I

Rom. J. Pct.ro Jogy, 1992, 75, p. 17-38

MAJOR AND TRACE ELEMENT GEOCHEMISTRY OF RHYOLITES FROM NORTHERN DOBROGEA. PETROGENETIC IMPLICATIONS

Ioan SEGHEDI, Alexandru SZAKĂCS, Constanţa UDRESCU, Gabriela GRABARI, Maria STOIAN, Anca TĂNĂSESCll

Institutul de Geologie ~i GeofizicX. Str. Caransebe'j 1, 78344 Bucul'C!jt.i 32.

Cat.rinel VLAD

Întreprinderea de Pr05pec~illni Geologice 'ii Geofizice. StI'. CIU'anseb~ 1, 78344 BuclIl'C1jti 32.

Q Kcy words: Rhyolites. Major elements. Minor elcmcnts. Pel.rology. Dobrogea­Northern Dohrogea - Măcin Arca, Tulcea and Consul-Niculitcl Area..,.

Resume: LCI geochimie de .• elements mCljellrs el eli tmce.~ des "hyoliff'l' de la Dobrogea .• eplent,.iona/e. Imp/icatioll petl'Ogenetiques . Tenant. comptc de la l1istribution dc leurs clemcnts majeurs el. eu traces, les volcauit.es el. sous-volcauitcs acidcs, "rcponderaut rhy­olitiques, des differcnt.cs unit.cs tectoniqucs de la Dobrogea septentrionalc sont divisees, scloll I'âge, 1<: cadre geotectonique, la sOllrce du magma el. I(·!; procc"'''I1~ de diffcrenciatioll, de la maniere suivante: (1) trachytes (lah~ozoÎqucs, associee" aUl( rocltf's basiques, dans I'unite de Tulcea, geuerees par la fusion partielle d'une source du lIIallteau au mitieu d 'un rift; (2) rhyolil.es carbouiferes, calco-alcalines, associees aUl( Mpâts terrigtmes, dans l'unite de Măcin. generees par la fusion partielle d'une source quarl.zo-feldspathique dans la erou te. situees dans un environemeul. geotcet.ouique de type 'back arc'; (3) rhyolites triasiCJues ealco-alcalines, associees aUl( basaltcs. daus les unites de M lkiu, Cousul ct Tulcea, geucrecs par la fusion partidle d'une source dan!' la eroÎtl.e ct, parfois, par la eristallisation fraet.ionce dans des chamhres lIIagmatiques interlllctliaircs, developpCc au debut d'une rift.ogcllese d'iutraplaque contineut.ale; (1) rltyolit.es d'â.ge I.riasiqlle-jurassique (?) ealco-alcalines-alcalillcs associees aux granites ayant. le!' IJIclIIes t.raits chimiques. sur 'I'alignement du sud' de I'unite cle Mlkin, gencrees par I'action d'un 'dome thermique', suivics, par places, par des processus de eristallisation fractionnee daus ul1e intraplaque continentale; (5) peu de rhyolites alcaliues jllrassi<Jue superieures associec8 aux basaltes, au sud-ouest de 1'11 uite de Măcin. genert:es par la cristallisation fractionuee d'lIl1 magma resultce de la fusioll partielle du materiei parental dans le manteau, ou il. Ia base de la crout.c.

1. Introductioll

The relationship between igneolls rock bodies and Paleozoic aud Mesozoir metamorphic and sedimentary formatious is oue of the major problems implied in the stlldy of the geologic st.ructure of Northern Dobrogea. Igneous rocks are represeuted by basalt.ic and rhyolitic volcanics and granitic plutouics. Taking int.o accollnt their wide areal extent and their gcneration within certain geotcctonic environment.s. a geochemical study of the igneous rocks is highly relevant to the rec.onstitution of the Hercynian and Alpine evolution of Northern Dobrogea.

Until presellt the petrographical and geochemical (mainly major elements) features of Northern Dobrogea rhyolites (" quartz porphyries") have been repeatedly investigated. Among t.he published works worth mentiou­ing are Peters (1867), Pascu (1904). Mrazec (1912), Cantuniari (1912), Murgoci (1914), Cădere (1925), Savul (1931.1935), Streckeisen (1931), Dimitrescu (1959), lanovici eL al. (1961), Mirăută, Mirăuţă (IH62), Mutihac (1964). Mirăuţă (1966a, 1966b), Stiopol et al. (19i8), Grădiuaru (1981), Savu el. al. (1982,1986), intorsureanu (1987), Seghedi et al. (1987). The unpublished studies (including tracI:' element dat.a. 1.00) lo be cit.ed belong to Bălan (1966, 1967), Stiopol et al. (1975), Constantinescu et al. (1 !Ji8, 1981, 1982), Caraveţeanu in Vîlceanu et al. (19i9) and in Manea et al. (198:.n, Baltreli et al. (1984), Ştefan, R.0liu in Berbel('ac et al.(1985) and in Nede\cu el al. (1986,1987,1988), Seghedi, Szakăcs in Seghedi et al. (1985,1986) and in Mirău~ă et al. (1986).

18 1. SEGHEDI et al.

A comprehensive study of both major and trace element geochemistry for rhyolites in Northern Dobrogea has not been carried out so far. The present study is the first attempt in this respect, involving both petrogenetic and geotectouic implications.

2. Geology

2.1 Geologic aud tectonic setting The Northern Dobrogea is an intracratonic orogenic belt completed during the Alpine orogeny. From west

to east the following major structural unils have been delimited (Mirăuţă, in Patrulius el al., 1973, unpubl. report): (1) Măcin Unit, (2) Consul-Niculitel Unit and (3) Tulcea Unit. They are overlapping one another and trend north-eastwards. These units were reconsidered by Săndulescu (1985) as (1) Măcin Nappe, (2) Niculi~e1 Nappe including (2a) Consul and (2b) Sarica digitations and (3) Tulcea Nappe.

The Măcin Unit consis!.s of several entities: pre-Silurian metamorphic formations, Silurian-Carboniferous very low grade metamorphic rocks (Carapelit Formation included) and Mesozoic (Triassic) sedimentary forma­tions with associated acid and basic igneous rocks.

The Consul Unit includes mainly calcareous Triassic sedimentary formations overlying a crystalline basement of Măcin type (Boclugea Series) and associated acid and basic eruptive rocks.

The Niculitel Unit consisls of a basic volcano-sedimentary formation aud U pper Triassic flysch deposits. The Thlcea Unit. is built of prealpine metamorphic formations (Precambrian, Silurian and Devonian) and

Mesozoic sedimentary formations, bot.h associated with acid and basic igneous rocks. An Upper Cretaceolls poSt.-tectonic sedimentary cover occurs in the southern part of aII these structural

units.

2.2 Igneous rocks: occurrence aud age

2.2.1 Măci1l U1Iit a) In the NW of the unit Paleozoic igneous rocks are represented by important granil,ic int.rusions, some older

than the Carapelit. Formation, others suhsequent to it (Rotman, 1914). Acid volcanics related t.o the Carapelit Formation are also known (Mrazec, Pascu, 1896; Pascu, 1904; Murgoci, 1914: Mirăuţă, Mirăuţă, 1962). They occur mainly in the Horia-Balabancea area and they are interlayered with continental terrigenous sedimentary deposits of alluvial fan type (Drăgănescu, in Russo-Săndulescu el al., 1975, unpubl. report; Seghedi, Oaie, 1986; Seghedi et al., 1987). Thcy include mainly subaerial pyroclastics (ignimbrit.es), epiclast.ics and subordinately lava flows. Their petrographic feat.ures were described by Russo-Săndulescu et al. (1975, unpubl. report), Seghedi et al. (1985, 1987), Ştefan, in Berbeleac et al. (1985, unpubl. report.) and in Nede\cu et al. (1986, unpubl. report).

b) The Mesozoic igneous rocks are represented by both acid (granites, alkalille rhyolites, rhyolit.es) and basic rocks. In the nort.h-eastern half of the Măcin Unit nllmerous basic dykes and veins piercing pre-Mesozoic formations have been mapped. Their trend is generally NW-SE; conformably to the main structural trends of this unit.. The Triassic age is illferred from both geometrical relations to the Paleozoic formations (Seghedi et al., 1980; Seghcdi, Oaie, 1986) aud their petrographical aud petrochemical similarity with the more accurately dat.ed rocks of t.he Consul Unit. (Seghedi et al., 1985). In lhe south-western area of the Măcin Unit the rhyolite occurrences show an elongated pattern (the "southern alignemellt") parallel to the Peceneaga-Camena fracture zone; from north to south they are grouped in three zones: Turcoaia, Cîrjelari and Camena.

In the Turcoaia-Iacobdeal area the rhyolites are intimately associated to grallites exhibiting similar general chemical feat.ures (alkaline rhyolites and granit.es). This genetic relation between these rhyolites and granites was unanimollsly accepted by the previous authors (Mrazec, 1899; Cantuniari, 1912; lanovici et al., 1969; Întorsureanu, 1987; Înt.orsureallu et al., 1989). Their common petrographic feature is the presence of alkaline mafic minerals, as riebeckitc aud aegirine. Rhyolites occur .as dykes, small shaIIow intrusive bodies or marginal facies in some granit.ic intrusions. Their age as well as the age of aII the acid igneous rocks in the NW of the Măcin Unit is wllt.roversial. They were initially considered as Hercyniall (Cantuniari, 1912; Mirăuţă, Mirăută, 1962) and t.his was also supported by some K/ Ar ages of rhyolite and alkaline granite samples from Turcoaia­Iacob deal (Mîllzat.u et al., 1975: Mure§an, 1975). According to more recent. result.s they seem to be younger ,

. i.e. Mesozoic. A Rh-Sr isochrone of 193±15 Ma points t.o the Triassic-Jurassic boundary as the time of their emplacement. (Pop et al., 1985; Întorsureauu el al., 1989) .

In the Cîrjelari area t.he rhyolites are associated with alkaline granitie rocks which occur in more limited areas than in the Turcoaia area. Their petrographic and chemical features are presented by Oimit.rescu (I959), Const.antinescu et al. (1985, unpubl. report), Stiopol, Orăghici (1978), Şt.efan, RoşII in 8erbeleae et al. (1985,

I

._ .. _---- .... _--------------------

GEOCHEM1STRY OF RJIYOUTES FROM NOHTllERV DOBRDGEA 19

unpubl. report) , in Lupli et al. (1986, I1I1Pllhl. reporl.) and in MOllnazih (HJ88, lInplIbl. thesis of doctor's degree). The rhyolit.es QCcur as both subvolcanic bodies and drusivc as \Vei I as explosive products. Like in the Turcoaia area, the age of the acid rocks is poorly e1ucidat.ed. Rb-Sr isochrones show 191±10 Ma (Pop et al., 1985), almost. ident.ical to t.he Rb-Sr age of Turcoaia rocks exhibit.iug similar chemical and petrogenetic features. According to Şt.efau anei Ro~u (in Nedelcu et al., 1987, 1988, lInpubl. reports) the Upper Jurassic age is very likely for thc Cirjelari rhyolit.es due to their rl'semblanccs t:o certain rhyolitcs from Camena. On the other hand rhyolitic tuff.'i are interlayered wit.h Upper Jurassic sedimcllt.ary deposits (Grădinaru, 1981).

In t.he Camena area the rhyolites occur along the Ppceneaga-Camena falllt. zone. According t.o several authors t.hese rhyolit.es are grouped in t.wo parallel aliglllllent.s of similar or dirrcrent cvolut.ion (Cădere, 1925; Constantinescu et aL, 1978, un[Jubl. report; Şt.efan in Berbeleac et. al., 1982, H185, lInpuhl. reports). Unlike the 1\ucoaia and Iacob deal areas here no granitic rocks oerur in association wit.h rhyolit.es. 1I0wevcr, a basaltic rock occurrence is not.iccd in t.1I!' lla.'ipumlr qllarry (Grr\{linaru el. al.. 1981). The lIlent.ioned itllt.hors consider the Camena rhyolitt's of bot.h sllbvolcallic and drllsiv!' or pyrodast.ie Iype.

Not. ali the aut.hors agree OII 1.11t' age of Ihe Camella rhyolit.es t'il.lwr . Former I":-AI' ages of 212-·232 Ma (Mînzat.u et, al., HJ75) diffcr from t.lw reccnt. ones of l4i--15:1 l\1a (ŞtdalJ in N"ddru et aL, 1988, unpubl. report.). Arcording to micropalcontologic inv"sligatiolls t.lw sedimcnl.ary dq>osits wit.h int.erlayered rlryolites in the lla.wunar quarry are Oxfordiall-Killlll1cridgiall in agc (GrăJinaru. 1981). In t.1U' Camena area t.lre rhyolit.cs associate wit.h Ladillian-Camian sediment.ary deposit.s (Grădinaru. HJ8 L) and other deposit.s ?f uncertain age, assigned - on palYllologic basis (Antonescu in Constant.inescu et aL, H185, unpubl. report.) - to tJte Spat.hian and the Middle Jura.'isir (Constantinescu el. aL, 1985, unpubl. report), t.he Middle-Upper Jurassic (Grădinaru, ]984), the Middle .Jura.'isic (MlIre~an el. aL, 1981, ulIPllhl. reporl), tlw Spat-hian (Mil'ăută. Visarion ill Nedelcu et aL, 1988, unpubl. report) .

2.2.2 COI/su/ U1II1

\Videspread rhyolite occurrences are rplat.t'd 10 the LO\\'cr Triassic Illainly carhonate deposit.s of this unit.. They prevail bet.wepn Valea Tcilor t.o t.he norllr afl(I Nicolae BălcesclI to ,IJ(' SOliI h. hOln Vall'a Teilor 1I0rthwards the rhyolites associate wit.h an increasing a 1lI01l nt. ofbasalt.ir rocks ofTriassic agl'. 'rIIe sOlltlwrlHllost occurrenccs are found at Mihai llravu.

There are several opinions 011 the nat.ure of tlw rhyolit.es . They are assigllcd as inl.rusiw (Murgoci, 1912; Ştefan in Nedelcu et aL, 1986, uuplIbl. reporl.), t'ffusivc (possihly partly int.rusive) and pyroclastic (Savul, 1935; SI,iopol el. aL, 1975, IInpubl. repOl't; Caraveţeanll in Vilc('anu el. al.. J!J80, lIupub!. report) or ignimbritic (Constantinescu et aL, 1918. l!J81 , l!)82, unpllbl. reports). The mainly "rrusive and sllbordinat.ely explosive character of the rhyolit.ic rocks ill t.his IInit. has been rerent.ly dt'lIlolIst.ral.cd (Scghedi ct. al. , 1990) . Their Lower Triassic (Spat.hian-Allisian) age is IInanilllollsly acc"pted al »rt'st'nl (St'ghcdi cI. aL . 19~6. IIllpubl . report; Mirăuţă et al., HIX6, IIlIpuhl. report).

2.2.8 Nlcu/itel Unit A typical fcature of this IIl1it. is I,he largI> <levl'lo»1I11'1I1 of lllt' t: PII"r Tri:Issic hasic i,c;IH'l.lllS rocks account.ed

as withinplate basalt.s (SavlI daI., 1985, 198(j) Of oJ>hiolilic rift hilsalts ill plate lIlargill ellvironmf'1I1. (Ciof1ica et al., 1980). Thc unit is practically devoid of rhyolilt's. Althong" 011 SOIIl(' IIlaps tllt' J;;;leCt~a rlryolit.es are reported t.o the Niculiţel lInit (Savu el al., 198!)), thf'ir assignlllcnt. 10 Ilw Tulcea (lnit is more appropriate considering thc geological background (Săudlllt~snl, 1!18"1) .

The rhyolites in the Trestenic area are relaled 10 Lower Triassir dt'»osil.salld eonst,il.ul.p an OUt.lit'f assigned to the COlIsul Unit., while th!' acid and hasil' dykl>s rrom t.IH~ Cilic area bl'long 10 an ollt.lier of the Măcin Unit. (Mirăuţă in Patrulius et al., 1973, unpub!. report).

2.2.4 Tu/cea Unit The Tulcea Unit contains a small Ilulllber of rhyolitic rock oul crops wlrich occllr mainly in the SOIllOV<l­

Mineri arca and easl. of Calaloi (in the Recliu and Uzumhair hills)(IVlirălltă. 1!16G; Seglwdi, Uricaru, 1985). Minor occurrences are fouud also al. Isaccca and al. Mllchia Păst.orullli. a plan' sit.uated bpl_ween Izvoarele and Nalbant.. Palt'ozoic granit.es are also reportf)<I . While 1.111' rllyolites from SOlllova-Milwri-ls<lcwa andMuehia Păst.orului are consider"e1 of Triassic agp because tlwy arI' associ<lted wit.h cilrholli-lll> roeks of the salTle age, t.hose east. of Cat-aloi whiclt pierce Pal,'ozoic forrnat.iolls arI' illlpiiritly assigut'd t.o t.11/' I'aleozoie (M irăuţă, Hl()()).

Th(' rhyolitt's from SOlllova-Milwri and Isa(,cl'a OU\IT in difl'en·nl. facies from slIl>volc-anic 10 efrusiw and ignimhritie (Si-IVltI . J!):!l : Sliopoll't. <IL, 19i.), 11111'111>1. rl ' porl. 1!)i8: Sa.vlI el. aL , 1!11<2 . l!ll-i!) . l!li!(i; Ştf'fan , Roşu ,

Gridan in Npdl'lcll (·t al.. I!liiG. III1Pllhl. ft')>orl). Tlwy art' rt ' I01II'<I Il) hasir of I'Onlillpntal within plat.e t.ype (Savll, I!l~G). AI '111C'hia Piistoruilli tI ... rhyolitl's arI' (,rrllsi\'<' and p.Hodastic.

20

ti.BLB 1 IUJOR ELEMEliT5

1. SEGHEDr et al.

GROUP. RO SAMPLE LOCATIOR 5i02

1. 159 41I0~'f! 77.59

2. 161 A" .. 7b.47 3. 161 B" .. 75.24 4.. 173 Crapou

6. 7. 8.

b1l177.70 324 4 Hotare

355 .. 358 .. )64 ..

b1l1 80.)0 .. 81.50 .. 79.10 .. 75.40

9. 40.1 Crapeea

0.24 11.24 2.23 0.19 0..0.2 0..43 0..17 6.30. 0..71 0..0.7 0..96 0..0.9 - 10.0..24 0..10. 12.24 1.26 0..31 0..0.2 0..23 0..11 B.25 0..60. 0..0.1 0..64 0..10. - 10.0..40. 0..13 11.90 1.)0 0.)8 0..0.1 0..19 0..12 B.70. 0..63 0..0.3 0..85 0..0.6 - 99.54

0..24 10..84 0..9& 0..40. 0..0.3 0..16 0..)0 &.64 1.54 0..0.) o..ba 0.08 - 99.52

0..22 9.64 1.77 0..29 0..0.4 0..40. 0..23 4.82 0..95 0..0.8 1.14 0..12 - 10.0..10. 0..2) 10..50. 1.08 0..14 0..0.3 0.~0.9 0..12 0..18 5.62 0..14 0..22 0..10. - 10.0.22 0..23 9.tlo. 0..59 0..220..0.6 0..18 0..'7 5.42 1.)8 0..16 0..69 0..)1 0..36 99.74 0..12 12.17 0..58 0..18 0..0.1 0..16 0..24 9.58 0..17 0..10 0..42 0..16 0.16 99.59

IA bil170.25 0..28 12.0.5 1.55 0.22 0.18 0. 24 3.00. 2.86 4.63 1.87 0..52 - 1.94 99.59

181

10..

u. 12. 13.

14. 15.

2405

3123

)124 3196

3197 3254

Cetate b1l1 80..0.0

MartiDa V.79.00 .. .. 79.50

Cetate . bUl 79.50

.. .. 78.70 Crapeea

b1l172.50

0.0.4 12.65 0..09 0..15 0.01 0.09 0.17 0.11 6.65 0.04

0.02 12.85 0.3) 0..52 0.03 0..)5 0..18 0.19 6.28 12.70 0..16 0.18 - 0..09 0.11 0.14 6.42 0.10.

0..04 12.45 0..16 0..18 0..0.2 0..15 0..15 0.11 6.45 0..04 0..0.2 12.80. 0..19 0..22 0..0.1 0.0.9 ~.25 0..11 6.11 0..0.8

0..2'{ 11.98 2.64 1.71 0..09 0..49 0..20. 7.83 0..19 0..0.6

0.08 0.0.9 - 100.17

0..21 0..14 0.02100.22

0.06 0.15 - 99.67

0..13 0.0.9 0.0.4 .99.50. 0..21 0..1) ·- 99.52

1 . 41 0..14 - 99.51 16. 325.~ .. .. 78.00. 0.18 11.)0. 0..9) 0..)0 0..0.2 0..3) 0..20. 7.61 0..60. 0.0.2 0.68 0..12 - 10.0.~19

17. )258 5eoaru bil1 76.9) 0..18 12.64 1.43 0..57 0..0.6 0..53 0..20 0..0.6 6.45 0..12 0..50. 0..0.7 - 99.76

1~. )259 It

19. 3260. " 20.. 3261 ..

..

..

.. 75.95 '(6.91 .:75. 0 9

0..14 13.60. 1.66 0..)9 0..0.1 0..16 0..12 0..0.4 7.24 0..0.5 o..le 0..0.6 -0.13 12.34 2.19 0..43 0..0.1 0..26 o..ac 0..0.6 6.56 0..0.3 0..50 0..0.6 -0..18 13.0.4 1.43 0..84 0..0.2 0..40. 0..15 0..16 6.01 0..0.5 o..~o. 0..0.5 -

99.64 99.70. 99.58

21. 320.0. Io.aneşul hill 79.50 0..0.3 12.35 0..0.6 0..)0 0..0.1 o..lb o..)ti 0..14 6.15 0..12 0..23 0..0.9 0..08 99.62

22.

23· 24. 25.

26. 27. 20. 29. )0..

31. 32. 33. 34.

35.

36. )7.

30. 39.

11) Geaterea Rusi 74.0.5

114 It It 74.13 120. "... 7,.29 122 Islam

Geaterea 75.60. 124 " " 13.91 120 fi ..

131 .. .. 133 " .. 1)6 Pietro.iu

70..21 74.9; 70..)2

lJare 7h0.4 13'( " • '7 4~77 136 " " 72.55 170 stipanuV.76.20.

0.39 1l.t34 0..61 1.69 0..0.6 0..59 0..)0 6.'() 2.87 0..0.8 0..50 0..12 -0..34 11.69 2.29 0..0.5 0..50. 0..28 6.91 2.69 0..0.3 0..60. 0..10. -0..35 11.64 1.53 0..b3 0..0.5 0..22 0..32 5.69 ).37 0..0.5 0..40. 0..0.6 -

0..31 11.14 1.42 0..49 0..0.) 0..14 o.l~ b.6b 0..18 0..0.4 0..70. 0..0.9 -0.45 11.44 1.16 0.92 0.04 0.51 0.24 8.48 1.00 0.10 0.55 0.05 ~ 0..62 12.24 1.0) 2.4b 0..0.5 0..77 0..31 9.20. 0..64 o..lb 1.0.0. 0..0.9 -0..37 11.70. 0..28 1.75 0..0., 0..44 0..25 7.44 1.41 0.:0.7 0..80 0.08 0..54 12.04 4.31 0..0.7 1, 48 0..40. 6.0.8 1.97 0..13 1.30. 0..1) ~

99.59 99.51 99.58 99~59

99.57

0.5, 12.0.0. 2.19c.tKS 0..0.3 0..74 0.2410.9'( 0..23 0..13 1.15 0..0.4 - 99.99 0..43 11.0.0. 2.0.1 0..0.4 0..59 0..18 9.59 0..22 0..0.9 0..50. 0..1~ - . 99.58 0..44 12.0.5 2.14 - 0..0.2 0..15 0..12 IL 10. 0..20. 0..10. 0..55 0..0.9 - 99.51 0..14 10."90. 1.70 0..31 0..0.2 0..47 0..20. 6.)8 2.36 0..76 o..cH - 99.60.

20.3 Cuanae hill 74.(j0. 0..19 11.35 1.18 0..79 0..0.2 0..65 0..20. 6.81 2.39 0..99 0..0.6 - 99.43

20.4 Pietro.iul hi1116.30. 0..22 12.20. 1.19 0..64 0..01 0.11 0..63 3.79 3.92 0..12 0.22 0..22 - 99.63

20.... " ti '76.0.0 213 Cuanae 76.90. 2905 MartinaV.76.10. 3053 Diteo.va

0..20. 12.10. 1.30. 0..26 0..0.2 0..11 0..32 7.0.9 1.85 0..11 - 0..13 0..35 99.84 0..22 11.40. o..bo. 1.26 0..0.2 0..59 0..19 6.45 1.90. 0.01 ~.26 0..0.6 - 100.06 0..10. 12.<t4 0..20. 0..50 0..01 0..120..)0 1.13 2.44 0..12 2.92 0..20 0..89 99.73

hill 75.8U 0..14 12.24 1.14 0..J4 0..0.2 0..2ti 0..14 5.0.4 3.52 0..0.3 0..70. 0..06 - 99.5~ ---4-0.-.-3:-:2:-U-::9-'---C:-:i-::l-::"ie==---::.:6b:....·.-::l-0.--0.-.:-82::--:iJ.3)"-4. 3-Ci. 14 0..15 0..U5 1.11 4.47 3.47 0..18 1.19 0..16 0..16]00..18 IB2

4_ţ . 3290. .. 62~ '(0. 1.14 1?40. 7.11 2.22 0..0.9 1.45 0. .• 99 5.83 2:49 0..17 0..98 0..12 0..72 99.53

GEOCJfEMTSTU1' OF HIIY01,l'fTS FlWM .vOUTJlI',R:Y POHlW(;f;/\

42. 214 Bujoru1 77.50 BulgAreso

bUl

0.2210.)0 1.77 1.)90.07 '0.110.13'4.053.710.08

:lI Tabk 1 (colltilllll'd)

0.11 0.)3 99.77

1 43. 215 Begolu V. 76.50 0.20 10.80 1.71 1.39 0.05 0.10 0.07 4.27 4.00 0.05 0.06 0.11 0.15 99.92 0.28 U.27 2.04 0.14 0.05 0.22 0.52 4.15 3.~ 0.09 0.10 . o.JU 0.34 99.~9

0.32 lL.80 1.45 0.29 0.03 0.24 0.22 5.18 2.54 '0.10 0.33 0.30 - 100.10 0.30 11.25 3.24 0.26 0.09 0.3J o.Jl 3.76 3.73 0.02 - 0.13 0.67 99.59

2

44. 31~ S.Turooa1a76.40 45. 31~ " r6.44 46. 325 ' Turooala 75.50 4'(. 158 Seoaru V. 7b.)3 0.13 12.09 1.77 0',62 0.02 0.11 0.10 3.~ 4.1J 0.14 0.10 48. 351 Cîrjelarl 73.20 0.33 11.95 2.51 0.40 0.01 0.25 0.05 9.40 0.28 0.08 0.75

0.1'2 -0.35 -

49: 3~2 " 7,.70 0.24 11.34 3.35 0.1~ 0.10 0.11 0.31 4.62' 3.50 0.07 0"35 0.19 0.04 100.27 '0. J.53 " 75.10 0.)9 12.45 1.85 0.66 0.07 0.25 0.10 5.16 3.38 0.16 0.26 0.25 - 100.38

__ ~5~1_; __ ~3~5~4~~ __ "~~7~b_.l~0~ __ 0~.)~0~172_.0~0~1~.~&4 __ 0_.~29~0_.~Q~3 __ 0_.1=b~0_·712~4~.~5~6_3_.~5~5~0_.1_0~0_·7'~5~_00-.·08~28~: 110000 •• 2

225 52. 150 A Tav Bair75.22 0.19 11.09 2.70 0.26 0.04 0.25 0.12 9.3) 0.26 0.04 0.67

:3

1

I

53. 150 B. "78.30 o.lti 10.54 2.16 0~16 0.04 0.33 0.14 7.49 0.32 0.07 0.66 0.08 - 100.47 54. 150 E " "72.00 0.32 13.34 3.82 0.~7 0.04 0.30 0.02 9.01 0.32 0.03 0.80 0.09 - 100.16 55. 151 Tall Balr .75.09 0.14 11.79 2.69 0'.07 0.02 0.35 0.068.56 0.31 0.10 0.57 0.06 - 100.41 56. 152 Camena 77.56 0.12 11.69 1.94 - 0.03 0.10 0.08 5.27 2.91 0.06 0.65 0.05 - 100.46 51. 155 A Bavpwnar68.58 0.12 13.34 3.51 0.19 o.oti 0.17 4.02 2.10 4.76 0.75 0.85 0.05 1.80 100.32 58. 156 'Slava RusiBo.02 - 10.540.95 - 0.090.090.19 6.46 1.23 0.13 0.70 0.09 - 100.49 59. 60. 61. 62. 63. 64. 65.

66~

67· 68.

3516 TaO Bair ,74.60 0.30 11.65 1.67 0.15 0.03 0.16 0.70 9.69 0.25 0.54 0.32 3517 Tav Bair E77.00 0.20 11.15,1.110.180.010.180.368.58 0.19 0.28 1.07 351ti TaV Bair E75.00 0.26 11.05 1.91 0.18 0.01 0.18 0.21 9.27 0.19 0.78 1.21 3521 TaO Bair W76.70 0.)0 10.35 2.09 0.15 0.03 0~11 0.43 8.67 0.18 0.57 0.14 3522 Tao Bair .76.00 0.16 12.35 0.26 0.48 0.02 0.18 0.98 6.79 2.34 0.46 0.24 3527 Tav BalrW76.00 0.21 11.850.650.360.030.110.70 7.12 1.81 0.36 o 353lHolduril1

Balr 3535 n

3538 n

75.00 75.00 76.50

3542 Callena V. '/5.00

3542 ACallenaV. 78.50

0.22 11.25 1.80 0.26 0.01 0.19 0.34 8.59 0.18 0.18 0.71 0.30 11.05 1.61 0.26 0.03 0.32 0.59 8.31 0.14 0.50 1.57 0.28 10.10 2.'17 0.11 0.03 0.26 1.38 7.51 0.16 0.44 0.42

0.)4 10.55 2.56 0.15 0.02 0.14 0.64 8.42 0.25 0.90 0.82 0.29 10.450.610.440.01 0.15 0~51 ~.18 0.18 0.36 0.42

0.08 o

0.06 o 100.14 100.31

0.08 o 100.15 o 0.05 99.'t7

0.08 o 100.34 0.05 0.09 99.34

0.06 1.09 99.88 0.04 o 99.72 0.07 o 100.03 o.oll o 99.87

0.08 o 100.W

10. 3543 PodarnlţaV.76.00 0'.24 11.65 1.77 0~'33 0.03 0,.21 0.43 5.4J 2.23 0.13 0.78 0.08 o 99.94

71. 3544 Uapenla V.78.00 0.26 8.80 3.10 0.18 0.01 0.06 0.22 7.bo 0.14 0.61 o 0.09 0.73 ~9.~0

'O.

74.

'(5.

7b.

77.

78.

19.

80.

86.

10

12

14

16

20

36

37

42

44

46

CODau1 19.1, nil!

II 11.09

II 19.90

" 74.96

l' 7~.40

Lozs>va bUL 77,90

" 75.43

" 77.61

" " 76.02

56 Conoul b~79.40

62 EachlbaUJc b1l1 76.20

18 Malciu h1U14.15

82 "

9b Coneul h!l18.92

81. 102" " -(5.0e!

8ti. 141 ~ecbibalik hUL 72.36

0.14 10.89 0.32 o.lJ 0.01 0.05 0.18 5.25 2.14 0.0'1 o.bo 0.06-

0.18 11.69 1.00 0.37 0.01 0.11 0.16 5.16 3.21 0.05 0.50 0.06-

0.20 10.b9 0.b9 0.65 0.02 0.2b 0.25 0.93 4.90 0.04 0.80 0.05 -

0.15 11.04 - 4.0~~.oU 0.60 0.12 4.66 2.44 0.05 .1.33 0.06-

0.13 10.34 0.72 0.25 0.02 0.13 0.17 4.66 3.00 0.04 0.60 0.07-

0.27 11.34 0.91 0.90 0.02 0.10 0.33 2.67 4.60 0.07 0.40 0.07 -

0.22 ·12.19 1.)3 0.48 0.01 0.16 0.1~ 6.32 2.64 0.06 0.55 0.05 -

0.22 10.89 0.47 0.67 0.01 0.24 0.16 7.41 0.67 0.06 1.09 0.06 -

0.43 12.14 0.59 1.06 0.02 0.16 0.22 5.59 4.2) 0.07 0.40 0.05-

0.20 11.U4 0.27 1.25 6.02 0.10 0.15 6.75 2.10 0.05 o.'{o 0.07 -

99.59

99.59

99. Sti

99.54

99.53

99.58

99.59

99.56

99.52

99.61

0.14 10.44 1.)2 0.35 0.02 0.13 0.23 4.5'( 2.)5 0.05 0.70 0.08 - 99.78

0.16 11.54 0.26 1.21 0.05 0.12 0.21 7.94 2.14 0.05 0.45 0.15 - 100.48

0.25 11.44 1.U3 0.6) 0.05 0.11 0.24 6.07 3.11 0.05 0.70 0.10 - 99.53

0.,3b 11.64 - 2.46 0.07 0.51 0.35 6.5) 1.D 0.04 1.10 0.07

0.32 10.59 0.)6 1.66 0.06 0.26 0.)0 ).03 3.10 0.05 o.Uo 0.09 ~

99.6)

99.54

0.34 11.59 - 2.25 0.06 0.)6 0.52 6.59 2.55 0.08 0.40 0.09 0.50 100.41

0.47 12.60 0.)9 2.15 o. oU 0.53 0.)1 U.45 4.52 0.09 0.55 o.oU -

mA

ma

I

22 1, SEGIIEDl et al,

Tahle 1 (COlltiIlIlCd)

89. 147 B.Bllc8aou 82.15 0.13 9.84 2.26 - 0.05 0.15 0.09 0.39 4.73 o.oU 0.53 0.06 - 100.46

90. 148 Il. Bravu 77.21 0.26 11.74 1.59 - 0.02 0.49 0.18 6.40 1.06 · 0.14 0.93 0.07 - 100.17

91. 149 I4.Bravu 76.36 0.25 12.19 1.57 0.04 0.02 0.51 0.113 4.93 2.39 0.02 1.17 0.05 - 99.60

92. 266 EEi'ob1balîk hUl 72.40 0.44 12.60 2.80 1.0~ 0.04 0'.59 0.34 5.71 3.12 0'.06 0.57 0.14 0.08 100.09

93. 293 Lozova h11174.35 0.20 11.98 1.30 0.90 0.04 0.72 0.34 ~.96 0.22 0.00 0.52 0.13 0.31 100.16

94. 312 Kalo1u h11176.45 0.20 ll.5o 1.69 0.22 0.02 0.09'0.24 5.90 2.t;2 0.06 0.10 0.14 0.10 99.65

95. 347 Delictav bUl 7~.3o 0.18 10.30 1.20 0.93 0.02 0.31 0.22 5.07 2.5~ 0.10 0.57 0.12 - 99.97

96. 146 IeaeeeR 72.46 0.54 13.24 1.90 o.bU 0.02 0.72 0.80 2.61 5.22 0.12 0.71 0.07 - 99.51

97. 367 l4uob1a Pila-. torulu1 b1ll15.20 0.21 ll.77 0.13 0.14 0.02 0.28 0.31 10.11> 0.23 0.10 0.26 0.12 0.29 100.14

~. 14:)A Cl,la bUl '/4.48 0.52 12.25 0.74 1.57 0.03 0.48 0.50 5.50 2.23 0.12 1.00 1.11 - 99.53

99. 143 B Cl,la bUl 71.22 0.54 13.30 2.72 0.03 0.57 0.24 7.14 2.21 0.13 1.30 0.15 - 99.55

100. 14 ~ A SOIlOva 77.20 0.15 10.04 1.10 0.18 0.04 1.oU 0.10 7.04 0.54 0.12 1.26 0.08 - 99.53

101. 145B • 79.30 0.13 8.44 J.05'7 0.01 0.05 0.06 0.01 8.91 0.23 0.14 0.70 0.06 - 99.79

102. 103 Tulcea loIonument 61.95 0.61 19.31 1.89 0.36 0.04 0.34 0.15 12.20 a.46, 0.09 1.90 0.20 0.36 99.46

103. 402 !.Iabmudla '(0.10 0.44 15.45 3.56 0.36 0.01 0.64 0.28 3.64 0.56 1.30 3.45 o o 99.79

The Tulcea Ullit also cOlltaills SOlii.' slIlall acid rock .\t'Clll'f"Ilt'.'S .. ,.; dykl's <illd \'"ills ellUillg thc Paleozoic format.ions al Tulcea (Monument) and OII t.lll' I\I<lhllludia hill . .;; tlll'Y arI' assigllt'd \.0 Ih" "porphyries" (M urgoci, 1914; lanovici cI. al., l!J61 ; !\'lirăuţă, l!)(iG) . BoUl mil\t'ralogieal and dWJllical feat.ures point 1.0 an intermediat.e alkaline composit.ion of these rocks. They are also <lssoeiated with hasic dyk.·s,

3. Petl'ochmuistl'Y

For a gcochemical charaett'risal iOIl of Nortllt'rIl Dobrogea rhyolit t'S Illt' followillg Hllalyst's have l>ccn carried out: (1) major e/elllellts fo/' lO:l salllplt,s (dll'llIical analys«,s)(Tah. 1): (2) sp.'elral allalyses (emissioll spectrog­raphy) for Pb, Cu, Zn , Ga, SlI, ~i, ('o. C'r, V, Se, Y, Yh, La. Nh, ZI', lle. Ba. Sr for U!) samples; (Tab. 2) (3) non dispersioll F X-ray s)leetl'onwlric Hllalyses for Y , Nh, ZI'. !lh , SI' for 91 salllplcs; (Tah. 2)(4) U, Th, K analyses by gamma spedrollwtry for \):~ salllpks; (Tab. 2)(5) ran' parth ('lc-IIlClltS (La, Ce, Sili, Eu, Tb, Lu) analyses by IIt'U!.I·OIl artivation IIwl hod for 2:ţ salllp"'s (Tah . :1) . A r"'('vanl IlIlIIllwr of analyscs \Vas envisaged for aII the struet.ural IIl1it.s and aII t.he OCelll'rt'lIe(' t,ypt'S both Palt'owie alld "ksozoic. Except.ing the Cirje/ari arca, ali t.11t~ ol.l\I'r rhyolitt, O('('!lrI't'lIet' art'as ill Nort.I\I'rtI. Dohrogea ar«~ r«'(>r('st'lIt "ci hy il numbcr of analyscs roughly proport.ional to th('ir OIl\.Crop arcas .

In oreler \0 facilitat(~ 111<' presl'n\.ation of analytieal dat il 11\1' following notatioll is propos('d : I. M ăcin V nit LA . Int.racarapclitie rhyolit.es I.B.!. Triassic rhyoliles dykes frolll t.11I' easl of tiI<' IIl1it LB .2. Triassic darite dykes from th!' Cilic oul,li<'r Le. TriassÎc-.Jurassie rhyolilt's rrom Ihe \Vest of the unii. ('southt'1'II aliglllllelll') LC.1. Turcoaia arca I.C.2. Cîrjelari arca 1. C .:l . Call1ell<l area II, ('onsul {)lIi t - Triassic rhyolitcs of the COlIsul Nappe III. Tulcea U lIil IIl.A . Triassic rhyolit.es

GEOCHEMISTRY OF RHYOLITES FROM NORTHERN DOBROGEA

III.B. Paleozoic trachytes

3.1 Major elements

23

As far as the major chemistry of rhyolit.es from the mail1 occurrence are as is generally known rrom prcvious studies, the chemi cal data will !le discussed by comparisoll . Thus, most. of the analyses are grouped on I.he TAS diagram (Fig. 1 a, b, c) in the rhyolite field. The plotting fields of rhyolites assigned to different units and to diffcrent areas within them are practically sllperposed with no relevant differellces between Paleozoic and Mesozoic rocks on the one halld and bct.wef~1l differC'llt stmctural ul1its and occurence areas on the other halld.

2

61

2

61

0 2 .3

65

65 69 73 C

+4 05

\ \

\

Si~%

77 Si~%

12

10

8

6 o

L.

2 \

61

\ \

69 73 77 b

Si~%

Fig. 1 - TAS (Na10+I\;lO _. 8i02) diagram for tII" rllyolite.s assigned to tllc differcnt stnlctural unil.s a"d 0""111"1"1'1\('" ar"as of NOI·thern Dobrogea. 1. i"lracaralwlil ir rh.volit,,,;; 2. Pal<x>zoic I"Ocks of ti", Tu!cea UniI.; :1, T"iassic rhyolitcs of the Mă­o:in l :"il : 4. Triassic' rhyolit..s of thl' Cilie outlier; ;;, r .. ia.~si .. rh.volilf!s of I itI' Cons,,1 Unit: (;, Trias­si. rhyolil"s of 1.1 ... Tul(.'"'' U"il.; 7, 8, 9, Triassic . .I"rassi .. rh~'olil"s o" ti ... ·soutlt",·" alignmenl.' of IIU' ~lă("ill liniI.; 7.1'tln :(.aia:u·(~a; 8, CÎl'jdari an~a; ~I . C,lflWIHl an'a .

MosI of tl)(' Ctnaly:-wd I:oeks an' mark. 'd hy high ,.;!linl wnt('nl.s and Ily ratlH'r high alkalinity. Tlwre is a llcgativt' wrrelation Iwlw('en Si02 cOl1leul. and t.11(' alllount. of alkalis. II' " '(' (')(l.I'lId t.!H' ealc-alkalillP/alkalillC' field ddimiting line to Ihe lichI of rhyolit.es on t.his diagram, Wf~ nnd that lIIOsl. of fhe roeks under disClIssioll cnter thc calc-alkalin(' field, but. dOSf' 1.0 the alkaline 011(', The Palf'owic rock,.; of 1.1 ... Tuk<:a uuit. are plot!.ed 011 the fidd of trachytcs (I'\'lonu111el1l,) or dacit./'s (MahlIIudia lIill). '1'11<' Triassic rocks of Ihe Cilic ollt.lier are marked by their 101\1 silic(\ and high alkaline composit.ion cnterillg t1H' lidd or I rachyda r ilf's. TIH'Y ohviollsly

24

Grou.p lfo. aaaple Pb Ou Zn Gs SI! Ii 00

1 2 } 4 5 6 7 8 9

1 159 19 10 140 22 6 <2 <2 2 161 A 15 1~ <}O 15 5.5<2 <2

1 161 B 70 40 17 6.5<2 <2 17} 60 44 }O 14 6 <2 _ 2.5

5 }24 <2 } 10 <2 4 2 6 }55 <2 4- } 2 6.5 <2 7 }58 <2 6.5 8.5 <2 ,2 <2 8 364- 10 6 8.5 <2 2 <2 9 401 5.5 2.5 6.5 <2 } <2

10 21105 <2 9.5 17 '.5 } <2 1 A 11 '12} 3.5 13 18.5 11 5 <2

12 }124 <2 2} 22 } 2 <2 1} }196 } -26 27 7 <2 <2 14 }197 , 9 19 4 2.5 <2 15 }200 3 10 26 c2 5.5 <2 16 }254 75 }7 • 16 5.5 4.5 4-17 }255 4 12 - 11 5 2 <2 18 '258 4.5 7 <}O 1} } 4.5 3.5 19 }259 100 50 }O 16 , <2 2 20 - }26O 5.5 II <}O 1} } 2.5 2.5 21 3261 36 10 <}O 165 5 4.5 6

22 11} 15 II 70 30 8- }.5 <2 2} 114 17 16 75 }1 6.5 4 <2 24- 120 6.5 8 <}O 27 6 4 <2 25 122 9 44 2}O 24- 6 2- <2 26 124- 10 25 2}O 18 4:5 2:5 2 27 128 10 70 <}O 24 3.5 4.5 5 28 131 28 27 40 27 5 2.5 2 29 1}3 26 23 38 32 9- 6 5 }O 136 13 13 }O 17 - 4.5 5 } }1 1}7 170 24- 60 15 4 }.5 2.5

1 B 1 32 138 15 12 35 15 4 3 }.5 }3 178 18 10 18 5.5 6.5 <2 }4 2O} 25 10 16.5 2 5 } 35 -204 }.5 24 30 6.5 4.5.:2 }6 208 14 18 18 5.5 }.5 <2 37 21} 1} 4 17 2.5 } 8 }8 2985 }.5 16 - 17 7 2.5 <2 }9 305} 19 9.5 }8 19 4 <2 <2 40 }199 20 25 20 } 2 <2

1B2 41 3289 12 20 18 5.5 5 6 42 3290 55 28 17 7.5 4.5 6

4} 214 20 14 28 7.5 4 <2 44 215 25 18 26 7 2.5 <2

10 1 45 }18 3 4 16 2.5 }.5 <2 46 }19 4 3.5 18 3.5 } <2 47 '256 } 6.5 18 4.5 6.5 <2

48 158 19 10 146 22 6 <2 <2 102 49 351 <2 9 20 2 ~ <2

50 352 5 }.5 19 3 5.5 <2

Or V Se

10 11 12

1 <2 <2 1 <2 2 1.5 2 }.5 1.5 5 } 1 8.5 4-5 10 4 2 9 4 2 5- 4 2 7.5 4 1.5 2.5 4.5 1.5 6 4 1.5 <2 4 2 S 12-2.5 5 }.5 } 12- } 2 8.5 7 1.5 }.5 3.5 4 10 4-1- 6 }.5 2.5 r, 2 7.5 12 4

2.5 4- 2.5 4- 4 2.5 2.5 5.5 } 2.5 4 2.5 5 15 5 5 28 6.5 4.5 8 4

10 28 8 5 2} 6-6.5 12 3.5 4.5 14 }.5 7 8- 2.5 5.5 1}.5 5 2 5 .:2 } 5 <2 2.5 65 -2' 8· 5 1.5 2.5 3 2.5 5.5 5

5.5 36 11 8.5 }8 11

3 2.5 <2 2 4 <2

-<2 5 6.5 2 3.5 5.5 4 5.5 2.5

1 <2 <2 <2 } 9 <2 }.5 4

Y

1}

110 24-44 50 68 60 55 }} 27 55 55 42

1}5 ?O 46 70 21 }4 19 24 52

53 56 72 47 40 40 55 50 45 28 20 65 72 ?O 75 8.6

60 }2 70

70 ?O

ll5 1}5 110 90

135

110 85

110

1. SEGllEDI et al.

y--

14

~~ 52 64 67 57 52 }7 29 65 67 7}

106 64 41 54 28 52

TAHLE

THAC\-:

n La lfb

15 16 17

7 67 2} 2"2 }2 1} }.4 }4 14 }.7 44 12; 6.8 45 28 6 42 2' 6.5 45 16 5.2 55 }2 3.6 50 14 7.2 36 44 6 72 22 6 <}O }4 8 }, 24 5.5 40 }6 4 }O 18 5- 95 10 2.6 40 15 } 40 l'

25.4 2.2 <}O 12 28 2.4 <}O 11 64 4 50 15 . 70 6 60 29 84- 7 62 }O 81 9 95 32 63 6 30 24 41 }.6 38 14 39 4 30 17 67 7 55 20 65 5.2 52 18 50 4 46 15 }9 2.6 }O l} 25 2.} }O 10

8.6 }O }6 - 7.2 68 19 75 65 6 24-64 7 60 }O }O 36- - }OO 61 5.2 30 30 46 2.6 56 15 51 5 52 28

51 4 55 14 50 }.7 45 14

84 14 70 30 92 8.5 52 }O 85 10.5 88 36 87 8.5 75 }4

110 14.5 90 40

91 7 67 23 7} 7.7 62 26 94 16 65 }4

I

GEOCHEMISTRY OF RHYOLITES FROM NORTHERN DOBROGEA

2 ELEMENTS

Jb iii

18

23 28 26 29 22 20 21 25 20 }1 32 50 21 32 19 15 19 24 18 14 16

28 33 33 28 17 21 25 23 20 18 17

-33 29 -}6 24 23

24 22

31 30 30 28 28

32 24 31

Zr

19

520 60 80

150 245 200 215 1}5 210 150 330

}Q 91

115 150 230

90 130 150 140 150

380 480 450 250 280 320 300 380 280 220 200 300 290 2?0 210

2,,6 80

100 220

320 280

1100 850 750 610

1000

520 435 740

Zr -

20

225 142 116 250 200

i~ 122 196 129 121

69 101 125 148 278 126 166 181 160 179

359 493 437 284 308 }42 }35 394 303 227 2}3

-}33 291 345 116 180 185

356 34?

1191 971 582 605 934

865 492 776

Be Ba

21 22

2 70 el 1250 <1 1300

1.6 1300 -

- -2.1 30 3.8 105 4.6 16

11 12 - 16

2.4 1000: 2.5 820

<1 , 28 1.1 55 l.} 26 1 42

3.8 300 3.8 260 4 400 1.6 950 1.3 1'700 1 · 1500 2.5 /20 1 800

<1 1800 <1 2900 <1 · 1400

2.6 }lll-5 3.2 480

310 - 380 17 -4.? 1300 1.} '700

1}5

3 '700 2.? 950

18 2?

2 '70

Sr Sr - Rb- Ti02C,,)· u

2} 24 25 26 27

26 24 237 0.260 3.3 40 60 }11 0.314 4.7 24 43 292 0.}23 4.3 28 54 217 0.242 5.9

13 148 0.356 2.2 61 4 0.317 -42 156 0.318 3.1 54 275 0.211 3.1 - 121 63 0.207 1.1

65 lb~ 0.}49 4.1 460 0.201

39 60 0.195 -32 48 O·m 2.6 40 71 - 0.3 2.2

100 1~ 3 · 0.114 2.2 <10 26 258 0.275 3.1 <10 27 242 0.220 0.71

42 79 10 0.222 1.7 48 101 6 0.;150 1 38 80 7 O.}45 3:2 40 99 13 0.152 1.8

36 47 235 0:227 4.9 211- 41 274 0,184 5 28 45 174 0:266 4-36 43 260 0,463 5.3 50 59 211 0.449 4 72 38 210 0.653 5.2

· 30 41 253 0;f,6i 6.5 46 50 259 O. 4 40 47 255 0;}6} 1.8 55 54 229 0.}40 2.9 20 29 264 0.}51 .4'

. 1? }.1 23 - - -44 82 144 0.423 6 36 5? 252 0.190 6 - - - -46 ?9 129 0.213 6 42 90 11;> 0.218 4.2 50 ')1 148 0.250 2.1

ao 103 159 0.?49 3.9 65 893 163 0.895 3.7

<10. 11.7 177 0.221 5.2 <10 10 209 0.258 4.1

32 n3 0.226 4.7 211- 163 0.261 3.1 22 132 0.<'84 1.8

?6 42 125 0.256 3.3 ;:>4 2?6 0.401 3.<' 17 l~G O.06~ 3

25

Th XC")

28 29

12.6 6.4-28.3 8.8 23.9 7.7 14.1 6.~ 14- 4.8_ - -12.3 4.~ 32.3 ~~5 11.9 35.2

21 38.5 14.6 ... 13 ' 7.8 2'7.4 7.5

9:7 8.7

12:2 13.2

25.5 5.3 24.2 5.3 23.3 4.4 27:3 8.6 22:5 8.2 18.7 8.7 23.6 7.7 22."- 6.9 23.4 9.1 18.7 8;5 18 10.1 11.8 3.4 -2V 4 27 6.8 - -36.9 3.8 27.11- 4.11-14.1 }.7

18.2 4.2 18.? 5.8

24;1 3.9 26.3 "-20 4 20.5 5.6 <>5.9 3.?

13.4 3.7 10.8 8.3 18.7 4.3

26 1. SEGHEDI et al.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

51 353 19 li'.5 - J5 ~·.5 3.5 <2 2.5 4.5 7 100 90 13.5 l/5 27 ~2 354 2.5 3 - 17 2~5 3 <2 4.5 3 3.5 78 82 7.7 60 25

53 150'· 16 25 48 18 li- <2 <2 2 7, 3 85 71 6.8 60 22 54 150 B 17 28 55 19 9.5 <2 <2 1.5 3.5 3 72 78 6.5 74 24 55 150 li: 95 34 63 22 5 <2 <2 <1 2 6 90 75 6 ?O 20 56 151 55 8 33 19 6 <2 <2 1 <2 <2' 115 96 8 ( 30 26 57 152 12 12 68 23 7.5 <2 <2 1 2 <2 90 98 9' < 30 26 58 155 A 30 9 55 15 5 5 <2 1.5 2 <2 42 46 3.7 60 40 59 156 24 12 30 18 6.5 <2 <2 1 . 2.5 <2 32' 48 3.2 (30 47 60 3516 15 8 12 5 5 5 2 3.5 8 6 85 9.5 46 19 61 3517 7.5 2.5 - 21 6' 2.5 <2 1 2.5 <'2 90 9 <30 30 62 3518 12 2.5 21 6 2 <2 <1 <2 < 2, 125 - 12 <30 33

10 :3 63 3521 20 3 17 6 2 <2 1 <2 3 82 9 <30 2? 64 3522 23 3 18 2.5 2 <2 <1 <2 3.5 32 4.5 ~ 44 65 3523 29 2.5 20 3.5 2.5 <2 <1 <2 3 23 ;1 <.30 42 66 3531 10 2 18 5.5 2.5 <2 1 <2 <2 98 - 10 33 30 67 3535 12 2.5 16 6 2 <2 1 .., 4 82 9 <30 27 "-68 ' .3538 24 3 22 6 2 <2 1 <2 . 3.5 115 - 13 <30 30 69 3542 17 5 18 4 <2 <2 1 3.5 4 90 9.5 52 24 70 3542 A 5 11 22 3.5' 2 2 1 2.5 3 72 8 36 19 71 3543 5 3 15 4 <2 <2 <1 <~ 5 85 - 10 4: 30 30 7? 3544 13 2.5 - II 4.5 <2 <2 <1 <2 3 120 8.5 <30 24

'8 10 14 20 42 20 6 2.5 <2 3 <2 <2 53 64 5.5 55 20 74 12 9.5 28 60 20 7.5 2 <2 3 2 <2 50 71 4.5 60 17 75 14 18 30 70 20 8.5 2 <2 2 . <2 . 2 ,l!,6 67 5.5 <'30 17 76 16 2l~ 21 85 24 6.5 2.5 '<2 1.5 2 <2 85 83 7 65 18 77 20 14 18 48 20 7.5 ,3 <2 3 2.5 <.2 c-",c. 74 5 58 20 78 36 42 22 <30 16 9 3 2 5 9 3.5 50 63 3.8 80 <.10 . 79 37 18 24 32 17 7 3 <2 4.5 8 3 29 31 2.6 I~O <10 80 42 12.5 19 <30 15 6.~ 3 <2 4 14 2.5 21 39 2 42 <'10 81 44 14 25 38 15 6 4 2 4 13 3 32 44 2.8 48 <10 82 46 34 24 92 20 7.5 2 <2 1.5 3 3 60 67 5.5 50 17 83 56 60 27 125 18 7.5 3 <2 2 2 <2 52 59 6 50 17 84 62 100 33 220 17 7 2.5 2 2.5 2 3 46 63 3.6 35 14

II 85 78 18 22 55 23 7 2.5 .::2 2.5 <.2 2.5 100 105 6.5 55 14 86 82 28 21 70 25 7.5 <2 <2 2 2 2.5 65 79 5.2 55 17 87 98 26 22 80 18 7.5 2.5 <.2 4 <2 2.5 60 73 r; 58 16 '" 88 102 13 16 5G l6 7 <2 <2 3 4.5 3 55 77 5.3 32 13 89 141 17 17 75 23 5 4.5 3.5 5.5 12 4 33 37 3.2 40 14 90 147 50 13 38 13 6.5 <2 <2 2 4.5 2.5 95 91 7 90 30 91 148 36 9 <30 16 4 <2 <2 3 8 " 4 55 61 5 52 17 92 149 26 13 32 18 5 2 <2 3 9 5 76 63 5.2 62 16 93 266 74 -911- 293 36 5.5 12 95 312' 57 9' J4? - 56

~ 143 A 38 21 <30 24 '1 5 3 7 26 6 50 51 5 60 15 143 B 30 19 38 24 6.5 6 4.5 13 26 7 ;'2 40 4.7 62 14

IIIA .99 145 A 20 18 30 14 8 <2 <2 1.5 5 2 27 38 2.5 56 18 100 145 B 180 28 38 7 3.5 <2 <2 1.5 3 <2 23 .% 2.2 47 16 101 146 30 8 44 16 5 <2 3 3 31 5.5 38 46 2.5 ~ 13 102 367 45 3 9 <2 7 <2 5.5 6 4.5 36 39 '4.5 17 ,

IIIB103 10~ 22 ' lh5 <JO 14 3.5 2.5 <2 3 75 'li 13 19 <1 50 <10 104 Lţ02 <2 5.5 - 16<2 16 3 4 20 4.5 18 27 ., c 50 22 L • ." .. analysed bJ non dispcrsion FX-ray 1"pectro::!ctry

GEOCHEMISTRl' OF RHYOLITES rROM NORTlIERN DOBROGEA 27

Table 2 (contillll<'d)

18 19 20 21 22 23 24 25 26 27 28 29

30 725 724 21 ,149 0.443 3.!) 2O~9 4:9 30 535 616 22 118 0.350 2.6 16.1 4.6

32 570 702 1.7 200 115 29 '321 0.2~2 3.7 19 9.2 32 400 1~7 2.2 150 110 11 381 0.323 2.1 20.1 7.9 28 520 681 2.'2 160 74 48 364 0.39~ 4 14.6 9.7 37 500 693 2.1 42 <10 13 . 324 0.22 3.8 18.8 8.4 37 560 774 3.2 36 <10 13 216 0.206, 4.2 21.1 4.3 58 140 191 2 170 16 , 197 80 0.058 4.5 20.8 1.7 61 120 180 2,6 130 13 22 289 0.239 7,5 20.9 5.97

440 1.4 _ 420 20 ~ 3.2 22.5 9.3 870 2 46 <10 2.7 25.2 8.8 900 2.3 36 <10 2.8 22.5 9.1 700 1.9 1300 <10 -120 2.1 100 elO 2.6 29 7.9 ,- 130 2.8 125 <10 2.1 28.5 7.8 800 2.3 42 <10 ,- "" 4.8 28.4 8.5 730 1.9 90 <10 30.7 ' 7 6.98 880 1.5 170 (10 620 1.5 320 <10, 480 1.4 200 < 10 - - 1.6 22 8.1 370 1.5, 280 <10

.. .. -650 1 380 (10 ...;

32 400 }68 2.8 165 38 32 204- OS?? 4.5 25.5 5 30 380 395 4 160 46 38 247 0.319 2~9 26.4 4.7 32 310 329 4.4 100 160 191 58 0.260 5.3 25.5 0.5 31 400 , 358 3.5 160 55 52 166 0.12~ 1.8 25.3 4.3 31 300 307 I~ 115 27 25 196 0.2'1 2.5 22.6 4;? 16 300 234 2.3 170 125 135 92 0.319 4.5 23.5 2 .. 4 16 220 210 1.4 450 62 ll9 272 0.141 4.9 22.5 5 19 320 239 2~3 950 115 55 223 0.349 2.9 19.5 6.7 20 240 248 1.9 625 48 54 237 0.320 4.5 24.6 4.9 25 '«)0 355 4 600 72 75 257 0.249 6.5 26 6.2 31 460 398 2;7 400 105 110 128 0.414 2.9 21.1 3.7 24 360 ,313 1.7 1100 43 29 229 0.107 63 21.6 6 31 460 367 , 3.2 5.60 66 53 140 0:275 3.3 23.7 5.2 32 540 423 4.2 280 240 199 300 0.321 5.2 24 5.6 26 360 305 4.6 }OO 60 62 162 0.347 5.4 24 . 5 2.5

290 350 331 3.8 280 35 26 253 0.376 5.7 25,7 5:3 18 200 .?28 1.4 1150 26 39 244 0.14C 4.2 21.2 7.2 36 750 757 1.7 44 20 37 18 0.229 5.8 ' 26 220 242 2.1 320 20 31 239 ' 0.588 3 15 5..2 28 250 ~44 2.2 11,60 19 4'. 195 0:346 3.9 1 30 520 40 203 0.374 4.4 21.3 5.5 16 200 17;:; 75 260 0.332 5.2 23.4 8.3 , 29 33'. ,36 1;)4 0.544 2- Z4 ' 5.4 29 275 39 170 0.238 6.4 24.5 4.9

18 320 327 3.3 700 65 89 251 0.488 7.2 31.4 5.3 18 370 331 4.6 500 55 57 256 0;385 5 33.9 ' 0.7 14 140 170 3;8 480 32 41 326 0.194 10.7 38.3 6.2 15 1;50 173 1.1 ' 2200 28 41 256 0.443 5.2 33 7.3 30 340 300 2.5 360 75 159 162 0.612 4.7 18.3 2.3 21 165 171 - 32 152 0.285

21 220 281 1 2000 80 60 514 0.412 4.5 80.4 12.9 23 260 300 72 165 0.505 4.2 " 31 3

I

28 - 1. SEGHEDI et al.

~~a 3

BARI BAlll'H BLJ:IIBITS

GIIXJP BO SAllPLB La c. SIlI • i'b tb Ia

1 159 24 66 6.2 0.75 0.38 . 7.0 0.611-2 173 4S 81 8.6 1.40 0.59 3.7 0.24

1 A 3 324- 37 59 7.7 1.00 0.81 6.8 0.57 4 364- 45 108 12.7 0.67 0.92 5.2 0.37 5 355 11 63 2.0 1.29 0.62 6.0 0.36

6 120 35 89 10.8 0.47 0.75 9.0 0.88 II 7 128 .36 79 & .. ' 0,80 0.32 4.0 0.41

1 01 8 318 48 122 6.1 1.20 1.04- 10.5 0.85

9 158 44- 116 11.6 0.60 0.76 7.0 0.95 1 C2 10 351 58 92 12.8 1.11 0.89 7.7 0.65

,11 354- 20 109 5.~ 0.90 1.00 7.7 0.74

12 1508 42 58 6.9 1.0 0.63 6.5 0.77 IC3 13 1551 37 85 6.5 0.30 0.61 3.7 0.52

:t4- 20 45 107 10.9 0.39 0.77 5.0 0.65 15 42 47 57 5.3 0.51 0.35 2.0 0.~9 16 78 36 94- 9~4 0.29 0.79 6.5 0.81

II 17 141 46 73 7.' 0.61 0.50 3.2 0.22 18 293 45 19 9.4 .0.48 0.90 5.5 0.41 19 147 ·'9 4' 11.6 0.92 0.86 7.0 0.62

20 14~ 52 81 6.4 0.87 0.75 5.0 0.34-III A 21 146 25 80 6.0 1.10 0.40 2.5 0.2' • 22 367 40 66 10.0 1.07 0.62 4.5 0.31

III I 23 103 46 57 '.0 1.13 0.52 1.0 0.22

differ frolJl III«' olllPr Triassic rhyolill' bodi<'s of 1.11«' i\Iărill Unit. 1.0 wliich t.lwy haVl' h(-~clI assigned OII tectonic. grollllds.

Quit.e pUllzling is 1111' plOI t.illg in t.he dacite field of ont' .'lampI/' collpc1.ed frolu t.lw Camella area, different from tiaI' ot.her rock.'l occurring t.herc. Tlai.'l sample is from OarjpllnlU qu<trry , tlw only place in which t.he Jurassie age of.'lollw acid vok<tnics on tiI!' ·sollt.hl'rtJ alignnwnt. ' has 1)('1'11 prowd (Crădinaru. 1981).

The K20/Na?O ralio rl'wals t.11(' u.'llIally pot.assic ch<traclc!' of mosi of tllI' stlldipd rhyolilt's. Thc only areas of rclat.ivt'ly high N (t:!O cont.eul. areat. Turcoaia and Clrjdari wllf'r«' 1111' rhyolil <'S arI' r\."social.<~d with comagmatic alkalill<' granile.'!. Thi.'l clacmical featur<' was poinll'd 0111 hy Ştl'fan (in N('<!<'lru C'I al.. HJ8() , IJllpubl. rcport.) ",ho calls 1.1\1'111 "aJkali-sodic rhyolit~s" in contrrt.<;1. 1.0 aII III«' 01111'1' thye,Iil ('S rrolll \orlll<'rt1 Dohrogea (" alkali­pol.assic rhyolil<,s"). '1'11<' suhullil.ary I\:!O/Na:!O ralin 1,; alslI eharacl('n"IÎc rol' snlll!' inlr<tcarapdit.ic rhyolites antl for SOliI<' rhyolitir rocks in tiI<' COlIs1I1 Unit. TII<' hi~Ili'''1 1" 10/\:1:,0 rallo C"\liHil('I, ·riz.,s 1lI0St. ofthe Camena. rhy(\li"'s. A I Yl'iC;ll f .. alllr .. for bol la Pal,'ozoic illld ." .. sozni .. rhynlil ('S rrolll \01'111<'1'11 Dohrogea is t.1lf' IIsually lo\\" F' ·:!():I . ,,'t{) \In{), \I)!:O alld ('aO ('olll"llls.

'1'1\1' sllldy of major chnlli('al COlllpositioll of t.1 .... Norll"'r11 Dohroll;('o rhyolil.(·" "hows t.ltal" except for the high

GEOCIfEMISTRY OF RIfYOLITES FROA! NORTIfERN DOBROGEA 29

potassic character ofthe Camena rhyolites and thc higher Na20 eOlItent. of Turcoaia anei Cîrjdari ones, there are no relevant chemieal differenees between t.he rhyolites assigned to various sl,ruct.ural unit.s or bct.wecn Palcozoic and Mesozoic rhyolit.es. It is worl.h mcnl;ioning t.hat, ,~ith respect. to the alkali sltm ami other dlcmical fcat.ures the Turcoaia and Cirjelari rhyolit.es do 1I0t diffcr from the other rhyolitc occurrences in Northern Dobrogea (Fig. 1 a, b, c).

3.2 Trace elements Becausc some trace elcmcnts sucll as Y, Yb, togct.her wit.h Ti02 , are less mobile during alteration, diagenesis

and met.amorphism, Winchester anei Floyel (1977) proposeel a syst.cmat.ir petrographic c1assification of old igneous rocks based on these element.s. The diagrams t.hus drawn (Figs. 2, 3) lead to the slight modificat ion of the rock assignment by comparison to their assignment. accordiilg to major chemistry, but point to certain differential.ion tenelencies among various rock groltps. Thercforc on thc Si02-Zr/Ti02 diagram (Fig. 2) most of the samples belong to rhyolites anei rhyodacit.es with a few cxceptions inferrcd from I.heir major chemical composit.ion. Only some samplt"s collcctcd from thc 'sollthern alignment.' of the Măcin Unit. belong to alkaline rhyolites, most of t.hem being assigned to rhyolites. Howcver, OII the Zr/Ti02-Nb/Y diagram (Fig. 3) almost aII the samples inclueling t.hose from !.he 'sout.hem alignment' are plotted in thc field of rhyolites and rhyodacites. On both diagrams the except.ions are reprcsent.ed by the Paleozoic volcanics ofthe Tulcea Unit (trachyandesites), the Jurassic rhyolit.e in the .Ba'lPunar quarry (the single samplc wit.h alkaline character) and the rocks of the Cilic outlier (dacit.es). This poinl.s out the calc-alkaline chemistry of mosI. of the Northern Dohrogea rhyolites,

. with special reference to the obvious alkaline tcndency of rocks from tltc 'sout.hern alignmcnt.' of t.IH~ Măcin

Unit ..

Si~

8L

72

58

~,-{~ t~ 0147 1" " ",.- ----, I ~~-ţ;o--~~ ___ ~_. ~ Il" ~010 \ ~' ,---- ,

.,-f, -s.., .... o~ I I ./ ~ "r°!ll.9'~# a / ,"".1. o It( 1 ,~ ",/IC

I • " I .e .," .... __ ---

" ·0 .... :... ....... " + ",-ti" , 18;11,', C-O'-'--____ _

I ,/ ~

54 : / t----...,t.-,<tC

o. , 0.10

Zr /Ti02

Fig. 2 - Si02-Z,-jTi02 diagratll; for symbols see Fig. 1.

1.00

ZrlTi~

,.'a .. , "G, IC ' 110

1 ... , '1,

~ • i,' .' ~1(P\

\ '1 IA: • , 0\ I o '

010

,'~JY. .~o \ - _ , ~_-_:"-I.. -- ~--. . .... "l'" ,

'ţ; ...... ~...... ,'IA \ .~"" ............ 1,

0.10

\ •• 8 1. ~ \ ..

. \ o : IB;B,InA \ -+ 1

\ ... \.;t./~

,.0

Fig. :3 - Zr/TiOr Nb/Y diagram; for sYllIbols see Fig. 1.

16 NbIV

The same diagrams (Figs. 2, 3) show t.he slight.ly different position of the Paleozoic and Mesozoic rhyolites inferred from the different correlation tendcncy of tlle element. ratios: the former are niarked by a relatively constant Zr/Ti02 ratio by comparison to the Mesozoic rhyolites with varying Zr/Ti02 ratio and \ow Nb/Y ratio. The Mesozoic rhyolites stand out by tlte superposil.ion of plotting are as of the rhyolites assigned to the dykes and veins of the Măcin Unit and t.hose of both Consul and Tulcea Unit. The rhyolites of t.he 'southern alignment' of the Măcin Unit. show the same correlatioll trelld alld have highest Zr/Ti02 ratio. This scems to be a common feature of aII three occurrence areas (Turcoaia, C.rjelari and Camena).

The rhyol: •. ir ro::ks fr<.. .n t.he differellt. structural unit.s of NorllH'rn Dobrogea al~o differ wil It rt"spl'(·t 'o t.lwir Rb aud Sr mntf'.llt.s. TIUt'; , the Sr- RI> diagram (Fig. 11) poillt.~ 10 usually low('r Sr "1)1111'111:- « lr I Il!' rll\ "IiI,',", of

I

30 I. SEGHEDI et al.

the 'southern alignment' by comparison 1.0 t,he others. Quite illustrative are thc Camena rhyolites exhibiting Rb contents higher than those from Turcoaia and Cîrjelari.

The Triassic rhyolites of groups 1 B 1, 1 B 2, II and III A have t.he same reverse correlation trend for Rb and Sr wil,hin their superposed plotting area..,. Unlike this, the int.racarapelitic rhyolites show positive Rb-Sr correlation with lower Sr values, partly overlapping t.he plotting arca of the Triassic rhyolites. Some of these rhyolites and the Triassic rhyolitic t.uff al, Nicolae Bălcescu are remarkahle by their anomalollsly low Rb values or even the lack of t.his element.; t.hc former have a higher Sr cont.enL It. is also t.o note t.he particular plotting of the Jurassic rhyolit.e from Ba.'lPunar quarry (high Sr) and of thc Paleozoi(' I.rachyte from Tulcea (maximum Rb).

Sr ppm

100

Zr

'\'" ~------,'A -- ........ , .: /' • 1;. -,,'

I~ ____ ~----~

0147

10

Fig. '. SI' Ilb cliagl'am; fOI' ~Ylll"ols see Fig. 1.

01L2

1.-----.-------'1. ................. ... ! ____ li.-",'

IA

10

Fig. S ZI' " Rb cliagl'am; foI' symbols see Fig. 1.

o o'l55A

100

082

1.00 500SXl 'KlOO Rb ppm

A more accentuated discrimination among the st.udied rock groups is obtained an the Zr-Rh diagram (Fig. 5) which shows: similar Rb vallles an the whole, exccpt for certain intracarapelit.ic rhyolites and the Camena rhyolites (low Zr cont.ent in the int.racarapelitic rhyolit.es, practically identical Zr content of Triassic rhyolites for groups IB . II, III), maximum Zr contents in tllC ·sout.hern aligllment' and the peculiar character of the Jura.'isic rhyolite and t.he l\Jlcea trachyte,

Significant. dirrerelln~s of tracI' element distribut.ion within the invest.igat.cd rock groups are also proved by the Rb-Y + Yb (Fig. Îl) and Nb-Y (Fig. 7) diagrams lISf><! by Pe arce do al. (\98'1) in order to discriminate the geotectonic environmellls of tlw gralliti\' rocks, OII hoth diagrams the Paleozoic I.rarhytcs of 1.1)(' Tulcea l Jnit.

GEOCHEMISTRY OF RllYOUTES FRDM NORTJlERN DOBRDGEA

Rb ppm 700

100

SYN- COLO

vĂo o

WPG

011.7

200 300 Y.Nb ppm

Nbppm

100 WPG

ORG

m Yppm

Fig. ti -- Hb Y+N" diagram (I'e,un, et aL, 1984) ; for symbols sec Fig. 1. AbbrevialÎolls: sYII-COLG, sYllcollisional granitl"'; VAG, volcanic arc grallit.es; WPG, withillplale grallill"; OIle, oceanic rift gralli"".

Fig. Î - N b -- Y diagram (Peal'ce el. al ., JH84) ; for sYIIlLois see Fig. 1, for abbre\'ial.ions sec Fig. G.

Uppm

10

5

" " "

A 1-''' O

.~ .... --10 20 30

Fig. M Li' - Th Iliagralll , for symbols SPoI' Fig. 1. Abbreviatiolls: CI, lower crust; CS, upper cros!..

Thppm

31

and the .sout.hemalignmenl .. l\1esozoicrhyolitesarest.andingaparl .. whiktheTriassicrhyolitegroups (IBl.IB2. II , IIIA) are al mosI. perfecl.ly superposed silJlilarly 1.0 the Ot.liN diagrilllls IIse<l. The intracarapelitic rhyolites are plollt->d on tii!' saliI«' area.

32

100

"­"­ ... ...

"- ... "- ...

"­ , "

.... -.... -..... ~/

'59 324

364 IA. -

IlIa ---113

"' ..... 103

20 147

n 293 42

141

120

18

143A Y!I1 III A 1t.6

s:" E'u Th

Fig. 9 - Chondrit.e normalized REE wstributioll diagrams. IA, intraca.rapelitic rhyolites; I B, Triassic J'hyolites of the Măcin Unit; II, rhyolites of the Consul Unit; III A. Trias­sic rhyolites of the Tulcea Unit; III B, Paleozoic rocks of the Tulcea Unit.

1. IC~2.

3D

1. SEGIIEDI et al.

GEOCHEMISTRY OF RHYOLITES FROM NORTHERN DOBROGEA 33

Ba is distributed , ... ithin a wide variation range (18-2900 ppm) which in the case of t.hese old ' rocks is accounted for by high Ba mobility during the geologicill proces..,es. 1'he 1'riassic rhyolit,es hav!:' higher Ba contents than the other rocks.

U and Th distribution showsthat the maximum cont.ellt of t.hese clement.s occurs in rhyolites assigned to the Tulcea Unit and the minimum ones within the Cîrjelari rhyolites, exccpt.ing t,he int,racarapelit.ic rhyolites with anomalous behaviour on most. of the diagrams. Rhyolites of the ot.her unit.s show intermeeliat.l' valueS similar for both U and Th; ollly the intracar<\pelit.ic rhyolit.es have a somewhat. lower 1'h content. (Fig. 8). The average U /Th ratios of the stuelieel rhyolit.e grollps, compared 1.0 thc lower and upper crust values rf.-1ylor and Mc Lennan, cited by Tischendorf et al., 1987) art' assigned approximat.dy to thc same correlation line which suggests their crust.al origin. .

The chondrite-normalized REE distrihution (Tab. 3. Fig. 9) points t.o major §imilaritics between the distinct rock groups and to less import.ant difTercnces . 1'he general REI'; disl.rihut.ion pat.t.ern is practically identical for aII rocks exccpt for the trachyte from Monllment. (Tulcea), showing t.he S;HlW light REf: fract.iollation alldlhe lack of fractionat.ion or even the slight. inncase of heavy IlEE. The high Yh wntent. is their common feal.llre. The trachyte sample from Monument. exhibit.s frartionat.ion of hoth ligllt. and heavy REE. There are sOlIle differences concerning Eu and 1'b behaviour: some samples show negat.ive Eu a.nomaly, 'others lacking this feature, depending on higher or lower Tb values. Tht"Sl' t.wo instances occur different.ly in aII the ment.ioned groups. Anothet difTerence regards the wider variatioll range of heavy REE, t.he maximum valucs bcing typical of the 'southern aligllmcnt' rhyolit.es. The Paleozoic int.racarapclit.ic rhyolit.es (IA) with a similar general REE distribution pat.tern differ however from t.he Mesozoic rhyolit.es duc to the absence of/or at.tenuated anomalies. Due to it.s low Sm conl.ent one of the samplcs (355) whirll was not plott.ed OII t.he diagrams shows a false positive Eu anomaly, difficuIt, to int.erpret .

The minor element dist.ribul,ion wit-hin Norlhern Dohrogea rhyolit.cs allows significant remarks and discrim­inations which seem 1.0 solve some cont.roversial problems of t.he chronological and genetic systematisation of acid rocks from different occurrcnces. Therefore, conside"ing t.lmt on most. of t.IH~ diagrams the rhyolit.es of groups IBl' IB2, II and UlA assigned to difTerent structural IIliits ploI OII t.he samp fields , t.heir strollg resem­blance is indicated and similar agcs and petrogenesis arI' slIggest.ed (t.he same source alld the same processes of magma generation and evolut.ion).

The rhyolite dykes and veins <?f the Măcin Unit intrlldillg only Paleozoic formations have been long time considered Palcozoic in age (Mirău~ă, Mirăută, 1962; Mllre12an, 1975). These rocks show similar chemical features as the rhyolites of t.he Consul Unit and their association with basic - usually contemporaneously -rocks suggest the same Triassic age for both.

As can be seen on the presented diagrams, most of the intracarapelit.ic rhyolites show similar trace element distribution to the Triassic rhyolites suggesting a commun crustal source. The divergent, trends of the plotting fields on some diagrams (Figs .. 2,3,4) as well as some difTercnces in the Sr, Zr, Th and Eu contents account however for different evolution trends of these common source magmas. Some of thc samples of these groups show strong differences of the trace element distribution wil,h respect 1,0 aII t.he acid rocks of Northern Dobrogea, as observed on sever al diagrams (Figs. 2, 4, 5, 8). This can be explained either as t,he result of some processes subsequent to rhyolite emplacement or of special processes connected wit.h t.he magma evolution.

The rhyolites of the 'southern alignment' (lC .. IC:? IC3 ) show a trace element, distribution slightly different from the Triassic rhyolites (the highest Zr and Y, the lowest Sr contellts). On mosi. of the diagrams these rhyolites plot on a distinct field. This could be a proof for their different. age and/or different source and genetic processes.

In the 'southern alignment' the rhyolites from Turcoaia aud Cîrjelari have similar trace element, dist.ributions while the Camena rhyolites are significantly different hy their high Rh cont,ents, the maximum vailles reported for the rhyolites of Northern Dobrogea.

The BR§punar rhyolite differs from hoth the 'southern alignment' rocks and the other rhyolitcs of Nort.hern Dobrogea throllgh its alkaline character, REE distribut.ion, the lowest. Rb, high Nb, t.he highest. Sr vailles. These features point. to sJightly different petrogenetic processcs than for the ot.her rhyolites and possihly a (Wferent sour,.e. The BR§punar rhyolite-associated with basalts - is t.he only occurl'ence. with a demonstrat.ed J urassic aţ" I '\inuneridgian-Oxfordian). The generalisation of t.his age for the other rhyolites with c1early different ch,. _lcal features is highlydoubtful.

Compared to the Paleozoic and Mesozoic rhyolitcs of Northern Dobrogea, the Paleozoic t.rachytf'S of the Tulcea Unit show a c1early different trace element distribution (sec aII the diagrams) suggcsting a difTt'rent origin.

34

Occurrences(groll ps) Măcin ConslIl Niculitcl Tulcea Deposit Unit Unit Unit Unit.

IIIB S,V IA P,E

IB S

II E,P

+ E,P,S IIlA E,P,S

ICI H,S

IC2 H,S,E,P

IC3 E.P

IC3 P,E

1. SEGHEDI et al.

Table Synthesis of petrogeneti<: dat.a on rhyolites

Acid Associated magmatites magmatites Age

Type Ser ies Type Series T A B Th pz R CA pz(C)

R,D CA B,D T,h TI

_2

R CA B,D Th T I - 2

B,D Tit T l -3

R,RD CA B,D Tit T l - 2

R CA-A G CA-A T!J(?)

R CA-A G CA-A TjJ(?)

H CA-A CA-A TjJ(?)

Ha A B Th h(Ox-Krn)

.. Oeposlt: V, volcamc; S, subvolcamc, E, effusl\'e, P, PYlOclasl.Jc, H, hypabyssal. Rock type: R, rhyolite;

TI, dacile; RO, l'hyndacite: Ra, a1kalille I'hyolite; B, basalt; 0, dolel'ite; G, granite; T, trachyte.

4. Petrogellesis

Enough geological aud geochemical data are now available to allow a petrogeuetic approach based on the presented petrochemica:l information. ParticlIlarly the trace element. dist.ribut.ion is relevant. to interpretations concerning t.he source of the rhyolit.e generat.ing magmas and the different. processes of magma evolution. Chemical data also enable liS to reconstl'uct t.he geotectonic setting in which t.hese magmas were generated and evolved.

Both major and mainly trace element geochemistry of the studied rocks suggest. the crustal origin of magmas which generated most of the acid igneous rocks of Northern Dobrogea. Arguments for such an origin are the low Sr and high Rb, Y, Yb, Nb, U, Th contents, the high REE sum, the low Fe203, FeO , MgO, MnO, Ni, Co, Cr, Se, V contents of rhyolites. Their derivation from basaltic melts throllgh fractional crystaIIisation and/or crustal contamination seems to contradict most of the analytical and observation data. Such a model could not explain the absence of rocks with intermediate composit.ion. A deeper magma source is easily acceptable only for the Paleozoic rhyolites of the Tulcea Unit and the Jurassic Ba.ljpunar rhyolite showing higher Sr contents and a specific REE distribution. For both the Triassic and intracarapelitic rhyolites (with 50-60 ppm Sr, Fig. 4) the correlation between Sr contents and magma generat ion depth (Hart et aL, 1970) points to a shallow source situated in the lower crust. For similar reasons the 'southem alignment' rhyolites may be related to a more shallow source located in the upper crust. Certain high Sr contents in the Triassic rhyolites of the Măcin, Consul and Tulcea units, closely associated to basic dykes could be explained by some magma mixing processes with deep seated basaltic melts. The presence of partly assimilated basic xenoliths in some rhyolites (Seghedi et al., 1990) supports this interpretation. The distinct plotting fields of the analysed rock groups in the Zr-Rb

, diagram (Fig. 5) can be interpreted as the result of the crustal source heterogeneity or, alternatively, due t.o different level of the source within the crusi..

As known the three main processes which control the trace element. distribution wit.hin magma derived rocks are partial melting, fractional cryst.aliisation aud magma mixing (All;~gre, Minster, 1978).

The t.race element. (Sr, Rh, Zr. Nh, Y. Yh and REE) distribut.ion sug!'s!s t.11I' illlport:tll('l' of hotII partial rndting and fracl ional ('fyst.allisat lOII i\.'i Ill'! fogellctic I"'()('('sses for thc sllJdipd rocb. Tlwl't· ,11'(' siglllrirrllll

GEOCIIEMlSTRY OF RH'{OLlTES FROM NORTHERN DOBROGEA

4 and their associated rocks in Nort.hern Dobrogea

Geotedonic Source Pct.rogenesis environmellt Acid volcanics Associated Processes

rock!! Rin-oceanic crust (?) malltle mantie TP

Intracontinental lower TP+CF back-arc crllst.

Intracontinental lowcr mantIe TP+CF+(AM) incipient. rift. crust.

Intracontinental lower mant.le TP+CF+(AM) incipient rift crllst

Intracont.inental rift. mantie Intracont.inent.al lower mantie TP+CF+(AM)

incipient rift crust Cont.inental upper upper TP+CF intraplate crllst crust

Cont.incnt.a1 upper upper TP+CF intraplat.e crust crust

Continental uppcr TP+CF intraplat.e crust

Intracontincntal lower mantie (TP)+CF incipient rift crust

( upper mant.le) ...

Magmalle SCl'lCS: A, alkalme; CA, calc-alkahnc; Th, tholellllc. Petrogcnelie pl'oecsses: TP, parl.ial melt; CF, f"aclional crystallisation; AM, magma mixt.urc.

35

differences with respect to the relative participation of t.hese t\Vo processes for different rock groups. For the il1tracarapelitic rhyolites the fractional cryst.aIIisation seems to have a less import.aut. cOl1tribution to t.heir petrogenesis because these rocks lack significallt. negative Eu anomali{>s whieh obviously result from plagioclase and K-feldspar ext.raction from the melt (Hanson, 1978). This is valid for t.he group IIIA rocks, too. Within the other groups (IA, IC, II) there are either cases of strong Eu al10malies or cases of no Eu anomaly. This suggests that fractional erystallisation occurrcd ollly within magma volumes with long residcnce tiJlles in intermediate magma chambers.

The trace element dist.ribut.ion is controlled by both partial melting and fractional crystallisation in 'southern a1ignment' rhyolites, too. A differcllt. dcgrec of part.ial mclt.illg for the Turcoaia and Girjelari rhyolites and the Camena ones is aecollnted for by their observed tracI' clemcnt distributions. AII these roeks derived from the same Sr deplet.ed and Zr enriehed source but. through a lower degree of part.ial melting for the Camena rhyolites. This also accounts for the K20jNa20 ratio: the "alkalisodie" composition of the Turcoaia and Cîrjelari rhyolites is not neeessarily explained by their different sourec, as suggested by Ştefan and Roliu (in Nedelctt et al., H)88, unpubl. report). Some information eonccrning the occurrence of feldspar and quartz xenocryst.s as restites in these rocks (Const.antinescu et al., 1981, ullpubl. report, Ştefan, Ro§u, in Berbeleac et al., 1985, unpubl. report, Nedelcu et al., 198"'6, 1988, unpubl. report.s) are in good agreement with the origin of the generat ing magma.'> within a felsic crustal source.

The petrogenetically significant t.race element dist.ribution in the Ba§punar quarry rhyolite suggests its formation by fractional crystallisation of a magma generated by a low degree of partial melting of an upper mantie or lower crustal source.

In the case of the Paleozoic trachyt.e from the Tulcea Unit t.he fractionation of both light and heavy REE can be explained only by the implication of gamet and, possibly, orthopyroxene, in t.he partial melt.ing process. This involves great depths and high pressure condition for magma genesis (Hanson, 1978).

5. DiscussiollS

Using t.he diagrams for discriminat.ing between different geotectonic setting of granitic rocks (Pearce et al., 1984; Figs. (i, 7) OHI:' ronfirms the withinplat.e set.t.ing of t.he investigat.ed rocks which is also suggest.ed by thE'ir

36 1. SEGHEDI et al.

regional geological cnvironment. The pet.rochemical study of the basaltic rocks of the Niculiţel and Tulcea Units associat.ed wit.h rhyolites in the other units (Savu et al., 1986) too revealed t.he same geotectonic setting, i. e. wit.hinplat.e, for a large part of them. The Carboniferous evolution of the Măcin Unit characterized by the association of rapid continental sedime.nt.atioll with calc-alkaline acid extrusive activity favours the interpretation in terms of an active continent.al margin with back-arc features as the result of subduction processes related to Hercynian orogeny.

The Triassic magmatic activity of polarised - acid and basic - character ('bimodal volcanic' activity' ac­cording to Vlad, 1984, Savu et al., 1986) was gcnerated during the Triassic riftogenesis. According to the geodynamic model proposed by' Liu (1980) riftogenesis occurs due .to the action of a 'thermal dome' situated in the mantIe generating magmatic processes and 'horst and graben' structures. A similar model may account for the riftogenesis and related magmatic activit.y in Northern Dobrogea as it has already been mentioned (Vlad , 1978, 1984, Savu et al., 1985, 1986). The ascent of a 'thermal dome' accounts for the almost simulta­neolls generation of basaltic magmas in t.he upper mant.le and of acid magmas by crustal anatexis, resulting in spatially and temporally associat.ed basic and acid eruptive arcas in agreement with the model proposed by Eichelberger (1978). A direct correlat.ioll between tlie amount of rhyolites generated by 'bimodal vo\canism' and the continental character of the crust. which supplies the magmas is suggested by this model. This seems to apply to Northern Dobrogea too, when cOllsidering t.he nllmerous rhyolit.e occurrences assigned t-o Măcin aud Consul Units by comparison to 1\tlcca Unit and thcir absence from .Nic;ulitel Unit.

The geotectonic significance of the Triassic/.J IIrassic rhyolit.es occurring in t.he area of t.he "southern align­menI," result.s from lhcir cak-alkalinc rhemistry, thc crustal sOllrce of magmas (with 87Sr/86Sr=0.707-0.708, Pop et aL, 1985), their continental (wit.hinplatc) locat.ion and their association with rogenetic granites. AII these features account. for crustal anat.exis initiatcd in fOlltincntal (withinplat.e) environment . Its causes are difficult to state according t.o the availablc data. However, t.aking into account. the small amounts of acid and basic igneous rocks generatcd in an incipient riftogencsis duriug the li pper Jurassic , as previously suggested (Grădinaru, 1981), a ' t.hermal dome' might. be supposed in the upper mantIe.

The occurrenc(")f ablludant illt.rusions II1ight be due to the act.ion of the 'thermal dome' during a compres­sion al tectouic stage. The variatiou of the relative amounts of rhyolites and grallites along the alignmellt scems to reflect the variat.iou of the comprcssional tectonic regime.

Table 4 is a synthetic representation of the sequence, evolution aud geotectonic background of the rhyolitic rocks from Nort.hern Dobrogea.

Acknowledgements We are indebt;ed to our colleague Antoaneta Seghedi for her sp.ecial help in elaborating t,he present study,

for the informationiupplied aud for fruitful discussions about the geology of Northern Dobrogea. The authors are also grateflll to dr. Elena Mirăuţă and Liviu Nedclcu for their kinduess in offering us thin

sections of numerous rhyolit.e samples, as well as for IIscful discussions.

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I

GEOCHEMISTRY OF RHYOLITES FROM NORTHERN DOBROGEA 37

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I

38 1. SEGHEDI et al.

, Uricariu V. (1!)85) Met.alllorphic Ieislury of Ihe UZUIll Hair Fornmt.ion. D. 8. In-il. Geol. Geofiz. , LXIX/l, p. 279-282, BUCUff~~ti. , Oaie G. (19S{j) Fonna~iul!eiL dl~ Campdit (Dohroj.\ciI de Nord): faciesuri 'îi sl,ruduei sedimentare. D. S. Inst. Oeol. Geofiz., 70-71/-1. p. 1 ~1:17, Bucure§ti. , Seghedi 1., Szakilcs A., Oai(~ G. (1 !lS7) Itdationschip helwcen sedilllenlation and vokanislII during deposition

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BlIcure!jti. (1984) Triassic milleralizat,ion in North Dobrogea (Romania), Rac/o!>; G('Oill,~liIlI/(/, .17, J>. 137·· 14:J, Bcograd.

Willchestcr J. A., Floyd D. A. (1977) Geochcmical discrirninal,ioll of different. milgmil scrie;; and t.heir differentiation producl.s nsing immobile demenl,s. Chem. (;eol., 20, p. 325- 3H,

Received: May 19, 1.988

Accepted: Junt' 6, 1988

P''esenled al tlle .~cicli/ific .~~·ssion of the II/stitute of Gwlog" alld GeolJI&ysics:

DecembH 9, 1988.

I 5EGHEDI e-t al. GE'ochimistry of Rhio(ites from Northern Dobrogea

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o

LU

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