Ura nium-rich monazite-(Ce) from the Krivá type gra nitic cob bles in con glom er ates of the Pieniny Klippen Belt, West ern Carpathians, Slovakia: com po si tion,
age de ter mi na tion and pos si ble source ar eas
Pavel UHER1, *, Dušan PLAŠIENKA2, Mar tin ONDREJKA1,
¼ubomír HRAŠKO3 and Patrik KONEÈNÝ3
1 De part ment of Min er al ogy and Pe trol ogy, Fac ulty of Nat u ral Sci ences, Comenius Uni ver sity, Mlynská dol ina G, 842 15 Bratislava, Slovakia
2 De part ment of Ge ol ogy and Pa le on tol ogy, Fac ulty of Nat u ral Sci ences, Comenius Uni ver sity, Mlynská dol ina G, 842 15 Bratislava, Slovakia
3 State Geo log i cal In sti tute of Dionýz Štúr, Mlynská dol ina 1, 817 04 Bratislava, Slovakia
Uher P., Plašienka D., Ondrejka M., Hraško L. and Koneèný P. (2013) Ura nium-rich monazite-(Ce) from the Krivá type gra - nitic boul ders in con glom er ates of the Pieniny Klippen Belt, West ern Carpathians, Slovakia: com po si tion, age de ter mi na tion and pos si ble source ar eas. Geo log i cal Quar terly, 57 (2): 343–352, doi: 10.7306/gq.1089
Monazite-(Ce) is a wide spread ac ces sory min eral in gra nitic cob bles of the Krivá type (Zástranie and Krivá lo cal i ties) in polymict con glom er ates of Cre ta ceous flysch se quences, the Pieniny Klippen Belt, West ern Carpathians, NW Slovakia. The gran ites show leucocratic mus co vite-bi o tite granodiorite com po si tion and peraluminous calc-al ka line, S-type char ac ter. The monazite con tains un usu ally high U, com monly 1 to 3, and in some places up to 6.6 wt.% UO2, to gether with 5 to 7.7 wt.%
ThO2. A cheralite-type sub sti tu tion [Ca(U,Th)REE–2] is the dom i nant mech a nism of U4+ + Th4+ in cor po ra tion into the monazite struc ture in the Zástranie sam ple, whereas both cheralite- and huttonite-type sub sti tu tion [(Th,U)SiREE–1P–1] are ev i dent in the Krivá gra nitic cob ble. Ura nium alone pre fers the CaU4+(REE)–2 mech a nism, whereas Th fa vours the huttonite sub sti tu tion. The chem i cal U-Th-Pb dat ing of monazite from both gra nitic cob bles show an Early Car bon if er ous age (346 ± 2 Ma), which is con sis tent with the main meso-Variscan, orogen-re lated plutonic ac tiv ity in the Cen tral Carpathian area (Tatric and Veporic superunits). Anal o gous U-rich monazites were de tected in some Variscan S-type leucogranites of the Rimavica mas sif (South Veporic Unit) and the Bojná and Bratislava mas sifs (north ern part of the Tatric Unit). On the ba sis of struc tural and palaeogeographic data, the North Tatric Zone is the most plau si ble source of the monazite-bear ing gra nitic boul ders in the Pieniny Klippen Belt. How ever, the source gra nitic body was most likely hid den by en su ing tec tonic short en - ing along the north ern Tatric edge af ter de po si tion of the Coniacian–Santonian Upohlav type con glom er ates.
Key words: ura nium-rich monazite, gran ite, con glom er ate, monazite dat ing, Pieniny Klippen Belt, West ern Carpathians.
INTRODUCTION
Gra nitic clasts are char ac ter is tic rocks in polymict con glom - er ate beds in Cre ta ceous flysch se quences of the Kysuce, Klape and Manín units in the West ern Carpathian Pieniny Klippen Belt (PKB). In ad di tion to dom i nant car bon ates and acid to ba sic vol ca nic rocks, in tru sive rocks (mainly gran ites) com pose 0.4 to 13 vol.% of the to tal clasts in PKB con glom er - ates (Marschalko, 1986). Two ba sic types of gra nitic clasts from peb ble and cob ble to boul der size have been dis tin guished in the PKB since early stud ies. These are the “Tatry-like” and “ex - otic” types (Zoubek, 1931; Andrusov, 1938). The first type
shows petrographic and geo chem i cal sim i lar i ties to com mon Variscan (Car bon if er ous) gra nitic rocks of the Tatric and Veporic superunits of the Cen tral West ern Carpathians, while the sec ond group con tains gra nitic rocks “ex otic” for the West Carpathian base ment (Wieser, 1958; Krivý, 1969; Kamenický et al., 1974; Šímová, 1985; Birkenmajer and Skupiñski, 1989).
De tailed min er al og i cal, geo chem i cal and geo-chro no log i cal stud ies of the “ex otic”, Upohlav type gra nitic rocks re veal their post-orogenic, A-type af fin ity and Perm ian age, whereas the
“Tatry-like“ (Krivá and Lubina types) magmatites show calc-al - ka line and orogen-re lated I/S-type trend, com pa ra ble with other West Carpathian Variscan (Car bon if er ous) suites (Uher and Marschalko, 1993; Uher et al., 1994; Uher and Pushkarev, 1994). Trace-el e ment geo chem is try, ac ces sory min er als and zir con iso to pic U-Pb dat ing in di cate the sim i lar i ties be tween the
“ex otic” Upohlav type gra nitic clasts and es pe cially the Turèok A-type gran ite in the Gemeric Superunit (Uher and Broska, 1996; Broska and Uher, 2001; Radvanec et al., 2009).
* Corresponding author: puher@fns.uniba.sk
Received: January 22, 2013; accepted: March 28, 2013; first published online: April 24, 2013
On the other hand, there is only re stricted data con cern ing the Krivá (“Tatry-like”) type gra nitic boul ders of the PKB. Pre lim - i nary elec tron-microprobe U-Th-Pb dat ing of monazite from one Krivá type gran ite clast sup ports their Variscan and Car - bon if er ous age (Fin ger et al., 2003). Our re cent microprobe dat - ing and anal y ses of monazite in these gran ites ini ti ated the more de tailed re search pre sented in this pa per. The aims of our study in clude (1) more pre cise age de ter mi na tion of the Krivá type gra nitic clasts, (2) the char ac ter iza tion and sig nif i cance of ura nium-rich monazite in stud ied gra nitic rocks, and (3) dis cus - sion of the pos si ble source area of the Krivá type gra nitic clasts in the PKB Cre ta ceous flysch, based on older and new re sults.
REGIONAL GEOLOGY AND LOCATION OF SAMPLES
The stud ied gra nitic boul ders were col lected from the Upohlav type con glom er ate beds of the Coniacian–Santonian Sromowce For ma tion in the Kysuca-Pieniny and/or Klape Unit of the Pieniny Klippen Belt in north west ern Slovakia. The Upo - hlav polymict con glom er ates oc cur in sev eral strati graphic ho ri - zons. They form stratiform bod ies usu ally 3 to 20 m thick, in ter - ca lat ing with polycyclic sand stone and mudstone beds as parts of deep-ma rine flysch turbidite se quences (e.g., Marschalko, 1986; Starek et al., 2010).
Palaeontological and strati graphi cal ev i dence in di cate Cre - ta ceous, Albian to Maastrichtian age of the Upohlav con glom er - ate beds in the west ern Slo vak seg ment of the Pieniny Klippen Belt (Marschalko, 1986; Mišík and Marschalko, 1988).
Two typ i cal con glom er ate oc cur rences con tain ing the Krivá type gran ite clasts (mainly of cob ble size) were se lected for our re search: Zástranie and Krivá (Fig. 1). The Zástranie lo cal ity (BP-8 sam ple) ex hib its con glom er ate out crops near Zástranie vil lage, in the vi cin ity of the town of Žilina, ap prox i mately 1500 m/322° from el e va tion point Straník (769 m a.s.l.). The Krivá lo cal ity (BP-10 sam ple) is a large con glom er ate out crop in the Orava Val ley, be tween the Krivá and Dlhá nad Oravou set - tle ments, ap prox i mately 1000 m/164° from the el e va tion point Vysoký grúò (849 m a.s.l.). De tailed in ves ti ga tion in di cated Turonian to Coniacian age for the Krivá (Klape Unit) and Coniacian age for the Zástranie (Pieniny Unit) con glom er ate beds (Marschalko, 1986).
ANALYTICAL METHODS
Rock-form ing min er als of the gra nitic cob bles were stud ied in pol ished thin sec tions. Stan dard modal anal y sis was counted on the ba sis of ap prox i mately 3,000 points for both sam ples.
The monazite in ter nal tex ture, chem i cal com po si tion and age were in ves ti gated by elec tron microprobe in pol ished thin sec tions us ing a Cameca SX100 elec tron microprobe at the Dionýz Štúr State Geo log i cal In sti tute, Bratislava. Monazite dat ing needs spe cific mea sur ing con di tions as re quired in trace el e ment dat ing. These in clude high sam ple cur rent and a long count ing pe riod. The sam ple cur rent of 180 nA and count ing times of 300 s for Pb, 80 s for U and 35 s for Th were used for more ac cu rate mea sure ment. The ac cel er at ing volt age of 15 kV was pre ferred to 20 kV, as it pro vides better spa tial res o - lu tion, it is less harm ful in sur face dam age and it re duces PAP ma trix cor rec tion fac tors. Large crys tals (LPET and LLIF) were used, as these are sev eral times more sen si tive than con ven - tional ones. El e ments were mea sured as fol lows: As, Si, Al
were mea sured with TAP: P, Pb, Th, U, Y, S, Ca, Sr with LPET and REE and Y with LLIF crys tal. The fol low ing stan dards and spec tral lines were used: bar ite (SKa), ap a tite (PKa), GaAs (AsLa), wollastonite (SiKa, CaKa), zir con (ZrLa), ThO2
(ThMa), UO2 (UMb), Al2O3 (AlKa), YPO4 (YLa), LaPO4 (LaLa), CePO4 (CeLa), PrPO4 (PrLa), NdPO4 (NdLb), SmPO4 (SmLb), EuPO4 (EuLb), GdPO4 (GdLa), TbPO4 (TbLa), DyPO4 (DyLa), HoPO4 (HoLb), ErPO4 (ErLb), TmPO4 (TmLa), YbPO4 (YbLa), LuPO4 (LuLb), faya lite (FeKa), SrTiO3 (SrLa), bar ite (BaLa), and PbCO3 (PbMa). Spe cial care was taken to en sure that line over laps were prop erly cor rected and that back ground po si - tions were clear of in ter fer ing lines among the REE. We used em pir i cally de ter mined cor rec tion fac tors ap plied to the fol low - ing line over laps: Th ® U, Dy ® Eu, Gd ® Ho, La ® Gd, Ce ® Gd, Eu ® Er, Gd ® Er, Sm ® Tm, Dy ® Lu, Ho ® Lu, Yb ® Lu, and Dy ® As (Koneèný et al., 2004). The ma trix ef fects were cor rected by the PAP pro ce dure. The re sul tant age was cal cu - lated us ing the math e mat i cal model pro posed by Montel et al.
(1996). The re sul tant age is the weighted av er age of a group of ap par ent ages (from point anal y sis). Anal y ses ac quired dur ing the monazite dat ing pro ce dure in volve a com plete set of el e - ments us able for study of ex change sub sti tu tion re ac tions in monazite. Anal y ses with non-dat ing pro ce dure were added to ex pand the dataset. These were ana lysed un der the mea sure - ment con di tions of 15 kV, 40 nA and 5–10 mm beam di am e ter, and the count ing time var ied from 20 to 120 s de pend ing on the sen si tiv ity of the given an a lyt i cal line.
The ac cu racy of HREE (es pe cially Ho to Lu) pres ent in monazite at low con cen tra tions (gen er ally <0.5 wt.% HREE2O3) is lower than de sired, and the geo log i cal in ter pre ta tion of such val ues can be am big u ous. The de tec tion limit for LREE is 0.09–0.12 wt.%, HREE (Dy to Lu) 0.15–0.19 wt.%, and for other el e ments it is less than 0.09 wt.%.
RESULTS
CHARACTERIZATION OF GRANITIC CLASTS
Both stud ied cob bles (ap prox i mately 10 cm in size) rep re - sent equigranular to slightly porphyric, me dium-grained gra nitic rocks with out pref er en tial min eral ori en ta tion or schis tose struc - ture. Anhedral quartz shows dis tinct un du la tory ex tinc tion.
Subhedral to euhedral plagioclase com monly oc curs in the cen - tral parts of larger in ter sti tial perthitic K-feld spar. Plagioclase is slightly al tered by fine-crys tal line white mica ag gre gates (sericitization). Subhedral to anhedral bi o tite is com monly al - tered to chlorite. In some places, mus co vite re placed bi o tite or plagioclase, while thin veinlets (<0.1 mm) of sec ond ary cal cite cut the rock-form ing min er als in oth ers. Ap a tite, rutile, zir con and gar net were iden ti fied as ac ces sory min er als.
1. The Zástranie sam ple (BP-8) is clas si fied as bi o tite leucogranodiorite. In vol.% it con tains: 37.1 – quartz, 43.9 – plagioclase, 14.9 – K-feld spar, 3.5 – bi o tite (±
chlorite), 0.5 – mus co vite and 0.1 – ac ces sory min er als.
2. The Krivá sam ple (BP-10) is two-mica granodiorite. In vol.% it con tains: 33.5 – quartz, 48.0 – plagioclase, 9.5 – K-feld spar, 7.6 – bi o tite (± chlorite), 1.3 – mus co vite and 0.1 – ac ces sory min er als.
The chem i cal com po si tion of both gra nitic rocks (BP-8 and BP-10.1; Uher et al., 1994) and also the zir con ty po logi cal and compositional study (Uher and Marschalko, 1993) in di cate their peraluminous, calc-al ka line and orogen-re lated char ac ter with S-type ten dency.
344 Pavel Uher, Dušan Plašienka, Martin Ondrejka, ¼ubomír Hraško and Patrik Koneèný
monazite-(Ce) from the Krivá type granitic boulders in conglomerates of the Pieniny Klippen Belt...345
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MONAZITE PROPERTIES AND ASSOCIATED MINERALS
Monazite-(Ce) is a char ac ter is tic ac ces sory min eral in both in ves ti gated gran ite sam ples. It usu ally forms euhedral to sub - hedral crys tals (ap prox i mately 15 to 200 mm across), in as so ci - a tion with plagioclase, quartz, bi o tite, mus co vite, ap a tite, zir - con, xeno time-(Y), ThSiO4-phase and py rite (Fig. 2). Un der back-scat tered elec tron mode (BSE), monazite crys tals re veal rel a tively ho mo ge neous in ter nal tex ture and oc ca sion ally they dis play reg u lar or ir reg u lar slightly brighter rim zones. Pri mary mag matic in clu sions of zir con and xeno time-(Y) in monazite- (Ce) were de tected (Fig. 2B, D). Some monazite crys tals show al ter ation phe nom ena (Fig. 2A, C). This is most likely due to their subsolidus par tial dis so lu tion and reprecipitation; with per - fo rated in ter nal tex ture, ir reg u lar rims, sec ond ary anhedral in - clu sions of ThSiO4-phase (thorite or huttonite), uraninite, ap a - tite or py rite, and in some places there are overgrowths of TiO2-phase (sec ond ary rutile, ana tase or brookite?), ap a tite, chlorite, and mus co vite.
MONAZITE COMPOSITION AND DATING
Elec tron-microprobe anal y ses (n = 64) show monazite-(Ce) com po si tions, where Ce3+ is a dis tinctly dom i nant REE cat ion and Ce>La,Nd>Pr>Sm dis tri bu tion of the REE`s oc curs (Ap -
pen dix 1*). The most char ac ter is tic fea ture of the monazite-(Ce) from the both gra nitic cob bles (es pe cially from Zástranie) is an un usu ally high ura nium con tent in ma jor ity of monazite crys tals, com monly 1 to 3, and in some places up to 6.6 wt.% UO2 (up to 0.06 apfu U), to gether with 5 to 7.7 wt.% ThO2 (up to 0.07 apfu Th). The U-rich monazite crys tals are rel a tively ho mo ge neous, with high U con tent de tected in the en tire crys tal. In con trast, U-poor monazite crys tals (£1 wt.% UO2) were also de tected in both in ves ti gated sam ples (Ap pen dix 1, anal y ses A31, A3, and A27). These U-poor monazites com monly show subsolidus al - ter ation and break down co ro nae with sec ond ary ap a tite and rutile. Compositional vari a tions of the main el e ments in monazite (P, Si, Th, U, REE, Ca) in di cate a cheralite-type, Ca+(U,Th) = 2REE sub sti tu tion as the dom i nant in cor po ra tion mech a nism of U4+ into the monazite struc ture in the Zástranie sam ple (BP-8), whereas both cheralite and huttonite type sub - sti tu tions [(Th,U)+Si = REE+P] are ev i dent in the Krivá (BP-10) gra nitic clast (Fig. 3A). More over, the U and Th con cen tra tions in both sam ples ex hibit no mu tual cor re la tion (Fig. 3B), thus in - di cat ing their dif fer ent geo chem i cal be hav iour. Ura nium ev i - dently pre fers cou ple-sub sti tu tion with cal cium in a CaU4+(REE)–2 mech a nism, whereas Th in clines rather to wards the Th4+Si(REE)–1P–1 trend (Fig. 3C–F). Two par al lel Si vs. Th trends in di cate two dif fer ent monazite com po si tions:
– Si-rich with dom i nant huttonite;
– Si-poor with mixed cheralite-huttonite sub sti tu tion, es pe - cially in the Krivá sam ple (Fig. 3F).
346 Pavel Uher, Dušan Plašienka, Martin Ondrejka, ¼ubomír Hraško and Patrik Koneèný
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1089 Fig. 2A–D – BSE pho to mi cro graphs of monazite-(Ce) and as so ci ated rock-form ing
and ac ces sory min er als in Zástranie (BP-8) and Krivá (BP-10) gra nitic clasts Ab – al bite, Ap – ap a tite, Bt – bi o tite, Mnz – monazite, Ms – mus co vite, Qtz – quartz,
Pl – plagioclase, Py – py rite, Th – ThSiO4 phase, Xnt – xeno time, Zrn – zir con
Fig. 3A–F – sub sti tu tion di a grams of monazite-(Ce) from Zástranie (BP-8) and Krivá (BP-10) gra nitic clasts (in at oms per for mula unit, apfu)
Con cen tra tions of Pb (~0.1 to 0.4 wt.% PbO) are pro por - tional to Th+U, and this in di cates the ra dio genic or i gin of lead (Ap pen dix 1). The con tents of other mea sured el e ments in clud - ing S, As, Fe, heavy REE’s, Sr and Ba are neg li gi ble in both sam ples.
The cal cu lated elec tron-microprobe (chem i cal) U-Th-Pb mo - nazite dat ing for the stud ied sam ples gave very sim i lar Variscan ages: 348 ± 2 Ma (n = 25) for Zástranie, and 338 ± 4 Ma (n = 22) for the Krivá gran ite boul der. There fore, we used all an a lyt i cal data in a uni fied isochron for both monazite sam ples with the re - sul tant age of 346 ± 2 Ma (n = 47, MSWD = 2.83; Fig. 4). This rel - a tively high an a lyt i cal pre ci sion of dat ing is partly due to el e vated con tents of U in monazite, be cause the con tri bu tion of U to the ra dio genic Pb is a few times higher than that of Th.
DISCUSSION
URANIUM-RICH MONAZITE
Nat u ral monazite-(Ce) usu ally con tains Th4+, a cat ion that partly re places REE`s by two ba sic heterovalent iso mor phic
mech a nisms: (1) ThSiREE–1P–1 (huttonite-type sub sti tu tion), and (2) CaThREE–2 (cheralite-type sub sti tu tion). There fore, the Th con tent in monazites com monly reaches up to 30 wt.%
ThO2, and monazite-huttonite and monazite-cheralite solid so - lu tions have been com monly de scribed in gra nitic monazites (e.g., Bea, 1996; Förster, 1998, 2006; Wil liams et al., 2007).
Both sub sti tu tions are pos si ble due to the sim i lar ity of cat ion ra - dii and other geo chem i cal prop er ties of Th4+ and light REE3+
(mainly La3+ and Ce3+). The monoclinic monazite-type struc ture with dom i nant large light REE’s (La3+ to Sm3+) in nine-fold co or - di na tion and ef fec tive ionic ra dii of 1.13 to 1.22 × 10–10 m (Shan - non, 1976), pre fers larger [9]Th4+ (1.09 × 10–10 m), than smaller [9]U4+ (1.05 × 10–10 m). On the other hand, tetragonal xeno - time-(Y) and zir con pre fer the en try of heavy REE’s and U4+ with smaller ionic di am e ters rather than the larger Th4+ cat ion. Con - se quently, U-rich monazite (UO2 > 1 wt.%) is rel a tively rare in nat u ral oc cur rences. The high est U con tents in monazite have been de scribed from gra nitic pegmatites of the Ital ian Alps, where UO2 oc ca sion ally reached 12 to 16 wt.% (Gramaccioli and Segalstad, 1978; Mannucci et al., 1986), and also from some gra nitic rocks (3 to 14 wt.% UO2; Gulson and Krogh, 1973; Bea, 1996; Förster, 1998; Ap pel et al., 2011). The en try of U4+ into the monazite struc ture is com pen sated mainly by Ca2+
via CaUREE–2 sub sti tu tion; and the ThSiREE–1P–1 mech a nism is neg li gi ble (Gramaccioli and Segalstad, 1978; Manucci et al., 1986; Bea, 1996; Förster, 1998; Ap pel et al., 2011). The pres - ence of Ca0.5U0.5PO4 com pound in nat u ral monazites is sup - ported by ex per i men tal syn the sis of their monoclinic, monazite-struc ture end-mem ber an a logue (Dusasoy et al., 1996; Bregiroux et al., 2007). How ever, the or i gin of such a high con tent of ura nium in nat u ral monazite is still un clear. Here, spe cific en rich ment by ura nium or in creased U/Th ra tio in pa - ren tal magma can pos si bly be as sumed.
POSSIBLE SOURCES OF U-RICH MONAZITES FROM THE PIENINY KLIPPEN BELT GRANITIC CLASTS
In ad di tion to the gran ites, var i ous other clasts of prob lem - atic prov e nance have been rec og nized in the Upohlav type con - glom er ates. These in clude the Tri as sic pe lagic lime stones, Up - per Ju ras sic plat form lime stones, Urgonian lime stones with ser - pen tin ite frag ments and Cr-rich spinel and glaucophane grains, Up per Ju ras sic blueschists and large amounts of ba sic, in ter - me di ate and acid vol ca nic rocks (for com pre hen sive re views of the clasts com po si tion see Mišík and Marschalko, 1988;
Birkenmajer, 1988; Marschalko and Rakús, 1997; Mišík and Reháková, 2004). There have been sev eral at tempts to iden tify the source ar eas of this vari able con glom er ate ma te rial. At the be gin ning of a sys tem atic study of the PKB struc ture in west ern Slovakia, Andrusov (1931, 1938) con sid ered the con glom er - ates to be transgressive and the rock clasts to be de rived from the orig i nal sub stra tum of the PKB “Pienidic” units. Later on, when the intra-formational char ac ter of the con glom er ates and their close re la tion ship to synorogenic flysch de pos its be came clear, the con cept of an “ex otic cor dil lera” (Ultrapienidic or Andrusov Ridge) was for mu lated, and this was main tained for the en tire sec ond half of the last cen tury (e.g., Andrusov, 1968, 1974; Mišík and Sýkora, 1981; Birkenmajer, 1986, 1988; Mišík et al., 1991). There have been sev eral con cepts of the na ture of this ridge: as an el e vated imbricated zone in front of the Cen tral Carpathian nappes (Andrusov, 1974), an accretionary com plex bear ing obducted ophiolites (Mišík, 1978; Mišík and Marschalko, 1988), tec tonic sliv ers within a large-scale trans - form zone (Marschalko, 1986; Rakús and Marschalko, 1997), a 348 Pavel Uher, Dušan Plašienka, Martin Ondrejka, ¼ubomír Hraško and Patrik Koneèný
Fig. 4A, B – re sults of chem i cal U-Th-Pb dat ing of monazite-(Ce) from Zástranie (BP-8) and Krivá (BP-10) gra nitic clasts:
monazite age his to gram (A) and Pb vs. Th* (wt.%) age monazite isochron di a gram (B)
Th* = Th + 3.15*U (Nagy et al., 2002)
Lower Cre ta ceous mag matic arc in an ac tive mar gin set ting (Marschalko, 1986; Birkenmajer, 1988), and Meliata-re lated su ture com plexes in the In ner Carpathian zones (Plašienka, 1995a, 2012; Kissová et al., 2005).
Sim i lar to the sit u a tion in other ex otic clasts, a di rect source of the ma jor ity of gra nitic ma te rial in the Upohlav con glom er ates is un known or ques tion able; at least at the pres ent ero sional sur face. In gen eral, gra nitic ma te rial oc curs in sev eral strati - graphic lev els of ex otic con glom er ates; namely in mid-Cre ta - ceous (Albian–Cenomanian), Up per Cre ta ceous (Conia - cian–Santonian), and in Maastrichtian–Eocene flysch se - quences. Con cern ing the true “ex otic” ma te rial, en com pass ing Perm ian A-type gran ites (Uher and Marschalko, 1993; Uher and Pushkarev, 1994; Uher et al., 1994), these are pres ent in all three strati graphic lev els. Based on zir con fis sion-track dat - ing, Kissová et al. (2005) pro posed their der i va tion from a con - tem po ra ne ous, ac tively de form ing and ex hum ing moun tain ous re gion in the south ern Cen tral Carpathian zones in mid-Cre ta - ceous times, while the youn ger oc cur rences rep re sent clasts re cy cled from the orig i nal Albian–Cenomanian con glom er ates.
How ever, the Krivá type gra nitic rocks of pre sum ably Tatric or Veporic prov e nance are only known from the sec ond, Coniacian to Santonian con glom er ates. There fore, their der i va - tion from sources dis tinct to those of the Perm ian A-type gran - ites must be con sid ered.
Pos si ble Cen tral Carpathian prov e nance of the “non-ex otic”
Krivá type gra nitic clasts is tested by the fol low ing com par i son.
A sys tem atic compositional study of monazites from the West Carpathian Variscan gra nitic and peg ma titic rocks (un pub lished ma te rial of the au thors) re veals their gen er ally low con cen tra - tions of U (usu ally 0.1 to 0.3 wt.% UO2). How ever, monazite from leucogranites and aplites of the Rimavica Mas sif (south - ern part of the Veporic Superunit), es pe cially from the Sinec body, con tained 3 to 8 wt.% UO2 in some places. More over, 1 to 3.7 wt.% UO2 was rarely de tected in monazite from gran ites of the Bojná Mas sif (Považský Inovec Mts.) and the Bratislava Mas sif (Malé Karpaty Mts.), both in the North Tatric Zone. All known oc cur rences of U-rich monazite in the West Carpathian gra nitic rocks (Rimavica, Bojná and Bratislava mas sifs) show sim i lar geo chem i cal fea tures to the Krivá gra nitic clasts, es pe -
cially S-type af fin ity and anal o gous ~350 Ma age (e.g., Broska and Uher, 2001; Fin ger et al., 2003; Kohút et al., 2009).
Two pos si ble sources of the ex am ined Krivá type gra nitic cob bles are now dis cussed. The first pos si bil ity is the South Veporic Rimavica Mas sif. The Cre ta ceous tec tonic evo lu tion of the Veporic Superunit was re cently re con structed by Janák et al. (2001) and Jeøábek et al. (2012). Ac cord ingly, the South ern Veporic zones ex pe ri enced up lift and ex hu ma tion in the lat est Cre ta ceous times, i.e. the Rimavica Mas sif might have been al - ready ex humed and was the o ret i cally able to pro vide clasts for the Coniacian to Santonian de pos its, not with stand ing that pub - lished cool ing ages (e.g., Putiš et al., 2009 and ref er ences therein) do not sup port this pos si bil ity. On the other hand, there are sev eral ob jec tions that cast doubt on this sup po si tion. First of all, the South ern Veporic do main is com posed of a great va ri - ety of rocks in ad di tion to the gra nitic rocks. These in clude base - ment am phi bo lites, gneiss es and micaschists and also var i ous meta mor phosed sed i men tary cover rocks (Perm ian and Tri as - sic sand stones and car bon ates). How ever, there is no clast of this prov e nance rec og nized in the Upohlav con glom er ates.
More over, there are no clasts of Veporic or i gin in the Senonian con glom er ates of the Gosau Group which were de pos ited just nearby the ex hum ing Veporic dome (e.g., Hovorka et al., 1990).
An other prob lem con cerns the dis tance be tween the pos si ble sources and the ac tual po si tion of the con glom er ates. This is ap prox i mately 100 km, and no traces of trans port pos si bil i ties are rec og niz able at the pres ent ero sional sur face (cf. Fig. 5).
The sec ond op tion is the Bojná and/or Bratislava Mas sif or sim i lar source lo cated at the outer Tatric mar gin, and there fore in the prox im ity of the PKB. This pos si bil ity seems more vi a ble for palaeotectonic rea sons. The north ern Tatric mar gin ex pe ri - enced short en ing and imbrication dur ing the Late Cre ta ceous (Plašienka, 1995b; Putiš et al., 2009). Coniacian-Santonian con glom er ates of the Belice Unit of pre sumed South Penninic- Vahic af fil i a tion con tain, in ad di tion to var i ous Me so zoic car bon - ates, also peb bles of aplit ic gran ites and es pe cially micaschists de rived from the over rid ing Tatric Inovec Nappe (Plašienka et al., 1994). It may be in ferred that a sim i lar tec tonic sit u a tion oc - curred in those parts of the Coniacian–Santonian Upohlav con - glom er ates which were lo cated nearby sources of the Krivá
Fig. 5. Con cep tual palaeotectonic cross-sec tion through the West ern Carpathians at ap prox i mately the Coniacian/Santonian bound ary (ca. 85 Ma)
The sec tion shows ac tive bas ins of this age (dot ted ar eas) in front of the Tatric sheet, filled with con glom er ates con tain ing gra nitic clasts ana - lysed in this work. Note that co eval con glom er ates de pos ited in Gosau-type bas ins ad ja cent to the Meliatic su ture in the south ern Carpathian zones do not con tain any ma te rial de rived from pos si ble Veporic sources, but are dom i nated by Meliatic ophiolitic de tri tus. Es ti mated po si - tion of the pos si ble sources dis cussed in the text is in di cated by BM (Bojná and Bratislava mas sifs) and RM (Rimavica Mas sif)
type gra nitic rocks in the Tatric base ment (Fig. 5). Dur ing sub - se quent short en ing, these sources could have been hid den by overthrusting of the higher Tatric nappes, in clud ing the Bojná and Bratislava base ment units.
CONCLUSIONS
Study of ac ces sory monazite-(Ce) in gra nitic cob bles from the Krivá type (Zástranie and Krivá lo cal i ties) polymict con glom - er ates of Cre ta ceous flysch se quences in the Pieniny Klippen Belt re veal their spe cific chem i cal com po si tion and age. The most char ac ter is tic fea ture of the stud ied monazite is its high ura nium con tent which is up to 6.6 wt.% UO2 in some places (£0.06 apfu U), to gether with an el e vated Th con cen tra tion. The ac ti nide el e ments (U+Th) are in cor po rated to gether in the monazite struc ture through both cheralite- and huttonite-type sub sti tu tions. How ever, while ura nium dis tinctly pre fers the cheralite-type, CaU4+(REE)–2 sub sti tu tion mech a nism, Th in - clines rather to wards the huttonite-type, Th4+Si(REE)–1P–1
trend. Un like the com monly high Th con cen tra tions, high U en - rich ment is nor mally scarce in nat u ral monazite. It has been re - ported rarely from some gra nitic rocks and pegmatites (e.g., Gramaccioli and Segalstad, 1978; Manucci et al., 1986; Bea, 1996; Förster, 1998; Ap pel et al., 2011). The or i gin of such note wor thy U en rich ment in monazite re mains un re solved; high
lo cal ized ura nium con tent or an in creased U/Th ra tio in pa ren tal gra nitic magma could have im por tant roles.
The chem i cal U-Th-Pb dat ing of monazite from both gra nitic sam ples gave a Variscan, Early Car bon if er ous age (346 ± 2 Ma). This re sult con curs with a main meso-Variscan, orogen- re lated gra nitic plutonic event in the Cen tral Carpathian area (Tatric and Veporic Superunits). Ura nium-rich monazite-(Ce) has only rarely been iden ti fied in the Cen tral Carpathian, Varis - can S-type gra nitic rocks; namely in some leucogranites of the Rimavica Mas sif (South Veporic Unit) and gran ites of the Bojná and Bratislava mas sif (north ern part of the Tatric Unit). Based on re cent struc tural and paleogeographic data, the North Tatric Zone is the most likely source of the monazite-bear ing gra nitic clasts of the Krivá type in the Cre ta ceous con glom er ate beds of the Pieniny Klippen Belt. How ever, the di rect pa ren tal gra nitic body was most likely hid den by en su ing tec tonic short en ing along the north ern Tatric edge af ter de po si tion of the Conia - cian–Santonian con glom er ates.
Ac knowl edge ments. The au thors thank A. Gawêda and B. Budzyñ for con struc tive re views which im proved the sci en - tific level of the manu script and R. Mar shall for cor rec tions of the Eng lish lan guage. This work was sup ported by the Slo vak Re - search and De vel op ment Agency un der con tracts No.
APVV-0557-06 and APVV-0465-06, and VEGA 1/0388/10.
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