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)

Invited presentation in the International Ophiolite Symposium in Oulu, Finland, 10-15 Aug.1998 (Akira Ishiwatari)


The eastern coastal terrain of Taigonos Peninsula faces the western coast of Kamchatka over Penzhina Bay. The terrain is composed of sediments and ophiolites of early Paleozoic to Cretaceous ages, comprising the southernmost part of the accretionary complexes of the Koryak Mountains. Elistratova ophiolite is exposed in the 6 x 20 km area (N61o35', E163o), and its 200 m-high coastal cliffs provide the best ophiolite outcrops among the circum-Pacific orogens. Its pre-Cretaceous age is confirmed by fossil-bearing lower Cretaceous conglomerate which unconformably covers the ophiolite. Jurassic chert and basaltic rocks in the northern part are inferred to be the upper members of this ophiolite. The northwestern margin of the ophiolite is in fault contact with Jurassic andesitic volcanics, and a serpentinite melange (1 km wide) with various blocks is developed along the fault.

The Elistratova ophiolite includes all members such as residual mantle peridotite, cumulate ultramafic rocks, gabbro, sheeted dikes, and basaltic rocks, but their geologic relationships are quite unusual. The ophiolite area is divided into three zones trending NE-SW; the northwestern and southeastern zones (1 and 0.5 km wide, respectively) are mainly occupied by ultramafic rocks, and the central zone (4 km wide) is composed of a layered gabbroic body, which displays a gentle synclinal structure. The gabbro body includes some ultramafic cumulate layers and ultramafic intrusions. Sheeted dike complexes up to 100% dike frequency are developed in several parts amid the gabbro body, especially in its northern and southeastern margins. The northern sheeted dike complex and the associated basaltic rocks partly shows tight folding and intense brecciation before being covered by the Cretaceous conglomerate. Basalt and dolerite are dominant, but plagiogranite is also common as a dike.

The southeastern margin of the gabbro is intrusive into the southeastern zone, which is a harzburgite body showing relatively fertile mineral chemistry (spinel Cr# 30-50) resembling that of ordinary abyssal peridotite. The northwestern margin of the gabbro body gradually changes into layered cumulate ultramafic rocks such as plagioclase lherzolite and websetrite of the northwestern zone, which also bears mantle harzburgite with fairly depleted mineral chemistry (spinel Cr# 65-75). The chromitite found as a block in the adjacent melange includes chromite with abundant hydrous inclusions (mainly Na phlogopite), suggesting hydrous secondary melting of the mantle in a supra- subduction zone setting. The gabbro body is mainly composed of olivine gabbronorite, gabbronorite and hornblende gabbronorite with abundant orthopyroxene. Mg# of its mafic minerals is normal for an ophiolitic mafic cumulate (75-85), while anorthite content of plagioclase is extremely high (90-95), indicating an arc-tholeiitic affinity of the magma.

These geologic and petrologic observations lead to our interpretation that the island-arc ophiolite forming the northwestern and central zones intruded into the oceanic mantle of the southeastern zone. The Mesozoic accretionary complexes of Koryak Mountains mainly consists of ophiolites and volcaniclastic sediments of island-arc origin, suggesting development of multiple intra-oceanic island arcs as in the present southwestern Pacific region for their original tectonic setting. Although juxtaposition of different types of ophiolites has been reported from some areas (e.g. Coto and Acoje blocks in Zambales ophiolite (Philippines), Lewis Hills and the other massifs of Bay of Islands ophiolite (Newfoundland), etc.), direct intrusive contact between different types of ophiolites has never been reported. The Elistratova ophiolite provides a vivid evidence for incipient island-arc magmatism which occurred in the ordinary suboceanic mantle above an intra-oceanic subduction zone.

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