Cr-rich allanite-(Ce) in the serpentinite-metapelite reaction layer in the Sanbagawa belt of Nushima, Hyogo Prefecture, Japan

Abstract

Cr-rich allanite-(Ce) has been found in a reaction layer between the serpentinite lens and the host metapelite of the Sanbagawa metamorphic belt in Nushima, Hyogo Prefecture, Japan. The reaction layer is divided into talc, tremolite, and chlorite zones in order from the inner to the outer regions of the layer. The Cr-rich allanite-(Ce) occurs as fine and spindle-shaped grains of 30-60 μm in length in the tremolite zone, and coexists with tremolite and subordinate amounts of chlorite, titanite, graphite, and zircon. The Cr-rich allanite-(Ce) is enriched in Cr₂O₃ (0.2-9.0 wt%) and total iron as FeO (6.0-9.0 wt%), with the following Cr-richest formula:

(Ca_0.98Na_0.01K_0.01)_1.00(REE_0.70Ca_0.28Th_0.01)_0.99(Al_1.40Cr_0.69Fe²⁺_0.63Fe³⁺_0.11Ti_0.09Mg_0.07Mn_0.03)_3.02(Si_2.94Al_0.06)_3.00O₁₂(OH),

where REE includes La = 0.17, Ce = 0.33, Pr = 0.04, Nd = 0.13 and Sm = 0.03 per formula unit. The core part of the Cr-rich allanite-(Ce) is partially enriched in TiO₂ of up to 2.3 wt%, suggesting the substitution of (Fe²⁺,Mg)Ti(Fe³⁺,Al)_-2 in the octahedral sites.