In the Hanaoka mining area of the “Green Tuff” region, the Miocene submarine lavas and pyroclastic rocks with basaltic to andesitic compositions were recrystallized at low temperatures to form various secondary minerals. The upper metabasites are characterized by the mineral assemblage of prehnite-pumpellyite facies; i.e. chlorite+epidote+prehnite+pumpellyite, although zeolites occur in amygudules and veinlets. The microprobe data of secondary minerals, in addition to the microscopic observations, revealed the existence of several local domains with different effective bulk compositions within a single thin section. The metabasites of the upper cores (about 300m below the surface) of two drill holes also show secondary mineral assemblages of prehnite-pumpellyite facies, but those of the lower cores (at 900m depth) show actinolitebearing assemblages such as chlorite+epidote+prehnite+actinolite± pumpellyite. This indicates the transition to the prehnite-actinolite facies with increasing depth.

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董青石,かんらん石,斜方輝石,スピネルを有する, Fe, A1に富んだホルンフェルスが,能郷白山花崗閃緑岩体周囲の接触変成帯に産する。この中に含まれているスピネルはヘルシナイト的なものであり,かっV₂O₃を最高2.2 wt. %含む。かんらん石と董背石からなるシンプレクタイトが斜方輝石周囲に観察され,これには少量のスピネルが伴うことがある。 Mg-Fe²⁺-Al-Si-O-H系における董青石,かんらん石,料方輝石,スピネルの相関係は,実験学的に既に求められている。この実験結果及び岩石記載から考えて,シンプレクタイトは後退変成作用時に,以下の反応によって形成されたと考えられる。すなわち,
10斜方輝石+ 2スピネル ?? 5かんらん石+董青石
P_total=1=P_fluld=2-3 kbと仮定した時に,最高変成温度は800°C以上であった。また後退変成作用時においては, f_o2が比較的低かったことが推定され,少なくとも赤鉄鉱-磁鉄鉱平衡曲線よりも低かったと考えられる。

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Petrological and geochemical study of volcanic rocks from Aguni-jima island, central Ryukyus has revealed their island-arc type characteristics. The volcanic rocks belong to Higashi Formation of early Pliocene age, and are divided into lower and upper units. Samples of lower unit are basaltic, whereas those of upper unit are andesitic. Trace element patterns are similar to those of island-arc volcanic rocks, and the rock suite may belong to the high-alumina basalt series. Major, trace and isotopic compositions show that the lower unit contains volcanic rocks of two different origins; one related to those of upper unit andesites, and the other unrelated ones. The basalt-andesite suite is possibly the result of low-pressure fractional crystallization of plagioclase, augite, magnetite, ±olivine and ±hypersthene.

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