Petrology of a peculiar amphibolite boulder rich in deep vivid green-colored amphibole porphyroblast from Hidaka Town, Hokkaido, Japan

Abstract

A peculiar amphibolite rich in deep vivid green colored amphibole porphyroblast was found as a boulder in the Chiroro River of Hidaka Town, Hokkaido, Japan. The amphibolite mainly consists of deep vivid green colored amphibole (porphyroblast), colorless amphibole (matrix) and plagioclase with a small amount of chlorite, epidote and chromian spinel. Deep vivid green colored amphibole porphyroblast is more than 20 modal % in the rock. Chromian spinel rarely occurs as inclusions in deep vivid green colored amphibole porphyroblast. The Cr 2 O 3 content of the deep vivid green colored domain is usually ~1 wt% (up to 2.5 wt% nearby chromian spinel) whereas that of colorless amphibole in the matrix is low, < 0.1 wt%. Deep vivid green colored amphibole porphyroblast is heterogeneous in chemical compositions within each grain and is divided into three domains as follows: (1) high Cr 2 O 3 (~1 wt%, up to 2.5 wt%) and Al 2 O 3 (> 10 wt%) green colored domain, (2) relatively high Cr 2 O 3 (~1 wt%) and low Al 2 O 3 (< 10 w%) green colored domain and (3) low Cr 2 O 3 (< 0.1wt%) and Al 2 O 3 (< 10 wt%) colorless domain. Chondrite normalized rare earth element (REE) patterns of both deep vivid green colored amphibole porphyroblast and colorless amphibole in the matrix are characterized by low light REE with positive Eu anomaly. The Cr#[= Cr/(Cr + Al) atomic ratio] and TiO 2 content of chromian spinel in the amphibolite are similar to those in a serpentinite complex of the Kamuikotan Zone. The amphibolite was formed by either amphibolitization of a rock containing chromian spinel derived from the surrounding serpentinite complex or extensively metasomatized serpentinite due to interaction with surrounding metamorphic rocks.