In–situ U–Pb zircon age dating deciphering the formation event of the omphacite growth over relict edenitic pargasite in omphacite–bearing jadeitite of the Itoigawa–Omi area of the Hida–Gaien belt, central Japan

摘要

Omphacite replacing after relic edenitic pargasite has been found in an omphacite–bearing jadeitite block of the Itoigawa–Omi area in the Hida–Gaien belt. Omphacite occurs sporadically as fine–grained aggregate reaching a few cm in length in a jadeite–albite matrix, and sometimes contains edenitic pargasite as a core. The edenitic pargasite is chemically and optically homogeneous and does not show direct contact with jadeite and albite. An irregular shaped omphacite–diopside mixed area occurs near edenitic pargasite in a coarse omphacite aggregate. The texture suggests that the breakdown of edenitic pargasite was triggered by the addition of a hydrothermal fluid, from which jadeite and albite were precipitated later, passing through diopside and omphacite by the reaction:? ?At the periphery of pseudomorphic omphacite, a hydrothermal fluid removed the breakdown components of the reaction other than omphacite.?New in–situ LA–ICP–MS U–Pb dating revealed that zircons in edenitic pargasite yield apparent age up to ~ 590 Ma, with mean ages of 560 ± 16 Ma, interpreted as the minimum age of a precursor rock. A zircon age of 519 ± 21 Ma from jadeitite without omphacite corresponds to a timing of crystallization of omphacite, jadeite, and albite. The studied jadeitite is a typical R–type jadeitite, and the nearly total replacement from a precursor rock to the omphacite–bearing jadeitite has been attributed to hydrothermal activity at Middle Cambrian times.