Geo log i cal Quar terly, 2009, 53 (3): 357–362
Palaeomagnetic age con straints in di cate post-Variscan orgin of mas sive brec cia in Wietrznia Quarry (Holy Cross Moun tains, Cen tral Po land)
Rafał SZANIAWSKI and Marek LEWANDOWSKI
Szaniawski R. and Lewandowski M. (2009) — Palaeomagnetic age con straints in di cate post-Variscan orgin of mas sive brec cia in Wietrznia Quarry (Holy Cross Moun tains, Cen tral Po land). Geol. Quart., 53 (3): 357–362. Warszawa.
Palaeomagnetic study has been car ried out to re solve the con tro versy over the age and or i gin of the mas sive brec cia body ex posed in Wietrznia Quarry in the Holy Cross Moun tains. The brec cia stud ied con sists of var i ously sized an gu lar frag ments of Frasnian and Fammenian lime stone and shale in fill ing a palaeodepression within Frasnian lime stone. For this pur pose, the con glom er ate test has been ap plied that en ables de ter mi na tion of the rel a tive age of remanence com po nents and for ma tion of the brec cia. Palaeomagnetic anal y sis of brec cia clasts re veals the pres ence of two char ac ter is tic com po nents of mag ne ti za tion. The ob served com po nents are akin to those pre vi - ously de scribed from in situ rocks at Wietrznia Quarry and from other De vo nian car bon ates from the Holy Cross Moun tains. Such com - po nents have been in ter preted as early synfolding and postfolding over prints of Viséan and Early Perm ian age. The palaeomagnetic con glom er ate test per formed in this study are pos i tive re sults, in di cat ing that the brec cia post dates both com po nents. This im plies that the brec cia is not of synsedimentary char ac ter and post dates Variscan de for ma tion.
Rafał Szaniawski, In sti tute of Geo phys ics, Pol ish Acad emy of Sci ences, Ks. Janusza 64, PL-01-452, Warszawa, Po land; e-mail:
rafsz@igf.edu.pl; Marek Lewandowski, In sti tute of Geo phys ics, Pol ish Acad emy of Sci ences, Ks. Janusza 64, PL-01-452, Warszawa, Po land, In sti tute of Geo log i cal Sci ences, Pol ish Acad emy of Sci ences, Twarda 51/55, PL-00-818 Warszawa, Po land (re ceived: April 24, 2008; ac cepted: July 07, 2009).
Key words: Holy Cross Moun tains, palaeomagnetism, con glom er ate test, palaeomagnetic dat ing.
INTRODUCTION
In this pa per, we fo cus on the age es ti ma tion of the mas sive brec cia ex posed in Wietrznia Quarry lo cated in the Holy Cross Moun tains (HCM; Fig. 1). This spec tac u lar megabreccia body is dis tin guished by its con sid er able size (500 ´ 150 ´ 50 m, ac - cord ing to Szulczewski, 1995) and is com posed of un sorted frag ments of Frasnian and Fammenian rocks of var i ous di men - sions, up to a few metres in di am e ter.
Based on field ob ser va tions, the struc ture was de scribed as
“fit ted megabreccia” formed as a re sult of Perm ian post-orogenic karstification (Szulczawski, 1995). A sim i lar or - i gin for the brec cia was sug gested by Lewandowski (1971) and Ur ban (2007). There are also al ter na tive in ter pre ta tions link ing the or i gin of brec cia with synsedimentary tec ton ics (Lamarche, 1999; Lamarche et al., 2003). There fore, de ter mi na tion of age con straints of this mas sive brec cia body is im por tant for un der - stand ing the Late Pa laeo zoic evo lu tion of the HCM since it con strains the scale of synsedimentary de for ma tion as well as the course of post-Variscan ex hu ma tion pro cesses.
This is sue is ex am ined here, with a palaeomagnetic con - glom er ate test be ing ap plied to de ter mi nate the time-re la tion - ships be tween the for ma tion of brec cia and ac qui si tion of in di - vid ual remanence com po nents. It is worth not ing that the con - glom er ate test is com monly used for age de ter mi na tion of geo - log i cal events (e.g., Con vert et al., 2006). This method is es pe - cially ef fec tive since the nat u ral remanent mag ne ti za tion (NRM) struc ture of the strata has been char ac ter ized in pre vi ous stud ies.
Palaeomagnetic stud ies per formed over the last three de cades on De vo nian car bon ates of the HCM re veal the oc cur rence of sev - eral com po nents of sec ond ary mag ne ti za tion (Lewandowski, 1981, 1985, 1999; Grabowski and Nawrocki, 1996, 2001;
Grabowski et al., 2006). More re cent out comes of de tailed stud ies of the De vo nian car bon ates of the HCM en able def i ni tion of two wide spread ep i sodes of re mag net isa tion shown by in di vid ual palaeomagnetic com po nents (Zwing, 2003; Szaniawski, 2008).
Fold tests per formed within the first or der Variscan folds of km-scale wave length doc u ment an early synfolding or i gin for the Viséan com po nent (B) and a postfolding or i gin of the Early Perm - ian (A) over print (Szaniawski, 2008). Thus, the re sults of the brec - cia test en able de ter mi na tion of the age of brec cia for ma tion rel a - tive to Variscan fold ing.
GEOLOGICAL SETTING AND METHODS
In Wietrznia Quarry, sit u ated within the Palaeozoic out crop of the HCM, that com prises mas sive Givetian–Frasnian car - bon ates over lain by Famennian basinal de pos its rep re sented by thin-bed ded car bon ate and marls (Szulczewski, 1971). The quarry is sit u ated on the south ern limb of the Kielce Syncline char ac ter ized by a re gional WNW–ESE tec tonic trend (Fig. 1).
Ac cord ingly, the strata seen in this ex po sure are char ac ter ized by a gen eral dip to wards the NNE. The brec cia stud ied is lo - cated in the cen tral part of the quarry.
To per form the palaeomagnetic con glom er ate test, six vari - ably ori ented blocks of the brec cia body were sam pled. From each block, three in de pend ent hand sam ples were col lected and the ori en ta tion of the bed ding plane was mea sured (with out the roof/till de ter mi na tion). It is worth noting that palaeomagnetic in ves ti ga tions of the in situ rocks were per formed in a pre vi ous study (Szaniawski, 2008) where seven hand sam ples were col - lected from the con tin u ous Frasnian lime stone suc ces sion bor - der ing the brec cia body.
The sam ples were sub se quently drilled in the lab o ra tory to give stan dard spec i mens of 24 mm di am e ter. Part of the sam - ples were uti lized for stud ies of mag netic min er al ogy in clud ing ac qui si tion of iso ther mal remanent mag ne ti za tion (IRM) and the Lowrie test (Lowrie, 1990). Such an a lyzes were per formed with a 2G SQUID cryo genic mag ne tom e ter plus Mag netic Mea sure ments MM-1 fur nace and pulse mag ne tizer. The same mag ne tom e ter and fur nace were uti lized for anal y sis of nat u ral remanent mag ne ti za tion (NRM) where a ther mal de mag ne ti za - tion tech nique was ap plied. The mag netic sus cep ti bil ity was mon i tored af ter sub se quent steps of ther mal clean ing us ing the KLY-2 bridge. To cal cu late the palaeomagnetic di rec tions, prin ci pal com po nents anal y sis (PCA) was em ployed (Kirschvink, 1980) uti liz ing the soft ware of Chadima and Hrouda (2006).
PALAEOMAGNETIC RESULTS
The IRM ac qui si tion curves ob tained show a typ i cally rapid in crease of mag netic remanence af ter ap pli ca tion of rel a tively low mag netic fields of 0–16 mT, in di cat ing the pres ence of low-coercivity mag netic min er als (Fig. 2A). The sam ples stud - ied were al most com pletely sat u rated in a field of 30–40 mT show ing prac ti cally no in crease of mag ne ti za tion up to the 3 T field. Re sults of Lowrie tests (Fig. 2B) con firm that the mag netic min er al ogy is dom i nated by low-coercivity min er als that show max i mum un block ing tem per a tures (Tub) of 450°C.
These re sults are gen er ally in line with the pub lished data of Grabowski et al. (2006). The low coercitivity mag netic phase of mod er ate block ing tem per a tures most probably rep re sents maghemite or non-stochiometric mag ne tite. The maximum block ing tem per a tures ob served in the Lowrie test are sim i lar to the un block ing tem per a tures of A and B com po nents de scribed by Zwing (2003) and Szaniawski (2008).
Re sults of ther mal de mag neti sa tion in di cate that the gen eral NRM struc ture ob served in the brec cia clasts is equiv a lent to those known from in situ de pos its from Wietrznia Quarry and from other car bon ates of the HCM (Szaniawski, 2008). Af ter de mag ne ti za tion of the soft mag ne ti za tion (up to 250°C), in ter - preted as a re cent vis cous over print, two com po nents of roughly op po site di rec tions are no tice able (Fig. 2C, D). They are dis tin guished by a char ac ter is tic in crease of mag netic sig nal in tem per a tures of 250–350°C and by lin ear seg ments of de - mag ne ti za tion plots within the in dic a tive spec tra of Tub.
Ac cord ing to their gen er ally op po site di rec tions and char - ac ter is tic Tub ranges of 250–375°C and 350–450°C, the com - po nents were rec og nized as A and B re spec tively, both known from pre vi ous stud ies (Szaniawski, 2008). Sep a ra tion and cal - cu la tion of these com po nents was fre quently dif fi cult due to par tial over lap ping of their un block ing tem per a tures spec tra as well as to thermochemical al ter ation (over 350°C) shown by
Fig. 1. Geo log i cal map of the Holy Cross Moun tains (af ter Czarnocki, 1938) show ing lo ca tion of Wietrznia Quarry
Palaeomagnetic age constraints indicate post-Variscan orgin of massive breccia in Wietrznia Quarry 359
Fig. 2. Palaeomagnetic re sults
A — stepwise ac qui si tion of IRM; B — ther mal de mag neti sa tion of a com pos - ite three-axis IRM, Lowrie tests (Lowrie, 1990); C and D — ther mal de mag ne - ti za tion re sults of spec i mens from block 3 (C) and 6 (D), red and blue colours mark Tub spectras char ac ter is tic of com po nent A (250–350°C) and com po nent B (over 375°C); E — over all di a gram show ing vari a tions of mag netic sus cep ti - bil ity dur ing the ther mal de mag neti sa tion
cha otic be hav ior of the remanance ac com pa nied by an in crease in mag netic sus cep ti bil ity (Fig. 2E). Fi nally, the ap pli ca tion of PCA en ables de ter mi na tion of the di rec tion of com po nent A in five blocks and com po nent B in three blocks (Ta ble 1; Fig. 3).
In or der to ver ify the pre sented re sults we de ter mined the an gles be tween com po nent B and the bed ding plane within in - di vid ual blocks. The val ues are: 7° for block 1, 23° for block 3 and 6° for block 6. These re sults are com pa ra ble with pre vi ous out comes de rived from in situ strata (Szaniawski, 2008) where the an gle be tween the bed ding plane and com po nent B is 21°.
The lat ter ob ser va tion, keep ing in mind that com po nent B is of early-synfolding or i gin (Szaniawski, 2008) and strata in the quarry are of gen er ally sim i lar ori en ta tion, ar gues in sup port of the re li abil ity of the re sults pre sented here.
INTERPRETATION AND DISCUSSION
The re sults of this study in di cate that com po nent A is well pre served and is pos si ble to ex tract in most of the brec cia blocks stud ied. In the in situ rocks of Wietrznia Quarry the oc - cur rence of com po nent A was also ob served but ca pa ble of sep a ra tion uti liz ing the PCA tech nique (Szaniawski, 2008).
This mi nor dif fer ence in the remanence re cord is not sur pris ing since the Up per Palaeozoic car bon ates from the HCM com - monly show var i ous NRM struc tures both be tween sam pling sites, as well as be tween in di vid ual spec i mens de rived from the same hand sam ple (Grabowski et al., 2006; Szaniawski, 2007).
Nev er the less, the postfolding com po nent A was pre cisely eval -
Fig. 3. Frag ment of the brec cia stud ied from Wietrznia Quarry with equal area di a grams show ing palaeomagnetic data (all di a grams be fore tec tonic cor rec tion)
Iso lated di a gram on the photo shows pre vi ous re sults of Szaniawski (2008), di rec tion of com po nent B from in situ rocks of Wietrznia Quarry (blue dots) and ref er ence site mean di rec tion of com po nent A (red cross). Di a grams be low show re sults of this study de rived from in di vid ual blocks (B1–B6). Solid (open) sym bols rep re sent down ward (up ward) palaeomagnetic di rec tions
u ated in other lo cal i ties dur ing a pre vi ous study (Szaniawski, 2008) and this data can by used as ref er ence for the brec cia test (Ta ble 1; Fig. 3).
The di rec tions of both com po nents, A and B, ob tained within the brec cia show good group ing within sep a rate brec cia blocks (Fig. 3). In con trast, di rec tions of com po nents A and B dif fer sig nif i cantly be tween in di vid ual brec cia blocks as well as dif fer ing from the ref er ence di rec tions known from in situ rocks. This clearly im plies the pos i tive re sult of the con glom er - ate test, e.g. the age of the brec cia post dates the ac qui si tion time of both com po nents, A and B.
The pre cise prop er ties of com po nents A and B were de - scribed pre vi ously based on re sults from 13 sam pling sites (Szaniawski, 2008). Out comes of the fold tests per formed within the first or der, km-scale Variscan folds in di cate the early synfolding or i gin of com po nent B (dated to the Viséan) and post-fold ing ac qui si tion of the Early Perm ian com po nent A.
Thus, re sults of con glom er ate test im ply that the brec cia stud - ied post dates the main Variscan fold ing and is youn ger than Early Perm ian.
The data ob tained con tra dict the hy poth e sis of a synsedimentary or i gin for this brec cia as sug gested by Lamarche (1999) and Lamarche et al. (2003) who ar gued that the brec cia body shows sed i men tary strat i fi ca tion. Con versely, the re sults of this study sup port mod els sug gest ing post- orogenic for ma tion of the brec cia body and its re la tion to karstification (Szulczewski, 1995; Ur ban, 2007). Szulczewski (1995) sug gested that the brec cia orig i nated as a re sult of abrupt “dis place ment of in tensely karstified De vo nian car bon -
ates along the fault sys tem” and no ticed that it cov ers an older gen er a tion of “rock-ma trix brec cia” of pre sum ably Perm ian age. This model was ex tended by Ur ban (2007) who inerpreted this brec cia as a coluvium de vel oped on the slope of the palaeokarst de pres sion. Ur ban (2007) re marks also that hy dro - ther mal cal cite “różanka” re places the ma trix of the brec cia stud ied. In con junc tion with the re sults of the pres ent study it im plies that “różanka” hy dro ther mal min er al iza tion post-dates the ac qui si tion of com po nent A.
CONCLUSIONS
1. Blocks of stud ied brec cia re cord two remanence com po - nents known from pre vi ous stud ies: com po nents A and B.
2. Re sults of the con glom er ate test are pos i tive, in di cat ing that the brec cia post-dates the ac qui si tion of both com po nents.
3. The stud ied brec cia is not of synsedimentary or i gin since it post-dates the Variscan fold ing and the Early Perm ian com - po nent A.
4. It is sug gested that the ac qui si tion of com po nent A post - dates the min er al iza tion of “różanka” hy dro ther mal cal cite.
Ac knowl edge ments. This work was sup ported by re search grants of the Pol ish Min is try of Sci ence and Higher Ed u ca tion (N307 037 31/2527). E. Márton and M. Narkiewicz the jour nal ref er ees, are thanked for care ful and con struc tive re views and H. Norberciak for tech ni cal as sis tance.
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Palaeomagnetic age constraints indicate post-Variscan orgin of massive breccia in Wietrznia Quarry 361
Site/block com po nent A com po nent B
Dir/dip D/I k a N1/n1 D/I k a N1/n1
Block 1 239/42 – – – – 315/3 151 6 3/6
Block 2 63/89 76/35 29 13 3/6 – – – –
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WC 39/39 – – – – 37/19 68 5 5/13
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