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Geochemical Journal, Vol. 33, pp. 369 to 377, 1999

K-Ar ages of the Aegean volcanic rocks and their implication for the arc-trench system

JUN-ICHI MATSUDA,1 KENJI SENOH,1 TERUYUKI MARUOKA,1 HIROKI SATO' and PANAGIOTIS MITROPOULOS2

'Department of Earth and Space Science , Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan DDepartment of Geology , University of , Panepistimiopolis, Ano Ilissia, Athens 15784,

(Received December 5, 1997; Accepted May 28, 1999)

K-Ar ages have been measured for calc-alkaline lavas from the Aegean volcanic arc in order to deter mine formation ages of the volcanic area of the . The samples from islands lying on the north side of the volcanic arc (e.g., , ) show older ages (2-3.6 Ma) than those from the islands lying on the south side of the volcanic arc (e.g., Methana, , ) showing younger ages up to 1.2 Ma. These ages seem to be in agreement with the model that the volcanic front has migrated to the south. The migration velocity has been estimated to be about 1 cm/yr. in this study, which is comparable to, but slightly less than previously reported values.

radially symmetric velocity model, the PM2 INTRODUCTION model, which is appropriate for the European The Aegean volcanic arc has formed in re Mediterranean mantle. Based on the 3-D velocity sponse to northeastward subduction of Mediter heterogeneity relative to the PM2 model, Wortel ranean seafloor of the African plate beneath and Spakman (1992) inferred that the deeper part and the southern Aegean, which are in the south of the subducted slab has become detached from end of the Eurasian plate (Fig. 1) (Hatzfeld, 1994). the shallower part at about 200 km depth beneath The magmas generated above the subducting plate the Dinarides-Hellenides (Hellenic/Aegean sub were spouted out, and formed volcanic islands duction zone and its extension in NW direction). The slab detachment started in the NW and mi (e.g., Mitropoulos and Tarney, 1992; Tatsumi, 1986). The volcanic islands (Aegina, Methana, grated in SW direction along strike. The gravita Poros, , Santorini, Kos and Nisyros) on a tional forces concentrated on the undetached part NW-SE arc stretching from the location near Ath of the slab caused slab roll-back and the south ens to the Turkish mainland consist of double ward movement of the Hellenic trench. The simi structure; Aegina and Kos form an inner (north lar geotectonic evolution of the Aegean arc has ern) volcanic arc, and others form an outer (south been described from the evidence of back-arc ern) arc (Fig. 1). These islands lie about 150 km volcanism indicating a discontinuous southward above the Wadati-Benioff zone, and the slab is migration of an arc-trench system (Boccaletti et thought to extend to some 600 km beneath the al., 1974; Papadopoulos, 1989). The caic-alkaline Aegean (Spakman et al., 1988). volcanics observed from Crommyonia (between Spakman (1991) and Spakman et al. (1993) Athens and Corinth) to Nisyros is clearly related investigated the 3-D P-wave velocity structure of to the active subduction. Quaternary andesitic the mantle beneath Europe, the Mediterranean re volcanism is observed in an internal position of the southern . Further north, the gion and a part of Asia. They used an improved

369 370 J. Matsuda et al.

N

•/ 0 t O

0 Greece • `' 1• . P b~. a Crommyonia Athens : a AG Q 3 4 ME` \ Z .o \ 0 0

. o PO MI\,I CS Q NI 0 SA ND 'f * Aegina 66 NDKos e~ . ME Methana MI Milos NI Nisyros PO Poros 0 100km SA Santorini Aegean i sland arc

Fig. 1. Map of the Aegean region.

volcanism becomes older and is of Oligocene time Ar age determination will be discussed in the next in (Fytikas et al., 1984; Pe-Piper section. The chemical compositions of major and and Piper, 1989). trace elements in these samples were given in that In order to discuss the Aegean arc develop paper (Table 2 in Mitropoulos et al., 1987). The ment, the radiometric ages of individual volcanic rock types ranging from basalts to rhyorites and islands of the Aegean arc are necessary. Here, we location of the samples are given in Table 1. The present K-Ar ages of volcanic rock samples of light REE enrichment in the eastern and western Aegina, Methana, Poros, Milos, Santorini, Nisyros sectors of the arc is greater than the central part . and Kos in the island arc area (Fig. 1). The concentrations of Ba and Sr were high and those of Th, K and Rb were relatively low in the western (Milos, Poros, Methana and Aegina) and SAMPLES eastern (Nisyros and Kos) sectors. Thus, it is Most of the samples used in this study were known that geochemical feature of Aegean arc is ground powders previously used for X-ray fluo different at the location of the arc, and is more rescence analyses for the determination of major limited in individual islands. and trace element chemistry (Mitropoulos et al., To examine the accuracy of the obtained K-Ar 1987). A problem to use powdered samples for K ages in our laboratory, we also measured the K K-Ar ages of the Aegean volcanic rocks 371

Table 1. Sample names, rock types and locations

Sample Rock type Location

Reference YZ-1,1 basalt Zao Volcano, Yamagata, Japan Aegina AG-2 andesite Marathon AG-5 andesite Profitis Ilias AG-6 andesite Perdlka Kos KO-6 dacite Vigla (Kephalos) KO-7 dacite Vigla (Kephalos) Methana ME-2 andesite Kammeno Vouno ME-3 andesite Kammeno Vouno ME-5 andesite Megalo Chorio Milos MI-1 dacite Plaka Village (north of Adamas) MI-3 dacite Plaka Village (north of Adamas) MI-4 dacite Plaka Village (north of Adamas) Nisyros NI-1 andesite south of Mandraki Village NI-2 andesite south of Mandraki Village Poros PO-1 andesite Poros Village PO-2 andesite Poros Village PO-3 andesite Poros Village Santorini SA-4 basaltic andesite Megalo Vouno (north Santorini) SA-5 basaltic andesite Megalo Vouno (north Santorini) SA-6 andesite Megalo Vouno (north Santorini) SA-7 andesite Megalo Vouno (north Santorini) SA-8 basaltic andesite Akrotiri (south Santorini) SA-9 basaltic andesite Akrotiri (south Santorini) SA-10 basaltic andesite Akrotiri (south Santorini) SA-I1 andesite Mikros Profitis Ilias (central Santorini, south of Megalo Vouno) SA-13 andesite Mikros Profitis Ilias (central Santorini, south of Megalo Vouno)

Ar age of the sample YZ-1 which is the Quater tained from the Ar analysis of each sample. A nary reference sample prepared at Yamagata Uni VG5400 noble gas sector-type mass spectrometer versity (Takaoka, 1988). YZ-1 is an andesite sam in Osaka University was used in this study. The pled from the volcano Zao in Japan. whole system to measure argon isotopic ratios consists of an extraction Ta furnace, a purifica tion line made of stainless steel (Maruoka and EXPERIMENTAL PROCEDURES AND RESULTS Matsuda, 1995) and the VG5400 mass The potassium concentrations of Aegean vol spectrometer. The samples (0.2 to 0.7 g) wrapped canic rocks studied were already determined by in aluminum foil were loaded in a vacuum line, X-ray fluorescence spectrometry (Mitropoulos et and were preheated at 150°C for about 12 hours al., 1987). The precision of K analysis was esti to remove adsorbed gases. The crucible was heated mated to be about 5%. The K concentration of YZ at 1800°C before the sample measurements. All 1 has been determined by flame photometry within the samples were melted completely at 1600°C for the error of 2%. 20 minutes. The gases extracted were purified with The concentration of radiogenic 40Ar is ob two Ti-Zr getters. The pressure of purified argon 372 J. Matsuda et al. was monitored by an ion gauge before introduc ratio of 296 (Nier, 1950). ing the extracted gases into the mass spectrometer The initial Ar isotopic ratios may have often to keep the moderate amount of argon. been mass-fractionated from the present atmos To calculate the concentration of radiogenic pheric ratio (0.188) as deduced from the Ar analy 40Ar , 36Ar intensity and 38Ar/36Ar and 40Ar/36Ar sis of historical lava and young volcanic rocks ratios were determined by the method comparing (Kaneoka, 1980; Matsumoto et al., 1989a). We peak intensity with the air standard. The concen plotted the 40Ar/36Arratios against the 38Ar/36Ar tration of radiogenic 40Ar ([40Ar(rad.)]) is calculated ratios in Fig. 2. Our data scatter along the as: fractionation line from the terrestrial air in both side with the addition of radiogenic 40Ar.In such [40Ar(rad.)] = [36Ar(total)](R R0) (1) cases, we can correct this fractionation effect by using the independently measured 38Ar/36Arratio where [36Ar(total)]is the concentration of total 36Ar, (Takaoka, 1988). Because the degree of R the measured 40Ar/36Ar ratio in the sample, and fractionation is proportional to the difference in R0 is the initial 40Ar/36Ar ratio which is usually atomic weight, we used the following equation: assumed to be equal to the present atmospheric

420

~ I

400

380 'L

360

Q

Q 340 i

320

'T T

300

Air Mass Fractionation Line 280

0.184 0.185 0.186 0.187 0.188 0.189 0.19 0.191 Fig. 2. 40Ar/36Arvs. 38Ar/36Ardiagram. The data points seem to lie on the fractionation line from the atmosphere with the addition of radiogenic 40Ar. K-Ar ages of the Aegean volcanic rocks 373

M 00 0 E) 0 0 0 O 0 0 0 M M '0 N M M d N -+ M N O M 0 0 0 0 O 0 o C C 0 O C 0 0 C 0 0 C o o a) v 0) O +1 +1 +1 +I C O 0 +1 +1 0 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 N In 0 O N 00 00 00 r` M O d 00 In O N 00 N N N M en 00 N I) Q U 0 N0 N en M N M O ~ O O C O O N N N O O C O C 0 O 0 x 0 0) O O0) Utd w 0) ^ N M 0' M O O M -+ 0 Vn ON O 00 00 00 In-InNMM 0 In vi ch vi ~i Ni 0C0 01 a\ a` 0\ m 01 01 00 0\ 0101 0\ 01 01 O101 01 ¢ 000 0000N O N rn 01 ON10000 0) Q O y CC Id In In v) O CO N O O O 0 C O O C N U 0 O 0 N M N N N N .- C '" n N N 0 0 0 C C 6" " c C 0 0 0 0 0 0 0 0 +I 0 0 +1 0 +1 +1 ao +1 +1 +1 1 +1 +1 +1 +1 +1 +1 +1 o) Id d tn NIn +1 +1 +1 +1 +1 00 +1 +1 M_ ±I+1 +1+1 +1 -HIn InN N O In 110 N \0 O f N f 01 - .-+ 10 N O ¢ 0 O N 0 N N N M 0 C 0 O N N N 0 0 0 0 0 0 0 0 O O U Q

O a\ b^i0 00 00 00 01 01 01 00 00 0' O 00 O 00 O O O O O O O 0 0 0 0 O 0 0 0 0 0 0 0 0 a 0 O 0 O W N W W 0 W w W W W 0 0) 00 1 N 00 N O WW)~01 0 N --M WIn M N 000W 00 0) 0 M 00 00 %C ~O ~O 0 0 M M 0 ~ 0 I In 0) N - it In .--i N N '0 00 00 00 In 00 M N 00M mot' Cr h 0 O h * 00 O, W-) 00 O M It v) --~ In - N In In O . N M V 0) 0 * F 0 . N r-i e-i .-:. N .~ N .-a +1 +1 +1 +1 +1 +1 +1 +1 +1 -H +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 0 N 0` In O '0 M 0 0 N 0 10 It O \C 01 In W) 00 00 C) Q O Oo v) O N 0\ N to 000,010M 01 '.0 00 vi a O, In O O N00NN-0001 01 01 0 0 0, O\ 0 en r 00 M qr + - M \ N O a Q M M m m M m N N N M M M M M M M N N N N M M N M M 0 0 +C'

\0 N 00 '0 00 InO v) M O S O 0 988 S 0008 g C) h O C) 0 C o O O 0 C O 0 0 0 0 0 0 o C C o w O0 0 O0 0 0 0 9 0 O 0 ¢ O OO 0 0 O 0 0 0666 0 O O O O O O +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +I O +1 +1 +1 +1 +1 +1 0 +I +1 +1 +1 +1 +1 +1 +I +1 In O~ M C\ M %0 00 01 N d' N M N tM C In 0 O 01 M In0 In 01N01 00 0 00 \ 00 00 Q 0 00 00 00000000 00 00 00 00 00 00 00 00 0000 0000000 00 00 00 00 00 00 00 00 00 O _0000000 rl e~/ rN rl 0 ra rrl 0 h O M U O O 0 O O O O O O O O O O O 0 O O 0 O 0 0 0 0 0 0 O 0 0 2 00 0 . V I. 0 0 0 00 0 0*1 0 010'01a,0 0 0 0 ONON C~ 0% 00 00 cl~ 00 0 0 0 0 0 0 0 0 0 0 0 O 0 W U 00 W W W W M W InW W 00 W W00 v) M WN W W WW W W W W W W W W W W W 0 W O G7 00 M vI N O '.0 M 01 00 N 1 0 cn, 00 0~ 00 N O In .-, 0O N 0000' O r~, N N 10 N D\ en to ON N 0 N --+ 00 .~ b N 0; N N N~DI0N M en N M M M N N M N c N N V Nv O w O h 0) O O 00 00 10 0 't 0 N C) O O 0 N M %O 0 0 0 01 Iq N r tn MM 00 W) 0 O N N 00 M 0) 1 O M N O d N N N ~0 00 0 - .-+ - rr r - N N U N N N N O N N N N r-+ N N N 0 3 M 0) V W OO O 0) CO U U CI3 * 0 N In N C: N M M N In ~0 N 00 01 M ,O C 0) N M O N s~ 0 4 4 r > N 0 W W W W0 ~' Q o) 0) N N 00 C7 Q ¢ Q Q Q ¢ CA Q x 2 z, z z oa.,PP,a 0CAIn(A00 w x * h 3 *

v0aNrMO* 374 J. Matsuda et al.

ally smaller than that of the 40Ar/36Ar ratio. This R0 RA )i(ro_rA'1i (2) is because we used the same collector system of RA 4 rA 2 LI the ion counting with electron multiplier for both 38Ar and 36Ar peaks , meanwhile Faraday collec where r is the measured 38Ar/36Ar ratio in a sam tor for 40Ar peak. Thus, the uncertainty of 40Ar/ ple. RA and rA are 40Ar/36Ar and 38Ar/36Ar ratios 36Ar ratio might be also responsible to the nega in the present-day atmosphere, respectively. This tive ages after making correction for mass equation is equivalent to: fractionation. The K-Ar ages making correction for fractionated initial Ar are generally in agree ment with the conventional K-Ar ages in many Ro=RA2r-1 (3) samples when their air 40Ar fractions are less than rA 98%. In the following discussion, we only use the K-Ar ages which were calculated from the mass Table 2 shows the two types of ages calculated fractionation corrected method and that agree with from the two different values of the radiogenic the conventional K-Ar ages within their errors. 40Ar amounts obtained by the conventional method The reference sample YZ I has been measured and the mass-fractionation corrected method. The in many laboratories in Japan (Takaoka, 1988; error of the concentration of measured 36Ar is es Matsumoto et al., 1989b; Nagao et al., 1991; Itaya timated to be within 5% including the error of the et al., 1991). Recently, Nagao et al. (1996) gives volume of the line and of the standard air pipette his best value of 0.227 ± 0.009 Ma for the aver etc. The errors of the K-Ar ages are calculated with age of his fifty four repeated analyses. Our results the error expansion formula with the same ways (0.26 ± 0.02, 0.28 ± 0.02, 0.24 ± 0.03 Ma) seem as those given in Nagao et al. (1996). to be slightly older than the value of Nagao et al. The air 40Ar fraction is the ratio of non-radio (1996). genic 40Ar to the total 40Ar in the sample. As seen in Table 2, most of the samples show very high air 40Ar fraction. This is because of young ages of DISCUSSION these samples studied and also because we had to Figure 3 shows the K-Ar ages of the present use the powdered samples prepared for the X-ray samples. They are generally uniform in each is fluorescence spectrometry. The adsorbed compo land. Although we are afraid that the rocks might nent would be predominant in fine grained sam have lost radiogenic 40Ar during the excessive ples and give mass-fractionation effect on Ar iso grinding during the sample preparation for X-ray topic ratios, which might be one of main causes fluorescence analyses (Dalrymple and Lanphere, of the variation of 38Ar/36Ar ratios shown in Fig. 1969), the uniform age in each island suggests that 2. Two samples (ME-2 and NI-2) show minus ages there were no severe Ar loss and that ages in this by the conventional method assuming the atmos study are more or less reliable. The obtained ages pheric ratio of 296, but give positive values when are all younger than 4 Ma, and are divided clearly we used the mass-fractionation corrected method. into two groups; about 2 to 3.6 Ma (Aegina, Kos Two samples (ME-3 and SA-6) give apparent and Poros) and less than 1.2 Ma (Methana, negative ages when we applied the mass Nisyros, Milos and Santorini). Milos gives slightly fractionation corrected method although they are older ages among the youngest group. We com hardly distinguishable from 0 Ma in taking the er pared our results with the K-Ar ages listed in rors into consideration. These calculated negative Fytikas et al. (1984). They are generally consist ages may be due to the difficulty of determining ent, but there are a few differences in the K-Ar the 38Ar/36Ar ratio with good precision. However, ages of Santorini and Milos island. As for the K the relative error of the 38Ar/36Ar ratio is gener Ar age of Milos, there are two groups; about 1 K-Ar ages of the Aegean volcanic rocks 375

compared to the arc line connecting Crommyonia Aegina ^T^ Methana-Milos-Santorini-Nisyros. As previously described, slab roll-back and

Kos back-arc basin formation.may result from the force pulling the slab. In the Aegean arc system, back

T arc extension allowed upwelling of hot materials Methana and thermal input from the asthenosphere. The Aegean volcanic front have gone to the south as -4' Milos the trench system goes to the south in relation with the lateral migration of slab detachment (Wortel Nisyros and Spakman, 1992). This is in agreement with the assertion by Laj et al. (1982), Kissel and Laj T (1988) and Taymaz et al. (1991). Kissel and Laj Poros T

T (1988) measured paleomagnetic declinations in Neogene rocks on the islands of the Aegean. On the basis of the paleomagnetic data, they suggested Santorini a geodynamical evolution that the original orien f tation of the arc was almost E-W direction between the Paleocene and the Late Burdigalian (Early 0 0.5 1 1.5 2 2.5 3 3.5 4 Miocene), so that the Early Miocene arc was al K-Ar age (Ma) most rectilinear along this trend. Then during the Fig. 3. K-Ar ages of the samples from the Aegean Vol Middle Miocene, the two rotations of opposite canicArc. The K-Arages listed are corrected for mass sense (clockwise in the west and counterclockwise fractionated initial 40Ar/36Arratio and cover the con in the east) documented in the western and east ventional K-Ar age within their errors. ern regions initiated the curvature of the arc and gave their present-day orientation. Taymaz et al. (1991) published first motion fault plane solutions Ma and about 2 Ma (Fytikas et al., 1984). Our of earthquakes in the region, and results are in agreement with the group of 1 Ma. constructed a model of the deforming zone, using As for Santorini, our results (<0.6 Ma) are for calc both P and SH-waveforms and first motion alkaline volcanism, on the other hand, the ages polarities of P-waves to constrain earthquake (~1.5 Ma) listed in Fytikas et al. (1984) are for source parameters. These mechanisms showed that tholeiitic volcanism. We compared the K-Ar ages the faulting in the western part of the Aegean re of each island with its geographic position in Fig. gion was mostly extensional in nature, character 1. These results clearly indicate that the north parts ized by normal faults with a NW to WNW strike of the arc (back-arc side; Aegina and Kos) give and with slip vectors in the direction of NNW to the older age than the south parts (trench side; NNE. In the central and eastern Aegean, and in Methana, Milos, Nisyros and Santorini), which is NW Turkey, distributed right-lateral strike-slip is compatible with the tectonic model of southward more prevalent, on the faults trending NE to ENE, migration of an arc-trench system stated previ and with slip vectors directed NE. The large scale ously. However, Poros island has old ages similar extension started in Miocene has continued to re to those of the northern part island in spite of that cent (Le Pichon et al., 1995). Poros island seems to be situated in the southern Our obtained result that the northern part of part of the volcanic arc (Fig. 1). Poros island is in the island arc is older than the southern part of it its subtle geographic position because it is in the seems to be compatible with the southward mi southern part, but slightly situated to the north gration of the island arc. On the simple assump 376 J. Matsuda et al.

tion that the age difference is due to the south REFERENCES ward migration of the island arc, the migration Boccaletti, M., Manetti, P. and Peccerillo, A. (1974) velocity of the volcanic front in late Tertiary to The Balkanides as an instance of back-arc thrust belt: Quaternary could be roughly calculated from the possible relation with the Hellenides. Geol. Soc. Am. K-Ar ages of two neighbor islands. We selected Bull. 85, 1077-1084. Kos and Nisyros that line vertically to the direc Dalrymple, G. B. and Lanphere, M. A. (1969) Potas tion of the plate boundary. The distance between sium-Argon Dating: Principles, Techniques and Ap Kos and Nisyros is about 30 km, and the K-Ar plications to Geochronology. W.H. Freeman and Company, 268 pp. age difference between them is about 3 m.y. Hence Fytikas, M., Innocenti, F., Manetti, P., Mazzuoli, R., the migration velocity of the arc-trench system Peccerilo, A. and Villari, L. (1984) Tertiary to Qua would be about 1 cm/yr. As the typical life span ternary evolution of volcanism in the Aegean region. of island arc volcanism is about 0.5 m.y. (Tomiya, Geol. Soc. London, Spec. Publ. 17, 687-699. 1991), the estimate of the migration velocity again Hatzfeld, D. (1994) On the shape of the subducting slab beneath the , Greece. Geophys. Res. Lett. include large error. Poros island does not fit this 21,173-176. model, because this island has old age even though Itaya, T., Nagao, K., Inoue, K., Honjou, Y., Okada, T. it situated in the south of the island arc. Thus, we and Ogata, A. (1991) Argon isotope analysis by a admit that this is very rough estimation only at newly developed mass spectrometric system for K the eastern side of the island arc. The obtained Ar dating. Mineral. J. 15, 203-221. Kaneoka, I. (1980) Rare gas isotopes and mass value is comparable to, but less than the plate fractionation: An indicator of gas transport into or migration velocity (5 cm/yr.) suggested by using from a magma. Earth Planet. Sci. Lett. 48, 284-292. geodetic, seismic and paleomagnetic data by Kissel, C. and Laj, C. (1988) The Tertiary geodynamical Hatzfeld (1994). Hatzfeld's velocity was calcu evolution of the Aegean arc; a paleomagnetic recon lated for the migration of the Aegean microplate struction. Tectonophysics 146, 183-201. relative to Eurasia (NE-SW direction). This direc Laj, C., Jamet, M., Sorel, D. and Valente, J. P. (1982) First paleomagnetic results from Mio-Pliocene se tion is about 45°--65° east from the direction of ries of the Hellenic sedimentary arc. Tectonophysics Kos to Nisyros (NNW-SSE). So, the trench mi 86,45-67. gration velocity to the direction of NNW-SSE from Le Pichon, X., Chamot-Rooke, N., Lallemant, S., his data is about 2.1-3.5 cm/yr. Noomen, R. and Veis, G. (1995) Geodetic determi (=5xcos(45°-65°)). Le Pichon et al. (1995) also nation of the kinematics of with re spect to Europe: Implications for eastern Mediterra showed that some of the SLR (Satellite Laser nean tectonics. J. Geophys. Res. 100, 12675-12690. Ranging) results were divided into the two veloc Maruoka, T. and Matsuda, J. (1995) Noble gas separa ity ingredients (perpendicular component to the tion by adsorptive phenomena in mass spectrometry. wedge and lateral component along the trench). J. Mass Spectrom. Soc. Jpn. 43, 1-8. They blamed this on shear partitioning, and the Matsumoto, A., Uto, K. and Shibata, K. (1989a) Argon vertical motion velocity of Kattavia near Kos or isotopic ratios in historic lavas-Importance of cor rection for the initial argon in K-Ar dating of young Nisyros was 2.3 cm/yr. We consider that these volcanic rocks-. J. Mass Spectrom. Soc. Jpn. 37, values are comparable to our study. 353-363 (in Japanese). Matsumoto, A., Uto, K. and Shibata, K. (1989b) K-Ar Acknowledgments-This work was supported with dating by peak comparison method-New technique Monbusho International Scientific Research Program applicable to rocks younger than 0.5 Ma-. Bull. "Geochemical Investigation of the Aegean Volcanic Geol. Surv. Jpn. 40, 565-579. Arc" (Representative: Dr. Kenji Notsu). We would like Mitropoulos, P. and Tarney, J. (1992) Significance of to thank Dr. T. Itaya for his critical reviewing. mineral composition variations in the Aegean Island Arc. J. Volcanol. Geotherm. Res. 51, 283-303. K-Ar ages of the Aegean volcanic rocks 377

Mitropoulos, P., Tarney, J., Saunders, A. D. and Marsh, Spakman, W. (1991) Delay-time tomography of the N. G. (1987) Petrogenesis of Cenozoic volcanic rocks upper mantle below Europe, the Mediterranean, and from the Aegean Island Arc. J. Volcanol. Geotherm. Asia Minor. Geophys. J. Int. 107, 309-332. Res. 32, 177-193. Spakman, W., Wortel, M. J. R. and Vlaar, N. J. (1988) Nagao, K., Ogata, A., Miura, Y., Matsuda, J. and The Hellenic subduction zone: a tomographic image Akimoto, S. (1991) Highly reproducible 13 and 17 and its geodynamic implications. Geophys. Res. Lett. ka K-Ar ages of two volcanic rocks. Geochem. J. 25, 15,60-63. 447-451. Spakman, W., Van der Lee, S. and Van der Hilst, R. Nagao, K., Ogata, A., Miura, Y. and Yamaguchi, K. (1993) Travel-time tomography of the European (1996) Ar isotope analysis for K-Ar dating using two Mediterranean mantle down to 1400 km. Phys. Earth modified-VG5400 Mass Spectrometers-I: Isotope Planet. Inter. 79, 3-74. dilution method. J. Mass Spectrom. Soc. Jpn. 44, 39 Takaoka, N. (1988) Absolute age determination of vol 61. canic rocks younger than 1 Ma. Report of the 1985 Nier, A. O. (1950) A redetermination of the relative 1987 Monbusho Scientific Research Program. Proj., abundances of the isotopes of carbon, nitrogen, oxy No. 60303011 (in Japanese). gen, argon!c"d potassium. Phys. Rev. 77, 789-793. Tatsumi, Y. (1986) Formation of the volcanic front in Papadopoulos, G. A. (1989) Cenozoic magmatism, deep subduction zones. Geophys. Res. Lett. 13, 717-720. tectonics, and crustal deformation in the Aegean tec Taymaz, T., Jackson, J. and McKenzie, D. (1991) Ac tonics, and crustal deformation in the Aegean area. tive tectonics of the north and central Aegean Sea. Paleomagnetic Rotations and Continental Deforma Geophys. J. Int. 106, 433-490. tion (Kissel, C. and Laj, C., eds.), 95-113, Kluwer, Tomiya, A. (1991) Volume of mantle diapir compat Dordrecht. ible with life span of a typical island-arc volcano. Pe-Piper, G. and Piper, D. J. W. (1989) Spatial and Tem Kazan 36, 211-221 (in Japanese). poral variations in Late Cenozoic back-arc volcanic Wortel, M. J. R. and Spakman, W. (1992) Structure and rocks, Aegean Sea region. Tectonophysics 169, 113 dynamics of subducted lithosphere in the Mediterra 134. nean region. Proc. K. Ned. Akad. Wet. 95, 325-347.