Ju-Chin Chen

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

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 by 40Ar–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 (≤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.

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

Abstract The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10 −13.4 to 10 −10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

Petrogenesis of two groups of pyroxenite from Tungchihsu, Penghu Islands, Taiwan Strait: implications for mantle metasomatism beneath SE China

Abstract Type II pyroxenite xenoliths are a common form of mantle material found in Miocene basaltic pyroclastic rocks at Tungchihsu in the Penghu Islands, Taiwan Strait. Two groups are identified from mineralogy, petrology and chemistry: Group I have mineral assemblages of clinopyroxene+amphibole (mosaic, generally kaersutite)±apatite±Ti–Fe oxide mineral±iron sulfide. These xenoliths are characterized by high abundances of large-ion lithophile elements (including Ba, K, Rb, Sr and Th), high field strength elements (Zr, Nb and Ti), and light rare-earth elements. Chemical compositions of this group of pyroxenite resemble those of Penghu alkali basalts. Likewise, the Sr–Nd isotopic compositions ( 87 Sr/ 86 Sr=0.70370–0.70385; 143 Nd/ 144 Nd=0.51285–0.51295) of clinopyroxene, amphibole and apatite separates from this group falls within the field for late Cenozoic Penghu basaltic rocks and megacrysts, and it is suggested that this group of pyroxenite was formed by the crystallization of alkali basaltic magma in the upper mantle. The second group pyroxenites (group II) contain clinopyroxene, garnet, spinel, amphibole (interstitial, generally pargasite/hastingsite) and/or orthopyroxene and a Ti–Fe oxide mineral. They have low Na 2 O, K 2 O, TiO 2 , LREE, and high Al 2 O 3 relative to group I pyroxenites, and compositionally are similar to tholeiitic picrites. From major- and trace elements evidence, we suggest that the second group of pyroxenites may have formed by crystallization of tholeiitic picritic magmas under high pressure. Isotopic differences between clinopyroxene, amphibole and garnet or apatite separates from the two groups (group II: 87 Sr/ 86 Sr=0.70460–0.70640; 143 Nd/ 144 Nd=0.51269–0.51290) indicate the two groups of pyroxenite are not related. The coexisting mineral assemblages (clinopyroxene, amphibole and garnet) of group II pyroxenite are not in isotopic equilibrium suggesting that the minerals crystallised from different melt phases, possibly in a magma chamber undergoing fractionation and recharge or by progressive crystallisation in a vein system. The pyroxenite thus formed was then subsequently fragmented and/or re-cemented by a subsequent volatile bearing-melt that crystallised the interstitial amphibole.

Geochemistry of late Cenozoic basalts from Wudalianchi and Jingpohu areas, Heilongjiang Province, northeast China

Late Cenozoic basaltic rocks in Wudalianchi and Chinpohu areas of Heilongjiang Province, northeast China include alkali–olivine basalt, leucite basalt and basanite. The alkali basalts from Wudalianchi are higher in Si, Mg, K, P, Ni, Zr, Rb, Ba and LREE but lower in Ti, Al, Fe, Ca, Na, Ta, Th, Sr and Y when compared with alkali basalts from East African province. The most striking chemical features in Wudalianchi alkali basalts are the high potassium (averaging 5.10%) and Ba (averaging around 1800 ppm) contents. The alkali olivine basalt and basanite from Jingpohu are higher in Fe, Mg, Ca and Ta but lower in K, P, Ni, Zr, Hf, Nb, Rb, Sr, Ba and LREE than alkali olivine basalt and basanite from Wudalianchi. Regular variations have been found in major and trace elements vs 100(MgO/MgO+FeOt) plots for alkali basalts from Wudalianchi and Jingpohu suggesting that fractional crystallization has occurred after the formation of the initial liquid. We suggest that the various alkali basalts from Wudalianchi and Jingpohu areas were derived from a mantle source which had been metasomatized and enriched in incompatible elements with a general characteristic similar to EM I defined by Weaver (l991, Earth Planet. Sci. Lett., 104, 381–397). The La/Ce ratios of Wudalianchi basanites tend to increase with younger ages which may be interpreted as due to the decrease of the degree of partial melting of the metasomatized mantle in the younger lavas.

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

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.

Taiwan: Geology, Geophysics, and Marine Sediments

The ocean margin island of Taiwan is a geodynamic body of young and complex build. Occupying an area of 36,000 km2 between the Chinese part of the Eurasian continent capping one lithospheric plate and the north¬western part of the Philippine Sea floored by another (Figs. 1, 7), this island, as a compression-plus-shear product, is elevated to a maximal height of almost 4000 m, higher than any other fold mountains on the northwest coast of the Pacific Ocean. It is an arcuate island extending its shorter arm eastward Open image in new window Fig. 1 Map of Taiwan and offshore areas. Isobaths (depth in 1000 m) are based on Chase et al. (1971). Heavy lines are locations of seismic reflection profiles. to the Ryukyus and its longer arm southward to the Philippines. The crestal zone of this mountainous island is the Central Range, which is fringed on its west side by the Foothill Zone and separated on its east from the Coastal Range by the corridor known as the Longitudinal Valley (Fig. 2). West of the Foothill Zone is a vast coastal plain with the very shallow Taiwan Strait farther west; east of the Coastal Ranage is the deep Philippine Sea whose floor exhibits arcs and trenches like the North Luzon Ridge and Trough. The dependent islands of Taiwan include the Penghu Islands in the Taiwan Strait and the islands of Lutao and Lanhsu off the southeast coast. It must be added that, in the less tightly compressed northeastern and southwestern parts of the mountain complex of the Central Range and Foothill Zone, there are, respectively, the Ilan Plain and the Pingtung Valley, each in the from of an intramontane trough wedging from the sea into the island.

Geochemistry and origin of tektites from the Penglei area, Hainan province, southern China

Ten tektites from the Penglei area in Hainan province were analyzed for the abundances of major elements, 31 trace elements, and the RbSr isotopic composition. These tektites can be divided into two compositional categories, which are distinguished by high (> 80 wt %) and low (<76 wt %) SiO2 contents. Tektite TK-1 (high SiO2) shows a vesicular massive body and has lower refractive index, density, major and trace elements. Its chemical composition closely resembles the average composition of Muong Nong-type indochinites. The others with low SiO2 either pitted or grooved surfaces, with schlieren structures on some surfaces, and splash-form have similar chemical compositions to those of indochinites. From the chemical composition, it is suggested that the tektites within the Indochina and the Hainan subfields are derived from similar parental material and are similar to the post-Archean upper crustal rocks. In addition, the tektites from Hainan (Hainanites) have large positive ϵSr(0) ratios, indicating that the parent material for these tektites resembles old terrestrial sedimentary rocks. From the Sr isotopic data, it is interpreted that the hainanites do not originate from continental material recently derived from the mantle or recent young sediments such as soil or loess. Based on RbSr isotopic data, it has been suggested by Blum et al. Geochim. Cosmochim. Acta 56, 483–492, 1992 that the depositional age of sedimentary target materials is close to 170 Ma (Jurassic). Mixing calculations for various amounts and combinations of target rocks indicate that the best fit for sample TK-1 tektite is a mixture of 2% shale, 38% sandstone, 50% greywacke and 10% quartzite, and the other splash-form tektite is a mixture of 41% shale, 2% sandstone, 20% greywacke and 37% quartzite.

Geochemistry of manganese nodules from offshore areas of Mariana Islands and Johnston Island

Abstract The manganese nodules near the Mariana Islands generally range from 2 to 4 cm in diameter and some samples have porous surfaces. The nodules near Johnston Island are larger in size (5–8 cm in diameter) and more compact than the Mariana nodules. The major FeMn minerals found in Mariana Islands samples are todorokite, birnessite and akaganeite (β-FeOOH) while, in the Johnston Island samples, only birnessite and akaganeite are found. The Mariana Islands nodules are higher in Mn, Mg, Na, K, Co, Ni, Pb and Th but lower in Fe, Sr, Zn, Ba, Zr, Y and REEs than the Johnston Island samples. The (Cu + Ni + Co) contents of the Mariana Islands samples are generally higher than 15,000 ppm with Co Zn ratios varying from 10 to 15, while the Johnston Island samples generally have (Cu + Ni + Co) between 8000 and 10,000 ppm with Co Zn ratios varying from 5 to 7. Therefore these nodules may not be related to hydrothermal activity (Toth, Geol. Soc. Am. Bull.91, 44–54, 1980). Both the Mariana Islands and Johnston Island nodules show similar LREE-enriched patterns with distinct positive Ce anomaly and negative Eu anomaly. The positive correlation between the Ce anomaly defined as log Ce 2 3 La+ 1 3 Nd and Mn Fe ratios found in the nodules studied suggest that a phosphorus-rich phase with REE pattern similar to that for biogenous apatite may be the third component in considering the source of REEs in the nodules. The growth rate determined by the excess 230Th method for Johnston Island nodule JA-1, from 0.2 to 0.45 mm depth, is 1.12±0.10 mm/Ma but a much higher rate (17.66 ± 7.61 mm/Ma) is observed from 0.45 to 1.8 mm depth. The growth rate of the nodule may be related to its Mn and Fe contents.

Petrogenesis of Cenozoic Basaltic Rocks from Jiangsu Province, China: Evidence from Geochemical Constraints

Cenozoic (Miocene to Pleistocene) basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt. We present major, trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area. On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province, we suggest that these basalts may belong to the same magmatic system. The alkali basalts found in Jiangsu province have higher σFeO, MgO, CaO, Na2O, TiO2 and P2O5 and incompatible elements, but lower Al2O3 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine, pyroxene and minor plagioclase. In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/σFeO ratios. The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB. Partial loss of the mantle Uthosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma. The 143Nd/144Nd vs. 87Sr/86Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EM1 component in the study area. According to Shaws equation, the basalts from Jiangsu province may be formed by 1%–5% partial melting of a depleted asthenospheric mantle source. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen (1982), we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts.

Geochemistry of Miocene basaltic rocks recovered by the Ocean Drilling Program from the Japan Sea

Ocean Drilling Program Leg 127 drilling at site 797 in the Yamato Basin of the Japan Sea indicated that the basement is composed of early Miocene (−19 Ma) basaltic-doleritic rocks, whereas at site 795 in the northern Japan Basin the basement is composed of middle Miocene (−15 Ma) calc-alkali basalt and basaltic andesite lava flows. The basaltic rocks from Hole 795B are characterized by moderate amounts of large-ion lithophile elements (including K, Rb and Sr), high-field strength elements (including Zr and Ti) and light rare-earth element (Ce around 2.5 ppm). The lithology of Hole 797C basaltic rocks can be divided into upper suite (unit 10 and above) and lower suite (unit 11 and below) based on the petrology and chemistry of these rocks (Tamaki et al., 1990). The 797C lower suite basalts are higher in Fe, Ti, Na, K, P, Ba, Hf, REEs, Cu, Cr and Ni when compared with the upper suite basalts. A distinct variation gap exists between Hole 797C upper suite and lower suite basalts in the Cr vs Y plot. The upper suite basalts are confined to a restricted area, while the lower suite basalts show a systematic variation trend. The upper suite and lower suite basalts may represent two distinct magmatic series. The mantle sources for Hole 797C lower suite basalts may be more enriched in incompatible elements than those of the upper suite basalts.