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: NaCa_2Mg_5(AlSi_7)O_(22)(OH)_2 → (NaAlSi_2O_6 + CaMgSi_2O_6) + CaMgSi_2O_6+ Mg_3SiO_5 + H_2O. 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.