The Petrology and Geochemistry of Volcanic Rocks on Jeju Island ...

JOURNAL OF PETROLOGY

VOLUME 46

NUMBER 3

PAGES 523–553

2005

doi:10.1093/petrology/egh087

The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin YOSHIYUKI TATSUMI1*, HIROSHI SHUKUNO1, MASAKO YOSHIKAWA1,2, QING CHANG1, KEIKO SATO1 AND MOON WON LEE3 1

INSTITUTE FOR RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN AGENCY FOR MARINE–EARTH SCIENCE

AND TECHNOLOGY ( JAMSTEC), YOKOSUKA 237-0061, JAPAN 2

INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 974-0907, JAPAN

3

DEPARTMENT OF SCIENCE EDUCATION, COLLEGE OF EDUCATION, KANGWON NATIONAL UNIVERSITY,

CHUNCHEON 200-701, SOUTH KOREA

RECEIVED OCTOBER 31, 2003; ACCEPTED OCTOBER 1, 2004 ADVANCE ACCESS PUBLICATION NOVEMBER 24, 2004 The incompatible element signatures of volcanic rocks forming Jeju Island, located at the eastern margin of the Asian continent, are identical to those of typical intraplate magmas. The source of these volcanic rocks may be a mantle plume, located immediately behind the SW Japan arc. Jeju plume magmas can be divided into three series, based on major and trace element abundances: high-alumina alkalic, low-alumina alkalic, and sub-alkalic. Mass-balance calculations indicate that the compositional variations within each magma series are largely governed by fractional crystallization of three chemically distinct parental magmas. The compositions of primary magmas for these series, using inferred residual mantle olivine compositions, suggest that the low-alumina alkalic and subalkalic magmas are generated at the deepest and shallowest depths by lowest and highest degrees of melting, respectively. These estimates, together with systematic differences in trace element and isotopic compositions, indicate that the upper mantle beneath Jeju Island is characterized by an increased degree of metasomatism and a change in major metasomatic hydrous minerals from amphibole to phlogopite with decreasing depth. The original plume material, having rather depleted geochemical characteristics, entrained shallower metasomatized uppermost mantle material, and segregated least-enriched low-alumina alkalic, moderately enriched high-alumina alkalic, and highly enriched sub-alkalic magmas, with decreasing depth.

INTRODUCTION

mantle

The eastern margin of the Asian continent is a site of intensive Cenozoic volcanism characterized by subduction-related arc–back-arc basin and mantle plume-related intraplate magmatism. The simultaneous occurrence of magmatism in association with both mantle downwelling and upwelling in the region provides a rare opportunity for investigating material circulation within the Earth’s mantle. Issues that may be addressed by analysing such magmatism include: (1) the contribution of subduction components extracted from the foundering oceanic lithosphere to the back-arc basin and further intraplate magmatism; (2) the contribution to continental margin magmatism of continental components, such as continental crust and subcontinental lithospheric mantle, with geochemical characteristics that are different from asthenospheric mantle; (3) the role of upwelling, asthenospheric mantle materials in causing such magmatism and controlling magma composition. Jeju Island is located between the Korean Peninsula of the Asian continent and Kyushu of the SW Japan arc, at the western margin of the Sea of Japan (or East Sea); the Sea of Japan is a back-arc basin built behind the SW and NE Japan arc–trench systems (Fig. 1). Jeju magmatism is thus distinct in that it takes place near the boundary between arc–back-arc basin and continental tectonic settings. Analytical study of Jeju magmatism may provide

*Corresponding author. E-mail: [email protected]

# The Author 2004. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@ oupjournals.org

KEY WORDS:

Jeju Island; magma genesis; mantle plume; subcontinental

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VOLUME 46

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MARCH 2005

(a) Tectonics 45°N ii Kur

eA

rc h

nc

Eurasian Plate

ile ur

S

35

W

Ja

p an

NE

Ja p

Sea of Japan

Trench

an Arc

K

40

Arc pan Ja

Pacific Plate

N

ch Tre n

yu uk Ry

Arc

25 115

120

125

130

(b) Geology

CJ31 CJ30

N

C11

135

CJ42 CJ13

140

CJ38

CJ14

Trench

-Mariana

Philippine Sea Plate

na Maria oninIzu-B

gh Trou kai an

Izu - B o n i n

Jeju Island

30

e Tr

CJ29

CJ15

145

150°E

CJ33 CJ32

CJ37

CJ12 CJ40

CJ1 CJ36 CJ2 CJ35 CJ3

CJ34

Stage 4 CJ26 CJ7 CJ8 CJ6 CJ5 CJ4

CJ27 CJ28 CJ10 CJ9 CJ16 CJ19 CJ17 CJ21 CJ20 CJ22 CJ18 CJ23 CJ24

CJ25

Stage 3-3 Stage 3-2 Stage 3-1

10 km

Stage 2

(c) Stratigraphy and samples Stage 4 3-4 3-3 Stage 3 3-2 3-1

Stage 2

Stage 1 Basement

Shell-sand Formation Scoria volcanic cones Backlockdam basalt Hallasan trachyte Hallasan basalt Seongpanak basalt Shiungri basalt Beobjeongri trachyandesite Hahyori basalt Sumangri basalt Jeju basalt Sinyangri Formation Sanbangsan trachyte Jungmun trachyandesite Hwasun-Seongsan hyaloclastite Seoguipo trachyandesite Pyosunri basalt Seoguipo Formation Basal basalt Granite

5, 33, 38, 42 30.1 30.2, 31 15, 16, 17, 26.2, 28 9, 10, 18, 19, 36, 40 6, 20 27 11, 12, 13, 14, 23, 24, 29 22

7, 8, 21 1, 2, 3, 25, 26.1, 32, 34, 35, 37 4 39, 41

Fig. 1. Tectonic and geological framework for Jeju Island. (a) Quaternary volcanism along the eastern margin of the Asian continent. Volcanic fronts, the trenchward limit of a volcanic arc, are shown by bold continuous lines. Along the convergent plate margins, extensive subductionrelated arc magmatism is taking place, whereas intraplate, possibly mantle-plume-related, volcanoes (stars) are built within the Eurasian plate. (b) Generalized geological map of Jeju Island after Lee (1982), showing sample localities for this study. (c) Stratigraphic relationships of Jeju volcanic rocks and samples for this study.

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TATSUMI et al.

JEJU PLUME MAGMATISM

constraints for assessment of the above-mentioned topics of continental margin magmatism. Voluminous volcanic piles accumulated on Jeju Island during the Late Cenozoic and include at least two chemically distinct magma series: alkalic and sub-alkalic series (Lee, 1982; Park, 1994). This paper presents petrography, major and trace elements, and Sr–Nd–Pb isotopic compositions for Jeju volcanic rocks. On the basis of this comprehensive dataset, the occurrence of three magma series, processes of magmatic differentiation, conditions of mantle melting, and the geochemical and mineralogical characteristics of the upper-mantle sources of the magmas will be discussed.

GEOLOGY The eastern margin of the Asian continent is characterized by the occurrence of extensive magmatism. Most Quaternary volcanoes in this region are built along the convergent plate margins where Pacific and Philippine Sea Plates are being subducted beneath the Eurasian Plate (Fig. 1a). These volcanic arcs form 100–200 km above the dipping seismic zone located near the surface of the foundering oceanic lithosphere (e.g. Tatsumi & Eggins, 1995). Far behind the volcanic arcs, on the other hand, are sporadically distributed ‘intraplate’ volcanoes such as Jeju volcano (Fig. 1a). Although no seismically active slab is observed, recent tomography results have revealed the presence of horizontally lying, subducted lithospheric material near the upper–lowermantle boundary beneath the intraplate volcanoes (Fukao et al., 2001). East of Jeju Island is the Sea of Japan, a back-arc basin formed behind the Japanese Islands at 30–15 Ma (Tamaki et al., 1992) with clockwise and anti-clockwise rotations of SW and NE Japan arc slivers respectively at 15 Ma (Otofuji et al., 1985). Although the principal cause of the back-arc rifting is controversial, back-arc basin formation results in, or is caused by, upwelling of asthenospheric material that ultimately creates new oceanic crust. It is therefore interesting to compare the chemical characteristics of upwelling asthenospheric material beneath the back-arc basin and the intraplate regions. Jeju Island is roughly elliptical in shape (80 km 40 km) and mainly comprises Holocene volcanic rocks. It is composed of thick piles of lava flows, minor pyroclastic rocks, hyaloclastites, and numerous parasitic scoria cones (Fig. 1b). These volcanic rocks are believed to have erupted onto a granitic basement, although granitic rocks are found only as xenoliths in both lavas and pyroclastics. The volcanic activity on this island can be divided into four stages (Fig. 1b and c), based on stratigraphic relationships (Lee, 1982). Radiometric age determinations for Jeju lavas indicate that the volcanic activity commenced

at 800 ka and continued to historical times (Lee, 1982). Stage 1 began with the eruption of basaltic lava flows that formed a shield volcano growing from the sea floor. Once the shield volcanic activity ended, the Stage 1 volcanic rocks were unconformably overlain by volcaniclastic sediments (Seoguipo Formation; Fig. 1c). The Stage 2 basaltic lavas (Pyosunri basalts; Fig. 1c) form the bulk of the exposed volcanic rocks as a lava plateau. Minor lava flows composed of trachyandesites and trachytes were also erupted during this stage. The Stage 3 volcanic rocks form the Halla shield volcano, with a peak height of 1950 m, and can be subdivided into four substages based on stratigraphic relationships and petrographical characteristics. The final volcanic activity on this island, Stage 4, yielded more than 360 parasitic scoria cones that are distributed along the axis of the island (Fig. 1b). Forty volcanic samples collected from the island cover almost all volcanic stages (Fig. 1b and c). To evaluate the role of the granitic basement in the formation of the Jeju magmas, two granitic xenoliths included in pyroclastic rocks were also analysed.

ANALYTICAL METHODS Major and trace element (Ni to Th in Table 1) compositions were measured using RIGAKU1 Simaltics 3550 and Rix 3000 X-ray fluorescence (XRF) spectrometers on fused glass beads and pressed powder pellets, respectively. Detailed analytical procedures have been described by Goto & Tatsumi (1994, 1996). Concentrations of rare earth elements (REE) and 11 other trace elements (Rb to U in Table 2) were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a VG Elemental1 PQ3 system enhanced with a chicane lens system, following the procedures described by Chang et al. (2003). Trace element data, except for the high field strength elements (HFSE: Zr, Nb, Hf and Ta), were obtained from HF–HClO4–HNO3 digestion. For HFSE, alkali fusion (LiBO2–Li2B4O7, SpectrofluxR 100B of Johnson Matthey) was applied to ensure a complete decomposition of refractory minor phases. Analytical accuracy and precision estimated from repeated measurements of international reference rocks were better than 10% and 2–5%, respectively. Rock samples for Sr–Nd–Pb isotope analysis were crushed to coarse chips (20 GPa; Fig. 15).

Alternatively, the characteristic incompatible element patterns in Fig. 16 can be understood as the result of buffering by residual phases. The High-Al ALK and SubALK magmas are depleted in amphibole and phlogopite components, respectively, suggesting the presence of those minerals as melting residues. The geochemical and petrographical characteristics of the Jeju magma source regions in the upper mantle, and their contributions to the generation of three different magma series, are schematically illustrated in Fig. 17.


TATSUMI et al.


Table 10: Inferred primary magma compositions Low-Al ALK

High-AL ALK

CJ-10

CJ-01

CJ-34

47.18 2.24

47.75 2.01

48.09 1.96

47.18 2.24

47.07 2.16

47.75 2.01

48.09 1.96

50.91 1.99

50.97 1.74

12.95 11.44

13.11 11.36

13.42 11.06

14.95 9.44

13.89 10.99

15.11 9.36

15.42 9.06

13.31 11.09

13.65 11.42

MnO

0.15

0.15 12.42 8.62

0.08

0.15

0.15

12.80 8.93

0.15 12.95 8.85

0.15

MgO

12.30 8.93

12.20 9.23

12.45 8.85

11.92 8.62

0.14 10.54 7.88

0.14 10.74 8.10

2.63 1.27

2.57 0.93

2.84 1.07

2.63 1.77

2.69 1.18

2.57 1.43

2.84 1.57

2.94 0.89

2.64 0.41

SiO2 TiO2 Al2O3 FeO*

CaO Na2O K2O

A-1

Sub-ALK

A-2

B

C

CJ-37

CJ-2

P2O5

0.40

0.31

0.37

0.40

0.50

0.31

0.37

0.31

0.21

Total

100.00 0.89

100.00 0.88

100.00 0.89

100.00 0.77

99.99 0.90

100.00 0.75

100.00 0.76

100.00 1.05

100.00 1.06

0.88

0.88

0.88

0.90

0.88

0.90

0.90

0.86

0.86

FeO*/MgO olivine1 *Total 1

iron as FeO. Mg-number of olivine in equilibrium with the primary magma.

plagioclase 2.0 GPa

1.0 GPa 1.5 GPa

2.5 GPa

3.0 GPa

High-Al ALK High-Al ALK (A-2) Low-Al ALK Sub-ALK

olivine

quartz

Fig. 15. Normative [using the method of Walker et al. (1979)] compositions of the Jeju primary magmas projected onto the plagioclase– olivine–quartz plane from the diopside apex. Compositions of A-2 and other primary magmas are given in Table 10. Partial melt compositions obtained in peridotite melting experiments at various pressures (Hirose & Kushiro, 1993) are also shown.

isotopically depleted, unmetasomatized asthenospheric material, which contributed significantly to producing Low-Al ALK magmas. What is the nature of the metasomatic agents that enrich the subcontinental upper mantle? This is an important but unsolved problem concerning the evolution of the continents. Hydrous phases such as amphibole and phlogopite that are inferred as metasomatic minerals in the magma source region in the upper mantle beneath Jeju Island may result from infiltration of either H2O-rich fluids or silicate–carbonate melts, because both agents can readily transport LILE. However, Fe/Mg values cannot be changed by fluid-dominant metasomatism, because of the fairly low solubility of these elements in an aqueous fluid. The vertical variation in the Mg-number of olivine in the upper mantle, which is inferred from the Jeju magma compositions, may, therefore, suggest the melt-dominant metasomatism for the Jeju upper mantle.

CONCLUSION The uppermost mantle beneath the region has been variably metasomatized and exhibits isotopically more enriched signatures with decreasing depths. Hydrous phases that probably crystallized as a result of the metasomatism include phlogopite, at rather shallow levels where Sub-ALK magmas segregated from the upwelling mantle material, and amphibole, at deeper levels from which the High-AL ALK magmas were derived. The major component of the Jeju mantle plume is likely to be

Although Jeju Island is located along the eastern margin of the Asian continent in the vicinity of the arc–trench system, the Jeju volcanic rocks were produced in association with intraplate, mantle plume-related magmatism. The major process responsible for differentiation of the Jeju magmas was fractional crystallization of mineral phases such as olivine, clinopyroxene, plagioclase, apatite and magnetite. The systematic difference in both incompatible element abundances and isotopic compositions



VOLUME 46

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Ionic Radius (pm)

Nb-normalised Low-Al ALK-normalised

1.6

60

80

100

120

1.4

140

160

Sr 2+

1.2 3+

Zr

Ba

La

Rb

1.0 Lu 4+

1+

Th

0.8 Ti

Na

K

High-Al ALK

Nb-normalised Low-Al ALK-normalised

0.6 3.0 Na

2.5

Lu

Ti 2.0

Pl Ph Am

1+ 3+

4+

Sr 1.5

2+

Zr

Ba

1.0 La

Th 0.5

60

80

100

K

Rb

Sub-ALK 120

140

160

Ionic Radius (pm) Fig. 16. Trace element characteristics of the Jeju volcanic rocks suggesting the residual mineral phases in the magma source region (see text for discussion). Sizes of cation sites for minerals (Matsui et al., 1977) are indicated by arrows. Pl, plagioclase; Ph, phlogopite; Am, amphibole.

observed for the Jeju magmas appears to reflect vertical compositional and mineralogical heterogeneity in the magma source region of the upper mantle: the upper mantle beneath Jeju Island tends to possess more enriched and metasomatized signatures with decreasing depth. Processes including upwelling of a rather depleted mantle plume into such a metasomatized and enriched upper mantle, subsequent interaction and mixing between these mantle components, and separation of magmas at different depths may reasonably explain the geochemical characteristics of the Jeju magmas. The presence of mantle plumes with depleted isotopic, especially Sr–Nd, signatures has been proposed as the cause of intraplate magmatism in NE China (Zhou et al., 1988; Song et al., 1990), as well as Jeju Island. However, the origin and location of such an isotopically depleted mantle component are unknown. Tatsumi & Eggins (1995) speculated that the harzburgitic portion of the

subducting lithosphere, which is the residue after extraction of oceanic crust MORB magmas and hence is likely to possess very depleted isotopic characteristics, could rise from the upper–lower-mantle boundary region owing to the density contrast between harzburgitic slab and fertile lherzolitic mantle material at those depths. A significant difference in the isotopic compositions of the Jeju and NE Chinese intraplate magmas is the rather high Pb isotopic ratios for the Jeju magmas, suggesting the contribution of a HIMU-like geochemical reservoir to the Jeju mantle plume. One plausible mechanism for creating a HIMU reservoir in the deep mantle is the accumulation of both fresh and dehydrated oceanic crust (Chauvel et al., 1992; Hauri & Hart, 1993; Brenan et al. 1995; Kogiso et al. 1997a, 1997b; Tatsumi & Kogiso, 2003). The location and the origin of such a HIMU reservoir in the Jeju mantle plume system, however, is a future problem to be addressed.


TATSUMI et al.


Granitic Lower Crust Upper Crust

Jeju Volcano

Fractional crystallization

phlogopite

entraining the surrounding mantle material

amphibole

High-Al ALK

Base of subcontinental metasomatised upper mantle

Low-Al ALK

increasing - degree of metasomatism - isotopic enrichment decreasing Mg# changing mineralogy

Upper Mantle

Sub-ALK

Depleted, original plume Asthenosphere

Jeju Plume Fig. 17. A schematic diagram illustrating the mineralogical and geochemical characteristics of the upper mantle and the melting regime beneath Jeju Island.

ACKNOWLEDGEMENTS

REFERENCES

We thank Takashi Sano, Ken Itoh and In-Seok Son for their help in sampling on Jeju Island, Yuka Yonezawa and Bogdan Vaglarov for analytical assistance, Miki Fukuda for preparing the manuscript and figures, and Richard Arculus and Monica Handler for constructive comments on the manuscript.

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