Geochemistry Of Lavas From Yap Trench-island Arcs And Its Geological Significance

The Yap trench-island arc system,where three tectonic plates(i.e.,the Philippine Sea Plate,Pacific Plate,and Caroline Plate)interact,has undergone a relatively complicated geological evolution history.Previous studies have mainly focused on the subduction activity of the Yap trench-island arc system.However,knowledge of the nature of the subducting plate's basement on the southeastern side of the trench and the Yap island arc lava is lacking,which limits our understanding of the complete tectonic evolution of the Yap island arc-trench system.In this study,we performed petrological,mineral geochemical and geochronologic analysis of newly acquired samples from the Sorol Trough segment(CJ09-82)and the Yap Island Arc(CJ09-72 and CJ09-126),and discussed the origin and magmatic processes of the rock and their geological significance.The main insights obtained are as follows:(1)The basement lava of Solor Trough phenocryst minerals are mainly olivine and plagioclase,with minor clinopyroxene microlites.The olivines are predominantly chrysolite,plagioclase phenocrysts are predominantly labradorite,with a few andesine plagioclase microlites.Plagioclase are enriched in light rare earth elements(LREEs),and most of plagioclase phenocrysts exhibit strong positive Eu anomalies.Plagioclase are enriched in large-ion lithophile elements(LILEs)(e.g.,Ba and Sr)and depleted in high field strength elements(HFSEs)(e.g.,Nb and Zr).Clinopyroxene microlites are mainly diopside and hedenbergite.Lavas from Sorol trough are mainly trachybasalt and belong to alkaline series.They are enriched in large-ion lithophile elements(LILEs)(Rb,Ba,K)and high field strength elements(HFSEs)(Nb,Ta,Zr),no obvious Eu anomalies,and show geochemical characteristics of ocean island basalt(OIB)-like.The discrimination diagrams for tectonic setting showed that they were formed in an intraplate setting.The simulation calculation shows that these Sorol Trough basalts may be formed by 5% to 10% partial melting of mantle spinel lherzolite.The potential temperature of the mantle at the subduction front were inferred to be 1331-1393°C,shows that there is no thermal anomaly in the mantle.In conclusion,the Sorol basalts were products of low-degree partial melting of relatively enriched mantle rock components that were passively upwelling during the formation of Sorol Trough separating Caroline Ridge into western-and eastern part.(2)The basalts from the sampling location of CJ09-126 have porphyritic texture,vesicular or block structure,and the phenocryst minerals are mainly clinopyroxene and plagioclase.Clinopyroxene phenocrysts are mainly diopside,and some clinopyroxene and plagioclase phenocrysts inlaid with each other and take on glomerophyric textures.Clinopyroxene are depleted in LREEs and LILEs(e.g.,Ba,Pb and Sr).Plagioclases are dominated by labradorite and bytownite,with obvious positive Eu anomalies in the phenocrysts,and they are enriched in LILEs(e.g.,Ba and Sr)and depleted in HFSEs(e.g.,Nb and Zr).These basalts are fore-arc basalts and belong to calc-alkaline series.They are enriched in LILEs(Rb,Sr,K)and HFSEs(e.g.,Ta),with no obvious Eu anomaly,and has geochemical characteristics of island arc basalt(IAB)-like.The simulation calculation shows that the source rock of these basalts is peridotite,and their parental magmas were formed by the 15% partial melting of the mantle spinel peridotite.During magmatic evolution processes,they didn't receive subduction component input.(3)The dacites from the sampling location of(CJ09-72)only have a few plagioclase microlites.They are enriched in LILEs(e.g.,Rb and K),and depleted in HFSEs(e.g.,Nb and Ta).It has the characteristics of island arc basalt(IAB)-like,and the ratio of Th/Nb and Ba/Nb reflects that it received subduction components released from subducted slab.Compared to basaltic lavas from CJ09-126,lavas at sampling location of CJ09-72 may represent the later-stage highly differentiated rocks and reflect that the island arc has developed the relatively mature crust.(4)K-Ar age of the Sorol basalts is 23.8±0.7Ma,indicating that the opening time of the Solor Trough was about 24 Ma or earlier;and that of lavas of the southern Yap island arc(CJ09-126)is 14.2±0.6 Ma,which may represent the forearc basalt(FAB)developed in the primitive Yap Trench.The K-Ar age of the dacites of the southern Yap island arc(CJ09-72)is 3.40±0.48 Ma,which may reflect that the Yap Island arc is still active at the time of 3.4 Ma.