The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

Mantle metasomatic enrichment in LILE of basalt magma sources beneath the Northeast Japan arc, as indicated by the LILE/Y-Zr/Y plots

OSAMU UJIKE

1987 Volume 82 Issue 7 Pages 245-256
Published: July 05, 1987
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.82.245

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The Nb/Y-Zr/Y plot implies that, before mantle metasomatism, magma sources for NE Japan arc basalts were more depleted in incompatible elements as a result of previous extraction of magmas than those for the N-type mid-oceanic ridge basalts. The LILE/Y-Zr/Y plots imply that the NE Japan arc basalts are derivatives from mantle regions metasomatically enriched in LILE probably by fluids released from the subduction zone. The degree of alkali (Rb and K) enrichment is lower in the trench side than in the back-arc side, but that of Sr enrichment is higher in the trench side. The alkalic volcanism in the back-arc region, with the exception of that belonging to the Chokai volcanic zone, is affected by mantle metasomatism to limited extents.

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Magma mixing model of the Tertiary granitic rocks of the southern Kyushu

Kenji Okamoto, Osamu Kinoshita, Kenji Nonaka, Shinjiro Yagi

1987 Volume 82 Issue 7 Pages 257-268
Published: July 05, 1987
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.82.257

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The Miocene granitic rocks in the southern Kyushu were analyzed by the INAA and gammaray counting for 17 trace elements, and by wet chemistry for major elements. Based on unusual differentiation trends of Na₂O and Th/U, the Th/U-K₂O/Na₂O diagram is proposed as a clue to study of the mixing process between two differently originated magmas. Present results show that all the differentiation trends direct towards a fixed point on that diagram, the Th/U and K₂O/Na₂O ratios of which are 6.5 and 1.7 respectively. This unfamiliar converging differentiation trend is concluded to indicate the mixing of two magmas, one of which has the composition of the fixed point. The point is explained to represent the minimum melt by partial melting in the Qtz-Or-Ab-An-H₂O system, based on comparison of its K₂O/Na₂O ratio with that of the Sierra Nevada batholith and melting experiments. The similarity of Th/U ratio between the fixed point and shales or sandstones of the Shimanto belt suggests that they have been source materials to be partly melted. The differentiation trends of the other elements are also explained by the mixingmodel.

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Salt crystallization decay and water-rock interaction of rock-cliff Budda sculptures at Yakushido Temple of Odaka Town, Fukushima, Japan

Yotaro Seki, Tomio Hirano, Kunio Watanabe

1987 Volume 82 Issue 7 Pages 269-279
Published: July 05, 1987
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.82.269

JOURNAL FREE ACCESS

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Rock-cliff Budda sculptures of Odaka Town, Fukushima Prefecture, Japan were carved about one thousand years ago on the Neogene dacitic tuffaceous sedimentary rocks.
These sculptures have been badly decayed by (1) the formation of scabs composed chiefly of gypsum, epsomite and thenardite with subordinate amounts of kogarkoite, langbeinite, zoherite, hydroglauberite, starkeyite, jurbanite, loeweite, hexahydrite, alunogen, mirabilite, glauberite, syngenite, hemihydrite and sylvite at the surface of the sculptures by the evaporation of groundwater percolated through the voids and joints in the Neogene volcanogenic sedimentary rocks and (2) the exfoliation of the these scabs and near-surface part of Neogene sedimentary rocks, particularly due to the heaving during the freezing-thawing cycle of seeped groundwater in winter season.

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