I

Rom. J. Petrology, 1994, 76, p. 85-90

INTRUSIVE BRECCIAS RELATED TO MICROGRANITIC RHYOLITES IN THE DUMBRAVA-CĂPUŞU MIC AREA (NW TRANSYLVANIA)

Avram ŞTEFAN, Emilian ROŞU, Viorica MILU, Anat,ol RUSU

Institutul Geologic al României. Str. Caransebe§ nr. 1, R 78344 Bucureşti 32

Key words: Breccia. Volcanic breccia. Rhyolite Brecciat.iou. Major elements. Miilor elcments. N\lV Transylvania zone.

Abstract: On the northern border of Gilău Mts, bctween Dumbra.va and Căpll~u Mic, an are a which is limited by two NW-SE orientccl fract.urcs stands out, in which the basemcnt consisting of crystalline schists is transgressively overIain by Scnonian' conglomcrates, sandstones and siltstones, pierced or covered by banatitic calc-all-aline rocks. The latll'r ones are rcpresented both by quartz andesitic volcanics with pyroxenc anei amp!tibolcs (Maastrichtian-Paleoccne, of the first cyde) and intrusivc rocks (Paleocene, seconel cyde): andesites witll amphiboles anei biotite, microgranitic rhyolites. Here basalt andesitcs with pyroxene ± olivinc crop out too, coming from another magmatic source. Microgranitic rhyolites represen ting final diITerentiatioll products of granitic-grauodioritic magma , are associateu with intrusive breccias, in whose generation f1uidization processcs have t,Lken place with diffcrent intcnsities from one place to another, being more substalltial lI ear t!tc Illaglllatic bouy. Intrusivc breccias with mixed composition always cOlltain lit!tic or 1110norniJlcrai fragmcnt~ which have been broken from t!te crystallille basement or from Sellonian deJlosits. Lruptivc rock fragments from the brcccia arc represented hot.!t hy quartz alllle~ites with pyroxene anei amphiboles (at, CăpuşlI Mic) . quartz alldesite,; with alllphibolcs anei biotitc (SE of Dumbrava) anei cspecially by abulldant alld oIllllipresellt microgranitic rhyolites, At Icast t\Vo generations of breccias witb thc same cOlllposit.ion arc pointed out, the latest one being fine-graineel (tuffaccous) with several trclldR allli pierces or desintegrates the first one. On the whole the brecciatioll process developed similar (,0 those [rom the breccia pipe, diatreme and pebble dyke systems, but less evolved in the case we are referring to, The evolution of the brecciation process was stopped by a rapidly consolidation of the microgranitic rhyolite body.

Introduction

In the region of t,he· Dumbrava and Căpu§u 1Iic 10-calities (Cluj county) on the northem border of the Gilău Mts, in the area framed by t\Vo NW-SE ori­ented fractures, there are banatitic magmatic rocks , piercing or covering the Senonian sedimelltary de­posits (Fig. 1). In this area, associated to rhyolites­microgranitic rhyolit,es which represent the latest dif­ferentiat,ion products of banatit,ic calc-alkaline magma, mixed intrusi ve breccias have been pointed out; these breccias contain fragments from bot,h Senonian sedi­mentary depostits and from crystalline schists (base­ment of region) , as well as from banatitic magmatit.es (from rhyolites to microgranitic rhyolites, inclusively) but as it seems, Hot from pyroxenic ±olivine andesi-

te. Senonian sedimentary deposits and banatitic tnag­mat,ic rocks are overIain by Paleogene fOflllations and Quaternary deposi ts.

Thc presence of intrusive breccias, yiclcled by failrd explosions and, more rarely. piercing to the smfacl', usuaIly accompanied by fluidizat,iOlr and inj ccl iOIl prf)­cesses, has been statecl by oile of thc authors (~«'fall. 1980; Ştefan , in Mantea et al., unpubl. ('pport) for I Ii<' VIădeasa eruptive massif. In every dist.ributioll <lr('a of t,he banatitic magmatites, hreccias are associat, 'd 1,0 the latest magmatic (acid) difrerellt.iatioll proc!lICls , enriched both in Si02 alld K 20. and in f1l1ids . [ i l'

to tbe present days it Ilas been consÎl!c-rpd Ill<tI t 11< ',"," breccias accompany only rhyolitcs in Ilw 111;. ill hod\' of' VIădeasa (Cycle 1, essf'lltially it \'olcilnic Olll') n(' l'li­crogranitic rhyolites fi'om IIH' irregular shajll-'d hodic 's,

I

c;; Dum brava·

·678

(\: ....•.........

•...... .

700 600 "" ... -...... ~ ... ,..+---~

500 400

300 200 100

O

Cetăţe,aua Hill

o SOOm I

D D . . . ::

3

4

5

6

rz2J " 1\ 7

O .... 8

G -- - 9 ---10

... ..... .. ..... 11

--13

1-- ~14

SE 2

Cionca Hill

i\ . S"/,r:FA Net al.

I· ,).!, . \ ;, ·olog,i, ·al ,k .. l .. h of II ... 1)1lIl,"rava-Căpll~1l :\li .. an·a. 1. Qual"rnary: allU\·ial. pmhl\' lal. ('<,lIlIvial alld d,·hl\·ial d"posils ; Palcu!!.,·,,,,: '1. •. Iibo" l:3('ds-CllIj Li lJIesl OJ"' : :l. basalt andesites with pyroxcne ± olivinc: -1, lIIil'lo­graniti/' rhyolit.es; 5, intrusive breC<'ia; coars" (a) and tulTaceous (b); 6, quartz anclesites with amphiboles and biot.itc; Paleocene-Maastrichtian; 7. quartz andesites wit.h pyroxene and amphiboles; 8 , Campaniall-Santonian: sanclst.ones, conglomerates, siltstones, 9, Cambrian-Upper Precambrian: Biharia Series: serieitie-ehlorite sehists, erystalline clolomites anei limestones, graphite sehists, amphibolites, granitoicls. Conventional signs: 10. geologieal linut in

general; 11 , unconfornuty lin1.it; 12, fault; 13, position of the geologieal seetion; 14 , geological seetion,

I

INTRUSIVE BRECCIAS MICROGRANITIC RHYOLITES 87

probably representing marginal faeies of greater gran­ite volumes from the depth (belonging to the 2nd eycle, only intrusive) .

Detailed observations in the fi eld , corrob6rated with microscopic stlldies, first pointed out breeeias whieh aeeompany final , generally vein magmatites , with rhyolite-granitie eomposition anei texture, enrieheel in alkalies, resulting from granoelioritie-granitie banatitie magma elifferentiatioll.

Geological Sctting

In the western part of the area, whieh is limited by the two fraciures, erystalline formations elevelop, being represented by: ehlorite sehists ± epidote, al­bite; serieitie-chlorite schists; erystalline limestones anei dolomites, ealcareous sehists; graphitie sehists and amphiboJites. These roeks belong to the Biharia Se­ries from the Lup§a Nappe (Dimitreseu, Dimitreseu , Săbău; in Rusu et al., 1991, ullpubl. report) whieh thrusts the formations of t he Some§ Series, elcveloped westwards .

The erystalline formations from both series are piereed by several dykes of banatitie magmat. ie roeks with a NW-SE trenel, represent.eel by porphyrie quartz mieromonzoeliori tes, daci te- porphyrie mierogranodior­ites, mierograu iti c rhyolites anei lamprophyres.

Within the main zone undeI' study, limiteel by thc two fraci. ures, the first. banatitie magmatic roeks em­plaeeel over tÎle Senonian seel il1lentary formation , re­presenteel hy sandstones, eonglomerates anei grey silt­stones, are quartz anelesites with pyroxene and horn­blende. These magmatites ean be eorrelated with those from the VIădeasa eruptive massif by whieh the ban ati tic volcanism from the first eycle begins. It must be remarkeel that. in the Dumbrava-Căpu§u Mic area daci tic anei rhyoJitie volcanies from the first cycle of banatitie magmatism do not oeeur.

Within the 2nel banatitie magmatie eycle two petrotypes are encountered in this area: 1) qllartz an­desites with hornblenele and biotite in thc Cerhat Ilill at Dumbrava, whieh includes several fragments of erys­talline sehists and bloeks of porphyrie microdiorites and 2) mierogranitic rhyolites in the Cetăteaua Hill (SE of Dumbrava), whieh c10se this evolution eycle of the banatitie magmatism.

The intrusi ve breeeias with changing granulation anei composition have been outlined both near Dum­brava, north of the mierogranitie rhyolite at the Cetăţeaua llill , alld to the south, at the Cionea Hil! (uorth of Căpu§lI Mic) and at Căpu§u Mic. The ex­tension of this breceia can be larger as it is figured on thc map (Fig. 1), as Paleogene anei Quaternary deposits h i neiel' ohservat.ions.

Basalt andesitcs \\'ith pyroxene ± olivinc, with dif­ferent. origin rrom pl'Oper banat itie roeks, in this case

seem to be emplaeed at least partly peneeon tempo­raneously with mierogranitie rhyolites (Ştefan et al., 1985).

The Cretaeeous sedimentary formations and the eruptive roeks entirely, including intrusi ve breeeias sit­uated between the two fraetures, are eliseordantly eov­ered by red continental deposits of the Jibou Beels (Paleoeene-Lower Lutetian). West of this area, Paleo­gene deposits transgressively eover a relief moulded on erystalline sehists piereeel by banatitie boelies (daci te­porphyrie mierogranodiorites type). East of the area whieh is Jimited by the two faults, only Paleogenc and Quaternary deposits develop .

In the Dumbrava-Căpu§u Mic area Quaternary de­posits oeeupy large spaees.

Physiography, composition and emplacement of breccias

Breeeias are massi ve, grey, redelish , white or green roeks with variable granulation (from few eentimetres to submilimetres). Lithie and mouomineral fragments are usually angular and rarely a little rounded.

The lithologie composition of breeeias is variable ae­cording to the nat.lIre of the formation that builds it up. Thus, in the Căpu§u Mic area one ean notiee that the t.ransition from Senonian eleposits (praetieaJly ulldisturbed) to mix:ed eomposition breeeias is aeeom­plisheel through a breeeia area eonsisting only of angu­lar or subrouuded sandstone and mieroeonglomerate fragments . Solitary banatitie rhyolite fragments oe­cur graelually and in the end turn into breeeias rieh in eruptive roeks (hornblcnde, pyroxene anelesites anei es­peeially rhyolitie fragments). In aiI cases erystaloclasts anei Jithoclasts from prebanatitie r()cks are obviously anei ean proeeed either from Senonian seelimentary roeks 01' from the erystalline sehist basement. Bree­eias were also encountered whieh were almost entirely eOllstituteel of banatitie magmatie (anelesitie and rhy­olitie) roeks, in whiel! one eannot see but mica f1akes anei rare smalJ quartz grains whieh haei been yielded from older formations.

The breeeias at Dumbrava eome into contact bo(.h with Senonian mieaeeous fine-grained sanelst.ones or siltstones anei with mierogranitie rhyolites from Cetăţeaua IIiII. As at Căpu~u Mic, basaltoid audesites from this are a e10 uot. seem to be involved in breecia formation .

Lithologically, the breec ias froll1 Dumbrava area are constituteel both of fragment.s taken over from oleler formations (Cjuartz , felelspar , seriei te, biotite , quartzites, ehlorite-I>cricitie sehists, gneisses, etc.) and of bauatitie magmatic roeks, representeJ by quartz an­elesites with hornblenele and biotite and espeeially by mierogranitic rhyolit.es. The eryst.alline sehist frag­ments with eentimetric sizes are probably e1ireetly

88

taken over from the basell1en t of the ll1etamorphic rocks of the Biharia and SOll1e~ Series. On the other hand, fine mica flakes and several fine quartz grains, probably come from Senonian sedill1entary formations.

l\[icrogranitic rhyolites, sometimes eutaxitic with variahle textllres of thc groulIdmass (granophyric, mi­crogranitic, cryptocrystalline), locally as blocks of a f('w cClltirnetres, are to be found as elements in the east of Dumhrava breccias, beside crystalline and quartz fragments (PI. 1, Fig. 1; PI. I, Fig. 5; PI. II , Fig. 1; PI. Il, Fig. 5). In other cases, andesite fragments occur beside microgranitic rhyolite fragments, being incluucd in a fine-grained matrix (PI. 1, Fig. 6).

TIH' granulation of the breccias east of Dumbrava and Cetăţeaua Hill is variable, as in case of those at Căpuşu Mic. In the first case, the tufaceous frac­tion wit.h the same compositioll with the corse brcccia becomes mort' important , especially !lear the micro­granitic rhyolite boely.

As we havc already stalt'd, tII<' ll1icrogranitic rhy­olites from t.he Cetătealla Tlill body repr('sent thc fi­nal dirrerentiation product of banatitic calc-alkalillc l1lagma. Prcvious data on these roeks from th c view­point of the general hanatit.ic ll1agll1atism evollltioll in the Apuseni l\l1s have been prt's(,'ltcd in recent papers (Ştefan et al., 1985; Ştefan el, al., 1988; Ştefan et al., 1992).

Rhyolite fragments from the breccias at CăpuşlI l\lic are similar to those which crop out in thc Cetăteaua Hill bouy, being oftf'n intensively altered. Feldspars ilre totally replaced, hot.h with clay minerals amI silica (PI. TI , Fig. 2).

l\1icrogranitir rhyolites from the breccia fragments at Dumbrava are similar to the ones from the body al. Cetăleaua Hill , bul, some differences are also no­ticed . Thus, in the rhyolite fragments mass, some­times with a banded or massive structure anei of ten with gt"anophyric texture of the grounclmass, biotite · and a greater fresh oligoclase ratio can be noticed.

The comparison of the Cetă~eaua IIill rhyolites with t.hose within the intrusive breccias was based 011 pre­viously published data (Ştefan et al., 1985; Ştefan et al., 1988) , as well as on the analyses of a rhyolite block from tl1(' breccia east of Dumbrava (Table). From the Table which shows the contents in major and minor elements for microgranitic rhyolites at Cetăţeaua Hill (1- 4) anei the values for the rhyolite block samples (5) from the breccia, comparable values result as for the analysed elements. The higher conLents in CaO, Na20 anJ Sr in rhyolite from breccia are probably due to the highcr ratio of fresh oligoclase . .

In 1 he clistribution diagram of t1w cont.f'llts in rare ('arth, rhonelrife norlllalizecl (Hendf'rson. 191<1), for micrograllitic rhyolilCS from tIl<' bocly at Cetăt('aua

lIill (:';3IllPif' 9L81) anei t.he sample rrom the br('ccia

A. STEFAN et al.

east of Dumbrava (sample 9407), the similarity of the two curves can be observed, excepting a smaller Eu anomaly for the microgranitic rhyolite from the brec­cia (Fig. 2). This suggests that blocks of rhyolites from the breccia come [roll1 a rhyolitic-microgranitic magma fradion which has ea rlier crystall i2ed, contain­ing a larger amount of plagioclase.

Table Major (%) and miIlor (pPlIl) elements in the Cet, ătealla

Hill rhyolitcs (1-4) and brcceias (5)

1 !. J '1 5 H08 B .'i7:3 7655 928] 9407

Si02 76.07 76.70 78.7'2 78 .76 71.80 Ti02 0.15 0.12 0.10 0.16 0.12 Al 2 0 J 12.85 11..69 9.30 11.'25 14.75 Fe2 0 J 1.17 0.62 0043 0.63 FeO 0.'21 1.14 0.08 O.OJ 0.26 MnO 0.01 0,02 0.04 I\lg0 0.51\ 0.01 0.6·') 0.10 0.19 CaO 004 :3 0.35 1.19 0.50 1.07 Na20 2.86 3.08 2.80 2.(i7 3.59 1\20 4.1 '1 4.55 4.40 4047 5.01 P 2 0 S 0.0.) 0.01 0.66 0.01 0.12 H2 O+ 1.48 0.96 1.0.5 1.07 1. 79 Pb 36.00 30.00 28.00 17.0 34.00 Cu 8.00 1 .. 50 5.00 5.0 3.60 Ga 1:3.00 17.00 J 3.00 14.0 IJ .OO Cr 2.00 :3.50 <1 8.0 2.00 V 4.00 4.00 <4 6.0 .5.00 Se 3.00 5.00 2.00 3.0 <2 Nb 20.5 16.00 Zr 145.0 100.00 Yb 4.20 1.00 2.90 2.4 <1 Y 20.00 17.00 21.00 19.0 D.OO Sr 22.00 <10 12.00 40.0 155.00 Ba 950.00 1250.00 820 .00 }400.0 1100.00 Rb 167.0 163.00 U 4.1 4.60 Th 21.0 21.30 La 37.00 50.00 26.0 33.00 Ce 71.0 81.00 Sm 3.5 4.80 Eu 0.5 1.02 Tb IA 1.00 Hf 5.5 7.l0

The lithology I granulation reIat ionshi p bd,ween di­verses breccia types, and implicitly tllf' gellcration pro­c('ss of the la!t('r , arp Vl'L'Y much silllilar 1O f Ilo,;p wlllch have been dcscribecl in sevf'fal case" from the hreeeia pipe, diatreJlles ilnd pehhle dyke syslems (1\[,C;lJlIIIII, .1985) . Tht> aci ion of SOI11<' flllid izat ion procpssps wil hi n t Il(' ahow-nwn t iOI1f'd systPIll~ has h('('Jl in\"okc'u by se\"­('fiii research(:rs. TIH' experilllf'lIlal proofs have been

I

INTR USI VE BRECCIAS MICROGRANITIC RHYOLITES 89

provided by McCallum (1985). In the region we re­fer to, although the produds of a Auidization process are obviously, the importance of this phenomenon is relatively smaller, as well as the evolution degree of breccias.

100

50

~E~ 00

"'''' u

10

92&1

SL--L-Q~C~.------~Sm~~Eu----~Tb~------------~~~

The fluid pressurc whi ch has resulte.d due to granitic magma crystallizatioll delcrmillcd both the fradure of the crystallized cllvclopc of t he l1lagmatic body and of the surroundi ng rocks, especia lly from its roof. Ex­plosioll processcs !tavI' taken place subsequent.ly, but cou lc! o llly r<lrcly piercc (o the surface, being followed by Auidization phenomcna. Coarse breccias, espe­cially at. Cetăteaua IIill anei Dumbrava, are piercecl by fine grained tllffaceous sequences which have resu lt ed by desintcgratiou of previolls fornwel breccias in fine (Iit.ho-cryst.alloclastic) grains , which have bcen driven by a gas strcal1l , characteristic of fluidization phenom­ena. Products of these processes can be noti ced both at macroscopic (PI. 1, Fig. 1) and microscopic sca le. In this case, eit her piercing phenomena of some micro­graui tic rhyolit e fragmcnts by fine-grai ned t uffaceolls breccias (PI. 11 , Fig. 3) , OI' on ly dismembcring trends of large r ell:' lll<.mts froll! tll(' same Illagmatites (PI. Il , Fig. 1), call 1)(' noliced.

Fine-grainpc] (ulTacpo tls hreecias wit,lI tll(' sa me fOlll­positioll and llluCI! Illor(' IlIliform from tlie viewpoint of tiI(' g ranulalion. ulldouhtly producl;; o f r1uidizalio n processps, intrude in tl](' sI ra t ifjcat ion planes of grey­blaeki"h siltslones Ilorth of ('('lil~ealla lIill. Such fine­grall1ed rock;; fali hp ('clsi ly IJli»! <lkpll for COIl1 IIJ OII lufa ­cpous roch, volc<lJloclast ics ( lf edelltj lli s t and HC'lll ey.

1985), or even for epiclastics (Keihaln, fide Torske,

1983). . ' Within tuffaceous breccias at Cetăţeaua lEII , in the

context of the presence of a fine grainecl material with a high c1ay ratio, spherical formations resem bling the accretionary lapillis have been pointecl out; these for­mations are characteristic f)r volcanic products, but <IrI' very likely to occur under intrusive conditions, as it has been experimentally demonstrated (McCallum , 1 D85).

On the whole , breccias from the investigated are a are less evolved and not accompanied by important hy­drometasomatic solutions. Brecciation, including fiu­ielization processes, is followed by the intrusion of the Cetăţeaua Hill microgranitic rhyolite, which stopped the brecciation processes. The latest rhyolite related solutions contributed to the already consoliclated rock si! ici fication .

In the area of Căpu§u Mic village there is also a dccp-seated rhyolitic body, next to which the partici­pation ratio of the fin e breccias resulted in fiuidization processes woulcl increase. In this case the presence of a more important hydrometasomat ic solution circula­ti o ll eanuot be ruled out.

References

Hedenquist J.W., Henley R.W. (1985) lIydrother­mal eruption in t he Wa.iotoplI geothermal system, New Zealanu: their origin, assocÎated ur('ccias aud re­lation to precious metal mineralization EcolI. Geol., 80, p. 1640-·1668. Lancaster.

Hcnderson P. (HJ84) Rare Earth element Geochem­istry. Ed.l'. Hellder:;on, Elsevier, Amst.erdam.

McCallulll M. E. (1985) Experimental Evidence for Fluiuizatioll Proces~es in Breccia Pipe Formation. EcolI. Gm/, 80, p. l 52:~-154:3. Lallcaster.

Ştefan A. (1980) Petrographic st lldy of the eastern part of the VIădeasa Er liptivC' Massif. An. Inst. Geol. Geofi:. , LV. p. 20i-325, Bucu reş t i.

- , Lazăr C., Intorsureanu 1., Horvath A., Gh<,orghită 1.. Dratosin 1., Şerbănescu A., Ci'ilinescu E. (198::;) Petrological study of the ba­lIatiti, e rllptiv(' rocks in IlIe p,tstern part. of the Gilău MOlllltaill~. /J.S. 1'18/. nml. Geofiz., LXIX/l, p .215-2·16. Bll c l)r p~ tl.

. Lazi\r C .. Dprbcleae 1.. Ud\lha~a G. (1988) E\'olllti o ii nf I>;III<ltitil' IlI ilglIIil li slII in t he ,\pllse lli ~Its alld aS'/H 'Î,-I I, 'J 111(·t;rllou.{' IH·~i, IJ S. It", (;(01 (,'f' ­

..fi= .. ; 2 / .1/2 (j 'II''). 1'1'\(,1. P J~ .) 21:3. HIi/'II['{'~ti.

I

90

,Roşu E., Andăr A., Robu L., Robu N., Bratosill 1., Grabari G., Stoian M ., Vâjdea E., Coli os E. (1992) Petrological and geochemical fea­tures of banatitic magmatites in Northern Apusen'i Mountains. Rom. Joum. of Petrology, 75, p. 97-115, Bucure§ti.

Received: June 4, 1991

Accepted: June 6, 1991

Presented at lhe scien/ific sessiol1 of the

Instituie of Geology anel Geophysics: December 4, 1992

A. STE.FAN et al.

Torske T. (1983) A fluidizat.ion breccia in granite at

Skaget, Svellingen , Fr0ya. N07'ges geol. Unders., 380, p. 107- 123, Trondh .·im.

I

PI. 1

A. ŞTEFAN et al. - INTRUSIVE llRECCIAS MICROGRANITW RHYOLITES

Fig. 1 - Coarse breccia pierced by tuffaceous breccia, SE of Cerhat RiU.

Fig. 2 - Coarse breccia with rhyolite fragments injected with tuffaceous breccia, Căpu§u Mic.

Fig. 3 - Breccia with iMtered rhyolite fragments, Căpu§u Mic.

Fig. 4 - TufI"aceous breccia in contact with black siltites. Cetăleaua RiU.

Fig. 5 - Breccia with micaceous quartzite (qtz) and rhyolites (p) elements, Cetăteaua Rill, N II, x 20 .

Fig. 6 - Tuffaceous breccia with andesite (il') aud rhyolite (p) fragments, SE of Cerhat RiU, NIl, x 20.

I

PI. II

A. ŞTEFAN et al. - INTRUSIVE BRECCIAS MICROGRANITIC RHYOLITES

Fig. 1 - 'l'ufraceOllS breccia with fluidal rhyolite (p) fraglllcnts, SI'; of Cerhill llill, N Il, x 2U.

Fig. 2 - Hydrothermally altered rhyolite fragment with feldspar relicts, CăPllŞll Mic, N II, x 20.

Fig. 3 - Tuffaceous breccia which pierces rhyolite fragments (p), Căpuşu Mic, N II, x 20.

Fig. 4 - Tuffaceous breccia desintegrating rhyolite (p) fragments, Cetătealla HiU, N II, x 20.

Fig. 5 - Fine-grained (tuffaceolls) breccia piercing and including a more coarse breccia generation, SE of Cerhat HiU, N II, x 20. .

Fig. 6 - Contact between black siltstones aud tuffaceous breccia, Cetăteaua HilI, N Il, x 20.