Japan Geoscience Union Meeting 2015

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT05] Hard-Rock Drilling: Oceanic Lithosphere to Island Arc Formation and Beyond

Wed. May 27, 2015 11:00 AM - 12:45 PM 303 (3F)

Convener:*Yoshihiko Tamura(R & D Center for Ocean Drilling Science, Japan Agency for Maine-Earth Science and Technology), Yildirim Dilek(Department of Geology & Environmental Earth Science Miami University), Tomoaki Morishita(School of Natural System, Colleage of Science and Technology, Kanazawa University), Takashi Sano(Department of Geology and Paleontology, National Museum of Nature and Science), Natsue Abe(R&D Center for Ocean Drilling Science Japan Agency for Marine-Earth Science and Technology), Susumu Umino(Department of Earth Sciences, Kanazawa University), Eiichi TAKAZAWA(Department of Geology, Faculty of Science, Niigata University), Shigeaki Ono(Japan Agency for Marine-Earth Science and Technology), Katsuyoshi Michibayashi(Institute of Geosciences, Shizuoka University), Chair:Natsue Abe(R&D Center for Ocean Drilling Science), Shigeaki Ono(Japan Agency for Marine-Earth Science and Technology), Katsuyoshi Michibayashi(Institute of Geosciences, Shizuoka University)

11:30 AM - 11:45 AM

[SIT05-14] Metasomatism during subduction initiation recorded in basal peridotites of the northern Fizh massif, Oman ophiolite

*Masako YOSHIKAWA1, Marie Python2, Shoji ARAI3, Akihiro TAMURA3, Tomoyuki SHIBATA1, Eiichi TAKAZAWA4, Akira UEDA5, Tsutomu SATO6 (1.Inst. Geotherm. Sci., Kyoto University, 2.Dep. Natural History Sci., Hokkaido University, 3.Dep. Earth Sci., Kanazawa University, 4.Dep. Geol., Niigata University, 5.Faculty of Science, University of Toyama, 6.Faculty of Engineering, Hokkaido University)

Keywords:mantle metasomatism, Oman ophiolite, basal peridotite, slab-derived fluid, trace element compositions, Nd-Sr isotopic compositions

The Oman ophiolite is one of the best preserved sections of oceanic lithosphere worldwide. It lies on more than 400 km along the north eastern coast of the Arabian Peninsula. The Oman ophiolite and its underling metamorphic sole are regarded as being direct analogues of obducted oceanic lithosphere and subducted oceanic crust that formed by overthrusting (e.g. Boudier et al., 1989, Ishikawa et al., 2002). This idea is supported by the presence of granitic rocks and boninite dykes in the northern part of the ophiolite, which were generated by the partial melting of the subducting plate sediment cover during metamorphic sole formation (Cox et al., 1999) or by mantle metasomatism by fluid dehydrated from metamorphic sole during subduction initiation (Ishikawa et al., 2002).

We focus on the basal clinopyroxene (Cpx)-rich peridotites in the northernmost Fizh massif in order to discuss the origin of the metasomatic agent and the degree of metasomatism, and to estimate the Cpx trace element and Nd-Sr isotopic compsositions.

The chondrite-normalised multi-element patterns for Cpxs in these rocks are significantly depleted in incompatible elements. The multi-element Cpx patterns were basically reproduced by 4-12% of melt extraction from a spinel peridotite source. However, the highly incompatible element (e.g., Ba, Nb, La, Ce, and Pb) characteristics of the basal Cpx-rich peridotites could not be reproduced by simple melting modelling. Ishikawa et al. (2005) proposed a trace element compositions for fluids released from the metamorphic sole beneath the Oman ophiolite. The enrichment of highly incompatible elements is generally reproduced by the addition of very small amounts of these fluids (≤ 0.3 %) to the residual peridotites.

The Sm-Nd isotopic data plotted on the gabbro isochron (100 Ma) of the Fizh block (as given by McCulloch et al., 1980, 1981) suggests that the basal Cpx-rich peridotites were formed by partial melting contemporaneously with the generation of oceanic crust. Initial Sr isotopic compositions of the Cpxs within the basal Cpx-rich peridotites cover a wide range (87Sr/86Sr = 0.7030-0.7074), in contrast to the rather constant initial Nd isotopic compositions. The initial Sr-Nd isotopic compositions consistently plot on the mixing line between Cretaceous seawater and MORB-type oceanic crust (presented by McCulloch et al., 1980), suggesting a contribution of seawater from the metamorphic sole.

Based on these observations, we propose that small amounts of fluids derived from the metamorphic sole (amphibolites and quartzose rocks) were added to the overlying residual peridotites during the initial stages of subduction.