夜久野オフィオライト朝来岩体における古生代海洋内島弧地殻の形成と進化過程

Genesis and evolutional processes of the Paleozoic oceanic island arc crust, Asago body of the Yakuno Ophiolite, Southwest Japan

Suda Yoshimitsu

Hayasaka Yasutaka

地質学雑誌

The Journal of the Geological Society of Japan

兵庫県朝来地域には，古生代海洋内島弧の中～下部地殻に由来する夜久野オフィオライト朝来岩体が露出する．本研究では夜久野古島弧の基盤地殻を構成する苦鉄質岩類の起源について，そしてSuda（2004）が報告したミグマタイトの産状分類に基づいた島弧花崗岩類の形成過程について，地球化学的手法を用いたモデル計算を行い検討した．結果，以下のことが示された．朝来岩体では背弧盆地殻に由来した基盤地殻が部分融解し，珪長質メルトが形成された．これら珪長質メルトは集積，上昇する過程で結晶分化しながらマグマへと成長し，キュームレイトに相当するものが主にミグマタイト中のリューコソムとして下部地殻相当域に，一方，残液に相当するものが岩脈，岩床として主に中部地殻相当域に残された．朝来岩体には，島弧下部地殻の部分融解過程と，島弧花崗岩質マグマの形成と発達による地殻の安山岩質化過程を示すまさにその現行段階が残されている．

A middle to lower crustal section of Paleozoic oceanic island arc is exposed in the Asago body of the Yakuno ophiolite. Based on field occurrence, petrography, and geochemical modeling, we investigated the evolution of the Asago body and relevant magmatic processes. The Asago body consists of two stages of rocks. The first-stage rocks consist of metagabbro and schistose amphibolite that represent the basement to the Permian Yakuno paleo-island arc. The second-stage rocks are mainly arc granitoids that intrude the first-stage rocks. Mafic migmatites occur in the lower crustal section of the Asago body. Field occurrences and petrographic data suggest that the migmatites formed by the anatexis of first-stage rocks, and that segregation and accumulation of the anatectic melt resulted in morphological changes in the migmatites toward the middle crustal section. Geochemical data indicate that the first-stage rocks were derived from a basaltic magma of back-arc affinity, suggesting in turn that the Yakuno paleo-island arc was developed within a back-arc basin. Moreover, a low-K series (hornblende gabbro, quartz diorite, and tonalite) and high-K series (quartz monzodiorite and granodiorite) within the second-stage rocks were generated by high and low degrees of partial melting of first-stage rocks, respectively. We conclude that the Asago body is an example of the transition from oceanic to continental crust, related to the anatexis of mafic lower crust in an oceanic island arc setting.

Yakuno ophiolite

oceanic island arc

lower crustal evolution

crustal anatexis

geochemical modeling

jpn

日本地質学会

© The Geological Society of Japan 2009

[ISSN] 0016-7630

[ISSN] 1349-9963

[NCID] AN00141768

[DOI] 10.5575/geosoc.115.266

[DOI] https://doi.org/10.5575/geosoc.115.266