb/206Pb isotope ratios of number of plutonic rocks crystallized in several stages, and the samples and ages are given in table 2 (Brockengranite formed cumulus shaped subsurface granites. According sample BG1dat) and 3 (Ramberggranite sample RAM1). to Baumann et al. (1991) the two plutons Brocken and To obtain the best age estimate the 206Pb/238U and the Ramberg intruded into the upper lithosphere between 207Pb/235U isotope ratios were correlated (Ludwig, 1998). 293 – 295 Ma. At the same time, such as the basin Only the concordant isotope ratios that overlap the con- and the basin developed as a result of the exten- cordia with a 2σ error ellipse were used to determine the sion of the crust. Th ese molasse basins are fi lled with up concordia age. Ages and errors were calculated by using to 800 m of siliciclastics with intercalated vulcaniclastic the Isoplot software of Ludwig (2001). Th e decay con- sediments and lava fl ows (Feldmann et al. 2002). stants by the IUGS (Steiger and Jäger 1977) were applied.

4.2. Geochemistry and thin sections 3. Sample description Seven samples from both plutons were analyzed concerning the geochemical composition and the thin sections in or- Totally, 15 samples were taken from the marginal facies der to characterize the lithology of the granites. Important (fi ne – middle grained) and from the core facies (por- parameters to distinguish diff erent granite types are major phyry – micropegamtitic grained) of the Brocken and element ratios, like Al2O3 / SiO2 or Na2O + K2O / SiO2. Ramberg granite. Th e exact co-ordinates of the samples Th is refl ects mainly the quartz en rich ment and the alkali are given in table 1. Samples were free of vegetation and concentration (Linnemann and Romer 2002). Th e ASI weathering features. according to Shand (1947: Al2O3/ (CaO+Na2O+K2O) re- fl ects the Al saturation degree (Markl 2004). K2O/Na2O refl ects the content of K-feldspar and mica relative to the plagioclase (Linnemann and Romer 2002). To character- 4. Analytical methods ize the chemical weathering, especially for feldspar and therefore the ratio of mobile elements to immobil ones Al2O3 vs. (CaO+Na2O) is decisive. Th e correlation of 4.1. Single zircon U/Pb-dating the trace elements and REE, e.g. Rb vs. Y + Nb is used to determine the geological setting (Pearce et al. 1984). One sample from the marginal facies of the Brocken Major, trace and rare earth elements (REE) were de- (sample BG1dat) and one of the Ramberg pluton (sam- termined by ACTLABS (Ancaster, Ontario, Canada) us-

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany ing lithium metaborate/tetraborate fusion and Inductive as D, P5 and S25 – idiomorphic, needle-shaped crystals Coupled Plasma Mass Spectroscopy (ICP-MS). Quality following the zircon classifi cation scheme of Pupin and tests for the measurements are available through reference Turco (1972). Additionally no inherited cores existed. Th e materials with known geochemical composition. Th e geo- zircons have been formed by a magmatic early crystalliza- chemical composition and the detectionlimits of all sam- tion by temperatures of 800 – 900 °C. Th ose conditions in ples are given in table 4. For data analysis and interpreta- the lithosphere suggest a doming of the asthenosphere and tion of the geochemical signatures the programm minpet a contamination of the meltings by an intruding fl uidal version 2.02 was used. phase out of the asthenosphere. Th e zircons of the popula- tion RAM1 are mainly G1 and P1 types following Pupin and Turco (1972) and featured traces of alteration. Th ese 4.3. Scanning electron microscopy shapes have been formed under temperature conditions between 600 – 650 °C, which refl ect the crystallization in Morphology of single zircon crystals, the fabric of thin sec- the upper lithosphere. tions and the chemical composition of single feldspar crys- tals were investigated by means of scanning electron micro- scope (SEM) ZEISS EVO 50 coupled with an energy dis- 5.2. Geochemistry persive micro analytical spectrometer (EDS) ROENTEC detector XFlash 3001 (Quantax 1.7.) and a cathodolumi- Th e geochemical results are shown in table 4 and fi g. 8 nescence detector (CL) SEM-CL-view (EMSystems). and 9. Th e concentrations of some elements, especially for several trace elements are lower then the detection limit. Brocken and Ramberg samples can be characterized as fel- sic A-type granites. According to the granite characteriza- 5. Results and discussion tion of Pearce et al. (1984) the samples of the Brocken Pluton (BG1dat, BG2, BG3, BG5) refer to within-plate granites (WPG), more specifi cally granites in attenuated 5.1. Single zircon U/Pb-dating continental crust. Th e samples RAM1 and RG5 of the Ramberg Pluton are postectonically cristallized collisional Six out of 24 zircons of BG1dat yielded mixed ages for the granites (syn-COLG) enriched in Rb and depleted in Y 206Pb/238U, 207Pb/235U and 207Pb/206Pb isotope ratios (table and Nb. Th e sample RG1 can be classifi ed as volcanic-arc 2) indicating recrystallization and/or alteration and were granite (VAG) indicating crystallization in spheres of at- therefore excluded from age calculations. Concerning the tenuated continental crust. Ramberg granite eight out of 46 zircons were excluded Considering the element composition within the sin- likewise because of mixed ages (table 3). Th e concordia- gle granite complexes the concentration becomes lesser ages (fi g. 2 and 3) calculated for the remaining zircons are from the core to the marginal facies. Th e ASI after Shand 283.0 ± 2.1 Ma (BG1dat) and 283.0 ± 2.8 Ma (RAM1), shows that all samples of both granite massifs are peralu- respectively. Th ese ages are interpreted to refl ect the timing minous. Although the two plutons are spatial near, ele- of the crystallization of both granites. A second concordia- ment compositions and concentration vary widely. Th e age for the Ramberg is 289.3 ± 0.59 Ma. Th is is likely due major elements, REE and trace elements are signifi cant to crystallisation of resulting from another, previ- enriched in the Brocken granite. To specify the deter- ous intrusion. Th e shape of the zircons from both Plutons mined concentration of the elements are up to three times is, however, very similar and can be divided into three higher then the measured ones of the Ramberg granite, subpopulations: i) needle shaped ii) lath prismatic and iii) excluding P2O5 and MgO. Both plutons have a decrease thick prismatic (fi g. 4). Reasons for that are i) the spa- of REE and a specifi c negative Eu anomaly in common. tial nearness, ii) the similar geological conditions during Typically for a-type granites are the low ratios of K2O/ crystallization and iii) a cogenetic formation through the Na2O (3.43 – 1.72) and a variable SiO2 concentration be- same geotectonical strength. Within this classifi cation the tween 73.34 % and 82.93 %. All samples are characterized dated crystallization ages of all zircons vary over the to- by an enrichment of Al2O3, especially the Brocken granite tal age range. Th e zircons of BG1dat can be characterized samples: Al2O3 > 11.62 %. Th is concentration decreas-

12 | GEOLOGICA SAXONICA Journal of Central European Geology 56/1 (2010) 9 – 24 232Th 232Th 208Pb/ 206Pb 207Pb/ 235U 207Pb/ 238U 206Pb/ 232Th 232Th 208Pb/ 206Pb 207Pb/ 235U 207Pb/ Pb isotope ratios and ages of the Brocken granite samples. Pb isotope ratios and ages of the Brocken 206 Pb/ 238U 207 206Pb/ U and 235 Pb/ ratio ratio ratio ratio ratio Age (Ma) ± 2s Age (Ma) ± 2s Age (Ma) ± 2s Age (Ma) ± 2s 0,227 0,04430,219 0,3142 0,05146 0,04630,222 0,01185 0,3438 0,0464 0,05388 279 0,01339 0,4094 3 0,06395 292 277 0,01643 3 8 292 300 260 4 3 800,265 3480,216 238 364 0,0463 22 0,0448 26 6 0,3414 740 0,3306 269 0,0535 142 0,05346 5 0,01304 329 0,01276 292 18 283 2 3 298 290 4 6 350 348 38 62 262 256 3 8 207 206Pb 0,2637 0,04520,26140,3085 0,3239 0,0436 0,0505 0,052010,1915 0,32170,1927 0,01166 0,7906 0,05344 0,047 0,047 0,11346 2850,2131 0,012 0,339 0,027430,2308 0,3616 2 0,0446 0,05233 2750,1296 318 0,0474 0,05582 285 0,3181 0,013630,2929 6 0,0467 0,01401 11 0,44970,2393 0,0517 5 0,0429 296 283 0,46730,2223 0,06882 592 296 0,0442 0,01346 2840,1314 0,3506 2 0,07255 0,01717 6 47 0,0438 40,2668 0,3857 281 48 0,05921 0,0474 0,023990,1589 296 1854 346 298 0,3325 0,0464 313 0,0633 0,01352 234 3 0,3496 130 0,0437 294 3 0,05503 96 4 0,3973 0,01318 80,2437 271 3 0,0535 280 547 0,3155 0,01281 7 241 2980,2524 0,06204 377 444 279 0,0458 0,012640,1628 6 41 0,05235 6 0,01418 276 26 389 0,0454 7 230,4106 44 0,3555 3 0,01193 0,0451 305 298 272 274 280,2184 3 0,3284 293 892 0,0579 281 0,05624 331 0,3342 276 15 62 4 1000 0,05249 0,0448 134 4 292 4 0,01328 1,4026 7 0,05372 23 574 130 5 270 0,01297 304 344 0,3228 5 0,17557 340 0,01316 289 718 132 479 278 5 5 0,05225 18 286 0,0401 412 7 162 2 272 284 51 0,01276 5 350 4 48 265 674 363 309 4 3 282 300 38 288 257 52 11 9 7 293 62 254 3 5 5 285 890 460 5 240 4 284 306 41 5 54 358 8 4 56 2610 267 52 88 260 296 5 264 795 54 6 7 43 256 6 208Pb/ U, 238 Pb/ 206 BG 19 BG 20 BG 21 BG 22 BG 23 age) (mixed BG 24 BG 2 BG 3 BG 4 age) (mixed BG 5 BG 6 BG 7 BG 8 age) (mixed BG 9 BG 10 age) (mixed BG 11 age) (mixed BG 12 BG 13 age) (mixed BG 14 BG 15 BG 16 BG 17 BG 18 BG 1 Sample BG 1 dat Tab. 2 : Tab.

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany ± 2 σ 232Th 232Th 208Pb/ Age (Ma) ± 2 σ 206Pb 207Pb/ Age (Ma) ± 2 σ 235U 207Pb/ 238U 206Pb/ 232Th 232Th 208Pb/ 206Pb 207Pb/ 235U 207Pb/ Pb isotope ratios and ages of the Ramberg granite samples. 206 Pb/ 238U 207 206Pb/ U and 235 Pb/ 0,16 0,0444 0,3309 0,05408 0,0113 280 4 290 7 374 56 227 6 ratio ratio ratio ratio ratio Age (Ma) ± 2s Age (Ma) 0,158 0,0458 0,3384 0,05352 0,01023 289 3 296 4 350 40 206 4 207 206Pb 0,10680,1336 0,04240,1651 0,0440,0976 0,3058 0,04370,1287 0,0449 0,3270,0502 0,05228 0,3175 0,04340,1688 0,3296 0,01132 0,05387 0,04810,1944 0,05268 0,3202 0,04440,1588 0,05324 0,01179 268 0,3438 0,01108 0,04510,1752 0,05344 0,3158 0,01143 0,04530,2539 278 0,05186 6 276 0,3297 0,01141 0,0448 0,05153 283 0,3376 0,01117 0,0436 40,1434 0,05301 271 4 274 0,3355 0,011730,1032 0,05399 3 303 0,3048 0,01172 287 0,044 70,0906 0,05431 280 4 280 0,01167 0,0450,1747 0,05069 289 5 284 0,01149 7 0,0448 296 0,4082 50,0772 282 7 286 0,01174 0,0453 0,318 7 300 4 0,06723 364 283 88 0,3305 0,0453 314 120,0824 279 4 275 0,3222 338 0,00796 0,0513 90,0697 72 0,05352 289 227 5 48 346 0,3288 0,0446 80,1154 0,05162 295 3 0,01083 58 0,01029 237 278 0,0441 278 4 7 1020,1237 0,05265 294 223 0,3274 0,01053 0,0455 264 9 270 2300,3283 283 282 86 0,3218 0,01065 229 6 9 0,0468 328 7 9 0,0532 285 74 0,3259 3700,0533 7 7 0,1208 0,05296 225 5 11 3 348 285 36 0,3503 384 0,01053 0,051980,0993 236 6 92 0,01071 0,2534 1,3916 226 280 9 0,05427 15 290 235 3 68 0,01055 281 0,0465 9 284 235 3,7433 0,08355 278 70 0,00983 844 8 6 7 289 231 287 2 0,3523 0,03434 0,10714 8 6 236 6 295 82 254 350 4 9 0,05493 4 0,05995 288 735 268 5 283 160 4 78 48 0,01154 312 286 1456 5 59 86 7 7 218 305 207 36 293 54 5 336 885 212 326 8 6 8 214 1581 7 48 71 284 9 80 36 382 6 306 1282 212 42 215 1750 48 110 8 212 4 16 10 198 683 408 5 1177 25 5 96 41 232 9 208Pb/ U, 238 Pb/ 206 RAM 1 RAM 2 RAM 3 RAM 4 RAM 5 RAM 6 RAM 7 RAM 8 RAM 9 RAM 10 RAM 11 RAM 12 RAM 13 RAM 14 RAM 15 RAM 16 RAM 17 RAM 18 RAM 19 RAM 20 RAM 21 RAM 22 RAM 23 age) (mixed RAM 24 age) (mixed RAM 25 Sample RAM 1 Tab. 3 : Tab.

14 | GEOLOGICA SAXONICA Journal of Central European Geology 56/1 (2010) 9 – 24 0,09 0,0460 0,3297 0,0520 0,01 290 4 289 9 284 84 225 9 0,045 0,04410,148 0,3268 0,0453 0,0537 0,3254 0,01 0,0521 278 0,01 4 286 2870,163 6 12 286 0,0463 356 9 124 0,336 232 288 0,05262 0,01078 10 92 215 292 6 5 294 9 312 90 217 7 0,1206 0,04500,1524 0,33140,1653 0,06280,1218 0,0534 0,0453 0,49090,1261 0,0616 0,36480,1788 0,01 0,0567 0,04560,0485 0,5159 0,0584 0,04600,1652 0,01 0,3340 284 0,0454 0,0608 0,32940,1562 0,01 0,0463 0,0531 4 0,32920,1365 393 0,02 0,0519 0,061 0,3537 286 291 0,0526 0,01 0,15630,0712 9 0,01 0,4595 0,05537 3850,1313 6 4 1,592 0,01 0,0439 4060,1724 288 0,01167 0,05464 25 0,04640,1235 290 346 316 0,07384 0,3226 13 0,0462 0,01464 3 286 422 2920,0425 0,3332 0,03436 0,047 9 52 4 0,05324 476 0,33390,1153 293 34 382 4 6 0,05204 0,047 0,011460,1081 544 289 214 936 0,3465 68 0,05244 0,0489 630 7 7 289 308 0,011 0,491 0,0454 24 9 70 277 0,01101 0,0535 6 290 0,3724 76 330 384 7 7 0,07579 0,3324 967 293 0,01125 280 245 4 291 7 0,0552 355 64 7 312 426 0,01391 0,05312 16 6 96 284 296 8 3 0,01174 25 222 396 62 0,01159 84 1036 296 292 223 9 4 293 308 58 8 36 232 234 286 45 13 338 6 302 5 5 294 11 683 406 9 286 4 84 9 304 321 11 49 25 106 291 230 348 46 9 1088 221 6 13 78 221 40 420 9 332 226 5 279 86 64 11 15 236 233 10 9 RAM 26 RAM 27 RAM 28 RAM 29 RAM 30 age) (mixed RAM 31 RAM 32 age) (mixed RAM 33 RAM 34 RAM 35 RAM 36 age) (mixed RAM 37 RAM 38 RAM 39 RAM 40 RAM 41 RAM 42 RAM 43 RAM 44 age) (mixed RAM 45 RAM 48 Tab. 3 : Continuation. Tab.

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany aapiterrelpe r 83 conf 68.3% are ellipses error data-point

0.049 data-point error ellipses are 68.3% conf. 0.0475 300 298 0.047

U 290 238 0.045 280 Pb/ 294 206 0.0465 0.043 270

0.041 290 0.295 0.305 0.315 0.325 0.335 0.345 0.355 207Pb/235U U 0.0455 286 238

Pb/ 282

206 0.0445 278

0.0435 274 Concordia Age = 283,0± ±2.1 Ma (95% confidence, decay-const. errs included) MSWD (of concordance) = 1,7, 270 Probability (of concordance) = 0,19 0.0425 0.30 0.31 0.32 0.33 0.34 207Pb/207235Pb/U 235U

Fig. 2: Concordia-diagram of the Brocken granite sample BG1dat. es from the core (BG5: 13.15 %) to the marginal facies ported by the thin sections in which no plagioclase could (BG1dat: 11.62 %). Additionally, the depletion of Na2O be found in the marginal facies samples and by the slightly is more marked in the Ramberg samples and follows also shattered zircons. Our results are in good agreement with a depletion gradient from the core (RG1: 5.12 %) to the Eidam and Seim (1971), who noticed a higher concen- marginal facies (RAM1: 1.58 %). Typical for upper lithos- tration of kalifeldpar then plagioclas in the core facies of pheric rocks the Ramberg granite is enriched in SiO2. Th e the Ramberg granite. Th e sample RG1 of the Ramberg diff erent distribution of the major elements of the samples granite has signifi cantly diff erent geochemical results for to the conclusion that inspite of the spatial nearness all analysed elements (SiO2 = 82.93%, Na2O = 5.12%, these plutons intruded and crystallized in diff erent Ba = 10 ppm, Rb = 2 ppm, La = 2,6 ppm, Ce = 6,4 ppm, and diff erent geochemical milieus of the lithosphere. eTh Nd = 5,0 ppm, Eu = 0,16 ppm, U = 1,4 ppm). Because granitic melt, which formed the Ramberg intruded in the there is only one sample we are hesitant to interprete this upper lithosphere. Th e Ramberg granite exhibit a typi- result. One possible explanation could be that this sample cal lithospheric element composition with enrichment in refers to a geochemical zonation in the core facies. Th e light metals, trace elements, like K, Cs Ba and SiO2. Th e geological setting of the Brocken granite took place in elements Na, K, Ca are depleted. Th is result could be sup- the lower crust in contamination to the doming asthe-

16 | GEOLOGICA SAXONICA Journal of Central European Geology 56/1 (2010) 9 – 24

0.068 420 conf 68.3% are ellipses error data-point

data-point e

0.3 1600 0.064 U 1200

238 0.2 380 Pb/ 800

0.060 206 0.1 Intercepts at 400 286 ± 22 & 1658 ± 86 [±87] Ma MSWD = 29

0.0 0 012345

U 0.056 207Pb/235U 340 238 0.052 Pb/

0.048 300 206 Concordia Age = 283,0 ±2.8 Ma (95% confidence, decay-const. errs included) MSWD (of concordance) = 3,0, 0.044 Probability (of concordance) = 0,085

260 Concordia Age = 289,3 ±0.59 Ma 0.040 (1s, decay-const. errs included) MSWD (of concordance) = 0,42, Probability (of concordance) = 0,52

0.036 0.26 0.30 0.34 0.38 0.42 0.46 0.50 207 235 207Pb/Pb/U235U Fig. 3: Concordia-diagram of the Ramberg granite sample RAM1. nosphere resulting from the extension and thinning of elements reacted volatile and a depletion of the cations the crust during the formation of the molasse basins. Th e Na, K, Ca and Eu as well as an enrichment of Al were enrichment in asthenospheric elements in the Brocken caused. Th e melt of the Ramberg intruded in phases so granite samples leads to the assumption that a resulting that the already crystallized plagioclases were fractionated heat fl ow intruded in the lower lithosphere. Th is led to a by the intruding fl uidal phase out of the asthenosphere. variety composition of lithosopheric and asthenospheric Moreover, the educts of both melts could already have a elements in the Brocken granite in accordance to an prim- negative Eu anomaly and the feldspar existed only subor- itive mantle rock with a high concentration of heavy met- dinate. als, especially of the elements Rb, Nb, Zr and Ti and REE. In addition this intrusion caused depletion of Eu and of the volatile elements Na, K, Ca, which explains the nega- tive Eu anomaly of all granitic samples. Furthermore the melt of the rocks were once crystallized and then melted again before recrystallization. In a primary melt the pla- gioclases were partial melted through anatexis. Th e mobil

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany

Brocken Ramberg

Needle Needle

Craned Craned

Rhombic Rhombic

Fig. 4: 3-D microscopic photographs of zircons from Brocken and Ramberg, Harz, Germany. Th e habitus description of the zircons is below the microphotograph.

296 +/- 2 296 +/- 2

BS-BG 6 CL-BG 6 281 +/- 3 281 +/- 3

BS-BG 9 CL-BG 9 286 +/- 4 286 +/- 4

BS-BG 21 CL-BG 21

Fig. 5: Electron microscopic photographs of the zircons BG 6, BG 9 and BG 21 from the Brocken granite sample. Zircon ages are in Ma. BS – back scatter, CL – ca thodo lu minescence.

18 | GEOLOGICA SAXONICA Journal of Central European Geology 56/1 (2010) 9 – 24

290 +/- 4 290 +/- 4

BS-RAM 27 CL-RAM 27 291 +/- 3 291 +/- 3

BS-RAM 42 CL-RAM 42 286 +/- 4 286 +/- 4

BS-RAM 35 CL-RAM 35

Fig. 6: Electron microscopic photographs of the zircons RAM 27, RAM 42 and RAM 35 from the Ramberg granite sample. Zircon ages are in Ma. BS – back scatter, CL – cathode-luminescence.

735 +/- 59 296 +/- 2

BS-RAM 23 CL-RAM 23 936 +/- 24 936 +/- 24

BS-RAM 38 CL-RAM 38 1.456 +/- 54 1.456 +/- 54

BS-RAM 24 CL-RAM 24

Fig. 7: Electron microscopic photographs of zircons RAM 23, RAM 38 and RAM 24 from the Ramberg granite sample. Zircon ages are in Ma. BS – back scatter, CL – cathodoluminescence.

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany

2000 30

1000 syn-COLG WPG 20

RG 5 O RAM 1 BG samples 2 O + Na 2

100 K 10

Rb 0 01234567 CaO 10

ORG K2O

VAG

RG 1 1 1 10 100 1000 2000 Y + Nb

Fig. 8: Rb vs. Y+Nb discrimination diagram after Pearce et al. (1989). BG – Brocken granite samples RAM, RG – Ram- berg granite samples.

Fig. 9: Concentration of the feldspar oxides Na2O, K2O and CaO. Red – Brocken granite samples. Blue – Ramberg granite samples. Na2O CaO

5.3. Thin sections acterized by myrmecitic growings between plagioclas and kalifeldspar (fi g. 10 top: kalifeldspar and quartz, bottom: Th e previous results are supported by thin section analy- plagioclase, kalifeldspar and quartz). Th is demonstrates ses. Th e analysed samples, excluding RG1, refer to the the contamination of the crystallizing melt with the fl ui- rock classifi cation granite. Th e marginal facies of the dal mantle magma. Th e enrichment of feldspar and mica Ramberg Pluton (sample RG1) belongs to alkali feld- are in accordance with the concentration of the mineral spar granite. Th e Ramberg granite samples are inherent forming elements of the geochemical signature. Th e dom- in micrographic growing between the kalifeldspars and inance of kalifeldspar refl ects the depletion of Na2O + the quartz minerals as it is typical for intrusions in the CaO / K2O. In the core facies (RG1) the dominance of upper lithosphere. Th e hypidiomorphic plagioclases are plagioclas is in agreement with the high concentration of often characterized by twin formations. Only in the core Na2O + CaO / K2O of the geochemical analyses. Th e ka- facies of the Ramberg (samples RAM1, RG5) both feld- lifelspars showed kaolinization and the plagioclases serici- spars could be identifi ed. Th e Brocken granites are char- tization feature.

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Fig. 10: Th e typical myrmecitic texture of the Brocken granite samples, documented by the thin section of sample BG 2. Scale = 200 μm.

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany MS MS ICP FUS- FUS- FUS- U MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP LOI Total Sc Be V Ba Sr Y Zr FUS- FUS- FUS- 5 O MS MS 2 ICP P FUS- FUS- FUS- 2 MS MS ICP FUS- FUS- FUS- O TiO 2 MS MS ICP FUS- FUS- FUS- OK 2 MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- MS MS ICP FUS- FUS- FUS- (T) MnO MgO CaO Na 3 O 2 FUS-MS FUS-MS FUS-ICP Fe 3 O MS MS 2 ICP FUS- FUS- FUS- Al 2 5 1.6 0.16 1.9 0.4 2.6 0.4 1.1 0.14 0.8 0.09 1.6 1 3 < 0.1 < 5 < 0.4 4.8 1.4 % % % % % % % % % % % % ppm ppm ppm ppm ppm ppm ppm 52 10 1.14 8.5 1.5 8.8 1.8 5.6 0.85 5.3 0.74 7.4 1.9 1 1.3 31 < 0.4 21.5 4.3 4050 2 < 140 < 20 < 10 < 1 < 20 < 30 < 10 < 20 18 40 < 10 19 2 50 < 5 20 2 291 14 2 11 275 6 < 2 18 < 0.5 259 < 0.2 < 2 21 < 0.5 7 < 0.2 < 2 < 0.5 1.3 3 < 0.2 5.1 1.2 12.7 5 4.8 27.7 1.6 47.1 3.37 4.3 97.6 43.9 11.5 101 10.4 20 1 20 10 30 1 1 5 2 1 2 0.5 0.2 1 0.5 0.5 0.1 0.1 0.05 50 2 < 20 < 10 < 30 15 2 14 289 8 < 2 < 0.5 < 0.2 5 1.4 6 8.5 17.1 2.17 Cr Co Ni Cu Zn Ga Ge As Rb Nb Mo Ag In Sn Sb Cs La Ce Pr 0.1 0.1 0.05 0.1 0.1 0.1 0.1 0.1 0.05 0.1 0.04 0.2 0.1 1 0.1 5 0.4 0.1 0.1 9.1 2.1 0.25 2.2 0.4 2.7 0.5 1.1 0.15 0.9 0.1 1.9 1.3 3 1.9 17 < 0.4 5.4 2.5 Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th MS MS ICP 13.742.1 3.241.1 7.3 0.3337.3 9 1.13 3.3 8 0.73 6 0.7 8.7 1.07 1.1 3.8 1.6 7.7 6.3 0.6 9.2 1.4 1.3 1.6 1.8 0.22 8.8 4.2 5.5 0.67 1.2 1.8 0.84 0.14 4.2 5.6 0.61 2.4 0.86 5 0.71 6.2 1.8 5.3 0.76 6.2 1.8 3 6.9 1.5 2 1.5 1.8 1 1.5 18 < 0.4 2 1.6 37 < 0.4 8.1 1.7 37 17.1 < 0.4 4.6 27 21.3 3.2 < 0.4 4.1 21 4.9 < 20 < 1< 20 < 20 2 < 10 < 20 80 < 10 20 30 2 19 9 2 266 5 21 277 < 2 20 < 0.5 < 0.2 2 < 0.5 2 < 0.2 1.9 10.3 4 57.6 1.4 104 17.2 12.4 64.4 125 14.9 75.9 11.62 2.2 0.047 0.06 0.57 2.9 4.99 0.154 0.03 0.78 99.27 5 3 17 514 37 38 194 < 20 < 1 < 20 < 10 < 30 13 1 < 5 2 7 < 2 < 0.5 < 0.2 5 1.3 < 0.5 2.6 6.4 0.9 0.01 0.01 0.01 0.001 0.01 0.01 0.01 0.01 0.001 0.01 0.01 0.0 11153224 ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm SiO FUS- FUS- FUS- 78.35 11.0482.9374.83 9.36 12.85 1.88 0.02175.37 1.43 12.29 2.23 0.1773.34 0.012 0.031 13.1575.87 0.19 0.11 12.26 2.45 0.2 1.58 0.26 0.038 2.64 0.34 5.43 5.12 0.036 2.14 0.13 < 0.01 0.094 2.43 0.033 0.29 0.081 0.2 0.09 5.36 0.09 2.78 0.114 0.1 1.09 0.24 1 5.31 99.93 0.15 0.36 2.76 0.187 2.88 99.77 1.05 5.68 0.05 3 5.37 99.58 < 1 0.222 0.16 1.08 < 1 99.97 0.07 0.02 4 2 < 5 0.76 0.73 6 99.68 < 5 99.97 2 184 10 2 31 7 6 5 < 5 49 168 14 3 2 344 14 36 49 < 5 7 57 43 21 535 530 54 64 50 82 196 53 55 218 239 Reference Reference Method: Detection Limit: Detection Units: Client I.D. RAM 1 1 RG 5 RG BG 1D BG 3 BG 5 BG 2 BG 1D BG 3 BG 5 BG 2 Element: RG 5 RG Element: Client I.D. RAM 1 1 RG 5 RG BG 1D BG 3 BG 5 BG 2 Element: Units: Limit: Detection Reference Reference Method: Client I.D. RAM 1 1 RG Reference Reference Method: Detection Limit: Detection Units: : Geochemical results of the Brocken granite and Ramberg samples. of the Brocken results 4 : Geochemical Tab.

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6. Conclusion ditionally the described geological conditions during the upper Rotliegend (Lower Permian) suggest a cogenetic geotectonic origin in the shear stresses and a cogenetic Th e concordia ages, geochemical signature and thin sec- intrusion. Th erefore our results are in good agreement tion microscopy to the following interpretation of with previous results (Benek 1965; Schoell et al. 1973; the granite formation in the Harz Mountains: Between Wachendorf and Krebs 1974; Baumann et al. 1991; Mohr 290 and 270 Ma two processes in the southern part of the 1993; Tietz 1996) assigning the intrusion after the varis- Rheno-Hercynian Zone of the Central European Variscides cian orogeny to the upper Rotliegend. A second, earlier dominated: (i) Extension and thinning of the lithosphere crystallization phase of the Ramberg granite was already caused the formation of the upper Rotliegend molasse ba- proposed by Eidam and Seim (1971) and could now be sins, such as the Ilfelder Becken, Meisdorfer Becken and dated to 289.3 ± 0.59 Ma. the Saalesenke; and (ii) Th e epirogenetic shear stress leads to a doming in the asthenosphere. Th at eff ects the intru- sion of a fl uidal phase of mantle magma and a partial ana- Acknowledgement texis of the lithosperic rock. Massiv plutons intruded along the subduction zone as linear belts and crystallized with a mineral admixture composition of asthenospheric and We thank Dr. K. Drost for help with the mineral separations lithospheric elements. Th e Brocken granite crystallized and preparation of the thin sections. under an asthenospheric-lithospheric mixed petrochemi- cal and petrophysical milieu with a geochemical signature of a primitive mantle rock. Th e Ram berg granite crystal- lized in the upper crust under litho spheric conditions. Th e References geochemical signature of the Brocken granite refl ects an el- ement compositions of the asthenosphere, like enrichment Baumann, A. et al (1991): Isotopic age determinations of crys- in and REE. Th e Ramberg granite corre- talline rocks of the Upper Harz Mountains, Germany. Inter- sponds with lithospheric rocks by having high concentra- national Journal of Earth Science 80(3): 669 – 690 tion on SiO2, light metals and trace elements and a lower Benek, R. (1965): Die Form des Ramberg-Granitkörpers im concentration of REE. Th e zircon types of both granites Harz. Monatsberichte der deutschen Akademie der Wissen- support those results. Th e dominating zircon shapes D, P5 schaf ten zu 7, Berlin: 823 – 826. and S25 of the Brocken granite samples imply temperature Eidam, J., Seim, R. (1971): Zur Geochemie und Genese des conditions between 800 – 900 °C during crystallization. Rambergmassivs (Harz). Chemie der Erde 29, : 277–333 Th e Ramberg granite zircons crystallized as G1 and P1 Feldmann, L. et al. (2002): Die geologische Entwicklung der types between 600 and 650 °C. Hence the intrusion and Tiefebene und der in Niedersachsen. Ver- öff ent lichungen der Akademie für Geowissenschaften Han- crystallization of the granite was cogenetic but in diff erent no ver e. V.: 9 – 13. levels in the lithosphere and under diff erent geochemical Geisler-Wierwille, T. (1999): U-Th-Gesamtblei-Datierung conditions. Th e epirogenetic shear stress caused fissures, von Zirkonen mit Hilfe der Elektronenstrahl-Mikrosonde in which magma intruded and crystallized fi ne-grained. Methodik und Anwendungsbeispiele. Dissertation, Ham- Th e typical volcanic lodes, like the “Mittelharzer Gänge” burg. or the Bodegang evolved. Tietz (1996) documented the Jeff ries, T. et al. (2003): Advances in U-Pb geochronology us- zircons of the granitoid-xenolith from the “Bodegang” and ing a frequency quintupled Nd YAG based laser ablation suggested a petrographical relation between those and the system (λ = 213 nm) and quadrupole based ICP-MS. J Anal zircons of the Ramberg granite. Th is could be verfi ed with At Spectrom 18: 847 – 855. the dominating zircon types G1 and P in both geological Linnemann, U., Romer, R. L. (2002): Th e in Saxo-Th uringia, Germany geochemical and Nd-Sr-Pb complexes. Additionally Tietz identifi ed the zircon types isotopic characterization of marginal basins with constraints P3 and P4 for the Brocken granite, which is also in good to geotectonic setting and provenance. Tectonophysics 352: agreement with our results. Th e equal crystallisation age 33 – 64. of the needle-shaped zircons of the Brocken 283.0 ± Ludwig, K. R. (1998) Calculation of uncertainties of U-Pb iso- 2.1 Ma and of the Ramberg I) 283.0 ± 2.8 Ma, and ad- tope data. Earth and Planetary Science letters 46: 212 – 220.

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U/Pb-dating and geochemical characterization Zech, J. of the Brocken and the Ramberg Pluton, Harz Mountains, Germany

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