Tomoaki Morishita

Widespread arc-related melting in the mantle section of the northern Oman ophiolite as inferred from detrital chromian spinels

We examined detrital chromian spinels, in recent river beds, derived from the mantle section to deterrmine whether the mantle peridotite of the northern Oman ophiolite is of oceanic or arc origin. The Cr-number (= Cr/(Cr + Al) atomic ratio) of the detrital chromian spinels mostly ranges from 0.4 to 0.8, and more than 60% of them have Cr-numbers higher than 0.6. This indicates a significant extent of island-arc nature because chromian spinels seldom have Cr-numbers higher than 0.6 in ocean-floor rocks. The high-Cr-number (>0.6) spinels, which are also low in TiO2 (<0.3 wt%), are probably derived from an arc setting. Lherzolite with spinel Cr-numbers <0.2 is absent except near the base of the section. The high-Cr-number detrital chromian spinels are more abundant from the mantle section with more frequent discordant dunite pods. Post-deformational discordant dunite and adjacent harzburgite include the high-Cr-number spinels in outcrops. The upper mantle of the northern Oman ophiolite was formed at a fast-spreading ridge and was later modified by island-arc magmatism in a subduction-zone environment. This switch of tectonic setting from a mid-oceanic ridge to an island arc is essential in the explanation of obduction of a slice of oceanic lithosphere onto a continental margin as an ophiolite.

High-pressure aluminous mafic rocks from the Ronda peridotite massif, southern Spain: significance of sapphirine- and corundum-bearing mineral assemblages

We firstly present detailed microtextural relationships in sapphirine- and corundum-bearing aluminous mafic rocks alternating with peridotites from the Ronda massif, southern Spain. Garnet and clinopyroxene are the main phases in the aluminous mafic rocks. Garnet is partially to completely kelyphitized. Clinopyroxene (CPX I) has high Al2O3 content in the core (up to 16 wt.%) and is partially converted to spherical symplectitic aggregates consisting of lower-Al2O3 clinopyroxene (CPX II) (<5 wt.%) and plagioclase. Corundum is associated with spinel and plagioclase. Sapphirine occurs in two different mineralogical associations, i.e., as elongated lamellae within CPX I and as very fine-grained crystals in plagioclase-rich domain. The sapphirine+plagioclase aggregate suggests the former presence of kyanite as one of reactants. Sapphirine is partially converted to symplectitic aggregate of spinel and plagioclase. The earliest metamorphic mineral assemblage recorded in the aluminous mafic rock was garnet+clinopyroxene±kyanite±corundum, i.e., eclogitic mineral assemblages, at P≥1.5 GPa and T≥900°C. Many reaction textures were developed during decompression possibly coupled with cooling. The latest P–T conditions recorded in the aluminous mafic rock were at P=1 GPa and T=800–900°C. This decompression had possibly followed the compression of plagioclase-rich, low-pressure cumulate or residue.

Petrology of an apatite-rich layer in the Finero phlogopite–peridotite, Italian Western Alps; implications for evolution of a metasomatising agent

We discovered an apatite-rich layer in the Finero phlogopite–peridotite body, Western Alps. Fine-grained apatite grains are concentrated, up to 10 modal percent locally, to form a lens within or adjacent to a thin layer, <1 cm in thickness, consisting of very fine-grained minerals of olivine, orthopyroxene, spinel, amphibole, phlogopite, sulfide minerals and apatite with small amount of carbonate and clinopyroxene. In the thin layer, apatite-rich part is richer in amphibole and carbonate minerals than apatite-poor part, suggesting that an alkali- and phosphate-rich carbonatitic metasomatising agent locally existed in the thin layer. On the other hand, film-shaped orthopyroxenes occur at grain boundaries of olivine in the thin layer. The film-shaped orthopyroxenes are distinctively low in Al2O3, Cr2O3 and CaO contents compared with those of the host peridotite, suggesting the consumption of a SiO2-rich metasomatising agent as the thin layer formed. These two distinctive metasomatising agents would be caused by the immiscibility of a homogeneous metasomatising agent which was evolved to be a high CO2/H2O with the result that hydrous minerals and other metasomatised minerals were formed by interaction between a parent metasomatising agent and host peridotite in the late stage of exhumation of the Finero complex. The parent metasomatising agent is probably derived from a subducted slab, possibly containing small amount of sediments.

Petrochemical characteristics of felsic veins in mantle xenoliths from Tallante (SE Spain): an insight into activity of silicic melt within the mantle wedge

Felsic and related veins within mantle-derived peridotite xenoliths from Tallante, Spain, were examined in order to understand the mantle-wedge processes related to the behaviour of Si-rich melt. The thickest part of the vein has a quartz diorite lithology, and is composed mainly of quartz and plagioclase, with pyroxenes, hydrous mineral, apatite, zircon and rutile present as minor phases. The thinner parts are free of quartz and predominantly composed of plagioclase. Orthopyroxene always intervenes between the internal part (plagioclase ± quartz) and host peridotite, indicating that it is a product of interaction between silica-oversaturated melt and olivine. This indicates that a sufficiently high melt/wall rock ratio enabled the melt to retain its silicaoversaturated character. The quartz diorite part has adakite-like geochemical signatures, except for negative Ba, Rb Eu and Sr anomalies, and positive Th and U anomalies. These negative anomalies indicate that fractionation of plagioclase and hydrous minerals was achieved between the upper most mantle and the slab melting zone. The shape of the rare-earth element (REE) pattern of clinopyroxene in quartz diorite is strikingly similar to that of clinopyroxene phenocrysts from Aleutian adakites. However, the former has one order higher REE contents than the latter, except for Eu which shows a prominent negative spike. This feature was caused by the precipitation of large amounts of plagioclase and small amounts of clinopyroxene from a fractionated adakitic melt before and during solidification. This adakitic melt was produced by partial melting of a detached and sinking slab beneath the Betic area in the Tertiary.

Closed-system geochemical recycling of crustal materials in alpine-type peridotite

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.

Occurrence and chemical composition of barian feldspars in a jadeitite from the Itoigawa-Ohmi district in the Renge high-P/T-type metamorphic belt, Japan

Abstract Coexisting Ba-rich (Cn56−59Or40−42Ab2An0−1) and Ba-poor (Cn7−15Or83−92Ab1−3An0−1) feldspars were found in a jadeitite collected from the Itoigawa-Ohmi district in the Renge high-P/T-type metamorphic belt, Japan. There is an apparent compositional gap in these barian feldspars at Cn15−56 in the ternary system BaAl2Si2O8−KAlSi3O8−NaAlSi3O8. Barian feldspars occur as interstitial phases between subhedral to euhedral jadeite and prehnite, and sometimes cut a jadeite crystal. The occurrence of barian feldspars combined with the texture and mineral assemblages of the host jadeitite suggest that barian feldspars were formed from residual fluids after the formation of jadeite and prehnite from primary aqueous fluids at low-T conditions (<350°C) and pressure of ~0.6 GPa. The Ba content in the residual fluids would increase locally during the formation of jadeite because Ba is incompatible with clinopyroxene. This study supports the suggestion that Ba-rich minerals are not uncommon in jadeitites and albitites/metasomatized rocks that occur as tectonic blocks in serpentinite-matrix mélanges. Barium is an important minor element in metasomatizing fluids related to the formation of jadeitites and might have been derived from subducted oceanic crust at the beginning of the subduction.

Model of layering formation in a mantle peridotite (Horoman, Hokkaido, Japan)

Abstract The Horoman peridotite complex exhibits a conspicuous layered structure. It is found, from the geological and petrological survey, that the pattern of layering has three characteristics: symmetry, asymmetry (subtly collapsed symmetry) and scale invariance. Especially, symmetric and asymmetric patterns clearly recognized in the sequence of mafic layers at the Northern ridge of Apoi-dake peak, and at the Western ridge of Bozu-yama peak are noticeable. We present a simple mathematical model describing stretching (thinning) and folding during deformation that accounts for the three characteristics. The model quantitatively reproduces the slope in cumulative frequency distribution of the width of mafic layers and indicates that the frequency distribution is strongly influenced by the spatial strain contrast. Applying the model result to the observational data for mafic layers, it is found that the strain contrast approximately ranges several to 10 times between regions with the highest and the lowest strain rates.