Ž .
Chemical Geology 182 2002 513–528
www.elsevier.comrlocaterchemgeo
Geochemical constraints on the petrogenesis of high-Mg basaltic
andesites from the Northern Taiwan Volcanic Zone
Kuo-Lung Wang
a,), Sun-Lin Chung
a, Chang-Hwa Chen
b, Cheng-Hong Chen
aa
Department of Geosciences, National Taiwan UniÕersity, 245 Choushan Road, Taipei, Taiwan
b
Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan
Accepted 1 June 2001
Abstract
Ž .
The Northern Taiwan Volcanic Zone NTVZ is a Late Pliocene–Quaternary volcanic field that occurred as a result of extensional collapse of the northern Taiwan mountain belt. We report here mineral compositions, major and trace element and SrrNd isotope data of high-Mg basaltic andesites from the Mienhuayu, a volcanic islet formed at ; 2.6 Ma in the
Ž .
central part of the NTVZ. The rocks are hypocrystalline, showing porphyritic texture with Mg-rich olivine Fo f 81–80 ,
Ž . Ž .
bronzite En f 82–79 and plagioclase An f 66–58 as major phenocryst phases. They have uniform whole-rock
composi-Ž .
tions, marked by high magnesium MgO f 5.9–8.1 wt.%, Mg value f 0.6 relative to accompanying silica contents
ŽSiO f 52.8–54.5 wt.% . The high-Mg basaltic andesites contain the highest TiO ; 1.5 wt.% and lowest K O ; 0.42 . 2Ž . 2 Ž .
wt.% among the NTVZ volcanic rocks. In the incompatible element variation diagram, these Mienhuayu magmas exhibit
Ž . Ž .
mild enrichments in large ion lithophile LILE and light rare earth elements LREE , coupled with an apparent Pb-positive
Ž .
spike. They do not display depletions in high field strength elements HFSE , a feature observed universally in the other
Ž .
NTVZ volcanics. The high-Mg basaltic andesites have rather unradiogenic Nd ´ Nd f q5.1–7.2 but apparently elevated
Ž87 86 .
Sr Srr Sr f 0.70435–0.70543; leached values isotope ratios. Their overall geochemical and isotopic characteristics are
Ž .
similar to mid-Miocene ; 13 Ma high-Mg andesites from the Iriomote-jima, southern Ryukyus, Japan. Despite these magmas have lower LILE and LREE enrichments and Pb positive spike, their Aintraplate-typeB incompatible element variation patterns are comparable to those of extension-induced Miocene intraplate basalts emplaced in the Taiwan–Fujian region. Therefore, we interpret the Mienhuayu magmas as silica-saturated melts derived from decompression melting of the ascended asthenosphere that had been subtly affected by the adjacent Ryukyu subduction zone processes. This interpretation is consistent with the notion that in the northern Taiwan mountain belt post-orogenic lithospheric extension started in Plio–Pleistocene time. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: High-Mg basaltic andesite; Geochemistry; Taiwan; Ryukyu subduction; Post-orogenic magmatism
)
Corresponding author. Present address: GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia. Tel.: q61-2-9850-9673; fax: q886-2-2363-6095.
Ž .
E-mail address: Kwang@els.mq.edu.au K.-L. Wang .
1. Introduction
High-Mg andesites, i.e. boninite and sanukite, represent a group of intermediate rocks that have been extensively studied for their unique petrological and geochemical features and geodynamic
signifi-Ž
cance Crawford et al., 1989; Tatsumi and Maruyama, 0009-2541r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
Ž .
.
1989 . These rocks, generally defined as having volatile-free recalculated compositions of SiO ) 532
wt.% and MgO ) 8 wt.%, occur exclusively in the fore-arc region of certain convergent margins. Their mantle sources are widely accepted to be consider-ably more refractory than those of mid-ocean ridge
Ž .
basalts MORB and typical arc magmas. Therefore, two important AingredientsB required for high-Mg
Ž .
andesite production would be: 1 a supply of hy-drous fluids into the refractory mantle source to
lower its solidus temperature and permit partial melt-Ž .
ing, and 2 a mechanism capable of producing and then maintaining a very high geotherm in the mantle
Ž .
wedge cf. Crawford et al., 1989 for review . The latter often involves formation of the back-arc basin. Investigations of high-Mg andesites, consequently, can provide pivotal information for understanding mantle dynamics and magma generation beneath the convergent margins in not only the fore-arc but also back-arc regions.
Ž . Ž .
Fig. 1. a Tectonic framework in the vicinity of Taiwan. OT: Okinawa Trough, RT: Ryukyu Trench. b Bathymetric map showing the
Ž .
location of Mienhuayu in the Northern Taiwan Volcanic Zone NTVZ . Open triangles and stars indicate Tertiary and Quaternary volcanoes, respectively. Thick line indicates the surface projection of the 100-km deep Wadati–Benioff zone of the subducting Philippine Sea plate
( )
K.-L. Wang et al.r Chemical Geology 182 2002 513–528 515
Ž .
Recently, Shinjo 1999 reported new geochemi-cal data for high-Mg andesites from several logeochemi-calities in the Ryukyu arc–Okinawa Trough back-arc basin system, which shed light on the nature of the mantle wedge under the Ryukyu subduction zone as well as the tectonomagmatic evolution of the Taiwan–
Ž .
Okinawa Trough areas Fig. 1 . In contrast to high-Mg andesites from the central Ryukyu and other
Ž .
fore-arc regions worldwide, mid-Miocene ; 13 Ma high-Mg andesites from the Iriomote-jima, southern
Ž . Ž .
Ryukyus Fig. 1b show ocean island basalt OIB geochemical and isotopic affinities that are compara-ble to Miocene intraplate basalts emplaced around
Ž .
the Taiwan–Fujian region Chung et al., 1994, 1995 .
Ž .
This new finding led Shinjo 1999 to conclude that at that time Ryukyu subduction did not exist in the Iriomote area and the westernmost part of the south-ern Ryukyu arc–trench system was established only after the collision between the northern Luzon arc and the Asian continent near Taiwan. In this paper, we report the occurrence of petrochemically similar
Ž .
but younger ; 2.6 Ma high-Mg basaltic andesites from the NTVZ, located ; 150 km behind the
pre-Ž sent-day southern Ryukyu arc–trench system Fig.
. Ž .
1 . The main objectives of our study include: 1 to document the petrological and geochemical features
Ž .
of these magmas; 2 to discuss petrogenetic pro-Ž . cesses involved in magma evolution; and 3 to better understand the geodynamic implications of development of the NTVZ and the particular colli-sionrextensionrsubduction tectonic setting in the Taiwan–Okinawa–Ryukyu region.
2. Geologic background of the NTVZ
The island of Taiwan is an active mountain belt created by oblique collision of the northern Luzon arc with the southeastern Asian continent starting
Ž .
; 12 Ma cf. Teng, 1990 for review . In central and southern Taiwan the tectonism reflects the ongoing collision, but in northern Taiwan extensional
col-Ž lapse began around Plio–Pleistocene time Teng,
.
1996 . Consequently, a series of onshore and off-shore volcanoes erupted during the Late Pliocene
Ž .
and Quaternary ; 2.8 Ma–recent , which altogether
Ž .
comprise the NTVZ Fig. 1b . The NTVZ has been regarded as the westernmost part of the Ryukyu
Ž .
volcanic arc e.g., Chen, 1990; Teng, 1996 . Such a conventional view was first questioned by Chen Ž1997 who suggested an extension-related instead of. a subduction-related setting for magma generation. To accommodate available geochemical, geophysical
Ž .
and geologic evidence, Wang et al. 1999 proposed a new model that the NTVZ resulted from post-colli-sional lithospheric extension owing to the collapse of the northern Taiwan mountain belt. This extension could, furthermore, have played a key role in the re-opening of the middle Okinawa Trough, and give way to its rapid southwestward propagation with associated development of the westernmost part of the southern Ryukyu arc–trench system. These tectonomagmatic processes may eventually have led to the present-day collisionrextensionrsubduction
Ž
context in the region of northern Taiwan Wang et .
al., 1999 .
Ž .
Wang et al. 1999 also noted that the NTVZ shows a spatial geochemical variation. From the northeast to the southwest, the magmas produced vary generally from low-K to calc-alkaline and then shoshonitic compositions. Most NTVZ volcanics have arc-like geochemical features, i.e., with signifi-cant enrichments in LILE and Pb and depletion in HFSE. This indicates that their mantle source regions must have been modified by processes associated with adjacent Ryukyu subduction zone. Volcanic rocks from two locations are particularly distinctive
Ž .
in their whole-rock chemistry. These are: 1 high-Mg Ž
potassic absarokites SiO f 48 wt.%; MgO f 152 .
wt.%; K O f 5 wt.% from the Tsaolingshan in the2
Ž .
westernmost part of the NTVZ Chung et al., 2001b ; Ž .
and 2 high-Mg basaltic andesites from the
Mien-Ž .
huayu in the central NTVZ see below .
3. Field occurrence and ages
The Mienhuayu islet consists mainly of mafic
Ž .
lava flows and subordinate scoria deposits Fig. 1c . Two types of lava, aXa and pahoehoe, were identified and both show oxidizing features indicative of
sub-Ž
aerial effusion of basaltic magmas Rowland and Walker, 1990; Paulick and Franz, 1997; Self et al.,
.
1998 . Detailed description for the field relations has
Ž .
been published separately Wang et al., 2000 . Juang Ž1993 reported K–Ar dates suggesting that the vol-.
canism occurred during 0.53–0.44 Ma. However, our recent 40Arr39Ar dating result reveals a
signifi-Ž .
cantly older age ; 2.6 Ma; Wang et al., 2000 . Therefore, we suggest that the Mienhuayu volcanic eruptions started around 2.6 Ma, and lasted in to the Quaternary. In this sense, onset of Mienhuayu vol-canism is broadly synchronous to that of the Tatun
Ž .
Volcanic Group ; 2.8 Ma; Wang and Chen, 1990 on Taiwan and Sekibisho volcanism to the north of
Ž . Ž
the Okinawa Trough Fig. 1 ; 2.6 Ma; Shinjo et .
al., 1991 , which represent the earliest phase of NTVZ activity, starting around the Plio–Pleistocene boundary.
4. Petrography and mineral chemistry
All volcanic rocks from Mienhuayu show hypocrystalline textures. They contain micro-pheno-crysts in an aphanitic matrix with abundant vesicles.
Ž Microscopically, they show porphyritic textures Fig.
. Ž .
2 with euhedral olivine 0.25–2 mm in size as the most abundant phenocryst, plus additional orthopy-roxene and plagioclase. Clinopyorthopy-roxene was not recorded as a phenocryst. Groundmass, which makes up ) 70% of the volume, consists mostly of plagio-clase, with subordinate olivine, orthopyroxene and iron oxides.
In Fig. 3, we show electron microprobe data Žfrom Chen, 1990. for representative phenocryst
Fig. 2. Thin section photo showing porphyritic texture with glassy matrix of the Mienhuayu sample MHY-2-6. The most abundant
Ž .
phenocryst phase is euhedral olivine OL .
Fig. 3. Plots of mineral compositions for the Mienhuayu volcanic
Ž .
rocks. a Pyroxenes and olivines, which mark with high Fo
Ž . Ž .
contents up to 82. b Plagioclases. Data are from Chen 1990 .
phases in the Mienhuayu volcanics, indicating their uniform and high-Mg compositions. Both olivine and orthopyroxene show restricted compositions and pla-gioclase ranges from An58 to An66.
5. Samples and analytical methods
Ž .
Ten lava flow samples and one scoria MHY-3
Ž .
sample collected from Mienhuayu Fig. 1c were subjected to whole rock major and trace element, and SrrNd isotope determinations. Powder samples were prepared using a jaw crusher and a corundum mill.
( )
K.-L. Wang et al.r Chemical Geology 182 2002 513–528 517 Table 1
Chemical and isotopic composition of the Mienhuayu high-Mg basaltic andesites, off NE Taiwan
MHH-01 MHY-1 MHY-2 MHY-3 MHY-4 MHY-5 MHY-7 MHY-8 MHY-9 MHY-2-1 MHY-2-2
( ) Major elements wt.% SiO2 54.50 53.30 53.70 53.28 49.44 49.99 53.41 53.56 52.82 53.45 54.21 TiO2 1.44 1.56 1.64 1.50 1.68 1.61 1.53 1.52 1.48 1.54 1.41 Al O2 3 14.60 14.52 15.01 14.35 14.26 14.81 14.35 14.50 14.58 14.49 14.24 a tFe O2 3 10.39 10.79 10.19 10.65 11.46 11.10 10.58 10.55 10.63 10.69 10.34 MnO 0.14 0.14 0.13 0.14 0.13 0.13 0.13 0.14 0.13 0.14 0.14 MgO 7.38 7.80 5.90 8.08 6.46 6.69 7.82 7.87 7.05 7.48 8.08 CaO 8.48 8.22 8.05 8.00 7.28 7.91 8.22 8.03 7.88 8.44 8.27 Na O2 2.14 2.47 2.66 2.72 2.47 2.53 2.67 2.66 2.73 2.45 2.34 K O2 0.40 0.41 0.47 0.42 0.55 0.45 0.43 0.42 0.53 0.42 0.59 P O2 5 0.13 0.54 0.69 0.23 3.41 2.06 0.14 0.14 0.65 0.51 0.29 L.O.I. 1.51 0.48 0.93 0.09 1.87 1.60 0.00 0.08 0.54 0.45 0.76 Total 101.11 100.22 99.38 99.46 99.02 98.87 99.27 99.47 99.01 100.06 100.67 b Mga 0.61 0.61 0.56 0.62 0.55 0.57 0.62 0.62 0.59 0.60 0.63 ( ) Trace elements ppm Sc 21.5 n.d. n.d. n.d. 21.8 60.6 67.9 18.8 54.4 21.1 13.5 V 170 83 89 142 137 149 141 143 141 143 139 Cr 371 241 188 411 422 424 425 434 429 381 414 Co 42.0 24.2 21.0 42.4 37.0 37.2 41.5 45.6 40.6 30.8 31.4 Ni 138 82 51 141 94 102 140 143 154 133 178 Cu 39.3 18.3 12.7 36.2 38.9 36.7 30.1 36.5 47.3 32.1 54.3 Zn 110 65 72 102 117 107 103 117 97 135 117 Ga 18.7 10.7 11.3 19.1 20.1 20.8 20.0 19.1 19.7 19.1 17.6 Rb 15.6 9.7 11.6 15.2 13.9 14.9 15.7 15.5 18.9 17.4 21.6 Sr 187 139 110 178 334 222 195 190 216 212 222 Y 22.2 12.3 13.6 19.4 13.8 21.9 20.2 19.7 23.4 20.5 18.4 Zr 92.0 50.8 56.9 82.9 90.4 90.7 85.0 84.6 83.6 83.5 80.6 Nb 6.45 3.51 4.37 5.25 5.84 5.98 5.37 5.33 6.43 5.62 7.94 Cs 0.50 0.26 0.27 0.53 0.49 0.48 0.51 0.51 0.64 0.38 0.52 Ba 137 102 91 131 151 140 139 136 172 94 124 La 5.58 3.72 4.22 5.03 4.75 5.23 4.57 4.86 6.18 4.90 7.10 Ce 12.4 6.8 7.6 11.3 11.4 12.0 10.6 10.9 13.6 11.0 15.3 Pr 1.73 1.13 1.26 1.63 1.63 1.81 1.59 1.64 1.99 1.59 2.15 Nd 8.85 6.30 7.06 8.25 8.24 9.12 8.05 8.27 9.48 8.36 9.99 Sm 3.15 2.10 2.32 3.15 2.96 3.39 3.04 3.09 3.27 2.94 3.05 Eu 1.22 0.83 0.88 1.24 1.17 1.38 1.26 1.27 1.30 1.06 1.03 Gd 4.07 2.64 2.81 3.57 3.25 3.78 3.41 3.56 3.73 3.45 3.31 Tb 0.66 0.44 0.47 0.69 0.64 0.75 0.69 0.69 0.70 0.65 0.59 Dy 3.90 2.60 2.86 3.89 3.48 4.26 3.87 3.85 3.98 3.80 3.57 Ho 0.75 0.48 0.53 0.78 0.70 0.83 0.78 0.77 0.80 0.66 0.62 Er 1.95 1.30 1.45 2.02 1.79 2.18 2.04 2.04 2.10 1.76 1.66 Tm 0.29 0.18 0.20 0.28 0.24 0.30 0.28 0.27 0.29 0.24 0.23 Yb 1.69 1.11 1.23 1.68 1.42 1.84 1.72 1.71 1.76 1.49 1.40 Lu 0.25 0.16 0.17 0.25 0.21 0.27 0.27 0.25 0.26 0.21 0.21 Hf 2.61 1.63 1.77 2.21 2.40 2.48 2.36 2.25 2.18 2.56 2.36 Ta 0.38 0.21 0.26 0.33 0.36 0.38 0.35 0.33 0.40 0.32 0.45 Pb 2.24 1.93 1.21 4.77 6.49 6.47 2.78 2.43 3.12 2.45 5.98 Th 1.15 0.86 0.88 1.26 0.80 1.47 1.44 1.21 1.66 0.84 0.93 U 0.32 0.21 0.21 0.40 0.68 0.46 0.35 0.33 0.41 0.29 0.31
Ž .
Table 1 continued
MHH-01 MHY-1 MHY-2 MHY-3 MHY-4 MHY-5 MHY-7 MHY-8 MHY-9 MHY-2-1 MHY-2-2
87 86 Srr Sr 0.70461 0.70453 0.70446 0.70722 0.70438 0.70440 0.70447 87 86 c Srr Sr 0.70448 0.70543 0.70451 0.70435 143 144 Ndr Nd 0.51298 0.51301 0.51290 0.51294 0.51299 0.51297 0.51293 d ´ Nd 6.6 7.2 5.1 5.9 6.8 6.4 5.6 n.d.: Not determined. a Total iron. bMga s Mg2qrŽMg2qqFe2q.. mo l mol mol c
Determined after acid leaching. d
wŽ143 144 . Ž143 144 . x 4
Ž143 144 .
´ Nd s Ndr Ndsampler Ndr NdCHURy1 = 10 ; Ndr NdCHURs0.51264.
Major element compositions were determined by
Ž . w
X-ray fluorescence XRF using a Rigaku RIX 2000 spectrometer at Department of Geosciences,
Ž .
National Taiwan University Lee et al., 1997 . The analytical uncertainties are generally better than 5% for all elements. Loss on ignition was determined by routine procedures. Trace elements were measured by inductively coupled plasma-mass spectrometry
Ž . w
ICP-MS using a Perkin Elmer Elan-6000 spec-trometer at Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences, which has a good stability range within ; 5% variation. Detailed
ana-Ž .
lytical procedures were reported by Liu et al. 1996
Ž .
and Li 1997 . Sr and Nd isotope ratios were mea-sured using VG354w
and Finigan MAT 262w
mass spectrometers, respectively, at the Institute of Earth Sciences, Academia Sinica, Taipei. Chemical and mass spectrometric procedures were described by
Ž .
Chen et al. 1990 . The isotopic ratios were corrected for mass fractionation by normalizing to 86Srr88Sr s0.1194 and 146Ndr144Nd s 0.7219. Long-term laboratory measurements for SRM 987 Sr and La
Ž .
Jolla UCSD Nd standards yield 0.71024 " 0.00004 Ž2 s and 0.51187 " 0.00003 2 s , respectively. All. Ž . analytical results are listed in Table 1.
Some of the Mienhuayu samples have uncom-monly high phosphorus contents, i.e., up to 2–3
Ž .
wt.% MHY-4 and MHY-5 . Therefore, rock chips Ž
of four samples MHY-1, MHY-4, MHY-5 and .
MHY-7 were leached using 1 N HCl, at ; 90 8C, for 20 min. The leached chips were then powdered for major element and Sr isotope determinations. The leachates were also collected for Sr isotope determi-nation. Experimental results of the four leached
sam-ples are given in Table 2, together with the composi-tions of unleached samples for comparison.
6. Whole-rock results 6.1. Major elements
Pre-leached Mienhuayu volcanic rocks show a Ž
large variation in their P O2 5 contents 0.13–3.41 .
wt.%; Table 1 . MHY-4 and MHY-5, two samples collected from the top sequence of the lava flows,
Ž .
have the highest P O2 5 3.4 and 2.1 wt.% but the
Ž .
lowest SiO2 49.4 and 50.0 wt.% contents. After acid treatment, however, all the four samples show a significant decrease in their P O contents, and their2 5 SiO contents increase to ; 54 wt.% on a volatile-2
Ž .
free basis Table 2 . Therefore, this AP-enrichedB feature is not primary but due to secondary addition of a high P O component. Despite the large varia-2 5
tion in P O contents and apparent P O elevation in2 5 2 5
certain samples, other major elements do not change
Ž .
significantly Table 2 . Based on the acid leaching experiments, we conclude that the Mienhuayu mag-mas have a rather uniform composition, with SiO2
content of ; 53.5–54.5 wt.% and P O2 5 of ; 0.1 wt.%.
These basaltic andesites plot in the subalkaline or tholeiite field in the total alkalis vs. SiO2 diagram ŽFig. 4a , and in the low-K tholeiite field in the K O. 2
Ž .
vs. SiO diagram Fig. 4b . In terms of SiO content,2 2
Ž .
they have relatively high MgO 5.9–8.1 wt.% , yielding Mg values of 0.56–0.62 that are consistent with the high-Mg nature of their phenocryst phases. In comparison with the rest of the NTVZ volcanics,
() K.-L. Wang et al. r Chemical Geology 182 2002 513 – 528 519 Table 2
Comparison for compositions of pre-leached and leached samples
a
MHY-1 MHY- MHY- Var. MHY-4 MHY-4L MHY- Var. MHY-5 MHY-5L MHY- Var. MHY-7 MHY-7L MHY- Var.
b c Ž .d Ž . Ž . Ž . 1L 1LN % 4LN % 5LN % 7LN % SiO2 53.30 53.49 54.28 2 49.44 51.60 54.16 8 49.99 52.25 54.33 7 53.41 54.62 54.40 1 TiO2 1.56 1.52 1.54 y1 1.68 1.67 1.75 3 1.61 1.59 1.65 1 1.53 1.54 1.53 0 Al O2 3 14.52 14.14 14.35 y1 14.26 12.85 13.49 y6 14.81 13.83 14.38 y4 14.35 14.45 14.39 0 tFe O2 3 10.79 10.51 10.66 y1 11.46 11.20 11.75 2 11.10 10.48 10.90 y3 10.58 10.61 10.57 y1 MnO 0.14 0.14 0.14 y1 0.13 0.14 0.14 13 0.13 0.13 0.14 6 0.13 0.14 0.14 3 MgO 7.80 7.65 7.76 0 6.46 6.81 7.15 10 6.69 6.62 6.88 2 7.82 7.73 7.70 y2 CaO 8.22 8.06 8.18 0 7.28 7.31 7.67 4 7.91 8.01 8.33 4 8.22 8.20 8.16 y1 Na O2 2.47 2.44 2.47 0 2.47 2.26 2.37 y5 2.53 2.47 2.57 1 2.67 2.61 2.59 y3 K O2 0.41 0.40 0.40 0 0.55 0.45 0.47 y16 0.45 0.43 0.44 y2 0.43 0.41 0.41 y5
P O2 5 0.54 0.20 0.20 y62 3.41 0.99 1.04 y70 2.06 0.37 0.38 y82 0.14 0.08 0.08 y46
Total 100.22 98.55 100.00 99.02 95.28 100.00 98.87 96.18 100.00 99.27 100.40 100.00
87 86
Srr Sr 0.70448 0.70722 0.70543 y0.25240 0.70451 0.70438 0.70435 y0.00426
87Srr Sr86 e 0.70910 0.70914 0.70902 0.70902
a
Values measured before acid leaching.
b
Values measured after acid leaching.
c
Volatile-free recalculated values of the leached samples.
d
Ž . Žw x w x . w x 4 w x
Var. % : Variation s leached sampleNyunleached sampleN runleached sampleN=100%; sample : volatile-free recalculated values.N e
Ž . Ž .
Fig. 4. a Total alkalis vs. SiO2 and b K O vs. SiO2 2 of the Mienhuayu lavas, showing their basaltic andesite and low-K tholeiitic natures. Data of the high-Mg andesites from southern
Ž .
Ryukyus Shinjo, 1999 and compositional range of the NTVZ
Ž .
magmas Wang et al., 1999; shaded areas are shown in both diagrams for comparison. Rock type boundaries are from Le
Ž . Ž . Ž . Ž .
Maitre et al. 1989 in a and Rickwood 1989 in b , respec-tively.
the Mienhuayu high-Mg basaltic andesites have
simi-Ž . Ž
lar Al O2 3 f13.5–15.0 wt.% , CaO f7.7–8.5
. Ž .
wt.% and tFe O2 3 f10.2–11.8 wt.% , but higher
Ž .
TiO2 f1.4–1.8 wt.% contents.
6.2. Trace elements
The Mienhuayu high-Mg basaltic andesites are generally homogeneous in their trace element com-positions. In the primitive mantle-normalized
incom-Ž .
patible element variation diagram Fig. 5a , they show smooth distribution patterns with moderate en-richments in LILE and an apparent positive spike in Pb, but are not depleted in HFSE. In comparison with other NTVZ volcanics that show significant
Ž .
HFSE depletion Wang et al., 1999 , the Mienhuayu Ž
lavas exhibit less enrichment in LILE e.g., Cs, Rb,
. Ž .
Ba, U and Th and Pb Fig. 5a , and a less
fraction-Ž .
ated REE pattern Fig. 6a . Except for sample MHY-2-2, all others show slight enrichment in LREE and a
Ž gradual slope change from LREE to MREE Fig.
.
6a , which leads to a AkinkB REE pattern. This unique feature is similar to that observed in mid-Miocene high-Mg andesites from the Iriomote-jima,
Ž .
southern Ryukyus Shinjo, 1999; Fig. 6b . In com-parison with high-Mg melts formed in the arc set-ting, the Mienhuayu lavas have higher trace element abundance than boninites, but a less fractionated
Ž .
variation pattern than adakites Figs. 5c and 6c .
6.3. Nd and Sr isotope ratios
The Mienhuayu high-Mg basaltic andesites have uniform Sr and Nd isotope ratios, with 87Srr86Sr f
Ž . 143 144
0.70435–0.70543 leached values and Ndr Nd
Ž .
f0.51290–0.51301 Table 1, Fig. 7 . Before the acid-leaching, MHY-4, the sample with
extraordi-Ž .
nary high P O content 3.4 wt.% , shows the high-2 5
Ž87 86 .
est Sr isotope ratio Srr Sr s 0.70722; Table 1 among the Mienhuayu lavas. However, the leaching experiment indicates that this ratio can be
signifi-Ž .
cantly reduced to 0.70543 Table 2 . The sample
Ž .
with the lowest P O2 5 MHY-7 , on the other hand, shows little variation between unleached and leached
Ž .
samples, from 0.70438 to 0.70435 Table 2 . More-over, Sr isotope ratios of the leachates from all four
Fig. 5. Primitive mantle-normalized variation diagram for the Mienhuayu high-Mg basaltic andesites. Normalizing values are from Sun and
Ž . Ž . Ž . Ž .
McDonough 1989 . Comparing examples shown include: a NTVZ volcanics Wang et al., 1999 , b Miocene intraplate basalts in NW
Ž . Ž . Ž .
Taiwan Chung et al., 1994, 1995 and high-Mg andesites in Iriomote-jima, southern Ryukyus IR-HMA; Shinjo, 1999 , c boninite
ŽCameron et al., 1983 , adakites Kay, 1978; Defant et al., 1991; Kay et al., 1993; Yogodzinski et al., 1995; Stern and Kilian, 1996 and. Ž .
Ž .
( )
Fig. 6. Chondrite-normalized REE patterns for the Mienhuayu high-Mg basaltic andesites. Data sources are the same as in Fig. 5. Chondrite
Ž .
normalizing values are from Sun and McDonough 1989 . Ž87 86
samples are nearly constant Srr Sr f 0.70902– .
0.70914; Table 2 , which is close to that of the Ž87 86
average modern seawater Srr Sr f 0.70923;
De-.
Paolo and Ingram, 1985 . These suggest a secondary increase in Sr isotope ratios of the Mienhuayu mag-mas associated with the phosphorous addition.
Inter-( )
K.-L. Wang et al.r Chemical Geology 182 2002 513–528 523
Fig. 7. The87Srr86Sr vs.143Ndr144Nd diagram for the Mienhuayu high-Mg basaltic andesites. Data of the Miocene high-Mg andesites
Ž . Ž
from southern Ryukyus are shown for comparison Shinjo, 1999 . Fields for the NTVZ volcanics including the Pengchiayu basalts Wang et
. Ž . Ž
al., 1999 , Miocene intraplate basalts from NW Taiwan Chung et al., 1994, 1995 , back-arc basalts from middle Okinawa Trough Wang,
. Ž .
1998; Shinjo et al., 1999 , and volcanics from central and northern Ryukyu Arc Shinjo et al., 1999, 2000 are also shown. Field of the East
Ž . Ž . Ž .
Taiwan Ophiolite ETO is from Jahn 1986 , Chung and Sun 1992 . The enriched mantle components of EMI and EMII are from Hart
Ž1988 ..
estingly, the leached Sr isotope values are still higher in comparison with their Nd isotope ratios so that the results plot to the right of the mantle array in the
Ž .
NdrSr isotopic correlation diagram Fig. 7 .
7. Discussion
7.1. The A excessB P and Sr isotope Õalues
Some Mienhuayu samples show unusually high phosphorous contents coupled with high Sr isotope ratios that, as shown in Table 2, can be reduced by acid treatment. Whereas degrees of the reduction are
Ž 87 86
variable P O f 0.08–1.04 wt.%,2 5 Srr Sr f .
0.70435–0.70543 , all the leachates display nearly Ž87 86
identical Sr isotope ratios Srr Sr f 0.70902– .
0.70914 which may be explained by seawater alter-ation. However, the associated AexcessB P feature precludes this explanation. Contamination by bio-genic phosphates from marine organisms can
ac-count for the high P and seawater-like Sr isotope Ž
ratios Staudigel et al., 1985; Ingram et al., 1994; .
Barrat et al., 2000 . However, the Mienhuayu mag-mas occurred subaerially at ; 2.6 Ma, and since then sea level has never risen sufficiently to
sub-Ž .
merge the islet Naish and Kamp, 1997 . Alterna-tively, biogenic phosphates from terrestrial organ-isms that may have Sr isotope ratios higher than that
Ž .
of seawater Barrat et al., 2000 are the most likely source. The secondary P and Sr isotope values could have been added from the source existing as an authigenic phase or simply absorbed onto the surface of the volcanic rocks. This interpretation is
consis-Ž
tent with the fact that the samples e.g., MHY-4, .
MHY-5 with high P content and Sr isotope ratios occurred near the surface of the upper lava sequence, in contrast to those having low P O content and low2 5 87
Srr86Sr which occur in the lower volcanic
succes-Ž .
sions e.g., sample MHY-7 . In addition, the upper volcanic sequence is composed of vesicle-rich lavas, whereas the lower part is more compact. Therefore,
we conclude that the extraordinarily high phospho-rous contents and Sr isotope ratios are derived from terrestrial biogenic phosphates, e.g., guano, that might have coated on the vesicle surface. This sec-ondary addition can be removed by appropriate acid treatment.
The leached Mienhuayu volcanic rocks still show elevated Sr isotope ratios relative to the associated
Ž .
Nd isotope ratios Fig. 7 . Three causes may account Ž .
for this feature: 1 the leaching experiments did not Ž . completely remove the secondary addition; 2 some degrees of the secondary alteration cannot be
re-Ž .
moved by the acid leaching; and 3 the elevation of Sr isotope ratios is a real feature of the Mienhuayu magmas. The first possibility may be applied to sample MHY-4 whose leached phosphorous content
Ž .
is high P O s 1.03 wt.%; Table 2 . However, with2 5
leached phosphorous content as low as 0.08 wt.%, sample MHY-7 still has a higher than expected Sr isotope ratio which decreases only from87Srr86Sr s
Ž . 87 86
0.70438 unleached to Srr Sr s 0.70435 Žleached . In addition, the leachate of sample MHY-7.
Ž87 86 .
yielded Sr isotope ratio Srr Sr s 0.70902 indis-tinguishable from leachates of the other three
sam-Ž .
ples Table 2 . Thus, we believe that in this sample the alteration effect for P O content and Sr isotope2 5 ratio has been totally removed by leaching. In this sense, the first and second possibilities are less likely, or at least insignificant, for the elevated Sr isotope ratio of leached MHY-7 that plots to the right of the
Ž .
mantle isotopic array Fig. 7 . It is hence implied that the mantle source of the Mienhuayu magmas is characterized by higher Sr isotope ratios relative to associated Nd isotope ratios.
Such an elevation of Sr isotope ratios in the mantle source may be ascribed to the nearby Ryukyu subduction zone processes. As discussed by Tatsumi
Ž .
and Eggins 1995 , convergent-margin magmas often show across-arc isotopic variations, with high Sr isotope ratios in the trench side, that can be ex-plained by interactions between the mantle wedge
Ž
and subduction components e.g., subducted sedi-.
ments or fluids . The Mienhuayu magmas, however, do not display this kind of co-variation with the rest of NTVZ volcanics. According to our recently
ob-Ž .
tained Pb isotope data Wang et al., unpubl. , the Mienhuayu rocks have nearly identical leached and
Ž206 204
unleached values Pbr Pb s 18.577–18.602,
207
Pbr204Pb s 15.599 –15.611, 208Pbr204Pb s .
38.729–38.767 marking with a DUPAL-type Pb isotope anomaly. Similar Pb isotope signatures have been reported in the Miocene intraplate basalts around
Ž .
NW Taiwan Chung et al., 1994, 1995 and the Quaternary arc volcanics from northern Ryukyus ŽShinjo et al., 2000 . It is widely recognized that a. DUPAL-like asthenospheric domain, or an Indian Ocean type convecting mantle, underlies the entire East Asian continent and marginal basins in the
Ž
western Pacific Flower et al., 1998; Smith, 1998; .
Chung et al., 2001a . A typical DUPAL mantle component, following the original definition by Hart Ž1984 , is commonly associated with high Sr isotope. ratios. We therefore speculate the elevated Sr isotope ratios observed in the Mienhuayu magmas to be derived from an asthenospheric mantle source with such isotopic features.
7.2. Petrogenesis of the Mienhuayu high-Mg basaltic andesites
High-Mg andesites are commonly observed in two tectonic environments, i.e., in the foarc re-gions of convergent margins, and intraplate rifts ŽCrawford et al., 1989 . The Mienhuayu high-Mg. basaltic andesites were emplaced in the back-arc region, distant from the Ryukyu fore-arc. The lavas are rich in the Abasaltic componentB, with CaOf 7.7–8.5 wt.%, tFe O f 10.2–11.8 wt.% and TiO2 3 2 f1.4–1.8 wt.%, reflecting a fertile mantle source. Their incompatible element contents are apparently
Ž higher and show different variation pattern Figs. 5c
.
and 6c from those of boninites. Thus, a fore-arc origin is considered unlikely.
The Mienhuayu volcanics have geochemical fea-Ž tures distinctive from the other NTVZ volcanics cf.
.
Wang et al., 1999 . The latter generally have a calc-alkaline nature similar to that observed in
con-Ž .
vergent-margin magmas Gill, 1981 . In terms of incompatible elements the Mienhuayu volcanics
dis-Ž .
play less enrichments in LILE and Pb Fig. 5a and lack the distinctive HFSE depletion shown by typical
Ž
subduction-related magmas McCulloch and Gamble, .
1991 . They also show the highest Nd isotopic ratios
Ž .
among the NTVZ volcanics Fig. 7 , virtually similar
Ž .
to those of Miocene ; 23–9 Ma intraplate basalts
Ž .
( )
K.-L. Wang et al.r Chemical Geology 182 2002 513–528 525
Ž . Ž .
Fig. 8. Plots of a NbrU vs. UrTh and b NbrLa vs. RbrBa ratios for the Mienhuayu high-Mg basaltic andesites. Values of N-MORB,
Ž . Ž .
E-MORB and OIB are from Sun and McDonough 1989 , the Ryukyu subducted sediments are from Plank and Langmuir 1998 . Fields for the NTVZ volcanics, NW Taiwan intraplate basalts, and magmas from central Ryukyu Arc and middle Okinawa Trough are according to
Ž . Ž . Ž .
higher than those of the high-Mg andesites from
Ž .
southern Ryukyus Shinjo, 1999 . Their high Nd
Ž .
isotope ratios ´ Nd up to q7.2 suggest an astheno-spheric mantle source. Moreover, the Mienhuayu volcanics show similar, though lower in LILE and LREE abundance, incompatible element variation pattern to those of the NW Taiwan intraplate basalts and almost the same pattern as the southern Ryukyus
Ž .
high-Mg andesites Figs. 5b and 6b . It is important to note that the latter two magma suites resulted from decompressional asthenospheric melting owing to intraplate lithospheric extension and attenuation that took place prior to the arc-continent collision in
Ž .
Taiwan Chung et al., 1994, 1995; Shinjo, 1999 . The intraplate geochemical affinities of the Mien-huayu volcanics are clearly illustrated in a NbrU vs.
Ž .
UrTh plot Fig. 8a . Among the NTVZ volcanics, the Mienhuayu rocks have the highest NbrU ratios Žf20 that plot close to southern Ryukyus high-Mg. andesites, NW Taiwan intraplate basalts and the average values of OIB and MORB. The E-MORBrOIB affinity is also shown in a NbrLa vs.
Ž .
BarRb plot Fig. 8b , which is consistent with the unradiogenic Nd isotopic ratios of the Mienhuayu lavas. Therefore, we propose that the Mienhuayu high-Mg basaltic andesites are silica-saturated melts that originated from ascending asthenospheric mantle having a composition similar to that of E-MORB. Such an asthenospheric mantle, however, may have been subtly affected by the Ryukyu subduction zone processes that account for the positive Pb spike.
Ž .
Kushiro 1969 reported that primary silica-saturated melts can be produced by partial melting of
Ž . Ž
mantle peridotites 1 at shallow depths i.e.,- 7 kb
. Ž .
or ; 20 km under anhydrous condition or 2 deeper Že.g., down to 60–70 km or 20 kb under hydrous. condition. Given that the continental crust is ; 30
Ž .
km thick beneath northern Taiwan Yeh et al., 1989 , the anhydrous melting scenario is unlikely for gener-ation of the Mienhuayu high-Mg basaltic andesites. Thus, the parental magmas for Mienhuayu volcano must have originated from deeper depths, which we suggest to have been the asthenospheric mantle un-der hydrous conditions. The depth of magma segre-gation may be constrained via certain trace element signatures. The Mienhuayu volcanics have low HREE and Y, implying residual garnet in the mantle source.
Ž . Ž .
Their LarYb f 2–3 and SrrY f 9.6–11 ratios
are lower than those of magmas marked by garnet Ž
fractionation e.g., adakite: LarYb ) 20, SrrY ) 40; . Defant and Drummond, 1990; Figs. 5c and 6c . It is therefore suggested that the mantle source of the Mienhuayu lavas, is not as deep as adakites and perhaps located near the upper limit of the garnet stability field. In other words, the mantle source resides at 60–70 km depth, around the transition
Ž zone between spinel and garnet stability fields
Wyl-.
lie, 1981 . The required hydrous condition can be obtained by dehydration of the subducting Philippine Sea plate in adjacent Ryukyu subduction zone. This subduction-related modification, however, is Asub-tleB because the Mienhuayu rocks show only moder-ate enrichments in LILE and Pb.
The REEAkinkB observed in the Mienhuayu vol-canics needs an explanation. Unlike typical boninites
Ž .
whose REE patterns are V-shaped Fig. 6c , the Mienhuayu lavas have a gradual slope change from
Ž .
LREE to MREE Fig. 6 and thus show the ‘kinkB feature. In comparison with E-MORB, they show a similar pattern of slight enrichment LREE, but lower
Ž .
HREE abundance Fig. 6c . The LREE enrichment, however, is apparently lower than that revealed by
Ž .
the NW Taiwan intraplate tholeiitic basalts Fig. 6b . Melt–wall rock interaction may occur during magma
Ž .
ascent cf. Kelemen, 1990 and this interaction could reduce LREE contents relative to HREE in the
as-Ž .
cending magmas Kelemen et al., 1993 . Following
Ž .
the interpretation by Shinjo 1999 for a similar AkinkB REE feature observed in the southern Ryukyus high-Mg andesites, we also adopt the melt–mantle interaction model to account for the REE pattern of the Mienhuayu volcanics.
8. Concluding remarks
The Mienhuayu islet, formed mainly by subaerial effusion at about 2.6 Ma, is composed of basaltic andesite lavas and subordinate scoria deposits. These rocks show high-Mg characters that may represent silica-saturated melts derived from a shallow mantle source. Compared with other NTVZ volcanics, the Mienhuayu high-Mg basaltic andesites exhibit dis-tinct geochemical features without HFSE depletion. Their overall geochemical affinities are similar to Miocene intraplate basalts from NW Taiwan and
( )
K.-L. Wang et al.r Chemical Geology 182 2002 513–528 527 high-Mg andesites from the southern Ryukyus. Such
similarities allow us to conclude that the Mienhuayu magmas were derived from an E-MORB type, as-cended asthenospheric mantle as a result of exten-sional collapse of the northern Taiwan mountain belt at the Plio–Pleistocene boundary. Eruption of the Mienhuayu lavas supports the notion that a substan-tial lithospheric extensional regime occurred before, and thus probably accounts for, the southwestward propagation of the Okinawa Trough.
Acknowledgements
We thank C.Y. Lee and X.H. Li for their help in arranging XRF and ICP-MS analysis, and F.T. Yang, C.H. Lo and Y.G. Chen for helpful discussion. We are most grateful for the detailed and constructive reviews provided by I. Graham, J. Gamble and jour-nal editor R. Rudnick, which significantly improved the content and presentation of this paper. This study benefited from financial support of the National Sci-ence Council and Institute of Earth SciSci-ences, Academia Sinica, Taiwan.
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