[SCG54-P12] Geochemistry of Wadi Tayin mantle section in the southern Oman mantle section with special reference to suprasubduction zone magmatism
キーワード：オフィオライト、マントル、中央海嶺、島弧、マントル-メルト反応
In this study, we report the lithology and the mineral compositions of peridotites from the Wadi Tayin massif in the southern Oman ophiolite to investigate the influence of arc related magmatism during oceanic thrusting. Many studies have been conducted in the Wadi Tayin mantle section in the southern Oman ophiolite (Kelemen et al., 1995; Godard et al., 2000; HanghØj et al., 2010). Kelemen et al. (1995) demonstrated that the REE abundances in clinopyroxenes in dunites from Wadi Tayin massif are in equilibrium with those of MORB although harzburgite did not. Godard et al. (2000) considered that the basal part of Wadi Tayin mantle section corresponds to the asthenosphere-lithosphere boundary based on whole rock compositions and modal abundance of clinopyroxene. HangØj et al. (2000) discovered that dunite and uppermost impregnated samples have high Os isotope ratio. Furthermore, they revealed that the peridotites from the shallowest part of the mantle section recorded higher metamorphic closure temperature relative to the deeper ones.
We systematically corrected harzburgites, dunites and lherzolites along wadis to cover the mantle section from the boundary between lower crust and uppermost mantle namely “Moho” to the basal thrust. Spinel Cr# [=Cr/ (Cr+Al) atomic ratio] of harzburgite varies from 0.22 to 0.58 and they are plotted on the abyssal peridotites area at compositional relationship diagrams. In addition, it tends to elevate from the basal thrust to the Moho. Spinel Cr# of dunites has a peak of frequency at 0.55 to 0.60, and some of it shows greater than 0.60. The highest spinel Cr# is 0.74. Those dunites occur both in the basal part of the mantle section and along NW-SE striking shear zone. The relationship between concordant dunites and discordant dunites (Arai et al., 2006) is also observed.
Abundances of trace elements in clinopyroxenes were analyzed by LA-ICP-MS. The chondrite-normalized patterns show that clinopyroxenes are depleted in LREE relative to HREE although abundance of LREE is more variable than that of HREE. Dunite with the highest spinel Cr# of 0.68 from the shear zone is the most enriched in MREE to LREE relative to other dunites. The trace element patterns for clinopyroxenes from this dunite show negative anomaly in high field strength (HFS) elements such as Nb, Ta, Zr and Hf. In addition, the diagram of Cr# vs Ce/Yb ratio for clinopyroxenes shows a positive correlation toward the compositional area of MTZ (Akizawa et al., 2012). We consider that these trends may be formed by flux-melting of harzburgites. Negative anomaly in the HFS elements indicates that fluid caused flux-melting in the mantle section might be ascended from metamorphic sole due to dehydration of oceanic crust during suprasubduction stage. Farther more, according to the distribution of dunites which contain high Cr# spinel, the fluid might have been locally infiltrated both in basal part and along share zone. On the other hand, we observed that clinopyroxenes that are contained uppermost mantle section and Moho transition zone dunites are equilibrium with N-MORB (Sun and McDonough, 1989) although harzburgites which exist in same area are depleted in LREE. This fact is consistent with that showed by Kelemen et al. (1995).
We systematically corrected harzburgites, dunites and lherzolites along wadis to cover the mantle section from the boundary between lower crust and uppermost mantle namely “Moho” to the basal thrust. Spinel Cr# [=Cr/ (Cr+Al) atomic ratio] of harzburgite varies from 0.22 to 0.58 and they are plotted on the abyssal peridotites area at compositional relationship diagrams. In addition, it tends to elevate from the basal thrust to the Moho. Spinel Cr# of dunites has a peak of frequency at 0.55 to 0.60, and some of it shows greater than 0.60. The highest spinel Cr# is 0.74. Those dunites occur both in the basal part of the mantle section and along NW-SE striking shear zone. The relationship between concordant dunites and discordant dunites (Arai et al., 2006) is also observed.
Abundances of trace elements in clinopyroxenes were analyzed by LA-ICP-MS. The chondrite-normalized patterns show that clinopyroxenes are depleted in LREE relative to HREE although abundance of LREE is more variable than that of HREE. Dunite with the highest spinel Cr# of 0.68 from the shear zone is the most enriched in MREE to LREE relative to other dunites. The trace element patterns for clinopyroxenes from this dunite show negative anomaly in high field strength (HFS) elements such as Nb, Ta, Zr and Hf. In addition, the diagram of Cr# vs Ce/Yb ratio for clinopyroxenes shows a positive correlation toward the compositional area of MTZ (Akizawa et al., 2012). We consider that these trends may be formed by flux-melting of harzburgites. Negative anomaly in the HFS elements indicates that fluid caused flux-melting in the mantle section might be ascended from metamorphic sole due to dehydration of oceanic crust during suprasubduction stage. Farther more, according to the distribution of dunites which contain high Cr# spinel, the fluid might have been locally infiltrated both in basal part and along share zone. On the other hand, we observed that clinopyroxenes that are contained uppermost mantle section and Moho transition zone dunites are equilibrium with N-MORB (Sun and McDonough, 1989) although harzburgites which exist in same area are depleted in LREE. This fact is consistent with that showed by Kelemen et al. (1995).