I

Rom. J. Petrology, 1994, 76, p. 77-84

PETROLOGICAL SIGNIFICANCE OF REE, U, Th, K, Rb, Sr, Hf AND OF THE 87Sr / 86Sr IN THE BASALTIC COMPLEX OF THE MUREŞ ZONE

Haralambie SAVU, Maria STOIAN, Ion TIEPAC, Gabriela GRABARI

Institutul Gcologic al României. Str. Caranscbc§ nr. 1, 78344 Bucurc§ti 32

Gheorghe POPESCU

IFIN-Măgurele

Key words: Basalts. Rare earths. Trace clemellt". '",:,-,/" ' '-'., \IIII"~ ZOIle'. Apuseni Mountains.

Abstract: The Mureş Zone is an Alpine ophiolit.ic slItllrc \Vith a complex evolution , laying in the sout.h and southeast of the Apuseni MOllntains aud in t.he north of the Banat. The ophiolitic rocks obelucted from the Mure§ Ocean in the axial zoue of t.he suture belong to the tirst two complexes of an ocean crust: the pillow lava basaltic complex (O!) anel the sheeteel dyke complex (02 ). This paper presents the distribution of REE, U, Th , K, Rb, Sr, Hf and the values of the Rb/Sr and 87Sr/86Sr ratios in the basaltic rocks of the tirst ophiolitic complex. These rocks have been sampled in the Cuiaş-Cillngani-Zam area. They formed an abyssal plain on the Mure§ Ocean floor, sitllated sou th-east of the median ridge of the ocean, whosc area is nowadays marked by the sheeted dykc complex. The values of the 87Sr/86Sr ratio (average value 0.702), the low contellts of Ti, Y, Ba and Rb, the chonclrite normalized REE di~tribut.ion and the values of the ratios between these element.s poinl. out clearly that th e basaltic complex (O}) developed, as the sheeted dyke complex, in a normal-type seg ment (N-type MORB) of the Mureş Ocean median ridge. In this respect therc are noteworthy the values of the Ba/La, (Ce/Yb)N, (Ce/Sm)N, and especially (La/Ce) N rat.ios, alld the diagrams in Figures 2, 3 alld 4. The rocks of the basaltic co mplex and those of the ~heeteel dyke complex are cO lI sanguineous. They resulted from the tholeiitic magma coming from the mantie, as showlI by the REE patterns chondrite normalized and by the common Tb negative anomaly. The basaltic CO lII plex originates in the lavas erupteel from the basalts anei normal elolerites elykes, which are prevailing in the sheeted dyke complex.

Introduction

The distribllt.iol1 of REE, raelioactive elements anei of other trace elements in tire Alpinc ophiolites from the ocean crust slab [rom the Mure§ Zone, which repre­sents an Alpine ophiolitic sut ur(' with a complex evolll­tion, has been dealt with by several researchers in their papers. Special attelltion has been given to the REE distribution in the sheeted-dykes comp lex (02 ) (Savu, Stoian, 1988). Lately, these elements have been also ana lysed in the Trascău Mt.s ophiol ites, rocks coming from the two ophiolitic complexes known iti the Mure§ Zone (Savu, Stoian, 1991).

For a global image of the geochemist ry of the l\1ure§ Zone ophiolites \Ve considered necessary a systcmatic study on the distribution of the abo\"e-menl ioned ele-

ments also in the ocean floor basalt complex (O d from this zone of ophiolitic suture.

Distribution of the basaltic complex- p etrographical aud petrochemical

aspccts

The ocean floor ba.'lalt complex (Ot}, a constituent of t.he ocean crust slab obducted in the M ure§ Zone, is of Liassic age (180 Ma - Herz et al., 1974), as the sheeted dyke complex (0 2 ), It. occurs in the Drocea Mts to the southeast and northeast of the sheeted dyke complex, which corresponds fo the median ridge arca of the Mureş Ocean. The strudures of the ocean crust slab are diagonal versus the trend of the M ureş ophi­olitic suture (Fig. 1). In ill(' southeastem part of

I

78

the sheeted-dyke complex, basalts probably formed ·and ahyssal plain on the Mureş flool'. The area ly­ing bd,wecil Cuia.~, Ciungani and ham represents tiI(' wid('st ;\1'('<1 of has;dh in t IJI' :\llIn''l ZOl1e (Fig;. 1).

1· i!,-.I SI I"el",..' of 11\1' ophi"lil i, rock, n"'gasl,," "b<l"I"("c! in

th!' '\Iurq ophiolilic ""ture arl'a (l\lurq Zone) am! its locali"/l

",'rSlIS the suture trend. 1, arca of the sheeted-dykE' complex

(meclian ridge of the Murc§ Ocean); 2, basaltic complex al'ca

(abyssal plain) with piJIow la\'a basaltic fiows; 3, approximate

trend of th,~ ophioli tic sut lIrc in this sf'gment.

East of the Vaţa-Zam lineament., whirh 11l:uks the boundary betwef'n the Liassic ophiolitic rocks, the ba'ialtic complex respeetively, and the Late Kimme­rian island arc \olcanics (.h Kd, basaltic ophiolites al80 oceur from under volcanics in t!le erosion inliers at Visea, Luncoi and Almaşu l\Iare in the l\.Ietaliferi

Mts, as well as in the Trasdiu :\1ts (SavlJ, 1983; SavlI,

1991 ). TIH' rocks of the ha><altic complex ar0 represented

by COttHllOn basalts. amygdaloidal basalts. anall1esites and spilites, in associatioll with rare intercalat ions of tachylites, volcanic breecias and agglollwrales, as well

as dolerite sills. Rodie" of gabbroic and llltnunafic roeks are also f'l1lplaced in t IIc eompl<,x.

The ]wtrography of t!le hasaltic rocks has thor­oughl) been stuelif'd by Sant ( 19(2), Sani d al. (1970, 1989). Thes(' papel'S are accompanieel by det aileel maps which collld be conslllLed for thc geologieal struc­ture. The basaltic rocks show an int<,rsert'll struc­ture specific to the tholeiitic basalts . Thry consis! of a network formeel of basic plagioclas<, laths (An 50-70); iLs mcshes include crystals of augilf', magnetite, and possibly pyrites , 'IS wP]1 as \·okan ic glass devit ri­fied anei partly chloritized undf'r conditions of thf' hy­drotllf'rmal mf'tamorphislJl on t.Iw \hIlT~ OCf'an noor, in lIw rpidote amphiholite, gr('0nschi:-;1 and 7,('olitc fa­ril's (Si1VU, IDG7; ('ol(,IlHtn, 1977). In spilit ... ". tlw pla­gioclase lat hs arc alhitir (An 1) ;"Iild aug;l! p is chl(Jrt­

tizrd, conromilalltly witlt tllt' int(>r;;1 it ial glas", il pro­

cess wllich yield(>d ll'l!('oxen(' and imll lIydrated oxides. Tachylliles are gla:-,~y !"Oeh t!pvitrifiul and part ly c1do-

Il. S'A VU ei al.

ritized within which plagioclase crystals are very r'lrely found. Dolerites display anei ophitic or sllbophitic strllcLure ami consist of ha.:;ic plagioclase laths (An 50-(iO), nystals of augite, oftCJt poikililic , anei rnagnetitf' "lId/or pyrite.

Table 1 Avecage chemical compof'itioH ('14 allalyses) of the rocks of the basaltic complex (according to Sa\'ll ct al., J994)

Oxides/ Elements Limit" Averagc Si02 (%) 39 .. J 1 5~.22 47.00

Ti02 O.tU ~.74 0.54

AI 2 0 3 13 .1,1 lti.56 14.i9

F'<~2 0 3 I.ti.) 1<.24 5.15

FeO 1.1~ 1 ~.79 '1.96 MgO :3./7 9.74 6.25

CaO 4 .. ')2 IG.12 IO.Oi

Na2 0 1. 7!) 6.20 3.1î4

K2 0 0,02 2. i ·l 0.5·1

P2 0 S 0.08 0.2!j 0.17

FeOt/MgO 0.50 3.·11 j n Ni (ppm) 2.00 210.()O 8·1. J I

Co 3.50 50 ()O 2!U:l

C'r 1.00 ~ 570.00 21fi.O{)

V :30.00 1h() 00 212.()()

Sc 15.0() ·1 '2 O() 10.79

Y lii .UO ()ii.O(l .!h 2')

Yb 1.2() Ij. C)O .l.G,)

Zr 4U.00 ·2 .!O.()O ,~:3. 7:;

Da 10.00 21:>.1)0 -) -.) ( _ .. )-Sr 65.00 (iOO.OO 221.25

Pb 2.00 l!<.OO 2.52

Cu 2.00 1 UO.()O 3642 Ca 9.00 22.00 14 .32

ZII 30.00 - 70.0f) 41.22

Aecording to Savu et al. (1991), thc rocks of lhe basaltie complex constitllte, he~ide those of lhe

sheeted-dyke complex, a t holei it ic maglll<1 tic s~'ries.

The chemieal composilioll (l'ab. 1) indicates COlll­

mon basalts, on the basis of the variation lil1lits anei

the Si02 3\if'rage vaJues. The Na20 con!.enls !tigher than 2.50 per cent a value typical of the t holei­itic hasalts (l\'fiyashiro, 1975) - point out IlIe spilite prcsence. The rock" with higher Ti02 anei V con­tents PllIphasize t he tendellcy of differentiation of the parental magma to the sep,Hatioll of a [i>rrogabbl'Oie (f .. rrohasaltic) llla)!;ltla (SavII"t al., UlM; Savu, Stoian, 1 9~8), 1 hrrefo]"(' I he iron cOllc,'nl raI ion (rell1l0T, 1929), alld impliCitly the COJlCl'tlt rat ion of tll(' gt'ochemically relat('d ell'f1wtlts. ~lIch il!' Ti aud \. 1I0W(,\"Pf, tlt(, differ­

Pili ialioll proces,.; Wilc. qllile 11'( ak a,., IlIdicakd 011 Lhe

diagralll ill Figur(' 2. \\"11(')"(' t 1](' rock;; of tIlP basaltic rOlllple:>. plot in it fi<'ld nloll!?, 11](> lin(' rrprf';;ellf ing: t he

DISTRIBUTlON OF REE AND TRACE ELEMENTS IN BASALTS 79

evolution of the basalts from the mid-ocean ridge seg­ments of normal or "N-type MORB". The tendency of the tholeiitic magma to be enriched in N a20 is quite obvious, as proved by the Na20 contents of about 6 per cent, a value quite clase ta that of S01111' sileeted dykl's.

4

3.5

3

2.5 o ;;.. N 2 O

t= 15

o 10 20 30 40

Yppm 50 60

Fig. '2 liO.-\" diagram (afler I'"rfit el al.. I!I~()). 1i/Y=;,0'2 , evollit iOIl lill<" of the ocean island-typf' vokattislll in ((awaii; E-type I\IORB. anomalous-typc basa!(,s line; N-type MORB,

normal-type tholeiitic magma evolllt.ioll line; a, basalt fields in

the Mure§ Zone.

The contents in other tracI' elements (Tab. 1) are also specific to the ocean floor t.holeiitic basalts, as indicated by the low Ba vailles. The value lower t hali 2 of the FeOt/MgO ratia points out this featme.

Rb, Sr distribution aud valuc of the 8iSr/86Sr ratio

The sall1ples of basalt.ic rocks analysed for Rb, Sr and other trace elements in the present paper (Ta bs . 2 and 3) have beell taken off mostly from the widl'st area of t.he basaltic complex, bcLwccn Cuia~, Ciungani anei Zam. Some basaIts originatr in the Visca inlier.

Rb and Sr content.s havc Iwen detl'fmi !led by Enl'rgy Dispersivl' X-ray Analysis. In hasaltic rocks Rb varie,; 1)('1\\'('1'11 5~6 and 16.1 ppm t.lle lo",cst vil.l­Uf'S Iwing oht ain('eI from lhl' hasil.lts in pillo", lava facil''; al Toc. ",I;n, ' Ifl'rz et CII. (ID7 i l) also deterrllilH'd la\\" [(h \·alul's. SI' prl',,<'J1h lo\\' COlltplltS. too. specific 10

tiu' ocean-flonr ha"alt~. 'fhis chal'i\cter is al"o p;i\'('11 by

the low value of the Rb/Sr ratia, which shows that the rocks of the basaltic complex, as well as those of the shected-dyke complex, originate in a magma formed in the mantie. The SI' ave rage value (187 pptn) poillts out, according to Hart's et al. diagl'am (1970. Fig. 4), that magma was formed at a depth of 125 km.

The conclusion is c1early evidencecl by t.11c quite low values of the 87Sr/86Sr ratia, ave rage valul' 0.702 ppm. Most of the values obtained are identical with those cletermined by IIerz et al. (1974)' on hasaltic rocks from the same complex; a similar sit lIatioll is that of the rocks from Toc, salllplecl from tbe sallle largc out­crop of pillow lava basalts an the DN 7 llighway. fIart (1971), O'Niolls el al. (197Î) and Cohell et al. (1980) pointed oul that the low valucs of the h7Sr/86Sr ra­tio are typical of tlJe ocean !loor hasalts. WooJ el al. (1979) mentioncd t hat !Il(> \'alul' 0.702 is specific lo N-type J\lORI3. Mosl of the data obtaineJ by liS are situated below the valul' 0.703, cOllsidered as typical MORB value. One of the \'alues is equal to it ano only two of them are higher (Tab. 2).

REE distribution ami petrological significance

The REE contents of the basaltic complex have been det.ermined by neutron activation. exccpt rOl' Yb which has becn analysed by C.Udrescll (in Savu et aL, 1994), using the emissioll spcctrography method. The data obtained are rendered in Table 3, in which the rocks allalysed are separated into 3 groups arteI' their Si02

contcnl , which varies between 39 aIld 52 per cent. The R EE conteIlts dctermined are similar with those

from normal basa It s of t11e sheeted-dyke complex in the Mure§ Zone (Savu, Stoian. 1988). formed in the same medi<ln ridge of the J\.lllrc~ Ocean. In the basaIt and shceted-dyke eOll1pl('x, t!](' UREE content. is lower in comparisoll with the LHEE content . which represents a characl.eristics of fhe ocean floor basalts series. The smaller contrnls of lTREE versus LREE are also em­phasized by t!te ratio of the total of the two groups of elements, which in tlte t1uee groups of basalts vary be­t.weell 4.06 'Ind 5.79 ppm (general average value 4.98). The difrerence between LREE and HREE is higher in case of t!te leas!. differentiated basalts from the first group, in wltich Si0 2 is lower than 48 per cent.

REE averages in basalts are clase to those presented by \Vemer (1984) for t.he ocean ridge tholeiites. As in case of the sheded-dykl' complex. the1'(" is a difference of lhe Tb CO!lt(,1l1 in basal1.s, too (Tab. 3). The val­ues al'p JO"'l'r and. eonseqllf'ntly, this p!elll("llt sltows a qllite ol)\'iou" llpp;alivp '111omaly (rig. :l). In out' ori­niol! tlw allolllaly J~ t Il<' t'('slllt of tiI<' partlal llH'lt ing of tllt' ,;Ollrc(' frolll \ 1](' lllilllll,· , '1" il W;IS ;11,,0 slIgp;f',;ter! I,y l1oLl.c:allll ('\ ill. (Jq,<J) ",ho COJl"id<'ri'd thal Ilie rrelc­

I iUllal ny~t ;tlliz;J1 iOIl or III<' P;H(·IIt.al \ liniei II ir magma

80

also participated in the process. As the differentia­tion process was weak in the basaltic complex rocks, as shown by the Si02 variation, aII the yielded rocks correspond to basalts. Conseqllently, the possibility that the strong Tb anomaly shollld have been deter­mined by the differentiation process is out of qllestion. A slighl tendency of Tb to form negative anomalies is also observed in case of the Pacific Ocean basalts (site 482 and 428) mentioned by Sallnders (1984), as examples of the N-type MORB tholeiites.

H . SA VU et al.

Ocean median ridge. In the field they occur ollly in the sheeted-dyke complex area or in associatioll with the gabbro bodies.

As already mentioned, there is a general trend ~f in­crease of the REE content, from basalts with a Si02 content lower than 48 per cent, to those with a Si02 content higher than 49.5 per cent; the only exception is that within the intermediary basalt group there is a decrease of the Ce and Sm contents as against the first grollp, althollgh in the third group (Si02>49.5 per

Table 2 Rb, Sr (ppm) coutents aud the values of the Rb/Sr and 87Sr/86Sr ratios

in the basaltic rocks

No. Rock Locality Rb Sr Rb/Sr " Sr/""Sr 1 13 asalt Toc (DN7)"t- 8.79 272 0.032 0.701 2 Hyalobasalt Toc (DN7) 5.86 162 0.036 0.702 3 Basalt Zam (quarry) 46.4 173 0.268 0.703 4 Basalt Zam (quarry) 13.4 138 0.097 0.704 5 Basalt Ro§ia Nouă 23.3 258 0.090 0.702 6 Tachylite Corbe§ti 14.5 241 0.060 0.702 7 Basalt Zam (quarry) 23.6 131 0.180 0.704 8 Basalt Almă§el 14.1 120 0.117 0.700

+ DN7 lIIdlcaJes NatIOnal Hll!;hway 110. 7, namely Bucure§tl- Arad hlghway

.... b

-'-'- " ..::< ...................... :::::;:.;:,..... .. ~: .... ... ~ 10 8

6 5 ~~--.-~--.-~--~-.--~-.--------------~

Le Ce Pr Nd Pm Sm Eu Gd .Tb Yb

I :i ,l!, .:~ ('IJolldritc 1JIII'IIlidi/l' d HI·:I< piLll('rJ1S in I)(t~alli( rtlt k .....

a. basalts wilil ~i()2<·I~ 'A: 1).1,;,,,"1., wilil Si02 belwepn 48

ami 4H.;; '10: c , IJiLSalts wil il Si02 >49.5 %.

Due to the slightly differentiation of the ·basalLic magma, the REE differellces bet.ween the basalts of the three grollps are qllite small (Tah. :n. Neverthe­less, a tendency of increase of the RE E contents COIl­

comitantly with the Si0 2 increase is clearly observed on the diagram in Figllre ;~, which presents the chon­drite normalized REE patterns (Nakamura, 1974), of the basalts from the tluee petrographic grollps. All the patterns range within the normalized vailles of 10 and 20, except for Tb which presellts much more lower normalized values. In this field plot the N-type MORB basalt patterns of the sheeted-dyke complex, which shows that the basalt complex includes only lavas cor­responding to basalt dykes. It is likely that. the basic and acid-albitic differentiates of the tholciitic magllJa remain al. depth in the axial zone of thc lV1l1rc~

""111) the behaviour of the two elements is again 110r­Illal.

According to their olltline, the patterns of the three l!,I'OlipS of basalts - with the common Tb anomaly ",Itich proves their consanguinil.y - are almost identical with the rock patterns from the sheeted-dyke complex in the Mure~ Zone (Savll, Stoian, 1988, Fig. 3). SlIch aspects point to the common origin of the t.wo ophi­oIi tic complexes - their formation in the median ridge of the Mure~ Ocean, of JlIrassic age.

. The (Ce/Yb)N - (Ce)N 2 diagram in Figllre 4, sim­plifted according to Saunders (1984), shows that. more than 70 per cent of the rocks of the basaltic complex in the Mure§ Zone plot in the basalts fieId from the median oceanic rielge segments of N-type MORB, ex­cept for a fe,," ones which raII olltsiJe of this field. This position of the ba'lalts on the diagram shows that, as on the diagrams in Figllres 2 and 3, the median ridge segment of the I\'IlIre~ Ocean , in whieh the two ophi­olitic complexes were formed , is of N-type similar 10 t.hat emphasized by Schilling (197.5). This conclusion is basf'd OII the average values of the ratios presenteJ in Tahl<' 3 . It is notable the smaller vallles of the Ba/La rat.io (2.99--4.11), (Ce/Yb)N ratio (0.90- 1.14) anei (Ce/SIll)N ratio (0.98- 1.20). According 1,0 Saun-

I

DISTRrBUTION OF REE AND TRACE ELEMENTS IN BASALTS 81

Table 3 REE contents in basaltic rocks

No. Rock Locality La Ce Sm Eu Tb Yb

a) Rocks with Si0 2 < 18 % 1 Basalt Visca 3.0 8 1.8 0.87 0.37 1.0 2 Baf'alt. ViRca 3.0 8 2.4 0.71 0.54 3.0

3 Basalt Ro§ia Nouă 6.0 13 3.0 0.90 0.38 -4 Dolerite Zam (quarry) 2.7 15 2.6 0.55 0.52 2.1 5 Rasalt Zam (quarry) 3.5 14 2.7 0.76 - 3 .6

6 Basalt Pietri§ 3.1 14 1.9 0.35 - 1.1 7 Basalt Almă§el 5.2 12 5.8' 0.72 0.86 1.6

'8 Basalt Piet.ri§ 4.6 14 1.8 0.80 0.37 2. 1 9 Basalt Pietri§ 3.6 10 1.8 0.68 0.39 3.0

10 Dole rite Pietri§ 4.6 14 2.9 0.67 - 1.1 11 Hyalobasalt Corbe§ti 4.6 10 2.2 0.61 - J.]

12 Tachylite Corbc§t.i 3.8 10 4.9 OAfi - ! .2

13 Babalt Toc IJN7 1.3 10 2.1 0.66 - IA

14 Basalt Ho~ia ;-Jouă 2.3 10 2.2 0.46 0.40 2.9 Averap;es 3 .66 11.57 2.94 0.66 0.42 2.21

LREE/IlREE = 5,79 ; La/'t'h = 1.G6; (La/SIl1)~ = 0.87; (Cc/Yb)N = 1.12; Ba/La = 4.11 ; (La/Ce)N = 0.89: (Cc/Sm)N = 0.98; La/Th = 3.02

b) Hocks witll Si0 2 bet weclI 48 and 49.5 % 15 Hyalobasalt Toc D:--n .'i.0 8 2.8 0.70 0.30 2.2 16 Basalt Toc D;-J7 4.9 10 2.3 0.71 - 2.8

17 Basalt Ro§ia Nouă 4.0 12 1.6 0.75 0.4-1

18 l3asalt. Visca '1 .0 9 3.8 1.18 0.12 7.0 19 AllaJI\('site Vata Vallcy 2.0 12 1.7 0.69 .- 2.8 20 Spilil(' Va~a \'allcy S.O 15 1.9 0.90 0.42 3.5

:n Ih .. 1 • .1 .,-. .11 ('!il. ii Ile§t i 3.0 11 2.2 0.91 0.48 3.4 0)) 1 ~ - ,1' ( ',i /ăuc§ti 4.0 12 2.1 0.71 0.43 3.5

\ \ 4 I d l!.' ... 1.99 11.12 2.30 0.83 0.11 3.60 ~:-- - .

L!{ 1· 1 j II \{ 1. 1, .:= LO G; La/Yb = 1.2i; (La/Sm)N = 1.11 ; (Ce/Yb)N = 0.90; Ilil/Ll = :1.11 ; (La/<'c)N = l.OO ; (C'cjSm)N = 1.20: La/Th = 8.2

c.) Rocks with Si02 > 49 .5 % 23 Dolc rite Căzălle§ti :!.O 12 1.a 0.70 0.10 2.9 24 Basalt. Vata Valle)' J.O 9 2.6 0.85 0.34 3.5 25 Dolerite Vata Valley :!.ll 8 2.0 0.80 0.32 2.5 26 Babalt. Vata Valley 1.0 10 2.9 0.93 0.13 4.7 27 Basalt Ro~ia Nouă 5.0 6 4.6 1.20 0.58 -

28 Basalt Ho§ia Nouă 8.0 18 4.4 1.05 0.18 -

29 Basalt Zam (q u arry) 4.5 20 2.7 0.86 0.58 4.4

30 Basalt ~Iicăne§ti 5.3 19 .1.0 0.62 0.48 3.2

Averages 4.35 ] 2.75 3.19 0 .88 O.4S 3.02

LREE/HRt:E = 5.10; La/Yb = 1.64: (La/Sm)N = 0.95; (Ce/Yb)N = 1.14 ; Ba/La = 2.99; (La/Ce)N = 0.95 ; (Ce/Sm)N = 1.00; La/Th = 2.6

ders (984), the values of the (La/Sm)N ratio, smaller or equal to one , point out the N-typ(' l\rORB basalt charaders of the rocks. Therefore t /te I'ocks fl'om the basaltic and shpeted-dyke eomplexes arI' N-type tholei­ites. The general a\'erage value of this ratio is 0.97. It also differentiates the N-type MORB basaltic rocks in the Murc~ Zone from the basalts formeJ in a "plume ridge" (P-type MORB), 01' in other words basalts of

anomalous t-ype (E-tYIW l\10RI3) . This deliillilatioll is c/early showlI Otl the diagrall1 in Figlll'l' '2 .

Quite characteristir in t.his fPspecl i" I I li' \'alll{ (l ftlH' (La/Ce):" ratio which varie" bel WH' IJ tU)!) ;1I1t! 1 for a li the three hasalt groups in I /tE' i\ll"'{'~ ZOlle , \ ':.1 11, ':-;

typical of the N-type l\lORI3 ba:-;alb.

I

82

2 3 45

•

10 20 304050 100

(CelN

Fig. 4 - (Ce/Yb}N - (Ce)N diagram (after SauJldcr", 1!)8·1). a,

normal-type basalt field «N-t.ypc MORS) del imited accordillg

to the mentioned author's data.

Distribution of r adioactive elements aud of Hf

The U, Th and K contents in the rocks of the basaltic complex (OI) (Tab. 4) have been determined by nat­ural garnma spedrometry. They presellt low values characteristic of the ocean fl oor basalts, as shown by Savu et al. (1989) in the Va~a and Visca areas. Thus, U contents vary between 0.3 and l.5 ppm (averagc

H. SA VU et al.

value 0.85 ppm); Th contents range from 0.2 to 3.2 ppm (average value l.21 ppm). The values of the Th/ U ratio are also low (0.17-3.5), the average value being l. 44 ppm. AH the data are specific to the basalts from the median ocean ridge basalts (MORB).

Potassium displays very low values, ranging from 0.03 to 0.75 per cent. The lowest Gontents occur in spilites, rocks from which this element, was removed during the spilitization process. The average value of the K content in basaltir rocks is 0.33 per cent, specific ' of tTle abyssal tholeiites (Miyashiro, 1975) or ocean floor tholeiites.

Hafnium var ies in the study basalts between 0.6 and 2.5 ppm, with an ave rage value of l.35 ppm Hf. The Zr/Hf ratio, with an average value of 78, is also a good indicator of the origin of the ocean floor basalts of the complex O).

Con~lusions

The rocks of the basalLic complex of the Mure§ Zone occurred in the I\lure§ Ocean probably in an abyssal plain, southeast of the median ridge, marked nowa­days .by the sheeted-dyke complex. The values of the 87Sr/86Sr ratio (average value 0.702), the low contents of Ti, Y .. Ba, as well as the REE distribution and the values of the rat-ios of these elements, point out clearly that the basaltic complex OI developed, as the sheeted-dyke complex O2 , in a N-type MORB segment of the median rid.ge of the Mureş Ocean, Liassic in age. The values of the rat.ios Ba/La, (Ce/Yb)N, (Ce/Sm)N anei especiaIly (La/Ce)N are quite obvious in this re­spect. This tectonic setting of t.he basalts is rendered clearly on the diagrams in Figures 2, 3 and 4. The

Table 4 Contents (ppm) of radioelements aud of Rf in basaltic rocks +

No. Rock Locality U Th Th/U K% Rf Zr/Rf 1 Basalt Zam (quarry) 0.4 1.0 2.5 0.75 2.5 58.0

2 Dolerite Zam (quarry) 0.4 0.8 2.0 0.46 - -

3 Basalt. (spi lite) Zam (quarry) 0.5 0.7 1.4 0.03 1.0 100.0 4 nasalt Pietriş 1.0 1.2 1.2 0.19 - -5 Basalt Almă.§el 1.2 0.2 0.17 0.06 1.1 145.0 6 Basalt Micăne§ti 1.2 1.9 1.6 0.37 1.3 80.0 7 nasal! (spilite) Pietri~ 0.5 0.2 0.4 0.03 1.7 52.9 8 Basalt. Pietri§ 1.3 1.0 0.8 0.55 0.6 125.0 9 Hyalobasalt Toc - 0.9 1.2 1.3 0.36 0.9 68.8

10 Dolerite with albite Pietri§ 0.9 3.2 3.5 0.03 1.4. 28.6 11 llyalobasal t Corbeşti l.l 1.4 1.3 0.55 - -12 Tachylite Corbe~ti 1.0 2.8 2.8 0.45 1.6 26.2 B Uasalt Toc DN7 1.5 2.0 1.3 0.34 0.8 50.0 l4 Basalt Toc DNî 0.6 0.1 0.7 0.12 0.6 12.5.0 1;; Basalt Ho§ia Nouă 0.:3 0.2 0.7 0.62 - .-

/\\,('rages 0.8;; 1.2J lA4 0.33 1.22 78.'2 , . + Zr ('o"t('nt~ llsed for til(' eak,ilatloll 01 the Zrjllf rallO were d e tenlll11ed b~' C. Udres('u

(ill Savll el al.. in pressj

I

DISTRIBUTION OF REE AND TRAGE ELEMENTS IN BASALTS 83

rocks of the basaltic complex are consanguineous with those of the sheeted-dyke complex. They have resulted from the tholeiitic magma coming from the mantie , a fact proved by the Tb common negative anomaly and by the similarity of the chondrite normalized REE patterns. The obtained data show that the basaltic complex is the result of the lavas erupted [rom the dykes of basalts and common dolerites of the sheeted­dyke complex.

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