Late stage Variscan magmatism in the Strzelin Mas sif (SW Po land): SHRIMP zir con ages of tonalite and Bt-Ms gran ite of the Gêsiniec intrusion

Transcription

1 Geo log i cal Quar terly, 2012, 56 (2): DOI: Late stage Variscan magmatism in the Strzelin Mas sif (SW Po land): SHRIMP zir con ages of tonalite and Bt-Ms gran ite of the Gêsiniec intrusion Teresa OBERC-DZIEDZIC and Ryszard KRYZA Oberc-Dziedzic T. and Kryza R. (2012) Late stage Variscan magmatism in the Strzelin Mas sif (SW Po land): SHRIMP zir con ages of tonalite and Bt-Ms gran ite of the Gêsiniec in tru sion. Geol. Quart., 56 (2): , doi: /gq.1017 The Gêsiniec com pos ite in tru sion in the north ern part of the Strzelin Mas sif (Fore-Sudetic Block, SW Po land) was formed in the course of three late Variscan mag matic ep i sodes: tonalitic I, granodioritic, and tonalitic II/gra nitic. The age of the Gêsiniec tonalite, Ma, is the same as that of an other tonalite body in the south ern part of the Strzelin Mas sif, the Kalinka tonalite. The youn ger bi o tite-mus co vite (Bt-Ms) gran ite, in a dyke cut ting the Gêsiniec tonalite, has an in dis tin guish able iso to pic age of Ma; it con tains, how ever, in her - ited zir cons with ages be tween ca. 5 Ga to 374 Ma, sim i lar to zir con ages from sur round ing gneiss es. This sug gests that the mag matic protolith of gneiss es and the magma of the Bt-Ms gran ite could have come from sim i lar sources, or that the magma of the Bt-Ms gran ite was con tam i nated by the gneiss es. Both the tonalite and Bt-Ma gran ite rep re sent a late stage of the granitoid magmatism in the east ern part of the Variscan orogen. Teresa Oberc-Dziedzic and Ryszard Kryza, In sti tute of Geo log i cal Sci ences, ni ver sity of Wroc³aw, M. Borna 9, Wroc³aw, Po - land, s: teresa.oberc-dziedzic@ing.uni.wroc.pl, ryszard.kryza@ing.uni.wroc.pl (re ceived: Ocotober 5, 2011; ac cepted: Jan u ary 11, 2012; first pub lished on line: May 6, 2012). Key words: Strzelin Mas sif, Gêsiniec com pos ite in tru sion, Variscan granitoids, SHRIMP zir con ages. INTRODCTION The Variscan orogenic belt in Cen tral Eu rope abounds in granitoids that were formed in sev eral mag matic events, be - tween 370 and 250 Ma (Fin ger et al., 1997) or 340 and 270 Ma (Schaltegger, 1997). In the West and Cen tral Sudetes, and in the adjoining Fore-Sudetic Block (NE part of the Bo he mian Massif), the Variscan granitoid magmatism oc curred be tween Ma and Ma, re spec tively (Mazur et al., 2007). The youn gest Variscan granitoids, Ma in age (Oberc-Dziedzic et al., 2010) are found in the Strzelin Mas sif in the east ern part of the Fore-Sudetic Block which, to gether with the East Sudetes, are cor re lated with Brunovistulicum (sensu Dudek, 1980; Fin ger et al., 1989). The Variscan granitoids of the Strzelin Mas sif are dif fer ent, in sev eral as pects, from other Sudetic granitoids. They do not form large mag matic bod ies, but nu mer ous, small in tru sions, com posed of rocks petro graphi cally var ie gated from quartz diorite to peraluminous gran ite. In many cases, petro graphi - cally similar rocks have different geochemical characteristics and ages. -Pb SHRIMP ages re veal three dis tinct stages of the Carboniferous early Permian granitoid magmatism: tonalitic I ca. 324 Ma, granodioritic ca. 305 Ma, and tonalitic II/gra - nitic ca. 295 Ma (Oberc-Dziedzic et al., 2010). In an ear lier contribution (Oberc-Dziedzic et al., 2010), we pro vided ev i - dence for two tonalitic stages: I 324 Ma and II 295 Ma in the south ern part of the Strzelin Mas sif, and we showed petrographic and geochemical differences between tonalites of both stages. Here, in this pa per, we pres ent new SHRIMP zir - con data from tonalite and biotite-muscovite (Bt-Ms) gran ite from the Gêsiniec com pos ite in tru sion (north ern part of the Strzelin Mas sif) and briefly dis cuss these within the con text of the Variscan granitoid plutonism in the Fore-Sudetic Block, at the NE edge of the Bo he mian Mas sif. GEOLOGICAL SETTING The Strzelin Mas sif (sensu Oberc-Dziedzic et al., 2010) is sit u ated in the east ern part of the Fore-Sudetic Block, 35 km south of Wroc³aw (Fig. 1). Two struc tural units, sep a rated by the Strzelin Thrust, are dis tin guished in this area: the lower unit (the footwall of the thrust) is com posed of the rocks of the Strzelin Com plex, and the up per unit (the hang ing wall of the

2 226 Teresa Oberc-Dziedzic and Ryszard Kryza Fig. Geo log i cal map of the Strzelin Mas sif (com piled by Oberc-Dziedzic and Madej, 2002, based on Wójcik, 1968; Wroñski, 1973; Badura, 1979; Oberc et al., 1988) The Strzelin Thrust (ST in the north ern part of the map) sep a rates the Stachów and Strzelin com plexes; in set map: Bo he mian Mas sif and Moravo-Silesian Zone (grey-shaded); open rect an gle shows the po si tion of the Strzelin Mas sif; Gêsiniec quarry lo ca tion: N , E thrust), com prises the rocks of the Stachów Com plex (Oberc-Dziedzic and Madej, 2002). The Strzelin Thrust is in - ter preted as the north ern ex ten sion of the bound ary be tween the East and West Sudetes (pres ently, the east ern part of the West Sudetes is de fined as the Cen tral Sudetes; Mazur et al., 2006), i.e. part of the tec tonic bound ary be tween the Brunovistulian and Moldanubian ter ranes in the NE part of the Bo he mian Mas - sif (Oberc-Dziedzic et al., 2005). The Strzelin Com plex is com posed of Neoproterozoic gneisses: the Strzelin orthogneiss, typ i cal of the north ern part of the Strzelin Mas sif, with zir con ages of and Ma (the first in ter preted as the mag matic age of the gneiss

3 Late stage Variscan magmatism in the Strzelin Massif (SW Poland) protholith, the sec ond as the age of pos si ble, later par tial melt - ing; Oberc-Dziedzic et al., 2003a), and the sillimanite Nowolesie gneiss, oc cur ring in the south ern part of the mas sif, with zir con ages of and Ma (Klimas, 2008) or Ma (Mazur et al., 2010). The Strzelin Com plex com - prises also, apart from the gneiss es, the older schist se ries of Neoproterozoic or early Pa leo zoic(?) age, com posed of am phi - bo lites, mica schists, calc-sil i cate rocks and mar bles, and the youn ger schist se ries (the Jeg³owa Beds; Oberc, 1966) of quartzites, quartz-seri cite schists and metaconglomerates. The Jeg³owa Beds have been cor re lated with biostratigraphically documented Lower Devonian metaquartzites of the Jeseniki Moun tains of the East Sudetes (Bederke, 1931; Oberc, 1966; Chlupaè, 1975). The Stachów Com plex con tains orthogneisses which yielded Early Or do vi cian (~500 Ma) zir con ages (Ol i ver et al., 1993; Kröner and Mazur, 2003; Oberc-Dziedzic et al., 2003b; Klimas, 2008; Mazur et al., 2010) and dark-col oured, fine-grained gneiss. The in ter ca la tions of the dark gneiss with mica schists and am phi bo lites are in ter preted as a Neoproterozoic or lower Pa leo zoic metasedimentary suc ces - sion, rep re sent ing the meta mor phic en ve lope of the ~500 Ma old granitoid protolith of the orthogneisses (Oberc-Dziedzic and Madej, 2002). The Strzelin and Stachów complexes underwent polyphase deformation and metamorphism that culminated during the Variscan orog eny (Oberc-Dziedzic et al., 2005, 2010). The Strzelin Mas sif was in truded by four groups of Variscan granitoids: granodiorites (~305 Ma, -Pb zir con SHRIMP method; Oberc-Dziedzic et al., 2010); tonalites and quartz diorites (I ~324 Ma and II ~295 Ma, -Pb zir con SHRIMP method; Oberc-Dziedzic et al., 2010); me dium- and fine-grained bi o tite gran ites; two-mica gran ites. The ages of the lat ter two groups of rocks were es ti mated us ing var i ous meth ods. The ear lier Rb-Sr whole-rock stud ies (Oberc-Dziedzic et al., 1996; Oberc-Dziedzic and Pin, 2000) yielded rel a tively old dates of Ma for the bi o tite gran - ites, and Ma for the two-mica gran ites. These dates have re cently been ques tioned as too old, in the light of Pb evap o ra tion zir con data for the bi o tite gran ite (301 7 Ma; Turniak et al., 2006), and new SHRIMP zir con ages, com - bined with geological evidence, indicating that the two-mica gran ites are youn ger than the ~295 Ma tonalites (Oberc-Dziedzic et al., 2010). THE GÊSINIEC COMPOSITE INTRSION The Gêsiniec composite intrusion comprises the largest and best ex posed tonalite body in the Strzelin Mas sif. Based on bore hole data, this in tru sion was in ter preted as a stock with a flat apophysis (Fig. 2), and this in ter pre ta tion has been con - firmed in the course of the ex ploi ta tion in the quarry. The tonalite body is cut by thin dykes of the fine-grained Fig. Geo log i cal cross-sec tion through the Gêsiniec in tru sion, mod i fied from Borek (1987) granodiorite and by dykes of leucocratic, fine-grained Bt-Ms granite (Fig. 3), tens of centi metres to 15 metres thick. The en - ve lope of the Gêsiniec in tru sion is com posed of the Strzelin Complex orthogneiss and am phi bo lite, the lat ter trans formed into pyroxene hornfelses at the con tact with the tonalite (Puziewicz and Oberc-Dziedzic, 1995; Oberc-Dziedzic, 2007). The petrography of the Gêsiniec tonalites, diorites and granodiorites, sources of their mag mas, pro cesses of magma mixing, mingling, and fractional crystallisation recorded in plagioclases were stud ied by Pietranik et al. (2006), Pietranik and Waight (2008) and Pietranik and Koepke (2009). Pietranik and Waight (2008) de fined sev eral types of tonalites and diorites, based on field and an a lyt i cal data. Here, in this pa per, we de scribe only the main fa cies of the Gêsiniec intrusion (Puziewicz and Oberc-Dziedzic, 1995; Oberc-Dziedzic, 2007). Our petrographic subdivision is simplified compared with that proposed by Pietranik and Waight (2008). The mar ginal fa cies of the in tru sion is formed of dark grey, very fine-grained quartz diorite (Table 1). This rock dis plays par al lel align ment of plagioclase grains and the pres ence of bi - o tite plates, up to 3 mm in size, sur rounded by white quartz-plagioclase rims in places. The quartz diorite is brecciated and cor roded by the tonalites. The in ner part of the in tru sion con sists of sev eral fa cies of the tonalites. Some of these have been com pletely ex ploited. The tonalite fa cies dif fer in tex ture and pro por tion of the main components (Table 1 and Fig. 4). The shapes of ap a tite, ti tan ite and plagioclase grains are highly vari able. Plagioclase grains usu ally show cor roded cores and a very com pli cated zon ing which al low de ci pher ing of pro cesses in the tonalite magma be - fore and af ter the em place ment (Pietranik and Waight, 2008). Dark min er als have ran dom or par al lel ar range ment and they form clus ters in places. The most com mon va ri ety, form ing the in ner part of the in tru sion, is pale grey, me dium-grained tonalite, with uni formly dis trib uted dark min er als. Lo cally, it shows an in dis tinct par al lel tex ture. The me dium-grained

4 228 Teresa Oberc-Dziedzic and Ryszard Kryza Fig. Dykes of Bt-Ms gran ite in tonalite, Gêsiniec quarry (2006) A bent dyke, south ern wall of the quarry; B dis rupted dyke, north ern wall of the quarry tonalite grades into dark, me dium-grained tonalite, with ir reg - ularly distributed dark minerals concentrated into clusters or spots, up to 5 mm in size. The dark min er als are, lo cally, ar - ranged into lay ers and schlieren. An other va ri ety of the dark tonalite con tains sin gle plates of bi o tite, up to 5 mm large. The pale grey, me dium-grained and dark va ri et ies of the tonalite can grad u ally change into pale tonalite. In the tran si tion zones, the dark tonalites dis play a schlieren struc ture. The pale tonalite con tains up to 1% microcline. Bi o tite is the only dark min eral. This type of tonalite also forms small dykes in the darker varieties. Pietranik and Waight (2008) showed that dif fer ent diorite-tonalite types are not re lated to each other by as sim i la - tion fractional crystallisation processes but they probably evolved as sep a rate magma batches. The sec ond mag matic in jec tion in the Gêsiniec intrusion is represented by grey, fine-grained granodiorite (Puziewicz and Oberc-Dziedzic, 1995). This rock forms ir reg u lar streaks or dykes within the tonalites. Bi o tite and plagioclase com pos ing Ta ble 1 Min er als Dark, fine-grained tonalite T 1 Min eral com po si tion of the Gêsiniec tonalites and Bt-Ms gran ite Pale grey, me dium-grained tonalite T 2 Dark, me dium-grained tonalite T 3 Pale tonalite T 4 Bt-Ms gran ite GT1 GT2 GT3 GT6 GT10 GT13 GT5 GT7 GT11 GT12 GT8 Quartz Plagioclase Microcline Hornblende Bi o tite Ti tan ite Ap a tite Opaque Chlorite Cal cite 0.5 Mus co vite not found or be low 0.1%

5 Late stage Variscan magmatism in the Strzelin Massif (SW Poland) SHRIMP ZIRCON STDY METHODS Fig. Gêsiniec tonalites and Bt-Ms gran ite on a QAP di a gram and sam ples an a lysed by the SHRIMP method the granodiorite are ar ranged par al lel to the bor ders of the dykes. A de tailed study of the granodiorite magma evo lu tion was given by Pietranik et al. (2006). The granodiorite is very rich in small, dark en claves of mica schist, up to 3 cm in size. Gneissic en claves are also very com - mon. They are, oc ca sion ally, sur rounded by thin quartz-feldspathic or bi o tite rims. Some en claves are di vided into parts of different orientation, suggesting their rotation in the magma. The con tacts of the granodiorite with the sur round ing me - dium-grained tonalite are usu ally sharp. How ever, the granodiorite can also pen e trate the tonalite. In that case, the con tacts of the two rocks be come ir reg u lar and small en claves of tonalite ap pear in the granodiorite. The third, youn gest mag matic in jec tion is rep re sented by pale, fine-grained Bt-Ms gran ite. This gran ite forms a dyke about 15 m in thick ness and sev eral thin ner dykes, m thick (Fig. 3). The gran ite is com posed of quartz, plagioclase, microcline, rare bi o tite and muscovite (Table 1 and Fig. 4), and con tains also small pinite pseudomorphs after cordierite. Seem ingly, there is a close spa tial re la tion ship be tween the granodiorite and the Bt-Ms gran ite. The mag mas of both rocks prob a bly used the same chan nels dur ing em place ment. The Gêsiniec tonalites of ten dis play a par al lel ar range ment of dark min er als which de fine lineation and fo li a tion. The mag - matic fo li a tion was fol lowed by late-mag matic shear zones. In con trast to the tonalites, the Bt-Ms gran ite does not dis play any foliation or lineation (Oberc-Dziedzic, 1999). The sam ples se lected for the SHRIMP zir con study, tonalite and two-mica gran ite, each ca. 3 5 kg in weight, were crushed and the heavy min eral frac tion ( mm) sep a rated us ing a stan dard pro ce dure with heavy liq uids and mag - netic separation. Zircons were hand picked un der a mi cro scope, mounted in ep oxy and pol ished. Transmitted and reflected light photomicrographs were made, along with CL im ages, in or der to se lect grains and choose sites for anal y sis. -Pb analyses were per formed on the Sensitive High Resolution Ion Microprobe (SHRIMP II) at the All Rus sian Geological Research Institute (VSEGEI) in St. Pe - tersburg, applying a secondary electron multiplier in peak-jump ing mode, fol low ing the pro ce dure de - scribed by Wil liams (1998) and Larionov et al. (2004). A pri mary beam of mo lec u lar ox y gen was em ployed to bom bard zir con in or der to sput ter sec - ondary ions. The elliptical analytical spots had a size of ca um, and the cor re spond ing ion current was ca. 4 na. The sput tered sec ond ary ions were ex tracted at 10 kv. The 80 mm wide slit of the sec ond ary ion source, in com bi na tion with a 100 um multiplier slit, allowed mass-resolution of M/AM >5000 (1% val ley) so that all the pos si ble iso baric in ter fer ences were re solved. One-min ute rastering over a rect an gu lar area of ca um was em ployed be fore each anal y sis in or der to re move the gold coat ing and pos si ble sur face com mon Pb con - tamination. The fol low ing ion spe cies were mea sured in se quence: 196 (Zr 2 O)- 204 Pb-background (ca. 204 AM)- Pb- Pb- 208 Pb Tho- 254 O with in te gra tion time rang ing from 2 to 20 seconds. Four cy cles for each spot ana lysed were ac quired. Each fifth mea sure ment was car ried out on the zir con Pb/ stan dard TEMORA (Black et al., 2003) with an ac cepted Pb/ 238 age of Ma. The zir con, with a con cen tra tion of 82 ppm and a Pb/ 238 age of Ma (Wiedenbeck et al., 1995), was ap plied as a -con- centration standard. The re sults col lected were then pro cessed with the SQID v12 (Lud wig, 2005a) and ISOPLOT/Ex 22 (Ludwig, 2005b) soft ware, us ing the de cay con stants of Steiger and Jäger (1977). The com mon lead cor rec tion was done us ing measured 204 Pb ac cord ing to the model of Stacey and Kramers (1975). The re sults of the zir con anal y ses are shown in Tables 2 and 3, and Figures 5 SAMPLE (TONALITE) Sam ple rep re sents the pale grey, me dium-grained tonalite, the most ho mo ge neous fa cies of the Gêsiniec tonalite

6 Spot Pb c [ppm] Th [ppm] Th/ Pb* [ppm] SHRIMP data for zircons from the 238 Pb/ Age Pb/ Pb Age Discordant G¹ s iniec t onalite ( G¹ s iniec quarry, sample ) Total 238 / Pb Total Pb/ Pb * / * * / 235 * / 2 38 T a b l e err. corr Teresa Oberc-Dziedzic and Ryszard Kryza Errors are 1 ; b P c * and Pb indicate the common and radiogenic portions, respectively; error in standard calibration was 0.78%; common Pb corrected using measured 204 Pb

7 T a b l e 3 SHRIMP data for zircons from the Bt-Ms granite ( G êsiniec quarry, sample ) Spot c [ppm] Th [ppm] 232 Th 238 / * 238 [ ppm] Pb/ Age Pb/ Pb Age Discor- d ant Total 2 38 / Pb Total Pb / Pb * / * * / 235 * / 2 38 err. corr For explanations see Table Late stage Variscan magmatism in the Strzelin Massif (SW Poland)

8 232 Teresa Oberc-Dziedzic and Ryszard Kryza Fig. Cathodoluminescence im ages of zir cons ana lysed from tonalite Sym bols of an a lyt i cal points cor re spond to those in Ta ble 2; Pb/ 238 ages and one er rors (rounded to in te gers) are given (Table 1 and Fig. 4) show ing a poorly vis i ble par al lel ar range - ment of dark min er als. The rock cor re sponds to leucocratic, me dium-grained quartz diorite in Pietranik and Waight (2008) classification. The tonalite is com posed of euhedral, zoned grains of plagioclase (An ) sur rounded by anhedral grains of quartz show ing a wavy ex tinc tion. Dark min er als are rep re sented by bi o tite and green hornblende oc cur ring as in di - vid ual grains or ag gre gates. Ap a tite and zir con are com mon ac - cessories. The zir cons of this sam ple (Fig. 5) are rel a tively large, euhedral, mostly short- and subordinately normal-prismatic. The ma jor ity of the crys tals dis play a reg u lar euhedral zonal pat tern: CL-dark cen tral parts over grown by a bright reg u larly zoned man tle, and dark and usu ally thin outer rims. Lon ger crys tals have usu ally less dis tinct in ter nal struc ture. Twenty points in eigh teen zir con grains in tonalite sam ple were ana lysed by SHRIMP. Prac ti cally, all the points ana - lysed be long to one ho mo ge neous group, with only two anal y - ses yield ing somewhat younger Pb/ 238 ages: 9.1 (276 5 Ma) and 11 (272 5 Ma). The Con cordia age for all the points is Ma, and with the two youn gest points ex - cluded Ma (Fig. 6). All the anal y ses con firm the homogeneity of the zircon population: Pb c val ues are mostly be low l% (max i mum value 46%)., Th and Pb con cen tra - tions are also sim i lar in most of the grains, with two ex cep tions (l2 and 11) which are much richer in (Table 2). The 232 Th/ 238 ra tios vary from low (0.06) to fairly high (19), with rather even dis per sion over the whole range; the low est val ues may cor re spond to late-mag matic crystallisation. Dis cor dance varies from 75 (one ex treme value 139 in grain 11) to +47, with sev eral points of con sid er ably lower val ues. Sum ming up, the zir con pop u la tion in the tonalite is ho mo ge - neous and rep re sents a mag matic crystallisation stage at ca Ma. The strong CL zonation is not re flected in age vari a tion. SAMPLE (TWO-MICA GRANITE) Sam ple was taken from a Bt-Ms gran ite dyke, 15 metres thick, sit u ated in the east ern part of the Gêsiniec quarry. It rep re sents a pale, fine-grained gran ite with rare, dis - persed, rounded or square pinite pseudomorphs after cordierite. The gran ite is com posed of euhedral or subhedral grains of plagioclase (An 24-6 ) with scarce muscovite inclusions, and anhedral grains of microcline and quartz. Brown bi o tite and muscovite form sparsely dis trib uted small plates. Ac ces sory zir con is not abun dant. The zir con grains in this sam ple have var i ous morphologies and other dif fer ent fea tures (Fig. 7). Most of them are euhedral, normal-prismatic, with a distinct internal recurrent zonation, well vis i ble in CL im ages (grains 1, 1, 1, 1, 6.1, 9.1, 11 and 11). su ally CL-bright in te ri ors have thin CL-dark euhedral overgrowths. A few of the grains ana lysed have rather dis tinct cores (e.g., 1 and 10.1) and oval inclusions (11). Sev eral crys tals are CL-dark and less clearly struc tured (7.1, 1, 11), and a few are bro ken (7.1, 11). Twenty points in 15 zir con crys tals were ana lysed by SHRIMP. Eight points yielded rel a tively old ages, be tween 373 Ma and 5 Ga, and were in ter preted as in her i tance. The core of one grain (6.2) dis plays a Mesoproterozoic, slightly dis - cor dant (D = 6%) Pb/ Pb age of Ma. Char ac ter - is ti cally, its man tle (6.1) is Ma old, and its rim (6.3)

9 Late stage Variscan magmatism in the Strzelin Massif (SW Poland) Fig. 6. Con cordia di a gram show ing re sults of SHRIMP zir con anal y ses from tonalite Fig. 7. Cathodoluminescence im ages of zir cons ana lysed from Bt-Ms gran ite Sym bols of an a lyt i cal points cor re spond to those in Ta ble 3; Pb/ 238 ages and one er rors (rounded to in te gers) are given

10 234 Teresa Oberc-Dziedzic and Ryszard Kryza Fig. Con cordia di a gram show ing re sults of SHRIMP zir con anal y ses from bi o tite-mus co vite gran ite Ma old (the lat ter two Pb/ 238 ages). Hav ing in mind this strong age zonation, the ages ob tained in this grain may partly over lap due to the large an a lyt i cal spot size. The Pb c con - tents in these points are low ( %), whereas the 232 Th/ 238 ra tio drops from 0.57 in the core to 0.04 in the rim. Rel a tively old (com pared with the main group 2) and gen er ally con cor dant ages were found in five other points: 1 ( Ma), 1 ( Ma), 9.1 (503 9 Ma), 11 (401 7 Ma) and 2 (374 7 Ma). They all represent grain interiors. Two of them have high Pb c val ues (1 36%, 11 48%), and most of them have low 232 Th/ 238 ra tios ( ; Table 3). Ten an a lyt i cal points (1, 2, 1, 1, 7.1, 1, 10.1, 11, 11 and 11) form the main age group of zir cons, with a con - cor dant age of Ma (Fig. 8). These points rep re sent both in ter nal parts (man tles) and outer rims of grains. Most of them are CL-dark and have var ied but pre dom i nantly mod er ate 232 Th/ 238 ratios ( ). However, three analytical points seem to be dis tinctly youn ger, with Pb/ 238 dates around 282 Ma, though based on a small num ber of points; it is dif fi - cult to de cide whether these are geo log i cally sig nif i cant ages or rather artefacts caused, e.g., by ra dio genic Pb loss. One of these points (2) dis plays high Pb c (7.39%), sim i larly to two other points (11 = 6.78, and 12 = 36%) that yielded un re al is ti - cally young dates of ca. and 263 Ma, re spec tively. These re sults sug gest a pos si ble dis tur bance of the -Pb sys tem af ter the mag matic crystallisation of the gran ite. Over all, the twenty points ana lysed in sam ple have re - vealed a wide di ver sity of ages, in clud ing a wide range of in her i - tance and a fairly co her ent mag matic pop u la tion. The abun dant in her ited zir cons, vary ing in age from ca. 374 Ma to ca. 5 Ga, represent crustal components of various ages, from Mesoproterozoic to mid-pa leo zoic. The mag matic zir con growth of the main pop u la tion is fairly well-con strained at Ma. DISCSSION The SHRIMP zir con study has shown that the Gêsiniec intrusives, rep re sented by the me dium-grained tonalite () and the Bt-Ms gran ite () in dykes cut ting the tonalite, are of the same age of 295 Ma. The same Rb-Sr age ( Ma) was ob tained ear lier by Pietranik and Waight (2008) from a WR+biotite+plagioclase isochron for the leucocratic me - dium-grained quartz diorite Gêsiniec tonalite. The sharp con tacts be tween the tonalite and the Bt-Ms gran ite dykes in di cate that the em place ment of the gran ite took place when the tonalite was completely solidified. After the em place ment, some the Bt-Ms gran ite dykes, as so ci ated with

11 Late stage Variscan magmatism in the Strzelin Massif (SW Poland) the granodiorite, were de formed bent or torn (Fig. 3). Ap par - ently, the pres ence of the (still hot?) granodiorite near the veins fa cil i tated their bend ing and heal ing of the cracks. These field relationships suggest that the emplacement of the tonalite, granodiorite and Bt-Ms gran ite mag mas took place over a short time spam, shorter than the er rors of the SHRIMP zir con ages. Al though the zir cons from the tonalite and Bt-Ms gran ite show the same age, they dif fer con sid er ably in their phys i cal and chemical features. The zircon population in the tonalite is ho mo ge neous and the grains are rel a tively large and euhedral. They show strong CL zonation which is not re flected in age variation. In con trast, the zir con grains in the Bt-Ms gran ite are much smaller than those in the tonalite, and have var i ous morphologies. Most of them are euhedral, with a dis tinct in ter - nal recurrent zonation. A num ber of grains have inherited cores with ages from ca. 374 Ma to ca. 5 Ga. The age spec trum of the inherited zircons is fairly similar to the ages re ported from orthogneisses of the Strzelin Mas sif (Oberc-Dziedzic et al., 2003a; Klimas, 2008). They may rep re sent a crustal com po - nent of the Bt-Ms gran ite magma and indicate that similar gneiss es could have been the source ma te rial for this magma. They also sug gest that the mag matic protolith of the orthogneisses and that the magma of the Bt-Ms gran ite could have come from sim i lar sources or that the magma of the Bt-Ms gran ite was con tam i nated by the gneiss es. The Ma age of the Gêsiniec tonalite is the same as the age obtained previously for the Kalinka tonalite from the south ern part of the Strzelin Mas sif (Oberc-Dziedzic et al., 2010). Both the tonalites show a similar geochemical composition (Bia³ek and Pietranik, 2006). How ever, the Kalinka tonalite is more ho mog e neous than the Gêsiniec tonalite. Moreover, the Kalinka tonalite is, like most of the tonalites from the south ern part of the Strzelin Mas sif, fine-grained, whereas the Gêsiniec tonalite is me dium-grained. The fine grain-size in the Kalinka tonalite in di cates a faster rate of its crystallisation, com pared with the Gêsiniec tonalite. This seems to be sup ported by the zir con hab its: the zir con crys tals from the Kalinka tonalite are long-pris matic, pen cil-like (Oberc- Dziedzic et al., 2010), typ i cal of fast crystallisation, whereas the zir cons from the Gêsiniec tonalite are mostly short- and, subordinately, nor mal-pris matic. In spite of the lo cal dif fer - ences, the to nal i ties from dif fer ent parts of the Strzelin Mas sif represent the same, ca. 295 Ma plutonic event re corded in the east ern part of the Fore-Sudetic Block. CONCLSIONS The Gêsiniec com pos ite in tru sion was formed in the course of three late Variscan mag matic ep i sodes at ~295 Ma, which oc curred shortly one af ter an other in the fol low ing se - quence: tonalite, granodiorite and gran ite. The Bt-Ms gran ite con tains in her ited zir cons varying in age from ca. 374 Ma to ca. 5 Ga, sim i lar to zir con ages from the surrounding orthogneisses, which could be, thus, the source of the gran ite magma or its con tam i nant. The age of the Gêsiniec tonalite is the same as the Kalinka tonalite in the south ern part of the Strzelin Mas sif, and both rep re sent the late-stage granitoid plutonism event in the Variscides of the NE mar gin of the Bo he mian Mas sif. Acknowledgements. The study was sup ported by in ter nal grants 1017/S/ING and 2022/W/ING of the ni ver sity of Wroc³aw. K. Dymna helped greatly in zir con sep a ra tion. S. Sergeev and other col leagues of VSEGEI are thanked for their as sis tance with SHRIMP anal y sis. We are also grate ful to M. Kohút and an anon y mous re viewer for their con struc tive com ments that helped to im prove this pa per. REFERENCES BADRA J. (1979) Szczegó³owa mapa geologiczna Sudetów, ark. Stolec 1: Inst. Geol., Wyd. Geol., Warszawa. BEDERKE E. (1931) Die moldanubische Überschiebung im Sudetenvorlände. Zentralblatt für Min eral. Geol. Paläont. Abt. B: BIA EK J. and PIETRANIK A. (2006) Geo chem is try and pe trol ogy of tonalite and granodiorite from Strzelin Crys tal line Mas sif com par i - son. Miner. Pol., Spec. Pap., 29: BLACK L. P., KAMO S. L., ALLEN C. M., ALEINIKOFF J. N., DAVIS D. W., KORSCH R. J. and FODOLIS C. (2003) TEMORA 1: a new zir con stan dard for Phanerozoic -Pb geo chron ol ogy. Chem. Geol., 200: BOREK Z. (1987) Kompleksowa dokumentacja geologiczna z³ó granitoidów Strzeliñski Okrêg Eksploatacji Surowców Skalnych. Arch. Przedsiêbiorstwa Geologicznego Proxima we Wroc³awiu. CHLPAÈ I. (1975) New finds of fauna in the meta mor phic De vo nian of the Hruby Jeseník Mts. (Moravia, Czecho slo va kia) (in Czech with Eng lish sum mary). Èas. Miner. Geol., 20: DDEK A. (1980) The crys tal line base ment block of the outer Carpathians in Moravia-Bruno-Vistulicum. Rozpravy Èeskoslovenské Akademie Véd, Øada matematických a pøirodnich Véd, 90: 1 8 FINGER F., FRASL G., HÖCK V. and STEYRER H. P. (1989) The granitoids of the Moravian Zone of north east Aus tria: prod ucts of a Cadomian ac tive con ti nen tal mar gin? Pre cam brian Res., 45: FINGER F., ROBERTS M. P., HANSCHMID B., SCHERMAIER A. and STEYRER H. P. (1997) Variscan granitoids of cen tral Eu rope: their typology, po ten tial sources and tectonothermal re la tions. Miner. Petrol., 61: KLIMAS K. (2008) Geo chron ol ogy and petrogenetical study of zir cons from se lected crys tal line rocks in the east ern part of the Fore-Sudetic Block (in Pol ish with Eng lish sum mary). niwersytet Wroc³awski, Inst. Nauk Geol., Wroc³aw. KRÖNER A. and MAZR S. (2003) Pro tero zoic and Palaeozoic crustal com po nents across the East/Cen tral Sudetes bound ary at the east ern mar gin of the Bo he mian Mas sif: new /Pb sin gle zir con ages from the east ern Fore-Sudetic block (SW Po land). J. Czech Geol. Soc. Ab stract Vol., 48 (1 2): 83 8 LARIONOV A. N., ANDREICHEV V. A. and GEE D. G. (2004) The Vendian al ka line ig ne ous suite of north ern Timan: on microprobe -Pb zir con ages of gab bros and syenite. Geol. Soc. Lon don Mem., 30: 69 7

12 236 Teresa Oberc-Dziedzic and Ryszard Kryza LDWIG K. R. (2005a) SQID 12 A ser s Man ual. A Geo chron ol - ogi cal Toolkit for Microsoft Ex cel. Berke ley Geochronol. Cen ter Spec. Publ.: 1 22, menu.html LDWIG K. R. (2005b) ser s Man ual for ISOPLOT/Ex 2 A Geo - chron ol ogi cal Toolkit for Microsoft Ex cel. Berke ley Geochronol. Cen ter Spec. Publ.: 1 71, MAZR S., ALEKSANDROWSKI P., KRYZA R. and OBERC-DZIEDZIC T. (2006) The Variscan Orogen in Po land. Geol Quart., 50 : MAZR S., ALEKSANDROWSKI P., TRNIAL K. and AWDANKIEWICZ M. (2007) Ge ol ogy, tec tonic evo lu tion and Late Palaeozoic magmatism of Sudetes an over view. In: Granitoids in Po - land (eds. A. Koz³owski and J. Wiszniewska). AM Mono graph, 1: MAZR S., KRÖNER A., SZCZEPAÑSKI J., TRNIAK K., HANŽL P., MELICHAR R., RODIONOV N. V., PADERIN I. and SERGEEV S. A. (2010) Sin gle zir con -Pb ages and geo chem is try of granitoid gneiss es from SW Po land: ev i dence for an Avalonian af fin ity of the Brunian microcontinent. Geol. Mag., 147 (4): OBERC J. (1966) Ge ol ogy of crys tal line rocks of the Wzgórza Strzeliñskie Hills, Lower Silesia (in Pol ish with Eng lish sum mary). Stud. Geol. Pol., 20: OBERC J., OBERC-DZIEDZIC T. and KLIMAS-AGST K. (1988) Mapa geologiczna Wzgórz Strzeliñskich w skali 1: (ed. J. Oberc). Inst. Nauk Geol. niwersytetu Wroc³awskiego, Przedsiêbiorstwo Geologiczne Wroc³aw. OBERC-DZIEDZIC T. (1999) Ge ol ogy of the Strzelin granitoids (Fore-Sudetic Block, SW Po land). Miner. Soc. Pol., Spec. Pap., 13: 22 3 OBERC-DZIEDZIC T. (2007) In ter nal struc ture of the gran ite and tonalite in tru sions in the Strzelin mas sif. In: Granitoids in Po land (eds. A. Koz³owski and J. Wiszniewska). AM Monograph, 1: OBERC-DZIEDZIC T. and MADEJ S. (2002) The Variscan overthrust of the Lower Palaeozoic gneiss unit on the Cadomian base ment in the Strzelin and Lipowe Hills mas sifs, Fore-Sudetic Block, SW Po land; is this part of the East-West Sudetes bound ary? Geol. Sudet., 34: 39 5 OBERC-DZIEDZIC T. and PIN C. (2000) The granitoids of the Lipowe Hills (Fore-Sudetic Block) and their re la tion ship to the Strzelin gran - ites. Geol. Sudet., 33: 17 2 OBERC-DZIEDZIC T., PIN C., DTHO J. L. and COTRIE J. P. (1996) Age and or i gin of the Strzelin granitoids (Fore-Sudetic Block, Po land): 87 Rb/ 86 Sr data. N. Jb. Miner. Abh., 171: OBERC-DZIEDZIC T., KLIMAS K., KRYZA R. and FANNING M. C. (2003a) SHRIMP -Pb zir con geo chron ol ogy of the Strzelin gneiss, SW Po land: ev i dence for a Neoproterozoic ther mal event in the Fore-Sudetic Block, Cen tral Eu ro pean Variscides. Int. J. Earth Sc. (Geol. Rundsch.), 92: OBERC-DZIEDZIC T., KLIMAS K., KRYZA R., FANNING M. C. and MADEJ S. (2003b) SHRIMP zir con ages from gneiss help lo cate the West-East Sudetes bound ary (NE Bo he mian Mas sif, SW Po land). J. Czech Geol. Soc. Abstr., 48 (1 2): 9 OBERC-DZIEDZIC T., KRYZA R., KLIMAS K., FANNING M. C. and MADEJ S. (2005) Gneiss protolith ages and tec tonic bound aries in the NE part of the Bo he mian Mas sif (Fore-Sudetic Block, SW Po land). Geol. Quart., 49 (4): OBERC-DZIEDZIC T., KRYZA R. and BIA EK J. (2010) Variscan mul - ti stage granitoid magmatism in Brunovistulicum: pet ro log i cal and SHRIMP -Pb zir con geo chron ol ogi cal ev i dence from the south ern part of the Strzelin Mas sif, SW Po land. Geol. Quart., 54 (3): OLIVER G. J. H., CORF F. and KROGH T. E. (1993) -Pb ages from SW Po land: ev i dence for a Cal edo nian su ture zone be tween Baltica and Gond wana. J. Geol. Soc. Lond., 150: PIETRANIK A. and KOEPKE J. (2009) In ter ac tions be tween dioritic and granodioritic mag mas in min gling zones: plagioclase re cord of mix ing, min gling and subsolidus in ter ac tions in the Gêsiniec In tru - sion, NE Bo he mian Mas sif, SW Po land. Contrib. Min er. Petrol., 158: PIETRANIK A. and WAIGHT T. (2008) Pro cesses and sources dur ing Late Variscan dioritic-tonalitic magmatism: in sights from plagioclase chem is try (Gêsiniec In tru sion, NE Bo he mian Mas sif, Po land). J. Petrol., 49 (9): PIETRANIK A., KOEPKE J. and PZIEWICZ J. (2006) Crys tal li za tion and resorbtion in plutonic plagioclase: im pli ca tions on the evo lu tion of granodiorite magma (Gêsiniec granodiorite, Strzelin Crys tal line Mas sif, SW Po land). Lithos, 86: PZIEWICZ J. and OBERC-DZIEDZIC T. (1995) Age and or i gin of granitoids of the Fore-Sudetic block (in Pol ish with Eng lish sum - mary). Ann. Soc. Geol. Pol. Spec. Vol., Przewodnik LXVI Zjazdu PTG: SCHALTEGGER. (1997) Magma pulses in the Cen tral Variscan Belt: ep i sodic melt gen er a tion and em place ment dur ing lithospheric thin - ning. Terra Nova, 9: STACEY J. S. and KRAMERS J. D. (1975) Ap prox i ma tion of ter res trial lead iso tope evo lu tion by a two-stage model. Earth Planet. Sc. Lett., 26: 22 STEIGER R. H. and JÄGER E. (1977) Subcommission on geo chron ol - ogy: con ven tion on the use of de cay con stants in geo- and cosmochronology. Earth Planet. Sc. Lett., 36: TRNIAK K., TICHOMIROVA M. and BOMBACH K. (2006) Pb-evap - o ra tion zir con ages of post-tec tonic granitoids from the Strzelin Mas sif (SW Po land). Miner. Soc. Pol., Spec. Pap., 29: WIEDENBECK M., ALLÉ P., CORF F., GRIFFIN W. L., MEIER M., OBERLI F., von QADT A., RODDICK J. C. and SPIEGEL W. (1995) Three nat u ral zir con stan dards for -Th-Pb, Lu-Hf, trace el e - ment and REE anal y ses. Geostandards Newslett., 19: 1 23 WILLIAMS I. S. (1998) -Th-Pb geo chron ol ogy by ion microprobe. Rev. Econom. Geol., 7: 1 3 WÓJCIK L. (1968) Szczegó³owa mapa geologiczna Sudetów, ark. Ciep³owody 1: Inst. Geol., Wyd. Geol., Warszawa. WROÑSKI J. (1973) Szczegó³owa mapa geologiczna Sudetów, ark. Ziêbice 1: Inst. Geol., Wyd. Geol., Warszawa.