G EOLOG ICA BALCANICA 19. 3, Sofia , Jun. 1989, p. 61 - 67.

Petrographic and geochemical ophiol itic rocks tn Northern Samothrace,

characteristics of the Greece

C. N . Kotopouli, K. Hat::.ipanagiotou, B. Tsikouras

Unirersily vf Pa1ras , D epart111ent of Geo logy, 261 10 Parras, Greece

( A ccepled /or pub lication D ecember 2, / 988)

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Bee nOpO,lbl HMefOT TO.leliTOBbiH COCTaB. KpOMe TOrO pacnpe-'l,C.leH He JJieMeHTOB B llOJJepHTaX xapaKTep­HO llJlll OCTPOBO.'lY)I(HbiX TOJJeHTOB. H aw fl HCCJlellOBaHHSI llOKa3biBafOT. 'ITO npH neTpOreHe3HCe 3THX llOPOll HMeJI MeCTO H cy6.J,yKUHOHHblii KOMOOHCHT. H a 0\.:HODaHHH Ony6nHKOBaHIIblX cpaBHHMblX llaHHbiX 0 CXO)lHblX llpOHB­JJeHHSI X B 0KOi !OpOLIO IICKOH 30He npe.i!llO.'IO)KeHa BOJ~IOlKHOCTb o6pa30BaH HH JTHX O<j}HO.'IHTOB B ycnOBHHX JOHb! cynpa-cy6.L()' KllH.OJ IHOro n 1na .

Abslracl . The ophiolite occu rence in . Samothracc consists of mafic plu tonic rocks in t ruded by W-trending dolerite dikes. The petrographic and geochemical examination suggests tha t they a re gabbros cor responding to the intermediate segment of an ophiolite sequence.

All rocks stud ied have tholeitic compositio n s. I n addition the dolerites y ield geochemical di stribution patterns exh ibiting characteri st ics o f i sland arc tholeite s. Our results indicate invo lvement of a subduction com­ponent i n the pet rogenesis o f these rocks. In view o f compa rable data repo rted from similar occurrences else­where in the C ircum-Rhodope Belt. the prob~bi li ty of a supra-subduction-zone - type tectonic environment is sugge st ed for these ophio l ites.

Introd uction

Th e isla nd of SLlmothrace. in the NE Aegl.!an Sea (Fig. I) , belongs to the Circum- Rh odope Bel t (K a u f ma n n et al .. 1976). In Greece, thi s belt extends from the Do ira ne La ke and through the Cha lkidike penin sula a nd part s of the Evros County, as far as the Greek-Bul­garian state boundary. Beca u se of it s geograp hic location th e isla nd of Samothrace is uni­que in the sen se that it forms the link between the western and eastern branches of the Cir­cum Rh od o pe Belt in the Aegea n : thu s a good under sta ndin g o f the island 's geological struc­ture and hi sto ry will co ntribute to a bette r kn owledge of th e geo logy of the reg1o n.

Geologica l se tting

Previou s work ( D av i s, 1963: H e i m mann et al.. 1972 ; F y t i k a s et a l. , 1979) has shown th at the i sla nd of Samothrace co nsist s of four main geological unit s (Fig. 1). The se a re a) op hi o litic rocks of mafic composition, contai ning loca lly inclu sio n s of greenschist­facie metamorpho sed rocks and occur rin g in the lower topographic pa rt s; b) a granitic

61

N

(]) S £ A

· · ~ --=~~=

-··---- - - -- ---------- - - -- ·--- ------ --- ----------------Fig. I . Simplified geological map o f Samo thrace (after D a v i s, 1963; H e i m m a n n, et a l. , 1972) CR : C ircum-Rhodope belt ; K : Kario tes; Pe: Pelagonia n zone; R h: Rhodope mass if; T h : T he rma; Y: Yard:u zone; solid black symbols in in set show o phiolite occurrences

intru sion o f Oligocene age (K y r i a k o p o u l o s, 1987) occ upying th e centra l pa rt of the isla nd : c) py roclast ic rocks o f rhyodacit ic and trachya ndesitic compo sition and d) Te r­tia ry and Quate rna ry clastic sed iment s occurring alo ng the periphe ral pa rt s of t he isla nd.

The mafic rocks outcropping a ro und the Oligocene gra nit ic in t ru sio n ma ke up a signi­fica nt pe rce ntage o f the isla nd 's a rea .

Thi s pa pe r report s o ur f ir st result s on the petrog rap hi c an d geochemical cha racte ri­za tion of the gabbro ic membe rs of t he ba sic se ri es ; in pa rti cul a r it co ncerns t he mos t typical occurence o f the se rock s, which is located in the northern pa rt of the isla nd . Thi s mass ive plutonic body extend s betwee n the localiti es K a riotes a nd Therm a. over a n outcro p area of about 5 km2 a nd is intruded by seve ral fin e gra ined dole rite di ke s. T he dike s a re up to a few meter s wide a nd have a ge neral no rtheaste rly directi o n. Repre sentati ve samples from these occurrences were coll ected fo r petrog raphi c a nd geoc hemica l wo rk.

Petrography

a) Gabbros They a re the p redo minate rock type. T he texture is hy pidi o mo rphic gra nula r a nd ra re ly cumulitic whe rein clino pyroxene is the cumulu s pha se a nd plagioc la se a nd Fe-Ti oxide s form the post-cumulus phases. Gra in size range s fro m 0.5 to 3 em a nd from 0.2 mm to 2cm in

62

the cumulu s and post cumulus crystals respectively. The idiomorphic to hypidiomorphic plagioc la se crystals (An ~~ - 70 ) ex hibit polysynthetic twinning and are often altered to seri­cite, clinozoisite, epidote, albite a nd calci te. Clinopyroxene forms short-pri smatic pa le green slightly pleochroic crysta ls, with extinction angles ny/c = 50°- 59° and ax ial angle s 2Vv = 40°-440. It contains inclu sions of apatite and rarely interlocking crystals of o rthopyroxene. The clinopyroxene have bee n subjected to a varying degree of uralitization and chloriti­zation that on some occasion s have completely transformed the primary crystals. b) Dolerite.\· These rocks exhibit porphyritic, ophitic and subophitic textures.Grain size ranges between 0.2 a nd 0.8 mm . As in gabbros, the minera l a ssem blages is clinopyroxene + plagioclase. Jdiom orphic to hypodiomorphic, twinned plagioclase (An3s-;r.) either occ urs as porphyro­crysta ls a nd in the grou ndmass of po rphyritic rocks or, together with clinopyroxene and magnetite, participates in the intergrowth texture s of the ophitic and subophitic members. Tran sformatio n of pl agioclase to an aggregate o f albite, clinozoisite, epidote and prehnite ha s been observed. The clinopyroxene forms idiomorphic to hypidiomorphic, pale green, occa sionall y twinned crystals wi th extinction angles nv/c=40°-44° and axial angles 2Vy= 50°-56°. I t occu r s both as porphyrocrystals and in the groundmass of the porphyritic rock s. The clinopyroxene s have been variously altered to amphibole and chlorite. Allotrio­morphic brownish-green ho rnblende is also observed in the ophitic rocks. Accessories are magnetite and sphene occa siona lly altered to xenotime. Some samples contain zeolites either as fracture filling materia l or in the g roundma ss; X-ray examination showed the presence of thomsonite and laumonti te.

The afo rementioned secondary assemblages are considered to be products of oceanic hydrotherm a l metamorphism that i s known to affect with variable intensity especially the upper part s of the ophiolitic complexes (G a s s, S m e w i n g, 1973 ; M i y a s hi r o, 1973 ; S p o o n e r. F y f e, 1973).

Geochemistry

Chemical analy sis for majo r, minor and trace was pefromed by ICP (ARL-OES 3520), after dige stion in HF-HN03-HCI0 4 , a t the Institute of Geology and Paleontology of Braun­schweigh. Silico n was analyzed from solutions of Li-tetraborate flux.

The chemical analyses of five gabbro s a re shown in Table I. From the relationships (Na ~O+ K~O) vs Si0 2 (J r vine, Barag ar, 1971), Al 20 3- Mg0-(Fe 20 3 + Fe0+ Ti0 2 ) (J en sen. 1976) and AFM it follows that the Samothrace gabbros have a subal­ka line tholeitic character. Figure 2, shows that the se rocks plot in the field of the mafic cu-

1\, ,(),

~""· "'' "

' - ----- ---~----------

(

, ~ CU iol liL A l £

-. ' \ . . ·'---· .----; -.

K O ... o\ lttl t ~/ / ~ I ~ M[ l A M Of\ Pili C

"~ .;: ~ ~:;;c--l_ ___ __::::-) "/ '"" • O '--------------==·'-' r.• .. o

Fig. 2. Al 20 3 - CO - MgO composition diagram (C o I e m a n, 1977) for the Samothrace gabbros

0 '

z, r ;

Fig. 3. MORB - normalized (Pea r c e, 1982) geochemical distribution pattern of the N. Samothrace dolerites (based on four samples)

63

,,

Tab I e I

Chemical analyses of representative rocks from N. Samothrace

.Mt % 3 4 5 8 II 12 2 6 7

------- -----~-- --- --Si02 48.1 49.6 50.3 48.6 50.2 17.9 18 7 50.9 50.0 Ti02 0.25 0.26 0.28 0.25 0.31 0.62 0.75 !.51 0.45 Al 20 01 17.0 15.6 12.7 16 .5 15.7 17.1 16.3 1·1.9 1.5.0 Fe20 3* 6.32 6.26 8.54 6.01 6.0 1 7.69 8.68 10.75 10.12 MnO 0.13 0.17 0.19 0.12 0.13 0.13 0. 15 0.17 0.16 MgO 10 35 11.37 12.82 10.25 10.46 8.2 ·1 9.30 7.lil 8.79 CaO 14.G9 11.39 11A6 14.39 12.58 11.84 1!.83 10.10 10.89 Na.,O 2.17 2.87 2.16 2.02 2.21 2.59 2 .94 3.67 3.25 Kl) 0 .. 31 0.43 0 20 0.18 0.52 0.25 0.34 0.24 0.14 P205 0.02 0.01 0.0·! O.o2 0.02 0.02 0.07 0.11 0.12 L. 0 . I. 2.02 1.65 1.65 I 38 2.39 3.4.5 1.78 0.92 !.32

Total I 01.36 99.61 100.3-! 99.72 100.53 99.83 100.8 I 100.88 100.2<4

Trace elements (ppm)

Zn 47 67 66 43 62 74 75 91 90 Ba 29 45 36 20 42 105 7S 74 37 Nb 8 8 13 9 10 21 20 23 18 Zr 18 17 20 26 20 30 25 31 28 y 8 9 6 10 10 18 19 30 29 Sc 38 45 37 39 38 32 34 34 3 1 Cr 190 161 572 431 304 218 204 22 1 199 Ni 81 78 186 130 96 109 14-1 84 79 Cu 24 15 68 27 20 62 65 57 50 Sr 89 90 80 92 73 212 199 178 1.38 Co 48 43 67 61 59 76 59 69 69

3, 4, S, 8, 11 = Gabbros; 12, 2, 6, 7 = Doleri les

* Total iron analysed as Fe20 3

mulates of the Al 20 3-Mg0-Ca0 diagram (C o I e m a n, 1977). The SiO~ vs FeO* / FeO ''' + MgO a nd AFM plots of the gabbros (not shown) yield similar results. From Table I is can be see n that the chemical composition of the Samothrace gabbros is characterized by rel a tively low Ti0 2 , P 20 5 , Sr, Ba, Zr and Y contents; a nd relatively high Al 20 3 and CaO. Similar compositions are reported from the cumulate parts of typical ophiolote complexes (Co I em an, 1977).

The chemical analyses of four representative dolerite samples are given in Table l. On the ba sis of the compositional parameters referred to a bove in the case of gabbros, the dolerite compositions are shown to have also a cubalkaline tholeitic character. The MORE­normalized (P e a r c e et a!., 1982) geochemical distribution pattern of the doierite s (avera­ge of four samples) is given in Fig. 3. Characteristic features of this pattern are: enrichment of the most incompatible elements (K, Ba and Nb) a nd Sr; depletion of the moderately and less incompat ible element s (P, Zr, Ti and Y), Sc and Cr; an d a negative Zr anomaly.

The enrichment of elements with low ionic potential (Sr, K, Ba) ha s been attributed by seve ral investigators (e. g. Best, 1975; Saunders, Tarn e y, 1979) to enrichment of the mantle source by aq ueou s fluids derived from dehydration of the subducted oceanic crust. This enrichment and the distribution of the elements with high ionic potential , with the exception of the high normalized Nb value exhibited by the dolerite patterns, arc typical characteristics of volcanic arc basalt s (Pearce et a!. , 1982). Due to the relative immo­bility of Nb in aqueous fluids, the high Nb content of th e dolerites probably repre sents and additional enrichment mechanism unrelated to subducti on. Figures 4 and 5 show plots of

64

~ ( f lQQ

10 )0 0

E 0. 0.

..: ..... oco

"

; QOO L-----------------~--------r-----------------~------~ 10 ~ 100 ~00 1000

Zr ppm

Fig. 4. Ti - Zr disc riminat ion diagram for theN. Samothrace dolerites. Fields after Pearce (1982): WP: within plate; MORB: mid-ocean ridge basalt

1000

E a. a.

~00

100

~0

••

r t 1::

- .""':"--

t 1\ \ If. ~\ v \ \

Y ppm

> ...... "" \ MORI

\ \ \ \ I I I I I I

-- :c_)

Fig. 5. Y-Cr discrimination diagram for the N . Samothrace dolerites. Fields after Pearce (1 982): IAT: island arc tholeite; WPB: within pla te basalt; MORB: mid-ocean ridge basalt

5 Geologica Balcanica, 19. 3 65

the Samothrace dolerites on the Ti-Zr and Cr-Y discrimination diagrams (P e a r c e et a!. 1982). Here also a subduction-related tectonic environment is indicated for these rocks; i~ addition , a particular affinity to isl and arc tholereites i s evident (Fig. 5).

Discussion and conclusions

The mafic plutonic rocks of N. Samothrace present petrographic and geochemical features appropriate to rocks from the gabbroic intermediate segment of an ophiolite complex.

The absence of ul t ramafic rocks in the study area render s identification of the likely setting of formation difficult. The accumulated data from other fragmentary ophi olite com­plexes in the Circum-Rhodope Belt include geochemical evidence for subduction zone invol­vement and geological evidence for extensional tectonics and marginal basin environment (Be bien, 1982; J u n g, M u s sa 1 I am, 1985 ; M us sa 11 am, J u n g, 1986; Be­b i e n et al., 1987). These data have been interpreted in terms of a tran sitory arc-marginal basin system involving island arc tholeitic volcanism, and oceanic crust developed as part of a short-lived Tethyan marginal basin during the Jurassic and Early Cretaceous (J u n g, M u s sa I I am, 1985; Be b i en et a!., 1987).

Marginal basin ophiolite s conta ining a geochemical component from an underlying subduction zone (supra-subduction zone ophiolites) a re di stinguished from mid-ocean ridge ophiolites which do not contain thi s component (Pe a r c e eta!., 1984). ln the context of a Circum Rhodope marginal basin evolution, the subduction-related geochemical cha­racter of the dolerite dikes intruding the Samothrace gabbros may represent incipient arc magmatism: in a supra-subduction setting, this event could succeed a subduction-related spreading stage.

Acknov>'ledgements

Chemical analyses were carried out at the geochemical laboratory of the Institute of Geo­logy and Paleontology of the Technical University of Braun schweig, under the supervision of Dr. Zachmann ; Mrs. D. Soldatou typed the manuscript .

References

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Best, M. G. 1975. Amphibole-bearing cumulate inclusions, Grand Canyon, Arizona and their bearing on silica-undersaturated hydrous magmas in the upper mantle. - J. Petrol., 16; 212-236.

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Kaufmann, G ., K o c k e I, F., M o II a t, H. 1976. Notes on the stratigraphi c and paleogeographic posi­tion of the Svoula Formation in the innermost zone of the Hellenides (Northern Greece). -Bull. Soc. Geol. France, 18. 2; 225-230.

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M u s s a I a m, K., J u n g, D. 1986. Petrology and Geotectonic Significance of Salic Rocks Preceding Ophio­lites in the Eastern Vardar Zone, Greece. - Tsclzermaks l\1itz . Petr . Mitt., 35; 217-242.

Pea r c c, J. A. 1982. Trace element characteristics of lavas from desrtuctive plate boundaries. - In: R. S. T h o r p e (ed.), Andesites; 525-548.

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