REE-bearing sector-zoned lawsonite in the Sanbagawa pelitic schists of the eastern Kii Peninsula, central Japan

Nagoya University, Department of Earth and Planetary Sciences, Nagoya, Japan

Rare-earth elements (REE)-bearing sector-zoned lawsonite was found in Sanbagawa pelitic schists in the Ise area of the eastern Kii Peninsula, central Japan. The pelitic schists consist mainly of lawsonite, chlorite, phengite, titanite, albite, quartz, calcite and weakly-ordered graphite. Metamorphic conditions are estimated at P = 3-8 kbar and T<350 degrees C based on stability of the lawsonite-albite-quartz-calcite assemblage. Lawsonite occurs as prismatic and euhedral crystals (up to 500 mu m long). Three sectors are well developed in the lawsonite grains. The {100} sector is more enriched in REE (La ₂ O ₃ up to 0.55, Ce ₂ O ₃ up to 1.18, and Nd ₂ O ₃ up to 0.58 wt%) and depleted in Ca than the {010} and {001} sectors. The {001} sector is more enriched in Ti and depleted in Al than the {100} and {010} sectors. Compositional variations in lawsonite grains suggest (super [6]) REE (super [4]) AlCa (sub -1) Si (sub -1) and (super [6]) Ti (super [4]) Al(Al, Fe (super 3+) ) (sub -1) Si (sub -1) substitutions. Lawsonite can preferentially concentrate REE, and constitutes a most important REE reservoir at high P-T metamorphic conditions, at which epidote group minerals are unstable.

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