Spinel and plagioclase peridotites of the Nain ophiolite (Central Iran): Evidence for the incipient stage of oceanic basin formation

The Nain ophiolites crop out along the western border of the central East Iran Microcontinent (CEIM) and consist of an ophiolitic mélange in which pargasite-bearing spinel and plagioclase mantle lherzolites are largely represented.Whole-rock and mineral chemistry data suggest that these rocks record the complex history of the asthenospheric and lithospheric mantle evolution. The spinel lherzolites have experienced low-degree (~5%) partial melting and contain clinopyroxenes with positive Eu anomalies (Eu/Eu⁎=1.10–1.48) suggesting that the partial melting occurred under oxidized conditions (fayalite–magnetite–quartz−0.8 to+1.3). The pargasite and coexisting clinopyroxene in these rocks are depleted in light rare earth elements (LREE) (mean chondrite-normalized CeN/SmN=0.045). The depleted chemistry of this amphibole reflects metasomatismduring interaction with H2O-rich subalkalinemaficmelts,most likely concurrentlywith or after the partial melting of the spinel lherzolites. The plagioclase lherzolites were subsequently formed by the subsolidus recrystallization of spinel lherzolites under plagioclase facies conditions as a result of mantle uprising, as evidenced by: (1) the development of plagioclase rims around the spinels; (2) plagioclase+orthopyroxene exsolution textures within some clinopyroxene grains; (3) an increase in plagioclase modal content coupled with an increase in modal olivine and a decrease in modal pyroxene and pargasite; (4) coincident decreases in Al, Mg, and Ni, and increases in Cr, Ti, and Fe in spinel, as well as decreases in Al and Ca, and increases in Cr and Ti in pyroxene and pargasite; and (5) the identical whole rock compositions of the spinel and plagioclase lherzolites, which rules out a magmatic origin for the plagioclase in these units. The Nain lherzolites have similar whole-rock and mineral geochemical compositions to subcontinental peridotites that are typically representative of Iberia-type rifted continental margins and ocean–continent transition zones (OCTZ), suggesting that they formed during the early stages of the evolution of the Nain oceanic basin. This means that the Nain lherzolites represent the Triassic–Jurassic western border of the CEIM or alternatively an associated OCTZ

Crustal and Paleo-Oceanographic Change of the Circum-Japan Sea area : A Review

金沢大学大学院自然科学研究科Scedule:17-18 March 2003, Vemue: Kanazawa, Japan, Kanazawa Citymonde Hotel, Project Leader : Hayakawa, Kazuichi, Symposium Secretariat: XO kamata, Naoto, Edited by:Kamata, Naoto

Role of dunite in genesis of primitive MORB

金沢大学理工研究域自然システム学系Dunite and related rocks from the Moho transition zone of the East Pacific Rise (EPR) and the Mid-Cayman Trough (MCT) are examined to know the spreading-rate dependence of their compositions. The rocks from both ridges are basically a reaction product between peridotite and primitive MORB. Despite the large difference of prevalent peridotite chemistry, depleted harzburgite for EPR and lherzolite for MCT, the dunite has less different compositional ranges. The Al2O3 content of spinel differs only by 5 wt%, which may be equivalent to the difference of about 1 wt% of Al2O3 in the melt. This is almost equal to the possible spreading-rate dependent compositional difference of observed Mg-rich MORB. The relatively unfractionated MORBs erupted are in equilibrium with the oceanic dunite of which composition is independent of or only slightly dependent on spreading rate. Dunite is calculated to be about half of the total oceanic mafic crust by weight. The dunite is concentrated as the Moho transition zone, and is sparsely distributed as small lenses within the uppermost mantle peridotite

Possible sub-arc origin of podiform chromitites

金沢大学理工研究域自然システム学系The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr no.) of their chromian spinel frequently is higher than 0.7. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr no. < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of HFSE to LILE, one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites. -from Author

Inhomogeneous spinel in chromitite from the Iwanai-dake peridotite complex, Hokkaido, Japan : variations of spinel unmixing texture and chemical composition

Graduate School of Natural Science and Technology, Kanazawa UniversityDepartment of Earth Sciences, Kanazawa Universit

Insight into Material Input from the Slab into the Mantle Wedge : an Application of In-Situ Trace-Element Analysis Minerals by La-ICP-MS

金沢大学大学院自然科学研究科Promoting Environmental Pesearch in Pan-Japan Sea Area : Young Researchers\u27 Network, Schedule: March 8-10,2006,Kanazawa Excel Hotel Tokyu, Japan, Organized by: Kanazawa University 21st-Century COE Program, Environmental Monitoring and Prediction of Long- & Short- Term Dynamics of Pan-Japan Sea Area ; IICRC(Ishikawa International Cooperation Research Centre), Sponsors : Japan Sea Research ; UNU-IAS(United Nations University Institute of Advanced Studies)+Ishikawa Prefecture Government ; City of Kanazaw

Geochemistry of clinopyroxene in peridotites from the Nukabira complex, Kamuikotan zone, Hokkaido, Japan : a LA-ICP-MS study

Department of Earth Sciences, Kanazawa UniversityDepartment of Earth Sciences, Kanazawa UniversityTrace-element compositions of clinopyroxene in peridotite from the Nukabira complex were examined to mainly discuss partial melting processes involved in their formation. The Nukabira complex in the Kamuikotan zone, Hokkaido, Japan, is mainly composed of mantle-derived peridotites with wide compositional variations. The Iherzolite (e.g., C# of spinel=0.18-0.34) and depleted harzburgite (e.g., Cr#=0.4-0.7) are predominant, and dunite is often surrounded by them. Samples of 8 Iherzolites, 3 harzburgites and 4 dunites from the complex were selected, and their clinopyroxene geochemical characteristics were determined by using LA-ICP-MS. Chondrite-normalized trace-element patterns of clinopyroxene in the Iherzolites except one sample are characterized by strong depletion of light-REE (LREE) and middle-REE (MREE) relative to high heavy-REE (HREE) (e.g., (Nd/Yb)_<0.004, Yb_N=2.7-6.1). One Iherzolite sample, on the other hand, has high REE abundances of clinopyroxene (e.g. Ce_N≈0.05, Yb_N≈5.3, (Ce/Yb)_N=0.02-0.06). The trace-element patterns of clinopyroxene in harzburgites are "spoon-shaped" being high in MREE and LREE and low in HREE abundance (e.g., (Nd/Yb)_N<0.09, Yb_N=1.0-1.7). The abundances of MREE and LREE are variable in each harzburgite sample (e.g., Ce_N=0.008-0.03, Nd_N=0.02-0.08). The dunites tend to be similar in clinopyroxene compositions to surrounding peridotites. The variations of Iherzolite can be ascribed not only to the difference of melting degree but also to conditions on melting, such as with or without residual garnet prior to spinel peridotite stability field melting. Most of the Iherzolites probably experienced the melting in the garnet peridotite stability field. The further melting of Iherzolites should generate the harzburgites. The clinopyroxene geochemistry, however, indicates that the harzburgite was produced by different melting processes from the Iherzolite formation. The melting assisted by LREE-rich hydrous fluid is more preferable to explain the harzburgite features and origin