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THE NORTHERN APOPHYSIS OF THE DITRĂU ALKALINE MASSIF - A PETROGRAPHIC APPROACH

Lucian IONESCU

GEOLEX S. A., str. Korosi Cs. 7,4100 Miercurea Ciuc

Key words: Alkaline pluton. East. Carpathians, Romania. Cont.act metamorphism.

Abstract: In the northern part of the Ditrău Massif, a 7.5 km long apophysis has been intruded in the Cambrian schists of t.he Tulgheş Group. Its sharp eastern limit is due 1.0 a directional fault. The apophysis is built up by alkalifeldspat.hic granitoids, alkaline granitoids, alkalifeldspathic syenites and roid syenites. Its up t.o 2.5 km - large thermal aureole comprises agreat variety of newly- formed minerals, from biotite zone 1.0 andalusite - cordierite - sillimanite - corundum - K-feldspar contact, assemblages. Mo and RE mineralizations are related to the apophysis.

Introduction

The studied apophysis is rooted in the midclle of the northern border of the DitrălI Alkaline Massif, represent.ing its singular structural element of this kincl, At its rool. it has a maximum wielth of 1.2 km anei it gradually thins out along a distanee of 7.5 km. The apophysis has been intruded in the Cambrian low-grade metamorphie formations of the Tulgheş Group, represented in the area by the Sandominic seqllence. Only Tg3 and Tg4 formations of the Sanclominic sequence crop out. These formations are foleleel, the folel hinges being parallel to the apophysis. The Tg3 Formation is representecl by acidic metavol­eanics t hat crop out across a restricteel are a on Parind Lung creek. The Tg 4 Formation crops out on a large territory, being built, lip by quartzitic- seriei tic schists ± chlorite ± graphite with intercalated blastoeletri­tai rocks , acidic metavolcanics, greenschists and black quartzites, whieh occur only westwards as thin anei discontinuous lithons. At its upper part (eastwards from the apophysis), the terminal sequence of the Tg4 Formation is represented by Mindra Porphy­roids, borelerc'eI by blastoeletrital rocks .

In the studied region the contact phenomena. generated by the Ditră.u Alkaline Massif occur in the Tulgheş Group westwarels from the apophysis anei in its northern extension up to a distance of 2.5 km. Inside the termally affected zone, small granitic intrusions (westwarels) anei syenitic intrusions (south - westwards) occur. The Ditrău Massif and its apophysis contain enclaves anei intercalations of hornfelseel regionally metamorphoseel rocks (Fig.).

Gellesis, structure and petrography of the apophysis

The intrusion of the apophysis is probably con tem­poraneous with the emplacement of the Ditrău Massif because it is built up by the same rocks that occur on the northern boreler of the massif. The intrusion seems to have tectonically been controlleel by an im­portant directional fault with respect to the meta­morphic structures. This fault is NNE - SSW eli­rectecl anei it limits the apophysis to the east. This olel tectonic fr act. u re , 1l0W healed, is sti Il reeognizable by t.he well-elevelopeel cataclastic features. The my­lonites are obvious only along the apophysis, north­warels this major tectonic rupture behaving as a sim­ple slip strike-fault. Because of this fault, tllf' shape of t.he east.ern boreler of the apophysis is even, while the west.ern one is inelented. The western compart,­ment of the fault was strongly uplift,ed as revealeel by the outcropping of Tg3 Formation and by the small granitic intrusions, as well as the extencled contact phenomena. The lower position of the eastern com­partment is shown by the outeropping of the upper sequences of Tg4 Formation and by the absence of the intrusions and hornfelses. Subsequently, a system of minor cross-cut faults , WNW - ESE orientated, has affected the metamorphics and the apophysis.

The apophysis is built up by granites and syenites , both alkaline and alkalifeldspathic. The t,ransition from one petrographic type to another is gradual. A few alkaline vein rocks occur in the southern and mid­dle parts of the apophysis. To the south - west of the root zone a monzonite complex occurs. The small

I

4 L. IONESCU

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Fig. - Geological map of the northern apophysis of the DitrălI alkaline massif: A. TlIlghe§ Grollp: 1, Tg4 Formation , a­Mândra porphyroidsj 2, Tg3 Formationj Ditrău alkaline massif: 3, granitoids, a - pegmatite facÎesj 4, syenites, a - qllartz syenites, b - foid syenitesj 5, monzonitesj Other symbols: 6, mineralizationsj i, hornfelses j 8, faultsj 9, mylonites. B. 10, medium grade metamorphitcs j 11, greenschist facÎes metamorphitcs j 12, Bucovinian Mesozoic sedimentarYj 13, Ditrău

alkaline massifj 14, Neogene andesitic volcanics.

intrusions of the western part are built up by granites. The eastern contact with the crystalline schists is achieved by a mylonite zone.

Gl'anites. These rocks crop out ai the northem bordel' of the massif (in the southem part of the apophysis) and yield small intrusions to the west.

I

THE APOPIIYSlS OF THE DITRĂU ALJ\.-\L1NE MASSIF 5

They are built up by K-feldspar (microcline. ortbose. pel't.hite = ;30 - .')4 %). quartz (21 - 30 %, as isolat.ed gl'aills or as aggregated crystals), tabular erystals or shol't. pl'isll1s of plagiodase (oligodase + albite = 5 -40 %), cbloritized biotite. muscovit.e with fine opaque gl'ains along cleavage planes (probably resulted from 1.lw expelled iron of a fOl'l11er biotite) and, rarely, epi­dote.

Alkalin granites. They crop out. mostly in the central-northel'll part. anei in the southem parI. of t.he apophysis. Tlwir composit.ion is dOl1linated by t.he 1\-fcldspars wit.h secondary sericite (pert.hit.e, micro­eline, ol't.hose = 33 - 64 %) and plagioclase (oligoelase + albite =9-31 %), sometimes int.ensely argillizeel anei sericitized. Su bordinately, hipidiol1lorphic quart.z grains (lO - 10 %) with unei ulatory extinction anei chloritized biot.ite (5 - 18%) occm.

Across t.wo small areas from the northwestern part of the a.popbysis t.hese granit.es show a pegmatoicl fa­ClE's.

Syenites. They OCCUl' in t.he northern part. of the DitrRlI iv1assif. in the southern anei middle part.s of t.he apophysis, and in some small isolateel intrusions in the met.amorphic schists from the south-eastern extremit.y of the stuelied zone. Their composition is dominateel by K-felelspars (l11icroeline, perthite, 01'­

those = 20 - 90 %) as well-eleveloped cryst.als, locally intensely sericitized, and short prismatic 01' tabu Iar plagioclase crystals (oligoclase + albite = 5 - i' %). The hipidiomorphic grains of quartz are less abun­dant (3 - 10 %). The chloritized biotite ftakes (1 - 8 %) have formed on riebeckite. Apatite , zircon , epi­do te anei museovite may oeeur sometimes.

Foid bearing syenites. These roeks crop out in the southern extremity of the apophysis. They are dominated by plagioclase (oligoclase + albit.e = 25 -58 %) and K- feldspar (mierocline + orthose = 20 -12 %), t.ha.t occllr as large aggregat.es which may contain clear riebeckite and biotite crystals. SOl11etinws small serieitized nepheline grains , analcite and canerinite oecur. Zircon, apat.ite anei orthite are very rare.

Vein rocks. In t.he southern anei middle parts of t.he apophysis t.wo bost.onit.e elykes anei a l11icrogranite dy ke were found.

The bostonites are built up by slight.ly orientated sericit.izeel plagioclase prisms (65 - 75 %). Chlorite oecurs subordinately, probably at the expense of for­n1('r mafics. Zircon is the common accessory.

Tlw microgranit.e , with an orientated texture, is built. up by quartz, plagioclase (oligodase, albite), muscovit.e and chloritized biotite. Cancrinite invades the rock. forllling elusters.

Mylonites . These rocks occur on the eastern bor­del' of the apophysis, which represents an old shear zone, now healed. Typical mylonites massively crop

out. on both sides of t.he Todera.'j creek, uear the joint with the Bat.ca C'l'eek (Fig.) . The rock looks a.s a greeuish - black t.eet.onized mass, wit.h cont.ort.ecl whit.ish IiU.le bands. UndeI' t.he microscope the quartz occurs as strongly flatteneel and deformeel porphyrob­I a.'3\.s , polygonized. crusheel. and enclosed in a mixecl mass built. up by sericite f1akes, chlorit.e and fine quart.z and feldspar crys(.als.

Contact phenomena

Arounel t.he apophysis t.he thermal ameole crops out only on the west.ern part, being cut, to the east, by the above mentionecl fault. The thermal inftu­ence can be observecl over a c1istance of 2.5 km from thc apophysis, its effeet beillg increa.'3ecl by the small granitoid illtrusions.

OUl' observations allowed the separation of three zones with respect to t.he intensity of the transfor­mat.ions , in good agreement with t.he facial zones de­seribed by Voicu el. al. (1990). Startillg from the massif. t.hey are as follows:

The intensely affected zone. It contains the fol­lowing contact. paragenesis: andalusite ± sillimanite­l11uscovite - biotit.e ± cordierite ± garnet ± corundum ± 1\ - feldspar. This zone ext.ends from the contact with t.he massif up 1.0 several hunclrecls meters, but sometimes it may thin out.. The formeI' rock is diffi­cult. if not. impossible 1.0 reeognize. These rocks are blackish due t.o t.he abunclance of biotite which, in the innermost. part.s of the aureole, may form monomin­erai separat.ions. The l11ica.-rieh bancls may alt.ernate with felclspathic ones, gellerating a baneled tcxture. The anclalusite anei (,he sillimanite may be seen wit.h the naked eye on the schistosity planes as elongated blackish-gray cl'ystals 01' a.., rounded, radiated spots. These hornfelses have a granolepicloblastic st.l'ucture, eOllt.aining quartz, felclspar anei anelalusite grains be­t ween the phyllosilicat.ic banels. Frequently, the an­elalusite is replaced by sericite ancl the biotite is re­placed by chlorite. The occurrence of two generations of ancla.lusite , biot.ite 01' l11uscovite, is COl11l11on. The younger mica genel'at.iolls are randomly ol'icntateel.

The alkaline l11eta..,omat.islll that has affected the regiona.lly metamorphosed rocks at t.he cont.act wit h the massif procluced import.ant quant i t.ies of pot.assic feldspars and only seldom some callerinite anei t.OU!'­maline.

The moderately affected zone. It is the best. represented, defined by t.he following contact. min­eral assemblage: muscovite - biotite ± andalusite ± K - feldspar. The formeI' I'egionally metamorphoseel rocks ea.n be easier (.0 reeognize. The abundanee of biot.ite anei muscovite varies very much. On the hile between Batea creek and Turcu ereek the hornfelses

6

are built, up almost exclusively by quartz and mus­covite, having the looking of metavolcanics in which the phenocrysts are missing. In these rocks. two biotite and muscovite generations may occur. The younger generation eleveloped along shear cracks that cross the schistosity. As Voicu et al. (1990) re­marked, the quartz-feldspar rocks are much less af­fected than the neighboUl"ing schists. The occurrence of the feldspat.hic rocks as continuous thick !ithons represents . arter the mentioned authors. lithological sereens which considerably diminish the contact. phe­Ilomena.

In the hornfelseel blast.odetrital rocks the quartz porphyroblasts are recrystallizeel, having usually anormal extinction, while the microgranular quartz has undulatory extinction. Feldspar porphyroblasts are sericitized to various degrees and the biotite ftakes are partly chloritizeel.

The weakly affected zone. It represents the outer zone of the aUl"eole, in which the only contact mineral is the biotite. Biotite has formed in schists while the quartz - feldspathic rocks are not affecteel. The biotite is chloritized (perhaps a retrograde pro­cess eluring the cooling of the massif). Sometimes the biotite occurs as rounded aggregates that may be seen on the schistosity planes, generating the spotted schists varieties.

Mineralizations

Ionescu et al. (199~) identified two epigenetic mineralizations in the studieel apophysis. They are high temperature metasomatic-hydrothermal mine-

Received: Febnwry 24, 1998

Acceptec/: Octobel' 30, 1998

L. IONESCU

ralizations, generated by the Ditrău magmatism. One mineralization, containing molybdenum anei

rare earths, represent the ea..,ternmost known OCCUl"­rence of Jolotca-type vein mineralization. In tl1f' syenites from the left side of the TUl"cU creek (Fig. 1). we identifieel fragments of a vein, intensely altereel. with the following contents: Mo = tens to 100 ppm: Yb - Sc - Nb = tens ppm; Y - Gel - La = hUllelreels ppm; Ce - Nd - Th = thousanels ppm.

The other mineralization occurs in the saelelle rrom the crest between the Batca creek anei the Paraul Lung creek southwards from Poalele Aramei Peak (Fig. 1), at the intersection of several faults. The mineralization occurs in hornfelseel schists, as blocks of intensely circulateel J'ocks that. leei to a strong al­teration of the sUl"rounding rocks. Uneler the vegetal soil, fault gauge occurs. The following contents have been founel: Au = 0.5 - 0.6 git; Ag = 100 - 160 git, Cu = 0.1.5 - 0.2 %; Pb = 1 - 2.5 %; Zn = 0.15 - 0.2 %; S = 0.1 - 2%; Bi = hundreels ppm; Sn = tens to 100 ppm. A similar but poorer mineralization occurs on the same crest at about 1.2 km southwarels.

References

Ionescu, L .. Kasztl. C., TOl'ok, T .. Zolya. L. (1992) Reporl. (in Romanian). Geolex SA Archive. Miercurea Ciue.

VoiclI. G .. Runceanu. M., Voicu, C .. Munteanu, M .• Popa. G. N. (19!JO) Report (in Romanian). Archivcs of the Geological Institute of Romania. Buchal'est.