Geochemical Investigation Of Arc Magmatism Of Tonga-kermadec Subduction Zone,Southwest Pacific

Detailed petrology,mineral and geochemical studies were carried out on modern Tonga-Kermadec arc lavas,late Eocene and late Miocene Tonga forearc lavas of DSDP Site 841,altered basalts and marine sediments of IODP Site U1365.Three questions were discussed:the alteration processes of oceanic basalt;control of subduction rate on Tonga-Kermadec arc magmatism;and the implication of forearc lavas on mantle source evolution.Altered oceanic crust is a significant source of the materials which subducted into the mantle and resulted in arc volcanisms.The variations in mineral and geochemical compositions that caused by alteration are crucial for understanding the chemical cycles in solid earth.The major elements of alteration products of 9 basalts were analysed by electronic probe.Combined with bulk rock geochemical data,we investigated the influencing factors of low temperature alteration,and discussed the control of alteration products on bulk rock geochemical variations.The major alteration products are recognized as saponite,celadonite,beidellite,zeolite,chlorite,palagonite,phyrite,calcite,and Fe-oxide/hydroxide,which represent typical low-temperature alteration.From the lava boundary or vein edge to the center of the rock,a progressive sequence of the dominant secondary mineral assemblage has been identified,changing from Fe-oxide/hydroxide to celadonite,to saponite+phyrite,indicating the transition from oxidizing condition to reducing condition.The chemical changes of oceanic basalts during alteration are mainly marked by rising of K₂O and Fe₂O₃,and the lost of FeO,CaO and Na₂O.The samples which are more available for sea water oxidation show assemblages of oxydic alteration products?such as Fe-oxide/hydroxide,palagonite,and celadonite?and more enriched K₂O and Fe₂O₃,indicates the control function of alteration types on oceanic basalt compositions.Dehydration/melting of oceanic crust during returning to the mantle in subduction zones are related to the origin of arc lavas.The factors that influence arc magmatism include compositions of the subducting slab and mantle wedge,and subduction rates.However,distinguishing these factors remains difficult and highly debated.Subducting rate is related to the total mass of inputs and controls thermal structure,thus plays a crucial role in arc magmatism.Here we explore the relationships between geochemical variations of arc lavas and convergence rates?increasing from 46mm/yr to the south to83mm/yr to the northward?in the Tonga-Kermadec arc system.Data of geochemistry for lava samples from nine islands of this arc system are collected and compiled to investigate the role of subduction rate in arc magmatism.It is suggested that the northern Tonga arc with a higher subduction rate has been influenced by a stronger role of subduction-released fluid,which results in stronger large-ion-lithophile elements?LILEs?and relatively weaker HFSEs contribution.It is interpreted that faster subduction rate tend to bring more cold subduction slab into the subduction zone,and create a cooler subduction zone due to cooling effects,leading to stronger dehydration subduction slab contribution with,thus,higher LILE/HFSE ratios of arc lavas.In addition to the influences from subduction inputs,the hydrous melting of mantle wedge is also a significant source for arc lavas.Therefore,arc lavas may be regarded as indicators for mantle wedge variation.Major elements,trace elements and Sr-Nd-Pb-Hf isotopes are analysed for forearc igneous rocks of late Eocene and late Miocene from DSDP Site 841,Tonga arc.Data are compared with other tectonic settings from this region.It is suggested that both oceanic crust and marine sediments undergone dehydration during subdction,and the later furtherly melt in some circumstances.Lavas from different periods show variations on fluid contributions,which increase from late Eocene to recent times.Moreover,lavas from back arc regions show weaker fluid signatures than those of forearc,indicating former dehydration of subduction plates under forearc region.Lavas from Tonga-Kermadec region show DUPAL anomaly on Pb isotopes,but no Indian mantle characteristics are shown in Hf-Nd isotope plots.Therefore,applications of Pb isotope anomalies for defining mantle domain boundaries are indicated to be improper.At least at this region,the mantle domain boundary is not under subduction zone.Hf-Nd isotopes show that different from IBM subduction zone,which has been applied by Indian type mantle,the mantle wedge beneath Tonga-Kermadec arc transformed from Pacific type to Indian type.Before late Eocene,the Tonga arc?or Vitiaz arc at that time?was underlain by Pacific type mantle;at late Miocene,the Pacific mantle was partly replaced and mixed by Indian type mantle;at recent time,the mantle wedge under Tonga arc is completed composed by Indian type mantle,while some Pacific type mantle remain under Kermadec arc,Lau basin and Lau ridge.