[S-IT41_28PM2] 海洋プレートの一生：誕生から解体，そして復活

[SIT41-21] Compositionally and genetically distinct domains found in the southernmost Salahi mantle section in the Oman ophiolite

*藤井 悟¹、高澤 栄一² (1.新潟大学大学院自然科学研究科、2.新潟大学理学部)

キーワード：オマーンオフィオライト, かんらん岩, 斜長石ダナイト, 斜長石レルゾライト, MORB, Boninite

We investigate spatial variability in mineral compositions in the southernmost part of the Salahi mantle section and discuss genetic relationship between highly refractory peridotites and less refractory plagioclase-bearing peridotites in this region. The study separates the study area into two domains based on spinel Cr# such as high Cr# domain in the eastern part that is closer to Moho and low Cr# domain in the central part. Concordant dunites commonly occur in the low Cr# domain whereas discordant dunites are common in the high Cr# domain. Plagioclase-bearing peridotites and wehrlite also occur in the low Cr# region. Highly refractory dunite with spinel Cr# >0.7 frequently occurs in the high Cr# domain. In the low Cr# domain, spinel Cr# is low and ranges from 0.47 to 0.57. We analyzed clinopyroxene (cpx) in dunites and harzburgite from both domains for REE abundances by LA-ICP-MS. The results show that harzburgite cpxs in the high Cr# domain and low Cr# domain are highly depleted in LREE ([Ce]CH =0.01~0.02) with [Yb]CH=2~3. Dunite cpxs in the low Cr# domain have REE abundances similar to the harzburgites in the same outcrop whereas those in the high Cr# domain are enriched in LREE relative to the harzburgite cpxs in the same outcrop. This implies that dunite cpxs in the high Cr# domain were reacted with LREE-enriched fluid infiltrated from the base of the ophiolite.In the low Cr# domain, plagioclase-bearing dunite, plagioclase-bearing lherzolite vein occur and phlogopite-bearing wehrlite discordantly cuts them. The spinel Cr# of these dunites are in a range from 0.46 to 0.56. Abundances of REE in a melt in equilibrium with cpx in plagioclase-bearing peridotites and associated dunites are similar to those of N-MORB. On other hand, a melt in equilibrium with wehrlite cpx resembles to those of boninitic dikes from the Fizh block in the northern Oman ophiolite (Yamazaki, 2013). From the field occurrence, plagioclase-bearing dunite and plagioclase-bearing lherzolite formed by a reaction with MORB melt beneath spreading ridge whereas cumulative wehrlite was crystalized from a boninitic melt.