Zircon SHRIMP U-Pb dating of the Neogene coral reefs,Xisha Islands,South China Sea:implications for tectonic evolution

TheXishaBlockisaminoroneintheSouthChinaSeaandanimportanttectonicunitinthenorthwesternpartoftheregion.ZirconSHRIMPU-PbagesforthreevolcanicintrusivecoresamplesfromXike-1,anexploratorywellpenetratingthebiohermsoftheXishaIslands.ThecoresamplesarefromtheMiocenereefcarbonatebedrockandarerecognizedasdark-graybiotite-hornblendegabbro,grayfne-grainedbiotitediorite,andgrayfine-grainedgranite,respectively.Zirconcathodoluminescence(CL)imagesandtraceTh,UandPbcompositionsofthezirconsshowthattheserocksareofvolcanicintrusiveorigin.ZirconSHRIMPU-Pbdatingyieldedsixgroupsofages,rangingfrom2451-1857MatoearlyCretaceous,whichindicatethattheformationandevolutionoftheXishaBlockwasaffectedbytheevolutionandclosureofNeotethysOcean,probablywithinitseasternextensionintoSouthChinaSea.Bothold,deepsourcedmaterial,includingfragmentsfromRodinasupercontinent,andrecentmantle-derivedmagmaproductscontributedtotheemergenceandformationoftheXishablock.TheSHRIMPU-PbresultsalsoprovedthatthisprocessdifferedfromthatoftheKontummassif,theHainanBlock,andtheSouthChinaBlock,butissimilartothatoftheNanshaandZhongshablocks.TheprocesswasassociatedwiththeeffectsofYanshanianmagmatisminducedbysubductionmechanismsofthePaleo-PacificPlateorthereworkingofthemultiplemagmatismssincetheEarlytomid-Yanshanian,possiblyjointlyexperiencedbytheXisha-Zhongsha-NanshaBlock.