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: NaCa2Mg5(AlSi7)O22(OH)2 →(NaAlSi2O6 + CaMgSi2O6) + CaMgSi2O6+ Mg3SiO5 + H2O. 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.
|Number of pages||15|
|Journal||Journal of Mineralogical and Petrological Sciences|
|Publication status||Published - Oct 1 2017|
- R-type jadeitite
- Subduction initiation
ASJC Scopus subject areas