Mass transfer associated with chloritization in the hydrothermal alteration process of granitic pluton

花崗岩体の熱水変質プロセス; 緑泥石化で生じる元素移動

湯口 貴史*; 松木 貴伸*; 五十公野 裕也*; 笹尾 英嗣   ; 西山 忠男*

Yuguchi, Takashi*; Matsuki, Takanobu*; Izumino, Yuya*; Sasao, Eiji; Nishiyama, Tadao*

花崗岩体で生じる熱水変質では、普遍的に緑泥石化、斜長石の変質、炭酸塩鉱物の晶出が生じる。本研究では、中部日本に分布する土岐花崗岩体を対象として、ホルンブレンドの緑泥石化、カリ長石の緑泥石化、脈状緑泥石の生成をもたらす熱水-鉱物間の元素移動について論じた。既往研究ならびに本研究の統合的な知見により、熱水変質に伴う岩体内の元素移動を包括的に捉えることが可能になった。本研究では、緑泥石化に伴い微小空隙が生じることを見出した。この知見と反応物と生成物の化学組成から、反応式を構築し、約180350Cの温度範囲で生じる熱水流体中の元素移動について論じた。

This study reveals the hydrothermal alteration processes in a pluton, with a focus on the mass transfer between minerals and hydrothermal fluid. Hydrothermal alteration of the Toki granite in Tono area, central Japan, progressed through the successive processes of chloritization, plagioclase alteration, and precipitation of a carbonate mineral. This paper describes the alteration process of hornblende chloritization, K-feldspar chloritization, and the formation of fracture-filling chlorite through petrography and mineral chemistry. Several types of chloritization reactions (including biotite chloritization) can be characterized by their reaction with the inflow of Al, Fe, Mn, and Mg and the outflow of HSiO, Ca, K+, and F. The reactions of chloritization and plagioclase alteration represent the sequential variations in fluid chemistry at temporal conditions from 68 Ma to 51 Ma as the temperature decreased from 350C to 180C.