Recent Research Interests of Akira Ishiwatari Laboratory
as seen from presentations in international scientific meetings
2006/12/18 Established, 2006/12/19 Updated
[Kanazawa University] [Department of Earth Sciences] [Ishiwatari Page] [Ishiwatari Lab Page] [Message Board]
Ophiolites, greenstones, LIPs, UHP/UHT metamorphic belts and superplumes in East Asia (060916, GSJ Annual Meeting in Kochi, Japan)
Ophiolite-blueschist belts from Japan to Russia and their genesis in intra-oceanic supra-subduction zones: Introduction and perspective (060304, Japan-Russia Ophiolite Symposium, Tokyo)
Ophiolites and oceanic plateau remnants (greenstones) in Japan and Far East Russia (051206, AGU Fall Meeting in San Francisco)
Late Paleozoic and Mesozoic superplume activities recorded in the accreted mafic-ultramafic volcanic rocks in the Asian margins and their tectonic implications (050910, Gondwana to Asia Symposium in Beijing)
Extensive melting of depleted mantle among circum-Pacific ophiolites: petrologic and Os-isotopic evidences (040823, IGC in Firenze)
Geological Society of Japan, 113th Annual Meeting (Kochi), Saturday, September 16, 2006, No. S-35
"Birth of Gondwana Supercontinent, Change of Surface Environment, and Explosive Evolution of Life in Cambrian" (presided by S. Maruyama and M. Santosh)
Ophiolites, greenstones, LIPs, UHP/UHT metamorphic belts and superplumes in East Asia
Akira Ishiwatari (Dept. Earth Sci., Kanazawa Univ.)
The earth has played a rhythm of ophiolite pulses through Phanerozoic (Abbate et al. 1985, Ofioliti, 10, 109-; Nicolas 1989, ISBN 0-7923-0255-9, Kluwer). The largest ophiolite pulse was in the Jurassic-Cretaceous time, and apparently coincided with the Cretaceous superchron (long-term absence of paleomagnetic reversals) and voluminous magmatic events that produced wide oceanic crust, oceanic plateaus, orogenic granites and continental large igneous provinces (LIPs). The next largest ophiolite pulse took place in Ordovician, but a smaller pulse probably existed in Permian (Ishiwatari 1994, Proc. 29th IGC, Part D, VSP). The circum-Pacific ophiolites such as Yakuno (Japan), Dun Mountain (New Zealand) and Canyon Mountain (Oregon, USA) represent the Permian pulse. These ophiolites were formed in supra-subduction zones (SSZs), and suggest vigorous plate convergence in the Permian time.
Our recent studies on Permian greenstones (altered fragments of oceanic volcanic edifices) in the Jurassic Mino-Tamba accretionary complex in Southwest Japan revealed their geochemical affinity to the basaltic rocks of oceanic plateaus (Koizumi & Ishiwatari 2006, Isl. Arc, 15, 58-). We also discovered superplume-related ultramafic volcanic rocks (meimechite and ferropicrite) among those basaltic rocks (Ichiyama & Ishiwatari 2005, Contr. Min. Petr., 149, 373-; Ichiyama et al. 2006, Lithos, 89, 47-). The Mino-Tamba greenstones structurally underlies the Yakuno ophiolite, and completely devoid of SSZ chemical signatures (such as Nb depletion), which are detectable in volcanic rocks of the Yakuno ophiolite. As oceanic lithosphere of the Permian age was completely swept away from the earth’s ocean floor by subduction, evidence of oceanic superplume magmatism in that time should be detected only among the accretionary complexes in continental margins and island arcs. The Permian greenstones are associated with pelagic sediments such as chert and limestone free from terrigenous material, and mid-oceanic environment is evident for their site of formation. Our findings add a new example of accreted Permian oceanic plateaus to those already reported from western North America (Wrangelia, Cache Creek, Angayucham, etc.), and confirm existence of the earth’s superplume pulse in the Permian time, helped by the presence of Permian LIPs (Siberia and Emeishan) and the Permian ophiolite pulse.
The major continental collision between Sino-Korean and Yangtze continental blocks happened in the Late Permian-Triassic time, where the coesite- and diamond-bearing, ultrahigh-pressure (UHP) metamorphic rocks were formed. It is possible that the superplumes in Siberia and Yangtze (Emeishan), as well as those in the Panthalassa ocean (e.g. Mino-Tamba), accelerated plate convergence and eventually resulted in the large-scale continental collision. A mafic granulite with coesite relict in garnet exists in the Su-Lu UHP belt, East China, suggesting isothermal (or slightly heated) decompression (Banno et al. 2000, Lithos, 52, 97-). Presence of intergranular coesite in the Yangkou eclogite (Su-Lu) indicates very quick exhumation (Ye et al. 1996, Chin. Sci. Bull. 41, 1407-). The Permo-Triassic ultrahigh-temperature (UHT) metamorphic rocks (>1,000ºC) in the Kontum massif (central Vietnam) also followed an isothermal (or heated) decompression path from the preceding eclogite stage (Osanai et al. 2004, J. Min. Petr. Sci. 99, 225-). This suggests heating of the subducted continental crust during its exhumation, possibly by underplating of voluminous basaltic magma or buoyancy of the mantle plume itself. The Permo-Triassic komatiites and basalts in the Song Da zone (northern Vietnam) may represent southeastern extension of the Emeishan LIP (Hanski et al. 2004, Contr. Min. Petr. 147, 453-), and mantle plumes related to this LIP may have been responsible for the quick (and often heated) exhumation of UHP metamorphic rocks and subsequent granulite-facies (often UHT) metamorphism. This implies that the mantle flow radiating from the Emeishan plume first facilitated subduction and collision of the overlying and nearby continental plates, but enlargement of the plume or passage of the subduction-collision system over the plume in turn caused forceful exhumation and heating of the subducted (UHP) continental crust.
Interaction between superplume and subduction system is also indicated from the Jurassic examples in East Asia. The Late Jurassic-Early Cretaceous volcanic rocks of the Sorachi belt (Hokkaido) and Aniva belt (Sakhalin) include HIMU basalt and abundant picritic rocks, and are interpreted as accreted oceanic plateau (Tatsumi et al. 1998, Geology, 26, 151-; Sakakibara et al. 1999, Mem. Geol. Soc. Jpn. 52, 1-). However, they are closely associated with the Late Jurassic ophiolites bearing strongly depleted mantle harzburgite (spinel Cr#>80) of undoubted SSZ origin (Ishiwatari et al. 2003, GSL Sp. Pub. 218, 597-). The Jurassic greenstones of the 1,000 km-long Mikabu belt (SW Japan) also include meimechite, but uniformly show olistostromal occurrence characterized by “gabbro sandstone” (Iwasaki 1984, Ofioliti, 9, 443-), and are partly affected by high-pressure metamorphism. Their quick accretion suggests their generation proximal to the subduction zone. In the Jurassic accretionary complex of Primorye (Russia) and Heilongjiang (NE China), meimechite lavas (partly pillowed) flowed on Jurassic mudstone and sandstone, which in turn were intruded by the Late Jurassic, TiO2-rich Alaskan-type plutons (Ishiwatari & Ichiyama 2004, Int. Geol. Rev. 46, 316-). Thus, these Jurassic superplume activities are thought to have taken place near convergent plate boundaries.
Ophiolites (SSZ lithosphere), greenstones (oceanic LIPs), continental LIPs and UHP/UHT metamorphic rocks may be connected to each other with a single key word; “superplume”.
Japanese-Russian Ophiolite Symposium (supported by JSPS and RFBR) (March 4, 2006. Ocean Research Institute, Tokyo)
Ophiolite-blueschist belts from Japan to Russia and their genesis in intra-oceanic supra-subduction zones: Introduction and perspective
Akira ISHIWATARIDept. Earth Sci., Fac. Sci., Kanazawa Univ. 920-1192 JAPAN
The ophiolite-blueschist belts in the northwestern margins of the Pacific Ocean are characterized by their extreme petrological diversity as well as by their wide temporal and spatial multiplicity (Ishiwatari, 1994; Proc. 29th IGC, D, 7-28). The former feature is represented by the common occurrence of highly depleted mantle harzburgite bodies, and the latter feature corresponds to the full-Phanerozoic span of formation ages of ophiolites and blueschists (Table 1; Ishiwatari et al. 2003; GSL Spec. Publ. 218, 597-).
Table 1. Ages of ophiolites and blueschists of the northwestern Pacific margins from Japan to Russia. O: ophiolite; B: blueschist; A: accretionary complex. *maybe Proterozoic age (Dobretsov, 1999; Petrology, 7, 410-).
This Japanese-Russian (JSPS-RFBR) co-operative research project “Origin of Paleozoic meta-ophiolites and Mesozoic accretionary complexes in the circum-Pacific orogenic belt of the Koyrak Mountains” (A. Ishiwatari and S.D. Sokolov, leaders) is aimed to clarify the geologic and petrologic nature of the peculiar Paleozoic (and older?) ophiolites and adjacent units in the Pekulney Range and Ust-Belaya area of northern Koryak Mountains (Fig. 1).
Fig. 1. Location of the Pekulney and Ust-Belaya ophiolites in the Koryak Mountains with the ophiolite-blueschist belts in the NW Pacific margins (Ishiwatari et al. 2003; GSL Spec. Publ. 218, 597-).
The Pekulney Range is mainly composed of garnet- and spinel-bearing mafic granulites and ultramafic rocks of ‘continental’ origin (Nekrasov, 1980; Dokl. Akad. Nauk SSSR, 238, 87-). However, it may represent a basal part of thick oceanic crust of a marginal basin or an island arc (e.g. Yakuno ophiolite; Ishiwatari, 1985, J. Petrol., 26, 1-; Suda, 2004, JMPS, 99, 339-; Hayasaka & Suda, this seminar; Tonsina ophiolite; DeBari & Coleman, 1989; JGR, 94, 4373-). The relationship between the Paleozoic (and older?) ophiolitic rocks and the associated Mesozoic accretionary complexes and ophiolites is also a target of our study in the Ust-Belaya and the Pekulney areas. The origin of multiple ophiolite-blueschist belts encased in the accretionary complexes of abundant clastic and volcaniclastic sediments is better understood in the light of the seafloor geology of the Japan arc and Izu-Bonin-Mariana arc areas (Fig. 2a). I suppose that an ophiolite formed in an area of intra-oceanic subduction initiation (Flower & Dilek, 2003; GSL Spec. Publ. 218, 21-), then blueschist form in the subduction zone (of Mariana Trench type; Fig. 2b), and then they are underplated by the oceanic and terrigenous sediments accreted to the subduction zone (of Nankai Trough type; Fig. 2c). The boninite- and adakite-bearing Hahajima Seamount in the Bonin forearc front at the Izu-Mariana junction may represent an ophiolitic thrust sheet emplaced on the subducting Ogasawara oceanic plateau (Ishiwatari et al. 2006; Island Arc, 15(1) in press). Our comparative study of the sea-floor and on-land ophiolites will better depict tectono-magmatic processes in the supra-subduction zones.
Fig. 2. (a) Tectonic framework of Japan and Izu-Bonin-Mariana areas. (b) Accretionary structure across the Shikoku - Nankai Trough (B-B’) section. (c) Cross section across the Mariana Arc and Mariana Trench (C-C’). (Ishiwatari, 2003; GSL Spec. Publ. 218, 597-).
Our studies on Permian greenstones of the Japanese Jurassic accretionary complexes (Mino-Tamba belt) provided evidences for their oceanic plateau (or large igneous province, or superplume) origin (Ichiyama & Ishiwatari, 2005, CMP, 149, 373-; Ichiyama et al. 2006; Lithos, in press) and the importance of oceanic plateau accretion in the development of the accretionary complex (Koizumi & Ishiwatari, 2006, Island Arc, 15(1), in press; this seminar). The Jurassic-Cretaceous greenstone bodies in SW Japan (Mikabu belt), Hokkaido (Sorachi belt), Sakhalin and Primorye (here, Alaskan-type zoned ultramafic plutons are associated) are also originated in an oceanic superplume magmatism (Tatsumi et al. 1998, Geology, 26, 151-), but their geologic occurrences indicate that the superplume magmatism took place at or near the trench (Ishiwatari & Ichiyama, 2004, Int. Geol. Rev., 46, 316-). Our study in the Koryak Mountains will provide implications for the Paleozoic-Mesozoic superplume magmatisms and their geotectonic impacts in the development of Pacific Ocean and its northwestern margins.
American Geophysical Union, Fall Meeting (San Francisco), Tuesday, December 06, 2005. T21E-07 (0935h) Marriott: Salon 5.
Session T21E "Links between ophiolites and the lost large igneous provinces record II" (presided by Y. Dilek and R. Ernst)
Ophiolites and oceanic plateau remnants (greenstones) in Japan and Far East Russia
Akira Ishiwatari, Yuji Ichiyama and Kazuto Koizumi (Dept. Earth Sci., Kanazawa Univ., Japan)
In Japan, an older ophiolite thrust onto younger ophiolite with tectonic intercalation of accreted oceanic sediments (chert, limestone, shale and sandstone forming “ocean plate stratigraphy” deposited on the basaltic basement) or their high-P/T metamorphosed varieties. For example, the Yakuno ophiolite (SW Japan) of early Permian igneous age and supra-subduction zone (SSZ) origin (Ichiyama & Ishiwatari, Island Arc, 13, 157-) is tectonically underlain by the Ultra-Tamba nappe (chert, shale, sandstone) accreted in Late Permian, which is further underlain by the Tamba nappes (greenstone, chert, limestone, shale and sandstone) accreted in Jurassic. Major occurrence of the greenstones (mainly Permian) in the Upper Tamba nappe (consisting of 3 sub-nappes) is more than 1 km thick intact sheet of >200 km extension forming the structurally basal part of each sub-nappe, originated in an oceanic edifice composed of pillow lava, massive lava, hyaloclastite and dikes (“Basal Type”). Another minor occurrence is greenstone fragments of a few cm to 100 m size in the muddy matrix (“Mixed Type”), constituting structurally upper part of each sub-nappe. The Basal Type greenstones show uniform E-MORB affinity, but the Mixed Type ones show diverse features such as N-MORB, OIT and OIA. This clear correlation between the occurrence of greenstones and their chemistries suggests the accretion of thick crust of oceanic plateau (E-MORB) to make Basal Type greenstones and the accretion of thin normal oceanic crust (N-MORB) with disseminated small seamounts (OIT and OIA) to make Mixed Type greenstones (Koizumi & Ishiwatari, Island Arc, in submission.). We discovered HFSE-rich picrite (meimechite) sills and hyaloclastites as well as ferropicrite and picritic ferrobasalt dikes emplaced in the Basal Type greenstones and its chert-dolomite cover of Late Permian age. Zr/Y and Ti/Al signatures of these ultramafic volcanic rocks are intermediate between Polynesian picrites and Siberian meimechites, suggesting their origin by deep (4-5 GPa) partial melting of a superplume (Ichiyama & Ishiwatari, 2005; CMP, 149, 373-; Lithos, in submission.). The Yakuno ophiolite and the Tamba greenstone thus represent coeval but unrelated SSZ and oceanic magmatisms, respectively, in the same Permian time. The Jurassic superplume-related volcanic rocks are also reported from Hokkaido (Japan) and Sakhalin (Russia), and Jurassic meimechite lavas and related Alaskan-type zoned ultramafic plutons are reported from Primorye (Russia), but their age of accretion is also Late Jurassic. The Jurassic ophiolites in Hokkaido and Sakhalin are characterized by unusually depleted harzburgite (Spinel Cr#80-90), indicating very high degree of melting (Ishiwatari et al. GSL Spec. Publ. 218, 597-). These facts suggests that the superplume was placed beneath the subduction zone (Ishiwatari & Ichiyama, 2004; Int. Geol. Rev., 46, 316-), and facilitated plate convergence and SSZ magmatism. This study provides a new evidence for Permian oceanic superplume magmatism that is coeval with Siberian and Emeishan LIPs, and postulates possibility of superplume-SSZ interaction in Jurassic NW Pacific margin.
Gondwana to Asia Symposium (Jingmin Hotel, Beijing), Sunday, September 10, 2005, 13:30
"Comparative study to geological evolution of eastern Asia" (Presided by Mingguo ZHAI)
Late Paleozoic and Mesozoic superplume activities recorded in the accreted mafic-ultramafic volcanic rocks in the Asian margins and their tectonic implications
Akira Ishiwatari (Department of Earth Sciences, Kanazawa University. 920-1192 Japan)
E-mail: firstname.lastname@example.org URL: http://earth.s.kanazawa-u.ac.jp/ishiwata/
Ophiolites, greenstones, and Alaskan-type zoned ultramafic plutons are major occurrences of the mafic-ultramafic rocks in the circum-Pacific orogenic belts.
Ophiolites occur as a few km to more than 100 km sized bodies of stratified sequence composed of ultramafic, gabbroic and basaltic rocks in ascending order, tectonically overlying accreted oceanic sediments of coeval or younger ages or their high-P/T metamorphosed varieties. In the circum-Pacific areas, an older ophiolite thrust onto younger one with or without intercalated oceanic sediments and their metamorphic derivatives, forming multiple Phanerozoic ophiolite belts with an oceanward younging trend. The circum-Pacific ophiolites are mostly of supra-subduction zone (SSZ) origin, as represented by the Yakuno ophiolite of SW Japan with thickly developed crustal section of a Permian oceanic island arc-backarc basin system (Ichiyama and Ishiwatari, 2004; Suda, 2004). The Jurassic ophiolites in Hokkaido (Japan) and Sakhalin (Russia) are characterized by very high melting degree of its mantle harzburgite, as shown by their very high spinel Cr# (80-90) (Ishiwatari et al., 2003).
“Greenstones” here denote oceanic volcanic rocks occurring in the accretionary complexes. The greenstones mostly occur as basal part of the so-called “ocean plate stratigraphy” (chert/limestone, shale and sandstone in ascending order) as an intact sheet or 1 km thick and 10-100 km wide (Basal Type). The other minor greenstones occur as a few cm to 100 m sized blocks in muddy mélange (Mixed Type). The Basal Type greenstones show homogenous E-MORB chemistry, suggesting plume-related, thicker oceanic crust (oceanic plateau) origin, while the Mixed type includes depleted N-MORB and enriched OIB (both OIT and OIA) together, suggesting their origin in normal oceanic crust with scattered oceanic islands and seamounts (Koizumi and Ishiwatari, in submission). The HFSE-rich picritic sills and hyaloclastites were found from the chert-dolomite beds covering the Basal Type basalts (Ichiyama and Ishiwatari, 2005), and ferropicrite and picritic ferrobasalt were discovered as dikes cutting the Basal Type basalts (Ichiyama et al. in submission). These peculiar ultramafic volcanic rocks show Zr/Y and Ti/Al ratios intermediate between Polynesian picrites and Siberian meimechites, indicating very deep (4-5 GPa) partial melting of the mantle source doped with recycled oceanic crust material (e.g. Fe, Ti-rich gabbro/eclogite). Ferropicrites have been reported only from continental large igneous provinces (LIPs). The occurrence of these peculiar ultramafic volcanic rocks with the Basal Type greenstones strongly suggests their origin in a Permian oceanic plateau produced by the superplume activity, as already proposed for Carboniferous greenstones (Tatsumi et al. 2000).
The Alaskan-type zoned ultramafic plutons, about 1 km in diameter, intruded into the accretionary complexes with contact metamorphic effects. They are present in the Jurassic accretionary complexes of Primorye (Russia) and Heilongjiang (China) areas, where these plutons are associated with the Jurassic picrite-meimechite lava flows sometimes with pillow structure (Ishiwatari and Ichiyama, 2004). The very Ti-rich nature of the gabbro and pyroxenite as well as occurrence of strongly alkalic rocks, carbonatite, and even diamond in some plutons suggest their very deep-seated origin and relation to the superplume. The superplume-related, Jurassic/Cretaceous high Zr/Y and Nb/Zr basalts are also reported from Hokkaido (Japan) and Sakhalin (Russia) (Tatsumi et al. 1998). These facts suggests that the superplume was emplaced beneath the subduction zone (Ishiwatari & Ichiyama, 2004), and facilitated rapid plate convergence and robust SSZ magmatism to make the highly depleted harzburgite residue. Presence of other Permian/Triassic oceanic plateaus accreted to the Pacific margins (Wrangellia, Cache Creek, etc.) suggests very large size of the superplume.
The Permian oceanic superplume inferred from the Japanese greenstones is coeval with Siberian (Russia) and Emeishan (SW China) LIPs, and the Late Permian East Asian margins were surrounded by these powerful superplumes. This situation must have resulted in a rapid plate convergence, causing continental collision, UHP metamorphism and vast development of accretionary complexes along sinuous plate boundaries (Ishiwatari and Tsujimori, 2003). The Jurassic superplume in turn appeared beneath the convergent East Asian continental margins, and caused superplume-SSZ interaction to form near-trench ultramafic lavas, Alaskan-type intrusions, and highly depleted harzburgite of the SSZ ophiolites.
Ichiyama, Y. and Ishiwatari, A. (2004) Petrochemical evidence for off-ridge magmatism in a backarc setting from the Yakuno ophiolite, Japan. Island Arc, v. 13, pp. 157-177.
Ichiyama, Y. and Ishiwatari, A (2005) HFSE-rich picritic rocks from the Mino accretionary complex, southwestern Japan. Contrib. Min. Petr., v. 149, pp. 373-387.
Ishiwatari, A. and Ichiyama, Y (2004) Alaskan-type plutons and ultramafic lavas in Far East Russia, Northeast China, and Japan. Int. Geol. Rev., v. 46, pp. 316-331.
Ishiwatari, A., Sokolov, S.D. and Vysotskiy, S.V. (2003) Petrological diversity and origin of ophiolites in Japan and Far East Russia with emphasis on depleted harzburgite. Geol. Soc. London Spec. Publ. v. 218, pp. 597-617.
Ishiwatari, A. and Tsujimori, T. (2003) Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis. Island Arc, v. 12, pp. 190-206.
Suda, Y. (2004) Crustal anatexis and evolution of granitoid magma in Permian intra-oceanic island arc, the Asago body of the Yakuno ophiolite, Southwest Japan. J. Min. Petr. Sci., v. 99, pp. 339-356.
Tatsumi, Y., Kani, T., Ishizuka, H., Maruyama, S. and Nishimura, Y. (2000) Activation of Pacific mantle plumes during the Carboniferous: Evidence from accretionary complexes in southwest Japan. Geology, v. 28, pp. 580-582.
Tatsumi, Y., Shinjoe,H., Ishizuka, H., Sager, W.W. and Klaus, A. (1998) Geochemical evidence for a mid-Cretaceous superplume. Geology, v. 26, pp. 151-154.
International Geological Congress, (Firenze, Italy) August 23, 2004. 14:15, Room 15, Session 127, T-27.05 Record of oceanic rocks in Precambrian and Early Phanerozoic times
Presided by Moores, E.M. (USA) and Spadea P. (Italy)
Extensive melting of depleted mantle among circum-Pacific ophiolites: petrologic and Os-isotopic evidences
The circum-Pacific orogenic belts are characterized by the spatial and temporal multiplicity of ophiolites, which are also diverse in their petrological and geochemical features. In southwestern Japan, the Ordovician Oeyama ophiolite with fertile mantle thrust over the Permian Yakuno ophiolite with depleted mantle and metamorphosed thick crust. In northeastern Japan, the Ordovician Miyamori ophiolite with “hydrous wedge mantle” and cpx-type cumulate thrust over the Jurassic accretionary complex, which is underlain by the Jurassic Horokanai ophiolite with highly depleted, cpx-free mantle and opx-type cumulate. Early Paleozoic ophiolites are also abundant in the intra-continental, collision-type orogens such as Appalachian, Caledonian, Uralian, Central Asian and Qilian belts, but they do not accompany Mesozoic or younger ophiolites as in Japan, E. Russia and W. USA. This reflects long-lasting, full-Phanerozoic orogeny of subduction-accretion type in the circum-Pacific areas in contrast to the short lifetime of the continent-collision type orogenic belts. The Horokanai-type, highly depleted mantle sections with opx-type cumulates are also present in the Krasnaya Mountain ophiolite in Koryakia (Russia), Shelting ophiolite in Sakhalin (Russia), Papua ophiolite in Papua New Guinea, and Adamsfield ophiolite in Tasmania (Australia), and are restricted to the western Pacific areas (Ishiwatari et al. 2003; Geol. Sod. London Spec. Publ. 218, 597-617). Os isotopic study of chromitites from 18 ophiolites of circum-Pacific and intra-continental belts revealed that initial 187Os/188Os ratios follow an evolution trajectory averaging at 0.1281-0.0058t (t is time before present in Ga), and deviations from this line are mostly less than 1% (Walker et al. 2002; Geochim. Cosmochim. Acta, 66, 329-345). This indicates that the ophiolitic mantle source has been a homogeneous DMM (MORB source) 1% more radiogenic than chondrite, and the effect of radiogenic fluids from subducting slab is scarce. The circum-Pacific ophiolitic chromites are lower in 187Os/188Os and Re/Os ratios than those of the intra-continental ophiolites, and the Re/Os ratio gradually decreases from 420 to 50 Ma. This suggests more depleted mantle sources for the circum-Pacific ophiolites and their secular depletion through removal of Re to the crust by partial melting. This is consistent with the common presence of the highly depleted harzburgite and metamorphosed thick crust among the circum-Pacific ophiolites.
Keywords: cpx-free harzburgite, opx-type cumulate, Western Pacific, Phanerozoic ophiolites, Osmium isotope