A hornblende-actinolite-cummingtonite composite grain from a quartz diorite porphyry in northeastern Shikoku.

Abstract

An unusual composite amphibole microphenocryst occurs in a quartz diorite porphyry of the Shirotori-Hiketa dike swarm in northeastern Shikoku. The composite grain is optically and chemically divided into four domains A, B, C and D, each of which consists of the following amphibole subspecies. Domain A: Ferro-edenite, ferro-hornblende, ferro-actinolitic hornblende and subcalcic ferro-actinolitic hornblende. Domain B: Subcalcic ferro-actinolite and calcian cummingtonite. Domain C: Calcian cummingtonite and cummingtonite. Domain D: Ferro-actinolite. Besides secondary actinolite crystallized as domain D, the chemical composition varies from hornblende, through subcalcic actinolite, to cummingtonite without any significant intermittence except for an abrupt increase in Mg/(Mg+Fe) at the boundary between domains A and B. Domain A crystallized from a quartz diorite porphyry magma prior to the dike intrusion, whereas domains B and C grew on domain A after emplacement of the dike magma. The internal compositional variation from domain B to domain C represents a metastable crystallization under diffusion-controlled conditions in the successively differentiating dike magma.