The Study Of Petrological Geochemistry Of Pumice From The Shikoku-Parece Vela Basin

The Shikoku - Parece Vela Basin and Okinawa Trough lying on the west Pacific are typical extension back-arc basins which are ideal for the researching of plate tectonics and Asthenosphere convecting.In this thesis,8 pumice samples from the study areas are examined by Petrology Geochemistry methods and the results show that:the pumice are calc-Alkalie series with the SiO₂ of about 55.26%～69.58%and Na₂O+K₂O of about 5.45%～10.39%;The minerals in the crystal of pumice are Plagioclase and Clinopyroxene with aphanitic-vitric matrix.The Spider Diagram of Trace Elements data shows the typical island arc pattern.The REE diagram shows a right-leaned pattern with La_N=44.84～206.61,(La/Yb)_N = 1.59～10.67,Pumices from both the two researching areas are enriched in LILE and LREE,while HFSE keeps its initial pattern of basaltic lava.The negative Eu anomaly indicates that the lava has the process of crystallization;The Ce anomaly indicates the lava has been effected by subduction composition or ocean water.The negative Ta,Sr anomaly also indicate the island arc pattern.The Petrological Geochemistry study of pumice samples from both the Shikoku - Parece Vela Basin and the Okinawa Trough proves that the lava of pumices coming from the extension back-arc basins is formed by the melting of subducting plate and effected by the subduction composition before its breaking out.So the lava expresses the property of both island arc pattern on both the Spider Diagram and REE diagram.