Magmatism of the westernmost (Komandorsky) segment of the Aleutian Island Arc

289

Tectonophysics, 199 (1991) 289-317 Elsevier Science publishers B.V., Amsterdam

Magmatism of the westernmost (Komandorsky) segment of the Aleutian Island Arc A.A. Tsvetkov Institute of Ore Deposit GeoIo@, Petrography, Mineralogy and Geochemistry, U.S.S.R Academy of Sciences,MC.I_WOW, USSR

(Received September 28,1989; revised version accepted July 15,199O)

ABSTRACT Tsvetkov, A.A., 1990. Magmatism of the westernmost (Komandorsky) segment of the Aleutian Island Arc. (In: L.P. Zonenshain (Editor), The Achievements of Plate Tectonics in the U.S.S.R. Tectonophysics, 199: 289-317. Four magmatic associations are distinguished in the westernmost (Komandorsky) segment of the Aleutian island arc: basalt-plagiorhyolite (45-28 Ma), trachybasalt-tescbenite (27-16 Ma), diorite-granitoid (12.5-8.5 Ma) and dacite-rhyolite (0.7-O Ma). The latter association is represented by the iavas of a recently discovered active submarine volcano (Piip volcano) about 40 km north of the Komandorsky Islands and 700 km west of the Quaternary center on Buldir Is. Detailed and 87Sr/*6Sr isotopic ratios, show that the parental melts of the petrogeochemical data, including REE and ‘“Nd/ltiNd Komandorsky Is. were of mantle origin and their sources remained isotopically homogeneous during the whole geological history of the Aleutian arc. “Be concentrations in lavas of the Piip volcano (2.7 x 10’ at/g) indicate subduction-related incorporation of young pelagic sediments in their magmatic sources or contamination of ascending melts by non-solidified sedimentary material directly beneath the volcano. The Komandorsky segment of the Aleutian arc evolved from a depleted arc tholeiitic series, through a markedly alkaline series to later talc-alkaline andesites and da&es, and finally to late-stage talc-alkaline dacites and rhyodacites erupted inboard of the older volcanic centers.

Introduction Among the fundamental achievements of the earth sciences is the recognition of a direct link between continental growth and island-arc processes. Markov (1975), Burchfiel and Davis (1975), Burchfiel (1983), Bogatikov and Tsvetkov (1988) showed that island arcs ~nt~u~ly evolve into fold belts which, upon accretion to the margins of ancient continental blocks, form new erogenic zones. Characteristic of this accretionary type of continental growth is the geological history of western North America (Rogers et al., 1974; Burchfiel, 1983) and New Guinea (Thorpe, 1982) where a number of arcs progressively younger in age have collided with the ancient silicic basement, considerably increasing the total volume of the continental mass. A major role in island-arc petrogenesis is played by mantle melts and fluids that rise from the subducted slab and mantle wedge to the upper

lithospheric levels. Each new magmatic episode is characterized by ~mposition~ly specific rock associations which can be used as tracers of successive tectono-magmatic stages in the evolution of the Earth’s lithosphere. In this paper, magmatic rocks of the Komandorsky Is. and some adjacent areas are described. Four magmatic episodes are distinguished which have temporal and compositional counterparts in other segments of the Aleutian arc. Geologkal and tectonic setting The Aleutian arc, over 3000 km long and 125235 km wide (165 km in average), with a 1200~km radius of curvature, is one of the most impressive island arcs of the world ocean. In terms of dimensions, structural framework and time of origin, it can be best correlated with the Kurile-Kamchatka arc described in the paper by Avdeiko et al. (this vol., pp. 271-287). The Aleutian arc stretches from

0040-1951/91/$03SO 6 1991 - Elsevier Science Publishers B.V. All riahts reserved

A.A.

‘1-SVEXKOV

Fig. 1. (a) Schematic map of the Aleutian arc. Solid lines show Miocene (lower line) and Quate~ (upper tme) volcanic tronts according to Scholl et al. (1975). (b) Schematic map showing major structures of the southern part of the Komandorsky Basin and its frame (Seliverstov et al., 1988). Key: 1 = water depth (m); 2 = isopachs (km, assuming VP in sediments 2 km/see); 3 = faults in the basement of the back-arc rise; 4 = greatest recent regional faults; 5 E axes of anticlinal structures within the back-arc rise; 6 = elements of Piip submarine volcanic massif: (a) margins of the massif, (h) volcanic centers and largest cones of the massif. Figures in circles: 1 = Komandorsky segment of the Aleutian Arc; 2-3 = respectively axial zones of the Aleutian and Kurile trenches; 4 = Shirshov Ridge; 5 = active fault within the Pacific slope of the Komandorsky segment; 6 = Bering shear zone boun~ng the Komandorsky Basin from the south; 7 = active submarine volcanic massif, 8 = Komandor graben; 9 = Alpha shear zone; 10 5*Betta Rise: I I = part of the Komandorsky Basin attached to the foot the Komandorsky segment of the Aleutian Arc.

MAGMATBM

OF THE

KOMANDGRSKY

SEGMENT

OF THE

ALEUTIAN

the Alaska and Kenai peninsulas in North America to Kamchatka, from which it is separated by a deep (over 3000 m) and wide (over 200 km) strait (Fig. I). Lately Fedorchuk (1990) has shown that “‘Aleutian-strike” structures can also be seen in Eastern Kamchatka. The Aleutian arc consists of five tectonic blocks along strike: Komandorsky, Near Islands, Adak, Unalaska and Alaska. In some places the segments are truncated by tectonic depressions. Investigations ac~mplish~ during certain geephysical projects (Scholl et al., 1975; Talwani and Pitman, 1977: Harbert et al., 1987) have shown that the Pacific plate is moving northwest at a rate of 6.4-7.4 cm/yr, nearly normal to the strike of the Central and Eastern Aleutians and practically oblique to the strike of the Komandorsky segment. Relative to the latter, the convergence rate decreases constantly and the plate is ~derth~st at about 30° to a depth of 40 km where its dip abruptly increases to 70 D (Jacob and Hamada, 1972). Deep seismic sections show that the crust,/ upper mantle boundary beneath the Komandorsky Is. occurs at depths of 20-25 km and that within 65-100 km and 125-I65 km intervals in the mantle low-velocity asthenospheric layers are located (Gainanov et al., 1968). The positions of Oligicene-Pliocene and Quate~a~ volcanic fronts in the Aleutian arc do not coincide (Kay et al., 1982; Marsh, 1987). As in the K&es, most of the historically active volcanoes here lie 5-8 km to the north of the older (Miocene) volcanic front. The Quatemary front is represented by numerous volcanic centers distributed more or less equidistantly (60-70 km) throughout the whole arc (Kay et al., 1982; Marsh, 1987). Groups of adjacent centers fall along straight lines which correspond to segmentation of the subducting oceanic plate (Marsh, 1979; Tsvetkov and Marsh, 1990). The four Komandorsky islands (Bering, Medny, Toporkov and At-ii Kamen) compose the westemmost flank of the Aleutian chain. Bering and Medny Is. are by far the largest, respectively 85 and 54 km long. The islands are built up of Paleogene-Neogene volcanic and sedimentary rocks dissected by a series of normal faults with offsets usually less than 100-200 m thick (Zhega-

ISLAND

ARC

291

lov, 1964; Schmidt, 1978). The geological structure of the Komandorsky Is. has been described by Moro~~ch (1912, 1925), ~s~h~ovsky (1963), Zhegalov (1964), Egiazarov (1969), Erlikh (1973), Sergeev (1976), Shapiro (19X), Schmidt (1978), Borsuk et al. (1982), Tsvetkov (1982, 1984), Ivaschenko et al. (1984), Frolova et al. (1985), Bogatikov and Tsvetkov (1988). Papers summarizing petrological and geochemical data on the magmatic rocks have appeared only following the initiation of field and laboratory investigations by two Soviet academic institutions - the Institute of Ore Deposit Geology, Petrography, Mineralogy and Geochemistry and the Geological Institute, as well as by Moscow State University in the late 1970’s. Naugler and Rea (1970) have shown that the acoustic basement to the north and to the south of the Komandorsky Is, is oceanic crust, apparently Late Cretaceous in age on the Bering Sea side and Paleogene in age on the Pacific side. Regional geological and geophysical investigations in the area of the Kurile arc - Aleutian arc junction undertaken by the Kamchatka Institute of Volc~ology have resulted in new data on the structure of the Komandorsky segment. Seliverstov (1987) has pointed out that the tectonic block underlying the Bering Sea slope of the Komandorsky Is. is slightly deformed. This is expressed in a low-angle (5-7 0 ) tilt of its sedimentary cover in the direction of the Central Aleutians and their “squeezing” at the southwestern edge of the block, The Pacific side of the Kom~dorsky Is. is also slightly deformed and is geophysically similar to the adjacent Pacific oceanic plate. Contrary to the Bering Sea slope, it is uplifted and possibly shifted towards the center of the arc. Sedimentary layers and the surface of the acoustic basement within the Pacific side of the arc are tilted to the southwest at an angle of lo-13*; the tilting continually increases to the northwest up to 20-22”. Such tectonic blocks in the basement of the Komandorsky segment are not unique (Seliverstov, 1987). Nearly all seismic sections across the trench show the same pattern. These tectonic blocks in the basement of the Komandorsky segment of the Aleutian arc seem to have been formed in the course of the breaking up and tectonic reworking of adjacent parts of the

292

AA.

TSVETKOV

TABLE 1 K/Ar ages of the magmatic rocks of Komandorsky Islands Island

Rock

K (W)

“OAr

“OArrad. ,,

(ng/g)

“OArtot.

1.15 It 0.15

3; 5

Age (Ma)

Location

Diorite-granitoid association

Medny

tonahte

Medny

andesite

55’7’N, 165”2’E

andesite

1.97 10.04

8.5 + 2.5 8.6 F 0.3 a

1.80

1.089

37; 40

8.8 f 0.4

Stock, Kuropatkin Mt., 13/15-77 Dyke, Yugo-Vostocbny C., 10/6-78 Lava, dredge haul (Scholl et al., 1976)

Depth 1600 m

Medny

dacite

Medny Medny

dacite granodiorite

1.25 lto.3 1.92 +0.03

1.1 _rtO.l 1.5 10.2

5; 15 13; 21

12.0 f 2 11 *3

Medny

aplite

4.32 10.04

3.5 rto.3

27; 32

11.5 * 2

Medny

granodiorite

1.53 Ito.

1.4 zko.2

16; 19

12.5 * 3

biotite alkaline feldspar

6.74 fO.05 5.27 f0.04

5.4 f0.3 4.4 f0.3

11; 21 14; 39

12 +2 12 *2

1.68 *0.03

1.8 f0.2

12; 15

16 *3

10.7 f 0.8 a

Lava flow, Glinka Cove, 9/l-78 Same, 9/2-78 Stock, Chemy C. 11/6-77 Vein, Cherny C., 11/7-77 Stock, Chemy C.. 8/l-78

Trachybasalt-teschenite association

Bering

trachydoierite

Medny

trachybasalt

Bering

teschenite

1.54 &0.03

2.3 f0.2

21; 25

21 +3

Bering

trachybasalt

1.51 kO.03

2.6 ho.3

19; 29

25 Ifr4

Bering Bering

trachydderite trachydolerite

2.27 z&O.03 1.51 *0.03

3.3 rto.3 2.2 f0.2

20; 30 26: 26

21 *3 20.5 * 3

Bering

teschenite

18.6 x!c0.6 a

27.0 f 0.8 ’

Lava dome, Nakovahrya Mt. 404,‘77 Dyke Sulkovski C., 3/19-78 Sill, Nickolskoye, 406/77 Lava flow, SeveroZapadny C., 414/17 Sill, Gaunta C., 415/77 Sill, Svinye Gori, 13/6-77 Dyke, Monati C., 17/3-78

~~~~t-plag~orhyolite association

Medny

plagiorhyolite

1.68 f0.03

3.5 kO.3

21; 28

30 +4

Medny

pl~orhyo~ite

1.50 +0.03

3.0 io.3

18; 25

28 zt4

Medny

plagiorhyolite

Medny Medny

pla~orhyolite plagiorhyohte

1.67 *0.03

4.0 +0.4

25; 28

31.4 * 1.6 a 34 +4

Medny Medny

plagiorhyolite basalt

2.14 i.O.04 1.35 +0.3

4.9 *0.4 2.9 ItO.

41; 53 32; 33

33 *4 31 13

31.3 f 0.9 a

lava flow, south of Kotenok C., 7/2-77 stock, rhyolite ridge, 5/2-77 lava flow, Preobrazhenskaya Cove, 2/10-78 same, 2/11-78 stock, rhyolite ridge, 4/2-77 same, 4/l-77 dyke, Suikovski C., 3/W78

MA~MATISM OF THE KOMAN~RSKY

293

SEGMENT OF THE ALEUTIAN ISLAND ARC

TABLE 1 (continued) Island

Rock

Km

‘%.r 0%/P) 2.2 QO.2

Location

X-K.

Ag (MaI

12; 20

38 *4

lava flow, Yushin C., F-15-86 same, F-16B-86 lava flow, Sammy C., F-25G-86 same, F-24-86

%r rad. b

Bering

basalt

0.80 Jro.03

Bering Bering

basalt basalt

1.34 f0.03 1.39 +0.03

3.6 zto.3 3.6 iO.3

10; 16 8; 10

38 *4 37 &4

Bering

basaltic andesite

1.198 * 0.03

3.7 f0.3

22; 34

45 jc5

’ See Appendix 1. h Two separate date~ations

of ‘?Ar rad./‘@Artotal.

Pacific and Bering Sea plates (Seliverstov, 1987). The northwestern termination of the Aleutian am can be considered as a structure composed of tectonically dismembered and reworked parts of the oceanic crust on which late arc volcanism is superimposed. Fragments of this crust represented by inclusions of peridotites, c~op~oxe~tes, gabbros and jasperites are sometimes found in the oldest tholeiitic basal& of the Komandorsky Is. (Tsevtkov and Schmidt, 1982). The geochemical features of basic inclusions differ from those of basic plutonic rocks exposed in the islands.

In the Komandorsky segment of the Aleutian arc, products of different ma~atic pulses are irregularly distributed and show some specific compositions trends. Presently available K/Ar data given in Table 1, combined with geological considerations, provide reasons for an objective timing of four successive stages (Fig 2). Gf?u/ogy The oldest ma~atic rocks in the Kom~dorsky Is, are incorporated into a basalt-plagiorhyolite ~s~iatjQ~ which consists mainly of basalts and plagiorhyolites with minor (less than 10 vol.%) andesites and dacites. Basalts are exposed in the northwest of Medny Is. and have recently also been found in the north of Bering Is. (Tsvetkov et al., 1990). In Medny Is. they are represented by lava flows, tuffs and tuffbreccias forming a steep Noah-dipp~g monocline

cut by basaltic dyke swarms and gabbroic stocks (up to 1000 m2). 30th pyroclastics and lavas, like the Japanese and the Kurile “‘green tuffs”, are metamorphosed in the greenschist facies. K/Ar ages of basaltic members are 47-45 f 4 Ma for Bering and 31 I 3 Ma for Medny Is. Xn spite of alteration, the ra~og~ic ages of the rocks seem not to differ considerably from their geological ages, as fossil findings in siltstones separating basaltic flows and tuff layers in both islands eorrespend to F!ocene, possibly Early Oligocene species (Tsvetkov et al., 1990). Thus, the basaltic magmatism of the earliest evolutional stage lasted from the Early Eocene or even the end of the Paleocene to the Oligocene, i.e. for more than 20 Ma, Pla~orhyolites and plagiorhyodacites outcrop only in the northwest of Medny Is., forming thick (50-70 m) and short (1-2 km) flows, domes, breccias, tuffs and tuff-breccias. In the extreme north of the island, the basaltic volcanics are cut by pla~orhyo~tic stocks, ~crola~o~~s, their marginal zones “stuffed” with basaltic xenoliths. In other localities (e.g., Peschanaya Cove), acid stocks include specific ma~atic bodies (basaltic “pseudodykes”), evidence of the early simultaneous co~t~sion of acid and mafic melts (Tsvetkov, 1982). Radiometric ages of plagiorhyolites fall within the range 34-28 Ma (Oligocene) while spatially associated basal& are 31 Ma old. Thus, at least some of plagiorhyolites were formed somewhat earlier than the basalts, though acid ma~ati~ generally postdated the basic type. Magmatic rocks of the Lower Miocene form a trachybadt-teschenite a.wociatiun{see Fig. 2). On Bering Is. it is represented by ~achyb~~tic lavas,

194

less frequently by sills and dykes of trachybasalts, tra~hydolerites and teschenites. A few dykes of trachy-andesites have been reported. The flows are 20-50 m thick and 2-5 km long. According to Schmidt (1978), ail the Lower-Miocene magmatic rocks in the north of Bering Is. formerly comprised a huge shield volcano, approximately 20 km

A.A.

i-SVETKOV

in diameter, that was later completely eroded. On Medny Is. the trachybasait-teschenite association is represented by numerous dykes found in the Zhirovaya and Gladkovskaya coves. A single “ beringitic” dyke discovered by Morozewich (1925) and Zhegalov (1964) in the southwest of Bering Is. is, according to our data, just one of

Fig. 2. Correlation of magmatic episodes in the Kornandorsky segment of the Aleutian Isiand Arc. (A) Bering Wand+ (8) Medny Island. 1 * da&e-rhyodacite association: F%p submarine volcano (shown only in geological column); 2-4 = diorite-granitoid association: 2 = and&tic and dacitic lavas and pyrodastics, 3 = granodiotitt stocks asid dykes, 4 = diorite, quartz diotite and tonalite and tr~y~~te laws, 6 = ~achyb~tic, trachystocks and dykes; 5-6 = ~acbyb~t-~e~e~te association: 5 = tr~hyb~t . 7 = basaltic laws, dykes and pyroclastics, 8 = doleritic and teschenitic sills and dykes; 7-8~ basa%plagiorhyolite JWoCkitiOn. plagiorhyolitic laws and pyroclastic~, 9 = plagiorhyolitic stodlrsand micrdlaccotiths; 10 = sedimexWuy rocks; II =i faults; 12 = K/Ar b, c). Q = Eocene-Oligocene, N: = Lower Miocene, NF3 = Middle-Upper data (Ma) for different magmatic a~socistio~~ (a, Miocene, Q = QmuWmary.

MAGMATISM

OF THE

KOMANDORSKY

SEGMENT

OF THE

ALEUTIAN

ISLAND

295

ARC

magmatic event lasted for almost 10 Ma, i.e. it was nearly twice as short as the first one. Middle-Upper Miocene magmatism in the Komandorsky segment is marked by a dioritegrunitoid association. It is known only in the south-central parts of Medny Is. and it incorporates tonalite, quartz diorite and granodiorite stocks, small laccolith-type bodies and dykes (see Fig. 2). Their radiometric ages fall between 12.5

many teschenitic dykes that outcrop both on the eastern shores of Medny and on the northern shores of Bering Is. K/Ar dating of six trachybasaits, trachy-andesites and teschenites gave ages of 25-16 Ma, corresponding to the Lower Miocene, whereas the seventh sample, representing the teschenite dyke in the extreme south of Bering Is. (Monati cape), gave 2’7 Ma, corresponding to the Upper Oligocene. Thus, the second Komandorsky

TABLE 2 Whole-rock chemical analyses of the bait-pla~orhyolite

association Plagiorhyolites

Basalts 3/7-78

31’2-78

5,‘3-78

5/l-77

440

3/15-78

5/5-77

6/4-78

2/10-78

2/X-78

12/21-78

47.47

49.20

49.25

49.00

51.72

53.17

72.49

75.16

75.61

78.06

84.73

TiOz

0.82

0.68

1.03

1.09

0.61

0.65

0.41

0.22

0.40

0.34

0.15

Al 20, FeO*

18.35

16.36

16.31

14.49

17.99

17.32

12.58

13.71

12.11

11.41

7.31

7.23

7.18

9.9s

13.92

8.83

7.26

4.21

0.87

2.39

2.10

2.16

MnO

0.15

0.10

0.05

0.20

0.20

0.41

0.09

0.08

0.07

0.05

0.03

MgO CaO

6.38

6.62

6.16

5.15

5.35

6.27

2.18

0.28

0.37

0.44

1.15

8.94

5.95

10.58

8.19

6.87

7.11

1.59

0.22

1.03

0.90

1.11

Na,O

3.13

5.52

2.22

3.08

3.67

3.14

4.50

5.88

5.10

5.17

2.01

K,O

1.17

0.91

0.45

0.48

0.77

0.57

1.73

2.11

0.96

0.29

H2O

4.99

4.42

3.14

4.76

3.85

3.33

n.d.

0.39

0.75

0.90

n.d.

p29

0.18

0.15

0.10

0.14

0.10

0.10

n.d.

0.05

0.08

0.06

n.d.

CO2

0.61

2.37

0.05

0.20

0.05

0.06

n.d.

0.10

0.16

0.09

n.d.

Total

99.42

99.46

99.29

100.70

100.01

99.99

99.78

99.07

99.03

99.81

99.99

Co (ppm) Ni

39.0

34.10

38.80

39.70

27.40

14.40

11.2

0.38

2.92

2.68

5.4s

50.0

31.43

41.64

18.07

43.60

28.00

10.9

7.86

7.85

7.85

2.34

Cr

28.15

56.70

56.90

29.10

65.30

11.1

1.66

2.20

1.91

8.38

Cs

0.95

0.22

1.20

0.05

0.18

0.15

0.588

SC

42.80

33.80

40.10

50.70

6.43

10.40

9.91

Sb

0.10

0.14

0.15

0.24

0.16

Zr

56.35

81.10

Ta

0.07

0.06

0.048

0.15

0.058

0.44

0.149

0.23

0.12

0.11

0.055

Th

0.20

1.12

0.886

0.26

0.814

OS10

1.20

2.12

1.17

1.07

0.468

U

0.18

0.893

1.41

0.19

0.522

0.293

0.571

1.60

1.50

1.9s

1.10

Hf

1.27

1.88

1.44

1.43

1.43

1.09

4.23

4.81

3.43

3.44

Rb

6

14

10

10

n.d.

Sr

229.6

306.0

408.0

230.0

n.d.

Ba

134

110

100

100

SiO, (wt.%)

0.135

n.d. 38.00

0.623 30.00

194.2 0.459 32.40

n.d.

n.d.

n.d.

69.00

n.d.

n.d.

n.d.

65.8

n.d.

n.d.

n.d. n.d.

1.37

6.26

5.43

2.75

5.07

3.81

Ce

3.93

13.70

12.10

7.51

12.20

9.44

Nd

3.06

10.40

lOSO

7.14

4.89

Sm

1.14

2.80

2.60

2.59

2.19

Eu

0.54 n.d.

0.802 n.d.

0.866 2.79

11.9

0.20

La

Gd

0.037

0.93 n.d.

0.656 n.d.

141.0

114.0

118.0

1.34

5.09 n.d. n.d.

1.67

10

10

10

n.d.

n.d.

100

10s

100

n.d.

571

428

21s

100

118

17.80

8.72

9.80

7.03

21.5

37.40

22.60

23.00

13.90

8.41

11.40

25.30

15.20

15.70

8.63

2.00

4.12

5.33

4.04

4.18

2.33

0.997

0.82

1.03

1.05

4.76

3.99

4.77

0.83

0.704

0.701

3.97

4.10

0.57 n.d.

7.46

n.d.

0.454 n.d.

Tb

0.275

0.425

0.420

OS92

DY Yb

2.41

2.83

2.80

3.87

1.34

1.9so

1.610

2.72

1.23

0.833

3.48

4.13

2.62

2.89

2.00

Lu

0.227

0.321

0.267

0.426

0.185

0.141

0.575

0.678

0.398

0.444

0.347

0.354 n.d.

0.281 n.d.

0.671 n.d.

SO4

0.368 n.d.

I

1

45

50

55

0-I

60

.-2

65

n-3

ID

a--b

13

SiOt

Fig. 3. RO* {total Fe as FeO)/MgO ratio versus Si&_ for the magma\ic associations of the Komandorsky segment. The de-alkaline (CA)-tholeiitic (TN) discriminant line is From Miyashiro (1974). Magmatic associations: 1 = basalt~lag~orhyolite, 2 = trachybasalt-teschenite, 3 = dioritegranitoid, 4 = dacite-rhy~a~ite.

and 8.5 Ma. During the 2nd and 26nd cruises of the Soviet RV ‘(~~~~~~~i~g“, 40-60 km to the north-east of the Komandorsky Is., a linear rise

of the Bering Sea basement was discovered. It follows the strike of the arc For more than 12Q km, its vertical amplitude from the foot to the top reaching 2500 m. Within the central part of this rise a huge submarine massif named the “Pip volcano” by the K~Ghatka volcanofogists was mapped bathymetrically. The total volume of the structure above the 3500 m isobath is more than 1100 k&. The upper part of the massif is built up of very fresh dacites and rhyodacites which we include in a dacite-rh.yodacite association. Their age as estimated from ~ai~ma~etic data is not older than 0.7 Ma {Seliverstov et al., 1986). This volcanic center is about 40 km north from Bering Is. Recently obtained geophysical data (Seliverstov et al., 1988) show the occurrence of some other basement rises on the Bering Sea side of the Komandorsky Is. Quite possibly they can be seamounts, or even active submarine volcanoes like the Piip volcano. ~ufortunately, at present these structures remain only poorly investigated.

#a,0 1

Fig. 4. Harker’s diagrams for the mastic

associations of the ~o~dorsky

segment. Symbols are the same as in Fig. 3.

MAGMATISM

OF THE KOMANDORSKY

SEGMENT

OF THE ALEUTIAN

Geochemistry

The basal& of the first magmatic phase are close to low-% tholeiites (IAB) occurring in the early evolutionary stages of other ensimatic arcs such as Eua Is. in the Tongas (Ewart et al., 1977) or Guam Is. in the Mariana (Reagan and Meijer, 1984) arcs though low-temperature alteration caused a considerable variability in concentrations of Fez03, FeO, MgO, CaO, Na,O and also of H,O and CO, (Table 2). In Myashiro’s diagram (Fe0 */ MgO-SiO*), the first Komandosrky basalts mainly plot within tholeiitic field: their F~*/MgO ratios range from 1.08 to 2.26 (Fig. 3). Variations of this ratio within the basaltic range of silica content are related to alteration. The basalt-plagiorhyolite association (Fig. 4) has a bimodal distribution on Harker’s plots, emphasizing the lack of inte~ediate members. Compared with rocks of later magmatic episodes, the basic rocks of the basalt-plagiorhyolite association have the lowest total amounts of REE (14.35-39.53 ppm) (Fig. 5) and are LREE depleted to slightly LREE enriched (La/Sm* = 0.60.7). Some basalts (e.g., 3/7-78, 19/3-78 and 19/4-78) have positive Eu-anomalies (Eu/Eu* = 1.lO- 1.26) indicative of plagioclase accumulation. The efficiency of such a process is proved by the plagioclase glomerocryst segregations (so-called “anorthosite” inclusions) in some Medny and Bering lavas (Tsvetkov and Schmidt, 1982). All the samples have a strong negative Ta-anomaly and Th/U values of 0.2-0.5. In relation to other Pacific-rim and ocean-floor structures, the REE dist~bution patterns in the Komandorsky basalts most closely match those of the interoceanic initial IAB. The REE plot in sample 3/17-78 practically repeats that in Eua basalt (Ewart et al., 1977) and is distinct from typical MORB (Kay, 1980). The acid members of this bas~t-pla~orhyolite asso& ation are more potassic than oceanic plagiogranites (Coleman and Peterman, 1975; Kay and Senechal, 1976). The abundances and distribution patterns

*

REE

and their ratios

were normalized

to Le&dy (Lo)

chondrite, for other elements normalizing coefficients were taken from Kay and Hubbard (1978).

ISLAND

ARC

297

of siderophile elements in the plagiorhyolites are compatible with IAB, including Medny basalts. On Miyashiro’s diagram (see Fig. 3) the Komandorsky acid rocks, unlike their spatially related basalts, plot exclusively within the talc-alkaline field, their TiOz, Al,O,, FeO, MgO, CaO contents decrease successively and Na,O, on the contrary, generally increases with growing silica content (see Fig. 4). While Fe0 */MgO ratios in the acid members of the association increase, TiO, remains practically at the same level, the basalts being somewhat richer in titania than plagiorhyolites. Total REE contents in Medny pla~orhyo~tes (35.06-109.02, av. 65.3 ppm) are considerably higher than in basalts (av. total REE = 29 ppm) and come close to those in Troodos ophiolitic plagiogranites (av. total REE = 52.5 ppm). At the same time total REE amounts in Komandorsky pla~orhyolites are lower than in most erogenic two-feldspar and two-mica granodiorites and granites, where they are in the range of 200-2500 ppm (Emmerman et al., 1973). The REE distribution shows a negative Eu-anomaly: Eu/Eu* = 0.50-0.89 (see Fig. 5). All pla~orhyolites, just as basalts, are characterized by a strong negative Ta-anomaly. The Miocene trachybasalt-teschenite association is of a continually differentiated type. Its major-element chemistry varies gradu~ly from trachybasalt with 50.17% SiO, to trachyandesite with 60.10% SiO,, though dominantly the SiO* contents are in the range of basalt-basaltic andesite compositions (Table 3). On Harker’s diagrams (see Fig. 4), a distinctive linear trend can be seen. Fe0 */MgO ratios are within the limits 0.79-2.58. With the increase of silica contents, K,O also rises and in some teschenites exceeds 3%. K,O/Na,O ratios are always < 1 and teschenites sometimes contain more than 7% of normative nepheline. Thus, the ~achybas~t-tesche~te association falls into the category of K-Na subalkaline - K-Na alkaline. An important point is that due to the high Ti contents it does not belong to arc shoshonite series such as those known, e.g., in Fiji (Gill, 1970) or New Guinea {Mo~son, 1980). Among the most significant geochemical features we should emphasize the enrichment in LJLE, especially in K, Rb and Ba. Komandorsky

A. 500

Basalts

MEOk”

1 SL

IU

, Cs

Rb L

El

II

lh

Ta

HI

Zr

U‘

(

Ba

U

Th

Ta Hi

Zt

I&

1

La

CP

Nd

Sm Eu

La

Ca

Nd

Sm

Tb

Vb Lu

lb

Yb Lu

Ia

g

5

/

I

2

***s... Er

Rb

Cs RR

II

K

Ba

U

Th

tr

Nf 2s

la

Ce

Id

Sn fu

Ea

Th

Vb

to

Fig. 5. Concentrations of some trace elements in rocks of the basalt-plagiorhyotite association plotted as multiples of chondritic concentrations (for REE, Th, U and Ba) and (for Cs, Rb and K) as multiples of values that yield the same normalized values in MORB KD-11 (Kay and Hubbard, 1978). Elements are arranged in order of increasing i~ompatibi~ty from right to left.

MAGMATISM

OF THE KOMANDORSKY

SEGMENT

OF THE

ALEUTIAN

trachybasalt-teschenitic rocks are also characteristically higher in U, Th, Ta, Hf and Zr than the basalt-plagiorhyolite association. Patterns of the REE distributions differ considerably from those of MORB and tholeiitic IAB (Fig. 6): all are substantially REE enriched (La/Sm > 2.3; La/ YB > 10). For all the rocks, a slight positive Eu-

ISLAND

299

ARC

anomaly is reported: Eu/Eu* varies from 1.06 to 1.37. On the La/Hf-La/Yb diagram the Komandorsky subalkaline and alkaline volcanics lie well apart from tholeiitic and talc-alkaline ones not only having higher La/Yb ratios but also La/Hf (3.3-8.2), forming a well-defined trend. The higher variability of the La/Hf ratios con-

TABLE 3 Whole-rock chemical analyses of the trachybasalt-teschenite Trachy-

Teschenites

basalt

18/l-78

17/3-78

20/l-78

19/4-79 60.10

3/19-78 SiO, (wt.%)

association

50.17

51.26

5.377

56.86

TiOz

1.19

2.04

1.23

2.08

0.89

Al 2% FeO*

16.15

15.68

13.67

17.63

17.64

7.51

8.46

6.15

5.35

5.42

MnO

0.12

0.14

0.09

0.10

0.11

Mt@ CaO

6.52

7.61

6.57

4.11

2.72

7.88

8.74

7.83

5.45

6.61 4.57

Na,O

4.69

3.67

4.48

5.11

K2O

0.75

2.40

3.56

3.31

H2O

3.96

n.d.

1.71

n.d.

n.d.

p205

0.49

n.d.

0.70

n.d.

n.d.

co2

0.13

n.d.

0.49

n.d.

n.d.

Total

99.56

100.00

100.25

29.00

31.8

32.30

Ni

153.23

167.0

267.18

Cr

168.00

194.0

Co(w-4

0.501

cs SC

18.2

0.180 20.7

Sb

n.d.

n.d.

Zr

n.d.

n.d.

100.00 15.40

12.2 36.2

3.52

4.34 0.182

0.058 230.0

99.77

26.7

0.12 18.80

1.71

11.7

66.6 0.500 8.71

n.d.

n.d.

n.d.

n.d.

Ta

0.162

0.722

0.39

1.13

Th

0.931

1.03

4.17

2.73

1.78

U

0.690

0.525

2.40

1.52

0.766

Hf

2.14

4.48

Rb

10

n.d.

Sr

1500

n.d.

Ba

460

428

5.17 32.6

n.d.

540

n.d.

n.d.

756

814

272

13.70

26.8

42.5

Ce

33.40

66.1

103.0

Nd

19.20

33.5

Sm

3.69

6.94

Eu

1.15

2.02

Gd

2.73

Tb

n.d.

0.610 1.94

n.d.

4.39

n.d.

La

n.d.

7.26

0.550

60.60 11.0 2.95 17.31 0.850 3.34

45.4 108 56.8 9.76 2.76 n.d. 0.850 n.d.

14.3 47.0 17.7 3.39 1.14 n.d. 0.467 n.d.

DY Yb

0.8

0.812

0.969

1.28

0.764

Lu

0.13

0.144

0.14

0.180

1.109

3UO

A.A. TSVETKOV

firms the suggestion

that it is the result of differ-

more

entiation

melts governed

Na,O,

tion

of primary

of major

mafic

silicate

by fractiona-

phases;

olivine

and

the of major-element

rocks

of

the

clearly

talc-alkaline

3). On Harker’s

trends;

MgO and CaO contents

fields

of

the

diorite-granitoid

association

are

ably, while

since on Myashiro’s

diagrams

the linear

the data

geochemistry,

diorite-granitoid

98% of them fall in the talc-alkaline along

towards

diagram

sociation

field (see Fig.

(see Fig. 4), they evolve

have rather anomaly

successively

decrease

ously

from

500

presented

the

varieties

while

somewhat.

associations

As

in Fig. 4 show that

basalt-plagiorhyolite

the whole

All rocks

Al,O,,

acid

increases

and

overlap

consider-

trachybasalt-teschenite

as-

is much richer in Ti.

the TiO,,

FeO,

more

on the contrary,

for TiO,,

clinopyroxene. In terms

basic

of the diorite-granitoid high concentrations

being much shallower formed

magmatic

association

of LILE,

the Ta-

than for the previ-

associations

(Table

A

t 200 -

IO0 -

50 -

r

s! L:

s +20-

. =.

z

IO-

5

t

200 t

B

100 -

50 -

z

s

20-

f

!: . p

‘O S-

2-

I

10

1

‘I

Cr Nb K

Na

U

L

I

111

Th

fr

Hf

Zr

La

1

Ce

1

Id

I

I

L

Sm

Eu

lb

Fig. 6. REE and some other trace elements in rocks of the tracahybasalt-teschtite

I

I

Vb Lu

(A) and diorite-granitoid (B) associations.

4).

MAGMATISM

OF THE

KOMANDORSKY

SEGMENT

OF THE

ALEUTIAN

The REE feature evolved distribution patterns and slightly positive Eu-anomalies (La/Sm > 2.2; La/Yb > 5.9; Eu/Eu * = 1.00-1.18). Apart from this general rule, there is only one analysis (andesite 10/14-78) whose concentrations of Ta and Sm are very low (0.047 and 0.4403 ppm respectively), causing the appearance of negative “peaks” in the

ISLAND

301

ARC

diagram (see Fig. 6). This rock is more enriched in light REE as compared to five other samples and has a high Eu/Eu* ratio (2.28). In the La/HfLa/Yb diagram the diorite-granitoid association field lies between those of the trachybasaltteschenite and basalt-plagiorhyolite associations. It is interesting to note that the Middle-Upper

TABLE 4 Whole-rock chemical analyses of the diorite-granitoid

association

Granodiorites 8/2-78

8/6-78

8/4-78

Basaltic

Andesite

Dacite

andesite

10/6-78

9/l-78

10/14-78 63.78

64.02

66.35

55.02

61.44

TiO*

0.45

0.52

0.46

0.71

0.50

0.46

AVJ, FeO*

17.29

17.18

16.87

17.88

16.53

15.62

4.10

3.88

3.71

7.56

5.29

2.90

MnO

0.05

0.11

0.17

0.21

0.07

0.03

Mi# CaO

2.26

2.89

2.16

3.85

2.76

1.12

4.95

4.79

4.37

8.67

4.83

4.37

Na,O

4.66

4.39

4.48

3.70

4.19

3.94

K2O

1.90

1.71

2.04

0.98

1.14

1.76

H2O

0.48

1.12

2.52

1.29

p205

0.23

co2

SiO, (wt.%)

Total

n.d.

n.d.

0.08

n.d.

n.d.

0.12

0.19

0.13

n.d.

n.d.

0.20

0.12

0.14

100.61

100.02

99.58

99.43

20.6

100.13

99.49

Co @pm) Ni

10.50

12.8

10.5

14.93

32.2

26.2

Cr

14.30

61.4

19.3

cs

0.18

SC

8.41

Sb

0.062

Zr

67.57

98.80

0.075 10.3

12.80

12.40

7.86

28.29

16.50 20.20

4.62

37.70

0.182

0.17

0.14

8.48

24.30

10.40

8.66 0.108

n.d.

n.d.

0.11

0.12

n.d.

n.d.

72.00

58.30

0.248

89.00

Ta

0.312

0.322

0.390

0.047

0.312

0.240

Th

1.73

1.53

2.43

0.979

1.88

1.55

U

1.39

0.899

1.06

1.11

1.04

1.25

Hf

2.53

2.55

3.02

1.84

2.27

2.36

Rb

15

n.d.

n.d.

Sr

651

n.d.

n.d.

652

618

746

Ba

272

348

255

280

268

300

La

8.02

Ce

16.80

8.43 19.1

11.7

15

12.7

11.7

10.5

8.91

27.6

22.5

19.10

22.5

16.6

10.0

12.0

9.85

Nd

8.91

8.56

9.52

Sm

2.00

1.90

2.47

0.403

2.10

2.68

Eu

0.658

0.675

0.710

1.23

0.702

0.768

Gd

1.80

Tb

0.271

DY Yb

3.49

1.50

2.07

0.883

0.764

0.975

2.13

0.919

0.983

Lu

0.137

0.122

0.143

0.337

0.157

0.172

1.41

n.d. 0.252 n.d.

n.d. 0.285 n.d.

4.03

1.68

1.92

0.598

0.273

0.334

302

AA

Miocene talc-alkaline granitoids of Medny Is. contain more Sr (618-746 ppm) than the Eocene-Oligocene plagiorhyolites (100-110 ppm), most probably due to the higher contents of feldspar in the former. Comparative analysis reveals a close compositional match between Komandorsky diorite-granitoid rocks and the Middle-Miocene to Quaternary talc-alkaline magmatic formations (lavas, pyroclastics, plutonics) of the East-Central Aleutians, just as in the case of similar formations of other Pacific island arcs (Perfit et al., 1980; Kay et al., 1982). Giant volumes of such rocks are typical also of intra-continental or marginal erogenic belts, e.g., South American Andes, the Caucasus, etc. (Spencer, 1974).

TSVETKOV

Although diorite-granitoid rocks have not yet been found on Bering Is., dredging performed by American Scientists on its submarine slopes recovered Upper-Miocene andesites with a K/Ar age of 8.8 Ma (Scholl et al., 1976), compositionally comparable with the Medny andesites. It is natural to assume that all these intermediate volcanics and plutonics correspond to just one tectono-magmatic impulse manifested in the western part of the Aleutian chain as much wider and stronger than could be judged from on-land exposures of the diorite-granitoid association on Komandorsky Is. As for the Quatemary dacite-rhiodacite association, a characteristic of the Piip volcano lavas is

TABLE 5 Whole-rock chemical analyses of dacite-rhyodacite B-26-G-5/1 SiO, (wt.%)

B-26-G-4/1

association

B-26-G-5/2

B-26G-3/2

B-26-G-4/2

B-26-G-3/1

B-26-G-3/3

B-26-G-23/1

64.18

64.78

65.28

65.80

65.89

66.00

66.31

TiO,

0.54

0.45

0.49

0.36

0.41

0.38

0.40

0.50

Al 203 Fe0

17.24

16.96

17.39

16.90

17.10

16.84

16.81

15.60

3.18

3.06

3.14

2.98

2.86

2.87

2.94

2.80

MnO

0.12

0.09

0.06

0.08

0.03

0.10

0.16

0.12

MtP CaO

2.75

2.96

2.42

2.94

2.82

3.02

2.82

0.87

5.49

5.21

5.30

4.84

4.85

4.77

4.81

2.82

Na,O

4.69

5.19

4.67

4.90

4.76

4.97

4.78

5.81

K,G Total

1.06

1.07

1.17

1.08

1.12

1.08

1.06

1.78

99.18

99.77

99.90

99.88

88.94

100.03

100.09

99.96

Co @pm) Ni

11

11

11

12

12

10

13

22

10

45

10

52

46

50

52

2

Cr

8

36

8

45

34

40

38

CS

0.16

SC Ta

10.4 0.330

0.21

0.19

0.17

0.16

0.19

0.20

69.66

3 0.28

8.9

8.6

8.7

8.7

7.0

0.39

0.29

0.34

0.36

0.36

0.48

0.48

9.1

10.3

Th

2.72

1.06

2.72

0.99

0.98

0.98

0.96

1.48

U

0.49

0.49

0.47

0.46

0.52

0.48

0.49

0.62

Hf

2.72

2.96

2.72

2.91

2.85

2.73

2.76

4.37

Rb

12

n.d.

15

n.d.

15

15

15

19

Sr

432

461

434

451

444

454

446

280

Ba

151

164

154

158

139

156

152

214

La

7.50

Cc

16.20

8.12

7.72

17.5

16.7

7.60

7.46

16.6

16.5

7.69

7.46

10.64

16.5

16.4

24.3 12.3

9.0

9.3

8.1

9.6

8.6

9.4

Sm

1.90

1.92

1.99

1.85

1.77

1.83

1.78

2.83

EU

0.576

0.551

0.569

0.509

0.511

0.511

0.497

0.707

Tb

0.34

0.30

0.33

0.29

0.29

0.29

0.28

0.48

Yb

1.31

1.28

1.31

1.14

1.22

1.18

1.11

1.80

LU

0.191

0.185

0.191

0.171

0.174

0.173

0.175

0.283

Nd

n.d.

MAGMATISM

OF THE KOMANDGRSKY

SEGMENT

OF THE ALEUTIAN

their considerably higher silica contents as compared with the Pliocene-Quatemary lavas of the East-Central Aleutians. All these lavas form a compact gxoup within the Sit& range of 64-708 and fall completely in the talc-alkaline field on Myashiro’s diagram (see Fig. 3), the Na,O contents being 4-5 times higher than that of K,O. In spite of the relative enrichment of the Piip volcano dacites and rhyodacites in alkalies, U and Th relative to the REE, the degree of such enrichment is lower (e.g., Ba/La = 19-20; Th/La = O-1390.127) and, in general, concentrations of all incompatible elements are lower than those of Aleutian arc dacites (Table 5). Hf/La (0.35-0.41) and Ta/La (0.37-0.64) ratios are higher than those of dacites from the Aleutian arc, while La/Yb

ISLAND

303

ARC

ratios (5.9-6.7) are similar to those in other Aleutian hornblende-bearing dacites. Dacites with similarly low REE concentration have been found on Medny Is. among rocks of the torte-~~toid association (see Fig. 6), in the Southern Andes (Stem et al., 1984) and in the Mariana Trench (Bloomer and Hawkins, 1987). Back-arc dacites from the Woodlark Basin, near New Guinea, have much higher REE and LILE concentrations (Johnson et al., 1987) (Fig. 7). It is known that covariations of Th, La, Ta and Hf in magmatic rocks can be used as indicators of their serial and geodynamic position (Pearce and Cann, 1973; Wood et al., 1979). In Th-Hf plots, the compositions of all Komandorsky magmatic associations are linearly arranged and mutually

CS

88

UThMTrLaCe

Nd

SmEuGdlb

Cs

Ba

UThtilTrLaCe

Nd

SmEuGdlbOyEr

DyEr VbLu

YbLu

Fig. 7. Extended trace-element diagrams normalized to chondrites (Romick et al., 1990). (A) Piip dacites compared to Aleutian-arc dacites. Data for UM-6 are from Kay et al. (1982), data for Sit-RK 5 are from Neuweld (1986). Kanaga is a Holocene pumice collected by J. Romick from Kanaga Island. (B) Piip dacites compared to Medny Island granodiorites. Also shown is the calculated trace elemeut pattern for a source from which the Piip dacites might have been derived (stippled region). (C) Boninitic (Mariana Trench, Bloomer and Hawkins, 1987) and back-arc (Woodlark Basin, Johnson et al., 1987) dacites from western Pacific settings, and hornblende da&e from the Southern Andes (Mt. Bumey, Stem et al., 1984). (D) Trace element concentrations of BABB from Scotia Sea (Saunders and Tamey, 1979) and Valu Fa Ridge (Jenner et al., 1987) and boninite lavas (Hickey and Frey, 1982) compared to the calculated source composition of Piip dacites (stippled field).

304

A A. TSVETKOV

Th wm

Lawm

Tappm .

I

IO

I

Th//

margins

Fig. 8. Th-Hf,

La-Ta

magmatic associations tions:

Ta

OcrM active .I

and Th-Hf/3-Ta of the Komandorsky

1 = basalt-plagiorhyolite,

3 = diorite-granitoid,

x2

l

3

04

diagrams

for the

segment. Associa-

2 = trachybasait-teschenite,

4 = dacite-rhyodacite.

Compositional

fields are given according to Wood et al. (1979).

correlated, evolutionary trends being practically parallel to each other (Fig. 8). The permanent enrichment of progessively younger rocks within the same SO, range in Th should be mentioned. Not a single basalt on this plot falls into MORB field (Hf/La > 12.5); this fact supports the previously drawn conclusion that none of them belong to oceanic crust entrapped in or superimposed on the structure of the Komandorsky segment. Compared with other magmatic products, we can see that the basalt-plagiorhyolite association plots somewhere in between the fields of tholeiitic and talc-alkaline basalts of Japan (1.25 < Hf/Th < 12.5) while major parts of the diorite-granitoid and the dacite-rhyodacite associations are in the latter field (1.25 < Hf/Th < 2.5). The trachybasalt-teschenite association, on the contrary, tends towards the field of oceanic islands, such as, e.g., the Azores and Iceland. In general, the Hf/Th ratio has a tendency to diminish with decreasing age of magmatic rocks. In La/Ta plots, similartype evolutionary patterns can be seen in the field restricted by the ratio lines La/Ta = 45 and La/Ta = 14. The most obvious correspondence of compositions exists between the diorite-granitoid and the dacite-rhyodacite associations, to a lesser extent with the basalt-plagiorhyolite association, and the magmatic series of Japan, although some points referring to the Komandorsky rocks fall above the ratio line La/Ta = 45. All Komandorsky Is. compositions in this diagram are also well separated from the MORB field. The locations of points corresponding to successive magmatic associations of the Komandorsky segment in the Th-Hf/ 3-Ta diagram proves its “evolved” character (Bogatikov and Tsvetkov, 1988). At the same time the trachybasalt-teschenite association falls partly outside the arc field, tending towards the fields of E-type MORB and hot-spot oceanic islands. Thus, since the Eocene in the Komandorsky segment we can distinguish three magmatic series: tholeiitic, talc-alkaline and subalkaline (K-Na alkaline), their geodynamic settings mostly being typical of island arcs; only the rocks of the trachybasalt-teschenite association are somewhat specific, possibly being related to other geodynamic regimes occurring locally in the Miocene in that particular part of the Northwest Pacific.

MAGMATISM OF THE KOMANDGRSKY SEGMENT OF THE ALEUTlAN ISLAND ARC

Isotopes Within the Komandorsky segment, 143Nd/ ‘*Nd and s7Sr/s%r ratios were determined for all the Tertiary magmatic associations (Zhuravlev et al., 1983). The obtained results are presented in Table 6 and plotted on a r,-87Sr/86Sr diagram (Fig. 9). The isotopic characteristics of Nd and Sr in the Komandorsky magmatic rocks fall entirely within the field of ensimatic arcs and ensimatic segments of continental erogenic belts (Mariana, New Britain and South Sandwich arcs and the Ecuadorian segment of the Andes) (DePaolo, 1988). High *7Sr/86Sr values for rocks of the initial magmatic episode can be attributed to the effects of isotopic exchange with seawater Sr (in the present-day seawater s’Sr/‘%r = 0.7093). Studies of this are now under way (Van Drach et al., 1986; DePaolo, 1988). The close match between the degree of alteration of tholeiitic basalts and plagiorhyolites and their 87Sr/86Sr ratios (Borsuk et al., 1982), the lack of covariation of Nd and Sr isotopes, the variability of Sr isotope values and the results of leaching experiments all strongly support Sr alteration during large-scale hydrother-

6Ud WI ALTERED

12 -

MORB rnG

O-

‘

0.;0?3

o,7;30

Fig. 9. CNd-*‘Sr/%r

0 7i370

o,;04,

o.;ou

’

s’Sr/ssSr

diagram for the magmatic rocks of the Komandorsky segment. Komandorsky magmatic associations (Zhuravlev et al., 1983): beet-p~~orhyo~te (45-28 Ma): 1 = plagiorhyolites; 2 = basalt; trachybasalt-teschtite (27-16 Ma): .? = trachybasalt; diotite-granitoid (12.5-8.5 Ma): 4 = granodiorite; dacite-rhyodacite (0.7-O Ma): 5 = da&e; 6 = Pli~ne-Quip talc-alkaline and tholeiitic andesi& and basalts of the Aleutian Arc (McCulloch and Perfit, 1981); 7 = high-Mg andesite (HMA) of Kay (1980).

305

mal metasomatic alteration of already crystallized arc rocks. The highest eNd value (up to 10) are reported for pla~orhyo~tes as compared with coeval tholeiitic basalts with eNd = 8.2 and later arc-related talc-alkaline andesites, dacites and granodiorites with fNd in the range of 8.9-9.2. This feature is of much interest and helps in constraining the genetic processes of the first acid melts of the arc, providing evidence for incorporation of a gabbro-basaltic part of the oceanic crust, with eNd > 10, in its magmatic source. In support to this conclusion, the results of REE mass-balance calculations clearly show that the first acid arc magmas could be derived by mixing of tholeiitic basaltic arc liquids with 25-30% of melts from quartz eclogite and separation of clinopyroxene, olivine, plagioclase and magnetite from the resulting melt (Tsvetkov, 1984). All the isotopic data for diorite-granitoid and dacite-rhyodacite associations fall within the mantle array. s7Sr/%r ratios (0.70273-0.70281) and ENd (9.5-9.8) values for the latter are lower and higher, respectively, than those observed for young Aleutian volcanic rocks and this also favors the idea of some contribution from crustal materal to their sources. It should be emphasized that the transition of the arc crust from oceanic type (Komandorsky and West-Central Aleutians) into a continental type (Eastern Aleutians and Alaska) does not affect the Nd and Sr isotopic parameters (Kay et al., 1978). This proves minimal degrees, if any, of crustal contamination by both the sialic and mafic parts of the crust during the later Aleutian evolutionary stages, contrary to the initial stage when such a process seems to have been rather efficient. Indeed, petrographically similar coeval Eocene-Oligocene basalts differ in the west-central and eastern segments of the arc in both major- and trace-element abundances. Thus, the rocks of the Unalaska Formation on Unalaska Is. (Perfit, 1977) are well separated from the Komandorsky basalt-plagiorhyolite association, possibly due to crustal contamination of primary melts; this seems reasonable in the east where the crust is thick and old but is practically negligible in the Komandorsky segment. We believe this is caused by a system of magmatic feeders that have

306

TABLE

A A TSVETKOV

6

~7Srpyr

an*

‘4’

Magmatic

association

Nd/j”Nd

isotopic

ratios

in the magmatic

Rock Sample

rocks of Komandorsky Age

Islands

%/%

‘43Nd/‘“Nd

CNd

(Ma)

dacite B-26-GS/l

0.7

0.10215 + 6

0.513136

* 4

9.1

dacite B 26-G-4/2

0.7

0.70273 f 6

0.513123

f

3

9.5

dacite B 26-G-3/3

0.7

0.70275 f 5

0.513136

5

3

9.7

rhyodacite

B26-G-23/1

0.7

0.70281 f 5

0.513138

f

4

9.8

Diorite-granitoid

granodiorite

11/6-77

11+3

0.70298 f 2

0.512298

f 21

8.9

Trachybasalt-te-

trachybasalt

415-77

21 f 3

0.70353 + 4

0.512344

f 27

9.9

schenite

teschenite

21 + 3

0.70315 + 4

0.512323

+ 33

9.5

Dacite-rhyodacite

406-77

Basalt-plagio-

plagiorhyolite

7/2-77

30 * 4

0.70371 * 5

0.512358

+ 19

10.1

rhyolite

plagiorhyolite

4/l-77

33 f 4

0.70478 + 8

0.512352

F 32

10.0

basalt 3/14-78

33 + 4

0.70363 f 4

0.512273

f 17

8.2

basalt 440

33 + 4

0.70363 + 4

0.512273

f 17

8.2

been more or less uniform throughout the whole “life” of the Aleutian arc. The wall-rocks of such feeding channels were becoming progressively depleted in LIL magmatophile elements with time and finally became isotopically equilibrated with the rising magmatic melt. The possibility of such process was outlined by Marsh (1987). As for the fate of subducting sediments, the available data are contradictory (Arculus and Johnson, 1981; Karig and Kay, 1981; Ping-Nan Lin and Stern, 1988; DePaolo, 1988; Goldstein et al., 1989). On the one hand, high K, Ba, Rb, U, Sr, Th and other incompatible-element abundances in arc igneous rocks seem to indicate their considerable involvement in the magmatic process (Kay, 1980; McCulloch and Perfit, 1981; Nohda and Wasserburg, 1981) but, on the other hand, recently obtained Nd and Sr isotopic results significantly constrains the volumetric percentage of sediments participating in melting reactions (Morris and Hart, 1983; Goldstein et al., 1989). A new but scientifically rewarding approach to the evaluation of contributions of sediments to islandarc magmatism is the investigation of the distribution of a short-living “Be radionucleide in arc lavas. Recently, it was convincingly shown by Tera et al. (1986) that its detection in Quaternary arc volcanics indicates the incorporation of a sedimentary component in their magmatic source, supplied there in the course of subduction. For the

Aleutian arc 1.96-15.3 X lo6 at/g of “Be were measured in historical lavas erupted from both the frontal and the back-arc zone volcanoes. “Be was also found in dacite B26-G3/17 dredged from the Piip submarine volcano. The sample was chosen for its utmost freshness and massive fabric, and prior to the analysis it was treated following the technique of Tera et al. (1986). Special care was taken of preliminary procedures because the isotope analysis was to be done on dredged rock. A number of experimental runs * gave the result 2.7 X lo6 at/g, thus in terms of “Be distribution the Piip volcano is within the same range as the rest of the Aleutian volcanoes. The question is what caused the appearance of this isotope in magmas erupted to far beyond the Aleutian volcanic front? If we assume that the subducting oceanic plate in the vicinity of the Komandorsky Is. has some component of convergence, then the occurrence of “Be in lavas of the Piip volcano can be explained, just as for other Aleutian volcanoes, by sediment incorporation in their magmatic source. Assuming (most optimistically) a 100% contribution of sub-

* Determinations Tera

from

Carnegie

were carried the

Institution

Department

out by Dr. J. Morris and Dr. F. of

of Washington

Terrestrial D.C., USA.

Magnetism.

MAGMATISM

OF THE

KOMAN~RSKY

SEGMENT

Oi= THE ALEUTIAN

ducted pelagic sediments to magmatic melts and using the formulas of Tera et al. (1986), we tried to evaluate some aspects of such a process. It was found that in order to have 2.7 lo6 at/g “Be in the melt, the subduction time (at a 6.5 cm/yr subduction rate} must have been 6 Ma, If the angle of underthrusting was 35”, the distance travelled by the plate edge with such geodynamic parameters must have been 390 km. In the case of a smaller sediment incorporation, the presently measured amount of “Be in Piip volcano lavas can be matched if a higher subduction rate is assumed, the latter idea is generally in accord with recent geodynamic concepts for this part of the Pacific (Zonenshain et al., 1984; Savostin et al., 1986; Harbert et al., 1987, 1989). An alternative model can certainly be introduced. As was shown by Seliverstov (1987), the Piip volcano lies in a tectonically complex rift type zone at the southern rim of the Komandorsky Basin (see Fig. 1). Taking into account the tectonic instability and high permeability of this structural unit and the thick sediment cover on the sea floor, contamination of talc-alkaline mantle melts by these mostly young pelagic sediments, both suitably fluid and enriched in “Be, and directly beneath the volcano, cannot be discarded. To choose between these two alternative models, one should obtain data on “Be in mineral separates from lavas of the Piip volcano, just as was done recently by Morris and Tera (1989) for the Bogoslof volcano. For the latter lavas it was found that all the “Be was acquired at the level of their magmatic source. Certain distinctions outlined above between the Quatemary acid volcanics in the rear of the Komandorsky segment and in the area of the recent Aleutian volcanic front can possibly be attributed to the hybrid origin of the former (Romick et al., 1990). Evidence for magma mixing includes: reversely zoned phenocrysts in Piip volcano Iavas, bimodal plagioclase (An,, and An,,) and hornblende (6% and 9.5% Al,O,) populations, high Cr (34-40 ppm) and Ni (46-52 ppm) concentrations. High-silica dacites-rhyodacites with Fe0 */MgO = 3.12-3.16 can be derived from these dacites by a closed-system crystallization of the observed mineral assemblage.

ISLAND

ARC

307

Discussion The considerable increase during the past decade in the amount of geophysical, geological, petro-geochemical and bathymetric data for the Aleutian-Being Sea re8ion provides constraints on geodynamic models for the North and Northwestern Pacific (Schmidt, 1978; Zonenshain and Savostin, 1981; Rea and Dixon, 1983; Zonenshain et al., 1984; Savostin et al., 1986; Bogdanov, 1988; Harbert et al., 1989). In the majority of these models the Aleutian submarine ridge, the predecessor of the later Aleutian arc megast~cture is considered to have begun rising soon after a lowangle subduction zone was formed in that region. The northern part of the Pacific plate in the vicinity of the present-day trench, a superficial indicator of this zone, is covered by a thick Paleogene-Quatemary volcano-sedimentary sequence of the arc “body”. Nevertheless, data exist which show that remnants of the Pacific plate are still preserved in the basement of certain segnnent of the Aleutian arc (Rubenstone, 1984; Seliverstov, 1987). Summarizing all available data on different segments of the Aleutian arc, we shall now try to analyse their evolutional patterns relative to the Komandorsky segment. We use published data on magmatic rocks of the central and eastern parts of the Aleutian Is. and Alaska in Burk (1965), Marsh (1976, 1979, 1987), Kay (1977, 1980), Marsh and Leitz (1979), Kay et al. (1978, 1982, 1983, 1986), Perfit et al. (1980), Citron et al. (1980), Myers et al. (1985, 1986) and Von Drach et al. (1986). Recently undertaken radiological dating of the Komandorsky (Bogatikov and Tsvetkov, 1988) and the Aleutian (Rubenstone, 1984) Is. magmatic rocks and available information on linear magnetic anomalies on the adjacent ocean floor (Cooper et al., 1976a) suggest that the Aleutian submarine ridge began growing as from at least the Late Cretaceous-Early Paleogene, though a former deep-seated structure, subsequently turning into a magmatic feeder, was probably formed even earlier in the Early Cretaceous (Scholl et al., 1975). The initial evolutionary stage throughout the whole Aleutian Island arc, including the Alaska peninsula, is characterized by predo~n~tly

A.A.

basalt-rhyolite sequences. In the central part of the Aleutian arc, corresponding to the basaltplagiorhyolite association of the Komandorsky Is. in time, composition and geological setting, magmatic suites are widely distributed in many islands: Adak and Ilak (Finger Bay Formation), Amchitka (Krugloi Point, Amchitka, and Banjo Point Formations}, Rat (Rat and Gunner Cove Formations). They are represented mainly by basaltic pillow lavas and tuffs which, as in the Komandorsky Is., have suffered ~eensc~st metamorphism. However, clinopyroxenes in these rocks (Wo,,_,,En,,_,,Fs,,_,,) are often significantly Fe-enriched and have more Al,O,, TiO, and CaO relative to the Komandorsky clinopyroxenes (De Long et al., 1978). As compared with the Central Aleutians, the average concentration of REE in the Komandorsky basic volcanics are usually lower though they feature the same chondrite-like pattern. It should be also noted that in the Central Aleutians, besides volumetrically predominant basaltic volcanic sequences, gabbro-granite and gabbro-monzonite plutons are also known: the examples are the 30-35 Ma Finger Bay (thole~tic) and Hidden Bay (talc-alkaline) plutons on Adak Is. (Citron et al., 1980; Kay et al., 1983). Within the eastern segment of the Aleutian arc, the Komandorsky basalt-pla~orhyolite association has much in common with the Unalaska Formation on Unalaska Is. (Perfit, 1977). The latter consists of fine-grained inte~ediate and acid pyroclastics, dacitic, andesitic and basaltic lavas, cut by sills, dykes and small stocks of the same composition. As already mentioned above, basalts from the Eastern Aleutians generally differ from the Komandorsky basalts in having higher concentrations of incompatible elements, including the REE. In Alaska, the time interval 38-26 Ma is marked by gabbro-quartz diorite-granitic plutons, compositionally varying from normal gabbros and gabbro-norites to monzonites, leucogranites and adamellites (Reed and Lanphere, 1973). Comparing general features of the Eocene-Oligocene stage magmatism throughout the whole length of the Aleutian arc, a few conclusions can be drawn. First, whereas within the ensimatic Komandorsky and West-Central Aleu-

WVE.TKOV

tians the older magmatic rocks belong mainly to the tholeiitic series and often are of a bimodal type, in ensialic Eastern segment of the arc, including Alaska, they are usually continually differentiated and dominantly belong to the calcalkaline series. Second, along the arc axis there is also a considerable variation in the relative volumes of acid and basic members within magmatic associations: for the Komandorsky segment, the ratio acid/basic rocks is approximately 1 : 3, for the Adak segment 1 : 5 and for the Unalaska segment 3 : 1. We believe that this feature may be attributed to the different styles of evolution of the magmatic melts. which are often less differentiated in the ensimatic part of the arc and strongly differentiated in its ensialic part, where magmas were quite often stored in crustal reservoirs for long periods of time and were affected by processes of wall-rock assimilation (Borsuk and Tsvetkov, 1982). Of interest is that plagiorhyolites are typical only for the Komandorsky segment of the arc. The geological and structural relationships of the older Komandorsky basalts (andesites, dacites and pla~orhyolites) their appearance within a very limited temporal range corresponding to the early stage of the Aleutian arc’s evolution, supplemented by some common geochemical features, allow us to consider them as members of the same basalt-plagiorhyolite association. However, we strongly doubt that plagiorhyolites can be obtained by means of fractionation ~fferentiation of a primary basaltic melt. This idea is not supported by mass-balance calculations (Tsvetkov, 1984), moreover, Komandorsky plagiorhyolites differ considerably from oceanic plagiogranites of ophiolite associations derived, as many authors believe (Coleman and Peterman, 1975; Kay and Senechal, 1976; Thorpe, 1982), from parental basaltic liquids. At the same time, the classical early experiments of Vincler and Von Platen (1961) clearly showed that plagiorhyolites usually can not be the result of anatectic melting of some hypothetical ancient sialic masses occurring, as Shapiro (1976) and Ivashchenko et al. (1984) believe, in the basement of the Komandorsky segment of the Aleutian arc. In this case, melting would have primarily yielded normal talc-alkaline acid rocks

MAGMATISM

OF THE

KOMANDORSKY

SEGMENT

OF THE ALEUTIAN

of granodiorite-granite type, not Na-rich (plagio) granites or (plagio) rhyolites. We would like to introduce here the following explanation of the origin of the Komandorsky plagiorhyolites. First, all presently available petrological, geochemical and isotopic data prove that the basalts forming the northern parts of both Medny and Bering Is. are of mantle origin and seem to be caused by the processes taking place in the subduction zone and mantle wedge. Their arc affinity is absolutely clear. Second, due to the thermal effects produced by basaltic masses rising to upper crustal levels, parts of the arc crust, composed mainly of basaltic tuff-turbidites, were melt; the resulting liquids fractionating in intermediate or near-surface conditions produced the whole gamut of Eocene-Oligocene acid-intermediate volcanic and plutonic rocks known in the Komandorsky Is. The theoretical acceptibility of such a model is proven by mass-balance calculations (Kay, 1980; Tsvetkov, 1984). The moment at which the first acid volcanics appeared in the structure of the Komandorsky segment (34 Ma) can be considered as the starting point for the transformation of primary oceanic crust in the arc basement into continental. This model also explains the fact that the greater part of the Komandorsky plagiorhyolites is somewhat younger than spatially associated basalts and andesites. It should be mentioned here that similar kinds of models describing the generation of basalt-plagiorhyolite or gabbro-plagiogranite magmatic associations of continental Phanerozoic fold belts are being widely discussed at present and accepted by many scientists (e.g., Gill, 1981; Thorpe, 1982; Bogatikov and Tsvetkov, 1988). Within the petrological and geochemical frame of reference these bimodal basic-acid magmatic associations like those from the Caucasus, the Western United States or the Ural Mountains can be considered as counterparts of the Komandorsky basalt-plagiorhyolite association. No doubt, the first (Eocene-Oligocene) magmatic episode was initiated by subduction of one segment of the Pacific plate beneath another. This process continued for almost 10 Ma, then abruptly stopped. The cessation of magmatism most probably was caused, as in the Aeolian arc (Barberi et

ISLAND

ARC

309

al., 1974), by “cracking” of the lower part of the subducting slab and its rapid sinking into the asthenospheric mantle, though there are other theories, e.g., relating the appearance of alkalic rocks in arc settings to shearing along the boundary of the two plates (Grenada for example), or to ridge subduction, or to aseismic ridge collision. The area adjacent to the zone of such “cracking” in the Komandorsky segment seem to be favorable for the ascent of magmatic diapirs, giving rise to subalkaline and alkaline magmas (trachybasaltteschenite association). Taking into account the high La/Hf and La/Yb ratios in rocks of this association, we can conclude that their parents most probably were low-degree partial melts of mantle origin. It is interesting to note that this, most likely non-island arc magmatism, took some time to develop in full, having been preceded by basaltic volcanism of normal alkalinity. Such a succession of events is often met with in ocean islands, like Hawaii, or in continental rifts where pre-rift and syn-rift stages are distinguished respectively by tholeiitic and subalkaline magmatism. As can be judged from the MORB distribution in the Komandorsky “pre-rift” gabbros and basalts, their parental melts were higher-percentage partial melts as compared to those of the trachybasalts and teschenites. They have flat distribution patterns like most tholeiitic intraplate basic volcanics. In easterly direction the total alkalinity and volume of material erupted during this evolutionary stage continually decrease. At the same time, subalkaline basal& trachybasalts, trachyandesites and teschenites, which are so typical and abundant in the Komandorsky Is. further eastward, give way to basalts of normal alkalinity (dolerites and gabbros), while in Alaska and adjacent islands in the ensialic part of the arc, this magmatic association is not distinguished at all. Among all the Aleutian alkaline rocks, limburgites are reported only in Kanaga Is. where they contain mantle inclusions of dun&es and peridodites (De Long et al., 1975). Compared with the products of subalkaline and alkaline magmatism in other geodynamic settings, we can find some common geochemical features between Komandorsky trachybasalts, trachyandesites, trachydolerites and

310

A A. TSV fTKOV

teschenites

and

the Hawaiian

increased

alkalinity,

reported

in Hawaii

arc

settings,

therefore,

although

teschenites

the geodynamic

as rather

are

submarine

segment)

uncommon; can be re-

Miocene,

active sub-

growth

with renewed of the Aleutian thereof

being

(in the Komandorsky associations.

age and composition

stricted

to those

seg-

that the westernmost Quatemary

only to Buldir

Komandorsky

volcanoes segments form-type

the andesite-dacite

magmatic

gists believed

of the Aleutian the

magmatism

ridge, the manifestations

and

For

of the Koman-

and persisted

significant

the diorite-granitoid ment)

are not

species.

also

of

specific.

was revived

force causing

teschenites

position

Later on, after the Middle duction

volcanics

as petrographic

dorsky subalkaline-alkaline garded

basic

within junctions

not older

than

and rhyodacitic concept.

total

the Komandorsky

lack

and Near

in

massif dacitic

urges us to reconsider

It seems likely that subduction,

Is.

trans-

plates

of a huge volcanic

0.7 Ma with talc-alkaline lavas

re-

of young

as due to oblique,

of the lithospheric

that area. The discovery

were

Is., 700 km to the east of

Is. The

was explained

manifestations

volcanism

this

at least of

some part of the Pacific plate in the Komandorsky

(in the Aleutian

segment,

Rocks similar

very small angle relative to the trench. Thus, taking into account the present-day trajectory of the Pacific plate, we can conclude that the Piip volcano

in

from the Koman-

dorsky Is. are widely spread in the Aleutian Is., particularly on Attu, Agattu, Semi&i, Kiska,

nevertheless

took

place,

although

at a

Adak, Kagalashka, Amchitka, Atka, Umnak, Unimak and Unalaska Is. They are represented by andesitic, dacitic, sparsely basaltic lavas, pyroclas-

now rests upon the very section that started underthrusting in the vicinity of Near Is. The distance from the Piip volcanic massif to the trench axis is 140-150 km, almost similar figures (av.

tics and large compositionally zones granodioritegranite plutons like the Kagalashka Pluton on

166 + 60 km) modern island

Kagalashka

front to the trench (Gill, 1981; Thorpe, 1982). The Pacific plate had to travel for about 400 km to get

Is. (Citron

et al., 1980) or the Captain’s

Bay and Scan Bay plutons on Unalaska Is. (Perfit et al., 1977). The most intensive volcanic activity the

Middle-Upper

geodynamic

parameter

is

close

boundary Magmatic

the time of pluton emplacement. rocks within ensimatic and ensialic parts

the depth to the Benioff zone. Its average for volcanic fronts, according to Gill (1981) is 124 f

actually show no petro-geochemical and they correspond to the calc-al-

38 km, for the Eastern Aleutians it is 110 km. The depth to the Benioff zone under the Piip volcano is difficult to evaluate due to the great uncertainty in the locations of earthquake hypocenters caused by the spatial position of seismic stations. It can

kaline series. The current tectono-magmatic the Pliocene one without any

Miocene

under the Piip volcano. Another important

occurred

of the arc distinctions

to

are obtained for the majority of arcs as distance from the volcanic

stage, following break, is char-

acterized mainly by talc-alkaline volcanism though, according to Kay et al. (1982) rocks of the tholeiitic (according to Myashiro’s understand-

be modelled as something in between a typical Benioff zone dipping beneath the arc and a fracture zone incorporating a set of normal seismically

ing) series, occurring primarily at the edges of tectonic segments, are also of considerable area1

active faults in the western termination of the arc (Seliverstov, 1987). These faults could be extension-related and could occur in a bending part of the downgoing slab. Extension vectors in this case are almost normal to the direction of a low-angle subduction. In the Komandorsky segment, earthquakes restricted to the trench feature that particular strain pattern (Stauder, 1968, 1969). Thus, recent magmatic manifestations in the rear of the Komandorsky Is. are, in geodynamic context, most probably of arc type but may correspond rather to a back-arc than to a frontal volcanic zone.

abundance. In the eastern part of the Aleutian back-arc zone, there are some small-scale manifestations of subalkaline basaltic magmatism (Bogoslof Is.); these are lacking within the rest of the Bering Sea side of the arc and geochemically resemble some of the Komandorsky Miocene trachybasalts. Of interest is the tectonic and geodynamic position of the Piip submarine volcano in the Komandorsky segment. Until lately, most geolo-

MAGMATBM

OF THE KOMANDGRSKY

SEGMENT

OF THE ALEUTIAN

Another possible scenario, similar in many aspects to that of Hickey and Frei (1982), was presented by Romick et al. (1990). According to this, the Komandorsky back-arc mantle was involved in more than one partial melting event which left a depleted mantle residue. Later introduction of a “metasomatic” component (presumably derived from the subducting slab) enriched the back-arc mantle in alkalies. Partial melting of this altered mantle resulted in a LREE-enriched, Mgrich magma which evolved to a dacitic composition before erupting. A relatively small degree of partial melting might be expected if the slab component acts as the fluxing agent. The alkali and isotopic data for Piip volcano lavas indicate that the slab components was very small, implying that the degree of partial melting it indicates would also be small. The tectonic history of the Komandorsky Basin (Savostin et al. (1986) make it possible to imagine that the slab component evident in Piip da&es was introduced during the Miocene. There has been no major episode of subduction-related volcanism since the Miocene, presumably because of the shift from convergence to strike-slip motion along the Western Aleutians-Komandorsky segment. This implies the storage of a slab component in the back-arc mantle over the last 5-10 Ma. The similar U/Th, Ba/Th and Ta/Th ratios in both Piip dacites and modern Aleutian-arc dacites suggests that the slab component was derived from a source similar to that seen in modem Aleutianarc volcanics. One possible model which explains the depleted character of the source of the Piip dacites is shown in Fig. 10. Opening of the Komandorsky Basin (Cretaceous? to Miocene) involved the eruption of back-arc basalts similar to the DSDP-191 basalt, leaving behind a residual depleted mantle. Subduction of the Pacific plate during the later stages of back-arc spreading introduced a slab component into the sub-arc mantle. Subsequent volcanism on Komandorsky Is. (Borsuk and Tsvetkov, 1982) during the Miocene reflects this slab component. Simultaneously, subduction introduced a small slab component into the (now) depleted back-arc mantle. The Pliocene to Recent evolution along the southern margin of the

ISLAND

311

ARC

A.

CrHaceous(?)4Mcene

B.

Pliocene(?)-Recent

I

W

I

Fig. 10. Cartoon of the magmatic evolution of the Kommdorsky back-arc mantle (Romick et al., 1990). (A) Subduction of the Pacific Plate, which postdated back-am spreading betwm the Cretaceous and Miccene and introduced a slab amponent into back-arc mantle depleted by the extraction of Komandorsky basin basalts. (B) Plio-Pleistocene to Recent back-arc spnading resultingin partial melting of this depleted mantle. Pending and fractionation at the crust-mantle boundary caused these partial melts to evolve to dacitic compositions which erupted along normal faults to form the Piip volcano.

Komandorsky Basin brought this altered, depleted mantle to shallow depths were it partially melted. Fractionation of this partial melt at the base of the back-arc crust produced dacitic magmas which rose along normal faults and erupted on the ocean floor to form the Piip volcano. This model is also capable of explaining the B of ‘*Be isotopes in Piip volcanics as the subduction time in this case is also about 6-8 Ma. The lack of basaltic or andesitic lavas at the Piip volcano and the presence of hornblende phe

312

nocrysts in the dacites distinguishes these lavas from volcanic rocks from other Pacific back-arc basins. Explemental studies (Romick et al., 1990) on dacites indicate that pressures in excess of 2 kbar are required for amphibole to achieve a liquidus phase at water contents less than 4.5%. This places the minimum depth at which Piip dacites could have fractionated at about 7 km. This is the appro~mate thickness of the oceanic crust in this region Conclusions Relatively short in the geological sense (about 50 Ma) as it is, the history of the KomandorskyAleutian island arc includes numerous tectonic and magmatic events. Among them are the formation of a primary deep-seated fault system in the Pacific Ocean lithosphere, later turning into a subduction zone; the accretion of oceanic, islandarc and back-arc lithospheric blocks; the deposition of terrigenous sediments produced in the course of subaeral and submarine erosion of the early arch-shaped ridge; and of pelagic sediments from the adjacent Pacific floor, etc. Finally, an interaction occurred between geochemically different lithospheric blocks and magmatic melts ascending from the mantle. Ail these events took place mainly in the frontal stnmtural zone of the Aleutian arc, the volcanic front not being stable in time but continually shifting to the north. The role of such lithospheric blocks in the structure of successive arc segments was also different: thus, in its western and central parts, the oceanic crustal type prevailed, while in the east, on the contrary, the thickness of accretionary complexes increased and the arc featured a continental-type crust. The available data show that the magmatic evolution of the Korn~~rs~-~eu~~ arc is more complex than was previously believed (Scholl et al., 1975). Among the magmatic prodw;ts are rocks of three series: tholeiitic, subalkaline (alkaline) and c&-alkaline, the latter predominating by far volumetrically. The least common are subalkaline and alkaline volcanics known only in the Miocene on the Komandorsky Is. and in the Quatemary on Bogoslof Is. Well known in island

A.A. TSVETKOV

arcs is the fact that initial tholeiitic volcanism later give way to talc-alkaline one. In the Komandorsky segment of the Aleutian arc the general evolutionary trend is evidently more complex. Quite often there these two kinds of volcanism occur together; sometimes even lavas of the same volcano belong to different magmatic series. This is possibly due to changes in the conditions of melting and c~stall~ation throughout the arc, to a lesser extent, the depth of origin of primary melts and geochemical differences in their source components, or to a changing geodynamic setting such as, e.g., in the Komandorsky segment in the Miocene where, along with a typical island-arc source, a source of a Hawaiian type was active for more than 10 Ma. We must stress here that the Komandorsky segment is specific in this respect and its magmatic evolutionary trend can be considered as “non-standard” (~gat~ov and Tsvetkov, 1988). The available geochemical and isotopic data show that all Komandorsky magmas were of mantle origin, the magmatic sources throughout the whole Aleutian arc have remained isotopically homogeneous for the whole period of its evolution (for more than 40 Ma). A crustal component very possibly played a contaminating role only during the early stages of arc development, though there is some evidence from “Be geochemistry that pelagic sediments may have contributed to the arc magmatic source, not only in the Aleutian but also in the Komandorsky part of the arc.

I thank my colleages from the Institute of Ore Deposit Geology, Petrography, Mineralogy and Geochemistry for numerous discussions on various aspects of the Kom~do~y-~euti~ magmatism, both in Moscow and in the field and to Dr. J. Romick from Cornell University, U.S.A. for providing analytical data for the Piip volcano. My special gratitude goes to Professor S. Bloomer for his comments on the paper and many editorial su~~tions which I tried to follow. The research was funded by the U.S.S.R. Academy of Sciences.

313

MAGMATISM OF THE KOMANDGRSKY SEGMENT OF THE ALEUTIAN ISLAND ARC

Appendix 1: Sample deswiption Basalt - plagiorhyolite association 3/7-78 - fine grained propilitized basalt, tuff, Kotenok C., Medny Is. 3/2-78, 3/3-78 - basalt, dykes, Sulkovski C., Medny Is. 5/3-78 - basalt, lava flow, Preobrazhenskaya Cove, Medny Is. 5/l-77 - basalt, xenodyke in plagiorhyolites, Peschanaya Cove, Medny Is. 3/14-78 - basalt, lava flow, Sulkovski C., Medny Is. 440 - pillow basalt, lava flow near Kotenok C., Me&y IS. 6/4-78 - glassy plagiorhyolite, lava flow, Trofimovski Cove, Medny Is. 2/10-78, 2/11-78, 12/21 - plagiorhyolite, lava flow, Preobrazhenskaya Cove, Medny IS. 4/l-77,5/5-78 - propilitized plagiorhyolite, lava flows, Rhyolite Ridge, Medny Is. 7/2-77 - plagiorhyolite, stock to the south of Kotenok C., Medny Is. Trachybasalt- teschenite association 3/19-78 - trachybasalt, dyke, Sulkovski C., Medny Is. 415/77 - trachydolerite, sill, Vhodnoi Reef C., Bering Is. 18/l-78 - teschenite, sill, Gaunta C., Bering Is. 17/3-78 - teschenite, dyke, Monati C., Bering Is. 406/77 - teschenite, sill, Sevemy C., Bering Is. 20/l-78 - teacher&e, sill, Nickolskaya Bay, Bering Is. 9/7-78 - teschenite, sill, Zhirovaya Cove, Me&y Is. Diorite - granitoid awciation 812-78, 8/6-78,8/4-78,11/6-77 - gmnodiorite, stock, Chemy C., Medny Is. 10/14-78 - basaltic andesite, lava flow, Yougo Vostocbny C., Medny Is. 10/6-78 - andesite, lava flow, Yougo Vostochny C., Medny Is. 9/l-78 - dacite, lava flow, Glinka Cove, Medny Is. Daeite - rhyodacite association B26-G3/1, B26-G3/2, GS/l, B26-G5/2 submarine volcano Island. B26-G23/1 - massive

B26-G3/3, B26-04/l, B26-G4/2, B26- massive da&es, lava flows of Piip located about 40 km to NW of Bering

analyses were done using inst~rn~~ neutron activation in the U.S. Geological Survey (Menlo Park, California) and Cornell University laboratories. Analysts: B. Lai, S. Ramage, D. M&own, R. Knight, J. Romick. K/Ar age determinations were carried out on a special IGEM argon device and Ml-1201 mass-spectrometer by the isotope dilution technique. With the addition of 38Aras a spike, K was determined by flame spectrophotometry. Radiometric ages were calculated using constants recommended by the International Subcomission on Geocbronology at the 27th International Geological Congress: Xk = 0.581x10-lo yrs-‘, h, = 4.962 x lo-” years- ‘, 4oK = 0.01167 at.%. Analyst M.M. Arakelian. A few deter~nations marked with asterisks were done in the U.S. Geological Survey laboratory at Menlo Park under supervision of Dr. L. Silberman. Microprobe analyses were performed on a “Cameca MS-46” housed in the I.G.E.M., Moscow. All analyses were done at 20 kV, and a current of 50 mA. The elements were counted for 70 s. (analyst M.V. Tsvetkova}. The results were refined using a special correction program, PUMA, based on the ZAF method for the Canon CX-1 computer. A unified I.G.E.M. collection of standards was used. The latter: chromdiopside B-6, shorlomite S-68, spessartine K-50, albite K-48, aegirine S-38, divine B-14, adular K-34, magnetite S-135 and rutile S-103 were obtained from the Fersmanian Min~~u~~l Museum of the U.S.S.R. Academy of Sciences and Mineralogical Museum of I.M.G.R.E. Institute in Moscow. Sr and Nd isotopic analyses were done in the I.G.E.M. by D.Z. Zburavlev, using a mass spectrometer (M&1320). The typical analytical uncertainty was rtO.OOS% for Nd and &0.007% for Sr (2 (I_a)_ Because the variations in isotopic ratios of Sr and especially Nd in Komandorsky lavas are very small, special care was taken to control the reproducibility of the isotopic measurements. Twelve analyses of Nd in the U.S. Geological Survey BCR-1 standard demonstrated good reproducibility and yielded a mean value of ‘43Nd/‘44Nd = 0,5I2629*0.~7 (95% confidence limit), which is in good agreement with the results of other laboratories. The estimate of the precision of an individual run at the 95% confidence limit is satisfactory, i.e. none of the results deviate from the mean value by more than their own uncertainty. The result of 8’Sr/sr’Sr m~urements were adjusted for instants bias to s7Sr/86Sr = 0.708~ for the Eimer and Amend standard. The mean measured value for this standard is 0.70808 f0.00002. Retailed descriptions of the analytical procedures were given in special publications (Zhuravlev et al., 1983). Measured isotopic ratios of Nd and Sr were age corrected. (‘43Nd/‘44Nd)m~r- (‘4’S~/‘~Nd)~~s

rhyodacite, lava flow of Piip volcano.

Appendix 2: Analytical techniques Major elements and some minor and trace elements were determined by X-ray fluorescence, FeG, H,O and CO, - by wet chemistry. REE and SC, Co, Ni, Ba, Hf, Ta, Th and U

x (e”-

(‘43Nd/‘QNd),Hua-(‘47Sm/‘elNd)rHUR(eXT-1)

1)

-’

1

x lo4 where T = age in years, (‘43Nd/‘~Nd)=~“a = 0.51187, (‘47Sm/1~Nd)~~~a = 0.1967 xzz = 6.54 x lo-l2 years; ( 87Sr/86Sr) t,a = 0.7045, (87Rd;86Sr)ua = 0.0827, ??i, =I.42 x lo-” years-‘.

314

A.A.

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