concomitant evolution of tectonic environment and magma...

Concomitant evolution of tectonic environment and magma geochemistry,

Ambrym volcano (Vanuatu, New Hebrides arc)

C. PICARD', M. MONZIER2, J.-P. EISEN3 & C. ROBIN2 ' Université Joseph Fourier, Institut Dolonzieu, rue Maurice Gignon, 38031 Grenoble,

France UR IF, ORSTOM, BP 76, Port Vila, Vanuatu

UR IF , ORSTOM, BP 70,29280, Plouzane, France

Abstract: Ambrym volcano lies in the central part of the New Hebrides island arc near the latitude where the D'Entrecasteaux Zone is colliding with the arc. It is a 35 km X 50 km, mainly basaltic stratovolcano constructed in four distinct parts: (i) well-preserved remnants of an old edifice in the north; (ii) a NlOO"-aligned, oval-shaped basal Hawaiian-type shield volcano; (iii) apyroclasticcone, cut by aconcentric 12 kmwide caldera; and (iv) post-caldera basaltic suites, both intra- and extra-caldera. Considering KzO, La and Zr variations, three major trends characterize the geochemistry of Ambrym; one medium-K (MK) to high-K (HK) tholeiitic to calc-alkaline basaltic trend, and two more evolved trends, respectively MK and HK, from basaltic to dacitic and rhyodacitic compositions. These magmatic suites correspond to several volcanic phases: (1) older MK basalts forming the Tuvio-Vetlam- Dalahum edifice; (2) MK to HK basalts forming the basal shield volcano; (3) MK andesites and rhyodacites forming the first pyroclastic sequence of the Ambrym Pyroclastic Series (APS), which probably initiated the formation of the caldera; (4) MK to HK basalts and andesites forming Surtseyan then Strombolian pyroclastic sequences 2,3 and 4 of the APS; and (5) post-caldera MK to HK basalts forming the recent olivine porphyritic- and plagioclase-porphyritic suites, locally associated with more evolved HK andesitic volcanic rocks in the eastern part of the caldera. (La/Yb)n and (La/K)n ratios (2-5.9 and 1.3-2.2, respectively) and Zr contents indicate that the parental magmas originated from incremental batch melting or fractional melting (21-25%) during uplift of a single spinel lherzolite source (from at least60 to 45 km depth) in response to modifications in the tectonicenvironment due to ongoing collision with the D'Entrecasteaux Zone. However, K and La contents require prior enrichments by possible mantle metasomatism associated with subduction. Fractionation of olivine, plagioclase, clinopyroxene and Fe-Ti oxides explain most of the major and trace element variations in the resulting liquids and interaction between magmas rising from two distinct chambers combined with massive introduction of seawater into the edifice appear to be the major causes of the giant eruption leading to caldera formation. More recent activity of the volcano is principally related to shallow incremental batch melting or fractional melting and to fractionation in magma chambers associated with N100" rifting.

Diverse schemes based on major and/or trace element geochemistry have been used to classify

systems have been generally well established in many calc-alkaline provinces. This is especially

island arc magmas and to propose petrological models for their origin. Among them, K content, LREE/HREE values and isotope data, particularly for Sr and Nd, provide useful information about depth of magma genesis, possible source heterogeneities, degree of partial melting, crustal contamination, and possible participation of subducted sediment during magma genesis, as well as about the differenti- ation processes operative in island arc magma

true-for studies of arc development ober the lifetime of the arc, or for large parts of an arc

scale of a single volcano over a short period (less than 0.1 Ma), examples showing large chemical 5 Q/ variations are scarce and most relate to con- e e tinental calc-alkaline volcanoes where the. difficulty of establishing primary magma com- t9 position makes the problem of magma genesis even more complicated.

system over a reduced period. In contrast, on the ST-

-90

- chambers.

Based on such geochemical studies, models for the spatial and temporal evolution of arc

Along the New Hebrides arc, distribution of=, 2, K 2 0 reveals important variations which are no- T:

well understood (Gorton 1974; MacFarlane eQ,' 3 ' 4 m

s 13e ?C Froin Smellie, J.L. (ed.), 1995, Volcanisni Associated with Extension at Consuming Plate Margins, Geological Society Special Publication No. 81,135-154. c 5 ' S Z

2 7 MARS 9995

136 C. PICARD ET AL.

0 L 1 5"-

NFB

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, c Fig. 1. Map showing central and northern Vanuatu islands (in black). Bathymetry in km from Chase & Seekins (1988). NLB, North Loyalty Basin; DEZ, D'Entrecasteaux Zone; WTM, West Torres Massif; SNHT, Southern New Hebrides Trench; NNHT, Northern New Hebrides Trench; NAB, North Aoba Basin; SAB, South Aoba Basin; ESV, Epi Submarine volcanoes; NFB, North Fiji Basin; CT, Coriolis troughs. New Hebrides convergence (1) and reverse back-arc thrusting (2) emphasized by thick lines and black triangles with relative motions in cm/y (Louat &Pelletier 1989). Volcanoes with activity during the last five centuries are marked by a black star (Simkin et al. 1981).

Y

al. 1988), although high-K compositions dominate only in the large volcanoes located at the latitudes where the D'Entrecasteaux Zone col- lides with the New Hebrides arc (Roca 1978; Eggins 1989). These variations are particularly significant in the Ambrym volcano, in which we

draw attention to the coexistence of high KzO (herein HK) and medium-K20 (MK) basalts and related andesitic to rhyodacitic lavas.

This paper deals with the petrological characteristics of successive pre-, syn- and post-caldera volcanic suites of Ambrym volcano and focuses

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3

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b'

EVOLUTION OF AMBRYM VOLCANO, VANUATU 137

especially on the event responsible for caldera formation, in which magmas with both HK and MK compositions took part. Their study allows us to propose a model explaining the chemical evolution of the volcano in relation to its tectonic setting.

Seismo-tectonic setting of Ambrym volcano Ambrym volcano (168"08' E; 16%' S) is situated in the central part of the New Hebrides (NH) island arc near the latitude where the D'Entrecasteaux Zone (DEZ, Fig. 1) is colliding with the arc. Eastward subduction of the Australian plate occurs along the NH trench. Convergent relative motion between the Aus- tralian plate and the NH island arc varies from 15-16 cmlyear in the north to 12 cm/year in the south (Louat &Pelletier 1989).

The D E Z is a high-relief, double-spined ridge on the Australian plate (Fig. 1). Collision and subduction of this E-W-trending ridge under the arc was probably initiated near 17"s about 3 Ma ago (Louat et al. 1988). According to Roca (1978), Collot et al. (1985), Collot (1989) and Eggins (1989), subduction of the DEZ and the progressive northern migration of the collision area (due to the obliquity of the DEZ compared with the general N65-75" azimuth of the relative plate convergence) explain most of 'the unusual characteristics of the arc between Vanua Lava and Efate islands. Present-day rates of subduction are least in front of the D E Z (9 cm/year) and back-arc thrusting (1 cm/year) occurs at the same latitude instead of the extensional regime characteristic of most of the back-arc areas of the NH (Louat & Pelletier 1989).

Intense tectonic erosion occurs in the fore-arc areas previously affected by the migrating collision zone (Efate-Malakula; Fig. l ) , whereas uplift of both forearc (SantwMalakula) and back-arc (Maewo-Pentecost) areas, together with subsidence of Aoba Basins and major faulting transverse to the arc, occur in front of the present collision zone (Collot 1989). The unusual shallow- and intermediate-depth seismicity between 14" and 18"s (Louat et al. 1988), and the intense volcanism along transverse fracture zones in the same area (the most active part of the volcanic arc, containing the large volcanoes of Gaua, Ambae and Ambrym; Macfarlane et al. 1988; Greene et al. 1988, and where high-K compositions clearly dominate; Roca 1978; Eggins 1989), appear to be strongly associated with past and present-day subduction of the DEZ.

The area between Ambrym and Tongoa

islands (Fig. 1) is at present the most active part of the arc (Simkin et al. 1981) and includes from north to south: the 12 km wide Ambrym caldera with the two active cones Marum and Benbow (Robin et al. 1993), the Lopevi active volcano (Warden 1967), three active submarine volcanoes near Epi island (Exon & Cronan 1983; Crawford et al. 1988), the Karua active submarine volcano in the caldera of Kuwae (Crawford et al. 1988), and lastly, some thermal springs on Tongoa (Warden et al. 1972). In addition, recently extinct volcanoes with well preserved cones are common on Epi, Tongoa and Emae (Warden et al. 1972), and Kuwae caldera is only 500 years old (from our unpublished data, and age given by Garanger 1972). Whereas Ambrym and Lopevi are clearly located on transverse fractures, interpreted by Greene et al. (1988) as a major active transcurrent wrench fault, other volcanoes do not display such a strong tectonic control.

Active volcanoes of the NH arc are generally located between 100 and 200 km above the Benioff zone (Louat et al. 1988). Nevertheless, a conspicuous gap in intermediate-depth seismicity occurs between Malakula and Efate (Marthelot et al. 1985; Louat et al. 1988), slightly west of the most active volcanic segment of the arc. This seismic gap may correspond to a window in the subducted lithosphere, generated between detached, sinking lithosphere and a short, newly established slab (Louat et al. 1988; Collot 1989). The high volcanicity of the area might be related to processes occurring around 200 km depth at the torn upper edge of the detached lithospheric slab.

Detailed present-day seismotectonics of the Ambrym area are shown in Fig. 2. The back-arc, west-dipping thrust zone of the central New Hebrides arc dies out to the south near the active Lopevi volcano, immediately east of Ambrym. The brief (3 days) 1987 superficial seismic crisis beneath eastern Ambrym, the only one recorded since 1977, included shallow earthquakes displaying both strike-slip and underthrust focal mechanisms which strongly support a N100" compressional stress for the crust in this area. Interestingly, the western and eastern rifts of Ambrym, from which numerous lava flows were erupted, display the same N100" trend and, thus, probably correspond to large scale tensional 'cracks' opened by a N100" regional compressive stress, rather than to active transcurrent faults (Greene et al. 1988). In addition, most of the recent intra-caldera lava flows were erupted along N100" fractures. As pointed out by Collot (1989), this regional compressive stress in the crust is a consequence of the collision between

138 C. PICARD ETAL.

Fig. 2. Present-day seismotectonics of the Ambrym area. (1) Boundary of the seismic crisis area of 26 to 28 November 1987 as recorded by the local seismic network in Vanuatu; CMT focal mechanism solutions are shown on equal-area projections of the lower hemisphere of the focal sphere, with focal depth in km (US Department of the Interior/Geological Survey 1987); (2) central New Hebrides back-arc west-dipping thrust zone (from Collot 1989); (3) western boundary (projected) of the active thrust belt; (4) western and eastern rifts of Ambrym; (5) regional compressive stresses through the crust and Ambrym island. B and M abbreviations correspond to the Benbow and Marum active cones in the Ambrym caldera.

the DEZ and the arc. The underlying west- dipping thrust zone in the crust does not appear to have influenced eruption from the volcano, as the western and eastern rifts have produced identical volumes of compositionally similar basaltic lava.

Characteristics of the Ambrym volcano. The summit of Ambrym volcano (1270 m) is about 1500 m above the surrounding seafloor (Chase & Seekins 1988). It is a 35 km x 50 km stratovolcano (MacFarlane 1976; Fig. 3) constructed in four distinct parts (Monzier et al. 1991; Robin etal. 1991,1993): (i) well-preserved remnants of an old edifice in the north; (ii) a NlOO"-aligned, oval-shaped basal Hawaiian- type shield volcano; (iii) a pyroclastic cone (the Ambrym Pyroclastic Series or APS: Monzier et al. 1991; Robin et al. 1993), 24 km in diameter,

cut by a concentric 12 km wide caldera (McCall et al. 1970); and (iv) post-caldera volcanic rocks, both intra- and extra-caldera.

The old edifice comprises three cones (Tuvio, Vetlam and Dalahum) aligned along a N10" direction in the northern part of the island, and is composed of subaerial basaltic lava flows and pyroclastic deposits. The lavas are glomeroporphyritic basalts with aggregates of plagioclase (Ansz cores to An55 rims), clinopyroxene (salite and augite), olivine and Fe-Ti oxides in a microlitic plagioclase- and pyroxene- rich groundmass.

The gently-dipping flanks (2-3") of the basal shield volcano are exposed only along the periphery of the island, and its oval shape may reflect incipient activity along NZOO" rifts. It is principally composed of low-viscosity (pahoehoe type) basaltic flows which typically have a microlitic texture formed by isolated or glomeroporphyritic phenocrysts of plagioclase (An91 cores to An67 rims), clinopyroxene (salite and augite), olivine (F084 cores to Fos1 rims) and Fe-Ti oxides.

The basal shield volcano is overlain by the APS which consists of: (i) daciticpyroclasticflow deposits related to plinian and phreatomagmatic eruptions (sequence 1, up to 60m thick); (ii) well-bedded, sometimes well-sorted, basaltic vitric tuffs with intercalated agglomerates, and ash flow deposits related to hydromagmatic (Surtseyan-type) eruptions and minor Pliniam eruptions (sequence 2, which represents the major part of the tuff cone); (iii) basaltic ash flow deposits, essentially composed of highly to extremely vesicular (6690%) droplets and basaltic pumiceous lapilli formed during a Plinian eruption (sequence 3, 1 6 2 5 m thick); and (iv) Strombolian basaltic deposits on the caldera rim (sequence 44, up to 250m thick around the Woosantapaliplip vent, Fig. 3, Robin et al. 1993). Plagioclase and clinopyroxene phenocrysts are abundant and form up to 50 YO of some beds as coarse ash or lapilli (sequence 2). The phenocrysts observed within the basaltic to basic-andesitic glasses of the APS are compositionally homogeneous: bytownite-anorthite (Anso-gl), augite-salite (w043-44; En40.5-44; Fsm 16), olivine (FO~Z-~O), and Fe-Ti oxides (4-6.5Y0 TioZ). The dacitic glasses include rare Fe-olivine microphenocrysts (F042-44) , two clinopyroxenes (wO4142; En34-36; Fsz1-24; and wO43-45; En3-0; FslG19), titanomagnetite (12-14% Tioz) and andesine with reverse zonation in a vitric glass with afew andesine microlites (An35). The thickness of the APS is estimated to be 20W50m near the caldera edge, which represents a calculated volume of 60-50 km3, which

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I I I I 1

U.LEWLEMBWI MB-MBUELESU ML-MARUMUGLUì

( VANUATU )

16"lO'S

16OX)'

O- 5 Km

1 6 8 O O O ' E

J (3 0' X

CALDERA I t

MI MARUM Y

Meters ... 1200 1000 8W 600 400 2w O 200 400

WWSAMAPALIPLIP

5 Km - PROTO. CALDE54

ante-caldera series APS post-caldera series

Fig. 3. (A) Topographical map of Ambrym island with sample locations (numbered). The main volcanic features (caldera, cones, maars, fissures) are shown in bold lines. Contour interval = 100 m. LL, Lewolembwi maar. (B) Composite section of the Ambrym volcano after Robin et al. (1993) showing the basal shield volcano (dark grey), tuff cone (white) and the post-caldera volcanic sequences (light grey). Vertical exaggeration X3. 3 APS, Ambrym Pyroclastic Series.

is more than 20 km3 of dense rock equivalent (Robin et al. 1993).

The caldera, which cuts the APS, is almost circular with a continuous scarp a few tens of metres to 450m high. Interestingly, its centre lies on the southern extension of the older N10" volcanic lineation and the N100" rifts clearly transect south of this centre (Fig. 3). On the north side of the tuff cone, an unusually viscous and glassy andesitic lava flow, 2CL25m thick overlies the APS. This flow was extruded near

t.

the caldera edge and flowed as far as the NW coast, forming a large 2 km wide lava tongue. Near the vent, part of this flow remained in the caldera and was intersected by the last collapse, indicating a close relationship to the terminal phase of the syn-caldera episode.

Post-caldera activity, dominantly from Marum (1270 m) and Benbow (1160 m) cones in the western part of the caldera (Fig.3), occurs along N100" fissures inside and outside of the caldera. Episodic lava lakes in the Benbow and

CI Table 1. Whole-rock analyses and average compositions of glasses (obtained by microprobe) from volcanic rocks of Ambrym P O

Tuvio and Vetlam basalts Ante-caldera basalts

N" Eg.66 Eg.M78 Eg.63 68 72 Eg.74 60D 61E 43A 33 50E 32 31 53 50F 42 36 48 35 55 59 MKBas MKBas MKBar MKBas MKBas MKBas MKBas MKBas MKBas MKBas MKBas MKBas MKBas HKBas HKBas HKBas HKBas HKBas HKBas MKAnd MKAnd

111 111 111 [Il

si02 TrO2 AID, Feo* MnO MfiO Ca0 Na20 K20 WO5 LOI 1050'

Rb Ba Nb I( La S, P Nd Zr EU Ti DY V

49.12 0.69

13.53 9.44 0.17

12.79 11.44 1.96 0.75 6.15

-0.16

13.0 210.0

1.0 6236

5.7 334.0 661.5

8.2 39.0

4138

15.0

299.0 801.0

223.0 44.0

0.75

210

Normolive eomporilion QZ 0.00 Or 4.43 Ab 16.56

Ne 0.M) Di-Wo 12.36 Di-En 8.09 Di-Fs 3.41 Hy-En 3.69 Hy-Fs 156 01-Fo 14.04 01-Fa 6.52

He 0.00 II 131

A" 25.88

Mt 1.81

49.16 0.62

14.06 9.03 0.17

12.24 11.91 1.99 0.63

0.00 10.0

231.0 0.9

5231

366.0 785.6

42.0

3717

13.0

0.18

250.0 801.0

173.0 40.0

0.75

0.00 3.72

27.55 16.83

0.00 12.66

3.50 3.49 1 A7

8.28

.. .. 13.10 6.10 1.72 0.W

49.29 0.73

14.30 9.54 0.17

12.04 10.79 2.17 0.84 0.12

-0.09

12.0 221.0

1.0 6999 4.7

360.0 531.9

8.0 42.0

4384

15.0

308.0 547.0

211.0 38.0

0.74

1.54

0.00 4.98

18.37 26.74 0.00

10.82 6.96 3.14

1.57 13.66 6 79

3.48

_. . ~

1.89 nnn

49.24 0.67

13.97 8.85 0.17

11.68 13.16 1.74

0.13 -0.03

4.1 142.0

1 3 3170 5.0

280.0 570.1

7.0 36.0 0.8

4036 2.6

15.0 1.6 1.4

245.0 620.0 50.0

130.0 48.0

0.38

0.75 2.62 3.63

0.00 2.25

14.69 29.15 0.W

14.71 9.57 4.13 5.20 2.24

10.01 4 77

48.78 0.80

14.84 9.69 0.19

10.22 12.57 2.07 0.64 0.19

-0.29

11.0

1.0 5344 4.2

304.0 834.0

7.5 44.0 0.9

4824 2.9

16.0 1.6 1.6

279.0 381.0 48.5

101.0 42.5

418.0

0.71 1.92 1.81

0.00 3.78

17.50 29.28 0.00

13.27 8.14 4.37 1.45

11.07 0.78

6.55 169 1 R5

48.50 0.78

15.25 10.06 0.18 9.85

11.74 2.83 0.67 0.14 0.14

11.0 187.0

1.0 5527 4.1

332.0 616.2

8.6 40.0 0.9

4656 3.2

16.0 2.0 1.6

290.0 324.0

97.0 44.0

0.69

1.71

0.00 3.93

26.89 3.07

12.68 7.60 4.41 0.00 0.00

7.57 1.92

1.89

18.28

11.85

0.00 1 47

50.95 0.61

16.84 9.29 0.19 6.30

12.54 2.22 0.87 0.20 0.02

17.0 244.0

1.3 7189

410.0

9.0 45.0 0.8

3682 3.1

16.5 1.6 1.6

310.0 198.0 40.0 56.0 39.0

5.8

878.8

0.61 2.55 1.84

0.00 5.11

33.40 0.00

11.44 6.00 5.11 7.44 6.33 1.57 1.50

0.00

18.72

1.78

1.17 0.48

47.17 0.23

22.98 6.32 0.12 6.20

15.37 1.38 0.12 0.10 0.10

2.0 87.0 0.7

1000 3.2

437.9 3.0 8.5 0.4

1384 1.1 5.5 0.7 0.7

144.0 83.0 36.0 43.0

430.0

28.0

0.69 3.48

0.00 0.71

11.67 56.16 0.00

4.80 2.91 3.13 1.90 5.26 3.52 1.21 0.00 0.44 0.24

8.11

49.90

16.72 12.00 0.23 5.07

11.15 2.52 1.29 0.26

-0.12

0.86 SOS1 0.79

10.09 0.19 4.95

11 2 9 2.53 1.52 0.28

-0.11

17.86

24.0 300.0

2.1 10670

540.0 1139.3

12.5 59.0 1.1

5177 3.3

19.0 2.0 1.8

370.0 16.0 44.0 26.0

8.8

35.0

26.0 323.0

2.0 12579

9.0 624.0

12343 13.0 64.0 1.0

4723 3.0

17.0 1.6 1.6

312.0 15.0 39.0 28.0 31.0

50.79 0.95

15.55 12.70 0.24 4.89

10.14 2.98 1.47 0.28 0.32

22.5 358.0

2.5 12204

9.1 528.0

1238.8 14.5 68.0 1.3

5713 3.5

22.0 2.1 2.0

420.0 5.5

45.0 17.0 35.0

49.98 0.69

9.52 0.18 4.71

12.13 2.40 1.22 0.23 0.02

18.94

19.0 287.0

1.4 IOlW

7.7

1017.6 11.0 51.0 1.0

4133 2.7

16.0 15 1.4

284.0 65.0 36.0 27.0 32.0

578.0

50.07 0.75

10.31 0.20

11.78 2.43 1.28 0.25

-0.11

18.34

4.58

20.5 3W.O

2.3 10664

565.0 1103.5

11.5 55.0

1.0 4487

2.9 17.0 1.6 1.5

323.0 183 38.0 23.0 31.6

8.1

51.29

16.93 9.87 0.20 5.24

10.63 2.77 1.84 0.34

-0.11

32.0 380.0

2.8 15304 10.5

620.0

14.0 76.0 1.1

3.3 20.0 2.2 1.8

340.0 59.0 38.0 41.0 30.5

0.88

1486.5

528s

51.23 0.89

17.32 10.53 0.20 4.51

10.53 2.86 1.61 0.32 0.29

29.0 370.0

2.7 13325

9.8 600.0

1409.7 14.0 70.0

1.1 5326

3.5 20.0

1.9 1.8

370.0 20.0 36.0 28.0 29.0

5239 0.97

16.73 10.10 0.19 4.46 9.35 3.08 2.32 0.41

-0.19

53.02 0.99

9.68 0.18 4.42

3.26 2.55

16.68

8.79

0.43 -0.18

45.0 450.0

3.9 19256 12.0

630.0

17.0 85.0 1.3

5794 3.6

20.0 2.2 1.8

365.0 16.0 35.0 29.5 25.0

1775.8

48.0

3.4

12.8 638.0

1858.0 17.5 93.0

1.3 5956

3.5 21.0 2.1 1.9

360.0 21.0 32.0 34.0 24.0

480.0

212087

52.64 0.92

18.02 BSI 0.16

9.46 3.20 2.41 0.41 0.04

47.0 475.0

3.7 19971

13.0 710.0

1800.9 16.5 87.0 1.1

5491 3.3

19.0 1.8

320.0 70.0 29.0 48.0 20.0

4.zn

1.8

52.72 0.85

7.93 0.15 4.01 9.26 3.21 2.57 0.42 -0.06

42.0 457.0

3.0 21342

11.7 745.0

1840.6 15.0 82.0 1.2

5117 3.1

18.0 1.9 1.6

297.0 69.0 26.0 49.0 18.0

18.88

55.19 0.88

16.09 10.35 0.23 3.73

3.46 1.32 0.30

-0.26

155 412.0

2.3 10972 10.4

400.0 1320.8

15.0 79.0

1.4 5262

1.5 275 2.9 2.6

248.0 14.0 29.6 11.7 29.5

8.44

55.46 0.90

16.15 10.06 0.23 3.57 8.34 3.41 1.56 0.30

-0.10

24.0 420.0

5.1 12989 23.5

398.0 1313.1

27.5 139.0

1 3 5411

5.0 31.0 3.5 2.9

245.0 15.0 30.5 13.0 30.0 ..

0.49 0.53 0.47 0.53 O S O 0.55 0.49 0.50 0.51 0.53 0.53 0.45 0.45

1.89 1.65 1.77 1.75 1.74 1.58 1.68 1.43 1.39 1.49 1.26 2.17 4.16 3.59 4.03 337 3.89 3.85 4.26 4.m 4.76 4.91 5.31 5.35 2.86 5.81

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.87 4.15 7.58 8.94 8.67 7.18 758 10.88 9.47 13.69 15.08 14.20 15.18 7.80 9.23

21.29 21.34 25.18 2031 20.51 23.37 2-1.14 26.05 27.59 27.06 27.16 29.24 28.82 30.46 32.88 24.67 37.25 35.30 28.31 29.66 24.94 23.28 27.66 29.47 24.44 24.10 om non 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -. -. . . . . 9.62 8.86 9.89 8.90 8.95 9.24 8.52 7.82 7.31 6.90 5.71 6.44 6.38 4.09 4.07 4.02 4.09 3.90 4.42 3.68 3.45 3.29 3.26 2.70 253 2.50 555 4.72 5.96 4.73 5.03 4.68 4.84 435 3.98 356 2.93 3.98 3.97 3.27 3.49 2.68 3.06 3.44 3.10 3.82 3.76 3.80 3.02 2.86 6.75 6.38 4.43 4.04 3.98 3.53 4.44 3.28 5.03 4.7s 4.59 3.30 3.10 10.60 10.15 3.67 3.33 3.81 3.21 2.84 3.86 2.61 2.71 2.74 3.06 3.09 0.00 0.00 5.48 4.25 6.24 4.08 4.03 4.51 3.79 3.78 3.65 3.67 3.69 0.00 0.00

~ .~ 2.30 1.93 2.43 1.82 1.97 1.89 2.01 1.93 1.86 1.63 1.52 1.98 1.93

1.18 1.39 1.28 1.52 .. 1.64 1.49 1.80 1.31 1.42 1.67 1.68 1.83 1.88 1.74 1.62 1.66 1.71 0.36 0.43 0.29 0.31 0.45 0.33 ~ ~ 0.62 0.67 0.67 0.55 0.60 0.81 0.76 0.96 1.01 0.98 1.00 0.72 0.71

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

AP

1 ' c c c L I L I -- c-- - '

,. r 2 n

Table 1. Continued

Pyroclastites: APSwquence 1 Pyroclastites: APS sequence 2

N" 57C 60A M)B1 60C 70 60C 29A2 29A1 50 39 39B 39 M 6012 60H 39 39A 39 AndMK AndMK DacMK DacMK DacMk DacMK MKBas MKBas HKBas HKBas HKBas MKAnd MKAnd MKAnd MKAnd HKAnd HKAnd HKDac

Ave. n=8 Ave. n=22 Ave. n = l l Ave. n=13 Ave. n=2 Ave. n=3 Aue. n=6 Ave. n=18

sioz 58.35 60.81 66.56 64.30 65.77 68.52 50.39 51.28 51.35 52.00 51.26 54.35 54.50 54.57 54.89 54.98 56.07 68.39 Ti02 0.72 0.62 0.43 0.55 0.50 0.33 0.73 0.79 0.94 0.97 0.93 1.09 1.25 0.80 0.87 0.93 0.78 0.41 AhOi 15.44 15.48 14.99 14.94 15.84 15.19 16.20 15.68 15.28 16.05 16.42 15.39 14.11 16.56 16.17 16.80 16.13 15.67 Feo' 9.51 8.09 5.34 6.92 5.04 4.30 11.59 11.43 12.41 11.21 11.54 11.14 11.90 1054 10.40 9.92 9.02 3.78 MnO 0.23 0.21 0.16 0.21 0.17 0.10 0.23 0.23 0.27 0.22 0.22 0.28 0.26 0.22 0.22 0.21 0.19 0.13 My0 3.02 2.49 1.11 1.38 0.91 0.22 5.98 5.88 5.14 5.03 4.96 4.65 4.65 3.93 3.91 3.65 3.67 0.53 Cao Na20 KzO PZOS LOI 1050"

Rb Ba Nb K

7.04 6.26 3.64 3.78 1.83 2.07 0.21 0.19 0.04 0.26

32.0 38.0 525.0 588.0

3.2 3.2

3.16 5.48 2.61 0.16 4.34

29.0 635.0

3.8

4.41 4.43 2.68 0.19 0.54

51.0 725.0

3.8

2.92 2.88 11.37 11.19 10.28 9.79 9.65 8.70 9.32 8.65 8.63 8.15 7.54 1.75 4.66 5.08 2.31 2.52 2.67 2.68 2.87 3.40 2.95 3.18 3.24 2.84 3.35 3.87 3.99 3.38 0.98 1.01 1.66 2.05 1.81 1.00 1.07 1.29 1.31 2.51 2.96 5.48

0.29 - 0.20 - 0.21 - 0.33 - 0.08 - 2.47 - 2.29 - -0.03 0.70 - 1.53 -

- - - 0.25 0.36 - - - - 76.0 18.0 31.0 24.0 22.6 54.0

8W.0 250.0 358.0 390.0 394.0 52.0 5.5 2 0 2 7 211 7 4 4 5

15201 17193 21661 22279 33147 8161 7.0

445.0 903.1

9.5 43.0

.. . 10719 10842

8.3 9.0 418.0 416.0

1100.5 1558.3 12.0 13.0 68.0 71.0

24543 13.5

488.0 1245.6

17.5

15015 10.3

572.0 1443.3

15.0 69.0

La Sr P Nd Zr

~ ~~ ~~~

12.3 13.1 16.0 15.8 19.0 360.0 335.0 255.0 288.0 310.0 922.0 841.7 697.1 839.6 877.8 17.0 18.0 22.0 19.0 23.0

105.0 114.0 120.0 142.0 182.0 112.0 1.1

4706 4.0

24.0 2.0 2.3

245.0 59.0 29.0 20.0 22.5

E" Ti DY

1.2 4342

5.3 33.5 3.3

1.2 3712 5.3

34.0 3.5 3.5

167.0 14.0 20.0 14.0 21.7

1.4 2554 5.7

32.0 3.6

1.3 3278 6.1

41.0 4.1 4.1

1.2 3015 5.0

31.0 3.2 3.3

24.0 2.0 8.0 3.0

10.5

0.9 4404 2 9

17.2 1.7 1.6

1.3 5576

1.2 0.9 4777 5199

3.9 4.2 23.5 24.5 2.9 3.0 2.5 2.6

299.0 244.0 5.0 14.0

32.0 38.0 13.0 11.5 29.0 29.0

3.4 20.5 1.7 1.8

330.0 155.0 41.0 lw.o

c O r

z O

F Er Yb V

3.4 260.0

2.0 26.0 9.0

26.5

3.2 29.5 1.6 6.0 4.5

12.0

70.0 31.0 12.0 17.0 17.0

~. 325.0 204.0 43.0

106.0 37.0

Cr CO Ni

sc 29.0 Mg# 0.42 0.41 0.32 0.31 0.29 0.10 0.54 0.54 0.48 0.50 0.49 0.49 0.47 0.46 0.46 0.46 0.48 0.24 (La). 2.59 2.71 3.59 2.78 4.19 3.24 4.15 2.36 2.50 4.27 W K ) . 1.86 1.75 1.70 1.63 1.32 1.97 1.57 1.78 1.90 1.26

QZ 7.81 11.34 14.51 14.45 13.70 18.09 0.00 0.00 0.00 0.00 0.00 2.30 4.07 3.67 4.20 2.79 1.95 17.90 Or 10.81 12.2) 15.41 15.84 23.58 19.92 5.80 5.97 9.80 12.12 10.67 5.89 6.33 7.62 7.71 14.83 17.45 32.37 Ab 30.78 31.92 46.38 37.41 39.38 42.88 19.49 21.30 22.53 22.67 24.28 28.70 24.95 26.85 27.42 24.03 28.35 32.80 A" 20.36 19.15 8.58 12.96 10.52 8.64 30.91 28.44 24.80 25.66 26.53 23.77 22.04 27.08 25.67 25.63 20.19 8.66 Ne 0.00 0.00 0.00 0.00 0.00 0.00 0.w 0.00 0.w 0.00 0.w 0.w 0.00 0.00 0.00 0.00 0.00 0.00 Di-Wo 5.49 4.44 2.53 3.18 1.10 2.35 10.05 11.29 10.92 9.54 7.99 8.07 10.07 5.91 6.16 6.15 6.40 0.00 Di-En 2.00 1.58 0.70 0.86 0.28 0.22 4.69 5.27 4.60 4.22 3.45 3.45 4.19 2.36 2.48 247 2.69 0.00 Di-Fs Hy-En Hy-Fs 01-Fo 01-Fa Mt He II AD

3.61 2.96 4.60 8.61 0.00 0.00 1.55 0.00 1.17 0.46

1.95 2.05 5.69 0.00 0.00 1.02 0.W 0.81 0.38

2.49 2.57 7.43 0.00 0.00 1.32 0.00 1.04 0.46

0.88 1.97

2.39 0.34

5.26 6.77 7.59 2.39 2.96 2.22 0.03 1.39 0.49

5.89 6.35 3.94

5.29 4.61 5.79 2.57 3.55 2.14 0.00 1.84 0.00

4.54 4.22 5.57 3.27 4.75 2.21

4.63 8.11

10.88 0.00 0.00 2.13

5.93 7.37

10.42 0.02 0.00 2.28

3.61 7.43

11.37 0.00 0.00 2.02

3.74 7.24

10.92 0.00 0.00

3.74 6.64

10.06 0.00 0.00

3.73 6.44 8.93 0.00 0.00 1.73 0.00 1.49 0.67

0.W 1.31 5.29 0.00 0.W 0.72 0.00

5.51 9.95 0.00 0.00 1.82 0.00 1.37 0.50

6.40 7.16 2.06 2.54 2.19 0.00 1.49

6.21 0.00 0.00

3.72 0.00 0.00 0.82 0.W

5.43 2.97 4.51 2.31 0.W 1.79

0.96 0.W 0.95 0.48

1.99 0.00 1.64 0.84

1.90 0.00 1.76 0.00

0.00 1.76 0.78

0.00 2.06 0.00

0.00 2.36 0.00

0.00 1.51 0.60

0.63 0.00

0.77 0.00 0.00 0.00

L

Table 1. Coritinued 8 APS sequence 4 Port-caldéra: 01-rich basalts Post-nldéra : PI-rich basalts Pyroclastites: APS sequence3

N" 65 65 65 65 65 4 B 61A 67 66 20 5 27 51 71 38 21 52 49 15A 47 8 6 MKBas MKBas MKBas HKBas HKBaa MKBas MKAnd MKBas MKBas MK MKBas MKBar HKBas HKBas HKBai HKBas HKBas HKBas HKBas HKBas HKBas HKBas

Ave. n=4 Ave. n=14 Ave. n=2 Ave. n=2

Si02 Ti02 AbOj Feo' MnO MfiO Ca0 Na20 K20 ROS LOI 1050'

Rb Ba Nb K La Sr P

51.64 0.96

15.61

0.22 5.06

10.80 3.14 0.90

11.68

52.06 1.06

14.59

0.28 5.02 9.96 2.84 1.34

12.86

50.65 0.80

18.05 11.14 0.21 4.65

10.82 2.46 1.00 0.23 0.70

51.13 52.38 1.47

50.94 0.77

14.81 10.23 0.19 8.32

11.11 2.27 1.12 0.23 0.49

54.89

16.42 I039 0.23

8.48 3.32 1.29 0.30

-0.31

0.87

3.81

49.64 0.70

10.94 0.21 8.42

11.68 2.22 1.W 0.22

-0.31

14.87

49.74 0.74

15.51 11.19 0.21 7.75

11.07 237

0.25 -0.41

20.5 310.0

15 9803

7.6 507.0

1100.7 12.0 50.0

0.9 4415

2.8 15.5 1.6

1.18

50.17 0.86

16.06 11-31 0.21 5.74

2.54 1.63 030

-0.59

11.18

28.0 342.0

50.78 0.76

13.26

51.02 0.71

14.65

51.57 0.94

16.56 11.03

51.39 0.86

17.77 9.65

4.80 10.46

0.18

51.09 0.94

16.66 11.61 0.23 4.64 9.83 2.83 1.84 0.34

-0.46 34.0 34.0

2.9 2.9 390.0 388.0

15248 15578 10.8

1489.2 613.0

14.5 71.0

1.1 5656

3.7 21.0 2.2 1.9

51.00 o u

51.10 ... 0.94

16.82 16.51 11.50 11.2 0.23 0.22 454 4.68 9.89 10.12 2.87 2.94 1.88 1.88 0.34 0.36

-0.52 -0.22

51.12 0.94

16.64 11.22 0.22 4.61

10.11

1.90 0.35 -0.34

2.89

51.16 0.95

51.54 0.96

51.11 0.94

16.12

51.19 0.96

16.24 11.65 0.23 4.80 9.90 2.75 1.94 0.35

-0.37

1.10 14.24 12.96 0.22 4.47 9.94 3.33 1.61

16.36 11.42 0.23 4.73

10.09

1.90 0.35

-054

2.80

16.23 11.41 0.23 451 9.69 3.14 1.92 0.36 0.00

14.43 14.68 0.21 3 . p a s o 3.00 2.14

11.44 0.22 4.97

10.14 2.78 1.93 0.34

-0.59

10.43 0.20 8.29

12.51

1.34 0.25

-0.36

24.0 293.0

1.9

2.18

9.81 0.19 9.73

1031 2.23 1.13 0.22 0.01

16.6 242.0

1.8

0.22 4.49

10.03 3.99 1.83 0.34

-0.31

34.0 390.0

15164 2.8

2.74 1.83 030

0.85

32.0 390.0

2.9 15177

10.1 655.0

1329.6 13.0 72.0 1.1

5175 3.3

17.0 1.7 1.6

315.0 66.0 35.0 40.0 27.0

35.0 392.0

2.5 15633 I l 3

37.0 395.0

2.9

11.0 15738

37.0 394.0

2.4 15801

10.7 635.0

36.0 416.0

2.6 15918

34.0 393.0

2.6 15995

11.3 615.0

1504.6 155 73.0

1.3

34.0 390.0

2.8 1Mw5

12.0 630.0

1542.2

19.0 151.0

1.6 a343

18.0

2.0 9267

7.2 460.0

268.0 23.0

392.0 2.4

10703

410.0 8.8

1318.5

19.5 305.0

1 4 2.4 13548

9.2

1318.8 12.8 60.0

588.0

9065 8.0

484.0

48.0

970.7 12.0

11157 8.1

488.0 1102.4

115 54.0

1.0 4543

9375 7 4 11.1

620.0 1497.3

15.0 80.0

10.1 11.1 610.0

7.8 482.0 984.5 11.0 51.0

1.1 4795 3.1

18.5 1.6 1.7

335.0 56.0

... 410.0

11.0 56.0

968.0 620.0

1496.9 15.0 72.0

1.3 5625

3.6 21.0 2.2

645.0

15.0 74.0

I 3 5614

3.7 21.0 2.3 2.0

350.0 48.0

1582.0 642.0

1540.1 15.0 73.0

1.3 5622

3.6 215 2.0 1.9

14.0 358.0

1538.2 155 73.0

1.4 5675

3.7 21.0 2 3 2.0

355.0 21.0 41.0 27.0 29.5

1549.6 16.0 76.0

io la5

1.n

11.0 56.0

16.0 75.0

1.4

Nd Zr E" Ti

13.0 72.0

1.3 1.3 5778

3.7 21.5 2.3 2.0

354.0 16.5

0.9 4182

1.0 5133

3.1

1.0 4231

1.4 5626 5654

3.7 21.0 2 5 1.9

350.0 15.0 40.0

5751

29.0 2.3 2.1

360.0 175 41.5

3.8 4623

3.0 17.0 2.0 1.6

344.0 437.0

5193 4.3

25.0 2.7 2.6

245.0 17.0 29.0

2.6 15.0 1.8 1.4

304.0 278.0 49.0

2.9 2.9 17.0

3.8 215 2.0

17.0 1.7 1.6

340.0 51.0 45.0 35.0

16.0 1.6 1.5

1 6 1.5

342.0 49.0

298.0 2.0

352.0 12.0 38.5 26.5 285

1.9 350.0

9 5 42.0 20.0 29.0

Yb Y Cr CO Ni SC

1.6 315.0 230.0 47.0 93.0

355.0 13.0 41.0 21.0 30.0

275.0 306.0 4X.O

180.0 33.0

40.0 25.0 29.0

39.0 23.0 28.0

41.0 185 28.0

35.0

31.0 38.0

45.0 133.0 34.0

28.0 32.0

27.0 30.0

13.0 30.0

109.0 37.0

69.0 43.0 35.0 32.0

050 0.47 0.49 0.44 0.33 0.65 0.45 0.64 0.61 0.54 0.64 0.69 0.48 0.53 0.48 0.47 0.49 0.48 0.48 0.47 oso 0.48 3.27 3.21 2.46 4.04 3.52 4.10 3.85 3.42 3.98 4.51 4.19 3.81 4.11 4.15 3.92 3.92 4.20 4.20 2.14 1.77 1.88 2.03 1.78 1.55 1.66 1.82 1.68 1.52 1.63 1.49 1.66 1.61 1.55 1.60 1.62 1.71

Mfi# ( L a p o " (LalK).

0.00 7.89

23.07 0.00

10.96 450 6.54 6.42 9.32 1.10 1.76 2.45 0.00

23.98

0.00 5.93

20.79 35.18 0.00 7.07 2.99 4.10 7.68

10.53 0.62 0.93 2.13 0.00 1.52 0.53 -

0.00 12.59 2532 19.59 0.00 9.40

7.01 4.59

0.41 1.14 2.81 0.W 2.78 0.00

2.78

11.58

-

0.00 659

19.21 26.89 0.00

11.12 6.27 4.39 7.98 5.59 4.52 3.49 1.96 0.00 1.46 0.55

3.97 7.61

28.09 26.04 0.00

0.00 6.44

18.79 27.33 0.00

12.14 6.68 5.01

0.00 6.97

20.03

0.00 10.46 553 4.60 3.47

7.20 6.60 2.14 0.00 1.40 0.60

28.14

2.89

-

0.00 9.63

21.45 27.56 0.00

10.80 5.03 5.65

0.00 7 91

0.W 6.67

26.60 0.00 9.63

3.34 9.99 5.77

18.83

5.78

0.00 0.00 10.79 23.18 30.75 0.00 7.97 3.71 4.18 4.13 4.65

3.57 1.85 0.00 1.64 0.72

2.88

-

0.00 11.07 24.21 27.45 0.00 8.06 3.33 4.78 3.08 4.43 3.42 5.41 2.20 0.00

0.00 11.11 24.84 26.26 0.00

3.82

2.23 3.03 3.92

2.15 0.00

0.86

8.98

5.18

5.86

1.78

-

0.00 11.23 23.65

0.00 1131 26.57 24.18 0.00

3.67 5.24

8.85

0.00 11.37 23.47 25.79 0.00

0.00 11.14 23.20 26.21

Normolive comporilion QZ 0.00 Or 5.30 Ab 26.49 An 25.81 Ne 0.00 Di-Wo 11.57 Di-En 5.02 Di-Fs 6.54 Hy-En 3.80

01-Fo 2.65 Ol-Fa 3.80 Mt 2.23 H e 0.W Il 1.82

Hy-Fs 4.95

AP 0.00

0.00 11.19 24.45 26.77 0.00

3.71 5.10 2.61 3.59 3.60 5.46 2.15 0.00 1.78

8.78

0.83

0.00 10.84 23.91 27.29 0.00 8.00 3.32 4.72 3.82 5.42 3.07 4.81 2.22 0.00 1.79 0.81

0.00 9.55

?XI2 19.12 0.00

12.56 4.85 7.89 1.59 2.59 3.28 5.88 2.43 0.00 2.10 0.00

10.78 25.24 26.34 0.00

3.70 5.14 336

8.81

18.44 22.38 0.00

15.85 8.85

26.43 0.00

3.76 5.14 3.38 4.61 3.25 4.88 2.19 0.00

0.83

8.88

1.80

-

0.00 8.56 3.62 4.98 4.06

9.27 4.02 5.24 3.07 4.01 3.69 5.30 2.19 0.00 1.79 0.81

5.85 232 359 6.37

2.99 2.15

2.26 3.23 3.72 5.85 2.18 0.00 1.83

7.15 11.06 0.00 0.00 1.99 0.00

2.99 2.24 7.91 6.54 2.09 0.00 1.32 0.53

1.63 558 2.98 4.52 2.23 0.00 1.82 0.84

1.83 5.34 6.60 2.16 0.00 1.62 0.71 -

4.67 2.87 439 2.11 0.00 1.78 o.ai

6.15 4.88 2.00 0.00 1.44 0.60

5.91 3.76

0.00 1.34 052

1.88

1.64 0.71

1.78 0.81

2.01 0.00 0.84

1 < I L r--

L-

'1. I- *,- F

Table 1. Continued

Lewolembwi maar Post-caldera : PI-rich basale

N" 16 18' 18 44 63 17 26 34 22 3 13 10 54 25A 41 23B 19D 12B 14 HKBas HKBas HKBas HKBas HKBas HKBas HKBas HKBas HKBas HKBas HKBaa HKBas H K B w HKBas HKBas HKBas HKbas HKAnd HKAnd

M6O 4.71 Ca0 10.02 Na20 2.79 &O 1.94 P203 O35 LOI 1050" -0.37

Rb 37.0 Ba 402.0 Nb 2.9 K 16141 La 10.6 Sr 625.0 P 1546.6 Nd 16.0 Zr 76.0 Eu 1.3 Ti 5767 DY 3.8

22.0

M6O 4.71 Ca0 10.02 Na20 2.79 K?O 1.94 Pl-o3 O35 LOI 1050" -0.37

Rb 37.0 Ba 402.0 Nb 2.9 K 16141 La 10.6 Sr 625.0 P 1546.6 Nd 16.0 Zr 76.0 Eu 1.3 Ti 5767 DY 3.8

22.0 E, 23

S C 30.0

Di-Wo 8.93 Di-En 3.74 Di-Fs 5.22 Hy-En 3.28 Hy-Fs 4.58 01-Fo 3.28 01-Fa 5.05 Mt 2.22 He 0.00 II 1.82

51.07 0.96

16.27 11.58 0.23 4.79

10.02 2.78 1.95 0.35

-0.45

30.0 392.0

3.0 16171 10.7

633.0 1541.5

15.5 73.0 1.4

5748 3.9

21.5 2.2 2.0

360.0 26.5 42.0 31.0 29.5

0.48 3.94 1.52

0.00 11.49 23.45 26.14 0.00 8.85 3.75 5.13 3.25 4.45 3.45 5.20 2.22 0.00 1.82

51.15 0.95

16.58 11.45 0.23 455 9.98 2.79 1.96 0.35

-0.36

30.0 400.0

2.9 16282 10.8

643.0 1544.1

16.0 74.0 13

5697 3.7

21.0 2.4 1.9

355.0 15.0 40.0 25.0 28.0

0.47 4.02 1.52

0.00 11.57 23.49 26.93 0.00 8.43 350 4.98 3.53 5.03 3.00 4.71 2.19 0.00 1.80

51.20 0.97

lk.98 11.70 0.23 4.76 9.98 2.85 1.97 0.36

-0.47

38.0 405.0

2.7 16335 11.5

620.0 1577.1

16.0 76.0 1.3

5838 3.6

22.0 2.1 2.0

8.0 42.0 24.0 30.0

311.0

0.48 4.21 1.61

0.00 11.61 24.09 24.96

0.00 9.23 3.88 539 2.93 4.08 3.52 5.40 2.24 0.00 1.85 0.85

51.28 0.98

15.92 11.84 0.25 4.70 9.89 2.78 1.97 0.39

-0.33

37.0 410.0

2.9 16341 11.1

626.0 1718.0

15.5 76.0

1.4 5870

3.9 21.0 2.3 2.0

365.0 6.5

39.0 21.0 30.0

0.47 3.96 1.55

0.00 11.61 23.45 25.12 0.00 8.90 3.69 5.26 4.00 5.71 2.81 4.42 2.27 0.00 1.86

51.18 0.95

16.70 1136 0.22 4.56 9.81 2.87 2.00 O35

-0.53

30.0 395.0

2.9 16579 11.4

648.0 1532.7

16.5 79.0

1.3 5715

3.7 21.7 2.2 2.0

355.0 11.5 39.0 23.0 28.0 0.48 4.15 1.58

0.00 11.78 24.25 26.74 0.00 8.18 3.41 4.80 3.03 4.27 3.42 531 2.17 0.00 1.81

51.24 0.97

16.05 11.72 0.23 4.70 9.89 2.83 2.01

. 0.35 -0.41

35.0 407.0

2 3 16679 11.0

1542.0 15.0 76.0

1.2 5810

3.9 22.0 2 3 2 0

375.0 9.5

41.0 24.0 30.0

618.0

0.48 4.01 152

0.00 11.85 23.88 25.14

0.00 9.00 3.76 5.29 3.16 4.45 3.35 5.21 2.24 0.00 1-84

51.74 0.97

16.17 11.57 0.23 4.55 9.39 2.97 2.04 0.35

-0.40

37.0 396.0

2.7 16931 11.5

616.0 1534.5

17.0 83.0

1.4 5842

3.9 22.0 2.6 2 0

355.0 4.0

39.0 17.0 29.0

0.47 4.12 1.56

0.00 12.03 25.12 24.72 0.W 8.15 3.36 4.83 4.00 5.74 2.77 4.39 2.21

52.10 0.84

18.75 8.68 0.17 4.13 9.75 3.06 2.14 0.38

-0.18

40.0 405.0

3.4 17755 10.6

720.0 1636.6

14.5 77.0

1.3 5044

3.2 18.0 2.1 1.7

305.0 52.0 30.0 41.0 21.6 0.52 4.61 1.37

0.00 12.62 25.87 31.07 0.00 6.18 2.82 3.31 3.62 4.24 2.68 3.45 1.66

52.09 1.00

15.95 11.54 0.23 4.39 9.13 3.13 2.14 0.39

-0.10

37.0 438.0

3.0 17769 11.7

613.0 1718.3

16.0 81.0 1.4

5992 4.0

23.0 2.5 2.2

360.0 5.0

37.0 19.0 29.0

0.46 3.90 1.51

0.00 12.63 26.44 23.12 0.00 8.15 3.31 4.90 3.77 5.57 2.69 4.39 2.21

51.95 0.98

16.48 11.29 0.73 4.25 9.22 3.04 2.18 0.37

-0.32

40.0 430.0

2.4 18072 12.6

645.0 1627.3

16.5 81.0 1.5

5861 3.9

23.0 2.5 2.1

345.0 6.0

38.0 18.0 27.0

0.46 4.25 1.60

0.00 12.84 25.71 24.83 0.00 7.69 3.11 4.65 3.77 5.64 2.60 4.28 2.16

ooo ono nnn nnn

52.06 0.99

16.31 11.36 0.24 4.23 9.16 3.06 221 0.38

-0.54

40.0 433.0

2.3 18307 12.0

645.0 1669.7

17.5 115.0

1.4 5916

4.0 23.5 2.4 2.3

352.0 5.0

39.0 18.5 27.0

0.46 3.78 1.50

0.00 13.01 25.86 24.22 0.00 7.80 3.13 4.74 3.85 5.84 2.48 4.14

52.13 0.97

16.61 10.05 0.19 4.67 9.36 3.32 2.29 0.40 0.12

42.0 452.0

3.8 19050

12.3 645.0

1756.8 15.5 88.0 1.2

5792 3.6

20.5 2.0 1.8

365.0 14.0 36.0 31.0 26.0

0.51 4.85 1.48

0.W 13.54 28.07 23.60 0.00 8.41 3.80 4.56 1.11 1.33 4.70

~ 6.20 2.17 1 97

52.37 0.88

18.40 8.78 0.17 4.20 9.39 3.10 2.32 0.39

-0.22

42.0 445.0

3.2 19220 11.8

710.0 1720.6

15.5 81.0 1.2

5273 3.3

18.5 1.8 1.7

315.0 54.0 31.0 41.0 21.0

0.52 5.05 1.41

0.00 13.67 26.23 29.40 0.W 6.08 2.79 3.24 3.77 4.37 2.72 9 'I7

52.82 0.99

16.63 9.87 0.19 4.46 8.95 3.18 249 0.42

-0.13 47.0

460.0 3.8

20671 12.9

635.0 1847.6

17.0 90.0

1.3 5922

3.6 21.0 2.1 1.9

3M.0 17.0 32.0 32.0 25.0

0.51 4.98 1.43

0.00 14.70 26.84 23.75 0.00 7.45 3.33 4.09 4.04 4.95 7 hn 3.52 1.89 nm

52.65 0.99

16.08 11.28 0.24 4.09 8.74 3.21 2.32 0.40

-0.59

40.0 455.0

3.1 19231

12.3 620.0

1750.5 17.0 87.0

1.4 5952

4.1 24.5

2.4 2.2

350.0 7.5

38.5 14.0 27.0

0.45 4.05 1.46

0.00 13.67 27.11 22.58 0.00 7.57 3.00 4.66 4.30 h hR ...- 2.02 3.45 2.16 0.00

53.02 1.00

16.18 11.05 0.24 3.93 8.50 3.29 236 0.42

-0.46

43.0 465.0

3.2 19629

13.0 625.0

1844.1 19.0 90.0

1.5 5971

4.1 25.0

2.6 2.2

340.0 7.0

36.0 11.0 26.0

0.45 4.18 1.52

0.00 13.95 27.80 22.36 0.00 7.10 2.79 4.40 4.84 7.64 1.52 2.64 2.11 0.00 1.89

59.47 0.82

16.30 7.66 0.21 2.11 5.26 4.09 3.58 0.50

-0.29

67.0 648.0

4.4 29738

18.5 514.0

2195.5 23.5

158.0 1.7

4886 4.8

31.0 3.2 3.0

136.0 5.0

20.0 6.4

14.5

0.38 4.42 1.43

5.72 21.15 34.54 15.55 0.00 3.03 1.03 2.08 4.22 8.51 0.00 0.00 1.47 0.00 1.55

61.53 0.74

16.22 6.73 0.21 1.58 4.29 4.29 3.97 0.43

-0.14

77.0 730.0

5.6 32966 19.3

460.0 1896.0

25.0 1M.O

1.5 4422

5.4 31.0

3.6 3.1

72.0 9 5

14.0 7.5

12.0

0.35 4.53 1.34

7.90 23.45 36.31 13.24 0.00 2.18 0.67 1.59 3.25 7.73 0.00 0.00 1.29 0.00 1.40

.. .. 1.68

~ ~ ~. .... 0.00 0.00 0.00 ~~ ~ . 1.85 1.60 1.90 1.85 1.87 1.83 1.67 1.87 1.88

AP 0.84 0.83 0.84 0.93 0.83 0.84 0.83 0.89 0.93 0.88 0.90 0.95 0.93 1.00 0.95 1.00 1.19 1.03

Analyst J. Colfen. BICIICIFC led fouranal wJIllrromIhCT"uio-vellam-Dalahumsuilr(Eggin~,unpublirhcddrfI). M~jorelEmenfrgeachemislryinwt%rummedlolWDlvolaf~lcIrre:FcO' = lolul~ronarFeO;l.Ol = lorionignition;lnieelemenlsinppn:Mg~o.mdnarmal~lrlianr determinedon Ihc brs¡sorFelP,(Fczi+ Feri) = O.Y. MK, Medium K HK. High K; Bas. harde And. andcsile: Dar. datile: Rhy. rhydacilc; APS. Ambiym Pymdas1icSetic.i: Avc.uuecage mmpo%ition:n = n u m b e r o l - r o p r o b c ~ " ~ l ~ ~ ~ d ï ~ ~ h ~ rvcrrgewl~ulrtian.SeeFlg. 3 forlocalion orthe %amples.


Marum craters drained out as basaltic lava flows onto the caldera floor, as occurred in 198&1989. Magma was also frequently ejected as ash and scoria, mantling the caldera and the western part of the volcano. In the eastern part of the caldera, a recent maar (Lewolembwi crater), located on a N100" fracture, is surrounded by a 2 km wide tuff ring, and older coalescent structures of the same type are infilled by 1986 lava flows. Extra- caldera activity is dominated by lava flows generated along the N100" rifts, and recent maars are present at both extremities of the island, where eruptions interacted with seawater. During the last two centuries, Ambrym has had a large number of eruptions, often with extra-caldera flows, produced by either over- flowing the caldera margins or by rift activity. These have occasionally been quite destructive

53, etc., Fisher 1957; Williams &Warden 1964; Gèze 1966), the latest being in 1986 and 1988-89 (Eissen et al. 1991). In 1990-92, no lava flows were erupted and the volcanic activity was moderately explosive and strongly fumarolic, and concentrated in the Mbuelesu area (Fig. 3). Lava flows are of aa or pahoehoe types and they are principally composed of plagioclase-rich basalts with glomeroporphyritic aggregates of zoned plagioclase (Ang2 to Ark5), Fe-Ti oxides -I- clinopyroxene (salite-augite) and olivine ( F O ~ ~ to Fo55) in a plagioclase (An7wo)- rich microlitic groundmass. Flows of olivine-rich basalts have been observed locally near the floor of the Lewolembwi maar or as widely distributed blocks. The olivine-rich basalts contain olivine ( F O ~ ~ to Foss), diopside-augite and rare plagioclase phenocrysts (Anss cores to An66 rims), in olivine (Fo424j) and clinopyroxene-rich microlitic groundmass.

(1888, 1894, 1913-14, 1929, 1937, 1942, 1952-

Geochemistry

Sampling and analytical techniques

64 new ICP whole-rock analyses (major, trace and rare earth elements) were carried out for this study (Table 1; Fig. 3): 4 samples from ankaramitic lava flows of the Tuvio-Vetlam-Dalahum suite; 13 samples from coas- tal lava flows of the pre-caldera shield; 13 samples from juvenile glasses of the APS; and 34 analyses from post-caldera lava-flows. 12 average compositions of the APS vitric clasts obtained from 105 new microprobe analyses and 4 whole-rock analyses of Eggins (unpublished data) from the Tuvio high-MgO basalts have also been selected for this study (Table 1).

Selected rock fragments were ground in agate. Powders were digested with a concentrated acid

T 40

si02 % Whole-rock analyses n = 68 Glasses: n = 137

Fig. 4. Si02 histogram of 137 microprobe analyses of vitric clasts from the syn-caldera Ambrym Pyroclastic Series (APS) and 68 whole-rock analyses from the entire volcano. Class interval: 1 YO.

s Rhyo- &Cite

4

3

a

1

O

si02 58 25

o , , I , ; , I , 4 ; * I , , : . - " : ' " ' 45 SO 55 60 65 70

Fig. 5. K20, La and Zr versus Si02 distribution for the Ambrym volcanic rocks. Major elements are expressed in wt% summed to 100% volatile free.

mixture (1 : 8 HN03:HF). Fluorides were then dis- solved and HF neutralized with an H3BOs solution. International standards (JB2, BEN, ACE, GSN, MICA-Fe) were used for calibration. Rb determi- nation was performed by flame Atomic Emission Snectrometrv. All other elements were determined bv

!

L EVOLUTION OF AMBRYM VOLCANO, VANUATU 145

12 .

10

b

\ e

3

'

I M R O "# .- @ \ A

Inductively Coupled Plasma Emission Spectrometry. Relative standard deviation for major oxides: SiOl = 1%; Tioz = 3%; Alzo3 = 2%; Fez03 = 2%; MnO = 5%; MgO = 2%; C a 0 = 2%; Na20 = 3%; K l o = 3%; P205 = 4%. Limits of detection and relative standard deviations for trace elements are as follows: Rb = 1 ppm, 5%; Sr = 0.2 ppm, 5%; Ba = 2 ppm, 5%; Sc = 0.25ppm,5%;V=2ppm,5%;Cr= lppm,5%;Co= 1 ppm, 5 to 10%; Ni = 2 ppm, 5%; Zr = 1 ppm, 5%; Nb = 0.8 ppm, 5 to 10%; Y = 0.5 ppm, 5%; La = 0.8 ppm,5%;Nd=2ppm, lO%;Eu=0.2ppm,5tolO%; Dy = 0.4 ppm, 5%; Er = 0.8 ppm, 10%; Yb = 0.2 ppm, 5%.

Results A histogram of SiOz content (Fig. 4) for all of the analysed Ambrym samples clearly shows a bimodal magmatic suite, with maxima at 50-53 YO (basalts and andesitic basalts) and 66% SiOz (dacites). Basaltic compositions are the most abundant (Table l ) , and thus Ambrym is principally a basaltic volcano. CIPW norm calculations (Table 1) show that all samples are olivine or quartz normative, and all have a sub-alkaline affinity. On a KzO v. SiOz diagram (Fig. 5), most of the basaltic samples range from MK to HK basalts, whereas the more differentiated samples form two separate MK and HK trends which evolve from basaltic to rhyodacitic compositions. These trends also appear in La v. Si02 and Zr v. SiOz diagrams (Fig. 5). Alzo3 varies from 13 to 17 YO, except in some plagioclase-rich samples (cumulates) having 17-19% Alzo3 (Fig. 6). MgO decreases rapidly (from 13 to 5%, Fig. 6) in the MK basalts of the Tuvio edifice and in the olivine porphyritic basalts of the post-caldera suite, suggesting important olivine fractionation. In the more evolved samples, the MgO contents decrease more slowly and reveal a second fractionation trend.

F e 0 and Tioz contents increase initially (from 8 to 12% for Feot; 0.75 to 1.5 YO for Tioz, Fig. 6), then decrease in the andesitic to dacitic samples indicating a tholeiitic affinity for the Ambrym volcanic suite, although the KzO behavior rather suggests a calc-alkaline affinity. In N-MORB normalized spider-diagrams (Fig. 7), the basalts appear very similar to N-MORB for the heavy rare earth elements (HREE), Zr and Y, while the light rare earth elements (LREE), Rb, Ba, K, Sr and P are more enriched suggesting an E-MORB or calc-alkaline affinity. These volcanic rocks also show the strong relative Nb depletion which characterizes island arc series. Thus Ambrym volcanic rocks are geochemically intermediate between tholeiitic

22 1 A1203 % Cumulaleï

14 ' + u + D + ++ +

si02 % 10 " " " ~ " ~ " " ! " " " " '

4 Fe0 % 8

4 1

- f TiO2% A

45 50 55 €4 65 70

Fig. 6. AlzOs, Feo', Tiol and MgO versus Sioz distribution for the Ambrym volcanic rocks. Major elements are expressed in wt YO summed to 100% volatile free. Feo' = total iron as Feo. Same legend as Fig. 5.

and calc-alkaline series, attesting to the difficulty in classifying arc magma (Gill 1981).

The Tuvio and Vetlam volcanic rocks range from LK to MK basalts (KzO = 0.38 to 0.87 Yo), show the highest MgO, Cr and Ni contents of the Ambrym volcano (MgO = 9.85 to 12.79 YO, Cr = 324 to 801 ppm; Ni = 97 to 223 ppm), the lowest Zr, Y and Nb (Zr = 36 to 45 ppm, Y = 13 to 17 ppm; Nb = 0.9 to 1.3 ppm) contents and have high Mg-numbers (Mg2+/[Fe:+ + Mg2+] = 0.69 to 0.75). These high-MgO basalts represent the most primitive magmas sampled on Ambrym.

The plagioclase glomeroporphyritic basalts of the basal volcano range from MK to HK basalts, are more evolved with lower MgO, Cr and Ni contents (MgO = 3.57-5.24%, Cr = 5.5-70


Fig. 7. N-MORB normalized incompatible element abundance patterns of Ambrym volcanic rocks. Elements are ordered in a sequence of decreasing incompatibility, from left to right, in oceanic basalts; order and concentrations in N-type MORB are from Sun & McDonough (1989). For comparison, the field covered by all samples (in grey) is reproduced on each diagram.

100

10

1

Post-Caldera Suite I A Andesites and dacites

Ambrym Pyroclastic Series ( U S ) Basal shield volcano or Sequence4

.x. Sequence 3 * Sequence2

100 d

1 'O

5! 4

3 o E L 1

0.1

100

P 10

B P l B

,

0.14 : ; : : : ; : i : : : : : :J 0.14 i i ; i i i i i i i i i : i '

Rb Ba Nb K La Sr P Nd Zr Eu Ti Dy Y Er Yb Rb Ba Nb K La Sr P Nd Zr Eu Ti Dy Y Er Yb

ppm; Ni = 12-49 ppm), lower Mg-number (0.45-0.55) and higher Zr, Y and Nb contents (Zr = 51-87 ppm, Y = 16-27ppm; Nb = 1.4-3.9

Samples of the lower APS (sequence 1) vary from MK andesites to MK rhyodacites. They represent evolved compositions with low MgO, Cr and Ni contents (Table l ) , low Mg-number (0.424.1) and high Zr contents (Zr = 105-182 ppm). They have low [LaTYbJn values (2.59- 4.19), fairly similar to those of the Tuvio-Vetlam volcano (1.89-3.48). Pyroclastic rocks of sequence 2 include MK to HK basaltic compositions and two magmatic trends: one from MK basalt to MK andesite; and another from HK basalt to HK rhyodacite. Sequence 3 is composed of MK to HK basalticpyroclastics and sequence 4 evolves from MK basaltic to MK andesitic compositions. All these evolved volcanic rocks have low MgO, Cr and Ni contents (Table l ) , and low Mg-number (0.54-0.24). In detail, the individual pyroclastic sequences are very heterogeneous, containing vitric clasts

P P d .

which vary from basaltic to rhyodacitic compositions in a single stratum (e.g. sample 39, Table 1). These observations indicate that the APS magmas have formed by mechanical mixing of a range of magmas from the MK and HK series, spanning basalt, andesite and rhyodacite compositions.

Most of the lava and pyroclastic rocks of the post-caldera suite evolved from MK to HK basaltic compositions (&O = 1.08-2.49%) in similar fashion to the pre-caldera glomeroporphyritic basalts. Only a few vitric samples from the -1986 lava flow and around the Lewolembwi maar range from HK basalt to HK andesite, indicating the presence of a HK differentiated magma chamber below the eastern part of the caldera. The more primitive olivine and pyroxene porphyritic basalts are exclusively MK compositions. They have relatively high MgO, Cr and Ni contents (MgO = 5.749.73 %, Cr = 51-342 ppm; Ni = 35-180 ppm), relatively high Mg-number (0.54-0.69) and low Zr, Y and Nb contents (Zr = 48-60

i

-

-

i EVOLUTION OF AMBRYM VOLCANO, VANUATU 147

4

3

2

1

. I K (wm) o " ' " " ' ' I ' ' " " ' " : ' " ' " " ' : " "

O loo00 20000 3oooo

Fig. 8. [Lab] , , versus K diagram for the Ambrym

Tuvio-Vetlam-Dalahum high-MgO MK basalts and evolved MK andesites, dacites and rhyodacites of the APS sequence 1 (lowest values of [La&%],); (B) MK and HK basalts from the basal shield volcano and from the post-caldera suites, MK - HK basaltic glasses of the APS, HK andesites-rhyodacites of the APS sequence 2, and HK andesites from the Lewolembwi area; and (C) several HK basalts from the basal shield volcano and from the post-caldera suites (highest values of [Lab] , ) . Same legend as Fig. 5.

I volcanic rocks. Trends A, B and C correspond to: (A)

)

ppm, Y = 15-17 ppm; Nb = 1.4-2.4 ppm). The plagioclase porphyritic basalts of this suite are more evolved, with HK compositions, lower MgO, Cr and Ni contents, higher Zr, Y and Nb contents and low Mg-number (0.45-0.53, Table 1).

Discussion

Evidence for three magma evolution trends and their temporal relationships Considering the KzO, La and Zr variations versus SiO2 (Fig. 5), three major trends characterize the geochemistry of Ambrym; one MK to HK basaltic trend, and two more evolved trends, respectively MK and HK, from basaltic to dacitic and rhyodacitic compositions. Basaltic volcanic rocks are present in all parts of the volcano, while the more evolved andesitic, dacitic and rhyodacitic rocks form pyroclastic sequences in the APS and locally around the Lewolembwi maar. Thus, the volcano shows several magmatic phases, well illustrated by the KzO and Sioz variations: (1) older MK basalts forming the Tuvio-Vetlam-Dalahum edifice; (2) MK to HK basalts forming the basal shield volcano; (3) MK andesites and rhyodacites forming the first pyroclastic sequence of the APS, which probably initiated the formation of the caldera; (4) MK to HK basalts and andesites forming Surtseyan then Strombolian pyroclastic

sequences 2, 3 and 4 of the APS; and (5) post-caldera MK to HK basalts forming the recent olivine porphyritic and plagioclase- porphyritic suites, locally associated with more evolved HK andesitic volcanic rocks in the eastern part of the caldera.

In agreement with previous observations, the volcanic rocks form different trends in the [LaNbIn v. K diagram (Fig. 8). Interestingly, the Tuvio-Vetlam-Dalahum high-MgO MK basalts and the evolved MK andesites, dacites and rhyodacites of the APS sequence 1 fall on the same lowest [LaNbIn trend (trend A). Most MK to HK basalts from the basal shield volcano and from the post-caldera suites form the second trend (B) which includes MK to HK basaltic glasses and HK andesitic to rhyodacitic pyroclastic rocks of the APS sequence 2, and HK andesites from the Lewolembwi area. Finally, some HK basalts from the basal shield volcano and from the post-caldera suites form a third group (C), characterized by the highest [La/Yb]n.

Genesis of the magmas and origin of the K and La contents The [LaNbIìz variations defining trends A and B in the more primitive volcanic rocks ([LaNbIn = 2 to 4, Fig. 8) are principally due to variations in La (La = 4.1-7.4 ppm), whereas Yb remains relatively constant (Yb = 1.4-1.7 ppm). This reflects probable changes in the conditions of melting and possible source effects. On the other hand, the significant increase of K contents over a limited range of [La/Yb]n values illustrates the role of fractionation processes in the magmatic evolution. These processes are discussed below in order to explain the chemical characteristics of the analysed samples.

Primary magmas. The clinopyroxene-rich and olivine-rich high-MgO MK basalts of the Tuvio- Vetlam-Dalahum cones (MgO = 12.79% in Eg.66; 10.22% in Amb72; Table l ) , and the olivine-rich basalts of the post-caldera suite (MgO = 9.73% in Amb 27; Table 1) represent the most magnesian lavas sampled from Am- brym volcano. Using relationships between mole %O of MgO and Fe0 at 1 atmosphere pressure (Roeder & Emslie 1970), these highest- MgO whole-rock compositions imply that olivine equilibrium compositions should be Foss for the Tuvio-Vetlam-Dalahum and Foss for the post-caldera suite. Actually, the most magnesian olivine crystals are Fos3 in the Tuvio- Vetlam-Dalahum edifice, Fos4 in the shield volcano, and F O ~ ~ in the post-caldera suite,

148 C. PICARD ETAL.

Jd+

B Basal shield volcano

Fig. 9. Plot of the more primitive basalts from the Ambrym volcano on 01-Jd t CaTs-Qz molecular normative projection following the method of Jaques &Green (1980), Falloon &Green (1988) and Falloon et al. (1988). Fine lines are cotectic grids for the Tinaquillo lherzolite after Falloon & Green (1988) and Falloon et al. (1988). Numerals indicate the cotectic pressures in kbar.

which reveals a lower MgO content in the equilibrium liquid and a possible effect of cumulates. However, Eggins (1989) reported higher-magnesian olivine phenocrysts (Fo93) in the shield volcano suggesting that the parental magmas had a picritic composition (13-1570 MgO, Eggins, unpublished data). Thus, we may conclude that the Ambrym primary magmas were probably high-MgO basaltic liquids with a minimum of 10-11 % MgO.

Sources arid melting processes. The flat HREE patterns of the more primitive lavas (Fig. 7) suggest that garnet was not residual during melting and that melting probably occurred at relatively shallow levels in the mantle. In order to estimate depthmof melting, we have plotted the most primitive basalts on the 01-Jd + CaTs-Qz molecular normative projection (Fig. 9) following the method of Jaques & Green (1980), Falloon & Green (1988) and Falloon et al. (1988). Thus, the most primitive basalts observed in the Tuvio-Vetlam-Dalahum suite appear to have last equilibrated at pressures between 20 and 15 kbar (60 to 45 km below the surface) during first stages of crystallization, while the most primitive basalts from the chronologically later basal shield volcano and from the post-caldera suites indicate equilibrium pressures between 15 and 10 kbar (45 to 30 km depth). These data suggest that the Tuvio- Vetlam-Dalahum primary melts segregated at a minimum depth of 60 km in the spinel lherzolite zone while the more recent primitive liquids may have segregated at shallower levels (about

45 km), suggesting possible uplift of the mantle source related to the DEZ collision, N100" rifting and doming of the volcano. Such a hypothesis may be supported by the increasingly Fe-rich compositions of the most magnesian olivines in successive volcanic suites of the volcano and by MgO decreasing in the more primitive volcanic rocks (Jaques & Green 1980). If true, such a model could also account for variations of K and La in the parental magmas and therefore some variations of (LaNn)n ratios (Fig. 8). However, these variations, principally related to variations in La (Yb remains relatively constant), may be also caused by incremental batch melting or fractional melting of the mantle. Such models are strongly supported by the frequent recurrence of primitive basalts in the APS and post-caldera series, which suggest new influxes of parental magmas probably resulting from renewed melting in the mantle. According to Watson & McKenzie (1991) and McKenzie & O'Nions (1991), who showed that >1% melts cannot remain in contact with their source region, such models seem more geologi- cally valid than a simple batch melting model. Nevertheless, using the simple equilibrium batch melting model (Hanson &Langmuir 1978; Steinberg et al. 1979; Hanson 1980) and the Zr contents of the high-MgO olivine-rich basalts (samples M78 and Amb 67, Tables 1-2), we calculate that the the primary melts of the Tuvio-Vetlam-Dalahum and post-caldera suites required 25 and 21% melting, respectively, from an appropriate mantle source.

Source effects. (LaYb)n variations, (La/K)n ratio (1.3-2.2; Table 1) and the observations given above are consistent with variable degrees of melting during possible uplift of a single spinel lherzolite source. However, K and La contents of the more primitive MK basalts (K = 5231 ppm and La = 4.1 ppm in the Tuvio samples; K = 9065 ppm and La = 7.4 ppm in the olivine porphyritic basalts of the post-caldera suite) indicate that degrees of melting are respectively 9% for potassium and 16% for lanthanum (calculated using the simplified equation of melting, Co/Cì, KdK-L, <0.1 and the mantle composition of Sun & McDonough 1989). These differences with the Zr calculations indicate that the mantle source may have been previously enriched in K relative to La, and in these elements relative to Zr, suggesting possible mantle metasomatism. Assessment of the relative roles of source effect and possible mantle metasomatism or crustal contamination on the K and La variations require further investigation.


Table 2. melting model using the equilibrium batch melting equation F = [(ColCl) - D]l(D - I ) of Hanson (1980) and Zr bulk partition coeficient of Pearce & Norry (1979) for the most primitive olivine-rich basalts of the Tuvio-Vetlain-Dalalzurn and post-caldera suites (sample M78 and Ainb 67, Table I ) and for a spinel lherzolite source (Wilson 1989)

Zr bulk partition coefficient Source: spinel lherzolite

Olivine OPX CPX Spinel 0.01 0.03 0.1 0.1

66% O1 4- 24% OPX 4- 8% CPX i- 2% Sp D(Zr) = 0.0238

Tuvio olivine porhyritic basalts: Higher MgO basalt: sample M78* Zr = 42.0 ppm F = 24.88%

Higher Mgo basalt: sample Amb 67 Post-caldera olivine porhyritic basalts: Zr = 48 ppm F = 21.46%

* Eggins, unpublished data Zr content in the mantle from Sun & McDonough (1989).

Isotopic studies of these suites are already in progress (Baize 1992).

Crystallizatiorz processes. The K and La variations for similar values of [LaNbIn in the more evolved volcanic rocks (Fig. 8) and the significant changes in SO2 , MgO, KzO, Feo , and C a 0 distributions (Figs 5, 6, 10) emphasize the important role played by crystallization processes. Using these variations and the least- squares mixing program of Mason (1987), we have modelled fractional crystallization processes for the different periods of the volcanic activity and for each segment of the fractionation path (Table 3).

Thus, the MK and the low [LaNbIn trends observed from the MK basalts of the Tuvio- Vetlam edifice to the evolved MK andesites, dacites and rhyodacites of the first sequence of the APS (trend 1 on Fig. 5; trend A on Fig. 8) can be explained by two phases of fractional crystallization (trends 1-3 on Fig..lO, Table 3): the first is characterized by a strong decrease of MgO while Feo,, Ti01 and K2O increase in the more primitive basalts (Figs 6, 10) and is related to olivine (16.97% Fo79), plagioclase (21.16% Anss) and clinopyroxene (20.6% augite) fractionation; the second is characterized by a more subdued decrease of MgO while Feo', Tio2 and C a 0 strongly decrease in the more evolved volcanic rocks and is principally controlled by plagioclase (29.61% An76), clinopyroxene (25.77% augite) and Fe-Ti oxide (8.65%) with minor olivine (3.82% F O ~ ~ ) fractionation.

Due to the observed compositional similari- ties between the basal shield volcano and the basaltic post-caldera suites (trend 2 on Fig. 5; trends B-C on Fig. 8, and trends 1-2 on Fig. lo), we have modelled the crystallization sequences of these two volcanic phases (Table 3) using the

O 2 4 6 8 10 12 14 16 14 I

O 2 4 6 8 10 12 14 16

Fig. 10. K20, Feo' and Ca0 versus MgO distribution for Ambrym volcanic rocks, illustrating crystallization trends described in the text. Major elements are expressed in wt YO summed to 100% volatile free. Feo' = total iron as Feo. Same legend as Fig. 5 .

more primitive olivine-rich basalt of the post- caldera suite. These volcanic rocks also show two fractionation paths (Figs 6, 10): the first, characterized by a strong decrease of MgO while Feo', T io2 and K20 increase in the more primitive basalts is principally related to olivine

Table 3. Resiilts of fractional crystallization modelling using the least-squares mixing program of Mason (1987) for different periods during the evolution of Ambrym volcano

MK trend: phare 1-Tuvio-APSsequence 1 MK trend phase 2 : basaltic to rhyodacitic APS seq. 1

O1 basalt PI basalt Fe-Ti PI basalt Rhyodacite Fe-Ti M78 = 29AI + Foe, + Augite + Amo + Oxide = Calc. Residuals 29AI = 60C + Foi0 + Augite + An76 + oxide = Calc. Residuals

Mars% lW.W 40.95 16.00 16.97 21.16 0.14 parent r lW.W 32.27 3.82 25.77 29.61 8.65 parent r

s i 0 2 49.16 51.28 42.52 48.63 51.08 0.10 49.18 -0.01 Ti02 0.62 0.79 0.W 0.62 0.04 6.18 0.45 0.17 A I ~ O ~ 14.06 15.68 0.00 6.63 29.43 5.10 14.05 0.01 Fe0 9.03 11.43 17.20 6.52 1.01 80.97 9.05 -0.01 MnO 0.17 0.23 0.43 0.15 0.04 0.35 0.21 -0.04 Me0 12.24 5.88 39.41 14.83 0.21 3.72 12.12 0.02 5.89 -0.01 5.08 0.22 40.97 14.87 0.17 3.72 Caö 11.91 11.19 0.36 21.19 1338 0.16 11.90 0.01 11.19 2.88 0.32 21.83 15.67 0.16 11.22 -0.1 Na20 1.99 2.52 0.03 0.39 3.76 0.06 1.92 0.07 2.52 5.08 0.W 0.26 2.67 0.06 2.50 0.03

I?= n.nh -i?= on2 K20 0.63 1.01 0.00 0.00 0.30 0.00 0.48 0.15 1.01 3.38 0.02 0.04 0.20 0.02 1.15 -0.14

Basal shield M I Q ~ : phare 2 Basal shield volcano :phare 1

O1 basalt PI basalt Fe-Ti PI basalt Andesite Fe-Ti Ambso = Amb3s + FOX + Augite + An,! + oxide = Calc. Residuals Amba = Ambio + Fon + Augite + Ann + Oxide = Calc. Residuals

1W.W 70.62 7 3 1 14.97 6.09 1.13 parent I 100.00 72.61 0.66 8.50 15.50 3.01 parent r

38.92 50.W 49.60 0.14 49.63 0.01 51.23 53.02 38.62 51.98 49.60 0.14 51.20 0.90 -0.01 0.03 0.08

49.64 0.03 0.55 o.on 4.46 0.77 -0.07 0.89 0.99 0.W 0.33

4.63 30.44 6.65 14.88 -0.01 1732 16.68 0.01 2.93 6.65 17.35 -0.02 30.44 0.70

10.53 0.00 0.02

0.18 0.03 10.94

4.03 4.54 -0.03 0.20 0.24 0.25 0.21

8.42 451 40.97 14.34

2x4 0.01 0.02 2.22 2.86 0.00 0.29 2.89 0.02 2.24 0.02 2.86 3.26 0.W 0.21 2.89 1.86 -0.25 11.68

0.W X?= 0.07

4.46 51.23 0.89

17.32 10.53 18.45 7.41 1.06 85.37 10.93 0.00 10.53 9.68 22.19 7.17 1.06 85.37

14.87

0.09 0.34 0.21 0.01 0.20 o.in 0.43 0.19 0.09 0.34 0.12 4.03 8.43 -0.01 451 4.42 38.86 10.47

0.32 22.19 14.49 0.05 11.69 -0.01 10.53 8.79 0.20 21.46 0.12

14.49 0.05 10.58 -0.05 10.53

1.09 1.61 0.02 0.00 0.W 0.00 1.14 -0.05 1.61 2.55 0.W 0.01 0.W X?= 0.01

Postcaldera series : phase 1 Postcaldera series : phase 2

O1 basalt Plbasalt Fe-Ti Ambs7 = Ambm + FOP) + Augite + Ann 1W.W 79.90 6.25 8.79 5.34 parent parent lW.W 69.24 1.58 13.08 13.59 2.41

- - Calc. Residuals parent * Calc. Residuals Ambio Ambto FOP) Augite Ann OP

O1 basalt O1 baralt

49.64 50.17 38.92 50.W 49.60 49.63 0.01 50.17 52.25 38.92 51.98 49.60 0.14 50.17 -0.01 0.70 0.86 0.03 0.55 0.08 0.74 -0.04 0.86 0.99 0.03 0.33 0.08 4.46 0.85 0.01

14.87 16.06 0.02 4.63 30.44 14.89 -0.01 16.06 16.37 0.02 2.93 30.44 6.65 16.05 0.01 10.94 11.31 18.45 7.41 1.06 10.91 0.03 11.31 11.41 18.45 7.17 1.06 85.37 11.31 0.00 0.21 0.21 0.24 0.25 0.09 0.21 0.W 0.21 0.24 0.2.4 0.19 0.09 0.34 0.22 0.00 8.42 5.74 40.97 14.34 0.12 8.44 -0.02 5.74 4.24 40.97 15.47 0.12 4.03 5.74 0.01

11.68 11.18 0.32 22.19 14.49 2.22 2.54 0.W 0.29 2.89 1.09 1.63 0.W 0.03 0.W

11.69 -0.02 2.21 0.01 1.30 -0.21

TJ= nnc

11.15 2.54 1.63

9.20 3.07 2.21

0.32 21.46 14.49 0.05 11.17 255 -0.01 0.01 0.W 0.21 2.89 0.02 0.W 0.00 0.W 0.03 1.58 0.05

Y& nnr I. 1."- -.

c s h !2

APS sequence 2 : HK basaltic to rhyodan'tictrend Postcaldera HK basaltic to dacitie Lewolembwi tTend O glass glass Fe-Ti PI basalt Andesite c

1W.W 34.20 2.93 22.57 31.32 8.83 parent )I

39A1 = 39A2 + F o ? ~ + Augite + Ana + Op Calc. Residuals Amboo = Ambrr + Fon + Augite + Ans + Op = Calc. Residuals 7.64 parent r 1W.W 48.72 1.34 16.97 25.09

52.W 0.97

16.05 11.21 0.22 5.02 9.79 2.68 2.05

68.39 0.41

15.67 3.78 0.13 0.53 1.75 3.87 5.48

38.92 50.45 51.29 0.03 0.39 0.02 0.02 4.27 29.80

18.45 7.17 1.29 0.24 0.10 0.03

40.97 14.87 0.07 0.32 21.83 13.48 0.W 0.26 4.10 0.02 0.04 0.32

0.11 52.01 -0.01 8.98 1.03 -0.06 4.76 16.05 0.00

11.20 0.01 0.33 0.11 0.10 2.56 5.02 0.W 0.08 9.78 0.01 0.01 2.67 0.02

1.99 0.07 0.02

82.15

52.25 0.99

16.37 11.41 0.24 4.24 9.20 3.07 2.21

61.53 0.74

16.22 6.73 0.21 1.58 4.29 4.29 3.97

38.92 51.98 50.08 0.03 0.33 0.M 0.02 2.93 29.43

18.45 7.17 1.01 0.24 0.19 0.04

40.97 15.47 0.21 0.32 21.46 13.38 0.00 031 0.76 0.W 0.W 0.40

0.44 0.18 0.06 2.86 4.23 0.02 0.05 9.17 0.02 0.09 3.10 -0.03 n nz 7 0 5 n 16

152 C . PICARD ET AL.

( F O ~ ~ ) , plagioclase (An72) and clinopyroxene fractionation; and the second, formed by the more evolved basalts and characterized by a more subdued decrease of MgO while Feot, Tioz and C a 0 decrease, is due to olivine (F075-79), plagioclase (AI I~~) , clinopyroxene and minor Fe-Ti oxide fractionation.

The differentiated suites from HK basalts to HK andesites of the Lewolembwi maar and the HK differentiated suite formed by any samples of the APS sequence 2 appear very similar (trend 3 on Fig. 5; trend B on Fig. 8, and trend2 on Fig. 10) and are characterized by a subdued decrease of MgO while Sioz, KzO , Zr and La increase and FeOt, Ti02 and C a 0 strongly decrease. For both suites, chemical variations are principally controlled by plagioclase (An6548) , clinopyroxene, Fe-Ti oxides and minor olivine ( F m ) fractionation (Table 3e-d).

Magmatic evolution of the volcano Except for some samples characterized by very high [LaNbIn (C on Fig. S), the volcanic rocks of Ambrym volcano seem to be derived from two magmatic series. The older MK series ((LaNb)n = 2; trend 1 in Fig. 5; A in Fig. S), including the Vetlam-Tuvio-Dalahum and MK volcanic rocks of the APS (sequence 1 and some glasses of sequences 2 and 3), formed by c.25% melting of a spinel lherzolite source at a moderately deep level (at least 60 km). This suite may correspond to an ancient episode of magma generation probably related to normal subduction, which formed a magma chamber below these edifices in which magma progress- ively differentiated according to the fractionation scheme modelled in Table 3a. The second series ((La/Yb)n = 3-5; trends 2-3 in Fig. 5; B-C in Fig. S), formed by a minimum of 21% melting of a similar spinel lherzolite source at probably shallower levels (about 45-30 km), includes volcanic suites of the basal shield volcano, some glasses of sequences 2 , 3 and 4 of the APS, and volcanic rocks of the post-caldera suite (B on Fig. 8). This second suite may correspond to a more recent magmatic episode mainly related to rifting and possibly responsible for the formation of a second, less evolved magma chamber lying along the rift axis.

Extension and uplift accompanying the formation of the rift system, after the Vetlam- Tuvio and Dalahum MK episode, might explain the compositional evolution of the volcano. The giant eruption responsible for the APS and the formation of the caldera could have been caused by emplacement of hot, primitive basaltic magma of the rift-related phase into the

differentiated MK andesitic to rhyodacitic magma of the Tuvio-Vetlam-Dalahum magmatic chamber. The mechanical mixing of mafic and felsic glasses and the range in composition of the juvenile clasts observed in the APS are strong arguments favouring such a model. The intrusion of hot basaltic magma probably took place between the Tuvio-Vetlam-Dalahum edifice and the N100" rift axis, thus explaining the location of the caldera centre half-way between the two volcanic structures. Simul- taneous introduction of seawater into the edifice, related to regional doming and frac- turing following uprise of the large basaltic magma body to shallow depth and/or seismic events affecting the N100" fracture zone, in- duced phreatomagmatic activity as demon- strated by Robin et al. (1993) and contributed to the paroxysmal eruption. After the violent emptying of the differentiated magma from the old chamber, massive emission of basaltic magma from the replenished reservoir progress- ively dominated the volcanic activity. Introduc- tion of seawater during this period is responsible for the basaltic surtseyan-like deposits of sequences 2 and 3 of the APS (Robin et al. 1993). The decreasing role of water at the end of the caldera episode explains the gradual changes in the dynamics of the eruptions, which became more strombolian with time (APS sequence 4 and post-caldera volcanism). Since the caldera- forming episode, the current magma chamber, located below the rift, appears to have been fed regularly by primitive magma and was responsible for all the recent magmatism. Lastly, the differentiated suite associated with the Lew- olembwi maar and the 1986 lava flow in the western part of the caldera indicate that differ- entiation processes are presently active in small secondary reservoirs of the present chamber.

Conclusions Sources uplifted from at least 60-45 km depth during extension and doming accompanying rift formation, together with high degrees of melting (about 25%) of spinel lherzolite and possible K-La enrichment by mantle metasomatism or crustal contamination can explain the compositions of the different high-MgO primitive liquids on Ambrym volcano, whereas olivine, plagioclase, clinopyroxene and Fe-Ti fractionation explain the variations of most of the major and trace elements in the associated MK and HK volcanic suites. The tectonic evolution of the region, mainly controlled by the ongoing collision between the DEZ and the arc, seems to be responsible for the"geochemica1 variations of

c

a

4

W


the magmatic series and for the changes in the volcanic regime. Thus, interaction between MK and HK magmas from an ancient and a more recent reservoir was probably the major cause of the giant eruption leading to the caldera formation during the APS episode, assisted by massive introduction of seawater into the system. The more recent volcanism is principally related to rifting. This model, featuring controls by the tectonic setting and interaction between two magmatic series aided by infiltration of seawater, can explain the characteristics of the Ambrym basaltic complex.

This work was financially supported by ORSTOM UR lF, French Foreign Affairs Ministry (MAE) and the ‘Direction de la Recherche et des Etudes Doctorales’ (DRED). We thank C. Douglas for useful participation during fieldwork. C. Mortimer, Director of the Department of Geology, Mines and Rural Water Supply of Vanuatu, and C. Reichenfeld, Director of the ORSTOM Centre of Port-Vila, constantly supported our investigations. We also express our thanks to Tony Crawford, S. Eggins, J. Scarrow and J.L. Smellie who reviewed our manuscript and perfected our English. Finally, J. Cotten of the Université de Bretagne Occidentale in Brest is thanked for his dedication to providing excellent ICP analyses.

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