Mantle Metasomatism At The Convergent Margin

Southwest Japan,as an active convergent margin,is one of the world's natural laboratories to investigate magmatic and metasomatic process associated with subduction tectonics.The metasomatic rocks in this region include serpentinite,rodingite and jadeite.This thesis presents results from detailed research on petrography,mineralogy including mineral chemistry,geochemistry and zircon U-Pb and Lu-Hf isotopes on serpentinite,pyroxenite and rodingite from the Kurosegawa belt in Kochi,SW Japan.The pyroxenite and rodingites occur as dykes and elliptical blocks within serpentinite mélange.Three stages of rodingitization are distinguished based on mineral assemblages and compositions.The first stage is represented by clinopyroxene + chlorite + hydrogarnet,the second stage by clinopyroxene + chlorite + hydrogarnet + vesuvianite and the third stage by vesuvianite + chlorite + hydrogarnet.The clinopyroxene is diopside,and the hydrogarnet is grossular-andradite solid solution,and also contain minor morimotoite end-member.The mineral chemistry of chromite is consistent with origin in a subduction mantle wedge rather than mid ocean ridge.The rodingites are generally Si O2 unsaturated(34.63%-40.32%),enriched in Ca O(22.15%-30.82%)and depleted in Mg O.Compared with the protolith(pyroxinite),the rodingite displays Ca O increase and Si O2 decrease indicating rodigitization is a process of Ca metasomatism and desilication.Pyroxenite is enriched in LILE and depleted in HFSE suggesting metasomatism triggered by fluids released from the subducted plate.In addition,compared to pyroxenite,the content of RRE and trace elements of rodingites show marked increase,suggesting that both REE and trace elements are mobile elements during the process of rodingitization.Zircon grains from the rodingites yield 238U/206 Pb age of around 501 Ma.The hydrothermal zircons in serpentinite and rodingite yield the timings of metasomatic events as 487-466 Ma and 483-468 Ma.In addition,the hydrothermal zircons in serpentinite and rodingite also yield minor age peaks at 379 Ma,295 Ma as well as 315 Ma,259 Ma and 195 Ma,indicating the long lasting metasomatism in southwest Japan which continued to Tertiary.The internal structures combined with negative ?Hf(t)reveal older xenocrystic zircon grains in the serpentinite and rodingite,possibly related to global sediment circulation.The zircon REE patterns can be divided into two groups,one is characterized by Ce positive anomaly and slight Eu negative anomaly.Combining with the wide range of grossular-andradite composition,this indicates changing f(O2)in fluid during rodingitization and serpentinization.This study provides insights into magmatic and metasomatic processes within a long-lasting suprasubduction zone mantle wedge.