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

Keitaro Kunugiza, Eizou Nakamura, Atsushi Goto, Katsura Kobayashi, Tsutomu Ota, Hiroshi Miyajima, Kazumi Yokoyama

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)256-270
Number of pages15
JournalJournal of Mineralogical and Petrological Sciences
Volume112
Issue number5
DOIs
Publication statusPublished - Oct 1 2017

    Fingerprint

Keywords

  • Diopside
  • Jadeite
  • Omphacite
  • R-type jadeitite
  • Subduction initiation

ASJC Scopus subject areas

  • Geophysics
  • Geology

Cite this