J o ur nal of Vo Lc anoLo gy and G e o the r maL Re s e ar c h, 3 7 ( 1989 ) 187 -202

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands187

PALEOGENE COLLISION-RELATED BASALTS AND BASALTICANDESITES IN THE EASTERN RHODOPES, BULGARIA

Y. YANEVI, B. MAVROUDCHIEV2 and R. NEDYALKOV3

'GeologicaL Institute, Bulgarian Academy of Sciences, Sofia 1 1 13, Bulgaria2Sofia Uniuersity "Kliment Ohridshi", Sofia 1000, Bulgaria

3Research Institute of Mineral Deposits, Committee of Geology, Sofia 1000, Bulgaria

( Received November 1 1, 1986; revised and accepted October 5, 1988 )

Abstract

Yanev, Y., Mavroudchiev, B., and Nedyalkov, R., 1989. Paleogene collision-related basalts and basaltic andesites inthe Eastern Rhodopes, Bulgaria. J. Volcanol. Geotherm. Res.,37 187 -202.

High-K calc-alkaline and shoshonitic basalts/basaltic andesites (SiOr=49-53 wt.% ) represented by a total of 10specimens and one absarokite sample are studied petrologically and geochemically (trace elements and REE ). Basaltphenocrysts include clinopyroxene, an ore mineral and plagioclase * olivine * biotite * sanidine, and absarokite phen-ocrysts are represented by olivine, clinopyroxene and orthopyroxene. Collisional basalts/basaltic andesites differ fromthose in the active continental margin settingin their increasedabundances of some Lll-elements (Rb, Cs, Ba) aswell as Th and U, in the high Kl"ti,KlP, K/Sr, Rb/Sr and Ba/Sr ratios, negative Nb anomaly and low abundancesof most Fe-Mg elements (Ti, Y, Yb, Sc, Ni, Co, Cr ). Their REE patterns are moderately fractionated and the rocksshow a negative Eu anomaly. Higher P and Cr abundances, lower Yb content and a positive Eu anomaly, however, arecharacteristic ofthe absarokite. The basalts/basaltic andesites represent highly crust-contaminated melts formed atmoderate /s, and high P11,6, whereas the absarokite shows some features indicative of mantle derived melts.

Introduction

This paper aims at describing basalts and re-lated basaltic andesites found in an environ-ment of continental collision, thus supplement-ing to descriptions of the great variety of basaltsformed in other plate-tectonic settings. Colli-sion magmatism is largely acid and intermedi-ate in composition as for instance that in thetypical collision orogenes, the Himalayas andTibet (Dewey and Burke, 1973; Baker, 1982;Gill, 1982; Leeman, 1983 ). In some cases, how-ever, very small amounts of basalts are also pro-duced as in the Rhodopes Paleogene collisiondescribed here, or in the Northwestern Alps

(Venturelli et al., 1984). Basaltic volcanismdominated the post-collision within-plate set-ting but it was entirely different, mostly alka-line in character (Magmatism and Metallo-geny.. . , 1983). This lat ter volcanism is notconsidered in this study.

Petrographic and geochemical data of theserare collision-type basalts and ofthe related ba-saltic andesites are compared to volcanics fromthe active continental margins (Andean tlpe).The rocks selected for comparison are high-Kbasalts to basaltic andesites and shoshoniticbasalts to shoshonites of similar SiOr content(50-54 wt.To on an anhydrous basis) from theAndes, Western United States and the Cascade

0377-0273/89/S03.50 O 1989 Elsevier Science Publishers B.V.

188

Mountains (after Ewart, 1982 - average anal-yses; Lopez-Escobar et al.,1977 ), from Mexico(Robin, 1982), Central America (Carr et al.,1982). and also collision basalts from thenorthwestern Alps (Venturelli et al., 1984).

Analytical techniques

Major elements were determined by classicalanalytical methods, and the trace elements wereanalyzed as follows: Sr,Zr, Y and Nb - by XRFin the Institute of Geology of Ore Deposits atthe Academy of Sciences of the USSR, Moscow;Rb, Ni and Co in samples 1, 2 and 10 - by AA inthe Sofia University; all other trace elementswere determined by INA in the Geological En-terprise for Laboratory Tests at the Committeeof Geology, Sofia.

General characteristics of the Rhodopestertiary magmatism

At the end of the Eocene and during theLower Oligocene (37-30 m.y., Lilov et al., 1987 )the Rhodopian and the Serbian-Macedonianmassifs, built up of metamorphic rocks and oldgranites, were affected by a volcanism mani-fested in several areas. One of them is the East-ern Rhodopes volcanic area (Fig. 1) which inturn consists of three volcanic regions - the Bo-rovitza, Momchilgrad-Arda ( Ivanov, 1960 ) andSushitza regions (the latter extends largely inGreece where it is known under the name ofKaloticho, Innocenti et al., 1984). In most ofthese areas the volcanics are acid whereas inthe Eastern Rhodopes area discussed here acidand intermediate rocks occur in approximatelyequal volumes (Ivanov, 1964, inset in Fig. 1).Six small intrusive bodies, some of them of dif-ferentiated composition (from gabbro to gra-nosyenites ), are intruded in some volcanoes aswell as in the metamorphic basement of the vol-canic area. Petrochemically, the volcanic rocksare predominantly transitional (shoshonites,Iatites, trachytes, trachydacites, trachyrhyod-acites and trachyrhyolites with ignimbrites)

(Ivanov, 1964; Yanev et al., 1984; Marchev,1985) with Iess common high K calc-alkaline(basaltic andesites, andesites, rhyodacites andrhyolites ). A petrochemical zonality has beendescribed in the intermediate volcanics (Iva-nov, 1964) consisting of transitional rocks oc-curring throughout the northernmost Borov-itza region; calc-alkaline and transitional onesbuilding the Momchilgrad-Arda region to thesouth; and calc-alkaline volcanics dominatingthe southernmost zone in North Greece (ac-cording to the data of Innocenti et al., 1984)nearest to the collision front where the effect ofjuvenile material is greatest (Gari6ry et al.,1985). Basic varieties are exceptions - one ab-sarokite flow only has been described so far( Ivanov .1978) .

The volcanic activity was preceded and ac-companied by a molassic (Ivanov and Kopp,1969), mostly marine and less frequently con-tinental sedimentation that took place in gra-ben basins. Volcanism was cyclic which is evi-denced by a succession ofthree or four series ofalternating intermediate and acid products(Ivanov, 1960; Ivanov and Kopp, 1969). Theyhave been called 1st intermediate volcanism(Priabonian), 1st acid volcanism (Early Oli-gocene ), 2nd intermediate volcanism (the sameage ), etc. So far basalts/basaltic andesites havebeen found in association with the 1st and 3rdintermediate volcanism only but they occur inall three volcanic regions (Fig. 1).

Geodynamic setting

The volcanism described above has been in-terpreted as orogenic, related to the collision ofthe Eurasian and the African plates (Yanev andBahneva, 1980). The latter took place duringthe Paleogene after the Tethys paleooceanclosed completely up (Aubouin, 1973). Thesame volcanism has been also interpreted as re-sulting from the underthrust of the oceanic plateof the Tethys southern branch (the so-calledSubpelagonian zone, Boccaletti et al., 1974 ); asa process in an active continental margin

189

Fig. 1. Schematic map of the Eastern Rhodopes Paleogene volcanic area. 1:Quaternary-Neogene cover; 2:intermediatevolcanoes; 3:acidvolcanoes (a) anddomeareas (b) accordingtoYanevetal. , 1984; 4:hypabyssal intrusions; 5:calderasassociated with the acid volcanism; 6 = Paleogene sediments and pyroclastic rocks; 7: metamorphic basement with a frontof nappes (after Ivanov, 1985); 8=basalts and basaltic andesites with sample numbers from Table 1. Top right corner -

estimates of the volume of volcanics in the cycles and in the volcanic regions (after Ivanov, 1964): 1o:1st intermediatevolcanism; Ib:1st acid volcanism: IIa=2nd intermediate volcanism, etc.; A:intermediate volcanics (SiOr=53-64%),

B=acidvolcanics (SiO:>64%):a-lavas,b:pyroclasticrocks; B.r.=Borovitzareglon;M.-A.r.=Momchilgrad-Ardaregion.

(Dimitrova et al., 1979); and as a postcollisionphenomenon (Ivanov, 1985 ). Recently the col-iision between the above two plates has beenalso related to the Paleogene and Miocene vol-canism in the Aegean area (Fytikas et al., 1984;Innocenti et al., 1984) analogous to that in theRhodopes Mountains, as well as to the volca-nism in West Turkey and Iran (Innocenti et al.,

1982; Leeman, 1983 ) and to the Oligocene vol-canism in the Northwestern AIns (Venturelli eta l . . 1984 ) .

Subduction ceases during collision due tocontinental blocking. Some authors suggest(Dewey and Burke, 1973; Aubouin, 1973; Mat-tauet, 1983) that during this process the lowercrust ("basaltic" layer) ofthe plate being sub-

190

A R N A

o6

A > *

MaffiwWrtNtrl@mv\zl r : 7 \ 21

E

os

0

t

@l t

i . , 1 ' ' , ' , ; 1 '

(

' so"i," ",--1 4- a u

T H E S S 0 N t K l

''t'1,/ \ .t'li?-,il

a6 {

,rt---- I -- --

o i t e r T u r c o t i e ( t S a Z )C ^ ,

L :

ductedbecomes detached and continues to movesome distance along the subduction zone whilein the upper ("granitic" ) Iayer of the crust thick

overthrust nappes are formed in the directionof collision (e.g, according to the collision modelof Turcotte, 1982). Such nappes, consistingmostly of metamorphic rocks, have been foundin the Rhodopes Mountains verging presum-

ably to the north (Ivanov, 1985). They wereformed before the beginning of volcanism whenthe collision zone was transformed into a com-plex many-storeyed edifice of piled up nappes.This resulted in a thickened crust reaching upto 40-45 km (Yosifov et al, 1980). Overthrustsinvolved Jurassic-Lower Cretaceous sedimen-tary rocks with basalts coming probably fromthe Tethys ocean floor. The origin of the met-amorphic material of the nappes is not known.It has been suggested (Ivanov, 1985) that the

Rhodopes area was a small continental massifmoving ahead of the African plate and thrustover the Eurasian one. On the other hand, theoverthrusts could have been formed behind thecollision front, i.e. at the fore-zone of the Eur-asian plate (similar to the model of Turcotte,F ie . 2 ) .

Crustal thickening and collision led to an or-ogenesis during which some blocks were liftedup while others sank thus forming graben bas-ins of molassic sedimentation.

1 9 1

Characteristics of the basalts

Location (Fig. 1, TabLe 1)

The Borovitza volcanic region is built up ofIarge stratovolcanoes covering an area of 40 km

X 25 km (1st and 2nd intermediate volca-nism). Two basaltic flows outcrop on thenorthern (sample 1) and southern (sarnple 2)slopes ofthese volcanoes. The southern flow hasbeen describedby Ivanov (1978) as an absarok-ite flow 5 m thick containing pyroclastics andepiclastics and beionging to the terminal part

of the section of the 1st intermediate volcanism(Priabonian).

In the Momchilgrad-Arda region the ba-salts/basaltic andesites are associated with theZvezdel volcano (Nedyalkov, 1986) belongingto the 3rd intermediate volcanism (Lower Oli-gocene). The volcano is about 15 km in diam-eter and the products described here form twoflows at the eastern (sample 3) and western(sample 4) periphery of the volcano, theirthickness reaching 10-15 m with scoria zonesin the top parts; a subvolcanic body (sample 5 )and a sill intruded into the rocks of the volcanobasement (sample 6). Several later dykes arealso of similar composition. They cut throughthe intermediate volcanics (sample 7 ) and theintrusion (samples 8 and 9) embedded in thecentre of the Zvezdel volcano. The dyke thick-ness varies from 0.5 to 3 m.

In the poorly studied Sushitza region, onlyone occurrence of basalt (sample 10) is known

Fig. 2. Schematic map and a cross section ofthe zone ofPaleogene collision and the associated volcanics in the eastern part

of the Balkan Peninsula. l :Remains of the Tethys paleoocean (after Dercourt, 19?0); 2:J-Cr, sediments and basic vol-

canics; 3=the Srednogorie zone of Upper Cretaceous island-arc magmatism (after Boccalletti et al., 1974): a:on the map,

b:in the section; Paleogene collision magmatism: 4:acidvolcanics.5: intermediate arrd acid volcanics: a:on the map

(hypabyssal intrusions marked with crosses), b:in the section; 6:naPPes in the metamorphic basement (after lvanov,

1985 ): a = on the map, b: in the section; 7: Neogene post-collision withiu-plate transitional and alkaline basalts and ultra-

basic volcanics. In the cross sectlon: 8=Paleogene graben basins; 9:the folded Stara Planina zone; l0:"upper" crust;

ll:"lower" crust. Tectonic units: SM:Serbian-Macedonian massif; /?h:Rhodopian massif; S:Srednogorie; SP:Stara

Planina; EA : Eurasian plate.Note :Thear rowin thesec t ionshowsthe increas ingvo iumeof theshoshon i t i cPa leogenevo lcan ics (CA:ca lc -a lka l ine ,

Sh: shoshonit ic ) .

r92

TABLE 1

Chemcial composition and C.I.P.W. norms of the basic volcanics from the Eastern Rhodopes volcanic area

Volcanicregions: Borovitza Momchilgrad-Ardino (.Zv ezdel volcano ) Sushitza (Kaloticho)

Samples No.: l 110

sio,Tio,AlrorFerO3FeO\,InO\'1goCaONaroKrOPrO.HrOH r o *CO,SO,

Total

aOrAbAn

Wo/DiEn/D iFs/DiEn/HypFs/HypMtIIApCcTen

51.46 47.560.49 0.28

16.96 11.544.40 5.503.69 2.030 .15 0 .144.99 6.868.91 12.613 .11 3 .092.55 3.270.42 1.151.08 1.021.61 .1.09

49.28 52.961.00 1.10

i8.55 16.354.60 3.785.04 5.570.20 0.164.77 6.707.38 5.462.52 3.802 . r2 1 .390.22 0.190.90 0.282 .30 L .641.60

50.76 51.721.10 0.92

18.12 16.693.43 4.246.05 4.680.12 0.074.09 5.198.85 8.913.10 2.802.18 r .64- 0.030.50 0.38

1.581.67 0.240.32 0.05

51.86 51.560.82 0.76

77.24 16.483.37 3.245.69 5.510.22 0.184.75 4.588.54 9.582.80 2.25i . 28 2 .18- 0 .190.46 1.502.63 0.960.79 0.440.20 0.30

62.98 51.160.97 0.51

t ] .27 17.852.56 3.565.40 4.130.13 0.153.41 3.697.99 9.402.26 3.022.79 2.380.24 0.370.88 0.83t .26 3.001.38- . 0 .14

5 1 . 2 11 . 1 0

77.744.544.840 . 1 94.017.603.512.380.38

II

| 2 .45I

-

99.99

0.9414.0729.7025.644.02.720.987.272.626.582 .770.90

0.07

99.89 99.95

0.96 *

15 .06 19 .3025.85 11 .4025.04 8.006.86 19 .705.11 17 .001.07 *

7 . 3 1 *

1 . 5 3 4 . 1 66.38 6 .670.93 1 .900.99 0.52

0 . 1 2

100.48 100.24

6.71 2.4012.53 8.2021.32 32.1025.06 23.500.90 1.000.60 0.700.00 0.20

11.38 16.004.76 5.306.67 5.501.90 2. t00.52 0.403.64

100.29 99.64

2.9 i 4.7412.88 9.6924.14 23.3634.24 28.321.31 5.920.76 4.200.50 1.209.43 8.726.17 2.494.97 6.152 . r r 1 .75

0.073.80 0.550.57 0.09

100.65 99.71 99.46

6.5C 6.01 8.757.56 12.88 16.49

22.38 17.07 t9.7231.39 29.48 28.592.50 5.85 0.321.51 3.53 0.170.86 2.01 0.13

10.32 7.88 8.325.85 4.49 6.304.89 4.10 3.7r1.56 L44 1.84- 0.45 0.571.80 1.00 3.140.35 0.53

100.19

2.0614.0724.6328.62

o . J I

4.2r1.864.982.205.160.970.88

0.25

*ne = 8 .00 fo :0 .10 fa :0

on Bulgarian territory and another one in NorthGreece (Innocent i et al . , 1984 ) (sample 11 ).

Petrographic characteristics ( TabLe 2)

The volcanics studied are porphyric rocks,some of them (sample 6) containing abundantphenocrysts. In all occurrences clinopyroxene(enstatite-diopside ) crystallized first followedby an ore mineral and plagioclase. Some of thevoicanics contain also olivine. and those from

the northern part of the Borovitza region haveearly biotite embayed in plagioclase and sani-dine. The groundmass is hyalopilitic, interser-tal to microdoleritic with microlites of clino-pyroxene, plagioclase, an ore mineral and, insample 1, apatite as well. In some volcanics(samples 5 and 6) there are voids fil led withchlorite or with carbonate and chalcedony. OI-ivine is partially or completely serpentinized.Absarokites (according to Ivanov, 1978) arealso porphyric rocks (with phenocrysts of ser-

TABLE 2

Mineral composition of the basic volcanics from the Eastern Rhodopes area

Samples No. 1 10

PhenocrystscpxP1olMtBiS

EnrrWorrFs, XAron Anru ,,

x(x)

XX

XX

XXX

EnorWo.,,Fs3 En.rWo.rFsrnADr13

"o Anrs o,

(x)X

X En'WorrFs" XAnr, 66 X Anr, ,,(x)

Percentage

Microlitescp*PIMtS

25-30%

(x)XX

8 - 1 6 % 60-70% 1 2 - 1 6 %

(x)X

XX X( X ) X

XX(x )

XX AnroX X

X(x )X

X

X - the mineral is present in the sample; (X) - the mineral is present in very small amounts in the sample.

{O

X

{:-O 1

Y+

z

Fig' 3. A.NarO * KrO vs. SiO, diagram showing the fields of calc-alkaline ( o ), transitional ( = shoshonitic ) ( b ) and alkaline(c) series in the family of basic rocks (45-53 + 1% ) according to Klassif ikatzia and Nomenklatura .. . , 1981.B. KrO vs. SiO, diagram (Pecceri l lo and Taylor, 1g?6).Note: On all diagrams the volcanics studied are marked by solid circles (1:lavas, and 2=later dykes from lheZvezdelvolcano ) their numbers corresponding to those in Table 1; absarokites are marked by solid triangles (without numbers = afterIvanov, 1978 ). Solid line = the Borovitza volcanic region; dotted line : the Momchilgrad-Arda region; double line : the Sush-itza resion.

r.5 50 5311 Si02"6 46 /.8

A r O z

52 Si 02V"

Pe tr o c he mic al c har acteris tic s

The volcanics described here are intermedi-ate between basalts and basaltic andesites, orshoshonites respectively, because of their highsilica content (49-53%, Table 1). As follows

pentinized olivine, zoned augite-diopside withcores of orthopyroxene, and biotite; and agroundmass of leucite (? ), sanidine, pyroxene,ore microlites and scarce apatite); epidote,chlorite, calcite, quartz and veinlets of analciteare secondary products.

A \ \

l^'-:*\I A .\c"--J' \ r = . \ Y - f l

*,oo viou"t'

o b s o r o k i t e

b o s o \ t

194

from the Sior/alkalies plot (Fig. 3A), the vol-canics from the Borovitza (sample 1) andSushitza (samples 10 and 11) regions and thelavas from the Zvezdel volcano in the Mom-chilgrad-Arda region (samples 3, 4 and 5) aretransitional (shoshonitic ) types while the laterdykes in the above volcano are calc-alkaline(samples 7, 8 and 9). The higher alkalinity ofthe Borovitza region lying more to the northand, correspondingly, at a greater distance fromthe collision front is clearly seen. The sametrend with respect to K is to be observed on thediagram of Peccerillo and Taylor (1976), Fig.3B.

Comparing the collision volcanics with thosefrom the active continental margins by themethod of Frolova et al. (1985) it is seen (Fig.4 ) that the former differ only in their slightlyhigher potassium and lower Fe2* and Ti con-tents (the SiO, content being the same). Theabsarokite, however, differs from the other vol-canics in its low Al and Fe'*, and high K, Caand Mg as well as P (Fig. 5 ) contents.

On the existing discrimination diagrams (e.9.FeO*-MgO-Al2Oir, TiO2 vs. P2Os, TiO2 vs. SiO2,KrO vs. MgO ) not shown here, the volcanicsformed in the two geodynamic setting fall in thesame field. They are clearly distinguished onlyon the K vs. Ti and K vs. P diagrarns becauseof the difference in their K/Ti and K/P ratios( F i g . 5 ) .

According to their normative composition(Table 2) the volcanics studied are mostly ba-saltic andesites comparable to those occurringin the destructive plate margins (Fig. 6, afterEwart, 1982). The Zvezdel volcanics and theIater dykes in particular show some specificfeatures: relatively high contents of normativequartz and ilmenite, presence of rock varietiesof extremely low contents of normative diop-side and, correspondingly, high hyperstheneconten ts (samples 3 ,4 ,5 ,7 and 9) . The absa-rokites are oiivine- and nepheline-bearing rocks.In their high diopside and low olivine contentthey differ from the shoshonitic basalts occur-ring at the destructive plate margins.

- v . t

- 0 6

- l

K20 Fe203 No20At203Si02CoO M90 FeO T i02

MoRB 0 .23 2 .46 2 .63 15 .67 49 .96 11 .36 7 .97 E.06 l .4E

Fig. 4. MORB-normalized composition of the EasternRhodopes collision basalts/basaltic andesites (7 ) and ab-sarokites (2) compared to the mean compositions of thesame t),?e of rocks from the Alps (3, Venturelli et al., 1984)and to the active continental margin basalts (4), data ofEwart ( 1982 ), Robin ( 1982 ) , Carr et al. ( 1982 ) and Lopez-Escobar et al. (1977).Note: The oxide contents (ri) in the volcanics are normal-ized by their contents in MORB (r" after Dmitriev et al.,1976 ) using the formula (x1- x.,) / x,, .

T r ac e - e le me nt abundunce s

This study considers mainly the trace-ele-ment abundances in the rocks of the Zvezdelvolcano (Table 3), the other samples beinganalyzed for part of the elements only. The ab-sarokite is characterizedby a single analysis (forsome of the elements in it the data of Ivanovand Stoyanova, 1966, are used).

?TITq), Ur"

.*3 * /

2tj,,ti

1 V

S i 0 2 o / o( 1 0 0 % d r

51.5 + 5/..

) J . 9 ; ) 6 . :

5 0 - 5 4

l0.60.60.40.3

0.2

195M 9 0 %

( t o o v " a r y )

I r 1 .6 : 5 .9

? . . 7 .1 : - to . l

3 * L . t - 5 . 5

a o 3 . 9 ; 6 . 8

R b p p m

0.1 0.2 0.3 0.4 0.5 v"

R b p p m

150

B o D D m1 5 0

i l00

9

600 1000

Fig. 5. Bivariant diagrams for some major and trace elements in collision basalts/basaltic andesites (dots) and absarokite(triangles) from the Eastern Rhodopes and the Alps (asterisks) compared with active continental margin basalts (data

from Ewart, 1982; Robin, 1982; Carr et al. , 1982; Lopez - Escobar et al. , 1977; Harmon et al. , 1984 ) .Note: For SiO,, contents see Fie. 4.

N e - D i

Fig. 6. Plot of the normative mineral composition of the Eastern Rhodopes collision volcanics compared to some volcanicsfrom the destructive plate margins (after Ewart, 1982).Note: S1'mbols as in Fig. 3.

' Ihe Zvezdel volcano basalts/basaltic ande-sites as well as the absarokite are characterizedby a great increase in the contents of some LILelements (Fig. 7 ) and particularly of Rb, Cs, aswell as Th and U, whereas the Sr concentra-

;1-\t ' l\^*/

tions correspond to the lowest contents foundin the active continental margin basalts (at thesame silica content ). That is the reason for thehigh K/Sr, Rb/Sr and Ba/Sr values observed(Fig. 5). The Th/Yb ratio is also high (for a

1,00 600 E00 1000 r 200

196

TABLE 3

Trace-element abundances in some ofthe basic volcanics from the Eastern Rhodopes area (in ppm)

ScBaSrHfThTaLuSmSampleNo.

17.8 33.331.3 52.3

2r.4 32.93 r .7 52 .819.2 34.134.1 50.8

1 . 3 0 . 1 4

i., o.rt2.r 0.391.9 0.26

1238 26.6956 25.2

755 14.0723 24 .6766 27.6869 17.6

0.3 r .4 6.9 2.60.5 2.8 10.5 4.0 445

4810.6 1.4 10.4 2. ;0.6 3.5 13.0 4.0 4570.4 3.0 10.4 2.8 4580.9 7. t 19.9 4.0 461

1 . 5 1 . 4

0 . 6 1 . 1t . 2 1 . 00.8 r .21 .3 t .4

4 . r6.4

7 . 15 .84 .88.9

39

1028

I235678I

10

Samplet \ o .

NbAuSbCrRbCsZr Ni Mn

2 5 23 1 2

2 6 42630

1 8 2 - 1 0 - 1 5 . 081 325 82 22.4 - 48 1.6

148 9.7 62 22 - 623 26.0 2.0 136 0.4154

- 0.9 51 36 2i 622 18.3 2.3 0.8154 0.2 47 20 - 554 26.7 2.5r23 5.3 57 48 30 578 24.1 4.4 84 0.2196 4.2 143 2t 8 719 23.4 6.8 114 0.3

t9 - 8 16.0

9.9 0.002r .2 0.005

r.4 0.0090.5 0.011.6 0.0074.6 0.015

1

235678I

10

given Ta/Yb ratio, Fig. 8A). AII these ratiosdistinguish the volcanics studied here fromthose in the active continental margins. TheNorthwestern Alps collision basalts (Ventu-relli et al., 1984) show the same geochemicalcharacteristics.

Among the other elements (MORB-normal-ized, Fig. 7 ) we should note the negative Nbanomaly (Ce"/Nb":5.7 and 11.3 ) not found inthe active continental margin basalts (Ce"/Nb"- 1.35-2.04) but observed in some island-arc volcanics ( Pearce, 1982, 1984 ). Besides, thecollision basaits/basaltic andesites describedhere show higher Sm contents than the activecontinental margin basalts whereas their Y, Yb,Sc, Cr and especially Ni contents (at the sameMg content) correspond to the lowest concen-trations found in the latter. The absarokiteshows a low content of Yb but high ones of Crand Ni corresponding to the highest contents

of these elements in the active continental mar-gin basalts.

As it is shown on Fig. 9, the REE patternsindicate a moderate fractionation (LalYb :

9.3*14.8 ) and do not differ from those in someactive continental margin basalts. "fhe Zvezdelvolcano basalts/basaltic andesites show a neg-ative Eu anomaly, and the absarokite a positiveone.

It can be seen that the trace-element pat-terns of the coilision volcanics differ onlyslightly from those in the active continentalmargin ones. That is why in most discrimina-tion diagrams not given here (Ti vs.Zr, Ti vs.P and logarithmic diagrams of various traceelements after Pearce, 1982 ) they lie in the fieldsof island-arc or active continental margin ba-salts. However, because of their high Th and"normal" Hf, Ta and Sr contents they can beset off these fields on the Th-Hf-Ta (Fig. 10)

197

r00807 060

rtfll'? ,tlH

- * 1*--;ir:lfl '

50

{,0

( I ) ? nt

: 1 ^

o

a ll1

0.5

u.a

0.10.05

S r K . 0M oRB 120 o . l t%

Th0 . 2

PrOc Zro:rtl. so

Ti 021.5 o/o

N i5 0 p p m

Fig. 7. MORB-normalized abundances (MORB, after Pearce, 1982 ) of trace and some major elements in the Zvezdel volcanobasalts/basaltic andesites (1 ) and absarokite (2) compared to the collision basalts from the Alps (3) and to the activecontinental margin basalts (4) , d,ata of Ewart ( 1982 ), Robin ( 1982 ), Carr et al. ( 1982 ), Lopez-Escobar et al. ( 1977 ).Note: For SiO, and MgO contents see Figs. 4 and 5.

'll'/"t / t

o.l ' rorvlo10 50

Co ppm

Fig. 8. A. Th/Yb vs. Ta/Yb plot after Pearce (1984) comparing MORB, some activa continental margin (hatchured) andthe Zvezdel volcano collision basalts/basaltic andesites. The vectors represent enrichment by subduction (s ), crustal con-tamination ( c ), within-plate ( u ) and fractional crystallization (/).B. Cr. vs. Co and Ni vs. Co plots for the Zvezdel volcano collision basalts/basaltic andesites (dots ) and absarokite (triangle )compared to active continental margin volcanics (open circles ).Note: For SiO, and MgO contents see Figs. 4 and 5.

Li " E

r \1t vI It ll l\J

Y Y b S c C r30 14 LD 2s0

H f S m2.1 33

T o N b C e0.16 3,5 l0

R b B o2 2 0

A

B

z o - o - o Xu o o ^ /

o 2

P- -.L.7 \

/ - o l

6 0000 ]\ t o o o I

H a i! /\ t

, . r i t? p /\ t /Yl./

L./

LL- 2

* - r - - l

+ $ + $ 5

Y b

Fig. 9. Chondrite-normalized REE patterns of the Zvezdelvolcano basalts/basaltic andesites (l ) and absarokite (2)

compared with the collision basalts from the NorthwesternAlps (3, Venturelli et al., 1984 ) , the active continental mar-gin basalts in the Chile Andes (4,after Lopez-Escobar etal. , 1977); 5:REE in the "andesite model" of the uppercrust (according to Taylor, 1977).

Fig. 10. Zvezdel volcano collision basalts/basaltic andesites(solid circles) and absarokite (solid triangle) on the Th-Hf-Ta discriminative diagram (Wood et al., 1979);M O RB : mid-ocean ridge basalts, I4IPB : within-plate ba-salts, DMB:destructive plate margin basalts. Asterisk-collision basalt from the Northwestern Alps (Venturelli eta l . , 1984 ) .

L e / 5 r

Fig. 11. Cr vs. Ce/Sr plot of the Zvezdel volcano basalts,/basaltic andesites compared to the field of volcanic arc ba-salts (after Pearce, 1982 ).

C t / 1 0 I O X I U

Fig. 12. Zvezdel volcano collision basalts/basaltic andesites( I ) and absarokite (2 ) from the Eastern Rhodopes plotted

on the Hf-Cr-Ta and Rb-Cr-Ta diagrams. For comparison:active continental margin basalts from (3) Mexico (data

of Robin, 1982) and (4) Lhe Chile Andes (data of Lopez-Escobar et al. . 1977): 5:mean abundances of these ele-ments in MORB, traps and continental rift basalts (after

Kravchenko et al. , 1983); 6:col l is ion basalts from theNorthwestern Alps (after Venturelli et a1., 1984 ).

C rr 0 0

EO1 A

6 0

5 0

4 A

l 0

IO9

61

6

5

=

a

E

Eu

80O Ur,0

H f l l

i b . ' ^ c o n t . r i f t s/ . O

199

and on the Cr vs. Ce/Sr diagrams (Fig. 11 ).Without being discriminative diagrams, those

of Ta-Hf-Cr and of Ta-Cr-Rb (Fig. 12 ) permitdistinguishing (following Kravchenko et al.,1983) the deeper derived basic magmas (for

comparison Fig. 12 shows MORB, traps andcontinental rift basalts ) from the shallower,probably much contaminated magmas (exem-plified by basalts from some continental mar-gins ). With the increasing depth of generationthe abundances of the most refractory elements(Ta, Hf, Nb, Ni, Cr, Co ) also increase. It is seenon the diagrams (Fig. 12 ) that the Zvezdel vol-canics come from depths even shallower thanthe levels where the basalts of the active con-tinental margins are generated and differ fromthe latter in their low CrlCo and Ni/Co values(Fig. 88). In contrast, the absarokite resem-bles the deeply derived basalts in its content ofrefractory elements.

Discussion

The very small amount of basalts comparedto the total volume of the Zvezdelvolcanics andthe pronounced differentiation of REE with anegative Eu anomaly lead us to assume that theydo not represent the source magma but proba-bly a low P-T fractionate of it.

Certain aspects of the conditions of forma-tion of the volcanics studied can be evaluatedfrom the petro- and geochemical data. Thus, theearlier crystallization of clinopyroxene and bio-tite than plagioclase suggests a relatively highPs,6. The oxygen fugacity as derived from theFe/Mg ratio was probably moderate (10 7-

10 - 8 bars ) in the basalts/basaltic andesites andhigh in the absarokite. With the absarokite asan exception, all volcanics have low Mg num-bers (45.5-56 ), lower than the boundary valuesfor mantle magmas (65-75 ). Furthermore, theZvezdelvolcano rocks have also low contents ofhighly refractory elements: Ni (8-10 ppm), Cr(2I-40 ppm) and low Cr/Co and Ni/Co ratios(Fig. 88), indicative of a formation at rela-tively shallow depths ( Kravchenko et al., 1983 ).

cent6;

l 0

2 0

l 0 o m P h . f u s i o n

O A

Fig. 13. Ce"/Yb" vs. Ce,, plot of the Zvezdel volcano ba-salts/basaltic andesites and an absarokite from the EasternRhodopes. The ]ines (according to Gill, 1981) show thechange in the Ce"/Yb" ratio during the partial fusion of acontinental crust of amphibolite composition (the segmentOA generates andesite melt), of eclogites and peridotites,

and during fractional crystallization.

On the other hand, similar to collision volcan-ics from elsewhere (Gill, 1982 ), the basalts/ba-saltic andesites described here are also enrichedin a number of incompatible elements such asCs, Rb, U and Th. The Rb/Sr ratio (Fig. 5)shows high, typically crustal values (after Ven-turelli et al., 1984). The high Th/Yb ratio re-flects, according to Pearce (1984), a substan-tial "contribution" of subduction and/or crustalcontamination (Fig. BA).

The REE distributional pattern of the Zvez'del volcanics is similar to the upper crust in the"andesitic" model of Taylor (1971). The IowCe"/Yb" ratio identifies the former on the dia-gram of Gill ( 1981) as possible products of am-phibolite fusion (Fig. 13). This is compatiblewith the petrological data - the only xenolithswith traces of melting found in the intermedi-ate Eastern Rhodopes volcanics consist of gab-bro and amphibolites ( Marchev. 1985 ).

The above evidence may be interpreted invarious ways which prevent us from reachingan unambiguous conclusion about the source ofmagma that produced the differentiates de-scribed and about the role of the thick conti-nental crust, whether it acted as a filter andcontaminator of mantle magma (Thorpe andFrancis, 1979 ), or it was the site of magma gen-

eration (Hawkesworth, 1982) under the action

200

of water- and potassium-rich fluids.Isotope studies of the volcanics described

have not been carried out. There are data onlyfor sample 11 from North Greece (875r/865r0.7063, Innocenti et al., 1984) and for the in-termediate and acid rocks from the Borovitzaregion (lVlarchev, 1985) which have practicallythe same values (0.7076-0.7096). They aresimilar to the isotope ratios in some active con-tinental margin volcanics. It is believed, how-ever, that in areas ofthick continental crust theIatter affects greatly the Sr isotope ratio and itis extremely difficult to solve the problem of themagma source (Hawkesworth, 1982; Balashov,1 v 6 5 t .

The limited geochemical data available for theabsarokite indicate some features of mantlemagmas: high Mg number (66-73.5), high Cr(325 ppm) and Ni (80 ppm) contents and cor-respondingly high Ni/Co and Cr/Co ratios. Likethe other collision volcanics, the absarokite isalso enriched in LIL elements, especially in Rband Cs (Ivanov and Stoyanova, 1966) as wellas in U and Th. The positive Eu anomaly can-not be attributed to the cumulative genesis sug-gested by Ivanov and Stoyanova (1966) sincethere is no plagioclase among the phenocrysts.An acceptable model is that of Shih (1972)which deals with the melting of a substrate be-longing to the plagioclase association and rela-tively enriched in Eu (i.e. existing at depthsshallower than 30 km and at presumably highfo").

Conclusion

The collision type of magmatism manifestedin areas with thick continental crust is charac-terized by the presence of large volumes of in-termediate and especially of acid rocks and,correspondingly, of insignificant amounts ofbasalts.

The collision basalts/basaltic andesites de-scribed have high SiO, content, thus forming atransition towards the intermediate membersof the volcanic series. Comnared to the active

continental margin basalts they are more en-riched in LIL elements, particularly in Rb andCs as well as in U and Th, showing correspond-ingly higher K/Sr, Rb/Sr, BalSr and Th/Tavalues, and, on the contrary, they have lowerTi, P, Y, Yb, Sc, Cr and especially Ni concen-trations ( at the same SiO, and MgO contents )and a negative Nb anomaly. All these specificfeatures in combination with some discrimi-native diagrams (Th-Hf-Ta) can be used todistinguish the rocks from the basalts origi-nated in the active continental margin settings.

Along with the basalts/basaltic andesites, thecollision volcanism contains also absarokites ofhigh Mg number enriched in LIL, P and in someferromagnesian elements (Ni and Cr), and de-pleted in Yb.

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