Akira Ishiwatari

The osmium isotopic composition of convecting upper mantle deduced from ophiolite chromites

Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γOs values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/188Os of a filtered suite yields a slope of −0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γOs value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory. Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab component.

Igneous Petrogenesis of the Yakuno Ophiolite (Japan) in the context of the diversity of ophiolites

An ophiolite complex includes three major members: basaltic volcanics, mafic-ultramafic cumulates, and residual peridotite. From the aspect of igneous petrology, three distinct types are recognized among the associations of the three members: (Liguria type) alkalic basalt, plagioclase-type cumulates, and lherzolite; (Yakuno type) high-alumina tholeiite, clinopyroxene-type cumulates, and clinopyroxene-bearing harzburgite; (Papua type) low-alumina tholeiite, orthopyroxene-type cumulates, and clinopyroxene-free harzburgite. In the light of recent experimental studies, the three types represent cogenetic, complementary products of low (<15%), moderate, and high (>30%) degrees of partial melting in the lherzolitic source mantle, respectively. The cumulates of the Yakuno ophiolite show structural and chemical continuity to the underlying residual peridotite, and were recrystallized at high pressures (5–10 kb). They originated in a deep, “soft-floored” magma chamber directly overlying the partially melted residual harzburgite, from which the magma was extracted. The three members of the Yakuno ophiolite were cogenetically formed through a magmatic event induced by a moderate degree of partial melting in the mantle.

Occurrence and field relationships of ultrahigh-pressure metagranitoid and coesite eclogite in the Su-Lu terrane, eastern China

Coesite eclogite is associated with metagranitoid in a 50×100 m2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

Decompression P–T path of coesite eclogite to granulite from Weihai, eastern China

Granulitized coesite-bearing eclogite from Weihai, northeastern part of the Shandong peninsula, eastern China was studied in detail to reveal the modification of mineral chemistry during decompression metamorphism. Considerable modification of chemical composition is recorded in clinopyroxene that occurs both as inclusions in garnet and as a matrix mineral. Careful examination of chemical variation with the change in microstructure made it possible to estimate the equilibrium composition of minerals at the coesite eclogite and garnet granulite stages. We were able to define three reference points on the P–T path, namely, coesite eclogite (3 GPa, 660±40°C), granulite (1 GPa, 700±30°C) and amphibolite (0.9 GPa, 600±20°C). The path thus obtained is similar to those obtained by previous workers and supports nearly isothermal decompression of coesite eclogite.

Petrological diversity and origin of ophiolites in Japan and Far East Russia with emphasis on depleted harzburgite

Abstract Ophiolites are divided into lherzolite-type (L-type) and harzburgite-type (H-type) by the lithology of their mantle peridotites. Rare depleted harzburgite-type (DH-type) is distinguished from the normal H-type by the more refractory nature of its mantle peridotite and the occurrence of orthopyroxene-type cumulate rocks including iron-rich harzburgite and orthopyroxenite. The Shelting (Sakhalin) and Krasnaya (Koryak Mountains) ophiolites in Far East Russia, which have both depleted harzburgite and orthopyroxene-type cumulate rocks, belong to this newly defined DH-type. The ophiolites in SW Japan-Primorye, NE Japan-Sakhalin, and the Koryak Mountains in the northwestern Pacific margin have diverse ophiolite types ranging from L- to DH-types. The wide petrological diversity, the common occurrence of DH-type, and the presence of thick crustal sections in these ophiolites suggest regionally inhomogeneous, commonly very high degrees of mantle melting over subduction zones, as in the modern Mariana forearc environment. The ophiolites of Japan and Far East Russia range in age from Early Palaeozoic to Cenozoic and are tectonically underlain by younger blueschists and accretionary complexes. The spatial association of these ophiolites with blueschists is analogous to the ophiolite-blueschist assemblages recovered from the Mariana forearc. This association might have formed in a period of non-accretion at an oceanic subduction zone that was followed by voluminous accretion of sediments, facilitating subsequent uplift of the ophiolites and blueschists.

Alpine ophiolites: product of low-degree mantle melting in a Mesozoic transcurrent rift zone

Abstract The Alpine ophiolites and their original sedimentary cover (Upper Jurassic radiolarite-limestone with ophiolite breccia) occur as dismembered tectonic slices (often inverted by Alpine tectonics) or olistoliths among the calc-schists (Cretaceous?). Original petrologic features of the ophiolites are largely masked by the Alpine metamorphism, but are discernible by the relic-mineral chemistry and petrographic reconstruction. The residual peridotite is generally plagioclase lherzolite resembling those from oceanic fracture zones, but spinel lherzolite is present at Davos (Switzerland) and Peas (France). The latter is characterized by higher Na 2 O (1.6%) of clinopyroxene and lower Cr 2 O 3 (8%) of spinel than the former ( 30%, respectively), and resembles subcontinental lherzolites. Geologic evidence indicates that these two lherzolites were together exposed on the Jurassic ocean floor. The mafic-ultramafic cumulates are of plagioclase-type characterized by the scarcity of orthopyroxene and high TiO 2 (avg. 0.8%) of clinopyroxene. The basaltic volcanics are alkalic rather than tholeiitic in respect to their relic clinopyroxene, which is as rich in TiO 2 and Na 7 O as that in Hawaiian alkali basalt. The Alpine ophiolite association of lherzolitic residual peridotite, plagioclase-type cumulates, and alkalic basalt is reasonable as cogenetic products of low-degree partial melting in the mantle. Geologic and petrologic features of the Alpine ophiolites are consistent with a geotectonic model postulating the generation of the Alpine ophiolites in an early Mesozoic rift zone cut by many transform faults.

Time-Space Distribution and Petrologic Diversity of Japanese Ophiolites

The Japanese ophiolites occur as nappes and melanges. Nappe-type ophiolites formed and were emplaced within 20-30 Ma in the Ordovician, Permian, and Jurassic-Cretaceous periods, corresponding to the circum-Pacific ophiolite pulses. The Paleozoic ophiolite nappes (Yakuno, Oeyama, Miyamori and others) are distributed in Honshu, while the Mesozoic ones (Horokanai, Poroshiri and others) are in Hokkaido. In southwestern Honshu, the Jurassic accretional complex is overthrust by the Permian Yakuno ophiolite which in turn is overridden by the Ordovician Oeyama ophiolite. This relationship is a mirror image of the superposing ophiolite nappes of corresponding ages in the Klamath Mountains. Downward (oceanward) younging of ophiolite nappes is a common feature on both sides of the Pacific.

High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al2O3. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.

Granulite facies relics in the Early Paleozoic kyanite-bearing ultrabasic metacumulate in the Oeyama Belt, the Inner Zone of southwestern Japan

Abstract Granulite facies relics are found in the early Paleozoic kyanite-bearing melanocratic metagabbro from the Fuko Pass metacumulate mass of the Oeyama belt, southwestern Japan. The granulite facies assemblage consists of relict Al-rich clinopyroxene (up to 8.5 wt.% Al 2 O 3 ) and pseudomorphs of spinel and plagioclase. The spinel pseudomorphs consist mainly of symplectic intergrowth of corundum and magnetite with minor gahnitic spinel. The plagioclase pseudomorphs are composed mainly of clinozoisite with minor kyanite. The symplectite suggests oxidation and Mg-depletion of the original spinel: hercynite (3FeAl 2 O 4 )+(O) = corundum (3Al 2 O 3 )+magnetite (Fe 3 O 4 ). This oxidation reaction may have taken place at 700-900°C temperature. The melanocratic metagabbro has later been hydrated to form the epidote amphibolite assemblage represented by clinozoisite+kyanite+paragonite. The clinozoisite+kyanite assemblage has further reacted to form margarite at a lower temperature. The first granulite facies assemblage implies that the metacumulate has originally constituted a basal part of thick oceanic crust, and then has experienced the high-P/T type metamorphism in a subduction zone. This indicates that the thick oceanic crust has been formed and accreted to the Circum-Pacific orogenic belt in the early Paleozoic time.

U-Pb zircon age of gabbroids of the Ust’-Belaya mafic-ultramafic massif (Chukotka) and its interpretation

The Ust’-Belaya mafic-ultramafic massif is assigned to the Western Koryak fold belt and largely composed of residual spinel peridotites, layered spinel and plagioclase peridotites, and gabbros. These rocks are crosscut by occasional plagiogranite and diorite veins and exhibit locally a close spatial association with basalts and carbonate-sedimentary deposits of Late Devonian and Early Carboniferous age. Based on this evidence, the massif was ascribed to the pre-Late Devonian ophiolite association. Our study presents new U-Pb SHPIMP II zircon ages and petrographic and mineralogical data on samples of the layered amphibole gabbro and vein diorite from the Ust’-Belaya massif. The approximate concordant U-Pb age corresponding to a timing of of amphibole gabbro crystallization is 799 ± 15 Ma, and the concordant U-Pb age reflecting a timing of of vein diorite crystallization is 575 ± 10 Ma. These ages coupled with geological studies of the massif, petrological and mineralogical investigations of the dated samples, as well as literature data on the petrology of peridotites and the age of formed plagiogranites suggest that the peridotites and layered gabbros of the Ust’-Belaya massif were formed by the Late Riphean, whereas the vein diorite and plagiogranite were resulted from a later (Vendian-Cambrian) magmatic stage. The peridotites and gabbros of the massif display no genetic relationship with spatially associated basalts and sedimentary rocks and, thus, they cannot be considered as members the pre-Late Devonian ophiolitic association. The results of this study will inevitably lead to a significant revision of geological and geodynamic interpretations of the Ust’-Belaya mafic-ultramafic massif. However, uneven study of the Precambrian complexes of the Koryak and Chukchi areas, their evolution in different structures of the region cannot yet be described by a single geodynamic scenario.