Geo log i cal Quar terly, 2020, 64 (1): 120–129 DOI: http://dx.doi.org/10.7306/gq.1515
Neoarchean to Paleoproterozoic frag ments in the Brunovistulia terrane, S Po land:
a com po nent of the Co lum bia Supercontinent?
Andrzej ¯ELAZNIEWICZ1, * and C. Mark FANNING2
1 In sti tute of Geo log i cal Sci ences PAS, Podwale 75, 50-449 Wroc³aw, Po land
2 Re search School of Earth Sci ences, the Aus tra lian Na tional Uni ver sity, Can berra, Aus tra lia
¯elaŸniewicz, A., Fan ning, C.M., 2020. Neoarchean to Paleoproterozoic frag ments in the Brunovistulia terrane, S Po land:
a com po nent of the Co lum bia Supercontinent? Geo log i cal Quar terly, 64 (1): 120–129, doi: 10.7306/gq.1515
The com pos ite terrane of Brunovistulia in cludes base ment of the Up per Silesia Block, south ern Po land. In its NE part, the base ment is el e vated by the Rzeszotary Horst. In the Rzeszotary 2 bore hole (Rz2) drilled in the horst, partly migmatized am - phi bo lites, fel sic gneiss es and gran ites oc cur. An Na-plagioclase-phengite-K-feld spar neosome con tained zir cons that yielded U-Pb SHRIMP ages ~2.75–2.6 Ga (cores and sin gle grains) and ~2.0 Ga (rims and sin gle grains). The older ages are in ter preted as the time of or i gin of the ig ne ous protolith of the migmatized am phi bo lites. The youn ger ages re corded meta - mor phism and migmatization that af fected both the mag matic pre cur sor of the am phi bo lites and ac com pa ny ing fel sic rocks dur ing a contractional tec tonic/orogenic event. Migmatization was greatly en hanced by an in flux of al kali-bear ing flu ids which her alded in tru sion of late-orogenic unfoliated K-gran ite in an extensional re gime, ter mi nat ing the 2.0 Ga event. It is pro posed that the en tire orogenic ed i fice, of which the Brunovistulian rocks drilled in Rz 2 are a small part, rep re sents frag ments of the Co lum bia Supercontinent that was as sem bled in the Paleoproterozoic and bro ken up in the Mesoproterozoic. In Ediacaran times, these frag ments be came even tu ally in cor po rated into the Cadomian orogen in the form of its fore land and con trib uted to the for ma tion of the com pos ite terrane of Brunovistulia. Such a sce nario ex plains why the U-Pb zir con age spec tra in the Rzeszotary ter rain dif fer dra mat i cally from those in the re main der of Brunovistulia, which is thought to be the Cadomian hin - ter land.
Key words: Brunovistulia, Cadomian, Co lum bia, migmatite, zir con.
INTRODUCTION
In the Up per Silesia Block (USB), south ern Po land, Pre - cam brian rocks are known from the subsurface as gran ites, gneiss es and phyllites dis cor dantly over lain by Lower Cam brian (sub-Holmia) sand stones (Bu³a et al., 1997, 2008; Bu³a and
¯aba, 2005). The crys tal line rocks are in ter preted as the base - ment of the Brunovistulia terrane (Dudek, 1980; Nawrocki et al., 2004) and as signed to the Cadomian orogen be cause of 580–545 Ma U-Pb ages of zir cons re trieved from the unfoliated (late/post-tec tonic) gran ites (¯elaŸniewicz et al., 2009). Be - sides the Ediacaran gran ites and gneiss es, in the east ern part of the USB, metabasites as so ci ated with a va ri ety of (meta)fel - sic rocks were found in the Rzeszotary 2 bore hole (Fig. 1) drilled in the con cealed Rzeszotary Horst (Konior, 1974; Heflik and Konior, 1974; Bu³a, 2000; ¯elaŸniewicz et al., 2009). Pre - lim i nary dat ing of gneiss es and am phi bo lites from the Rzeszotary 2 bore hole re vealed a U-Pb zir con age of 2.7 Ga for the ig ne ous protolith of fel sic rocks of the for mer (Bylina et al.,
2000). Such data in di cated the pres ence of an ed i fice of at least Neoarchean age, or rather a frag ment of it, in the USB base - ment, which thus con tains the old est rocks in Po land. To check this no tion, U-Pb zir con SHRIMP anal y ses were made of an unfoliated gran ite vein and neosome rock from migmatized am - phi bo lites drilled in the Rzeszotary 2 bore hole (Rz2). The re - sults ob tained have im pli ca tions for the prov e nance of Bruno - vistulia and for fea tures of the Cadomian orogen.
GEOLOGICAL FRAMEWORK
Brunovistulia (Dudek, 1980; Nawrocki et al., 2004) is a com pos ite terrane, with mainly Neoproterozoic base ment, accreted to the south west ern mar gin of Baltica in cen tral Eu - rope (Fig. 1A). It is com posed of crustal pieces of Gondwanan de scent which once be longed to the Cadomian orogen of An - dean type (Fin ger et al., 2000; Kalvoda et al., 2003, 2008;
¯elaŸniewicz et al., 2009; Hanžl et al., 2019). Brunovistulia is iden ti fi able up to the Bal kans mainly by subsurface in ves ti ga - tions (Haydutov and Yanev, 1995; Kalvoda et al., 2008). In its Up per Silesian part, frag ments of both the Cadomian hin ter - land and fore land have been rec og nized (Bu³a et al., 2008;
¯elaŸniewicz et al., 2009). A fore land base ment (Fig. 1B) was en coun tered in the Rzeszo tary Horst. In 1909, only a few metres of greenschist and mus co vite orthogneiss were drilled
* Corresponding author, e-mail: pansudet@pwr.wroc.pl
Received: October 18, 2019; accepted: December 17, 2019; first published online: February 27, 2020
in the Rzeszotary 1 bore hole and thus the prov e nance of these rocks has been var i ously in ter preted (Petrascheck, 1909; Nowak, 1927). In 1960–1961, the Rzeszotary 2 bore - hole was drilled 200 m east of the ear lier one. In Rzeszotary 2, a 118 m thick frag ment of the crys tal line base ment (top at
~845 m b.t.l.) was pen e trated be low a Mid dle Ju ras sic–Cre ta - ceous plat form cover in turn over lain by Mio cene de pos its of the West Carpathian Foredeep and overthrust by nappes of the Al pine sys tem (Burtan, 1962). Fel sic crys tal line rocks of the Rzeszotary Horst were pen e trated by an other dozen bore - holes (Fig. 1C) though at greater depths (Konior, 1974). Am - phi bo lites were found only in the Rzeszotary 2 bore hole, which cor rob o rates po ten tial field data. On mag netic maps (Cieœla et al., 1993), the horst area co in cides with a dis tinct low (~ –50 nT). On the gravimetric maps (Królikowski and Petecki,
1995), it over laps with a small lo cal high (3–4 mgal) that fits well the in ferred base ment el e va tion in the Rzeszotary Horst.
Tim ing of the horst or i gin re mained un clear. Crys tal line rocks found in Rzeszotary 2 were in ter preted as a pre- Hercynian suc ces sion of “…more or less diaphtoretically al - tered am phi bo lites and gneiss es in jected with feld spars and sub jected to granitization, likely form ing a man tle to a more deeply seated gran ite body…” (Burtan, 1962). Re cent re-ex am - i na tion of still avail able Rzeszotary 2 core re vealed a wider va ri - ety of gneiss es, gran ites and migmatized am phi bo lites (Fig. 2).
Plagioclase-mus co vite (pl-ms1) gneiss es (Fig. 3A), pre dom - i nant at a depth of ~845–887 m, are com posed of al bite (An
<10)2 and quartz with sub or di nate phengitic mus co vite (Si apfu
<3.5), mi nor cal cite, rare epidote (Ps >10), and rel ics of gar net and Al2SiO5 (kyan ite?) phase. The plagioclase is gen er ally poor Fig. 1A – lo ca tion of Brunovistulia (box) within a tec tonic sketch of cen tral Eu rope (¯elaŸniewicz et al., 2009; age data af - ter Bogdanova et al., 2008); EEC – East Eu ro pean Craton, D – Dobrogea, KLF – Kraków-Lubliniec Fault Zone, MTZ – Moldanubian Thrust Zone, M – Moesia, VF – Variscan Front; B – Brunovistulia; C – Rzeszotary Horst (light grey) with api - cal part (white) ex posed at the Mid-Ju ras sic palaeosurface, flanked by De vo nian car bon ates (grid); Rzeszotary 2 bore - hole (dot), other bore holes (squares), mod i fied af ter Konior (1974)
1Mineral abbreviations after Whitney and Evans (2010);
2Chemical compositions of mineral phases were determined with the CAMECA SX-100 electron microprobe facility at the Interinstitutional Laboratory of Microanalysis at the Institute of Geochemistry, Mineralogy and Petrology, Faculty of Geology, Warsaw University
122 Andrzej ¯elaüniewicz and C. Mark Fanning
in in clu sions. Fo li a tion is ex pressed by mica flakes and al ter nat - ing plagioclase-rich (pl>qz) and quartz-rich (qz>pl) lay ers.
These rocks were de scribed ear lier as “mus co vite schists and feld spar-mus co vite gneiss es” (Burtan, 1962).
Epidote (ep) gneiss es (Fig. 3B) con sist of plagioclase and quartz ac com pa nied by abun dant phengitic mica and epidote whereas K-feld spar, cal cite, gar net, bi o tite, rutile, ti tan ite and hornblende are sparse. Plagioclase (An13-6) is poikiloblastic ow - ing to the abun dance of pref er en tially ori ented epidote in clu - sions. Low-Fe epidote also oc curs as par al lel ar ranged grains in the groundmass. Gar net (Alm56–54 -Grs25–30 -Prp18–11) is tec toni - cally dis persed within polymineral lay ers. Epidote gneiss oc curs as interlayers in the am phi bo lites.
Pink ish gran ite (Fig. 3C) is com posed of K-feld spar/al kali feld spar, al bite (An5–1), phengitic mica, a few relicts of Ca-low almandine-pyrope gar net (Alm60–61 -Prp32–35 -Grs2–5), scarce ap - a tite and al la nite. The three lat ter min er als form oc ca sion ally large, densely packed ag gre gates. Epidote is very sparse or ab - sent. In the al kali feld spar, the K-com po nent in creases to ward the rims. Kfs also forms embay ments or ex ten sively re places al bite, ap pears as in clu sions in phengitic mica and in al bite and de vel ops inter growths with quartz. On the other hand, kfs it self con tains in clu sions of al bite, mica and quartz. The pink ish gran - ite is an unfoliated and prac ti cally ductilely un strained rock, ev i - dently the youn gest lithological vari ant in Rz2. It pen e trated the coun try rocks both as sills and dykes along frac ture zones.
Vol u met ri cally dom i nant am phi bo lite is a mas sive coarse to me dium-grained gar net-poor rock (type 1), pos si bly de rived from a gabbroic protolith (Fig. 4A). It con sists mainly of Ca-am - phi bole, plagioclase (An22–5), low-Fe epidote both in the groundmass and as in clu sions in plagioclase, ti tan ite and ± almandine-grossular gar net (Alm58–56-Grs29–27-Py12–11), compo - sitio nally sim i lar to that in the epidote gneiss es.
Am phi bo lite type 2 is a rel a tively fine-grained rock (Fig. 4B), pre sum ably de rived from ba saltic protolith, also com posed of Ca-am phi bole, plagioclase (larger grains An20–10; small ground - mass grains An6–3) and abun dant gar net (Alm59–55-Grs23–20- -Py18–16) but scarce epidote and ti tan ite. Con ven tional P-T es ti - mates for am phi bole-plagioclase pairs show that meta mor - phism oc curred at tem per a tures of 570–615°C (geothermo - metry of Hol land and Blundy, 1994) and pres sure of 9–10 kbar (Plyusnina, 1982; Schmidt, 1992) in the case of the am phi bo lite types stud ied (un pub lished data, in prep., 2019).
Am phi bo lites, mainly type 1, pass into striped and migma - titic am phi bo lites due to the in creas ing con tents of fel sic min - er als and white mica (Figs. 3D and 5). The neosome lay ers are rel a tively coarse-grained rocks com posed of quartz and feld - spars (Pl + Kfs) ac com pa nied by ms and hbl in vari able amounts, thus grad ing to hbl-gneiss es/granodiorites. In the migmatites, mesosome, melanosome and leucosome por - tions are dis cern ible (Fig. 5A).
The neosome in the migmatized am phi bo lites gen er ally formed prior to in tru sions of K-gran ite. Scarce yet coarse Mg- hornblende blasts, rid dled with drop-like quartz, phengite and gar net in clu sions are dis persed in the leucocratic ground - mass. Large, densely twinned, poikilitic plagioclase (An10–2) grains are as so ci ated with thick quartz lay ers made also of coarse sub polygonal, al most un strained grains or of mark edly elon gate grains. There is sub or di nate phengite and gar net (Alm58–55 -Grs30–27-Py16–11) ± epidote (Ps7–5). The fel sic min er - als are cor roded or coated by kfs which also forms ir reg u lar grains and veinlets.
The data col lected show that meta mor phism slightly pre - ceded migmatization. Migmatization in the am phi bo lites was ac com plished by de vel op ment of stripes com posed/en riched in quartz, al bite, epidote and phengite while the mafic ground - mass con sists of more densely packed am phi bole but less feld - spar (oligoclase). Striped am phi bo lites were pro duced by syn - tectonic meta mor phic seg re ga tion. The pro cess was re mark - ably en hanced by pen e tra tion of al kali-bear ing flu ids and fa cil i - Fig. 2. Lithological log of the Rzeszotary 2 bore hole
with lo ca tion of geo chron ol ogi cal sam ples (Rz17, Rz18)
Fig. 3. Fel sic rocks
A – pl-ms gneiss, depth of 885 m; B – ep-gneiss depth of 928 m;
C – pink ish K-gran ite, depth of 955 m; D – neosome in migmatized am phi bo lite, depth of 920 m
Fig. 4. Mafic rocks
A – epidote am phi bo lite (type 1), depth of 895 m;
B – gar net am phi bo lite (type 2), depth of 943 m
Fig. 5. Tec tonic de for ma tion in migmatitic am phi bo lite
A – mesosome, fine-grained melanosome and coarse -grained leucosome/neo some (grad ing to
®hbl gneiss) lay ers, while a later extensional shear zone cross -cuts the migmatitic lay er ing (red ar row), depth of 918 m; B – syntectonic thin leucosome lay ers seg re gated with fluid as sis tance in a contractional re - gime (up per ar rows: T-gash, lower ar rows: thrust), depth of 898 m; C – thicker syntectonic segregations with fluid-pro moted coarse-grained (re)crys tal lized lay ers cut by S–C’ nor mal shear (arrrow), depth of 936 m; mel – melanosome, mes – mesosome, leuc – leucosome; ar rows – shear sense
tated by shear-pro moted chan nels. Large fel sic and mafic min - eral blasts and com mon en rich ment in al bite com po nent in migmatitic neosomes as well as albitic/al kali-feld spar rims on plagioclase grains in all these rocks in di cate a gen eral ac tiv ity of volatiles and K-car ry ing flu ids in the for ma tion of the neosome (un pub lished data, in prep., 2019). The K-car ry ing flu ids were pre sum ably con nected with the same source that re leased the K-rich pink ish gran ite. Some 2–5 m thick gran ite lay ers that oc - cur at the up per and lower por tions of the bore hole pre sum ably rep re sent apophyses which prob a bly stemmed from a larger, more deep-seated, unfoliated gran ite body.
In the sec tor drilled, sev eral dis crete shear zones oc cur at var i ous depths. These were ear lier de scribed as signs of tec - tonic move ments (Burtan, 1962). In the metabasites, zon ally more in tense strain pro duced strongly pro nounced fo li a tion up to mylonites (Fig. 5B, C). Later de for ma tion in semi-brit tle con - di tions pro duced cataclastic zones. Fel sic lay ers, spe cif i cally ep-gneiss es, were rel a tively rheolo gically weaker and thus ca - pa ble of lo cal iz ing the im posed duc tile de for ma tion. Mica fishes, small folds and min eral lineation with up-slip ki ne mat ics tes tify to a contractional re gime and syntectonic meta mor - phism/migmatization (Fig. 5B). An extensional over print is doc - u mented by the S–C’ fab ric, nar row shear zones (Fig. 5A) and by small-scale nor mal faults (Fig. 5C).
U-Pb GEOCHRONOLOGY
METHODS
Zir con grains were sep a rated from two sam ples (Fig. 2), Rz17 (neosome) and Rz18 (pink ish gran ite), us ing stan dard crush ing, desliming, heavy liq uid and para mag netic pro ce - dures. Hand-se lected zir con grains were placed onto dou - ble-sided tape, mounted in ep oxy to gether with chips of the ref - er ence zir cons (Du luth Gab bro – FC1), ground to ap prox i - mately half-thick ness and pol ished. Re flected and trans mit ted light pho to mi cro graphs were pre pared for all zir cons, as were cathodoluminescence (CL) Scan ning Elec tron Mi cro scope (SEM) im ages. These CL im ages were used to de ci pher the in - ter nal struc tures of the sec tioned grains and to en sure that the
~20 µm SHRIMP spot was wholly within a sin gle age com po - nent within the sec tioned grains. The U-Th-Pb anal y ses were made us ing a Sen si tive High Res o lu tion Ion MicroProbe (SHRIMP II) at the Re search School of Earth Sci ences, The Aus tra lian Na tional Uni ver sity, Can berra, Aus tra lia, fol low ing pro ce dures given in Wil liams (1998, and ref er ences therein).
Each anal y sis con sisted of 6 scans through the mass range, with FC1 ref er ence zir con grains ana lysed for ev ery three un - known anal y ses. The data have been re duced us ing the SQUID Ex cel Macro of Lud wig (2001). The Pb/U ra tios have been nor mal ised rel a tive to a value of 0.01859 for the FC1 ref - er ence zir con, equiv a lent to an age of 1099 Ma (Paces and Miller, 1993). Un cer tain ties given for in di vid ual anal y ses (ra tios and ages) are at the one sigma level (Ap pen dix 1*). Wether ill con cordia plots, prob a bil ity den sity plots with stacked his to - grams and weighted mean 207Pb/206Pb age cal cu la tions, con - cordia ages and discordia re gres sion in ter cepts were car ried
out us ing ISOPLOT/EX (Lud wig, 2003). All cal cu lated ages are re ported with un cer tain ties at 95% con fi dence lim its.
For this study, in or der to get more in for ma tion about tim ing of the above-de scribed migmatization and K-gran ite in tru sion into the mafic rocks, zir cons were sep a rated from a neosome sam ple (Rz17) and from a pink ish gran ite (Rz18).
ZIRCON SAMPLES
The zir cons from the migmatitic leucosome sam ple Rz17 are elon gate, yel low-brown grains, with subrounded ter mi na - tions. Most of the grains are cracked, but they do con tain clear ar eas. The cen tral parts are of ten dark and metamict, and there is a clear dis tinc tion be tween the clearer outer ar eas and the more metamict cen tral parts to the grains. The CL im ages fur - ther high light that dis tinc tion and re veal com plex in ter nal struc - tures (Fig. 6A). In gen eral, the outer parts of most grains are rel - a tively ho mo ge neous to weakly sec tor zoned. The cen tral ar - eas to grains 1, 7 and 10, for ex am ple, are very dark, un der CL sub dued, of ten oval in shape. Thin, very bright CL rims are pres ent on many grains, but are <10 µm in width and too nar row to be ana lysed in the cur rent study.
The zir cons from the unfoliated K-gran ite sam ple Rz18 are no ta bly equant to ir reg u larly shaped grains and frag ments of grains that are very clear un der trans mit ted light. The CL im - ages show a mostly sub dued, rel a tively ho mo ge neous in ter nal struc ture, with mi nor os cil la tory and sec tor zon ing pres ent in some grains (Fig. 6B).
RESULTS
SAMPLE Rz17
The anal y ses for the neosome sam ple Rz17 fur ther high - light the com plex na ture of the zir con pop u la tions (Ap pen dix 1 and Fig. 7A). Twenty-one ar eas have been ana lysed on 19 zir - con grains. The anal y ses re cord a range of 207Pb/206Pb ages, with a scat tered older group ing be tween ~2570 and ~2765 Ma, and a near to con cor dant youn ger group ing at ~2000 Ma. The two anal y ses on each of grains 5 and 12 pro vide a key to the un - der stand ing of the U-Pb sys tem at ics. Grain 12 has an es sen - tially un zoned outer area which re cords a 207Pb/206Pb age of
~2010 Ma. The dark CL, higher U cen tral area has a 207Pb/206Pb age of ~2694 Ma. The low U, thin outer rim to grain 5 and the high U (dark CL) cen tral com po nent both re cord 207Pb/206Pb ages of ~2575 Ma. In con trast to the re sults for grain 12, the lower U outer com po nents to grains 1, 4, 6, 9 and 10 all re cord
207Pb/206Pb ages that are within the scat tered Late Archean group ing. The rims and outer ar eas on grains 7, 8, 11, 13, 15, 17 and 19, to gether with that for grain 12 all re cord 207Pb/206Pb ages ~2000 Ma. Those anal y ses range from con cor dant to ~5%
dis cor dant and a re gres sion line fit ted to the 8 anal y ses gives an up per in ter cept at 2005 ±6 Ma (MSWD = 1.3; Fig. 7A). The lower in ter cept is within un cer tainty of the pres ent day, and the weighted mean of the 207Pb/206Pb ages for these 8 anal y ses is 2002.9 ±3.1 (MSWD = 1.2; Fig. 7A).
In terms of the older ar eas/grains, the ma jor ity of the anal y - ses are dis cor dant, the old est anal y sis is of grain 16 at
124 Andrzej ¯elaüniewicz and C. Mark Fanning
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1515
~2765 Ma and is ~10% dis cor dant. On the con cordia plot there ap pear to be sev eral pos si ble discordia lines, but no clearly de - fined lines of best fit. The ar eas ana lysed are con sid ered to have un der gone an cient and mul ti ple ra dio genic Pb loss ep i - sodes that gave rise to the scat ter ing. How ever, there can be no doubt that there was a prom i nent Late Archean zir con crys tal li - za tion event(s). The anal y sis of the coarser grain 2 to gether with the anal y ses of the struc tured grain 5 can be used to im ply that the zir con crys tal lized ~2570–2575 Ma.
The Th/U ra tios for sev eral of the ar eas giv ing the ~2570 to
~2765 Ma dates are no ta bly low, ~0.02 (anal y ses 14.1 and
18.1) to ~0.08 (grain 4). These may im ply that some of the zir - con has a meta mor phic paragenesis. In terms of the ~2000 Ma zir con com po nent, the Th/U ra tios are be tween ~0.22 to ~0.36, and these val ues are com mon to crustal ig ne ous zir con.
SAMPLE Rz18
The mor phol ogy and CL struc ture of the zir cons from this sam ple of unfoliated gran ite show rel a tively sim ple ig ne ous fea tures (Fig. 6B). The 20 ar eas ana lysed on 18 zir con grains range from con cor dant to ~10% dis cor dant, with 207Pb/206Pb Fig. 6. BSE im ages of an a lysed zir cons with an a lyt i cal spots
A – neosome Rz17; B – K-gran ite Rz18
126 Andrzej ¯elaüniewicz and C. Mark Fanning
ages that are be tween ~1930 to ~2010 Ma (Ap pen dix 1 and Fig. 7B). The ar eas ana lysed have mod er ate to high U con - cen tra tions (up to ~1235 ppm) and the Th/U ra tios are con sis - tent with crustal ig ne ous zir con (~0.18 to ~0.40). Un for tu - nately, de spite the ap par ently sim ple in ter nal struc ture, there is some scat ter in the discordia ar ray, and a re gres sion line fit - ted to all 20 anal y ses has ex cess scat ter (MSWD = 5.7; plot
not shown). This dis per sion is high lighted on the prob a bil ity den sity plot, with stacked his to gram (see in set Fig. 7B), where the main group ing shows a no ta bly asym met ric dis tri bu tion, with tail ing on both the older and youn ger age sides; mostly on the youn ger side. This dis tri bu tion im plies that there has been ra dio genic Pb loss prior to the pres ent day. An ar bi trary se lec - tion of 13 of the 20 anal y ses in di cates a pos si ble weighted Fig. 7. Con cordia plots of zir cons
A – neosome Rz17; B – K-gran ite Rz18
mean 207Pb/206Pb date of 1998 ±3 Ma (MSWD = 1.5 Ma). This pro vides a con straint on the time of ig ne ous crys tal li za tion in the K-gran ite which in truded into the Rzeszotary gneiss-am - phi bo lite do main.
DISCUSSION
The new U-Pb zir con data for the Rzeszotary rocks sug gests that they orig i nated at ~2.75–2.6 Ga. In an ear lier study re ported in an ab stract by Bylina et al. (2000), zir con grains from a pl-ms gneiss yielded a U-Pb TIMS up per in ter cept age of 2.7 Ga, which cor re sponds with our re sults. Com plete re cy cling of still older (meta mor phosed?) crust can not be en tirely ex cluded, yet there is no ev i dence of U-Pb zir con re set ting. Al though the geo chem is - try of the am phi bo lites points to man tle or i gin (un pub lished data, in prep., 2019), the lim ited core ma te rial does not al low one to de cide upon the pri mary re la tion ships be tween pre cur sors of the am phi bo lites and pl-ms gneiss es. The ob served pres ence of a few thin in ter ca la tions of am phi bo lite in the gneiss es is am big u - ous. These might rep re sent mafic xe no liths en trapped by fel sic magma on in trud ing or al ter na tively veins of mafic rocks that pen e trated the ear lier de vel oped fel sic ed i fice, whether gra nitic or gneissic. Such an ed i fice may have been de rived via par tial melt ing from a prim i tive ba saltic proto lith which was pro duced by ear lier man tle melt ing (see Barker, 1979; Mar tin, 1993). In any case, the 2.75–2.6 Ga event must have pro duced zir con-car ry - ing fel sic rocks that con trib uted to the for ma tion of the crust in Neoarchean times.
In Rzeszotary 2, more leg i ble are the re cords of a sub se - quent ma jor event that oc curred at 2.0 Ga. The bore hole cores re vealed ev i dence of contractional de for ma tion con cur rent with broadly syntectonic meta mor phism and migmatization. Striped am phi bo lites were pro duced by syntectonic meta mor phic seg - re ga tion no ta bly en hanced by pen e tra tion of al kali-bear ing flu - ids that as sisted in the for ma tion of the neosome. The ep- gneiss es were also strongly af fected by migmatization which was both pre ceded and fol lowed by the lo cal ized duc tile shear - ing. Such fea tures along with the large grain-size in di cate an ac tive role of volatiles and K-car ry ing aque ous flu ids. The meta - mor phic segregations were most likely ac com plished in an open sys tem, as sisted by flu ids and par tial melt ing sim i lar to the man ner de scribed by Bowes and Park (1966). Wa ter and K-ions may have come from al kali-car ry ing fluid in fil tra tions pre - sum ably con nected with the for ma tion of K-gran ite deeper in the crust, which in truded the Rzeszotary do main at 2.0 Ga con - cur rently with migmatization. The com po si tion of the gran ite points to a fel sic pre cur sor, prob a bly meta mor phic as in di cated by Th/U ra tios in zir con. This no tion is in line with relicts of pyrope-almandine gar net which has re mained pre served in the gran ite. Such re mains sug gest that ear lier HP fel sic granulitic crust, or one in clud ing fel sic granu lites, was par tially melted and the melt gave rise to gra nitic magma. Al ter na tively the for ma tion of the gran ite may have been ac com plished by de hy dra tion par - tial melt ing of hbl-grt am phi bo lites, which seems less likely.
The po ten tial field data and bore hole ob ser va tions sug gest that the Rzeszotary Horst is built of pre dom i nantly fel sic rocks.
Our zir con ages show that these rocks did not un dergo any leg i - ble ther mal event af ter 2.0 Ga. In both the sam ples ana lysed, the zir cons suf fered ra dio genic Pb loss at un con strained times. This might be at least partly con nected with Permo-Car bon if er ous mag matic ac tiv ity in the area (Nawrocki et al., 2010; Mikulski et al., 2019), which would con cur with the lower in ter cept at 290 Ma (Fig. 7) and ex plain thin bright CL rims on some zir con grains.
The 2.75–2.6 Ga ages make a sig nif i cant con trast to the re main - der of the terrane of Brunovistulia. In tec tonic terms, the Rzeszo - tary do main clearly be longed to the fore land of the Cadomian orogen pre served in Brunovistulia (Bu³a and ¯aba, 2005; Bu³a et al., 2008; ¯elaŸniewicz et al., 2009). On the other hand, the Cadomian hin ter land in Brunovistulia is built of Gond wana (Amazonia)-de rived frag ments that re corded (tectono)ther mal events at ~1.0, 1.2, 1.4–1.5 Ga and ~ 1.65–1.8 Ga (Fin ger et al., 2000; Friedl et al., 2000; ¯elaŸniewicz et al., 2005, 2009). In view of that, the Rzeszotary part of Bru no vistulia may be con sid ered as an ex otic frag ment of an other con ti nen tal en tirety which was com posed of Neoarchean and Paleoproterozoic crust. Such con ti nen tal crust af ter pro found re work ing in the course of the
~2.0 Ga event was then bro ken up and the re sul tant frag ments be came re as sem bled in Neo proterozoic times (Nawrocki et al., 2004; ¯elaŸniewicz et al., 2009). In view of the above, the an ces - tors of the Rzeszotary crustal pieces should be looked for among cratons that have car ried re cords of both ~2.75–2.6 Ga and 2.0 Ga orogenic events. Re cords of events of such ages have been re ported from some cratons. How ever, in the pres ent-day state of knowl edge, it is dif fi cult to tell which craton the Rzeszotary do - main was ac tu ally de rived from, to be even tu ally in cor po rated into the Cadomian belt. The Neorchean-Paleoproterozoic age spec tra found in the Rzeszotary 2 bore hole cores are com pat i ble with those rec og nized in ar eas as re mote to day as Tan za nia and Dharwar, In dia but pos si bly also at shorter dis tances in the Osnitsk–Mikashevychi belt of the East Eu ro pean Craton.
In Tan za nia, Khoza et al. (2013) sug gested that subduction of the oce anic litho sphere and col li sion be tween the Zim ba bwe and Kaapvaal cratons at 2.7–2.5 Ga were then con tin ued at 2.2–1.9 Ga by transcurrent move ments and crustal ex hu ma tion de ci phered in the Limpopo belt (Manya et al., 2006).
From the Dharwar craton, In dia, is land arc tholeiitic bas alts of ~2.7 Ga age, compositionally akin to mod ern arc bas alts (Manikyamba et al., 2004) and a still poorly known tectono - thermal event at 2.0 Ga (Bhaskar Rao et al., 1992) were re - ported too.
Still an other can di date to con trib ute to the Rzeszotary ter - rain, lo cated much closer in its pres ent-day po si tion, could be the 2.0–1.95 Ga Osnitsk–Mikashevychi ig ne ous belt which em - braced an is land arc as so ci a tion of am phi bo lite fa cies rocks that were later cut by K-gran ite in tru sions. The belt is a junc tion be - tween two large cratonic seg ments of the East Eu ro pean Plat - form. It ini tially de vel oped on the Sarmatia craton mar gin which even tu ally col lided with Fennoscandia af ter subduction of the Belarus oce anic plate (Bogdanova et al., 2001, 2008; Garetsky and Karatayev, 2011). How ever, the 2.75–2.65 Ga units oc cur away from this mar gin (Lobach-Zhuchenko et al., 2017). What - ever the provenence of the Rzeszotary rocks was, they were very likely in volved in the 2.1–1.8 Ga global event that led to the amal gam ation of the Paleoproterozoic supercontinent of Co - lum bia (Zhao et al., 2002, 2011).
CONCLUSION
Search ing for a cra dle of the Rzeszotary Horst rocks re - quires more data about the ge ol ogy of po ten tially par ent ter - rains. Nev er the less, the re cords rec og nized of the 2.0 Ga event in Rzeszotary may have very likely re mained in the di rect con - nec tion with the 2.1–1.8 Ga global event, which gave rise to the Co lum bia Supercontinent that was as sem bled in the Paleo - proterozoic and bro ken up in the Mesoproterozoic (Zhao et al., 2002, 2011). It is sug gested that (meta)fel sic-mafic rocks of the
Rzeszotary ter rain evolved in the course of this pro cess. To sum ma rize, the Rzeszotary Horst is built of 2.75–2.0 Ga rocks that are in ter preted as de rived from an an cient mag matic arc, as sug gested by geo chem i cal sig na tures (un pub lished data, in prep., 2019), and a sub se quent collisional/orogenic zone em - braced by the Co lum bia Supercontinent. In Ediacaran times, frag ments of Co lum bia, af ter the Rodinian ep och, be came even tu ally in cor po rated into the Cadomian orogen as part of the fore land of the lat ter, and thus con trib uted to the for ma tion of
the com pos ite terrane of Brunovistulia and now rep re sent its old est Neoarchean-Paleoproterozoic com po nents.
Ac knowl edge ments. We grate fully ac knowl edge sup port for this work that was avail able through the pro ject of the Na tional Sci ence Cen tre, Po land, No. 2017/25/B/ST10/02927, though the ini ti a tion was owed to “Palaeozoic Ac cre tion of Po land” PCZ-07- 21 (Min is try of En vi ron ment). F. Fin ger and P. Poprawa are thanked for thought ful and con struc tive com ments.
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