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40Ar–39Ar dating and geochemical characteristics of late Cenozoic basaltic rocks from the Zhejiang–Fujian region, SE China: eruption ages, magma evolution and petrogenesis

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40

Ar–

39

Ar dating and geochemical characteristics of late Cenozoic

basaltic rocks from the Zhejiang–Fujian region, SE China:

eruption ages, magma evolution and petrogenesis

Kung-suan Ho

a

, Ju-chin Chen

b,

*, Ching-hua Lo

c

, Hai-ling Zhao

d a

Department of Geology, National Museum of Natural Science, Taichung 404, Taiwan, ROC b

Institute of Oceanography, National Taiwan University, P.O. Box 23-13, Taipei 106, Taiwan, ROC c

Department of Geosciences, National Taiwan University, Taipei 106, Taiwan, ROC d

China University of Geosciences, Beijing 100083, China Received 2 May 2002; accepted 12 November 2002

Abstract

The Zhejiang – Fujian late Cenozoic volcanism took place sporadically in four volcanic belts and constitutes an important diffuse continental rift basalt province in eastern China. The volcanic rocks consist predominantly of basanite, alkali olivine basalt, olivine tholeiite and quartz tholeiite with subordinate nephelinite and rare alkali picrite basalt. Twenty-four new ages of the basaltic rocks and amphibole megacryst determined by40Ar –39Ar incremental heating experiments demonstrate that the basaltic lava erupted from 0.9 to 26.4 Ma. Although some basanite dikes and nephelinite pipes in Inner and Inner Middle belts of the Zhejiang area are early Miocene, almost all the late Cenozoic basaltic volcanism occurred following cessation of South China Sea seafloor spreading ( V 16 Ma). Based on these results, most late Cenozoic intraplate magmatism surrounding the South China Sea margins may be related to the migration of the South China Sea mid-ocean ridge system beneath SE China since mid-Miocene. However, in the Zhejiang – Fujian region, volcanic activities terminated gradually and propagated westward and northward due to the collision of the Luzon arc with the eastern edge of the Eurasia continent during late Miocene to Pleistocene.

The Zhejiang – Fujian basalts exhibit trace element and isotopic affinities with OIB. Sr and Nd isotope compositions range from 0.703264 to 0.704235 and 0.512725 to 0.512961, respectively, similar to the composition of the Leiqiong basalts in South China. The enrichment of LREE coupled with depleted Sr – Nd isotopic compositions in the basaltic rocks imply that recent mantle metasomatism occurred shortly before the Cenozoic magmatism. The Sr – Nd – Pb isotopic compositions as well as high LILE/HFSE ratios found in some basaltic rocks showed that an EM2-type lead isotope signature existed in the continental lithospheric mantle in the Zhejiang – Fujian region. This character may have resulted from mantle metasomatism due to a paleo-subduction event. The Zhejiang – Fujian basaltic rocks were generated by partial melting and mixing of different proportions of depleted asthenospheric mantle (DMM or MORB) with EM2-type lithospheric mantle and have undergone different degree of fractional crystallization when the magma ascended to the surface.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords:40Ar –39Ar age; Petrogenesis; Magmatism; Late Cenozoic; South China

0009-2541/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2541(02)00399-6

* Corresponding author. Fax: +886-2-23626092. E-mail address: jcchen@oc.ntu.edu.tw (J. Chen).

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1. Introduction

Zhejiang, Fujian, Guangdong and Hainan provin-ces of SE China lie on the East Asiatic continental margin with abundant Cenozoic basalts belonging to the circum-Pacific volcanic belt. Volcanism is gener-ally attributed to mantle upwelling induced by passive extension or asthenosphere extension following the collision of India into Eurasia, before the termination by a return to a more compressional tectonic regime

(Chung et al., 1994; Smith, 1998).

According to the timing of cessation of the South China Sea floor spreading (f 16 Ma, Chung et al., 1997), the late Cenozoic volcanic activity in SE China may be divided into two periods: (1) volcanic rocks of the 1st period (>16 Ma)—the sparse volcanic activ-ities occurred and produced small amounts of lavas and pyroclastics. Some of the basalt lavas, basanite dikes and nephelinite pipes have been dated including the surface outcrop samples in Puning, Guangdong province, Zhejiang Inner volcanic belt and borehole samples in the northern part of Hainan Island, and radiometric ages yielded 34.3 – 16.3 Ma (Sun, 1991; this study). (2) Volcanic rocks of the 2nd period ( V 16 Ma)—the eruption process of this period was partic-ularly active, and a large amount of basaltic lavas with minor pyroclastic rocks erupted to form a diffuse volcanic province.

In Zhejiang – Fujian area, the volcanic rocks are mainly distributed near the coastal area, including Shengxian, Xinchang, Ninghai, Linhai and Longhai. Occurrences of basaltic rocks in other areas are gen-erally small and scattered (Fig. 1). Development of these volcanic rocks might have been essentially controlled by a series of northeast – southwest trending fault system, where many isolated tectono-volcanic basins, calderas and volcanic rents of different sizes occurred. According to the relation between the vol-canic rock crop out and deep fault distribution

(Bureau of Geology and Mineral Resources of Zhe-jiang Province, 1989; Bureau of Geology and Mineral

Resources of Fujian Province, 1985), four NE

trend-ing volcanic belts can be divided from east to west

(Fig. 1), namely, Xiamen – Longhai – Dongshan belt

(Outer belt), Ninghai – Linhai – Minqing – Puning belt (Outer Middle belt), Shengxian, Xinchang – Jinyun – Songxi – Yongding belt (Inner Middle belt), Zhuji – Longyou – Quzhou belt (Inner belt of Zhejiang) and Mingxi – Qingliu – Changting belt (Inner belt of Fujian). Based on the geophysical data(Ma and Wu,

1987), the crustal and lithospheric thickness gradually

increased from the coastal to inland areas.

Previous age dating for late Cenozoic basaltic rock in Zhejiang – Fujian area was mainly by K – Ar method

(Wang and Yang, 1987; Liu et al., 1992). Although

more than 10 age data of the Zhejiang basaltic rocks have been published in the last two decades, whereas the K – Ar age determinations on the basaltic speci-mens were mainly concentrated in the Shengxian and Xinchang regions. The virtual absence of age data in some volcanic regions such as Inner belt of Zhejiang province has limited the study of the magmatic evolution. In addition, it is worth noting that the Ar – Ar ages of the basaltic rocks in some regions are always younger than the ages obtained by the K – Ar method. Owing to some problems in dating vol-canic rocks by the K – Ar method, e.g., loss of Ar or gain of K associated with alteration and inherited argon residing in phenocryst can be overcome by the40Ar –39Ar technique(Lo et al., 1994). Therefore, we used40Ar –39Ar method to determine the eruption ages of basaltic rocks from four volcanic belts in the Zhejiang – Fujian region.

In addition, we report the major and trace element abundances, and Sr – Nd isotope data for basaltic rocks from Zhejiang – Fujian volcanic province. These re-sults are used to (1) determine the temporal and spatial distribution of the volcanic activity, and (2) identify the geochemical characteristics of the magma source regions and understand the genesis of late Cenozoic basalts. Besides, the Zhejiang – Fujian region offers a good opportunity to investigate the route of magma migration that is essentially intraplate in origin, being formed after the cessation of the South China Sea

Fig. 1. Map of the SE China showing major outcrops of late Cenozoic extension-related basaltic rocks and sample sites in Zhejiang – Fujian area. Four volcanic belts: 1A, Inner belt of Zhejiang; 1B, Inner belt of Fujian; 2, Inner Middle belt; 3, Outer Middle belt; 4, Outer belt are also shown. Distribution of basaltic rocks modified afterBureau of Geology and Mineral Resources of Zhejiang Province (1989)andBureau of Geology and

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seafloor spreading in mid-Miocene. Therefore, we combine the radiometric age and geochemical data to monitor the migration route of the late Cenozoic magmatism in SE China.

2. Analytical methods

Twenty-three basaltic rocks and one amphibole megacryst were dated by40Ar/39Ar incremental heat-ing method. Samples were carefully selected to avoid alteration, on the bases of petrographic examination and chemical composition, which shows low loss-on-ignition content (L.O.I. < 3 wt.%). An 80 – 120-mesh sieve-sized fraction of the samples was chosen and ultrasonically cleaned in acetone and distilled water baths. Weighted aliquots of samples (0.5104 – 1.6021 g) were wrapped in aluminum foil packet and stacked in aluminum canister with LP-6 biotite standard (127.7 F 1.4 Ma,Odin et al., 1982) and HDB-1 biotite (24.7 F 0.4 Ma,Fuhrmann et al., 1987), and irradiated in position VT-C of the THOR Reactor, with neutron flux of 1.566  1013 n/cm2s, at National Tsing-Hua University for 8 h. After irradiation, samples were loaded in degassed fused silica boats, and placed into a degassed fused quartz tube, baked at 250 jC for 24 h, and then incrementally heated following a 30-min per step schedule using a resistance furnace. The purified gas was analyzed with a Varian-MAT GD150 mass spectrometer. The concentrations of36Ar,37Ar,38Ar,

39

Ar and 40Ar were corrected for system blanks, for radioactive decay of the nucleogenic isotopes, and for minor interference reactions involving Ca, K and Cl, following procedures outlined in detail byLo and Lee

(1994). The gradient of the neutron flux across the

irradiated canister was less than 0.5% as reflected in the J-value variation between the standards at the top and the bottom of the canister. A weighted mean of J values obtained from the analyses of irradiation stand-ard minerals is adopted in the age calculations. Inte-grated dates were calculated from the sum total of the peak heights and their errors from the square root of the sum of squares of the peak height errors, for all of the temperature steps. Plateau dates were calculated by the same approach, but utilizing only those temper-ature steps yielding dates on the plateau. As discussed

byLanphere and Dalrymple (1978), a plateau should

meet the following three requirements: (1) at least four

successive temperature steps with dates that fall within 2r of the average; (2) the gas fractions for these plateau steps, which must comprise more than 50% of total39Ar released; and (3) the plateau date should be concordant with its respective intercept date, with reasonable 40Ar/36Ar intercept value, obtained from the isotope correlation plots. Data were further plotted on age spectra and36Ar/40Ar –39Ar/40Ar isotope cor-relation diagrams. The40Ar/39Ar dates and 40Ar/36Ar intercept values were calculated from the intercept of the regressed line, respectively. The cubic least-square fitting scheme outlined byYork (1969)was employed in regressing the data. Because the39ArK,38ArCl and 37

ArCa release data potentially reflect the chemical

compositions (K, Cl and Ca, respectively) of the samples, the data are also plotted as Ca/K and Cl/K spectra for each sample. Ca/K and Cl/K ratios are calculated according to the relationships Ca/K = 1.82 ( F 0.17) 37ArCa/39ArK and Cl/K = 0.22 ( F 0.04)  38

ArCl/39ArK, as described byLo and Lee (1994). The

detailed 40Ar/39Ar analytical data are available on request from the senior author.

A set of 85 samples was analyzed for major and trace element abundances. All elements except Si and Al were determined using solutions prepared by dis-solving 0.5 g of rock powder in a mixture of ultrapure HF and HNO3 in Teflon beakers under clean room

condition. Solutions were analyzed using a Perkin-Elmer Model 5100PC atomic absorption spectropho-tometer (Fe, Mg, Ca, Na, K, Mn, Co, Cr, Cu, Li, Ni, Rb, Sr, Zn) and inductively coupled plasma mass spectrometry (Ba, Hf, Nb, Sc, Th, V, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Yb, Lu) at National Museum of Natural Science and National Tsing-Hua Univer-sity. For Si and Al determinations, the solutions were prepared by NaOH fusion of 0.05 g rock powder in nickel crucibles followed by water leaching and HCl acidification. The analyses of Si, Al, Ti and P were carried out by the colorimetric method using an SP-2000 spectrophotometer at the National Taiwan Uni-versity. Calibration curves were constructed using US Geological Survey rock standards AGV-1, BCR-1, W-1 and G-2 and Geological Survey of Japan rock standard JB-1. Values for these rock standards were adapted from compilations by Govindaraju (1994). The precision is estimated to be around F 2% for atomic absorption and colorimetric methods and bet-ter than F 5% for all ICP-MS analyses(Table 1). The

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details of the analytical techniques have been dis-cussed byHo (1998).

Eleven representative basaltic rocks were analyzed for Sr – Nd isotopic composition following the proce-dure ofSmith and Huang (1997). Isotopic composi-tions were measured using a MAT 262 mass spec-trometer at National Cheng Kung University. Nd isotopic ratios have been normalized to146Nd/144Nd = 0.7219 and are reported relative to 0.511855 for the La Jolla standard. Sr isotopic ratios are normalized to

86

Sr/88Sr = 0.1194 and are relative to 0.710240 for the SRM987 standard. Precision of Nd and Sr isotopic compositions is better than F 0.000010 (2r).

3. Petrography and principal rock types

Basaltic lava flow is the major type of volcanic activities in Zhejiang – Fujian region, and sometimes basanitic dikes or breccia pipes also occurred. The basaltic rocks are mostly massive, and exhibit por-phyric, fine granular and aphyric textures. The phe-nocrysts consist predominantly of lath plagioclase and interstitial clinopyroxene in tholeiite; of clinopyrox-ene, olivine and plagioclase in alkali olivine basalt; and of olivine and clinopyroxene in basanite, alkali picrite basalt and nephelinite. The groundmass in tholeiite and alkali olivine basalt is rather variable,

Table 1

Precision test for international rock standard W-1

Element Analytical Literature Replicate analyses by present study Average S.D.d C.V.e

methoda valueb W101 W102 W103 W104 W105 value c (%) (wt.%) SFeO 1 10.05 9.97 9.91 10.23 9.87 9.81 9.96 0.15 1.46 MgO 1 6.62 6.55 6.54 6.44 6.59 6.67 6.56 0.08 1.14 CaO 1 10.99 11.23 11.15 10.82 10.78 10.78 10.95 0.20 1.79 Na2O 1 2.16 2.13 2.14 2.18 2.18 2.16 2.16 0.020 0.95 K2O 1 0.64 0.64 0.63 0.65 0.66 0.65 0.65 0.010 1.58 MnO 1 0.17 0.165 0.164 0.168 0.172 0.171 0.168 0.003 1.88 (ppm) Li 1 12.8 12.3 12.3 12.8 12.8 12.7 12.6 0.23 1.84 Cr 1 119 116 115 118 120 121 118 2.28 1.93 Cu 1 113 110 113 113 111 112 112 1.17 1.04 Zn 1 84 87 85 86 84 84 86 1.17 1.37 Rb 1 21.4 22.1 21.9 21.1 21.2 22.0 21.7 0.42 1.95 Sr 1 186 185 188 183 182 186 185 2.14 1.16 Y 2 26 25 25 24 24 25 25 0.49 1.99 Nb 2 9.9 10.1 9.4 9.1 9.7 9.1 9.5 0.38 4.03 Ba 2 162 166 161 165 163 162 163 1.86 1.14 La 2 11 10.8 11.0 10.9 10.6 10.7 10.8 0.14 1.31 Ce 2 23.5 23.7 23.0 23.6 22.8 23.3 23.3 0.34 1.47 Nd 2 14.6 14.8 15.2 14.6 14.0 14.7 14.7 0.39 2.65 Sm 2 3.68 3.45 3.55 3.53 3.44 3.56 3.51 0.051 1.45 Eu 2 1.12 1.15 1.13 1.13 1.09 1.09 1.12 0.024 2.15 Tb 2 0.63 0.67 0.67 0.64 0.62 0.63 0.64 0.021 3.19 Yb 2 2.03 2.17 2.15 2.18 2.07 2.09 2.13 0.044 2.06 Lu 2 0.317 0.35 0.35 0.32 0.34 0.33 0.34 0.012 3.45 Hf 2 2.5 2.3 2.6 2.4 2.5 2.4 2.4 0.10 4.18 Th 2 2.4 2.5 2.6 2.5 2.4 2.4 2.5 0.08 3.02 a

Present study: 1, AAS; 2, ICP-MS. b Govindaraju (1994). c Present study. d Standard deviation¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N Xn i¼1 ðXi X Þ2 s . eCoefficient of variation=(S.D./X )  100.

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and is mainly composed of fine- to medium-grained plagioclase, clinopyroxene, olivine and Fe – Ti oxide minerals. Nephelinites and a few basanites with essentially no feldspar phase, but with abundant nepheline and accessory analcite, occur as pipe or lava flow. In addition, alkali feldspar is present in minor amount in most of the alkali basalts.

Based on Ne – Ol – Di – Hy – Qz tetrahedron of

Yoder and Tilley (1962), the basaltic rocks can be

classified into three types, e.g., quartz tholeiite, oli-vine tholeiite and alkali basalt(Fig. 2A). The chemical compositions of the Zhejiang – Fujian alkali basalts vary considerably (e.g., SiO2= 39.62 – 50.33 wt.%,

Na2O + K2O = 2.52 – 7.83 wt.%,Table 2), accordingly

Fig. 2. (A) Normative compositions of basaltic rocks from the Zhejiang – Fujian area. Nomenclature of volcanic rocks afterYoder and Tilley

(1962). (B) Plots of normative nepheline against normative olivine compositions for alkali basalts. Field boundaries afterChih (1988). Zhejiang

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Table 2

Major and trace element compositions of representative basaltic rocks from Zhejiang – Fujian area Province Zhejiang

Volcanic belt Outer Middle Inner Middle

Locality Linhai Ninghai Jinyun Tiantai Xinchang

Sample no. Z-6A Z-7 Z-8 Z-10 Z-11B Z-3A Z-5A Z-12B Z-15

Rock typea OT QT OT OT QT B N B B (wt.%) SiO2 49.47 50.86 49.22 50.19 50.79 45.42 42.04 41.51 48.66 Al2O3 13.60 13.87 13.62 13.41 13.65 12.33 11.46 12.12 14.60 SFeOb 11.86 11.34 10.90 11.14 10.65 12.21 14.36 13.65 11.34 MgO 7.35 6.85 8.27 7.85 7.32 9.95 6.94 10.38 3.50 CaO 9.53 9.36 9.05 9.28 9.09 10.82 12.95 11.19 6.65 Na2O 2.92 2.87 2.93 2.86 2.85 3.06 4.30 4.11 4.38 K2O 0.98 0.84 1.02 0.88 0.65 1.77 2.31 1.25 3.45 TiO2 2.92 2.66 2.50 2.58 2.16 2.60 2.77 2.91 2.44 P2O5 0.64 0.48 0.43 0.44 0.30 0.82 1.48 0.84 1.12 MnO 0.154 0.154 0.159 0.163 0.166 0.180 0.236 0.206 0.169 L.O.I. 0.70 0.39 1.34 1.31 1.69 1.19 0.96 1.74 2.95 Total 100.124 99.674 99.439 100.103 99.316 100.350 99.806 99.906 99.259 MGc 57.99 57.37 62.83 61.09 60.49 64.48 51.85 62.88 40.74 (ppm) Li 4.5 4.5 6.1 4.9 8.3 13.0 22.5 9.8 33.6 Sc 21.2 21.3 21.0 22.1 20.8 18.8 14.6 19.7 9.4 V 182 171 155 158 167 191 144 208 78 Cr 226 220 286 263 234 272 122 224 36 Co 56.3 50.7 51.1 51.2 48.4 61.3 49.5 61.4 36.3 Ni 168 135 199 185 151 233 107 180 51 Cu 91 75 76 69 65 137 43 62 29 Zn 114 114 166 113 108 113 183 130 190 Rb 18.2 15.7 15.5 9.3 6.5 49.4 49.7 40.9 4.47 Sr 502 487 535 574 420 924 1948 1167 1621 Y 25 24 24 25 21 25 44 29 38 Zr 143 139 157 172 123 156 322 209 406 Nb 18.9 18.0 19.9 21.9 14.7 37.7 91.9 60.3 80.9 Ba 295 238 273 285 211 601 737 691 945 La 16.0 13.6 18.2 19.3 11.9 37.5 83.2 45.3 62.8 Ce 36.6 31.1 38.3 40.1 25.8 68.7 154.1 81.8 117.7 Nd 26.0 22.3 25.9 27.8 17.8 36.1 81.8 46.1 69.8 Sm 7.36 6.72 7.35 7.94 5.55 7.94 17.69 11.08 17.49 Eu 2.64 2.54 2.73 3.09 2.16 2.59 6.58 4.45 7.29 Tb 0.65 0.68 0.71 0.77 0.70 0.61 0.92 0.89 1.10 Yb 1.43 1.57 1.66 1.85 1.73 1.33 1.84 1.88 2.11 Lu 0.20 0.26 0.27 0.31 0.29 0.12 0.24 0.30 0.38 Hf 4.0 4.3 5.1 5.8 4.2 3.6 8.5 6.8 12.6 Th 1.4 1.6 2.6 2.8 2.1 5.1 11.5 7.1 10.2 U 0.82 1.24 1.66 1.80 1.89 1.24 3.07 3.16 4.90 Ba/Rb 16.2 15.2 17.6 30.7 32.5 12.2 14.8 16.9 211.4 Zr/Nb 7.6 7.7 7.9 7.9 8.4 4.1 3.5 3.5 5.0 Zr/Y 5.7 5.8 6.5 6.9 5.9 6.2 7.3 7.2 10.7 La/Ce 0.44 0.44 0.48 0.48 0.46 0.56 0.54 0.55 0.53 Ce/Sm 5.0 4.6 5.2 5.1 4.7 8.7 8.7 7.4 6.7 (La/Yb)N 8.0 6.2 7.9 7.5 4.9 20.2 32.4 17.3 21.4

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Province Zhejiang Volcanic belt Inner Middle

Locality Xinchang Shengxian

Sample no. Z-16 Z-18A Z-19 Z-20 Z-21 Z-22 Z-23A Z-24 Z-25

Rock typea B AOB OT B AOB QT OT OT B

(wt.%) SiO2 44.45 47.24 50.86 40.80 48.56 50.92 51.99 51.23 39.62 Al2O3 14.70 14.52 14.28 11.92 13.53 14.20 14.12 13.89 11.87 SFeOb 12.11 11.29 10.99 13.64 11.60 10.68 11.24 10.86 13.89 MgO 7.66 7.65 7.04 11.08 8.52 6.52 6.99 7.15 10.43 CaO 9.28 9.14 8.92 11.05 9.13 8.89 8.78 8.94 11.07 Na2O 3.43 3.01 3.04 3.91 3.35 3.09 3.40 3.39 3.64 K2O 1.65 1.53 1.17 1.15 1.21 0.70 1.24 1.15 1.14 TiO2 2.66 2.38 2.05 2.69 2.41 2.07 2.18 2.10 2.85 P2O5 0.69 0.59 0.35 1.09 0.47 0.36 0.39 0.38 1.19 MnO 0.171 0.187 0.160 0.205 0.178 0.168 0.16 0.16 0.217 L.O.I. 2.77 2.22 0.77 2.03 1.30 2.60 0.01 0.15 3.53 Total 99.571 99.757 99.630 99.565 100.258 100.198 100.50 99.40 99.447 MGc 58.49 60.15 58.80 64.41 62.07 57.63 58.08 59.46 62.59 (ppm) Li 26.3 5.7 4.6 10.0 6.8 7.5 7.0 7.1 12.4 Sc 16.9 18.3 20.3 14.9 18.5 19.5 20.0 20.5 14.6 V 176 181 179 169 169 168 185 182 177 Cr 175 182 188 281 244 212 184 183 222 Co 51.8 50.2 49.3 59.8 53.5 45.3 51.1 50.8 60.0 Ni 162 126 121 244 188 123 148 148 202 Cu 82 60 74 70 75 69 79 75 72 Zn 141 117 110 141 113 111 120 116 159 Rb 31.8 32.9 22.1 41.2 23.5 11.1 18.6 17.3 27.7 Sr 1036 678 502 1402 641 496 503 527 1250 Y 28 24 22 26 21 22 24 23 29 Zr 260 167 133 211 122 124 130 120 242 Nb 62.0 39.7 23.4 71.8 27.4 20.5 21.7 21.3 83.5 Ba 559 432 263 697 334 264 301 305 218 La 38.5 26.2 17.6 47.9 20.7 17.2 15.5 15.2 57.1 Ce 75.1 51.0 35.6 90.3 40.0 33.5 31.6 30.5 101.9 Nd 38.2 26.4 19.7 45.1 22.0 19.0 18.5 18.2 52.3 Sm 8.74 6.14 4.98 9.68 5.37 5.14 5.01 5.00 10.95 Eu 2.83 2.04 1.66 3.09 1.75 1.69 1.71 1.77 3.41 Tb 1.18 0.92 0.81 1.32 0.88 0.85 0.89 0.88 1.37 Yb 1.59 1.54 1.48 1.58 1.57 1.58 1.67 1.71 1.52 Lu 0.22 0.22 0.21 0.25 0.25 0.24 0.24 0.29 0.19 Hf 5.8 3.8 3.1 5.0 3.1 3.3 3.3 3.2 5.7 Th 6.0 4.2 2.8 8.0 3.1 2.6 2.2 2.2 8.8 U 1.50 1.03 0.65 2.01 0.88 0.65 0.59 0.84 2.23 Ba/Rb 17.6 13.1 11.9 16.9 14.2 23.8 16.2 17.6 7.9 Zr/Nb 4.2 4.2 5.7 2.9 4.5 6.1 6.0 5.6 2.9 Zr/Y 9.3 7.0 6.1 8.1 5.8 5.6 5.4 5.2 8.3 La/Ce 0.51 0.51 0.49 0.53 0.52 0.51 0.49 0.50 0.56 Ce/Sm 8.6 8.3 7.2 9.3 7.5 6.5 6.3 6.1 9.3 (La/Yb)N 17.4 12.2 8.5 21.8 9.5 7.8 6.7 6.4 27.0 Table 2 (continued)

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Province Zhejiang Fujian

Volcanic belt Inner Middle Inner Inner

Locality Shengxian Zhuji Longyou Quzhou Mingxi

Sample no. Z-26A Z-28B Z-1A Z-1C Z-31B Z-32A Z-33A F-1B F-2A

Rock typea AOB AOB B AOB B N N N N

(wt.%) SiO2 50.33 49.27 46.86 46.62 42.51 40.56 39.57 41.4 43.35 Al2O3 13.53 14.50 13.68 13.17 13.52 11.22 11.67 12.7 12.52 SFeOb 10.83 10.53 11.60 12.01 11.50 12.42 13.34 12.8 11.64 MgO 7.44 6.35 8.83 8.85 9.43 12.07 9.69 9 11.45 CaO 8.66 8.30 9.90 9.48 10.12 11.23 11.15 11.2 10.75 Na2O 3.78 4.13 3.37 3.29 4.57 5.03 3.91 3 3.92 K2O 1.50 2.34 1.78 1.58 1.84 2.21 1.44 2 2.20 TiO2 2.31 2.86 2.34 2.29 2.50 2.47 3.12 3 2.47 P2O5 0.56 0.79 0.70 0.59 0.91 1.42 1.38 1 1.14 MnO 0.156 0.154 0.17 0.177 0.175 0.194 0.184 0.1 0.18 L.O.I. 0.31 0.42 0.72 1.35 2.39 0.63 4.23 1 0.14 Total 99.406 99.644 99.95 99.407 99.465 99.454 99.684 9.9 99.76 MGc 60.48 57.33 62.91 62.15 64.62 68.41 61.81 63.20 68.67 (ppm) Li 9.0 11.5 7.3 8.1 15.2 15.9 109.7 14.5 9.1 Sc 18.3 15.1 17.3 18.5 18.7 14.0 15.5 21.6 19.0 V 178 216 179 179 165 156 143 188 160 Cr 217 48 227 225 212 373 208 159 326 Co 48.8 40.4 56.2 53.5 52.5 57.3 59.8 62.3 57.4 Ni 179 76 194 192 190 330 186 182 307 Cu 62 65 80 79 54 46 64 52 56 Zn 117 122 121 116 122 143 137 138 112 Rb 28.9 47.8 33.8 27.8 21.5 38.4 25.9 87.8 62.5 Sr 633 901 788 762 1318 1539 1384 1604 1302 Y 24 31 23 24 29 32 32 38 30 Zr 156 234 157 157 261 320 293 387 301 Nb 33.7 55.2 39.1 40.1 96.9 157.2 116.5 48.0 52.7 Ba 388 695 473 462 937 956 77 1068 965 La 22.0 35.0 25.7 25.1 64.3 98.0 89.4 97.5 81.1 Ce 43.0 66.6 49.3 47.7 117.6 164.6 150.5 179.7 145.8 Nd 23.9 33.5 24.9 25.3 55.3 67.8 86.5 86.4 65.8 Sm 6.08 8.07 6.29 6.01 10.72 13.20 16.62 15.69 11.62 Eu 2.01 2.68 2.27 2.03 3.39 4.29 4.58 3.67 2.89 Tb 0.93 1.17 0.65 0.57 1.27 1.60 1.46 1.83 1.42 Yb 1.62 1.98 1.43 1.28 1.72 1.70 2.00 2.91 2.22 Lu 0.20 0.28 0.22 0.18 0.21 0.21 0.20 0.38 0.30 Hf 3.8 5.3 3.9 3.6 5.8 7.2 9.8 7.9 6.2 Th 3.8 5.6 3.9 3.2 10.4 16.2 13.9 13.2 12.0 U 0.79 1.29 1.56 0.88 2.32 3.56 3.01 2.99 2.64 Ba/Rb 13.4 14.5 14.0 16.6 43.6 24.9 3.0 12.2 15.4 Zr/Nb 4.6 4.2 4.0 3.9 2.7 2.0 2.5 8.1 5.7 Zr/Y 6.5 7.6 6.8 6.5 9.0 10.0 9.2 10.2 10.0 La/Ce 0.51 0.53 0.52 0.53 0.55 0.60 0.59 0.54 0.56 Ce/Sm 7.1 8.3 7.8 7.9 11.0 12.5 9.1 11.5 12.6 (La/Yb)N 9.7 12.7 12.9 14.1 26.8 41.4 32.1 24.0 26.2 Table 2 (continued)

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Province Fujian

Volcanic belt Inner Outer Middle Outer

Locality Mingxi Qingliu Minqing Longhai

Sample no. F-3A F-4A F-5 F-9B F-6 F-10B F-11 F-12 F-14B

Rock typea QT APB AOB OT OT AOB AOB AOB QT

(wt.%) SiO2 53.92 42.01 46.87 51.85 51.75 48.01 47.81 47.58 52.13 Al2O3 14.34 11.98 12.95 14.48 14.15 13.15 13.40 13.25 15.41 SFeOb 9.64 12.15 11.29 9.68 9.61 9.52 9.69 10.24 9.96 MgO 6.60 13.08 9.93 7.23 8.09 10.46 9.06 9.68 5.09 CaO 8.53 11.57 9.26 8.08 7.95 8.95 9.05 8.96 7.75 Na2O 3.05 1.34 2.99 3.25 3.11 2.94 2.58 2.71 3.24 K2O 1.04 1.18 1.93 1.20 1.25 2.19 2.51 2.50 1.61 TiO2 1.87 2.87 2.66 1.88 1.88 2.04 2.06 2.11 1.95 P2O5 0.40 1.11 0.84 0.47 0.54 0.79 0.81 0.84 0.54 MnO 0.131 0.188 0.174 0.130 0.152 0.156 0.147 0.15 0.128 L.O.I. 0.66 3.31 0.63 0.88 1.35 1.65 2.83 1.67 1.80 Total 100.281 100.788 99.524 99.130 99.832 99.856 99.947 99.59 99.608 MGc 60.40 70.57 66.21 62.46 65.22 71.00 67.56 67.80 53.24 (ppm) Li 4.8 8.1 7.7 7.3 6.6 8.1 6.1 7.1 3.6 Sc 17.9 19.3 21.0 17.7 17.9 16.9 17.6 18.0 16.8 V 116 174 174 127 128 139 143 127 129 Cr 257 369 272 294 332 386 372 322 130 Co 48.5 62.3 61.8 42.2 80.2 49.4 49.1 49.0 37.7 Ni 176 333 251 207 250 304 287 252 95 Cu 71 52 58 81 85 67 65 62 53 Zn 103 118 120 114 143 106 109 109 142 Rb 17.6 127.3 44.1 25.4 25.6 65.9 60.3 55.9 48.7 Sr 513 1560 971 522 561 1181 1134 1191 570 Y 22 32 27 29 23 25 26 24 22 Zr 138 307 239 153 163 279 296 247 226 Nb 16.3 90.6 78.5 27.6 38.1 68.4 67.0 69.8 43.0 Ba 272 903 710 333 361 850 774 735 428 La 20.8 85.1 56.6 32.3 29.6 58.8 58.9 53.0 32.4 Ce 41.3 154.2 106.5 49.4 58.3 110.0 110.9 100.2 59.9 Nd 23.1 73.7 52.4 38.8 29.5 50.5 52.5 55.0 33.9 Sm 5.57 13.19 9.84 9.26 6.40 9.34 9.79 10.32 7.70 Eu 1.04 2.26 2.49 2.32 1.59 1.70 1.77 2.80 2.43 Tb 0.74 1.59 1.17 1.20 0.80 1.12 1.21 0.97 0.91 Yb 1.65 2.33 2.05 2.37 1.64 1.86 2.00 1.52 1.31 Lu 0.24 0.31 0.26 0.30 0.24 0.24 0.23 0.20 0.17 Hf 3.4 6.0 5.9 3.6 4.0 5.9 6.3 6.5 5.7 Th 3.0 10.4 7.9 3.3 4.8 9.3 8.2 7.5 4.0 U 0.74 1.92 1.74 0.86 1.13 2.25 2.06 1.83 0.83 Ba/Rb 15.5 7.1 16.1 13.1 14.1 12.9 12.8 13.2 8.8 Zr/Nb 8.5 3.4 3.0 5.5 4.3 4.1 4.4 3.5 5.3 Zr/Y 6.3 9.6 8.9 5.3 7.1 11.2 11.4 10.3 10.3 La/Ce 0.50 0.55 0.53 0.65 0.51 0.53 0.53 0.53 0.54 Ce/Sm 7.4 11.7 10.8 5.3 9.1 11.8 11.3 9.7 7.8 (La/Yb)N 9.0 26.2 19.8 9.8 13.0 22.7 21.1 25.0 17.7 Table 2 (continued)

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Province Fujian Volcanic belt Outer

Locality Longhai Zhangpu Jinmen

Sample no. F-18A F-20 F-22 F-24 F-28 F-30 F-31 F-33 F-34

Rock typea OT OT QT AOB QT OT QT QT QT

(wt.%) SiO2 52.09 49.15 53.60 50.01 53.74 50.58 50.79 53.30 52.50 Al2O3 15.21 14.60 15.38 15.31 15.89 14.98 14.92 14.86 15.38 SFeOb 9.89 10.27 9.17 9.98 8.99 9.86 10.44 9.59 10.03 MgO 6.46 7.77 7.11 6.43 6.45 7.10 5.98 7.15 7.34 CaO 7.32 8.61 8.02 7.57 9.31 7.56 7.49 9.59 9.33 Na2O 3.24 2.88 2.71 3.55 2.58 3.13 2.94 2.72 2.75 K2O 2.04 1.80 0.64 2.54 0.39 2.00 1.55 0.89 0.48 TiO2 1.97 2.00 1.21 2.12 1.20 2.10 2.46 1.70 1.49 P2O5 0.52 0.43 0.19 0.59 0.14 0.52 0.59 0.24 0.19 MnO 0.129 0.137 0.121 0.122 0.129 0.131 0.145 0.148 0.151 L.O.I. 0.54 2.20 1.36 1.52 0.66 2.18 1.99 0.21 0.34 Total 99.409 99.847 99.511 99.742 99.479 100.141 99.295 100.398 99.981 MGc 59.27 62.76 63.33 58.94 61.51 61.60 56.07 62.42 61.98 (ppm) Li 5.1 4.3 3.0 6.3 3.0 6.5 4.7 5.0 4.0 Sc 16.2 22.6 21.0 15.4 22.1 15.9 19.5 21.8 22.6 V 141 181 133 153 125 125 154 154 153 Cr 222 268 258 120 207 230 175 228 206 Co 41.6 47.5 45.7 46.0 46.4 46.0 59.5 52.1 51.0 Ni 151 175 157 119 139 151 185 200 197 Cu 48 58 54 40 84 54 46 73 72 Zn 116 101 86 120 84 113 129 108 103 Rb 40.1 46.8 15.0 62.7 8.2 51.9 36.2 17.0 11.0 Sr 645 543 254 764 261 614 606 301 257 Y 19 22 14 22 16 21 28 20 21 Zr 245 216 98 268 91 235 281 122 112 Nb 30.6 43.2 16.2 52.4 13.2 55.2 46.2 17.5 15.6 Ba 533 466 158 587 137 521 371 163 129 La 37.5 33.9 11.6 45.4 8.9 38.8 31.5 12.6 9.7 Ce 72.0 65.0 23.1 91.2 19.0 70.3 65.8 26.2 21.2 Nd 36.1 29.7 11.8 39.7 11.6 33.7 37.1 15.9 13.4 Sm 6.98 5.90 3.43 7.71 3.28 6.64 8.01 4.00 3.50 Eu 2.01 2.37 1.22 2.97 1.19 2.05 2.17 1.38 1.21 Tb 0.71 0.85 0.47 0.95 0.49 0.85 0.98 0.65 0.61 Yb 1.30 1.47 1.07 1.37 1.15 1.38 1.55 1.66 1.56 Lu 0.13 0.17 0.12 0.17 0.14 0.18 0.23 0.21 0.20 Hf 6.2 4.9 2.1 5.9 2.3 5.9 6.3 2.9 2.7 Th 4.3 4.5 1.7 5.8 1.1 4.7 3.1 1.3 1.0 U 1.05 0.91 0.37 1.35 0.24 1.08 0.79 0.50 0.39 Ba/Rb 13.3 10.0 10.5 9.4 16.7 10.0 10.3 9.6 11.7 Zr/Nb 8.0 5.0 6.1 5.1 6.9 4.3 6.1 7.0 7.2 Zr/Y 12.9 9.8 7.0 12.2 5.7 11.2 10.0 6.1 5.3 La/Ce 0.52 0.52 0.50 0.50 0.47 0.55 0.48 0.48 0.46 Ce/Sm 10.3 11.0 6.7 11.8 5.8 10.6 8.2 6.6 6.1 (La/Yb)N 20.7 16.5 7.8 23.8 5.6 20.2 14.6 5.4 4.5 Table 2 (continued)

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based on CIPW normative data; they can be further subdivided into alkali olivine basalt, basanite, alkali picrite basalt and nephelinite (without normative albite) by the classification scheme proposed byChih

(1988) (Fig. 2B). In a survey of 85 samples (only 45

representative samples are presented inTable 2), the most abundant rock type is quartz tholeiite which constitutes about 27% of the rock samples recovered. The olivine tholeiite, alkali olivine basalt and basanite comprise about 25%, 20% and 18%, respectively, of the total samples. Only few samples analyzed fall within the nephelinite (9%) and alkali picrite basalt (1%) fields. Nephelinite and alkali picrite basalt occur as small isolated pipe or dike, and are only found in the Inner and Inner Middle belts.

In the Zhejiang province, tholeiitic rocks appear to be the most voluminous in the Outer Middle belt, but alkali basalts are the dominant type in the Inner Middle- and Inner belts. It is obvious that there is a general tendency of increase in alkali contents from the coastal areas to the interior of the continent(Fig. 3). However, it is worth noting in the Fujian province that these are coeval, but volumetrically major tho-leiitic rocks in the Mingxi and Qingliu areas of Inner belt.

4.40Ar –39Ar dating results

The40Ar/39Ar dating results for 24 samples from the Zhejiang – Fujian area are plotted as age spectrum and36Ar/40Ar –39Ar/40Ar isotope correlation diagrams

inFigs. 4 and 5. A summary of the results is given in

Table 3.

Fig. 4 shows the apparent age, Ca/K and Cl/K

spectra, and the isotope correlation diagrams for two analyzed samples (Z6A and Z8) from Outer Middle belt of Zhejiang province. Both samples (Z6A and Z8) yield identical integrated dates of 10.4 F 0.5 and 10.5 F 0.5 Ma, respectively. The apparent dates for temperature steps >420 jC (Z6A) and >470 jC (Z8) are all consistent with each other within 2r forming a perfect flat plateau over 95% of total gas released

(Fig. 4), with plateau date of 10.4 F 0.5 Ma for Z6A

and 10.5 F 0.5 Ma for Z8. Both plateau dates are all identical to their respective integrated dates. Never-theless, there are some anomalously old dates appear-ing in the first few steps for both samples. The corresponding Ca/K and Cl/K ratios for these low temperature steps are also slightly higher than those of plateau steps. These anomalously old dates are most likely due to the outgassing of alteration phases, since alteration phases often exhibit higher Cl and Ca contents with respect to ordinary constitute minerals in volcanic rocks, as discussed by Lo et al. (1994). However, considering the facts that most of the plateau steps show consistent Ca/K and Cl/K ratios and that the gas fraction released from these discord-ant steps at low temperatures is relatively low ( < 5%)

(Fig. 4), the possible effects of alteration in the present

samples seem to be minor and can be negligible. Regression of the data for plateau steps in the

36

Ar/40Ar –39Ar/40Ar correlation diagram indicates an intercept date of 10.1 F 0.5 Ma, and a 40Ar/36Ar initial value of 297.4 F 0.9, with the value of 0.77 for the mean sum of weighted deviate (MSWD), for Z6A. Whereas, Z8 sample yields an intercept date of 10.3 F 0.5 Ma and a 40Ar/36Ar initial value of 296.8 F 0.6, with an MSWD value of 1.05. These intercept date and the 40Ar/36Ar initial values are perfect in agreement with their corresponding plateau date and the atmospheric composition with 40Ar/36Ar ratio of 295.5, with MSWD values close to unity(Fig. 4). These would indicate that the K – Ar isotopic systematics in the samples has been kept in closed system since solidification, showing almost identical radiometric clocks in the samples.

Fig. 5shows integrated and plateau dates, and age

spectra for the other 22 samples analyzed. Similar to Z8 and Z6A, all the samples also display a fairly flat profile for the age spectra, exhibiting well-defined plateau over >68% of the total gas released, with some disturbed dates in the low- and high-temperature steps

(Fig. 5). The disturbed steps usually exhibit Ca/K and

Cl/K ratios distinct from those from the plateau steps, which may indicate the effects of outgassing from

Notes to Table 2: a

Rock types: QT, quartz tholeiite; OT, olivine tholeiite; AOB, alkali olivine basalt; B, basanite; N, nephelinite; APB, alkali picrite basalt. b

SFeO = SFe2O3/1.1, and assuming that Fe2O3= 0.2 FeO(Middlemost, 1989). c

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alteration phases and some phenocrysts, similar to those described previously byLo et al. (1994).

Regressions of plateau step data also suggest

40

Ar/36Ar initial values in the range of 290.8 – 322.0, which are close to the atmospheric values, and intercept dates of 0.8 – 26.8 Ma, which are also concordant with their respective plateau dates in the range of 0.9 – 26.4 Ma, with statistically meaningful MSWD values of < 2.83(Table 3). All these perfectly meet the criteria of meaningful plateau dates, pro-posed by Lanphere and Dalrymple (1978), strongly indicating that all the obtained plateau dates can be considered as the best estimate for the ages of vol-canic eruptions.

4.1. Zhejiang Segment

The oldest two samples dated are basanite Z31B from Longyou dike and nephelinite Z32A from Quz-hou pipe in Inner belt, 26.4 F 0.3 and 23.7 F 0.3 Ma, respectively. Based on the stratigraphic relationships and relevant dating information of the Fujian Inner belt, the volcanic activity in the Quzhou region has previously been considered to be Pliocene byLiu et al.

(1992). However, the present40Ar/39Ar dating results

show that the volcanic activity in this area may have taken place largely in early Miocene (Table 3). Vol-canic breccia in the Jinyun area has entrained varied types of mantle xenoliths and megacrysts. The amphib-ole megacryst (17.0 F 0.8 Ma) has similar radiometric age with the basanitic pipe (15.6 F 0.7 Ma), suggesting that the amphibole megacryst may represent the prod-uct of earlier or simultaneous magmatic activities, and was captured during eruptive event.

The most extensive basaltic lava activity in the Zhejiang province is later than Jinyun basanitic pipe, Quzhou nephelinitic pipe and Longyou basanitic dike intrusion. The respective Ar – Ar ages of 10 basaltic lavas fall between 10.5 F 0.5 and 2.5 F 0.1 Ma. These results show that a magmatic quiescent period appears

Fig. 3. Histogram showing distribution of basalt types for various volcanic belts in (A) Zhejiang and (B) Fujian Provinces. Numbers in longitudinal axis represent percentage of different rock types of the analyzed samples in each volcanic belt. Total amount of analyzed samples in each volcanic belt is labeled in parentheses. Analyzed data of the Inner Middle belt of Fujian afterSun and Lai (1980)and

Liu et al. (1994). Abbreviations of rock types same as inTable 2.

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to have occurred from 15.6 to 10.5 Ma, and the mag-matic activity was resumed at Shengxian – Xinchang region in the late Miocene. The Inner belt of Zhejiang is largely devoid of young volcanic activity except in the Zhuji area. The youngest age (2.5 F 0.1 Ma) is regis-tered in Z1A basanite from Zhuji lava, which may have recorded the latest volcanic activity in the Zhejiang province.

4.2. Fujian Segment

An alkali olivine basalt from Longhai, sample F24, exhibits a40Ar/39Ar age of 15.7 F 0.6 Ma. Four tholei-ites from the Outer belt yield ages from 17.1 F 0.6 to 14.8 F 0.6 Ma (Table 3). It is clear that no obvious relationship exists between the eruptive age and rock type for the Outer belt basalts.

Fig. 4. Cl/K and Ca/K, and 40Ar/39Ar age spectra for two samples from Outer Middle belt, are shown on the left-hand side. The 36Ar/40Ar –39Ar/40Ar correlation diagrams and the regression results for these samples are shown on the right-hand side. In the spectrum plots, the bars represent F 1r. All the errors are given in 1r.

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Appa re n t A g e (Ma) Appa re n t A g e (Ma) Appa r e nt Age (Ma) Appa r e nt Ag e( M a ) Appa r e nt Age (Ma) Appa r e nt Ag e (Ma) Appa re n t A g e (Ma) Appa re n t A g e (Ma) Appa r e nt Age (Ma) Appa r e nt Ag e (Ma) F1B F14B F18A F20 F24 F30 F2A F5 F9B F12 0 0 5 10 15 20 25 0 5 10 15 20 25 0 0 0 5 10 10 10 5 15 20 20 20 25 30 30 30 40 35 25 15 35 30 35 40 0 0 0 0 0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1 0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1 5 5 10 10 15 20 25 15 1 2 2 2 4 4 6 6 3 4 5 1.8 0.1 Ma Integrated date = 1.8 0.1 Ma Integrated date = 1.2 0.1 Ma 0.9 0.1 Ma 2.2 0.1 Ma Integrated date = 2.2 0.1 Ma Integrated date = 2.3 0.1 Ma 2.0 0.1 Ma 11.9 0.4 Ma

Integrated date = 11.9 0.4 Ma Integrated date = 14.8 0.6 Ma 14.9 0.6 Ma Integrated date = 15.7 0.6 Ma 15.4 0.6 Ma Integrated date = 15.3 0.6 Ma 15.3 0.6 Ma Integrated date = 14.9 0.6 Ma 14.9 0.6 Ma Integrated date = 17.1 0.6 Ma 17.1 0.6 Ma Fraction of39ArKReleased

Cumulative CumulativeFraction of39ArKReleased

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The plateau age of an Outer Middle belt alkali olivine basalt yields 11.9 F 0.4 Ma, which is younger than the previous conclusion of 14.1 F 0.3 Ma byLiu

et al. (1992)deduced from K – Ar dating method.

40

Ar/39Ar dating of one olivine tholeiite, one alkali olivine basalt, and two nephelinites from the Inner belt suggests ages ranging from 0.9 F 0.1 to 2.2 F 0.1 Ma. These ages are generally concordant with the younger ages (0.7 F 0.1 – 3.0 F 0.1 Ma) measured previously by K – Ar method (Liu et al., 1992).

However, the K – Ar ages of two basanites and one limburgite, ranging from 4.5 F 0.1 to 5.0 F 0.2 Ma

(Liu et al., 1992), appear to be older than the Ar – Ar

ages obtained by this study. Without information for the samples dated byLiu et al. (1992), it is difficult to further interpret the discrepancies. Given that the present samples are fresh and that all the plateau dates are considered to be meaningful as discussed earlier, the 40Ar/39Ar ages presented in the study should be reliable.

Table 3

Summary of40Ar/39Ar dating results for the basaltic rocks and an amphibole megacryst from Zhejiang – Fujian area Sample no. Rock/minerala Occurrence Locality Integrated

dateb(Ma) Plateau dateb(Ma) Intercept dateb(Ma) (40Ar/36Ar) ic MSWD Zhejiang—Inner belt

Z-1A B Lava Zhuji 2.6 F 0.1 2.5 F 0.1 2.4 F 0.1 295.6 F 11.0 0.59

Z-32A N Pipe Quzhou 23.7 F 0.3 23.3 F 0.3 23.4 F 0.3 290.8 F 5.0 1.83

Z-31B B Dike Longyou 26.4 F 0.3 26.4 F 0.3 26.8 F 0.3 303.4 F 1.9 1.09

Zhejiang—Inner Middle belt

Z-26A AOB Lava Shengxian 3.0 F 0.1 2.9 F 0.1 2.9 F 0.2 298.5 F 0.3 1.18

Z-28B AOB Lava Shengxian 3.3 F 0.1 2.9 F 0.1 2.9 F 0.1 309.0 F 1.0 0.77

Z-23A OT Lava Shengxian 3.5 F 0.1 3.5 F 0.1 3.6 F 0.1 298.4 F 0.5 0.17

Z-21 AOB Lava Xinchang 5.2 F 0.3 4.9 F 0.3 4.9 F 0.3 302.7 F 0.2 0.41

Z-12B B Lava Tiantai 5.4 F 0.3 5.4 F 0.3 5.4 F 0.3 295.0 F 1.0 0.48

Z-19 OT Lava Xinchang 8.4 F 0.4 8.4 F 0.4 8.5 F 0.4 303.7 F 6.5 0.69

Z-15 B Lava Xinchang 9.4 F 0.1 9.4 F 0.1 9.4 F 0.1 299.9 F 4.2 2.64

Z-3A B Pipe Jinyun 15.5 F 0.7 15.6 F 0.7 15.6 F 0.7 305.5 F 7.7 2.83

Z-5B Amphi Meg. Jinyun 17.4 F 0.8 17.0 F 0.8 17.0 F 0.1 299.0 F 0.3 0.38

Zhejiang—Outer Middle belt

Z-6A OT Lava Linhai 10.4 F 0.5 10.4 F 0.5 10.1 F 0.5 297.4 F 0.9 0.77

Z-8 OT Lava Ninghai 10.5 F 0.5 10.5 F 0.5 10.3 F 0.5 296.8 F 0.6 1.05

Fujian—Inner belt

F-2A N Lava Mingxi 1.2 F 0.1 0.9 F 0.1 0.8 F 0.1 321.1 F 1.4 0.36

F-1B N Lava Mingxi 1.8 F 0.1 1.8 F 0.1 1.8 F 0.1 295.1 F 4.4 1.73

F-5 AOB Lava Mingxi 2.2 F 0.1 2.2 F 0.1 2.4 F 0.1 295.5 F 8.0 2.79

F-9B OT Lava Qingliu 2.3 F 0.1 2.0 F 0.1 1.9 F 0.3 305.9 F 1.1 0.29

Fujian—Outer Middle belt

F-12 AOB Lava Minqing 11.9 F 0.4 11.9 F 0.4 12.5 F 0.5 294.7 F 4.0 2.34

Fujian—Outer belt

F-30 OT Lava Zhangpu 14.8 F 0.6 14.9 F 0.6 14.7 F 0.6 298.2 F 7.5 2.68

F-18A OT Lava Longhai 14.9 F 0.6 14.9 F 0.6 15.3 F 0.6 308.3 F 2.7 0.47

F-20 OT Lava Longhai 15.3 F 0.6 15.3 F 0.6 15.0 F 0.6 317.0 F 10.3 1.87

F-24 AOB Lava Longhai 15.7 F 0.6 15.4 F 0.6 15.4 F 0.6 302.1 F 9.6 1.14

F-14B QT Lava Longhai 17.1 F 0.6 17.1 F 0.6 17.1 F 0.6 322.0 F 3.4 1.54

aAbbreviations of rock types same as inTable 1. Amphi: amphibole. bAll age errors are shown in 1r.

c(40Ar/36Ar)

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In summary,40Ar –39Ar dating of 10 samples in the Fujian province yields eruption ages ranging from 17.1 to 0.9 Ma (Table 3), corresponding to middle Miocene to late Pleistocene. The eruption ages, 17.1 – 14.8, 11.9 and 2.2 – 0.9 Ma, indicate that there are three periodic volcanic eruptions, migrating from east to west with geological time, and the first stage magmatism is the most voluminous.

5. Bulk chemistry

5.1. Major and trace elements

The abundance of major and trace (including the rare earths) elements of the representative basaltic rocks in Zhejiang – Fujian area are listed in Table 2. The analyzed samples show wide range in chemistry

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Fig. 7. Chondrite-normalized REE distribution of representative basaltic rocks from Zhejiang – Fujian area. Note that the samples show strong LREE-enriched patterns with (La/Yb)N= 4.9 to 42.9. REE ranges of Leiqiong basalts are from Ho et al. (2000a); E-MORB data and the normalizing values are fromSun and McDonough (1989).

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and vary from strongly undersaturated nephelinites and alkali picrite basalts through mildly undersatu-rated basanites and alkali olivine basalts to olivine tholeiites and quartz tholeiites. All analyzed samples have SiO2 ranging from 39.57% to 53.92%. The

silica contents are generally good indicators of differ-ent basaltic suites. Most of the tholeiitic rocks have higher SiO2contents than the alkali suites.

In Zhejiang – Fujian area, only a few nephelinites (e.g., Z32A, F2A) have Mg-values (100 Mg/(Mg + Fe+ 2)) between 68.4 and 68.9, MgO 11.45 – 12.65%, Ni 306 – 379 ppm and Cr 326 – 405 ppm which match the composition of primary basalt defined byHart and

Allegre (1980). Most of the basaltic rocks have low

Mg-values, and low and varied concentrations of MgO, Ni and Cr indicate that they may not represent primitive basalts and probably have undergone certain degree of fractional crystallization. Based on the systematic increase of Al2O3, and decrease ofSFeO,

MgO, CaO and MnO with increasing SiO2contents

(Fig. 6), the Zhejiang – Fujian basalts may have

expe-rienced fractional crystallization of ferromagnesian minerals such as olivine and clinopyroxene after the formation of the initial liquid. In addition, the abun-dances of K2O and TiO2are negatively correlated with

SiO2 contents of the Fujian tholeiites (Fig. 6). It is

suggested that fractional crystallization of feldspar and Ti-bearing opaques may have occurred during mag-matic evolution.

The total alkali contents (Na2O + K2O) of the

basaltic rocks from Zhejiang – Fujian area are always

higher than 2.5%, and the Na2O/K2O ratios of all

basalts studied are more than unity indicating their alkali-enriched and high-sodium nature. However, the Na2O content in alkali picrite basalt (F4A) is around

1.34%, which is lower than those in the other rock types (Na2O = 2.25 – 5.03%). Alkali picrite basalt has

high-Mg and low alkali contents which correspond to picrobasalt of IUGS classification system (Le Bas,

2000).

The chondrite-normalized rare earth element (REE) patterns of basaltic rocks from Zhejiang – Fujian area show moderate to steep sloping with light rare earth element (LREE) enrichment (Fig. 7). The (La/Yb)N

ratios increase with the undersaturated character from 4.9 to 17.7 in the quartz tholeiites, 6.0 to 25.6 in the olivine tholeiites, 9.5 to 35.0 in the alkali olivine basalts, 12.2 to 27.0 in the basanites, and 23.6 to 42.9 in the nephelinites and alkali picrite basalt. The LREE abundance in tholeiitic basalts is higher than that of E-type mid-ocean ridge basalts (Sun and McDonough,

1989), but is comparable with Leiqiong basalts(Ho et

al., 2000a) (Fig. 7).

5.2. Sr – Nd isotopic composition

Basaltic rocks of Zhejiang – Fujian area show a wide range of isotopic composition with 87Sr/86Sr = 0.703264 – 0.704235 and 143Nd/144Nd = 0.512725 – 0.512961 (Table 4). These results are similar to the Sr and Nd isotopic compositions of the Leiqiong basalts (87Sr/86Sr = 0.703101 – 0.704624; 143Nd/

Table 4

Sr – Nd isotopic compositions of late Cenozoic basaltic rocks in the Zhejiang – Fujian area

Volcanic belt Sample no. Locality Rock typea 87Sr/86Srb 143Nd/144Ndb eNdc

Outer Middle belt F12 Minqing AOB 0.703833 F 8 0.512845 F 18 + 4.0

Z6A Linhai OT 0.703264 F 9 0.512961 F 13 + 6.3

Z11B Ninghai QT 0.704140 F 12 0.512892 F 9 + 5.0

Inner Middle belt Z5A Jinyun N 0.703557 F 7 0.512890 F 16 + 4.9

Z12B Tiantai B 0.703678 F 10 0.512902 F 9 + 5.2

Z26A Shengxian AOB 0.703736 F 9 0.512910 F 11 + 5.3

Z24 Shengxian OT 0.704235 F 11 0.512873 F 10 + 4.6

Z22 Xinchang QT 0.704189 F 7 0.512725 F 26 + 1.7

Inner belt Z32A Quzhou N 0.703704 F 7 0.512864 F 6 + 4.4

Z31B Longyou B 0.703817 F 8 0.512875 F 17 + 4.6

Z1C Zhuji AOB 0.703885 F 10 0.512915 F 9 + 5.4

aAbbreviations of rock types same as inTable 1. bErrors of87Sr/86Sr and143Nd/144Nd are shown in 2r. ceNd=((143Nd/144Nd)

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144

Nd = 0.512736 – 0.513039, Zhu and Wang, 1989;

Zhao, 1990; Tu et al., 1991). On the143Nd/144Nd vs.

87

Sr/86Sr diagram (Fig. 8), the Zhejiang – Fujian basalts plot within the OIB field and most of samples are closely related to the mantle array.

6. Discussion

6.1. Geochemical characteristics of the Zhejiang – Fujian basaltic rocks and implications for their mantle sources

The Zr/Nb ratios of the Zhejiang – Fujian basaltic rocks analyzed vary from 2.0 to 10.7, and the Zr/Y ratios vary from 5.0 to 16.2. These incompatible element ratios of basaltic rocks are higher than those

of MORB, but are similar to those observed in ocean island basalts (average Zr/Nb and Zr/Y ratios of OIB are 5.8 and 9.7, respectively, Sun and McDonough, 1989). In general, LREE are highly incompatible in the mantle – melt system, the LREE ratios should be close to those of the mantle source under a batch melting condition(Sun and Hanson, 1975). The La/Ce (0.44 – 0.65) and Ce/Sm (4.6 – 12.6) ratios of the Zhejiang – Fujian basaltic rocks appear to be higher than those of primitive mantle (0.39 and 4.0, respec-tively, Sun and McDonough, 1989), strongly reflect-ing their derivation from a fertile mantle source.

In chondrite-normalized REE patterns (Fig. 7), except exhibiting strong LREE enrichment, a few basaltic rocks also have slightly positive or negative Eu anomalies. Philpotts and Schnetzler (1968) sug-gested that fractional crystallization of 18% or more

Fig. 8. Plots of143Nd/144Nd vs.87Sr/86Sr diagram for the Zhejiang – Fujian basalts. The fields for the Tungchihsu group II pyroxenites(Ho et al.,

2000b), and the late Cenozoic basalts of Leizhou, Hainan, Penghu, and Kuanhsi – Chutung (NW Taiwan) regions(Peng et al., 1986; Zhu and

Wang, 1989; Zhao, 1990; Basu et al., 1991; Tu et al., 1991; Zhang and Chen, 1992; Lan et al., 1994; Lee, 1994; Chung et al., 1995)are shown

for comparison. The fields of DMM, HIMU, EM1 and EM 2 are fromZindler and Hart (1986); the East Taiwan Ophiolite (ETO) N-type basalts field is fromJahn (1986)andChung and Sun (1992); the East Pacific Rise (EPR) MORB field is fromWilson (1989); the OIB field is from

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plagioclase from the magma could cause 5% or more negative Eu anomaly in the basaltic rocks. Therefore, the analyzed samples such as F3A, F10B and F11 have REE spectra with slightly negative Eu anoma-lies, which can be attributed to the fractionation of plagioclase playing an important role during the gen-eration and evolution of these magmas. On the other hand, the Eu enrichment of the basaltic rocks (e.g., Z1A, Z7, Z10, Z12B, F20, F24) can be considered as the result of a natural consequence of partial melting of the mantle under reducing conditions (Sun and

Hanson, 1975). In addition, the plagioclase

mega-crysts in basaltic rocks have been found in some localities such as at Xinchang and Longhai (Chih,

1988), if the early crystallized plagioclase megacrysts

mixed into the magma source, it could also produce a positive Eu anomaly in the melt.

The primitive mantle-normalized incompatible ele-ment diagrams of Zhejiang – Fujian basalts are quite similar to that of the average oceanic island basalts, although a few alkali basalts show Ba and K ‘‘troughs’’ and Li ‘‘peak’’ (Fig. 9). The nephelinite and alkali picrite basalt have slight negative K anomalies, but their Ba/Rb ratios (except Z33A sample) are similar to those of the intraplate basalts (f 12,Sun and

McDo-nough, 1989), which may be interpreted as due to

K-bearing minerals in the residual phases. In general, if degree of partial melting of mantle source increased, then the percentage of partial melting of K-bearing minerals in the mantle source will also increase. Therefore, the tholeiitic basalts, which have lower incompatible element contents than the alkali basalts and show no distinct negative K anomalies, may have been derived from larger degree of partial melting. It is suggested that the incompatible elements such as LREEs contents generally increase with decreasing SiO2saturation (Fig. 7). These regular compositional

variations may reflect the chemical characteristics and different degree of partial melting of a garnet- or spinel-lherzolite mantle source. Therefore, it is believed that the quartz tholeiite or olivine tholeiite were derived from relative shallow mantle source through a larger degree of partial melting, while the alkali basalts were formed from deeper mantle source through a smaller degree of partial melting.

Unlike other late Cenozoic intraplate basalts in SE China, the alkali-rich nephelinite and magnesium-rich alkali picrite basalt have been found in the Inner and

Inner Middle belts in Zhejiang – Fujian region. Anal-yses of five nephelinite samples in this study show that they have restricted incompatible compositions and are strongly enriched in Hf, Sr, Th and LREEs typical of alkaline rocks (Table 2). However, sample Z33A has abundant clinopyroxene, analcite and rare olivine, titano-magnetite, with a lower chrome content (208 ppm) and Mg-value (61.8) which is similar to the melanephelinite found on the western Kenya area, Africa (Le Bas, 1987). The most striking chemical feature of melanephelinite is the strong depletion in barium (Ba = 77 ppm) and the enrichment in lithium (Li = 110 ppm) relative to those found in the neph-elinites (Ba = 671 – 1068 ppm; Li = 9 – 23 ppm) (Fig. 9). It should be noted that the melanephelinite dike hosts numerous Ti-mica megacrysts, which contain very high amount of barium (Ba = 734 ppm, Ho, 1999). The Ti-mica megacrysts belonging to Group

B (Irving, 1984) were considered to be xenocrysts

derived from a relatively evolved magma, and were brought up to the earth’s surface by ascending basaltic magma at a later stage. According to the experiment

of LaTourrette et al. (1995), the barium and lithium

partition coefficients between mica and basaltic melt under mantle condition are 3.68 and 0.064, respec-tively. Crystallization of large amounts of mica from the parental magma may therefore affect the barium and lithium signatures in the residual magma (Ho,

1999).

6.2. Petrogenesis of the late Cenozoic Zhejiang – Fujian basalts

6.2.1. Mantle metasomatism beneath SE China Alkali basalts in SE China contain varied amounts of mantle xenoliths and high-pressure megacrysts that can provide a good opportunity to reveal chemical compositions and processes in the lithospheric mantle. Detailed petrographic and geochemical investigations

(Fan and Hooper, 1989; Qi et al., 1995; Ho et al.,

2000b) showed that the lithospheric mantle beneath

the SE China is compositionally and isotopically heterogeneous, reflecting possible experience of depletion and enrichment events.

In general, the metasomatically enriched xenoliths are characterized by hydrous and other exotic acces-sory phases. In SE China, amphibole and phlogopite megacrysts were found in Jinyun and Quzhou,

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Zhe-Fig. 9. Primitive mantle-normalized incompatible element patterns for selected basaltic rocks from Zhejiang – Fujian area. Note that the samples are enriched in highly incompatible trace elements similar to those of OIB. OIB data and the normalizing values are fromSun and McDonough

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jiang province and Tungchihsu, Penghu Islands in the Taiwan Strait. In addition, some mantle lherzolites, pyroxenites and composite xenoliths in Mingxi, Fujian province; Puning, Guangdong province and Tungchihsu of Penghu Islands contain certain amount of amphibole or mica, commonly interstitial and secondary in origin (Zhao, 1990; Ho et al., 2000b;

and unpublished data). These evidences show that the

depleted lithospheric mantle beneath SE China was affected by metasomatism.

Late Cenozoic basalts in Zhejiang – Fujian, Lei-qiong area and South China Sea Basin have high

208

Pb/204Pb (37.610 – 39.260) and 2 07Pb/2 04Pb (15.437 – 15.649) ratios which plot above the Northern Hemisphere Reference Line indicating Dupal anom-aly toward EM2 direction(Peng et al., 1986; Zhu and

Wang, 1989; Basu et al., 1991; Tu et al., 1991, 1992;

Lan et al., 1994; Zou et al., 2000). It is generally

believed that the EM2 component of the basalts may be derived from continental lithospheric mantle such as Tungchihsu group II pyroxenitic rocks (Fig. 8).

Chung et al. (1994) suggested that the EM2

compo-nent of conticompo-nental lithospheric mantle is related to the subduction of the Pacific plate beneath the Eurasian plate in Mesozoic.Zhang et al. (1996)also concluded that the mantle source of Leiqiong basalts might have been affected by sediments associated with a paleo-subduction zone. The negative Nb anomaly observed in the spidergram of some basalts (Fig. 9) indicated that the influence of a paleo-subduction zone derived component could not be excluded in considering the genesis of the basalts from the Zhejiang – Fujian area.

Fig. 10.87Sr/86Sr vs.206Pb/204Pb and143Nd/144Nd vs.206Pb/204Pb diagrams for the Zhejiang – Fujian basalts. The fields for the Leiqiong(Zhu

and Wang, 1989; Tu et al., 1991)and South China Sea seamount(Tu et al., 1992)are shown for comparison. The fields of DMM, EM1 and

EM2 are adopted fromZindler and Hart (1986). Sr, Nd and Pb isotope compositions of Zhejiang – Fujian basalts are fromPeng et al. (1986),

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Fig. 11. A petrogenetic model for the late Cenozoic basaltic rocks from Zhejiang – Fujian region, SE China. Abbreviation: PC: plagioclase; OL: olivine; CPX: clinopyroxene; MT: Ti – Fe oxide minerals.

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6.2.2. The isotopic characteristics and generation of basalts

Sr, Nd and Pb isotope compositions of Zhejiang – Fujian basalts were presented byPeng et al. (1986),

Zhao (1990), Basu et al. (1991), Zhang and Chen

(1992), Lan et al. (1994), Zou et al. (2000) and this

study. Limited isotopic data have shown spatial chem-ical and isotopic variations in the basalts. In the Zhejiang province, except one olivine tholeiite Z6A, tholeiitic basalts have higher 87Sr/86Sr (0.704140 – 0.704235) and lower 143Nd/144Nd (0.512725 – 0.512932) ratios than those of alkali basalts which have87Sr/86Sr ranging from 0.703292 to 0.703885 and

143

Nd/144Nd ranging from 0.512864 to 0.512990. In the Fujian province, except Inner belt, tholeiite has a wide Sr isotope range, in general 87Sr/86Sr and

206

Pb/204Pb ratios of the basalt increase progressively from the Inner to the Outer belts. In addition, Sr and Nd isotopic compositions of Outer belt alkali basalts overlap with those of tholeiites.

In 143Nd/144Nd vs. 206Pb/204Pb and 87Sr/86Sr vs.

206

Pb/204Pb diagrams(Fig. 10), the plots show that the Zhejiang – Fujian basalts can be produced by DMM (depleted MORB mantle) and EM2 (enriched mantle with high206Pb/204Pb, high87Sr/86Sr and intermediate

143

Nd/144Nd, Zindler and Hart, 1986) components

mixing. Based on the genetic model proposed by

Chung et al. (1994), the SE China lithosphere thinning

occurred during the Miocene and that the lithospheric mantle was thermomechanically eroded by convective upwelling of the asthenosphere, and created a plum-pudding type convecting mantle domain. Therefore, the spatial chemical and isotopic variation in the Zhejiang – Fujian basalts can be explained by different degrees of decompression melting of the convecting mantle compounded by variable contributions from the continental lithospheric mantle (CLM)-derived plume (i.e., enriched) component. The voluminous

tholeiites in the Outer belt owe their generation largely to lithospheric mantle which has undergone extensive melting near the locus of upwelling.

The Sr – Nd isotope geochemistry of Zhejiang – Fujian and other SE China basalts showed that some basalts exhibit considerable deviation from the MORB-OIB array (Fig. 8), a feature which may be accounted for by involving Tungchihsu group II pyroxenite components (with high 87Sr/86Sr) in the magma evolution. We suggest that the SE China basalts were produced through mixing of various proportions of two mantle components. The depleted asthenosphere component is represented by the East Taiwan Ophiolite (ETO) N-type basalt(Chung et al.,

1994; Lan et al., 1994), while the enriched lithosphere

component is partly represented by the Tungchihsu group II pyroxenite.

A simple petrogenetic model for late Cenozoic Zhejiang – Fujian basaltic rocks is sketched inFig. 11. In general, the mantle source of tholeiitic magma may have mixed with a more enriched component and un-dergone a relatively higher degree of partial melting than that of the alkali basalt magma. In addition, low-pressure fractionation has also occurred in Zhejiang – Fujian basaltic magma. Based on major and trace elements studies, the alkali basalts have mainly fractio-nated olivine and clinopyroxene, while the tholeiitic basalts have fractionated plagioclase, clinopyroxene and/or minor amounts of Fe – Ti oxide minerals and olivine.

6.3. Late Cenozoic volcanic activities in the Zhe-jiang – Fijian area: eruption ages and tectonic impli-cations

In the Zhejiang – Fujian area, sparse volcanic activ-ities occurred and produced small amounts of basanite dikes and nephelinite pipes in the Zhejiang Inner

Fig. 12. Sketch map of South China Sea and adjacent areas exhibiting spatial and temporal distribution of late Cenozoic intraplate basaltic flows and major tectonic features. Solid lines in Zhejiang – Fujian – Guangdong area represent four volcanic belts as labeled inFig. 1. Marine magnetic anomalies in the South China Sea afterTaylor and Hayes (1983)andBriais et al. (1993). Identified marine magnetic anomalies are indicated by Anomaly number. Hatched lines depict the boundary of oceanic crust and double dashed lines represent inactive spreading centers. Arrows indicate the direction of thermal center migration since the beginning of mid-Miocene as discussed in the text. Radiometric ages, given in Ma, for late Cenozoic basalts from Zhejiang – Fujian area are compiled fromWang and Yang (1987),Liu et al. (1992)and this study. For Penghu Islands and NW Taiwan, the data are fromChen (1990),Juang and Chen (1992),Lee (1994)andJuang (1996). For Leizhou Peninsula and Hainan Island, the data are fromZhu and Wang (1989),Ge et al. (1989),Sun (1991)andHo et al. (2000a). For Indochina Peninsula, the data are

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volcanic belt before cessation of the South China Sea floor spreading. By comparison, the eruption process was comparatively active after cessation of the

spreading of the South China Sea, and a large amount of basaltic lavas with minor pyroclastic rocks erupted to form a diffuse volcanic province (Fig. 12).

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The timing of the initial volcanic eruptions in Fujian Outer belt is still under debate. Some of the K – Ar dates, which are older than the eruption ages determined by the40Ar/39Ar technique, may reflect the

results related to alteration and inherited argon(Lo et

al., 1994).40Ar/39Ar dating in this study yielded the

oldest eruption date at 17.1 F 0.6 Ma(Table 3), which was considered to be the initial eruptive age. If this

Fig. 13. Histogram showing ranges of radiometric ages for late Cenozoic basalts from four volcanic belts of Zhejiang – Fujian area, western foothills of Taiwan, Penghu Islands in the Taiwan Strait, Leizhou Peninsula, Hainan Island and Indochina Peninsula. Also shown are the major tectonic events in SE China. Age data of the basaltic rocks are compiled from the present study and the sources listed inFig. 12as well.

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Ar – Ar age is accepted, it appears that the volcanic activity in the Zhejiang – Fujian area may start prior to 16 Ma.

Overall, volcanic activity began approximately 17 Ma ago at the eastern part of the Fujian province near the Longhai – Zhangpu region. Nine Ar – Ar ages (10.5 – 2.9 Ma) from the Outer Middle and Inner Middle belts of Zhejiang province are obtained in this study, which are roughly consistent with the previ-ously reported K – Ar ages (10.4 – 4.8 Ma;Wang and

Yang, 1987; Liu et al., 1992). These results support

the observation that the volcanic rocks of these regions are younger than those of the Outer and Outer Middle belts of Fujian province (17.1 – 11.7 Ma). The volcanic activity of the Zhejiang province ceased about 2.5 Ma ago in the Zhuji area, while volcanism of Fujian Inner belt continued to be active until Quaternary (f 0.72 Ma).

Generally speaking, the magmatism commenced and stopped, and eruption center migration in these regions may be influenced by the changing of tectonic setting. Intraplate melting could have resulted from rift-induced asthenospheric decompression during stress redistribution(Latin and White, 1990). Late Cenozoic intraplate basalts are widespread in the areas surround-ing the South China Sea includsurround-ing Zhejiang – Fujian – Taiwan – eastern Guangdong, Leiqiong (including the Leizhou Peninsula and the northern of the Hainan Island), and Indochina Peninsula(Fig. 12). However, regional extensive intraplate magma occurred approx-imately following cessation of the South China Sea opening in these areas(Fig. 13). This implies that the eruption center of the basalts from the South China Sea mid-ocean ridge system might move northward (to Fujian Outer volcanic belt and Penghu Islands), north-westward (to the Leiqiong area) and north-westward (to south-central Vietnam) since the beginning of mid-Miocene(Fig. 12).

On the basis of the dating results and geographic distribution of the volcanic rocks, the inception and/or cessation of magmatic activity in the Zhejiang – Fujian region followed a rough time – space pattern, which appears to reflect progressive activation from Fujian Outer belt to western and northern-oriented rifts(Fig. 12). The western sector includes Minqing (14.1 – 11.9 Ma) and Mingxi (4.96 – 0.72 Ma), while the northern sector includes Shengxian – Xinchang (10.5 – 2.9 Ma) and Zhuji (2.5 Ma). Therefore, it is suggested that the

cessation of magmatism and migration of eruption center in the Zhejiang – Fujian area may be related to the collision of Philippine Sea Plate with the Eurasian Plate during late Miocene to Quaternary.

About 12 Ma ago, the northern tip of the Luzon Arc might have begun overriding the Asian continental margin(Teng, 1992), and the present Taiwan area also began to experience compressional process. During Pliocene to Pleistocene, the collision event and the accompanying continental crust compression caused an active orogeny, which developed regional uplifting to form the mountain ranges in Taiwan. In late Cen-ozoic time, the regional extensional stress in the Taiwan area and its neighboring South China conti-nental margin had been changed to compressive stress

(Sun, 1985; Angelier et al., 1990). Therefore, although

folding and thrusting diminished in intensity from east to west, and the orogenic records by arc – continental collision such as metamorphism and structural defor-mation were only restricted to the Taiwan area, whereas the intensified collision between the Luzon Island Arc and the continent of Asia during the on-going convergence of the Eurasian and Philippine Sea plates might affect deep stress system of neighboring South China continental margin including Zhejiang – Fujian region. The youngest radiometric age of basal-tic lava in each region might constrain the closure time of the magma-ascending related to compressive stress process. The igneous activity in the Fujian Outer belt ceased earlier than the Penghu Islands and western foothills of Taiwan which may be due to the fact that the coastal region of Fujian Province lies on the locus of upwelling, where the thickness of lithosphere (f 80

km; Ma and Wu, 1987) is thinner than around the

Penghu Islands (f 100 km;Chung et al., 1994).

7. Conclusions

Late Cenozoic basaltic rocks in Zhejiang – Fujian area show wide variations in chemistry, ranging from tholeiitic to alkalic and strongly alkalic affinity. The chemical trends of major and trace elements in the basaltic rocks reveal that the fractional crystallization of clinopyroxene and olivine or plagioclase is the most important process during magma evolution. The chon-drite-normalized REE abundances in the basaltic rocks have LREE-enriched patterns and the (La/Yb)Nratios

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range from 4.9 to 42.9, which show strong resemblance to oceanic island basalts. In addition, the Sr – Nd – Pb isotopic component of basaltic rocks in Zhejiang – Fujian area showed an EM2 Dupal-type Pb signature. These geochemical features of the Zhejiang – Fujian basalts resemble those of other basaltic rocks occurred in SE China region. The chemical and isotopic data suggested that basaltic rocks were generated by mixing of different proportions of depleted asthenosphere (N-MORB) with enriched (EM2) mantle component. Ar – Ar dating of 24 basaltic samples from the Zhejiang – Fujian area shows ages ranging from 0.9 to 26.4 Ma, providing good constraints on the timing of magmatic activity. These data, except some neph-elinitic and basanitic pipes or dikes in the Inner belt of Zhejiang province, indicate that most basaltic magma-tism occurred from 0.9 to 17.1 Ma in Zhejiang – Fujian area. According to the dating results, most late Cen-ozoic intraplate magmatism of SE China may be related to the northward migration of the South China Sea mid-ocean ridge system beneath SE China since the beginning of mid-Miocene, i.e., following cessa-tion of the South China Sea opening. In Zhejiang – Fujian area, mid-Miocene volcanic activity is believed to start in the Outer belt, close to Longhai area in Fujian province. Owing to the collision of Philippine Sea Plate with the Eurasian Plate around Taiwan area in late Miocene (f 12 Ma), the thermal centers grad-ually migrated westward and northward with time. The Zhuji (2.5 Ma) and Mingxi – Qingliu (0.7 – 5.0 Ma) volcanic eruptions are likely to be the latest.

Acknowledgements

We thank C. Hu for his assistance in the field. The Sr – Nd isotopic analysis was carried out by A.D. Smith and L.Y. Huang. The authors are most grateful for their kind assistance. Thanks are also due to P.L. Wang and L.H. Lin for their helpful assistance in the Ar – Ar dating and M. W. Hong for preparing the figures and typing the tables. We would like to thank Dr. Haibo Zou and an anonymous reviewer for their penetrating reviews that lead to significant improve-ment of the manuscript. This research was supported by the National Science Council, R.O.C., and the National Museum of Natural Science, Taichung, Taiwan. [RR]

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Chung, S.L., Cheng, H., Jahn, B.M., O’Reilly, S.Y., Zhu, B., 1997. Major and trace element, and Sr – Nd isotope constraints on the origin of Paleogene volcanism in South China prior to the South China Sea opening. Lithos 40, 203 – 220.

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數據

Fig. 2. (A) Normative compositions of basaltic rocks from the Zhejiang – Fujian area. Nomenclature of volcanic rocks after Yoder and Tilley (1962)
Fig. 3. Histogram showing distribution of basalt types for various volcanic belts in (A) Zhejiang and (B) Fujian Provinces
Fig. 4. Cl/K and Ca/K, and 40 Ar/ 39 Ar age spectra for two samples from Outer Middle belt, are shown on the left-hand side
Fig. 5. Age spectra, integrated and plateau dates for 22 samples studied.
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