Akira ISHIWATARI (Dept. Earth Sci., Kanazawa Univ. 920-1192 JAPAN)*, Sergei D. SOKOLOV (Geol. Inst., Rus. Acad. Sci., Moscow 109017 RUSSIA), Daichi SAITO*, Tatsuki TSUJIMORI* and Sumio MIYASHITA (Dept. Geol., Niigata Univ. 950-2181 JAPAN)

Superplume International Workshop (Jan. 28-31, 1999 at Inst. Phys. Chem. Res., Wako City, Japan) Abstract p. 155 (Poster P-31)


Superplume magmatism may have unique geochemical and isotopic signatures such as high Ti/Al and HIMU. However, most ophiolites have been proved to be of supra-subduction zone (SSZ) origin, and there may be little chance to find big fragments of superplume-induced oceanic edifices among them, although some small fragments of such edifices may occur as clastic fragments, melange blocks or high-pressure schists in the accretionary complexes. On the other hand, the superplume magmatism increases production rate of the oceanic crust, accelerates plate movement, and activates supra-subduction zone magmatism. Higher degrees of mantle melting and thicker development of mafic (and silicic) crust are expected for island arcs and marginal basins produced during the superplume periods. Ophiolites, like granites, may provide such "indirect" evidences for ancient superplumes.

The Koryak Mountains comprise an accretionary (Pacific-type) orogenic belt, which has been developed aside the eastern margin of the Kolyma-Omolon continental block. The Okhotsk-Chukotka volcanic belt, a volcanic arc with abundant granite intrusions, formed along the southeastern continental margin during Cretaceous time. However, the granite intrusions are scarce, and ophiolite complexes are abundant in the adjacent Koryak Mountains. The accretionary complexes are mainly composed of dark-colored clastic fragments of mafic-intermediate igneous rocks originated in oceanic crust, oceanic island arcs and seamounts, and often include exotic blocks of older chert and limestone. Nappe structure is developed among the accretionary complexes, where older rocks tend to occupy upper structural position. Turbidite (flysch) and molasse sedimentation migrated toward the southeast from late Jurassic to Tertiary.

The distribution, age and petrologic characteristics of ophiolites in the Koryak Mountains were first summarized by Palandzhjan (1986; Tectonophysics, 127, 341-). He stressed wide range of ophiolite ages (at least, early Paleozoic through Cretaceous) and wide petrologic diversity of ophiolitic mantle peridotite (lherzolite [type B; Mg#90], pyroxene-rich harzburgite [type A; Mg#91], and pyroxene-poor harzburgite [type C; Mg#92]) and gabbro (troctolite series and norite series) in this area. We have carried out several field expeditions to the Koryak Mountains during the recent nine years. Due to logistic difficulties, our field studies were limited in the three areas; Mainits zone, Taigonos Peninsula, and Anyui zone. Our observation and previous data on the ophiolite bodies in the Koryak Mountains are summarized below.

1. Mainits zone, central Koryak Mountains [studied by A.I. and S.D.S., helped by Dr. A. Stavsky during the Soviet "Geodyanmics Seminar" in 1990 ] (from NW to SE):

(1) Tamvatney lherzolite body. Not yet examined by us, but occurrence of "eclogite" and "diamond" was noted. Fertile lherzolite (spCr#20) and lherzolitic harzburgite (spCr#35) are present (Dmitrenko et al. 1990).
(2) Yagel serpentine melange. lherzolite (spCr#35), gabbro, sheeted dikes, pillow lava.
(3) Krasnaya Gora harzburgite nappe. Depleted harzburgite (spCr#75-80) with opx-rich ultramafic cumulates (Dmitrenko et al. 1985)
(4) Srednaya Gora residual dunite body. Strongly foliated dunite with Cr#85 spinel, associated with island-arc gabbronorite-amphibolite bodies.

2. Taigonos Peninsula, southern Koryak Mountains [studied by A.I. and S.D.S. in 1995 and by all of us in 1997] (from NE to SW along the eastern coast):

(1) Elistratova ophiolite: an island-arc ophiolite (spCr#55 harzburgite, opx-rich cumulate, arc-type gabbronorite with An>90 plagioclase, arc-tholeiite sheeted dikes) intruding into southern oceanic mantle (spCr#40 harzburgite).
(2) Kengeveem River gabbro: MORB gabbro with Mg#80 cpx and An=60 plagioclase.
(3) Cape Nablyudeniy: depleted harzburgite (spCr# 80) and meta-pillow lava
(4) Cape Povorotnyi: ophiolite melange with lherzolite bodies (spCr#20) and garnet amphibolite blocks (Silantユev et al., 1996; Gelman and Nekrasov, 1968).

These ophiolites belong to Penzhina-Pekulney belt (Palandzhjan, 1986), and were accreted to the Uda-Murgal island arc by the early Cretaceous time (Sokolov, 1992). Kuyul ophiolite is the biggest in this belt, and Pekulney ophiolite in the northern end contains layered garnet-bearing metagabbro and ultramafic rocks (Nekrasov, 1978).

3. Anyui zone, a northeastern margin of Kolyma-Omolon continental block which was collided by Chukotka-Wrangelia microcontinent [studied by A.I. and S.D.S. in 1998 ]

(1) Paleozoic or Mesozoic Aluchin ophiolite, cpx-bearing harzburgite with Cr#82 chromitite-wehrlite-noritic gabbro-sheeted dikes-basalt. (Lychagin, 1985)
(2) Middle Paleozoic(?) Gromadnenskiy layered gabbronorite complex associated with late Paleozoic(?) Vurguveem tonalite complex and Cretaceous(?) troctolite bodies. (Lychagin et al, 1991). However, the troctolite bodies look like layers in the gabbro.

Characteristics of the circum-Pacific Phanerozoic multiple ophiolite belts, as exemplified above by the Koryak Mountains, are summarized as follows.

1. Juxtaposition of ophiolites of widely varying ages ranging from early Paleozoic to late Cretaceous (to Tertiary in Japan, Taiwan, Chile, Philippines, Indonesia, etc.).
2. Juxtaposition of ophiolites of diverse petrologic nature (fertile lherzolite-type and highly depleted harzburgite-type) as in the Mainits zone and Taigonos Peninsula.
3. Presence of highly depleted mantle peridotite (HDMP; cpx-free harzburgite or dunite) with Cr#>=80 spinel in association with orthopyroxene-type cumulate. Similar HDMPs are known from only a few places such as Papua New Guinea, Tasmania and Hokkaido, and are almost restricted to the western Pacific margins.
4. Occurrence of thick gabbroic nappes including spinel- or garnet-bearing mineral assemblages (e.g. Pekulney), or accompanying abundant tonalite (Gromadnenskiy-Vurguveem). Such gabbroic nappes are better developed in southwestern Japan and Sikhote Alin, and are also reported from Alaska.

These features may be related to the superplume surges and subsequent global magmatic pulses of at least two times in early Paleozoic and late Mesozoic.

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