Geochronology and geochemistry of Penghu basalts, Taiwan Strait and their tectonic significance

Journal of Southeast Asian Earth Sciences, Vol. 7, No. 2/3, pp. 185-193, 1992 Printed in Great Britain

0743-9547/92 $5.00+ 0.00 Pergamon Press Ltd

Geochronology and geochemistry of Penghu basalts, Taiwan Strait and their tectonic significance W. S. JUANG* and J. C. CnEr~? *National Museum of Natural Science, Taichung, Taiwan, R.O.C.; tInstitute of Oceanography, National Taiwan University, Taipei, Taiwan, R.O.C. (Received 28 June 1991; accepted for publication 9 September 1991) Abstract--The Penghu Islands are located in the Taiwan Strait and consist of 64 islets. These islands, except for Huahsu, are composed mainly of basaltic lavas with minor amounts of sedimentary rocks. The basaltic lavas have effused from many vents rather than from fissure eruption. K-Ar datings of the basalts, and on planktonic foraminifers and calcareous nannofossils in the sedimentary rocks revealed that the volcanic activity was from Middle Miocene to Late Miocene. The basalts in the Penghu Islands are mainly alkalic and tholeiitic. The presence of many half-graben-type basins in the Tertiary sequence in western Taiwan and the paleostress analysis on the Penghu Islands indicate a continental rifting environment in which the intraplate volcanism occurred. REE data and other evidence suggest that the alkali basalt may have derived from relatively deeper mantle which had been metasomatized by LILE enriched fluid through partial melting. But the tholeiite may have originated from unmetasomatized mantle lherzolite at relatively shallow level by 5-10% equilibrium batch partial melting. Contemporaneous volcanism occurred sporadically during the deposition of Miocene sediments in the western foothills of Taiwan. The volcanic rocks of the western foothills of Taiwan and those of the Penghu Islands are similar in geochronology and geochemistry. They are closely related to Cenozoic rift tectonism along the Asiatic continental margin caused by the third heating and rifting episodic evolution of the South China Sea.

INTRODUCTION ThE PENGHU Islands, previously called Pescadores, are located in the Taiwan Strait between Taiwan and mainland China. The Penghu Islands consist of 64 islets, scattered between latitudes 23°09'N and 23°45'N and longitudes 119°18' E and 119042' E. They belong to the Eurasian passive margin, west of the collision zone of Taiwan. The islands, except for Huahsu, are composed mainly of basaltic lavas with minor amounts of sedimentary rocks. The activity of the Penghu basalts can be divided into more than three phases and in each phase several lava flows had taken place. The basaltic lavas are found to have effused from many vents rather than from fissures. However Huahsu consists mainly of porphyrite with subordinate quartz porphyrite which are hydrothermally altered. The geology of the Penghu Islands had been studied by Deguchi (1912), Hayasaka (1933), Linet al. (1957) and Yen (1958, 1965, 1987). The age of the basalts was considered by these authors to be Late Pliocene or Pleistocene. In 1966 a well coded TL-1, was drilled by the Chinese Petroleum Corporation at Tungliang in Paisha to a depth of 899 m. The well section passes through shallow water Miocene Miogypsina-bearing coquinoid limestone directly overlying the basement composed of siliceous shale and quartzite, presumably Aptian because of its close resemblance to the Aptian ammonite-bearing sequence on the western coast of Taiwan (Chou 1969). In the Tungliang well it has been reported that the Pliocene-Pleistocene section is present between the surface to 320 m depth, and a possible Mesozoic section between 500 and 899m (Huang 1967). Three basaltic core samples were collected

from the well, augite-bearing hypersthene basalt from 116 to 116.45 m depth, basaltic-tuff from 150 to 185 m and hypersthene-augite dolerite at 303 m depth. Both the augite-bearing hypersthene basalt and hypersthene--augite dolerite belong to the tholeiitic magma type. Based on phenocrystic and groundmass minerals (primary analcite, olivine and hypersthene) and the grain size of the groundmass minerals, the Penghu basalts were classified into five types by Yen (1965, 1987)--Type I, alkali basalt with analcite as primary mineral in the groundmass; Type II, mainly alkali basalt with minor tholeiite; Type III, tholeiite with both olivine and hypersthene as phenocrysts; Type IV, tholeiite always with hypersthene as one of the phenocrystic minerals and Type V, tholeiite without phenocrystic olivine and hypersthene. Chen (1973) considered that the alkali basalt probably represented the partial melting product of mantle peridotite at a deeper level, which is characterized by higher Ti, Cr, Ni, K, Rb and Sr contents and higher MgO/Y.FeO, Ni/Co and K/Rb and lower Na~O/K20 and Cr/Ni ratios as compared with the tholeiite. The volcanic activity in the Penghu Islands has previously been thought to be Late Pliocene and Pleistocene (Lin et al. 1957, Yen 1987), though the basalts have not been radiometrically dated. To establish the tectonic history of the Penghu Islands, a crucial point is the precise age of the basalt flows and the interbedded sediments. It is the purpose of this paper to present recent chronological and geochemical data of the Penghu basalts. These data, together with other geologic information, may shed some light on the tectonic evolution of the Penghu Islands.

185

186

W . S . JUANG a n d J. C. CHEN

GEOLOGICAL SETTING

corals distributed sporadically at higher places. Unconformity Upper Miocene-Lower Miocene: Penghu Formation basalt lavas, dikes, sands, clay and sandstone. Miogypsina limestone (Tungliang well) Unconformity

Based on logging of the stratigraphy of 6 exploration wells dug, the Tungliang well (TL-1), field mapping and radiometric dating of the basalts (this paper), a stratigraphic succession applicable to most of the islets is proposed as follows: Recent: Alluvium Alluvium, lateritic soil, and coral reefs on the shallow sea bottom.

Pre-Tertiary: Porphyrite Porphyrite, quartz porphyrite with quartz vein. Siliceous shale and quartzite (Tungliang well)

Unconformity Pleistocen~Recent: Coquina limestone Coquina composed of foraminifers and

A geologic map and typical columnar sections of the Penghu Formation are shown in Figs 1 and 2.

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Geochronology and geochemistry of Penghu basalts, Taiwan Strait Q

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The geographic distributions of basalts in the Penghu Islands, the Penghu basalts, can be divided into two groups, Northern Group, composed of alkali and tholeiitic basalts and Southern Group, consisting only of alkali basalts. The ultramafic nodules in alkali basalt have been found in the northeastern district. Mineralogical and petrological studies have revealed that these inclusions represent the residual fragments of upper mantle materials (Yang et al. 1987). It should be mentioned that ultramafic xenolith-bearing alkali basalts occurred in most continental rift systems (Basu 1975, Huckenholz 1973). Thus, we tentatively suggest that a rift zone

trending NW-SE exists in the northern part of the Penghu Islands. Aeromagnetic survey of the Penghu region indicated the existence of either fault zones or lineaments trending WNW-ESE, that were partly filled with volcanics (Bosum et al. 1970).

GEOCHRONOLOGY OF THE PENGHU FORMATION Volcanic and sedimentary rocks are major lithologies of the Penghu Islands and they belong to the "Penghu Formation" which has been described by Yen (1987).

188

W . S . JUANG a n d J. C. CHEN

The basaltic lava flows in the Penghu Formation are intercalated with marine strata of sandstone and mudstone containing plant fragments. The sands are loosely-packed with no effect of metamorphism. Based on the mollusca and benthic foraminifera found in the mudstone and sandstone, Lin et al. (1957) and Yen (1987) suggested that the Penghu Formation may be correlated with the Plio-Pleistocene Toukoushan Formation widely developed in the western part of Taiwan. However, their suggestion is somewhat unsatisfactory as evident below.

was that described by McDougall (1966). A highly enriched 38Ar tracer (38/36 > 104) was dispensed from a spike pipette system and the isotope ratios were measured with a modified MS-10 mass spectrometer. Potassium content in the whole rock samples and mineral separates was determined by atomic absorption following the procedures used by Juang (1981). Decay constants and conversion factor are those recommended by Steiger and J/iger (1977). The error of the K-Ar dates reported here is around ___3% (1 a). Table 1 and Figs 1 and 2 show the analytical results and the locations of the dated samples.

K - A r dating The Northern Group The basalts from Penghu Formation were dated by the K-Ar method for 24 whole rock samples, 1 hornblende and 1 phlogopite. Most whole rock samples dated in this work are dense, aphyric and free from xenoliths and xenocrysts. They are homogeneous when viewed both as a hand specimen and in thin section, except for two samples, 910 and 411 which are ultramafic nodulebearing alkali basalt. Small xenocrysts are present in these two samples which may be suspected to contain excess 4°Ar, a possible cause of the older ages. For these two samples special precaution was taken to cut the hand specimens thinly and then remove the xenoliths carefully under binocular microscope. The argon analysis was carried out on samples crushed to - 2 2 + 36 BS mesh (0.4-0.7 mm) and prebaked at 220°C for 12 h, the argon extraction procedure

Typical outcrops of the basaltic lavas and sandstone beds occur in the southernmost part of Hsihsu (Fig. 1) where three well developed lava flows with two sandstone beds in alternation can be observed. But in the middle and northern part of the island two lava flows and one sandstone bed are commonly found. The lower and middle lava flows are composed mainly of alkali basalts. However, part of the middle flow and the upper lava flow are tholeiite. The ages of these three lava flows are 14.7, 10.4, 9.2 Ma, respectively (Table 1 and Fig. 2). From field observations, there are two different basaltic lava flows in Mutouhsu (Fig. 2). The lower alkali basalt layer (Sample 411, 13.8 _ 0.4 Ma) is characterized by a lot of ultramafic nodules. The upper layer is tholeiite (sample 409, 12.0 + 0.3 Ma).

Table 1. K-Ar ages of the Penghu basalts

Sample locality

Weight analyzed (g)

K (%)

502 (T) 501A (T) 501B (T) 1021 (A) 503 (T) 1032 (A) 936 (A) 948 (T) 949 (T) 1014 (T) 913 (T) 910 (A) 534 (A) 919 (A) 533 (A) 409 (T) 411 (A)

3.111 3.283 2.610 2.656 2.865 2.780 1.001 2.404 3.614 3.623 1.5959 1.002 2.527 1.012 3.148 3.233 2.776

0.374 0.125 0.141 0.548 0.365 1.229 1.088 0.232 0.299 0.266 0.292 0.913 0.548 1.021 0.863 0.241 1.254

513 403 404 973 970 971 971 959 405

3.061 3.527 3.346 1.6343 3.2142 0.228 1.523 2.943 3.135

1.278 1.270 1.635 0.717 0.958 7.405 0.349 0.913 1.635

Sample No.

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Age (Ma)

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0.331 0.067 0.074 0.263 0.587 0.474 0.450 0.142 0.204 0.232 0.306 0.560 0.479 0.540 0.556 0.168 0.699

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Geochronology and geochemistry of Penghu basalts, Taiwan Strait

189

basal~

The Southern Group 41

Typical outcrops of two lava flows separated by a sandstone bed are found on Hsichihsu (Figs 1 and 2). Alkali basalts are the predominant rock type here. Lava flows with columnar joints constitute the upper group while vesicular lavas with platy joints occur in the lower group. The ages of the basalts are 9.7 and 11.6 Ma, respectively (Table 1 and Fig. 2). In the Southern Group, the lower vesicular platy lava flows contain analcite or other secondary minerals, which are unsuitable for K-Ar dating. Accordingly the study is concentrated on the upper columnar basaltic lavas. Based on the K-Ar ages, we suggest that the volcanic activity of Penghu Islands occurred from Middle Miocene to Late Miocene. This conclusion is different from earlier reports which indicated that the volcanic activity is Late Pliocene or Pleistocene (Lin et al. 1957, Huang 1967, Yen 1987). Nannofossils andforaminifers. In the Southern Group, sandstone and mudstone containing nannofossils and foraminifers occur in Chimeihsu and Tunghsuping. The calcareous nannofossil assemblage comprises the following: Catinaster coalitus, Discoaster exilis, D. neohamatus, Sphenolithus pacificus, S. abies, D. quinqueramus, Calcidiscus macintyrei, Coccolithus pelagicus, and Braarudosphaera bigelowi (Juang 1988). Thus the age may belong to NN8-9 (upper Middle Miocene; Hsieh and Chi 1989, personal communication). The planktonic foraminifers identified by K. S. Hsieh, Central Geological Survey, include Globoquadrina dehicens, Globorotalia merotumida--G, plesiotumid, Neogloboquadrina acostaensis--N, humerosa, Dentoglobigerina globosa, Sphaeroidinellopsis seminulina and Globigerinoides obliquus indicating a Middle-Upper Miocene (N16-17) age (Blow 1969).

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The REE spectra of these rocks are marked by a typical LREE enrichment; the (La/Yb)N ranges from 6.4 to 28.9 in alkali basalts, and 2.7 to 6.4 in tholeiites. These values are comparable with basalts from continental environments (Ratcliffe 1987, Auchapt et al. 1987, Dupuy and Dostal 1984).

(b ) Magmatic evolution of the Penghu basalts

Figure 5 shows the Sr/Ca vs Ba/Ca plots for the Penghu basalts. Note that the plots define a roughly linear trend with alkali basalts having relatively higher Sr/Ca and Ba/Ca ratios. The Sr-Ba systematics defined by a series of the most primary tholeiite and alkali basalts from volcanoes in the Izu Islands region (Onuma et al. 1983) are also plotted for comparison. It is clear that the data points of the Penghu basalts have a different slope from the Izu Islands partial melting line. It may be interpreted that the upper mantle under the Penghu Islands does not have a chondritic composition in terms of Sr/Ca and Ba/Ca ratios. However, an alternative argument is that the Penghu tholeiite and alkali basalts have undergone limited fractional crystalGEOCHEMISTRY OF THE PENGHU BASALTS lization which leads to the lower Sr/Ca and Ba/Ca ratios as compared with the Izu partial melting line. The slope The Penghu basalts consist mainly of alkali basalt and of Sr-Ba systematics may have been controlled by the tholeiite of Middle to Late Miocene age. These basalts ratio of plagioclase/clinopyroxene crystallizing in the crop out on the Penghu Islands but may extend further magma chamber (Onuma et al. 1983). to the southwest. The LREE contents of the Penghu basalts are plotted against each other in Fig. 6. It is obvious that the La-Ce (a) Evidence for continental intraplate volcanism slope is steeper whereas the Sm-Ce slope is gentler than the chondritic ratio. As the LREE are highly incompatThe chemistry of the Penghu basalts has been studiecL ible in the mantle-melt system, the ratios should be close by atomic absorption and instrumental neutron acti- to those of the mantle source under a batch melting vation analysis. Figure 3 shows TiO2 vs (Ba/La)N plots condition (Sun and Hanson 1975). The Ce vs K20 and for the Penghu basalts, suggesting that they fall within Ce vs P205 plots (Fig. 6) display a linear relationship, the intraplate basalt field. It should be mentioned that strongly reflecting that LREE and other incompatible Juan et al. (1984) and Sun (1982) suggested that the elements are closely related during the evolution of the existence of many half-graben-type basins in the Tertiary basalts. sequence in western Taiwan indicates a continental Calculations using Shaw's (1970) equation and rifting environment where intraplate volcanism, such as partition coefficients suggested by Hanson (1980) the Penghu basalts are expected to occur. indicate that the alkali basalts of the Penghu Islands The chondrite-normalized REE patterns of the cannot be derived from partial melting of a particular Penghu alkali basalts and tholeiites are shown in Fig. 4. mantle source with REE three times of chondrite. Chen SEAES

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(1973) argued that Penghu alkali basalt originated at a relatively deeper level than the tholeiite. It is suggested that the deeper part of the mantle beneath the Penghu Islands may have been metasomatized by LILE enriched fluid and enriched in LREE (Chen 1988), then 10-15% partial melting from this metasomatized upper mantle would yield the observed REE abundances of the Penghu alkali basalts (Fig. 7). Lee (1985) proposed that the tholeiite in the Penghu Islands was formed by the mixing of alkali basalt with a theoretical melt which was generated by 20% partial melting of the primary mantle. However, the theoretical basalt has not been found in the Penghu Islands. The present chemical data (Figs 5 and 6) tend to support the conclusion reached by Chen (1973) that the tholeiite and

alkali basalt may represent two independent magma types. If the argument that the Penghu tholeiite originated at a relatively shallower level than the alkali basalt is accepted, it could be assumed that the mantle peridotite possessed REE 2-3 times that of chondrites. If the unmetasomatized mantle at relatively shallow level is composed of lherzolite with 55% olivine, 25% orthopyroxene, 15% clinopyroxene and 5% garnet and has 2 times chondritic REE abundances, then 5-10% equilibrium batch partial melting with a melting mode of 10:20:40: 30, respectively, could generate the Penghu tholeiites (Fig. 8).

TECTONIC IMPLICATION OF PENGHU VOLCANISM

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Contemporaneous volcanism took place sporadically during the deposition of the Miocene sediments in the western foothills of Taiwan. Volcanic activity was most extensive in the early sedimentary cycle. Volcanism of this phase has been called the Kungkuan volcanic stage (Yen 1950, 1958, Ho 1988). In the middle sedimentary cycle, the volcanic activity was very limited. However, volcanic activity was active again during the late sedimentary cycle. The latest Miocene volcanic phase is named the Chiaopanshan Stage by Yen (1950, 1958). Miocene volcanic activities have also been recognized under the western foothills and coastal plain, and in offshore region from subsurface well data (Yang et al. 1981, Yuan 1981). Briefly, the onshore and offshore geology of Taiwan during the Late Miocene is characterized by the onset of paralic deposition with intensive basaltic volcanism of

Geochronology and geochemistry of Penghu basalts, Taiwan Strait 10C

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faulted basins with very thick Neogene sediment infill along the South China Sea margin, SW of Taiwan (Tainan Basin), about 50 km SSE of the Penghu Islands. Normal faults with similar WSW-ENE trends bound the Penghu Basin with Neogene sediment infill about 1300 m thick, about 50 Km NNW of the Penghu Islands. As a result, the Penghu Islands represent the top of a horst 100 km wide that trends parallel to the passive margin of the South China Sea.

the Chiapanshan stage and rifting tectonism (Yen 1958, Sun 1982, Liu and Pan 1984). Geochemical study of these basalts indicates that they belong to the continental rifting type (Juang 1981). Seismic profiles in the Taiwan Strait off Hsinchu and on the Peikang basement strongly indicate the development of normal faults, grabens and horsts (Sun 1982, Liu and Pan 1984). The rifting event and the accompanying continental crustal extension developed a series of W S W - E N E trending normal 30

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192

W . S . JUANG a n d J. C. CHEN by Dr A. W. Webb of the AMDEL of Australia. For their kind assistance we are very thankful. Thanks are also due to Prof. J. G. Lo, National Tsing Hua University, for helpful assistance in the neutron activation analysis. We would also like to thank Dr T. P. Yen, Mr C. C. K. Fong and two anonymous reviewers for their critical reading of the manuscript. This study was supported by the Central Geologic Survey, MOEA.

c/ A

REFERENCES

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[

ioo (Ce)N

Fig. 8. Log-log plot of (Ce/Yb)N ratio vs CeN. Fractionation trends modified from Tarney et al. (1980). A: closed system low pressure fractional crystallization; B: open system low pressure fractional crystallization (O'Hara 1977); C: open-system high pressure eclogite fractionation; D: equilibrium batch partial melting (garnet Iherzolite source, 55% ol, 25% opx, 15% cpx, 5% gt, melting mode of 10:20:40:30, respectively, and two times chondritic REE abundances). Dots indicate Penghu tholeiites (this study). N-type MORB from Henderson (1984). Based on the K - A r dates a n d T u n g l i a n g well d a t a , we suggest t h a t the volcanic activity in the P e n g h u I s l a n d s m a y be c o r r e l a t e d with the M i o c e n e volcanic activity in western T a i w a n . T h e basalts o f the western foothills o f T a i w a n a n d o f the P e n g h u I s l a n d s are similar in petrology, g e o c h e m i s t r y a n d g e o c h r o n o l o g y (Juan et al. 1984). It s h o u l d be m e n t i o n e d t h a t the w i d e s p r e a d basaltic flows o f L a t e C e n o z o i c age f o u n d in I n d o c h i n a , H a i n a n , s o u t h e a s t e r n c o a s t o f m a i n l a n d China, Pearl River M o u t h Basin, the P e n g h u I s l a n d s a n d western T a i w a n m a y be related to the s a m e C e n o z o i c rift tectonism a l o n g the Asiatic c o n t i n e n t a l m a r g i n caused by the third h e a t i n g a n d rifting episodic e v o l u t i o n o f the S o u t h C h i n a Sea ( R u a n d P i g o t t 1986, Y u 1988). F u r t h e r m o r e , it m u s t be m e n t i o n e d t h a t paleostress analysis in the P e n g h u I s l a n d s e n a b l e d us to reconstruct the o r i e n t a t i o n o f L a t e C e n o z o i c stresses (Angelier et al. 1988). T w o m a i n extensional tectonic events were thus identified: a N - S extension d o m i n a t e d d u r i n g the M i d d l e - L a t e Miocene, a n d a m o r e recent N W - S E extension p r e v a i l e d d u r i n g the L a t e Miocene. T h e characteristics o f the N e o g e n e extensional tectonics are i n t i m a t e l y related to m a j o r rifting processes in the S o u t h C h i n a Sea. O w i n g to the fact t h a t the P e n g h u I s l a n d s are n e a r to, b u t o u t s i d e of, the T a i w a n O r o g e n ( H o 1988), the m a j o r P l i o - Q u a t e r n a r y collision processes m a y n o t affect the P e n g h u Islands. Therefore, the P e n g h u F o r m a t i o n has r e m a i n e d h o r i z o n t a l a n d u n d i s t u r b e d , a n d except for rare fault scarps, m a p p a b l e tectonic features are a l m o s t absent. Acknowledgements--Some of the Ar isotope analyses were completed

by Prof. H. Bellon of University of Bretagne Occidentale of France and

Angelier, J., Bergerat, F., Chu, H. T., Juang, W. S. & Lu, C. Y. 1988. Paleostress analysis as a key to margin extension: the Penghu Islands, South China Sea. In: Intern. Syrup. Geodyn. Evolution Eastern Eurasian Margin, Paris, September 1988, Abstr. Vol. 23. Auchapt, A., Dupuy, C., Dostal, J. & Kanika, M. 1987. Geochemistry and petrogenesis of rift-related volcanic rocks from south Kivu (Zaire). J. Vole. Geotherm. Res. 31, 33-46. Basu, A. R. 1975. Hot-spots, mantle plumes and a model for the origin of ultramafic xenoliths in alkali basalts. Earth Planet. Sci. Lett. 28, 261-274. Blow, W. H. 1969. Late middle Eocene to Recent planktonic foraminiferal biostratigraphy. Internal Conference on Planktonic Microfossils, Geneva, 1967, Proc. Vol. 1, 199-422. Bosum, W., Burton, G., Hsieh, S. H., Kind, E. G., Schreiher, A. and Tang, C. H. 1970. Aeromagnetic survey of offshore Taiwan. CCOP Tech. Bull. 3, 1-34. Chauvel, C. and Jahn, B. M. 1984. Nd-Sr isotope and REE geochemistry of alkali basalts from the Massif Central, France. Geochim. cosmochim. Acta 48, 93-110. Chen, J. C. 1973. Geochemistry of basalts from Penghu Islands. Proc. Geol. Sci. China 16, 23-36. Chen, J. C. 1988. Geochemistry of basalts from Niu-Hsin-Shan area, Penghu Islands. Proc. Geol. Soc. China 31, 53-66. Chou, J. T. 1969. A petrographic study of the Mesozoic and Cenozoic rock formation in the Tungliang well TL-I of the Penghu Islands Taiwan, China. CCOP Tech. Bull. 2, 97-115. Deguchi, Y. 1912. Report on the geology and mineral resources of the Boko Islands. Government-General of Taiwan: 93 pp. (in Japanese). Dupuy, C. and Dostal, J. 1984. Trace element geochemistry of some continental tboleiites. Earth Planet. Sci. Lett. 67, 61-69. Hanson, G. N. 1980. Rare earth elements in petrogenetic studies of igneous systems. Ann. Rev. Earth Planet. Sci 8, 371-406. Hayasaka, I. 1933. Geologic data about the Boko Islands, Taiwan. Tigaku Kizi 4, 73-78 (in Japanese). Henderson, P. 1984. Rare Earth Element Geochemistry. Elsevier, New York. Ho, C. S. 1988. An Introduction to the Geology o f Taiwan: Explanatory Text o f the Geologic Map o f Taiwan. Central Geological Survey, MOEA: 192 pp. Huang, T. Y. 1967. Foraminiferal study of the Tungliang well TL-1 of the Penghu Islands. Petrol. Geol. Taiwan 5, 131-149. Huckenholz, H. G. 1973. The origin of fassaitic augite in alkali basalt suite of Hocheifei area, West Germany. Contr. Miner. Petrol 40, 315-326. Juan, V. C., Chen, C. H. and Lo, H. J. 1984. Basaltic rock types in various tectonic setting in Taiwan. Proc. Geol. Soc. China 27, 11-24. Juang, W. S. 198 I. Analysis of international rock standards by atomic absorption spectroscopy. Proc. Geol. Soc. China 24, 28-39. Juang, W. S. 1988. Geochronology and chemical variations of late Cenoozic volcanic rocks in Taiwan. Ph.D. thesis, Inst. Oceanography, NTU 231 pp. Lee, C. Y. 1985. Trace element geochemistry of major basaltic rocks from Penghu area: M.S. thesis, Inst. of Geology, National Taiwan University, 83 pp. Lin, C. C., Chiu, Y., Lu, H. C. and Huang, T. Y. 1957. The geology and mineral resources of the Penghu Islands. Taiwan Min. Ind. 9(3-4), 26-38. Liu, C. H. and Pan, Y. S. 1984. Seismic stratigraphic study on the Tertiary sequences in the Hsinchu Basin Taiwan. Petrol. Geol. Taiwan 20, 97-112. McDougall, I. 1966. Precision methods of potassium-argon isotopic age determinations on young rocks. Methods and Techniques in Geophysics, Vol. 2. Wiley, London. O'Hara, M. J. 1977. Geochemical evolution during fractional crystallization of a periodically refilled magma chamber. Nature 266, 503-507. Onuma, N., Hirano, M. and Isshiki, N. 1983. Genesis of basalt magmas and their derivatives under the Izu Islands, Japan, inferred from Sr/Ca-Ba/Ca systematics. J. Vole. Geotherm. Res. lg, 511-529.

Geochronology and geochemistry of Penghu basalts, Taiwan Strait Perfit, M. R., Gust, D. A., lknce, A. E., Arculus, R. J. and Taylor, S. R. 1980. Chemical characteristic of island arc basalts, implications for mantle source. Chem. Geol. 30, 227-256. Ratcliffe, N. M. 1987. High TiO~ metadiabase dikes of the Hudson Highlands, New York and New Jersey: possible late Protozoic rift rocks in the New York recess. Am. J. Sci. 287, 817-850. Ru, K. and Pigott, J. P. 1986. Episodic tiffing and subsidence in the South China Sea. Bull. AAPG 70, 1136-1155. Shaw, D. M. 1970. Trace element fractionation during anatexis. Geochim. cosmochim. Acta 34, 237-243. Steiger, R. H. and J/iger, E. 1977. Subcommission on geo~hronology convention on the use of decay constants: In geo-~ and cosmochronology. Earth Planet. Sci. Lett. 36, 359-362. Sun, S. C. 1982. The Tertiary basins of offshore Taiwan. Proceedings of the Second ASCOPE Conference and Exhibition, pp. 125-135. Sun, S. S. and Hanson, G. N. 1975. Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites. Contr. Miner. Petrol. 52, 77-106. Tarney, J., Wood, D. A., Saunders, A. D., Cann, J. R. and Varet, J. 1980. Nature of mantle heterogeneity in the North Atlantic: evidence

193

from deep sea drilling. Phil. Trans. R. Soc. Lond. Set. A, 297, 179-202. Yang, H. J., Chen, J. C. and Yang, H. Y. 1987. Geochemistry of ultramafic nodules in basaltic rocks from Peiliao, Penghu Islands and Liutsu, northern Taiwan. Proc. Geol. Soc. China 30, 40-57. Yang, H. Y., Wu, H. C. and Chou, T. F. 1981. Igneous rocks in the well in the west-central Taiwan. Rep. Explor. Prod. 4, 123-124 (in Chinese). Yen, T. P. 1950. Neogene and Quaternary basalts in Taiwan. Q. JI Taiwan Mus. 3(1), 33-54. Yen, T. P. 1958. Cenozoic volcanic activity in Taiwan. Taiwan Min. Ind. 10, 1-39. Yen, T. P. 1965. Some geological and petrological features of the Penghu basalts. Proc. Geol. Soc. China 8, 52-60. Yen, T. P. 1987. Geology of the Penghu Islands, Taiwan. Bull. Geophys. 27-28, 1-44. Yu, H. S. 1988. Tectonic evolution of the Pearl River Mouth Basin off the South China Coast. Acta Oceanogr. Taiwanica 20, 79-94. Yuan, J. W. 1981. Radiomettic ages of the igneous bodies in the wells in the western central Taiwan. Rep. Explor. Prod. 4, 363-366.