[SCG60-P12] Magma processes of the plutonic rocks in the Susuma region, Yamaguchi Prefecture, SW Japan : Implications for Cretaceous magmatism in active continental margin
Keywords:Granodiorite, Gabbro, Granite, Sanyo belt
[Susuna-Nagao complex] Granodiorite shows massive and medium grained and contains plagioclase, hornblende, biotite, quartz with small amounts of K-feldspar. Gabbro is subdivide into wo types; Gabbro-A and Gabbro-B. Gabbro-A is medium grained, consisting mainly of plagioclase and hornblende. Gabbro-B is also medium grained. The constituent minerals are plagioclase, hornblende, orthopyroxene, clinopyroxene and olivine.
[Shimokubara granite] The Simokubara granite is characterized by euhedral K-feldspar up to 5 cm in length and is composed of Quartz, K-feldspar, Plagioclase and biotite.
The boundary between the Susuma-Nagao complex and Shimokubara granite is generally unclear and locally including each other. Samples collected from such boundary have mixing and mingling texture, e.g., acicular apatite, acicular biotite / hornblende, and dusty zoned plagioclase. These features suggest that the Susuma-Nagao complex and Shimokubara granite are the coeval intrusive rocks and locally mixed with each other.
In the Harker diagram, Granodiorite and Gabbro-B make monotonous trends. Chemical compositions of Gabbro-B are plotted off this trend. Gabbro-A geochemically shows cumulous features in the SiO2/Al2O3 vs. Mg# diagram. In addition, Gabbro-B represents high-Mg# values similar to a primitive basalt. The Shimokubara granite makes different trends from those of Granodiorite and Gabbro-B.
The Sr-Nd isotopic compositions corrected with 92 Ma for the Susuma-Nagao complex are plotted in the same domain. However, the Shimokubara granite isotopically shows high-εSrI and Low-εNdI rather than those of the Susuma-Nagao complex. In addition, Sr-Nd isotopic compositions of the Shimokubara granite are influenced by those of host rocks.
Considering the geochemical features, the Susuma-Nagao complex could be derived from cognate magma series due to fractional crystallization from Gabbro-B as a parent to Granodiorite as a daughter leaving Gabbro-A as a cumulate. Geochemical characters combined with petrography of the Susuma-Nagao complex and Shimokubara granite reveal that they are originally derived from different magma with limited magma mixing / mingling along their boundary.
An wedge mantle is generally metasomatized by water and melt released from a subducted oceanic plate and partial melting takes place into the mantle, giving rise to an arc basaltic magma. Such basaltic magma becomes as the enriched Gabbro-B magma in the Susuma-Nagao complex. On the other hand, the Shimokubara granite is likely to be produced by partial melting of the middle to lower crust because the Sr-Nd isotopic compositions are affected by those of continental crust.