L Rom. J. Pctrology, 1992; 75, p. 117-130

PETROLOGY, GEOCHEMISTRY AND ORIGIN OF THE LARAMIAN VOLCANICS OF THE MUREŞ COULOIR,

BETWEEN ZAM AND GURASADA

Harahmbie SAVU, Constanta UDRESCU Institutul de Geologie §i Geofizică. Str. Caransebe§ 1, 78344 Bucureşti 32.

Vasilica NEACŞU intreprinderea de Prospectiuni Geologice §i Geofizice. Str. Caransebe§ 1, 7834,1 Bucure§ti 32.

Maria STOIAN Institutul de Geologie §i Geofizică. Str. Caranscbe§ 1,78344 Bucureşti 31.

Key words: Volcanic rocks. Island ates. Magmas. Magmatic differentiation . Calc­a1kalie eomposition: Subduetion. Major elements. Miilor elelllents ... \.plIseni Mouulains - Laramian eruptive - Sîrbi-Lăpugiu-BlIlza awa.

Resume: Petro/ogie, geochimie el origine des ,mlcanites laram,e,mF,' du Couloir du Mureş, cntl'e Zam et Gurasada. Les vlllcanites laramiennes du Couloir du Mureş

representent le resllltat cI'un voJcanisme d'arc continent.al intern e des Carpalhes Me­ridionales, determine par la subduction de la plaque moesienne i'ttl-d es!5ous de la plaquE: transylvaine. Elles reposent sur un soubassement tectonise pol)' phasiquemen/', appar­tenant â. Ia par tie sud de la Zone de Mureş. Les roehes form ent line serie volcanique ealco-alcaline a. tendenees alcalines ou on separe trois groupes r~Î rographiques: (1) basaltes â. olivine et andesites basaltiqlles; (2) andesites et trachyandesites ; (3) roches leucoerates acides et a1ealines. Les roehes preselltent les caractcres geoehimiques des vul­eanites d'arc insulaire. Les roches basiques el eelles intenllt!diaires reprf:se nte nt ~orogel1ic high-K andesites", aspect qui souligne la. tendence alcalillc de la serie el les separe des vulcanites de l'arc volcanique exlerne qui manifestent des earactcristiques de "medium-I\ andesites". La presence des roches alcalines, lant dans le groupe andesitique qne dans celui leueoeratique, souligne aussi la consangllinite des vulcallites. Le magma patental s'est forme â. des profolldeurs de 200 km, dans le manteau surjacent, au plan de Bellioff, par la fusion du substratum cclogitiquc mctasomatise par des solutions emanees de la erou te oeeanique enfoneee par subduction, en voie d'etre transforlllee en cclogite. Le magma a eu line eomposition dioritique ii 58 % Si02 , mais il a ete fnrtement differentie, en resultant andesites, basaltes â. olivine el. roches acides et alcaline".

IntroductÎon

Along the Mure~ Couloir, cropping out be(,ween Gro~i and Sîrbi, on the northern margin of the Lăpllgiu posttectonic basin, the products of a Laramian continental arc volcanism are t.o be found vis-a-vis the Pietroasa­Tomeşti volcanics on the southern margin of the basin. Up till 1983 the mentioned rocks were coHsidered Neogene volcanics by different workcrs, e.g, Kadic (1906), Papiu (1954), Savu (1962), Peltz and Peltz (1965) , Ghj~u)escu, Borcoş (1966), Rădulescu . Borco§ (1968), Jallovici et al. (1969) etc,. who studied thern rrom different, points of view.

In 1983-l984, Popescu (in Borcoş et al. , 1986) established that the microfaulla of the sedimentary deposits associated to the "Neogene" volcanics of the Mure§ Couloir, al Sirbi, are of Uppermost Cretaceous-Paleocene age. At the same time , Savu el al. (1984, unpubl. report) obtained the radiometric age of 59.6±2.4 Ma on a biotite tuR" at Vica. On the basis of these da.ta Savu (1984) presented the volcanics as products of a. Laramian (banatitic) volcanism like those in the Rusca Montană Basin. In the years' 1984-1985 R.o~u (unpubl. reports) studied the Laramian volcanics in the out.skirts of the loeality of Gurasada with a view to e1aborating the

I

118 H. SAVUet al. ...

geologica.1 map seale 1:50000, sheet Gurasada (1986). In 1984 13oreo~ et al. (unpubl. report.) mentioned that in the Lăpugiu Basin both Paleogene and Neogene voleanic roeb are to bc found.

Although thc above-mentioncd papers presented lllllTlerous pet.rographical and petrochelllieal data on t.he volcanics of the Mureş Couloir, no modern, complex study has been c1aborat.ed up t.il! now. That is why the present paper gives a detailcd descriplion of the voleanie rocks Iying betweell Zalll, Boit! de Sus and (.jurasada, north of the Mureş (Pl. 1), which comprise a complete suite of the Laramiau voleanies in thc region. This study is based on the dat.a obt.ained in the per iod 1983- 1986 as a resull. of the surveys carri ed out. for t.hc elaboration of the geological map sca-Ie 1 :50 000, sheet Vorţa (Savu et, al., 1987, unpubl. repart), anel its correlation with the adjacent zones.

PrevolcRnic Bascmellt of the R.egim}

The Laramian voleanics overlie a prevokanic basement. comprising the whole Lăpugiu Basin. Tlie geological formations of t.his basement belong to two tectonic units: (1) Căpilna.~-Tcdlereu Unit. and (2) Tisa Unit (PI. 1).

1. Căpîlnaş-Techereu Unit (Lupu, 1975) is post-Neocomian in agc and bclongs to theMureş Zone. Its formations crop out from under the Laramian volcanics and Neozoic deposit.s ill the northern part of the regioll. Their hasement is represent.ed by a Liassic oc .. ~an crust reslllt.illg frorll Uw sp litting of the ophiolitic shect (megaslab) of t.he Mureş, obducted at the end of the Jurassic froJlI tbe \lul'cş Ou'an over the convergent plates, north and south of it (Savu, 1983). The ocean crust. basemelil is .'xposed alollg a llarrow strip between Glodghileşti and Boiu de Sus. It. represents a segment of the Glodghile§ti-Săli~ lioaril tectoni c rise occurring along a sllb-Hercynian OI' Laramian longitudinal fault. trending E-W , which put.s illta tectonic contact the Liassic ophiolites of the nappe ba.'Sement with the Barremian-Aptian depasits (Savu , lD83,1984 ; Savu el al.,1987). The ophiolitic bast'ment collsists of the ocean ftoor basalt complex (01) , usually in pillow lava fari es, representcd by hyalobasalts, anamesitcs, basalts - of ten amygdaloidal - and very rarely doleritcs.

The ophiolitic basement is unconformably overlain by postophiolitic Late Kimmmerian island ar!; volcanics (J 3-Crt) in association with jaspers and red argillitcs which pass laterally anel vertically to Stramberg reef limestones (h-Crl) and to Tithonian-Neocomian flysch formations. The erosioll period which probably suc­ceeded to the emplacement of thc Căpîlnaş-Techereu Nappe is followed lIncollformably by Barremian-Albian wildflysch deposits. They form sever al folds, south of the tectonic rise arca of the basemcnt ophiolites, anel an asymmetrical syncline consisting of Upper Apt.ian-Albian wildftyseh formations (Lupu, in Savu et aL, 1987, unpubL report), occurring unconformably in the north of the regioll, . approximately in the axial zone of the sOllthern Late Kimmerian islalld arc of the Mure~ Zone (Pl. 1). It is to be noled that thc Late Kimmerian voleanies IInconformably overlie the ocean crust basement like the Laramian volcanics at Boin ele Sus (PI. 1) and the Neogclle ones al. Obîrşia, north of the area represented OII (.he allncxed map. As the latter ones, they do 1I0t belong to t.he ocean erl.ist because dnrillg the manifestation of the Late Kimmerian island arc vokanism in t!le eugeosYllcline the ocean crust. was to bc obducted, blocks being broken from it, which were included as olistoliths in t.he .h-Crl flysch (Drocea Mt.s), OI' even in the Neokimmerian vo\canics in tlte Metaliferi Mountains (Savu, HJ84) . AII the mellt.ioned scries of island arc voleanics are posi.ophiolitic (Rocei el. al., 1980). Provided that hy definition t1w ophiolites represent obducted ocean crust (Penrose Conference. 1972; Nlesorian et aL , 19i3; Moskow Confercllce, 1974) the Lat.e Kimmerian voleanics, likc the ol.her ment.ioned voleanics, cannot be referred to this cat.egory.

The Căpilna.'?-Teehereu Unit and its southern autoehthon were affeeled by lat.er t.ectonic movements and sectiolleel by longitudinal faults strongly dipping northwards - e.g . that in the sOllth of th~ Glodghile~ti­Sălişt.ioara tedonic rise - a.., well a.., by t.hrust plalles, e.g. that which determined the thrust of the Tisa Unit over the Căpillla~ -- Techereu Unit.

2. Tisa Unit (Savu, in Andrei et al., 1987, unpubl. report) was probably determined by the sub-Hercynian Orogeny. It lies bctwcen Gro~i, Tisa aud Vărmaga, an alignment a long which t.he crystalline schists (retro­morphic phyllit,es aud amphibolites) of (.he Poian a RlIscă Mountains t,hrust from thc sout.h !-o the nort.h over t.he formations sit.llated on ti\(' Că.p'ilna:j-Techcrcu Unit. , t,he la.')t thrust. formation being the 13arremian-Aptian wildftysch deposits al Tisa, sout.h of (he l\fure~ (PI. 1). The thrust plane was affeeted by t.he regional crustal fracture pointed out. by Andrei and Cristescu (1966) betwecn Cuieş on thc .Mure~ and Recaş in the Banat, along about 45 km . The cont.act bel.wcen "Ite t.wo tectonic units is mostly masked by Laramian voleanics a.ml by recent. sediment.s (PI. 1, geologi ral scction) , rrom under which crop out in SOI11C places , like t.hat on the Şendreasca Val­ley, at Tisa, strongly lectonized crystalline schist.s (Peltz, Peltz, 19(5) thrust. over Barremiall-Apt.ian deposits (Savu, in .Andrei el al., IU87, lItlpubl. report).

LARAMIAN VOLCANICS OF THE MUIlEŞ COULOIR 119

This tectonic accident is marked by gravimetric anomalies OII whose basis Andrei el al. (1975) refel'red the formations on this alignment to the "Mure~ Cordillera". It. is obviolls t.hat the Tisa Nappe began its rising in the Cretaceous Sea as il cordillera which at present is eroeleel. From this Cret.aceous cordillera were probably torn off the crystalline schisls olislolith on the Toma (Dobîrle~ti) Brook and the microclille granite olistoliths insedimented in t.he Barremian·· Aptian depasits south of Căprioara (Savll, L966).

Distl'ibution of Volcanics and Theil' Petrography

Like the Pietroasa eruptive rocks '- on the southenl bordel' of the Lăpugiu Basin (Savu, 19(2) - the Laramian vo\canics in t.he Zam-Gurasada region belong to two eruption phases : (1) phase of mostly leucocrate rocks; (2) phase of intermediary and melallocrate rocks.

1. The eruptive rocks of the first phase are rarely found and are reprcsented especially by stratifieel tllffs, some of them with an ignimbritic character, situateu al, t.he base of t.he Laramian volcanies. Tlley oecur aL Gurasada, Tătărăşti and Vica (PI. 1) anei are represellted by leuccc rate hiotite tuffs with feldsp ar, rarely quartz, phenocryst.alloc\ast.s whose composition fI:scmblcs t,hat of rhyodacitcs, dacit.es, trachydacit.es , trachyles and bioti te andesites, as shown in Table 1 and on TAS diagram (Le Bas et al. , lP86) in Figllre 1. It is wort.h mentioning that since Kadic (1906) the leucocrate rocks in the M ureş Couloir have been considered tlS biotite andesites and trachytes. R.hyodacites and dacites are quite similar with t.he acid rocks described by Savu (1962) at Pietroasa anei Tomeşti. Trachytes are found as blocks within an agglomerate of biotit.e andesites on thc Tătăr~ti Valley. They consist. of a groundmass with trachytic texture includillg phenocrysts of potash fddspar anei biotit.e. It. is to be noted that potash feldspar phenocryst.s are rarely founel in tuffs of alkaline lellcocrate roeks; it indi cates that. K20 is mostly cont.ained in the glassy mass. Riotite andesites are grey leucoerate rocks consisti ng of a groundmass formed of oligocla.se microcrysts, within which ocellr phellocrysts of zoned alldesine-oligoc:lase plagioclase and biotite.

IS

13

11 . .. 'i9 <1-. :oc: • 7

i 3

37 41

Ultrabasic

45 49 I

4S Basic I

53 51 61 65 69

5'2 Int.rmediary 613 Acid

I I I I

\ \ \

73 17

Si02 wr/.

Fig. 1 - TAS diagram (Le Bas el. al., 198G) for t.he Larrunian volcanics of tlle r'v!ure~ CouJoil'

The biotite tuffs at Viea cOllsist of a vitrocrystal\oclastic mass which includes fragments of glassy rock with sinuous flow lines.The phenocryst.s are represented by oligoclase (An:?8) anei biot.it.e At Gurasada the leucocrate rock tuffs, partieularly the dacite ones, have been affected by bentonit.ization proee'ises, at the expense of t.heir groundmass secondary minerals being formed, such as montmorillonit.e and cristobalite, det.ermined by Vanghelie (I.G.G .) by X-ray analyses. For this reason t.he tuffs are exploited in qllarries.

..

120

No 1 2 3 4 5 Si02 50.78 52.90 56.82 57.20 57.24 Ti02 1.50 1.04 1.20 1.18 1.04 Ab0 3 15.24 13.92 16.46 19.11 16.06 FC203 4.33 2.63 3.16 3.63 3.05 FeO 5.37 5.67 3.49 3.39 3.57 MnO 0.17 0.15 0.15 0.17 0.17 MgO 6.18 9.76 3.58 2.90 4 .66 CaO 11.21 9.78 6.96 7.13 7.41 Na20 2.43 2.55 3.27 3.61 2.71 K20 1.53 0.98 3.41 1.50 3.3.5 P20 5 0.23 0.11 0.20 0.08 0.31 S 0.07 0.27 0.23 0.18 0.22 CO2 H2 O+ 1.05 0.75 0.64 0.78 0.69 H2 O-

Total 100.11 100.51 99 .57 100.86 100.48 Ni 17 21 19 7.5 24 Co J.t 25 20 15 22 Cr 100 260 4:0 1.5 110 V 300 100 210 160 190 Se 44 17 17 14 20 Y 21 12 26 19 23 Yb 2.8 1.3 2.1 2.3 2.0 La <30 <30 44 <30 38 Zr 110 75 210 105 190 Ba 280 300 600 360 600 Sr 440 720 700 560 480 Pb 7.5 Il 20 14 14 Cu 23 80 30 8.5 20 Ga 13 15 17 20 12 Sn 2 <2 3 <2 2.5

6 7 57.49 57 .60

1.16 1.02 16,41 16.51

2.61 2.30 4.02 4.44 0.16 0.17 4.08 4.17 7.24 7.13 2.80 2.87 2.86 2.i4 0.10 0.16 0.29 0.19

1.25 1.38

100,47 100.68 17 13 15 16 80 48

190 210 16 19 20 23

2.2 2.3 36 42

190 180 650 650 530 680

25 25 20 22 18 20

2.5 3

H. SAVU et al. ....

Table Chemieal eompositioJl of the Laramian

8 !) 10 11 12 57.77 58.15 58 . .37 58.38 58.42

0.84 1.14 1.18 0.88 0.86 16.11 16.31 li.31 17.11 16.81 4.28 3.63 6.34 2.71 3.99 2,42 3.:J6 0..10 3.76 2.15 0.17 0.16 0.08 0 .14 0.01 3.69 2.88 2.47 2,47 3.70 6.97 6.i.3 5.98 6.S9 6.69 2.67 2.82 3.25 3.28 3.12 3.55 2.97 3.64 2.35 2.20 0.33 0.16 0.32 0.15 0.10 0.25 0.25 0.25 0.23 0.27

1.53 1.19 1.19 1.43 2.08

100.58 99.75 100.78 99 .i/l 100.40 ]8 12 15 8.5 24 23 19 14 15 18 47 16 30 4.5 65

180 210 200 175 180 17 17 16 15 15 23 29 22 20 24

1.8 2.8 2.1 1.8 3.1 42 60 50 <30 <30

180 200 200 175 180 600 150 600 500· 500 600 720 680 620 720

18 2:J 16 12 15 19 30 22 12 23 13 20 li 16 14

3 <2 3 <2 <2

* The ana.lyscs in this table represent: basalt, l-Peh'e§ti; basaltic andesitc, 2-Runc§or; andt.'Sites, 6, 12,14, 15-Runc!jor;

5, 8-Glodghilc§ti; ll-Voia Vallcy; 17-Pctrc§ti Valley; trachyandesites , 4,7, 9, 16- HuIlCljol'; 3-Ch.ihu Swmnit; 10, 13-Măgura

Summ.it, lS .. Glodgh.ilc§ti; 20, 24-Gurasada quarry; l'hyodacite, 2S-Pil'iul Bisericii .

2. After this short extrusive episode with leucocrate rocks, the voleani c activity continues with volcanic breccias, agglomerates and stratified tuffs (Glodghileşti), and rarely lava flows. The volcanic products are represented by basalts, basaltic andesites, andesites and trachyandesites (Fig. 1), rocks constituting the main mass of the Laramian vokanics in the region (PI. 1). According to the t-ype of the phenocrysts, abOlit seven petrographic varieties can be distinguished among the second phase volcallics: olivine basalts, basaltic an­desi tes , plagiodase-pyroxene andesites, hornblcnde-pyroxene andesites, hornblende-pyroxclIe-plagiocln!ic an­desites, hornblende andesites, trachyandesites, and their tuffs.

Olivine basalts are rarcly fOUlld. They are aphanitic rocks consisting of a hyalopilit.ic. lip to pilot.axitic groundmass, represellted by microcrysts of plagioclase, rarely of allgite and maglletite (PI. II, Fig . 1), in which augite and oii viile micropheuocrysts, as well as idillgsit.e pseuJolllorphoses aftcr the la.t.t.er, are f1oat.ing. The augite mierophellocrysts (ct\Ng=430-480) are twinned aft.er (100). They are groupcd determin ing t.lw glomeroporphiritic texture of the rock. AlIgite appears as idiomorphic crystals, locally resorbed by tlle magma. Olivine microphenocryst.s are morI'! rare; they arc idiomorphic and are replacedby idingsite on fissurcs, OI'

entirely. A fine magnetit.e powder is spread in the wholc rock. Basaltic andesites can be separated into augite andesitcs and augite hypersthene andesit.es. Thcy :W'

porphyritic rocks consisting of a hyalophilitic groundmass, rcpresent.ed by plagioclase mierolit.es oriented parall .. 1 to the lava flow trend, within whidl augite and hyperst.hene phenocryst.s are to he found . 1'111' fir!;i an' polysinthetically twinned after (100) anei are grouped forming il. glorneroporphirit.ic t.ext(JJ'('. Tlw 0xtinctinn

an'gle (ct\Ng=450-550) shows that. IllI' llIineral is it Ti-allgite, with an hOlll' glass st.rllctlll·C . II is to 1)(' nokd tlll'

I

LARAMIAN VOLCANICS OF TllE MUREŞ COULOlR

1 the laramian vo\canics of the Mure§ Couloiro

13 14 15 16 17 18 19 20 21 22 23 24 25··

58.45 59.14 59.18 59.53 ,59.62 60.79 61.,50 63.16 63.34 65.47 67.47 68.,57 72.05 0.76 1.04 0.76 1.16 0.98 0.96 0.36 0.78 0.19 0.,54 0.i8 0.19 0.30

17.36 17.16 17.56 17.51 14.41 16.56 19.19 18.61 12.85 . 19.19 16 .92 12.72 11.55 1.91 2.99 3.47 4.46 2.15 3.46 4.14 1.81 2.13 1.84 1.24 3.75 3.94 2.62 2.57 1.12 2.95 ' 1.95 1.66 0.18 1.86 0.18 0.35 0.39 0.22 0.14 0.12 0.12 0.0,5 0.13 0.10 0.04 0.05 0.07 0.04 0.05 0.03 0.05 2.45 3.56 2.49 1.25 2.28 2.93 1.10 0.35 3.25 0.50 1.00 2.60 0.50 5.76 6.64 6.26 7.03 6.40 .'l.74 2.48 5.56 1.76 4.81 2.30 1.63 2.00 3.16 3.59 3.42 3.60 3.51 2.90 2.84 4.06 1.12 2.45 3.20 1.05 1.50 3.93 2.28 1.74 2.65 2.43 3.45 9.20 2.40 1.17 2.94 4.90 1.17 2.50 0.29 0.10 0.08 0.16 0.10 0.16 0.10 0.08 0 .02 0.15 0 .06 0.03 tr. · 0.29 0 .20 0.29 0.26 0.27 0.35 0.01 0.25 0.01 0.34 0.30 0.08 abs.

0.43 0.62 0.19 1.24 1.17 1.73 0.97 1.24 1.64 ~.04 0.64 5.19 1.69 2.22 4.75 3.93

6.59 5.66 1.50 99.68 100.61 99.67 99.75 99.47 100.89 99.98 100.26 99.85 100.46 100.79 100.20 99.85

7.5 21 5 16 7.5 21 9.5 6.5 2 14 . 5.5 2 11 13 10 9 13 14 2 5 2 4.5 3.5 2

6.5 44 1 54 4 75 17 2.5 2.5 95 2 4 150 145 120 250 180 135 54 120 8.5 80 14 17

12 13 12 20 13 14 8.5 8 5 8 3.5 7.5 21 16 17 24 19 23 22 15.5 18 12 12 18

2.0 2.3 2 .) 2.3 2.3 1.8 1.5 1.7 1.5 1.2 1.5 1.5 45 <30 <30 36 < 30 60 52 30 30 33 50 30

150 145 100 115 180 135 96 120 87 80 140 130 620 500 370 630 420 600 lIOO 530 440 380 920 380 650 620 600 550 470 430 250 730 85 360 580 75

23 20 9.5 21 15 22 21.5 6 24 18 23 20 14 31 5 22 10 22 7.5 2 3.5 18 17 19 16 20 14.5 20 17 17 16 9 30 17 18 10

3 2 <2 <2 2 2 2 2 2 2 3 2

,.,. The analysis tlo. 25 was otrered to us by E. Ro§u, for which we arc very grateful.

Remarks: Nb is lowerthan 10 ppm, excepting samples 20 (12 ppm) and 24 (20 ppm). Mo is present in the samples

20 (3 ppm) and 24 (57 ppm).

121

presence of "opacite" pseudomorphoses after a resorbed amphibole. Plagioclase phcnocrysts (An50-S4) show a poorly zoned structure aud polysyuthetic twins; they of ten contain glassy inclusions. I1ypersthene phenocrysts are light pleochroic: Ng=greenish; Nm=brown-greenish; Np=yellowish.

Plagioclase-pyroxel1e andesites can be divided, according to the character and amount of phenocrysts, into plagioclase-augit.c 3ndesites and plagioclase-augite-hypersthene andesites . The first. variet.y is frequently found . The rocks with a porphyritic texture collsist of the same hyalophilitic groulldmass and unoriented or oriented plagioclase phenocryst.s, rarely resorbed, with a zonary strudHrp. and polysynthctieally twinned. Loeally they display glassy indusions. The anorthitie eOIltent. varies in different zones from An56 to An50. The pyroxene phenocrysts are rare and they are represented by augite and more rarely hypersthene. In some eases the melanocrat.e minerals have been replaced by a semiopaque reddish mat.erial, formed of a powder of iron aud titanium oxidcs, or by grcen-bluish or yellowish chlorite. In places the magnetite crystals, usually small-sized, are larger thall th<, plagioclase microcrysts, looking like microphenocrysts. Within t.hese andesit.e varieties pyroxene eryst.als are grouped, formiug a glomeroporphyritic texture.

Hornbleude-pyroxelle andesit.es display a porphyrit.ic texture aud consist. of a high-glass groulldmass within which the plagioclase mic:rocryst.als arc of ten parallel t.o the margins of t.lw augite, hornblende alld plagio­c1a.<;t, phellocrysts. Hornblende phenocrysts are qllite frequently round; they display the following pleochroism: Ng=gret'n-brownish; Nm=brown-greenish ; Np=yellow-greenish and are locally resorbeu and usually opaeitized OII margins (PI. II, Fig. 2). Augit.e phenocryst.s (ct\Ng=55() ) are idiolllorphic and t.hey locally present a zonary

...

I

122 H. SA VU et al. ....

or hour-glass strudure. Plagioclase phenocrysts (An30-4S) are polysyntheticaIly twinned and display glassy inclusions. Locally they are partly resorbed.

Hornblende-pyroxene-plagioclase andesites are rocks with a porphyrit.ic OI' glomeroporphyritic text.ure, origi­nating in a glassy gl'Oundmass with a fluidal structure in which the plagioclase micl'Ocryst.als and t.he magnetite grains are oriented parallel to the litva f10w trend, surrounding t.he phenocryst.s of plagioclase, h~rnblende, augite and hypersthene. Plagioclase phenoci:'ysts (AU40-4S) present a pom zonary structure; they are polysyn­thetically twinned and contain glassy inclusions (PI. II, Fig. 3). Rarely they are partly resorhed . In places they form crystal agglomeratiolls in which t.he composition of different zones var ies rrom An40 to An2S. Horn­blende phellocryst.s (usually 5 mm long) are Ilsually idiomorphic aud opacit.ized OII margins, microphenocrysts being of ten entirely replaced by iron oxides. They display t.wins aft.er (l00) and include plagiocla<;e and mag­netit.e crystals. Locally the amphibole substitution is made (I) by opacitization and (2) by its alterations on margins into aII aggregate of plagioclase, maglletite and augite microphcnocrysts, in wllich plagioclase moulds the oLher minerals . Augite phenocrysts (cANg=4GO) oecur less frequel,t1y. Th ey are idioJllorphic or resorbed by the magma and prescnt polysynthetic I.wins aHer (100). llypersthenC' phenocrysl.s are idiomorphic and light pleochroic: Ng=greenish; Np=light brown-redish. ln same rocks one can obserw~ small aggregates of plagiocla<;e, clinopyroxene , hyperstl!ene, magnetite aud , lllor,~ rarely, hornblclHle crystals displa.ying a micro­gabbro-diorite aspect (PI. II, Fig. 4), in which pyroxenes are Illoulded by plagiodase. They have been torn off from an intermediary magmatic chamber.

HornbJende andesÎtes are widely porphyritic , wit.h homblendc phenocryst.s [rom 2 t.o 10 mm long, resorbed on margins and opacitized. They are parallel 1.0 the trending of t.he f10w lilles frorn the groundmass, which displays a hyalophilitic text urc in which the plagiocla<;e microlit.es tel'd (o !.le parallC'1.

Andesite tuffs are rarely found. They usually occur in the base of the volcanics pile (Glodghile~ti), pointing out that the volcanic activity began alsoin the second phase with erupt.ions whose pressure was very high. Plagioclase-pyroxenC' andesite tuffs appear at Glbdghileşt.i. They are stratified, with a crystallovitroclastic structure, and consist. of a glassy groundlT)aSs including plagioclase microphenocrystallocias ts, sedimented par­allel 1.0 tlte tuff st.ratification. The different lel1els or bands occurring in tufE; vary as regards t.he amount. of microphenocrystalloclasts. .

Trachyandesites include rocks with il high K:!O cont-en!: (> a %, Tah. 1), falling 011 the diagram in Figlire 1 in the trachyandcsite field . However, it is 1.0 be not.ed that no pota<;h felelsp.u phenocrysts have beell observed within thern, so that in pctrographic respect t,hey do not diffcr l11uch from andesites

Geochemistry and Tectonic Settillg

In the Zam-Gurasada arca the rocks form 11 strongly differentiatcd volcanic series - from basalts to rhyo­dacites (Tab. 1) . Theil,' SiO:! contenl.s vary a<; follows : basalts - SiO:! < 52 per cent, andesites and trachyan­desites - SiO:! = 53-63 per cent, Icucocrate rocks - Si02 > G3 per cent. , t./ms being inchlded in Gill's (1981) and Ewart.'s (1982) c1assificat.ions. Diagrams in Figures 1 aud 2 show Il calc-alkaJine scries, as in case of the andesite group where trachyandesitcs occur and in case of the leucocratf' rocks,associated with trachyandesites and trachytes.

Basalts and andesites present high MgO contents ranging between 6.18 and 9.76 per cent, aud CaO conl.cnts varying from 9.78 t.o 11.21 per cent. In t.rachyandesites the alkali content.s are higher than in andesites, their amount bcing higher than 3 per cent. In rhyodacites Si02 is higher than 72 per cent, and in trachytes it is lower (61.50 per cent.), but K20 is much higher, reaching 9.20 per cent .

The Si02 variation from 50.78 per cent to 72.05 per cent, as well as thc variation of other oxides, show that the parental magma underwent an intense differentiation process. It was a dioritic calc-alkalinc magma, wit.h a Jittle higher alkali cOlItent. The presence of alkaline rocks, both in the andesites group anei in the leucocrate rocks, point.s out t.he consanguinity of the volcanics.

Trace e1ement.s (Tab. 1) are characteristic of the calc-alkaline ser ies. Tahiel shows that andesites and trachyandesites differ c1carly from basalts and leu coc rate rocks. Basalts display higher content.s of Co, V and Sc and lower cont.ent.s of Ba. LCllcocrate rocks are poor in siderophile e1emcnts anei are emiched in Ba. Duc to their trace element.s contents, the andesitic rocks belong to the suite of " high-K" orogenic andesites (Gili, 1981), as also indicateel by the high Ba values (300-750 ppm, mean value 530 ppm) and high Sr values (430-720 ppm, mean value 600 ppm). Thesc averages can be compared with those present.ed by Gill (1981) for this t.ype of andesites. The value of the Ni/Co ratio :::; 1, obtained in mosi. of t.he rocks, is characterist.ic of t.he andesites. Chrome from andesites shows content.s varying within a wide interval. as compared with Ni whose values range

LARAMIAN VOLCANICS OF TIfE MURES COULOIR 123

from 5 ta 24 ppm. This aspect is also rendered evident an the diagram in Figme 3, an which the rocks faH into the islancl arc field. The appurtenance of the Laramial1 rocks ta the island arc volcanics is also emphasizecl on the diagram in Figure 4, on which the leucocrate rocks plot, too.

Ti/Cr

5000·

2000

1000

500

200

100

50

20

10

5

2

. . . .

\ \ \ \ \

(sland arc Tholeiites

Ocean Iloor TholeiikS

FeOtot ,-.-------,.60

20 30 40 50 60 70 80

Fig. 2 ~ FeOtot - MgO ~ Na20+K2 0 diagralll (Irvine, 8aragar, 1971; Hulchinson. 1982).

Ti Ippm)

50.000

30.000

I

/' /

",.-.-.-- ..........

\

\

'IlOOO

/ /

Within-plate ~, lavas . . ::" •• 2- \" 5.000

3.000

I I

• o ':. I "' I . I \ I \ I

I __

I ,/ -- ...... Arc ,-'/ " o \ tavas

o

\ 0\ 00

\

\

\ \

.1 02

5 10 20 50 m 200 SOO Ni(ppml 30 50 100 200 300 500 Zr ( ppm)

Fig. 3 - Ti/Cr - Ni diagl'am (8eccaluva el. al., 1979).

Fig. 4 ~ Ti-~Zl' diagram (Pearce, 1980). 1, basalts anei a.ndesites; 2, leucocrate rocks.·

The values of the Rb/Sr ratio determineel by Grabari (LG.G ~-Bucharest) on samples 2 anel 23 (Tab. 1) are 0.051 for the basaltic andesite and 0.226 for trachyanelesitc. On the same rocks, Popescu (LF.I.N.- Măgurele)

eletermineel for the 87Sr/86Sr ratia values of 0.710 for the former and 0.706 for the latter. Ali values are characteristic of the arc volcanics.

The REE contents present.ed in Table 2 are higher for LREE aud lower for HREE, typical of the arc calc­alkaliue series, as iudicatedalso on the diagram in Figure 5. As Table 2, this diagram poillts out an increase of the REE contents from the melanocrate rocks to the leucocrates ones~ The paUerns of the three rock groups are approximately parallel anel they are similar to those presented by Gill (1981) for the orogenic andesites of the "high-K" suite. An exceptiou is the pattern of the leucocrat.e rocks, which poillts to a positive anomaly for Ce anel to a negative one for Yb. Thel'e is 110 negative anomaly for Eu, which is typical of the rocks with an alkaline tendency, e.g. mOllzogranites and syenogranites (Cullers, Graf, 1984). The values of the Eu/Sm ratio (0.16-0.39) and of the La/Lu ratio (8.5--5.0) are wit.hin t.he limits indicated by Cullers anei Graf (1984)

...

124 H. SA VU et al. ...

for the continental orogenic andesites, showing that the Laramian magmatic rocks in the Mure§ Couloir are arc volcanies formed in a continental subduction zone, as shown on the diagrams in Figures 3 and 4. This conc\usion also results from the diagram in Figure 6, on which the rocks are situated both in the field of the plate margin basalts (62 per cent) and in that of the intraplate basalts (38 per cent) , pointing out t,hat some rocks from the continental volcanics ares present higher Ti.and Zr content.s than those from intraocean island arcs. The average values of Ule Ba/Lai ratio are of 17-24 and they are placed wit.hin the ilmits given by Gill (1981) for the arc volcanics in New Zealand (12-25).

m 70

50

Î 1

JO

20

Q

Table 2 REE cOlltents (ppm) in the volcanic rocks

No· La Ce Sm Eu Tb Yb Lu 1 12 16 4.1 0.64 0.46 2.8 0.22 2 17 26 3.6 1.00 0.47 1.3 0.20 4 15 30 3.3 1.00 0.74 2.3 0.26 5 26 68 6.0 1.50 0.43 2.0 0.28

13 32 82 6.2 1.70 1.00 2.0 0.52 16 25 43 4.2 1.30 0.60 2.3 0.22 21 15 56 4.0 1.23 0.87 1.7 0.35 23 32 100 4.3 1.70 0.98 1.5 0.58

* The number of the allalyses corresponds to that 11\ Table 1.

> ... '" ...... ',e b·· ... "

"" '" ,.~,

".~

"'-'" ';: .... ... .:...,:...-----....... '-'- ..... -..... . .-.-._._ . _ . _'""':~. .. " . ~-.-.-.- . .J.-.

.... -.... '

. ~ 5~-T __ ~~ __ ~ __ ~~ __ ~~--~~r--r--~~--~~ La Ce Pr Ne! Pm Sm Eu Gel Tb Oy Ho Er Tm Yb Lu

Fig. 5 - REE distribution, nonnalized by chondrite, in the volcallic l'Ocks. a, basalts and basaltic andesites; b, an"desitcs aud trachyandesites; c, leu­cocrale rocks.

Origin of the Volcanics

The Laramian volcanics in the Mur~ Couloir occur along an arc of gravimetric anomalies spread from the Laramian granitoid intrusions in Banat. towards ENE, beyond Sirbi (Mure§ Couloir) , thus indicating the extension of the volcanic arc which generated them. This volcanic arc is paraJlel to the Severin ophiolitic suture (Fig. 7), that is to a curve line of the collisional conta<;t between the Moesiall Plate aud the Transilvallian

1 When calculat ing t.he Ba/La rat.io, for La wcre used only the valucs present.cd in Table 2, obt.ained by neutron .activation. hecause the cOlltents <lct.enn.ined by emission spectrogl'aphy (Tab. 1) , being doSe 1.0 the detcction limit, can present greater errors of analysis.

I

LARAMIAN VOLCANICS OF THE MUREŞ COULOIR

Zr/y

7

Plate 5 margin

basalts ( 70·'.)

3

100

. •

. . • •

. . •

300

Wlthin plate basalts ( 30·'.)

500 700 Ti/V

"" ...

Fig. 6 - Zr/Y-Ti/Y diagram (Pearce. Gale, 1971).

O ., o .... _10".;;. _20 ..... _ ... ~ km

.,. J. ...

4-O ~.

O ., .,. O .... •

Fig. 1 - Position oC the three volcanic arca as against the Severin ophiolitic suture. EVA, externa! volcanic arc; IVA I , first intemal volcanic arc; IVA 2 , second interna! volcanic arc.

125 ....

126 H. SAVU et al.

Plate (Savu et al., 1985), on whose extern al margin also occur Laramian volcanics belonging 1.0 an external volcanic arc (EVA), and to the axis of the South Carpathians a first internal volcanic arc (IVAţ), in the Rusca Montană Basin (Savu et al., 1987) . The first internal volcanic arc (IVAd extends south-westwards with the Laramian intrusions from the second and the third banatitic (Laramian) alignment established by Giu~că et al. (1966). The second intern al continental volcanic arc (IVA2 ) - which we are dealing with in this paper -extends south-westwards with Laramian intrusions from the first banatitic alignment (Fig. 7), generated by subduction (Rădulescu, Săndulescu, 1973). We mention that on this aligriment, like in the Mure§ Couloir,

. t.here are igneous rocks with a more alkaline tendency than in the other two volcallic arcs (EVA, IVA.t) or the two mentioned Laramian alignrnents in Banat . These volcanic ares extend in Yugoslavia and Bulgaria.

The position of the three continental volcanic arcs as against the Severin ophiolitic suture, the margin of the peri-CarpathianDepiession ('frough) respectively, shows that their formation is connected with the subduction of the Moesian Plate' under the Transylvanian Pl ate (Savu et al., 1985, 1987); it reminds us of the volcanic ares sueeession in the Andes described by Palacios (1984). It isa Chilean-type volcanism (Byeda, 1981), more compressive and with porphyry copper mineralizatiolls. 'fhe volcanic arcs occurred successi vely, concomitantly with the WNW advallce ofthe subduction process; the extern al arc volcanics (EVA) erupted during the lJpper Thronian-Senoniau (Stan et al., 1979; Savu et al., 1987), the volcanics from the first internal volcanic arc (IVA!) were formed duriug the Maastrichtian (Maier et al, 1975) aud those of second volcanic arc (IVA2 ) occurred during the Paleocene (Popescu in Borco§ et al., 1986; Savu, 1984). The volcanics from the extern al volcanic arc erupted in the Carpathian miogeosyncline in which the Upper CretaceOlls wildflysch waS being deposited aud the rocks from the two int.ernal volcanic arcs erupted in smaller epicontinental sedimentary basins.

La (ppm)

~

io

'K)

/

• • OROGENIC

• • ANDESITES'

High-K

• --- ---

200 400 600 800 Ba (ppm) .

Fig. 8 - La-Ba diagram (GiIl, 1981). For La, the values pl'esent.ed in Table 2 were used.

1000

/ /

IVA / ~200km

/ IVA1 ~

, 180Km

/

/ /

/

05~--~~------5~5------~60~------~65

wr'/.Si02

Fig. 9 - K 2 0 - Si0 2 diagJ'am (Ninkovich, Hayes, 1971).

The mentioned succession in the formation of the continental volcanic arcs and the regional zonality as against the Severin ophiolitic suture line determined a petra- and geochemical polarity of the volcanics. Within the external volcanic arc (EVA), the rocks are more basic and display "medium-K andesites" characteristics (Savu et al. , 1987), whereas those of the second internal volcanic arc (IVA2 ) correspond to "high-K" andesites (Fig. 8). As regards the first intern al arc (IVAt) there are no geochemical data necessary for the establishing of these characteristics . However, the position of the rocks on the diagram in Figure 9 and the value of the K20jSi02 ratio , as we shall see furtiler on, show that these rocks must have an intermediary position versus the rocks from the other two volcanic arcs. 'fhe K20 and Si02 contents of the volcanics increase concomitantly with the distance from the margin of the Transylvanian Plate and the sinking of the Benioff plane, as shown on the diagram in Figure 9, on which occurs the plotting (average values ) of the rocks froril thc three volcanic arcs. The value of the K20jSi02 ratio increases from 0.024 in the extern al volcanic arc to 0.041 , in the first intern al volcanic arc, and to 0.046 in the second intern al volcanic arc.

In the Zam-Gurasada area the parental magma of the volcanic rocks, sit.uated in t.he second internal volcanic arc (IVA:? ) , was a dioritic magma with about. 58% Si02 formed at a depth of 200 km (Fig. 9) . Similarly occurred the parental magmas of the other two vokanic arcs, the parental magma of extern al volcanic arc displaying a

,.

LARAMIAN VOLCANICS OF THE MUREŞ COULOIR 127 .

basaltic character with an average Si02 content of 51-52 per cent. Those magmas were formed at even smaller d~pths (180 km and 150 km, respectively) in the mantie, decreasing concomitantly with the lowering of the depth of the Benioff plane (Fig. 10).

If the fOflllation depth of the magmas rendered on the diagralll in Figure 9 is compared with the actual small distance bet.ween the three continent.al volcanic arcs and the Severin ophiolitic suture one must admit either that the dipping of the Benioff plane was great.er than 45° or that the distance between the volcanic arcs and the subduction line was longer. In fact, the spatial relationships observed nowadays (see Fig. 7) represent the result of the evolution of a geotectonic. syst.em from an oceanic zone, by subduction processes, to the collision stage, with crust collsumption, therefore those elements must have initially been farther one from another.

EVA MP Collilion TP

200Km

Manlle

Mantie

Benioff

Fig. 10 - Formation of t.he calc-aJkaline parental magmas in the three continental volcanic al'CS of the South Car'pathjans by the melting of the metasomatized mantie undeI' conditiolls of the subduction of the Moesian Plate und" .. the Transylvanian Plate. MP, Moesiail Plate; TP, Transylvanian Plate; EC, Earth's crust; MC, magmatic chambers, MS, metasomatic solutions; OC, subdllcted oceanic crust which is being eclogitized.

There are several opinions as regards the magma formation in the subduction zones. Taylor (1968) consid­ered that magma resulted from the Illelting of the subducted oceanic crust, but Gill (1981) stated that the Illerţ melting of the oceanic qust could not generate a calc-alkahne magma. Mysen and Boettcher (1975) and Green (1976) showed that the andesites carne from hydrated meltings of the peridotites at pressures of ca. 10 kbar. Considering the above-mentioned opinions, as weB as those published by Minear and Toksoz (1970), Mysen (1979) aud Roden anei Murthy (l985), wecame to the conclusion that the parental magma from the second intern al volcanic arc (IVA:Jl was formed in' the mantie suprajaceilt to the Benioff plaue - like the magma of the other two volcanic arcs - at. a depth of about 200 km, as a result of thepartial melting of the eclogitic source. that had been affected by metasomatic processes with substallces emallated from the oceanic crust of the Moesian Plate sllbduded on the Benioff plane (Fig. 10). The average vallle of the (La/Ce)cn ratio (1.16) suggests that. ihe parental magma was generated by a Soul'ce that underwent a pl'ocess of LREE depletion (Gill, 1981) or , according to liS , it origillates in the met.asomatized mantIe. This magma generated "high-K andesites" aud trachyandesit.f's. Small amounts of olivine basalt,s anei basaltic andesites, as well as acid aud

128 H. SAVU et al. ....

alkaline leucocrate rocks were formed by partial melting and differentiation . The percentages of the three rock groups point t.o those from t.he Brokeoff vo\cano in California (USA) studied by Fountain (1979). The small amollnts of basalts and leucocrate rocks in comparison with the great amount of andesitic rocks in the Mure§ Couloir are also comparable with the percentages established by Sugimura (1968) for the island arc volcanics in Japan.

ConclusioDS

Laramian volcanics are products of a continent.al arc vo\canism, formed in coril1f~ct.ion with the subduction of the Moesian Plate un<fer the Transylvanian Plate. .

The rocks constitute a calc-alkaline vo\canic series with obvious alkalinc tcndcncies, within which three pet.rographic groups can be distinguished : (1) olivine basalts and basalt.ic andesites; (2) andesites and tra­chyandesites; (3) acid and alkaline leucocrate rocks.

The rocks display geochemical features of island arc vo\canics. The basic and intermediary rocks belong to the "high-K andesites" series; it points out their alkaline trend, differentiating them from the rocks of the external volcanic arc of the South Carpathians, which display " medium-K andesites" characteristics.

The presence of the alkaline rocks both in the andesite group and in leucocrate group emphasizes the vo\canics consanguinity.

The parental magma was formed at a depth of 200 km in the mantie suprajacent to the Benioff plane by the partial melting of the ecJogitic source metasomatized by substances emanat.ed from the subducted !lnd ecJogitized oceanic crust.

Magma displayed a dioritic composition with 58 per cent SiO:? but it strongly differentiated generating rilainly andesites and sporadically olivine basalts and leucocrate rocks.

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Moscow, 1974.

Receivt:d: Mag 12, 1988

AcceJlled: Mag 16, 1988

Presented al Iilescientific sessioll of Ilie Institute of Geology alld Geopllysic!/: May 28, 1988

...

H. SAVU et al. - LARAMIAN VOLCANICS OF THE MUREŞ COULOIR

1 2

3 4

Plate II

Fig 1 - Structure of the olivine basalt groundma.ss. Petrqti. Nic. II, x 44. F'.,;. 2 - Pyroxene homblende andesite within whiCh the amphibole phenocrysts are opacitized on margms. Gloaghile§ti.

Nic. II, x 34. Fig. 3 - Homblende p)'J:Oxene plagioclase andesite within which the plagioclase phenocrysts display glassy inclusions with a

zon8l'Y disposition. Pîrîul Negru. Nic. II, x 50.

Fig. 4 - Microgabbrodiorite xenolith within a homblende pyroxene plagiociase andesite. Petreljti. Nic. II, x 20.