Mineralogy,Geochemistry And Petrogenesis Of The Late Cenozoic Basalt From The Thailand And Its Geodynamical Significance

Shortly after the cessation of seafloor spreading,intraplate magmatism affected large areas in the South China Sea(SCS)region,covering not only the SCS basin,but also Leiqiong Peninsula and Indochina Peninsula.The origin and geodynamic setting of the post-spreading volcanism is still in debate,although many previous studies have focused on petrogenesis and mantle source of the late Cenozoic basalts from the SCS region.Until now,there reach a consensus on the geodynamic setting of these volcanism that they could be closely related to Hainan mantle plume.However,the influential extent of Hainan plume in the SCS region has not been well constrained.In Thailand,Indochina Peninsula,there also distribute many post-spreading basaltic flow centers.A comprehensive mineralogical and petrological study on the late Cenozoic basalts of Thailand is not only helpful to understanding their petrogenesis,mantle source and magmatic process,but also to indictating tectonic evolution of Indochina Peninsula and the whole SCS region.Thailand and its adjacent regions can be divided into three tectonostratigraphic units: the west Sibumasu Block(Sino-Burma-Malaya-Sumatra),the middle Sukhothai arc terrane and the east Indochina Block,which are separated by two sutures(Chiangmai-Chanthaburi,Nan-Uttaradit Sra Kaeo).Petrographic characteristics suggest that these basaltic rocks have porphyritic texture and vesicular structure,and phenocryst minerals are mainly olivine,plagioclase and a few of clinopyroxene,and the microlites minerals are plagioclase,olivine and clinopyroxene,and the accessory mineral is magnetite.Olivine phenocrysts are predominantly chrysolite and hyalosiderite,and they are magmatic in origin.Clinopyroxene are mostly sahlite and augite with a few diopside and endiopside.Clinopyroxenes are enriched in light rare earth elements(LREEs)(La N/Yb N=1.93-4.27).Relative to primitive mantle,they are depleted in large-ion lithophile elements(LILEs)and show obvious Nb/Ta fractionation.The clinopyroxene discrimination diagrams exhibit that the host magma formed in an intraplate tectonic setting environment.Plagioclases are predominantly labradorite,with a few andesine and bytownite.Plagioclases are enriched LREEs,Ba,Sr and Pb,and most of the plagioclase exhibit strong positive Eu anomalies.During its acent to the surface,parental magma of late Cenozoic basaltic rocks from Thailand has experienced multi-stage mineral fractional crystallization.Based on thermobarometer developed from the experimental petrological data,the crystallization pressure and temperature of clinopyroxenes in late Cenozoic basaltic rocks in Thailand have been calculated,ranging from 0.4–0.9 GPa and 1145–1214 °C,respectively.Correspondingly,those of plagioclase ranged from 0.3-0.9 Gpa and 1067°C to 1133°C,respectively.Moreover,the plagioclase had a high An(100Ca/(Ca+Na))content,suggesting that it was formed during the rapid upwelling of magma.The order of mineral crystallization of the late Cenozoic basaltic rocks of Thailand is olivine,clinopyroxene,plagioclase,and Fe-Ti oxide.The major element compositions show that Thailand basaltic rocks are mainly trachybasalt and basaltic trachyandesite,with a few basanite and basalt,and they belong to alkali magmatic series.The bulk-rock trace element compositions show that they are enriched in Large ion lithophile elements(LILEs)(Rb,Sr,Ba and K)and high field strength elements(HFSEs)(Nb,Ta and Zr),which is similar to those contemporaneous basalts from other areas(southern Vietnam,northern Hainan,and SCS seamounts)in the SCS region.The clinopyroxene discrimination diagrams for tectonic setting showed that these late Cenozoic basaltic rocks from Thailand were formed in an intraplate setting.Based on modeling calculation methods,the basaltic rocks of Thailand could be produced by 5%-20% partical melting of garnet pyroxenite.The mantle potential temperatures beneath Thailand are in the range 1448–1467°C,which can be comparable to those beneath southern Vietnam and northern Hainan,and the latter two have been proposed to be produced by the Hainan mantle plume.The slightly lower Tp of late Cenozoic basaltic rocks from Thailand than those from northern Hainan Island and southern Vietnam may reflect that some energy is consumed when the mantle plume material flows along the lithosphere rheology boundary layer westward to farther Thailand region(relative to Vietnam and Hainan).Specifically,the Tp of northern Hainan Island,near the center of mantle plume,is higher than that of the Indochina Peninsula,which is more distant from the center of the mantle plume.In conclusion,the deep geodynamic setting of the late Cenozoic basaltic rocks from Thailand is similar to that of the whole SCS region during post-spreading,in which the Hainan mantle plume plays a significant role in the formation of these basaltic rocks.