Water Contents Of Cenozoic Basalts In Leiqiong Area And The Implications For The Genesis Of Large Igneous Provinces

Intraplate basalts refer to basalts erupting in continents or oceans away from plate boundaries,and their genesis can be hardly explained by the classic theory of the Plate Tectonics.Large igneous province(LIP),as the product of intensive intraplate magmatism,is generated by the emplacement of massive mafic magmas in a short period.The large-scale formation of magma usually requires the following four conditions:abnormally high temperature,substantial decompression,addition of fusible components,and remarkable enrichment in volatiles(especially water).Recent studies have revealed that all the four conditions,especially high source water contents,occur in the formation of many Panerozoic LIPs.However,it still remains unclear whether the water enrichment is indispensable or not.In order to investigate whether the high water content in the source area is the necessary condition for the formation of LIP,it is necessary to study such a case where all other three conditions were met but no LIP was generated.The Cenozoic basalt province in the southeastern Asia,displaying such characteristics,is a showcase for serving the purpose.Geophysical observations showed that the Hainan mantle plume is located beneath the northern Hainan Island.Therefore,we selected the Cenozoic basalts distributing in the northern Hainan Island and the Leizhou Peninsula(Leiqiong basalts)as the representative.We studied the geochemical compositions,melting conditions,and source water contents of these basalts,which further constrains the role of water in the formation of LIPs.The results of major and trace elements,and Sr-Nd-Pb isotopic compositions of bulk rocks indicate that most of the Leiqiong basalts have been little affected by later surficial alteration and crustal contamination.The fractional crystallization of olivine is the main process that controls the compositional evolution of the Leiqiong basalts.The trace element compositions of the Leiqiong basalts are similar to those of oceanic island basalts(OIBs).The major and trace element compositions of bulk rocks,and chemical compositions of olivine phenocrysts suggest that pyroxenite contributes to the mantle source of the Leiqiong basalts.Sr-Nd-Pb isotopic compositions signify the mantle sources of the Leiqiong basalts as a mixture of depleted and enriched mantle endmembers,DMM and EM II.Moreover,143Nd/144Nd ratios display a good correlation with trace element ratios such as Th/La and Rb/Nb,and the result of mixing modeling points to the presence of ancient oceanic crust and subducted sediments in the mantle sources.The water contents of Cpx phenocrysts in the Leiqiong basalts measured by Fourier transform infrared spectroscopy,range from 12.4 to 254.4 ppm,with an average of 68 ± 39 ppm.Based on these results,the water content in primary melts of Leiqiong basalts is estimated to be 0.28-1.4 wt.%,with an average of 0.74 ± 0.33 wt.%.Our calculated water contents fall within the range of OIBs,but are much lower than those of most Phanerozoic LIPs.According to the partial melting models,water contents in the mantle sources of Leiqiong basalts were estimated to be 83-465ppm,with an average of 234 ± 105 ppm,which is much lower than those of typical Phanerozoic LIPs.Based on the bulk compositions of Leiqiong basalts,the mantle potential temperature beneath the northern Hainan Island were estimated to be 1515 ± 18 ℃,which is higher than that of the depleted mid-oceanic ridge basalt(MORB)souce mantle(1350± 50 ℃).Our results show that the mantle souce of the Southeastern Asian Basalt Province(SABP)has abnormally high temperature and the enrichment of fusible components,but much lower water content than those in mantle sources of most Phanerozoic LIPs.Compared to the effects of the high water contents,the low water content in LIPs’ sources can significantly enhance the viscosity of materials in mantle plume,which may cause the stagnation of Hainan mantle plume at depths of 350-550 km.At the same time,high viscosity can also reduce the rising rate of mantle plume,so as to reduce the supply rate of mantle plume material.In addition,low water content will also result in relatively low melt production.Hence the low water content of the Hainan mantle plume would induce the relatively lower production rate and smaller eruption scale of the melts compared with those of the typical Phanerozoic LIPs,although the SABP was inferred to form at the conditions of the high temperature,substantial decompression and the incorporation of fusible components related to the Hainan mantle plume and ambient multiple subduction zones.Accordingly,the SABP,as a counterexample,corroborates the view that water enrichment in the mantle source can play a significant role in facilitating the generation of LIPs.Additionally,the highly variable H2O/Ce ratios(50-425)of Leiqiong basalts display a good correlation with 143Nd/144Nd,(Rb/Nb)n and(Th/La)n(n represents the normalized result of the primitive mantle)ratios of bulk rocks,indicating the incremental incorporation of recycled oceanic crust and sediment in the sources.The recycled materials incorporated in the Hainan plume source may have undergone variable degrees of dehydration or deep rehydration by fluids released from subcrustal hydrous minerals at greater depths during subduction.