Volcanic evolution in eastern Papua

Twtonophysics,

87 (1982) 315-333

Elsevier Scientific

VOLCANIC

Publishing

315

Company,

EVOLUTION

Amsterdam-Printed

IN EASTERN

in The Netherlands

PAPUA

IAN E.M. SMITH Departmmt (Fmal

of GeoioR): University

version

received

November

of Auckland,

Private Bag, Auckland

(New Zealand)

4, 1980)

ABSTRAC’I

Smith,

I.E.M.,

1982. Volcanic

the India-Pacific Late Mesozoic tectonic

evolution

Plate Boundaries. and Cenozoic

in eastern

volcanic

events which reflect interaction

formations

of Upper

sea floor spreading

Cretaceous centers

spreading

in the Coral

Cenozoic

but shows

development magmas.

of thickened

and uplift associated this environment

and are thought

with crustal

basalt

(Editor),

are comparable volcanism

event.

crust/mantle Papua

The Evolution

of

The presence

activity

was prominent

An alternative

Basement

associated during

explanation

with the generation

the late Cenozoic

of Quaternar~

series of volcano-

sea plates.

to those characteristic

volcanic

interaction

during

a complex

and Solomon

type and&tic

to a subduction

of eastern tension.

record

during

crust and consequent

is now being replaced

Papua

to have originated

Arc-trench

environment

Packham

the Indo-Australian

and Eocene submarine

Sea basin.

In: G.H.

rocks in eastern

between

no clear relationship

The tectonic

Papua.

Tectonoph.vsics, 87: 315~333.

of with

the late links the

of andesitic

was one of block faulting

peralkaline

rhyolites

suggests

that

by active rifting,

INTRODUCTION

Eastern Papua has been an area of interaction between major crustal plates since the late Mesozoic and its geology reflects changes in tectonic environment from extension late

(sea floor spreading,

Mesozoic

and

Cenozoic

rifting) volcanic

to compression rocks

which

(abduction, dominate

subduction). the geology

The of the

Papuan peninsula and extend into the east Papuan island archipelagos provide one record of these tectonic events. This paper traces the volcanic evolution of eastern Papua and uses the succession of characteristic magmatic associations in the area as a key to unravelling the complex tectonic history of the Papuan peninsula and offlying

islands

(Fig. 1).

Tecronic setiirrg

Present day seismicity in eastern Papua is confined to a diffuse zone of mainly shallow seismicity extending along the northeast Papuan coast, through the D’En0040- I95 1,/82/~-~/~02.75

b 1982 Elsevier Scientific

Publishing

Company

Fig. I. Eastern Papua

trecasteaux Islands and eastward across the Solomon Sea (Johnson and Molnar, 1972; Curtis, 1973). Earthquake focal mechanism solutions suggest radically changing stress patterns within the adjacent plates over short distances (Ripper. 1982; Weissel et al., 1982). In the east the focal mechanisms are consistent with extension about an east-west axis; over-thrust solutions have been obtained in the central part of the area and there are indications of a southwesterly dipping seismic zone beneath the western end of the Papuan peninsula (Dent, 1976). To the east of the D’Entrecasteaux Islands the plate boundary traverses the Woodlark Basin which has been identified as a recent spreading center (e.g. Milsom, 1970; Luyendyk et al., 1973). In a recent inte~retation of available data Weissel et al. (1982) suggest that the ~ummencement of sea floor spreading in the Woodlark Basin has been time-transgressive, beginning prior to 3.5 m.y. in the east and at successively later times to the west, and that eventually the Woodlark plate boundary will propagate westward through the D’Entrecasteaux Islands into the Papuan peninsula. The interpretation of the Woodlark Basin as a recent spreading center is consistent with evidence for block faulting, minor rifting and major late Cenozoic uplift in eastern Papua and the D’Entrecasteaux Islands (Davies and Ives, 1965; Smith, 1970; Smith and Davies, 1976). Relative plate motion west of the Dawson Strait area is left lateral shear (Johnson and Molnar, 1972) with a component of convergence (Ripper, 1982). At least part of the tectonics of the area can be explained in terms of

317

diapiric

uplift of relatively

Papuan

ultramafic

Current dominated Davies,

low density

material

belt (cf. Ollier and Pain,

interpretations

of

early

by the concept

of overthrusting

Emplacement

tectonism

Papuan

and

ultramafic

Timing

Jaques,

1971; Hamilton,

of the Papuan

belt clearly occurred

ultramafic

in response

eastern

are

belt

(e.g.

other models

belt was presumably

block and island arcs lying New Guinea (cf. Hamilton, the emplacement

to a convergent

Papua is uncertain.

Papua

ultramafic

1979) although

1980). In this interpretation

of the event in eastern

in

of the Papuan

linked to the collision between the Australian continental to the north which is recorded in the geology of northern 1979; Johnson

by the thrust sheet of the

1980).

Cenozoic

1971, 1977; Davies and Smith,

have been suggested.

overridden

tectonic

of the regime.

Davies ( 197 1, 1977) and Davies

and Smith (1971) among others, have suggested on geological grounds that emplacement of the Papuan ultramafic belt took place early in the Eocene. However, this evidence is not conclusive and a later date is indicated by the timing of events elsewhere in Papua New Guinea (e.g. Johnson and Jaques, 1980). The late Eocene and early Oligocene was a time of major tectonic upheaval and metamorphism in northern New Guinea, and similar events in eastern Papua may be correlative. The evidence from DSDP holes on the Queensland plateau and in the Coral Sea basin suggests that the basin opened during the early Eocene (Mutter, 1975) rather than the late Eocene-early Oligocene (Gardner, 1970; Davies and Smith, 197 1). Deformation of Eocene sediments in the Coral Sea basin during late Eocene-Oligocene (Mutter, 1975) is logically associated with a compressive event. Volcanic rock associations The volcanic temporal comparable world.

to associations

petrographic island

and

associations

identified

geochemical

alkali-rich

and

Papua have spatial

of southeastern

tectonic

significance

features

can be grouped

in southeastern

evolution

in similar

has tectonic

arc and intra-plate

Ocean-floor

recognised

in the volcanic

Each association

volcanic

in eastern Papua

rock associations

significance

times

settings

under three main headings, volcanic

and

and are

in other parts of the

and is recognised by distribution

Papua

by characteristic

of rock namely,

types.

The

ocean-floor,

rocks.

volcanic rocks

Tholeiitic volcanic rocks occur widely in the late Mesozoic and early Tertiary basement formations of the Papuan peninsula. In the western part of the peninsula late Mesozoic basalts overlie peridotites of the Papuan ultramafic belt (Davies, 197 1). Field relationships and available geochemical data are consistent with Davies’ (1971) interpretation of these rocks as former oceanic crust thrust southward as part of an ophiolite sheet. The volcanic rocks are thought to have originated at a late Jurassic,

or early

Cretaceous

oceanic

spreading

center

(Hamilton,

1979) and

are

allochthonous

in their present

geologic setting.

To the south of the Papuan

ultramafic

volcanics

(Fig. 2) which has been referred

although

the volcanic

Goropu

Mountains

ultramafic sheared

rocks

belt lies a belt of predominantly to as the Milne ophiolite

rocks do not show an association Cretaceous

basalt with minor

of the Papuan

and metamorphosed

ultramafic

up to greenschist

(Hamilton.

with ultramafic

microgabbro

belt. These

basaltic 19791

rocks. In the

and gabbro

underlie

have been

variabt?

rocks

facies: the presence

of sporadic

blue

amphibole and lawsonite in some specimens from the northern Goropu Mountain:, is indicative of comparatively high P/T conditions (Davies, 1980). To the south ol the Goropu Mountains lies an extensive area (> 5000 km’ ) of volcanic rocks which on the basis of fossils present

in sporadic

intercalated

sediments

are partly

Upper

Cretaceous and partly Middle Eocene. Over 70% of this formation consists of basalt> and the remainder is made up of microgabbro (25%). minor gabbro and associated differentiates.

and

pelagic calcareous

sediment. sediment

Pillow

structures

and

the presence

show that these east Papuan

basalts

of interbedded

are submarine.

West of the Goropu Mountains a large sheet of basalt. microgabbro and gabbro mapped by Yates and De Ferranti (1967) as an Oligocene intrusive mass. has been reinterpreted by Hamilton (1979) as a part of the late Cretaceous Eocene Milnr ophiolite. This latter interpretation is consistent with the notion of a belt of late Cretaceous and Eocene submarine basalt and associated rocks lying to the south of the Papuan ultramafic belt and in part separated from them by Mesozoic metasedimentary rocks-the Owen Stanley metamorphic belt (cf. Davies and Smith, 1971). Small basement

0

Fig. 2. Volcanic

100

inliers within

a thick late Tertiary

sequence

w

200

rock associations

sedimentary

in eastern

Papua.

on Cape

Vogel peninsula

are made up of pillow lavas, lava flows and inter-bedded

are predominantly position

(Smith

(Dallwitz

of basaltic and

composition

Davies,

1976).

et al., 1966) indicates

possibility

exists

that

formations

in eastern

they

but include

A single

whole

rock

that these rocks are Upper

are

comparable

tuff which

rocks of intermediate K-Ar

determination

Oligocene

in age to other

com-

although

basement

the

volcanic

Papua.

The volcanic basement of Woodlark Island off the northeastern side of the Papuan peninsula is also made up of pillow lavas with associated tuff and thin bedded agglomerate (Trail. 1967). These volcanics Miocene limestone but apart from this relationship, spatially

remote

from the Mime ophiolite:

form basement beneath lower their age is unknowI1. They are

their significance

lies in the fact that they

are the oldest rocks exposed on the Woodlark Rise and so provide some insight into the nature of the crust to the northeast of the Papuan mainland. The petrography of the basaltic rocks of the Mime ophiolite and the Woodlark Island basement can be generalised from published descriptions (Trail, 1967; Pieters, 1974; Smith and Davies. 1976). Fresh specimens consist of labradorite, typically An sO_sz(40--60~). chnopyroxene (20-30s) and iron-titanium oxides (5%); divine is a minor phase in some specimens. monly contain the assemblage glaucophane and lawsonite. Representative Mime

ophiolite

analyses and

from

Metamorphosed basaltic specimens chlorite-albite-epidote-actinolite

of basaltic

rocks from the central

the Woodlark

Island

basement

most comwith rare

and eastern

parts of the

are given

in Table I:

further geochemical data is available in Smith and Davies (1976) and Smith ( 1976). As a group the rocks show variations in composition that can be explained by processes of secondary alteration, metamorphism and fractionation. General characteristics unaltered ranging (l-2%)

are relatively low Al,O, (typically 13--140/o), low K,O (less than 0.2% in specimens). The rocks are all partly oxidised with Fe,O,/FeO ratios from 0.3 to 0.9. TiO, contents

to relatively

high values (2.5-3s)

range from typical

tholeiitic

in highly fractionated

of the trace-element abundances is the extremely elements (Rb, Ba, La, Ce, U, Th, Pb); abundances

basaltic

specimens.

values

A feature

low content of incompatible of these elements greater than

average can be correlated with alteration or metamorphism. Sr typically lies within the range 100-200 p.p.m. and values significantly outside this range can be related to secondary alteration. Chondrite normalised La, Ce and Y abundances indicate typically unfractionated REE abundance patterns (Fig. 3); this is supported by other available REE data (Smith 1976). The east Papuan basalts are tholeiitic on the basis of normative mineralogy, relative alkali content, Fe-Mg variation and trace element ratios (Fig. 3). The low Al 2O, and incompatible element content. and high Na,0/K20 (generally > IO but lower in clearly altered specimens) and K/Rb (300- 1100) ratios are comparable to oceanic basalts as described by Engel et al. (1965) and Kay et al. (1970). REE, abundances are higher than those attributed to island-arc tholeiites (Fig. 3). On the

99.80

99.72

Total 99.83

99.92

0.13

0.00

I

0.1

0.19

co2

0.76

1.18

0.52

0.96

H,O-

0.19

9.13

0.11

6.11

7.87 12.04 3.52

0.22

0.21

0.32

8.62

2.22

5.15

5.75 6.41

0.05

13.34

1.79

1.29 13.49

48.86

48.00

577

4

---

99.57

0.00

0.57

1.42

0.21

0.42

2.46

10.64

7.09

0.20

7.08

4.11

14.56

1.80

49.0 1

576

5

of the Milne ophiolite.

I .78

1.71

I .48

-

564

3

specimens

1.21

0.10

0.12

H20+

K2O

p205

Na,O

2.58

8.07

Il.33

7.47

11.66

WJO CaO

0.24

0.22

0.2 1

MnO

2.46

8.05

0.09

4.21

5.22

13.73

Al 2%

6.94

13.29

1.34

Fez4 Fe0

1.47

47.85

SiO,

48.00

2

563

1

566

of representative

TiO,

wt.%)

No.

Major and trace element analyses

TABLE I

99.73

0.33

0.15

99.73

99.88

0.39

0. I4

0.17 0.06

2.89

0.20

99.26

0.04

0.2X

2.74

0.19

3.69 0.09

2.87

10.21

7.02

0.18

7.25

4.32

13.46

1.78

48.0 1

584

Q

0.71

10.17

6.71

0.20

7.06

4.97

13.81

1.-I8

47.92

587

8

3.06

0.19

0.20 3.11

0.14

0.90

3.81

9.82 2.45

6.86

7.33

0.18

7.04

4.6 I

14.39

1.75

47.45

585

7

Papua

IO.76

0.19

7.06

5.09

14.10

I .95

46.11

586

6

southeast

IO

99.40

0.20

0.88

0.26

0.12

0.07

2.44

12.78

6.21

0.18

6.27

4.54

14.84

1.34

49.21

-.-_II__-____

579

115

8

Sr

Pb

A.N.U.

6

119 17

96 17

1-4 unaltered

19

83

130

69

170

285

36

16

7

28

7

120

(5

<5

9

215

4

49

5-6 altered

and gravimetry

specimens,

(Na,O)

55

66

52

401

167

101

131

382

48

9

43

4

5

31

6

89

<5

(5

2

24

4

69

(5

8

151

121

(5

5

16 1

(5

(FeO), flame photometry

15

of Geology,

by XRF, titration

in the Department

* Analyses

16

Ga

101

95

156

Zn

361

V

Cr

95

44

SC

234

227

3

La

Ce

89

458

3

Y

162

49

24

Nb

cu

1

67

4

15

U

Zr

Ni

21

3

is

51

4

(5

7

128

5

<5

(5

Th

1

-i5

Rb

Ba

( PPm) 1

, CO,).

IS

109

90

62

145

328

37

10

4

28

6

99

<5

<5

7

181

<5

Specimen

7-9 metamorphosed

H,O

specimens,

(H20+,

18

117

103

58

191

383

43

10

4

32

9

129

(5

<5

8

173

21

40

specimens,

numbers,

18

118

98

65

230

360

45

11

4

31

6

110

<5

<5

8

148

15

26

c5

-

-

-

-

-

-

-

-

-

-

10 Woodlark

Island.

Prefix 33 refer to material

17

111

106

43

194

330

39

8

4

28

4

105

(5

<5

<5

105

(1

housed

m:d-Ailanttc

mid-Atlantic

Japonesetsland tholeiltes

arc

li; r

Fig;. 3. Characterisation

(a) Normative

of tholeiitic

mineralogy,

basalts

in eastern

Fez0,/FeO=0.2,

altered

Papua. and fractionated

samples

omitted.

fb) F.M.A. (c) Totai dkalies/SiC$ fd) ka,/Y,. (e) Chondrite tholeiires (f) Ternary

(after Macdonaid

normalised

from Japan

plots Ti-Zr-Sr

LKT =iow potassium Tiwrich basaltic

REE

(Masuda

of east Papuan and Aoki.

and TX&-Y

tholeiites,

basalts

(Smith

et at., 1982) compared

after Pearce and Cann (1471,

the defined basalt,

1964). with island-arc

1978).

CAB =calc-atkah

rocks plot outside

Open squares = metamorphosed

and Katsura.

basal&, fields.

solid circles=

1973). UFB =oeean

WPB = within plate basalts.

basalt.

Boor basalt,

Highfy fractionated

323

basis of these geochemical the basaltic

rocks which

ocean basins-the abyssal (Bryan et al., 1976). Typical

members

criteria

the Papuan

form a major tholeiites

(Engel

of the volcanic

basalts

component

basement

are considered of sample

to

from the

I ocean

et al.. 1965) or group inliers

comparable

collections

basalts

on Cape Vogel peninsula

are

made up of labradorite and clinopyroxene with subordinate iron-titanium oxides: olivine or more commonly quartz is accessary (Smith and Davies, 1976). A minor proportion

of the Cape Vogel rocks are clino-enstatite-bearing

(Dallwitz

et al., 1966).

The volcanic rocks of Cape Vogel peninsula differ from those of the Mime ophiolite in showing a greater SiO, range, higher Al,O, and Fe,O,/FeO. and lower TiO?; trace element abundances are not presently available. These volcanics are tholeiitic in character although

compositionally

distinct

from

those of the Milne ophiolite; Cameron et al. (1979) have suggested that they may be termed boninites. The use of the term boninite causes some confusion because although boninites are almost exclusively associated with ophiolites (Cameron et al., 1979) their type area is an island arc-the Bonin Islands. However, since the Cape Vogel volcanics may be younger than the basaltic basement elsewhere in eastern Papua it is possible that they represent a transitional magmatic early Tertiary sea-floor basalts and the island-arc type andesitic characterise

episode between volcanics which

the late Cenozoic.

Islund arc volcanic rocks Tonalite and diorite of Eocene age (SO-55 m.y. based on K-Ar determinations) intrude the northwestern part of the Papuan ultramafic belt (Davies, 1971). Eocene andesitic volcanics the northern end Jaques

unconformably overly basaltic volcanics of the ultramafic belt at data (Davies, 1971; of the complex (Davies, 1977). Available

and Chappell,

interpreted

1980) show these rocks to be of arc-trench

to represent

the products

of a northward

dipping

type and they are arc-trench

system

active during the Eocene prior to the emplacement of the Papuan ultramafic belt. A major episode of arc-trench type volcanism commenced during the middle Miocene and has continued to the present time producing the east Papuan volcanic province. In the earlier part of this episode activity was partly submarine but during the Pliocene and Quaternary, activity became entirely subaerial; this reflects the emergence of eastern Papua as a landmass during the latter part of the Cenozoic (Smith, 1970). Volcanic rocks of of the Papuan peninsula and Louisiade Archipelago (Pieters, oldest rocks in the province are

the east Papuan volcanic province extend the length occur in the off-lying islands as far east as the 1974; Smith, 1976a; Smith and Davies. 1976). The those associated with mid-Miocene intrusive rocks

(Smith, 1972) to the southwest of Milne Bay at the eastern tip of the peninsula. Upper Miocene volcanic rocks are recorded from both eastern and western ends of the province

(Pieters,

1974; Smith,

1976a); Pliocene

and Pleistocene

volcanism

was

324

widespread available

over much radiometric

of the area. The pattern dates suggests

eastern

part of the area, although

central

part of the province

Although

the volcanic

show a continuous

of eruptive

that arc-trench

eruptions

activity

type activity

during

together

with

has ceased in the

the past 100 years show that the

is still active. rocks of the late Cenozoic

spectrum

of compositions

east Papuan

(Johnson

volcanic

province

et al., 1978a), variations

in

the temporal pattern of volcanic activity and in the fine compositional details allow a division into two partly overlapping volcanic belts referred to as the northern and southern

volcanic

belts respectively

(Fig. 2).

The southern volcanic southeast Papuan coast

belt comprises volcanic as well as minor lava

associated

sediments

volcanic volcanic areas

with Pliocene

rocks which are exposed on the flows and pyroclastic interbeds

in the Musa

Valley

(Smith

and Davies,

1976)

rocks on Managlase Plateau (Ruxton, 1966; Smith and Davies, 1976) and cappings in the Owen Stanley Range (Pieters, 1974; Blake, 1976). These

define

a gently

curving

trend

extending

southeast

from

the Owen

Stanley

Range. Small plutons in the Milne Bay area which are comparable in chemical composition to nearby volcanic rocks and which are interpreted as intrusive equivalents to them, have yielded dates of 12 to 16 m.y. (Smith, 1972). These ages are supported by the presence of volcanic detritus in Miocene sedimentary rocks which outcrop in the extreme southeast of the Papuan peninsula (Smith and Davies, 1976) and which were penetrated by two exploration holes drilled on the Trobriand platform to the north of the peninsula (Stoen and Garside, 1973). In the Musa Valley area basaltic rocks have been dated as mid-Pliocene 1966; Smith and Davies, ridge cappings

and valley

Blake, 1976). Geochemical

1976) and further fill deposits

west lavas of Recent

in the Owen Stanley

data on rock types in the southern

age are found

Range

belt is limited.

(Ruxton,

(Pieters,

as

1974;

Major and trace

element data are available from the eastern end of the belt (Kesson and Smith, 1972; Smith and Davies, 1976; Table II) but only limited major element data are available from the western

part (Ruxton,

1966; Blake,

1976; P.E. Pieters,

unpubl.

analyses,

1974). The most common rock type in the southeast is a basaltic rock high in CaO, K,O, Ba and Sr but characteristically relatively low in TiO,, Zr, Nb and Ni. Rock types in the west show more variation in major elements but in general these too are relatively low in SiO, and have high K,O/Na,O ratios. Available 87’86Sr data show scattered values in the range 0.7036 to 0.7049 (Smith, 1976a). In earlier work the therm shoshonite association has been applied to these rocks because of their characteristically high K,O contents (Kesson and Smith, 1972; Smith and Davies, 1976). The characteristic low TiO,, Zr, Nb, Ni and Cr of the high-K basaltic rocks in southeastern Papua is shared by the basaltic rocks associated with andesites in the northern volcanic belt. The main differences between the volcanic rock suites in

325

TABLE

II

Major and trace element in southeastern No.

Papua

analyses

of representative

specimens

of late Cenozoic

arc-trench

type volcanism

*

1

2

3

4

5

6

7

8

597

589

596

600

650

653

660

673

(wt%) SiO,

46.83

47.90

50.20

59.00

52.80

53.60

55.45

TiO

0.73

0.62

0.97

0.55

1.26

I .08

1.18

0.87

15.55

10.20

19.10

17.80

18.00

IX.51

17.84

16.02 2.95

Al ztJ1

64.63

Fe@, Fe0

5.78

5.85

4.20

4.15

2.43

2.45

2.49

4.55

4.55

2.85

0.90

4.40

5.30

3.84

1.46

MnO

0.18

0.16

0.13

0.06

0.13

0.14

0.1

I

0.09

MgO CaO

5.76

10.60

3.95

1.90

5.01

3.99

3.93

1.51

9.75

12.50

7.20

4.30

9.17

8.28

7.71

4.02

Na,O

2.40

1.40

2.85

4.15

3.80

3.43

4.00

4.60

K,O

3.58

2.25

3.75

4.10

I .27

I .66

I .67

2.76

P,O,

0.56

0.46

0.69

0.38

0.39

0.31

0.30

0.27

“20

2.86

2.10

1.45

1.Ol

0.37

0.55

0.9

I

0.23

H,O-

0.25

1.09

1.59

1.17

0.42

0.2 1

0.33

0.62

co2

0.69

0.47

0.69

1.11

0.00

0.0

1

0.08

0.03

99.47

100.15

99.62

100.64

99.45

99.84

100.06

Total

99.52

(PPm)

Ba

114

482

1223

705

519

582

586

Rb

81

37

85

142

23

20

36

71

846

616

205 1

935

666

941

594

486

56

32

9

13

16

21

9

Sr Pb

20

9

Th

(5

<5

U

<5

(5

Zr

13 <5

(5 (5

<5

<5

(5

153

161 5

42

35

200

Nb

3

2

8

Y

12

12

16

15

825

9

5 <5

<5

174

189

201

4

5

4

22

19

21

30

La

8

8

39

27

26

22

23

52

Ce

16

12

82

42

47

39

50

65

SC

27

50

12

9

25

21

19

9

V

321

280

209

103

186

198

153

85

Cr

108

398

36

33

46

Ni

30

98

26

27

37

(5

43

3

26

8

CU

181

115

187

28

49

56

28

II

Zn

81

70

76

56

45

75

73

61

Ga

15

11

19

I2

18

19

20

17

methods

as in Table I. Specimen

* Analytical Department

of Geology,

A.N.U.

1-4 southern

numbers, volcanic

Prefix

33 refer

to material

belt. 4-8

northern

volcanic

belt.

housed

in the

326

northern

and southern

basaltic than

belts appear

to be that

rocks which are lower in Al,O,

those of the northern

compositions

of these

and

the southern

and generally

belt. Nevertheless,

detailed

other

rock

comparable

(Johnson

et al.,

1978a)

has shown

that

southeast

Papua

and in the New Guinea

high-K

belt contains

mainly

have higher K20,/Na,0 analysis

types

in Papua

basaltic

Highlands.

rocks

to Recent

element

New

such

Guinea

as those

are trapsitional

composition to high-K andesitic suites. Predominantly andesitic volcanoes of late Miocene

ratios

of major

in

in chemical

age forming

a belt

from the northeast Papuan coast through the D’Entrecasteaux Islands to the Louisiade Archipelago constitute the northern volcanic belt. The andesitic volcanic rocks are found in six distinct areas namely the western end of the Calvados chain. Egum Atoll, Normanby Island. Amphlett Islands, Moresby Strait area and northeast Papuan coast. Activity on the mainland appears to have been essentially a Quaternary phenomenon and is continuing at a moderate level at the present time. Volcanic activity earlier activity

in the D’Entrecasteaux (late Miocene in the western

rocks are late Miocene. andesitic

activity

Islands,

to Quaternary)

Amphlett

Islands

part of this area. In the extreme These ages provide

through

and

and there is geomorphic southeast

some evidence

the late Cenozoic

similar

Egum

Atoll

evidence

was

of Recent

of the belt volcanic

for westward

to that observed

migration

of

in the southern

volcanic belt. All of the centers of the northern volcanic belt have erupted andesite; in most centers andesite is accompanied by basaltic andesite with or without subordinate basalt and dacite. Rhyolite is locally abundant in the Moresby Strait area, and on Managlase plateau. Considered as a whole the rocks form an andesite association characterised by high alkali contents and comparatively high K,O/Na,O ratios. Incompatible trace elements are typically high, ferro-magnesian trace elements show wide variations but are also high relative to oceanic crust; *‘/s6Sr ratios show a close

5-

5102

45-50wt.%

‘ _ 510~

.

. .

3-

.

I

NORTH axis of Trobrland Trough

I

1

150 200 kilometres

.! 8 l @ ;i

2-

t

. . . I 100

.

55-60wt.%

8 8 t 8

a

lI

1

250

300

I NORTH ax is of Trobriand Trough

I

100

1

I

150 200 kilometres

I

I

250

300

Fig. 4. Variation in K,O content of late Cenozoic volcanic rocks across southeast Papua.

327

grouping

around

comparison island

an

average

with continental

chain andesites

If the positions

of 0.7042 margin

characteristic

of volcanic

allow for late Cenozoic

extension

all lie within

ward from Mount volcanic belt. This

Lamington paired belt

volcanic

sandman)

colcmism

data

rather

indicate

than

a

with the

volcanic

belt are adjusted

in the eastern part of the area (e.g. Luyendyk 25 km of a curvilinear and running is considered

which can be represented

Quufernaty

These

type andesites

in the northern

arc and as such it shows a weakly

polarity

1976a).

of many West Pacific arcs.

centers

1973) these centers

(Smith.

trend extending

et al..

southeast-

essentially parallel to the southern to represent a single late Cenozoic developed

but systematic

in terms of K,O variations

in the Lusuncqy

to

geochemical

across the arc (Fig. 4).

Islands

The Lusancay Island group is part of a reef complex lying on a basement high (the Woodlark Rise) between the Trobriand platform and the Trobriand Trough which marks the southern boundary of the Solomon Sea basin. Islands on this basement high are composed mainly of corai limestone although aeromagnetic data (C.G.G.. 1973) indicate the presence of near surface basement. Volcanic rocks are exposed on a few small (< 0.5 km2) islands in the group. These have been dated by K-Ar techniques as 1-2 m.y. old (Smith, 1973). The volcanic rocks in the Lusancay Islands have SiO, contents dacites

typical

within

the field of trachytes

striking

of arc-trench

differences

al., 1979). Initial are slightly

defined

in trace-element K7Sr/XhSr ratios

higher

than

(0.7042 i- 0.0003). The relationship Papuan

systems

by Johnson contents

et ai. (1978a).

compared

of the Lusancay

those for many

between

comparable

but are much richer in total alkalies to typical

trachytes

Further. dacites

(0.7044,

the Lusancay

trachytes

and the andesitic

arc 70 km to the south is unknown

although

because

there are (Smith

Smith,

of the rocks in the northern

of isotopic

to

and plot et

1976a)

volcanic

belt

rocks of the similarities

it has been suggested that they may be derived from similar sources (Smith et al.. 1979). Nonetheless because the Lusancay trachytes are contemporaneous with the andesitic volcanoes their existence has important implications for the volcanic evolution fntm-plate

of eastern alkali-rich

Papua. volcanic rocks

A suite of Quaternary peralkaline rhyolites outcrops in the Dawson Strait area of the D’Entrecasteaux Islands. These rhyolitic rocks are predominantly fragmental but include a small proportion (about 5%) of glassy and crystalline comendite. Boulders and inclusions of basaltic and intermediate rocks are associated with the rhyolitic rocks. There are well preserved eruptive centers in the Dawson Strait area suggesting recent activity,

32x

possibly

within

The

the last 600 yrs (Taylor

Dawson

Strait

basalt-peralkaline

comendites

rhyolite

in Davies,

which

association

1973).

appear

overlap

to form

rocks of the western

D’Entrecasteaux

have been described

by Smith (1976b) and are typical

with transitional continental

and mildly

Islands

alkaline

basaltic

part

of a transitional

in time with the Quaternary and east Papuan

mainland.

of those found

andesitic The rock5

in association

rocks in the ocean basins

and in some

rift structures.

Volcanic evolution Volcanic

in southeast Papua

associations

in eastern

Papua

define

a pattern

of evolution

from

geochemically primitive magma types to highly evolved arc-trench type and alkalirich magmas. It can be argued that in fact the volcanic rocks which now make up southeastern Papua have developed not at one plate boundary but at several. Nevertheless it is suggested that the sequence of volcanic rocks observed in eastern Papua is representative of the variety of magmas which can develop in a single complex zone of interaction between major crustal plates. The basement volcanic rocks in eastern Papua are tholeiitic submarine basalts comparable to those of the ocean basins. It is logical to link eruption of these rocks with sea floor spreading in the area which is now the Coral Sea basin. The age of these

rocks

middle

is not

Eocene

tholeiites

well known

basaltic

are allochthonous

moved into spatial

and

rocks indicates

association

ing in the Coral Sea basin. emplacement of the ultramafic

the presence

of both

Upper

a complex

divergent

event. These basement

to the present

east Papuan

with the Papuan This interpretation belt.

ultramafic

plate

Cretaceous

boundary

and

and were

belt by sea floor spread-

is independent

of the

time

of

At present there is uncertainty regarding the age, magmatic affinity and tectonic position of the volcanic rocks on Cape Vogel peninsula. Although clearly different from the late Cenozoic arc-trench arc-trench

arc-trench

type volcanoes

the possibility

that they may have

affinities blurs the otherwise clear temporal separation of late Cenozoic type volcanism from earlier volcanic activity linked to sea floor spread-

ing. Detailed work on the Cape Vogel volcanics Walker, A.N.U.) may resolve this problem.

which is currently

in progress

(D.

Arc-trench type volcanic activity in the east Papuan arc commenced during the middle Miocene with eruption of the high-K rocks at present forming the eastern end of the southern volcanic belt. Typically andesitic volcanism started at the eastern end of the northern volcanic belt in late Miocene times. Regional compositional and temporal variations define a volcanic arc in which there has been migration of activity westward. The observed increase of potassium and related elements southward defines a geochemical polarity within the arc albeit a weakly developed one. Further, the high-K rocks to the south are in fact older than the rocks in the north and this is contrary to the trend predicted by some models of

island

arc evolution

Andesitic associated

with

lithosphere.

comparable

Benioff

Current

component material

(e.g. Jakes and White,

rocks

to those

zones

and

petrogenetic

of this lithosphere (e.g. Ringwood,

1972). in eastern

Papua

by implication

with

models

for andesitic

with, or without,

subduction

rocks invoke

admixture

1977). There is little evidence

beneath the Papuan peninsula, arcs elsewhere, the hypothesis

are elsewhere

nevertheless by analogy. that andesitic volcanoes

of oceanic recycling

of mantle for present

typically

and/or

of a crustal

day subduction

with active andesitic volcanic in eastern Papua are related

to a subduction event should be a reasonable one on which to base a paleotectonic history. If the origin of andesitic magmas is a direct consequence of subduction. then continuing andesitic activity in eastern Papua (recorded eruptions in 1890’s, 1943. 1951; Smith, ceased

1982) may be explained

and

that

present

activity

magmatism.

The geochemical

subduction.

Following

underly

the volcanoes

the surface

by the argument

represents

polarity

Dickinson

that subduction

a final

in eastern

burst

to the north

(1975) a southward

of the Woodlark

of subduction

Papua indicates dipping

at a depth of 150 km and, assuming

has recently

southward

subducting

related dipping

slab would

a dip of 55” would reach

Rise in the Trobriand

Trough.

In essence

this is Karig’s (1972) interpretation of the area; Hamilton (1979) also suggests that the Trobriand Trough is a feature related to slow or recently inactivated subduction. The model of the late Cenozoic subduction dipping southward beneath the Papuan peninsula encounters of the fact that since volcanic m.y. the episode apparent

cannot

geochemical

In thermal genetic

models

of relatively hypotheses

have been

polarity

reverse of that predicted thrusting

a number of important difficulties particularly in view activity has so far extended over a period of at least 16 a minor

is weakened

by subduction

because

based

upon

by the fact that age relationships

controlled

for active subduction cold oceanic

one. The argument

are the

models of island arc development.

isotherms

crust. This feature

it allows subsolidus

the

are depressed

by the under-

is important

to some petro-

dehydration

of the subducted

slab

but it also predicts that the area adjacent to the trench will be relatively cold. If southward dipping subduction existed beneath southeastern Papua and gave rise to late Cenozoic arc-trench type volcanoes the presence of Quaternary volcanoes requiring relatively high heat flow in the Lusancay Islands adjacent to the Trobriand Trough is anomalous. It has been suggested (Davies, 1977) that the east Papuan volcanoes are related to northward dipping subduction from the Moresby Trough. Northward dipping subduction resolves the dilemma of the Quaternary volcanism in the Lusancay Islands. The unusual trace-element abundances in the Lusancay Island trachytes resemble those predicted as characteristic of high pressure melts of subducted oceanic basalt although Smith et al. (1979) argue that oceanic basalt could not have been their source. However, apart from this there is no evidence to support substantial northward dipping subduction beneath eastern Papua during the late Cenozoic.

The east Papuan arguably formed

peninsula

the result early

is surrounded

of tensional

tectonic

in the Tertiary;

spreading

center.

spreading

although

Cenozoic,

interaction

eastward

The Solomon data

on three sides by young events.

the Woodlark

basin

Sea may also be the result

to substantiate

between

To the south

this is lacking.

the Australian

oceanic

the Coral

is a currently of Tertiary

At least

and Pacific plates

during

basin4

Sea basin active

bea floor the late

has been predomi-

nantly along the outer arc system stretching southeastward from Bougainville Island. It is argued that in this context of late Cenozoic regional extensional tectonics a model involving justify.

a major episode

of subduction

beneath

eastern

Papua

is difficult

to

Because evidence for late Cenozoic subduction beneath eastern Papua is lacking, Johnson et al. (1978b) suggested that the Miocene-Recent arc-trench type volcanics in the area may have originated by delayed partial melting of mantle modified during an earlier subduction event. The emplacement of the Papuan ultramafic belt is widely recognised as the result of the freezing of a northward dipping subduction zone by collision with sialic crust (e.g. Davies, 1977). Andesitic volcanoes in eastern Papua are closely associated with. or lie to the south of a discontinuous belt of ultramafic rocks extending east from the Papuan rocks represent the outcrop of a lower Tertiary position of the volcanoes from this zone.

cannot

be related

ultramafic belt. If the ultramafic subduction/abduction zone the

to a subducted

slab dipping

northward

Clearly the east Papuan volcanic arc is not readily explained by contemporaneous subduction nor can it be related to subduction linked to the only Tertiary compressive event for which there is good evidence, namely, emplacement of the Papuan ultramafic belt. However, the east Papuan region has suffered very complex events during the Cenozoic and it is suggested that in complex situations of this kind, subduction as normally envisaged would be a gross simplification. One effect of abduction leading to the emplacement of the Papuan ultramafic belt has been to produce a strip of crustal material estimated to be at least 25-30 km in thickness (Milsom and Smith, 1975; Drummond et al., 1979). This strip lying between the Coral Sea and Solomon Sea basins is exposed as the Papuan peninsula and offlying islands. The presence of high grade metamorphic rocks including eclogites in the D’Entrecasteaux Islands (Davies and Ives, 1965) indicates that crustal rocks have been subjected to relatively high pressures and temperatures. Non-peralkaline late Cenozoic rhyolites on western Fergusson Island may represent crustal melts (Smith, 1976a) and thus provide further evidence of relatively high crustal temperatures. In eastern Papua an alternative to conventional subduction leading to the source for andesitic magmas is that tectonic thickening of crust has provided the means whereby lower crust and upper mantle material can interact to produce a geochemitally evolved source which may then produce andesitic melts. The upper mantle diapirism proposed on the basis of gravity anomalies beneath

331

the eastern

tip of the Papuan

in the process.

peninsula

The details

more geochemical

by Milsom

of the process

contortions

and Smith (1975) may play a role

remain

than petrogenetic

to be worked models

out but require

which require

no

conventional

subduction. Throughout acterised

the late Cenozoic,

by block

extensional Dawson

faulting

tectonics. Strait

the geology

accompanying

The

presence

of eastern

major

vertical

of Quaternary

area of the D’Entrecasteaux

Papua

movements,

peralkaline

Islands

has been

suggests

char-

indicating

volcanism

a change

in the

to an active

rifting environment by analogy with the geological setting of peralkaline volcanoes elsewhere (Smith et al., 1977). Thus the sequence of magma types in southeast Papua provide support for the suggestion by Weissel et al. (1982) that the Woodlark spreading center is propagating westward on to the Papuan mainland. The tectonic development of eastern Papua during the Cenozoic has been extremely complex and many of the details remain to be worked out. Magmatic evolution in the area appears to have been sensitive to tectonic changes imposed by changes in the relative motion of adjoining crustal plates. Eastern Papua provides an example of a complex west Pacific arc in which several distinctive magmatic associations are juxtaposed and in which island arc type volcanism replaced by intra-plate alkaline volcanism.

is currently

being

ACKNOWLEDGEMENT

This work forms part of a regional study of Papuan Australian Bureau of Mineral Resources and carried

volcanoes supported by the out in the Department of

Geology, Australian National University. The paper has benefitted constructive reviews by T.H. Green and R.W. Johnson.

greatly

from

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