Early Car bon if er ous (~337 Ma) gran ite in tru sion in Devonian (~400 Ma) ophiolite of the Cen tral-Eu ro pean Variscides
Ryszard KRYZA
Kryza R. (2011) – Early Car bon if er ous (~337 Ma) gran ite in tru sion in Devonian (~400 Ma) ophiolite of the Cen tral-Eu ro pean Variscides. Geol. Quart., 55 (3): 213–222. Warszawa.
The Cen tral-Sudetic ophiolites com prise mafic-ultra mafic com plexes around the E and S edges of the Góry Sowie Mas sif in SW Po land and are rec og nized as frag ments of De vo nian (~400 Ma old) oce anic crust. They con tain small rodingite bod ies and tectonized gran ite dykes that po ten tially can high light the ig ne ous, meta mor phic and struc tural de vel op ment of the ophiolitic suites. The gran ite dykes have been ten ta tively cor re lated with the Variscan granitoids of the Strzegom–Sobótka Mas sif to the north. How ever, new U-Pb SHRIMP zir - con data for gran ites from the ser pen tin ite quarry at Jordanów show a con cordia age of 337 ±4 Ma for the main zir con pop u la tion, and of 386 ±10 Ma for mi nor in her i tance. Thus, the age of the gran ite is con sid er ably older than the ages of the Strzegom–Sobótka granitoids, dated at ~310–294 Ma. The gran ite dyke has a sim i lar age as some other granitoids found near the ophiolitic frag ments, e.g., the Niemcza granitoids to the south, dated at 338 +2/–3 Ma; these older granitoids all rep re sents a rel a tively early stage of granitoid magmatism re - corded in that part of the Variscan Orogen. The age of the granitoid dyke within serpentinites con firms that the Pa leo zoic ophiolites were in cor po rated into the con ti nen tal crust al ready in early Visean times.
Ryszard Kryza, In sti tute of Geo log i cal Sci ences, Uni ver sity of Wroc³aw, Cybulskiego 30, PL-50-205 Wroc³aw, Po land, e-mail:
ryszard.kryza@ing.uni.wroc.pl (re ceived: No vem ber 01, 2010; ac cepted: June 14, 2011).
Key words: Variscides, ophiolites, Variscan granitoids, SHRIMP zir con geo chron ol ogy.
INTRODUCTION
The Cen tral-Sudetic ophiolites (CSO) com pris ing sev eral out crops of mafic-ultra mafic com plexes around the E and S edges of the Góry Sowie Mas sif in the Cen tral Sudetes and Fore-Sudetic Block (SW Po land) have been rec og nized as broadly com plete and well-pre served frag ments of Pa leo zoic oce anic crust (Majerowicz, 1979; Pin et al., 1988) in cor po rated into the struc tural mo saic of the NE part of the Bo he mian Mas - sif. These ophiolites have been used as key ar gu ments for con - struct ing palaeotectonic mod els for the evo lu tion of the east ern part of the Variscan belt of Eu rope (e.g., Jamrozik, 1981; Matte et al., 1990; Pin, 1990; Narêbski, 1993; Fin ger and Steyer, 1995; Cymerman et al., 1997; Höck et al., 1997; Franke, 2000;
Franke and ¯elaŸniewicz, 2000; Aleksandrowski and Mazur, 2002; Kryza et al., 2004; Mazur et al., 2006; Kryza and Pin, 2010; Nance et al., 2010).
The pe trol ogy of the ophiolites is well-stud ied, and in par - tic u lar the pre dom i nant MORB char ac ter is tics of the mafic rocks, as well as the man tle af fin ity of the ultra mafic rocks, are well-doc u mented (e.g., Majerowicz, 1979, 1994; Pin et al.,
1988; Gunia, 1992, 2000; Dubiñska, 1997; Dubiñska and Gunia, 1997; Abdel Wahed, 1999; Kryza and Abdel Wahed, 2000; Floyd et al., 2002; Kryza and Pin, 2010). De tailed geo - chem i cal stud ies of rodingites from these ophiolites (Dubiñska, 1997) sug gest that the rodingite protoliths could have orig i - nated in a su pra-subduction set ting, pos si bly in a fore-arc en vi - ron ment, dur ing obduction of the oce anic crust.
The age of the mag matic em place ment of the mafic rocks, af ter two de cades of con tro ver sies (see Pin et al., 1988; Ol i ver et al., 1993; Dubiñska et al., 2004), is now fairly well con - strained at ca. 400 Ma (Kryza, 2010; Kryza and Pin, 2010).
More vague re mains the meta mor phic and struc tural evo lu tion of the ophiolitic suite. The mafic rocks are usu ally se lec tively (non-penetratively) de formed, but in one out crop (the Braszowice Mas sif, south of the Œlê¿a ophiolite), the gab bros are ex cep tion ally highly de formed. Dziedzic (1989) in ter preted this strong de for ma tion as a re sult of “dy namic crys tal li sa tion”
af ter the em place ment of the gab bros.
The model of meta mor phic evo lu tion pro posed by Dubiñska (1995) re fers to pet ro log i cal con straints from ultra mafic rocks and as so ci ated rodingites. In this model, the ini tial serpentinisation and rodingitisation oc curred in an oce anic set -
was ex ten sive, pro moted by in tense frac tur ing and deep sea-wa - ter pen e tra tion (Jêdrysek et al., 1989). The sub se quent con ti nen - tal stage in volved “tec tonic gran u la tion” (in tense brecciation) and for ma tion of late meta mor phic min er als, and was broadly con tem po ra ne ous with in tru sion of Variscan granitoids. The main points of this model are broadly in line with the se quence of meta mor phic events in ferred by Kryza (in Majerowicz et al., 2000) for the gab bros of Œlê¿a: (1) mag matic em place ment of gab bro; (2) early low-T meta mor phism (prob a bly ocean-floor meta mor phism); (3) peak-T re gional meta mor phism as so ci ated with lo cal ized, non-pen e tra tive shear ing of the gab bro; (4) wan - ing (late) meta mor phism, un der de creas ing T, and partly re plac - ing the ear lier parageneses, and as so ci ated with lo cal ized de for - ma tion in semi-brit tle to brit tle con di tions.
Ac cord ing to Mierzejewski and Abdel Wahed (2000), the serpentinites show two deformational ep i sodes: ear lier top-to-WNW thrust ing along listric thrust faults, and later top-to-ESE nor mal fault ing due to re lax ation of the thrust blocks.
The con tact be tween the serpentinites and the gabbroic mem ber higher in the ophiolitic pseudostratigraphy is strongly tectonized.
As men tioned above, im por tant pet ro log i cal is sues of the CSO con cern the rodingites found in serpentinites of the struc - tur ally low er most mem ber of the ophiolite (Majerowicz, 1979, 1984; Heflik, 1982; Dubiñska and Szafranek, 1990; Dubiñska, 1995), which bear ev i dence of im por tant ep i sodes in the geo - log i cal, and par tic u larly the meta mor phic, evo lu tion of the ophiolitic suite. The rodingites are lo cally as so ci ated with leucocratic rocks of gra nitic (or less fre quently plagiogranitic) char ac ter (called “weibstein” in older lit er a ture), dis play ing very com plex min er al ogy and struc tural re la tion ships. The most fa mous lo cal ity of such “leucocratic zones” is the quarry at Jordanów, in the cen tral part of the Fore-Sudetic Block. The quarry has been known in the min er al og i cal lit er a ture for over 120 years (Traube, 1888), mainly for the fa mous oc cur rence of neph rite. Many pa pers de scrib ing the min er al ogy and pe trol - ogy of this lo cal ity, pub lished within the past few de cades, pro - vided a range of in ter est ing min er al og i cal data, and con trib uted much to our un der stand ing of the meta mor phic evo lu tion of the Œlê¿a ophiolie (for a re view of the ex ten sive lit er a ture, see Heflik, 1982; Majerowicz, 1984; Dubiñska and Szafranek, 1990; Dubiñska, 1995, 1997).
One of the key prob lems in the leucocratic zone at Jordanów are gran ite bod ies which form a very com plex con - tact zone against serpentinites and as so ci ated calc-sil i cate rocks. The gran ites have been ten ta tively cor re lated with the Variscan granitoids of the Strzegom–Sobótka Mas sif (Dubiñska and Szafranek, 1990; Dubiñska, 1995); how ever, their true age has been unconstrained.
This study pro vides a U-Pb SHRIMP zir con age for the gran ite from the leucocratic zone at Jordanów. Based on this new geo chron ol ogi cal data, some re gional age cor re la tions of
GEOLOGICAL FRAMEWORK AND PREVIOUS STUDIES
Small leucocratic bod ies and veins have been re ported from a num ber of lo cal i ties in the Gogo³ów–Jordanów ser pen tin ite mas sif, which is the out crop of the low er most, ultra mafic mem - ber of the Œlê¿a ophiolite (Fig. 1; Majerowicz, 1979, 1984, 1994; Dubiñska and Wiewióra, 1988; Pin et al., 1988;
Majerowicz and Pin, 1989; Dubiñska and Szafranek, 1990;
Dubiñska, 1995, 1997). Typ i cally, they are strongly tectonized and weath ered, but show dis tinct con tact zones, taken as ev i - dence that they rep re sent apophyses of the Variscan Strzegom–Sobótka granitoids (Dubiñska and Szafranek, 1990, and ref er ences therein).
Majerowicz (1984) and Majerowicz and Pin (1989) clearly dis tin guished be tween typ i cal rodingitic rocks and aplit ic rocks, the later ap par ently re lated to the Variscan granitoids.
The rodingites are com posed of grossular, diallage, sheridanite and vesuvianite and are in ter preted as boudinaged and metasomatically rodingitised (strongly inriched in cal cium) frag ments of gabbroic dykes of the ophiolite. These rocks have been con sid ered as com mon com po nents of ophiolitic com - plexes, be ing trans for ma tion prod ucts of sur round ing rocks at var i ous stages of serpentinisation. On the other hand, the aplit ic rocks, which are light-col oured, fine-grained vein rocks lo cally found within serpentinites, cor re spond mostly to gran - ite-granodiorite in com po si tion, and have been in ter preted as re lated to the granitoids.
The best stud ied “leucocratic zone” in the serpentinites is ex posed in the fa mous neph rite quarry at Jordanów. Heflik (1967, 1982) de scribed the min er al ogy of this leucocratic zone and in ter preted the al tered rocks as rodingites or rodingite-like rocks de vel oped at the ex pense of gabbroic dykes. Lis and Sylwestrzak (1981) re ported “small in clu sions of gra nitic peg - ma tite”, with beryl, gar net, tour ma line, colum bite, gahnite etc., within a quartz-zoisite vein in the Jordanów quarry, and they con sid ered these pegmatites as ge net i cally re lated to the Strzegom–Sobótka granitoids.
Dubiñska and Wiewióra (1988) de scribed in de tail layer-sil i cates in the con tact zone be tween gran ite and ser pen - tin ite at Jordanów and ten ta tively con cluded that the con tact zone can be re garded as a rodingite backwall, formed dur ing serpentinisation of ultra mafic rocks. In a sub se quent pa per, Dubiñska and Szafranek (1990) cor re lated the for ma tion of layer sil i cates: chlorite, ver mic u lite, talc and so on, with tex - tural changes of the rocks of the con tact zone be tween an apophyse of Variscan gran ite and older ser pen tin ite.
A sys tem atic de scrip tion of the rodingites of the E part of the Gogo³ów–Jordanów Mas sif is given by Dubiñska (1995),
who states that the rodingites are de rived from two types of protoliths: mafic rocks, and albitites or plagiogranites.
Rodingites, and the en clos ing serpentinites, form the highly tectonized and min er al og i cally very com plex con tact zone against youn ger gran ites in the Jordanów Quarry.
FIELD RELATIONSHIPS AND PETROLOGICAL NOTES
Field re la tion ships be tween var i ous rocks in the Jordanów Quarry and their min er al ogy, pe trol ogy and geo chem is try were de scribed by Dubiñska and Szafranek (1990). The serpentinites dom i nant in the quarry are rep re sented by an antigoritic va ri ety, with no rel ict mafic min er als nor pseudomorphic tex tures. How - ever, opaque min er als in di cate bastite pseudo morphs, and the tex ture is lo cally oblit er ated by fine-grained talc and colour less monoclinic am phi bole (op. cit.).
Ac cord ing to Dubiñska (1995), the var i ous fel sic rocks (i.e., rodingites and metagranites – R. Kryza) in the Jordanów Quarry oc cur in two ad ja cent zones, 20–25 m and ~5 metres wide, both at the con tact be tween gran ite and ser pen tin ite. In a sche matic sketch of the con tact zone (Dubiñska, 1995, fig. 4), the coun try rocks, antigorite serpentinites are as so ci ated with sev eral va ri et ies of rodingites (zoisite ± gar net, zoisite ± gar - net and di op side), ridingite brec cias and plagiogranite, as well as tectonized leucocratic gran ite. Along the main con tact, vermiculite–chlorite–tremolite ± talc con tact schists are de vel - oped, with lo cal con cen tra tions of neph rite and tremolite rocks.
Within the leucocratic zone, frag ments of brecciated leucogranite are com mon. Ac cord ing to Dubiñska (1995), the light-col oured and fine-grained gran ite is com posed of quartz, plagioclase (An18–22), microcline and mi nor bi o tite, fine-grained white mica and ac ces sory zir con. No over print ing of the gran ite by Ca-sil i cates of the “rodingitic” as sem blage has been found, ex cept for “...small sheafs of actinolite formed af -
Fig. 1. Geo log i cal sketch of the Œlê¿a ophiolite (based on Majerowicz, 1979, com put er ized by A. Wójcik and M. Kryza)
Tec tonic units (zones) of the Cen tral-Eu ro pean Variscides: MO – Moldanubian, MS – Moravosilesian, RH – Rhenohercynian, ST – Saxothuringian, TB – Tepla-Barrandian; lo ca tion of sam ple J8 from Jordanów Quarry is in di cated
gran ite (con tain ing K-feld spar, al bite, quartz and ac ces sory zir - con and ap a tite) are strongly over printed by sec ond ary parageneses, with abun dant grossular and subordinate actinolite-type mineral.
Ac cord ing to Dubiñska and Szafranek (1990), the leucocratic rocks of the Gogo³ów–Jordanów Mas sif are rel a - tively rich in po tas sium (up to 9.90 wt.%) and thus their com - po si tions cor re spond to the field of “con ti nen tal granophyre” of Coleman (1977, vide Dubiñska and Szafranek, 1990). Their two gran ite sam ples ana lysed from the Jordanów Quarry have the main com po nents within the fol low ing ranges (in wt.%):
SiO2 68.58–75.22, Al2O3 13.10–16.39, Fe2O3 0.02–0.38, FeO 0.35–0.85, MgO 0.31–0.35, CaO 0.38–0.46, K2O 5.12–9.90, Na2O 2.94–4.12. Two other ana lysed sam ples of “feld spar-rich rocks” con tain less SiO2 and K2O, and more Al2O3, CaO and Na2O; their com po si tion cor re sponds to oligoclasite and they are sug gested to be pos si ble pa ren tal ma te rial for the calc-sil i - cate rocks (Dubiñska and Szafranek, 1990).
A model for the de vel op ment of the calc-sil i cate rocks from the leucocratic zone at Jordanów is com plex and com prises sev - eral stages (Dubiñska and Szafranek, 1990). The early rodingites de vel oped at the ex pense of albitite and/or plagiogranite bod ies.
The sub se quent ep i sodes of the evo lu tion of this zone were strongly in flu enced by the em place ment of gra nitic veins, pro - duc ing a con tact zone be tween ser pen tin ite (con tain ing a rodingite body) and gran ite. A sort of mélange of calc-sil i cate rocks was formed in the small leucocratic zone as a re sult of tec - tonic dis rup tion and dis place ment (Dubiñska, 1995).
METHODS
The sam ple se lected for SHRIMP anal y sis was thin-sec - tioned for petrographic in ves ti ga tion, us ing rou tine po lar iz ing
Wroc³aw, do nated by the Free Uni ver sity of Am ster dam).
The sam ple for zir con sep a ra tion, ca. 3 kg in weight, was crushed and the heavy min eral frac tion (0.06–0.25 mm) sep a - rated us ing a stan dard pro ce dure with heavy liq uids and mag - netic sep a ra tion. Zir cons were hand-picked un der a mi cro - scope, mounted in ep oxy and pol ished. Trans mit ted and re - flected light pho to mi cro graphs were made to gether with CL im ages in or der to se lect grains and choose sites for anal y sis.
The Sen si tive High Res o lu tion Ion Microprobe (SHRIMP II) at the Cen tre of Iso to pic Re search (CIR) at the All-Rus sian Geo - log i cal Re search In sti tute (VSEGEI) in St. Pe ters burg was used to de ter mine zir con ages in the sam ple se lected. De tails of the SHRIMP pro ce dures and an a lyt i cal de tails are given in the Ap - pen dix. Un cer tain ties for in di vid ual anal y ses (ra tios and ages) are at the one s level; how ever, the un cer tain ties in cal cu lated con cordia ages are re ported at the 2 s level.
SAMPLE DESCRIPTION
The sam ple for SHRIMP zir con dat ing was col lected from the main leucocratic body, at the base of the north ern wall of the Jordanów quarry (Fig. 2). The rock is leucocratic, whit - ish-cream in col our, with prac ti cally no mafic min er als discernable. The tex ture is fine-grained and mas sive, dom i - nated by a creamy mass, intergrown with grey ish patches of min eral ag gre gates dif fi cult to rec og nize with the na ked eye.
Lo cally, joints are cov ered by colour less “drop lets” of opal.
Un der the mi cro scope, the rock ap pears to be dom i nated by gar net of grossular com po si tion (Ta ble 1). The gar net is colour - less, com monly cloudy, dis play ing weak bi refrin gence in places. The mass of grainy gar net is intergrown with stripes and nests of microaggreagtes com posed of quartz and feld spars.
The lat ter are rep re sented by nearly pure al bite and K-feld spar;
Fig. 2. Leucocratic rocks within serpentinites on the north ern wall of the Jordanów Quarry (A) Sam ple 8 was col lected on the right-hand edge of photo B
a few anal y ses show sig nif i cant pro por tions of both al bite and orthoclase end-mem bers (Ta ble 2). The rock also con tains patchy segregations of microaggregates com posed of a cloudy min eral mass, yel low ish to brown ish-red in col our, of ten with a fi brous tex ture. These are highly inhomogeneous in BSE im a - ges, con tain ing an acicular or fi brous min eral intergrown with Fe-rich sec ond ary phases. In spite of usu ally poor pol ish ing, the chem i cal com po si tion of these ag gre gates, tested with EDS, is fairly con stant and cor re spond to actinolite-type am phi bole (Fig. 3). Ac ces so ries are rep re sented by ap a tite and zir con. The main com po nents: grossular, quartz, al bite, and K-feld spar, have been con firmed by X-ray dif frac tion anal y sis, whereas mi nor and ac ces sory com po nents have not been de tected.
Sum ming up, the sam ple con tains, apart from ev i dently meta mor phic calc-sil i cate min er als such as grossular and actinolite, com po nents typ i cal of gran ite com po si tion: quartz, al - bite, K-feld spar, and ac ces sory zir con (fairly abun dant) and ap a - tite, which to gether with field ev i dence, sug gest that we are deal - ing with a strongly al tered granitoid dyke in trud ing serpentinites.
SHRIMP RESULTS
ZIRCON CHARACTERISTICS
The main zir con pop u la tion is rather ho mo ge neous: the euhedral and subhedral crys tals are short-pris matic, trans par ent, with a char ac ter is tic pink-brown ish tint (Fig. 4). In trans mit ted light, they show del i cate, mag matic zonation, rather poorly vis i - ble in CL and BSE im ages. No dis tinct cores are discernable.
SHRIMP DATA AND AGES
Nine points in 9 zir con crys tals ana lysed yielded a gen er ally clear pic ture of age dis tri bu tion (Ta ble 3, Figs. 4 and 5).
Two fairly con cor dant points (1.1 – dis cor dance D + 6, and 3.1 – D + 1) are dis tinctly older and give an av er age con cordia age of 386 ±10 Ma. These two older zir cons have dif fer ent mor phol ogy: subhedral, short-pris matic, with poorly de vel o - ped prisms. Also, in con trast with zir cons of the main pop u la - tion, they are nearly colour less and clear. Fur ther more, they con tain rel a tively low amounts of U and Th, com pared with the zir cons of the main population.
Seven points in seven grains all seem to be long to the main pop u la tion of zir cons and they dis play sim i lar chem i cal iso to - pic sig na tures: high U (2877–6433 ppm) and Th (160–920 ppm), low 232Th/238U ra tios (0.06–0.17), and low com mon lead, 206Pbc con tents, be tween 0 and 0.45%. All are con cor dant to slightly dis cor dant (Ta ble 3). The av er age con - cordia age for six points, ex clud ing one sig nif i cantly older, is 336.5 ±4.5 Ma (Fig. 5).
Two grains, 2.1 and 8.1, of sim i lar phys i cal fea tures (pris - matic habit and char ac ter is tic pink ish tint) and chem i cal sig na - tures, have sig nif i cantly older ages (Ta ble 3). If these two
Anal y sis Gr1 Gr2 Gr6 Mean
SiO2 38.666 40.314 39.143 39.374 TiO2 0.009 0.081 0.058 0.049 Al2O3 22.460 21.962 22.612 22.345 FeO 0.513 0.785 0.234 0.511 MnO 0.000 0.000 0.000 0.000 MgO 0.071 0.057 0.064 0.064
CaO 36.549 34.832 36.106 35.829
To tal 98.268 98.031 98.217 98.172
Si IV 2.960 3.069 2.986 3.005 Al IV 0.040 0.000 0.014 0.000 T site 3.000 3.069 3.000 3.005 Al VI 1.987 1.970 2.018 2.010 Ti VI 0.001 0.005 0.003 0.003 O site 1.987 1.975 2.022 2.013 Fe+2 0.033 0.050 0.015 0.033 Mn+2 0.000 0.000 0.000 0.000 Mg+2 0.008 0.006 0.007 0.007 Ca+2 2.998 2.841 2.951 2.930 T a b l e 1 Microprobe anal y ses and for mulae of grossular
gar net from sam ple S-8
Anal y sis NaFs7 NaFs5 NaKFs3 KFs4 SiO2 68.575 68.060 66.810 64.704 TiO2 0.000 0.000 0.000 0.019 Al2O3 19.166 18.933 18.476 18.197 Fe2O3 0.064 0.051 0.091 0.108 MnO 0.000 0.000 0.061 0.026 MgO 0.026 0.030 0.000 0.000 CaO 0.178 0.056 0.024 0.019 Na2O 11.543 9.948 4.611 0.197 K2O 0.087 2.281 9.677 15.739 To tal 99.639 99.359 99.750 99.009
Si+4 3.005 3.011 3.018 3.009 Ti+4 0.000 0.000 0.000 0.001 Al+3 0.990 0.987 0.984 0.998 Fe+3 0.002 0.002 0.003 0.004 Mn+2 0.000 0.000 0.002 0.001 Mg+2 0.002 0.002 0.000 0.000 Ca+2 0.008 0.003 0.001 0.001 Na+1 0.981 0.853 0.404 0.018 K+1 0.005 0.129 0.558 0.934 To tal 4.992 4.986 4.970 4.965
K+Na+Ca 0.994 0.985 0.963 0.953 or 0.005 0.131 0.579 0.980 ab 0.987 0.867 0.420 0.019 an 0.008 0.003 0.001 0.001 T a b l e 2 Microprobe anal y ses and for mulae of feld spars
from sam ple S-8
points were in cluded, the av er age con cordia age would in - crease to 339 ±4 Ma, and if both were ex cluded, the mean con - cordia age would be by ca. 3 My years youn ger: 334 ±5 Ma.
The o ret i cally, the two older dates, around 350 Ma, could be in - ter preted as mixed ages, be tween the in her ited zir cons of ca.
386 Ma and the main pop u la tion zir cons of ca. 337 Ma. How - ever, there are no in di ca tions of such “mix ing” in the crys tals.
An al ter na tive is that they be long to the main pop u la tion and the true mag matic age is around 339 ±4 Ma.
DISCUSSION AND CONCLUSION
The new SHRIMP re sults from the gran ite dyke in serpentinites at Jordanów in di cate that the mag matic cristallization age of the gran ite was 337 ±4 Ma. The rock con - tains a few con sid er ably older in her ited zir cons, ca.
386 ±10 Ma in age. These have dif fer ent mor phol ogy, with subhedral, short-pris matic habit, and poorly de vel oped prisms.
Also, in con trast with zir cons of the main pop u la tion, the in her - ited grains are nearly colour less and clear. Fur ther more, they
con tain rel a tively low amounts of U and Th com pared with the zir cons of the main pop u la tion.
In ter est ingly, the age of the in her ited zir cons is sim i lar to the age of the Œlê¿a gab bro (Kryza, 2010). How did these older zir cons get into this gran ite sam ple? One pos si bil ity is con tam i - na tion of gra nitic magma with the coun try rocks (gabbro).
Some of the gran ite dykes found within the ophiolite suite have been ten ta tively cor re lated with the lo cal Variscan granitoids, such as those in the Strzegom–Sobótka Mas sif to the north. Dubiñska and Szafranek (1990) con vinc ingly ar gue that “...the gra nitic rock from Jordanów, which is ex tremely rich in po tas sium, should not be in cluded into the ophiolite se - quence, since granitoids ge net i cally re lated to ophiolites are mostly K-poor. More over, the tex ture of slightly tectonized gran ite frag ments is sim i lar to that of one of the va ri et ies of Variscan Strzegom–Sobótka gran ites.” Also, Nb-Ta min er als dis cov ered in the con tact schists have been taken as an other pos si ble ar gu ment for this cor re la tion, since sim i lar phases were re ported by Lis and Sylwestrzak (1981) from peg ma tite bod ies at Jordanów, that were con sid ered as re lated to the Strzegom–Sobótka granitoids.
Fig. 3. Elec tron-microprobe data from sam ple S-8
A – BSE im age show ing the main min eral com po nents of the leucocratic rock: Ab – al bite, Ac – actinolite?, Gr – grossular, Kf – K-feld spar, Qz – quartz; B and C – dis tri bu tion of Si and Ca in the field shown in A; D – EDS spec trum of actinolite-type
Fig. 4. Trans mit ted light and CL im ages of the zir cons from dated sam ple S-8 from the Jordanów Quarry SHRIMP an a lyt i cal spots are in di cated by el lip ses; sym bols of spots cor re spond to those in Ta ble 3
Spot
206Pbc
[%] U
[ppm] Th [ppm]
232Th/
238U
206Pb*
[ppm]
206(1) Pb/238U
age
207 (1) Pb/206Pb
age
Dis- cor- dant [%]
238(1)
206U/
Pb* ±
[%]
207(1)
206Pb*/
Pb* ±
[%]
207(1)
235Pb*/
U ±
[%]
Error corr
1.1 0.42 486 162 0.34 25.9 386.8 ±7.5 411 ±120 6 16.17 2.0 0.05500 5.2 0.4690 5.6 0.359
2.1 0.07 6232 705 0.12 297 347.9 ±5.8 346 ±26 0 18.03 1.7 0.05341 1.1 0.4084 2.1 0.829
3.1 0.45 1524 250 0.17 80.9 384.9 ±6.7 396 ±82 3 16.25 1.8 0.05460 3.6 0.4630 4.1 0.444
4.1 0.10 5045 507 0.10 228 330.8 ±5.6 343 ±33 4 18.99 1.7 0.05334 1.5 0.3873 2.3 0.765
5.1 0.18 5285 552 0.11 239 329.5 ±5.5 329 ±36 0 19.07 1.7 0.05301 1.6 0.3833 2.3 0.729
6.1 0.00 4059 650 0.17 188 338.3 ±5.7 343 ±28 1 18.56 1.7 0.05333 1.3 0.3961 2.1 0.810
7.1 0.01 4437 491 0.11 203 333.9 ±5.6 332 ±28 –1 18.81 1.7 0.05307 1.2 0.3890 2.1 0.818
8.1 0.43 2877 160 0.06 140 353.2 ±6.0 326 ±91 –8 17.75 1.8 0.05290 4.0 0.4110 4.4 0.399
9.1 0.03 6433 920 0.15 297 337.6 ±5.6 327 ±24 –3 18.60 1.7 0.05296 1.1 0.3926 2.0 0.851
Er rors are 1-sigma; Pbc and Pb* – the com mon and ra dio genic por tions, re spec tively; er ror in stan dard cal i bra tion was 0.68% not in cluded in above er rors but re quired when com par ing data from dif fer ent mounts; (1) – com mon Pb cor rected us ing mea sured 204Pb
T a b l e 3 SHRIMP data for zir cons from gran ite, sam ple S-8 from Jordanów
The new U-Pb SHRIMP zir con data for the gran ite from the well-known ser pen tin ite quarry at Jordanów put in doubt the cor re la tion of such gran ites with the Strzegom–Sobótka granitoids. The mag matic age of 337 ±4 Ma of the gran ite is much older than the re cently ver i fied ages of the Strzegom–Sobótka granitoids, most of which fall within the range of 310–294 Ma (Turniak et al., 2005). How ever, the gran ite dyke from Jordanów has a sim i lar age to some other granitoid bod ies that oc cur close to ophiolitic frag ments in that area, e.g., the Niemcza granitoids to the south, dated at 338+2/–3 Ma (Ol i ver et al., 1993). All these older granitoids rep re sent a rel a tively early stage of granitoid magmatism re - corded in that part of the Variscan Orogen. Within the wider con text of the Bo he mian Mas sif, the most wide spread mag mas of that time (around 338 Ma) are of durbachitic af fin ity (Fin ger et al., 2009). These granitoids are de fined by very high-K con - tents but, nev er the less, are com monly in ter preted as man tle mag mas (melts from en riched man tle), likely re lated to postcollisional slab break-off (Janoušek and Holub, 2007; Fin - ger et al., 2007). Many durbachite plutons of the Bo he mian Mas sif are in ter me di ate magmatites. They are syenitic, melagranodioritic or quartzmonzonitic, in terms of the Streckeisen clas si fi ca tion, but fel sic end-mem bers with high-K gra nitic com po si tion are lo cally also as so ci ated (e.g., in the Rastenberg pluton in Aus tria; Fin ger et al., 2009).
The tim ing of par tic u lar petro gen etic events in the evo lu - tion of the Cen tral-Sudetic ophiolites re mains, at least in part, prob lem atic. Dubinska et al. (2004) ar gued that their SHRIMP zir con age of 400+4/–3 Ma from rodingite cor re sponds to the rodingitization and serpentinization pro cesses, rather than to the mag matic em place ment age of the gab bros. How ever, new SHRIMP zir con ages from gab bros from sev eral dif fer ent lo -
serpentinisation pro cesses must have oc curred fairly soon af ter the mag matic em place ment of the mafic rocks of the ophiolites.
On the other hand, the strong al ter ation of the 337 Ma old metagranite at Jordanów in di cates that metasomatic pro cesses may have been ac tive, at least lo cally, much later than the mag - matic pro cesses in the ophiolite. This would be in line with the model of a multi-stage PT path for the Œlê¿a gab bros pro posed by Kryza (in Majerowicz et al., 2000; see above).
The re gional frame work and struc tural po si tion of the CSO re main con tro ver sial (e.g., ¯elaŸniewicz, 1995; Mazur et al., 2006; Kryza and Pin, 2010). How ever, the granitoids of the early plutonic phase of the Variscan orog eny in this part of the Bo he mian Mas sif, ~340 Ma in age, are in ter preted as synorogenic in tru sions (Mazur et al., 2007, and ref er ences therein), thus the em place ment of these magmatites into the ophiolite com plex must have taken place still within a tectonothermally ac tive set ting. The clear spa tial co in ci dence of granitoid sills with zones of strong pen e tra tive de for ma tion in the Niemcza Shear Zone has re cently been em pha sized by Lorenc and Kennan (2007). The small granitoids in that area were emplaced syntectonically dur ing the re gional shear ing that af fected the rocks of the Niemcza Zone, where the lineations and foliations in some of the intrusives, as well as the elon ga tions of their en claves, are subparallel to the pen e tra tive fab rics in the coun try rocks (Dziedzic, 1963). Parts of the ophiolite com plex, such as that ex posed at Jordanów, could have been lo cated out side zones of in tense shear ing, though the orig i nal spa tial re la tion ships have been ob scured by tec tonic dis place ments.
De spite con tro ver sial struc tural in ter pre ta tions, the ca.
337 ±4 Ma age of the granitoid dykes of con ti nen tal-crustal af - fin ity (e.g., the high-K gra nitic com po si tion of these rocks) within serpentinites shows that the ophiolites were in cor po - rated into the con ti nen tal crust al ready in early Visean times.
This geo chron ol ogi cal-palaeotectonic con straint should be kept in mind when mod els of the Variscan orogenic col li sion are refined.
Ac knowl edge ments. K. Dymna is thanked for zir con sep a - ra tion, W. Raczek for X-ray anal y sis, and J. Æwi¹kalski for as - sis tance with microprobe work. Thanks are due to S. Sergeev and I. Paderin (CIR, VSEGEI, St. Pe ters burg), who helped in the SHRIMP an a lyt i cal work. The study was sup ported by Uni - ver sity of Wroc³aw in ter nal grants: 2022/W/ING and 1017/S/ING. The au thor is grate ful to F. Fin ger and A. ¯elaŸniewicz for their con struc tive reviews.
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APPENDIX
SHRIMP an a lyt i cal pro ce dure
The Sen si tive High Res o lu tion Ion Microprobe (SHRIMP II) at the Cen tre of Iso to pic Re search (CIR) at the All-Rus sian Geo log i cal Re search In sti tute, VSEGEI, in St. Pe - ters burg was used to de ter mine zir con ages in the sam ples se - lected. In situ U-Pb anal y ses were per formed ap ply ing a sec - ond ary elec tron mul ti plier in peak-jump ing mode fol low ing the pro ce dure de scribed in Wil liams (1998) or 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 ond ary ions. The el lip ti - cal an a lyt i cal spots had a size of ca. 27 ´ 20 mm, and the cor re - spond ing ion cur rent 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 mm mul ti plier slit, al lowed mass-res o lu tion of M/DM ³5000 (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. 60 ´ 50 m
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 tam i na tion.
The fol low ing ion spe cies were mea sured in se quence:
196(Zr2O)–204Pb–back ground (ca. 204 AMU)–206Pb–207Pb–
208Pb–238U–248ThO–254UO with in te gra tion time rang ing from 2 to 20 sec onds. Four cy cles for each spot analysed were ac - quired. Each fifth mea sure ment was car ried out on the zir con Pb/U stan dard TEMORA 1 (Black et al., 2003) with an ac - cepted 206Pb/238U age of 416.75 ±0.24 Ma. The 91 500 zir con with a U con cen tra tion of 81.2 ppm and a 206Pb/238U age of 1062.4 ±0.4 Ma (Wiedenbeck et al., 1995) was ap plied as a
“U-con cen tra tion” standard.
The col lected re sults were then pro cessed with the SQUID v1.12 (Lud wig, 2005a) and ISOPLOT/Ex 3.22 (Lud wig, 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 mea sured
204Pb ac cord ing to the model of Stacey and Kramers (1975).
