Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches

Earth and Planetary Sctence Letters, 80 (1986) 265-280 Elsevier Science Publishers B V, Amsterdam - Pnnted an The Netherlands

265

[21

Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches A.J. Crawford 1, L. Beccaluva

2,

G. Serri 3 and J. Dostal 4

i Geology Department, Universtty of Tasmanta, G P 0 Box 252C, Hobart, Tasmama 7001 (Austraha) 2 Istttuto dt Petrologza, Untverslta dt Napoh, Vta Mezzacannone, 9, Napoh (Italy) s Istttuto dz Petrologta, Umverstta dt Plsa, Vza S Maria, 53, Ptsa (Italy) 4 Geology Department, St Mary's Unwerszty, Hahfax, Nova Scotia (Canada)

Received June 6, 1986, revised version received August 18, 1986 Rocks dredged from the forearc very close to the intersection of the Yap and Manana trenches include a state of highly depleted arc tholentes, and several samples of transitional to shghtly alkaline basalt The tholentes range from magnesian quartz tholeutes with 046-06% T102, to andesltes with up to 62% S102 and 82% FeO* All show pronounced LREE depletion and halve very low contents of Ba and Sr They are postulated to have been produced by partml melting of upper mantle pendotlte residual after MORB extraction, following influx of hydrous flmds from the subducted slab Whale these flmds were responsible for small enrichments m Ba, K, Rb and Sr m melts generated, LREE were not involved m the metasomalasm, and the strong LREE depletion probably reflects the unmodified, depleted source pendotlte The second lava suite includes shghtly Ne-normatave, Tl-auglte-beanng basalts with convex-upward REE patterns, showing slight LREE depletion ((La/Sm)N = 0 76) The chemical features of these basalts support affimtms with basalts erupted dunng the earhest stages of backarc basin opemng A K-At age on one sample (7 8 _+ 1 3 m y ) ms m good agreement with the lmttal opemng of the Manana Trough The tectomc slgmficance of the dredged arc tholente state is less obwous A K-At age of 10 8 _+0 4 My on one andeslte, and the occurrence of similar lavas m dredges from at least 300 km along the length of the Yap arc, suggest that subductton was occurnng beneath the Yap arc an the Late Miocene, after overthrustmg of the Yap greenschast allochthon, and whale calc-alkahne arc magmatlsm was occumng further north on the West Manana Radge We suggest that the depleted arc tholentes m dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the Yap forearc following subductlon beneath thas area of young, hot Sorol Trough crust These arc tholentes represent a magma type transitional between more typical arc tholeutes (e g Tongan) and hagh-Mg andesltes and bommtes

1. Introduction V e r y little is k n o w n o f the n a t u r e a n d c o m p o s i tion of magmatism accompanying the earhest stages o f a r c v o l c a m s m . S u n l l a r l y , w i t h t h e e x c e p tion of llrmted data from the Mariana forearc [ 1 - 5 ] , n o t h i n g is k n o w n o f t h e a r c - r e l a t e d r o c k s e r u p t e d o n t o o c e a n i c c r u s t in t h e f o r e a r c r e g i o n s o f o c e a n i c arcs. F u r t h e r m o r e , w h i l e it is w e l l e s t a b h s h e d t h a t i s l a n d arcs, b o t h l n t r a o c e a m c [ 6 - 8 ] a n d a s t n d e c o n t i n e n t a l m a r g i n s [ 9 - 1 1 ] m a y split to f o r m i n t e r a r c o r b a c k a r c basins, f e w d a t a are a v a i l a b l e p e r t i n e n t to the c o m p o s i t i o n o f l a v a s erupted during the earhest phase of arc rifting and b a c k a r c b a s i n o p e n i n g [9] I n this p a p e r , w e p r e s e n t m i n e r a l c h e m i c a l a n d 0012-821x/86/$03 50

© 1986 Elsevier Science Pubhshers B V

whole-rock major and trace element data for a s u i t e o f andesltlC a n d b a s a l t i c r o c k s d r e d g e d f r o m t h e f o r e a r c o f a n o c e a n i c arc, c l o s e to t h e i n t e r s e c tion of the Yap and Manana trenches, dunng the 1976 cruise of t h e R.V. " D n u t r y M e n d e l e e v " Detailed characterization of rocks from such mode m t e r r a n e s is a n e c e s s a r y p r e r e q u i s i t e to the i d e n t i f i c a t i o n o f s i m i l a r s e q u e n c e s in P h a n e r o z o l c f o l d b e l t s , a n d u l t i m a t e l y , to the u n r a v e l h n g o f t h e tectonic history and polarity of plate colhslons which produced such foldbelts In addition, an u n d e r s t a n d i n g o f t h e p e t r o g e n e s l s of l a v a s o c c u r r i n g in f o r e a r c s s h o u l d f u r t h e r c o n s t r a i n a n d i m prove schemes for the petrogenetlc evolution of a r c - b a c k a r c b a s i n systems.

266

2. Tectonic setting and sampling The rocks described here were dredged from " D m i t r y Mendeleev" site 1438 on the inner trench wall of the Mariana Trench (Fig. 1) at depths between 6400 and 6700 m, close to the bowl-shaped depression which marks the southern termination of the Marlana Trench [1,3]. In this region the M a n a n a Trench swings to an east-west trend and plate m o t m n is largely strike-shp [12]. Unlike further north in the same arc, in this regmn the Bemoff zone is poorly defined, the arc itself is disrupted, and locally, the inner slope of the trench directly abuts the southern end of the M a n a n a Trough backarc basin [5]. The southern end of the Mariana Trench is intersected by the north-south trending Yap Trench (Fig. 1), which extends northward, possibly as a fracture zone into the

I P O D Trough. The latter feature represents the median valley of the extinct spreading centre of the Parece Vela Basin, which was active from 32 to 17 Ma [13]. In this complex triple junction setting, site 1438 might represent Parece Vela Basin crust, West M a n a n a Ridge island arc volcanics, or crust of the southern M a n a n a Trough (Fig 1) Beccaluva et al. [3] presented K-Ar ages for two site 1438 lavas, basalt 1438 D 3added an age of 7.8 + 1 3 My, and andeslte 1438E gave an age of 10.8 + 0.4 My These Late Miocene ages rule out the possibility that the site 1438 lavas were produced during opemng of the Parece Vela backarc basin ( > 17 Ma) The West M a n a n a Ridge remnant arc has no physlograptuc expression m the area where the rocks were dredged, and Beccaluva et al. [3] point out that the site 1438 tholeutes are chemically quite unlike the calc-alkallne lavas of

20 ° PARECE

0447 "290

~

o)b

1398~A

448

~ o

0 4~

0449

'

452

2

VEL 4

15"

>I "'"" 1438

~43o ~

135 o

10~

Jooo ~

140 °

145 °

150 °

Fig 1 Map of the West Pl~hppme Sea-Yap arc-Manana arc area, showing locations of "Dmltry Mendeleev" dredge sites (stars) and DSDP sites (filled circles), bathymetnc contours are m meters

267

the West Manana Ridge [14]; they concluded that the site 1438 lavas probably represent magmas generated during the early stages of formaUon of the Marlana Trough This topic is readdressed m more detail in a later secUon of this paper.

3. Petrography On the basis of petrography and maneral and whole-rock chermcal data, rocks dredged at site

TABLE 1 Petrographic descnptlons of "Dmitry Mendeleev" dredge site 1438 igneous rocks Group A M Quench-textures, largely glassy lava contmmng rare euhedral phenocrysts of plag~oclase and orthopyroxene, and abundant, zoned pyroxene nucrohtes Q Autobrecciated basalt with abundant chnopyroxene phenocrysts and less common phenocrysts of orthopyroxene (some with chromite inclusions) and plagloclase E Almost aphync, vesicular lava (13% vesicles), rare microphenocrysts of plagtoclase (2%), chnopyroxene (1%) and orthopyroxene (0 5%) m a hyalophihtlc groundmass composed of plag~oclase laths, granular chnopyroxene and Fe-T1 oxade B Vesicular, porphyritic andeslte (13% vesicles) with 7 modal % pla~oclase phenocrysts, 3% chnopyroxene phenocrysts and 2% orthopyroxene phenocrysts Crystalhne groundmass of elongate plagloclase needles, mterstmal granular pyroxenes and Fe-TI oxides C Vesicular porphyrmc basalt (12% vesicles) with phenocrysts of chnopyroxene (7 5%) and orthopyroxene (4%), and rare altered ohvlne phenocrysts (0 9%), some with chronute inclusions Alteration products include chlorophae~te, montmonllomte, and zeolite nunerals Plagloclase occurs only as groundmass laths, together with chnopyroxene, Fe-T1 oxides, and altered glass F Basaltic agglomerate or hyaloclasttte with quench texture Almost aphync Rare phenocrysts of plag~oclase and orthopyroxene, and altered ohwne (somettmes with chromite inclusions) Glassy groundmass charged with aclcular, zoned pyroxene microhte Group B N Altered dolermc-textured basalt probably from dyke or sdl, or interior porlaon of thick flow Aphyrlc, with pinkish au~te and fresh to partly altered plagtoclase Mesostasls altered to green chlorophaelte-smect~te-chlonte nunerals and zeohtes D Fresh dolentic-textured basalt with p~nk augtte plates and subophitxcally mtergrown plag~oclase Fe-TI oxade granules, anhedral pla~oclase and laths and overgrowths of primary amphibole occur m mterstmal areas Scarce ohwne replaced by montmonllomte group minerals

1438 may be divided into two groups (Table 1) Group A includes orthopyroxene-bearmg basalts and andesltes, and breccias derived from these rocks, while Group B is represented by only two samples, both dolentlc-textured basalts which lack orthopyroxene and contain abundant pink chnopyroxene.

4. Mineral chemistry 4 1 Opaque oxides Small chromite euhedra occurring as discrete microphenocrysts or as mchislons m orthopyroxene phenocrysts have been analysed from basalts C, F and Q (Table 2). Compositions of these chrormtes are slgmflcant m three respects (1) Their C r / ( C r + A1) ratios (0 77-0 84) are notably greater than for chromltes from MORB and Hawauan tholentes (Fig. 2A), due to their higher Cr203 and lower A1203 contents, and are also greater than C r / ( C r + AI) ratios in chromates from prlmltwe arc basalts [15,16]. The octahedral substitutions in chromate are probably complex functions of hquld A1203, Cr203 and Fe203 contents [17,18], liquid SIO2 contents [19] and fo~ [20]. However, it is hkely that the residual perldotite from which the Group A parent magma segregated was more refractory than residual perldotItes from either MORB- or prlmltwe arc magma-producing melting events. This may reflect higher degrees of melting mvolved m the Group A-producing partial melting, or more depleted, refractory source mantle. The C r / ( C r + A1) ratios of Group A chromites overlap w~th those of bomnlte chromates, suggesting derivation from a depleted source which had ywlded one or more magma batches in previous melting events. (2) Unlike most basalt chromates, and chromites from bommtes, those m Group A lavas are decldely Fe3+-rlch (Fig. 2B) Chromates m calc-alkahne basalts from New Georgia (Solomons) and Aoba (Vanuatu) [15], and in tholentlc basalts on Bamus (New Britain) [21], are all also notably enriched m Fe 3+ relative to those m MORB or mtraplate basalts, ~mpl3nng that arc basalts commence crystallisation under higher fo2 conditions than do most other basalts The same apphes to chromltes in arc alkah ohvine basalts [22] and arc basamtes [23]. (3) The TiO 2 contents of Group A chromates

268 TABLE 2 Electron macroprobe analyses of chromltes from Group A lavas, FeO and Fe203 were calculated on the basis of stolcluometry, assurmng 24 cations and 32 oxygens

T102 m1203 Fe203 FeO MnO MgO Cr203 N10 Total C r / ( C r + A1) M g / ( M g + Fe 2+ ) F e 3 + / ( F e 3+ + C r + A 1 )

C

C

Q

Q

F

F

0 22 10 9 61 16 0 0 45 11 2 54 4 0 12 99 4

0 33 95 65 17 1 0 41 10 6 55 9 0 21 100 5

0 24 89 47 11 5 0 41 13 9 59 2 0 11 99 0

0 19 77 52 11 9 0 43 13 7 60 7 0 06 99 9

0 38 100 56 10 6 0 39 15 1 58 5 100 6

0 43 101 55 10 9 0 35 15 0 58 4 100 7

0 77 0 56 0 25

0 80 0 53 0 25

0 82 0 68 0 27

0 84 0 67 0 28

0 80 0 72 0 33

0 80 0 71 0 31

4 2 Plagtoclase Plagloclase occurs as phenocrysts in samples F, Q, B and E, and as groundmass nucrolites m all samples except M. Phenocrysts display nuld oscd-

are lower than for all other chromates except those from bonlmtes. Ttus again reflects the depleted sources of these lavas relative to MORB, mtraplate and most arc basalts

10

-(B)

10

08

08

t.,)

+06

~06

<

+

4-

~

0.4

o4

g.

02

02

oo

08

06

04 Mgl(Mg ~- Fe 2+)

02

__1 oc

oo

08

oe

04 Mg/(Mg + Fe 2+)

02

oo

Fig 2A C r / ( C r + A1) versus M g / ( M g + Fe 2 + ) for chromttes in Group A lavas (fdled circles), also shown are composlUonal fields for chromates from other lava states, including b o m m t e s [71], Bamus hagh-Mg andeslte [21], New Georgm plcntes [15], H a w a n a n basalts [41], Grenada basamtes [23], M O R B [16] and opbaohte harzburgltes from Solomon Is (R Ramsay, personal commumcataon) B Fe 3 +/(Fe 3+ + Cr + A1) versus M g / ( M g + Fe 2 +) for chrormtes from Group A lavas, also shown are fields for chroImtes from other states (for sources, see captaon to Fig 2A, and for Osluma [22]

269

latory zoning superimposed upon a general core to rim decrease in An content (Table 3); phenocryst cores range from An96 to An90 in all samples, and rims are slightly more sodic extending to An s5 (Fig. 3). Mlcrohtes and quench needles of plagloclase m glassy groundmass vary from Ans2 to An86 (F), An78 to Ans3 (B), Anal to An87 (Q) and Ansi to An9i in sample C. Slgrnflcantly, the most calclc rmcrolites occur m sample C, which lacks phenocrystal plagloclase. Iron contents of plagloclase (measured as FeO with the macroprobe) are lowest in phenocryst cores (0.7-0.8%) and increase to around 1.0% in rims; mlcrohte iron contents are even higher, extending to 1.7%. The K 2 0 contents of all plagloclases analysed are less than 0.2% Highly calclc plagioclase phenocrysts such as those in Group A lavas are not uncommon in island arc basalts and andesltes [24]. However, the highly anorthitic groundmass mlcrohtic plagloclases m Group A basalts and andesltes are unique among stu&ed arc lavas, for which groundmass plagloclase composiuons are virtually always less than An80 [25-27]. The high An content of plagloclase phenocrysts in arc lavas is generally attributed to the presence of dissolved H 2 0 in arc magmas [28], although the mechanism of this reaction is poorly understood. Group A lavas are particularly vesicular, suggesting that they crystalhsed from a hydrous magma at quench temperatures above the appearance temperature of amphibole. However, a distinctly low Na 20/CaO ratio in the Group A parent magma would also result in the crystalhsatlon of highly anorthitic plagloclase. The high CaO and low Na 2° contents

An

An

An

An

90

80

r 70

Fig 3 Plagaoclase composttlons m Group A lavas Phenocryst range with horizontal ruhng, groundmass range shaded Orthoclase contents are always less than 1 tool%, but are exaggerated here to better show range m Ab and An

of Group A andesites (Table 7) when compared to average andesltes from clrcum-PaclfiC arcs [24] support the latter contention 4 3 Pyroxenes Calclc pyroxenes, as phenocrysts and mlcrohtes, are always more modally abundant than orthopyroxene in Group A lavas; the phenocrysts are normally zoned and display maid Fe-enrlchment from cores to nms (Table 4, Fig. 4). The most magnesian chnopyroxenes are cores of phenocrysts m sample C ( M g / ( M g + Fe2+), herein Wl'ltten as m g ' = 0.91), and the most Fe-nch are rims on phenocrysts in andesltes B and M (rag' = 0 67). Most chnopyroxene phenocrysts have less than 2 5% A1203, and TIO 2 contents are usually less than 0 3%, covering the same range of Al203 and TiO 2 contents as chnopyroxene phenocrysts from Tongan arc tholeutes [25], but lower than A1203 and TIO 2 contents of clinopyroxene pheno-

TABLE 3 Representatwe electron microprobe analysis of ptagloclase m Group A lavas Q

Q

F

F

F

C

B

B

Pc

Pr

Pc

Pr

M

M

Pc

Pr

$102 AI203 FeO CaO Na20, Total

45 5 33 9 0 73 19 0 0 74 99 9

49 2 31 2 1 02 16 4 1 88 99 7

45 1 34 5 0 78 19 1 0 49 100 0

45 4 34 1 0 77 18 9 0 89 100 1

50 1 31 1 0 93 15 9 2 00 100 0

48 3 31 4 0 81 16 9 1 59 99 0

46 1 33 8 0 93 18 4 0 86 100 1

46 4 33 4 1 00 18 2 0 94 99 9

An Ab

93 8 62

83 6 164

95 8 42

92 6 74

82 3 177

86 3 137

92 2 78

91 9 81

P = phenocryst, M = mlcrohte, c = core, r = n m

270 TABLE 4 Representative electron microprobe analyses of chnopyroxene phenocrysts in Group A lavas C

$102 T102 A1203 FeO MnO MgO CaO Na20 Cr203 Total

rag' Wo Fs En

Q

B

M

lc

lr

2c

2r

3c

3r

4c

4r

5c

5r

53 6 0 06 1 66 3 46 0 13 19 1 21 3 0 07 0 75 100 2

53 6 0 16 1 84 4 51 0 13 18 5 20 5 0 10 0 49 99 8

52 1 0 26 2 65 5 13 0 14 17 8 20 8 0 11 0 40 99 3

53 0 0 18 2 28 5 89 0 14 19 8 18 0 0 08 0 31 99 7

53 9 0 04 1 27 3 54 0 07 18 8 21 5 0 08 0 53 99 9

53 5 0 11 2 31 4 75 0 19 19 1 19 0 0 09 0 50 99 7

52 2 0 25 2 52 6 92 0 22 16 4 20 1 0 14 0 61 99 4

51 8 0 18 1 80 9 10 0 21 15 7 20 0 0 22 0 06 99 1

52 4 0 18 1 58 8 30 0 24 16 2 20 0 0 12 0 11 99 2

51 8 0 41 1 81 12 1 0 33 15 4 17 8 0 15 0 09 100 0

0 91

0 88

0 86

0 86

0 90

0 88

0 81

0 76

0 78

0 69

42 1 53 52 6

c = core, r = rim,

41 1 71 51 8

rag' =

42 0 81 49 9

35 8 93 54 9

42 6 56 51 8

38 5 75 54 0

41 6 112 47 2

40 9 145 44 6

40 8 132 46 0

36 6 194 44 0

M g / ( M g + Fe 2+ )

DI

^

oo

^

^

^

Hd

"

i

Sa Phenocrysts Mlcro|ltes

En

v

A

.

*

•

•

v

y

~

• Fs

, Hd " i t

•

•

B

/ Sa Phenocrysts Mtcroiaes

En

y

Y

,,

Y

-(

• • ¥

•

.

Y

~ Fs

Fig 4 A Pyroxene quadnlateral showing compositions of zoned pyroxenes m Group A basalts, arrows indicate core to rim zomng direction B Pyroxene quadnlateral showing compositions of pyroxenes m Group A andesltes, and Group B basalt 1438D Arrows indicate core to n m zoning direction

271 crysts f r o m m o s t o t h e r arc b a s a l t s (e.g. [15,29-32]). O r t h o p y r o x e n e p h e n o c r y s t s are n o r m a l l y zoned, b u t c o m p o s i t i o n a l z o n i n g r a r e l y exceeds 4 mol% E n in i n d i v i d u a l crystals ( T a b l e 5). I n b a s a l t s C a n d Q, o r t h o p y r o x e n e p h e n o c r y s t s range f r o m mg' = 0.87 to 0.83, while they e x t e n d to less m a g n e s i a n c o m p o s i t i o n s in the andesites (rag'= 0.80-0.70). T h e A1203 c o n t e n t s o f bronzate-hypersthene p h e n o c r y s t s are low (1 7-0.8%), a n d N a 2° c o n t e n t s are f r e q u e n t l y b e l o w the m i c r o p r o b e detection h m l t (0.08%). G r o u n d m a s s p y r o x e n e s in G r o u p A lavas show e x c e p t i o n a l a n d c o m p l e x c o m p o s i t i o n a l variation, b u t within each s a m p l e they define c o h e r e n t zoning trends (Fig. 4 A a n d B). F u r t h e r studies of these g r o u n d m a s s p y r o x e n e s are in progress a n d will b e r e p o r t e d elsewhere. A p p h c a t l o n o f the Wells [33] t w o - p y r o x e n e g e o t h e r m o m e t e r to coexisting p y r o x e n e macrophen o c r y s t cores, a n d p h e n o c r y s t n o n - q u e n c h e d r i m c o m p o s i t i o n s yields t e m p e r a t u r e s which cluster around ll00°C for b a s a l t s C a n d Q, a n d 1 0 0 0 - 1 0 3 0 ° C for andesltes B a n d M. Obviously, p y r o x e n e p h e n o c r y s t cores c o m m e n c e d crystallizat i o n at t e m p e r a t u r e s a b o v e l l 0 0 ° C . P i n k i s h c h n o p y r o x e n e s u b o p t u t i c a l l y interg r o w n with p l a g l o c l a s e in G r o u p B b a s a l t D is Tl-sahte, with lower S1 a n d higher t e t r a h e d r a l A1, T1 a n d N a t h a n p y r o x e n e p h e n o c r y s t s in G r o u p A lavas. R e p r e s e n t a t i v e analyses are g w e n in Bec-

c a l u v a [3], a n d a r e p l o t t e d in Fig. 4B T h e c o m p o s i t i o n o f these p y r o x e n e s clearly reflects the t r a n s i t i o n a l to slightly alkaline n a t u r e of the m a g m a f r o m winch they crystallized

5. Geochemistry 5 1 Group A R e p r e s e n t a t i v e analyses of G r o u p A lavas, inc l u d i n g b r o a d b e a m p r o b e analyses of clasts in the hyaloclastites a n d brecclas, are given in T a b l e 6, a n d R E E analyses are given in T a b l e 7. T h e low KEO , high F e O * a n d C a O a n d L R E E d e p l e t i o n of the G r o u p A lavas are all typical features of p n m t t l v e arc tholellte ( I A T ) suites [39]. T h e i r S10 E c o n t e n t s increase with increasing F e O * / M g O ( T a b l e 6), b u t the S10 E c o n t e n t o f s a m p l e C (50.3%) is low c o m p a r e d to o t h e r o r t h o p y r o x e n e - p h y n c arc basalts (e.g. [35]). B r o a d b e a m scan analyses o f the g r o u n d m a s s (glass, a l t e r a t i o n p r o d u c t s of glass, a n d microhtes) of C y i e l d e d the following average analysis: S102 -- 37.2%, T i O E = 0.49%, A1EO 3 = 17 9%, F E O = 4 7 % , MGO=6.4%, CAO=74%, N a E O = 0.9%, KEO = 0.8%, with a very low total of 76%. T h e original glass of u n k n o w n c o m p o s i tion has clearly altered to low-SiO E chlorltlC altera t i o n p r o d u c t s , resulting in a significant d r o p in the w h o l e - r o c k SiO E c o n t e n t of s a m p l e C. A l t h o u g h it is i m p o s s i b l e to r e c o n s t r u c t the original c o m p o s i t i o n of C, it is h k e l y that its pristine $10 E

TABLE 5 Representative electron microprobe analyses of orthopyroxene phenocrysts m Group A lavas C 510 2 ZlO 2

AI203 FeO MnO MgO CaO CrEO3 Total

mg' Wo Fs En

Q

F

M M

lc

lr

2c

2r

3c

3r

4c

4r

55 6 0 11 1 62 89 0 10 31 9 0 47 0 60 99 3

55 8 0 10 1 51 98 0 19 31 0 1 51 0 24 100 1

55 4 0 11 0 93 93 0 11 31 7 1 70 0 26 99 5

55 1 0 11 1 31 11 0 0 19 30 3 1 40 0 06 99 5

55 7 0 07 1 64 96 0 12 31 4 1 50 0 27 100 3

54 9 0 05 1 51 100 0 19 30 6 1 99 0 30 99 5

55 0 0 12 1 49 12 9 0 18 29 0 1 61 0 20 100 4

54 9 0 15 1 55 12 7 0 21 29 3 1 58 0 14 100 5

53 5 0 27 1 64 182 0 25 24 0 2 07

0 85

0 86

0 83

0 85

0 85

0 80

0 80

0 70

0 87 20 13 4 85 6

29 14 6 82 5

32 13 6 83 2

27 16 5 80 8

28 14 3 82 9

M = mlcrophenocryst, c ffi core, r ffi nm, rag'ffi Mg/(Mg+ Fe 2+ )

38 14 9 81 3

31 19 5 77 4

30 19 1 77 9

99 9

42 28 8 670

M 53 8 0 25 0 85 182 0 28 24 8 1 89 1000 0 71 37 28 3 68 0

272 TABLE 6 Whole-rock analyses of Group A lavas (C, Q, F, B, E, M) and Group B lava (D) from " D m l t r y Mendeleev" dredge site 1438

S102 T102 A1203 Fe203 FeO* MnO MgO CaO Na20 K20 P205 Total

C

Q

50 3 0 46 15 5 5 72 1 73 0 11 12 7 10 7 1 86 0 88 0 03 100 0

51 7 0 59 16 6 92 0 20 89 10 2 1 76 0 75 99 9

4 61 0 77

0 63

Loss

mg' NI Cr V Sc Zr Y Sr Rb Ba Th Hf Nb

F 54 6 0 59 14 7 10 9 0 21 73 91 1 99 0 59 100 0

0 54

204 879 262 47 16 11 36 8 21 <03 06 <1

B

E

59 5 0 68 14 9 6 08 2 70 0 15 3 97 9 06 2 27 0 62 0 05 100 0

62 0 0 73 14 4 5 13 3 59 0 13 2 47 8 20 2 66 0 56 0 08 100 0

1 79 0 46

1 06 0 35

33 73 311 24 21 54 1

M

D

64 0 0 84 14 3 86 012 2 33 7 50 2 07 0 24 100 0

033

50 2 1 64 179 6 20 2 85 016 4 65 11 28 4 53 0 25 0 24 1000 1 81 0 50 83 230 261 32 124 27 289 9 37 04 27 6

17 15 321 33 33 27 69 10 38 03 12 1

Analyses with trace elements done by XRF, others are averages of electron microprobe scan analyses

content was several percent higher, than the present value. Similar orthopyroxene-bearmg arc basalts with low T102 contents (<0.5%) have 52-55% S102 (e.g. [34,35]), whale a smular range of $102 contents (52-54%) is shown by Bndgeman Island backarc basalts from the lmtxal stages of Bransfield Stratt backarc basin opening [9], and dredge 24 in the East ScoUa Sea [36], It is asTABLE 7 Rare earth element contents (ppm) of Group A basalt 1438C and andeslte 1438E, and Group B basalt 1438D

sumed, therefore, that sample C was a saturated tholentlc basalt with 52-55% S 1 0 2 The fractlonatlon trend defined by Group A lavas can probably be accounted for by separation of orthopyroxene, chnopyroxene and plagtoclase from the most primitive sample, but no attempt to quantify the fractlonatlon scheme has been attempted due to the unconstrained starting hquld composition. However, it is possible to use the REE data for sample C and fresh aphync andeslte E to gauge approximately the amount of fractlonatxon involved in driving the composition from

La Ce Nd Sm Eu Tb Yb Lu

1438C

1438E

1438D

0 38 1 16 1 33 0 73 0 30 0 25 1 05 0 18

1 09 3 27 3 80 1 99 0 77 0 65 2 92 0 48

4 51 15 0 11 7 3 64 1 39 0 85 2 71 0 43

C to E

Two

hrmtlng cases may be tested

The first, which ywlds the minimum amount of fracuonatlon, a s s u m e s K D ( b u l k fractxonating assemblage/hquld) = 0, m whach case the Raleigh fractionatlon equation reduces to F = C°/C L. Using measured partition coefficients for La, Sm and Yb [37], composition E represents the residual liquid after separation of 65-67 wt.% sohds from composition C The second llrmtlng case assumes

273

a bulk fractlonatmg sohd/hqmd K D equal to tha~t of the fractionating phase with the highest KD, which for tins assemblage is chnopyroxene (KD(La) = 0.05, KD(Sm ) = 0 26, KD(Yb ) -- 0.27). With tins assumption, andesite E represents residual liquid after 68 wt.% (La) to 76 wt.% (Yb) removal of sohds from composition C. It may be concluded that andesite E could be derived from basalt C by removal of around 70 wt.% sohds composed of clinopyroxene, orthopyroxene and plagioclase; however, the relative proportions of these phases m the bulk fractionate cannot be determined. The evolved andesitlC compositions, such as E and M have 8-9% FeO* at 2 3-2.4% MgO, considerably more than in calc-alkaline andesites of similar MgO contents. Only pnrmtive andesltes of the IAT suite in young oceamc arcs possess such ingh contents of FeO* at low MgO contents [24,38-40]. The A1203 contents of Group A lavas are low but not uniquely so. Island arc tholentes (IAT) from the Palau-Kyushu Ridge [40], Manam Island [35] and some Tongan andesites [38,39] have less than 15% A1203. The ingh CaO contents (7-9%) of Group A andesltes are also characteristic of IAT suites. At 2-3% MgO, the great majority of Andean, Mexican, New Zealand and Antarctic Pemnsula calc-alkahne andesltes have less than 5% CaO. Group A andesites have low N a 2 0 contents (1.6-2.6%) and their C a O / N a 2 0 ratios are almost double those of typical calc-alkahne andesltes of silmlar MgO contents. Only IAT andesites erupted in youthful arcs on very tinn crust display the same low C a O / N a 2 0 ratios as Group A lavas. The K 2 0 contents of Group A lavas are variable due to alteration of glassy groundmass, the least altered samples (B, E, M and F) have K 2 0 contents less than 0.61%, indicating affinities with !aw-K IAT

Trace element geochemistry. Alteration of glassy mesostasis in the basaltic Group A lavas (Q, F and C) has resulted in significant ennchment of K in these samples. Tins is clearly shown by a comparison of the K 2 0 contents of the basalts relative to the fresh, far more evolved andesites (Table 6). The relative enrichment in Rb, Ba and Sr during alteration of the mesostasis of the basalts, while being apparently less than enrichment in K 2 0 , was most pronounced for Rb, winch is only 2 ppm

more abundant i n fresh andesite E than altered basalt C. Further discussion of the abundances of K, Rb, Ba and Sr in these lavas is presented in the next section Transition elements, especially Sc, V and Cr are essentially immobile during low-grade alteration of basaltic rocks, and their abundances in basalt C are probably pristine. During fractionatlon from basalt to andeslte, Ni and Cr contents decrease as expected, winle Sc contents also decrease significantly and V contents increase This reflects pyroxene-dommated fractionation, w~th little or no fractionation of Fe-Ti oxides, winch would rapidly deplete V. The high T1/Zr ratios of Group A lavas (130-170) are typical of island arc tholentes from young arcs, erupted through very tinn (< 20 km) crust [39,41]. These Ingh T i / Z r values relative to MORB and chondrltes (= 110) [42] probably reflect the fact that T1 becomes more incompatible than Zr at Ingh degrees of melting, or during

2o

20 10

i.

'

.~

' s'm E'.

rb

Yb t.

=.. z o = 2O ,~

10

~5

5 4 3 2

B I a Ce "

N;,

. . Sm . . Eu

T

Yb

Lu

Fzg 5 A Chondnte-normahzed REE patterns for Group A basalt C and andeslte E Also shown are REE patterns for an average N-type MORB [41], two pnnutzve arc tholezzte basalts from Tonga (dash-dot hne, and hght continuous lane) [39] and two from New Bntmn (long dashes sample IA-1, short dashes sample IA-2 [41] B Chondnte-normalzzed REE patterns for Group A basalt C and andeszte E, showing range for typical N-type ORB (hachured), and patterns for two pnmztave IAT andesztes (dashed hne and dash-dot hne) from Tonga [39] Normalzzmg values are those of [66]

274

second stage melting, as witnessed by the high T 1 / Z r ratios of m o d e m (Gorgona) and Archaean pendoUtlC komatiltes (130) [40,43] Group A Z r / H f rattos (2-7) are close to the average for LREE-depleted MORB, and the V / S c ratio of basalt C (5.6) ts very close to values for MORB (6-7). Rare earth elements. REE patterns deternuned for

basalt C and andeslte E are shown in Fig. 5A and B, in winch they are compared wath some other depleted arc tholentes and MORB. The patterns for C and E are perfectly parallel, suggesting that the REE remained immobile during alteration of the mesostasis of C. The REE patterns of both Group A lavas are strongly LREE-depleted, far more so than any other recorded IAT suite or MORB. This IS well shown by the fact that the LREE contents of andesite E, winch has 62% SiO 2 and 2.5% MgO, are notably less than those for typical LREE-depleted MORB wath 7-9% MgO (Fig. 5) Further aspects of the REE geochemistry of these lavas are discussed in the following section.

Recogmtlon that large volumes of upper ocean crust are enriched during sea-floor alteration in K, Rb, Ba and to a lesser (but nevertheless still significant) degree in LREE and Sr led to petrogenetic hypotheses for arc basalts revolving LILE-ennched hydrous fluids denved by dehydration or partial melting or both of altered basalts m subducted ocean crust [46,47]. Bonmltes erupted in forearc settings in some oceamc arcs also show pronounced LILE enrichment (Fig. 6). However, in the case of bomnites, tins LILE metasomatism was superimposed on a very depleted, probably harzburgltic source winch had yielded MORB and probably arc lavas in prior melting events [49,50] It was pointed out earher that alteration of basalt C has produced enrichment in K, Rb and Ba, however, the parallelism of REE pattern for

30

i

BASALTS

~ .

N

5

qt

I ~\

•

•

t,-

o

\\

5.2 Dzscusston

Although not umversally accepted, most current models for IAT magma genesis are extensions of a model developed by Rlngwood [43] and revaewed by Green [44], m winch melting of subarc mantle shallower than about 70 km is tnggered by reflux of hydrous fluids derived from dehydratton of the subducted slab. At depths less than 70 km, suboceamc upper mantle is probably composed of fertile MORB-source perldotite at lower levels, and more depleted pendotlte which has yielded MORB magmas, at shallower levels closer to the Moho [45]. If etther type of pendotlte were to partmlly melt beneath an intraoceamc arc wxthout mtroduction of metasomatlc fluids from the subducted slab, resultant parttal melts would necessarily have LILE contents equal to or less than those measured in normal LREE-depleted MORB Examination of REE patterns (Fig 5) and MORB-normahsed dement abundance patterns (Fig. 6) for representative arc tholentes shows LILE abundances close to or even greater than for the average MORB; tins indicates that these IAT were generated from upper mantle which had undergone metasomat~c enrichment in LILE.

O

Rb Ba

Th U K Nb La

Ce

St Nd P Sm Ht Zr Ti Tb Dy Y Yb

50

20 ~10

-, ~

ANDESITES

~

,,

"--\

o

//

0

1

I

I

Rb Ba

I

V / "'- ~?~...

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

Th U K Nb Lit Ce Sr Nd P Sm Hf Zr TI Tb Dy Y Yb

Fig 6 A MORB-normahzed element patterns for (1) Group A basalt C, (2) calculated parental arc basalt, Aoba, Vanuatu [15], (3) IAT basalt 10380 Kermadec arc [39], (4) Cape Vogel bommte [67], (5) average Sunda arc tholenttc basalt [68] Normah~ng values are those of [55] B MORB-normahzed element patterns for (1) Group A andeslte E, (2) Tongan arc tholenttc andeslte NT64 [39], (3) average Sunda arc tholentlc andeslte [68]

275 basalt C with that for perfectly fresh andeslte E suggests that the L R E E were not sigmfxcantly affected during alterauon of C. Therefore, m order to compare the L I L E contents of C with other primitive arc tholelites, its MORB-normahsed pattern of elements more incompatible than N b (i.e. Rb, Ba, Th, K) has been drawn parallel to that for andeslte E This assumption is only an approxlmat~on of the truth, since fract~onaUon paths from basalt to andeslte usually produce increasing enrichments m progressively more incompatible elements. Therefore, the pattern given for basalt C is hkely to be an upper hmlt for the elements Rb to N b However, as our aim is to emphasise the highly depleted nature of the G r o u p A parent magma, the assumption used m no way invalidates the conclusions reached Slrmlar conclusions m a y be obtained using measured pristine element abundances m andeslte E, and c o m p a n n g these with other I A T andesltes (Fig. 6B). In c o m m o n with other primitive IAT, basalt C has HREE, Y, TI and Zr contents less than prlmltwe MORB. Such magmas are hkely to be derived from upper mantle more depleted than MORBsource mantle, perhaps mantle residual after extraction of M O R B [51]. The most important and obvious feature of the MORB-normahzed element patterns for G r o u p A lavas (Fig 6A and B) is their pronounced depletion in LREE, implying derivaUon from a source winch had been depleted m L I L E m one or more prior melting events. The enrichment in L R E E that appears to have affected the sources of all other arc suites, whether of IAT, boninlt~C or calc-alkahne afflmty, was not revolved m the petrogenesls of G r o u p A lavas. Like most arc basalts, however, sample C shows a distract p o s m v e Sr anomaly or spike [52]. As alteration of mesostasls of C resulted in enriched K, Rb and Ba contents, the same may be true for Sr. If it was a primary feature, the original m a g m tude of the Sr spike cannot be determined, as plagtoclase fractlonatlon passing from basalt to andeslte d~rmnlshes Sr contents m residual magmas Its former existence ~s verified, however, by the persistence of the Sr spike in andeslte E. Note the very low Sr contents of both C and E relatwe to other arc lavas; this is well shown by the fact that G r o u p A lavas appear to be the only arc lava state studied for winch MORB-normahzed Sr always falls below H R E E and Tl (Fig 6A and B).

Despite the pronounced L R E E depletion of the G r o u p A lavas, both C and E show distract enrichments in Rb, Ba, Th and K relative to MORB, and m tins respect are slrmlar to other arc lava suites. The smooth decrease from Rb to Th shown by Group A lavas on the MORB-normalized diagrams (Fig. 6A and B) is unlike most arc lavas, which normally have pronounced positive Ba anomahes [52], but hke bonimte patterns Even so, the Rb, Ba, Th and K contents of Group A lavas are notably lower than for other IAT at equivalent fracUonaUon stages. In summary, the source of Group A lavas appears to have been perldotlte more depleted m T1, Zr and H R E E than MORB-source upper mantle, winch was metasomatized prior to or during the melting event winch generated the Group A parent magma. Tins metasomat~sm differed from that involved in the petrogenes~s of other arc lava suites m two important respects (1) the extent of enrichment of the source perldotlte m Ba, Rb, Th, K and Sr was quite small relative to other arc suites, including primitive IAT, and ( 2 ) L R E E were not involved in the metasomat~sm. Thus the low L R E E abundances and strong L R E E depletion of the G r o u p A parent magma, subsequently propagated through the state during fractlonatlon, probably reflects the orlgmal, unmodified nature of the severely depleted source perldotlte

5.3 Group B Unfortunately, only one sample (1438 D) of basalt from G r o u p B was considered fresh enough to analyze. Tins is a fairly evolved (mg'= 0.50), shghtly Ne-normatlve (3.8% Ne), Tl-sahte beanng basalt clearly unrelated to Group A arc tholentes. G w e n the tectomc setting of dredge 1438, sample D could, m principle, have been produced m e~ther the West Marlana Radge island arc, the Marlana Trough backarc basra, the currently active M a n a n a arc, an ocean ~sland or seamount scraped from the downgolng Pacific plate (such as those accreted onto the M a n a n a forearc further north [5]), or m an off-axis lntraplate setting m which the lavas were erupted through arc or backarc basra crust. To choose among these possible scenarios, both the K-Ar age and trace element geochermstry of sample D need to be considered. The K-Ar of 7 8 _ 1 3 My [3] and the trace element chemistry

276 both rule out the hkehhood that D was produced m either the West M a n a n a Ridge (active 18-9 Me) or the presently actwe M a n a n a arc Below, we compare the trace element geochemistry of 1438D with ocean island basalts (OIB) and backarc basra basalts (BAB) m an effort to constrain its origin. The REE pattern of 1438D (Fig. 7) is convex-up, with shght L R E E depletmn ( ( L a / S m ) N = 0 7 6 ) and shght H R E E depletmn ( ( S m / Y b ) y = 1 44; ( L a / Y b ) N = 1.10). Similar patterns are shown by basalts erupted during the early stages of backarc basin opemng, Indeed, such patterns may charactenze early BAB, as basalts from the M a n a n a Trough, Bransfield Strait, the South Sandwich spreading centre and the Guaymas Basra at the head of the Gulf of Cahfornla all show similar convex-up patterns (Fig. 7) Also, basalts from the narrowest section of the autochthonous Rocas Verdes backarc basra ophlohte m South Chile display similar, or shghtly LREE-enrlched patterns [53,54]. In contrast, BAB from mature backarc basins (e.g. Lau and Parece Vela Basins, and the wider southern section of the Rocas Verdes ophiohte) have typical N-type MORB LREE-depleted patterns [7,40,54]. MORB-normahzed element patterns for 1438D and some early BAB show that general enrichments m Rb, Be, Th, K and L R E E relative to M O R B are charactensUc of early BAB, but these enrichments do not persist throughout the spreadmg hfe of the basin (Fig. 8). Basalts erupted during all but the earhest stages of backarc spreading are httle different from N-type MORB, denwng from N-type MORB source mantle &aplrs [8,56] One explanation [8,36,56] of tins temporal

14oF ~- 3o

change in the L I L E contents of BAB is that the N-type MORB source mantle dlaplr ascendmg beneath the arc interferes w~th normal arc m a g m a generation processes, and enters the metasomatlc " h a l o " of the dehydrating, subductmg slab. Early BAB are consequently enriched xn L I L E from the slab, and show a "calc-alkahne" lmpnnt [14] relatwe to more voluminous N-type MORB erupted from a steady state dIaplr later m backarc basra development, when the source dlaplr has moved away from the trench, beyond the metasomatlc halo of the slab. Some mtraplate basalts, including ocean island tholentes, tholentes from the early stages of contlnental rifting (e.g British Tertiary Volcamc Province [57] and the Faeroes [58]), and also transitional (T-type) M O R B show similar R E E and MORB-normahzed element patterns to 1438D, but m general, absolute abundances of LILE, R E E and high field strength elements such as Zr, T1 and H f are greater In the intraplate laves and T-type M O R B than in 1438D at equivalent rag' values The MORB-normallzed ratio most &agnostic m determining the affimtles of 1438D ~s ( K / N b ) N , winch is 0.97 for 1438D, a value typical of N-type MORB, compared to s~gmficantly lower values (0.4-0.7) for the mtraplate or T-type M O R B tholeiltes. For those early BAB where data is available ( K / N b ) N rauos are always close to, or greater than 1.0, due to the generally lower N b contents of the latter relatwe to mtraplate or T-type M O R B tholentes. On this basis, any affinity of 1438D with OIB or transmonal M O R B is considered unlikely, and a tectomc setting of eruption during early backarc basin opening would seem most appropriate It is of interest to note that a large percentage

s

N :E

10t, LI

,

,

Ce

Nd

, Sm

, Eu

,

L

~

~

T

Gd

Tb

Dy

Ho

Er

" Tm

", Yb

Fig 7 Chondnte-normallzed RI~E pattern for (1) Group B

basalt 1438D compared with patterns for other backarc basra basalts, (2) Guaymas Basra (head of Gulf of Cahfornm) basalt 477-2c [69], (3) Sarmlento ophxohte dolente PA28B [53], (4) Bransfield Strmt, Decepuon Is basalt B103-3 [9], (5) Manana Trough basalt 46-11 [48], (6) South Sandwich spreading centre basalt D23 5 [36]

2

"

\\

o

I

L

I

Rb

Ba

Tn

i_

K

L

I

I

Nb

Ls

Ce

J

h

I

I

i

i

Sr

Nd

P

Sm

Hf

Zr

i

Ti

J

i

Tb

Y

Yb

Fig 8 MORB-normahzed element patterns for (1) Group B basalt 1438D, (2) South Sandwich spreading centre basalt D23 5, (3) DecepUon Island basalt D103 3, (4) Guaymas Basra basalt, average of 4 samples For sources, see caption to Fig 7

277

of early BAB show llrmted but obvious HREE depletion (Fig. 7) that N-type MORB and later BAB do not show. The most sunple explanation of this feature is that early BAB include the first increments of melt tapped from the diaplr of MORB-source mantle ascending beneath the arc, and these liquids equdibrated with a garnetbearing residuum. During subsequent continued melting, garnet IS eliminated from the residuum, and magmas approach N-type MORB compositions. If these early melt Increments were produced by very small degrees of partial melting [57], anomalous enrichments in LILE relative to N-type MORB would be generated, thus obviating the need for subductlon zone metasomatlsm involving input from the subducted slab. Tins posslbihty remains unproven and deserves further study.

6. Tectonic implications It is generally agreed that the opemng of the Manana Trough commenced sometime shortly before 5 Ma [59,60], after the dechne of arc volcamsm on the West Manana Ridge around 9 Ma Therefore, the 7.8 _ 1.3 My age determined for 1438D by Beccahiva et al. [3], and our interpretation of this rock as having formed during the earhest stages of Martini Trough opemng, fit well the pre-estabhshed tectomc scenario. The origin of the highly depleted Group A IAT is more problematical, and a discussion of their tectonic slgmficance should be previewed by a brief synthesis of available petrochermcal and tectonic data from the Yap region. Behind the Yap arc, the Parece Vela Basin is only half the width it attains further north, and the Pilau arc at the southern end of the PalauKyushu arc appears never to have split. Opening of the Parece-Vela Basin around 32-30 Ma drove the forearc block of the rafted section of the Palau-Kyushu Ridge oceanward, and It is preserved on Guam, Saipan and the current Manana forearc [8,59]. The Yap arc is unusual in several features; it has a very narrow arc-trench gap relative to other oceanic arcs, and lacks seIsmlcxty. Ollgocene (Palau-Kyushu arc) volcanics are not exposed on Yap, which is composed of greenschists derived from mafic and ultramaflc rocks [61] overlain by

breccxas derived from Slrmlar hthologies and some plutomc rocks The youngest unit exposed in Yap IS a deeply weathered, fragmentary volcanic fortuition, the Tomll Formation, of unknown but presumably post-Eocene age [62]. Samples dredged from the Yap forearc both northeast and south of the Yap Islands include amphibohte derived from subalkahc basalts, marbles, and greenschists sirmlar to those exposed on Yap [61]. In a dredge (HB 2, Fig. 1) north-northeast of Yap, at depths between 4 and 1.5 km, Hawhns and Battza [61] record feldspathic basalts that "have a relatively fresh appearance (which) suggests that they are younger (than the schists) and did not participate in the deformation event which developed the schists" This basalt has 9.5% MgO, only 0 44% TIO2, and K 2 0 , CaO and N a 2 0 contents very close to those of 1438C. "Dmltry Mendeleev" dredges 1427 and 1429 were from close to the Yap Trench some 100 km south of Yap, in the forearc (Fig. 1), and yielded IAT, K-Ar dating of the two freshest samples from dredge 1427 gave ages of 7.6 _+ 1 and 10.9 + 3 m.y [31. McCabe and Uyeda [63] suggest that emplacement of the Yap greenschist allochthon was preMiocene, and may have occurred when the Eocene-Ollgocene subduction zone fronting the Yap arc was blocked by seamounts on the Pacific plate, and the Caroline Ridge. Continued westward motion of the Pacific plate may have resuited in Parece-Vela backarc basin crust being thrust over the Yap arc, and metamorphosed [61]. The presence of LREE-depleted IAT erupted between approximately 10 and 7 Ma along at least 300 km of the Yap arc requires that subductlon was occurring beneath the Yap arc at this time, which is post-emplacement of the Yap greenschists. We suggest that after initial blocking of the trench by the Carohne Ridge perhaps about 20 Ma, continued northwest movement of the Pacific and Caroline plates may have led to hrmted subductlon beneath the Yap allochthon, leachng to generation and eruption of IAT in the Yap forearc and maybe through the Yap allochthon (If it had been emplaced by this time). If part of the crust being subducted at this time was hot, recently generated Sorol Trough crust (two MORB-hke basalts dredged at "Dmltry Mendeleev" site 1440, where the Sorol Trough abuts the Yap Trench have been

278 dated at 7 My [3]), m a g m a productmn at abnormally shallow depths m this area would generate I A T Source pendotlte at these shallow levels, perhaps less than 50 kin, would be more depleted than source pendotlte for arc magmas generated at greater depths, thus explalmng the low T102, Zr and L R E E contents of the Group A lavas Further north, below the West M a n a n a Ridge, old, cold Pacific ocean crust was being subducted, and m a g m a generation at greater depths, probably 80-150 km, produced calc-alkahne parental magmas [40]. The hypothesis that Group A magmas were generated at abnormally shallow levels beneath the Yap forearc may also account for two other unusual chenucal features of the Group A lavas. Basaltic hqmds produced dunng perxdotlte meltmg experiments [51,64] at pressures less than 10 kbar, either anhydrous or under H20-undersaturated conditions, show relatively bagh $102 contents (51%), a feature of the Group A basalts. Secondly, after subductmn to around 50 km depth, hot, young Sorol Trough crust might dehydrate, but would not be deep or hot enough to parually melt. DehydraUon flmds mxght transport water soluble elements such as Ba, Rb and K, but not the more immobile REE; thus the weak but slgmficant enrichment of Ba, Rb and K in the Group A lavas (and also bomnltes), but lack of enrichment m REE. Normal arc volcamcs, whether I A T or calc-alkalme, show a slgmficant ennchment m both the L R E E and elements such as K, Rb and Ba. Tlus m a y imply that both a slab-derived dehydratmn fired, and a slab-derived partial melt effectwely metasomatlze the source of most arc volcamcs, at depths probably in excess of 50 km.

7. Conclusion Rocks dredged from the forearc close to the lntersectmn of the Yap and M a n a n a trenches include basaltic and andesltxC representaUves of a htghly LREE-depleted island arc tholeute state, and transitional to shghtly alkaline basalts unrelated to the IAT. The latter group, dated at 7 8 + 1 3 My, have chemacal features suggesting they represent the earhest manifestation of backarc basra magmaUsm which gave nse to the still actively-spreading M a n a n a Trough. The depleted I A T are postulated to have been generated by

shallow ( < 50 km)partlal melting of upper mantle perldotxte residual after M O R B extraction, followm g influx of slab-derived hydrous flmds enriched m Ba, K and Rb, but not REE These lavas erupted along the Yap forearc between 7 and 10 Ma, following subductlon of young, hot Sorol Trough crust, at the same Ume that typically calcalkahne arc magmas were being erupted further north along the West M a n a n a Radge. The depleted I A T dredged at site 1438 probably represent a m a g m a type transmonal between typical island arc tholenuc basalts, generated at greater depths, and lugh-Mg andesites and bonlnites, generated at shallower levels and hagher temperatures.

Acknowledgements A.J.C. is very grateful to the Itahan C N R for financial support to work m Italy in 1982. This work was fmanclally supported by M P.I (40%) to L.B. and G.S. Mxcroprobe work was performed in the Central Science Laboratory of the Umverslty of Tasmama, with valuable assistance from W~esclaw Jablonskl. Major and trace element X R F analyses were done m lhsa, and REE, Th and Sc analyses by I N A A in Hahfax. Anne Crawford and June Pongratz helped with typing and drafting, respectwely. Finally, we thank the Russian sclenUric and shap crew of the "Dmatry Mendeleev" for making this study possible.

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