西モンゴル, アルタイ地域のハンタイシルオフィオライトの岩石学的特徴と形成場のテクトニックセッティング
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概要
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Precambrian-Cambrian Hantaishir ophiolite complex locates in the Altai region of the West Mongolia. The complex is approximately 50 km long and 12 km wide, and its entire configuration is elongated from northwest to southeast. The complex consists of two major massifs: they are Naran massif and Taishir massif. Although the massifs are continuous and no boundary is recognized, both consist of altered harzburgite and dunite with cumulated websterite, clinopyroxenite (Taishir massif) and gabbro with volcanic rocks. Pillow lavas are dominant both in the Naran and Taishir massifs. Sheeted dikes are also major rocks in both massifs, but they are apparently subordinate volcanics in the Taishir massif to the pillow lavas. Sheeted dikes of the Taishir massif consist of boninites, tholeiitic andesites and tholeiitic basaltic andesites. A small amount of the sheeted dikes of the Naran massif are solely tholeiitic andesite in composition. Both massifs are sandwiched in the large serpentinite melange. The dominant ultramafic rock is harzburgite in the ophiolite complex and it is highly altered. Chromian spinel commonly remains as fresh mineral in the harzburgite, and they have similar composition to the chromian spinel in the harzburgite occurring in the other supra-subduction zone (SSZ) ophiolites. The mineral chemistries of rarely found highly magnesian olivine in the harzburgite and dunite are consistent with residual upper mantle origin in combination with the highly chromian spinel chemistry. The ultramafic body of the Hantaishir ophiolite could be formed in the mantle below the zone of plate collision. The REE patterns of the gabbro and volcanic rocks of the Naran massif seem almost flat and significantly differ from those of the Taishir massif: both the tholeiitic andesites and boninites of the latter are substantially depleted in LREE. The spider diagrams for both massifs show clearly different patterns as well. No significant chemical difference between boninites and tholeiitic andesites from the Taishir massif was noticed as far as trace elements are concerned. The chemical characteristics of the boninite from the ophiolite are common to the modern boninites from the West Pacific island arcs excluding REE patterns: geochronologically recent boninites have U-shaped forms. On the basis of the discrimination diagrams, the gabbros and volcanic rocks from the Hantaishir ophiolite complex can be considered to have originated near plate margin.
- 2007-03-30