Magmatism in the South China Basin

Chemical Geology, 97 (1992) 47-63 Elsevier Science Publishers B.V., Amsterdam

47

[31

Magmatism in the South China Basin 1. Isotopic and trace-element evidence for an endogenous Dupal mantle component Kan T u a, Martin F.J. Flowera, Richard W. Carlson b, Guanghong Xiec, Chu-Yung Chen a and Ming Zhanga ~Department of Geological Sciences, University oflllinois, P.O. Box 4348, Chicago, 1L 60680, USA hDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015. USA "Institute of Geochemistry, Academia, Guangzhou 510640, People's Republic of China aDepartment of Geology, University of Illinois, Urbana, IL 61801, USA (Received February 5, 1990; revised and accepted September 18, 1991 )

ABSTRACT Tu, K., Flower, M.F.J., Carlson, R.W., Xie, G., Chen, C-Y. and Zhang, M., 1992. Magmatism in the South China Basin, 1. Isotopic and trace-element evidence for an endogenous Dupal mantle component. Chem. Geol., 97: 47-63. A widespread episode of intraplate volcanism followed the cessation of sea-floor spreading in the South China Basin ( ~ 32-17 Ma BP ), affecting large parts of southern China and Indochina, and penetrating oceanic basement and stranded microcontinent fragments. Geochemical data for post-spreading seamount and island lavas define suites of quartz tholeiite, olivine tholeiite, alkali olivine basalt and nephelinite, characterised by OIB-type incompatible-element distributions. High-K alkalic lavas show extreme enrichment in large-ion lithophile and high-field-strength elements relative to N-MORB. S7Sr/86Srand 143Nd/144Ndratios are depleted relative to bulk Earth values and partially overlap with Central Indian Ridge MORB and associated OIB. In contrast, 2°SPb/2°4pb and 2°7pb/z°4pb ratios are variable and surprisingly radiogenic for given MORB-like z°6pb/2°4pb. The isotopic and trace-element systematics confirm source heterogeneity but appear to be decoupled, implying complex mantle enrichment histories. At least two heterogeneous source components are required: a depleted but "contaminated" Indian Ocean MORB type, and an EM-2 reservoir whose isotopic composition corresponds to continent-derived sediment. Dupal-like Pb isotopic compositions (A7/4Pb = 2-13, A8/4Pb = 45-73 ) are shared by intraplate basalts from Hainan Island, the Penghu Islands, northern Taiwan and post-collision arc basalts from the Philippines. It is proposed these reflect endogenous mantle processes related to disaggregation of the south China margin rather than a northward extension of the southern hemisphere Dupal anomaly.

I. Introduction

The South China Basin (SCB) formed through successive episodes of rifting, crustal attenuation and sea-floor spreading (Karig, 1971; B. Taylor and Hayes, 1980, 1983; Holloway, 1981; Ru and Piggott, 1986; Briais et al., 1989 - - cf. Ben-Avraham and Uyeda, 1973) and presents a natural laboratory for studying relationships between magmatism, lithosphere extension and mantle dynamics (R.S. White et al., 1987; McKenzie and Bic0009-2541/92/$05.00

kle, 1988 ). Preliminary geochemical studies of intraplate lavas from the SCB appear to define a regional mantle domain characterised by Dupal-like Pb isotopic compositions and K-, Th- (etc.) enriched trace-element distributions (Flower et al., 1988; Tu et al., 1988). Since recognition of the southern hemisphere Dupal anomaly, two contrasting model sources for Dupal-type mantle have been proposed: ( l ) deep mantle reservoirs, perhaps comprising ancient subducted sediment (Hart, 1984, 1988; Castillo, 1988); and (2) relatively shallow-

© 1992 Elsevier Science Publishers B.V. All rights reserved.

48

level reservoirs derived from the accumulation of thermally-eroded subcontinental lithosphere (e.g., Hawkesworth et al., 1986). This dichotomy allows for the interpretation of enriched mantle reservoirs in terms of either global or local ("endogenous") dynamic models. This paper is the first of a pair addressing the geochemistry and petrogenesis of SCB magmas in the context of the early to mid-Tertiary disaggregation of south China. Together with Part 2 in this issue, they present evidence for the existence of Dupal mantle in the northern hemisphere and evaluate global vs. endogenous models for its origin. The first paper documents the nature and extent of the SCB domain with reference to the southern ocean Dupal province, and active subduction systems representing potential sedimentary sources for sub-SCB mantle. The second paper (Flower et al., 1992 in this issue) evaluates data for Hainan Island basalts (a microcosm of the SCB data set) and the suggestion that refractory megacrysts in Hainan Island alkali basalts may represent the crystallisation products of supra-subduction sedimentary melts. In this paper we report data for major and trace elements and Sr, Nd and Pb isotopic ratios for post-spreading lavas from the Scarborough Seamounts (marking the extinct SCB spreading center), Reed Bank Seamounts, Paracel Islands, and ophiolite pillow lavas from Mindoro and Palawan (Philippines) representing possible relics of an early spreading episode, as part of a broader investigation of Cenozoic magmatism in the region. 2. Tectonic evolution of the South China Sea region

Opening of the SCB followed the collision of India and Eurasia, and began with rifting of an Andean-type marginal arc in the Paleocene (B. Taylor and Hayes, 1980, 1983; Ru and Piggott, 1986; Tapponnier et al., 1986; Briais et al., 1989). The m a i n opening phase appears to

K. T U ET AL.

have coincided with the clockwise rotation of Indochina relative to South China assisted by left-lateral motion along the Red River fault (Achache et al., 1983; Tapponnier et al., 1986; Briais et al., 1989). Crustal attenuation was dominated by pull-apart basins that enabled the separation and migration of numerous microcontinent fragments now manifest as the Paracel Islands, Dongzha Islands, Reed and Macclesfield Banks, Dangerous Grounds, Luconia Shoals (all in the SCB), and the North Palawan Continental Terrane (NPCT) in the Philippines (Fig. 1 ). Opening of the basin culminated in at least one sea-floor spreading episode forming southwest and eastern subbasins (Fig. 1 ) and was terminated in the midto late Miocene by the collision of microcontinents with obstructions at its eastern margin: the NPCT with the Western Philippine arc, Dangerous Grounds and Luconia Shoals with Borneo and Taiwan with mainland China (McCabe et al., 1985; Rangin et al., 1985; Stephan et al., 1986). Several models (B. Taylor and Hayes, 1980, 1983; Pautot et al., 1986; Hayes et al., 1987) concur that at least the early phase of eastern subbasin opening was characterised by N-S spreading, with microcontinents migrating along N-S-trending transform factures. In contrast, others (e.g., Pautot et al., 1986; Briais et al., 1989) proposed that spreading occurred along an essentially SW-NE-trending axis, albeit at variable rates, and explained the apparent E-W anomaly pattern (Taylor and Hayes, 1983) as an effect of NW-SE transform fracturing at a scale below resolution of the interpolated sea-floor magnetic survey data. These models follow from the proposed southeastward "extrusion" of Indochina and Borneo interpreted to result from the Indo-Eurasian collision (Tapponnier et al., 1986). A third scenario (Ru and Piggott, 1986) involves more than one episode of sea-floor spreading, each separating a phase of crustal stretching and heating. Evidence suggesting that subduction

49

M A G M A T I S M IN T H E S O U T H C H I N A B A S I N , 1.

105o

'

110 °

'

115 °

120 °

E

I~,ENGHLIIS//

CIH , N A

%

oo,,~ .°°'~",`~''~' ~I

•

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MACCLES

/' Y

IE

25 ° N

20 °

AR

:;?;~;' -v ,0

l \_\

15 °

10°

....

/,_/"

t+77 ,

Fig. 1. Map of the South China Basin showing principal tectonic features and locations of analysed samples (filled circles'). Two fine lines bound normal oceanic crust of the basin. Dashed and solid barbed lines represent inactive and active subduction zones, respectively. Also shown are: eastern and southwestern subbasins, Hainan Island, Penghu, Dongzha and Paracel Islands, Scarborough Seamounts, Reed Bank, Macclesfield Bank, ]le des Cendres, and Mindoro and Palawan (Philippines) ophiolites. NPCT= North Palawan Continental Terrane (after B. Taylor and Hayes, 1983 ).

at the Borneo-Palawan trough was terminated by mid- to late Miocene collision of the Dangerous Grounds and Luconia Shoals blocks (Hinz and Schluter, 1985 ) suggests that earlyformed SCB oceanic crust was either consumed by subduction or obducted (e.g., Rangin et al., 1985; Pautot and Rangin, 1989). While early rifting and lithospheric stretching probably involved intrusive and extrusive activity (K. Ru, pers. commun., 1990), there is no indication that this was extensive. However, the SCB is distinguished from other basins by prolific post-spreading volcanism that affected large parts of southeast Asia (Barr and McDonald, 1981; Wu et al., 1985 ). This activity pierced oceanic basement and stranded microcontinents such as the Paracel Islands, and Macclesfield and Reed Banks, and produced thick intraplate basalt sequences in northern

Hainan Island and the Leizhou Peninsula in addition to large parts of Indochina and Thailand. Basin opening may have been more complex than for other documented basins (Ru and Piggott, 1986). Heat flow and gravity surveys (Ben-Avraham and Uyeda, 1973; Anderson et al., 1978 ) suggest that either the rate of subsidence was anomalously low or thermal flux continues to be high (Ru and Piggott, 1986). The chronologic and spatial relationships of postspreading activity are diffuse and geophysical data equivocal concerning the question of whether mantle upwelling or lithospheric extension was the principal factor in producing magma (cf. R.S. White et al., 1987; McKenzie and Bickle, 1988; R.S. White and McKenzie, 1989). These factors are critical to interpreting the origin of Dupal-like source compositions in the SCB region.

50

3. Sampling and analytical techniques Samples used in this study were obtained from collections dredged during cruises of the R/V "Vema" (B. Taylor and Hayes, 1983 ) and "Sonne-II" (Kudrass et al., 1986) from sites on the Scarborough and Reed Bank Seamounts, respectively. Samples were also obtained from the Paracel Islands and from ophiolite pillow lavas from Mindoro and Palawan. Of the three Scarborough Seamount sites dredged (Fig. 1 ), D9 and DIO are close to or within the axial rift, while D8 is located close to a transform fracture zone (B. Taylor and Hayes, 1983). The samples are manganese-coated pillow fragments in some cases with brown glass coatings, and yield averaged K-Ar and Ar-Ar ages of 13.9 Ma (D8), 9.9 Ma (Dg) and 3.5 Ma ( D / 0 ) (Wang et al., 1984 ). They range from sparsely-phyric to plagioclase-phyric types and are slightly to moderately altered. The four samples dredged from Reed Bank Seamounts are less altered, showing a quenched glass matrix with large clear olivine phenocrysts. Reported K - A r ages for these samples range from 2.7 to 0.5 Ma (Kudrass et al., 1986). The Paracels sample is a fresh olivine-phyric nephelinite lava associated with volcanic breccia, while samples drilled from ophiolite pillow lava sequences on Mindoro and south Palawan are metabasaltic. In preparing samples for analysis, vesiclefilling material was removed before crushing and pulverising in a ceramic disk mill. Major elements and the trace elements Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb and Ba were analysed by X-ray fluorescence (XRF) at Northern Illinois University (Table 1). Rare-earth elements (REE), Th, Ta and Hf contents were determined by instrumental neutron activation analysis (INAA) at the University of Illinois (Urbana-Champaign) (Table 2). Precision and accuracy of XRF and INAA were evaluated by replicate analysis of reference standards BHVO-1 and BE-N [Flower et al., 1992 in this issue, table 2, (a) ].

K. TU ET AL.

Sr, Nd and Pb isotopic measurements were carried out on unleached and leached samples at the Department of Terrestrial Magnetism (DTM), Carnegie Institution of Washington (Table 3). Whole-rock powders were leached by using the multiple-step acid-leaching procedure (Mahoney, 1987). This extensive leaching method is believed to effectively remove all isotopic effects of seawater even for extremely altered lavas (Mahoney, 1987). St, Nd and Pb were separated for isotopic measurement using standard ion-exchange techniques (Carlson, 1984). Sr and Pb isotopic compositions were measured on a multi-collector VG-354 ® mass spectrometer while Nd isotopic compositions were measured on the DTM 15" (38.1 cm) mass spectrometer. Ratios of SVSr/86Sr were determined on both leached and unleached whole-rock powders (Table 3). After acid leaching the measured 87Sr/86Srdecreased significantly, reflecting removal of secondary Sr associated with seawater alteration. The largest decrease was observed for the ophiolite metabasalts, followed by Scarborough Seamount olivine tholeiite and Reed Bank samples, with relatively little change observed for Scarborough Seamount alkali basalt and trachybasalt. Three samples, one of which (S023-35-6) included leached, unleached and leachate separates, were analysed for Nd isotopic composition to monitor weathering effects on the S m - N d system. The results (Table 3) show random variation of 143Nd/144Ndwithin analytical error, indicating that the effect of submarine weathering on Nd isotopic composition is minimal for SCB samples. Variations between the Pb isotopic ratios measured on leached and unleached splits of two samples (Table 3) are random, slightly greater than the analytical uncertainties, but smaller compared to the total observed variations of the sampled SCB lavas.

4. Major- and trace-element compositions Chemical data and CIPW norms of the analysed samples (Table 1 ) define a range ofmag-

37 317 21 63 53 89 148 26 273 40 166 22 n.d.

38 288 32 54 38 94 167 19 321 34 157 23 32

7.82 5.81

52.6

49,20 2.14 15.82 2.52 9.07 0.19 5,65 11.03 3.09 0.74 0.54

D8-2 OT

31 359 40 64 31 75 185 20 326 39 205 30 147

7.17 6.39

49.6

49.44 2.80 15,07 2.72 9.79 0.22 5,40 9,69 3.46 0.96 0.45

D8-3 OT

35 289 175 60 48 85 129 13 314 30 150 21 15

9.70 5.04

55.5

49.73 2.09 15,97 2,42 8.71 0.13 6.10 10.77 3, I5 0.59 0.34

D8-4 OT

24 306 50 52 17 29 154 40 783 49 350 55 438

1.94 5.36

39.6

49.14 3,77 16.39 2.36 8.48 0.15 3.13 9.15 3.61 2,46 1.36

D9 TB

42 298 200 50 43 87 152 34 301 39 156 22 n.d.

9.53 4,55

52.0

49,13 2.19 16.08 2.51 9.05 0,14 5.50 11.00 3.02 0.77 0.61

D9-1 OT

24 308 15 54 15 27 161 54 749 45 344 53 441

2,96 5.96

39.3

48,91 3.67 15.77 2.58 9.30 0.14 3.38 8.99 3,51 2.42 1.33

D9-2 TB

42 296 41 55 44 93 151 34 304 34 154 21 18

9.35 4,97

52.9

49.48 2,13 15,86 2.51 9,04 0.15 5.69 10.75 3.02 0,89 0.48

D9-3 OT

23 249 146 58 77 48 118 61 819 43 349 84 854

12.29 7.07

56.8

49.31 3.36 16.53 2,38 8,57 0.20 6.32 9.37 3.42 2.67 0.89

DIO AOB

29 254 200 70 133 52 142 14 481 30 180 29 140

13.67 2.70

57.1

47.65 2.61 15.69 2.54 9.41 0.18 6,82 0.09 3.60 1.05 0.63

S02340 OT

27 214 294 94 151 87 123 27 481 27 158 34 285

6,69 9.54

59.8

50.11 2.14 14,46 2.44 8.77 0.17 7.33 9,68 3,28 1.15 0.47

S02337-31 OT

30 247 675 79 153 70 118 48 651 34 225 55 427

7,85 6,81

57.6

48.39 2,72 14.96 2.21 7.96 0.16 6.08 11.17 3.75 1,92 0.68

S02337-7 AOB

Reed Bank Seamounts

29 253 405 74 240 73 118 50 774 31 253 66 600

13.23 9,23

64.8

47.44 2.59 13,95 2,28 8.19 0.16 8.46 10.13 3.86 2.15 0,79

S02335-6 AOB

22 264 247 84 312 56 140 30 891 49 543 103 504

23.01 12.65

71.0

40.59 3.86 11.14 2.78 10.01 0.02 13.76 11.37 2.76 1.77 1.75

75 31-1 NE

Paracal

33 306 157 40 73 66 77 3 74 34 100 6 n,d.

10.64 6,47

61.2

51,61 1,44 15.12 2.28 8.22 0.19 7.26 9.89 3.78 0,12 0.09

M 32183 OT

Mindoro

36 306 183 41 47 60 81 7 535 33 81 4 24

8,19 8.05

59.8

51,93 1.38 16.14 2.19 7.88 0.23 6.57 9.09 4.05 0.45 0.08

M 32283 OT

32 321 29 57 32 66 86 9 155 30 76 5 23

3.30 17.29

5.10 16.91

36 327 103 47 46 54 91 20 166 33 94 4 n.d.

56.0

53.63 1.42 15.57 2.22 7.98 0.21 5.69 9.24 3.35 0.56 0,14

PALAWAN5QT

57.2

53.98 1,11 16.25 2.07 7,44 0,17 5.58 9.87 3.03 0.37 0.14

PALAWAN-3 QT

Palawan

Values for oxides are in percentage and normalized to 100%. Fe203 calculated assuming Fe3+/Fe 2+ =0.20. M g - n u m b e r = M g 2 + / ( M h 2+ + F e z+ ), Values tbr trace elements are in ppm. n, d. = not detected. QT = quartz tholeiite; OT = olivine tholeiite; AOB = alkali olivine basalt; TB = trachybasalt; NE = nephelinite, CIPW = normative molecules; Qz = quartz; Hy = hyperstene; O1 = olivine; Ne = nephelinite.

Sc V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba

10.76 5,79

51.4

Mgnumber

Qz Hy O1 Ne

48.62 2.24 15.71 2.76 9.93 0,24 5.88 10,51 2.77 0.89 0.44

SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO N%O K20 P20s

Samples D8-1 V36 OT Rocks Locations

Scarborough Seamounts

Major- and trace-element abundances a n d CIPW normatives

TABLE 1

,"e

,.....I ,-e

~r

,.q

£3

52

K. TU ET AL.

TABLE 2 INAA results (in ppm) of REE, Hf, Ta and Th Sample

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

V36D8-2 V36D8-4 V36D9-1 V36D9-2 V36DlO S023-40 S023-37-31 S023.37-07 S023-35-6 75-31-1 M32283 PALAWAN-5

17.47 17.53 16.20 51.21 51.29 19.45 23.21 36.00 48.04 81.13 2.86 3.21

34.10 40.54 33.23 106.44 109.01 41.35 47.03 73.006 94.41 157.26 9.51 10.32

15.56 20.06 23.42 54.89 57.74 22.82 22.37 35.17 43.32 72.17 9.40 7.88

5.12 5.12 4.80 11.24 11.56 6.00 5.61 7.33 8.21 14.85 2.74 2.95

1.61 1.78 1.74 3.76 3.81 2.16 1.99 2.68 2.82 4.84 1.14 1.13

0.81 0.75 0.88 1.47 1.86 1.09 1.14 1.54 1.33 1.20 0.77 0.82

2.33 1.91 1.96 2.64 2.46 1.71 1.26 1.77 1.46 2.28 2.57 2.56

0.38 0.37 0.36 0.43 0.43 0.27 0.24 0.34 0.29 0.34 0.48 0.49

3.2 3.7 3.3 7.4 8.1 4.7 3.9 5.1 5.8 9.6 2.3 2.4

1.4 1.2 1.4 3.7 4.0 1.7 2.2 3.7 4.3 6.8 0.1 0.3

1.6 2.2 2.2 4.4 5.4 2.5 3.3 5.3 6.6 10.7 0.2 0.2

TABLE 3 Sr, Nd, and Pb isotopic compositions 2°6pb/204Pb

2°Tpb/2°4Pb

2°8pb/204Pb

5.7

18.704

15.609

38.33

9.0

58.5

0.512916

5.4

18.600

15.632

38.85

12.5

73.4

0.704433 0.703796

0.512922

5.5

18.667

15.535

38.68

2.0

48.2

V36D9-2 Leached

0.703967 0.703897

0.512813

3.4

18.954

15.588

38.99

4.2

44.9

V36DlO Leached

0.704007 0.703909

0.512805 0.512791

3.3 3.0

18.875 19.029

15.593 15.600

38.93 39.10

5.6 4.6

48.4 46.7

S023-40 Leached

0.703814 0.703430

0.512952

6.1

18.601

15.557

38.63

5.0

51.3

S023-37-31 Leached

0.703991 0.703893

0.512898

5.1

18.543

15.606

38.60

10.5

55.3

S023-37-7 Leached

0.703936 0.703666

0.512894 0.512882

5.0 4.8

18.481 18.486

15.567 15.537

38.62 38.49

7.3 4.2

64.7 51.3

S023-35-6 Leached Leachate

0.704355 0.704038 0.704453

0.512913 0.512896 0,512909

5.4 5.0

18.411

15.575

38.55

8.8

66.8

75-31-1 Leached

0.703689 0.703218

0.513035

7.7

18.521

15.520

38.40

2.1

38.3

M32283 Leached

0.704222 0.703264

0.513184

10.7

17.864

15.447

37.61

2.2

39.0

PALAWAN-5 Leached

0.704453 0.703754

0.513129

9.6

17.886

15.449

37.65

1.9

39.7

Sample

87Sr/86Sr

143Nd/j44Nd

V36D8-2 Leached

0.703594 0.703192

0.512929

V32D8-4 Leached

0.703561 0.703176

V36D9-1 Leached

~Nd

d7/4Pb

d8/4Pb

Sr: ffactionation corrected to 87Sr/86Sr = 0.1194, data reported relative to 875r/86Sr = 0.71025 for NBS 98 7 Sr standard. Nd: fractionation corrected to 146Nd/~44NdO = 0.722250, corresponding to 146Nd/144Nd = 0.7219, data reported relative to ~43Nd/ ~4Nd -- 0.511860 for the La Jolla Nd standard. 143Nd/~44Nd = 0.512636 corresponds to ~r~d= 0. The 2a measurement range is _+0.000025 for both NBS 987and La Jolla at DTM. Within-run errors are much lower than external uncertainties. Pb: data reported relative to 2°6pb/2°4pb= 16.937, 2°7pb/2°4pb-- 15.491 and 2°8pb/2°4Pb= 36.71 for NBS 98~ Ph standard. The external uncertainties on this standard are ~ _+0.05% ainu- ~. Calculated A-values based on the method of Hart (1984).

MAGMATISM

IN THE SOUTH

53

C H I N A B A S I N , 1.

350 -

3

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I

[] •

300

~ TH]

AOB|

A

250

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[] 2.5-

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24

150

~

100

50

0-

0Cx/

1.5-

%

1-

0.5" M

T

•

35

A

J

200

A Mg #

350 -

.

P

.

0

g

35

75

4~5

55

6'5

75

Mg # 10

300

J/

250

J

200 -

150 -

100-

Z

50-

0

i

100

i

200

i

i

300

400

500

Zr

1 0.02

0 .1

0 m5

Th/Nb

Fig. 2. Selected elemental variation of South China Basin and Philippines ophiolite lavas: Ni ( p p m ) and K20 % vs. Mgnumber; Ni vs. Zr (both in p p m ) ; and C e / N b vs. T h / N b (after Saunders et al., 1988). S.S. = Scarborough Seamount; R.B. = Reed Bank Seamount; P.I. = Paracel Island; M.P. = Mindoro and Palawan ophiolites; C.C. = continental crust.

mas that includes quartz tholeiite, olivine tholeiite, alkali olivine basalt, trachybasalt and nephelinite, associated as follows: Scarborough Seamounts (olivine tholeiite, alkali basalt and trachybasalt); Reed Bank Seamounts (olivine tholeiite and alkali basalt); Paracel Islands (nephelinite); and Mindoro and Palawan ophiolite (olivine tholeiite and quartz tholeiite). While the plots of Ni vs. Mg-numher (Fig. 2) clearly reflect varying degrees of magmatic fractionation, with Reed Bank Seamounts and the Paracel nephelinite samples being relatively primitive and trachybasalt from site D9 the most fractionated, diverse parental magma types are indicated by the positive or scattered covariation of (e.g.) Ni vs. Zr

(Fig. 2 ) and the extensive range of SiO2 saturation. The intraplate SCB lavas can be divided into moderate-K tholeiitic and high-K alkalic groups that broadly correlate with SiO2 undersaturation. The high-K alkalic group includes samples with both high and low Mg contents (Fig. 2), which cannot be explained by fractional crystallisation. This distinction is matched by abundances of large-ion lithophile, high-field-strength and light rare-earth elements (LILE, HFSE and LREE, respectively ), (Figs. 3 and 4 ) (most HFSE and LREE being effectively resistant to weathering), and is also observed for the Hainan Island association of quartz tholeiite, olivine tholeiite, alkali basalt and basanite (Flower et al., 1992 in this

54

K. T U ET AL.

4 ~

,

SOUTH

CHINA

BASIN

J

10 2-

i-o:: r~, z O T U

O CE !0 ~

3 ~ 10o

Lcl

Ce

Nd

Sm Eu

Tb

YD

Lu

Fig. 3. Chondrite-normalised REE distributions for South China Basin and Philippine ophiolite samples; normalised to ordinary chondrites (Boynton, 1984, table 3.3, "recommended chondrite abundances" ). Symbolsas in Fig. 2. Note relative enrichment in LREE of high-K alkalic cf. moderate-K tholeiitic samples, and depletion of Yb in Reed Bank samples.

issue). The ophiolite metabasalts have LREEdepleted chondrite-normalised REE patterns, which are similar to those of MORB-type pillow basalt glasses from the East Taiwan Ophiolite (Jahn, 1986). The remaining SCB samples show LREE-enriched patterns with the high-K alkalic group being extremely enriched (Fig. 3 ). The MORB-normalised incompatible-element distributions for SCB samples are typical for oceanic island basalt (OIB), and comprise moderately (tholeiitic) and highly (alkalic) enriched variants (Fig. 4). Ba depletion observed in Scarborough Seamount olivine tholeiite and trachybasalt samples is possibly due to weathering. Both lava types are enriched relative to "normal" OIB such as Hawaii (Loubet et al., 1988) for equivalent SiO2 saturation (Clague and Frey, 1982; C.-Y. Chen and Frey, 1985) and resemble southern hemisphere high-K OIB associations with Dupaltype isotopic signatures, e.g., Gough (Sun and

McDonough, 1989), Kerguelen (Storey et al., 1988) and Samoa (Palacz and Saunders, 1986 ), whose MORB-normalised patterns are characterised by enrichment in Nb, Ta, Th and Ba. Moreover, plots of Ce/Nb vs. Th/Nb for the seamount samples vary significantly and coincide with the high Th/Ce array observed for Dupal-type OIB (Saunders et al., 1988 ). In contrast, the ophiolite samples have lower Th/ Nb and higher Ce/Nb and correspond to EMORB. These elements are highly incompatible with similar bulk distribution coefficients and are unlikely to be affected by post-genetic fractionation processes. 5. Sr, Nd and Pb isotope systematics Plots of eNd (unleached) vs. 87Sr/86Sr (leached) ratios for seamount samples (Fig. 5 ) show a negative correlation within the mantle array extending from MORB to bulk Earth values. While 87Sr/86Sr and 1 4 3 N d / J a 4 N d re-

MAGMATISM IN THE S O U T H CHINA BASIN, 1.

55

102

d] o:: 0 I

101

5 0

100

3 x 10

1 Sr"

K

Rb

B(:]

Th

To

Nb

Ce

P

Zr"

Hf

Sm

Ti

Y

Yb

Fig. 4. Distributions of large-ion lithophile, high-field-strength and rare-earth element contents of South China Basin and Philippine ophiolite samples, normalised to average N1-MORB (Viereck et al., 1989). Note enriched distributions of high-K alkalic cf. moderate-K tholeiitic samples, also, some LILE variation due to seawater alteration.

flect time-averaged depletion in Rb and LREE, Rb/Sr and S m / N d ratios are greater and less than bulk Earth values, respectively, indicating that enrichment in LILE and LREE was relatively recent. Ophiolite metabasalt samples have high 1 4 3 N d / l a 4 N d ratios, within the ~ya range of the East Taiwan Ophiolite ( + 13.3 to +8.7) (Jahn, 1986), characteristic of MORB, although Sr isotopic compositions are more radiogenic than expected. The most undersaturated incompatible-element-enriched sample, the Paracels nephelinite, has MORBtype Sr and Nd isotopic ratios and falls in the field of Central Indian Ridge (CIR) MORB. Most seamount samples have lower ~Nd-Values, between + 5.0 and + 5.7 and higher 87Sr/ 865r, from 0.7032 to 0.7040, and partially overlap the field of Central Indian Ocean (CIO) OIB. The Scarborough Seamount alkali basalt and trachybasalt show the lowest end of

+3.3 and relatively high 87Sr/86Sr ratios (0.7039). On plots of 2°Tpb/2°apb vs. 2°6pb/2°apb and 2°8pb/2°apb vs. 2°6pb/2°apb (Fig. 6) all data points lie above the northern hemisphere reference line (NHRL) of Hart (1984). In terms of their A8/4Pb and A7/4Pb values, i.e. vertical deviation from the N H R L (after Hart, 1984), they have variable A7/4Pb ( 1.9-12.5 ) and high A8/4Pb, ranging from 38 to 40 for ophiolite samples and nephelinite, and 45 to 73 for seamount samples. Ophiolite samples have low 2°6pb/2°apb and plot towards the lower extreme of the East Taiwan Ophiolite field. The Paracels nephelinite, again, falls in the CIR MORB field if the more radiogenic Pb data for the latter are included. Of the Scarborough Seamount samples, trachybasalt D9 and alkali basalt DIO have the highest 2°6pb/2°apb ratios, within the field of CIO OIB. Olivine

56

K. TU ET AL. 12-

6. Discussion CIR M O R B

~ .

lO-

6.1. Source heterogeneities and mixing

BZ

o

6

N~O

OIB

2

0

0.702

0.703

87Sv/ 865r

7

0.r04

BULK EARTH

0.705

Fig. 5. Plots of end (unleached) vs. STSr/S6Sr (leached) for South China Basin and Philippine ophiolite samples. Error bar represents 2~r uncertainties. Data fields for Central Indian Ridge (CIR) MORB and associated Central Indian Ocean (CIO) OIB (Mahoney et al., 1989, and references therein) shown for comparison. SCB data extend from CIR MORB towards CIO OIB. Ophiolites outlined by dashed line displace from the main array.

tholeiite D8 shows high 2 ° 7 p b / 2 ° 4 p b and 2 ° s p b / 2°4pb ratios, trending toward to Java Trench sediment compositions. In contrast, olivine tholeiite D9 has the lowest d7/4Pb value, plotting close to the NHRL. Reed Bank data are relatively less scattered and show lower 2°6pb/ 2°4pb values (18.41-18.60), with 37/4Pb ranging from 5.0 to 10.5 and 38/4Pb from 51 to 67 (Table 3). Collectively, the sampled SCB lavas exhibit variable Sr, Nd and Pb isotopic compositions, which are not attributable to alteration and are therefore considered intrinsic to the mantle source. Although exchange with seawater may increase 87Sr/86Sr ratios significantly it is unlikely to affect Nd and Pb isotopic systems in view of the extremely low concentrations of these elements in seawater. Olivine tholeiites D8 and D9 show similar degrees of alteration but have significantly different Pb and Sr isotopic compositions.

Variable Sr, Nd and Pb isotopic compositions (Figs. 5 and 6) confirm the existence of inter- and intra-seamount heterogeneities for SCB mantle sources, comprising variable hybrids and depleted MORB and an enriched component. In particular, the SCB samples reflect high and variable 2°7pb and 2°8pb, defining Dupal-like trends towards an enriched EM2 type component (Zindler and Hart, 1986; Hart, 1988 ). These compositions are shared by those of intraplate basalts from Hainan Island (Tu et al., 1991), northern Taiwan and the Penghu Islands (Sun, 1980), and alkaline arc basalts from the Philippines (Mukasa et al., 1987), which appear to define a circum-SCB isotopic domain. There is general agreement that the isotopic and elemental compositions of oceanic basalt sources are influenced by mantle recycling of lithospheric materials, viz. oceanic lithosphere (by subduction of MORB and underlying refractory peridotite ), continental crust (by subduction of continentally derived sediments), and subcontinental mantle lithosphere (SCML) (by thermal erosion or delamination) (e.g., McKenzie and O'Nions, 1983; W.M. White, 1985; Zindler and Hart, 1986), the latter two of which may contribute Dupaltype chemical and isotopic signatures. Of the analysed radiogenic isotopic systems, Pb is the most sensitive to crustal input. Because of the exceedingly low Pb concentrations in depleted mantle and relatively high crustal abundances (e.g., Hofmann, 1988), the Pb isotopic composition of depleted mantle rapidly approaches that of the crustal end-member before equivalent changes in Nd or Sr isotopic composition can occur. Two possible endmembers for the SCB mantle source can be projected in Pb-Pb space (Fig. 6). While a depleted component can be represented by MORB from the northern Mid-Atlantic Ridge

M A G M A T 1 S M I N T H E S O U T H C H I N A B A S I N , 1.

57

40

+

Java Sed. ~ C I O Gough

/f

[B]

39

CIR M t'L?

OIB

E

O

R

B

~

~

Scarborough

~

oT • TB AOB Reed B a n k 0 OT • AOB Paracel NE Philippines N OT,QT

~

38

/'I 37

i . 18.0

t7.5

158t

{

.

.

. :'06

i

pbl;.~ 0 4 p b

I9.o

19.5

JavaSed

~

~f-~

/

ClOOIB

/

IR MO

15.41

'/. . . . . . . 17.5

18.0

1

206p18.504 b / z pb

19o

195

Fig. 6. Plots of 2°7pb/2°apb and2°Spb/2°4pb vs. 2°6pb/z°4pb for South China Basin and Philippine ophiolite samples, shown for comparison with fields for Central Indian Ridge (CIR) MORB (Mahoney et al., 1989), Central Indian Ocean (670) OIB (Hamelin et al., 1986), Gough Island (Sun, 1980), East Taiwan Ophiolite (ETO) (Jahn, 1986), and Java Trench sediments (Othman et al., 1989). Leached sample data shown by starts. Error bar represents 2a uncertainties. The Northern Hemisphere Reference Line (NHRL) (Hart, 1984) and zero-age geochron are shown for comparison. Mixing end-member compositions are: (A) MAR-EPR MORB: 2°6pb/2°apb= 18.5, 2°7pb/z°apb= 15.5, 2°8pb/2°4Pb= 38.0 (calculated such that A7/4Pb, 38/4Pb=0 (Hart, 1984), Pb=0.034 ppm (calculated such that Ce/Pb= 25, Ce from (Wood el al., 1979 ); (B) average Java Trench sediments: 2°6pb/2°4pb = 18.82, 2°Tpb/2°4pb= 15.68, 2°8pb/2°apb = 39.03, Pb = 27 ppm: and (C) average CIR MORB: 2°6pb/2°4pb= 18.115, 2°7pb/2°4pb= 15.49, 2°Spb/2°4pb=38.025, Pb=0.034 ppm. Plusses on curve represent the addition of 0.1, 0.5, 1 and 2 wt% sediment Pb.

( M A R ) , the East Pacific Rise (EPR) or CIR, there are at least two candidates for an enriched end-member: an EM-2-type component as represented by crustal sediment, and a CIR OIB hotspot type. Although we have no Pb iso-

topic data for SCB sediment, Java Trench sediments are most likely derived from Eurasian crust and thus provide a reasonable analogue for crustal Pb in the SCB source. It is noted that that the Pb composition of Miocene pelagic

58

limestone from the Palawan shelf, used by Mukasa et al. ( 1987 ) to represent SCB sediments, also falls within the Java Trench sediment field. A simple mixing calculation for Pb isotopic systems between M A R - E P R MORB mantle and Java Trench sediment is illustrated in Fig. 6, showing that addition of < 1% sediment Pb to depleted mantle could produce the 2°7pb/ 2°4pb range observed in SCB lavas. The addition of 1-2% sediment Pb would effectively shift the depleted mantle composition to that of the sediment end-member. However, this mixing array fails to account for the observed range of 2°Spb/2°4pb, which is essentially subparallel to the NHRL. If a similar calculation is made using average CIR MORB as the depleted-mantle end-member, the resulting mixing array accomodates variation of both 2°7pb/ 2°4pb and 2°Spb/Z°4pb, suggesting the requirement of a "contaminated" Indian Ocean MORB-like component prior to mixing. This model is supported by the observation that west Philippine pre-collision and post-collision arc lavas have Pb isotopic compositions comparable to Indian Ocean MORB and some OIB (Mukasa et al., 1987 ), suggesting these lavas were derived from a MORB-like source characterised by a high T h / U over several hundred million years. Interestingly, pillow basalts from Philippine ophiolites and the East Taiwan Ophiolite, both likely to represent early SCB oceanic crust, have similar isotopic compositions to Indian Ocean MORB.

6.2. Origin ofSCB Dupal-like character There has been considerable debate about whether Dupal signatures are confined to the southern hemisphere and whether they reflect deep or shallow mantle phenomena (e.g., Hawkesworth et al., 1986; Castillo, 1988; Hart, 1988; Sun and McDonough, 1989). The majority of Dupal suites satisfying the requirements of being old, deep and (semi-) global in character clearly occur in the southern hemisphere, such that the SCB region appears to be

K. TU ET AL.

one of the few Dupal-like domains reported from northern latitudes. Questions are thus raised as to its continuity with the main Dupal province, and the possibility of an endogenous evolution associated with the break-up of south China. One "global" and two "endogenous" models for the evolution of sub-SCB mantle are evaluated. While not mutually exclusive each has profound and contrasting geodynamic implications. ( 1 ) If the SCB domain is a lobate extension of the southern Dupal province it may involve deep mantle reservoirs tapped by vigorous whole-mantle convection as suggested for southern hemisphere OIB suites (Castillo, 1988; Silver et al., 1988; Hart, 1988). However, there is no strong suggestion of deep mantle convection from geophysical evidence, e.g. heat flow, bathymetry, gravity and geoid in this region (cf. Watts et al., 1985 ), and despite extensive syn- and post-tectonic SCB magmatism, precursive flood basalts of the type associated with plume-induced crustal splitting, e.g. flanking the Atlantic and Indian Oceans (R.S. White et al., 1987; R.S. White and McKenzie, 1989) are absent. By the same token, plume components like those feeding Indian Ocean hotspot OIB (e.g., Kerguelen, R6union) are also absent, and accordingly it appears unlikely that SCB basalts reflect a northern extension of the Indian Ocean Dupal maxima. (2) A second possibility is that MORB-like sub-SCB mantle received additions of sediment at relatively shallow levels via subduction. Model ages for oceanic sediments fall in the range 2.6-3.0 Ga, and high values of A7/ 4Pb, A8/4Pb, Th/Ta, Ba/Nb, etc., in alkaline arc eruptives have been ascribed to sediment subduction rather than to the pre-subduction mantle character (e.g., W.M. White and Dupr6, 1986; Mukasa et al., 1987). However, landward-directed subduction (i.e. toward Eurasia) ceased prior to opening of the SCB, and subduction along the active eastern margins has since been east- and southeastward directed.

MAGMATISM IN T H E S O U T H C H I N A BASIN, 1.

Northeastward-directed subduction at the Java Trench remains a possible source of sediment contamination although it would appear capable of reaching SCB mantle at extreme depths only. As observed above, SCB Pb isotopic ratios (18.95, 15.63 and 38.99) can be reconciled with mixing of modern sediment (e.g., Othman et al., 1989) and a CIR MORB source of relatively high 2°8pb/2°4pb (Fig. 6 ). However, SCB source ratios for Ba/Nb (7.6 + 1.9, average with 1cruncertainties, excluding low-Ba olivine tholeiite samples), La/Nb (0.75 + 0.10) and Th/La (0.12 + 0.02), and 87Sr/86Sr are too low to be ascribed to sediment addition alone (e.g., Sun and McDonough, 1989). (3) A third scenario may involve SCML as has been suggested responsible for small-scale heterogeneities in some MORB and OIB suites (McKenzie and O'Nions, 1983; Shirey et al., 1986) and to dominate intraplate continental basalt sources (e.g., Carlson, 1984; Hawkesworth et al., 1986; Lightfoot and Hawkesworth, 1988). However, the range of Sr, Nd and Pb isotopic compositions in SCML overlaps much, if not all, of that of intraplate oceanic basalts, and there is little purpose in constructing mixing curves that involve hypothetical compositional averages for SCML (e.g., Shirey et al., 1986; McDonough, 1990). In some cases, SCML-derived lavas display HFSE depletions considered typical of the continental crust (Dud~is et al., 1987), while in others, incompatible-element patterns lack HFSE depletions and resemble those of intraplate oceanic basalts (e.g., Palacz and Saunders, 1986; Weaver et al., 1986; McDonough, 1990). On the basis of data for Chinese Cenozoic basalts (Tu et al., 1989) SCML isotopic compositions appear to be related to lithospheric age and thickness and reflect the complex timeintegrated effects of metasomatising melts and fluids. Therefore, it is not surprising that SCML beneath young, thinned, SCB has more radiogenic Pb than that yielding intracratonic ba-

59

salts associated with Archean/Proterozoic terrains in northern China (Tu et al., 1989). A broad negative correlation of 87Sr/86Sr and 2°6pb/2°4pb and positive correlation of ~43Nd/ ~44Nd and 2°6pb/z°4pb observed for the latter intersect the corresponding SCB data fields, implying the involvement of SCML. Especially low eNd-values were also considered typical of SCML by Hawkesworth et al. (1990) (an example being lavas and included harzburgite xenoliths from the Luzon-Taiwan arc, with end up to --6.2; Vidal et al., 1989; C.h. Chen et al., 1990) although McDonough ( 1990 ) suggests this may be incorrect. The apparent confinement of Dupal-type MORB, OIB, and associated continental flood basalts to the southern hemisphere has been interpreted to reflect remobilisation of continental lithosphere during the Paleozoic break-up of Gondwanaland (e.g., Hawkesworth e t a | . , 1986; Klein et al., 1988; Mahoney, 1988; Mahoney et al., 1989 ), and entrapment of delaminated SCML at the core-mantle interface (Castillo, 1988; Hart, 1988). Paleontologic (Ridd, 1971), paleoclimatologic (Nie et al., 1990) and paleomagnetic ( L i n e t al., 1985) evidence suggests that much of southeast Asia and south China belonged to Gondwanaland, having drifted north after separating from Australia. Considering the lack of evidence for deep mantle upwelling beneath the SCB region, it is not surprising that the asthenosphere beneath SE Asia (perhaps disturbed by tectonic effects of the Indo-Eurasian collision, e.g., Tapponnier et al., 1986) is isotopically similar to CIR MORB. While lithosphere stretching provides a mechanism for the recent asthenospheric contamination of SCML, continued subduction of the Pacific plate prior to opening of the SCB may have contributed EM-2 and high Th/ Ta, and Th/Ce signatures to the already-enriched SCML. The apparent lack of mantle plumes (on the one hand) and contemporaneous subduction (on the other) beneath the present-day SCB supports the geochemical ar-

60

K. TU ET AL.

guments in favour of a relatively shallow SCML source for SCB post-spreading magmas. Despite the resemblance of discrete source mixing spectra in intraplate SCB and Hainan basalts with those of the southwestern Luzon alkaline arc, the lack of proximal subduction systems appears to preclude the recent injection of sediment through subduction.

gation of the south China margin and do not represent a Dupal maximum of the type characterising the southern hemisphere. Nevertheless, the relatively high 2°8pb/2°4pb ratios of SCB basalts suggest that endogenous enrichment of SCB sources was superimposedon CIR MORB-type mantle, itself representing the northern periphery of the Dupal domain.

7. Conclusions

Acknowledgements

(1) Basalts from the South China Basin (SCB) represent part of a widespread postspreading magmatic episode, penetrating stretched continental lithosphere, new oceanic basement and relict microcontinent fragments. Compositionally they range between olivine tholeiite and nephelinite, most representatives comprising transitional tholeiite and alkali olivine basalt, characterised by Dupallike OIB-type incompatible-element distributions. High-K alkalic lavas show extreme enrichment in LILE and lack HFSE depletions compared to MORB. (2) Sr, Nd and Pb isotopic compositions for SCB lavas reflect strong source heterogeneity intermediate in character between that of Central Indian Ridge (CIR) MORB and Dupallike OIB. It is proposed that the SCB mantle sources comprise both asthenospheric and lithospheric components, including a lower region of accreted asthenospheric melt (isotopically resembling CIR MORB) overprinted by radiogenic melts of subducted sediment. In the probable absence of a mantle plume beneath this region decompression melting of the subcontinental lithosphere and asthenospheric mantle may have resulted from lithosphere stretching as a regional response to the IndoEurasian collision. ( 3 ) Dupal-like Pb isotopic compositions are shared by intraplate basalts from Hainan Island, Penghu Islands and northern Taiwan, and post-collision arc basalts from the Philippines. It is proposed that these largely reflect endogenous mantle processes related to the disaggre-

We thank Dennis Hayes (LDGO), Karl Hinz (Hannover, F.R.G.), Xuechang Yang (Guiyang, P.R.C. ), and Jose Almasco (UIC) for donating samples used in this study, Toshiaka Hasenaka for use and assistance with XRF analysis, and S.-s. Sun and S.-1. Chung for discussions. Martin Menzies and Pat Castillo are thanked for comments on an earlier version of the paper, and Bill McDonough and Bill White for constructive reviews. The work was supported by grants from NSF (INT8617805), the American Chemical Society PRF (20094-AC2) and Sigma Xi. Kan Tu acknowledges a pre-doctoral fellowship at the Carnegie Institution of Washington, Department of Terrestrial Magnetism. References A c h a c h e , J., Courtillot, V. and Besse, 1983. Paleomag-

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MAGMAT1SM IN THE SOUTH CHINA BASIN, 1.

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