The Basic Characteristics Of Mesozoic-Cenozoic Rhyolites In The Northern Margin Of The South China Sea

The South China Sea,which is located in the intersection of several plates has experienced a complex geological evolution in history.Since late-Mesozoic it experienced the close of proto-south china sea and the cracking of modern South China Sea.The north margin of SCS,owns a completely different tectonic character and shifts from active continental margin to passive in the late-Mesozoic to Cenozoic.In this paper,we selected the Mesozoic and Cenozoic rhyolites in the north margin of SCS as the main research object,carry on the detailed analysis of the petrology,geochemistry,chronology and the rock's background and evolution process.After that,conclusions are draw as following:There are many types of rhyolite in the northern margin of the South China Sea.The rhyolite have different chronology,the one in the Qiongnan area is generally produced in the Early Cretaceous(121Ma ~ 98Ma),Maoming basin and Changtang basinis belong to the late Cretaceous(100Ma ~ 96.8Ma),which may be correspond to the active continental margin background.Only the rhyolite samples in the Sanshui area are analyzed by the Cenozoic(63.9Ma ~ 28.5Ma),which is equivalent to the passive continental margin pyrolysis of the times.The rhyolites found in maoming basin,Hainan island and sanshui basin are all porphyritic texture,with more quartz,plagioclase in maoming basin and rhyotaxitic structure in hainan island,while the sanshui basin samples with less quartz and more acidic than the maoming basin,the feldspar is brokened and plagioclase is directed in a semi-directional arrangement.The rhyolites of maoming basin and Hainan island are peralkaline sodium and high-potasium type,have a high fraction of heavy and light rare earths and light Eu negative anomaly,enriched in Th,U,K,depleted in Nb,Ta.While the sanshui samples are peraluminous to peralkaline,with more REE and a heavy Eu negative anomaly,has a good consistency with the typical continental rift alkaline rhyolite,enriched in Rb,Th,U and Ta,Nb,Zr,Hf.The rhyolites of maoming basin and Hainan island both have typical characteristics of volcanic rock in subduction zone,while Sanshui basin produced in the plate or orogenic background.Although two series of rocks are the same as the shell source,but the former's source depth deeper than the latter,and Maoming Basin and Hainan Island magma source area is not uniform,and a slight mixing or mixed,which reflect that the complexity in subduction zone.Compared with the rhyolite in Zhejiang and Fujian under the condition of the famous ancient Pacific dive,the rhyolite of Maoming and Hainan Island is differenr from it in the aspect of systematic geology,rock formation,and tectonic evolution stage.