Abstract

Oki-Dogo, southwest Japan is one of the best fields to study the development oflower crust on the representative convergent margin, because mafic to ultramafic
xenoliths composed of gabbro, pyroxenite and peridotite were transported by Pliocene
alkaline basalt from the deep crust. In order to understand formation and evolution
processes of the lower crust, petrological, geochemical and isotopic studies of morethan 200 rock samples including basalts and exposed gneisses have been studied. Major
classification of xenoliths is based on lithology: peridotite suites (modal olivine > ~40 %), pyroxenite suites and gabbro suites (modal plagioclase > 5 %).

These suites

can be also subdivided I-type and G-type xenoliths. I-type xenoliths are characterized
by cumulus texture and exsolution lamellae in pyroxenes, and olivine and plagioclase
are observed in all lithologies.

On the other hand, G-type xenoliths are characterized

by recrystallized granoblastic and equigranular texture and no lamellae in pyroxenes. Gtype gabbros, moreover, consist of no olivine, and G-type ultramafic xenoliths areplagioclase absent. Thermometric calculations based on major element composition ofpyroxenes indicate that the both I- and G-type xenoliths were equilibrated under thegranulite facies condition at temperatures range as 950 - 1050ºC. Co-existing mineral
assemblage of G-type xenoliths also supports that G-type xenoliths underwent
granulite-facies metamorphism at pressure range of 0.7 – 1.2 GPa.
Whole rock geochemistry of the both I- and G-type xenoliths shows that I-type
xenoliths are relatively enriched in highly incompatible elements comparing with Gtype xenoliths. Additionally, depletion of Zr and Hf of G-type xenoliths is significant.
Correlation among the modal abundances of constituent minerals, major and traceelement concentrations, suggest that both I- and G-type xenoliths would be formed bycrystal accumulation processes from basaltic magma(s). In order to investigate theorigin of I- and G-type xenoliths, the author examined Sr, Nd, Hf and Pb isotopes forwhole rock samples. As a result, I-type xenoliths have depleted in Sr, Nd and Hf isotope
compositions (87Sr/86Sr: 0.70457 ~ 0.70498; εNd: -1.5 ~ +0.1; εHf: -0.1 ~ +2.7) andenriched in Pb isotope compositions (206Pb/204Pb: 17.86 ~ 18.50) compared with G-typexenoliths (87Sr/86Sr: 0.70578 ~ 0.70826; εNd: -5.4 ~ -2.9; εHf: -6.1 ~ -2.7; 206Pb/204Pb:
17.72 ~ 17.93).
The Rb-Sr and Sm-Nd mineral isochron age cannot be defined from G-type gabbroic
sample. This result is implied that these xenoliths underwent cryptic metasomatism after
the accumulation and recrystallization processes. One of the evidences of thisimplication is that the xenoliths from Oki-Dogo generally include platy fluid inclusion
trails in the constituent minerals. In order to examine the effect of metasomatic event for
the geochemical signature of the xenoliths, separated plagioclase (PL) andclinopyroxene (CPX) grains (< 1 mm) from G-type gabbro and pyroxenite were sliced
into 2 or 3 pieces (1 – 8 mg for each), and curried out Sr and Nd isotope analyses for theeach slice.

As a result, isotopic heterogeneities correlating with relative amount of

fluid inclusions were observed in both separated PL and CPX grains.

The isotopic

variation, moreover, define as a mixing line among the inclusion-free mineral andinclusion-bearing minerals. The mixing lines converge in the direction of isotopic
compositions of the late Cenozoic alkaline basalts, and alkaline magmas would beconsidered to be a source of the metasomatic agent. The leaching experiment by HBr +HNO3 mixed acid for whole-rock powder also support that the lower crustal xenoliths,
not only G- but also I-type, underwent the metasomatism by the Cenozoic alkaline
basalts.
Based on these experiments, the author examined to estimate the metasomatic
influence for trace element and isotopic compositions of the whole-rock samples. Inconsequence, the extent of the metasomatism might be limited as a few percentages for
the Sr, Nd and Hf isotopes. Even though the corrected isotope values are applied,therefore, systematic difference of the geochemical signatures between I- and G-type
xenoliths still exists. The isotopic compositions of I-type xenoliths are plotted withinlate Cenozoic alkaline basalts around Japan Sea, and strongly suggest that I-type
xenoliths were formed by crystal accumulation from underplating these alkaline
magmas. On the other hand, isotopic variations of G-type xenoliths expand widely
toward the isotopically enriched felsic gneisses field (Oki gneiss). These variations
could be interpreted that assimilation of pre-existing felsic crust by basaltic magma
played the important role to form the G-type lower crustal xenolith. Moreover, it isconsidered that the involved magma would be subduction-related rather than the lateCenozoic alkaline basalt, because significant depletion of HFSE is observed in G-typexenoliths.
The comprehensive geochemistry of xenoliths from Oki-Dogo reveals that the lower
crust in convergent margin could be explained by two-stage underplating model ofbasaltic magma. In the first stage, cumulates from subduction-related arc type magma
would be formed together with the assimilation of the pre-existing felsic crust. Theexistence of calc-alkaline type lava, Kori basalt, which has early Miocene of theeruption age, in Oki-Dogo also supports this model. These mafic to ultramafic cumulate
had subsequently been underwent granulite-facies metamorphism and recrystalized.
After evolving of this arc-type lower crust, back-arc magmatism has been created new
magmatic lower crust by accumulation of underplated alkaline magmas. It could beconsidered that I-type xenoliths correspond with this back-arc type lower crust. In thisstudy, the author elucidates the two-stage development history of the lower crust
beneath Oki-Dogo. This type of crustal evolution process on the dynamic convergent
margin would be essentially important to assess the geochemical evolution of the Earth,because the geochemical structure developed by the multiple-stage model would differ
from that by simple magma-underplating. The research scheme based on thecomprehensive geochemical approach is inevitably important to reconsider the crustal
evolution processes through the Earth’s history.