Isotopic ages on ophiolites from the eastern Mediterranean

Proceedings Chapter

Reference

Isotopic ages on ophiolites from the eastern Mediterranean

DELALOYE, Michel, et al.

Abstract

In the context or Eastern Mediterranean geology, the three ophiolitic massifs of Troodos (Cyprus), Hatay (Turkey) and Baër-Bassit (Syria) can be considered, in many ways, as a coherent ensemble. Lapierre et al. (1972) have shown that the Paphos region of Cyprus and Baër-Bassit are similar, while Parrot (1976) has described the close relationship between Baër-Bassit and the Hatay. Many similarities are also evident in the geochronological data.

The gabbros, in particular, have given similar·Cretaceous rediometric ages. On the other hand, dates from the Hatay pillow-lavas lncidate that they have been affected by more recent events than those of Troodos. Good radiometric ages, in aggrement with paleomagnetic and stratigraphic data, are essential to the correct paleogeographic reconstructions of the region concerned. However, the problem in dating ophiolitic rocks is that the majority have been transformed mineralogically and chemically, either on the ocean floor or during emplacement to their present positions. The interpretation of the dates measured, is therefore, difficult. The scatter of dates obtained in our study [...]

DELALOYE, Michel, et al . Isotopic ages on ophiolites from the eastern Mediterranean. In:

Panayiotou, A. Ophiolites: proceedings International Ophiolite Symposium . [S.l.] : Cyprus Geological Survey Department, 1980. p. 292-295

Available at:

http://archive-ouverte.unige.ch/unige:143467

Disclaimer: layout of this document may differ from the published version.

1 / 1

Isotopic ages on ophiolites from the eastern Mediterranean

M. Delaloye, H. de Souza, J.-J. Wagner and I. Hedley

Department of Mineralogy, The University, 1211 Geneva 4, Swit zerland

Abstract

In the context or Eastern Mediterranean geology. the three ophiolitic massifs of Troodos (Cyprus). Hatay (Tur- key) and Baer-Bass il (Syria) can be considered, in many ways. as a coherent ensemble. Lapierre et al. (1972) have shown that the Paphos region of Cyprus and Baer-Bassil are similar. while Parrot (1976) has described the close relationship between Baer-Bassit and the Hatay.

Many similarities are also eviden t in the geochrono- logical data. The gabbros. in particular. have given similar·

Cretaceous rediometric ages. On lhe other hand. dates from the Hatay pi llow- lavas lncidate that they have been affected by more recent events than those of Troodos.

Good radiometric ages, in aggrement with paleomag- net·ic and stratigraphic data, are essential to the correct paleogeographic reconstructions of the region con- cerned. However, the problem in dating ophiofitic rocks is thal the majority have been transformed mineralogically and chemically, either on the ooea n floo r or during emplacement to thei r presen t positions. The interpreta- tion of the dates measured. is therefore, difficul t.

The scatter of dates obtained in our study can be explained by variable radiogenic argon loss or perhaps, by addition ot potassium probably from sea-water (hal- myrolysis) . The presence of excess argon need not be invoked to ex plain these variations.

Introduction

Tr.e NE-SW trending Hatay ophiolitic massif (Vuagnat and Cogulu, 1967) occurs at the SW end of the Amanos Mts. between the towns of Antakya and lskenderun (Fig.

1 ). The massif is particularly well exposed along the coast wh ere all the elements characteristic of an ophiolile are seen:

- an ultramafic complex of harzburgites and dunites (sometimes serpentinized and tectonized),

- layered and homogenous gabbros.

- a sheeted dyke complex.

- and pillow lavas.

Sedimentary units above and below the massif are also

Problems of dating ophiolitic rocks

Dating o phiolitic rocks by the Potassium-Argon method poses a number of problems which are discussed briefly below.

1. As many of the opl1iolitic roc ks are line-grained, minera l separations are not easy, obliging the use of whol e-rock samples, the dates or which are difficult to interpret.

2. The potassium contents of sam ples are usually low - less than 0.3% K. This leads lo greater errors in the measurement of the low quantities or radiogenic argon and thus in the ages themselves . 3. The ubiquitous alteration of the ophiolitic rocks often

results in anomalous ages being recorded.

4. These rocks have usually been metamorptwsed lo greenschist facies on the ocean floor often resulting in the partial or complete resetting o f their ages.

In consequence, the ages measured on ophiolitic rocks and minerals need to be interpreted wil11 great cau- tion .

These problems have deterred isotopic dating studies in the past. In the absence of rocks and minerals that can be reliably dated. our approach has been to analyse a large number or samples to see if significant age pallern s.

in relation to other geological and geophysical data, can be obtained.

Previous age determinations

Previous age studies on the Eastern Mediterranean ophio- lites have concentrated on the Troodos and Hatay com- plexes. At present. no dates are available from the Baer- Bassit complex . Vine et al. (1970) obtained ages between 70 and 90 m.y. from the Troodos. partly con flrn1ing the

T U R K E Y

seen in the coastal section (Dubertret, 1953). 1 - - - --.-·ne Ralay massif is part of a wider ophiolitic belt in

the Eastern Mediterranean (Fig. 1) which includes the Troodos and Baer-Bassil ophiolites. The former has been intensively studied (see Coleman. 1977, for a detailed bibliography) while the latter has been described by Par- rot in a series of papers (e.g. Parrot, 1974). Similarities between the ophiolites of the Hatay/Baer-Bassit area and Cyprus are well esta bli shed and they are considered to be fragments of the Tethyan ocean floor (e.g. Mesorian et al., 1973). Particularly striking are the correlations between the dismembered Baer-Bassil complex and the Paphos region of Cyprus, (Lapierre and Parrot, 1972) on the one hand and between the Hatay and Troodos complexes on the other (Baroz et al. , 1976).

This paper reports some new ag es from the Hatay gabbros and seeks a wider interpretation of already published isotopic ages.

MEDITERRANEAN SEA BAER-

BASSIT

0 ,_____, 50 km

4

0::

>- en Fig. 1 Geographical situation of East Mediterranean ophiolites.

pre-Campanian age of the complex indicated by the pal- aeontology (Moores and Vine, 1971 ). The K-Ar isochron age of 79 m.y. (Campanian as defined by Odin, 1978) obtained by Desmet et al. ( 1978) for the sheeted dyke complex has been supplemented by nine new K-Ar whole-rock ages, ranging between 70 and 105 m.y. (Delal- oye et al., in preparation). The Troodos pillow lavas have recently beerr <..h:1t&d by Delaloye and De3mct (1!J79) who obtained a K-Ar isochron age of 75 m.y. They interpret this as representing the age of emplacement, in agree- ment with the dates from the dyke complex, and restrict- ing Troodos volcanic activity to the Upper Cretaceous.

The Hatay dykes and pillow-lavas were dated by Del- aloye et al. (1977) . Ages from the latter (44 to 74 m.y.) may be tectonic ages while those of the former (isochron age 94 m.y.) could be near their true age. Two ages from the gabbros (116 and 71 m.y.) were published by Cogulu et al.

(1975) .

Apart from those with very low potassium contents, all of the ages cited above lhave been included In the his- tograms (Fig. 2). All ages quotes in this paper have been calculated or recalculated with the new decay constants (Steiger and Jager, 1977).

KIZIL DAG PILLOW - LAVAS

-

I n

30 40 50 60 70 80 90 100 110 120 130 140 150 160 My.

KIZIL DAG DOLERITES .

30 4"0 50 Go 70 80 90 100 110 120 130 140 150 160 M.y.

KIZIL DAG GABBROS.

R.T.

RT. R.T. RT.

R.T. R.T. M M R.T. I M M 30 40 50 60 70 80 90 100 110 120 130 140 150 160 M.y.

b

'New age determinations

Eight new K-Ar whole-rock and hornblende dates have been obtained for lhe Hatay cumulate-gabbros by one of us (M.0 .) (Table 1). Several other samples were analysed but are considered unreliable because of their very low K values (<0.1%); it is hoped to measure their K values by isotope dilution. Analytical methods are those described by Delaloye and Wagner (1974). ·

The dates from the gabbros (all of which show evi- dence of greenschist facies metamorphism) span a simi- lar range as those from the Hatay dolerites (see Fig. 2b) . Apart from one sample (KA 1107), all the hornblende dates are higher than the whole-rock dates. This may be the result of the higher closure temperature of amphib- oles compared to other mlnerals. The different intervals between the hornblende and whole-rock dates could indi- cate different cooling histories for each sample but other more complicated factors may be involved.

Discussion

All reliable K-Ar age data from the Troodos and Hatay complexes have been plotted in histogram form in figure 2. Data from each unit in the ophiolites have been plotted

TROODOS PILLOW - LAVAS.

-

-

,....__ ,....__

L B u L

u L 8 L u u

30 <10 50 60 70 80 90 100 110 120 130 140 150 160 M.y.

3 3 TROODOS DOLER/TES.

.

3 3 3 L 4 2 2 4 2 B 2 3 3 B

B 2 2 2 5

lil

30 40 so 60 10 8·o 90 100 110 120 130 140 150 100 M.y.

TROODOS GABBROS .

M I R.T.

30 40 50 60 70 80 90 100 110 120 130 140 150 160 M.y.

Q

Fig. 2 Histogram of Ages from the Troodos and Hatay (Kizil Dag) ophiolites.

U

=

=

=

1-4

=

=

M

=

Table 1 Analytical data on Hatay gabbros HBL

=

=

"Ar rad

Analysls Sample 'loK moles/g %"Ar rad

No. x 10-¹⁰

KA-1100 HBL 0.23 0.671 99.7 KA-1100 WR 0.27 0.552 44.7 KA-1105 HBL 0.18 0.294 38.3 KA-1105 WR 0.28 0.418 58.8 KA-1107 HBL 0.11 0.197 24.0 KA-1107 WR 0.09 0.173 19.5 KA-1108 HBL 0.13 0.300 32.8 KA-1108 WR 0.18 0.347 ~.2

Age in m.y.

157.6 ± 6.1 113.8±18.3

91.8± 8.3 83.9 ± 4.9 101.5±13.1 104.3±15.7 128.6±11.3 105.2 ± 6.9

separately. The age distnoution for both ^complexe~

seems similar and both are probably Cretaceous in age.

The age spread can be aftributed to argon loss caused by alteration and reheating.

For the Troodos, the apparent ages of pillow-lavas overlap those of the dykes but are generally lower. Only two gabbro dates are avai lable but they are similar to those of the dvkes.

. Like the Troodos, pillow lava dates in the Hatay are generally lower than those of the dykes. Dates from the dykes and gabbros are approximately the same although those of the dykes are slightly higher. This may be related to the cooling of the ocean crust as the gabbros would have cooled more slowly than the dykes. In both the Troo- dos and the Hatay the lower dates of the pillow lavas may be related to leakage of radiogenic argon from the altered minerals and devitrified glass.

The dates from the Hatay dolerites were analysed using the radiogenic •OAr v. •°K isochron diagram (Harper, 1970). These dykes were from a small zone in the dyke complex (- 500 m) and therefore amenable to this type of

· analysis. On the isochrom plot, (Fig. 3) three lsochrons

..

••

0 2

0' ~

••

Fig. 3 ⁴⁰Ar/K lsochron plot for the Hatay Sheeted Complex Dykes.

can be distinguished corresponding to ages of 140 ± .18 m.y., 150 ± 10 m.y. and ·110 ± 10 m.y. (MSWD 3.2, 0.3, 1.2 respectively). The apparent ages of the points on the 140 rn.y. and 110 m.y. 1sochrons (Groups I and 111) are similar to tne1r 1socnron ages. However, the points on the 150 m.y. lsochron (Group II), which has a negative intercept on the ⁴⁰Ar axis, have apparent ages between 120 m.y. and 130 m.y. Indicating that these samples have lost argon.

We interpret the ages from the lsochron diagram as implying that the first two groups of dykes were intruded at about the tim e of formation of the dyke complex (150·

130 m.y.) . The Group I dykes have, however, retained much of their radlogenic argon. The third group is younger, intruded at about 110-120 m.y. This is consistent with field evidence of two or more generations of dykes.

However, major element chemical differences between the dykes are not apparent because of their alteration (possibly deuteric in the youngest) .

Although the evidence form the isochron diagram cannot be considered significant because the number of points is insufficient, the age differences are supported by petrographic and magnetic data. The youngest dykes are slightly fresher with fresh pyroxenes, and it is paradoxical that the most altered samples should give the oldest ages.

The age division into three groups is mirrored by the mag- netic data (obtained on the same samples) showing an increase in magnetic susceptibility from oldest to youn- gest (Fig. 4). This trend can be extrapolated to the Troo- dos dykes which in the histograms appear younger than the Hatay dykes and In addition have slightly higher mag- netic susceptibilities (Vine et al., 1973). The intensity of remanence shows a similar trend.

150 140 130

t

^~

HATA Y I

HATAY II

120 HATAY m

.. o

100 TROOO O S

9 0

10- 4 10-3 10- 2

Magnetic Susceptibility (Gauss I Oe)

Fig. 4 Apparent Age

v.

It has been noted in the N. Atlantic (e.g. Irving et al ..

1970) that the intensity of remanent magnetization dec- reases away from the mid-ocean ridge and the above isot- opic and magnetic data strongly suggests that the Hatay complex is older than the Troodos.

Conclusions

The setting of the Troodos and Hatay ophiolites in the Eastern Mediterranean indicates that they once formed part of the Tethyan ocean crust. The distribution of the isotopic data suggests that the Hatay ophlolite (formed between 150-1 10 m.y., late Jurassic-Cretaceous) is older than the Troodos (110-80 m.y., Mid to Upper Creteceous).

This is supported by the magnetic data.

The Jurassic-Cretacous age of these ophiolites is consistent with the plate tectonic reconstructions for this area (e.g. Parrot, 1977) which imply early Jurassic- Cretaceous sea-floor spreading.

The distribution of the gabbro dates from the Hatay in relation to those of the dolerites, and the dolerite dates themselves, indicate that further K-Ar dating of this and other complexes, together with field, petrographic and magnetic studies, could reveal considerable Information on the formation and thermal evolution of the Tethys ocean crust.

Acknowledgements

We are grateful to A. Coleman, M. Lanphere and C. Tin- kler for discussions and for reviewing the manuscript.

This work has been supported by the Swiss National Science Foundation (Grant no. 2.207-079).

References

Baroz, F .. Desmet, A. and Lapierre, H., 1976, Les traits dominants de la , gllologle de Chypre. Bull. Soc. Geol. Fr., 18, 419-427. : Cogulu, E., Delaloye, M., Vuagnat, M. and Wagner, J.J .. 1975, Some geo·

chemical, geochronologlcal and petrophysical data on lhe ophiolllic mas- sif from the Klzll Dag (Ha1ay. Turkey). C.R. Soc. P/iys. Hist. N;it., ^Gen~ve, 10/2, 141-150.

Coleman, R., 1977, Ophiolites. Springer-Verlag, Berlin, 220p.

Delaloye, M. and Wagner, J.J., 1974. Potassium-argon dating; an auto·

matlc equipment with digital output for computer processing. C.R. Soo.

Phys. Hist. Nat., Gen~ve 9, 66.

Delaloye, M., Vuognat, M. and Wagner, J.J., 1977, K-Ar ages lrom the Klzil Dag optilolltlc complex (Hatay. Turkey) and their Interpretation. inl. Syrop.

Struct. Hist. Medit. Basins, Eds. Blju-Duval, Band Monladert, L .. Split, 73·

78.

Oelaloye, M. and Desmet, A .. 1979, Nouvelles donnees rediometriques sur les pillows lavas du Troodos. C.R. Acad. Sc/.. Paris, 288, 461-464.

Desmet, A., Lapierre, H., Roccl, G., Gagny, C.C., Parrot, J.F. and Delaloye, M., 1978, Constitution and slgnillcance of the Troodos sheeted complex.

Nature, 273, 527-530.

Oubertret, L., 1953, G<lotogle des roches vertes du nordouest de la Syrle et du Hatay. NotrM et Men. Moy. Orient, 6, 2·179.

Harper, C.T., 1970. Graphical solutions to the problem or radiogenic Argon-40 loss trom metamorphic minerals. Eclogae, Geol. Helv., 63. 119·

140.

Irving, E., Park, J.K., Haggerty, S.E., Aumento, F. and Loncarevic, B., 1970, Magnetism and opaque mineralogy of basalts from the mid-Atlantic ridge at 45° N. Nature, 228, 97 4-976.

Lapierre, H. and Parrot, J.F., 1972, ldentite geologique. des rllgions de Paphos (Chypre) et du Baer-Bassit (Syrie). C.R. Acad. Sci., Paris, D, 274, 1999-2002.

Mesorlan, H., 1973, ldees actuelles sur la constitution, l'orlgine et !'evolu- tion des assemblages ophiolitiques mesogllens. Bull. Soc. Geol. Fr .. )5, 478-493.

Moores, E.M. and Vine, F.J., 1971 , The Troodos massif, Cyprus and other ophiolites as oce·anlc r.rust: evaluation and implications. Roy. Soc. Lon- don, Phil. Trans .. A. 268, 443·468.

Odin, G.S., 1978, Sur .la duree des lltages et biozones du Ctlltacll. Bull.

Liaison lnf. /GCP Projet 1G3/4, 9/ 18.

Parrot, J.F., 1974, L'assemblage ophiolitlque du Baer-Bassil (nord-ouesl de la Syria): Elude petrogrephique et geochlrnique du complexe filonien, des laves en coussins qui lul sont assootees, el d'une partledes formatlons effuslves du volcano-slAdimentalre. Cah. Orstom, Ser. Gdol., 6, 97-126.

Parrot, J.F., 1977, Ophlolites du nord-ouest syrien et evolution de la croute oceanlque tllthysienne au cours du Mesozoique. Tactonophy- sics, 41 , 251-268.

Steiger, R.H. and Jager, E., Subcommisslon on geochronology: conven·

tion on the use of deoany constants in gee· and cosmochronology. Earth

Planet. Sci. Lett., 36-359. .

Vine, F.J.₁ Poster, C.K. and Gau, l.G., 1973, Aeromagnetic survey of the Troodos igneous massit, Cyprus. Nar. Phys. Sc/., 244, 34-38.

Vuagnat, M. and Cogutu, E., 1967, Quelques rllflexions sur le massif baslque-ultrabaslque du Kizll-Dagh, Haiay, Turqule. C.R. Soc. Phys. Hist.

Nat., Geneve, 2, 210-216.