| Co-Investigator(Kenkyū-buntansha) |
HENMI Chiyoko Okayama University, Faculty of Science, Assistant Professor, 理学部, 助教授 (80124791)
MATSUDA Toshihiko Okayama University, Faculty of Science, Assistant Professor, 理学部, 助教授 (10033359)
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| Research Abstract |
The present study aims at clayfing the genesis of alkali rocks accompanied with skarns, which are formed as high-temperature metasomatic products at Fuka, Okayama Prefecture and Kushiro, Hiroshima Prefecture.
1. Occurrence of alkali rocks : In the two districts, many quartz monzonite dikes occur in limestones. Skam minerals are primarity formed on both side of the dikes, and show a zonal arrangement : (quartz monzonite) - (gehlenite zone) - (spurrte zone) - (crystaline limestone). The alkali rocks occur along the boundary between quartz monzonite and gehlenite zone, and are always accompained with granet and vesuvianite sub-zones.
2. Transfer of elements during the skarn formation : The elements are supplied from quartz monzonite. The distance of movement of each elment is related to its ionic radii. The small ions as Si move up to 20 meters, and the lager ions as Na moveless. It is clear that the elements are moved by the diffusion mechanism.
3. Chemical features of alkali rocks : The Al_2O_3 contents are almost the same as those of quartz monzonite. Gains (+) and losses (-) during the change in alkali rocks from quartz monzonite, assuming constant aluminum (g/100cm^3) , are as follows ; SiO_2 : -36--60, Na_2O : -2, K_2O : -5, FeO^* : -3.9-20.5, MgO : 3-17, CaO : 9-30g/100cm^3, La : -3.5-2.2, Ce : 5.1--2.0, Pr : -0.4-0.2.Nd : -0.5-0.5, Ba : 87, Li : 33, Sr : 69, V : 66, Zn : 81, Zr : 28mg/100cm^3 mg/100cm^3.
4. Formation of alkali rocks : Much silica, 52g/100cm^3 on an average, leached from Quartz monzonite, and reacted to the gehlenite zone. As a consequence, quartz monzonite magma was deficient in silica. It is very likely that the alkali rocks was formed by desilication of quartz monzonite.
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