オフィオライトから推定するかんらん岩の島弧成熟過程

抄録

  Ophiolites are tectonically exposed oceanic crust and part of the upper mantle on land. Although tectonic settings for the formation of ophiolites are still in debates, some lithologies of many ophiolites are believed to be formed in subduction settings. This paper summarizes results from two ophiolites (the northern Oman ophiolite of Oman and the Mirdita Ophiolite of Albania) and two ophiolitic localities (the Izu-Bonin-Mariana fore arcs and the Talkeetna massif, Alaska) in order to investigate the maturing processes of arc systems. Based on variations and frequency of volcanic rocks and mafic-ultramafic cumulates coupled with crust thickness, arc maturing processes are classified into ‘Nascent stage (northern Oman and the early stage of the Izu-Bonin-Mariana magmatism)’ characterized by the presence of boninitic magmatic activity, ‘Young stage’ (Mirdita and the Izu-Bonin-Mariana magmatism between 53 and 45 Ma) characterized by increasing of calc-alkalic silica-rich volcanic rocks and arc-type plutons, such as high-Mg and high-Cr gabbronorites, coupled with the increasing of crust thickness (up to 25 km), and “Mature stage (Talkeetna and the Izu-Bonin magmatism <45 Ma)” characterized by high ratio of silicic volcanic rocks and the increasing of crust thickness, up to 30 km, resulting in the formation of garnet-bearing lithologies, then probably followed by delamination. The frequency of highly refractory harzburgite (+dunite and chromitite) caused by influx of slab-derived fluids and silica enrichments resulting in increasing of pyroxenites in peridotites can be good indicators of arc-related magmatic modifications in the mantle.