U-Pb zircon geochronology of basaltic pyroclastic rocks from the basement beneath the Xisha Islands in the northwestern South China Sea and its geological significance

As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding of the pre-Cenozoic basement of the Xisha Islands. Well CK-1, a kilometer-scale major scientific drill in the Xisha Islands in the northwestern SCS, penetrated thick reefal limestone(0–888.4 m) and the underlying basement rocks(888.4–901.4 m). In this study, we present the zircon U-Pb ages of basement basaltic pyroclastic rocks from Well CK-1 in the Xisha Islands of the northwestern SCS to investigate the basement nature of the Xisha microblock. The basement of Well CK-1 consists of basaltic pyroclastic rocks on the seamount. The zircon grains yielded apparent ages ranging from ca. 2 138.9 Ma to ca. 36 Ma. The old group of zircon grains from Well CK-1 was considered to be inherited zircons. Two Cenozoic zircons gave a weighted mean 206Pb/238U age of(36.3 ± 1.1) Ma,Mean Squared Weighted Deviations(MSWD) = 1.2, which may represent the maximum age of the volcano eruption. The Yanshanian inherited zircons(116.9–105.7 Ma and 146.1–130.2 Ma) from Well CK-1 are consistent with the zircons from Well XK-1, indicating that the basement of Chenhang Island may be similar to that of Well XK-1. We propose that the Xisha micro-block may have developed on a uniform Late Jurassic metamorphic crystalline basement, intruded by Cretaceous granitic magma.

Ferroan dolomites in Miocene sediments of the Xisha Islands and their genetic model

Carbonate rocks are important reservoirs for global petroleum exploration.The largest oilfield in the South China Sea,Liuhua 11-1,is distributed in the massive carbonate reef area of the Zhujiang(Pearl)River Mouth Basin.Previous studies showed that one 802.17-m-long core from well Xichen-1 in the South China Sea mainly consisted of white and light gray-white organic reefs.Recently,a Miocene whole core(161.9 m long)of well Xiyong-2,near well Xichen-1,was found to contain six layers of yellowish brown,light yellowish gray,iron black,or light yellowish gray-white organic reefs.Scanning electron microscope images of these layers reveal a typical ferroan dolomite rich in Fe(up to 29%),with the high concentrations of Mn,Cu,W,Zn,Cr,Ni,and Co.Systematic X-ray powder diffraction analysis yields a 1.9–6.1 match in phase ratio with ankerite,5.4–26.9 with dolomite,and zero with calcite,which indicate that the samples can be classified as ferroan dolomite.The iron and heavy metals are inferred to be originated from multiple volcanic eruptions of Gaojianshi Island in the Dongdao Atoll during the middle-late Miocene.These elements were dissolved in seawater,likely as a sol,and carried to Yongxing Island in the Xuande Atoll by sea currents and tides enhanced by prevailing winds,and deposited as a part of the sedimentation process in the study area.The ferroan dolomite has Sr content of (125–285)×10^(-6),which is lower than the accepted Sr boundary value of dolomite.This finding suggests that dolomitization occurred during large-scale global glacial regression in the late Miocene.The isolated Xisha carbonate platform,exposed to air,underwent freshwater leaching and dolomitization induced by mixed water,and caused the extensive Fe-Mg exchange along the organic reef profile to form ankerite and ferroan dolomite.These results may help to understand paleoceanographic environmental changes in the South China Sea during the Miocene.

Taxonomy and toxin profi le of harmful benthic Prorocentrum(Dinophyceae)species from the Xisha Islands,South China Sea

Some benthic Prorocentrum can produce okadaic acid(OA)and dinophysistoxins(DTXs)that cause diarrheic shellfish poisoning(DSP)in humans.The diversity and toxin profi les(OA and DTXs)of benthic Prorocentrum were investigated in the Xisha Islands,South China Sea.The benthic Prorocentrum was identified by both morphological features and molecular phylogenies.Morphologies were examined by light,fluorescence,and scanning electron microscopy,and phylogenetic analyses were based on partial large subunit(LSU)rDNA and ITS1-5.8S-ITS2(ITS)region.Seven Prorocentrum species including P.borbonicum,P.caipirignum,P.concavum,P.elegans,P.cf.emarginatum,P.lima complex,and P.rhathymum were identified in Xisha Islands.Among them,P.borbonicum and P.elegans were recorded in Chinese waters for the first time.OA and DTXs contents of seven benthic Prorocentrum species were evaluated based on liquid chromatography-tandem mass spectrometry(LC-MS/MS).All Xisha Islands strains of P.lima complex produced OA at contents ranging from 1663 to 3816 fg/cell.P.caipirignum also generated OA at 407 fg/cell,but other five species had no detectable toxins.Besides,interestingly,two strains of P.lima complex produced DTX-1 only(74 and 183 fg/cell)and another two strains generated an isomer of OA and DTX-2.Our findings provided insight into the biodiversity of benthic Prorocentrum in the Xisha Islands and pointed out the potential risk of DSP in this area.

Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession at the Xisha Islands,South China Sea

The late Quaternary shallow-water carbonates have been altered by a variety of diagenetic processes,and further influenced by high-amplitude global and regional sea level changes.This study utilizes a new borehole drilled on the Yongxing Island,Xisha Islands to investigate meteoric diagenetic alteration in the late Quaternary shallowwater carbonates.Petrographic,mineralogical,stable isotopic and elemental data provide new insights into the meteoric diagenetic processes of the reef limestone.The results show the variation in the distribution of aragonite,high-Mg calcite(HMC)and low-Mg calcite(LMC)divides the shallow-water carbonates in Core SSZK1 into three intervals,which are UnitⅠ(31.20-55.92 m,LMC),UnitⅡ(18.39-31.20 m,aragonite and LMC)and UnitⅢ(upper 18.39 m of core,aragonite,LMC and HMC).Various degrees of meteoric diagenesis exist in the identified three units.The lowermost UnitⅠhas suffered almost complete freshwater diagenesis,whereas the overlying UnitsⅡandⅢhave undergone incompletely meteoric diagenesis.The amount of time that limestone has been in the freshwater diagenetic environment has the largest impact on the degree of meteoric diagenesis.Approximately four intact facies/water depth cycles are recognized.The cumulative depletion of elements such as strontium(Sr),sodium(Na)and sulphur(S)caused by duplicated meteoric diagenesis in the older reef sequences are distinguished from the younger reef sequences.This study provides a new record of meteoric diagenesis,which is well reflected by whole-rock mineralogy and geochemistry.

El Ni?o/Southern Oscillation during the 4.2 ka event recorded by growth rates of corals from the North South China Sea

The 4.2 ka event that occurred during the period from 4 500–3 900 a BP was characterized by cold and dry climates and resulted in the collapse of civilizations around the world. The cause of this climatic event, however, has been under debate. We collected four corals(Porites lutea) from Yongxing Island, Xisha Islands, South China Sea, dated them with the U-series method, and measured the annual coral growth rates using X-ray technology. The dating results showed that the coral growth ages were from 4 500–3 900 a BP, which coincide well with the period of the4.2 ka event. We then reconstructed annual sea surface temperature anomaly(SSTA) variations based on the coral growth rates. The growth rate-based SSTA results showed that the interdecadal SSTA from 4 500–3 900 a BP was lower than that during modern times(1961–2008 AD). A spectral analysis showed that the SSTA variations from4 500–3 900 a BP were under the influence of El Nino-Southern Oscillation(ENSO) activities. From 4 500–4 100 a BP, the climate exhibited La Nina-like conditions with weak ENSO intensity and relatively stable and lower SSTA amplitudes. From 4 100–3 900 a BP, the climate underwent a complicated period of ENSO variability and showed alternating El Nino-or La Nina-like conditions at interdecadal time scales and large SSTA amplitudes. We speculate that during the early and middle stages of the 4.2 ka event, the cold climate caused by weak ENSO activities largely weakened social productivity. Then, during the end stages of the 4.2 ka event, the repeated fluctuations in the ENSO intensity caused frequent extreme weather events, resulting in the collapse of civilizations worldwide. Thus, the new evidence obtained from our coral records suggests that the 4.2 ka event as well as the related collapse of civilizations were very likely driven by ENSO variability.

New insights on the origin of the basement of the Xisha Uplift,South China Sea

The study of basement geochronology provides crucial insights into the tectonic evolution of oceans. However, early studies on the basement of the Xisha Uplift were constrained by limited geophysical and seismic data; Xiyong1 was the only commercial borehole drilled during the 1970 s because of the huge thickness of overlying Cenozoic strata on the continental margin. Utilizing two newly-acquired basement samples from borehole XK1, we present petrological analysis and zircon uranium(U)-lead(Pb) isotope dating data in this paper that enhance our understanding of the formation and tectonic features of the Xisha Uplift basement. Results indicate that this basement is composed of Late Jurassic amphibole plagiogneisses that have an average zircon 206 Pb/238 U age of 152.9±1.7 Ma. However, the youngest age of these rocks, 137±1 Ma, also suggests that metamorphism termination within the Xisha basement occurred by the Early Cretaceous. These metamorphic rocks have adamellites underneath them which were formed by magmatic intrusions during the late stage of the Early Cretaceous(107.8±3.6 Ma). Thus, in contrast to the Precambrian age(bulk rubidium(Rb)-strontium(Sr) analysis, 627 Ma) suggested by previous work on the nearby Xiyong1 borehole, zircons from XK1 are likely the product of Late Mesozoic igneous activity. Late Jurassic-Early Cretaceous regional metamorphism and granitic intrusions are not confined to Xisha; rocks have also been documented from areas including the Pearl River Mouth Basin and the Nansha Islands(Spratly Islands) and thus are likely closely related to large-scale and long-lasting subduction of the paleo-Pacific plate underneath the continental margins of East Asia, perhaps the result of closure of the Meso-Tethys in the South China Sea(SCS). Controversies remain as to whether, or not, the SCS region developed initially on a uniform Precambrian-aged metamorphic crystalline basement. It is clear, however, that by this time both Mesozoic compressive subduction and Cenozoic rifting and extension had significantly modified the original basement of the SCS region.