Search

High arsenic abundance of 50–700 μg/L in the groundwater from the Chianan Plain in southwestern Taiwan is a well-known environmental hazard. The groundwater-associated sediments, however, have not been geochemically characterized, thus hindering a comprehensive understanding of arsenic cycling in this region. In this study, samples collected from a 250 m sediment core at the centre of the Chianan Plain were analyzed for arsenic and TOC concentrations (N = 158), constituent minerals (N = 25), major element abundances (N = 105), and sequential arsenic extraction (N = 23). The arsenic data show a prevalence of > 10 mg/kg with higher concentrations of 20–50 mg/kg concentrated at 60–80 and 195–210 m. Arsenic was extracted mainly as an adsorbate on clay minerals, as a co-precipitate in amorphous iron oxyhydroxide, and as a structural component in clay minerals. Since the sediments consist mainly of quartz, chlorite, and illite, the correlations between arsenic concentration and abundances of K 2 O and MgO pinpoint illite and chlorite as the major arsenic hosts. The arsenic–total iron correlation reflects the role of chlorite along with the contribution from amorphous iron oxyhydroxide as indicated by arsenic extraction data. Organic matter is not the dominant arsenic host for low TOC content, low arsenic abundance extracted from it, and a relatively low R 2 of the arsenic–TOC correlation. The major constituent minerals in the sediments are the same as those of the upriver metapelites, establishing a sink–source relationship. Composition data from two deep groundwater samples near the sediment core show Eh values and As(V)/As(III) ratios of reducing environments and high arsenic, K, Mg, and Fe contents necessary for deriving arsenic from sediments by desorption from clay and dissolution of iron oxyhydroxide. Therefore, groundwater arsenic was mainly derived from groundwater-associated sediments with limited contributions from other sources, such as mud volcanoes. [ABSTRACT FROM AUTHOR]

The Late Cretaceous to Early Cenozoic magmatism in the South China coastal area produced some amounts of rhyolitic rocks in two phases, which may be used to unravel the geohistory of the epilog of the paleo-Pacific plate subduction system. Essence of the Phase I rocks is the high temperature rhyolite (A-type)-trachydacite association in north Fujian (95–91 Ma) that was coeval with regional A-type granites. They succeeded the vast rhyolite–dacite–andesite (RDA) associations and I-type granitoids (113.5–96 Ma) and preceded the silicic-dominating rhyolite/basalt bimodal suites or monolithologic rhyolite in Zhejiang (89–86 Ma). Phase II rocks include (a) the RDA association or rhyolite alone in some drifted continental fragments nearby (83–56 Ma) and (b) the following rift-basin related rhyolite–trachyte/basalt bimodal suites in Guangdong and west Taiwan (56–38 Ma). The silicic volcanism, spatially changed from a NE–SW to the nearly E–W direction after 83 Ma, may reflect tectonic-driven eruptions occurred in the post-orogenic extensional (Phase I), resumed plate subducting (Phase IIa) and continental margin rifting (Phase IIb) stages. Rhyolitic rocks basically are shoshonitic to high-K calc-alkaline affinities while the Phase IIa RDA associations are mostly concentrated in the high-K to medium-K calc-alkaline series. All these rocks generally possess a continental arc character in tectonic discrimination diagrams, except shoshonitic rocks that have within-plate signatures. Based on the trace element and Nd–Pb isotope data, A-type rocks are suggested to have derived from mixing between trachydacitic (or syenitic) magmas and crustal melts of various sources under the high temperature condition (±metasomatism), and the succeeding silicic rocks are derivatives of the contaminated lithospheric mantle melts through crystal fractionation. On the other hand, Phase II silicic rocks are mainly the fractionation products of mafic magmas originated either from the lithospheric or the asthenospheric mantle. In reflection to such magmatotectonics with time, break-off of the subducted slab shortly before 113.5 Ma, change of the plate-moving direction between 86 and 83 Ma, and termination of plate movement probably at ∼56 Ma signify the epilog of the paleo-Pacific plate. [ABSTRACT FROM AUTHOR]

Abstract: There is an ongoing debate concerning whether the vast Nanling Mountain granitoids of South China are the result of Jurassic paleo-Pacific subduction. We address this question by examining two Mesozoic basalt successions in South China, one parallel and the other oblique to the convergence boundary between the paleo-Pacific plate and South China continent. The geochemical characteristics of these basalts are used to constrain the influence of this presumed subduction system on mantle composition. 40Ar/39Ar age, major element and trace element abundances, and Sr, Nd and Pb isotope data indicate that the northeast-southwest trending basalts in the Southeast Coast Magmatic Belt that formed during 101–76 Ma exhibit arc-like signatures and are derived from subduction-modified mantle. However, the east–west trending basalt array extending from the Cathaysia Interior to the Cathaysia Folded Belt, which decreases in age from 175 to 98 Ma in an eastward direction, is characterized by lithosphere-modified OIB-like asthenosphere composition, except for a few younger rocks from Huichang which resemble Southeast Coast Magmatic Belt basalts. These results suggest that the Jurassic mantle beneath the Cathaysia Interior and the Cathaysia Folded Belt was rather homogeneous and undisturbed, and was not affected by the paleo-Pacific subduction system. Alternatively, post-orogenic (Indosinian) extension accompanied by mafic underplating may have been the cause of vast Jurassic granitic magmatism in South China. [Copyright &y& Elsevier]

Abstract: In this study, the geochemical characteristics of mud volcanoes were investigated to seek their possible linkage with arsenic mobilization in the region. Among the mud volcanoes of southern Taiwan, the largest and well-preserved active mud volcanoes are situated in Wushanting (WST) and Hsiaokunshui (HKS). These two selected active mud volcanoes are exhaling mainly methane (CH4 >95%) along with other gases like O2, CO2, CO, SO2, H2S, NO, and N2. The content of CH4 in the HKS active mud volcano is very close to that of the WST mud volcano. High Na+ and Cl− concentrations in mud liquids and mudstone deposits indicate their origin in a marine depositional source. However, a newly erupted active mud volcano in WST reflects the presence of a high level of SO4 2−. Significant variations (p ≤0.05) in trace and transition element (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Mo, Cd and Ba) concentrations in the liquids and sediments of mud volcanoes are noticed at different eruption periods, suggesting different elemental compositions and release mechanisms in different locations. The weathering of micas as well as clay minerals seems to be responsible for the enrichment in trace metals in mud volcano sediments and fluids. Several trace elements (V, Cr, Mn, Fe, Co, Ni, and Zn) are strongly bound to the sediments, but concentrations of Cu, Se, and Sr are significantly higher in the fluids. The presence of higher As contents in the sediment of HKS was observed in contrast to its fluid. As a reducing agent, humic substances seem to play an important role in binding with trace elements and particularly As in the sediments. Fourier Transform Infrared (FTIR) spectra of humic substances from muds showed the presence of possible functional groups of primary and secondary amines, ureas, urethanes (amide) and silica. [Copyright &y& Elsevier]

Abstract: The widespread Mesozoic granitoids in South China (∼135,300km2) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS (n =7) and FZC (n =6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO2 content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI=0.97–1.08 and εNd(T)=−8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI>1.1 and εNd(T)<−11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI<1.0 and εNd(T)>−6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd(T)>−8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd(T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites. [Copyright &y& Elsevier]

Abstract Major and trace elements and Sm–Nd isotopic data are presented for metabasites that are present as lenses within Paleozoic metasediments in the Chenxing and Bangxi regions, central Hainan Island, Southeast (SE) China. Most metabasites are metamorphosed cumulated gabbroic rocks tholeiitic in nature, and characterized by varying degrees of depletion in Th, Nb, Ta and light rare earth elements (LREE). Moreover, they show high positive ∈Nd(T) values of approximately +7, similar to those of mid-ocean ridge basalts (MORB). A Sm–Nd isochron age of 333 ± 12 Ma obtained for the metabasites is interpreted as their crystallization age. The combined geochemical and Sm–Nd isotopic data suggest that the metabasites were generated by dynamic partial melting from a MORB-like mantle source in an oceanic regime. These rocks probably represent remnants of fragmented oceanic crust of the eastern part of Paleo-Tethys. They were obducted onto the continental crust as part of the ‘Shilu Mélange’ in earliest Mesozoic time when southern Hainan (part of the Indochina block) collided with northern Hainan (part of South China). Alternatively, they could be formed in a volcanic rifted passive margin at the sea-floor spreading stage as part of MORB-like seaward-dipping reflector break-up packages. [ABSTRACT FROM AUTHOR]