Geochemical Study Of Volcanic Materials In Sediments Of The Middle Okinawa Trough

The Okinawa Trough is a nascent back-arc basin located in the eastern margin of the East China Sea,which is still in the early stage of expansion.Frequent volcanic activity has given rise to numerous volcanic rocks in the Okinawa Trough.Because of the continuous sedimentation since the late Quaternary,the Okinawa trough is covered by a thick layer of sediments.The volcanic materials in the sediments provide valuable materials for the study of the magma system in the back-arc basin during the geological history.In this dissertation,the volcanic materials in the sediment core obtained from S9 site in the middle of Okinawa trough is taken as the main research object,and a systematic geochemical study has been carried out.The pumice clasts selected from the sediments were analyzed for major,trace element and Sr-Nd-Pb isotope compositions.The volcanic glass collected from the sediments and the clinopyroxene in pumice clasts were performed for in situ major and trace element analysis.In addition,the planktonic foraminifera in the sediments were separated for radiocarbon dating.The petrogenesis and magmatic evolution of the grayish-white pumice,the physical properties and chemical composition of the pumice with different colors,the crystallization temperature and pressure of clinopyroxene in the pumice,and the source of the volcanic glass and its sedimentary significance were discussed on the basis of the analysis results.The major findings are as follows:The grayish-white pumice clasts in Core S9 belongs to dacite in lithology,which are the products of felsic volcanic activity in the middle Okinawa trough during 13?11.2cal.ka BP.The parent magma of pumice has the characteristics of subduction related magma,which was generated from hybrid magma through extensive fractional crystallization.The Indian MORB type mantle was first metasomatized by the subducted Philippine Sea sediments to form the primitive magma,and then contaminated with a small amount of lower crust material in the lower crust to generate the hybrid magma.The geochemical characteristics of grayish-white pumice indicated that a millennial scale magma chamber might exist in the lower crust of the middle Okinawa Trough,and provided magma supply during 13?11.2 cal.ka BP.The felsic volcanism in the middle of Okinawa Trough has produced white,gray and black pumice.The three kinds of pumice were derived from similar primitive magma but with different degree of magma evolution.The white pumice has the highest magma evolution degree,followed by grayish-white pumice and black pumice.The variations in pumice color might be related to the content of Fe-enriched phenocryst mineral.Pumice with higher content of such mineral tend to have darker color.The characteristics of pumice vesicles might be associated with the physical properties of the magma.The magma with high viscosity and continuous decompression is easier to form pumice with dense vesicular and flowing structure,while the magma with low viscosity and staged decompression are more likely to generate pumice with large and loose pores.The clinopyroxene in grayish-white pumice of Core S9 were originated from the crystallization differentiation process of tholeiitic magma.The crystallization temperature for clinopyroxene is 1079?1108? with the average value of 1096 ?.The geochemical characteristics of clinopyroxene indicate that minerals such as plagioclase,clinopyroxene,orthopyroxene,magnetite/titanomagnetite,and apatite were crystallized during the evolution of magmatic system.The volcanic glass deposited in the core site were erupted during 12.2?6.5 cal.ka BP.There was a potential felsic volcanic glass source in the Okinawa Trough,which provided continuous volcanic glass supply during the period of 12.2?7.3 cal.ka BP.The volcanic glass from Kyushu Island(Kikai volcano)might contribute significantly to the volcanic glass in Core S9 during the period of 7.3?6.5 cal.ka BP.Volcanic glass has a long-term impact on the geochemical composition of terrigenous sediments in the middle of Okinawa trough.Therefore,the impact of volcanic materials should be fully considered and evaluated when using the geochemical composition of silicate fractions of sediments in the study area to establish proxies for source discrimination and regional silicate weathering history reconstruction.The variabilities of paleoenvironment in the study area may have a great influence on the sedimentation of volcanic glass in Core S9.During the period of 12.2?7.3 cal.ka BP,because the climate was dry and cold and the sea level was in a lowstand period,the products of volcanic activity in Okinawa Trough were the dominant sources of volcanic glass in Core S9,which has played a positive role in the sedimentation of the middle Okinawa Trough.Due to the rise of sea level during the period of 7.3?6.5 cal.ka BP,the volcanic material flux from Kyushu Island to the middle of Okinawa Trough has enhanced significantly,which led to the acceleration of sedimentation rate and the increase of mean grain size.